Abstract
An excellent single-atomic photocatalyst, Ti@C4N3, is theoretically found to effectively convert CO2 to C2H6 by density functional theory (DFT) calculations and non-adiabatic molecular dynamics (NAMD) simulations. The Ti@C4N3 photocatalyst has remarkable stability both thermally, chemically, and mechanically. Electronically, it has strong absorption properties (λ = 327.77 and 529.61 nm), suitable band positions, and a long photogenerated electron lifetime (τe = 38.21 ps), allowing photogenerated electrons to migrate to the surface. Notably, the high-valence active site effectively activates two CO2 through dual activation: Under light irradiation, the weakly adsorbed CO2 undergoes photo-induced activation by the photoelectron of conduction band minimum (CBM); without light, the high Lewis acidity of the Ti site induces CO2 activation through back-donating π-bond. Contrast simulation results uncovered that dual activation of CO2 is attributed to the thermal and photonic synergy. Furthermore, two activated CO2 species under light easily couple to form oxalate with the barrier of 0.19 eV, and further reduced to C2H6 with a low activation energy of 1.09 eV.
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Introduction
The latest few years has witnessed a significant global push towards achieving carbon neutrality as a response to the escalating impact of greenhouse gases (e.g. CO2) on the planet1,2. Compared to other strategies (directly reducing CO2 emission, CO2 capture and storage, chemical conversion of CO2 triggered by heat or electric energy), the conversion of solar-powered CO2 photoreduction to value-added chemicals or fuels3,4 (e.g. CH3OH, CH4, C2H4, and C2H6) is an appealing and green solution to ameliorate energy shortages as well as reduce CO2 emission. Essentially, however, activation of CO2 is both thermodynamically and kinetically difficult because of the two delocalized \({{\rm{\pi }}}_{3}^{4}\) bonds with the bond length of 1.16 Å. Thus, it is challenging to transform CO2 selectively into other chemicals at mild reaction conditions.To this end, much effort has been made to investigate the developments of photocatalysts for CO2 reduction reaction (CO2RR), e.g. metal nanoparticle5, metal oxides6 and single site catalysts7.
Photocatalysis is a light-driven process that uses a semiconductor catalyst to generate excited electrons and holes when exposed to photons with sufficient energy. These charge carriers trigger various redox reactions, resulting in the formation of end products. Within this context, CO2 photoreduction involve the C1 products8 (e.g. CO7, CH3OH9, CH410), C2 products11,12 (C2H613, C2H5OH14, CH3COOH15, C2H43 and so on) and C3 products16 (C3H817, CH3COCH318 and so on). Compared to C1 product, however, rational design of high-performance (e.g. high selectivity) photocatalysts for multi-carbon product is highly challenging. In this vein, single-atom catalysts (SACs)19, featuring catalytic activity adjustability as well as designed synthesis diversity20, shown good photocatalytic performance for CO2RR, e.g. Er1/CN-NT21, Fe SAS/Tr-COF22. Generally, an active site on heterogeneous catalysts can activate only one CO2, leading to C2 coupling steps requiring the synergistic collaboration of adjacent sites. Thus, C2 coupling step during CO2RR process is of great difficult to trigger in catalysts with highly discrete sites (e.g. SACs) due to limited reductive electronic sources as well as high atomically dispersion of active sites23,24,25,26.
Herein, we theoretically designed an outstanding single-atom photocatalysts Ti@C4N3 by a combination of DFT and NAMD simulations. The supporting Ti@C4N3 photocatalyst features excellent stability, good absorption properties, suitable band positions and long photocarrier lifetime. Notably, real-time time dependent DFT (rt-TDDFT) simulation reveal that high-valence Ti site renders the dual activating CO2 on Ti@C4N3: high Lewis-acidity of Ti site can thermally induce activation of CO2 by π back-donating bond without light; and under visible light irradiation the weak-adsorbed CO2 undergoes photo-induced activation by the photoelectron from CBM. Catalytic studies reveal that the two activated CO2 is easily coupled to be oxalate, which is further reduced to be C2H6.
