Applied Catalysis A: General, Vol.572, 107-114, 2019
Enhancement of Mn/Ce/W/Ti catalysts through control of pH and oxygen mobility during their preparation
The purpose of this study is to investigate the effects of pH control on the oxidation state and oxygen storage capacity of catalysts, and the correlation with low-temperature selective catalytic reduction (SCR) activities using Mn/Ce/W/Ti catalysts. The activity of the catalysts showed increases in low-temperature SCR reaction activity at (120-180) degrees C, when the pH of the slurry of active metal and support TiO2 was controlled to low levels during the preparation of Mn/Ce/W/Ti catalysts. To characterize these catalysts, NH3-TPD, FT-IR DRIFT, XPS, and O-2-chemisorption were analyzed. In the DRIFT experiment using FT-IR, the NH3 adsorption on acid sites of the Mn/Ce/W/Ti catalysts, which exhibited excellent reaction activity in the SCR reaction, was increased, compared with those of the Mn/Ce/Ti catalysts. In particular, the NH4+ adsorbed on Bronsted acid site was greatly increased. When the adsorbed NH3 was reacted with injected NO + O-2, the adsorbed NH3 species was rapidly decreased on Mn/Ce/W/Ti catalysts whose pH was controlled to low levels, and the reaction rate was greatly increased. According to XPS analysis, the increase of the O-2 storage capacity of the catalyst is due to the decrease of the lattice oxygen present on the surface, and the increase of adsorbed oxygen species as the nonstoichiometric species Ce3+ increases. In addition, the O-2-chemisorption showed that oxygen mobility was increased in the catalysts whose pH was controlled to low levels. These results indicate that oxygen mobility was increased due to the increases in the adsorbed oxygen species, which was found by XPS analysis. Therefore, the control of pH in the preparation of Mn catalyst increases the Bronsted acid site on the catalyst surface and increases oxygen mobility, due to the increase of adsorbed oxygen species, resulting in an increase in the SCR activity.