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.     

Ethylene adsorption on Pt/Au/SiO2 catalysts

Ethylene adsorption on a Pt/Au/SiO₂ catalyst (2 wt% Pt; Au/Pt atomic ratio of 10) was studied using adsorption microcalorimetry and FTIR spectroscopy. Ethylene adsorption at 300 K on Pt/Au/SiO₂ produced π‐bonded, di‐σ‐bonded, and ethylidyne species with an initial heat of 140 kJ/mol, compared to a heat of 157 kJ/mol for Pt/SiO₂ on which only ethylidyne species formed. At 203 and 263 K, ethylene adsorbed on Pt as well as on Au surface atoms for the Pt/Au/SiO₂ catalyst. Quantum chemical, DFT calculations indicate that Au exerts a significantly smaller electronic effect on Pt than does addition of Sn to Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solvent effects in the kinetics of the enantioselective hydrogenation of ethyl pyruvate

The influence of solvent on the kinetics of enantioselective hydrogenation of ethyl pyruvate by Pt/Al₂O₃/dihydrocinchonidine is reported. In a non‐polar solvent, toluene, the reaction is approximately zero order in substrate at constant hydrogen pressure, while under the same conditions and at the same substrate concentration, in the polar solvents ethanol and propylene carbonate the reaction shows a first‐order substrate concentration dependence. Fits to a Michaelis–Menten rate expression show that these differences are the expression of the relative magnitudes of the adsorption term in the rate expression, which in turn reflects the influence of the solvent on the adsorption–desorption processes which take place at the catalyst surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Examination of catalytic behavior and origin of the initial transient period for Pt nanoclusters modified with cinchonidine

Nanoclusters prepared by a novel water-free method are compared directly with nanoclusters prepared by the known aqueous preparation as well as conventional Pt/Al₂O₃ for the enantioselective hydrogenation of ethyl pyruvate. The catalytic behavior of cinchonidine on colloidal Pt was investigated during ethyl pyruvate hydrogenation in acetic acid under 10 bar of hydrogen at 22 °C with (1 mmol L⁻¹) and without addition of free cinchonidine. The effect of hydrogen pressure, cinchonidine concentration, ethyl pyruvate and catalyst loading on the enantiomeric excess (EE) with time were also studied. Through these studies, we propose that the nature of the observed initial transient period (ITP) for these “quasi-homogeneous” systems may be explained by desorption of the weakly adsorbed tilted “N lone pair bonded” cinchonidine species from the Pt surface due to interaction with hydrogen.     

The thermal activation of propyl groups on Pt(111)

The thermal chemistry of 1‐ and 2‐propyl moieties on Pt(111) was studied by using temperature‐programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). The propyl intermediates were prepared via thermal activation of the C–I bond of 1‐ and 2‐iodopropane adsorbed precursors, respectively. It was determined that the subsequent thermal activation of those propyl groups results in a competition between reductive elimination to propane, β‐hydride elimination to propene, and complete decomposition to propylidyne (and eventually to hydrogen and surface carbon). It was found that while the 2‐propyl intermediate favors propene production, 1‐propyl also yields significant amounts of propane. The formation of propene via β‐hydride elimination was identified by isotopic labeling TPD experiments, and directly about 200 K by RAIRS. Coadsorption experiments with hydrogen and deuterium were used to characterize hydrogenation and H–D reactions. All possible propene and propane isotopomers are formed from both 1‐ or 2‐iodopropane on the D/Pt(111) surface, indicating that exchange is likely to occur via a cyclic propyl–propene–propyl mechanism involving the formation of both 1‐ and 2‐propyl intermediates. Relative rates for 1‐ versus 2‐propyl conversion were estimated. This revised version was published online in August 2006 with corrections to the Cover Date.     

