Enhanced CO2 photocatalytic reduction performance on alkali and alkaline earth metal ion-exchanged hydrogen titanate nanotubes

Accepted Manuscript Full Length Article Enhanced CO2 photocatalytic reduction performance on alkali and alkaline earth metal ion-exchanged hydrogen titanate nanotubes Qijun Tang, Zhuxing Sun, Penglu Wang, Qian Li, Haiqiang Wang, Zhongbiao Wu PII: DOI: Reference:

S0169-4332(18)32388-2 https://doi.org/10.1016/j.apsusc.2018.08.245 APSUSC 40282

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

8 June 2018 9 August 2018 27 August 2018

Please cite this article as: Q. Tang, Z. Sun, P. Wang, Q. Li, H. Wang, Z. Wu, Enhanced CO2 photocatalytic reduction performance on alkali and alkaline earth metal ion-exchanged hydrogen titanate nanotubes, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.08.245

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Enhanced CO2 photocatalytic reduction performance on alkali and alkaline earth metal ion-exchanged hydrogen titanate nanotubes Qijun Tanga,b, Zhuxing Suna,b, Penglu Wanga,b, Qian Lia,b, Haiqiang Wang*a,b, Zhongbiao Wua,b a

Key Laboratory of Environment Remediation and Ecological Health, Ministry of

Education, College of Environmental & Resources Science, Zhejiang University, Hangzhou 310058, P.R. China b

Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace

Flue Gas Pollution Control, Hangzhou, 311202, P. R. China * Corresponding author: (H. Wang) E-mail: [email protected] Tel. / Fax: +86-571-87953088.

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Abstract: In this work, the photocatalytic reduction performance of carbon dioxide (CO 2) was investigated on the alkali and alkaline earth metal ions (Mg2+, Na+, K+) exchanged hydrogen titanate nanotubes (H-TNTs) under simulated sunlight. The experimental results verified that carbon monoxide (CO) and methane (CH4) were the main products in the studies. Compared to the pure H-TNTs, 0.1M-Mg-H-TNTs was the most excellent one among the Mg2+ modified samples, 5-h CO and CH4 generation amount of which promoted to 3.8 times and 16.5 times, respectively. It was found that the tubular structure of H-TNTs was well maintained after Mg2+ ion-exchange, according with the characterization results of XRD and TEM. The result of UV-Vis spectra, PL spectra, Photocurrent measurement, CO2-TG and CO2-TPD indicated that Mg-H-TNTs had better transmission efficiency of photogenerated electron and stronger CO2 adsorption capability. What’s more, the CO2 photocatalytic reduction test on Na-H-TNTs and K-H-TNTs indicated that alkali and alkaline earth metal ions-exchange method was a feasible process to enhance photocatalytic reduction performance of H-TNTs. This study not only provide a simple approach to modify photocatalysts for the enhanced CO2 photocatalytic reduction performance, but also inspire more practical and feasible modification for CO2 photoreduction. Keywords: CO2 photocatalytic reduction; hydrogen titanate nanotubes; ion exchange

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1. Introduction In the past decades, both greenhouse effect and energy shortage have attracted much attention around the world because of their emergency and harm to human beings[1]. As a major greenhouse gas which does harm to the global climate, carbon dioxide (CO2) can also be used as a cheap carbon energy sources to produce high-level chemicals. Hence, converting CO2 to fuels by using renewable energy sources is of great importance to solve the two challenges simultaneously[2]. Photocatalytic reduction of CO2, scilicet artificial photosynthesis, is regarded as an innovative technology using inexhaustible solar energy in CO2 reduction field[3–5]. However, there are quite a few drawbacks so far in photocatalytic reduction of CO2 using semiconductors such as low utilization efficiency of the solar energy, poor CO2 adsorption and photogenerated electron–hole pairs separation and so on[6–9]. Titanium dioxide (TiO2), the most studied photocatalyst, also suffers from quite low solar energy utilization efficiency because of its wide energy band gap (Eg= 3.2 eV), high ratio of photogenerated electron-hole recombination and small specific surface area[10]. Although great efforts have been made to overcome these shortcomings, there is still a long way to realize a high solar energy utilization efficiency[10–14]. Prepared from TiO2, hydrogen titanate nanotubes (H-TNTs) are regarded as a promising material, which have drawn much attention lately for its excellent property such as large surface area and high ion exchange capacity for cations of different metals[15–18]. When compared to H-titanate nanosheets (H-TNS) and nanofibers (H-TNF), H-TNTs showed the highest photocatalytic activity in 3

