In situ observation of methoxide hydrogenation on the Ni(111) surface

The surface chemistry of methoxide (CH₃O-) on the Ni(111) surface has been studied in the presence of hydrogen pressures up to 2 Torr. During heating in vacuum methoxide decomposes to H₂ and CO, which desorb at 380 and 445 K, respectively. The CH₃O-decomposition process is rate limited by CH bond breaking and exhibits a strong deuterium kinetic isotope effect in CD₃O-. In the presence of ambient hydrogen pressures of 0.02–2.0 Torr both CH₃O- and CD₃O-are hydrogenated directly to methanol at 310 K. Methoxide is hydrogenated by adsorbed hydrogen, which nearly saturates the surface at these pressures and temperatures.     

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.     

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.     

The kinetics of ethylene hydrogenation catalyzed by metallic palladium

The activity of Pd(111) for ethylene hydrogenation is measured using a high-pressure reactor incorporated into an ultrahigh vacuum chamber for temperatures between 300 and 475 K, ethylene pressures between 50 and 300 Torr and hydrogen pressures from 45 to 600 Torr. The reaction rate is found to be rapid with turnover frequencies up to 400 reactions/site/s (where rates are referenced to the atom site density on the (111) face of palladium). The measured activation energy is 35 kJ/mol. A hydrogen reaction order of 1.02 was found at a reaction temperature of 300 K and an ethylene pressure of 100 Torr, where the hydrogen reaction order was found to depend on temperature. A negative reaction order of –0.22 was found in ethylene pressure at a reaction temperature of 320 K and a hydrogen pressure of 100 Torr. The reaction rates are in good agreement with values obtained on silica-supported palladium and with other work on palladium single crystals.     

States of surface hydrogen under reaction conditions of the Fischer-Tropsch synthesis

The reaction of hydrogen with Fe surfaces was observed by the ellipsometric method at 77 K to 500 ° C under H₂ pressures of 10^–3 Torr (0.1 Pa) to 500 Torr (7×10⁴ Pa). The ellipsometric analysis reveals no existence of adsorbed hydrogen on the surface above 400 °C; hydrogen seems to be alternatively absorbed into the subsurface region of two to three atomic layers. It is concluded that the subsurface hydrogen is specific to the high temperature and the high pressure owing to strained and roughened Fe surfaces as well as the equilibrium with gas phase hydrogen. Absence of adsorbed hydrogen is indicated as well in the ellipsometric response to the hydrogenation reaction of surface carbon species on Fe surfaces.     

Hydrogenation of 1,3-butadiene on platinum surfaces of different structures

1,3-butadiene hydrogenation is studied on platinum foil and Pt(111), Pt(100), Pt(755) single-crystal surfaces at 300--375 K. The results are compared with the data of alkene hydrogenation reactions. Similar to the hydrogenation kinetics of butenes, 1,3-butadiene hydrogenation exhibits near zeroth-order dependence on hydrocarbon and near first-order dependence on hydrogen pressure. With the same hydrocarbon (3.5-70 Torr) and hydrogen (14-140 Torr) pressure, the rate of 1,3-butadiene hydrogenation is one order of magnitude lower than for the rates for n-butenes. The hydrogenation products include 1-butene, trans- and cis-2-butene, and n-butane. The reaction selectivity is independent of reactant mixture and platinum surface structure, but changes slightly as a function of reaction temperature. The butene product distribution is determined by surface reaction kinetics. While 1-butene is thermodynamically less stable than trans- and cis-2-butene, it is the major butene product in 1,3-butadiene hydrogenation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Varied Thermal Chemistry of Hydrocarbons on Nickel Single-Crystal Surfaces

Temperature programmed desorption (TPD) results are reported for the conversion of styrene, bromocyclopentane, 1-pentene, 1,5-pentadiene, and 1,5-diiodopentane on Ni(100), and of iodoethane and 1- and 2-iodopropane on Ni(110) single-crystal surfaces. The purpose of these experiments was to illustrate the versatility of nickel in promoting a variety of surface steps for the conversion of adsorbed hydrocarbons. Our examples include the selective hydrogenolysis of styrene to toluene, the migration of carbon?Ccarbon double bonds in cyclopentene and 1-pentene, the ring closure of C₅ metallacyclic surface intermediates, the coupling of alkyl groups, and the growth of hydrocarbon chains starting from ethyl and propyl surface intermediates. Additional information is reported on the relative rates of the hydrogenation and dehydrogenation surface steps responsible for multiple H?CD exchange steps.     

