OXIDATION OF CARBON-MONOXIDE, PROPENE, PROPANE AND METHANE OVER A PD AL2O3 CATALYST - EFFECT OF THE CHEMICAL-STATE OF PD/

A 1 wt.-% Pd/gamma-Al2O3 catalyst was prepared (38% dispersion) and si ntered under a flow of 3 vol.% O-2 in N-2 at 900 degrees C (9% dispers ion). The temperature-programmed oxidation of CO, C3H6, C3H8 and CH4 i n substoichiometry of O-2 was carried out on the fresh and, in some ca ses, on the sintered catalyst. The chemical state of palladium in the sintered catalyst was determined by XRD after interruption of the reac tion at different temperatures. The temperatures at which a 20% conver sion was obtained on the fresh pre-oxidised catalyst were: CO, 210 deg rees C < C3H6, 260 degrees C < C3H8, 300 degrees C < CH4, 335 degrees C corresponding to an activity (in mmol h(-1) g(-1)) at 300 degrees C of: CO, 403 > C3H6, 66 > C3H8, 15.3 > CH4, 7.9. Pre-reduction did not change the catalyst activity in CO and in C3H6 oxidation while it decr eased the activity by a factor 2.4 for C3H8 and 3.2 for CH4. Except fo r a definite decrease of activity, the above conclusions remained vali d for the sintered catalyst. XRD measurements showed that PdO initiall y present in the pre-oxidised sample was reduced before the oxidation of CO (< 180 degrees C) and C3H6 (< 210 degrees C) started, which expl ains why both pre-oxidised and pre-reduced samples had the same activi ty. In the case of C3H8, the reduction occurred during the reaction an d led to a temporary decrease of activity. For CO and C3H8, the Pd deg rees structure was clearly identified while a new structure PdO xi (0 < xi much less than 1) was formed with C3H6 This compound has the cfc structure of Pd degrees with a higher lattice parameter (3.990 Angstro m instead of 3.889 Angstrom in Pd degrees). (C) 1997 Elsevier Science B.V.