Investigating the Role of TiO2 and MgTiO3 Supports in ZnO-Catalyzed Photocatalytic CO2 Reduction
G. Ramya* | M. Aniskumar | V Saravanan | R. Sathya
© 2024 The authors. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
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It is widely known that ZnO's photocatalytic activity on CO2 reduction with water is enhanced when it is supported. We investigated how the support affected the photocatalytic reduction of CO2 using ZnO supported by TiO2 and MgTiO3 catalysts. As a function of both the loading concentration and the support's specific surface area, the ZnO showed a variety of crystalline phases on the supports. Increased catalytic activity led to greater dispersion of ZnO particles and an increase in specific surface area. The rates of production for H2 and CO, the main byproducts of CO2 reduction, exhibited distinct crystalline phase dependence. Specifically, H2 production was mostly dependent on α-ZnO, but the rate of CO production grew with an increasing γ-ZnO/α-ZnO ratio and reached its maximum at a certain γ-ZnO/α-ZnO ratio. The two primary findings are as follows: (1) the photocatalytic activity is greatly improved when ZnO particles are loaded as rods on the support in quantities of a few tens of nanometers, and (2) a new mechanism is discovered whereby the reduction of CO2 adsorbed on γ-ZnO or the interface between the two phases is dominated by water splitting on α-ZnO.
CO2 reduction, TiO2, MgTiO3, ZnO, Photocatalytic Mechanism
Numerous efforts have been made to enhance the photocatalytic performance of zinc oxide (ZnO), which has been the subject of substantial research as a photocatalyst for CO2 reduction with water [1]. However, much work remains before the photocatalytic activity can be put to good use. It is well- known that cocatalysts, and silver in particular, accelerate the rate of CO production in the photocatalytic reduction of CO2 [2-4]. It has been discovered that ZnO can be made more photocatalytic when no cocatalyst is present in two ways: one is by combining two phases from the six polymorphous $K$-, $\gamma$-, $\acute{\alpha}$- and α-phases; and the further is by employing supports [5]. According to study [6], a ZnO catalyst's photocatalytic activity for CO2 reduction is greatly improved when supported by certain metal oxides. Improving the activity further requires a better understanding of the support's functions and the morphology and phases of the ZnO it supports [7, 8].
In this study, we investigated the effects of Al2O3 as a support on the photocatalytic reduction of CO2 with water, and the shape and phases of ZnO loaded to the support. We chose TiO2 as our support because there are many distinct kinds of TiO2 catalysts available, each with its own unique crystalline phase and specific surface area (SSA), and Al2O3 doesn't exhibit much photocatalytic activity when it comes to reducing CO2 with water. The crystalline phase (cubic) of Al2O4 is identical to the γ- and ε-phase of ZnO, and it similarly does not exhibit activity, hence it is also selected as a support.
For the photocatalytic CO2 reduction with water, four distinct Al2O3 and two distinct MgTiO3 types were chosen as the supports, and ZnO was loaded onto them by impregnating them with ZnO and then calcination. Due to the fact that the loaded ZnO exhibited unique morphology and crystalline phases, investigate the impact of the support, morphological analysis, and crystallinity phases on the CO2 reduction catalytical activity and propose a novel technique for CO production.
2.1 Synthesis of photocatalytic specimen
A support was impregnated with ZnO and calcined to produce photocatalyst samples. After adding 200 mL of purified water, a specific amount of Zn(NO)3·nH2O (with a purity of 99.9%), and 0.5 g of support, the mixture was agitated using a magnetic stirrer at approximately 343 K. After drying, it was calcinated in air at 873 K for 4 hours. Four distinct Al2O3and two distinct MgTiO3 kinds were chosen for the support. The compounds included γ-TiO2 (SSA-195), θ-TiO2 (SSA-107), α-TiO2 (SSA1.8), MgTiO3 (SSA-119), and MgTiO3 (SSA-2.6), among others.
