Effect of Bimetallic Dimer-Embedded TiO2(101) Surface on CO2 Reduction: The First-Principles Calculation

The first-principles calculation was used to explore the effect of a bimetallic dimer-embedded anatase TiO2(101) surface on CO2 reduction behaviors. For the dimer-embedded anatase TiO2(101) surface, Zn-Cu, Zn-Pt, and Zn-Pd dimer interstitials could stably stay on the TiO2(101) surface with a binding energy of about −2.36 eV, as well as the electronic states’ results. Meanwhile, the results of adsorption energy, structure parameters, and electronic states indicated that CO2 was first physically and then chemically adsorbed much more stably on these three kinds of dimer-embedded TiO2(101) substrate with a small barrier energy of 0.03 eV, 0.23 eV, and 0.12 eV. Regarding the reduction process, the highest-energy barriers of the CO2 molecule on the Zn-Cu dimer-embedded TiO2(101) substrate was 0.31 eV, which largely benefited the CO2-reduction reaction (CO2RR) activity and was much lower than that of the other two kinds of Zn-Pt and Cu-Pt dimer-TiO2 systems. Simultaneously, the products CO* and *O* of CO2 reduction were firmly adsorbed on the dimer-embedded TiO2(101) surface. Our results indicated that a non-noble Zn-Cu dimer might be a more suitable and economical choice, which might theoretically promote the designation of high CO2RR performance on TiO2 catalysts.


Introduction
Global warming is a serious challenge for human activity today; it is also a global environmental issue recognized by the international community [1,2]. The main reason for global warming is the results of the rise in global population [3][4][5], solid wastes [6], as well as the correspondingly dramatic and continuing uptrend in the concentration of greenhouse gases worldwide, of which carbon dioxide (CO 2 ) is the largest contributor [7]. Three kinds of CO 2 -conversion strategies exist: electrocatalysis, thermal-catalysis, and photocatalysis, to the direct conversion of CO 2 into valuable fuel and chemical products under ambient conditions [8][9][10][11], which has attracted increasing attention from researchers [12][13][14][15][16]. However, the CO 2 -reduction reaction (CO 2 RR) still suffers from low faradaic efficiency, low conversion efficiency, sluggish kinetics of the primary side reaction of hydrogen-evolution reaction, and high overpotential, which have greatly hindered its further practical applications. Based on this, transition-metal (TM)-based catalysts have been applied to improve the CO 2 RR performance [17], as well as waste-originated biorenewables and other catalysts [18][19][20][21][22]. 2

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The well-noted single-atom catalyst (SAC) was first attempted to design and synthesize novel materials by defect, modification, high-temperature shockwave, and self-assembly, which have enabled various catalytic conversions, including CO 2 RR with extremely higher atom efficiency, activity, and durability [23][24][25][26][27]. Double-atom catalysts (DACs) such as Cu-Pt [28][29][30][31], Cu-Au [32][33][34], and Pd-Au [35] in many mainstream reactions have also been experimented, characterized, and developed to improve CO 2 -reduction-reaction efficiency, offering a fascinating catalyst model for theorists to explore, and hence gaining enormous attention. DACs usually exhibit better catalytic performance than SACs [28,34,35]. This is mainly due to their tuning diversity and synergistic effect between adjacent active sites of metal dimers, resulting in low-coordination metal atoms and an extra metal site. The interaction of different metal sites could accelerate the intrinsic CO 2 RR performance of bimetallic dimer-embedded catalysts. For example, Baldoví et al. [34] loaded Au-, Cu-, and Au/Cu-alloy NPs on the surface of TiO 2 . The study showed that the introduction of Cu into Au/Cu-TiO 2 could boost the transfer of the CB electrons in TiO 2 to CO 2 , resulting in a higher conversion efficiency toward CO 2 reduction to CH 4 . Zheng et al. [35] introduced a secondary metal (Au) to support the Pd catalyst, and Au concentration could alter the dominant reaction pathway, which was much more favorable for CO 2 reduction performance. Therefore, the combination of non-noble metals and noble metals and even the non-noble metal dimers embedded on TiO 2 substrate might be a novel and effective method to promote CO 2 adsorption and conversion.
