Analysis of Urea Electrolysis for Generation of Hydrogen
A thesis presented to
the faculty of
the Russ College of Engineering and Technology of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Science
Deepika Singh
November 2009
2
This thesis titled
Analysis of Urea Electrolysis for Generation of Hydrogen
by
DEEPIKA SINGH
has been approved for
the Department of Chemical and Biomolecular Engineering
and the Russ College of Engineering and Technology by
Gerardine G. Botte
Associate Professor of Chemical and Biomolecular Engineering
Dennis Irwin
Dean, Russ College of Engineering and Technology
3
ABSTRACT
SINGH, DEEPIKA, M.S., November 2009, Chemical Engineering
Analysis of Urea Electrolysis for Generation of Hydrogen (100 pp.)
Director of Thesis: Gerardine G. Botte
The oxidation of urea was studied as a means of remediating urine-rich waste
water to produce hydrogen and simultaneously denitrificating the waste water. The
proposed reaction mechanisms, preferred pathway and rate determining steps have been
predicted using Density Functional Theory calculations. Both the electro-oxidation
reaction as well as the chemical oxidation reaction mechanisms have been postulated on
the surface of the active catalyst NiOOH. The preferred pathway for electro-oxidation
was found to be: *CO(NH2)2→ *CO(NH.NH2)→ *CO(NH.NH)→
*CO(NH.N)→*CO(N2) → *CO(OH) →*CO(OH.OH) →*CO2 with desorption of CO2
as the rate limiting step. From the thermodynamic calculations of the chemical oxidation
reactions, it was evident that the presence of OH- catalyzes the reaction. Experimentally,
the effects of varying concentrations of KOH and urea were investigated. The
experimental results supported the argument that a higher concentration of OH- is more
favorable for the reaction.
Approved: _____________________________________________________________
Gerardine G. Botte
Associate Professor of Chemical and Biomolecular Engineering
4
ACKNOWLEDGMENTS
Firstly I would like to acknowledge the guidance, motivation and tremendous
support of my advisor, Dr. Gerardine Botte. She will continue to be a source of
inspiration for me throughout my career. I would also like to thank my colleagues at the
Electrochemical Engineering Research Laboratory for their guidance and patience in
helping me learn the basic laboratory techniques. In particular, I want to acknowledge the
tremendous contribution of Damilola Daramola who has helped me from the initial stages
of teaching computational techniques right untill the end for editing and formatting of the
submitted publications, apart from mentoring me at all stages in my research. Without his
support none of this would have possible. I would also like to extend my heartiest
gratitude to the Ohio Super Computing Center for providing valuable resources and
computing time for the computational calculations.
I would also like to thank all friends, especially Saurin Shah and Santosh Vijapur
for being there for me all the time. For all the emotional and moral support and for
believing in me, for giving me the strength to persevere, I can never thank both of you
enough. Finally I’d like to thank my immediate family who has always been there with
me mentally, if not physically. For their undying belief, the kind words and unfailing
support. For being with me through thick and thin, at all times of the day. Thank you so
much. I could not have done this without you all.
5
TABLE OF CONTENTS
Page
Abstract ............................................................................................................................... 3 Acknowledgments............................................................................................................... 4 List of Tables ...................................................................................................................... 7 List of Figures ..................................................................................................................... 8 Chapter 1 : Introduction .................................................................................................... 10 1.1 Project Overview .............................................................................................. 10 1.2 Statement of Objectives .................................................................................... 13 1.3 Significance of Research................................................................................... 14 Chapter 2 : Literature Review ........................................................................................... 15 2.1 Theoretical ........................................................................................................ 15 2.2 Experimental ..................................................................................................... 18 Chapter 3 : Computational Methods ................................................................................. 19 Chapter 4 : Electro-Oxidation Mechanisms ...................................................................... 23 4.1 Reaction Mechanism Path 1.............................................................................. 25 4.2 Reaction Mechanism: Path 2 ............................................................................ 36 4.3 Reaction Mechanisms: Path 3 ........................................................................... 41 4.5 Conclusion ........................................................................................................ 49 Chapter 5 : Chemical Oxidation Mechanisms .................................................................. 51 5.1 Different Orientations of Urea towards NiOOH ............................................... 51 5.2 Urea decomposition with NiOOH ................................................................... 53 5.3 Urea and NiOOH in the presence of OH- ion: .................................................. 55 5.4 Conclusion ........................................................................................................ 58 6
Chapter 6 : Experimental .................................................................................................. 59 6.1 Experimental Methods: Electroplating and Preliminary Results ...................... 59 6.2 Potentio-dynamic Tests ..................................................................................... 60 6.3 Results and Discussion ..................................................................................... 61 6.4 Conclusion ........................................................................................................ 64 Chapter 7 : Conclusions and Recommendations .............................................................. 65 References ......................................................................................................................... 67 Appendix A: Supporting Information for Urea Electro-Oxidation Reaction ....................72
Appendix B: Supporting Information for Urea Chemical Oxidation Reaction .................93
7
LIST OF TABLES
Page
Table 3.1: Bond lengths and bond angles of urea as computed using different basis sets
with the B3LYP correlational functional ...........................................................................20
Table 3.2 : Experimental bond lengths for NiOOH kept constant for the reaction
mechanisms ........................................................................................................................20
Table 4.1: Proposed reaction mechanisms for urea electro oxidation reaction .................25
Table 4.2: Sum of free energies for all the intermediate steps...........................................48
Table 4.3: Kinetics of the reaction pathways and rate constants for intermediate steps ...49
Table 5.1: Binding Energies of different orientations of urea towards NiOOH ................53
Table 5.2: Free Energies differences for Equations 5.1 and 5.2 ........................................55
Table 5.3: Free Energies differences for Equations 5.3 and 5.4 ........................................58
8
LIST OF FIGURES
Page
Figure 1.1:Global Energy Systems Transition1 ................................................................ 10 Figure 2.1: Mechanism of urease catalyzed urea hydrolysis14 ......................................... 16 Figure 4.1: Initial state (a) Transition state (b) and Final structure (c) for
Equation 4.5 ...................................................................................................................... 26 Figure 4.2: Initial state (a), transition state (b) and final structure (c) for
Equation 4.6 ...................................................................................................................... 27 Figure 4.3: Initial state (a), transition state (b) and final structure (c) for
Equation 4.7 ...................................................................................................................... 29 Figure 4.4: Initial state (a), transition state (b) and final structure (c) for
Equation 4.8 ...................................................................................................................... 30 Figure 4.5: Initial state (a), transition state (b) and final structure (c) for
Equation 4.9 ...................................................................................................................... 31 Figure 4.6: Initial state (a), transition state (b) and final structure (c) for re-arrangement
of nitrogen atoms .............................................................................................................. 32 Figure 4.7: Initial state (a), transition state (b) and final structure (c) for Equation 4.10..
.......................................................................................................................................... .34 Figure 4.8: Initial state (a), transition state (b) and final structure (c) for
Equation 4.11 .................................................................................................................... 35 Figure 4.9: Initial state (a), transition state (b) and final structure (c) for
Equation 4.12 .................................................................................................................... 36 Figure 4.10: Initial state (a), transition state (b) and final structure (c) for
Equation 4.13 .................................................................................................................... 37 Figure 4.11: Initial state (a), transition state (b) and final structure (c) for
Equation 4.14 .................................................................................................................... 38 9
Figure 4.12: Initial state (a), transition state (b) and final structure (c) for
Equation 4.15 .................................................................................................................... 39 Figure 4.13: Initial state (a), transition state (b) and final structure (c) for
Equation 4.16 .................................................................................................................... 40 Figure 4.14: Initial state (a), transition state (b) and final structure (c) for rearrangement
of amine groups................................................................................................................. 42 Figure 4.15: Initial state (a), transition state (b) and final structure (c) for Equation
4.17……………………………………………………………………………………….42 Figure 4.16: Initial state (a), transition state (b) and final structure (c) for
Equation 4.18 .................................................................................................................... 44 Figure 4.17: Initial state (a), transition state (b) and final structure (c) for Equation 4.17.
........................................................................................................................................... 45 Figure 5.1: Optimized structures for different orientations of urea towards NiOOH ....... 52 Figure 5.2: Optimized structures for Equations 5.1 and 5.2 ............................................. 54 Figure 5.3: Optimized Transition States for Reactions 2 and 3 ........................................ 55 Figure 5.4: Adsorption of OH- onto NiOOH .................................................................... 56 Figure 5.5: Optimized structures for Equations 5.3 and 5.4. ............................................ 56 Figure 5.6: Transition State Structures for Equations 5.3 and 5.4 .................................... 57 Figure 6.1: Preliminary experiment. Different concentrations of KOH at 20g L-1 urea to
determine lower setpoint. .................................................................................................. 62 Figure 6.2: Urea concentration of 5 g L-1 varying KOH concentrations. Scan rate:
20mV s-1. Speed of rotation: 1000rpm.............................................................................. 63 Figure 6.3: Urea Concentration of 10 g L-1 with varying KOH concentrations. Scan Rate:
20mV s-1. Speed of rotation 1000rpm. .............................................................................. 63 Figure 6.4: Urea concentration of 20 g L-1 with varying KOH concentrations. Scan rate
20mV s-1. Speed of rotation 1000 rpm. ............................................................................. 64 10
Chapter 1 : INTRODUCTION
1.1
Project Overview
The use of hydrogen as an alternative fuel has been the focus of attention for many
decades now, especially as the demand for fuels from renewable energy sources is
constantly on the rise. The transition from liquid petroleum to gases over a span of 200
years is as shown in Figure 1.1. According to this trend, there will be a complete shift
from petroleum to hydrogen for meeting world energy requirements by the turn of the
next century1.
Figure 1.1:Global Energy Systems Transition1
11
The devices that produce hydrogen for the purpose of electricity generation are
called fuel cells. They operate on the principle of recombination of hydrogen with oxygen
to release energy and produce water as a byproduct. Although there are five different
types of fuel cells being developed for commercial applications, proton exchange
membrane fuel cells are considered as viable, low temperature operating devices for both
transportation and stationary applications2, 3.
Proton exchange membrane (PEM) fuel cells working on water electrolysis are
based on the mechanism of splitting up of the water molecule into hydrogen and oxygen
with the liberation of energy in an exothermic reaction. The water electrolysis reaction is
as follows:
Anode: H2 → 2H+ + 2e-
(1.1)
Cathode: 4H+ + 4e- + O2→ 2H2O
(1.2)
Heat Output T∆S= 48.7 kJ mol-1
4
The Electrochemical Engineering Research Laboratory (EERL) at Ohio
University has recently devised a new alternative to the water electrolysis reaction:
ammonia electrolysis in which the ammonia molecule dissociates to give nitrogen as
follows:
Anode: 2NH3 + 6OH-→ N2 + 6H2O + 6e-
(1.3)
Cathode: 2H2O + 2e- → H2 + 2OH-
(1.4)
12
This process produces hydrogen to power PEM fuel cells and is a self sustainable
source of energy. It eliminates the problems associated with storing hydrogen as it
produces hydrogen on demand5-7.
An alternative to the above mentioned ammonia electrolysis process is urea
electrolysis, by means of which urine from animal farms and waste water lagoons can be
directly utilized to produce hydrogen to power fuel cells8. Urea is known to naturally
decompose to ammonia, hence is a major issue among farmers regarding taxes on
ammonia emissions9. This process once commercialized will not only help farmers
receive tax cuts for reduced ammonia emissions, but it will also decrease their
dependence on fossil fuels for power generation.
Urea electrolysis in alkaline medium is being investigated at the Electrochemical
Engineering Research Laboratory (EERL) at Ohio University as a novel technique for
hydrogen production. This project is of importance as it addresses the need to remediate
waste water from poultry farms as well as residential and commercial areas and using it
as a tool to solve one of the world’s impending energy crises. This following project was
undertaken to understand urea electrolysis for the purpose of generation of hydrogen for
fuel cells using theoretical and experimental methods.
Experimental methods will be combined with molecular modeling to gain a better
understanding of the electrolysis process. The reaction mechanisms are being studied
both chemically and electrochemically using the Gaussian 03 software. Experimental
techniques involve studying the effect of reaction parameters using a rotating disk
electrode.
13
1.2
Statement of Objectives
The purpose of this thesis is to gain a better understanding of the urea electrolysis
process in order to aid further development of the technology to make it commercially
viable for fuel cells. The following objectives are proposed to be accomplished with the
completion of the thesis.
1) Postulating reaction mechanisms for urea electrolysis using molecular modeling
techniques along with activation energies and rate constant calculations both in terms
of chemical oxidation and electrochemical oxidation. Under this objective, two
possible scenarios were considered:
i) Chemical Decomposition: Urea is known to dissociate naturally to ammonia
and carbamates. This reaction has been studied in the presence of the catalyst
nickel oxyhydroxide.
ii) Electrochemical Oxidation: Urea has also been found to undergo
electrochemical oxidation as found by the Electrochemical Engineering
Research Laboratory according to the following reactions:
Anode: CO(NH2)2(aq) + 6OH-→ N2(g) + 5H2O(l) + CO2 (aq) + 6e-
(1.5)
Cathode: 6H2O (l) + 6e-→ 3H2(g) + 6OH-
(1.6)
Overall: CO(NH2)2(aq) + H2O(l) → N2(g) + 3H2(g) + CO2 (aq)
(1.7)
The elementary steps involved in the anodic reaction have been studied.
2) Determining the effect of reaction parameters such as concentration of potassium
hydroxide (KOH) and urea on process.
14
With the help of these reaction mechanisms and by determining the preferred
pathway as well as the rate determining step, measures can be taken to improve
efficiency of the process experimentally.
