Mechanistic study of Ni/CeO₂-catalyzed CO₂/CH₄ reaction using flow and static methods

Ni/CeO₂ catalysts with different Ni loadings (5, 7, 10, 12, and 14 wt% Ni) were prepared by an impregnation method and examined for the CO₂ reforming of methane using flow and static reactors. Their catalytic activities and selectivities were measured under CO₂/CH₄/Ar (=5/5/40 cm³/min) flow at 450-800°C using a flow reactor system with an on-line gas chromatography. At fixed temperature, the CO₂ and CH₄ conversions varied only slightly with the Ni wt%, whereas the H₂/CO ratio increased with increasing Ni wt%. The conversions increased with temperature, reaching 98% at 800°C. The H₂/CO ratio varied with temperature in the range of 450-800°C, from less than 1 below 550°C to close to 1 at 550-600°C and then back to less than 1 above 600°C. The apparent activation energies were determined to be 43.1 kJ/mol for the CO₂ consumption and 50.2 kJ/mol for the CH₄ consumption based on the rates measured for the reforming reaction over 5 wt% Ni/CeO₂ catalyst at 550-750°C. Additionally, the catalytic reforming reaction at low pressure (40 Torr) was investigated by a static reactor system by using a differential photoacoustic cell, in which the rates were measured from the CO₂ photoacoustic signal data at early reaction times over the temperature range of 460-610°C. Apparent activation energies of 25.5-30.1 kJ/mol were calculated from the CO₂ disappearance rates. The CO₂ adsorption on the Ni/CeO₂ catalyst was investigated by the CO₂ photoacoustic spectroscopy and Fourier transform infrared spectroscopy. Feasible side reactions during the catalytic CO₂/CH₄ reaction were suggested on the basis of the kinetic and spectroscopic results.