Photocatalytic conversion of CO2 and H2O to useful fuels by nanostructured composite catalysis

Accepted Manuscript Photocatalytic conversion of CO2 and H2O to useful fuels by nanostructured composite catalysis

Fares Almomani, Rahul Bhosale, Majeda Khraisheh, Anand Kumar, Muhammad Tawalbeh PII: DOI: Reference:

S0169-4332(19)30935-3 https://doi.org/10.1016/j.apsusc.2019.03.304 APSUSC 42259

To appear in:

Applied Surface Science

Received date: Revised date: Accepted date:

3 December 2018 23 March 2019 27 March 2019

Please cite this article as: F. Almomani, R. Bhosale, M. Khraisheh, et al., Photocatalytic conversion of CO2 and H2O to useful fuels by nanostructured composite catalysis, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.03.304

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ACCEPTED MANUSCRIPT PHOTOCATALYTIC CONVERSION OF CO2 AND H2O TO USEFUL FUELS BY NANOSTRUCTURED COMPOSITE CATALYSIS Fares Almomani1, Rahul Bhosale1, Majeda Khraisheh1, Anand Kumar1, Muhammad Tawalbeh2 1) Department of Chemical Engineering, College of Engineering, Qatar University, P. O. Box 2713, Doha, Qatar

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Abstract

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27272 Sharjah, UAE

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2) Sustainable & Renewable Energy Engineering Department, College of Engineering, University of Sharjah, P.O Box

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Cu-TiO2 nano-catalysis were successfully prepared using sol-gel method and used for solar photo-

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reduction of CO2 in gas and liquid phase. Adding Cu toTiO2 matrix modify its crystalline structure, improved its optical property and increased the photo-catalytic activity towards CO2 reduction.

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Incorporating Cu at an oxidation state of 2+ into TiO2 matrix generated a mixture of Anatase and Rutile

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structure with high surface area, increased oxygen vacancies and enhanced atomic mobility which improved CO2 photo-reduction in both phases. The highest CO2 photoreduction rate was observed to occur

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for Cu-TiO2 nano-catalyst with Cu loading of 1.5wt%. Methanol was the most produced hydrocarbon

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amongst the products with a production rate of 4.0 µmol.g-cat-1.h-1, followed by methane. Gas phase solar CO2 photo-reduction was effective and dependent on the gas relative humidity. CO2 and H2O mixture

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with relative humidity (%RH) ≤ 30 % generated CH4 and CO as the main products. At higher %RH, the main products were methane, hydrogen, methanol, ethanol, and acetaldehyde. Gas phase solar CO2 photoreduction is more effective than liquid phase in terms of hydrocarbons production rate, space yield, and quantum efficiencies. Results showed that solar photo-catalytic reduction can be successfully applied to reduce CO2 from the atmosphere.

*Corresponding

author: Professor. Department of Chemical Engineering, Qatar University, Tel: +974 4403 4140 Fax: +974 4403 4131, Mobile: +974 6641 3198. P.O. Box: 2713 - Doha, Qatar.E-mail: [email protected]

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ACCEPTED MANUSCRIPT Keyword: Global warming, Greenhouse gases, emissions, sustainability, nano-catalysis, nano-energy

1- INTRODUCTION

There has been a significant rise in the concentrations of greenhouse gases (water vapor, CO2, methane, NOx, O3 and CFCs) in the atmospheric as results of excessive fossil fuel combustion [1-3]. Although the

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presence of these gases in the atmosphere is vital, the unbalancedrise in their concentrations has resulted

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in several environmental problems such as climate change, the global increase in the average earth surface

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temperatures and increase in the chances for acid rain formations [4, 5]. CO2 is considered one of the main

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compounds contribute to these environmental problems. Continuous measurements of CO2 in the atmosphere have shown an increase in its concentration by 2.1 ppm per years combined with an increase

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in the global average temperature of 0.658 °C per century. Because of the serious problems associated by increasing the concentrations of CO2 in the atmosphere, several countries have proposed strategies to

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decrease its emissions and eventually reverse the process using technologies converting it to useful fuel

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[6, 7].

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Naturally, plants demonstrate the most direct strategy to maintain atmospheric CO2 at equilibrium levels by converting it into energy-rich products using solar renewable energy in the photosynthesis process as

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presented in reactions (1-3) [8]. Photo-reduction of CO2 to usual energy products showed promises in

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recent years as an effective alternative that mimics the natural photosynthesis process, reduces the CO2 from the atmosphere and generates useful fuel[9-13]. CO2 + H2O → CH4 + O2

(1)

CO2 + H2O → CH3OH + O2

(2)

XCO2

+ ½ yH2O → CxHy + (x+2y)O2

(3)

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ACCEPTED MANUSCRIPT The efficiency of CO2 photo-reduction processes is highly dependent on the utilization of light intensities and the type of photo-catalyst. Research on this area has shown that the process can work effectively if appropriate catalysis with high quantum yields, activity, and selectivity toward CO2 reduction is used. TiO2 is usually used in photo-catalytic processes due to its low cost, thermal stability and high response to light irradiation, making it a good candidate for CO2 photoreduction technologies [14, 15]. However,

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the productivity of TiO2 still below expectations as the maximum CO2 photo-reduction rate cannot exceed

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25μmol.gcat−1.hr−1 [16, 17]. Mixing TiO2 with metals or oxide can increase its lifetime and improve its

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photo-catalytic activities by promoting the separation of photo-excited charge to enhance CO2 photo-

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reduction[18-22]. Other types of photocatalytic materials with enhanced visible-light photocatalytic activity were also recently prepared and used in treating different gaseous compounds such as nitric oxide

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(NO) [23-26].