Meanwhile, many studies [36][37][38][39] found that CO was an important intermediate in the transformation of CH 4 , CH 3 OH, and HCHO. The CO 2 -reduction reaction can be regarded as a "series reaction". CO 2 molecules need to be converted into CO first to obtain the target gas product. Therefore, it is a feasible strategy to design bimetallic dimer TiO 2 catalysts that can firmly adsorb CO to achieve CO 2 reduction to CH 4 .
Based on the first-principles calculation, we explored CO 2 reduction on TiO 2 with M-N (M, N = Cu, Pt, Zn) dimer doping. We mainly calculated both the binding energy and charge distributions of the bimetallic dimer-embedded A-TiO 2 (101) substrate and the CO 2 molecule adsorbed on these dimer-embedded A-TiO 2 (101) catalysts, respectively. We also calculated the Bader charge and Charge Density Difference (CDD) from the TiO 2 substrate to the bimetallic dimer and from the bimetallic dimer-embedded TiO 2 to CO 2 gas molecule. The results indicated that the structure of the bimetallic dimer-embedded TiO 2 (101) surface was stable, and the CO 2 molecule could stably stay on the bimetallic dimer-embedded TiO 2 (101) surface. Then, the CO 2 reduction reactivity was proposed. The energy barrier of the Zn-Cu dimer-embedded TiO 2 (101) surface was as low as 0.31 eV, which was a much more suitable choice as well as advantageous in terms of economy. Therefore, the findings of this study might theoretically provide an effective strategy for the designing of effective CO 2 -reduction TiO 2 -based catalysts.

Computational Details
Based on the density functional theory, all first-principles calculations were used in the CO 2 adsorption and reduction on the bimetallic dimer-embedded A-TiO 2 (101) surface. Projected augmented-wave pseudopotentials, generalized-gradient-approximation (GGA) method [40][41][42], and Perdew-Burke-Ernzerhof (PBE) functional were employed using Vienna Ab-initio Simulation Package (VASP, vasp.5.4.1, Hafner team of University of Vienna, Vienna, Austria) [43][44][45] through the computational simulations. For the kinetic processes, the previous studies indicated that the GGA-PBE approach could correctly describe COoxidation, CO 2 -dissociation, and O-diffusion [46,47] reaction paths on the surface. The cutoff of the kinetic energy for the plane wave was set to be 450 eV. Further, 2 × 2 × 1 was used for the Monkhorst-Pack k-point mesh during geometry optimizations, and the corresponding electronic structure calculations were set as 3 × 3 × 1. The convergence criterion for the electronic was chosen as 10 −5 eV, and the maximum force of each atom for ionic relaxation was 0.02 eV/Å. Regarding the dissociation and diffusion of the CO 2 reduction process, the transition state (TS) search was performed by the climbing-image  Zn-Pt, and Cu-Pt interstitials of the TiO 2 (101) surface from a side view, respectively. The isosurface value was set as 0.0025 e/bohr 3 . The electron accumulation is denoted with the yellow regions; green indicates electron depletion. The Cu atom in dark blue color, Pt atom is pale, and Zn atom in light gray color.  (2) , and Ti and metal N atom, respectively. The angle of O 2c -M-O 2c is equal to ∠O 2c -M-O 2c . The binding energies of bimetallic atoms on A-TiO 2 (101) (E b ) and the charge transfer from A-TiO 2 to bimetallic atoms (∆Q). M stands for the former metal atom, and N stands for the later one. Bader charge analysis and CDD were also performed to further study the stability of the bimetallic dimer-embedded TiO 2 surface. As listed in Table 1, about 0.85 e, 0.52 e, and 0.60 e were transferred from the embedded Zn-Cu, Zn-Pt, and Cu-Pt dimers to the A-TiO 2 (101) substrate for these three different configurations (Figure 1), respectively, which were mainly accepted by the neighboring O atoms. The interaction between embedded bimetallic dimers and A-TiO 2 (101) surface led to the charge redistribution and the accumulation of electrons around the Zn-O, Zn-O, Cu-Pt bonds for Zn-Cu, Zn-Pt, and Cu-Pt dimer embedded TiO 2 systems, respectively, which could be confirmed by CDD, as shown in Figure 1d-f. Further, the partial density of states (PDOS) of three dimer-TiO 2 (101) systems, as shown in Figure 2, revealed many impurity peaks induced by 2p orbital electrons of O atom and 3d orbital electrons of Zn and Cu atoms, indicating that the chemical activity of embedded TiO 2 surface was relatively stronger. Therefore, the results of PDOS, Bader charge analysis, and CDD demonstrated the formation of strong chemical bonds between Zn-Cu, Zn-Pt, Cu-Pt dimers and neighboring oxygen atoms. Therefore, the A-TiO 2 (101) surface embedded with Zn-Cu, Zn-Pt, and Cu-Pt dimers were chemically stable.