Promotion of the electrochemical hydrogenation of nitrobenzene at hydrogen storage alloys studied using a solid electrolyte method

The electrocatalytic properties of an AB₅-type hydrogen storage alloy towards the electrochemical hydrogenation of unsaturated organic compounds have been studied by a solid electrolyte method using electrochemical hydrogenation of nitrobenzene as a model reaction. Voltammetric studies reveal that the kinetics of the nitrobenzene electro-reduction on the hydrogen storage alloy electrode is similar to that on a Ni electrode. Aniline and p-aminophenol are produced as the reaction products. Compared to the Ni electrode, the production of aniline is considerably promoted on the hydrogen storage alloy electrode. Modifying the alloy surface with a thin layer of Cu enhances the reaction selectivity and current efficiency for aniline formation. Compared to a Cu electrode, the electrochemical hydrogenation of nitrobenzene to aniline is promoted on the Cu-modified alloy electrode. The hydrogenation promotion effect is attributed to the chemical reaction between nitrobenzene and metal hydrides that are electrochemically generated in situ. Hydrogen storage alloys therefore make it possible to intensify the electrochemical hydrogenation process of unsaturated organic compounds.     

Poisoning and Regeneration of an Ni/SiO2 Catalyst

The poisoning effect of thiophene during the hydrogenation of styrene on an Ni/SiO₂ catalyst, and the regenerating role of both pure hydrogen and 2-butyne in the presence of hydrogen on the poisoned catalyst, were studied. The treatments induced the elimination of sulfur and promoted an important recovery of the catalytic activity and selectivity. XPS analyses show that the sulfur species adsorbed during the catalyst poisoning is thiophene. Part of the sulfur remained irreversibly adsorbed after the regeneration treatments carried out at 473 K; a modification on the adsorbed sulfur electronic state was detected, which can be ascribed to thiophene hydrogenolysis induced by the regenerating temperature. © 1997 SCI.     

On the effect of hydrogen on the palladium-catalyzed formation of benzene from acetylene

An infrared spectrum of a Pd(111) surface collected in the presence of 5 Torr of acetylene as a function of hydrogen pressure reveals that the ethylidyne coverage increases with hydrogen pressure (P(H₂) between zero and 20 Torr). The amount of CO that can be accommodated onto the surface at a pressure of 5 Torr, measured after evacuating the acetylene and hydrogen, increases linearly with hydrogen pressure, and this effect is ascribed to the presence of a more open surface produced by the formation of ethylidyne. It is found that acetylene adsorbs in ultrahigh vacuum on ethylidyne-covered Pd(111) and reacts to form benzene, where the benzene desorbs at 280 K. This effect is mirrored in the catalytic chemistry where the rate of benzene formation from acetylene in the presence of hydrogen increases linearly with hydrogen pressure.     

裂解汽油一段加氢用Ni/Al_2O_3催化剂CS_2中毒研究

采用固定床反应器研究了Ni/Al2O3上CS2对裂解汽油原料油中主要化合物芳烃、单烯烃和共轭烯烃加氢活性的影响,其对加氢抑制的顺序为:芳烃单烯烃共轭烯烃。XRD、XPS和IR表征分析表明,Ni/Al2O3催化剂失活的可能原因是CS2吸附在活性相表面,部分CS2碳硫键断裂发生氢解反应产生H2S和CH4,H2S与镍活性中心作用形成镍硫化合物。原料油中部分CS2吸附在催化剂表面,催化剂对共轭烯烃加氢也失去活性。     

Stoichiometric hydrogenation of ethene on Rh(111); mechanism,importance of weakly adsorbed ethene,and relationship to homogeneous catalysis

The hydrogenation of ethene is an important reaction in heterogeneous catalysis and, despite its apparent simplicity, many aspects of the reaction mechanism remain unclear. By contrast, the mechanism using homogeneous catalysts such as Wilkinson's catalyst [(RhCl(PPh₃)₃] is thought to be well understood. To allow a comparison between the homogeneous and heterogeneous reactions we have studied ethene/hydrogen interactions on the (111) plane of rhodium in the temperature range 160–500 K. Under UHV conditions no catalytic reaction was detected. However, we have been able to observe stoichiometric hydrogenation and exchange in the chemisorbed layer. A mixed adlayer of either ethene/deuterium (or perdeuteroethene and hydrogen) was formed at ca. 160 K, and allowed to warm up. From previous spectroscopic studies, ethene is adsorbed at 165 K as partially rehybridised, bonded species with a C-C bond order of ca. 1.5, similar to ethene in Zeise's salt. At 190–210 K we observe coincident desorption of undeuterated ethene — the major species — together with much smaller quantities of deuterated ethane and partially deuterated ethenes. The influence of both hydrogen and ethene pre-coverage has been studied as has the relative extent of hydrogenation and exchange. The ethane formation results parallel those reported by other authors on Pd(110) and Pt(111) and Pt(110). We propose that on all three metals both hydrogenation and exchange follow the same pathway, with a common intermediate for exchange and hydrogenation. This isa weakly held, bonded species formed during the desorption process, which can be convertedreversibly into an adsorbed ethyl species. A detailed comparison indicates that the mechanism of heterogeneous hydrogenation closely parallels that in the homogeneous phase.     

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.     