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To date, few research works have been conducted to investigate and optimize the effect of adding simple

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and cheap metals (e.g. Ni, Cu and Zn ) to TiO2 on CO2 photo-reduction or to understand their effect on improving the TiO2 photo-catalytic properties [27-29]. According to our literature review, there is no

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work, yet, investigated the use of solar irradiation for photo-reduction of CO2 and determined the

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efficiency of using solar energy to mimic the natural photosynthetic process in the conversion of CO2 into fuel molecules, such as CH4, CH3OH. Most of the work done in this research area utilized artificial UV

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or visible light. Thus, the present work aims to investigate the effect of adding Cu to TiO2 on the photocatalytic conversion of CO2 to fuel. To this end, an efficient and selective nano-structured photo-catalysts were synthesized and used in the artificial photosynthesis process. The effects of different operational conditions on the systems performance and the reactions products were studied. Detailed catalysis characterization was presented and discussed. Experiments were conducted in gas and liquid phase pilot plant using solar irradiation. 2. MATERIALS AND METHODS

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ACCEPTED MANUSCRIPT 2.1 Catalysis preparation 2.1.1 Cu-TiO2 nanoparticles Cu-TiO2 nano-crystallines were prepared using sol-gel methods. Initially, a specific amount of copper acetate dihydrate (Cu(CH3COO)2·H2O) was dissolved in a mixture of 0.91 g mono-ethanolamine (MEA),

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10 mL deionized water and 15 mL isopropanol alcohol (Solution A). Another solution of 10 mL of

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titanium butoxide (Ti(OBu)4 was dispersed in 40 mL ethyl alcohol using ultra-sonication for 15 min

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(Solution B). Solution B was added to solution A in a stepwise fashion and vigorously stirred at room temperature for 3 hrs to produce Mixture-AB. The resulting transparent mixture (Mixture AB) was aged

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for another 6 h at room temperature, then dried with dry air at 75 °C for 40 hr. The dry material was

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calcined at 500 °C for 1.5 h. Nanoparticles with Cu weight percentage of 0.5%, 1%, 1.5% and 2.0% were prepared and compared with original TiO2 nanoparticles toward CO2 reduction. TiO2 nanoparticles loaded

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with specific %wt. of Cu will be denoted as wt % Cu -TiO2. Thus, the particles prepared in this study are

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0.5%Cu-TiO2, 1%Cu-TiO2, 1.5%Cu-TiO2 and 2%Cu-TiO2.

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2.1.2 Cu-TiO2 loaded on support media

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Mesoporous silica was prepared following the procedure outlined in Zhao et al.,[30]. Initially, 5 g of Pluronic (Sigma-Aldrich) was dissolved in 130 g of 1.8 M HCl at 30°C and gently mixed at 30 rpm.

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Mixing was continued until all the Pluronic dissolved. Next, tetraethyl orthosilicate (Sigma Aldrich) was added into the solution and the generated mixture was mixed at 60 RPM for 24 hr under a controlled temperature of 35 °C. The mixture was aged for another 24h at 80 °C in a closed container. The generated nano-powder was filtrated, air-dried at 30°C for a period of 24hr and calcinated at 500 °C for 6 h. CuTiO2 nano-crystalline was loaded on Mesoporous silica by mixing the generated nano-powders with Mixture AB. The procedure used for aging, drying, and calcination of the loaded material is the same as

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ACCEPTED MANUSCRIPT above. Copper and TiO2 loaded on mesoporous silica will be denoted as L-wt%-Cu-TiO2. Thus, the loaded particles used in this study are L-0.5%Cu-TiO2, L-1%Cu-TiO2, L-1.5%Cu-TiO2 and L- 2%Cu-TiO2

2.1.3 Nano-catalysis characterizations XRD (Hiltonbrooks) was used to determine the structure of the nano-catalyst. The surface area of the

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produced nano-catalyst was estimated using Brunauer–Emmett–Teller (BET) analysis. XPS (Kratos Axis

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Ultra) was used to determine the chemical composition and electronic states of the elements present in the

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nano-catalysts. All binding energies were charge corrected to the internal standard of carbon (1s, 285 eV).

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Porosimetry analyzer (Micromeritics Autopore IV 9500 V1.05) was used to measure the pore size distribution and porosity changes in the Cu-TiO2. Measurements were carried out using nitrogen

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adsorption-desorption isotherm (AD-DE-ISO) at a pressure in the range of 0.1–20,000 psia. The band gap

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and the absorbance changed for Cu-TiO2 as a function of the transmittance were measured using UV

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spectrophotometer (VARIAN 100 Bio UV-visible spectrophotometer, USA).

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2.2 Experimental setup

Photo-catalytic reduction of CO2 was conducted in liquid and gas phase solar reactors. Figure 1a presents

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the schematic diagram of the liquid phase solar reactor (LPSR). The LPSR consist of five quartz solar

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elements (ID= 5cm and L=80cm). The setup has an internal liquid circulation loop where deionized water is circulated through the tube at a specific flow rate (1.5, 3, 6, 9, 20 and 30 L/min) to maintain the reaction temperature constant and prevent the nano-catalyst precipitation. The gas line consisted of gas flowmeter, pressure gauge, gas diffusers, outlet gas lines and gas analysis system. The gas analysis system consisted of gas chromatography (GC) equipped with an automated gas-sampling valve and connected to thermal conductivity detector (TCD) and flame ionization detector (FID). The liquid in the LPSR was analyzed regularly with HPLC and TOC analyzers to verify if any of the CO2 photo-reduction products are soluble in water. The solar elements are hosted around a compound parabolic collector (CPC) to collect as much 5

ACCEPTED MANUSCRIPT as possible from solar radiation (direct and reflected) and make them available for reaction. The surface of the solar reactor has an angle of 45° with the horizontal level to optimize solar irradiation (400–1000 nm). The sun’s movements were tracked manually during each experiment and a radiometer (Macam Q102 PAR) was used to measure the solar light intensity during the solar reactions. The LPSR was operating at semi-batch flow mode and the liquid and gas effluents were analyzed continuously by a gas

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chromatograph (GC-FID-TCD, Agilent 7890A) and HPLC-MS (Xevo TQD-Waters, Germany).

