Kinetics and mechanism of the reduction of nitric oxides by H2 under lean-burn conditions on a Pt-Mo-Co/a-Al2O3 catalyst

The kinetics and the mechanism of the selective redn. of nitric oxides (NOx) by hydrogen is studied on a trimetallic Pt-Mo-Co/a-Al2O3 catalyst under oxidizing conditions. This system is interesting in view of an exhaust gas control of power plants or lean-burn cars. It can be shown that the NO dissocn. is the crucial step, dominating the overall reaction behavior and that it depends on temp. and on the partial pressure of H2. With increasing temps. the reaction reveals an autocatalytic behavior resulting in Bi-stability and hysteresis. At higher temps., where no Bi-stability is found, the NO/H2 as well as the competing O2/H2 reaction occur only above a certain crit. partial pressure of H2. The kinetics of the NO/H2/O2 reaction are established using a modified Langmuir-Hinshelwood model which takes into account the crit. H2 partial pressure. The model describes the exptl. data within +-15%. The detd. activation energies are: 63 kJ/mol for the NOx consumption, 77 and 45 kJ/mol for the N2 and N2O formation, resp., and 130 kJ/mol for the O2/H2 reaction. Adsorption enthalpies are detd. to -59 kJ/mol for the adsorption of H2, -77 kJ/mol for the adsorption of NO and -97 kJ/mol for the adsorption of O2. An interesting feature of the reaction is the enhancement of the NO/H2 reaction by oxygen for low partial pressures of O2. This appears to be the first study where a promoting effect of oxygen for the NO/H2 reaction is found on a platinum supported catalyst. [on SciFinder (R)]