Highly efficient photocatalytic reduction of CO2 on surface-modified Ti-MCM-41 zeolite

Article abstract of DOI:10.1016/j.cattod.2018.11.046

Three dimensional microporous zeolites have attracted much attention in catalysis because of their large BET surface area and abundant pores. Herein, Ti-MCM-41 zeolites with different molar ratio of Si/Ti were synthesized, and the surface acidity and active sites were adjusted by the modification of basic-earth metal oxides, and noble metal nanoparticles. The photocatalytic activity and selectivity were evaluated by the reduction of CO₂ to CH₄ and CO. The results indicated that the efficiency and selectivity of CO₂ photoreduction to CH₄ of Ti-MCM-41 zeolites were 93 ppm g^?1 h^?1 and 29% in the ratio Si/Ti = 10 under keeping the zeolite structure, and which is much higher than that of P25 with the value of 24 ppm g^?1 h^?1 and 23%. When the surface of Ti-MCM-41 zeolites was modified by different basic-earth metal oxides, the photo-reduction activity of CO₂ to CH₄ increased about 1.5 times with MgO modification, and the selectivity of CO₂ to CH₄ improved to 42%. Moreover, the photoactivity and selectivity enhanced to 8835 ppm g^?1 h^?1 and 93% when the surface was further decorated by Pt nanoparticles, while the selectivity of CO₂ to CO reached 94% when Pd was loaded. Our work reveal reasonably that adjusting the acidity and active site of the microporous zeolites is a highly efficient way to improve the photocatalytic efficiency of CO₂ reduction.

Full text of DOI:10.1016/j.cattod.2018.11.046

Accepted Manuscript
Title: Highly efficient photocatalytic reduction of CO2 on
surface-modified Ti-MCM-41 zeolite
Authors: Wenhui Jia, Taifeng Liu, Qiuye Li, Jianjun Yang
PII:
S0920-5861(18)31500-1
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.11.046
CATTOD 11780
To appear in:
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Revised date:
Accepted date:
4 September 2018
6 November 2018
20 November 2018
Please cite this article as: Jia W, Liu T, Li Q, Yang J, Highly efficient photocatalytic
reduction of CO2 on surface-modified Ti-MCM-41 zeolite, Catalysis Today (2018),
https://doi.org/10.1016/j.cattod.2018.11.046
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Highly efficient photocatalytic reduction of CO2 on surface-modified
Ti-MCM-41 zeolite
Wenhui Jia, Taifeng Liu, Qiuye Li*, Jianjun Yang*
National & Local Joint Engineering Research Center for Applied Technology of Hybrid
Nanomaterials, Henan University, Kaifeng, 475004, China.
*Corresponding author. TEL and Fax:+86-371-25152066, E-mail address:qiuyeli@henu.edu.cn,
yangjianjun@henu.edu.cn.
Graphical abstract
Highlights


Ti-MCM-41 zeolites with different ratio of Si/Ti were synthesized.
Surface acidity and active sites were adjusted by basic-earth metal oxides and
noble metals.



The activity increased 56 times and the selectivity of CO2 to CH4 reached 93%
by Pt loading.
The highly product selectivity of CO2 to CO reached 94% when Pd loading.
Abstract: Three dimensional microporous zeolites have attracted much attention in
catalysis because of their large BET surface area and abundant pores. Herein,
Ti-MCM-41 zeolites with different molar ratio of Si/Ti were synthesized, and the
surface acidity and active sites were adjusted by the modification of basic-earth metal
oxides, and noble metal nanoparticles. The photocatalytic activity and selectivity were
evaluated by the reduction of CO2 to CH4 and CO. The results indicated that the
efficiency and selectivity of CO2 photoreduction to CH4 of Ti-MCM-41 zeolites were
93 ppm·g-1·h-1 and 29% in the ratio Si/Ti=10 under keeping the zeolite structure, and
which is much higher than that of P25 with the value of 24 ppm·g-1·h-1 and 23%.
When the surface of Ti-MCM-41 zeolites was modified by different basic-earth metal
oxides, the photo-reduction activity of CO2 to CH4 increased about 1.5 times with
MgO modification, and the selectivity of CO2 to CH4 improved to 42%. Moreover,
the photoactivity and selectivity enhanced to 8835 ppm·g-1·h-1 and 93% when the
surface was further decorated by Pt nanoparticles, while the selectivity of CO2 to CO
reached 94% when Pd was loaded. Our work reveal reasonably that adjusting the
acidity and active site of the microporous zeolites is a highly efficient way to improve
the photocatalytic efficiency of CO2 reduction.
Key words: Ti-MCM-41 zeolite; CO2 photoreduction; surface acidity; active sites.
1. Introduction
The environmental and energy crises, which are partially due to the increasing of
CO2 concentration caused by fossil fuels, is becoming significant issues of global
concern. From the perspective of sustainable development, photocatalytic reduction of
carbon dioxide into hydrocarbon fuels is an ideal way to realize CO2 recycling [1-4].
However, few materials are known to photocatalyze the reduction of CO2 to fuels
with highly efficiency and selectivity. Semiconductor-based photocatalysts, such as

