Methanation of CO2 on iron based catalysts

In this paper, several bare iron oxide samples were investigated for catalytic hydrogenation of CO2 to CH4 and H2O under atmospheric pressure. The fresh samples were thoroughly characterised by N-2 physisorption, X-ray diffraction (XRD) and Moessbauer spectroscopy. Hydrogenation studies were performed with a CO2 fraction of 1000 ppm and a molar H-2/CO2 ratio of 200 to accurately monitor gas phase species providing reliable mass balance of carbon. The obtained results evidenced reduction of the iron oxides followed by significant accumulation of carbon. In situ XRD and temperature-programmed hydrogenation (TPH) demonstrated formation of iron carbide (theta-Fe3C)as well as carbonaceous deposits implying surface and bulk carbon entities. The quantity of each carbon species depended on the physical-chemical properties of the catalysts with fast carburisation of relatively large iron crystallites originated from iron oxide reduction. The best methanation activity was found for nano-sized. gamma-Fe2O3 indicating maximum CH4 yield of ca. 60% at 400 degrees C. TPH analyses suggested that the hydrogenation efficiency of this catalyst was mainly associated with reactive surface carbon species. Furthermore, for more practical assessment of ni-Fe2O3,CO2 hydrogenation was conducted with a CO2 proportion of 10 vol.% and stoichiometric H-2/CO2 ratio leading to decreased yield of CH4 and predominate formation of CO. Based on XRD, TPH and Moessbauer spectroscopic analyses the catalytic activity under these conditions was ascribed to O-Fe3C and X-Fe5C2 entities, with some contribution of carbon species deposited on the catalyst. (C) 2017 Elsevier B.V. All rights reserved.