Reactivity of copper nanopowders in a model reaction of isopropylbenzene oxidation

Article abstract of DOI:10.1007/s11167-005-0385-x

The reactivity of copper nanopowders produced by an electric explosion of a conductor or mechanochemically was studied. Oxidation of isopropylbenzene was used as a model reaction. The dependence of the oxygen uptake rate on the specific surface area of a copper nanopowder and on the method used for its production is discussed. A possible mechanism of isopropylbenzene oxidation in the presence of copper nanopowders is suggested.

Full text of DOI:10.1007/s11167-005-0385-x

Russian Journal of Applied Chemistry, Vol. 78, No. 5, 2005, pp. 753 757. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 5,
2005, pp. 767 771.
Original Russian Text Copyright
2005 by Skorokhodova, Kobotaeva, Sirotkina.
CATALYSIS
Reactivity of Copper Nanopowders in a Model Reaction
of Isopropylbenzene Oxidation
T. S. Skorokhodova, N. S. Kobotaeva, and E. E. Sirotkina
Institute of Petrochemistry, Siberian Division, Russian Academy of Sciences, Tomsk, Russia
Received July 27, 2004; in final form, December 2004
Abstract The reactivity of copper nanopowders produced by an electric explosion of a conductor or
mechanochemically was studied. Oxidation of isopropylbenzene was used as a model reaction. The depen-
dence of the oxygen uptake rate on the specific surface area of a copper nanopowder and on the method used
for its production is discussed. A possible mechanism of isopropylbenzene oxidation in the presence of
copper nanopowders is suggested.
The intermediate position occupied by nanopar-
ticles in passing from a bulk metal to a separate atom
predetermines the deviation of their physicochemical
properties from those of bulk metals, on the one hand,
and from the properties of isolated atoms, on the
other. This deviation is manifested in electronic, mag-
netic, optical, and other properties of nanoparticles
with a characteristic size of several tens of nanometers
to several nanometers [1]. Numerous summarizing
papers concerning methods for preparation and stabili-
zation of nanoparticles and their optical and other
properties have already appeared, whereas studies
of chemical transformations involving nanoparticles
are only in their initial stage [2].
EXPERIMENTAL
As objects of study served copper NP obtained by
an electric explosion of a conductor in the atmosphere
of nitrogen, argon, and xenon, as well as copper NP
produced mechanochemically on an AGO-2 installa-
tion in the presence of organic and inorganic addi-
1
tives. The additives were used to raise the specific
surface area of the mechanochemically produced NP
[7]. Copper nanopowders were produced by mechani-
cal treatment (MT) in AGO-2 planetary-centrifugal
high-energy-intensity mills in the presence of an or-
ganic surfactant (2,2,3,3,4,4,5,5-octafluorovaleramide)
in an amount of 10% for 10 (Cu I), 20 (Cu II), and
30 min (Cu III). Copper powders were also prepared
by mechanical treatment in the presence of 15 wt %
CuCl (CuCl was washed out with ethanol after MT)
on an AGO-2 installation for 10 (Cu 1), 20 (Cu 2),
and 30 min (Cu 3). The physicochemical parameters
and the preparation conditions of copper NPs used
in this study are listed in Table 1.
It is known [3] that nanosize powders (NPs) of
metals show an increased reactivity, compared to
compact metals. For example, reactions that do not
occur in the presence of some metals or require severe
conditions readily proceed in the presence of metal
nanopowders. In particular, synthesis of complex
compounds (phthalocyanines and tetraphenylpor-
phyrins) with the use of copper and indium NP [4, 5]
is performed under milder conditions (room tempera-
ture and atmospheric pressure) than the conventional
synthesis with metal salts. Use of metal NPs as cata-
lysts for oxidation of organic compounds is known.
It was shown in [6] that the uptake of oxygen in oxi-
dation of isopropylbenzene (IPB) in the presence of
NPs of copper and cobalt proceeds at a sufficiently
high rate even at 30 C without any initiator.
2
2
IPB was oxidized on a gasometric installation [8].
A 0.7-M portion of IPB and 0.12 wt % copper NP
were placed in the reactor. The reaction was per-
formed at 30 and 60 C without initiator (azodiiso-
butyronitrile, AIBN). The reaction was stopped at the
1
minimum rate of oxygen uptake (10 15 l min ),
which is determined by the scale factor of the buret in
the gasometric installation. In some cases, repeated
oxidation was performed 24 h after the end of the first
1
Electric-explosion Cu NPs were obtained at the Institute of
In this study we examined the reactivity of copper
NP, obtained by an electric explosion of a conductor
(EEC) or mechanochemically, in a model reaction of
IPB oxidation.
