Insights on Hydrogen Liberation From Water Using
Anionic Transition Metal Oxide Clusters
Raghunath O. Ramabhadran
Indiana University, Bloomington
1
Research Overview
Collaborative study of metal-oxide clusters
Electronic structure &
Reaction mechanisms
Anion PES &
Mass spectrometry
Recent focus on Anionic Mo/W oxide clusters &
their reactivity with small molecules
2
An experimental conundrum
Mo3O4 + D2O → Mo3O5 + D2
Mo3O5 + D2O → Mo3O6D2
Ion Intensity (arbitrary units)
Mo3O6 + D2O → NO PRODUCT
Mo3O7,8 + D2O → Mo3O8,9D2
Mo3O9 + D2O → NO significant product
W3O4 + D2O → W3O5 + D2
W3O5 + D2O → W3O6 + D2
W3O6 + D2O → W3O7D2
Mass/charge (amu/e)
W3O7 + D2O → W3O8D2
3
The key questions
• Why does Mo3O5− form a kinetic-trap?
• What is the structure of the kinetic-trap?
• Why does Mo3O5− not get oxidized to liberate H2?
• Why does W3O5− get oxidized to liberate H2?
• Is the oxidized product the lowest energy isomer of W3O6− ?
• Why does the W3O5− not form a kinetic trap?
Overall, are there new insights on H2 liberation from
cluster models of these Transition Metal Oxides?
4
Our approach
a)
Start with lowest energy isomers of the clusters
b)
Map out the initially formed electrostatic complexes
c)
Study the reaction pathway starting with each of the
complexes
d)
Determine the processes that are kinetically or freeenergetically favorable
e)
Identify the salient structural features responsible for the
reactivity pattern
f)
Account for experimental observation
g)
Insights from processes happening and not happening
5
Computational methods
• Geometries: B3LYP
• Stuttgart-Dresden relativistic pseudopotentials for Mo, W
• Augmented Double zeta and Triple zeta basis sets – added diffuse
and polarization functions
• cc-pVTZ basis sets for first-row and second-row elements
• B3LYP, M06 and other density functionals
• Vibrational analyses to confirm minima and transition states
• Intrinsic Reaction Coordinate calculations to confirm reaction paths
• Carefully study different spin-states
• Reaction paths from initial complexes to rearranged pathways
6
Lowest energy states of M3O5− and M3O6−
•
Ground state – doublet electronic
states
•
2 bridging and 3 terminal oxygens
(M3O5−)
•
3 bridging and 3 terminal oxygens
(M3O6−)
• Legend: Gray - Mo
Purple - W
7
Electrostatic complexes: Free-energy window
Free-Energy
scale
(kcal/mol)
(0.4%)
(0.3%)
(3.3%)
(2.4%)
(3.6%)
(3.5%)
(6.4%)
(3.6%)
(12.1%)
(8.5%)
(15.5%)
Mo-oxide
(11.7%)
(26.6%)
(31.0%)
(32.1%)
(39.0%)
W-Oxide
8
C2 provides access to higher energy complex C8
Free-Energy
scale
(kcal/mol)
9
Kinetic-trap formation with Mo-oxide:
Possibilities
• Dihydrides (M−H) and hydride-hydroxides (M−H & M−OH)
– ruled out due to high computed ADE/VDEs (3 – 3.7 eV)
• Dihydroxides: several possibilities (1.9 – 2.4 eV)
(kcal/mol)
10
DH3 can be obtained from C5
Free-Energy
scale
(kcal/mol)
3 kcal/mol barrier
in step 2
11
DH4 can be obtained from C2 via C8
Step 2
Free-Energy
scale
(kcal/mol)
Step 1
Step 3
6 kcal/mol barrier in step 3
12
Kinetics vs. Thermodynamics :
DH4 is the kinetic-trap
• DH3 = Kinetically more favored
• DH4 = Thermodynamically favored but kinetically not disfavored greatly
• Therefore, DH4 is likely.
