Computational Identification of High-Efficiency Dual-Metal-Atom-Modified Ti-Metal–Organic Frameworks for Photocatalytic CO ₂ Reduction to Value-Added Chemicals

Open AccessCCS ChemistryRESEARCH ARTICLES29 Aug 2024Computational Identification of High-Efficiency Dual-Metal-Atom-Modified Ti-Metal–Organic Frameworks for Photocatalytic CO2 Reduction to Value-Added Chemicals Shuang Wang, Xiaowa Nie, Chunshan Song and Xinwen Guo Shuang Wang State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Materials, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 , Xiaowa Nie *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Materials, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 , Chunshan Song *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Faculty of Science, The Chinese University of Hong Kong, Shatin, NT, Hong Kong 999077 and Xinwen Guo *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Materials, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 Cite this: CCS Chemistry. 2024;0:1–12https://doi.org/10.31635/ccschem.024.202404583 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Selective photocatalytic reduction of carbon dioxide (CO2) to specific C2 and C1 products is highly desirable. The present work is a computational screening of the photocatalytic performance of a series of Cu-M dual-metal-atom-modified Ti-MOF catalysts (Cu-M/Ti-BPDC, M = Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, Sn, and Bi; BPDC = 2,2′-bipyridine-5,5′-dicarboxylic acid) for CO2 reduction to value-added chemicals by means of density functional theory (DFT) calculations. Among the screened Cu-M/Ti-BPDC catalysts, Cu-In/Ti-BPDC shows the highest activity and selectivity for CO2 reduction to C2 products (especially C2H4). The Cu-In/Ti-BPDC catalyst has a relatively small limiting free energy change of 1.37 eV, over which the *CO2-*COOH → *COOH-*COOH is identified as the rate-determining step. Compared with other screened candidates, the moderate binding strength of the *CO2 reactant and *COOH-*COOH intermediate with Cu-In/Ti-BPDC is responsible for excellent catalytic activity for C2 products formation, which can be better understood by the corresponding structure-activity relationship analysis. In addition, the Cu-Os/Ti-BPDC and Cu-Sn/Ti-BPDC catalysts are identified as promising candidates for CO2 reduction to CH3OH and HCOOH, respectively. Therefore, by carefully tuning the composition of dual-metal-atom active centers as well as their interactions with the MOF, the catalytic activity and product selectivity of CO2 photoreduction can be effectively regulated. Download figure Download PowerPoint Introduction The massive consumption of fossil fuels has led to a sharp increase in the amount of carbon dioxide (CO2) in the atmosphere, which contributes to a series of environmental problems such as the greenhouse effect and climate change.1,2 Hydrogenation of CO2 with H2 derived from renewable energy sources to produce value-added chemicals and fuels is of great interest because of its promising role in a sustainable carbon cycle. This process not only reduces the level of CO2 in the atmosphere, but it also decreases our dependence on fossil fuels.3 Among the various technologies for CO2 conversion, photocatalytic reduction is particularly attractive because it makes full use of solar energy, a renewable energy source, which can proceed under mild reaction conditions and has the remarkable advantage of its energy efficiency.4,5 Reduction of CO2 to C2 products such as ethanol (C2H5OH) and ethylene (C2H4) is economically more attractive but technically challenging with low yield and selectivity.6 This is due to the complex multielectron transfer, multiproton activation, and C–C coupling processes involved in CO2 photoreduction.7 Therefore, efficient photocatalysts with tailored performance are urgently needed to improve the activity and selectivity of CO2 reduction to C2 products. Single-atom catalysts (SACs) have attracted increasing attention in the