ZnO-Incorporated ZSM-5 for Photocatalytic CO₂ Reduction into Solar Fuels under UV–Visible Light ^†

Abstract

Direct conversion of CO₂ into chemical compounds has become a prospective pathway to transform CO₂ into valuable chemical compounds. The introduction of porous materials with high uptake into photocatalytic systems can enrich the CO₂ absorption on the surface of the photocatalyst for catalytic conversion. In this regard, another feasible strategy can be accomplished via combining commercial photocatalyst materials with porous supporting materials. The present study investigated a series of ZnO-incorporated ZSM-5 catalysts to produce solar fuels under UV–visible light irradiation. ZnO/ZSM-5 was synthesized using the wet impregnation method using Zn(CH₃COO)₂ as a reagent, followed by calcination. Various characterizations were also conducted to study morphology, structure, absorbance, and physiochemical properties of the photocatalysts. SEM-EDX images showed that ZnO was successfully incorporated into ZSM-5 surfaces with a particle size of around 50 nm. The optical properties of ZnO/ZSM-5 correspond to 3.36 eV, showing an increase in the bandgap value compared to pure ZnO, with 3.18 eV. The solar fuel production, including formic acid (HCOOH), formaldehyde (HCOH), and methanol (CH₃OH) evolution was evaluated under UV–visible light irradiation. The designed composite ZnO/ZSM-5 catalyst achieved methanol and formic acid yields of 39.2 µmol/g·h and 0.72 µmol/g·h µmol/(g·h), respectively, which are about 1.5- and 2.5-fold higher than the neat ZnO catalyst. The improved yield and selectivity towards methanol products are attributed to the greater light absorption, more efficient charge transfer, nanostructure morphology, and more active sites available for CO₂ adsorption.

1. Introduction

In recent years, CO₂ conversion into valuable chemicals, such as CH₃OH, H₂CO, HCOOH, CO, and CH₄, via photocatalysis has been presented as an alternative technology in overcoming the current global emission [1,2]. As a result, the design of photocatalyst materials has become a major challenge, particularly for producing highly active and selective products. Among various photocatalysts, zinc oxide (ZnO) is one of the most widely investigated due to its high photoactivity, high chemical and thermal stability, low cost, and non-toxicity [3,4]. However, the applications of ZnO are heavily limited by the wide range of its bandgap (E_g = 3.27 eV), rapid recombination, poor solar light utilization, and photochemical corrosion. Moreover, ZnO particles can also easily agglomerate, leading to poor industrial application performance [5].

Therefore, many different approaches have been adopted to overcome the disadvantage of pristine ZnO, which mainly focuses on improving the specific surface area and creating more reaction active sites to enhance the photocatalytic activity [6]. From this point, another feasible approach can be accomplished via the combination of ZnO with other inorganic porous materials, such as graphene oxide [7], single-walled carbon nanotubes [8], fullerenes [9], and Pd [10], which can successfully improve its photocatalytic activity. For this purpose, zeolite is considered a good candidate for the support of photocatalysts. The porous structure of zeolite can confine small molecules such as CO₂ to enhance its photocatalytic activity further. Moreover, zeolite has been applied as supporting material for various hetero-structured photocatalysts, such as TiO₂/Clinoptilolite [11], Ag–TiO₂/Zeolite-Y [12], ZnO/Zeolite-Y [13], and SnO₂/Clinoptilolite [14]. However, these reported works mainly focused on air and pollutant degradation, exhibiting higher efficiency than bare materials. Among the different types of zeolites, ZSM-5 zeolite is considered the most widely applied material as catalyst support due to its high surface area, surface acidity, ion exchange capacity, strong adsorption, and chemical stability [15,16]. It was also important to note that immobilization of metal oxides over ZSM-5 zeolites in a well-dispersed form can affect the increased bandgap energy and decrease the rate of electron–hole recombination [16,17].

In this study, zinc acetate (Zn(CH₃COO)₂) was used as a Zn source to prepare a ZnO/ZSM-5 photocatalyst via the wet impregnation method. The crystal structure, functional group, morphology, and pore structure of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), diffuse reflectance ultraviolet–visible spectrometry (DRUV–Vis), and N₂ adsorption/desorption and pore size distribution analysis. In addition, the product yield and selectivity were also investigated.

2. Materials and Methods

2.1. Materials

ZSM-5 (provided by Meiqi Industry & Trade Co., Ltd., Gongyi, China) was used as the catalyst support in all experiments. Chemicals were analytical grade and used as supplied: Zn(CH₃COO)₂.2H₂O (>99%, Merck) and ZnO (>99%, Sigma-Aldrich^®, St. Louis, MO, USA).

2.2. ZnO/ZSM-5 Synthesis

The impregnation method was used to synthesize ZnO/ZSM-5, as reported in the previous literature [18]. The concentration of Zn(CH₃COO)₂.2H₂O was matched to obtain 5% wt of ZnO content. Then, a suitable amount of commercial ZSM-5 powder was dispersed in the mixture. The mixtures were stirred for 24 h. The slurry was then dried at 110 °C for 6 h. Calcination was performed at 350 °C under N₂ gas flow with heating flow rates of 5 °C/min for 5 h.

