2.1 Roles on AACs for Raising Photocatalytic Ethylene/Ethane Evolution

The photocatalytic conversion of CO₂ to value-added ethylene/ethane products is mainly hindered by several key factors. First, a significant energy barrier (−1.9 V vs RHE) is required to break the thermodynamically stable CO₂ molecules and produce the ^•CO₂⁻ intermediate, which is the initial product in the PRCO₂ process.^[53] This highly negative potential indicates the substantial energy input required for achieving effective CO₂ activation (Equation 1). Once the ^•CO₂⁻ is formed, a further reduction step to form the CO, which is a common intermediate in the subsequent reductions to hydrocarbons (e.g., ethylene and ethane), also requires a considerable amount of energy (−0.10 V vs RHE; Equation (2)).^{[32, 39, 54]} Additionally, the wide variety of possible gaseous/liquid products that can possibly emerge from the PRCO₂ and the complexity of generating C2 products, especially ethylene/ethane, through slow kinetics of C–C coupling and hydrogenation processes pose great challenges to achieving high yield and high selectivity for these products (Figure 1).^{[16, 24, 29]}

And remarkably raise the efficiency/selectivity/robusteness of photocatalytic CO₂ conversion to renewable chemical/fuel. Contrary to the traditional photocatalysts that often consist of nanoparticles/clusters, AACs anchored photocatalysts exhibit the presence of AACs highly scattered on the surface of the support catalyst. This distinctive structure has equipped them with outstanding catalytic performance. One of the main merits of AACs is the maximized atomic utilization as the high exposure of AACs over the support surface allows each atom to be exposed and act as catalytic centers to interact with CO₂ molecules. This is in remarkable contrast to the conventional nanoparticle/cluster scattered photocatalysts, which often get buried in the bulk of photocatalysts. This obviously raised atomic utilization not only improves the efficiency of overall photocatalytic CO₂ reduction but also reduces the amounts of metals required to prepare photocatalysts, resulting in reduced economic costs.^[56-59]

Moreover, the characteristics of distinctive electronic structures of AACs anchored photocatalysts can regulate the electronic densities distributed around the active centers to affect the accumulation of photoexcited charge carriers around them, arousing raised CO₂ adsorption/activation, reducing the energy barriers for the thermodynamically disadvantageous reactions along with forming favorable intermediates for the subsequent desired reactions over these centers.^{[60, 61]} For instance, Cao et al. reported that anchoring Au single atoms (SAs) on Cd_1−xS photocatalysts aroused a remarkable enhancement for CO₂ photoreduction compared to anchoring Au nanoclusters over the same catalyst. This is because adding Au SAs manipulates the electron density of the surface through the strong hybridization of Au 5 d and S 2p orbits, which accelerates the photogenerated electron transfer to the surface of the photocatalyst, resulting in the availability of more electrons to the PRCO₂, reduced energy barriers and raised overall efficiency. Also, introducing the Au SAs to the Cd_1−xS system aroused the formation of the H₃CO* intermediate, which is advantageous for the successive reaction. However, the Au nanoclusters prompt the formation of HCO* intermediate for the same reaction.^[62] Therefore, incorporating SAs is regarded as an innovative strategy to raise the formation rates of desired intermediates required to achieve higher production yields of C₂H₄ and C₂H₆.^{[30, 59, 63]}

Furthermore, the regulable coordination environments of AACs anchored photocatalysts are another merit, as the AACs usually coordinate with the surrounding atoms of various supports (e.g., metal oxides, 2D materials, and MOF) in a well-defined order. These coordination environments can stabilize the AACs, prevent their agglomeration, and create highly active catalytic centers for controlling the hydrogenation of the PRCO₂ process.^{[64, 65]} For instance, Yang et al. have reported that the hydrogenation pathway of CO₂ reduction intermediates over Cu–N₄ active centers is completely different from the hydrogenation route over Cu–N₃ active centers. They revealed that the Cu–N₄ coordination structure prefers to catalyze the photoreduction and hydrogenation of CO₂ to CH₃OH via the formate pathway and achieving 95.5% selectivity, whilst the Cu–N₃ coordination structure induces the CO₂ reduction to CO with 94.3% CO selectivity. These results give prominence to the importance of tuning the coordination environment for raising the desired products yields/selectivity.^{[66, 67]} Besides, the unsaturated coordination of AACs anchored photocatalysts facilitates the adsorption/activation of CO₂ and promotes the rates of sluggish C–C coupling process and the hydrogenation competitive steps (Figure 2a–b). Consequently, through controlling these coordination environments, we can achieve the rational design of AACs anchored photocatalysts, which can enhance the interaction with CO₂ reduction intermediates and improve the selectivity toward ethylene/ethane.^{[66, 68]} Additionally, for the C2 products to be formed, the CO₂ molecules ought to be adsorbed and activated over a single active site. The resulting C1 intermediates then need to migrate to other active sites with other C1 intermediates for the C–C coupling process to occur, which significantly reduces the selectivity of the produced C2 products.^{[69, 70]} However, the highly exposed adjacent AACs dispersed over the photocatalyst support can reduce the migration distance for the C1 intermediates, owing to the availability of plenty of adjacent active sites in the AACs anchored photocatalysts, which promotes the C–C coupling and increases selectivity for ethylene/ethane.^[68]

Finally, AACs can act as efficient light-absorbing centers owing to their unique properties. These properties extend the light-absorption abilities of photocatalysts compared to traditional nanoparticles/clusters, generating more photoexcited electrons/holes and raising the PRCO₂ efficiency. Also, the built-in electric field in AACs can facilitate charge carrier separation, promote the transfer of electron/hole to the reactant and reduce the charge recombination rates, which are major challenges in photocatalysis. This efficient separation can provide more electrons for CO₂ reduction and enhance the overall reaction rate.^{[61, 67]}

Overall, AACs anchored photocatalysts have been extensively investigated for the photocatalytic conversion of CO₂ to value-added chemicals, providing an atomic-scale understanding of the structure–performance relationship of photocatalysts and how they can be engineered to achieve higher efficiencies. But the complexity of reaction pathways, sluggish kinetics, high energy barriers, and intermediate competition are restricting the massive utilization of AACs in PRCO₂ to form renewable C2 hydrocarbons, that is, ethylene and ethane. Thus, further investigations are still required to explore these obstructions and fill the existing gap toward achieving higher selectivity/efficiency/robustness for photocatalytic ethylene/ethane evolution.

2.2 Routes on Anchoring AACs onto Photocatalysts

Effective routes for anchoring AACs are of prime importance for exploring highly efficient photocatalysts to transform CO₂ molecules into renewable hydrocarbons. Achieving precise control over the engineering process can raise the feasibility, reproducibility, stability, and overall photocatalytic performance. This accurate control is also propitious for: i) tailoring catalytic centers at the atomic level to enable selective interactions with reactants; ii) achieving maximum atomic utilization by ensuring that all the atomic active centers are available for reaction; and iii) achieving higher catalytic activities and selectivity for the desired products. A variety of techniques have been developed to ensure the optimum scattering of AACs over photocatalysts. The most common routes reported for photocatalytic CO₂ conversion into ethylene/ethane are: i) wet chemistry preparation, for example, wet impregnation and up-bottom ion-cutting (solvothermal); ii) high-temperature incorporation, for example, hydrothermal, calcination, and thermal polymerization; iii) atomic layer deposition; and iv) photoreduction loading.

