Showing papers in "Applied Catalysis A-general in 2006"

Photocatalytic degradation of organic contaminants in water by ZnO nanoparticles: Revisited

Abstract: Nanoscale photocatalysts have attracted much attention due to their high surface area to volume ratios. This work investigates the photodegradation of organic contaminants using the fluorescence emission characteristics of ZnO nanoparticles (ZnO-nano) in aqueous solutions. This is accomplished by preparing nanocrystalline ZnO; the presence of organic contaminants in water is readily detected from the quenching of fluorescence observed from ZnO semiconductor films. Photolysis of ZnO thin films immersed into an aqueous system containing organic contaminants results in the degradation of the contaminants. A comprehensive study has been done involving several organic contaminants in water (like aliphatic and aromatic chloro compounds as well as some commonly used aromatic solvents) to check the suitability of ZnO-nano as an efficient photocatalyst. The ZnO nanoparticles not only serve as a better catalytic system compared to bulk ZnO and commercially available Degussa TiO2 in achieving degradation of the added contaminants, but unlike other semiconductor systems can also act as a non-specific sensor for the presence of these common contaminants in water. A total cleanup of a cocktail of contaminants in water was also achieved using the ZnO-nano.

Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst

Abstract: Biodiesel fuel, consisting of methyl esters of long chain fatty acids produced by transesterification of vegetable oils or animal fats with methanol, is a promising alternative diesel fuel regarding the limited resources of fossil fuels and the environmental concerns. In this work, an environmentally benign process for the transesterification of soybean oil to methyl esters using alumina loaded with potassium as a solid base catalyst in a heterogeneous manner was developed. The catalyst loaded KNO 3 of 35 wt.% on Al 2 O 3 , after being calcined at 773 K for 5 h, it was found to be the optimum catalyst, which can give the highest basicity and the best catalytic activity for this reaction. The effects of various reaction variables such as the catalyst loading, oil to methanol ratio, reaction time and temperature on the conversion of soybean oil were investigated. The catalysts were characterized by means of XRD, IR and Hammett titration method. The results indicated that K 2 O derived from KNO 3 at high temperature and that the Al–O–K groups were, probably, the main reasons for the catalytic activity towards the reaction. The catalyst activity was correlated closely with its basicity as determined by the Hammett method.

Review on methods to deposit catalysts on structured surfaces

Abstract: The methods used to deposit a catalyst on structured surfaces are reviewed. Physical methods such as PVD and chemical methods (sol–gel, CVD, direct synthesis, etc.) are described. The coating of catalysts based on oxide, zeolite or carbon support is detailed on various surfaces such as silicon or steel microstructured reactors, cordierite monoliths or foams, fibres, tubes, etc.

Effect of supports and Ni crystal size on carbon formation and sintering during steam methane reforming

Abstract: Steam methane reforming was studied at 823 K, a total pressure of 20 bar and steam to carbon ratios from 0.08 to 2.4 over conventional NiO/α-Al 2 O 3 and NiO/CaO-Al 2 O 3 catalysts as well as catalysts supported on hydrotalcite derived materials. Catalyst activity, coke formation and deactivation at steam reforming conditions were studied using the tapered element oscillating microbalance (TEOM). Nickel supported on hydrotalcite derived materials had a smaller crystal size and a higher resistance to coke formation than the conventional NiO/α-Al 2 O 3 and NiO/CaO-Al 2 O 3 . The higher resistance to carbon formation could be due to a higher saturation concentration of carbon in the smaller nickel crystals. Sintering experiments were performed at 903 K and 20 bar on the hydrotalcite derived catalysts and compared with an industrial NiO/CaAl 2 O 4 catalyst. The particle growth for the hydrotalcite derived catalysts was larger than for the industrial catalyst, but the hydrotalcite derived catalysts had the smallest size of stabilized Ni crystals.

