10,000 publications from this institution
Single-atom catalysts are often considered as the ultimate design principle for supported catalysts, due to their unique geometric and electronic properties and their highly efficient use of precious materials. Here, we report a single-atom catalyst, Cu/UiO-66, prepared by a covalent attachment of Cu atoms to the defect sites at the zirconium oxide clusters of the metal–organic framework (MOF) UiO-66. Kinetic measurements show this catalyst to be highly active and stable under realistic reaction conditions for two important test reactions, the oxidation of CO at temperatures up to 350 °C, which makes this interesting for application in catalytic converters for cars, and for CO removal via selective oxidation of CO in H2-rich feed gases, where it shows an excellent selectivity of about 100% for CO oxidation. Time-resolved operando spectroscopy measurements indicate that the activity of the catalyst is associated with atomically dispersed, positively charged ionic Cu species. Density functional theory (DFT) calculations in combination with experimental data show that Cu binds to the MOF by –OH/–OH2 ligands capping the defect sites at the Zr oxide clusters.
The cyclic fatigue and fracture toughness behavior of reactive hot‐pressed Ti 3 SiC 2 ceramics was examined at temperatures from ambient to 1200°C with the objective of characterizing the high‐temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti 3 SiC 2 materials with fine‐ (3–10 μm) and coarse‐grained (70–300 μm) microstructures. Results indicate that fracture toughness values, derived from rising resistance‐curve behavior, were significantly higher in the coarser‐grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, Δ K th fatigue thresholds were found to be essentially unchanged between 25° and 1100°C; however, there was a sharp decrease in Δ K th at 1200°C (above the plastic‐to‐brittle transition temperature), where significant high‐temperature deformation and damage are first apparent. The substantially higher cyclic‐crack growth resistance of the coarse‐grained Ti 3 SiC 2 microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack‐tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes.
First Page