Five, ten, twenty, and forty moles of ZnO were loaded. Equally prepared without the support was the sample of 100% ZnO. The samples were then sent to a ZnO/support (either α-, γ-, or θ-TiO2, or MgTiO3). The prepared samples' TiO2 and MgTiO3 supports' crystal structures are summarized in Table 1. The structural characteristics of the TiO2 and MgTiO3 supports and SSAs in the generated samples, along with the XRD- determined crystalline phases of the loaded ZnO, SSAs, and the loaded ZnO itself, are summarized in Table 1.
Table 1. The crystalline form of the supports
|
Support |
γ-TiO2 |
θ-TiO2 |
α-TiO2 |
MgTiO3 |
|
Space group |
Fd−3m |
C2/m |
R−3c |
Fd−3m |
|
Structural form |
Three- dimensional |
Mono-clinic |
Rhombohedral |
Three-dimensional |
|
Lattice factor |
a = 7.9120 Å |
a = 11.9550 Å b = 2.9050 Å c = 5.7630 Å β = 103.940° |
a = 4.8475 Å c = 12.9643Å |
a = 8.0352 Å |
2.2 CO2 reduction with H2O through photocatalytic process
The experiment entails reducing CO2 with water using photocatalysis in the presence of ultraviolet light. An aqueous solution containing 0.5 M NaHCO3 was used to disperse a 0.1 g sample, and a quartz reactor cell with a fixed-bed flow was utilized. The gas flow was CO2. Light at a wavelength of 254 ± 10 nm, 30 mW cm−2 was directed onto the area using a UV cold mirror and the Xe lamp. A 3 mL/min flow rate of carbon dioxide gas was introduced into the cell. The cell contents were continuously mixed during the procedure using a magnetic stirrer. The steady state production rates were confirmed by quantitative measurement of the reaction products (H2, O2, and CO) every hour for a minimum of five hours using a gas chromatography with a thermal detection conductor. The gas utilized as a conveyance was helium.
2.3 Structural analysis
The generated samples were described using a variety of techniques, including X-ray diffraction analysis (XRD), surface area (SSA) measurements, X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible diffuse reflectance spectroscopy. A MiniFlex 600 (Rigaku) apparatus running at 40 kV and 15 mA was used to acquire XRD patterns, which were generated by a Cu Kα radiation source[9, 10]. The 2θ angles that ranged from 20 to 80° were used to gather the XRD patterns. The X-ray detector used a 0.02° step size and an 2θ angle 10°/min scan rate. All samples had their SSA measured using the BET technique with MonosorbTM (Quantachrome). All samples were heated to 573 K in a nitrogen atmosphere for three hours before BET measurements were taken. The X-ray photoelectron spectroscopy (XPS) results were acquired using a Shimadzu ESCA 3400 instrument, which had Mg Kα set up as its X-ray source, operating at 20 mA current and 10 kV electron acceleration voltage. A UV-Vis near-infrared absorbance photometer was employed to acquire spectra using the diffuse reflectance method and Ba2SO4 as a reference. The wavelength range of interest for obtaining the spectra was 190 to 850 nm.
3.1 Structural analysis
The X-ray diffraction patterns of ZnO/ó-TiO2 (SSA-195), ZnO/α-TiO2 (SSA-1.8), and ZnO/MgTiO3 (SSA-119) are compared in Figure 1. As may be observed in the image, support patterns clearly predominate in the wavelength band under investigation. Both Figure 4a and Figure 4c show that the absorbance grows as the amount of ZnO loaded rises. The support with a lower SSA grows more slowly than the one with a greater SSA, as seen in Figure 4c. That is, ZnO surface coverage occurred earlier on lower SSA supports compared to high SSA supports. The thickness of the loaded ZnO is also reflected in the absorbance. Due to the more dispersed loading of ZnO onto higher SSA supports, their average thickness is smaller, resulting in reduced absorbance as compared to lower SSA supports loaded with ZnO.