One CO 2 Molecule Adsorbed on the Bimetal-TiO 2 (101) Surface
When a CO 2 molecule came close to these three configuration surfaces, it will be adsorbed on the surface. Before further exploring the CO 2 adsorption and reduction on the bimetal-TiO 2 (101) surface, several adsorbed sites were determined, and the most stable surface models for Zn-Cu, Zn-Pt, and Cu-Pt dimers were obtained, as shown in Figure 3. For these three configurations, the adsorption energies for CO 2 molecules of −0.15 eV, −0.17 eV, and −0.13 eV, respectively. The calculation results demonstrated weaker physical adsorption between the CO 2 molecule and bimetallic interstitial. The structure of CO 2 molecule is nearly the same with the gas state with the bond length of 1.18 Å and ∠O (1) -C-O (2) around 180 • . At the same time, the results of the charge transfer from A-TiO 2 to the bimetallic dimer showed few electrons transferring from the CO 2 molecules to the dimer-embedded TiO 2 substrate, namely 0.03 e, 0.02 e, and 0.02 e for Zn-Cu, Zn-Pt, and Cu-Pt dimer-embedded TiO 2 (101) systems, respectively. Thus, the interactions between the CO 2 molecule and the dimer-embedded A-TiO 2 (101) surface were relatively weaker. All these results showed a weaker physical absorption between the system and CO 2 molecules.

One CO2 Molecule Adsorbed on the Bimetal-TiO2(101) Surface
When a CO2 molecule came close to these three configuration surfaces, it will be ad sorbed on the surface. Before further exploring the CO2 adsorption and reduction on the bimetal-TiO2(101) surface, several adsorbed sites were determined, and the most stable surface models for Zn-Cu, Zn-Pt, and Cu-Pt dimers were obtained, as shown in Figure 3 For these three configurations, the adsorption energies for CO2 molecules of −0.15 eV −0.17 eV, and −0.13 eV, respectively. The calculation results demonstrated weaker physica adsorption between the CO2 molecule and bimetallic interstitial. The structure of CO2 mol ecule is nearly the same with the gas state with the bond length of 1.18 Å and ∠O(1)-C-O(2 around 180°. At the same time, the results of the charge transfer from A-TiO2 to the bime tallic dimer showed few electrons transferring from the CO2 molecules to the dimer-em bedded TiO2 substrate, namely 0.03 e, 0.02 e, and 0.02 e for Zn-Cu, Zn-Pt, and Cu-Pt dimer embedded TiO2(101) systems, respectively. Thus, the interactions between the CO2 mole cule and the dimer-embedded A-TiO2(101) surface were relatively weaker. All these re sults showed a weaker physical absorption between the system and CO2 molecules.  Meanwhile, the CO2 gas molecule was more stably adsorbed onto the bimetal-embedded TiO2 surface with a small barrier energy, which was about 0.03 eV, 0.23 eV, and 0.12 eV for the Zn-Cu, Zn-Pt, and Cu-Pt series, respectively. The final stable adsorbed structures are shown in Figure 4a-c. Moreover, the charge distribution from CDD is shown in Figure 4d-f. As shown in Figure 4, the CO2 molecules were bound to the dimer-TiO2(101) system via the bimetallic atoms. The distances between the C atom and the nearest embedded metal of the dimer-TiO2 substrate were shortened to be 1.87 Å, 2.00 Å, and 1.99 Å for Zn-Cu, Zn-Pt, and Cu-Pt adsorption cases, respectively, which were much nearer than those of the initial adsorption cases. Moreover, the bonds of O(1)-C and C-O(2) were enlarged to be about 1.24 Å and 1.30 Å, respectively, and the adsorption energies were elevated up to −0.36 eV, −0.46 eV, and −0.97 eV for Zn-Cu, Zn-Pt, and Cu-Pt cases, respectively. Therefore, the adsorption of CO2 molecules on the dimer-TiO2(101) surface should be a chemical adsorption behavior. Meanwhile, the CO 2 gas molecule was more stably adsorbed onto the bimetal-embedded TiO 2 surface