COMPUTATIONAL STUDIES OF THE
PROPERTIES OF BARE AND ZEOLITE
SUPPORTED GOLD CLUSTERS
Abstract
With its alluring golden yellow hue, gold has fascinated mankind since
ages, mostly for its widespread use in jewellery. In addition, it finds
application in numerous fields ranging from investment and monetary
exchange, electronics, dentistry, medical and chemical field, aerospace
industry and as a symbol of status. Gold possess excellent properties that it does not tarnish on exposure to atmosphere and retains
its beautiful lustre undiminished for years. Its resistance to oxidation
and corrosion in air as well as its chemical inertness towards various
reactions contributes largely to the noble behaviour of gold.
However, the nobility diminishes when bulk gold is reduced to the
nano dimension, thereby making it one of the excellent catalysts for
reactions such as low temperature CO oxidation, direct formation of
hydrogen peroxide etc. This makes the study of gold particularly interesting. The use of nanogold dates back to ancient ages and the most
famous example is the Lycurgus Cup which was manufactured in the
5th to 4th century B.C and is now exhibited in the British Museum.
In 1857, the legendary work of Michael Faraday on the formation of
deep red solutions of colloidal gold by reduction of an aqueous solution
of chloroaurate AuCl−
4 using phosphorus was the first scientific investigation on finely divided gold. Since, then synthesis and application
of gold nanoparticle in fields of catalysis, biological labelling, optical
and electronic devices etc have been steadily growing.
Abstract
The earliest report on the use of gold as a catalyst dates back to 1906
when Au gauzes were reported as catalysts for H2 oxidation. The work
of Bone and Andrew in 1925 is one of the earliest reference where Au
had been reported as catalyst for CO oxidation. Years later, Huber et
al. in 1977 reported CO oxidation on Au atom based model and attempted to interrelate the results with actual heterogeneous oxidations
of CO to CO2 . In the subsequent years, pioneering work of Haruta et
al. demonstrated the catalytic activity of transition metal oxide supported ultrafine gold towards low temperature CO oxidation. This
opened up new dimension towards catalytic activity of supported gold
clusters. The catalytic activity has been attributed to various physicochemical parameters such as cluster size of gold particles, morphology,
site-specificity and electronic state of gold clusters. Small gold particles
differ significantly from the bulk due to the presence of low coordination atoms and the adoption of geometries which lead to more reactive
electronic structures. Thus, catalytic properties of gold nanoparticles
can be attributed partly to their geometric structures. Modification
of the electronic properties by the support has been considered as an
important factor in supported gold clusters.
The concept of single atom catalysis has recently gained impetus in
the field of contemporary catalytic research so as to maximize the
efficiency of metal utilization. It is believed that catalysts with welldefined single active centres are necessary to understand the catalytic
mechanisms better unlike the multiple active sites of subnanoclusters
which are not always the most desirable centres for catalytic processes.
Single atom catalysts have evolved as an effective way to utilize each
and every metal atom of supported metal clusters.
In this thesis, we have employed density functional theory to explore
the catalytic activity of neutral and charged gas phase gold clusters
towards CO oxidation. Further, the catalytic activity of zeolite supii
Abstract
ported neutral and charged gold monomer towards CO oxidation and
water gas shift reaction has been studied with hybrid DFT method.
The adsorption of CO and O2 on zeolite supported Au and the effect
of the presence of moisture on their adsorption has been investigated.
We also explore the structure and electronic properties of gas phase hydrogenated gold clusters with the aid of density functional theory. The
reactivity properties have been investigated with DFT based global reactivity descriptors. The structure of zeolite supported hydrogenated
gold and palladium clusters obtained due to reverse hydrogen spillover
from bridging OH groups of the zeolite to the metal clusters has been
studied.
The thesis consists of seven (7) chapters:
Chapter 1 gives a brief introduction about clusters with a special
emphasis on gold clusters and its unique properties such as presence of
aurophilic interactions and large relativistic effect. Gold clusters undergo substantial structural transition with increase in cluster size such
that the small clusters are planar while larger clusters exhibit compact
core-shell structure. The size and the charge state of the host gold
cluster and presence of specific low coordination binding sites have
been identified as some factors that on which the adsorption of O2
and CO depends. On oxide supported gold clusters, O2 preferentially
adsorbs on the metal-support interface rather than the gold particles
and with sufficient charge transfer, activation of O2 to superoxo state
O−
2 is observed. Gold clusters as small as a single Au atom is an
excellent catalyst towards carbon monoxide oxidation. However, the
inevitable presence of moisture in practical catalytic systems can affect
the adsorption behaviour of CO and O2 and consequently the catalytic
activity towards CO oxidation. One of the emerging trends in the field
of catalytic research is the concept of single atom catalysis. Single Au
atoms supported on zeolites, oxides, BN-monolayer exhibit potential
as catalysts for CO oxidation, water gas shift reaction etc. We have
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Abstract
included these aspects in the introductory chapter of the thesis. The
objectives of the present work are included in this chapter.
