Dual Activation and C-C Coupling on Single Atom Catalyst for CO₂ Photoreduction

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

npj Computational Materials 9: 220 (2023)

doi.org/10.1038/s41524-023-01177-3

Published online:11 December 2023

编辑概述

光照下单个催化位点能活化多个CO2分子吗?

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。

图1两个CO₂分子耦合示意图：有光与无光下CO₂耦合过程

   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。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（如图1）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）(如图2)，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 )。

图2 C₄N₃和Ti@C₄N₃的能带结构

图3可见光照下CO₂分子活化机理

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。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。

图4可见光照下CO₂分子催化还原过程

Editorial Summary

Can a Single Catalytic Site Activate Multiple CO₂ Molecules 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. 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 C₄N₃ carrier 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.