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.     

Molecular dynamics simulations of surface oxide-water interactions on Pt(111) and Pt/PtCo/Pt3Co(111)

Classical molecular dynamics simulations of the interactions of water with oxidized Pt(111) and Pt/PtCo/Pt(3)Co(111) surfaces are performed by modeling water with the CF1 central force model that allows molecular dissociation and therefore the presence of other intermediates of the oxygen reduction reaction different from atomic oxygen. It is found that the water-surface oxide interactions do not affect the overall structure of the catalyst represented by an extended periodic slab. However, such interactions are affected by changes in the electrochemical potential which are simulated by higher values of the surface and atomic oxygen charges at increased oxygen coverage. Thus, electrochemical potential as well as the presence of protons and anions products of acid dissociation define the identity and the amount of oxygen reduction reaction intermediates such as OH or H(3)O. We observe agglomerations of water molecules over regions of the surface and the presence of OH and H(3)O in their vicinity. Our simulation model is able to qualitatively reproduce features of the degradation of the catalyst surface after oxidation and reduction cycles.     

Sulfate-enhanced catalytic destruction of 1,1,1-trichlorethane over Pt(111)

The catalytic destruction of 1,1,1-trichloroethane (TCA) over model sulfated Pt(111) surfaces has been investigated by fast X-ray photoelectron spectroscopy and mass spectrometry. TCA adsorbs molecularly over SO4 precovered Pt(111) at 100 K, with a saturation coverage of 0.4 monolayer (ML) comparable to that on the bare surface. Surface crowding perturbs both TCA and SO4 species within the mixed adlayer, evidenced by strong, coverage-dependent C 1s and Cl and S 2p core-level shifts. TCA undergoes complete dechlorination above 170 K, accompanied by C-C bond cleavage to form surface CH3, CO, and Cl moieties. These in turn react between 170 and 350 K to evolve gaseous CO2, C2H6, and H2O. Subsequent CH3 dehydrogenation and combustion occurs between 350 and 450 K, again liberating CO2 and water. Combustion is accompanied by SO4 reduction, with the coincident evolution of gas phase SO2 and CO2 suggesting the formation of a CO-SOx surface complex. Reactively formed HCl desorbs in a single state at 400 K. Only trace (<0.06 ML) residual atomic carbon and chlorine remain on the surface by 500 K.     

Preoxidation of CO on Os-modified Pt(111): a comparison with Ru-modified Pt(111)

The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 × 2)-α to (√19 × √19) via the transient structures of (2 × 2)-β and (1 × 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 × 2)-β and (1 × 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (√19 × √19), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.     

NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.     

Selectivity control for the catalytic 1,3-butadiene hydrogenation on Pt(111) and Pd(111) surfaces: Radical versus closed-shell intermediates

The hydrogenation of 1,3-butadiene to different C4H8 species on both Pd(111) and Pt(111) surfaces has been studied by means of periodic slabs and DFT. We report the adsorption structures for the various mono- and dihydrogenated butadiene intermediates adsorbed on both metal surfaces. Radical species are more clearly stabilized on Pt than on Pd. The different pathways leading to these radicals have been investigated and compared to those producing 1-butene and 2-butene species. On palladium, the formation of butenes seems to be clearly favored, in agreement with the high selectivity to butenes observed experimentally. In contrast, the formation of dihydrogenated radical species seems to be competitive with that of butenes on platinum, which could explain its poorer selectivity to butenes and the formation of butane as a primary product.     

Methanol oxidation on a Pt(111)-OH/O surface

The methanol oxidation on a hydroxylated Pt (Pt(111)-OH) surface has been investigated by means of infrared reflection absorption spectroscopy (IRAS) in ultra-high vacuum (UHV) and in acidic solution. The Pt(111)-OH surface in UHV was prepared by introducing water molecules on a Pt(111)-(2 x 2)-O surface and annealed at temperature higher than 160 K. Methanol was then, introduced to the Pt(111)-OH surface to show the dependence of the reaction intermediate on the annealing temperature. At an annealing temperature below 160 K, IR bands assignable to methanol overlayer were observed and no detectable intermediates, such as CO, formaldehyde and formate, were formed, suggesting that methanol molecules remain stable on Pt(111) surface without dissociation at this temperature region. At an annealing temperature above 160 K, on the other hand, CO and formate were observed. In addition, the oxidation of CO on Pt(111)-OH showed no sign of formate formation, indicating that formate is not derived from CO, but from a direct oxidation of methanol. Methanol oxidation was carried out in 0.1 mol dm(-3) HClO(4) solution on Pt(111) with a flow cell configuration and showed the formation of formate. These results indicate that the formate is the dominant non-CO intermediate both in UHV and in acidic solution, and the preadsorbed oxygen-containing species, in particular OH adsorbates, on Pt(111) surface plays a very important role in the formate formation process in methanol oxidation reaction.     

