Catalytic water formation on platinum: a first-principles study

Hot precursor reactions during the collisions of gas-phase oxygen atoms with deuterium chemisorbed on Pt(100)

We utilized direct rate measurements and temperature programmed desorption to investigate reactions that occur during the collisions of gaseous oxygen atoms with deuterium-covered Pt(100). We find that both D2O and D2 desorb promptly when an oxygen atom beam impinges upon D-covered Pt(100) held at surface temperatures ranging from 90 to 150 K, and estimate effective cross sections of 12 and 1.8 A2, respectively, for the production of gaseous D2O and D2 at 90 K. The yields of D2O and D2 that desorb at 90 K are about 13% and 2%, respectively, of the initial D atom coverage, though most of the D2O product molecules (approximately 80%) thermalize to the surface rather than desorb at the surface temperatures studied. Increasing the surface temperature from 90 to 150 K causes the D2O desorption rate to decay more quickly during O atom exposures to the surface and results in lower yields of gaseous D2O. We attribute the production of D2O and D2 in these experiments to reactions involving intermediates that are not thermally accommodated to the surface, so-called hot precursors. The results are consistent with the production of hot D2O involving first the generation of hot OD groups from the reaction O*+D(a)-->OD*, where the asterisk denotes a hot precursor, followed by the parallel pathways OD*+D(a)-->D2O* and OD*+OD(a)-->D2O*+O(a). The final reaction contributes significantly to hot D2O production only later in the reaction period when thermalized OD groups have accumulated on the surface, and it becomes less important at higher temperature due to depletion of the OD(a) concentration by thermally activated D2O production.

Dissociation of water buried under ice on Pt(111)

Water on Pt(111) is generally thought to be nondissociative. However, by adsorbing a thick ice film [>150 monolayers (ML)], substantial (approximately 0.16 to 1 ML) dissociation of the "buried water" occurs for T>151 K. New temperature-programmed desorption peaks signal the dissociation (after careful isothermal predesorption of the overlying ice films). The buried water likely dissociates via the elevated temperatures and/or solvation changes experienced under the ice. Dissociation charges the growing ice film (up to +9 V) due to trapping of approximately 0.007 ML H3O+ at the vacuum-ice interface.

An interaction model for OH + H2O-mixed and pure H2O overlayers adsorbed on Pt(111)

A model potential for the adsorbate-adsorbate interaction among OH and H2O molecules adsorbed on a Pt(111) surface has been developed solely based on first-principle calculations. By combining this directional-dependent model potential for the lateral interaction with a lattice model of Ising type, large length scale structure calculations can be made. The strength of different hydrogen bonds can be analyzed in detail from this model potential. It is found that the hydrogen bond between OH and H2O molecules is stronger than that between two H2O molecules (0.4 eV per pair as compared to 0.2 eV per pair, respectively). Via the computed chemical potential for water in mixed OH + H2O overlayers the water uptake as a function of oxygen precoverage on Pt(111) has been determined. The results compare very well with recent experiments.

Surface Composition of Materials Used as Catalysts for Methanol Steam Reforming: A Theoretical Study

PdZn (1:1) alloy is assumed to be the active component of a promising catalyst for methanol steam reforming. Using density functional calculations on periodic supercell slab models, followed by atomistic thermodynamics modeling, we study the chemical composition of the surfaces PdZn(111) and, as a reference, Cu(111) in contact with water and hydrogen at conditions relevant to methanol steam reforming. For the two surfaces, we determine similar maximum adsorption energies for the dissociative adsorption of H(2), O(2), and the molecular adsorption of H(2)O. These reactions are calculated to be exothermic by about -40, -320, and -20 kJ mol(-1), respectively. Using a thermodynamic analysis based on theoretically predicted adsorption energies and vibrational frequencies, we determine the most favorable surface compositions for given pressure windows. However, surface energy plots alone cannot provide quantitative information on individual coverages in a system of coupled adsorption reactions. To overcome this limitation, we employ a kinetic model, from which equilibrium surface coverages of H, O, OH, and H(2)O are derived. We also discuss the sensitivity of our results and the ensuing conclusions with regard to the model surfaces employed and the inaccuracies of our computational method. Our kinetic model predicts surfaces of both materials, PdZn and Cu, to be essentially adsorbate-free already from very low values of the partial pressure of H(2). The model surfaces PdZn(111) and Cu(111) are predicted to be free of water-related adsorbates for a partial H(2) pressure greater than 10(-8) and 10(-5) atm, respectively.

