Mechanistic Studies of Methanol Synthesis Reaction over Cu and Pd–Cu Catalysts

Article abstract of DOI:10.1134/S0023158420040035

Abstract: Monometallic copper and bimetallic palladium–copper catalysts supported on ZnO–Al₂O₃, CeO₂–Al₂O₃ and ZrO₂–Al₂O₃ were prepared by conventional impregnation method and tested in the methanol synthesis reaction in a gradient less reactor under elevated pressure (3.5 MPa) at 220°C. The physicochemical properties of prepared catalytic systems were studied using BET, TPR-H₂, TPD-NH₃, XRD, SEM-EDS and FT-IR techniques. The results of XRD and SEM-EDS measurements showed the formation of Pd–Cu alloy during the activation of bimetallic catalysts. It was found that the formed alloy was responsible for the improved activity and selectivity of catalysts in the studied reaction. Among investigated catalysts, the highest formation rate of methanol was observed with 2%Pd–20%Cu/ZnO–Al₂O₃ system. Based on the results of FT-IR measurements it can be concluded that hydrogen molecules adsorb dissociatively on the metallic copper surface to form hydrogen atoms, increasing the hydrogen spillover effect on the metal-support interface. In contrast, CO₂ adsorb on the oxygen vacancies of the support to form carbonates, which can further undergo hydrogenation to methanol.

Full text of DOI:10.1134/S0023158420040035

ISSN 0023-1584, Kinetics and Catalysis, 2020, Vol. 61, No. 4, pp. 623–630. © Pleiades Publishing, Ltd., 2020.
Mechanistic Studies of Methanol Synthesis Reaction over Cu
and Pd–Cu Catalysts
a,
a
a
a
a
R. Ciesielski *, O. Shtyka , M. Zakrzewski , J. Kubicki , W. Maniukiewicz ,
a
a
A. Kedziora , and T. P. Maniecki
Lodz University of Technology, Lodz, 90-924 Poland
e-mail: radoslaw.ciesielski@p.lodz.pl
a
*
Received December 6, 2019; revised January 10, 2020; accepted January 21, 2020
Abstract—Monometallic copper and bimetallic palladium–copper catalysts supported on ZnO–Al O ,
2
3
CeO –Al O and ZrO –Al O were prepared by conventional impregnation method and tested in the meth-
2
2
3
2
2
3
anol synthesis reaction in a gradient less reactor under elevated pressure (3.5 MPa) at 220°C. The physico-
chemical properties of prepared catalytic systems were studied using BET, TPR-H , TPD-NH , XRD, SEM-EDS
2
3
and FT-IR techniques. The results of XRD and SEM-EDS measurements showed the formation of Pd–Cu
alloy during the activation of bimetallic catalysts. It was found that the formed alloy was responsible for the
improved activity and selectivity of catalysts in the studied reaction. Among investigated catalysts, the highest
formation rate of methanol was observed with 2%Pd–20%Cu/ZnO–Al O system. Based on the results of
2
3
FT-IR measurements it can be concluded that hydrogen molecules adsorb dissociatively on the metallic cop-
per surface to form hydrogen atoms, increasing the hydrogen spillover effect on the metal-support interface.
In contrast, CO adsorb on the oxygen vacancies of the support to form carbonates, which can further
2
undergo hydrogenation to methanol.
Keywords: methanol synthesis, CO hydrogenation, palladium-copper catalysts, Pd–Cu alloy, binary oxide,
2
reactive intermediates
DOI: 10.1134/S0023158420040035
INTRODUCTION
The atmospheric concentrations of CO have been
increasing steadily for last decades. The combustion of
fossil fuels is the largest contributor to the CO emis-
CO + H = CO + H O,
ΔH298K = 41.0 kJ/mol.
2
2
2
(2)
2
Thermodynamically, CO2 hydrogenation to
CH OH is less favorable than hydrogenation of CO
2
3
sion [1]. Without proper emission management, this
[
6]. At 523ꢀK and 4.0ꢀMPa, the CH OH yield at equi-
could lead to significant climate changes. Therefore,
3
librium via CO /H is 14%, while equilibrium yield
the effective utilization of CO and mitigating the fos-
2
2
2
through CO/H is as high as 50% [7]. Such a striking
sil fuel dependence are of prime interest of the scien-
tific community [1–4].
