New catalysts and adsorbents on the basis of the InSb-CdTe semiconducting system

Article abstract of DOI:10.1134/S0036024407040061

The acid-base properties of solid solutions and binary components of the InSb-CdTe system were studied by IR spectroscopy, pH isoelectric point measurements, and conductometric titration; adsorption properties with respect to CO, O₂, NO₂, NH₃, CO + O₂, and NO₂ + NH₃, by piezoquartz microweighing; and catalytic properties in the oxidation of carbon(II) oxide and reduction of nitrogen(IV) oxide with ammonia, by the pulsed and circulation flow methods. The nature, strength, and concentration of acid centers were determined. Changes in the concentration of acid centers under the action of gases (NO₂ and NH₃), gamma irradiation, and composition variations were estimated. The experimental dependences, thermodynamic and kinetic adsorption characteristics, the electrophysical, acid-base, and other physicochemical characteristics of the adsorbents, and adsorption characteristic-composition phase diagrams were analyzed taking into account the electronic nature of adsorbate molecules to determine the mechanism and characteristics of adsorption processes depending on the conditions of adsorption and the composition of the system. The results of adsorption studies were used to preliminarily determine the temperature regions of the occurrence and the mechanism of the reactions studied. A shock mechanism was suggested. Separate components (predominantly, solid solutions) of the InSb-CdTe system showed high catalytic activity at comparatively low temperatures. Along with behavior common to the system and its binary compounds (InSb and CdTe), solid solutions exhibited features characteristic of multi-component systems. These were the presence of extrema in the pH_iso-composition, adsorption characteristic-composition, and catalytic activity-composition diagrams. The use of these diagrams allowed us to discover system components most active with respect to the gases and reactions studied and create high-sensitivity and selective sensors and high-activity and selective catalysts on the basis of these components. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036024407040061

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2007, Vol. 81, No. 4, pp. 535–543. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © I.A. Kirovskaya, 2007, published in Zhurnal Fizicheskoi Khimii, 2007, Vol. 81, No. 4, pp. 627–636.
CHEMICAL KINETICS
AND CATALYSIS
New Catalysts and Adsorbents
on the Basis of the InSb–CdTe Semiconducting System
I. A. Kirovskaya
Omsk State Technical University, Omsk, Russia
E-mail: phiscem@omgtu.ru
Received December 23, 2005
Abstract—The acid–base properties of solid solutions and binary components of the InSb–CdTe system were
studied by IR spectroscopy, pH isoelectric point measurements, and conductometric titration; adsorption prop-
erties with respect to CO, é2, Né2, NH3, CO + O2, and NO2 + NH3, by piezoquartz microweighing; and cata-
lytic properties in the oxidation of carbon(II) oxide and reduction of nitrogen(IV) oxide with ammonia, by the
pulsed and circulation flow methods. The nature, strength, and concentration of acid centers were determined.
Changes in the concentration of acid centers under the action of gases (NO2 and NH3), gamma irradiation, and
composition variations were estimated. The experimental dependences, thermodynamic and kinetic adsorption
characteristics, the electrophysical, acid–base, and other physicochemical characteristics of the adsorbents, and
adsorption characteristic–composition phase diagrams were analyzed taking into account the electronic nature
of adsorbate molecules to determine the mechanism and characteristics of adsorption processes depending on
the conditions of adsorption and the composition of the system. The results of adsorption studies were used to
preliminarily determine the temperature regions of the occurrence and the mechanism of the reactions studied.
A shock mechanism was suggested. Separate components (predominantly, solid solutions) of the InSb–CdTe
system showed high catalytic activity at comparatively low temperatures. Along with behavior common to the
system and its binary compounds (InSb and CdTe), solid solutions exhibited features characteristic of multi-
component systems. These were the presence of extrema in the pHiso–composition, adsorption characteristic–
composition, and catalytic activity–composition diagrams. The use of these diagrams allowed us to discover
system components most active with respect to the gases and reactions studied and create high-sensitivity and
selective sensors and high-activity and selective catalysts on the basis of these components.
DOI: 10.1134/S0036024407040061
INTRODUCTION
used to reveal extremal effects, which indicate the way
for seeking new materials. This is substantiated by the
results already obtained [1].
The InSb–CdTe system is one of the complex semi-
conducting systems studied by us to find effective
materials for modern technologies, primarily sensor
electronic devices, adsorbents, and catalysts.
The passage to such systems is a difficult task, there
is no theory for them because of the scarcity of experi-
mental data.At the same time, the discovery of the quite
unexpected properties of new systems is evidence that
they offer much promise for practical applications and
therefore require systematic studies.
