Fe–TiO2 Photocatalyst for CO2

Journal of Inorganic and Organometallic Polymers and Materials https://doi.org/10.1007/s10904-019-01092-5

A Novel ­Ag2O/Fe–TiO2 Photocatalyst for ­CO2 Conversion into Methane Under Visible Light N. R. Khalid1   · M. Khalid Hussain1 · G. Murtaza2 · M. Ikram3 · M. Ahmad4 · A. Hammad1 Received: 29 October 2018 / Accepted: 30 January 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019

Abstract Modified ­TiO2 based nanomaterials have attained significant interest because of their unique morphology and excellent optical and photocatalytic properties. In this research, a very novel and highly efficient A ­ g2O/Fe–TiO2 porous structure was developed by simple hydrothermal method. The structural and morphological properties of the photocatalysts were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The surface areas of the samples were measured by Brunauer–Emmett–Teller theory (BET). The chemical composition and optical properties were investigated using X-ray photoelectron spectroscopy (XPS) and UV–visible spectroscopy. The optical absorption measurements show a clear red-shift ­ g2O varying ratio (0–15 at.%) in absorption edge of Fe–TiO2 after loading of ­Ag2O ­(Ag2O/Fe–TiO2 composite). Moreover, A has also enhanced the efficiency of A ­ g2O/Fe–TiO2 photocatalyst for C ­ O2 conversion into methane under visible light illumination (λ ≥ 420 nm). The optimum ratio of A ­ g2O loading which exhibited maximum performance is 10 at.%. Moreover, the 10%Ag2O/Fe–TiO2 composite synthesized at 180 °C hydrothermal temperature showed an excellent increase in photocatalytic activity than other composites synthesized at 150 and 210 °C. This excellent performance of photocatalyst can be attributed to the highly porous petal-like structure of composite. Therefore, it is expected that the present study will be a good addition in literature for designing highly active photocatalytic materials for reduction of ­CO2 into useful hydrocarbons. Keywords TiO2 · Hydrothermal method · Photocatalysis · CO2 conversion · Methane formation

1 Introduction The energy crisis and environmental pollution have emerged as two major challenges due to growth of population and urbanization recently [1, 2]. The major cause of environmental pollution is the consumption of fossil fuels which unfortunately produces harmful greenhouse gases (GHG) in our society [3]. In order to resolve this great challenge, * N. R. Khalid [email protected] * M. Ikram [email protected] 1



Department of Physics, University of Gujrat, HH Campus, Gujrat 50700, Pakistan

2



Centre for Advanced Studies in Physics, GC University, Lahore 54000, Pakistan

3

Department of Physics, GC University, Lahore 54000, Pakistan

4

Department of Physics, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan



the development of clean and sustainable alternative energy sources has attracted a lot of attention. Owing to clean and cost effective properties, the photocatalytic reduction of C ­ O2 into useful hydrocarbon is an efficient technique because it utilizes earth abundant solar energy [4–7]. For this purpose, many semiconductor photocatalysts such as T ­ iO2, ZnO, ­Fe2O3, ­SnO2, ­Bi2O3 and ­V2O5 have been used for ­CO2 reduction into hydrocarbon fuels [8–11]. Among them, titanium dioxide ­(TiO2) is green and highly efficient semiconductor photocatalyst used for water splitting, pollutants degradation and conversion of ­CO2 into useful hydrocarbons like CO, ­CH3OH, ­C2H5OH and C ­ H4 etc. [12, 13]. Unfortunately, the extensive practical applications of ­TiO2 have certain limitations due to some problems such as electron–hole recombination and low light utilization efficiency because of its wide band gap (3.2 eV) [14]. Therefore, several attempts have been made to improve its photocatalytic efficiency including the development of black ­TiO2 structure, cationic and anionic elements doping, morphological modification and semiconductor coupling [15–21].