TiO2 [5-7], Zn2GeO4 [8], BiOX [9, 10], Cu2O [11, 12], WO3 [13] and W18O49 [14]
have been reported. According to these studies, high specific surface area and more
active sites are the important factors for the highly efficient CO2 photo-reduction.
Comparing to conventional semiconductor photocatalyst, molecular sieves are
intrinsic porous structure which could provide high specific surface area and more
active sites for photocatalytic reaction [15]. We previously reported that the Eosin
Y-sensitized Ti-MCM-41 had a very high activity of hydrogen production under
visible light irradiation, which may be due to the larger specific surface of
Ti-MCM-41 and the better dispersed TiOx clusters in zeolite [16]. Anpo et al. found
that the introduction of transition metal atoms allowed the molecular sieves to exhibit
photocatalytic activity, and showed that metal-oxygen units could be excited by
UV-irradiation to form charge transfer excited states. They also found that the
mesopores surface of the highly dispersed TiO2 molecular sieves showed a higher
photocatalytic activity of CO2 reduction than the bulk TiO2, and the higher dispersion
state of the titanium oxide species play a vital role in photocatalytic activity [17-21].
The porous molecular sieves containing titanium had TiO4 units, and the
photo-generated electrons and holes were present in the same sites such as a pair of
Ti3+ and O-, which worked together result in very high reactivity. This was in stark
contrast to TiO2 semiconductor photocatalysts [22, 23].
Besides the intrinsic structure of the photocatalysts, the surface construction is
also important for the improvement of photoactivity. Wang et al. prepared NaOH
modified TiO2 photocatalyst without any noble metal cocatalyst by impregnation
method, and found that the basic surface of TiO2 promoted CO2 chemisorption and
increased photocatalytic performance [24]. Xie et al. reported that the addition of
alkaline metal oxide to TiO2 photocatalyst enhanced the activity of photocatalytic
reduction of CO2 [25]. Liu et al. reported the photocatalytic activity of surface
La-modified TiO2 nanoparticle was 13 times of that over commercial P25, one of the
reasons for the enhanced photocatalytic performances were attributed to the basic
characteristics of La2O3 for CO2 adsorption [26]. Therefore, it is important to adjust
the surface active sites of the photocatalysts for the highly efficient CO2
photoreduction.
In this work, we aim to adjust the content of Ti atoms in the framework