First, wet impregnation and up-bottom ion-cutting are regarded as the facile wet chemistry preparation routes for the incorporation of AACs into materials. These routes are easily scalable for industrial application and can be adapted to a wide range of elements. Typically, the process involves the interaction between the photocatalysts and salt precursors, followed by calcination at ambient conditions to assure strong bonding with the support surface if required. For example, one research^[68] reports implanting Au AACs on the red phosphorous (RP) surface using the wet impregnation technique. This involves mixing RP with certain amounts of HAuCl₄ as a precursor for Au AACs in methanol, followed by sonication and stirring. Subsequently, the photocatalyst was freeze-dried and calcined at 125 °C for 2 h in the H₂ atmosphere to acquire the Au/RP photocatalyst. Another study reports anchoring Mo AACs over TpBpy COF through mixing Mo(CO)₆ with TpBpy COF in a toluene solution followed by heating at 110 °C for 6 h under an N₂ atmosphere.^[71] On the contrary, the up-bottom ion-cutting method is quite similar to the wet impregnation method but with some differences. For instance, CuAu diatomics (DAs) have been anchored on TiO₂ photocatalyst using this technique through: i) mixing TiO₂, PVP, KBr, and L-ascorbic acid in an oil bath at 80 °C for 10 min; ii) adding CuCl₂·2H₂O and HAuCl₄ solutions to the mixture; iii) continuing the heating process at 80 °C for another 3 h to successfully construct the Cu₅Au₁–TiO₂. After that, 100 mg of the as-prepared Cu₅Au₁–TiO₂ was scattered in a solution of FeCl₃·6H₂O and 0.1 m HCl bubbled with Ar gas. Then, the suspension mixture was etched for 7 h in an oil bath at 50 °C to obtain the E₇–Cu₅Au₁–TiO₂ photocatalyst.^[61] However, it is challenging for these approaches to achieve full control of loading AACs, which affects the reproducibility of the catalyst. Besides, they involve using hazardous materials.

Second, the high-temperature incorporation routes (e.g., hydrothermal treatment, calcination, and thermal polymerization) involve heating the precursors and supports to high temperatures in an inert or reducing atmosphere to facilitate the precursor decomposition and the subsequent scattering of AACs over the catalyst surface. These routes are beneficial for improving the stability of the as-prepared photocatalyst by creating a strong interaction between the support surface and anchored AACs. This can result in the creation of robust AACs anchored photocatalysts with improved temperature stability and long-term catalytic robustness. For example, one of the above references reports the process of anchoring Cu AACs onto phosphorous-modulated carbon nitride photocatalyst (Cu AACs/PCN) through a two-step thermal polymerization.^[64] Two other studies outline the preparation of Cu^δ+ active centers/CeO₂-TiO₂ and Cu₁/TiO₂ photocatalysts through calcination.^{[59, 63]} The Cu AACs/PCN photocatalyst is created by dissolving CuCl₂·2H₂O and melamine in the HCl solution. The product is then separated, dried, and undergoes a sequence of treatments, including the first thermal polymerization step, where it is calcined with NaH₂PO₂·H₂O at 500 °C for 4 h in the N₂ atmosphere. After that, the formed product is ground with LiCl and KCl, and calcined again at 550˚C for another 4 h in an N₂ atmosphere, followed by leaching using H₂SO₄ and boiling water to obtain the final catalyst.^[64] In addition to this, the Cu^δ+/CeO₂–TiO₂ photocatalyst is prepared by adding Cu⁺² and Ce⁺³ solution into the MIL-125-NH₂ precursor, followed by calcination at 450 °C for 3 h in air.^[63] Nevertheless, the Cu₁/TiO₂ photocatalyst is fabricated by adding a calculated amount of CuCl₂·2H₂O to a colloidal solution of SiO₂@TiO₂ nanoparticles and stirring for 3 h at room temperature. The resulting product is then centrifuged, washed, and dispersed in water. PVP is added to the mixture, and after its adsorption, the product is collected by centrifuge and redispersed in an ethanol and water mixture solution. Aqueous ammonia and TEOS are added to the mixture, and after 4 h, the product is separated, washed, and dried at 80 °C. The resulting powder is then calcined at 950 °C for 2 h in air. The SiO₂ nanoparticles are etched using a NaOH solution to acquire the final Cu₁/TiO₂ product.^[59] Besides, one study^[72] reports the synthesis of CA/Ni₁Cu_NP N–C photocatalysts by adding Cu⁺² and Ni⁺² solutions to ZIF-8 precursor followed by calcination at 800–950 °C under N₂ atmosphere. Then, a heterojunction with layered doubled hydroxide (LDH) (i.e., CoAl-LDH (CA)) is acquired using ultrasonication. Furthermore, there are two reports for preparing Cu₁/W₁₈O₄₉ and Ag/CIS photocatalysts using hydrothermal treatment at 200 °C for 12 h and 180 °C for 24 h, respectively.^{[56, 58]} On the contrary, only one reference reports the combination of both hydrothermal and calcination treatment to prepare P/Cu SAs@CN catalyst. In this report, melamine and NaH₂PO₂ were mixed in hydrothermal treatment at 80 °C for 6 h. The acquired product was dispersed in 1.0 M Cu(NO₃)₂·3H₂O ethanol solution as the copper precursor, followed by drying at 80 °C and calcination of the resulting powder at 550 °C for 2 h.^[65] Yet, the increased energy consumption associated with these methods is making them less energy efficient compared to other methods.

Third, one unique study reports the utilization of an atomic layer deposition route to integrate Cu AACs onto TiO₂ nanosheets, assuring an even scattering of Cu AACs across the support surface. The process involved the use of a viscous-flow stainless-steel tube reactor system with N₂ as a carrier gas at 300 °C, employing Cu(hfac)₂ and formaldehyde as precursors.^[67] On the contrary, the complexity and equipment requirements necessary for this method still challenge its applicability in large-scale applications.

Finally, a photoreduction loading route was employed in one report to anchor Cu AACs and Cu–Au alloy nanoparticles on TiO₂. P25 TiO₂ nanoparticles were scattered in ethanol, deoxygenated with N₂, and varying amounts of Cu(NO₃)₂·3H₂O solution were added. The mixture was then irradiated using a 300 W mercury lamp for 30 min. Then, specific quantities of HAuCl₄ were added, and the suspension was further irradiated for an additional 30 min to complete the loading.^[57] This incorporation method can be limited by the different reactivity of the multiple AACs, resulting in difficulties in controlling the output of the process. Finally, Figure 3 exhibits a schematic diagram of various preparation routes for acquiring AACs anchored photocatalysts for ethylene/ethane evolution.

3.1 Cu AACs Boosting Photosynthesis of Ethylene/Ethane

The aforementioned results elaborate on the synergetic effect of coloading Cu AACs and Cu–Au alloy NPs on TiO₂ support, which enhances the CO₂ adsorption, lowers energy barriers for the overall reaction, and promotes the slow C–C coupling for PRCO₂ into ethylene.