2
difference makes the route to convert CO into meth-
2
anol more challenging than syngas conversion. Over
the past decades, significant efforts have been directed
to developing Cu–Zn-based catalysts for CH OH
synthesis from both syngas conversion and CO2
hydrogenation [9, 10]. On the other hand, palladium
systems gained attention as catalysts for CO hydroge-
Methanol (CH OH) is an important chemical
3
feedstock, as well as an energy storage medium [5]. The
production of CH OH from CO involves the exother-
3
3
2
mic CO hydrogenation (Eq. (1)) and endothermic
reverse water gas shift reaction (RWGS, Eq. (2))
2
2
CO + 3H = CH OH + H O,
2
2
3
2
nation due to their high activity in CO hydrogenation
2
(
1)
ΔH298K = 49.4 kJ/mol,
to methanol in comparison with Cu–ZnO based cata-
lysts [11, 12]. Numerous data confirmed that sup-
ported Pd nanoparticles are actually active in the syn-
Abbreviations: RWGS, reverse water gas shift reaction; TPR-H ,
2
H temperature-programmed reduction; TCD, a thermal con-
2
thesis of CH OH from H /CO [13–17].
ductivity detector; TPD-NH , NH temperature programmed
3
2
2
3
3
desorption; SEM, scanning electron microscopy; EDS – energy
dispersive X-ray spectroscopy; GC, gas chromatography; XRD,
X-ray diffraction; FT-IR, Fourier-transform infrared spectros-
copy.
The main goal of this study was to investigate the
physicochemical and catalytic properties of the copper
(10, 20 and 40 wt % of Cu) and palladium–copper
623

6
24
CIESIELSKI et al.
(
2 wt % of Pd and 20 wt % of Cu) catalysts supported reduced in hydrogen stream (5% H –95% Ar) with a
2
3
on ZnO–Al O , ZrO –Al O , CeO –Al O binary
2
3
2
2
3
2
2
3
volumetric flow rate of 40 cm /min. Hydrogen con-
oxides Their activity in the reaction of methanol syn- sumption was monitored by a thermal conductivity
thesis from a CO /H mixture was evaluated. Com- detector (TCD).
2
2
pared to monoxide supports such systems are charac-
terized by a well-developed specific surface and
improved stability [18]. By a proper selection of both
the qualitative and quantitative composition of the
support it is possible to control its physicochemical
properties such as specific surface, acid–base nature
of the surface, and susceptibility to reduction. The
composition of the support and the active phase has a
significant impact on mechanical and thermal resis-
tance, carbon deposition tolerance, specific surface
area and hydrophobicity of the catalyst. Such parame-
ters shape the properties of the supported catalyst, pri-
marily its selectivity and activity in the studied cata-
lytic process.
The procedure used to characterize the catalyst
surface acidity includes the purification of catalyst
surface, adsorption of ammonia and NH temperature
3
programmed desorption (TPD-NH ). The method
3
applied to purify the catalyst could lead to dihydroxyl-
ation of the catalyst surface. The sample (about 0.15 g)
3
was flushed with pure argon stream (40 cm /min,
4
00°C, 2 h) to remove water from the system. After
cooling the sample to 50°C in argon, a gaseous ammo-
nia flow (0.5 h) was introduced into the sample tube to
entirely saturate the catalyst surface. After NH
3
adsorption, the excessive amount of weakly and phys-
ically adsorbed ammonia on the catalyst surface was
removed by flushing the catalyst in pure argon stream
3
with (40 cm /min). TPD-NH measurements were
3
EXPERIMENTAL
made in a temperature range of 100–600°C with a lin-
ear heating rate of 25°C/min using TCD detector.
Copper and palladium-copper catalysts were pre-
pared by wet aqueous impregnation. In order to pre-
The diffraction patterns were recorded using CuKα
pare ZnO–Al O , CeO –Al O and ZrO –Al O sup- radiation on a PANalytical X’Pert Pro diffractometer
2
3
2
2
3
2
2
3
in the 2θ range from 5° to 90° with steps of 0.167° and
Δt = 10 s. Since the initial diffraction data may contain
interference, the background during the analysis was
recorded using the Sonneveld–Visser algorithm [19].
