REACTIVITY OF UNSATURATED LINEAR C6 HYDROCARBONS
251
hydrogenation to 1,3-hexadiene-h2, mass 83 is from hydro- Hexatriene adsorbed on the hydrogen-saturated surface
genation/deuteration to 1,3-hexadiene-h1d1 and mass 84 is produced hexadiene and hexene. Selective hydrogenation
from deuteration to 1,3-hexadiene-d2. It is also possible that of the hexadienes and hexatriene is clearly related to the
contributions to masses 83 and 84 could come from ex- interaction of the additional double bonds with the sur-
change products from 1,3-hexadiene-h2 and -h1d1. Masses face which allows insertion of hydrogen into the half-
observed equal to 86 and greater are the result of hydro- hydrogenated intermediate produced.
genation and deuteration of hexadiene to hexene, similar
to the reaction scheme proposed for 1,3-hexadiene, and ex-
change products of hexene.
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From the results of this work, it is clear that 1,5-hexa-
diene, 1,3-hexadiene, and 1,3,5-hexatriene can be hydro-
genated selectively to 1-hexene. Also, the conjugated mole-
cules, 1,3-hexadiene and 1,3,5-hexatriene are more easily
hydrogenated than are the isolated double bonds found in
1,5-hexadiene. However, to hydrogenate the C6 hydrocar-
bons to hexane, the intermediate species (alkyl) must go
through reductive elimination. The strength of the bonding
of the alkyl species or the adsorbed hydrogen atom to
the surface or both may influence the rate of this step.
Alkenes adsorbed on Pd(100) also do not hydrogenate
to the alkane under similar conditions (6), though on Pt
(2) and Rh (3) hydrogenation to the alkane is observed.
The origin of the difference between these metals and Pd
has been attributed to the strength of the metal–hydrogen
bond because a distinctly different reaction mechanism is
not likely (6). The occurrence of hydrogenation follows
the trend in the H–metal binding energies.
V. SUMMARY
The adsorption and reactivityof1-hexene, 1,5-hexadiene,
1,3-hexadiene and 1,3,5-hexatriene on clean Pd(111) and
H(D)/Pd(111) were investigated using TPRS. The low-
temperature adsorption configuration for the linear C6 hy-
drocarbons appears to be a weak ꢀ-bonded species. Upon
heating, a fraction of the ꢀ-bonded species bonds more
tightly with the surface and can either desorb molecularly
at higher temperatures through hydride elimination of a
half-hydrogenated intermediate, dehydrogenate, dehydro-
cyclize, or selectively hydrogenate. Dehydrogenation to ad-
sorbed carbon and hydrogen initiates through the loss of hy-
drogen from the carbon backbone of the adsorbed species
at ꢂ400 K. Above 400 K, fragmentation of the adsorbed
species and dehydrogenation of these fragments occurs.
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Hydrogenation to hexane was not observed for any of
the adsorbed molecules. This lack of alkane formation is
–
attributed to the strength of the H Pd bond, which inhibits
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1,5-hexadiene adsorbed on hydrogen-saturated Pd(111).
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