Results
Stability of Ti@C4N3. Via three-step strategies (binding energy, band gap and adsorption energy of CO2), we systematically screen out Ti@C4N3 as a potential photocatalyst (Detailed information see Supplementary Information file). Furthermore, we conduct a comprehensive stability investigation of Ti@C4N3 in three aspects: thermal stability, chemical stability in aqueous liquids, and mechanical stability. Firstly, Ab-initio molecular dynamic (AIMD) simulations were firstly carried out for a duration of 10 ps at a temperature of 500 K and indicate that both the energy and bonds of the structure fluctuates within a constant range, shining light on excellent thermal stability up to 500 K (see Fig. 1a). To further identify the thermal stability of SACs, an essential factor is the potential for single-atom agglomeration on the substrate. Studying the aggregation process of Ti atoms on the C4N3 surface, the climbing image-nudged elastic band (CI-NEB) method was employed to identify diffusion pathway and calculate diffusion barriers (see Supplementary Fig. 9). The results reveal that, when a single Ti atom aggregates with its nearest neighboring Ti atom, the first Ti atom initially diffuses from the current N3 hollow site to a middle site between them, encountering a barrier of Ea = 2.38 eV. Subsequently, the other Ti atom also migrates to the vicinity of this middle site to agglomerate with the first Ti atom, forming a Ti-Ti distance of d(Ti-Ti) = 2.603 Å, with the uphill barrier being Ea = 3.93 eV, which represents the rate-determining step (RDS) for aggregation. The entire process of aggregation between the two Ti atoms is endothermic, involving an enthalpy change of 4.79 eV. These results indicate that the aggregation of single Ti atoms on C4N3 substrate is exceptionally challenging both thermodynamically (∆H = 4.79 eV) and kinetically (Ea = 3.93 eV). Consequently, Ti@C4N3 remains thermodynamically robust under photocatalytic conditions. To gauge chemical stability on aqua liquid environment, a solid−liquid interface model is constructed in Supplementary Fig. 10. The equilibrium structure was used to calculate the kinetic activation energy of Ti atom leaching from N3C site. The “slow-growth” method27 involves selecting the reaction coordinate in which the Ti-N bonds are elongated (see Fig. 1b). The analysis reveals that the free energy progressively increases with the displacement of the Ti atom from the surface, culminating in a value of 2.47 eV at the reaction endpoint, shedding light on the extreme difficulty of the three Ti−N bonds breaking under aqua environment28. Next, mechanical stability are performed by calculating the independent elastic stiffness tensor components. For this two-dimensional (2D) lattice with tetragonal symmetry, the independent elastic constants are C11 = C22 = 136.08 GPa and C12 = 8.96 GPa, fully meeting the Born−Huang criteria (C11 > 0 and C11 > | C12 | )29. Moreover, Ti@C4N3 feature the smaller Young’s modulus of 136.25 GPa than those of many other 2D materials (1000 GPa for graphene30, 330 GPa for MoS231 and 279 GPa for h-BN32). Meanwhile, the Poisson’s ratio (0.066) of Ti@C4N3 reflects its isotropically higher stiffness than that of Zn metal (0.27). Generally, Ti@C4N3 catalyst has excellent mechanical stability.
Optical properties and Photocarrier Dynamics of Ti@C4N3: Electronically, compared to the main contribution of N atoms for valance band maximum (VBM) and CBM of C4N3, the VBM of Ti@C4N3 is also concentrated on the 2p states of N atoms, while the CBM is accumulated at 3d state of Ti atom (see Fig. 2a, b). The pristine C4N3 features conductor property, where the 2p states of N atoms occupy across the Fermi level, while for Ti@C4N3 the 3d state of Ti atom occupy the Fermi level above instead of 2p states in N atoms and effectively reduce band dispersion at the vicinity of Fermi level, rendering to open band gap of Ti@C4N3 (see Fig. 2c). It can be seen from the band gap that Ti@C4N3 (Eg = 0.97 eV) from HSE-06 calculations has two distinct absorption peaks (327.77 and 529.61 nm) show in Fig. 2d, which are redshifted by 52.37 and 254.21 nm compared to the maximum absorption peak of pristine C4N3 (λ = 275.40 nm). Therefore, the Ti atoms loading enables the enhancement of C4N3 in the absorption of visible light, fully improving the ability to capture sunlight.
a VBM and b CBM distributions of C4N3 and Ti@C4N3, respectively. c Distribution of C, N and Ti atoms on the energy band structure of C4N3 and Ti@C4N3 respectively. The coordinates of the highly symmetric points are as follows: G (0.00 0.00 0.00), M (0.50 0.00 0.00), K (0.33 0.33 0.33). d Optical adsorption spectra of pristine C4N3 and Ti@C4N3, their absorption coefficients are illustrated by blue and pink lines, respectively.
