Surface and bulk structural response of Pt black upon its hydrogen treatment and catalytic reaction with n-hexane

Abstract

Pt black has been studied by X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS) and XRD after reduction, after presintering and after subjection to various in situ treatments. All samples contained carbon and oxygen impurities. Sintering decreased the abundance of C. Samples introduced from the air contained a large amount of oxygen whose amount dropped markedly upon keeping the Pt in UHV for several hours. XPS and UPS revealed considerable amounts of nondissociatively chemisorbed CO after this treatment. An in situ hydrogen treatment at 603 K decreased the O content further but increased the concentration of surface carbon. The Pt 4f regions showed some oxidized Pt both before and after sintering. Pt reached an almost clean metallic state after UHV treatment. This state seemed to remain unchanged after further manipulations. Hardly any electronic interaction could thus be observed between Pt and its main impurity: C, which was present mostly as graphite and C_xH_y polymer after treatment with n-hexane plus hydrogen. n-Hexane alone produced mostly “disordered” surface carbon species. The intensity of higher-order X-ray reflections of Pt was suppressed upon sintering. This anisotropy was reversed after in situ H₂ treatments, inducing recrystallization with preferential formation of higher Miller-index planes, (220) and (311). These reflections were again suppressed after exposure to n-hexane. Thus, adsorbate-induced solid-state rearrangement occurred as a result of the interplay of surface and subsurface impurities. The catalytic reactions of n-hexane over Pt black subjected to different pretreatments were different: abundant (220) and (311) reflections promoting isomerisation, C₅-cyclisation and also hydrogenolysis. Carbon accumulation decreased the “intrinsic” activity of the Pt fraction detected by XPS. Selectivities, in turn, were governed by the crystallite structure as well as by the presence of composite platinum–carbon sites. “ Disordered” surface carbon species poisoned all skeletal reactions and favoured dehydrogenation to hexenes.