Palladium-Phosphorus Nanoparticles as Effective Catalysts of the Chemoselective Hydrogenation of Alkynols

Article abstract of DOI:10.1134/S0023158420030209

Abstract: The effect of the composition of the catalytic system and reaction conditions on the properties of phosphorus-modified palladium catalysts in hydrogenations of alkynols was studied. Modification with phosphorus increased the activity and turnover number of palladium catalysts in the hydrogenation of the model compound 2-methyl-3-butyn-2-ol (MBY) without any reduction in the selectivity to 2-methyl-3-butene-2-ol at 95–98percent MBY conversion. The promoting effect of phosphorus on the properties of the palladium catalyst is caused not only by an increase in the particle size, but also, probably, by a change in the energy of interaction of reagents with the active sites. Hypotheses on the nature of the carriers of catalytic activity in Pd–P particles were discriminated using kinetic methods with the differential selectivity of catalytic systems as the main measured parameter under the conditions of competition between two alkynols. The hydrogenation of acetylenic alcohols involves only one of the two potentially active forms in Pd–P nanoparticles—Pd(0) clusters, whereas the hydrogenation of the resulting allyl alcohols involves both Pd(0) clusters and palladium phosphides.

Full text of DOI:10.1134/S0023158420030209

ISSN 0023-1584, Kinetics and Catalysis, 2020, Vol. 61, No. 4, pp. 575–588. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Kinetika i Kataliz, 2020, Vol. 61, No. 4, pp. 526–539.
Palladium-Phosphorus Nanoparticles as Effective Catalysts
of the Chemoselective Hydrogenation of Alkynols
N. I. Skripova, L. B. Belykha, *, T. P. Sterenchuka, K. L. Gvozdovskayaa, V. V. Zherdeva,
T. M. Dashabylovaa, and F. K. Schmidta
aIrkutsk State University, Irkutsk, 664003 Russia
*е-mail:belykh@chem.isu.ru
Received September 18, 2019; revised November 8, 2019; accepted December 24, 2019
Abstract—The effect of the composition of the catalytic system and reaction conditions on the properties of
phosphorus-modified palladium catalysts in hydrogenations of alkynols was studied. Modification with phos-
phorus increased the activity and turnover number of palladium catalysts in the hydrogenation of the model
compound 2-methyl-3-butyn-2-ol (MBY) without any reduction in the selectivity to 2-methyl-3-butene-2-ol
at 95–98% MBY conversion. The promoting effect of phosphorus on the properties of the palladium catalyst is
caused not only by an increase in the particle size, but also, probably, by a change in the energy of interaction of
reagents with the active sites. Hypotheses on the nature of the carriers of catalytic activity in Pd–P particles were
discriminated using kinetic methods with the differential selectivity of catalytic systems as the main measured
parameter under the conditions of competition between two alkynols. The hydrogenation of acetylenic alcohols
involves only one of the two potentially active forms in Pd–P nanoparticles—Pd(0) clusters, whereas the hydro-
genation of the resulting allyl alcohols involves both Pd(0) clusters and palladium phosphides.
Keywords: hydrogenation, palladium, phosphorus, nanoparticle, phosphide, cluster, 2-methyl-3-butyn-2-ol,
3-methyl-1-pentyn-3-ol, activity, differential selectivity
DOI: 10.1134/S0023158420030209
INTRODUCTION
etc. [1]. Increased selectivity is generally achieved at a
sacrifice of catalytic activity, which is reduced. The
presence of toxic elements (primarily, Pb) in the che-
moselective catalyst and low catalytic activity impose
serious restrictions on future applications of these
materials. In connection with modern environmental
safety standards [9], the goal is to create highly selec-
tive and environmentally more friendly catalysts that
meet the sustainable development strategy and are
characterized by a reasonable proportion of selectivity
and activity [10]. Therefore, studies in this field have
gained a new impetus in the last decade [1].
Modern approaches aimed at creating selective
catalysts of chemoselective hydrogenation of alkynes
and alkynols are based on the size and shape control of
transition metal nanoparticles [10–12]; isolation of
active sites on the surface of hybrid materials and
development of catalysts based on individual palla-
dium atoms [1, 13]; the use of new bimetallic alloys [5,
14–16]; and surface modification of the active com-
ponent of the catalyst [6, 17] or support [18].
The selective hydrogenation of multiple bonds,
including the chemoselective hydrogenation of
alkynes to alkenes [1–4] and of alkynols [5, 6] or
unsaturated carbonyl compounds [7] to allyl alcohols
without their further hydrogenation to alkanes or
alkanols, respectively, is an important reaction in
chemical industry; it is mainly used in the production
of polymers, pharmaceuticals, fat-soluble vitamins
(e.g., E and K) and agrochemicals. Thus, the level of
phenylacetylene in the monomer—styrene with a
polymerization degree of purity—should be below
10 ppm [8]. In the synthesis of many pharmaceuticals,
vitamins, flavors, and agrochemicals, however, the
catalysts developed over 60 years ago are used [1]. The
typical catalysts for selective hydrogenations are most
often obtained by “partial poisoning of the active
metal” (Pd, Pt, Rh, or Ni); the metal that is active in
catalysis is protected and selectively deactivated by
various compounds: Pb, V, Cu, Ag, CO, quinolone,
Abbreviations: DMF, N,N-dimethylformamide; MBY, 2-methyl-3-
butyn-2-ol; MPY, 3-methyl-1-pentyn-3-ol; MBE, 2-methyl-3-
butene-2-ol; MBA, 2-methylbutan-2-ol; XRD analysis, X-ray
powder diffraction analysis; XPS, X-ray photoelectron spectros-
copy; GLC, gas-liquid chromatography; TON, turnover num-
ber; TOF, turnover frequency.
The palladium catalysts of hydrogenation are gen-
erally modified by Lewis bases [1] and alkali metal cat-
ions in the form of salts and bases [6]. The introduc-
tion of nitrogen-containing Lewis bases leads to an
increase in selectivity due to a decrease in catalytic
575

576
SKRIPOV et al.
activity (blocking of active sites) [1], while alkali metal was subjected to two vacuum distillations (8 Torr) at a
cations at certain concentrations promote both selec- temperature of up to 42°C. For deeper drying, benzene
tivity and activity of palladium catalysts in the hydro- was additionally distilled over LiAlH4 using a distilla-
genation of alkynols [6]. Sodium and potassium
hydroxides facilitate the reduction of Pd(II) precur-
sors to Pd(0). Alkali metal cations can interact with the
surface of Pd nanoparticles, leading to site separation
and changing the adsorption energy of reagents, and
they also affect the mobility of Pd nanoparticles on
solid supports, preventing the sintering during the heat
treatment. The activity of heterogeneous palladium
catalysts modified with alkali metal cations in the
hydrogenation of alkynols does not exceed 426 min–1
at rather high (90°C) temperatures [6].