selectively oxidating alcohols under visible light irradiation[19]. Unfortunately, similar to TiO2, H-TNTs also have low response to visible light, poor photogenerated electron–hole pairs separation. Besides the above obstacles, other technical challenges like poor CO2 adsorption need to be solved but are seldomly addressed in the literatures[20–22]. Recently, adding basic metal oxides like MgO on TiO2-based catalyst was prone to enhancing CO2 adsorption and promoting the conversion efficiency of CO2[21,23]. It is believed that the chemisorbed CO2 molecules on MgO become destabilized and its reactivity will be higher than the liner CO 2 molecules in the following CO2 photocatalytic reduction[21]. Inspired by the fact that H-TNTs are easy to be cation-exchanged, alkali and alkaline earth metal ions (Mg2+, Na+, K+) are chosen to exchange protons(H+) on H-TNTs to realize the uniformly alkali and alkaline earth metal doping in this work as alkali and alkaline earth metals are earth-abundant, cheap to obtain from industry. After H+ is ion-exchanged by Mg2+, Na+ and K+, the modified TNTs may adsorb more CO2 and destabilize the adsorbed CO2 easily, resulting a higher CO2-to-fuel efficiency. Different from other study, high-temperature calcination was not employed in this work after the ion exchange of H-TNTs, which could maintain its large specific surface area to the utmost extent. The experiment results verified that ion-exchange could promote the CO2 photocatalytic reduction performance of H-TNTs. We hope this work reported here can provide a new insight into improving the efficiency of CO2 photocatalytic reduction. 2. Experimental 4

2.1 Chemical Mg(NO3)2·6H2O, KNO3, NaNO3, NaOH and HCl were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). P25 TiO2 was purchased from Evonik-Degussa Co. Ltd in Germany. All the chemicals used in the experiment were of analytical purity and used without any further purification. 2.2 Sample preparation Hydrogen titanate nanotubes were synthesized by a hydrothermal treatment in which P25 TiO2 was mixed with 10 M NaOH solution in a Teflon lined autoclave at 150 oC for 24 h[24–26]. Afterwards, the precipitates were treated by following steps: First, washed by 0.1 M HCl solution for several times with a twenty minute interval, the precipitates reached a pH value approach to 1.6; Second, they were washed by deionized water with a thirty minute interval until reached a pH value close to 7; Third, they were dried at 70oC for 12 h before use. Ion-exchanged hydrogen titanate nanotubes were synthesized as follows[27]: First, 0.8 g hydrogen titanate nanotubes and 50 mL of 0.1 M metallic salt solution were mixed and then stirred for 2 h; Second, after filtration and washing, the precipitates were dried at 70 oC for 12 h. These samples were labelled as M-H-TNTs (M= Mg2+, K+, Na+). Different concentrations of H-TNTs modified by Mg2+ were synthesized by a similar method. The only difference is the concentrations of Mg(NO3)2, which is 0.005 M, 0.025 M, 0.1M and 0.2M, separately. The obtained samples were labelled as XM-Mg-H-TNTs (X=0.005, 0.025, 0.1, 0.2). 2.3 Characterization of photocatalysts 5

