Catalytic activity of CuS nanoparticles in hydrosulfide ions air oxidation

Article abstract of DOI:10.1016/j.molcata.2003.11.010

The most efficient technique for H₂S removal from the wastewaters is the catalytic aeration of the wastewaters, i.e., H₂S oxidation in air-saturated solutions in the presence of catalysts. Photophysical characteristics of colloidal CuS nanoparticles synthesized in various conditions and stabilized in aqueous solutions with sodium polyphosphate were studied. Hydrosulfide ions air oxidation in aqueous solutions at room temperatures and 1 atm proceeded with small rates in the absence of catalysis and increased, substantially upon the injection of CuS nanoparticles into a reacting mixture. The rate of HS^- catalytic oxidation grew at an increase in molar CuS concentration, initial concentration of Na₂S, volume fraction of oxygen in gas mixture bubbled into the reactor, and pH of a solution (≤ 11.9). A scheme for the mechanism of HS^- catalytic oxidation was proposed. According to the scheme, HS^- oxidation is a chain radical reaction initiated on the surface of CuS nanoparticles and propagated further in the bulk of a solution.

Full text of DOI:10.1016/j.molcata.2003.11.010

Journal of Molecular Catalysis A: Chemical 212 (2004) 259–265
Catalytic activity of CuS nanoparticles in hydrosulfide
ions air oxidation
A.E. Raevskaya, A.L. Stroyuk∗, S.Ya. Kuchmii, A.I. Kryukov
L.V. Pysarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prosp. Nauky, 03028 Kyiv, Ukraine
Received 8 August 2003; received in revised form 31 October 2003; accepted 1 November 2003
Abstract
Photophysical characteristics of colloidal CuS nanoparticles synthesized in various conditions and stabilized in aqueous solutions with
sodiumpolyphosphate(SPP)werestudied. AcorrelationbetweenthebandgapofCuSnanoparticlesandtheiraveragediameterwasestablished.
Catalytic activity of colloidal CuS nanoparticles in sodium sulfide air oxidation in aqueous solutions at atmospheric pressure and room
temperatures was established and thoroughly investigated. Kinetics of HS− oxidation and the nature of principal products of this reaction
(thiosulfate and sulfate ions) were elucidated. A scheme for the mechanism of hydrosulfide ions catalytic oxidation was proposed. Accordingly
to the scheme, HS− oxidation is a chain-radical reaction initiated on the surface of CuS nanoparticles and propagating further in the bulk of a
solution.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Copper sulfide; CuS nanoparticles; Hydrogen sulfide oxidation
1. Introduction
In the present paper we discuss the results of the investi-
gation of catalytic activity of nanometer colloidal copper(II)
sulfide particles stabilized with sodium polyphosphate (SPP)
in the oxidation of hydrosulfide ions by molecular oxygen
in aqueous solutions at room temperatures and atmospheric
pressure. Attempts of the interpretation of the kinetics of this
process stimulated an investigation of optical characteristics
of CuS nanoparticles, synthesized in various conditions, in-
cluding the substitution of Cd2+ by Cu2+ ions in the lattice
of CdS nanoparticles of known diameter. The data obtained
in that way let us establish a correlation between the band
gap of CuS nanoparticles (Eg) and their average diameter
(2R) analogous to the well-known correlation Eg −(2R) for
CdS nanoparticles (see for example [6]).
It is worthy of notice that along with the plenty of pa-
pers concerning the photophysical and photochemical prop-
erties of some semiconductor nanoparticles, especially CdS,
ZnO, TiO2, WO3 and MoS2 [6–8], as well as their capa-
bility to catalyze various photochemical reactions [6], there
are virtually no literature examples of the investigation of
the properties of semiconductor nanocrystals which are not
directly bound with the light absorption, in particular, their
catalytic activity in redox-transformations proceeding with-
out the irradiation. The present paper is a part of a cycle of
the investigations of photophysical properties and catalytic
Hydrogen sulfide and its water-soluble salts are wide-
spread impurities of the waste waters of metallurgy, coke,
shale-processing, asphalt, cellulose, food, pigments, artifi-
cial silk, leather and other industries [1–3]. In cities hy-
drogen sulfide is formed in substantial amounts in drainage
systems at biocatalytic reduction of sulfates and through
biomass decomposition.
