Understanding the Origin of Structure Sensitivity in Nano Crystalline Mixed Cu/Mg?Al Oxides Catalyst for Low-Pressure Methanol Synthesis

Article abstract of DOI:10.1002/cctc.202100488

Cu nanoparticles of size 5–10 nm supported on Mg?Al mixed oxide were prepared by the sol-gel method. Cu loading was varied from 2.5 to 10 wt % on the support to investigate the effect on particle size and activity/selectivity of the catalyst. The Cu/Mg?Al catalysts containing small copper nanoparticles favor high selectivity of methanol, while the rate of CO formation was higher for larger copper particles. The high methanol selectivity (～99 %) and methanol formation rate (0.016 mol g_Cu^?1 h^?1) over the 4.8Cu/Mg?Al catalyst was due to the combined effect of the presence of high Cu dispersion, Cu surface area, and strong interaction between small Cu particles with Mg?Al support. The high stability of the catalyst was attributed to the strong binding of the Cu cluster (?179.7 kJ/mol) to the MgO/γ-Al₂O₃ support, as shown by the DFT study. Additionally, the adsorption energy calculated using DFT showed preferential adsorption of CO₂ and H₂ at the Cu/MgO(100) active site (?120.9 kJ/mol, ?130.4 kJ/mol) compared to the Cu/γ-Al₂O₃(100) (?64.2 kJ/mol, ?85.7 kJ/mol)active site. The high selectivity of the catalyst towards methanol can be attributed to the higher stability of the formate (HCOO) intermediate (?257.2 kJ/mol) compared to the carboxylate (COOH) intermediate (?131.0 kJ/mol).

Full text of DOI:10.1002/cctc.202100488

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Understanding the origin of structure sensitivity in nano
crystalline mixed Cu/Mg-Al oxides catalyst for low-pressure
methanol synthesis
Authors: Rajaram Bal, Sachin Kumar Sharma, Bappi Paul, Mukesh
Kumar Poddar, Chanchal Samanta, Tuhin Suvra Khan, M. Ali
Haider, and Piyali Bhanja
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To be cited as: ChemCatChem 10.1002/cctc.202100488
Link to VoR: https://doi.org/10.1002/cctc.202100488
01/2020

10.1002/cctc.202100488
ChemCatChem
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Understanding the origin of structure sensitivity in nano
crystalline mixed Cu/Mg-Al oxides catalyst for low-pressure
methanol synthesis
Sachin Kumar Sharma, [a,b] Bappi Paul,[a,c] Piyali Bhanja,[d] Mukesh Kumar Poddar,[a] Chanchal
Samanta,[e] Tuhin Suvra Khan,[a] M. Ali Haider,[f] and Rajaram Bal*[a,b]
[a] Light Stock Processing Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India. Fax: +91 135 2660202; Tel: +91 135 2525917.
[b] Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India
[c] Department of Chemistry, National Institute of Technology Nagaland, Dimapur, Nagaland 797103, India
[d] CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India
[e Bharat Petroleum Corporation Ltd., Greater Noida, Uttar Pradesh 201306, India.
[f] Renewable Energy and Chemicals Laboratory, Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India.
Supporting information for this article is given via a link at the end of the document.
Abstract: Cu nanoparticles of size 5-10 nm supported on Mg-Al
mixed oxide were prepared by the sol-gel method. Cu loading
was varied from 2.5 to 10 wt% onto support to investigate the
effect on particle size and activity/selectivity of the catalyst. The
Cu/Mg-Al catalysts containing small copper nanoparticles favor
high selectivity of methanol, while the rate of CO formation was
higher for larger copper particles. The high methanol selectivity
major challenge due to thermodynamic difficulties in the CO2
activation 10-13 since CO2 is a very stable molecule (ΔHf = -393.5
kJ/mol CO2(g).
Cu/ZnO/Al2O3 based catalyst prepared through conventional co-
precipitation method is being used commercially in the
production of methanol under the operating temperature of (473-
573 K) and high pressure (7-10 MPa) 1, 14, 15. Such a catalyst has
also been investigated extensively for methanol synthesis from
CO2 hydrogenation 16-18. However, low selectivity for methanol
has been encountered in the CO2 hydrogenation because of the
large quantities of CO formation as a by-product 19. Two
competitive reactions which suppress CO2 conversion to
methanol are reverse water gas shift (RWGS) reaction for the
formation of CO and also the formation of methane via
hydrogenation through C-O bond dissociation 20, 21. It has been
-1
(~99%) and methanol formation rate (0.016 mol gCu h-1) over
the 4.8Cu/Mg-Al catalyst was due to the combined effect of the
presence of high Cu dispersion, Cu surface area, and strong
interaction between small Cu particles with Mg-Al support. The
high stability of the catalyst was attributed to the strong binding
of the Cu cluster (-179.7 kJ/mol) to the MgO/γ-Al2O3 support, as
shown by the DFT study. Additionally, the adsorption energy
calculated using DFT showed preferential adsorption of CO2 and
H2 at the Cu/MgO(100) active site (-120.9 kJ/mol, -130.4 kJ/mol)
compared to the Cu/γ-Al2O3(100) (-64.2 kJ/mol, -85.7
kJ/mol)active site. The high selectivity of the catalyst towards
methanol can be attributed to the higher stability of the formate
(HCOO) intermediate (-257.2 kJ/mol) compared to the
carboxylate (COOH) intermediate (-131.0 kJ/mol).
shown that catalyst with moderate activity for non-selective
22, 23
RWGS reaction is suitable for high methanol selectivity
.
Although there is still significant doubt about the actual reaction
mechanism, under specific industrial conditions, methanol is
produced mostly through CO2 hydrogenation, with CO acting as
a
CO2 source and water producing oxygen atoms as
a
scavenger, which acts as an inhibitor of active metal sites 24-27
.
Introduction
Most of the earlier studies report low conversion of CO2, poor
catalytic stability, and low selectivity of methanol, where copper
alone found ineffective to produce methanol via CO2
hydrogenation 28, 29. It is also reported that single oxide support
cannot improve the performance of the active Cu particles
Global atmosphere concentration of CO2 has reached an
unprecedented level of 400 ppm in recent times due to
excessive use of fossil resources creating global warming and
serious environmental problems 1, 2. It has become imperative to
adopt a circular economy through reduce, reuse, and recycle
approaches, and significant thrusts are now being given to
reducing CO2 concentration through carbon capture and
utilization (CCU) routes 2-4. CO2 is a non-toxic, cheap, and
widely available C1 Feedstock that can be converted into
important chemicals and fuels like methanol, dimethyl ether
(DME), and synthetic natural gas (SNG) 3-5. Therefore, chemical
utilization of CO2 has attracted great attention, which can reduce
the effect of excess CO2 in the natural environment and
transform the economy towards carbon neutrality. Catalytic
conversion of CO2 to methanol is especially becoming attractive
because of the potential uses of methanol as both chemical
feedstock and fuel and fuel components such as direct methanol
fuel cells 6-9. However, obtaining a high yield of methanol, i.e.,
high CO2 conversion and high methanol selectivity, remains a
significantly, but the presence of second metal oxide such as
30, 31
MgO, ZrO2, SiO2, TiO2 can stabilize Cu nanoparticles
.
Arena et al. found that after the introduction of ZrO2 in the Cu-
ZnO catalyst prepared by different methods showed high activity
for methanol production, and the stability of this catalyst was
also improved 26, 32. It is also reported that the presence of Ga
28, 33, 34
also improved the activity and stability of the catalyst
.
