(A) study on the characteristics for the catalytic decomposition of 1,2-dichlorobenzene and dioxins over titania supported metal oxide catalysts

Abstract

Titania supported transition metal oxides were investigated to develop new catalysts for the catalytic decomposition of toxic polychlorinated aromatic pollutants such as polychlorinated benzenes, polychlorinated biphneyls, polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) from waste flue gases. The oxidative decomposition of 1,2-dichlorobenzene ($Cl_2Bz$) was chosen as a model reaction for preliminary catalyst screening since it is one of the toxic polychlorinated benzenes and structurally similar to the most toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin. The $VO_x$/$TiO_2$ catalyst showed the highest activity and stability among the transition metal oxides such as vanadium, chromium, copper, manganese and iron oxide on the same support even in the presence of water vapor. The optimum loading of vanadia was 5wt%, corresponding to half a monolayer, suggesting that the active species are isolated, tetrahedrally coordinated vanadyl species on titania. Decrease in the activity with higher loading than 5wt% $V_2O_5$ would be attributed to the polymerization of tetrahedral vanadyl species, which is believed to be the active species for the complete decomposition of $Cl_2Bz$. The activity of $VO_x$/$TiO_2$ catalyst and $CO_2$ selectivity were remarkably increased with the addition of water to the feed stream at low temperature as well as $CO_2$ selectivity. It was suggested that surface hydroxyl group or surface hydrophilic character of catalytic surface might play an important role in this reaction. The addition of water vapor, however, reduced the $Cl_2Bz$ conversion significantly over the other transition metal catalysts and also inhibited the formation of higher chlorinated benzenes over $CuO_x$/$TiO_2$ catalyst. Titania supported platinum and palladium catalysts were also tested for the oxidative decomposition of $Cl_2Bz$ in the presence of water. The platinum catalysts showed a higher conversion and $CO_2$ selectivity than palladium catalysts at hi...