Experimental and theoretical studies of mercury oxidation over CeO2 − WO3/TiO2 catalysts in coal-fired flue gas

Ceria-based catalyst has been regarded as a very promising material for mercury oxidation due to its high catalytic oxidation activity. A series of CeO2 − WO3/TiO2 (CeW/Ti) catalysts were synthesized by an ultrasound-assisted impregnation method and tested for their catalytic activity of mercury oxidation in coal-fired flue gas. CeW/Ti catalysts with a CeO2:WO3:TiO2 mass ratio of 0.5:0.5:1 exhibited the highest catalytic activity for mercury oxidation in the wide temperature window of 200-350 °C. O2 promoted mercury oxidation by regenerating the chemisorbed oxygen and lattice oxygen of the catalysts. NO and SO2 could enhance mercury oxidation in the absence of HCl. Mercury oxidation over CeW/Ti catalysts was significantly enhanced when HCl was added to the simulated flue gas, over 95% mercury oxidation efficiency was obtained. H2O weakly inhibits mercury oxidation in flue gas. Mercury oxidation by HCl over CeW/Ti catalysts follows the Eley-Rideal mechanism, in which gas-phase Hg0 reacts with active surface chlorine species generated from HCl dissociation. First principles calculations based on the density functional theory (DFT) were used to elucidate the process of mercury oxidation over CeW/Ti catalysts. Compared to the one-step mercury oxidation process (Hg0→HgCl2), the dominant reaction pathway of mercury oxidation is a two-step process (Hg0→HgCl→HgCl2) in which the second step (HgCl→HgCl2) with an energy barrier of 50.11 kJ/mol is the rate-limiting step.