Mechanism of methanol conversion on ZrO22 and 5% Cu/ZrO 2 according to in situ IR spectroscopic data

Article abstract of DOI:10.1134/S002315841003016X

It is demonstrated by in situ IR spectroscopy that, in methanol conversion on ZrO₂ and 5% Cu/ZrO₂ catalysts, methoxy groups are present on the catalyst surface, which result from O-H or C-O bond breaking in the methanol molecule. Two types of formate complexes, localized on ZrO ₂ and CuO, are also observed. The formate complexes form via the oxidative conversion of the methoxy groups. There are two types of linear methoxy groups. First-type linear methoxy groups condense with the formate complex located on CuO to yield methyl formate and then CO and H₂. Second-type methoxy groups appear as intermediate products in the formation of dimethyl ether. The main hydrogen formation reactions are the recombination of hydrogen atoms (which result from the interconversion of surface complexes) on copper clusters and the decomposition of methyl formate. The source of CO ₂ in the gas phase is the formate complex, and the source of CO is methyl formate. The effect of water vapor and oxygen the surface reactions and product formation is discussed. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S002315841003016X

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 3, pp. 428–437. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © V.A. Matyshak, O.N. Sil’chenkova, I.T. Ismailov, V.F. Tret’yakov, 2010, published in Kinetika i Kataliz, 2010, Vol. 51, No. 3, pp. 447–456.
Mechanism of Methanol Conversion on ZrO2
and 5% Cu/ZrO According to In Situ IR Spectroscopic Data
2
a
a
b
b
V. A. Matyshak , O. N. Sil’chenkova , I. T. Ismailov , and V. F. Tret’yakov
a
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 119991 Russia
b
eꢀmail: matyshak@polymer.chph.ras.ru
Received September 7, 2009
Abstract—It is demonstrated by in situ IR spectroscopy that, in methanol conversion on ZrO and 5%
2
Cu/ZrO catalysts, methoxy groups are present on the catalyst surface, which result from O–H or C–O bond
2
breaking in the methanol molecule. Two types of formate complexes, localized on ZrO and CuO, are also
2
observed. The formate complexes form via the oxidative conversion of the methoxy groups. There are two
types of linear methoxy groups. Firstꢀtype linear methoxy groups condense with the formate complex located
on CuO to yield methyl formate and then CO and H . Secondꢀtype methoxy groups appear as intermediate
2
products in the formation of dimethyl ether. The main hydrogen formation reactions are the recombination
of hydrogen atoms (which result from the interconversion of surface complexes) on copper clusters and the
decomposition of methyl formate. The source of CO in the gas phase is the formate complex, and the source
2
of CO is methyl formate. The effect of water vapor and oxygen the surface reactions and product formation
is discussed.
DOI: 10.1134/S002315841003016X
INTRODUCTION
mate and carbonate complexes, and the source of CO
is MF. The introduction of water vapor into the reactꢀ
ing stream exerts a favorable effect on the outcomes of
the reaction, reducing the CO formation rate without
changing the hydrogen yield.
In recent years, much attention has been focused
on obtaining H ꢀcontaining gas mixtures from methaꢀ
2
nol. This is largely due to the development of hydrogen
fuel cells. Hydrogenꢀrich mixtures can be obtained by
the catalytic decomposition and steam and autotherꢀ
mal reforming of methanol [1–4]. Obviously, inforꢀ
mation concerning the mechanism of surface reacꢀ
tions would be helpful in the design of efficient methꢀ
anol decomposition catalysts.
Here, we report a similar study of the role of surface
complexes in methanol conversion for copperꢀconꢀ
taining catalysts supported on ZrO2.
As follows from the literature [5–9], the interaction
of methanol with oxide catalysts typically yields methꢀ
oxy groups and formate, aldehyde, and carbonate
EXPERIMENTAL
Zirconium dioxide was synthesized via a standard
complexes on the catalyst surface. However, the role of procedure [11]. According to Xꢀray diffraction data,
these complexes in the formation of products, such as the resulting zirconia was a mixture of monoclinic (mꢀ
carbon monoxide and dioxide and hydrogen, has not ZrO ) and tetragonal (tꢀZrO ) phases. Its specific surꢀ
2
2
2
been elucidated, apparently because there has been no face area was 116 m /g. The sample containing 5%
comparison between the surface complex conversion copper was prepared by impregnating zirconia with a
and product formation rates.
copper nitrate solution followed by drying at 100°С
Earlier, we studied methanol conversion on CeO2 and calcination at 500°С in air for 2 h.
and 5% CuO/CeO catalysts by in situ IR spectrosꢀ
2
The state of copper in the catalyst was characterꢀ
copy [10]. Formate and carbonate complexes and
bridging and linear methoxy groups were identified on
the surface of these catalysts, and the reaction prodꢀ
ucts were H , methyl formate (MF), CO, CO ,
ized by CO adsorption from the 1% CO + He mixture,
temperatureꢀprogrammed reduction (TPR), and difꢀ
fuse reflectance spectroscopy. The spectra of powder
samples were recorded on a Hitachi 330 spectrophoꢀ
tometer.
