In situ FTIR spectroscopic study of ethanol oxidation on Pt(111)/Rh/Sn surface. The anion effect

In situ Fourier Transform Infrared (FTIR) spectroscopy was used to study the anion effect in ethanol oxidation on Pt (111) surface modified by rhodium and tin, Pt(111)/Rh/Sn. The in situ FTIR spectra showed that ethanol oxidation reaction pathway is strongly influenced by the nature of the electrolyte anion. In perchloric and sulfuric acid electrolytes were observed the formation of acetaldehyde, acetic acid and CO₂; however in phosphoric acid the acetic acid is not observed. The sulfuric acid is the most favorable electrolyte for acetaldehyde and CO₂ formation.     

NO adsorption on Pt (111)/Bi surfaces

The adsorption of nitric oxide (NO) on a Pt (111) surface modified with irreversible adsorbed bismuth adatoms is reported. While the voltammetric results reveal a close interaction between the two co-adsorbed compounds. In-situ infrared spectroscopy and scanning tunnelling microscopy indicate the formation of segregated adlayers. Formation of compressed Bi adlayers with modified redox properties is proposed to reconcile both results. This agrees with the observation of Bi islands in the STM images when NO is coadsorbed, not observed in the absence of NO.     

Hydrogen and deuterium exchange on Pt(111) and its poisoning by carbon monoxide studied by surface sensitive high-pressure techniques

Catalytic hydrogen/deuterium exchange on a platinum (111) single crystal and its poisoning with carbon monoxide was studied using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), sum frequency generation vibrational spectroscopy (SFG), and mass spectrometry under reaction conditions at pressures in the mTorr to atmospheric range. At 298 K and in the presence of 200 mTorr of hydrogen and 20 mTorr of deuterium the surface is catalytically active, producing HD with activation energy of approximately 5.3 kcal/mol. Addition of 5 mTorr of CO stops the reaction completely. In situ STM images reveal an ordered surface structure of chemisorbed CO. At 353 K the addition of 5 mTorr of CO slows the reaction by 3 orders of magnitude, but HD production continues with an activation energy of 17.4 kcal/mol. Changes in coverage and adsorption site of CO were followed by XPS and SFG up to a temperature of 480 K. From these data, a CO dominated, mobile and catalytically active surface model is proposed.     

Examination of Pt(111)/Ru and Pt(111)/Os surfaces: STM imaging and methanol oxidation activity

Ruthenium and osmium were deposited in submonolayer amounts on Pt(111) single crystal surfaces using the previously reported ‘spontaneous deposition’ procedure [Chrzanowski et al., Langmuir, 13 (1997) 5974]. Such surfaces were first explored using ex situ scanning tunneling microscopy (STM) to image the deposition characteristics of ruthenium and osmium islands on Pt(111). It was found that, using the spontaneous deposition procedure, a maximum coverage of 0.20 ML ruthenium is formed on the surface after 120 s of exposure to a RuCl₃ solution in 0.1 M HClO₄. A homogeneous deposition on the Pt(111) surface was found, with no observed preferential deposition on step edges or surface defect sites. In contrast, in the spontaneous deposition of osmium, osmium clusters form preferentially at, though not limited to, surface defect sites and step edges. Osmium island deposition occurs at a greater rate than ruthenium on Pt(111), and possible explanations are presented. Methanol activity on the Pt(111)/Ru and Pt(111)/Os surfaces is also studied, using the coverage values determined to yield the highest activity for methanol electro-oxidation (0.20 ML coverage for Ru and 0.15 ML for Os). At potentials more negative than 0.40 V vs. RHE, the Pt(111)/Ru surface yields a higher surface activity than Pt(111)/Os. However, at potentials more positive than 0.04 V, Pt(111)/Os exhibits demonstrably higher surface activity. The relevance of this data is discussed and future avenues of interest are indicated.     

