Showing papers in "Applied Catalysis A-general in 1997"

Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts

Abstract: Catalyst productivity and selectivity to C5+ hydrocarbons are critical design criteria in the choice of Fischer-Tropsch synthesis (FTS) catalysts and reactors. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons from CO/H2 mixtures. Optimum catalysts with high cobalt concentration and site density can be prepared by controlled reduction of nitrate precursors introduced via melt or aqueous impregnation methods. FTS turnover rates are independent of Co dispersion and support identity over the accessible dispersion range (0.01–0.12) at typical FTS conditions. At low reactant pressures or conversions, water increases FTS reaction rates and the selectivity to olefins and to C5+ hydrocarbons. These water effects depend on the identity of the support and lead to support effects on turnover rates at low CO conversions. Turnover rates increase when small amounts of Ru (Ru/Co<0.008 at.) are added to Co catalysts. C5+ selectivity increases with increasing Co site density because diffusion-enhanced readsorption of α-olefins reverses, β-hydrogen abstraction steps and inhibits chain termination. Severe diffusional restrictions, however, can also deplete CO within catalyst pellets and decrease chain growth probabilities. Therefore, optimum C5+ selectivities are obtained on catalysts with moderate diffusional restrictions. Diffusional constraints depend on pellet size and porosity and on the density and radial location of Co sites within catalyst pellets. Slurry bubble column reactors and the use of eggshell catalyst pellets in packed-bed reactors introduce design flexibility by decoupling the characteristic diffusion distance in catalyst pellets from pressure drop and other reactor constraints.

Structure and reactivity of surface vanadium oxide species on oxide supports

Abstract: Supported vanadium oxide catalysts, containing surface vanadia species on oxide supports, are extensively employed as catalysts for many hydrocarbon oxidation reactions. This paper discusses the current fundamental information available about the structure and reactivity of surface vanadia species on oxide supports: monolayer surface coverage, stability of the surface vanadia monolayer, oxidation state of the surface vanadia species, molecular structures of the surface vanadia species (as a function of environment and catalyst composition), acidity of the surface vanadia species and reactivity of the surface vanadia species. Comparison of the molecular structure and reactivity information provides new fundamental insights into the catalytic properties of surface vanadia species during hydrocarbon oxidation reactions: (1) the role of terminal VO, bridging VOV and bridging VO-support bonds, (2) the number of surface vanadia sites required, (3) the influence of metal oxide additives, (4) the influence of surface acidic and basic sites, (5) the influence of preparation methods and (6) the influence of the specific oxide support phase. The unique physical and chemical characteristics of supported vanadia catalysts, compared to other supported metal oxide catalysts, for hydrocarbon oxidation reactions are also discussed.

Oxidative dyhydrogenation of short chain alkanes on supported vanadium oxide catalysts

Abstract: This paper summarizes the main data appeared in the last years on the oxidative dehydrogenation (ODH) of short chain alkanes on supported vanadium oxide catalysts. The acid-base character of metal oxide support influences the dispersion of vanadium on the surface of the support, as well as the nature of the vanadium species. The reducibility and structure of surface vanadium oxide species and the acid-base character of catalysts, in addition to their catalytic properties in the ODH of C2–C4 alkanes, strongly depend on the metal oxide used as support and the vanadium loading. In this way, it appears that tetrahedral V5+-species are active and selective sites in the ODH of C2–C4 alkanes. The effect of the coordination number and aggregation state of surface vanadium oxide species, and the presence of acid/base sites on the catalytic behavior of supported vanadium oxide catalysts are discussed. It is concluded that these are important factors that must be considered to develop selective catalysts in ODH reactions.

