NATURE OF ACTIVE SPECIES IN COPPER-BASED CATALYSTS AND THEIR CHEMISTRY OF TRANSFORMATION OF NITROGEN-OXIDES

Copper-based catalysts are active in a wide range reactions of transfo rmation of nitrogen oxides and represent an useful model system to bet ter understand the fundamental aspects of the chemistry and mechanism of reaction of catalytic transformation of these pollutants. After an introduction on the reactivity of copper-based catalysts (supported an d unsupported copper oxide, Cu-zeolites, cuprates and other copper com pounds) in various reactions of conversion of nitrogen oxides, four ma in sub-topics are discussed in detail: (i) nature of copper species, ( ii) chemisorption and surface transformations of NO, (iii) relationshi p between copper species and activity in the conversion of nitrogen ox ides and (iv) mechanism of reduction of nitrogen oxides to N-2. Five r eactions of transformation of nitrogen oxides are discussed in detail. (i) decomposition of NO, (ii) reduction of NO with ammonia in the pre sence or not of oxygen, (iii) reduction of NO with hydrocarbons in the presence of oxygen, (iv) reduction of NO with CO and (v) decompositio n of N2O. The mechanism of reduction of nitrite and N2O by copper enzy mes is also discussed, with a view to provide some useful insights on the chemistry of transformation. In this review particular attention i s directed towards controversial points in the literature, underestima ted questions, and hypothesis and theories which do not allow interpre tation of all sets of experimental data. Discussion is also focused on the presence of multiple and competitive pathways of transformation, the relative roles of which depend on reaction conditions.