The role of surface oxygen on copper metal in catalysts for the synthesis of methanol

Discrepancies in experimental measurements of adsorbed oxygen coverage on copper metal surfaces in working Cu/ZnO/Al₂O₃ catalysts are interpreted in terms of two types of adsorbed oxygen. The first, O(a), is identical with that observed in studies of single‐crystal copper surfaces. The second, O^*(a), not seen in single‐crystal studies, is more strongly bonded to the metal surface. It is suggested that the adsorption sites of O^*(a) contain Zn as well as Cu, from surface α‐brass (copper/zinc alloy) formation during catalyst reduction. Earlier experimental results on O(a) coverages on various supported copper catalysts are re‐assessed. Only catalysts containing Zn (or Ga) gave abnormally high coverages: with other supports, basic or acidic, O(a) coverages are less than ∼0.1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface composition and possible rearrangement of disperse Pt and Rh catalysts: does the presence of carbon and oxygen contribute to different catalytic properties?

The surface composition of Rh and Pt blacks (as determined by XPS) shows carbon and oxygen impurities in the untreated state. Oxygen on Pt is present as adsorbed O as well as OH/H₂O groups and oxidized carbon. Rh was partly oxidized to Rh₂O₃, in agreement with UPS showing hardly any Fermi‐edge intensity in untreated Rh as opposed to untreated Pt. High Fermi‐edge intensities indicated a predominant metallic surface after an in situ treatment with H₂ at 483 K, increasing the purity (XPS) to ∼90%. This treatment reduced Rh to metal and removed its C impurity. Pt, in turn, retained much carbon after H₂ treatment, present mainly as graphitic carbon. A minor amount of CO was also detected, some of the O 1s peak belonging to it. The two metals were tested in methylcyclopentane reactions. Considering the necessity of carbon for nondegradative reactions and oxygen enhancing fragmentation, a correlation is suggested between the typical impurities of Pt and Rh and their respective catalytic propensities: the high fragmentation activity of Rh and the predominant nondegradative reactions to C₆ “ring opening products” on Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Raman and infrared spectroscopies as in situ probes of catalytic adsorbate chemistry at electrochemical and related metal–gas interfaces: some perspectives and prospects

Some recent progress in the utilization of infrared and especially Raman spectroscopies for the in situ vibrational characterization of adsorbates at electrochemical interfaces having relevance to catalytic chemistry is briefly outlined, and illustrated by means of examples culled chiefly from our laboratory. The primary factors responsible for the differences as well as similarities in the experimental strategies pursued for metal–solution interfaces as compared with metal surfaces in gas‐phase and ultrahigh vacuum (UHV) environments are discussed, and the distinct virtues of surface‐enhanced Raman scattering (SERS) and infrared reflection‐absorption spectroscopy (IRAS) for scrutinizing the first interfacial type are assessed. The detailed influences of the electrochemical double layer on chemisorbate vibrational properties at ordered metal–solution interfaces as gleaned by in situ IRAS data in comparison with spectra for analogous “model electrochemical” interfaces in UHV are described, and briefly illustrated for carbon monoxide on Pt(111) and Ir(111). The significance of the surface potential φ in controlling chemisorbate properties on metal surfaces in gaseous and UHV as well as electrochemical environments is pointed out. Evidence for the occurrence of “redox pinning” of φ by gaseous species in ambient‐pressure systems is outlined, along with possible catalytic implications. The burgeoning prospects for utilizing SERS as a versatile as well as uniquely sensitive vibrational probe of catalytically significant, especially transition‐metal, interfaces in both electrochemical and gas‐phase environments are delineated. Emphasis is placed on the typically richer vibrational spectra attainable for SERS compared to IRAS, arising from differing surface selection rules along with the greater sensitivity and wider wavenumber ranges accessible to the former method, and exemplified by benzene adsorption on rhodium and palladium electrodes. The anticipated power of SERS for assessing the reactivity as well as identity of adsorbed intermediates in ambient catalytic systems by means of transient in situ spectral measurements is noted, and illustrated briefly for ambient‐pressure methanol oxidation on rhodium. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chemisorption measurements on polymer‐stabilized colloidal platinum and rhodium nanoclusters in liquid dispersion

A simple method is reported for the precise and accurate differential barometric measurement of gas adsorption into liquid dispersions of colloidal metal catalysts. Using both hydrogen adsorption and hydrogen/oxygen titration the fraction of the total surface of polyvinylpyrrolidone‐stabilised Pt and Rh colloids available for adsorption is found to be approximately 44%. This method is also applicable to supported metal catalysts in liquid slurries. This revised version was published online in July 2006 with corrections to the Cover Date.     