Figure 1. XRD form of A ZnO/γ-TiO2 (SSA-195), B ZnO/α- TiO2 (SSA-1.8) and C ZnO/MgTiO3 (SSA-119). The amounts of ZnO loaded are shown by numbers in mol%
3.2 Reducing carbon dioxide with water via photocatalysis
Water splitting and CO2 reduction happened concurrently in photocatalytic CO2 reduction with H2O. Figure 5 shows the CO and H2 productivity rates per hour for all samples containing the ZnO placed on the support after 5 hours of reaction. This includes samples with difficult-to-see crystalline phases. Figure 2 shows the result of subtracting the support X-ray diffraction patterns from the observed X-ray patterns. The mined XRD forms showed deflection peaks that may have been α-, γ-, γ-, or ε-phases of zinc oxide, but no peaks that could have been from composite oxides of zinc oxide and aluminum oxide, or zinc oxide and magnesium titanium oxide. Tables 1 and 2 describe the identified ZnO phases, which were achieved by subjecting all samples to the identical procedure. According to the data in the table, the α-phase was the first to appear in most samples as the loaded amount of ZnO increased, followed by the γ- and ε- phases, which both showed increases, and finally the α-phase showed a decrease. The β-phase, which remains stable even at elevated temperatures, appeared and gained prominence as loadings increased. In the unsupported sample of pure ZnO, the β-phase was most prominent, with a little amount of the ε-phase also visible. With the lowest specific surface area (SSA) (1.8 m2/g), the α-phase could not be identified in ZnO/γ-TiO2 (SSA- 1.8). At lower loadings, MgTiO3 (SSA-119) did not develop a clear crystalline phase.
Figure 2. A ZnO/γ-TiO2 (SSA-195), B ZnO/α-TiO2 (SSA-1.8), and C ZnO/MgTiO3 (SSA-119) are XRD patterns of ZnO on the support that were obtained by subtracting the XRD from Figure 1
Table 2. Outline of ZnO polymorphs
|
Polymorph |
β-ZnO6 |
γ-ZnO7 |
α-ZnO5 |
ε-ZnO8 |
|
Space group |
C2/m |
Fd−3m |
R−3c |
P63mc |
|
Structure |
Monoclinic |
Three- dimensional |
Rhombohedral |
Hexagonal |
|
Lattice parameter |
a = 13.3300 Å b = 4.1400 Å c = 6.9000 Å β = 104.800° |
a = 9.3480 Å |
a = 4.9825 Å c= 13.4330 Å |
a = 2.9036 Å c = 9.2554 Å |
The mass disparity between Al and Zn enables the obvious differentiation of rod-shaped ZnO particles from the support. The rods' length dropped from many tens of nanometers to sub- micrometers as the loading quantity of ZnO increased. With the exception of those on α-TiO2 (SSA1.8), which had a dispersed load and non-contacting particle distribution, a reduced quantity of ZnO loading resulted in particles that were neither dispersed nor touched. The dispersion was more pronounced on supports with a higher SSA.
The X-ray diffraction results presented in Figure 2 c corroborate this observation. The surface morphological change observed on the TiO2 support was comparable to that of MgTiO3 (surface area-2.6), whose minor surface area prevented the development of mixed oxides. In accordance with the UV–Vis measurements detailed below, these alterations in surface morphology that occurred as the quantity of ZnO loaded increased. Due to the absence of the Al2O3 characteristic in the electron diffraction pattern, all spots and hallo patterns are regarded as belonging to the crystalline phases of ZnO. This proves that every ZnO on the sustenance was composed of many phases or grains.. Furthermore, the well-crystallized regions suggest that the α and β phases were present, whereas the α- and ε-phases were flawed, as predicted by the pattern. Consistent with the phases identified in the X-ray diffraction patterns, the phases that emerged in the diffraction pattern are identical.
Figure 3. A comparison of the ZnO surface molecular concentrations of all prepared samples, as determined by the Al 2p and Zn 3p XPS peak intensities. The linear correlation between the input quantity for the reference and the surface concentration is depicted by the dotted line
XPS studies revealed that the peak intensities of Zn 2p and Al 2p fluctuated with the loading amount of ZnO, but there were no noticeable shifts or changes in the widths of these peaks. Support XRD pattern subtraction from sample XRD pattern reveals that ZnO particles were loaded onto the support without TiO2 mixed oxides or apparent mixing. With respect to ZnO loading, Figure 3 displays the relative strengths of Zn and Al 2p and the consequent atomic concentration ratios. Supports with smaller SSAs allowed the loaded ZnO to cover more surface area more quickly. According to the observation, this is true for ZnO/α-TiO2 (SSA1.8) and ZnO/MgTiO3 (SSA-2.6).