35
Schlögl et al. showed the role of ZnO in the Cu/ZnO catalyst
prepared by using a hydroxyl carbonate precursor. Based on
the literature reports, it is generally accepted that the Cu0
species in catalysts are the active phase for CO2 hydrogenation,
and the metal oxide supports can disperse the active copper
species on the surface. On the other hand, the presence of H2
favours the sintering of the Cu particles, which deactivates the
catalyst. Typically, the Cu oxide of the synthesized catalyst is
reduced by H2 and CO during the reaction. Researchers also
1
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reported that the conversion of CO2 is affected by the metallic
Cu surface area, and the selectivity of methanol highly depends
on the dispersion of basic sites on the catalyst surface 36. So,
the presence of high copper surface area and basicity of the
catalyst is the key factors for selecting the methanol synthesis
catalyst via CO2 hydrogenation 37. Dasireddy et al. 38. reported
and (222) planes of magnesium oxide (MgO) (JCDPS card
no.89-4248). In addition, it was observed that the diffraction
peak for spinel species such as MgAl2O4 (JCPDS No: 21-1152)
and CuAl2O4 (JCPDS No: 01-1153) was found, which exhibited
40,
the 2θ values at 65.28° and 31.27°
41. Low intensity for
CuAl2O4 and MgAl2O4 spinel species suggest that the
the significant role of alkaline-earth metal oxide (MgO, BaO, SrO, hydrotalcite-like precursor was converted into mixed metal oxide
and CaO) on the copper-based catalyst, which enhanced the
number of CO2 and H2 adsorption active sites and also
of both CuAl2O4 and MgAl2O4 spinel species due to the high
dispersion of copper 42. Furthermore, γ-Al2O3 was not found to
be a distinct phase due to it is in amorphous materials. On the
other hand, XRD analysis for the spent catalysts was carried out
to check the crystallinity and the thermal stability of the catalysts
after the reaction, and it was found that the spent catalysts did
not show any major change in their crystallinity and phase
behavior. However, the spent 9.6Cu/Mg-Al and 7.3Cu/Mg-Al
catalysts showed metallic copper species at a 2θ value of
44.31°. It was observed that due to high dispersion and smaller
copper particle size resulted in inhibition in sintering through
strong metal-support interaction between copper nanoparticles
and MgO support in the 4.8Cu/Mg-Al catalyst during the
reactions.
increased the metal-support interaction with high metallic Cu
39
surface area. Ren et al.
showed that the presence of MgO
increases the formation of small metallic copper species, which
enhanced the catalytic activity of the Cu-based methanol
synthesis catalyst.
In the current investigation, we have prepared highly dispersed
sinter resistant Cu-nanoparticles in the range between 5-10 nm
supported on nanocrystalline Mg-Al mixed oxide catalyst, which
has been found to provide superior selectivity (~99%) methanol
and moderate activity at low pressure (3 MPa) methanol
synthesis. In addition, DFT was to used to understand the high
stability and high methanol selectivity of Cu/MgO/Al2O3 catalyst.
Results and Discussion
Powder XRD pattern of all synthesized fresh catalyst are shown
in Figures S1a to S1f, respectively (supporting information).
Interestingly, the 2.3Cu/Mg-Al and 4.8Cu/Mg-Al catalyst
exhibited no distinctive peaks of any Cu oxides in the XRD
pattern (Figures S1c and S1d), suggesting the presence of a
highly dispersed small nanocrystalline Cu-particles. It has to be
noted that XRD has the limitations of analyzing the crystallites
below 5 nm 40. Whereas, the 7.3Cu/Mg-Al and 9.6Cu/Mg-Al
catalyst showed a peak at 38.75º, which is assigned for CuO
(JCPDS Card no. 89-5896). The peak at 38.75° in the XRD
pattern was used to determine the crystallite size of the CuO
(111) phase using Scherrer’s equation (Table S1, supporting
information). Moreover, powder X-ray diffraction (XRD) patterns
of the reduced and spent catalysts are presented in Figure 1A
and 2B. All reduced Cu/Mg-Al catalysts showed uniformly
resolved diffraction peaks. The 4.8Cu/Mg-Al does not show any
Cu peaks due to very high Cu dispersion and presence of
smaller Cu particles. The XRD peak at 2θ value of 44.3°
represents the metallic Cu (111). The intensity of any peak in the
XRD pattern indicates the extent of crystallinity of a particular
plane. The XRD peak at 44.3° for 4.8Cu/Al shows very intense
because the metallic Cu species present in the γ-Al2O3 support
are highly crystalline in nature compare to the other supports.
We believe that in presence of Mg the dispersion of Cu
increases at the same time the Cu particles size also decreases,
so the intensity of the Cu peaks decreases (crystallinity is low).
In general, the smaller the particles the more is the XRD peak
broadening and vice versa. However, in the XRD patterns of
reduced 7.3Cu/Mg-Al and 9.6Cu/Mg-Al samples, an additional
low intense peak at 43.31° can be seen for the (111) plane of
Cu0 species (JCPDS card no.89-2838). The peak at 43.31° in
the XRD pattern was used to determine the crystallite size of the
Cu (111) phase using Scherrer’s equation (Table 1). The
diffraction peaks at 2θ values of 36.94°, 42.92°, 62.31°, and
78.64° are assigned to the diffraction from (111), (200), (220),
Figure 1. Powder XRD patterns of Figure (A) reduced, (B) spent, (C) Isotherm
plot and (D), BJH plot for 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al and
9.6Cu/Mg-Al catalysts.
The physicochemical properties of the reduced and used
catalysts are given in Table1. The BET-Surface area of catalysts
was determined by N2 adsorption-desorption measurements. It
can be seen that the sample exhibits a type-IV adsorption-
desorption isotherm with a type H2 hysteresis loop in the P/P0
range from 0.80 to 0.95 (Figure 1). The pores are predominantly
in the range from 2 to 12 nm as confirmed by the pore size
distribution which is justified from the TEM images (inset of
Figure. 2). The surface area is decreased when the amount of
copper loading was increased. The surface area of the catalysts
was different with different supports. The specific surface area of
4.8Cu/Al catalyst was 60 m2/g, whereas 4.8Cu/Mg catalyst has a
surface area 64 m2/g.
The 2.3Cu/Mg-Al, 4.8Cu/Mg-Al,
7.3Cu/Mg-Al, and 9.6Cu/Mg-Al catalysts exhibited surface area
of 72, 70, 61, and 57 m2/g, respectively. It was found that when
Cu loading was less than 5 wt% (in case of 2.3Cu/Mg-Al and
4.8Cu/Mg-Al) the catalyst does not show any deactivation and it
2
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is stable as shown in the time-on-steam study. But when the Cu
loading was more than 5 wt% (in case of 7.3Cu/Mg-Al and
9.6Cu/Mg-Al) the catalysts deactivate with time. In the surface
area analysis also, it was found that surface area of 2.3Cu/Mg-Al
changes from 72 to 68.3 and for 4.8Cu/Mg-Al catalyst it changes
from 70 to 67.8 m2/g during catalysis, whereas it changes from
61 to 50.3 m2/g for 7.3Cu/Mg-Al and for 9.6Cu/Mg-Al it changes
to 42.5 from 57 m2/g during catalysis. BET analysis of spent
2.3Cu/Mg-Al and 4.8Cu/Mg-Al catalysts revealed that there was
only a negligible change in surface area, indicating good thermal
stability of the catalyst. Whereas, the spent 7.3Cu/Mg-Al and
9.6Cu/Mg-Al catalysts showed further decrease in surface area
due to agglomeration of catalyst particles during catalysis.
morphology. Whereas the 4.8Cu/Mg-Al catalyst showed around
7 nm of metallic Cu species, and the lattice fringes for Cu (111)
with a d-spacing value of 2.08 nm can be seen. The TEM
images of 7.3Cu/Mg-Al and 9.6Cu/Mg-Al showed 20–40 nm of
MgO nanoparticles (Figure 2c and 2d). The dispersion of
4.8Cu/Mg-Al nanocrystalline catalysts was determined by the
STEM elemental mapping, and the result indicates the
homogenous distribution of Cu-nanoparticles on nanocrystalline
Mg-Al support (Figure S4, supporting information). The
morphologies of spent catalysts were also investigated by TEM
analysis, and the images are shown in Figure 2e, 2f, 2g, and 2h,
respectively. The 2.3Cu/Mg-Al, 4.8Cu/Mg-Al catalysts retained
its spherical morphologies after catalysis (Figures 2e and 2f),
whereas the 7.3Cu/Mg-Al and 9.6Cu/Mg-Al catalysts showed
agglomerated particles after catalysis (Figures 2g and 2h). The
analysis indicated that larger particles unable to resist sintering
during catalysis.