Coverage dependence and hydroperoxyl-mediated pathway of catalytic water formation on Pt (111) surface

Hydrogen oxidation on Pt (111) surface is modeled by density functional theory (DFT). Previous DFT calculations showed too large O2 dissociation barriers, but we find them highly coverage dependent: when the coverage is low, dissociation barriers close to experimental values (approximately 0.3 eV) are obtained. For the whole reaction, a new pathway involving hydroperoxyl (OOH) intermediate is found, with the highest reaction barrier of only approximately 0.4 eV. This may explain the experimental observation of catalytic water formation on Pt (111) surface above the H2O desorption temperature of 170 K, despite that the direct reaction between chemisorbed O and H atoms is a highly activated process with barrier approximately 1 eV as previous calculations showed.

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.

A molecular dynamics simulation of the adsorption of water molecules surrounding an Au nanoparticle

This study uses molecular dynamics simulations performed in a parallel computing environment to investigate the adsorption of water molecules surrounding Au nanoparticles of various sizes. An observation of the oxygen and hydrogen atom distributions reveals that the adsorption of the water molecules creates two shell-like formations of water in close vicinity to the Au nanoparticle surface. These shell-like formations are found to be more pronounced around smaller Au nanoparticles. The rearrangement of water molecules in this region reduces the local hydrogen bond strength to below that which is observed in the bulk region. Finally, the simulation results indicate that the absolute value of the interaction energy between the water molecules and the Au nanoparticle is reduced when the water molecules surround a nanoparticle of larger diameter. This observation implies that a stronger adsorption effect exists between smaller Au nanoparticles and water molecules. Hence, the value of the adsorption constant increases for smaller Au nanoparticles.

Water desorption from an oxygen covered Pt(111) surface: Multichannel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample.

Thermodynamic Analysis of the Temperature Dependence of OH Adsorption on Pt(111) and Pt(100) Electrodes in Acidic Media in the Absence of Specific Anion Adsorption

The effect of temperature on the voltammetric OH adsorption on Pt(111) and Pt(100) electrodes in perchloric acid media has been studied. From a thermodynamic analysis based on a generalized adsorption isotherm, DeltaG degrees , DeltaH degrees , and DeltaS degrees values for the adsorption of OH have been determined. On Pt(111), the adsorption enthalpy ranges between -265 and -235 kJ mol(-1), becoming less exothermic as the OH coverage increases. These values are in reasonable agreement with experimental data and calculated values for the same reaction in gas phase. The adsorption entropy for OH adsorption on Pt(111) ranges from -200 J mol(-1) K(-1) (low coverage) to -110 J mol(-1) K(-1) (high coverage). On the other hand, the enthalpy and entropy of hydroxyl adsorption on Pt(100) are less sensitive to coverage variations, with values ca. DeltaH degrees = -280 kJ mol(-1) and DeltaS degrees = -180 J mol(-1) K(-1). The different dependence of DeltaS degrees with coverage on both electrode surfaces stresses the important effect of the substrate symmetry on the mobility of adsorbed OH species within the water network directly attached to the metal surface.

