The role of Ce addition in catalytic activity enhancement of TiO 2 -supported Ni for CO 2 methanation reaction.

In this work, various amounts of Ce were added to TiO 2 to form a mixed oxide support; Ce x Ti 1- x O 2 ( x = 0, 0.003, 0.05, 0.10 and 0.15) and then those synthesized supports were impregnated by 10 wt% Ni to produce a catalysts. The 10 wt% Ni-Ce x Ti 1- x O 2 ( x = 0, 0.003, 0.05, 0.10 and 0.15) catalysts were tested for CO 2 methanation reaction by using a fixed-bed reactor in the temperature range of 100-500 °C. The sample was pretreated at 450 °C under H 2 and then a mixed feed gas of CO 2 and H 2 was switched into the reactor to start the reaction. The results showed that 10 wt% Ni-Ce 0.003 Ti 0.997 O 2 catalyst (the lowest Ce content) exhibited the highest CO 2 conversion and CH 4 yield. Moreover, 10 wt% Ni-Ce 0.003 Ti 0.997 O 2 showed highly stable during the stability test (50 h.). The results indicated that upon addition of small amount of Ce into TiO 2 -supported Ni, the surface, structural, electrical and redox properties of the catalyst were improved to the extent that these properties can promote the catalytic activities for CO 2 methanation. The Ce addition can improve the CO 2 methanation catalytic activity by several ways. First, higher dispersion of Ni on catalysts surface upon addition of Ce was observed which resulted in higher adsorption rate of H 2 on this metal active site. Second, formation of a larger amounts of oxygen vacancies as well as basicity improvement upon addition of Ce were occurred which can increase the CO 2 adsorption on catalyst surface. Third, incorporation of Ce resulted in improving of a starting reduction temperature of Ni 2+ to Ni 0 for TiO 2 -supported Ni catalyst which can indicate that the reducibility of Ce-doped TiO 2 -supported Ni catalyst was enhanced and then alter its catalytic activity. However, increasing of Ce content led to lowering of CO 2 methanation activities which resulted from increasing of basicity by Ce addition. The excess amounts of adsorbed CO 2 would lead to competitive adsorption to H 2 and then lead to a decrease of catalytic activity. Therefore, an appropriate amount of H 2 and CO 2 adsorption ability on catalyst surface was a prominent factor to dominate the catalytic activity.