Coprecipitated CuO-MnOx catalysts for low-temperature CO-NO and CO-NO-O2 reactions

Article abstract of DOI:10.1006/jcat.1998.2347

An analysis of XRD, FTIR spectra, and other data showed that at ambient temperature (25°C) both adsorption and interaction of CO and NO became more intense with increasing copper atomic fraction (Cu/(Cu + Mn)) ≤ 0.53. Using 0.53 Cu (the most active catalyst), the degree of the conversion of NO in NO-CO-Ar-O₂ to N₂ was almost the same as that in NO-CO-Ar. A Langmuir-Hinshelwood mechanism was proposed for both (CO-NO and CO-NO₂) interactions at 25°C. With increasing temperature for CO-NO-Ar, reduction of the catalyst surface started. This led in turn to depression of the CO-NO reaction at ≤ 130°C to progressive acceleration of the CO-NO reaction at > 130°C. In the presence of O₂, CuO-MnO_x had an even higher activity at 75°-300°C. The formation of a disordered mixed oxide with a spinel-like structure was the cause of CuO-MnO_x's high activity.

Full text of DOI:10.1006/jcat.1998.2347

Journal of Catalysis 185, 43–57 (1999)
Article ID jcat.1998.2347, available online at http://www.idealibrary.com on
Coprecipitated CuO–MnO Catalysts for Low-Temperature CO–NO
x
and CO–NO–O Reactions
2
1
Ivanka Spassova, Mariana Khristova, Dimitar Panayotov, and Dimitar Mehandjiev
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Received June 11, 1998; revised September 17, 1998; accepted November 9, 1998
are necessary to complete a catalytic cycle. The oxidation of
The activity of the mixed copper–manganese oxides CuO–MnOx carbon monoxide at ambient temperature over amorphous
�
(
1.5 < x < 2), obtained by coprecipitation and activated at 300 C, mixed “hopcalite”-type catalysts has long been established
has been studied for CO–NO and CO–NO–O2 reactions. As shown
by magnetic susceptibility data, X-ray diffraction, FTIR spectra,
and catalytic tests, an interaction between CuO and MnOx with for-
mation of a disordered mixed oxide with a spinel-like structure is
the cause of the high activity of CuO–MnOx samples.
(
1) and this type of catalyst is still the general choice for
respiratory protection. Formation of amorphous copper–
manganese spinel is found to be crucial for ambient temper-
ature oxidation of carbon monoxide by molecular oxygen
(
2–4). Amorphous CuMn2O4 has been stated to catalyze, at
�
At ambient temperature (25 C) both adsorption and interaction
�
temperatures up to 500 C, the combustion of some organic
substances, such as hydrocarbons and hydroxy-, halide- and
nitrogen-containing compounds (5–7).
of CO and NO become more intense with increasing atomic fraction
of copper content Cu/Cu + Mn up to 0.53 (catalyst C5M5). For the
most active sample C5M5, the degree of NO conversion to N2 at-
tained with a CO + NO + O2 + Ar mixture is almost the same as that
Reduction of NO by CO has been studied over Mn3O4
obtained with a CO + NO + Ar mixture. A Langmuir–Hinshelwood (8), and that by CO, H2, and NH3, over MnO (9) and MnO2
mechanism is proposed for both (CO–NO and CO–NO2) interac- (10). Activity and selectivity of pure manganese oxides
tions at ambient temperature.
(
MnxOy) in the SCR of NO with NH3 have been inves-
With increasing temperature under the conditions of a CO +
NO + Ar mixture, reduction of the catalyst surface starts. This
leads in turn to depression of the CO–NO reaction at temper-
tigated by Kapteijn et al. (11). Several studies have been
published on the decomposition of NO over the manganese
oxides MnO2 (12), Mn3O4 (13), Mn2O3, and Mn3O4 (14).
The decomposition of N2O has also been studied over the
manganese oxides MnO, Mn3O4, Mn2O3, and MnO2 (15),
and over manganese-containing perovskites (16). Compos-
ite manganese–cerium oxide is studied (17) for oxidation of
CO and decomposition of N2O. It is found that cerium pro-
�
atures up to 130 C (low-temperature region) and to progres-
sive acceleration of the CO–NO reaction at temperatures above
�
1
30 C (middle-temperature region). In the presence of oxygen (a
CO + NO + O2 + Ar mixture), CuO–MnOx catalysts have an even
�
higher activity over a wide temperature range (75–300 C).
�c 1999
Academic Press
Key Words: NO; CO; O2; low-temperature; reduction; nitric ox- vides oxygen to manganese oxide at low temperatures and,
ide; carbon monoxide; oxygen; copper–manganese; oxide; spinel; on the contrary, withdraws oxygen at high temperatures.
catalyst.
Some recent studies are directly related to the applica-
tion of manganese oxides in catalyst systems that are devel-
oped for environmental pollution control. Exploratory re-
search to develop Rh-free, “Pt-only” automotive catalysts
INTRODUCTION
is described in (18). Oxides of Co, Fe, Mn, Ti, and V show
During the past several years the catalytic abatement of
promising results when added as promoters or cocatalysts
NOx has attracted the attention of many laboratories world-
wide because of stringent regulations for NOx emissions.
to Pt/SiO2 to facilitate the reduction of NO by CO. Selective
catalytic reduction (SCR) of NO has been studied (19) on a
Transition metal oxides in addition to noble metal catalysts
highly dispersed oxidic manganese layer over alumina. SCR
are of particular interest, specifically the achievement of
of NO by decane under an oxidizing atmosphere has been
low-temperature activity and selectivity with respect to NO
carried out (20) on mechanical mixtures of sulfated zirconia
reduction to N2. Manganese oxides are of interest as cata-
with MnO2 or Cu5Mn13Ox. A high reductive activity (NO
reduction to N2) is obtained as a result of combining the acid
lysts because they have various types of labile oxygen that
and oxidizing properties of sulfated zirconia and MnO2 or
Cu Mn O , respectively. Manganese oxide (MnO ) is used
5 13 x x
(21) as a novel oxygen storage component (OSC) for NO
1
To whom correspondence should be addressed. E-mail: metkomeh@
bas.bg. Fax: (+359-2)705024.
43
0021-9517/99 $30.00
Copyright �c 1999 by Academic Press
All rights of reproduction in any form reserved.