Catalysis in the Production and Reactions of Dimethyl Carbonate, an Environmentally Benign Building-Block

Abstract: Dimethyl carbonate (DMC) is a unique molecule having versatile chemical reactivity. In many aspects, DMC is an environmentally benign building block. DMC is a safe substitute for dimethyl sulfate or methyl halides. These conventional methylating agents are toxic and corrosive and give a stoichiometric amount of inorganic by-products. Methylation with DMC is often very selective. The mono-methylation of phenylacetonitrile, O -methylation of phenol and N -methylation of aniline are such examples. The transesterification of DMC with phenol yields methyl phenyl carbonate, which gives diphenyl carbonate by further transesterification with DMC or the disproportionation. Diphenyl carbonate is an essential starting material for polycarbonates resins by “non-phosgene” process. DMC serves also as a methoxycarbonylation agent. The methoxycarbonylation of amines with DMC gives methyl carbamates, which can be converted into isocyanates by decomposition. DMC reacts with silica at 550–600 K to afford tetramethoxysilane. This gas-solid reaction gives a simple and convenient method to depolymerize silica. In this review, various aspects of the reactions of DMC and the catalysts involved in the reactions will be overviewed.

Methanol oxidation as a catalytic surface probe

Abstract: The goal of this review is to present some aspects of the use of a test reaction, i.e., methanol oxidation, as a tool to characterize oxidation catalysts. The selectivity pattern and the formation rates of the reaction products are used to characterize both structural (dispersion) as well as chemical properties (acid-base and redox) on supported oxide catalysts, especially for molybdenum-based systems supported on silica and vanadia on titanium oxide. This highly sensitive technique which gives information on the catalytically active sites at the molecular level characterizes a catalyst at work and is particularly well-adapted to the study of supported catalysts.

TPR and TPD studies of CuOCeO2 catalysts for low temperature CO oxidation

Abstract: Copper oxide supported on cerium dioxide ( CuO CeO 2 ) catalysts were prepared and used for carbon monoxide oxidation in stoichiometric carbon monoxide and oxygen. The catalysts were characterized by means of XRD, H2-TPR and CO-TPD studies. The CuO CeO 2 catalysts exhibit high catalytic activity in CO oxidation, showing markedly enhanced catalytic activities due to the combined effect of copper oxide and cerium dioxide. The activity of the CuO CeO 2 (15%) catalyst prepared by impregnation is higher than that prepared by co-precipitation. CeO2 promotes the hydrogen reduction activity of copper, so that CuO CeO 2 catalysts show a different behavior with respect to pure CuO. Two reducible copper species were observed in all CuO CeO 2 catalysts. CO-TPD experiments revealed that CuO CeO 2 catalysts can adsorb CO, while pure CuO and CeO2 cannot. Combining the results of TPR, TPD study, and the catalytic activity measurements, it is proposed that the well dispersed CuO which can adsorb CO and which is reducible at low-temperature is responsible for low-temperature CO oxidation. The bulk CuO which cannot adsorb CO and which is reducible at high-temperature contributes little to the oxidation activity.

Synthesis of dimethyl ether (DME) from methanol over solid-acid catalysts

Abstract: The catalytic conversion of methanol to dimethyl ether (DME) has been studied over a series of solid-acid catalysts, such as γ-Al2O3, H-ZSM-5, amorphous silica-alumina, as well as titania modified zirconia. All the catalysts are active and selective for DME formation. The apparent activation energy for DME formation over γ-Al2O3 is ca. 25 kcal/mol, a value that increases to ca. 37 kcal/mol upon the addition of 23 Torr of H2O to the reagent. The rate of methanol dehydration decreases with increasing acidity (silica content) over the amorphous silica-alumina catalysts. Although H-ZSM-5 with Si/Al = 25 is the most active among the catalysts tested, the DME selectivity is only 20% at 280°C, a typical temperature used in the syngas-to-methanol process. An amorphous silica-alumina catalyst with 20 wt.-% silica content (SIRAL20) exhibits the best catalytic performance of those tested at 280°C.

