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.     

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.     

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.     

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.     

Reforming of C6 Hydrocarbons Over Model Pt Nanoparticle Catalysts

Size-controlled model Pt nanoparticle catalysts, synthesized by colloidal chemistry, were used to study the hydrogenative reforming of three C₆ hydrocarbons in mixtures with 5:1 excess of H₂: methylcyclopentane, n-hexane and 2-methylpentane. We found a strong particle size dependence on the distribution of different reaction products for the hydrogenolysis of methylcyclopentane. The reactions of 50?Torr methylcyclopentane in 250?Torr H₂ at 320 °C, using 1.5 and 3.0 nm Pt nanoparticles produced predominantly C₆ isomers, especially 2-methylpentane, whereas 5.2 and 11.3 nm Pt nanoparticles were more selective for the formation of benzene. For the hydrogenolysis of n-hexane and 2-methylpentane, strong particle size effects on the turnover rates were observed. Hexane and 2-methylpentane reacted up to an order of magnitude slower over 3.0 nm Pt than over the other particle sizes. At 360 °C the isomerization reactions were more selective than the other reaction pathways over 3.0?nm Pt, which also yielded relatively less benzene.     

Preparation and Characterization of Ru/Al2O3/Cordierite Monolithic Catalysts for Selective Hydrogenation of Benzene to Cyclohexene

A series of Ru/Al₂O₃/cordierite monolithic catalysts were prepared and characterized by BET, XRD, TPR, TEM and SEM-EDAX. The catalytic performances in selective hydrogenation of benzene to cyclohexene were investigated in a continuous fixed-bed reactor. The preparation conditions significantly influence morphology, particle size, and surface area of the catalyst, subsequently affecting the catalytic performances. It was found that higher calcination temperature of the Ru-based monolithic catalyst led to the conglomeration and crystallite growth of the t-RuO₂, which will decrease the catalytic activity. The lower thickness and the larger pore size of the alumina washcoating layer are the preferential choices to obtain higher cyclohexene selectivity due to the improved internal mass transfer of cyclohexene. It was also found that high ruthenium loading resulted in deep hydrogenation of cyclohexene. Moreover, the reduction temperature was optimized to 473 K and excess high temperature led to the deterioration of both activity and cyclohexene selectivity.     

Thermodynamic Equilibrium Analysis of Propane Dehydrogenation with Carbon Dioxide and Side Reactions

A thermodynamic analysis of propane dehydrogenation with carbon dioxide was performed using constrained Gibbs free energy minimization method. Different reaction networks corresponding to different catalytic systems, including non-redox and redox oxide catalysts, were simulated. The influences of CO₂/C₃H₈ molar ratio (1–10), temperature (700–1000 K), and pressure (0.5–5 bar) on equilibrium conversion and product composition were studied. In the presence of CO₂ with a molar ratio of CO₂/C₃H₈ = 1, the temperature of dehydrogenation can be 30 K lower than that of dehydrogenation in the presence of steam (H₂O/C₃H₈ = 1) and about 50 K lower than that of simple dehydrogenation without dilution to achieve 60% propane conversion. It was found that the occurrence of dry reforming of propane and coke-forming side reactions could strongly impact the equilibrium product composition of the multireaction system and, therefore, these reactions should be kinetically controlled. Comparison of the simulated reactant conversions with those reported in the literatures revealed that the experimental conversion levels of propane are far below the corresponding equilibrium values due to rapid catalyst deactivation by coke, implying that research efforts should be directed toward formulation of more active and selective catalysts.     

Kinetics of ethylene oxidation on plane Pt/SiO2 catalysts in the viscous pressure regime: evidence of support activity

C₂H₄ oxidation on plane Pt/SiO₂ model catalysts with various Pt loadings was studied atT = 373–473 K and in the pressure ranges 10^–6–10² Torr C₂H₄ and 0.3–1500 Torr 02 (1 Torr = 133.3 Pa). Mass spectrometry combined with spatially resolved gas sampling enabled kinetic data to be collected far into the viscous pressure regime. Reaction orders and activation energies were similar to those of a macroscopic Pt surface. However, under fuel-lean conditions the global reaction rate decreases faster than the decrease in metal area. On the other hand, the global rate wasindependent of Pt loading and metal surface area in fuel-rich gas mixtures. This is interpreted in terms of a spillover effect.     