Density functional theory study of water dissociation in a double water bilayer with or without coadsorption of CO on Pt(111)

Using density functional theory, we investigate the structure of the double water bilayer with or without coadsorption of CO on Pt(111). The double water bilayer consists of two bilayers. Each bilayer is buckled with every second water molecule being closer to the surface than every other water molecule. CO is found to adsorb most strongly when substituting in the first bilayer, the water molecule closest to the surface. Dissociation of H2O in the water bilayer (with or without CO) is further considered. A great number of pathways for the dissociation are studied. These include homolytic pathways where both dissociation products end up adsorbed on the Pt surface and heterolytic pathways where only the OH is adsorbed, while a proton is transferred to the water adlayers. We find that the heterolytic dissociation pathways are energetically more favorable than the homolytic ones, yet they are all rather endothermic. The most favorable pathways found have reaction energies of 0.60 and 0.52 eV without and with CO present. The corresponding activation energies are 0.99 and 0.53 eV, respectively.

A density functional theory study of sulfur poisoning

Density functional theory calculations have been used to investigate the chemisorption of H, S, SH, and H(2)S as well as the hydrogenation reactions S+H and SH+H on a Rh surface with steps, Rh(211), aiming to explain sulfur poisoning effect. In the S hydrogenation from S to H(2)S, the transition state of the first step S+H-->SH is reached when the S moves to the step-bridge and H is on the off-top site. In the second step, SH+H-->H(2)S, the transition state is reached when SH moves to the top site and H is close to another top site nearby. Our results show that it is difficult to hydrogenate S and they poison defects such as steps. In order to address why S is poisoning, hydrogenation of C, N, and O on Rh(211) has also been calculated and has been found that the reverse and forward reactions possess similar barriers in contrast to the S hydrogenation. The physical origin of these differences has been analyzed and discussed.

Density Functional Theory Study of Water Activation and COads + OHads Reaction on Pure Platinum and Bimetallic Platinum/Ruthenium Nanoclusters

A density functional theory study of the elementary steps that lead to the removal of CO(ads(Pt)) over alloyed and sequentially deposited Pt/Ru bimetallic nanoclusters is presented. The reaction energies and activation barriers for the H2O(ads(Ru)) dissociation and CO(ads(Pt)) + OH(ads(Ru)) reaction are estimated in solid-gas interface and in a microsolvated environment to determine which surface morphology is more tolerant to COads poisoning. On the basis of the energetics, the sequentially deposited Pt/Ru nanocluster is predicted to be a much more promising anode catalyst than the alloy cluster surface in fuel cell applications.

The structure of the mixed OH+H2O overlayer on Pt{111}

The structure of the mixed p(3 x 3)-(3OH + 3H2O) phase on Pt[111] has been investigated by low-energy electron diffraction-IV structure analysis. The OH + H2O overlayer consists of hexagonal rings of coplanar oxygen atoms interlinked by hydrogen bonds. Lateral shifts of the O atoms away from atop sites result in different O-O separations and hexagons with only large separations (2.81 and 3.02 angstroms) linked by hexagons with alternating separations of 2.49 and 2.813.02 angstroms. This unusual pattern is consistent with a hydrogen-bonded network in which water is adsorbed in cyclic rings separated by OH in a p(3 x 3) structure. The top-most two layers of the Pt atoms relax inwards with respect to the clean surface and both show vertical buckling of up to 0.06 angstroms. In addition, significant shifts away from the lateral bulk positions have been found for the second layer of Pt atoms.

Water Production Reaction on Rh(110)

By means of scanning tunneling microscopy and density functional theory calculations, we studied the water formation reaction on the Rh(110) surface when exposing the (2 x 1)p2mg-O structure to molecular hydrogen, characterizing each of the structures that form on the surface during the reaction. First the reaction propagates on the surface as a wave front, removing half of the initial oxygen atoms. The remaining 0.5 monolayers of O atoms rearrange in pairs, forming a c(2 x 4) structure. Second, as the reaction proceeds, areas of an intermediate structure with c(2 x 2) symmetry appear and grow at the expense of the c(2 x 4) phase, involving all the oxygen atoms present on the surface. Afterward, the c(2 x 2) islands shrink, indicating that complete hydrogenation occurs at their edges, leaving behind a clean rhodium substrate. Two possible models for the c(2 x 2) structure, where not only the arrangement but also the chemical identity is different, are given. The first one is a mixed H + O structure, while the second one resembles the half-dissociated water layer already proposed on other metal surfaces. In both models, the high local oxygen coverage is achieved by the formation of a hexagonal network of hydrogen bonds.

