Synthesis, properties, and activity of nanosized palladium catalysts modified with elemental phosphorus for hydrogenation

Article abstract of DOI:10.1134/S0023158410010088

The applicability of elemental phosphorus as a modifier of palladium catalysts for hydrogenation was demonstrated, and the conditions for the synthesis of nanoparticles that are highly efficient in hydrogenation catalysis were optimized. The modifying eff

Full text of DOI:10.1134/S0023158410010088

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 1, pp. 42–49. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © L.B. Belykh, N.I. Skripov, L.N. Belonogova, V.A. Umanets, F.K. Schmidt, 2010, published in Kinetika i Kataliz, 2010, Vol. 51, No. 1, pp. 47–55.
Synthesis, Properties, and Activity of Nanosized Palladium Catalysts
Modified with Elemental Phosphorus for Hydrogenation
L. B. Belykh, N. I. Skripov, L. N. Belonogova, V. A. Umanets, and F. K. Schmidt
Irkutsk State University, Irkutsk, 664033 Russia
eꢀmail: belykh@chem.isu.ru
Received July 10, 2008
Abstract—The applicability of elemental phosphorus as a modifier of palladium catalysts for hydrogenation
was demonstrated, and the conditions for the synthesis of nanoparticles that are highly efficient in hydrogeꢀ
nation catalysis were optimized. The modifying effect of elemental phosphorus depends on the P/Pd ratio; it
is associated with changes in the catalyst dispersity and the nature of the formed nanoparticles containing varꢀ
ious palladium phosphides (PdP2, Pd5P2, and Pd6P) and Pd(0) clusters. The main stages of the formation of
palladium catalysts for hydrogenation were determined, and a model of an active catalyst, in which the Pd6P
phosphide is the core of a nanoparticle and Pd(0) clusters form a shell, was proposed.
DOI: 10.1134/S0023158410010088
INTRODUCTION
EXPERIMENTAL
Reagents
White phosphorus is the most reactive species of eleꢀ
mental phosphorus; it is a starting material for the synꢀ
thesis of various organophosphorus compounds [1, 2].
Many studies devoted to the coordination chemistry of
white phosphorus have been reported in the last few
years [3–8]; the molecule of white phosphorus can
The solvents (benzene and
N,Nꢀdimethylformaꢀ
mide (DMF)) were purified by standard procedures
used in operations with organometallic compounds
[14]. For drying and removing amine impurities,
DMF was kept over anhydrous copper sulfate until the
formation of a green solution and twice subjected to
vacuum distillation (8 Torr) at a temperature of no
higher than 42°С. For deeply drying, benzene was
additionally distilled from LiAlH4 on a rectification
column and kept in an atmosphere of argon in sealed
ampoules over molecular sieve 4A. The concentrations
serve as a
η
1 or 2 ligand [7]. The activation of the white
η
phosphorus molecule in the coordination sphere of a
transition metal can result in P–P bond cleavage folꢀ
lowed by the recombination of small fragments into
dinuclear complexes and polyatomic aggregates [3–6]
or in hydrolytic oxidation in the presence of water [5, 8].
Interest in the use of elemental phosphorus as a of water in benzene and DMF were 1.1
×
10–3 and
promoter for palladium catalysts for hydrogenation is 0.8 mol/l, respectively, as measured by the Fischer
based on currently available data on the nature of method [15].
nanosized catalysts. The formation of particles cataꢀ
lytically active in hydrogenation reactions based on
palladium complexes with organic phosphines [9–11]
or phosphine (PH3) [12] in an atmosphere of hydroꢀ
gen resulted in the formation of microheterogeneous
systems because of the stepꢀbyꢀstep degradation of
phosphorusꢀcontaining ligands. Depending on the
nature of phosphine and the P/Pd ratio, the resulting
nanosized particles can consist of polynuclear pallaꢀ
dium complexes with phosphide and phosphinidene
ligands or palladium phosphides on which Pd(0) clusꢀ
ters are immobilized. We were the first to demonstrate
[13] that not only organic phosphines or PH3 but also
elemental phosphorus can be used for the synthesis of
highly efficient hydrogenation catalysts.
Bis(acetylacetonato)palladium was prepared in
accordance with a published procedure [16]. Its H
NMR spectrum in benzene contained signals with the
1
chemical shifts
(СН3) = 1.76 ppm (s, 6H).
White phosphorus was mechanically cleaned to
δ(СН) = 5.04 ppm (s, 1H) and
δ
remove surface oxidation products and washed with
anhydrous benzene immediately before use. The soluꢀ
tion of white phosphorus in benzene was prepared and
kept in an inert atmosphere in a fingerꢀtype vessel,
whose design allowed us to evacuate it and fill with
argon. 31P NMR spectrum:
= –522 ppm (s).