Sonochemical synthesis of nanocrystallite Bi2O3 as a visible-light-driven photocatalyst

Abstract: The synthesis of Bi2O3 by a simple sonochemical route is investigated. Surfactant polyvinylpyrrolidone (PVP) has strong effects on the grain sizes and morphologies of Bi2O3. Bi2O3 single crystallite with grain size of 40–100 nm is obtained in the presence of 0.5 g PVP. X-ray diffraction (XRD) pattern shows that the nanocrystallite Bi2O3 is monoclinic and has a high degree of crystallinity. Optical characterizations show that the nanocrystallite Bi2O3 presents the photoabsorption properties from UV light region to visible light shorter than 470 nm and the band gap of the nanocrystallite Bi2O3 is 2.85 eV. The photocatalytic performance of the nanocrystallite Bi2O3 is evaluated using methyl orange (MeO) as a model pollutant. The photocatalytic results show that such nanocrystallite Bi2O3 can effectively degrade 86% MeO within 100 min under visible light illumination (λ > 400 nm). The action spectrum of MeO degradation over nanocrystallite Bi2O3 further confirms that the photocatalytic reaction can be driven by visible light. The photocatalytic mechanism is also studied based on electronic structure calculations using density functional theory (DFT).

Comparison of reducibility and stability of alumina-supported Ni catalysts prepared by impregnation and co-precipitation

Abstract: Three Ni–Al 2 O 3 catalysts, with nickel loadings of 10–13 wt.%, were prepared by co-precipitation (Ni–Al co-precip), impregnation on an in-house sol–gel derived alumina (Ni/sol–gel Al 2 O 3 ), and impregnation on a commercial γ-Al 2 O 3 (Ni/γ-Al 2 O 3 ). The catalysts were characterized by N 2 physisorption, H 2 chemisorption, TPR, XRD, SEM and TEM. The Ni species, Ni particle size, and Ni reducibility depended on the preparation method. The Ni–Al co-precip and Ni/sol–gel Al 2 O 3 catalysts contained NiAl 2 O 4 species after calcination, while the Ni/γ-Al 2 O 3 catalyst contained NiO and NiAl 2 O 4 species after calcination. Although the Ni/γ-Al 2 O 3 catalyst was the easiest to reduce, according to TPR, this catalyst had the lowest hydrogen uptake over a 100 h temperature-staged reduction experiment. The Ni–Al co-precip and Ni/sol–gel Al 2 O 3 catalysts had Ni dispersions of over 7% with reduction at 550 °C for 31 h, and maximum dispersions of ∼10%, after reduction at 650 °C for 7 h. After reduction at 550 °C, the Ni particles were not evident by TEM examination. The results suggest that the formation of a surface NiAl 2 O 4 spinel phase during preparation is beneficial for a high Ni dispersion in the reduced catalyst.

Synthesis, characterization and photocatalytic activity of different metal-doped titania systems

Abstract: Different titania-based systems doped with diverse transition metals (Ag, Fe, Pd, Pt, Zn and Zr) were synthesized by the sol‐gel method. Samples were characterized by a wide range of physical techniques (ICP-MS, N2-isotherms, TGA‐DTA, TEM, XRD, FT-Raman and UV‐vis spectroscopies). The effect of aging of the gel either under magnetic stirring (ST) or ultrasonic irradiation (US) on the characteristics of the resulting solids was studied. Sonication ensured the obtention of pure anatase nanoparticles and generally led to an increase in surface area. All the solids were tested for gas phase selective photooxidation of 2-propanol. Irrespective of the aging method, doping with Pd, Pt or Ag resulted in an increase in molar conversion as compared to bare-TiO2, whereas the presence of Fe and Zr had a detrimental effect. Those solids for which sonication hadthegreatestimpactontexturalandstructuralcharacteristics (bare-TiO2andZn-containingsamples)exhibitedthegreatestdifference in molar conversion. Moreover, high selectivities to acetone were achieved in most of the cases. # 2006 Elsevier B.V. All rights reserved.

New biocomponents from glycerol

Abstract: Glycerol is a by-product of biodiesel production, for which new uses are being sought. Etherification of glycerol with isobutene in liquid phase with acidic ion exchange resin catalyst gave five product ethers and, as a side reaction, isobutene reacted to C8–C16 hydrocarbons. The effect of the reaction conditions on the system was studied and conditions for optimal selectivity toward ethers were discovered near with isobutene/glycerol molar ratio of 3 at 80 °C. The conditions controlling the distribution of the product ethers were studied and it was found that the extent of the etherification reaction and thus the main ether products can be changed by varying the reaction conditions.