Tertiary, secondary, and primary phosphines,
unlike nitrogen-containing Lewis bases, undergo deg-
radation in the coordination sphere of Pd(0), which
gives rise to polynuclear palladium complexes with
phosphide, phosphynidene ligands, and even palla-
dium phosphides [19, 20]. Taking into account the
formation of palladium phosphides during the forma-
tion and functioning of active particles based on palla-
dium phosphine complexes, we proposed the most
reactive form of elemental phosphorus—white phos-
phorus—as a modifier instead of phosphines, in order
to develop effective catalysts of hydrogenation [21].
The reduction of palladium compounds with
hydrogen in the presence of P4 under mild conditions
makes it possible to obtain highly dispersed palladium
catalysts, whose particles contain Pd(0) clusters and
palladium phosphides. The size, composition, and
state of the surface layer of particles can be varied by
varying the P : Pd ratio [21], the acido ligand at palla-
dium [22, 23], or the solvent [24]. Of the two chemical
forms of palladium (Pd0, Pdδ+), Pd(0) prevails on the
surface of nanoparticles at low initial P : Pd ratios
(≤0.3); its electron binding energy is 0.7 eV lower than
that of bulk palladium [25]. As the hydrogenation of
alkynols is a size-sensitive reaction [11, 12], the above-
mentioned physical characteristics of Pd–P nanopar-
ticles must have a positive effect on their properties in
the hydrogenation of alkynols.
tion column. The solvents were stored in an argon
atmosphere in sealed ampules over 4A molecular
sieves.
Palladium bis-(acetylacetonate) Pd(acac)2 was pre-
pared by the procedure described in [27] followed by
recrystallization from acetone. Palladium diacetate
(Pd(OAc)2) and palladium dichloride (PdCl2)
((chemically pure)) were used without additional
purification.
White phosphorus was mechanically cleaned from
the surface oxidation products and washed in anhy-
drous benzene immediately before use. A solution of
white phosphorus in benzene was prepared and stored
in an inert atmosphere in a finger-type vessel, which
was previously evacuated and filled with argon.
2-Methyl-3-butyn-2-ol (MBY) (Sigma-Aldrich,
>98%) and 3-methyl-1-pentyn-3-ol (MPY) (Sigma-
Aldrich, >98%) were used without additional purifi-
cation. Phenylacetylene was kept over previously cal-
cinated calcium dichloride for 2 weeks and distilled
over calcium hydride (40°C/10 Torr). Styrene was
purified by shaking it with a 5% alkali solution until a
portion of alkali became colorless. Then it was washed
with distilled water, dried over anhydrous calcium
chloride, and distilled in vacuum (32°C/10 mmHg).
Triethylaluminum (AlEt3) (Sigma-Aldrich) was
distilled in vacuum, collecting a fraction boiling at 48–
1
49°C/1 Torr, and used as a solution in benzene. H
1
NMR: δ(СН3) = 1.22 ppm, (t, 3H, J = 8.24 Hz);
δ(СН2) = 0.45 ppm (q, 2H, 1J = 8.24 Hz).
Examples of Experiments
Alkynols were hydrogenated in a thermostatted
glass vessel of the duck type at 30°C and initial hydro-
gen pressure of 2 atm in the presence of a Pd–P cata-
lyst formed in situ.
A solution of phosphorus in benzene (0.1 × 10–5–
1.0 × 10–5 mol based on the atomic form of phospho-
rus) (1 mL) was added dropwise to a solution of
Pd(аcac)2 (0.00304 g, 1 × 10–5 mol) in DMF (9 mL)
placed in a thermostatted duck vessel. The solution
was stirred for 7 min at room temperature. Then the
temperature was raised to 80°C, and the reaction mix-
ture was stirred in hydrogen for 40–45 min up to quan-
titative conversion of Pd(аcac)2. The resulting black
and brown colloidal solution was cooled to 30°C, a
hydrogen pressure of 2 atm was created, and the
In order to develop effective catalysts of the che-
moselective hydrogenation of acetylenic compounds
and determine the nature of catalytic activity carriers,
we studied the hydrogenation of 2-methyl-3-butyn-2-
ol and the competitive hydrogenation of alkynols
under the action of Pd–P particles formed by the low-
temperature method.
EXPERIMENTAL
Materials
The solvents (benzene, N,N-dimethylformamide) 2-methyl-3-butyn-2-ol substrate was syringed through a
were purified by the standard procedures [26]. N,N- teflon stopper with a rubber gasket. MBY was hydroge-
Dimethylformamide (DMF) was kept over anhydrous nated with vigorous stirring, precluding the reaction in
copper sulfate until a green solution was obtained, for the diffusion region, while periodically collecting ali-
dehydration and removal of amine impurities; then it quots of reaction mixture for GLC analysis.
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PALLADIUM-PHOSPHORUS NANOPARTICLES AS EFFECTIVE CATALYSTS
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The catalytic systems were formed in the same way the density of palladium; NA is the Avogadro number;
at Pd(аcac)2 concentrations higher than 1 mmol/L.
The properties of Pd–P particles at catalyst concen-
trations of 0.25 and 0.5 mmol/L were studied by diluting
and dTEM is the surface-average particle diameter;
3
nd
i
i
i
the preformed catalyst system (
= 1 mmol/L).
СPd асас
(2)
dTEM
=
,
2
i
(
)
The catalytic experiments using other2 palladium pre-
cursors (PdCl2, Pd(OAc)2) were performed in a simi-
nd
i
i
lar way. The differences were in the temperature and where ni is the number of particles with a diameter di;
time of catalyst formation. In the case of PdCl2 and
A
DTEM
Pd(OAc)2, the catalyst systems formed in hydrogen at
(3)
TOF =
,
30°C for 5 min.
where A is the catalytic activity calculated based on all
palladium (mol of substrate (mol of Pdtot min)–1); and
DTEM is the dispersity determined from the transmis-
sion electron microscopy data.