2
2
and H O.
2
This earlier study demonstrated that hydrogen
results mainly from the recombination of hydrogen
atoms on copper clusters and from MF decomposiꢀ helium, and up to 3% oxygen. Helium contained 0.2%
The feed gas was composed of 14% methanol,
tion. The sources of CO in the gas phase are the forꢀ oxygen as an impurity.
2
428

MECHANISM OF METHANOL CONVERSION ON ZrO AND 5% Cu/ZrO
429
2
2
1
115
A
.0
.8
.4
0
2
812
1
160
1060
2
922
1
367
3
670
1
0
0
1581
1
382
3
730
5
4
3
2
1
3000
2000
1500
1000
1
ν
, cm−
Fig. 1. IR spectra recorded for methanol (14 vol % MeOH + He) decomposition on ZrO at T = (1) 180, (2) 242, (3) 280, (4) 347,
2
and (5) 375°С.
Spectrokinetic measurements under methanol detector. CO, H , and CO were quantified using
2
2
conversion conditions are described elsewhere [12].
molecular sieve 5A and Porapak Q columns and a
thermalꢀconductivity detector.
The experimental setup included a Bruker IFS 45
3
FTIR spectrometer, a heated 1ꢀcm reactor cell, a gas
preparation unit, and a product and reactant analysis
system. The sample as a 20ꢀ to 30ꢀmg pellet with a surꢀ
face area of 2 cm was placed in the cell, which also
served as a catalytic flow reactor. Prior to measureꢀ
ments, the sample was heatꢀtreated at 450°С in flowꢀ difference IR spectra recorded under the methanol
ing He for 1 h and was then cooled to the preset temꢀ
perature. Thereafter, the reaction mixture was passed
through the cell at a rate of 30 ml/min.
RESULTS
ZrO2
2
Steadyꢀstate measurements. Figure 1 shows the
(
4% MeOH in He) conversion conditions. The spectra
show absorption bands in the region of C–O vibraꢀ
tions in the linear methoxy groups (1160 and
⎯
1
⎯1
1
1
120 cm ) and in the bridging one (1060 cm ) [13–
6] and bands due to the monodentate formate comꢀ
In unsteadyꢀstate spectrokinetic experiments, we
measured the concentration of surface species by in
⎯
1
plex (1580, 1382, and 1367 cm ) [17]. In the 2800–
situ IR spectroscopy as the methanol conversion reacꢀ 3000 cm⎯ range, there are a number of absorption
1
tion was coming to a steady state or after the eliminaꢀ bands due to C–H stretching vibrations in the methꢀ
tion of methanol from the feed.
oxy groups and formate complex [13–17]. The methꢀ
oxy group and formate vibration frequencies decrease
as the reaction temperature is raised.
Negative absorbance with peaks at 3670 and
730 cm is observed in the region of the stretching
vibrations of surface OH groups (Fig. 1). These negaꢀ
tive band intensities in the difference spectra indicate
that surface hydroxyl groups disappear in the interacꢀ
tion between methanol and the catalyst surface.
The intensity of absorption bands was measured in
difference spectra in units of absorbance (A). These
spectra were obtained by subtracting, from the specꢀ
trum of the sample in flowing reaction mixture, the
spectrum of the same sample in flowing inert gas at the
same temperature. The number of coadditions was
⎯
1
3
–1
usually 64, and the spectral resolution was 4 cm .
Reactant and product concentrations were meaꢀ
Figure 2 plots the temperature dependences of the
sured on a Model 3700 chromatograph with heated concentrations of methanol and its conversion prodꢀ
pipelines using helium as the carrier gas. Dimethyl ucts and those of band intensities measured in the
ether (DME), methanol, and methane were identified same run. Clearly, H , CO, DME, and CH formation
2
4
using a Porapak Q column and a flameꢀionization takes place only above 300°С (Fig. 2b). The weakenꢀ
KINETICS AND CATALYSIS Vol. 51
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2010

430
MATYSHAK et al.
accompanied by an increase in the product concentraꢀ
tions (Fig. 2).