The Surface Chemistry of 1-Iodopropane Adsorbed on Pt(111)

On Pt(111) at 110 K, 1-iodopropane, C₃H₇I, adsorbs molecularly, but for doses below 1.7 × 10¹⁴ molecules cm⁻², only H₂ and I appear in thermal desorption. C–I bond cleavage occurs between 160 and 220 K, forming adsorbed n-propyl, C_(a)H₂CH₂CH₃, and atomic iodine, based on temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). n-Propyl undergoes β-hydride elimination forming propylene, with desorption peaks at 185 and 240 K. At 240 K, hydrogenation to propane also occurs. Some di-σ bonded propylene, C_(a)H₂C_(a)HCH₃, remains at 240 K and it rearranges to propylidyne near 300 K. Atomic H, bound to Pt, recombines and desorbs at ca. 260 K. Further desorption of H₂ is limited by C–H bond breaking and occurs over a broad temperature range with local maxima at ca. 280, 320, and 420 K, typical of propylidyne fragments on Pt. Atomic iodine desorbs in a broad feature at 825 K.     

Particle Size Effect on CH4 Oxidation Over Noble Metals: Comparison of Pt and Pd Catalysts

Intrinsic catalytic activities (TOF values) in CH₄ complete oxidation under lean conditions were estimated as a function of Pt and Pd particle sizes (d_m) for two series of Pt/Al₂O₃ and Pd/Al₂O₃ catalysts. Comparison of TOF ~ f(d_m) relationships revealed significant difference between Pt and Pd catalysts. For Pt catalyst TOF showed tendency to increase by 2–3 times with increasing particle size from 1 to ca 3 nm and remained constant, when Pt particles became larger than 3 nm. On the other hand, for Pd catalyst TOF increased almost linearly when particle size grew from 1 to 20 nm. These different tendencies were attributed to the different mechanisms of CH₄ oxidation over Pt and Pd catalysts: Langmuir–Hinshelwood and Mars-Van Krevelen respectively.     

Preparation of Highly Active Alumina-Pillared Clay Montmorillonite-Supported Platinum Catalyst for Hydrodesulfurization

Effect of Pt precursor and pretreatment on hydrodesulfurization (HDS) activity of Pt/Al-PILM catalyst was examined to prepare highly active Pt-supported HDS catalyst. The order of HDS activities of Pt/alumina-pillared clay montmorillonite (Al-PILM) catalysts prepared by various Pt precursors was Pt(C₅H₇O₂)₂ > H₂PtCl₆ · 6H₂O > [Pt(NH₃)₄](NO₃)₂ > [Pt(NH₃)₄]Cl₂ · H₂O > H₂Pt(OH)₆. This order was in accordance with that of Pt dispersion. Thus, high Pt dispersion is essential factor to prepare highly active Pt/Al-PILM catalyst for HDS reaction. On the other hand, the effect of pretreatment on the HDS activities of Pt/Al-PILM catalysts prepared by various Pt precursors was also evaluated. The UC-TPS Pt/Al-PILM catalyst showed the highest HDS activity among various pretreated Pt/Al-PILM catalysts, in which uncalcined catalyst was sulfided by temperature programmed sulfidation (TPS). We assumed that high HDS activity of UC-TPS Pt/Al-PILM catalyst is caused by partly sulfided Pt particle with high dispersion. It is concluded that the highly active Pt/Al-PILM catalyst for the HDS reaction could be prepared by using Pt(C₅H₇O₂)₂ as a precursor and UC-TPS treatment.     

