Microwave dielectric heating of the gas phase decomposition of H 2 S catalysed by metal sulfides on a g-Al 2 O 3 support results in significant apparent shifts in the equilibrium constant, which have been attributed to the development of hot-spots in the catalytic beds; X-ray diffraction and electron microscopy measurements have indicated the formation of hot-spots with dimensions of 90-1000 mm and which involve not only the active phase, but also the support.The acceleration of heterogeneous catalytic processes under microwave dielectric heating conditions has attracted both academic and industrial interest. [1][2][3][4][5] The majority of systems studied involve a catalyst which preferentially absorbs the microwaves relative to the support material and the reasons for the observed rate enhancements have been the subject of some controversy. The measurement of temperature under conditions where there are strong electric fields is problematical, but may be overcome, for example, by using optical fibre technology. Such measurements are still only capable of providing average temperatures. Both differential coupling abilities of materials and distribution of electromagnetic fields may result in localized temperature distribution in catalytic beds, but the contribution of these effects is difficult to quantify. Stuerga and Gaillard 6,7 have given a thorough theoretical analysis which has conclusively demonstrated that specific athermal effects are implausible given the small energies associated with the microwave quanta and the classical nature of the heating phenomenon and have suggested that localized enhancements of the reaction rates may be responsible for non-isothermal and heterogeneous kinetic phenomena. The majority of the experimental studies on catalytic reactions under microwave conditions have dwelled on the kinetic aspects and noted that the rates of reaction and product distributions are more consistent with a temperature 300-400 K higher than that measured for the bulk of the catalyst bed. However, for supported metal catalysts, calculations 8,9 have shown that the rate of heat transfer at 1 atm gas pressure is so high that the formation of hot-spots localised at the catalytically active sites are implausible. Here we report significant circumstantial evidence for the formation of hotspots in the microwave experiments and have demonstrated for the first time that these hot-spots are not localised exclusively on the active catalyst, but also involve the support material. We have also estimated the dimensions of these hot-spots.The catalytic conversion of H 2 S into hydrogen and sulfur, which is commercially important for the coal and petrochemical industry, 10,11 has recently been investigated in our laboratories using parallel microwave and conventional heating conditions. The experimental set-up is illustrated schematically in Fig. 1 and the reactions were performed under continuous flow conditions using quartz reactors. The temperature in the microwave cavity was monitored using an Accufibre optica...

The application of microwave dielectric heating in a range of environment-related heterogeneous catalytic reaction systems has been reviewed. The reactions investigated include the decomposition of hydrogen sulfide, the reduction of sulfur dioxide with methane, the reformation of methane by carbon dioxide, the hydrodesulfurization of thiophene, and the oxidative coupling of methane. The interaction of microwave irradiation with heterogeneous catalytic systems and its consequence for the microwave heating behaviour of catalysts have been examined. The effect/mechanism of microwave dielectric heating on heterogeneous catalytic reaction systems has also been discussed.

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The catalytic reduction of sulfur dioxide with methane to form carbon dioxide and sulfur has been studied over MoS 2 /Al 2 O 3 catalysts. The reaction has been found to occur with microwave (2.45 GHz) heating at recorded temperatures as much as 200 • C lower than those required when conventional heating was used. An activation energy of 117 kJ mol −1 has been calculated for the conventionally heated reaction, but an Arrhenius analysis of the data obtained with microwave heating was not possible, probably because of temperature variations in the catalyst bed. The existence of hot spots in the catalysts heated by microwave radiation has been verified by the detection of ␣-alumina at a recorded temperature some 200 • C lower than the temperature at which the ␥-to ␣-alumina phase transition is normally observed. Among four catalysts prepared in different ways, a mechanically mixed catalyst showed the highest conversion of SO 2 and CH 4 for microwave heating at a given temperature. Supported catalysts, sulfided either by conventional heating or under microwave conditions, showed little difference in the extent of SO 2 and CH 4 conversions. The highest conversions to carbon dioxide and sulfur, combined with low production of undesirable side products, was obtained when the molar ratio of SO 2 to CH 4 was equal to two, the stoichiometric ratio.

Microwave heating was applied in the oxidative coupling of methane to higher hydrocarbons over alumina supported La 2 O 3 /CeO 2 catalysts. It was found that microwave heating had a dramatic effect on the reaction when methane was converted into C 2 hydrocarbons in the absence of oxygen, with products produced at measured temperatures about 250 • C lower than those observed with conventional heating. It was shown that the reaction in the absence of oxygen occurred primarily in the gas phase and it was the localised heating of the methane that was responsible for the increased reaction rate. However, it was observed that microwave heating did not produce significant "special microwave effects" which affected the product species formed when the oxidative coupling reaction occurred in the presence of oxygen.

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Dielectric, ultrasonic and ' 3C nuclear magnetic resonance relaxation measurements are reported on mixtures of t-butyl alcohol and water over the temperature range 27S308 K. Supporting measurements of the refractive index, density and viscosity are also presented. Deviations from ideal mixing behaviour are observed in the volumes of mixing, permittivities and adiabatic compressibilities of the mixtures. The location of the minima in the excess occurring at different compositions depend upon the method of measurement. Similarly the relaxation behaviour, as observed using different techniques, reveals different aspects of the motion of the molecules in the fluid. The various effects observed are rationalized in terms of a model in which it is assumed that t-butyl alcohol dimers and trimers and various hydrated forms play an important role in determining the structure of these mixtures.