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CCI Solar Events - December, 2017 Pick date

1 PM

CCI Cybermeeting - Joe Elias, Nocera Group - Harvard

01:00 PM to 02:00 PM, December 14, 2017

Location: eZuce

Electrochemical Reduction of P(V)
The activation and cleavage of stable bonds to oxygen is a fundamental obstacle to efficient electrocatalysis pertaining to solar fuels. Processes such as carbon dioxide and oxygen reduction reactions involve the transfer of multiple electrons and protons, which leads to large overpotentials — and sluggish kinetics — for these reactions.

Like the reduction of CO2 to CO, the reduction of phosphate (PO43-) to phosphite (PO33-) ions in aqueous solution is a two-electron and two-proton process that occurs at potentials more negative than the competing HER (E0 = -0.28 V vs RHE).

Strategies for the selective activation and breaking of the strong phosphoryl bond (D0 = 544 kJ mol-1) in PO43- — and the suppression of hydrogen evolution — is therefore of considerable practical importance for multi-electron electrocatalysis in general.

Furthermore, the process industry currently employs for the production of synthetically-useful P(III) compounds is inefficient. For PCl3, phosphate rock is first reduced by five electrons to P4 at high temperatures (1500–2000 °C, ΔHrxn = 700 kcal mol-1) before being oxidized by three electrons to PCl3 in the presence of Cl2Hrxn = -304 kcal mol-1), with much of the energy stored in P4 being wasted to heat.

An attractive strategy, then, involves the direct two-electron reduction of PO43- to PO33-, which doesn’t require accessing P4. Electrochemically-generated PO33-, in turn, can be converted to phosphine (PH3), an important intermediate in the production of synthetically relevant compounds such as PCl3, PCl5, and POCl3.

We studied the electrochemical reduction of the related phosphorus(V) compound, triphenylphosphine oxide (TPPO) — itself an important waste byproduct of numerous organic reactions — to gain insight into the reduction of phosphate. We’ve found that the addition of simple Lewis acids, notably triphenyl borate, leads to the two- and four-electron reduction of TPPO to triphenylphosphine and diphenylphosphine at Faradaic efficiencies of 30 and 60%, respectively.

Cyclic voltammetry suggests that the electrochemical reduction of TPPO proceeds through a radical-substrate dimerization/disproportionation mechanism in which the one electron reduced TPPO radical (TPPO●–) transfers an electron to a reactive association complex of the Lewis acid and TPPO●–, ultimately forming triphenylphosphine. The further two-electron reduction of triphenylphosphine to diphenylphosphine is not accelerated by the presence of the Lewis acid, but rather the TPPO●–/Lewis acid complex.

These results suggest that a possible route towards the room-temperature reduction of phosphate to phosphite may include the stabilization of reduced phosphate intermediates through their association with simple Lewis acids.