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 reduction of nitric oxides (NOx) by hydrogen is studied on a trimetallic Pt± Mo±Co/a-Al2O3 catalyst under oxidising 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 dissociation is the crucial step, dominating the overall reaction behaviour and that it depends on temperature and on the partial pressure of H2. With increasing temperatures the reaction reveals an autocatalytic behaviour resulting in bistability and hysteresis. At higher temperatures, where no bistability is found, the NO/H2 as well as the competing O2/H2 reaction occur only above a certain critical partial pressure of H2. The kinetics of the NO/H2/ O2 reaction are established using a modi®ed Langmuir±Hinshelwood model (Tˆ1428C±1608C, yO2 >4%) which takes into account the critical H2 partial pressure. The model describes the experimental data within 15%. The determined activation energies are: 63 kJ/mol for the NOx consumption, 77 and 45 kJ/mol for the N2 and N2O formation, respectively, and 130 kJ/ mol for the O2/H2 reaction. Adsorption enthalpies are determined to ÿ59 kJ/mol for the adsorption of H2, ÿ77 kJ/mol for the adsorption of NO and ÿ97 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 ®rst study where a promoting effect of oxygen for the NO/H2 reaction is found on a platinum supported catalyst. # 1998 Elsevier Science B.V. All rights reserved.