The hydrogenation of alkynols proceeds in two
stages. At the first stage, the triple bond is predomi-
nantly hydrogenated; at the second stage, the resulting
allyl alcohol is reduced to saturated alcohol. The
amount of the product of complete hydrogenation
(alkanol) at the first stage was up to 5%. Therefore, the
The above approach was used only for the Pd–P
particles (initial ratio P : Pd ≤ 0.3) whose volume and
surface were enriched with palladium Pd(0) according
to the XRD and XPS data [25].
reaction rate r
at each stage was deter-
mol min
(
)
H2
mined from the kinetic curve in the following way. The
rate of MBY hydrogenation to allyl alcohol was calcu-
lated from the slope of the straight sections of the
curves in the hydrogen absorption range 0.1–0.5 mol
H2 per mol MBY. The rate of hydrogenation of allyl
alcohol to saturated alcohol was found from the slope
of the straight sections of the curves in the hydrogen
absorption range 1.1–1.3 mol H2 per mol MBY. The
selectivity of Pd–P catalysts for 2-methyl-3-butene-2-
ol was calculated at 95–98% conversion of the starting
acetylene alcohol. In order to determine the highest
TON and the stability of the Pd–P catalyst, hydroge-
nation was repeated by introducing an aliquot of fresh
substrate in the reaction vessel after the hydrogenation
of the previous portion of MBY to 2-methylbutan-2-ol
was completed.
Methods of Investigation
The UV spectra of the catalyst solutions were
recorded on an SF-2000 spectrophotometer (Spektr,
Russia) in quartz cells with an absorbing layer thick-
ness of 0.1 cm. The conversion of Pd(acac)2 was mon-
itored according to the 330 nm absorption band (ε330
=
10 630 L cm–1 mol–1).
The hydrogenation products were analyzed by
GLC on a Crystal 5000 chromatograph (Chromatec,
Russia) equipped with a 30 m long capillary column
(phase: 5% diphenyl, 95% dimethylpolysilphenylene
siloxane) and a flame ionization detector (FID) by the
internal standard method using temperature program-
ming: 100°C (1.5 min), 270°C (10 min), heating rate
30°C/min. The pressure of the carrier gas (nitrogen)
was 20 kPa.
The competitive hydrogenation of alkynols in the
presence of Pd–P particles obtained from Pd(acac)2
and P4 in hydrogen was performed as in the case of the
noncompetitive hydrogenation of alkynols. For this, a
mixture of alkynols: 2-methyl-3-butyn-2-ol (MBY)
(3.3535 mmol, 0.31 mL) and 3-methyl-1-pentyn-3-ol
(MPY) (3.3644 mol, 0.38 mL) was injected with a
syringe into a colloidal solution of Pd–P particles
formed in situ (as described above) and hydrogenated
with vigorous stirring. The reaction was monitored by
GLC analysis of the intermittently collected samples.
The hydrogenation products were additionally
analyzed on a GCMS-QP2010 Ultra chromato-mass
spectrometer (Shimadzu, Japan) with a 30 m long
GsBP-5MS capillary column; the phase was poly(5%
diphenyl, 95% dimethylpolysilphenylene siloxane).
Ionization was performed by electron impact; the ion-
ization energy was 70 eV. The obtained mass spectra
were compared with published data (Wiley, NIST,
NIST05 databases).
The TEM images were obtained on a Tecnai G2
electron microscope (FEI, United States) with an
accelerating voltage of 200 kV. A drop of the reaction
solution was applied to a carbonized copper grid and
dried in a box in an inert atmosphere at room tempera-
ture. The images were recorded using a CCD camera
(Soft Imaging System, Germany). To find the average
particle size, a section containing at least 100–
150 particles was processed.
The dispersion (DTEM) and TOF of Pd black and Pd–
P particles were calculated by the following equations:
6M
ρ d A N
Pd TEM Pd
Pd
(1)
DTEM
=
,
A
where MPd is the atomic mass of Pd (g/mol); APd is the
2
surface area of the Pd atom (
/atomPd surf); ρ is
mPd surf
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

578
C, M
SKRIPOV et al.
2-methyl-3-butene-2-ol—2-methylbutan-2-ol—forms
(Fig. 1). The products of dimerization of alkynol C10
and hydrogenation of the resulting dimer (2,7-
dimethyl-3,5-octadiyn-2,7-diol to 2,7-dimethyl-5-
octen-3-yn-2,7-diol) were detected in trace amounts
by chromato-mass spectrometry and GLC. The prod-
ucts of isomerization of 2-methyl-3-butyn-2-ol to
unsaturated aldehyde were absent.
0.6
0.5
0.4
0.3
0.2
0.1
1
2
3
The activity of Pd–P particles in the hydrogenation
of 2-methyl-3-butyn-2-ol depends on the composi-
tion of the catalytic system and reaction conditions. As
a result of the modification of the palladium catalyst
with phosphorus introduced before the stage of the
reduction of Pd(acac)2 with hydrogen, the rates of
hydrogenation of both the triple bond of MBY and the
double bond of the 2-methyl-3-butene-2-ol product
increase without any decrease in selectivity for allyl
alcohol over the whole range of P : Pd = 0.1–1 at 99–
98% conversion of the substrate (Fig. 2). Even when
small amounts of elemental phosphorus are intro-
duced (P : Pd = 0.1), the activity of Pd–P particles (A)
calculated for all palladium increases from 58.5 to
185 min–1, i.e., almost threefold compared with that of
Pd black (Fig. 2a). At a Pd(acac)2 concentration of
1 mmol/L, the greatest promoting effect of phospho-
rus is observed at a ratio of P : Pd = 0.3: the activity of
Pd–P particles in MBY hydrogenation is 285 min–1
(Fig. 2a).
0
5
10
15
20 25 30 35 40
t, min
Fig. 1. Kinetic curves of hydrogenation of (1) 2-methyl-3-
butyn-2-ol and (2), (3) formation of (2) 2-methyl-3-
butene-2-ol and (3) 2-methylbutan-2-ol in the presence of
Pd–P particles; P : Pd = 0.3. Reaction conditions: С
=
=
Pd
0.25 mmol/L, С
2 atm.
