Visible-Light-Assisted Photocatalytic CO2 Reduction over InTaO4

Oct 25, 2017 - The InTaO4 materials were prepared by a facile sol–gel route, and Ni nanoparticles were loaded by the photodeposition method. Prepare...
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Visible light assisted photocatalytic CO2 reduction over InTaO4: Selective methanol formation Nikita Singhal, Reena Goyal, and Umesh Kumar Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02123 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Visible light assisted photocatalytic CO2 reduction over InTaO4: Selective methanol formation Nikita Singhal [ab], Reena Goyal[ab] and Umesh Kumar [ab]* [a] Chemical Science Division, CSIR-Indian Institute of Petroleum, Dehradun-248005, India [b] AcSIR-Academy of Scientific & Innovative Research New Delhi, India CSIR-Indian Institute of Petroleum, Dehradun-248005, India Corresponding Author Umesh Kumar, Chemical Science Division, CSIR- Indian Institute of Petroleum, [email protected]

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ABSTRACT: The InTaO4 materials were prepared by a facile sol-gel route, and Ni nanoparticles were loaded by photodeposition method. Prepared materials were characterized by XRD, UV, SEM, TEM, EDX, and XPS. InTaO4 shows CO2 reduction under visible light irradiation. By using the Ni/InTaO4 as heterogeneous catalysts, the photocatalytic performance of the CO2 to methanol conversion was remarkably enhanced. Loading of Ni slightly reduces the band gap and may help to slow down the recombination process, which provides a higher yield of methanol.

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Introduction The consequences caused by a significant rise in the CO2 emissions due to fossil fuel combustion drag the attention towards CO2 utilization.1 Photocatalytic reduction of CO2 by using sunlight is a promising pathway to address the global challenges of alternative energy needs and environmental sustainability.2 After pioneering research of CO2 photocatalytic reduction,3 various efforts have been attempted to develop efficient photocatalysts for CO2 reduction to

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fuel.4 Till date, TiO2 is known as benchmark catalyst due to its properties such as nontoxic nature, abundance, suitable optoelectronic properties, and stability.5 However, new catalysts are continuously being developed as TiO2 can work only under UV irradiation and fastest recombination of electron-hole pair limits its efficacy. For effective utilization of solar energy, it is indispensable to develop visible light photocatalysts with high stability.6 InTaO4 is a visible light responsive photocatalyst. It has 2.6 eV band gap, belongs to the family of ABO4 compounds with A3+B5+O4 wolframite structure.7 Its conduction band is made up of 5d orbital of Ta and valence band is formed by 2p orbitals of oxygen of InO6. In1-xNixTaO4 was reported to split water into H2 and O2 under visible light irradiation with a quantum yield of about 0.66%.8 Hiroshi et al. found that InTaO4 doped with vanadium could decompose gaseous isopropyl alcohol into acetone and CO2 with ultraviolet radiation but showed little activity under visible light irradiation.9 A series of metal-doped InTaO4 was prepared and tested for H2 evolution from the aqueous methanol solution. The rate of H2 formation on In0.8Ni0.2TaO4 is about 180% than non-doped InTaO4.7 Chiou et al. stated that InTaO4 prepared by sol-gel technique demonstrated higher photocatalytic activities for H2 production because of a smaller band gap, particle size (0.05-0.1 µm) and better crystalline phase compared to the one prepared by solid-state reaction.10 Zeng et al. prepared InTaO4 having three different morphologies using the sol-gel route and tested for H2 evolution from an aqueous solution of 10% TEOA. Among the three, InTaO4 nanofibers have shown highest light utilization efficiency, specific surface area and photocatalytic activity for H2 evolution.11 Pan et al. prepared InTaO4 by solid-state reaction, and NiO was loaded as co-catalyst. The material has shown activity for CO2 reduction to methanol in KHCO3 solution under a 500W halogen lamp. 1.0 wt% NiO-InTaO4 R500-O200 (after pretreatment) has 1.394 µmol g-1 h-1 of methanol formation rate.12 Wang et al. prepared NiO/InTaO4