Synthesis of silver nanoshell-coated cationic polystyrene beads: A solid phase catalyst for the reduction of 4-nitrophenol

Abstract: Silver nanoshell-coated cationic polystyrene beads have been synthesized at room temperature through immobilization of specific silver precursor ions, followed by wet chemical reduction. The electrostatic field force has been taken into consideration for the immobilization of precursor ions onto the resin beads. The as-synthesized particles were characterized by XRD, XPS, SEM, TEM, EDX, and FTIR studies and have been exploited as a solid phase catalyst for the reduction of 4-nitrophenol in the presence of sodium borohydride. The detailed kinetics of the reduction process was monitored under varied experimental conditions. At the end of the reaction, the catalyst particles remain active. They can thus be separated from the product, 4-aminophenol, and can be recycled a number of times after the quantitative reduction of 4-nitrophenol. The activity of the solid catalyst particles has also been examined to promote the reduction of other nitrophenols, e.g., 2-, 3-nitrophenol. The synthesis of the solid catalyst particles, their applications and detailed kinetic aspects of the reduction of 4-nitrophenol have been reported.

Reforming reactions of acetic acid on nickel catalysts over a wide temperature range

Abstract: Catalytic steam reforming of bio-oil, a liquid derived from pyrolysis of biomass, may be a viable process of renewable hydrogen production. Acetic acid is one of the major constituents of bio-oil, and for this reason, it is used as a model compound to study its reaction network under steam reforming conditions over Al2O3 and La2O3, and Ni catalyst supported on La2O3/Al2O3 carrier, employing transient and steady-state techniques. It is found that acetic acid interacts strongly with the Al2O3 carrier and less strongly with La2O3. Decomposition reactions as well as the ketonization reaction take place, especially at intermediate temperatures. In the presence of Ni, catalytic activity is shifted toward lower temperatures. Nickel promotes steam reforming reactions and retards the rate of carbon deposition onto the catalyst surface. It is also found that carbon formation is affected by reaction temperature, the HAc/H2O ratio and catalyst composition. Carbon deposition is favoured at low reaction temperatures and at high HAc/H2O ratio.

Effect of metal oxide additives on the activity and stability of Cu/ZnO/ZrO2 catalysts in the synthesis of methanol from CO2 and H2

Abstract: The Cu/ZnO/ZrO2 catalyst has been modified by adding small amounts of B, Ga, In, Gd, Y, Mn and Mg oxides. Two series of catalysts were obtained: series A: by the co-precipitation of basic carbonates and series B: by complexing with citric acid. The oxide additives were found to influence the catalytic activity in the reaction of methanol synthesis from CO2, dispersion of Cu, surface composition of the catalyst, and the stability of catalysts during their operation. In both series of catalysts, the Ga2O3 additive was especially useful. The mechanism of copper sintering and the performance of the oxide additives are discussed.

Review of absorption and adsorption in the hydrogen–palladium system

Abstract: The hydrogen–palladium system has been the subject of much study, both experimentally and computationally. In this review article the authors have set out to draw a comparison between the experimentally determined thermodynamic data for this system and the calculated energies, in order to attempt to bridge the gap between computational chemistry and experimental work and so gain insight into the absorption and adsorption of hydrogen on palladium. Rigorous thermodynamic analysis of the data for the absorption of hydrogen into palladium metal shows that although constant volume measurements have been made, the analysis that has been applied in the literature in several instances is valid only for a constant pressure system. Re-analysis of the data has lead to a heat of formation for β-palladium hydride which is not a function of composition and a weak function of temperature. Values for the internal energy of absorption of −36.7, −35.2 and −34.4 kJ/mol of H 2 were obtained at 0 °C and in the temperature ranges from 200 to 313 °C and from 366 to 477 °C, respectively. There is a good agreement between these values and the calculated values. The implicit assumptions that underpin the integrated form of the Clausius–Clapeyron equation are that an isobaric system is being analyzed, and that the enthalpy is not a function of composition or temperature. Since heat of adsorption is known to be a function of surface coverage and is generally measured in a constant volume system, the application of the integrated Clausius–Clapeyron equation to determine the enthalpy of adsorption as a function of surface coverage has been questioned and an alternative thermodynamic analysis has been proposed that enables one to calculate the differential change in internal energy of adsorption with surface coverage. It has been found that the internal energy of adsorption varies with increasing surface coverage in a similar manner to the way in which internal energy varies as two atoms approach each other. It is noted that the variation in internal energy with surface coverage (0.1 θ The computationally determined energies of adsorption do not reflect this trend and appear to under estimate the electrostatic repulsion (or over estimate the attraction) between gas phase molecules and atoms that are already adsorbed on the surface for this system.

