Hydroxyl-mediated ethanol selectivity of CO2 hydrogenation

Article abstract of DOI:10.1039/c8sc05608k

Oxide-supported Rh nanoparticles have been widely used for CO₂ hydrogenation, especially for ethanol synthesis. However, this reaction operates under high pressure, up to 8?MPa, and suffers from low CO₂ conversion and alcohol selectivity. This paper describes the crucial role of hydroxyl groups bound on Rh-based catalysts supported on TiO₂ nanorods (NRs). The RhFeLi/TiO₂ NR catalyst shows superior reactivity (≈15% conversion) and ethanol selectivity (32%) for CO₂ hydrogenation. The promoting effect can be attributed to the synergism of high Rh dispersion and high-density hydroxyl groups on TiO₂ NRs. Hydroxyls are proven to stabilize formate species and protonate methanol, which is easily dissociated into *CH_x, and then CO obtained from the reverse water-gas shift reaction (RWGS) is inserted into *CH_x to form CH₃CO*, followed by CH₃CO* hydrogenation to ethanol.

Full text of DOI:10.1039/c8sc05608k

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Hydroxyl-mediated ethanol selectivity of CO2 hydrogenation
Chengsheng Yang1, Rentao Mu1, Guishuo Wang, Jimin Song, Hao Tian, Zhi-Jian Zhao, and Jinlong
Gong*
Received 00th January 20xx,
Accepted 00th January 20xx
Oxide-supported Rh nanoparticles have been widely used for CO2 hydrogenation, especially for the ethanol synthesis.
However, this reaction operates under high pressure up to 8 MPa and suffers low CO2 conversion and alcohols selectivity.
This paper describes the crucial role of hydroxyl groups bound on Rh-based catalysts supported on TiO2 nanorods (NRs). The
RhFeLi/TiO2 NRs catalyst shows superior reactivity (≈15% conversion) and ethanol selectivity (32%) for CO2 hydrogenation.
The promoting effect can be attributed to the synergism of high Rh dispersion and high-density hydroxyl groups on TiO2 NRs.
Hydroxyls are proven to stabilize formate species and protonate methanol, which is easily dissociated into *CHx, and then
*CHx is inserted by CO obtained from the reverse water-gas shift reaction (RWGS) to form CH3CO*, followed by CH3CO*
hydrogenation to ethanol.
DOI: 10.1039/x0xx00000x
www.rsc.org/
For example, 5 wt% RhFe10 and RhLi11 supported on SiO2
showed ethanol selectivities of 16.4% and 15.5%, respectively.
However, there are still some limitations for the alcohols
Introduction
Carbon dioxide (CO2) is one of the major components of
greenhouse gases, which can result in climate change and ocean
acidification. Among the different approaches explored for
controlling CO2 emission, the chemical conversion of CO2 to
high-value-added fuels (oxygenates, alcohols, and olefin et al.)
has attracted extensive attention.1-4 Compared with C1 product
(CO, CH4 and CH3OH), the higher alcohols (C2+OH, especially
C2H5OH), which are mostly produced from biological
fermentation, have been widely applied in industries as
indispensable higher-energy-densities engine fuel and fuel
additive.1 According to the thermodynamic analysis, the
formation of ethanol from CO2 is limited enormously at 1-30 bar
due to the preferential production of CO or CH4, thus the
selectivity of ethanol is relatively low.4
Therefore, the production of C2+OH by CO2 hydrogenation is
appalling but remains grand challenging. Previous studies have
shown that a Pt/Co3O4 catalyst5 achieved 27.3% selectivity of
C2+OH with H2O/DMI as a solvent. Multi-functional composite
catalysts, such as CoMoS6 (5.5% selectivity of ethanol),
physically mixed Fe-based and Cu-based catalysts7 (17.4%
selectivity of ethanol) and K/Cu-Zn-Fe catalysts8,9 (19.5%
selectivity of C2+OH and CH3OH) were also used for alcohols
synthesis. Particularly, Rh-based catalysts have been evaluated
as promising catalysts for the selective synthesis of ethanol.4,10-
11 In general, promoters such as Fe and Li are frequently used in
enhancing ethanol selectivity via changing the electronic state
of Rh and increasing the intensity of bridged-bond CO species.
