PRETREATMENT EFFECT ON THE CATALYST ACTIVITY AND ON THE ENTHALPY CHANGES DURING EXPOSURE OF PD/SNO2 AND PD METAL TO CO, O-2, AND CO+O-2

The paper reports on the adsorption and the enthalpy changes observed during exposure of a Pd(2%)/SnO2 catalyst and polycrystalline Pd powder to CO, O-2 and CO + O-2 (2:1) at different temperatures in the 300-470 K range and as a function of an oxidizing (ox) or a reducing (red) pretreatment given to the samples. A larger fraction of CO was adsorbed/reacted as compared to that of O-2 when a Pd/SnO2(ox) sample was exposed to successive CO + O-2 (2:1) pulses, The fraction of adsorbed O-2 and the yield of CO2 increased progressively with the rise in sample temperature, On the other hand, a negligible amount of CO and mainly O-2 was adsorbed/reacted on a Pd/SnO2(red) sample. Even though the amount of adsorbed O-2 increased, no measurable CO2 was formed with the rise in sample temperature. On the average, about 205, 360, 420, and 448 kJ of heat was evolved per mol of CO +/- O-2 adsorbed/reacted over Pd/SnO2(red) at the respective temperatures of 300, 370, 420, acid 470 K, the corresponding values in the case of the Pd/SnO2(ox) sample being around 75, 125, 150, and 170 kJ mol(-1). In the case of Pd metal, the adsorption behaviour was almost independent of pretreatment and the fraction of CO or O-2 adsorbed from a CO + O-2 pulse depended on the catalyst temperature. The heat evolved per mol of CO + O-2 was found to be 171 +/- 3 kJ mol(-1) irrespective of the sample temperature. Similarly, the adsorption and the enthalpy changes during CO pulse exposures and during subsequent O-2 dozing depended on the surface species present at the time of interaction. The data provide evidence for the important role played by the lattice oxygen of support in addition to the oxygen chemisorbed at Pd sites. Whereas the CO was found to react in both the gaseous and the adsorbed forms, the oxygen participated in CO oxidation only in the adsorbed state. The thermochemical data indicate that the CO and O-2 molecules compete for metal sites and the preponderance of one of the reactions, namely CO(g) + O(ad) --> CO2(g), CO(ad) + O(ad) --> CO2(ad), or CO(g) + O(lattice) --> CO2(g) is decided by the temperature and the chemical nature of the catalyst surface at the time of interaction. The effect of H-2 pretreatment on the adsorption characteristics and also on the observed CO oxidation activity of Pd/SnO2 is attributed to the formation of Pd-O-Sn moieties at the catalyst surface. (C) 1995 Academic Press, Inc.