CO2 reforming of CH4 over La–Ni based perovskite precursors

Abstract: LaNiO3 and La2NiO4 type perovskites were prepared by the “self-combustion” method and were used as catalyst precursors for the CO2 reforming of CH4 reaction at 700 °C. The catalysts were tested in reduced and non-reduced form. High CH4 and CO2 conversion were obtained without carbon deposition. This result was explained by the occurrence of the RWGS (reverse water gas shift) reaction. The La2NiO4 perovskite used as precursor presents the smallest nickel particles after the reduction treatment. Consequently the catalytic activity is higher than that obtained with Ni/La2O3 or LaNiO3. When La2NiO4 is used without treatment prior to the reaction high methane and carbon dioxide conversions are reached but a carbon deposition is observed. The perovskite structure is not completely transformed and the presence of metallic nickel particles at the surface of La2NiO4 would be responsible for the carbon deposition. It is assumed that the role of the support is to allow the activation of carbon dioxide, which is favoured over La2O3 whereas it is limited over La2NiO4. Consequently the reaction between the complex C–Ni species (resulting from methane activation at the surface of the nickel particle) and gaseous CO2 is inhibited over Ni/La2NiO4 leading to a carbon accumulation at the surface of the catalyst. As soon as the perovskite structure is completely transformed, after reductive treatment or during the reaction, a high activity is reached and no carbon deposition was further observed, the catalytic performances being optimal when the average nickel particles size is the smallest.

Effect of potassium content in the activity of K-promoted Ni/Al2O3 catalysts for the dry reforming of methane

Abstract: In this paper the effect of the potassium content in the structure and properties of the Ni active phase and in the activity and selectivity of the NiK/Al 2 O 3 catalysts for dry reforming of methane has been studied. The following characterization techniques were used: SEM, TEM, temperature programmed reduction (TPR-H 2 ) and reaction (TPR-CH 4 ), temperature programmed oxidation (TPO) and XAFS. The reforming of methane with CO 2 was carried out at 973 K using 0.18 g of catalyst and a mixture CH 4 :CO 2 (50:50, 60 ml/min, space velocity of 22,500 h −1 ). Catalytic tests for 6 and 24 h have been developed. TPR-H 2 and XAFS results reveal that potassium does not modify the arrangement of Ni atoms although facilitates the reduction of nickel species by H 2 due to the modification of the interaction between metallic species and the alumina support. The activity data indicate that the addition of a low amount of potassium (0.2 wt.% K 2 O) allows to obtain a catalyst with an acceptably high activity (over 63% methane conversion, close to thermodynamic equilibrium) and very low coke deposition (below 30 mg C/g cat. during 6 h reaction). Independently of the amount of potassium, the catalytic activity remains almost constant during at least 24 h.

Comparison of commercial solid acid catalysts for the esterification of acetic acid with butanol

Abstract: Esterification of acetic acid with butanol has been studied in a heterogeneous reaction system, using a variety of solid acid catalysts. Comparative esterification experiments have been carried out using the homogeneous catalysts sulphuric acid, p -toluenesulphuric acid and a heteropolyacid. The catalysts have been characterised using gas adsorption analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed decomposition (TPD) techniques. The weight-based activity of the heterogeneous catalysts decreases in the following order: Smopex-101 > Amberlyst 15 > sulphated ZrO 2 > H-USY-20 > H-BETA-12.5 > H-MOR-45 > Nb 2 O 5 > H-ZSM-5-12.5. The low activity of ZSM-5 is a result of internal diffusion limitations in the medium sized pores of this zeolite type material. For H-MOR the low activity can be explained by pore blocking in the one-dimensional H-MOR channels. For the H-USY-type zeolites, the influence of the Si/Al ratio on catalytic activity has been examined. Although the amount of acid sites decreases with an increase in the Si/Al ratio, an optimum Si/Al ratio of 20 has been found. The activity of sulphated zirconia shows an optimum calcination temperature, even though both the amount and acidity of the acid sites increase monotonically with calcination temperature. Most likely, at higher calcination temperatures Lewis acid sites are formed and Bronsted acid sites are removed. As Bronsted acid sites are essential for catalysis of esterification reactions this explains the decrease in activity.