1.3
Significance of Research
The most important technological impact of this project arises from the utilization
of the most abundant waste on earth, urine, to produce cheap electricity. Apart from
being a significant source of hydrogen production, this technology can also be used to denitrificate waste water, thus saving a huge amount of expenditure on waste water
remediation. At present, the permissible nitrate concentration in water is 10 mg L-1
however, most denitrification processes are expensive and ineffective10. Natural
hydrolysis of urea to ammonia leads to the formation of ammonium sulfate and
ammonium nitrate in the atmosphere which pose significant health hazards11. Hence, by
electrolyzing urea to useful products, we are able to bypass the formation of the hazardcausing products.
Another important aspect is that the electrolytic cell potential required for the
overall reaction to occur is only 0.37 V at standard conditions. When this is compared to
the cell potential required to produce hydrogen (1.23 V), it amounts to generation of 70%
cheaper hydrogen theoretically12.
These factors emphasize the need for a better understanding of the ongoing
process, which has been achieved in this project by means of the invaluable tool of
molecular modeling.
15
Chapter 2 : LITERATURE REVIEW
2.1
Theoretical
Urea electrolysis is a modification of the ammonia electrolysis technology for the
purpose of generating hydrogen for fuel cells. Urea hydrolysis and decomposition
mechanisms have generated interest in the past in varied fields including removal of urea
from the blood using dialyzers and also formulation of urease inhibitors for better soil
fertility. When urea dissociates in the presence of the bio enzyme urea amidohydrolase 1315
(urease), it leads to a sudden increase in pH of the soil due to the liberation of ammonia,
leading to its decreased fertility thus rendering it ineffective for agricultural purposes16, 17.
For this reason, biocatalytic decomposition of urea by urease which catalyzes the
reaction has been given considerable attention in the literature18-20. Urease decomposes
urea to ammonia and carbon dioxide under specific reaction conditions according to the
following reactions21:
urease
CO(NH 2 )2 ⎯⎯
⎯→ NH 3 + HNCO
NH3 + HNCO +H2O → 2NH3 + CO2
(2.1)
(2.2)
The enzyme urease comprises of two pseudo-octahedral Ni(II) ions as its active
sites. Suarez et al.13 have proposed the reaction mechanisms for urea hydrolysis.
According to their work, urea binds to the two active nickel sites in urea in a bidentate
manner. The more electrophilic nickel attaches itself to the carbonyl group of urea, while
the other nickel atom attacks one of the amino groups. They have considered a bridging
hydroxide group between the two nickel atoms, which donates a proton to the amino
group that is attached to the second nickel atom as shown in Figure 2.1.
16
Figure 2.1: Mechanism of urease catalyzed urea hydrolysis13
The mechanism of urea decomposition in aqueous phase has been further studied
by different authors 15, 22, 23. This study was performed with the presence of urea in water
alone without the urease enzyme. In the aqueous solution, the elimination mechanism
yields isocyanic acid and ammonia, whereas intramolecular proton transfer gives cyanic
acid and ammonia18. It was also concluded that elimination mechanism greatly disrupts
the resonance stabilization of urea. However, Alexandrova and Jorgenson 18 have
analyzed the activation energies of elimination and hydrolytic mechanism pathways of
urea in aqueous solution and conclude that urea prefers to eliminate ammonia rather than
undergo hydrolysis.
These mechanisms are relevant in the context of this study because at EERL at
Ohio University, studies are being conducted for the electrolytic dissociation of urea for
the purpose of generating hydrogen for proton exchange membrane fuel cells24. Nickel
has been identified as the active catalyst for the reaction, which is supported by the fact
17
that urea undergoes natural hydrolysis in the presence of the urease enzyme which has
nickel as its active site.
An alkaline medium is used to carry out the electrolysis, and nickel undergoes
oxidation to its active state: nickel oxyhydroxide (NiOOH) in this medium by the
following reaction:
Ni(OH)2(s) + OH- → NiOOH(s) + H2O (l) + e-
(2.3)
Nickel oxyhydroxide plays the role of an active catalyst in many alkaline
batteries, and has thus received considerable attention in electrochemical research.
This reaction is hypothesized to occur on the surface of the nickel electrode in the
presence of urea as well. As a result, it is important to study the interaction of the nickel
oxyhydroxide molecule with urea to come up with feasible reaction mechanisms for the
nature of interactions on the electrode surface.
The above mentioned modeling calculations have been performed using Gaussian
03 softwares. In an analogy to experimental operating conditions, theoretical calculations
comprise of basis sets which are the pre-defined parameters within the confines of which
the calculations are performed.
In the past, quantum chemical calculations have been performed using Linear
Combination of Atomic Orbitals Molecular Orbitals (LCAO MO). These molecular
orbitals exist as a linear combination of atomic orbitals as follows:
௡
߰௜ ൌ ෍ ܿఓ௜ ߶ఓ
ఓୀଵ
18
Where ψi is the ith molecular orbital, cμi are the coefficients of linear combination
andΦμis the μth atomic orbital and n is the number of atomic orbitals.25 These atomic
orbitals (AO) are solutions of the wave functions for a single electron in an atom. A basis
set is a set of these wave functions within the framework of which quantum chemical
calculations are performed. Basis sets play a crucial role in the binding energies obtained
from the molecular modeling calculations.
2.2
Experimental
Experimentally, urea electrolysis has been traditionally applied in techniques such
as dialysis and synthesis of carbon nitride thin films. These applications have been
studied in acidic medium with noble metals like platinum, iridium, ruthenium. Simka et
al.26 have investigated different compositions of Ti/(Pt-Ir), Ti/RuO2, Ti/(Ta2O5-IrO2) to
produce non toxic products with this reaction. But it is for the first time that alkaline
electrolysis of urea is being considered for the purpose of hydrogen production. Here we
have considered nickel as the catalyst for this reaction as it is cheap, economically
feasible and shows high activity for urea electrolysis. Currently, high concentrations of
alkali potassium hydroxide (KOH) are being used in the reaction. Typical concentration
used is 5 M (280 g L-1). It is important to examine the effect of KOH concentration to
investigate if lower concentrations can be used under the given operating conditions.
19
Chapter 3 : COMPUTATIONAL METHODS
With the primary purpose of elucidating the reaction mechanism, single molecule
interactions of NiOOH with urea have been considered. DFT calculations were carried
out using the Gaussian 03 program27 with the B3LYP correlation functional28. A mixed
basis set was used comprising of Los Alamos National Laboratory of double zeta quality
(LANL2DZ)29-31 and 6-31g*32 for carbon, nitrogen hydrogen and oxygen atoms, also
referred to as the LACVP* basis set. The comparison of the relevant geometrical
features of the urea molecule was reported on the B3LYP level in literature33 (Table 3.1).
These values of bond angles and bond lengths reported using the 6-31g* basis set were
found to be reasonably accurate in comparison with the experimental values. Considering
its requirement of less processing time, 6-31g* was chosen as a building block for these
calculations.
20
Table 3.1: Bond lengths and bond angles of urea as computed using different basis sets
with the B3LYP correlational functional.
Intra molecular bond lengths and angles (in Å and degree)
6-31g(d,p)
DZP
6-311g(d,p)
TZP
exp
C-O
1.271
1.273
1.263
1.260
1.262
C-N
1.349
1.352
1.349
1.349
1.345
N-H1
1.014
1.022
1.012
1.011
1.009
O-C-N
121.4
121.4
121.6
121.6
121.4
N-C-N
117.1
117.2
116.9
116.7
117.2
C-N-H
119.9
119.9
119.9
120.0
119.1
For the electro oxidation reaction mechanisms the bond lengths for atoms of
NiOOH were kept constant at their experimental values34, 35 for the electrochemical
oxidation reactions as follows in Table 3.2:
Table 3.2 : Experimental bond lengths for NiOOH kept constant for the reaction
mechanisms
X-Y Pair
Bond Length (Å)
Ni-O
1.88
Ni-OH
1.91
O-H
0.956
21
No further geometry constraints were placed on the system. The Gaussian 03
algorithm was used to calculate the vibrational frequency and analytical force constant
calculations on all structures. The transition states for all elementary steps were located
implementing the default Gaussian 03 method. The transition state geometry possessed
two imaginary frequencies: one corresponding to the geometry constraint placed on
NiOOH (the O-H bond) and the other corresponding to the transition state (TS) structure.
Animation of the particular transition state negative frequency verified that the TS
corresponded to the interacting atoms for the particular step under consideration.
The rate constant calculations based on the transition state theory36 were performed using
partition functions as shown in the following equation:
⎛
⎞
k BT ⎜ q # ⎟ ⎛ −E i ⎞
exp⎜
k=
⎟
h ⎜⎜ ∏n q ⎟⎟ ⎝ RT ⎠
⎝ j=1 j ⎠
(3.1)
where,
k= rate constant (L mol-1 s-1)
qt=partition function for transition state (Hartrees)
qr=partition function for reactant (Hartrees)
Ei= difference in zero point energies of reactants and transition state structures(J mol-1)
kb =Boltzmann’s constant= 1.38x10-23 J K-1
h= Planck’s constant= 6.63x10-34 J s
T= 298 K
R= Universal gas constant=8.314 J K-1 mol-1
22
On solving the above equation for a second order reaction, the rate constant value
is obtained in L mol-1 s-1 upon multiplying by a unit concentration term.
The free energies in Gaussian 03 are evaluated from the vibrational frequency analysis,
which is in turn used to determing the partition function based on the harmonic oscillator
model. Therefore, the underlying assumption in this analysis is that the second derivative
matrix is evaluated at a point on the potential energy surface where the gradient is zero.
As such, since the gradient is zero, the coupling between the nuclear degrees of freedom
and the molecular orbital coefficients can be ignored.
When using geometry constraints, the non-zero forces are ignored while
evaluating the optimization criteria. As a result, the energy values change and these
changes cannot be measured legitimately with the implementation of geometry
constraints during optimization. Hence for the thermodynamic calculations, individual
intermediate structures were considered with the entering OH- and leaving H2O
molecules with no geometry constraints on the NiOOH molecule. . For the chemical
oxidation mechanisms no geometry constraints were placed on the system.
23
Chapter 4 : ELECTRO OXIDATION MECHANISMS
The urea electro-oxidation reactions are as follows12:
Anode: CO(NH2)2(aq) + 6OH-→ N2(g) + 5H2O(l) + CO2 (aq) + 6e-
(4.1)
Cathode: 6H2O (l) + 6e-→ 3H2(g) + 6OH-
(4.2)
Overall: CO(NH2)2(aq) + H2O(l) → N2(g) + 3H2(g) + CO2 (aq)
(4.3)
The anodic reaction is proposed to be taking place on nickel which undergoes
oxidation according to the following reaction in an alkaline medium:
Ni(OH)2(s) + OH- → NiOOH(s) + H2O (l) + e-
(4.4)
Within this context the objective in this study is to use Density Functional Theory
(DFT) methods to predict the mechanism and rate determining step of the anodic urea
oxidation reaction on the NiOOH surface. This study is significant in order to understand
and improve overall efficiency of the experimental process of urea electro oxidation. In
order to predict the reaction mechanisms, the electronic energy barriers for the
elementary steps were estimated. Based on these steps, three reaction mechanisms have
been predicted. The proposed reaction mechanisms are as shown in Table 2. To
summarize the pathways, the first step involved the adsorption of urea onto the NiOOH
catalyst, and was common for all three mechanisms. From here onwards, path 1
demonstrated the initial loss of protons from the amino group H1-N1-H2, while path 2
involved the initial loss of protons from the second amino group H3-N2-H4. In path 3,
the amino groups bonded together by the rotation of the group H1-N1-H2 towards N2H3, whereas in paths 1 and 2 this rotation takes place only after the elimination of all
protons from the adsorbed species.
24
After the withdrawal of all the protons of urea by the approaching hydroxyl ions in steps
2 to 6, the final adsorbed structure at the end of step 6 is identical for all the pathways,
rendering a common mechanism from step 7 onwards. These steps have been discussed
in further detail later.
Solvent effects have been excluded as a first approximation. The calculation of
rate constants have been carried out using partition functions obtained from the transition
states and reactants to estimate the rate constants and hence predict the rate limiting step,
as will be discussed in detail later. The initial, transition and final states for all the
reaction pathways are shown in Table 4.1. Each reaction step is illustrated with four
figures: the first structure (a) is the optimized geometry for the initial state. The second
(b) and third figures (c) are the transition and final states respectively.
25
Table 4.1: Proposed reaction mechanisms for urea electro oxidation reaction
Steps
Path 1
1
Path 2
CO (NH2)2 + M → [M.CO (NH2)2]ads
[M.CO(NH2)2]ads + OH→ [M.CO(NH2.NH)]ads
+H2O + 1e[M.CO(NH2.NH)]ads
+OH- → [M.CONH2N]ads
+ H2O + 1e-
M.CO(NH2NH)ads +
OH-→ [M.CONH2.N]ads
+ H2O
4
[M.CO(NH2N)]ads + OH→ [M.CONHN]ads + H2O
+ 1e-
[M.CO(NH2.N)]ads +
OH-→ [M.CONHN]ads
+ H2O + 1e-
5
[M.CONHN]ads +OH→[M.CO.N2]ads +H2O
+1e-
[M.CO.NHN]ads + OH→ [M.CO.N2]ads + H2O
+ 1e-
2
3
Path 3
6
[M.CO(NH2)2]ads + OH- → [M.CO(NH2.NH)]ads
+H2O + 1e-
Not an Elementary Step
Not an Elementary Step
[M.CO.NH2NH]ads +
OH-→
[M.CONH.NH]ads +
H2O + 1e[M.CO.NHNH]ads + OH→ M.CO +NH.N + H2O
+ 1e[M.CO.NHN]ads + OH→ M.CO.N2 + H2O +
1e-
7
[M.CO.N2]ads + OH-→ [M.CO.OH]ads + N2 + 1e-
8
[M.CO.OH]ads + OH-→ [M.CO2]ads + H2O + 1e-
9
[M.CO2]ads→ M + CO2
4.1
Reaction Mechanism Path 1
Step 1
Figure 4.1 shows the first step of the reaction: adsorption of urea onto NiOOH by
the following reaction:
CO(NH2)2 + M → [M.CO (NH2)2]ads
(4.5)
M is the catalyst NiOOH. The Gibb’s free energy was calculated as the sum of
energies of the two separate structures of NiOOH and urea. b shows the Transition State
26
(T
TS) whereass c illustrattes urea adsoorbed onto niickel from oxygen
o
(O2) as the site of
o
addsorption. The
T interactioon of NiOOH
H with urea changes
c
the NH2 (H4-N2-H3) bond
anngle in c, which
w
in the case of the urea
u moleculle is 118.8o and
a in the finnal structuree
w NiOOH,, it decreasess to 109.2o. The differennce in structuures betweenn the transitiion
with
sttates and thee final products is the varriation the Ni-O2-C
N
bonnd angle whicch is 97.55o in
thhe transition state and 1000.58 degreees in the finaal structure. The
T rate connstant calculaated
foor this reactiion was 6.81 s-1.