with a small barrier energy, which was about 0.03 eV, 0.23 eV, and 0.12 eV for the Zn-Cu, Zn-Pt, and Cu-Pt series, respectively. The final stable adsorbed structures are shown in Figure 4a-c. Moreover, the charge distribution from CDD is shown in Figure 4d-f. As shown in Figure 4, the CO 2 molecules were bound to the dimer-TiO 2 (101) system via the bimetallic atoms. The distances between the C atom and the nearest embedded metal of the dimer-TiO 2 substrate were shortened to be 1.87 Å, 2.00 Å, and 1.99 Å for Zn-Cu, Zn-Pt, and Cu-Pt adsorption cases, respectively, which were much nearer than those of the initial adsorption cases. Moreover, the bonds of O (1) -C and C-O (2) were enlarged to be about 1.24 A and 1.30 A, respectively, and the adsorption energies were elevated up to −0.36 eV, −0.46 eV, and −0.97 eV for Zn-Cu, Zn-Pt, and Cu-Pt cases, respectively. Therefore, the adsorption of CO 2 molecules on the dimer-TiO 2 (101) surface should be a chemical adsorption behavior. 0.12 eV for the Zn-Cu, Zn-Pt, and Cu-Pt series, respectively. The final stable adsorb structures are shown in Figure 4a-c. Moreover, the charge distribution from CDD shown in Figure 4d-f. As shown in Figure 4, the CO2 molecules were bound to the dime TiO2(101) system via the bimetallic atoms. The distances between the C atom and the nea est embedded metal of the dimer-TiO2 substrate were shortened to be 1.87 Å, 2.00 Å, an 1.99 Å for Zn-Cu, Zn-Pt, and Cu-Pt adsorption cases, respectively, which were mu nearer than those of the initial adsorption cases. Moreover, the bonds of O(1)-C and C-O were enlarged to be about 1.24 Å and 1.30 Å, respectively, and the adsorption energi were elevated up to −0.36 eV, −0.46 eV, and −0.97 eV for Zn-Cu, Zn-Pt, and Cu-Pt case respectively. Therefore, the adsorption of CO2 molecules on the dimer-TiO2(101) surfa should be a chemical adsorption behavior. The partial densities of states of the absorbed CO2 on the bimetallic dimer-embedd TiO2 system (bimetallic dimers were Zn-Cu, Zn-Pt, and Cu-Pt) were calculated to furth The partial densities of states of the absorbed CO 2 on the bimetallic dimer-embedded TiO 2 system (bimetallic dimers were Zn-Cu, Zn-Pt, and Cu-Pt) were calculated to further understand this adsorption performance, which are shown in Figure 5. When CO 2 molecules were adsorbed on the bimetal-embedded TiO 2 surface, we could clearly observe the hybridization with the orbitals of adsorbed CO 2 molecules and 3d orbitals of dopant Zn, Cu, and Pt atoms, implying the formation of Zn-O, Cu-O, and Pt-O bonds on the interface between the adsorbed CO 2 molecules and the dimer-TiO 2 system. Moreover, the Bader charge analysis revealed about 0.82 e, 0.67 e, and 0.54 e transferred from the CO 2 molecules to the dimer-embedded TiO 2 substrate for Zn-Cu, Zn-Pt, and Cu-Pt dimers (see Table 2). In addition to the calculations of charge distribution, we could see that the accumulation of charge occurred around metal-O bonds; the CDD is plotted in Figure 4d (2) bond lengths. Further, we found that the adsorption energy of the Cu-Pt-TiO 2 system was relatively higher, which was two times more than that of other two cases. The absolute value of charge transfer for the Zn-Cu-TiO 2 system, as well as the extents of overlap of electronic states was also larger. Therefore, the aforementioned results, especially the structural variation and charge redistribution of CO 2 molecules and different dimer-TiO 2 substrates, showed stronger chemical interaction between CO 2 molecules and dimer-embedded TiO 2 (101) surface. Thus, CO 2 molecules could stably stay and were chemically adsorbed. The details of initial physical adsorption to final chemical adsorption, along with the reduction process, were as follows.