2.3. ZnO/ZSM-5 Characterization

An X-ray diffractometer (Shimadzu, Tokyo, Japan) was used to analyze the crystallography profile of the samples. The morphology was observed by SEM (JEOL JSM-6510LA (JEOL Company, Musashino, Tokyo, Japan)). The light response and bandgap of the samples were evaluated by a dual-beam UV–Vis diffuse reflectance spectrophotometer (Shimadzu, Tokyo, Japan). The N₂ adsorption/desorption isotherms of the samples were measured using a Quantachrome Nova 4200E (Kanagawa, Japan) instrument. The FTIR instrument (PerkinElmer) investigated the functional group in the wavenumber range of 500–3750 cm⁻¹.

2.4. Photocatalytic Activity Assessment

The photocatalytic reduction of CO₂ into solar fuels was measured in a photoreaction system under UV–Vis light irradiation (λ = 300–1100 nm) emitted from a 300 W Xe lamp. Before the mixture was added to the reactor, the as-prepared sample (1 g/L) was dispersed in deionized water (400 mL) and continuously mixed for 30 min. Next, the photoreaction system was purged with high-purity nitrogen for 30 min before turning on the lamp. The temperature and pressure of the reactor were controlled at room temperature and ambient conditions, respectively. The final products were analyzed after 4 h of reaction using a gas chromatography system equipped with a thermal conductivity detector (TCD) for gas products and a high-performance liquid chromatography system (HPLC) for liquid products.

3. Results and Discussion

3.1. Structural Analysis

The crystal structure of the bare and synthesized materials was investigated by X-ray powder diffraction. Figure 1a depicts the diffractogram patterns of neat ZnO, ZSM-5, and ZnO/ZSM-5. A glance at the XRD patterns reveals that impregnated samples display ZnO and ZSM-5 peaks. The presence of ZSM-5 can be confirmed from peaks at 2Ө = 7.67, 8.61, 14.7, 23.1, 23.35, 23.72, and 23.96 (JCPDS 00-044-0002) in the tetragonal phase. Moreover, ZnO has peaks at 2Ө = 31.89, 34.55, 36.39, 47.69, 56.77, 63.03, and 68.22 (JCPDS 01-076-0704) in the hexagonal phase. By comparing with the bare ZnO and ZSM-5 data, it is also observed that after incorporating ZnO into ZSM-5, the peaks related to ZnO gradually decrease, and the intensity of the ZSM-5 peaks decrease. The crystallite size and relative crystallinity of ZnO/ZSM-5 were 11.53 nm and 79%, respectively, as shown in

Table 1. Summary of structural properties of ZnO, ZSM-5, and ZnO/ZSM-5.

Samples	Crystallite Size (nm)	Relative Crystallinity (%)
ZnO	26.37	-
ZSM-5	11.53	87.53
ZnO/ZSM-5	9.94	68.33

.

Figure 1b presents the FTIR spectra of ZnO/ZSM-5 in a wavenumber range of 400–4000 cm⁻¹. As shown in the graph, the FTIR patterns of ZnO/ZSM-5 have a similar trend to bare ZSM-5 and show no significant change. The interacting O-H or bridging O-H groups can be observed from the broad peaks at 3616 and 1551 cm⁻¹ [18]. Catalysts also possess water absorption properties from the air, which can be seen from the other recorded peak at 1551 cm⁻¹ [19]. Moreover, the broad peaks in the wavenumber range of 400–1200 cm⁻¹ correspond to the Si-O(Si) and Si-O(Al) vibrations in the tetrahedral or alumina and silica-oxygen bridges, respectively [20]. Furthermore, the stretching vibration of Zn-O can be attributed to the peak in the range of 400–500 cm⁻¹, where it can also be attributed to the overlapping peak with the Zn-O bond in the region [21]. It is also important to note that incorporating transition metal oxide cations does not change the main zeolite structure observed from the spectra [22].

3.2. Morphological Analysis

Figure 2a,b represent the SEM micrographs of the samples. The morphology of ZSM-5 has a hexahedral structure, in agreement with the ZSM-5 characteristics from a previously reported study [23]. By incorporating ZnO into ZSM-5, the surface was covered with a small amount of nano-scale ZnO particles, which led to the differentiation of ZSM-5 particles. In accordance with the relative crystallinity, the incorporation also affects the decreases in the relative crystallinity of ZSM-5. Additionally, the grain size also changes, attributed to the electronegativity of Zn (II) in the zeolite pore structure, which is stronger than the Si-O-Al framework [24]. This result confirms the irregular crystal grains due to the clustering phenomenon over the crystal [25]. A deeper examination of the SEM results revealed that a low amount of agglomeration could be observed from the synthesized samples. Nevertheless, agglomeration is considered a disadvantage of excessive loading, leading to worse catalytic performance. Furthermore, these images also show that impregnated ZnO sample catalysts have nanometric surface particles. Thus, they can provide more reactive and reducible sites and higher catalytic performance of the photocatalyst [26].

