Synergistic effect of HCl and NO in elemental mercury catalytic oxidation over La2O3-TiO2 catalyst

Abstract

Elemental mercury (Hg0) oxidation over a low-temperature La2O3-TiO2 (LaTi) catalyst through a fixed-bed reactor system is reported here. Despite the presence of oxygen (O2), Hg0 was not effectively oxidized by either hydrogen chloride (HCl) or nitrogen oxide (NO). However, Hg0 oxidation over the LaTi catalyst was greatly improved when both HCl and NO were present in the simulated flue gas. Several methods, including X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and temperature-programmed decomposition (TPD), were used to study the inherent mechanisms involved in the Hg0 oxidation by HCl and NO. The results indicated that the combined presence of HCl and NO exhibited great synergy for Hg0 catalytic oxidation over the LaTi catalyst. The reaction mechanism is proposed as follows: 1) NO was adsorbed and oxidized on the catalyst surface, forming NO2; 2) the reactive NO2 then oxidized gas phase Hg0 to mostly HgO, and; 3) HgO reacted with gas phase HCl to produce HgCl2, which escaped from the catalyst’s surface. Differing from previously proposed mechanisms, La2O3 was first studied as an active substance in Hg0 oxidation. These findings are beneficial for those seeking economical and efficient Hg0 oxidation catalysts for coal-fired power plants.