PT-BLACK CATALYSTS SINTERED AT DIFFERENT TEMPERATURES - SURFACE-ANALYSIS AND ACTIVITY IN REACTIONS OF N-HEXANE

Pt-black catalysts sintered at 473 and 633 K (''Pt-473'' and ''Pt-633' '), respectively, have been characterized by X-ray diffraction and ana lyzed by XPS, UPS, and AES after carrying out n-hexane reactions. The analysis has been repeated after O-2-H-2 regeneration in the preparati on chamber of the UHV apparatus. The surface of the blacks contains so me carbon and oxygen impurities even after regeneration. Both Pt-473 a nd Pt-633 show high Pt 4f line intensity. Decomposition of the C 1s li ne reveals a higher amount of oxidized carbon polymers on Pt-633. This sample is free from lattice strain. The likely higher abundance of ex posed hexagonal symmetry faces, namely the (111) plane and analogous s tepped and kinked structures, may be one of the reasons why fragmentat ion and aromatization are favored on Pt-633, as opposed to the higher selectivity of isomerization and C-5-cyclization on Pt-473. The higher amount of oxidized carbonaceous polymer overlayer (serving as ''hydro gen catcher'') on Pt-633 as well as the less-retained hydrogen by its crystallites without lattice strain may also contribute to the differe nt selectivity and hydrogen pressure response of the n-hexane reaction over the two samples. Product ratios are suggested as an additional d iagnostic tool for characterizing the hydrogen availability on the cat alysts. (C) 1995 Academic Press, Inc.