Abstract:

Methane, as an abundant resource, serves not only as an excellent fossil fuel but also as a pivotal feedstock for synthesizing high-value-added chemical products. Solar-driven methane conversion offers a highly promising pathway for the direct production of high-value chemicals such as methanol (CH₃OH) and formaldehyde (HCHO) under mild conditions. However, the core challenge of this conversion process lies in the tendency of target products to undergo over-oxidation, resulting in low selectivity—a critical bottleneck that urgently requires breakthrough in this field. Herein, we constructed an Ir-modified CdS (Ir_x/CdS) photocatalytic system and proposed that regulating the generation types of key reaction intermediates via metallic Ir is an effective strategy to enhance the selectivity of target products and suppress over-oxidation. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) confirmed that the types of key intermediates generated during methane activation differ, which decisively influences the product distribution. On pure CdS surfaces, the key intermediate *CH₃O tends to participate in subsequent deep oxidation reactions via its O atom, ultimately leading to over-oxidized products like CO₂. In contrast, after Ir loading, the key reaction intermediate shifts to *CH₃. The Ir sites facilitate the conversion of *CH₃ to ‧CH₃ radicals through localized electron transfer, and the generated ‧CH₃ radicals rapidly combine with ‧OH radicals to selectively form CH₃OH. The performance evaluation of photocatalytic methane conversion demonstrated that under conditions of 60 ℃, 0.1 MPa, and molecular oxygen as the oxidant, the 0.50 wt% Ir-loaded Ir_0.50/CdS sample exhibited optimal performance: the yield of oxygenated liquid products (CH₃OH and HCHO) reached 509.2 μmol g⁻¹ h⁻¹, with overall selectivity enhanced to 88%. Characterization techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) revealed the coexistence of two valence states of Ir on the catalyst surface (metallic Ir⁰ and oxidized Ir⁴⁺), with the metallic state being dominant. The strategy proposed in this work—regulating intermediate species generation via metal modification to inhibit over-oxidation—provides a novel approach for the efficient conversion of methane into high-value oxygenated chemicals.

Key words: Methane conversion, Photocatalysis, Metal loading, Generation of key intermediates