Showing papers in "Applied Catalysis A-general in 2003"

Carbon nanotubes and nanofibers in catalysis

Abstract: This review analyses the literature from the early 1990s until the beginning of 2003 and covers the use of carbon nanotubes (CNT) and nanofibers as catalysts and catalysts supports. The article is composed of three sections, the first one explains why these materials can be suitable for these applications, the second describes the different preparation methods for supporting metallic catalysts on these supports, and the last one details the catalytic results obtained with nanotubes or nanofibers based catalysts. When possible, the results were compared to those obtained on classical carbonaceous supports and explanations are proposed to clarify the different behaviors observed.

Selective CO oxidation over CuO-CeO2 catalysts prepared via the urea-nitrate combustion method

Abstract: CuO-CeO2 catalysts have been proposed as a promising candidate catalytic system for CO removal from reformed fuels via selective oxidation. In this work, the performance of CuO-CeO2 catalysts, synthesized via the urea–nitrate combustion method, in the reaction of selective CO oxidation in the presence of H2 has been investigated. The combustion method was found to be a simple and fast route for the synthesis of ultrafine, nanocrystalline CuO-CeO2 catalysts. The influence of the fuel/oxidant (urea/nitrate) ratio and the Cu content on the catalytic properties of CuO-CeO2 catalysts has been studied and optimal values of these parameters have been determined. Compared to CuO-CeO2 catalysts prepared with other techniques, the catalysts prepared via the combustion method exhibited similar catalytic performance, remaining very active and stable, remarkably selective and with good tolerance towards CO2 and H2O.

Synergism between rutile and anatase TiO2 particles in photocatalytic oxidation of naphthalene

Abstract: Photocatalytic oxidation of naphthalene was investigated in a mixed solution of acetonitrile and water using various kinds of titanium dioxide (TiO 2 ) powders as the photocatalysts and molecular oxygen as the electron acceptor. The main product from naphthalene is 2-formylcinnamaldehyde. For this reaction, anatase small TiO 2 particles, which are commonly used as photocatalyst, are inactive, probably because band bending is necessary for the oxidation of naphthalene. If the particles are not extremely small, pure rutile and pure anatase powders show fairly high activity, and those containing both anatase and rutile phases show the highest activity. When a pure anatase powder is partly (about 90%) converted to the rutile form by heat treatment, the activity is largely enhanced. The activity of pure rutile particles is also enhanced by physically mixing them with a small amount of small-sized anatase particles, which are inactive for this reaction. These results can be explained by the synergism between rutile and anatase particles. We consider that electrons are transferred from rutile particles to anatase particles, i.e. naphthalene is mainly oxidized on rutile particles and oxygen is mainly reduced on anatase particles. This electron transfer process is supported by electrochemical properties of TiO 2 electrodes for reduction of oxygen.

Metal carbides and nitrides as potential catalysts for hydroprocessing

Abstract: The carbides and nitrides of Mo and W can adsorb and activate hydrogen. The effects of particle size and surface area on the total amount of adsorbed hydrogen differ from those observed for transition metal sulfides. In the former case, this amount increases with increasing particle size and/or decreasing surface area. This is consistent with the involvement of the sub-surface regions of the crystallites during hydrogen adsorption. The activity for hydrogenation, hydrodesulfurization and hydrodenitrogenation exhibits similar trends. The high HDN activity of the Mo(W) carbide and nitride based catalysts has been noted. These catalysts are stable under typical hydroprocessing conditions although a partial sulfidation of their surface during HDS cannot be avoided.

Acid catalysis by heteropoly acids

Abstract: The achievements in the field of acid catalysis by heteropoly acids having different structures HxPW11LO40 (L = ZrIV, TiIV, ThIV), HxZW12O40 (Z = SiIV, PV, BIII, CoIII, GeIV), H21B3W39O132, H6P2W18O62, H6P2W21O71, H6As2W21O69 are reviewed. The data on the acidity of HPAs are generalized. The rules of homogeneous and heterogeneous acid catalysis by HPAs are discussed.