Furthermore, the temperature dependence of the optical properties of Ti@C4N3 was investigated. According to the electron-phonon interaction33, we utilize one-shot method to sample the cartesian coordinates of atoms at different temperatures34,35,36, and finally obtain the average values of samples at different coordinate sets at a given temperature ranging from 273.15 K to 373.15 K with the step size of 10 K. The band gap (see Supplementary Fig. 11a) of Ti@C4N3 calculated by the GW method37 indicated that the band gap decreases with the increase of temperature, compatible with most of semiconductors. In addition, the temperature does not have a notable impact on the probability of electron transition from VBM to CBM (see Supplementary Fig. 11b), as evidenced by the variation trend in sum result of the squares of the dipole transition matrix elements (P2) at different k points. Furthermore, the effect of electron-phonon interaction on carrier mobility was indirectly assessed by the effective mass of the carriers (see Supplementary Fig. 11c, d). Photogenerated electron effective mass is temperature-insensitive with around 4mo, whereas photogenerated hole effective mass decreases with temperature. The effective masses of photogenerated electrons and holes tend to approach each other with increasing temperature, resulting in faster carrier recombination during transport and contributing to the low quantum efficiency commonly observed in semiconductor photocatalytic materials. Essentially, non-radiative relaxation and molecular thermal vibrations in semiconductors can lead to photothermal effects, which can reduce the apparent activation energy of photocatalysis and promote carrier mobility or reactant mass transfer. However, excessive temperature can lead to a stronger photothermal effect, causing the effective mass of photogenerated electrons to exceed that of holes, which can further accelerate carrier recombination and reduce photocatalytic activity. Thus, the lower me* than mh* demonstrated Ti@C4N3 keeps high photocatalytic activity up to 373 K.
The dynamics of photogenerated carriers is an important factor affecting the quantum efficiency of photocatalytic reaction. Ab-initio NAMD simulations show that after light excitation the photogenerated electrons accumulate on the CBM of Ti@C4N3, due to the separation of photogenerated carriers. By fitting the long-time evolution curve with P(t) = exp(-t/τ)38 (see Supplementary Fig. 12), the lifetime (τe) of the photogenerated electron is 38.21 ps. The picosecond-range lifetime of photogenerated electrons in Ti@C4N3 semiconductor suggests that they have ample time to migrate to the catalyst surface and engage in the reduction reaction. These results demonstrated that the generation of Ti@C4N3 photogenerated carriers is very fast (femtosecond magnitude), much higher than the carrier trapping and compounding (picosecond magnitude), which is consistent with the characteristic time of photocatalysts reported in other literatures39,40. Thus, ensuring enough photogenerated carriers to migrate to the surface and undergo surface charge transfer for the photocatalytic reaction. To sum up, Ti@C4N3 photocatalyst has excellent photocatalytic activity.
Activation Mechanism of CO2. As a crucial prerequisite for catalytic reaction, the reactant adsorption properties were of great importance for the activation41. In general, the adsorption strength is associated with activation degree of reactant. The CO2 molecule, featuring linear configuration with two \({{\rm{\pi }}}_{3}^{4}\) bonds, possesses the non-bonding highest occupied molecular orbital (HOMO) with energy level −10.36 eV and the σ-type and π-type antibonding lowest unoccupied molecular orbital (LUMO) with energy level of 0.36 eV and 0.65 eV respectively (see Supplementary Fig. 15). Activating CO2 poses a challenge as transferring electrons from the deep-energy level HOMO to the active site of the catalyst incurs a significant energetic penalty. Conversely, the active site of the catalyst donates electrons to the LUMO of CO2 to facilitate the activation process.
