A scanning tunneling microscope study of single platinum atoms,platinum dimers and trimers on highly-oriented pyrolytic graphite

We report a topographic study of platinum clusters on highly-oriented pyrolytic graphite (HOPG) using a scanning tunneling microscope operating in air. The particles were produced by evaporation of platinum onto the graphite-surface in high vacuum. The simultaneous finding of single platinum atoms, clusters and small particles on an otherwise clean and atomically flat surface shows that the platinum-HOPG surface interaction is strong enough to yield stable images of Pt atoms and yet is not strong enough to annihilate the Pt-Pt interaction. Small flat platinum clusters on HOPG can be imaged with atomic resolution of the cluster and the surrounding graphite lattice. We show the adsorption site distribution for the monomers. The Pt-dimers show a very broad bond length distribution on graphite with an average of 2.46 Å. We found two types of Pt-trimers, one which is almost linear and one of triangular form. The average nearest neighbour distance of the trimers is 2.61 Å.     

Structure of ethene adsorption sites on supported metal catalysts from in situ XANES Analysis

The structures of the catalytically active sites in supported metal catalysts are a long sought after goal. In this study, XAS has been used to establish these structures. The ethene-induced changes in the XAS spectra as a function of temperature and pressure were correlated to changes in the adsorption mode of the hydrocarbon. At low temperature, ethene was adsorbed in on-top (pi) and bridged (di-sigma) sites on small platinum clusters. Below room temperature, the adsorbed ethene was dehydrogenated to an ethylidyne species, which was adsorbed in threefold Pt sites. On larger clusters the dehydrogenation proceeded at higher temperature indicating a different reactivity. EXAFS results showed that changes in the geometrical structures were mainly due to (co)adsorbed hydrogen. Our results for platinum agree with those obtained using other techniques proving that detailed shape analysis of the L3 edge XANES is a practical tool to determine the structure of the sites that are involved in bonding to reactants and intermediates. Application to gold and alloy catalysts showed that ethene induced a significant change in the electronic structure of gold nanoclusters that could be interpreted as ethene adsorbed on top of single gold atoms or in bridged sites. Ethene adsorbed on both platinum and gold in the bimetallic clusters.     

Preparation of polymer-protected gold/platinum bimetallic clusters and their application to visible light-induced hydrogen evolution

The dispersions of polymer-protected gold/platinum bimetallic clusters were easily and reproducibly prepared by refluxing the mixed solutions of tetrachloroaureic(III) acid and hexachloroplatinic(IV) acid in ethanol/water (1/1) at 90 ∼ 95 °C for 2 h in the presence of a protective polymer such as poly(N-vinyl-2-pyrrolidone) (PVP). The gold/platinum bimetallic clusters thus obtained were very small, well dispersed and very stable. The UV-Vis spectra and the transmission electron micrographs have indicated that each bimetallic particle has an alloy structure consisting of both gold and platinum atoms, and that the surface of the cluster particle is rich in platinum atoms and the inner core in gold atoms. The gold/platinum bimetallic clusters were used as the multi-electron redox catalysts for visible light-induced hydrogen evolution from water. The rate of hydrogen evolution depended on the mole ratio of the gold/platinum bimetallic clusters. The bimetallic clusters at the mole ratio of Au/Pt = 2/3 were the most active catalyst. The in-situ UV-Vis spectra during the reaction have indicated that the order of the aggregation in the two kinds of metal atoms is very important for structure determination of the Au/Pt bimetallic clusters. The protective polymer PVP plays a role not only in protecting hydrophobic colloidal particles in an aqueous solution, but also in determining the metal composition of the cluster surface.     

Activation of molecular hydrogen on platinum nanoparticles: Quantum-chemical modeling

The interaction of molecular hydrogen with platinum clusters of different size has been modeled by the density functional theory method within the generalized gradient approximation (GGA). The cluster size turns out to have little effect on the interaction energy, whereas the effect of the cluster structure is rather significant. The most efficient interaction with hydrogen is observed for clusters with a structure resembling the crystal structure of platinum metal. In such clusters, the hydrogen molecule is attached to its surface without a barrier. Configurations with the bidentate hydrogen coordination are the most stable ones. The H atoms can migrate over the cluster surface, overcoming moderate potential barriers of ∼0.3–0.4 eV.     

