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.