| The selective catalytic reduction of nitric oxide by propylene, in the presence of excess oxygen, has been investigated over a series of platinum catalysts, utilizing alumina and silica as the supports.; Detailed kinetic studies were conducted over 0.8 wt.-% Pt/Al2O 3 and 0.9 wt.-% Pt/SiO2 catalysts. The Pt/Al2O 3 catalyst exhibits a maximum NO reduction temperature of 300°C whereas, the corresponding temperature for Pt/SiO2 is 285°C. The two catalysts exhibit similar kinetic behavior indicating that the reaction proceeds, on both, through a similar mechanism. The results further indicate the presence of two kinetically distinct regions below and above the temperature of maximum NO conversion. In particular, the reaction was found to be zero order in NO at temperatures below that of maximum NO conversion, and first order in NO at temperatures above the maximum. Additional kinetic experiments were also carried out at variable oxygen concentrations and in the presence of water and NO2; The reaction mechanism was further investigated by the use of in situ Fourier transform infrared (FTIR) spectroscopy at 250°C. Experiments conducted under reaction conditions, over the Pt/SiO2 catalyst, resulted in the formation of a surface cyanide (-CN) species, and a gas phase/weakly adsorbed nitrous oxide (N2O) species. The surface cyanide is the product of the reaction between an oxygen-activated hydrocarbon derivative and NO, and is reactive towards NO, NO2 and O2, resulting in the formation of N2 and N2O and CO2.; Infrared studies conducted over the Pt/Al2O3 catalyst revealed the presence of surface cyanide and isocyanate species, under reaction conditions. The surface cyanide in the case of Pt/Al2O 3 can be excluded as a reactive intermediate. This species is very strongly adsorbed on the catalyst surface and is not reacting in O2, NO, and NO2 environments. On the contrary, the surface isocyanate species is present only under SCR conditions (i.e. it is not present in the absence of O2), is held weakly on the catalyst surface, and reacts with O2, NO and NO2.; In light of the results of the kinetic and the FTIR spectroscopy studies, a reaction mechanism is proposed. |