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by sol-gel and the material was dip coated on optical fibers and calcined at 1100 °C tested for CO2 reduction under visible light. At 25 °C the rate of methanol production was 11.1 µmol g-1 h-1 with a light intensity of 327 mW cm-2. Increasing the reaction temperature to 75 °C increased the production rate to 21.0 µmol g-1 h-1.13 Liou et al. also showed CO2 reduction with water in the gas phase over NiO/InTaO4 loaded over PMMA optical fibers in a monolith reactor under UVA and visible light irradiation. 0.3 µmol g-1 h-1 acetaldehyde was obtained with a loading of 2.6 % NiO by simulated sunlight AM1.5G at 70 °C.14 It is reported that InTaO4 produces methanol via CO2 reduction under visible light irradiation. Moreover, InTaO4 prepared by esterification process shows better activity than those by solid-state fusion. On increasing calcination temperature from 850 °C to 1100 °C, the methanol yield increased due to uniform crystallinity.15 In this paper, the sol-gel route was followed to synthesize InTaO4, and Ni nanoparticles were loaded using photodeposition method. The prepared photocatalysts were evaluated for CO2 reduction under the visible light radiation. Results and discussion The as-prepared InTaO4 catalysts were characterized by XRD, UV-Vis, SEM, TEM, and XPS. XRD was used to determine crystallinity, phase structure. As shown in Figure 1, the prepared catalysts exhibited sharp diffraction peaks indexed as monoclinic InTaO4 (JCPDS No.-01-0811196). No additional diffraction peaks were detected, indicating the high phase purity of the material. Further, Ni loading does not alter the parent crystalline phase of the material. Figure 2 shows diffuse reflectance spectra of the InTaO4 sample. The compound showed obvious absorption in the visible light region up to 475 nm. The band gap of InTaO4 was estimated ca. 2.6 eV from the onset of absorption spectra and correlated with literature.8 Ni

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loading over InTaO4 was able to shift the band position toward higher wavelength to 488 nm corresponding to 2.54 eV. The morphology of the prepared material was examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). Figure 3 shows the SEM image of catalysts and agglomerated in nature. EDX confirms the presence of elements, i.e., In, Ta and O Figure 3b, 3d. Loading of Ni over InTaO4 has also been confirmed by EDX Figure 3d. TEM reveals the particle size of materials ca. 50-80 nm Figure 4. HRTEM shows the crystallinity of InTaO4 which is in good agreement with XRD pattern. The d spacing of 0.20 nm and 0.24 nm corresponds to [111] plane of Ni and NiO respectively Figure 4e, 4f.16 () To elucidate chemical states of constituting elements, XPS measurements of InTaO4 and Ni/InTaO4 were conducted and presented in Figure 5. Ni 2p spectrum has four peaks at the binding energy of 854.3, 860.7, 870.4 and 878.9 eV which are assigned to NiO 2p3/2, NiO 2p3/2 satellite, Ni 2p1/2 and NiO 2p1/2 satellite.17 The results show the presence of Ni and NiO, which are in good agreement with XRD pattern. The peaks observed at 452.3 and 444.7 eV corresponding to 3d3/2 and 3d5/2 respectively, are due to In3+ ions.18 Further, the presence of Ta5+ is confirmed with peaks at 25.9 and 27.8 eV. O1s peak is present at 530.6 eV is ascribed to lattice oxygen.19 The catalytic activity of prepared material was examined for photocatalytic reduction of CO2 in acetonitrile/water mixture by using TEOA as sacrificial agent under visible light irradiation. In order to analyze the reaction progress, gas and liquid samples were analyzed at regular time intervals by gas chromatograph. The results show that there was no carbonaceous product in gas samples but methanol as the selective product in liquid. Figure 6 shows CO2 reduction to

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methanol as a function of time under visible light (20 W, white LED). During photocatalytic reduction of CO2, 107 µmol g-1 and 200 µmol g-1 methanol was formed over InTaO4 and Ni/InTaO4 respectively, after 72 h of continuous irradiation. It has also been observed that Ni loading improved the catalytic activity by 1.9 fold than bare InTaO4. It has been reported that NiO helps to promote the efficiency for photocatalytic water splitting.20 Loading of Ni facilitated the effective electron transfer and enhanced absorption in the visible radiation and resulted in enhanced methanol yield.21 The effect of Ni loading was also monitored and the increase in Ni metal concentration from 1 to 2%. The nickel loading amount has a reciprocal effect on the methanol production. A decrease was observed with Ni2%/InTaO4 140 µmol g-1 methanol; it may be due to coverage of the larger surface by Ni and less space for photons interaction to generate photoelectrons to carry out the redox reaction Figure 7. Therefore 1% Ni loading is sufficient over InTaO4 for photocatalytic CO2 reduction.22 Quantum yield of methanol (ΦMeOH) is obtained 1.01% and 1.85% for InTaO4 and Ni/InTaO4 respectively. Conclusions In summary, the InTaO4 was synthesized by sol-gel route followed by calcination at 950 °C. 1wt% Ni was loaded over InTaO4 using photodeposition method. The band gap of InTaO4 and Ni/InTaO4 catalysts was observed 2.6 eV and 2.54 eV respectively. Therefore, they are capable of utilizing visible part of the solar spectrum. The prepared catalysts were tested for the photocatalytic reduction of CO2 under visible light LED. InTaO4 produce methanol in acetonitrile, water and TEOA mixture. Ni/InTaO4 has improved photocatalytic reduction of CO2 by 1.9 fold.

