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Applied Catalysis A, General (v.326, #1)

Contents (pp. iii-vi).

Editorial Board (pp. co2).

2-Propanol photodegradation over lead niobates under visible light irradiation by Xiukai Li; Tetsuya Kako; Jinhua Ye (pp. 1-7).

Two binary lead niobates, viz. Pb₃Nb₂O₈ and Pb₃Nb₄O₁₃, had been developed as visible-light-sensitive photocatalysts. Pb₃Nb₄O₁₃ showed much higher activity than Pb₃Nb₂O₈ for 2-propanol photodegradation in gas phase. Compared to the solid-state reaction method (SSR), the co-precipitation method (CP) created a catalyst of greater surface area, smaller particle size, and much better photocatalytic activity. ▪Two binary lead niobates, viz. Pb₃Nb₂O₈ and Pb₃Nb₄O₁₃, had been developed as visible-light-sensitive photocatalysts. Their physical characteristics were examined by techniques such as XRD, UV–vis reflectance spectroscopy and SEM; the photocatalytic activities of several samples were investigated for 2 -propanol degradation in gas phase. Compared to pure Nb₂O₅, the hybridization of Pb 6s² and O 2p⁶ orbitals notably pushed up the position of the valence band top, and narrowed the band gap of lead niobates. Due to the appropriate crystal structure and energy band structure, Pb₃Nb₄O₁₃ performed much better than Pb₃Nb₂O₈ for 2 -propanol photodegradation. Different preparation methods were also adopted to prepare the Pb₃Nb₄O₁₃ samples. Compared to the solid-state reaction method (SSR), the co-precipitation method (CP) created a high homogeneity in the precursor powder, which notably decreased the calcination temperature required for crystal formation. The so-obtained Pb₃Nb₄O₁₃(CP) photocatalyst possesses much greater surface area than its Pb₃Nb₄O₁₃(SSR) counterpart, and exhibited notably improved photocatalytic activity for 2 -propanol degradation as a result.

Keywords: Photocatalysis; Lead niobates; Visible light; 2-Propanol

Effect of Ce-doping on Rh/ZrO₂ catalysts for partial oxidation of methane by S. Eriksson; S. Rojas; M. Boutonnet; J.L.G. Fierro (pp. 8-16).

The partial oxidation of methane over supported (ZrO₂, CeO₂–ZrO₂) rhodium catalysts was investigated at atmospheric pressure. The effect of temperature, CH₄/O₂ ratio, catalyst composition and pre-treatment was studied. Ceria doping of the support material resulted in significant improvements concerning the methane conversion and syngas selectivity, which could be related to a higher noble metal dispersion on the Rh/CeO₂–ZrO₂ catalyst. In addition, the light-off temperature was decreased by 128°C when using CeO₂–ZrO₂ as support. X-ray photoelectron spectroscopy revealed the presence of different Rh oxidation states depending on catalyst composition and pre-treatment. A stabilization of partially oxidized (Rh^δ+) species by ceria could be detected. An active and stable catalyst behavior could be observed for Rh/CeO₂–ZrO₂, irrespectively of catalyst pre-treatment, whereas an activation period was required for stabilizing the activity of the Rh/ZrO₂ catalyst. The activity tests indicate that the indirect reaction mechanism, consisting of methane combustion followed by steam and dry reforming, prevails under the experimental conditions studied.The partial oxidation of methane over supported (ZrO₂, Ce–ZrO₂) rhodium catalysts was investigated at atmospheric pressure. The effect of temperature, CH₄/O₂ ratio, catalyst composition and pre-treatment was studied. Ceria doping of the support material resulted in significant improvements concerning the methane conversion and syngas selectivity, which could mainly be related to a higher noble metal dispersion on the Rh/Ce–ZrO₂ catalyst. ▪

Keywords: Partial oxidation of methane; Rh catalysts; Catalytic combustion; Ceria; XPS

Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres by W.K. Jozwiak; E. Kaczmarek; T.P. Maniecki; W. Ignaczak; W. Maniukiewicz (pp. 17-27).

