Experimental Study of CO 2 Conversion into Methanol by Synthesized Photocatalyst (ZnFe 2 O 4 / TiO 2 ) Using Visible Light as an Energy Source

: Ozone layer depletion is a serious threat due to the extensive release of greenhouse gases. The emission of carbon dioxide (CO 2 ) from fossil fuel combustion is a major reason for global warming. Energy demands and climate change are coupled with each other. CO 2 is a major gas contributing to global warming; hence, the conversion of CO 2 into useful products such as methanol, formic acid, formaldehyde, etc., under C, and optimum yield was at a 1:1 w / w coupling ratio of ZnFe 2 O 4 / TiO 2 . It was observed that due to the enhancement in the electron–hole pair lifetime, the methanol yield at 141.22 µ mol / g cat · h over ZnFe 2 O 4 / TiO 2 was found to be 7% higher than the earlier reported data.


Introduction
Depletion of the ozone layer is not only a serious threat due to the extensive release of greenhouse gases; the emission of CO 2 is also a major reason for global warming. Current global demand for energy is almost 17 TW (Terawatt) and is forecasted to be doubled in upcoming years [1]. The climate change committee i.e., the IGP (Intergovernmental Panel), recommends that emissions of CO 2 must be reduced by up to half of the total 85% reduction target by 2050 [2]. Reports of the Kyoto Protocol revealed that the countries under the Kyoto Protocol produce 22.5% less CO 2 compared to production back in 1990. The carbon capture and storage techniques (CCS) are highly uneconomical for medium and small plants, and that is a major drawback [3].
For the effective conversion of CO 2 into useful products such as methane, methanol, formic acid, etc., many researchers/scientists suggested different techniques from electrochemical cell to thermal decomposition. Thus, electrochemical cells have a superior advantage over thermal decomposition due to sustainable energy utilization i.e., visible light/sunlight, etc.
There are several processes where CO 2 can be converted into market useful products; CO 2 can be captured by using following techniques: (a) before the combustion capturing of CO 2 , (b) in-process capturing by adjusting the oxygen-fuel ratio, and (c) after combustion capturing of CO 2 . Considering all of the said techniques, the most researched and discussed option is post combustion CO 2 capturing and conversion. Many catalytic/photocatalytic materials have been utilized for the conversion of CO 2 into useful products i.e., TiO 2 /SB (SB/Antimony) used for the conversion of carbon dioxide into CH 3 OH, Ag Br/CNT (CNT, or carbon nanotubes) used for the conversion of carbon dioxide into CH 3 OH and C 2 H 5 OH, CdSe/Pt/TiO 2 used for CH 4 , CH 3 OH, etc. Factors such as photo corrosion resistance, valance and conduction band positions, accessibility, cost, surface area, ease of availability, etc. are the most important factors in the selection of photocatalysts, in which our selected photocatalyst i.e.,ZnFe 2 O 4 /TiO 2 is ahead of the other visible light-induced photocatalysts. The most viable catalyst is ZnFe 2 O 4 /TiO 2 in terms of valance and conduction band position, resistance toward corrosion, cheap cost, and stability; all of these above-mentioned factors make this photocatalyst most suitable for conversion within a visible range and maximum yield of the specific product i.e., methanol.
Different products can be obtained through the reduction of CO 2 , as shown in Table 1. Despite the market demanding products from the CO 2 reduction reactions, still much work needs to be done in this field. The utilization of value-added products from CO 2 conversion such as methanol seems to be a most promising technique to reduce the effect of major greenhouse gases (CO 2 ) on the atmosphere. The photocatalytic reduction of CO 2 depends upon the hydrogen produced during water splitting, because water is the most common reductant used for this reaction. During the reduction reaction, more than one product obtained formaldehyde or formic acid. The photocatalytic splitting of water is a simple process because only two products are obtained by this reaction: hydrogen and oxygen. On the other hand, the photocatalytic reduction of CO 2 is a complex reaction because more than one product is obtained during the reaction, and still, no full complete explanation for the product selectivity is available. Researchers found that during the reduction of CO 2 , the first product that was formed was C T E D Catalysts 2020, 10,163 3 of 17 formic acid, which converts into formaldehyde, and the formaldehyde finally converts into methanol. However, on the other hand, some studies indicated that product selectivity can be controlled by the position of the conduction band of the photocatalyst. Other products can also be produced during the reaction such as formaldehyde, hydrogen, ethanol, and CO, but they are not detected in this study.
The purpose of the current research is to convert CO 2 into useful fuel i.e., methanol. For this objective to be achieved, we first synthesize the ZnFe 2 O 4 /TiO 2 photocatalyst, and afterwards characterize a ZnFe 2 O 4 /TiO 2 heterojunction photocatalyst to check whether our desired specified catalyst has been synthesized or not. Then, we evaluate the activity of the prepared photocatalyst for CO 2 conversion into methanol under visible light (initially, a tungsten bulb is used, but we aim to get conversion through sunlight/renewable energy). Starting from the energy of +805 kJ/mol is required for the bond breakage of chemically stable CO 2 due to its carbon-oxygen bonds. A carbon-free energy source such as solar light can be used for this purpose under the presence of a photocatalyst. The photocatalysis process continues only if there is suitable photon energy of light UV/visible light. Research works in the literature reported that the obtained yield and selectivity of the product was very low in CO 2 photocatalysis under visible light [7]. The conversion of carbon into useful fuel required a sustainable amount of hydrogen i.e., some suitable source of hydrogen such as H 2 and H 2 O, etc. The cheapest and most easily available source is water. During the photocatalysis of CO 2 with water, water splitting also competes with CO 2 reduction reaction because water splitting is much simpler than CO 2 reduction. This is because the splitting of water into H 2 and O 2 is a two-electron process, but CO 2 conversion into methanol required six electrons and six protons [8]. Moreover, the position of the conduction band plays an important role for CO 2 conversion into the desired product. Different types of photocatalysts were prepared and tested by various research groups in the last few decades for the conversion of CO 2 into methanol such as metal oxides, layered double hydroxides and metal ferrite [9,10]. ZnFe 2 O 4 works under visible light due to its smaller band gap and transferred electrons to TiO 2 because it is a p-type semiconductor and TiO 2 is n-type based; based on the previous study, that the ZnFe 2 O 4 coupled with TiO 2 can be synthesized as an efficient visible light drove the photocatalyst for CO 2 reduction. However, TiO 2 is identified as one of the best photocatalysts due to its cost and stability [3]. The major problem in TiO 2 is its large band gap of approximately 3.2 eV [11]. The large band gap of TiO 2 makes it a UV-Vis-driven photocatalyst.
In order to obtain the high selectivity of a given product and the maximum yield of the desired product, water and CO 2 used as raw feed, oxidation also has the same importance as CO 2 . Both components are stable, and their bond breakage is possible by using a carbon-free sustainable source of energy as a solar light in the presence of a photocatalyst. The maximum Faradaic efficiency of hydrocarbons was observed at −1.9 V with 40.3%; similarly, hydrogen formation could be depressed to 34.7%. During the reduction of CO 2 with water, the most favorable reaction is water reduction instead of CO 2 due to this huge difference in their reduction potential. This problem of water reduction instead of CO 2 can be solved by the oxidation of water instead of reduction. For water oxidation, the photocatalyst must have a valence band edge that is more positive than water oxidation, which is +0.82 eV. The most common reaction that happened during the reduction of CO 2 with water at 7pH is tabulated below in Table 2. The most common problem associated with photocatalysis by these catalysts is the conversion of CO 2 to more than one product [12]. Thus the reaction mechanism, the effect of the amount of catalyst loading, the influence of calcination temperature, and the effect of the recycled catalyst during photoactivity are investigated thoroughly.