Hydrophobic, solid acid catalysts for production of biofuels and lubricants

Abstract: A novel application of Fe–Zn double-metal cyanide (DMC) complexes as solid catalysts in the preparation of fatty acid alkyl esters (biodiesel/biolubricants) from vegetable oils is reported. The catalysts are hydrophobic (no H 2 O adsorption at reaction temperatures) and contain only Lewis acidic sites (NH 3 and pyridine adsorption). Bronsted acid sites are absent (absence of 1546 and 1639 cm −1 bands on adsorption of pyridine). Basic sites are also absent (no CO 2 adsorption). Unlike the homogeneous or other solid catalysts (like ZnO–Al 2 O 3 , for example), the Fe–Zn, DMC catalysts are highly active even for the simultaneous transesterification of triglycerides and esterification of the free fatty acids (FFA) present in unrefined and waste cooking oils as well as non-edible oils. They are also tolerant of water, probably, due to their surface hydrophobicity. A relationship between the transesterification activity and the concentration of strong Lewis acid sites has been observed. Coordinatively unsaturated Zn 2+ ions in the structure of the Fe–Zn complex are the probable active sites.

Photocatalytic activities of a novel ZnWO4 catalyst prepared by a hydrothermal process

Abstract: ZnWO 4 catalysts have been successfully synthesized by a hydrothermal crystallization process. ZnWO 4 catalyst synthesized at 180 °C for 24 h exhibited relatively high photochemical activity for the decomposition of rhodamine-B, and showed better activity for the degradation of formaldehyde than that of TiO 2 (P-25). The generations of the photocurrent and H 2 evolution from pure water were also realized. UV bands observed in the catalyst were assigned to the band transition from the occupied O 2p orbitals to the empty W 5d orbitals. On the basis of the analysis of the energy levels and the band structures, the photocatalytic mechanism of the catalyst was also discussed.

Structural features of La1-xCexNiO3 mixed oxides and performance for the dry reforming of methane

Abstract: Mixed oxides La 1− x Ce x NiO 3 ( x = 0, 0.05, 0.4 and 0.7) have been prepared by the citrate method and tested, after reduction activation, in the CO 2 reforming of methane reaction into synthesis gas. The compounds were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), specific surface area measurements, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and temperature-programmed oxidation (TPO). The LaNiO 3 perovskite exhibited activity to methane reforming, but suffered a slow deactivation with time-on-stream. Nevertheless, substitution of the A site metal ion with a tetravalent metal cation (Ce) led to an increase in catalytic activity. Moreover, the insertion of Ce increased the stability of the catalysts with respect to the reforming reaction. The La 0.95 Ce 0.05 NiO 3 catalyst showed the highest activity, with CO 2 conversion of 62% at 1023 K. The XRD and TPR analyses confirmed that at high Ce contents, ceria appears as segregated CeO 2 phase and interferes with the rate of perovskite structure formation, so that NiO and La 2 NiO 4 are produced. As a consequence of the low solubility of cerium oxide, its insertion in the perovskite structure is also possible in the low Ce-content regions. This low amount of cerium incorporated is responsible not only for the enhancement of catalytic performance of the perovskite after its activation by reduction, but also for the inhibition of carbon formation.

Sintering of nickel catalysts: Effects of time, atmosphere, temperature, nickel-carrier interactions, and dopants

Abstract: Supported nickel catalysts are widely used in the steam-reforming process for industrial scale production of hydrogen and synthesis gas. This paper provides a study of sintering in nickel-based catalysts (Ni/Al 2 O 3 and Ni/MgAl 2 O 4 ). Specifically the influence of time, temperature, atmosphere, nickel-carrier interactions and dopants on the rate of sintering is considered. To probe the sintering kinetics, all catalysts were analyzed by sulfur chemisorption to determine the Ni surface area. Furthermore selected samples were further analyzed using X-ray diffraction (XRD), mercury porosimetry, BET area measurements, and electron microscopy (EM). The observed sintering rates as a function of time, temperature, and P H 2 O / P H 2 ratio were consistent with recent model predictions [J. Sehested, J.A.P. Gelten, I.N. Remediakis, H. Bengaard, J.K. Norskov, J. Catal. 223 (2004) 432] over a broad range of environmental conditions. However, exposing the catalysts to severe sintering conditions the loss of nickel surface area is faster than model predictions and the deviation is attributed to a change in the sintering mechanism and nickel removal by nickel-carrier interactions. Surprisingly, alumina-supported Ni particles grow to sizes larger than the particle size of the carrier indicating that the pore diameter does not represent an upper limit for Ni particle growth. The effects of potassium promotion and sulfur poisoning on the rates of sintering were also investigated. No significant effects of the dopants were observed after ageing at ambient pressure. However, at high pressures of steam and hydrogen (31 bar and H 2 O:H 2 = 10:1) potassium promotion increased the sintering rate relative to that of the unpromoted catalyst. Sulfur also enhances the rate of sintering at high pressures, but the effect of sulfur is less than for potassium.