The hydrogen reduction behavior of Fe₂O₃, Fe₃O₄ and FeO is strongly influenced by time–pressure dependent process. The reduction of hematite takes place according to scheme: 3Fe₂O₃→2 Fe₃O₄→6 FeO→6 Fe. The same pathway of hydrogen reduction for magnetite and wüstite is postulated. First, reduction of magnetite to wüstite Fe₃O₄→FeO and second step of wüstite disproponation 4FeO→Fe₃O₄+Fe. Only after the disappearance of Fe₂O₃ phase, the reduction of Fe₃O₄ to Fe can be observed. The appearance of FeO crystal phase as an intermediate compound of iron(III) oxide reduction was experimentally confirmed by XRD method above 560°C. The complete reduction of hematite into metallic iron phase can be accomplished even at low temperature of up to 380°C.▪The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPR_H2 and TPR_CO), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “ in situ” XRD methods. Five different compounds of iron oxides were characterized: hematite α-Fe₂O₃, goethite α-FeOOH, ferrihydrite Fe₅HO₈·4H₂O, magnetite Fe₃O₄ and wüstite FeO. In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300°C. Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe₂O₃→2 Fe₃O₄→6 Fe, three-stage mechanism 3Fe₂O₃→2Fe₃O₄→6FeO→6Fe is postulated especially when temperature of reduction overlaps 570°C. Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe²⁺⇌2Fe³⁺+Fe, occurring at both the atomic scale on two-dimensional interface border Fe₃O₄/Fe or stoichiometrically equivalent and thermally induced, above 250°C, phase transformation—wüstite disproportionation to magnetite and metallic iron, 4FeO⇌Fe₃O₄+Fe. Above 570°C, the appearance of wüstite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “ in situ” XRD method. In the case of FeO–H₂ system, instead of one-step simple reduction FeO→Fe, a much more complex two-step pathway FeO→Fe₃O₄→Fe up to 570°C or even the entire sequence of three-step process FeO→Fe₃O₄→FeO→Fe up to 880°C should be reconsidered as a result of the accompanying FeO disproportionation wüstite⇌magnetite+iron manifesting its role above 150°C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide. The disproportionation reaction of FeO does not consume hydrogen and occurs above 200°C much easier than FeO reduction in hydrogen above 350°C. The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions. The disproportionation reaction wustite⇌magnetite+iron makes simple wüstite reduction FeO→Fe a much more complicated process. In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wüstite→magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe₃O₄. Depending on TPR_H2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380°C. Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction.

Keywords: Iron oxide; Reduction process

Direct oxidation of H₂ to H₂O₂ over Pd/Ga₂O₃ catalyst under ambient conditions: Influence of halide ions added to the catalyst or reaction medium by Chanchal Samanta; Vasant R. Choudhary (pp. 28-36).

H₂O₂ yield/selectivity in the direct H₂O₂ synthesis over Pd/Ga₂O₃ catalyst is markedly increased with the addition of Br⁻ ions (at an optimum concentration) to the catalyst or reaction medium. ▪The formation of H₂O₂ from H₂ and O₂ and H₂O₂ decomposition/hydrogenation have been investigated in an acidic aqueous reaction medium over halide-assisted Pd/Ga₂O₃ catalyst system. The halide ions were introduced in the catalytic system by directly adding the halide ions to the reaction medium or by indirectly through the catalyst. The H₂O₂ formation selectivity was found to be influenced significantly with the addition of halide ions to the catalytic system. The H₂O₂ formation and H₂O₂ decomposition/hydrogenation studies over halide-modified Pd/Ga₂O₃ catalysts (pretreated under different gaseous atmosphere viz. air and nitrogen) revealed that bulk Pd oxidation state and nature of halide ions incorporated in the catalyst cooperatively promote the selective H₂O₂ formation. The decomposition and hydrogenation H₂O₂ on the Pd/Ga₂O₃ catalyst were also found strongly depend upon the nature of the halide ions introduced in the catalytic system. The selectivity for the conversion of H₂ to H₂O₂ was enhanced markedly due to the presence of Br⁻ ions in the catalytic system, irrespective of the bulk palladium oxidation state (PdO or Pd⁰) in the catalyst. The beneficial role of Br⁻ ions is attributed to the inhibition of the parallel H₂ to water oxidation and the consecutive H₂O₂ decomposition and hydrogenation reactions over the catalyst.

Keywords: Selective H; ₂; oxidation; Pd/Ga; ₂; O; ₃; catalyst; H; ₂; O; ₂; decomposition; H; ₂; O; ₂; hydrogenation; Bromide promoter

PtSn/SiO₂ catalysts prepared by controlled surface reactions for citral hydrogenation in liquid phase by I.M. Vilella; I. Borbáth; J.L. Margitfalvi; K. Lázár; S.R. de Miguel; O.A. Scelza (pp. 37-47).

