New approaches to the preparation of highly efficient chromium-containing oxide catalysts for the water gas shift reaction

Article abstract of DOI:10.1134/S002315840906007X

It was found experimentally that the solutions of Cr³⁺ nitrate and the nitrates of other metals that are the constituents of Cr-containing catalysts can be prepared by dissolving a corresponding metal (for example, cast iron and electrolytic co

Full text of DOI:10.1134/S002315840906007X

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2009, Vol. 50, No. 6, pp. 837–850. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © A.A. Khassin, T.P. Minyukova, M.P. Demeshkina, N.A. Baronskaya, L.M. Plyasova, G.N. Kustova, V.I. Zaikovskii, T.M. Yurieva, 2009, published in
Kinetika i Kataliz, 2009, Vol. 50, No. 6, pp. 871–885.
New Approaches to the Preparation of Highly Efficient
ChromiumꢀContaining Oxide Catalysts
for the Water Gas Shift Reaction
,b
A. A. Khassina , T. P. Minyukovaa, M. P. Demeshkinaa, N. A. Baronskayaa,
L. M. Plyasovaa, G. N. Kustovaa, V. I. Zaikovskiia, and T. M. Yurievaa
a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
b Novosibirsk State University, Novosibirsk, 630090 Russia
eꢀmail: aakhassin@catalysis.ru
Received October 21, 2008
Abstract—It was found experimentally that the solutions of Cr3+ nitrate and the nitrates of other metals that
are the constituents of Crꢀcontaining catalysts can be prepared by dissolving a corresponding metal (for
example, cast iron and electrolytic copper) in a solution of chromic anhydride and nitric acid to reach the
quantitative reduction of Cr6+ without the formation of nitrogen oxides. Analogously, the oxidation of Fe2+
cations to Fe3+ coupled with the reduction of hexavalent chromium can be performed. The precipitation of
Fe3+, Cr3+, and Cu2+ ions at a ratio of Fe : Cr = 9 and a concentration of Cu2+ to 20 at % can result in the
formation of a partially hydrated oxide with the hydrohematite structure—a dispersed and highly defective
oxide structure with a high specific surface area more than 300 m2/g and a higher thermal stability, as comꢀ
pared with the goethite phase ( ꢀFeOOH). The dehydration of hydrohematite occurred at a noticeable rate
α
at temperatures higher than 400°C. Hydrohematite promoted with copper cations exhibited high activity
below 400°C; this can decrease the starting temperature of the adiabatic highꢀtemperature WGSR to 300°C
or below.
DOI: 10.1134/S002315840906007X
INTRODUCTION
the formation of oxo–hydroxo clusters containing
support cations on the metal surface (for example, the
formation of metal particles of size 4–8 nm stable to
It is well known that the specific catalytic activity of
the majority of oxide catalysts correlates with the speꢀ
cific surface area, and the relationship between the
catalytic activity and the specific surface area of an
oxide is adequately described by a direct proportionalꢀ
ity for many catalysts. One of the most wellꢀknown
procedures for the preparation of oxides with a high
specific surface area is the calcination of correspondꢀ
ing hydroxides or hydroxo carbonates at relatively low
temperatures to provide only incomplete removal of
2−
sintering up to 650–800°C upon the reduction of
mixed oxo–hydroxo compounds of Co–Al, Co–Si,
and Ni–Al–Si [3, 4]). In other cases, this was due to
the retention of a defect oxide structure, which proꢀ
vided the epitaxial metal–support bond, for example,
in catalysts prepared by the reduction of copper
chromite or copper–zinc oxide. The epitaxial bond of
copper metal and oxide particles was retained up to the
temperatures at which the complete dehydration of
the support oxide occurred [5, 6]. Note that, in many
cases, the role of admixture anions is not limited by an
increase in the stability of an active nanodispersed
metal phase: the catalytic properties of active metal
particles decorated with oxide clusters can be considꢀ
erably different from the properties of undecorated
particles. This phenomenon has been described in
many scientific publications, and it is well known as
the strong metal–support interaction.
ОН– and
anions. The structure of these anionꢀ
CO3
modified oxides contains a considerable amount of
defects. In addition to a developed surface, the presꢀ
ence of defects stabilizes cations in a more preferable
coordination to increase the mutual solubility of
oxides. For example, it provides an opportunity to
increase the solubility of Cu2+ cations in spinel [1] or
zinc oxide [2].
The presence of admixture anions has also a conꢀ
siderable effect on the structure and catalytic properꢀ
The removal of admixture anions from the oxide
ties of catalysts whose active components are metal structure results in the relaxation of structural defects,
particles formed as a result of reduction. In particular, exothermal oxide crystallization, and, as a conseꢀ
the presence of admixture anions stabilizes nanosized quence, a dramatic decrease in the specific surface
metal particles to temperatures at which the complete area or, in some cases, in the decomposition of a mixed
dehydration of the system occurs. This can be due to oxide into individual oxides, that is, in a dramatic
837

838
KHASSIN et al.
change in the catalyst structure. In this context, it tion of cations and homogeneous supporting⎯precipꢀ
seems very important to increase the thermal stability itation procedure [9]) imply the use of salts of chroꢀ
of anionꢀmodified oxides, which are catalysts or cataꢀ mium(III) and other metals, which are catalyst conꢀ
lyst precursors. The stability of admixture anions as stituents, for example, nitrates, as raw materials.
oxide constituents can be increased by introducing Unfortunately, these raw materials are expensive so
promoter cations forming stable hydroxo compounds that catalysts prepared by coprecipitation cannot
or partially hydrated oxides, such as Mg2+, Al3+
and Zr4+
,
compete in factors of price with less expensive but
more hazardous catalysts prepared with the use of
chromic anhydride.
.
The Cr3+ cation is one of the most effective proꢀ
moters, which improve the stability of ОН– anions in
oxide structural defects. The efficiency of stabilization
of hydroxyl groups in the defects of chromiumꢀconꢀ
taining oxides can be illustrated by a special mechaꢀ
nism of reduction of copper cations from wellꢀcrystalꢀ
lized copper chromite (CuCr2O4), a mixed copper and
chromium oxide with the tetragonally distorted spinel
structure. The reduction of this compound in a flow of
hydrogen is accompanied by only the partial removal
of oxygen anions with the formation of water moleꢀ
cules, and it mainly occurs by hydrogen oxidation to
protons. The resulting protons are absorbed by the
oxide to replace reduced copper cations in its structure
as ОН– groups, as found recently with the use of neuꢀ
tron inelastic scattering [7]. The structural dehydraꢀ
tion of reduced copper chromite, which is accompaꢀ
nied by the formation of chromium oxide and the rupꢀ
ture of an epitaxial bond between copper metal
particles and an oxide support, occurs only at temperꢀ
atures higher than 450°C. The absorption of hydrogen
in the course of reduction of copper cations was also
observed previously in other copper oxides, for examꢀ
ple, containing Zn2+ and Al3+ cations [8]. However,
the steps of hydrogen absorption and dehydration
occur at very different temperatures only in the reducꢀ
tion of copper chromite.
Because of the above, the preparation of the soluꢀ
tions of Cr3+ and other metal (including Fe3+, Cu2+,
and Ni2+, which are catalyst constituents) nitrates
from inexpensive and available raw materials is a probꢀ
lem of considerable current interest. In this study, corꢀ
responding metals, chromic anhydride, and nitric acid
were chosen as these raw materials. In this work, we
considered the preparation of nitrate solutions without
the release of higher nitrogen oxides and the preparaꢀ
tion of catalysts by the coprecipitation of cations from
the resulting solutions and studied the catalytic propꢀ
erties of these catalysts. For the Fe–Cr(–Cu) oxo–
hydroxide system, the preparation of partially
hydrated Fe–Cr oxide with the hydrohematite strucꢀ
ture, which exhibited high activity in the range of
300–400°C, was demonstrated.
EXPERIMENTAL
Catalyst Preparation
P2 pig iron of second category with respect to sulꢀ
fur, chromic anhydride (CrO3), copper oxide (CuO),
and a solution of nitric acid were used for the preparaꢀ
tion of Fe–Cr and Fe–Cr–Cu catalysts. Electrolytic
copper, chromic anhydride, zinc oxide, pseudoboehꢀ
mite, AlOOH, and a solution of nitric acid were used
for the preparation of Cu–Zn–Al–Crꢀcontaining
samples.
The efficiency of catalyst promotion with chroꢀ
mium cations in order to increase the specific activity
and thermal stability of catalysts was demonstrated by
longꢀfactor experience in the development and operaꢀ
tion of Fe–Cr catalysts for highꢀtemperature WGSR,
Cu–Zn–Al–Cr catalysts for lowꢀtemperature
WGSR, Ni–Cr catalysts for hydrogenation, etc. Howꢀ
ever, along with undoubted advantages, chromiumꢀ
containing catalysts have a number of disadvantages.
First of all, this is due to the high toxicity of Cr(VI)
Hydroxo compounds with specified compositions
were precipitated from prepared working solutions
with a solution of sodium carbonate. The precipitate
was washed by decantation. The catalyst mass was
dried and calcined in a flow of nitrogen at 200–300°C
.
The hydroxo compounds of iron rather than carꢀ
compounds, which poses rigid requirements on the bonates are formed on precipitation from a solution of
preparation of these catalysts: the Cr(VI) content of iron and chromium nitrates because of hydrolysis.
the catalyst should be no higher than 0.05 wt %. In Depending on the composition of this solution and the
spite of these requirements, many chromiumꢀconꢀ conditions of precipitation, phaseꢀhomogeneous
taining catalysts are currently manufactured with the hydroxo compounds with the structures of goethite
use of chromic anhydride as a raw material, in particꢀ and hematite (frequently referred to as 2ꢀline ferrihyꢀ
ular, using the wet mixing method. This method canꢀ drite, see the discussion below) or Fe2O3 with the
not ensure the absence of hexavalent chromium in the hematite structure can be obtained. An analysis of
catalyst and the complete and uniform interaction of published data on the properties of the hydroxo comꢀ
chromium cations with other catalyst components pounds of iron and procedures for their preparation
(i.e., the formation of a homogeneous oxide system). [10, 11] made it possible to develop procedures for the
The procedures used to prepare a homogeneous cataꢀ preparation of iron–chromium and iron–chromium–
lyst mass with the complete inclusion of chromium copper hydroxo compounds with the goethite and
cations into the mixed oxide structure (coprecipitaꢀ hydrohematite structures.
KINETICS AND CATALYSIS Vol. 50
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2009