Water formation reaction on Pt(111): Role of the proton transfer

The catalytic water formation reaction on Pt(111) was investigated by kinetic Monte Carlo simulations, where the interaction energy between reaction species and the high mobility of H(2)O molecule was considered. Results obtained clearly reproduce the scanning tunneling microscopy images which show that the reaction proceeds via traveling the reaction fronts on the O-covered Pt(111) surface by creating H(2)O islands backwards. The reaction front is a mixed layer of OH and H(2)O with a (square root 3 x square root 3)R30(o) structure. Coverage change during the reaction is also reproduced in which the reaction consists of three characteristic processes, as observed by the previous experiments. The simulation also revealed that the proton transfer from H(2)O to OH plays an important role to propagate the water formation.

Theoretical Study of the Adsorption and Dissociation of Oxygen on Pt(111) in the Presence of Homogeneous Electric Fields

The effect of homogeneous electric fields on the adsorption energies of atomic and molecular oxygen and the dissociation activation energy of molecular oxygen on Pt(111) were studied by density functional theory (DFT). Positive electric fields, corresponding to positively charged surfaces, reduce the adsorption energies of the oxygen species on Pt(111), whereas negative fields increase the adsorption energies. The magnitude of the energy change for a given field is primarily determined by the static surface dipole moment induced by adsorption. On 10-atom Pt(111) clusters, the adsorption energy of atomic oxygen decreased by ca. 0.25 eV in the presence of a 0.51 V/A (0.01 au) electric field. This energy change, however, is heavily dependent on the number of atoms in the Pt(111) cluster, as the static dipole moment decreases with cluster size. Similar calculations with periodic slab models revealed a change in energy smaller by roughly an order of magnitude relative to the 10-atom cluster results. Calculations with adsorbed molecular oxygen and its transition state for dissociation showed similar behavior. Additionally, substrate relaxation in periodic slab models lowers the static dipole moment and, therefore, the effect of electric field on binding energy. The results presented in this paper indicate that the electrostatic effect of electric fields at fuel cell cathodes may be sufficiently large to influence the oxygen reduction reaction kinetics by increasing the activation energy for dissociation.

General insight into CO oxidation: a density functional theory study of the reaction mechanism on platinum oxides

CO oxidation on PtO2(110) has been studied using density functional theory calculations. Four possible reaction mechanisms were investigated and the most feasible one is the following: (i) the O at the bridge site of PtO2(110) reacts with CO on the coordinatively unsaturated site (CUS) with a negligible barrier; (ii) O2 adsorbs on the bridge site and then interacts with CO on the CUS to form an OO-CO complex; (iii) the bond of O-OCO breaks to produce CO2 with a small barrier (0.01 eV). The CO oxidation mechanisms on metals and metal oxides are rationalized by a simple model: The O-surface bonding determines the reactivity on surfaces; it also determines whether the atomic or molecular mechanism is preferred. The reactivity on metal oxides is further found to be related to the 3rd ionization energy of the metal atom.

Density-functional based modeling of the intermediate in the water production reaction on Pt(111)

A model based on density-functional calculations has been developed for the overlayer formed by dissociation of water on an oxygen covered Pt(111) surface. The directional dependent interaction within the overlayer is treated by means of a lattice model of Ising type. Stable large length scale structures are found for two compositions proposed in the literature: a hydroxyl-water and a hydroxyl-hydrogen mixed composition, respectively. The water containing composition produces an overlayer structure in very good agreement with the structures seen in scanning tunneling microscopy experiments.