4
4
SPASSOVA ET AL.
reduction and CO and CH4 oxidation in natural-gas-fueled times with distilled water until reading pH = 6 and allowed
vehicles. The catalysts contain MnOx supported on an inert to stand in this medium for 18 h. After filtering, the pre-
�
�
LaAlO3 perovskite and a noble metal component (Pd) sup- cipitate was dried at 100 C for 4 h and calcined at 300 C
ported on a separate high surface area lanthana stabilized for 4 h to give the final catalyst sample. Reference samples
Al2O3. The removal of NO by absorption on manganese of the simple oxides CuO and MnOx were prepared by an
zirconium oxide (Mn–Zr oxide) has been investigated (22) analogous procedure.
in both the presence and absence of gaseous O2. The re-
moval of NO appears to proceed by the oxidation of NO Physicochemical Characterisation
on the Mn sites and a subsequent absorption at the Zr sites
�
2
The copper content in the samples was determined by
standard iodometry, using starch as indicator. Both copper
and manganese were estimated by the EDTA method in the
presence of hydroxylammonium chloride (A.r. grade) and
hexamethylenetetramine (A.r. grade) using xylenol orange
as indicator at pH = 6.
as NO3 species. High surface area (90–400 m ) manganese
nodules show (23) a high catalytic activity in CO oxidation,
and NOx reduction by H2, CO, or NH3.
Selective catalytic reduction of NO by hydrocarbons in
the presence of oxygen over copper ion-exchanged zeo-
lites has been studied by Iwamoto et al. (24). Selective re-
duction of NO by water-soluble oxygen-containing organic
compounds over CuZSM-5 zeolite is proposed by Monreuil
and Shelef (25). Selective catalytic reduction of nitric oxide
with ammonia (SCR) over supported vanadia catalysts has
also been studied (26).
The catalyst samples were characterised by FTIR
(
Br u¨ ker, model IFS 25), X-ray powder diffraction (XRD)
using CoK� radiation (TUR, Germany), magnetic mea-
surements (Faraday-type magnetic balance), and BET sur-
face area methods.
We have studied the activity of Cu–Mn spinels, both
unsupported (27) and � -Al2O3-supported (28), towards
NO–CO and NO–CO–O2 interactions. The unsupported
spinel CuMn2O4 (27) exhibits an activity that is several
times higher than that of Ni and Co manganites (MMn2O4,
M = Cu, Ni, or Co). Maximal activity of supported Cu–Mn
spinels is observed for samples with a Cu : Mn atomic ratio
close to that of the stoichiometric CuMn2O4.
Catalytic Test
The catalytic experiments were carried out in a flow ap-
paratus described previously (31). About 1 cm of a catalyst
3
(
a 0.3–0.6 mm fraction) was placed into the reactor (quartz
tube, d = 8 mm). Argon (purity 99.999% vol.) supplied by
CHIMCO Vratza was used as a carrier gas at a total gas
3
� 1
flow rate of 350 cm /min (space velocity 26,000 h ).
The activity of CuO–MnOx catalysts is substantially
dependent on the preparation method (2, 3, 29), most
active being catalysts prepared by a coprecipitation pro-
cedure (29). It has also been found (3, 30) that the best low-
temperature activity towards the CO–O2 reaction might be
obtained with a copper–manganese oxide catalyst thermally
activated at 300 C.
The purpose of the present paper is to study the behav-
ior of the mixed copper–manganese oxides CuO–MnOx
The concentrations of NO, NOx (NO + NO2), N2O, CO,
and CO2 were continuously measured by gas analysers and
the data were collected by a CSY-10 personal data sta-
tion. The inlet concentration of CO was controlled by Infr-
alyt 2106, NDIR gas analyser. The outlet concentrations
of NO and CO were controlled by UNOR 5 (Maihak)
and that of CO2, by Infralyt 2106. A thermal converter
was applied to NOx (NO + NO2) analysis. A Specord 75
IR spectrophotometer with a 1-m folded path gas cell
�
(
1.5 < x < 2), obtained by coprecipitation and activated
(
Specac) was used for determination of the outlet N2O
�
at 300 C, towards CO–NO and CO–NO–O2 reactions. To
the best of our knowlege this is the first report on low-
temperature activity of amorphous CuO–MnOx catalysts
for NO reduction by CO.
content. A Pye Unicam gas chromatograph with a cali-
brated volume sampler and a column with a molecular sieve
were used for the analysis of O2 and N2 under steady-state
conditions.
Temperature-programmed desorption (TPD) experi-
ments were carried out on the same catalytic apparatus at
EXPERIMENTAL
�
3
a heating rate of 13 C/min in an Ar flow (350 cm /min).
Before a catalytic test each catalyst sample was treated
Preparation of Catalyst Samples
�
for 1 h in an Ar flow at 300 C. Following the pretreatment
Samples of CuO–MnOx were prepared using a copre- procedure, the temperature waslowered to the desired level
�
cipitation procedure. Aqueous solutions of the nitra- (e.g., to 25 C). The concentration step change Ar/CO +
tes (Ferak, Berlin) Cu(NO3)2 � 3H2O (0.5 mol/liter) and NO + Ar, CO + NO + Ar/CO + NO + O2 + Ar was then
Mn(NO3)2 � 6H2O (0.5 mol/liter) were mixed in different enforced.
ratios. The mixed nitrate solution was precipitated with an
The data for steady-state activity of the catalyst samples
NaOH (Ferak, Berlin) solution under vigorous stirring until were collected after 8 h testing under the same conditions
pH = 9. The resulting precipitate was then washed several as those of transient experiments.

COPRECIPITATED CuO–MnOx CATALYSTS
45
The following competitive reactions take place when the corporated oxide. According to Kanungo (2), during the
mixture of CO + NO + O2 is contacted with the catalyst incorporation of a small quantity of copper, the metal ions
surface:
interchange their coordination sites, which leads to an un-
equal charge distribution and an increase in metal–oxygen
[1] bond lengths of both oxides along the distortion axes (2).
2
NO + 2CO → N2 + CO2
O2 + 2CO → 2CO2.
We suggest that incorporation of copper ions into the lat-
[2]
tice of MnOx would alter the exchange interaction. For this
reason, an increase in magnetic susceptibility and specific
surface area (2) of the oxides is observed with the transi-
[3] tion from MnOx to C1M9. On further increase of copper
content in the catalysts, both Mn and Cu gradually shift to
In Ref. (32) we introduced the so-called redox index RO,
RO = (CO)inlet/{(NO)inlet + 2(O2)inlet},
to describe the deviation of a gas flow composition (CO + their regular coordination sites and, as a consequence, the
NO + Ar or CO + NO + O2 + Ar) from the stoichiometric magnetic susceptibility decreases again.
one (RO = 1) that corresponds to the full interaction of the
reductant CO with the oxidants NO and O2 to form N2 and
CO2 according to Eqs. [1] and [2].
X-Ray Powder Diffraction Pattern
XRD patterns of as-prepared MnOx, CuO–MnOx, and
CuO catalysts (finally calcined in air for 4 h at 300 C)
�
RESULTS AND DISCUSSION
indicate X-ray amorphous samples. The FTIR spectra of
these as-prepared samples are shown in the next section.
We carried out an additional prolonged calcination of the
samples for a total time of 160 h under the same condi-
Chemical Analysis, Surface Area,
and Magnetic Susceptibility
�
tions (300 C, in air). A certain crystallization of the sam-
The catalyst samples, CuO–MnOx, were obtained under
different copper–manganese ratios. The data on metal con-
tent, specific surface area, and magnetic susceptibility are
shown in Table 1. The CuO–MnOx samples are denoted as
C1M9, C3M7, C5M5, C7M3, and C9M1 according to their
atomic fraction of copper content (Cu/Cu + Mn). The as-
signments C10 and M10 are used for the reference samples
of CuO and MnOx, respectively.
A volcano-type relationship between the BET surface
area of the CuO–MnOx catalysts and their atomic fraction
of copper content is observed. The surface area gradually
increases and has a maximum at Cu/Cu + Mn = 0.53, with
C5M5 having an area 4 times higher than that of the MnOx
sample. The incorporation of copper into the matrix of the
MnOx sample also leads to a sharp increase in the magnetic
ples was evident after the first 90 h of calcination by
the appearance of very broad and low intensity lines.
Table 2 shows the effect of prolonged calcination (160 h)
on the X-ray diffraction patterns of CuO–MnOx samples.
The data in Table 2 (concerning 160-h treated samples)
reveal that addition of a small fraction of copper (C1M9,
Cu/Cu + Mn = 0.11) leads to a substantial change in the
host MnOx. A series of lines disappear and/or shift to an-
other position. With increasing copper content, the pres-
ence of a Cu–Mn spinel is observed with samples C3M7
and C5M5. With further addition of copper, new lines for
CuO appear in addition to the lines corresponding to the
spinel.
susceptibility of the catalyst samples. According to Selwood FTIR Spectra
(
33), this increase in magnetic susceptibility is due to the
� 1
The FTIR spectra (800–400 cm ) of as-prepared MnOx
valence induction effect of the host metal oxide on the in-
and CuO are presented in Fig. 1a, and those of the mixed
oxide samples CuO–MnOx are shown in Fig. 1b. Spectra
of analytical grade commercial CuO (Ferak, Berlin) and a
sample of Cu–Mn oxide spinel obtained in Ref. (34) are also
shown in Fig. 1. The FTIR spectra of samples treated for
TABLE 1
Chemical Analysis, BET Surfaces Area and Mass
Magnetic Susceptibility
�
a long time in air at 300 C do not differ substantially from
Cu
(at% )
Mn
(at% )
Cu/Cu + Mn
S (BET)
(m g )
�
those of the as-prepared samples. Only the background ab-
sorption of treated samples is less pronounced.
2
� 1
(EMU/g 106)
Sample
(a.u.)
It is not possible to make definite assignments of bands
in this state of the phase due to the lack of a definite crystal
structure of the CuO–MnOx samples. Kanungo (2) has stud-
MnOx
C1M9
C3M7
C5M5
C7M3
C9M1
CuO
—
0.64
1.10
0.84
0.54
0.28
0.13
—
—
29
49
77
128
105
48
34.3
56.8
47.1
35.5
20.8
9.9
0.14
0.38
0.61
0.84
1.09
0.76
0.11
0.31
0.53
0.75
0.89
—
ied thoroughly the structure of some MnO –CuO mixed ox-
x
ides prepared by thermal decomposition of carbonates (co-
precipitated from a mixed nitrate solution by ammonium
carbonate) and by thermal decomposition of nitrates. The
5
—

