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

Citation
Z. Paal et al., Surface and bulk structural response of Pt black upon its hydrogen treatment and catalytic reaction with n-hexane, PHYS CHEM P, 3(11), 2001, pp. 2148-2155
Citations number
61
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
ISSN journal
1463-9076 → ACNP
Volume
3
Issue
11
Year of publication
2001
Pages
2148 - 2155
Database
ISI
SICI code
1463-9076(2001)3:11<2148:SABSRO>2.0.ZU;2-6
Abstract
Pt black has been studied by X-ray and ultraviolet photoelectron spectrosco pies (XPS, UPS) and XRD after reduction, after presintering and after subje ction to various in situ treatments. All samples contained carbon and oxyge n impurities. Sintering decreased the abundance of C. Samples introduced fr om 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 conside rable amounts of nondissociatively chemisorbed CO after this treatment. An in situ hydrogen treatment at 603 K decreased the O content further but inc reased the concentration of surface carbon. The Pt 4f regions showed some o xidized Pt both before and after sintering. Pt reached an almost clean meta llic state after UHV treatment. This state seemed to remain unchanged after further manipulations. Hardly any electronic interaction could thus be obs erved between Pt and its main impurity: C, which was present mostly as grap hite and CxHy 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 H-2 treatments, inducing recrystalli zation 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 differ ent: abundant (220) and (311) reflections promoting isomerisation, C-5-cycl isation and also hydrogenolysis. Carbon accumulation decreased the "intrins ic'' activity of the Pt fraction detected by XPS. Selectivities, in turn, w ere governed by the crystallite structure as well as by the presence of com posite platinum-carbon sites. "Disordered'' surface carbon species poisoned all skeletal reactions and favoured dehydrogenation to hexenes.