Characterization by CO/FTIR spectroscopy of Pd/silica catalysts and its correlation with syn‐gas conversion

Well‐dispersed Pd catalysts, supported on two morphologically different silicas (meso‐ and microporous Davison G59 and G03 grades, respectively) and used for syn‐gas activation at T=493–523 K and P=1–4 MPa (CO/H₂= 1/2.5), have been studied by CO chemisorption using FTIR spectroscopy. The long‐term exposure to 760 Torr CO(g) at 298 K produces deep changes on the surface structure of Pd particles on both supports. The Pd particles become rougher and/or show more open crystal planes. This phenomenon of surface restructuring seems to depend both on the exposed metal fraction (FE) of palladium and the morphology of the support. The rate of surface restructuring but not its extent, is a function of the superimposed CO(g) pressure. On the microporous G03 silica CO chemisorbs in multicoordinated or “hollow” sites (H band), but these signals are not shown by preparations of the supported metal of comparable dispersity on mesoporous G59 grade. Terminal (L band) and di‐coordinated CO_s (B band) appear in both types of catalysts. The high‐loading preparations on the microporous support showed a higher proportion of Pd(111) planes than those of low Pd loading, which seems to contribute to the high TOF_CH4 and high selectivity to methane in syn‐gas activation of these catalysts, while the remaining ones show excellent capability for methanol production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dechlorination of 1,2‐dichloroethane catalyzed by Pt–Cu/C: unraveling the role of each metal

The effect of Pt–Cu/C catalyst composition on the activity and selectivity in the reaction of 1,2‐dichloroethane dechlorination in a H₂‐containing atmosphere has been investigated. For monometallic Pt catalysts and those with Cu/Pt atomic ratio ⩽1, the reaction products are almost entirely ethane and monochloroethane. However, increasing the Cu/Pt ratio increases the selectivity towards ethylene. Approximately 90% selectivity towards ethylene is obtained for catalysts with Cu/Pt ratio ⩾9. Monometallic Cu/C produce only ethylene, but the activity is two orders of magnitude lower than that of other catalysts studied. With time on stream during the initial 2–30 h there is a continuous increase in selectivity towards ethylene at the expense of ethane. This behavior was rationalized in terms of equilibration of bimetallic particle surface composition exposed to the reaction mixture. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acid function of Al‐MCM‐41 supported platinum catalysts in hydrogenation of benzene, toluene o‐xylene

A series of Al‐MCM‐41 mesoporous materials with different Si/Al ratios are synthesized. The acidities of catalysts are measured by the temperature‐programmed desorption of ammonia (NH₃‐TPD) and IR spectra of pyridine. Their catalytic activities for hydrogenation of benzene, toluene and o‐xylene are investigated on a pulse reactor system. NH₃‐TPD and IR results show that only weak and medium acid sites could be observed on the catalysts, and the number of total acid sites decreases obviously with the increase of the Si/Al ratio whereas the medium acid sites are somewhat constant. The introduction of platinum onto Al‐MCM‐41 material decreases the total acid number by a small amount. The hydrogenation activities of the 1% Pt/HAl‐MCM‐41 catalysts are found to correspond well with the ratio of medium acid sites to total acid sites in the employed catalysts. It is proposed that, in addition to metal sites, the acid sites in the metal–acid interfacial regions are also the active sites for hydrogenation and the medium acid sites play important roles in the reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Activities of unsupported second transition series metal sulfides for hydrodesulfurization of sterically hindered 4,6‐dimethyldibenzothiophene and of unsubstituted dibenzothiophene

The applicability of transition metal sulfides (TMS) from the second transition series in deep hydrodesulfurization (HDS) was examined and compared to that of a traditional, supported CoMo/Al₂O₃ catalyst. Sulfides of Nb, Mo, Ru, Rh and Pd were studied for HDS of dibenzothiophene (DBT) and 4,6‐dimethyldibenzothiophene (4,6‐Me₂DBT). Measurements were carried out with unsupported TMS samples at different temperatures and H₂S partial pressures. The trend in DBT HDS activities agreed quite well with those found by previous authors. It was furthermore found that the activities of the metal sulfides towards the sterically hindered molecule 4,6‐Me₂DBT closely followed those for DBT. This is somewhat surprising since the direct sulfur abstraction route was of major importance for DBT while the prehydrogenation route, in which ring‐hydrogenation in the DBT skeleton precedes desulfurization, was prevalent for 4,6‐Me₂DBT. This suggests that common steps are involved in the two routes. For the unsupported metal sulfides, ring‐hydrogenated but not desulfurized DBT and 4,6‐Me₂DBT products were found in much larger amounts than for supported and promoted MoS₂‐based catalysts. This can be rationalized as being due to a relatively higher hydrogenation/desulfurization selectivity ratio for the different transition metal sulfides. Inhibition by H₂S was found to be most pronounced near the center of the transition series. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Ba and rare earths cations on the properties and dehydrocyclization activity of Pt/K‐LTL catalysts

