Researchers Construct Isolated Active Sites In Porous Single-Crystalline Structures

Porous single-crystal monolithic catalytic materials offer the benefits of a clear lattice structure, long-range ordered lattice structure, precise chemical composition, disorderly interconnected pore structure, and clear surface composition.

Image Credit: ImageFlow/Shutterstock.com

These characteristics can help develop a continuous highly distorted active surface and fine active structure. It is of high importance for the analysis of the catalytic mechanism and surface structure in different practical catalytic reactions.

A group of researchers headed by Professor Kui Xie at Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences recently developed porous single-crystalline Mn₂O₃ and Mn₃O₄ monoliths at centimeter-scale to make well-defined surface structures. They restricted the atomically dispersed Pt at Mn sites at the twisted surfaces in lattice to create isolated active sites in the porous single-crystalline architectures.

The researchers exhibited the considerably improved activation of lattice oxygen associated with the isolated Pt ions in lattice at the well-defined surfaces within the local structures. The research has been published in the CCS Chemistry journal.

First, the team developed the parent single crystals of (014) MnCO₃ and (001) Mn₂P₂O₇, and subsequently eliminated the periodic target atomic P-O or C-O structures by a lattice reconstruction approach through which the Mn-O single crystal’s framework structure was rebuilt into the mesoporous single crystal.

At higher temperatures, the rebuilding of lattice structures in a crystallization process would result in interconnected pore formation. The shrinking of lattice from parent carbonate and phosphates single crystals to oxide single crystals predominantly led to the formation of pore structure and porosity.

Moreover, the team restricted the isolated Pt at the Mn site at the top layer of porous Mn₂O₃ and Mn₃O₄ single crystals in lattice using an atomic layer deposition technique. The high-sensitive low-energy ion scattering (HS-LEIS) spectra demonstrated effective deposition of Pt at the top layer of porous single crystals.

By calculating the Fourier transform of k3-weight of Extended X-Ray Absorption Fine Structure spectrum for the isolated Pt₁/PSC-Mn₃O₄, the researchers identified a radial distribution at approximately 1.62 Å for the Pt-O coordination. At the same time, the Pt-Pt distribution at about 2-3 Å was not observed.

Through X-ray absorption near edge structure measurements, it was verified that the valence states of isolated Pt species lied between the Pt⁰ and Pt⁴⁺ at the top layer of PSC-Mn₃O₄. A comparative analysis of the Cs-HRTEM image and lattice structures helped clarify the coordination structures of the PtO_1.5 and PtO_1.4 active sites.

The researchers constructed isolated and fine structure active centers of PtO_1.4 and PtO_1.5 in the surface of crystal lattice by loading Pt. The lattice oxygen linked to Pt ions can be more easily activated in the local structure, and there is an approximately seven to eight times increase in the surface oxygen exchange coefficient.

The team demonstrated the complete CO oxidation with air at 65 °C without any degradation even after 300 hours of operation with the porous single-crystalline monoliths.

This research offers an efficient approach to modify the CO₂ electroreduction selectivity by developing single types of active sites and stabilizing crucial intermediates with hydrogen bond, thereby enhancing the current density by using conductive framework materials.

Journal Reference:

Lin, G., et al. (2021) Identifying and engineering active sites at the surface of porous single-crystalline oxide monoliths to enhance catalytic activity and stability. CCS Chemistry. doi.org/10.31635/ccschem.021.202000740.