The AIMD simulation results provide insight into the activation process of CO2 by Ti@C4N3. When CO2 approaches the catalyst, its bond angle decreases from 180° to about 135° within 30 fs, and the two C-O bond lengths gradually elongate from 1.16 Å to 1.30 Å and 1.20 Å, respectively (see Supplementary Fig. 16a, b). The chemisorption characteristic is revealed by the obtained adsorption energy of −1.05 eV, indicating complete activation of the CO2 molecule both energetically and structurally through thermal-induced activation. Bader charge analysis indicates that the catalyst donates 0.65 |e| electrons to the adsorbed CO2 by back donating π bond (see Fig. 3a). The energy levels of Ti d orbitals and CO2 π* orbitals are matched, leading to partial occupation of the formed d-π* orbitals and spin-polarization near the Fermi level. This results in a 0.2 eV energy level reduction in spin-up d-π* states compared to spin-down d-π* states (see Supplementary Fig. 17a). The well-dispersed π* states of CO2 below the Fermi level also validate activation states. Despite the adsorption of CO2, the Ti site retains a charge of 2.31 |e| due to the high valence of Ti (IV) ions.
Optimised structure of CO2 adsorption configuration, differential charge density, bader charge and the orbital interaction of CO2 and Ti@C4N3, a the adsorption of only one CO2 molecule, b the adsorption of two CO2 molecules. The yellow and blue areas show the accumulation and depletion of charges with the isosurface of ±0.025 e bohr−3. The bond angle of CO2-2 molecule on Ti@C4N3 evolve with time at c light-300K, d light-0K and e no-light-300K, respectively. Snapshots at three representative times (0, 20, 60 and 80 fs) during the CO2-2 activation process on Ti@C4N3 are shown in the insets ①~④; Time evolution of electron populations for the atoms at the reaction center in the period of 0~80 fs at f light-300K, g light-0K and h no-light-300K, respectively; i Piecewise temporal evolution of electron populations at light-300K. The O atom near the Ti atom is labeled O1, and the O atom far from the Ti atom is labeled O2.
In photocatalytic reactions, the gas hourly space velocity (GHSV) of CO2 is typically high, which increases the probability of multiple collisions between the substrate and catalyst. Therefore, it is necessary to investigate the multiple adsorptions of CO2 on the catalyst. The AIMD simulation results of dual CO2 adsorption on Ti@C4N3 indicate that after 10 ps, the first adsorbed CO2 (CO2-1) still features structural bending of about 45° and bond-length stretching of about 0.13 Å (see Supplementary Fig. 19a, b). However, the second CO2 (CO2-2) molecule maintains a linear configuration with weak physisorption of −0.39 eV adsorption energy. Electronically, the CO2-2 only involves 0.03 |e| electrons from the substrate and therefore attains a non-activated state (see Fig. 3b). Supplementary Fig. 17b shows that the Ti d orbitals can only partially match the π* orbitals of the CO2-2 above the Fermi level, revealing its weak adsorption on the catalyst. In summary, for high GHSV of CO2 gas, Ti@C4N3 activates only one CO2 molecule, accompanied by weak adsorption of another CO2 molecule.