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Recently, black T ­ iO2 (reduced T ­ iO2) has attained great interest in the field of photocatalysis due to its narrow band gap [15–18]. Fan et al. [16] reported their work about engineering disordered layers on T ­ iO2 nanocrystals via hydrogenation, which created amorphous ­TiO2 with varying degree of blackness and a substantial enhancement of solar-driven photocatalytic activity. Many studies have proved that hydrogenation treatment of ­TiO2 induces oxygen vacancies and ­Ti3+ sites in ­TiO2 structure, resulting in the separation of photo-created charge carriers and the band gap narrowing, which remarkably enhanced the photocatalytic efficiency of ­TiO2 [17, 18]. Secondly, it is well-known that doping ­TiO2 with suitable metal or nonmetal ion has significant influence on the narrowing of its band gap and reduction of charge carrier’s recombination. Recently, many researchers have modified T ­ iO2 by doping with different elements which extended its light absorption response in visible light region and improved photocatalytic efficiency [22, 23]. For example, Harifi et al. [24] incorporated ­Fe3+ into T ­ iO2 crystal structure and found greater photocatalytic performance under visible light illumination for pollutants degradation. The improved activity was attributed to the process of interfacial charge transfer between T ­ iO2 and iron metal. Moreover, the coupling of T ­ iO2 with semiconductor of small band gap is also an effective method to enhance its efficiency as photocatalyst [25–28]. ­Ag2O has small band gap (1.2 eV) and is p-type semiconductor, which was found suitable as visible light responsive photocatalyst previously [29–31]. Secondly, A ­ g2O nature of visible light sensitizer makes it attractive candidate for heterogeneous photocatalysis [31–33]. For example, ­Ag2O/TiO2 nanobelts based photocatalyst showed superior performance under visible light during the degradation of dyes compare to bare A ­ g2O and T ­ iO2 alone [34]. Since T ­ iO2 is a n-type semiconductor, therefore, the p-n heterojunction formed between ­Ag2O and ­TiO2 will be an efficient approach for the inhibition of charge carrier’s recombination and maximum visible light absorption. It is known that the p-n junction not only creates a potential gradient but also transfers electrons easily from the excited low band gap semiconductor ­Ag2O to ­TiO2, in order to promote the light utilization effectively. In this work, highly porous ­Ag2O/Fe–TiO2 composites were synthesized via simple hydrothermal method. The prepared photocatalysts exhibited superior visible light responsive photocatalytic efficiency for the reduction of C ­ O2 into methane ­(CH4).

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Journal of Inorganic and Organometallic Polymers and Materials

2 Experimental 2.1 Synthesis of Fe–TiO2 In the typical procedure, titanium isopropoxide and acetic acid with equal proportion were slowly mixed into 50 ml isopropanol using constant magnetic stirring for 20 min to attain the homogeneity of the solution. Then the required quantity of iron nitrate (Fe ­(NO3)3·9 ­H2O) was dissolved in 25 ml isopropanol with continuous stirring for 10 min. After this both solutions were mixed and stirred for further 30 min. The obtained powder was grinded and calcined at 400 °C for 3 h to get the required Fe–TiO 2 samples. The atomic ratio of Fe to ­TiO2 was to 2 at.% because it was previously optimized in our study [35]. The bare ­TiO2 was also prepared using the same conditions except adding Fe source.

2.2 Synthesis of ­Ag2O/Fe–TiO2 Composite The synthesis of A ­ g2O/Fe–TiO2 composites was accomplished via hydrothermal process. During the synthesis reaction, 1.7 g of ­AgNO3 was added into 80 ml ­H2O solution containing Fe–TiO 2 or T ­ iO 2 at room temperature under continuous stirring of 30 min. Then, 0.1M solution of NaOH was dropwise added into above solution under magnetic stirring to obtain pH 13. Then the obtained dispersion was hydrothermally treated in 100 ml Teflon-lined stainless steel autoclave at different temperature 150, 180 and 210 °C for 24 h under fixed conditions. Followed by washing with distilled water to obtain neutral pH, the precipitates were dried in microwave oven at 80 °C for 12 h. After this, the prepared samples were calcined at 400 °C for 3 h to achieve desired composite samples. The atomic ratios of ­Ag2O to Fe–TiO2 was varied (0%, 5%, 10% and 15 at.%). For comparison, pure A ­ g2O was also synthesized by the same method without using ­TiO2 or Fe–TiO2.