The selection of the chemical reagents (CO, O2,
NO2, and NH3) as adsorbates and possible participants
of catalytic reactions was determined not only by their
electronic nature, but also by toxicity considerations
and the possibility of solving ecological problems.
EXPERIMENTAL
Our interest in the objects of study selected in this
work, (InSb)x (CdTe)1 – x solid solutions, as in similar
systems, stems from the unique properties of the initial
binary compounds AIIIBV and AIIBVI (primarily electro-
physical, photo- and piezoelectric, and optical proper-
ties) already used in technology and semiconducting
catalysis.
In each of the areas specified, an important role is at Tcond = 298 K and p = 1.33 × 10–3 Pa onto glass plates,
played by the surface. It follows that we must thor- KBr single crystals, and electrode surfaces of piezo-
oughly study its physicochemical properties. As far as quartz resonators. After deposition, the films were
data on the character of composition-dependent annealed in initial component vapors [3]. Film thick-
changes in these properties are concerned, they can be nesses were determined interferometrically and from
The objects of study were powders (ssp = 0.405–
0.62 m2/g) and films (d = 0.25–0.35 µm) of InSb and
CdTe and their solid substitution solutions
(InSb)x(CdTe)1 – x (x = 0.01–0.05 and 0.94–0.99) pre-
pared by isothermal diffusion in the regions of the
mutual solubility of the binary components [2]. The
films were prepared by discrete deposition in a vacuum
535

536
KIROVSKAYA
Table 1. pH isoelectric points (pHiso) of the surface of the
InSb and CdTe binary compounds and (InSb)x(CdTe)1 − x
solid solutions subjected to various treatments
rence of carbon(II) oxide oxidation and nitrogen(IV)
oxide reduction with ammonia was controlled by mea-
suring CO and NO2, NO [10] content changes in gas
mixtures.
x
I
II
III
IV
V
Studies of the adsorption of pure reaction partici-
pants and their mixtures gave valuable information that
was used to preliminarily determine the temperature
regions of the occurrence of the reactions under consid-
eration and study their mechanisms. Conclusions con-
cerning changes in the electronic state of the surface
during adsorption were drawn from changes in conduc-
tivity measured by the probe method [5].
1.0
6.7
6.63
–
6.91
5.82
–
5.62
5.65
5.8
0.99
0.98
0.97
0.95
0.05
0.04
0.03
0.01
0.0
6.57
7.57
7.2
7.10
–
–
–
–
–
–
5.65
–
6.56
7.47
7.53
7.60
7.43
7.34
–
–
–
7.08
7.11
7.12
7.10
7.05
–
7.47
7.54
7.56
7.43
7.34
7.51
7.35
7.3
–
The reproducibility of the results was checked by
conducting repeat experiments. Calculations and statis-
tical data processing were performed on a PC.
–
–
6.98
6.95
7.55
RESULTS AND DISCUSSION
Note: I, storage in air; II and III, exposure to NO and NH , respec-
2
3
tively, at 300 K (17 h); and IV and V, gamma irradiation with
doses of 0.18 and 0.54 Mrad, respectively.
A Study of Acid–Base Properties
IR Spectroscopic Study. According to the IR data
[6], the surface of the binary components and solid
solutions of the InSb–CdTe system exposed to air pre-
dominantly contained adsorbed water molecules and
hydroxyl groups (bands at 1850 and 730–750 cm–1),
carbon-containing compounds (bands at 2350 (ëé2),
1100 (C–O), and 2950 and 980 cm–1 (C–H)), and intrin-
sic oxides (a band at 610 cm–1 (B–O)).
changes in the piezoquartz resonator frequency [4].
Conclusions on the structure of the films were drawn
from X-ray data, which were, together with forbidden
gap and conductivity measurements, used to certify
solid solutions [2].
The acid–base properties of the surface were studied
by IR spectroscopy, hydrolytic adsorption (the determi-
nation of the pH isoelectric point), mechanochemistry,
and nonaqueous conductometric titration [5, 6].
Adsorption was studied by piezoquartz micro-
weighing (sensitivity 1.23 × 10–11 g/(cm2 Hz)). Films-
adsorbents were deposited on the electrode surfaces of
lens-shaped AT-cut piezoquartz resonators with an
intrinsic vibrational frequency of 7–8 mHz [4]. The
same samples were used to simultaneously study
changes in conductivity and, accordingly, surface
charging under the influence of the gases adsorbed. The
temperature and pressure ranges were 253–473 K and
0.5–20 Pa, respectively.
Catalytic studies were preformed by the gradientless
pulsed and flow-circulation methods [5] under the con-
ditions that excluded the influence of mass and heat
transfer at T = 290–473 K and p = 101–103 kPa; the
carrier gas flow rate was 8–26 ml/min, and the pulse
volume was 0.5–1.5 ml. To ensure ideal displacement
conditions and remove outside diffusion deceleration in
the flow reactor, special rules and techniques were fol-
lowed [7, 8].
