Showing papers in "Applied Catalysis A-general in 2007"

Manganese oxide catalysts for NOx reduction with NH3 at low temperatures

Abstract: Manganese oxide catalysts prepared by a precipitation method with various precipitants were investigated for the low temperature selective catalytic reduction (SCR) of NOx with NH 3 in the presence of excess O 2 . Various characterization methods such as N 2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and X-ray absorption near edge structure (XANES) were conducted to probe the physical and chemical properties of MnOx catalysts. The active MnOx catalysts, precipitated with sodium carbonate and calcined in air at moderate temperatures such as 523 K and 623 K, have the high surface area, the abundant Mn 4+ species, and the high concentration of surface oxygen on the surface. The amorphous Mn 3 O 4 and Mn 2 O 3 were mainly present in this active catalyst. The carbonate species appeared to help adsorb NH 3 on the catalyst surface, which resulted in the high catalytic activity at low temperatures.

Processing biomass in conventional oil refineries: Production of high quality diesel by hydrotreating vegetable oils in heavy vacuum oil mixtures

Abstract: Renewable liquid alkanes can be produced by hydrotreating of vegetable oils and vegetable oil‐heavy vacuum oil (HVO) mixtures at standard hydrotreating conditions (i.e. 300‐450 8C) with conventional hydrotreating catalysts (sulfided NiMo/Al2O3). The reaction pathway involves hydrogenation of the C C bonds of the vegetable oils followed by alkane production by three different pathways: decarbonylation, decarboxylation and hydrodeoxygenation. The straight chain alkanes can undergo isomerization and cracking to produce lighter and isomerized alkanes. The carbon molar yield of straight chain C15‐C18 alkanes was 71% on a carbon basis (the maximum theoretical yield for these products is 95%) for hydrotreating of pure vegetable oil under optimal reaction conditions. The rate of alkane production from pure sunflower oil is greater than the rate of hydrodesulfurization of a HVO with a 1.48 wt% sulfur content (e.g. 100% conversion of sunflower oil at 350 8C compared to 41% conversion of sulfur). The yield of straight chain alkanes increases when sunflower oil is mixed with HVO, illustrating that dilution of HVO can improve the reaction chemistry. For example, with a 5 wt% sunflower oil‐95 wt% HVO feed the maximum theoretical straight chain C15‐C18yield from the sunflower oil was higher (87%) than it was with the pure sunflower oil (75%). Mixing the sunflower oil with HVO does not decrease the rate of desulfurization indicating that sunflower oil does not inhibit the hydrotreating of HVO. # 2007 Elsevier B.V. All rights reserved.

Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

Abstract: The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPR H2 and TPR CO ), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “ in situ ” XRD methods Five different compounds of iron oxides were characterized: hematite α-Fe 2 O 3 , goethite α-FeOOH, ferrihydrite Fe 5 HO 8 ·4H 2 O, magnetite Fe 3 O 4 and wustite FeO In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300 °C Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe 2 O 3 → 2 Fe 3 O 4 → 6 Fe, three-stage mechanism 3Fe 2 O 3 → 2Fe 3 O 4 → 6FeO → 6Fe is postulated especially when temperature of reduction overlaps 570 °C Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe 2+ ⇌ 2Fe 3+ + Fe, occurring at both the atomic scale on two-dimensional interface border Fe 3 O 4 /Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wustite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe 3 O 4 + Fe Above 570 °C, the appearance of wustite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “ in situ ” XRD method In the case of FeO–H 2 system, instead of one-step simple reduction FeO → Fe, a much more complex two-step pathway FeO → Fe 3 O 4 → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe 3 O 4 → FeO → Fe up to 880 °C should be reconsidered as a result of the accompanying FeO disproportionation wustite ⇌ magnetite + iron manifesting its role above 150 °C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide The disproportionation reaction of FeO does not consume hydrogen and occurs above 200 °C much easier than FeO reduction in hydrogen above 350 °C The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions The disproportionation reaction wustite ⇌ magnetite + iron makes simple wustite reduction FeO → Fe a much more complicated process In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wustite → magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe 3 O 4 Depending on TPR H2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380 °C Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction

Recent developments in titanium oxide-based photocatalysts

Abstract: Recent development in titanium oxide-based photocatalysts was reviewed concerning the development in the highly dispersed titanium oxide photocatalysts prepared on or within zeolites and the visible light-responsive TiO 2 photocatalysts. The unique and high reactivities of titanium oxide species anchored or incorporated in the zeolite for various photocatalytic reactions such as reduction of CO 2 with H 2 O and direct decomposition of NO x into N 2 and O 2 were discussed focusing on the relationship between the reactivity and local structures of the catalysts. Moreover, the preparation of the visible light-responsive TiO 2 photocatalysts by applying ion-engineering techniques such as an ion-implantation and an RF magnetron sputtering deposition method was discussed focusing on its unique reactivity for the decomposition of water into H 2 and O 2 with a separate evolution under sunlight irradiation.

