Faces Of Platinum Research Articles

Excitation of the shear horizontal mode in a monolayer by inelastic helium atom scattering

Inelastic scattering of a low-energy atomic helium beam (HAS) by a physisorbed monolayer is treated in the one-phonon approximation using a time-dependent wave packet formulation. The calculations show that modes with shear horizontal polarization can be excited near high symmetry azimuths of the monolayer, in agreement with recent experiments. The parameters of the calculations are chosen to match the conditions of HAS experiments for triangular incommensurate monolayer solids of xenon, krypton, and argon adsorbed on the (111) face of platinum, and the results show many of the systematic experimental trends for relative excitation probability of the shear horizontal and longitudinal acoustic phonon branches. The inelastic scattering at beam energies near 8 meV is exceedingly sensitive to small misalignment between the scattering plane and the high symmetry directions of the monolayer solid. The diffraction and inelastic processes arise from a strong coupling of the incident atom to the target and the calculated results show large departures from expectations based on analogies to inelastic thermal neutron scattering.

Dynamics of monolayer xenon adsorbed on Pt(111)

Effects of substrate dynamics for xenon adsorbed on the (111) face of platinum are calculated using realistic xenon interatomic potentials, force constants for the platinum that reproduce the bulk lattice dynamics, and the Barker-Rettner semiempirical $\mathrm{Xe}\ensuremath{-}\mathrm{Pt}$ potential. There is only a minor change in the platinum density of states when the coupling of harmonic normal modes of the xenon monolayer solid to the platinum substrate is included. Effects of the dynamic coupling on the xenon are small, apart from the previously known Brillouin-zone-center mixing, both for the simply commensurate $\sqrt{3}\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ lattice and for a possible denser $\sqrt{19}$-commensurate unit cell with seven Xe atoms. The frequency spectra are calculated for four other hypothetical higher-order commensurate xenon lattices with average lattice spacings close to those for experimentally observed triangular incommensurate $\mathrm{Xe}∕\mathrm{Pt}(111)$ lattices, but assuming a static substrate. Compared to the $\sqrt{3}$ Xe lattice, the spectrum ${\ensuremath{\omega}}_{\ensuremath{\perp}}(q)$ of perpendicular Xe vibrations of the other lattices is shifted to lower frequencies and is greatly broadened. This significant calculated dispersion of ${\ensuremath{\omega}}_{\ensuremath{\perp}}(q)$ is counter to experiment for incommensurate monolayer xenon. Also, the coefficient of Brownian friction for a single Xe adatom on Pt(111) is evaluated.

INFLUENCE OF STEPS ON ATOMIC ADSORPTION AND SEQUENTIAL GROWTH ABOVE VICINAL SURFACES

Two different situations corresponding to out-of-equilibrium growth of an adsorbate and to equilibrium adsorption of a gas are discussed using the vicinal face of a transition metal as the substrate. Monte-Carlo simulations in the kinetic and equilibrium regimes are applied to the ordering of Xe and Ag atoms close to the monatomic steps of the (997) vicinal face of platinum. It is shown that the steps can favor the growth of one or two rows of adatoms (row-by-row growth) in the submonolayer domain, depending on the temperature and on intrinsic parameters connected to the magnitude of the interactions between the adspecies themselves and between these adspecies and the substrate. Such a behavior is consistent with very recent helium atom scattering experiments.

