Molecular surface studies of adsorption and catalytic reaction on crystal (Pt,Rh) surfaces under high pressure conditions (atmospheres) using sum frequency generation (SFG) – surface vibrational spectroscopy and scanning tunneling microscopy (STM)

Sum frequency generation (SFG) – surface vibrational spectroscopy and the scanning tunneling microscope (STM) have been used to study adsorption and catalyzed surface reactions at high pressures and temperatures using (111) crystal surfaces of platinum and rhodium. The two techniques and the reaction chambers that were constructed to make these studies possible are described. STM and SFG studies of CO at high pressures reveal the high mobility of metal atoms, metal surface reconstruction, ordering in the adsorbed molecular layer, and new binding states for the molecule. CO oxidation occurs at high turnover rates on Pt(111). Different adsorbed species are observed above and below the ignition temperature. Some inhibit the reaction, and others are reaction intermediates since their surface concentration is proportional to the reaction rate. The dehydrogenation of cyclohexene on Pt(100) and Pt(111) proceeds through a 1,3‐cyclohexadiene surface intermediate. The higher dehydrogenation rate is related to the higher surface concentration of these molecules on the (100) crystal face. This revised version was published online in August 2006 with corrections to the Cover Date.     

Gas‐phase pinacol conversion on AlPO4, γ‐Al2O3 and SiO2 catalysts

Pinacol (2,3‐dimethyl‐2,3‐butanediol) conversion over AlPO₄ (Al/P = 1) and γ‐Al₂O₃ catalysts proceeded in two parallel reaction pathways with formation of 2,3‐dimethyl‐1,3‐butadiene (by 1,2‐elimination) and 3,3‐dimethyl‐2‐butanone (by rearrangement), the latter being the main reaction product. The activity was in accordance with the surface acidity data as measured versus cyclohexene skeletal isomerization reaction. Thus, AlPO₄ showed the highest activity (almost total conversion at 523 K). The 1,2‐elimination/rearrangement ratio depended on the type of catalyst used and diene formation increased with reaction temperature. Moreover, pinacolone was a reaction intermediate for diene production. This revised version was published online in July 2006 with corrections to the Cover Date.     

TPD study about the surface modification of some Ni/spinel catalysts in the hydrodechlorination of 1,2,4‐trichlorobenzene. Influence on hydrogenation ability

NiAl₂O₄ supports and fresh and reactivated Ni/NiAl₂O₄ catalysts were tested in the gas phase hydrodechlorination of 1,2,4‐trichlorobenzene. Fresh catalysts hydrogenate 1,2,4‐trichlorobenzene to cyclohexane in the first 30 min of reaction at 523 K. An irreversible partial chlorination of the catalytic surface makes the hydrogenation of the aromatic ring difficult. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nonoxidative dehydrogenation and aromatization of methane over W/HZSM‐5‐based catalysts

Highly active and heat‐resisting W/HZSM‐5‐based catalysts for nonoxidative dehydro‐aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W−H₂SO₄/HZSM−5 catalyst prepared from a H₂SO₄‐acidified solution of ammonium tungstate (with a pH value at 2–3) displayed rather high DHAM activity at 973–1023 K, whereas the W/HZSM‐5 catalyst prepared from an alkaline or neutral solution of (NH₄)₂WO₄ showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of (WO₆)^n- groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H₂‐TPR results showed that the reduction of precursor of the 3% W–H₂SO₄/HZSM‐5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W⁵⁺ and W⁴⁺, whereas the reduction of the 3% W/HZSM‐5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W⁵⁺. The results seem to imply that the observed high DHAM activity on the W–H₂SO₄/HZSM‐5 catalyst was closely correlated with (WO₆)^n- groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W–H₂SO₄/HZSM‐5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W–1.5% Zn–H₂SO₄/HZSM‐5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV=1500 ml/(h g−cat.), methane conversion (XCH₄) reached 23% with the selectivity to benzene at ∼96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Thiophene hydrodesulfurization over modified alumina-supported molybdenum sulfide catalysts

Methanethiol has been synthesized by one‐step catalytic reaction from H₂S‐content syngas on K₂MoS₄/SiO₂ catalyst with selectivity over 95% under the optimum reaction conditions of 563 K, 2.0–3.0 MPa and 5–6% H₂S content in the feed syngas. The results of XRD and XPS showed that Mo–S–K phase on the surface of the catalyst K₂MoS₄/SiO₂ was responsible for the high activity and selectivity to methanethiol, and which may be restrained by the existence of (S–S)^2- species. This revised version was published online in July 2006 with corrections to the Cover Date.     

