On the kinetics of the methanation of carbon dioxide on coprecipitated NiAl(O)_x

A series of NiAl(O)_x catalysts with varying Ni/Al ratio has been prepared by coprecipitation of the metal nitrate solutions with NaOH/Na₂CO₃ at constant pH. The samples have been analyzed by XRD, N₂ physisorption, temperature programmed reduction and H₂ chemisorption. For kinetic characterization, apparent activation energies and reaction orders are determined. The correlation of the catalytic acitivity and metallic surface area indicates a linear relationship. A data set comprising over 200 data points with varying gas composition, temperature and pressure has been recorded as a basis to develop a kinetic model that captures the intrinsic kinetics of the methanation of carbon dioxide under process relevant conditions. As rate equations power laws, power laws with inhibition, for instance by water, and Langmuir-Hinshelwood-Hougen-Watson (LHHW) approaches are evaluated. Modeling results emphasize that the kinetics at differential conversions and pure H₂/CO₂ feed markedly differ from the regime closer to equilibrium. Power laws with inhibition and adequate LHHW approaches are capable to reflect the kinetics over a wide range of conditions from differential conversion to thermodynamic equilibrium.