Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold

Theoretical Insights on O2 and CO Adsorption on Neutral and Positively Charged Gold Clusters

With the aim of understanding the elementary steps governing the oxidation of CO catalyzed by dispersed or supported gold nanoclusters, the adsorption of molecular species, such as O2 and CO, on model neutral and positively charged clusters (Au(n)(m+) n = 1, 9, and 13; m = 0, 1, and 3) has been studied using an ab initio approach. The computed structural and thermodynamic data related to the binding process show that molecular oxygen interacts better with neutral clusters, acting as an electron acceptor, while CO more strongly binds to positively charged species, thus acting as an electron donor.

A density functional theory study of sulfur poisoning

Density functional theory calculations have been used to investigate the chemisorption of H, S, SH, and H(2)S as well as the hydrogenation reactions S+H and SH+H on a Rh surface with steps, Rh(211), aiming to explain sulfur poisoning effect. In the S hydrogenation from S to H(2)S, the transition state of the first step S+H-->SH is reached when the S moves to the step-bridge and H is on the off-top site. In the second step, SH+H-->H(2)S, the transition state is reached when SH moves to the top site and H is close to another top site nearby. Our results show that it is difficult to hydrogenate S and they poison defects such as steps. In order to address why S is poisoning, hydrogenation of C, N, and O on Rh(211) has also been calculated and has been found that the reverse and forward reactions possess similar barriers in contrast to the S hydrogenation. The physical origin of these differences has been analyzed and discussed.

Identifying an O2 Supply Pathway in CO Oxidation on Au/TiO2(110): A Density Functional Theory Study on the Intrinsic Role of Water

Au catalysis has been one of the hottest topics in chemistry in the last 10 years or so. How O2 is supplied and what role water plays in CO oxidation are the two challenging issues in the field at the moment. In this study, using density functional theory we show that these two issues are in fact related to each other. The following observations are revealed: (i) water that can dissociate readily into OH groups can facilitate O2 adsorption on TiO2; (ii) the effect of OH group on the O2 adsorption is surprisingly long-ranged; and (iii) O2 can also diffuse along the channel of Ti (5c) atoms on TiO2(110), and this may well be the rate-limiting step for the CO oxidation. We provide direct evidence that O2 is supplied by O2 adsorption on TiO2 in the presence of OH and can diffuse to the interface of Au/TiO2 to participate in CO oxidation. Furthermore, the physical origin of the water effects on Au catalysis has been identified by electronic structure analyses: There is a charge transfer from TiO2 in the presence of OH to O2, and the O2 adsorption energy depends linearly on the O2 charge. These results are of importance to understand water effects in general in heterogeneous catalysis.

Stabilizing CO on Au with NO2: electronegative species as promoters on coinage metals?

CO adsorption on NO(2)-predosed Au[111] reveals an unexpected attractive coadsorbate interaction, associated with an unprecedented blueshift of the CO stretch frequency, a sizeable attenuation of the infrared NO(2) symmetric stretch band, and a (sq.rt(7) x sq.rt(7))R19 degrees structure characterized by scanning tunneling microscopy and low energy electron diffraction. Density functional calculations allow us to rationalize these observations, and point towards a general pattern of behavior for electronegative coadsorbates on coinage metals, with important implications for catalytic promotion.

Unique CO Chemisorption Properties of Gold Hexamer: Au6(CO)n- (n = 0−3)

Elucidating the chemisorption properties of CO on gold clusters is essential to understanding the catalytic mechanisms of gold nanoparticles. Gold hexamer Au(6) is a highly stable cluster, known to possess a D(3)(h) triangular ground state structure with an extremely large HOMO-LUMO gap. Here we report a photoelectron spectroscopy (PES) and quasi-relativistic density functional theory (DFT) study of Au(6)-CO complexes, Au(6)(CO)(n)(-) and Au(6)(CO)(n) (n = 0-3). CO chemisorption on Au(6) is observed to be highly unusual. While the electron donor capability of CO is known to decrease the electron binding energies of Au(m)(CO)(n)(-) complexes, CO chemisorption on Au(6) is observed to have very little effect on the electron binding energies of the first PES band of Au(6)(CO)(n)(-) (n = 1-3). Extensive DFT calculations show that the first three CO successively chemisorb to the three apex sites of the D(3)(h) Au(6). It is shown that the LUMO of the Au(6)-CO complexes is located in the inner triangle. Thus CO chemisorption on the apex sites (outer triangle) has little effect on this orbital, resulting in the roughly constant electron binding energies for the first PES band in Au(6)(CO)(n)(-) (n = 0-3). Detailed molecular orbital analyses lead to decisive information about chemisorption interactions between CO and a model Au cluster.