After thermal treatment in a vacuum until gas
release was minimized, the surface became virtually
free of adsorbed impurities and, to a substantial extent,
oxides, as in [4, 5].
The Determination of the Isoelectric Point of the
Surface (pHiso). The main results obtained in the deter-
mination of pHiso on the surface of the InSb–CdTe sys-
tem components stored in air for a long time, exposed
to nitrogen dioxide and ammonia, and subjected to
gamma irradiation are listed in Table 1.
According to the pHiso values, the surface of InSb
and solid solutions enriched in InSb is weakly acid,
whereas the surface of CdTe and solid solutions
enriched in CdTe is weakly basic. In both cases, the
composition dependence of pHiso, like the composition
dependences of structural characteristics (a, dhkl, and
ρr) and energy gap width (∆E), contained extrema.
Similar shapes of property–composition dependences,
the analysis performed in [11], and the data obtained in
[12, 13] are an additional piece of evidence in favor of
the formation of substitution solid solutions in the
InSb–CdTe system.
Argon (and air for comparison) was predominantly
used as a carrier gas. Gases-adsorbates (CO, é2, NO2,
and NH3 reagents) were prepared following standard
procedures [5, 9], and reaction components were mixed
in the ratios CO : é2 = 1 : 2 and NO2 : NH3 = 1 : 2.
The character of the composition dependence of
pHiso remains the same for samples subjected to the
action of gases (NO2 and NH3) and gamma irradiation.
In NO2 and under gamma irradiation, the surface is
The specific catalytic activity of samples was evalu- acidified, and in NH3, it becomes alkalized. The behav-
ated from the specific reaction rate at the given temper- ior of pH in gases can be explained in terms of the
ature and reaction mixture composition. The occur- known concepts of the mechanism of interaction of var-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007

NEW CATALYSTS AND ADSORBENTS
537
ious gases with the surface of diamond-like semicon-
ductors [14].
The same concepts also explain the behavior of the
pH
5
surface of the system components under gamma irradi-
ation. In agreement with these concepts, the NO2, NH3,
and similar molecules largely interact with coordina-
tion-unsaturated surface atoms to produce donor–
acceptor or dative bonds. These formations can be
sources of radical ions as a result of complete ionization
when they additionally capture free electrons or holes,
which is favored by the participation of closely situated
defects. Gamma irradiation creates fairly stable radia-
tion defects (primarily, structure defects) in the bulk
and on the surface. They act either as direct adsorption
centers or as donors and acceptors of electrons partici-
pating in adsorption bonding. This results in the dis-
charging of ionized impurities present on the real sur-
face, which is accompanied by weakening or the disap-
pearance of the corresponding bands in the IR spectra
(C–O, B–O, and éç– group). In addition, this facili-
tates the dissociation of adsorbed water with the forma-
tion of Me–H bonds and surface dehydration. Indeed,
the observed decrease in pHiso under the action of
gamma irradiation is evidence of a decrease in the con-
tent of OH groups and an increase in the content of
hydrogen ions.
1
4
2
~
~
6
1'
4
0
2'
10
20
30
τ, min
Fig. 1. Kinetic isotherms of the dispersion in water of
coarse-grained (1, 2) InSb and (1', 2') CdTe powders (1,
1') exposed to air and (2, 2') subjected to gamma irradiation,
dose 0.54 Mrad.
tion-unsaturated atoms (Lewis centers) and adsorbed
ç2é molecules and éç– Broensted centers can be
responsible for them. It is pertinent to mention that the
pHiso point tends to shift (monotonically or through an
extremum) to the alkaline region in the series InSb
(InSb)x(CdTe)1 – x
CdTe.
Mechanochemical Studies
The exposure of the samples to NO2 or NH3 results
in the appearance of additional differential conducto-
metric titration curve peaks and, therefore, new, likely
Broensted, acid centers [6].
The absence of any change caused by small-dose
(0.18 Mrad) gamma irradiation leads us to suggest that
the acid–base properties of the surface remain virtually
unchanged at this dose.A similar conclusion was drawn
from pH isoelectric point measurements.
The dependence of the total concentration of acid
centers (cac) calculated from all peaks on the composi-
tion of the InSb–CdTe system contains extrema, a min-
imum and a maximum at x = 0.5 and 2 mol % CdTe,
respectively (Fig. 3). In other words, solid solutions
have the lowest and highest (21.48 × 10–4 and 25.22 ×
10–4 g-equiv/g, respectively) concentrations of acid
centers at these compositions.
The results of these studies performed with freshly
prepared surfaces [1] substantiated and augmented
those obtained in pH isoelectric point measurements.
