Formation and role of Cu+ species on highly dispersed CuO/SBA-15 mesoporous materials for SOx removal: An XPS study

Abstract

In the present work, copper species were incorporated in SBA-15 organized mesoporous silica (“OMS”) through wet impregnation of copper nitrate and the corresponding material was evaluated as SO2 adsorbent. The calcination under a high flow of synthetic air leads to the formation of highly dispersed copper species in strong interaction with the support (Cu2+OSi). Indeed, the characterization of this material by X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), do not indicate the presence of crystalline CuO. A comprehensive X-ray Photoelectron Spectroscopy (XPS) study allowed the identification of the active species in SOx trapping and underlined the importance of a thermal pretreatment on the performances of the adsorbent in SOx trapping. It reveals that a pretreatment at 600°C leads to the formation of active Cu+ species in higher amount ([Cu+]=79.0%) than at 400°C ([Cu+]=37.6%), which strongly increases the adsorbent efficiency. These species were found to be stable in the presence of [O2]=10% (balance nitrogen) at 400°C but unstable at ambient atmosphere at room temperature (presence of water). The treatment under SO2 at 400°C leads to the reoxidation of the Cu+ species into Cu2+ ones, which are partially sulfated (formation of CuSO4). Additionally, the sulfation step causes the sintering of a part of the Cu2+OSi species into CuO. A reaction scheme was proposed according to which, in a first step Cu+ species catalyze the SO2 oxidation to SO3. In the second step, Cu(I) is reoxidized into Cu(II) as CuSO4.