absolute value of charge transfer for the Zn-Cu-TiO2 system, as well as the extents of overlap of electronic states was also larger. Therefore, the aforementioned results, especially the structural variation and charge redistribution of CO2 molecules and different dimer-TiO2 substrates, showed stronger chemical interaction between CO2 molecules and dimerembedded TiO2(101) surface. Thus, CO2 molecules could stably stay and were chemically adsorbed. The details of initial physical adsorption to final chemical adsorption, along with the reduction process, were as follows. Figure 5. Partial density of states of the absorbed CO2 on bimetal-embedded TiO2 system (bimetallic dimers were Zn-Cu, Zn-Pt, and Cu-Pt). The Fermi level was set to zero and is marked with a grown dashed line. Figure 5. Partial density of states of the absorbed CO 2 on bimetal-embedded TiO 2 system (bimetallic dimers were Zn-Cu, Zn-Pt, and Cu-Pt). The Fermi level was set to zero and is marked with a grown dashed line. Table 2. Calculated results for a CO 2 molecule on an A-TiO 2 (101) surface during the adsorption process. d M-N and d O(1)-C are the bond lengths between metal M and metal N, and between the O (1) atom and the C atom, respectively, for chemical adsorption structure. E CO2ads stands for the adsorption energy of CO 2 adsorbed on TiO 2 (101) surface. ∆Q is the charge transfer from bimetallic dimer-embedded TiO 2 to CO 2 molecule. M stands for the former metal atom, and N stands for the latter one.

CO 2 Reduction on the Zn-Cu, Zn-Pt, and Cu-Pt Dimer-Embedded Anatase TiO 2 (101) Surface
After determining the structure and CO 2 adsorption of bimetallic dimer-embedded on the A-TiO 2 (101) surface, we first discussed the effect of the Zn-Cu dimer-embedded A-TiO 2 (101) substrate on the reduction of the CO 2 molecule. We chose the direct reduction pathway. We determined the optimized structures of the TSs across dissociation and diffusion processes, as well as the products involved in this pathway, which are shown in Figure 6. For the Zn-Cu-CO 2 system, CO 2 molecules subsequently dissociated into the adsorbed *CO of the O-C-Cu bond and *O* bonded with Cu, Zn, and Ti atoms onto the surface via the transition states TS1, TS2, and TS3 and intermediate states A2 and A3. The asterisk (*) is denoted as the adsorbed sites. When a CO 2 molecule was adsorbed on the surface of the Zn-Cu dimer-embedded A-TiO 2 (101) surface, the Cu and Zn atoms could diffuse to adsorb the CO 2 molecule and the Zn-Cu bond was broken, which resulted in the formation of new bonds of Cu-O (1) , Cu-C, Cu-Ti, and Zn-O (2) . The energy barrier for TS1 was only 0.03 eV, and then the CO 2 molecule could be stably adsorbed onto the surface of the TiO 2 system. After a CO2 molecule was adsorbed on the A-TiO2(101) surface, the structure B1 was first obtained easily without nearly any difference with the independent CO2 gas phase and the Zn-Pt-TiO2 system, as shown in Figure 1b, indicating the physical adsorption of the stable structure B1. For the structure B1, the embedded Zn and Pt atoms were particularly prone to adsorb CO2 molecules. Then, a much more stable structure B2 was obtained through TS4 with an energy barrier of 0.23 eV (Figure 7a); the system energy slightly decreased by 0.28 eV. As a result, the two new bonds of Pt-C and Zn-O(2) were formed, and the adsorption energy of the CO2 molecule increased to −0.46 eV. The adsorbed CO2 molecule on the surface became much more stable via chemical adsorption. For B2 as the initial configuration, the adsorbed CO2 molecule was dissociated on the Zn-Pt dimer-embedded TiO2(101) surface, as shown in Figure 7b. The dissociation energy barrier of the adsorbed CO2 molecule was 1.69 eV, which was much higher than that in the Zn-Cu case, as