Gas Phase Photocatalytic CO₂ Reduction, “A Brief Overview for Benchmarking”

Abstract

Photocatalytic CO₂ reduction is emerging as an affordable route for abating its ever increasing concentration. For commercial scale applications, many constraints are still required to be addressed. A variety of research areas are explored, such as development of photocatalysts and photoreactors, reaction parameters and conditions, to resolve these bottlenecks. In general, the photocatalyst performance is mostly adjudged in terms of its ability to only produce hydrocarbon products, and other vital parameters such as light source, reaction parameters, and type of photoreactors used are not normally given appropriate attention. This makes a comprehensive comparison of photocatalytic performance quite unrealistic. Hence, probing the photocatalytic performance in terms of apparent quantum yield (AQY) with the consideration of certain process and experimental parameters is a more reasonable and prudent approach. The present brief review portrays the importance and impact of aforementioned parameters in the field of gas phase photocatalytic CO₂ reduction.

1. Introduction

Photocatalytic products, as a consequence of CO₂ photoreduction, are industrially desirable with the additional benefit of normalizing anthropogenic CO₂ [1]. It is inevitable to develop and design efficient photoreactors and optimize the photoreaction conditions in order to scale up the photocatalytic process, all in congruence with synthesis of robust photocatalysts [2]. Although there have been many studies pertaining to the synthesis of stable and efficient photocatalysts, only few studies are dedicated upon reaction engineering so as to ascertain the optimum reaction conditions and photo reactor design [3]. Both of these factors have significant influence on photocatalytic yield. For instance, product yield will be different for photoreactors with batch and continuous flow operation under different conditions of reaction parameters, feed type and concentration ratio, photocatalyst loadings, and sources of irradiation [4,5]. In particular, irradiation sources and its mode of irradiation over the photocatalyst have vital importance. As most of the frequently used photocatalysts are Ultraviolet (UV) light active, irradiation from UV source as compared to solar light will significantly enhance the product yield [6,7,8]. Further, solar concentrator technology with utilization of Fresnel lens for photocatalyst irradiation leads to enhanced light intensity and photon flux, which in turn renders a significant enhancement in yield of hydrocarbon products [8,9]. Although, most of the photocatalytic reactions are carried out at room temperature, concentrating solar light will increase the temperature of the system, which will induce the thermal effect that alters the yield [6,7].

Another decisive factor is the exposed area of the photocatalyst to interact with the light irradiations. Even for the different types of photocatalysts, when tested under similar conditions and with yields reported per gram of photocatalyst, a comparison can be misleading because the exposure area, per gram of the photocatalyst, to light will be different for powder catalysts and thin films [10,11]. Moreover, irrespective of photoreaction under standard conditions, yields have been reported in different customary units i.e., ppm cm⁻² h⁻¹, μmol g⁻¹ h⁻¹ and μmol cm⁻² h⁻¹ which further complicates the comparison [12,13,14,15]. Thus, photocatalysts that are tested under such different conditions and reporting consequent yield in different units lead to ambiguity for fair performance comparison. It is inappropriate to compare the activities of the photocatalysts on the basis of their intrinsic performance without considering effects of optimum conditions and reactor geometries because of these implications.

The aforementioned bottlenecks strongly advocate the necessity for standardized protocols in order to compare the results for photocatalytic CO₂ reduction on equal grounds. Therefore, reporting yields on the basis of photonic efficiency, which incorporates the radiation source and exposure area of the photocatalyst, is an appealing approach [16]. This review specifically focuses on the parameters that influence the actual yield of the photocatalytic CO₂ reduction reactions and outlines the standard testing practices for comparison and evaluation of performance. Moreover, we have calculated the apparent quantum yield (AQY) of different research works, on the basis of data available, for comparing the efficiencies of the photocatalysts.

2. Role of Organic Contaminations

Photocatalysts synthesis predominately involves organic materials as reaction reagents. The residues of such organic materials are not easily removed, even with calcination at higher temperatures. Photocatalytic CO₂ reduction products mainly hydrocarbons, using such catalysts, originate concomitantly from these organic residues and CO₂ as well [1,2,3]. Thus, yields from such photocatalysts are overestimated when compared to the actual yield originating from photocatalytic CO₂ reduction. However, isotopic labelling by ¹³CO₂ is carried out to rule out the possible involvement of these organics in overall photocatalytic yield. To understand the effects of organic contamination, Yang et al. performed Fourier Transform Infrared Spectroscopy (FTIR) investigations for photocatalytic CO₂ reduction over Cu(I)/TiO₂ photocatalyst synthesized by two different ways: one with polyethylene glycol (PEG) and another without PEG. The FTIR results showed that Cu(I)/TiO₂ with PEG produces more CO (photocatalytic reduction product) as compared to Cu(I)/TiO₂ without PEG as shown in Figure 1. This CO originates from organic contaminants even without the introduction of CO₂. The possible mechanism of formation of CO is represented by Equations (1) and (2),

CO₂ + C → 2CO

(1)

H₂O + C → CO + H₂

(2)

Calcination at high temperature and illumination under dry He/Ar is incapable of completely wiping out these contaminants [4,5]. However, UV treatment in the presence of H₂O vapors is propitious in removing these contaminants [4]. Furthermore, they extended this study to elucidate the effects of prolonged and repeated pretreatment, for four cycles (7 h each) under He/H₂O, for abolishing organic contaminants. In another study involving the synthesis of Ti-SBA-15 while using P123 (Pluronic acid) and TEOS (Tetraethylorthosilicate) by calcining at 550 °C for 6 h, they revealed the generation of significant amounts of hydrocarbon products (CH₄, C₂H₆, C₂H₄) under He/H₂O environment. However, the yield of such photocatalytic products was significantly decreased after the first cycle but not diminished completely as shown in Figure 2a–d [5]. Moreover, the control experiments for photocatalytic reduction of CO with H₂O exhibited transition in enhanced selectivity from C₁ to C₂. Thus, CO produced as a consequence of organic contamination, as shown in Equations (1) and (2) has profound effects over yield and selectivity of photocatalytic CO₂ reduction [5].

In a similar study by Busser et al., they reported the adsorption of CO₂ over the surface of the photocatalyst during photodeposition of Cu/Cr upon Ga₂O₃ in the presence of CH₃OH [1]. Their study confirmed that the CO₂ originates from photooxidation of methanol as shown in Equation (3).

Cu²⁺ + CH₃OH + H₂O → Cu⁰ + CO₂ + 6H⁺

(3)

Thus, the assessment of possible contribution of these organic contaminations is indispensable for gauging the actual yield.