Kinetic study of hydrogen sulfide decomposition on Pt(111) surface

In this work, kinetic of H₂S conversion to H₂ molecule on the surface of Pt(111) is studied using kinetic Monte Carlo simulation. The results of simulation were fitted to the experimental temperature-programed desorption spectra. The good agreement between the empirical and the simulated data confirms the proposed mechanism and kinetic data (activated energies and pre-exponential factors). The influence of variables such as temperature and concentrations of H₂S and H₂ on the overall results of hydrogen production is studied. The condition is proposed in which the best yield of reaction at minimum temperature is obtained. Results show that platinum is a perfect catalyst for converting H₂S to H₂ and it has a perfect performance (98%) after 5 μs at low temperature of 227°C.     

On the dynamics of H2 adsorption on the Pt(111) surface

The dynamics and kinetics of the dissociation of hydrogen over the hexagonal close packed platinum (Pt(111)) surface are investigated using Car–Parrinello molecular dynamics and static density functional theory calculations of the potential energy surfaces. The calculations model the reference energy‐resolved molecular beam experiments, considering the degrees of freedom of the catalytic surface. Two‐dimensional potential energy surfaces above the main sites on Pt(111) are determined. Combined with Car–Parrinello trajectories, they confirm the dissociative adsorption of H₂ as the only adsorption pathway on this surface at H₂ incindence energies above 5 kJ/mol. A direct determination of energy‐resolved sticking coefficients from molecular dynamics is also performed, showing an excellent agreement with the experimental data at incidence energies in the 5–30 kJ/mol range. Application of dispersion corrections does not lead to an improvement in the prediction of the H₂ sticking coefficient. The adsorption reaction rate obtained from the calculated sticking coefficients is consistent with experimentally derived literature values.     

Bulk CO oxidation on platinum electrodes vicinal to the Pt(111) surface

Bulk CO oxidation has been studied on platinum stepped surfaces belonging to the series Pt(S)[n(111) × (111)], using a hanging meniscus rotating disk electrode (HMRDE) configuration. The general shape of the voltammograms is not significantly affected by the presence of the steps. However, the curves shift towards negative values as the step density increases. Thus, in the positive-going scan, a linear relationship is observed for the dependence of the potential for the ignition peak vs the step density for surfaces with terraces wider than five atoms, shorter terraces deviate from this behavior. In the negative-going scan, a similar situation is observed for the potential where the current drops to zero. In this case, Pt(111) electrode also deviates from the expected behavior because of the formation of the ordered bisulfate adlayer on the electrode. The anion readsorption process is also observed by recording the HRMDE voltammograms at a high scan rate. All these results have been analyzed in light of a common mechanism, discussing the possible role of the steps in the stability and reactivity of the CO adlayer. In memoriam of Francisco C. Nart, an excellent scientist, colleague, and friend.     

n-alkanes on Pt(111) and on C(0001)Pt(111): chain length dependence of kinetic desorption parameters

We have measured the desorption of seven small n-alkanes (C(N)H(2N+2), N=1-4,6,8,10) from the Pt(111) and C(0001) surfaces by temperature programed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption preexponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.     

Inelastic scattering and chemisorption of CO on a Pt(111) surface

A semiclassical model is used to calculate energy transfer in collisions between CO and a Pt(111) surface. The sticking probability is found to be as large as 0.7–0.8 for small collision energies (≈0.14 eV). At higher energies (≈5 eV) it decreases to ≈0.3. Strong interaction between the adsorbed molecule and the phonons is observed.     

Pt oxide and oxygen reduction at Pt(111) studied by surface X-ray diffraction

The influence of the oxygen reduction reaction on the oxidation of Pt(111) is studied by surface X-ray diffraction. The oxygen reduction reaction does not significantly influence the place-exchange process during the initial stages of oxidation and there is no change in the onset potential and kinetics.     