The crystal phases of the as-prepared samples were analyzed by X-ray powder diffraction (XRD, Shimadzu XRD-6000, Japan) with Cu Kα irradiation. The applied current and accelerating voltage were 30 mA and 40 kV, respectively, with a 2θ ranged from 5° to 80° and a step size of 0.02°. To investigate the surface chemical compositions，X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA) measurements were performed with a monochromatized Al Kα source (150 W, hν=1486.6 eV, 500 μm). The specific surface area was determined on a nitrogen adsorption apparatus (JW-BK 132F, Beijing JWGB Sci & Tech Co., China) by the Brunauer–Emmett–Teller (BET) method. With a scan UV–vis spectrophotometer (TU-1901, China) equipped with an integrating sphere, the UV–visible diffuse reflection spectra (UV–vis DRS) was obtained and BaSO4 was used as a reference. The photoluminescence spectra were detected with a fluorescence (FLS920, Edinburgh Instruments, England) using 325 nm lasers as excitation source. The morphologies, structures and grain sizes of the as-prepared samples were examined by transmission electron microscopy (JEM-2010, Japan). With a scanning electron microscope (SU-8010，Hitachi，Japan), field emission scanning electron microscopy (FE-SEM) images were recorded to observe the morphology of the samples. Using SDT Q600 DTA－TG (TA instruments, USA), CO2 thermogravimetry (CO2-TG) was applied to detect the CO2 adsorption amount of the samples. CO2 temperature programmed desorption (CO2-TPD) was used with a mass spectrometer (Hiden QGA, U.K.) using 10 mg sample. Samples was pretreated in pure He at room temperature for 1h prior to the experiment to remove physically adsorbed molecules on the surface 6

and then saturated with pure CO2 for 30 min. Helium was used again to thoroughly blow away the residual CO2 until the mass spectrometry signal of CO2 remained unchanged[2,28]. Then desorption was carried out by heating the samples in He (30 mL min-1) from 30℃ to 800℃ at a heating rate of 10℃/min. With the equipped mass spectrometer, the amount of CO2 in the effluent gas was recorded. Photocurrents of as-prepared catalysts were obtained in a three-electrode system where Ag/AgCl electrode, platinum wire and the FTO glass with some sample were used as the reference electrode, counter electrode and working electrode, respectively, with 0.2 M Na2SO4 as electrolyte. The sample film on FTO glass was made as follows: First, 20 mg sample was dispersed in 1 mL of dimethyl formamide (DMF); Second, 20 μL Nafion solution was added and the mixture was sonicated till well dispersed; Finally, 40 μL of the suspension was dipped to a 1×1 cm2 FTO glass and dried at 60 °C for 12h. The group change on the sample surface before and after illumination were studied by Fourier transform infrared spectroscopy (Nicolet 5700, USA). 2.4 Photocatalytic activity experiments Using a continuous-flow system as introduced in our pervious paper, photocatalytic reduction of CO2 was carried out[2]. With a quartz window on the top of the stainless steel-made reactor, 40 mg catalyst was dispersed at the bottom of it. A 300W Xe lamp (PLS-SXE300UV, Beijing Trust-tech Co. Ltd., China) was employed as the light source to provide simulated sunlight without any light filters. At the beginning of the experiment, pure CO2 gas was pumped into the reactor at 120 mL/min through a water bubbler for 1 h to get rid of the impurity gases. Then 7

after the CO2 flow rate changed to 3 mL/min, the system stabilized for one more hour before the activity test. During the reaction process, a gas chromatography (Agilent 7890A, USA) was used to detect the instant concentration of CO, CH4 and other hydrocarbons in the effluent gas through auto-valves with a twenty-minute interval. 3. Results and discussion 3.1 Photocatalytic CO2 reduction performance CO2 photocatalytic reduction tests were carried out under simulated sunlight with as-prepared catalysts. The main products were CO and CH4 with other negligible hydrocarbons. To confirm CO and CH4 were generated from CO2, controlled experiment was conducted, which showed there were few products without light or catalysts. The CO and CH4 productions of H-TNTs and XM-Mg-H-TNTs (X=0.005, 0.025, 0.1and 0.2) were exhibited in Fig. 1. Compared to the pure H-TNTs, the CO evolution on XM-Mg-H-TNTs (X=0.005, 0.025, 0.1 and 0.2) increased to 2.27, 2.68, 3.76 and 2.97 times, respectively. To our surprise, 5-h accumulated production of CH4 on XM-Mg-H-TNTs (X=0.005, 0.025, 0.1 and 0.2) were 5.39, 8.80, 16.49 and 11.18 times, respectively, as high as that of pure H-TNTs. The generation of CH4 increased greatly when the ion-exchange concentration of Mg2+ was 0.1 M. 3.2 Crystal phase and morphology Fig.2 shows XRD patterns of XM-Mg-H-TNTs (X=0.005, 0.025, 0.1, 0.2) and pure H-TNTs. All the samples had peaks at 24.3°, 28.0° and 48.2°, corresponding to (110), (211) and (020) of H2Ti3O7 (PDF-ICDD 47-0561), respectively[29,30]. And the crystal structure of titanate nanotubes well remained after ion-exchange, which agreed 8