High-Current Electronics, Siberian Division, Russian Acad-
emy of Sciences, Tomsk; mechanochemical Cu NPs, at the
Institute of Solid-State Chemistry and Mechanochemistry,
Siberian Division, Russian Academy of Sciences, Novosibirsk.
1070-4272/05/7805-0753 2005 Pleiades Publishing, Inc.

754
SKOROKHODOVA et al.
Table 1. Physicochemical parameters and preparation conditions of copper nanopowders
Preparation
technique
Ssp,
Time of
MT, min
Wmax,**
l min
Induction
period, min
Powder
Cu 8
Cu 11
Cu 13
Cu 18
Cu 18(2)
Cu 19
Cu I
Cu II
Cu III
Cu 1
Cu 2
Cu 3
E/Es*
Medium
Additive
1
1
m2 g
EEC
EEC
EEC
EEC
EEC
EEC
MT
MT
MT
MT
MT
1.0
1.2
0.8
2.0
2.0
1.0
8.7
6.0
2.2
2.1
9.7
5.6
3.2
3.9
5.2
4.3
6.2
6.9
Nitrogen
Xenon
Argon
Nitrogen
Nitrogen
Nitrogen
Air
335
300
100
100
365
280
H(CF2CF2)2CONH2
H(CF2CF2)2CONH2
H(CF2CF2)2CONH2
CuCl2
10
20
30
10
20
30
19
17
6
Air
Air
Air
Air
CuCl2
CuCl2
MT
Air
* E, supplied energy; E , energy of metal sublimation.
s
** W
, maximum rate of oxygen uptake.
max
oxidation. Liquid reaction products (acetophenone,
AP, and dimethylphenylcarbinol, DMPC), were ana-
lyzed by gas liquid chromatography on a Perkin
X-ray diffraction analysis was made on a DRON-1
instrument with a cobalt radiation source. The X-ray
diffraction patterns were recorded by point-by-point
2
Elmer Sigma 2B chromatograph with a flame-ioniza- scanning with a step of 0.05 , with recording into
tion detector on an SE-52 column (column length
33 m) in the programmed-temperature mode: initial
thermostat temperature 80 C, temperature raised at
a file for further computer processing.
The efficiency of copper NPs as catalysts for IPB
oxidation was judged from the rate of oxygen uptake
by a system constituted by IPB and Cu NP. Figure 1a
shows the most typical kinetic curves of oxygen up-
take in oxidation of IPB in the presence of 0.12 wt %
Cu NP produced by EEC in various inert gases.
Curves 1 and 2 are characteristic of Cu NP possessing
a sufficiently large specific surface area [Cu 18(2),
Cu 8, Cu 19, and Cu 11]. The run of the kinetic
curves is about the same irrespective of the medium in
which a powder was obtained: an exceedingly high
rate at the beginning of the reaction and an abrupt fall
in 10 15 min (Fig. 1). In the system constituted by
IBP and Cu 13 NP with a small specific surface area
1
a rate of 3 deg min to 150 C. Carrier gas helium.
Quantitative analyses were performed with an internal
reference (n-hexane). IPB hydroperoxide (IPB HP)
was determined iodometrically by the standard proce-
dure [9].
1
(a)
W, l min
2
1
(S = 2.2 m g ), oxygen is taken up at a low rate
sp
during the entire reaction course (Fig. 1, curve 3). For
Cu NP produced by EEC, the following tendency can
be noted: the larger the specific surface area of Cu
NP, the higher the rate of oxygen uptake by the sys-
tem constituted by IBP and Cu NP (Table 1).
1
t, min
W, l min
(b)
A somewhat different behavior is exhibited in IBP
oxidation by Cu NP produced mechanochemically.
Irrespective of an additive, the rate of oxygen uptake
gradually increases with time (Fig. 1b), with this type
of curves characteristic of all mechanochemical Cu
t, min
Fig. 1. Rate W of oxygen uptake vs. time t in oxidation
of IPB in the presence of nanopowders. Nanopowder:
(a) (1) Cu 8, (2) Cu 11, and (3) Cu 13; (b) Cu II.
Temperature 60 C; the same for Fig. 2.
