Showing papers in "Applied Catalysis A-general in 1999"

Short history and present trends of Fischer–Tropsch synthesis

Abstract: Due to the large volume of existing literature on Fischer–Tropsch (FT) synthesis, the diversity of the subject and the actually reoriented interest, it seemed indicated to write a historical sketch about the process, putting also emphasis on present trends and future options. The matter is complicated and may be regarded from different positions. Thus history and trends have been divided into several lines which are elaborated individually: • The changing environment of demand and supply of fossil energies and the more stringently upcoming aspects of pollution control, of cleanliness of the automotive fuels and of energy saving. • The development of FT-reactors and processes, an area of highest present industrial activity and progress. • Preparation and performance of catalysts with particular concern about cobalt as the base metal. • FT intermediates and elemental reactions, difficult subjects as the FT multistep conversion features on the catalyst surface in the adsorbed state and kinetic principles as selective inhibition and spatial constraints seem to rule the regime. • Kinetic modelling of FT synthesis which has made great progress recently, particularly on the basis of newly provided detailed experimental data. Of course, presenting history and trends of FT synthesis on a few pages means generalising from many individual investigations and developments and also selection of only a few citations. So I want to apologise for all the contributions to science and technology around FT synthesis which I have missed to include into the article.

Industrial application of solid acid–base catalysts

Abstract: A statistical survey of industrial processes using solid acid–base catalysts is presented. The number of processes such as alkylation, isomerization, amination, cracking, etherification, etc., and the catalysts such as zeolites, oxides, complex oxides, phosphates, ion-exchange resins, clays, etc., are 127 and 180, respectively. The classification of the types of catalysts into solid acid, solid base, and solid acid–base bifunctional catalysts gives the numbers as 103, 10 and 14, respectively. Some significant examples are described more in detail. On the basis of the survey, the future trend of solid acid–base catalysis and the fundamental research promising for industrial success are discussed.

The multiple roles for catalysis in the production of H2

Abstract: This review describes a number of different, largely catalytic approaches for producing H2. Since a major fraction of the world's H2 is produced by catalytic processes, involving multiple steps with different types of catalysts, it is clear that catalysis plays a critical role in the production of H2. This review is focused on the use of catalysis for the current and future production of H2. Some background will be provided to give a perspective of the dramatic change in the supply and demand for H2 in the past decade, followed by a review of how it is produced commercially, with a view to how multiple types of catalysis contribute to the total process for H2 production. Steam methane reforming, the major approach for H2 manufacture, will be a focal point for most of the discussion in pointing out the large number of catalytic steps that are used in this major technology. Finally, some alternative catalytic approaches for H2 production will be described.

Methanol–steam reforming on Cu/ZnO/Al2O3 catalysts. Part 2. A comprehensive kinetic model

Abstract: Surface mechanisms for methanol–steam reforming on Cu/ZnO/Al 2 O 3 catalysts are developed which account for all three of the possible overall reactions: methanol and steam reacting directly to form H 2 and CO 2 , methanol decomposition to H 2 and CO and the water-gas shift reaction. The elementary surface reactions used in developing the mechanisms were chosen based on a review of the extensive literature concerning methanol synthesis on Cu/ZnO/Al 2 O 3 catalysts and the more limited literature specifically dealing with methanol–steam reforming. The key features of the mechanism are: (i) that hydrogen adsorption does not compete for the active sites which the oxygen-containing species adsorb on, (ii) there are separate active sites for the decomposition reaction distinct from the active sites for the methanol–steam reaction and the water-gas shift reaction, (iii) the rate-determining step (RDS) for both the methanol–steam reaction and the methanol decomposition reaction is the dehydrogenation of adsorbed methoxy groups and (iv) the RDS for the water-gas shift reaction is the formation of an intermediate formate species. A kinetic model was developed based on an analysis of the surface mechanism. Rate data were collected for a large range of conditions using a fixed-bed differential reactor. Parameter estimates for the kinetic model were obtained using multi-response least squares non-linear regression. The resultant model was able to accurately predict both the rates of production of hydrogen, carbon dioxide and of carbon monoxide for a wide range of operating conditions including pressures as high as 33 bar.