field of catalysis because of their definite active sites, maximum atomic utilization, and minimization of metal usage. Their unique advantages in enhancing the light collection, charge transfer rate, and surface reaction have promoted the exploration of SACs in the field of photocatalysis.8,9 Metal–organic frameworks (MOFs) can be used as an ideal support for stabilizing single atoms (SAs).10,11 The unique composition and structure of MOFs can provide abundant coordination sites to anchor the SAs and prevent them from aggregating, thus improving the dispersion of SAs and catalyst stability. The single-metal-atom-modified MOFs have become promising photocatalysts for CO2 reduction to value-added chemicals.12,13 For example, Wang et al.14 introduced isolated Cu into UiO-66-NH2 (Cu SAs/UiO-66-NH2) to reduce CO2 to methanol with a formation rate of 5.33 μmol g−1 h−1 under visible light irradiation. Experimental characterization combined with density functional theory (DFT) calculations revealed that the isolated Cu on UiO-66-NH2 is conducive to the enrichment of electrons on the catalyst and facilitates the CO2 reduction to CH3OH by reducing the barriers to the formation of *CO and *CHO intermediates.14 In addition, Ye et al.15 successfully synthesized a Co-porphyrin-based MOF catalyst (MOF-525-Co) for CO2 photoreduction to C1 chemicals, with which the yields of CO (200.6 μmol g−1 h−1) and CH4 (36.67 μmol g−1 h−1) from CO2 are significantly increased compared to those with the parent MOF. The excellent activity of this catalyst is mainly attributed to the atomically dispersed Co sites embedded in the porphyrin ring which are more favorable for CO2 adsorption and activation, leading to enhanced photocatalytic activity.15 Recently, our group successfully synthesized a novel Fe/Ti-BPDC (BPDC = 2,2′-bipyridine-5,5′-dicarboxylic acid) MOF photocatalyst with atomic dispersion of Fe sites, which exhibited high activity for CO2 reduction to HCOOH, whose yield reached 703.9 μmol g−1 h−1 and selectivity greater than 99.7%.16 Inspired by this experimental work, we proposed a new strategy for designing efficient single-metal-atom-modified Ti-BPDC photocatalysts for CO2 reduction to C1 products through structural and electronic modulation by performing DFT calculations. Computational results on the structure-activity volcano curves revealed that in addition to the superior candidate Fe/Ti-BPDC, the single-metal-atom Ag/Ti-BPDC, Cr/Ti-BPDC, and Mn/Ti-BPDC are also excellent candidates for CO2 photoreduction to HCOOH with smaller limiting free energy changes, while the Pd/Ti-BPDC is an ideal candidate for CH3OH production.11 Although SACs have been widely used for CO2 reduction, the activity and selectivity of SACs are still insufficient for producing C2 products because it is difficult for a single active metal site to complete the complex catalytic process of multielectron and multiproton reaction.17 Therefore, how to design an efficient photocatalyst possessing multiple functions with multiple sites to achieve synergistic effect has become an attractive focus in photocatalytic studies. For example, two different metal atoms in dual metal site catalysts (DMSCs) can be used as active sites at the same time to cooperatively catalyze a series of complex processes such as CO2 adsorption, activation, proton activation, electron transfer, chemical bond cleavage, and reconstruction.18–20 The DFT work from Jiang and coworkers21 showed that the dual metal sites coordinated with N atoms have strong synergistic effects and exhibit high catalytic activity compared to a single metal site. CuIn bimetallic catalysts have been considered as promising catalysts for CO2 reduction. The recent work in our group successfully synthesized a bimetallic photocatalyst consisting of atom-dispersed In and Cu anchored on polymeric carbon nitride (InCu/PCN), over which the selectivity to C2H5OH from CO2 photoreduction reached 92%. The DFT calculations combined with photocatalytic experiments