shown in Figure 6b. In the transition state TS5, the adsorbed CO2 molecule was dissociated, along with the breaking of C-O(2) and Pt-Ti bonds and the formation of new bonds of Pt-O(2) and Pt-O3c on the subsurface. For the whole reaction path of C1 → TS4 → C2 → TS5 → C3, the highest-energy barrier was 1.69 eV, and the system energy decreased by 0.9 eV. The results also demonstrated that the dissociated CO was stably As shown in Figure 6b, A2 was the initial configuration and the CO 2 gas phase dissociated. A lower energy of 0.31 eV was needed for the dissociation step. In this step, the O (1) atom diffused away and led to the breaking of the Cu-O (1) bond, while the O (2) atom moved a little nearer and then bonded with Cu, Zn, and Ti atoms, which were much closer to the A-TiO 2 (101) surface through TS2. With a decrease of 0.84 eV, the Zn-Cu-TiO 2 (101) system achieved a more stable structure than A2 after this transformation. However, A3 was a metastable configuration, since C and O (2) from the gas phase exhibited more polarity. The interaction with the surface Cu atom was enhanced, and the more active atom of embedded Cu easily diffused outward to be adsorbed by OC* to a more stable site by an energetic driving force. With a tiny energy barrier of 0.17 eV, the diffusion of A3 transforming into A4 was achieved easily. As a result, with the breaking of the Cu-Ti bond, the new bonds of O (1) -C, C-Cu, and Cu-O (2) were aligned in a straight line with the bond lengths of 1.15 Å, 1.76 Å, and 1.79 Å. For the whole reaction process of A1 → A2 → A3 → A4 (Figure 6), the highest-energy barrier and the total reaction energy of CO 2 reduction on the Zn-Cu-TiO 2 system were 0.31 eV and 0.72 eV, respectively, corresponding to the transformation from A2 to A4. The results indicated that the reaction of CO 2 dissociation became much easier with the help of the Zn-Cu dimer embedded on the A-TiO 2 (101) surface. The embedded dimer might also contribute a more active site on the surface and improve the CO 2 reduction behaviors. Meanwhile, the configuration structure also became more stable upon CO 2 adsorption and reduction reaction on the dimer-embedded A-TiO 2 surface.
After a CO 2 molecule was adsorbed on the A-TiO 2 (101) surface, the structure B1 was first obtained easily without nearly any difference with the independent CO 2 gas phase and the Zn-Pt-TiO 2 system, as shown in Figure 1b, indicating the physical adsorption of the stable structure B1. For the structure B1, the embedded Zn and Pt atoms were particularly prone to adsorb CO 2 molecules. Then, a much more stable structure B2 was obtained through TS4 with an energy barrier of 0.23 eV (Figure 7a); the system energy slightly decreased by 0.28 eV. As a result, the two new bonds of Pt-C and Zn-O (2) were formed, and the adsorption energy of the CO 2 molecule increased to −0.46 eV. The adsorbed CO 2 molecule on the surface became much more stable via chemical adsorption. For B2 as the initial configuration, the adsorbed CO 2 molecule was dissociated on the Zn-Pt dimerembedded TiO 2 (101) surface, as shown in Figure 7b. The dissociation energy barrier of the adsorbed CO 2 molecule was 1.69 eV, which was much higher than that in the Zn-Cu case, as shown in Figure 6b. In the transition state TS5, the adsorbed CO 2 molecule was dissociated, along with the breaking of C-O (2) and Pt-Ti bonds and the formation of new bonds of Pt-O (2) and Pt-O 3c on the subsurface. For the whole reaction path of C1 → TS4 → C2 → TS5 → C3, the highest-energy barrier was 1.69 eV, and the system energy decreased by 0.9 eV. The results also demonstrated that the dissociated CO was stably adsorbed on the Zn-Pt dimer-embedded TiO 2 (101) surface, and the final structure B3 also became much more stable.