Table 1: Physico-chemical properties of Cu/Mg-Al catalysts.
Cu
loading
(wt.%)
Average
Sample
SBET(m2g-1)
Reduced Spent
crystallite sizea (nm)
Cu
MgO
(200)
ND
ND
ND
24.8
22.8
24.2
24.8
25.3
γ-Al2O3
(111)
(100)
MgO
γ-Al2O3
4.8Cu/Al
4.8Cu/Mg
2.3Cu/Mg-Al
4.8Cu/Mg-Al
7.3Cu/Mg-Al
9.6Cu/Mg-Al
0
0
76
68
60
64
72
70
61
57
ND
ND
ND
ND
ND
ND
4.8
4.8
2.3
4.8
7.3
9.6
58.2
55.6
68.3
67.8
50.3
42.5
8.8
11.3
ND
27.5
ND
ND
ND
ND
9.2
ND
12.4
ND
ND- Not determined; a determined from XRD using Scherrer formula
N2O titration method was carried out to find out Cu metal
dispersion and Cu particle size in the reduced catalysts. It was
found that with increasing Cu loading, the dispersion of Cu
decreases gradually, and for 2.3Cu/Mg-Al, 4.8Cu/Mg-Al,
7.3Cu/Mg-Al, and 9.6Cu/Mg-Al catalysts, the dispersion values
were 19.4%, 18.7%, 14.8%, and 11.8%, respectively. The
maximum copper dispersion was found for the 4.8Cu/Mg-Al
catalyst. The decreasing trend of metal dispersion was due to
the formation of much bigger Cu particles with increasing Cu
loading. The number of active particle sites estimated from metal
dispersion analysis is provided in Table 2.
Figure 2. TEM images of reduced and spent catalysts (a,e) 2.3Cu/Mg-Al, (b,f)
4.8Cu/Mg-Al, (c,g) 7.3Cu/Mg-Al and (d,h) 9.6Cu/Mg-Al.
The reduction profiles of all samples prepared with different
loading of the amount of copper exhibit a broad peak of H2
consumption in the range of 160-280 °C. The catalyst having
lower Cu loading (2.3Cu/Mg-Al and 4.8Cu/Mg-Al catalysts) with
smaller crystallites size of Cu showed the reduction at relatively
lower temperatures compare to their higher Cu loading
(7.3Cu/Mg-Al and 9.6Cu/Mg-Al catalysts) counterparts. The
lower temperature reduction peaks indicate the reduction of
relatively smaller CuO particles, while the higher temperature
peaks are attributed to the reduction of comparatively larger
CuO particles 43. From XRD analysis it was found that when Cu
loading was less than 5 wt% XRD does not show any Cu peaks
whereas when Cu loading was more than 5 wt%, in the XRD
pattern different Cu species like CuO and Cu2Al2O4 were present
In addition, strong metal-support interaction is also present
(Figure 3A), 44, 45. The 2.3Cu/Mg-Al catalyst exhibited a single
board reduction peak at 190 °C, which is attributed to the
reduction of the Cu oxide species of the catalyst in Figure 3A
and after deconvolution it shows two peaks at 174 and 196 °C
(Figure S2, supporting information). We believe that the high-
temperature reduction peak corresponds to the reduction of bulk
CuO with relatively large particle size, while the low-temperature
reduction peak corresponds to the reduction of dispersed CuO.
On the other hand, 4.8Cu/Mg-Al catalyst showed two reduction
peaks in the range 160-208 °C, which was ascribed into different
aggregation states of CuO, Cu2O, and their combination
Table 2: Copper metal dispersion (MD) and Copper particle sizes obtained by
the N2O decomposition method.
Sample
Cu
dispersion
(%)
Cu SA
(SCu
m2g-1)
Cu
particle
size (dCu
nm)
Number
of the
,
,
active Cu
atoms/g
2.24 x 1011
4.8Cu/Al
8.4
9.7
12.3
4.8Cu/Mg
10.6
19.4
18.7
14.8
11.8
12.3
21.6
20.8
17.1
14.2
9.7
5.3
5.5
6.8
8.7
6.87 x 1012
2.97 x 1015
5.90 x 1015
1.10 x 1014
3.78 x 1013
2.3Cu/Mg-Al
4.8Cu/Mg-Al
7.3Cu/Mg-Al
9.6Cu/Mg-Al
TEM images of 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al, and
9.6Cu/Mg-Al are shown in Figure 2. Several black dots are seen
in Figure 2, which may be attributed to Cu particles. The TEM
images of 7.3Cu/Mg-Al and 9.6Cu/Mg-Al catalysts are showing
agglomerated particles. This is maybe due to high Cu loading.
As illustrated in Figure 2a, 2.3Cu/Mg-Al indicates the formation
of about 6 nm metallic Cu species, which possessed a spherical
3
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2, 38, 46
indicates different reduction properties of copper species
.
The reduction peak at a lower temperature for 4.8Cu/Mg-Al
catalyst indicates the higher dispersion of Cu species with strong
metal-support interaction. In addition, with increasing copper
loading, the 7.3Cu/Mg-Al and 9.6Cu/Mg-Al catalysts showed the
reduction peak shift to higher temperatures. The 7.3Cu/Mg-Al
catalysts exhibited the reduction temperature between 162 to
238 °C, while the 9.6Cu/Mg-Al catalysts exhibited the reduction
temperature between 168 to 251 °C. The reduction peak for
these two catalysts was shifted to higher temperatures due to
the presence of larger (agglomerated) Cu particles with weak
metal-support interaction. In addition, the reduction peak area
increases with increasing Cu loading over Cu/Mg-Al catalysts.
The total hydrogen consumption values (mmol/g) are 0.33963,
0.38936, 0.77511 and 0.92081 for 2.3Cu/Mg-Al, 4.8Cu/Mg-Al,
7.3Cu/Mg-Al and 9.6Cu/Mg-Al, respectively. The H2
consumption values during the TPR analysis in the Table S2
(supporting information). The reduction peak area for 4.8Cu/Mg
is less than 2.3Cu/Mg-Al because the amount of reducible Cu
species is less for 4.8Cu/Mg compare to 2.3Cu/Mg-Al. From
Figure S3, (supporting information) the TEM analysis it was
found that 4.8Cu/Mg catalyst produced agglomerated Cu
species (big Cu particles) whereas 2.3Cu/Mg-Al catalyst
produced highly dispersed Cu species (small Cu particles). As
the amount of copper loading increased, the reduction peaks of
agglomerated larger Cu particles were shifted to higher
temperatures, which was not easily reduced in compare to
smaller Cu particles during the reduction process.
Figure:3 (A) H2-TPR patterns and (B) CO2-TPD profile of synthesized
catalysts.
X-ray photoelectron spectroscopic (XPS) analysis was carried
out to examine the chemical state of Cu, Mg, Al and O present
on the surface of the fresh, reduced (H2 pre-treated) and spent
2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al
catalysts. The XPS spectra of the reduced catalysts are given in
Figure 4, whereas the XPS spectra of the fresh and spent
catalysts are given in Figures S5, (supporting information).
