The chemistry of methane reforming with carbon dioxide and its current and potential applications

Abstract

The reforming of CH 4 with CO 2 produces synthesis gas with a lower H 2/CO ratio than that generated by the widely employed steam/CH 4 reforming reaction. The two reactions have similar thermodynamic characteristics except that in the case of CO 2/CH 4 reforming there is a greater potential for carbon formation, primarily due to the lower H/C ratio of this system. Thermodynamic analysis of the CO 2/CH 4 reforming reaction system shows that carbon formation is possible over a wide range of reaction conditions of possible commercial interest. Whilst technology has been developed to enable CO 2/CH 4 and steam/CH 4 reforming to be carried out simultaneously, the former reaction has to date had no significant commercial application by itself. However, there is now renewed interest in C 1-chemistry to produce chemicals and fuels requiring synthesis gas with a 1/1 H 2/CO ratio. Conducted in the absence of steam/CH 4 reforming, CO 2/CH 4 reforming has a number of major advantages over alternative chemical reactions for the thermochemical storage and transmission of renewable energy sources such as solar energy. Hence it is likely to become an increasingly important industrial reaction in the future. A review of the literature on the catalysis of CO 2/CH 4 reforming shows that Group VIII metals, when distributed in reduced form on suitable supports, are effective catalysts for this reaction. Rh appears intrinsically to be the most suitable, and considering the relative material costs, Ni catalysts deserve closer attention. In the latter case the emphasis should be on developing catalysts which are capable of carbon-free operation under practical reaction conditions. Of the various supports studied to date, alumina and magnesia or combinations thereof are most promising. Analysis of the reaction mechanism indicates that the effective catalysts are those metal-support combinations which actively dissociate CH 4 into CH x residues including carbon, whilst at the same time also activating CO 2 to generate CO and an adsorbed O species on the catalyst surface. The O thus produced is consumed in the conversion of CH x and C to CO. Net carbon formation becomes a problem when the CH 4 dissociation and CO 2 activation steps are out of balance. Considering the current status of catalyst development and the likely future large-scale applications for CO 2/CH 4 reforming, significant scope exists for further work in optimising both catalysts and reactor design for this reaction.