Steady State Isotopic Transient Kinetic Analysis of Flameless Methane Combustion over Pd/Al2O3 and Pt/Al2O3 Catalysts

The SSITKA measurements were performed in the steady state of complete methane oxidation on the Pd/Al₂O₃ and Pt/Al₂O₃ catalysts. It was found that the number of intermediates and their average life-time on the catalyst surface changes with the increase of reaction temperature. On the Pd/Al₂O₃ catalyst there is larger number of active centres than on Pt/Al₂O₃ catalyst which permits the course of methane oxidation at lower temperatures.     

A novel solid superbase of Eu2O3/Al2O3 and its catalytic performance for the transesterification of soybean oil to biodiesel

A novel solid superbase catalyst of Eu₂O₃/Al₂O₃ was prepared and its basic strength reached 26.5 measured by indicators according to Hammett scale. The catalytic activity of Eu₂O₃/Al₂O₃ was evaluated for the transesterification of soybean oil with methanol to biodiesel in the fixed bed reactor and under atmospheric pressure. The results show that Eu₂O₃/Al₂O₃ is an excellent catalyst for the transesterification of soybean oil, and the conversion of soybean oil can reach 63.2% at 70 °C for 8 h.     

Controlling the Sulfur Poisoning of Ag/Al2O3 Catalysts for the Hydrocarbon SCR Reaction by Using a Regenerable SOx Trap

The deactivation of a silver-based hydrocarbon selective catalytic reduction catalyst by SO_x and the subsequent regeneration under various operating conditions has been investigated. Using a sulfur trap based on a silica-supported catalyst it was found that, for a Ag/SiO₂ + Ag/Al₂O₃ combination, the negative effect of SO₂ on the n-octane-SCR reaction can be eliminated under normal operating conditions. The trap can be regenerated by hydrogen at low temperatures or at higher temperatures using a hydrocarbon reductant.     

The role of silver species on Ag/Al2O3 catalysts for the selective catalytic oxidation of ammonia to nitrogen

The role of Ag species on Ag/Al₂O₃ catalyst for the selective catalytic oxidation (SCO) of NH₃ to N₂ was studied using 10 wt% Ag/Al₂O₃ catalysts prepared with impregnation, incipient wetness impregnation and sol–gel methods. The catalyst characterization was preformed using N₂ adsorption–desorption, UV/Vis, TEM and XRD. O₂-chemisorption and H₂–O₂ titration were measured to confirm the metal dispersion on the catalyst. The Ag species state and Ag particle size have significant influence on the Ag/Al₂O₃ activity and N₂ selectivity of the SCO of NH₃ at low temperature. Ag⁰ is proposed to be an active species on the H₂ pretreated catalyst at low temperature (<140 °C). It is evident that well-dispersed and small particle Ag⁰ enhances catalytic activity at low temperature, whereas large particle Ag⁰ is relate to a high N₂ selectivity. In contrast, Ag⁺ could also be the active species at temperatures above 140 °C.     

Enhanced NOx Conversion with Decane Over Ag/Al2O3 and Metal Supported ZSM-5 Composite SCR Catalysts

A combination of Ag/Al₂O₃ and a partial oxidation catalyst M/ZSM-5, M being different metal cations, were evaluated for selective catalytic reduction of NO with decane. Physical mixture of Ag/Al₂O₃ and M/ZSM-5 catalysts showed significant increase in NOx conversion compared to single component Ag/Al₂O₃ catalyst. M/ZSM-5 as a second catalyst component was found to generate more reactive hydrocarbons, such as unsaturated small chain hydrocarbons and oxygenates in situ, and enhance the NOx conversion over Ag/Al₂O₃ HC-SCR catalyst.     

