An in situ XAS study of Cu/ZrO2 catalysts under de-NOx reaction conditions

This paper reports an X-ray absorption spectroscopy investigation of the chemical state of copper in Cu/ZrO2 catalysts under reaction conditions. The catalysts were subjected to different treatments with hydrocarbons (CH4, C3H8, or C3H6) and hydrocarbon/NO and hydrocarbon/NO/O2 mixtures. The analysis was done for samples heated in situ at temperatures ranging from 298 to 773 K. The first derivative of the X-ray absorption near-edge structure (XANES) spectra of the Cu K edge was analysed by factor analysis (FA). This analysis provided an accurate means of estimating the percentages of Cu2+, Cu+, and Cu0 under each reaction condition. Before reaction, the original catalyst consisted mainly of Cu2+ species. Heating the catalyst with the hydrocarbon to 573 K led to its partial reduction with formation of a significant amount of Cu+. At the higher temperature, and depending on the hydrocarbon, Cu0 species were dominant. When the catalyst was treated at 773 K with mixtures consisting of hydrocarbons plus NO, there was still a significant concentration of Cu+ and Cu0 species, although the relative concentration of Cu2+ generally increased. For these gas mixtures, it was also found that the relative concentration of the three different oxidation states of copper was highly dependent on the type of hydrocarbon. In general, the concentration of reduced species was related to the reactivity of the hydrocarbon (CH4 < C3H8 < C3H6) and was particularly high with C3H6. Finally, the analysis showed that most copper remained as Cu2+ when the sample was heated in a hydrocarbon/NO/O2 mixture at 773 K, although some Cu+ can persist under these conditions. Some quantitative discrepancies were observed by comparing the results obtained by XANES/FA and those obtained by fitting analysis of the extended X-ray absorption fine structure spectra. However, the higher concentration of Cu+ species detected by XANES/FA analysis under working conditions is congruent with temperature-programmed reduction and X-ray photoelectron spectra experiments on this sample. It is concluded that the combination of these two procedures may yield more reliable information about the physicochemical state of the different Cu2+, Cu+ and Cu0 species existing in the Cu/ZrO2 catalyst under de-NOx reaction conditions.