production through CO2 hydrogenation reaction, such as the
difficulties in CO2 activation, high energy barrier for C-O bond
scission and the formation of C1 by-products.1 Therefore, the
design of efficiently heterogeneous catalysts for ethanol
production is of great importance. Tuning the particle size of
noble metals can often increase the CO2 conversion and product
selectivity.12-16 For example, the suitable reducible metal oxide
supports, such as TiO2, ZrO2, have been extensively applied to
tune the particle size.17-18 The Au/TiO2 catalyst with abundant
oxygen vacancies exhibited high selectivity to ethanol from CO2
reduction in DMF solvent.19 Bimetallic Pd2Cu/P25 catalyst also
presented the excellent yield of ethanol with the help of
water.20 On the other hand, promotion strategy via hydroxyl
groups has been also proved to be an efficient approach
towards improving alcohols selectivity in CO hydrogenation.21-
23
This work shows that the high yield of ethanol under low
pressure can be achieved by the introduction of hydroxyls onto
TiO2 support. Firstly, 1wt% RhFeLi (Rh:Fe:Li =1:1:1) supported
on a series of reducible oxides were prepared. The catalysts
supported on TiO2 nanorods (NRs) display highest selectivity of
ethanol. Since TiO2 NRs have been extensively used in a variety
of catalytic systems, such as photocatalytic water splitting, CO2
photoreduction and dissociation of CO2 to CO,24-26 we
synthesized TiO2 NRs by the modified hydrothermal method.
More importantly, high-density surface hydroxyls can be
introduced to the catalytic system after reduction of TiO2 NRs-
supported catalysts in H2. A significant improvement of ethanol
yield is observed over Rh-based catalysts supported on TiO2
NRs, which have not been reported in the previous works.
Furthermore, the hydroxyl-mediated mechanism of ethanol
formation over RhFeLi/TiO2 NRs catalysts is investigated.
Key Laboratory for Green Chemical Technology of Ministry of Education, School of
Chemical Engineering and Technology, Tianjin University; Collaborative Innovation
Center of Chemical Science and Engineering, Tianjin 300072, China. Email:
jlgong@tju.edu.cn
†
Electronic Supplementary Information (ESI) available: Details in Experimental
section and supporting figures and tables. See DOI: 10.1039/x0xx00000x
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shows that FeOx species are well dispersed on TiO2 NRs (Fig. 1g).
View Article Online
DOI: 10.1039/C8SC05608K
We note that no Raman shift of TiO2 can be observed for all
catalysts (Fig. S2†), indicating FeOx species are deposited on
surface rather than doping into TiO2 bulk. H2 temperature-
programmed reduction (H2-TPR) results of FeOx/TiO2 show that
Results and discussion
Catalyst structure
the reduction temperature of FeOx over TiO2 NRs is 100
than the reduction temperature of FeOx over TiO2 Com (Fig. S3a
), suggesting FeOx species are better dispersed on TiO2 NRs
̊C higher
The morphology of synthesized TiO2 NRs is shown in Fig. 1a and
Fig. S1.† The length and diameter of TiO2 NRs are 50-200 nm
and 10-20 nm, respectively. The specific surface area of TiO2
NRs is determined to be 23.6 m2·gcat-1, which is close to that of
commercial TiO2 (TiO2 Com, 18.2 m2·gcat-1) (Table S1
†). The Rh-
†
compared with TiO2 Com.
H2-TPR studies of RhFeLi/TiO2 NRs and RhFeLi/TiO2 Com were
also carried out to investigate the interfacial interaction
based catalysts supported on TiO2 NRs and TiO2 Com were
prepared by incipient wetness impregnation. The Rh
nanoparticles on TiO2 NRs present uniform size distribution with
an average diameter of 2.3 ± 1.0 nm (Fig. 1a, c, e). The high-
resolution images of Rh nanoparticles in Fig. 1c and 1d present
a lattice distance of 0.23 nm, corresponding to Rh (111)
planes.27 In contrast, two-times-larger (~ 4.0 nm) Rh
nanoparticles in the range of 1-7 nm are observed on TiO2 Com
(Fig. 1b, d, f). TEM studies indicate TiO2 NRs can prevent the Rh
nanoparticles from severe agglomeration after the reduction at
between Rh and oxide promoters (Fig. S3b
previous studies,10 the peaks below 200
C can be ascribed to the
reduction of Rh2O3, while the broad peak appearing at higher
temperature (300-500 C) can be assigned to the reduction of
Fe2O3. We find that the reduction temperature of Rh2O3 for
RhFeLi/TiO2 NRs is 50 C higher than that on RhFeLi/TiO2 Com,
illustrating smaller Rh size on TiO2 NRs.14 As such, the higher
dispersion of Rh-based nanoparticles on TiO2 NRs should
increase the number of interfacial sites between Rh and oxide
promoters, where C-C coupling occurs via reaction between CO
and *CHx.5-7
†). According to
̊
̊
̊
400 ̊C in H2 atmosphere.
