Spectroscopic evidence of surface species during CO2 methanation catalyzed by supported metals: A review

The CO2 methanation catalyzed by supported metals has been widely investigated. In spite of its apparent simplicity, there are still open questions regarding its mechanism, with apparently contradictory proposals that rely mainly on infrared (IR) spectra of functioning catalysts. Here we review the various mechanistic proposals for the reaction. Special focus is made on the following questions: (a) what is the actual physical evidence of adsorbed species present on the catalysts?, (b) how can reaction intermediates be distinguished from spectators?, (c) what is the identity of the active site(s) for the reaction, and (d) could multiple reaction routes occur simultaneously on the catalysts? Kinetic models of the reaction and a survey of the apparent contradictions in the interpretation of IR spectra characterizing functioning catalysts are also presented.