Bimetallic PtSn/SiO₂ catalysts with broad Sn/Pt atomic ratios were prepared by controlled surface reactions (CSRs). The catalytic behavior of these catalysts was studied in citral hydrogenation in liquid phase carried out at 70°C and atmospheric pressure. Results of citral hydrogenation show that the contribution of different reaction routes strongly depends on the metallic loading of both Pt and Sn, thus giving either a high selectivity to citronellal (CAL) or to unsaturated alcohols nerol+geraniol (UA), for monometallic and bimetallic catalysts, respectively.▪Bimetallic PtSn/SiO₂ catalysts with broad Sn/Pt atomic ratios were prepared by an organometallic method using a controlled surface reactions (CSRs) between hydrogen adsorbed on silica-supported Pt and tin tetraethyl. The catalytic behavior of these catalysts was studied in citral hydrogenation in liquid phase carried out at 70°C and atmospheric pressure. The catalysts were characterized by test reactions of the metallic phase (cyclohexane dehydrogenation (CHD), and cyclopentane hydrogenolysis (CPH)), CO chemisorption and Mössbauer spectroscopy. All these results clearly showed that tin was introduced exclusively into platinum. Results of citral hydrogenation show that the contribution of different reaction routes strongly depends on the metallic loading of both Pt and Sn, thus giving either a high selectivity to citronellal (CAL) or to unsaturated alcohols nerol+geraniol (UA), for monometallic and bimetallic catalysts, respectively. The citronellal cyclization resulting in the formation of isopulegol, was almost negligible because of the low concentration of acidic sites of the support. It was observed that the higher the Pt loading, the lower the Sn/Pt atomic ratio required to get higher UA selectivities. Products of undesirable side reactions were detected in trace amount. The results of CO chemisorption and CHD test reaction provided unambiguous prove that, due to the addition of tin, the catalytic properties of the metallic phase have been modified. Thus, results show that modified catalysts are highly selective for the hydrogenation of the carbonyl group.

Keywords: PtSn/SiO; ₂; catalysts; Selective hydrogenation; Citral hydrogenation; Controlled surface reactions

Aging investigation on catalysts for hydrofluorocarbons synthesis by Stefania Albonetti; Laura Forni; Paolo Cuzzato; Paolo Alberani; Sergio Zappoli; Ferruccio Trifirò (pp. 48-54).

This study is aimed at verifying whether the thermal effects occurring during the catalyst oxidative regeneration may deal with the irreversible catalyst deactivation industrially observed for Cr-AlF₃-based catalysts. It was proved that due to the aging treatments, Cr³⁺ ions interact with the support, and lead to the formation of less reducible Cr³⁺ species.▪This study aims to verify whether the thermal effects occurring during catalyst oxidative regeneration may have to do with the irreversible catalyst deactivation industrially observed for CrO_xF_y-AlF₃-based catalysts. Deactivation phenomena can be due to modifications occurring on both the active phase and the support; therefore, the aging treatment effects on the support (AlF₃) and fresh and stabilized catalysts CrO_xF_y/AlF₃ were compared. It was proved that at temperatures higher than 600°C, the support undergoes to release of HF molecules and sintering phenomena correlated with the irreversible evolution to the thermally stable α-phase. The presence of Cr ions has an inhibiting effect on the support crystals growth, but strongly catalyses the surface area decrease at a relatively low temperature. In addition, the evolution of HF from chromium-containing catalysts starts at a very low temperature and is related to the formation of the crystalline Cr₂O₃ phase. Moreover, UV–vis and TPR analysis demonstrated that, due to aging treatments, chromium ions tend to interact with the support, and lead to the formation of less reducible chromium species, which are probably responsible for the irreversible catalytic activity degradation observed in commercial processes.

Keywords: Catalyst aging; Chromium; Aluminum-trifluoride; TPR; DR–UV–vis

Precipitated iron Fischer-Tropsch catalyst: Effect of carbidization on the morphology of iron oxyhydroxide nanoneedles by Amitava Sarkar; Alan K. Dozier; Uschi M. Graham; Gerald Thomas; Robert J. O’Brien; Burtron H. Davis (pp. 55-64).