Several Pt/L‐zeolite catalysts were prepared by the impregnation method from L‐zeolite samples previously exchanged with Ba²⁺, La³⁺, Pr³⁺, Nd³⁺ and Sm³⁺. Lanthanides (Ln³⁺) increase the overall dispersion of platinum, measured by H₂–O₂ titration. TPR and CO/FTIR measurements indicate that these ions modify the distribution of Pt in the zeolite lattice, increasing the fraction of metal on the external surface. Additionally, both CO/FTIR and competitive hydrogenation of toluene–benzene mixtures indicate that, in the presence of Ln³⁺, the electron density of the platinum decreases in comparison with Pt/KL‐zeolite. On the other hand, Ba²⁺ does not substantially modify neither the distribution nor the electronic state of Pt. The reactivity measurements in the hydroconversion of n‐heptane show that Pt/BaKL and Pt/KL exhibit similar catalytic behaviour with a high dehydrocyclization activity. However, the exchange of K⁺ by Ln³⁺ increases the production of heptane isomers, while selectivities to aromatic and terminal hydrogenolysis products significantly decrease. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pd/polyaniline as the catalysts for 2‐ethylanthraquinone hydrogenation. The effect of palladium dispersion

By doping of polyaniline (PANI) in PdCl₂ aqueous and ethanol solutions the catalysts containing crystalline and colloidal Pd particles of different sizes were prepared. The size of palladium particles present in Pd/PANI catalysts (characterised by SEM and XRD methods) influenced the course of 2‐ethylanthraquinone (eAQ) hydrogenation, a key step in the industrial production of H₂O₂. The presence of large palladium particles promotes reactions leading to the formation of the so‐termed “degradation products” not capable of hydrogen peroxide formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isothermal, non‐oxidative, two‐step CH4 conversion over unsupported and supported Ru and Pt catalysts

The isothermal, non‐oxidative, two‐step conversion of CH₄ to C₂₊ hydrocarbons was investigated over unsupported and supported Pt and Ru catalysts at moderate temperatures and elevated pressures. The single‐cycle specific activity (μmol C₂₊/g_cat) and total product yield (μmol C₂₊/μmol surface metal) for Ru powder at 430 K were significantly higher than for Pt powder at 503 K. The activity and total product yield for mixed metal oxide (MMO)‐supported Ru were significantly less than for Ru powder; however, Pt/MMO exhibited similar activity and product yield in comparison to Pt powder. The formation of methylbutane and methylpentane increased with increasing pressure over Ru/MMO. However, increasing pressure favored the formation of C₄ species over Pt/MMO. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The role of lanthanum oxide on Pd‐only three‐way catalysts prepared by co‐impregnation and sequential impregnation methods

The effect of lanthanum oxides on the catalytic performance and the physicochemical properties of Pd‐only three‐way catalysts prepared by co‐impregnation and sequential impregnation methods was studied by using hydrogen chemisorption, BET surface area, X‐ray diffraction and X‐ray photoelectron spectroscopy. It was found that the roles of La closely depended on the order of La introduction in the preparation of the Pd catalysts. Pd–La/Al₂O₃ prepared by co‐impregnation of La and Pd, kept its superior activity in spite of the significant loss of surface area of the alumina support after thermal aging at 1273 K, indicating that the primary role of La was a Pd stabilizer through the intimate interaction between La and Pd. However, on Pd/La/Al₂O₃, in which Pd was consecutively impregnated after the impregnation of La, La preferentially interacted with the alumina support as a form of La_xAl_yO₂, resulting in the stabilization of the alumina support during thermal aging. XPS results indicated that lanthanum oxide suppressed the formation of PdO interacting with alumina during thermal aging. In the case of Pd/La–Ce/Al₂O₃, the formation of the solid solution of (Ce_xLa_1−x)O₂ was not strong enough to maintain the high activity and the good textural property after thermal aging. This revised version was published online in July 2006 with corrections to the Cover Date.     