In XRD measurements, no characteristic peak of Fe2O3 can be
found over RhFeLi/TiO2 Com and RhFeLi/TiO2 NRs when the
loading of Fe is relatively low (~ 1 wt%) (Fig. 2). When the
loading of Fe is increased to 2.5 wt%, the diffraction peak of
Fe2O3 at 33.2° can be seen on TiO2 Com. However, diffraction
peak of Fe2O3 does not appear on TiO2 NRs even though the
loading of Fe is increased to 5 wt% (Fig. 2). XRD results indicate
that the dispersion of FeOx over TiO2 NRs is higher than that
over TiO2 Com. EDS elemental mapping of RhFeLi/TiO2 NRs also
To further illustrate the surface structure of RhFeLi/TiO2 NRs
and RhFeLi/TiO2 Com catalysts, CO titration experiments were
conducted. The active loop volume of CO for RhFeLi/TiO2 Com
(0.09 cm3/g), which is consumed by CO adsorption on Rh, is
much smaller than that for RhFeLi/TiO2 NRs (0.69 cm3/g) (Table
S2
† ). The observed low adsorption amount of CO on
RhFeLi/TiO2 Com may be due to the partially encapsulation of
Rh sites by oxide overlayers. TEM image also shows the Rh
nanoparticles are decorated by oxide overlayers over
RhFeLi/TiO2 Com catalyst (Fig. 1d). Quantitative XPS analysis
was also conducted to investigate the surface structure of Rh-
based catalysts (Table S3†). The surface molar ratio of Rh:Fe
Fig. 1. (a, b) TEM; (c, d) HRTEM images, particle size distributions (Inset figures)
and (e, f) HAADF-STEM images of catalysts after reaction. 2.5 wt% RhFeLi
supported on TiO2 NRs (a, c, e), 2.5 wt% RhFeLi supported on TiO2 Com (b, d, f).
(g) STEM-EDS elemental map of 2.5 wt% RhFeLi supported on TiO2 NRs.
Fig. 2. (a) XRD spectra of 2.5 wt% RhFeLi/TiO2 NRs with different Fe loadings. (b)
XRD spectra of 2.5 wt% RhFeLi/TiO2 Com with different Fe loadings. The Figures
on right of (a) and (b) show the enlarged XRD patterns.
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over RhFeLi/TiO2 NRs is determined to be 58:42, which is close
to the bulk molar ratio of Rh:Fe measured by ICP-AES (48:52)
and the initial feed ratio. For RhFeLi/TiO2 Com, XPS
investigations show that the surface molar ratio of Rh:Fe is
29:71. However, the bulk molar ratio of Rh:Fe determined by
ICP-AES (Rh:Fe = 50:50) still agrees with the initial feed ratio,
suggesting the Rh nanoparticles should be partially covered by
FeOx species. Note that the binding energy (BE) of Rh 3d5/2
reactivity and long-term stability of RhFeLi/TiO2 NRs catalysts
provide an inspiration for their potentially industrial
application.
Promotion effects of the hydroxyl
View Article Online
DOI: 10.1039/C8SC05608K
Firstly, the hydroxyl groups were introduced by pre-reduction
of RhFeLi/TiO2 NRs and RhFeLi/TiO2 Com catalysts in H2
atmosphere at 400
experiments were carried out to characterize the surface
hydroxyl groups on RhFeLi/TiO2 catalysts. As shown in Fig. S8
̊C. The fourier transform infrared (FTIR)
†
,
locates at ~ 307.0 eV (Fig. S4b
state of Rh.28
†), corresponding to the metallic
the broad band at 3450 cm-1 and the sharp peak at 1640 cm-1
are assigned to the stretching and bending vibrations of
associative hydroxyls, respectively.29 The density of hydroxyl
groups on RhFeLi/TiO2 NRs is much higher than that on
RhFeLi/TiO2 Com (Fig. 4a), suggesting that high-density hydroxyl
groups could be introduced to catalytic system by using TiO2
NRs. Subsequently, the catalytic properties of hydroxyls on TiO2
NRs is characterized by CO temperature-programmed
desorption (CO-TPD). A sharp signal peak of CO2 (m/z=44) at ~
Catalytic performance
The catalytic performance to CO2 hydrogenation is studied over
Rh-based catalysts supported on different oxide supports. The
TiO2 NRs-supported catalysts present highest ethanol yield (Fig.