3. Flow versus Batch Reactors

Photocatalysts that were tested under different reaction parameters and/or in different reactor geometries can have variability in reaction rate, yield and selectivity [6,7]. Literature suggests variety of reaction conditions and set-ups for photocatalytic CO₂ reduction. Of these conditions, two type of reaction modes are extensively applied in photocatalytic CO₂ reduction: (i) flow reaction system and (ii) batch reaction system [5,9,10,11,12]. However, the largely reported use of batch reactors obtains smaller yields of the photocatalytic products. Thus, the activity of the photocatalysts, tested under these different conditions, is imprecise to compare [2,10,13,14,15,16,17].

For batch reactors, it is difficult to understand and control the reaction mechanism and also product composition as well. This is because the products generated in the photocatalytic reaction may get re-adsorb over the surface of the photocatalysts or they can participate in side reactions to yield different products [9,18]. The key limitations of the batch reactor system include an accumulation of the products inside the reactor for a certain defined time which can lead to changes in the concentration of reactants by photocatalytic reactions itself, re-adsorption of the intermediate species or products, and the initiation of side reactions such as hydrogenation or re-oxidation to CO₂. For instance, O₂ that is produced during photocatalytic reduction when adsorbed on the surface of the photocatalyst might compete with CO₂ for electron intake. Thus, batch reactors for CO₂ photoreduction under various experimental conditions make it very difficult to compare the photocatalytic performance and they are not a suitable option for extended time and for industrial scale applications [12]. To resolve such issues, Pipelzadeh et al. designed a pressure swing reactor for facilitated product (CO mainly) desorption. In their configuration, the products were continuously recycled, thus periodic injection/evacuation of gases generated turbulence, which enhanced the CO production yield to 30–80%, depending upon flow rates as shown in Figure 3. Thus the continuous desorption of products is imperative which may surmount mass transfer limitations and the deactivation problems of the photocatalysts [19].

Contrary to batch reactors the re-adsorption of products and other aforementioned issues can be curtailed in continuous flow reactor system [9]. Despite that, the yield reported is still inadequate since these types of systems only allow for short residence time of reactants i.e., restricting reactants to make proper contact with photocatalysts. However, better performance can be obtained by optimizing the reaction conditions and using robust photocatalysts [20,21]. In another study, a twin reactor, as shown in Figure 4, was designed to avoid the possible re-oxidation of the photocatalytic products. The photocatalytic CO₂ reduction results revealed that the production of hydrogen and oxygen in separate compartments and then use of as produced hydrogen in CO₂ reduction increased the yield of the products [22].

As evident from

Table 1. Summary of operating parameters and yields reported by In et al. for photocatalytic CO₂ reduction.

Photocatalyst	Synthesis Method	Pre-Treatment of Reactor	Light Source	Reducing Agent	Reaction Parameters	Reactor	Main Product	AQY (%)
Degussa P25 standard titania [23]	store-bought	purged with high purity CO2 gas, at least five times	UVP, UVGL-58 lamp with λ = 365 nm; 1200 μW cm−2	H2O	50 mg catalyst on a 30 mm diam. glass disk; 15.4 cm3 reactor; CO2 flowrate @ 10 cm3 min−1; 500 μL sample gas extracted; ambient temperature and pressure; 1 h irradiation	Batch reactor	CH4 @ 0.021 µmol g−1 h−1	0.0021
CZTS−TiO2 hybrid mesoporous [24]	hot injection and annealing	purged with CO2 gas (1000 ppm in He) and vacuum	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	50 mg catalyst on a 30 mm diam. glass disk; 15.4 cm3 reactor; CO2 flowrate @ 10 cm3 min−1; 500 μL sample gas extracted; ambient temperature and pressure; 1 h irradiation	Batch reactor	CH4 @ 118.75 ppm g−1 h−1	0.0057
CZTS-ZnO nanoparticles [14]	hydrothermal treatment	three times purged with CO2 gas (1000 ppm in He) and vacuum	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	50 mg catalyst on a 30 mm diam. glass disk; 15.4 cm3 reactor; CO2 flowrate @ 10 cm3 min−1; 500 μL sample gas extracted every 1 h; ambient temperature and pressure; 1 h irradiation	Batch reactor	CH4 @ 0.0954 µmol g−1 h−1	0.0128
CuxO−TiO2 mesoporous p-type/n-type heterojunction material [25]	thermal decomposition then calcination	purged with CO2 gas (1000 ppm in He) and vacuum	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	50 mg catalyst on a 30 mm diam. glass disk; 15.4 cm3 reactor; CO2 flowrate @ 10 cm3 min−1; 500 μL sample gas extracted; ambient temperature and pressure; 1 h irradiation	Batch reactor	CH4 @ 221.63 ppm g−1 h−1	0.0177
Pt-x-RT nanoparticles [15]	magnesio-thermic reduction	five times purged with CO2 gas (1000 ppm in He) and vacuum	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	70 mg catalyst on a 30 mm diam. glass disk; 15.4 cm3 reactor; CO2 flowrate @ 10 cm3 min−1; 500 μL sample gas extracted; ambient temperature and pressure; 1 h irradiation	Batch reactor	CH4 @ 1.13 µmol g−1 h−1	0.1234
C,N-TNT06 nanotubes [26]	alkaline hydrothermal technique then calcination	purged with CO2 gas (1000 ppm in He) and vacuum	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	100 mg catalyst on a 30 mm diam. glass disk; 15.4 cm3 reactor; CO2 flowrate @ 10 cm3 min−1; 500 μL sample gas extracted; ambient temperature and pressure; 1 h irradiation	Batch reactor	CH4 @ 9.75 µmol g−1 h−1	1.0532
Pt-XG/RBT nanoparticles [8]	facile vacuum treatment and photodeposition	1 h purging with moist CO2 gas @ 40 mL min−1	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	40 mg catalyst on a 4.9 cm2 porous disk; 26.57 cm3 reactor; CO2 flowrate @ 1 mL min−1; sample gas analyzed every 30 min; ambient temperature and pressure; 7 h irradiation	Continuous flow reactor	CH4 @ 37.0 µmol g−1 h−1 (AQYCH4 = 5.248) C2H6 @ 11.0 µmol g−1 h−1 (AQYC2H6 = 2.73)	7.978
Pt-BT-X nanoparticles [11]	facile low-temperature synthesis, annealing and photodeposition	1 h purging with moist CO2 gas @ 40 mL min−1	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	40 mg catalyst on a 4.9 cm2 porous disk; 26.57 cm3 reactor; CO2 flowrate @ 1 mL min−1; sample gas analyzed every 30 min; ambient temperature and pressure; 6 h irradiation	Continuous flow reactor	CH4 @ 80.35 µmol g−1 h−1	12.357
Cux%–Pty%–BT nanoparticles [27]	facile low-temperature synthesis, annealing and photodeposition	1 h purging with moist CO2 gas @ 40 cc min−1	100 W Xe solar simulator with an AM 1.5 filter; 100 mW cm−2	H2O	40 mg catalyst on a 2.5 cm diam. porous disk; 26.57 cm3 reactor; CO2 flowrate @ 1 mL min−1; sample gas analyzed every 30 min; ambient temperature and pressure; 6 h irradiation	Continuous flow reactor	CH4 @ 3.0 mmol g−1 (AQYCH4 = 79.14) C2H6 @ 0.15 mmol g−1 (AQYC2H6 = 6.92)	86