δ
Reaction between Bis(acetylacetonato)palladium
and White Phosphorus
In this work, we report the results of a study of the
formation, nature, and properties of palladium cataꢀ
The reaction was performed in an inert atmosphere
lysts for hydrogenation modified with elemental phosꢀ at various ratios between the initial components in a
phorus. fingerꢀtype vessel.
42

SYNTHESIS, PROPERTIES, AND ACTIVITY OF NANOSIZED PALLADIUM CATALYSTS
43
For example, to obtain the ratio P/Pd = 3 (the 400 cm–1, absorption bands were absent from the IR
ratios between the reagents are given in terms of the spectrum. The sample was Xꢀray amorphous. A diffuse
atomic form of phosphorus), 10 ml of a solution of halo appeared in the reflection angle range
2θ = 35°–
white phosphorus (4.5 mmol) in benzene was added 45°; the size of the coherentꢀscattering region was
dropwise to a solution of 0.4566 g (1.5 mmol) of 3.13 nm. To convert the sample into a crystalline state,
Pd(acac)2 in 50 ml of DMF, and the contents were it was thermostated in an inert atmosphere at 400°С
stirred at room temperature for 24 h. After 2–3 min for 4 h. XRD data
upon the onset of reaction, the color of the solution 2.225, 2.210, 2.037, 1.926, 1.868, 1.852, 1.784, 1.573,
changed from yellow to dark brown. Filtrate samples 1.456, 1.419, 1.342, 1.284, 1.244 (Pd6P) [18].
(d/n): 2.993, 2.570, 2.362, 2.259,
Å
were analyzed at regular intervals using spectroscopic
methods (NMR and UV spectroscopy). After compleꢀ
tion of reaction, the solution was concentrated in a
vacuum to 1/5 volume (40°C/1 Torr), and 10 ml of
benzene was added. The resulting dark brown precipiꢀ
tate was filtered off, washed with benzene, and dried in
a vacuum (30°C/1 Torr). The yield was 0.17 g. The
sample was Xꢀray amorphous. The size of the coherꢀ
entꢀscattering region was 1.4 nm, as calculated from
the Selyakov–Scherrer formula. A diffuse halo
The catalyst with the ratio P/Pd = 1.2 was prepared
in an analogous manner. The yield was 0.21 g. The
sample was Xꢀray amorphous. A diffuse halo appeared
in the reflection angle range
the coherentꢀscattering region was 1.96 nm. XRD data
): 3.351, 2.733, 2.525, 2.491, 2.458, 2.289, 2.251,
2.123, 2.091, 2.050, 1.989, 1.930, 1.845, 1.720, 1.681,
1.488, 1.466, 1.434, 1.375, 1.361, 1.341 (Pd5P2)
2 = 35°–45°; the size of
θ
(d/n
Å
[18]; 2.926, 2.884, 2.491, 2.050, 1.993, 1.681, 1.466,
1.361 (PdP2) [17]. The Pd5P2 palladium phosphide
was predominant; the PdP2 phosphide was present in
small amounts.
appeared in the reflection angle range
2 = 35°–45°,
θ
where the hkl (111) reflections of both crystalline palꢀ
ladium and palladium phosphides were observed. To
convert it into a metal state, the sample was thermoꢀ
stated at 400°С in an inert atmosphere for 4 h. XRD
Catalytic Hydrogenation
data
1.681, 1.466, 1.361
2.491, 2.458, 2.289, 2.251, 2.123, 2.091, 2.050, 1.989,
1.930, 1.845, 1.720, 1.681, 1.488, 1.466, 1.434, 1.375,
1.361, 1.341 (Pd5P2) [18].
The experiments at other ratios between the
reagents were performed in an analogous manner.
(
d
/
n
): 2.926, 2.884, 2.733, 2.491, 2.050, 1.993,
The reaction was performed in a thermostated glass
longꢀnecked flask at 30оС and an initial hydrogen
pressure of 1 atm in the presence of an in situ formed
catalytic system. A 1ꢀml portion of a solution of phosꢀ
Å
(PdP2) [17]; 3.351, 2.733, 2.525,
phorus in benzene (0.3
wise to a solution of 0.00304 g (
×
10–5 mol) was added dropꢀ
10–5 mol) of
Å
1
×
Pd(аcac)2 in 9 ml of DMF in the thermostated longꢀ
necked flask in a flow of hydrogen, and the contents
were stirred at room temperature for 5 min. Then, the
temperature was increased to 80°C, and the reaction
mixture was stirred in an atmosphere of hydrogen.
