可見光催化CO₂還原制高附加值化學品是一種極具吸引力且環保的方法，不僅解決能源短缺問題，同時減少二氧化碳排放。然而，目前這種人工光還原CO₂的效率，遠遠達不到工業上大規模生產的要求。在多相催化中，一般一個 CO₂ 分子只能被一個催化位點活化。

因此，制備C₂等多碳化學品，需要相鄰多位點間協同作用。因此，由于有限的還原能力和活性位的高度分散性，單位點催化劑（例如：單原子催化劑）觸發C-C偶聯顯得非常具有挑戰性。該研究基于從頭算–非絕熱分子動力學模擬發現了：光照下單一催化位點發生有趣的雙重活化（熱致活化和光致活化）CO₂分子，并誘導C-C偶聯生成高附加值C₂H₆。

Fig. 2 Optical properties.

來自浙江工業大學化工學院工業催化研究所莊桂林教授團隊針對光催化CO₂還原反應（如本文壓題圖）成功地設計了一種穩定且高效的單原子催化劑Ti@C₄N₃，并提出了單位點雙重活化的概念。即該研究發現負載高價態Ti⁴⁺到C₄N₃載體，打開能帶發生導體到半導體轉變，形成具有平帶特征且由Ti-3d態組成導帶底（CBM），這種高度局域的CBM有效提高光生電子從活性位到反應底物的傳輸效率和壽命( 38.21 ps )。

進一步原位光照下含時密度泛函理論(rt-TDDFT)模擬研究（如圖3）揭示：高空速的CO₂流過Ti@C₄N₃催化劑表面經歷了2個活化過程：(1) 無光照時，高Lewis 酸位的Ti通過給反饋π鍵以熱誘導方式活化一個CO₂, 部分CO₂ 分子以范德華力作用弱吸附在位點附近。( 2 )可見光照時，催化劑的價帶附近電子被激發到導帶上；而CBM上電子態主要布居在Ti-3d態上。

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

因此某個弱吸附CO₂容易通過Ti@C₄N₃的CBM從Ti位點獲得光電子，從而發生光誘導活化。催化機理研究表明如圖4，在E_a?= 0.19 eV能壘下，兩個活性的CO₂非常容易偶聯為草酸鹽，進一步經過多步還原高選擇性產生C₂H₆(?E_a?= 1.09 eV )。該研究在CO₂雙重活化的發現將對可見光催化劑的設計提供一種新的思路。相關論文近期發布于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及其翻譯

Dual Activation and C-C Coupling on Single Atom Catalyst for CO₂Photoreduction (光照下單原子催化劑雙重活化CO₂和C-C偶聯)

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.

摘要結合密度泛函理論（DFT）計算和非絕熱分子動力學（NAMD）模擬結果發現了一種優異的單原子光催化劑Ti@C₄N₃可以有效地將CO₂轉化為C₂H₆。 Ti@C₄N₃光催化劑具有顯著的熱穩定性、化學穩定性和機械穩定性。在電子性質方面，它具有很強的光吸收特性（l = 327.77和529.61 nm）、合適的帶邊位置和較長的光生電子壽命（τ_e = 38.21 ps），且允許光生電子有效遷移到表面。值得注意的是，高價態活性位點通過雙重模式有效地活化了兩個CO₂分子，即：在光照射下，弱吸附的CO₂受到導帶底（CBM）光電子的誘導活化；在無光照下，Ti位點的高路易斯酸性通過反饋π鍵誘導CO₂活化。對比模擬結果發現，CO₂的雙重活化歸因于熱能和光子的協同作用。此外，兩個被活化的CO₂分子在光照下很容易耦合形成草酸鹽，反應能壘低至0.19 eV；并進一步經過多步還原生成C₂H₆，且速控步的活化能僅為1.09 eV。