Showing papers in "Catalysis Letters in 1998"

Photocatalytic O2 evolution under visible light irradiation on BiVO4 in aqueous AgNO3 solution

Abstract: BiVO4 with a 2.3 eV band gap showed an activity for O2 evolution from aqueous solutions containing Ag+ as an electron scavenger under visible light irradiation (λ > 520 nm). The quantum yield was 0.5% at 450 nm.

Chemical vapor deposition of gold on Al2O3, SiO2, and TiO2 for the oxidation of CO and of H2

Abstract: In order to clarify the effect of metal oxide support on the catalytic activity of gold for CO oxidation, gold has been deposited on SiO2 with high dispersion by chemical vapor deposition (CVD) of an organo-gold complex. Comparison of Au/SiO2 with Au/Al2O3 and Au/TiO2, which were prepared by both CVD and liquid phase methods, showed that there were no appreciable differences in their catalytic activities as far as gold is deposited as nanoparticles with strong interaction. The perimeter interface around gold particles in contact with the metal oxide supports appears to be essential for the genesis of high catalytic activities at low temperatures.

Structure sensitivity of CO oxidation over model Au/TiO 2 2 catalysts

Abstract: Model catalysts of Au clusters supported on TiO2 thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO2 catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)-1 × s-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO2(001)/Mo(100) is structure sensitive.

FT-IR study of NO + O2 co-adsorption on H-ZSM-5: re-assignment of the 2133 cm-1 band to NO+ species

Abstract: Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O2 to NO led to the appearance of bands at 2133 and 977 cm-1. Concomitantly, the number of acidic zeolite OH groups decreased while H2O hydrogen-bonded to zeolite OH groups developed. Introduction of small amounts of 18O2 did not change the 2133 cm-1 band wavenumber, nor the use of a partly deuteroxylated D–H-ZSM-5 sample. In such a case, HOD formation was detected. The results obtained evidence that the 2133 cm-1 band, generally considered as characterizing NO+2 species, is, in fact, due to NO+ species occupying cationic positions in the zeolite. The 977 cm-1 band is attributed to the Olattice–NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.

New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Abstract: Pd and Pt supported on ZnO, Ga2O3 and In2O3 exhibit high catalytic performance for the steam reforming of methanol, CH3OH+H2O→CO2+3HH2, and the dehydrogenation of methanol to HCOOCH3, 2CH3OH→HCOOCH3+2HH2. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.

The effect of ZnO in methanol synthesis catalysts on Cu dispersion and the specific activity

Abstract: The effect of ZnO in Cu/ZnO catalysts prepared by the coprecipitation method has been studied using measurements of the surface area of Cu, the specific activity for the methanol synthesis by hydrogenation of CO2, and XRD. Although the Cu surface area increases with increasing ZnO content (0–50 wt%) as is generally known, the specific activity of the Cu/ZnO catalysts with various weight ratios of Cu:ZnO is greater than that of a ZnO-free Cu catalyst. These facts clearly indicate that the role of ZnO in Cu/ZnO catalysts can be ascribed to both increases in the Cu dispersion and the specific activity. The XRD results indicate the formation of a Cu–Zn alloy in the Cu particles of the Cu/ZnO catalysts, leading to the increase in specific activity. It is thus considered that the Cu–Zn surface alloy or a Cu–Zn site is the active site for methanol synthesis in addition to metallic copper atoms that catalyze several hydrogenation steps during the methanol synthesis. Furthermore, the advantage of the coprecipitation method through a precursor of aurichalcite is ascribed to both improvements in the Cu surface area and the specific activity.

Synthesis of glycerol carbonate by direct carbonatation of glycerol in supercritical CO2 in the presence of zeolites and ion exchange resins

Abstract: Glycerol carbonate is a key bifunctional compound employed as solvent, additive, monomer, and chemical intermediate. We have synthesized it on a pilot scale in the laboratory in cyclic or alicyclic organic carbonate medium. In this study, we examined the use of supercritical CO2 as a reaction medium and as a source of carbonate for carbonatation of glycerol. Glycerol carbonate could be obtained by direct reaction of carbon dioxide with an organic carbonate in the presence of heterogeneous catalysts. Carbonatation of glycerol into glycerol carbonate went to equilibrium in supercritical CO2 medium.