Low-temperature oxidation of carbon monoxide on Pd(Pt)/CeO2 catalysts prepared from complex salts

Catalysts containing cerium oxide as a support and platinum and palladium as active components for the low-temperature oxidation of carbon monoxide were studied. The catalysts were synthesized in accordance with original procedures with the use of palladium and platinum complex salts. Regardless of preparation procedure, the samples prepared with the use of only platinum precursors did not exhibit activity at a low temperature because only metal and oxide (PtO, PtO₂) nanoparticles were formed on the surface of CeO₂. Unlike platinum, palladium can be dispersed on the surface of CeO₂ to a maximum extent up to an almost an ionic (atomic) state, and it forms mixed surface phases with cerium oxide. In a mixed palladium-platinum catalyst, the ability of platinum to undergo dispersion under the action of palladium also increased; as a result, a combined surface phase with the formula Pd _x Pt _y CeO_{2 ? δ}, which exhibits catalytic activity at low temperatures, was formed.     

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.     

Adsorption of carbon monoxide and oxygen on Pt(110)

Adsorption of CO and O₂ on Pt(110) was studied by XPS, LEED and TDS methods to elucidate the role of O_ads states and structural rearrangements of the surface under the action of CO_ads in the appearance of self-oscillations in the rate of CO oxidation on Pt(110).

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, CO O₂ Pt(110) O CO CO Pt(110).

     

The adsorption and co-adsorption of oxygen and carbon monoxide on Pt3Ni(111): a vibrational study

High-resolution electron energy loss spectroscopy has been used to investigate the adsorption and co-adsorption of oxygen and CO on the Pt(3)Ni(111) surface. For the sake of comparison, similar measurements have also been performed on the Pt(111) surface. We find that CO adsorbs at the same manner on both surfaces. By contrast, significant differences between the two surfaces exist concerning the adsorption of O and the co-adsorption of O with CO.     

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.     

Acetaldehyde on Pt(111) and Pt/Sn(111): a DFT study of the adsorption structures and of the vibrational spectra

The relative stability of the eta1mu1 (atop) and eta2mu2 (di-sigma) geometries of acetaldehyde are compared on Pt(111) and on two PtSn alloys ((2 x 2) and (square root(3) x square root(3))R30 degrees) by means of density functional theory (DFT) calculations. At low coverage on Pt (1/9 ML), the two forms are equivalent in energy, with eta1mu1 being slightly more stable. At high coverage (1/4 and 1/3 ML), eta2mu2 is less competitive and acetaldehyde is adsorbed through the aldehydic hydrogen. The evolution of the adsorption energy with the coverage and the apparition of the structure adsorbed through the aldehydic hydrogen are explained by the existence of attractive dipole-dipole interactions. On PtSn, only the eta1mu1 geometry is stable with an adsorption energy equal to that on Pt, in agreement with temperature-programmed desorption (TPD) experiments. The calculated vibrational spectra allow us to conclude that the experimental spectrum corresponds to a mixture of eta1mu1 (majority) and eta2mu2 (minority) structures on Pt and to only eta1mu1 on PtSn. The various interactions and the relative stability of the species on Pt and PtSn are explained by the density of states (DOS) curves.     

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.     

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.     

Chemisorption and oxidation of carbon monoxide on palladium alloys

Kinetic and adsorption data (programmed thermal desorption in situ) for the oxidation of carbon monoxide on palladium alloys with silver and gold have been discussed. It has been shown that the local reaction plays a determining role and that active cluster sites are best for catalysis which are monoatomic with respect to palladium, weakly binding the chemisorbed CO molecule. The role of silver is to activate the second component: oxygen. The cluster model makes it possible to predict the optimum composition of Pd-Ag catalysts for the oxidation of CO and to explain the dependence of Pd-Au activity on the composition.Kiev University. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 5, pg. 574–578 September–October, 1991. Original article submitted July 12, 1991.     

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.     