, studies by In et al. clearly vindicate the efficacy of transition from batch to continuous flow reactor along with the use of robust photocatalysts. Their well-designed flow reaction system, as shown in Figure 5 consists of a mass flow controller (50 standard cm³, 20 °C, 1 atm) that regulates the flow of gases, a water bubbler (diameter = 3 cm, length = 15 cm) to maintain the desired humidity, a vacuum pump (pumping speed = 100 L min⁻¹) to obtain high purity conditions by degassing the ultra-sealed photoreactor under a vacuum of 3.5 × 10⁻³ torr, and a gas chromatography unit for an automatic product intake and analysis (Shimadzu GC-2014, Restek Rt-Q Bond column, internal diameter = 0.53 mm, length = 30 m). Moreover, they used a ceramic porous disc (pore size = 1~1.6 µm) to support the catalyst and a quartz glass (diameter = 5 cm, thickness = 2 mm) to ensure efficient light transfer and the sealing of the reactor. Their research group also optimized the photoreactor dimensions to further improve the reaction process, (as shown in Figure 6a–c).

4. Reactor Geometry and Catalyst Support

Even the distribution of light over the surface of the photocatalyst is essential in reaping its full potential. In most of the reactor geometries where light impinges over the surface of the photocatalyst, from center or side, a shadow is casted on the opposite side. Consequently, a major portion of the photocatalyst is not activated. The contact of the light and photocatalyst might be enhanced by fabricating effective reactor geometries to achieve a uniform distribution of light and better photocatalyst dispersion [6,28,29]. Higher dispersion of the photocatalyst results in enhanced contact with reactants, better mass transfer and guarantees the maximum utilization of the light which all eventually translate to higher quantum yield [30]. To achieve this, a variety of approaches are reported in literature including the utilization of different reactor geometries and catalyst supports, as discussed in detail below.

4.1. Monolith Reactor

Tahir et al. studied the effect of photocatalyst dispersion by comparing performance of TiO₂ coated micro channel monolith and cell type support (dispersed as single layer over stainless steel cell). Their study revealed a significant enhancement in CO production by TiO₂ coated monolith as shown in Figure 7. This increment was mainly attributed to a broader exposed photocatalyst surface available for photocatalytic reaction [6]. In their study with gold-indium TiO₂ dispersed over monolith, higher yields were reported particularly formation of C₂ and C₃ products. This enhancement was linked to effective utilization of photons owing to larger illuminated area of monolith [31]. A similar result of enhancement in CO₂ reduction to CH₄ was reported for montmorillonite modified TiO₂ that was loaded over monolith as compared to the bare one [32].However, their performance is still restricted by limited light penetration despite the high flow rates, minimal pressure drop, and large surface area [33,34,35].

4.2. Fiber Optic Reactor

Fiber optic reactors are advantageous when compared to packed bed reactors owing to a better dispersion of photocatalyst and the spreading of light on large surface area [36]. Nguyen et al. compared the yield of the photocatalytic CO₂ reduction carried over the photocatalyst coated on optical fiber and glass plate. Their study demonstrated ~15.2 time enhancement in CH₄ and 11.6 times in C₂H₄ yield, for same amount of photocatalysts. This may be attributed to the synergistic effects of catalyst dispersion and effective light utilization [29]. Wang et al. also carried out CO₂ photoreduction while using fiber optic reactor and they attributed the enhancement in yield to the gradual and uniform distribution of light upon irradiation [37]. Although optical fibers accompany the features of catalyst support and effective light distribution, they are saddled with the constraints of limited utilization of reactor volume and the shorter transportation distance of light from the tip of incidence. They occupy 20–30% of reactor volume, but a limited catalyst coated area restricts the effective use of incident light [33,35,37,38].

4.3. Monolith Fiber Optic Combined Reactor

Ola et al. combined the mutual effects of higher surface area of monolith and effective light distribution of fiber optics to fabricate internally illuminated monolith reactor, and compared the CO₂ reduction performance of this system with slurry reactor. It was found that internal illumination, by optical fibers, of the monolith reactor enhanced quantum efficiency to 23 times owing to the higher surface area of monolith and even the distribution of light by optical fibers [39]. Liou et al. inserted carved polymethylmethacrylate (PMMA) made optical fibers into NiO/InTaO₄ coated monolith (honey comb structure) as shown in Figure 8. This reactor when applied for photocatalytic CO₂ reduction enhanced the yield of products (methanol and acetaldehyde). An enhancement in the yield can be attributed to increased surface area, higher photocatalyst loading and effective utilization of the light [35].

5. Light Irradiations

Light intensity and the type of irradiations are the most influential parameters for photocatalytic CO₂ reduction [40]. Thus, to spur the solar chemical/fuel yielding reactions an effective contact, particularly at the microscopic level, between light and catalyst is imperative. As reported in the literature, a majority of the photocatalysts work efficiently in the UV range thus they are capable of only harnessing a limited range of solar spectrum [41]. To resolve this bottleneck, the modification of semiconductor is carried out for harvesting a wide range of solar spectrum. Other than that, light can be concentrated and channeled to obtain higher photon flux of irradiations [42]. Higher photon flux is not only conducive in propelling apparent quantum efficiency (AQE) but it also shores up the selectivity of multi-electron photoreactions to yield ethane and other long chain solar products. These claims were further vindicated by a study by Nagpal et al. while using TiO₂ and CuInS (copper indium sulfide) nanocrystals as the photocatalyst. An enhanced AQY of 4.3% with a higher ethane selectivity of 70% was observed under concentrated sunlight owing to availability of a higher number of electrons [43]. Han et al. performed CO₂ photoreduction by concentrating light over TiO₂ and Pt/TiO₂. The authors performed CO₂ photoreduction experiments with different concentrating ratio (CR), which is defined as the ratio of concentrated light flux (amount of energy per unit time per unit area) on the photocatalyst surface to the ambient flux (under non-concentrated conditions). The light irradiation is concentrated and varied by changing the distance between the Fresnel lens (placed in between light source and photocatalyst) and photocatalyst surface, resulting in different light intensities with different light concentrated focal areas. Their study confirmed that the optimum concentration ratio (CR) significantly increased the AQY by 4.0 and 3.17 times for TiO₂ and Pt/TiO₂ respectively [42]. Similar results were reported by Li et al. where photocatalytic CH₄ yield was improved by 29.5 and 6.2 times under optimum CR for untreated and pre-treated samples, respectively [44]. In their other study, they reported that CH₄ yield, for photocatalytic CO₂ reduction, over g-C₃N₄ at CR10 (10 times concentrating the light) was enhanced by factors of ~11.9 and ~16.0 for untreated and pre-treated samples respectively [45]. Tan et al. also reported enhanced AQY up to optimum light intensity but beyond that, it decreases as the number of photons exceeds the requirement for photocatalytic reaction. They also reported that reaction yields were significantly higher for AM 1.5 filter as compared to the UV cut-off filter which could be attributed to higher photon energy for UV leading to the generation of more photogenerated charges [46]. Based on these studies it can be gauged that better contact of light with photocatalyst under optimally concentrated light may lead to an exceptional yield increase.