The effect of alloying on the H-atom adsorption on the (100) surfaces of Pd-Ag,Pd-Pt,Pd-Au,Pt-Ag,and Pt-Au. A theoretical study

The effect of alloying on the adsorption of atomic hydrogen was studied using density functional theory (DFT). In the study the (100) surfaces of Pd-Ag, Pd-Pt, Pd-Au, Pt-Ag, and Pt-Au alloys were considered by means of a cluster model. The structural and energetic properties of the H atom adsorbed on the Pd₄Me (Me = Ag, Pt, Au) and Pt₄Me (Me = Pd, Ag, Au) clusters were calculated and compared with the H-atom adsorption on monometallic clusters. The effect of alloying on the H-atom adsorption is evident for all the investigated bimetallic systems. However, it strongly depends on the second metal atom, Me, is placed in the surface layer or in the subsurface one. In general, the H atom adsorbed in a site containing the second metal exhibits different properties from those characteristic of its adsorption on Pd(100) and Pt(100). Hence, the modified interaction between atomic hydrogen and the alloyed surfaces may increase the selectivity of the catalytic hydrogenation reactions on such surfaces.     

A density functional theory study of the dissociation of H2 on gold clusters: importance of fluxionality and ensemble effects

Density functional theory was employed to calculate the adsorption/dissociation of H2 on gold surfaces, Au(111) and Au(100), and on gold particles from 0.7 (Au14) to 1.2 nm (Au29). Flat surfaces of the bulk metal were not active towards H2, but a different effect was observed in gold nanoclusters, where the hydrogen was adsorbed through a dissociative pathway. Several parameters such as the coordination of the Au atoms, ensemble effects and fluxionality of the particle were analyzed to explain the observed activity. The effect of the employed functional was also studied. The flexibility of the structure, i.e., its adaptability towards the adsorbate, plays a key role in the bonding and dissociation of H2. The interaction with hydrogen leads to drastic changes in the structure of the Au nanoparticles. Furthermore, it appears that not only low coordinated Au atoms are needed because H2 adsorption/dissociation was only observed when a cooperation between several (4) active Au atoms was allowed.     

The effect of hydrogen adsorption on the electronic structure of gold nanoparticles

It has been demonstrated that hydrogen adsorption has an effect on the electronic structure of gold nanoparticles. The physicochemical properties of separate gold nanoparticles have been studied under an ultrahigh vacuum scanning tunneling microscope. The structure and electronic structure of gold–hydrogen clusters were modeled by the quantum-chemical density functional theory method. Hydrogen adsorption onto gold nanoparticles 4–5 nm is size at room temperature was experimentally revealed, and the lower limit of 1.7 eV for the Au–H bond energy was determined. The interaction of hydrogen with gold leads to a considerable rearrangement of the electronic subsystem of nanoparticles. The experimentally observed effects were supported by quantum-chemical calculations. The rearrangement mechanism is related to strong correlations in the electronic subsystem.     

Structures of active sites for alkane transformations over the Pt/HZSM-5 and Pt/NaZSM-5 catalysts

The structure and reactivity of Pt₆ particles in the sodium and hydrogen forms of the ZSM-5 zeolite were investigated by the DFT method. Upon adsorption on the sodium form, the interaction energy is 15 kcal mol⁻¹ and a negative charge appears on the metal cluster. In channels of the hydrogen form, the adsorption energy of the metal particle increases to 45 kcal mol⁻¹ and the oxidized states of platinum are formed. The formation of an active site in the H-form of the zeolite involves the interaction of the platinum particle with the acid site resulting in the suppression of the acidity of the support. An alternative mechanism of alkane transformations avoiding acid site participation was proposed. A possibility to envisage the direction of the transformation of alkanes adsorbed on the metal particles was shown. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1139–1144, June, 2008.     