S5†). Additionally, a series of Rh-based catalysts with different
promoters were synthesized. Compared with mono-component
Rh/TiO2 catalyst, the selectivity of ethanol over RhFe/TiO2 is
improved significantly (16% for RhFe/TiO2 Com and 25% for
0
̊
RhFe/TiO2 NRs in Fig. S6
CO2 conversion and the selectivity of ethanol and CH4 decrease
while the selectivity of CO increases (Fig. S7 ). Since FeOx could
†). With the loading of Fe increasing,
†
CO2 may origin from water gas shift process, in which the CO
adsorbed on Rh sites reacts with hydroxyl groups.29-30 However,
†
catalyze the reverse water-gas shift reaction (RWGS) to produce
CO, the changes of CO2 conversion and product distribution
indicate that the excess Fe species has a passive effect on CO2
conversion and ethanol synthesis by blocking the active Rh
sites.10 This blocking effect may be caused by the encapsulation
of Rh sites by FeOx species, which have been proven by XPS
measurements combined with CO chemisorption (Table S2†).
On the other hand, the addition of Li can increase the CO2
conversion of Rh/TiO2 by 5%, while the selectivity of ethanol
small CO2 peak at ~ 0
̊
pure TiO2 Com due to the lack of hydroxyls (Fig. 4c, Fig. S8a
†
The TOF of ethanol formed over RhFeLi/TiO2 NRs is
determined to be 0.12 h-1, which is much higher than that over
RhFeLi/TiO2 Com or RhFeLi/SiO2 (0.08 h-1) (Table S2
† ).
Therefore, we suggest that the surface hydroxyl may play an
important role in ethanol formation via CO2 hydrogenation. To
verify the role of hydroxyls on ethanol formation, the
RhFeLi/TiO2 NRs catalyst was pre-treated in CO atmosphere
(RhFeLi/TiO2 NRs-CO). The removal process of hydroxyl groups
does not change (Fig. S6c † ). Based on these results, we
conclude the addition of Fe can promote the ethanol selectivity,
while the addition of Li accelerates the CO2 conversion as
electronic promoter. As such, higher ethanol selectivity and CO2
conversion are obtained by adding binary promoters i.e., Fe and
Li (Fig. S6d†).
More interestingly, we show the ethanol selectivity and CO2
conversion over RhFeLi/TiO2 NRs are much higher than that
over RhFeLi/TiO2 Com at 250 ̊C (Fig. S7†). For example, 2.5 wt%
RhFeLi/TiO2 NRs (Rh:Fe:Li=1:1:1) catalyst presents more than
30% ethanol selectivity and 15% CO2 conversion, which is about
seven-fold higher for ethanol yield than 2.5 wt% RhFeLi/TiO2
Com (Fig. 3). The catalytic performance of RhFeLi/TiO2 NRs
keeps stable in 20 h stability test (Fig. S7 † ). The superior
Fig. 4. (a) The peak area of hydroxyls in FTIR normalized by SBET (×
103) over 2.5
wt% RhFeLi/TiO2 NRs, 2.5 wt% RhFeLi/TiO2 Com and 2.5 wt% RhFeLi/TiO2 NRs-CO.
(b) CO-TPD profiles of 2.5 wt% RhFeLi/TiO2 NRs. Pretreatment: H2 reduction at
400
reduction at 400 C
CO reduction at 350
̊
C. (c) CO-TPD profiles of 2.5 wt% RhFeLi/TiO2 Com. Pretreatment: H2
. (d) CO-TPD profiles of 2.5 wt% RhFeLi/TiO2 NRs. Pretreatment:
C.
Fig. 3. (a) The CO2 conversion (Grey) and the product selectivity of 2.5 wt%
RhFeLi/TiO2 NRs and 2.5 wt% RhFeLi/TiO2 Com. (b) The ethanol yield of Rh-based
catalyst supported on TiO2 NRs and TiO2 Com.