In-situ scanning tunneling microscopy of carbon monoxide adsorbed on Au(111) electrode

In-situ scanning tunneling microscopy (STM) coupled with cyclic voltammetry was used to examine the adsorption of carbon monoxide (CO) molecules on an ordered Au(111) electrode in 0.1 M HClO4. Molecular resolution STM revealed the formation of several commensurate CO adlattices, but the (9 x radical 3) structure eventually prevailed with time. The CO adlayer was completely electrooxidized to CO2 at 0.9 V versus RHE in CO-free 0.1 M HClO(4), as indicated by a broad and irreversible anodic peak which appeared at this potential in a positive potential sweep from 0.05 to 1.6 V. A maximal coverage of 0.3 was estimated for CO admolecules from the amount of charge involved in this feature. Real-time in-situ STM imaging allowed direct visualization of the adsorption process of CO on Au(111) at 0.1 V, showing the lifting of (radical 3 x 22) reconstruction of Au(111) and the formation of ordered CO adlattices. The (9 x radical 3) structure observed in CO-saturated perchloric acid has a coverage of 0.28, which is approximately equal to that determined from coulometry. Switching the potential from 0.1 to -0.1 V restored the reconstructed Au(111) with no change in the (9 x radical 3)-CO adlattice. However, the reconstructed Au(111) featured a pairwise corrugation pattern with two nearest pairs separated by 74 +/- 1 A, corresponding to a 14% increase from the ideal value of 65.6 A known for the ( radical 3 x 22) reconstruction. Molecular resolution STM further revealed that protrusions resulting from CO admolecules in the (9 x radical 3) structure exhibited distinctly different corrugation heights, suggesting that the CO molecules resided at different sites on Au(111). This ordered structure predominated in the potential range between 0.1 and 0.7 V; however, it was converted into new structures of (7 x radical 7) and ( radical 43 x 2 radical 13) on the unreconstructed Au(111) when the potential was held at 0.8 V for ca. 60 min. The coverage of CO adlayer decreased accordingly from 0.28 to 0.13 before it was completely removed from the Au(111) surface at more positive potentials.     

The electrochemical oxidation of carbon monoxide adsorbed on Pt(111) in aqueous electrolytes as monitored by in situ potential step-second harmonic generation

The dynamics of electrooxidation of adsorbed CO on Pt(111) microfacets was examined in CO-saturated 0.1 M HClO4 aqueous solutions by in situ time resolved second harmonic generation (SHG). Analysis of the temporal dependence of the intensity of the SHG signals recorded for experiments in which the potential was stepped to values high enough for adsorbed CO oxidation to ensue, was found to be consistent with the mean field theory model, yielding rate constants somewhat higher than those reported by Lebedeva et al. (N. P. Lebedeva, M. T. M. Koper, J. M. Feliu and R. A. v. Santen, J. Electroanal. Chem., 2002, 524-525, 242-251) in sulfuric acid solutions. The smaller rates observed by these authors may be ascribed to the presence of anions, ie. in all likelihood bisulfate, which are capable of competing effectively for Pt sites thereby blocking formation of oxygenated species on the surface. Also discussed in this work are the virtues and limitations of in situ SHG for monitoring fast surface processes.     

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.     

Effect of the Random Defects Generated on the Surface of Pt(111) on the Electro-oxidation of Ethanol: An Electrochemical Study

In the present work, the Pt(111) surface was disordered by controlling the density of {110}- and {100}-type defects. The cyclic voltammogram (CV) of a disordered surface in acid media consists of three contributions within the hydrogen adsorption/desorption region: one from the well-ordered Pt(111) symmetry and the other two transformed from the {111}-symmetry with contributions of {110}- and {100}-type surface defects. The ethanol oxidation reaction (EOR) was studied on these disordered surfaces. Electrochemical studies were performed in 0.1 M HClO₄+0.1 M ethanol using cyclic voltammetry and chronoamperometry. Changes in current densities associated to the specific potentials at which each oxidation peak appears suggest that different surface domains of disordered platinum oxidize ethanol independently. Additionally, as the surface-defect density increases, the EOR is catalysed better. This tendency is directly observed from the CV parameters because the onset and peak potentials are shifted to less positive values and accompanied by increases in the oxidation-peak current on disordered surfaces. Similarly, the CO oxidation striping confirmed this same tendency. Chronoamperometric experiments showed two opposite behaviors at short oxidation times (0.1 s). The EOR was quickly catalyzed on the most disordered surface, Pt(111)-16, and was then rapidly deactivated. These results provide fundamental information on the EOR, which contributes to the atomic-level understanding of real catalysts.     

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.     

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.