Reverse hydrogen spillover on and hydrogenation of supported metal clusters: insights from computational model studies

"Reverse" spillover of hydrogen from hydroxyl groups of the support onto supported transition metal clusters, forming multiply hydrogenated metal species, is an essential aspect of various catalytic systems which comprise small, highly active transition metal particles on a support with a high surface area. We review and analyze the results of our computational model studies related to reverse hydrogen spillover, interpreting available structural and spectral data for the supported species and examining the relationship between metal-support and metal-hydrogen interactions. On the examples of small clusters of late transition metals, adsorbed in zeolite cavities, we showed with computational model studies that reverse spillover of hydrogen is energetically favorable for late transition metals, except for Au. This preference is crucial for the chemical reactivity of such bifunctional catalytic systems because both functions, of metal species and of acidic sites, are strongly modified, in some cases even suppressed - due to partial oxidation of the metal cluster and the conversion of protons from acidic hydroxyl groups to hydride ligands of the metal moiety. Modeling multiple hydrogen adsorption on metal clusters allowed us to quantify how (i) the support affects the adsorption capacity of the clusters and (ii) structure and oxidation state of the metal moiety changes upon adsorption. In all models of neutral systems we found that the metal atoms are partially positively charged, compensated by a negative charge of the adsorbed hydrogen ligands and of the support. In a case study we demonstrated with calculated thermodynamic parameters how to predict the average hydrogen coverage of the transition metal cluster at a given temperature and hydrogen pressure.     

Selected properties of Pt(111) modified surfaces: A DFT study

Density functional theory calculations were employed to investigate the electronic properties of a Pt(111) surface modified with foreign atoms. The effects of alloying platinum with molybdenum, palladium, and tin changed the interaction between adsorbate orbital and metal d band. This letter discusses the interaction between metal atoms and adsorbate and its influence on electronic structure rearrangement of the species—changes that must be taken into account to explain the behavior of catalytic systems and sensors. Mo/Pt(111) and Sn/Pt(111) exhibited lower susceptibility to poisoning by CO, compared with pure platinum. Both Pt-based materials are expected to find utility in electrodes for alcohol and hydrogen oxidation.     

Electroreduction of carbon dioxide: Part III. Adsorption and reduction of CO2 on platinum

The main properties of reductional adsorption of CO₂ on the platinum metals are studied. Chemisorbed particles are found to be produced only on platinum and rhodium. Electroreduction of CO₂ on these metals proceeds as a result of the interaction of CO₂ molecules activated in the course of adsorption on the metal surface with chemisorbed hydrogen. As a result, strongly chemisorbed particles are obtained on the surface which are the products of more profound reduction of CO₂ than to formic acid. The further reduction of these chemisorbed particles, accompanied by their desorption into the solution, is very slow due to very strong coupling of sorbed particles with the surface and to very fast backward adsorption of the reduction products. Neither reductional chemisorption of CO₂ nor interactions of CO₂ with adsorbed hydrogen were observed for iridium, palladium, osmium or ruthenium.     

Preparation, characterization, and reactivity of Pt/SDBC catalysts for the hydrogen-water isotopic exchange reaction

Pt/SDBC catalysts, which are used for the hydrogen-water isotopic exchange reaction, were prepared. TGA experiments showed that the treatment temperature of Pt/SDBC catalysts in inert gas is limited to 400 °C and the maximum allowable heat treatment temperature in oxygen is 200 °C. From nitrogen adsorption and hydrogen chemisorption measurements, it was shown that the dispersion of platinum particles depended on the physical properties, i.e., specific surface area and pore structure of SDBC. It was found that the heat treatment could not impact the structure of SDBC and the oxygen treatment at 150 °C improved the platinum dispersion. It was shown by XPS analysis that the oxygen treatment of impregnated Pt/SDBC increased the fraction of platinum metal state and platinum dispersion. As the supported platinum area increases, the catalytic activity of Pt/SDBC for the hydrogen-water vapor isotopic exchange reaction increases. It indicates that the hydrogen chemisorption measurement can be used to estimate the catalytic activities of Pt/SDBC catalysts. It was not observed that the particle size of supported platinum affected the specific reaction rate at 60 °C. It implies that this reaction is structure insensitive.     