As is well known, photocatalytic reaction proceeds under continuous light irradiation42. Whether it is possible for weakly adsorbed CO2-2 molecule to be activated under light? Thus, ab initio rt-TDDFT molecular dynamics simulation under light illumination43 (see Supplementary Fig. 22) was used to study the kinetically activation process of dual CO2 adsorption on Ti@C4N3 catalyst. Fig. 3c–i depicts the dependence of geometrical factors (bond angles) and electronic populations over irradiation time. To investigate the underlying mechanism for dual activation under light, we divide the electron density into each reaction center atom by Hirshfeld charge analysis. The whole reaction process involves three distinct stages (see Fig. 3f): (1) Electron excitation (0~20 fs, white span): Under the activation of the laser pulse, C4N3 substrate lose about 0.03 e− charge (at t = 17 fs). As shown in Fig. 3i, the Ti site initially acquires around 0.01 e− charge from C4N3 substrate at t = 15 fs. Simultaneously, the weakly-adsorbed CO2-2 molecule gains 0.01 e− charge, while the Ti atom experiences a decrease of approximately 0.02 e− from 17 fs to 20 fs. This suggests a potential charge transfer between Ti atom and CO2-2. During this process, the bond angle of CO2-2 becomes bent to 165° (see Fig. 3c). (2) Charge transfer (20~60 fs, pink span): In this stage, electronic populations of related species undergo significant changes compared to the first stage. Firstly, a Ti atom transfers approximately 0.05 e− charge to a CO2-2 molecule, causing the CO2-2 bond angle to bend from 165° to 157° within 20~40 fs. This indicates that the singular Ti site in Ti@C4N3 functions as a carrier bridge, providing reductive photoelectrons to activate CO2-2. Subsequently, C4N3 substrate also contributes about 0.05 e− charge to CO2-2 within 40~60 fs, resulting in a maximum gain of approximately 0.10 e− charge at t = 60 fs. This reveals a charge transfer process from Ti@C4N3 support to CO2-2. Concurrently, the CO2-2 bond angle further bends from 165° to 152° at 20~60 fs, and the C-O1 bond gradually lengthens to around 1.35 Å with oscillations. This behavior indicates that the CO2-2 molecule undergoes vibrational excitation following the charge transfer in this stage. (3) CO2 activation deeply (60~80 fs, bule span): During the initial 10 fs (t = 60~70 fs) of this stage, the geometrical structure of CO2-2, including bond angles and bond lengths, remains unchanged, and the electronic population of CO2-2 shows minimal changes, displaying only slight oscillations. During this time, Ti@C4N3 acquires approximately 0.15 e− charge at 67 fs. Meanwhile, thermal-activated CO2-1 experiences a loss of approximately 0.22 e− charge. As a result, the bond angle of CO2-1 bends to 148° (see Supplementary Fig. 24), and the C-O bonds elongate to 1.50 and 1.40 Å, respectively (see Supplementary Fig. 23a). These changes indicate that CO2-1 further amplifies the activation process by providing electronic feedback to Ti@C4N3. Furthermore, at t = 70~80 fs the CO2-2 acquired another 0.15 e− charge, resulting in the bond angle bending from 152° to 145° and the bond length of C-O elongating to 1.37 Å. Throughout this process, both Ti@C4N3 and CO2-1 experience a reduction in their charges. Hence, the substantial activation of CO2-2 can be attributed to the electron transfer from both CO2-1 and Ti@C4N3 to CO2-2, leading to strengthened interaction between the two CO2 molecules and enabling CO2 coupling. Furthermore, the attached simulation video (see Supplementary Video) visually illustrates the dynamic processes of CO2 activation. This provides evidence that the Ti@C4N3 photocatalyst can potentially activate two CO2 molecules simultaneously when exposed to light.
To examine thermal effects in the photocatalytic reaction, we conducted additional simulations in two conditions: (1) with light and at 0 K (light-0K); (2) without light at 300 K (no-light-300K), alongside the previously mentioned conditions under light and 300 K (light-300K). Compared to that under light-300K, the light-0K simulation features different results (see Fig. 3g). In 0~20 fs, C4N3 substrate has almost no electron transfer to Ti atom, Ti transfers about 0.02 e− charge to CO2-2 in 20~40 fs, CO2-2 acquires the electron around 0.04 e− charge from C4N3 substrate at 40~60 fs. At this point CO2-2 gradually begins to activate and bends to approximately 160°. At 66~70 fs, C4N3 substrate begins to transfer some electrons to Ti atom (about 0.15 e− charge). Subsequently, Ti provides charge to CO2-2 around 0.13 e− charge during 70~80 fs (see Supplementary Fig. 25). The bond length of CO2-2 during this whole process fluctuated around 1.25 and 1.30 Å (see Supplementary Fig. 23b), respectively, and the maximum curvature was 158° at t = 45 fs, after which it fluctuated in the range of 160°. Thus, the slower electron-transfer rate at light-0K makes the activation of CO2-2 worse than at light-300K. Under the no-light-300K, the electron densities of CO2 and Ti@C4N3 do not change significantly, fluctuating roughly around 0 e− charge because the transitions between vibrational levels are insufficient to induce electron transfers. Structurally, there were minimal changes in both the bond angle and bond length of CO2-2. Consequently, activating two CO2 molecules under the no-light-300K condition proved to be highly challenging. The combined results from light-300K, light-0K, and no-light-300K simulations reveal that concurrent activation of two CO2 molecules occurs. Under light-300K, CO2-2 undergoes significant charge transfer and structural changes compared to the no-light-300K condition, whereas it maintains an approximately linear structure under no light. Comparing light-300K and light-0K, both conditions can activate two CO2 molecules simultaneously, but charge transfer is slow pronounced under light-0K, and the catalyst-CO2-2 interaction is weaker. These findings suggest that a synergistic effect between photoexcitation and thermal effects plays a crucial role in the simultaneous activation of two CO2 molecules. Light conditions are key to this dual activation, while thermal effects facilitate charge transfer via electron-phonon coupling, further enhancing CO2-2 activation15,44,45.