In particular, the dependences of medium pH on the
duration of dispersing large-sized InSb and CdTe pow-
der particles exposed to air and subjected to gamma
irradiation in water are shown in Fig. 1. The dispersion
of InSb and CdTe exposed to air is accompanied by
medium acidification and alkalization, respectively.
When the same samples preliminarily subjected to
gamma irradiation are dispersed, substantial medium
acidification is observed in both cases.
To summarize, the use of freshly prepared surfaces
allowed us to more clearly demonstrate surface acid–
base properties and the tendency toward increasing
acidity under the action of gamma irradiation.
The results of these studies were used to elucidate
the mechanism of adsorption and catalytic processes on
the objects specified.
Conductometric Titration
The conductometric titration of the InSb–CdTe sys-
tem components allows us to reveal several types of
acid centers and determine their total concentration and
changes in this concentration under the action of gases
(NO2 and NH3).
Adsorption Studies
Typical results obtained in studies of the adsorption
Typical differential conductometric titration curves of pure gases (CO, NO2, NH3, and é2) and gas mixtures
(for binary semiconductors and solid solutions) shown are shown in Figs. 4 and 5 and listed in Table 2. These
in Fig. 2 contain four peaks. This is evidence of the gases were of interest because of their different elec-
presence of acid centers of four types on the surface. As tronic natures and as participants of the catalytic reac-
with other diamond-like semiconductors [5], coordina- tions under study.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007

538
KIROVSKAYA
∆σ/∆V, mS/m
6
4
2
0
1
8
4
2
0
0.5
1.0
1.5
2.0
V, ml
Fig. 2. Differential conductometric titration curves of CdTe powder exposed to (1) air and (2) NO .
2
9
9
25
24
23
22
21
80
70
60
8
7
6
5
4
8
7
6
5
4
1
3
2
0
1
2
3
4
5
95
96
97
98
99 100
InSb
CdTe
InSb, mol %
Fig. 3. Composition dependences of the (1) total concentration of acid centers on the surface of the InSb–CdTe system components
exposed to air, (2) adsorption values, (3) adsorption activation energy, and (4) heat of adsorption at p = 8 Pa and T = 300–358 K.
init
α–logt coordinates are straight lines with different
slopes and positions with respect to the axis of ordi-
nates. Since the kinetic adsorption isotherms can be lin-
earized in the logarithmic coordinates, the Roginskii
logarithmic law
The experimental dependences of the adsorption of
pure gases αp = f (T), αT = f (p), and αT = f (t) were sim-
ilar for all system components (e.g., see Fig. 5). The
results obtained lead us to conclude that physical
adsorption occurs below 273–300 K and chemical acti-
vated adsorption, at higher temperatures. These conclu-
sions are substantiated by the results of thermodynamic
and kinetic analyses. For instance, the heats of adsorp-
tion calculated by the Clausius–Clapeyron equation for
the descending αp = f(T) curve portions and by the
semiempirical equation suggested in [4] for the whole
temperature range studied are 6–12 kJ/mol at various T
and α, which corresponds to the chemical nature of
adsorption on diamond-like semiconductors [14].
α = Blnt
b
holds. Accordingly, the surface of adsorbents must be uni-
formly heterogeneous and have the distribution function
ρ(E) = ∂N(E)/∂E = H
(B = HRT). We used the “controlling band” method
suggested by Roginskii for heterogeneous surfaces and
the ensuing equation [4]
The kinetic adsorption isotherms measured at vari-
ous temperatures (above 273–300 K) and plotted in the
Ea = RT ln(t + t0)/τ0
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007

NEW CATALYSTS AND ADSORBENTS
539
a × 104, mol/m2
20
An increase in the activation energy and a decrease
in the heat of adsorption as the degree of surface filling
increases substantiate the inhomogeneous character of
the surface and the presence of active centers on it that
differ in strength and energy state. The same follows
from the results of studies of the acid–base properties of
the surface of the adsorbents under consideration (see
above). They are indicative of the presence of acid cen-
ters of at least four types. As with other diamond-like
semiconductors, coordination-unsaturated atoms with
vacancy defects surrounding them (Lewis centers) and
adsorbed ç2é molecules and éç– groups (Broensted
centers) should be responsible for them.
15
10
5
1'
2'
3'
a × 106, mol/m2
4
1
Considering the acid–base properties of the surface
of adsorbents and the electronic structure of adsorbate
molecules, we can suggest that the adsorption of CO,
NO2, and Nç3, like the adsorption of ëé2 [1, 14], is
governed by the donor–acceptor mechanism with the
predominant participation of surface A atoms (with
stronger metallic properties), vacancies B, and adsor-
bate molecules as electron pair donors (acceptors are
surface atoms with free d- and p-orbitals). In particular,
NO2 molecules were found to play the role of electron
donors. This follows from conductivity σ measure-
ments under the conditions of NO2 adsorption (in [15],
we performed similar measurements for adsorption on
PbS). Indeed, conductivity increased in an NO2 atmo-
sphere compared with a vacuum.
