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., 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/ZrO2 catalysts (3% copper). With no oxygen in the reaction flow, complete reduction of NO to N-2 occurs at T > 623 K with C3H8. 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 + C3H8 mixture, provided that carbonaceous species are previously formed by heating the catalyst with C3H8 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/ZrO2 catalysts, an FTIR experiment has been carried out. Exposure of the catalyst to C3H8 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 :S 623 K. Above this temperature, the bands decreased in intensity to disappear at T = 723 K. The implications of the formation of Cu+ and Cu2+ states and that of the adsorbed oxygenated carbonaceous species in de-NO, processes over Cu/ZrO2 catalysts are discussed.