Electrolytic Synthesis of White Phosphorus Is Promoted in Oxide-Deficient Molten Salts.

Elemental white phosphorus (P4) is a key feedstock for the entire phosphorus-derived chemicals industry, spanning everything from herbicides to food additives. The electrochemical reduction of phosphate salts could enable the sustainable production of P4; however, such electrosynthesis requires the cleavage of strong, inert P-O bonds. By analogy to the promotion of bond activation in aqueous electrolytes with high proton activity (Brønsted-Lowry acidity), we show that low oxide anion activity (Lux-Flood acidity) enhances P-O bond activation in molten salt electrolytes. We develop electroanalytical tools to quantify the oxide dependence of phosphate reduction, and find that Lux acidic phosphoryl anhydride linkages enable selective, high-efficiency electrosynthesis of P4 at a yield of 95% Faradaic efficiency. These fundamental studies provide a foundation that may enable the development of low-carbon alternatives to legacy carbothermal synthesis of P4.

Graphite Conjugation Eliminates Redox Intermediates in Molecular Electrocatalysis.

The efficient interconversion of electrical and chemical energy requires the intimate coupling of electrons and small-molecule substrates at catalyst active sites. In molecular electrocatalysis, the molecule acts as a redox mediator which typically undergoes oxidation or reduction in a separate step from substrate activation. These mediated pathways introduce a high-energy intermediate, cap the driving force for substrate activation at the reduction potential of the molecule, and impede access to high rates at low overpotentials. Here we show that electronically coupling a molecular hydrogen evolution catalyst to a graphitic electrode eliminates stepwise pathways and forces concerted electron transfer and proton binding. Electrochemical and X-ray absorption spectroscopy data establish that hydrogen evolution catalysis at the graphite-conjugated Rh molecule proceeds without first reducing the metal center. These results have broad implications for the molecular-level design of energy conversion catalysts.

Chemiresistive Detection of Gaseous Hydrocarbons and Interrogation of Charge Transport in Cu[Ni(2,3-pyrazinedithiolate)2] by Gas Adsorption.

The development of new chemiresistive materials for use in chemical sensors that operate near ambient conditions could potentially reduce the costs of implementation, encouraging their use in new areas. Conductive metal-organic frameworks represent one intriguing class of materials for further investigation in this area, given their vast structural diversity and the specificity of adsorbate interactions afforded by their crystallinity. Here, we re-examine the electronic conductivity of the desolvated and acetonitrile-solvated microporous framework Cu[Ni(pdt)2] (pdt2- = 2,3-pyrazinedithiolate), and find that the conductivity in the pristine material is 200-fold greater than in the solvated state, highlighting the sensitivity of sample conductivity to guest inclusion. Additionally, the desolvated material is demonstrated to selectively adsorb the gaseous hydrocarbons ethane, ethylene, acetylene, propane, propylene, and cis-2-butene at ambient temperature. Investigation of the effect of gas adsorption on conductivity using an in situ measurement cell reveals a chemiresistive response for each adsorbate, and the change in conductivity with adsorbate pressure closely follows an empirical model identical in form to the Langmuir-Freundlich equation. The relative sensitivity of the framework to each adsorbate is, surprisingly, not correlated with binding strength. Instead, the differences in chemiresistive response between adsorbates are found to correlate strongly with gas phase specific heat capacity of the adsorbate. Nanoconfinement effects, manifesting as a relative deviation from the expected chemiresistive response, may influence charge transport in the case of the largest adsorbate considered, cis-2-butene. Time-resolved conductance and adsorption measurements additionally show that the chemiresistive response of the sensor equilibrates on a shorter time scale than gas adsorption, suggesting that interparticle contacts limit conduction through the bulk material and that conductivity at the crystallite surfaces is most responsive to gas adsorption.