with the previous study[15,30]. Moreover, crystal structure of magnesium oxide had not been detected even in the XRD spectrum of 0.2M-Mg-H-TNTs sample. The morphology of the catalysts was investigated by transmission electron microscope (TEM), as shown in Fig.3. No apparent difference could be observed between H-TNTs and 0.1M-Mg-H-TNTs. After Mg2+ ion-exchange, tubular structure of Mg-H-TNTs was well maintained, which was consistent with the XRD result. 3.2.1 Optical property of Mg-H-TNTs UV–Vis diffuse reflection spectra of XM-Mg-H-TNTs (X= 0.1, 0.2) and pure H-TNTs showed slight difference with an absorption started from about 408 nm, as displayed in Fig.4. The color of ion-exchanged samples remained white, whilst there were no obvious visible photo-response enhancement after ion-exchange which were different with works reported before[15,31]. However, 0.1M-Mg-H-TNTs and 0.2M-Mg-H-TNTs exhibited a stronger absorption than pure H-TNTs in the region between 200-300nm. The band gap (Eg) of an indirect-gap semiconductor could be determined by the equation: (αhν)2 =A(hν -Eg)[32]. The corresponding (αhν)2 ∼ hν curves were showed to estimate their band gap energies in Fig. 4 (insert). Accordingly, the band gap energies of as-prepared samples were almost the same as presented in Table 1. To study the recombination rate of photo-generated holes and electrons on photocatalysts, Photoluminescence (PL) spectroscopy is typically used. The intensity of the PL signals indicates the ability to stabilize photo-generated holes and electrons. A weaker PL spectrum intensity reflected a lower recombination rate[33]. 9

Photoluminescence spectra of XM-H-Mg-TNTs (X= 0.1, 0.2) and pure H-TNTs excited by wavelength of 325 nm light were displayed in Fig. 5A. All the tested catalysts could be excited by 325 nm light. The PL signals intensity were slightly quenched after Mg2+ ion-exchange, and indicated Mg-H-TNTs had lower e--h+ recombination rate. Photocurrent measurement (I-t curve) was employed to investigate the efficiency in separation and transfer of photo-generated electrons and holes of the catalysts. As shown in Fig. 4B, the photocurrent intensity of 0.1M-Mg-H-TNTs was about twice as that of pure H-TNTs, indicating Mg2+ ion-exchange could suppress the recombination rate of the photo-excited charges. Combined with the above UV-Vis diffuse reflection and PL spectrum result, Mg-H-TNTs was superior to pure H-TNTs in generation, separation, and transfer of photogenerated charge carriers due to Mg2+ ion-exchange. 3.2.2 Physical and chemical property of Mg-TNTs The BET surface area of the XM-Mg-H-TNTs（x= 0.1, 0.2）and pure H-TNTs catalysts were given in Table 2. It showed that BET surface areas of XM-Mg-H-TNTs （x=0.1, 0.2）were less than that of the pure H-TNTs after Mg2+ ion-exchange. It could contribute to that Mg2+ with a relatively large ion radius of 0.072nm can enter the layers in H-TNTs’ wall and lead the decrease of surface area. To investigate the surface elemental distribution and atomic concentration over H-TNTs and 0.1M-Mg-H-TNTs, XPS characterization was employed. As shown in the Fig. S1, XPS peaks in 1303.9 eV indicated the appearance of Mg in 0.1M-Mg-H-TNTs. Table 3. summarized surface atomic concentration of Ti, O and 10