Methanol–steam reforming on Cu/ZnO/Al2O3. Part 1: the reaction network

Abstract: On-board generation of hydrogen by methanol–steam reforming on Cu/ZnO/Al 2 O 3 catalyst is being used in the development of fuel-cell engines for various transportation applications. There has been disagreement concerning the reactions that must be included in the kinetic model of the process. Previous studies have proposed that the process can be modelled as either the decomposition of methanol followed by the water-gas shift reaction or the reaction of methanol and steam, to form CO 2 and hydrogen, perhaps followed by the reverse water-gas shift reaction. Experimental results are presented which clearly show that, in order to explain the complete range of observed product compositions, rate expressions for all three reactions (methanol–steam reforming, water-gas shift and methanol decomposition) must be included in the kinetic analysis. Furthermore, variations in the selectivity and activity of the catalyst indicate that the decomposition reaction occurs on a different type of active site than the other two reactions. Although the decomposition reaction is much slower than the reaction between methanol and steam, it must be included in the kinetic model since the small amount of CO that is produced can drastically reduce the performance of the anode electrocatalyst in low temperature fuel cells.

Catalyst and process technologies for ultra low sulfur diesel

Abstract: The production of clean diesel by hydrotreating and deep hydrodesulfurization (HDS) has attracted increased attention recently due to the introduction of new environmental legislation regarding fuel specifications. In order to meet the specifications there is a need to modify and improve existing reactors and processes and to introduce more active and selective catalysts. The removal of sterically hindered sulfur-containing molecules is observed to be a key issue for deep HDS. Also the choice of operation conditions and reactor internals play an important role for deep HDS. The present article will focus on key hydrotreating options available to obtain ultra low sulfur diesel levels and some of the theoretical and experimental structure–activity relationships which may aid catalyst developments.

Effects of the support on the morphology and electronic properties of supported metal clusters : modern concepts and progress in 1990s

Abstract: Modern trends in studying the influence of the support on the electronic state, morphology, and catalytic properties of supported metal particles are reviewed. The analysis of new developments in techniques for catalyst characterization is given. Metal–support interaction effects are discussed in terms of electronic modification of the supported clusters and their morphological transformations induced by the support. Special attention is paid to the recent concepts of the structure of the metal–support interface. The support effects on the catalytic properties of metal particles are revealed as (1) changes due to metal particle charging, (2) effects related to variations in metal particle shape and crystallographic structure and, (3) appearance of the specific active sites at the metal–support boundary.

Comparative study of Fischer–Tropsch synthesis with H2/CO and H2/CO2 syngas using Fe- and Co-based catalysts

Abstract: Hydrogenation of CO, CO2 and their mixtures has been comparatively studied with a Co–MnO–Aerosil–Pt and a Fe–Al2O3–Cu–K catalyst at the University of Karlsruhe. With iron catalysts as promising for CO2 hydrogenation, their composition was varied: (1) several supports (SiO2, TiO2, Al2O3), (2) alkali promotion (Li, Na, K, Rb), (3) usage of Zeolite Y as catalyst component. The catalysts were characterised by adsorption methods, XRD, TPR and temperature programmed decarburisation after a H2/CO2 treatment (Korea Research Institute of Chemical Technology). Iron and cobalt catalysts behaved differently in CO2 hydrogenation. With the alkalised iron catalyst the same hydrocarbon product composition was obtained from a H2/CO2 and from a H2/CO synthesis gas in spite of the CO partial pressure remaining low, specifically due to water gas shift equilibrium constraints. With the cobalt catalyst at increasing CO2 and respectively decreasing CO content of the syngas, the product composition shifted from a Fischer–Tropsch type (mainly higher hydrocarbons) to almost exclusively methane. These basically different catalyst behaviours are explained by different modes of formation of the kinetic regime of FT synthesis—selective inhibition of methane formation and the selective inhibition of product desorption as a prerequisite for chain growth—in the case of iron through irreversible carbiding and alkali surface coverage and in case of cobalt through strong reversible CO adsorption. Investigation of the various modified iron catalysts showed alumina to be the best support for CO2 hydrogenation and potassium to act as a powerful promotor. With the Fe–Y–zeolite–alkali catalysts, a decrease of methane selectivity was observed in the order Li < Na < K < Rb being applied as promotors.