3
2
2
1
0
3
250
300
350
400
450
T, K
Fig. 4. Temperature dependences of the adsorption of (1) CO,
(2) CO : é = 1 : 2, (3) é , (2') NO , (3') NH , and
(1') NO : NH = 1 : 2 on CdTe at p = 10 Pa.
2
2
2
3
2
3
init
(t0 is the correction found from the slope of kinetic iso-
therms in the α–logt coordinates and τ0 = 1/K0 equals
the inverse of the preexponential factor and has the
dimension of time) to calculate the mean activation
energies at various surface fillings (α values). They
were of from 43 to 86 kJ/mol.
Several bonds of various strengths and degrees of
electron delocalization can be formed when surface
atoms differ in the degree of coordination unsaturation
(especially in multicomponent systems). This corre-
sponds to the presence of four differential conducto-
α × 104, mol/m2
2''
2'
12
10
8
1''
1'
2
1
6
4
2
0
275
20
300
40
325
10
350 T, K
12 p, P‡
80 τ, min
4
6
8
0
60
Fig. 5. (1, 2) Temperature dependences and (1', 2') equilibrium and (1'', 2'') kinetic isotherms of adsorption on (1, 1', 1'') CdTe and
(2, 2', 2'') the (InSb) (CdTe) solid solution at p = 8–10 Pa and T = 303 K.
0.03
0.97
init
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007

540
KIROVSKAYA
Table 2. Main adsorption characteristics of cadmium tellu-
ride (powder with ssp = 0.405 m2/g)
It was interesting to compare the binary and quater-
nary (solid solution) components of the InSb–CdTe
system as adsorbents with respect to the gases studied.
Their behaviors exhibit features common to all adsor-
bents and only characteristic of solid solutions. The
former include similar αp = f(T), αT = f(p), and αT = f (t)
experimental dependences, adsorption values, thermo-
dynamic and kinetic adsorption characteristics, adsorp-
tion center natures, and mechanisms and rules govern-
ing adsorption interactions. The special feature of solid
solutions as multicomponent systems is the presence of
extrema in adsorption characteristic–composition dia-
grams (e.g., see Fig. 3).
According to these diagrams, the solid solution with
3 mol % InSb has the highest adsorption activity. The
same solid solution exhibits a deviation from the Veg-
ard [16] composition dependence of the lattice con-
stant. According to [5], this can be related to the great-
est degree of structure imperfection and coordination
unsaturation of surface atoms. Indeed, it was shown in
[5] that the degree of structure ordering and imperfec-
tion of the initial binary compounds changes in the for-
mation of solid solutions under the influence of substi-
tuting atoms. This in turn changes the degree of coordi-
nation unsaturation of surface atoms and, accordingly,
the number of active centers and strength of their bonds
with adsorbates.
α × 106,
q,
kJ/mol
Adsorbate
T, K
Tinit, K
mol/m2
CO
O2
360
360
360
303
303
303
3.2
0.99
2.6
298
–
6.1
4.4
CO + O2 (1 : 2)
NO2
298
273
303
273
6.2
6.76
3.23
17.52
6.32
4.80
7.45
NH3
NO2 + NH3 (1 : 2)
Note: T is the temperature of measurements, α is the adsorption
value, T is the temperature of the beginning of chemical
init
adsorption, and q is the heat of adsorption.
metric titration curve peaks for the adsorbents studied
(e.g., see Fig. 2).
The adsorption isobars of CO + é2 mixtures are
similar to the adsorption isobars of CO (Fig. 4). We
observe virtually complete coincidence of physical
(243–298 K) and activated chemical (298–450 K)
adsorption. Oxygen is adsorbed reversibly over the
temperature range 298–450 K. The chemical nature of
adsorption over the temperature intervals specified fol-
lows from the heats of adsorption (Table 2) with correc-
tions for the special features of diamond-like semicon-
ductors as adsorbents [14] and the observed surface
charging [1, 4, 14]. The equilibrium adsorption iso-
therms of CO, é2, and a 1 : 2 CO–é2 mixture are linear
and obey the Henry law
The system components that are most active with
respect to CO, NO2, and Nç3 were found by analyzing
adsorption characteristic–composition diagrams. They
were used as materials for sensors designed to detect
microimpurities of the corresponding gases.
α = Kp ,
Catalytic Studies
where α is the adsorption, K is the adsorption coeffi-
cients, and p is the adsorbate pressure.