Effect of calcination temperature on the catalytic activity of Au colloids mechanically mixed with TiO2 powder for CO oxidation

Abstract: In order to elucidate the role of the contact structure between gold and metal oxide support in low-temperature CO oxidation, a mechanical mixture of colloidal gold with TiO2 powder was prepared and calcined at different temperatures. The sample calcined at 473 K, which is composed of spherical gold particles with a mean diameter of 5.1 nm and TiO2 powder, is poorly active for CO oxidation at temperatures up to 473 K. The catalytic activity appreciably increases with an increase in calcination temperature up to 873 K even though gold particles grow to larger ones, reaching a level with almost the same turnover frequency as that of Au/TiO2 prepared by a deposition–precipitation method.

Characterization of surface carbon formed during the conversion of methane to benzene over Mo/H-ZSM-5 catalysts

Abstract: During the conversion of methane to benzene in the absence of oxygen over a 2 wt% Mo/H-ZSM-5 catalyst at 700°C, three different types of surface carbon have been observed by X-ray photoelectron spectroscopy: adventitious or graphitic-like C (284.6 eV), carbidic-like C (282.7 eV), and hydrogen-poor sp-type C (283.2 eV), where the C 1s binding energies for the respective forms of carbon are given in parentheses. Pretreatment of the catalyst at 700°C in CO also resulted in a strong signal at 283.2 eV; thus, the species responsible for this signal appears to be different from the usual aromatic-type coke. The coke with dominantly sp hybridization is concentrated on the external surface of the zeolite and is responsible for the gradual deactivation of the catalyst.

Enhancement in methanol oxidation by spontaneously deposited ruthenium on low-index platinum electrodes

Abstract: Using Auger electron spectroscopy and electrochemistry we confirm that ruthenium is spontaneously deposited on platinum single-crystal faces from ruthenium(III) chloride solutions in perchloric acid The deposit is catalytically active towards methanol present in electrolytic media On the Pt(111) surface - the most active catalyst for the oxidation process - the data show that the packing density of ruthenium is 99±15% and increases with ruthenium concentration in the electrolytic bath, confirming the previous electrochemical coverage estimate In the most favorable instances the ruthenium enhancement factor, the current ratio due to methanol oxidation with and without ruthenium on the surface, is equal to15 (at 0490 V vs RHE reference on Pt(111)) We believe this study is an important addition to our long-range efforts in investigating surface structure effects in platinum-ruthenium heterogeneous electrocatalysis

Partial oxidation of methane to syngas over Ni/MgO, Ni/CaO and Ni/CeO2

Abstract: Partial oxidation of methane to syngas at atmospheric pressure and 750°C was examined over Ni/MgO, Ni/CaO and Ni/CeO2 catalysts with nickel loading of 13 wt%. All catalysts had similar high conversion of methane and high selectivity to syngas, which nearly approached the values predicted by thermodynamic equilibrium. However, only Ni/MgO showed high resistance to carbon deposition under thermodynamically severe conditions (CH4/O2 = 2.5, a higher CH4 to O2 ratio than the stoichiometric ratio). Its catalytic activity remained stable during 100 h of reaction, with no detectable carbon deposition. The oxidation of carbon deposited from pure CH4 decomposition and from pure CO disproportionation was investigated by in situ TPO-MS study which showed that both were effectively inhibited over Ni/MgO. In addition, the catalysts were characterized by TPR, XRD and XPS. It was revealed that the excellent performance of Ni/MgO resulted from the formation of an ideal solid solution between NiO and MgO.

Aromatization of methane in the absence of oxygen over Mo-based catalysts supported on different types of zeolites

Abstract: The catalytic performance of Mo-based catalysts supported on various zeolites has been studied for methane aromatization in the absence of oxygen in a fixed-bed continuous-flow quartz reactor, and their catalytic properties are correlated with features of zeolite structure. It was found that H-type silica–alumina zeolites, such as ZSM-5, ZSM-8, ZSM-11 and β possessing two-dimensional structure and pore diameter equaling the dynamic diameter of a benzene molecule (about 6 A) simultaneously, are fine supports for methane activation and aromatization catalysts. Among them, MoO3/H-ZSM-11 has the best activity and stability; for instance, a methane conversion of 8.0% and selectivity higher than 90% was obtained at 973 K. The catalytic performance of MoO3/H-ZSM-8 is somewhat lower than that of MoO3/H-ZSM-5, while activity of MoO3/H-β is lower than that of MoO3/H-ZSM-8. Catalysts supported on H-MCM-41 and H-SAPO-34 exhibit low activity for methane aromatization and those supported on H-MOR, H-X and H-Y give only a little amount of ethylene. Over MoO3/H-SAPO-5 and MoO3/H-SAPO-11 no hydrocarbons were detected.