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.     

First principles study of O2 dissociation on Pt(111) surface: Stepwise mechanism

We have performed first‐principle density functional theory calculations to investigate O₂ dissociation on Pt(111) surface. A stepwise mechanism has been proposed. First, the adsorbed O₂ dissociate into two oxygen atoms to get adsorbed on the nearby adsorption sites. Then, oxygen atoms further migrate to other more stable adsorption sites. The influence of solvent water on oxygen dissociation was also examined. The results show that the co‐adsorption of water has little impact on O₂ dissociation. However, when water participates in the reaction, the energy barriers were reduced greatly. These results have very important significance to understand the mechanism of oxygen reduction. © 2016 Wiley Periodicals, Inc.     

The influence of substrate temperature on the surface chemistry of ketene on Pt(111)

Processes occurring during dosing of ketene on a heated Pt(111) surface were investigated using high-resolution electron energy loss spectroscopy and temperature-programmed desorption. During dosing at 400 K, CO and H₂ are evolved and ethylidyne accumulates on the surface. In contrast, at 350 K a C_xH_y species, not ethylidyne, is formed and some CO accumulates. It is postulated that small amounts of adsorbed carbon monoxide stabilize the C_xH_y species and inhibit ethylidyne formation.     

On the adsorption and formation of Pt dimers on the CeO2(111) surface

The direct adsorption of Pt(2) dimers on CeO(2)(111) and their formation from isolated adsorbed Pt atoms have been studied using periodic slab model calculations based on density functional theory and including the so-called on-site Hubbard parameter (GGA + U). In the most stable configuration Pt(2) is found to be almost parallel to the surface; the electronic ground state is closed shell and there is no evidence of charge transfer towards or from the surface. The formation of Pt(2) from two single adsorbed Pt atoms involves a rather small energy barrier of ~0.10 eV only. On the contrary, dissociation of adsorbed Pt(2) requires to overcome a considerable barrier of ~1.43 eV. This indicates that once Pt(2) is formed it will remain on the surface, thus likely triggering the growth of larger supported Pt particles.     

Reversible hydrogenation of surface N atoms to form NH on Pt(111)

The formation and dissociation chemistry of the NH species on Pt(111) was characterized with reflection absorption infrared spectroscopy and temperature programmed desorption. Irradiation of a chemisorbed bilayer of ammonia with a 100 eV electron beam at 85 K leads to a mixture of NH, N, and H on the surface. Annealing to temperatures in the range of 200-300 K leads to reaction of N and H to form additional NH. The NH species has an intense and narrow NH stretch peak at 3320 cm(-1), while no peak due to the PtNH bend is observed above 800 cm(-1). The NH species is stable up to a temperature of approximately 400 K. The surface N atoms produced from NH dissociation are readily hydrogenated back to NH by exposure of the surface to H2. However, NH cannot be further hydrogenated to generate adsorbed NH2 or to NH3 under the conditions used here. Exposure of the NH/Pt(111) surface to D2 at 380 K produces the ND species. Comparison with the results of density functional theory calculations based on small Pt clusters indicates that NH occupies three-fold hollow sites with the molecular axis perpendicular to the surface.     

Molecular dynamics of hydrogen dissociation on an oxygen covered Pt(111) surface

The dissociation of hydrogen on a Pt(111) surface covered with a p(2 x 2) oxygen phase was investigated using quasiclassical, six-dimensional molecular dynamics. The potential energy surface (PES) used in these simulations was obtained by an iterative novelty sampling algorithm. Compared to molecular beam experiments performed under similar conditions, the simulations give an accurate prediction of the reaction probability via a direct dissociation pathway. When compared to previously reported reaction probability curves for the clean Pt(111) surface, we find that the presence of an oxygen overlayer inhibits the direct pathway to hydrogen dissociation. This inhibition is a function of incident energy and cannot be described by a simple site blocking model. An indirect pathway to dissociation, which was observed in experiments, is not properly captured by the PES. Spatially resolved "reaction maps" indicate that the preferred site for hydrogen dissociation on an oxygen covered Pt surface is the top site of the Pt atom farthest from the adsorbed oxygen atom.