All the calculations were carried out using the X’Per-
tHighScore Plus computer program.
ports with the targeted molar ratio of Zn : Al, Ce : Al
and Zr : Al = 0.5, nitrates of zinc, cerium, zirconium and
3
aluminum were used. Aqueous solutions of 1 mol/dm
3
zinc/cerium/zirconium nitrate and 1 mol/dm alumi-
num nitrate were mixed in appropriate quantity under
vigorous stirring at 80°C. A concentrated ammonia
solution was then added dropwise to the solution until
The SEM measurements were made by scanning
the solution pH reached values between 10 and 11 and electron microscopy (SEM) and energy dispersive
then the mixtures were stirred for another 30 min. The X-ray spectroscopy (EDS), respectively, on the equip-
resulting fine precipitates were washed two times with ment JSM-6010PLUS/LA (JEOL). The EDS method
deionised water and then dried at 120°C for 15 h and made it possible to qualitatively determine elements
calcined for 4 h at 400°C in air. Metal phase Cu and Pd present in the studied micro-area of the sample sur-
was introduced on the previously prepared supports face layer on the basis of the obtained characteristic X-
surface by wet impregnation using aqueous solutions ray spectra. A map of the distribution of elements on
of copper and palladium nitrates. The supported cata- the studied micro-area was made. The accelerating
lysts were then dried in air at 120°C for 2 h and finally voltage was 20 kV. For the analysis, the samples were
calcined for 4 h in air at 400°C. Copper and palladium coated with a carbon target using a Cressington 208
loadings were 10–40 and 2 wt %, respectively.
HR system.
The IR spectra were taken on a Shimadzu
The specific surface areas of supports and catalysts
were determined by nitrogen adsorption (77 K)
employing the BET method with an automatic Sorp-
tomatic 1900 apparatus.
IRTracer-100 equipped with a mercury cadmium tel-
luride detector. Prior to the analysis, the catalysts were
reduced in situ at 300°C in an atmosphere of 5% H and
2
9
5% argon for 2 h. After reduction, the samples were
The temperature-programmed reduction (TPR-H2)
measurements were carried out in an automatic TPR
system AMI-1 in the temperature range of 25–900°C
with a linear heating rate of 10°C/min. Transfer line
between reactor and detector was heated up to 120°C.
Before analysis, the samples were heated in an argon
cooled to 220°C and the gas stream was switched over to
the reaction mixture of H and CO (3 : 1 mole ratio).
2
2
Activity tests in methanol synthesis reaction were
carried out in the gradient less reactor using a gas mix-
ture of H and CO with a molar ratio of 3 : 1. The total
2
2
3
stream at 400°C for 2 h to remove water and impurities flow rate of the reaction mixture was 80 cm /min. Pro-
from catalytic systems. Samples (about 0.1 g) were cess was carried out under pressure (3.5 MPa) at
KINETICS AND CATALYSIS
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MECHANISTIC STUDIES OF METHANOL SYNTHESIS REACTION
20°C and products were analyzed by gas chromatog- RESULTS AND DISCUSSION
625
2
raphy (GC). The accuracy of the chromatographic
measurements was at a precision level of 5%. The tem-
perature was measured using Class A thermoelectric
elements, placed directly in the catalyst bed, which
ensures accuracy of 1°C of the range of measured tem-
peratures. Before activity tests all catalysts were
Table 1 contains the results of activity tests in meth-
anol synthesis carried out by converting CO and
2
2
H over copper and bimetallic palladium–copper cat-
alysts supported on binary oxides ZnO–Al O , ZrO –
2
3
2
Al O , CeO –Al O .