Mechanism of photocatalytic CO2RR. To evaluate the catalytic mechanism of the CO2RR on Ti@C4N3 catalysts, transition state calculations were performed utilizing DFT + U (Ueff = 3.79 eV) calculations. Both proton transfer and proton-coupled electron transfer (PCET) is utilized to describe the reduction mechanism of CO224,46,47. Several possible reaction pathways for the CO2 reduction, including single CO2 reduction and dual CO2 reduction, were identified on the Ti@C4N3 (see Fig. 4). In the following *, Ea, Eads and Edes denote the adsorption site, barrier energy, adsorption energy and desorption energy respectively. Firstly, the reduction mechanism of single CO2 on Ti@C4N3 photocatalyst was studied. CO2 molecule is activated on the catalyst to form a CO2* anionic intermediate. Via the first PCET, the adsorbed CO2* binds with H proton with two different pathways of HCOO* and COOH*. Subsequently, the more stable COOH* intermediate experiences the C-O bond rupture with the uphill trend of 0.23 eV, resulting in the formation of CO* and OH* species co-binding with Ti site. Then through an energy barrier of 0.40 eV, the OH* generates H2O* by the PCET process and desorbed from the catalyst, while under the larger energetic penalty of 1.51 eV, the CO* is desorbed from the catalyst, which is the RDS for CO generation (see Fig. 5). Thus, it is accessible for subsequent hydrogenation on CO*. The CO* is hydrogenated to produce two possible species: COH* and CHO* with the associated barrier of Ea = 1.73 eV (COH*) and Ea = 0.11 eV (CHO*) (see Supplementary Fig. 26), implying the optimal product of CHO* rather than COH*. During the fourth PCET process, the CHO* will involve these two processes: CHO* + H+ + e− → HCHO* and CHO* + H+ + e- → CHOH* (see Supplementary Fig. 27). The hydrogenation of CHO* to HCHO* (Ea = 0.30 eV) is more favorable than that of CHOH* (Ea = 1.21 eV) from the perspective of kinetics. The larger adsorption energy (Eads = −2.26 eV) of HCHO* on Ti@C4N3 leads to the desorption difficulty, while via the barrier of 0.70 eV the hydrogenation of HCHO* intermediate generates the CH2OH*, which further form CH3OH* product via the 6th PCET step with the barrier of 0.65 eV. Under the uphill barrier of 0.40 eV, the adsorbed CH3OH* enables hydrogenated (7th PCET) and dehydrated to CH3*, which is easily hydrogenated to CH4 under the 8th PCET. The CH4 is more readily desorbed (Edes = 0.56 eV) from the Ti@C4N3 catalyst surface than other C1 products (such as CO, HCHO, CH3OH), and HCHO* + H+ + e− → CH2OH* is a RDS for CH4 generation. In general, single adsorbed CO2 molecule undergoes a series of hydrogenation steps to form C2 intermediate species through four common coupling pathways: CO*-CO*, CO*-COH* or CHO*, CO*-CHn* (n = 1, 2, 3), and CHn*-CHn*11,12,16. Among the four pathways, a common feature is the formation of CO* intermediates before C-C coupling. However, for SACs, the nearly identical charge distributions between neighbouring C1 intermediates inevitably result in a strong dipole−dipole repulsion and thereby hinders C − C coupling. Apart from that as the discrete active sites and strong adsorption for crucial intermediates (−1.51 eV for CO*, −2.74 eV for COH*, −2.98 eV for CHO*, −4.38 eV for CH*, −3.70 eV for CH2* and −3.08 eV for CH3*), desorption and diffusion of these intermediates are too difficult to achieve C-C coupling through Langmuir–Hinshelwood (L-H) or Eley–Rideal (E-R) mechanisms. In summary, the generation of C2 products via the four coupling pathways described earlier is particularly challenging when using Ti@C4N3 catalysts. Thus, the reduction of individual CO2 molecule is more inclined to generate C1 species via the eight PCET steps, where the barriers of RDS are 1.51 eV for CO, 2.26 eV for HCHO, 1.59 eV for CH3OH and 0.70 eV for CH4, respectively, Thereinto, CH4 features the highest kinetic selectivity.