A
.8
0
(a)
0
0
0
0
0
0
0
.7
.6
.5
.4
.3
.2
.1
0
Unsteadyꢀstate measurements. After the establishꢀ
ment of a steadyꢀstate concentration of surface comꢀ
pounds under the 14% MeOH + He flow, methanol
was eliminated from the feed and the surface comꢀ
pounds were desorbed isothermally into the helium
flow. The intensities of the IR absorption bands of the
surface compounds were monitored in the 150–350°С
range.
The kinetic curves obtained in this way allowed the
kinetic order of the surface complex disappearance
process and the effective rate constants to be deterꢀ
mined. The equation describing the time variation of
the methoxy group concentration is firstꢀorder.
Table 1 lists temperatureꢀdependent methoxy group
disappearance rate constants.
The kinetic curves for formate formation on the
surface at low temperatures were also processed in the
coordinates of a firstꢀorder equation. The formate forꢀ
mation rate constants are presented in Table 1. As the
temperature is raised, a maximum appears in the forꢀ
mate concentration versus time curve. This means that
both the formation and disappearance of the formate
complexes take place at elevated temperatures.
1
2
5
3
4
40
100 160 220 280 340 400
130 190 250 310 370
, °C
70
T
C
, %
.3
C
, %
(b)
0
1
1
1
4
1
2
0
0.2
4
8
6
0.1
2
4
3
5
2
6
It is clear from the data presented in Table 1 that the
sum of the methoxy group disappearance rate conꢀ
stants at 21°С us close to monodentate formate forꢀ
mation rate constant (absorption band at 1580 cm ).
Note also that the methoxy group disappearance rate
constants increase sharply on passing from 290 to
0
0
400
4
0
100
160
220
250
, °C
280
340
7
0
130
190
310
370
⎯
1
T
Fig. 2. Temperature dependences of the (a) intensities of
the absorption bands of surface compounds and (b) prodꢀ
uct concentrations for methanol decomposition on ZrO :
3
46°С owing to the high rate of the reaction at 346°С
2
5)
(
Fig. 2b).
(
1
(
a)
581 cm ; (b) (
) CH , and ( ) DME.
ν
= (
1) 1060, (
2) 1120, (
3
) 1160, (
4
3
) 1380, and (
) CO, ( ) CO2,
⎯
1
1
) CH OH, (
2) H , (
4
3
2
5
6
4
5% Cu/ZrO2
The same surface complexes, but with somewhat
ing of the absorption bands of the methoxy groups
Fig. 2a, curves ) in the 190–280°С range is
accompanied by the strengthening of the bands of the
formate complexes (curves 4, 5). The weakening of the Cu/ZrO surface demonstrated that, along with the
2
different vibration frequencies, are observed for the
copper oxide–containing catalyst. A shape analysis of
the absorption band of the formate complex on the 5%
(
1–3
bands of the formate complexes and methoxy groups is complex described for ZrO , there is another one on
2
Table 1. Rate constants of methoxy group disappearance and formate formation in methanol conversion on ZrO2
Rate constant, min–1 (n = 1)
methoxy group disappearance
formate formation formate disappearance
T, °C
Absorption bands
1160 cm–1
1
060 cm–1
1120 cm–1
1580 cm–1
145
210
290
346
0.012
0.021
0.027
0.148
0.16
0.014
0.023
0.032
0.176
0
–
–
0.025
0.028
0.152
0.065
–
–
–
0.070
KINETICS AND CATALYSIS Vol. 51
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2010

MECHANISM OF METHANOL CONVERSION ON ZrO AND 5% Cu/ZrO
431
2
2
⎯
1
this sample, which shows itself at 1595 cm . Since this
species is observed only in the presence of copper, it is
natural to assume that it is localized on copper oxide
clusters.
A
(a)
0
0
0
0
0
0
.6
.5
.4
.3
.2
1
Figure 3 plots the concentrations of MeOH and
reaction products (H , MF, CO, CO , and CH ) and
band intensities as a function of temperature. Clearly,
the reaction over the copperꢀcontaining catalyst yields
a different product composition: MF is observed here.
In addition, the reaction products form at lower temꢀ
peratures and their concentrations are considerably
higher than in the case of ZrO (Figs. 2b, 3b). It is
interesting that, on passing from 250 to 300°С, the
outlet methanol concentration somewhat increases
2
2
4
5
2
3
.1
0
4
2
4
0
100
160
220
190
T, °C
280
340
400
70
130
250
310
370
(
Fig. 3b, curve
1), while the H and CO concentraꢀ
2 2
tions decrease (curves
2
and 4, respectively). This fact
Concentration, %
Concentration, %
0.05
can be explained by the occurrence of the reverse reacꢀ
tion (methanol synthesis from H and CO ) in this
temperature interval. A similar effect was observed for
(
b)
2
2
16
CeO ꢀsupported catalysts [18]. Note that the formaꢀ
14
2
0.04
tion of H , CO , and MF (curves
2
,
4
,
6
) begins at a
lower temperature than the formation of CO, CH4,
and DME (curves ).
