A comparative theoretical study of Au,Ag and Cu adsorption on TiO<Subscript>2</Subscript> (110) rutile surfaces

The adsorption properties of Au, Ag and Cu on TiO₂ (110) rutile surfaces are examined using density functional theory slab calculations within the generalized gradient approximation. We consider five and four different adsorption sites for the metal adsorption on the stoichiometric and reduced surfaces, respectively. The metal-oxide bonding mechanism and the reactivity of metal atoms are also discussed based on the analyses of local density of states and charge density differences. This study predicts that Au atoms prefer to adsorb at the fourfold hollow site over the fivefold-coordinated Ti(5c) and in-plane and bridging O(2c) atoms with the adsorption energy of ≈0.6 eV. At this site, it appears that the covalent and ionic interactions with the Ti(5c) and the O(2c), respectively, contribute synergistically to the Au adsorption. At a neutral F _s ⁰ center on the reduced surface, Au binds to the surface via a rather strong ionic interaction with surrounding sixfold-coordinated Ti(6c) atoms, and its binding energy is much larger than to the stoichiometric surface. On the other hand, Ag and Cu strongly interact with the surface bridging O(2c) atoms, and the site between two bridging O(2c) atoms is predicted to be energetically the most favorable adsorption site. The adsorption energies of Ag and Cu at the B site are estimated to be ≈1.2 eV and ≈1.8 eV, respectively. Unlike Au, the interaction of Ag and Cu with a vacancy defect is much weaker than with the stoichiometric surface. °This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Creation of Low-Coordination Gold Sites on Au(111) Surface by 1,4-phenylene Diisocyanide Adsorption

The adsorption of CO on a saturated overlayer of 1,4-phenylene diisocyanide (PDI) adsorbed on a Au(111) surface at 300 K is studied using scanning tunneling microscopy (STM), density functional theory (DFT) calculations and reflection absorption infrared spectroscopy (RAIRS). The PDI forms closed-packed rows of gold-PDI chains by extracting gold atoms from the Au(111) substrate. They are imaged by STM and the structure calculated by DFT. The adsorption of CO is studied on the low-coordination gold sites formed on the PDI-covered surface where it adsorbs exhibiting a CO stretching frequency of 2004 cm^?1, consistent with adsorption on an atop site. It is found that CO is stable on heating the sample to ~150 K and is only removed from the surface by heating to ~180 K. Since low-coordination gold atoms are suggested to be the active catalytic sites on supported gold nanoclusters, ??embossing?? the surface to form similar low-coordination sites using PDI might offer a strategy for tailoring the catalytic activity of gold.     

Density functional study on the reaction of CO molecules with MgO surfaces

Adsorption of CO to the MgO surface modeled by Mg_nO_n (n = 4, 6, 9, 10) clusters was investigated employing the density functional method, and modes of bonding, adsorption energies, and CO exchange mechanisms were discussed. The atoms at the low coordination sites possess the small amount of charges consistent with the crystal field theory. The adsorption to such sites results in large stabilization though the magnitude is less remarkable than in the case of hydrogen adsorption. A possible mechanism for the CO exchange reaction, observed experimentally, is presented based on the energetics calculated. The CO's adsorbed and in the gas phase are exchangeable through the two-molecule adsorption state, which is realized at the edge site or the O-atom defect site. For the latter, the structure of the intermediate is more consistent with the IR measurement.     