NEW APPROACHES TO THE PREPARATION OF HIGHLY EFFICIENT
839
The preparation of a hydroxo compound with the a moist gas mixture at 320–330°C for 1 h at a heating
goethite structure is based on the formation of rate of 2 K/min.
α
ꢀ
FeOOH upon the oxidative hydrolysis of iron(II).
Therefore, in the dissolution of cast iron, conditions
were chosen to obtain a solution of a bivalent iron salt,
which was subsequently oxidized by precipitation with
atmospheric oxygen blown through the solution. To
prepare samples with the hydrohematite structure, a
solution of trivalent iron nitrate was used. If necessary,
copper was introduced into the prepared solutions.
Catalytic Data Processing
The initial catalytic experimental data contained
information on the composition of the starting reacꢀ
tion mixture (SRM) and final reaction mixture
(FRM) after a flow reactor. The composition of the
starting and final reaction mixtures was determined by
chromatography with the use of a flameꢀionization
detector. After separation on a chromatographic colꢀ
umn with activated carbon 0.5 m in length, carbon
oxides were methanized in a reactor with a commerꢀ
cial Ni–Cr catalyst before analysis.
To evaluate the characteristics of specific catalytic
activity (for example, the rate constants of catalytic
reactions), it was necessary to chose a kinetic model of
the process. A few rate equations have been described
in the literature; among them, Eq. (1), which correꢀ
sponds to the model of a stepwise reaction on the
assumption that the rates of the steps of CO oxidation
and hydrogen reduction from water are approximately
equal, is most accurate and reliable [13]:
The concentration of chromium(VI) in solutions
was determined by chemical analysis (qualitatively,
with the use of phenylanthranilic acid as an indicator
for chromium or, quantitatively, by titration with
Mohr’s salt) or spectrometric analysis (based on
absorption line intensities at 22500 and 28500 cm–1).
Extracts from catalysts calcined in a flow of an inert
gas were also analyzed. The sensitivity of the used
methods to Cr(VI) was <10–3 wt %.
Phase analysis was performed on Dꢀ500 (Siemens)
and Dꢀ8 (Bruker) Xꢀray diffractometers with the use
of filtered Cu
K radiation (graphite monochromator
α
in the reflected beam).
The IR spectra were measured in the range of 350–
4000 cm–1 using a Bomem MBꢀ102 spectrometer.
Catalyst powders were diluted in KBr or CsI matrices
and pressed as pellets.
−1
⎛
→ ⎜
⎝
⎞
⎟
⎠
PCOP − K p P
P
H2O
CO2 H2
.
(1)
W = k
APH O + PCO
2
2
At the same time, Semenova et al. [14] recommended
the use of the following simplified rate equation,
which corresponds to first order of reaction with
respect to CO, over a wide range including industrial
catalyst operation conditions:
The differential thermal analysis (DTA) of sample
dehydration was performed on a Netzsch STAꢀ409
thermal analyzer in a flow of argon at a heating rate of
10 K/min.
The electronꢀmicroscopic study of the samples was
performed on a JEMꢀ2010 transmission electron
microscope (JEOL) (resolution, 1.4 Å; accelerating
voltage, 200 kV). The samples were supported onto
perforated carbon substrates from alcohol suspensions
subjected to ultrasonic dispersion.
⎛
⎜
⎝
⎞
⎟
⎠
−1 P PH
CO2
2
.
(2)
W = k
PCO − Kp
→
P
H2O
The difference between Eqs. (1) and (2) consists in the
factor
⎛
⎜
⎝
⎞
⎟
⎠
P
H2O
,
(3)
APH O + PCO
2
2
Catalytic Tests
which depends on the reaction mixture composition,
the temperature (because the parameter depends on
temperature in accordance with the Arrhenius equaꢀ
tion with Ea = 36.7 kJ/mol), and the nature of cataꢀ
lysts.
The use of simplified kinetics (2) facilitates data
processing, but it can lead to errors for the following
two reasons:
(1) The dependence of factor (3) on mixture comꢀ
position in solving an integral rate equation for a flow
reactor (integration over a catalyst bed) is ignored.
А
The catalytic activity of the samples in the water gas
shift reaction was determined in a catalytic flow sysꢀ
tem over a temperature range of 240–480°C at a nearꢀ
atmospheric pressure. The tests were performed in a
reaction mixture containing 12 vol % CO, 5.5 vol %
CO2, and 82.5 vol % H2 at a steam : gas ratio of 0.6–
0.9 on a catalyst fraction of 0.25–0.50 mm mixed with
quartz glass of the same particle size in a ratio of 1 : 1
and on heatꢀconducting catalyst plates as disc segꢀ
ments of size 25 × 2.5 mm (the preparation procedure
and testing of heatꢀconducting catalyst plates was
described in detail elsewhere [12]). The weight of the
catalyst fraction was varied from 0.05 to 0.50 g. The
space velocity of the reaction mixture was varied in the
range of 400–40000 h–1.
(2) The temperature dependence of factor (3) is
ignored; because of this, the apparent activation
energy is much different from the true activation
energy of a rateꢀlimiting step.
An analysis of changes in factor (3) under changes
Before the tests, the catalyst was activated in nitroꢀ in the composition of the reaction mixture in the
gen to 200°C at a heating rate of 5 K/min and then in region of CO conversion to 40%, which was reached in
KINETICS AND CATALYSIS Vol. 50
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2009

