The development of efficient catalysts for converting water and CO₂ selectively to hydrocarbons using renewable (electrical) energy is considered the “holy grail” of a sustainable energy economy. Electrochemical CO₂ reduction has been demonstrated to form CO, HCOOH, C1 and C2 alcohols on noble metals,^1,2 and alkanes on copper.³ These technologies are still pre-commercial and limited by three main problems: 1) atom efficiency: significant H₂ production at the expense of the desired product; 2) selectivity: low for a single hydrocarbon product, and 3) electrical efficiency: high overpotential for the reaction. Among the proposed mechanisms on copper, two steps have been postulated as rate-limiting: 1) reductive binding to form adsorbed CO₂^-* and, 2)  hydrogen addition to adsorbed CO* forming HCO*⁴.

To overcome these barriers, we used three strategies which involve use of catalyst surfaces comprised of binary solids (M_xP_y) in which: 1) phosphorous is chosen for its high P-O bond energy needed to stabilize side-on binding of the HCO* and CO₂^-* intermediates via O atom; 2) the M-C bond energy is controlled (choice of M); and 3) an adjustable M-P interatomic spacing that allows C to adopt its favored trigonal binding geometry (ideal sp² C hybridized orbitals).  We have synthesized four distinct binary transition metal phosphides, having pure and stable crystalline phases and evaluated their performance as catalysts for CO₂ reduction using low surface area (flat) electrodes. This allows for direct observation of catalytic activity on the most stable crystal termination and is directly comparable to Cu-foils reported in the literature. Herein we present results that show CO₂ reduction to CH₄ at -0.6 V, the lowest potential seen thus far to our knowledge. Furthermore, using metal phosphides there is no formation of carbon monoxide, a poisonous gas that is produced in large quantities when copper is employed as catalyst.

Supported by Rutgers University and the Science Without Borders CAPES fellowship.

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