Ti-MCM-41 zeolite as large as possible under the condition of keeping the zeolite
intrinsic structure, to increase the photogenerated charge carriers to participate in the
photocatalytic reaction. And through the surface modification of Ti-MCM-41zeolite
with baisc-earth metal oxide, the adsorption sites of CO2 molecules would be
increased accordingly. And further loading the noble metal of Pt or Pd nanoparticles
as the active sites, the product selectivity of CO2 photoreduction would be improved.
Through the structure designing and surface modification of Ti-MCM-41zeolite, the
photoactivity and selectivity of CO2 to CH4 was improved from 93 ppm·g-1·h-1 and
29% to 8835 ppm·g-1·h-1 and 93%. The reason for the remarkably enhancement of the
photocatalytic activity was investigated in detail.
2. Experimental
2.1. Preparation of photocatalysts
Ti-MCM-41 zeolite was prepared by the previously reported hydrothermal
synthesis [27]. In a typical procedure, 5.7 g of Na2SiO3.9H2O and 1.85 g of CTAB
were dissolved in water, and the pH value was slowly adjusted to 7 with 1 mol.L-1
H2SO4. A certain amount of TiCl3 aqueous solution was diluted with water, and then
slowly dripped into the above formed silicone gel. The pH value was adjusted to 9.5
with ammonia, and then continued to stir for 2 hours at room temperature. Finally, the
resulting mixed gel was transferred into a PTFE-lined autoclave and hydrothermally
reacted at 120 ℃ for 72 h. After the reaction, it was naturally cooled to room
temperature. The product was filtered, washed with water and ethanol, and dried. At
last, the product was calcined at 550 °C for 5 h, a series of mesoporous molecular
sieve Ti-MCM-41 with different Si/Ti molar ratios were obtained.
Then Ti-MCM-41 molecular sieve (nSi/nTi=10) was loaded by basic-earth metal
oxides (MO) by the impregnated method, the metal nitrates was the precursor as the
oxides. The mixture was calcined at 500 ℃ for 5 h, and the samples was denoted as
Ti-xMO (x represents the concentration of nitrate, mol·L-1). At last, the noble metals
(Pt, Pd) were loaded by the photo-deposition method, and the products were denoted
as Ti-xMO-Pt or Ti-xMO-Pd.
2.2. Characterization

The X ray diffraction (XRD) pattern of the samples was measured by the X'Pert
Philips diffractometer (Bruker D8 Advance). FTIR experiments were carried out on a
VERTEX 70 IR spectrometer with the standard KBr pellet method. The morphology
of the samples was observed by transmission electron microscopy (TEM, JEOL
JEM-2100) and scanning electron microscopy (SEM, Sigma500, Carl Zeiss). The UV
visible diffuse reflectance spectra (DRS) were obtained by using the UV visible
spectrophotometer (UV-vis, UV-2600), using BaSO4 as the reference. The
photoluminescence (PL) spectrum was tested by a fluorescence spectrometer (JY
HORIBA FluoroLog-3). The specific surface area of the sample was calculated
according to the Brunauer-Emmett-Teller (BET) method by the nitrogen
adsorption-desorption test (Quadrasorb SI-4). The surface acid-basic property of the
photocatalyst was conducted by CO2 temperature-programmed desorption (CO2-TPD,
Autochem Ⅱ2920) [28]. 0.1g sample was placed in the U glass tube and preheated 1
h at 773K in He flow, and then cooled to room temperature. CO2 was introduced into
the glass tube at room temperature for 30 min, and evacuated at 373K for 30 min in
He flow. Finally, the TPD measurements were carried out at the heating rate of
10K·min-1 from 373K to 773K.
2.3. Evaluation of photocatalytic activity
Photocatalytic reduction of CO2 was conducted in a 200 mL quartz reactor. 0.1g
photocatalyst and 125 mL H2O were put into the reactor and then the catalyst was
evenly dispersed with the assistance of the ultrasound. The high purity CO2 gas was
introduced to the solution for 1 hour under magnetic stirring to obtain the CO2
saturated solution. The photocatalytic reaction was performed at room temperature for
6 h using a high-pressure Hg lamp. The products were analyzed by a gas
chromatography GC-2018 (SHIMADZU, Japan), which was equipped with a flame
ionization detector (FID), a thermalconductivity detector (TCD) and a methanation
reactor.
3. Results and discussion