840
KHASSIN et al.
with nitric acid, the metal is oxidized to the state M2+
with the formation of a corresponding nitrate solution
and gaseous nitrogen compounds in accordance with
the reactions
Table 1. Standard electrode potentials of redox reactions with
the participation of the nitrate anion
Standard elecꢀ
Half reaction
trode potential, V
2M + 6HNO3
3M + 8HNO3
4M + 10HNO3
5M + 12HNO3
4М+10HNO3
2M(NO3)2 +NO2
3M(NO3)2 + 2NO
4M(NO3)2 + N2O
5M(NO3)2 + N2
4M(NO3)2 +NH4NO3
↑
+ NO + H2O;(I)
↑
0.775
N
N
N
O3– + 2H+ + e– = NO2 + H2O
O3– + 3H+ + 2e– = HNO2 + H2O
O3– + 4H+ + 3e– = NO + 2H2O
↑
+ 4H2O; (II)
0.930
0.957
1.116
1.244
↑
+ 5H2O; (III)
↑
+ 6H2O; (IV)
+3H2O.(V)
↑
2NO3– + 10H+ + 8e– = N2O + 5H2O
2NO3– + 12H+ + 10e– = N2 + 6H2O
In this case, reaction (I) occurs through the formaꢀ
tion of nitrous acid
M + 3HNO3
M(NO3)2 + НNO2 + H2O, (VI)
which decomposes with the formation of higher nitroꢀ
gen oxides by reaction (VII) or is further reduced to
result in the occurrence of the process in accordance
with overall reactions (II)–(V).
our experiments, demonstrated that the integration of
the rate equation over the reactor length ignoring the
dependence of factor (3) on the composition of the
mixture introduced an error of no higher than 20 rel. %
or less than 10 rel. % in the region of high temperaꢀ
tures. On the other hand, the use of accurate kinetics
(1) for calculating constants requires experimental
data on the concentrations of all components in the
starting and final reaction mixtures. Each of these
experimental values is determined with an error of
~5–7 rel. %; because of this, the introduced error can
be as high as 15%.
The activation energy determined from the true
reaction constants, which can be determined from Eq.
(1), is considerably different from the apparent activaꢀ
tion energy estimated from data using simplified
kinetics (2). However, for the purpose of this work, the
apparent activation energy at a nearꢀindustrial mixture
composition rather than the true activation energy of
reaction is of crucial importance.
2HNO2
NO↑
+ NO2
↑
+ H2O.
(VII)
A decrease in the concentration of nitric acid and
the reaction temperature resulted in a decrease in the
rate of reaction (VII) and a further decrease in the oxiꢀ
dation state of nitrogen. In this connection, a metal
can be dissolved to nitrate without the formation of
higher nitrogen oxides by properly choosing the temꢀ
perature and nitric acid concentration.
Note that iron is converted into corresponding Fe2+
salts only under the action of dilute inorganic acids.
Under the action of 13 wt % HNO3, iron metal forms
a mixture of iron(II) and iron(III) nitrates, whereas
concentrated nitric acid (25–30 wt %) reacts with iron
on heating in accordance with the equations
Fe + 4HNO3 = Fe(NO3)3 + NO↑ + 2H2O; (VIII)
Fe + 6HNO3 = Fe(NO3)3 + 3NO2↑ + 3H2O. (IX)
Table 1 summarizes the standard electrode potenꢀ
tials of the corresponding halfꢀreactions of nitrogenꢀ
containing anion reduction [15].
It is well known that nitrous acid is oxidized to
nitric acid in the presence of an oxidizing agent in the
system; for example, this occurs in the presence of
hydrogen peroxide:
Because of this, in this work, we used simplified
kinetic model (2) and characterized the specific activꢀ
ity of catalysts in the WGSR by the apparent rate conꢀ
stant of the forward reaction, which was evaluated with
consideration for the reversibility of reaction assuming
first order with respect to CO in accordance with the
equation
−
−
.
(X)
NO2 +H2O2 = NO3 +H2O
⎛
⎜
⎝
⎞
⎟
⎠
C
CO−humid − CCO−eq
−1
1
τ
,
, s
(4)
k = ln
→
It is also well known that the standard electrode
potential of the halfꢀreaction
C
CO−FRM − CCO−eq
where ССО–moist is the mole fraction of CO in the humid
initial reaction mixture, ССО–FRM is the mole fraction
in the reaction mixture at the reactor outlet, and
ССО is the equilibrium mole fraction of CO at the
Cr2O27− + 14H+ + 6e− = 2Cr3+ + 7H2O
(XI)
is 1.333 V versus the standard hydrogen electrode.
Comparing this value with potentials in Table 1, we
note that the reduction of chromium coupled to the
⎯
eq
reaction temperature.
NO3−
oxidation of HNO2 to
is thermodynamically
more preferable than the formation of gaseous nitroꢀ
gen oxides even at a very low concentration of dichroꢀ
mate anions. Thus, we believe that the dichromate
anion is an efficient oxidizing agent for the nitrite
anion in accordance with the reaction
RESULTS AND DISCUSSION
Preparation of the Nitrate Solutions of Cr3+
and Other Metals That Are Catalyst Constituents
(Fe3+, Cu2+, and Ni2+)
3HNO2 + Cr2O27− + 5H+
= 2Cr3+ + 3 NO3− + 4 H2O.
Dissolution of metals in the solutions of nitric acid
and chromic anhydride. It is well known that, in the
reactions of iron metal, nickel metal, or copper metal
(XII)
KINETICS AND CATALYSIS Vol. 50
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2009

NEW APPROACHES TO THE PREPARATION OF HIGHLY EFFICIENT
841
3[Cr3+]/(2[Cu2+ 3[Cr3+])
] −
Consequently, a metal can be dissolved in a soluꢀ
tion of nitric acid and chromic anhydride in accorꢀ
dance with the overall reaction
3M + Н2Cr2O7 + 12HNO3
(XIII)
= 3M(NO3)2 + 2Cr(NO3)3 + 7H2O
to obtain a solution of M2+ and Cr3+ nitrates without
the formation of nitrogen oxides.
10
Note that the redox reaction between a metal and
the dichromate anion does not occur at a noticeable
rate in the absence of nitric acid (for example, in a
solution of acetic acid and chromic anhydride). That
is, nitric acid is a catalyst for the interaction of a metal
with the dichromate anion. The catalytic cycle of overꢀ
all reaction (XIII) consists of two redox steps. At initial
step (XIV), the oxidation of a metal in an acidic
−
−
0.6
−
0.5
−
0.4
−
0.3
−0.2
pH
medium is accompanied by the reduction of the
NO3
anion (see also reaction (VI)):
Fig. 1. Dependence of the ratio between the concentraꢀ
tions of Cr and Cu cations in solution on the pH of a
solution of nitric acid and chromic anhydride.
3+
2+
−
M +
+ 3H+
M2+ + HNO2 + H2O. (XIV)
NO3
At the second step, which is described by Eq. (XII),
nitrous acid reduces the chromium of the dichromate
anion to Cr3+ to be oxidized to the nitrate ion and to
close the catalytic cycle.
release of higher nitrogen oxides was not observed in
the course of dissolution. This fact can be explained by
a low steadyꢀstate concentration of nitrous acid,
which is an intermediate in overall reaction (XIII).
If we consider processes occurring with the particiꢀ
pation of nitrous acid, the concentration ratio shown in
Fig. 1 corresponds to the ratio of the rate of reduction of
the dichromate ion by reaction (XII) to the sum of the
rates of nitrous acid decomposition by reaction (VII)
and interaction with copper metal, which leads to transꢀ
formations described by reactions (II)–(V). The reducꢀ
tion of dichromate occurs with the participation of proꢀ
tons; consequently, its rate depends on the pH of soluꢀ
tion, whereas the rate of decomposition of nitrous acid
is independent of pH. Consequently, the above ratio
would be expected to increase with decreasing pH. This
is obviously supported by data in Fig. 1. In accordance
with our experimental data, the dependence on the pH
of solution is expressed by the relationship
Our experiments on the dissolution of nickel metal
and copper metal in a solution of nitric acid and chroꢀ
mic anhydride demonstrated that the rate of reaction
(XIII) was much higher than the rate of metal dissoluꢀ
tion in reactions (I)–(V) at sufficiently low values of
pH (lower than –0.5) and a low temperature (e.g., at
room temperature). Thus, Fig. 1 shows experimental
data on the cationic composition of solution upon the
complete dissolution of copper metal powder in a
solution of nitric acid and chromic anhydride at a conꢀ
stant solution temperature of 22°C. The concentraꢀ
tion of nitric acid was varied from 8 to 17.5 wt %. The
initial amounts of chromic anhydride and copper
metal strictly corresponded to the stoichiometry of
reaction (XIII), that is, Cu : Cr = 3 : 2. Upon compleꢀ
tion of the reaction, the concentrations of Cu2+ and
Cr3+ cations and
2− anions in solution were deterꢀ
Cr2O7
log{3 [Cr3+]/(2[Cu2+] – 3[Cr3+])} = –0.37 – 3 pH.(6)
mined. Figure 1 shows the results as the dependence of
the 3Cr3+/(2Cu2+–3Cr3+) ratio on the pH of solution.
The above ratio between the concentrations of
cations in the final solution corresponds to the ratio
of the amount of copper (nCu) dissolved by reaction
(XIII) to the amount of copper dissolved without the
participation of chromic anhydride, that is, by reacꢀ
tions (I)–(V):
The third order of reaction (XII) with respect to the
concentration of H+ in solution may be indicative of
the mechanism of this step; however, currently availꢀ
able data are insufficient to hypothesize this mechaꢀ
nism.
The results obtained show that the solutions of
nitric acid and chromic anhydride are promising for
the preparation of Cr3+ and other metal nitrate soluꢀ
tions by interactions accompanied by the release of
nitrogen oxides in an insignificant amount. Another
important test for the applicability of this method to
the preparation of the stock solutions of nitrates conꢀ
sists in the completeness of the reaction of hexavalent
chromium.
nCu,(13)
nCu,(13)
nCu,(1−5) nCu, Σ − nCu,(13)
3[Cr3+]
2[Cu2+] − 3[Cr3+]
=
(5)
3/2nCr,(13)
nCu, Σ − 3/2nCr,(13)
=
=
.
It can be clearly seen that, at pH < –0.5, more than
95% of dissolved copper was oxidized by the reduction
Note that, if the starting metal is taken in an excess,
of hexavalent chromium. In this case, note that the the occurrence of reaction (XIII) results in the comꢀ
KINETICS AND CATALYSIS Vol. 50
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2009