Disproportionation and transalkylation of alkylbenzenes over zeolite catalysts

Abstract: Disproportionation and transalkylation are important processes for the interconversion of mono-, di-, and tri-alkylbenzenes. In this review, we discuss the recent advances in process technology with special focus on improvements of para-isomer selectivity and catalyst stability. Extensive patent search and discussion on technology development are presented. The key criteria for process development are identified. The working principles of para-isomer selectivity improvements involve the reduction of diffusivity and the inactivation of external surface. In conjunction with the fundamental research, various practical modification aspects particularly the pre-coking and the silica deposition techniques, are extensively reviewed. The impact of para-isomer selective technology on process economics and product recovery strategy is discussed. Furthermore, perspective trends in related research and development are provided.

Heterogeneous catalytic transformations for environmentally friendly production

Abstract: Progress towards environmentally responsible processing is marked by the elimination of waste and by-product generation and reduced dependence on hazardous chemicals. The key to both is often provided by catalytic as alternatives to stoichiometric processes. Heterogeneous catalysis, long established in bulk-chemicals processing, is beginning to make inroads into the fine-chemicals industry also. This tendency is helped by the availability of novel catalytic materials and modern techniques of creating and investigating specific active sites on catalyst surfaces. In this overview, examples from the areas of acid/base and redox catalysis are chosen to illustrate the trend from non-catalytic to catalytic processes.

Low temperature Fischer–Tropsch synthesis from a Sasol perspective

Abstract: The characteristics of a slurry phase reactor are contrasted with those of a conventional tubular fixed bed reactor (TFBR) for the conversion of synthesis gas to long chain hydrocarbons. Hydrodynamic information needed for the design of a commercial scale slurry phase Fischer–Tropsch (FT) reactor were obtained from experiments carried out on a 1 m internal diameter pilot plant reactor. The kinetics, selectivities and deactivation mechanisms of Fe and supported Co FT catalysts are compared for both slurry phase and fixed bed operation. The combined advantages of the slurry phase reactor and a very active Co catalyst create the opportunity to convert remote natural gas to high quality middle distillates in a cost effective manner.

Preparation and characterization of Fischer-Tropsch active Co/SiO2 catalysts

Abstract: Silica supported cobalt catalysts have been prepared by a new method combining the precipitation of the metal precursor (cobalt nitrate) by oxalic acid with the hydrolysis and condensation of the silicium precursor (tetraethoxysilane). Depending on the pH during preparation, the textural properties (BET specific surface area, porosity) of the Co/SiO 2 catalysts can be modified. In an acid medium (pathway A), the resulting silica is constituted by a polymeric net with few branchings, the catalysts are microporous. In basic medium (pathway B) silica is composed of more branched polymers leading to mesoporous catalytic systems. After calcination, the only crystallized phase detected by XRD is the Co 3 O 4 spinel. At 773 K, the surface degrees of reduction of the catalysts at cobalt isocontent (25 wt.% Co) as determined by XPS is of 81% and 69% for systems prepared by pathways A and B, respectively. The presence of small unreduced Co II suggests the existence of non-crystallized cobalt silicate formed during the reduction by reaction of CoO with silica. The activity for the CO + H 2 reaction for the 25 wt.% catalysts prepared by pathway A increased with the specific surface area which can be controlled by the preparation parameters. The reduction degree has a direct influence on the selectivity for the Co/SiO 2 catalysts. The presence of a part of unreduced cobalt (in interaction with the support) results in a better selectivity to the C 5 –C 13 fraction (gasoline), whereas a higher reduction degree of cobalt favors the production of higher molecular weight hydrocarbons (waxes) (C 22 + selectivity > 40%).