There are many methods for hydrogen sulfide removal
from the waste waters, in particular reagent and sorption
water purification, bio-, electro-, photo- and radiochemical
decomposition (or oxidation) of H2S and sulfide ions, but
the most efficient technique is the catalytic aeration of the
waste waters, i.e. H2S oxidation in air-saturated solutions in
the presence of catalysts. Activated carbon, nature and arti-
ficial zeolites, polymeric fibers and cellular polymers, mod-
ified with transition metals ions are widely used as aeration
catalysts [1–5].
∗
Corresponding author. Tel.: +38-44-265-6295;
fax: +38-44-265-6295.
E-mail addresses: photochem@ukrpost.net (A.E. Raevskaya),
alstr@ukr.net, photochem@ukrpost.net (A.L. Stroyuk), stephan@ukr.net
(S.Ya. Kuchmii), photochem@ukrpost.net (A.I. Kryukov).
1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.11.010

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A.E. Raevskaya et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 259–265
to 6 × 10−3 M) at 0–25 ◦C (method (a)) leads to the for-
mation of CuS sols which are stable to aggregation. The
colloids absorb the light with λ < 700 nm as a result of
indirect interband transitions in copper(II) sulfide nanopar-
ticles (see Fig. 1a), which is confirmed by the linearity
of spectral curves in (αEhν)0.5 − Ehν coordinates (see
Fig. 1b) [9], where Ehν is quantum energy (eV), α(λ) =
2.303×103D(λ)ρC−1d−1 the CuS absorption coefficient on
the wave length corresponding to the energy Ehν (cm−1),
D(λ) the optical density of colloidal CuS solution, ρ the
density of colloidal CuS nanoparticles which was adopted
to be equal to the density of bulk covellite (4.6 g cm−3) [16],
C the CuS concentration (g cm−3), d the optical path. Ex-
trapolation of a linear section of the spectral curve plotted in
coordinates (αEhν)0.5−Ehν down to the axis of abscissae let
us determine the minimal energy of indirect interband tran-
sition in CuS nanoparticles Eg + Ef (where Eg is the band
gap of CuS nanoparticles, Ef is phonon energy [8,9,17]).
Taking into account small values of Ef in semiconductors
(Ef ∼ 10−2 eV [9,17]), we adopted Eg + Ef ≈ Eg.
activity of some metal-sulfide semiconductor nanoparticles,
in particular CuS, Cu2S and Ag2S [9–12], which we hope
could fill the above-mentioned gap.
2. Experimental
Copper(II) sulfide colloids were prepared by two differ-
ent methods: (a) in a reaction between diluted aqueous so-
lutions of CuSO4 (reagent grade) and Na2S (reagent grade,
Aldrich) in the presence of a stabilizer—sodium polyphos-
phate (NaPO3)n (reagent grade, Aldrich) at intense stirring;
(b) through the complete substitution of Cd2+ cations with
Cu2+ in CdS nanoparticles obtained by mixing of diluted
aqueous solutions of Cd(CH3COO)2 (reagent grade) and
Na2S in the presence of SPP [13]. Sodium sulfide solutions
were prepared immediately before the synthesis of CdS and
CuS in distilled water bubbled with argon. Electronic absorp-
tion spectra were recorded with the use of Specord UV-Vis
spectrophotometer.
Concentrations of sulfur-containing species (HS−,
SO42−, SO32−, S2O32−) in solutions were determined
spectrophotometrically using correspondingly (a) methy-
It is known [6–8] that one of the quantum confine-
ment effects in ultrasmall semiconductor crystals lies in a
change in their electronic characteristics (band gap width
and allowed bands positions) with a change of particles
dimensions in the 1–10 nm region. We established that for a
number of CuS colloids synthesized by method (b) through
the substitution of Cd2+ with Cu2+ in CdS nanoparticles,
one can observe a distinct dependence of Eg on the av-
erage nanoparticles diameter, 2R (see Fig. 1c). X-axis of
the correlation given in Fig. 1c corresponds to the average
diameter of original CdS nanoparticles, whereas Y-axis
corresponds to Eg values of the resulting CuS nanoparti-
cles. When calculating the correlation given in Fig. 1c we
suggested that no substantial change of nanoparticles di-
ameter occurs during the transformation of CdS into CuS.