842
KHASSIN et al.
−
1
Cr3+ and corresponding metal (Cu2+, Ni2+, or Fe3+)
k
, s
→
nitrates without the formation of a detectable amount
of nitrogen oxides. The solution of chromic anhydride
and nitric acid can also be used for the oxidation of the
Fe2+ nitrate to the Fe3+ nitrate and the preparation of
a solution of Cr(NO3)3 and Fe(NO3)3. Nitric acid is a
catalyst for the interaction of metals or Fe2+ cations
with the dichromate anion. A decrease in pH considꢀ
erably increases the contribution of reaction (XIII) to
metal oxidation. This method for the preparation of
nitrate solutions for the subsequent preparation of catꢀ
alysts has a number of serious advantages:
80
70
60
50
40
30
20
1
(1) The use of inexpensive and available raw mateꢀ
rials—a metal, chromic anhydride, and nitric acid.
(2) The absence of considerable nitrogen oxide
emissions.
(3) The complete reduction of hexavalent chroꢀ
mium.
10
2
0.19
0.20
0.21
0.22
0.23
1000/RT, mol/kJ
0.24
(4) The low concentrations of sulfur and other
impurities. The concentrations of impurities depend
on the purity of the metal used. For example, P2 pig
iron of first and second category with respect to sulfur
contains no more than 0.02 wt % sulfur, which comꢀ
pletely meets all of the requirements imposed on ironꢀ
containing catalysts.
Fig. 2. The temperature dependence of the rate constant of
the forward reaction in the Arrhenius coordinates for Cu–
Zn–Al
⎯Cr catalysts prepared by (1) coprecipitation and
(
2
) mixing. Reaction conditions: 12% CO and 5.5% CO
2
in the dry initial reaction mixture; steam/gas = 0.9; and
= 1.5 atm.
P
Because of these advantages, methods for the prepꢀ
aration of catalysts with the use of nitrate solutions, for
example, a coprecipitation method, can be impleꢀ
mented on a commercial scale.
plete reduction of hexavalent chromium because the
thermodynamic equilibrium is strongly shifted to the
formation of a solution of Cr3+ and M2+ nitrates and
the standard Gibbs potential of reaction at 25°C is very
0
high
kJ/mol). We failed to detect
(−Δ(13, Ni)G298 > 900
hexavalent chromium in solution using available
analytical techniques. After completion of converꢀ
sion in accordance with reaction (XIII), the subseꢀ
quent dissolution of the remaining metal occurred by
reactions (I)–(V) in accordance with chosen condiꢀ
tions (nitrate anion concentration, pH, and temperꢀ
ature).
Cu–Zn–Al–Cr Catalyst Prepared by Coprecipitation
for the LowꢀTemperature Conversion of CO
Figure 2 clearly illustrates that a coprecipitation
method is preferable to a wet mixing method, which is
commonly used in industrial practice for the preparaꢀ
tion of catalysts for the lowꢀtemperature conversion of
CO (such as the NTKꢀ4 catalyst). This figure shows
the temperature dependence of the rate constants of
reactions measured in the kinetic region for a Cu–
Zn–Al–Cr catalyst prepared by coprecipitation from
a solution of nitrates and an analog of a commercial
Cu–Zn–Al–Cr catalyst prepared by mixing. The
samples were calcined at 350°C in an inert gas flow.
From Fig. 2, it follows that the catalyst prepared by
coprecipitation was more active than the catalyst preꢀ
pared by mixing by a factor of 1.5–2.5. This is most
likely due to the complete involvement of chromium
and copper in the composition of a mixed oxide and
the homogeneous phase composition of the catalyst.
Oxidation of Fe2+ cations to Fe3+ in the solutions of
nitric acid and chromic anhydride. The oxidation of
iron metal in a solution of weak nitric acid under conꢀ
ditions that prevent the formation of higher nitrogen
oxides occurs only to the oxidation state of Fe2+. The
further oxidation of Fe2+ to Fe3+ can be performed by
interaction with air in accordance with the reaction
2Fe2+ + 1/2O2 + 2H+
2Fe3+ + H2O.
(XV)
However, the dichromate anion can also be used as
an oxidizing agent:
6Fe(NO3)2 + Н2Cr2O7 + 12HNO3
(XVI)
= 6 Fe(NO3)3 + 2Cr(NO3)3 + 7H2O.
Analogously to reaction (XIII), reaction (XVI)
Fe–Cr and Fe–Cr–Cu Catalysts Prepared
by a Coprecipitation Method
occurs rapidly, and the standard affinity is high
0
for the HighꢀTemperature WGSR
= 325.3 kJ/mol); this provides the complete
(−Δ16G298
interaction of Fe2+ and hexavalent chromium.
Preparation conditions, including temperature,
Thus, the use of chromic anhydride as an oxidizing the pH of the mother liquor, the Fe3+/Fe2+ ratio in the
agent in a solution of nitric acid allows us to dissolve initial nitrate solution, and precipitate aging condiꢀ
copper, nickel, or iron and to obtain the solutions of tions and time, affect the phase composition and
KINETICS AND CATALYSIS Vol. 50
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2009

