Surface composition of Pt-Rh alloys : the role of lattice vibrational entropy

The bulk and surface composition of a polycrystalline Pt-Rh alloy, equilibrated at 1000 K., have been analysed by use of Auger electron spectroscopy. The results are discussed in relation to experimental data in the literature. Although the enthalpy of sublimation and the atomic diameters of Pt and Rh do not differ very much, a significant enrichment of Pt in the surface is observed. It is shown that the surface segregation behaviour in atomically clean Pt-Rh alloys can be understood quantitatively by taking into account the difference in vibrational entropy of bulk and surface atoms of the constituent metal

Influence of phosphate on the structure of sulfided alumina supported cobalt-molybdenum catalysts

The influence of phosphate on the sulfiding rate of alumina-supported cobalt, molybdenum and cobalt-molybdenum catalysts has been studied by means of Temperature Programmed Sulfiding (TPS). Combination of the TPS results with recently published Temperature Programmed Reduction (TPR), Mössbauer spectrometry and nitrogen oxide adsorption measurements results in a detailed picture of the sulfidic catalyst. The sulfiding of cobalt ions in phosphorus-containing alumina-supported cobalt catalysts shifted to somewhat higher temperatures with increasing phosphorus content, most likely due to the presence of Co-AlPO4 phases. In the temperature region from 1000-1270 K, part of the cobalt sulfide species reacted with AlPO4 to cobalt phosphides. The amount of cobalt which reduced to phosphides increased strongly with increasing phosphorus content and became about 70% of the total amount of cobalt present. The sulfiding pattern of phosphorus-containing Co-Mo/Al catalysts is only slightly influenced by increasing phosphorus content below 1000 K. All components including the Co-Mo-O-P phase, present in the oxidic precursor, are relatively easy to sulfide. After sulfiding at 673 K, the Co-Mo-S phase is most likely still to be associated with the AlPO4 species. Analogous to cobalt catalysts, the major part of the cobalt in Co-Mo-P/Al catalysts ultimately reacted to cobalt phosphides above about 1000 K. The sulfiding rate of the molybdenum ions is not changed by the presence of phosphates. MoS2 does not react with the phosphates to molybdenum phosphide probably because this reaction is thermodynamically restricted. Based on the surface structures, deduced from a combination of TPR and TPS data, the striking difference between the effect of phosphates on the hydrodenitrogenation (HDN) and on the hydrodesulphurisation (HDS) reactions is discussed

Interaction of NO with Pt-Rh alloy single crystal surfaces

The clean surfaces of Pt0.25-Rh0.75 alloy single crystals showed Pt surface enrichment that increased with increasing equilibration temp. The activity of the alloy for NO dissocn. is dependent on both the surface structure and the surface compn. (as varied by a proper choice of the equilibration temp.). The NO dissocn. activity was in the following order: Rh-rich (410) ³ Rh-rich (210) ³ Pt-rich (410) > Rh-rich (321) > Rh-rich (111) > Pt-rich (210) > Pt-rich (321) > Pt-rich (111