The SEM-EDX mapping analysis shows the existence of Al, Si, and Zn elements, which also confirms the absence of impurities in the samples, as shown in Figure 2c. This is additional evidence that ZnO/ZSM-5 was successfully synthesized and had well-dispersed Zn species in the ZSM-5 supports. Overall, the EDX analysis results also show that the dispersion of Zn was achieved at about 5% wt, as theoretically expected (

Table 2. Summary of elemental analysis of ZnO, ZSM-5, and ZnO/ZSM-5.

Samples	ZnO (%-wt)	Al2O3 (%-wt)	SiO2 (%)	Si/Al Ratio
ZnO	86.12	-	-	-
ZSM-5	-	0.92	46.62	50.67
ZnO/ZSM-5	4.92	0.86	35.11	40.82

).

3.3. Optical Analysis

Figure 3a demonstrates the diffuse reflectance UV–visible spectra of the photocatalysts. The absorption peaks of bare ZSM-5 and ZnO are observed at 220 nm and 292 nm, respectively. The absorption peak near 292 nm may be attributed to a small sub-ZnO cluster, while the absorption peak near 220 nm may be attributed to the ZnO and ZSM-5 interaction [27]. When the ZnO content increases, the intensity of the absorption peak increases gradually, which indicates that the response range expands in the visible spectrum and the photocatalytic activity is improved. Therefore, the indirect energy bandgap is also narrowed based on the Tauc plot. The bandgap value of ZnO/ZSM-5 was 3.36 eV, as the blue shifts show, as shown in Figure 3b. The shifting from red to blue in the ZnO/ZSM-5 samples is caused by the ZSM-5 absorption in the visible range (4.3 eV).

3.4. Textural Analysis

The N₂ adsorption isotherm and physicochemical properties including surface area, pore size and pore volume of as-prepared samples are shown in Figure 4 and

Table 3. Summary of textural properties of ZnO, ZSM-5, and ZnO/ZSM-5.

Samples	SBET (m2/g)	Pore Size (nm)	Pore Volume (cm3/g)
ZnO	8.43	11.34	0.023
ZSM-5	314.68	3.05	0.178
ZnO/ZSM-5	220.49	3.02	0.237

, respectively. In agreement with previous studies, bare ZSM-5 and ZnO exhibited a surface area of 314.68 and 8.43 m²/g, which are in the range of 300–400 m²/g and 5–40 m²/g, respectively [28,29]. Furthermore, the metal oxide loading into bare ZSM-5 decreased the surface area by 1.4-fold, attributed to the deposition of zinc species into micropores. The loss of ZSM-5 crystallinity also supports this hypothesis. This result can also be justified from the agglomerations of the supported samples, which correspond to the SEM images.

3.5. Photocatalytic Activity

The photocatalytic performance of the prepared catalysts was evaluated for CO₂ reduction in the liquid phase under UV–visible light illumination. The obtained products, including HCOOH and CH₃OH, were observed, as shown in Figure 5. It can be seen that CH₃OH was the main product of the photocatalytic CO₂ reduction, while HCOOH became the minor product when using the neat ZnO and ZnO/ZSM-5 catalysts. The incorporation of ZnO into ZSM-5 exhibited remarkable photocatalytic activity, reaching a product yield of CH₃OH and HCOOH of up to 1.5 and 2.5 times higher than the neat ZnO.

The improved photocatalytic activity of ZnO/ZSM-5 can be associated with a greater charge transfer efficiency, higher surface area, and enhanced light absorption than the neat ZnO catalyst [30]. Moreover, the presence of zeolite was attributed to the enhancement of CO₂ adsorption, as reported in a previous study under similar conditions. The adsorbed CO₂ is necessary because it can provide an electron trap for numerous electrons to generate light hydrocarbon products further. Ultimately, the production of CH₃OH in the present work was more competitive than previous works [30,31], illustrating the possibility of using ZnO/ZSM-5 composites in large-scale applications photocatalytic CO₂ conversion to methanol.

4. Conclusions

This work presented ZnO incorporated into ZSM-5 prepared by wet impregnation for photocatalytic CO₂ reduction into value-added chemicals. SEM analysis indicated a good dispersion of nanometric ZnO onto the external surface of ZSM-5. The FTIR and XRD results confirm the phase formation of ZnO/ZSM-5. The UV–Vis results reveal a noticeable improvement in the absorbance of the UV–visible light region, while the bandgap of the modified ZnO was increased, as compared to the bare ZnO. The remarkable photocatalytic activity was exhibited for CH₃OH generation (39.2 µmol/g·h) and HCOOH (0.72 µmol/g·h) by the ZnO/ZSM-5 composite. These results demonstrate an alternative pathway for obtaining highly effective and low-cost catalysts to produce renewable solar fuels.