Impact of transition metal incorporation on the photocatalytic CO2 reduction activity of polymeric carbon nitride

Incorporation of transition metals in polymeric carbon nitride (CN) is an effective strategy to enhance its photocatalytic CO 2 reduction activity, however, the difference of activity enhancement by incorporating different metals is not well understood. Herein, CN is modified with different transition metals by pyrolyzing the mixtures of urea and metal-organic frameworks (MOFs) to obtain MCN (M = Cu, Co, Ti or Fe). For each given type of metal-modified CN, the photocatalytic CO 2 reduction activity is optimized by controlling the content of MOF precursor during pyrolysis. The optimized MCN delivers significantly enhanced CO evolution rate than pure CN, in the order of CN (83 μmol g −1 h −1 ) < CuCN (246 μmol g −1 h −1 ) < CoCN ( 326 μmol g −1 h −1 ) < TiCN (454 μmol g −1 h −1 ) < FeCN (490 μmol g −1 h −1 ). It is revealed that for CuCN and CoCN, Cu and Co are doped in CN. In contrast, for TiCN and FeCN, Ti and Fe exist as TiO 2 and Fe 2 O 3 forming Z-scheme heterojunctions with CN. The progressively improved photocatalytic activity corresponds to the increased specific surface area, CO 2 adsorption capacity, visible light absorption as well as charge separation and transfer efficiency. Furthermore, we design and prepare bimetal incorporated CN through combining metal doping with heterojunction construction strategies, i.e., Cu doped CN/TiO 2 and Co doped CN/Fe 2 O 3 , which exhibit further enhanced CO 2 photoreduction performance with CO evolution rates of 613 and 718 μmol g −1 h −1 , respectively. This work provides insight into the design and preparation of highly efficient CN-based photocatalytic materials. • Transition metals incorporated CN (MCN, M = Cu, Co, Ti or Fe) were prepared. • The CO 2 photoreduction activity of MCN was optimized by tuning the metal content. • The photocatalytic activity is in the order of CN< CuCN < CoCN < TiCN < FeCN. • Cu and Co are doped in CN, whilst Ti and Fe exist as oxides. • Bimetal incorporation is an effective strategy to design CN photocatalyst.