Promotional SMSI Effect on Supported Palladium Catalysts for Methanol Synthesis

High-temperature reduction (HTR) of palladium catalysts supported on some reducible oxides, such as Pd/CeO₂, and Pd/TiO₂ catalysts, led to a strong metal-support interaction (SMSI), which was found to be the main reason for their high and stable activity for methanol synthesis from hydrogenation of carbon dioxide. But low-temperature-reduced (LTR) catalysts exhibited high methane selectivity and were oxidized to PdO quickly in the same reaction. Besides palladium, platinum exhibited similar behavior for this reaction when supported on these reducible oxides. Mechanistic studies of the Pd/CeO₂ catalyst clarified the promotional role of the SMSI effect, and the spillover effect on the HTR Pd/CeO₂ catalyst. Carbon dioxide was decomposed on Ce₂O₃, which was attached to Pd, to form CO and surface oxygen species. The carbon monoxide formed was hydrogenated to methanol successively on the palladium surface while the surface oxygen species was hydrogenated to water by spillover hydrogen from the gas phase. A reaction model for the hydrogenation of carbon dioxide was suggested for both HTR and LTR Pd/CeO₂ catalysts. Methanol synthesis from syngas on the LTR or HTR Pd/CeO₂ catalysts was also conducted. Both alcohol and hydrocarbons were formed significantly on the HTR catalyst from syngas while methanol formed predominantly on the LTR catalyst. Characterization of these two catalysts elucidated the reaction performances.     

Effect of promoters for n-butane oxidation to maleic anhydride over vanadium–phosphorus‐oxide catalysts: comparison with supported vanadia catalysts

The oxidation of n-butane to maleic anhydride was investigated over model Nb‐, Si‐, Ti‐, V‐, and Zr‐promoted bulk VPO and supported vanadia catalysts. The promoters were concentrated in the surface region of the bulk VPO catalysts. For the supported vanadia catalysts, the vanadia phase was present as a two‐dimensional metal oxide overlayer on the different oxide supports (TiO₂, ZrO₂, Nb₂O₅, Al₂O₃, and SiO₂). No correlation was found between the electronegativity of the promoter or oxide support cation and the catalytic properties of these two catalytic systems. The maleic anhydride selectivity correlated with the Lewis acidity of the promoter cations and oxide supports. Both promoted bulk VPO and supported vanadia catalysts containing surface niobia species were the most active and selective to maleic anhydride. These findings suggest that the activation of n-butane on both the bulk and supported vanadia catalysts probably requires both surface redox and acid sites, and that the acidity also plays an important role in controlling further kinetic steps of n-butane oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Modifiers on the Reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 Catalysts for the Oxidative Dehydrogenation of Propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (T_max and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.     

Temperature-programmed reduction of NiOWO3/Al2O3 Hydrodesulphurization catalysts

Temperature-programmed reduction patterns are reported of NiO/Al₂O₃, WO₃/Al₂O₃, and NiOWO₃/Al₂O₃ catalysts, and of bulk oxides relevant to these catalysts. At loadings below 10% NiO and 19% WO3 nickel and tungsten are present as disperse species and no bulk oxides are found on the support. Tungsten forms a stable monolayer species which is extremely difficult to reduce. At high temperatures of reduction tungsten metal is formed without the intermediate formation of WO₂. Several nickel species were detected in the supported catalysts, and their characters and amounts depend on the loading and the temperature of calcination. After calcination at low temperatures and at low loadings (2% NiO) nickel is incorporated in the surface layers of the support; at higher loadings a NiO like species is formed which is more difficult to reduce than bulk NiO because of the interaction with the aluminium ions of the support. A high temperature of calcination favours the formation of a diluted spinel phase at the expense of the surface phases. In NiOWO₃/Al₂O₃ catalysts a fraction of nickel is present in a mixed NiWOAl phase which is formed by incorporation of nickel in a tungsten containing surface layer. At high temperatures of calcination some disperse NiWO₄ is formed. It is shown that there are two causes for a strong interaction between nickel species and the support: incorporation of nickel ions in the surface layers of the support during impregnation, and solid-state diffusion during calcination of the catalysts. The nickel containing species which are the precursor to the active phases in sulfided HDS catalysts are identified as the nickel species in the surface layers of the support in NiO/Al₂O₃ samples, and the NiWOAl phase in NiOWO₃/Al₂O₃ catalysts.     