First-Principles Study of CO Adsorption and Vibration on Au Surfaces

A CO stretching frequency analysis is presented for the adsorption of CO on various Au(110) surfaces from density functional theory calculations. The structure sensitivity of the adsorption has been studied by considering the unreconstructed (1 x 1) surface, the missing-row reconstructed (1 x 2) surface, the vicinal stepped (20) surface, and the adsorption on adatoms deposited on the (110)-(1 x 2) surface. The calculated CO stretching frequencies are compared with infrared reflection-absorption spectroscopy (IRAS) measurements carried out at room temperature and pressure below 1 atm. The overall stability of the systems is discussed within the calculations of surface free energies at various coverages. At room temperature, the adsorption of CO on the ridge of the missing-row reconstructed surface competes in the high pressure regime with more complex adsorption structures where the molecule coadsorbs on the ridge and on adatoms located along the empty troughs of the reconstruction. This result is supported by the CO stretching frequency analysis.

Formation of Molecularly Chemisorbed Oxygen on TiO2-Supported Gold Nanoclusters and Au(111) from Exposure to an Oxygen Plasma Jet

We present results of an investigation into the low-temperature formation of molecularly chemisorbed oxygen on a Au/TiO(2) model catalyst and on a Au(111) single crystal during exposure to a plasma jet of oxygen. Through the use of collision-induced desorption measurements and isotopic mixing experiments we show evidence suggesting that at least some of the molecular oxygen is formed as a result of recombination of oxygen atoms on the samples during the plasma exposure. Of course, adsorption of excited molecular oxygen directly from the gas phase may also take place. We also present evidence showing that the adsorption of oxygen atoms on the surface assists in the molecular chemisorption of oxygen on the Au/TiO(2) model catalyst samples. Thus, oxygen molecules impinging on the samples during plasma-jet exposures (plasma jet has approximately 40% dissociation fraction) could have an enhanced probability of adsorption due to simultaneous oxygen atom adsorption.

In-situ scanning tunneling microscopy of carbon monoxide adsorbed on Au(111) electrode

In-situ scanning tunneling microscopy (STM) coupled with cyclic voltammetry was used to examine the adsorption of carbon monoxide (CO) molecules on an ordered Au(111) electrode in 0.1 M HClO4. Molecular resolution STM revealed the formation of several commensurate CO adlattices, but the (9 x radical 3) structure eventually prevailed with time. The CO adlayer was completely electrooxidized to CO2 at 0.9 V versus RHE in CO-free 0.1 M HClO(4), as indicated by a broad and irreversible anodic peak which appeared at this potential in a positive potential sweep from 0.05 to 1.6 V. A maximal coverage of 0.3 was estimated for CO admolecules from the amount of charge involved in this feature. Real-time in-situ STM imaging allowed direct visualization of the adsorption process of CO on Au(111) at 0.1 V, showing the lifting of (radical 3 x 22) reconstruction of Au(111) and the formation of ordered CO adlattices. The (9 x radical 3) structure observed in CO-saturated perchloric acid has a coverage of 0.28, which is approximately equal to that determined from coulometry. Switching the potential from 0.1 to -0.1 V restored the reconstructed Au(111) with no change in the (9 x radical 3)-CO adlattice. However, the reconstructed Au(111) featured a pairwise corrugation pattern with two nearest pairs separated by 74 +/- 1 A, corresponding to a 14% increase from the ideal value of 65.6 A known for the ( radical 3 x 22) reconstruction. Molecular resolution STM further revealed that protrusions resulting from CO admolecules in the (9 x radical 3) structure exhibited distinctly different corrugation heights, suggesting that the CO molecules resided at different sites on Au(111). This ordered structure predominated in the potential range between 0.1 and 0.7 V; however, it was converted into new structures of (7 x radical 7) and ( radical 43 x 2 radical 13) on the unreconstructed Au(111) when the potential was held at 0.8 V for ca. 60 min. The coverage of CO adlayer decreased accordingly from 0.28 to 0.13 before it was completely removed from the Au(111) surface at more positive potentials.