4
6
SPASSOVA ET AL.
TABLE 2
�
X-Ray Powder Diffraction Patterns of CuO–MnOx Catalysts after Prolonged Calcination (160 h, 300 C, in Air)
�
Mn2O3
-0540
� MnO2
14-644
CuO
5-0661
CuMn2O4
Ref. (3)
Cu1.2Mn1.8O4
35-1029
Cu1.5Mn1.5O4
35-1172
a
6
C1M9
C3M7
C5M5
4.82 w
C7M3
4.79 w
C9M1
4.80 w
4
.93 w
4.82 w
4.03 w
4.82 m
4.01 w
4.82 (6.1)
4.81 vw
4.79 vw
3
.96 vs
3
.75 w
3.75 w
2.93 w
3
2
2
.09 vw
.89 w
.82 w
2.93 m
2.93 w
2.92 vw
2.96 (46)
2.95 w
2.937 w
2
2
.77 s
2.78 s
2
.73 w
2.73 w
2.50 vs
2.38 vw
2.31 s
2.751 w
2.53 m
2.523 vs
2
.60 s
.52 vs
2.49 vs
2.46 s
2.49 vs
2.39 w
2.50 vs
2.38 w
2.52 (122)
2.41 (4.9)
2.515 vs
2.407 vw
2.495 vs
2.39 vw
2
.46 s
2
2
.42 vs
.32 s
2.43 s
2.32 m
2.32 w
2.323 s
2.312 w
2
.31 w
2.31 s
2
.22 w
2.12 s
.05 m
2.19 w
1.87 s
2.17 w
2.07 w
2.20 w
2.07 w
2.23 vw
2.06 w
2
2.06 w
2.08 (17)
2.084 w
2.064 w
1
.96 vw
1.959 vw
1.866 w
1
.86 w
1.86 w
1.69 w
1.59 m
1.87 w
1.69 vw
1.59 m
1.86 m
1
.79 vw
1.778 vvw
1.714 vw
1
.72 s
1.69 w
1.65 w
1.59 s
1.69 w
1.71 w
1.70 (16)
1.60 (49)
1.701 vw
1.604 w
1.689 vw
1.592 m
1
.637 s
1
1
1
.605 ms
1.59 m
1.58 mw
1.50 m
1.581 w
1.505 mw
1
.50 w
.486 w
1.48 vw
1.46 m
1
.45 m
1.46 w
1.41 w
1.46 m
1.46 w
1.47 (54)
1.473 m
1.458 m
.422 ms
1.43 w
1.418 w
1.41 w
1.375 w
1
.41 m
1.41 w
1.39 w
1.41 mw
1.37 mw
1.31 vw
1.398 vvw
1
.38 vw
1.362 m
.306 w
1
1.31 (49)
1.318 vw
1.271 vw
1
1
.27 (24)
.20 (2.4)
1.26 mw
1.262 w
1.262 vw
Note. s, strong; m, medium; w, weak; v, very.
a
Data taken from JSPDS PDF, reference numbers are given.
�
�
1
IR spectra of the samples obtained in this work resemble 530 cm , disappear and a very broad band emerges around
1
� 1
the spectra of samples prepared by thermal decomposition 545 cm . The bands below 450 cm gradually decrease in
of nitrates in Ref. (2). intensity. These changes indicate complete collapse of the
In Ref. (2) the bands of �-MnO2 (pyrolusite) are ten- structure and this continues up to 84 at% of copper. It is
tatively assigned according to Farmer (35) as follows: plausible that not only Mn4 and Cu interchange their co-
+
2+
�
1
� 1
3
u
� 1
2
u
� 1
7
(
20 cm (A2u), 615 cm (E ), 400 cm (E ), 340 cm
ordination with respect to oxygen but also some incipient
E ), and 530 cm (B ). The spectrum of the MnOx sam- formation of a solid solution takes place during the precip-
1
� 1
1
u
u
ple (Fig. 1a-1) has bands at close positions: 720, 634, 530, itation. The C9M1 shows three well-resolved bands at 608,
�
1
� 1
� 1
and 400 cm . The sharp band at 668 cm is analogous 509, and 440 cm . These bands differ in both position and
to that appearing in the spectra of �-MnO2 (Ref. (2)) at intensity from those observed for sample C10 (Fig. 1b-2,
�
1
6
75 cm . Incorporation of about 11 at% of copper (sample CuO prepared by the same procedure as that used for the
�
1
C1M9) shifts the peak at 634 to 618 cm , a new broad band CuO–MnOx samples). The spectrum of sample C10 (CuO)
�
1
appearing around 580 cm . This indicates possible dis- is very close to that shown in Ref. (2) for CuO. However,
placement of Mn4 from the octahedral site by Cu , which the spectrum of the catalyst C9M1 is very close to that ob-
results in considerable distortion or disorder in the host lat- tained for CuO powder and resembles that of the Cu–Mn
tice, as explained by Kanungo (2). On further addition of spinel (Fig. 1b-3 and -4). Therefore, the FTIR spectra of
+
2+
�
1
copper (C3M7), the band at 618 cm decreases in inten- catalyst C9M1 together with the XRD pattern (of a 160-h
sity. At 61 at% of copper (C5M5) both bands, at 618 and treated sample) indicate the presence of a mixture of CuO