2
3
2
2
3
As can be seen from the obtained results, the main
prereduced for 2 h in a flow of 5% H –95% Ar mixture
2
reaction products were methanol, carbon monoxide
and small amounts of methane. Additionally, the for-
mation of trace amounts of dimethyl ether and form-
aldehyde was also observed. It is worth mentioning
that no byproducts were formed when the process was
performed without catalyst. Also, the activity mea-
at 300°C under atmospheric pressure. The steady-
state activity measurements were taken after at least
1
2 h on the stream. The analysis of the gaseous reac-
tion products were carried out by an on-line GC
equipped with a flame ionization detector (FID) and
1
0% Carbowax 1500 on Graphpac column. The trans- surement showed that an increase in copper content
fer line from the outlet of the reactor to the GC was leads to deterioration of the catalytic activity of copper
heated to 120°C to prevent condensation of reaction systems regardless of the support used. The similar
products. The CO and CO concentrations were moni- results were obtained by Ahouari et al. [20]. They
2
investigated the effect of copper concentration on the
tored by GC chromatograph equipped with TCD detec-
tor (120°C, 130 mA) and Carbosphere 60/80 (65°C) col-
umn. The catalytic activity of the investigated systems
was expressed as the amount of product formed on the
activity of CuO–ZnO–Al O catalyst in the reaction
2
3
of methanol synthesis (250°C at 3.0 MPa). It was
found that the catalyst with the copper load of 51% was
the most active while the further increase in Cu con-
centration (up to 72%) resulted in decreasing rate of
−
cat
1
–1
mass of catalyst per unit of time (gCH3OH kg h ).
Table 1. The activity results for methanol synthesis from a mixture of H and CO over copper and palladium-copper cat-
2
2
alysts supported on binary oxides ZnO–Al O , ZrO –Al O and CeO –Al O
2
3
2
2
3
2
2
3
Selectivity, %
CH4 CH OCH
3
MeOH yield, gCH3OH
Catalyst
gCH3OH kg h–1
−1
CO
CH OH
HCHO
cat
3
3
Empty reactor
0.00
30.42
21.74
0.00
100
0.00
2.80
1.95
5.17
0.00
0.50
0.92
0.93
0.61
0.00
0.00
0.70
0.58
0.00
0.02
0.02
0.02
0.08
0.07
0.03
0.05
0.03
0.02
0.02
0.02
0.02
0.00
0.01
0.01
0.01
0.02
0.03
0.01
0.02
0.01
0.00
0.01
0.01
0.02
19
430
325
280
336
681
636
619
715
687
637
640
667
1
2
4
2
0%Cu/CeO –Al O
66.75
76.28
94.80
98.77
70.85
73.21
94.96
98.20
78.46
79.90
84.90
90.08
2
2
3
0%Cu/CeO –Al O
2
2
3
0%Cu/CeO –Al O
2
2
3
%Pd–20%Cu/CeO –Al O
3
1.13
2
2
1
2
4
2
0%Cu/ZnO–Al O3
28.55
25.83
4.04
2
0%Cu/ZnO–Al O3
2
0%Cu/ZnO–Al O3
2
%Pd–20%Cu/ZnO–Al O3
1.15
2
1
2
4
2
0%Cu/ZrO –Al O
21.52
20.07
14.37
9.30
2
2
3
0%Cu/ZrO –Al O
2
2
3
0%Cu/ZrO –Al O
2
2
3
%Pd–20%Cu/ZrO –Al O
3
2
2
Reaction conditions: H /CO ratio in the feed = 3, temperature = 220°C, total pressure = 3.5 MPa, weight of catalyst = 0.2 g, total flow
2
2
3
of the reaction mixture = 80 cm /min.
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

6
26
CIESIELSKI et al.
(a)
(b)
(c)
Δ2θ = 0.49°
Δ2θ = 0.57°
Δ2θ = 0.37°
Cu
Cu
Cu
PdCu
PdCu
PdCu
2
1
2
1
2
1
42
43
2
44
θ, deg
45
42
43
2θ, deg
44
45
42
43
2θ, deg
44
45
Fig. 1. XRD profiles of Cu (1) and Pd–Cu (2) catalysts supported on CeO –Al O (a), ZnO–Al O (b) and ZrO –Al O (c)
2
2
3
2
3
2
2 3
after reduction at 300°C for 2 h.