Characterization
The following characterization techniques have been used on order to confirm the exact formation of the desired photocatalyst, but these techniques have been limited to some extent, as our main focus is to study the experimental effects of the different parameters.

FTIR (Fourier Transform Infrared Spectroscopy)
A Shimadzu spectrometer in the range of 4000-450 cm −1 used for recording the FTIR spectra. The catalyst was in powdered form, and the formation of ZnFe 2 O 4 was confirmed by using FTIR data as a preliminary result. The FTIR spectra recorded using a spectrometer in the range of 4000-450 cm −1 , which is comparable to previously reported results [13]. The catalyst was in powdered form, and the formation of ZnFe 2 O 4 was confirmed i.e., the spectrum band at 592.5 cm −1 and 487.3 cm −1 represents Zn 2+ and Fe 3+ ions, as shown in Figure 1. Moreover, the FTIR spectrum of ZnFe 2 O 4 -TiO 2 exhibits peaks having main characteristics of TiO 2 at 487.3 cm −1 and ZnFe 2 O 4 at 592.5 cm −1 . In the same way, the band at 1488.6 cm −1 points toward the sample containing a slight amount of water. The band at 1253.1 cm −1 is an indication of the presence of nitrates. The presence of water is mainly due to the modified method of synthesis; nitrates were present because the synthesis starts from the nitrates of metals. The last peaks at 2401.7 cm −1 and 3248.8 cm −1 indicate the presence of CO 2 that was absorbed into the catalyst synthesized from the environment. Some peaks can be witnessed, which are major and show good agreement with those of ZnFe 2 O 4 -TiO 2 nanocomposites, ensuring that the presence of functional groups on the surface of ZnFe 2 O 4 -TiO 2 are due to octane residues, as evident in Figure 1. The most common problem associated with photocatalysis by these catalysts is the conversion of CO2 to more than one product [12]. Thus the reaction mechanism, the effect of the amount of catalyst loading, the influence of calcination temperature, and the effect of the recycled catalyst during photoactivity are investigated thoroughly.

Characterization
The following characterization techniques have been used on order to confirm the exact formation of the desired photocatalyst, but these techniques have been limited to some extent, as our main focus is to study the experimental effects of the different parameters.

FTIR (Fourier Transform Infrared Spectroscopy)
A Shimadzu spectrometer in the range of 4000-450 cm −1 used for recording the FTIR spectra The catalyst was in powdered form, and the formation of ZnFe2O4 was confirmed by using FTIR data as a preliminary result. The FTIR spectra recorded using a spectrometer in the range of 4000-450 cm −1 , which is comparable to previously reported results [13]. The catalyst was in powdered form, and the formation of ZnFe2O4 was confirmed i.e., the spectrum band at 592.5cm −1 and 487.3cm −1 represents Zn 2+ and Fe 3+ ions, as shown in Figure 1. Moreover, the FTIR spectrum of ZnFe2O4 -TiO2 exhibits peaks having main characteristics of TiO2 at 487.3 cm −1 and ZnFe2O4 at 592.5 cm −1 . In the same way, the band at 1488.6 cm −1 points toward the sample containing a slight amount of water. The band at 1253.1 cm −1 is an indication of the presence of nitrates. The presence of water is mainly due to the modified method of synthesis; nitrates were present because the synthesis starts from the nitrates of metals. The last peaks at 2401.7 cm −1 and 3248.8 cm −1 indicate the presence of CO2 that was absorbed into the catalyst synthesized from the environment. Some peaks can be witnessed, which are major and show good agreement with those of ZnFe2O4-TiO2 nanocomposites, ensuring that the presence of functional groups on the surface of ZnFe2O4-TiO2 are due to octane residues, as evident in Figure  1.

