Fourier Transform Infrared Spectroscopy Study of the Nature and Reactivity of NOx Compounds Formed after Coadsorption of NO and O₂ on Cu/ZrO₂

Adsorption of CO on Cu/ZrO 2 leads to the formation of Cu + −CO (2126 cm - 1 ) and Zr 4+ −CO (2192 cm - 1 ) carbonyls and of carbonato−hydrogen-carbonate structures (bands in the 1650−1000 cm - 1 region). Adsorption of NO on the same sample produces bands characterizing N 2 O (2246 cm - 1 ), NO + (2122 cm - 1 ), mononitrosyls coordinated to Cu + (1759 cm - 1 ) and Cu 2+ (1872 cm - 1 ) ions, respectively, and nitro (1527, 1327 cm - 1 ) and nitrito compounds (1185 cm - 1 ). Introduction of oxygen into the system results in an initial increase in concentration of NO 2 - compounds followed by oxidation of these complexes to nitrates (1630, 1580, 1554, 1287, 1225, 1031, and 1004 cm - 1 ). The nitrates thus obtained are stable up to 573 K. They enhance the Lewis acidity of the neighboring Zr 4+ sites on the support as a result of which mononitrosyls of the Zr 4+ (NO 3 )−NO type (1932 cm - 1 ) are formed. The copper nitrates formed on the surface begin to react with C 2 H 4 at 523 K, yielding an organic nitro-containing deposit which decomposes to reactive cyanides (2142 cm - 1 ). The mechanism of selective catalytic reduction of nitrogen oxides with hydrocarbons on Cu/ZrO 2 and the oxidation state of the copper ions during the process are discussed.