indicated that there is a strong interaction between In, Cu, and PCN support, and the electronic structure of Cu is modulated by the In-N-Cu coordination, which is conducive to the adsorption of reactants and key intermediates, reducing the C–C coupling barrier.22 Dual metal sites have a synergistic effect on the bonding of C2 intermediates, inducing specific pathways in the conversion of CO2, providing a viable pathway to enhance the yield and selectivity to C2 products.23,24 Rational design of high-efficiency DMSCs requires a clear understanding of catalyst structure, active sites, and their interactions with the reactant molecules, especially the relationship between the synergistic effect of dual metal sites and the catalytic performance.25,26 It is well known that Cu-based catalysts show particular advantages for C–C coupling to C2+ products in some CO2 reduction reactions.27–32 Inspired by the experimental and computational results of single-metal-atom-modified Ti-BPDC MOF catalysts for CO2 photoreduction to C1 chemicals and the potential application of DMSCs in CO2 reduction to C2 products, in this work, we systematically investigate the photocatalytic performance of a series of Cu-based dual-metal-atom-modified Ti-BPDC MOFs (Cu-M/Ti-BPDC, M = Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, Sn, and Bi) for CO2 reduction to C2 products using DFT calculations. The computational results correlate the Gibbs free energies of key reaction intermediates in CO2 reduction pathways with limiting free energy changes to identify the structure-activity relationships. Based on the calculated reaction energy diagrams, we found that the reduction steps of *CO2-*COOH → *COOH-*COOH and *CO-*CHO → *CHO-*CHO on Cu-M/Ti-BPDCs have large free energy changes. By screening these candidates, Cu-In/Ti-BPDC was identified as the most active photocatalyst for CO2 reduction to C2 products (especially C2H4) with a limiting free energy change of 1.37 eV associated with the *CO2-*COOH → *COOH-*COOH step. The good performance of Cu-In/Ti-BPDC is due to the fact that the difference in electronegativity between Cu and In results in the local charge redistribution, which facilitates the adsorption and conversion of important C2 intermediates. In addition, the calculation results also show that the selectivity of CO2 reduction to C2 on Cu-In/Ti-BPDC is higher than that of C1 products by comparing their formation-limiting free energy changes, which further validates that Cu-In/Ti-BPDC is more selective to C2 products. Finally, we predict that Cu-Sn/Ti-BPDC and Cu-Os/Ti-BPDC are more active and selective to HCOOH and CH3OH products, respectively among the screened candidates, better than previously identified single-metal-atom-modified Ti-BPDC photocatalysts (Fe/Ti-BPDC and Pd/Ti-BPDC). Therefore, by carefully modulating the bimetallic combination and coordination interaction with the MOF support, Cu-M/Ti-BPDCs are expected to be promising candidates for selective CO2 reduction. Computational Methods Based on previous work on single-atom catalysts by He and coworkers,33 as shown in Supporting Information Figure S1. Cu was chosen as the host metal and other metals including Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, Sn, and Bi were separately introduced as the second metal component to construct the dual-metal-atom-modified Ti-BPDC MOFs (denoted as Cu-M/Ti-BPDC). The optimized structures of Cu-M/Ti-BPDC catalysts are shown in Supporting Information Figure S2. The computational parameters used in this work are the same as those adopted in our previously published work on single-metal-atom-modified Ti-MOFs for CO2 photocatalytic reduction.11 Details of computational methods are provided in the Supporting Information Tables S1 and S2. In this work, we investigate the reaction pathways for the formation of different C2 products, including C2H4 and C2H5OH over the Cu-M/Ti-BPDC catalysts. The process involves multiple net proton-coupled electron transfer (PCET) steps, including