2.3 Characterization XRD (Bruker AXN model) furnished with Cu Kα (λ = 0.15406 nm) over the range of 10 to 80° (2θ) with step size of 0.02°/s was used to investigate crystal size and inter planer spacing of synthesized samples. Scanning electron microscope (SEM) (model JEOL JSM-6330F) was used to characterize surface morphology of synthesized samples. The Brunauer–Emmett–Teller (BET, ASAP 2020) surface areas (SBET) of the samples were calculated by nitrogen physisorption on a nitrogen adsorption apparatus at 77 °K (Micrometrics Instruments). XPS (ESCALAB250

Journal of Inorganic and Organometallic Polymers and Materials

having monochromatic Al ­Kα X-ray source) was utilized for obtaining chemical composition determination. The Shimadzu UV-1800 spectrometer was used to get the UV–visible absorption spectra of photocatalysts.

2.4 Photocatalytic Measurement The ­CO2 conversion into methane via photocatalysis was carried out using gas closed circulation system with controlled temperature and pressure. A xenon lamp (Perfect Light PLSSXE300) of 300 W was utilized for visible light irradiation (λ ≥ 420 nm). The suspension containing 50 mg catalyst and 100 ml distilled water was ultrasonically treated in the reactor. After evacuating the system, the ­CO2 gas was bubbled into the cylindrical tube of quartz reactor. The resulting samples of gas products were systematically analyzed with Agilent 7890A gas chromatograph online.

3 Results and Discussion The crystal structure of T ­ iO2, ­Ag2O, Fe–TiO2 and A ­ g2O/ Fe–TiO2 composites was studied by XRD and results are shown in Fig. 1. In the patterns of ­TiO2 and Fe–TiO2, the characteristic peaks are observed at 2θ of 25.3° (101), 37.9° (004), 48° (200), 53.8° (105), 55° (204), 62.8° (211), 70.6° (220) and 75.1° (213) which belong to ­TiO2 anatase phase consistent with JCPDS card no. 21-1272. Secondly, in the XRD pattern of pure ­Ag2O, four sharp peaks are seen at 2θ of 32.8° (111), 37.9° (200), 54.9° (220) and 65.3° (311), showing the presence of cubic crystal phase of A ­ g 2O (JCPDS 41-1104) [33]. Moreover, in ­Ag2O/Fe–TiO2 composite samples, there is an extra peak of A ­ g2O (111) at 2θ

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TiO

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Fig. 1  XRD patterns of different samples

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32.8° along with anatase ­TiO2 peaks. Furthermore, it can be seen that the intensity of A ­ g2O peak in composite sample is also increased after increasing the concentration of A ­ g 2O from 5 to 15 at.%. Interestingly, no other peaks are observed in the composite samples, which indicates the high purity of these prepared photocatalysts. The morphology of as-synthesized 10%Ag2O/Fe–TiO2 composite at different hydrothermal temperature (150, 180 and 210 °C for 24 h) was examined by SEM as shown in Fig. 2. The results indicate that ­Ag2O/Fe–TiO2 composite photocatalyst exhibited two kinds of porous structure due to varying hydrothermal temperature. The samples hydrothermally synthesized at 150 °C (Fig. 2a) displays the porous and sponge-like structure. Furthermore, Fig. 2b shows the highly porous petal-like structure at 180 °C. It is noteworthy that when hydrothermal temperature was further increased up to 210 °C (Fig. 2c), the petal-like structure was converted into sponge-like structure again. These porous and higher surface area structures will be highly capable for the maximum absorption of light and also adsorption of ­CO2 gas on catalyst surface because these parameters are critical for higher efficiency of the photocatalyst. It is well-known that the high specific surface area and porous structure of the photocatalyst play vital role to improve its photocatalytic properties due to increase adsorption and transportation of reactant molecules [36]. Figure 3 shows the nitrogen adsorption–desorption isotherms of four samples. All curves showed H ­ 2 hysteresis isotherm, indicating mesoporous nature of the pore structure. The surface areas were calculated from the low pressure portion of adsorption isotherm using the BET method. The calculated surface areas are 131, 139, 143 and 161 m2 ­g−1 for T ­ iO2, Fe–TiO2, ­Ag2O/TiO2 and ­Ag2O/Fe–TiO2 samples respectively. The results show that Fe and A ­ g2O incorporation into ­TiO2 have increased its surface area, which will be beneficial to enhance photocatalytic performance by providing more active sites on the surface of catalyst. For the identification of surface chemical composition of 10%Ag2O/Fe–TiO2 sample synthesized at 180 °C for 24 h, XPS was employed and results are shown in Fig. 4. The high resolution spectrum of Ti2p (Fig. 3a) shows two signals at 458.9 ­(Ti2p3/2) and 464.6 eV ­(Ti2p1/2) which corresponds to T ­ i4+ state [37]. Figure 3b displays the O1s corelevel XPS spectrum having two peaks centered at 530.2 and 532.2 eV corresponds to oxygen atoms in T ­ iO2 and A ­ g 2O [38]. In Fe2p core level spectrum (Fig. 3c), it can be seen that ­Fe2p3/2 and F ­ e2p1/2 peaks binding energies are located at 712.4 and 725 eV respectively attributed to ­Fe3+ state [39]. Furthermore, in highly-resolved spectrum of Ag3d (Fig. 3d), the signals at 368.5 and 374.5 eV are due to ­Ag3d5/2 and ­Ag3d1/2, respectively. These peaks confirm the presence of silver in ­Ag2O state and these observations are in good agreement with the reported literature [37, 38].

