X-ray Photoelectron Spectroscopy and Infrared Study of the Nature of Cu Species in Cu/ZrO2 de-NOx Catalysts

The present paper aims to clarify some of the still controversial aspects of the chemistry of copper in Cu-based catalytic systems active for de-NO x processes by using hydrocarbons as reductants. Reactivity tests, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) experiments have been carried out with Cu/ZrO 2 catalysts (3% copper). With no oxygen in the reaction flow, complete reduction of NO to N 2 occurs at T > 623 K with C 3 H 8 . In the presence of oxygen, complete oxidation of the hydrocarbon occurs without any significant conversion of NO. XPS shows that Cu + species can be formed at 423 < T ≤ 623 K when heating with a NO + C 3 H 8 mixture, provided that carbonaceous species are previously formed by heating the catalyst with C 3 H 8 at 773 K. To clarify the chemical interaction occurring between carbonaceous residues, copper and NO on the surface of real catalysts, experiments have been carried out with a model system consisting of copper particles deposited by evaporation on the surface of graphite. It was found that 100% Cu + is obtained when the Cu−graphite system is exposed to NO between 298 and 773 K. Simultaneously, it was found that C−NO (N1s binding energy, BE 403.3−404.8 eV), C−N (N1s BE 398.5 eV), and different C−O species are formed on the surface of graphite. −C−N and −C−O species are stable up to 773 K, whereras the −C−NO features disappear from the spectra at T > 473 K. To understand the chemical processes that occur on the Cu/ZrO 2 catalysts, an FTIR experiment has been carried out. Exposure of the catalyst to C 3 H 8 at 773 K leads to the appearance of a series of bands between 1300 and 1600 cm -1 attributed to adsorbed carbonaceous species. Also, a very little band coming from −CN species is detected in these conditions. These species were stable when contacting the sample with NO at T ≤ 623 K. Above this temperature, the bands decreased in intensity to disappear at T = 723 K. The implications of the formation of Cu + and Cu 2+ states and that of the adsorbed oxygenated carbonaceous species in de-NO x processes over Cu/ZrO 2 catalysts are discussed.