Infrared spectroscopy studies of cyclohexene hydrogenation and dehydrogenation catalyzed by platinum nanoparticles supported on mesoporous silicate (SBA-15). Part 1: The role of particle size of Pt nanocrystals supported on SBA-15 silicate

Platinum nanoparticles were prepared from colloidal solution in the 5–16 nm range. SBA-15 mesoporous silica was impregnated with particles small enough to enter the 10 nm pores until 0.1 wt% loading was reached. Characterization of the catalysts was carried out by XRD, TEM and BET. Cyclohexene hydrogenation/dehydrogenation was monitored using a reaction cell that permitted infrared spectroscopy monitoring of the gas phase as well as the catalyst surface that was pressed in a wafer and inserted in the reactor. The surface hydroxyl groups on the mesoporous silica show shifts in the 3633–3705 cm^–1 range characteristic of the presence of cyclohexene, 1,3- and 1,4-cyclohexadienes. Reaction studies using 10 Torr of cyclohexene and 100 Torr of hydrogen in the 298–473 K range showed that hydrogenation occurs readily at room temperature while dehydrogenation takes place only at higher temperatures as expected. The small platinum nanoparticles carry out reactions at the highest rates while the largest size metal particles of the lowest. There is no apparent change of metal particle size during the reactions.     

Effect of Carbon Deposits on Reactivity of Supported Pd Model Catalysts

Alumina-supported Pd model catalysts were prepared by Pd evaporation onto a thin alumina film grown on a NiAl(110) substrate. Adsorption and co-adsorption of ethene, CO and hydrogen on Pd/Al₂O₃/NiAl(110) covered by carbon species, formed by ethene dehydrogenation at 550 K, was studied by temperature programmed desorption (TPD). TPD results show that carbon deposits do not prevent adsorption but inhibit dehydrogenation of di- bonded ethene. Carbon species suppress CO adsorption in the highly coordinated sites and also suppress the formation of hydrogen ad-atoms on the surface. The ethene hydrogenation reaction performed by co-adsorption of hydrogen and ethene is inhibited by the presence of carbon deposits. The inhibition is independent of particle size studied (1-3 nm). The effects are rationalized in terms of a site-blocking behavior of carbon species occupying highly coordinated sites on the Pd surface.     

Adsorption of reaction species for hydrogenation of nitrobenzene on copper chromite at catalytic conditions

Adsorption of nitrobenzene, aniline and water on copper chromite has been investigated at the temperatures (483–558 K) and partial pressures (0–40 kPa) involved in the catalytic process (viz. hydrogenation of nitrobenzene), using the gas chromatographic pulse technique. The adsorption of reaction species was found to follow the Freundlich adsorption isotherm. The data on isosteric heats of adsorption (at different surface coverages) were obtained from the adsorption isotherms. The results indicated that nitrobenzene and water are physically adsorbed, whereas aniline is chemisorbed on the catalyst. The variation in the heat of adsorption with surface coverage for the adsorption of aniline indicated the presence of surface heterogeneity on the copper chromite.     

Kinetics of the reaction of hydrogen with thin films of carbon

The kinetics of the reaction of hydrogen with thin films of carbon has been studied over the temperature range 870–1150K and at pressures of hydrogen from 50–300 Torr (6.7–40 KPa). Thin films of carbon of average thickness about 30 nm were deposited on the surface of a quartz reactor by the pyrolysis of methane at 1100 K and the kinetics were studied in a static system. The products of the reaction were methane, ethane and ethylene, formed in successive hydrogenation steps, which in the low temperature region occurred largely on the surface of the carbon. In this region the activation energy of the rate of formation of methane was 6.5 kcal/mole. At temperatures above about 1050 K the thermal dissociation of hydrogen provided a source of radicals which caused a rapid increase in the rate of hydrogenation, both heterogeneous and homogeneous, giving an activation energy for the rate of formation of methane of 51 kcal/mole. A self-inhibition was observed, probably caused by a heterogeneous polymerization reaction leading to the formation of higher molecular weight products which remained adsorbed on the surface.     

Reaction pathway studies of C6 hydrocarbons on palladium model catalysts: effects of hydrogen/hydrocarbon ratio, molecular structure, surface structure and temperature

Reaction pathways of C₆ hydrocarbons, such as hydrogenolysis, dehydrogenation, aromatization and dehydrocyclization, have been studied on small surface area palladium model catalysts, including Pd(111) single crystal and polycrystalline palladium foil. These reactions were performed in a batch reactor, at atmospheric pressure and at two different temperatures (573 and 673 K). This revised version was published online in July 2006 with corrections to the Cover Date.     

The first measurement of an absolute surface concentration of reaction intermediates in ethylene hydrogenation

The first measurement of a turnover rate with respect to surface intermediate concentration in a high pressure heterogeneous catalytic reaction is reported. By using infrared-visible sum frequency generation to study the hydrogenation of ethylene on Pt(111), it was found that the surface concentration of -bonded ethylene, the key reaction intermediate, represented approximately 4% of a monolayer. Thus the absolute turnover rate per surface adsorbed ethylene molecule is 25 times faster than the rate measured per platinum atom. To explain these results, we propose a model of weakly adsorbed ethylene intermediates reacting on atop sites.     

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).