REACTIVITY OF STEAM IN EXHAUST-GAS CATALYSIS .3. STEAM AND OXYGEN STEAM CONVERSIONS OF PROPANE ON A PD/AL2O3 CATALYST/

A 1% Pd catalyst (38% dispersion) was prepared by impregnating a gamma -alumina with palladium acetylacetonate dissolved in acetone, The beha viour of this catalyst in oxidation and steam reforming (SR) of propan e was investigated, Temperature-programmed reactions of C3H8 with O-2 or with O-2 + H2O were carried out with different stoichiometric ratio s S (S = [O-2]/5 [C3H8]). The conversion profiles of C3H8 for the reac tion carried out in substoichiometry of O-2 (S < 1) showed two discret e domains of conversion: oxidation at temperatures below 350 degrees C and SR at temperatures above 350 degrees C. The presence of steam in the inlet gases is not necessary for SR to occur: there is sufficient water produced in the oxidation to form H-2 and carbon oxides by this reaction, Contrary to what was observed with Pt an apparent deactivati on between 310 and 385 degrees C could be observed with Pd in oxidatio n. This is due to a reduction of PdOx into Pd-0, which is much less ac tive than the oxide in propane oxidation. Steam added to the reactants inhibits oxidation while it prevents the reduction of PdOx into Pd-0. Compared to Pt and to Rh, Pd has a higher thermal resistance: no deac tivation occurred after treatment up to 700 degrees C and limited deac tivation after treatment up to 900 degrees C, provided that the cataly st is maintained in an oxygen-rich atmosphere during the cooling.