= 0.598 mmol/L, Т = 30°С,
Р
MBY
Н2
RESULTS AND DISCUSSION
Hydrogenation of 2-methyl-3-butyn-2-ol in the
presence of Pd black and Pd–P particles in DMF is an
example of a typical sequential reaction. At the first
stage of the reaction (before the absorption of 1 mol H2
per mol MBY), 2-methyl-3-butyn-2-ol is predomi-
nantly hydrogenated to 2-methyl-3-butene-2-ol; at
Under the isothermal conditions of the process,
the activity of Pd–P particles is determined not only
by the P : Pd ratio, but also by hydrogen pressure and
MBY and palladium precursor concentrations
the second stage, the product of hydrogenation of (Fig. 3). The rate of hydrogenation of the MBY triple
(а)
(b)
А, min–1
300
Selectivity, %
100
А, min–1
180
Selectivity, %
100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
3
3
160
140
120
100
80
250
200
150
100
50
1
2
1
2
60
40
20
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
P : Pd
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
P : Pd
Fig. 2. Effect of the phosphorus modifier on the (1, 2) activity and (3) MBE selectivity of Pd–P nanoparticles in hydrogenation
of (1) 2-methyl-3-butyn-2-ol and (2) 2-methyl-3-butene-2-ol at a Pd(acac) concentration of (a) 1 or (b) 2 mmol/L. Reaction
2
–3
conditions: ν
= 6.64 × 10 mol, DMF solvent, 10 mL, T = 30°C;
= 2 atm. The selectivity was measured at 97–98%
Р
MBY
Н2
conversion of MBY.
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PALLADIUM-PHOSPHORUS NANOPARTICLES AS EFFECTIVE CATALYSTS
579
bond (r1) depends linearly on the hydrogen pressure
(а)
r, molMBY L–1 min–1
(Fig. 3a), and the dependence of the reaction rate on
the MBY substrate concentration passes through a
maximum (Fig. 3b). The MBY hydrogenation rate
nonlinearly increases when the Pd(acac)2 concentra-
tion increases from 0.25 to 5 mmol/L, reaching a pla-
0.14
0.12
0.10
0.08
0.06
0.04
0.02
teau at
= 2 mmol/L (Fig. 3c). The depen-
СPd acac
(
)
dence of the ac2tivity of Pd–P particles on
in
)
2
СPd acac
(
the indicated range of concentrations is antibatic
(Fig. 3c). At an optimum concentration of Pd(acac)2
(0.25–0.5 mmol/L), the activity of Pd–P particles in
MBY hydrogenation reaches 450 min–1 at 30°C and
= 2 atm. In their activity in MBY hydrogenation,
РН
2
the Pd–P particles surpass the palladium clusters in
the form of a cube or cuboctahedron with sizes of 6
and 5.5 nm (АPd total = 153 and 135 min–1, respectively;
0
0.5
1.0
(b)
1.5
2.0
PH2, atm
r, molMBY L–1 min–1
T = 60°С,
= 3 atm) [10], bimetallic alloys
РН
2
Pd7Bi/SiO2 (АPd total = 12.6 min–1; T = 50°С,
=
РН
2
0.30
1 atm) [14], and PdZn/TiO2 (АPd total = 180 min–1; Т =
40°С, = 1 atm) [14] and are comparable to Ziegler
0.25
0.20
0.15
0.10
0.05
РН
2
type systems (Table 1). For Ziegler type systems, as
well as for Pd–P particles, the catalytic activity
increases after dilution of the Pd(acac)2–n AlEt3 sys-
tem (n = 4, 6) (from 2 to 0.5 mmol/L) (Table 1). The
reasons for the effect of dilution on the activity of Pd–P
particles and palladium nanoclusters in Ziegler sys-
tems, in our opinion, may be as follows.
According to the TEM data, the Pd–P nanoparti-
cles are aggregates of particles (Figs. 4b–4e), while the
palladium nanoclusters in Ziegler systems are repre-
sented by individual high-contrast particles (Fig. 4f).
The reason for the aggregative stability of palladium
nanoclusters is their stabilization with AlEt2(acac) and
AlEt3 [28–30]. The inhibitory effect of organoalumi-
num compounds as a result of adsorption on the palla-
dium active sites was previously proved experimentally
[29, 30]. In view of this, the increase in the activity of
Ziegler systems in the hydrogenation of MBY after
dilution of the reaction solution is mainly due to the
elimination of the inhibitory effect of the stabilizers
(AlEt2(acac), AlEt3) as a result of hydrolysis of the sur-
face compounds with water impurities in the solvent.
The reason for the effect of dilution on the activity of
Pd–P particles may be prevented aggregation of parti-
cles, which is known to occur by a second order reac-
tion [28]. The reaction occurring in the kinetic region
was proved by additional experiments.
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4
CMBY, mol/L
(c)
A, min–1
450
400
350
300
250
200
150
100
r, molMBY L–1 min–1
0.3
0.2
0.1
0
50
0
0.5
1.0
1.5
2.0
CPd, mmol/L
The introduction of phosphorus before the stage of
Pd(acac)2 reduction with hydrogen significantly
increases not only the activity but also the productivity
of palladium catalysts. In MBY hydrogenation under
the action of the Pd–P catalyst (P : Pd = 0.5), TON is
almost two times higher (10 650 mol MBY per mol Pd)
than that of Pd black (5470 mol MBY per mol Pd)
(Fig. 5).
Fig. 3. Dependence of the hydrogenation rate (r) of
2-methyl-3-butyn-2-ol or activity (A) on the (a) hydrogen
pressure and (b), (c) concentration of the (b) substrate and
(c) Pd(acac) in the presence of Pd–P particles (P : Pd =
2
0.3). Reaction conditions: (a) С = 0.5 mmol/L, С
=
Pd
MBY
0.598 mol/L, Т = 20°С; (b) С = 0.5 mmol/L, Т = 30°С,
Pd
–3
= 2 atm; (c) ν
= 2 atm, DMF solvent, 10 mL.
= 6.64 × 10 mol, Т = 30°С,
Р
Н2
Р
Н2
MBY
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580
SKRIPOV et al.
