Mercury oxidation by hydrochloric acid over TiO 2 supported metal oxide catalysts in coal combustion flue gas

Abstract

Mercury oxidation by hydrochloric acid over the metal oxides supported by anatase type TiO 2 catalysts, 1 wt.% MO x/TiO 2 where M = V, Cr, Mn, Fe, Ni, Cu, and Mo, was investigated by the Hg 0 oxidation and the NO reduction measurements both in the presence and absence of NH 3. The catalysts were characterized by BET surface area measurement and Raman spectroscopy. The metal oxides added to the catalyst were observed to disperse well on the TiO 2 surface. For all catalysts studied, the Hg 0 oxidation by hydrochloric acid was confirmed to proceed. The activity of the catalysts was found to follow the trend MoO 3 ~ V 2O 5 > Cr 2O 3 > Mn 2O 3 > Fe 2O 3 > CuO > NiO. The Hg 0 oxidation activity of all catalysts was depressed considerably by adding NH 3 to the reactant stream. This suggests that the metal oxide catalysts undergo the inhibition effect by NH 3. The activity trend of the Hg 0 oxidation in the presence of NH 3 was different from that observed in its absence. A good correlation was found between the NO reduction and the Hg 0 oxidation activities in the NH 3 present condition. The catalyst having high NO reduction activity such as V 2O 5/TiO 2 showed high Hg 0 oxidation activity. The result obtained in this study suggests that the oxidation of Hg 0 proceeds through the reaction mechanism, in which HCl competes for the active catalyst sites against NH 3. NH 3 adsorption may predominate over the adsorption of HCl in the presence of NH 3.