Mg, which displayed 0.1M-Mg-H-TNTs had 5.64 at% Mg on its surface. Since the thickness of the H-TNTs’ wall was close to the detected depth (2-3 nm) of XPS[26], only the exterior Mg and part of the interior Mg in the H-TNTs could be detected. To cut the wall of the H-TNTs, the catalysts were etched via ion sputtering. Thus, the Mg species inside the H-TNTs would be exposed and then detected. In general, for the sample of Mg located inside the H-TNTs, the ion sputtering would lead to enlarged exposure of the Mg species, which would result in an increase of Mg content. In fact, the XPS results confirmed this assumption (Table 4.). Additionally, the etching treatment was operated with a rate of 1 nm/10 s. Since the outside diameter of the H-TNTs wall was 10–12 nm[26,34], so after 150 s ion etching, the detected Mg was on the other side of the H-TNTs and showed a slight decline, indicating that most of the Mg species were located inside the H-TNTs. To determine the distribution of elements on Mg-H-TNTs, TEM EDX spectra and elemental mapping depicted in Fig.S2. Ti, O, and Mg were expectedly observed in the elemental analysis and mapping of 0.1M-Mg-H-TNTs. And Mg element was evenly distributed on the selected area. CO2 adsorption capability is of great importance in photocatalytic reduction of CO2, because the adsorption is the first step of the catalysis process[35]. CO2 thermogravimetry (CO2-TG) was carried out to find whether CO2 adsorption was enhanced after Mg2+ ion-exchange. As displayed in Fig.6, adsorbed CO2 per surface area of 0.1M-Mg-H-TNTs and 0.2M-Mg-H-TNTs was 6.40×10-5 and 6.66×10-5 g/m2, respectively, while that of pure H-TNTs was 5.77×10-5 g/m2, implying about 15% 11

enhancement of CO2 adsorption after H+ was exchanged by Mg2+. Even though Mg2+ ion-exchange lowered the surface area of modified samples (Table 2.), CO2 adsorption amount enhanced. CO2

temperature

programmed

desorption

(CO2-TPD)

curves

of

0.1M-Mg-H-TNTs and pure H-TNTs from 30℃ to 800℃ were displayed in Fig.7. Carbon dioxide desorption temperature was related with the intensity of basic sites[36]. 0.1M-Mg-H-TNTs and pure H-TNTs had desorption peaks below 100℃ (78℃ and 90℃， respectively), corresponding to the strongly physical adsorbed CO2[2,37,38]. This low CO2 desorption temperature was corresponding to a relatively weak basic sites. Because of weak covalent bonds between CO2 molecules and the basic sites[39], 0.1Mg-H-TNTs could capture higher content of CO2 than that of H-TNTs, which was in accordance with the CO2-TG result. Moreover, the desorption peaks above 600℃ were ascribed to the existence of the super basic sites[40,41]. The peaks at higher temperature (718℃) of 0.1M-Mg-H-TNTs could be ascribed to the decomposition of structural carbonate[40,42,43]. What’s more, the in-situ DRIFT spectra of H-TNTs and 0.1M-Mg-H-TNTs with absorbed forms of H2O and CO2 after 60-min light irradiation were presented in Fig.S3. Based on the in-situ DRIFT spectra and the previous reported work, CO2 adsorption mode presented in the sample was displayed in Fig. 8[44]. Furthermore, according to Fig. S3, the absorbance of HCO32-(1663 cm-1) on the surface of 0.1M-Mg-H-TNTs showed a certain decrease after 60-min irradiation. The decreased amount of HCO32- would participate in the following reaction, leading to the promotion of photocatalytic reduction of CO 2. 12

3.3 Mechanism for the photocatalytic activity enhancement The main processes that contributed to the CO2 reduction performance on Mg-H-TNTs are schematically illustrated in Figure 9. Firstly, CO 2 molecules were adsorbed and immobilized on the catalyst surface. Due to the Mg2+ ion exchange, a larger amount of CO2 could be adsorbed on the basic sites of Mg-H-TNTs than pure H-TNTs as confirmed by CO2-TG and CO2-TPD test. Secondly, electrons on the valance band of Mg-H-TNTs were excited under light irradiation and transferred to the conduction band. Then the adsorbed CO2 specises were reduced at the adsorption site with electrons and protons.

