The inhibitory effect of graphene oxide on photocatalytic hydrogenation from organic fatty acids

Abstract

Two catalysts, carbon doped TiO2 (C‐TiO2) and C‐TiO2 conjugated with reduced graphene oxide (rGO)–C‐TiO2/rGO were synthesized and compared with respect to the photocatalytic hydrogen (H2) production using three organic fatty acids (OFAs) as electron donors under visible light irradiation. Our results show that the hybridization between C‐TiO2 and rGO did not subtaintially improve the production of H2 but instead elicited inhibitory effects compared to C‐TiO2. For instance, the specific H2 production on C‐TiO2 was 214 and 336 μmol·h−1·g−1 for acetic and propionic acids, respectively, whereas C‐TiO2/rGO only achieved 155 and 161 μmol·h−1·g−1, respectively. With butyric and lactic acids as electron donors, C‐TiO2 and C‐TiO2/rGO led to almost the same H2 production rates (233 and 155 μmol·h−1·g−1, respectively). The inhibitory effect of rGO on photocatalytic activity could be due to the potential deposition of OFAs on graphene sheets that may reduce the mobility and accessibility of OFAs toward catalytic reactions with C‐TiO2. The electron spin resonance spectrometry (ESR) tests revealed that normalO2·− and CH3• were the major radicals responsible for OFA degradation. Moreover, the produced concentration of CH3• was consistently higher in C‐TiO2 suspension than that in C‐TiO2/rGO suspension. The estimated aqueous/solid mass transfer rate constant (s−1) was lower on the catalyst surface of C‐TiO2/rGO than C‐TiO2. Consequently, the decoration of rGO sheets on C‐TiO2 may not always promote photocatalytic H2 production especially when using OFAs as electron donors. The results demonstrated hereby aim to provide new evidence on potential drawbacks of graphene‐based photocatalysis. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 410–416, 2019