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.     

Synergistic Effect in the Dehydrogenation of Cyclohexene on C/W(111) Surfaces Modified by Submonolayer Coverage Pt

The dehydrogenation and decomposition of cyclohexene on the Pt-modified C/W(111) surfaces have been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). The objective of the current study is to investigate how the surface reactivity of tungsten carbide is modified by the presence of submonolayer Pt. Similar to that observed on Pt(111), Pt(100) and C/W(111) surfaces, the characteristic reaction pathway on Pt/C/W(111) is the selective dehydrogenation of cyclohexene to benzene. At a Pt coverage of 0.52 monolayer, the selectivity to the gas-phase benzene product is 86±7%, which is slightly higher than that on Pt(111) (75%) and on C/W(111) (67±7%). More importantly, the desorption of benzene on Pt/C/W(111) is a reaction-limited process that occurs at 290 K, which is much lower than the benzene desorption temperature of 400 K from Pt(111).     

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.     

Molecular ethane adsorption dynamics on oxygen-covered Pt(111)

The dynamics of ethane trapping on Pt(111)-p(2×2)-O were investigated by supersonic molecular beam techniques at a surface temperature of 100 K. The initial trapping probability was measured in the range of incident energy from 10 to 45 kJ/mol and incident angles from 0^° to 60^°. A broad angular distribution of scattered ethane and total energy scaling (E_T cos^0.2) for ethane trapping indicated a corrugated gas–surface potential. Stochastic trajectory simulations employing a potential developed from the trapping of ethane on Pt(111) gives quantitative agreement of the measured initial trapping probabilities over entire ranges of incident energies and angles. Calculations of energy transfer for ethane after the first bounce on Pt(111) and Pt(111)-p(2×2)-O clearly indicate that interconversion of parallel and perpendicular momentum and energy transfer to lattice vibrations account primarily for the differences in trapping probabilities between ethane on the two surfaces. At glancing incidence trapping is not significantly reduced on the oxygen-covered Pt(111) because the parallel momentum appears to be transferred partially to phonons.     

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.     

Surface characterization of molecules at pt(111) using leed, auger, hreels and electrochemistry in ultrahigh vacuum

Reviewed in this article is surface characterization of organic molecules adsorbed at well-defined Pt(111) electrode surfaces from aqueous solution. Among the metal electrodes and surface-structure sensitive properties studied in this work are those involving electrocatalytic activity, such as platinum metal. Platinum is one of the most interesting materials for study in view of its exceptional spectrum of catalytic properties and its good stability in various electrolytic media. The compounds investigated were: (1) 4-Phenylpyridine (4PPY), (II) 4,4′-Bipyridyl (44BPY), (III) 2-Phenylpyridine (2PPY), (IV) 2-2′ Bibyridyl (22BPY), (V) Pyridazine (PD), and (VI) 4-Pyridazinecarboxylic acid (4PDCA). For (I) and (II), the pyridine ring binds to the Pt(111) surface in a tilted, nearly vertical orientation having a pendant phenyl ring, and is virtually unreactive toward electrochemical oxidation. However, (III) and (IV) attach to Pt(111) with their pyridine ring vertically oriented and N-attached, while the other aromatic ring is oriented parallel to the electrode surface. Similarly, (V) and (VI) are attached to Pt(111) through their ring nitrogen atom, with a tilted-vertical orientation and average ring-to-surface angles ranging from 73° to 86°. Compounds (I-VI) were studied in order to understand the influence of molecular structure on surface bonding, molecular orientation, electronic structure, and electrochemical reactivity at well-defined surfaces such as Pt(111), as a function of pH, electrode potential and adsorbate concentration.     

Site Selective Detection of Methane Dissociation on Stepped Pt Surfaces

Topics in Catalysis - We report a combined experimental and theoretical study comparing methane dissociation on three different platinum surfaces Pt(111), Pt(211), and...     

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.     

静电雾化沉积制备PbTiO3薄膜的研究

采用静电雾化沉积技术制备了PbTiO3薄膜,探索了沉积温度和沉积时间对所制备薄膜的结构和形貌的影响.通过调节制备工艺,制备了多孔和致密的PbTiO3薄膜.测试了所制备钛酸铅薄膜的介电频率特性,在100kHz的介电常数和介电损耗分别为222和0.0247.     