Table 1. Catalytic properties of Ziegler type systems Pd(acac)2–nAlEt3 in the hydrogenation of 2-methyl-3-butyn-2-ol*
Activity (А), min–1
AlEt3 : Pd СPd, mmol/L
А1 : А2
Conversion, %
Selectivity for MBE, %
А1
А2
−С≡С−
−С=С−
4
6
4
4
6
2
2
1
0.5
0.5
0.5
194
184
395
370
367
332
–3
441
373
528
772
744
557
0.4
0.5
0.7
0.5
0.5
0.6
94.2
95.8
91.8
96.6
97.8
97.1
93.9
92.7
95.6
94.1
93.7
92.9
10
* Reaction conditions: ν
= 6.64 × 10 mol; toluene solvent, volume 10 mL, Т = 30°С;
= 2 atm.
Р
Н2
MBY
The replacement of the acetylacetonate ligand at 2-methyl-3-butyn-2-ol, while retaining high selectiv-
ity at 96–98% conversion of the substrate.
palladium by acetate has almost no effect on the prop-
erties of Pd–P nanoparticles in the hydrogenation of
2-methyl-3-butyn-2-ol (Table 2). The lower activity
of Pd–P particles obtained from PdCl2 is the result of
poisoning of the surface palladium atoms by chloride
anions that formed as a result of hydrogenolysis. Thus,
phosphorus mainly affects the activity and productiv-
The promoting effect of phosphorus on the activity
of Pd–P nanoparticles in the hydrogenation of
2-methyl-3-butyn-2-ol may be the consequence of
the increased proportion of active sites and/or the for-
mation of new, more active catalyst sites. To distin-
guish between these two factors, it is necessary, first of
all, to take into account the fraction of surface atoms
ity of the palladium catalyst in the hydrogenation of in Pd black, palladium clusters in Ziegler systems, and
50 nm
50 nm
50 nm
(а)
(b)
(c)
50 nm
50 nm
20 nm
(d)
(e)
(f)
Fig. 4. TEM images of the catalysts: (a) Pd black; (f) Pd(acac) –4AlEt system, СPd = 5 mmol/L; (b) Pd–P particles formed in
2
3
hydrogen, P : Pd = 0.3; (c) P : Pd = 1.0, СPd = 1 mmol/L; (d) P : Pd = 0.1, СPd = 1 mmol/L; (e) СPd = 2 mmol/L.
KINETICS AND CATALYSIS Vol. 61 No. 4
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PALLADIUM-PHOSPHORUS NANOPARTICLES AS EFFECTIVE CATALYSTS
581
Pd–P particles. The results of TEM determination of
A, min–1
the sizes of Pd black formed from Pd(acac)2 in hydro-
gen, palladium clusters in Ziegler systems, and Pd–P
particles depending on the initial Pd(acac)2 concen-
tration and the amount of introduced modifier are
presented in Table 3. The particle size of the Pd–P
catalyst decreased almost fourfold compared with the
size of Pd black (Fig. 4). The number average size of
Pd–P particles changed slightly when the P : Pd ratio
increased: from 5.9 1.04 (P : Pd = 0.1) to 5.5 0.92
(P : Pd = 0.3) and 5.3 1.01 nm (P : Pd = 1.0). The
size of Pd–P particles and Pd black increases with the
concentration of the precursor (Pd(acac)2). For the
Pd–P catalyst (P : Pd = 0.1), it increases from 5.9
300
250
200
150
100
50
1.04 nm (СPd = 1 mmol/L) to 10.7 2.3 nm (СPd
=
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
2 mmol/L) (Fig. 4).
MBY portion no.
The turnover frequencies of palladium catalysts
calculated for the surface palladium atoms are pre-
sented in Table 3. The highest TOF values character-
istic of Pd black and Pd–P particles in MBY hydroge-
nation differ 1.8-fold (TOFPd–P : TOFPd black = 1.8).
According to TOF, the Pd–P particles surpass not
only palladium nanoclusters in the form of a cube
(TOF = 918 min–1), octahedron (TOF = 1050 min–1),
and cuboctahedron (TOF = 876 min–1) [10], which
are close in size, but also Ziegler type systems (Table 3).
Fig. 5. Hydrogenation of the first 16 portions of 2-methyl-
3-butyn-2-ol (gray column) and the resulting 2-methyl-3-
butene-2-ol (black column) under the action of the Pd–P
catalyst, P : Pd = 0.5. Reaction conditions: С
2 mmol/L; one portion of MBY is 6.64 × 10 mol; DMF
=
Pd
–3
solvent, 10 mL, T = 30°C,
= 2 atm.
Р
Н2
The preserved high alkenol selectivity of Pd–P
nanoparticles at 96–98% conversion of the substrate
shows that the selectivity depends not only on the ratio
of hydrogenation rates of the competing substrates,
but also on the ratio of the constants of adsorption
equilibrium of the competing substrates (alkynol,
alkenol) and the catalyst. As is known, the selectivity
of palladium catalysts during triple bond hydrogena-
tion is determined by the adsorption of alkyne, which
is stronger than that of alkene or alkane [3]. For hydro-
genation to occur, the adsorption of the reacting sub-
stances should not be too strong. According to the
XPS data, at low P : Pd ratios (P : Pd = 0.3), the Pd(0)
clusters that prevail on the surface of Pd–P particles
are characterized by lower binding energies
(Еb(Pd3d5/2) = 334.5 eV) [25] compared with the bind-
ing energy of the bulk metal (Еb(Pd3d5/2) = 335.2 eV)
Additional experiments on hydrogenation of phe-
nylacetylene and styrene in the presence of Pd–P par-
ticles showed similar results (Table 3). According to
TOF in hydrogenation of both phenylacetylene and
styrene, Pd–P particles surpass both Pd black and pal-
ladium nanoclusters in Ziegler systems. The signifi-
cantly higher TOF values of Pd–P particles (TOF =
2380 min–1) relative to TOFs for Pd black (Table 3)
and palladium nanoclusters of various forms [10] sug-
gests higher reactivity of Pd–P nanoparticles (P : Pd =
0.3) in hydrogenation of alkynols, alkynes, and
alkenes.
