Heats of adsorption of linear CO species adsorbed on the Au degrees and Ti+delta sites of a 1% Au/TiO2 catalyst using in situ FTIR spectroscopy under adsorption equilibrium

The heats of adsorption of two linear CO species adsorbed on the Au degrees particles (denoted L(Au degrees)) and on the Ti(+delta) sites (denoted L(Ti+delta)) of a 1% Au/TiO(2) catalyst are determined as the function of their respective coverage by using the AEIR procedure (adsorption equilibrium infrared spectroscopy) previously developed. Mainly, the evolutions of the IR band area of each adsorbed species (2184 cm(-1) for L(Ti+delta) and at 2110 cm(-1) for L(Au degrees)) as a function of the adsorption temperature T(a), at a constant CO adsorption pressure P(CO), provide the evolutions of the coverages theta(LTi+delta) and theta(LAu degrees ) of each adsorbed CO species with T(a) in isobar conditions that give the individual heats of adsorption. It is shown that they linearly vary from 74 to 47 kJ/mol for L(Au degrees ) and from 50 to 40 kJ/mol for L(Ti+delta) at coverages 0 and 1, respectively. These values are consistent with literature data on model Au particles and TiO(2). In particular, it is shown that the mathematical formalism supporting the AEIR procedure can be applied to literature data on Au-containing solids (single crystals and model particles).     

The mechanism of emerging catalytic activity of gold nano-clusters on rutile TiO2(110) in CO oxidation reaction

This paper reveals the fact that the O adatoms (O(ad)) adsorbed on the 5-fold Ti rows of rutile TiO(2)(110) react with CO to form CO(2) at room temperature and the oxidation reaction is pronouncedly enhanced by Au nano-clusters deposited on the above O-rich TiO(2)(110) surfaces. The optimum activity is obtained for 2D clusters with a lateral size of ～1.5 nm and two-atomic layer height corresponding to ～50 Au atoms∕cluster. This strong activity emerging is attributed to an electronic charge transfer from Au clusters to O-rich TiO(2)(110) supports observed clearly by work function measurement, which results in an interface dipole. The interface dipoles lower the potential barrier for dissociative O(2) adsorption on the surface and also enhance the reaction of CO with the O(ad) atoms to form CO(2) owing to the electric field of the interface dipoles, which generate an attractive force upon polar CO molecules and thus prolong the duration time on the Au nano-clusters. This electric field is screened by the valence electrons of Au clusters except near the perimeter interfaces, thereby the activity is diminished for three-dimensional clusters with a larger size.     

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.     

CO adsorption and reaction on clean and oxygen-covered Au(211) surfaces

We have used primarily temperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRAS) to investigate CO adsorption on a Au(211) stepped single-crystal surface. The Au(211) surface can be described as a step-terrace structure consisting of three-atom-wide terraces of (111) orientation and a monatomic step with a (100) orientation, or 3(111) x (100) in microfacet notation. CO was only weakly adsorbed but was more strongly bound at step sites (12 kcal mol(-1)) than at terrace sites (6.5-9 kcal mol(-1)). The sticking coefficient of CO on the Au(211) surface was also higher ( approximately 5x) during occupation of step sites compared to populating terrace sites at higher coverages. The nu(CO) stretching band energy in IRAS spectra indicated that CO was adsorbed at atop sites at all coverages and conditions. A small red shift of nu(CO) from 2126 to 2112 cm(-1) occurred with increasing CO coverage on the surface. We conclude that the presence of these particular step sites at the Au(211) surface imparts stronger CO bonding and a higher reactivity than on the flat Au(111) surface, but these changes are not remarkable compared to chemistry on other more reactive crystal planes or other stepped Au surfaces. Thus, it is unlikely that the presence or absence of this particular crystal plane alone at the surface of supported Au nanoparticles has much to do with the remarkable properties of highly active Au catalysts.     

First-principles study of interaction of cluster Au32 with CO, H2, and O2

First-principles calculations are performed to study the interaction of cluster Au(32) with small molecules, such as CO, H(2), and O(2). The cagelike Au(32)(I(h)) shows a higher chemical inertness than the amorphous Au(32)(C(1)) with respect to the interaction with small molecules CO, H(2), and O(2). H(2) can only be physically adsorbed on Au(32)(I(h)), while it can be dissociatively chemisorbed on Au(32)(C(1)). Although CO can be chemically adsorbed on Au(32)(I(h)) and Au(32)(C(1)) with one electron transferred from Au(32) to the antibonding pi* orbit of CO, it is bound more strongly on Au(32)(C(1)) than on Au(32)(I(h)). Spin polarized and spin nonpolarized calculations result almost identical ground state structures of Au(32)(I(h))-O(2) and Au(32)(C(1))-O(2), in which O(2) is dissociatively chemisorbed.     

