Abstract
Two kinds of temperature-programmed technology, temperature-programmed reduction (TPR) and temperature-programmed desorption(TPD), were used as exploratory techniques to characterize theproperties of metallic catalysts. Samples of platinum metalcrystallites (Pd, Pt and Rh) finely dispersed on alumina, silicaand silica-aluminate were prepared by impregnating Al2O3, SiO2or a series of SiO2-Al2O3 with H2PdCl4, PtCl4 or RhCl3solutions. Reduced samples were oxidized over a wide temperaturerange to examine the variation of metal oxides with theoxidation temperatures (To). Oxide species formed arecharacterized according to their reduction temperature (Tr) andH2 consumption found in the TPR experiments. Besides, chemicalkinetics of oxygen desorption from metal-oxide species were alsostudied from experimental and literature TPD spectra. In TPRstudies, five kinds of oxidation products formed on Rh/Al2O3, i.e., oygen-chemisorbed rhodium (RhOc), surface rhodium oxide(RhsO), bulk rhodium-oxides (RhOx), interacting with support(RhiOx) and rhodium aluminate [Rh(AlO2)y]. The rhodium atoms inthe subsurface Rh(AlO2)y structure would segregate to thesurface of alumina upon reduction at Tr above 750 C. For Pt/Al2O3, four PtOx species, i.e., PtOc, PtO, PtO2 and platinumaluminate are formed as raising To. The optimized To for theformation of each PtOx species varied with dispersion of Pt andduration of oxdiation. Both PtO and PtO2 species are verified toco-exist on SiO2 support according XPS, TPR and ICP-MSexperiments. A Graphic Analysis method (GA method) is proposedto determin all the kinetic parameters, i.e., reaction order(n), activation energy (Ed) and pre-exponential factor (v). Thedesorption order of O2 from Pt/Al2O3 was found to vary with thespecies of platinum oxide : n=2 for PtO species, but n=1 forPtO2. On Pd/Al2O3 samples, the O2 desorption behaves in a secondorder kinetics. An obvious size-dependent relation of metalparticles (or dispersion of metal, D) is obtained for theactivation energy evolved upon O2 desorption from Pd/Al2O3according to :Ed=175+174D kJ mol-1. The chemical kinetics ofoxygen desorption from the surface of Pd crystallites were alsopursued. The desoprtion order was determined to be n=2 wheninitial coverage was less 0.04 monolayer but generally shiftedto n=1 on increasing the coverage to 0.19. As coverage largethan 0.40, the migration of adsorbed oxide ions into subsurfaceof palladium induced a surface reconstruction and a formation ofPd-O payer. Desorption of O2 from these oxide-layer structureappears a second order and keeps a constant Ed value around 165kJ mol-1. The variation in the v values with the Ed can beinterpreted by "compensation effect". The GA method is a directand time-saving technique, and would be a convenient method foranalyzing kinetic parameters for simple desorption processes.