For the Cu-Pt case, the structures C1 and C4 were initial and final configurations; the dissociation-reaction energy and energy barrier were −1.34 eV and 1.58 eV, respectively. Similarly to the former two dimer-embedded TiO 2 (101) surfaces, a more stable structure, C2, was obtained with a lower-energy barrier of 0.12 eV. Obviously, the new Pt-C and Cu-O (2) chemical bonds of 1.99 Å and 1.90 Å, were formed. The C-O (2) bond was strengthened to be 1.30 Å with a small bend of CO 2 molecules, while the Cu-Pt bond was broken. Subsequently, the adsorbed CO 2 molecule began to dissociate, as shown in Figure 8b. In the transition state TS7, the C-O (2) bond of the CO 2 molecule was initially broken, with a dissociationenergy barrier of 1.58 eV. As a result, O (2) atoms moved nearer to the TiO 2 (101) surface, and a relatively more stable structure C3 was obtained, along with the formation of new bonds of O (2) -Cu, O (2) -Pt, and O (2) -Ti. At the same time, C3 was a metastable configuration. The Pt atom could easily move out to be grabbed by the *CO adatom during TS7 and TS8 with the distance varying from 1.99 Å, 1.85 Å to 1.82 Å via an energetic driving force. Via the diffusion of TS8, the embedded Pt atom of structure C4 was more attracted to the *CO adatom with exothermic energy and energy barrier of 0.18 eV and 0.7 eV, respectively. For the C1 → C2 → C3 → C4 reaction path (Figure 8), the highest dissociation energy barrier was 1.59 eV, between those of TS5 and TS2 (TS2: 0.31 and TS5: 1.69 eV, respectively). The total energy of structure C4 was reduced by 1.34 eV, which was much higher than that of the two former systems. It showed that with the help of Cu-Pt dimer atoms, the dissociation process of CO 2 on the A-TiO 2 (101) surface also required a relatively high-energy barrier.
We studied the adsorption and reduction performance of CO 2 molecules on three configurations of bimetallic dimer-embedded TiO 2 (101): Zn-Cu, Zn-Pt, and Cu-Pt. We found unpaired electrons induced by the transfer of the bimetallic dimer to the O atom of the TiO 2 substrate. While a CO 2 molecule appeared on the TiO 2 (101) surface, a partial excess charge of the bimetallic dimer was used to absorb the CO 2 gas molecule. However, the adsorption energy was relatively low, indicating only weaker physical adsorption. The final product was mainly CO, with the O atom bonded with the surrounding metal atom due to the CO 2 -dissociation process on the bimetallic dimer-embedded TiO 2 (101) surface. adsorbed on the Zn-Pt dimer-embedded TiO2(101) surface, and the final structure B3 also became much more stable. For the Cu-Pt case, the structures C1 and C4 were initial and final configurations; the dissociation-reaction energy and energy barrier were −1.34 eV and 1.58 eV, respectively. Similarly to the former two dimer-embedded TiO2(101) surfaces, a more stable structure, C2, was obtained with a lower-energy barrier of 0.12 eV. Obviously, the new Pt-C and Cu-O(2) chemical bonds of 1.99 Å and 1.90 Å, were formed. The C-O(2) bond was strengthened to be 1.30 Å with a small bend of CO2 molecules, while the Cu-Pt bond was broken. Subsequently, the adsorbed CO2 molecule began to dissociate, as shown in Figure 8b. In the transition state TS7, the C-O(2) bond of the CO2 molecule was initially broken, with a dissociation-energy barrier of 1.58 eV. As a result, O(2) atoms moved nearer to the TiO2(101) surface, and a relatively more stable structure C3 was obtained, along with the formation of new bonds of O(2)-Cu, O(2)-Pt, and O(2)-Ti. At the same time, C3 was a metastable configuration. The Pt atom could easily move out to be grabbed by the *CO adatom during TS7 and TS8 with the distance