Just as aforementioned, given for high GHSV of CO2 gas, the Ti@C4N3 can activate two CO2 molecules through both thermal and photonic excitation. Thus, it is whether two activated CO2* directly undergoes C-C coupling reaction under light irradiation. However, it is very difficult to use the ab-initio NAMD simulation to identify the whole real-time coupling process under light. Thus, we hypothesize that photon is only beneficial for the activation rather than the diffusion of intermediate. Based on CI-NEB strategy (see Supplementary Fig. 29), we found that direct coupling of two CO2* proceeds with the tiny barrier of 0.19 eV, which is thermodynamically easier than the coupling via previous four pathways (see Fig. 6). Such result is different from the consensus that CO* is necessary for the formation of multicarbons (C2+) products11,48,49. Supplementary Fig. 30 indicates that the kinetically CO2* to CO2* coupling (Ea = 0.19 eV) capacity is higher than that of CO2* to H* proton (Ea = 0.23 eV). Furthermore, crystal orbital hamilton population (COHP) results in Supplementary Fig. 31 showed that negative ICOHP of C-C bond (−3.99) and Ti-O bonds (−2.82 and −2.79) sheds light on the stability of OOCCOO* on the surface of Ti@C4N3 catalyst and the existence of strong C-C bond50,51. Thus, the adsorbed OOCCOO* intermediate is able to participate in the subsequent reduction to generate multiple C2 products (e.g. (COOH)2, CH ≡ COH, C2H2, C2H4 and C2H6). Via the first PCET step (OOCCOO* + H+ + e− → OOCCOOH*), the barrier of 0.64 eV is required for the formation of OOCCOOH*, which binds with one proton to form cis HOOCCOOH* through the next elementary step (see Fig. 5) with the barrier of Ea = 0.86 eV.
Then, via the intramolecular proton transfer, the cis HOOCCOOH* can undergo a structural transformation resulting in the production of trans HOOCCOOH* with an associated energetic penalty of 0.94 eV. The obtained trans HOOCCOOH* prefers to continue hydrogenation to produce HOHOCCOOH* (Ea = 1.01 eV) rather than the desorption with energy of 2.56 eV. Next, HOHOCCOOH* underwent proton transfer to produce HOHOCCOHO* with the barrier of 0.77 eV. The HOHOCCOHO* generates species HOHOCCOHOH* is demands an energy barrier of 1.09 eV, which is further reduced to HOCCOHOH* by C-O bond fracture dehydration (Ea = 0.27 eV). Due to the structural instability of HOCCOHOH*, the proton transfer from the hydroxyl group to the C atom with the formation of carbonyl group (Ea = 0.69 eV), resulting in OHCCOHOH* intermediate. Subsequently, OHCCOHOH* undergoes PCET dehydration with 0.56 eV barrier to yield OHCCOH*, which must overcome a 0.60 eV barrier to undergo hydrogenation and form an O-H bond, resulting in the generation of HOHCCOH*. Rapid hydrogenation of HOHCCOH* leads to C-O bond breakage to emerge CHCOH* and H2O (Ea = 0.09 eV). On Ti@C4N3 catalyst, the stronger adsorption of CHCOH* (Eads = −1.71 eV) benefits further hydrogenation and dehydration to obtain CHC* through the barrier of 0.53 eV. The successive hydrogenation of CHC* → CHCH* → CH2CH* → CH2CH2* → CH3CH2* → CH3CH3*occurs under the barriers of 0.24, 0.06, 0.07, 0.19 and 0.03 eV, respectively. Despite the formation of C2H2 and C2H4 products, the subsequent hydrogenation process leads to the final production of C2H6 due to their adsorption energy (Eads = 2.02 eV for C2H2, 1.75 eV for C2H4) compared to the hydrogenation barrier. Generally, during the coupled hydrogenation of two CO2 on Ti@C4N3, various products are formed with different energy barriers of 2.56 eV for (COOH)2, 1.71 eV for CH ≡ COH, 2.02 eV for C2H2, 1.75 eV for C2H4 and 1.09 eV for C2H6, respectively. Therefore, C2H6 is determined to be the optimal product via fourteen-electron reduction, and the generation of the HOHOCCOHOH* intermediate is the RDS for the entire reaction in both kinetics (Ea = 1.09 eV) and thermodynamics (ΔGmax = 1.57 eV).