In another report, Wang et al.^[63] implanted atomically scattered Cu^δ+ species over CeO₂–TiO₂ heterostructure to acquire the Cu^δ+/CeO₂–TiO₂ photocatalyst. The resulting Cu^δ+/CeO₂–TiO₂ photocatalyst featured the presence of porous TiO₂, which acted as a light absorber to generate charge carriers being efficiently separated over the CeO₂–TiO₂ heterostructure interface. This would reduce charge recombination and raise catalytic performance. In addition, the Cu and Ce active centers were explored to stimulate the adsorption/activation of CO₂ molecules, and accelerate the slow kinetic C–C coupling process. These resulted in astonishing activity for CO₂ conversion into ethylene in a saturated solution of CO₂ irradiated for 5 h by a xenon lamp (320 nm<λ<850 nm) at room temperature. Figure 4e displays that the optimized Cu^δ+/CeO₂–TiO₂ achieved a high ethylene production rate of 4.51 μmol g⁻¹ h⁻¹ with a selectivity of 47.5%, with both CO and CH₄ products detected. In comparison, bare TiO₂ exhibited no activity for CO₂ reduction; while CeO₂–TiO₂ presented only a low CO yield rate of 1.08 μmol g⁻¹ h⁻¹. Notably, the Cu^δ+/TiO₂ exhibited a lower ethylene evolution rate of 1.91 μmol g⁻¹ h⁻¹ and a selectivity of 35.9%, compared to the as-prepared Cu^δ+/CeO₂–TiO₂. The effect of Cu and Ce active centers on the optical characteristics of Cu^δ+/CeO₂–TiO₂ was evaluated by steady-state PL, UV-Vis DRS, and transient photocurrent measurements. The steady-state PL peak intensity for the Cu^δ+/CeO₂–TiO₂ sample was found to be lower than those of TiO₂, CeO₂–TiO₂, and Cu^δ+/TiO₂, respectively. These results indicate that the Cu and Ce active centers significantly reduced the charge recombination, resulting in improved electron–hole separation and consequently higher activities compared to other catalysts.

Moreover, UV-Vis DRS results exhibited an upshift in the absorption peak of Cu^δ+/CeO₂–TiO₂ to the visible light region, compared to pristine TiO₂ or Cu^δ+/TiO₂. The efficient charge separation over the optimized Cu^δ+/CeO₂–TiO₂ photocatalyst was further explored by transient photocurrent measurements. The results revealed more effective charge separation in Cu^δ+/CeO₂–TiO₂ with light irradiation than those in other catalysts. Moreover, the atomic structure and coordination environment of anchored Cu AACs in the Cu^δ+/CeO₂–TiO₂ catalyst were explored using XANES and FT-EXAFS spectroscopy. The normalized Cu K-edge XANES spectra of Cu^δ+/CeO₂–TiO₂ exhibited an absorption edge at 8996.5 eV and a smaller shoulder at 8989.3 eV, assigned to the presence of Cu²⁺ atomic species (Figure 4f). Moreover, the FT-EXAFS spectra of the Cu K-edge for the Cu^δ+/CeO₂TiO₂ exhibited a small peak at ≈3.2 Å ascribed to the CuCe bonding. This confirmed that the anchored Cu species in Cu^δ+/CeO₂TiO₂ were atomically scattered Cu^δ+ cations with no metallic copper or CuO present. Furthermore, the chemical environment of the active centers for Cu^δ+/CeO₂TiO₂ was further explored using the CO-absorbed diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) technique. The CO-DRIFTS spectra for Cu^δ+/CeO₂TiO₂ exhibited three distinct peaks at 2217, 2055, and 2127 cm⁻¹, corresponding to Cu²⁺CO, Cu⁺CO, and Ce³⁺CO bonding modes, respectively. In contrast, the Cu^δ+/TiO₂ sample exhibited only the Cu²⁺CO and Cu⁺CO bonding peaks, while the Ce³⁺CO was the sole peak observed in the CeO₂TiO₂ catalyst (Figure 4g). In situ DRIFTS was conducted to further track the role of CuCe dual centers in promoting the CO₂ photoreduction to ethylene over Cu^δ+/CeO₂TiO₂ with light irradiation. The analysis revealed the presence of characteristic IR bands corresponding to *CO intermediates and *COOH. Besides, the *COCO intermediate resulting from the coupling of the *CO was detected as well. These results revealed the pivotal functions of the Cu and Ce atomic centers: i) reducing the activation barriers of adsorbed CO₂ molecules on Cu^δ+/CeO₂TiO₂, and ii) boosting the formation of *COCO intermediate, which further transformed into ethylene through reduction and hydrogenation steps, as illustrated in the proposed reaction pathway mechanism (Figure 4h). In another report, Lee et al.^[59] reported the integration of several transition metal (e.g., Cu, Ni, Co, and Rh) AACs over anatase TiO₂ (M₁/TiO₂) photocatalysts for photocatalytic CO₂ reduction. They found that TiO₂ can stabilize the anchoring of AACs over the TiO₂ surface through the electronic interaction between the TiO₂ and the anchored AACs, creating spontaneous oxygen vacancies (TiO_2−x) adjacent to the integrated AACs. These integrated AACs are preferred to be scattered in the Ti vacancies of TiO₂. This site-preference can stabilize the anchored AACs and prevent their agglomeration. The synergetic effect of O vacancies and the incorporated AACs can further promote the photocatalytic CO₂ conversion for the as-prepared (M₁/TiO₂) catalyst compared to bare TiO₂ by eliminating the CO₂ activation barriers and stabilizing the formed CO₂ intermediates. This research also exhibits that the preparation route allows for the uniform scattering of metallic AACs and precise control of charge localization disturbance. The addition of various metallic AACs at a loading of 0.21% significantly improved the CO₂ reduction performance of anatase TiO₂ under simulated solar irradiation. The Cu₁/TiO₂ catalyst exhibited the highest CO₂ conversion rate, producing 1416.9 ppm g⁻¹ h⁻¹ of CH₄ and a substantial rate of 64.2 ppm g⁻¹ h⁻¹ of ethane, while other M₁/TiO₂ catalysts only produced CH₄. Theoretical results revealed that the anatase TiO₂ would be partially reduced to Ti³⁺ upon anchoring Cu AACs, with the Cu AACs preferring to be located near the Ti³⁺ sites, which could stabilize the added Cu AACs and create intrinsic O vacancies. A series of characterization techniques further confirmed these results. For example, the XPS Ti 2p peaks of optimized Cu₁/TiO₂ exhibited a movement to the lower binding energy position compared to those of anatase TiO₂. These results confirmed the partial reduction of Ti⁴⁺ to Ti⁺³ over the TiO₂ surface after integrating the Cu AACs. The coordination environment and the atomic bonding nature of the anchored Cu AACs in the as-prepared Cu₁/TiO₂ photocatalyst were explored by the X-ray absorption spectroscopy (XAS) technique. The X-ray absorption near-edge structure (XANES) analysis revealed the oxidation state of the Cu AACs to be (+2) with the absence of any other Cu species. Furthermore, the extended X-ray absorption fine structure (EXAFS) for the Cu₁/TiO₂ catalyst is in agreement with its XANES result, confirming the stabilization of Cu AACs via the Ti vacancies in the as-prepared TiO₂. Considering all the aforementioned theoretical and characterization results, it is concluded that the naturally reduced anatase TiO₂ stabilizes the anchoring of Cu AACs in Ti vacancies, leading to the creation of spontaneous O vacancies nearby. This synergistic effect raises the PRCO₂ performance of the Cu₁/TiO₂ photocatalyst by reducing energy barriers and creating favorable active centers for CO₂-formed intermediates, ultimately generating renewable ethane. However, this research does not provide accurate mechanistic insights into the photosynthesis of ethane.

