Ethylene dimerization over a model Sn/Pt(111) catalyst

The dimerization reaction of ethylene was studied over Pt(111) and (3×3)R30°-Sn/ Pt(111) model catalysts at moderate pressures (20–100 Torr). The catalyst surfaces were prepared and characterized in a UHV surface analysis system and moderate pressure catalytic reactions were conducted with an attached batch reactor. The overall catalytic activity of the (3×3)R30°-Sn/Pt(111) surface alloy for C₄ products was slightly higher than that at Pt(111). In addition to the dimerization reaction, hydrogenolysis of ethylene to propane and methane was also observed, with the (3×3)R30°-Sn/Pt(111) surface alloy less active than Pt(111). Among the C₄ products, butenes andn-butane were the major components. Carbon buildup was observed to be significant above 500 K with the (3×3)R30°-Sn/Pt(111) surface alloy much more resistant than Pt(111). The dimerization of ethylene was not eliminated by the presence of surface carbonaceous deposits and even at significant surface coverages of carbon the model catalysts exhibited significant activities. The results are discussed in terms of the surface chemistry of ethylene and the previously reported catalytic reactions of acetylene trimerization andn-butane hydrogenolysis at these surfaces.     

n-butane hydrogenolysis on planar and faceted Pt/W(111) surfaces

The Relationship between surface structure and reactivity is investigated by means ofn-butane hydrogenolysis, a known structure sensitive reaction, for planar and faceted Pt/ W(111) surfaces. The W(111) surface reconstructs to form pyramidal facets with [211] orientation upon vapor deposition of Pt (>1.3 ML) and annealing above 750 K. The hydrogenolysis kinetics over the planar and the faceted surface are found to be quite different. The planar surface has a higher selectivity towards ethane formation and a higher reaction rate. The apparent activation energies are found to be 33 ± 4 kJ/mol for the planar surface and 76 ± 6 kJ/mol for a surface covered with 20 nm facets. There appears to be a correlation between the concentration of fourfold coordination (C₄) sites on the surface and the amount of ethane produced. The C₄ concentration is altered by changing the facet size (annealing temperature). The results indicate the presence of a different intermediate on the C₄ sites as evidenced by the differences in the apparent activation energy, the reaction rate and the overall selectivity.     

Molecular surface studies of adsorption and catalytic reaction on crystal (Pt,Rh) surfaces under high pressure conditions (atmospheres) using sum frequency generation (SFG) – surface vibrational spectroscopy and scanning tunneling microscopy (STM)

Sum frequency generation (SFG) – surface vibrational spectroscopy and the scanning tunneling microscope (STM) have been used to study adsorption and catalyzed surface reactions at high pressures and temperatures using (111) crystal surfaces of platinum and rhodium. The two techniques and the reaction chambers that were constructed to make these studies possible are described. STM and SFG studies of CO at high pressures reveal the high mobility of metal atoms, metal surface reconstruction, ordering in the adsorbed molecular layer, and new binding states for the molecule. CO oxidation occurs at high turnover rates on Pt(111). Different adsorbed species are observed above and below the ignition temperature. Some inhibit the reaction, and others are reaction intermediates since their surface concentration is proportional to the reaction rate. The dehydrogenation of cyclohexene on Pt(100) and Pt(111) proceeds through a 1,3‐cyclohexadiene surface intermediate. The higher dehydrogenation rate is related to the higher surface concentration of these molecules on the (100) crystal face. This revised version was published online in August 2006 with corrections to the Cover Date.     

The interaction of sulfur with Cu/Pt(111) and Zn/Pt(111) surfaces: copper-promoted sulfidation of platinum

The interaction of sulfur with Pt(111), Zn/Pt(111) and Cu/Pt(111) has been examined using X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and thermal desorption mass spectroscopy (TDS). At temperatures between 300 and 600 K, the exposure of Pt(111) to S₂ gas produces a chemisorbed layer of sulfurwithout the formation of bulk sulfides. Exposure of S₂ to a Zn/Pt(111) alloy, at room temperature, results in a breakdown of the alloy and formation of a zinc-sulfide film on Pt(111). Further S₂ exposure at 550 K sulfidizes the remaining metallic zincwithout affecting platinum. For the Cu/Pt(111) surface alloy, on the other hand, exposure to S₂2 at 550 K leads to sulfidation of the platinum. Platinum can effectively compete for sulfur atoms bonded to copper but not for those bonded to zinc. The reaction of S₂ gas with Cu/Pt(111) surfaces produces copper sulfides that promote the sulfidation of Pt by providing surface sites for the dissociation of S₂, and by favoring the diffusion of S into the bulk of the system.     