Special Sites at Noble and Late Transition Metal Catalysts

An overview of recent advancements in density functional theory modeling of particularly reactive sites at noble and late transition metal surfaces is given. Such special sites include sites at the flat surfaces of thin metal films, sites at stepped surfaces, sites at the metal/oxide interface boundary for oxide-supported metal clusters, and sites at the perimeter of oxide islands grown on metal surfaces. The Newns–Anderson model of the electronic interaction underlying chemisorption is described. This provides the grounds for introducing the Hammer–N?rskov d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. A reactivity change described by this model is characterized as an electronic structure effect. Br?nsted plots of energy barriers versus reaction energies are discussed from the surface reaction perspective and are used to analyze the trends in the calculated changes. Deviations in the relation between energy barriers and reaction energies in Br?nsted plots are identified as due to atomic structure effects. The reactivity change from pure Pd surfaces to Pd thin films supported on MgO can be assigned to an electronic effect. Likewise for the reactivity change from flat Au surfaces, over Au thin films to Au edges and the Au/MgO interface boundary. The reactivity enhancement at atomic step sites is of both electronic and atomic structure nature for NO dissociation at Ru, Rh and Pd surfaces. The enhancement of the CO oxidation reactivity when moving from a CO+O coadsorption structure on Pt(111) to the PtO₂ oxide island edges supported by Pt(111) is, however, identified as mainly an atomic structure effect. As such, it is linked to the occurrence of favorable pathways at the oxide island edges and is occurring despite of stronger adsorbate binding of the oxygen within the oxide edge, i.e. despite of an opposing electronic effect. As a final topic, a discussion is given of the accuracy of density functional theory in conjunction with surface reactions; adsorption, desorption, diffusion, and dissociation. Energy barriers are concluded to be more robust with respect to changes in the exchange-correlation functional than are molecular bond and adsorption energies.     

Insights into the reactivity of supported Au nanoparticles: combining theory and experiments

The origin of the extraordinary catalytic activity of gold nanoparticles is discussed on the basis of density-functional calculations, adsorption studies on single crystal surfaces, and activity measurements on well characterized supported gold particles. A number of factors are identified contributing to the activity, and it is suggested that it is useful to consider low-coordinated Au atoms as the active sites, for example, CO oxidation and that the effect of the support can be viewed as structural and electronic promotion. We identify the adsorption energy of oxygen and the Au-support interface energy as important parameters determining the catalytic activity.     

Dendrimer templates for heterogeneous catalysts: Bimetallic Pt–Au nanoparticles on oxide supports

Polyamidoamine (PAMAM) dendrimers were used to template Pt, Au, and bimetallic Pt–Au dendrimer encapsulated nanoparticles (DENs) in solution. Adjusting the solution pH allowed for slow, spontaneous adsorption of the nanoparticles onto silica, alumina, and titania. After dendrimer removal, the catalysts were characterized with infrared spectroscopy of adsorbed CO and tested with CO oxidation catalysis. Infrared spectroscopy of the monometallic Pt catalysts showed a slight shift in the CO stretching frequency for the different supports. For the bimetallic catalysts, infrared spectra showed CO adsorbed on both Pt and on Au sites. Spectra collected during CO desorption showed substantial interactions between the two bands, confirming the presence of bimetallic particles on all the supports. The bimetallic catalysts were found to be more active than the monometallic catalysts and had lower apparent activation energies. The titania supported Pt–Au catalyst was resistant to deactivation during an extended treatment at 300 °C. Correlations between IR spectra and catalytic activity showed differences between the mono- and bimetallic materials and implicated a bimetallic Pt–Au ensemble at the catalytic active site. This is the first study to show that DENs are appropriate precursors for studying support effects on catalysis by metal nanoparticles, although the magnitude of the effects were small.     

Insights into the active sites of dual-zone synergistic catalysts for semi-hydrogenation under hydrogen spillover

A well-defined catalyst with platinum (Pt) and gold (Au) encapsulated in micropore and mesopore of micro-mesoporous zeolite (Pt-Au/TMSN), respectively, was designed to investigate the original active sites of semi-hydrogenation. Specifically, hydrogen molecules are dissociated on Pt nanoclusters (NCs) to form hydrogen atoms that migrate to the surfaces of TMSN zeolite and Au nanoparticles (NPs) through hydrogen spillover effect. The characterization and catalytic results demonstrate that the Au NPs and zeolite surface are both identified as the semi-hydrogenation sites. Especially, the Au active site with low adsorption ability of alkene possesses preferable selectivity in the semi-hydrogenation of phenylacetylene and 1,5-cyclooctadiene. Noteworthy, the Pt-Au/TMSN exhibits higher selectivity of phenylethylene and cyclooctene than Pt/TMSN, as well as higher turnover frequency than Au/MSN. This work creates an effective regulation strategy of active sites working with a tandem mechanism for improving the semi-hydrogenation performance.     