Ultra-deep oxidative desulfurization of diesel fuel with H2O2 catalyzed under mild conditions by polymolybdates supported on Al2O3

Abstract: The evaluation of the catalytic oxidative desulfurization (OD) activity of alumina-supported polymolybdates (Mo/Al2O3) was carried out using organosulfur model compounds and diesel fuel. Hydrogen peroxide was the oxidizing reagent. Ultra-deep desulfurization, 97.8% removal of sulfur in diesel fuel, was achieved by reaction under mild conditions followed by solvent extraction. The catalyst was prepared by equilibrium adsorption and characterized by infrared, Raman, X-ray photoelectron, 31 P and 27 Al MAS NMR spectroscopies. Our results indicate that the phosphomolybdate used during synthesis of the catalysts decomposes and forms hydrated hepta- and octamolybdates as well as phosphate ions on the surface of alumina. # 2006 Published by Elsevier B.V.

Effects of crystal size and Si/Al ratio on the surface properties of H-ZSM-5 zeolites

Abstract: Highly crystalline H-ZSM-5 zeolite samples with crystal sizes from 0.5 to 5 μm and Si/Al ratios ranging from 20 to 70 have been prepared and characterized by IR spectroscopy, SEM and XRD techniques. The surface properties of these materials have been compared with those of conventional small crystal size materials by using FT-IR spectroscopy of the surface hydroxyl groups and of adsorbed carbon monoxide. The external surface has been investigated using pivalonitrile (2,2-dimethyl-propionitrile) as an adsorption probe. Data show that the basic structure of the zeolite active sites do not depend significantly on the crystal size. Bronsted acid sites of high acid strength are constituted by bridging OH groups located inside the zeolite channels, while Lewis acid sites and weakly acidic silanol groups are found on the external surface in all cases.

State-of-the-art in the monolithic catalysts/reactors

Abstract: According to some authors development of monolithic catalysts and/or reactors has been one of major achievements in the field of heterogeneous catalysis and chemical reaction engineering of the recent years. This work is aimed at pointing out the advantages of monolithic catalysts and/or reactors with respect to the conventional ones, with particular focus on the integral approach to the catalyst and reactor design. The paper is divided into several parts. The first part gives basic definitions and classification of monolithic catalysts, including basic features of the monoliths and factors that have proceeded to the development and application of the monolith structures. It is explained that monoliths belong to the class of the catalytic reaction systems, where usual differences between catalysts and reactors, arising from their action level, are diminishing. Second part of paper is devoted to the preparation of monolithic catalysts. Next part deals with their commercial application with particular emphasis on the less known applications, and those which are still under development. The paper concludes with forecast of potential monoliths applications and ends up with the future research priorities and directions.

Low temperature water–gas shift: Characterization and testing of binary mixed oxides of ceria and zirconia promoted with Pt

Abstract: A series of Pt promoted ceria–zirconia mixed oxides was prepared, characterized, and tested for the low temperature water–gas shift reaction. An enhancement in the water–gas shift rate was observed by doping zirconium atoms into ceria to form a binary oxide for Pt promoted catalysts. By characterization using TPR and XANES, doping zirconia to ceria decreased the temperature for the surface reduction step. However, the total number of bridging OH group defect sites decreased, as Zr remained to a great extent in the Zr 4+ oxidation state. This was confirmed by CO adsorption, whereby the density of total surface formates was found to decline with increased Zr concentrations. However, the formate forward turnover rate in steam was increased by zirconia addition, and was found to be higher than either Pt/ceria or Pt/zirconia alone. Both the overall rate of the formate decomposition and the water–gas shift rate, as measured by the CO conversion, passed through a maximum with increasing Zr content. Two types of formates were observed, those associated with a ceria-rich surface phase, and those associated with a zirconia-rich surface phase. The relative amounts of the two formates correlated with the Zr/Ce atomic ratios obtained by XPS. EXAFS results provided direct evidence that a solid solution was present in the mixed oxide, as a distinct peak in the Fourier transform magnitude corresponding to the Zr–Ce interaction was observed, increasing with increasing Ce/Zr ratio. The sensitivity to added carbon dioxide in the feed of the undoped and a Zr doped catalyst was also explored.