Supported Ni–W–O Mixed Oxides as Selective Catalysts for the Oxidative Dehydrogenation of Ethane

Mixed Ni–W–O catalysts (with a W/(Ni + W) atomic ratio of 0.3) supported on γ-Al₂O₃ or on mesoporous alumina have been prepared, characterized and tested in the oxidation of ethane. For comparison unsupported and supported NiO as well as bulk Ni–W–O mixed oxides catalysts have also been studied. Supported Ni–W–O materials show interesting catalytic performances in the oxidative dehydrogenation of ethane. They show similar catalytic activities than the corresponding unsupported Ni–W–O catalysts. However, the selectivity to ethylene over supported catalysts was higher than that achieved over unsupported samples (the selectivity to ethylene followed the trend: mesoporous-supported > γ-Al₂O₃-supported > unsupported Ni–W–O). In addition, it has also been observed that Ni–W–O catalysts are more efficient than the corresponding W-free NiO catalysts. The discussion of the catalytic results will be undertaken on the basis of the modification of active sites of NiO when incorporating WO₃ and/or metal oxide supports.     

Oxidation of o-Xylene to Phthalic Anhydride on Sb-V/ZrO2 Catalysts

Zirconia-supported and bulk-mixed vanadiumantimonium oxide catalysts were used for the oxidation of o-xylene to phthalic anhydride. X-ray diffraction, Raman spectroscopy and photoelectron spectroscopy were used for characterization. It was found that vanadium promotes the transition of tetragonal to monoclinic zirconia. The simultaneous presence of Sb and V on zirconia at low coverage led to a preferential interaction of individual V and Sb oxides with the zirconia surface rather than the formation of a binary Sb-V oxide, while at higher Sb-V contents the formation of SbVO₄ took place. Sb-V/ZrO₂ catalysts showed high activity for o-xylene conversion and better selectivity to phthalic anhydride as compared to V/ZrO₂ catalysts. However, their selectivity to phthalic anhydride was poor in comparison to a V/TiO₂ commercial catalyst. The improved selectivity of the Sb-containing catalysts is attributed to the blocking of non-effective surface sites of ZrO₂, the decrease of the total amount of acid sites and the formation of surface V-O-Sb-O-V structures.     

Selective deposition of Au-Pt alloy nanoparticles on ellipsoidal zirconium titanium oxides for reduction of 4-nitrophenol

Au-Pt alloy nanoparticles that are selectively anchored on TiO₂ surface of the ellipsoidal zirconium titanium composite oxides were successfully prepared by a facile two-step method: prefabricated binary composite oxides on the ellipsoidal Fe₂O₃@SiO₂ by a versatile cooperative template-directed coating method, and then in situ formation of Au-Pt alloy NPs with Sn²⁺ as the reduction agent. The alloy catalysts were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The result suggested that highly dispersive and ultrafine Au-Pt alloy nanoparticles were deposited onto TiO₂ surface of the binary oxides solely. The particle size of nanoalloys was closely related to the ratio of Zr: Ti in the composite oxides shell. Increasing the content of Zr element led to a growth in the size of alloy nanoparticles. When used as catalysts for the reduction of 4-nitrophenol, the prepared supported alloyed catalysts exhibited high catalytic activity, and the sample could be easily recycled without a significant decrease of the catalytic activity.     

A Novel Method for Measuring Active Sites of Fe3O4 for WGS Reaction

Phase transformation among iron oxides was investigated. Pure magnetite material was obtained by reducing iron oxide with diluted hydrogen in a narrow temperature window and with steam to prevent over-reduction. A pulse chromatographic method with N₂O decomposition over magnetite surface to determine active sites of iron oxide based catalysts for water gas shift reaction has been developed. N₂O decomposes over activated Fe₃O₄ surface to N₂ and leaves oxygen species at oxygen vacancy on the catalyst surface, which is the same site for water gas shift reaction. Lower temperature for N₂O decomposition is required to avoid magnetite bulk oxidation. An oxygen coverage on the active sites θ = 1 corresponded to a surface stoichiometry of N₂O/Fe²⁺ = 0.5 was estimated. A linear correlation between water gas shift reaction rate and the quantity of decomposed N₂O over the corresponded catalysts was observed.     