Hydrogen bonding in mixed OH+H2O overlayers on Pt(111)

The stability of OH on Pt(111) has been investigated to determine the role of hydrogen bonding in stabilizing the overlayer. We find that the optimal structure is a mixed (OH+H2O) phase, confirming recent density-functional theory predictions. The reaction O+3H(2)O forms a hexagonal (sqrt[3]xsqrt[3])R30 degrees -(OH+H2O) lattice with a weak (3x3) superstructure, caused by ordering of the hydrogen bonds. The mixed overlayer can accommodate a range of H(2)O/OH compositions but becomes less stable as the H2O content is reduced, causing defects in the hydrogen-bonding network that lift the (3 x 3) superstructure and destabilize the overlayer.

The mechanism of N[sub 2]O formation via the (NO)[sub 2] dimer: A density functional theory study

Catalytic formation of N(2)O via a (NO)(2) intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)(2) and cis-(NO)(2) configurations. A possible pathway involving (NO)(2) formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N(2)O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO + H(2) reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV.

A Systematic Study of CO Oxidation on Metals and Metal Oxides: Density Functional Theory Calculations

CO oxidation on Ru(0001), Rh(111), Pd(111), Os(0001), Ir(111), Pt(111), and their corresponding metal oxides is studied using density functional theory. It is found that (i) the reactivity of metal oxide is generally higher than that of the corresponding metal, and (ii) on both metals and metal oxides, the higher the chemisorption energy is in the initial state, the larger the reaction barrier. The barriers are further analyzed by decomposing them into electronic and geometric effects, and the higher reactivity of metal oxides is attributed mainly to the surface geometric effect. Moreover, the electronic effect on both metals and metal oxides follows the same pattern: the shorter the OC-O bond distance in the TS, the higher the barrier.

New insights into ethene epoxidation on two oxidized Ag[111] surfaces

Reaction mechanisms and activation energies for the complete conversion of ethene to ethene epoxide on two recently characterized oxidized Ag{111} surfaces have been determined from density functional theory. On both surfaces, epoxidation proceeds through a two-step nonconcerted mechanism via an oxametallacycle intermediate. The key implications are that both surfaces are active and that epoxidation can take place over a wide O coverage regime.

Identification of general linear relationships between activation energies and enthalpy changes for dissociation reactions at surfaces

The activation energy to reaction is a key quantity that controls catalytic activity. Having used ab inito calculations to determine an extensive and broad ranging set of activation energies and enthalpy changes for surface-catalyzed reactions, we show that linear relationships exist between dissociation activation energies and enthalpy changes. Known in the literature as empirical Brønsted-Evans-Polanyi (BEP) relationships, we identify and discuss the physical origin of their presence in heterogeneous catalysis. The key implication is that merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.

Different surface chemistries of water on Ru[0001]: from monomer adsorption to partially dissociated bilayers

Density functional theory has been used to perform a comparative theoretical study of the adsorption and dissociation of H(2)O monomers and icelike bilayers on Ru[0001]. H(2)O monomers bind preferentially at atop sites with an adsorption energy of approximately 0.4 eV/H(2)O. The main bonding interaction is through the H(2)O 1b(1) molecular orbital which mixes with Ru d(z)2 states. The lower-lying set of H(2)O molecules in an intact H(2)O bilayer bond in a similar fashion; the high-lying H(2)O molecules, however, do not bond directly with the surface, rather they are held in place through H bonding. The H(2)O adsorption energy in intact bilayers is approximately 0.6 eV/H(2)O and we estimate that H bonding accounts for approximately 70% of this. In agreement with Feibelman (Science 2002, 295, 99) we find that a partially dissociated OH + H(2)O overlayer is energetically favored over pure intact H(2)O bilayers on the surface. The barrier for the dissociation of a chemisorbed H(2)O monomer is 0.8 eV, whereas the barrier to dissociate a H(2)O incorporated in a bilayer is just 0.5 eV.