The adsorption of CO on group 10 (Ni, Pd, Pt) transition-metal clusters

The adsorption of a single CO molecule on clusters of the Group 10 transition metals is characterized by infrared multiple photon dissociation (IR-MPD) spectroscopy. The cationic, neutral, and anionic carbonyl complexes contain between 3 and up to 25 metal atoms. The C-O stretching frequency nu(CO) shows that while both nickel and platinum clusters adsorb CO only in atop positions, palladium clusters exhibit a variety of binding sites. These findings can be rationalized by considering the increasing role relativistic effects play in the electronic structure of the cluster complexes going down the group. Conclusions for the cluster-support interactions for size-selected supported particles are drawn from the charge dependence of nu(CO) for the gas-phase species.     

Gold catalysts for pure hydrogen production in the water-gas shift reaction: activity, structure and reaction mechanism

The production of hydrogen containing very low levels of carbon monoxide for use in polymer electrolyte fuel cells requires the development of catalysts that show very high activity at low temperatures where the equilibrium for the removal of carbon monoxide using the water-gas shift reaction is favourable. It has been claimed that oxide-supported gold catalysts have the required high activity but there is considerable uncertainty in the literature about the feasibility of using these catalysts under real conditions. By comparing the activity of gold catalysts with that of platinum catalysts it is shown that well-prepared gold catalysts are significantly more active than the corresponding platinum catalysts. However, the method of preparation and pre-treatment of the gold catalysts is critical and activity variations of several orders of magnitude can be observed depending on the methods chosen. It is shown that an intimate contact between gold and the oxide support is important and any preparative procedure that does not generate such an interaction, or any subsequent treatment that can destroy such an interaction, may result in catalysts with low activity. The oxidation state and structure of active gold catalysts for the water-gas shift reaction is shown to comprise gold primarily in a zerovalent metallic state but in intimate contact with the support. This close contact between small metallic gold particles and the support may result in the "atoms" at the point of contact having a net charge (most probably cationic) but the high activity is associated with the presence of metallic gold. Both in situ XPS and XANES appear unequivocal on this point and this conclusion is consistent with similar measurements on gold catalysts even when used for CO oxidation. In situ EXAFS measurements under water gas shift conditions show that the active form of gold is a small gold cluster in intimate contact with the oxide support. The importance of the gold/oxide interface is indicated but the possible role of special sites (e.g., edge sites) on the gold clusters cannot be excluded. These may be important for CO oxidation but the fact that water has to be activated in the water gas shift reaction may point towards a more dominant role for the interfacial sites. The mechanism of the water gas shift reaction on gold and other low temperature catalysts has been widely investigated but little agreement exists. However, it is shown that a single "universal" model is consistent with much of the experimental literature. In this, it is proposed that the dominant surface intermediate is a function of reaction conditions. For example, as the temperature is increased the dominant species changes from a carbonate or carboxylate species, to a formate species and eventually at high temperatures to a mechanism that is characteristic of a redox process. Similar changes in the dominant intermediate are observed with changes in the gas composition. Overall, it is shown that reported variations in the kinetics, structure and reaction mechanism for the water gas shift reaction on gold catalysts can now be understood and rationalised.     

Growth of a Pt film on non-reduced ceria: a density functional theory study

The growth of platinum on non-reduced CeO(2) (111) surface is studied by means of calculations based on the density functional theory. Particles of increasing size are formed on the oxide surface by incorporating the platinum atoms one by one until multilayer films are obtained. The main conclusion is that platinum atoms tend to maximize the number of metallic bonds and to approach the situation of the bulk, hence preferring films to particles, particles to isolated atoms, and a three-dimensional growth to a two-dimensional one. The supported particles and the films exhibit a contraction of the Pt-Pt distances, with respect to those of the Pt bulk, in order to match the ceria lattice. The density of states projected on the film surface platinum atoms shows important differences in shape and energy (lower d-band center) compared to the Pt(111) reference surface, which could be the major reason for the observed changes in catalytic reactivity when deposited particles are compared with single crystal surfaces.     

