Relating catalyst structure and composition to the water–gas shift activity of Cu–Zn-based mixed-oxide catalysts

In order to investigate the effect of cerium oxide on Cu–Zn-based mixed-oxide catalysts four catalyst samples were characterized by means of XRD, in situ XANES and thermogravimetric analysis. The activity of the catalyst samples was tested for the forward water–gas shift reaction. Cerium oxide was found to increase the crystallinity of the ZnO phase indicating a segregation of the Cu and ZnO phases. The TOF of the water–gas shift reaction based on chemisorption data was found to be independent of composition and preparation conditions of the four catalyst samples. In contrast, the catalyst stability depends on composition and preparation conditions. Cerium oxide impregnated before calcination of the hydrotalcite-based Cu–Zn precursors leads to a more stable water–gas shift catalyst.     

The effect of synthesis gas composition on the Fischer–Tropsch synthesis over Co/γ-Al2O3 and Co–Re/γ-Al2O3 catalysts

The Fischer–Tropsch synthesis over Co/γ-Al₂O₃ and Co–Re/γ-Al₂O₃ was investigated in a fixed-bed reactor at 20 bar and 483 K using feed gases with molar H₂/CO ratios of 2.1, 1.5 and 1.0 simulating synthesis gas derived from biomass. With lower H₂/CO ratios in the feed, the CO conversion and the CH₄ selectivity decreased, while the C₅₊ selectivity and olefin/paraffin ratio for C₂–C₄ increased slightly. The water–gas shift activity was low for both catalysts, resulting in high molar usage ratios of H₂/CO (close to 2.0), even at the lower inlet ratios (i.e. 1.5 and 1.0). For both catalysts, the drop in the production rate of hydrocarbons when shifting from an inlet ratio of 2.1 to 1.5 was significant mainly because the H₂/CO usage ratio did not follow the change in the inlet ratio. The hydrocarbon selectivities were rather similar for inlet H₂/CO ratios of 2.1 and 1.5, while significantly deviating from those for an inlet ratio of 1.0. With the studied catalysts, it is possible to utilize the advantages of an inlet ratio of 1.0 (higher selectivity to C₅₊, lower selectivity to CH₄, no water–gas shifting of the bio-syngas needed prior to the FT reactor) if a low syngas conversion is accepted.     

Oxidative dehydrogenation of ethane on Pt–Sn impregnated monoliths

The oxidative dehydrogenation of ethane was studied over Pt–Sn impregnated monoliths at 1 bar, 600–900 °C and with different contents of oxygen, hydrogen and steam in the feed gas. As expected a decrease in oxygen in the feed led to a decrease in the conversion of ethane due to lower temperatures in the reactor. Adding steam to the feed showed no effect on the ethane conversion or the ethene selectivity. When the hydrogen/ethane ratio in the feed was varied from 0 to 0.5 at 700 and 850 °C, it resulted in a significant increase in the selectivity to ethene while the ethane conversion remained relatively unchanged. At 700 °C the selectivity increased from about 50% to 93% (carbon basis) with only a small decrease in the conversion of ethane. The results clearly show that both Pt and Sn have a catalytic effect. Pt caused the ethane conversion to rise and addition of Sn resulted in much better ethene selectivity. However, even though Sn alone showed some catalytic effect at lower temperatures, it cannot explain the great difference between the Pt and Pt–Sn catalysts. A reasonable assumption is therefore that there exist interactions between Pt and Sn that gives the Pt–Sn catalysts excellent properties for oxidative dehydrogenation of ethane, in particular upon addition of hydrogen.     

Small-scale hydrogen production from propane

Partial oxidation and oxidative steam reforming of propane were investigated over 0.01 wt.% Rh/Al₂O₃ foam catalysts. High selectivity to hydrogen was obtained for both reactions, but addition of steam to the reactant mixture gave higher selectivity to hydrogen. Stability tests over 7 h revealed that the catalytic activity of Rh was quite stable under partial oxidation conditions. Higher loss in Rh activity was observed when steam was present in the reactant mixture. FE-SEM images showed that Rh particle size and distribution are modified under partial oxidation and oxidative steam reforming conditions. However, these changes were more distinct on the catalyst used for oxidative steam reforming.     