Observation of anomalous reactivities of Ni/Pt(111) bimetallic surfaces

We have investigated the surface reactivities of Ni/Pt(111) bimetallic model catalysts using ethylene and cyclohexene as probing molecules. The bimetallic surfaces were generated by evaporating Ni onto a Pt(111) single- crystal surface held at 600 K. The surface chemistry was investigated using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). The reactivities of the bimetallic surfaces were compared with those of the clean Pt(111) surface and a thick Ni(111) film on the Pt(111) substrate. Formation of the bimetallic surface led to a significantly reduced reactivity towards the decomposition of ethylene when compared to either Pt(111) or Ni(111)/Pt(111) surfaces. Furthermore, although the surface reactivity towards cyclohexene was retained for the bimetallic surface, the decomposition mechanism was distinctly altered from that of either Pt(111) or Ni(111)/Pt(111) surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dehydrogenation of Cyclohexene on Platinum Nanoclusters on a Thin Film of Al2O3/NiAl(100)

Using a scanning-tunneling microscope, reflection high-energy electron diffraction and photoelectron spectra with synchrotron radiation, we investigated the temperature dependence of the dehydrogenation of cyclohexene (C₆H₁₀) adsorbed on Pt nanoclusters supported on an ultra-thin film of Al₂O₃/NiAl(100). The Pt clusters, grown by vapor deposition, are structurally ordered and exhibit a mean diameter 2.2 nm and height 0.4 nm. The progress of dehydrogenation was monitored through the temporal variation of C 1s photoelectron spectra; analysis of these features revealed that the dehydrogenation of cyclohexene with increasing sample temperature occurs as a sequential process beginning around 150 K, a temperature significantly less than that for Pt single-crystal surfaces. The dehydrogenation behavior, particularly the decomposition into elemental carbon, is found to vary with Pt coverage.     

Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O_sub) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1×1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O_ads) state in an on-top or bridge position. It was also inferred that subsurface oxygen O_sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O_ads atoms. Experiments with ¹⁸O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO₂ at T 140–160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non–linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as 200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction–diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.     

Selective Modification of Pt Active Sites on Pt–Au/C Catalysts Prepared by Surface Redox Reactions

This study focused on the selective deposition of Au⁰ onto (111), (100) faces and (111)/(100) edges of cuboctahedral Pt particles present on the Pt/C(graphite) model system. The Pt–Au/C catalysts were prepared by novel surface redox methods involving the direct reduction (DR) of AuCl ₄ ^– species onto the Pt particles or reducing these species on the Pt–H interface, i.e., the refilling (RE) method. The presence of Au on the Pt particles was verified by means of high-resolution energy dispersive spectroscopy (EDS), and, after treatment at 300°C in H₂, the formation of crystalline Au⁰ aggregates was verified by X-ray wide-angle diffraction; further treatments at 500°C in H₂ led to a true Pt–Au solid solution. The Monte Carlo simulation methods indicated the selective deposition of Au⁰ onto the (111)/(100) edges of the Pt cuboctahedral particles when the relative Au concentration varied from 10 to 50 wt% Au. The catalytic conversion of n-heptane on the Pt–Au/C (DR and RE solids) catalysts presented an oscillatory behavior with respect to Pt/C, indicating modification of the active Pt ensembles, driven by the energy released during the exothermic n-C₇ dehydrogenation and cracking reactions, which should enhance the Au⁰ mobility at the Pt particle surface level.     

