Interaction of N 2O (as gas dope) with nickel molybdate catalysts during the oxidative dehydrogenation of propane to propylene

Abstract

We report effects brought about by the introduction of N 2O as gas dope in the reaction feed during the oxidative dehydrogenation of propane to propylene on nickel molybdate catalysts. The addition of N 2O modifies the catalytic performances. At low concentration (300 ppm) of N 2O, the conversion of propane decreases, the yield in propylene increases and the selectivity to propylene is improved. Decreases in the oxygen consumption, in the yield and in the selectivity in CO 2 were also observed. N 2O induces the reduction of Mo at the surface of the catalyst. This is supposed to occur through an inhibition of the adsorption of O 2 because of the presence of N 2O at the surface of the catalyst, which thus slows down the reoxidation rate of the catalyst. In presence of N 2O, catalytic sites thus work in a more reduced state. At higher concentrations of N 2O (1–10% in volume), the selectivity in propylene decreases, the oxygen consumption and the selectivity to CO 2 decrease further. The catalyst is more deeply reduced and carbonaceous products is formed. These observations are interpreted as being due to the inhibition effect evoked at low concentration but additionally to a direct reduction of the catalytic surface brought about by N 2O. Propylene is produced from propane in presence of pure N 2O. But this probably occurs only under transitory conditions as N 2O probably needs the presence of molecular oxygen to produce propylene durably. This work shows that adjusting the concentration of promoters, like N 2O and CO 2, could be a useful tool to modulate the oxidation state of oxide catalysts at work during the catalytic reaction, and thus their catalytic performances.