Effect of preparation method on the hydrogen production from methanol steam reforming over binary Cu/ZrO2 catalysts

Abstract: Several methods (impregnation, oxalate gel–coprecipitation and conventional aqueous coprecipitation) have been comparatively examined for the preparation of binary Cu/ZrO2 catalysts for the catalytic production of hydrogen by steam reforming of methanol (SRM). A variety of techniques including N2 adsorption, XRD, N2O chemisorption, XRD, H2-TPR, and XPS were used to characterize the physical and chemical properties of the as-obtained catalysts. The results show that the preparation method significantly affects the component dispersion, microstructural properties and the resulting catalytic performance with respect to methanol conversion, H2 production and CO concentration. The catalyst with higher specific copper surface area and component dispersion shows higher activity for methanol conversion at lower temperature. The best Cu/ZrO2 catalyst has been prepared by an oxalate gel–coprecipitation method, which shows much higher catalytic activity and enhanced long-term stability in the SRM reaction as compared to the catalysts prepared by conventional aqueous–coprecipitation and impregnation methods.

Factors influencing the catalytic activity of SBA-15-supported copper nanoparticles in CO oxidation

Abstract: Copper nanoparticles were deposited onto mesoporous sba-15 support via two different routes: post-grafting method and incipient wet impregnation method. both xrd and tem reveal that the post-grafting can make cu particles very small in size and highly dispersed into channels of sba-15, while the impregnation method mainly forms large cu particles on the external surface of sba-15. tpr experiments show that cuo species formed by the post-grafting method is more reducible than that prepared by the impregnation method. the catalytic activity tests for co oxidation manifests that the sample prepared by the post-grafting method has a much higher activity than that prepared by the impregnation method, with a lowering of 50 degrees c for t-50, showing a strong dependence of catalytic activity on the size and dispersion of cu particles. besides the preparation procedure, other factors including calcination temperature, reduction treatment, copper loading as well as the feed composition, have an important effect on the catalytic activity. the best performance was obtained when the catalyst was calcined at 500 degrees c and reduced at 550 degrees c. the calcination and reduction treatment at high temperature have been found to be necessary to completely remove the organic residue and to generate active metallic copper particles. (c) 2005 elsevier b.v. all rights reserved.

Water-gas shift reaction over Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by homogeneous precipitation

Abstract: Both binary Cu/ZnO and ternary Cu/ZnO/Al2O3 catalysts were prepared by homogeneous precipitation (hp) using urea hydrolysis. The structure and the activity for the water-gas shift reaction of these catalysts were studied compared with those prepared by coprecipitation (cp). The binary precursors contained hydroxycarbonates such as malachite and aurichalcite phases, whereas the ternary precursors were composed of hydrotalcite, malachite and aurichalcite phases depending on the metal composition. After thermal decomposition, both catalysts contained apparently CuO and ZnO as crystalline phase. No phase derived from Al was observed, since the amount of Al was small as 10 at.% in the ternary catalysts. After reduction pretreatment with hydrogen, the catalysts were tested for the shift reaction between 150 and 300 °C. The activity of hp-catalysts was higher than that of cp-catalysts; binary hp-Cu/ZnO showed higher activity than ternary hp-Cu/ZnO/Al2O3 catalysts none the less the surface area was larger for the latter than for the former. The activity apparently depended on the surface area of Cu metal formed on the surface of hp-catalysts and a good correlation was observed between the Cu metal particle size and the activation energy of the shift reaction. However, more precise evaluation of the activity based on turn-over frequency strongly suggested the formation of Cu+ species as the active sites at the boundary between Cu metal particles and ZnO particles. Even after the pre-reduction at the high temperature, 250 °C, hp-Cu/ZnO catalyst showed no significant deactivation as well as no detectable sintering of the Cu metal particles during 50 h of the reaction, indicating that the hp-preparation method afforded the Cu catalysts with high sustainability in the shift reaction.