On the role of promoters in hydrogenations on metals; α,β-unsaturated aldehydes and ketones

Abstract: Promoters of metal catalysts play an essential role in the hydrogenation of unsaturated bonds, such as in carbonylic compounds. This promotion is a key to a high selectivity to unsaturated alcohols in hydrogenation of α,β-unsaturated aldehydes. Analysis of the information available in the literature on the adsorptions through the C—C and C—O groups, respectively, together with the information on the catalytic behaviour of the promoter-free and promoted catalysts, allows us to suggest a model of surfaces active in selective hydrogenation of unsaturated aldehydes.

TAP-2: An interrogative kinetics approach

Abstract: The TAP-2 reactor system and the theoretical basis of TAP pulse response experiments are discussed. On the basis of the TAP system, an alternative to the traditional kinetic approach in heterogeneous catalysis is developed. This new approach, termed ‘interrogative kinetics’ involves a combination of two types of kinetic experiments called ‘state-defining’ (friendly) and ‘state-altering’ (typical) experiments. The state defining kinetic parameter, or ‘kinetic parameter of the catalyst’ (KPC) is proposed and compared with the kinetic characteristic ‘turnover number’. The theory of TAP pulse response experiments is developed. Two deterministic models based on partial-differential equations are analyzed for the cases of diffusion, irreversible adsorption/reaction and reversible adsorption. The ‘standard diffusion curve’ that can be used for distinguishing the Knudsen flow regime is described, and simple criteria of the Knudsen regime are proposed. The concept of relative flow is described, and different fingerprints of TAP kinetics for irreversible adsorption/reaction and reversible adsorption are presented. TAP-2 experimental results on the selective oxidation of n-butane are used to illustrate the methodology of interrogative kinetics.

Removal of carbon monoxide from hydrogen-rich fuels by selective oxidation over platinum catalyst supported on zeolite

Abstract: Special catalysts — Pt supported zeolites — for the selective oxidation of carbon monoxide in reformed fuels from methanol or natural gas were proposed. They can be applied for the application to polymer electrolyte fuel membrane cells of which anode Pt catalysts suffer serious poisoning by the presence of trace carbon monoxide. The proposed Pt-supported zeolite catalysts can oxidize carbon monoxide much more selectively in a large excess of hydrogen with the addition of a low concentration of oxygen than a conventional Pt-supported alumina catalyst. The selectivity was affected by supports, in the order A type zeolite mordenite X type zeolite alumina. With decreasing oxygen content, an enhanced selectivity was obtained on Pt-zeolite catalysts (approaching 100%). Pt supported on mordenite showed the highest selectivity, with high conversion from carbon monoxide to carbon dioxide among the catalysts examined. It was demonstrated that the oxygen addition can be minimized almost to the stoichiometric amount required for the complete oxidation of carbon monoxide in a large excess of hydrogen.

Electrochemical investigations on carbon supported palladium catalysts

Abstract: Highly dispersed palladium catalysts on carbon were prepared by reduction of their salts with various reducing agents. The crystallite parameters and metal surface area were evaluated by XRD and electrochemical techniques. The electrochemical activity for the oxidation of hydrogen, methanol, formaldehyde and ethylene glycol and reduction of oxygen was evaluated in alkaline solutions by making PTFE bonded carbon electrodes. The electrochemical activity was correlated with the catalyst properties. Pd dispersed catalysts on carbon supports exhibit high activity for the oxidation of hydrogen and ethylene glycol in alkaline medium. Their electrochemical activity is dependant upon the crystallite properties like surface area, dispersion and crystallite size. For oxygen reduction reaction, the activity can be increased by alloying with other elements.

Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics

Abstract: Supported Ru catalysts for NH3 synthesis were prepared from Ru3(CO)12 and high-purity MgO and Al2O3. In addition to aqueous impregnation with alkali nitrates, two non-aqueous methods based on alkali carbonates were used to achieve alkali promotion resulting in long-term and high-temperature stable catalysts. For the reliable determination of the Ru particle size, the combined application of H2 chemisorption, TEM and XRD was found to be necessary. The power-law rate expressions were derived at atmospheric pressure and at 20 bar which were shown to be efficient tools to investigate the degree of interaction of the alkali promoter with the Ru metal particles. The following sequence with respect to the turnover frequency (TOF) of NH3 formation was found: Cs 2 CO 3 Ru MgO > CsNO 3 Ru MgO > Ru MgO > Ru KAl 2 O 3 > Ru Al 2 O 3 . The Cs-promoted Ru MgO catalysts turned out to be more active than a multiply-promoted Fe catalyst at atmospheric pressure with an initial TOF of about 10−2 s−1 for the non-aqueously prepared Cs 2 CO 3 Ru MgO catalyst at 588 K. The strong inhibition by H2 was found to require a lower molar H2:N2 ratio in the feed gas than 3:1 in order to achieve a high effluent NH3 mole fraction. The optimum ratio for Cs 2 CO 3 Ru MgO at 50 bar was determined to be about 3:2, resulting in an effluent NH3 mole fraction which was just a few percent lower than that of a multiply-promoted Fe catalyst operated at 107 bar and at roughly the same temperature and space velocity. Thus, alkali-promoted Ru catalysts are an alternative to the conventionally used Fe catalysts for NH3 synthesis also at high pressure.

In-situ surface and gas phase analysis for kinetic studies under transient conditions The catalytic hydrogenation of CO2

Abstract: Transient measurement techniques are applied for the kinetic investigation of the reaction mechanism of the carbon dioxide methanation, using diffuse reflectance infrared spectroscopy and mass spectrometry. The coupled information of the surface intermediates and the gas phase components time evolution leads to accurate identification of spectator species on the surface. Reaction intermediates, carbon monoxide and formates have been identified. The former is a key intermediate, and its hydrogenation leads to methane formation. The latter is fixed on the support, in equilibrium with a active formate species on the interface metal-support. A reaction mechanism is proposed including the formation step of the formates through a carbonate species.

Partial oxidation of methanol to produce hydrogen over CuZn-based catalysts

Abstract: In this work CuZnO and Cu/ZnO/Al2O3 catalysts have been studied for the partial oxidation of methanol with oxygen to produce hydrogen. These CuZn based catalysts showed high activity for the partial oxidation of methanol and it was found that the catalytic activity is directly related to the copper metal surface area. In the series CuZn with copper relative content of 20–70 wt%, the catalyst Cu40Zn60 (Cu 40 wt% and Zn 60 wt%) which showed the highest copper area gave the best results for the partial oxidation of methanol. The activation energies and TOF (turnover frequencies) varied with the CuZn catalyst composition. For catalysts with low copper loading very high Ea and TOF were obtained (for Cu30Zn70 Ea=482 kJ/mol and TOF ca. 200 min−1 at 497–499 K) whereas for higher copper contents the Ea and TOF decreased tending to constant values (for Cu70Zn30 Ea=71 kJ/mol and TOF=160 min−1 at 497–499 K). These results are discussed in terms of a possible effect of the CuZnO interaction which depends on the catalyst composition. Catalytic experiments with Cu40Zn55Al5 (Cu 40 wt%, Zn 55 wt% and Al 5 wt%) showed that the presence of aluminium has an inhibiting effect producing slightly lower methanol conversion. On the other hand, higher selectivities for H2 and CO2 were obtained with only traces of the undesirable carbon monoxide. Moreover, the Al is very important for catalyst stability and life-time experiments showed that Cu40Zn55Al5 is stable during the partial oxidation of methanol with no significant change in activity and selectivity even after 110 h operation at 503 K. The catalyst Cu40Zn60 with no Al, deactivates rapidly after 20 h reaction at 503 K. Experiments using N2O as oxidant showed higher activity to convert methanol but producing large amounts of H2O and CO. The impregnation of catalyst with Na produced similar effect increasing the selectivity for H2O and CO. The results presented seem to indicate that the copper metal is active for partial oxidation of methanol to H2 and CO2 whereas Cu+1 favour the formation of H2O and CO. Cu+2 as CuO shows very low activity for methanol conversion producing only CO2 and H2O.