̊
̊
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in CO was monitored by in situ diffuse reflectance infrared
fourier transform spectroscopy (DRIFTS, Fig. S9a
).31 We find
that the IR peak of hydroxyl stretching vibrations (3450 cm-1)
disappears gradually in CO at 350 C. The hydroxyls can be
removed completely after ~ 12 minutes of CO-feeding (Fig. S9b
). After the pre-treatment in CO flow at 350 C, the CO2 peak at
~ 0 C isn’t observed in CO-TPD (Fig. 4d), and RhFeLi/TiO2 NRs-
CO catalysts with hydroxyl-deficient surface are prepared (Fig.
4a, Fig. S8b ). Compared with RhFeLi/TiO2 NRs reduced with H2,
View Article Online
DOI: 10.1039/C8SC05608K
†
̊
†
̊
̊
Fig. 6. (a) The absorbance intensity of hydroxls at 3600 cm-1 in DRIFTS of 2.5 wt%
RhFeLi/TiO2 NRs and 2.5 wt% RhFeLi/TiO2 Com versus time after switching the CO2+H2+Ar
̊C. (b) The absorbance intensity of formate at
1595 cm-1 in DRIFTS of 2.5 wt% RhFeLi/TiO2 NRs and 2.5 wt% RhFeLi/TiO2 Com versus
†
RhFeLi/TiO2 NRs-CO catalyst performs much lower CO2
conversion (4.7%) and produces almost no ethanol (Fig. 5a). The
(CO2:H2=1:3) flow to pure CO2 flow at 250
selectivity of CO reaches ~ 80% among the products and the time after switching the CO2+H2+Ar (CO2:H2=1:3) flow to pure CO2 flow at 250 ̊C.
selectivity of CH4 decreases from 53.9% to 9.6%. In TEM images
(Fig. S10†), RhFeLi/TiO2 NRs and RhFeLi/TiO2 NRs-CO catalysts
show similar size distribution. Besides, there is no BE shift of XPS
Rh 3d peak for RhFeLi/TiO2 NRs-CO compared with RhFeLi/TiO2
NRs (Fig. S11†, Table S4†). Therefore, the influences of size
effect and chemical state of Rh can be excluded. Instead, the
decrease of the selectivity of CH4 and ethanol should be
attributed to the lack of hydroxyls. When hydroxyl groups are
stretching region between 1650 and 1200 cm-1, the bands at
1520 and 1390 cm-1 are assigned to carbonate, while the rest
peaks may stem from adsorbed formate (1595, 1370 cm-1, Table
S5†
).32-34 The absorbance intensities of the dissociated hydroxyl
stretching vibrations at 3600 cm-1 and OCO asymmetric
stretching vibration at 1595 cm-1 in DRIFTS are used to
represent the amount of hydroxyls and formate, respectively.34
As shown in Fig. 6, the changes of the hydroxyl amount and the
formate amount are plotted as a function of time when
re-introduced by H2 exposure (Fig. 5b, Fig. S8b†), the promoted
catalytic performance is achieved (35% ethanol selectivity and
18% CO2 conversion), which is very similar with that over fresh
RhFeLi/TiO2 NRs reduced with H2. These results further indicate
hydroxyls play an important role in tuning product distribution
and promoting ethanol synthesis through CO2 hydrogenation.