Additional experiments showed that the optimum
Synthesis of a Catalyst Based on Pd(acac)2
and Elemental Phosphorus in a Hydrogen Atmosphere
The catalyst was prepared in a thermostated longꢀ
time of the formation of the Pd(acac)2–nP catalytic
necked flask in an atmosphere of hydrogen at 80°C. A
10–4 mol)
system at 80°С was 25–30 min [13]. The resulting
blackish brown “solution” was cooled to 30°C, and a
substrate was injected into it with a syringe. Hydrogeꢀ
nation was performed under conditions of intense stirꢀ
ring to exclude the occurrence of reaction in the diffuꢀ
sion region. The course of the reaction was monitored
using volumetry and GLC analysis.
In the experiments on catalyst poisoning with merꢀ
cury, 0.03 ml of mercury (Hg/Pd = 200) was introꢀ
duced into the reaction mixture with a syringe 2 min
after the onset of the catalytic hydrogenation of styꢀ
rene.
2ꢀml portion of a solution of phosphorus (5.1
in benzene was added dropwise to a solution of
0.51748 g (1.7
10–3 mol) of Pd(аcac)2 in 60 ml of
×
×
DMF (P/Pd = 0.3). The reaction mixture was stirred
at a hydrogen pressure of 1 atm and a temperature of
25°C for 5 min. After 1 min upon the addition of a
white phosphorus solution, the color changed from
yellow to dark brown. Then, the reaction mixture temꢀ
perature was increased to 80°C, and stirring was conꢀ
tinued for 15 min at an excess hydrogen pressure of
1 atm. According to UVꢀspectroscopic data,
Pd(acac)2 was completely converted in this time.
After completion of the reaction, the blackish
brown solution was cooled to room temperature and
transferred to a fingerꢀtype vessel in an inert atmoꢀ
sphere. Then, the solvent was distilled in a vacuum
Spectroscopic and Electron Microscopic Studies
The UV spectra were measured on a Specord UV
(2/3 volume), and benzene was added until the formaꢀ VIS spectrometer in a quartz cell. The concentrations of
tion of a precipitate. The precipitate was washed three bis(acetylacetonato)palladium and acetylacetone were
times with 10ꢀml portions of benzene in an atmoꢀ calculated from absorption at 330 nm (ε330
=
sphere of argon and dried in a vacuum (50°С/1 Torr). 10630 l mol–1 cm–1) and 290 nm (ε290 = 5000 l mol–1 cm–1
The yield was 0.19 g. Elemental analysis data, %: Pd, for Hacac or ε290 = 3090 l mol–1 cm–1 for Pd(acac)2),
87.88; P, 3.59; C, 0.99; H, 0.25. In the region of 4000– respectively.
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

44
BELYKH et al.
1
The samples were studied by transmission electron
w
, (mol H2)(mol Pd)−1 min−
microscopy (TEM) on a Philips EMꢀ410 microscope.
A drop of an in situ formed catalyst solution was supꢀ
ported onto a support grid coated with a carbon film
and dried in an atmosphere of argon. The analytical
conditions excluded the melting and degradation of
the test samples under the action of an electron beam.
300
250
200
150
100
50
The procedure for the determination of Pd(0) was
described elsewhere [19].
RESULTS AND DISCUSSION
Catalytic Properties of Palladium Catalysts for
Hydrogenation Modified with Elemental Phosphorus
The modifying effect of elemental phosphorus on
the properties of hydrogenation catalysts based on
Pd(acac)2 formed in a hydrogen atmosphere at 80°С
depends on the P/Pd ratio (Fig. 1).
0
0.2
0.4
0.6
0.8
1.0
1.2
P/Pd
The addition of small amounts of elemental phosꢀ
phorus to bis(acetylacetonato)palladium (P/Pd = 0.3
in terms of phosphorus atoms) before the stage of the
reduction of the latter with hydrogen dramatically (by
a factor of >9) increased the catalyst activity in the
reaction of styrene hydrogenation (Fig. 1). The proꢀ
moting effect of elemental phosphorus on the pallaꢀ
dium catalyst with the ratio P/Pd = 0.3 also maniꢀ
fested itself in the hydrogenation of other substrates:
phenylacetylene, benzaldehyde, and nitrobenzene
(Table 1). At the same time, at the ratios P/Pd > 0.75,
the almost complete suppression of hydrogenation
catalytic activity was observed (see Fig. 1).
Fig. 1. Dependence of the activity of the Pd(acac) –P catꢀ
alytic system in the reaction of styrene hydrogenation on
the ratio between reagents. Pd content, 1 mmol/l; [subꢀ
2
strate]/[Pd] = 870; solvent, DMF;
T = 30°С; and PH =
2
1 atm.