UV-Vis Spectroscopy
The UV-Vis spectroscopy of synthesized catalysts ZnFe 2 O 4 /TiO 2 , ZnFe 2 O 4 , and TiO 2 was done at a wavelength range of 200-800 nm. This absorption spectrum of the catalyst revealed that the synthesized catalyst is the product of visible light active range of the catalyst. A wavelength of 200-800 nm has been used for studying the UV-Vis spectroscopy of the synthesized catalysts ZnFe 2 O 4 /TiO 2 (1:1), ZnFe 2 O 4 , and TiO 2 . The absorption spectrum of the synthesized catalyst revealed that it is a visible light active photocatalyst; the UV-Vis spectra of synthesized ZnFe 2 O 4 , TiO 2 , and ZnFe 2 O 4 coupled with TiO 2 in different w/w ratios in the wavelength range of 200-800 nm is shown in Figure 2. ZnFe 2 O 4 shows a good absorbance pattern from 400-800 nm, which reveals that it is an excellent photocatalyst [14]. TiO 2 illustrates absorbance only from 200 to 400 nm, which concludes that it is a UV active photocatalyst. When ZnFe 2 O 4 is coupled with TiO 2 in a 1:1 weight/weight ratio, it clarifies visible light region maximum absorbance from 400 to 800 nm. The absorbance pattern of the remaining two ratios is also illustrated in Figure 2. Moreover, another two ratios also display absorbance under visible light region, but the absorbance is comparatively lower than a 1:1 ratio. These three ratios show transparency above 516 nm, 442 nm, and 427 nm, which indicates that all three ratios have shifted the band gap of commercialized TiO 2 from an ultraviolet region to visible light. The most suitable ratio that covers the majority of the visible light region is 1:1. The coupling of a low band gap ZnFe 2 O 4 with TiO 2 reduces the band gap, and the proper ratio of both semiconductors is 1:1. This ratio efficiently minimizes the band gap of coupled semiconductors and absorbs a maximum portion of visible light. TiO 2 shows an absorbance pattern only under the UV region. After extrapolation, a line has been produced that shows the photon energy axis and produces the semiconductor band gap.

UV-Vis Spectroscopy
The UV-Vis spectroscopy of synthesized catalysts ZnFe2O4/TiO2, ZnFe2O4, and TiO2 was done at a wavelength range of 200-800 nm. This absorption spectrum of the catalyst revealed that the synthesized catalyst is the product of visible light active range of the catalyst. A wavelength of 200-800 nm has been used for studying the UV-Vis spectroscopy of the synthesized catalysts ZnFe2O4/TiO2(1:1), ZnFe2O4, and TiO2. The absorption spectrum of the synthesized catalyst revealed that it is a visible light active photocatalyst; the UV-Vis spectra of synthesized ZnFe2O4, TiO2, and ZnFe2O4 coupled with TiO2 in different w/w ratios in the wavelength range of 200-800 nm is shown in Figure 2. ZnFe2O4 shows a good absorbance pattern from 400-800 nm, which reveals that it is an excellent photocatalyst [14]. TiO2 illustrates absorbance only from 200 to 400 nm, which concludes that it is a UV active photocatalyst. When ZnFe2O4 is coupled with TiO2 in a 1:1 weight/weight ratio, it clarifies visible light region maximum absorbance from 400 to 800 nm. The absorbance pattern of the remaining two ratios is also illustrated in Figure 2. Moreover, another two ratios also display absorbance under visible light region, but the absorbance is comparatively lower than a 1:1 ratio. These three ratios show transparency above 516 nm, 442 nm, and 427 nm, which indicates that all three ratios have shifted the band gap of commercialized TiO2 from an ultraviolet region to visible light. The most suitable ratio that covers the majority of the visible light region is 1:1. The coupling of a low band gap ZnFe2O4 with TiO2 reduces the band gap, and the proper ratio of both semiconductors is 1:1. This ratio efficiently minimizes the band gap of coupled semiconductors and absorbs a maximum portion of visible light. TiO2 shows an absorbance pattern only under the UV region. After extrapolation, a line has been produced that shows the photon energy axis and produces the semiconductor band gap. The band gaps of TiO2, ZnFe2O4, and ZnFe2O4coupled with TiO2 (1:1) are 3.15, 1.86, and 2.79 eV, respectively. ZnFe2O4 shows maximum absorbance at 624 nm and is well matched with previously reported results. The coupling of ZnFe2O4 with TiO2 in a proper ratio shifts the band gap of TiO2 from the UV region to the visible region and is used efficiently for the reduction of CO2 into methanol under visible light irradiation.