NEW APPROACHES TO THE PREPARATION OF HIGHLY EFFICIENT
843
2
6
1
3
C
1
4
5
2
3
20
30
40
50
60
70
, deg
2
θ
Fig. 3. Diffraction patterns of the catalyst samples (
FeCrꢀ1, ( ) FeCrCuꢀ2, and ( ) FeCrCuꢀ3 and ( ) a comꢀ
mercial Fe–Cr–Cu catalyst for highꢀtemperature WGSR
and the model diffraction patterns of ( ) (ICDD 00ꢀ033ꢀ
0664) hematite ꢀFe O and ( ) (ICDD 00ꢀ029ꢀ0713)
goethite ꢀFeOOH structures.
1)
2
3
4
5
1800
1400
1000
Wavenumber, cm
600
−
1
α
6
2
3
α
Fig. 4. IR spectra of catalyst samples: (
FeCrCuꢀ2, and ( ) FeCrCuꢀ3.
1) FeCrꢀ1, (2)
3
structure of the precipitate prepared by the interaction
of a solution of Cr3+ and iron nitrates with a solution
of sodium carbonate. It was found that catalyst samꢀ
ples containing three oxo hydride phases with various
degrees of Fe–Cr oxide hydration can be obtained by
varying preparation conditions. To study the structure
and catalytic properties of these phases, we prepared
three corresponding phaseꢀhomogeneous samples
FeCrꢀ1, FeCrꢀ2, and FeCrꢀ3 with the cationic ratio
Fe3+ : Cr3+ = 90 : 10 and analogous samples with the
cationic ratio Fe3+ : Cr3+ : Cu2+ = 88 : 8 : 4. Figures 3
and 4 show the diffraction patterns and IR spectra of
samples. Note that the diffraction patterns and IR
spectra of Fe–Cr samples and corresponding Fe–Cr–
Cu samples promoted with copper are very similar.
absorption bands at 289, 370 (shoulder), 416 (shoulꢀ
der), 472, 617 (shoulder), 893, and 806 cm–1 were
observed; they are consistent with published data [17]
on the IR spectrum of goethite. Absorption bands at
1336, 1384, 1465, and 1633 cm–1 are related to the
−
vibrations of
2− and
ions and the deformation
CO3
NO3
vibrations of water. A shift in the positions of diffracꢀ
tion maximums and difference from data (ICDD 00ꢀ
029ꢀ0713) on intensity distribution should be associꢀ
ated with the presence of Cr3+ and Cu2+ cations in the
structure.
The structure of samples FeCrꢀ3 and FeCrCuꢀ3 is
essentially different from the structures discussed
above. These samples are poorly crystallized. The difꢀ
fraction patterns are described by two broad diffusion
The structure of samples FeCrꢀ1 and FeCrCuꢀ1 is
similar to the structure of
α
ꢀFe2О3 (hematite).
According to published data [16], peaks at 324, 462,
561, and 633 cm–1 observed in the IR spectra are charꢀ
lines in the range of
with maxima at θ1
2
=
θ
angles of 25 and 55
35.0° 0.1° and 2θ2 = 61.6°
°
–49
°
°–70°
acteristic of ꢀFe2O3, and the positions of the Xꢀray
α
2
diffraction lines of sample FeCrꢀ1 closely correspond
to data given in the ICDD database for hematite and
somewhat shifted to greater angles for the sample proꢀ
moted with copper. This is unambiguously related to
the decrease in the unit cell parameters due to the
presence of Cu2+ cations in the oxide structure. Note
that the commercial catalyst for the highꢀtemperature
WGSR is a mixed Fe–Cr–Cu oxide with the hematite
structure and with a considerably greater admixture of
a spinel phase than that in sample FeCrCuꢀ1, and it
contains a graphite phase introduced into the catalyst
to facilitate pelletization.
0.1°. Two halos in the diffraction pattern can characꢀ
terize a twoꢀlayer hexagonal packing of anions, in the
octahedral or tetrahedral voids of which cations are
arranged. Correspondingly, in factors of a hexagonal
system, the indices (002) and (110) can be ascribed to
the diffraction maximums; on this basis, the following
unit cell parameters can be calculated:
а = 2.98
0.01 Å and = 5.04 0.01 Å. According to the closest
с
packing rule [18], the closest hexagonal packing of
c
a
ions has the ratio
1.63. For the test sample, this
=
ratio is 1.69, which suggests a disorder in the layers of
The samples FeCrꢀ2 and FeCrCuꢀ2 have the strucꢀ
ture of
α
ꢀFeOOH (goethite). In the IR spectrum, this structure.
KINETICS AND CATALYSIS Vol. 50
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2009

844
KHASSIN et al.
The IR spectra of these samples exhibited absorpꢀ under discussion, as proposed by Wolska. This factor
tion bands at wavenumbers of 330, 462, 578, 710 reflects the essence of the discussed phase as a highly
defect structure stabilized by admixture ОН– anions,
in which the local environment of Fe3+ cations is close
to that characteristic of the hematite structure.
(shoulder), and 920–930 cm–1, which are close to
absorption bands reported by Wolska [19] for the
structure of a partially hydrated Fe3+ oxide—hydroheꢀ
matite. The spectra also contained absorption bands in
Electronꢀmicroscopic data additionally justified
this interpretation of the hydrohematite structure.
Figure 5 shows a highꢀresolution electron micrograph
of sample FeCrCuꢀ3 thermally treated at 400°C for
2 h in an atmosphere of argon. According to thermal
analysis data, which will be discussed below, the dehyꢀ
dration of the hydrohematite phase at this temperature
occurred at a rate that provided the removal of only a
portion of admixture ОН– groups for 2 h. To the right
of the micrograph, Fourier filtration data are given for
the micrograph of regions marked in the left figure and
having the hematite and hydrohematite structures,
respectively. From a comparison between the figures,
it is evident that, on the one hand, these structures are
related to each other and, on the other hand, they are
different, as manifested itself in the high imperfection
of hydrohematite.
the region of 1340–1540 cm–1 due to the vibrations of
−
CO32−
NO3
, ОН–, and
anions and an absorption band
at 1630 cm–1 corresponding to the deformation vibraꢀ
tions of water.
The physicochemical properties of iron oxides with
the hematite and goethite structures are well studied
(e.g., see [11]). It is well known that hematite is
reduced to the Fe(Fe,Cr)2O4 oxide with the magnetite
structure under the reaction conditions of the WGSR
[20, 21]. At a low steam/gas ratio, the further reducꢀ
tion of iron cations with the formation of carbide is
also possible; this is extremely undesirable in factors of
a catalytic process because it results in the formation
of hydrocarbons in the Fischer–Tropsch synthesis.
Promotion with chromium and copper cations
increases the stability of the oxide with the magnetite
structure and thus the stability of the catalyst for highꢀ
temperature WGSR to fluctuations in the composition
of the initial reaction mixture.
The particle size was estimated from Xꢀray diffracꢀ
tion data using the Scherrer formula [30]
⎛
⎞
nλ
βcosθ
; it was found that the size of coherent
D =
⎜
⎝
⎟
⎠
A structure disordered with respect to the cationic
sublattice was also observed in a number of studies;
based on IR spectra, we attributed this structure to the
partially hydrated oxide Fe2 + xO3(OH)3x (hydrohemaꢀ
scattering regions in the direction [002], that is, the
nanoparticle thickness, was ~14 Å, and the size of
coherent scattering regions in the direction [110], that
is, the layer length was also ~14.5 Å. Thus, the primary
particle of a coherent scattering region consisted of
~3 unit cells in thickness and ~5 unit cells in length.
These particles can have an uncompensated
charge; therefore, they strongly retained adsorbed
tite). It was ascribed to the Fe5HO8 ⋅ 4H2O ferrihydrite
nanostructure [22, 23]. Evidently, in these publicaꢀ
tions, twoꢀline ferrihydrite is considered; this is a
nanomaterial with a crystallite size smaller than 2 nm,
whose Xꢀray diffraction patterns [24, 25] are fully conꢀ
sistent with the Xꢀray diffraction patterns of samples
FeCrꢀ3 and FeCrCuꢀ3.
H O, OH–,
2−, and other groups on their surface to
CO3
2
prevent sintering and to facilitate ordering.
Note that twoꢀline ferrihydrite should not be conꢀ
The calcination of Fe–Cr and Fe–Cr–Cu oxo
fused with the mineral ferrihydrite [26, 27], which has hydroxides results in dehydration and the formation of
six Xꢀray diffraction lines and hence is referred to as a phase with the hematite structure. Figure 6 shows
sixꢀline ferrihydrite. Recently, Michel et al. [28] proꢀ data obtained by in situ Xꢀray diffraction under condiꢀ
posed the structure of this mineral, which contains tions of the calcination of sample FeCrCuꢀ2 in a flow
80% iron cations in an octahedral coordination and of argon. From these data, it follows that the decomꢀ
20% in a tetrahedral coordination, corresponding to position of goethite promoted with copper and chroꢀ
the stoichiometry Fe10O14(OH)2. A detailed review of mium cations occurs at a temperature of 200°C or
experimental data and proposed interpretations [11] higher, and it is accompanied by the formation of a
makes it possible to clearly distinguish between these phase with the hematite structure. It follows from
two phases, which historically have the same name. In thermal analysis data, which are shown in Fig. 7, that
this context, note that, more recently, Chukhrov [29], the dehydration of the hydrohematite structure occurs
who is the author of the initial factor ferrihydrite, proꢀ at much higher temperatures, although this structure
posed the factor protoferrihydrite for twoꢀline ferrihyꢀ is much more imperfect. The onset of dehydration at a
drite; however, the latter factor has not received wide heating rate of 10 K/min was detected at a temperaꢀ
acceptance although it has been used in other publicaꢀ ture of about 400°C, and a maximum rate of dehydraꢀ
tions. To exclude terminological confusions and takꢀ tion was observed at 580°C. In this case, the dehydraꢀ
ing into consideration the absence of a strict stoichiꢀ tion of hydrohematite was exothermic as a conseꢀ
ometry from the twoꢀline ferrihydrite phase (accordꢀ quence of a considerable structure relaxation, whereas
ing to Wolska [19], the degree of oxide hydration can the goethite structure was converted into hematite in
vary over a wide range), we assume that hydrohematite the course of an endothermic process. These data are
is the most correct factor to determine the structure consistent with published data [22–24] on the high
KINETICS AND CATALYSIS Vol. 50
No. 6
2009