6. Temperature

Concentrating solar light also increases the temperature depending upon the CR, due to long wavelength irradiations [42,47,48]. Photocatalytic CO₂ reduction at an elevated temperatures is promising as it overcomes the thermal barriers that lead to slow reaction rate and marginal yields [48]. The efficacy of temperature rise for photoreaction can be underscored on the basis of enhanced effective collisions among photogenerated charges and reactants that directly relate to the reaction rate [30,46]. Furthermore, elevated temperature also facilitates the desorption of the products providing the way for the adsorption of the CO₂ on vacant sites leading to increased reaction rate [49,50]. Wang et al. found that production rate almost doubled when the temperature was increased from 25 °C to 75 °C [37]. Desorption of methanol from the photocatalyst surface increases due to increase in temperature, thus providing more active sites for CO₂ photoreduction. The AQY in turn also increased due to more efficient utilization of the incident light and enhanced CO₂ adsorption as result of methanol desorption. However, the reaction temperature should not increase too much as it might also desorb the CO₂ thus decelerating the photoreduction process. Similarly, Guan et al. attained a temperature up to 590 K for photocatalytic reduction of CO₂ and they found temperature rise to be an effective factor for enhancing the yield of solar products [47]. They suggested that such an enhancement was due to the increased collision frequency of photons and diffusion rate of CO₂ towards the surface active sites. Hence the thermal energy along with light irradiation can significantly improve the solar products yield by efficiently overcoming the kinetic barrier for CO₂ photoreduction reactions. In another study Alxneit et al. revealed the rate of CH₄ formation via photocatalytic CO₂ reduction increase when the temperature was increased from 25 to 200 °C. Upon further increasing the temperature, the reaction rate decreased due to desorption of the reactants. However, with the aforementioned temperature range, the reaction rate increases with increasing temperature which indicates the importance of thermal step tending to decrease the surface coverage from intermediate species and products. These results clearly suggest that temperature dependence is one of the pivotal factors that influences the photocatalytic reaction rate and thereof the product yield [48]. In another study, Tahir et al. also confirmed the efficacy of temperature rise in photocatalytic CO₂ reduction for enhanced CO production over In/TiO₂ as shown in the Figure 9 [6]. Such an observation was again explained on the basis of adsorption-desorption phenomenon. Upon increasing the reaction temperature, the mass transfer of CO₂ on the active sites is increased, which leads to increased CO₂ adsorption, resulting in increased reaction rates. They also observed the raising temperature also transformed the product selectivity towards longer chain hydrocarbons. Similarly Zhang et al. reported, when temperature increased from 323 K to 343 K, the yield of the photocatalytic reaction became twice [50]. They also suggested that enhancement in the product yield is due to desorption of products on the photocatalyst surface, providing more chances of collisions between the excited states and adsorbed reactants. Although raising temperature is an effective strategy to obtain higher yields, different studies suggest that there is always an optimum temperature range to get the best AQY [42].

7. Effect of H₂O/CO₂ Feed Ratio

An established mixture of water vapors and CO₂ gas (H₂O/CO₂) is considered to be a cost effective and invulnerable feed for photocatalytic reduction into chemicals/fuels. The feed ratio is another crucial factor with a profound impact over reaction rate and product yield. In addition, the affinity of the photocatalyst for H₂O/CO₂ may also lead to well tuning of the product selectivity. Yamashita et al. found highly selective methanol formation over the surface of hydrophilic Ti-Beta(OH) zeolites when compared to conventional Ti-Beta(F) zeolites [51]. Tahir et al. studied the effect of varying H₂O/CO₂ ratio by manipulating the CO₂ flow rates. In their study they reported that at lower concentration of CO₂, water could adsorb over the photocatalyst to react efficiently and give better yields. However, at higher CO₂ concentrations, H₂O has to compete for adsorption which may influence the yield. The same authors also studied the variation in H₂O/CO₂ ratio and its influence on the yield [6]. Zhang et al. also reported similar results mentioning enhancement in yield with increasing H₂O/CO₂ ratio [50]. In another study by Tahir et al., which reports enhancement in CH₄ yield with increasing H₂O/CO₂ ratio. This is attributed to adsorption of excess water molecules over the photocatalyst and resulting in incremented ability to reduce CO₂. Further increase in CO₂ decelerated the yield, shown in Figure 10, due to competition between water and CO₂ molecules on the photocatalyst active sites [30]. As the H₂O/CO₂ feed ratio is increased more water molecules will cover the photocatalyst surface due to hydrophilic nature of the material, which competes with the CO₂ molecules to get adsorb on the photocatalyst active sites during the photoreduction process. Therefore, an optimum feed ratio is required for moderate adsorption of both water and CO₂ molecules which in turn lead to the maximum CH₄ yield. The importance of optimum H₂O/CO₂ ratio was further vindicated by Wu et al. in which they reported optimum H₂O/CO₂ ratio is essential for enhanced yield of methanol [52]. The investigation of Tan et al. also stressed the need for adsorption of optimum number of molecules of CO₂ and H₂O over the catalyst surface to obtain enhanced yields. Moreover it is important to note that, enhanced adsorption of any reactant will certainly hinder the adsorption of others [46]. Thus, a tradeoff should exist between CO₂ and H₂O to mitigate their competitive adsorption over the surface of the photocatalyst. Thus, due to rivalry in adsorption, an optimum feed ratio is indispensable to get improved yield.

8. Other Factors

In addition to the above mentioned parameters, there exists several other parameters which also alters the photocatalytic yield. A reported work investigated the performance of photocatalyst in cell type and monolith photoreactor which revealed that broader exposed area of photocatalyst, as for monolith catalyst support, improves the yield (shown in Figure 11). This can be related to enhancement in the illumination area of photocatalyst i.e., exposure of same weight of the catalyst in a larger area [29,30,39,42,53]. In addition to the conditions of high purity, ultra-high vacuum also ensures the absence of external impurities [9]. Moreover, by improving the surface properties like CO₂ adsorption capability, photocatalytic yield can also be improved [44,45,54,55,56,57,58].

