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
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.