The role of Ce addition in catalytic activity enhancement of TiO₂-supported Ni for CO₂ methanation reaction

In this work, various amounts of Ce were added to TiO₂ to form a mixed oxide support; CeₓTi₁₋ₓO₂ (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–CeₓTi₁₋ₓO₂ (x = 0, 0.003, 0.05, 0.10 and 0.15) catalysts were tested for CO₂ methanation reaction by using a fixed-bed reactor in the temperature range of 100–500 °C. The sample was pretreated at 450 °C under H₂ and then a mixed feed gas of CO₂ and H₂ was switched into the reactor to start the reaction. The results showed that 10 wt% Ni–Ce₀.₀₀₃Ti₀.₉₉₇O₂ catalyst (the lowest Ce content) exhibited the highest CO₂ conversion and CH₄ yield. Moreover, 10 wt% Ni–Ce₀.₀₀₃Ti₀.₉₉₇O₂ showed highly stable during the stability test (50 h.). The results indicated that upon addition of small amount of Ce into TiO₂-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 CO₂ methanation. The Ce addition can improve the CO₂ 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 H₂ 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 CO₂ adsorption on catalyst surface. Third, incorporation of Ce resulted in improving of a starting reduction temperature of Ni²⁺ to Ni⁰ for TiO₂-supported Ni catalyst which can indicate that the reducibility of Ce-doped TiO₂-supported Ni catalyst was enhanced and then alter its catalytic activity. However, increasing of Ce content led to lowering of CO₂ methanation activities which resulted from increasing of basicity by Ce addition. The excess amounts of adsorbed CO₂ would lead to competitive adsorption to H₂ and then lead to a decrease of catalytic activity. Therefore, an appropriate amount of H₂ and CO₂ adsorption ability on catalyst surface was a prominent factor to dominate the catalytic activity.