(a)
(b)
(c)
F
Figure
4.1: Initial state (a) Transitioon state (b) and
a Final struucture (c) foor Equation 4.5
4
27
(a)
(b)
(c)
nitial state (aa), transition state (b) andd final structture (c) for Equation
E
4.6
Figure 4.2: In
Step 2
Figuree 4.2 illustraates the initiaal (a) transitiion state (b) and the finaal structure (cc) for
g reaction:
thhe following
[M
M.CO(NH2)2]ads + OH - →[M.CO(N
→
NH2.NH)]ads +H2O + 1e-
(4.6)
In the initial structure the N1--H1 bond lenngth for the dissociating
proton is 1.002Å
d
w
whereas
in th
he TS, it increases to 1.122Å. The N2--C distance is
i noted as 1.59Å and the
N
N2-Ni
bond length
l
is 1.922 Å .The rate constant foor this reactiion was calcuulated as
2.7x1011 L mol
m -1 s-1.
28
Step 3
In step 3 of path 1as shown in Figure 4.3, the NH2 attached is deprotonated by the
approaching OH-, according to the following reaction:
[M.CO(NH2.NH)]ads +OH- → [M.CONH2N]ads + H2O + 1e-
(4.7)
Due to the vacant site on N1-H2, it serves as a point of attachment for the free
OH- ion in the initial optimized structure (Figure 4.3a). The N1-(O4H6) bond length in
the initial structure is 1.44 Å, N1-H2 being 1.02 Å. In the TS (Figure 4.3b), H is 1.29 Å
away from N. This OH- then withdraws a proton to leave the system as water.
Subsequently, the N1-C bond becomes shorter in the final structure (1.29 Å) versus the
initial structure (1.43Å). The rate constant for this reaction is 2.8x10-23 L mol-1 s-1.
29
(a)
(b)
(c)
Figure 4.3: In
nitial state (aa), transition state (b) andd final structture (c) for Equation
E
4.7
Step 4
In step
p 4, the folloowing reactioon occurs:
M.CO(NH2N)]
N ads + OH-→ [M.CONH
HN]ads + H2O + 1e[M
(4.8)
W
When
the OH
H- approachees the secondd NH2 (H3-N
N2-H4) grouup in Figure 4.4a, it pullss
aw
way the entiire NH2 grouup towards it (O4-N2: 1..44 Å, N2-H
H4: 1.02 Å). In the transiition
sttate the proto
on moves tow
wards OH-, and the NH (N2-H3) grooup reattachhes back to nickel
n
ass shown in th
he final statee (Figure 4.44c). N2-H3 bond
b
length in the TS inccreases to 1..13
Å The rate co
Å.
onstant for this reaction in Equation 4.8 was calcculated to bee 1.1x10-15
L mol-1 s-1.
30
(a)
(b)
(c)
nitial state (aa), transition state (b) andd final structture (c) for Equation
E
4.8
Figure 4.4: In
Step 5
p 5 (Figure 4.5),
4
similar to step 3, duue to a vacannt site on NH
H (N2-H3), OH
O In step
O4-H6) adso
orbs onto thee surface, acccepts a protoon and detacches as waterr according to
t
(O
thhe following
g reaction:
[M
M.CONHN]ads +OH-→[M.CO.N2]adds +H2O +1e-
(4..9)
The O4-N2,
O
N2-N
Ni and N2-H
H3 bond lengtths in the iniitial state aree 1.43Å, 1.844Å
annd 1.03Å resspectively (F
Figure 4.5a).. In the TS, the
t N2-H3 distance
d
increeases to 1.155Å.
Inn the final sttate, as obserrved in step 3 before, thee N2-Ni bonnd length deccreases to 1.559Å.
T rate consstant for this reaction froom Equation 4.9 is 2.7x10-24 L mol-1 s-1.
The
31
(a)
(b)
(c)
nitial state (aa), transition state (b) andd final structture (c) for Equation
E
4.99
Figure 4.5: In
Step 6
From step 5 (Figuure 4.5c), it is evident thaat the two N (N1 and N22) atoms are at a
mation of N2 and the
siignificant distance from each other, thus preventting the form
suubsequent deesorption off N2 from thee catalyst’s surface.
s
In orrder to faciliitate the bondd
foormation bettween the tw
wo atoms, steep 6 was inclluded whereein the diheddral angle
between N2-N
Ni-C-N1 waas changed frrom -177.75o to -0.24o. As
A can be seeen from the final
sttructure in th
his step, therre is a bond between
b
the two Ns (Figgure 4.6c) whhich will enaable
N2 formation as can be seeen later. Aftter having doonated all itss protons to the
t approachhing
OH- ions, the remaining steps
O
s
(7, 8, and
a 9) are thee same for both parts 1 and
a 2.
32
(a)
(b)
(c)
Figure 4.6: In
nitial state (aa), transition state (b) andd final structture (c) for re-arrangeme
r
ent of
nitrogen atom
ms
Step 7
In step
p 7 (Figure 4.7),
4
the appproaching OH
H- ion is adsorbed on thee surface whhile
orbed simultaneously wiith the follow
wing reactionn:
thhe N2 is deso
[
[M.CO.N
M.CO.OH]adds + N2 + 1e2]ad
ds + OH → [M
(4.110)
The OH
O - gets adsoorbed onto carbon
c
whereeas the N2 molecule
m
getss desorbed frrom
nickel. The raate constant for this step is 7.3 x108 L mol-1 s-1.
Step 8
The next
n reaction in step 8 (F
Figure 4.8) iss as follows:
[
[M.CO.OH]
ads
a + OH → [M.CO2]ads + H2O + 1e
(4.111)
33
The last approaching OH- gets adsorbed onto nickel. The Mulliken charge on Ni
reduces from 0.415 in the initial state (Figure 4.8a) to 0.372 in the final state (Figure
4.8c). As a result of this, Ni exhibits a tendency to form a bond with O4, which loses a
proton(H6) to the detaching OH (O5-H7). The rate constant for this step is 1.6 L mol-1 s-1.
Step 9
The final state is CO2 (O1-C-O4) adsorbed onto NiOOH. This gets desorbed in the
final step (Figure 4.9) as follows:
[M.CO2]ads→ M + CO2
The rate constants for the step is 4.3x10-65 L mol-1 s-1 respectively.
(4.12)
34
(a)
(
(b)
(c)
nitial state (aa), transition state (b) andd final structture (c) for Equation 4.10
Figure 4.7: In
35
(a)
(b)
(c)
nitial state (aa), transition state (b) andd final structture (c) for E
Equation 4.11
Figure 4.8: In
36
(a)
(b)
(c)
nitial state (aa), transition state (b) andd final structture (c) for Equation
E
4.12
Figure 4.9: In
4.2
Reaction Mechhanism: Pathh 2
Step 1
This step
s is comm
mon for all paathways.
Step 2
In an alternative mechanism
m
f step 2, wee consideredd the possibility of the NH
for
N 2
atttached to niickel (H3-N22-H4) donatiing its protons to the OH
H- ion (Figurre 4.10)
acccording to the
t followinng reaction:
[M
M.CO(NH2)2]ads + OH- → [M.CO(N
NH2.NH)]ads +H2O + 1e-
(4.13)
37
The kinetics
k
of thiis step impliied it was fasster than patth 1as the ratte constant value
v
iss 1.38x1017 L mol-1 s-1 veersus the steep 2 of path 1 with a ratee constant vaalue of 2.7 x11011
L mol-1 s-1. Th
his is most likely
l
due to the positionn of the OH- molecule whhere it can
obbtain an H from
fr
either N1
N or N2 as shown
s
in thee transition state.
s
In the TS,
T the two N2H bond leng
H3
gths are 1.1Å
Å and N2-H44 is 1.15Å. The
T initial strructure (Figuure 4.10a) iss the
saame as consiidered for paath 1. The traansition state and final structures
s
aree as shown in
Figure 4.10b and Figure 4.10c
4
respecctively.
(a)
(bb)
(c)
I
state (a),
( transitionn state (b) annd final struccture (c) for Equation 4.13
Figure 4.10: Initial
Step 3
The reeaction in thhis step is as follows:
M
M.CO(NH
N ads + OH-→ [M.CONH
H2.N]ads + H2O + 1e2NH)
(4.114)
38
T N2-H4 bond lengths in the initiall structure (F
The
Figure 4.11bb) and TS (Fiigure 4.11c) are
1.02Å and 1.2
28Å respectiively. The rate
r constantt is 2.3x10-21 L mol-1 s-1.
(b)
(a)
(c)
Figure 4.11: Initial
I
state (a),
( transitionn state (b) annd final struccture (c) for Equation 4.14
Step 4
Next,, the approacching OH- taakes up a hyddrogen from
m the second NH2 (H1-N1H group according to thhe followingg reaction:
H2)
[M
M.CO(NH2.N
N)]ads + OH-→ [M.CON
NHN]ads + H2O + 1e-
(4.15)
It can also be seenn that due to presence off excess electrons on N, it
i forms a rinng
liike structure with nickel and oxygenn. The N-H bond
b
length increases
i
froom 1.02Å in the
innitial state to
o 1.05Å in thhe final state. (Figure 4.112) The rate constant forr this step waas
caalculated as 4.1 x107 L mol
m -1 s-1.
39
(a)
(b)
(c)
I
state (a),
( transitionn state (b) annd final struccture (c) Equuation 4.15
Figure 4.12: Initial
Step 5
p 5 of path 2 the NH (H1-N1) groupp loses its lasst proton to the
t approachhing
In step
OH- with the following reeaction:
O
[M
M.CO.NHN]ads + OH-→ [M.CO.N2]ads + H2O + 1e-
(4.16)
N bond lenngth changess from 1.02Å
Å to 1.14Å between
b
the initial
i
and
The N-H
trransition stattes (Figure 4.13).
4
The reemaining struucture is idenntical to the one obtained in
sttep 6 of path
h 1 where rottation of nitrrogen needs to be accom
mplished in order
o
to faciliitate
40
desorption off the N2 moleecule. The raate constant for this step is 8.8x1015 L mol-1 s-1.
n the steps foor paths 1,2 and
a 3 are ideentical.
From here on
(a)
(b)
(c)
I
state (a),
( transitionn state (b) annd final struccture (c) for Equation 4.16
Figure 4.13: Initial
41
4.3
Reaaction Mechhanisms: Pathh 3
Steps 1 and 2
They are the samee as path 2.
Step 3
p 3 of path 2,
2 NH2 (H1-N
N1-N2) grouup detaches from
f
carbon and attaches to
In step
N (N2-H3),, forming a cyclic
NH
c
structuure as shownn in Figure 4.14.
4
This faacilitates
desorption off N2 in the laater steps witthout the neeed to rotate the
t N-atoms towards eacch
otther.
(a)
(c)
(b)
42
Figure 4.14: Initial
I
state (a),
( transitionn state (b) annd final struccture (c) for rearrangemeent
ups
of amine grou
Step 4
In the next step 4,, (Figure 4.15) the NH2 group
g
loses a proton to thhe approaching
OH- ion, form
O
ming water according
a
to the reaction:
[M
M.CO.NH2NH]
N ads + OH-→ [M.CON
NH.NH]ads + H2O + 1e-
(4.17)
As sho
own in the TS
T (Figure 4.15b), there is a possibillity of the OH
H- withdraw
wing a
prroton from either
e
of the amine groupps. As a resuult, this is thee fastest stepp in this pathhway,
thhe rate consttant being 1.12x1017 L/m
mol.s. The N1-H2 bond lengths
l
for thhe dissociatiing
prroton are 1.0
02Å and 1.266Å in the iniitial (Figure 4.15a) and transition
t
staate (Figure
4.15b) respecctively. The N1-N2
N
bondd length decrreases from the
t initial to final step frrom
9Å.
1.43Å to 1.39
(a))
(b)
(c)
Figure 4.15: Initial
I
state (a),
( transitionn state (b) annd final struccture (c) for Equation 4.17
43
Step 5
Thereafter, the next approaching OH- takes up a proton from the –N1H1 group
(Figure 4.16). The rate constant for this step is 2.5x10-4 L mol-1 s-1 in the following
reaction:
[M.CO.NHNH]ads + OH-→ M.CO +NH.N + H2O + 1e-
(4.18)
44
(a)
(b)
(cc)
I
state (a),
( transitionn state (b) annd final struccture (c) for Equation 4.18
Figure 4.16: Initial
Step 6:
p 6, (Figure 4.17) when the approachhing OH- takkes up the laast proton
In step
t followinng reaction:
acccording to the
[M
M.CO.NHN]ads + OH- → M.CO.N2 + H2O + 1e
(4.19)
The fiinal structuree is the samee as the one obtained froom path 1 steep 6. In this
pathway sincee the nitrogeen atoms bouund togetherr initially in step
s 3, it neggates for theiir
rootation towaards each othher as in pathh 1. The N2-H3 bond lenngths for the initial (Figuure
4.17a) and traansition statees (Figure 4..17b) are 1.001Å and 1.022 Å. The N1-N2 bond length
45
becomes shorrter in the finnal step (1.43Å) from initial step (1.223Å). The raate constant for
thhis reaction is
i 3.6x10-7 L mol-1 s-1 froom Equationn 4.9. The nitrogen
n
atom
ms are in thee
vicinity of each other in this
t step, hennce eliminatiing the needd for rotationn as was requuired
inn path 1. Thu
us this path has
h least resiistance as coompared to path
p 1.