Chemisorption sites of CO on small gold clusters and transitions from chemisorption to physisorption

Gold clusters adsorbed with CO, Au(m)(CO)(n) (-) (m=2-5; n=0-7), were studied by photoelectron spectroscopy (PES). The first few CO adsorptions were observed to induce significant redshifts to the PES spectra relative to pure gold clusters. For each Au cluster, a critical CO number (n(c)) was observed, beyond which the PES spectra of Au(m)(CO)(n) (-) change very little with increasing n. n(c) was shown to correspond exactly to the available low coordination apex sites in each Au cluster. CO first chemisorbs to these sites and additional CO then only physisorbs to the chemisorption-sautrated Au(m)(CO)(n) (-) complexes.

Catalytic Properties of Silver Nanoparticles Supported on Silica Spheres

In this work, we investigate the catalytic properties of silver nanoparticles supported on silica spheres. The technique to support silver particles on silica spheres effectively avoids flocculation of nanosized colloidal metal particles during a catalytic process in the solution, which allows one to carry out the successful catalytic reduction of dyes. The effects of electrolytes and surfactants on the catalytic properties of silver particles on silica have been investigated. It is found that the presence of surfactants depresses the catalytic activity of the silver particles to some extent by inhibiting the adsorption of reactants onto the surface of the particles. Electrolytes either increase the migration rate of reactants in the solution resulting in an increase in the catalytic reaction rate or inhibit the adsorption of reactants onto the surface of the silver particles leading to a loss in the activity of the metal particles.

The nature of the oxidation states of gold on ZnO

The interaction between gold in the 0, i, ii and iii oxidation states and the zinc-terminated ZnO(0001) surface is studied via the QM/MM electronic embedding method using density functional theory. The surface sites considered are the vacant zinc interstitial surface site (VZISS) and the bulk-terminated island site (BTIS). We find that on the VZISS, only Au(0) and Au(i) are stable oxidation states. However, all clusters of i to iii oxidation states are stable as substitutionals for Zn2+ in the bulk terminated island site. Au(OH)(x) complexes (x= 1-3) can adsorb exothermically onto the VZISS, indicating that higher oxidation states of gold can be stabilised at this site in the presence of hydroxyl groups. CO is used as a probe molecule to study the reactivity of Au in different oxidation states in VZISS and BTIS. In all cases, we find that the strongest binding of CO is to surface Au(i). Furthermore, CO binding onto Au(0) is stronger when the gold atom is adsorbed onto the VZISS compared to CO binding onto a gas phase neutral gold atom. These results indicate that the nature of the oxidation states of Au on ZnO(0001) will depend on the type of adsorption site. The role of ZnO in Au/ZnO catalysts is not, therefore, merely to disperse gold atoms/particles, but to also modify their electronic properties.

Synergistic Effect in an Au−Ag Alloy Nanocatalyst: CO Oxidation

Au-Ag alloy nanoparticles supported on mesoporous aluminosilicate have been prepared by one-pot synthesis using hexadecyltrimethylammonium bromide (CTAB) both as a stabilizing agent for nanoparticles and as a template for the formation of mesoporous structure. The formation of Au-Ag alloy nanoparticles was confirmed by X-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, and transmission electron microscopy (TEM). Although the Au-Ag alloy nanoparticles have a larger particle size than the monometallic gold particles, they exhibited exceptionally high activity in catalysis for low-temperature CO oxidation. Even at a low temperature of 250 K, the reaction rate can reach 8.7 x 10(-6) mol.g(cat.)(-1).s(-1) at an Au/Ag molar ratio of 3/1. While neither monometallic Au@MCM-41 nor Ag@MCM-41 shows activity at this temperature, the Au-Ag alloy system shows a strongly synergistic effect in high catalytic activity. In this alloy system, the size effect is no longer a critical factor, whereas Ag is believed to play a key role in the activation of oxygen.

Reaction of CO with Molecularly Chemisorbed Oxygen on TiO2-Supported Gold Nanoclusters

In this study we present results of an investigation into the reactivity of molecularly chemisorbed oxygen species on a Au/TiO2 model catalyst. We have previously shown that a Au/TiO2 model catalyst sample can be populated with both atomically and molecularly chemisorbed oxygen species following exposure to a radio frequency-generated oxygen plasma-jet. To test the reactivity of the molecularly chemisorbed oxygen species, we compare the CO2 produced from a sample that is populated with both oxygen species to the CO2 produced from a sample that has been given an identical exposure but has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. We observe that samples that are populated with both oxygen species consistently result in greater CO2 production. For the data presented in this paper, we observe a difference of 41% in the CO2 production. We interpret this result to indicate that molecularly chemisorbed oxygen can react directly with CO to form CO2.