Pt-Fe催化剂次表层Fe结构表面的CO氧化反应性能

利用紫外光电子能谱等表面科学方法,研究了Pt-Fe模型催化剂的次表层Fe结构[即Pt/Fe/Pt(111)结构]在不同条件下CO的吸附及其氧化反应。结果表明,Pt/Fe/Pt(111)结构在H_2气氛或者超高真空中是种稳定结构,最外层的原子与Pt(111)相同,是密排的铂原子面;但次表层的原子中有约0.5单层的铁原子,使费米边附近(0~2.0)e V的电子态密度明显低于Pt(111)表面,从而改变表面的CO和O_2吸附以及反应性能。程序升温的紫外光电子能谱结果显示,Pt/Fe/Pt(111)表面在(100~300)K,CO的吸附受温度的影响不明显,且O_2能够吸附、活化并使共吸附的CO发生氧化反应;当温度为300 K时,O_2无法在Pt/Fe/Pt(111)表面吸附、活化,所以CO氧化反应无法进行。Pt/Fe/Pt(111)结构虽然能有效地减弱CO的吸附从而避免CO毒化的问题,但O_2的吸附和活化也受到显著抑制并影响到一定条件下CO的氧化反应。     

Surface relaxation at the metal/electrolyte interface

X-ray diffraction is an ideal technique for the in situ study of single crystal metal surfaces in an electrochemical environment. In this paper, measurements of the low-index surfaces of Au and Pt are described, in particular with reference to surface expansion effects. Surface expansion can be probed potentiodynamically to correlate expansion with the adsorption/desorption of solution species. In general, the results are in good agreement with recent theoretical calculations. The X-ray technique can also give insight into electrocatalytic reactions as shown by the results for the adsorption and oxidation of CO on Pt(111).     

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.     

On the correlation between crystallinity of platinum bottom electrode and that of MOD derived PZT thin films

Growth of well(111)-oriented Pt and Pt/Ti films on SiO₂/Si(111) substrates and crystallinity of PZT films grown on the Pt(111)/SiO₂/Si(111) and Pt(111)/Ti/SiO₂Si(111) substrates have been investigated. It was found by X-ray diffraction analysis that well (111)-oriented Pt film with a best full-width at half maximum (FWHM) of 0.28° was grown by the DC sputtering method. PZT films were prepared by metallo-organic decomposition (MOD) on Pt(111)-coated SiO₂Si(111) substrates. The crystallinity of the PZT films improved as the FWHM of the Pt(111) diffraction peak decreased. The best FWHM obtained for a PZT film grown on a Pt(111)/Ti/SiO₂/Si(111) substrate was 0.33°.     

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.     

Oxygen Reduction on Structurally Well Defined,Bimetallic PtRu Surfaces: Monolayer PtxRu1−x/Ru(0001) Surface Alloys Versus Pt Film Covered Ru(0001)

The electrocatalytic activity of different, structurally well defined bimetallic PtRu surfaces in the oxygen reduction reaction was investigated by a combination of scanning tunnelling microscopy and electrochemical measurements performed under controlled mass transport conditions in a flow cell. We compare the effect of pseudomorphic Pt cover layers, mimicking the situation in a core–shell Pt/Ru nanoparticle, and of mixed Pt_xRu_1?x monolayer surface alloys, reflecting the situation in an alloyed nanoparticle. The results unambiguously demonstrate that these bimetallic surfaces can reach activities well in excess of that of Pt(111), both for the film surfaces and the surface alloys, by optimizing the Pt surface content (surface alloys) or the Pt film thickness (film surfaces). The results are compared with simulated kinetic current–potential profiles based on existent density functional theory calculations (Greeley and Nørskov, J Phys Chem C 113:4932, 2009; Lischka et al., Electrochim Acta 52:2219, 2007) revealing very good agreement in trends. Potential and limits of this approach are discussed.     

Graphene growth on Pt(111) and Au(111) using a MBE carbon solid-source

In this work we present a Molecular Beam Epitaxy (MBE) growth method to obtain graphene on noble metals using evaporation of carbon atoms from a carbon solid-source in ultra-high vacuum conditions. We have synthesized graphene (G) on different metal surfaces: from a well studied substrate as platinum, to a substrate where it can only be formed using innovative methods, as is the case of gold. For the characterization of the graphene layers we have used in situ surface science techniques as low energy electron diffraction (LEED), auger electron spectroscopy (AES) and scanning tunneling microscopy (STM).One of the main advantages of our methodology is that low surface temperatures are required to form graphene. Thus, by annealing Pt(111) and Au(111) substrates up to 650 °C and 550 °C respectively during carbon evaporation, we have obtained the characteristic LEED diagrams commonly attributed to graphene on these surfaces. STM results further prove the formation of graphene. For the case of G on Pt(111), STM images show a long range ordering associated with moiré patterns that correspond to a monolayer of graphene on (111) platinum surface. On the other hand, G/Au(111) STM results reveal the formation of dendritic islands pinned to atomic step edges. This method opens up new possibilities for the formation of graphene on many different substrates with potential technological applications.     

