Role of NO and SO2 in mercury oxidation over a La2O3/Fe2O3 catalyst with high thermal stability

Abstract

In this study, the thermal stability of a ferric oxide catalyst for mercury oxidation was found to be considerably promoted by doping with La2O3. The catalysts doped with La2O3 maintained a higher surface area when subjected to high-temperature calcination, with lower average pore size and a narrower pore size distribution. X-ray diffraction (XRD) results revealed that La2O3 doping hinders the growth of catalyst particles and crystallization of the material at high temperatures. Both NO and SO2 inhibited Hg0 oxidation over the La2O3/Fe2O3 catalyst. Fourier transform infrared (FTIR) spectra revealed that SO2 reacts with O2 over the catalysts to form several species that are inert for mercury oxidation, such as SO42−, HSO4−, or other related species; these inert species cover the catalyst surface and consequently decrease Hg0 oxidation capacity. In addition, NO or SO2 competed with Hg0 for active sites on the La2O3/Fe2O3 catalyst and hindered the adsorption of mercury, thereby inhibiting subsequent Hg0 oxidation. Hg0 oxidation on the La2O3/Fe2O3 catalyst mainly followed the Eley–Rideal mechanism. Moreover, the inhibition effects of NO and SO2 were at least partially reversible, and the catalytic activity was temporarily restored after eliminating NO or SO2.