3.1. Structure and morphology analysis of catalyst
b
a
Si/Ti=20
Si/Ti=20
Si/Ti=10
Si/Ti=10
Si/Ti=2
Si/Ti=2
10
20
30
40
50
60
70
2
4
6
8
10
2 theta (Degree)
2 theta (Degree)
Fig. 1. Low angle XRD (a) and XRD pattern, (b) of Ti-MCM-41 with different Si/Ti molar ratio.
Si/Ti=20
Si/Ti=10
1084
806
Si/Ti=2
960
464
4000 3500 3000 2500 2000 1500 1000
Wavenumber/cm-1
500
Fig. 2. FTIR of different Si/Ti molar ratio of molecular sieves.
The low angle XRD diagram of the sample was shown in Fig. 1a. The
characteristic peak at 2.3º can be indexed the diffraction peak of (100) plane of
Ti-MCM-41 zeolite, which indicated that the sample had a long-range and orderly
hexagonal mesoporous structure. With the increase of the amount of the introduced
titanium, the peaks representing the highly ordered mesostructure gradually broaden
and the strength gradually decreased, indicating that the long-range mesoporous
structure was to some extent affected with the increase of the amount of titanium
introduced. When the molar ratio of silicon to titanium reached 2, it had basically
become an unordered structure. Moreover, as the amount of titanium increased, the
diffraction peak of the representative (100) surface shifted to a high angle, indicating
that titanium had entered in the molecular sieve structure[29]. In the wide-angle XRD
pattern of the samples in Fig. 1b, a characteristic peak of amorphous silicon appeared

at 22.5°, and no diffraction peak of TiO2 appeared. This indicated that titanium did not
form an independent TiO2 phase, it should enter into the framework of MCM-41
zeolite. Besides, as shown in the FTIR spectra of Fig. 2, the peak around 1084 cm-1 is
the asymmetric stretching vibration characteristic peak of [SiO4] tetrahedron, the peak
near 806 and 464 cm-1 is the characteristic peak of Si-O bending vibration of [SiO4]
tetrahedron, and the peak near 960 cm-1 belongs to the vibration characteristic peak of
Ti-O-Si bond for titanium-containing molecular sieve. From the above XRD and
FTIR analysis, we can conclude that Ti has entered into the framework of Ti-MCM-41
zeolite.
Fig. 3. (a) N2 adsorption-desorption curves of Ti-MCM-41 with different Si/Ti ratios, (b) pore Size
Distribution of Ti-MCM-41 with different Si/Ti Ratios.
The BET surface areas and the pore size distribution of the Ti-MCM-41 was
tested by the N2 adsorption-desorption isotherm. As shown in Fig. 3a, we could
conclude that the adsorption and desorption curves of Ti-MCM-41 zeolites with
different Ti contents all belonged to the typical type IV isotherms [30]. It indicated
that the mesoporous structure of zeolites were well synthesized, which was in
consistent with the low-angle XRD results. The BET surface areas of Ti-MCM-41
with the Si/Ti ratio of 10 and 20 are similar, but the one with Si/Ti ratio of 2 decreased
to 625 m2g-1. That implies that when the content of Ti in MCM-41 zeolite is too large,
the uniform framework would be destroyed to some extent. The pore size distribution
diagram in Fig. 3b showed that the pore sizes of all three samples were about 3.8 nm,
indicating that the pore size was not influence by the different Si/Ti ratio under

keeping the framework of the Ti-MCM-41 zeolites.
The morphologies of Ti-MCM-41, and Pt-loaded MgO-modified Ti-MCM-41
were shown in Fig. 4. Fig. 4a showed that the Ti-MCM-41 molecular sieve had a
clearly arranged and highly ordered pore structure. It could be roughly estimated that
the pore size was about 3-4 nm, which was in consistent with the value of the 3.8 nm
calculated by the BJH model of the N2 adsorption-desorption isotherm. Fig. 4b
indicated that the Pt nanoparticles were loaded uniformly on the surface of
Ti-MCM-41 zeolites, and the particle size is about 3-5 nm. But the MgO nanoparticles
were hard to observed in the TEM images. This should be due to the little content and
amorphous phase of MgO. Therefore, the high resolution secondary electron image
and back-scattered electron image by the field emission scanning electron microscope
(FESEM) were taken to analyze the existence of the MgO and Pt nanoparticles.
Through comparison the two images, we can clearly observed that the metallic Pt and
MgO nanopartilces were loaded successfully on the surface of Ti-MCM-41 zeolites.
In addition, the EDS elemental mapping images were taken to analyze the distribution
and content of the different elements. As shown in Fig. 5, Mg and Pt elements were
dispersed uniformly on Ti-MCM-41 zeolites, and the content of Mg atom is ca. 1%.

