Optimally Selecting Photo- and Electrocatalysis to Facilitate CH₄ Activation on TiO₂(110) Surface: Localized Photoexcitation versus Global Electric-Field Polarization

Abstract

Photo- and electrocatalytic technologies hold great promise for activating inert chemical bonds under mild conditions, but rationally selecting a more suitable method in between to maximize the performance remains an open issue, which requires a fundamental understanding of their different catalytic mechanisms. Herein, by first-principles calculations, we systematically compare the activation mechanisms for the C–H bond of the CH 4 molecule on TiO 2 (110) under the photo- and electrocatalytic modes without or with water involved. It quantitatively reveals that the activation barrier of the C–H bond decreases dramatically with a surprising 74% scale by photoexcitation relative to that in thermocatalysis (1.12 eV), while the barrier varies with a maximum promotion of only 5% even under −1 V/Å external electric field (EEF). By detailed geometric/electronic analysis, the superior photocatalytic activity is traced to the highly oxidative lattice O br •– radical excited by a photohole ( h + ), which motivates the homolytic C–H bond scission. However, under EEF from −1 V/Å to 1 V/Å, it gives a relatively mild charge polarization on the TiO 2 (110) surface region and thus a limited promotion for breaking the weakly polar C–H bond. By contrast, in the presence of water, we find that EEF can facilitate CH 4 activation indirectly assisted by the surface radical-like OH* species from the oxidative water cleavage at high oxidative potential (>1.85 V vs SHE), which explains the high energy cost to drive electrocatalytic CH 4 conversion in experiment. Alternatively, we demonstrate that more efficient CH 4 activation could be also achieved at much lower oxidative potential when integrating the light irradiation. In such a circumstance, EEF can not only promote the h + accumulation at the catalyst surface but also help H 2 O deprotonation to form hydroxide, which can serve as an efficient hole-trapper to generate OH • radical (OH – + h + → OH • ), unveiling an interesting synergistic photoelectrocatalytic effect. This work could provide a fundamental insight into the different characteristics of photo- and electrocatalysis in modulating chemical bond cleavage.

Photocatalytic C–H Bond Activation of Toluene on Rutile TiO2(110)

“…Therefore, the barrier of C−H bond cleavage of CH 4 via photocatalysis is lower than that via thermocatalysis (0.29 eV vs 1.12 eV). 50 Similarly, the barrier of photocatalytic dissociation of toluene (0.32 eV) is also far lower than that of thermocatalytic process (1.10 eV) in this work. Furthermore, the hole transfer is coupled with the H atom transfer as shown by spin density distribution in Figure 4, suggesting that the C−H cleavage is homolytic, which is similar to that of the C−H bond activation of CH 4 on R-TiO 2 (110).…”

“…Furthermore, the hole transfer is coupled with the H atom transfer as shown by spin density distribution in Figure 4, suggesting that the C−H cleavage is homolytic, which is similar to that of the C−H bond activation of CH 4 on R-TiO 2 (110). 50 The radical nature of CH 3 species in the transition state during the photocatalysis of CH 4 is confirmed by a Bader charge of 0.090|e| and a magnetic moment of 0.43μ B , indicating that the photocatalytic process follows the hydrogen atom transfer (HAT) mechanism (homolytic cleavage mode). 50 In this work, the formation of radical-like TS 2 type indicates that toluene may also follow a similar photoreaction pathway.…”

“…•− ) (IS 2 ). 20,26,50 Then, the hole is distributed on both the initial hole-trapping O br atom and the toluene as shown in TS 2 . As a result, the TS 2 changes from the original dihapto configuration in the thermocatalytic case to the monohapto configuration, in which only a methyl H atom in the toluene interreacts with the O br…”

“…•− formed via photoexcitation holds fewer electrons than O br 2− (Bader charge: −0.690 vs −1.082| e|). 50 As a result, it would be easier for the dissociated H atom to transfer electrons to the O br . Therefore, the barrier of C−H bond cleavage of CH 4 via photocatalysis is lower than that via thermocatalysis (0.29 eV vs 1.12 eV).…”

Striking Size and Doping Effects of Ti−Si−O Clusters on Methane Conversion Reactions

“…Titanium and silicon continue to play an important role in the conversion of methane to value‐added chemicals, and they are commonly employed as the support of active centers [1] . In fact, these two elements themselves may already serve as active species mediating for example photocatalysis and plasma reactions for low‐temperature methane conversion [2] . For example, the TS‐1 molecular sieves facilitates the reaction of methane and ammonia to HCN under plasma conditions at 400 °C; [3] light alkanes were generated in plasma‐assisted methane conversion with TiO 2 /macroporous SiO 2 catalyst [4] .…”