FE-SEM Analysis
The possible accumulation and morphologies due to the tiny crystallites contact of synthesized catalysts were determined by using FE-SEM (field emission electron scanning electron microscope). This technique also pointed out the particle size of the synthesized catalyst. The magnification was done from 10 to 150. The following results shows that the ZnFe2O4:TiO2 nanocomposites are regularly shaped nanoparticles. The ZnFe2O4 synthesized formed well-defined crystalline irregular spherical shapes with particle sizes in the range of 200-400 nm (Figure 3a, b). The anatase TiO2 is also observed

FE-SEM Analysis
The possible accumulation and morphologies due to the tiny crystallites contact of synthesized catalysts were determined by using FE-SEM (field emission electron scanning electron microscope). This technique also pointed out the particle size of the synthesized catalyst. The magnification was done from 10 to 150. The following results shows that the ZnFe 2 O 4 :TiO 2 nanocomposites are regularly shaped nanoparticles. The ZnFe 2 O 4 synthesized formed well-defined crystalline irregular spherical shapes with particle sizes in the range of 200-400 nm (Figure 3a,b). The anatase TiO 2 is also observed C T E D Catalysts 2020, 10, 163 6 of 17 to be more homogeneous with high crystallinity. The particle size of TiO 2 nanoparticles is found to be in the range of 50-300 nm (Figure 3c,d). However, the ZnFe 2 O 4 /TiO 2 was attributed to the magnetic property of ZnFe I 2 O 4 particles. It is observed that the ZnFe 2 O 4 /TiO 2 heterojunction with a (1:1) ratio shows high homogeneity and porosity (Figure 3e,f). It is observed that a hydrothermally developed ZnFe 2 O 4 :TiO 2 (1:1) heterojunction followed by annealing at 500 • C instigates higher homogeneity in the photocatalyst. The corresponding elements of the heterojunction such as Ti, O, Zn, and Fe show better homogeneity in the structure, while post annealing may promote the interfacial contact between ZnFe 2 O 4 and TiO 2 nanostructures ( Figure 3). Moreover, an interface/boundary is witnessed between two nanoparticles, indicating the presence of a heterojunction between ZnFe 2 O 4 and TiO 2 which can further enable the charge transfer more freely to expressively reduce electron-hole pairs recombination in addition to enhancing the visible light-driven photocatalytic activity of ZnFe 2 O 4 :TiO 2 nanocomposites. It illustrates the importance of the development of a heterojunction by hydrothermal modification followed by post-sintering treatment.
Catalysts 2020, 10, x FOR PEER REVIEW 6 of 18 to be more homogeneous with high crystallinity. The particle size of TiO2 nanoparticles is found to be in the range of 50-300 nm (Figure 3c,d). However, the ZnFe2O4/TiO2was attributed to the magnetic property of ZnFe I 2O4 particles. It is observed that the ZnFe2O4/TiO2 heterojunction with a (1:1) ratio shows high homogeneity and porosity (Figure 3e,f). It is observed that a hydrothermally developed ZnFe2O4:TiO2 (1:1) heterojunction followed by annealing at 500 °C instigates higher homogeneity in the photocatalyst. The corresponding elements of the heterojunction such as Ti, O, Zn, and Fe show better homogeneity in the structure, while post annealing may promote the interfacial contact between ZnFe2O4 and TiO2 nanostructures ( Figure 3). Moreover, an interface/boundary is witnessed between two nanoparticles, indicating the presence of a heterojunction between ZnFe2O4 and TiO2 which can further enable the charge transfer more freely to expressively reduce electron-hole pairs recombination in addition to enhancing the visible light-driven photocatalytic activity of ZnFe2O4:TiO2 nanocomposites. It illustrates the importance of the development of a heterojunction by hydrothermal modification followed by post-sintering treatment.

XRD Spectra
The synthesized catalyst is characterized by XRD for the identification of crystallinity. CuKα (1.542Å) radiation is used. The XRD scan range is 10-80°, the accelerating potential was 40 KVA, and the scan step size was 0.02°. The XRD pattern of ZnFe I 2O4, TiO2, and ZnFe2O4/TiO2 (1:1 w/w ratio) is shown in Figure 4. The XRD pattern of TiO2 illustrates that it is available in anatase phase and all the peaks are well matched. ZnFe I 2O4peaks prove that it is in the spinel phase. No peak shift or extra peak are observed after coupling, which means that no impurity is induced into it after the coupling of both semiconductors, and the pure catalyst retained its previous phase. The crystalline sizes of ZnFe2O4, TiO2, and ZnFe2O4:TiO2 (1:1) are 44.8, 65.2, and 46.9 nm.

XRD Spectra
The synthesized catalyst is characterized by XRD for the identification of crystallinity. CuKα (1.542Å) radiation is used. The XRD scan range is 10-80 • , the accelerating potential was 40 KVA, and the scan step size was 0.02 • . The XRD pattern of ZnFe I 2 O 4 , TiO 2 , and ZnFe 2 O 4 /TiO 2 (1:1 w/w ratio) is shown in Figure 4. The XRD pattern of TiO 2 illustrates that it is available in anatase phase and all the peaks are well matched. ZnFe I 2 O 4 peaks prove that it is in the spinel phase. No peak shift or extra peak are observed after coupling, which means that no impurity is induced into it after the coupling of both semiconductors, and the pure catalyst retained its previous phase. The crystalline sizes of ZnFe 2 O 4 , TiO 2 , and ZnFe 2 O 4 :TiO 2 (1:1) are 44.8, 65.2, and 46.9 nm.

Conversion of CO 2 into Methanol Using Photocatalyst
Approximately 7 h has been taken in order to study the photocatalytic conversion of CO 2 into methanol by using ZnFe 2 O 4 /TiO 2 photocatalysts. The ratio used here is the one that gives the maximum yield of methanol, i.e., ZnFe 2 O 4 /TiO 2 (1:1 w/w); moreover, 7 h has been taken by considering previous research and literature [2,15]. The experimental results reveal that methanol is a major product in C T E D Catalysts 2020, 10, 163 7 of 17 liquid, while the photoactivity of metal ferrite decreases, which is a clear indication of the exhaust of active sites due to photo corrosion. The other catalysts such as metal, non-metal, or S-S doped TiO 2 or other than TiO 2 also show the deactivation after 7 h. Hence, a 7 h irradiation time was chosen based on the previous work. In a current study, the catalyst has shown deactivation after 5.5 h. The highest yield is obtained after 5.5 h, which is 531.6 µmol/L·g cat , but after that, the yield of methanol decreases, which is due to the conversion of CO 2 into other products or the occurrence of a backward reaction [16,17].