In another study, Wang et al.^[67] introduced an innovative route for immobilizing photo-induced metastable asymmetric Cu AACs onto TiO₂ nanosheets (Cu₁/TiO₂) using atomic layer deposition. With light irradiation, the photogenerated electrons migrated from TiO₂ nanosheets to the Cu 3d orbits of Cu AACs, resulting in the rearrangement of the energy levels of Cu-3d orbitals. These rearrangements aroused a distortion of the previously symmetric Cu₁–O₄ coordination environment of the incorporated Cu AACs in the dark, resulting in a unique metastable asymmetric Cu₁–O₂₊₁ coordination structure that can revert back to the original Cu₁–O₄ coordination upon removal of the light stimulus. As presented in (Figure 5a), The Cu₁/TiO₂ photocatalyst changes color from watery blue in the dark to chartreuse in the light and back. This unique behavior is not seen in pristine TiO₂ nanosheets and is not affected by temperature changes, indicating it's not driven by a photothermal effect but rather by reversible photoinduced structural changes. In order to gain insights into the fundamental properties of the Cu₁/TiO₂ and the metastable asymmetric Cu₁–O₂₊₁ coordination structure, a range of physicochemical characterizations were employed. For example, the HAADF-STEM images of the Cu₁/TiO₂ sample exhibited a uniform scattering of individual Cu atoms on the TiO₂ nanosheets. Also, the electronic characteristics of the Cu₁/TiO₂ catalyst were investigated using XPS, XANES, and Fourier-transformed EXAFS (FT-EXAFS) techniques. The analysis of the O 1s XPS spectrum revealed the presence of oxygen vacancies. The Ti³⁺ oxidation state peak is attributed to structural distortions caused by incorporating Cu AACs. The Cu 2p XPS spectra show peaks indicating the presence of Cu²⁺ and Cu⁺ or Cu⁰ species. XANES confirms the oxidation state of Cu in Cu₁/TiO₂ to be ≈2+. Further analysis with FT-EXAFS in R space exhibited the presence of CuO bonding spectra. The well-fitted results confirmed the presence of atomic Cu species in a CuO₄ coordination environment. The UV-Vis (DRS) results exhibited that the anchoring of Cu AACs expanded the light absorption range of the TiO₂ nanosheets. Nevertheless, the steady-state photoluminescence (PL) spectra for Cu₁/TiO₂ presented a reduction in peak intensity compared to those of pristine TiO₂ nanosheets, indicating lower electron–hole (e–h) recombination rates and improved charge separation in the Cu₁/TiO₂ photocatalyst. On the contrary, the photoinduced structural changes in Cu₁/TiO₂ with light irradiation were monitored using several state-of-art in situ techniques. The in situ XANES analysis for the corresponding Cu K-edge suffered from a blue shift with light, indicating a partial reduction of the atomic Cu^δ+ species to a lower valence state (0< δ <2) when exposed to light. The in situ wavelet transform (WT)-EXAFS contour graphs demonstrated that the CuO bond was stretched after light irradiation. The combination of these results with the in situ EXAFS fitting results confirmed the destruction of the primary symmetric Cu₁O₄ coordination structure in the Cu₁/TiO₂ with light irradiation and the formation of metastable asymmetric Cu₁O₂₊₁ coordination structure intermediate. Similarly, the energy rearrangements of the Cu-3d orbitals of the incorporated Cu AACs were explored using the in situ soft-XAS technique. The results in (Figure 5b) exhibited that the peaks at 931.2 and 951.2 eV were identified as the transitions from 2p_3/2 to 3 d (L₃) and from 2p_1/2 to 3 d (L₂) states of Cu. Upon light exposure, the absorption edge of the Cu L_2,3 peaks is moved to a lower energy position, revealing a partial reduction of Cu. Moreover, a new peak appeared at ≈934.7 eV, possibly originating from the rearrangement of energy levels in the Cu 3 d orbitals. This arose from the migration of photogenerated electrons from TiO₂ nanosheets to the d-orbits of anchored Cu AACs. The optimized Cu₁/TiO₂ achieved a remarkable evolution rate of 60.4 μmol g⁻¹ h⁻¹ for ethylene with an outstanding 75.2% selectivity (Figure 5c). In contrast, TiO₂ only produced CO and CH₄, which increased in yield with the utilization of Cu₁/TiO₂ photocatalyst. These results reveal the role of Cu AACs over TiO₂ nanosheets in promoting the C–C coupling and reducing the overall energy barriers for the value-added multicarbon synthesis. Furthermore, the ethylene-generation rate remained preserved after six catalytic cycles (Figure 5d). The reactant molecules and successive hydrogenation of the formed intermediates were tracked over the CO₂ reduction pathway to ethylene using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurement on Cu₁/TiO₂. The spectra showed rising characteristic peaks of intermediates such as *CO, *CHO, *CH₃O, *COOH, and *CH₂ (Figure 5e). The *C–COH intermediate which is essential for forming the C2 products, was also detected. The evolution of ethylene over the Cu₁O₂₊₁ active center with prolonged irradiation was also observed, and the calculated energy barriers of the CO₂ photoreduction pathway over the Cu₁O₂₊₁ structure were found to be lower compared to the Cu₁O₄ structure (Figure 2a). These results corroborate the key role of the metastable asymmetric Cu₁O₂₊₁ coordination structure in exceptionally raising the photocatalytic activity of the as-prepared Cu₁/TiO₂ photocatalyst for ethylene evolution.

In another research, Xie et al.^[64] utilized a distinct approach by integrating Cu AACs on phosphorus-modulated carbon nitride (PCN), a nonmetallic 2D photocatalyst. This route contrasts with previous reports, which primarily used metal oxides. This preparation strategy enabled the regulation of the coordination environment of anchored Cu AACs with the surrounding atoms in the as-prepared photocatalyst (Cu AACs/PCN) to create active centers with accurate CuN coordination structure (CuN₄). These CuN₄ centers effectively lower the energy levels of the formed intermediates in the CO₂ photoreduction pathway. As a result, it raised the CO₂ adsorption/activation to form *CO and accelerated the subsequent stepwise CC coupling and hydrogenation processes of the produced intermediates, ultimately resulting in the generation of ethylene. The optimal Cu AACs/PCN photocatalyst was prepared utilizing a two-step thermal polymerization process to convert the salt precursors of Cu, P, and carbon nitride into the final catalyst. In comparison, the Cu AACs/CN photocatalyst was prepared utilizing the same route but without the P precursor to reveal the role of P in raising the photocatalytic activity of Cu AACs/PCN. The P content was regulated to create two other samples, which were annotated as Cu AACs/PCNH and Cu AACs/PCNL, with higher and lower P-loading contents, respectively. Initial characterization results confirmed the successful integration of the Cu AACs into the PCN matrix. As shown in Figure 6a, the HAADF-STEM images revealed a uniform scattering of Cu AACs (circled in red) over the PCN nanosheets, with no observation of any Cu NPs/clusters/aggregation. This finding was further supported by energy dispersive X-ray spectroscopy (EDX) elemental mapping images for the as-prepared Cu AACs/PCN, which exhibited highly scattered Cu AACs throughout the PCN (Figure 6b). The intrinsic electronic state and chemical structures of the Cu AACs were explored using XPS, XANES, and FT-EXAFS. The high-resolution XPS Cu 2p spectrum was deconvoluted into four distinct peaks. Two of them at 932.7 and 952.6 eV were associated with Cu⁺ species; while the other two at 935.2 and 955.1 eV were attributed to Cu²⁺ species (Figure 6c). This revealed that the oxidation state of the Cu AACs in the Cu AACs/PCN system is between (+1) and (+2). Furthermore, the P 2p XPS spectra exhibited a single peak at 133.2 eV, indicating the presence of the PN bonding mode. The XANES absorption edge for the CuK edge in Cu AACs/PCN was found to be positioned between the absorption edges of Cu₂O and CuO. This is consistent with the XPS results and indicates that the oxidation state of the Cu AACs is between +1 and +2. The fitting results of FT-EXAFS confirmed the existence of atomically scattered Cu AACs coordinated with 4 N atoms, forming the CuN₄ coordination spectra measurement of all the as-prepared samples (Figure 6d). Then the impact of CuN₄ active centers on the properties of Cu AACs/PCN was further explored. The TSPL of all the samples exhibited the highest average charge lifetime (τ_ave = 3.87 ns) for Cu AACs/PCN (Figure 6e), compared to those of the other photocatalysts. These results highlight the obviously improved electron–hole separation for Cu AACs/PCN, resulting in higher catalytic performance. Figure 6f exhibited the calculated conduction band (CB) and valence band (VB) positions for the as-prepared photocatalysts against a normal hydrogen electrode (NHE) along with the measured optical bandgap energies. The results revealed that the addition of Cu AACs and P to the CN nanosheets aroused a reduction of the bandgap energy and an extension of its light absorption range. Moreover, the CB band positions confirmed that the photoreduction of CO₂ to ethylene is thermodynamically feasible by all the as-prepared catalysts. Then the photocatalytic performances of all the catalysts for CO₂ photoreduction were evaluated with light irradiation in the presence of [Ru(bpy)₃]²⁺ as a photosensitizer and TEOA as a hole scavenger. The Cu AACs/PCN exhibited an impressive ethylene generation of 30.51 μmol g⁻¹ with a selectivity of 53.2% after 3 h light irradiation (Figure 6g). Finally, the Gibbs energies all over the reaction pathway of CO₂-to-ethylene conversion (Figure 6h) revealed the obviously reduced energy barriers for Cu AACs/PCN, compared to those of Cu AACs/CN. These results corroborated the key role of combining the Cu AACs and loaded P to reduce the energy barriers for CO₂ reduction and boost the sluggish C–C coupling of the formed intermediates to generate value-added ethylene (Figure 6h).

