Distinct Pathways in Visible-Light Driven Thermo-Photo Catalytic Methane Conversion

Abstract

The utilization of visible light for direct photocatalytic methane conversion remains a huge challenge. Here, we developed a thermo-photo catalytic process with a visible-light-responsive Pt/WO3 catalyst and realized highly efficient visible-light driven methane conversion for the first time. The conversion efficiency was enhanced by 4.6 and 14.7 times compared to room-temperature photocatalysis and thermal catalysis at 150 °C, respectively. Furthermore, the production of liquid oxygenates (mainly CH3OH) was found to proceed via photocatalysis with high apparent quantum efficiencies of 5.9%, 4.5%, and 1.9% at 350, 420, and 450 nm, respectively, while CO2 evolution was contributed by photoassisted thermal catalysis. Solid isotope evidence further confirmed that CH3OH, HCHO, and CO2 were produced via parallel rather than sequential reactions. These observations provide a valuable guide for designing a visible-light driven system for methane conversion with high efficiency and controllable selectivity.

Thermo-photo catalysis: a whole greater than the sum of its parts

“…Firstly, thermo-photo catalysis can lead to a significantly enhanced catalytic activity compared with room-temperature photocatalysis (Table 1) due to the improved reaction kinetics. 21,28,39,40,90 Impressively, both the H 2 evolution rate and apparent quantum efficiency were found to be enhanced by three orders of magnitude in thermo-photo catalytic water splitting over Rh/black TiO 2 catalyst. 40 Moreover, very recently, an unprecedentedly high CH 3 CH 2 OOH yield of 5.6 mmol h À1 g À1 was achieved in visible-light driven thermo-photo catalytic partial oxidation of C 2 H 6 over Au/WO 3 catalyst, which is 6 times higher than that at room temperature.…”

“…90 Secondly, the introduction of thermal energy enables the effective utilization of visible light in photocatalytic processes. 21,22,28,39,40,90 In 2015, Hu and co-workers first revealed the intrinsic issue in visible-light photocatalysis and demonstrated their design of thermo-photo catalysis for enhancing the contribution of visible light to H 2 production from 0 to B85% of the entire solar spectrum. 21 Besides, the absolute H 2 yield under visible light was enhanced from 0 to 497 mmol h À1 g À1 .…”

“…This inhibits the over-reaction of reactants such as the over-oxidation of CH 4 into CO 2 due to the timely desorption of the desired mildly oxidized oxygenates. 28,95 More importantly, this realizes the efficient refresh of active sites for subsequent adsorption of reactants.…”

“…66 Moreover, one can control catalytic selectivity by inhibiting the generation of undesirable products based on the thermodynamic design. 28,90 For charge-carrier-mediated catalysis involving either photo carriers or hot carriers, the efficient separation and transfer of charge carriers should be ensured. 63,141 This principle is opposite to that of thermal-energy-driven (photo-thermal) catalysis.…”

“…21 Since then, a rapid and exciting development in thermo-photo catalysis has been witnessed with an emphasis on catalyst design and catalytic performance enhancement. So far, thermo-photo catalysis, as a whole greater than the sum of its parts (individual thermal catalysis and photocatalysis), has been applied in a wide range of chemical processes such as CO 2 reduction, [22][23][24][25][26] CH 4 oxidation, [27][28][29][30][31][32][33] CO conversion, [34][35][36][37][38] production, 21,[39][40][41][42] N 2 fixation, [43][44][45][46] pollutant degradation, [47][48][49][50][51] and organic synthesis, [52][53][54][55][56] showing high activity, tuneable selectivity, and excellent stability under mild conditions at low cost, owing to the synergetic effects of thermal and photo energies (Fig. 2).…”