the following reactions: 2CO2 + 12H+ + 12e− → C2H4 + 4H2O; 2CO2 + 12H+ +12e− → C2H5OH + 3H2O, in which protons and photogenerated electrons are added to the reaction intermediates progressively in the presence of a catalyst, ultimately resulting in the formation of C2 products. The equations for calculating the adsorption energy (Eads) and free energy (G) of adsorbed species are given in the Supporting Information. The formula for calculating the free energy change (ΔG) of each elementary step involved in CO2 reduction is given and explained in the Supporting Information. Results and Discussion Structures and electronic properties of Cu-M/Ti-BPDC catalysts Figure 1a shows a schematic illustration of the geometry of Cu-M/Ti-BPDC studied in this work, which is characterized by the formation of two stable metal-N bonds for each single-atom metal with the BPDC linker. According to previous experimental and theoretical studies,11,16,33 influenced by the solvent in photocatalytic reaction, H2O-derived species such as OH* can coordinate with the metal center in single-atom-modified Ti-BPDCs, thus modifying the coordination environment and electronic structure of active sites. Transition metals are important for the catalytic reactions of CO2. For example, PdAg nanoparticles loaded on TiO2 can effectively promote CO2 hydrogenation to HCOOH.34 By introducing Cu and Ni SAs into a metal–organic framework, the MOF-808-CuNi is synthesized, which gave a selectivity up to 99.4% to CH4 in the solar-light-driven conversion of CO2.35 In this work, Cu is used as the host metal; representative 3d metals (Sc, Mn, Fe, and Cu), 4d metals (Nb, Ru, Rh, Pd, In, and Sn), and 5d metals (Os, Ir, Pt, and Bi) are selected as the second components to construct the Cu-M/Ti-BPDC photocatalysts, since these metals are commonly used in CO2 reduction reactions (Figure 1b). In the catalyst structure, the Cu atom shares an OH species with the second component metal, and the two metal centers are ultimately stabilized through the formation of a Cu–O–M bond which separately coordinates with different numbers of the OH species to maintain the system charge balance. The model construction operation of how active metals coordinate with OH species is consistent with our previous work for single-metal-atom-modified Ti-MOFs.11,16 The optimized structures of examined Cu-M/Ti-BPDC catalysts (M = Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, Sn, and Bi) are shown in Supporting Information Figure S2, while Supporting Information Table S3 summarizes the bond parameters of all Cu-M/Ti-BPDC structures. Figure 1 | (a) Schematic illustration of the structure of Cu-M/Ti-BPDC (M = Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, Sn, and Bi). (b) Representative 3d (Sc, Mn, Fe, Cu), 4d (Nb, Ru, Rh, Pd, In, and Sn), and 5d (Os, Ir, Pt, and Bi) metals selected for second component metal screening with Cu as the host metal in Cu-M/Ti-BPDCs.(c) The adsorption-free energies of the most stable configurations of CO2 on Cu-M/Ti-BPDC photocatalysts. Download figure Download PowerPoint In order to gain a better understanding of the interactions between the two metal sites and the Ti-BPDC framework structure as well as the mechanism of electronic regulation, we analyzed the Bader charge, differential charge density, and partial density of states (PDOS) of Cu-M/Ti-BPDC systems. Taking Cu-In/Ti-BPDC as an example ( Supporting Information Figure S3), based on the calculated PDOS, we find that the main peak of the Cu-3d orbital is located in the region near the Fermi level, and there appears an obvious hybridization among the Cu-3d, In-5p, and O-2p orbitals ( Supporting Information Figure S3a). According to the Bader charge analysis, the Cu and In atoms respectively transfer about 0.94 |e| and 1.67 |e| to the bridged O1 atom between the two metals, which leads to the accumulation of positive charge around Cu and In. This result indicates that there is a significant charge transfer within the Cu-O1-In structure. The