Mechanism of nitrite hydrogenation over Pd/γ-Al₂O₃ according a rigorous kinetic study

The kinetics of nitrite hydrogenation over a Pd/γ-Al₂O₃ catalyst was studied in a semi-batch slurry reactor at atmospheric pressure, in absence of any mass transfer effects. The hydrogen concentration and pH were kept constant during an experiment by continuously flowing a gas mixture containing hydrogen and 10% v/v CO₂. The kinetic experiments were performed in an unprecedented wide concentration window of nitrite and hydrogen, revealing extreme variation in the apparent orders in hydrogen and nitrite, including reaction orders in hydrogen between 2 and 0.3, whereas the order in nitrite varied between 0.4 and −0.9. The rate of reaction is almost exclusively determined by the rate of formation of N₂ as the selectivity to ammonia is very low. A Langmuir-Hinshelwood mechanism with competitive adsorption is in operation. Several mechanistic pathways, as well as possible rate determining steps in those pathways, are discussed based on these observations in combination with prior knowledge on the mechanism in literature, resulting in a revised mechanistic scheme. It is concluded that formation of NH via dissociative hydrogenation of HNOH is the rate determining step, whereas molecular N₂ forms via reaction of NH with either NO, NOH or HNOH. N-N bond formation via dimerization of adsorbed NO or adsorbed N can be excluded.