NO reduction by C3H6 in excess oxygen over fresh and sulfated Pt‐ and Rh‐based catalysts

The selective catalytic reduction (SCR) of NO with C₃H₆ was studied over three noble‐metal‐based catalysts: 2% Pt/γ‐Al₂O₃, 2% Rh/γ‐Al₂O₃ and 1.5% Rh/TiO₂(4% WO₃). The SO₂ effect on the catalyst activity was examined using sulfated samples of the above catalysts and SO₂‐containing feeds. Temperature‐programmed desorption and oxidation studies were carried out to examine the adsorption characteristics of NO and C₃H₆, respectively, in the absence or the presence of SO₂. The adsorption data were linked to variations in the NO reduction rates over fresh and sulfated samples. Modification of the support surface as a result of the SO₂ presence affects the NO and propene sorption characteristics, the NO oxidation and the propene consumption rates. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the conversion of 1‐butene over Pt‐ZSM5

The conversion of 1‐butene over Pt‐ZSM5 was studied in the presence and absence of hydrogen. In the absence of hydrogen, the catalytic activity of the metal was rapidly poisoned and in the steady state the catalyst showed identical properties as the parent ZSM5. In the presence of hydrogen, the skeletal isomerization of butene was not affected, but the rate of oligomerization/cracking was enhanced. From the by‐products butadiene, propane, ethane and methane, ethane and propane originated mainly from oligomerization/cracking on the acid sites and not from hydrogenolysis on Pt. The contribution of the metal to the overall by‐product formation was small. Thus, an adaptation of the acid rather than of the metal sites is necessary to improve the performance of the bifunctional catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Steady‐state N2O decomposition on Pt and Rh surfaces using a free‐jet molecular‐beam excited by nozzle heating

Steady‐state N₂O decomposition reaction on polycrystalline Pt and Rh surfaces has been studied using a supersonic free‐jet molecular beam (2.1 × 10¹⁸ molecules/cm² s). The energy of the incident N₂O beam was controlled by a nozzle heating technique in conjunction with a seeding technique. The decomposition rate shows both translational and vibrational energy dependence on the Pt surface. However, there is also the surface temperature dependence of the decomposition rate even varying the incident beam energy, indicating precursor‐mediated dissociation of N₂O on the Pt surface. On the other hand, no energy dependence was observed on the Rh surface, suggesting that the decomposition dynamics are different between Pt and Rh surfaces. This revised version was published online in August 2006 with corrections to the Cover Date.     

Enantioselective hydrogenation of α-ketoesters using cinchona modified platinum catalysts and related systems: A review