Photocatalytic reactivity for O2•- and OH• radical formation in anatase and rutile TiO2 suspension as the effect of H2O2 addition

Abstract: Effect of crystalline structure, anatase and rutile, on the production of OH and O2 − by TiO2 photocatalytic reaction was investigated. The OH radical free from the TiO2 surface was monitored by the fluorescence intensity of 2-hydroxyl terephthalic acid produced by the reaction with terephthalic acid. Superoxide radical was detected by the chemiluminescence probe method with luminol. Formation rate of OH with rutile photocatalysts was significantly lower than that with anatase photocatalysts. By the addition of H2O2, the formation rate of OH was significantly increased for rutile and for anatase mixed with rutile by 10–20%, while pure anatase showed an opposite tendency. We suggest that the adsorption structure of H2O2 on the rutile TiO2 surface is preferable to produce OH . In photocatalytic production of O2 −, rutile surpassed anatase in stabilizing the produced O2 −. On H2O2 addition, anatase surpassed rutile in the photocatalytic activity to produce O2 − from H2O2.

Fischer-Tropsch synthesis : Temperature programmed EXAFS/XANES investigation of the influence of support type, cobalt loading, and noble metal promoter addition to the reduction behavior of cobalt oxide particles

Abstract: TPR-XANES/EXAFS carried out using a novel multi-sample holder provided key information for verifying the nature of the chemical transformations occurring during cobalt Fischer–Tropsch synthesis catalyst activation in hydrogen. In the past, assumptions had to be made regarding the nature of the cobalt species present along the trajectory of a standard TPR experiment. The new technique directly provided insight into (a) the nature of the reduction process of cobalt oxide species and (b) the resulting cobalt crystallite size, as a function of the strength of the catalyst support interaction with the cobalt oxide species. A two-step reduction process involving Co 3 O 4 to CoO and CoO to Co 0 transformations over standard calcined catalysts was observed and quantified over all catalysts exhibiting both weak interactions (e.g., Co/SiO 2 ) and strong interactions (e.g., Co/Al 2 O 3 ) with the support. Noble metal promoter (e.g., Pt) addition strongly improved the reducibility of cobalt oxide species, most likely via a H 2 dissociation and spillover mechanism. Increasing cobalt loading, on the other hand, led to a measurable, but lesser, improvement on reducibility, due to the larger resulting particle size that resulted in less surface contact with the support. Higher reduction temperatures were needed to effectively reduce cobalt oxide particles deposited on strongly interacting surfaces in comparison with unsupported Co 3 O 4 or only weakly interacting supported cobalt catalyst. Nevertheless, despite lower extents of reduction, the smaller resulting Co particles on the more strongly interacting catalysts generally led to higher Co 0 active site densities. The addition of the noble metal promoter to strongly interacting supported catalyst significantly decreased the temperature required to reduce the cobalt oxides to Co 0 particles; this allows one to take advantage of the higher Co 0 surface areas arising from the combination of a smaller average Co 0 particle size and a higher extent of reduction.

The acid properties of H-ZSM-5 as studied by NH3-TPD and 27Al-MAS-NMR spectroscopy

Abstract: The acid properties of H-ZSM-5 zeolites with different SiO2/Al2O3 ratios (30, 50, 80, 150, 280 and 1000) were examined by means of the temperature programmed desorption of ammonia (NH3-TPD). Different pretreatments together with a comparison of different curve-fitting methods after desorption of ammonia were used to differentiate between different adsorption sites. 27Al-MAS-NMR spectroscopy was applied to distinguish the framework (AlF) from the extra-framework aluminum (AlEF). Both techniques reveal that the concentration of Bronsted acid sites is lower than expected when considering the aluminum content. The strength of the Bronsted acid sites, determined by a theoretical model assuming the free readsorption of ammonia, is found to be in the range of ΔH = 129–161 kJ mol−1 and shows variations in the acid strength to be less than σ = 12 kJ mol−1.