COPRECIPITATED CuO–MnOx CATALYSTS
47
FIG. 1. FTIR spectra of as-prepared samples of (a) MnOx, CuO; (b) CuO–MnOx. Spectra of CuO powder (Ferak, Berlin) and Cu–Mn spinel
(
Ref. (29)) are given for comparison.
and a highly disordered spinel phase. The band at 720 cm 1 of the pure oxides are considerably broadened. On other
�
observed for the host MnOx sample is present in the spectra hand, the observed shift of the absorption bands to lower
of all CuO–MnOx samples.
frequencies is due to the increase in metal–oxygen bond
The broadening of IR bands with increasing copper con- length. It is well known that in the vibrational spectrum
tent of the CuO–MnOx samples could be explained in terms the value of the wavenumber is directly proportional to the
of Farmer’s concept (35) about the IR spectra of substi- force constant of the bond and, hence, it increases with the
tuted oxides. According to Farmer, when the vibration fre- increase in valence and decreases with the increase in co-
quencies of a substituent ion and its coordinated oxygen lie ordination number, i.e., in bond length. The increase in the
within or close to those of the host oxide, the sharper bands metal–oxygen bond length after addition of copper is also

4
8
SPASSOVA ET AL.
�
FIG. 2. Activity of MnOx and CuO samples at ambient temperature (25 C). Transient responses of NO, CO, and CO2 obtained during Ar/CO +
NO + Ar/Ar concentration step changes, and temperature-programmed desorption (TPD) experiments.
supported by the increase in magnetic susceptibility of the on the CuO sample are more heterogeneously distributed
CuO–MnOx system discussed above.
by the activation energy of NO adsorption than are those
on MnOx.
Ambient Temperature Activity Towards CO–NO
and CO–NO–O2 Interactions
CuO–MnOx catalyst system. The catalytic experiments
at ambient temperature carried out with the CuO–MnOx
MnOx and CuO catalysts. The activity of the MnOx and catalystscomprise the Ar/CO + NO + Ar/CO + NO + O2+
�
CuO samples at ambient temperature (25 C) is studied Ar/Ar reaction stages. The CuO–MnOx catalysts are re-
towards CO–NO interaction. Following the pretreatment markably more active than are simple oxides towards the
�
procedure (300 C, Ar flow) the temperature is lowered to CO–NO interaction, as shown by the steady-state data in
2
�
5 C and the concentration step change Ar/CO + NO + Ar Table 3.
is enforced. After about 30 min of reaction time (in a
Incorporation of 11 at% of copper into MnOx (sample
CO + NO + Ar mixture), the Ar flow is switched back and C1M9, Cu/Cu + Mn = 0.11) increases the conversion of NO
the isothermal desorption of reaction gases is followed. from 3–5 to 13% . On further addition of copper, the ac-
For both catalysts the CO responses (Fig. 2, left-hand side, tivity of CuO–MnOx catalysts rises sharply and is about
curves 2) are of an instantaneous type, while monotonous 60–70% for samples with an atomic fraction of copper con-
responses are observed for CO2 (Fig. 2, left-hand side, tent (Cu/Cu + Mn) around 0.3–0.7. In Fig. 3, the transient
curves 3). In the case of MnOx, a delay is registered for responses tracing this interaction are presented.
the response of NO, indicating the adsorption of NO on the
The instantaneous responses for CO2 and the material
catalyst surface. The isothermal desorption actually shows balance for the consumption of CO and NO indicate that
that NO is reversibly adsorbed on the surface of MnOx. The CO interacts first with the oxygen from the catalyst surfaces
attained conversions of CO and NO at this low temperature during the initial transient period. The delayed responses
are fairly low (3–5% ).
of NO and the corresponding desorption curves indicate
The data from TPD experiments (Fig. 2, right-hand side) the adsorption of NO on the catalyst surfaces. The reactive
show that both oxides differ clearly by their reactivity to- oxygen during the initial transient period comes from the
wards NO and CO2. In the case of MnOx, two peaks with catalyst surfaces. After depletion of this “excess oxygen,”
NO
NO
�
NO
�
close T
are observed, i.e., T1 = 86 C and T = 132 C. reduction of NO starts. A coincidence between CO and
max
2
The TPD spectra of NO in the case of CuO may be approx- NO responses is observed for more active catalysts (Fig. 3,
imated by four Gaussians, the first and the last of which C7M3). Simultaneous adsorption of CO and NO occurs
differ in Tmax by more than 220 C, i.e., T
T4 = 357 C, respectively. This implies that the active sites ysis of N2 during the transient period was impossible).
�
NO
1
�
= 125 C and with production of CO2 (gas chromatographic (GCh) anal-
NO
�

COPRECIPITATED CuO–MnOx CATALYSTS
49
TABLE 3
Activity of CuO–MnOx Catalysts towards CO–NO and CO–NO–O2 Interactions
�
at Ambient Temperature (25 C)
Isothermal
desorption
Steady-state conversions attained under the conditions of gas mixtures
CO + NO + Ar
NO/N2 CO/CO2
CO + NO + O2 + Ar
Ar (10 min)
Catalyst
sample
NO(total)
(% )
NO/N2
(% )
CO/CO2
(% )
O2
(% )
NO
2
(% )
(% )
(�mol/m g)
C1M9
C3M7
C5M5
C7M3
C9M1
13
61
66
76
9
16
65
70
80
13
39
57
98
65
35
7
24
58
24
4
15
54
96
58
12
83
100
100
100
92
0.44
0.38
0.37
0.23
0.42
When added to the gas mixture, (CO + NO + O2 + Ar
stage (Fig. 3)), oxygen does not inhibit the surfaces of CuO–
MnOx catalysts towards the CO–NO interaction, as is the
case with CuO (36) and Mn3O4 (8). Initially oxygen has
some positive effect on the CO–NO reaction. However,
gradual deactivation of the surfaces of CuO–MnOx sam-
ples with time is observed. The overshoot type of the CO2
response curves implies that the slow regeneration of active
sites (37) may be the reason for this deactivation, i.e., the
rate limiting step could be related to the slow desorption of
the reaction product CO2.
The data in Table 3 show that with all CuO–MnOx cata-
lysts the steady-state degree of total conversion of NO is
higher than that of NO conversion to N2. At the same time,
oxygen is almost fully reacted and the degree of CO con-
version to CO2 is higher than that of NO to N2. As shown
by NOx/NO analyses, fast homogeneous gas-phase oxida-
tion of NO to NO2 occurs at ambient temperature under
the conditions of a CO + NO + O2 + Ar mixture. The dif-
ference NO(total) � NO/N2 (NO converted to N2, Table 3)
corresponds to the conversion of NO to NO2 as measured
by NOx/NO analyses. Thus, the NO2 formed reacts with CO
to give N2. The material balance for the consumption of CO,
NO, NOx, and O2 and the production of CO2 and N2 under
steady-state conditions may be described correctly by the
following balance equations:
2
NO + O2 → 2NO2
[4]
[5]
2
NO2 + 4CO → 4CO2 + N2.
For the most active sample C5M5 the steady-state de-
gree of NO conversion to N2 (Table 3) attained after about
2
5 min with CO + NO + O2 + Ar is slightly lower than that
with a CO + NO + Ar mixture. In the cases of less active
catalysts (C1M9 and C9M1), NO is mostly consumed by
the NO–O2 interaction.
FIG. 3. Activity of CuO–MnOx catalysts at ambient temperature
�
(
25 C): (a) C1M9 and C3M7; (b) C7M3 and C9M1. Transient responses
Kapteijn et al. (38) have found that NO interacts very
weakly with the surface of manganese oxide supported on
of NO, CO, and CO2 obtained during Ar/CO + NO + Ar/CO + NO +
O2 + Ar/Ar concentration step changes.