The effect of carbidization on the morphology of iron oxyhydroxide particles has been studied. Homogeneous oxidation of iron(II) sulfate results in predominantly γ-FeOOH particles which have a rod-shaped crystalline structure. Carbidization of the prepared iron oxyhydroxide particles at 270°C and atmospheric pressure without mechanical stirring was found to produce iron carbides with spherical layered structure residing in close proximity. HRTEM, EDS and electron microdiffraction analysis revealed that carbidization alters the initial nanoneedle-type morphology and generates ultrafine carbide particles with irregular spherical shape. When carbidization was performed under severe mechanical stirring, formation of spherical carbide particles completely separated from each other was obsrved. Initial activity of iron oxyhydroxide for Fischer-Tropsch synthesis was found to be high (a CO conversion of about 84%) which declined steadily with synthesis time (26% CO conversion after 340h of synthesis).The effect of carbidation on the morphology of iron hydroxide oxide (γ-FeOOH) particles with a three dimensional crystalline nanoneedle structure has studied. Carbidation of γ-FeOOH with CO at atmospheric pressure produced iron carbides with spherical layered structure. HRTEM and EDS analysis revealed that carbidation changes the initial nanoneedles morphology and generates ultrafine carbide particles with irregular spherical shape. ▪

Keywords: Fischer-Tropsch synthesis; Iron oxyhydroxides (FeOOH); Iron carbide; Morphology; Carbidization; Nanoneedles

Fischer–Tropsch synthesis on mono- and bimetallic Co and Fe catalysts in fixed-bed and slurry reactors by V.A. de la Peña O'Shea; M.C. Álvarez-Galván; J.M. Campos-Martín; J.L.G. Fierro (pp. 65-73).

Mono- and bimetallic cobalt-iron silica supported catalysts were tested in a fixed-bed reactor and stirred-tank slurry reactor, for the Fischer–Tropsch reaction. For the fixed-bed reaction, the monometallic iron catalyst presented the highest activity. In the bimetallic catalysts an increase in alcohol selectivity is produced. In the slurry studies, CO conversion is greater for the cobalt catalyst and lower for bimetallic systems.▪Mono- and bimetallic cobalt and iron oxide nanoparticles deposited on the surface of a silica substrate were tested in a fixed-bed reactor and stirred-tank slurry reactor under different process conditions, for the Fischer–Tropsch reaction, this comparison is presented for the first time, being presented in this work a comparison of above systems in both reactor types. For the fixed-bed reaction, the monometallic iron-based catalyst presented the highest activity at low reaction temperatures. By contrast, the monometallic cobalt catalyst is the less active for all reaction temperatures. This catalyst records high values for the α parameter, whereas the iron catalyst reveals major selectivity to alcohols. Moreover, the introduction of iron produces a decrease in hydrocarbon chain growth and an increase in alcohol selectivity. This behaviour might be due to the formation of a Co–Fe alloy, observed by several characterization techniques. In the slurry studies, CO conversion is greater for the cobalt-based catalyst and lower for bimetallic systems, whilst the iron catalyst deactivates in a few hours, which is explained by the formation of carbonaceous deposits. The bimetallic catalysts record a decrease in CO conversion and an enhancement in the selectivity of methane and light hydrocarbons with the increase in iron amount. The absence of alcohols in this type of reactor is explained by the greater solubility of hydrogen, as well as by the greater contact time of reactants, which decreases the possibility of non-dissociative CO insertion.

Keywords: Fischer–Tropsch; Cobalt-iron catalyst; Slurry reactor; Fixed-bed reactor

Esterification of acrylic acid with but-1-ene over sulfated Fe- and Mn-promoted zirconia by N. Essayem; V. Martin; A. Riondel; J.C. Védrine (pp. 74-81).

The role of Mn and Fe as promoters of sulfated zirconia (ZS) has been studied in the reaction of acrylic acid (AA) esterification by but-1-ene to sec-butyl acrylate (SBA). The main result is that Mn and Fe did not improve the catalytic properties of sulfated zirconia as far as activity and selectivity to sec-butyl acrylate (90–95%) are concerned but strongly improved its resistance to deactivation.▪The role of Mn and Fe as promoters of sulfated zirconia (ZS) has been studied in the reaction of acrylic acid (AA) esterification by but-1-ene to sec-butyl acrylate (SBA) at 343K. The main result is that Mn and Fe did not improve the catalytic properties of sulfated zirconia as far as activity and selectivity to sec-butyl acrylate (90–95%) are concerned but strongly improved its resistance to deactivation, which makes this system attractive for substituting Amberlyst resins or sulfuric acid. The deactivation was shown not to be due to sulfated species and/or promoters leaching or to zirconia tetragonal phase transformation. It is proposed that in all cases acid sites are poisoned by the accumulation of unsaturated species primarily issue from the acrylic acid. Pyridine adsorption and n-butane isomerisation reaction at 473K showed that the acidity of sulfated promoted zirconia was not increased by the addition of the promoters. Mn was observed by XPS to be slightly reduced during reaction while the oxidation state of the Fe was not changed. But this was shown not to be related to the deactivation process.