The role of Co–Mo–S type structures in hydrotreating catalysts

Abstract: The Co–Mo–S model is used widely to understand and control the catalytic properties of hydrotreating catalysts. Depending on the nature of the support interactions, the Co–Mo–S structures may be present as either Type I or II which has different catalytic properties. Most current high activity industrial hydrotreating catalysts are based on Type II structures. From a fundamental point of view, it has for many years been difficult to understand in detail the catalytic properties of the different types of Co–Mo–S structures since direct atom-resolved insight has not been available. Recently, such insight has been provided by scanning tunneling microscopy (STM) and from the application of density functional theory (DFT) and together with other advances, this has contributed greatly to improve our fundamental knowledge of MoS2 and Co–Mo–S structures. Many surprising features were discovered. For example, it has been observed that key catalytic steps may involve active sites which are not sulfur vacancies. Of particular interest, it was found that fully sulfur-coordinated sites with metallic-like properties play a role in hydrogenation reactions of such catalysts. The new insight has played a role in the recent introduction of the BRIM™ family of improved industrial hydrotreating catalysts.

The hydrothermal synthesis of mesoporous TiO2 with high crystallinity, thermal stability, large surface area, and enhanced photocatalytic activity

Abstract: Well-defined spherical mesoporous TiO2 was prepared from a poly(ethylene glycol)-poly(propylene glycol)-based triblock copolymer and titanium isopropoxide mixed with 2,4-pentanedione by using a simple sol–gel approach in aqueous solution. Hydrothermal treatment was performed to increase the crystallinity, thermal stability, surface area, and photocatalytic activity of the mesoporous TiO2. The hydrothermally treated mesoporous TiO2 materials were found to have a high crystallinity with a nanocrystalline anatase structure even in the as-synthesized state, whereas untreated materials were found to have an amorphous or semicrystalline phase prior to calcination at 300 °C. The surface area of hydrothermally treated mesoporous TiO2 was found to exceed 395 m2 g−1, whereas the areas of the untreated materials were less than 123 m2 g−1. The pore size distributions of the hydrothermally treated mesoporous TiO2 materials were found to be narrower than those of untreated materials; the average pore size increased from 5.7 to 10.1 nm with increases in the calcination temperature. The photocatalytic activity of hydrothermally treated mesoporous TiO2 is significantly higher than the activities of untreated materials, with a maximum decomposition rate that is three times faster than that of a commercial TiO2, P25. The high photocatalytic activity of mesoporous TiO2 is due to the large surface area and high crystallinity with a nanocrystalline anatase that is induced by the hydrothermal treatment.

High temperature methanation sintering and structure sensitivity

Abstract: Manufacture of substitute natural gas (SNG) using high temperature methanation of synthesis gas is becoming important due to high energy prices, a wish for a stable energy supply, and diminishing natural gas in some areas. Maintaining the activity and stability of the catalyst after exposure to high temperatures is crucial for the process. At 600 °C, loss of active surface area proceeds via the atom migration sintering mechanism. The methanation reaction is structure sensitive and it is suggested that atomic step sites play the important role as the active sites of the reaction.

Transesterification of poultry fat with methanol using Mg–Al hydrotalcite derived catalysts

Abstract: The synthesis of biodiesel from poultry fats provides a way to convert the by-product of a renewable resource to a very important value-added biofuel. In this work, the use of heterogeneous base catalysts derived from Mg–Al hydrotalcite was investigated for the conversion of poultry lipids to biodiesel. This solid base showed high activity for triglyceride (TG) transesterification with methanol without signs of catalyst leaching. Catalytic performance was significantly affected by pretreatment and operating conditions. Calcination at optimum temperature was key in obtaining the highest catalyst activities. Rehydration of the calcined catalyst before reaction using wet nitrogen decreased catalytic activity for the transesterification of poultry fat, opposite to what has been reported for condensation reactions. Also, methanol had to be contacted with the catalyst before reaction; otherwise, catalyst activity was seriously impaired by strong adsorption of triglycerides on the active sites. Both temperature (60–120 °C) and methanol-to-lipid molar ratio (6:1–60:1) affected the reaction rate in a positive manner. The use of a co-solvent (hexane, toluene, THF), however, gave rise to a change in TG conversion profile which cannot be explained solely by a dilution effect. The catalyst underwent significant deactivation during the first reaction cycle probably due to deactivation of the strongest base sites. Subsequent reaction cycles showed stable activity. By re-calcination in air, complete catalyst regeneration was achieved.

