Sulphur poisoning and regeneration of precious metal catalysed methane combustion

In many of the applications of catalytic combustion small amounts of sulphur compounds are present in the feed gas. In the case of natural gas combustion, and solid fuel syngas, the sulphur compounds are in reduced forms. The present work investigates the influence of small quantities of reduced sulphur compounds on the combustion of methane over alumina-supported precious metal catalysts. The kinetics of the methane combustion in the presence of low concentrations of a mixture of sulphur compounds (ethyl mercaptan, methyl mercaptan, carbonyl sulphide and hydrogen sulphide) are compared with those in the absence of sulphur compounds. The ease of regeneration of the poisoned catalysts, via low temperature reduction with hydrogen, is also examined. In the conditions studied all catalysts have reduced activity in the presence of the sulphur-based gas mixture, but Pt/Al2O3 and Rh/Al2O3 are more strongly poisoned than Pd/Al2O3, Qualitative studies using gas chromatography with atomic emission detection of the exhaust gases, and FTIR spectroscopy of the spent Rh/Al2O3 catalyst, suggest that the catalysts experience a mixture of reduced and oxidised species under reaction conditions, and that sulphating of the support occurs. The regeneration step facilitates metal mobility and meets with varying success depending upon the metal. Pt/Al2O3 in particular is difficult to regenerate by reduction in hydrogen (400degreesC for 0.5 h), and agglomeration is observed by TEM. Rh/Al2O3 regenerates well, and low metal particle size is maintained. A non-linear deactivation model is tested to separate sulphur-induced deactivation from 'natural' deactivation in the reaction mixture and preliminary results are presented. (C) 2003 Elsevier B.V. All rights reserved.