Such assumption is, in our opinion, quite justified, since in
the course of the substitution of cadmium(II) by copper(II)
the sulfur sublattice of nanocrystals remains practically
undisturbed, so the average particle size should remains
unchanged. It is known that the average diameter of CdS
nanoparticles remains unchanged also at the complete Cd2+
lene blue dye formation (λmax
=
670 nm, ε670
=
24600 M−1 cm−1) in the reaction between H2S and
N,N-dimethyl-n-phenylenediamine in the presence of Fe(III)
in strong-acid media [14]; (b) free chloranile acid (λmax
=
2−
320 nm) generation in the reaction of SO4 with an ex-
cess of barium chloanilate [15]; (c) formation of a dye
2−
(λmax = 590 nm) in a reaction between SO3 and fuchsine
in the presence of formaldehyde in water–alcohol mix-
−
tures [15]; (d) reduction of I3 complex (λmax = 360 nm,
ε360 = 24000 M−1 cm−1) by S(II–IV) compounds resulting
−
in a decrease in the optical density of I3 solution [14].
2−
S2O3 concentrations were calculated by the subtraction
of S2− and SO3 concentrations, which were determined
2−
independently, from the total S(II–IV) content.
Kinetic regularities of Na2S photocatalytic air oxidation
were studied with the use of thermostatically controlled
10.0 ml glass reactor where oxygen, air or mixture of oxy-
gen and argon were bubbled with constant rate through a
peristaltic pump. Reacting mixtures were stirred with mag-
netic stirrer. Diminution of HS− concentration after 60 s of
gas bubbling (V60, M s−1) was used as hydrosulfide ions ox-
idation rate unit.
substitution by other metals cations, for example Pb2+
,
even in spite of the difference between crystal structures of
the original CdS nanoparticles (blende structure) and the
resulting PbS nanoparticles (rock-salt structure) [18]. The
average radii of CdS nanoparticles used for the preparation
of CuS nanoparticles, were determined using equation (I)
[8,13]:
3. Results and discussion
h¯ 2π2
(2R)2
3.1. Synthesis and photophysical properties of CuS
nanoparticles stabilized in aqueous solutions with sodium
polyphosphate
ꢀEg =
((m∗e)−1 + (m∗h)−1
)
where ꢀEg is a gap between Eg values of nanoparticles
and bulk crystal of cadmium sulfide, h¯ the reduced Planck
constant, m∗e and m∗ are the effective masses of CdS con-
duction band electrohns and valence band holes, respectively.
It was established that the interaction between equimo-
lar amounts of CuSO4 and Na2S in aqueous solutions
(1 × 10−4 to 5 × 10−3 M) in the presence of SPP (5 × 10−4

A.E. Raevskaya et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 259–265
261
Fig. 1. (a) Electronic absorption spectra of colloidal CdS solution stabilized with sodium polyphosphate (SPP) (1) and CuS colloid obtained from CdS
after the complete substitution of Cd2+ with Cu2+ (2). [CdS] = 5 × 10−4 M, [CuS] = 5 × 10−4 M, [SPP] = 5 × 10−3 M, optical path d = 1 cm. (b)
0.5
Linearization of the absorption spectrum of CuS colloid synthesized at 18 ◦C and stabilized with SPP in coordinates (αEhν
)
−Ehν. [CuS] = 5×10−4 M,
[SPP] = 5 × 10−3 M. (c) Correlation between the band gap of colloidal CuS nanoparticles, synthesized form CdS (Eg) and the average diameter of
parental CdS nanoparticles (2R).
It should be noted that since we did not accomplish any di-
rect determination of the average size of CuS nanoparticles
of different size, the correlation Eg(R) can be used only for
evaluation of CuS nanoparticles diameter. Fig. 1c shows that
an increase in electronic transition energy induced by spatial
exiton confinement [6,8] becomes especially noticeable for
copper(II) sulfide particles with 2R < 5 nm. Using the cor-
relation given in Fig. 1c we established that the average di-
ameter of CuS nanoparticles synthesized by method (b) and
used as catalysts of HS− oxidation, did not exceed 10 nm.