Kinetic Study of a Direct Water Synthesis over Silica-Supported Gold Nanoparticles

The reaction mechanism of water formation from H2 and O2 was studied over a series of silica-supported gold nanoparticles. The metal particle size distributions were estimated with TEM and XRD measurements. Hydrogen and oxygen adsorption calorimetry was used to probe the nature and properties of surface species formed by these molecules. DFT calculations with Au5, Au13, and Au55 clusters and with Au(111) and Au(211) periodic slabs were performed to estimate the thermodynamic stability and reactivity of surface species. Kinetic measurements were performed by varying the reactant partial pressures at 433 K and by varying the temperature from 383 to 483 K at 2.5 kPa of O2 and 5 kPa of H2. The measured apparent power law kinetic parameters were similar for all catalysts in this study: hydrogen order of 0.7-0.8, oxygen order of 0.1-0.2, and activation energy of 37-41 kJ/mol. Catalysts with Si-MFI (Silicalite-1) and Ti-MFI (TS-1 with 1 wt % Ti) exhibited similar activities. The activities of these catalysts with the MFI crystalline supports were 60-70 times higher than that of an analogous catalyst with an amorphous silica support. Water addition in the inlet stream at 3 vol % did not affect the reaction rates. The mechanism of water formation over gold is proposed to proceed through the formation of OOH and H2O2 intermediates. A rate expression derived based on this mechanism accurately describes the experimental kinetic data. The higher activity of the MFI-supported catalysts is attributed to a higher concentration of gold particles comparable in size to Au13, which can fit inside MFI pores. DFT results suggest that such intermediate-size gold particles are most reactive toward water formation. Smaller particles are proposed to be less reactive due to the instability of the OOH intermediate whereas larger particles are less reactive due to the instability of adsorbed oxygen.

Car Exhaust Catalysis from First Principles: Selective NO Reduction under Excess O2 Conditions on Ir

Combining energetic data from density functional theory with thermodynamic calculations, we have studied in detail selective NO reduction under excess O2 conditions on Ir. We show that excess O2 can readily poison the Ir catalyst for NO reduction and the poisoning starts from a low O coverage on the surface. The adsorbed O switches the reaction selectivity from reduction (N2 production) to oxidation (NO2 production). As the O coverage is built up, Ir metal can eventually be oxidized to IrO2, which is predicted to be thermodynamically possible under reaction conditions. To prevent O poisoning the surface, the presence of reductants is thus essential. We demonstrate that NO reduction is sensitive to the choice of reductant, and that alkenes are the most effective, mainly because they are able to produce surface C atoms that can selectively remove O atoms from Ir steps. On the basis of our analyses of the electronic structures, the mechanism of O-poisoning is elucidated and the reactant sensitivity in NO reduction is also discussed in terms of the bonding competition effect. We found that for different adsorbates, such as NO, O, and N, their bondings with surface d-states are remarkably similar. This gives rise to an indirect repulsion between adsorbates whenever they may bond with the same metal atoms. This energy cost can be qualitatively correlated with the valency of the adsorbate, and this is the key to understand the O-poisoning effect and the structure sensitivity in NO reduction.

Investigation of Alumina-Supported Au Catalyst for CO Oxidation by Isotopic Transient Analysis and X-ray Absorption Spectroscopy

Alumina-supported Au particles (1.16 wt %) were prepared by a deposition-precipitation method involving a HAuCl4 precursor. X-ray absorption spectroscopy at the Au L(III) edge was used to monitor the evolution of the Au oxidation state and atomic structure during pretreatment in He up to 623 K. Although the as-prepared material had Au present in a +3 oxidation state, thermal treatment at 623 K facilitated autoreduction of Au cations to metal particles. Analysis of the EXAFS revealed a coordination number (Au-Au) of 7.2, which is consistent with spherical particles of 1.2 nm in average diameter. Steady-state isotopic transient kinetic analysis was used to evaluate the intrinsic turnover frequency (TOF intr) and the surface coverage of carbon-containing species (theta COx) on the gold catalyst during CO oxidation at 1.2 atm total pressure and 296 K. The artifacts in the kinetic parameters caused by re-adsorption of product carbon dioxide were removed by varying the total flow rate. The values of TOF intr and theta COx determined from the intrinsic lifetime of surface intermediates at infinite flow rate were 1.6 s(-1) and 4.9%, respectively. The intrinsic turnover frequency was nearly independent of temperature, indicating a very low activation energy for the reaction. However, the rate was significantly accelerated by the presence of water.