Promoted Fe/ZSM-5 catalysts prepared by sublimation : de-NOx activity and durability in H2O-rich streams

Abstract: Fe/ZSM-5 catalysts with an Fe/Al ratio 1:0, were prepared by sublimation of FeCl3 into H/ZSM-5 They display high activity and durability for the selective catalytic reduction of NOx to N2, both in dry and wet gas flows These catalysts have now been modified by exchanging a second cation into the zeolite Mere neutralization of zeolite protons by Na+ lowers the selectivity for NOx reduction to N2, but the cations Ce3 + and La3 + act as true catalyst promoters With isobutane as the reductant in a simulated vehicular emission gas, almost 90% of NOx is reduced to N2 at 350°C over the La-promoted catalyst The presence of 10% H2O in the feed does not impair the catalyst performance at high temperature; in the temperature region below 350°C it even increases the N2 yield The beneficial effect of La is due to its lowering of the catalyst activity for the undesired combustion of the hydrocarbon No signs of zeolite destruction are evident after 100 h TOS in a wet gas flow at 350°C Carbonaceous deposits causing a slight deactivation are easily removed in an O2/He flow at 500°C; this in situ regeneration fully restores the original activity

Characterization study of CeO2 supported Pd catalyst for low-temperature carbon monoxide oxidation

Abstract: Catalysts consisting of palladium supported on cerium dioxide (Pd/CeO2) were prepared and used for carbon monoxide oxidation in a stoichiometric mixture of carbon monoxide and oxygen. Pd/CeO2 exhibits high catalytic activity for the oxidation of CO, showing markedly enhanced catalytic activities due to the combined effect of palladium and cerium dioxide. The Pd/CeO2 catalyst is superior to Pd/ZrO2, Pd/Al2O3, Pd/TiO2, Pd/ZSM-5 and Pd/SiO2 catalysts with regard to the activity under the conditions examined. The catalysts were characterized by means of XRD and TPR. The position of the H2-TPR peak shifts to lower temperature with increasing Pd loading from 0.25 to 2.0%. CeO2 inhibits the hydrogen reduction of PdO. CO-TPR measurements have shown the existence of three peaks. The low-temperature peak (α) is due to the Pd hydroxide species. The β peak has been attributed to finely dispersed PdO. The high-temperature peak (γ) has been attributed to crystal phase PdO. Crystal phase PdO is more difficult to reduce by CO than finely dispersed PdO. On the basis of the catalytic activity and CO-TPR results, we conclude α species (Pd hydroxide) mainly contribute to the catalytic activity for low-temperature CO oxidation.

CO adsorption and oxidation on Au/TiO2

Abstract: Preoxidized Au/TiO2 showed no initial activity during a first heating stage up to 70°C, while prereduction yielded a high initial CO conversion at room temperature. With FTIRS, two different CO absorption bands were detected. One band is usually attributed to CO on an oxidic gold species (2151 cm-1), the other one is characteristic of CO on metallic gold (2112 cm-1). The presence of the first species appears to have a detrimental effect on the CO oxidation by O2. The present results do not support a model in which the activity of supported gold catalysts in CO oxidation is ascribed to ionic Au particles.

Stable carbon dioxide reforming of methane over modified Ni/Al2O3 catalysts

Abstract: CO2 reforming of methane was studied over modified Ni/Al2O3 catalysts. The metal modifiers were Co, Cu, Zr, Mn, Mo, Ti, Ag and Sn. Relative to unmodified Ni/Al2O3, catalysts modified with Co, Cu and Zr showed slightly improved activity, while other promoters reduced the activity of CO2 reforming. Mn-promoted catalyst showed a remarkable reduction in coke deposition, while entailing only a small reduction in catalytic activity compared to unmodified catalyst. The catalysts prepared at high calcination temperatures showed higher activity than those prepared at low calcination temperature. The Mn-promoted catalyst showed very low coke deposition even in the absence of diluent gas and the activity changed only slightly during 100 h operation.