Conversion of CO2 into Methanol Using Photocatalyst
Approximately 7 h has been taken in order to study the photocatalytic conversion of CO2 into methanol by using ZnFe2O4/TiO2 photocatalysts. The ratio used here is the one that gives the maximum yield of methanol, i.e., ZnFe2O4/TiO2 (1:1 w/w); moreover, 7h has been taken by considering previous research and literature [2,15]. The experimental results reveal that methanol is a major product in liquid, while the photoactivity of metal ferrite decreases, which is a clear indication of the exhaust of active sites due to photo corrosion. The other catalysts such as metal, non-metal, or S-S doped TiO2 or other than TiO2 also show the deactivation after 7h. Hence, a 7h irradiation time was chosen based on the previous work. In a current study, the catalyst has shown deactivation after 5.5 h. The highest yield is obtained after 5.5h, which is 531.6μmol/L·gcat, but after that, the yield of methanol decreases, which is due to the conversion of CO2 into other products or the occurrence of a backward reaction [16,17].
The Figure 5 indicates that the yield is increasing with a continuous passage of time, but after 5.5 h, the yield declines suddenly. Methanol oxidation occurred instead of water oxidation, which is one of the major reasons for yield diminution [17]. Similarly, as Figure 5 indicates, after 7h of visible light irradiation, the photoactivity of metal ferrites reaches its high point, indicating that all the active sites are exhausted [18]. The Figure 5 indicates that the yield is increasing with a continuous passage of time, but after 5.5 h, the yield declines suddenly. Methanol oxidation occurred instead of water oxidation, which is one of the major reasons for yield diminution [17]. Similarly, as Figure 5 indicates, after of visible light irradiation, the photoactivity of metal ferrites reaches its high point, indicating that all the active sites are exhausted [18].

Effect of Different Coupling Ratios of ZnFe2O4 with TiO2
A different coupling ratio effect has been studied and finally plotted on the graph as shown in Figure 6, which reveals that ZnFe2O4/TiO2 in a 1:1 w/w ratio produces a higher yield. This has also been evident from the FTIR analysis when 1:1 coupling shows maximum absorbance, too. Other coupling ratios have also been tested i.e., ZnFe2O4 is coupled with TiO2 in a 1:2 ratio, while the yield

Effect of Different Coupling Ratios of ZnFe 2 O 4 with TiO 2
A different coupling ratio effect has been studied and finally plotted on the graph as shown in Figure 6, which reveals that ZnFe 2 O 4 /TiO 2 in a 1:1 w/w ratio produces a higher yield. This has also been evident from the FTIR analysis when 1:1 coupling shows maximum absorbance, too. Other coupling ratios have also been tested i.e., ZnFe 2 O 4 is coupled with TiO 2 in a 1:2 ratio, while the yield of methanol is decreased upto 30% as compared to a 1:1 ratio. In the case of a 1:2 ratio, it can be suggested that the majority of the catalyst is TiO 2 , which can work only under UV light such that only a small portion ofZnFe 2 O 4 is sensitized to TiO 2 . This is the main reason behind the yield depression. Similarly, when ZnFe 2 O 4 is coupled with TiO 2 in a 2:1 ratio, then the yield is much less. Correspondingly, the yield for the ratios of 1:3 and 3:1 has also been so minimal that it has also vanished. We can say the optimum methanol yield in comparison with the 1:1 ratio is 702.11µmol/g cat , and the yield is decreased up to 30% by using a 1:2 ratio. Up to 70% less methanol is obtained when a 2:1 ratio is used. Moreover, the yield has drastically decreased i.e., by 86.9% and 87.3% for 1:3 and 3:1, respectively.

Variable Calcination Temperature of ZnFe2O4 Effects
For synthesized photocatalysts calcined at different temperatures, the effect of different calcination temperatures on the methanol yield is illustrated in Figure 7. The catalyst was exposed under visible light irradiations for 5.5 h to check the photoactivity. The trend reveals that the amount of methanol is directly proportional to calcination temperature from 600 to 1000 °C. However, a sudden decrease in yield is observed when the calcination temperature is increased from 900 to 1000 °C. The increase in methanol yield with the increase in calcination temperature from 700 to 900 °C is due to the removal of all the impurities and volatile matters. The methanol yield obtained at 900 °C calcination temperature is 51% higher in comparison to a catalyst calcined at 700 °C and 36% greater than the catalyst calcined at 800 °C. Moreover, as the calcination temperature raise from 900to 1000 °C, an 85% decline in the yield is achieved in comparison with 900 °C calcination temperature. It is recommended that at >900 °C, the ZnFe2O4obtains impurities and the majority of active sites are deactivated, which results in a sudden dispersion in yield. As the temperature increases, the peak intensity increases and negligible peaks become clearer such that the crystallinity and shape of the

Variable Calcination Temperature of ZnFe 2 O 4 Effects
For synthesized photocatalysts calcined at different temperatures, the effect of different calcination temperatures on the methanol yield is illustrated in Figure 7. The catalyst was exposed under visible light irradiations for 5.5 h to check the photoactivity. The trend reveals that the amount of methanol is directly proportional to calcination temperature from 600 to 1000 • C. However, a sudden decrease in yield is observed when the calcination temperature is increased from 900 to 1000 • C. The increase in methanol yield with the increase in calcination temperature from 700 to 900 • C is due to the removal of all the impurities and volatile matters. The methanol yield obtained at 900 • C calcination temperature is 51% higher in comparison to a catalyst calcined at 700 • C and 36% greater than the catalyst calcined at 800 • C. Moreover, as the calcination temperature raise from 900to 1000 • C, an 85% decline in the yield C T E D Catalysts 2020, 10, 163 9 of 17 is achieved in comparison with 900 • C calcination temperature. It is recommended that at >900 • C, the ZnFe 2 O 4 obtains impurities and the majority of active sites are deactivated, which results in a sudden dispersion in yield. As the temperature increases, the peak intensity increases and negligible peaks become clearer such that the crystallinity and shape of the crystals change [19,20].
Catalysts 2020, 10, x FOR PEER REVIEW 10 of 18 Figure 7. Effect of calcination temperature on yield.