2
X-ray studies were carried out at the Institute of Strength
Physics and Materials Science, Siberian Division, Russian
Academy of Sciences, Tomsk.
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REACTIVITY OF COPPER NANOPOWDERS
755
Table 2. Composition of products formed in isopropylbenzene oxidation in the presence of a copper nanopowder
Yield, %
Concentration of
copper powder, %
Oxidation tem-
Powder
Conversion, %
perature,
C
IPB HP
AP
DMPC
Cu 8
Cu 11
Cu I
Cu II
Cu III
Cu 0
0.53
0.6
0.59
0.8
0.30
0.32
0.77
0.88
0.5
0,35
0.44
1.00
1.25
0.64
1.18
1.36
2.36
2.93
1.83
0.12
0.13
0.12
0.12
0.14
60
60
60
60
60
60
80
60
0.69
Cu 0
Initiator
Trace amounts
1.75
0.12
NPs. The powders are as if shaken up in the course
of time. The oxygen uptake in IPB oxidation in the
presence of mechanochemical Cu NP begins only
after a certain induction period (Table 1).
of the reaction, the amount of reaction products is in-
significant, in contrast to those characteristic of mech-
anochemical Cu NPs, at the same oxygen uptake. For
example, the conversion is 1.36% (0.60 IPB HP, 0.32
AP, 0.44 DMPC) for electric-explosion copper Cu 11
with a specific surface area of 6 m g and 2.9%
(0.80 IPB HP, 0.88 AP, 1.25 DMPC) for mechanical-
ly treated Cu II NP. An increase in the mass of Cu NP
by a factor of nearly 10 does not lead to any sig-
nificant rise in the yield of the reaction products.
The time in which the minimum rate of oxygen
2
1
1
uptake (10 15 l min ) is reached depends on the
method by which Cu NPs are obtained: for NP pro-
duced by EEC, the minimum oxidation rate is reached
rather rapidly (in 30 40 min), and this occurs the
faster, the larger the specific surface area of the
powder. For mechanochemical Cu NP, the time in
which the minimum rate of oxygen uptake is reached
is long and independent of the specific surface area of
the powder. After the minimum rate of oxygen uptake
is reached, the reaction may continue during an in-
finitely long time until the whole amount of IPB
enters into the reaction or the reaction terminates
because of recombination processes.
It is known [11] that IPB oxidation in the presence
of a copper catalyst occurs by the radical-chain mech-
anism, with the primary radicals appearing on the
surface of copper. This makes it possible to suggest a
heterogeneous-homogeneous mechanism of the reac-
tion by the scheme
(M) + O2
(M+) (O O) + RH
(M+) + (OOH)
(M+) (O O) ,
IPB oxidation in the presence of a Cu powder with
a particle size of 40 m and a specific surface area of
(M+) + (OOH) + R ,
.
2
1
.
0.016 m g (Cu 0) deserves separate consideration.
Copper with such a specific surface area does not
catalyze the IPB oxidation at 60 C. At 80 C, the reac-
tion starts in 15 min, and then oxygen is adsorbed
at a very low, nearly constant rate.
(M) + HO2,
(1)
.
.
HO2 + RH
H2O2 + R .
The performance of a catalyst depends on the rate
at which the active centers on the catalyst surface can
adsorb reactants, retain them during the chemical
transformation, and then release the reaction products
to be free to perform a new catalytic cycle [1].
A chromatographic analysis of the reaction prod-
ucts formed in IPB oxidation in the presence of Cu
NP demonstrated the presence of not only IPB HP,
but also AP and DMPC (Table 2), irrespective of the
preparation method, specific surface area, preparation
medium, and other physicochemical properties of NP
(Table 1). When used in the form of shavings, metal-
lic copper decomposes hydroperoxides [10]; probably,
Probably, fast sorption of oxygen at the first instant
of time is natural for Cu NPs with a sufficiently large
specific surface area. After the surface of copper is
saturated with oxygen, the rate of uptake decreases
the same behavior is characteristic of Cu NPs because significantly because three processes start to compete:
it was impossible to selectively obtain only IPB HP
in the presence of Cu NP. For electric-explosion Cu
NPs, which have a large specific surface area and
exhibit a high rate of oxygen uptake at the beginning
oxygen uptake, formation of reaction products on the
surface, and desorption of the reaction products into
solution. Chromatographic analysis for the reaction
products, made immediately after the reaction ter-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 5 2005

756
SKOROKHODOVA et al.
means that the surface is not freed of oxygen even
1
(a)
W, l min
in the course of time. The curve of oxygen uptake in
the repeated oxidation in the presence of mechano-
chemical Cu NP runs somewhat higher than that of
the first oxidation (Fig. 2b). It was assumed that oxy-
gen is consumed for oxidation of copper, and an
X-ray diffraction analysis was carried out. However,
an X-ray diffraction analysis did not reveal any addi-
tional formation of copper oxides after the reaction.