2
2
1
12
7
0.03
.02
3,
5,
7
10
8
Figure 3a shows the temperature dependences of
band intensities measured for the surface compounds
in the same run. As the temperature is raised, the conꢀ
centration of linear methoxy groups with a vibration
0
6
6
4
2
5
0.01
0
⎯
1
3
frequency of 1160 cm passes through a maximum at
00°С, the concentration of bridging methoxy groups
2
0
4
4
3
decreases steadily, and the concentration of linear
methoxy groups with a vibration frequency of
0
100
160
220
250
, °C
280
340
400
70
130
190
310
370
⎯
1
1
120 cm increases (Fig. 3a, curve 2). The formate
T
concentration passes through a maximum at 200°С
and begins to increase again above 300°С (Fig. 3a,
Fig. 3. Temperature dependences of the (a) intensities of
the absorption bands of surface compounds and (b) prodꢀ
uct concentrations for methanol decomposition on 5%
curve
5). This increase concurs with the decrease in
the concentration of linear methoxy groups with a
⎯
1
Cu/ZrO : (a)
ν
= (
1
) 1060, (
2
) 1120, (
) CH OH, (
3
) 1160, ( ) 1380,
4
vibration frequency of 1160 cm (curve
3).
2
⎯
1
and (5) 1581 cm ; (b) (
1
2
) H , (3) CO,
3
2
(
4
) CO , (
5
) CH , ( ) MF, and (7) DME.
6
4
An analysis of the spectroscopic and catalytic data
Figs. 3a, 3b) suggests that product formation takes
2
(
place in the same temperature range as the conversion
of the formate complexes and methoxy groups.
In the presence of water vapor, the H concentraꢀ
tion remains practically invariable (Fig. 4a), the conꢀ
2
Effects of water vapor and oxygen. Figure 4 presents centrations of CO (Fig. 4b) and methane (Fig. 4c) are
the temperature dependences of the MeOH and prodꢀ somewhat lower, and the CO concentration (Fig. 4b)
2
uct concentrations and band intensities obtained is higher. Under the action of water vapor, MF formaꢀ
under the methanol conversion condition in the presꢀ
ence of various concentrations of water vapor. The
introduction of water vapor into the system in the temꢀ
perature range of the reaction decreases the intensity
and frequency of the linear methoxy group band at
tion stops (Fig. 4a) and the DME concentration
decreases markedly (Fig. 4c, curves ).
3, 4
Figure 5 plots the same temperature dependences
recorded under methanol conversion conditions in the
presence of various amounts of oxygen. Oxygen also
decreases the concentration of linear methoxy groups
⎯
1
1
160 cm (Fig. 4d, curves
1, 2). In addition, water
vapor increases the intensity of the formate band at
⎯
1
⎯1
1
580 cm (Fig. 4d, curves
3
,
4
). The position and (1160 cm ) and increases the formate concentration
120 cm and those of the bridging methoxy group increases the CO formation rate and reduces the forꢀ
2
intensity of the linear methoxy group band at (Fig. 5d) over the temperature range of the reaction. It
⎯
1
1
⎯
1
band at 1060 cm change only slightly. The temperaꢀ mation rates of CO, MF (Fig. 5b), CH , and DME
ture dependences of the intensities of these bands were (Fig. 5c). These data indicate that water vapor and
4
discussed above (Fig. 3).
oxygen exert similar effects on the reaction.
KINETICS AND CATALYSIS Vol. 51
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432
MATYSHAK et al.
Concentration, %
Concentration, %
6
(a)
4
3
2
(b)
4
2
5
4
3
2
3
1
4
3
1
2
1
0
1
0
5
4
0 70 100 130 160 190 220 250 280 310 340 370 400
T, °C
40 70 100 130 160 190 220 250 280 310 340 370 400
, °C
T
A
.7
Concentration, %
0
0
0
(d)
0
0
0
0
0
.05
.04
.03
.02
(c)
.6
.5
3
0.4
1
4
3
0
0
0
.3
2
2
.2
4
.01
1
.1
0
0
4
0 70 100 130 160 190 220 250 280 310 340 370 400
T, °C
40 70 100 130 160 190 220 250 280 310 340 370 400
, °C
T
Fig. 4. Temperature dependences of (a–c) product concentrations and (d) absorption band intensities for methanol (14 vol %
MeOH + He) decomposition on 5% Cu/ZrO at water vapor concentrations of ( ) 0 and ( ) 3%: (a) ( ) CH OH,
) DME; (d) ( ) methoxy groups (1160 cm
1,
3
,
5
2,
4
1, 2
2
3
⎯
1
(
3
,
4
) H , and (
5
) MF; (b) (
1
,
2
) CO and (
3
,
4
) CO ; (c) (
1
,
2
) methane and (
3
,
4
1,
2
)
2
2
⎯
1
and (3, 4) formate structures (1580 cm ).