charge transfer feature can be further verified by the differential charge density calculation ( Supporting Information Figure S3b). An obvious charge redistribution occurs between Cu-O1-In and the surrounding ligand. There is charge accumulation and depletion between the ligand and metal atoms due to the interaction between the 2p orbitals of O atom in the ligand, the 5p orbitals of In atom, and the 3d orbitals of Cu atom. In addition, we also investigate the electronic properties of Cu-Mn/Ti-BPDC (3d), Cu-Fe/Ti-BPDC (3d), Cu-Ru/Ti-BPDC (4d), Cu-Rh/Ti-BPDC (4d), Cu-Os/Ti-BPDC (5d), and Cu-Ir/Ti-BPDC (5d), and the obtained PDOS profiles are shown in Supporting Information Figure S4. Similar to the case of Cu-In/Ti-BPDC, there is a significant hybridization of the In-5p and Cu-3d orbitals with the 2p orbitals of oxygen atoms, indicating strong interactions between the metal atoms and the nearby O atoms in the MOF. Overall, appropriate bond lengths, significant charge transfer, and strong hybridization between M-d/M-p and O-p orbitals demonstrate that the two metal atoms can be stably anchored to the Ti-BPDC framework. CO2 reduction mechanism on Cu-M/Ti-BPDC catalysts After clarifying the structural properties of Cu-M/Ti-BPDC (M = Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, Sn, and Bi), we further investigate the adsorption of CO2 reactant on these candidate materials, which is considered the first and crucial step in the photocatalytic reduction process. Since two CO2 molecules are required to initiate the photocatalytic reduction to C2 products, two OH species coordinated with the metal centers of Cu-M/Ti-BPDC are removed in order to provide adequate adsorption sites for CO2 molecules. It is worth noting that CO2 molecules can be adsorbed on the metal sites through C or O atoms, thus producing two different adsorption configurations, namely *COO and *OCO. After a comprehensive structural examination and optimization, the most stable adsorption structures of CO2 on Cu-M/Ti-BPDC candidates are identified, as shown in Supporting Information Figure S5. We observed that on Cu-M/Ti-BPDC (M = Sc, Fe, Cu, and Bi) both CO2 molecules end up far away from the metal sites and do not bind to the metal centers. However, for Cu-M/Ti-BPDC (M = Mn, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, and Sn), the CO2 molecule does not bond to Cu, but adsorbs on the second component metal site through a M–C bonding (adsorbed as *COO configuration). The calculated adsorption free energies of CO2 on Cu-M/Ti-BPDC catalysts in the most stable configuration are given in Figure 1c. We note that the adsorption energy values of the most stable adsorption configurations of CO2 on Cu-M/Ti-BPDCs are in the range of −1.77 to 0.93 eV. According to previous work by Shi and coworkers and Sun and coworkers,22,36,37 CuIn bimetallic catalysts are promising materials for CO2 reduction. For example, Sun et al.37 used a MOF derivatization preparation strategy to introduce both Cu and In atoms into ZIF-8 for the high-selective conversion of CO2 to methanol. In our recent work, we discover the superior catalytic performance of the InCu/PCN catalyst for the selective production of ethanol from CO2 photoreduction.22 In addition, the correlation between the adsorption strength of the catalyst towards the reactant and its catalytic activity has been identified in the literature.38 Through evaluating the CO2 adsorption strength over Cu-M/Ti-BPDCs and considering the computational efficiency, the Cu-In/Ti-BPDC catalyst with medium CO2 adsorption strength is initially selected as a typical representative to explore the reaction mechanism of photocatalytic CO2 reduction to C2 products. Based on the adsorption configuration of CO2 over the Cu-In/Ti-BPDC catalyst, all possible reaction pathways and intermediates for the reduction of CO2 to C2H4 or C2H5OH products are considered, as illustrated in Figure 2. By comparing the reaction Gibbs free energies, the favorable pathway for CO2 