Oxygen sorbed on the surface does not oxidize copper,
but, probably, can be rather firmly retained on the sur-
face of powder particles, without being involved in
IPB oxidation to give reaction products; for example,
it may form charge-transfer complexes with copper.
t, min
1
W, l min
(b)
Figure 3 shows X-ray diffraction patterns of Cu 11
NP before and after the oxidation. The patterns reveal
no difference between the structures of the initial NP
and that isolated after the reaction. For example, the
lattice constants (fcc unit cell) differ by less than
0.01% and the size of crystallites, determined from
the angular broadening of the lines, was 60 and 62 nm
in the initial and treated states, respectively. The only
difference that could be revealed in the X-ray diffrac-
tion patterns is that the intensity of weak lines at 40
48 , which, according to a phase analysis made on the
basis of the ASTM file, belong to CuO (tenorite struc-
ture, 41-254 card), changes: after treatment of the
powder, their intensity increases.
t, min
Fig. 2. Rate W of oxygen uptake vs. time t in oxidation
of IPB in the presence of (a) Cu 8 and (b) Cu I nano-
powders. Oxidation: (1) first and (2) repeated in 24 h.
I, arb. units
It was of interest to reveal the nature of the active
center in Cu NPs, which can activate oxygen by
scheme (1) and give rise to radical-chain transforma-
3
tions. A study was carried out in order to elucidate
2 , deg
Fig. 3. X-ray diffraction patterns of Cu11 powder (1) before
and (2) after the oxidation. (I) Intensity and (2 ) Bragg
angle.
the reasons for the increase in the reactivity of a
mechanochemically treated copper powder, compared
to the same powder in the initial state. It was demon-
strated by X-ray photoelectron spectroscopy (XPS)
that the intensity of oxygen peaks with a binding
energy of 530.9 531.1 eV, which is not characteristic
of adsorbed oxygen or copper oxides, increases in
mechanochemically treated copper NPs [12]. The
authors of this study believe that this peak may belong
to the lattice, or suboxide, oxygen bound with defec-
tive sites in copper powder particles. In view of the
fact that the bonding energy of the lattice oxygen
minated, reveals trace amounts of AP and DMPC.
This means that the oxygen sorption is faster than
formation of the reaction products and their desorption
from the surface into the bulk. Nevertheless, after
a certain time, when all the products would have dif-
fused from the surface into the bulk, the conversion in
the oxidation in the presence of electric-explosion Cu
NPs with a large specific surface area is considerably
lower than that with mechanochemical NPs. Probably,
part of oxygen involved in oxidation is irreversibly
sorbed on these NPs. Repeated oxidation of the sys-
tem constituted by IPB and Cu NPs was performed in
24 h. The rate of oxygen uptake in the repeated oxida-
tion of the system constituted by IPB and Cu NP
prepared by the EEC method is always lower than that
in the first oxidation of the same system, irrespective
of the specific surface area of the NP (Fig. 2a). This
exceeds that in the oxides CuO and Cu O, it may be
2
stated that the bonding of copper atoms with the lat-
tice oxygen is largely covalent, i.e., the suboxide
oxygen donates its excess electron density to a copper
atom bonded with it. The authors assume that the role
of active centers in a copper sample activated by
3
At the Institute of Solid-State Chemistry and Mechanochemis-
try, Siberian Division, Russian Academy of Sciences.
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REACTIVITY OF COPPER NANOPOWDERS
757
mechanical treatment or other methods leading to
saturation with the lattice oxygen is played by oxygen
atoms covalently bonded to copper atoms. Therefore,
they suggest the following model of interaction of
such an active center with molecular oxygen:
peated oxidation runs exceeds that in the first oxida-
tion of isopropylbenzene, which may be due to the
high degree of caking of these powders and their
shake-up in the course of time.
(3) The final products are isopropylbenzene hydro-
peroxide, acetophenone, and dimethylphenylcarbinol.
The relative amounts of the reaction products depend
on the method used to produce the nanopowders.