1
31
The H and P NMR spectra were obtained on a
VXRꢀ500S Varian pulse spectrometer. The chemical
31
shifts of P were measured with reference to 85%
phosphoric acid. Positive values correspond to a
downfield shift. In the analysis of the samples by NMR
spectroscopy, the solution was sealed in a preevacuꢀ
ated insert ampoule filled with argon.
A comparison between the modifying effects of eleꢀ
mental phosphorus and previously studied modifiꢀ
ers—phenylphosphine [11] and phosphine (РН3
)
The XRD analysis of the catalyst samples was perꢀ
[12]—on the properties of palladium catalysts for
hydrogenation allowed us to reveal the following reguꢀ
larities: the strongest promoting effect of phosphines
formed on a DRONꢀ3M diffractometer (Cu
K radiaꢀ
α
tion).
Table 1. Comparison between the catalytic properties of the Pd(acac)2–0.3P system and Pd black in the hydrogenation reactions
of various substrates
Catalyst activity
w,
Pd black
product
Pd(acac)2–0.3P
(
mol Н2) (mol Pd)–1 min–1
Substrate
content, mmol
Substrate
Pd black* Pd(acac)2–0.3P
yield, %
product
yield, %
Styrene
8.7
9.1
4.9
2.4
9.9
30
68
–
280
157
64
Ethylbenzene
Ethylbenzene
–
100
100
–
Ethylbenzene
Ethylbenzene
Aniline
100
100
71
Phenylacetylene
Nitrobenzene
36**
1
128
15
Aniline
40
16
2
100
95
Benzaldehyde
Benzyl alcohol
Toluene
Benzyl alcohol
Toluene
5
Note: Concentration of Pd, 1 mmol/l; solvent, DMF; DMF volume, 10 ml; temperature, 30°C; and H pressure, 1 atm.
2
* Prepared under analogous conditions by the reduction of Pd(acac) with hydrogen.
** The concentration of Pd was 5 mmol/l.
2
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

SYNTHESIS, PROPERTIES, AND ACTIVITY OF NANOSIZED PALLADIUM CATALYSTS
Productivity,
45
(
mol styrene)/(mol Pd)
12000
10000
8000
6000
4000
2000
0
1
w
, (mol H2)(mol Pd)−1 min−
350
300
250
200
150
100
50
(a)
(b)
0
P4 PH3 PH2Ph PHPh2 PPh3
Pdꢀ
black
Pdꢀ
black
P4
PH3 PH2Ph
Fig. 2. Effect of the nature of the phosphorusꢀcontaining modifier on the (a) activity and (b) productivity of palladium catalysts
in the reaction of styrene hydrogenation.
and phosphorus was reached at the same ratio the lowest oxidation states results in the formation of
P/Pd = 0.3. The test catalytic systems were close to
each other in activity and selectivity (Fig. 2). The use
of elemental phosphorus as a modifier simplified the
formation of a highly active catalyst because the synꢀ
thesis of phosphine ligands became not necessary.
Triphenylphosphines and diphenylphosphines exhibꢀ
ited a much smaller promoting effect that of elemental
phosphorus.
new complexes, whereas redox processes accompaꢀ
nied by the release of metals [2] and metal phosphides
[23] can occur on the interaction of Р4 with transition
metal compounds in high oxidation states.
Analysis performed using 1H NMR, 31P NMR, and
UV spectroscopy suggests that Pd(acac)2 readily reacts
with white phosphorus in an inert atmosphere at room
temperature in a DMF solution to form acetylacetone.
Palladium catalysts modified with elemental phosꢀ
phorus and formed in an atmosphere of molecular
hydrogen are much better in terms of activity than palꢀ
ladium nanoparticles prepared under the action of
other stronger reducing agents: AlEt3 [20], NaH2PO2
[21], and LiAlH4 [22]. Thus, it is reasonable to study
the nature of the catalytic test systems not only theoꢀ
retically but also practically.
1
This is evident from the H NMR spectra, which
exhibited the following chemical shifts , ppm: enol
δ
form, 15.87 (s, 1H, OH), 5.74 (s, 1H, CH), 2.10 (s,
6H, СН3); keto form, 3.82 (s, 1H, СН2), 2.26 (s, 3H,
Concentration of Pd, %
100
80
The Nature of Palladium Catalysts Modified
with Elemental Phosphorus
60
40
20
2
The reduction of Pd(acac) with hydrogen in a
DMF solution is an autocatalytic reaction, which
occurs at a sufficiently high rate at 80°С. Modification
with elemental phosphorus increased the induction
period from 5 to 15–20 min (Fig. 3). This suggests that
palladium atoms formed as a result of hydrogenolysis
do not aggregate into Pdx clusters, which are capable
of activating molecular hydrogen, but react with white
phosphorus or its conversion products.