Fig. 4. TEM images of Ti-MCM-41 (a) and Pt-loaded MgO-modified Ti-MCM-41 (b); high
resolution secondary electron image (c) and back-scattered electron image (d) of Ti-0.05MgO-Pt
(The bright spot in the red circle are Pt nanoparticles, and the one in the blue circle is MgO).
Fig. 5. SEM and EDS elemental mapping images of Ti-0.05MgO-Pt (the scale bar is 2 μm).
3.2. Optical absorption properties

1.5
1.2
0.9
0.6
0.3
0.0
P25
Si/Ti=2
Si/Ti=10
Si/Ti=20
200
300
400
500
600
700
800
Wavelength(nm)
Fig. 6. UV-vis absorption spectra of Ti-MCM-41 zeolites with various Si/Ti molar ratio.
As shown in the UV-vis absorption spectra of Fig.6, all the zeolites with different
mole ratio of Si/Ti represent strong absorption in the UV light region. And the onset
absorption edge red shifted as the increase of the introduced titanium content in the
MCM-41 zeolites. As the design idea of this work, we wish to increase the introduced
titanium content under keeping the MCM-41 zeolite framework. So more excited
charge carriers can participate into the CO2 photoreduction reaction. As well known,
the pure MCM-41 zeolite only contains -Si-O-Si- chain, it almost belongs to the
insulator and often used as a supporter [31, 32]. When the titanium atom introduced to
the framework, it seems that some TiOx clusters embedded into the long-range
ordered -Si-O-Si- chain. So the material would represent the semiconductor properties
of TiO2 and the quantum size effect of the clusters. Therefore, the band gap of
Ti-MCM-41 is larger than that of P25, and the onset absorption edge red shifted as the
increase of the introduced titanium content.
3.3. Photocatalytic activity of CO2 reduction and analysis
100
93
b
157
a
160
140
120
100
80
80
60
40
20
0
108
93
34
34
60
46
24
40
24
20
0
Ti-0.05MgO
P25
Ti-0.01BaO
Si/Ti=20
Si/Ti=2
Si/Ti=10
P25
Ti-0.01SrO
Ti-MCM-41

c
157
160
140
120
100
80
152
146
146
60
40
20
0
1
2
3
4
Cycle
Fig. 7. Photoactivity of CO2 reduction to CH4 on Ti-MCM-41 with different Si/Ti ratio (a) and
basic-earth metal oxides modified Ti-MCM-41 (nSi/nTi=10) (b); the stability of Ti-0.05MgO (c).
Fig. 7a showed the photocatalytic activity of Ti-MCM-41 molecular sieves with
different Si/Ti molar ratios under ultraviolet light, and the production rate of CH4 on
P25 was 24 ppm·g-1·h-1. All the Ti-MCM-41 zeolites exhibited higher photoactivity
than that of P25. As the increase of the introduced Ti contents, the activity increased
from 34 to 93 ppm·g-1·h-1, and then decreased. The highest activity of Ti-MCM-41
with the nSi/nTi=10 reached the highest activity, which was 3.8 times that of P25. This
can be mainly attributed to the fact that as the content of Ti increases, more TiO4
tetrahedral unit can participate in the photo-excited process, and thereby take part in
the photoreduction of CO2. When the introduced content of Ti increased to 1:2, the
BET surface areas increased a lot, and the uniform framework of zeolite was not good
than the other Ti-MCM-41 samples, so the activity decreased. The production rate of
photocatalytic reduction of CO2 to CH4 was changed after surface modification by
basic-earth metal oxides, as shown in Fig. 7b. The highest CH4 production rate of 157
ppm·g-1·h-1 was obtained on the Ti-MCM-41 zeolite modified by MgO. When BaO
was modified on Ti-MCM-41 zeolite, the activity decreased a lot. This should be due
to that the alkalinity of BaO is poor, that can be verified by the following CO2-TPD
diagram. In addition, part of BaO particles would shield some incident light, and
decrease the light absorption efficiency. Moreover, the stability of photocatalyst was
measured in Fig. 7c, it was found that the photocatalytic production rate of CH4 was