Organized mesoporous alumina: synthesis, structure and potential in catalysis

Abstract: Organized mesoporous alumina represents a very interesting molecular sieve exhibiting a narrow pore size distribution with higher surface areas compared to conventional aluminas, used as a support for catalytically active species in numerous large-scale industrial processes. This review encompasses various synthesis approaches to organized mesoporous aluminas, description of their structures and properties, and characterization by various experimental techniques. The potential of the mesoporous alumina with respect to use in catalysis is also outlined. Surface areas up to 800 m2/g and pore sizes ranging from 2.0 to more than 10 nm are characteristic for organized mesoporous aluminas prepared by neutral, anionic and cationic synthesis routes. Although utilization of mesoporous aluminas as a support in catalysis has not been reported frequently, they have a certain potential in hydrodesulfurization and metathesis reactions.

Oxidative desulfurization of fuel oil - Part I. Oxidation of dibenzothiophenes using tert-butyl hydroperoxide

Abstract: The oxidation of sulfur compounds in kerosene was conducted with tert-butyl hydroperoxide (t-BuOOH) in the presence of various catalysts. The oxidation activities of dibenzothiophene (DBT) in kerosene for a series of Mo catalysts supported on Al2O3 with various Mo contents were estimated. The results show that the oxidation activity of DBT increased with increasing Mo content up to about 16 wt.% and decreased when Mo content was beyond this value. The oxidation of benzothiophene (BT), dibenzothiophene (DBT), 4-methyl dibenzothiophene (4-MDBT), and 4,6-dimethyl dibenzothiophene (4,6-DMDBT) dissolved in decalin was also carried out on 16 wt.% Mo/Al2O3 catalyst with t-BuOOH to investigate the oxidation reactivities of these sulfur compounds. The results indicated that the oxidation reactivities of these sulfur compounds decreased in the order of DBT>4-MDBT>4,6-DMDBT⪢BT. Analyses of the oxidative reactions of the sulfur compounds suggested that the oxidative reaction of each sulfur compound can be treated as a first-order reaction. The oxidation mechanism is then discussed.

Catalytic autothermal reforming of methane and propane over supported metal catalysts

Abstract: Catalytic autothermal reforming of methane and propane over supported metal catalysts has been investigated in the present study. The carbon deposition region and the heat balance of the reaction have been determined from the equilibrium calculations. The sequence of the activities of the 2 wt.% metal on alumina support for autothermal reforming of methane was Rh>Pd>Ni>Pt>Co. The catalytic activity of 10 wt.% Ni/Al 2 O 3 was higher than that of the 2 wt.% Rh/Al 2 O 3 . The activity of Ni was significantly lowered by the preferential oxidation of the catalyst in the reactant gas at low temperatures. Although little carbon deposition was observed for the autothermal reforming of methane in the deposition-free region expected from the equilibrium, a large amount of carbon deposition was observed for the propane autothermal reforming even in the steam-rich conditions. The deposited carbon possessed fibrous morphology. The catalytic autothermal reforming appears to be initiated by decomposition of hydrocarbon at the inlet zone; then the reforming reaction subsequently proceeded in the catalyst bed.

Catalytic carbon monoxide oxidation over CoOx/CeO2 composite catalysts

Abstract: CoOx/CeO2 composite catalysts of different cobalt/ceria ratios have been prepared and tested for carbon monoxide oxidation in mixtures of carbon monoxide and oxygen. The small activity decay observed for them is due to carbon dioxide retention. The CoOx/CeO2 composite catalyst showed good resistance to water vapor poisoning. The catalysts were investigated by X-ray diffraction (XRD), temperature-programmed reduction (TPR), CO-TPD and XPS. The CoOx/CeO2 composite catalysts exhibit high catalytic activity in carbon monoxide oxidation, showing markedly enhanced catalytic activities due to the combined effect of cobalt oxide and ceria. Combining the results of XRD, TPR and XPS, we propose that the finely dispersed and higher valence state CoOx species mainly contribute to the catalytic activity.