Oxidation of ketones over metal oxide catalysts: I. Catalytic synthesis of biacetyl from methyl ethyl ketone

The gas-phase oxidation of methyl ethyl ketone was studied on metal oxide catalysts in the presence of water vapor. Two types of competitive partial oxidations, i.e., biacetyl formation and oxidative scission reaction leading to acetaldehyde and acetic acid, took place on every oxide studied at 400–500 K. An approximate linear relationship was observed between the selectivity of each reaction and the acid-base property of the oxides; the former reaction was accelerated on the basic oxides such as Co₃O₄, while the latter reaction became predominant on the acidic oxides. As Co₃O₄ was the most effective biacetyl former of single-component oxides, modification of Co₃O₄ was examined to develop more effective catalysts for biacetyl synthesis. Scission reaction took the place of biacetyl formation over a catalyst where Co²⁺ ions were located in Y zeolite by an ion-exchanged method. Scission reaction was suppressed when Co oxide was supported on basic oxides such as MgO or CaO; however, the selectivity to biacetyl was slightly decreased due to the occurrence of a new reaction, acetone formation. The addition of Na₂O or Li₂O to Co₃O₄ was found to improve the selectivity to biacetyl without loss of catalytic activity. The maximum efficiency (13%) in biacetyl formation was attained at a Li content of ca. 7 at.%.     

Propane Oxidative Dehydrogenation on V–Sb/ZrO2 Catalysts

The catalytic properties of V–Sb/ZrO₂ and bulk Sb/V catalysts for the oxidative dehydrogenation of propane were studied. Samples were characterized by nitrogen adsorption, temperature-programmed reduction, temperature-programmed pyridine desorption and photoelectron spectroscopic techniques. Vanadia promotes the transition of tetragonal to monoclinic zirconia and the formation of ZrV₂O₇. Surface V and Sb oxide species do not appear to interact among them below monolayer coverage, but SbVO4 forms above monolayer. Simultaneously the excess of antimony forms α-Sb₂O₄. Activity and selectivity show no dependence on the acidity of the catalysts. However, there is a strong dependence of activity/selectivity on composition; surface vanadium species are active for propane oxidative dehydrogenation and the presence of Sb, affording rutile VSbO₄ phase makes the system selective to C₃H₆, this is believed to be related to the redox cycle involving dispersed V⁵⁺ species and lattice reduced vanadium site in the rutile VSbO₄ phase.     

Perovskite-type oxides in methane dry reforming: Effect of their incorporation into a mesoporous SBA-15 silica-host

A series of Ni-based perovskite-type oxides LaNiO₃, La_0.8Ca_0.2NiO₃ and La_0.8Ca_0.2Ni_0.6Co_0.4O₃, were synthesized as catalyst precursors both bulk and built-in a highly ordered mesoporous SBA-15 silica-host with the aim of using them as heterogeneous catalysts in syngas production by the methane dry reforming with CO₂. The solids were characterized by means of FT-IR, XRD, BET surface area and TPR techniques. All synthesized oxides showed a perovskite-type structure. Incorporation of the oxides into the mesoporous silica-host generates a higher metal–support interaction increasing the Ni reduction temperature. A decrease in CH₄ and CO₂ conversion was observed when a second cation in A- and/or B-site was added to the bulk perovskite. The built-in of these solids in the SBA-15 mesoporous silica-host allows working at lower temperature with an increase in conversions and selectivity towards syngas which represent an attractive perspective for industrial application.     

Sb-V-Ox catalysts—Role of chemical composition of MCM-41 supports in physicochemical properties

SbVO_x binary oxides were loaded on MCM-41 matrices of various chemical compositions: silicate (MCM-41), Nb-silicate (NbMCM-41), and Al-silicate (AlMCM-41). Vanadium and antimony were introduced by the post synthesis wetness impregnation carried out step by step (first V next Sb). The materials were characterised by N₂ adsorption/desorption, XRD, UV–vis, ESR, H₂-TPR, FT-IR combined with pyridine adsorption, and test reaction–hydrosulphurisation of methanol. SbVO_x dispersion was much higher when the support contained transition metal (NbMCM-41). Tetrahedrally coordinated vanadium(IV) species were deduced on all prepared samples from UV–vis spectra and were the only registered species on SbV/NbMCM-41 and SbV/AlMCM-41, whereas octahedrally ones were also present on SbV/MCM-41 and SbV/SiO₂. In bulk SbVO_x octahedral coordination dominated. The chemical composition of mesoporous support determined acidic–basic properties of SbVO_x catalysts and influenced the activity and selectivity in methanol hydrosulphurisation. The presence of Lewis acid–base pairs in the SbV/AlMCM-41 and SbV/NbMCM-41 catalysts strongly activated thiol formation in the reaction between methanol and hydrogen sulphide, whereas bulk binary oxides and SbVO_x loaded on silicate MCM-41 were less active and exhibited different selectivity.     

