Apparent equilibrium shifts and hot-spot formation for catalytic reactions induced by microwave dielectric heating

Abstract

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...

Microwave assisted catalytic reduction of sulfur dioxide with methane over MoS2 catalysts

“…This conclusion is in keeping with that of a previous study of the catalytic decomposition of H 2 S over MoS 2 catalysts [14], where microwave heating caused an acceleration in reaction rates and an apparent shift in the position of equilibrium. As in the current study, the results could be explained only by the formation of hot-spots, which were estimated to have temperatures 100-200 • C higher than the average bulk temperature measured in the catalyst bed.…”

“…10 mm) which could be placed either in a cylindrical microwave cavity or in a conventional furnace [14]. A directional coupler was inserted into the microwave guide system so that the amount of microwave power reflected from the cavity could be measured.…”

“…Recent studies [13][14][15] have shown that the rates of many heterogeneous catalytic reactions can be considerably enhanced by using microwave radiation to heat the catalyst. This effect is thought to arise because the reaction sites are at a considerably higher temperature than the bulk of the catalyst.…”

Infrared and microwaves at 5.8 GHz in a catalytic reactor

“…Microwaves are known to lead to hot spots on powder solids. On supported alumina catalysts, the spot size has been measured to be 90 mm, and 100-200 K hotter than the average surrounding alumina 23 . The temperature is homogenised by conduction in the sample after the microwaves stop.…”

“…17,18 Studies of heterogeneous catalysts under conditions of microwave stimulation have shown that microwaves are capable of producing a rapid increase of reaction temperature and can modify selectivity. [19][20][21][22][23][24][25][26][27][28][29] The impact of microwaves on the reaction can be due to a thermal effect 20,23,26 or to an influence on the electronic structure of the reactants, [27][28][29] but this is still a matter of debate. 30,31 A new tool for studying surface chemistry under microwave irradiation can help understanding.…”