Reduction property and catalytic activity of Ce1-XNiXO2 mixed oxide catalysts for CH4 oxidation

Abstract: Ce 1− X Ni X O 2 oxides with X varying from 0.05 to 0.5 were prepared by different methods and characterized by XRD and TPR techniques. Ce 0.7 Ni 0.3 O 2 sample prepared by sol–gel method shows the highest reducibility and the highest catalytic activity for methane combustion. Three kinds of Ni phases co-exist in the Ce 1− X Ni X O 2 catalysts prepared by sol–gel method: (i) aggregated NiO on the support CeO 2 , (ii) highly dispersed NiO with strong interaction with CeO 2 and (iii) Ni atoms incorporated into CeO 2 lattice. The distribution of different Ni species strongly depends on the preparation methods. The highly dispersed NiO shows the highest activity for methane combustion. The NiO aggregated on the support CeO 2 shows lower catalytic activity for methane combustion, while the least catalytic activity is found for the Ni species incorporated into CeO 2 . Any oxygen vacancy formed in CeO 2 lattice due to the incorporating of Ni atoms adsorbs and activates the molecular oxygen to form active oxygen species. So the highest catalytic activity for methane combustion on Ce 0.7 Ni 0.3 O 2 catalyst is attributed not only to the highly dispersed Ni species but also to the more active oxygen species formed.

X-ray diffraction study of nickel oxide reduction by hydrogen

Abstract: Hydrogen reduction of porous bulk NiO particles has been studied with in situ hot-stage X-ray diffraction (XRD) in the temperature range 175–300 °C. This technique has the ability to measure NiO disappearance and Ni appearance simultaneously, together with the crystallite size of each. Since the sample was a very thin, 50-μm slab of dispersed 20-μm diameter grains, textural and morphological features normally encountered during studies with fixed beds of NiO particles were absent and measurements reflected only the chemical mechanism and kinetics. The results indicated that reduction in the absence of water added to the reducing gas followed a series of steps: (1) an induction period associated with the initial reduction of NiO and the appearance of Ni metal clusters; (2) acceleration of the reduction rate as the size of the clusters increase; and (3) a pseudo-first-order (excess H 2 ) process in which NiO disappeared and Ni appeared in concert until reduction slowed at a fractional conversion of about 0.8. Crystallite size measurements showed NiO crystallites of about 3 nm in size were transformed into Ni crystallites of more than 20 nm, implying that Ni 0 ion transport following reduction was very fast due to the close proximity of the NiO crystallites being reduced. When 2.2×10 −2 atm of H 2 O was added to the reducing gas, induction times increased by approximately a factor of two and reduction rates decreased (increasingly at lower temperatures) with an apparent activation energy of 126±27 kJ mol −1 compared to 85±6 kJ mol −1 without added water. The lag between NiO reduction and Ni growth observed in previous studies was not seen, indicating that textural and morphological factors are very important in establishing the role of water vapor in the reduction process.

Characterization of Ca-promoted Ni/α-Al2O3 catalyst for CH4 reforming with CO2

Abstract: In CH4 dry reforming, the influence of Ca on the activity of supported Ni catalyst depended on the properties of the supports (SiO2, α-Al2O3, γ-Al2O3) and on the amount of Ca added. Small amounts of Ca increased the activity and stability of Ni/α-Al2O3. Temperature-programmed reduction with H2 (H2-TPR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterizations of Ca-promoted Ni/α-Al2O3 catalysts indicated that Ca improved the dispersion of Ni, strengthened the interaction between Ni and Al2O3, and retarded the sintering. A higher amount of Ca covered the surface of α-Al2O3 and enhanced the decomposition of CH4.