reduction to C2H4 or C2H5OH over Cu-In/Ti-BPDC is identified, as shown in Figure 3a, and the optimized configurations of intermediates involved are shown in Supporting Information Figure S6. Other pathways considered and examined are provided in Supporting Information Figure S7. Figure 2 | Possible reaction pathways for the reduction of CO2 to C2 (C2H5OH and C2H4) products on Cu-M/Ti-BPDCs (M = Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, In, Sn, and Bi). Download figure Download PowerPoint First, the adsorbed CO2 at the In site goes through a hydrogenation step to form the *CO2(Cu)-*COOH(In) intermediate (Supporting Information Figure S6:2) via PCET, which is slightly exothermic by 0.24 eV in the free energy diagram. Formation of the *CO2(Cu)-*COOH(In) species is thermodynamically more favorable than the formation of *COOH(Cu)-*CO2(In) on the Cu side (Supporting Information Figure S7a). Subsequently, the hydrogenation of *CO2(Cu)-*COOH(In) species on the Cu side leads to the formation of *COOH-*COOH intermediate (Supporting Information Figure S6:3), and the ΔG value of this elementary step is calculated to be 1.37 eV (Figure 3a). Immediately thereafter, two protons attack the *COOH-*COOH species, leading to the formation of *CO-*CO species (Supporting Information Figure S6:4), accompanied by the generation of two H2O molecules. The ΔG of this elementary reaction is determined to be −2.50 eV, which is thermodynamically more favorable (Figure 3a). After the release of formed H2O, the *CO-*CO complex can be further reduced in three possible ways: (1) Coupling of CO Species: Two *CO species can couple to form a C2 intermediate (Figure 2). Figure 3 | (a) The calculated Gibbs free energy diagram of the optimal path for CO2 reduction to C2 (C2H5OH and C2H4) products on Cu-In/Ti-BPDC and (b) optimized structures of all states involved in high free-energy change steps (2→3 and 5→6) within the path. Download figure Download PowerPoint (2) Proton Attack on the C Atom: A proton can attack the carbon atom of *CO-*CO on the indium (In) side, forming *CO-*CHO, or on the copper (Cu) side, forming *CHO-*CO ( Supporting Information Figure S7b). (3) Proton Attack on the O Atom: A proton can attack the oxygen atom of *CO-*CO on the In side to form *CO-*COH (Figure 2), accompanied by electron transfer. The calculation results show that the formation of *CO-*CHO ( Supporting Information Figure S6: 5) is slightly endothermic by 0.27 eV (Figure 3a); however, the C2 intermediate (*COCO) formed from *CO-*CO coupling and the intermediate *CO-*COH are unstable over the Cu-In/Ti-BPDC catalyst and thus are excluded in the following study. Then, the resulting *CO-*CHO species continues to proton addition, which has three possible binding sites, i.e., forming *COH-*CHO, *CHO-*CHO, or *CO-*CHOH intermediates, as shown in Supporting Information Figure S7b. Given, the free energy diagram given in Supporting Information Figure S7b, we observe that the formation of *CHO-*CHO, *CO-*CHOH, or *COH-*CHO has uphill free energy of 1.31, 1.96, and 2.17 eV, respectively, indicating that *CO-*CHO reduction to *CHO-*CHO is energetically more favorable than the other two intermediates. It is worth mentioning that the C–C coupling of *CO-*CHO to generate C2 intermediate (*COCHO) is very difficult to achieve on Cu-In/Ti-BPDC due to the structural instability of this species. In the next step, the two *CHO species adsorbed on both metal sites undergo C–C coupling to form the important C2 intermediate *CHOCHO ( Supporting Information Figure S6: 7), which releases the energy of 0.98 eV. Then, the further reduction of *CHOCHO to *CHOCHOH is slightly uphill with a ΔG value of 0.47 eV (Figure 3a). Subsequent conversion of *CHOCHOH species leads to the formation of four possible intermediates, namely *CHOCH+*H2O, *CH2OCHOH, *CHOCH2OH, and *CHOHCHOH. The calculation results on Gibbs free energies in Supporting Information Figure S7c show that *CHOCH2OH is the thermodynamically most favorable