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 Ti3AlC2conductive cocatalyst anchored with g-C3N4/TiO2to construct 2D/0 D/2D heterojunction has been explored for enhanced CO2photoreduction in a fixed-bed and monolith photoreactor. The TiO2particle sizes(NPs and MPs) were systematically investigated to determine effective metalsupport interaction with faster charge carrier separation among the composite materials. When TiO2NPs were anchored with 2D Ti3AlC2MAX structure, 10.44 folds higher CH4production was observed compared to anchoring TiO2MPs. Maximum CH4yield rate of 2103.5 μmol g-1 h-1 achieved at selectivity96.59% using ternary g-C3N4/TiO2/Ti3AlC22D/0 D/2D composite which is 2.73 and 7.45 folds higher than using binary g-C3N4/Ti3AlC2MAX and TiO2NPs/Ti3AlC2samples, 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 CO2conversion to CO and CH4. More importantly, when light intensity was increased by 5 folds, CH4production 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 CH4production was noticed. The lower photocatalytic activity in a monolith photoreactor was due to lower visible light penetration into the microchannels. Thus, 2D MAX Ti3AlC2composite catalyst can be constructed for selective photocatalytic CO2methanation under visible light in a fixed-bed photoreactor.