methanol formation. The authors noted that the cata- Cu/ZrO –Al O showed the presence of two peaks (T
2
2
2
3
lytic activity increased with an increase in the surface and T in Table 2, Fig. 2) attributable to the reduction
3
area of metallic copper, but a linear relationship was
of copper(II) and copper(I) oxides, respectively. The
not observed. From this result a suggestion was made same peaks associated with the reduction of copper
that the size of copper metallic surface and the specific were observed for Cu/ZnO–Al O systems. A high-
2
3
interaction between copper and support are the main
factors that affect the catalytic activity. Based on the
results presented in Table 1, it is easily to conclude that
the increase in the Cu content in the tested catalytic
system leads to increasing selectivity towards the main
product – methanol. Also, it can be seen, that the
addition of palladium to monometallic copper cata-
lysts resulted in enhanced formation of methanol.
Such effect can be attributed to the formation of an
alloy Pd–Cu during the activation process in hydrogen
mixture as evidenced by X-ray diffraction (XRD) pat-
terns (Fig. 1).
temperature shift of the maxima of the second reduc-
tion peak with increasing Cu content also merits con-
sideration. In contrast, the TPR profile of Cu/CeO –
2
Al O showed three reduction peaks (Fig. 2a). The
2
3
first two peaks were due to the reduction of highly dis-
persed CuO and copper ions interacting with the sup-
port (T , T ), respectively. The last peak of hydrogen
2
3
uptake (about 850°C) was attributed to the reduction
of large clusters of CuO in the bulk of the support.
Similarly, as in the previous case, a shift of the reduc-
tion peak towards higher temperatures with increasing
copper content was observed. It is worth emphasizing
that the catalysts least susceptible to the reduction
were also characterized by the lowest activity in the
investigated reaction (Tables 1 and 2).
In order to confirm this assumption, the physico-
chemical measurements of the investigated catalysts
were performed (Table 2). From these results, it can be
inferred that the activity of catalysts is closely related to
their susceptibility to the reduction. The TPR profile et al. [21] and Huang et al. [22]. They investigated the
acquired from the measurements performed for reducibility of copper catalysts (5–70 wt %) supported
The similar TPR results were observed by Zhang
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MECHANISTIC STUDIES OF METHANOL SYNTHESIS REACTION
627
Table 2. Activity results and physicochemical properties of supported mono- and bimetallic catalysts
TPR
Total acidity
Specific
MeOH yield,
(temperature of the maximum
reduction rate)
Catalyst
of the surface, surface area,
gCH3OH kg h–1
−1
cat
mmol/gcat
2
m /g
T1, °C
T2, °C
T3, °C
CeO –Al O
3
–
–
–
380
201–213
200–216
202–213
180
–
0.59
0.64
0.45
0.31
0.44
0.99
0.98
0.78
0.57
0.86
1.65
0.85
0.66
0.42
0.73
144
111
128
63
2
2
1
2
4
2
0%Cu/CeO –Al O
430
325
280
336
–
265
295
330
240
–
2
2
3
0%Cu/CeO –Al O
–
2
2
3
0%Cu/CeO –Al O
–
2
2
3
%Pd–20%Cu/CeO –Al O
3
130
–
116
251
191
185
113
172
252
160
164
105
159
2
2
ZnO–Al O3
–
2
10%Cu/ZnO–Al O3
681
636
619
715
–
–
203
248
271
296
193
–
2
2
4
2
0%Cu/ZnO–Al O3
–
206
2
0%Cu/ZnO–Al O3
–
203–250
141
2
%Pd–20%Cu/ZnO–Al O3
105
–
2
ZrO –Al O
3
–
2
2
1
2
4
2
0%Cu/ZrO –Al O
687
637
640
667
–
214
251
266
320
233
2
2
3
0%Cu/ZrO –Al O
–
215
2
2
3
0%Cu/ZrO –Al O
–
214
2
2
3
%Pd–20%Cu/ZrO –Al O
3
120
187
2
2
on SiO and ZnO/ZrO . It was reported that increas- that the addition of Pd into the copper catalyst facili-
2
2
ing in the metal load results in an increase in the tates the reduction of copper oxide species by the spill-
reduction temperature of catalysts.
over effect developed between Pd and CuO [23, 24].