NEW APPROACHES TO THE PREPARATION OF HIGHLY EFFICIENT
845
1 nm
10 nm
1 nm
Fig. 5. (left) Electron micrograph of sample FeCrCuꢀ3 thermally treated in Ar at 400°C for 1 h. (right) Results of the Fourier
filtration of micrograph regions corresponding to the (top) hematite and (bottom) hydrohematite structures.
^
α
ꢀFeOOH
ꢀFe2O3
*
^
α
^
^
^
^
^
^
^
^
^
^
5
^
^
^
^
^
^
^
4
3
*
*
*
^
^
*
*
^
^
*
*
*
*
*
*
*
*
*
*
*
2
*
*
*
*
*
*
1
10
20
30
40
50
60
70
, deg
2
θ
Fig. 6. Data of in situ Xꢀray diffraction on the calcination of sample FeCrCuꢀ2 in a flow of argon at the following temperatures:
) 20, ( ) 130, ( ) 200, ( ) 250, and ( ) 300°C
(1
2
3
4
5
.
stability of unpromoted hydrohematite. Promotion
with Cr3+ and Cu2+ cations did not exert a considerꢀ
able effect on the range of temperatures at which the
dehydration of oxo hydroxides occurred. However,
unlike the dehydration of unpromoted hydrohematite
[24], we did not detect the intermediate formation of
Catalytic Properties of Fe–Cr and Fe–Cr–Cu Oxo
Hydroxides in the WGSR
Table 2 summarizes the specific surface areas of
Fe–Cr and Fe–Cr–Cu oxo hydroxides determined by
the BET method for the samples calcined at 200°C in
an atmosphere of argon.
γ
ꢀFe2O3 (maghemite).
KINETICS AND CATALYSIS Vol. 50
No. 6
2009

846
KHASSIN et al.
1
2
280
580
DTA
400
0
100
200
300
400
500
600
700
800
Temperature, °С
Fig. 7. Thermal analysis data for the decomposition of samples in an atmosphere of argon: (1) FeCrCuꢀ2 and (2) FeCrCuꢀ3.
Heating rate: 10 K/min.
The catalytic activity of Fe–Cr catalysts correlates content did not cause the formation of phases other
well with the specific surface area of the oxide. The than oxo hydroxide with the hydrohematite structure.
introduction of copper increases the activity of all of Figure 8 shows the results of catalytic activity tests at
the catalysts; however, the effect of copper promotion 300, 330, and 450°C depending on the copper content
is much higher for catalysts with the hydrohematite of hydrohematite. From these data, it can be clearly
structure. Thus, the activity of catalyst FeCrCuꢀ3 is seen that, in the range to 12% copper, the rate constant
higher than the activity of catalyst FeCrCuꢀ1 by one of forward reaction of the WGSR (
k ) linearly
→
order of magnitude. Taking into account electronꢀ depended on (moreover, was directly proportional to)
microscopic data, we assume that this is due to a high the amount of introduced copper in the temperature
concentration of defects accessible to gasꢀphase molꢀ range of 300–330°C. Only upon the introduction of
ecules, in which admixture copper cations are stabiꢀ 20% copper, a considerable decrease in the specific
lized. It is likely that these copper cations are active activity with respect to a linear trend was observed.
sites at temperatures of 300 and 330°C. However, note
that we did not study the nature of active copper sites
and this problem is of considerable interest for future
studies.
The activity of catalysts at 450°C nonlinearly
depended on the amount of copper introduced into
the catalyst composition; it is likely that it was only
indirectly related to the copper content through the
An increase in the cooper content of Fe–Cr–Cu effect of the copper content on the oxide structure
samples to 7, 12, and 20 at % under the preparation formed after the dehydration of hydrohematite at eleꢀ
conditions of sample FeCrCuꢀ3 at the ratio Fe/Cr = vated temperatures. After longꢀterm calcination at
9 : 1 resulted in an increase in the specific activity of 450°C, the hydrohematite structure was converted
the catalyst. In this case, note that, according to XRD into a hematiteꢀtype structure. In this case, the difꢀ
and IRꢀspectroscopic data, an increase in the copper fraction pattern of the oxide formed in a sample conꢀ
Table 2. Experimental data on the specific surface areas of the test catalysts and on the activity of these catalysts in the WGSR
Rate constant of the forward reaction
k
, s–1
Structure and
copper content, at %
Specific surface
of the WGSR
→
Sample
area SBET, m2/g
300
°
C
330
°
C
450°C
FeCrꢀ1
Hematite, 0
67
75
0.2
1.0
1.8
2.2
2.2
1.0
2.2
3.9
7.0
5.5
21
60
FeCrCuꢀ1
FeCrꢀ2
Hematite, 4
Goethite, 0
147
167
310
295
56
FeCrCuꢀ2
FeCrꢀ3
Goethite, 4
73
Hydrohematite, 0
Hydrohematite, 4
62
FeCrCuꢀ3
14
24
127
KINETICS AND CATALYSIS Vol. 50
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2009

NEW APPROACHES TO THE PREPARATION OF HIGHLY EFFICIENT
847
taining 4% copper was different from that of hydroheꢀ
matite in only a decrease in the cell parameter (
8.383 Å, as compared with 8.396 Å for ꢀFe2O3
according to published data [31]). However, in the
samples with high copper concentrations, a tetragonal
distortion of the hematite structure was observed,
which manifested itself in the splitting of the [400]
reflection into two ([400] and [004]). The lattice
parameters of the tetragonally distorted structure for
−
1
k
, s
→
a
=
α
80
60
40
20
the sample containing 20 at % copper were
8.444 Å and = 8.372 Å.
а =
с
Difference in the dependence of the activity of catꢀ
alysts on copper content in the regions of low temperꢀ
atures (300–330°C) and elevated temperatures
(450°C) suggests that the seam conversion of CO
occurs at different active sites in different temperature
ranges. The temperature dependence of catalytic
activity was studied in detail for a hydrohematite samꢀ
ple containing 7 at % copper. To provide an isothermal
catalytic bed, this catalyst was tested in a structured
reactor with an increased thermal conductivity of the
catalyst bed, which was reached by introducing the
catalyst mass into heatꢀconducting composite plates
25 mm in diameter and 2.5 mm in thickness. The proꢀ
cedure for the preparation of heatꢀconducting comꢀ
posite catalysts as plates, the characterization of their
thermal conductivity and efficiency, and the proceꢀ
dures of testing were described in detail elsewhere
[12]. Figure 9 shows data on the catalytic activity
(expressed in factors of the apparent rate constant of
forward reaction) of the catalyst with the hydrohemaꢀ
tite structure containing 7 at % copper in the course of
a continuous series of experiments performed for
150 h. These data indicate that the steadyꢀstate occurꢀ
rence of the reaction was reached in 15–20 h after
increasing the reaction temperature. Henceforth, all
of the discussed values that characterize the activity of
catalysts are given for steadyꢀstate reactions. Figure 9
also indicates that the activity of the catalyst at low
temperatures (330°C) decreased many times after
heating the catalyst to 460°C in a reaction atmoꢀ
sphere. The temperature dependence of the catalytic
activity of the catalyst with the initial hydrohematite
structure after performing the catalytic reaction in the
region of temperatures to 480°C was studied in
another series of continuous tests with a total duration
of 275 h. Table 3 summarizes the initial experimental
data and the calculated apparent rate constants of forꢀ
ward reaction; they support the conclusion on a
change in the catalytic properties of the catalyst after
1
2
3
0
1
2
3
4
5
6
7
8
9 10 11 1219 20 21
Copper content, at
%
Fig. 8. Dependence of the activity of catalysts with the
hydrohematite structure on the copper content at various
reaction temperatures: (
Catalyst grain size: 0.14–0.25 mm.
1) 310, (2) 330, and (3) 450°C.
From Fig. 10, it follows that the temperature
dependence of the steadyꢀstate rate constant is comꢀ
plicated. The rate constant depends only slightly on
temperature in the range of 310–420°C; the apparent
activation energy can be estimated at 10
2 kJ/mol.
Above 420°C, the apparent activation energy has a sigꢀ
nificantly larger value of 54 4 kJ/mol. The catalyst
kept at 480°C for 15 h or longer exhibited a lower catꢀ
alytic activity, and the temperature dependence of its
activity over the entire test temperature range obeyed
the Arrhenius equation with activation energies close
to the activation energy observed for the commercial
catalyst (~60 kJ/mol). Taking into account the above
results, including the effect of copper content on the
catalytic activity of copperꢀcontaining catalysts with
the hydrohematite structure, we can explain data
shown in Fig. 10 in the following manner:
(1) In the region of temperatures to 400–420°C
,
the hydrohematite structure is stable (at least, the rate
of dehydration is low so that the conversion of hydroꢀ
hematite into hematite does not occur within the
experiment time of about 100 h). The activity of the
copperꢀpromoted catalyst with the hydrohematite
structure depends on copperꢀcontaining sites. It is
likely that the reaction proceed by a concerted mechꢀ
anism [32].
operation in the temperature range of 460–480°C
.
(2) In the region of temperatures higher than
Figure 10 shows data in the Arrhenius coordinates 420°C, hydrohematite is dehydrated to hematite. This
on the steadyꢀstate activity of the above catalyst over structure is also thermodynamically stable in the
the entire temperature range as the temperature was region of low temperatures; therefore, a decrease in
gradually increased to 480°C followed by a decrease in the temperature below 400°C does not lead to the
the reaction temperature. This figure also shows the recovery of the initial degree of oxide hydration. Iron
temperature dependence of the steadyꢀstate activity of cations are the active sites of this catalyst at which the
a commercial Fe–Cr–Cu catalyst with the hematite WGSR occurs with a higher activation energy most
structure (copper content of 2.8 at %).
likely by a stepwise mechanism [33].
KINETICS AND CATALYSIS Vol. 50
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2009

