Propane
hydrogenolysis on Pt catalysts did not show serious deactivation over periods of 8 h below 630 K, but
Arrhenius analyses were complicated. Selectivities to methane (S1) and ethane (S2) remained constant at 1.00 ± 0.02,
with S1/S2 just above 1, when less than 10 mmol propane were converted per g catalyst per hour. However,
S1/S2 rose as CH4 was formed more selectively at higher conversions as the temperature increased. Thus
at lower temperatures propane hydrogenolysis occurred by single C–C bond rupture, before switching to multiple
bond rupture at higher temperatures. Transient analysis on Pt/SiO2 using alternating H2 and C3H8 pulses, similar
in size to the adsorption capacity of the catalyst, enabled study of the surface at close to its working coverages of
(θH) and H-deficient hydrocarbon species, such as
m
n (θCmHn). C3H8 pulses at 573–773
K (to a catalytic surface that had just seen hydrogen) resulted in H2
emergence as a result of dehydrogenation
rather than displacement. H2 pulses under the same conditions (onto the silica-supported Pt that had just seen C3H8)
released C3H6 and C2H4 from previously accumulated surface
m
n on the Pt, even though these
are less favourable products than alkanes. Such observations may suggest a relationship between catalysed hydrogenolysis and dehydrogenation and a potential for catalytic fine-tuning through reaction coupling. Pulse
work shows that hydrogenolysis activity at intermediate temperatures can involve multiple C–C scission initially,
but that the build-up of carbonaceous deposits deactivates such sites only leaving those able to support
single C–C bond rupture in propane. Such maturation effects on hydrogenolysis selectivity need
to be better
understood by a combination
of pulse and steady-state experiments.
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