The reducibility of catalysts strongly correlates with
the activity in the studied reaction. For instance, the
catalyst 2%Pd–20%Cu/ZnO–Al O with the easiest
reducibility was characterized by the highest forma-
tion rate of methanol.
The TPR profiles of catalysts promoted by palla-
dium exhibited additional reduction peak (T in Table 2)
in the low temperature range of 105–130°C, which can
be attributed to the reduction of palladium oxide par-
ticles. A low-temperature shift of maxima of reduction
peak of copper oxide species compared with those of
1
2
3
The results of reducibility measurements were in a
unpromoted catalysts is worth noting. This indicates good correlation with those obtained by XRD tech-
(a)
(b)
(c)
T2
T2
T2
T1
T3
T3
T1
2
1
T1
T3
2
1
2
1
T2
T2
T3
T3
T2
T3
0
200 400 600 800
Temperature, °C
0
200 400 600 800
Temperature, °C
0
200 400 600 800
Temperature, °C
Fig. 2. TPR profiles of Cu (1) and Pd–Cu (2) catalysts supported on CeO –Al O (a), ZnO–Al O (b) and ZrO –Al O (c) after
2
2
3
2
3
2
2 3
calcination in air atmosphere at 400°C for 4 h.
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

6
28
CIESIELSKI et al.
Table 3. Surface characteristics of mono- and bimetallic catalysts
Pd/Cu atomic
ratio
on the catalyst
surface*
Amount on the surface, Amount in the active phase, Crystallite size,
at %* at %** nm**
Catalyst
Pd
Cu
PdCu
Cu
Cu
PdCu
2
2
2
2
2
2
0%Cu/CeO –Al O
–
0.03
–
–
0.90
–
14.78
29.90
14.61
24.08
19.59
18.57
–
12.6
–
–
86.4
–
71
68
74
52
87
62
–
6
2
2
3
%Pd–20%Cu/CeO –Al O
3
2
2
0%Cu/ZnO–Al O3
–
10
–
5
2
%Pd–20%Cu/ZnO–Al O3
0.50
–
12.22
–
42.8
–
57.2
–
2
0%Cu/ZrO –Al O
3
2
2
%Pd–20%Cu/ZrO –Al O
3
0.05
0.94
28.6
71.4
2
2
*
Determined by EDS measurements, after reduction in a mixture of 5%H –95%Ar at 300°C.
2
*
* Calculated based on XRD measurements, after reduction in a mixture of 5%H –95%Ar at 300°C.
2
nique. It was found that catalysts with the smallest size sample had the lowest relative concentration of Pd–
of copper particles underwent reduction readily while Cu alloy and the largest size of copper particles. On
the samples with the biggest particle size were the least the other hand, the 2%Pd–20%Cu/ZnO–Al O cata-
2
3
susceptible to the reduction (Tables 2 and 3). The
order of decreasing reducibility for the investigated
lyst, which was proved to be the most active system,
was characterized by the highest surface Pd/Cu ratio
(0.50), highest relative concentration of Pd–Cu alloy
catalysts was found to be: 2%Pd–20%Cu/ZnO–Al O
2
3
(
the average size of Cu particles was 52 nm) > 2%Pd–
(
42.8%), and, at the same time, by the smallest size of
2
0%Cu/ZrO –Al O (62 nm) > 2%Pd–20%Cu/
2 2 3
copper crystallites. The similar results were also
obtained by other investigators. Jiang et al. [25, 26]
reported that the atomic ratio of Pd and Cu has a cru-
CeO –Al O (68 nm).
2
2
3
The acidity measurements carried out for mono-
and bimetallic catalysts showed that the deposition of
cial influence on the course of hydrogenation of CO2
copper oxide onto the surface of supports bring about to CH OH. Hu et al. [27] investigated the activity of
a decrease in their total acidity. The opposite effect PdCu–ZnO catalysts (with different Pd loading) in
3
was observed when palladium was added into the cop-
per catalyst. It is worth noting that the highest total
the same process. The authors observed the same rela-
tionship between the methanol yield and the Pd/Cu
ratio in the catalytic system. It was reported that the
most active catalyst produced a system containing 1%
Pd (Pd/Cu = 0.01) since the yield of methanol was
amount of desorbed NH was observed for 2%Pd–
3
2
0%Cu/(ZnO–Al O ) catalyst (0.86 mmol NH /g ).
