Sulfur poisoning of co-precipitated Ni-Al catalysts for the methanation of CO2

This study provides deep insights into the mechanism and kinetics of sulfur poisoning of co-precipitated Ni catalysts for the methanation of CO2. A large number of catalysts with different Ni loadings were poisoned with 5 ppm of H2S and SO2 at equilibrium conditions (H-2/CO2/Ar = 4/1/5, 400 degrees C, 1 bar). Prior to the complete loss of activity, thermography reveals a moving reaction front through the fixed-bed microreactor. The stability of catalysts depends on available Ni surface atoms. H-2 chemisorption and post-mortem CHNS analysis show an average S/Ni-* surface atom ratio of 0.73 +/- 0.02. Based on this stoichiometry, a model for predicting catalyst lifetimes is derived and extrapolated to different H2S partial pressures. In an ex situ poisoning approach, liquid (NH4)(2)S was used to adjust sulfur coverages between 0 and 0.73. Activity measurements under differential conditions reveal an activity loss of more than 80% at coverages as low as theta(S) = 0.2. A kinetic description based on a Maxted-type correlation is derived. The strong dependence of activity on sulfur coverage is explained by the space requirements of CO2 adsorption on Ni-0. Activation energies of non-poisoned and poisoned samples are similar and in the range of 80-87 kJ/mol. Sulfur poisoning is therefore ascribed to site blockage rather than electronic effects.