Steps 7, 8 an
nd 9
Thesee subsequentt elementary steps will now be same as in path 1.
(a)
(b)
(c))
Figure 4.17: Initial
I
state (a),
( transitionn state (b) annd final struccture (c) for Equation 4.17
46
The free energies values as well as the rate constants for the steps for the different
pathways are summarized in Tables 4.2 and 4.3.
As can be seen from the rate constant values, the desorption of CO2 from NiOOH
is the rate limiting step with a value of the order of magnitude of -65. This conclusion is
supported by the fact that the urea electrolysis reaction rate decreases with time which
can be attributed to a surface blockage of the catalyst when a build up of CO2 takes place
over a period of time8. Thermodynamic calculations also suggest the largest contribution
to the free energy change of the reaction is from the last step. The total free energy
change (ΔG) required is 1227.7 kJ mol-1, whereas the requirement for CO2 desorption
alone is 1242.2 kJ mol-1. Using this value of ΔG, the theoretical value of of the cell
potential was calculated using the Nernst Equation
ΔG=-nFEocell
(4.20)
where,
ΔG= Change in Gibbs Free Energy (kJ mol-1)
n= Number of electrons transferred per mole of reactant
F= Faraday’s Constant (96485 coulombs mol-1)
Eocell= Standard Potential (V)
The calculated Standard Potential was calculated as -2.12V at 298 K and 1
atmosphere pressure. The difference between this calculated potential and the theoretical
potential versus SHE8 of -0.46V can be attributed to two main factors. Firstly, in this
system one NiOOH site per molecule of urea has been considered. This system of single
molecule interactions limits the available active catalyst sites per molecule of urea. As a
47
result the overall energy required to desorb the final product CO2 is expected to be higher
in such a system, as compared to systems with larger NiOOH cluster sizes where a
greater number of catalytic sites are available per molecule of reactant. This in turn
explains the higher value of calculated standard potential. Secondly, gas phase
calculations have been performed without using solvent effects in order to simplify the
system. This has also possibly accounted for deviation in calculation of cell potential.
However, since the objective of this study was to gain a relativistic view of the kinetics of
the elementary steps, this has been well accomplished using a considerably simple model
of single molecule gas phase interactions.
Furthermore, we considered the possibility of other causes of surface blockage,
mainly from the preferential adsorption of OH- onto the surface of NiOOH. The binding
energies of CO2 and OH- calculated to be 9.2 kJ mol-1 and 18.0 kJ mol-1 respectively. This
suggested that in excess of OH- ions, the hydroxyl group is more preferentially adsorbed
onto the catalyst’s surface than CO2. This competition between adsorbed OH- and CO2 on
the catalyst’s surface leads to an increased tendency of surface blockage which could
further explain the decreased rate of reaction with time.
48
Table 4.2: Sum of free energies for all the intermediate steps
Reactions
∆G (kJ mol-1)
CO (NH2)2 + M → [M.CO (NH2)2]ads
66.2
[M.CO(NH2)2]ads + OH- → [M.CO(NH2.NH)]ads +H2O + 1e-
-28.9
[M.CO(NH2.NH)]ads +OH- → [M.CONH2N]ads + H2O + 1e-
-185.1
[M.CO(NH2N)]ads + OH-→ [M.CONHN]ads + H2O + 1e-
75.4
[M.CONHN]ads +OH-→[M.CO.N2]ads +H2O +1e-
-178.2
[M.CO.N2]ads + OH-→ [M.CO.OH]ads + N2 + 1e-
392.7
[M.CO.OH]ads + OH-→ [M.CO2]ads + H2O + 1e-
-156.6
[M.CO2]ads→ M + CO2
1242.2
Total
1227.7
49
Table 4.3: Kinetics of the reaction pathways and rate constants for intermediate steps
Steps
1
Rate Constants (L mol-1 s-1)
Path 1
Path 2
6.8
Path 3
2
2.3x1011
1.38x1017
3
2.8x10-23
2.3x10-21
Not an Elementary
Step
4
1.1x10-15
4.1x107
1.1x1017
5
2.7x10-24
8.8x1015
2.5x10-4
6
Not an Elementary Step
3.6x10-7
7.3x108
1.6
4.3x10-65 (rds)
7
8
9
4.5
Conclusion
Based on the calculations summarized in Table 4.3, it can be deduced that
kinetically, the last step, i.e. desorption of CO2 from the catalyst’s surface is the rate
limiting step. This indicates the occurrence of surface blockage in the presence of a larger
number of reacting molecules of urea than what is considered here. Thermodynamic
calculations ( Table 4.2)also suggest a large contribution to the free energy change
occurring from the last step. Total free energy change required is 293.1 kcal mol-1,
whereas the requirement for CO2 desorption alone is 296.7 kcal mol-1. Using the free
50
energy, the theoretical value of the cell potential was calculated to be -2.12V. The
difference between this theoretical and experimental value8 of -0.46V obtained can be
attributed to the limitation of using gas phase calculations as well as single molecule
interactions in the system.
Also, the path of least resistance is path 2, wherein the NH2 migrates to bond with
the NH group initially in step 3, before the remaining proton transfer could take place. As
a result of this migration, it involves no further need to rotate the N molecules towards
each other to bring about N2 desorption, as the atoms are already in the vicinity of each
other. This makes path 2 the preferred pathway. However, even if the reaction progresses
via any given mechanism, the rate limiting step, i.e, desorption of CO2 is common to all
the mechanisms.
51
Chapter 5 : CHEMICAL OXIDATION MECHANISMS
The objective of this study was to investigate the thermodynamics and kinetics of
the urea decomposition reactions occurring on NiOOH. The first consideration was urea
adsorbed onto the surface of NiOOH, and secondly the inclusion of a hydroxide ion in the
system was considered, to investigate the catalytic effects of OH- in the reaction.
5.1
Different Orientations of Urea towards NiOOH
The electrophilic atoms of urea were oriented towards the nucleophilic atoms of NiOOH
and vice versa (Figure 5.1). The configurations obtained are tabulated along with their
binding energies Table 5.1. In Figure 5.1 a, the interaction of Ni with N1 was considered,
wherein no significant interaction between the two species was observed. In Figure 5.1b
the interaction of Ni with O1 was considered which resulted in a similar output of
NiOOH separated from urea as in Figure 5.1a. In Figure 5.1c, double atomic interactions
of Ni with N2 and OH with C were considered. The resulting output as shown consisted
of detached ammonia and isocyanic acid attached to NiOOH. In Figure 5.1d, Ni
interacted with N2 while O2 interacted with C. The resulting structure has urea attached
to NiOOH through the point of attachment of carbon. In Figure 5.1 e the interaction of Ni
with N2 and OH- with C was considered, which resulted in breakage of OH from NiOOH
and attachment of Ni-O to urea.
The binding energies of structures in Figure 5.1 b and Figure 5.1 d were the least
in the group, suggesting that these structures were most likely to undergo dissociation.
Since structure in Figure 5.1 d shows a more plausible scenario of multiple atomic
52
innteractions (b
between Ni--N and O-C)), it was conssidered for thhe complete dissociationn
m
mechanism
of NiOOH wiith urea.
(a)
(c)
(
(b)
(d))
(e))
O
strructures for different
d
orieentations of urea towardds NiOOH
Figure 5.1: Optimized
53
Table 5.1:Binding Energies of different orientations of urea towards NiOOH
Binding Energies (kJ mol-1)
5.2
a.
11.7
b.
5.9
c.
11.3
d.
8.3
e.
12.6
Urea decomposition with NiOOH
The proposed reaction mechanism for dissociation of urea in the presence of
NiOOH is as follows:
(NH2)2CO + NiOOH Æ (NH2)2CO. NiOOH
(5.1)
(NH2)2CO. NiOOH Æ HNCO.NiOOH + NH3
(5.2)
The optimized reactants, products and transition states are shown in Figure 5.2
and Figure 5.3. Upon attachment to NiOOH, the urea molecule which initially resides in
a single plane has now been changed as the Hs on the amides bend closer to each other
and out of the original plane. The N1-C-N2 bond angle changes from 115.4o in urea
(Figure 5.2a) to 110.4o in intermediate 1(Figure 5.2b). The H1-N1-H2 and H3-N2-H4
bond angles in urea are 118.8o and they reduce in intermediate 1 from 109.2o to 107.9o
respectively. Between the transition state TS I in Figure 5.3a, and intermediate 1(Figure
54
5.2b), the Ni--O-C angle changes
c
from
m 97.6o to 1000.6o. There is no signifiicant differennce
inn the other parameters.
In step
p 2, N2 donaates its protoon to the leavving ammonnia molecule which detacched
inn the final prroduct at a distance of 2..02 Å from NiOOH
N
(Figgure 5.2c). Thhe isocyanicc acid
reemains boun
nd to NiOOH
H from the pooints of attacchment of Ni
N and O. Thee Ni-N2 bonnd
annd C-O2 bon
nd lengths arre 1.87 Å annd 1.36 Å, reespectively. NH3 is seenn to rotate aroound
thhe bound HC
CNO group at
a the Ni-N1 distance off 2.01 Å.
The values of ratee constants for
fo the forwaard reactions were calcullated as k1= 6.81
6
s-1
, k2= 1.54x1
10-6 s-1, indiccating that libberation of ammonia
a
is the
t rate limiting step in this
t
reeaction. Table 5.2 showss the changes in free eneergy in the tw
wo steps of thhe reaction.
(a)
(c)
O
strructures for Equations
E
5..1 and 5.2
Figure 5.2: Optimized
(b)
55
(a)
(b)
Figure 5.3: Optimized
O
Transition Stattes for Reacttions 2 and 3
Table 5.2: Free Energyy differencess for equatioons 5.1 and 5.2
5
5.3
Sttructures
Freee energies chhanges (kJ mool-1)
Equuation 5.1
-666.9
Equuation 5.2
-2009.3
O
Overall
-2776.3
Urea and NiOOH
N
in the presence of
o OH- ion:
When
n OH- was opptimized witth NiOOH, itt resulted in the adsorptiion of the
hyydroxide ion
n onto Ni as shown in Fiigure 5.4. Thhe output struucture was then
t
further
opptimized witth urea.
T proposed
The
d reaction meechanism is as follows:
(N
NiO.OHOH))ads + CO(NH
H2)2→ (Ni(O
OH)3.CONH.NH2)ads
(N
Ni(OH)3CON
NH.NH2)ads →(Ni(OH)3.C
CNO)ads + NH
N 3
(
(5.3)
(5.4)
56
Figure 5.4: Adsorption
A
off OH- onto NiOOH
N
(a)
(b)
(c)
(
O
strructures for Equations
E
5..3 and 5.4.
Figure 5.5: Optimized
Figuree 5.5a illustrrates the optiimized reacttants for Equuation 5.3 annd 5.4. The N1N
N distance iss 2.06Å. In thhe intermediiate structuree (Figure 5.55b), the ureaa molecule
Ni
unndergoes rottation to such that both NH
N 2 groups face downw
wards. Due too the close
prroximity of N1-H1
N
to O2, O2 withdrraws the prooton from N11 while N1 bonds
b
to Ni at
a the
saame time. Th
he N1-Ni boond length reeduces to 1.996Å. The N11-C-N2 bondd angle increeases
57
frrom 111.1o in the reactannts to 115.8o in the interm
mediate due to the N1-N
Ni bonding. In
I
thhe transition state for this step (Figurre 5.6a), the urea molecuule is not rottated, and thee
prroton H5 facces away froom the N2 am
mine group.
TS I (aa)
TS II (b))
Figure
F
5.6: Transition
T
Sttate Structurres for Equattions 5.3 andd 5.4
In equ
uation 5.4, am
mmonia is formed
fo
and desorbed,
d
leaaving CNO- still adsorbeed
onnto Ni(OH)3 (Figure 5.5c) at a distannce of 2.09 Å from O2. The
T Ni-N1 distance
d
does not
chhange signifficantly at 1.95Å. The Nii-N1-C bondd angle is 1334.4o. The traansition statee for
thhis step (Figu
ure 5.6b) illuustrates the displacemen
d
nt of proton H2
H between the two amiine
grroups. H2 iss at a distancce of 1.28 Å from the leaaving NH grooup and 1.322Å from the
acccepting NH
H2 group. The rate constaant calculatioons yield thee rate constaant for reactioon
5.3 as 3.02 x 104 L mol-1 s-1 and for reeaction 5.4 as
a 1.37x10-266 L mol-1 s-1. In this casee, as
inn the previou
us mechanism
m too, the ellimination of ammonia is
i the rate deetermining sttep.
T reaction profile
The
p
as a function
f
of the
t reaction coordinates is shown in Figure 5.8. The
frree energies for the two steps are givven in Table 5.3.
58
Table 5.3: Free Energy differences for reactions 5.3 and 5.4
Structures
Free energy changes (kcal mol-1)
Equation 5.3
-581.9
Equation 5.4
-16.7
Overall
-598.7
5.4
Conclusion
The free energy differences between the reactants and products in the both the
mechanisms suggest that the reaction occurs spontaneously. The free energy difference
between the two steps is higher in the presence of OH- rather than without it, suggesting
that the reaction occurs more spontaneously in the presence of OH-. This is validated by
experimental results, as there are traces of ammonia present as a result of the urea
decomposition reaction.