Recently, Mao et al.^[56] presented an excellent ethylene production rate (4.9 μmol g⁻¹ h⁻¹) with high selectivity (72.8%) using Cu (I) AACs anchored W₁₈O₄₉ nanowires (Cu₁/W₁₈O₄₉) (Figure 7a). The photocatalyst was prepared by a facile one-step hydrothermal approach. The intrinsic mutual conversion of the W⁵⁺ and W⁶⁺ ions in W₁₈O₄₉ facilitated the reduction of Cu²⁺ to Cu¹⁺ cations, providing higher stability for the incorporated Cu (I) AACs in the reaction. In addition, the Cu (I) AACs reduced the energy barriers for CO₂ reduction and stabilized the formed *CO intermediates. Meanwhile, the asymmetric dual active centers (Cu–W) that emerged from the interaction between Cu AACs and the W₁₈O₄₉ support can boost the C–C coupling of two adjacent *CO intermediates and their subsequent hydrogenation to ethylene. The oxidation states of the synthesized 3.6-Cu₁/W₁₈O₄₉ were investigated using XPS tests. The Cu 2p XPS spectrum confirmed the presence of Cu (I) based on the appearance of the Cu 2p_3/2 peak at 932 eV. Additionally, no peaks related to Cu⁰ or Cu²⁺ were observed, confirming the atomic nature of the incorporated Cu in 3.6-Cu₁/W₁₈O₄₉ and all the Cu²⁺ precursors were reduced to Cu¹⁺ within the modified W₁₈O₄₉ (Figure 7b). Besides, the XPS spectrum of the W 4f showed a notable 2.57 % reduction in the content of W⁵⁺ cations, providing further evidence for the involvement of W⁵⁺ centers in the reduction of Cu²⁺ to Cu¹⁺. The coordination environment of the active centers over the optimized Cu₁/W₁₈O₄₉ was explored using XANES and FT-EXAFS analyses. The normalized XANES spectra for the Cu K-edge exhibited an absorption edge close to that of Cu₂O, suggesting a coordination state of ≈1 for the Cu AACs in 3.6-Cu₁/W₁₈O₄₉. Moreover, the FT-EXAFS results presented a major peak located at ≈1.5 Å, corresponding to the CuO bonding, and the fitting results from the second shell exhibited a considerable amount of the CuW bonding path (Figure 7c). To further explore the influence of active centers on the CC coupling of the intermediates and the evolution of ethylene, in situ DRIFTS measurements were performed for the 3.6-Cu₁/W₁₈O₄₉ under light irradiation in the CO₂ atmosphere (Figure 7d). The results revealed the observation of IR bands attributed to *CHO, *COOH, and *CO intermediates, indicating the boosted CO₂ adsorption/activation on 3.6-Cu₁/W₁₈O₄₉. Furthermore, the vibrational peaks observed at 1567 and 1517 cm⁻¹ were attributed to the *COCO intermediate, which is pivotal in the CC coupling process. Interestingly, the *CHOCO intermediate was found at 1576 cm⁻¹, produced through the hydrogenation of *COCO intermediate. On the contrary, bare W₁₈O₄₉ exhibited no peaks related to the CC coupling-related intermediates, revealing the key role of Cu (I) AACs and the asymmetric dual CuW active centers in boosting the conversion of CO₂ to ethylene over 3.6-Cu₁/W₁₈O₄₉.

3.2 Au or Ag AACs Boosting Photosynthesis of Ethylene/Ethane

Apart from the Cu AACs, researchers have recently explored Au or Ag AACs anchored photocatalysts as promising candidates for photocatalytic CO₂ conversion to ethylene/ethane. This research is based on the unique merits of Au or Ag AACs, such as their significant reduction capabilities, ability to enhance charge separation/transfer and promotion of CC coupling interactions. Besides, the intrinsic localized SPR of Ag and Au AACs can remarkably extend the light absorption capabilities of the as-prepared AACs anchored photocatalysts. This results in the generation of more photogenerated electrons for the multielectron PRCO₂ to ethylene/ethane and further increases the overall yields. For example, Ou et al.^[68] reported the regulation of the C–C coupling process in PRCO₂, resulting in a remarkable selectivity of 96% for ethane as a product. This steering was achieved by utilizing highly scattered Au AACs over nonmetallic red phosphorous (RP) nanosheets as support. The intrinsic properties of the RP photocatalyst render it an excellent choice as support for CO₂ photoreduction. For example, its single-element structure, which can cause a homogenous coordination environment for all of the incorporated AACs. Moreover, unlike the other common favorable centers for the AACs bonding over support materials, such as O, N, and S, the reduced electronegativity of the P elements allows for precise control of the electron density around the AACs. In addition, the rich-electron environment of RP can facilitate CO₂ adsorption/activation to produce the necessary C1 intermediates for the later CC coupling interactions. Besides, the loaded Au AACs raised the photogenerated electrons transfer, and the coupling of the formed C1 intermediates to generate ethane. The Au₁/RP photocatalyst was prepared utilizing a wet impregnation route followed by calcination in the H₂ atmosphere to complete the anchoring of Au AACs. The morphology of the incorporated Au AACs was confirmed by various characterization techniques, such as TEM and HAADF-STEM. The HAADF-STEM images exhibit highly scattered isolated bright spots, ascribed to the anchored Au AACs over the RP (Figure 7e). Besides, the elemental mapping images revealed that the Au AACs were homogeneously scattered over the surface of RP. The chemical state of the optimized Au₁/RP was further tested by XPS analysis. The P 2p XPS spectrum exhibited a main peak centering at 130 eV related to the PP bonding in the RP, and a smaller peak at 135 eV, attributed to the PO boding resulting from the oxidized surface of the RP. The Au 4f spectrum was deconvoluted into two peaks at 85 and 89 eV, corresponding to Au 4f_7/2 and Au 4f_5/2, respectively. This indicates that the oxidation state of the atomic Au species in Au₁/RP ranged between 0 and +3. The coordination environment of anchored Au AACs in Au₁/RP was verified using XANES and FT-EXAFS analyses. The absorption edge in the normalized Au K-edge XANES spectrum of Au₁/RP was positioned between the absorption intensities of the Au Foil and Au₂O₃, revealing that the valence status of the Au AACs is between 0 and +3. The FT-EXAFS analyses of the optimized Au₁/RP revealed a major peak at 1.83 Å with the absence of a characteristic peak of the AuAu bonding at >2.7 Å (Figure 7f), suggesting the absence of Au NPs in the optimized Au₁/RP sample. Furthermore, the WT-EXAFS contour map of the Au foil unveiled two prominent peaks at 2.7 and 1 Å in R space, corresponding to AuAu and AuO coordination structures, respectively. Meanwhile, the contour map of Au₁/RP exhibited a single peak at 1.8 Å in R-space, correlated with AuP bonding (Figure 7g). Notably, the EXAFS fitting results indicated that the immobilized Au AACs in Au₁/RP adopted an AuP₂ coordination structure with an average bond length of 2.34 Å. Nevertheless, the photocatalytic performance of the optimal Au₁/RP photocatalyst was evaluated for PRCO₂ using light irradiation without a sacrificial agent. The Au₁/RP photocatalyst exhibited an outstanding production rate of 1.32 μmol g⁻¹ h⁻¹ for ethane and an astonishing selectivity of 96%; while RP only showed small yields of CO and CH₄, highlighting the key role of Au AACs in promoting the generation of ethane. To confirm this, various AACs, such as Pt, Pd, Ir, and AuPd, were loaded on the RP and tested under the same conditions. Interestingly, only Au AACs produced ethane, confirming the key function of anchored Au AACs in the CC coupling process. In the proposed mechanism for ethane evolution over Au₁/RP, the RP support is identified as the active center for adsorbing and activating CO₂ molecules to generate C1 intermediates. Meanwhile, Au AACs minimize the migration distance of C1 intermediates, boosting their interaction with other C1 intermediates on a different RP active center and promoting CC coupling to generate ethane (Figure 7h).