General rules for predicting where a catalytic reaction should occur on metal surfaces: a density functional theory study of C-H and C-O bond breaking/making on flat, stepped, and kinked metal surfaces

To predict where a catalytic reaction should occur is a fundamental issue scientifically. Technologically, it is also important because it can facilitate the catalyst's design. However, to date, the understanding of this issue is rather limited. In this work, two types of reactions, CH(4) <--> CH(3) + H and CO <--> C + O on two transition metal surfaces, were chosen as model systems aiming to address in general where a catalytic reaction should occur. The dissociations of CH(4) --> CH(3) + H and CO --> C + O and their reverse reactions on flat, stepped, and kinked Rh and Pd surfaces were studied in detail. We find the following: First, for the CH(4) <--> Ch(3) + H reaction, the dissociation barrier is reduced by approximately 0.3 eV on steps and kinks as compared to that on flat surfaces. On the other hand, there is essentially no difference in barrier for the association reaction of CH(3) + H on the flat surfaces and the defects. Second, for the CO <--> C + O reaction, the dissociation barrier decreases dramatically (more than 0.8 eV on Rh and Pd) on steps and kinks as compared to that on flat surfaces. In contrast to the CH(3) + H reaction, the C + O association reaction also preferentially occurs on steps and kinks. We also present a detailed analysis of the reaction barriers in which each barrier is decomposed quantitatively into a local electronic effect and a geometrical effect. Our DFT calculations show that surface defects such as steps and kinks can largely facilitate bond breaking, while whether the surface defects could promote bond formation depends on the individual reaction as well as the particular metal. The physical origin of these trends is identified and discussed. On the basis of our results, we arrive at some simple rules with respect to where a reaction should occur: (i) defects such as steps are always favored for dissociation reactions as compared to flat surfaces; and (ii) the reaction site of the association reactions is largely related to the magnitude of the bonding competition effect, which is determined by the reactant and metal valency. Reactions with high valency reactants are more likely to occur on defects (more structure-sensitive), as compared to reactions with low valency reactants. Moreover, the reactions on late transition metals are more likely to proceed on defects than those on the early transition metals.

Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold

Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO(2) has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O(2) dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O(2) dissociation on Au (O(2)-->2O(ad)) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O(2) on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O(2) can adsorb on the interface of Au/TiO(2) with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O(2) dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O(2)-->CO(2)+O, and CO+O-->CO(2); and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.

A New Insight into Fischer−Tropsch Synthesis

Fischer-Tropsch (FT) reaction is an important synthetic route to convert CO and H(2) to fuels and chemicals in industry. To date, its reaction mechanism remains uncertain. With extensive density functional theory studies on FT reactions on Ru, we compare quantitatively several C/C coupling mechanisms that are likely to be involved. We found that a well-regarded CH(2) + CH(2)R (R = H or alkyl) mechanism possesses high reaction barriers, and a stepwise C + CR mechanism has been identified that may be relevant to FT synthesis.

A first-principles study of methanol decomposition on Pt(111)

A periodic, self-consistent, Density Functional Theory study of methanol decomposition on Pt(111) is presented. The thermochemistry and activation energy barriers for all the elementary steps, starting with O[bond]H scission and proceeding via sequential hydrogen abstraction from the resulting methoxy intermediate, are presented here. The minimum energy path is represented by a one-dimensional potential energy surface connecting methanol with its final decomposition products, CO and hydrogen gas. It is found that the rate-limiting step for this decomposition pathway is the abstraction of hydroxyl hydrogen from methanol. CO is clearly identified as a strong thermodynamic sink in the reaction pathway while the methoxy, formaldehyde, and formyl intermediates are found to have low barriers to decomposition, leading to very short lifetimes for these intermediates. Stable intermediates and transition states are found to obey gas-phase coordination and bond order rules on the Pt(111) surface.