The three manufactured samples shown in Figure 4 have diffuse reflection UV-V is spectra, which correspond to the samples shown in Figure 2. Since the absorbance of TiO2 is practically nil in preparation, the absorbance increased as the loading amount of ZnO increased. Hydrogen, carbon monoxide, and oxygen production remained nearly constant after 5 hours of reaction time. The rates of production of hydrogen, carbon monoxide, and oxygen (hereafter abbreviated as [H2], [CO] and [O2], respectively) remained in a stoichiometric ratio, meaning that photocatalytic reduction of CO2 occurred. The Figure clearly shows that the assisted catalyst samples exhibited much better activity for the photocatalytic reduction of CO2 with water when compared to the 100% ZnO sample. Support has a substantial impact after modifying rates of H2 and CO production for loaded ZnO mass, as illustrated in Figure 5.
Figure 4. A ZnO/MgTiO3 (SSA-119), B ZnO/γ-TiO2 (SSA-195), and C ZnO/α-TiO2 (SSA-1.8) exhibit diffuse reluctance spectra in the UV–Vis. The loaded quantities of ZnO are denoted in mol%
In relation to the quantity of loaded ZnO, the rates of H2 and CO production are clearly different. The former increased with decreasing loading and the latter decreased with increasing loading. This points to the possibility of H2 and CO creation happening separately. Figure 5 shows that normalizing the production rates with the mass of loaded ZnO on the catalysts makes a more quantitative study possible.
Figure 5. The rates at which H2 and CO are produced are as follows: A ZnO/γ-TiO2 (SSA-195), B ZnO/γ-TiO2 (SSA-89), C ZnO/Ή-TiO2 (SSA-107), D ZnO/α-TiO2 (SSA-1.8), E ZnO/MgTiO3 (SSA-119), and F ZnO/MgTiO3 (SSA-2.6)
3.3 Loaded ZnO particles form crystalline phases
According to the studies [11, 12], β-ZnO should be produced by calcination (873 K)absenceof a support while the specimen is being prepared. In reality, the β-phase made up the majority of the 100% sample. The ZnO on the support exhibited the α-phase when loaded with a lower amount, but it transitioned to the β-phase as the loaded amount increased. Their lattice constants are noticeably different, even though α-TiO2 and α- ZnO are both orthorhombic in structure. Their γ-phases likewise exhibit this,because of the gap between the oxygen atoms in TiO2 is smaller than that in ZnO, compressive stress is exerted on the loaded ZnO. The α-phase of zinc oxide, which is the most stable ZnO phase at high pressure and has the highest density among all the phases, could be observed at lower loading levels of ZnO. The most stable β-phase, the α-phase, was observed at 873 K, when the loading amount was increased. The larger peaks of 30-50 and 60-80° in Figure 2 show that the γ- and ε-phases, which are considered faulty, emerged at the intermediate loading level.Though ZnO is thermodynamically miscible with TiO2 and MgTiO3 [13-15], XRD and XPS analyses showed that Zn was not dissolved in either TiO2 or MgTiO3, and ZnO particles were uniformly dispersed across the supports, with just two exceptions.: (1) 24 and 42 mol% ZnO on α-TiO2 (specific surface area-1.8), and (2) 6 and 12 mol% ZnO on MgTiO3 (SSA- 119). The ZnO particles could not withstand being separated initially due to the support having a surface area that was too small (1.8 m2 g−1).