1
0
10
20
30
40
Time, min
It is well known that white phosphorus, which is a
highly reactive elemental phosphorus species, readily
reacts with transition metal compounds in both low
and high oxidation states [2–8]. The interaction of
white phosphorus with transition metal complexes in
Fig. 3. Kinetic curves of the conversion of (
and ( ) the Pd(acac) –0.3P catalytic system in an atmoꢀ
sphere of hydrogen. Pd content, 1 mmol/l; solvent, DMF;
= 10 ml; 80°С; and PH = 1 atm.
1) Pd(acac)
2
2
2
V
T =
DMF
2
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

46
Concentration, %
BELYKH et al.
The subsequent treatment of the Pd(acac)2–nР
catalytic system (P/Pd = 0.3–1) with hydrogen at
80°С resulted in the quantitative conversion of
Pd(acac)2 to form acetylacetone. According to TEM
data, a disperse system was formed as a result of the
hydrogenolysis of bis(acetylacetonato)palladium in
the presence of elemental phosphorus, and the partiꢀ
cle size of this system depended on the ratio P/Pd. At
P/Pd = 0.3, highly contrast particles 5.2 nm in diamꢀ
eter were predominant in the system; at P/Pd = 1.0, a
more highly dispersed system was formed (Figs. 5, 6).
In both cases, the detected nanoparticles had a much
100
90
80
70
60
50
40
30
20
10
1
2
smaller size than that of Pd black (d = 25–30 nm),
0
1
2
3
4
P/Pd
which was formed under analogous conditions in the
absence of elemental phosphorus [11]. The ratio P/Pd
had no effect on the supramolecular structure: nanoꢀ
particles underwent aggregation to produce a loose
branched structure characteristic of fractal clusters
[25]. The formation of these clusters was related to the
selfꢀorganization (aggregation) of nanoparticles with
close sizes in the absence of stabilizing factors when
the diffusion of nanoparticles was a rateꢀlimiting step.
The nature of the nanoparticles formed depended
on the ratio P/Pd. The black precipitate (sample I)
separated from the Pd(acac)2–0.3P–H2 reaction sysꢀ
tem can be described by the empirical formula
Pd9.1P1C2.2H6.7. The fraction of Pd(0) in the sample
found by chemical analysis was ~27%. Diffraction
maximums due to only the Pd6P phosphide were
detected in a study of sample I by XRD analysis. In our
opinion, the apparent inconsistence between the
results of elemental, chemical, and XRD analyses was
due to the fact that Pd(0), which was present in samꢀ
Fig. 4. Dependence of the concentrations of (
and ( ) acetylacetone on the ratio [P]/[Pd(acac) ] in
DMF (the initial concentration of Pd(acac) was
5 mmol/l).
1) Pd(acac)
2
2
2
2
СН3). Phosphorous acid (Н3РО3) (31Р NMR:
4.42 ppm, JP–H = 531 Hz) and phosphoric acid
31Р NMR:
= 3.35 ppm) were also formed. Special
experiments with the use of D2O showed that water
occurring in solution served as a source of protons in
the formation of acetylacetone; the concentration of
water in DMF was as high as 0.8 mol/l.
The quantitative analysis of 1H NMR and UV specꢀ
tra suggests that the complete conversion of Pd(acac)2
was reached at a threefold excess of white phosphorus
(Fig. 4). At P/Pd < 3, unreacted Pd(acac)2 remained
in the reaction mixture.
The dark brown precipitate separated from the
Pd(acac)2–P reaction system (P/Pd = 3) consisted of
the palladium phosphides PdP2 and Pd5P2, and the
former species was predominant. The ratio between
PdP2, Pd5P2, and unreacted Pd(acac)2 increased as the
concentration of white phosphorus was increased.
The set of the above data suggests that the reaction
of Pd(acac)2 with elemental phosphorus in an inert
atmosphere is the redox process that can be described
by the following reaction scheme:
δ
=
(
δ
ple I along with the palladium phosphide Pd6P
,
occurred in a highly dispersed amorphous state.