Insights from Theory on the Relationship Between Surface Reactivity and Gold Atom Release

Density functional theory, informed by experimental studies, is used to investigate the interplay of surface morphology, the adsorption site of reactants, the nature of the interaction between adsorbates and the surface, the potential energy landscape for adsorbates on the surface, adsorbate coverage, temperature, and the dynamic evolution of these factors during adsorption and reaction. We summarize our current understanding of Au atom release on the (111) surface and the corresponding effects on adsorption and reactivity. Gold was selected for these investigations because of the recent intense interest in the activity of gold nanoparticles for several important catalytic reactions. Fundamental experimental studies on Au single-crystal surfaces have established that atomic O is extremely active for oxidation of CO and olefins, that the local bonding of O is an important factor in determining the reactivity and selectivity for oxidation, and that Au atom release is induced by electronegative adsorbates, such as O, Cl, and S. These experimental results guided our theoretical studies. Density functional theory is an extremely useful tool since it evaluates the energetics associated with the incorporation of gold into the adsorbate layer, while providing fundamental physical insight into the underlying cause of gold incorporation. We use our results from static DFT calculations along with ab initio molecular dynamics simulations to understand the effect of surface morphology on the activity of gold for CO oxidation. Our investigation of Au atom release and incorporation induced by electronegative atoms clearly illustrates the importance of using experiments in combination with theory to establish the importance of and the underlying reasons for metal atom release and the affect on bonding and reactivity.     

On the Importance of Gradient-Corrected Correlation for van der Waals Density Functionals

The van der Waals density functional (vdW-DF) family of exchange?Ccorrelation functionals is a promising step towards accounting for van der Waals interactions in density functional theory. This approach consists of a nonlocal correlation term in addition to semilocal generalized gradient approximation exchange and local density approximation correlation. It has proven useful for describing vdW bonded complexes but unfortunately deteriorates the prediction of solid-state properties such as bulk lattice parameters and cohesive energies, as compared to the underlying GGA functional. By considering a broad range of different condensed matter systems including weakly interacting complexes as well as strongly bonded molecules and bulk solids, we show that inclusion of gradient-corrected correlations in vdW-DF-type calculations may not only improve the accuracy for vdW bonded systems, but also amend vdW-DF deficiencies in predicting structural properties of solids. Based on this insight we construct a prototype vdW-DF which demonstrates high accuracy in describing the dispersive interactions responsible for benzene adsorption on the noble Au(111) surface.     

Formic acid decomposition on Au catalysts: DFT,microkinetic modeling,and reaction kinetics experiments

A combined theoretical and experimental approach is presented that uses a comprehensive mean‐field microkinetic model, reaction kinetics experiments, and scanning transmission electron microscopy imaging to unravel the reaction mechanism and provide insights into the nature of active sites for formic acid (HCOOH) decomposition on Au/SiC catalysts. All input parameters for the microkinetic model are derived from periodic, self‐consistent, generalized gradient approximation (GGA‐PW91) density functional theory calculations on the Au(111), Au(100), and Au(211) surfaces and are subsequently adjusted to describe the experimental HCOOH decomposition rate and selectivity data. It is shown that the HCOOH decomposition follows the formate (HCOO) mediated path, with 100% selectivity toward the dehydrogenation products (CO₂ + H₂) under all reaction conditions. An analysis of the kinetic parameters suggests that an Au surface in which the coordination number of surface Au atoms is ≤4 may provide a better model for the active site of HCOOH decomposition on these specific supported Au catalysts. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1303–1319, 2014     

