Oxidation state of palladium as a factor controlling catalytic activity ofPd/SiO2-Al2O3 in propane combustion

The oxidation state of palladium on SiO2-Al2 O-3 used for propane combustio n was examined by XPS and XRD, and the correlation of the catalytic activit y with the oxidation state of palladium was systematically studied. The pro pane conversion over 5 wt% Pd/SiO2-Al2O3 was measured in the range I.0 less than or equal to S less than or equal to 7.2 (S is defined as [[O-2]/5[C3H 8] based on stoichiometric ratio). The propane conversion strongly depended on the S value and reached the maximum at S=5.5. The oxidation state of pa lladium also changed with the S value; palladium particles were more oxidiz ed under the reaction mixture of higher S value. On the sample used for the reaction at S=5.5, both of metallic palladium. and palliadium oxide were f ound. It is concluded that partially oxidized palladium which has optimum r atio of metallic palladium to palladium oxide shows the highest catalytic a ctivity in propane combustion. (C) 1998 Elsevier Science B.V. All rights re served.