CO2 hydrogenation and ethanol steam reforming over Co/SiO2 catalysts: Deactivation and selectivity switches

Description

Two 20 wt.% Co/SiO2 catalysts have been prepared using silica gel as a support and cobalt acetate or nitrate as precursors. They were characterized before and after reaction using XRD, skeletal IR and DR-UV–vis-NIR spectroscopies and FE-SEM microscopy. Fresh catalysts are active in CO2 methanation at atmospheric pressure but rapidly deactivate at 623–673 K, due to formation of encapsulating carbon. However, they retain a more stable activity in producing CO by the reverse water gas shift (rWGS) reaction. Methanation and rWGS appear to occur on independent sites. The Co/SiO2 catalyst produced from Co-acetate precursor shows stable activity in Ethanol Steam Reforming (ESR) at 873−973 K, producing also carbon whiskers (nanotubes) that, however, do not cause catalyst deactivation at the laboratory conditions and timescale. The catalyst produced starting with Co-nitrate is even more active in ESR but it deactivates fast. At lower temperature or when deactivated with respect to ESR, both catalysts shift to high activity in ethanol dehydrogenation to acetaldehyde, showing that the ability to activate water is mostly lost. The differences observed among catalysts produced from cobalt acetate and cobalt nitrate precursors can be associated to the different pH of the impregnating solution. The higher pH of cobalt acetate solution results in surface dissolution / hydrolysis of silica and this may be at the origin of the formation of cobalt silicate surface species, which can also stabilize the support against high temperature reactivity. This might favor larger Co particles formation, less active, but also giving rise to inert cobalt species.

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