Performance of Rh/Al2O3 catalyst in the steam reforming of ethanol: H2 production for MCFC

Abstract: The steam reforming of ethanol on 5% Rh/Al2O3 catalyst has been investigated to produce H2 adequate to feed a molten carbonate fuel cell (MCFC). The influence of reaction temperature, H2O/EtOH molar ratio, gas hourly space velocities (GHSV) and O2 were investigated in order to maximize the hydrogen content at the outlet of ethanol reformer. Endurance tests to assess the feasibility of a FC system fed by simulated bio-EtOH stream (H2O/EtOH molar ratio=8.4) were also performed. Experiments carried out at MCFC operative conditions allowed to obtain a H2 reach mixture close to 60% on dry basis. In steam reforming (SR) conditions an extensive formation of encapsulated carbon was registered while a great benefits, both in terms of catalyst stability and coke formation were evidenced adding 0.4 vol.% of oxygen in the reaction stream. Oxygen however promoted metal sintering. Consideration on reaction mechanism, mainly investigated to distinguish the reaction pathway influencing the H2 production, is reported.

Effect of CeO2 loading on the surface and catalytic behaviors of CeO2-Al2O3-supported Pt catalysts

Abstract: Pt catalysts supported on mixed CeO2-Al2O3 carriers with different CeO2 loading (0.5–10.3 wt.%) were prepared by wetness impregnation method. The catalysts were characterized by SBET, X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and thermogravimetric analysis (TG). It was shown that pretreatment temperature and the concentration of CeO2 in the support influences significantly on the morphology of Pt. XRD showed the formation of nanocrystallites of Pt on the surface of alumina and low-loaded CeO2 (≤6 wt.%) samples at higher temperature of calcination (1073 K). Amorphous Pt was observed in all reduced samples. XPS spectra showed the presence of interaction between Pt and Ce, which leads to easy surface reduction of both, ceria and platinum, as revealed by TPR patterns. The effect of CeO2 loading on the catalytic behavior of supported Pt catalysts in the reaction of CO2 reforming of CH4 was determined. Addition of cerium oxide results in improvement of catalytic performance for the reforming of methane with CO2. Pt catalyst with 1 wt.% of CeO2 exhibited the highest specific activity and stability, due to the increase in the metal–support interface area, caused by the higher Pt dispersion.

On the potential role of hydroxyl groups in CO oxidation over Au/Al2O3

Abstract: The deuterium isotope effect in the steady state CO oxidation rate over Au/-Al2O3 in the presence of H2 or H2O and the effect of pretreatment on an uncalcined catalyst were studied. In a reaction feed containing 1% CO, 0.5% O2, and 40.5% H2 at room temperature, CO oxidation exhibited a deuterium isotope effect (kH/kD )o f 1.4 ± 0.2. The rate of D2 oxidation was also slower than the oxidation of H2, such that the selectivity for CO oxidation was 86% in the presence of D2 versus 77% in the presence of H2. In contrast, there was no deuterium isotope effect in a feed containing 1% CO, 0.5% O2, and 1.5% H2O. H2 was also more effective in regenerating a CO oxidation reaction deactivated catalyst than D 2, whereas H2O and D2O were equally effective. The difference was attributed to the different mechanisms with which H 2 or H2O prevented deactivation of the catalyst during CO oxidation. An uncalcined Au/-Al2O3 was rather inactive. It could be activated by treatment with a mixture of H2 and H2 Oa t 100 ◦ C, although treatment by either H2 or H2O alone was ineffective. The observations are consistent with the model of the active site consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group.

Low temperature water–gas shift: in situ DRIFTS-reaction study of ceria surface area on the evolution of formates on Pt/CeO2 fuel processing catalysts for fuel cell applications

Abstract: Steady state infrared (IR) measurements for adsorption of only CO and under water–gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, and that their concentrations vary with surface area of partially reduced ceria. Under steady state WGS, the concentrations of surface formates are strongly limited at high CO conversions. However, at low temperatures and conversions, the formates are close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. Comparisons at constant temperature indicate that formate bands from IR may provide an indication of the number of active sites present on the catalyst surface, as the rates varied accordingly. The IR results favor a formate intermediate mechanism to explain WGS. However, more kinetic studies are required, and over a broad range of temperatures, to verify this conclusion. Previous low temperature kinetic studies at a relatively high CO/H2O ratios have produced a zero-order dependency for CO and the authors related this to a mechanistic scheme involving reaction of Pt-CO with CeO2 to yield CO2, followed by reoxidation of Ce2O3 by H2O, with liberation of H2. The zero-order was suggested to be due to saturation of noble metal surface with CO during WGS. Saturation of ceria with carbonates was also reported. In this study, a high H2O/CO ratio was used where the CO rate dependency was first-order. This criteria requires that the surface coverage of the adsorbed CO intermediate should be reaction rate limited. Therefore, the formates are suggested to be the intermediates.