A density functional theory study on the active center of Fe-only hydrogenase: characterization and electronic structure of the redox states

We have carried out extensive density functional theory (DFT) calculations for possible redox states of the active center in Fe-only hydrogenases. The active center is modeled by [(H(CH(3))S)(CO)(CN(-))Fe(p)(mu-DTN)(mu-CO)Fe(d)(CO)(CN(-))(L)](z)() (z is the net charge in the complex; Fe(p)= the proximal Fe, Fe(d) = the distal Fe, DTN = (-SCH(2)NHCH(2)S-), L is the ligand that bonds with the Fe(d) at the trans position to the bridging CO). Structures of possible redox states are optimized, and CO stretching frequencies are calculated. By a detailed comparison of all the calculated structures and the vibrational frequencies with the available experimental data, we find that (i) the fully oxidized, inactive state is an Fe(II)-Fe(II) state with a hydroxyl (OH(-)) group bonded at the Fe(d), (ii) the oxidized, active state is an Fe(II)-Fe(I) complex which is consistent with the assignment of Cao and Hall (J. Am. Chem. Soc. 2001, 123, 3734), and (iii) the fully reduced state is a mixture with the major component being a protonated Fe(I)-Fe(I) complex and the other component being its self-arranged form, Fe(II)-Fe(II) hydride. Our calculations also show that the exogenous CO can strongly bond with the Fe(II)-Fe(I) species, but cannot bond with the Fe(I)-Fe(I) complex. This result is consistent with experiments that CO tends to inhibit the oxidized, active state, but not the fully reduced state. The electronic structures of all the redox states have been analyzed. It is found that a frontier orbital which is a mixing state between the e(g) of Fe and the 2 pi of the bridging CO plays a key role concerning the reactivity of Fe-only hydrogenases: (i) it is unoccupied in the fully oxidized, inactive state, half-occupied in the oxidized, active state, and fully occupied in the fully reduced state; (ii) the e(g)-2 pi orbital is a bonding state, and this is the key reason for stability of the low oxidation states, such as Fe(I)-Fe(I) complexes; and (iii) in the e(g)-2 pi orbital more charge accumulates between the bridging CO and the Fe(d) than between the bridging CO and the Fe(p), and the occupation increase in this orbital will enhance the bonding between the bridging CO and the Fe(d), leading to the bridging-CO shift toward the Fe(d).

Microscopic aspects of pattern formation on surfaces

Recent scanning tunneling microscopy (STM) work gives insight into microscopic processes of surface reactions that play a role for spatio-temporal pattern formation. STM allows to resolve adsorbed particles, follow their surface motion, and monitor reactions with other particles on the atomic scale. The data reveal pronounced deviations from the implicite assumptions of the reaction-diffusion equations traditionally used to model spatio-temporal patterns. In contrast to these descriptions, particles are often not randomly distributed, but cluster in islands because of attractive interactions, and particle hopping can be highly correlated. It is shown that such phenomena can even affect the macroscopic kinetics. The article also discusses a case where the atomic processes inside propagating reaction fronts could be resolved. Here particular strong interaction effects were observed, caused by hydrogen bonds between the reacting species. (c) 2002 American Institute of Physics.

Spatiotemporal self-organization in a surface reaction: from the atomic to the mesoscopic scale

Scanning tunneling microscopy data revealed the atomic processes in propagating reaction fronts that occur in the catalytic oxidation of hydrogen on Pt(111). The fronts were also characterized on mesoscopic length scales with respect to their velocity and width. Simulations on the basis of a reaction-diffusion model reproduce the experimental findings qualitatively well. The quantitative comparison reveals the limitations of this traditional approach to modeling spatiotemporal pattern formation in nonlinear dynamics.