Promoting effect of Pt, Pd and Rh noble metals to the Ni0.03Mg0.97O solid solution catalysts for the reforming of CH4 with CO2

Abstract: Reforming of CH4 with CO2 to produce synthesis gas was studied over Ni0.03Mg0.97O solid solution catalyst and its bimetallic derivative catalysts which contained small amounts of Pt, Pd and Rh, respectively (the atomic ratio of M/(Ni+Mg) was varied from 0.007 to 0.032%, M=Pt, Pd or Rh). It was found that there was a significant promotion in both activity and stability due to the addition of noble metals. The optimum loadings of these noble metals were observed at the atomic ratio of M/(Ni+Mg) ≈0.021%. Temperature programmed hydrogenation (TPH) of carbonaceous species formed during the catalytic reaction indicated that the good resistance of Ni0.03Mg0.97O solid solution to carbon deposition was retained on the bimetallic catalysts. Furthermore, we obtained a clear evidence showing the formation of Pt—Ni alloy particles by means of TEM and EDS observation. The kinetic study and temperature programmed reaction (TPR) of CH4 suggest that CH4 decomposition as the rate-determining step on the Ni0.03Mg0.97O is accelerated by the alloy formation. The improved stability was attributed to the promotion in catalyst reducibility.

Metal-support boundary sites in catalysis

Abstract: In highly-dispersed metal catalysts, the supporting oxide stabilises the metal particles, but may also promote individual reaction steps. In particular, many reactions are believed to proceed, preferentially or exclusively, at the metal-oxide interface which contains sites with distances between metal atoms and support in the range of atomic dimensions. In this paper, the physicochemical characterisation of these so-called adlineation sites and their possible catalytic effects are reviewed. It will be shown in various examples that they may have a particular activity and can be poisoned selectively. They may contribute to several particle size effects observed in hydrocarbon reactions, e.g. to ring opening and isomerization, but they may also be responsible for CO activation on noble metals supported by reducible oxides. Furthermore, they may play a role in selectively adsorbing chlorine which is an important component in catalyst restructuring. It will be attempted to detect and to relate common features of the observed phase boundary effects and to seek for a common explanation. The paper is, however, not intended as a review of strong metal-support interaction phenomena. The ring opening of methylcyclopentane on platinum is particularly sensitive to phase-boundary effects. Starting out from the authors' results it will be shown how this reaction is influenced by modifications of the support, by changes of hydrogen pressure and reduction state, and by selectively poisoning the interface. Further on, the CO hydrogenation on supported Rh and the CH4CO2 reforming reaction on Pt ZrO 2 serve as examples for CO activation, and the influence of Lewis acid sites at the boundary on the catalytic activity is discussed in more detail. The last chapter is dedicated to the role of adsorbed chlorine and to the partly controversial results concerning the effect of halogen additives on the properties of supported catalysts, including the possible existence of ‘soluble platinum’.

Hydrotalcites as base catalysts. Kinetics of Claisen-Schmidt condensation, intramolecular condensation of acetonylacetone and synthesis of chalcone

Abstract: The activity of hydrotalcites has been investigated by means of several reactions able to evidence their basic properties and the mechanism involved, and also to show their interest in the synthesis of fine chemicals of great practical interest for pharmaceutical applications. With this aim, the Claisen-Schmidt condensation of benzaldehyde with excess acetone has been performed in solid-liquid conditions. The kinetic study suggests a Langmuir-Hinshelwood mechanism, in which the rate-determining step would be the condensation of benzaldehyde and acetone, in agreement with former results of Hammett correlations. The reaction rate goes through a maximum as a function of calcination temperature of the catalyst, or the water content of the solvent, suggesting that the active sites are basic hydroxyls. These results are obtained by the intramolecular condensation of acetonylacetone as test reaction giving selectively 3-methyl-2-cyclopentanone, and showing, indeed, the presence of mainly basic sites on the hydrotalcite. Taking these results into account, the synthesis of chalcone has been performed over a suitably activated hydrotalcite.