FTIR study of low-temperature water-gas shift reaction on gold/ceria catalyst

Abstract: Chemisorption and reactivity of the molecules involved in the water-gas shift (WGS) reaction on gold/ceria catalyst have been studied at 90 and 300 K by FTIR spectroscopy. Forward and reverse WGS reaction at 300 K and up to 573 K have been investigated, too. The FTIR results show that gold causes a strong modification of the surface properties of the support. The nanosized metallic gold particles in close contact with defective ceria play an essential role for the genesis of high catalytic activity in WGS reaction at low temperature and appear to be of crucial importance in explaining the remarkably high stability of this catalytic system. An electronic interaction between small gold metallic nanoparticles and ceria has been evidenced.

Catalytic decomposition of methane over Ni-Al2O3 coprecipitated catalysts: Reaction and regeneration studies

Abstract: The catalytic decomposition of methane over nickel catalysts is a potential alternative route to steam reforming or partial oxidation for the production of hydrogen from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of a Ni(30%)/Al2O3 catalyst during the catalytic decomposition of methane. The influence of the operating and reduction temperatures and feed composition on the methane conversion, hydrogen production and coking rate has been studied. The effects of the regeneration cycles with oxygen on activity and carbon formation are also investigated. It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. An increase in the reaction temperature increases both the deactivation rate and the growth rate of filaments. However, at high reduction temperatures, there is a decrease in the number of filaments formed due to sintering of the Ni particles. A kinetic model has been developed for the prediction of H2 production and of carbon, taking into account both stages of carbon formation, nucleation and filament growth.

Supported Keggin type heteropolycompounds for ecofriendly reactions

Abstract: New supported Keggin heteropolyacid (HPA) catalysts were prepared in order to make a contribution to the field of catalyzed acid reactions by means of ecofriendly technologies. The esterification of isoamyl alcohol and acetic acid in gaseous phase was studied using tungstophosphoric acid (TPA) supported on different carriers, silica materials, carbon and alumina. These catalysts were characterized by FT-IR, DRS, TPR and their acidity was evaluated through the isopropanol dehydration test reaction. The higher acidic catalysts prepared with silica materials led to the greatest activities due to the higher proton availability which, in turn, was related to the lower interaction and dispersion of the active species. In order to perform this acid reaction in heterogeneous liquid phase and to avoid the tungstophosphoric or molybdophosphoric acid (MPA) solubilization during reaction, functionalized silica and SiMCM-41, and polyvinyl alcohol hydrogel–polyethylenglycol (PVA-PVG) beads were used as new supports. The physical–chemical characterization of the supported catalysts by FT-IR, XRD, 31 P MAS NMR and TG-DTA showed mainly undegraded primary Keggin structure. The solids exhibited low or negligible solubilization and their acidity, measured by potentiometric titration with n-butylamine, was high. The SiMCM-41 and polymeric bead-supported catalysts lead to obtain attractive activity in the isoamyl acetate synthesis through esterification reaction.