Kinetic studies of methane steam reforming on ceria-supported Pd

Abstract: Steady-state, steam-reforming rates for methane were measured on model and high-surface-area, ceria-supported Pd catalysts and compared to rates on Pd/silica between 600 and 825 K. Specific rates on Pd/ceria were higher than those on Pd/silica by a factor of 10 5 and were higher even than rates reported in the literature for Ni catalysts. Model catalysts prepared with Pt or Rh on ceria also showed very high rates, essentially identical to rates for Pd/ceria. However, catalysts prepared by addition of Pd to ceria which had been calcined to high temperatures (>1175 K) before the addition of Pd were not active. Based on these results, we suggest that steam reforming on ceria-supported catalysts occurs through a bifunctional mechanism in which oxygen from ceria reacts with dissociated methane on the precious metal. High-temperature calcination of ceria significantly changes its reducibility, so that the reaction is not enhanced.

Ti-containing MCM-41 catalysts for liquid phase oxidation of cyclohexene with aqueous H2O2 and tert-butyl hydroperoxide

Abstract: Framework Ti-substituted and Ti-grafted MCM-41 mesoporous material has been prepared by direct hydrothermal synthesis and a post-synthesis grafting method. The materials have been tested as catalysts for cyclohexene oxidation with aqueous H2O2 and tert-butyl hydroperoxide (TBHP). With aqueous H2O2 in methanol, the major products were cyclohexene diol and its methyl ethers. No cyclohexene oxide was produced. Titanium leaching was a serious problem, and the catalyst lost its activity irreversibly after only one cycle of reaction. With TBHP, the selectivity for cyclohexene oxide was near 100%, titanium leaching was negligible, and the catalyst could be repeatedly used after regeneration without suffering significant activity loss. However, the reaction rate was lower than when H2O2 was used. Framework substituted material and catalysts prepared by Ti-grafting onto a MCM-41 support behaved similar, but the Ti-grafted MCM-41 is somewhat more active. The turnover frequency (TOF) per mole of Ti decreases with an increase of Ti content in the catalyst. This is caused by a reduced Ti dispersion within the silica matrix.

Reduction of lean NOx by ethanol over Ag/Al2O3 catalysts in the presence of H2O and SO2

Abstract: The reduction of lean NOx using ethanol in simulated diesel engine exhaust was carried out over Ag/Al2O3 catalysts in the presence of H2O and SO2. The Ag/Al2O3 catalysts are highly active for the reduction of lean NOx by ethanol but the reaction is accompanied by side reactions to form CH3CHO, CO along with small amounts of hydrocarbons (C3H6, C2H4, C2H2 and CH4) and nitrogen compounds such as NH3 and N2O. The presence of H2O enhances the NOx reduction while SO2 suppresses the reduction. The presence of SO2 along with H2O suppresses the formation of acetaldehyde and NH3. By infrared spectroscopy, it was revealed that the reactivity of NCO species formed in the course of the reaction was greatly enhanced in the presence of H2O. The NCO species readily reacts with NO in the presence of O2 and H2O at room temperature, being converted to N2 and CO2 (CO). Addition of SO2 suppresses the formation of NCO species and lowers the reactivity of the NCO species. However, the reduction of NOx is still kept at high conversion levels in the presence of H2O and SO2 over the present catalysts. About 80% of NOx in the simulated diesel engine exhaust was removed at 743 K.

Enantiodifferentiation in asymmetric sonochemical hydrogenations

Abstract: The effect of sonochemical pretreatment on the enantioselectivity of Pt/Al2O3–cinchonidine-catalyzed ethyl pyruvate hydrogenation was studied at different hydrogen pressures in various solvents, mainly in acetic acid. The sonochemical pretreatment of a commercial Pt/Al2O3–cinchonidine catalytic system in acetic acid resulted in enhanced enantioselectivity providing excellent ee values (97% ee) under mild and widely varied experimental conditions. Moreover, the application of ultrasonics provides a possibility of the catalyst recycling without regeneration. The catalyst was tested by transmission electron microscopy to determine the effect of the sonication on the metal particle size morphology.