5
0
SPASSOVA ET AL.
alumina (2–8.4 wt% Mn). Strongly bonded oxidized species
are formed in the presence of oxygen, resulting in NO2,
nitrito, and nitrate groups. These species decompose, giv-
ing back NO gas. Hence, oxygen can oxidize gas-phase NO
to NO2 as well as the catalyst surface, thus favoring NO
adsorption.
TPD spectra of NO and CO2 after CO–NO–O2 interac-
tion on CuO–MnOx catalysts. After isothermal desorp-
tion following the CO + NO + O2 + Ar experiments, TPD
was carried out. With all catalysts the TPD spectra of
CO were barely discernible; i.e., a very low surface cov-
erage by CO was found. The TPD spectra of NO and
CO2 obtained with some of the catalysts are presented in
Fig. 4.
FIG. 5. Dependence of specific surface area, steady-state degree of
NO conversion (total and to N2), and amount of desorbed NO on atomic
fraction of copper content (Cu/Cu + Mn) of CuO–MnOx catalysts.
The TPD spectrum of NO observed with the C1M9
catalyst (Fig. 4a, curve 2) may be approximated by two
NO
�
NO
�
Gaussians with Tmax, i.e., T1 = 86 C and T = 132 C.
2
These maxima coincide with those obtained with MnOx
(
Fig. 4a, curve 1). In the cases of C3M7 and C5M5, a new
�
peak appears at Tmax around 190 C. The peak is more in-
tense with C5M5, which is the most active catalyst.
It is logical to suppose that a relation could exist be-
tween the attained degree of NO conversion during the
CO + NO + O2 + Ar stage and the amount of NO desorbed
after this stage. As Fig. 5 shows, a volcano-type curve is ob-
served for the dependence of the steady-state degree of
NO conversion (total and to N2) on the Cu/Cu + Mn ratio.
This is also valid for the dependence of the amount of des-
orbed NO on the Cu/Cu + Mn ratio. An analogous volcano-
type curve is observed with the SBET vs Cu/Cu + Mn depen-
dence. Hence, it may be deduced that the desorption of NO
originates from the active sites on the catalyst surface for
CO–NO and CO–NO–O2 reactions. The number of active
sites is proportional to the specific surface area of the cata-
lysts and has a volcano-type dependence on the atomic frac-
tion of their copper content (Cu/Cu + Mn). The maximum
activity of the CuO–MnOx samples (around 50 at% of cop-
per) might be attributed to the formation of a highly disor-
dered copper manganese spinel-like phase. It has been sug-
2
+
3+
gested that the charge transfer reaction Cu + Mn
⇔
+
4+
Cu + Mn ensures the high activity of the amorphous
copper–manganese spinel (having a Mn : Cu ratio of 1 : 1)
in the CO + O2 reaction (29, 39). If we plot the calculated
2
amount of NO converted to N2 per m of surface area vs the
Cu/Cu + Mn ratio, a volcano-type curve is observed with a
maximum around Cu/Cu + Mn � 3–7. This implies that the
activity of CuO–MnOx samples depends not only on the
surface area but also on the Mn : Cu ratio of the catalysts.
FIG. 4. Temperature-programmed desorption (TPD) spectra of NO
(
a) and CO2 (b) obtained after interaction of catalysts with CO + NO + Ar
�
and CO + NO + O2 + Ar gas mixtures at ambient temperature (25 C).

COPRECIPITATED CuO–MnOx CATALYSTS
51
With increasing copper content of the CuO–MnOx cata-
lysts, new peaks appear in the TPD spectra of CO2 (Fig. 4b).
CO2
1
�
These are a low-temperature peak with T
= 98 C and a
CO2
�
high-temperature peak with T4 = 410 C. The first peak
of CO2 corresponds to the low-temperature peak of NO
desorption. Hence, with increasing copper content in the
CuO–MnOx samples, two types of new active sites are
formed on the catalyst surfaces. On the first type of new
active sites, both the reagent (NO) and the product (CO2)
are adsorbed. During TPD, they are desorbed in the same
�
temperature region, around 100 C. On the second type of
active sites, only the product CO2 is strongly adsorbed (des-
�
orbed above 350 C). It is the strong adsorption of CO2 on
these sites that may be the reason for the deactivation of
�
catalysts at ambient temperature (� 25 C).
Behaviour of MnOx and CuO Catalysts during
the Thermal Activation of the CO–NO Reaction
After the TPD experiments, the temperature has been
�
decreased to 50 C and a study of CO–NO interaction has
been carried out. The same set of concentration steps of
Ar/CO + NO + Ar/Ar are performed at 50, 75, 100, 130,
�
1
70, 200, 250, and 300 C. The temperature has been ele-
vated stepwise. The catalysts MnOx and CuO are almost
�
not active at temperatures up to 100 C. The behaviour of
�
both oxides in the temperature region 50–75 C is similar
to that observed at ambient temperature: low outlet con-
�
centrations of N2 and CO2 are registered. At 100 C for
�
MnOx and at 170 C for CuO, intense interaction of CO with
oxygen from the catalyst surface starts. The behaviour of
CuO is analogous to that previously observed in this labo-
ratory (36) and also discussed in detail by Rozovskii et al.
�
FIG. 6. Activity of some CuO–MnO catalysts in the low-temperature
region (50–100 C). Transient responses of NO, CO, and CO2 obtained
x
(
40). The behaviour of MnOx above 100 C and that of CuO
�
�
above 170 C resemble that of other simple oxides (8, 9, 36)
and oxide spinels (27, 32, 41). The surface reduction leads to
activation of the CO–NO reaction. High degrees of NO to
N2 conversion (up to 100% ) are observed when increasing
during Ar/CO + NO + Ar/CO + NO + O2 + Ar/Ar concentration step-
changes.
�
the temperature up to 300 C.
is observed for all CuO–MnOx catalysts. In the second
�
(
middle-temperature) region of 130 to 300 C, the catalysts
Behaviour of CuO–MnOx Catalysts during Thermal
Activation of CO–NO and CO–NO–O2 Interactions
show some difference in behaviour towards the CO–NO
reaction.
The study of CO–NO and CO–NO–O2 interactions on
CuO–MnOx catalysts is carried out after the TPD exper- Figure 6presentsthe resultsobtained at 50–100 C with cata-
Low-temperature activity of CuO–MnOx catalysts.
�
iments (shown in Fig. 4). The temperature is decreased lysts C1M9 and C7M3.
�
�
to 50 C and the concentration steps Ar/CO + NO + Ar/
At 50 C, immediatelyafter the concentration step change
CO + NO + O2 + Ar/Ar are performed. The same exper- Ar/CO + NO + Ar, an interaction of CO with the catalyst
iments are carried out at 50, 75, 100, 130, 170, 200, 250, and surface oxygen (CO–Osurf.) starts first, evolving CO2 in the
�
3
00 C. Some of these results are presented in Figs. 6 and 7. effluent gas (Fig. 6a). The extent of CO to CO2 conversion
Two temperature regions may be distinguished for the depends on the Cu content in the CuO–MnOx samples.
catalytic behaviour of CuO–MnOx catalysts towards the For catalysts with a Cu/Cu + Mn ratio above 0.3 (C5M5,
CO–NO and CO–NO–O2 interactions. In the first (low- etc.), complete conversion of CO to CO2 is observed. The
�
temperature) region of 50 to 100 C, activation of the cata- CO–Osurf. reaction leads in turn to progressive activation of
lyst surfaces to both reactions (CO–NO and CO–NO–O2) the catalyst surface towards the CO–NO reaction.