Ethanol reforming and partial oxidation with Cu/Nb2O5 catalyst

Ethanol reforming and partial oxidation were studied on Cu/Nb₂O₅ and Ni/Al₂O₃ catalysts. Compared to the Ni/Al₂O₃ catalyst, the Cu/Nb₂O₅ catalyst presents conversion as high as Ni/Al₂O₃ catalyst, however, for the same level of formation of hydrogen it occurs at much lower temperature on the Cu/Nb₂O₅ catalyst, 200 °C lower than for the Ni/Al₂O₃ catalyst, with remarkable little formation of CO, which can be attributed to the strong interaction between copper and niobia. Temperature-programmed desorption (TPD-ethanol) and surface reactions (TPSR) of partial oxidation of ethanol showed formation of ethylene, acetaldehyde, ethane and mainly H₂ and CO₂ besides little methane. DRIFTS results are in accordance with TPD analysis and the formation of acetate species at room temperature suggests reactivity of the surface and its oxidative dehydrogenation capacity. The adsorption of ethanol gives rise to ethoxide species, which form acetate and acetaldehyde that can be oxidized to CO₂ via carbonate. A comparison with reported results for Cu/Al₂O₃ this catalyst is promising, yielding high level of H₂ with little CO production during reforming and partial oxidation reaction. The maximum H₂ formation for the partial oxidation of ethanol was 41% at ratio (O₂/Et) 0.8, increasing to 50% at ratio 1.5. The H₂/CO is around 10 for the partial oxidation and 7 for steam reforming, which is excellent, compared to the Ni/Al₂O₃ catalyst with a factor 4–8 lower.     

Complete Oxidation of Methane at Low Temperature over Pt Catalysts Supported on High Surface Area SnO2

The catalytic activity of Pt catalysts supported on high surface area tin(IV) oxide in the complete oxidation of CH₄ traces under lean conditions at low temperature was studied in the absence and in the presence of water (10 vol.%) or H₂S (100 vol.ppm). Their catalytic properties were compared to those of Pd/Al₂O₃ and Pt/Al₂O₃. In the absence of H₂S in the feed, Pt/SnO₂appears as a very promising catalyst for CH₄ oxidation, being even significantly more active under wet conditions than the best reference catalyst, Pd/Al₂O₃. Catalysts steamed-aged at 873 K were also studied in order to simulate long term ageing in real lean-burn NGV exhaust conditions. To this respect, Pt/SnO₂ is slightly less resistant than Pd/Al₂O₃. In the presence of H₂S, Pt/SnO₂catalysts are rapidly and almost completely poisoned, comparably to Pd/Al₂O₃and the catalytic activity is hardly restored upon oxidising treatment below 773 K. A synergetic effect between Pt and specific surface SnO₂sites active in CH₄oxidation is proposed to explain the superior catalytic behaviour of Pt/SnO₂.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.     

Role of the surface state of Ni/Al2O3 in partial oxidation of CH4

The effect of gas phase O₂ and reversibly adsorbed oxygen on the decomposition of CH₄ and the surface state of a Ni/Al₂O₃ catalyst during partial oxidation of CH₄ were studied using the transient response technique at atmospheric pressure and 700°C. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H₂ can be produced from non‐selective oxidation of CH₄ by reversibly adsorbed oxygen which is more active in oxidizing CH₄ completely than NiO via the Rideal–Eley mechanism and both the conversions of CH₄ and O₂ and the selectivities to CO and H₂ are very low. Therefore, keeping the catalyst surface in the reduced state is the precondition of high conversion of CH₄ and high selectivities to CO and H₂. The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH₄. During partial oxidation of CH₄, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduced state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH₄, and the indirect oxidation mechanism is not able to gain dominance in the reaction under the experimental conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Activity and characterization of Rh/Al2O3 and Rh-Na/Al2O3 catalysts for the SCR of NO with CO in the presence of SO2 and HCl

SO₂ and HCl are major pollutants emitted from waste incineration processes. Both pollutants are difficult to remove completely and can enter the catalytic reactor. In this work, the effects of SO₂ and HCl on the performance of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts for NO removal were investigated in simulated waste incineration conditions. The characterizations of the catalysts were analyzed by BET, SEM/EDS, XRD, and ESCA. Experimental results indicated the 1%Rh/Al₂O₃ catalyst was significantly deactivated for NO and CO conversions when SO₂ and HCl coexisted in the flue gas. The addition of between 2 and 10 wt.% Na promoted the activity of the 1%Rh/Al₂O₃ catalyst for NO removal, but decreased the CO oxidation and BET surface area. The catalytic activity for NO removal was inhibited by HCl as a result of the formation of RhCl₃. Adding Na to the Rh/Al₂O₃ catalyst decreased the inhibition of SO₂ because of the formation of Na₂SO₄, which was observed in the XRD and ESCA analyses. SEM mapping/EDS showed that more S was residual on the surface of the Rh-Na/Al₂O₃ catalyst than Cl.     