Based on the data obtained, we believe that the
modifying effect of phosphorus on the properties of
Pd–P particles in hydrogenation is due not only to the
geometrical, but also to the electronic factor, which
changes the energy of interaction of the active sites of [31]. Pd catalysts with a high electron density on pal-
Pd–P particles with the reaction participants. This ladium are generally more active in the hydrogenation
assumption is based on the following facts.
of unsaturated compounds [32, 33]. On the one hand,
Table 2. Effect of the precursor on the activity and selectivity of Pd−Р nanoparticles in the hydrogenation of 2-methyl-3-
butyn-2-ol*
Activity (А), min–1
А1 : А2
Precursor
Conversion, %
Selectivity for MBE, %
−С≡С−
−С=С−
Pd(OAc)2
166
91
127
91
1.3
1.0
1.4
97.2
98.0
96.2
95.6
95.5
96.1
PdСl2
Pd(acac)2
163
113
–3
* Reaction conditions: СPd = 2 mmol/L, P : Pd = 0.5, ν
= 6.64 × 10 mol; DMF solvent, volume 10 mL, Т = 30°С;
= 2 atm.
Р
Н2
MBY
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

582
SKRIPOV et al.
Table 3. Turnover frequencies of palladium catalysts formed from Pd(acac)2 in the hydrogenation of MBY*
TOF, min–1
PhA**
Particle size, nm
СPd acac
mmol/L
,
)
2
(
DTEM
Catalyst
Pd black
P : Pd
dTEM
d
MBY
styrene
0
0.1
0.1
0.3
0.3
0.3
0
1
22.2 4.5
5.9 1.0
23.4
6.0
0.045
0.169
0.090
0.187
0.187
0.187
0.418
0.418
0.418
0.604
0.604
1300
1189
1444
1497
2326
2380
430
444
911
942
850
−
666
1118
−
Pd−P particles
1
2
10.7 2.3
5.5 0.9
11.7
1
5.65
1497
−
0.5
0.25
5
5.5 0.9
5.65
5.65
2.53
2.53
2.53
1.65
1.65
5.5 0.9
−
−
Pd(acac)2−4AlEt3
Pd(acac)2−6AlEt3
2.45 0.64
2.45 0.64
2.45 0.64
1.55 0.57
1.55 0.57
279
−
430
−
0
2
464
0
0.5
5
794
−
−
0
382
201
−
264
−
0
0.5
607
The dash means that hydrogenation of PhA and styrene was not performed at these concentrations.
*For conditions, see Tables 1 and 2 and Fig. 2.
** PhA is phenylacetylene.
an excess of electrons on palladium atoms facilitates
the dissociative chemisorption of the hydrogen mole-
cule [32, 33]. On the other hand, it reduces the
adsorption of both alkynes and alkenes, while the
adsorption of alkenes is suppressed to a greater extent
than that of alkynes [3]. Monosubstituted alkynes and
alkynols are characterized by an extremely high
adsorption affinity for palladium. Therefore, an
increase in electron density on Pd(0) in Pd–P particles
(P : Pd = 0.3) and, as a consequence, a decrease in the
sorption affinity of alkynes (alkynols) should promote
the growth of the catalytic activity. An increase in the
selectivity and activity of Pd–Zn bimetallic alloys in
the hydrogenation of 2-methyl-3-butyn-2-ol as a
result of a decrease in the absolute values of the
adsorption equilibrium constants KMBY and KMBE and
a 1.4-fold increase in the KMBY/KMBE ratio compared
with the corresponding values of the Lindlar catalyst
was reported in [34].
Figure 3 presents the results of the formal kinetic
study of MBY hydrogenation in the presence of the
most active Pd–P particles (P : Pd = 0.3). The
extremal dependence of the hydrogenation rate on the
MBY concentration in the range 0.6–1.4 mmol/L
indicates the competitive adsorption of MBY and
hydrogen molecules at the active sites of Pd–P parti-
cles. The nearly zero concentration order of the reac-
tion for MBY in the concentration range 0.6–1.05 M,
as well as the first order for hydrogen (n(Н2) = 1.04),
under mild reaction conditions (1–2 atm) suggest that
the catalyst surface is saturated with the substrate and
hydrogen is involved in the limiting stage of the reac-
tion. Therefore, to describe the heterogeneous cata-
lytic hydrogenation of 2-methyl-3-butyn-2-ol in the
presence of Pd–P particles, we used the Langmuir–
Hinshelwood mechanism, according to which activa-
tion of both hydrogen and MBY is necessary for the
chemical reaction to proceed. The hydrogenation of
2-methyl-3-butyn-2-ol according to the Langmuir–
Hinshelwood mechanism implies the competitive [38,
39] or noncompetitive [34] adsorption between hydro-
gen and MBY molecules at the active sites of the cata-
lyst. The Rideal–Eley mechanism, which includes the
activation of only one reagent [40], was not considered
in this case.
This conclusion can explain the high TOF values of
Pd–P particles only if the carriers of the catalytic
activity are Pd(0) clusters. However, the Pd–P parti-
cles obtained by the low-temperature method contain
both Pd(0) clusters and palladium phosphides, which
contain the electron-deficient form of palladium
(Pdδ+). Palladium-enriched palladium phosphides
can also be active in hydroprocesses [35–37]. In order
to determine which of the two forms (or both) is
responsible for the catalytic properties of Pd–P
nanoparticles in the hydrogenation of alkynols under
mild conditions, additional experiments were per-
formed.
Various kinetic models of the Langmuir–Hinshel-
wood mechanism for hydrogenation were presented in
[41]. As the surface of Pd–P nanoparticles contains
two potentially active forms of palladium, the follow-
ing kinetic models are most likely (Table 4):
1. Hydrogenation proceeds between the previously
adsorbed MBY and hydrogen molecules at active sites
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PALLADIUM-PHOSPHORUS NANOPARTICLES AS EFFECTIVE CATALYSTS
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584
SKRIPOV et al.
of the same nature (competitive adsorption); the lim-
iting stage is the surface reaction (model 1).
2. Hydrogenation occurs between the preliminarily
adsorbed MBY and hydrogen molecules at different
active sites (noncompetitive adsorption), but hydro-
gen can be adsorbed at the same sites as MBY; the lim-
iting stage is the surface reaction (model 2).
3. Hydrogenation proceeds between the prelimi-
narily adsorbed MBY and hydrogen molecules at dif-
ferent active sites (noncompetitive adsorption), but
MBY can be adsorbed at the same sites as hydrogen;
the limiting stage is the surface reaction (model 3).
4. The limiting stage is chemisorption of the hydro-
gen molecule (model 4).