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Materials and methods Indium Nitrate (Alfa Aesar, 99.99%), tantalum ethoxide (Sigma Aldrich, 99.98%), Nickel nitrate hexahydrate (≥97%, Sigma Aldrich) Glacial acetic acid (Merck, 98%), Ethanol (Fischer Scientific, >99%), Nitric acid (Merck, 69%) CO2 (Sigma Gases, 99.9995%) and HPLC grade water. (a) Synthesis of catalysts InTaO4 was synthesized by sol-gel method. In a typical experiment, Indium nitrate (1.5 g, 5 mmol) was dissolved in 10 mL ethanol followed by addition of 10 mL ethanolic solution of tantalum ethoxide (2.03 g, 5 mmol). Nitric acid was used to maintain acidic pH ≈ 2. Then after 10 mL glacial acetic acid was added to the above solution and kept under continuous stirring overnight to get the viscous gel. The obtained gel was dried in air oven at 80 °C for 24 h followed by grinding in mortar pestle and calcination under air atmosphere at 950 °C for 6 h with ramp rate 2 °C min-1. 1 wt% Ni was loaded over InTaO4 using photodeposition method. 0.0123 g of Ni(NO3)2.6H2O was also dissolved in 20mL of 1% methanol containing water then 0.250 g InTaO4 was also dispersed in this solution. The reaction vessel was evacuated and kept under visible light (20 W, LED) for 4 h to deposit Ni nanoparticles over InTaO4. The obtained material was washed three times with water and dried in an oven at 80 °C for 3 h. (b) Characterization methods

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The obtained InTaO4 and Ni/InTaO4 catalysts were well characterized with different techniques. The crystallinity of catalysts was evaluated by Powder X-ray diffraction pattern. XRD analysis was performed on Bruker D8 advance X-ray diffractometer equipped with CuKα radiation (λ= 1.54 Å) at a scanning rate 0.04 s-1 in a 2θ range 20-70°. The acceleration voltage of instrument was 40 kV, and the emission current was 40 mA. Scanning electron microscope (SEM) (FEI Quanta 200F) system equipped with energy-dispersive X-ray (EDX) spectroscopy and Transmission electron microscope (TEM) (JEOL) were used to characterize the morphology and elemental composition of the catalysts. The catalysts sample were sonicated in ethanol and dropped over carbon coated grid to determine TEM. The solid UV-vis spectrum was obtained to find optical properties with Perkin Elmer Lambda 950 instrument using BaSO4 as reference material. The surface characterization of the prepared samples was conducted by X-ray photoelectron spectroscopy (XPS) model S/N-10001 (Prevac Poland) equipped with a VG Scienta-R3000 hemispherical energy analyzer. (c) Photocatalytic CO2 reduction Photocatalytic activity of prepared catalysts was evaluated by CO2 reduction under visible light irradiation at ambient conditions. 20 W white LED (Siskin) was used as a visible light source. The reaction was done in a 100 mL pyrex made round bottom flask. Typically, 0.02 g of synthesized photocatalyst was suspended in 20 mL solution consists of acetonitrile, water, and triethanolamine (3:1:1). The reaction mixture was sealed by silicon rubber septum and purged with N2 for 0.5 h to remove dissolved gases and then with CO2 for 1 h to saturate the mixture before irradiation. A CO2 balloon was placed over round bottom flask for continuous availability of CO2 in the system. Next, the reaction setup was exposed to visible light by LED kept at 5 cm distance. During reactions, the system was continuously stirred. Liquid and gaseous samples

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were collected at regular time interval from reaction system and injected into Perkin gas chromatogram equipped with FID (Plot-Q, 30 m x 0.53 mm) and TCD (shincarbon packed column, ST 80/100, 2 m, 2 mm, 1/8”) Quantum yield of methanol was calculated as follows:

AQY (%) (ΦMeOH) =

.    ∗               

∗ 100

Figure 1. XRD pattern of InTaO4 and Ni/InTaO4

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Figure 2. UV-Vis Diffuse Reflectance Spectra of InTaO4 and Ni/InTaO4

Figure 3. SEM of InTaO4 a) and corresponding EDX b) SEM of Ni/InTaO4 c) and corresponding EDX d)

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Figure 4. TEM a-b) InTaO4 and c-f) Ni/InTaO4

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Figure 5. XPS of Ni/ InTaO4 a) Ni 2p b) O 1s c) In 3d d) Ta 4f

Figure 6. Production of methanol as function of time under visible light irradiation over InTaO4 and Ni/InTaO4

Figure 7. Optimum metal concentration over InTaO4 for CO2 photoreduction.

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