9. Benchmarking for Performance Evaluation

Based on the above discussion it can be well acknowledged that reactor geometries, reaction parameters, light intensity and wavelength are important parameters that play a pivotal role to control the product yields and selectivity. Thus photocatalysts that were tested under different conditions cannot be compared on equal grounds [42]. Like, Tan et al. reported that a maximum yield of 3.14 µmol g⁻¹ was obtained at a light intensity of 177.2 mW cm⁻² but in terms of AQY, the performance is better at 81 mW cm⁻², as shown in Figure 12, for a given amount of photocatalyst [46]. This clearly reveals that although the photocatalyst is producing maximum yield at 177.2 mW cm⁻² it still underperforms in capitalizing photogenerated charges. Thus, reporting performance of the photocatalytic reaction only on the basis of intrinsic capability of photocatalyst to yield the products is not a rationalized approach. On the contrary, evaluating the performance of the photocatalyst in terms of AQY, which incorporates reactor area, incident and harvested light, can be a more adequate approach [59]. For meaningful performance comparison, AQYs of the different photocatalysts are calculated and presented in Table 1,

Table 2. Summary of operating parameters and yields used by other research works for photocatalytic CO₂ reduction using Batch reactor.

Photocatalyst	Synthesis Method	Pre-Treatment of Reactor	Light Source	Reducing Agent	Reaction Parameters	Reactor	Main Product	AQY (%)
Bi2WO6/Au/CdS Z-scheme [61]	bath deposition method	vacuum-treated several times, and then filled with high purity CO2 gas	300 W Xe lamp (λ > 400 nm)	0.4 mL of DI water	100 mg catalyst; 230 mL reactor; 1 mL sample gas; ambient pressure; 8 h irradiation	Batch reactor	CH4 @ ~0.75 µmol g−1 h−1	0.012 b
rGO-CuO hybrid structure [62]	covalent grafting	purged with nitrogen gas for 15 min then purged with CO2 for 30 min under continuous stirring	20 W white cold LED flood light (200 < λ < 700 nm); 85 W m−2	mixture of DMF (45 mL) and H2O (5 mL)	100 mg catalyst; 100 mL reactor; 20 μL total sample gas analyzed; 24 h irradiation	Batch reactor	CH3OH @ 1282 µmol g−1	0.013 b,c
Cd1−xZnxS solid solution [63]	two-step self-templated synthesis	purged with argon for 1 h, then 2 mL of deionized water was injected, and purged with ultra-pure CO2 for 30 min	100 W LED plate with collimating lens; visible light (λ = 450 nm); 285 mW cm−2	2 mL of DI water	45 mg catalyst; 130 cm3 reactor; 250 μL sample gas extracted every 1 h; 1 atm; 25 °C; 5 h irradiation	Batch reactor	CO @ 2.90 µmol g−1 h−1 (AQYCO = 0.016) CH4 @ 0.22 µmol g−1 h−1 (AQYCH4 = 0.005)	0.02 a
Co-ZIF-9/TiO2 nanostructure [64]	in situ growth method	purged with high-purity CO2 gas	300 W Xe lamp (200 < λ < 900 nm); 494 mW cm−2	3 mL DI water	50 mg catalyst; 390 mL reactor; 0.5 mL sample gas extracted; 70 kPa; 10 h irradiation	Batch reactor with gas circulation system	CO @ 17.58 µmol g−1 h−1	0.053 a
Pt/TiO2 mesoporous structure [65]	soft-template method	purged with high purity CO2 bubbled through DI water for more than 1 h	350 W Xe-lamp with 420 nm cutoff filter; UV light @ 34.8 mW cm−2	H2O	100 mg catalyst; 159 mL tubular reactor (length: 28 cm, Ø: 3 cm); 60 ± 2 °C; 2 h irradiation	Batch reactor	CH4 @ 5.7 µmol g−1	0.064 a
In2O3–C3N4 hybrid structure [66]	simple solvothermal method	purged with high-purity CO2 gas	500 W Xenon lamp; 1200 mW cm−2	0.1 mL ultrapure H2O	20 mg catalyst; 90 mL reactor; 4 h irradiation	Batch reactor	CH4 @ 159.2 ppm	0.082 a
Pd/(10 wt.% LDH/C3N4) hybrid structure [67]	electrostatic interaction	introduction of 200 torr CO2 into the system	500 W Hg (Xe) lamp without filter	100 mL H2O	200 mg catalyst; 200 μL sample gas extracted; AQY @ λ = 420 nm; 200 torr; 72 h irradiation	Batch reactor	CH4 @ 6.5 µmol	0.093 b
In/TiO2-monolith [6]	sol–gel single step method	continuous passing of CO2, He and H2O mixture through the reactor for about 1 h	200 W Hg lamp for UV irradiations (λ < 252 nm); 150 mW cm−2	H2O	50 mg catalyst; 150 cm3 reactor; 1000 μL sample gas extracted; PCO2 = 0.20 bar; PH2O = 0.074 bar; 10 h irradiation	Batch reactor	CO @ 962 µmol g−1 h−1	0.10 b
TiO2−x/CoOxhybrid structure [68]	(own method)	blown with CO2 for 20 min	150 W UV lamp; 20 mW cm−2	2 mL of DI water	50 mg catalyst; 100 mL reactor; 1.01 bar; room temperature; 4 h irradiation	Batch reactor	CO @ 1.247 µmol g−1 h−1 (AQYCO = 0.0817) CH4 @ 0.0903 µmol g−1 h−1 (AQYCH4 = 0.0237)	0.105 a
Ag-Au/TiO2 nanowires [69]	facile hydrothermal synthesis	compressed CO2 and H2 were continuously passed through the reactor	35 W HID Xe lamp; 20 mW cm−2	H2	10 mg catalyst; 108 cm3 reactor; 0.20 bar; 4 h irradiation	Batch reactor	CO @ 1813 µmol g−1 h−1	0.1108 b
LaPO4–Pt nanorods [70]	hydrothermal method and photo deposition	reactor was evacuated and filled with CO2 for 1 h with stirring	125 W high-pressure Hg lamp (λ < 365 nm)	70 mL H2O	50 mg catalyst; 200 mL reactor; 1 atm; 20 °C; 4 h irradiation	Batch reactor	CH4 @ 0.62 µmol g−1	0.15 b
Zn2GeO4 micro/mesoporous [71]	simple ion exchange	vacuum-pumped and washed with high purity CO2 gas	300 W Xe arc lamp (λ = 251 ± 16 nm)	0.5 mL DI water	200 mg catalyst; 360 mL reactor; 0.5 mL sample gas extracted; ambient pressure; 12 h irradiation	Batch reactor	CH4 @ 9.5 ppm g−1 h−1	0.20 b
ZnIn2S4 one-unit-cell atomic layers [72]	(own method)	vacuum-treated three times, then pumped with high-purity CO2	PLS-SXE300/ 300 UV Xe lamp; 100 mW cm−2	2 mL DI water	100 mg catalyst; atmospheric pressure; 298 ± 0.2 K; 1 h irradiation	Batch reactor	CO @ 33.2 µmol g−1 h−1	0.23 b
Cu2O/x% RGO composites [73]	microwave-assisted hydrothermal reaction	CO2 purged	150 W Xe lamp; 540 µW cm−2	3 mL DI water	500 mg catalyst; 120 mL reactor; sample gas extracted every 30 min; 20 h irradiation	Batch reactor	CO @ 50 ppm g−1 h−1	0.34 b
