ABSTRACT
In situ and operando investigation of photocatalysts plays a fundamental role in understanding the processes of active phase formation and the mechanisms of catalytic reactions, which is crucial for the rational design of more efficient materials. Using a custom-made operando photocatalytic cell, an in situ procedure to follow the formation steps of Pd/TiO2 photocatalyst by synchrotron-based X-ray absorption spectroscopy (XAS) is proposed. The procedure resulted in the formation of â¼1â nm Pd particles with a much narrower size distribution and homogeneous spreading over TiO2 support compared with the samples generated in a conventional batch reactor. The combination of in situ XAS spectroscopy with high-angle annular dark-field scanning transmission electron microscopy demonstrated the formation of single-atom Pd(0) sites on TiO2 as the initial step of the photodeposition process. Palladium hydride particles were observed for all investigated samples upon exposure to formic acid solutions.
ABSTRACT
An interplay between Pd and PdO and their spatial distribution inside the particles are relevant for numerous catalytic reactions. Using in situ time-resolved X-ray absorption spectroscopy (XAS) supported by theoretical simulations, a mechanistic picture of the structural evolution of 2.3 nm palladium nanoparticles upon their exposure to molecular oxygen is provided. XAS analysis revealed the restructuring of the fcc-like palladium surface into the 4-coordinated structure of palladium oxide upon absorption of oxygen from the gas phase and formation of core@shell Pd@PdO structures. The reconstruction starts from the low-coordinated sites at the edges of palladium nanoparticles. Formation of the PdO shell does not affect the average PdâPd coordination numbers, since the decrease of the size of the metallic core is compensated by a more spherical shape of the oxidized nanoparticles due to a weaker interaction with the support. The metallic core is preserved below 200 °C even after continuous exposure to oxygen, with its size decreasing insignificantly upon increasing the temperature, while above 200 °C, bulk oxidation proceeds. The PdâPd distances in the metallic phase progressively decrease upon increasing the fraction of the Pd oxide due to the alignment of the cell parameters of the two phases.
ABSTRACT
We compared the H2 production from glycerol photoreforming for different TiO2 polymorphs, highlighting an increase of activity in the order Pt-rutile < Pt-P25 ≈ Pt-anatase < Pt-brookite with a different distribution of the reaction intermediates. We show that the highest ability to adsorb water and the different distribution of Pt active sites in brookite can positively influence its photoactivity.
ABSTRACT
Infrared spectroscopy (IR) is a widely used technique enabling to identify specific functional groups in the molecule of interest based on their characteristic vibrational modes or the presence of a specific adsorption site based on the characteristic vibrational mode of an adsorbed probe molecule. The interpretation of an IR spectrum is generally carried out within a fingerprint paradigm by comparing the observed spectral features with the features of known references or theoretical calculations. This work demonstrates a method for extracting quantitative structural information beyond this approach by application of machine learning (ML) algorithms. Taking palladium hydride formation as an example, Pd-H pressure-composition isotherms are reconstructed using IR data collected in situ in diffuse reflectance using CO molecule as a probe. To the best of the knowledge, this is the first example of the determination of continuous structural descriptors (such as interatomic distance and stoichiometric coefficient) from the fine structure of vibrational spectra, which opens new possibilities of using IR spectra for structural analysis.