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Journal of Inorganic and Organometallic Polymers and Materials

Fig. 2  SEM images of hydrothermally synthesized 10%Ag2O/Fe–TiO2 composite at a 150 °C, b 180 °C and c 210 °C

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Fig. 3  N2 adsorption–desorption isotherms of a ­TiO2, b Fe–TiO2, c 10%Ag2O/TO2 (180 °C) and d 10%Ag2O/Fe–TiO2 (180 °C)

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Figure 5 demonstrates the absorption spectra of ­TiO2, Fe/ TiO2 and 10%Ag2O/Fe–TiO2 (180 °C) photocatalysts. The pure ­TiO2 (Fig. 4a) shows absorption edge at about 387 nm wavelength which corresponds to band gap energy of 3.2 eV. It can be seen from the Fig. 4b that Fe doping into ­TiO2 shifted the absorption edge towards longer wavelengths around 400 nm (3.1 eV) due to the creation of traps energy levels between the conduction and valence band of T ­ iO2 [39]. Moreover, the incorporation of ­Ag2O into Fe–TiO2 (10%Ag2O/Fe–TiO2) has broadened the absorption range of Fe–TiO2 into the visible light region, which might be due to narrow band gap of ­Ag2O [32]. The band gap energy of 10%Ag2O/Fe–TiO2 composite sample has value 1.93 eV. This significant enhancement in visible light absorption could be attributed to the presence of A ­ g2O, which acted as visible light sensitizer in the composite sample [32, 33]. The efficiency of ­Ag2O, ­TiO2, Fe–TiO2, ­Ag2O/TiO2 and ­Ag2O/Fe–TiO2 photocatalysts were investigated for the conversion of C ­ O2 into hydrocarbons under visible light irradiation (≥ 420 nm) for 8 h as shown in Fig. 6. The gas product analysis (Fig. 6a) shows that methane ­(CH4) gas was major product in the present study under these experimental conditions. In addition to methane, ­CH3OH, ­C2H5OH and CO

Journal of Inorganic and Organometallic Polymers and Materials

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Fig. 4  Core-level XPS spectra of a Ti2p, b O1s, c Fe2p and d Ag3d of 10%Ag2O/Fe–TiO2 (180 °C)

(a) TiO2

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Fig. 5  UV–visible absorption spectra of different samples

