Kinetics and mechanism of the reduction of nitric oxides by H-2 under lean-burn conditions on a Pt-Mo-Co/alpha-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/alpha-Al2O3 catalyst under oxidising conditions. This system is interesting in view of an exhau st 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 behavi our and that it depends on temperature and on the partial pressure of H-2. With increasing temperatures the reaction reveals an autocatalytic behaviou r resulting in bistability and hysteresis. At higher temperatures, where no bistability is found, the NO/H-2 as well as the competing O-2/H-2 reaction occur only above a certain critical partial pressure of H-2. The kinetics of the NO/H-2/ O-2 reaction are established using a modified Langmuir-Hinsh elwood model (T=142 degrees C-160 degrees C, y(O2)>4%) which takes into acc ount the critical H-2 partial pressure. The model describes the experimenta l data within +/-15%. The determined activation energies are: 63 kJ/mol for the NOx consumption, 77 and 45 kJ/mol for the N-2 and N2O formation, respe ctively, and 130 kJ/ mol for the O-2/H-2 reaction. Adsorption enthalpies ar e determined to -59 kJ/mol for the adsorption of H-2, -77 kJ/mol for the ad sorption of NO and -97 kJ/mol for the adsorption of O-2 An interesting feat ure of the reaction is the enhancement of the NO/H-2 reaction by oxygen for low partial pressures of O-2 This appears to be the first study where a pr omoting effect of oxygen for the NO/H-2 reaction is found on a platinum sup ported catalyst. (C) 1998 Elsevier Science B.V. All rights reserved.