The role of the support and dispersion in the catalytic combustion of chlorobenzene on noble metal based catalysts

Polychlorinated benzenes (PhClx) are formed as byproducts in the combustion of chlorobenzene on Pt supported on gamma-Al2I3, SiO2, SiO2-Al2O3, or ZrO2 . The congener and isomer distribution of the PhClx differs for the various supports. The amounts of PhClx correlate with the dispersion of platinum. Thus, a Pt/gamma-Al2O3 catalyst calcined at 500 degrees C to yield very sma ll Pt crystallites was more active in PhClx formation than Pt/gamma-Al2O3 c alcined at 800 degrees C. In all cases T-50% for chlorobenzene conversion i s close to 300 degrees C and appears to be independent of the crystallite s ize of the platinum. Replacing platinum by palladium led to lower rates of combustion and to more byproducts. These results lead us to propose that, i n the presence of Cl and higher oxygen concentrations, small Pt crystallite s are converted more easily into Pt(IV) species. These are less efficient i n combustion, but can be more active in chlorination. (C) 2000 Elsevier Sci ence B.V. All rights reserved.