Unsteadyꢀstate measurements. The time depenꢀ considerably and these dependences obey a secondꢀ
dences of the methoxy group band intensities for isoꢀ order equation. Examples of the linear fit of the time
thermal desorption (below 250°С) can be fitted to a dependence of band intensity to a secondꢀorder equaꢀ
firstꢀorder equation. At 250°С and higher temperaꢀ tion are given in Fig. 6. The rate constants of the corꢀ
tures, the methoxy group conversion rate increases responding processes are listed in Table 2.
Table 2. Rate constants of the conversion of surface compounds in methanol conversion on 5% Cu/ZrO2
Rate constants of the conversion of surface compounds, min–1
Frequency,
Surface compound
–1
cm
n
= 1
*n
= 2
Т
,
°
C
140
170
210
250
11.45
7.69
2.73
250 (H O) 250 (O2)
2
340
–
Methoxy group
1155
0.026
0.024
0.024
0.035
0.038
0.040
0.057
0.064
0.059
18.78
8.50
3.12
24.91
19.68
5.01
1115
055
–
1
–
n
= 1
Formate
1580
–
–
–
0.0838
0.074
0.090
0.35
–
Rate constant of formate formation, min–1
0
.066
–
–
–
–
–
*
The rate constants were determined spectroscopically, using concentrations estimated from absorbance (A) data.
KINETICS AND CATALYSIS Vol. 51
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MECHANISM OF METHANOL CONVERSION ON ZrO AND 5% Cu/ZrO
433
2
2
Concentration, %
Concentration, %
6
1
0
9
8
7
6
5
4
3
2
1
0
(a)
(b)
2
5
4
3
2
1
0
1
1
4
2
3
4
5
3
6
4
0 70 100 130 160 190 220 250 280 310 340 370 400
T, °C
40 70 100 130 160 190 220 250 280 310 340 370 400
, °C
T
Concentration, %
A
.8
.7
.6
.5
.4
0
0
0
.15
.10
.05
0
(c)
(d)
0
0
0
0
0
1
2
4
1
3
0.3
.2
0
2
4
0.1
0
3
4
0 70 100 130 160 190 220 250 280 310 340 370 400
T
40 70 100 130 160 190 220 250 280 310 340 370 400
, °C
, °C
T
Fig. 5. Temperature dependences of (a–c) product concentrations and (d) absorption band intensities for methanol (14 vol %
MeOH + He) decomposition on 5% Cu/ZrO at oxygen concentrations of ( ) 0 and ( ) 3%: (a) ( ) CH OH, (
) DME; (d) ( ) methoxy groups (1160 cm ) and
1
4
,
3,
5
2
1, 2
,
4
,
6
1,
2
3, 4)
2
3
⎯
1
H ; (b) (
1
,
2
) CO and (
3
,
4
) CO , and (
5,
6
) MF; (c) (
1,
2
) methane and (
3,
2
2
⎯
1
(3, 4) formate structures (1580 cm ).
The introduction of water vapor into the reaction interactions). As was found by TPR, copper oxide in
system at 250°С increases the disappearance rate conꢀ this sample is in two states, undergoing reduction at
stants of the linear methoxy groups, and a still stronger 130 and 200°С. According to the literature [19, 20],
effect is produced by oxygen (Table 2). The disappearꢀ
ance rate constant of the bridging methoxy group is less
sensitive to the presence of water vapor and to an
−
1/A0 + 1/A
increase in the oxygen concentration. As was reported
above, the MF and DME formation rates decrease
under the action of oxygen and water vapor (Figs. 4, 5).
