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Membrane reactors are of great interest in the chemical industries because they offer the possibility of improved yields, improved selectivities and more compact plant. However, a significant barrier to their uptake is the unavailability of membrane systems having the required performance at an acceptable cost. In this paper we will explore the use of one class of membrane that has the potential to deliver high performance at reasonable cost. Ion‐conducting ceramic membranes can be used in a wide range of high temperature applications including fuel cells, advanced reactors and even smart catalytic systems.

Membrane reactors are of great interest in the chemical industries because they offer the possibility of improved yields, improved selectivities and more compact plant. However, a significant barrier to their uptake is the unavailability of membrane systems having the required performance at an acceptable cost. In this paper we will explore the use of one class of membrane that has the potential to deliver high performance at reasonable cost. Ion-conducting ceramic membranes can be used in a wide range of high temperature applications including fuel cells, advanced reactors and even smart catalytic systems.

Shake flask experiments were conducted to determine the biodegradability of aqueous linear alkylbenzene sulfonate (LAS) and LAS (1600 mg dm^-3) subjected to wet air oxidation (WAO), to assess the suitability of WAO as a pre-treatment for biological degradation. The effects of WAO temperature (180-240 °C) and the concentration of the orthophosphoric acid catalyst (0-1.0 mol dm^-3) were investigated. Results showed that a higher WAO temperature increased the biodegradability of the WAO effluent. This was due to a greater removal of both recalcitrant sulfonated organics and organic concentration (TOC and COD). Conversely, greater orthophosphoric acid concentrations decreased the biodegradability of the WAO effluents. This was because the higher acid concentration increased the ionic strength and changed the WAO intermediate and product distribution, inhibiting microbial action. Nevertheless, the effluents from both variations of WAO were still more biodegradable than LAS at equivalent concentrations. However, since higher WAO temperatures can substantially increase capital costs, future work should focus on developing a WAO catalyst that both desulfonates and mildly oxidises LAS at moderate temperatures (200 °C).© 2002 Society of Chemical Industry

A novel temperature-programmable electrochemical cell was designed and constructed. The cell consisted of a CaO-stabilised zirconia solid oxide electrolyte, an Ag cathode exposed to air, and an Ag, Au, or solid oxide anode. Electrochemical oxygen ion pumping (EOP) from the cathode could be performed in both constant-current and constant-potential-difference modes at a constant temperature or during a temperature programme. The gas exiting the anode chamber was analysed by a quadrupole mass spectrometer (QMS). During EOP, the O₂ detected by the QMS was found to be related faradaically to the charge passed across the electrolyte. Oxygen species formed on the anode surface during EOP were investigated by performing O₂ temperature-programmed desorption experiments with working anodes. Oxygen species derived electrochemically gave rise to a low-temperature desorption peak not seen for species formed from gaseous O₂. This electrochemically generated oxygen species may be related to the surface species postulated to be responsible for the non-faradaic electrochemical modification of catalytic activity effect (Vayenas, C. G., Bebelis, S., Yentekakis, I. V., and Lintz, H.-G., Catal. Today 11, 297(1992)).Copyright 1995, 1999 Academic Press, Inc.

Nickel solid oxide fuel cell anodes are unsuitable for operation with dry natural gas because of carbon deposition. Results from steady state and temperature programmed experiments over an alternative anode material, La0.8Ca0.2CrO3−x, indicate that both complete oxidation of methane and surface methane dissociation occur at temperatures above 500°C. Negligible carbon deposition occurs during complete oxidation.

Methanol production from CO/CO2/H2over a Cu/ZnO/Al2O3catalyst has been compared in an internal recycle reactor (finite conversion) and a differentially operated tubular reactor. From experiments at differential conversion it was found that the intrinsic rate of CO2hydrogenation was approximately 20 times faster than the intrinsic rate of CO hydrogenation. The rate of methanol production at differential conversion increased almost linearly with respect to the CO2concentration, indicating that there is negligible inhibition by CO2in the absence of products. With 1–4% CO2in the synthesis gas, the methanol production rate was found to be substantially greater at finite conversion than at differential conversion, which is evidence for a promoting influence by the product water. With >4% CO2in the synthesis gas there was increasing loss of methanol production at finite conversion, and under CO2/H2in particular there was an order of magnitude difference between methanol production at finite and differential conversions. This effect was due to inhibition of CO2hydrogenation associated with the presence of product water.