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Figure 1b presents the gas phase solar reactor (GPSR) that consisted of inlet CO2 line connected to the

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mass flow controller, deionized water saturator, inlet pressure gauge and humidity meter. CO2 bubbles were passed through deionized water to create CO2 and water vapour mixture. The GPSR had five quartz

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solar elements (ID= 2cm and L=50cm), contained the mesoporous silica with Cu-TiO2 nano-catalyst. The

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catalyst support was placed in the center of the solar element and supported by a ceramic ring. The light intensity received at the surface area of the solar reactor was measured by a radiometer (Macam Q102

Figure 1

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continuously by the GC-FID-TCD.

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PAR). The GPSR was operating at a continuous-flow mode and the effluent gas sample was analyzed

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3. RESULTS AND DISCUSSION

3.1 Characteristics of the nano-catalysis Figure 2 presents the XRD analysis of the TiO2, wt%Cu-TiO2 and L-wt%Cu-TiO2 at different percentage of Cu (0.5%, 1%, 1.5%, 2.0%) after calcination process. The XRD measurements on TiO2 before calcination (Results not shown), showed an amorphous structure with diffraction peaks at 25.6, 40.2, 47.0, 53.1 and 63.8° which was confirmed to be the properties of tetragonal anatase phase. A 35-37% weight, loss in nano-powder was reported after calcination because of drying and organic combustion. After calcination at 500°C, the thermal treatment of the TiO2 surface changed to crystalline to pure rutile phase 6

ACCEPTED MANUSCRIPT with diffraction peaks at 23.1, 35.4, 42.8, 44.6, 50.6, 53.6, 63.0 and 63.2°( line # 1, Figure 1a). The peaks are narrow, suggesting a bigger crystalline size. The crystal size and surface area of TiO2 were calculated from the highest peak in the XRD at Rutile (110) using Scherrer equation with a factor of 0.92 and found to be 145.1 nm and 7.45 m2.mg-1, respectively. Similar observations and results were reported by Yang et

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al [12].

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Figure 2

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The XRD for the wt%Cu-TiO2 nano-powders ( lines 2-5, Figure 1a) showed the same diffraction peaks with a maximum intensity at 23.1, 35.4 and 50.6°. It was confirmed that all peaks in the XRD

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spectrum belong to TiO2 and no peaks were observed for Cu suggesting that copper is highly dispersed

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within the TiO2 crystallite or Cu+2 with ionic radius 0.72 A° was incorporated within the structure of Ti+4 which has a comparable ionic radius of 0.68 A° [31]. Similar trends were observed by Nasution et al.,[32]

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and Slamet et al.,[33]. The crystal structure of the wt%Cu-TiO2 nano-powders was found to contain both

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the Anatase and Rutile phases, the condition that reduced the crystal sizes of these nano-powders. Measurements showed that the crystal size of wt%Cu-TiO2 ranged from 5.9 to 19.6 nm, 8.5 to 18.7nm,

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8.7 to 18.3nm and 16.4 to 25.6 nm for nano-powders with Cu loading of 0.5wt%, 1%, 1.5% and 2%,

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respectively. It was also observed that the ratio Anatase to Rutilse increased by increasing the Cu-loading as a result of enhancing particles sintering. The surface areas of wt%Cu-TiO2 nano-powders were in the

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range 35.3 to 92.6 m2.mg-1. In reference to TiO2 surface area after calcination (7.45 m2.mg-1), adding Cu to TiO2 increased the surface area of the nano-powder by a factor of 5 to 10. XRD for L-wt%Cu-TiO2 (Figure 2b) showed that for samples with Cu loading ≤ 1 wt%, the peaks were similar to the original TiO2 nano-powder. TiO2 loaded with Cu higher than 1% showed additional peaks of rutile at 27.44◦ and 36.08◦. These peaks were not observed at lower loading percentages most likely due to the similarity in ionic radii of Cu+2 and Ti+4 or that copper as well dispersed and easily incorporated into the crystal lattice of TiO2 [34]. The very law metal content can be another reason for not observing

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ACCEPTED MANUSCRIPT additional rutile peaks. The diffraction peak intensity of the main anatase peak (110) of L-wt%Cu-TiO2 loaded on support media was observed to be higher than the intensity of the peak on nanoparticles. Figure 3 The XPS measurements on TiO2 nano-powder are presented in Figure 3. Titanium atom showed two peaks

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in the 2P status, which was confirmed to belong to 2P3/2 and 2P1/2 with corresponding binding energy at

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458.3eV and 464.1 eV, respectively. Oxygen on the same nano-powders showed peaks on (O1S) status at

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530.5 eV. The ratio Ti to O in TiO2 structure was estimated to be 1.45. The observed binding energies in this study were in agreement with values reported in the literature at 458.8eV and 539.0 eV [12, 35, 36].

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XPS measurements were also conducted on wt%Cu-TiO2 samples with different Cu loading percentage.