Cu2p3 2 XPS peaks of the fresh catalysts with binding energy
/
values of 933.2 eV, 933.1 eV, 933.2 eV and 933.2 eV
respectively, for 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al and
9.6Cu/Mg-Al catalyst confirmed the presence of Cu2+ in the
sample. The Cu XPS spectra of the fresh 2.3Cu/Mg-Al,
4.8Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al catalysts are shown
in Figure S5 (supporting information). All the catalysts showed
peaks at ~933.1 eV for the Cu2p3/2 and peaks near ~953.1 eV
for the Cu 2p1/2, followed by shake-up satellite peak at ~942 eV
and 962 eV, confirming the presence of Cu2+. These satellites
were due to the charge transfer between the transition metal 3d
and surrounding ligand oxygen 2p orbitals, and the reduced and
spent catalyst also showed the absence of satellites in the
spectra, confirmed that no considerable quantity of Cu2+ species
is present on the surface 49. All the catalysts (2.3Cu/Mg-Al,
4.8Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al) showed Cu2p3/2
The surface basicity as well as strength of interaction of CO2
with the catalyst surface of various Cu/Mg-Al catalysts were
examined by CO2-TPD. The CO2-TPD profile of Cu/Mg-Al
catalysts (Figure 3B) exhibited three types of peaks, which are
attributed to weak, moderate, and strong basic sites,
respectively 2, 46. As shown in Figure 3B, the low temperature
(50-120 °C) peak corresponds to a weak basic site, which can
occupy the adsorbed acidic CO2 on the surface. The moderate
basic sites at a temperature range between 120-200 °C may be
assigned to metal-oxygen pair (Mg-O and Al-O) and surface
oxygen anions, whereas the strongly basic site is appearing at
the temperature between 200-250 °C due to the unsaturated O2-
38, 47,
binding values ~932.2 eV and Cu2p1/2 binding energy values of
ions and the lattice oxygen anions
48. The 4.8Cu/Mg-Al
50-53
~ 952 eV
confirming the presence of metallic Cu after the
catalyst showed strong basic sites, and the reason may due to
increasing the metal dispersion and copper surface area, which
helps to increase the amount of basic sites of the catalyst 39. In
addition, the formation of CuAl2O4 and MgAl2O4 spinel phases
can also increase the amount of basic sites. However, the
significant increase in the basic sites in the 4.8Cu/Mg-Al catalyst
is beneficial for the adsorption of CO2.
reduction as shown in Table 3 (supporting information). The Mg
2p XPS spectra for 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al,
and 9.6Cu/Mg-Al catalysts contain binding energy peaks at 50.8,
51.0, 51.1, and 51.2 eV, respectively, indicating that the Mg
species exist as Mg2+ (Figure 4B). Moreover, the XPS spectra of
Al 2p (Figure 4C) in all the synthesized catalysts are found in the
binding energy value of ~74.7 eV confirming that Al is present as
Al3+ in the prepared catalysts and the results are summarized in
Table 3 (Figure 4C). The O1s XPS spectra(Figure 4D) for
2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al
catalysts show two types of peaks, whereas peak near 530 eV
(529.8, 530.0, 530.3 and 530.3 eV, respectively for 2.3Cu/Mg-Al,
4.8Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al) belongs to lattice
O (Mg-O and Cu-O), whereas the peak near 531eV (531.3,
531.4, 531.1 and 531.0 eV, respectively for 2.3Cu/Mg-Al,
4.8Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al) is due to the
presence of O-H in the catalysts 53-55. The XPS spectra of spent
2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al, and 9.6Cu/Mg-Al
catalysts are shown in Figure S5 (Supporting information) and
(Table 3). For all spent Cu-containing samples showing Cu 2p3/2
4
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binding energy peaks at ~ 932.2 eV and Cu 2p1/2 binding energy
peaks at ~ 952 eV 53-55 as shown in Table 3.
MgO-γ-Al2O3 support also plays a crucial role for the high
methanol selectivity and catalyst stability. Additionally, γ-Al2O3
acts as a stabilizer for the Cu-particles preventing the Cu-
particles against sintering during reaction when Cu loading was
less than 5wt%. The Cu/MgO interface has been proposed to
generate the active site responsible for the high CH3OH
selectivity 56. The Cu/MgO interface has been proposed to
generate the active site responsible for the high CH3OH
56
selectivity
. So, γ-Al2O3 acts as a stabilizer for the Cu/MgO
catalyst against sintering during catalysis as observed by earlier
researchers also 30, 56. The conversion of CO2 and high yield of
methanol was found over 4.8Cu/Mg-Al catalyst at a lower
temperature (200 °C) compared to 2.3Cu/Mg-Al, 7.3Cu/Mg-Al,
and 9.6Cu/Mg-Al catalysts. The lowest conversion of CO2 was
found for the 9.6Cu/Mg-Al catalyst due to the presence of less
number of active sites for the adsorption of reactive molecules
during catalysis. When copper loading was increased, Cu
dispersion decreased, and CO2 conversion was also decreased.
The catalyst 4.8Cu/Mg-Al exhibited 19.4% of Cu dispersion,
while the 2.3Cu/Mg-Al, 7.3Cu/Mg-Al, and 9.6Cu/Mg-Al catalyst
showed dispersion value of 18.7, 14.8, and 11.8%, respectively.
Figure 4. XPS analysis of 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al and.
6Cu/Mg-Al catalysts. (A) Cu 2p (reduced), (B) Mg 2p (reduced), (C) Al 2p
(reduced), and (D) O 1s (reduced).
Table 3: Binding energy/kinetic energy of peaks in the XPS spectra.
Samples
Cu 2p/eV
2p1/2
Mg 2p/eV
Al 2p/eV
O 1s/eV
Reduced
2p3/2
Spent
2p1/2
2p3/2
FWHM = 1.4
932.1
FWHM = 0.9
932.2
FWHM = 2.5
50.8
FWHM = 2.6
74.6
FWHM = 1.6
529.8, 531.3
2.3Cu/Mg-Al
4.8Cu/Mg-Al
7.3Cu/Mg-Al
9.6Cu/Mg-Al
951.9
952.0
952.1
952.1
952.0
952.0
952.1
952.1
FWHM = 1.5
932.2
FWHM = 0.9
932.2
FWHM = 2.5
51.0
FWHM = 2.7
74.7
FWHM = 1.7
530.3, 531.4
FWHM = 1.6
932.3
FWHM = 1.3
932.3
FWHM = 2.4
51.1
FWHM = 2.7
74.7
FWHM = 1.8
530.3, 531.1
FWHM = 1.6
932.3
FWHM = 1.2
932.3
FWHM = 2.5
51.2
FWHM = 2.8
74.8
FWHM = 1.8
530.3, 531.0
Catalytic activity for CO2 hydrogenation to methanol
The 2.3Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al catalysts
showed 5.4%, 6.2% and 5.2% conversion of CO2 with 95.6%,
93.4% and 90.3% of selectivity of methanol at 200 °C and
12.5%,12.7% and 11.6% of CO2 conversion with 53.6%, 55.7%
and 53.8% selectivity of methanol, respectively at 280 ºC. The
required activation energy for CO2 activation over the active
particles reached at 200 ºC, and the maximum CH3OH yield was
achieved at 260 ºC. Further increase in temperature resulted in
the drop of methanol selectivity and yield. The continuous
decrease in CH3OH selectivity is due to the fact that with
increasing temperature, the RWGS reaction became dominant
over methanol formation. Figure 5C shows CO selectivity during
CO2 hydrogenation reaction over 4.8Cu/Mg-Al catalyst. The
7.3Cu/Mg-Al and 9.6Cu/Mg-Al catalyst showed very low CO2
conversion and methanol selectivity during the catalysis. This
We tested the reaction over all the prepared catalysts (4.8Cu/Al,
4.8Cu/Mg, 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al, and
9.6Cu/Mg-Al) at the same reaction conditions, where the
temperature was between 200-280 ºC, Pressure =30 bar, WHSV
= 7200 ml gcat-1 h-1 and Feed composition of CO2: H2: N2 = 1:3:1.