After preliminarily estimating the temperature inter-
vals of the adsorption of pure gases and gas mixtures
(participants of the reactions of interest), we performed
direct catalytic experiments. The main results for the
oxidation of CO are shown in Figs. 6 and 7 and listed in
Table 3. A comparative analysis of CO conversion
curves at 381 and 423 K showed that the oxidation of
CO occurred most effectively at 423 K irrespective of
the carrier gas (argon or air). The maximum conversion
at the initial CO content of (1.63–2.05) × 10–5 mol was
82 and 96% in argon and air, respectively.
A comparison of the adsorption of pure gases and
gas mixtures (Table 2) leads us to conclude that the dis-
placement rule holds. For instance, the adsorption val-
ues for the CO + é2 mixture are smaller (larger) than
those for the more active CO (less active (é2)) compo-
nent,
αO < αCO + O < αCO
.
2
2
The adsorption of a 1 : 2 NO2 : NH3 mixture over the
Because of outside diffusion deceleration possible at
low gas flow rates (up to 13 ml/min), the CO conversion
curves contained poorly resolved maxima, which disap-
peared as flow rates increased. The reaction stopped as
the content of CO increased to 3.3 × 10–5 mol in the ini-
tial mixture, likely because the removal of desorption
products from the surface of catalyst grains was then
hindered.
temperature range 290–310 K is superadditive (Table 2);
that is, α(NO
> αNO > αNH ; therefore, NO2 and
2 + NH3)
2
3
NH3 present together are not independent but interact
with each other [5]. The more active component under
the conditions studied is nitrogen(IV) oxide.
The heats of adsorption (Table 1) typical of dia-
mond-like semiconductors [14] correspond to the
chemical nature of adsorption processes described
The kinetic data on the catalytic oxidation of CO
above. Some understanding of these processes can be with air oxygen, in particular, on CdTe at 423 K and a
developed already from an analysis of adsorption iso- circulation rate of 56 mol/min, are shown in Fig. 7 and
bars (Figs. 4 and 5).
presented in Table 3.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007

NEW CATALYSTS AND ADSORBENTS
541
As has been mentioned, the reaction rate was deter-
χ, %
mined from a decrease in the concentration of CO, and
the total and partial reaction orders were calculated
according to Van’t Hoff [17]. The oxidation of CO in
the circulation loop occurred at a constant concentra-
tion of oxygen present in excess. This allowed us to
determine the rate of the process and partial reaction
order by varying the initial concentration of carbon(II)
oxide in the gas flow. The partial reaction order was
found to be nëé = 1.
100
1
60
20
2
The solid solution with 5 mol % InSb was most active
in the oxidation of CO. The conversion at as low as 378 K
was 83% on this catalyst and only 38% on the CdTe
binary component. The coincidence of the adsorption
values for CO and a CO + é2 mixture at the temperature
at which the reaction was most intense (423 K) leads us
to conclude that the catalytic oxidation of CO does not
depend on the concentration of oxygen in the gas phase.
0
1
2
3
4
nCO × 105, mol
Fig. 6. Dependences of the degree of conversion of CO on
(1) the (InSb) (CdTe)
solid solution and (2) cadmium
0.05
0.95
telluride on the amount of CO introduced into the reactor at
T = 380 K and v = 26 ml/min; carrier gas argon.
vol
Conductivity changes and accompanying surface
charging during the adsorption of CO are evidence of
the dependence of the reactivity of CO molecules not
only on the local factor (the chemical properties of mol-
ecules and active centers) but also on the electronic fac-
tor (collective surface properties). The latter is deter-
mined by the position of the Fermi level and the char-
acter of the energy spectrum of the surface [4, 14]. Such
special features of the donor–acceptor bonds formed
are evidence of the participation of free carriers in an
elementary catalytic event. The selectivity of catalytic
action is then determined by local surface properties, as
on other diamond-like semiconductors [5].
C
CO, vol %
1.2
1
2
3
0.4
4
0
2
4
6
τ, min
The coincidence of the total and partial reaction
orders (the partial reaction order in CO is one) and the
independence of the rate of CO oxidation from the con-
centration of oxygen in the gas phase show that the order
of the reaction with respect to oxygen is zero. The formal
kinetic reaction equation therefore takes the form
Fig. 7. Time of complete CO conversion on CdTe at initial
CO concentrations of (1) 1.49, (2) 1.0, and (3) 0.31 vol %.