59
Chapter 6 : EXPERIMENTAL
The experimental section is a brief study carried out in order to study the effect of
varying concentrations of KOH and urea on the current density obtained. The urea electro
oxidation reaction has been analyzed by conducting potentiodynamic tests with a rotating
disk electrode. The rotating disk electrode offers several advantages over stationery
electrode experiments. With the disk in constant motion, reaction rates are not diffusion
limited. Hence it throws light on the nature of the reaction taking place37. With
conventional experiments conducted using a rotating disk electrode, a steady state current
profile is obtained with changing potentials. However this is not the case with the urea
electro oxidation reaction, where reactions of a more complex nature seem to be going on
at the surface of the electrode, as will be discussed later.
The different operational parameters studied are the concentration of KOH, urea
and temperature effects on the current density of the reaction. Preliminary tests were
conducted to identify the lowest possible concentrations of KOH at which a response is
obtained. Once the lowest concentration was established, the current density at 5 levels of
concentration of KOH tested for 3 levels of concentration of urea were carried out at
room temperature.
6.1
Experimental Methods: Electroplating and Preliminary Results
Catalyst preparation was performed in two stages: one for the preliminary tests
and another for the main set of experiments. The chemicals were obtained from Alfa
Aesar or Fisher Scientific. Electroplating was carried out in a 200 mL beaker at 45° C
60
against a platinum foil counter electrode. The bath composition was as follows:
NiSO4.6H2O (280 g L-1), NiCl2.6H2O (40 g L-1), H3BO3 (30 g L-1).
The Rotor and Motor for the Rotating Disk Electrode Model 616 were obtained
from Pine Industries. A 5mm diameter Titanium removable disk was fitted into a Teflon
block which was then mounted onto the shaft of the rotating disk.
6.2
Potentio-dynamic Tests
In the preliminary study, a 2 cm by 2 cm Titanium foil electrode was electroplated
with nickel in the plating bath described above. The electro deposition was carried out at
-0.7 Volts versus Ag/AgCl electrode at 45o C for 15 minutes giving a catalyst loading of
100 mg, effectively 12.5 mg cm-2.. This nickel plated titanium electrode was used to
determine lowest concentration of KOH at which a response was obtained. Urea electro
oxidation to determine the response was carried out at room temperature starting with 2
M KOH, decreasing in steps of 0.5 M.
For the second stage, the titanium removable disk electrode of diameter 5 mm was
electroplated with under the same operating conditions as mentioned above in the same
plating bath. Electroplating was carried out again for 15 minutes giving a deposition of
1.5 mg± 0.2 mg, effectively 5.1 mg cm -2. A platinum ring arrangement was used as the
counter electrode around the removable rotating disk working electrode.
The preliminary set of experiments was conducted at 4 levels of KOH
concentration. The concentration of urea was kept constant at 20 g L-1 (composition of
the human urine) and all experiments were performed at room temperature. The 4 levels
of concentration of KOH tested were 1 M, 0.5 M, 0.25 M and 0.1M. This study was
61
conducted to purely select the lowest concentration at which a response is obtained. The
upper limit of the experiment matrix is set at 5 M KOH.
The second stage of experiments was carried out at five levels of concentration of
KOH and three levels of concentration of urea namely 0.5 M, 1 M, 2 M, 3 M, 5 M KOH.
The speed of rotation of the rotating disk electrode was set at 1000 rpm, with the HgHgO reference With 0.5 M KOH solution, the three levels of concentration of urea were
tested by performing cyclic voltammetry on the Solartron potentiostat. Then the
concentration of KOH was changed and the three concentrations of urea were again
tested. All experiments were performed at room temperature. The scan rate used was
20mV s-1.
6.3
Results and Discussion
Figure 6.1 represents the set of experiments performed initially to determine the
lower set point of KOH concentrations. 4 concentrations of KOH were tested starting
with 0.1 M. There was no response peak at 0.1 M. The lowest concentration of KOH that
gave a response was 0.25 M. There was not a significant difference between the
maximum current obtained with 0.25 M and 0.5 M. Hence 0.5 M was chosen as the lower
set point.
62
Figure 6.1: Preliminary experiment. Different concentrations of KOH at 20g L-1 urea to
determine lower setpoint.
The peaks given in Figure 6.2, Figure 6.3 and Figure 6.4 were obtained for
different concentrations of KOH at different urea concentrations. Baseline KOH
represents a solution with 1M KOH with no urea present. It is evident from all three
figures that the current density corresponding to the peak is the highest in the case of 5M
KOH. There is not a significant increase in the current density in case of 1 M, 2 M and 3
M KOH for all three concentrations of urea used.
63
Figure 6.2: Urea concentration of 5 g L-1 varying KOH concentrations. Scan rate: 20mV
s-1. Speed of rotation: 1000rpm.
Figure 6.3: Urea Concentration of 10 g L-1 with varying KOH concentrations. Scan Rate:
20mV s-1. Speed of rotation 1000rpm.
64
Figure 6.4: Urea concentration of 20 g L-1 with varying KOH concentrations. Scan rate
20mV s-1. Speed of rotation 1000 rpm.
6.4
Conclusion
This experimental study was strongly indicative of the fact that the concentration
of KOH plays a significant role in catalyzing the oxidation reaction. The maximum
current density obtained at 5 M KOH supports the argument that a higher concentration
of KOH is more favorable towards the oxidation reaction. However, as can be seen from
the oxidation peak and the rapid decrease of current from potentials of 0.55V to 0.7V,
which is uncharacteristic of rotating disk electrode experiments, it is an indication of an
adsorption-desorption reaction occurring on the electrode surface. There is a possibility
of adherence of CO2 or the OH- onto the NiOOH surface which causes this rapid rise and
fall of the electrode current.
65
Chapter 7 : CONCLUSIONS AND RECOMMENDATIONS
In summary, the electro oxidation reaction mechanisms studied indicate the
desorption of CO2 as the rate limiting step with the calculated rate constant value of
4.32x10-65 L mol-1s-1 . The desorption step also contributes to the maximum energy
requirement of the path (1242.2 kJ mol-1). Also based on the kinetics of the reaction,
*CO(NH2)2→ *CO(NH.NH2)→ *CO(NH.NH)→ *CO(NH.N)→*CO(N2) → *CO(OH)
→*CO(OH.OH) →*CO2 has been identified as the preferred pathway among the three
mechanisms. This has been discussed in Chapter 4 as Path 2. In this pathway, the bonding
between the NH2-NH group occurs initially versus the rotation of the nitrogen atoms
towards each other in the later stages as in Path 1. This facilitates easy desorption of the
nitrogen molecule.
Another important finding of this study is the investigation of causes of surface
blockage, mainly from the preferential adsorption of OH- onto the surface of NiOOH.
The binding energies of CO2 and OH- calculated to be 9.2 kJ mol-1 and 18.0 kJ mol-1
respectively. This suggested that in excess of OH- ions, the hydroxyl group is more
preferentially adsorbed onto the catalyst’s surface than CO2. This competition between
the molecules leading to adsorption onto the NiOOH surface leads to an increased
tendency of surface blockage which explains the decreased rate of reaction as time
progresses.
In the chemical oxidation mechanisms, the thermodynamic feasibility of the
reaction mechanisms both without and in the presence of OH- has been discussed.
Change in free energy for the oxidation mechanism without OH- is -276.3 kJ mol-1
66
whereas with OH- it is -598.7 kJ mol-1. This indicates a greater spontaneity of the
reaction in the presence of hydroxide ions, which is known to catalyze the reaction. In
both the reaction mechanisms, the desorption of ammonia is the rate limiting step. In
mechanism 1 it was calculated as 1.54x10-6 s-1 and in mechanism 2 it is 1.37x10-26
L/mol.s.
The experimental potentio-dynamic tests carried out with 3 levels of
concentration of KOH (0.5 M, 1 M, 2 M, 3 M, 5 M) indicate a significant increase in
current density of the anodic reaction with the highest KOH concentration in the matrix
of 5M in case of all three levels of concentration of urea of 5, 10 and 15 g L-1. There is no
significant difference in current densities for the lower concentrations of KOH. This is in
agreement with the modeling results which also indicate a greater favorability of the
reaction in presence of OH-.
After the role of OH- and the rate limiting step in the oxidation mechanism has
been established with this study, it is now recommended to look into improvements in the
rate constant approximations with inclusion of solvent effects in the system. Greater basis
sets can also be used to carry out similar calculations. At the same time, an experimental
model should be developed to calculate experimentally the kinetic parameters of the
model which can then be compared to the theoretical values.
67
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Stefanov,B.B.; Liu,G.; Liashenko,A.; Piskorz,P.; Komaromi,I.; Martin,R.L.; Fox,D.J.;
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72
APPENDIX A
SUPPORTING INFORMATION FOR ELECTROCHEMICAL OXIDATION OF UREA
Figure 4.1a.
Cartesian Co-ordinates
C
-1.82316000
O
-2.55328600
N
-1.97383000
H
-1.12406800
H
-2.72889600
H
-0.59547900
N
-0.60348500
H
-0.59210900
O
0.67900800
Ni
1.16539200
H
1.30277400
O
1.88514900
-0.24239000
-1.13695200
1.09072400
1.67220000
1.37104700
-1.56764400
-0.56182000
-0.06419400
1.75585200
0.02173500
2.39748300
-1.45208300
0.01975200
-0.35074500
-0.14353600
-0.09544600
-0.75445700
0.95055800
0.78547000
1.67783600
0.08121300
-0.05718900
-0.28939300
-0.29295200
Sum of electronic and zero-point Energies=
-545.482798
Number of Imaginary Frequencies= 0
Figure 4.1b.
Cartesian Co-ordinates
C
1.41239600
O
1.81069200
N
0.38580700
H
0.52624400
H
0.39711300
H
3.34913000
N
2.44541400
H
2.36721100
O
-2.74998500
Ni
-1.03769300
H
-3.14380200
O
0.59761700
0.01851800
-0.21337200
1.16294000
1.79385800
1.68465100
0.14233900
0.26800800
-0.43278900
0.19414100
-0.18616900
0.60065900
-1.06873400
0.08323000
1.34003300
0.06431400
-0.72441100
0.94067000
-0.43878800
-0.89024100
-1.62530200
-0.05149200
-0.08272000
0.73432400
-0.19956800
Sum of electronic and zero point Energies= -545.458333
Number of Imaginary Frequencies= 1
73
Figure 4.1c.
Cartesian Co-ordinates
C
1.41709000
O
1.83677600
N
0.39217400
H
0.52026600
H
0.42578200
H
3.22910400
N
2.47127100
H
2.15179700
O
-2.75475400
Ni
-1.03883900
H
-3.16555000
O
0.59040700
-0.01699400
-0.71645200
1.04688300
1.94794500
1.17902500
-0.08598300
0.58917500
0.80659800
0.19854100
-0.13894400
0.21392000
-0.92227900
0.08075200
1.13680100
0.49994200
0.03881700
1.51094400
-0.75036700
-0.67591300
-1.61984000
0.01374500
-0.13879000
0.89076100
-0.58015900
Sum of electronic and zero point energies= -545.458736
Number of Imaginary Frequencies= 0
Figure 4.2a.
Cartesian Co-ordinates
C
1.20477200
O
1.85529500
N
0.30534900
H
0.67818100
H
0.26404500
H
3.02345900
N
2.11110400
H
2.19992300
O
-2.99382400
Ni
-1.21216700
H
-3.44559700
O
0.21439700
H
1.03501000
O
1.67936100
-0.76897600
-1.68279900
0.14547400
1.11212400
-0.27581900
0.25160400
0.28811100
0.07273300
0.53908400
-0.12352700
-0.01897600
-1.07721100
2.35076700
2.41406100
0.05966100
0.64009700
1.01417700
1.01899500
1.94032500
-0.15776700
-0.60411100
-1.59702100
0.05754000
-0.12886100
0.69175900
-0.89684500
-0.63881400
0.08948300
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
-621.290518
74
Figure 4.2b.
Cartesian Co-ordinates
C
-1.00853200
O
-1.79221700
N
-0.00754200
H
-0.18298600
H
-0.02112300
H
-1.93274400
N
-1.64546300
H
-2.59039100
O
3.23163000
Ni
1.46783800
H
3.60446200
O
-0.02117600
H
-3.37570400
O
-3.79058100
0.38715800
0.91899500
-0.67147100
-1.64336300
-0.54339800
0.22917700
-0.46021200
-0.92884500
-0.62758900
0.09830500
-0.30301000
1.16827400
-0.00699700
-0.70433700
0.09992600
0.93547700
0.75841700
0.50613300
1.76945600
-1.71882000
-1.02067400
-0.63632000
-0.00731400
-0.10838500
0.81330000
-0.52359800
0.54014400
-0.02992300
Sum of electronic and zero-point Energies=
-621.287915
Number of Imaginary Frequencies= 2
Figure 4.2c.
Cartesian Coordinates
C
-0.98825400
O
-1.89510300
N
-0.28348700
H
-0.65390500
H
-0.47033000
H
-1.84394100
N
-1.62676500
H
-3.11019500
O
3.17859200
Ni
1.44396400
H
3.46707600
O
0.17247500
H
-3.05429100
O
-3.38897900
-0.61353200
-0.78725900
0.80947600
1.46341900
1.17283700
-1.62838700
-0.63177300
0.94708400
0.75770900
-0.02770900
1.02064700
-1.40458300
0.68477300
1.37822600
-0.08341100
-0.99862700
-0.17768400
0.51023900
-1.11154800
1.34744900
1.21994800
0.86349900
0.16281800
0.01353500
-0.71205600
-0.13455200
-0.58135600
0.03403700
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 2
-621.319782
75
Figure 4.3a
Cartesian Coordinates
C
1.45975300
O
1.52767800
N
-1.41977700
H
-2.23479800
H
-0.59904700
H
2.66707100
N
2.69410500
O
-3.17691400
Ni
-1.36679900
H
-3.16970400
O
0.43525600
H
3.32800200
O
3.78898500
0.00492500 -0.04374900
1.10600600 0.53073600
1.46060300 -0.65833300
1.85756100 -0.17860800
1.87629000 -0.20172900
-1.60381100 -0.22294400
-0.60726300 -0.42678900
-0.50928800 0.50907400
-0.26902300 -0.05115700
-0.16791400 1.40415900
-0.68025000 -0.39450700
0.72784200 0.70572700
-0.06150100 0.32771400
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.3b
C
O
N
H
H
H
N
O
Ni
H
O
H
O
-1.46935900
-1.97486000
1.18200300
2.09753600
0.57108700
-3.96278600
-1.52080100
3.35823800
1.45144900
3.62261800
-0.32334900
-3.89480800
-4.54584000
0.06621100
1.07859000
1.44064700
1.89108600
1.91237200
-0.92103600
-0.91249800
-0.33933700
-0.24042900
0.04461000
-0.66667900
0.50498900
-0.17186100
-620.758842
-0.10310400
-0.55952800
0.62516000
0.53065700
-0.05207400
0.31696000
0.73525900
0.00554800
-0.04419700
-0.83138200
-0.49453300
-0.15340800
0.11382100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 2
-620.682434
76
Figure 4.3c.
