Photocatalytic CO2 conversion is vital technology to realize global carbon neutrality and generate future energy supplies. This review proposes fundamentals, challenges, strategies, and prospects for photocatalytic CO2 conversion research.

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electrochemical, [7][8][9] thermo-, [10,11] and biocatalysis. [12,13] Converting CO 2 using readily available sunlight and water is one of the most promising strategies due to the following advantages: i) it utilizes solar light, a renewable energy, to drive the reduction of CO 2 , ii) it can convert CO 2 into valuable carbonaceous products like CO, CH 4 , C 2 H 6 , and CH 3 OH, iii) it operates under atmospheric conditions, iv) it is a controllable process, and v) it is scalable for industrial application. [14,15] Thus, converting undesirable CO 2 into value-added chemicals using artificial photosynthesis is a plausible way to address the current environmental and energy issues associated with the burning of fossil fuels. [16] Given the significance of photocatalytic CO 2 conversion, researchers have extensively researched various catalysts, such as metal oxides, alloys, chalcogenides, carbon-based structures, organic polymers, inorganic complexes, and MXenes. [17][18][19][20][21][22][23][24][25][26] However, low productivity, poor product selectivity, poor long-term stability, sluggish mechanisms, and economic unviability limit the application of these conventional catalysts. Furthermore, the solar to fuel conversion efficiency of most reported catalysts is less than 1%, which is insufficient for large-scale applications. Accordingly, the need for high-performance and economically viable catalysts has prompted research organizations to design and synthesize a range of materials that can advance CO 2 reduction technology and its commercial viability.It has been recognized that the downsizing of nanoparticles (NPs) of a catalyst exposes more active sites, leading to the optimization of its electronic properties. [27] Recently, catalysts based on size-and shape-controlled, atomically dispersed metals have garnered the attention of the scientific community for various energy and environmental applications. [28,29] Single-atom (SA) catalysts (SACs) are obtained once the size and aggregation of metal particles are reduced to isolated metal atoms, which form low coordination states and enable the use of almost 100% of active metal sites; the electronic properties of metal SAs differ from those of corresponding bulk materials. [30,31] In nature, the magnesium porphyrin complex in chlorophyll has an SA-like structure necessary for photosynthesis; the iron porphyrin complex in cytochrome has a similar SA-like structure which plays a vital role in the molecular transfer of CO 2 . [32][33][34] In recent years, single-atom catalysts (SACs) have attracted the interest of researchers owing to their suitability for various catalytic applications. For instance, their optoelectronic features, site-specific activity, and costeffectiveness make SACs ideal for photocatalytic CO 2 reduction. The activity, product selectivity, and photostability of SACs depend on various factors such as the nature of the metal/support material, the interaction between the metal atoms and support, light-harvesting ability, charge separation behavior, ...

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.