Recently, Xu and coworkers^[58] investigated the impact of Ag AACs in raising the photocatalytic performance of bimetallic sulfide support, such as CuInS₂ (CIS), for CO₂ photoreduction to ethylene. They reported the integration of highly scattered Ag AACs over CIS using a one-pot hydrothermal preparation route. The anchored Ag AACs induced the formation of sulfur vacancies (Vs) over the CIS surface, creating a symbiotic effect. This symbiotic effect transformed the original dual active centers of Cu and In in the bare CIS to new Vs and In dual centers in the optimized (2%Ag/CIS) photocatalyst. Besides, the similar electronic structure of Cu and In dual centers in the bare CIS resulted in high dipole–dipole repulsion, increasing energy barriers for the CC coupling and resulting in the production of only C1 products over the bare CIS. However, the unique electronic structure of the Vs and In dual centers on the 2%Ag/CIS reduced the dipole–dipole interaction, thus reducing the energy barriers for the CC coupling and ultimately boosting the selective production of ethylene. The TEM and HAADF-STEM images for the 2%Ag/CIS revealed the uniform scattering of Ag AACs over the surface of CIS, and no Ag NPs or agglomerations were observed, indicating the success of the hydrothermal preparation route for anchoring the highly scattered Ag AACs over CIS. The valence state of anchored Ag AACs in the 2%Ag/CIS was explored using XANES and EXAFS analyses. The XANES spectrum for the Ag K-edge of the 2%Ag/CIS exhibited a peak intensity located between the Ag foil and Ag₂O, suggesting the partial positive charge of the loaded Ag species. Moreover, the EXAFS fitting results revealed that the Ag AACs are homogeneously scattered in 2%Ag/CIS, and each Ag atom is coordinated with three surrounding S atoms. EPR measurements were conducted to confirm the presence of structural defects. The emerged EPR signal from the Vs located at g = 2.03 exhibited the lowest intensity for the bare CIS, suggesting the low content of Vs in CIS. Interestingly, upon the addition of Ag AACs to the CIS, the signal intensity was significantly raised until it reached its peak at 2%Ag/CIS. These results highlight the key role of Ag AACs in inducing Vs formation and creating a symbiotic effect. The impact of Ag AACs and Vs upon the photo-generated electron movement was elucidated by kelvin probe force microscopy (KPFM) and in situ XPS techniques. The KPFM measurements exhibited that the contact potential difference (ΔCPD) increased from 12.11 mV for the bare CIS to 64.43 mV for 2%Ag/CIS after light irradiation, indicating that the combined Ag AACs and Vs enhanced the charge carrier transfer to the surface of 2%Ag/CIS, thereby raising the photocatalytic activity. Furthermore, the in situ XPS spectra revealed the movement in the binding energies of Cu 2p and In 3d to higher values with light irradiation; while the Ag 3d spectrum is moved to lower binding energies. These results reveal that the CIS generated electrons, which migrated to the Ag AACs. The optimized 2%Ag/CIS exhibited substantial activity for converting CO₂ to ethylene using visible light irradiation without applying sacrificial agents. It achieved an ethylene production rate of 53.8 μmol g⁻¹ h⁻¹ with a selectivity of 98.8%. In contrast, the bare CIS only produced small amounts of lower-value C1 products. These performance results confirmed the symbiotic effect of combining the Ag AACs and the Vs on enhancing the production rate and selectivity of ethylene. Finally, the in situ DRIFTS results revealed specific intermediate and reaction products associated with the CC coupling mechanism over the 2%Ag/CIS catalyst. The identification of characteristic vibrational peaks, including *COCHO, *CH₂=, *CH₂ and *C₂H₄, confirmed the occurrence of CC coupling reactions resulting in ethylene production over the 2%Ag/CIS catalyst.

3.3 Mo or Ni ACCs Boosting the Photosynthesis of Ethylene/Ethane

The intrinsic properties of MOFs and COFs photocatalysts, such as extended-π conjugation, visible light absorption capabilities, low recombination rates, stability, and large surface area, have resulted in extensive research on their applications in PRCO₂. These exceptional properties position them as promising options as support photocatalysts to anchor multiple AACs. Moreover, anchoring AACs on MOFs and COFs supports can improve photoelectrons migration, create modulated active sites for CO₂ adsorption and activation, enhance charge carrier separation efficiency, and promote the CC coupling of the formed intermediates. Consequently, this facilitates the complex multielectron CO₂ reduction process to produce the desired C₂ products. Kou et al.^[71] successfully incorporated Mo AACs onto TpBpy COF using the impregnation method, creating an efficient Mo–COF photocatalyst. The resulting MoCOF photocatalyst showcased the presence of MoN₂ active sites as catalytic centers for CO₂ photosynthesis to C₂H₄. Characterization via TEM, elemental mapping, and HAADF-STEM images revealed the uniform dispersion of Mo AACs across the TpBpy COF without any evidence of agglomerations, clusters, or nanoparticles. The local structure of the MoN₂ active sites was explored using XPS measurements, which demonstrated the Mo 3d spectrum deconvoluting into two peaks at 232.27 and 235.32 eV, corresponding to Mo in a (+6) oxidation state. The N 1s spectrum of the MoCOF exhibited an extra peak at 397.54 eV compared to the TpBpy COF N 1s spectrum, confirming the formation of MoN₂ active sites (Figure 8a). The coordination structure of the optimized MoCOF was verified through XANES and EXAFS analyses, which revealed an absorption edge close to that of MoO₃, indicating the Mo oxidation state to be +6, consistent with the XPS results. Furthermore, both FT-EXAFS and WT-EXAFS confirmed the coordination between the Mo AACs and the pyridinic nitrogen of the COF matrix, leading to the formation of MoN₂ active sites. The presence of MoN₂ sites in the MoCOF significantly enhanced its photocatalytic activity for PRCO₂ to C₂H₄. Under visible light irradiation, the optimized MoCOF yielded 3.57 μmol g⁻¹ h⁻¹ of ethylene with selectivity of 32.92%, while the COF and g-C₃N₄ as a reference photocatalyst produced only CO under the same conditions (Figure 8b). Additionally, the influence of anchoring Mo AACs on the charge carriers kinetics of the COF was investigated using transient-state surface photovoltage (TSSPV), TSPL, and transient photocurrent measurements. The TSSPV results showed that the Mo-COF had a higher response than the reference g-C3N₄ (Figure 8c), while the TSPL indicated an increased lifetime of 1.12 ns compared to 0.86 ns for the TpBpy COF. The transient photocurrent measurements showed that the Mo-COF had a higher response than the COF, suggesting improved charge carrier separation efficiencies and higher photocatalytic activities. The incorporation of Mo AACs also reduced the energy barriers for CO₂ activation and promoted the CC coupling of the formed intermediates to obtain the desired C₂H₄ product. However, further research is needed to investigate the influence of different AACS on different MOFs and COFs over ethylene/ethane photosynthesis yields and selectivity, as research on AACs anchored to MOFs and COFs is still limited.