Fischer–Tropsch synthesis over silica supported cobalt catalysts: mesoporous structure versus cobalt surface density

Abstract: The effect of support mesoporous structure and cobalt content on cobalt dispersion and reducibility was studied using two series of Fischer–Tropsch (FT) silica supported cobalt catalysts. The first series of the catalysts was supported by an SBA-15 periodic mesoporous silica with narrow pore size distribution, the second series was supported by a commercial mesoporous silica with broader pore size distribution. It was shown that in a wide range of cobalt surface densities (0–50 Co/nm 2 ), cobalt dispersion in silica supported catalysts was largely influenced by support texture. Cobalt dispersion was higher in Co catalysts supported by the SBA-15 silica with a pore diameter of 9.1 nm than in the commercial mesoporous silica with an average pore diameter of 33 nm. A more than 10-fold increase in cobalt surface density did not result in any noticeable sintering of Co 3 O 4 particles in SBA-15 periodic mesoporous silicas; the cobalt dispersion seems to be maintained by catalyst mesoporous structure. The effect of support pore diameter on cobalt dispersion was less significant for the catalysts supported by commercial silicas with broader pore size distribution. At the range of cobalt surface densities from 5 to 15 Co/nm 2 , higher Fischer–Tropsch reaction rates were observed over cobalt catalysts supported by the SBA-15 periodic mesoporous silica. This effect was attributed to higher cobalt dispersion in these catalysts. An increase in cobalt surface densities did lead to any significant changes in hydrocarbon selectivities and in chain growth probabilities for both series of supported catalysts.

Carbon capacious Ni-Cu-Al2O3 catalysts for high-temperature methane decomposition

Abstract: Steady and efficient decomposition of methane can be achieved at 625–675 °C over copper-promoted (8–15 wt.% of copper) nickel catalysts prepared from a Feitknecht compound precursor. Such catalysts permit one to increase the yield of catalytic filamentous carbon (CFC) and control both microstructural and textural properties of CFC by varying the copper concentration in the catalyst. The maximal conversion of methane into hydrogen and carbon reaches 40% at 675 °C at the carbon capacity not lower than 700 g/g Ni under optimal conditions. The BET surface area of the CFC is 285.9 m 2 /g. The influence of promotion of nickel-containing catalysts with copper on the catalytic activity is discussed.

Study of reverse water gas shift reaction by TPD, TPR and CO2 hydrogenation over potassium-promoted Cu/SiO2 catalyst

Abstract: The reverse water gas shift (RWGS) reaction over Cu/SiO2 with and without potassium promoter was studied by means of CO2 hydrogenation, temperature programmed reduction (TPR) and temperature programmed desorption (TPD). After addition of even a little amount of potassium, Cu/K2O/SiO2 obviously offers better catalytic activity than Cu/SiO2. The coverage of formate species increases on Cu/K catalyst. TPD of CO2 adsorbed on Cu/K2O/SiO2 and TPR profile of Cu/K2O/SiO2 showed that new active sites could be created at interface between Cu and K. The main role of K2O was to provide catalytic activity for decomposition of formates, besides acting as a promoter for CO2 adsorption. A new reaction mechanism was suggested. Hydrogen was dissociatively adsorbed on Cu and could spill over to K2O to associate with CO2. This resulted in the formation of formate species for the production of CO.

Structural studies on NiO-CeO2-ZrO2 catalysts for steam reforming of ethanol

Abstract: The influence of Ce/Zr ratio on the redox behavior of Ni in a series of NiO-CeO2-ZrO2 catalysts was investigated using in situ electron paramagnetic resonance (EPR), diffuse reflectance UV-visible (DRUV-visible) and X-ray photoelectron spectroscopy (XPS). At all concentrations, a small amount of Ni (species I) substitutes in the fluorite lattice. Superparamagnetic, nanosize Ni crystallites (species II) were found in samples with 1–5 wt.% NiO and ferromagnetic, larger Ni crystallites (species III) were detected in samples with 20 wt.% or more NiO when contacted with hydrogen. Ce promoted the reduction of Ni. The reducibility of Ni decreased in the order: I>III>II. At steam reforming conditions (in the presence of H2+H2O+hydrocarbon/alcohol at 773 K), the extent of Ni reduction varies in the order: H2+alkane>H2+alcohol>H2 alone. Catalytic activity and especially stability in the steam reforming of bio-ethanol (containing 5 ppm S) correlates with the type III Ni species and is influenced by both the Ni-content and the Ce/Zr ratio in the support. A catalyst of composition NiO (40 wt.%)-CeO2 (30 wt.%)-ZrO2 (30 wt.%) maintained its activity for more than 500 h without deactivation.

