0428同步年報-2021-全

022 ACTIVITY REPORT 2021 M ethane (CH 4 ), the predominant component of natural gas, shale gas and combustible ice, is not only a widely used energy resource but also an important building block to produce value-added chemical substances. Substantial energy input ( i.e. , high operating temperature and pressure) is currently required for the conversion of CH 4 because of its strong C–H bonds, small polarizability and negligible electron affinity. Photocatalysis, which requires only solar energy as energy input, is regarded as an appealing approach to achieve conversion of CH 4 under mild conditions (ambient temperature and pressure). Despite the potential, this approach has faced a great challenge in terms of product selectivity. The preferable products of CH 4 conversion are multicarbon (C 2+ ) compounds, with light olefins, mainly ethylene (C 2 H 4 ), as key chemical feedstocks, having greater added value than alkane (C 2 H 6 ). With methyl radicals as the sole reaction intermediate, the current C 2+ products are, however, dominated by C 2 H 6 , with a negligible selectivity towards C 2 H 4 . A rationally designed photocatalyst to regulate the reaction intermediates potentially involved in CH 4 conversion is hence urgently needed to realize a direct conversion of CH 4 to C 2 H 4 . Yujie Xiong (University of Science and Technology of China, China) and his co-workers recently reported a Pd-modified ZnO-Au hybrid that implements a path mediated by alkoxy intermediates for the photocatalytic conversion of CH 4 to C 2 H 4 . Employing X-ray-absorption fine-structure (XAFS) spectra at TPS 44A , 1 the team found that the introduced Pd sites are atomically dispersed in the surface lattice of the Au nanorods supported on ZnO ( Fig. 1 ). Various characterizations in situ revealed that the Pd-induced dehydrogenation capability of the catalyst enabled the formation and transformation of methoxy and ethoxy intermediates. During the reaction, CH 4 molecules are first dissociated into methoxy on the ZnO surface with the assistance of Pd. These methoxy intermediates are then further dehydrogenated and coupled with a methyl radical into ethoxy, which can be subsequently converted into C 2 H 4 through dehydrogenation ( Fig. 2 ). As a result, the optimized ZnO-AuPd hybrid with atomically dispersed Pd sites in the Au lattice achieved a methane conversion 536.0 μmol g −1 with C 2+ compound selectivity 96.0% (39.7% C 2 H 4 and 54.9% C 2 H 6 in total produced C 2+ compounds) after irradiation with light for 8 hours. Methane Valorization by Design of an Atomically Precise Photocatalyst A design for dehydrogenation at an atomic level achieves direct conversion of methane to ethylene of high added value under mild conditions. Fig. 1 : (a,b) Images from a transmission electron microscope (TEM) of ZnO-AuPd hybrids. (c) Scanning TEM image of ZnO-AuPd and corresponding EDS elemental mapping of Zn (blue), Au (yellow) and Pd (cyan). (d) Normalized Pd K-edge X-ray-absorption near-edge structure (XANES) spectra of ZnO-AuPd. (e,f) Fourier-transformed (FT) magnitudes of k 2 -weighted Pd K-edge extended X-ray absorption fine structure spectra of ZnO-AuPd and corresponding fitting analysis. [Reproduced from Ref. 1]