Cyclohexene dehydrogenation and hydrogenation on Pt(111) monitored by SFG surface vibrational spectroscopy: different reaction mechanisms at high pressures and in vacuum

The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(111) crystal surfaces were investigated by surface vibrational spectroscopy via sum frequency generation (SFG) both under vacuum and high pressure conditions with 10 Torr cyclohexene and various hydrogen pressures from 30 up to ~600 Torr. At high pressures, the gas composition and turnover rate (TOR) were measured by gas chromatography. In vacuum, cyclohexene on Pt(111) undergoes a change from π/σ‐bonded, σ‐bonded cyclohexene and c‐C₆H₉ surface species to adsorbed benzene when the surface was heated from 130 to 330 K. A site‐blocking effect was observed at saturation coverage of cyclohexene that caused dehydrogenation to shift to somewhat higher surface temperature. At high pressures, however, none of the species observed in vacuum conditions were detectable. 1,4‐cyclohexadiene (1,4‐CHD) was found to be the major species on the surface at 295 K, even with the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable reaction at the temperature range between 300 and 400 K with 1,3‐cyclohexadiene (1,3‐CHD) on the surface, as revealed by SFG. Further increasing the surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation reaction rate and both 1,3‐CHD and 1,4‐CHD were present on the surface simultaneously. The simultaneous observation of the reaction kinetic data and the chemical nature of surface species allows us to postulate a reaction mechanism at high pressures: cyclohexene hydrogenates to cyclohexane via a 1,3‐CHD intermediate and dehydrogenates to benzene through both 1,4‐CHD and 1,3‐CHD intermediates. Isomerisation of the 1,4‐CHD and 1,3‐CHD surface species is negligible. This revised version was published online in July 2006 with corrections to the Cover Date.     

Metal effects in the Fischer-Tropsch synthesis: Bond-order-conservation-morse-potential approach

We have applied the BOC-MP method to theoretically analyze the metal effects in the Fischer-Tropsch (FT) synthesis by calculating the energetics of conceivable elementary steps (the relevant heats of chemisorption and the reaction activation barriers) during CO hydrogenation over the periodic series Fe(110), Ni(111), Pt(111), Cu(111). The basic steps such as dissociation of CO, hydrogenation of carbidic carbon, C-C chain growth by insertion of CH₂ versus CO into the metal-alkyl bonds, and chain termination leading to hydrocarbons (alkanes versus -olefins) or oxygenates are discussed in detail. It is shown that the periodic trends in the ability of metal surfaces to dissociate chemical bonds and those to recombine the bonds are always opposite. In particular, we argue that metallic Fe is necessary to produce the abundance of carbidic carbon from CO but the synthesis of hydrocarbons and oxygenates can effectively proceed only on carbided Fe surfaces which resemble the less active metals such as Pt. More specifically, we project that the C-C chain growth should occur predominantly via CH₂ insertion into the metal-alkyl bond and the primary FT products should be -olefins. These and other model projections are in agreement with experiment.     

Prospects for detecting metal–adsorbate vibrations by sum‐frequency spectroscopy

Sum‐frequency spectroscopy (SFS) was used in an attempt to detect the platinum–carbon vibration of CO adsorbed on Pt(111). The international free‐electron laser FELIX at the FOM Institute, Rijnhuizen, provided the required tunable far‐infrared (19–23 μm) source, while complementary measurements in the C–O stretch region (4.7–5.1 μm) were performed at the University of Oxford with a conventional nanosecond laser system. Ordered Pt(111) surfaces were prepared by the H₂/O₂ flame annealing approach and CO monolayers were produced by exposure of the Pt crystal to gaseous CO in a flow reactor. The monolayers were characterized by sum‐frequency (SF) measurements of the v _C-O vibrational frequency. The CO adsorbed primarily in the terminal (atop) configuration, with a v _C-O frequency of around 2078 cm⁻¹. In the far‐IR region, the non‐resonant background from the Pt substrate could readily be detected by SFS, but there was no evidence for the v _Pt-CO mode. Direct laser‐induced desorption and thermal desorption of CO are unlikely under the experimental conditions. It is therefore probable that the intrinsic cross‐section of the Pt–CO mode is too low for easy detection by SFS. The implications for the use of SFS to detect metal–adsorbate vibrational modes are discussed in light of these findings. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlating Low-temperature Hydrogenation Activity of Co/Pt(111) Bimetallic Surfaces to Supported Co/Pt/γ-Al2O3 Catalysts