5
2
SPASSOVA ET AL.
It should be recalled here that after the CO + NO + O2 mixtures, the state of the catalyst surface varies between
�
stage at 25 C, remarkable amounts of NO are left strongly reduced and oxidised.
adsorbed on the catalyst surface. This NO is then desorbed
in the temperature region 50–100 C during the TPD exper- interaction of CO from the gas phase with the surface oxy-
Therefore, during the CO + NO + Ar stages an intense
�
iment (Figs. 3 and 4). Hence, it would be logical to examine gen that causes depletion of adsorbed NO (thus depres-
the influence of preadsorbed NO on the CO–NO interac- sing its adsorption and reduction) is characteristic for the
tion, i.e., without inclusion of an intermediate TPD stage. CO–NO reaction in the low-temperature region. During
The data shown in Fig. 6b for catalysts C7M3 are obtained the CO + NO + O2 + Ar stage, the gas-phase oxygen causes
by such a procedure (a TPD stage is not included between an enhancement of NO reduction, although oxidising con-
�
the concentration steps carried out at 25 and 50 C). Des- ditions are realised. It is reported in the literature that the
orption of large amounts of NO and simultaneous complete catalytic reduction of NO2 by CO (43, 44) or by HC (45)
conversion of CO to CO2 is observed immediately after the is much faster than the NO reduction. We suggest that it is
�
Ar/CO + NO + Ar concentration step at 50 C (Fig. 6b). It is fast oxidation of NO to NO2 followed by fast reduction of
not Ar that would initiate desorption of NO as the tempera- NO2 by CO that outlines the probable reaction route for
�
ture is elevated from 25 to 50 C with Ar still passing through N2 formation. This might explain the higher degrees of NO
the reactor. Therefore, NO is displaced from the catalyst conversion to N2 observed during the CO + NO + O2 + Ar
surface and thus desorbed by the adsorption of another gas stage as compared to the conversion detected during the
(
or gases) during the setting up of the Ar/CO + NO + Ar CO + NO + Ar stages.
concentration step. If NO is adsorbed as nitrito and/or ni-
tro species (according to Refs. (38, 42)), then the stronger
adsorption of CO and its interaction with surface oxygen the middle-temperature region (130 to 300 C) the interac-
Middle-temperature activity of CuO–MnOx catalysts. In
�
(
giving CO2) are the probable reasons for the displacement tion of CO with catalyst surface oxygen (CO–Osurf.) also
of NO. Indeed, CO2 is instantaneously evolved in the gas proceeds during the CO + NO + Ar stages. However, NO
phase after the Ar/CO + NO + Ar concentration step.
reduction to N2 is not depressed as much as it is at 75 and
�
After addition of oxygen (CO + NO + Ar/CO + NO + 100 C. In the case of the catalyst C1M9 (Fig. 7a), at a tem-
�
�
O2 + Ar concentration step, 50 C), initial considerable con- perature of 170 C and above, the surface reduction leads in
sumption of NO is registered with all catalysts. Hence, the turn to gradual enhancement of NO reduction to N2.
addition of oxygen to CO + NO stimulates the adsorption
It is the competition between the catalyst surface oxygen
of NO. Part of this NO is desorbed during the isother- and NO for interaction with CO that defines the rate of N2
mal desorption (Fig. 6b, catalyst C7M3). More strongly production. This oxygen is accumulated on the surface dur-
adsorbed NO desorbs during the next Ar/CO + NO + Ar ing the CO + NO + O2 + Ar stages, because an excess of
�
�
concentration step carried out at 75 C. At 50 C, the con- oxidants is applied (a redox potential RO = 0.4–0.5). The
version of NO to N2 during the CO + NO + O2 + Ar stage is amount of oxygen depleted from the catalyst surface dur-
remarkable. For catalysts with Cu/Cu + Mn > 0.3 it is even ing the CO + NO + Ar stage and that transferred back to
higher than that observed during the CO + NO + Ar stage; the surfaces during a subsequent CO + NO + O2 + Ar stage
i.e., oxygen from the gas phase has a promoting effect on are close (equal duration of stages is used). In the cases of
the NO to N2 conversion.
the most active catalysts C5M5 and C7M3, the amounts of
�
At 75 and 100 C the extent of CO–Osurf. interaction dur- transferred oxygen are within the range 15–20% of mono-
ing the CO + NO + Ar stage increases, and the NO to N2 layer surface coverage. The amount of oxygen present in
reduction is strongly depressed. On the contrary, during the one monolayer of the oxide surface has been approximated
CO + NO + O2 + Ar stage the rate of NO reduction to N2 in Ref. (41) to about 1019 atom/m . For the less active cata-
is enhanced. For the most active C5M5 and C7M3 catalysts, lysts C1M9, C3M7, and C9M1, the amounts of transferred
2
conversion degrees of NO to N2 over 90% are attained at oxygen are about 30–35, 20–25, and 15–20% of monolayer
�
1
00 C during the CO + NO + O2 + Ar stage. These results surface coverage, respectively.
are remarkable, bearing in mind that under these conditions
In this temperature region, measurable steady state con-
the amount of oxidants exceeds by >50% that required by versionsofNO to N2 are attained duringthe CO + NO + Ar
the stoichiometry of the CO–NO–O2 interaction (a redox stages; i.e., the reduction of NO to N2 is fast enough
potential RO = 0.43 is used). With all the catalysts, com- to compete with the CO–Osurf. reaction. Analogous be-
�
plete conversion of CO to CO2 is attained at 75 and 100 C. haviour has been observed with simple oxides (36) and
According to the material balance for the gas phase, an ad- manganites (MMn2O4, M = Cu, Ni, Co) (27) and cobaltites
ditional reaction of catalyst surface reoxidation also occurs (CuxCo(3� x)O4 (41)). However, these catalysts lose com-
during the CO + NO + O2 + Ar stage at the expense of ex- pletely (CuO, (36)) or to some extent (MMn2O4 (27) and
cess oxidants. Thus, when the gas-phase composition is cy- CuxCo(3� x)O4 (41)) their activity in NO reduction when
cled between the CO + NO + Ar and CO + NO + O2 + Ar O2 is added to the gas phase. As may be seen in Figs. 6