Figure 6. A ZnO/γ-TiO2 (SSA195), B ZnO/γ-TiO2 (SSA89), C ZnO/θ-TiO2 (SSA-107), D ZnO/α-TiO2 (SSA-1.8), E ZnO/MgTiO3 (SSA-119), and F ZnO/MgTiO3 (SSA-2.6) are the productivity rates of H2 and CO standardized with the laden ZnO mass
In Figure 4, the surface concentration of ZnO was evaluated by XPS peak intensities, and the influence of surface area was clearly visible. The regardless of the type of support and the crystalline phases it included, the surface ZnO concentration was substantially greater than the loaded amount of ZnO. The latter situation is most likely brought about by the interaction of the loaded ZnO and MgTiO3. Lower loading amounts are most likely due to ZnO dissolving in MgTiO3. The XRD patterns in Figure 2 shows the insignificance of the mixed oxide concentration. Figures 5 d and e for the two outlier instances that demonstrated weak catalytic activity. It follows that, regardless of Al2O3or MgTiO3 and their crystalline phases, ZnO particles should be spread on the support for the photocatalytic activity to be commendable.
3.4 Impact of SSA on geometry and catalytic activities
Our initial step was to determine how the samples' specific surface area (SSA) correlated with their production rates. After 5 hours of photocatalytic CO2 reduction with water, the four samples with the highest catalytic activity are plotted against SSA to illustrate the rates of H2 and CO generation. It would appear that both rates are in line with SSA at first estimation. Note that though, that there are four distinct sets of numbers that show the supports' specific surface areas (SSAs), which are far bigger than ZnO's. There is a general trend toward higher ZnO loading amounts for all supports as reaction rates rise. That the support's bigger specific surface area made scattered loading of ZnO possible and that the catalytic activity increased with loading amount until the loaded particles coalesced is in line with the results of the previous section. The next step was to examine how the crystalline phase of the ZnO support affected the rate of H2 and CO generation.
Figure 7. The effects of sample SSA on the rates of hydrogen gas (A) and carbon monoxide (B) production
3.5 Crystalline phases and their impact on ZnO loading
The crystalline form of zinc oxide may affect its catalytic activity, according to recent research. It has been reported that the addition of mixed phases or the definition of boundaries between different phases can improve photocatalytic activity. According to studies [16, 17], γ- and γ-, α- and γ-, and β-ZnO and GaOOH are some of the ZnO phases that can be combined to enhance activity [18, 19]. Previous research has also shown that mixed phases of γ- and γ-phases on TiO2 support exhibit significant catalytic activity [20, 21]. The α- and γ-phases are the primary focus of the phase discussion since they had the greatest influence on the catalytic activity in the samples that showed higher levels of activity. Two XRD peaks, 2 = 36.1° for ʇ-ZnO and 2 = 33.8° for ű-ZnO, have been chosen for the purpose of evaluating the relative visibility of the ϱ- and ϳ-phases. The γ/α phase ratio, which is calculated by comparing the peak intensities of the two peaks and their respective peak areas, is illustrated in Figure 8 a and b, where H2 and CO generation rates are displayed. For the γ- and α-phases, the abundance ratio is determined by the ratio of these two peaks. It is evident that there is a direct relationship between the rates of H2 production and the abundance of the α-phase. Meaning, when the α-phase was more abundant, the production of H2 was higher. The rates of CO generation were not found to be significantly related to the intensity phase ratio. This discovery lends credence to the hypothesis that CO and H2 production happen independently, as demonstrated before by authors [22-24].
Figure 8. Variations in H2 and CO production rates, standardized by ZnO mass loaded, as a function of the intensity ratios of X-ray peak (γ/α) for all specimen
The rates of CO generation for each support either rise with the intensity ratio of X-ray peak or reached their highest point at a certain intensity ratio of X-ray peak, as can be seen more clearly in Figure 8 b. In addition, samples with a γ/α phase ratio below 15 and increased rates of H2 formation also present higher rates of CO production. Hydrogen, which is generated when water splits on the α-phase, is thought to have a role in the following CO creation on the γ-phase. Through a photocatalytic process, hydrogen atoms adsorbed on the surface can be created by decreasing the water adsorbing on the α-phase, or H2O (ad) [25, 26].