Probing the interface in vapor-deposited bimetallic Pd-Au and Pt-Au films by CO adsorption from the liquid phase

Bimetallic Pd-Au and Pt-Au and monometallic Pd, Pt, and Au films were prepared by physical vapor deposition. The resulting surfaces were characterized by means of XPS, AFM, and CO adsorption from the liquid phase (CH2Cl2) monitored by ATR-IR spectroscopy. CO adsorption combined with ATR-IR proved to be a very sensitive method for probing the degree of interdiffusion occurring at the interfaces whose properties were altered by variation of the Pd and Pt film thickness from 0.2 to 2 nm. Because no CO adsorption was observed on Au, the evaporation of Pt-group metals on Au allowed us to study the effect of dilution on the adsorption properties of the surfaces. At equivalent Pd film thickness, the evaporation of Au reduced the amount of adsorbed CO and caused the formation of 2-fold bridging CO, which was almost absent in monometallic surfaces. Additionally, the average particle size on Pd-Au surfaces was smaller than that on monometallic Pd surfaces. The results indicate that a Pd/Au diffuse interface is formed that affects the Pd particle size even more drastically than the simple decrease in Pd film thickness in monometallic surfaces. Pt-Au surfaces were less sensitive to CO adsorption, indicating that the two metals do not mix to a significant extent. The difference in the interfacial behavior of Pd and Pt in the bimetallic gold films is traced to the largely different Pd-Au and Pt-Au miscibility gaps.     

Effect of adsorption site, size, and composition of Pt/Au bimetallic clusters on the CO frequency: a density functional theory study

Density functional theory (DFT) calculations were performed to investigate the C-O stretching frequency changes when a CO molecule was adsorbed to Pt/Au clusters of 2-4 atoms. Our calculations show that the adsorption site is the most sensitive quantity to the C-O stretching frequency shifts. All the bridge site adsorptions yield a CO frequency band of 1737-1927 cm-1 with the CO bond distance of 1.167-1.204 A regardless of cluster composition and size, and all the atop site adsorptions yield a CO frequency band of 2000-2091 cm-1 with the CO bond distance of 1.151-1.167 A. More detailed analysis of the two frequency bands shows that each band may consist of two emerging subbands with the lower frequencies corresponding to the CO adsorption to Pt atoms and the higher frequencies to the CO adsorption to Au atoms. The insensitivity of the CO frequency shift to the cluster size indicates that the trend discussed here for small clusters may be used to interpret the experimental observations for nanoparticles. Our results also illustrated that the Fourier transform infrared spectroscopy measurement may be used as a sensitive tool to identify adsorption sites of the Pt/Au nanoparticles using CO adsorption as the probe.     

Low-temperature decomposition of methanol on Au nanoclusters supported on a thin film of Al2O3/NiAl100

With a variety of surface probe techniques, we investigated low-temperature decomposition of methanol on Au nanoclusters formed by vapor deposition onto an ordered Al(2)O(3)/NiAl(100) thin film. Upon adsorption of methanol on the Au clusters (with mean diameter 1.5-3.8 nm and height 0.45-0.85 nm) at 110 K, some of the adsorbed methanol dehydrogenates directly into carbon monoxide (CO); the produced hydrogen atoms (H) begin to desorb near 125 K whereas most of the CO desorbs above 240 K. The reaction exhibits a significant dependence on the Au coverage: the produced CO increases in quantity with the Au coverage, reaches a maximum at about 1.0-1.5 ML Au, whereas decreases with further increase of the Au coverage. The coverage-dependence is rationalized partly by an altered number of reactive sites associated with low-coordinated Au in the clusters. At least two kinds of reactive sites for the low-temperature decomposition are distinguished through distinct C-O stretching frequencies (2050 cm(-1) and 2092 cm(-1)) while the produced CO co-adsorbs with H and methanol.     

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.     

CO adsorption on pure and binary-alloy gold clusters: a quantum chemical study

We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters.     