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For Space limitation, XPS results for Cu-TiO2 with Cu loading of 2wt% Figure 3(b-d) will be discussed. The XPS Spectrum of Cu on the status 2P showed two consecutive peaks corresponding to 2P3/2 and 2P1/2

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at a binding energy of 933.1, 934.1, 951.9 and 954.4 eV which are related to Cu in oxidation state [37,

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38]. Colon et al., [39] showed the presence of satellite peaks at the high binding side between Cu+1 and Cu+2 responsible for legend shakeup in the 3d orbit. In the present study, such a peak was not observed

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suggesting that the 3rd orbit of Cu was filled and Cu exist in only Cu+2 form. Figure 4c shows that Ti 2p

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has a clear peaks at 458.6 and 465.1eV corresponding to Ti4+ in the TiO2 structure. Another peak at 457.6 and 463.9 eV were corresponding to Ti3+ which was generated to overcome the difference in binding

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energy due to introducing Cu with Ti. Zhang et al., [40, 41] observed the same trends for Cu-TiO2 composite. The XPS of oxygen (O1S) in the TiO2 structure shows that oxygen existed at three overlapped peaks with the corresponding binding energies 530.1, 530.6 and 531eV. The highest peak was observed at 530.1 which belongs to oxygen in TiO2 structure, a similar observation was noticed by Rumble et al., [37]. The peak at 530.6 is related to oxygen in Cuo structure and the peak at 531 represented other forms of oxygen in the structure such as hydroxide [19, 42].

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ACCEPTED MANUSCRIPT Absorption spectrum measurements showed that TiO2 had the highest absorption in the wavelength range of 200-389nm. Adding Cu to TiO2 showed a significant increase in the absorbance and a slight shift in absorption wavelength. The cutoff wavelengths were 392, 400, 207 and 414 nm for TiO2 loaded with 0.5. 1.0, 1.5 and 2.0wt% Cu. The band gap energy for TiO2 was estimated using equation (1) as outlined by Usubharatana et al., [43].

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1240 𝜆

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𝐸𝑔 =

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Where 𝜆 is the cutoff wavelength, TiO2 has a band gap energy of 3.05 eV. Adding Cu to TiO2 reduced

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the band gape to the range 2.50-2.85 eV. Similar results were reported by Colon et al.,[39] and Liu et al.,[44].

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3.2 Morphology of the Nano-catalysts

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The surface property of the nano-catalyst was investigated using nitrogen adsorption-desorption isotherm

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(AD-DE-ISO) (Figure 4a). The AD-DE-ISO of TiO2 and wt%Cu-TiO2 nano-particles followed H1-type hysteresis loop confirming the generation of mesophorous structure during the formation of nano-powders

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aggregates. The pore size distribution of TiO2 and Cu-TiO2 with different Cu loading is presented in figure

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4b. TiO2 had an average pore size diameter of 7.5 nm, wt%Cu-TiO2 nano-powders with Cu loading of

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0.5wt%, 1%, 1.5% and 2% have an average pore size diameter of 7.1, 6.8, 5.0 and 4.3 nm, respectively. Figure 4

The AD-DE-ISO of L-wt%Cu-TiO2 loaded on amorphous silica exhibit the same trends as for pure nanopowder (results not shown) suggesting mesoporous materials characteristics. The pressure ratio (P/Pi) of the hysteresis loop of pure amorphous silica started from 0.55 to 0.75, while for L-wt%Cu-TiO2 the range was from 0.41 to 0.66. It has been reported that a hysteresis loop with lower starting relative pressure indicates a small pore size diameter [45]. The obtained AD-DE-ISO suggest that L-wt%Cu-TiO2 loaded on amorphous silica has small pore size as compared with pure amorphous silica indicating that the nano-

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ACCEPTED MANUSCRIPT particles were uniformly dispersed on the surface of the silica channels. The average pore size diameter of amorphous silica was estimated to be 6.3 nm. L-wt%Cu-TiO2 loaded on amorphous silica showed a pore size 5.5 nm for L-0.5wt% Cu-TiO2, increasing the Cu-TiO2 content has resulted in a further decrease in the pore size diameter being 5.0, 4.8 and 4.1 nm for L-wt%Cu-TiO2 with Cu of 1, 1.5 and 2 wt%, respectively. A possible reason for the decrease in pore size diameter is the increase in the number of

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nanoparticles deposition on the silica the condition that block the pores of its surface, thus the pore size

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distribution converged to the size that represents the Cu-TiO2 nanoparticles.

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3.3 Solar reduction of CO2 3.3.1 Liquid phase solar CO2 reduction

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Preliminary experiments were conducted in the LPSR to identify the main byproduct generated from the

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solar reduction of CO2; hydrogen, methane, acetaldehyde, ethanol and methanol were the main chemicals identified. Figure 5 presents the solar reduction of CO2 in LPSR using TiO2 and wt%Cu-TiO2

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at different Cu loading percentages. Experiments were conducted under three solar irradiation intensities

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(SOL-INTs): (1) Low SOL-INTs with irradiation in the range 135 to 143μE.m−2 s−1 and an average value of 138 μE.m−2 s−1(Denoted as set-1)(2), Medium SOL-INTs in the range 165 to 188 μE.m−2 s−1 and an

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average value of 175 μE.m−2 s−1 (Denoted as set-2), and (3) high SOL-INTs in the range 185 to 210 to

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μE.m−2 s−1, and an average value of 195 μE.m−2 s−1, (Denoted as set-3). TiO2 alone showed negligible CO2 solar reduction performance. During the 6 hour reaction time, the % of CO2 underwent photoreduction did not exceed 2.5 %. Adding Cu to TiO2 improved the solar photo-reduction activity and more CO2 were converted to by-product. It was observed that CO2 photo-reduction increased by increasing the weight percentage of Cu in the wt%Cu-TiO2 nanoparticles up to 1.5 wt%, after that the reduction of CO2 decreased. The % CO2 reduction increased by 45.0% to 46.5% by increasing the Cu loading from 0.5 to 1wt%, another 23.2% to 25.9 % increase in CO2 reduction was observed for 1.5wt%Cu-TiO2. However, increasing the Cu loading to 2wt% Cu-TiO2 showed a 59% decrease in CO2 10

