Insights into the role of SO2 and H2O on the surface characteristics and de-N2O efficiency of Pd/Al2O3 catalysts during N2O decomposition in the presence of CH4 and O2 excess

Abstract

The catalytic abatement of nitrous oxide (N2O), a powerful greenhouse and ozone depletion gas, is an efficient end-of-pipe technology for N2O emissions control. However, de-N2O performance is notably suppressed by SO2 and H2O presence on the flue gases, whereas little is known about their influence on catalyst surface chemistry. In the present study, the impact of sulfur dioxide and water vapor on the catalytic performance of Pd/Al2O3 catalysts during the N2O decomposition in the presence of CH4 and O2 excess is investigated, with particular emphasis on the corresponding surface chemistry modifications. Catalytic activity and stability measurements, in conjunction with a kinetic study, were carried out to elucidate the individual effect of each molecule on de-N2O performance. X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and Fourier transform infrared spectroscopy (FTIR) of pyridine adsorption are employed to evaluate the impact of SO2 and H2O molecules on catalyst surface chemistry, which is appropriately correlated with the achieved catalytic performance. The results revealed that the de-N2O efficiency can be substantially improved by CH4 under reducing (absence of O2) conditions, due to the scavenging of strongly adsorbed Oads species by the hydrocarbon; however, under O2 excess conditions the beneficial effect of CH4 is marginal. Water vapor in the feed has a detrimental influence on both N2O and CH4 conversions, which, however, is totally reversible; the latter is mainly ascribed to the competitive adsorption of H2O molecules on catalyst surface. In contrast, SO2 addition in feed stream results in a severe, irreversible deactivation; SO2 leads to the creation of Brönsted acid sites on Al2O3 support, which in turn results in highly oxidized Pd entities, inactive for N2O decomposition.