Pt/MgAl-LDO/TiO2 hybrid structure [74]	in-situ deposition, calcination and photo deposition	degassed for 30 min, and then bubbled with CO2 till the pressure reaches 1 atm	300 W Xe lamp; 1.1 mW cm2	H2O	20 mg catalyst; AQY @ λ = 365 nm; 1 atm; 20 °C; 8 h irradiation	Batch reactor	CH4 @ 0.11 µmol	0.35 b,c
LDH/RGO/CN hybrid structure [75]	hydrothermal synthesis and in situ loading	vacuum-treated several times, and then flowed with high purity CO2 gas	300 W Xe arc lamp; 1.8 mW cm−2	4 mL DI water	50 mg catalyst; 420 mL reactor; 1 mL sample gas extracted; AQY @ λ = 385 nm; ambient pressure; 5 h irradiation	Batch reactor	CO @ 10.11 µmol g−1 h−1	0.45 b
Cu2O/WO3 nanosheets [76]	modified method	vacuum treated, and then purged several times with high purity CO2 gas	300 W Xenon arc lamp with a UV cutoff filter (λ > 400 nm)	H2O	85 mg catalyst; 18 h irradiation	Batch reactor	CO @ 0.56 µmol g−1 h−1	0.503 b
TiO2 microsphere [77]	sol−gel approach	introduction of pressurized CO2 @ (50 psi)	40 W Hg UV lamp (λ = 254 nm); 20 mW cm−2	100 μL H2O	200 mg catalyst; 39 mm diameter and 9 mm depth reactor; 10 mL sample gas extracted; 50 psi; 24 h irradiation	Batch reactor	CO @ 0.56 µmol g−1 h−1 (AQYCO = 0.204) CH4 @ 0.94 µmol g−1 h−1 (AQYCH4 = 0.34)	0.54 b,c
RGO-CdS nanorod composites [78]	microwave hydrothermal route	degassed with nitrogen for 30 min	300 W Xe arc lamp with a UV-cutoff filter (λ ≥ 420 nm); 150 mW cm−2	10 mL distilled water	100 mg catalyst; 200 mL reactor; 1 mL sample gas extracted every 1 h; atmospheric pressure and ambient temperature; 3 h irradiation	Batch reactor	CH4 @ 2.51 µmol g−1 h−1	0.80 b
HCP-TiO2-FG composite [79]	in situ growth	-	300 W Xe lamp (λ ≥ 420 nm); 433 mW cm−2	H2O	20 mg catalyst; standard atmospheric pressure; 5 h irradiation	Batch reactor	CH4 @ 27.62 µmol g−1 h−1 (AQYCH4 = 1.14) CO @ 21.63 µmol g−1 h−1 (AQYCO = 0.2227)	1.36 a
Co/Palheterostructure [80]	in situ electrostatic adsorption deposition process	filled with high purity CO2 gas	300 W Xe lamp	5 mL acetonitrile/H2O (4:1)	9 mg photosensitizer + 1 mg co-catalyst + 1 mL TEOA; 80 mL reactor; AQY @ λ = 420 nm; 1 atm; 25 °C; 6 h irradiation	Batch reactor	CO @ ~86 µmol	1.38 b
CuO−TiO2 hollow microspheres [81]	one-pot template-free synthesis	introduction of pressurized CO2 @ (50 psi)	40 W Hg UV lamp(λ = 254 nm); 20 mW cm−2	200 μL H2O	10 mg catalyst; reactor diameter of 39 mm and a depth of 9 mm; 50 psi; 24 h irradiation	Batch reactor	CO @ 14.5 µmol g−1 h−1 (AQYCO = 1.285) CH4 @ 2.1 µmol g−1 h−1 (AQYCH4 = 0.747)	2.03 b
Pt-TiO2 spheres [77]	microwave-assisted solvothermal method	introduction of pressurized CO2 @ (50 psi)	40 W Hg UV lamp (λ = 254 nm); 20 mW cm−2	100 μL H2O	200 mg catalyst; 39 mm diameter and 9 mm depth reactor; 10 mL sample gas extracted; 50 psi; 24 h irradiation	Batch reactor	CO @ 18.9 µmol g−1 h−1 (AQYCO = 1.632) CH4 @ 3.6 µmol g−1 h−1 (AQYCH4 = 1.315)	2.95 b,c
PdxCu1-TiO2 hybrid structures [82]	in situ growth	filled with 0.2 MPa CO2 for 60 min	300 W Xe lamp (λ < 400 nm); 2 mW cm−2	H2O	5 mg of TiO2 + 0.01 mmol of metal atoms for catalyst; 100 mL reactor; 0.2 MPa; 2 h irradiation	Batch reactor	CH4 @ 19.6 µmol g−1 h−1	12.53 a
In/TiO2 nanoparticles [60]	sol–gel single step method	purged with CO2 and He for an hour	500 W mercury flash lamp (λ = 365 nm); 40 mW cm−2	H2O	0.25 mg catalyst; 106 cm3 reactor; 1000 μL sample gas extracted; 0.20 bars, 373 K; 8 h irradiation	Batch reactor	CH4 @ 244 µmol g−1 h−1 (AQYCH4 = 42.39) CO @ 81 µmol g−1 h−1 (AQYCO = 3.52)	45.91 a
ZnV2O4 microspheres [83]	one-step hydrothermal process	purged with CO2 gas carrying H2O for 30 min	35 W HID Xe lamp; 20 mW cm−2	H2O	100 mg catalyst; CO2 flowrate @ 20 mL min−1; 108 cm3 reactor; 0.20 bar; 100 °C; 4 h irradiation	Batch reactor	CO @ 485 µmol g−1 h−1 (AQYCO = 31.92) CH3OH @ 100 µmol g−1 h−1 (AQYCH3OH = 19.75)	51.67 a
NiO/InTaO4 monolith coated structure [35]	impregnation method and sol-gel method	purged overnight using a flow of He then switched to pure CO2 with saturated water vapor for 1 h	300 W Xe arc lamp with AM 1.5 filter; 100 mW cm−2	H2O	88.7 mg catalyst; 216 cm3 reactor; 1 bar; 70 °C; 2 h irradiation	-	CH3OH @ 0.16 µmol g−1 h−1 (AQYCH3OH = 0.012) CH3CHO @ 0.3 µmol g−1 h−1 (AQYCH3CHO = 0.058)	0.07 b
MAT nanofibers [84]	(own method)	blown with nitrogen for 30 min	300 W simulated solar Xe arc lamp	H2O	200 mL reactor; 1 mL sample gas extracted every 1 h; atmospheric pressure and ambient temperature; 3 h irradiation	-	CH4 @ 0.86 µmol g−1 h−1	0.091 b
BiOI few-layered nanosheets [85]	(own method)	thoroughly vacuum-treated	300 W high pressure Xe lamp	5 mL H2SO4 & 1.712 g NaHCO3	150 mg catalyst; 500 mL reactor; 0.15 mL sample gas extracted; 20 °C AQY @ λ = 420 nm; 4 h irradiation	-	CO @ 0.615 µmol h−1 CH4 @ 0.063 µmol h−1	0.140 b
CdS–WO3 heterostructure [86]	simple precipitation method	blown with nitrogen for 30 min	300 W Xe arc lamp with a UV-cutoff filter (λ ≥ 420 nm); 6.0 mW cm−2	H2O	100 mg catalyst +10 mL of distilled water to form films; 200 mL reactor; 1 mL sample gas extracted every 1 h; AQY @ λ = 420 nm; atmospheric pressure and ambient temperature	-	CH4 @ 1.02 µmol g−1 h−1	0.40 b
CeOx-S/ZnIn2S4 hybrid structure [87]	one-pot hydrothermal method	introduction of high purity CO2 gas into the reactor for 3 min	9.0 W (455 nm LEDs)	0.5 mL H2O	10 mg catalyst; 6.98 mL reactor; 1 bar; below 42 °C; 10 h irradiation	-	CO @ 0.18 mmol g−1 h−1	1.34 b
Pt/TiO2 [42]	vacuum impregnation	reactor was cleaned with nitrogen for half an hour then it was replaced and saturated with CO2 gas for at least 30 min	300 W UV light; 10 mW cm−2	2 mL H2O	100 mL reactor; sample gas analyzed every 1 h; 0.1 MPa; 7 h irradiation	-	CH4 @ 20.55 µmol g−1	10.03 b