Formation of hydronium and water–hydronium complexes during coadsorption of hydrogen and water on (2×1)Pt(110)

In the previous literature, people have found that a new species forms when hydrogen and water coadsorb on various faces of platinum. The new species was identified as “hydronium” based on a preliminary analysis of electron energy loss spectroscopy (EELS) spectra. At present, this assignment is not well accepted in the literature since thermodynamically a bare hydronium ion would not be stable on platinum. In this paper we do the measurements and the calculations to tie down the identity of the species. Ab initio methods are used to estimate the vibrational spectrum of H 3O +, and [(H 2O) x H +] with x=2, 3 or 4, H 3O radicals, H 2–H 2O complexes and H 3O −. We then measured better quality EELS spectra than had appeared in the previous literature for coadsorption of hydrogen and water on Pt(110). By comparing our experimental spectra and calculated spectra, we found the calculated spectrum of H 5O + 2 and H 7O + 3 fit our experimental data the best. These results suggest that the species which forms when hydrogen and water coadsorb on platinum is a cationic species similar to hydronium. It is probably not H 3O +. However, it is likely that the surface includes a mixture of hydrated intermediates such as H 5O + 2, H 7O + 3 and H 9O + 4.

Surface properties of fractured and polished platinum microelectrodes

An investigation of the surface properties related to the method of microelectrode fabrication is presented and discussed. The voltammetric response in acidic media (sulphuric and formic acids) of fractured 25 μm diameter wire which had been sealed in soft glass was used to investigate the surface microstructure. SEM studies showed that the crystallite size of the fractured wire, which had previously been sealed in glass, was large, and that a single crystallite may be exposed to the solution surface. The effect of sealing the wire in glass on the bulk properties of the wire is related to the voltammetric response in acid media. Thus, the surface of a polished platinum microelectrode sealed in glass cannot be considered as truly polycrystalline. The responses of fractured platinum wire sealed in soft glass in sulphuric acid and formic acid are predominantly Pt(111), when compared by non-linear regression and congruent analysis with the three low index faces of platinum in the same acidic medium under the same conditions. This is explained by the process of slip deformation, which involves the slipping of Pt(111) planes (close packed planes or planes of highest atomic density) forming steps with the exposed surface predominantly Pt(111).

Surface reconstruction of platinum and gold and the embedded-atom model.

We investigate the surface reconstructions of various faces of gold and platinum theoretically with a number of embedded-atom potentials. The potentials examined include conventional models plus a sequence of models where the shape of the embedding function has been varied for electron densities typical of the surface. We find that whereas all models predict virtually equivalent bulk properties, only potentials with a sufficient curvature in the embedding function predict the experimentally observed buckled quasihexagonal reconstruction for the (100) faces. Furthermore, the models that predict this reconstruction also give surface energies in close agreement with experiment, unlike most of the conventional models. Almost all embedded-atom potentials considered predict the observed missing-row structure for the (110) faces, and some the observed missing-row structure for the (311) face of platinum. We take the best combination of Au and Pt potentials and calculate the structure of a Pt(100) substrate in the presence of Au adatoms. In good agreement with experiment, the quasihexagonal reconstruction is lost for coverage greater than 0.4 monolayers.

Dissociative adsorption process of acetaldehyde on Pt ( h,k,l) basal surfaces : An EHMO study

The adsorption of species arising from the dissociative adsorption process of acetaldehyde on the (100) and (111) faces of platinum were studied. Three-layer clusters consisting of 28 platinum atoms were employed. Adsorption by the carbonylic carbon atom is energetically favoured in both faces. The adsorption of didehydrogenated species on the platinum (100) face favours the atop configuration, whilst adsorption on the (111) face favours the bridged configuration. Competitive processes can occur on the platinum (111) face between monodehydrogenated species adsorbed by the carbonylic carbon atom and the didehydrogenated species. A significant negative difference overlap population was calculated for the C-C bond of the carbonylic carbon atom in the adsorbed species.