In sequential experiments, the mixture of CO2 and H2
switching the CO2+H2 flow to pure CO2 flow at 250 ̊C. We find
that the amounts of hydroxyls and formate species over
RhFeLi/TiO2 NRs keep almost unchanged under pure CO2 flow
within 40 mins (Fig. 6, Fig. S12a, c
and formate adsorbed on RhFeLi/TiO2 Com disappears rapidly
within 20 min (Fig. 6, Fig. S12b, d ). We suggest that the
†). In contrast, the hydroxyls
†
abundant hydroxyl groups over RhFeLi/TiO2 NRs can stabilize
formate species, which has been proposed to be one of
intermediates for methanation via formate hydrogenation and
then scission of C-O in *CHx-O.18,35
In situ DRIFTS was further carried out to investigate the
catalytic role of hydroxyls in ethanol formation. In contrast to
RhFeLi/TiO2 Com, additional bands at 1470 cm-1 and 1746 cm-1
are observed on RhFeLi/TiO2 NRs under CO2+H2 atmosphere
(CO2:H2=1:3) are firstly introduced to the in situ cell at 250 ̊C,
followed by a switch to pure CO2 flow to investigate the stability
of hydroxyls and formate species. In CO2+H2 atmosphere
(CO2:H2=1:3) at 250 ̊
C, the bands at 3016, 2965 and 2880 cm-1 in
the νC-H region appear stemming from gaseous CH4 (3016 cm-1)
and adsorbed formate species respectively. In the O-C-O
(CO2:H2=1:3) at 250
̊C (Fig. 7a). The existence of the band at
35
36
1746 cm-1 has been reported for Rh/Al2O3 and Ru/Al2O3
catalysts, which can be attributed to adsorbed formyl (CHO*)
species. It is believed that the formation of CHO* is the rate-
limiting step of ethanol synthesis (Scheme S1).12 Also, CHO* is
more favored to be dissociated into *CHx in thermodynamics
than CO.21-23,37 As expect, significant amounts of *CH3 species
(1470 cm-1) are observed on the surface of RhFeLi/TiO2 NRs (Fig.
7a).17-18 Subsequently, these abundant adsorbed *CH3 species
on RhFeLi/TiO2 NRs can be inserted by CO, which may be
responsible for the high ethanol yield.38 Based on the above
analysis, a mechanism that hydroxyls stabilize the formate and
accelerate the scission of CHx-O to produce *CH3 species is
proposed (Scheme 1).
With increasing the calcination temperature from 400 to
600 ̊C, the normalized peak area of associative hydroxyl
vibration bands over RhFeLi/TiO2 NRs decreases gradually,
indicating the density of hydroxyls is decreased (Fig. 5c, Fig. S8c
). In addition, summed selectivity and TOF of CH4 and ethanol
Fig. 5. (a) CO2 conversion and product selectivity versus time obtained from 2.5
wt% RhFeLi/TiO2 NRs. CO reduction was conducted at 350
reduction in H2 was carried out at 400 C for 1 h. Reaction conditions: P = 30 atm,
T = 250 C
̊C for 0.5 h. Re-
̊
̊
, GHSV= 6000 h-1, CO2/H2 = 1/3. (b) The peak area of hydroxyls in FTIR
normalized by SBET (×103) over 2.5 wt% RhFeLi/TiO2 NRs pretreated under different
reduction atmospheres. (c) The peak area of hydroxyls in FTIR normalized by SBET
(×103) over 2.5 wt% RhFeLi/TiO2 NRs calcined at different temperatures.
†
show a downward trend with increasing the calcination
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Scheme 1. Schematic of CO2 hydrogenation over Rh-based catalyst with or without
hydroxyl groups on TiO2. The hydroxyls play an important role in accelerating the scission
of CHx-O* and promote the formation of ethanol.
temperature of TiO2 NRs (Table S2 and S6
nanoparticles show similar size distribution (Fig. S10
electronic state of Rh and Fe over TiO2 NRs calcined at various
temperatures (Fig. S11 , Table S4 ), the differences in catalytic
†
). Since the RhFeLi
†
) and same
†
†
performance should be attributed to the changes of surface
hydroxyls. It is noteworthy that the catalytic performance of
RhFeLi/TiO2 NRs-600 ̊C is comparable to that of RhFeLi/TiO2
Fig. 7. (a) In situ DRIFTS over 2.5 wt% RhFeLi/TiO2 NRs, 2.5 wt% RhFeLi/TiO2 Com and 2.5
wt% RhFeLi/TiO2 NRs-CO under CO2+H2+Ar (CO2:H2=1:3) atmosphere at 250 ̊C. (b) The
NRs-CO catalyst, because there are few hydroxyls on their
surfaces. Similarly, changing the support from TiO2 NRs to TiO2
Com can also generate hydroxyl-deficient surface, causing the
summed selectivity of CH4 and ethanol as a function of the peak area of hydroxyls
normalized by SBET of the samples obtained from 2.5 wt% RhFeLi supported on different
TiO2. (c) In situ DRIFTS over 2.5 wt% RhFeLi/TiO2 NRs, 2.5 wt% RhFeLi/TiO2 Com and 2.5
wt% RhFeLi/TiO2 NRs-CO after CH3OH+Ar adsorption followed by H2 adsorption at 250
(d) In situ DRIFTS over 2.5 wt% RhFeLi/TiO2 NRs, 2.5 wt% RhFeLi/TiO2 Com and 2.5 wt%
RhFeLi/TiO2 NRs-CO under CO+H2+Ar (CO:H2 = 1:2) atmosphere at 250 C.