848
KHASSIN et al.
−
1
k
, s
460°C
→
10
9
440°C
8
7
6
320°C
380°C
400°C
420°C
5
4
3
2
1
330°C
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Time on stream, h
Fig. 9. The temperature dependence of the apparent rate constant of forward reaction of the WGSR for the Fe–Cr–Cu catalyst
with the hydrohematite structure containing 7 at % copper.
The observed activation energies are lower than the diffusion limitations in catalytic tests on grains with a
corresponding values known for the WGSR on Feꢀ characteristic size of ~2.5 mm. According to published
containing catalysts (Ea 90 kJ/mol on Feꢀcontaining data [34], the efficiency is 35–60% for heatꢀconductꢀ
≈
catalysts [33]). This may be explained by the effect of ing plates. However, the occurrence of diffusion limiꢀ
Table 3. Results of the tests of the catalytic activity of the catalyst with the hydrohematite structure containing 7 at % copper
depending on temperature obtained in the course of a continuous series of experiments for 350 h*
Composition of the wet initial
reaction mixture, vol %
Composition of the reaction
mixture after the reactor, vol %
VSRM
nl/h
(NTP)
,
equil
CO
CO conversion,
%
C
,
T,
°
C
τ
, s
k
, s–1
→
vol %
H2O
CO
CO2
CO
CO2
320
310
350
380
480
460
440
420
400
380
350
330
50.2 0.119 38.7
50.6 0.120 38.4
49.8 0.114 37.5
49.9 0.109 37.9
49.8 0.095 37.7
49.8 0.097 37.6
49.6 0.100 37.4
49.6 0.103 38.1
44.6 0.118 31.1
50.2 0.108 38.3
49.8 0.114 38.2
48.9 0.120 37.4
7.4
7.4
7.5
7.4
7.4
7.4
7.6
7.5
8.3
7.6
7.4
7.3
3.5
3.4
3.5
3.4
3.5
3.4
3.4
3.4
3.8
3.5
3.4
3.4
0.89
0.78
1.23
1.60
3.16
2.82
2.54
2.23
1.90
1.67
1.22
0.96
4.78
5.10
4.67
4.41
5.23
5.31
5.57
5.72
6.55
6.37
6.54
6.63
6.2
5.7
6.3
6.3
5.6
5.5
5.4
5.2
5.5
4.7
4.3
4.1
35.6
31.5
38.1
40.1
29.4
28.3
26.7
24.0
20.8
16.6
12.1
9.7
4.4
3.6
5.3
6.6
7.6
6.3
5.1
4.0
2.7
2.2
1.37
0.98
* The catalyst was tested as heatꢀconducting plates (see the text); the order of rows in the table corresponds to the sequence of temperature
changes in the course of tests; tests were performed to reach a steadyꢀstate course of reaction at each temperature.
KINETICS AND CATALYSIS Vol. 50
No. 6
2009

NEW APPROACHES TO THE PREPARATION OF HIGHLY EFFICIENT
849
−
k [s ]
1
than that in the absence of chromic anhydride. The
resulting solutions are characterized by high purity in
factors of sulfur determined by the sulfur content of
the initial raw materials (the S/Fe weight ratio is no
ln
→
2.5
2.0
1.5
1.0
0.5
0
higher than
2
×
10–4 with the use of grey cast iron of
1
first and second category with respect to sulfur).
Moreover, it is of importance that it is inexpensive, as
compared with the price of individual nitrates, in
combination with practical usability.
1
'
The precipitation of Fe3+, Cr3+, and Cu2+ cations
in the ratio Fe/Cr = 9 at a concentration of Cu2+ to
20 at % can result in the formation of a partially
hydrated oxide with the hydrohematite structure—a
disperse and highly imperfect oxide structure with an
ordered anionic sublattice and the absence of an order
in the cationic sublattice observed by Xꢀray diffracꢀ
tion. The hydrohematite phase is characterized by a
high specific surface area of >300 m2/g and a higher
thermal stability, as compared with the phase of goetꢀ
2
−
0.5
1.0
0.15
−
0.16
0.17
0.18
0.19
0.20 0.21
1000/RT, mol/kJ
Fig. 10. The temperature dependence of the apparent
steadyꢀstate rate constant of forward reaction of the
WGSR for (
matite structure containing 7 at % copper and (
mercial Fe–Cr–Cu catalyst containing 2.8 at % copper.
The tests were performed in the following orders: (
320–330–340–380–400–420–440–460–480°C
(1') 480–460–440–420–400–380–350–330°C.
1
) the Fe–Cr–Cu catalyst with the hydroheꢀ
2
) the comꢀ
hite,
occurs at a noticeable rate at temperatures higher than
400°C
α
ꢀFeOOH. The dehydration of hydrohematite
1
) 310–
and
.
Hydrohematite promoted with copper ions exhibits
high activity in the region of temperatures lower than
400°C. The activity of Fe–Cr–Cu hydrohematite is
proportional to the amount of copper introduced into
the catalyst, whereas it is likely that copper cations are
the active sites of the catalyst. After the dehydration of
hydrohematite, which occurs at a noticeable rate at
temperatures higher than 400°C, the activity of the
catalyst decreases and the catalyst approaches cataꢀ
lysts with the hematite structure in its catalytic properꢀ
ties. After dehydration, the activity is restricted to the
sites characteristic of Fe–Cr catalysts over the entire
temperature range.
The high activity of the Fe–Cr–Cu catalyst with
the hydrohematite structure in the region of temperaꢀ
tures lower than 350–400°C can decrease the starting
temperature of an adiabatic process of highꢀtemperaꢀ
ture WGSR to 300°C or lower.
However, note that the dehydration of hydrohemaꢀ
tite is an exothermic process; consequently, it is therꢀ
modynamically favorable at lower temperatures. In
this context, to evaluate the operation stability of a
catalyst for the WGSR in the region of temperatures
lower than 400°C, the kinetics of dehydration of
hydrohematite should be studied in detail, and the
results of these studies will be published elsewhere.
tations in the course of the experiment has no crucial
effect on the above conclusions.
CONCLUSIONS
The above experiments demonstrated that the soluꢀ
tions of Cr3+ nitrate and the nitrates of other metals
that are the constituents of Crꢀcontaining catalysts
can be prepared by dissolving a corresponding metal
(such as cast iron and electrolytic copper) in a solution
of chromic anhydride and nitric acid. Nitric acid is a
catalyst for the redox reaction between a metal and the
dichromate anion with the formation of M2+ (Fe3+ in
the case of iron) and Cr3+ cations. Under conditions of
this catalytic reaction, the steadyꢀstate concentration
of nitric acid in solution is very low; this determines
the low rate of side reactions of metal oxidation with
nitric acid to form nitrogen oxides. A decrease in the
pH of solution results in an increase in the rate of the
catalytic reaction between the metal and the dichroꢀ
mate anion and, consequently, in an increase in the
fraction of the metal dissolved in the target reaction, as
compared with the fraction of the metal dissolved
without the participation of the dichromate anion.
Analogously, the oxidation of Fe2+ cations to Fe3+
coupled with the reduction of hexavalent chromium
can be performed.
ACKNOWLEDGMENTS
The proposed method for the preparation of nitrate
This work was performed at the Boreskov Institute
solutions is characterized by the complete interaction of Catalysis, Siberian Branch, Russian Academy of
of a metal or Fe2+ cation with the dichromate anion; Sciences in 2005–2008; it was supported in part by
the residual concentration of Cr6+ in solution is no OAO AZKiOS. We are grateful to Cand. Sci. (Eng.)
higher than 10–3 wt %; that is, the fraction of reduced I.D. Reznichenko, M.I. Tselyutina, O.M. Posokhova,
chromium is higher than 99.95%. In addition, the and L.G. Volchatov (OAO AZKiOS) for helpful disꢀ
amount of nitrogen oxides released in the course of cussions, constructive criticism, and thorough examiꢀ
metal dissolution is lower by two orders of magnitude nation of the results of laboratory studies a portion of
KINETICS AND CATALYSIS Vol. 50
No. 6
2009