Adsorption of O2 and oxidation of CO at Au nanoparticles supported by TiO2(110)

Density functional theory calculations are performed for the adsorption of O2, coadsorption of CO, and the CO+O2 reaction at the interfacial perimeter of nanoparticles supported by rutile TiO2(110). Both stoichiometric and reduced TiO2 surfaces are considered, with various relative arrangements of the supported Au particles with respect to the substrate vacancies. Rather stable binding configurations are found for the O2 adsorbed either at the trough Ti atoms or leaning against the Au particles. The presence of a supported Au particle strongly stabilizes the adsorption of O2. A sizable electronic charge transfer from the Au to the O2 is found together with a concomitant electronic polarization of the support meaning that the substrate is mediating the charge transfer. The O2 attains two different charge states, with either one or two surplus electrons depending on the precise O2 adsorption site at or in front of the Au particle. From the least charged state, the O2 can react with CO adsorbed at the edge sites of the Au particles leading to the formation of CO2 with very low (approximately 0.15 eV) energy barriers.

Interactions of Au cluster anions with oxygen

Experimental and theoretical evidence is presented for the nondissociative chemisorption of O2 on free Au cluster anions (Aun-, n=number of atoms) with n=2, 4, 6 at room temperature, indicating that the stabilization of the activated di-oxygen species is the key for the unusual catalytic activities of Au-based catalysts. In contrast to Aun- with n=2, 4, 6, O2 adsorbs atomically on Au monomer anions. For the Au monomer neutral, calculations based on density functional theory reveal that oxygen should be molecularly bound. On Au dimer and tetramer neutrals, oxygen is molecularly bound with the O-O bond being less activated with respect to their anionic counterparts, suggesting that the excess electron in the anionic state plays a crucial role for the O-O activation. We demonstrate that interplay between experiments on gas phase clusters and theoretical approach can be a promising strategy to unveil mechanisms of elementary steps in nanocatalysis.

Saturated adsorption of CO and coadsorption of CO and O2 on AuN− (N=2–7) clusters

A first-principles quantum chemistry method, based on the Kohn-Sham density-functional theory, is used to investigate the adsorption of CO and O2 on small gas-phase gold cluster anions. The saturated adsorption of carbon monoxide on gold cluster anions AuN- (N=2-7) is discussed. The adsorption ability of CO reduces with the increase of the number of CO molecules bound to gold cluster anions, resulting in saturated adsorption at a certain amount of absorbed CO molecules, which is determined by geometric and electronic properties of gold clusters cooperatively. The effect of CO preadsorption on the electronic properties of gold cluster anions depends on the cluster size and the number of adsorbed CO, and the vertical detachment energies of CO-adsorbed gold cluster anions show a few changes with respect to corresponding pure gold cluster anions. The results indicate that the impinging adsorption of CO molecules may lead to geometry structure transformation on Au3- cluster. For the coadsorption of CO and O2 on Au2-, Au3- isomers, Au4-, and Au6-, we describe the cooperative adsorption between CO and O2, and find that the O2 dissociation is difficult on gas-phase gold cluster anions even with the preadsorption of CO.

Step-enhanced selectivity of NO reduction on platinum-group metals

The conversion of NO to N2 is a key issue encountered in the control of emission from vehicles. The selectivity of NO reduction on two platinum group metals, Ir and Pt, including the close-packed flat surface and the monatomic steps are extensively studied within the first-principles density functional theory framework. A stepped Ir surface is found to possess high selectivity for NO reduction, which is attributed to both the electronic and geometric structures of the Ir steps. The other surfaces considered fail to combine both attributes, activity and selectivity. In particular, a stepped Pt surface has a poor N2 selectivity with a large tendency for N2O production. The results explain the observed metal dependency and the structure sensitivity of the NO reduction under excess O2 conditions.

Evidence for Molecularly Chemisorbed Oxygen on TiO2 Supported Gold Nanoclusters and Au(111)

In this study, we present evidence for the existence of a molecularly chemisorbed oxygen species on a Au/TiO2 model catalyst and a Au(111) single crystal following exposure of these samples to an oxygen plasma-jet molecular beam. We present evidence for the molecularly chemisorbed oxygen species from thermal desorption, collision-induced desorption, and heat of adsorption/reaction-induced desorption measurements. Thermal desorption measurements reveal a peak desorption temperature at approximately 145 K which corresponds to an activation energy for desorption of approximately 0.35 eV.