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

Abstract: 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.

Redox solid catalysts for the selective oxidation of cyclohexane in air

Abstract: The selective oxidation of cyclohexane to cyclohexanol, cyclohexanone and adipic acid using molecular oxygen as the oxidant and at moderate temperatures (403 K) has been investigated over four different cobalt‐containing aluminophosphate (AlPO) molecular sieves. There is a correlation between catalytic activity and the fraction of (framework) Co(II) ions that is first oxidised to Co(III) in air. CoAlPO‐36 (pore aperture 6.5 x 7.5 A) exhibits significant activity for the oxidation of cyclohexane in contrast to CoAlPO‐18, which, although it has the highest fraction of oxidisable cobalt, does not show any activity chiefly because of its smaller pores.

On the nature of the active site for the ethylbenzene dehydrogenation over iron oxide catalysts

Abstract: The dehydrogenation of ethylbenzene to styrene was studied over single-crystalline iron oxide model catalyst films grown epitaxially onto Pt(111) substrates. The role of the iron oxide stoichiometry and of atomic surface defects for the catalytic activity was investigated by preparing single-phased Fe3O4(111) and α-Fe2O3(0001) films with defined surface structures and varying concentrations of atomic surface defects. The structure and composition of the iron oxide films were controlled by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrahigh vacuum experiments were combined with batch reactor experiments performed in water–ethylbenzene mixtures with a total gas pressure of 0.6 mbar. No styrene formation is observed on the Fe3O4 films. The α-Fe2O3 films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenzene dehydrogenation over unpromoted α-Fe2O3. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits with a photoelectron emission microscope (PEEM). It starts at extended defects and exhibits a pattern formation after further growth. This indicates that the deactivation is a site-selective process. Post-reaction LEED and AES analysis reveals partly reduced Fe2O3 films, which shows that a reduction process takes place during the reaction which also deactivates the Fe2O3 films.

Application of zeolite titanium Beta in the rearrangement of α-pinene oxide to campholenic aldehyde

Abstract: Zeolite titanium Beta is found to be an effective catalyst for the industrially relevant rearrangement of α-pinene oxide to campholenic aldehyde, giving selectivities of up to 89% in the liquid phase. In the gas phase, up to 94% selectivity is obtained at conversions above 95%, which exceeds the best values obtained with homogeneous Lewis acid catalysts. The selectivity obtained is profoundly influenced by the solvent or co‐adsorbate applied.

Solid-state NMR and FT-IR investigation of 12-tungstophosphoric acid on TiO2

Abstract: The chemical and dynamic nature of the tungstophosphoric acid (TPA) species adsorbed on pressed, extruded, and powdered TiO2 support was studied by 31P and 1H solid-state NMR spectroscopy and diffuse reflectance FT-IR. From 31P NMR and FT-IR results the TPA appeared to be present in at least five forms on the titania surface: a bulk salt phase, two weakly bound intact Keggin species, a range of partially fragmented clusters such as the 11-“defect” Keggin ion, and a range of species formed by high or complete fragmentation of the Keggin ion. The relative amount of these species varied depending on the form of the support. Adsorption was incomplete on extruded titania resulting in bulk salt formation. There was almost complete adsorption on pressed titania pellets, yielding predominantly fragmented Keggin units, along with a small amount of bulk salt. However, complete adsorption (and fragmentation) without bulk salt formation was attained by impregnation of powdered TiO2. The acidic nature of the catalysts and the degree of TPA dispersion were investigated by solid-state 1H high-speed MAS-NMR. Pure TPA salt yielded resonances at about 6.5 ppm. The catalysts yielded acidic proton resonances at about 7–8 and 10–11.5 ppm. The broad resonance observed at 7–8 ppm is due to a dispersion of acidic sites on the surface. The intensity of the acidic resonance at around 11 ppm increased with adsorption effectiveness and could, therefore, be related to the formation of hydroxonium ions upon fragmentation of adsorbed Keggin ions.