Molecular structure and reactivity of the Group V metal oxides

The physical, electronic and reactivity properties of bulk and supported Group V metal oxides (V, Nb, Ta and Db) were compared at the molecular level. Dubnium is a very short-lived element, 60 s, whose properties have not been extensively studied, but can be predicted from knowledge of the other members of the Group V metal oxides. Bulk V₂O₅ possesses platelet morphology with the active surface sites only located at the edges: primarily surface redox sites and some surface acidic sites. Bulk Nb₂O₅ and Ta₂O₅, as well as to be expected for bulk Db₂O₅, possess isotropic morphologies and the active surface sites relatively homogeneously dispersed over their surfaces: only surface acidic sites. However, the bifunctional bulk V₂O₅ was found to exhibit a much higher specific acidic catalytic activity than the acidic bulk Nb₂O₅ and Ta₂O₅, the latter being almost identical in their specific acidic catalytic activity. The bulk properties of the Group V metal oxides were essentially transferred to the analogous supported Group V metal oxides, where the active Group V metal oxides were present as a two-dimensional monolayer on various oxide supports (e.g., Al₂O₃, TiO₂, ZrO₂ as well as Nb₂O₅ and Ta₂O₅). For supported vanadia catalysts, the active surface sites were essentially redox sites, with the exception of supported V₂O₅/Al₂O₃ that also contained strong acidic sites. For supported niobia and tantala catalysts, as well as to be expected for supported dubnia catalysts, the active surface sites were exclusively acidic sites. However, the TOF_redox for the supported vanadia catalysts and the TOF_acidic for the supported niobia and tantala catalysts varied over several orders of magnitude as a function of the specific oxide support with the electronegativity of the oxide support cation. However, the TOF_redox varied inversely to that of the TOF_acidic variation because of the opposite requirements of these active surface sites. Surface redox sites are enhanced by reduction and surface acidic sites are enhanced by stabilization (lack of reduction). The current fundamental understanding of the Group V metal oxides allows for the molecular engineering of their metal oxide applied catalytic materials.     

A corrected procedure and consistent interpretation for temperature programmed reduction of supported MoO3

Experiments in which bulk MoO₃ has been diluted in silica or alumina reveal a corrected TPR pattern in which the bulk oxide reduces at the same temperature as the supported crystalline phase. The reduction patterns for diluted bulk MoO₃, in conjunction with an XRD study of the reduction process and a recent comprehensive speciation theory have greatly assisted the assignment of peaks for silica and alumina supported samples. A simple interpretation of TPR patterns has been applied with good agreement to the body of MoO₃ TPR literature. With consistent interpretation, temperature programmed reduction might be employed with increased confidence to characterize supported MoO₃ and perhaps other supported oxides.     

Catalytic Properties of Cr2O3 Doped with MgO Supported on MgF2 and Al2O3

Catalytic properties of Cr₂O₃ supported on MgF₂ or Al₂O₃ have been modified by magnesium oxide. The catalysts have been obtained by the co-impregnation method and characterised by: BET, XRD and TPR. As follows from the results, the oxides supported on magnesium fluorine react with each other already at 400 °C, leading to formation of an amorphous spinel-like phase. On the Al₂O₃ support such an MgCr₂O₄ spinel has appeared at much higher temperatures. The addition of magnesium oxide has a significant effect on the activity and selectivity of the catalysts studied in the CO oxidation reaction at room temperature and in the reaction of cyclohexane dehydrogenation. The magnesium–chromium catalysts supported on MgF₂ have been found to show much higher activity and selectivity than the analogous systems supported on Al₂O₃.     

The effect of sodium on the catalytic activity of ZnO–Al2O3/ZSM-5 and SnO–Al2O3/ZSM-5 for the transesterification of vegetable oil with methanol

In order to elucidate the effect of sodium on the activity of ZSM-5 supported metal oxides catalysts (ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5) for the transesterification of soybean oil with methanol, ZSM-5 supported metal oxides were prepared with and without sodium hydroxide by impregnation. The metal compositions of the ZSM-5 supported metal oxide catalysts and the metal concentrations dissolved from the catalysts to the methylester phase were measured by SEM-EDS and inductive coupled plasma spectroscopy, respectively. The catalytic activity of ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5 containing sodium did not originate from surface metal oxides sites, but from surface sodium sites or dissolved sodium leached from the catalyst surface.     