Estimating vibrational and thermodynamic properties of adsorbates with uncertainty using data driven surrogates

In this work, we propose a strategy to develop data driven local surrogate models of ab initio potential energy functions describing the interaction of adsorbates on heterogeneous catalytic materials. We show that these multivariable surrogate models, based on orthogonal polynomial expansion and trained on sampled ab-initio energies/forces, can be used to compute harmonic vibrational frequencies and the entropy of adsorbates. Further, we show that the errors in our surrogate model can be estimated and propagated to calculate the uncertainty in the computed properties. We show proof-of-concept illustrations of our method to calculate the vibrational frequencies of ethene on 1D edges of molybdenum sulfide (MoS₂), (b) 2D surfaces of Pt(111), and (c) 3D micropores of a HZSM-5 zeolite; the entropy of ethane adsorbed on Pt(111); and the associated uncertainties in all the cases.     

Deactivation of hydrodechlorination catalysts: I. Experiments with 1,1,1-trichloroethane

In order to understand the cause(s) of catalyst deactivation, microreactor studies were carried out on the catalytic hydrodechlorination (HDC) of 1,1,1-trichloroethane (111-TCA) using a fixed-bed reactor divided into three segments. The catalysts studied were: ηδ-alumina, α-alumina, Pt/α-alumina, Pt/η-alumina, and Pt/ηδ-alumina. Experiments were carried out at atmospheric pressure over a temperature range of 423–623 K and at H₂/111-TCA molar ratios between 10 and 99. Only the Pt-containing catalysts were able to remove all three Cl atoms from 111-TCA. However, increasing amounts of partially-dechlorinated compounds were formed as the Pt/ηδ-alumina and the Pt/η-alumina catalysts deactivated. The only product with ηδ-alumina was 1,1-dichloroethylene (11-DCE). The conversion of 111-TCA decreased more rapidly with time for Pt/ηδ-alumina than for ηδ-alumina without Pt. Much larger quantities of coke were formed on the Pt/ηδ-alumina than on the ηδ-alumina, at similar conditions. The ηδ-alumina was essentially completely regenerated by heating in flowing He at 773 K. The Pt/η-alumina was only partially regenerated by this technique. The apparent stability of the Pt/ηδ-alumina catalysts increased with increasing Pt concentration. Poisoning by hydrochloric acid, a reaction product, did not cause significant deactivation of either the ηδ-alumina or the Pt/ηδ-alumina catalysts.     

Structural effects in electrocatalysis: electrooxidation of carbon monoxide on Pt3Sn single-crystal alloy surfaces

The kinetics of the electrochemical oxidation of carbon monoxide (CO) and CO/hydrogen mixtures (0.1 and 2% CO) in sulfuric acid electrolyte at 25–62°C was studied on different surfaces of the ordered single crystal Pt₃Sn alloy. Characterization of the surface composition and structure was determined in UHV using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low energy ion scattering (LEIS) prior to determining the electrode kinetics using the classical rotating disk method (RDE) with CO dissolved in the electrolyte. Clean annealed and sputtered-cleaned but not-annealed surfaces of (110) and (111) orientation were studied. A remarkable difference in activity was observed between the annealed (111) surface and the sputtered but not-annealed (110) surface, with both surfaces having the same nominal surface composition, 20–25 at% Sn, but different local structures. The onset potential for CO oxidation on the (111) surface was shifted cathodically by 0.13 V relative to that for the sputtered (110) surface, and the onset comes remarkably close to 0 V on the reversible hydrogen potential scale. Relative to pure Pt surfaces (of any crystal structure), the potential shift is more than 0.5 V, corresponding to a catalytic activity that is higher by more than four orders of magnitude. Comparable shifts were observed for the oxidation of CO/H₂ mixtures. Both the structure sensitivity and the high catalytic activity of the Pt₃Sn surface are attributed to an adsorbed state of CO unique to this alloy and occurs at relatively high coverage on the (111) surface.     

Why Au and Cu Are More Selective Than Pt for Preferential Oxidation of CO at Low Temperature

Self-consistent, periodic density functional theory (DFT) calculations and micro-kinetic modeling are used to compare selectivity for the preferential oxidation of CO (PROX) with respect to H₂ based on studies of elementary reaction steps on the (111) facet of Au, Cu and Pt. The first step of H oxidation (OH formation) has a higher activation barrier than the second step (H₂O formation) on all three metal surfaces, indicating that OH formation competes with CO oxidation for the removal of trace amounts of CO from a typical reformate gas. The activation energy barrier for CO oxidation is found to be 0.18eV on Au(111), 0.82eV on Cu(111) and 0.96eV on Pt(111), whereas the barrier for OH formation is 0.90, 1.28 and 0.83eV respectively. A micro-kinetic model based on the DFT results shows that trends in the selectivity of these metals at different temperatures is due to (i) differences in the rate constants of the competitive CO and H oxidation reactions, and (ii) differences in the CO and H surface coverages. Our results explain why Au and Cu are more selective PROX catalysts compared to Pt at low temperatures. At higher temperatures, Pt and Cu lose some of their selectivity to CO oxidation, whereas the selectivity on Au decreases substantially primarily because of the significantly weaker CO adsorption.