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 analyzed by XPS, UPS, and AES after carrying out n-hexane reactions. The analysis has been repeated after O2-H2 regeneration in the preparation chamber of the UHV apparatus. The surface of the blacks contains some carbon and oxygen impurities even after regeneration. Both Pt-473 and Pt-633 show high Pt 4f line intensity. Decomposition of the C 1s line reveals a higher amount of oxidized carbon polymers on Pt-633. This sample is free from lattice strain. The likely higher abundance of exposed hexagonal symmetry faces, namely the (111) plane and analogous stepped and kinked structures, may be one of the reasons why fragmentation and aromatization are favored on Pt-633, as opposed to the higher selectivity of isomerization and C5-cyclization on Pt-473. The higher amount of oxidized carbonaceous polymer overlayer (serving as "hydrogen catcher") on Pt-633 as well as the less-retained hydrogen by its crystallites without lattice strain may also contribute to the different selectivity and hydrogen pressure response of the n-hexane reaction over the two samples. Product ratios are suggested as an additional diagnostic tool for characterizing the hydrogen availability on the catalysts