Chapter 2 describes the computational tools like molecular mechanics (MM), quantum mechanics (QM), density functional theory (DFT),
and various basis sets and functionals used within the realm of DFT.
Hybrid quantum mechanics/molecular mechanics (QM/MM) methods
are discussed in detail. We have also included the concepts of potential
energy surface, electron correlation etc in this chapter. DFT, hybrid
QM/MM methods have been utilized in our calculations.
Chapter 3 is divided into two sections: Section 3.1 presents the
results of the DFT investigation of the structures, electronic and reactivity properties of Au6 Hn (n = 1 − 12) clusters. In this study, we have
calculated different parameters like bond lengths, average Hirshfeld
charges, electronic properties like HOMO-LUMO gap, chemical hardness, vertical ionization potential, adiabatic ionization potential and
binding energy. DFT based reactivity descriptors have been used to
calculate the reactivity properties of the hydrogenated clusters. Section 3.2 summarizes the results of reverse hydrogen spillover from
bridging OH groups of the faujasite support to Pd6 and Au6 clusters
using DFT. We utilize the distorted octahedral structures for Pd6
and Au6 and the geometries of the model clusters are optimized without imposing symmetry constraints. To investigate the reactivity of
the metal clusters with the OH groups of the zeolite fragment, we
have calculated the energy of reverse hydrogen spillover from bridging
OH groups to the metal cluster, M6 and Hirshfeld population analysis
(HPA) has been used to characterize the electronic charge distribution.
Chapter 4 investigates the catalytic activity of neutral and charged
gold hexamer, Au6 cluster towards CO oxidation which elucidates the
effect of cluster charge state on the catalytic activity. We also present
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Abstract
the results of CO and O2 adsorption on neutral, anionic and cationic
Au6 clusters. For our study, we have considered the conventional
bimolecular Langmuir-Hinshelwood mechanism with co-adsorbed CO
and O2 at the neighbouring sites in all the clusters.
Chapter 5 presents the results of hybrid quantum mechanics molecular mechanics calculations on the adsorption of small molecules such
as CO and O2 on faujasite supported gold monomer in three oxidation
states viz. 0, +1 and +3. We have considered three different modes
viz. top, bridge and dissociative for O2 adsorption. The parameters
such as bond length, binding energy, amount of charge transferred
to O2 and CO are calculated. These results are summarized in Section 5.1. We have also considered the adsorption of CO and O2 on
Aun /FAU (n = 0, +1and + 3) in the presence of pre-adsorbed water
using QM/MM method and the results are summarized in Section
5.2. The effect of the pre-adsorbed H2 O on CO and O2 adsorption
accompanied by the changes in binding energies, mode of adsorption
and structural changes encountered are presented. We have also investigated the process of proton sharing between H2 O and O2 and consequently forming a hydroperoxyl-hydroxyl species on the supported Au
monomer. The barrier involved in H2 O dissociation is calculated.
Chapter 6 discusses the catalytic activity of faujasite supported gold
monomer towards reactions like CO oxidation, water-gas shift reaction using hybrid quantum mechanics molecular mechanics method.
Section 6.1 focuses on the activity of faujasite supported neutral
(Au0 ) and cationic (Au+ andAu3+ ) gold monomer towards CO oxidation (2CO + O2 → 2CO2 ).
We have considered different CO-
O2 co-adsorption configurations and their interaction energies have
been determined. Based on that different oxidation pathways have
been appraised for this study and the catalytic activity of neutral and
cationic Au monomer has been compared. The results of hybrid quanv
Abstract
tum mechanics molecular mechanics calculations on the activity of
neutral and cationic gold monomer towards water-gas shift reaction
(CO+H2 O → CO2 +H2 ) are summarized in Section 6.2. We have initially considered two different CO − H2 O co-adsorption configurations
for all the oxidation states viz. Au0 /FAU, Au+ /FAU and Au3+ /FAU.
The parameters such as bond length, binding energy of CO/H2 O in
presence of pre-adsorbed H2 O/CO have been calculated. The reaction
mechanism has been investigated with the more favourable co-adsorbed
configuration and the catalytic activity of the different Au oxidation
states has been compared.
Chapter 7 recapitulate the salient observations of the entire work
presented in the thesis and presents future prospects to refine and
reinforce the research work.
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