The reaction pathways for HSCH3 adsorption on Au(111): a density functional theory study

Density functional theory was used to investigate the reaction pathways for HSCH(3) adsorption on Au(111) at low coverage. A molecular adsorbed state was found with the S atom bond on Top sites (E approximately -0.38 eV) and molecular adsorption is nonactivated. The H-SCH(3) dissociation process is energetically less favorable and becomes slightly exothermic only when surface relaxation is considered (DeltaE approximately -0.2 eV). All the reaction pathways present a sizable activation energy barrier, with the lowest being approximately 0.52 eV (0.41 eV taking into account slab relaxation). In the corresponding saddle point of the potential energy surface, the S atom of the methylthiolate molecule is placed on Top sites and the H near a Bridge site. The high barrier obtained explains the complete absence of reactive methanethiol dissociation found in recent experiments.     

Metal catalyzed ethylene epoxidation:A comparative density functional theory study

Ethylene epoxidation on Ag(111), Pt(111), Rh(111) and Mo(100) has been studied by density functional theory (DFT) calculations. The results show that the adsorption energies of possible adsorbed species involved in the ethylene epoxidation increase in the order: Ag相似文献   

Selective oxidation of styrene on an oxygen-adsorbed Au(111): A density functional theory study

The reaction mechanism for the styrene selective oxidation on the oxygen preadsorbed Au(111) surface has been studied by the density functional theory calculation with the periodic slab model. The calculated results showed that the process of reaction includes two steps: forming the oxametallacycle intermediate (OMME) and then producing the products. It was found that the second step, from OMME to product is the rate-controlling step, which is similar to ethylene selective oxidation on Ag. Importantly, the present density-functional-theory calculation results suggested that the mechanism via the OMME (2) (i.e. the preadsorbed atomic oxygen bound to the CH2 group involved in C6H5--CH=CH2) to produce styrene epoxide is kinetically favored than that of OMME (1).     

Oxidation of carbon monoxide on Rh(111): a density functional theory study

The adsorption of CO on the oxygen precovered Rh(111) and the oxidation of CO are investigated with density functional theory. The preferred adsorption site and the possible reaction channel are determined. The experimentally observed inertness of the (2 x 2)-O-CO structure is explained by the reaction barrier and the stability of the reactants.     

Charging energy and barrier height of pentacene on Au(111): a local-orbital hybrid-functional density functional theory approach

We analyze the pentacene/Au(111) interface by means of density functional theory (DFT) calculations using a new hybrid functional; in our approach we introduce, in a local-orbital formulation of DFT, a hybrid exchange potential, and combine it with a calculation of the molecule charging energy to properly describe the transport energy gap of pentacene on Au(111). Van der Waals forces are taken into account to obtain the adsorption geometry. Interface dipole potentials are also calculated; it is shown that the metal/pentacene energy level alignment is determined by the potential induced by the charge transfer between the metal surface and the organic material, as described by the induced density of interface states model. Our results compare well with the experimental data.     

Density functional study of ethylene oxidation on an Ag(111) surface

The mechanism of ethylene epoxidation on Ag surfaces has been investigated using the density functional method and Ag _n clusters (n = 3 to 10) modeling the Ag(111) surface. The adsorption energy of O₂ to the Ag clusters was strongly dependent on the HOMO level of the cluster, and the clusters with higher HOMO levels afforded larger O₂ adsorption energies. The energetics was investigated for both the molecular and atomic oxygen epoxidation mechanisms. For the atomic oxygen mechanism, epoxidation was found to proceed without an activation energy, whereas a small amount of activation energy (about 5 kcal/mol) was calculated for the molecular oxygen mechanism. Received: 2 July 1998 / Accepted: 9 September 1998 / Published online: 8 February 1999     

Acrolein hydrogenation on Pt(211) and Au(211) surfaces: a density functional theory study

Partial hydrogenation of acrolein, the simplest α,β-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.     