In addition, Pan et al.^[72] investigated the synergetic effect of Ni AACs and Cu NPs on ZIF-8-derived nitrogen-doped carbon NC for enhancing the PRCO₂ to ethane via the fabrication of (Ni₁Cu_NP NC) photocatalyst through calcination, followed by the formation of heterojunction with CoAl-LDH (CA) using ultrasonic to synthesize the optimized (CA/Ni₁Cu_NP NC) composite. The incorporation of Ni AACs into the NC resulted in the creation of NiN₄ active sites with enhanced pore structures and electrical conductivity, leading to improved adsorption of reactants and intermediates, as well as enhanced charge carrier migration during the CC coupling reactions. Moreover, the inclusion of Cu NPs led to increased electron density at Ni atomic sites, providing improved charge distribution and separation, thereby promoting the formation of C2 products. Additionally, the formation of a heterojunction with CA maximized the hydrogenation of intermediates, primarily by accelerating the dissociation of water, which significantly boosted the production rate of C₂H₆. The coordination environment of the synthesized CA/Ni₁Cu_NP NC has been confirmed using XPS, XANES, and EXAFS analyses. The HR-XPS Ni spectra showed peaks at 852.9 and 856.0 eV, indicating the presence of Ni⁰ and Ni²⁺ species. The N 1s XPS spectra confirmed the presence of NiN bonding at 399.36 eV, suggesting the successful construction of the Ni N₄ coordination sites. Additionally, the XANES results showed that the CA/Ni₁Cu_NP NC has an adsorption edge between that of Ni foil and NiO, and the FT-EXAFS fitting confirmed the existence of Ni AACs and Ni N₄ active centers. Furthermore, the coexistence of both Ni AACs and Cu NPs in the CA/Ni₁Cu_NP NC composite extended its light-absorption capabilities compared to CA/Ni₁ NC, CA/Cu_NP NC, and CA photocatalysts. The UV-Vis results indicated an enhanced light absorption in the infrared (IR) region due to SPR induced by the Cu NPs in the synthesized samples (Figure 8d). Figure 8e displays the PL spectra with a remarkable reduction in intensity for the CA/Ni₁Cu_NP NC compared to other samples, demonstrating the notable role of the Ni AACs and Cu NPs in suppressing the recombination rates and enhancing the charge carrier separation. The optimized CA/Ni₁Cu_NP NC exhibited a notable ethane production rate of 25.328 μmol g⁻¹ h⁻¹ with a selectivity of 50.61% under visible light irradiation (Figure 8f). Additionally, it was observed that lower CA content led to reduced C₂H₆ generation, indicating the hydrogenation role of CA. The photocatalytic activities, along with the characterization results, highlighted the synergetic effect of Ni AACs and Cu NPs in enhancing the separation of photogenerated carriers and promoting the CC coupling to remarkably improve the photosynthesis of ethane.

3.4 Dual AACs Boosting Photosynthesis of Ethylene/Ethane

The anchoring of dual AACs over photocatalysts is raised as a creative design strategy for preparing the composite photocatalysts. This approach aims to address the challenges associated with the process of PRCO₂ to produce valuable ethylene/ethane. The dual AACs can provide better regulation of the microstructure environment and enhance the photogenerated carrier separation, which could achieve higher production rate and selectivity for ethylene/ethane. For example, Wang et al.^[65] reported the construction of PN and CuN₄ active centers through anchoring metallic Cu AACs and nonmetallic P AACs onto carbon nitride (CN) nanosheets. The as-prepared P/Cu AACs@CN photocatalyst was prepared by applying hydrothermal and polymerization synthesis routes. The photogenerated charge kinetic results exhibited that both the Cu and P AACs could efficiently capture the photoexcited charge carriers to boost the charge separation over the CN structure with light irradiation. This enhanced separation resulted in the creation of a charge-enriched environment around the Cu centers. The Cu charge-enriched centers can adsorb CO₂ and activate its reduction to *CO intermediate, followed by the subsequent coupling and hydrogenation of the adsorbed *CO to produce ethane. The optimal P/Cu AACs@CN photocatalyst exhibited substantial performance for PRCO₂ to ethane, realizing a remarkable ethane evolution rate of 616.6 μmol g⁻¹ h⁻¹ and ethane selectivity of 33% after 10 h light irradiation. Notably, the Cu AACs@CN sample only exhibited 23.7 μmol g⁻¹ h⁻¹ yield of ethane, which was 26 times less than the measured activity of P/Cu AACs@CN. The P@CN and CN catalysts only produced minimal amounts of ethane. These findings underscore the importance of the created PN and CuN₄ dual active centers in maximizing the CC coupling process for ethane production. Several characterization techniques were employed to investigate the morphological properties, microstructure, and coordination environment of the incorporated dual centers in the P/Cu AACs@CN. For example, HAADF-STEM images of P/Cu AACs@CN revealed the high scattering of bright isolated Cu AACs with no sign of any larger particles, and elemental mapping images confirmed the homogeneous scattering of Cu and P species over the CN nanosheets. Furthermore, the XPS P 2p spectrum for the P/Cu AACs@CN exhibited two peaks at 133.5 and 134.4 eV, corresponding to the PN and PN coordination bonding modes, respectively. XANES and EXAFS analyses were applied to gain insights into the valence state and coordination structure of the Cu active centers in the P/Cu AACs@CN. The XANES spectra for the Cu K edge of P/Cu AACs@CN and other Cu-based references revealed the presence of an absorption shoulder of P/Cu SAs@CN between the absorption shoulders of Cu₂O and CuO, indicating that the valence state of the Cu AACs is between +1 and +2. The FT-EXAFS analysis exhibited a main peak at 1.5 Å related to the CuN bonding mode with no sign of the CuCu bonding, confirming the atomic state of the incorporated Cu species. Moreover, the WT-EXAFS contour graphs for the P/Cu AACs@CN revealed one peak located at 3.8 Å⁻¹, also attributed to the presence of the CuN bonding. The fitting results of the FT-EXAFS revealed the coordination number of the Cu AACs to be 4 in the Cu-N₄ structure. PL and TRPL techniques were applied to analyze the charge kinetics of the P/Cu AACs@CN. The reduced intensity of the PL peak for the P/Cu AACs@CN and the extended average lifetime (τ_ave = 7.13 ns) compared to the as-prepared Cu AACs@CN, P@CN, and CN samples corroborated the superior charge separation by the PN and Cu-N₄ dual centers. The transfer of photogenerated electrons and holes of the P/Cu AACs@CN was explored using in situ XPS analysis. The P 2p spectrum moves to higher binding energies after irradiation, revealing the accumulation of holes around the P AACs. Meanwhile, the Cu 2p spectrum moves to lower binding energies after light exposure, revealing the accumulation of photogenerated electrons over the Cu active centers. These findings, in conjunction with the co-existence of the vibrational band of the key intermediate *OCCOH in the in situ DRIFTS spectra, have proved the effectiveness of the inherent characteristics of the combined Cu and P AACs in altering the microstructure environment of the CN. Consequently, creating charge-enriched centers raises the charge separation and boosts the CC coupling reactions, ultimately achieving higher rates of ethane evolution.