n‐octane reforming over alumina‐supported Pt, Pt–Sn and Pt–W catalysts

Pt, Pt–Sn and Pt–W supported on γ‐Al₂O₃ were prepared and characterized by H₂ chemisorption, TEM, TPR, test reactions of n‐C₈ reforming (500°C), cyclohexane dehydrogenation (315°C) and n‐C₅ isomerization (500°C), and TPO of the used catalysts. Pt is completely reduced to Pt⁰, but only a small fraction of Sn and of W oxides are reduced to metal. The second element decreases the metallic properties of Pt (H₂ chemisorption and dehydrogenation activity) but increases dehydrocyclization and stability. In spite of the large decrease in dehydrogenation activity of Pt in the bimetallics, the metallic function is not the controlling function of the bifunctional mechanisms of dehydrocyclization. Pt–Sn/Al₂O₃ is the best catalyst with the highest acid to metallic functions ratio (due to its lower metallic activity) presenting a xylenes distribution different from the other catalysts. The acid function of Pt–Sn/Al₂O₃ is tuned in order to increase isomerization and cyclization and to decrease cracking, as compared to Pt and Pt–W. 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.     

Formation of a liquid film of AgNO3 on a silver surface

The behavior of a AgNO₃/Ag₂O/Ag “sandwich” upon heating in vacuum was studied by in situ X‐ray photoelectron spectroscopy (XPS) and ex situ scanning electron microscopy (SEM). The AgNO₃/Ag₂O/Ag “sandwich” was prepared by exposure of a silver foil to a NO : O₂ mixture. The upper layer of the “sandwich” consists of AgNO₃ crystals of a mean size between 0.1 and 0.4 μm. Heating at 550 K in vacuum results in melting of the AgNO₃ crystals. A liquid film of AgNO₃, readily wetting the silver, covers the surface. Cooling below the melting point of AgNO₃ leads to the agglomeration of silver nitrate to long islands with a size reaching a few tens of micrometers (μm). The possible effects of AgNO₃ liquid‐phase formation on surface processes are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

An in situ infrared study of NO reduction by C3H8 over Fe‐ZSM‐5

The interactions of NO, O₂ and NO₂ with Fe‐ZSM‐5, as well as the reduction of NO by C₃H₈ in the presence of O₂, have been investigated using in situ infrared spectroscopy. The sample of Fe‐ZSM‐5 (Fe/Al =0.56) was prepared by solid‐state ion exchange. NO adsorption in the absence of O₂ produces only mono‐ and dinitrosyl species associated with Fe²⁺ cations. Adsorbed NO₂/NO₃ species are formed via the reaction of adsorbed O₂ with gas‐phase NO or by the adsorption of gas‐phase NO₂. The reduction of NO in the presence of O₂ begins with the reaction of gas‐phase C₃H₈ with adsorbed NO₂/NO₃ species to form a nitrogen‐containing polymeric species. A reaction pathway is proposed for the catalyzed reduction of NO by C₃H₈ in the presence of O₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbonyl‐precursor‐based W/Al2O3 and CoW/Al2O3 catalysts: characterization by temperature‐programmed methods

Carbonyl‐precursor‐based W/Al₂O₃ and bimetallic CoW/Al₂O₃ catalysts were prepared by gas‐phase adsorption in a fluidized‐bed reactor. The surface species formed during the gradual and controlled preparation process were studied by temperature‐programmed methods. Interactions on the surface were investigated as a function of metal loading by temperature‐programmed oxidation (TPO) and oxygen pulse chemisorption (PCO). A clear relationship was observed between decarbonylation treatment and the tungsten species formed. Total acidity of the samples was determined by temperature‐programmed desorption of ammonia (NH₃‐TPD). The NH₃‐TPD measurements, together with previous activity studies, suggest a relationship between total acidity and hydrotreating activity. The results of PCO and NH₃‐TPD measurements indicate that when the controlled gas‐phase preparation method is applied to zerovalent carbonyl precursor, the unfavourable formation of tungsten oxide can be minimized. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization and catalytic properties of Ti‐ZSM‐5 prepared by chemical vapor deposition

Ti‐ZSM‐5 is prepared via a chemical vapor deposition method by reacting HZSM‐5 with TiCl₄ at temperatures of 200–400°C. Ti‐ZSM‐5 is characterized by skeletal‐ and surface hydroxy‐FT‐IR, XPS, and XANES spectroscopy. It seems that Ti is incorporated in the zeolite surface with tetrahedral coordination. Contents of incorporated Ti atoms in Ti‐ZSM‐5 zeolite increase with increasing SiO₂/Al₂O₃ ratio and a CVD reaction temperature of 400°C is optimal. Cyclohexanone ammoximation was used as test reaction and Ti‐ZSM‐5 catalysts show similar catalytic activities compared to TS‐1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis, characterization and cyclohexene hydrogenation activity of high surface area molybdenum disulfide catalysts