Spectroscopic and Kinetic Studies of the Reaction of CO+H(2)O and CO+O(2) and Decomposition of HCOOH on Au/H-Mordenite Catalysts

The surface species formed from the reaction of CO+H(2)O and CO+O(2) and decomposition of HCOOH on Au incorporated into H-mordenite zeolite have been studied by means of in situ FTIR spectroscopy. On H-mordenite, a bidentate formate species (2912, 1536, and 1390 cm(-1)) is produced upon exposure to the CO+H(2)O gas mixture at 323 K, as well as different carbonate-like species (1956, 1852, 1705, and 1360 cm(-1)). The latter species was extensively formed in a short time and was responsible for hindering the CO(2) adsorbed species. However, Au/H-mordenite presented different vibration modes of formate species with a high emphasis on the monodentate ones (2950, 2916, 2896, 1690, and 1340 cm(-1)). The HCOOH adsorption on Au/H-mordenite showed two bands at 1622 and 1590 cm(-1) of the nu(as)(OCO) species, suggesting the formation of two types of formate species. The decomposition rate of the formate species formed on Au moieties was faster than that formed on H-mordenite. This was consistent with the calculated activation energies of CO(2) formation that showed a lower value (40.1 kJ/mol) on the former sample than on the latter one (63.3 kJ/mol). A dehydrogenation mechanism is proposed (HCOOH-->H(2)+CO(2)) for the decomposition of HCOOH on the Au/H-mordenite catalyst. On the other hand, the Au/H-mordenite catalyst activated the CO oxidation reaction. This reaction proceeded mainly through the formation of carboxylate species at first, which tended to obviate with time, preferring the formate species. The latter species resulted from the interaction of CO with OH stretching of the zeolite assisted by the presence of gas phase O(2). The formate species is further decomposed with time to carbonate species. Copyright 2000 Academic Press.     

CO在M55（M=Cu,Ag,Au）团簇上吸附的密度泛函研究

在全电子相对论BVP86/DNP水平下对CO在Au55,Ag55和Cu55团簇上的吸附进行了比较研究,并考察了电荷对吸附的影响.计算结果表明,CO在Au55团簇上吸附能最大,其次为Cu55团簇,最弱的为Ag55团簇.团簇电荷对C—O键活化和CO与团簇表面原子成键影响较小.金团簇的电荷对吸附能影响较大,而银和铜团簇的电荷对吸附能影响较小.CO吸附到团簇上导致团簇上电子向CO转移.C—O键活化强度与吸附位置密切相关,其中孔位吸附导致C—O键活化程度最大,最弱的为顶位吸附.CO在金团簇上吸附具有较好选择性,而在银和铜团簇上吸附无选择性.     

In situ transient FTIR and XANES studies of the evolution of surface species in CO oxidation on Au/TiO2

The adsorption of CO and its reaction with oxygen were investigated using a combination of in situ Fourier transform infrared spectroscopy, step response measurements in a microreactor, (18)O isotopic labeling, and X-ray absorption near edge structure spectroscopy. An as-prepared sample in which Au is present as a surface oxyhydroxy complex does not adsorb CO. On an activated sample in which only metallic Au is detected, 0.18 +/- 0.03 mol CO/(mol Au) are adsorbed on Au at -60 degrees C, which shows an IR band at 2090 cm(-1). When oxygen is present in the gas phase, this species reacts with a turnover rate of 1.4 +/- 0.2 mol CO(mol Au min)(-1), which is close to the steady-state turnover rate. In contrast, there is a very small quantity of adsorbed oxygen on Au. A small IR peak at 1242 cm(-1) appears when an activated sample is exposed to CO. It reacts rapidly with oxygen and is shifted to 1236 cm(-1) if (18)O is used. It is assigned to the possible intermediate hydroxycarbonyl.     

Cinchonidine adsorption on gold and gold-containing bimetallic platinum metal surfaces: an attenuated total reflection infrared and density functional theory study

Adsorption of cinchonidine on monometallic Au and bimetallic Pt-Au and Pd-Au thin model films prepared by physical vapor deposition has been investigated with attenuated total reflection infrared (ATR-IR) spectroscopy. On Au the alkaloid forms an adsorbed layer that shows higher stability against desorption than the corresponding adsorption on Pt. In this adsorption layer the intermolecular interactions dominate over metal-adsorbate interactions as indicated by the absence of the spectroscopic features attributed to strongly flat adsorbed species. This behavior is further supported by Density Functional Theory (DFT) calculations indicating that flat and tilted orientations of the quinoline ring have comparable adsorption energy on Au but lower (7-10 kcal/mol) compared to adsorption on Pt (ca. 40 kcal/mol). As a consequence, the creation of a metal surface with isolated chiral sites is prevented by formation of an adsorbed structure formed by intermolecularly bound cinchonidine molecules on Au. While the binding to Pt is due to the formation of sigma-bonds to surface atoms, such aggregates are bound to Au mainly by van der Waals forces. Given this different nature of bonding of cinchonidine to Au and Pt, addition of Au to Pt and Pd films could be used to probe the changes of fractional coverage of the different adsorbed species of cinchonidine on the platinum metals. It is demonstrated that the lowering of the domain size of the platinum group metal by Au can simulate the effect of particle size on the distribution of the surface conformations of the alkaloid on a metal surface.     

