Catalyst characterization

The crystallographic structure of all the samples was determined by Powder XRD (Rigaku Ultima IV, Japan) patterns using a Cu Kα X-ray source (λ = 1.54056 Å). The morphology of all the samples was imaged by transmission electron microscope (TEM, JEM-1400, JEOL Co., Japan). The HRTEM, line-scan analysis, and elemental mapping were carried out on FEI Tecnai F30 electron microscope with an acceleration voltage of 200 kV. STEM images were obtained on an FEI Titan Themis Z, operating at 60-300 kV to observe the surface morphology of the materials. Before microscopy examination, the catalyst powders were ultrasonically dispersed in ethanol and then a drop of the solution was put onto a carbon-coated copper grid. The elemental content of all the samples was examined by scanning electron microscopy energy dispersive spectrometer (SEM-EDS, ZEISS Sigma, Germany) and inductively coupled plasma-atomic emission spectrometry (ICP-AES, iCap 7000, USA). The chemical compositions and valence states of all the samples were analyzed by XPS (K-Alpha, USA). NAP-XPS (SPECS Surface Nano Analysis GmbH, Germany) measurements were carried out on a SPECS system equipped with a differentially pumped Phoibos hemispherical electron energy analyzer using monochromatic Al Kα radiation (1486.6 eV). The H₂ or O₂ (99.999%) flow was introduced into the NAP cell and the total pressure was kept constant at 0.2 mbar via the electronic back-pressure regulator. X-ray Absorption Fine Structure (XAFS) of all samples was analyzed by TLS-01C beamline of the National Synchrotron Radiation Research Center (NSRRC, Hsinchu, Taiwan), and data were processed according to standard procedures using the Demeter program package (Version 0.9.24)³⁹.