Exploration of cinnamaldehyde hydrogenation in Co–Pt/γ‐Al2O3 catalytic membrane reactors

The feasibility of Co–Pt/γ‐Al₂O₃ catalytic membrane reactors for cinnamaldehyde hydrogenation has been explored. The results of hydrogenation in four membrane reactors with different configurations indicate that the effect of the gas transport limitation is more important than the liquid diffusion limitation. The membrane with the catalytic layer situated on the gas side shows the highest activity due to the minimized gas transport limitation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of hydrogen on methane conversion to hydrocarbons in “one‐step” reaction under non‐oxidative conditions at low temperature over Pd–Co/SiO2 catalysts prepared by the sol/gel method

Methane conversion to higher hydrocarbons in a “one‐step” process under non‐oxidative conditions at low temperature was here first introduced and investigated over Co–Pd/SiO₂ catalysts at 250°C as a function of hydrogen concentration in helium and of catalyst composition. A maximum in the production of C₂₊ hydrocarbons including aromatics (benzene and toluene) was observed at 1.3 vol% H₂/He mixture in which one pulse of methane was introduced. Additional hydrogenation with the same H₂/He mixture at 400°C was efficient to remove the larger hydrocarbon fragments already existing on the surface. On pure Pd/SiO₂ the one‐step process is not so efficient as on cobalt‐rich samples, but in the latter case the hydrocarbon removal is the most efficient during high‐temperature hydrogenation. It was found that methane conversion in the one‐step process is at least 2.5 times greater than that measured in the “two‐step” process and, in some cases, 80% of the methane introduced is converted to larger hydrocarbons. The results are discussed in terms of the hydrogen coverage ensuring the optimum hydrogen content in the surface CH_x species leading to chain growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

EXAFS characterization of bimetallic Pt–Pd/SiO2–Al2O3 catalysts for hydrogenation of aromatics in diesel fuel

Bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts exhibited much higher activities in aromatic hydrogenation of distillates than monometallic Pt/SiO₂–Al₂O₃ and Pd/SiO₂–Al₂O₃ catalysts. The studies of extended X‐ray absorption fine structure (EXAFS) indicated that there was an interaction between Pt and Pd in the Pt–Pd/ SiO₂–Al₂O₃ catalyst. Furthermore, from the EXAFS, it was assumed that the active metal particle on the Pt–Pd/SiO₂–Al₂O₃ catalysts is composed of the “Pd dispersed on Pt particle” structure. Regarding both the activities of aromatic hydrogenation and the EXAFS results, it was concluded that the Pd species dispersed on Pt particles were responsible for the high activity of the bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

A novel palladium complex catalyst for carbonylation of alkynes under mild conditions

Carbonylation of alkynes has been carried out using a catalyst system consisting of Pd(OAc)₂, a monophosphine, p‐toluene sulphonic acid and semilabile anionic bidentate ligands such as pyridine or piperidine carboxylic acids. Turnover frequencies (TOF) upto 3500 h⁻¹ and 98% selectivity to 2‐substituted 2‐propenoic acid/ester have been achieved under mild CO pressures of 1–3 atm at 373 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and characterization of alumina‐supported metal‐carbonyl‐derived model catalytic systems in UHV conditions

Active catalysts for metathesis of alkenes, hydrodesulfurization, and hydrogenation can be prepared by exposing a high‐surface‐area alumina support to molybdenum hexacarbonyl at room temperature. This strategy is mimicked in ultrahigh vacuum by adsorbing molybdenum hexacarbonyl onto an ultrathin hydroxylated alumina film grown onto a Mo(100) substrate. In contrast to results found on high surface area, no Mo(CO)₆ is found to adsorb on alumina at 300 K, and significant molybdenum deposition is only found by heating the sample to above 670 K. Alternatively, molybdenum hexacarbonyl adsorbs on alumina when cooled to 80 K. In this case the majority of the carbonyl desorbs intact and temperature‐programmed desorption and X‐ray photoelectron spectroscopy indicate that ∼2% of a monolayer of the carbonyls undergoes decarbonylation. Auger and X‐ray photoelectron spectroscopy measurements reveal that molybdenum carbide (MoC) is deposited onto the alumina surface heated to 700 K forming a monolayer after an exposure of ∼50 L. This layer is reduced to the metal by heating to ∼1500 K by reaction with the alumina substrate to evolve CO and form metallic molybdenum. The carbide can be reformed by heating the metal‐covered alumina sample in ethylene at 900 K, and the carbide can once again be reduced to the metal by heating to 1500 K. This process can be repeated so that the carbide can be regrown by reaction with ethylene and reduced by annealing in vacuo to 1500 K. Subcarbonyl species are detected after adsorbing Mo(CO)₆ on hydroxylated alumina at 80 K as the sample is heated to ∼200 K. At higher temperatures, the molybdenum is oxidized to an approximately 4+ oxidation state and deposits primarily oxalate species on the surface. The adsorbed oxalates thermally decompose at ∼300 K to evolve CO to form surface formates. These are stable to ∼560 K and react to evolve CO at this temperature. It is also found that the extent of decarbonylation depends on the degree of alumina hydroxylation so that heating hydroxylated alumina to 900 K, which removes ∼50% of the surface hydroxyls, decreases the both CO desorption yield and the oxalate coverage by 50%. This revised version was published online in June 2006 with corrections to the Cover Date.     