Discussion
In summary, by utilizing DFT calculation and ab-initio NAMD simulation, we build the computational frameworks to screen an outstanding single-atom photocatalysts Ti-supported 2D C4N3 material, Ti@C4N3. Structurally, the Ti@C4N3 catalyst shown the excellent stabilities both thermally, chemically and mechanically. Electronically, such catalyst has great potential as a photocatalyst for CO2 reduction: two main absorption peaks (327.77 and 529.61 nm), suitable band positions (0.002 eV for VBM and −0. 968 eV for CBM) and long photocarrier lifetime (38.21 ps for electron), ensuring that enough photogenerated electrons migrate to the surface and participate in photocatalytic CO2RR. Essentially, such properties are intimately tied with the doping Ti atom, embodying the fundamental transformations from conductor to semiconductor. Interestingly, under the high GHSV of CO2, such high-valence of doping Ti ion renders the dual activation of CO2: Without light, high Lewis-acidity of Ti site thermally induces activation of CO2 by back-donating π-bond; and under visible light irradiation another weak-adsorbed CO2 attains the photoelectron from Ti site through CBM and thereby undergoes photo-induced activation. Catalytic mechanism studies systematically reveal that with the barrier of 0.19 eV the two activated CO2 is easily coupled to be oxalate, which is further reduced to be C2H6 (Ea = 1.09 eV). Our finding reveals the possibility of dual activations (thermally induced activation and photo-induced activation) during the photocatalytic CO2 reductions.
Methods
Ground-State Calculations. DFT calculations (geometrical optimization, electronic properties and catalytic mechanism) were carried out by VASP package52 (6.3.1) using Perdew−Burke−Ernzerhof53 (PBE) functional and projector-augmented wave54 (PAW) pseudopotential. Detailed computational information was presented in Supplementary Data file.
Excited-state Dynamic calculations. The photocarrier dynamics was simulated by using the ab initio NAMD program (Hefei-NAMD)55. Dual activation of CO2 under light irradiation was identified by rt-TDDFT MD simulation utilizing the TDAP program56. Detailed computational information was presented in Supplementary Data file.
Data availability
The computational results, including structure files, transition state calculations, excited state calculations and so on, can be found at https://archive.materialscloud.org/record/2023.178. Additional supporting data for this study is available in the article, its supplementary information files, or upon reasonable request from the corresponding authors.
Code availability
VASP 6.3.1 software was obtained through commercial purchase (https://www.vasp.at/). Two open-source software (TDAP software and Hefei-NAMD software) can be obtained from http://tdap.iphy.ac.cn/ and https://github.com/QijingZheng/Hefei-NAMD, respectively.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No: 22022108, 22072135, 21671172), Zhejiang Provincial Natural Science Foundation of China (Grant No: LTGY23B010001), Natural Science Foundation of Shandong Province (ZR2020ZD35).
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F.L.S. and G.L.Z. conceived and designed the whole study; C.B.L. performed carrier dynamic calculation; Q.C. provide PBE + U based TDAP software and discuss with corresponding calculations; W.Z., W.X.C. and X.N.L. discussed dual activation of CO2 and analyze CO2 coupling mechanism.
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Sun, Fl., Lin, Cb., Zhang, W. et al. Dual activation and C-C coupling on single atom catalyst for CO2 photoreduction. npj Comput Mater 9, 220 (2023). https://doi.org/10.1038/s41524-023-01177-3
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DOI: https://doi.org/10.1038/s41524-023-01177-3
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