The low-temperature self-hydrogenation (disproportionation) of cyclohexene was used as a probe reaction to correlate the reactivity of Co/Pt(111) bimetallic surfaces with supported Co/Pt/γ-Al₂O₃ catalysts. Temperature-programmed desorption (TPD) experiments show that cyclohexene undergoes self-hydrogenation on the ~1 ML Co/Pt(111) surface at ~219 K, which does not occur on either pure Pt(111) or a thick Co film on Pt(111). Supported catalysts with a 1:1 atomic ratio of Co:Pt were synthesized on a high surface area γ-Al₂O₃ to verify the bimetallic effect on the self-hydrogenation of cyclohexene. EXAFS experiments confirmed the presence of Co–Pt bonds in the catalyst. Using FTIR in a batch reactor configuration, the bimetallic catalyst showed a higher activity toward the self-hydrogenation of cyclohexene at room temperature than either Pt/γ-Al₂O₃ or Co/γ-Al₂O₃ catalysts. The comparison of Co/Pt(111) and Co/Pt/γ-Al₂O₃ provided an excellent example of correlating the self-hydrogenation activity of cyclohexene on bimetallic model surfaces and supported catalysts.     

Decomposition of NO on Tungsten Carbide and Molybdenum Carbide Surfaces

The decomposition of ¹⁵NO on C/W(111), C/W(110), and on monolayer and bulk C/Mo/W(111) surfaces is compared based on temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) measurements. Our results indicate that the decomposition of ¹⁵NO occurs readily over all surfaces, and the only ¹⁵N-containing reaction products are ¹⁵N₂ and ¹⁵N₂O under our experimental conditions. Much higher surface reactivity for ¹⁵NO decomposition was observed over the more open-structured C/W(111) surface, with a value of 0.68 ¹⁵NO/W, in contrast to the surface reactivity of 0.24 ¹⁵NO/W over the close-packed C/W(110) surface. The selectivity of these two ¹⁵N-containing reaction products depends on the structure of the substrates as well. The more open-structured C/W(111) surface favors the production of ¹⁵N₂, with a product selectivity of ¹⁵N₂ being approximately 87%. In contrast, the selectivity to ¹⁵N₂ is only about 52% on C/W(110). In addition, we have investigated the decomposition of ¹⁵NO on C/Mo surfaces that were epitaxially grown on W(111). The selectivity of ¹⁵N₂ on C/Mo/W(111) surfaces is 88%, which is very similar to that observed on C/W(111). Finally, the general similarity between the DeNOx chemistry on carbides and on Pt-group metals will also be discussed.     

Compensation Effect of Benzene Hydrogenation on Pt(111) and Pt(100) Analyzed by the Selective Energy Transfer Model

Kinetic measurements at low temperatures (310–360 K) using gas chromatography (GC) for benzene hydrogenation on Pt(100) and Pt(111) single crystal surfaces have been carried out at Torr pressures. These kinetic measurements demonstrated a linear compensation effect for the production of cyclohexane. A detailed application of the model of selective energy transfer to the experimentally obtained results yields the vibrational frequency of the adsorbate leading to reaction. This frequency is attributed to ring distortion modes. The vibrational frequency of the heat bath, or catalyst, is ascribed to a Pt-H mode. An approximate heat of adsorption of the reacting molecule is also calculated from the model.     

Chemical dynamics at surfaces

The current status of molecular dynamics simulations of chemical processes at surfaces is assessed. Limitations of the method are discussed, and recent progress towards overcoming the limitations is described. The reliability and depth of understanding achievable through interplay between simulation and experiment is illustrated by a simple example, the trapping of Ar on a Pt(111) crystal surface. The prognosis for extending these techniques to chemically reacting systems in real environments is addressed.     

Structure sensitivity of oxide surfaces: the adsorption and reaction of carbon monoxide and formic acid on NiO(100) and NiO(111)

The adsorption and reaction of CO and HCOOH on the NiO(100)/Mo(100) and NiO(111)/Mo(110) surfaces have been studied using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Significant differences have been found for the two faces of NiO regarding the adsorption and reaction of HCOOH. While molecularly adsorbed formic acid is stable up to 200 K on the NiO(100) surface, formic acid decomposition to formate occurs on the NiO(111) surface at 100 K. Upon heating to 700 K, most of the formate on the NiO(111) surface dehydrogenates or dehydrates, while 70% of the formate species on the NiO(100) surface desorbs as molecular formic acid. With respect to CO adsorption, the NiO(111) surface shows a slightly higher binding energy than does the NiO(100) surface.     