160
140
120
100
T
= 250°C (H O 0°C)
2
The conversion of the formate complex obeys a
firstꢀorder equation with a rate constant almost indeꢀ
pendent of the presence of water vapor and oxygen
y
R
= 11.463x
2
= 0.9923
(
Table 2).
y
R
= 8.5359x
2
8
6
4
2
0
0
0
0
= 0.9807
1
DISCUSSION
2
3
Model of the Active Surface
y
R
= 3.3427x
= 0.9761
2
The diffuse reflectance spectrum of 5% Cu/ZrO
Fig. 7) shows a charge transfer band at 36360 cm
2
1
⎯
(
⎯
1
2
2
→
and d–d transition bands at 12500 cm (
B
B2g
A1g). This
0
2
4
6
8
10
12
14
t, min
1g
2
2
⎯
1
2
2
→
and B1g
→
Eg) and 5200 cm
(
B1g2
+
spectrum was attributed to isolated Cu ions with an
octahedral, tetragonally distorted coordination sphere
19] (large copper oxide particles are unobservable by
Fig. 6. Time dependences of the intensities of the absorpꢀ
tion bands of the methoxy groups in the coordinates of the
[
secondꢀorder equation for 5% Cu/ZrO at 250°С
:
ν
=
2
(1) 1150, (2) 1110, and (3 .
) 1074 cm⎯
1
diffuse reflectance spectroscopy because of exchange
KINETICS AND CATALYSIS Vol. 51
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434
MATYSHAK et al.
groups. Bianci et al. [16] described a single type of
A
⎯
1
0
0
0
0
0
0
0
0
0
.9
.8
.7
.6
.5
.4
.3
.2
36360 cm−
1
methoxy group (1144 cm ). Other authors [14, 15,
⎯
1
2
(
3] reported linear (1163 cm ) and bridging
⎯
1
1056 cm ) methoxy groups resulting from the interꢀ
action of methanol with coordinatively unsaturated
4+
Zr ions. In the latter case, the absorption band of the
bridging methoxy group is broadened on its highꢀfreꢀ
quency side, so that a band due to another kind of
2500 cm−
1
1
⎯
1
5208 cm−
1
methoxy group can be distinguished near 1100 cm .
These discrepancies arise from the variation of the
phase composition of the zirconia samples. It was
demonstrated that two types of methoxy groups occur
on the surface of monoclinic zirconia and only one
type is observed for the tetragonal modification [24].
This is confirmed by the observation of two methoxy
142 cm−
1
7
.1
0
2
00
600
1000
1400
1800
2200
⎯
1
4
00
800
1200
1600
2000
group vibration frequencies—1155 and 1051 cm —
Wavelength, nm
for monoclinic ZrO [25]. Our sample consisted of the
2
tetragonal and monoclinic phases in approximately
equal proportions. In view of the above, it is evident
that the linear and bridging methoxy groups with
Fig. 7. Diffuse reflectance spectrum of 5% Cu/ZrO2 .
⎯
1
vibration frequencies of 1160 and 1056 cm , respecꢀ
tively, are localized in the monoclinic phase. The other
these states may correspond to finely dispersed and
clustered copper ions. The totality of the data preꢀ
sented here demonstrates that the copper oxide in the
copperꢀcontaining catalyst is just in these two states—
finely divided and clustered. It is interesting that the
⎯
1
type of linear methoxy group (1115 cm ) is localized
in the tetragonal phase.
The following three mechanisms of methoxy group
formation were described in the literature (see, e.g.,
IR spectrum of adsorbed CO molecules (40–170°С
)
[
26]): C–O bond breaking in the methanol molecule
⎯
1
exhibits an absorption band at 2100 cm , which is due
involving acidic hydroxyl groups of the surface; O–H
bond breaking in the methanol molecule involving
basic hydroxyl groups of the surface; and methanol
adsorption on coordinatively unsaturated metal ions,
which yields hydroxyl groups. In our case, negative
0
to vibrations in the Cu –CO complex [18, 19]. This
can be explained by the fact that, in the presence of
CO, the finely dispersed copper ions undergo reducꢀ
tion even at 40°С. The ease of their reduction is eviꢀ
2+
dently due to the coordination sphere of Cu having
anionic vacancies [20, 21] occupied by active forms of
atomic oxygen. The existence of this kind of sites on
the surface was proved by experiments on the isotherꢀ
mal desorption of surface compounds (after the elimꢀ
⎯1
absorbance with maxima at 3670 and 3730 cm is
observed in the region of the stretching vibrations of
surface OH groups (Fig. 1). The OH groups with such
vibration frequencies are localized in the monoclinic
and tetragonal phases, respectively [24]. The negative
absorbances in the difference spectra (Fig. 1b) suggest
that these hydroxyl groups, characterized by acid and
base properties, are involved in the formation of methꢀ
ination of methanol from the feed). For ZrO and 5%
2
Cu/ZrO , we observed a bathochromic shift of the
2
absorption bands of the methoxy groups in time. This
was reported to be due to the oxidation of the surface
14, 15]. Indeed, in the presence of methanol, active
oxygen in the anionic vacancies is spent for methanol
oxidation, as is indicated by the appearance of CO in
the gas phase (Fig. 2). As a result, the surface underꢀ
goes reduction. (Data concerning copper oxide reducꢀ
tion in the presence of methanol are presented in
⎯
1
oxy groups vibrating at 1056 and 1115 cm . In addiꢀ
tion, as follows from the data presented in Figs. 2–5,
the concentration of methoxy groups with the vibraꢀ
[
2
⎯1
tion frequency 1160 cm increases as the temperature
is raised to 300°С. Since the number of hydroxyl
groups on the surface decreases and the number of
4+
coordinatively unsaturated Zr sites increases with
[
22].) After methanol elimination from the feed, the
increasing temperature, it is believed that this type of
methoxy group results from methanol binding to the
anionic vacancies are occupied by oxygen present in
the carrier gas as an impurity and the surface is thus
oxidized.