COPRECIPITATED CuO–MnOx CATALYSTS
53
its subsequent interaction with CO might be suggested as
key steps for the mechanism of NO reduction to N2 on
the surface of CuO–MnOx catalysts under the oxidising
conditions of the CO + NO + O2 + Ar gas mixture. It was
previously concluded by Shelef et al. (46) that the catalyst
(
H-ZSM5, Cu-ZSM5, or Cu/Al2O3) which has the highest
activity in NO oxidation to NO2 also has the highest ac-
tivity towards NO decomposition. Using isotope labeling
technique, Chang and McCarty (47) have established that
NOx reduction over Cu-ZSM5 and Co-ZSM5 proceeds via
�
NO2 intermediates at very low temperatures (<200 C).
Adsorption properties and activity of a C5M5 catalyst at
ambienttemperature. In order to gain a better understand-
ing of the interaction of the gas phase with the surface of
CuO–MnOx catalysts, we have carried out a set of sequen-
tial stages of reaction and TPD with the most active catalyst
C5M5. The reaction stages are carried out at ambient tem-
perature with NO + Ar, CO + Ar, NO + O2 + Ar, CO +
NO + Ar, and CO + NO + O2 + Ar gas mixtures. The re-
sults are presented in Fig. 8. The sample of catalyst C5M5
has not been withdrawn from the reactor after the exper-
iments shown in Fig. 7; i.e., the same sample has been
utilized.
The first stage of the experiments includes the interaction
of a NO + Ar mixture with the surface of the used C5M5
catalyst (Fig. 8a).
The material balance shows that about 329 �mol NO are
consumed during the reaction stage, 49 �mol are desorbed
during the isothermal desorption stage and 169 �mol are
desorbed in the TPD experiment (Fig. 8b). The two latter
amounts correspond to approximately 2 and 8% , respec-
tively, of NO surface coverage. Hence, about 111 �mol of
NO consumed during the reaction stage is not registered
FIG. 7. Activity of some CuO–MnOx catalysts in the middle-tem-
perature region (130–300 C). Transient responses of NO, CO, and CO2
�
obtained during concentration step changes Ar/CO + NO + Ar/CO + in the TPD stage. Gas chromatographic analysis has indi-
NO + O2 + Ar/Ar.
cated the evolution of a trace amount of N . However, the
sensitivity of the GCh analysis has not been sufficient for
2
and 7, CuO–MnOx catalysts have an even higher activity in evaluation of the material balance.
the presence of oxygen. Moreover, this behaviour of CuO–
During the CO + Ar stage (Fig. 8c), remarkable conver-
MnOx catalysts is observed over a wide temperature range sion of CO to CO2 is observed even at a low temperature
�
�
(25 C). The material balance has shown that 435 �mol CO
(
75–300 C).
At temperatures above 200 C, some decrease in the rate is consumed and 345 �mol CO2 is produced. During the
�
of NO reduction is observed for all catalysts during the isothermal desorption, 18 and 13 �mol of CO and CO2,
CO + NO + O2 + Ar stages (Fig. 7). If the oxidation of NO respectively, are evolved, while 7 and 53 �mol of CO and
to NO2 and the subsequent reaction of NO2 with CO are ac- CO2 are desorbed during the TPD (Fig. 8d). These data
cepted as stages of an appropriate route for NO reduction, show full equilibration of the amounts of consumed CO
then the inhibition of the NO–O2 gas-phase reaction with and the sum CO(desorbed) + CO2(produced), (about 440 �mol).
increasing temperature may account for the partial depres- The amount of surface oxygen involved in the CO2 pro-
sion of NO reduction. It is known that the NO–O2 reaction duction is 411 �mol, which corresponds to approximately
has a negative temperature coefficient.
19.6% of monolayer coverage. This implies the existence of
The most important point here is that for all catalysts the veryreactive oxygen left on the surface ofthe C5M5catalyst
addition of O2 at the CO + NO + O2 + Ar stages leads to after the NO + Ar/TPD stages. The fact that both CO and
an increase in the rate of NO reduction. Formation of NO2 CO2 are desorbed simultaneously during the TPD stage
(
via homogeneous or even catalytic oxidation of NO) and indicates the participation of CO and surface oxygen in

5
4
SPASSOVA ET AL.
FIG. 8. Sequential reaction and TPD stages carried out with the most active catalyst C5M5. The reaction stages are performed at ambient
�
temperature (25 C) with NO + Ar, CO + Ar, NO + O2 + Ar, CO + NO + Ar, and CO + NO + O2 + Ar gas mixtures. Transient responses of NO, CO,
and CO2.
the same (probably carbonate-like) surface species. These manner analogous to that in the stage I (matching NO re-
species are easily destroyed even at ambient temperature to sponses are observed). Addition of oxygen at stage IV leads
give CO2. Desorption of CO2 is complete at temperatures to an abrupt increase in the consumption of NO. With time
�
as high as 170 C.
the outlet concentration of NO is monotonously increased.
The subsequent stages (III–V, NO + Ar/NO + O2 + Ar/ The transient response of NO could reflect a fast homo-
NO + Ar) show (Fig. 8e) some interesting features. In geneous NO–O2 reaction followed by adsorption of NO2
stage III, nitric oxide reacts with the catalyst surface in a on the catalyst surface. In the steady state (after 20–25 min

COPRECIPITATED CuO–MnOx CATALYSTS
55
reaction time), O2 is completely consumed (GCh analysis) specificsurface area. In the present work, an analogousrela-
and about 30% of NOx belong to NO2 (NOx/NO analysis). tionship is found between the ambient temperature activity
Hence, in the steady state NO and O2 are stoichiometrically of CuO–MnOx catalysts towards NO reduction and the spe-
reacted. It has been reported that oxidation of NO by O2 cific surface area. As shown by magnetic susceptibility data
to form NO2 is effectively catalyzed by some zeolite-based and XRD and FTIR spectra, an interaction between oxides
catalysts (Ref. (47), and the references therein) at a rela- with formation of a disordered mixed oxide (with probable
tive low temperature, e.g., ambient temperature. The TPD spinel structure) is the cause for the high activity of CuO–
spectra of NO (Fig. 8f) comprise three peaks (Gaussian fit) MnOx samples. The greatest activity of catalyst C5M5 might
as observed in the TPD spectra after the adsorption of NO be attributed to the highly disordered copper–manganese
(
Fig. 8b). The Tmax of those three peaks are practically at spinel phase. Metal ions of different oxidation states will
the same positions but the height and the area of the second be created on the surface of such a disordered phase, and
peak are increased. This may be attributed to the partial re- thus the adsorption of CO and NO will be facilitated. This
oxidation of the catalyst surface during the NO + O2 + Ar is clearly shown by the correlation between the amount of
stage. The material balance for NO has shown that half desorbed NO and the activity of the catalysts (Fig. 5). It
of the consumed NO is left as adsorbed on the catalyst has been reported recently (48) that two types of Cu–Mn
surface. As established in Ref. (46), formation of NO2-like spinels, i.e., Cu2 Mn O4 and Cu [Cu Mn ]O4, may be
+
3+
+
2+
4+
2
0.5
1.5
species, part of which is desorbed as NO, could also be sug- formed in the CuO–MnOx oxide system. It may be sug-
gested that a redox couple, Cu2 + Mn ⇔ Cu + Mn
+
3+
+
4+
,
gested.
The last two reaction stages (VI and VII) comprise ex- formed on the surface of CuO–MnOx samples may play a
periments with CO + NO + Ar and CO + NO + O2 + Ar certain role in the catalytic reaction mechanism of NO–CO
gas mixtures (Fig. 8g). In stage VI the amounts of CO re- reaction.
acted and CO2 produced coincide well and show that about
Yamashita and Vannice (4) have suggested that NO
5
4 �mol of CO are reacted with NO. Comparison of the NO decomposition over Mn2O3 proceed via a second-order,
response with those of CO and CO2 shows that under these rate determining surface reaction between NO molecules
conditions NO is more strongly adsorbed on the catalyst or NO-related intermediates, with retardation effects of
surface than are CO and CO2.
oxygen or oxygen-related intermediates. Adsorption of
Addition of O2 (stage VII) leads to precipitous changes NO on an active site, designated as �, and a Langmuir–
in the concentrations of all gases. The response curves Hinshelwood (L-H) model are proposed for NO decom-
�
for CO and CO2 synchronize well. Obviously, even at position. The suggested NO-related intermediates are NO
�
�
ambient temperature oxygen steps immediately in the or N2O , where N2O species are rapidly decomposed to
interaction and the CO2 production is sharply increased. give N2.
The steady-state concentrations of CO, NO, and CO2
In the case of CO–NO interaction on CuO–MnOx cata-
(
oxygen is completely consumed) and the material balance lysts, we have observed simultaneous adsorption of CO and
indicate that NO is oxidised to NO2, which reacts with CO NO at ambient temperature and formation of CO2 (GCh
to give CO2. The TPD spectrum of NO (Fig. 8h) is close analysis of N2 during the transient period was impossible).
to that observed in the stage I-TPD (Fig. 8b), i.e., after the The coincidence between CO and NO responses is better
NO + Ar reaction stage I. The amount of desorbed NO in for more active catalysts (Fig. 3). Adsorbed CO is mainly
VII-TPD is about 166 �mol. The calculated (on the basis of desorbed as CO2, and NO and CO2 have close Tmax around
�
material balance) total amount of adsorbed NO during the 100 C (Figs. 4 and 8f). The foregoing implies the formation
reaction stages VI and VII corresponds to about 145 �mol. of both CO-related and NO-related intermediate species
These values for adsorbed NO are close enough to support for the CO–NO interaction. We have proposed (49), on the
the suggestion that NO2 is an intermediate product of NO basis of IR spectra, and kinetic and TPD data, formation
conversion to N2.
of a surface carbonate complex for CO adsorption and of
nitro- or nitrito-like complexes for NO adsorption in or-
�
Proposed reaction pathways of the mechanism of ambient der to explain the low-temperature (25–50 C) activity of
temperature CO–NO and CO–NO–O2 interactions. The the CuCo2O4 catalyst. Hence, the steps compose the
foregoing data show that amorphous CuO–MnOx oxides path of the CO–NO reaction mechanism that occurs
have much greater ability to adsorb and activate CO and on the surface of CuO–MnOx catalysts at ambient tempera-
NO molecules in their catalytic interaction than do the sim- ture. Hence, by analogy with the scheme for NO decompo-
ple oxides MnOx and CuO. In Ref. (2) it has been estab- sition (4) it might be suggested that an L-H model would be
lished that the activity of CuO–MnOx catalysts can be corre- plausible for describing the CO–NO interaction. Thus, after
lated better with surface excess oxygen than with active (or being formed on the catalyst surface, CO- and NO-related
available) oxygen which is a bulk property. This correlation intermediates interact to give finally N2 and CO2. On less
is supported by the direct relationship between activity and active catalysts (e.g., C1M9 and C9M1), production of CO2