ACCEPTED MANUSCRIPT reduction compared with 1.5wt% Cu-TiO2. The observed trends can be related to incorporating of Cu into TiO2 matrix generating Anatase structure with high surface area. The XRD measurements have confirmed the presence of the TiO2 anatase phases and rutile with the major part as anatase. It was also confirmed that Cu at an oxidation state of 2+ which increase oxygen vacancies and atomic mobility. These conditions increased CO2 photo-reduction. Zhang et al., [46] reported that the electron hole

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recombination can be delayed via electron traps generated during the change from rutile to anatase. The rutile/anatase phase junction also facilitated charge separation, improved photo-reduction conditions and

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increased in catalysis lifetime [47]. Tseng et al., [18] showed that as the concentration of Cu increase

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above 2wt% the amount of photon absorbed on the surface of TiO2 decreased the conditions that decrease the performance of CO2 reduction.

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Figure 5

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3.3.2 Product formation

Figure 6 presents the production rate of methane, hydrogen, methanol, ethanol and acetaldehyde over

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various Cu loaded TiO2 nanoparticles. Experiments carried out with TiO2 showed negligible production

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rate of these products. However, adding Cu to TiO2 improved the solar photo-reduction process compared with pure TiO2. The highest production rate between the products was observed to occur when 1.5wt%Cu-

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TiO2 is used. Methane, methanol and ethanol are the most produced hydrocarbon amongst the products.

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The maximum production of methane, methanol and ethanol were found to be 4.0, 2.3 and 0.78 µmol.gcat-1.h-1, respectively, when 1.5wt% Cu-TiO2 nano-catalyst was used. Higher Cu loading showed a decrease in the production of hydrocarbons confirming the observed trends during CO2 photo-reduction. The general trends for CO2 photo-reduction products under all studied conditions show that the production rate of H2, methanol, ethanol, acetaldehyde and methane increased by factors of 8.8, 3.3. 4.3 and 3.3 as the percentage of Cu in the wt%Cu-TiO2 nano-particles increased from 0.5 to 1.5%. Higher Cu percentage resulted in at least 48% reduction in the production of all products.

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ACCEPTED MANUSCRIPT Previous works confirmed that Cu+2 and CuO are the primary phases responsible for CO2 photo-reduction [48-50]. CuO can trap the photo-excited electron from the TiO2 conduction band to be used in reduction reaction and prevent electron-hole recombination. Based on that, the redox potential will only occur if the potential of the valance band (EVB) is more positive than the corresponding redox potential. It is known that the redox potentials required to convert CO2 to hydrogen, methanol, ethanol at neutral pH and 25 °C

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are - 0.40, -0.38 and -0.33V Vs NHE. Comparing this potential value with CuO potential in the conduction band ECB=-0.36V, suggests that it is enough to reduce CO2 to these compounds. Another reaction that can

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occur in the solar system is the oxidation of CO2 to O2 supported by the high positive EVB of CuO (2.84).

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CuO was efficiently used for a similar application such as degradation of rhodamin B, and water splitting [51, 52].

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3.3.3 Effect of Solar irradiation

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The sustainability of the solar CO2 photo-reduction was investigated by following the effect of variation of solar irradiation intensities (SOL-INTs) on the photo-reduction of CO2 (Figures 5 and 6). Although

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the effect of varying the SOL-INTs needed further detailed investigations, the objective of this section

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was to explore whether CO2 photo-reduction was feasible under different solar irradiation values. For the purpose of comparison, the reaction temperature was held constant and the only variable to be

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considered was the SOL-INTs. As indicated before, three sets of experiments with low (set-1), medium

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(set-2) and high (set-3) SOL-INTs were conducted. The obtained results showed that Cu-TiO2 nanocatalyst had significant CO2 photo-reduction under all studied SOL-INTs. Compared with TiO2 alone, adding Cu to TiO2 created impurity energy levels within the TiO2 band gap that enhanced visible light harvesting, delayed the electron-pairs recombination rate and increased the interfacial electron-transfer rate. Thus, the CO2 photoreduction rate and fuel products formations increased. An increase in the average light intensity by 25.0% (138 to 185 μE.m−2 s−1 ) between Set-1 and 2 resulted in a slight increase of ~ 3.3% in CO2 reduction. The same trends were observed for the solar intensity between Set1 and Sets 3. For 0.5wt% Cu-TiO2, increasing the average SOL-INTs from 138 to 175 and then to 195 12

ACCEPTED MANUSCRIPT μE.m−2 s−1 has resulted in an increase in the CO2 photo-reduction by 2.9% and 1.5%, respectively. The same increase in SOL-INTs has resulted in an increase of 2.12% and 4.3 %, respectively for the nanocatalyst with 2wt% Cu-TiO2. The increment in CO2 photoreduction is not high suggesting that under all the studied conditions the solar photoreduction of CO2 is the same order of magnitude. Although the measured SOL-INTs were considered low, the photo-generated holes in the TiO2 valence band (EVB)

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were more positive than O2/H2O oxidation potential and led to the generation of H+ and electrons in the conduction band (ECB) thereby resulting in more CO2 reduction to hydrocarbon. The CO2 reduction to

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hydrocarbon (Figure 6) increased by a factor of 1.2 to14 as the SOL-INTs changed from low to high.