An analysis of the results of competitive hydroge-
nation of two substrates of the same type presented in
the form of phase trajectories will make it possible to
differentiate the above-proposed models 1 and 3 at
high probability. The coincidence of phase trajectories
for different palladium catalysts indicates that the dif-
ferential selectivities coincide and the nature of active
sites is the same [43]. It should be borne in mind, how-
ever, that this result is still not an unambiguous proof
of the identical nature of the catalyst. If the selectivity
proved insensitive to a change in the nature of the cat-
alyst, the result would be apparent constancy of selec-
tivity with allowance for the experimental accuracy of
its determination. If, however, the phase trajectories
again coincide when the process parameters are var-
ied, the nature of the active sites is the same. The non-
coincidence of the phase trajectories of competitive
hydrogenation of two substrates of the same type indi-
cates that the nature of the active sites of the catalysts
differs [43]. In view of the foregoing, two related acet-
ylene alcohols were selected for competitive hydroge-
nation: 2-methyl-3-butyn-2-ol and 3-methyl-1-pen-
tyn-3-ol.
The preliminary parallel experiments of competi-
tive hydrogenation of MBY and MPY in the presence
of various palladium catalysts showed good reproduc-
ibility. The almost complete coincidence of the phase
trajectories of the parallel experiments on competitive
hydrogenation of two acetylene alcohols in the pres-
ence of Pd black (Fig. 6a), colloidal solutions of Pd–P
nanoparticles (Fig. 6b), or Ziegler systems (Fig. 7)
allows us to exclude the effect of reproducibility of the
heterogeneous catalytic experiments on discrimina-
tion of hypotheses and to proceed to the analysis of
phase trajectories for various palladium catalysts. It
should be emphasized that the phase trajectories con-
structed according to the accumulation of the prod-
ucts of competing reactions (allyl or saturated alco-
hols) and according to the decrease in the concentra-
tions of competing acetylene alcohols coincided in the
parallel experiments (Fig. 6b). At the same time, the
similar phase trajectories of competitive hydrogena-
tion of MBY and MPY in the presence of Pd–P
nanoparticles and Pd black diverge (Fig. 8). At a first
glance, it could be suggested that of the two potential
forms (Pd(0) clusters and palladium phosphides), pal-
ladium phosphides are active in the hydrogenation of
alkynols. However, as hydrogenation of alkynols is a
size-sensitive reaction [11, 12] and the sizes of Pd–P
nanoparticles (d = 5.9 nm) and Pd black (d = 22 nm)
significantly differ, additional experiments were per-
formed on the competitive hydrogenation of MBY and
MPY in the presence of smaller palladium nanoclus-
ters (Fig. 4). The catalysts were Ziegler systems, in which
the size of palladium nanoclusters can be relatively easily
adjusted by varying the ratio of the organoaluminum
cocatalyst to the palladium precursor [30].
5. The limiting stage is MBY adsorption (model 5).
At initial MBY concentrations higher than 0.6 M, a
decrease in the reaction rate makes it possible to
exclude kinetic models 2 and 5 from consideration; for
these models, the initial reaction rate should increase
continuously with an increase in the initial MBY con-
centration (linearly for model 5 and nonlinearly for
model 2) (Table 4). For models 1, 3, and 4, the initial
reaction rate decreases in the range of certain MBY
concentrations [41]. The hydrogen chemisorption
(model 4) evidently cannot be the limiting stage of
MBY hydrogenation in the presence of Pd–P particles
as the reaction rate in this case will decrease with
increasing initial MBY concentrations. Thus, of the
five proposed models, model 1 or 3 remains.
To answer the question of whether both potentially
active forms of palladium (palladium nanoclusters and
palladium phosphides) or only one of them (models 1
and 3) are involved in the hydrogenation of 2-methyl-
3-butyn-2-ol, a combination of kinetic methods was
proposed, which use the differential selectivity of cat-
alytic systems as the main measured parameter under
conditions of competition of several substrates.
In contrast to the catalytic activity, the differential
selectivity at the same degrees of conversion is inde-
pendent of the concentration of the active component
of the catalyst and is determined by its nature [42, 43].
However, this condition is satisfied only when the
products formed on one catalyst have the same orders
of magnitude in the catalyst concentration [43]. The
postulate about the identity of active sites on which the
products are formed is fulfilled if competitive hydro-
genation is performed for two substrates of the same
type that differ in the substituent that is remote from
the reaction site [43]. A simple method for evaluating
the differential selectivity was proposed in [42, 43], in
which it was suggested that the phase trajectories of
the reaction that represent the mutual dependences of
the product yields of the competing reactions should
be used. The slope to any point on the phase trajectory
equals the ratio of the product accumulation rates in
the competing reactions, which unambiguously
defines the differential selectivity.
A comparison of the phase trajectories of the com-
petitive hydrogenation of acetylene alcohols (MBY
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PALLADIUM-PHOSPHORUS NANOPARTICLES AS EFFECTIVE CATALYSTS
585
(а)
(b)
CMBY (MBE), mol/L
CMBY (MBE), mol/L
0.35
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.30
0.25
0.20
0.15
0.10
0.05
3, 4
3, 4
1, 2
1, 2
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
CMPY (MPE), mol/L
CMPY (MPE), mol/L
Fig. 6. Phase trajectories of parallel experiments on the (1, 2) competitive hydrogenation of two alkynols (MBY, MPY) and (3, 4)
formation of allyl alcohols (MBE, MPE) in the presence of (a) Pd black and (b) Pd–P nanoparticles (P : Pd = 0.3). Reaction
conditions: (a) C = 1.0 mmol/L, (b) C = 0.5 mmol/L, T = 30°C, = 2 atm, DMF solvent, 10 mL.
Р
Н2
Pd
Pd
and MPY) in the presence of Pd–P nanoparticles and hols differ (Fig. 9). The coincidence of the phase tra-
jectories of the competitive hydrogenation of acety-
lene alcohols suggests, at high probability, that the
nature of active sites in these catalysts is identical.
Therefore, of the two potentially active sites contained
in Pd–P nanoparticles, the Pd(0) clusters exhibit
activity in the hydrogenation of alkynols.