Vapour phase O‐methylation of 2‐naphthol over the solid bases alkali‐loaded silica and Cs‐loaded MCM‐41

O‐alkylation of 2‐naphthol has been investigated in the vapour phase over alkali‐loaded fumed silica and Cs‐MCM‐41. Both SiO₂ and MCM‐41 had low C‐alkylation activities and no O‐alkylation activity. The introduction of alkali ions considerably increases 2‐naphthol conversion with 2‐methoxynaphthalene being the major product. The activity of the catalysts increases with alkali loading and the basicity of the metal (Cs > K > Na > Li). Very high conversion (∼99%) of 2‐naphthol and selectivity (>95%) for 2‐methoxynaphthalene are obtained over Cs‐loaded fumed silica and MCM‐41. A small amount of 1‐methyl‐2‐hydroxynaphthalene is also formed over the Li‐, Na‐ and K‐loaded silica. This revised version was published online in July 2006 with corrections to the Cover Date.     

Highly efficient catalyst system for the synthesis of 2‐arylpropionic acids by carbonylation

Carbonylation of 1‐(4‐isobutylphenyl)ethanol has been studied using a homogeneous catalyst system consisting of PdCl₂(PPh₃)₂/ TsOH/ LiCl. Higher reaction rates with TOF up to 1200 h⁻¹ and ibuprofen selectivity >95% have been achieved at 388 K under a CO partial pressure of 5.4 MPa. The reaction proceeds through the formation of 4‐isobutylstyrene and 1‐(4‐isobutylphenyl)ethyl chloride as intermediates. The same catalyst system is shown to be effective for carbonylation of various α-arylethanols, vinyl aromatics and corresponding chloro derivatives. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pulse‐MS studies on CH4/CD4 isotope effect in the partial oxidation of methane to syngas over Pt/α‐Al2 3

By replacing CH₄/+O₂ with CD₄+O₂, the deuterium isotope effect in the partial oxidation of methane over Pt/α‐Al₂O₃ was studied in the temperature range of 550–650 °C using the pulse‐MS method. The effect of space velocity of carrier gas and CO₂ reforming of CH₄ to syngas were also investigated. No deuterium isotope effect was observed for CH₄ conversion whereas CO formation showed a normal deuterium isotope effect. The surface reaction between adsorbed hydrocarbon species and adsorbed oxygen species to CO formation may be a relatively slow step. The results support the parallel mechanism, namely CO and CO₂ are simultaneously formed in parallel from the direct oxidation of methane. 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.     

On the oxidation–reduction kinetics of palladium

The oxidation–reduction kinetics of Pd was investigated on polycrystalline Pd foils using XPS and sputter‐depth profiling over a wide range of temperatures. The observed behavior can be explained in terms of a desorption‐controlled process at lower temperatures (T > 230° C) and a diffusion‐controlled process at higher temperatures (T > 450° C). In the intermediate range (230 < T < 450°C) a transition between the two processes is observed which gives rise to a pronounced shoulder in the reduction time trace. The interpretation is qualitatively confirmed using a simple mathematical model for a coupled bulk diffusion/surface reaction system. This revised version was published online in July 2006 with corrections to the Cover Date.     

Redox and photo‐redox properties of isolated Mo5+ ions in MoH‐ZSM‐5 and MoH‐beta zeolites: in situ ESR study

Redox and photo‐redox properties of isolated Mo⁵⁺ ions stabilized in H‐ZSM‐5 and H‐beta zeolites are studied by in situ ESR in flowing O₂, NO, H₂, and C₃H₆. Upon oxidation of pre‐reduced samples at 20 °C, NO demonstrates a higher oxidative ability, as compared with O₂. Interaction of Mo⁵⁺ ions with propene at 20 °C results in formation of a chemisorption complex with enhanced reactivity of Mo(V) toward NO. Illumination of the Mo⁵⁺/HZSM‐5 sample with UV‐visible light causes measurable acceleration of Mo(V) oxidation by NO at 20 °C. Therefore, photochemical activation of the oxidation step could be realized, in principle, for Mo/zeolite catalysts. At 500 °C in the reaction mixture NO + H₂, the step of the catalytic site reduction is fast, and the dynamic equilibrium of the redox reaction Mo(VI) ↔ Mo(V) for MoH‐ZSM‐5 and MoH‐beta seems to be strongly shifted to Mo⁵⁺. This revised version was published online in July 2006 with corrections to the Cover Date.