a = AQY computed; b = AQY was computed by authors of the reference paper; c = AQY was computed by authors of the reference paper multiplying it with mass of photocatalyst.

and

Table 3. Summary of operating parameters used by other research works for photocatalytic CO₂ reduction using Continuous flow reactor.

Photocatalyst	Synthesis Method	Pre-Treatment of Reactor	Light Source	Reducing Agent	Reaction Parameters	Reactor	Main Product	AQY (%)
TiO2/NRGO-300 nanocomposites [88]	one-step urea-assisted hydrothermal method	purged with CO2 at 16 mL min−1 for 40 min	400 W Xe lamp (250 < λ < 400 nm); 11.5 mW cm−2	H2O	10 mg catalyst; CO2 flowrate @ 3 mL min−1; sample gas extracted every 1 h; 8 h irradiation	Continuous flow reactor	CO @ 356.5 µmol g−1	0.0072 b,c
5GO–OTiO2 (UV light) hybrid heterostructure [46]	facile wet chemical impregnation technique	purged with wet CO2 at 30 mL min−1 for 30 min	500 W Xe arc lamp with a UV filter (λ > 400 nm); 81.0 mW cm−2	H2O	CO2 flowrate @ 5 mL min−1; Quartz column reactor (ID = 9 mm, OD = 11 mm, length = 250 mm); sample gas extracted every 0.5 h; 1 bar; 25 ± 5 °C; 8 h irradiation	Continuous flow reactor	CH4 @ 2.7 µmol g−1	0.0103 b
TiO2 nanofibers [89]	sol-gel method and electrospinning technique	firstly, degassed under vacuum and then purged with Ar for 1 h, then fed with CO2/H2O mixture in dark for 1 h, then reactor was pressurized and kept at a reaction flow rate of 2 mL min−1 for another 1 h.	four 6 W UV lamps (λmax = 365 nm)	H2O	100 mg catalyst; 190 mL reactor; 7.25 CO2:H2O molar ratio; sample gas analyzed every 22 min; 2 bars; 50 °C; 20 h irradiation	Continuous flow reactor	CO @ 203.91 μmol gcat−1	0.04 b
Cu/GO-2 hybrid structure [90]	one-pot microwave process	purged with nitrogen gas for 1 h then followed by pure CO2 for another 1 h	300 W halogen lamp; 100 mW cm−2	H2O	100 mg catalyst; 300 mL reactor; CO2 flowrate @ 4 µL/min; 25.0 ± 0.5 °C; 2 h irradiation	Continuous flow reactor	CH3OH @ 2.94µmol g−1 h−1 (AQYCH3OH = 0.0296) CH3CHO @ 3.88 µmol g−1 h−1 (AQYCH3CHO = 0.065)	0.095 a
G/TiO2-001/101 nanocomposites [91]	one-pot solvothermal method	purged with the CO2 + H2O mixture at 200 mL min−1 for 1 h and then reduced to 5 mL min−1 for 30 min	300W Xe arc lamp (300 < λ < 400 nm); 20.5 mW cm−2	5 mL DI water	10 mg catalyst; 85 mL reactor; sample gas analyzed every 30 min; atmospheric pressure; 120 °C; 4 h irradiation	Continuous flow reactor	CO @ 70.8 µmol g−1 h−1	0.0864 b,c
BWO-OV/BiOI binanosheets [92]	simple self-assembly approach	purged with the CO2/H2O gas mixture at 50 mL min−1 for 30 min	500 W Xenon arc lamp with UV cut-off filter (to remove λ < 400 nm)	H2O	CO2 flowrate @ 5 mL min−1; sample gas analyzed every 1 h; atmospheric pressure and ambient temperature; 8 h irradiation	Continuous flow reactor	CO @ 320.19 µmol g−1 CH4 @ 18.32 µmol g−1	0.432 b
Pt2+–Pt0/TiO2 nanoparticles [93]	sol–gel method	purged with CO2 + H2O mixture at 200 mL min−1 for 1 h and then at 3 mL min−1 for another 30 min.	300 W Xe arc lamp UV light irradiation (320 < λ < 420 nm); 32.5 mW cm−2	H2O	200 mg catalyst; 85 mL reactor; sample gas extracted every 40 min; 50 °C; 7 h irradiation	Continuous flow reactor	CH4 @ 264.5 µmol g−1 (AQYCH4 = 1.35) CO (AQYCO = 0.07)	1.42 b
(Pt/TiO2) @rGO core-shell-structured [94]	hydrothermal method	vacuum-treated, then purged with CO2 gas @ 50 cm3 min−1 for 30 min	300 W Xe lamp (320 < λ < 780 nm); 80 mW cm−2	2.0 mL H2O	100 mg catalyst; sample gas extracted every 1 h; 0.1 MPa; 4 °C; 8 h irradiation	Continuous flow reactor	CH4 @ 41.3 µmol g−1 h−1	1.93 b,c
NiO/Ni-GR nanoparticles [95]	pyrolysis and incipient wetness impregnation	photoreactor was heated at different temperatures	300 W Xe lamp; 2236 W m−2	H2	40 mg catalyst; 51 mL reactor; 1.3 bar; 200 °C; 2 h irradiation	Continuous flow reactor	CH4 @ 642 µmol gNi−1 h−1	1.98 b
Pt-TiO2 nanostructured films [96]	aerosol chemical vapor deposition	purged with CO2 and water vapor at 100 mL min−1 for 1 h, and then reduced and maintained at 3 mL min−1	400 W Xe lamp (250 < λ < 388 nm); 19.6 mW cm−2	H2O	0.7 mg catalyst; atmospheric pressure and room temperature; 5 h irradiation	Continuous flow reactor	CH4 @ 1361 µmol g−1 h−1 (AQYCH4 = 2.33) CO @ 179.34 µmol g−1 h−1 (AQYCO = 0.077)	2.41 b,c