The CO2 conversion, methanol selectivity, CO selectivity, and
methanol formation rate are presented in Figure 5 (A), 5 (B), 5
(C), and 5 (D), respectively. Methanol, carbon monoxide, and
water were the major products detected during the reaction. We
have explored the individual role of the MgO and γ-Al2O3 also
studied and their catalytic activity in the hydrogenation reaction.
In addition of Cu/Mg-Al catalysts, we have also performed the
catalytic activity of Cu/MgO and Cu/γ-Al2O3. The MgO-free
binary Cu/γ-Al2O3 catalyst showed low activity, similarly γ-Al2O3-
free binary Cu/MgO catalyst also showed low activity. We
believe that dispersion of Cu increases over MgO-γ-Al2O3 when
Cu loading was less than 5 wt% and it form small Cu particles at
the same time synergetic effect between small Cu-particles and
reason may be due to the uncontrolled deposition of Cu resulted
44, 56
in larger sized Cu-species
with a very less number of
smaller active Cu-species present in the catalyst.
5
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2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al, and 9.6Cu/Mg-Al
catalysts also showing a similar trend and methanol formation
-1
rate of 0.012, 0.016, 0.011 and 0.010 mol gCu h-1, respectively
at the optimum temperature (200 °C). So, it can be seen that the
4.8Cu/Mg-Al catalyst exhibited the best performing with high
methanol production rate, which has the maximum copper
dispersion with the highest copper surface area. On the contrary,
it was also observed that the methanol formation rate is
increasing with increasing temperature for all the prepared
catalysts. Most investigators have observed a similar linear
relationship between copper surface area and catalytic activity
69-73. The presence of the active copper atom affects the
methanol formation rate, which was calculated from the Cu
particle size for the various catalysts, shown in Figure 6B, where
a high methanol formation rate was achieved with smaller Cu
particles. The 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al, and
9.6Cu/Mg-Al catalysts showed the methanol production rates of
0.012, 0.016, 0.011 and 0.010 mol gCu-1 h-1, and the Cu particles
size was 5.3, 5.5, 6.8, 8.7, respectively. Although a direct linear
correlation between the methanol formation rates and catalyst
Cu surface area or Cu particle size was not found, a good linear
correlation was observed for the methanol formation rate vs
number of active site Cu as shown in Figure S6 (supporting
information). In addition, for the 4.8Cu/Mg-Al, 7.3Cu/Mg-Al, and
9.6Cu/Mg-Al catalysts the catalyst activity (rate of methanol
formation) was found to be in good correlation the 2.3Cu/Mg-Al
catalyst was found to be an outlier. It was because although the
2.3Cu/Mg-Al catalyst have a higher surface area (21.6 m2g-1)
ompared to 4.8Cu/Mg-Al catalyst (20.8 m2g-1), the number of
active Cu particles were found to be almost same (2.97 x 1015
atom/g for 2.3Cu/Mg-Al and 5.90 x 1015 atom/g for 4.8Cu/Mg-Al).
Figure 5. (A) CO2 conversion, (B) Methanol selectivity, (C) CO selectivity, and
(D) Methanol formation rate from CO2 hydrogenation over a different of
catalysts. Reaction condition: Temperature (200-280 ºC), Pressure (30 bar),
WHSV (7200 ml gcat-1 h-1) and Feed ratio (CO2: H2: N2 = 1:3:1).
Cu is an active metal for promoting methanol synthesis via CO2
hydrogenation, where the size of the copper particle plays an
important role, which affects the activity of the catalyst 57, 58. As
the size of the active Cu species increases, the reducibility of the
catalyst decreases, affecting the activity of the catalyst 59-63. It
was observed that small Cu particle size and the nature of the
support plays an important role in increasing the activity of the
catalyst 64-66. For 4.8Cu/Mg-Al catalyst, TEM images show that
the Cu particle size is about 5-8 nm, which was further
confirmed by the metal dispersion analysis via the N2O titration
method, where the Cu particle size is 5.5 nm. The difference in
the active species particle sizes can be explained by the fact
that in TEM images, only a portion of the catalyst is analyzed,
and very small Cu-particles were not detected by the technique.
On the other hand, during metal dispersion analysis, very small
Cu-species particles were also contributed to H2 chemisorption
67. The high activity of 4.8Cu/Mg-Al catalyst is due to the
presence of very small Cu-particles, which lowered the
activation energy of CO2. It was observed that with increasing
Cu loading, the Cu particle size increases, and metal dispersion
is decreasing. The rate of methanol formation of all synthesized
catalysts against different temperatures is shown in Figure 5D.
The 2.3Cu/Mg-Al, 4.8Cu/Mg-Al, 7.3Cu/Mg-Al and 9.6Cu/Mg-Al
catalysts showed methanol formation rate of 0.012, 0.016 and
Figure 6. (A) Methanol formation rate vs. copper surface area, and (B)
Methanol formation rate vs. Cu particle size.
To examine the long-term stability of the most efficient
4.8Cu/Mg-Al catalyst under optimized reaction conditions, CO2
hydrogenation to the desired product methanol was carried out
for 100 h time stream, during which the reactor was operated
continuously under test conditions. Figure S7 indicates both
conversion of CO2 and selectivity of methanol at 200 °C for 100
h. The CO2 conversion and methanol selectivity of the
4.8Cu/Mg-Al catalyst was unchanged after 100 h reaction,
confirming the very high stability of the catalyst. It was clear that
methanol production and CO2 conversion is quite high over
4.8Cu/Mg-Al catalyst. The TEM images in Figure 2b shows no
change of copper particles size of 4.8Cu/Mg-Al during catalysis.
-1
0.011 and 0.010 mol gCu h-1, respectively at 200 °C and 0.012,
0.016, 0.013 and 0.011 mol gCu-1 h-1, respectively at 280 °C.
It was found that the activity in terms of the methanol formation
rate of the different catalysts is directly correlated with the Cu
surface area, and Cu particle size, as shown in Figure 6A. It is
clear that the Cu surface area and activity are strongly
associated where catalysts with large copper surface areas
show a high methanol production rate. It was found that the Cu
surface area increases expectedly with increasing metal
dispersion because the dispersion of metal is opposite to the
radius of a spherical particle 68. Figure 6A, the 2.3Cu/Mg-Al,
4.8Cu/Mg-Al, 7.3Cu/Mg-Al, and 9.6Cu/Mg-Al catalysts exhibited
19.4, 18.7, 14.8, and 11.8% metal dispersion with the 21.6, 20.8,
17.1, and 14.2 m2g-1 Cu surface area, respectively (Table 2).
6
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DFT calculation for hydrogenation of CO2 to methanol
The single Cu atom binds to both the surface oxygen and Al
atoms of γ-Al2O3 (100), as shown in Figure 9(a). The Cu-O bond
in the Cu/γ-Al2O3 (100) was calculated to be 2.06Å, whereas the
Cu-Al bond length was calculated to be 2.60Å (Figure 9(a)). The
Cu4 nanocluster also binds to the γ-Al2O3(100) surface forming
two Cu-O bonds (2.07Å and 2.01Å) and one Cu-Al bond (2.53 Å),
as shown in Figure 9(b). The bigger Cu13 cluster form multiple
Cu-O bonds with the γ-Al2O3 (100) surface, as shown in Figure
9(c). The four Cu-O bonds in the Cu13/γ-Al2O3 (100) were
calculated to be 2.23Å, 2.13Å, 2.11Å, and 2.10Å, and the Cu-Al
bond calculated 2.55 Å, as shown in Figure 9(c).