The observations described above also lead us to
conclude that the oxidation of CO predominantly
occurs under kinetic control conditions. This follows
from the chemical activated adsorption of the CO + é2
–dcCO/dt = kcCO
,
mixture with the formation of the ëé–, CO3– , and é–
where c is the concentration of CO at time t and k is the
rate constant.
intermediate particles, the desorption of the products in
the form of ëé2 molecules, and the dependence of the
Considering the enhanced adsorbability of car-
bon(II) oxide in CO + é2 mixtures, its donor properties
(positive adsorbent surface charging), the first order of
the reaction (with respect to CO and total), and the
mechanism of adsorption (with the formation of CO–2 )
Table 3. Kinetic characteristics of the oxidation of CO on
CdTe
k × 103, vol %/s
N
and desorption (in the molecular form) of carbon diox-
ide [14], which is the expected reaction product close in
nature and behavior to CO, the conclusion can be drawn
that the adsorption of the CO + é2 mixture and, accord-
ingly, oxidation of CO are predominantly governed by
the shock mechanism similar to that suggested in [4],
cCO, vol %
0.31
0.37
0.4
0.98
2.16
2.18
1.46
0.6
1.59
1.05
0.97
1.19
0.94
0.36
1.04
0.59
0.72
1.0
ëé(g) + Ä
ëé+δA–δ(ads),
ëé+δA–δ(ads)
CO–(ads) + é2(g)
CO–(ads),
CO–3 (ads),
0.30
0.95
1.49
Note: k is the rate constant for the reaction and N is the total reac-
CO–3 (ads)
ëé2(ads) + O–(ads).
tion order.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007

542
KIROVSKAYA
Table 4. Catalytic reduction of NO2 with ammonia on CdTe
and (InSb)x(CdTe)1 − x solid solutions; initial mixture compo-
sition NO2 : NH3 = 1 : 2
num–vanadium catalyst for the selective reduction of
nitrous gases in the production of nitric acid. This cata-
lyst gives a conversion of 96% only at 578 K. The larg-
est degree of NO2 conversion at x = 0.03 corresponds to
its largest adsorption value under the same conditions
(T, P).
The conversion of NO2 increases during catalyst
functioning. This can be explained by the accumulation
of Broensted centers on the surface of catalysts. The
role of Broensted centers is in all probability played by
adsorbed water molecules formed in the reaction. Their
presence on the surface together with Lewis centers
(coordination-unsaturated atoms and vacancy defects
[1, 14]) has a promoting action on the catalysts. These
conclusions were substantiated by the conductometric
titration of the catalysts after the reaction and after their
subsequent heating at 463 K.
The acid center concentrations corresponding to
jumps in the curves of differential conductometric titra-
tion with a nonaqueous solution of potassium ethoxide
are listed in Table 5, in particular, for CdTe.
An analysis of these results showed that, after the
catalytic conversion of NO2 and subsequent catalyst
heating, additional jumps appeared in differential con-
ductometric titration curves. These jumps can reason-
ably be related to the appearance of new acid centers
mentioned above.
m2
β, %
x = 0,
m2
β, %
m2
β, %
x = 0.03,
m1 = 36.20 µg
x = 0.05,
m1 = 35.40 µg
m1 = 16.49 µg
8.51
4.26
4.04
48.39
74.19
75.48
13.68
8.69
5.68
3.51
1.60
1.45
62.2
62.2
84.3
90.3
95.6
96.0
21.17
21.17
12.89
12.39
12.32
40.2
40.2
63.6
65.0
65.2
x = 0,
m1 = 24.47 µg
5.85
4.79
4.79
76.08
80.43
80.43
Note: m and m are the contents of NO at the entrance and exit
1
2
2
of the reactor and β is the degree of NO conversion.
2
Table 5. Acid–base center concentrations on CdTe samples
after various treatments (1 through 6 are the numbers of
jumps)
c × 104, g-equiv/g
Treat-
ment
1
2
3
4
5
–
6
–
I
1.94
2.00
1.96
3.64
3.25
5.74 28.00
The adsorption and direct catalytic studies per-
formed in this work allow us to suggest the shock
mechanism of catalytic reduction of NO2 with ammo-
nia similar to that suggested for the catalytic oxidation
of CO. First, nitrogen dioxide is adsorbed (this is the
most reactive gas in the reaction mixture) with the for-
II
III
5.00 10.61 17.62 24.40
8.64 12.30 19.62 28.00
–
Note: I, storage in air; II, after the reaction 6NO + 8NH = 7N +
2
3
2
12H O; and III, after the reaction and heating in an air flow
2
at 463 K.
mation of donor–acceptor NO+2δ – A–δ complexes,
where A is a coordination-unsaturated atom. Next, rad-
ical ions NO–2 can be formed as a result of the complete
rate of the reaction on the specific surface area of the
catalyst.
delocalization of electrons in these complexes. Lastly,
radical ions interact with ammonia molecules in the gas
phase.
