Support effects in metal catalysis: a study of the behavior of unsupported and silica-supported cobalt catalysts in the hydrogenation of CO2 at atmospheric pressure

Two 20 wt.% Co/Sio(2) catalysts were prepared using silica gel as a support and cobalt acetate and nitrate as precursors, respectively. As a comparison unsupported amorphous cobalt nanoparticles (10-80 nm) were synthesized by decomposing cobalt carbonyl. They were characterized before and after reaction using XRD, IR spectroscopy in the range of framework vibrations, DR-UV-vis-NIR spectroscopies and FE-SEM microscopy. They are active in CO2 methanation at atmospheric pressure. However, unsupported cobalt and the silica-supported catalyst prepared using cobalt acetate deactivate fast at 623-673 K, also due to formation of encapsulating carbon. On the other hand, they retain more stable activity in the reverse water gas shift (rWGS) reaction, producing CO. Methanation and rWGS appear to occur on independent sites. The silica-supported catalyst produced from cobalt nitrate show more stable methanation activity, attributed to the presence of smaller cobalt particles (10-20 nm) that resist deactivation. For both unsupported CoNPs and Co/SiO2, the significant parameters for catalysts deactivation have been determined for CO2 methanation. The differences observed among catalysts produced starting from cobalt acetate and cobalt nitrate, can be associated to the different pH of the impregnating solution.