Constructing S-scheme 2D/0D g-C3N4/TiO2 NPs/MPs heterojunction with 2D-Ti3AlC2 MAX cocatalyst for photocatalytic CO2 reduction to CO/CH4 in fixed-bed and monolith photoreactors

Exfoliated 2D MAX Ti3AlC2 conductive cocatalyst anchored with g-C3N4/TiO2 to construct 2D/0D/2D heterojunction has been explored for enhanced CO2 photoreduction in a fixed-bed and monolith photoreactor. The TiO2 particle sizes (NPs and MPs) were systematically investigated to determine effective metal-support interaction with faster charge carrier separation among the composite materials. When TiO2 NPs were anchored with 2D Ti3AlC2 MAX structure, 10.44 folds higher CH4 production was observed compared to anchoring TiO2 MPs. Maximum CH4 yield rate of 2103.5 µmol g−1 h−1 achieved at selectivity 96.59% using ternary g-C3N4/TiO2/Ti3AlC2 2D/0D/2D composite which is 2.73 and 7.45 folds higher than using binary g-C3N4/Ti3AlC2 MAX and TiO2 NPs/Ti3AlC2 samples, respectively. A step-scheme (S-scheme) photocatalytic mechanism operates in this composite, suppressed the recombination of useful electron and holes and provides higher reduction potential for efficient CO2 conversion to CO and CH4. More importantly, when light intensity was increased by 5 folds, CH4 production rate was increased by 3.59 folds under visible light. The performance of composite catalyst was further investigated in a fixed-bed and monolith photoreactor and found monolithic support increased CO production by 2.64 folds, whereas, 53.99 times lower CH4 production was noticed. The lower photocatalytic activity in a monolith photoreactor was due to lower visible light penetration into the microchannels. Thus, 2D MAX Ti3AlC2 composite catalyst can be constructed for selective photocatalytic CO2 methanation under visible light in a fixed-bed photoreactor.