Coadsorption of CO and O(2) on selected gold clusters: evidence for efficient room-temperature CO(2) generation

Spurred by the recent demonstrations of the size- and support-dependent reactivity of supported gold clusters, here we present results on the coadsorption of CO and O(2) on selected anionic gold clusters, Au(N)(-), in the gas phase. O(2) adsorbs in a binary (0,1) fashion as a one-electron acceptor on the Au(N)()(-) clusters, with even-N clusters showing varying reactivity toward O(2) adsorption, while odd-N clusters show no evidence of reactivity. CO shows a highly size-dependent reactivity for Au(N)(-) sizes from N = 4 to 19, but no adsorption on the gold dimer or trimer. When the gold clusters are exposed to both reactants, either simultaneously or sequentially, interesting effects have been observed. While the same rules pertaining to individual O(2) or CO adsorption continue to apply, the preadsorption of one reactant on a cluster may lead to the increased reactivity of the cluster to the other reactant. Thus, the adsorbates are not competing for bonding sites (competitive coadsorption), but, instead, aid in the adsorption of one another (cooperative coadsorption). New peaks also arise in the mass spectrum of Au(6)(-) under CO and O(2) coadsorption conditions, which can be attributed to the loss of a CO(2) molecule (or molecules). By studying the relative amount of reaction, and relating it to the reaction time, it is found that the gas-phase Au(6) anion is capable of oxidizing CO at a rate 100 times that reported for commercial or model gold catalysts.     

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.     

Theoretical study of CO adsorption on gold/alumina substrates

Aiming to understand the role of the substrate in the adsorption of carbon monoxide on gold clusters supported on metal-oxides, we have started a study of that process on two different alumina substrates: an amorphous-like fully relaxed stoichiometric (Al2O3)20 cluster and the Al terminated (0001) surface of alpha-(Al2O3) crystal. In this paper, we present first principles calculations for the adsorption of one Au atom on both alumina substrate and the adsorption of Au8 on (Al2O3)20. Then, we study the CO adsorption on the minimum energy structure of these three different gold/alumina systems. A single Au adsorbs preferably on top of an Al atom with low coordination, the binding energy being higher in the case of Au/(Al2O3)20. CO absorbs preferably on top of the Au atom, but in the case of Au/(Al2O3)20, Au forms a bridge with the Al and O substrate atoms after CO adsorption. We find other stable sites for CO adsorption on the cluster but not on the surface. This result suggests that the Au activity toward CO may be larger for the amorphous cluster than for the crystal surface substrate. For the most stable Au8/(Al2O3)20 configuration, two Au atoms bind to Al and a O atoms respectively and CO adsorbs on top of the Au which binds to the Al atom. We find other CO adsorption sites on supported Au8 which are not stable for the free Au8 cluster.     

Tuning cluster reactivity by charge state and composition: experimental and theoretical investigation of CO binding energies to Ag(n)Au(m)(+/-) (n + m = 3)

Temperature-dependent gas-phase reaction kinetics measurements and equilibrium thermodynamics under multicollision conditions in conjunction with ab initio DFT calculations were employed to determine the binding energies of carbon monoxide to triatomic silver-gold binary cluster cations and anions. The binding energies of the first CO molecule to the trimer clusters increase with increasing gold content and with changing charge from negative to positive. Thus, the reactivity of the binary clusters can be sensitively tuned by varying charge state and composition. Also, multiple CO adsorption on the clusters was investigated. The maximum number of adsorbed CO molecules was found to strongly depend on cluster charge and composition as well. Most interestingly, the cationic carbonyl complex Au(3)(CO)(4)(+) is formed at cryogenic temperature, whereas for the anion, only two CO molecules are adsorbed, leading to Au(3)(CO)(2)(-). All other trimer clusters adsorb three CO molecules in the case of the cations and are completely inert to CO in our experiment in the case of the anions.     

Agglomeration, sputtering, and carbon monoxide adsorption behavior for Au/Al(2)O(3) prepared by Au(n)(+) deposition on Al(2)O(3)/NiAl(110)

Size-selected gold clusters, Au(n)(+) (n = 1, 3, 4), were deposited on an ordered Al(2)O(3) film grown on NiAl(110), and changes in morphology and electronic properties with deposition/annealing temperature and cluster size were investigated by X-ray photoelectron spectroscopy (XPS) and ion-scattering spectroscopy (ISS). Extensive agglomeration was observed by ISS for annealing temperatures above 300 K, accompanied by large shifts in the Au XPS binding energy. Agglomeration is more extensive in room-temperature deposition, compared to samples prepared by low-temperature deposition, then annealed to room temperature. Agglomeration is also observed to be dependent on deposited cluster size. CO adsorption was studied by ISS and temperature-programmed desorption, and we looked for CO oxidation under conditions where substantial activity is seen for Au(n)/TiO(2). No activity was observed for Au(n)/Al(2)O(3). The differences between the two systems are interpreted in terms of the nature of the metal-support interactions.     