C
-1.46935900
O
-1.97486000
N
1.18200300
H
2.09753600
H
0.57108700
H
-3.96278600
N
-1.52080100
O
3.35823800
Ni
1.45144900
H
3.62261800
O
-0.32334900
H
-3.89480800
O
-4.54584000
0.06621100
1.07859000
1.44064700
1.89108600
1.91237200
-0.92103600
-0.91249800
-0.33933700
-0.24042900
0.04461000
-0.66667900
0.50498900
-0.17186100
-0.10310400
-0.55952800
0.62516000
0.53065700
-0.05207400
0.31696000
0.73525900
0.00554800
-0.04419700
-0.83138200
-0.49453300
-0.15340800
0.11382100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.4a.
C
O
N
H
H
N
O
Ni
H
O
H
O
2.02492500
1.99230200
-0.61064000
-1.36543800
0.16339300
3.10461800
-2.81349600
-0.91949600
-3.23848300
0.86501000
0.87111100
-0.08032800
-0.38489000
0.91605100
1.43887000
2.04508800
1.45952600
-1.09422100
-0.74968000
-0.52005600
0.10260600
-1.07525700
1.60390300
2.06478900
-0.00241700
0.08180300
0.56088100
0.88167600
1.23586300
0.09749400
-0.07955800
0.01063900
0.02417000
-0.19354400
-0.56236200
-0.61762000
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
-620.698557
-620.105700
77
Figure 4.4b.
C
O
N
H
H
N
O
Ni
H
O
H
O
2.01170900
2.18295300
-0.97253500
-0.90336700
-0.07156000
3.21320000
-2.76489000
-0.89091900
-3.16585000
0.92016600
0.66233400
-0.25457200
-0.35367700
0.86382800
1.00690500
1.85766900
1.03695000
-0.94561200
-0.75587600
-0.51640500
-0.08689400
-1.01324400
2.05342800
2.31669100
0.00509000
0.29183800
1.05888200
0.31385400
1.55617200
-0.18820800
-0.29843000
-0.01738600
0.25785900
-0.10412700
-0.57228000
-0.78853600
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 2
-620.043488
Figure 4.4c.
C
1.96280600 -0.43774300 0.00690100
O
2.15911500 0.75955800 0.29045100
N
-1.01634500 0.81115600 1.12483600
H
-0.52846700 2.35796200 -0.09843000
H
-0.07393700 0.78283100 1.55640600
N
3.10837800 -1.22417600 -0.20758200
O
-2.82974900 -0.56562900 -0.38717700
Ni
-0.95133400 -0.54084800 -0.04215000
H
-3.13908700 0.18970600 0.11436400
O
0.85033100 -1.07380200 -0.10819300
H
0.94280400 2.18654700 -0.37593500
O
0.19717400 2.77291100 -0.60488200
Sum of electronic and zero-point Energies=
-620.073313
Number of Imaginary frequencies= 1
Figure 4.5a.
C
1.85531900 -0.51445200
O
2.02630300 0.33721900
N
-1.00502600 1.39784300
H
-1.15329900 1.59437300
N
3.14067500 -0.54648500
O
-2.72238400 -0.61415200
Ni
-0.91312400 -0.44327300
H
-3.18626000 -0.03964600
O
0.81996400 -0.96466100
H
0.91654400 1.69123500
-0.04685200
0.96322400
-0.11606600
0.88437800
-0.09955600
0.49622800
-0.09154700
-0.11412000
-0.60042600
0.19153900
78
O
0.23974600 2.02820400 -0.43503000
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.5b.
C
-1.80307200
O
-2.06590800
N
1.40802000
H
1.18727100
N
-3.05916200
O
1.84845500
Ni
0.88835200
H
1.15059400
O
-0.73322100
H
-0.49570300
O
0.40822500
-0.43587900
0.82757900
1.26871100
2.05879800
-0.53880500
-1.49960600
-0.20613000
-1.81839300
-1.10888000
1.86977900
1.92683000
-619.463527
-0.03111000
-0.32615400
-0.77548000
0.03327100
0.22860600
0.69189800
-0.33436800
1.26559500
-0.03446300
0.71847700
1.08868700
Sum of electronic and zero-point Energies=
-619.383245
Number of Imaginary Frequencies= 2
Figure 4.5c.
C
-1.55801100 -0.86827500 -0.01634100
O
-2.12518300 0.17772100 -0.59183200
N
1.10723600 1.45677200 -0.74826800
H
-0.14275900 2.44646300 0.36703300
N
-2.75314600 -1.28669700 0.20188800
O
2.26529900 -0.99142700 0.85525700
Ni
1.07705900 -0.08490500 -0.33403100
H
1.64146200 -1.41536100 1.44591000
O
-0.35094200 -1.19375100 0.18140300
H
-1.52060900 1.80655600 0.47647100
O
-0.94746300 2.45230400 0.92844100
Sum of electronic and zero-point Energies=
-619.398179
Number of Imaginary Frequencies= 1
Figure 4.6a.
C
-1.82804700 0.19467100 -0.00024900
O
-2.23271100 -1.05845200 0.00616400
N
1.97273000 -1.44818500 -0.00202400
N
-3.07606300 0.50789200 -0.00490500
O
1.81955400 1.48553400 0.00312700
Ni
0.94313600 -0.21151400 -0.00089600
H
1.04159700 2.04459100 0.00486400
O
-0.68156600 0.73439600 -0.00051200
79
Sum of electronic and zero-point Energies=
-542.992980
Number of Imaginary Frequencies= 1
Figure 4.6b.
C
-1.63484600
O
-2.83955600
N
1.11450600
N
-1.06448400
O
2.02456000
Ni
0.70852500
H
1.65641400
O
-0.68952800
-0.09523300
-0.37128000
1.52061700
0.94766600
-1.19952400
0.17304800
-1.86311300
-0.89029800
-0.00410300
0.01681500
0.62042600
-0.61927200
-0.27830400
-0.09887300
0.30640300
0.57131000
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=2
Figure 4.6c.
C
-1.77365400
O
-2.97989100
N
-0.00409200
N
-1.22717800
O
2.76664500
Ni
0.88179200
H
2.96961400
O
-0.83662600
-0.14105900
-0.30110800
1.34012200
1.28134900
0.03632900
-0.27035900
0.44625400
-1.03273900
0.00890900
0.03424500
-0.02902100
-0.01027800
-0.05209600
-0.01508000
0.78964600
-0.00037100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.7a.
C
O
N
H
H
H
N
H
O
Ni
H
O
1.20477200
1.85529500
0.30534900
0.67818100
0.26404500
3.02345900
2.11110400
2.19992300
-2.99382400
-1.21216700
-3.44559700
0.21439700
-0.76897600
-1.68279900
0.14547400
1.11212400
-0.27581900
0.25160400
0.28811100
0.07273300
0.53908400
-0.12352700
-0.01897600
-1.07721100
-543.003947
-543.202457
0.05966100
0.64009700
1.01417700
1.01899500
1.94032500
-0.15776700
-0.60411100
-1.59702100
0.05754000
-0.12886100
0.69175900
-0.89684500
80
H
O
1.03501000 2.35076700 -0.63881400
1.67936100 2.41406100 0.08948300
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.7b.
C
O
N
H
H
H
N
H
O
Ni
H
O
H
O
1.11893100
1.92178300
0.31997700
0.79280000
0.29309100
2.20324600
2.03676700
2.72240500
-2.91919800
-1.24860500
-3.32197200
0.08690300
1.38512900
1.86994300
-0.73269800
-1.43583100
0.37140100
1.35376400
0.14710300
1.06996200
-0.02633800
-0.64937200
0.74401700
-0.17519500
0.46847600
-1.41727000
2.75363600
2.32691200
0.05283800
0.82940900
0.75932000
0.57014000
1.75441800
-0.52331000
-0.82170100
-1.23704700
0.00072800
-0.11003000
0.82511500
-0.56617600
-0.67991600
0.04742200
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=2
Figure 4.7c.
C
O
N
H
H
H
N
H
O
Ni
H
O
H
O
0.99746100
1.81397600
0.38280100
1.45038100
0.30536100
2.56283500
1.98359200
2.53471600
-2.92928800
-1.28226400
-3.35673400
-0.05528100
1.82922200
2.17410300
-0.78442200
-1.49510900
0.42593500
1.81702300
0.23990700
0.41764900
-0.35347000
-1.14934600
0.86073800
-0.10614900
0.65211500
-1.48195200
2.65544200
2.43365500
-621.290518
-621.301259
0.08853800
0.90042200
0.66379300
0.39054500
1.66850400
-0.60344900
-0.94270900
-1.25722700
-0.05867100
-0.08192800
0.77291200
-0.45081800
-0.82412500
0.05506900
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
-621.317789
81
Figure 4.8a.
C
O
N
H
H
N
H
O
Ni
H
O
O
H
2.07533200
1.91428500
-0.98069100
-0.19066600
4.03864900
3.39767200
3.44676700
-2.52425600
-0.68445500
-2.59765000
1.18239400
-2.09995500
-2.68296300
0.15923400
1.35279200
1.13176900
1.64968800
0.18640400
-0.35319800
-1.34977100
-1.00716700
-0.59698500
-0.95643800
-0.73350200
1.63084400
0.83655000
0.05206000
0.33248100
-0.60469600
-0.19527700
0.54568700
-0.02314400
0.15291500
0.24593200
-0.06226100
1.19976900
-0.23729300
0.16401700
0.08163100
Sum of electronic and zero-point Energies=
-620.767669
Number of Imaginary Frequencies= 1
Figure 4.8b.
C
O
N
H
H
N
H
O
Ni
H
O
O
H
-2.01971700
-1.63251600
1.44529700
2.54472200
-3.96335600
-3.38071700
-3.58822800
1.52765100
0.33632700
1.48228400
-1.18267700
3.38207700
3.01739700
-0.04641800
1.14885200
-1.24220100
-0.97774800
0.38467800
-0.36026500
-1.27189000
1.48282000
0.08971000
1.85457200
-1.01696300
-0.53705400
0.37303700
0.03517700
0.05996500
0.00133000
0.60593200
-0.29700100
0.06348300
-0.32512200
-0.11058600
-0.03435800
0.78646500
0.01359700
-0.00544700
-0.13324000
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 2
-620.710005
82
Figure 4.8c.
C
O
N
H
H
N
H
O
Ni
H
O
O
H
1.92185800
1.31353900
-0.21271600
-2.82797000
3.73100600
3.28794200
3.76579300
-2.38315200
-0.49067200
-2.54290500
1.34181800
-2.72928200
-1.78824100
0.18612900 -0.06240300
1.18693000 0.44695700
0.89510200 0.67233400
0.76435300 -0.40241700
1.00943800 0.30147100
0.33039700 -0.30551700
-0.56249900 -0.32411500
-0.94589000 0.14991600
-0.73092200 0.00705700
-0.87886600 1.09212100
-0.89563300 -0.37927400
1.73246700 -0.56262400
1.81513400 -0.29776600
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
Figure 4.9a.
C
-1.10122000
O
-0.55989300
N
0.80418900
H
-2.83943900
N
-2.40089700
H
-3.02709300
O
2.73944200
Ni
1.47603800
H
3.62369500
O
-0.30589800
O
-4.08180100
H
-3.63676600
0.39373100
1.65287000
1.66410000
1.29120000
0.38732100
-0.40148200
-1.48309000
-0.05218100
-1.14916200
-0.58011800
-1.23072900
-1.87328000
-0.18453500
0.10855100
0.23462000
-0.18867400
-0.36498100
-0.20352100
-0.03380100
0.03261500
0.12225100
-0.25090900
0.24513900
0.82461900
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
Figure 4.9b.
C
O
N
H
N
H
O
1.03551900
0.72848200
-0.60113600
3.09045600
2.29896700
2.79023400
-2.99256100
0.06890400
1.41223800
1.66506400
0.40864900
-0.15961800
-1.05148900
-1.18496100
-620.710005
-620.078074
0.27798200
0.05818700
-0.15052500
0.15097200
0.55800600
0.28024500
-0.07399000
83
Ni
H
O
O
H
-1.56030900
-2.61596100
0.08957200
4.34828800
4.93573800
0.07859600
-2.05287100
-0.76574800
-0.67666300
-0.73543900
-0.08735300
0.07657200
0.22788900
-0.56188500
0.21621400
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=2
Figure 4.9c.
C
O
N
H
N
H
O
Ni
H
O
O
H
1.22622900
0.32081600
-1.02153600
2.63257000
2.49045500
3.42709400
-3.03500200
-1.19376300
-3.36695000
0.68449700
3.36652200
2.39816700
-620.068123
0.73729100 -0.00670100
1.80642100 0.01919000
1.44997900 0.00145100
1.99918400 0.03851600
0.98969800 0.00967500
-0.79707400 0.03125800
-0.86549300 -0.03561600
-0.35769400 -0.02680700
-0.65886100 0.83895700
-0.43699000 -0.04275500
-1.77574000 0.03426500
-1.85489400 0.00353100
Sum of electronic and zero-point Energies=
-620.134964
Number of Imaginary Frequencies=1
Figure 4.10a.