a = AQY computed; b = AQY was computed by authors of the reference paper; c = AQY was computed by authors of the reference paper multiplying it with mass of photocatalyst.

, using Equations (4)–(8) [11,59].

$$\mathrm{AQY}(\%)=\frac{\mathrm{number}\mathrm{of}\mathrm{reacted}\mathrm{electrons}}{\mathrm{effective}\mathrm{number}\mathrm{of}\mathrm{incident}\mathrm{photons}}\times 100\%,$$

(4)

$$\mathrm{number}\mathrm{of}\mathrm{reacted}\mathrm{electrons}=\left[\begin{array}{c}\mathrm{mole}\mathrm{of}\mathrm{product}\\ \mathrm{produced}\mathrm{in}\mathrm{time},\mathrm{t}\end{array}\right]\times \left[\begin{array}{c}\mathrm{number}\mathrm{of}\mathrm{electrons}\\ \mathrm{required}\mathrm{to}\mathrm{produce}\\ 1\mathrm{mol}\mathrm{of}\mathrm{product}\end{array}\right]{\times \mathrm{N}}_{\mathrm{A}},$$

(5)

$$\mathrm{effective}\mathrm{number}\mathrm{of}\mathrm{incident}\mathrm{photons}=\frac{\mathrm{light}\mathrm{absorbed}\mathrm{by}\mathrm{the}\mathrm{photocatalyst}}{\mathrm{average}\mathrm{photon}\mathrm{energy}}\times \mathrm{t},$$

(6)

$$\mathrm{light}\mathrm{absorbed}\mathrm{by}\mathrm{the}\mathrm{photocatalyst}=\mathrm{H}\times \mathrm{A},$$

(7)

$$\mathrm{average}\mathrm{photon}\mathrm{energy}=\frac{\mathrm{hc}}{\mathsf{\lambda }},$$

(8)

where H is the apparent light input (W m⁻²), A is the geometric irradiation area (m²), h is Planck’s constant (6.626 × 10⁻³⁴ J·s), c is speed of light ($3\times {10}^8$ m s⁻¹), λ is the average wavelength of light source (nm) and N_A is Avogadro’s number ($6.022\times {10}^{23}$ atoms mol⁻¹).

Values for the calculations of AQYs included in this review are given in detail in Supplementary Tables S1 and S2. Moreover, prior to photocatalytic testing under CO₂, repeated photocatalytic reduction tests with H₂O and inert gas can rule out the possible role of organic contamination. Further confirmation of the carbon source of photocatalytic products can be ascertained by an isotopic labeling test while using ¹³CO₂. Additionally, optimization of the reaction parameters, condition of high purity and effective photocatalyst and light contact can provide an even better judgment of the performance [1,4,9,60].

10. Conclusions

Photocatalytic CO₂ reduction is a fascinating approach owing to its two-fold benefits of CO₂ abatement and its subsequent conversion to renewable fuels/chemicals. However, optimizing this process has been a challenge for researchers as a variety of process parameters and reaction conditions indistinctly influence the product yield. One parameter is the reaction temperature which by increasing overcomes the kinetic barrier; but on the contrary, excessive temperature might also lead to decomposition. The reactants feed ratio also affects the product yield: at an optimum value, it synergizes the product yield and selectivity but beyond which, the non-stoichiometric ratios of H₂O/CO₂ are competing to get adsorbed on the photocatalyst, resulting in decreased productivity. In addition, optimal light intensity is imperative to produce stoichiometric amount of photogenerated charges for efficient transformation of adsorbed H₂O/CO₂ to solar fuel/chemicals; surplus of these charges, produced at higher light intensities, will recombine and eventually reduce the AQY. Moreover, productivity enhancement also depends on the choice of reactor type: continuous flow reactors have shown potential in overcoming the low yield, re-adsorption, and decomposition of photocatalytic products accustomed to batch reactors. Additionally, the reactor geometry with better light and photocatalyst contact notably affects the production yield. Knowing how these parameters affect the product yield, performance of the photocatalysts tested under different conditions are inappropriate to compare in frequently used customary units i.e., μmol g⁻¹ or ppm cm⁻². Hence, with the consideration of various process parameters and reaction conditions i.e., reaction temperature, feed ratio, irradiation source, reactor type and geometry, reporting yield of the photocatalysts in terms of AQYs rather than production rate is a more appropriate and pragmatic approach. Additionally, the removal of organic contamination, isotopic labelling, and photocatalytic CO₂ reduction under inert and ultra-sealed condition can provide more realistic assessment of performance.

The overall aim of the present review article is to highlight the influence of key process and reaction parameters on CO₂ photoreduction process to be used for benchmarking. While considering all the aforementioned parameters, a continuous and ultra-sealed gas phase reactor purged under high vacuum with an optimum blend of temperature, feed ratio, and offering larger contact between light and photocatalyst can be an effective approach for achieving maximum yield with the effective utilization of incident photons. Moreover, in order to compare the photocatalytic system performance, a unified parameter should be recognized; reporting yield in terms of AQY standardizes the photocatalytic performance analysis by normalizing the effect of the most influential parameters. Our aforementioned photoreaction setup and studies, well matching with the concluded benchmarking criteria, can be followed for future studies in the field of gas phase CO₂ photoreduction.