(4) A possible mechanism of isopropylbenzene
oxidation in the presence of Cu nanopowders was
suggested. This mechanism is associated with the
presence of suboxide oxygen, which is involved in the
oxidation and activates molecular oxygen.
The lattice oxygen donates its excess electron
density to the neighboring Cu atom, whose charge
rapidly reaches the surface atom. The presence of an
excess electron density on surface atoms leads to
activation of molecular oxygen, which can then ini-
tiate radical-chain transformations in the catalytic
oxidation of IPB.
REFERENCES
1. Bukhtiyarov, V.I. and Slin’ko, M.G., Usp. Khim.,
2001, vol. 70, no. 2, pp. 167 181.
The data of [12] can be used to explain the results
obtained in this study. The copper NPs mechanically
treated in ball mills in air probably contain a larger
amount of suboxide oxygen capable of formation of
active centers giving rise to radical chain reactions,
because a 2 times greater amount of reaction products
is formed. In addition, it is possible that the mechano-
chemical Cu NP treated in the presence of organic
additives has on its surface a smaller number of free
active sites at which oxygen not involved in the oxi-
dation could be adsorbed. This is so because these
sites, represented by various dislocations and struc-
tural surface defects, are already occupied in part by
surfactants (e.g., by an organic surfactant, 2,2,3,3,
4,4,5,5-octafluorovaleramide), which can themselves
form complexes with copper.
2. Sergeev, G.B., Usp. Khim., 2001, vol. 70, no. 10,
pp. 915 933.
3. Gusev, A.I., Nanokristallicheskie materialy: metody
polucheniya i svoistva (Nanocrystalline Materials:
Preparation Methods and Properties), Yekaterinburg:
Ural’sk. Otd. Ross. Akad. Nauk, 1998.
4. RF Patent 2103995.
5. RF Patent 2157373.
6. Sirotkina, E.E., Kobotaeva, N.S., Svarovskaya, N.V.,
et al., in 32nd Int. Annual Conf. of ICI, Karlsruhe
(Germany), July 3 6, 2001, pp. 97 105.
7. Polyboyarov, V.A., Lapin, A.E., Korotayeva, Z.A.,
et al., Abstracts of Papers, Int. Conf. Fundamental
Bases of Mechanochemical Technologies, Novosi-
birsk (Russia), August 16 18, 2001, pp. 67 69.
8. Emanuel’, N.M., Gladyshev, G.P., Denisov, E.T.,
et al., Poryadok testirovaniya khimicheskikh soedine-
nii kak katalizatorov polimernykh materialov (Proce-
dure for Testing of Chemical Compounds as Catalysts
for Polymeric Materials), Chernogolovka: Otd. Inst.
Khim. Fiz. Akad. Nauk SSSR, 1976.
9. Houben-Weyl, Methoden der organischen Chemie,
vol. 2: Analytische Methoden, Stuttgart: Georg
Thieme, 1953.
10. Lebedev, N.N., Khimiya i tekhnologiya osnovnogo
organicheskogo i neftekhimicheskogo sinteza (Chemis-
try and Technology of Basic Organic and Petrochemi-
cal Synthesis), Moscow: Khimiya, 1981.
11. Shalya, V.V., Kolotusha, B.I., Yampol’skaya, F.A.,
and Gorokhovatskii, Ya.B., Kinet. Katal., 1972,
vol. 13, no. 2, pp. 404 408.
For copper NP produced by EEC in an inert at-
mosphere, presence of a large amount of suboxide
oxygen is unlikely, and, therefore, it may be assumed
that most part of absorbed oxygen is not activated and
is not involved in the oxidation, being irreversibly
sorbed at defective sites on the surface.
CONCLUSIONS
(1) In oxidation of isopropylbenzene in the pres-
ence of electric-explosion copper nanopowders ob-
tained in an inert atmosphere, the maximum rate of
oxygen uptake depends on the specific surface area of
the nanopowders. The rate of oxygen uptake in the
first oxidation run exceeds that in the case of a re-
peated oxidation run.
12. Polyboyarov, V.A., Lapin, A.E., Korotayeva, Z.A.,
et al., Abstracts of Papers, Int. Conf. Mechanochemi-
cal Synthesis and Sintering, Novosibirsk (Russia),
June 14 18, 2004, pp. 122 123.
(2) With mechanochemically produced copper
nanoparticles, the rate of oxygen uptake in the re-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 5 2005