Effect of Different Catalyst Loading Ratios
To understand the effect of catalyst loading on the reduction of CO2 into methanol, different loading ratios were used ranging from 0.5 to 3 g/L, and the results are represented in Figure 8. It is apparent that the amount of methanol (μmol/L) increases as the amount of loaded catalyst increases from 0.5 to 3 gm/L.

Effect of Different Catalyst Loading Ratios
To understand the effect of catalyst loading on the reduction of CO 2 into methanol, different loading ratios were used ranging from 0.5 to 3 g/L, and the results are represented in Figure 8. It is apparent that the amount of methanol (µmol/L) increases as the amount of loaded catalyst increases from 0.5 to 3 gm/L.

Effect of Different Catalyst Loading Ratios
To understand the effect of catalyst loading on the reduction of CO2 into methanol, different loading ratios were used ranging from 0.5 to 3 g/L, and the results are represented in Figure 8. It is apparent that the amount of methanol (μmol/L) increases as the amount of loaded catalyst increases from 0.5 to 3 gm/L.

R E T R A C T E D
Catalysts 2020, 10, 163 10 of 17 Table 3 will further clarify that at which loading ratio quantity, Methanol yield will be optimum etc. The trend in Figure 8 shows an incremental increase; however, it is not exactly proportional to the amount of the catalyst loading ratio. The yield of methanol is 240.60 µmol/g cat by using 0.5 g/L catalyst. As the amount of catalyst is increased from 1 to 2 g/L, the amount of methanol should be doubled theoretically. However, only 36.2% higher methanol is obtained by using 2 g/L and vice versa. Moreover, the ratio percentage shows that the maximum yield is at 1 g/L, i.e., at 2 g/L, it must be double or near to that, but in the original, it is much lesser than that of 1 g/L, and the reverse is true until 3 g/L. Thus, at 1 g/L, the yield is 57.5%; at 2 g/L, the yield is 45.8%; and at 3 g/L, the yield is 41.7%. So, the optimum loading ratio is 1g/L, as the maximum yield has been obtained as per the loading ratio.

Effect of Catalyst Recycling
The catalyst has been recycled five times, and experimentation afterwards shows the trend of the recycling effect, as illustrated in Figure 9. After the first run, the whole solution present in a reactor including the catalyst was centrifuged at 1700 rpm for 18 min, and the catalyst was separated during the centrifuged process. The obtained catalyst was dried in an oven for 14 h at 120 • C. It is observed that activity of the catalyst decreases almost up to 55% during the second run. During the third run, a further decrease in the yield is observed, but this is not a straight-line depression in yield. For further standing, calculations of the trend of the decrease in the fourth and fifth run of the catalyst have also been done, which show that the yield has exponentially dropped by the fifth time run. It can be suggested that after 5.5 h, all the active sites are deactivated, and after recycling again and again, only a few sites are able to become active. It can be concluded that the catalyst is not useable again efficiently. Secondly, it is not possible to separate all the catalysts; however, the major portion is separated effectively. The deficiency of the catalyst is fulfilled by adding a fresh catalyst. The results clearly indicate that the catalyst can be effectively used only for one run. The metal ferrites do not work efficiently after recycling, because photo corrosion is a basic problem associated with this class [21,22].
Thus, it can be said that a similar trend has been observed in the current study. Methanol production via a CO 2 reduction mechanism through a ZnFe 2 O 4 /TiO 2 photocatalyst is illustrated in Figure 10.
It can be clearly understood from the yield experiment that after 5.5 h, the active sites become deactivated and after recycling, only just a few sites become active; thus, the main sites remain deactivated. Finally, it can be concluded that the catalyst is non-usable again and again effectively and efficiently. Thus, we can conclude from the said findings that the catalyst could only be used once efficiently. Moreover, the ferrites of metal did not work efficiently after recycling, the reason behind which could be the photo corrosion, etc.

C T E D
Catalysts 2020, 10, 163 11 of 17 Catalysts 2020, 10, x FOR PEER REVIEW 12 of 18 Figure 9. Effect of recycling a catalyst on the yield of methanol.
Thus, it can be said that a similar trend has been observed in the current study. Methanol production via a CO2 reduction mechanism through a ZnFe2O4/TiO2 photocatalyst is illustrated in Figure 10. It can be clearly understood from the yield experiment that after 5.5 h, the active sites become deactivated and after recycling, only just a few sites become active; thus, the main sites remain deactivated. Finally, it can be concluded that the catalyst is non-usable again and again effectively and efficiently. Thus, we can conclude from the said findings that the catalyst could only be used  Thus, it can be said that a similar trend has been observed in the current study. Methanol production via a CO2 reduction mechanism through a ZnFe2O4/TiO2 photocatalyst is illustrated in Figure 10. It can be clearly understood from the yield experiment that after 5.5 h, the active sites become deactivated and after recycling, only just a few sites become active; thus, the main sites remain deactivated. Finally, it can be concluded that the catalyst is non-usable again and again effectively and efficiently. Thus, we can conclude from the said findings that the catalyst could only be used

