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212
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Kalenchuk et al.
is an additive value reflecting the changes of the strucꢀ
ture of each benzene ring constructing the PAH molꢀ
ecule and the surface geometry. With increasing number
of the benzene ring in the hydrogenated substrate,
the substrate adsorption energy increases; however, acꢀ
cording to the above equation for E°ads this increase is not
linear, which correlates with the order of the substrate
reactivity.
The enthalpy of the reaction can serve as an objective
quantitative estimate of the difference of the reactivity of
the terphenyl isomers to hydrogenation. Since the stoiꢀ
chiometry of the reaction is the same for all three terpheꢀ
nyl isomers, the differences in the heat effects are mainly
predetermined by the standard molar enthalpies of forꢀ
mation of the starting terphenyl isomers and reflect
the structural differences between isomers. From Table 1,
it is clearly seen that the standard molar enthalpies of
formation of pꢀ and mꢀterphenyls are close to each other
but noticeably different from the value known for oꢀterꢀ
phenyl. A comparison of the thermodynamic values
with the experimental data indicates that the enthalpies
of formation simbatically increase with increasing inꢀ
tramolecular interaction between benzene rings of all three
isomers.
In summary, the directions of hydrogenation of all
studied substrates until formation of completely saturatꢀ
ed products were experimentally investigated. For the
studied substrates, the relationships reflecting the strucꢀ
tural effects on activity in hydrogen consumption were
determined. It was found that the rate of hydrogenation
decreases on going from benzene to terphenyl and with
increasing the degree of substrate hydrogenation. The rate
of hydrogenation of the terphenyl isomers decreases in
the order pꢀterphenyl > mꢀterphenyl > oꢀterphenyl. No
cracking products were found in all experiments when
examining analytical samples. This is due apparently to
the use of activated carbon support lacking acidic centers
on the surface. The revealed relationship between the
structures of the studied substrates and their reactivity
towards hydrogenation can be of use in designing the
efficient catalyst for hydrogenation.
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This work was financially supported by the Russian
Science Foundation (Project No. 14ꢀ50ꢀ00126).
Received December 21, 2016;
in revised form April 6, 2017