Dimerisation of ethanol to butanol over solid-base catalysts

Abstract: Ethanol is converted into 1-butanol over alkali earth metal oxides and modified MgO catalysts (1–20% yield). The MgO catalyst exhibits the highest reaction activity and 1-butanol selectivity amongst the catalysts studied. Reaction of various intermediates (acetaldehde, crotonaldehyde, crotylalcohol, butanal) and butanol over MgO (1 bar, 450 °C) revealed that the dimerisation reaction does not proceed primarily through the aldol condensation reaction. The reaction is proposed to proceed through a mechanism (Yang and Meng) in which a CH bond in the β-position in ethanol is activated by the basic metal oxide, and condenses with another molecule of ethanol by dehydration to form 1-butanol.

Carbon deposition and catalyst stability over La2NiO4/γ-Al2O3 during CO2 reforming of methane to syngas

Abstract: The use of 9% La2NiO4/γ-Al2O3 (9NLA) as a catalyst for CO2 reforming of methane to syngas has been investigated in a fixed-bed reactor. The results revealed that the yields of CO and H2 over a 800 °C calcined 9NLA catalyst were remarkably higher than those over a 500 °C calcined one. The BET data confirmed that the properties, such as specific surface area, pore volume and average pore diameter, of a used and a regenerated catalyst were similar to those of a fresh catalyst. Temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) results indicated that the activity of the catalyst depended largely on the degree of catalyst reduction, as well as on the dispersion of metallic nickel. The results of X-ray diffraction (XRD) and temperature-programmed oxidation (TPO)-MS studies demonstrated that the formation of carbon species on the catalyst could be related to the structure and surface composition of the catalyst. With the rise in temperature for catalyst calcination, there was a decline in coke formation due to the formation of a stable NiAl2O4 spinel structure.

Efficient hydrogen production using cyclohexane and decalin by pulse-spray mode reactor with Pt catalysts

Abstract: Highly efficient production of hydrogen without CO 2 emission is achieved in the dehydrogenation of cyclic hydrocarbons under a non-steady spray pulse operation over supported Pt and Pt-M (M=Re, Rh, Pd) catalysts. Cyclohexane, methylcyclohexane, tetralin and decalin were efficiently dehydrogenated by the Pt-containing catalysts supported on thin active carbon cloth (CFF-1500S) sheets and alumite (anodized aluminum) plates. Production rate of hydrogen under the spray pulse mode is higher than the conventional batch-type liquid phase reaction and the steady state gas phase reaction in the flow system. The highest rate, 3800 mmol g Pt −1 min −1 , was obtained in the dehydrogenation of cyclohexane over Pt/alumite heated at 375 °C and cyclohexane feed of 190 mmol min −1 with 3.5 mmol pulse at 1.0 s interval. Bimetallic Pt-Rh/CFF-1500S catalyst showed a higher activity than monometallic Pt/CFF-1500S. Production rate of hydrogen is greatly dependent on the rate of reactant feed, the reaction temperature, and the support. Retardation by products adsorbed on the catalysts was negligible under the spray-pulse operation.

Properties of ceramic foam catalyst supports: mass and heat transfer

Abstract: Mass and heat transport properties have been determined for 30 PPI α-Al 2 O 3 ceramic foam containing 6 wt.% γ-Al 2 O 3 washcoat. The foam was loaded with 5 wt.% platinum and the rate of carbon monoxide oxidation measured for a 0.3 cm cylindrical segment of the foam operating with mass transfer controlling at 550 °C. This gave a mass transfer factor versus Reynolds number correlation that was equivalent to a packed bed of particles. A correlation for the radial heat transfer coefficient in a bed of ceramic foam was determined by measuring outlet temperatures achieved when air at varying flow rates and inlet temperatures was passed through a bed of foam pellets. Correlation parameters of a 1D model were fitted from 700 to 1000 °C using a Simplex optimization routine. Radial heat transfer coefficients were two to five times higher than those predicted from packed bed correlations.