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The change from low to the medium was accompanied with at least 14 to 17 % increase in hydrocarbon production.

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Figure 6

3.3.4 Effect of circulation flow rate

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Another set of experiments was conducted by varying the water circulation flow rate in the LPSR and

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changing the flow regime from laminar to turbulent flow. Results are summarized in Table 1, where the CO2 photo-reduction rate increased by increasing the flow rate from 650 ≤ Re ≤ 3900, after that

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increasing the liquid circulation flow rate showed a decrease on the solar photo-reduction of CO2. The

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obtained results may be attributed to the limited efficiency of the received solar irradiation to create redox potentials required for CO2 reduction. It is known that the photo-generated electron-hole pairs have very short recombination time in order of 10-9s [53, 54], at the same time the surface chemical reaction has a longer time in the range 10-8 to 10-3 s. Therefore, increasing the degree of mixing would increase the chance by which the reactants are mixed with photo-excited electrons and holes to enhance the CO2 reduction. Higher velocity may also decrease the visible light harvesting, increase electron-pairs recombination rate and limits the interfacial electron-transfer rate. Thus, CO2 conversion decreased. Table 1 13

ACCEPTED MANUSCRIPT 3.4 Liquid phase Mechanism Based on the detected products, the mechanism for solar CO2 reduction can be summarized by reactions 4-15 and Figure 7. The photo-reduction started by electrons transfer from TiO2 conduction band (CB) to Cu, and a chemical reaction on the surface of TiO2 surface where H2O dissociated into O2 and proton (H+) (Reaction 4). The electrons transferred to the Cu surface reacted with CO2 generating

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𝐶𝑂2.− intermediate radicals that led to the formation of CO and HCOOH (Reactions 5and 6). The

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electrons also reacted with CO2 in the presence of H+ generating methane (Reaction 7), methanol (Reaction 8), ethanol (Reaction 9), carbon monoxide(Reaction 10) and other products also produced

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(Reaction 11-14) . The generated products depended on the number of electrons and protons involved in each chemical reaction [55]. Hydrogen produced via water dissociation (Reaction 15). A similar results

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mechanism was reported in previous works [56-58]. A control experiment was conducted to verify

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whether the produced hydrocarbon detected in the reactor effluent were generated from CO2 photoreduction or via the reaction with surface carbon impurities in the presence of water. The experiment

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was run by passing circulating deionized water through the solar system in the absence of CO2. Control

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experiments were conducted under all the test conditions and resulted in negligible hydrocarbon production confirming that the detected hydrocarbons were resulted from CO2 photo-reduction. 1

𝐸 ° = 0.82

(4)

𝐶𝑂2 + 𝑒 − → 𝐶𝑂2. → 𝐶𝑂

𝐸 ° = −1.90

(5)

𝐶𝑂2 + 𝑒 − → 𝐶𝑂2. → 𝐻𝐶𝑂𝑂𝐻

𝐸 ° = −1.90

(6)

𝐶𝑂2 + 8𝐻 + + 8𝑒 − → 𝐶𝐻4 + 2𝐻2 𝑂

𝐸 ° = −0.24

(7)

𝐶𝑂2 + 6𝐻 + + 6𝑒 − → 𝐶𝐻3 𝑂𝐻 + 𝐻2 𝑂

𝐸 ° = −0.38

(8)

2𝐶𝑂2 + 9𝐻 + + 12𝑒 − → 𝐶2 𝐻5 𝑜𝐻 + 3𝐻2 𝑂

𝐸 ° = −0.33

(9)

𝐶𝑂2 + 2𝐻 + + 2𝑒 − → 𝐶𝑂 + 𝐻2 𝑂

𝐸 ° = −0.53

(10)

2𝐻 + + 2𝑒 − → 𝐻2

𝐸 ° = −0.41

(11)

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𝐻2 𝑂 → 2 𝑂2 + 2𝐻 + + 2𝑒 −

14

ACCEPTED MANUSCRIPT 𝐶𝑂2 + 𝐻 + + 2𝑒 − → 𝐻𝐶𝑂2.

𝐸 ° = −0.49

(12)

𝐶𝑂2 + 4𝐻 + + 4𝑒 − → 𝐻𝐶𝐻𝑂 + 𝐻2 𝑂

𝐸 ° = −0.48

(13)

2𝐶𝑂2 + 8𝐻 + + 12𝑒 − → 𝐶2 𝐻4 + 2𝐻2 𝑂

𝐸 ° = −0.34

(14)

2𝐻2 𝑂 + 2𝑒 − → 𝐻2 (𝑔) + 2𝐻𝑂−

(15) Figure 7

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3.4.1 Gas phase solar reduction

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Experiments were conducted using L-wt%Cu-TiO2 to investigate the solar-photo-reduction of CO2 to valuable products in GPSR. Preliminary tests showed that the products of CO2 photo-reduction in GPSR

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depends on relative humidity. For GPSR operated with %RH ≤ 30 % the main product was methane. At higher %RH, Methane, hydrogen, methanol, ethanol and acetaldehyde were detected in the effluent gas.

