456
neqv, mmol
KRASNOV et al.
the nitro group into the amine group. Processing the
obtained dependences within linear coordinates con-
firmed the retention of the zero order of the reaction
with respect to hydrogen in all cases.
8
Concentration curves that illustrate the change in
the amounts of all participants during the hydrogena-
tion of CNA over time are presented in Figs. 2 and 3.
The shape of the obtained dependences shows that the
transformation of CNA can proceed in a sequential
network of transformations, in agreement with the
data of [5]. In the case of supported palladium cata-
lysts with low active metal contents compared to skel-
etal nickel, a sharp decrease in the concentration of
CPDA accompanied by a growth in PDA is observed
at high degrees of the conversion of initial CNA. In
contrast, a rise in the concentration of PDA is
observed right from the onset of the reaction in the
case of the 10% Pd/C and especially the 5% Pd/BaSO4
catalysts.
6
4
2
5% Pd/BaSO4 10% PdC
0.8% PdC 0.5% PdC Niskel
τ, s
0
1000
2000
3000
Fig. 1. Curves of the absorption of hydrogen in the hydro-
genation of CNA in an aqueous solution of 2-propanol
over different samples of catalysts; T = 298 K, 1 0.03 g of
CNA, and 1 0.03 g of the catalyst.
RESULTS AND DISCUSSION
According to the dependences presented in Fig. 1
and the calculated values of the rate constants of the
hydrogenation of CNA, the catalysts used in this work
can be ranged as follows with respect to their catalytic
activity: 5% Pd/BaSO4 > 10% Pd/C > Niskel > 0.8%
Pd/C > 0.5% Pd/C. The first two points were not
used in calculating the rate of the transformation of
CNA, since the loss of CNA from the solution at the
onset of the reaction was determined not only by the
interaction with hydrogen but also to a great extent
by the adsorption of the initial compound. The
increase in the observed rate constants of the trans-
formation of the nitro group for the series of sup-
ported palladium catalysts could be due to the rise
in the number of active centers on the surface of the
catalysts in the case of increasing dispersion of
metal particles (DPd) and specific surface area of the
catalyst Ssp (table).
Skeletal nickel has a fundamentally different
structure, so the amount of the active phase in skel-
etal nickel is substantially higher at a relatively lower
surface area, compared to supported palladium cat-
alysts. We should also remember that the presence
of weakly bound molecular and strongly bound
atomic forms of adsorbed hydrogen is characteristic
of skeletal nickel, while the surface of supported
palladium catalysts is more energetically uniform as
a result of the high concentration of dissolved
hydrogen [6].
0.03 mmol. During the experiment, the volumes of
absorbed hydrogen were determined by means of vol-
umetry, while the concentrations of the initial com-
pounds and reaction products, 2-chloro-1,4-phenyl-
enediamine (CPDA) and 1,4-phenylenediamine
(PDA), were calculated using spectrophotometric
data. A LEKISS 2110 UV scanning spectrophotometer
was used to analyze the initial compound and reaction
products. The sensitivity of our method was no lower
than 10–2 mmol.
The values of the initial rates and rate constants of
r 0
the absorption of hydrogen
, the formation of
H2
CPDA and PDA, and the transformation of CPDA at
high degrees of conversion of the initial CNA were
used as the main kinetic characteristics of the reaction
under study. Statistical analysis of the experimental
results showed that the errors in determining the rates
did not exceed 10%. In determining the current con-
centrations of CNA, CPDA, and PDA, the accuracy
was 5%, and in determining the rate constants, it was
15% of the measured quantity. Each experiment was
repeated at least three times in order to obtain conver-
gent results.
The kinetic features of the hydrogenation of CNA
over skeletal nickel and supported palladium catalysts
in an aqueous solution of 2-propanol with the azeo-
tropic composition (х2 = 0.68 molar parts) were stud-
ied. Figure 1 shows the curves of the absorption of
hydrogen over time, which were subsequently used in
calculating the rates of the absorption of hydrogen and
the observed rate constants of the reaction. The
kinetic curves with respect to hydrogen are provided
with allowance for the stoichiometry of the reaction:
3 mol Н2/mol CNA, based on the transformation of
The ratio of the amounts of initial CNA and the
catalyst was selected so as to exclude the nonreversible
oxidation of the surface and minimize the contribu-
tion from the diffusion resistance with respect to
hydrogen to the total rate of the reaction [7]. With
skeletal nickel and low-percentage palladium catalysts
(Fig. 2), the amounts of absorbed hydrogen and
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 91
No. 3
2017