Density functional investigation of the interaction of acetone with small gold clusters

The structural evolution of Au(n) (n=2, 3, 5, 7, 9, and 13) clusters and the adsorption of organic molecules such as acetone, acetaldehyde, and diethyl ketone on these clusters are studied using a density functional method. The detailed study of the adsorption of acetone on the Au(n) clusters reveals two main points. (1) The acetone molecule interacts with one gold atom of the gold clusters via the carbonyl oxygen. (2) This interaction is mediated through back donation mainly from the spd-hybridized orbitals of the interacting gold atom to the oxygen atom of the acetone molecule. In addition, a hydrogen bond is observed between a hydrogen atom of the methyl group and another gold atom (not involved in the bonding with carbonyl oxygen). Interestingly, the authors notice that the geometries of Au(9) and Au(13) undergo a significant flattening due to the adsorption of an acetone molecule. They have also investigated the role of the alkyl chain attached to the carbonyl group in the adsorption process by analyzing the interaction of Au(13) with acetaldehyde and diethyl ketone.     

Model electrodes with defined mesoscopic structure

Model electrodes with defined mesoscopic structure were either generated by adsorption of surfactant stabilized metal clusters from colloidal solution on a support of gold or by electrochemical deposition of platinum on gold substrates. Both types of model electrodes were characterized by STM (scanning tunnelling microscopy), cyclic voltammetry and electrooxidation of adsorbed CO. The supported colloidal Pt as well as the electrochemically deposited Pt revealed different reactivities regarding the CO monolayer electrooxidation as compared to a polycrystalline Pt bulk electrode. In addition, in-situ FTIR (Fourier transformed infrared) spectroscopy was applied to characterize CO adsorbed on electrochemically deposited Pt on gold. Combined with the structural information from STM it seems likely that the differences regarding the catalytic properties of the model electrodes are due to different coverages of the substrate with catalyst particles. Received: 24 June 1996 / Revised: 29 November 1996 / Accepted: 4 December 1996     

Hydrogen chemisorption on Al2O3-supported gold catalysts

Hydrogen is dissociatively adsorbed on the gold particles in Au/Al(2)O(3) catalysts, as demonstrated by a combination of in-situ X-ray absorption spectroscopy, chemisorption, and H/D exchange experiments. This chemisorption of hydrogen induces changes in the Au L(3) and L(2) X-ray absorption near-edge structures. The gold atoms on corner and edge positions dissociate the hydrogen, which does not spill over to the face sites. Therefore, the average number of adsorbed hydrogen atoms per surface gold atom increases with decreasing particle size. With temperature, the hydrogen uptake by supported gold increases or remains constant, whereas it decreases for platinum. Furthermore, in H/D exchange experiments, the activity of Au/Al(2)O(3) increases strongly with temperature. Thus, the dissociation and adsorption of hydrogen on gold is activated.     

Effects of adsorbates on supported platinum and iridium clusters: Characterization in reactive atmospheres and during alkene hydrogenation catalysis by X-ray absorption spectroscopy

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.     

Structural, energetic, and electronic properties of hydrogenated titanium clusters

Hydrogen undergoes dissociative chemisorption on small titanium clusters. How the electronic structure of the cluster changes as a function of the number of adsorbed hydrogen atoms is an important issue in nanocatalysis and hydrogen storage. In this paper, a detailed theoretical investigation of the structural, energetic, and electronic properties of the icosahedral Ti13 cluster is presented as a function of the number of adsorbed hydrogen atoms. The results show that hydrogen loaded Ti13H20 and Ti13H30 clusters are exceptionally stable and are characterized by hydrogen multicenter bonds. In Ti13H20, the dissociated hydrogen atoms are bound to each of the 20 triangular faces of Ti13, while in Ti13H30, they are bound to the 30 Ti-Ti edges of Ti13. Consequently, the chemisorption and desorption energies of the Ti13H20 (1.93 eV, 3.10 eV) are higher than that of Ti13H30 (1.13 eV, 1.95 eV). While increased hydrogen adsorption leads to an elongation of the Ti-Ti bonds, there is a concomitant increase in the electrostatic interaction between the dissociated hydrogen atoms and the Ti13 cluster. This enhanced interaction results from the participation of the subsurface titanium atom at higher hydrogen concentrations. Illustrative results of hydrogen saturation on the larger icosahedral Ti55 cluster are also discussed. The importance of these results on hydrogen saturated titanium clusters in elucidating the mechanism of hydrogen adsorption and desorption in titanium doped complex metal hydrides is discussed.