Unfortunately,

serious recombination of

photo-induced electron-hole pairs took place because of the poor conductivity of pure H-TNTs (see Fig.5B, photocurrent test). Thanks to the Mg 2+ ion-exchange, not only the transmission of the charge carriers but also the separation of excited electron-hole pairs (see Fig.5A, PL measurement) promoted. Thus, the electron density around CO 2 adsorption sites increased. Obviously, two protons and two electrons were needed to produce CO, while for CH4 production, eight protons and eight electrons were required (Equations (1) and (2)). Due to the enhanced electron conductivity and decreased recombination rate on Mg-H-TNTs, the yield of CO and CH4 were greatly enhanced on Mg-H-TNTs than that of pure H-TNTs, especially for the increased CH4 selectivity. CO2 + 2H+ + 2e- → CO + H2O

(1)

CO2 + 8H+ + 8e- → CH4 + H2O

(2)

To verify our assumption that alkali and alkaline earth metal ion-exchanged H-TNTs have superior CO2 photocatalytic activity, K-H-TNTs and Na-H-TNTs were 13

also prepared and tested under the same conditions mentioned above, of which result is showed in Fig.10. Compared to pure H-TNTs, 5-h accumulated production of CO of K-H-TNTs and Na-H-TNTs were 1.71 and 1.39 times, respectively. To our surprise, CH4 yield of K-H-TNTs and Na-H-TNTs were as 25.2 and 11.5 times as much as that of pure H-TNTs. The above results strongly supported our conjecture that simply exchanging H+ of H-TNTs with alkali and alkaline earth metal cation can significantly enhance the CO2 photocatalytic reduction performance. 4. Conclusion In this study, the CO2 photocatalytic reduction performance of Mg-H-TNTs under simulated sunlight with water vapor as proton provider was investigated. And CO and CH4 were the main products with negligible other hydrocarbons. Moreover, all as-prepared samples showed more excellent performance in CO2 photocatalytic reduction than pure H-TNTs. Among all the catalysts, 5-h CO and CH4 amount of 0.1M-Mg-H-TNTs was the most excellent one and was about 3.76 times and 16.49 times as much as that of the pure H-TNTs, respectively. From the characterization results of XRD and TEM, it was concluded that ion-exchanged with Mg2+ wouldn’t destroy the tubular structure of H-TNTs. The key factors that led to the improved photocatalytic reduction performance were as follows: (1) efficient photogenerated hole-electron pairs generation, separation and transfer, (2) improved CO 2 adsorption and activation by the alkali and alkaline earth metal ion-exchange. Further study on Na-H-TNTs and K-H-TNTs proved that simply exchanging H+ of H-TNTs with alkali cation could indeed significantly enhance CO2 photocatalytic reduction performance. The findings provide a simple method to obtain ion-exchanged H-TNTs with 14

remarkable CO2 photocatalytic performance and inspire further development of more efficient photocatalysts for CO2 photoreduction to solve greenhouse effect and energy crisis simultaneously.

Acknowledgements This research is financially supported by National Natural Science Foundation of China (NSFC-51578488), Zhejiang Provincial “151” Talents Program, the Program for Zhejiang Leading Team of S&T Innovation (Grant No. 2013TD07) and Changjiang Scholar Incentive Program (Ministry of Education, China, 2009).