As can be seen from the data given in Table 1, variations
in the reagents and stabilizer concentrations, as well as in
temperature and pH at which the synthesis of CuS colloid is
performed, allow to obtain purposefully CuS nanoparticles
with certain band gap (from 1.25 to 1.75 eV) and the average
diameter (from 2R ≥ 5 nm).
of CuS nanoparticles towards O2 molecules determines ap-
parently their high catalytic activity in hydrosulfide ions air
oxidation.
3.2. Catalytic activity of CuS nanoparticles in HS−
oxidation by molecular oxygen
Hydrosulfide ions air oxidation in aqueous solutions at
room temperatures and atmospheric pressure proceeds with
small rates in the absence of catalysis and increases substan-
tially upon the injection of CuS nanoparticles into a reacting
mixture (see Fig. 2 and Table 2, row 1). To establish pos-
sible mechanism of HS− catalytic oxidation we performed
detailed investigation of the kinetics of this reaction and
studied dependences of oxidation rates on reagents concen-
trations, temperature and pH of the medium and determined
also composition of a mixture of HS− oxidation products. It
was found that the rate of HS− catalytic oxidation grows at
an increase in molar CuS concentration (see Table 2, row 1),
initial concentration of Na2S (see Table 2, row 2), volume
fraction of oxygen in gas mixture bubbled into the reactor
(see Table 2, row 3) as well as pH of a solution—up to 11.9
Air bubbling through CuS colloids results in a decrease in
the intensity of the absorption band of a solution. Since the
absorption band of CuS colloid restores to the initial form
after an addition of some excess of sodium sulfide, so such
behaviour of a solution was connected with the partial oxi-
dation of CuS particles by dissolved oxygen. Such reactivity

262
A.E. Raevskaya et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 259–265
Table 1
Table 2
Dependence of Eg of colloidal CuS particles upon the synthesis conditions
Rates of hydrosulfide ions air oxidation in various conditions
T·(◦C)
V × 104
No.
[CuS] ×
[SPP] ×
[Na2S] ×
pH
T·(◦C)
Eg·(eV)
No.
[CuS] ×
[Na2S] ×
ν(O2)
(%)
pH
104 (M)
103 (M)
(M min−1
)
104 (M)
103 (M)
103 (M)
1
0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
19
19
19
19
19
19
19
19
11.9
11.9
11.9
11.9
11.9
11.9
11.9
11.9
18
18
18
18
18
18
18
18
0.25
0.70
1.20
1.40
1.65
1.80
2.05
2.10
1
2.0
5.0
10.0
20.0
5.0
5.0
5.0
5.0
–
–
–
–
6.5
18
18
18
18
1.69
1.58
1.54
1.38
2.0
3.5
5.0
6.0
8.0
10.0
12.5
6.5
6.5
6.5
2
3
1.0
2.0
5.0
10.0
20.0
5.0
5.0
5.0
5.0
5.0
10.0
10.0
10.0
10.0
10.0
11.9
11.9
11.9
11.8
11.8
18
18
18
18
18
1.75
1.60
1.53
1.44
1.40
2
5.0
5.0
5.0
5.0
5.0
5.0
5.0
2.0
4.0
6.5
19
19
19
19
19
19
19
10.2
10.6
11.2
11.7
11.9
12.1
12.6
18
18
18
18
18
18
18
0.35
0.50
0.90
1.15
1.40
1.50
2.10
2.0
2.0
2.0
2.0
5.0
5.0
5.0
5.0
–
–
–
–
6.8
6.8
6.8
6.8
3–5
10
25
1.72
1.69
1.66
1.56
8.0
10.0
12.0
16.0
70
4
5
2.0
2.0
2.0
5.0
5.0
5.0
0.5
1.0
5.0
–
–
–
18
18
18
1.58
1.58
1.50
3
4
10.0
10.0
10.0
10.0
12.0
12.0
12.0
12.0
19
36
60
84
12.1
12.1
12.1
12.1
20
20
20
20
2.20
4.40
5.20
7.70
5.0
5.0
5.0
5.0
5.0
0
–
–
–
–
–
6.8
6.8
6.8
6.8
7.2
18
18
18
18
18
1.41
1.69
1.71
1.75
1.75
0.5
1.0
2.0
6.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
12.0
12.0
12.0
12.0
12.0
12.0
12.0
19
19
19
19
19
19
19
10.2
11.2
11.5
11.7
11.9
12.2
12.8
18
18
18
18
18
18
18
0.25
0.95
1.40
1.60
1.70
1.35
0.40
6
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
–
–
–
–
11.6
6.5
4.8
18
18
18
18
1.70
1.69
1.70
1.63
3.5
5
0
0
0
5.0
5.0
5.0
12.0
12.0
12.0
12.0
12.0
12.0
19
19
19
19
19
19
12.1
12.1
12.1
12.1
12.1
12.1
16
30
50
16
30
50
0.20
0.30
0.60
1.60
1.35
1.10
(see Table 2, row 4). Linearization of these dependences in