Keywords: Sulfated zirconia; Esterification; Sec-butyl acrylate; Acrylic acid; Carboxylic acids; Solid acids

Catalytic isosafrol oxidation mediated by impregnated and encapsulated vanadyl-Y-zeolite under microwave irradiation by Heiddy Márquez Alvarez; Luiz Fernando Brum Malta; Marcelo H. Herbst; Adolfo Horn Jr.; O.A.C. Antunes (pp. 82-88).

The paper reports the characterization of vanadyl-Y-zeolites produced via impregnation and encapsulation methods. Their catalytic activities for isosafrol oxidation, promoted via microwave irradiation, were evaluated in contrast of well-known catalysts, as V₂O₅/Y-zeolite composites and non-exchanged zeolite. ▪Piperonal is an important intermediate in the synthesis of drugs likel-DOPA. Vanadyl-Y-zeolites have been tested as potential catalysts for this process. The use of encapsulated vanadyl compounds in Y-zeolite pores led to excellent rates of isosafrol oxidation under microwave irradiation. Native Y-zeolite better allowed the diffusion of the oxidant agent (hydrogen peroxide) through the catalyst pores compared to the VO-exchanged catalysts. This led to piperonal, which was also obtained due to the presence of V⁵⁺ supported on one of the catalysts. These factors aided the extension of the oxidation process of isosafrol. On the other hand, vanadyl-zeolites, containing V⁴⁺ species, promoted the reaction to intermediate (epoxide) with high selectivity under microwave radiation. Finally the impregnation of VO(Salen) on Y-zeolite showed better selectivity for epoxide formation than did the incorporated VO(Salen)-zeolite. Catalyst's surface area and availability of the vanadyl group were the main factors which allowed this superior performance.

Keywords: Y-zeolite; Ion exchange; Isosafrol; Oxidation; Microwaves

Low-temperature CO oxidation on Au/fumed SiO₂-based catalysts prepared from Au(en)₂Cl₃ precursor by Haoguo Zhu; Zhen Ma; Jason C. Clark; Zhengwei Pan; Steven H. Overbury; Sheng Dai (pp. 89-99).

Highly dispersed gold nanoparticles on Cab-O-Sil fumed SiO₂, which were prepared using Au(en)₂Cl₃ (en=ethylenediamine) as the precursor, were found to be highly active for CO oxidation below 0°C. Effects of catalyst pretreatment, gold loading, post-treatment, catalyst deactivation, storage, regeneration, and surface coating of other metal oxides were systematically studied.▪Many gold catalysts have been actively surveyed, but Au/SiO₂ catalysts that are highly active for CO oxidation still remain evasive. In this work, gold nanoparticles well dispersed on Cab-O-Sil fumed SiO₂ were prepared using Au(en)₂Cl₃ (en=ethylenediamine) as the precursor, and found to be very active for CO oxidation below 0°C. The catalyst pretreatment via reduction and calcination, effect of gold loading, post-treatment in acidic and basic media, catalyst deactivation, storage, regeneration, and effect of surface modification by other metal oxides were explored. The results provide new perspective on the activation and promotion of active Au/SiO₂-based catalysts.

Keywords: Gold catalysis; Nanoparticles; CO oxidation; Silica; Au(en); ₂; Cl; ₃

Chiral borane of layered α-Zirconium- N-( m-solfophenyl)-l-Valine-Phosphonate Methanphosphonate promoters for the asymmetric Mukaiyama Aldol reaction by Umberto Costantino; Francesco Fringuelli; Morena Nocchetti; Oriana Piermatti (pp. 100-105).