Carbon monoxide adsorption on silver doped gold clusters

Well controlled gas phase experiments of the size and dopant dependent reactivity of gold clusters can shed light on the surprising discovery that nanometer sized gold particles are catalytically active. Most studies that investigate the reactivity of gold clusters in the gas phase focused on charged, small sized clusters. Here, reactivity measurements in a low-pressure reaction cell were performed to investigate carbon monoxide adsorption on neutral bare and silver doped gold clusters (Au(n)Ag(m); n = 10-45; m = 0, 1, 2) at 140 K. The size dependence of the reaction probabilities reflects the role of the electronic shells for the carbon monoxide adsorption, with closed electronic shell systems being the most reactive. In addition, the cluster's reaction probability is reduced upon substitution of gold atoms for silver. Inclusion of a single silver atom causes significant changes in the reactivity only for a few cluster sizes, whereas there is a more general reduction in the reactivity with two silver atoms in the cluster. The experimental observations are qualitatively explained on the basis of a Blyholder model, which includes dopant induced features such as electron transfer from silver to gold, reduced s-d hybrization, and changes in the cluster geometry.     

Unraveling the mechanisms of O2 activation by size-selected gold clusters: transition from superoxo to peroxo chemisorption

The activation of dioxygen is a key step in CO oxidation catalyzed by gold nanoparticles. It is known that small gold cluster anions with even-numbered atoms can molecularly chemisorb O(2) via one-electron transfer from Au(n)(-) to O(2), whereas clusters with odd-numbered atoms are inert toward O(2). Here we report spectroscopic evidence of two modes of O(2) activation by the small even-sized Au(n)(-) clusters: superoxo and peroxo chemisorption. Photoelectron spectroscopy of O(2)Au(8)(-) revealed two distinct isomers, which can be converted from one to the other depending on the reaction time. Ab initio calculations show that there are two close-lying molecular O(2)-chemisorbed isomers for O(2)Au(8)(-): the lower energy isomer involves a peroxo-type binding of O(2) onto Au(8)(-), while the superoxo chemisorption is a slightly higher energy isomer. The computed detachment transitions of the superoxo and peroxo species are in good agreement with the experimental observation. The current work shows that there is a superoxo to peroxo chemisorption transition of O(2) on gold clusters at Au(8)(-): O(2)Au(n)(-) (n = 2, 4, 6) involves superoxo binding and n = 10, 12, 14, 18 involves peroxo binding, whereas the superoxo binding re-emerges at n = 20 due to the high symmetry tetrahedral structure of Au(20), which has a very low electron affinity. Hence, the two-dimensional (2D) Au(8)(-) is the smallest anionic gold nanoparticle that prefers peroxo binding with O(2). At Au(12)(-), although both 2D and 3D isomers coexist in the cluster beam, the 3D isomer prefers the peroxo binding with O(2).     

CO adsorption on gold clusters stabilized on ceria-titania mixed oxides: comparison with reference catalysts

Fourier transform infrared spectra of CO adsorption from 120 K up to room temperature on two gold catalysts supported on different mixed ceria-titania oxides are discussed in comparison with those obtained on Au/TiO(2) and Au/Fe(2)O(3) reference catalysts provided by the World Gold Council. The spectra of adsorbed CO, run on the different samples before preliminary treatment, are shown and compared with those of the untreated catalysts and of the samples reduced either in CO or in hydrogen. Big differences have been found between the ceria-titania supported samples and the reference ones: unusual absorption bands, irreversible to outgassing, have been detected after CO interaction on the untreated and oxidized ceria containing samples. These absorptions are assigned to CO on Au(n)(+) small clusters stabilized at the ceria defects. By reduction in hydrogen, negatively charged Au(n)(-) species are produced on the same sample. Oxidized small particles are present on the reference catalysts, but only on the untreated samples; after treatment, only metallic step sites are evident.     

Direct evidence of oxidized gold on supported gold catalysts

Supported gold catalysts have drawn worldwide interest due to the novel properties and potential applications in industries. However, the origin of the catalytic activity in gold nanoparticles is still not well understood. In this study, time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has been applied to investigate the nature of gold in Au (1.3 wt %)/gamma-Al2O3 and Au (2.8 wt %)/TiO2 catalysts prepared by the deposition-precipitation method. The SIMS spectrum of the supported gold catalysts presented AuO-, AuO2-, and AuOH- ion clusters. These measurements show direct evidence for oxidized gold on supported gold catalysts and may be helpful to gaining better understanding of the origin of the catalytic activity.     

