Fundamental aspects of catalysis on supported metal clusters (original) (raw)
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Catalysts, 2011
It is demonstrated that the support effects play a crucial role in the gold nanocatalysis. Two types of support are considered-the "inert" support of hexagonal boron nitride (h-BN) with the N and B vacancy defects and the "active" support of rutile TiO 2 (110). It is demonstrated that Au and Au 2 can be trapped effectively by the vacancy defects in h-BN. In that case, the strong adsorption on the surface defects is accompanied by the charge transfer to/from the adsorbate. The excess of the positive or negative charge on the supported gold clusters can considerably promote their catalytic activity. Therefore gold clusters supported on the defected h-BN surface can not be considered as pseudo-free clusters. We also demonstrate that the rutile TiO 2 (110) support energetically promotes H 2 dissociation on gold clusters. We show that the formation of the OH group near the supported gold cluster is an important condition for H 2 dissociation. We demonstrate that the active sites towards H 2 dissociation on the supported Au n are located at corners and edges of the gold cluster in the vicinity of the low coordinated oxygen atoms on TiO 2 (110). Thus catalytic activity of a gold nanoparticle supported on the rutile TiO 2 (110) surface is proportional to the length of the perimeter interface between the nanoparticle and the support.
Journal of the American Chemical Society, 2002
Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO2 has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O 2 dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O 2 dissociation on Au (O2f2Oad) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O 2 on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O2 can adsorb on the interface of Au/TiO2 with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O2 dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O2fCO2+O, and CO+OfCO2; and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.
Factors in gold nanocatalysis: oxidation of CO in the non-scalable size regime
Topics in Catalysis, 2007
Focusing on size-selected gold clusters consisting of up to 20 atoms, that is, in the size regime where properties cannot be obtained from those of the bulk material through scaling considerations, we discuss the current state of understanding pertaining to various factors that control the reactivity and catalytic activity of such nanostructures, using the CO oxidation reaction catalyzed by the gold nanoclusters adsorbed on MgO as a paradigm. These factors include the role of the metal-oxide support and its defects, the charge state of the cluster, structural fluxionality of the clusters, electronic size effects, the effect of an underlying metal support on the dimensionality, charging and chemical reactivity of gold nanoclusters adsorbed on the metal-supported metal-oxide, and the promotional effect of water. We show that through joined experimental and first-principles quantum mechanical calculations and simulations, a detailed picture of the reaction mechanism emerges.
Theoretical study of CO adsorption and oxidation on the gold–palladium bimetal clusters
The Au 3-5 clusters supported on silico-aluminophospates (SAPO-11) were studied using quantum chemistry calculations, by means of the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au 3 /SAPO-11, Au 4 /SAPO-11, Au 5 /SAPO-11, CO-Au 3 /SAPO-11, CO-Au 4 /SAPO-11 and CO-Au 5 /SAPO-11 aggregates to analyze the geometries of small clusters of Au 3 , Au 4 and Au 5 on SAPO-11 support. Au 3 cluster presents a triangle structure in Au 3 /SAPO-11. Au 4 cluster shows a ''Y shaped'' structure in Au 4 /SAPO-11. Au 4 as a rhombus structure is also studied but it is a less stable structure. Au 5 cluster displays a ''pentagon shaped'' structure as the most stable. The CO interaction with Au 3 , Au 4 and Au 5 /SAPO-11 is studied; this CO adsorption is different from that reported in the literature. The formation energy DE F of the aggregates, the CO adsorption energy DE ads on them and the change of energy of the oxidation reaction are presented. Some relevant charges and bond indexes were calculated. On supporting all Au 3-5 there is a charge transfer from the Au cluster to the binding O atoms. On CO adsorption in all Au 3-5 clusters there is a charge transfer from the cluster to the CO. j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p t c
Topics in Catalysis, 2011
We calculate the energetics of CO oxidation on extended surfaces of particular structures chosen to maximize their reactivity towards either O 2 dissociation, after which CO ? O to CO 2 is a facile reaction, or to CO 2 from molecular O 2 and CO. We identified two configurations of Au atoms for which the energetics of these reactions are feasible. A site consisting of four Au atoms in a square geometry appears well suited for dissociating oxygen.
Effect of Silicon Doping on the Reactivity and Catalytic Activity of Gold Clusters
The Journal of Physical Chemistry C, 2014
Doping is known to be an excellent and simple way of catalyst design. Although notable progress has been made in understanding the reactivity and catalytic activity of gas-phase and supported gold clusters, very few studies have been carried out on the doped gold clusters. In the present work, we have carried out density functional theory calculations to investigate the effect of silicon doping on the reactivity and catalytic activity of gold nanoclusters. The present work particularly focuses on the adsorption and activation of molecular oxygen on the pristine and silicon-doped gold clusters. The results confirm that the silicon-doped Au 7 Si cluster shows considerable binding and activation of the O 2 molecule in comparison to the pristine Au 8 cluster as reflected in the relevant geometrical parameters (O−O and Au−O bond lengths) and O−O stretching frequency. However, silicon doping has no contrasting effect on the reactivity and catalytic activity of the Au 7 cluster. In addition to the stronger binding and activation of the O 2 molecule, the doped Au 7 Si cluster leads to a significant reduction in the activation barrier (0.57 eV) for the environmentally important CO oxidation reaction in contrast to the catalytically inactive pristine Au 8 cluster (1.22 eV). Thus, our results highlight the critical role of doping foreign impurities for future endeavors in the field of gold nanocatalysis.
Oxygen Molecule Activation on Single-Atom Catalysts with Cu, Ag, and Au: A Cluster Model Study
2021
2 1 MSi O n n+ (M = Au, Ag, Cu; n = 1, 2, 3) clusters were used as a cluster model to study the activation of oxygen molecules on single-atom catalysts. Structures of-2 1 MSi O n n+ clusters were studied by density functional calculations with global optimization. For each n, the most stable structures are quite similar for different metal types, and the oxygen molecule prefers to be adsorbed onto M atoms. It is found that the activation degree of oxygen is higher on clusters with non-noble metal Cu than that of Ag or Au containing clusters, by comparing the changes of O-O bond length and vibrational frequency, natural charge population analysis, Fuzzy bond order analysis, and energy barriers of O 2 dissociation. CO oxidation was used as a probe reaction to study the reactivity of Cu-containing clusters, and it is found that the reactivity decreases with the increase of the size of silicon-oxygen clusters. Our results give a new aspect to understand the reaction mechanism of non-precious metal single-atom catalyst for oxygen activation with high efficiency.
CO adsorption on transition metal clusters: Trends from density functional theory
Surface Science, 2008
This work reports for the first time the trends for carbon monoxide (CO) chemisorption on transition metal clusters present in supported metal catalysts. In particular, the energetic, structural and infrared adsorption characteristics of linearly (atop) CO adsorbed on transition metal nano-clusters of less than 10 Å in size were explored. Spin-unrestricted density functional theory (DFT) calculations were employed to explore the trends of CO adsorption energy (A M-CO ) and C-O vibrational frequency (m CO ) for clusters composed of Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au. The effects of the transition metal electronic structure onto the adsorption energy of CO and the vibrational stretching frequency of C-O, and how these chemical parameters can be correlated to the catalytic activity of transition supported metal catalysts that involve the adsorption, surface diffusion, and C-O bond dissociation elementary steps in heterogeneous catalytic surface reactions, are discussed. Our findings show that an increase of the electronic d-shell occupancy and the principal quantum number (n) in transition metals causes an increase in the vibrational stretching frequency of the C-O bond. This trend is inconsistent with the classical Blyholder model for the metal-carbonyl bond.