Interaction of SO2 with CeO2 and Cu/CeO2 catalysts: photoemission, XANES and TPD studies

CeO2 and Cu/CeO2 are effective catalysts/sorbents for the removal or destru ction of SO2. Synchrotron-based high-resolution photoemission, X-ray absorp tion near-edge spectroscopy (XANES), and temperature-programmed desorption (TPD) have been employed to study the reaction of SO2 with pure and reduced CeO2 powders, ceria films (CeO2, CeO2-x, Ce2O3+x) and model Cu/CeO2 cataly sts. The results of XANES and photoemission provide evidence that SO4 was f ormed upon the adsorption of SO2 on pure powders or films of CeO2 at 300 K. The sulfate decomposed in the 390-670 K temperature range with mainly SO2 and some SO3 evolving into gas phase. At 670 K, there was still a significa nt amount of SO4 present on the CeO2 substrates. The introduction of O vaca ncies in the CeO2 powders or films favored the formation of SO3 instead of SO4. Ceria was able to fully dissociate SO2 to atomic S only if Ce atoms wi th a low oxidation state were available in the system. When Cu atoms were a dded to CeO2 new active sites for the destruction of SO2 were created impro ving the catalytic activity of the system. The surface chemistry of SO2 on the Cu-promoted CeO2 was much richer than on pure CeO2. The behavior of cer ia in several catalytic processes (oxidation of SO2 by O-2, reduction of SO 2 by CO, automobile exhaust converters) is discussed in light of these resu lts.