An ammonium tetrathiomolybdate (ATTM) catalyst precursor is synthesized and then thermally decomposed at different temperatures in N₂ or H₂ atmosphere. Characterization of the resulting compounds by powder X-ray diffraction (XRD) and surface area analysis indicates the formation of MoS₂–2H with a surface area of 5–9 m²/g. When ATTM is treated with cetyltrimethylammonium chloride and then decomposed in N₂ at 723 K, the resulting material has a surface area nearing 200 m²/g. If treatment also includes hydrazine, the surface area of the resulting MoS₂–2H reaches 215 m²/g. Analysis by XRD and electron microscopy shows a noticeable dispersion in the layers of the resulting MoS₂. The catalytic activity of the materials is tested in a batch reactor for cyclohexene hydrogenation, where the highest activity sulfides are those obtained by thermal decomposition of the chemically treated precursors in N₂.     

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.     

Coke formation on an industrial reforming Pt–Sn/γ‐Al2O3 catalyst

The characterization of the coke deposited on an industrial Pt–Sn/γ‐Al₂O₃ catalyst, used in a continuous reforming process, was performed with AFM, XRD, FTIR, EPR, NMR, TG‐DTG and DTA techniques. Composition, structure and location of the coke on the catalyst were investigated. The coke was predominantly deposited on the catalyst surface and in the interstices between the catalyst particles. Its content increased along the reactor from top to bottom. Coke was deposited in the form of uniform films and clusters of three‐dimensional disks with diameters between 0.12 and 0.18 μm. It had a pseudo‐graphite structure produced by the dehydrogenation and polymerization of the aromatic precursor compounds. The coked catalyst showed a good combustion behavior; it was regenerated below 550°C. These results are important to elucidate the coke formation mechanism, to generate new continuous reforming catalysts, and to optimize the reactor operation parameters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modification of the alumina‐supported Mo‐based hydrodesulfurization catalysts by tungsten

The incorporation effect of tungsten as an activity‐promotional modifier into the Ni‐promoted Mo/γ‐Al₂O₃ catalyst was studied. Series of W‐incorporated catalysts with different content of tungsten were prepared by changing the impregnation order of nickel and tungsten onto a base Mo/γ‐Al₂O₃. Catalytic activities were measured from the atmospheric reactions of thiophene hydrodesulfurization (HDS) and ethylene hydrogenation (HYD). The HDS and HYD activities of the WMo/γ‐Al₂O₃ catalysts (WM series) initially increased and subsequently decreased with increasing content of tungsten as compared with those of their base Mo/γ‐Al₂O₃. The maximal activity promotion occurred at the W/(W + Mo) atomic ratio 0.025. For the Ni‐promoted Mo/γ‐Al₂O₃ catalysts, the effect of W incorporation was greatly dependent on the impregnation order of tungsten. The catalysts prepared by impregnating Ni onto the WMo/γ‐Al₂O₃ catalysts showed the same trend of activity promotion as for the WM series, while those by impregnating W onto a NiMo/γ‐Al₂O₃ catalyst resulted in lower activities than their base NiMo/γ‐Al₂O₃ catalyst. To characterize the catalysts, temperature‐programmed reduction and low‐temperature oxygen chemisorption were conducted. The effects of W incorporation on the NiMo‐based catalysts were discussed in reference to those on the CoMo‐based catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Highly active and moisture‐insensitive solid catalysts – GaCl3 and InCl3 supported on montmorillonite‐K10 and Si‐MCM‐41 for benzylation of benzene

InCl₃ and GaCl₃ supported on montmorillonite‐K10 or on high silica mesoporous MCM‐41 show very high activity in the benzylation of benzene by benzyl chloride (at 80°C) with little or no effect in the presence of moisture in the catalyst or in the reaction mixture on their benzylation activity; the supported InCl₃ catalyst shows superior performance in the benzylation reaction in regard to both the activity and moisture insensitivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Theoretical studies of CO hydrogenation to methanol over Cu, Pd, and Pt metals

Theoretical studies of CO hydrogenation to methanol over Cu, Pd, and Pt metals have been carried out using a quasi‐relativistic density‐functional method. The metal surface is simulated by a M₁₀ cluster model. Reaction energies for the elementary steps involved are determined. The activation energies are estimated by the analytic BOC‐MP formula. The results support that these metals are active in CO hydrogenation to methanol. The rate‐determining steps are shown to be different for the metals. The highest activation energies of reaction on the metals fall in the order Cu < Pd < Pt, which corresponds to the order of the catalytic activities of the metals in CO hydrogenation. 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.