Fischer-tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts

Abstract: Abstract The unpromoted and promoted Fischer–Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer–Emmett–Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al2O3 catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.

Cu-Ni-K/γ-Al2O3 supported catalysts for ethanol steam reforming: Formation of hydrotalcite-type compounds as a result of metal–support interaction

Abstract: The interaction of Cu2+, Ni2+ and Al3+ ions during the impregnation step of K/γ-Al2O3 support was studied. Cu-Ni catalyst precursors (“just impregnated solids”), just reduced precursors (H2, 300 °C), and calcined precursors in the range of 400–800 °C were characterised by X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) measurements. In addition, the catalytic behaviour of reduced precursors and calcined catalysts was analysed in the ethanol steam reforming reaction at 300 °C and atmospheric pressure. XRD results of different precursors indicated that, after the impregnation step, copper is present in two different phases: a copper basic nitrate and a CuAl and/or CuNiAl hydrotalcite-type compound (HT). In the sample containing only nickel, this metal is present as a NiAl-HT compound. At constant copper content of 6 wt.%, the ratio between the copper phases depends on the nickel content. Adding nickel favours the formation of HT compounds. While calcination of Cu-Ni precursors in the range of 400–800 °C produces a CuO segregated phase and/or a phase of copper called “surface spinel”, nickel is always found as a nickel aluminate after the calcination treatment. The catalytic behaviour of the samples strongly depends on the conditions of the thermal treatments. Thus, the increase in the calcination temperature of the precursors produces a strong interaction between nickel and aluminium, decreasing nickel reducibility and selectivity to C1 compounds. On the other hand, the just reduced precursors showed to have the best catalytic performance for the ethanol steam reforming reaction at 300 °C.

The state of the art on Wells–Dawson heteropoly-compounds: A review of their properties and applications

Abstract: The scientific literature concerning the structure, hydrolytic stability in solution, thermal stability in the solid state, redox-acid properties and applications of heteropoly-compounds (HPCs) with Wells–Dawson structure is summarized in the present work. Wells–Dawson heteropoly-anions possess the formula [(X n+ )2M18O62] (16−2n)− where X n+ represents a central atom (phosphorous(V), arsenic(V), sulfur(VI), fluorine) surrounded by a cage of M addenda atoms, such as tungsten(VI), molybdenum(VI) or a mixture of elements, each of them composing MO6 (M-oxygen) octahedral units. The addenda atoms are partially substituted by other elements, such as vanadium, transition metals, lanthanides, halogens and inorganic radicals. The Wells–Dawson heteropoly-anion is associated with inorganic (H + , alkaline elements, etc.) or organic countercations forming hybrid compounds. Wells–Dawson acids (phospho-tungstic H6P2W18O62·24H2O, phospho-molybdic H6P2Mo18O62·nH2O and arsenicmolybdic H6As2Mo18O62·nH2O) possess super-acidity and a remarkably stability both in solution and in the solid state. These properties make them suitable catalytic materials in homogeneous and heterogeneous liquid-phase reactions replacing the conventional liquid acids (HF, HCl, H2SO4, etc.). Although, the application of the acids in heterogeneous gas-phase reactions is less developed, there is a patented method to oxidize alkanes to carboxylic acids on a supported Wells–Dawson catalyst that combines acid and redox properties. Wells–Dawson anions possess the ability to accept or release electrons through an external potential or upon exposure to visible and UV radiation (electro and photochemical reactions). Additionally, Wells–Dawson HPCs catalyze the oxidation of organic molecules with molecular oxygen, hydrogen peroxide and iodosylarenes; epoxidation and hydrogenation in homogeneous and heterogeneous liquid-phase conditions. The ability of transition metal substituted Wells–Dawson HPCs to be reduced and re-oxidized without degradation of the structure is promising in the application of those HPCs replacing metalloporphyrins catalysts in redox and electrochemical reactions.