The geometry optimized structures of the MgO (100) and γ-Al2O3
(100) surface slabs were shown in Figure 7. The oxygen
terminated γ-Al2O3 (100) surface was found to be more stable
compared to the Al terminated γ-Al2O3 (100) surface. CO2
molecule binds weakly to both the MgO (100) and γ-Al2O3 (100)
surfaces, -49.4 kJ/mol, and -35.5 kJ/mol, respectively (Figure 7).
Preferable adsorption at the MgO (100) surface indicates higher
basicity of the MgO support. The CO2 TPD spectra, shown in
Figure 3B, also show similar trends as observed from DFT.
Figure 7. Adsorption geometry of CO2 over the (a) MgO (100) and (b) γ-Al2O3
(100) surfaces. Color code: Mg (green), Al (pink), O (red) and C (black).
Figure 9. DFT optimized geometry of Cu, Cu4, and Cu13 nanoclusters
adsorbed at the γ-Al2O3 (100) surface. Color code: Al(pink), O (red) and Cu
(marron).
To understand the high metal dispersion and high stability
against agglomeration at high temperatures, the adsorption of
Cu atom and three small Cu clusters (Cu1, Cu4 and Cu13) were
studied over the MgO (100) and γ-Al2O3 (100) surfaces. DFT
optimized geometry of Cu, Cu4, and Cu13 clusters adsorbed on
the MgO (100) surface is shown in Figure 8. The single Cu atom
binds to the surface oxygen atom of MgO (100) at the on-top
position, as shown in Figure 8(a). The Cu-O bond in the Cu/MgO
(100) was calculated to be 2.17Å. The Cu4 nanocluster also
binds to the MgO (100) surface in a similar configuration forming
two Cu-O bonds (2.17Å and 2.0Å), as shown in Figure 8(b). The
bigger Cu13 cluster forms multiple Cu-O bonds with the
MgO(100) surface, as shown in Figure 8(c). The geometry of
Cu13 nanocluster remained intact upon adsorption at the MgO
(100) surface. The Cu-O bonds in the Cu13/MgO (100) were
calculated to be (2.11Å, 2.12Å, 2.12Å, and 2.19Å), as shown in
Figure 8(c).
The Cu single atom and both the Cu4 and Cu13 nanoclusters
bind very strongly to the MgO (100) surface, as can be observed
from the high adsorption energy, shown in Figure 10A. The
adsorption energy of Cu, Cu4, and Cu13 clusters were calculated
to be -71.7 kJ/mol, -201.9 kJ/mol, and -179.7 kJ/mol,
respectively (Figure 10A). The Cu4 nanocluster binds
comparatively stronger compared to a single Cu atom and a
bigger Cu13 cluster. The weaker adsorption energy of Cu13
nanocluster can be attributed to the high stability of Cu13 cluster
in gas-phase due to the presence of magic number Cu atoms.
However, the adsorption energy per atom was found to
decrease monotonically as the cluster size increases Cu (-71.7
kJ/mol) > Cu4 (-50.5 kJ/mol) > Cu13 (-13.8 kJ/mol) (Figure 10A).
Due to the strong metal-support interaction in the Cu/MgO
catalyst, the small catalyst nanoparticle will be stable against
agglomeration, as observed experimentally.
Figure 8. DFT optimized geometry of Cu, Cu4, and Cu13 nanoclusters
adsorbed at the MgO (100) surface. Color code: Mg (green), O (red), and Cu
(marron).
Figure 10. (A) Adsorption energy of Cu, Cu4, and Cu13 nanoclusters adsorbed
at the MgO (100) surface and (B) Adsorption energy of Cu, Cu4, and Cu13
nanoclusters adsorbed at the γ-Al2O3 (100) surface.
Similarly, the adsorption of Cu atom and Cu nanoclusters were
studied over the γ-Al2O3 (100) surface, as shown in Figure 9.
7
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Similar to the MgO (100) surface, the Cu single atom and both
the Cu4 and Cu13 nanoclusters bind very strongly to the γ-Al2O3
(100) surface, as shown in Figure 10B. The adsorption energy of
Cu and Cu4 clusters were calculated to be -91.6 kJ/mol and -
229.1 kJ/mol, respectively (Figure 10B), which are comparatively
higher compared to the adsorption energy values obtained over
the MgO (100) surface. The corrugated nature of the γ-Al2O3
(100) surface may be the reason for the high adsorption of the
small Cu cluster. However, the bigger Cu13 cluster adsorbed
weaker (-168.5 kJ/mol) at the γ-Al2O3 (100) surface compared to
the Cu13 cluster adsorption at the MgO (100) surface (-179.7
kJ/mol). Similar to the trends observed over the MgO (100)
surface, over the γ-Al2O3 (100) surface the Cu4 nanocluster
binds comparatively stronger compared to single Cu atom and
bigger Cu13 clusters. Similarly, the adsorption energy per atom
was found to decrease monotonically as the cluster size
increases Cu (-91.6 kJ/mol) > Cu4 (-57.3 kJ/mol) > Cu13 (-13.0
kJ/mol) (Figure 10B). The higher adsorption energy of the Cu13
cluster at the MgO (100) compared to the γ-Al2O3 (100) surface
is due to the higher basicity of the MgO system compared to γ-
Al2O3. Due to higher basicity, the electron transfer from the MgO
to the Cu13 cluster is higher compared to Lewis acidic γ-Al2O3
surface, which makes the Cu-O(support) interaction much stronger.
Though the agglomeration of the Cu clusters is
thermodynamically still favorable over MgO/γ-Al2O3 support, due
to the strong metal-support interaction in the Cu/MgO/γ-Al2O3
catalyst will be less energetically favorable compared to the gas-
phase agglomeration, as shown in Figure S10. Similar
observation was made by Wang et al. for growth of Cu particle
on γ-Al2O3 surface 74. The strong metal-support interaction in the
Cu/MgO/γ-Al2O3 catalyst also stabilizes the small catalyst
nanoparticle against diffusion at the catalyst surface also may
reduce the tendency for forming agglomerate, as observed
experimentally.
through two distinct intermediates, the carboxylate (COOH*) and
formate (HCOO*) surface intermediates. The adsorption
geometry of the COOH* and HCOO* intermediates were shown
in Figure 11(b) and 14(c). Similar to CO2 adsorption, Both the
COOH* and HCOO* intermediates bind preferably to the
Cu13/MgO (100) interface forming bond with both the Cu13
nanocluster and MgO (100) surface. The C-Cu and O-Mg bond
distances for the COOH* adsorption (Figure 11(b) were
calculated as 1.99Å and 2.21Å. Whereas for the HCOO*
adsorption the C-Cu, O-Mg and O-Cu bond distances (Figure
11(c) were measured to be 2.13Å, 2.19Å and 2.04Å,
respectively. The high selectivity observed in this reaction
possibly indicate that the formate pathway is preferable here.
The main side product CO can be formed from the CO-OH bond
dissociation of COOH intermediate, whereas the formate
pathway is not known to produce CO. As can be observed in the
formation energy values given in Table 4, the formation HCOO*
surface intermediate (-257.2 kJ/mol) is more favorbale
compared to the COOH* intermediate (-131 kJ/mol) by nearly -
126 kJ/mol at the Cu13/MgO (100) active site. Similar
observations also made for the Cu13/γ-Al2O3 (100) active site,
however, the adsorption energies of both the HCOO* and
COOH* intermediate were much weaker compared to the
Cu/MgO (100) site. The preferable formation of the HCOO*
intermediate over the COOH* intermediate at the Cu/MgO (100)
active site may be the reason for the high methanol selectivity
seen of the Cu/MgO/γ-Al2O3 catalyst.
Table 4. Adsorption energies (Eads) of surface intermediates CO2, H, COOH,
HCOO at the Cu13/MgO (100), and Cu13/γ-Al2O3 (100) catalyst interface. The
gas-phase CO2 and H2 were used as a reference.