Oxygen adsorption at anionic free and supported Au clusters

The structure, stability, and O2 adsorption properties of anionic Au(n) (n=1-11) clusters either free or supported at defected MgO100 surfaces are investigated using density-functional theory. O2 adsorption is strong whenever unpaired electrons are present, except for at some small, supported, planar, high-band-gap clusters. These clusters have the unpaired electrons pinned by the Madelung potential of the support. Larger clusters (starting at Au7-Au8) become three dimensional and metallic. This ensures that while one cluster orbital is pinned to the defect, another orbital at comparable energy can undergo depletion, thus binding O2 with charge transfer.     

金团簇二十面体结构融合过程中其催化活性的演变

近年来,由有机配体保护的原子精确金属团簇在合成方面已取得了重要进展,其独特的原子结构对一些化学反应产生独特的催化效果.原子精确的团簇催化剂明显不同于纳米颗粒催化剂和单原子催化剂,是一种关联均相和多相的、原子数目确定、尺寸均一、结构精确的新型催化剂.从原子尺度上精确构筑团簇催化剂,探究亚纳米尺度的微观结构对催化性能的影响...     

Adsorption of small Au(n) (n = 1-5) and Au-Pd clusters inside the TS-1 and S-1 pores

We used a hybrid quantum-mechanics/molecular-mechanics (QM/MM) approach to simulate the adsorption of Au(n)() (n = 1-5), AuPd, and Au(2)Pd(2) clusters inside the TS-1 and S-1 pores. We studied nondefect and metal-vacancy defect sites in TS-1 and S-1 for a total of four different environments around the T6 crystallographic site. We predict stronger binding of all clusters near Ti sites in Ti-substituted framework compared to adsorption near Si sites-consistent with the experimental finding of a direct correlation between the Ti-loading and the Au-loading on the Au/TS-1 catalysts with high Si/Ti ratio. The cluster binding is also stronger near lattice-metal vacancies compared to fully coordinated, nondefect sites. In all the cases, a trend of binding energy (BE) versus Au cluster size (n) shows a peak at around n = 3-4. Our results show that there is enough room for the attack of H(2)O(2) on the Ti-defect site even with Au(1-4) adsorbed-a result that supports the possibility of H(2)O(2) spillover from the Au clusters to the adjacent Ti-defect sites. Mulliken charge analysis indicates that in all the cases there is electron density transfer to adsorbed clusters from the zeolite lattice. In the case of both gas-phase and adsorbed Au-Pd clusters, all the Pd atoms were positively charged, and all the Au atoms were negatively charged due to the higher electron-affinity of Au. We also found a correlation between the BE and the charge transfer to the clusters (the higher the charge transfer to the clusters, the higher the BE), and a universal correlation was found for Au(2-5) when BE and charge transfer were plotted on a per atom basis. A relatively larger charge transfer to the adsorbed clusters was found for the Ti sites versus the Si sites, and for the defect sites versus the nondefect sites. The trends in the BE were corroborated using Gibbs free energy of adsorption (DeltaG(ads)), and the implications of DeltaG(ads) in sintering of Au clusters are also discussed. Our results confirm that electronic factors such as cluster-charging are potentially important support effects for the Au/TS-1 catalyst.     

CO oxidation on Aun/TiO2 catalysts produced by size-selected cluster deposition

Planar model catalysts were prepared by deposition of size-selected gold clusters containing up to seven atoms on rutile TiO2 (110). Molecular oxygen is observed to bind inefficiently to the surface, probably at oxygen vacancies, and some oxygen also appears to bind to the gold clusters. Stable CO binding is observed atop gold for catalysts prepared by Au and Au2 deposition, but not for larger Aun. CO oxidation activity is strongly dependent on cluster size, with Au7-prepared samples >50 times more reactive than samples prepared by Au or Au2 deposition     

Adsorption energies of molecular oxygen on Au clusters

The adsorption properties of O(2) molecules on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2-0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au(3) and Au(5) clusters can adsorb one O(2) molecule.     

Size-dependent structural evolution and chemical reactivity of gold clusters.

Ground-state structures and other experimentally relevant isomers of Au(15) (-) to Au(24) (-) clusters are determined through joint first-principles density functional theory and photoelectron spectroscopy measurements. Subsequent calculations of molecular O(2) adsorption to the optimal cluster structures reveal a size-dependent reactivity pattern that agrees well with earlier experiments. A detailed analysis of the underlying electronic structure shows that the chemical reactivity of the gold cluster anions can be elucidated in terms of a partial-jellium picture, where delocalized electrons occupying electronic shells move over the ionic skeleton, whose geometric structure is strongly influenced by the directional bonding associated with the highly localized "d-band" electrons.     

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.     

