Efficient photocatalytic oxidation of CH4 over Ag-modified ZnO nanorods

Abstract

Photocatalysis is a highly promising strategy for the direct conversion of inert methane (CH₄). In this study, a ZnO semiconductor with a nanorod morphology (r-ZnO) was synthesized using a template-directed hydrothermal method, which was further used to prepare nAg/r-ZnO photocatalysts through impregnation and reduction. The structure, composition, and microstructure of nAg/r-ZnO were characterized using techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and UV-visible absorption spectroscopy (UV-vis). The in-depth mechanism of the photocatalytic conversion of CH₄ was revealed through electron paramagnetic resonance (EPR) and radical capture experiments. The results showed that the introduction of Ag species promoted the separation of photoinduced electron (e⁻) and hole (h⁺) pairs, which facilitated the generation of reactive oxygen species (ROS). Therefore, the performance of photocatalytic CH₄ oxidation can be significantly enhanced. Interestingly, formaldehyde (HCHO) was generated as the primary product instead of low-value methanol (CH₃OH). Through optimization of Ag loading and reaction conditions, an excellent oxygenate (CH₃OOH, CH₃OH, and HCHO) yield of 15 551.60 μmol g⁻¹ h⁻¹ with a high selectivity of 98.46% was achieved simultaneously, which has been rarely reported in previous studies.