Recent advances in methane dehydro-aromatization over transition metal ion-modified zeolite catalysts under non-oxidative conditions

Abstract: The effective activation and direct conversion of methane to higher hydrocarbons is a topic of great challenge in catalysis science. Besides oxidative activation such as oxidative coupling of methane to C2+, non-oxidative activation of methane to produce aromatics over Mo/HZSM-5 catalysts in a continuous flow mode has attracted significant interest since 1993. This paper reviews the recent advances in catalytic dehydro-aromatization of methane over Mo/HZSM-5 catalysts without the use of oxidants. The catalysts and reaction conditions are presented. Emphasis has been focused on the modification of catalysts and optimization of reaction conditions. The results of investigations on the interactions between Mo and/or transition metal ions and the zeolite support, as well as, the reaction mechanisms of the formation of aromatics and carbonaceous deposits are discussed.

Study of the effect of water on alumina supported cobalt Fischer–Tropsch catalysts

Abstract: The effect of water on Co/Al 2 O 3 and CoRe/Al 2 O 3 catalysts has been studied by adding water to the synthesis gas feed and by model studies exposing the catalysts to H 2 O/H 2 feeds using several characterization techniques such as TPR, gravimetry, XPS, TPD and pulse adsorption. It was found that the CoRe/Al 2 O 3 catalyst deactivates when water is added during Fischer–Tropsch synthesis and model-studies showed that this catalyst oxidizes in H 2 O/H 2 mixtures with a ratio much lower than expected for oxidation of bulk cobalt. The reoxidation increases with increasing H 2 O partial pressure and H 2 O/H 2 ratio. TPR and gravimetry showed only small amounts of bulk reoxidation, while pulse adsorption and TPD indicated large decreases in Co-surface metal. It is suggested that oxidation of highly dispersed phases or surface oxidation are the cause for the observed deactivation. Significant differences in behavior of the Co/Al 2 O 3 and the CoRe/Al 2 O 3 catalyst when exposed to H 2 O/H 2 /He were found by gravimetry, TPR, pulse adsorption and XPS. The CoRe/Al 2 O 3 catalyst was reoxidized more easily in H 2 O/H 2 /He mixtures than the Co/Al 2 O 3 catalyst. This is probably a result of the higher dispersion of the CoRe/Al 2 O 3 catalyst, but a direct influence of Re on the reoxidation cannot be excluded. A phase interacting strongly with the alumina support was found in both catalysts after H 2 O/He exposure, but also another oxide phase was formed. This second phase is reduced at lower temperature for the CoRe/Al 2 O 3 catalyst than for the Co/Al 2 O 3 catalyst.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups

Abstract: Activated carbons were used as catalysts for the oxidative dehydrogenation of ethylbenzene. The type and amount of functional groups on the surface of the carbon catalysts was modified by oxidative treatments in the gas or in the liquid phase, while heat treatments at different temperatures were used to selectively remove some of the functional groups. The performance of these catalysts was evaluated in terms of conversion, styrene yield and selectivity. The results show that the gas phase treatments lead to improved performance associated with an increase in the amount of carbonyl/quinone groups on the surface, which were identified as the active sites for the reaction. A good correlation between catalytic activity and the concentration of these surface groups was obtained.

Fundamentals and selection of advanced Fischer-Tropsch reactors

Abstract: Fischer-Tropsch synthesis on a large scale is of interest as a means for conversion of remote natural gas to high-quality products, particularly liquid transportation fuels. Recent developments have resulted in reactors of advanced design having production capacities of 2500 bbl/day or higher, which is more than two orders of magnitude higher than the productivity of classical reactors operated before or during World War II. Some fundamental aspects of these reactors, which belong to the classes of gas-solid fluidized beds, multitubular trickle-beds, and slurry bubble columns are discussed to aid selection and design of reactors for a specific application. Special attention is given to scaling up of slurry bubble columns. A scaling-up strategy is proposed which might obviate the inclusion of a costly demonstration stage in the development of a novel process using bubble columns.