̊C.
selectivity of ethanol and CH4 reduced largely (Table S6†). To
display the relationship between hydroxyl groups and catalytic
performance directly, we take the summed amount of CH4 and
ethanol as the total amount of *CH3, because these two
products stem from *CH3 hydrogenation and CO insertion,
respectively.39 As shown in Fig. 7b, the amount of *CH3 exhibits
linear correlation with the density of hydroxyls, illustrating
hydroxyls may accelerate the scission of C-O bond to form *CH3
species.
̊
Previous works have proposed that ethanol can be
synthesized by CO insertion into *CH3 species to form CH3CO*,
followed by CH3CO* hydrogenation.37-38 To verify this route of
ethanol formation, the DRIFTS of RhFeLi/TiO2 NRs and
RhFeLi/TiO2 NRs-CO were obtained after pre-treatment with
CH3OH and subsequent feeding with CO+H2 (CO:H2=1:1) at
The reactions of CH3OH and H2 over RhFeLi/TiO2 were
conducted as well to elucidate the role of hydroxyls. The DRIFTS
were obtained after pre-treatment with CH3OH and subsequent
250
̊
†
). Both gaseous and liquid products were
C (Fig. S13b ),
analyzed in the reaction of CH3OH+CO+H2 at 250
̊
†
and ethanol is the only C2+ product. Hence, the appearance of
methylene peak (2858 cm-1) over RhFeLi/TiO2 NRs indicates the
C-C coupling and formation of ethanol.43 However, the
methylene, i.e., ethanol is not formed over RhFeLi/TiO2 NRs-CO.
According to these data, the high ethanol selectivity over
RhFeLi/TiO2 NRs might be attributed to the high-density
hydroxyls, which enhances the C-O bond scission to produce
*CHx intermediates for the CO insertion.
Surface functionalization with hydroxyls is frequently applied
to promote the catalytic performance of catalysts.44-47 The role
of surface hydroxyls has often been considered to modulate the
local concentration of hydrophilic reactants, such as alcohols,
around the active sites. For example, the hydrophobic
treatment of Pd/MOF improves the catalytic activity of styrene
hydrogenation by increasing the interaction between
hydrophobic reactants and Pd sites.48 In formaldehyde
oxidation reaction, the abundant hydroxyl groups nearby the Pt
active sites can also facilitate formate oxidation through the
formation of Pt/Ni(OH)x interface.49 In CO2 hydrogenation
reaction, hydroxyl groups on hydrophilic SiC quantum dots can
promote the methanol formation via a H-transfer mechanism,
in which the diffusion of H from hydroxyl groups to CO2 assists
the formation of intermediate HCOO*.50 Here, our work clearly
demonstrates the catalytic role of hydroxyl groups in ethanol
feeding with H2 at 250 ̊C (Fig. 7c). Upon the feeding of CH3OH,
all the samples show similar CH3O* (2825, 2927 cm-1) species.40-
42
After the feeding of H2, the IR peak intensity of CH3O* over
RhFeLi/TiO2 NRs decreases. Simultaneously, obvious IR peak of
gaseous CH4 (3016 cm-1) can be observed over RhFeLi/TiO2 NRs.
The formation of CH4 should be attributed to the C-O bond
scission in CH3O* followed by *CH3 hydrogenation. Hydroxyl
groups over RhFeLi/TiO2 NRs are suggested to promote the C-O
bond scission in CH3O* to produce *CH3 intermediate. On the
contrary, the IR peak intensity of CH3O* does not decrease, and
CH4 is hardly found over RhFeLi/TiO2 Com and RhFeLi/TiO2 NRs-
CO after H2 feeding. Therefore, it is reasonable to infer that
hydroxyls on RhFeLi/TiO2 NRs could protonate CH3O*, i.e.,
promoting C-O bond scission in CH3O* to form *CH3. Similar
phenomenon is observed under CO and H2 at 250 ̊C (Fig. 7d).