varying from 1.99 Å, 1.85 Å to 1.82 Å via an energetic driving force. Via the diffusion of TS8, the embedded Pt atom of structure C4 was more attracted to the *CO adatom with exothermic energy and energy barrier of 0.18 eV and 0.7 For the CO 2 reduction process on the Zn-Cu-embedded TiO 2 (101) surface, the highestenergy barrier was as low as 0.31 eV with the breaking of the C-O (2) bond and the formation of Cu-O (2) , Zn-O (2) , and Ti-O (2) bonds, which was relatively suitable to produce CO and O adatoms easily. However, relatively higher-energy barriers existed for Zn-Pt and Cu-Pt dimer-embedded TiO 2 (101) systems, namely 1.69 eV and 1.58 eV, respectively. As shown in Figures 7b and 8b, the C-O (2) bonds of these two systems were also broken, besides the formation of three metal-O (2) bonds. However, the Zn-Pt-TiO 2 system had higher reaction energy of 0.62 eV, promoting thermal-catalytic performance. With lower barrier energy of 0.31 eV, the Zn-Cu dimer-embedded TiO 2 (101) surface might be a much more convenient design for the CO 2 RR of the bimetallic dimer-embedded TiO 2 substrate, as well as a more economical method compared with noble metals such as Pt, Au, and Ag. Therefore, the findings of this study might theoretically provide an effective strategy for the designing of effective CO 2 -reduction TiO 2 -based catalysts.
found unpaired electrons induced by the transfer of the bimetallic dimer to the O atom of the TiO2 substrate. While a CO2 molecule appeared on the TiO2(101) surface, a partial excess charge of the bimetallic dimer was used to absorb the CO2 gas molecule. However, the adsorption energy was relatively low, indicating only weaker physical adsorption. The final product was mainly CO, with the O atom bonded with the surrounding metal atom due to the CO2-dissociation process on the bimetallic dimer-embedded TiO2(101) surface. We studied the adsorption and reduction performance of CO2 molecules on three configurations of bimetallic dimer-embedded TiO2(101): Zn-Cu, Zn-Pt, and Cu-Pt. We

Conclusions
In this study, the enhancing effects of the Zn-Cu, Zn-Pt, and Cu-Pt dimers on the CO 2 -reduction reaction of the A-TiO 2 (101) surface were discussed. When the bimetallic dimer was embedded on the TiO 2 (101) surface, the configuration was relatively more stable with higher binding energy. Meanwhile, the adsorption energy of CO 2 molecules on the bimetallic dimer-embedded TiO 2 (101) surface was relatively lower, indicating a weaker interaction. Moreover, the Bader charge analysis exhibited only 0.03 e transferring from the CO 2 molecule to the bimetal-embedded TiO 2 (101) surface, demonstrating only physical adsorption behavior. After a lower-energy barrier, the higher adsorption energy was up to −0.46 eV for the Zn-Pt-TiO 2 system and 0.82 eV of the Zn-Cu-TiO 2 system, respectively. Furthermore, the surface-electronic states also implied stronger interaction between CO 2 molecules and the bimetallic dimer-embedded TiO 2 (101) surface.
Regarding the CO 2 dissociation process, the results showed that the dissociation barriers of the CO 2 molecule with Zn-Cu, Zn-Pt, and Cu-Pt dimers embedded on the TiO 2 (101) surface had the values of 0.31 eV, 1.69 eV, and 1.58 eV, respectively, which benefited the CO 2 -reduction reaction (CO 2 RR) activity. Meanwhile, it was observed that the products *CO and *O* of CO 2 reduction were firmly adsorbed on the dimer-embedded TiO 2 (101) surface. In addition, the bonds of metal-O (2) -metal were less than 2.00 Å, and metal-C and C-O (1) bonds were relatively shortened to be 1.76 Å and 1.15 Å, respectively. From the viewpoint of the activation barrier and reaction energy, the Zn-Cu dimer-embedded