Comparison of patented Co F–T catalysts using fixed-bed and slurry bubble column reactors

Abstract: This paper addresses the current generation of patented cobalt-based Fischer–Tropsch catalysts. It provides an overview of the patent literature and an experimental comparison at the same reaction conditions of a number of the more important patented catalysts. Based on information provided in patents assigned to Gulf, Shell, Exxon, and Statoil, a series of catalysts was prepared consisting of 12–20 wt% cobalt, a second metal promoter (Ru or Re), and an oxide promoter such as lanthana, zirconia, or alkali oxide, the support being alumina, silica, or titania. All catalysts were extensively characterized by different methods. In addition, the catalysts were evaluated in terms of their F–T activity and selectivity, both in a fixed-bed reactor and in a slurry bubble column reactor. The reaction results are correlated with their physico-chemical properties.

Reactions of α-olefins of different chain length added during Fischer–Tropsch synthesis on a cobalt catalyst in a slurry reactor

Abstract: Based on the picture of Fischer–Tropsch synthesis as an ideal polymerization reaction it is easily noticeable that primarily formed product olefins can undergo secondary reactions, which generally lead to carbon number dependencies of certain olefin reaction pathways and thereby to modifications of product distributions. To obtain more insight into the role of secondary olefin reactions during Fischer–Tropsch synthesis and the reason for their chain length dependence, extensive studies on their extent and selectivity and particularly their chain length dependence at different reaction conditions (variation of CO partial pressure and reaction temperature) were performed by co-feeding α-olefins (C2–C11) during FT synthesis with cobalt catalysts in a gradientless slurry reactor. Conversion of the added olefins revealed a strong carbon number dependence exhibiting a sharp minimum at C3. The observed chain length dependencies of individual reaction pathways (hydrogenation and incorporation) are in agreement with the selectivities observed in the base case experiments without olefin addition and mainly due to carbon number dependent solubility of the olefins in the liquid reaction product.

Monomolecular cracking of n-hexane on Y, MOR, and ZSM-5 zeolites

Abstract: The activity and activation energy for cracking of n-hexane were measured over three zeolites: HZSM-5, H-MOR, H-USY (ultrastable Y zeolite), and CDHY (AHFS dealuminated Y zeolite), under conditions where the product selectivities were nearly identical and there was no deactivation of the catalysts. The identical selectivities implied that the same reaction mechanism for cracking was occurring over all three catalysts. Within experimental error, the differences in apparent activation energies could be entirely attributed to differences in heats of n-hexane adsorption, such that the intrinsic activation energies were identical. These results suggest that the kinetics of cracking is insensitive to any possible differences in acid strength among these catalysts, and the large enhancement in activity at lower temperatures observed upon steam dealumination of Y zeolite is due to effects other than changes in acid strength.

High temperature Fischer–Tropsch synthesis in commercial practice

Abstract: The commercial application of high temperature Fischer–Tropsch (HTFT) technology is described. The types of reactor used are discussed with emphasis on the advantages of using the Sasol Advanced Synthol (SAS) reactor. A reactor replacement project at Secunda, South Africa will result in a total installed capacity of 124 000 barrel per day of products from the new SAS reactors. The preparation of the catalyst used in these reactors is described. The changes to the catalyst structure during synthesis are illustrated, making it clear that the actual catalyst consists of small carbide/magnetite particles embedded in a continuous carbon phase. The changes in catalyst morphology which occur during synthesis are discussed with reference to the effect on the observed post sulphur poisoning behaviour. The SAS process can be considered to be a mature technology which has been applied in large-scale commercial plants. Future applications should take advantage of the considerable potential to recover chemical feedstocks from the primary Synthol products. The potential amounts of these chemical feedstocks are quantified.

Selective saccharides dehydration to 5-hydroxymethyl-2-furaldehyde by heterogeneous niobium catalysts

Abstract: Selective dehydration of different substrates, such as fructose, sucrose and inulin, to 5-hydroxymethyl-2-furaldehyde has been studied in aqueous medium by using heterogeneous niobium-based catalysts. Batch experiments have been performed in the presence of both commercial niobium phosphates and catalysts prepared by treatment of niobic acid with phosphoric acid. Flow experiments on catalyst packed beds have been also examined. Finally, batch catalytic experiments, characterized by combination of reaction and products extraction steps, have been also carried out.