Among the InSb–CdTe system components studied,
the (InSb)0.05(CdTe)0.95 solid solution was most active
in the oxidation of CO. The degree of CO conversion on
this catalyst was 83% at 378 K (Fig. 6). It also com-
pares favorably with the other catalysts [18–21].
REFERENCES
Studies of the adsorption of pure NO2 and NH3 and
their mixture showed that the gases were not indepen-
dent at 290–310 K and interacted with each other, NO2
being the more active component. This was substanti-
ated by direct studies of the catalytic properties of the
InSb–CdTe system components with respect to the
selective reduction of nitrogen(IV) oxide with ammo-
nia. The main results of these studies are presented in
Table 4.
1. I. A. Kirovskaya, Surface Phenomena (Omsk. Gos.
Tekhn. Univ., Omsk, 2001) [in Russian].
2. I. A. Kirovskaya, O. P. Azarova, E. G. Shubenkova, and
O. N. Dubina, Neorg. Mater. 38 (2), 135 (2002) [Inorg.
Mater. 38 (2), 91 (2002)].
3. V. A. Kas’yan, P. I. Ketrush, Yu. A. Nikol’skii, and
F. I. Pasechnik, Eds., Thin Films of Indium Antimonide:
Preparation, Proprties, and Application (Shtiintsa, Chi-
sinau, 1989) [in Russian].
The high catalytic activity of CdTe and solid solu-
tions containing 3 and 5 mol % InSb attracts attention.
Already at room temperature, the degrees of conversion
of NO2 on these catalysts under stationary conditions
were 80.4, 96.0, and 65.2%, respectively. This can be
compared with the activity of industrial AVK-10 alumi-
4. I. A. Kirovskaya, Adsorption Processes (Irkutsk. Gos.
Univ., Irkutsk, 1995) [in Russian].
5. I. A. Kirovskaya, Surface Properties of Diamond-like
Semiconductors: Chemical Composition of Surface and
Catalysis (Irkutsk. Gos. Univ., Irkutsk, 1988) [in Rus-
sian].
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007

NEW CATALYSTS AND ADSORBENTS
543
6. I. A. Kirovskaya and E. V. Mironova, Zh. Fiz. Khim. 79 13. L. A. Skorobogatova and E. N. Khabarov, Fiz. Polupro-
(4), 755 (2005) [Russ. J. Phys. Chem. 79 (4), 649
vodnikov, No. 2, 401 (1974).
(2005)].
14. I. A. Kirovskaya, Surface Properties of Diamond-like
Semiconductors: Adsorption of Gases (Irkutsk. Gos.
Univ., Irkutsk, 1984) [in Russian].
15. V. F. Markov and L. N. Maskaeva, Zh. Anal. Khim. 56
(8), 846 (2001) [J. Anal. Chem. 56 (8), 754 (2001)].
16. E. V. Mironova, Abstracts of Papers, XL International
Conference “Student and Scientific-Technological
Progress” (Novosibirsk, 2002), pp. 172–173.
17. I. A. Kirovskaya, Chemical Kinetics and Chemical Equi-
librium (Omsk. Gos. Tekhn. Univ., Omsk, 2005) [in Rus-
sian].
7. Theory and Practice of Studies in the Field of Catalysis,
Ed. by V. A. Roiter (Naukova Dumka, Kiev, 1973) [in
Russian].
8. I. P. Mukhlenov, V. N. Anokhin, V. A. Proskuryakov,
et al., Catalysis in Boiling Bed (Khimiya, Leningrad,
1978) [in Russian].
9. F. M. Rapoport and A. A. Il’inskaya, Laboratory Meth-
ods for Obtaining Pure Gases (Goskhimizdat, Moscow,
1963) [in Russian].
10. I. I. Kirpatovskii, Environment Protection: A Handbook
18. P. M. L’dokova, N. M. Popova, K. Zh. Kaigaltyrova,
et al., RF Patent No. 2046653 (1995).
(Khimiya, Moscow, 1980) [in Russian].
11. I. A. Kirovskaya, Surface Properties of Diamond-like
Semiconductors: Solid Solutions (Irkutsk. Gos. Univ.,
Irkutsk, 1984) [in Russian].
19. V. M. Mukhin, S. G. Kireev, N. P. Vasil’ev, et al.,
RF Patent No. 2054322 (1996).
20. L. A. Voropanova and O. G. Lisitsyna, RF Patent
No. 2156164 (2000).
12. V. A. Brodovoi, N. G. Vyalyi, and L. M. Knorozok,
Neorg. Mater. 33 (3), 303 (1997) [Inorg. Mater. 33 (3), 21. V. I. Kononenko, I. A. Chupova, V. G. Shevchenko, et al.,
248 (1997)].
RF Patent No. 2171712 (2001).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 4 2007