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Figure 8a shows the % CO2 photo-reduction at different L-wt%Cu-TiO2 at low, medium and high SOL-

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INTs and %RH = 30. The solar photo-catalytic activity for CO2 was highly dependent on Cu loading on amorphous silica, wherein the best photo-reduction was observed to occur for L-2wt%Cu-TiO2 compared

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with unloaded TiO2. Higher Cu loading was tested and showed lower CO2 photo-reduction. The

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percentage CO2 photo-reduction in GPSR using TiO2 ranged from 2.6 to 5.3%. Up to 50.3%, 62.2% and 73.1% CO2 photo-reductions were achieved in Set-1, 2 and 3 using L-0.5wt%Cu-TiO2, respectively. An

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increase in the percentage of CO2 photo-reductions by 15.3%, 8.3% and 11.5% was observed when L-

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1.0wt%Cu-TiO2 was used in the respective sets. CO2 photo-reduction up to 80.4%, 85.6% and 89.4% was obtained for L-2.0wt%Cu-TiO2. The increase in photo-reduction activity by adding Cu to TiO2 may be attributed to the fact that Cu improved the surface properties to absorb more photon that resulted in higher photo-reduction. As indicated before, L-wt%Cu-TiO2 were uniformly dispersed on the surface of the silica channels and the increase in % Cu in Cu-TiO2 content had resulted in a decrease in the pore size diameter, changed the titania phase structure the conditions that increased the level of oxygen vacancies via valance reduction resulting in more CO2 photo-catalytic conversion [59]. In addition, Cu reduces the Cu-TiO2 crystal size and affected the atom arrangement to better transformation from Rutile to Anatase which 15

ACCEPTED MANUSCRIPT enhanced the photon activities as reported in previous works [39, 60, 61]. The change Cu implemented on TiO2 was confirmed by UV analysis which showed that introducing Cu to the conduction band of TiO2 led to the formation of a new unoccupied molecular orbit that narrow the band gap and enhance the CO2 photo-reduction. The presence of two TiO2 phases as a result of adding Cu to the supported nano-catalyst may be another reason for the enhancing charge separation that increased the reactivity towards CO2

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reduction.

Figure 8a also shows the effect of SOL-INTs on CO2 photo-reduction in GPSR. For all the studied L-

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wt%-Cu-TiO2, increasing the received SOL-INTs resulted in an increase in the % CO2 reduced. The

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incremental increase in the amount of CO2 reduced ranged from 5.3 to 24.5 % and 4.1 to 17.4% as the SOL-INTs changed from low to medium ( Set-1 to 2) and medium to high ( Set-2 to 3), respectively.

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The rate of production of methanol, ethanol, hydrogen and acetaldehyde over L-2wt% Cu-TiO2

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supported catalyst is presented in Figure 8b. Hydrocarbon production increased by increasing the reaction time from 0 to 2.5 hours after that, the production rate leveled off. The rate of production of

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methane was found to be the highest at 5 µmol.g-cat-1 h-1, followed by H2, ethanol, acetaldehyde and

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lastly methanol at 4.6, 4.2, 3.9 and 3.9 µmol.g-cat-1 h-1, respectively. As indicated before, the type and amount of hydrocarbon produced depend on the electrons generated in the system and the available by-

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products (Reactions 1-10). At some stage hydrocarbon may be oxidized to other chemicals: methanol

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can be converted to acetaldehyde. Thus, the cumulative production rate of the later is higher than methanol. The slight decrease in the cumulative hydrocarbon production rate after 2.5 hours may attributed to partial catalyst deactivation and/or insufficient photons to sustain CO2 reduction as some of these photons can be consumed in hydrocarbon oxidation. The effect of relative humidity on the solar photo-reduction of CO2 for the experiments carried out at high SOL-INT ( Set-3) and L-2wt%Cu-TiO2 is presented in Figure 8c. Noticeably, CO2 photo-reduction started at %RH =15 and increased by increasing the % RH up to 50. After that, the CO2 photo-reduction decreased significantly, suggesting catalytic inhibition by excess H2O in the moisture. The decrease in the photo16

ACCEPTED MANUSCRIPT reduction efficiency at higher %RH can be attributed to the decrease in CO2 adsorption on L-2wt% CuTiO2 as a result of the presence of high H2O vapor at the solid-gas interface as outlined by Teramura [62]and Xie [63]. The low and limited desorption of the formed products from the catalyst surface is also another factor that can affect the photo-reduction efficiency. Figure 8d shows that the % RH in GPSR determine the type of products generated from the solar photo-reduction process. As the % RH increased,

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the number of products detected in the reactor effluent also increased. The general trends for all the L-

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wt%Cu-TiO2 catalysis showed that at lower %RH the main reaction products were methane and hydrogen,

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as the %RH increased, the products changed and methanol, ethanol and acetaldehyde were detected as

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products. 3.5 Comparison between liquid and gas phase reactors

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LPSR and GPSL used in the present study have the same configurations; therefore, the comparison

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between the two setups is beneficial to provide an understanding of their performance with respect to CO2 solar reduction to energy fuels. The comparison was based on % CO2 reduction, quantity, type of

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products formed, quantum efficiency and space-time. Table 2 presents a summary of the results in both

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systems.

Table 2

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Hydrocarbon production, space yield and quantum efficiencies for GPSR were higher than LPSR. As the

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solar intensities received by both systems are almost the same it may be concluded that the performance of GPSR toward CO2 photo-reduction to hydrocarbon was better than LPSR. For all the studied conditions, the amount of hydrocarbon produced in GPSR was at least 3 times higher than LPSR. The quantum efficiency of the GPSR in the range of 0.04% to 0.37 % was 10- 16.8 times higher than that of LPSR. The obtained results may be related to the difference in light utilization efficiencies between LPSR and GPSR, which may be affected by the water circulation in the first setup. The liquid-gas mass transfer resistance and solubility limitation in the LPSR may be another reason that affects the first system [64]. Conversely, high CO2 photo-reduction was observed in GPSR due to the existence of one phase reaction that allowed 17

ACCEPTED MANUSCRIPT maximum utilization of Cu-TiO2 high surface area, received solar photons and low mass transfer resistance due to high gas mixing. 4. Conclusion Copper-TiO2 nano-catalysts synthesized using sol-gel synthesis methods, involved highly dispersed Cu within the TiO2 crystallite anatase and rutile phases. The Cu-TiO2 was uniformly dispersed on