logarithmic coordinates enabled us to calculate kinetic or-
ders of HS− oxidation on CuS, Na2S, O2 and HO− con-
centrations, which proved to be equal to 0.5, 1.0, 0.7 and
Note: The rate of gas bubbling, 100 ml min−1; reactor volume, 10 ml.
0.5 correspondingly. An increase in the rate of catalytic ox-
idation at the increase of pH of reacting mixture up to 11.9
may be connected with the participation of hydroxide ions
in certain steps of this complex process, while a decrease
in the oxidation rate observed at higher pH (see Table 2,
row 4) may be explained, in our opinion, by the competition
between HS− and HO− anions for adsorption sites on the
surface of CuS nanoparticles in strong-alkaline media.
An increase in the temperature of a reacting mixture exerts
contrariwise influence upon the rates of heterogeneous cat-
alytic and homogeneous non-catalytic oxidation of sodium
sulfide. Thus, HS− oxidation rate in the presence of CuS
nanoparticles decreases at an increase in temperature of a
solution, while usual Arrhenius dependence of the oxidation
rate on the temperature is observed in the absence of cata-
lysts (see Table 2, row 5). The study on the influence of the
temperature on optical properties of CuS colloids allowed
us to suppose at least two different reasons for a reduction
in the catalytic activity of CuS nanoparticles at an increase
in the temperature of a reacting mixture. The first reason
lies in the agglomeration of CuS particles upon heating of
Fig. 2. Change in sodium sulfide concentration in a solution due to HS−
oxidation by air in the absence of catalysts (1), in the absence of catalysts
by pure oxygen (2), by air in the presence of CuS nanoparticles (3), by
pure oxygen in the presence of CuS nanoparticles (4). [CuS] = 1×10−3 M;
[SPP] = 5 × 10−3 M; gas mixture bubbling rate, 100 ml min−1; reactor
volume, 10 ml.

A.E. Raevskaya et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 259–265
263
Fig. 3. Electronic absorption spectra of colloidal CuS solution synthesized
at 18 ◦C (1), and heated for 15 min at 70 ◦C (2). [CuS] = 5 × 10−4 M,
[SPP] = 5 × 10−3 M.
the colloidal solution, which is indicated by a decrease in
Eg value (and subsequently in the average diameter of CuS
nanoparticles) at the synthesis or post-synthesis ageing of a
colloid at raised temperatures. The second reason consists,
in our opinion, in an alteration of the state of the surface of
CuS nanoparticles upon heating of a colloidal solution. An
increase in the temperature at the synthesis of CuS nanopar-
ticles as well as heating of already formed colloidal particles
in aerobic conditions leads to the evolution of a new absorp-
tion band with a maximum lying in near infra-red region of
the spectrum at λ ∼ 1200 nm (see Fig. 3) [9]. In literature
one can find different interpretations of the nature of spec-
tral changes observed in the near-IR section of the spectra
of CuS colloids [8,9,19–21]. The most probable reason for
the evolution of the IR-band is, in our opinion, formation of
copper(I, II) oxides layer on the surface of CuS nanoparticles
and generation of chromophoric centers within this layer,
which are analogous to polynuclear mixed-valence copper(I,
II) complexes with oxygen-containing ligands having simi-
lar intervalence charge transfer absorption bands in near IR
spectral region [20,21]. Transformation of a surface layer of
CuS nano-particles which essentially mediates HS− oxida-
tion should, as we suppose, hamper adsorption of hydrosul-
fide ions from the bulk of a solution and inhibit subsequent
electron transfer with the participation of CuS valence band
holes and adsorbed HS− ions.