The state of the art for the heterogeneous enantioselective hydrogenation of α-ketoesters using cinchona modified Pt catalysts and related systems is reviewed. The effect of the following elements of the catalytic system are well known: Catalyst. Supported Pt catalysts with relatively low dispersion (particle diameter >2 nm) are preferred for the hydrogenation of α-ketoacid derivatives, Pd catalysts for functionalized olefins. Most support materials are suitable. Substrate. The reacting function is preferentially a ketone or a C=C bond, a carbonyl group in a-position is necessary for good optical yields. Modifier. The minimal requirements for an efficient modifier for the hydrogenation of α-ketoesters is the presence of a basic nitrogen atom close to one or more stereogenic centers and connected to an extended aromatic system (preferentially quinolyl or naphthyl). The presence of an alcohol or ether in β-position to the basic nitrogen often gives better enantioselectivities. Solvent. Solvents with adielectric constant between 2 and 10 give best selectivities for a-ketoesters with best e.e.'s in acetic acid. For the hydrogenation of substrates with a free acid function aqueous polar solvents are preferred. The highest optical yields for the different substrate types: 95% e.e. for α-ketoesters, 85% for a-ketoacids and 70% for α,)-unsaturated acids. Practical problems for the use of the catalytic system are low e.e.'s at the start of the reaction, the instability of the modifier and some side reactions as well as the purity of the ethyl pyruvate. Mechanistic investigations have established interactions between substrate and modifier in solution and adsorption of the ethyl pyruvate and cinchonidine on the catalyst. The dependence of rate and e.e. on catalyst, cinchonidine, ethyl pyruvate and hydrogen concentration has been established for ethyl pyruvate hydrogenation using Pt/Al₂O₃-cinchona. A Langmuir-Hinshelwood scheme is well suited for explaining the observed kinetic results. Based on the kinetic results, the effect of modifier and substrate structure, and molecular modeling studies, the following mechanistic model has been developed: On the unmodified catalyst, the a-ketoester and hydrogen are reversibly adsorbed and the addition of the first hydrogen atom is rate determining. A modified active site is formed by adsorption of one cinchona molecule. It is postulated that a protonated adsorbed modifier interacts with the α-ketoester and forms a stabilized half hydrogenated intermediate. The rate determining step for the preferred enantiomer is the addition of the second hydrogen. The rate acceleration and the enantiodiscrimination is therefore due to the preferential stabilization of one of the two diastereomeric intermediates. Alternative mechanisms are discussed but considered to be less satisfying.     

Microcalorimetric studies of interactions of ethene, isobutene, and isobutane with silica-supported Pd, Pt, and PtSn

Microcalorimetric measurements were made of the interaction of hydrogen, ethene, isobutene and isobutane at 300 K with silica- supported Pt, Pd, and PtSn catalysts. The initial heats of hydrogen adsorption on silica-supported Pd and Pt are 104 and 95 kJ/mol, respectively. The presence of Sn decreases the saturation uptake of hydrogen on the PtSn sample. The initial heats of ethene interaction with Pd/silica and Pt/silica are 170 and 145 kJ/mol, respectively. The presence of Sn decreases the initial heat to 115 kJ/mol on the PtSn sample. The initial heats of isobutene interaction with silica-supported Pd and Pt are 160 and 190 kJ/mol, respectively. The presence of Sn decreases the initial heat to 125 kJ/mol on the PtSn sample. It appears that ethene and isobutene adsorb dissociatively on silica-supported Pd and Pt to form alkylidyne species at 300 K, with an average strength of carbon-metal bonds for these species near 230 kJ/mol. Ethene and isobutene adsorb on silica-supported PtSn to form di- σ- and π-bonded alkene species at 300 K, with an average strength of carbon-metal bonds for these species near 190 and 130 kJ/mol, respectively. Isobutane appears to adsorb dissociatively on a small number of sites on silica-supported Pd and Pt, and this dissociation is also inhibited by Sn on PtSn samples. This revised version was published online in July 2006 with corrections to the Cover Date.     

FTIR study of CO adsorption on coked Pt–Sn/Al2O3 catalysts

Infrared spectra of adsorbed CO have been used as a probe to monitor changes in Pt site character induced by the coking of Pt/Al₂O₃ and Pt–Sn/Al₂O₃ catalysts by heat treatment in heptane/hydrogen. Four distinguishable types of Pt site for the linear adsorption of CO on Pt/Al₂O₃ were poisoned to different extents showing the heterogeneity of the exposed Pt atoms. The lowest coordination Pt atoms (ν(CO) < 2030 cm⁻¹) were unpoisoned whereas the highest coordination sites in large ensembles of Pt atoms (2080 cm⁻¹) were highly poisoned, as were sites of intermediate coordination (2030–2060 cm⁻¹). Sites in smaller two‐dimensional ensembles of Pt atoms (2060–2065 cm⁻¹) were partially poisoned, as were sites for the adsorption of CO in a bridging configuration. The addition of Sn blocked the lowest coordination sites and destroyed large ensembles of Pt by a geometric dilution effect. The poisoning of other sites by coke was impeded by Sn, this effect being magnified for Cl‐containing catalyst. Oxidation or oxychlorination of coked catalyst at 823 K followed by reduction completely removed coke from the catalyst surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enantioselective hydrogenation of ethyl pyruvate over Pt/alumina: systematic variation of the modifier structure