The catalytic behaviour of the perovskite oxide La_0.5Sr_0.5MnO_3−δ was investigated during carbon monoxide oxidation at 500-550°C. The technique of solid electrolyte potentiometry was employed to monitor the oxygen activity of the solid catalyst under reaction conditions. At 550°C, the reaction rate was found to be a function of the gas phase oxygen (or carbon monoxide) partial pressure and the catalyst oxygen activity. At 500°C, the rate tended to depend only on the gas phase composition and not on the catalyst state. The results suggested that the reaction mechanism changes from an ionic redox model (550°C), where the reoxidation of the reduced sites occurs by lattice oxygen, to a Mars-van Krevelen redox model (500°C), where the reoxidation occurs by gas phase oxygen. The hysteresis at 550°C was attributed to diffusional limitations appearing during either the reoxidation of the reduced sites or the reoxidation of the oxide bulk. The change in the reaction mechanism in going from 500 to 550°C was considered to be the result of an increase in the mobility of lattice oxygen, in agreement with the oxygen temperature programmed desorption profile of the catalyst.Copyright 1995, 1999 Academic Press, Inc.

The perovskite oxide La0.8Ca0.2CrO3 was studied for application as a direct methane oxidation anode in solid oxide fuel cells (SOFCs) using temperature-programmed (TP) techniques. Activity for methane oxidation, redox behaviour, methane adsorption and extent of deactivation by carbon deposits for the oxide were compared with those for the conventional anode material, NiZrO2 cermet, studied previously. The NiZrO2 exhibited a much higher activity for the formation of carbon deposits than the oxide. Furthermore, the temperature of onset of carbon deposition was much lower on the cermet than on the oxide. Over the oxide, two main methane reaction regimes were identified: complete oxidation at intermediate temperatures (over the pre-oxidised catalyst only) and methane dissociation at high temperatures. Each reaction took place via two mechanisms. Complete oxidation involved reaction with either surface or absorbed oxygen species. Methane dissociation appeared to occur via catalysed (probably at oxygen ion vacancies) and uncatalysed mechanisms. At approximately 400–600°C, where complete oxidation of methane occurred over the oxide, no carbon deposition was detected. This is in contrast with NiZrO2 where carbon deposition was observed over the entire active temperature range of the catalyst. This suggested that a methane direct oxidation SOFC with such an oxide anode material may be operated at these temperatures without becoming deactivated by carbon deposits. The effect of the addition of water to the methane feed was studied and was seen to substantially reduce the amount of carbon deposition.

A combination of temperature-programmed (TP) and isothermal experiments were performed on the candidate solid oxide fuel cell (SOFC) anode materials, La0.8Ca0.2Cr0.9X0.1O3 (X = Ni, Co), to investigate their suitability for use with dry methane feeds. Comparing with the behaviour of the unsubstituted material, La0.8Ca0.2CrO3 [1], the Ni and Co doping were found to have a marked modifying effect on the ease and extent of reduction of the materials. The total extent of reduction was higher than for the unsubstituted oxide. Two main reaction regimes were identified, as for the unsubstituted material, involving complete oxidation and methane dissociation. Complete oxidation only occurred over the pre-oxidised material. When reduced at high temperatures, the Ni-substituted material displayed behaviour characteristic of both La0.8Ca0.2CrO3 and Ni-YSZ cermet [2] indicating that segregation of Ni occurs during high temperature reduction.

The technique of solid electrolyte potentiometry (SEP) was employed to monitor in situthe oxygen activity (the oxidation state) of La(Sr)MnO3catalyst during CO oxidation under steady-state and transient conditions. The reaction at 550°C was found to proceed through an ionic redox mechanism; the reoxidation and the reduction reactions occur at different sites on the catalyst surface and oxygen diffuses through the oxide lattice from the ox-sites, where it is adsorbed, to the r-sites, where it reacts with CO. Under reaction conditions it cannot be assumed that oxygen adsorbed at the r-sites is in equilibrium with oxygen adsorbed at the ox-sites. The rate was accordingly found to be a function of the catalyst oxygen activity and the gas-phase oxygen (reoxidation determined reaction) or carbon monoxide (reduction-determined reaction) partial pressure. Transient data can also be interpreted within the framework of the ionic redox model.