157, 152, 146 and 146 ppm·g-1·h-1 by the sample of Ti-0.05MgO through the four
cycle experiments. That is to say, 93% of the photoactivity could be kept after a
long-term reaction, indicating that the catalysts possessed a very good stability.
Ti-MCM-41
Ti-0.05MgO
Ti-0.01SrO
Ti-0.01BaO
100
200
300
400
500
Temperature(°C)
Fig. 8. CO2-TPD diagram of basic-earth metal oxide modified molecular sieves.
The photoreduction activity strongly depends on the adsorption of CO2
molecules on the surfaces of the photocatalyst. Adjusting the surface basicity will
promote the adsorption of CO2, and thereby improving the photoactivity. As shown in
the CO2-TPD diagram in Fig. 8, the unmodified Ti-MCM-41 sample showed one
desorption peak centered at 145℃, which was attributed to the weak adsorption of
CO2 molecule on the surface of catalysts. After the modification of Ti-MCM-41
surface by BaO, SrO and MgO, the position of the main peak shifted to 156, 178, and
258℃, respectively. This indicated that the three kinds of basic-earth metal oxides
could enhance the adsorption strength of CO2 molecule, because they need more high
temperature to desorb CO2 molecules. In addition, comparing the peak area of
Ti-MCM-41, those of BaO, SrO and MgO modified Ti-MCM-41 increased from 7
μmol·g-1 to 20, 33, and 80 μmol·g-1, respectively. Based on the CO2-TPD results, we
can conclude that the basicity and adsorption ability for CO2 increased with the
modification of basic-earth oxides, and the basic sites were beneficial in facilitating
the adsorption of CO2 [33].

a
8835
b
CH4
CO
CH4
CO
8000
6000
4000
2000
0
8000
6000
4000
2000
0
2499
1623
933
882
319
157
42
1%Pd
93
Ti-MCM-41
24
0.5%Pt
1.5%Pt
1%Pd 1.5%Pd
0.5%Pd
1%Pt
P25
Ti-0.05MgO
1%Pt
Fig. 9. Activity of CO2 photoreduction to CH4 and CO on different samples.
The activity and product selectivity of CO2 photoreduction over Ti-0.05MgO
with different Pt and Pd cocatalyst loadings were shown in Fig. 9a and Table. 1. The
results indicated that Pt showed a high selectivity of 93% for CH4 conversion and Pd
had a higher selectivity for CO. As the loading amount of Pt/Pd cocatalysts, the
production rate of CH4 and CO increased first, and then decreased, the highest activity
obtained at the loading amount of 1%. Besides, as shown in Fig. 9b, we can see that
the production rate of CH4 increased from 157 to 8835 ppm·g-1·h-1 when Pt loading
on Ti-0.05MgO surface, and the product selectivity of CH4 was up to 93%. This result
may be due to the rapid transfer of photogenerated electrons from molecular sieve to
Pt cocatalyst, which greatly improved the separation rate of the photoinduced electron
and hole pairs. From Table 1, we can clearly observe the positive role of Pt and Pd
cocatalysts contribute to the photoactivity and product selectivity. The highest product
selectivity of CH4 and CO reached 93% and 94%, respectively.

Ti-MCM-41
Ti-0.05MgO
Ti-0.05MgO-Pt
350
400
450
500
Wavelength(nm)
Fig. 10. PL spectra of different basic-earth metal oxide modified Ti-MCM-41.
Photoluminescence (PL) emission spectroscopy was used to understand the
transfer and recombination of photogenerated charge carriers[32]. As shown in Fig.
10, Ti-MCM-41 and Ti-0.05MgO exhibited broad signal located ca. 403 and 436 nm
due to the recombination of photogenerated electron and hole pairs. The peak
intensity of Ti-MCM-41 and Ti-0.05MgO are high, indicating that the electron-hole
recombination efficiency are much higher. Comparing the two curves, we found that
the modification of MgO would not influence the separation efficiency of
electron-hole pairs. However, when Pt loaded on Ti-0.05MgO surface as the
cocatalyst, the PL intensity decreased a lot, indicating that Pt played a very important
role to improve the separation of photogenerated charge carriers.
3.4. Photocatalytic mechanism of CO2 reduction