A series of enantiomerically pure amino alcohols and amines has been prepared and tested as modifiers in the enantioselective hydrogenation of ethyl pyruvate over Pt/alumina. Systematic variation of the modifier structure revealed that an extended aromatic π–system is necessary for the function of the modifier and that the enantioselectivity crucially depends on the structure of the amino function. Surprisingly simple modifiers such as 1–(1–naphthyl)ethylamine and 1–(9–anthracenyl)–2–(1–pyrrolidinyl)ethanol proved to be nearly as effective as 10,11–dihydrocinchonidine. This revised version was published online in June 2006 with corrections to the Cover Date.     

Restructuring during pretreatment of platinum/alumina for enantioselective hydrogenation

The influence of reductive and oxidative heat treatment on the enantioselectivity of chirally modified Pt/alumina has been reinvestigated, using the hydrogenation of ketopantolactone as a test reaction. Enhancement in ee by 39–49% has been observed after treatment in hydrogen at 250–600°C, as compared to untreated or preoxidized catalysts. The changes in ee after reductive and oxidative treatments are reversible, and always the final treatment is decisive. A HRTEM study indicates that adsorbate‐induced restructuring of Pt crystallites during hydrogen treatment at elevated temperature can play a role in the selectivity improvement, but the changes are superimposed by the strong structure‐directing effect of the alumina support. The possible contribution of other effects (complete reduction of Pt ⁿ⁺ surface species, removal of impurities, or change of Pt particle size) could be excluded. This revised version was published online in July 2006 with corrections to the Cover Date.     

Metal-Organic Framework Constructed by Copper(I) Cyanide and Ethyl Isonicotinate Through Hydrogen Bonding

The reaction of K₃[Cu(CN)₄] and ethyl isonicotinate (EIN) in the presence of Me₃SnCl in H₂O/acetonitrile medium at room temperature affords the 3D-supramolecular coordination polymer (SCP), ∞ ³[CuCN·(EIN)], 1. The structure of 1 consists of 1D-zig-zag chains, which extend via the EIN ligands through hydrogen bonds organized in AB 2D-layers. The infinite AB···AB layers are further extended to form a 3D-network via hydrogen bonds and π–π stacking interactions, thus creating wide channels. The emission spectrum of 1 in the solid state reveals a set of high energy and low energy distinct peaks in the visible region at 400–580 nm upon excitation at 280 nm. The luminescence excitation in 1 could be caused by different possible transitions including metal-to-ligand charge transfer (MLCT) or single-metal-centered (MC) transitions.     

Enantioselective hydrogenation of carbonyl compounds over Pd-Pt/alumina catalysts

A series of Pd-Pt/alumina catalysts were prepared by consecutive deposition of Pd onto supported Pt particles. Electrochemical model studies indicated that under very mild conditions in aqueous acetic acid the reduction of Pd²⁺ ions occurred partly via the ionization of hydrogen adsorbed on Pt and partly by the slow oxidation of acetic acid. There was only a moderate change in the surface Pd/Pt atomic ratio during heat treatment in flowing hydrogen at 400°C, as determined by XPS analysis. The catalytic performance of the bimetallic catalysts was tested in the enantioselective hydrogenation of ethyl pyruvate and ketopantolactone, in the presence of cinchonidine. Pd was virtually inactive and acted as a site blocker, which decreased the size of Pt ensembles and hindered the formation of the bulky transition complex between the reactant and the chiral modifier. The intrinsically low activity and selectivity of Pd are discussed in the light of H/D exchange studies in deuterated ethanol. This revised version was published online in July 2006 with corrections to the Cover Date.