Reaction heat-driven CO2 desorption during CO oxidation on Au(997) at low temperatures

Adsorption and reaction of CO and CO₂ were studied on oxygen-covered Au(997) surfaces by means of temperatureprogrammed desorption/reaction spectroscopy. Oxygen atoms (O(a)) on Au(997) enhances the CO₂ adsorption and stabilizes the adsorbed CO₂(a), and the stabilization effect also depends on the CO₂(a) coverage and involved Au sites. CO₂(a) desorption is the rate-limiting step for the CO+O(a) reaction to produce CO₂ on Au(997) at 105 K and exhibits complex behaviors, including the desorption of CO₂(a) upon CO exposures at 105 K and the desorption of O(a)-stabilized CO₂(a) at elevated temperatures. The desorption of CO₂(a) from the surface upon CO exposures at 105 K to produce gaseous CO₂ depends on the surface reaction extent and involves the reaction heat-driven CO₂(a) desorption channel. CO+O(a) reaction proceeds more easily with weakly-bound oxygen adatoms at the (111) terraces than strongly-bound oxygen adatoms at the (111) steps. These results reveal complex rate-limiting CO₂(a) desorption behaviors during CO+O(a) reaction on Au surfaces at low temperatures which provide novel information on the fundamental understanding of Au catalysis.     

Cluster size-dependent mechanisms of the CO + NO reaction on small Pdn (n < or = 30) clusters on oxide surfaces

The CO + NO reaction (2CO + 2NO --> N(2) + 2CO(2)) on small size-selected palladium clusters supported on thin MgO(100) films reveals distinct size effects in the size range Pd(n) with n < or = 30. Clusters up to the tetramer are inert, while larger clusters form CO(2) at around 300 K, and this main reaction mechanism involves adsorbed CO and an adsorbed oxygen atom, a reaction product from the dissociation of NO. In addition, clusters consisting of 20-30 atoms reveal a low-temperature mechanism observed at temperatures below 150 K; the corresponding reaction mechanism can be described as a direct reaction of CO with molecularly adsorbed NO. Interestingly, for all reactive cluster sizes, the reaction temperature of the main mechanism is at least 150 K lower than those for palladium single crystals and larger particles. This indicates that the energetics of the reaction on clusters are distinctly different from those on bulklike systems. In the presented one-cycle experiments, the reaction is inhibited when strongly adsorbed NO blocks the CO adsorption sites. In addition, the obtained results reveal the interaction of NO with the clusters to show differences as a function of size; on larger clusters, both molecularly bonded and dissociated NO coexist, while on small clusters, NO is efficiently dissociated, and hardly any molecularly bonded NO is detected. The desorption of N(2) occurs on the reactive clusters between 300 and 500 K.     

Spectroscopic Identification of Adsorbed Intermediates Derived from the CO+H(2)O Reaction on Zeolite-Encapsulated Gold Catalysts

Identification of reaction intermediates in the water-gas shift reaction (WGSR: H(2)O+CO-->H(2)+CO(2)) on Au(n+) (1相似文献   

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.     

Fullerene adlayers on various single-crystal metal surfaces prepared by transfer from L films

Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.     

Theoretical study of the CO catalytic oxidation on Au/SAPO‐11 zeolite

Quantum chemistry calculations were carried out, using ONIOM2 methodology, to investigate the CO adsorption and oxidation on gold supported on Silicoaluminophospates (SAPO) molecular sieves Au/SAPO‐11 catalysts. Two models were studied, one containing one Au atom per site (Au? SAPO‐11), and the other with two Au atoms per site (Au₂? SAPO‐11). The results reveal that the CO adsorption and oxidation are exothermic on Au/SAPO11 with an ΔE of ?41.0 kcal/mol and ΔE = ?52.0 kcal/mol, for the adsorption and oxidation, respectively. On the Au₂? SAPO‐11 model, the CO adsorption and oxidation reaction occur, with a ΔE of ?29.7 kcal/mol and ?52 kcal/mol, respectively. According to our results, the oxidation reaction exhibits an Eley‐Rideal type mechanism with adsorbed CO. The theoretical calculations reveal that this type of material could be interesting to disperse Au and consequently to strengthen its catalytic use for different reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2573–2582, 2010