4+
coordinatively unsaturated Zr ions. Thus, all of the
three methoxy group formation mechanisms take
place on the 5% Cu/ZrO surface.
2
The IR spectra recorded for the ZrO ꢀcatalyzed
Conversion of Surface Compounds
2
reaction (Fig. 1) show absorption bands at 1580, 1367,
⎯
1
The data reported for methoxy groups on the ZrO2 and 1382 cm , which are due to vibrations in the surꢀ
surface are controversial. Alekseev et al. [13], who face formate complex (see, e.g., [26–30]). A shape
studied the interaction of methanol with hydroxyl analysis of the bands of the formate complex on 5%
groups on the ZrO surface, identified linear Cu/ZrO (Fig. 3) demonstrated that, along with this
2
2
⎯
1
⎯1
(
1170 cm ) and bridging (1060 cm ) methoxy complex, there is another formate complex, characꢀ
KINETICS AND CATALYSIS Vol. 51
No. 3
2010

MECHANISM OF METHANOL CONVERSION ON ZrO AND 5% Cu/ZrO
435
2
2
⎯
1
terized by a vibration frequency of 1595 cm , on the natively unsaturated site (in other words, O–H bond
surface of this sample. Since this shape of absorption breaking in the methanol molecule). Both of these
band is observed only in the presence of copper, this conditions are satisfied with the 5% Cu/ZrO catalyst,
2
complex is likely localized on copper oxide clusters.
as distinct from ZrO . As a consequence, MF forms
2
The weakening of the absorption bands of the only over the copperꢀcontaining catalyst. MF formaꢀ
methoxy groups with time in isothermal desorption tion as a result of condensation between the formate
from ZrO obeys a firstꢀorder equation. As follows complex and a methoxy group was also reported by
2
from the data presented in Fig. 2, no reaction products other authors [28–30].
or surface compounds form at 145°С. In this case, the
weakening of the bands of methoxy groups with time
indicates that the main methoxy group conversion
route is desorption and methanol formation in the gas
phase [16]. However, as the temperature is raised, the
methoxy groups begin to participate in formate formaꢀ
tion in addition to being desorbed. The marked
increase in the methoxy group disappearance rate
constants at 350°С (Table 1) is due to methanol conꢀ
version into a hydrogenꢀcont6aining gas mixture
occurring at a noticeable rate (Fig. 2).
According to our earlier studies [10, 31, 32], DME
results from the recombination of bridging methoxy
groups. The fact that only traces of DME form on
ZrO and 5% Cu/ZrO (Figs. 2, 6) and that water
vapor slows down DME formation (Fig. 4) suggests
that the diffusion of bridging methoxy groups on the
catalysts is hampered.
2
2
As the temperature is raised, the MF concentration
decreases (Fig. 5b) because of MF decomposition to
H and CO. Another source of hydrogen (and CO ) in
2
2
the case of 5% Cu/ZrO is the conversion of the forꢀ
2
The expression for the methoxy group conversion
rate is written as follows [10]:
mate complex. This is indicated by the fact that the
simultaneous appearance of CO and hydrogen in the
2
d
(
CH OZ
)
/dt = −θCH3OZ(k θ + k θ ).
(1) gas phase takes place at the same temperature as the
3
3
O
7 ZH
decrease in the formate concentration on the surface
Here, k3 and k7 are the rate constants of formate comꢀ
plex formation and methoxy group desorption,
respectively.
(
Figs. 3–5).