5
6
SPASSOVA ET AL.
�
is favored via interaction of CO with surface oxygen (O ).
8. Alkhazov, T. G., Gassan-zadeh, G. Z., Osmanov, M., and Sultanov,
Yu., Kinet. Katal. 16, 1230 (1975).
cat
Formation of an unstable intermediate surface carbonate
complex via a primary and rate controlling step of CO
combination with surface oxygen has been proposed by
Brittan et al. (50) and also by Kanungo (3) for ambient
9
. Shelef, M., and Otto, K., J. Catal. 10, 408 (1968).
0. Echigoya, E., Niiyama, H., and Ebitani, A., Nippon Kagaku Kaishi,
22 (1974).
1
1
1
1
1
2
1. Kapteijn, F., Singoredjo, L., Andreini, A., and Moulijn, J. A., Appl.
Catal. B 3, 173 (1994).
2. Yur’eva, T. M., Popovski, V. V., and Boreskov, G. K., Kinet. Catal. 6,
temperature oxidation of CO. Formation of Cu2 (O )(NO)
species is supposed as a primary step for NO decomposition
on Cu-ZSM5 catalysts (47, 51).
+
�
9
41 (1965).
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Under addition of O2 at ambient temperature, fast ho-
mogeneous gas-phase oxidation of NO to NO2 occurs, as
shown by NOx/NO analysis. The data in Table 3 and NO re-
sponses observed in the CO + NO + O2 + Ar stage (Fig. 3)
imply some contribution of catalytic oxidation of NO on the
catalyst surfaces as observed on other Cu-containing cata-
lysts (46, 47). Oxygen has no strong inhibition effect on NO
reduction, although oxidizing conditions are realised (re-
dox index RO � 0.4–0.5), as previously observed for base
metal oxide catalysts (8, 27, 36, 41). Presumably, the reduc-
tion of NO2 by CO is fast enough (43, 44) to compete with
the CO–O2 reaction. Thus, interaction between CO- and
NO2-related intermediates to produce finally N2 and CO2
is suggested for the CO–NO–O2 interaction. The slow deac-
tivation of the catalyst surfaces is most probably due to the
formation of strongly held carbonates, which decompose at
high temperature (above 250 C, Figs. 4 and 8f). Thus, we
suggest that the L-H model can also be accepted for de-
scription of CO–NO2 interaction at ambient temperature.
With increasing temperature (from 25 to 50 C), adsorbed
NO2 decomposes with elution of NO or reacts with CO to
liberate NO (Figs. 6).
The results of the present work show that effective cata-
lysts for low-temperature CO–NO and CO–NO–O2 reac-
tions can be obtained on the basis of the CuO–MnOx oxide
system. Further study on the stability of these catalysts to-
wards poisons such as water and SO2 are now in progress.
(
15. Yamashita, T., and Vannice, A., J. Catal. 161, 254 (1996).
1
1
1
1
2
2
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8
7. Imamura, S., Shono, M., Okamoto, N., Hamada, A., and Ishida, S.,
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9. Kijlstra, W. S., Poels, E. K., Bliek, A., Weckhuysen, B. M., and
Schoonheydt, R. A., J. Phys. Chem. B 101, 309 (1997).
0. Delahay, G., Coq, B., Ensuque, E., and Figueras, F., C. R. Acad. Sci.
Ser. II B 322, 881 (1996).
1. Chang, Y. F., and Mccarty, J. G., Catal. Today 30, 163 (1996).
22. Eguchi, K., Watabe, M., Ogata, S., and Arai, H., Bull. Chem. Soc.
Jpn. 68, 1739 (1995).
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Solymosi, and P. Tetenyi, Eds.), p. 213, Akad e´ miai Kiad o´ , Budapest,
2
2
�
1993.
�
25. Monreuil, C. N., and Shelef, M., Appl. Catal. B 1, L1, (1992).
2
2
6. Forzatti, P., and Lietti, L., Heterogeneous Chem. Rev. 3, 33
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Burgas, Bulgaria, 1991.
(
2
2
8. Panayotov, D., React. Kinet. Catal. Lett. 58, 73 (1996).
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State Ionics 32/33, 206 (1989).
3
3
3
0. Spasova, I., and Mehandjiev, D., React. Kinet. Catal. Lett. 58, 57
(
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ACKNOWLEDGMENTS
Financial support of this work from the Foundation for Scientific In-
vestigations at the Ministry of Science and Education, Project X-621, is
gratefully acknowledged. The help of Dr. D. Stoilova with the FTIR study
and Dr. D. Kovacheva with the XRD analysis is greatly appreciated.
33. Selwood, P. W., in “Advances in Catalysis” (W. G. Frankenburg,
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New York, 1952.
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416 (1997).
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