Kinetic modelling of Fischer–Tropsch product distributions

Abstract: Starting from the idea of Fischer–Tropsch (FT) synthesis as an ideal polymerisation reaction it is easily realised, that product olefins undergo secondary reactions and thereby modify the product distribution. This generally leads to chain length dependencies of certain olefin reaction possibilities, which are again suited to serve as a characteristic feature for the kind of olefin conversion. By extending an existing model and accounting for olefin readsorption, incorporation, hydrogenation and isomerisation via double bond shift and the chain length dependence of product solubilities, typical deviations from ideal distributions can be simulated and experimentally observed data with cobalt and iron catalysts can be satisfactorily described, suggesting the correctness of the assumptions made. Furthermore, other existing models dealing with the same issue are briefly discussed in this paper.

Studies on product distributions of iron and cobalt catalyzed Fischer–Tropsch synthesis

Abstract: The dependencies of hydrocarbon product distributions of iron and cobalt catalyzed Fischer–Tropsch synthesis on partial pressures of reactants have been studied. For cobalt catalysts particular attention has been focussed on the modification of distributions by secondary chain growth of readsorbed 1-alkenes while for iron catalysts secondary chain growth has been proved as negligible. The widely discussed concept of two superimposed Anderson–Schulz-Flory distributions has been applied for the representation of product distributions for both iron and cobalt catalysts. Based on 1-alkene and ethene cofeeding experiments with cobalt catalysts and the promoter effect of alkali on iron catalysts the conclusion has been drawn that superimposed distributions with different chain growth probabilities are the result of different chain growth mechanisms. These results and the dependence of product distribution on the partial pressures of hydrogen and carbon monoxide have lead to the conjecture that products associated with the lower growth probability are formed by the well accepted CH2 insertion mechanism.

Fischer–Tropsch reactions and the environment

Abstract: Suitable economic conditions given, the Fischer–Tropsch (FT) process is an alternative route to liquid fuels and chemicals (in particular linear 1-alkenes). Being S and N free and low in aromatics, the fuels are more environment friendly than those produced from crude oil. In particular, the production of environment friendly high quality diesel fuel is an attractive application of the FT process. Relatively large amounts of CO2 are produced in the gasification processes, but whether this will really contribute to global warming is a disputed question. The water effluent from an FT complex is zero.

Methane partial oxidation over NiO/MgO solid solution catalysts

Abstract: The following issues regarding the partial oxidation of methane to synthesis gas, over NiO/MgO solid solution catalysts, at a very high space velocity of the feed gas (CH 4 /O 2 =2/1, GHSV=720 000 cm 3 /g h), were investigated: (i) the effect of the solid solution formation between NiO and MgO on the reaction; (ii)the effect of the composition of the catalyst; and (iii) the reaction mechanism. It was found that the mechanical mixtures of NiO and MgO have low activity and selectivity; however, their activity and selectivity became higher with increasing calcination time, due to the formation of a NiO–MgO solid solution. In contrast, the solid solution catalysts prepared via impregnation provided high activity and selectivity, as well as high stability over a large range of NiO concentrations. However, the activity was low for too low concentrations of NiO in NiO/MgO and the catalyst was no longer stable for too high concentrations of NiO. By comparing, under the same reaction conditions, the rate of partial oxidation of methane for CH 4 /O 2 =2/1 to that of the reforming reaction of a mixture CH 4 /H 2 O/CO 2 obtained from the former by complete combustion, one could conclude that the pyrolysis mechanism was dominant at 700°C and 750°C, but at 850°C, the combustion reforming mechanism also played a part.

High temperature partial oxidation reactions over silver catalysts

Abstract: The exceptional catalytic activity of silver for a number of partial oxidation reactions has been known for nearly a century. Despite the wid espread use of silver in heterogene ous catalysis, there still remain unresolved questions about the mechanistic details of reaction. The ethylene epoxidation and formaldehyde synthesis reactions are the two industrially-relevant reactions which have received, by far, the most attention. The importance of these reactions cannot be underestimated. Both ethylene epoxide and formaldehyde serve as primary chemicals for a wide variety of materials which find use in an enormous number of products. There is, therefore, a great scientific and economi c motivation for understanding th

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Abstract: Mixed LaNixFe(1−x)O3 perovskite oxides (0≤x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi0.3Fe0.7O3.