Eads (kJ/mol)
CO2*
Cu13/MgO (100)
-120.9
Cu13/γ-Al2O3 (100)
-64.2
-85.7
-89.9
-197.8
H*
-130.4
COOH*
HCOO*
-131.0
To understand the activity and selectivity trends of the Cu/MgO/
γ-Al2O3 catalyst, DFT method was used to calculate the
formation energies of surface adsorbed intermediate CO2*,
COOH*, HCOO* and H* at the Cu13/MgO (100) and Cu13/γ-Al2O3
(100) metal/support interface, as shown in Figure 11 and Figure
12. The adsorption of the CO2 molecule at the Cu13/MgO active
site (-120.9 kJ/mol) is found to be preferable by ~ 57 kJ/mol
compared to the Cu13/γ-Al2O3 (100) active site (-64.2 kJ/mol).
The basic nature of the MgO support enhances the adsorption of
CO2 molecules evident from the above observation. The CO2
molecule bind to the Cu13/MgO (100) interface by forming bonds
with both the Cu13 nanoparticle as well as the MgO support, as
shown in Figure 11 (a). The C-Cu(1), C-Cu(2), O-Mg and O-Cu
bond distances were calculated as 2.05Å, 2.39Å, 2.12Å and
2.08 Å, respectively, (Figure 11(a)). This is not the case for the
CO2 adsorption at the Cu13/γ-Al2O3 (100) active site, where the
CO2 only formed a bond with the Cu13 metal cluster (Figure
12(a)). The C-Cu and O-Cu bond distances were calculated as
2.11Å and 2.07Å, respectively (Figure 12(a)). Similar trends
were also observed for the adsorption energy of the H atom,
which preferentially binds to the Cu13/MgO (100) active site (-
130.4 kJ/mol) compared to the Cu13/γ-Al2O3 (100) active site (-
85.7 kJ/mol). The favourable adsorption energy trends for the
CO2 and H at the Cu13/MgO (100) interface indicate Cu/MgO
(100) to be the prominent active site for this reaction. Formation
of methanol from the hydrogenation of CO2 mainly commence
-257.2
In this study, we have used energies of CO2(g) and H2(g) as
reference gas-phase energies to calculate the adsorption
energies of HCOO* and COOH*, as given in equation 3 in DFT
75
Method (supporting information). Previous studies by Liu et al.
,
Marvikakis et al. 76, Liu et al. 77, the authors have used the gas-
phase energies of the intermediate species HCOO(g) and
COOH(g) for the calculation of the adsorption energies of their
respective adsorbed species. To compare with the literature, we
have calculated the adsorption energies of H*, COOH* and
HCOO* relative to their gas-phase species. The adsorption
energies obtained in this study (given in the Table S4) are
75
comparable to the adsorption values obtained by Liu et al. at
the Custrip-ZrO2 interface. The adsorption energies obtained for
the adsorbate species CO2*, H*, COOH* and HCOO* over the
Cu13/MgO (100) are higher compared to the values obtained by
75
Liu et al.
at the Custrip/ZrO2 interface, as can be seen in the
table S4. At the Cu13/γ-Al2O3 (100) surface, the CO2 adsorption
energy was found to be comparable to the value obtained by Liu
et al. 75, whereas the adsorption energies for COOH* was
weaker by 30 kJ/mol at the Cu13/γ-Al2O3 (100) surface. The
adsorption energies of H* and HCOO* were calculated to be
higher at the Cu13/γ-Al2O3 (100) surface compared to the
Custrip/ZrO2 interface studied by Liu et al. 75 as given in Table S5
(supporting information). The co-adsorption of H with surface
8
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adsorbate CO2, COOH and HCOO were studied as shown in
Figure S8 (supporting information). The adsorption energy of co-
adsorbed H+CO2 was calculated to be -135.8 kJ/mol (Table S5),
which is much smaller compared to the sum of the adsorption
energy of H (-130.4 kJ/mol, Table 4) and CO2 (-120.9 kJ/mol,
Table 4). Similarly, the adsorption energy of co-adsorbed
H+COOH (-117.3 kJ/mol, Table S5) and H+HCOO (-235.9
kJ/mol, Table S5) were also calculated to be smaller compared
to the sum of the individual species adsorption energies. The
weaker adsorption energies calculated for the co-adsorbed state
can be attributed to the adsorbate-adsorbate interaction and
The CO2 molecule adsorbs at the Cu1/MgO (100) active site (-
51.4 kJ/mol, Table 5) forming bond with both the metal Cu and
Mg of MgO support, as shown in Figure 13 (a). The C-Cu and O-
Mg bond distances were calculated as 2.00Å, and 2.26Å,
respectively (Figure 13(a)). Similar binding configuration was
also observed at the Cu4/MgO (100) active site, where the CO2
formed two Cu-C bonds and a Mg-O bond (Figure 13(e)). The C-
Cu(1), C-Cu(2) and O-Cu bond distances were calculated as
2.02Å, 2.17 Å and 2.24Å, respectively (Figure 13(e)). The
binding energy of CO2 at the Cu4/MgO (100) active site was
calculated to be -72.4 kJ/mol (Table 5), which is ~ 20 kJ/mol
stronger compared to Cu1/MgO (100) active site, but ~ 50 kJ/mol
weaker compared to Cu13/MgO (100) active site. The adsorption
energy of CO2 increases following the trend: Cu1/MgO (100) <
Cu4/MgO (100) < Cu13/MgO (100). The adsorption of H at the
Cu1/MgO (100) active site was found to be on-top of the Cu
atom (Cu-H bond distance 1.50 Å), as shown in Figure 13(d).
The adsorption geometry of H was found to be bridging in
Cu4/MgO (100) active site, forming two Cu-H bonds (1.69Å and
1.64Å), as shown in Figure 13(h). The adsorption energy of H at
the Cu1/MgO (100) and Cu4/MgO (100) active sites were
calculated to be -117.5 kJ/mol and 8.2 kJ/mol, respectively
(Table 5). The hydrogenation of adsorbed CO2 will give two
important surface intermediates COOH* and HCOO*. The DFT
optimized geometries of COOH* and HCOO* intermediates at
the Cu1/MgO (100) active site were shown in Figure 13(b) and
14(c), respectively. The COOH* intermediate bind only to the Cu
atom at the Cu1/MgO (100) interface (Cu-C bond length 1.89Å),
whereas the HCOO* intermediate bind to both the Cu atom and
MgO (100) surface, as shown in Figure 13(c), forming C-Cu and
O-Mg bonds (1.91Å and 2.22Å, respectively). As can be
observed in the formation energy values given in Table 5, the
formation HCOO* surface intermediate (-221.5 kJ/mol, Table 5)
is more favorbale compared to the COOH* intermediate (-143.8
kJ/mol, Table 5) by nearly -77 kJ/mol at the Cu1/MgO (100)
active site. Similar observations also made for the Cu4/MgO
(100) active site, where the formation HCOO* surface
intermediate (-114.1 kJ/mol, Table 5) is more favorbale
compared to the COOH* intermediate (-6.7 kJ/mol, Table 5) by
nearly -107 kJ/mol, however the adsorption energies of both the
HCOO* and COOH* intermedia were much weaker compared to
the Cu1/MgO (100) site. The COOH* and HCOO* intermediates
bind both to the Cu atom and the MgO support as shown in
Figure 13(f) and 14(g), respectively. The C-Cu and O-Mg bonds
for the COOH* intermediate at the Cu4/MgO (100) interface was
measured to be 1.96Å and 2.15Å (Figure 13(f)). The HCOO*
intermediate bind to the Cu4 cluster forming two O-Cu bonds
(2.15Å and 2.01Å) and also bind the MgO support forming O-Mg
bond (2.31Å), as shown in Figure 13(g). The preferable
formation of the HCOO* intermediate over the COOH*
intermediate at the all the three surfaces with different Cu cluster
Cu1/MgO (100), Cu4/MgO (100) and Cu13/MgO (100) active sites
further indicate to the high methanol selectivity seen of the
Cu/MgO/γ-Al2O3 catalyst.
crowding of the adsorbate in the small Cu13 metal cluster 78-81
.