In brief, the most significant surface reactions
The oxygen coverage of the surface is constant occurring in methanol conversion can be written as
= const) because impurity oxygen (up to 0.2%) is follows (a more detailed reaction network is presented
(
θ
O
always present in the gas stream, and the H or hydroxyl elsewhere [10]):
coverage of the surface changes insignificantly during
CH OZ + ZO
3
→
HCOОZ + Z + H2,
desorption. This is why the methoxy group concentraꢀ
tion varies according to a firstꢀorder equation. The
rate constants for different methoxy groups are fairly
similar (Table 1), and it is, therefore, impossible to
identify the type of methoxy group responsible for forꢀ
mate formation. However, based on the rate constant
ratios at 210°С and Eq. (1), it can be deduced that all
of the three types of methoxy groups are involved in
formate formation and that the contribution from desꢀ
orption to the disappearance of methoxy groups at this
temperature is small.
CH OZ+ HCOОZ
3
→
HCOOCH Z + ZO,
3
HCOOCH Z
3
→
2CO + 2H + Z,
2
HCOOZ
→
CO2(g) + ZH,
ZH + ZH
→
H2(g) + 2Z.
In addition, deep methanol oxidation takes place
on ZrO in the presence of oxygen in the reaction mixꢀ
2
ture. This is indicated by the fact that CO is the only
reaction product at comparatively low temperatures
Fig. 2). The oxidizing power of surface oxygen is
obviously due to the oxygen species occupying anionic
vacancies.
2
(
This is also true for the 5% Cu/ZrO catalyst up to
10°С (Table 2). At 250°С, the methoxy group conꢀ
2
2
version in the efficient reaction zone passes from the
firstꢀ to the secondꢀorder rate law (Fig. 6). The expresꢀ
sion for the methoxy group conversion rate in this case
appears as
Role of Cu in Methanol Conversion on 5% Cu/ZrO2
The presence of copper in the catalyst significantly
enhances its activity and changes the product compoꢀ
is sition, while the composition of the surface complexes
d
(
CH OZ
)
/dt = −θCH3OZ(k θ + k θ + k θ ). (2)
3
3 O 4 F 7 ZH
Here, k4 is the MF formation rate constant and
θ
F
the formate coverage of the surface. The second order remains nearly the same (Figs. 2, 3). Therefore, new
of methoxy group disappearance is likely due to the surface reactions between the same complexes become
fact that the main contribution to the rightꢀhand side possible in the presence of copper. The most imporꢀ
of Eq. (2) is from the k4
θ
term (MF formation rate).
tant of these reactions are hydrogen atom recombinaꢀ
F
As follows from our earlier data [10, 31, 32], the tion on copper clusters (hydrogen atoms result from
following two conditions are necessary for MF formaꢀ the interconversion of surface compounds, such as
tion: the presence of the formate complex localized on HCOOZ
→
CO (g) + ZH [5]) and the reaction
2
copper clusters and the presence of a linear methoxy between the linear methoxy group and the formate
group that has resulted from the interaction of a methꢀ complex located on copper, which yields MF. Thus,
anol molecule with a basic hydroxyl group or a coordiꢀ there are two main hydrogen formation routes—
KINETICS AND CATALYSIS Vol. 51
No. 3
2010

436
MATYSHAK et al.
hydrogen atom recombination on copper oxide partiꢀ face, the oxidative conversion of methoxy groups into
CO and H via formate dominates over methoxy
2
2
2
cles and MF decomposition (WH2 = k7θMF
+ k θ ).
13 H
group–formate condensation.
The decrease in the formation and disappearance
temperatures of the surface formate complexes sugꢀ
gests that copper markedly raises the oxidizing power
of the catalyst. This property of the catalyst is due to
oxygen activation on supported copper.
It is interesting that the introduction of oxygen and
water vapor decreases the MF and DME formation
rates (Figs. 4, 5). According to spectroscopic data, the
introduction of oxygen and water vapor diminishes the
CONCLUSIONS
The above analysis demonstrates that the main
hydrogen formation reactions are hydrogen atom
recombination on copper clusters and MF decompoꢀ
sition, and the source of CO is MF. It is appropriate to
introduce oxygen and water vapor into the feed: this
will decrease the CO formation rate without affecting
the hydrogen yield.
⎯
1
concentration of linear methoxy groups (1160 cm )
and increases the concentration of formate complexes
(
Figs. 4d, 5d). Apparently, surface oxidation takes
ACKNOWLEDGMENTS
place when water vapor and oxygen are present simulꢀ
taneously. Oxidation (increase in the surface oxygen
concentration) changes both the state of copper partiꢀ
cles and the state of the copperꢀmodified ZrO surface
(
demonstrated in [33–35]). On the oxidized surface,
the oxidative conversion of methoxy groups dominates
over the condensation of methoxy groups with formate
complexes.
This work was support by the Russian Foundation
for Basic Research, grant no. 07ꢀ03ꢀ00373.
2
0
+1
the oxidation of surface Cu or Cu with water was
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