Characterization of surface processes at the Ni-based catalyst during the methanation of biomass-derived synthesis gas: X-ray photoelectron spectroscopy (XPS)

Abstract: Surface modifications during the production of methane from reformer synthesis gas over a commercial Ni/Al2O3 catalyst were investigated by quasi-in situ X-ray photoelectron spectroscopy (XPS) and other surface analytical techniques. Experiments of methanation reaction were done in the high pressure cell (HPC) integrated in the XPS system and under fixed-bed conditions. The interaction of the different reformer biomass-derived synthesis gas on the surface properties of the catalyst and on its activity under methanation conditions were studied on a nanoscopic level. Detailed description of changes in metal particle morphology and carbon deposit is presented. A mechanism of C-whiskers formation during long exposure to methanation conditions is described. The role of additional components in the gas mixture, such as CO2, H2O, CH4, C2H4, C2H2, C3H6 and C2H6, were studied in details. Furthermore, role of different nickel compounds such as oxides, hydroxides, carbides and carbonates are discussed.

CO methanation over supported bimetallic Ni–Fe catalysts: From computational studies towards catalyst optimization

Abstract: DFT calculations combined with a computational screening method have previously shown that bimetallic Ni–Fe alloys should be more active than the traditional Ni-based catalyst for CO methanation. That was confirmed experimentally for a number of bimetallic Ni–Fe catalysts supported on MgAl 2 O 4 . Here, we report a more detailed catalytic study aimed at optimizing the catalyst performance. For this purpose, two series of mono and bimetallic Ni–Fe catalysts supported on MgAl 2 O 4 and Al 2 O 3 , respectively, were prepared. All catalysts were tested in the CO methanation reaction in the temperature interval 200–300 °C, and characterized using elemental analysis, N 2 physisorption measurements, XRD and TEM. Optimization of the catalyst performance was made by varying the Ni:Fe ratio, the total metal loading and the support material. For both support materials, the bimetallic catalysts with compositions 25Fe75Ni and 50Fe50Ni showed significantly better activity and in some cases also a higher selectivity to methane compared with the traditional monometallic Ni and Fe catalysts. A catalyst with composition 25Fe75Ni was found to be the most active in CO hydrogenation for the MgAl 2 O 4 support at low metal loadings. At high metal concentrations, the maximum for the methanation activity was found for catalysts with composition 50Ni50Fe both on the MgAl 2 O 4 and Al 2 O 3 supports. This difference can be attributed to a higher reducibility of the constituting metals with increasing metal concentration. The maximum of the catalytic activity and the highest selectivity to methane were observed for the sample with 20 wt% total metal loading. It appears that it is possible to increase substantially the efficiency of Ni-based methanation catalyst by alloying with Fe.

Development of a Ru/C catalyst for glycerol hydrogenolysis in combination with an ion-exchange resin

Abstract: The combination of Ru/C and Amberlyst ion-exchange resin is effective for the dehydration and hydrogenation (denoted as hydrogenolysis) of glycerol to 1,2-propanediol under mild reaction conditions (393 K). A Ru/C catalyst prepared by using active carbon with a low surface area (∼250 m2/g) showed better performance than that prepared by using active carbon with a high surface area. In addition, treatment of Ru/C catalysts prepared from Ru(NO)(NO3)3 with Ar flowing at the appropriate temperature enhanced the performance compared to that of the commercially available Ru/C catalysts. This temperature treatment can be influenced by the decomposition of Ru precursor salt and aggregation of Ru metal particles. In addition, the degradation reaction as a side-reaction to C1 and C2 compounds of glycerol hydrogenolysis was more structure-sensitive than the hydrogenolysis reaction, and the selectivity of hydrogenolysis was lower on smaller Ru particles. The combination of Ru/C with the Amberlyst resin enhanced the turnover frequency of 1,2-propanediol formation drastically, and this indicates that 1,2-propanediol can be formed mainly by dehydration of glycerol to acetol catalyzed by Amberlyst and subsequent hydrogenation of acetol to 1,2-propanediol catalyzed by Ru/C.

Glycerol hydrogenolysis to 1,2-propanediol catalyzed by a heat-resistant ion-exchange resin combined with Ru/C

Abstract: The combination of Ru/C with Amberlyst ion-exchange resins was effective to the hydrogenolysis of glycerol to 1,2-propanediol The difference between Amberlyst15 and 70 in performance was small at 393 K, however, it became larger at higher reaction temperature This can be related to the highest operating temperature of the resins This can be explained by the poisoning of Ru/C with the sulfur compounds originated from the thermal decomposition of the resins The Ru/C + heat-resistant Amberlyst70, whose highest operating temperature is 463 K, showed higher conversion, selectivity to 1,2-propanediol and stability at 453 K

Synthesis and characterization of ZrO2–TiO2 binary oxide semiconductor nanoparticles: Application and interparticle electron transfer process