Surface reaction of alkynes and alkenes with H-Si(111): a density functional theory study

Recent experiments on the addition of alkene and alkyne molecules to H-terminated silicon surfaces have provided evidence for a surface chain reaction initiated at isolated Si dangling bonds and involving an intermediate carbon radical state, which, after abstraction of a hydrogen atom from a neighboring Si-H unit, transforms into a stable adsorbed species plus a new Si dangling bond. Using periodic density functional theory (DFT) calculations, together with an efficient method for determining reaction pathways, we have studied the initial steps of this chain reaction for a few different terminal alkynes and alkenes interacting with an isolated Si dangling bond on an otherwise H-saturated Si(111) surface. Calculated minimum energy pathways (MEPs) indicate that the chain mechanism is viable in the case of C(2)H(2), whereas for C(2)H(4) the stabilization of the intermediate state is so small and the barrier for H-abstraction so (relatively) large that the molecule is more likely to desorb than to form a stable adsorbed species. For phenylacetylene and styrene, stabilization of the intermediate state and decrease of the H-abstraction barrier take place. While a stable adsorbed species exists in both cases, the overall heat of adsorption is larger for the alkyne molecules.     

Methanol oxidation on the PtPd(111) alloy surface: A density functional theory study

The mechanisms of methanol (CH₃OH) oxidation on the PtPd(111) alloy surface were systematically investigated by using density functional theory calculations. The energies of all the involved species were analyzed. The results indicated that with the removal of H atoms from adsorbates on PtPd(111) surface, the adsorption energies of (i) CH₃OH, CH₂OH, CHOH, and COH increased linearly, while those of (ii) CH₃OH, CH₃O, CH₂O, CHO, and CO exhibited odd‐even oscillation. On PtPd(111) surface, CH₃OH underwent the preferred initial C H bond scission followed by successive dehydrogenation and then CHO oxidation, that is, CH₃OH → CH₂OH → CHOH → CHO → CHOOH → COOH → CO₂. Importantly, the rate‐determining step of CH₃OH oxidation was found to switch from CO → CO₂ on Pt(111) to COOH → CO₂ + H on PtPd(111) with a lower energy barrier of 0.96 eV. Moreover, water also decomposed into OH more easily on PtPd(111) than on Pt(111). The calculated results indicate that alloying Pt with Pd could efficiently improve its catalytic performance for CH₃OH oxidation through altering the primary pathways from the CO path on pure Pt to the non‐CO path on PtPd(111).     

Why copper is intrinsically more selective than silver in alkene epoxidation: ethylene oxidation on Cu(111) versus Ag(111)

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically important. Although large-scale ethylene epoxidation is universally carried out with Ag catalysts, recent laboratory studies on single crystal surfaces show that Cu is intrinsically much more selective than Ag in the epoxidation of a variety of terminal alkenes. The reasons for this striking difference between Ag and Cu have been investigated by means of density functional theory. It is found that the fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation (the latter being the first step on the route to combustion). On Cu, epoxide formation is less activated than aldehyde formation; the opposite is true on Ag. This behavior is associated with a late transition state to epoxidation on Cu (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on Ag.     

Reaction mechanism of CO oxidation on Cu(2)O(111): A density functional study

The possible reaction mechanisms for CO oxidation on the perfect Cu(2)O(111) surface have been investigated by performing periodic density functional theoretical calculations. We find that Cu(2)O(111) is able to facilitate the CO oxidation with different mechanisms. Four possible mechanisms are explored (denoted as M(ER1), M(ER2), M(LH1), and M(LH2), respectively): M(ER1) is CO((gas))+O(2(ads))→CO(2(gas)); M(ER2) is CO((gas))+O(2(ads))→CO(3(ads))→O((ads))+CO(2(gas)); M(LH1) refers to CO((ads))+O(2(ads))→O((ads))+CO(2(ads)); and M(LH2) refers to CO((ads))+O(2(ads))→OOCO((ads))→O((ads))+CO(2(ads)). Our transition state calculations clearly reveal that M(ER1) and M(LH2) are both viable; but M(ER1) mechanism preferentially operates, in which only a moderate energy barrier (60.22 kJ/mol) needs to be overcome. When CO oxidation takes place along M(ER2) path, it is facile for CO(3) formation, but is difficult for its decomposition, thereby CO(3) species can stably exist on Cu(2)O(111). Of course, the reaction of CO with lattice O of Cu(2)O(111) is also considered. However, the calculated barrier is 600.00 kJ/mol, which is too large to make the path feasible. So, we believe that on Cu(2)O(111), CO reacts with adsorbed O, rather than lattice O, to form CO(2). This is different from the usual Mars-van Krevene mechanism. The present results enrich our understanding of the catalytic oxidation of CO by copper-based and metal-oxide catalysts.     