The sharp CH4 peak emerges on RhFeLi/TiO2 NRs, which is
accompanied by formate (2880, 2965 cm-1) and CH3O* (2825,
2927 cm-1). In contrast, we find only CH3O* species adsorbed
over RhFeLi/TiO2 NRs-CO. The IR peaks of gaseous CH4 and
formate are undetectable over RhFeLi/TiO2 NRs-CO and
RhFeLi/TiO2 Com, which should be attributed to the removal of
hydroxyls after CO treatment.
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synthesis via CO2 hydrogenation. We show the surface hydroxyl Hydrogenation of CO2
View Article Online
DOI: 10.1039/C8SC05608K
species on RhFeLi/TiO2 NRs can protonate methanol and reduce
All the catalytic reactions were carried out in a fixed-bed micro-
reactor. In a typical experiment, 300 mg of each catalyst with a
20-40 mesh size distribution was mixed with 2.0 g of quartz
particles (SiC: granulation of 0.075-0.4 mm) to avoid hot spots
and pressure drop across bed and packed in the stainless steel
(ɸ 8 × 400 mm) tubular reactor. Prior to each experiment, the
catalyst was activated by reducing in H2 atmosphere (99.99%)
with the flow rate of 30 mL/min and the temperature of 400
for 1 h. The RhFeLi/TiO2 NRs-CO sample was obtained from the
RhFeLi/TiO2 NRs-500 C reduced under CO flow at 350 C for 0.5
the energy barrier for C-O bond scission, facilitating the
generation of *CH3 species. Accordingly, abundant *CH3 species
can be inserted by CO obtained from RWGS to form CH3CO*,
followed by CH3CO* hydrogenation to ethanol.
Conclusions
̊C
In conclusion, we have demonstrated the crucial role of surface
hydroxyls over the RhFeLi/TiO2 NRs catalyst to synthesize
ethanol from CO2 hydrogenation. Based on in situ spectroscopic
characterizations, we propose two advantages of TiO2 NRs
support for CO2 hydrogenation to ethanol: (i) Rh-based
nanoparticles are highly-dispersed on TiO2 NRs due to the
strong interaction between catalysts and TiO2 NRs support, thus
displaying high activity; (ii) abundant hydroxyls over TiO2 NRs
can protonate methanol, which is easily dissociated into *CHx,
thus favored the formation of ethanol upon CO insertion. This
work not only provides the detailed understanding of the
catalytic role of hydroxyls in heterogeneous catalysis but also
opens an avenue for developing efficient catalysts for CO2
conversion.
̊
̊
h. After the reduction of catalyst, the reactor was cooled down
to reaction temperature. Then the reactant gases (CO2 and H2
with molar ratio of 1:3, 30 bar) were introduced into the
reactor. The gas hourly space velocity (GHSV) was set at 6000 h-
1
.
The product gas was analyzed with an online gas
chromatograph (GC, Agilent 7890B) equipped with two
detectors. One is flame ionization detector (FID) with a HP-FFAP
column using H2 as a carrier gas to analyze the organic species
such as alcohols, oxygenate and hydrocarbons. The other one is
thermal conductivity detector (TCD) with columns of MS-5A
column and Hayesep Q using He as a carrier gas to monitor the
incondensable gas species including H2, CO2, N2, CO and CH4. All
the flows between the reactor and the GC were heated and kept
beyond 150
products.
̊C, to avoid the liquefaction of the alcohols
Experimental section
Chemicals
TiO2 NRs were prepared by hydrothermal treatment of a
mixture of titanium tetrachloride, nitric acid and water.51-52
Briefly, titanium tetrachloride (TiCl4, Shanghai Chemical Agent
Co., 98%) was dissolved in ultrapure water within an ice-water
bath to obtain the 3 M TiCl4 solution. Subsequently, 35 mL
aliquot of concentrated nitric acid (HNO3, 15 M) was refluxed in
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
a silicone oil bath and heated to 200 ̊C gradually, and then 20
This work was financially supported by the National Key R&D
Program of China (2016YFB0600901), the National Natural
Science Foundation of China (21525626, 21603159, 21676181),
the Program of Introducing Talents of Discipline to Universities
(B06006) for financial support.
mL 3 M titanium tetrachloride solution was rapidly injected into
nitric acid under vigorously stirring. After aging for 20 h, the
autoclave was cooled to room temperature. The obtained
precipitates were centrifuged and washed several times with
deionized water and ethanol. The filtered solid was dried at
100
600
̊
̊
References
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