C
1.28372000
O
0.77513400
N
-0.83279900
H
3.14490000
N
2.68242300
O
-3.12296400
Ni
-1.22047900
H
-3.30299800
O
0.60760900
O
3.15887000
H
2.33261900
0.36211600 -0.05855400
1.51590400 0.06553200
1.44016300 0.03944000
0.93437400 0.41503000
0.26792300 -0.19951600
-0.43741700 0.02286800
-0.28288700 -0.04619700
-0.25129400 0.94520700
-0.71283000 -0.13363700
-1.03249500 0.20545400
-1.55683400 0.12340100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
-619.445090
84
Figure 4.10b.
C
1.07048800
O
1.87619200
N
-2.82397300
H
2.15024300
N
1.50094800
O
0.37911500
Ni
-1.28692600
H
0.81515100
O
-0.24553100
O
2.14338100
H
2.68153000
1.22872700
2.11851100
-0.22638400
-0.70207600
0.00940000
-0.93155500
-0.17631400
-1.65751900
1.37622300
-2.06472300
-2.54474400
0.01385300
-0.23583100
-0.41218900
-0.07420300
0.53803600
0.61282500
0.06326300
0.16492600
-0.13554800
-0.60101200
0.05039800
Sum of electronic and zero-point Energies=
-619.418264
Number of Imaginary Frequencies=2
Figure 4.10c.
C
-1.03104100
O
-2.01134700
N
3.16613400
H
-3.62230600
N
-1.20226600
O
0.14225200
Ni
1.63771900
H
0.07969200
O
0.21561600
O
-4.15372800
H
-3.41670700
-0.61339600
-1.38501600
-0.32873800
-0.35434900
0.73400000
1.31385500
0.12580200
2.19215700
-1.10288000
0.47130500
1.10516200
0.02962100
0.06616600
0.14334800
0.00499100
0.12515900
-0.06078000
-0.04638300
0.31662700
-0.09498200
-0.03992200
-0.04402100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.11a.
C
-1.43331000
O
-2.46176500
N
-1.54662400
H
-0.73200100
H
-3.22462500
N
-2.76397700
H
-3.35303500
O
3.06344400
Ni
1.40847700
H
3.69522900
-0.57440100
-1.22281500
0.82412800
1.34066500
0.72246100
1.44029300
1.49551600
0.79265900
-0.15263700
0.19496600
-619.431958
0.05632800
-0.17251000
0.27775200
-0.04021800
-0.71904600
-0.14895600
0.68451700
-0.13256200
-0.00766600
0.26915300
85
O
-0.23278600 -1.05538400 0.15266100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.11b.
C
1.67138600
O
2.85490100
N
0.81855100
H
0.95356300
H
-0.82075600
N
-0.74859600
H
0.06312900
O
-2.45950900
Ni
-0.66286500
H
-2.64313900
O
0.91578500
-0.23508800
-0.05748600
-1.23915400
-1.46292600
2.18524000
2.70981400
2.25077100
-0.75164800
-0.28575000
-1.55311300
0.52125300
-544.993594
-0.01004800
0.25548100
0.45123400
1.43577200
1.23917600
0.35181500
-0.09945700
0.25640100
-0.19427500
-0.23522800
-0.81958300
Sum of electronic and zero-point Energies=
-544.942875
Number of Imaginary Frequencies=2
Figure 4.11c.
C
O
N
H
H
N
H
O
Ni
H
O
-1.32900000
-2.27563200
-0.33787300
-0.28957900
-2.71759000
-2.21435400
-2.88588400
2.86966300
1.02069900
3.15880300
-0.59474800
0.05968900
0.50500300
1.04021700
1.73150100
0.52720100
-0.30173600
-1.00297700
0.20088300
-0.17470700
0.63677800
-1.02191300
-0.08359900
-0.95108900
0.35109000
-0.40713600
1.38973300
1.07425000
0.75874600
0.25633600
-0.04092500
-0.54625900
-0.49586800
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
Figure 4.12a.
C
1.20228000 -1.00064900
O
2.34234600 -1.34082300
N
0.66153000 0.33815300
H
0.81933100 1.07303700
-544.904115
-0.14041600
0.10842900
0.12692400
-0.56260100
86
H
N
H
O
Ni
H
O
O
H
2.00659000
1.12199600
1.33025200
-2.67560200
-1.06992100
-2.82520500
0.18564800
1.21941500
0.35400700
0.51053300
0.96965600
1.88828700
0.55203300
-0.38037800
1.17051000
-1.66911600
2.45273900
2.89877200
1.54305200
1.32602600
0.93468600
0.38811800
-0.05965500
-0.32803000
-0.60472400
-0.93044000
-0.93598600
Sum of electronic and zero-point Energies=
-620.736065
Number of imaginary frequencies=1
Figure 4.12b.
C
0.88637500 1.22595500 -0.01745800
O
1.99165100 1.71551000 -0.00849800
N
0.59656000 -0.23096000 0.03473600
H
1.05761700 -0.74606800 -0.76922600
H
0.45076000 -1.62838200 1.39209800
N
1.14750600 -0.89842700 1.20609900
H
1.97771500 -1.51529200 0.48205000
O
-2.54868000 -1.23192000 -0.00602000
Ni
-1.23352100 0.15195500 -0.06386700
H
-2.57693400 -1.61157200 -0.88512800
O
-0.29129900 1.77796500 -0.11590400
O
2.46372800 -1.88274000 -0.65093700
H
3.17952500 -1.23396800 -0.76173300
Sum of electronic and zero-point Energies=
Number of imaginary frequencies= 2
Figure 4.12c.
C
0.88245100 1.03228600
O
2.03898900 1.40571300
N
0.47536800 -0.41708700
H
0.75418100 -0.81002300
H
0.51667900 -2.09565200
N
0.90114300 -1.16285800
H
2.61812300 -1.41212200
O
-2.75976600 -1.09439300
Ni
-1.33230600 0.16404200
H
-2.92282100 -1.16293500
O
-0.22666400 1.67589400
O
3.22228400 -1.23539000
-620.733923
0.04788700
-0.02163800
-0.00890200
-0.91980100
0.89504600
1.09303700
0.20598900
-0.20277400
-0.03906800
-1.14430400
0.12298000
-0.55894600
87
H
3.20939900 -0.26113200 -0.53627800
Sum of electronic and zero-point Energies=
-620.745255
Number of Imaginary Frequencies= 1
Figure 4.13a.
C
-1.05145600
O
-2.03201400
N
-1.25927300
H
-0.59857500
H
-2.59701900
N
-2.60321200
O
3.38847000
Ni
1.73762200
H
3.13720800
O
0.18726700
O
-3.05519400
H
-3.15712200
0.82665700
1.53156600
-0.51796800
-1.14814600
-1.90944500
-0.92134600
-0.84863400
0.11055000
-1.74107200
1.16947800
-0.99596900
-0.03301700
-0.15465900
0.10256500
-0.62364400
-0.16008800
-0.82555400
-0.57442200
0.09637700
0.04379800
0.33759300
-0.05353100
0.82535900
0.96996400
Sum of electronic and zero-point Energies=
-620.159263
Number of Imaginary Frequencies= 1
Figure 4.13b.
C
0.90139800
O
1.75293600
N
1.31962600
H
0.56100200
H
2.67775400
N
2.52076000
O
-3.19803000
Ni
-1.71774300
H
-2.80602100
O
-0.36915000
O
3.22145900
H
4.11525200
1.23664100 -0.02634000
2.09188700 -0.15191200
-0.11823100 0.43495000
-0.81719800 0.41730300
-1.46271800 0.74953700
-0.41539300 0.71600300
-1.19682900 0.09359400
-0.00042700 -0.06600200
-2.06904500 0.15131600
1.30236700 -0.20174300
-1.92062400 -0.58611700
-1.53795900 -0.59929400
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=2
-620.122504
88
Figure 4.13c.
C
0.45338500
O
1.06372000
N
1.30628900
H
0.64645600
H
3.43696500
N
2.51444900
O
-2.84091100
Ni
-1.72880600
H
-2.23087000
O
-0.79682800
O
4.09381200
H
4.93020100
1.62193200
2.66592200
0.29921100
-0.53745400
-1.44664900
0.23624500
-1.81938300
-0.26656300
-2.55803800
1.36617000
-2.18429600
-1.70119900
0.00034500
0.00334700
-0.00983600
-0.01105300
-0.01457500
-0.01471100
-0.00510000
0.00337600
-0.00675700
0.00380000
0.00245500
0.07159100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
Figure 4.14a.
C
1.87237300
O
3.10232700
N
1.15502300
H
1.68057300
N
-0.15046500
O
-2.59182200
Ni
-0.71911900
H
-2.81104200
O
1.09370600
O
-0.99680800
H
-1.85957300
-0.12034900
-0.11537100
1.10443100
1.86216600
0.98959200
-0.31285700
-0.68791800
-0.31614500
-1.17198500
1.89219600
1.44374200
-620.167276
-0.06347700
-0.06692000
-0.07318500
0.35722700
0.50297200
0.01764400
0.03788800
-0.91493100
-0.07935100
-0.25532200
-0.05923000
Sum of electronic and zero-point Energies=
-619.562461
Number of Imaginary Frequencies=1
Figure 4.14b.
C
1.47486200
O
2.58927600
N
1.07900000
H
1.82346400
N
-0.13958400
O
-2.98581800
Ni
-1.14870600
H
-3.04988400
O
0.43474400
-0.66919400
-1.11176000
0.61611900
1.17489400
0.87589900
0.26133800
-0.25355200
1.07419400
-1.18296600
0.06258000
0.23345300
0.72037600
1.13909100
0.94022500
-0.00660300
-0.09609000
-0.50951900
-0.50016200
89
O
H
2.05541500 1.58250900 -0.83761800
1.21617500 2.02845200 -1.05129600
Sum of electronic and zero-point Energies=
-619.519978
Number of Imaginary Frequencies=2
Figure 4.14c.
C
-0.97219500
O
-1.86348000
N
-1.34638900
H
-2.89407200
N
-0.38306600
O
2.62666500
Ni
1.26491900
H
2.57281700
O
0.30860600
O
-2.96390500
H
-2.02021400
-1.46887400
-2.29451100
0.00638600
1.46493500
0.76807900
1.30733700
-0.03170300
1.74824200
-1.65025500
2.43822500
2.66010900
0.01016600
0.04041400
-0.01146900
0.01137300
-0.03326900
-0.04417700
-0.01733200
0.80462600
-0.00497000
0.00474700
-0.04664700
Sum of electronic and zero-point Energies=
-619.603755
Number of Imaginary Frequencies=1
Figure 4.15a.
C
1.85361000 -0.69527000 0.00091400
O
2.69658600 -1.58546300 0.05749100
N
-2.19381500 -0.69743100 -0.01696000
N
-3.03606200 -1.43428000 0.02505900
O
0.06399500 2.18771100 -0.03204300
Ni
-0.74856200 0.45923400 -0.01727400
H
0.13435100 2.48004500 0.87754700
O
0.55852900 -0.89189500 0.00180700
O
2.28225800 0.60559500 -0.07008500
H
1.50191500 1.22742000 -0.11339900
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.15b.
C
2.07737000 -0.45788100
O
3.08608700 -1.14307300
N
-2.37830500 -0.55203600
N
-3.43377200 -0.92670700
O
-0.33935300 1.97861600
Ni
-0.68252400 0.10037600
-619.102835
0.01378600
0.13593200
0.09565100
0.19530400
-0.12792400
-0.07744500
90
H
O
O
H
-0.53759300
0.88501100
2.18771100
1.31294000
2.38683300
-0.91719300
0.90001700
1.35420300
0.71577200
-0.28187000
0.18733400
0.02549500
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=2
Figure 4.15c.
C
-1.51305700
O
-2.61158000
N
-1.01175700
N
-1.48718000
O
3.02054600
Ni
1.25514200
H
2.99958700
O
-0.44890900
O
-1.35425600
H
-0.41907400
-0.99905100
-1.40528200
2.42502400
2.67041300
0.28074300
-0.32156200
0.47718500
-0.90362500
-0.57776200
-0.29979600
0.02089600
-0.32088400
0.37036200
-0.59690900
0.22431900
-0.18638400
1.16172800
-0.72657900
1.33634100
1.41190700
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
Figure 4.16a.
C
1.95169500
O
3.05976200
O
-2.76653900
Ni
-0.90896200
H
-2.77721800
O
0.88502900
O
1.77194400
H
0.83480500
O
-0.78521400
H
-1.63667100
-0.14060800
-0.64582300
-0.39470300
-0.30726900
-0.26993600
-0.78408700
1.17261900
1.43149500
1.46528700
1.77926800
-0.00963800
-0.19572800
-0.23536600
-0.42748000
-619.094919
0.04005900
0.15361100
0.34836600
-0.08737100
1.29814700
-0.38509100
0.35679900
0.15065600
-0.37543900
-0.02875300
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
Figure 4.16b.
C
1.79641700
O
3.01358100
O
-2.54966500
Ni
-0.76634400
-619.096785
-585.380718
0.13052900
0.16389200
0.55097800
-0.10549700
91
H
O
O
H
O
H
-2.45825200
1.00250900
1.14975300
0.06349600
-0.78610100
-1.56673800
-0.21433500
-0.83668200
0.94479900
1.51776400
1.45640600
1.65641300
1.50437500
-0.59346600
0.76174000
-0.14201400
-0.75672100
-0.20300100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=2
Figure 4.16c.
