KINETICS OF THE HYDROGENATION
1775
spectrophotometric analyses and are given in the kinetic regularities of the hydrogenation of individual
table. As follows from the data in the table, before the oxygen-containing compounds on the skeletal nickel
maximum content of A is attained in the solution vol- catalyst in [12]. As the process finishes, the actual
ume, the amount of hydrogen absorbed from the gas amount of hydrogen absorbed in the reaction is even
phase ( abs) is smaller than the one necessary for the
somewhat greater than the one necessary for the trans-
n
H
2
formation of NB and AB. The overabsorption of
hydrogen is due to the saturation of the catalyst surface
as a result of its removal in the initial reaction phase,
and possibly to the reduction of the partially oxidized
surface catalyst.
The close values of the analytically determined
amounts of A in the phase volume and the values cal-
culated from the reduction in NB and AB (see table)
indicate that at the chosen ratio of the amounts of cat-
alyst NB and AB, the latter are selectively reduced to
A, there is no accumulation of intermediate products
in the phase volume, and the irreversible oxidation of
the catalyst surface is prevented.
conversion of NB and AB to A ( nec).
n
H
2
nec
abs
The maximum divergence of these values (
–
)
n
n
H
H
2
2
corresponds to the initial moment in time. This differ-
ence diminishes rapidly upon moving from negative to
positive values in attaining the complete conversion of
NB and at the conversion of AB higher than 0.5.
As follows from the data in Fig. 3, the time depen-
dence of the amount of A passes through a maximum.
A drop in the amount of A at high conversions was also
observed for the hydrogenation of phenylhydroxyl-
amine [10] and azobenzene [11]. We may assume that
the adsorbabilities of NB, AB, phenylhydroxylamine,
and azobenzene are higher than those of A. The recov-
ery of hydrogen on the surface begins simultaneously
with a reduction in the amount of A in the solution
volume (table). This could be experimental proof that
hydrogen and organic compounds are adsorbed inde-
pendently of each other and do not compete for active
sites on the catalyst’s surface.
The drop in the amount of A from the solution to
the completion of the reaction was 0.89 mmol or
1.78 mmol/g. It was established from the data of
adsorption measurements and processing the obtained
isotherms in different model approximations that the
experimental data is best described by the Dubinin–
Radushkevich equation [10]. The authors conclude
that the adsorption of A on skeletal nickel in aqueous
solutions of propan-2-ol can be described using the
theory of the volume filling of micropores (TVFM),
and the amounts adsorbed of A ranged from 0.45
0.05 to 12.7 0.10 mmol/(g of catalyst). The data from
our kinetic and adsorption measurements are consis-
tent and do not contradict the literature data.
Prior to the moment when the maximum amount
of A is attained in the solution volume, the hydrogen
uptake in the reaction is lower than the theoretically
necessary amount, even if we allow for the adsorption
of the hydrogenated compound (see table). The exces-
sive amounts of adsorbed NB and AB in an aqueous
azeotropic solution of propan-2-ol on skeletal nickel
at the chosen amount of the initial NB cannot exceed
0.40 0.05 mmol/(g of catalyst) [8]. We may therefore
assume that the azoxy and especially nitro groups are
reduced under hydrogen deficient conditions in the
initial reaction phase at high degrees of filling of the
porous space of the catalyst with an organic com-
pound. This was mentioned earlier when analyzing the
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Translated by E. Yablonskaya
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 89 No. 10 2015