Knowing the total palladium and phosphorus concenꢀ
trations, the fraction of palladium in a reduced state,
and the composition of the palladium phosphide, we
can express the averaged composition of sample I by
the formula {Pd6P}1.0{Pd(0)}3.1. It is most likely that
carbon and hydrogen occurred in the sample resulted
from the solvent in which the synthesis was performed,
that is, DMF. Then, the composition of sample I can
be better described by the empirical formula
{Pd6P}1.0{Pd(0)}3.1{DMF}0.6. The following fact should
be noted: PdP2 and Pd5P2 were formed as a result of a
redox process before treatment with hydrogen with the
predominance of PdP2, whereas the composition of
palladium phosphides changed after the interaction of
the components of the Pd(acac)2–0.3P reaction sysꢀ
tem with hydrogen. Consequently, palladium atoms,
3Pd(acac)2 + 2P + 6H2O
= 3Pd(0) + 6Hacac + 2H3PO3.
The subsequent interaction of Pd(0) atoms with
elemental phosphorus leads to the formation of the
palladium phosphides PdP2 and Pd5P2
:
2Pd + P4 = 2PdP2;
PdP2 + 4Pd = Pd5P2.
which resulted from the hydrogenolysis of Pd(acac)2
,
reacted with the PdP2 and Pd5P2 phosphides present in
solution to form the Pd6P phosphide enriched in palꢀ
ladium:
The reaction scheme proposed for the formation of
palladium phosphides in an inert atmosphere is conꢀ
sistent with data published by Henkes et al. [24], who
experimentally found that the synthesis of transition
metal nanophosphides from M(acac)n and P(C8H17)3
at 300–360°С occurs through the step of the formaꢀ
tion of metal nanoparticles.
Pd(acac)2 + H2 = Pd(0) + 2Hacac;
Pd5P2 + 7Pd = 2Pd6P;
PdP2 + 11Pd = 2Pd6P.
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

SYNTHESIS, PROPERTIES, AND ACTIVITY OF NANOSIZED PALLADIUM CATALYSTS
47
(а)
(b)
Fraction of particles, %
18
16
14
12
10
8
6
4
2
0
2.2
3.1
3.9
4.8
5.7
6.5
7.4
8.3
9.2
2.6
3.5
4.4
5.2
6.1
, nm
7.0
7.8
8.7
9.6
50 nm
d
Fig. 5. (a) TEM image and (b) bar diagram of the Pd(acac) –0.3P–H catalytic system.
2
2
(а)
(b)
Fraction of particles, %
25
20
15
10
5
0
1.9
2.9
3.8
4.8
5.7
6.8
7.6
8.6
50 nm
2.4
3.3
4.3
5.2
6.2
7.1
8.1
9.0
d, nm
Fig. 6. (a) TEM image and (b) bar diagram of the Pd(acac) –1.0P–H catalytic system.
2
2
This resulted in an experimentally observed of the Pd6P palladium phosphide and the shell consists
of Pd(0):
increase in the induction period of Pd(acac)2 reducꢀ
tion with hydrogen in the presence of elemental phosꢀ
phorus. After the quantitative conversion of the Pd5P2
and Pd2P phosphides into Pd6P, palladium(0) atoms,
which were formed from Pd(acac)2 once again,
formed Pd(0) clusters as a result of aggregation:
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd6P
.
mPd → Pdm
Because insoluble palladium phosphide particles
were present in the reaction system, the heterogeneous
nucleation of palladium clusters should be considered
the most likely mechanism in which nucleus–shell
Pd
The occurrence of particular species in the reaction
nanoparticles are formed, where the nucleus consists system does not mean that these species are responsiꢀ
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

48
Hydrogen volume, ml
BELYKH et al.
1
r
, mol l−1 min−
0.35
400
350
300
250
200
150
100
50
Hg
0.30
0.25
0.20
0.15
0.10
0.05
2
1
0
2
4
6
8
10
12
Time, min
0
2
4
6
8
Pd concentration, mmol/l
Fig. 7. Kinetic curves of styrene hydrogenation in the presꢀ
ence of the Pd(acac) –0.3P catalytic system: ( ) with no
) with a mercury additive. Pd conꢀ
tent, 1 mmol/l; [substrate]/[Pd] = 1740; solvent, DMF;
= 30°С; and PH = 1 atm.
Fig. 8. Dependence of the rate of styrene hydrogenation (r)
1
2
in the presence of the Pd(аcac) –0.3P catalytic system on
mercury additive and (
2
2
palladium concentration. Substrate content, 8.7 mmol;
solvent, DMF;
Т = 30°С; and PH = 1 atm.
Т
2
2
at Pd(acac) concentrations higher than 3 mmol/l
(Fig. 8) are consistent (together with the above facts)
with the microheterogeneous nature of the catalyst.
ble for catalytic activity. Widegren and Finke [26] proꢀ
posed seven tests for recognizing homogeneous and
nanosized catalysis: the detection of the presence or
absence of an induction period, the filtration and subꢀ
sequent testing of the reaction solution, the evaluation
of the addition of poisons that affect homogeneous or
heterogeneous catalysts, the TEM identification of
nanoparticles, etc.