intermediate, whose formation releases the energy of 0.65 eV. Then, the formed *CHOCH2OH intermediate is further hydrogenated by PCET to form *CH2OCH2OH, *CHOCH2+*H2O or *CHOHCH2OH species, and the ΔG value for the formation of *CHOHCH2OH is −0.40 eV, which is lower than that for the formation of the other two intermediate species ( Supporting Information Figure S7c), suggesting that *CHOHCH2OH is the preferred intermediate from *CHOCH2OH reduction. In the next step, further reduction of *CHOHCH2OH leads to *CHOH*CH2+*H2O, *CHCH2OH+*, or *CH2OHCH2OH. As observed in Supporting Information Figure S7c, the formation of *CHCH2OH+*H2O and *CH2OHCH2OH species are thermodynamically unfavorable with ΔG values of 0.88 and 1.83 eV, respectively. In contrast, the *CHOHCH2OH reduction to *CHOHCH2+*H2O is thermodynamically more favorable, with a ΔG value of −0.40 eV. For the resulting *CHOHCH2 intermediate, two possible hydrogenation paths lead to the generation of different C2 intermediates, either releasing a H2O molecule to generate the *CHCH2 species (ΔG = −0.09 eV), which in turn continues to reduce to the *CH2CH2 (ΔG = −0.45 eV), or continuing to convert to *CH2OHCH2 species (ΔG = −0.33 eV), which then transforms into the *CH3CH2OH (ΔG = 0.10 eV). Finally, the formed *CH3CH2OH or *CH2CH2 desorbs from the Cu-In/Ti-BPDC catalyst with a desorption-free energy of −0.64 and −0.33 eV, respectively (Figure 3a). Overall, the optimal pathway for photocatalytic reduction of CO2 to C2 (C2H5OH and C2H4) products on Cu-In/Ti-BPDC goes through 2*CO2 → *CO2(Cu)-*COOH(In) → *COOH-*COOH → *CO-*CO → *CO-*CHO → *CHO-*CHO → *CHOCHO → *CHOCHOH → *CHOCH2OH → *CHOHCH2OH → *CHOHCH2 → *CHCH2 or *CH2OHCH2 → *CH2CH2 or *CH3CH2OH → * + C2H4 or * + C2H5OH. According to the free energy diagram in Figure 3a, the two hydrogenation steps of *CO2-*COOH → *COOH-*COOH (ΔG = 1.37 eV) and *CO-*CHO → *CHO-*CHO (ΔG = 1.31 eV) have substantial free energy changes (optimized structures of states involved in these two steps are provided in Figure 3b). Moreover, on the Cu-In/Ti-BPDC catalyst, the C–C coupling is not difficult to achieve, but the hydrogenation reactions before C–C coupling proceed slowly. In addition to the calculations on reaction-free energy changes for CO2 reduction to C2 products, kinetic barriers for crucial steps involved in the optimal pathway are also considered. As shown in Supporting Information Figure S8, the kinetic barriers for the two reduction steps of *CO2-*COOH → *COOH-*COOH and *CO-*CHO → *CHO-*CHO are calculated to be 1.96 and 1.48 eV, respectively, thus the reduction of CO2* to COOH* intermediate determines the overall rate for CO2 to C2 products over Cu-In/Ti-BPDC. With respect to the C–C coupling step of *CHO-*CHO → *CHOCHO, the kinetic barrier is calculated to be 0.54 eV, thus proceeding relatively quickly over Cu-In/Ti-BPDC. In general, the obtained trend based on the kinetic barrier agrees well with the prediction according to reaction-free energy change. CO2 reduction activity and selectivity on Cu-M/Ti-BPDC catalysts From the above calculation results, the reduction steps of *CO2-*COOH → *COOH-*COOH and *CO-*CHO→*CHO-*CHO both have large kinetic barriers and highly uphill free energy changes. Here, we further evaluate the potential of other Cu-M/Ti-BPDCs (M = Sc, Mn, Fe, Cu, Nb, Ru, Rh, Pd, Pt, Os, Ir, Sn, and Bi) as a CO2 reduction photocatalyst by calculating the free energy changes for generation of *COOH-*COOH and *CHO-*CHO intermediates ( Supporting Information Figure S9). The calculation results show that Cu-In/Ti-BPDC has the smallest free energy change values for *COOH-*COOH and *CHO-*CHO formation of the Cu-M/Ti-BPDC candidates. Thus, it is predicted that Cu-In/Ti-BPDC exhibits the highest CO2 reduction activity toward C2 products formation among all the candidates studied. To better understand the difference in CO2 reduction activity of these Cu-M/Ti-BPDC candidates, the adsorption strength of *COOH-*COOH species is further analyzed, which is a key reaction