Kinetics of Hg° oxidation over a V2O5/MoO3/TiO2 catalyst: Experimental and modelling study under DeNOX inactive conditions

In this work, a kinetic study of mercury oxidation over a commercial plate-type V/Mo/Ti/O catalyst was addressed by investigating the effects of key operating variables (temperature, HCl and Hg degrees concentration). The study was performed under reference DeNO(x)-inactive conditions, that is inthe absence of NOx, NH3, SOX, recognizing that surface coverage effects can greatly affect the kinetics of this complex reacting system. Aim of the work was in fact to better clarify the nature of the intrinsic reaction (in particular its temperature dependence), that, despite the increasing interest toward this reaction, is still debated with a very large scatter among observations. The experiments were performed over slabs. The combined effects of temperature and HCl concentration evidenced that the conversion of Hg degrees was independent from the HCl content below 300-325 degrees C, where the reaction showed a positive temperature dependence; at higher temperature, the conversion of Hg degrees became highly sensitive to the HCl concentration, which determined the observed temperature dependence (negative at 2.5 ppm HCl, positive at 50 ppm HCl). Thus the data suggest a strong dependence of the reaction kinetics on the surface coverage of HCl. A model of the reactor cell was developed and an engineering analysis of data was performed to decouple the effects of inter-phase and intra-porous mass transfer limitations. A rate expression was derived from the formalization of a redox reaction scheme wherein a step-wise oxy-chlorination process (Hg degrees -> HgClads -> HgCl2) is assumed in line with recent theoretical findings. By assuming that the surface coverages of the HgCI intermediate and the reduced sites are negligible with respect to the coverage of free and chlorinated oxidized sites, a simple rate expression is derived which accounts for a quadratic dependence on HCl coverage and a linear dependence on the gas-phase concentration of elemental mercury (in line with the experimental evidence). The whole bulk of data could be well described, capturing the key observed trends. The activation energy of the reaction steps was found to be 14000 cal/mole while the enthalpy of HCl activation was estimated at -20000 cal/mole, coherently with the energetics of V-sites chlorination. (C) 2016 Elsevier B.V. All rights reserved.