Carbon-oxygen bond scission during methanol decomposition on (1.times.1)platinum(110)

Recently there has been some controversy whether C-O bond scission occurs during methanol decomposition on the (111) faces of platinum and palladium. Here, the decomposition of methanol on Pt(110) is examined by TPD and EELS. It is found that, on the (1×1) reconstruction of Pt(110), the methanol adsorbs molecularly at 100 K. When the layer is heated to 140 K, the C-O bond breaks to yield water and a mixture of CH xad species. The CH xad species then further react to form CH 4 , H ad , C ad ) and some as yet unidentified higher hydrocarbons

Structure sensitivity of methanol decomposition on (1×1) and (2×1) Pt(110)

Recently, there has been some controversy about the mechanism of methanol decomposition on the (111) faces of platinum and palladium. Here, the decomposition of methanol on (1×1) and (2×1) Pt(110) is examined by temperature‐programmed desorption and electron‐energy‐loss spectroscopy. It is found that on the (1×1) reconstruction of Pt(110), the methanol adsorbs molecularly at 100 K. Upon heating to 140 K, the C–O bond breaks to yield water and a mixture of CHx ad species. The CHx ad species then further react to form CH4, Had, Cad, and some acetylenic complexes. By comparison, if the sample preparation procedure is changed to produce a (2×1) reconstruction, no water, methane, carbon, or higher hydrocarbons are detected. Instead, the methanol decomposes via a methoxy intermediate to yield CO and hydrogen. These results demonstrate that the mechanism of methanol decomposition on platinum is strongly affected by small changes in the sample preparation procedure and surface structure of the sample. This may explain some of the apparent discrepancies in the literature.

Surface Reconstruction of Platinum and Gold and the Embedded Atom Model

ABSTRACTWe investigate the surface reconstructions of various faces of gold and platinum theoretically with a number of embedded atom potentials. We find that whereas all potentials examined predict equivalent bulk properties, only those with a relatively steep embedding function at surface electron densities predict the experimentally observed buckled quasihexagonal reconstruction for the (100) faces and surface energies in close agreement with experiment. Almost all embedded atom potentials considered predict the observed 2×1 missing row structure for the (110) faces, and some the observed missing row structure for the (311) face of platinum.

The equilibrium shape and surface energy anisotropy of clean platinum

We have measured the equilibrium shape of clean platinum by monitoring the changes in the shape of a series of micrometer-sized platinum droplets during annealing at 1200°C in 10 −7 Torr of oxygen. The droplets start out with a rounded shape and a wavy surface. Upon annealing for 24 h or more, the particles are converted to nearly spherical shapes with distinct facets in the (100) direction and smaller facets in the (111) direction. At equilibrium, the facets only cover 16% of the particle's surface. The rest of the surface is stepped or kinked. The equilibrium particle shape is also independent of particle size for particles in the size range explored here ( d > 1 micrometer). Initially, the particles show a series of hill and valley structures around the (100) facets. However, the hill and valley structures disappear after a long anneal. After a long anneal, the particles also assume an epitaxial relation with the support. From the data, we estimatethat the surface energy of platinum only varies by about 92 erg/cm 2 with crystal face, with Pt(100) having the lowest surface energy, Pt(111) having a slightly higher surface energy, and all of the other faces of platinum having essentially the same surface energy. By comparison, previous workers have shown that heats of adsorption of simple gases often vary by 350 erg/cm 2 with crystal face. The implication, therefore, is that in platinum the variations in surface energy with orientation are small enough that particle shape control in supported metal catalysis is feasible. Consequences to the stability of various single crystal faces are also discussed.

Adsorption of acetaldehyde on Pt(100), (110) and (111) faces: A semi-empirical quantum chemical study

The adsorption of acetaldehyde on three Pt faces (100), (110) and (111) has been investigated using the semi-empirical quantum chemical EHMO method and 3-layer thick clusters consisting of 28 Pt atoms. The modes of chemisorption interactions are studied for a variety of configurations on the platinum faces. Charge transfer from the MO's of adsorbates to the surface is important in the initial steps for C-H bond breaking. The role of the 6s,p-like orbitals in Pt-H bond formation is pointed out.

