Charlynn Sher Lin Koh

Favoring the unfavored: Selective electrochemical nitrogen fixation using a reticular chemistry approach

Originally unfavored nitrogen-to-ammonia electroconversion is now preferred over competing reaction using reticular chemistry. Electrochemical nitrogen-to-ammonia fixation is emerging as a sustainable strategy to tackle the hydrogen- and energy-intensive operations by Haber-Bosch process for ammonia production. However, current electrochemical nitrogen reduction reaction (NRR) progress is impeded by overwhelming competition from the hydrogen evolution reaction (HER) across all traditional NRR catalysts and the requirement for elevated temperature/pressure. We achieve both excellent NRR selectivity (~90%) and a significant boost to Faradic efficiency by 10 percentage points even at ambient operations by coating a superhydrophobic metal-organic framework (MOF) layer over the NRR electrocatalyst. Our reticular chemistry approach exploits MOF’s water-repelling and molecular-concentrating effects to overcome HER-imposed bottlenecks, uncovering the unprecedented electrochemical features of NRR critical for future theoretical studies. By favoring the originally unfavored NRR, we envisage our electrocatalytic design as a starting point for high-performance nitrogen-to-ammonia electroconversion directly from water vapor–abundant air to address increasing global demand of ammonia in (bio)chemical and energy industries.

Nanoporous Gold Bowls: A Kinetic Approach to Control Open Shell Structures and Size-Tunable Lattice Strain for Electrocatalytic Applications.

Controlling sub-10 nm ligament sizes and open-shell structure in nanoporous gold (NPG) to achieve strained lattice is critical in enhancing catalytic activity, but it remains a challenge due to poor control of reaction kinetics in conventional dealloying approach. Herein, a ligament size-controlled synthesis of open-shell NPG bowls (NPGB) through hetero-epitaxial growth of NPGB on AgCl is reported. The ligament size in NPGB is controlled from 6 to 46 nm by varying the hydroquinone to HAuCl4 ratio. The Williamson-Hall analysis demonstrates a higher lattice strain in smaller ligament size. In particular, NPGB with 6 nm (NPGB 6) ligament size possess the highest strain of 15.4 × 10(-3) , which is nearly twice of conventional 2D NPG sheets (≈8.8 × 10(-3) ). The presence of high surface energy facets in NPGBs is also envisaged. The best electrocatalytic activity toward methanol oxidation is observed in NPGB 6 (27.8 μA μg(-1) ), which is ≈9-fold and 3-fold higher than 8 nm solid Au nanoparticles, and conventional NPG sheets. The excellent catalytic activity in NPGB 6 is attributed to the open-shell structure, lattice strain, and higher electro-active surface area, allowing efficient exposure of catalytic active sites to facilitate the methanol oxidation. The results offer a potential strategy for designing next generation electrocatalysts.

Driving CO2 to a Quasi-Condensed Phase at the Interface between a Nanoparticle Surface and a Metal–Organic Framework at 1 bar and 298 K

We demonstrate a molecular-level observation of driving CO2 molecules into a quasi-condensed phase on the solid surface of metal nanoparticles (NP) under ambient conditions of 1 bar and 298 K. This is achieved via a CO2 accumulation in the interface between a metal-organic framework (MOF) and a metal NP surface formed by coating NPs with a MOF. Using real-time surface-enhanced Raman scattering spectroscopy, a >18-fold enhancement of surface coverage of CO2 is observed at the interface. The high surface concentration leads CO2 molecules to be in close proximity with the probe molecules on the metal surface (4-methylbenzenethiol), and transforms CO2 molecules into a bent conformation without the formation of chemical bonds. Such linear-to-bent transition of CO2 is unprecedented at ambient conditions in the absence of chemical bond formation, and is commonly observed only in pressurized systems (>105 bar). The molecular-level observation of a quasi-condensed phase induced by MOF coating could impact the future design of hybrid materials in diverse applications, including catalytic CO2 conversion and ambient solid-gas operation.

Microchemical Plant in a Liquid Droplet: Plasmonic Liquid Marble for Sequential Reactions and Attomole Detection of Toxin at Microliter Scale

Miniaturizing the continuous multistep operations of a factory into a microchemical plant offers a safe and cost-effective approach to promote high-throughput screening in drug development and enforcement of industrial/environmental safety. While particle-assembled microdroplets in the form of liquid marble are ideal as microchemical plant, these platforms are mainly restricted to single-step reactions and limited to ex situ reaction monitoring. Herein, we utilize plasmonic liquid marble (PLM), formed by encapsulating liquid droplet with Ag nanocubes, to address these issues and demonstrate it as an ideal microchemical plant to conduct reaction-and-detection sequences on-demand in a nondisruptive manner. Utilizing a two-step azo-dye formation as our model reaction, our microchemical plant allows rapid and efficient diazotization of nitroaniline to form diazonium nitrobenzene, followed by the azo coupling of this intermediate with target aromatic compound to yield azo-dye. These molecular events are tracked in situ via SERS measurement through the plasmonic shell and further verified with in silico investigation. Furthermore, we apply our microchemical plant for ultrasensitive SERS detection and quantification of bisphenol A (BPA) with detection limit down to 10 amol, which is 50 000-fold lower than the BPA safety limit. Together with the protections offered by plasmonic shell against external environments, these collective advantages empower PLM as a multifunctional microchemical plant to facilitate small-volume testing and optimization of processes relevant in industrial and research contexts.