Abstract: A series of nanocrystalline mesoporous ZrO2–TiO2 binary oxide photocatalysts with different wt% of ZrO2 and TiO2 were prepared by a sol–gel method. These binary oxide photocatalysts were characterized by XRD, N2 adsorption–desorption, DRS, FTIR, Raman spectroscopy, photoluminescence and TEM analyses. Detailed investigations revealed that the ZrO2–TiO2 catalysts are highly micro-crystalline in nature with a larger surface area than that of the pure TiO2 or ZrO2 catalysts since the added ZrO2 plays an important role in promoting the formation of nanoparticles with an anatase structure, high surface area and acidity. The photocatalytic reactivity of the catalysts was investigated for the degradation of 4-chlorophenol in an aqueous phase in which the ZrO2–TiO2 photocatalysts were found to exhibit remarkably higher photocatalytic reactivity than that of pure TiO2 and ZrO2. The catalytic activity of the binary oxide photocatalysts for the degradation of 4-chlorophenol was observed to be gradually enhanced with an increase in the ZrO2 content and reached an optimum at 12 wt% of ZrO2 while maintaining the same percentage degradation with further loading of ZrO2 until 50 wt%. Such high reactivity is due to the easy transfer of the photo-formed electrons from the conduction band surface trap states of ZrO2 to the conduction band of TiO2 through strong chemical interactions, thereby, preventing the radiative recombination of the photo-formed electrons and holes. The ZrO2–TiO2 catalysts were, thus, found to be highly active for the efficient degradation of 4-chlorophenol and, in fact, exhibited just as efficient activity as the commercial P-25, Degussa TiO2 catalysts, and a new reaction mechanism has, hereby, been proposed.

Effect of sulphiding agents on the hydrodeoxygenation of aliphatic esters on sulphided catalysts

Abstract: In hydrodeoxygenation (HDO) on sulphided hydrotreating catalysts, addition of a sulphiding agent is typically required to maintain the catalyst activity. The effects of H2S and CS2 on the HDO of aliphatic esters on sulphided NiMo/γ-Al2O3 and CoMo/γ-Al2O3 catalysts were investigated in a fixed-bed flow reactor. The model compounds studied were methyl heptanoate, ethyl heptanoate, heptanol and heptanoic acid. The HDO produced C7 and C6 hydrocarbons in reactions where oxygen- and sulphur-containing compounds were formed as intermediates. Unlike CS2, H2S had a promoting effect on the HDO of the aliphatic oxygenates. The addition of H2S stabilised the selectivities as a function of time, and shifted the main products from C7 to C6 hydrocarbons, but did not prevent catalyst deactivation. The promoting effect of H2S was attributed to the increased catalyst acidity, which enhanced the acid-catalysed reactions (hydrolysis, esterification, dehydration, E2 elimination and SN2 nucleophilic substitution). Both H2S and CS2 suppressed the hydrogenation reactions on the NiMo catalyst but did not affect them significantly on the CoMo catalyst. H2S induced less hydrogen consumption and coke formation than CS2, but the carbon efficiency suffered in the presence of H2S. Thus, the use of H2S as sulphiding agent in the HDO of the aliphatic oxygenates was concluded to be more beneficial than the use of CS2.

Carbon deposition on Ni/YSZ composites exposed to humidified methane

Abstract: Carbon formation on Ni/YSZ cermets in high-temperature processes involving hydrocarbons can be a severe problem. Understanding the mechanisms of carbon formation is necessary for developing strategies to avoid or minimize the problem. In this study, Ni/YSZ pellets (70/30 NiO/YSZ) have been exposed to humidified methane at temperatures between 773 K and 1073 K. The carbon formed on these pellets and/or any structural changes in the pellets was studied with X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO). At temperatures below 873 K, carbon fibres dominated. At higher temperatures, the majority of the carbon dissolved into the Ni particles. After TPO and removal of the carbon, the Ni structure was damaged to various extents depending on the exposure temperature. The amount of carbon deposited was significantly reduced by placing zirconia-doped ceria pellets on each side of the Ni/YSZ pellet. The addition of a ceria layer did not completely eliminate the carbon formation but the carbon that was formed was weakly attached to the Ni/YSZ such that the structure was not irreversibly changed. The role of the ceria is beyond simply reforming the methane.

