Interfacial Structures of Heterogeneous Systems Determined by Using in-Situ XAFS

Abstract

During chemical reactions and catalysis, the surface and interfacial structural properties of materials are considerably important to understand and control the reactions. However, in-situ measurements have many limits during reactions. We used in-situ X-ray absorption fine structure (XAFS) to understand the local structural properties at the surface and the interface of metal catalysts on transition−metal−oxide supports. Pt nanoparticles (NPs) with a mean size of ~1 nm on TiO2 supports are synthesized using an incipient wetness method. In-situ XAFS measurements are performed from the Pt NPs/TiO2 at Pt L3 and Ti K edges during heating from room temperature to 500 ℃ in an H2 environment. Fourier−transformed extended X-ray absorption fine structure (FT−EXAFS), which is a unique tool to quantitatively determine the local structural properties of materials around a selected species element, shows a lack of oxygen atoms as the nearest neighbors of Pt atoms at T = 250 ℃; meanwhile, the wavelet-transformed EXAFS (WT-EXAFS) with enhanced resolution shows a weak but distinguishable oxygen signal around Pt atoms. Pt−O bonds likely exist at the interface of Pt/TiO2, although oxygens on the outer surfaces of Pt NPs are mostly dissociated at T > 250 ℃ in an H2 environment. This finding strongly suggests that the combination of FT−EAXFS and WT−EXAFS can more accurately describe small amounts of light elements bonding with heavy elements than FT−EXAFS can alone. DFT calculations confirm that the presence of extra oxygen atoms at the interfaces of Pt NPs/TiO2, playing a critical role in the stability and the dispersion of Pt NPs.