The preparation of several layered α-Zirconium- N-( m-solfophenyl)-l(+)-Valine-Phosphonate Methanphosphonate samples and their use as heterogeneous chiral ligands for the in situ preparation of chiral oxazaborolidine catalysts for the asymmetric Mukaiyama Aldol reactions were reported. ▪Several α-Zr[O₃PC₆H₄SO₂NHCH(CH(CH₃)₂)COOH)]2−_x(O₃PCH₃)_x· nH₂O samples, containing chiral pendent groups, were prepared by allowing Zr fluoro complexes to decompose in the presence of N-( m-solfophenyl)-l(+)-Valine-phosphonic (PVal) and methanphosphonic (PMe) acids. The layered compounds were characterized by different techniques and left to react with BH₃·THF reagent to obtain heterogeneous chiral oxazaborolidinone. These materials were found to be efficient catalysts of the typical asymmetric Mukaiyama Aldol reaction of 1-methoxy-2-methyl-1-trimethylsiloxy-1-propene and benzaldehyde in CH₂Cl₂ and THF at 0°C, with enantiomeric excess up to 50%.

Keywords: Heterogeneous asymmetric catalysis; Mukaiyama Aldol reactions; Zirconium phosphonate

Synthesis of β-MoO₃ through evaporation of HNO₃-added molybdic acid solution and its catalytic performance in partial oxidation of methanol by Takanori Mizushima; Kazuya Fukushima; Hironobu Ohkita; Noriyoshi Kakuta (pp. 106-112).

Pure β-MoO₃ was successfully synthesized through evaporation of a molybdic acid solution containing HNO₃, whereas α-MoO₃ was selectively produced from the HNO₃-free solution. Compared to the α-form, β-MoO₃ had the larger amount and higher acidity of Lewis acid sites on the surface, and thereby exhibited much higher catalytic activity in partial oxidation of methanol. ▪Two types of molybdenum trioxide were synthesized by evaporating the molybdic acid solution prepared via cation-exchange of an aqueous solution of Na₂MoO₄·2H₂O. When the molybdic acid solution was evaporated to dryness at 323K under reduced pressure and then calcined at 573K in a stream of oxygen, α-MoO₃ was exclusively produced. However, addition of HNO₃ to the solution before the evaporation resulted in formation of a bright yellow powder which was identified as β-MoO₃. The phase ratio of β/α was dependent on the quantity of HNO₃; pure β-MoO₃ was successfully synthesized when the molar ratio of HNO₃/Mo in the solution was in the range of 1–2. It was also found that the HNO₃ addition made no structural change of the isopolymolybdates in the solution and the dried precursor, but did affect the dehydration of the dried precursor to induce the crystallization of the β-phase. In partial oxidation of methanol, β-MoO₃ exhibited much higher methanol conversion than the α-form. IR spectra of chemisorbed pyridine elucidated that the higher catalytic activity was caused by the relatively large number and high acidity of Lewis acid sites on the surface of β-MoO₃.

Keywords: β-MoO; ₃; α-MoO; ₃; Selective synthesis; Methanol oxidation; Acid property

Mössbauer and XPS spectroscopies studies of SMSI effect on Fe/Nb₂O₅ catalysts for the Fischer–Tropsch synthesis by M.V. Cagnoli; A.M. Alvarez; N.G. Gallegos; J.F. Bengoa; C.D. Duarte de Souza; M. Schmal; S.G. Marchetti (pp. 113-119).

SMSI effect on the FTS was studied using two Fe/Nb₂O₅ catalysts prepared at two different reduction temperatures. The lower reduction temperature led to a partial covering of the Fe° crystals by NbO_x species. The higher temperature produced an important activity decrease, methane production increase and a shift of the products to higher molecular weight hydrocarbons, “breaking” the Anderson–Schultz–Flory (ASF) distribution.▪In order to study if the SMSI effect produces benefitial results on the Fischer–Tropsch (FT) reaction using the Fe/Nb₂O₅ system we prepared two catalysts by reduction at two different temperatures. The solids were characterized by Mössbauer spectroscopy, XRD, XPS, N₂ adsorption, atomic absorption, CO chemisorption and volumetric oxidation. It was demonstrated that the support is partially reduced leading to a covering of the Fe° crystals by NbO_x species. At the higher reduction temperature studied (773K) this effect produced a very important activity decrease, an increase in methane production and a shift of the product distribution towards to higher molecular weight hydrocarbons.

Keywords: SMSI effect; Fischer–Tropsch synthesis; Fe/Nb; ₂; O; ₅; catalysts; Mössbauer spectroscopy

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Applied Catalysis A, General (v.326, #1)