光照下，1个催化位点能活化几个CO2分子?

可见光催化CO₂还原制高附加值化学品是一种极具吸引力且环保的方法，不仅解决能源短缺问题，同时减少二氧化碳排放。然而，目前这种人工光还原CO₂的效率，远远达不到工业上大规模生产的要求。在多相催化中，一般一个 CO₂ 分子只能被一个催化位点活化。

因此，制备C₂等多碳化学品，需要相邻多位点间协同作用。因此，由于有限的还原能力和活性位的高度分散性，单位点催化剂（例如：单原子催化剂）触发C-C偶联显得非常具有挑战性。该研究基于从头算–非绝热分子动力学模拟发现了：光照下单一催化位点发生有趣的双重活化（热致活化和光致活化）CO₂分子，并诱导C-C偶联生成高附加值C₂H₆。

Fig. 2 Optical properties.

来自浙江工业大学化工学院工业催化研究所庄桂林教授团队针对光催化CO₂还原反应（如本文压题图）成功地设计了一种稳定且高效的单原子催化剂Ti@C₄N₃，并提出了单位点双重活化的概念。即该研究发现负载高价态Ti⁴⁺到C₄N₃载体，打开能带发生导体到半导体转变，形成具有平带特征且由Ti-3d态组成导带底（CBM），这种高度局域的CBM有效提高光生电子从活性位到反应底物的传输效率和寿命( 38.21 ps )。

进一步原位光照下含时密度泛函理论(rt-TDDFT)模拟研究（如图3）揭示：高空速的CO₂流过Ti@C₄N₃催化剂表面经历了2个活化过程：(1) 无光照时，高Lewis 酸位的Ti通过给反馈π键以热诱导方式活化一个CO₂, 部分CO₂ 分子以范德华力作用弱吸附在位点附近。( 2 )可见光照时，催化剂的价带附近电子被激发到导带上；而CBM上电子态主要布居在Ti-3d态上。

Fig. 4 Reaction pathways. Schematic illustration of the possible reaction pathways for CO2RR on Ti@C4N3.

因此某个弱吸附CO₂容易通过Ti@C₄N₃的CBM从Ti位点获得光电子，从而发生光诱导活化。催化机理研究表明如图4，在E_a = 0.19 eV能垒下，两个活性的CO₂非常容易偶联为草酸盐，进一步经过多步还原高选择性产生C₂H₆( E_a = 1.09 eV )。该研究在CO₂双重活化的发现将对可见光催化剂的设计提供一种新的思路。相关论文近期发布于npj Computational Materials 9: 220 (2023)。

Editorial Summary

Can a Single Catalytic Site Activate Multiple CO₂Molecules in Photocatalysis: One or More?

Visible light catalysis for CO₂ reduction to high-Value chemicals is an attractive and environmentally friendly approach, not only addressing energy shortages but also mitigating carbon dioxide emissions. However, the current efficiency of artificial light-driven CO₂ reduction falls far short of the requirements for large-scale industrial production. In heterogeneous catalysts, typically, only one CO₂ molecule can be activated by one catalytic site. Therefore, the preparation of multi-carbon chemicals, such as C₂, requires cooperative interactions between adjacent catalytic sites. Due to limited reduction capacity and the high dispersion of active sites, triggering C-C coupling in single-site catalysts, such as single-atom catalysts, proves to be highly challenging. This study, based on first-principles non-adiabatic molecular dynamics simulations, discovered intriguing dual activation (thermal and photoinduced) and C-C coupling at a single site during the photo-reduction of CO₂. 

Fig. 6 Schematic diagram of two CO2 molecules coupled.

Professor Zhuang Guilin’s team from the Institute of Industrial Catalysis, School of Chemical Engineering, Zhejiang University of Technology, successfully designed a stable and efficient single-atom catalyst, Ti@C₄N₃, for photocatalytic CO₂ reduction, introducing the concept of dual activation at the single site. The study revealed that loading a high-valence Ti⁴⁺ onto the C₄N₃carrier induces a band transition from a conductor to a semiconductor and forms a Ti-3d component with flat-band characteristics, effectively enhancing the efficiency and lifetime of photo-generated electrons (38.21 ps). Real-time time-dependent density functional theory (RT-TDDFT) simulations under in situ light exposure found that, as high-speed CO₂ passed over the catalyst surface, two processes occur: (1) Without light, CO₂ at the high Lewis acid site Ti activates one CO₂ through thermally induced feedback to the π bond, and some CO₂ molecules weakly adsorb near the site through van der Waals forces. (2) Under visible light, electrons in the valence band are excited to the catalyst’s CBs; since the electron states on the CBM are mainly localized on the Ti-3d states, a weakly adsorbed CO₂ easily gains photogenerated electrons from the Ti@C₄N₃‘s CBM at the Ti site, leading to photoinduced activation. Mechanistic studies suggest that under a low energy barrier (E_a = 0.19 eV), coupling two active CO₂ molecules to form oxalate is highly facile, and oxalate is further selectively reduced to C₂H₆ (E_a= 1.09 eV). The discovery of dual activation in CO₂ opens up new avenues for the design of visible-light catalysts. This research was recently published in npj Computational Materials 9: 220 (2023).

原文Abstract及其翻译

Dual Activation and C-C Coupling on Single Atom Catalyst for CO₂Photoreduction (光照下单原子催化剂双重活化CO₂和C-C偶联)

Fu-li Sun, Cun-biao Lin, Wei Zhang, Qing Chen, Wen-xian Chen, Xiao-nian Li & Gui-lin Zhuang* 

Abstract An excellent single-atomic photocatalyst, Ti@C₄N₃, is theoretically found to effectively convert CO₂ to C₂H₆by 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 (l= 327.77 and 529.61 nm), suitable band positions, and a long photogenerated electron lifetime (τ_e= 38.21 ps), allowing photogenerated electrons to migrate to the surface. Notably, the high-valence active site effectively activates two CO₂through dual activation: Under light irradiation, the weakly adsorbed CO₂undergoes photo-induced activation by the photoelectron of conduction band minimum (CBM); without light, the high Lewis acidity of the Ti site induces CO₂activation through back-donating π-bond.Contrast simulation results uncovered that dual activation of CO₂is 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.

摘要结合密度泛函理论（DFT）计算和非绝热分子动力学（NAMD）模拟结果发现了一种优异的单原子光催化剂Ti@C₄N₃可以有效地将CO₂转化为C₂H₆。 Ti@C₄N₃光催化剂具有显著的热稳定性、化学稳定性和机械稳定性。在电子性质方面，它具有很强的光吸收特性（l = 327.77和529.61 nm）、合适的带边位置和较长的光生电子寿命（τ_e = 38.21 ps），且允许光生电子有效迁移到表面。值得注意的是，高价态活性位点通过双重模式有效地活化了两个CO₂分子，即：在光照射下，弱吸附的CO₂受到导带底（CBM）光电子的诱导活化；在无光照下，Ti位点的高路易斯酸性通过反馈π键诱导CO₂活化。对比模拟结果发现，CO₂的双重活化归因于热能和光子的协同作用。此外，两个被活化的CO₂分子在光照下很容易耦合形成草酸盐，反应能垒低至0.19 eV；并进一步经过多步还原生成C₂H₆，且速控步的活化能仅为1.09 eV。