SUPPORTED PALLADIUM NANOMATERIALS AS CATALYSTS
109
involves the reactions of disproportionation of Pd(II)
compounds and Pd(0) to give Pd(I) compounds.
It was found that the scheme given in Table 1
describes well the kinetic features of the Pd(II) diaceꢀ
tate reaction with hydrogen on silica gel. The rootꢀ
meanꢀsquare deviations of the calculated from the
experimental data are 4.5 and 5.8% for 30 and 50°C,
respectively. Figures 1 and 2 illustrate good fit of these
data.
An identifiability analysis of the estimates obtained
showed that only the constant of the step limiting the
reaction rate during the initial period of time, i.e., the
trimer dissociation (step 1), is unambiguously deterꢀ
mined for both the carbon support and silica gel. The
other constants given in Table 1 appear in “comꢀ
plexes” of estimates of quite complicated character
and, therefore, cannot be determined unambiguously
[4].
Y
, %
80
70
60
50
40
30
20
10
9.1% Pd
9.1% Pd + treatment
0
40
80
120
160 200
140 180
20
60
100
Time, min
Fig. 3. Reaction kinetics at 30°C and 1 atm of H : the
2
lower (a) and (b) upper curves respectively refer to the
usual experimental procedure and the reduction with
hydrogen after preliminary heating of the sample at 90°C
for 3 h in a nitrogen flow.
The calculated activation energy for step 1 is 56
16 kJ/mol; its rate increases about fourfold with an
increase in temperature by 20°C.
related to the effect of the support surface on the therꢀ
modynamic and kinetic parameters of trimer dissociꢀ
ation should be expected on silica gel. Here one can
see some analogy with the effect of solvent on the reacꢀ
tivity of metal complex compounds including Pd(I)
compounds [14].
The data obtained reveal a number of differences in
the mechanism of Pd(II) diacetate reduction with
hydrogen between silica gel and the carbon support.
First of all, the kinetic data show that the first step of
the process (dissociation of the Pd(II) trimer) proꢀ
ceeds more slowly on silica gel than on the carbon supꢀ
port and does not reach equilibrium (Table 1). It
seemed reasonable to confirm the conclusion
obtained experimentally from the kinetic model that
this step determines the rate of the initial period of the
reaction on silica gel. For this purpose, the following
experiment was conducted. We attempted to accelerꢀ
ate step 1 and to “accumulate” the dimer and monoꢀ
mer forms of Pd acetate on the support by the sample
To explain the difference in Pd(II) diacetate interꢀ
action with hydrogen on silica gel and carbon, we perꢀ
formed quantumꢀchemical simulation of the interacꢀ
tion of the precursor with these supports and of the
effect of the supports on the Pd–O bond distances,
which play a key role in the reaction of hydrogen with
coordination compounds [15].
The results of the simulation showed the following.
The energies of interaction of different Pd(II) acetate
species (monomer, dimer, and trimer) with the carbon
support surface are low (3–5 kcal/mol), and their
structures slightly differ from those not bound with the
support. Unlike the case of carbon support, the interꢀ
action energy of the Pd(II) diacetate trimer with the
silica gel surface is 14.5 kcal/mol, promoting its higher
thermodynamic stability. It is likely for this reason that
the Pd(II) diacetate trimer is less prone to degradation
into the dimer and monomer on silica gel than on the
carbon support. This assumption is consistent with the
kinetic observation that the trimer dissociation step
(step 1) is slower on silica gel compared with the carꢀ
bon support (Table 1).
(9.1% Pd) heating for 3 h at 90
Then, nitrogen was replaced by hydrogen and Pd diacꢀ
etate was reduced at 30 . As expected, the rate of
°C in a nitrogen flow.
°
C
precursor reduction considerably increased (Fig. 3).
The kinetic data also show that one of the main difꢀ
ferences of carbon from silica gel as a support is that
the monomer form is more reactive to hydrogen on the
former and the dimer of Pd(II) diacetate is more reacꢀ
tive on the latter. Furthermore, noncatalytic (with
molecular hydrogen) and catalytic (with palladium
hydrides) reduction of Pd(I) compounds is more
active on silica gel. Note that among various palladium
acetate species, the Pd(II) diacetate monomer is
reduced most slowly on silica gel, unlike the Pd(I)
acetate dimer on carbon.
According to the kinetic data, the trimer itself is
It is likely that difference in the reactivity of pallaꢀ slightly reactive toward hydrogen. This is presumably
dium acetates toward hydrogen on silica gel and carꢀ due to the fact that the terminal acetate groups effecꢀ
bon is due to the nature of the silica gel surface made tively screen the Pd atoms from hydrogen attack
by irregular packed silicon–oxygen tetrahedra, whose because of the symmetric structure of the trimer [10],
surface layer contains hydroxyls [13]. Therefore, and the reaction of interest mainly proceeds via the
stronger interaction of Pd(II) diacetate with the surꢀ step of its dissociation. Quantumꢀchemical simulation
face; a change in geometrical parameters of its differꢀ also allows us to speculate why the Pd(II) diacetate
ent forms; and, as consequence, another reactivity dimer preferably reacts with hydrogen on silica gel and
PETROLEUM CHEMISTRY Vol. 54
No. 2
2014