镍掺杂对氧化铝纳米片负载金催化剂在CO氧化反应中的促进作用（英文）

负载型金催化剂在CO氧化反应中具有良好的低温活性,受到了研究者的广泛关注,其催化性能与载体的性质密切相关.氧化铝具有廉价易得、比表面积大和热稳定性好等优点.然而,作为一种非还原性载体,氧化铝提供活性氧物种的能力差,与还原性载体相比催化剂的CO氧化活性较低.理论计算和实验结果表明,在金催化剂中引入过渡金属镍能够有效促进氧分子在催化剂表面的吸附和活化,从而提升金催化剂活性.此外,过渡金属的存在能够提高金的分散度,增加活性位数目,防止在高温预处理过程中金颗粒的烧结,从而提高催化剂的活性和稳定性.基于上述考虑,本文在氧化铝纳米片合成过程中原位引入硝酸镍,以实现对氧化铝载体的改性,然后负载金并应用于CO氧化反应.结果表明,当载体中的Ni/Al摩尔比为0.05,金负载量为1wt%时,采用还原性气氛对催化剂进行预处理可以得到具有CO氧化性能优良的金催化剂, 20 oC下CO转化率即可达100%.预处理气氛能够显著影响催化活性,采用还原性气氛预处理后催化剂活性明显优于氧化性气氛预处理.采用X射线衍射(XRD)、高分辨透射电镜(HRTEM)、氢气程序升温还原(H2-TPR)、氧气程序升温脱附(O2-TPD)、CO吸附原位红外光谱(CO-DRIFT)和X射线光电子能谱(XPS)等表征手段进一步研究了镍掺杂对Au/Al2O3催化剂上CO氧化反应的促进作用机制.XRD测试未观察到明显的金或镍衍射峰,表明金或镍物种均为高分散.HRTEM结果进一步证实,引入镍物种后金颗粒的粒径由3.6 nm减小为2.4 nm,表明镍掺杂有助于提高金的分散度.而XPS结果显示,镍掺杂催化剂中金与镍存在电子转移,而镍仍以Ni O为主.H2-TPR结果表明,镍掺杂的催化剂前驱体中的金物种更容易被还原.O2-TPD结果证实,镍掺杂催化剂能够引入更多的氧空位,促进氧分子的吸附和活化,从而促进CO氧化反应的进行.CO-DRIFT结果表明,相比于氧化性气氛,采用还原性气氛预处理后金物种的电子云密度增加, CO吸附增强.而对于镍掺杂的催化剂,金物种吸附CO分子的能力进一步提高,有利于CO氧化反应的进行.综上,镍掺杂能够有效提高催化剂中金的分散度,增强催化剂对CO的吸附,促进氧气分子的吸附和活化,从而提高了催化剂的CO氧化活性.     

Oxidation of CO on gold supported catalysts prepared by laser vaporization: direct evidence of support contribution

Gamma-Al2O3, ZrO2, and TiO2 gold supported model catalysts have been synthesized by laser vaporization. Structural characterization using Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy experiments have shown that the gold clusters deposited on the different supports have similar distribution of size centered around 3 nm and are in the metallic state. However, X-ray photoemission measurements also indicate lower binding energies than the usual Au 4f(7/2) at 84.0 eV for both alumina and titania supported catalysts, indicating a modification of the electronic structure of the metal. One has taken benefit of these features to study the influence of the nature of the support toward CO oxidation activities without being hindered by particle size or gold oxidic species effects. By comparing the activities of the different catalysts, it is concluded that the nature of the support directly affects the activity of gold. The following tendency is observed: titania and zirconia are superior to alumina as supports, titania being slightly better than zirconia. From XPS and activity results we can conclude that the existence of negatively charged clusters is not the key point to explain the high activity observed for Au/ZrO2 and Au/TiO2 catalysts and also that metallic Au is the major catalytically active phase. Hence, due to their very nature, titania and to a less extent zirconia should participate to the catalytic process.     

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.     

Size-dependent carbon monoxide adsorption on neutral gold clusters

We report on experiments probing the reactivity of neutral Au(n) clusters, n = 9-68, with carbon monoxide. The gold clusters are produced in a pulsed laser vaporization cluster source, operated at room temperature (RT) or at liquid-nitrogen temperature (LNT), pass through a low-pressure reaction cell containing CO gas, and are subsequently laser ionized. The reaction probabilities are determined by recording mass abundance spectra with time-of-flight mass spectrometry. The main observations are a strong temperature dependence and a remarkable size dependence. Upon cooling of the cluster source to LNT, the reactivity increases substantially. At LNT, the reaction probabilities for Au(n) with the first CO molecule are about a factor 10 higher than at RT. Moreover, adsorption of two, three, and even four CO molecules is observed, in contrast to RT clusters which at most adsorb one CO molecule. This temperature dependence is related to the lifetime of the cluster-molecule complexes, being much longer for cold clusters. The observed striking size dependence is similar at both temperatures and is discussed in terms of the electronic structure effects.