It should be noted that colloidal CuS nanoparticles retain
its catalytic activity in the consecutive oxidation of several
(at least four) portions of Na2S centimolar solutions. Spec-
tral properties of CuS nanoparticles characterizing their size
and surface state (intensity and edge of the fundamental ad-
sorption band, Eg values) remain unchanged at multifold
utilization of colloidal solution.
Analysis of reacting mixtures performed in the course of
HS− catalytic oxidation showed that the main products of
the reaction are thiosulfate and sulfate ions (see Fig. 4). At
initial stages of the process small amounts of polysulfides
are permanently present in solution. Accumulation of poly-
sulfides is accompanied by the evolution of an absorption
band with maximum at 360–370 nm, which is characteristic
Fig. 4. HS− concentration decrease (1) and simultaneous accumulation of
the oxidation products: SO3 (2) and SO4 (3) in a solution containing
[CuS] = 5 × 10−4 M, [Na2S] = 1 × 10−2 M, [SPP] = 5 × 10−3 M. Air
bubbling rate, 100 ml min−1; reactor volume, 10 ml.
2−
2−
2−
for Sx
(where x = 2–4) in aqueous solutions at room
temperatures [22]. Formation of sulfite ions in the course of
HS− oxidation was not observed.
3.3. A scheme for a mechanism of hydrosulfide ions air
oxidation in the presence of CuS nanoparticles
It is known that hydrosulfide ions oxidation in aqueous
solutions at room temperatures in the absence of catalysts
is a chain-radical process [1–3,23–25]. HS• and O2 radi-
•−
cals forming in the chain initiation stage at the interaction of
oxygen molecules with HS− ions, are the primary intermedi-
ates responsible for the propagation of chain oxidation. The
process of initiation has small rate (k = 1.5 × 10−3 M−1 s
[25]) in the absence of catalysts and determines apparently
the overall rate of homogeneous hydrosulfide oxidation.
Copper(II) sulfide is a degenerated p-type semiconductor
with Fermi level lying within the valence band [19]. Con-
ductivity of CuS in ground state is therefore determined
by delocalized holes, which are able to oxidize adsorbed
substrates. At the same time a decrease in the intensity of
the absorption band of colloidal CuS upon air bubbling in
the absence of an excess of Na2S indicates the possibility
of CuS nanoparticles oxidation by molecular oxygen. In
the presence of excessive hydrosulfide ions the intensity of
the absorption band of colloidal CuS remains unchanged
at prolonged air bubbling, thus indicating the stabilizing
role of HS− ions in the oxidation of colloidal CuS [19,26].
Taking into account the above-stated considerations, the
initiation of hydrosulfide ions oxidation on the surface
of CuS nanoparticles may be described by the following
reactions:
−
(CuS)n + O2 → (CuS)n−1(CuS)•+ + O2
k1
k2
(1)
(2)
•
(CuS)n−1(CuS)•+ + HS− → (CuS)n + HS•

264
A.E. Raevskaya et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 259–265
analogously to [23], gives us final expression for the HS−
catalytic oxidation rate:
Unfortunately we could not find the values of k1 and k2 in
literature sources. Available data, in particular [26], where
an investigation of the interaction of CuFeS2 nanoparticles
with one-electron oxidants both in the absence and in the
presence of HS− excess was accomplished and the values
of k1 and k2 were shown to be as high as107 M−1 s, indicate
indirectly that reactions (1) and (2) do have sufficiently high
rates.