Structural and surface features of PtNi catalysts for reforming of methane with CO2

Abstract: The effect of Ni content (1–12 wt%) on the surface and catalytic behavior of bimetallic PtNi catalysts supported on ZSM-5 for reforming of methane with CO2 was studied. The properties of the catalysts, before and after exposure to reaction conditions, were investigated employing N2 adsorption–desorption isotherms, XRD, TGA/DTA, FTIR spectroscopy of framework vibrations, DRIFT spectroscopy of adsorbed CO, XPS and TPR. It was shown that addition of a small amount of Pt (0.5%) to Ni catalyst leads to formation of small nano-sized NiO particles and easy reduction of NiO. It was found that the amount of Ni precursor plays an important role on the surface and catalytic properties of bimetallic catalysts. The improvement of catalytic activity and stability observed for bimetallic catalyst was attributed to an increase of the nickel metallic dispersion caused by an intimate contact between nickel and platinum at Ni-loading of 6 wt%.

Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2

Abstract: Ni and Pt catalysts supported on α-Al2O3, α-Al2O3-ZrO2 and ZrO2 were studied in the dry reforming of methane to produce synthesis gas. All catalytic systems presented well activity levels with TOF (s−1) values between 1 and 3, being Ni based catalysts more active than Pt based catalysts. The selectivity measured at 650 °C, expressed by the molar ratio H2/CO reached values near to 1. Concerning stability, Pt/ZrO2, Pt/α-Al2O3-ZrO2 and Ni/α-Al2O3-ZrO2 systems clearly show lower deactivation levels than Ni/ZrO2 and Ni or Pt catalysts supported on α-Al2O3. The lowest deactivation levels observed in Ni and Pt supported on α-Al2O3-ZrO2, compared with Ni and Pt supported on α-Al2O3 can be explained by an inhibition of reactions leading to carbon deposition in systems having ZrO2. These results suggest that ZrO2 promotes the gasification of adsorbed intermediates, which are precursors of carbon formation and responsible for the main deactivation mechanism in dry reforming reaction.

Etherification of glycerol and ethylene glycol by isobutylene

Abstract: The influence of catalyst, solvent and temperature on the etherification of glycerol and ethylene glycol with isobutylene in the liquid phase catalysed by strong acid ion-exchange resins of Amberlyst type (Amberlyst 15 and 35), p-toluenesulfonic acid and by two large-pore zeolites H-Y and H-Beta was studied. Reactions were carried out in the temperature range from 50 to 90 °C at autogenous pressure in solvent (dioxane, dimethyl sulfoxide and sulfolane). The basic kinetic parameters for complex of 11 equilibrium reactions for glycerol and 5 equilibrium reactions for ethylene glycol were estimated. The highest conversion of glycerol was achieved on H-Beta, but on this catalyst the formation of tri-tert-butyl glycerol was sterically hindered. The highest amount of di- and tri-ethers was formed over Amberlyst 35. Over H-Y the reaction was slower due to its lower acidity, and final concentration of di- and tri-ethers was not achieved in monitored reaction time. p-Toluenesulfonic acid provides satisfactory results only when sulfolane was used as a solvent. The solvent plays an essential role because it can affect the investigated etherification reaction with its polarity and homogenization of reaction mixture. The concentration of glycerol and ethylene glycol higher ethers in end product can decrease as temperature increases showing that the side reaction of isobutylene dimerisation is more sensitive to temperature.

Hydroprocessing catalysts regeneration and recycling

Abstract: The three typical causes of deactivation of hydroprocessing catalysts are coke, sintering and contamination. The two first can be eliminated by regeneration, the ex situ method being the rule nowadays for a better performance recovery. Regeneration consists in a controlled oxidation which eliminates coke and converts sulfides back to oxides. The main limits to catalyst reuse are the decrease of activity and mechanical properties. Non-contaminated regenerated catalysts can recover activities rather similar to fresh ones, as assessed by a statistical study performed at Eurecat on CoMo catalysts. Nevertheless some new generation catalysts require additional treatments to recover full activity. The handling and transport of spent catalysts to an off-site regeneration facility is currently performed, but requires some precaution as the material is classified as self-heating. At the end of the cycle, spent non-reusable catalysts have to be recycled for metals reclamation. This can be performed either by hydrometallurgical or pyrometallurgical routes.

Unsupported transition metal sulfide catalysts: From fundamentals to industrial application

Abstract: In the open literature, there are numerous papers related to the chemical and physical properties of unsupported transition metal sulfides. Besides characterization studies, the performance of a large variety of mono-, bi- and multi-metallic materials in various test reactions has been examined. The performance studies identified several unsupported materials with higher activity and/or selectivity than the traditional γ-Al 2 O 3 supported Ni/Co–Mo/W catalysts. This has resulted in numerous patents disclosing the preparation of unsupported transition metal sulfides and their use as hydroprocessing catalysts. However, for several reasons (high price and too high activity for existing refinery process equipment), the commercial use of unsupported catalysts in refinery processes has so far been limited to the NEBULA ® catalysts. Catalysts based on NEBULA technology provide unprecedented activity for hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and aromatics saturation (HDA).

