TPD-TPR-MS MECHANISTIC STUDY OF THE AMMOXIDATION OF 2-METHYLPYRAZINE OVER SB-V-MN-O CATALYST

The setting up of a TPD-TPR-MS apparatus particularly suited to the study of the mechanisms of heterogeneous catalytic reactions is described. The system has been employed for the study of the ammoxidation of 2-methylpyrazine. The information collected showed that all the reactants and products except oxygen adsorb competitively on at least two types of site on the catalyst surface. From sites of the first type (the lower-energy sites) they desorb unaltered, while sites of the second type are higher-energy sites on which the reactants are activated both for the desired reaction to 2-cyanopyrazine and for the decomposition to by-products. Ammonia adsorbs also on a third type of site (very high-energy sites), which are able to activate the molecule for the oxidation to N2 and NOx Oxygen does not compete in adsorption with any of the other species. Very likely it dissolves in the solid, rapidly diffusing through the bulk and is involved in the reaction through a Mars-van Krevelen mechanism. A Rideal-Eley type of mechanism, with either oxygen or ammonia coming from the gas phase, seems not to be supported by the present results. © 1991.