Ziegler systems gave an unexpected result. The phase
trajectories of competitive hydrogenation constructed
according to the decrease in the MBY and MPY con-
centrations coincide, while those constructed accord-
ing to the concentrations of the reaction products
diverge; i.e., the ratios of the hydrogenation rates of
alkynols coincide, while the ratios of the accumulation
(formation and consumption) rates of ethylene alco-
The noncoincidence of the phase trajectories of the
formed ethylene alcohols could be the result of the
CMBY (MBE), mol/L
CMBY (MBE), mol/L
0.40
0.35
0.35
0.30
3, 4, 5, 6
3
0.30
0.25
4
1, 2
0.25
0.20
0.20
0.15
0.10
0.05
2
0.15
0.10
0.05
1
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
CMPY (MPE), mol/L
CMPY (MPE), mol/L
Fig. 7. Phase trajectories of parallel experiments on the
competitive hydrogenation of (1, 2) two acetylene alcohols
(MBY, MPY) and formation of (a) (3, 4) allyl (MBE,
MPE) and (b) (5, 6) saturated (MBA, MPA) alcohols in
the presence of the Pd(acac) –6AlEt system. Reaction
Fig. 8. Phase trajectories of the (1, 2) competitive hydro-
genation of alkynols (MBY, MPY) and (3, 4) formation
of allyl alcohols (MBE, MPE) in the presence of (1, 3) Pd
black and (2, 4) Pd–P nanoparticles (P : Pd = 0.3). Reac-
2
3
conditions: C = 0.5 mmol/L, T = 30°C,
= 2 atm,
tion conditions: Т = 30°С,
= 2 atm, DMF solvent,
Р
Н2
Р
Н2
Pd
toluene solvent, 10 mL.
10 mL.
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

586
SKRIPOV et al.
(а)
(b)
CMBY (MBE), mol/L
CMBA, mol/L
0.35
0.35
0.30
0.25
0.20
0.15
0.10
0.05
6
0.30
0.25
0.20
0.15
0.10
0.05
4
3
5
1, 2
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
CMPY (MPE), mol/L
CMPA, mol/L
Fig. 9. Phase trajectories of the (1, 2) competitive hydrogenation of alkynols (MBY, MPY), and (3, 4) formation of (a) allyl (MBE,
MPE) and (b) (5, 6) saturated alcohols (MBA, MPA) in the presence of (1, 3, 5) Pd–P nanoparticles (P : Pd = 0.3) and (2, 4, 6)
Pd(acac) –6AlEt system. Reaction conditions: T = 30°C, = 2 atm.
Р
Н2
2
3
conversion of alkynols into the by-products (of hydrogenation catalysis, but more resistant to deacti-
dimerization and isomerization). However, this is
excluded in view of the coincidence of the phase tra-
jectories constructed according to the sums of the
conversion products of alkynols (allyl and saturated
alcohols). Therefore, the observed difference between
the phase trajectories of the competitive hydrogena-
tion of acetylene alcohols (MBY and MPY) con-
structed according to allyl alcohols is associated with
their further saturation. If the unsaturated alcohols
remained the final products, the phase trajectories
constructed according to both the decrease in the con-
centration of acetylene alcohols and accumulation of
their hydrogenation products would coincide.
vation [46, 47]. The hydrogenation requires the activa-
tion of both molecular hydrogen and substrate. As the
partial positive charge on palladium in palladium
phosphide (Еb(Pd3d5/2) = 336.1 eV) [25] is unfavorable
for the activation of molecular hydrogen, hydrogen
chemisorption evidently predominantly occurs on
Pd(0) clusters, while the adsorption of allyl alcohols
occurs on both Pd(0) clusters and palladium phosphi-
des. Palladium phosphides show activity in the pres-
ence of the more active form—Pd(0) clusters—proba-
bly because of the partial transfer of hydrogen by the
spillover mechanism from Pd(0) clusters to allyl alco-
hol adsorbed on palladium phosphide and its subse-
quent saturation.
As is known, hydrogenation can involve not only
hydrogen of the surface layer, but also hydrogen of the
near-surface layer called “nonselective” [1], which
saturates alkynes to alkanes without their desorption
stage. The contribution of “nonselective” hydrogen
can be neglected in view of the lower solubility of
hydrogen in small palladium clusters [44] and palla-
dium phosphides compared with that in bulk palla-
dium [45], and also taking into account the low pro-
portion of 2-methylbutan-2-ol and 3-methylpentan-
3-ol during the hydrogenation of alkynols (Fig. 1).
CONCLUSIONS
A method was proposed for the preparation of an
effective catalyst for hydrogenation of alkynols, which
surpasses the known palladium nanoclusters in its
activity and retains high selectivity (95–96%) at 97–
98% substrate conversion. The promoting effect of
phosphorus on the properties of the palladium catalyst
is due not only to an increase in dispersion, but also to
higher reactivity of Pd–P particles. An analysis of the
differential selectivity of various palladium catalysts
shows that of the two types of active sites contained in
Pd–P particles, hydrogenation of alkynols most likely
occurs on Pd(0) clusters, and further saturation of allyl
alcohols occurs on both Pd(0) clusters and palladium
An analysis of the phase trajectories of the compet-
itive hydrogenation of MBY and MPY suggests that
the hydrogenation of alkynols in the presence of Pd–P
nanoparticles most likely proceeds on Pd(0) clusters,
and further saturation of allyl alcohols occurs on both
types of active sites: Pd(0) clusters and palladium
phosphide. Of the two types of active sites, the reac-
tion mainly proceeds on those sites that are character-
ized by the lower activation energy. The d metal phos-
phides are generally less active than metal catalysts in phosphide.
KINETICS AND CATALYSIS
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PALLADIUM-PHOSPHORUS NANOPARTICLES AS EFFECTIVE CATALYSTS
587
FUNDING
16. Mashkovsky, I.S., Baeva, G.N., Stakheev, A.Y., Var-
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Temerev, V.L., and Tsyrul’nikov, P.G., Kinet. Catal.,
2019, vol. 60, no. 4, p. 446.
This study was performed under the government con-
tract in the field of research and development of the Russian
Ministry of Education and Science (no. 4.9489.2017/8.9).
The electronic images of the catalyst samples were obtained
on the electron microscope of the Multiaccess Center, Bai-
kal Center for Nanotechnology, Irkutsk National Research
Technical University.
18. Guo, M., Li, H., Ren, Y., Ren, X., Yang, Q., and Li, C.,
ACS Catal., 2018, vol. 8, p. 6476.
19. Vargaftik, M.H., Kozitsyna, N.Yu., Cherkashin, N.V.,
Rudyi, R.I., Kochubei, D.I., Novgorodov, B.N., and
Moiseev, I.I., Kinet. Catal., 1998, vol. 39, no. 6, p. 740.
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