Coke formation during cracking of hydrocarbons. I. The effect of pre-sulphiding on coke formation on a nickel-chromium-iron alloy under steam cracking conditions

Studies have been carried out on the effect of pre-sulphiding a nickel-chromium-iron alloy on coke formation under steam cracking conditions. Pre-sulphiding has no great effect on the gaseous product spectrum as long as some sulphur is present, but the coking rate is dependent on the amount of sulphur pre-deposited. Small amounts of sulphur reduce the coke formation, but the coking rate increases with increasing amounts of pre-deposited sulphur. As the sulphur content increases still further, coke formation passes through a maximum. A tentative explanation of these observations is advanced in terms of the effect of metal sulphides on the established mechanism of coke formation. Initial sulphiding is suggested to increase grain boundary penetration by coke and to result in the accumulation of iron sulphide on the surface. Eventually this may act to seal off grain boundaries, thereby inhibiting further coke formation.     

Methane activation in exchange and oxidative dimerization reactions : a study using isotope steady-state and transient techniques

A study of methane activation (reversible for exchange reaction or irreversible for oxidative dimerization) is carried out on alkali promoted and unpromoted basic oxides (magnesia and lanthana) using isotopic steady-state and transient techniques. Rates of CH₄/CD₄ scrambling, isotopic effect between CH₄ and CD₄ coupling, surface concentration of reversibly adsorbed methane, of C₂ and CO_x precursors are determined. Mechanistic propositions focused on the role played by the surface basicity and the poisoning effect of alkali carbonate are formulated.     

A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts

Catalytic partial oxidation of methane has been reviewed with an emphasis on the reaction mechanisms over transition metal catalysts. The thermodynamics and aspects related to heat and mass transport is also evaluated, and an extensive table on research contributions to methane partial oxidation over transition metal catalysts in the literature is provided.Presented are both theoretical and experimental evidence pointing to inherent differences in the reaction mechanism over transition metals. These differences are related to methane dissociation, binding site preferences, the stability of OH surface species, surface residence times of active species and contributions from lattice oxygen atoms and support species.Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of active species may be reduced by direct interaction with methane or from the reaction with H, H₂, C or CO.The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. Another essential property for the formation of H₂ and CO as primary products is a low surface coverage of intermediates, such that the probability of O–H, OH–H and CO–O interactions are reduced.The local concentrations of reactants and products change rapidly through the catalyst bed. This influences the reaction mechanisms, but the product composition is typically close to equilibrated at the bed exit temperature.     

CO hydrogenation over supported cobalt catalysts: FTIR and gravimetric studies

The hydrogenation of CO over supported cobalt catalysts has been studied using in situ FTIR spectroscopy and gravimetry at P = 6 bar, T = 473–723 K and H₂/CO = 2–3. On both silica- and alumina-supported catalysts IR absorption bands corresponding to linearly adsorbed CO on metallic cobalt were observed. On alumina an additional pair of bands at lower frequencies was attributed to bridge-bonded CO. Absorption bands corresponding to adsorbed hydrocarbons (3050–2700 cm^?1) and to oxygen containing species (1800–1200 cm^?1) were found to correspond to adsorbed products or unreactive species. The gravimetric studies showed a significant difference between the supports. On the silica-supported catalyst the weight uptake decreased with increasing temperature (473–573 K). The weight increase during reaction was attributed to adsorbed hydrocarbon reaction products. On the alumina-supported catalyst the weight uptake increased with increasing temperature, and there was also a significant weight increase with the support alone. Most of the weight uptake can be attributed to the formation of stable formate and carbonate species on the alumina support. At 723 K the deposits formed were stable in H₂, and the shape of the curves indicated different mechanisms for deposition of material. In particular the Co/Al₂O₃ sample showed a very high and linear rate of weight gain, which was an order of magnitude higher than for the other samples.     

Effect of composition and composition inhomogeneity on the strain sensitivity of plated wire memory elements

An analysis was made of the relation between the strain sensitivity of plated wire memory elements and the chemical composition in the film. According to this analysis, torsion sensitivity, which measures the easy-axis skew when the wire is twisted, depends on theaverage permalloy composition in the film. On the other hand, tension sensitivity, which measures the change in anisotropy field,H _k, when the wire is stretched, should depend on both the average composition and the degree of variation, or composition inhomogeneity. In general, the inhomogeneity contribution is negative. Experimental results supporting this analysis showed that 1) films with nearly zero torsion strain sensitivity invariably exhibit negative tension strain sensitivity and 2) the measured tension strain sensitivity is in approximate agreement with that predicted from electron microprobe measurements of composition inhomogeneity. The strain sensitivity analysis provides a means of estimating the discrepancy between torsion and tension zeromagnetostrictive compositions.