Figure 11. Adsorption geometry of CO2, COOH, HCOO, and H surface
intermediates at the Cu13/MgO (100) active site. Color code: Mg (green), O
(red), C (black), H (white) and Cu (marron).
Figure 12. Adsorption geometry of CO2, COOH, HCOO and
H surface
intermediates at the Cu13/γ-Al2O3 (100) active site. Color code: Al (pink), O
(red), C (black), H (white) and Cu (marron).
To understand the effect of Cu nanocluster size, additional DFT
calculation were done to obtain the adsorption energies of CO2,
COOH, HCOO and
H
surface intermediates over the
Cu1/MgO(100) and Cu4/MgO(100) active site. The DFT obtained
geometry optimized structures of CO2, COOH, HCOO and H
adsorbed at the Cu1/MgO(100) and Cu4/MgO(100) surface
shown in Figure 13.
Figure 13. Adsorption geometry of CO2, COOH, HCOO, and H surface
intermediates at the (a-d) Cu1/MgO(100) and (e-f) Cu4/MgO(100) active sites.
Color code: Mg (green), O (red), C (black), H (white) and Cu (marron).
9
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Table 5. Adsorption energies (Eads) of surface intermediates CO2, H, COOH,
HCOO at the Cu1/MgO (100), and Cu4/MgO (100) catalyst interface. The gas-
phase CO2 and H2 were used as the reference state.
the carboxylate (COOH*) route, high selectivity for methanol was
observed when MgO was added to the Cu/γ-Al2O3 catalytic
system.
Eads (kJ/mol)
CO2*
Cu1/MgO (100)
-51.5
Cu4/MgO (100)
-72.4
Experimental Section
H*
-117.5
8.2
COOH*
HCOO*
-143.9
-6.71
Catalysis synthesis
-221.5
-114.1
Details of materials and methods used in the catalysts synthesis
and, catalyst characterization details are presented in the
supporting information.
Previous studies have in general shown the CO2 hydrogenation
barrier to be smaller for HCOO formation in comparison to
COOH formation. Huš et al.82 calculated the activation barrier for
CO2*+H*
→ HCOO* over four different catalyst surfaces
Hydrogenation of CO2 to methanol
Zn3O3/Cu, Mg3O3/Cu, Cr3O3/Cu and Fe3O3/Cu to be 56.9 kJ/mol,
45.4 kJ/mol, 68.5 kJ/mol and 11.6 kJ/mol which were smaller
compared to the CO2*+H* → COOH*, which were 68.5 kJ/mol,
73.3 kJ/mol, 84.9 kJ/mol and 61.8 kJ/mol. Similarly, Zhang et
al.83 calculated the activation barrier for CO2 hydrogenation to
HCOO to be 93.1 kJ/mol over Cu4/Al2O3 surface, whereas the
activation barrier for COOH formation calculated to be higher
(212.2 kJ/mol). Similar activation energies were also observed
by Grabow et al.76 over Cu(111) surface where the HCOO*
formation activation barrier (84.0 kJ/mol) was found to be
smaller compared to COOH* formation activation barrier 171.8
kJ/mol. Frei et al.84 also found the COOH* formation to have a
higher barrier compared to the HCOO* formation.
The activity measurement of CO2 hydrogenation to methanol
over Cu/Mg-Al catalyst was conducted in a continuous fixed-bed
tubular reactor (Figure 14). 0.5 g of the catalyst was loaded into
the reactor, and a K-type thermocouple was inserted at the
centre of the furnace. Before each reaction, the catalyst was
initially treated (heating rate 2 C/min) under a flow of 10 ml/min
H2 (10% H2 in Helium) at 250 °C for 2 h under atmospheric
pressure. The reactor was allowed to cool down to 190 °C, and
a gas mixture of CO2/H2/N2 = 1: 3: 1 (flow rate:60 ml/min) was
passed over the catalyst bed in the reactor to reach the reactor
pressure of 3.0 MPa. The corresponding weight hourly space
velocity (WHSV) was 7200 ml. gCat-1. h-1, and the flow of
gaseous mixture was passed over the catalyst bed for 18 hours
at 200 °C. During this time, steady-state of the reaction was
reached. Subsequently, the products were quantitatively
analyzed by using online gas chromatography (Agilent 7890A),
equipped with a 0 0er98765.two-column system connected to a
thermal conductivity detector (TCD) using porapack-Q column
(for analyzing H2, N2, CO, CO2, and H2O), and HP-Plot Q (for
analyzing CH3OH and CH4). In all the experiments, CH4
concentration in the outlet gas mixture was found to be nil.
Carbon balance was estimated from the conversion of CO2 and
selectivity of CH3OH, CH4, and CO using N2 as the internal
standard. The material balance and carbon balance were
calculated with an accuracy of ± 3% (between 97% and 103%).
Conclusion
Nanocrystalline Cu/Mg-Al oxide catalysts were prepared by the
sol-gel method, where ~ 5wt% Cu was the optimum loading
showing the formation of copper nanoparticles with size ~ 5 nm
with high Cu surface area, and dispersion, which directly
influence the catalytic performances. Smaller particles consist of
significantly more active Cu species, showing very high Cu
dispersion favouring higher adsorption of CO2, giving > 99%
methanol selectivity. H2-TPR shows that the metal support
interaction is present for 5 wt% Cu supported Mg-Al catalyst,
which favours the stability of active Cu particles against
sintering. 5 wt% Cu supported Mg-Al catalyst with a methanol
formation rate of 0.016 mol/gCu /h. DFT studies also revealed
that strong metal-support interactions in Cu/MgO/γ-Al2O3
catalysts would stabilize small catalytic nanoparticles against
agglomeration. The catalyst was very stable and did not show
any deactivation even after 100 h time on stream. In Addition,
DFT studies also revealed that the weaker adsorption energy (-
179.7 kJ/mol) of Cu13 nanocluster could be attributed to the high
stability of Cu13 cluster in gas-phase due to the presence of
magic number Cu atoms. The nanostructure catalyst will be
stable against agglomeration due to the strong metal-support
interaction in the Cu/MgO/γ-Al2O3 catalyst. As shown in the DFT
study, in addition to the increase in CO2 adsorption capacity,
MgO was found to play the key role in the highly selective
production of methanol. DFT calculation showed that the
Cu/MgO (100) catalyst system highly favors the HCOO* route
compared to the COOH* route. On the Cu13/MgO (100) surface,
the HCOO* intermediate was found to be nearly 130 kJ/mol
more stable compared to the COOH* intermediate. The
difference is only ~ 110 kJ/mol on the Cu13/γ-Al2O3(100) surface.
Due to the highly favorable formate (HCOO*) route compared to
The CO2 conversion (XCO2), selectivity for methanol (SCH3OH
)
were calculated according to the following equations:
Figure 14. Schematic diagram of continuous fixed bed reactor set-up for CO2
hydrogenation to methanol production.
10
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Acknowledgements
SKS acknowledges the Petrotech Society of India for providing a
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between Bharat Petroleum Corporation Ltd. (BPCL) and CSIR-
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Keywords: CO2 hydrogenation • Methanol • Sol-gel method •
Cu-nanoparticles • DFT-study
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Table of Contents
Copper nanoparticles size supported on Mg-Al supported catalyst for CO2
hydrogenation to methanol with high methanol production rate with the range
at 200-280 °C temperature.
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14
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