The occurrence of an induction period in our
experiments only suggests that the initial complex was
a precursor of catalytically active species, but these
species were not necessarily nanoparticles. Additional
experiments were performed to discriminate hypotheꢀ
ses on the nature of the catalyst formed in the
Thus, the above results suggest that the palladium
catalyst promoted with elemental phosphorus conꢀ
sisted of nanoparticles containing the palladium phosꢀ
phide Pd6P and Pd(0) clusters (see the above model).
The formation of particles that were more highly disꢀ
persed than palladium black particles and the accessiꢀ
bility of Pd(0) clusters to the coordination and activaꢀ
tion of substrates is a reason for the promoting action
of elemental phosphorus.
The formation of nanoparticles also occurred at the
Pd(acac)2–0.3P system. It is universally recognized ratio P/Pd = 1 (see Fig. 6); however, the Pd(acac)2–
that mercury is a poison for heterogeneous catalysts 1.0P–H2 system did not exhibit catalytic activity in
[27]. The introduction of an excess of mercury into the hydrogenation reactions. We experimentally found
Pd(acac)2–0.3P reaction system suppressed the cataꢀ that an increase in the concentration of elemental
lytic activity (Fig. 7). However, it is our opinion that phosphorus affected the composition of nanoparticles
only this single test cannot be unambiguous evidence (Table 2). According to XRD data, the black precipiꢀ
for the microheterogeneous nature of the catalyst tate separated from the reaction system (sample II)
because cases are known in which mercury also inhibꢀ consisted of the Pd5P2 and PdP2 palladium phosꢀ
ited homogeneous catalytic processes [28]. At the phides; however, unlike the sample prepared by the
same time, the nonlinear dependence of the reaction interaction of components in an inert atmosphere, the
rate
r
on palladium concentration and a decrease in
r
Pd5P2 phosphide was predominant in it. The fraction
Table 2. Characteristics of the products of Pd(acac)2 conversion in the presence of elemental phosphorus in an atmosphere
of hydrogen at 80°C
XRD data
Sample
P/Pd
Pd(0) fraction, %
sample
after calcination
initial sample
I
0.3
1
Xꢀray amorphous (coherentꢀscattering region, 3.13 nm)
Xꢀray amorphous (coherentꢀscattering region, 1.96 nm)
Pd6P
27
8
II
Pd5P2, PdP2
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

SYNTHESIS, PROPERTIES, AND ACTIVITY OF NANOSIZED PALLADIUM CATALYSTS
49
12. Shmidt, F.K., Belykh, L.B., Skripov, N.I., Belonoꢀ
gova, L.N., Umanets, V.A., and Rokhin, A.V., Kinet.
Katal., 2007, vol. 48, p. 685 [Kinet. Catal. (Engl.
Transl.), vol. 48, p. 640].
of Pd(0), which was measured using a chemical
method, was no higher than 8%; that is, in this case,
Pd(0) atoms formed upon the hydrogenolysis of
Pd(acac)2, initially reacted with the PdP2 phosphide
present in the reaction system to form the Pd5P2 phosꢀ
phide enriched in palladium. The PdP2 content of this
catalytic system was higher than that at the ratio
P/Pd = 0.3; therefore, the major portion of Pd(0)
atoms was consumed in the formation of palladium
phosphides. Because of this, the fraction of Pd(0) also
decreased. Since the hydrogenation of alkenes does
not belong to structurally sensitive reactions [29], the
detected small decrease in the catalyst particle size
with increasing P/Pd ratio would only increase the rate
of hydrogenation because of an increase in the fraction
of surface atoms. However, an opposite effect was
really observed. Consequently, the main reason for the
inhibiting effect of phosphorus at P/Pd > 0.75 was a
change in the catalyst composition and a decrease in
the fraction of Pd(0) as a result of its conversion into
palladium phosphides.
13. RF Patent 2323776, 2008.
14. Gordon, A.J. and Ford, R.A., A Handbook of Practical
Data, Techniques, and References, New York: Wiley,
1972.
15. Mitchell, J. and Smith, D., Aquametry, New York:
Wiley, 1948.
16. US Patent 3474464, 1969.
17. Raub, Ch.J., Zachariasen, W.H., Geballe, T.H., and
Matthias, B.T., J. Phys. Chem. Solids, 1963, vol. 24,
p. 1093.