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Table and Figure captions: Table 1. Band gap of XM-Mg-H-TNTs（x=0.1,0.2）and pure H-TNTs Table 2. BET surface area of XM-Mg-H-TNTs（x= 0.1, 0.2）and pure H-TNTs Table 3. Surface atomic concentration of Ti, O and Mg obtained with XPS Table 4. Atomic concentration with XPS characterization Fig. 1. 5-h accumulated production of CO and CH4 on H-TNTs and XM-Mg-H-TNTs (X=0.005, 0.025, 0.1 and 0.2) Fig. 2. XRD patterns of XM-Mg-H-TNTs (X=0.005, 0.025, 0.1, 0.2) and pure H-TNTs Fig. 3. TEM images of H-TNTs (A) and 0.1M-Mg-H-TNTs (B) Fig. 4. UV–Vis diffuse reflection spectra of XM-Mg-H-TNTs (X= 0.1, 0.2) and pure H-TNTs Fig. 5. (A) Photoluminenscence spectra with 325 nm excitation light and (B) photocurrent responses of XM-Mg-H-TNTs (X= 0.1, 0.2) and pure H-TNTs Fig. 6. Adsorbed CO2 per surface area of XM-Mg-H-TNTs (X= 0.1, 0.2) and pure H-TNTs Fig. 7. CO2-TPD curves of 0.1M-Mg-H-TNTs and pure H-TNTs Fig. 8. CO2 adsorption mode on Mg-H-TNTs: bicarbonate Fig. 9. Schematic illustration of the mechanism for the promotion of CO 2 photocatalytic reduction with Mg-H-TNTs Fig. 10. 5-h accumulated production of CO and CH4 on H-TNTs and X-H-TNTs (X=Mg, K, Na) 19

Table 1. Band gap of XM-Mg-H-TNTs（x=0.1,0.2）and pure H-TNTs Sample

Band gap/eV

H-TNTs

3.05

0.1M-Mg-H-TNTs

3.01

0.2M-Mg-H-TNTs

3.03

Table 2. BET surface area of XM-Mg-H-TNTs（x= 0.1, 0.2）and pure H-TNTs Sample Surface area(m2/g)

H-TNTs 0.1M-Mg-H-TNTs 0.2M-Mg-H-TNTs 442

332

310

Table 3. Surface atomic concentration of Ti, O and Mg obtained with XPS Surface elemental concentration Samples

Ti 2p

O 1s

Mg 1s

H-TNTs 0.1M-Mg-H-TNTs

25.47 23.82

74.53 70.54

/ 5.64

Table 4. Atomic concentration with XPS characterization Catalysts

Treatment

O (at%)

Ti (at%)

Mg (at%)

0.1M-Mg-H-TNTs

Before etching 50 s etching 100 s etching 150 s etching No etching

72.96 68.07 67.54 67.32 72.16

21.17 25.61 25.53 26.63 27.84

5.87 6.32 6.92 6.05 /

H-TNTs

20

Fig. 1. 5-h accumulated production of CO and CH4 on H-TNTs and XM-Mg-H-TNTs (X=0.005, 0.025, 0.1 and 0.2)

Fig. 2. XRD patterns of XM-Mg-H-TNTs (X=0.005, 0.025, 0.1, 0.2) and pure H-TNTs

21

Fig. 3. TEM images of H-TNTs (A) and 0.1M-Mg-H-TNTs (B)

Fig. 4. UV–Vis diffuse reflection spectra of XM-Mg-H-TNTs (X= 0.1, 0.2) and pure H-TNTs

22

Fig. 5. (A) Photoluminenscence spectra with 325 nm excitation light and (B) photocurrent responses of XM-Mg-H-TNTs (X= 0.1, 0.2) and pure H-TNTs

Fig. 6. Adsorbed CO2 per surface area of XM-Mg-H-TNTs (X= 0.1, 0.2) and pure H-TNTs

23

Fig. 7. CO2-TPD curves of 0.1M-Mg-H-TNTs and pure H-TNTs

Fig. 8. CO2 adsorption mode on Mg-H-TNTs: bicarbonate

24

Fig. 9. Schematic illustration of the mechanism for the promotion of CO 2 photocatalytic reduction with Mg-H-TNTs

Fig. 10. 5-h accumulated production of CO and CH4 on H-TNTs and X-H-TNTs (X=Mg, K, Na) 25



Mg2+ ions exchanged hydrogen titanate nanotubes significantly promoted the production of CO and CH4.



Better transmission efficiency of photogenerated electron and stronger CO 2 adsorption were found to be the main reason of increased activity.



K+ and Na+ ion exchange results indicated alkali and alkaline earth metal ion-exchange method was a feasible way to promote CO2 photocatalytic performance.

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