Decorated Ru/Au(111) and Os/Au(111) surfaces: An in situ STM and electrochemistry study

The structure and reactivity of bimetallic electrodes obtained by spontaneous deposition of Ru and Os on Au(111) single-crystal surfaces are studied. In situ electrochemical STM and cyclic voltammetry are used to characterize a wide range of surface morphologies thus produced. The STM results on Ru/Au(111) demonstrate a pronounced step decoration, while a random distribution of Ru nuclei, quite uniform in size, occurs on terraces. Osmium deposits show a slight preference for deposition on steps, but it also occurs readily on terraces. However, many of the Os islands grow into multilayer heights. The coverage of the Au(111) by the deposited Ru or Os islands for a particular solution concentration depends on the deposition time. Nanostructures of Ru and Os are tested for catalytic behavior and correlated to CO oxidation activity as measured by CO stripping voltammetry. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1385–1392. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Density functional theory investigation of benzenethiol adsorption on Au(111)

We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61 degrees from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2 square root of 3 x square root of 3)R30 degrees herringbone structure is stabilized against the (square root 3 x square root 3)R30 degrees structure by large steric relaxation. In the most stable (2 square root 3 x square root 3)R30 degrees SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21 degrees, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.     

Density functional theory study of the adsorption of alkanethiols on Cu(111), Ag(111), and Au(111) in the low and high coverage regimes

The structure, the surface bonding, and the energetics of alkanethiols adsorbed on Cu(111), Ag(111), and Au(111) surfaces were studied under low and high coverages. The potential energy surfaces (PES) for the thiol/metal interaction were investigated in the absence and presence of externally applied electric fields in order to simulate the effect of the electrode potential on the surface bonding. The electric field affects the corrugation of the PES which decreases for negative fields and increases for positive fields. In the structural investigation, we considered the relaxation of the adsorbate and the surface. The highest relaxation in a direction perpendicular to the surface was observed for gold atoms, whereas silver atoms presented the highest relaxation in a plane parallel to the surface. The surface relaxation is more important in the low coverage limit. The surface bonding was investigated by means of the total and projected density of states analysis. The highest ionic character was observed on the copper surface whereas the highest covalent character occurs on gold. This leads to a strong dependence of the PES with the tilt angle of the adsorbate on Au(111) whereas this dependence is less pronounced on the other metals. Thus, the adsorbate-relaxation and the metal-relaxation contributions to the binding energy are more important on gold. The adsorption of thiols on gold was investigated on the 111 surface as well as on a surface with gold adatoms in order to elucidate the effect of thiols on the surface diffusion of gold. The CH(3)CH(2)S radical adsorbs ontop of the gold adatom. The diffusional barrier of the CH(3)CH(2)SAu species is lower than that for a bare gold adatom and is also lower than that for the bare thiol radical. The adsorption of the molecular species CH(3)SH and CH(3)CH(2)SH was also investigated on Au(111). They adsorb via the sulfur atom ontop of a gold atom. On the other hand, the adsorption of the alkanethiol radicals on the perfect 111 surfaces occurs on the face centered cubic (fcc)-bridge site in the low coverage limit for all metals and shifts toward the fcc site at high coverage on copper and silver.     

4-Mercaptopyridine on Au(111): a scanning tunneling microscopy and spectroscopy study