• Most stable dihydroxide
• Cs symmetry
• Has the same metal-oxide framework as Mo3O6−
13
Franck-Condon simulations (PESCAL) of DH4
Simulation Parameters
Anion (cm-1)
Neutral (cm-1)
ΔQ (Å·amu1/2)
285
287
-0.71
626
700
-0.30
357
380
-0.24
484
466
0.26
701
734
-0.22
925
994
-0.13
480
502
0.15
214
219
-0.19
• No changes to calculated
frequencies or displacements
• 10 active vibrational modes
predicted
• Two lowest frequency modes
removed (106/116 and
103/106)
•
•
•
•
Anion T, 500 K (top), 300 K (bottom)
Onset 1.940 eV
0.007 eV Gaussian width1414
8 modes
14
Experimental observation:
H2 liberation with W3O5−
• Electrostatic requirement for most facile H2 liberation:
Protonic hydrogen + Hydridic hydrogen
i.e., Metal hydroxide + Metal hydride
• Previous experience with other systems (e.g., Al oxides, etc.) has
shown that hydrogen migrations to reach the hydride can involve
high activation barriers
• Are such barriers accessible for W?
• Are such barriers not accessible for Mo?
15
H2 liberation for W3O5− : Key steps shown
Free-Energy
scale
(kcal/mol)
16
More complex H2 liberation pathways for W3O5−
Free-Energy
scale
(kcal/mol)
17
Processes not happening:
Key steps for getting H2 from the Mo-oxide
Free-Energy
scale
(kcal/mol)
18
Comparison between Mo & W oxide clusters:
Key steps for getting H2
Mo-oxide
W-Oxide
Y-axes in terms of free-energies
(kcal/mol)
19
Answers to the questions: The Mo-oxide part
1.
Why does Mo3O5− form a kinetic-trap?
Ans: Because, once the stable kinetic-trap is formed it has to
cross over a barrier. So, it reacts no further.
2. What is the structure of the kinetic-trap?
Ans: DH4
3. Why does Mo3O5− not get oxidized to liberate H2?
Ans: Same as the answer for question 1.
20
Answers to the questions: The W-oxide part
1.
Why does W3O5− get oxidized to liberate H2?
Ans: Because, once the right geometry or electrostatics are
obtained, there are no barriers for hydrogen liberation
2.
Is the oxidized product the lowest energy isomer of W3O6− ?
Ans: The most favorable pathway results in the lowest energy
isomer
3.
Why does W3O5− not form a kinetic trap?
Ans: The kinetic-trap formed readily reacts further with no barrier
to result in hydrogen liberation
21
Key factors in H2 generation from TMOs
• Metal-Oxygen bond strengths:
Stronger W−O bond resulted in oxidation and the weaker Mo−O
bond resulted in a kinetic-trap.
• Electrostatic and geometric factors:
Recognize the important electrostatic requirements and facial
selectivity aspects – can result in disparate oxidized products.
• Insights from processes that do not occur:
A full picture emerges only after processes that do not occur are
accounted for as well, not just the processes experimentally
observed
22
Future directions
• Further computational and experimental work on the reactions of
sulfides
• Compare reactivity of oxides vs. sulfides
•
Understanding other modes for H2 elimination
•
How much excess energy does it take to make further reactions
possible?
•
Understanding the progression from suboxide to hyperoxide
•
Catalyst regeneration: How can we reduce the metal oxides with
other reagents?
•
Theoretical studies of larger clusters difficult to study
experimentally
23
Acknowledgments
• Research Advisor: Prof. Krishnan Raghavachari
Arjun Saha, Manisha Ray, Benjamin Gamoke, Vicki Erdely,
Arka Sengupta, Jovan Jose & Alex Krukau
• Collaborator: Prof. Caroline Jarrold
Jenny Mann
Sarah Waller
David Rothgeb
• Funding
DOE-CPIMS
• Thank you for listening!