Vanadium pentoxide I. Structures and properties

Abstract: Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Structure and properties of V 2 O 5 are described and the role of three types of oxygen atoms present in the lattice: vanadyl oxygen atoms O(1) coordinated only to one vanadium atom, and bridging oxygen atoms O(2) and O(3) coordinated to two and three vanadium atoms, respectively, is discussed. Equilibration of gas phase oxygen with vanadium oxides results in the formation of the intrinsic defect structure of V 2 O 5 consisting of oxygen vacancies. Discussion of the properties of vacancies as deduced from measurements of electrical conductivity, EPR and IR spectra and the stability of different oxygen vacant sites as obtained from quantum-chemical calculations is given. Mechanism of the reduction of V 2 O 5 through crystallographic shear, resulting in the formation of V 6 O 13 or V 4 O 9 , is illustrated. It is shown that because of a pronounced anisotropy of V 2 O 5 crystal structure sensitivity of catalytic reactions appears. Two types of crystal planes are exposed. The (0 1 0) basal plane has all chemical bonds almost fully saturated. The non-bonding d-orbitals of V ions have the LUMO character and act as Lewis acid sites, whereas the lone electron pairs of bridging oxygen atoms have the HOMO character and behave as Lewis basic sites. On the (1 0 0) and (0 0 1) planes cleavage leaves coordinatively unsaturated vanadium and oxygen ions, which develop Bronsted acid-base interactions with reacting molecules, causing the heterolytic chemisorption. Oxygen vacancies in the lattice are replenished through oxidation by gas phase oxygen, which sometimes is considered as oxygen chemisorption.

Kinetic study of the reverse water-gas shift reaction over CuO/ZnO/Al2O3 catalysts

Abstract: The kinetics of the reverse water-gas shift (RWGS) reaction over CuO/ZnO/Al2O3 catalysts was studied by use of CO2H2 cycles, hydrogen chemisorption and catalytic tests performed in both differential and integral plug flow reactors. The effect of the reactant composition on the reaction rate was specifically studied by changing the PH20/PCO20 ratio between 9.0 and 0.3. It was found that different reagents become rate limiting depending upon pressure. While in a H2-rich region the rate increases strongly with CO2 partial pressure and is zero order in hydrogen, under low PH20/PCO20 ratios the reaction is less active and is strongly positive order in hydrogen and low order in carbon dioxide. The experimental data were modeled by considering that the reaction proceeds through a surface redox mechanism, copper being the active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the dissociative CO2 adsorption (H2-rich region) or both the CO2 dissociation and the water formation (H2-lean region) determine the rate of the overall reaction. Based on previous studies performed on copper crystal surfaces, such a change in kinetics may be explained by assuming that under H2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociation.

On the flexibility of the active phase in hydrotreating catalysts

Abstract: This review deals with the structure of the active phase in sulfidic Co Mo and Ni Mo hydrotreating catalysts. Various models describing the catalyst and its interaction with the reaction environment are discussed in the light of the evolution of the active phase during the catalyst life cycle. Special attention is paid to the contribution of Mossbauer Emission Spectroscopy (MES), Extended X-ray Absorption Fine Structure (EXAFS), High Resolution Transmission Electron Microscopy (HR-TEM) and Molecular Modeling to the unraveling of the active phase structure. It is concluded that MoS 2 sintering and Co 9 S 8 (Ni 3 S 2 , NiS) segregation during the catalyst life cycle force the shift of the actual reaction mechanism from that involving a single site or an ensemble of sites to that of a close cooperation between segregated components. The adsorption of reactants can take place in many diferent ways, through the heteroatom or through the ring, on the Mo sites as well as on the Co (Ni) sites. The exact state of each active site depends on the reaction environment. If the catalyst is S deficient, the classical adsorption mechanism involving the S vacancy is dominant. With a fully sulfided catalyst surface the adsorption takes place through S S bonds. The catalyst is a dynamic evolving system adapting itself to its ever changing reaction environment. Each catalyst component fulfills multiple functions in determining the catalyst structure as well as its interaction with the reactant molecules.