Proposed Reaction Mechanism
In Figure 10, a reaction mechanism of the carbon conversion into methanol has been proposed, the pH was 6.2 during the experimentation such that CO 2 could not only exist as dissolved CO 2 but also in the form of HCO 3 − . The most abundant form of CO 2 at this pH is the dissolved CO 2 and HCO 3 − . [ The coefficient 2 in above expression indicates that 2 moles of H + are required for the conversion of CO 3 2− into HCO 3 − , which is considered as a reference level. The pH during the reaction was C T E D Catalysts 2020, 10, 163 12 of 17 maintained at 6.2, which is acidic, such that all the bicarbonates that can be produced in CO 2 and water mixture can be neglected. The conversion of CO 2 occurs on this surface of TiO 2 and water oxidizes on the surface of ZnFe 2 O 4 because holes are transferred from TiO 2 to ZnFe 2 O 4 due to the position of its valence band.
The CB (Conduction Band) position of ZnFe 2 O 4 is more negative than the CB of TiO 2 , and its position is also close to the reduction potential of CO 2 into methanol. So, under the presence of visible light irradiation, the excited electrons are transferred from the VB (Valance Band) of ZnFe 2 O 4 to the CB of ZnFe 2 O 4 . These excited electrons are easily transferred to TiO 2 because of the formation of a heterojunction between these two semiconductors. Secondly, TiO 2 is also available in the Ti 3+ state, which is more active than the Ti 4+ state. Therefore, the efficiency also increases due to this reason. During the photoexcitation process, the holes are also generated that are transferred to the VB of ZnFe 2 O 4 , as illustrated in Figure 10. The heterostructure increases the e-and h+ separation time. Therefore, the recombination rate of e-and h+ is very slow, which is very suitable for photocatalytic reactions. The conversion of CO 2 occurs on this surface of TiO 2 , and water oxidizes on the surface of ZnFe 2 O 4 because holes are transferred from TiO 2 to ZnFe 2 O 4 due to the position of its valence band. The yield of methanol is slightly higher than the recently reported results, which are 651µmol/g cat . The increase in yield is suggested due to the proper alignment of ZnFe 2 O 4 with TiO 2 . Moreover, the constant temperature during the reaction and higher intensity of light are other reasons to get a higher yield.

Comparison with Previous Results
As TiO 2 cannot be used only for visible light because it has a higher band gap and its electron-hole pair recombination is much faster than the reduction, another catalyst must be added in order to make it a visible light-induced photocatalyst; some previously used photocatalyst results have also been shown with the current research in the following Table 4. The above table shows that the current research shows a maximum yield of 141.22 µmol/g cat ·h of methanol as compared to other catalysts using the same source of light i.e., a 500-W tungsten Bulb.
Photocatalyst used for the conversion of carbon dioxide into useful fuel with enhanced yield by using visible light could be a most important technique of current era because it can give us Carbon reduction with a fuel. Moreover selective product could be achieved by controlling conduction band position as illustrated by previous research [23,24]. Researchers also suggest that maximum yield still couldn't be achieved due to the reason of formation of other products like formaldehyde, formic acid as well as backward reaction [25]. was formed. A similar procedure was followed with TiO 2 , which was dissolved in 2.5 M of HNO 3 and nutrient agar solution, stirred, and heated continuously until an orange gel was formed. Then, to form the desired band gap, the photocatlyst ZnFe 2 O 4 /TiO 2 was sonicated and calcined at different temperatures for approximately 4.5 h in the presence of N 2 gas. Figure 11 explains the steps involved in the synthesis of the photocatalyst.

Synthesis Procedure
Altered process conditions were used to synthesized the ZnFe2O4/TiO2photocatlyst for which initially Zinc nitrate, Zn(NO3)2·4H2O, and iron nitrate Fe(NO3)3·9H2O were dissolved in 2.5 M of HNO3 and nutrient agar solution, at continuous stirring and heating continuously until a brownblackish gel was formed. A similar procedure was followed with TiO2, which was dissolved in 2.5 M of HNO3 and nutrient agar solution, stirred, and heated continuously until an orange gel was formed. Then, to form the desired band gap, the photocatlyst ZnFe2O4/TiO2 was sonicated and calcined at different temperatures for approximately 4.5 h in the presence of N2 gas. Figure 11 explains the steps involved in the synthesis of the photocatalyst.

Electron and Hole Pair Formation
The following equations represent concepts of electron and hole pair formation.

Electron and Hole Pair Formation
The following equations represent concepts of electron and hole pair formation.
where Eg = band gap energy; Ec = energy of the conduction band; and Ev = energy of the valance band.