Pt-Promoted Cu/SBA-15 Catalysts with Excellent Performance for Chemoselective Hydrogenation of Dimethyl Oxalate to Ethylene Glycol

Cu/SBA-15 catalysts containing a small amount of Pt (Cu–Pt/SBA-15) were prepared by sequential adsorption–reduction method and examined for chemoselective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). The Cu–Pt/SBA-15 catalyst with an optimal Cu/Pt atomic ratio of 10 showed a DMO conversion close to 100 % with a 98 % selectivity to EG at a temperature as low as 463 K. The results showed that the best Cu–Pt/SBA-15 enhanced the space time yield of EG by about 1.47 times compared with Cu/SBA-15. The introduction of Pt with stronger ability for H₂ adsorption and activation substantially enhanced the reducibility of the Cu²⁺ species and further promoted the chemisorption capacity of H₂. After reduction, a portion of Cu was alloyed with Pt, which was beneficial for the generation and stabilization of a balanced Cu⁺/Cu⁰ ratio during the hydrogenation process.     

Cyclohexanone Conversion Catalyzed by Pt/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Evidence of Oxygen Removal and Coupling Reactions

Abstract  

The conversion of cyclohexanone, often identified as an intermediate in the conversion of lignin-derived compounds, was catalyzed by Pt/γ-Al₂O₃ in the presence of H₂ at 573 K. Dehydrogenation was a kinetically significant reaction, indicated by a high selectivity for phenol. Oxygen-removal reactions are indicated by products including benzene, cyclohexene, and cyclohexene. Bimolecular reactions involving cyclohexanone and/or products of its conversion led to the formation of bicyclic C₁₂ compounds, with 2-cyclohexylcyclohexan-1-one and 2-phenylphenol being the most abundant. Increasing the H₂ partial pressure led to increased oxygen removal and faster formation of monocyclic and bicyclic hydrocarbons. At temperatures higher than 573 K, dehydrogenation became the dominant reaction class.     

In situ d electron density of Pt particles on supports by XANES

The dependency of d electron density of Pt in Pt/SiO₂ catalysts on the particle size was investigated by means of in situ X-ray absorption near-edge structure (in situ XANES) spectroscopy. The d electron density of Pt particles was measured under vacuum, H₂ and ethene, to gain information about ethene hydrogenation on Pt/SiO₂. The intensities of the white lines at L_III and L_II edges in XANES spectra, which are regarded to reflect the unoccupied density of state, varied with the change of particle size under both vacuum and reaction gas atmospheres. The interaction between Pt particle and adsorbates was weak with small particles below 1.5 nm. A new peak induced by Pt-H bonding in the XANES spectra under H₂ was observed for the samples with Pt particle size 1.5 nm. This is related to the change of the turnover frequency and activation energy for ethene hydrogenation by Pt particle size.     

Structure-Sensitivity of Propylene Hydrogenation over Cluster-Derived Bimetallic Pt-Au Catalysts

A series of Pt and Pt-Au catalysts supported on TiO₂ has been studied using C₃H₆ hydrogenation as a probe reaction to determine the composition of the active catalytic surface. The catalysts were characterized by H₂ chemisorption and TEM analysis to determine concentrations of surface Pt sites for TOF calculations and metal particle size distributions, respectively. Similar TOF values for C₃H₈ formation (approximately 30 sec⁻¹) were observed for a monometallic Pt/TiO₂ and a bimetallic Pt–Au/TiO₂ sample prepared by impregnation from individual salt precursors. In contrast, the TOF for C₃H₈ formation over a Pt₂Au₄/TiO₂ sample prepared from an organometallic Pt₂Au₄ cluster precursor was decreased to 0.07 sec⁻¹, suggesting strong structure sensitivity for the hydrogenation reaction over this catalyst. Characterization results indicate that Pt on the surface of the Pt₂Au₄/TiO₂ catalyst is heavily diluted by Au atoms. In combination with the kinetic results, this suggests that the highly diluted surface ensembles of Pt are too small to effectively catalyze C₃H₆ hydrogenation, although electronic effects induced by the presence of Au adjacent to Pt sites can not be excluded.     

Decomposition of Propane and Its Reactions with CO2 Over Alumina-Supported Pt Metals

Propane underwent dehydrogenation and cracking on alumina-supported Pt metals at 823-923 K. Propylene formed with highest selectivity (50-80%) on Pt/Al₂O₃. The deposition of carbonaceous species was observed on every sample, the hydrogenation of which occurred only above 700-750 K with a peak temperature of 900-950 K. The presence of CO₂ basically altered the reaction pathway of propane, and the formation of CO and H₂ came into prominence. The highest specific rates for the production of H₂ and CO were measured for the Ru and Rh catalysts.     

