Exceptional stability and chemical mechanism over spinel ZnCr2O4 catalyst for HCl oxidation to Cl-2

Deacon process, one-step catalytic oxidation of HCl to Cl-2, is the most effective way to realize the recycling utilization of chlorine element in industry since 1868. However, the stability of non-noble metal catalysts, e.g. Cr-based catalysts, is problematic to be commercialized due to the Cr loss in reaction atmosphere. Herein, we reported a spinel ZnCr2O4 catalyst exhibiting a high activity as well as an unprecedented long-term stability, with a sustainable HCl conversion rate (chi(Hcl)) of (similar to)78% and the space time yield (STY) of 2.25 g(cl2) g(cat)(-1) h(-1) even after 300 h run at 390 degrees C and feed O-2/HCl (1:1). In addition, we conceived the control experiments and density functional theory (DFT) calculations to deeply understand the chemical mechanisms over spinel CoCr2O4 and ZnCr2O4. The XPS, O-2-TPD and H-2-TPR characterizations suggested the different surface redox behaviors of both catalysts. DFT calculations indicated that a defect surface of ZnCr2O4 was more active in HCl catalyzed oxidation and HCl preferred to be absorbed on the Zn top site and Cl* coupling to Cl-2 was the Delta G-determining step, which led to the excellent stability of ZnCr2O4 by efficiently avoiding the Cr loss. On the contrary, the higher reaction energy of HCl adsorption on the Co-Cr bridge sites of CoCr2O4 caused the decreasing of activity and stability. Consequently, the different stability performance over CoCr2O4 and ZnCr2O4 was well correlated with the two distinct chemical mechanisms and therefore an outstanding agreement was obtained from the theoretical calculations and experimental behaviors