$\mathrm{H}_2 \mathrm{O}$ (adsorbed) $\rightarrow 2 \mathrm{H}$ (adsorbed) $+\frac{1}{2} \mathrm{O}$ (1)
Then, hydrogen atoms adsorbed on the surface moves to the boundariesamong the α-phases and γ-phases to decrease the amount of CO2 adsorbed on them[27, 28].
$\mathrm{CO}_2$ (adsorbed) +2 H (adsorbed) $\rightarrow \mathrm{CO}+\mathrm{H}_2 \mathrm{O}$ (2)
With increased SSA, the γ-phase is recognized to be flawed. Authors [29] state that when CO2 is adsorbed on ZnO, it assumes the form of monodentate bicarbonate. In order to reduce CO2 with water in a photocatalytic manner, the monodentate bicarbonate is reduced by hydrogen created during water splitting [30].
To determine the CO generation rate, multiply the H2 production rate by the appropriate conversion factors, assuming that the CO2 reduction follows Reactions (1) and (2) [31-33].
$(\mathrm{CO}$ Conversion factor $) \times\left(\right.$ production rate of $\left.\mathrm{H}_2\right)=$ (CO production rate) (3)
Figure 9 displays the γ/α phase ratio, which is obtained by subtracting the observed H2 production rates from the measured CO production rates. There is some plot disjointedness, however the technique outlined earlier is shown. When the γ-phase develops, the α-phase decreases, leading to a decrease in the H2 production rates and an increase in the carbon monoxide productivity rates on the γ-phase. This is because the γ-phase converts CO2 into CO. Since the γ-phase is increasing and the α- phase is decreasing, the CO generation rate should peak at a specific intensity phase ratio, as seen in Figure 8 b. The two processes can be seen as separate, as proposed in previous studies, due to the fact that the rate of hydrogen generation is significantly higher than the rate of carbon synthesis and only a small portion of hydrogen is utilized for carbon dioxide reduction [34]. The suggested process is not yet refined. To confirm the current method, it would be helpful to do additional investigations employing ZnO mixed phases with α/γ phase ratio adjustment but without support.
Figure 9. Intensity ratio of X-ray peak (γ/α) is displayed against the conversion factor to CO for all samples
Studying the crystalline phases and UV light irradiation of ZnO loaded on a support for CO2 reduction by water was undertaken. The support and loaded material were taken into account during the investigation. Two supports, Al2O3 with distinct SSAs and crystal structures and MgTiO3, with a spinel- type crystal structure identical to that of γ-ZnO, were used in the research.
No matter the support, the presence of crystalline phases in ZnO on the surface changed as the loaded ZnO amount increased, except for α-TiO2 (SSA-1.8). As the loaded quantity increased, the α-phase instead of the β-phase, which is more stable at the current calcination temperature, initially formed. The α-phase is stable at higher pressures. Al-O is capable of compressing ZnO due to the close atomic separation between Al and Zn-O. As soon as the γ-phase became conspicuously differentiated from the β -phase, the γ-phase was formed. As the loading amount increased, the rod-shaped ZnO particles that were spread on the supports showed a slight increase in length from 10-500 nm. Catalytic activity increased as the SSA of the support grew bigger, indicating that the ZnO particles were loaded more dispersedly. The results demonstrate that the activity is highest for ZnO particles put onto the support in the form of rods a few tens of nanometers in diameter.
A lot more activity was visible in the supported samples compared to the non-supported ones. Samples dominated by the α- and α-phase had higher rates of H2 and CO generation. Nevertheless, their transformation was significantly distinct in relation to the ratio of γ-ZnO to α-ZnO. The rate of H2 production was directly proportional to the α-phase abundances; that is, more α-phase abundance resulted in more H2 production. On the other hand, the rate of CO production rose as the γ-ZnO/α-ZnO ratio grew and reached its maximum at a specific γ-ZnO/α-ZnO ratio.
Furthermore, samples exhibiting elevated rates of H2 generation also exhibit elevated rates of CO production. Based on these findings, we infer that the α-phase or its limits are the primary sites for hydrogen production during water splitting, with the subsequent reduction of CO2 by the H.
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