Growth kinetics of the Pt(111) surface during low temperature vacuum deposition

The competition between advancement of monatomic steps and two-dimensional nucleation on the (111)-face of platinum during vacuum deposition in the temperature range of 130 to 400 K has been considered under the assumptions that nucleation follows a “non-classical” mechanism and layer propagation is controlled by surface diffusion. Previous experimental results [M. Alnot, J.J. Ehrhardt and J.A. Barnard, Surface Sci. 208 (1989) 285] showing that at temperatures of about 400 K the surface between steps remains atomically smooth during growth, while at the temperature of 130 K the surface becomes kinetically rough, are thus interpreted.

ChemInform Abstract: The Hydrogenolysis of Ethane over Re‐Pt(111) and Pt‐Re(0001) Bimetallic Crystal Surfaces

Abstract Platinum-rhenium bimetallic surfaces were prepared by condensing both rhenium on the (111) face of platinum, and platinum on the (0001) face of rhenium from the vapor under ultrahigh vacuum. Using a high-pressure microreactor, the hydrogenolysis of ethane was investigated over the small-area catalysts in the temperature range 300–350 °C, with pressures of 5–10 Torr ethane and 100–1000 Torr hydrogen. The reaction is structure sensitive on both platinum and rhenium as indicated by the large increase in the initial rates of reaction observed after the Pt(111) and Re(0001) surfaces were roughened by argon ion bombardment. The Re(0001) surface was two orders of magnitude more active than the Pt(111) surface, yet a bimetallic surface of the stoichiometry Re2Pt was found to be the most active surface for ethane hydrogenolysis—about one order of magnitude more active than the Re(0001) surface. Thus the activity of the bimetallic catalyst is not a linear combination of the activities due to the two metallic components. Hydrogen pressure dependence studies show that a bimetallic surface composed of 0.3 monolayer of rhenium on Pt(111) had an activity close to that of a monometallic Re(0001) surface, yet displayed a hydrogen partial pressure dependence closer to that of a Pt(111) surface. These results suggest that an electronic interaction exists between platinum and rhenium metals that strongly influence the catalytic hydrogenolysis of ethane. The accumulation of carbonaceous deposits was fairly insensitive to temperature and hydrogen pressure on rhenium and bimetallic PtRe surfaces. However, the accumulation of carbonaceous deposits on monometallic Pt(111) surfaces was highly sensitive to the reaction conditions, and adsorbed on this surface more tenaciously than on bimetallic PtRe surfaces.

The hydrogenolysis of ethane over RePt(111) and PtRe(0001) bimetallic crystal surfaces

Platinum-rhenium bimetallic surfaces were prepared by condensing both rhenium on the (111) face of platinum, and platinum on the (0001) face of rhenium from the vapor under ultrahigh vacuum. Using a high-pressure microreactor, the hydrogenolysis of ethane was investigated over the small-area catalysts in the temperature range 300–350 °C, with pressures of 5–10 Torr ethane and 100–1000 Torr hydrogen. The reaction is structure sensitive on both platinum and rhenium as indicated by the large increase in the initial rates of reaction observed after the Pt(111) and Re(0001) surfaces were roughened by argon ion bombardment. The Re(0001) surface was two orders of magnitude more active than the Pt(111) surface, yet a bimetallic surface of the stoichiometry Re 2Pt was found to be the most active surface for ethane hydrogenolysis—about one order of magnitude more active than the Re(0001) surface. Thus the activity of the bimetallic catalyst is not a linear combination of the activities due to the two metallic components. Hydrogen pressure dependence studies show that a bimetallic surface composed of 0.3 monolayer of rhenium on Pt(111) had an activity close to that of a monometallic Re(0001) surface, yet displayed a hydrogen partial pressure dependence closer to that of a Pt(111) surface. These results suggest that an electronic interaction exists between platinum and rhenium metals that strongly influence the catalytic hydrogenolysis of ethane. The accumulation of carbonaceous deposits was fairly insensitive to temperature and hydrogen pressure on rhenium and bimetallic PtRe surfaces. However, the accumulation of carbonaceous deposits on monometallic Pt(111) surfaces was highly sensitive to the reaction conditions, and adsorbed on this surface more tenaciously than on bimetallic PtRe surfaces.