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Catalysts 2020, 10, 163 14 of 17 The photocatalyst when exposed under light irradiation produced the excited electrons and holes, as shown in Equation (1). The electrons start recombining with each other if no absorbed species are present, as shown in Equation (2). This indicates that electron and hole pairs can combine with each other without taking part in oxidation and reduction reactions. As a result, reaction heat is released. If electron and hole pairs have enough energy, they move toward the surface and take part in the reaction. The electron and hole pairs can also be formed on the catalyst surface and produce unproductive heat. Equation (3) represents the band gap calculations as a difference between the valence band and conduction band. The activity of the catalyst depends upon the arrangement of the reaction medium, the absorption capacity of chemicals on the semiconductor's surface, the type of semiconductor and its morphology, and the capacity of the catalyst to absorb UV-Vis light irradiation. Surface improvement and reducing the band gap of a photocatalyst are two major factors.
Methanol production via a CO 2 reduction mechanism through a ZnFe 2 O 4 /TiO 2 photocatalyst is described as shown in Equations (4)- (8). pH has been maintained around 5.9-6.2 such that CO 2 could not only exist as dissolved CO 2 but also in the form of HCO 3 − . The most abundant form of CO 2 at the said pH is dissolved CO 2 and HCO 3 − . The proton balance for this is shown by Equation (4).
The coefficient 2 in Equation (4) indicates that 2 moles of H + are required for the conversion of CO 3 2− into HCO 3 − and is considered as a reference level.
The pH of the reaction and during the whole reaction was kept and maintained at around 5.9-6.2, which are acidic values such that all the bicarbonates that can be produced in CO 2 and water mixture can be deserted.
So, it results in another relation as expressed by Equation (5).
Potassium hydroxide was used to increase the absorbance of CO 2 into water, so the proton balance is shown as per Equation (6).
The pH of the solution was very close to a neutral value, so the [H + ] and [OH − ] can be ignored easily. So, it results in another relation as expressed by Equation (7). [ The conversion of CO 2 occurs when the reduction reaction on the surface of the synthesized catalyst is done as per Equation (8).
The reduction potential of CO 2 into methanol falls between the conduction band and valance band of ZnFe 2 O 4 /TiO 2 , which means that the coupled semiconductor can reduce CO 2 into methanol. The band gap of ZnFe 2 O 4 is lower, that is, 1.9 eV; hence, it works under visible light irradiation in comparison with TiO 2 , which has a band gap of approximately 3.2 eV.

Experimental Setup and Operating Conditions
An experimental setup includes a continuous flow reactor followed by a 500 Watt Xenon lamp, which is used as a visible light/radiation source. NaNO 2 (2 M) solution was used to cut the UV light. First of all, 0.1 M sodium sulfite, potassium hydroxide, and 0.1 M sodium sulfide were added in 500 mL of distilled water. In the next step, CO 2 fizzed from the solution in the reactor for 1 h to assure that O 2 C T E D Catalysts 2020, 10, 163 15 of 17 (dissolved) is approaching toward elimination and the pH of the solution is maintained at 6.2. Moreover, the catalyst should be mixed to the solution, making the concentration of catalyst 1 g/L. Afterwards, the photoreaction started as soon as the 500-watt bulb was switched on. The reactor temperature was maintained at 25 • C with the help of a chiller during the reaction. The CO 2 continuously flowed through the solution during irradiation. Three liquid samples were collected and analyzed in GC-FID for product detection. The photocatalytic reaction was carried out for 6 h. The experimental results of the methanol yield have been confirmed by repeating it three times, and the average yield was taken. Methanol was analyzed by GC-FID, and a DB-WAX 123-7033 column was used for the detection of methanol. Helium gas was introduced as the shipper gas, and the flow rate was 35 cm/s. The oven temperature was adjusted at 40 • C, and inlet temperature was fixed at 200 • C. The split ratio was 1:50 and the FID temperature was 300 • C. Nitrogen gas was used as the makeup with a flow rate of 30 mL/min at the FID detector. The yield was calculated by using the following equation.
Y = yield C = methanol concentration, µmol/L V = volume, L W = mass of catalyst, g

Conclusions
The primary object of the research was to synthesize a photocatalyst (ZnFe 2 O 4 /TiO 2 ) as a visible light-induced photocatalyst for the conversion of CO 2 into methanol under visible light irradiation. ZnFe 2 O 4 andZnFe 2 O 4 /TiO 2 heterojunction photocatalysts were synthesized and used for the reduction of CO 2 into methanol. The most optimized catalyst loading amount was 1g/L to get a maximum yield of methanol. The maximum yield observed was 141.22 µmol/g cat ·h. The prominent feature of this study is to analyze the effect of different parameters on the methanol yield such as the effect of calcination temperature, the intensity of light, the catalyst loading ratio, the effect of different coupling ratios, and the effect of recycling the catalyst. However, as the pH of the reaction solution was maintained at 5.9-6.2; hence, the pH and the most dominant species were 2 and HCO 3 − . The reported yield of methanol obtained by ZnFe 2 O 4 /TiO 2 is quite better because of e − and H I having a slow recombination rate due to the formation of the heterojunction. A 7% increase in the yield indicates that the coupling of ZnFe 2 O 4 with a TiO 2 increases the photocatalytic activity. The coupling of ZnFe 2 O 4 /TiO 2 resulted in a new band gap and made it visible light-responsive. Moreover, the maximum yield has been obtained at a calcination temperature of 900 • C and the coupling ratio is 1:1 w/w. Hence, after considering all the essential parameters, the results indicate that coupling of ZnFe 2 O 4 with TiO 2 is effectively enhanced under visible light irradiation. Thus, the following recommendations still need to be considered for future studies.

Recommendations
There are some suggestions for the future studies on the synthesis of ZnFe 2 O 4 and ZnFe 2 O 4 /TiO 2 for the reduction of CO 2 into methanol or some other photocatalytic applications. Despite the fact that ZnFe I 2 O 4 is a visible light-induced catalyst and forms a heterojunction with TiO 2 , which proved itself a very suitable technique for CO 2 reduction, other metal ferrites should also be tested.
Photoreactors of different types should be designed and tested in order to identify the most suitable photoreactor.
Author Contributions: Authors contributed a lot and despite of the busy schedule, gather as when required basis to discuss the conceptualization of the said research. N.M. and M.S. wrote the manuscript, investigate and validate the results. N.M. and U.S. performed data analysis and data curation. S.R.N. supervised, editing and