Simultaneous catalytic oxidation of CO and Hg⁰ over Au/TiO₂ catalysts: Structure and mechanism study

Au/TiO₂ catalysts with different morphologies were prepared by the modulation of chlorine via a hydrothermal method, which were used for the simultaneous catalytic removal of CO and Hg⁰. Results showed that the Au/TiO₂ catalyst with a structure of submicron sphere and ascorbic acid modification (denoted as AuV/2 M T) showed 38.2% (41.1%, without H₂O) CO conversion at 50 °C and nearly 100% CO conversion at 275-350℃ (225-350℃, without H₂O). This catalyst also exhibited stable catalytic efficiency of approximately 88% (95.7%, without H₂O) for Hg⁰ removal at 300℃. The high catalytic activity of the AuV/2 M T was ascribed to a series of superior properties, including a high specific surface area, a large amount of adsorbed oxygen species and a large brookite/rutile TiO₂ ratio. Moreover, the AuV/2 M T also exhibited excellent stability. The AuV/2 M T continued to exhibit nearly 90.5% CO conversion efficiency and 84.5% Hg⁰ conversion efficiency after continuous testing with 6 vol.% H₂O for 120 h. Therefore, this catalyst is expected to be used in coal-fired power plants. The mechanism of CO removal over AuV/2 M T was in-depth investigated. The presence of chlorine promoted the catalytic oxidation of Hg⁰ while the presence of CO has no great effect on desorption of Hg⁰. Moreover, catalytic oxidation of CO over AuV/2 M T catalyst was found to follow L-H mechanisms. The formate was the intermediate species for CO catalytic oxidation, and it transformed to CO₂ quickly by consuming the oxygen species (O₂⁻, O⁻ and −OH) adsorbed onto the catalyst surface. The oxygen species consumed on the surface of catalyst was supplied by oxygen, and it was the key factor in determining the reaction rate.