Aerobic oxidation of glucose: II. Catalysis by colloidal gold

Abstract: The selective oxidation of d -glucose to d -gluconic acid was performed in aqueous phase at atmospheric pressure, controlled pH value and different glucose and oxygen concentrations, in the temperature range from 303.2 to 333.2 K, using a colloidal metal gold catalyst (average gold diameter 3.5 nm). Initial rate was measured as a function of glucose and oxygen concentration: in the experimental conditions it was found that gluconic acid is produced together with hydrogen peroxide, which later decomposes in a fast way, due to the presence of alkali. The measurements were interpreted by considering different models based on different reaction pathways. Among the considered models, the experimental data fit with an Eley–Rideal mechanism where a glucose molecule, adsorbed on the catalyst, interacts with an oxygen molecule coming from the liquid phase. The model includes a kinetic parameter k cat and the equilibrium constant K G for the adsorption of glucose on the gold surface. The activation energy for k cat was found to be 47.0 ± 1.7 kJ mol −1 . It has been observed that K G decreases when temperature is increased, but the experimental uncertainty did not allow to obtain a precise value of the adsorption enthalpy. The values of the rate parameters here calculated for the colloidal gold catalyst have been compared with those previously obtained using the homogeneous enzymatic catalyst Hyderase under similar experimental condition. Considering geometric constraints, the specific activity of gold catalysis resulted quite similar to the enzymatic one.

Catalytic polymeric membranes: Preparation and application

Abstract: The study of catalytic membranes is a multidisciplinary activity, which in recent years has attracted the attention of scientists in a number of disciplines, including material science, chemistry and chemical engineering. Membrane based reactive separation processes, which seek to combine two distinct functions, i.e. reaction and separation, have been around as a concept since the early stages of the membrane field, itself, but have only attracted substantial technical interest the last decade or so. According to the literature, most studies combining membranes and catalysts concern gas phase reactions at relatively high temperature. In most of these applications inorganic membrane made from ceramic or metals are applied. Polymeric membranes (porous or dense) are used when the reaction temperatures are lower, i.e. in the field of fine chemicals or when biocatalysts are present. Dense polymeric membranes in use to separate gases or liquids from mixtures by a sorption–diffusion mechanism can be coupled to catalytic reaction and then be used to separate and react in one step. The polymeric membrane should be not only highly selective, but it should also be permeable enough to give a sufficient separation. Liquid phase catalytic reactions are involved in numerous industrial processes ranging from fine to bulk chemical synthesis; polymeric membranes may also play a significant role in this field. In this paper, a review on the preparation and application of polymeric membranes in the field of fine chemicals with adequate performance in catalysis both in gas phase and liquid phase reactions is presented and discussed.

Synthesis of dimethyl ether (DME) on modified HY zeolite and modified HY zeolite-supported Cu–Mn–Zn catalysts

Abstract: Synthesis of dimethyl ether (DME) via methanol dehydration were investigated over HY zeolite and over Fe-, Co-, Ni-, Cr-, or Zr-modified HY zeolite, and via direct CO hydrogenation over modified HY zeolite-supported Cu–Mn–Zn catalysts. Zr- and Ni-modified HY zeolite exhibited higher activity and stability for methanol dehydration, while Fe-, Co-, and Cr-modified HY zeolite deactivated quickly due to carbon deposition. For DME synthesis directly from CO hydrogenation, it was found that the amount of dehydration component had an important influence on the performance of the dual catalyst. Zr–HY supported Cu–Mn–Zn catalyst was more active and stable than Cu–Mn–Zn/HY in the “synthesis gas to dimethyl ether” process.

Effects of Ce addition on the Pt-Sn/γ-Al2O3 catalyst for propane dehydrogenation to propylene

Abstract: The effects of Ce addition on the Pt-Sn/gamma-Al2O3 catalysts for propane dehydrogenation to propylene have been investigated by reaction tests and some physicochemical characterizations like XRD, BET, NH3-TPD, H-2-TPD, H-2-TPR, H-2-chemisorption, CO-FTIR and TPO The results show that the Ce addition could greatly improve the catalytic performance and catalytic stability of the Pt-Sn/gamma-Al2O3, which is reported as the optimal catalyst for propane dehydrogenation to propylene We could keep > 38% of propane conversion, > 98% propylene selectivity and > 37% propylene yield over 50 h in the reaction of propane dehydrogenation to propylene over Pt-Sn/Ce-Al2O3 catalysts at 576 degrees C, 3800 h(-1) and H-2/C3H8/Ar = 1/1/5 The presence of Ce in the Pt-Sn/Ce-Al2O3 catalysts could not only stabilize the active states of Pt, Sn and support, but could also suppress the coke accumulation on the catalyst during reaction, and further improve the catalytic performance of Pt-Sn/gamma-Al2O3 (c) 2006 Elsevier BV All fights reserved