Laser Raman spectroscopy of MoO3 and NiO–MoO3 supported on gallia and gallium–aluminum mixed oxides

The laser Raman spectra of MoO₃ and NiO–MoO₃ supported on gallia and gallium–aluminum mixed oxides are presented and correlated to the type of support and the hydrodesulfurization (HDS) activity of the catalysts. The results show that the support affects the species (orientation of catalyst) that are formed on its surface. For example, MoO₃ forms mainly HDS inactive tetrahedral species on gallia; on alumina surface, it forms the HDS active polymeric octahedral species. Also, bulk oxide is formed more readily on gallia than on alumina. On addition of NiO, as a promoter, to the MoO₃ catalyst supported on gallia, a band that can be assigned to HDS inactive species was also formed. The intensity of this band is low in the spectra of NiO–MoO₃ catalysts supported on alumina. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbide phases on iron-based Fischer-Tropsch synthesis catalysts part I: Characterization studies

The characterization of iron carbide phases formed on iron-based Fischer-Tropsch catalysts is reviewed in this part with particular emphasis on Mössbauer and XPS studies of selected iron and supported iron catalysts, in combination with some specific results in activity and selectivity. Mössbauer studies have shown clearly the evolution of bulk carbide species on iron catalysts under synthesis reaction conditions; the relationship of these to the nature of near-surface species and their temporal evolution under synthesis conditions is indicated by XPS results. Comparisons among reduced, unreduced and carbided iron catalysts are given; XPS and reaction results suggest that Fe₃O₄ is active for synthesis even in the absence of carbide phases.     

Effect of chlorine on the selectivity of a fischer-tropsch catalyst composed of iron/vanadium oxides

Fischer-Tropsch catalysts (Fe/V oxides with ZnO and K₂CO₃ as promoters) were exposed to CHCl₃, thereby producing surface and bulk chlorides. The effect of this exposure on activity and selectivity was studied in a continuous recycle reactor at a total pressure of 10 bar (CO/H₂ in most experiments: ca 1:1) in a temperature range between 200 and 343°C. CHCl₃ was introduced in amounts of up to 1 × 10^?2 mol chlorine per g catalyst. The catalyst samples were characterized by internal surface area, pore-size distribution and adsorption capacities for CO, H₂ and C₂H₄. Prior to synthesis, the catalysts were reduced by H₂. Catalyst exposure to CHCl₃ resulted in a decrease of activity and considerable changes in product distribution. Hydrogenation and isomerization of 1-olefins were partly suppressed; the chain length of the products was slightly increased. Deactivation of the catalysts due to chlorine addition was partly reversible during operation, while olefin formation was not significantly altered with time-on-stream. The effect of chlorine on activity and selectivity is explained by dissociation of CO as the chain initiating step and CO insertion into a carbon/metal bond as a possible chain propagation step. Since adsorption capacity for H₂ decreases on the addition of chlorine, this may also contribute to lower activity and change in selectivity, compared to the unexposed catalyst.     

A comparative study on the selective hydrogenation of α,β unsaturated aldehyde and ketone to unsaturated alcohols on Au supported catalysts

The hydrogenation of trans,4-phenyl,3-buten,2-one (benzalacetone) and trans,3-phenyl, propenal (cinnamaldehyde) was carried out on Au supported on iron oxides catalysts. Commercial goethite (FeOOH), maghemite (γFe₂O₃) and hematite (αFe₂O₃) were used as supports. The catalytic activity of Au/Fe₂O₃ reference catalyst, supplied by the World Gold Council, was also investigated. Gold catalysts and the parent supports were characterized by BET, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of ammonia (NH₃-TPD) and high resolution transmission electron microscopy (HRTEM).Among the catalysts investigated Au supported on FeOOH shows the highest activity and selectivity to UA in the hydrogenation of unsaturated carbonyl compounds whereas Au supported on αFe₂O₃ are the less active and selective catalysts.The catalytic activity and selectivity to unsaturated alcohols (UA) in the hydrogenation of benzalacetone and cinnamaldehyde are less influenced by the morphology of gold particles and are mainly influenced by the nature of the support.A correlation between the reducibility of the catalysts and the activity and selectivity to UA has been found. Increasing the reducibility of the catalysts both the activity and selectivity to UA increase. These results let us to argue that active and selective sites are formed by negative gold particles formed through the electron transfer from the reduced support to the metal.