The propagation of chain oxidation takes place in the bulk
of a solution with the participation of HS• and O2•− radicals
forming in the stages (1) and (2):
d[HS−]
k1k4
= √
ꢀ
−
[HS−] [CuS][O2][HO−]
(IV)
dt
k5k6
Exponents values at CuS, HS−, O2, HO− concentrations in
equation (IV), equal to 0.5, 1.0, 0.5, 0.5, well coincide with
corresponding experimental values of kinetic orders of hy-
drosulfide ions air oxidation catalyzed by CuS nanoparticles.
So, the proposed scheme for the mechanism of the process
under investigation reflects adequately all observed kinetic
regularities, although the lack of the values of rate constants
of the initiation stage (reactions 1 and 2) do not allow us to
use expression (IV) for direct calculation of the rates of cat-
alytic HS− air oxidation in various conditions. Accordance
of the scheme of HS− chain-radical air oxidation with ex-
perimental results indicate also that CuS nanoparticles play
in this process principally the role of initiators and do not
affect appreciably the propagation of chain oxidation in the
bulk of a solution. However, in contrast to “classical” initia-
tors, which are usually consumed in the course of initiation,
in solutions, containing sufficiently high amounts of hydro-
sulfide ions, CuS nanoparticles do not undergo any changes
during oxidation, i.e. they display the features of true
catalyst.
HS• + O2 + HO → O2 − + S + H2O
k3
(3)
−
•
−
−
−
•
O2 + HS + 2H2O → H2O2 + HS• + 2HO
k4
(4)
(5)
O2 − + H2O ↔ HO2• + HO
k5
−
•
O2 − + HO2• + H2O → H2O2 + O2 + HO
k6 (6)
−
•
In the foregoing scheme, the stages (1) and (2) describe the
initiation of chain-radical HS− oxidation on the surface of
copper(II) sulfide nanoparticles, reactions (3) and (4) cor-
respond to the propagation of chain HS− oxidation in the
bulk of a solution, reaction (6) is the principal chain termi-
nation route [23,25]. Oxidation products (Sx2−, S2O32− and
SO42−) accumulate in the following reactions:
4. Conclusions
2−
(x − 1)S + HS− → H+ + Sx
(x = 2–4)
(7)
(8)
(9)
In the paper we discussed the results of the investigation of
photophysical characteristics of colloidal CuS nanoparticles
stabilized in aqueous solutions with sodium polyphosphate
and synthesized in various conditions, including the sub-
stitution of Cd2+ by Cu2+ in CdS nanoparticles of known
average diameter. The study of optical properties of CuS
particles synthesized from CdS nanocrystals of various av-
erage size allowed us to establish a correlation between the
band gap of CuS nanoparticles and their average diameter.
Catalytic activity of colloidal CuS nanoparticles in the hy-
drosulfide air oxidation in aqueous solutions at atmospheric
pressure and room temperatures was established and thor-
oughly investigated. We studied in details kinetics of the ox-
idation and found that the principal products of this reaction
are thiosulfate and sulfate ions. We proposed a scheme for
the mechanism of hydrosulfide ions catalytic oxidation. Ac-
cordingly to the scheme, HS− oxidation is a chain-radical
reaction initiated on the surface of CuS nanoparticles and
propagated further in the bulk of a solution.
S22− + O2 + H2O → S2O32− + 2H+
2S + 2O2 + 2H2O → 2SO42− + 4H+
An analysis of the aforecited scheme leads to the following
expression for the rate of sodium sulfide air oxidation in the
presence of CuS nanoparticles:
d[HS−]
= k2[CuS][HS−] + k4[HS−][O2
]
(I)
−
•
−
dt
Using an approximation of stationary concentrations we ob-
tain the following expressions:
−
•
d[O2
]
−
= k1[CuS][O2] + k3[HS•][O2][HO−]
dt
−
− k4[O2 −][HS−] − k6[O2 ][HO2 ] = 0
(II)
•
•
•
d[HS•]
= k2[CuS•−][HS−] + k4[O2 −][HS−]
•
−
dt
− k3[HS•][O2][HO−] = 0
(III)
References
Solution of the equations (I)–(III) with the assumption of
sufficiently long oxidation chains (when we may neglect
the initiation rate in comparison with the contribution of
reactions (4) and (5) in the overall HS− oxidation rate),
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