Redox properties of Pd/NbOx/Al2O3 catalysts

PdMb(2)O(5)/Al2O3; catalysts were prepared with several niobia loadings. XP S was used for determination of the monolayer coverage formation on Pd/Nb2O 5/Al2O3 samples. The spectra suggest that monolayer coverage occurred betwe en 10 and 20 wt% of Nb2O5. Temperature-programmed surface reaction (TPSR) m easurements showed three distinct reactions for the oxidation of propane on palladium surface: complete oxidation, steam reforming and propane hydroge nolysis. Niobia addition inhibited the last two reactions and this was ascr ibed to the ensemble effect produced by NbOx reduced species on palladium. Even the propane oxidation has begun later on Pd/Nb2O5/ Al2O3 catalysts, bu t this was related to the Pd-0 surface instead of Pd2+ observed on Pd/Al2O3 . Infrared spectroscopy of adsorbed carbon monoxide was used to investigate the palladium surfaces before and after propane reaction. Immediately afte r reduction by Hz, the spectra showed the linear and bridge adsorbed CO spe cies on the Pd-0 surface. After reaction, one could observe a new band at 2 130 cm(-1) related to CO adsorption on Pd2+ species. This oxidized site was more intense on Pd/Al2O3. It occurred with lower intensity on Pd/5%Nb2O5/A l2O3 and could not be observed on Pd/20%Nb2O5/Al2O3 catalyst. Carbon monoxi de species adsorbed on Pd-0 were also observed in all samples after reactio n. Our results suggest that niobia addition promoted different surface rati os of Pd-0/PdO during the propane oxidation. Thus, this result could explai n the decrease in the combustion reaction rate as opposed to morphological, poisoning or support effects.