Characterization of highly active silver catalyst for NOx reduction in lean-burning engine exhaust

A new Ag/Al₂O₃ catalyst for removing NOx in lean exhaust gas was developed. Oxidized Ag/Al₂O₃ catalyst is highly active for reduction of NOx with ethanol and propene, whereas reduced Ag/Al₂O₃ catalyst is less active for these reactions. Selectivity to N₂ is also high on the oxidized Ag/Al₂O₃ compared to that on the reduced Ag/Al₂O₃. XRD and SEM studies of these two types of Ag catalysts suggest that oxidation induces an interaction between Ag and the support, where the particles are grown in large size. In contrast, the metallic Ag particles are finely dispersed by the reduction process. Although dispersion of Ag particles is decreased by the oxidation process, the catalytic activity is increased. This suggests that the Ag-alumina sites created in the high temperature oxidizing environment are active in catalytic reduction of NOx. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cooperation of Ag/Al2O3 and Sn/Al2O3 Catalysts for the Selective Reduction of NO by Propene

Compared to the Ag/Al₂O₃ catalyst, a two‐stage catalyst composed of an Ag/Al₂O₃ layer followed by a Sn/Al₂O₃ layer shows higher low‐temperature activity and a wider temperature window. Its activity below 350 °C is enhanced in the presence of SO₂. Even in the presence of H₂O and SO₂, the performance of the same catalytic system is still satisfactory.     

Low-Temperature CO Oxidation over a Pt/Al2O3 Monolith Catalyst Investigated by Step-Response Experiments and Simulations

The ignition–extinction processes for CO oxidation over a Pt/Al₂O₃ monolith catalyst have been studied by flow-reactor experiments and simulations. The study was performed by stepwise changes of the inlet O₂ concentration ranging 0–20 vol% while the CO concentration and the inlet gas temperature were kept constant at 1.0 vol% and 423 K, respectively. Several features observed experimentally are qualitatively simulated with our model: (i) the ignition of the CO oxidation demands 8.0vol% O₂ (ii) corresponding to a catalyst ignition temperature of 433 K (due to the exothermicity of the reaction) and (iii) occurs in the rear part of the monolith where (iv) a local reaction zone is formed which (v) moves towards the reactor inlet as a function of time on stream. Additionally, the simulations show first order kinetic phase transitions, i.e. rapid adsorbate concentration changes, where the catalyst surface is predominantly CO covered in the low reactive state and almost completely oxygen covered in the high reactive state. For the ignition process the kinetic phase transition occurs after the actual catalytic ignition. However, the extinction process is more difficult to simulate dynamically without changing the model parameters for O₂ adsorption in the low and high reactive state, respectively. The influence of diffusion limitations and the role of formation of a less reactive Pt state under oxidising conditions is discussed.     

Conditions and mechanism of interconvertibility of compounds 12CaO.7Al2O3 and 5CaO.3Al2O3

Based on a critical analysis of the data reported in the literature and our own experimental studies relating to the melt and glass formation from compositions corresponding to 12CaO.7Al₂O₃ under various atmospheric conditions, the liquid structure of the melt phase and conditions for stabilization of 12CaO.7Al₂O₃ and 5CaO.3Al₂O₃ in crystalline state have been highlighted. The role of “stabilizers” like O^2?, F^?, Cl^?, S^2? for the crystalline structure of 12CaO.7Al₂O₃, effect of temperature and atmosphere on melt structure and properties, and conditions favouring the appearance of 12CaO.7Al₂O₃ and 5CaO.Al₂O₃ crystalline phases from the given melt have been specifically dealt with.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Effects of pore diameter and catalyst loading in hydroliquefaction of coal with CoO/MoO3/Al2O3 catalysts

The effect of catalyst pore size has been studied for the hydroliquefaction of a West Virginia coal in the presence of Co/Mo/Al₂O₃ catalyst. The alumina supports used for catalyst preparation had relatively sharp, unimodal pore size distribution with average pore diameters in the range of 100 Å to almost 1000 Å. Loading of MoO₃ and CoO on the Al₂O₃ supports was in the constant weight ratio of 5:1, but the absolute loading was in direct proportion to the surface area of the support. Two series of catalyst were studied: “High loading”, with 9.7 × 10^?4 g MoO₃/m² Al₂O₃, and “low loading”, with 4.5 × 10^?4 g MoO₃/m² Al₂O₃; both loadings were less than the amount necessary for monolayer distribution of MoO₃ on Al₂O₃. The weight of catalyst charged in each autoclave run was varied so that the same weight of MoO₃ and CoO was present for each experiment.The principal results were: (1) Al₂O₃ alone is not catalytic, even in large amount; (2) conversion of coal increases as catalyst pore diameter increases; from 100 Å to at least 500 Å; (3) the increased conversion with increasing pore size is manifested mainly as increased yield of asphaltenes at 400°C, so the ratio of oil to oil-plus-asphaltenes decreases as pore diameter increases; and (4) catalysts with “low loading” of MoO₃ and CoO on the Al₂O₃ surface give higher liquefactions than their counterparts with “high loading”. Most of the results are consistent with an expected low diffusion rate of large, coal-derived molecules through the catalyst pore system. The higher liquefaction with “low loading” of the Al₂O₃ surface might result from slow desorption of large product molecules (asphaltenes) exhibiting multiple-site adsorption to Mo neighbors on the surface.     