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gases were detected along with very small traces of other gases. However, the main produced gas is methane, therefore, we have evaluated the photocatalytic efficiency of all samples for the production of methane in this study. It is the fact that ­CO2 conversion into different gases significantly depends on the properties of catalyst surface and reaction system [33]. The photocatalytic performance evaluation results of different samples are shown in Fig. 6b, which demonstrate that the pure ­Ag2O and ­TiO2 showed very poor performance for methane production and ­CO2 conversion rate is 2.5 and 4.0 µmolh−1gcat.−1 respectively, while Fe–TiO2 and ­Ag2O/TiO2 photocatalysts exhibited higher efficiency such as 6.5 and 7.2 µmolh− 1gcat.−1 respectively. Interestingly, the incorporation of ­Ag2O into Fe–TiO2 sample has dramatically improved the photocatalytic activity for methane formation and production rate increases to 11.3 µmolh−1gcat.−1 for 5%Ag2O/Fe–TiO2 composite and 14 µmol h­ −1gcat.−1 for 10%Ag2O/Fe–TiO2 composite. However, further increase in

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(a)

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Fig. 6  Photocatalytic activity for ­CO2 conversion into methane under visible light illumination (λ ≥ 420 nm) using different photocatalysts a ­Ag2O, b ­TiO2, c Fe–TiO2, d 10%Ag2O/TiO2 (150 °C), e 5%Ag2O/Fe–TiO2 (150 °C), f 10%Ag2O/Fe–TiO2 (150 °C), g 15%Ag2O/Fe–TiO2 (150 °C), h 10%Ag2O/Fe–TiO2 (180 °C) and i 10%Ag2O/Fe– TiO2 (210 °C)

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­Ag2O concentration into the Fe–TiO2 system (15%Ag2O/ Fe–TiO2) has diminished the performance to 12.6 µmol ­h−1gcat.−1. This decrease in photocatalytic activity for methane production at higher ­Ag2O concentration could be ascribed to the enhanced electron–hole pair’s recombination due to excessive loading of A ­ g2O into the composite. In order to further explore the morphological influence on activity of 10%Ag2O/Fe–TiO2 photocatalyst for methane production, the hydrothermal temperature was varied to prepare this novel photocatalyst. It can be seen that sample prepared at 180 °C hydrothermal temperature showed

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the best performance for methane production (16.1 µmol ­h−1gcat.−1) than other prepared at 150, and 210 °C temperature. This enhance photocatalytic efficiency could be attributed to highly porous petal-like structure of the prepared photocatalyst as shown in Fig. 2d. In order to study the stability of the photocatalyst, a sequence of four cycles of ­CO2 photocatalytic conversion into methane using 10%Ag2O/Fe–TiO2 sample were carried out and results are shown in Fig. 7a. In each cycle, the initial methane evolution rate was achieved and there is no obvious change observed in methane production rate. The

Journal of Inorganic and Organometallic Polymers and Materials

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Fig. 7  a Stability test of ­CO2 photocatalytic conversion using 10%Ag2O/Fe–TiO2 (180 °C) under visible light irradiation (λ ≥ 420 nm) and b XRD patterns of 10%Ag2O/Fe–TiO2 (180 °C) sample before and after photocatalysis

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2θ ( degree ) study reveals that 10%Ag2O/Fe–TiO2 (180 °C) composite photocatalyst is stable under present experimental conditions even after 8 h irradiation time. Moreover, Fig. 7b shows the XRD patterns of 10%Ag 2O/Fe–TiO2 (180 °C) sample before and after photocatalysis. It can be seen from

results that there is no change in the crystal structure of photocatalyst. This confirms that the visible light irradiation did not produce silver metal ions ­(Ag+) or silver nanoparticle and silver is present in metal oxide form ­(Ag2O) in the composite.

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4 Conclusion The ­Ag2O/Fe–TiO2 photocatalyst have been successfully prepared using hydrothermal method and utilized for ­CO2 conversion into methane. The optical absorption and C ­ O2 conversion measurements show that the incorporation of Fe and A ­ g2O not only broadens the light absorption range of ­TiO 2 into visible region but also enhances the C ­ O2 conversion into methane due to highly porous structure. Moreover, the 10%Ag2O/Fe–TiO2 synthesized at 180 °C demonstrated excellent photocatalytic efficiency for methane production among all samples. The improved photocatalytic activity can be attaributed to extended visible light absorption, highly porous structure of the composite and synergistic effects between ­Ag2O, Fe and ­TiO2.

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