Catalytic ammonia decomposition over Ru/carbon catalysts: The importance of the structure of carbon support

Abstract: Catalytic ammonia decomposition has been of increasing interests as a means of supplying pure hydrogen for fuel cells. In this study, Ru catalysts supported on different carbons were used for catalytic ammonia decomposition. The influences of the porous and graphitic structures of carbon supports on the activities of the catalysts were examined. The catalytic activity over supported Ru catalysts is ranked as Ru/GC (graphitic carbon) > Ru/CNTs (carbon nanotube) > Ru/CB-S (carbon black) > Ru/CB-C > Ru/CMK-3 approximate to Ru/AC. The samples are characterized by XRD, N-2 adsorption, Raman spectra and H-2 chemisorption. Ru particles are highly dispersed on carbon supports. The optimum range of Ru particle sizes is around 3-4 nm. On the support side, the graphitic structure of the carbons is critical to the activity of the supported Ru catalyst, while the surface area of carbons is less important. (c) 2007 Elsevier B.V. All rights reserved.

Methanol conversion on SAPO-34 catalysts prepared by mixed template method

Abstract: Effects of templating on the catalytic performance of SAPO-34 catalyst have been investigated in conversion of methanol to olefins (MTO). SAPO-34 catalysts were synthesized using a single agent or a mixture of morpholine or tetraethyl ammonium hydroxide of (TEAOH) as the template during synthesis of gel with nominal composition as 1Al2O3:1P2O5:0.6SiO2:x morpholine:(2 − x)TEAOH:52H2O. Compared with using single template, using a mixed template leads to the particle size reduction and the morphology change to spherical type formed by the aggregation of nano-sized crystals. Although all the catalysts showed similar activity and product distribution in the MTO reaction, the catalyst obtained using the mixture of 75% morpholine and 25% TEAOH gave the longest lifetime.

Influence of structural features on the photocatalytic activity of NaTaO3 powders from different synthesis methods

Abstract: Perovskite-like NaTaO 3 photocatalyst powders have generally been synthesized with a solid-state method, which formed the orthorhombic phase that has a direct band gap and a Ta–O–Ta bond angle of ca. 163°. The present work reports a sol–gel synthesis, in which NaTaO 3 nanoparticles were obtained at a temperature as low as 500 °C by using sodium acetate and tantalum chloride as the starting materials and citric acid as the complexing agent. Because of the low-temperature condition used in the synthesis, the sol–gel NaTaO 3 was of the monoclinic phase that has an indirect band gap, high densities of states near the band edges, and a Ta–O–Ta bond angle close to 180°. Thanks to the surface area as well as the electronic and crystalline structures, the sol–gel NaTaO 3 was considered more favorable to photocatalytic reactions than the solid-state material. This interpretation was supported by the finding that the sol–gel NaTaO 3 exhibited a remarkably higher photocatalytic activity in water splitting than the solid-state material.

Cr-containing magnetites Fe3-xCrxO4: The role of Cr3+ and Fe2+ on the stability and reactivity towards H2O2 reactions

Abstract: A series of Cr-containing magnetites, Fe3−xCrxO4 (x = 0.00, 0.07, 0.26, 0.42 and 0.51) has been prepared using conventional co-precipitation method. Mossbauer and powder X-ray diffraction measurements suggested the formation of the spinel crystalline phase with initial substitution of F e oct 3 + by Cr3+ and for higher Cr contents, chromium also replaces F e oct 2 + and F e tet 3 + in the crystalline structure. N2 adsorption/desorption revealed that the presence of Cr has a remarkable effect on the texturial properties of the material decreasing the pore diameter from meso- to micropore with a significant increase on the BET surface area. Thermal analyses (TG and DTA) coupled with XRD and Mossbauer showed that thermal treatment up to 270 °C with O2 leads to the oxidation of F e oct 2 + producing Cr-substituted maghemite but at 600 °C the cubic maghemite is converted to the hexagonal hematite and Cr is expelled from the iron oxide structure. The reactivity of Fe3−xCrxO4 was investigated using two H2O2 reactions, i.e. the decomposition to O2 and the oxidation of the dye methylene blue used as model contaminant. The obtained results showed that the presence of Cr directly promoted the H2O2 decomposition. On the other hand, the presence of small concentration of Cr, i.e. Cr0.07 and Cr0.21, caused a significant increase on the activity for the oxidation of the dyes with complete discoloration and high degree of mineralization. The higher activity of Fe2.93Cr0.07O4 was discussed in terms of a coupling of the redox pairs Fe3+/Fe2+ and Cr2+/Cr3+ producing a more efficient regeneration of the Fenton active specie Fe2+.

