Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide IV. Diffuse reflectance FTIR study of NO adsorption and reaction

Abstract

Adsorption of NO onto the oxidized surfaces of both crystalline and amorphous chromia results in the formation of nitrate complexes, by the reaction of NO with surface oxygen. Up to six differently bound species exhibiting different thermal stabilities have been identified. As the catalyst surfaces are saturated with adsorbed oxygen, the bound NO molecules are not decomposed: Upon heating, NO is molecularly desorbed, and neither formation of N 2O nor of N 2 is observed. Reduced chromia surfaces, which are partially depleted in surface oxygen, are more reactive towards NO. Decomposition of NO, to form N 2O and N 2, is taking place already at room temperature and is accompanied by a partial reoxidation of the catalyst surface. Nitrato complexes are the most abundant surface species also on the reduced chromia; signals of adsorbed mono- and dinitrosyl complexes are comparatively weak. This observation shows that NO preferentially binds to the Lewis basic surface oxygen ions. As a consequence of the decreased surface oxygen concentration, the coverage of bridging surface nitrate species (requiring two adjacent surface oxygens) is smaller than on the oxidized catalysts. The reaction of NO with surface hydroxyl groups, producing N2 and NO 2 −, results in a minor concentration of Cr-NO 2 surface nitro complexes. On the reduced amorphous catalyst, extensive reoxidation of the surface accompanied by N 2O formation is observed between 311 and 411 K in temperature-programmed reaction and desorption (TPRD) experiments, while this process is observed to a much smaller extent on reduced crystalline chromia. Above 473 K, bridging and bidentate surface nitrates are the most abundant species on both morphologies. Further heating results in the decomposition of these nitrates yielding N 2, N 2O (on crystalline chromia only), and surface oxygen.