In situ high-temperature ESR characterization of FeZSM-5 and FeSAPO-34 catalysts in flowing mixtures of NO, C 3 H 6 , and O 2

Abstract: In situ ESR of FeHZSM-5 shows that samples made by sublimation of FeCl3 contain mainly ferric ions in tetrahedral and distorted tetrahedral sites. Above 200°C these ions do not chemisorb water, which confirms the resistance to activity deterioration by water vapor claimed for FeHZSM-5, when used in SCR of NOx. The number of ESR-active Fe3+ ions decreases when moving from strongly oxidizing conditions towards stoichiometry at 500°C. In more reducing atmospheres, i.e., when excess reductant is present, the exposure produces an agglomerated ferromagnetic species, presumably magnetite. This process is irreversible. As the trivalent dispersed iron ions are the catalytically active sites, this transformation makes Fe-containing catalysts vulnerable to accidental but irreversible transformation induced by even mildly reducing conditions. The dispersion of ferric ions in FeSAPO-34 is not as good as in FeHZSM-5 and the reactive Fe3+ in distorted tetrahedral sites is absent. Catalytic oxidation of ethane allows correlation of the EPR results with activity and stability.

Mo incorporation in MCM-41 type zeolite

Abstract: Mo-incorporated MCM-41 has been prepared by direct hydrothermal synthesis. XRD and N2-adsorption measurements showed the characteristics of MCM-41. IR, FT-Raman and UV-VIS DR spectroscopic analyses gave the evidences for the incorporation of Mo in the framework of MCM-41. They are found to be stable and active for cyclohexanol and cyclohexane oxidation reactions with H2O2 as oxidant. Activity of this system has been compared with that of Ti-MCM-41 and molybdena impregnated on pure siliceous MCM-41.

Effect of Ag‐promotion on Pd catalysts by XANES

Abstract: Pd L3 near‐edge absorption measurements (XANES) were performed on four commercial acetylene hydrogenation catalyst samples, with and without the Ag promoter. The Pd L3 edge XANES spectra showed that the Ag‐promoted catalysts have relatively weaker absorption peaks and they follow the same order as the relative commercial performances of the four catalysts studied in terms of selectivity in ethylene purification, which indicates that there are increases in the Pd d‐band electron densities due to the addition of Ag. These results provide a reasonable explanation for the observed improvement in selectivity of the Ag‐promoted acetylene hydrogenation catalysts. The Ag L3 XANES spectra of the supported Pd–Ag catalysts indicate the absence of a white‐line feature which seems to suggest that the charge transferred from Ag to Pd may not be the d‐type.

Preparation of MoS2 and WS2 catalysts by in situ decomposition of ammonium thiosalts

Abstract: The in situ decomposition of ammonium thiometallates during the hydrodesulfurization (HDS) of dibenzothiophene (DBT), to obtain molybdenum disulfide and tungsten disulfide catalysts, was investigated. It was found that very efficient catalysts for the HDS of DBT were obtained by in situ decomposition. Mechanical uniaxial pressing of the precursors (ammonium thiometallates) affected both textural and catalytic properties of the catalysts. Surface areas of molybdenum and tungsten disulfides increased as a function of uniaxial pressing, while catalytic activities went through a maximum. For MoS2, the hydrogenation selectivity was much higher for in situ catalysts than for ex situ ones. For WS2 catalysts, the hydrogenation selectivity was less sensitive to the condition of decomposition (ex situ/in situ). The surface S/M (M = Mo, W) atomic ratio from the Auger signal decreased as a function of uniaxial pressing, while the C/M ratio remained almost constant at 1.6. The best catalyst showed an experimental S/Mo ratio that is slightly higher than the stoichiometric value. The effect of in situ decomposition and mechanical deformation of thiometallate precursors is discussed.

Problems in preparation of FeZSM-5 catalysts

Abstract: FeZSM-5 catalysts for SCR of NO to N2 can be made in a highly durable form that may be useful for automotive exhaust cleanup, but not presently reproducible. Herein we outline the properties and characteristics of good versus poor catalysts and provide results of experiments that effect the properties of the final preparation. It is shown how Mossbauer characterization will be useful in understanding the changes that occur as the catalysts age. The problem has not yet been resolved. This letter is meant to challenge research by others.