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We are developing the in-situ techniques for identifying the catalytic properties for various reactions. Currently, we are successful to conduct the measurement of in-situ X-ray absorption spectroscopy, in-situ high-energy-resolution fluorescence-detected X-ray absorption spectroscopy, in-situ X-ray diffraction, and in-situ Raman spectroscopy.
In-situ X-ray absorption spectroscopy
X-ray absorption spectroscopy (XAS) contains X-ray absorption near edge spectra (XANES) and extended X-ray absorption fine structure (EXAFS). The former allows us to recognize the average oxidation number and the electronic configuration of the specific elements in materials while the latter provides the information of interatomic distance and the coordination number. The local atomic environment can be described for each type of atom in a chemical compound, so that the geometrical sites of the occupied atoms, e.g., tetrahedral or octahedral sites, can be identified. Besides, the material does not require an ordered structure, which means that atom in amorphous materials or molecular samples can be investigated. These natures make XAS as a powerful manner to in-situ probe the structural and chemical variation in the electrocatalysts during the catalysis.
In-situ XAS provides abundant informative evidence regarding the detailed mechanism of catalysis, including the identification of the active elements for the catalysis, the verification of the authentic catalysts, and observation of steady reaction intermediates in single-atom catalysts. XAS via hard X-ray averages the signals from bulk and surface parts of the materials, weakening the information through the catalysis, so promoting the signals on the catalytic surface by reducing the X-ray penetration depth can boost and widen the development and versatility of the in-situ XAS for catalysis.
Ref: Hung, S.-F. Catal. Sci. Technol. 2022, accepted.
In-situ high-energy-resolution fluorescence-detected X-ray absorption spectroscopy
In-situ probing and analyzing the information regarding the evolution of d orbitals in central metal sites during the reaction helps understand the overall catalytic mechanism. In-situ soft X-ray absorption spectroscopy exerts its advantages as above mentioned. Sometimes, the conditions that materials are required to be deposited onto specific silicon nitride window and the signal is acquired from the substrate side rather than the side of catalytic surface significantly hinder the extensive applications in diverse catalytic systems. In-situ high-energy resolution fluorescence detected X-ray Absorption Spectroscopy (HERFD-XAS, 1s → 3d) allows us to investigate 3d orbitals in a typical catalytic environment.
Ref: Hung, S.-F. J. Am. Chem. Soc. 2018, 140, 17263-17270.
In-situ X-ray diffraction
X-ray diffraction (XRD) offers precise crystallography information, including the lattice parameters and the preferred orientation. Thus, the strain, the crystal facet, the doping effect, and the phase separation can be analyzed through XRD. XRD distinguishes the atomic arrangement with long-range order, meaning the amorphous phase could not be recognized, but the peak sets in XRD can be employed to reconstruct the whole crystal structure. In-situ surface XRD can clarify the lattice-related information, i.g., phase identification, strain, grain size, which also provides a crucial segment to understand the catalytic mechanism.
Ref: Hung, S.-F. ACS Energy Lett.2019, 4, 2813-2820.
In-situ Raman spectroscopy
Raman spectroscopy allows us to identify various vibration modes of the chemical bonds, which are the fingerprint of specific materials. In-situ Raman spectroscopy can detect the material phase and the surface intermediates during the catalysis.
Ref: Hung, S.-F. ACS Energy Lett.2019, 4, 2813-2820.