A new catalyst material based on niobia/iron oxide composite on the oxidation of organic contaminants in water via heterogeneous Fenton mechanisms

Abstract: The present work describes the preparation of novel materials based on niobia (Nb2O5)/iron oxides and their use as catalyst on oxidizing reactions of organic compounds in aqueous medium with hydrogen peroxide. These new composites were prepared by mixing natural niobia and iron oxides and were characterized with powder X-ray diffraction (XRD), chemical analyses, scanning electron microscopy (SEM), and 57Fe Mossbauer spectroscopy. Results showed that the main iron oxides so formed were goethite (αFeOOH) and maghemite (γFe2O3) with small particle sizes. The decomposition study was realized with a basic dye as a model molecule: the methylene blue. The analysis of the products, with electrospray ionization mass spectrometry (ESI-MS), showed that the methylene blue was successively oxidized (hydrolyzed) through different intermediate species. These results strongly suggest that highly reactive hydroxyl radicals generated from the H2O2 on the surface of the 1:1 niobia:iron oxide composite act as an efficient heterogeneous Fenton catalyst

Production of hydrogen from catalytic steam reforming of bio-oil using C12A7-O--based catalysts

Abstract: The production of hydrogen from the catalytic steam reforming of bio-oil, generated from fast pyrolysis of biomass, was investigated by using novel metal-doped catalysts of [Ca 24 Al 28 O 64 ] 4+ ·4O − /M (C12A7-O − /M, M = Mg, K, Ce). The features of the steam reforming of the bio-oil, including the effects of temperature, the metal-doped content and the S/C ratio (the ratio of mol steam to mol carbon fed) on the hydrogen yield, carbon conversion (mol carbon in production gases to mol carbon fed) and the distributions of the products were measured in the fixed-bed continuous flow reactor. It was found that the C12A7-O − /18% Mg catalyst gave the highest yield of hydrogen and the best carbon conversion among our tested catalysts. For the C12A7-O − /18% Mg catalyst, a hydrogen yield as high as 80% was obtained, and the maximum carbon conversion is up to 96% under the steam reforming condition (S/C > 4.0, GHSV = 10,000 h −1 , T = 750 °C). The catalyst deactivation was mainly caused by the deposition of carbon (coke-formation). Initial catalyst activity can be partly maintained through periodic regeneration via the cleaning of the catalyst and the gasification of the carbon deposits. The catalyst characteristics and the intermediate species formed in the steam reforming processes were investigated by the XRD, XPS, ICP-AES and FT-IR measurements. The mechanism of the bio-oil steam reforming was addressed according to the above investigations.

Steam reforming of ethanol on supported nickel catalysts

Abstract: The catalytic behavior of Ni/Al2O3 catalysts modified with La and Ag was investigated in the steam reforming of ethanol. The catalysts were characterized by SBET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and Fourier transform infrared spectroscopy (FTIR) of CO adsorption. The reaction rate for noncatalytic decomposition of ethanol in the homogeneous gas phase becomes significant only at high temperatures (T ≥ 890 K). FTIR results revealed that Ag strongly modifies the Ni surface, decreasing the intensity ratio of the bands for adsorbed CO in the bridging mode at low frequency (LF) and the linear mode at high frequency (HF). Similar but smaller effect was observed in the La-containing catalyst. The activity and stability against carbon depostion for steam reforming of ethanol of Ni/Al2O3 catalyst was only slightly sensitive to Ag but the activity was strongly dependent on the presence of La. The reaction data suggest that at temperatures lower than 650 K the Ni/Al2O3 catalyst was active for decomposition of ethanol via the acetaldehyde intermediate, showing high selectivity to methane and CO. The rate of steam reforming of ethanol became significant at temperature higher than 650 K. Comparing Ni/Al2O3 and Ni/La–Al2O3 catalysts the results pointed out that the Ni became more susceptible to modification by water in La-containing Ni catalyst. The Ni/La–Al2O3 catalysts become inactive at low temperatures, and the activity could be regenerated with reduction of NiO by ethanol on raising the reaction temperature. Differently from the steam reforming of methane, the coking cannot be suppressed by Ag promotion in the case of steam reforming of ethanol. Inversely, the La has a positive effect to decreasing coking on Ni catalysts. A scheme for ethanol reactions with H2O on Ni surfaces is proposed based on reaction tests.