The performance of alumina supported unpromoted and cerium promoted nickel catalysts in CO 2 methanation reaction was investigated. It was found that the activity of catalysts in CO 2 methanation reaction at low reaction temperatures can be improved by the increase in nickel loading and introduction of cerium promoter. The catalysts showed high resistance for sintering and coking at high reaction temperatures. A few stages of catalysts deactivation by H 2 S in the methanation reaction carried out at 475 °C with the time on stream were identified. It was found that an introduction of H 2 S to the stream (8 ppm) did not induce rapid decrease of activity. Slight and then strong drop of CO 2 conversion and simultaneously the loss of methane selectivity was observed after specific time, depending on the catalysts composition. Deactivation of catalysts was related to the nickel content and the presence of cerium. X-ray diffraction studies indicated small changes of crystallite size with the time on stream. Raman spectroscopy studies pointed out that deactivation of catalysts was not connected with formation of carbon deposits. An in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy studies showed that exposition of catalysts to the reaction mixture containing the traces of H 2 S led to the blocking of nickel active sites responsible for CO 2 and H 2 activation and successive transformation to carbonyl and formate species.

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Direct methanation of CO 2 over ceria-and alumina-supported nickel catalysts in the feed stream containing methane and traces of H 2 S is reported. Stability tests for 20 h at 350 and 600°C with a packed-bed reactor showed high resistance of the catalysts to sintering processes. Higher conversion at 350°C was observed for ceria supported nickel catalyst. Thermodynamic analysis indicated that CO 2 contained in biogas can be converted to methane without carbon formation under specific reaction conditions. An introduction of CH 4 to CO 2 À H 2 feed stream led to the decrease in CO 2 conversion and CH 4 selectivity. An introduction of the trace amounts of H 2 S into the feed stream led to the fast drop of CO 2 conversion and CH 4 selectivity; higher durability (20 %) was observed for Al 2 O 3 than CeO 2 supported catalysts. An improved performance of catalysts coated onto high-pressure microchannel microreactors for the direct CO 2 methanation reaction applying model biogas mixture was demonstrated.

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