Preparation and properties of vanadia/titania monolayer catalysts

Abstract: Work performed on vanadia/titania catalysts at Brunel University over the past 20 years is summarised, and the thinking that led to the concept of reactive oxide monolayers is described. Both impregnation and grafting techniques lead to monolayers, and their structure, and that of the supramonolayer region is revealed through characterisation by a number of physical techniques. Factors responsible for the stability of monolayer systems are considered, and the basis of their activity in selective oxidations is discussed.

Mixed manganese oxide/platinum catalysts for total oxidation of model gas from wood boilers

Abstract: Mixed manganese oxide/platinum catalysts in the form of monoliths have been prepared by the deposition-precipitation method. The influence of the platinum content and the calcination temperature has been investigated by activity measurements for total oxidation of methane, naphthalene and carbon monoxide in the presence of steam and carbon dioxide, and by the temperature programmed reduction (TPR) technique. Manganese oxide behavior is influenced by the presence of even very small amounts of platinum especially when the catalyst is calcined at a higher temperature due to favorable synergetic effects.

Liquid phase oxidation reactions by peroxides in the presence of vanadium complexes

Abstract: The reaction of vanadium (V) derivatives with hydrogen peroxide or with alkyl hydroperoxides leads to peroxovanadium complexes which are effective and selective oxidants of organic and inorganic substrates either under stoichiometric or catalytic conditions. Examples of synthetically significant oxidations of various classes of organic compounds are reported together with details on the commonly accepted mechanisms operating. When possible a comparison is made between peroxovanadium complexes of other do metal derivative, e.g. Ti(IV), Mo(VI) and W(VI).

Partial oxidation of methane to syngas over nickel-based catalysts modified by alkali metal oxide and rare earth metal oxide

Abstract: The NiO/Al2O3 catalyst was modified by alkali metal oxide (Li, Na, K) and rare-earth metal oxide (La, Ce, Y, Sm) in order to improve the thermal stability and the carbon-deposition resistance during the partial oxidation of methane to syngas (POM) reaction at high temperature. The reaction performance, thermal stability, structure, dispersity of nickel and carbon-deposition of the modified NiO/Al2O3 catalyst and unmodified NiO/Al2O3 catalyst were investigated by a series of characterization techniques including flow-reaction, BET, XRD, CO chemisorption and TG analysis. The results indicated that the modification with alkali metal oxide and rare-earth metal oxide improves the dispersion of active component nickel and the activity for the POM reaction over the nickel-based catalysts, and enhances their thermal stability during high temperature reaction and the ability to suppress the carbon-deposition over the nickel-based catalysts during the POM reaction. The nickel-based catalysts modified by alkali metal oxide and rare-earth metal oxide have excellent POM reaction performance (CH4 conversion of 94.8%, CO selectivity of 98.1%, 2.7×104l/kg·h), excellent stability and carbon-deposition resistance.

In-situ FT-IR study on CO2 hydrogenation over Cu catalysts supported on SiO2, Al2O3, and TiO2