Direct conversion of methane to synthesis gas through gas-solid reaction using CeO2-ZrO2 solid solution at moderate temperature

Abstract: A series of CeO2–ZrO2 composite oxides (Ce1−xZrxO2) was characterized and tested for the gas-solid reaction with CH4 in the absence of gaseous oxidant. A solid solution with fluorite structure was formed for the samples with Zr content below 50% (x ≤ 0.5). For Ce1−xZrxO2 both desorption of O2 and reduction by H2 took place at lower temperatures as compared with that for CeO2 alone. Like CeO2, the reaction of Ce1−xZrxO2 with CH4 selectively produced synthesis gas with a H2/CO ratio of 2, but the formation rates of H2 and CO were increased and the activation energy was remarkably decreased due to the incorporation of ZrO2 into CeO2. The reaction was further accelerated by the presence of Pt catalyst. The conversion of CH4 to H2 and CO could be achieved at a temperature as low as 500°C by using Ce0.8Zr0.2O2 in the presence of Pt. The reduced Ce0.8Zr0.2O2−y could be oxidized with H2O back to Ce0.8Zr0.2O2, producing pure H2 simultaneously.

Catalytic properties of early transition metal nitrides and carbides: n-butane hydrogenolysis, dehydrogenation and isomerization

Abstract: Phase-pure early transition metal nitrides and carbides were prepared via the temperature programmed reaction of metal oxides with NH3 or a CH4/H2 mixture. The nitrides and carbides were mostly mesoporous with surface areas up to 81 m2/g. Their gravimetric butane conversion rates were generally higher than those for a Pt–Sn/Al2O3 catalyst. Activities for the nitrides and carbides ranged from 0.4×1012 to 10×1012 molecules/cm2 s at 723 K and decreased as follows: γ-Mo2N>W2C≈WC>β-W2N≈WC1−x>β-Mo2C>VN≈V8C7≫NbC≈Nb4N3.92. The metal atom type had the most significant effect on the activity and selectivity. The Group VI metal nitrides and carbides were much more active than the Group V metal compounds. In general, the Group VI metal compounds catalyzed butane hydrogenolysis and dehydrogenation with similar selectivities while the vanadium compounds had dehydrogenation selectivities in excess of 98%. The β-W2N catalyst also catalyzed butane isomerization possibly as a consequence of the presence of oxygen on the surface. The effect of lattice structure was significant and obvious for the tungsten carbides where WC (hex) was almost twice as active as WC1−x (fcc) despite having similar C/W ratios. Nitrides and carbides of the same metal and lattice structure had similar activities suggesting that the effect of the non-metal atom type was small. We believe variations in the catalytic properties of the nitrides and carbides were the result of differences between their electronic structures.

Promotion of the hydrogen peroxide decomposition activity of manganese oxide catalysts

Abstract: Pure (MnO x ), mixed (A y Mn 1− y O x ) and composite ((A,B) y Mn 1− y O x ) manganese oxide based catalysts were prepared, where A is either Ni II , Cu II , Bi III or Ce IV , A and B are various combinations of them, and y =0.4. Subsequently, they were characterized for the crystalline bulk structure by X-ray powder diffractometry, the electron availability by magnetic susceptibility measurements, the surface area by BET-analysis of nitrogen adsorption isotherms, and the surface chemical composition and oxidation state by X-ray photoelectron spectroscopy. The H 2 O 2 decomposition activity was determined by oxygen-gasometry of the reaction kinetics at 20–35°C. Results thus obtained have helped to conclude that (i) the mixed and composite catalysts are generally more active than the pure catalyst, and (ii) of the mixed and composite catalysts, those assuming mixed bulk phases with the host oxide are relatively more active. The optimal catalytic surface has been found to be that exposing ionic sites of different oxidation states and residing on a crystalline bulk of ca. 50% electron availability. These properties were found to be accomplished by surfaces of NiMnO x , on which the H 2 O 2 decomposition required a much lower activation energy (11.6 kJ mol −1 ) as compared to that (77.1 kJ mol −1 ) necessary for the reaction on the pure catalyst (MnO x ).