Abstract
The photocatalytic activity of NiO/Ni nanoparticles (NPs) supported on defective graphene (NiO/Ni-G) has been tested for the photoassisted CO2 reduction with H-2. NiO/Ni-G was prepared by H-2 reduction of NiCl2 adsorbed on few-layers defective G and storage under air. An optimal Ni loading of 23 wt% was found, reaching the maximum specific CH4 formation rate (642 mu mol CH4 g(Ni)(-1) h(-1) at 200 degrees C) and quantum yield of 1.98%. Under the same conditions Ni NPs supported on silica-alumina or NiO NPs exhibit notably lower specific CH4 production rates than NiO/Ni-G. It was found that H2O formed in the reaction has a detrimental influence on the photocatalytic activity and evidence supports that H2O desorption is one of the reasons why the system requires heating. Under continuous flow operation, undesirable water molecules were easier desorbed from the NiO/Ni-G photocatalyst than in the batch mode, reaching a steady specific CH4 production rate of 244.8 mu L h(-1) for 50 mg of NiO/Ni-G catalyst with a residence time of 3.1 s. Quenching experiments with electron donor of different oxidation potential (dimethylaniline, anisole and p-xylene) are compatible with a mechanism involving photoinduced charge separation.