18. Powder Diffraction File Q, Hanawalt Search Manual:
Inorganic Phases, Swarthmore: Joint Committee on
Powder Diffraction Standards, 1992, pp. 1–42
19. Shmidt, A.F. and Mametova, L.V., Kinet. Katal., 1996,
vol. 37, p. 431 [Kinet. Catal. (Engl. Transl.), vol. 37,
p. 406].
20. Belykh, L.B., Goremyka, T.I., Skripov, N.I., Umaꢀ
nets, V.A., and Shmidt, F.K., Kinet. Katal., 2006,
vol. 47, p. 373 [Kinet. Catal. (Engl. Transl.), vol. 47,
p. 367].
21. Shmidt, F.K., Belykh, L.B., Umanets, V.A., Cherenꢀ
kova, T.V., and Belonogova, L.N., Kinet. Katal., 1999,
vol. 40, p. 466 [Kinet. Catal. (Engl. Transl.), vol. 40,
p. 418].
22. Titova, Yu.Yu. and Kusovskaya, E.E., Materialy Vseꢀ
rossiiskoi konf. “Sovremennye podkhody k problemam
fizikokhimii i kataliza (Proc. AllꢀRussia Conf. on
PresentꢀDay Approaches to Physicochemical Probꢀ
lems of Catalysis), Novosibirsk, 2007, p. 212.
23. Nekrasov, B.N., Osnovy obshchei khimii (Fundamental
of General Chemistry), Moscow: Khimiya, 1974,
vol. 1.
24. Henkes, A.E., Vasquez, Yo., and Schaak, R.E., J. Am.
Chem. Soc., 2007, vol. 129, p. 1896.
REFERENCES
1. Trofimov, B.A., Rakhmatulina, T.N., Gusarova, N.K.,
and Malysheva, S.F., Usp. Khim., 1991, vol. 60, p. 2619.
2. Dorfman, Ya.A., Aleshkova, M.M., Polimbekova, G.S.,
Levina, L.V., Petrova, T.V., Abdreimova, R.R., and
Doroshkevich, D.M., Usp. Khim., 1993, vol. 62, p. 928.
3. Milikov, V.A., Budnikova, Yu.G., and Sinyashin, O.G.,
Usp. Khim., 2005, vol. 74, p. 859.
4. Peruzzini, M., Abdreimova, R.R., Budnikova, Yu.,
Romorosa, A., Scherer, O.J., and Sitzmann, H.,
J. Organomet. Chem., 2004, vol. 689, p. 4319.
5. Peruzzini, M., Consalvi, L., and Romerosa, A., Chem.
Soc. Rev., 2005, vol. 34, p. 1038.
6. Budnikova, Y.H., Yakhvarov, D.G., and Sinyashin, O.G.,
J. Organomet. Chem., 2005, vol. 690, p. 2416.
7. Akbaeva, D.N., Koord. Khim., 2006, vol. 32, p. 344
25. Schmidt, F.K., Fraktal’nyi analiz v fizikoꢀkhimii geteroꢀ
gennykh sistem i polimerov (Fractal Analysis in the
Physical Chemistry of Heterogeneous Systems and
Polymers), Irkutsk: Irkutsk. Gos. Univ., 2001.
[Russ. J. Coord. Chem. (Engl. Transl.), vol. 32, p. 329].
8. Barbaro, P., Viara, M.D., Peruzzini, M., Costantini, S.S.,
and Spopioni, P., Angew. Chem., Int. Ed. Engl., 2008,
vol. 47, p. 4425.
26. Widegren, J.A. and Finke, R.G., J. Mol. Catal., 2003,
vol. 198, p. 317.
9. Shmidt, F.K., Belykh, L.B., and Cherenkova, T.V., 27. Collman, J.P., Hegedus, L.S., Norton, J.R., and
Kinet. Katal., 2001, vol. 42, p. 182 [Kinet. Catal. (Engl.
Transl.), vol. 42, p. 163].
10. Schmidt, F.K., Belykh, L.B., and Goremyka, T.V.,
Finke, R.G., Principles of Organotransition Metal
Chemistry, Mill Valley, Calif.: Univ. Science Books,
1987, vol. 2.
Kinet. Katal., 2003, vol. 44, p. 683 [Kinet. Catal. (Engl. 28. Schmidt, A.F., Halaiqa, A.Al., and Smirnov, V.V., Synꢀ
Transl.), vol. 44, p. 623].
lett., 2006, no. 18, p. 2861.
11. Belykh, L.B., Goremyka, T.V., Belonogova, L.N., and 29. Bukhtiyarov, V.I. and Slin’ko, M.G., Usp. Khim., 2001,
Schmidt, F.K., J. Mol. Catal., 2005, vol. 231, p. 53. vol. 70, p. 167.
KINETICS AND CATALYSIS Vol. 51
No. 1
2010