The adsorption of 4-mercaptopyridine (4MPy) molecules on reconstructed Au(111) is investigated by Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) at low temperature and under ultra-high vacuum (UHV) conditions. As made visible by STM, at low coverage (<10%) 4MPy adsorbs preferentially at elbow sites of the Herringbone reconstruction and at step edges of the Au(111). Increasing coverage (but still <30%) results in formation of molecular chains followed, at even higher coverage, by a 3-dimensional growth. Detailed analysis of z-V spectroscopy (ramping the tunneling bias V while keeping the tunneling current constant) provides information on the bias dependent apparent height of a single 4MPy/Au(111) as well as on the local density of states (LDOS) of single and chain 4MPy molecules in comparison to the bare Au(111) surface revealing a significant shift of the lowest unoccupied molecular orbital (LUMO) towards lower energy for molecules within chains. Additionally, the data provide no evidence that for these samples prepared in UHV the adsorption of 4MPy on Au(111) requires mediating Au adatoms. Also, clear indications are given that the adsorption does not induce a strong reduction of the Au DOS close to its Fermi energy. Finally, in context of the apparent STM height of 4MPy molecules, the behavior of the differential barrier height Φ(diff)(V) = (?(z)?(V)I/?(V)I)(2) on bare Au(111) and 4MPy/Au(111) is analyzed and the corresponding experimental values are applied to recover the LDOS of the molecule for unoccupied states according to a previously published numerical recipe [B. Koslowski, H. Pfeifer and P. Ziemann, Phys. Rev. B, 2009, 80, 165419 and M. Ziegler, N. Néel, A. Sperl, J. Kr?ger, and R. Berndt, Phys. Rev. B, 2009, 80, 125402]. In this way, one obtains a spectrum comprising a constant DOS of the Shockley-like surface state of Au(111) and a Lorentzian line attributed to the LUMO of 4MPy.     

Catalytic activity of Pd ensembles over Au(111) surface for CO oxidation: a first-principles study

Employing the first-principles pseudopotential plane-wave methods and nudged-elastic-band simulations, we studied the reaction of CO oxidation on Pd-decorated Au(111) surface. We found that the contiguous Pd ensembles are required for the CO + O(2) reaction. Interestingly, Pd dimer is an active site for the two-step reaction of CO+O(2)→OOCO→CO(2)+O, and a low energy barrier (0.29 eV) is found for the formation of the intermediate metastable state (OOCO) compared to the barrier of 0.69 eV on Pd trimer. Furthermore, the residual atomic O in the CO + O(2) reaction can be removed by another CO on Pd dimer with the barrier of 0.56 eV close to the value of 0.52 eV on Pd monomer via Langmuir-Hinshelwood mechanism. The higher energy barriers (0.96 and 0.64 eV) are also found for the CO + O reaction on Pd trimers. The calculated results indicate Pd dimer is highly reactive for CO oxidation by O(2) via association mechanism on Pd-decorated Au(111) surface.     

Phenyl-coinage metal (Ag, Au) complexes: an anion photoelectron spectroscopy and density functional study

The important intermediate phenyl-coinage metal complexes (Ag(m)C6H5(-), Au(m)C6H5(-)), which are produced from the reactions between coinage metal clusters formed by laser ablation and the benzene molecules seeded in argon carrier gas, are studied by PES (photoelectron spectroscopy) and DFT (density functional theory). The EAs (adiabatic electron affinities) of these complexes are obtained from PES at both 308 and 193 nm photon energies and show odd-even alternation. Calculations with DFT are carried out on the structural and electronic properties of Ag(m)C6H5(-) and Au(m)C6H5(-); the adiabatic detachment energy and the calculated DOS (density of states) for the ground state of a given anion are in good agreement with the experimental PES results. The observed spectra are also compared with those of the pure coinage metal clusters, which reveal that there are some similarities between them and the phenyl acts like an additional metal atom in the clusters. Furthermore, the bonding between phenyl and metal is analyzed, suggesting that phenyl group binds perpendicularly on metal clusters through C-M sigma bond.     

Mechanistic study of the electrochemical oxygen reduction reaction on Pt(111) using density functional theory

Density functional theory (DFT) was used to study the electrolyte solution effects on the oxygen reduction reaction (ORR) on Pt(111). To model the acid electrolyte, an H(5)O(2)(+) cluster was used. The vibrational proton oscillation modes for adsorbed H(5)O(2)(+) computed at 1711 and 1010 cm(-1), in addition to OH stretching and H(2)O scissoring modes, agree with experimental vibrational spectra for proton formation on Pt surfaces in ultrahigh vacuum. Using the H(5)O(2)(+) model, protonation of adsorbed species was found to be facile and consistent with the activation barrier of proton transfer in solution. After protonation, OOH dissociates with an activation barrier of 0.22 eV, similar to the barrier for O(2) dissociation. Comparison of the two pathways suggests that O(2) protonation precedes dissociation in the oxygen reduction reaction. Additionally, an OH diffusion step following O protonation inhibits the reaction, which may lead to accumulation of oxygen on the electrode surface.