Hetero-metallic metal-organic frameworks for room-temperature NO2 sensing

Metal-organic frameworks (MOFs) with exceptional features such as high structural diversity and surface area as well as controlled pore size has been considered a promising candidate for developing room temperature highly-sensitive gas sensors. In comparison, the hetero-metallic MOFs with redox-active open-metal sites and mixed metal nodes may create peculiar surface properties and synergetic effects for enhanced gas sensing performances. In this work, the Fe atoms in the Fe₃ (Porous coordination network) PCN-250 MOFs are partially replaced by transition metal Co, Mn, and Zn through a facile hydrothermal approach, leading to the formation of hetero-metallic MOFs (Fe₂^IIIM^II, M = Co, Mn, and Zn). While the PCN-250 framework is maintained, the morphological and electronic band structural properties are manipulated upon the partial metal replacement of Fe. More importantly, the room temperature NO₂ sensing performances are significantly varied, in which Fe₂Mn PCN-250 demonstrates the largest response magnitude for ppb-level NO₂ gas compared to those of pure Fe₃ PCN-250 and other hetero-metallic MOF structures mainly attributed to the highest binding energy of NO₂ gas. This work demonstrates the strong potential of hetero-metallic MOFs with carefully engineered substituted metal clusters for power-saving and high-performance gas sensing applications.

Unusual Moisture-Enhanced CO2 Capture within Microporous PCN-250 Frameworks

Developing metal-organic frameworks (MOFs) with moisture-resistant feature or moisture-enhanced adsorption is challenging for the practical CO₂ capture under humid conditions. In this work, under humid conditions, the CO₂ adsorption behaviors of two iron-based MOF materials, PCN-250(Fe₃) and PCN-250(Fe₂Co), were investigated. An interesting phenomenon is observed that the two materials demonstrate an unusual moisture-enhanced adsorption of CO₂. For PCN-250 frameworks, H₂O molecule induces a remarkable increase in the CO₂ uptake for the dynamic CO₂ capture from CO₂/N₂ (15:85) mixture. For PCN-250(Fe₃), its CO₂ adsorption capacity increases by 54.2% under the 50% RH humid condition, compared with that under dry conditions (from 1.18 to 1.82 mmol/g). Similarly, the CO₂ adsorption uptake of PCN-250(Fe₂Co) increases from 1.32 to 2.23 mmol/g, exhibiting a 68.9% increase. Even up to 90% RH, for PCN-250(Fe₃) and PCN-250(Fe₂Co), obvious increases of 43.7 and 70.2% in the CO₂ adsorption capacities are observed in comparison with those under dry conditions, respectively. Molecular simulations indicate that the hydroxo functional groups (μ₃-O) within the framework play a crucial role in improving CO₂ uptake in the presence of water vapor. Besides, partial substitution of Fe³⁺ by Co²⁺ ions in the PCN-250 framework gives rise to a great improvement in CO₂ adsorption capacity and selectivity. The excellent moisture stability (stable even after exposure to 90% RH humid air for 30 days), superior recyclability, as well as moisture-enhanced feature make PCN-250 as an excellent MOF adsorbent for CO₂ capture under humid conditions. This study provides a new paradigm that PCN-250 frameworks can not only be moisture resistant but can also subtly convert the common negative effect of moisture to a positive impact on improving CO₂ capture performance.