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.