Synthesis, Characterization of Ag/MCM-41 and the Catalytic Performance for Liquid-phase Oxidation of Cyclohexane

Nano-scale silver supported mesoporous molecular sieve Ag/MCM-41 was directly prepared by one-pot synthesis method. The prepared sample was characterized by XRD, TEM, and N₂ sorption. The results showed that the sample of Ag/MCM-41 had no appreciable incorporation of silver into the mesoporous matrix of MCM-41 with good crystallinity, and silver nanoparticles were dispersed inside or outside of the channels in the mesoporous host. The catalytic performance of the sample for the cyclohexane liquid-phase oxidation into cyclohexanone and cyclohexanol by oxygen in the absence of solvents without inducing agents was investigated. The 83.4% selectivity to cyclohexanol and cyclohexanone at 10.7% conversion of cyclohexane was obtained over Ag/MCM-41 catalyst at 428 K for 3 h. The turn over numbers (TONs) of Ag/MCM-41 was up to 2946. The catalytic activity of Ag/MCM-41 was also compared with Ag/TS-1 as well as Ag/Al₂O₃. The results indicated that Ag/MCM-41 showed superior activity to both Ag/TS-1 and Ag/Al₂O₃. A calcined Ag/MCM-41 was found to be an efficient catalyst for the cyclohexane oxidation into cyclohexanol and cyclohexanone using oxygen as oxidant.     

Influence of low concentration of SO2 for selective reduction of NO by C3H8 in lean-exhaust conditions on the activity of Ag/Al2O3 catalyst

The effect of SO₂ for the selective reduction of NO by C₃H₈ on Ag/Al₂O₃ was investigated in the presence of excess oxygen and water vapor. The NO_x conversion decreased permanently even in the presence of a low concentration of SO₂ (0.5–10 ppm) at <773 K. The increase in SO₂ concentration resulted in a large decrease in NO_x conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al₂O₃ remained constant even in the presence of 10 ppm of SO₂. The sulfate species formed on the used Ag/Al₂O₃ were characterized by a temperature programmed desorption method. The sulfated species formed on silver should mainly decrease the deNO_x activity on the Ag/Al₂O₃. The sulfated Ag/Al₂O₃ was appreciably regenerated by thermal treatment in the deNO_x feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO₂ for long time by the heat treatment at every 20 h intervals.     

Fixed- and fluidised-bed synthesis of coiled carbon fibres on an in situ generated H2S-modified Ni/Al2O3 catalyst from a NiSO4/Al2O3 precursor

NiSO₄/Al₂O₃ and NiO/Al₂O₃ catalyst precursors were formed by calcination of NiSO₄·6H₂O/Al₂O₃ at 500 and 800 °C, respectively. The catalyst precursor was reduced under H₂ and N₂ and then reacted under C₂H₂, H₂ and N₂ at 650 °C. Coiled carbon fibres were formed in fixed- and fluidised-bed reactors using the NiSO₄/Al₂O₃ catalyst precursor. Thermodynamic modelling using an infinite equilibrium stage construction predicted complete reduction of NiSO₄ to Ni and simultaneous H₂S formation occurs in both fixed- and fluidised-bed systems. XRD measurements confirmed that Ni was the only catalytically active crystalline species present at concentrations >0.5 wt.% (XRD detection limit) post-reduction, however XRF and XPS measurements additionally detected the presence of small quantities (<0.9 wt.% S) of S species. S is adsorbed onto the Ni surfaces during reduction when H₂S is released and dissociates on the Ni surface. Non-coiled carbon fibres produced on the Ni/Al₂O₃ catalyst formed from the NiO/Al₂O₃ precursor demonstrated that modification of Ni/Al₂O₃ with S is required for coiled carbon fibre synthesis.