C
1.74801200
O
2.80315800
O
-2.19391200
Ni
-0.29431100
H
-2.38395800
O
1.54657900
O
0.54518500
H
-1.26952200
O
-2.22596700
H
-2.39422600
0.34345300
0.94982800
-0.81692000
-0.72541800
-0.87326100
-0.90880200
0.83864600
1.99212100
1.95512400
0.98911600
-585.354161
0.03470600
0.08126800
0.14615900
-0.03060100
1.08342800
-0.36510600
0.39620700
-0.10830200
-0.27659400
-0.18200300
Sum of electronic and zero-point Energies=
-585.389147
Number of Imaginary Frequencies= 1
Figure 4.17a.
C
-1.57006600
O
-2.78384100
O
2.60967800
Ni
0.72906000
H
2.87080600
O
-0.88717500
O
-0.67167200
-0.05627200
-0.18973200
-0.20197200
0.12738300
-0.37518500
1.08717500
-1.05221100
0.00681100
0.02369600
-0.07037200
-0.01651300
0.83493100
0.01487600
-0.01987800
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
-508.978193
92
Figure 4.17b.
C
-1.83991200
O
-2.40824700
O
2.84993300
Ni
1.03944500
H
3.50864600
O
-0.73053400
O
-2.40785800
0.00009500
-1.06644000
0.00044600
-0.00029500
0.00181000
-0.00034400
1.06707400
0.02852300
-0.32254400
-0.50413500
0.10433300
0.19146500
0.73803200
-0.32184500
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=2
Figure 4.17c.
C
-2.08348900 -0.53288100
O
-3.18104700 -0.15537300
O
2.30042100 -1.06685600
Ni
0.95452600 0.28833100
H
1.93702600 -1.95309600
O
-0.16030000 1.80210800
O
-0.97942600 -0.94523900
-508.809218
0.00291700
-0.01856600
-0.01230200
-0.00066100
0.00460900
0.00571400
0.02470100
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=1
-508.806195
93
APPENDIX B:
SUPPORTING INFORMATION FOR CHEMICAL OXIDATION OF UREA
Figure 5.1a.
Cartesian Co-ordinates
C
O
N
H
H
H
N
H
O
Ni
H
O
1.79999000
2.47628600
1.99579300
1.17481100
2.76085800
0.51542400
0.57107600
0.62807100
-2.00042900
-1.10904300
-1.53119700
-0.63370500
-0.27177200
-1.24210400
1.02149400
1.64682200
1.22065900
-1.43418000
-0.44743500
0.11146100
-1.32354100
0.16156700
-2.12569400
1.77430300
0.01907400
-0.24614600
-0.29783000
-0.26584000
-0.92750300
1.09125100
0.83845100
1.69279800
-0.34514300
-0.01248300
-0.61949500
0.02623100
Sum of electronic and zero-point Energies=
-545.478109
Number of Imaginary Frequencies= 0
Figure 5.1b.
C
-1.79914400
O
-0.69068500
N
-2.94461500
H
-3.80994000
H
-2.81538600
H
-1.01264300
N
-1.90334600
H
-2.77604700
O
2.59110700
Ni
1.11318100
H
3.02872500
O
0.71793200
-0.20186200
-0.79472300
-0.93478100
-0.58804200
-1.93683600
1.64306800
1.12072100
1.60150300
-0.93860000
0.02580200
-1.16549900
1.68744200
-0.00811300
0.08449100
0.13396200
-0.25565400
0.10789800
-0.16896100
-0.23359600
-0.07094700
-0.17292700
-0.02517600
0.66263400
0.23544500
Sum of electronic and zero point Energies= -545.482146
Number of Imaginary Frequencies= 0
94
Figure 5.1c.
C
O
N
H
H
H
N
H
O
Ni
H
O
1.87733800
3.05747600
-2.18224100
-2.27774300
-2.33964100
1.30893300
1.00353700
-2.87274100
0.98987200
-0.60526100
1.33476600
-1.69976800
0.03577100 -0.01829500
-0.11991500 0.17401700
-1.08778200 0.05514600
-1.68407000 0.87698900
-1.65468600 -0.77825000
1.98016200 -0.02933900
1.02227400 -0.13759700
-0.33059300 0.09564000
-1.15452300 -0.17801600
0.09471300 -0.01894200
-1.86541000 0.38892400
1.41775900 0.08691800
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 0
Figure 5.1d.
C
O
N
H
H
H
N
H
O
Ni
H
O
1.41709000
1.83677600
0.39217400
0.52026600
0.42578200
3.22910400
2.47127100
2.15179700
-2.75475400
-1.03883900
-3.16555000
0.59040700
-0.01699400
-0.71645200
1.04688300
1.94794500
1.17902500
-0.08598300
0.58917500
0.80659800
0.19854100
-0.13894400
0.21392000
-0.92227900
0.08075200
1.13680100
0.49994200
0.03881700
1.51094400
-0.75036700
-0.67591300
-1.61984000
0.01374500
-0.13879000
0.89076100
-0.58015900
Sum of electronic and zero-point energies=
Number of Imaginary Frequencies= 0
-545.492283
95
Figure 5.1e.
C
O
N
H
H
H
N
H
O
Ni
H
O
1.71291800
2.62526100
1.94385400
0.60634200
1.81155000
0.33158300
0.44408500
2.90604400
-0.40794400
-1.15224800
-0.58575300
-2.19230600
-0.44840700
-1.11562200
0.94163000
1.72396200
1.05630900
-1.84568400
-0.85921000
1.17816700
1.85223100
0.05584300
2.25804500
-1.21422200
0.01029800
-0.45310900
0.38932100
-0.12224000
1.39489600
0.04647300
0.26100300
0.15586200
-0.22389400
-0.00267500
-1.08795400
0.06122900
Sum of electronic and zero-point Energies=
-545.473262
Number of Imaginary Frequencies= 0
Figure 5.2a.
C
-1.82316000
O
-2.55328600
N
-1.97383000
H
-1.12406800
H
-2.72889600
H
-0.59547900
N
-0.60348500
H
-0.59210900
O
0.67900800
Ni
1.16539200
H
1.30277400
O
1.88514900
-0.24239000
-1.13695200
1.09072400
1.67220000
1.37104700
-1.56764400
-0.56182000
-0.06419400
1.75585200
0.02173500
2.39748300
-1.45208300
0.01975200
-0.35074500
-0.14353600
-0.09544600
-0.75445700
0.95055800
0.78547000
1.67783600
0.08121300
-0.05718900
-0.28939300
-0.29295200
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 0
-545.482798
96
Figure 5.2b
C
O
N
H
H
H
N
H
O
Ni
H
O
1.41709000
1.83677600
0.39217400
0.52026600
0.42578200
3.22910400
2.47127100
2.15179700
-2.75475400
-1.03883900
-3.16555000
0.59040700
-0.01699400
-0.71645200
1.04688300
1.94794500
1.17902500
-0.08598300
0.58917500
0.80659800
0.19854100
-0.13894400
0.21392000
-0.92227900
0.08075200
1.13680100
0.49994200
0.03881700
1.51094400
-0.75036700
-0.67591300
-1.61984000
0.01374500
-0.13879000
0.89076100
-0.58015900
Sum of electronic and zero-point Energies=
-545.458736
Number of Imaginary Frequencies= 0
Figure 5.2c
C
O
N
H
H
H
N
H
O
Ni
H
O
1.80583200
2.99247700
1.06771100
-1.99019300
1.29348200
-1.88257900
-1.98112700
-2.86411400
-1.75674700
-0.51757500
-1.35873700
0.87091400
-0.10139700
-0.25106700
1.07737400
-1.86640800
1.78728400
-1.93002200
-1.29915900
-0.79402500
1.34830200
0.09400300
2.22838400
-1.08428800
-0.02411800
0.13320400
-0.19009400
-0.80621600
0.50805800
0.83565300
0.04096500
0.11557100
0.06994200
-0.01451300
-0.03206100
-0.08139800
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 0
-545.538248
97
Figure 5.3a
C
O
N
H
H
H
N
H
O
Ni
H
O
1.42994700
2.22621200
0.26590400
0.93513500
0.13619100
2.90432300
1.94848300
1.88734900
-2.61170700
-1.00679800
-2.75577800
0.51083600
-0.20637300
-0.80468300
0.76317100
1.55074100
0.70011700
1.27488800
1.05870700
0.93076800
0.30015600
-0.21065100
1.02453200
-0.88269000
0.15512900
0.86427400
0.77391800
0.02319100
1.78818100
-0.40529800
-0.68477700
-1.69438200
0.21530900
-0.17878900
0.84312400
-0.71752000
Sum of electronic and zero-point Energies=
-545.458333
Number of Imaginary Frequencies= 1
Figure 5.3b
C
O
N
H
H
H
N
H
O
Ni
H
O
1.42994700
2.22621200
0.26590400
0.93513500
0.13619100
2.90432300
1.94848300
1.88734900
-2.61170700
-1.00679800
-2.75577800
0.51083600
-0.20637300
-0.80468300
0.76317100
1.55074100
0.70011700
1.27488800
1.05870700
0.93076800
0.30015600
-0.21065100
1.02453200
-0.88269000
0.15512900
0.86427400
0.77391800
0.02319100
1.78818100
-0.40529800
-0.68477700
-1.69438200
0.21530900
-0.17878900
0.84312400
-0.71752000
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 1
-545.419332
98
Figure 5.5
Ni
O
H
O
O
H
-0.05202400
1.75106900
2.08459300
-1.69139300
-0.01596700
-0.97759800
-0.25123300
-0.44884600
0.46194700
-0.50479200
1.56139300
1.71055100
-0.00064900
0.00038900
0.00616200
0.00123000
-0.00045500
0.00271200
Sum of electronic and zero-point Energies=
-396.138570
Sum of Imaginary frequencies= 0
Figure 5.6a.
Ni
O
H
O
O
H
H
N
C
H
O
N
0.90498000
0.39570700
1.09816700
1.27548100
2.53639900
2.85236700
-0.81141500
-0.96630800
-2.08702600
-0.96213900
-2.83660300
-2.16520100
-0.06165300
1.73806900
2.19952300
-1.74192400
0.39394300
-0.51901000
-1.60335800
-0.59524000
-0.16361900
-0.20557000
-0.96115600
1.19565200
0.07464000
0.28008400
-0.20057000
-0.00175800
-0.55680600
-0.68349500
0.72194200
0.74210900
-0.05256800
1.68171900
-0.60019600
-0.09250800
H
H
-1.22145600 1.67176100 -0.02410500
-2.82010200 1.53032900 -0.78781200
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies=0
-621.369304
99
Figure 5.6b.
Ni
O
H
O
O
H
H
N
C
H
O
N
H
H
0.92295100
1.39352400
2.36208900
0.53345700
2.70598700
2.64403800
-0.45139900
-0.91081200
-2.06835900
-0.95967000
-2.21020700
-3.25590600
-3.19725600
-4.04533000
-0.03617000
-1.76795800
-1.67892600
1.69433000
0.38481100
1.35359800
1.74977300
-0.63003800
0.02977300
-1.63836300
1.27173100
-0.76759000
-1.62560900
-0.20627500
0.00837800
-0.02652200
-0.04308500
0.00480500
0.00399100
-0.01291100
-0.00781700
0.03209900
-0.01341700
-0.06357300
-0.01610800
-0.09154100
0.44898100
0.21107900
Sum of electronic and zero-point Energies=
Number of Imaginary Frequencies= 0
Figure 5.6c.
Ni
O
H
O
O
H
H
N
C
H
O
N
H
H
0.92295100
1.39352400
2.36208900
0.53345700
2.70598700
2.64403800
-0.45139900
-0.91081200
-2.06835900
-0.95967000
-2.21020700
-3.25590600
-3.19725600
-4.04533000
-0.03617000
-1.76795800
-1.67892600
1.69433000
0.38481100
1.35359800
1.74977300
-0.63003800
0.02977300
-1.63836300
1.27173100
-0.76759000
-1.62560900
-0.20627500
0.00837800
-0.02652200
-0.04308500
0.00480500
0.00399100
-0.01291100
-0.00781700
0.03209900
-0.01341700
-0.06357300
-0.01610800
-0.09154100
0.44898100
0.21107900
Sum of electronic and zero-point Energies=
Number of imaginary frequencies= 0
-621.403613
-621.403613
100
Figure 5.7a
Ni
O
H
O
O
H
H
N
C
H
O
N
H
H
0.83634700
0.60221200
1.45141100
1.08594600
2.53840800
2.85374500
0.22323800
-0.95152800
-2.05826300
-0.99293300
-3.07267200
-1.92630000
-1.05107600
-2.63888500
-0.07634100
1.68834200
2.02002800
-1.80350200
0.34014500
-0.52784100
-2.14191100
-0.56450300
-0.07983900
-0.11575600
-0.74841000
1.18710800
1.69437000
1.51686500
0.11034500
0.44563300
0.10458000
-0.25112100
-0.48601700
-0.78337000
0.05900000
0.74511300
-0.01519700
1.66394400
-0.21582500
-0.52384700
-0.37504200
-1.15781800
Sum of electronic and zero-point Energies=
-621.387529
Number of Imaginary Frequencies= 1
Figure 5.7b.
Ni
O
H
O
O
H
H
N
C
H
O
N
H
H
-0.93043700
-1.26439500
-2.23848300
-0.73286400
-2.74834800
-2.77467400
0.21258900
0.98126500
1.91307700
1.88717500
2.34208300
3.18627700
3.77160800
3.77095200
-0.06461400
-1.81582100
-1.79526700
1.71621700
0.19486200
1.16558100
1.94261500
-0.45667800
0.40346300
-1.35368000
1.52997900
-1.12884200
-1.23666800
-1.23743100
-0.00014300
0.00026000
0.00061900
-0.00039500
0.00034600
0.00031100
-0.00036300
-0.00037500
0.00013700
-0.00058600
0.00060400
-0.00011100
-0.82965500
0.82973600
Sum of electronic and zero-point Energies=
Number of imaginary frequencies= 1
-621.318147