Recently, Xie et al.^[61] exploited a novel strategy for preparing diatomic photocatalysts for efficient PRCO₂ to ethylene. Contrary to the extensively used single atom catalysts (SACs), diatomic photocatalysts involve the incorporation of two atoms, which can maximize the utilization of atomic active centers and cooperatively alter their steric/electronic properties to regulate the reaction kinetics/pathways. They reported the anchoring of CuAu diatomic active centers over TiO₂ NPs using a novel up-bottom ion-cutting preparation route. The conventional preparation strategies for bimetallic photocatalysts usually arouse the accumulation or agglomeration of atoms in preparation, resulting in a reduction in available active centers for the reaction. In comparision, this innovative up-bottom ion-cutting route can manage the vectored etching of certain atoms in an alloy, distort the alloy structure and uniformly scatter diatomic active centers over the support surface. The optimized Et₇Cu₅Au₁TiO₂ (E₇-C₅A₁) photocatalyst was constructed after etching the Cu₅Au₁ alloy in the Cu₅Au₁TiO₂ catalyst (C₅A₁) for 7 h. The HAADF-STEM images of the Et₇Cu₅Au₁TiO₂ photocatalyst confirmed the even scattering of diatomic CuAu over the TiO₂ NPs, revealing the successful vectored etching of the Cu₅Au₁ alloy (Figure 9a–c). The atomic configuration and coordination structure of the diatomic CuAu in the Et₇Cu₅Au₁TiO₂ sample were further investigated using XANES, FT-EXAFS, and WT-EXAFS analyses. The XANES spectrum for the Cu K-edge of Et₇Cu₅Au₁TiO₂ is located at a higher photon energy than that of Cu foil (Figure 9d), revealing that some Cu AACs could be bonding with the O atoms of TiO₂. Similarly, the Au L-edge absorption peak resembled that of the Au foil, indicating the presence of Au⁰ species (Figure 9e). Additionally, the FT-EXAFS spectra for the Cu K-edge and Au L-edge revealed distinct peaks, providing information about the coordination environment of diatomic CuAu in the sample. The Cu K-edge FT-EXAFS spectrum exhibited peaks at 1.49 Å and 2.53 Å, corresponding to CuAu and CuO coordination; while the Au L-edge spectrum exhibited peaks at 1.52 and 2.49 Å, attributed to AuO and AuCu/AuAu coordination, respectively (Figure 9f,g). The coordination environments of Et₇Cu₅Au₁TiO₂ were further confirmed by the WT-EXAFS colored maps, as they revealed the existence of maxima intensities related to CuAu, CuO, AuO, and AuCu/AuAu coordination structures. Besides, the XPS results exhibited that the Au 4f peaks of Et₇Cu₅Au₁TiO₂ are moved to lower binding energies, this is attributed to the migration of photogenerated electrons from TiO₂ to the diatomic CuAu, which is advantageous for the CC coupling reactions. These results reveal that diatomic CuAu is the main catalytic center for PRCO₂ and CC coupling processes. Nevertheless, charge kinetics was explored using the PL and TRPL spectroscopy. The PL emission peak of Et₇Cu₅Au₁TiO₂ at 425 nm exhibited the lowest intensity compared to that of pure TiO₂ or Cu₅Au₁TiO₂, revealing the key role of diatomic CuAu in reducing the electron–hole recombination in Et₇Cu₅Au₁TiO₂. Also, the shorter calculated τ₃ lifetime of Et₇Cu₅Au₁TiO₂ in the TRPL measurements compared with those of the other catalysts suggested that the photogenerated electrons preferred to transfer from TiO₂ to the diatomic CuAu rather than recombining in the Cu and diatomic CuAu recombination centers. Nevertheless, Et₇Cu₅Au₁TiO₂ exhibited exceptional performance for CO₂ reduction to C2 products with light irradiation (320 nm < λ < 780 nm). It achieved a magnificent ethylene evolution rate of 71.6 μmol g⁻¹ h⁻¹ with a selectivity of 68.3%, outperforming the other as-prepared photocatalysts. The addition of Cu₅Au₁ to bare TiO₂, along with optimizing the etching time, contributed to the obvious improvement in ethylene generation and selectivity. And the optimal performance was achieved by Et₇Cu₅Au₁TiO₂ (Figure 9h–i). Interstingly, the generation of ethane was observed in the reaction, with Et₇Cu₅Au₁TiO₂ exhibiting the highest ethane evolution rate (8.5 μmol g⁻¹ h⁻¹) with a selectivity of 9.4%. These photocatalytic performances highlight the key role of incorporating diatomic CuAu over TiO₂ for achieving higher performance in ethylene production. Moreover, Figure 9j exhibited the free energy diagram for PRCO₂ to ethylene over Et₇Cu₅Au₁TiO₂ alongside the structure of the formed intermediates. The above results revealed that the diatomic centers regulated the electronic/optical properties of the support, which can reduce the overall energy barriers of the CO₂ reduction reaction and raise the sluggish CC coupling rates to acquire higher production rates and selectivity for ethylene/ethane.

The aforementioned references revealed the significance of anchoring Cu,^{[56, 57, 59, 63, 64, 67]} Au,^[68] Ag,^[58] Mo,^[71] Ni,^[72] P/Cu,^[65] and CuAu^[61] AACs over a range of support photocatalysts to raise the PRCO₂. This enhancement results in higher production rates and selectivity of valuable ethylene/ethane. The integrated AAcs can regulate the coordination environments and microstructures of the photocatalysts, creating novel active centers at the atomic level with unique electronic properties. These characteristics can help overcome the complex reaction pathways involved in the photosynthesis of ethylene/ethane. Through regulating the electronic densities distributed over the active centers and their coordination structures, we can accumulate photogenerated charge carriers to obtain highly active AACs. These AACs can reduce the thermodynamic energy barriers of CO₂ adsorption/activation, resulting in desired intermediates for subsequent reaction pathways. Moreover, these AACs can boost the slow kinetics of CC coupling and competitive hydrogenation steps, resulting in higher evolution rates and selectivities of the targeted renewable hydrocarbons. However, the utilization of AACs anchored photocatalysts for the photosynthesis of ethylene/ethane is not extensively reported. Further insightful mechanistic research is urgently required to fill the current gap of very low production rates and selectivities of ethylene/ethane in realistic conditions. Besides, the exploration of various AACs anchored photocatalysts with remarkable performances will greatly facilitate industrial-scale applications, and meet the high demand in this area.