Fig. 11. The proposed photocatalytic mechanism of CO2 reduction.
The photocatalytic mechanism of CO2 reduction on Pt-loaded MgO-modified
Ti-MCM-41 zeolite was proposed in Fig. 11. The large BET surface areas and
regularly ordered porous structure of Ti-MCM-41 zeolite are favourable for the
adsorption of CO2, and the CO2 molecules can be confined in the pores of the
molecular sieve as a micro-reactor. During the photoinduced reaction, TiO4 tetrahedral
units in the the framework of molecular sieve are excited to form electrons and holes,
wherein electrons transferred from O2- to Ti4+, resulting in the transition charge
transfer excited state formation of a pair of hole center O- and electron center Ti3+,
that is [Ti3+-O-]∗[32]. CO2 molecule is first chemisorbed on the MgO layer, and the
chemisorbed CO2 molecule becomes destabilized and its reactivity is considered to be
higher than that of the linear CO2 molecules [35, 36]. On the other hand, the
photogenerated electrons on Ti-MCM-41 can be easily trapped by the Pt cocatalysts,
the enriched electron density on Pt nanoparticles would favour the formation of CH4
[37]. Due to the synergistic effect between the surface MgO and Pt cocatalysts, the
higher photocatalytic acitvity of CO2 reduction was achieved on Ti-MCM-41.
4. Conclusions
In summary, Ti-MCM-41 zeolites with different Si/Ti molar ratios were
synthesized. When the mole ratio of Si/Ti equals to 10, the zeolites showed a

relatively higher photoactivity for CO2 reduction to CH4 under keeping the regular
mesoporous zeolite structure. When the surface basicity of Ti-MCM-41 zeolite was
adjusted by different basic-earth metal oxides, the photoreduction activity of CO2 to
CH4 increased 1.5 times by MgO modification, and the product selectivity rose to
42%. CO2-TPD measurement indicated that MgO decoration was beneficial for
enhancing CO2 adsorption, and thereby improving the photoactivity. When Pt
nanoparticles were further loaded as the cocatalyst, the CH4 evolution rate and
selectivity increased to 8835 ppm·g-1·h-1 and 93%, and this activity is ca. 56 times
than that of MgO-modified Ti-MCM-41. While Pd nanoparticles loaded as the
cocatalyst, CO evolution selectivity rose to 94%. Our work reveals reasonably that
adjusting the surface basicity and active site of the microporous zeolites is a highly
efficient way to improve the photocatalytic efficiency of CO2 reduction.
Acknowledgments
The authors gratefully acknowledge the support of the National Natural Science
Foundation of China (Nos. 21673066, 21703054, 51702087), Program for Science &
Technology Innovation Talents (15HASTIT043).
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Table
Table 1. Activity and product selectivity of CO2 photoreduction over (Pt/Pd)-loaded Ti-0.05MgO
CH4 production
CO production
Selectivity for
CH4 (%)
Selectivity for
CO (%)
rate(ppm·g-1·h-1) rate(ppm·g-1·h-1)
Photocatalyst
Ti-0.05MgO-0.5%Pt
Ti-0.05MgO-1%Pt
Ti-0.05MgO-1.5%Pt
Ti-0.05MgO-0.5%Pd
Ti-0.05MgO-1%Pd
Ti-0.05MgO-1.5%Pd
3281
8835
4831
34
3084
2499
3221
152
81
93
86
47
6
19
7
14
53
94
71
42
1623
399
36
29
(The selectivity of production is evaluated based on the required electrons using the following
equation: Selectivity (%) =[8ν(CH4)]/[2ν(CO)+8ν(CH4)]×100%, where ν(CO) and ν(CH4) stand
for the production rate of CO and CH4 [34]. )