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amorphous silica and resulted in smaller pore size diameter compared to pure amorphous silica indicating that the nanoparticles were on the surface of the silica channels. The average pore size diameter of Cu-

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TiO2 loaded on amorphous silica was in the range of 4.1 to 5.5 nm. The Cu mixed with TiO2 powerfully

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promoted solar CO2 photo-reduction to fuel hydrocarbon. This promotion depended on the % Cu in CuTiO2 nano-powder that changed the surfaces morphology of the catalysis and enhance photon reception

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from solar irradiation. Solar reduction of CO2 produced mainly methane, hydrogen, methanol in liquid

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phase reactions. Solar photo-reduction may be applied as sustainable technology to convert CO2 in the

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atmosphere to useful fuel products.

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Re

% CO2

Average light

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Experiment

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Table 1: Effect of water circulation flow rate on CO2 photo-reduction in Liquid solar reactor (LPSR).

Reduction (±1) intensity(±1)

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μE.m−2 s−1

650

80

138

2

1300

83

139

3

2600

89

141

4

3900

90

140

5

8700

87

139

13000

80

140

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1

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6

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Parameter

Catalysis

Catalyst

Products, production rate (µmol/g-cat.h)(3)

Space Yield (µmol.h-1 m-3)4

Quantum

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Table 2: Comparison between LPSR and GPSL efficiency(4)

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weight (g) (2) % Cu

H2

Methanol

Ethanol

acetaldehyde

Methane

H2

Methanol

Ethanol

acetaldehyde

Methane

0.5

0.059

0.12

1.17

0.59

0.02

0.63

Wt%cu-

1.0

0.117

0.39

2.34

1.17

0.07

1.13

TiO2

1.5

0.177

1.04

4.00

2.34

0.78

1.9

2.0

0.234

0.46

2.47

1.17

0.39

1.32

(1)

GPSR

0.5

0.046

0.76

0.95

1.25

1.10

1.29

Wt%cu-

1.0

0.094

1.27

1.58

2.09

1.83

2.15

TiO2

1.5

0.139

2.02

2.53

3.34

2.94

3.43

0.18

2.0

0.185

2 .30

2.88

3.79

3.33

3.90

0.37

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LPSR(1)

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33

43

34

0.004 0.009

38

28

32

0.015 0.023 0.04

58

66

90

53

0.10

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(1) Solar intensities (SOL-INTs); 185 to 210 to μE.m−2 s−1,avg. 185 μE.m−2 s−1. (2) Catalyst weight was calculated as the difference between the weigh of amorphous silica before and after Cu-TiO2 loading (3) Products, production rate was calculated from the average values during the solar reduction process (4) Calculated based on the volume of solar elements in the LPSR and GPSR n∗Mole of hydrocarbon produced (5) %Quantum efficiency = , where n is the number of electrons

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𝑆𝑂𝐿−𝐼𝑁𝑇𝑆

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Figue 1: Experimental set up; (a) liquid phase solar reactor (LPSR) and (b) gas phase solar reactor

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(GPSR)

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Figure 2: XRD patterns of (A) TiO2 and wt%Cu-TiO2 nano powder (b) L-wt%Cu-TiO2 loaded on support media. (1) TiO2, (2) 0.5 wt% Cu, (3) 1 wt% Cu, (4) 1.5 wt% Cu and (5) 2 wt% Cu.

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Figure 3: XPS spectra of (a) TiO2 nano-powder; Outer graph (Ti 2p) and inner graph ( O 1s) (b) Cu 2p

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of 2wt% Cu-TiO2 (c) Ti 2p of 2wt% Cu-TiO2 and (d) O 1s of 2wt% Cu-TiO2

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Figure 4: (a) nitrogen adsorption-desorption isotherm (AD-DE-ISO) (b) pore size distributions

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Figure 5: Evolution of CO2 as a function of time during the solar reduction in LPSR at different Cu-

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TiO2. Solar intensities for Set-1: 135 to 143μE.m−2 s−1, Set-2: 165 to 188 μE.m−2 s−1 and set-3: 185 to

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Figure 6: Hydrocarbon production rate over wt%Cu-TiO2 nanoparticles under solar oxidation. Set-1:

135 to 143μE.m−2 s−1, Set-2: 185 to 210 to μE.m−2 s−1 and set-3: 165 to 188 μE.m−2 s−1 .

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Figure 7: Surface mechanism of CO2 photo-reduction in liquid phase

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Figure 8: (a) percentage CO2 photo-reduction in gaseous phase reaction at L-wt% Cu-TiO2 and %RH =

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30%, (b) The rate of production of methanol, ethanol, hydrogen and acetaldehyde over 2wt% Cu-TiO2

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supported catalyst as function of reaction time in set- 3 and %RH = 30. (C) Effect of relative humidity on the solar photo-reduction of CO2 on L-wt% Cu-TiO2 and set- 3 (D) Effect of relative humidity on product formation over 2wt% Cu-TiO2 in GPSR in set-3. Set-1: 135 to 143μE.m−2 s−1, Set-2: 185 to 210 to μE.m−2 s−1 and set-3: 165 to 188 μE.m−2 s−1

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Highlights  Solar photo-catalytic reduction technology was applied to reduce CO2 from atmosphere  Cu-TiO2 nano-catalysis were prepared and used for CO2 solar photo-reduction.  Cu modify TiO2 crystalline structure, increase oxygen vacancies and atomic mobility  Cu-TiO2 structure enhanced visible light harvesting and electron-transfer rate.

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