Theoretical Study On Several Important Reactions Catalyzed By Gold-based Catalysts

Catalysis by gold has attracted significant attention in the last decades, since Haruta's discovery that these catalysts have ultrahigh catalytic activity in the low temperature CO oxidation and that the activity is even higher in humid atmosphere. This discovery changes the traditional concept that gold does not have the catalytic activity, so a research fever is raised all over the world on the nano-gold and its alloy clusters. The studies on their structures and properties are the foundation on which the studies on the chemical and physical properties of gold and its alloy clusters are based and have very significant scientific meanings and application values for conducting related experimental and theoretical studies.In this dissertation, we studied the interaction and reaction of molecules with Au-based catalysts with density functional theory (DFT) calculations. Our purposes are to (a) shed light on the mechanistic details of the Au-based catalysts and hence obtain a better interplay between theory and experiment, (b) understand the intrinsic catalytic activity of gold-based catalysts, (c) provide a general profile of the catalytic reaction by gold-based catalysts. Our results provide detailed information on the transition states of gold catalyzed reactions and on the behavior of small molecules adsorption. This should be helpful for the designing the new efficient gold-based catalysts.The valuable results in this dissertation can be summarized as follows:1. The research history and current state on gold-based catalysts have been briefly reviewd. A number of important chemical reactions at low temperature catalyzed by gold-based catalysts are reported. Different explanations have been proposed to account for the apparently high catalytic activity of gold-based catalysts. Moreover, the theory of quantum chemistry and the calculation methods of this paper are summarized. The contents of these reports were the basis and background of our studies and offer us with useful and reliable quantum methods.2. While nanoscale gold particles show exceptional catalytic activity towards the water-gas-shift (WGS) reaction, not much is known about the detailed reaction mechanism and the influence of charge state of Au nanoparticles on the reactivity. We here report a systematic theoretical study by carrying out density functional theory calculations for the WGS reaction promoted by cationic, neutral, and anionic Au dimers, which represent three simplest prototypes of Au nanoparticles with different charge states. To better understand the catalytic activities of the Au dimers towards the WGS reaction, we first study their complexes with CO and H2O molecules. We find that the calculated values of the binding energies of H2O and CO molecules on Au dimers are closely related to the oxidation state of gold. From the positively charged to neutral and to the negatively charged dimers, the value substantially decreases. The reaction mechanism is explored along two possible entrances:one involves the complexes of the dimers with CO and the other is related to the complexes of the dimers with H2O. In all cases, it is found that the catalytic cycle proceeds via the formate mechanism and involves two sequential elementary steps:the rupture of the O-H bond in H2O and the formation of H2 molecule. Great efforts have also been made to locate the intermediates and first-order saddle points proposed in the redox mechanism, however, our requests were always unsuccessful. It is found that the atomic O intermediates and transition states supposed in the redox mechanism are led to either the reactant-like intermediates or product-like intermediates in our calculations. This fact proposes that the WGS reaction mediated by small Au clusters may not adopt this mechanism. The calculated results show that the reaction mediated by Au2+ is energetically most favorable compared to those promoted by Au2 and Au2-, indicating that the charge state of Au dimers plays an essential role for the catalysted WGS. The notable catalytic activity of Au2+ may originate from the action of the cation, which stabilizes the intermediates and transition states by trsnsferring its charge to the ligand molecules. The present theoretical study rationalizes the early experimental findings well and enriches our understanding of the catalytic WGS by Au-based catalysts.3. Density functional theory (DFT) is used to study the NO reduction by H2 on Au4+ and Au4 clusters. The reaction mechanism is explored along two possible entrances:one involves the complexes of the clusters with H2 and the other is related to the complexes of the clusters with NO. In all cases, it is found that the catalytic cycle involves two sequential elementary steps:the rupture of the H-H bond in H2 and the formation of H2O and N2O molecule. The calculated results show that the reaction mediated by Au4+ is energetically most favorable compared to that promoted by Au4, indicating that the charge state of Au clusters plays an essential role for the catalysted NO reduction. The present theoretical study rationalizes the early experimental findings well and enriches our understanding of the catalytic NO reduction by Au-based catalysts.4. Density functional theory calculations have been performed to elucidate the mechanism of N2O formation over Au(111) surface during NO reduction. Initial adsorption manner of a molecule on a metal surface is expected to affect the following surface reaction. To better understand the reactivity of NO on Au(111) surface, we examine the NO adsorption behavior on Au(111) surface by considering three possible adsorption manners:the N atom