2 3 3 cat
The differences in the surface acidity of mono- and
bimetallic catalysts also correlate with their activity in
the investigated reaction. It can be suggested that the
catalysts with the highest acidity stronger stabilize the
intermediates formed during the methanol synthesis
and in this way improve the catalytic activity.
2
.5 times that reported for the non-promoted Cu–
ZnO catalyst. Such observation can be explained by
enhanced interactions between Pd and Cu that inten-
sify the hydrogen spillover effect during the hydroge-
nation of CO to methanol. Also, the formation of
2
The surface content of active metals was estimated Pd–Cu alloy modifies the redox properties of the Cu
by means of SEM-EDS measurements (Table 3). The surface, preventing the oxidation of copper metal by
obtained results suggested that the distribution of
metal particles deposited on the surface of supports
was not homogeneous, as evidenced by large differ-
CO during the process [28].
2
In order to investigate the reaction mechanism,
ences in the atomic percentage of individual metals. in situ Fourier-transform infrared spectroscopy (FT-IR)
Interestingly, 2%Pd–20%Cu/CeO –Al O catalyst measurements were performed to identify intermedi-
2
2
3
which had the lowest Pd/Cu ratio was also the least ate species formed on the surface of both catalysts and
active among the bimetallic catalysts. Moreover, this supports during the reaction (Fig. 3).
KINETICS AND CATALYSIS
Vol. 61
No. 4
2020

MECHANISTIC STUDIES OF METHANOL SYNTHESIS REACTION
Oxygen vacancy
629
Cu0
Cu0
Cu0
O–2
Ce+4 Vo
Ce+4 O–2
Ce+4
O–2 Ce+4
O–2 Ce+4
H2
Dissociatively adsorbed
CO2
H atoms
Spillover
Spillover
Spillover
O
C
O
O
Adsorbed
C + H O
CH OH + OH H O
3 2
C
2
O
O
O
H
H H H
Scheme 1. Scheme of proposed mechanism for CO hydrogenation to methanol.
2
The IR spectra of CeO showed adsorption bands trum obtained for CeO –Al O showed additional
2
2
2
3
absorption bands at 1570 and 2856 cm– associated
1
attributable to cerium formates (b-HCO -Ce: 1335,
520 and 2958 cm and m-HCO -Ce: 2867 cm ), as with the corresponding formate species (b-HCO -Al).
well as methoxy groups (1065, 2824 cm ). The spec- Furthermore, the addition of copper led to the appear-
2
–1
–1
1
2
2
–1
2
856 cm–1
b-HCOO-Al
2
958 cm–1
–1
1570 cm–1
b-HCOO-Al
b-HCOO-Ce
1595 cm
b-CO3
2–
2
175 cm–1
4
CO (g)
3
2
1
1
065 cm–1
CH3O-Ce
2
928 cm–1
m-HCOO-Cu
2
117 cm–1
1335 cm–1
b-HCOO-Ce
2
867 cm–1
m-HCOO-Ce
CO (g)
2
824 cm–1
CH3O-Ce
520 cm–1
b-HCOO-Ce
1425 cm–1
CO3
1
CO2 (g)
2–
3000
2500
2000
Wavenumber, cm
1500
1000
–1
Fig. 3. IR spectra of adsorbed forms of the intermediates formed in the synthesis of methanol from a mixture of H and CO at
2
2
2
20°C on the systems: CeO (1), CeO –Al O (2), 20%Cu/CeO –Al O (3), 2%Pd–20%Cu/CeO –Al O (4).
2 2 2 3 2 2 3 2 2 3
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

6
30
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1
4
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2
–1
3
5
6
7
8
. Räuchle, K., Plass, L., Wernicke, H.-J., and Bertau, M.,
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2
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vation of hydrogen due to the spillover effect, as evi-
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2
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2020