The bonding in thiolate protected gold nanoparticles from Au4f photoemission core level shifts

Density functional theory calculations are used to evaluate Au4f core level shifts of methyl thiolate protected Au(25), Au(102) and Au(144) nanoparticles. The shifts are found to provide sensitive fingerprints of the chemical environment. In particular, Au atoms in protective gold-thiolate complexes have higher binding energies than Au atoms with solely metal neighbors. The core level shifts for the nanoparticles are compared to the corresponding results for methyl thiolates adsorbed on Au(111) and implications for the understanding of the gold-sulfur bond is discussed.     

Understanding the mechanism of hydrogen adsorption into metal organic frameworks

Hydrogen adsorption mechanism into MOF-5, a porous metal-organic framework (MOF) has been studied by density functional theory calculation. The selected functionals for the prediction of interaction energies between hydrogen and potential adsorption sites of MOF-5 were utilized after the evaluation with the various functionals for interaction energy of H₂C₆H₆ model system. The adsorption energy of hydrogen molecule into MOF-5 was investigated with the consideration of the favorable adsorption sites and the orientations. We also calculated the second favorable adsorption sites by geometry optimization using every combination of two first adsorbed hydrogen molecules. Based on the calculation of the first and the second adsorption sites and energies, it has been suggested that the hydrogen adsorption into MOF-5 follows a cooperative mechanism in which the metal sites initiate the propagation of the hydrogen adsorption on the whole frameworks. In addition, the interaction mode between the simple benzene ring with hydrogen is significantly changed when the benzene ring has been incorporated into the framework of MOF-5.     

Finite-Size Effects in O and CO Adsorption for the Late Transition Metals

Gold is known to become significantly more catalytically active as its particle size is reduced, and other catalysts are also known to exhibit finite-size effects. To understand the trends related to finite-size effects, we have used density functional theory to study adsorption of representative adsorbates, CO and O, on the late transition metals Co, Ni, Cu, Ir, Pd, Ag, Rh, Pt and Au. We studied adsorption energies and geometries on 13-atom clusters and compared them to the fcc(111) and fcc(211) crystal facets. In all cases, adsorbates were found to bind significantly more strongly to the 13-atom clusters than to the extended surfaces. The binding strength of both adsorbates were found to correlate very strongly with the average coordination number of the metal atoms to which the adsorbate binds, indicating that the finite-size effects in bonding are not specific to gold.     

Making gold less noble

Self‐consistent density functional calculations for the adsorption of O and CO on flat and stepped Au(111) surfaces are used to investigate effects which may increase the reactivity of Au. We find that the adsorption energy does not depend on the number of Au layers if there are more than two layers. Steps are found to bind considerably stronger than the (111) terraces, and an expansive strain has the same effect. On this basis we suggest that the unusually large catalytic activity of highly‐dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au–support interface. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO and O2 adsorption on model Au-TiO2 systems

Au/TiO₂ catalysts are active for CO oxidation at room temperature and lower. To probe the surfaces of these catalysts, CO and O₂ adsorption and coadsorption on model Au-TiO₂ systems were examined under UHV conditions using TPD, ASE and XPS. No chemisorption of molecular O₂ was detected, as previously reported for clean Au single-crystal surfaces. A low concentration of CO adsorption sites associated with Au was observed; however, no unique interfacial sites could be unambiguously identified on these surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