850
KHASSIN et al.
13. Shchibrya, G.G., Morozov, N.M., and Temkin, M.I.,
which is considered in this paper. A.A. Khassin
acknowledges the support of the Russian Science Supꢀ
port Foundation.
Kinet. Katal., 1965, vol. 6, no. 6, pp. 1057, 1115.
14. Spravochnik azotchika (Nitrogen Engineer’s Handꢀ
book), Moscow: Khimiya, 1986, p. 147.
15. http://www.chem.msu.ru/rus/handbook/redox/.
REFERENCES
16. The Sadtler Infrared Spectra Hanbdook of Minerals and
1. Minyukova, T.P., Plyasova, L.M., Yurieva, T.M., Litꢀ
vak, G.S., and Ketchik, S.V., Kinet. Katal., 1989,
vol. 30, no. 2, p. 415.
Clays
, Ferraro, J.R., Ed., Philadelphia: Sadtler
Research, 1982.
17. Mineralspektren, Moenke, H., Ed., Berlin: Akadeꢀ
2. Yurieva, T.M., React. Kinet. Catal. Lett., 1983, vol. 23,
mische, 1962, Spectrum 2.29.
no. 3, p. 267.
18. Belov, N.V., Struktura ionnykh kristallov i metalliꢀ
cheskikh faz (Structure of Ionic Crystals and Metallic
Phases), Moscow, 1947.
3. Khassin, A.A., Yurieva, T.M., Kaichev, V.V., Bukhtiꢀ
yarov, V.I., Budneva, A.A., Paukshtis, E.A., and Parꢀ
mon, V.N., J. Mol. Catal. A: Chem., 2001, vol. 175,
nos. 1–2, p. 189.
19. Wolska, E., Z. Kristallogr., 1981, vol. 154, p. 69.
20. Baranski, A., Lagan, M., Pattek, A., and Reizer, A.,
4. Khassin, A.A., Yurieva, T.M., Kaichev, V.V., Zaiꢀ
kovskii, V.I., Demeshkina, M.P., Minyukova, T.P., Baꢀ
ronskaya, N.A., Bukhtiyarov, V.I., and Parmon, V.N.,
Kinet. Katal., 2006, vol. 47, no. 3, p. 420 [Kinet. Catal.
(Engl. Transl.), vol. 47, no. 3, p. 412].
React. Kinet. Catal. Lett., 1980, vol. 15, p. 285.
21. Spitzer, R.H., Manning, F.S., and Philbrook, W.O.,
Trans. Met. Soc. AIME, 1966, vol. 236, p. 725.
22. Mitov, I., Paneva, D., and Kunev, B., Thermochim.
Acta, 2002, vol. 386, p. 179.
5. Plyasova, L.M., Solovyeva, L.P., Krieger, T.A.,
Makarova, O.V., and Yurieva, T.M., J. Mol. Catal. A:
Chem., 1996, vol. 105, p. 61.
23. Oliveira, A.C., Marchetti, G.S., and Do Carmo Ranꢀ
gel, M., J. Therm. Anal. Calor., 2003, vol. 73, p. 233.
6. Yurieva, T.M., Plyasova, L.M., Zaikovskii, V.I.,
Minyukova, T.P., Bliek, A., van den Heuvel, J.C.,
Davydova, L.P., Molina, I.Yu., Demeshkina, M.P.,
Khassin, A.A., and Batyrev, E.D., Phys. Chem. Chem.
Phys., 2004, vol. 6, no. 18, p. 4522.
24. Eggleton, R.A. and Fitzpatrick, R.W., Clays Clay
Miner., 1988, vol. 36, p. 111.
25. Drits, V.A., Sakharov, B.A., Salyn, A.L., and
Manceau, A., Clays Clay Miner., 1993, vol. 28, p. 185.
7. Khassin, A.A., Yurieva, T.M., Plyasova, L.M., Kusꢀ
tova, G.N., Jobic, H., Ivanov, A., Chesalov, Yu.A.,
Zaikovskii, V.I., Khassin, A.V., Davydova, L.P., and
Parmon, V.N, Ross. Khim. Zh., 2008, vol. 52, no. 1,
p. 32.
26. Chukhrov, F.V., Zvyagin, B.B., Gorshkov, A.I.,
Ermilova, L.P., and Rudnevskaya, E.S., Izv. Akad.
Nauk SSSR, Ser. Geol., 1971, no. 1, p. 3.
27. Chuckrov, F.V., Zvyagin, B.B., Gorshkov, A.I., Yerꢀ
milova, L.P., and Balashova, V.V., Int. Geol. Rev., 1973,
vol. 16, p. 1131.
8. Trunov, V.A., Sokolov, A.E., Lebedev, V.T., Smirnov, O.P.,
Kurbakov, A.I., Heuvel, J., Batyrev, E., Yurieva, T.M.,
Plyasova, L.M., and Torok, G., Phys. Solid State, 2006,
vol. 48, no. 7, p. 1291.
28. Michel, F.E., Ehm, L., Antao, S.M., Lee, P.L., Chuꢀ
pas, P.J., Liu, G., Strongin, D.R., Schoonen, M.A.A.,
Phillips, B.L., and Parise, J.B., Science, 2007, vol. 316,
no. 5832, p. 1726.
9. Dillen, J.A, Geus, J.W, Hermans, L.A, and van der
Meijden, J., Proc. 6th Int. Congr. on Catalysis, London,
1976, p. 677.
29. Chukhrov, V.F., Chem. Geol., 1973, vol. 12, p. 67.
30. Guinier, A., Theorie et technique de la radiocristallograꢀ
phie, Paris: Dunod, 1956.
10. Schwertmann, U. and Cornell, R.M., Iron Oxides in
Laboratory, New York: VCH, 1991, p. 137.
31. Michael, F.E., Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 1984, vol. 40, no. 9, p. 1491.
11. Jambor, J.L. and Dutrizac, J.E., Chem. Rev., 1998,
vol. 98, p. 2549.
32. Yurieva, T.M., Boreskov, G.K., and Gruver, V.Sh.,
12. Baronskaya, N.A., Minyukova, T.P., Sipatrov, A.G.,
Demeshkina, M.P., Khassin, A.A., Dimov, S.V.,
Kozlov, S.P., Kuznetsov, V.V., Terentiev, V.Ya., Khrisꢀ
tolyubov, A.P., Brizitskiy, O.F., and Yurieva, T.M.,
Chem. Eng. J., 2007, vol. 134, p. 195.
Kinet. Katal., 1969, vol. 10, no. 4, p. 862.
33. Boreskov, G.K., Yurieva, T.M, and Sergeeva, A.S.,
Kinet. Katal., 1970, vol. 11, no. 6, p. 1476.
34. RF Patent 2281805, 2006.
KINETICS AND CATALYSIS Vol. 50
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2009