Partial hydrogenation of benzene catalyzed by Pt/N‐n‐propyl chitosan hybrid membrane

The partial hydrogenation of benzene by a Pt nano‐cluster/N‐n‐propyl chitosan hybrid membrane was investigated in this article. Monodispersed Pt nano‐clusters were prepared by the reduction of H₂PtCl₆ with ethylene glycol under microwave conditions. TEM, FTIR, XRD, ¹H‐NMR, and XPS were used to characterize the structure of Pt nano‐particles, N‐n‐propyl chitosan and Pt/N‐n‐propyl chitosan hybrid membrane, respectively. Experimental results showed that Pt/N‐n‐propyl chitosan hybrid membrane catalyst gave a high selectivity for cyclohexene of 85.2% in the liquid phase hydrogenation of benzene, while the selectivity of cyclohexene was only 58.2% over the Pt/chitosan hybrid membrane catalyst. It was worth noting that there was no cyclohexene in the product when the catalyst was only Pt nano‐particles without chitosan hybrid membrane. So the chitosan or modified‐chitosan membranes played an important role in the controlling to the hydrogenation of benzene, and the relationship of the swelling degree and the catalytic activity was discussed in detail. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Cobalt Particle Size Effects in the Fischer–Tropsch Synthesis and in the Hydrogenation of CO2 Studied with Nanoparticle Model Catalysts on Silica

We have investigated the effect of cobalt nanoparticle size in Fischer–Tropsch synthesis (CO/H₂) and have compared it to data obtained for carbon dioxide hydrogenation (CO₂/H₂) using model catalysts produced by colloidal methods. Both reactions demonstrated size dependence, in which we observed an increase of the turnover frequency with increasing average particle size. In both case, a maximum activity was found for cobalt particles around 10–11 nm in size. Regarding the selectivity, no size-dependent effect has been observed for the CO₂ hydrogenation, whereas CO hydrogenation selectivity depends both on the temperature and on the size of the particles. The hydrogenation of CO₂ produces mainly methane and carbon monoxide for all sizes and temperatures. The Fischer–Tropsch reaction exhibited small changes in the selectivity at low temperature (below 250 °C) while at high temperatures we observed an increase in chain growth with the increase of the size of cobalt particles. At 250 °C, large crystallites exhibit a higher selectivity to olefin than to the paraffin equivalents, indicating a decrease in the hydrogenation activity.     

Different Catalytic Reactions of n-Hexane and 1-Hexene on Molybdenum Based Catalysts

Controlled reduction by hydrogen, of the equivalent five monolayer of MoO₃ deposited on TiO₂ as a function of the reduction temperature up to 873 K, enabled us to obtain three Mo₂O₅, bifunctional (metal-acid) MoO₂(H_x)_ac and the metallic Mo(0) phases. Characterization of these phases was made by employing surface XPS-UPS techniques in parallel with catalytic reactions. Hydroisomerization of n-hexane occurs on the bifunctional phase, while hydrogenation/ dehydrogenation and benzene formation were performed by the metallic Mo state.     

Catalytic Activity of a Thermoregulated, Phase-Separable Pd(II)-perfluoroalkylphthalocyanine Complex in an Organic/Fluorous Biphasic System: Hydrogenation of Olefins

A fluoroalkene-soluble tetrakis[heptadecafluorononyl]-substituted Pd(II)-phthalocyanine complex has been studied for olefin (styrene, 1-octene, trans-2-octene and cyclohexene) hydrogenation with molecular hydrogen in an organic/fluorous biphasic system [n-hexane/perfluoromethylcyclohexane (PFMCH)]. The palladium complex was found to be an active catalyst for styrene (100% conversion, TON = 634) and 1-octene (92%, TON = 596) at 80 °C and 15 bar of H₂ after 6 h of reaction time. The catalyst was recycled in nine consecutive reactions for the hydrogenation of styrene without the loss of activity or metal contamination.