负载金属的ZIF-8催化活性的计算化学研究

金属-有机骨架材料（metal-organic frameworks,MOFs）是一类新型纳米多孔功能材料。由于其独特的结构特征,在催化方面展现出潜在的应用价值。采用分子模拟结合密度泛函理论的计算方法系统地研究了ZIF-8在负载金属Pd、Ag、Pt、Au后的催化活性。结果表明,金属与材料之间主要有3种作用方式,其中以“碳-金属-碳”（C-M-C）形式最为稳定。并且对于同一种方式,ZIF-8负载金属后的稳定性顺序为：Pd >Ag >Pt >Au。同时,利用CO作为探针分子,系统地研究了负载金属后ZIF-8的催化活性,发现这些金属原子的Lewis酸性强弱与其电子接受能力有关,其催化活性顺序为：Pd >Pt >Ag >Au。这为研究利用MOF材料负载金属用于催化提供了参考信息。     

Synergistic activity of gold-platinum alloy nanoparticle catalysts

The understanding of the composition–activity relationship is essential for the exploitation of the synergistic properties of multimetallic nanoparticles in catalytic reactions. This paper focuses on the discussion of findings from the investigation of bimetallic gold-platinum (AuPt) nanoparticles of different compositions. Infrared spectroscopic data for CO adsorption on silica-supported AuPt nanoparticles reveal that the surface binding sites are dependent on the bimetallic composition. The analysis of this dependence further led to the conclusion that the relative Au-atop and Pt-atop sites for the linear CO adsorption on the nanoparticle surface are not only correlated with the bimetallic composition, but also with the electronic effect as a result of the d-band shift of Pt in the bimetallic nanocrystals, which is the first demonstration of the nanoscale core–surface property correlation for the bimetallic nanoparticles over a wide range of bimetallic composition. A further examination of the electrocatalysis data for methanol oxidation reaction on carbon-supported AuPt nanoparticle catalysts reveal important insights into the participation of CO or OH adsorption on Au sites and the catalytic activity of Pt in the AuPt alloys with relatively high Au concentration. Implications of these findings to synergistic correlation of the bifunctional activity of the bimetallic nanoparticle catalysts with the bimetallic composition are also discussed.     

Halogen adsorption on crystallographic (1 1 1) planes of Pt, Pd, Cu and Au, and on Pd-monolayer catalyst surfaces: First-principles study

Halogen (Cl, Br and I) adsorption on crystallographic (1 1 1) planes of Pd, Pt, Cu, Au and on palladium monolayer catalysts surfaces was investigated by DFT calculations. Palladium monolayer catalyst here denotes either the Pd monolayer deposited over (1 1 1) crystallographic plane of Pt, Cu and Au monocrystals (Pd_ML/Me(1 1 1)), or the (1 1 1) crystallographic plane of Pd monocrystal with inserted one-atom thick surface underlayer of Pt, Cu and Au (Me_UND/Pd(1 1 1)). The adsorption on the 3-fold sites was found to be the strongest, and adsorption energies decreased if the size of the halogen atoms increased. For the case of Pd-monolayer catalysts it was demonstrated that energy of adsorption of halogen atoms could be correlated to the position of the d-band of surface atoms. Charge states of halogen adatoms and work function changes were evaluated. On the basis of calculated data and both experimental and theoretical data available in the literature, the changes in the catalytic activity toward oxygen reduction reaction of the Pd_ML/Pt(1 1 1) surface, caused by chloride adsorption, were discussed.     

Brønsted–Evans–Polanyi Relations for H<Subscript>2</Subscript>O<Subscript>2</Subscript> Synthesis on Gold Surfaces

Abstract  

By means of density functional theory calculations, the reaction mechanisms of H₂O₂ synthesis on three low index and two stepped Au surfaces have been investigated in detail. This study shows the activation energies of five elementary reaction steps of H₂O₂ synthesis, which include two hydrogenation and three decomposition steps of key species, are a function of reaction energies, which observe the Br?nsted–Evans–Polanyi rules on both the flat Au surfaces and the step edge sites of stepped Au surfaces. This study not only provides a simple method to estimate the reaction barriers of elementary steps of H₂O₂ synthesis by the reaction energies but also predicts the catalytic performances of Au nanoparticles applied in real catalysis.