close to the surface, the O atom close to the surface, and both the N and O atoms close the surface. It is found that NO adsorption occurs with the N atom close to the surface. It is noted that in all situations the NO molecular axis is tilted with respect to the surface normal and the most stable adsorptions occur at the top site. In addition, NO binds weakly to the Au(111) surface. During the catalytic NO reduction on metal surfaces, the formation of N2O via the direct dissociation mechanism is considered as the most straightforward pathway. At such, we first investigate the possibility of the direct dissociation mechanism. Our calculations show that the dissociation of NO into an N atom and an O atom involves a barrier as high as 3.9 eV, and the final states are more unstable than the initial states by 3.03 and 2.91 eV, respectively. These extremely high barriers and the strong endothermicity of the reaction indicate that the NO dissociation over the Au(111) surface is both kinetically and thermodynamically very unfavorable. We thus rule out the possibility of direct NO dissociation on Au(111) and hence the direct dissociation mechanism for N2O formation during NO reduction on Au(111) surface. Alternatively, we find that the reaction may occur via a dimer mechanism, i.e. two NO molecules initially associate into a dimeric (NO)2, which then dissociate into a N2O molecule and a N atom. We find that the formation of dimer (NO)2 over the Au(111) surface is a thermodynamically favorable process. We have scanned the potential energy surface forming N2O along different pathways, which involve trapezoid OadNNOad dimer, inverted trapezoid ONadNadO dimer, zigzag ONadNOad dimer, or rhombus ONadOadN dimer. The trapezoid dimer, OadNNOad is found to be a necessary intermediate for the formation of N2O, and the calculated barrier for the rate-determining step along the energetically most favorable pathway is only 0.34 eV. The present results rationalize the early experimental findings well, and enriches our understanding of the reduction of NO on Au surface.5. The adsorption of gas molecules on transition metal surfaces is the first elementary step in a heterogeneous catalysis and it is fundamental to the understanding of catalytic mechanism. Nowadays it has being one of key subjects in the field of surface science and numerous studies have been carried out in experiments as well as theories. The first principles density function theory (DFT) calculation plays more and more important role in understanding of adsorption mechanics and explaining of experiment phenomenon in atomic scale. Here, DFT calculations are performed to study NO adsorption on neutral, anionic, and cationic Au(111), Au(100), Au(310), and Au/Au(111). We carefully study the NO adsorption at different adsorption sites on each surface, and find that NO prefers to bond at the top site with the NO molecular axis tilted to the surface normal. The adsorption energy of NO on the surfaces increases as the coordination number of Au atoms decreases:NO binds weakly to the Au(111) surface, while the adsorption energy of NO on Au/Au(111) is as high as 0.89 eV. In addition, the charge state of Au surfaces has a very strong effect on the Au activity:the cationic surfaces generally present stronger reactivity towards the NO molecule than the neutral and anionic surfaces. We have also provided detailed evidence for the origin of these trends:the low coordinated gold atoms and the surface with the concentration of positive charges have d states closing to the Fermi level, resulting in the high activity toward NO adsorption. The N—O bond length are also taken into account. For the cationic surfaces, the NPA charge on the NO molecule in all situations is positive. This indicates that electrons transfer from NO to the cationic surfaces, which will reduce the occupation of theπ* orbital of NO. Thus, the N-O bonding will be enhanced and the N-O bond length will shorten. In contrast, for the anionic and neutral surfaces, electron transfer occurs from the surfaces to molecule and the transferred electrons enter the 2π* orbital of NO. As a result, the N-O bonding becomes weaker, as indicated by the calculated longer N-O bond lengths. The present results enrich our understanding of the adsorption of NO on Au surfaces.6. Here configurations of different Pd-containing Au(111) bimetallic surfaces with Pd substituents varying from one to three atoms have been studied using density functional theory within the generalized gradient approximation. The stability of the so-formed Pd atoms in the surface of a Au(111)-(2×2) unit cell and their influence on the adsorption of CO molecule have been investigated. The influences of surface-ligand effect and lattice strain effect on activity were demonstrated. We have furthermore analyzed local trends by considering different adsorption sites on the different surfaces. The catalytic efficiency of Pd-Au bimetallic systems depends largely on the surface composition of Pd and Au. The addition of Pd significantly improves the activity of a Pd-Au bimetallic slab on CO adsorption. The surface Pd atoms are active and serve as independent attractive centers towards CO. The results can be rationalized within the d-band model. The Pd-d band becomes narrow and well below the Fermi level, very different from those in a bulk Pd. The work provides an effective method which can be used to link experiment and theory result.In this dissertation, the study rationalizes the early experimental findings well and enriches our understanding of the catalytic activity of gold-based catalysts. The valuable results have provided reliable verification and theoretical guide for the development of gold-based catalysts.