Reaction kinetics and in situ sum frequency generation surface vibrational spectroscopy studies of cycloalkene hydrogenation/dehydrogenation on Pt(111): Substituent effects and CO poisoning

The influence of substituent effects and CO poisoning were examined during the hydrogenation/dehydrogenation of cycloalkenes (cyclohexene and 1- and 4-methylcyclohexene) on a Pt(111) single crystal. Reaction rates for both hydrogenation and dehydrogenation decreased when a methyl group was added to the cycloalkene ring. The location of a methyl group relative to the CC double bond was influential in the overall kinetics for both reaction pathways. All cycloalkenes demonstrated “bend-over” Arrhenius behavior, after which rates for hydrogenation and dehydrogenation decreased with increasing temperature (inverse Arrhenius behavior). This is explained in terms of a change in surface coverage of the reactive cycloalkene. The potential importance of hydrogen effects is discussed. Introduction of CO in the Torr pressure range (0.015 Torr) led to a decrease in turnover frequency and increase in apparent activation energy for both the hydrogenation and dehydrogenation of all cycloalkenes. Sum frequency generation (SFG) surface vibrational spectroscopy revealed that upon adsorption, the three cycloalkenes form a surface species with similar molecular structure. SFG results under reaction conditions in the presence of CO demonstrated that the cycloalkene coverage is low on a CO-saturated surface. Substituted cyclohexenes were more sensitive than cyclohexene to the presence of adsorbed CO, with larger increases in the apparent activation energy, especially in the case of dehydrogenation. A qualitative explanation for the changes in activity with temperature and the increase in apparent activation energy for cycloalkene hydrogenation/dehydrogenation in the presence of CO is presented from a thermodynamic and kinetic perspective.     

Aniline hydrogenolysis on nickel: effects of surface hydrogen and surface structure

Fluorescence yield near-edge spectroscopy (FYNES) above the carbon K edge and temperature programmed reaction spectroscopy (TPRS) have been used as the methods for characterizing the reactivity and structure of adsorbed aniline and aniline derived species on the Ni(100) and Ni(111) surfaces over an extended range of temperatures and hydrogen pressures. The Ni(100) surface shows appreciably higher hydrogenolysis activity towards adsorbed aniline than the Ni(111) surface. Hydrogenolysis of aniline on the Ni(100) surface results in benzene formation at 470 K, both in reactive hydrogen atmospheres and in vacuum. External hydrogen significantly enhances the hydrogenolysis activity for aniline on the Ni(100) surface. Based on spectroscopic evidence, we believe that the dominant aniline hydrogenolysis reaction is preceded by partial hydrogenation of the aromatic ring of aniline in the presence of 0.001 Torr of external hydrogen on the (100) surface. In contrast, very little adsorbed aniline undergoes hydrogen induced C-N bond activation on the Ni(111) surface for hydrogen pressures as high as 10^–7 Torr below 500 K. Thermal dehydrogenation of aniline dominates with increasing temperature on the Ni(111) surface, resulting in the formation of a previously observed polymeric layer which is stable up to 820 K. Aniline is adsorbed at a smaller angle relative to the Ni(111) surface than the Ni(100) surface at temperatures below the hydrogenolysis temperature. We believe that the proximity and strong -interaction between the aromatic ring of the aniline and the surface is one major factor which controls the competition between dehydrogenation and hydrogen addition. In this case the result is a substantial enhancement of aniline dehydrogenation relative to hydrogenation on the Ni(111) surface.     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.     

Poisoning of Pt/C catalysts by CO and its consequences over the kinetics of hydrogen chemisorption

The initial rate of hydrogen dissociation was studied as a function of irreversible CO coverage at 353 K on 30 wt.% Pt/carbon catalysts (Pt/C) prepared according to different processes. The Pt/C catalysts exhibit similar Pt dispersion (D  0.07) and mean Pt particles size (dp  16 nm). The turnover frequency (number of hydrogen molecules dissociated per CO-free surface Pt atom) was determined as a function of CO coverage from 0.0 to 0.8. The evolution of TOF as a function of CO coverage is in agreement with the model of CO adsorbing on low coordination sites (edges, corners) and then spreading across the faces to grow islands as Brandt suggested in the past (R.K. Brandt, M.R. Hughes, L.P. Bourget, K. Truszkowska, R.G. Greenler, Surf. Sci. 286 (1993) 15–25). At high CO coverage (0.8), TOF depends on the process by which the Pt/C catalyst was prepared. In particular, a Pt/C elaborated according to a colloidal process exhibits a low sensitivity to CO poisoning with an increase of TOF by one order of magnitude.