Ammonia adsorption and decomposition on several faces of platinum

This paper is part of an ongoing study of the structure sensitivity of ammonia adsorption on platinum. In the work here, the adsorption of ammonia on Pt(111) and Pt(210) are compared using temperature programmed desorption (TPD), single-reflection infrared (IR), electron energy-loss spectroscopy (EELS), and molecular-beam techniques. In addition, the adsorption of ammonia on Pt(111), (5×20)Pt(100), (2×1)Pt(110), Pt(210), and Pt(211) are compared with TPD. It is found that the TPD spectrum of ammonia is similar on Pt(111), Pt(100), Pt(110), Pt(210), and Pt(211). There was always a molecular peak between 310 and 370 K, another peak at 170 K, and a multilayer peak at 120 K. The sticking probability was found to vary by a factor of 10 from Pt(210) to Pt(111). However, the infrared spectrum of ammonia adsorbed on Pt(111) is virtually identical to the EELS of ammonia on Pt(210) after accounting for a difference in the sensitivity of IR and EELS. Analysis of the TPD data indicates that the surface concentration of ammonia is a factor of 100–200 higher on Pt(210) than on Pt(111) at 670 K. By comparison, Pt(210) was found to be at least a factor of 100 more active than Pt(111) for ammonia dissociation at 670 K. There is more work to be done. However, the results so far indicate that the vaariations in sticking probability and residence time play an important role in the variations in rate.

Summary Abstract: The adsorption of thiophene and tetrahydrothiophene on several faces of platinum

Summary Abstract: The adsorption of thiophene and tetrahydrothiophene on several faces of platinum

The adsorption of thiophene and tetrahydrothiophene on several faces of platinum

The adsorption of thiophene and tetrahydrothiophene on several faces of platinum

The adsorption of thiophene and tetrahydrothiophene on several faces of platinum

X-ray photoelectron spectroscopy, TPD, and HREELS were used to investigate the adsorption and desulfurization of thiophene and tetrahydrothiophene on Pt(111), Pt(100), and Pt(210). On all of these surfaces, thiophene was found to desulfurize at 350 K to form a multiply-bound cyclic structure which dehydrogenated above 400 K. Tetrahydrothiophene appears to undergo ring opening upon adsorption on Pt(100)5×20 at 175 K. Heating to 350 K causes desulfurization of the molecule and the formation of a C 4 species identical to that produced by thiophene desulfurization on platinum. Implications of these results are discussed.

Coadsorption of sulphur and hydrogen on platinum (110) studied by radiotracer and electrochemical techniques

The influence of chemisorbed sulphur on the adsorption of hydrogen on the (110) face of platinum in acid medium was quantitatively studied. Sulphur was pre-adsorbed in a gaseous phase (H 2S/H 2) and the sulphur coverage measured by using the 35S radioisotope. The hydrogen was underpotentially deposited and its coverage measured by cyclic voltametry. The relation between the coverages of sulphur ( θ S) and hydrogen ( θ H) coadsorbed on the platinum surface was established. On the sulphur-free surface the hydrogen coverage at saturation is 1.5, consistent with a reconstructed surface (110)−(1×2). The adsorption of hydrogen is totally inhibited when the surface is saturated by sulphur ( θ S = 0.8). The θ S versus θ H curve is not linear. At low θ S, θ H decreases sharply when increasing θ S. The number of adsorption sites blocked by one sulphur atom is 12±3. This indicates that all hydrogen adsorption sites containing at least one platinum atom in nearest-neighbour position with a sulphur atom are deactivated. With increasing θ S, the influence of each sulphur atom decreases. A model is suggested for the adsorption of hydrogen in sulphur overlayer structures, which accounts for the observed poisoning effect. The shape of the isotherms for hydrogen adsorption on surfaces partially covered by sulphur indicates that sulphur weakens the PtH bond at θ H → 0 and has a screening effect on the interactions between H atoms at higher θ H.