Abstract: For methanol synthesis from CO 2 hydrogenation, TiO 2 -supported Cu catalyst (Cu/TiO 2 ) showed the highest turnover frequency (specific rate per one active site) among three catalysts (Cu/Al 2 O 3 , Cu/SiO 2 , and Cu/TiO 2 ). According to in-situ FT-IR observations, the peaks of adsorbed formate species observed during the reaction were suppressed for Cu/TiO 2 , although they were dominant for both Cu/Al 2 O 3 and Cu/SiO 2 . In order to understand the relationship between IR spectra and the activity, as well as to find out the essential factors that control the activity, we investigated the reactivity of intermediate species mainly by in-situ FT-IR spectroscopy. Each Cu site itself had little ability to adsorb CO 2 ; however, Al 2 O 3 and TiO 2 readily adsorbed CO 2 by weak bondings on them. CO 2 species adsorbed on Cu sites were rapidly converted to formate species under reaction conditions. Formate species generated on Cu sites of Cu/SiO 2 remained under H 2 at 473 K. On Cu/Al 2 O 3 the formate species adsorbed on the support firmly, so it was difficult to distinguish the formate on Cu from that on the support (Al 2 O 3 ), and they were stable under H 2 at 473 K. On Cu/TiO 2 , hydrogenation of formate species occurred at 423 K. The behavior of IR spectra during reaction over Cu/TiO 2 could be explained by the promotion of formate hydrogenation by the synergetic effect between Cu and TiO 2 .

Oxidative dehydrogenation of alkanes over vanadium-magnesium-oxides

Abstract: Vanadium-magnesium-oxides are among the most selective and active catalysts for the oxidative dehydrogenation of alkane. The selectivity for dehydrogenation depends strongly on the alkane, the Mg vanadate phase, the presence of modifiers, and to a lesser extent, the method of preparation. The results of studies on these variables are reviewed and discussed with respect to the current understanding of the nature of the active site and requirements for selective dehydrogenation.

Gas phase hydrogenation of crotonaldehyde over Pt/Activated carbon catalysts. Influence of the oxygen surface groups on the support

Abstract: Three activated carbons were prepared with different content of oxygen surface complexes and impregnated with aqueous solutions of [Pt(NH34]Cl2. The catalysts were characterized by H2 and CO chemisorption at room temperature, temperature-programmed decomposition (TPD) and X-ray photoelectron spectroscopy, and their catalytic behavior in the vapor phase hydrogenations of benzene and crotonaldehyde (trans-2-butenal) was determined. Metal dispersion is highly dependent on the degree of support oxidation, being lower for the catalyst support containing the highest amount of surface acidic complexes. This is attributed to the decomposition of the surface complexes, which act as anchoring centers for the platinum precursor, upon the reduction treatments at which the catalysts are subjected. The specific catalytic activity in the gas phase hydrogenation of crotonaldehyde is higher as the starting support is more oxidized, and the activity per gram of platinum increases with reduction temperature. The selectivity for the hydrogenation of the carbonilic CO bond instead of the olefinic CC bond is also improved when an oxidized support is used, and the production of the unsaturated alcohol, crotyl alcohol, is enhanced when catalysts are reduced at higher temperature, especially those prepared with oxidized supports.

Effect of the carbon pre-treatment on the properties and performance for nitrobenzene hydrogenation of Pt/C catalysts

Abstract: This paper reports a study of the effect of the purification and functionalization treatments of a peach pit derived carbon on the properties and performance for nitrobenzene hydrogenation reaction of Pt/C catalysts. Results show that the elimination of inorganic impurities, mainly sulphur, enhances the nitrobenzene hydrogenation rate. Moreover, the functionalization treatments of purified carbon with ozone and hydrogen peroxide have a positive effect both on the Pt dispersion and on the hydrogenation capacity of the catalyst, while the HNO3-treatment has a lower effect. The effect of the different oxidants can be related to the nature of the functional groups developed on the carbon surface. Thus, HNO3-treated carbon displays a high density of both strong and weak acid sites, while H2O2- and O3-treated carbons show an important concentration of weak acid sites but a low concentration of strong acid sites, according to the TPD results. Moreover, the H2PtCl6 isotherms in liquid phase at 298 K show a stronger interaction of the metallic precursor with the carbons of low acidity (like those treated with H2O2 or O3) than with the most acidic carbon (treated with HNO3). Carbons functionalized with weak oxidants, which develop acidic sites with moderate strength and show strong interaction with H2PtCl6 during impregnation, would favour the Pt dispersion on the carbon surface and consequently the catalytic behaviour.