Small and salty CO ₂ reduction scheme: Most artificial photosynthesis approaches focus on making hydrogen. Modifying CO ₂ , as plants and microbes do, is more chemically complex. Asadi et al. report that fashioning WSe ₂ and related electrochemical catalysts into nanometer-scale flakes greatly improves their activity for the reduction of CO ₂ to CO. An ionic liquid reaction medium further enhances efficiency. An artificial leaf with WSe ₂ reduced CO ₂ on one side while a cobalt catalyst oxidized water on the other side. Science , this issue p. 467

Reduction of CO₂ to CO on Fe, Co, Ni, and Cu surfaces has been studied using density functional theory (DFT) methods. Three reaction steps were studied: (a) adsorption of CO₂ (M + CO₂ = CO₂/M) (M = transition metal surface), (b) decomposition of CO₂ (CO₂/M = (CO + O)/M), and (c) desorption of CO ((CO + O)/M = O/M + CO). Binding energies and reaction energies were calculated using the generalized gradient approximation (GGA) via the Perdew–Burke–Ernzerhof (PBE) functional. Calculations show an interesting trend for reaction energies and total reaction barriers, as a function of metal: from Fe to Cu, reactions tend to be less exergonic; the metals earlier in the 3d series have lower total barriers for CO₂ reduction. However, “overbinding” of CO₂ on Fe causes a thermodynamic sink on the reaction coordinate, and Co and Ni are more favorable in terms of a smaller fluctuation in reaction energies/barriers for these elementary catalytic steps. A Brønsted–Evans–Polanyi (BEP) relationship was analyzed for C–O bond scission of CO₂ on the metal surfaces. Heterogeneous catalysis is also compared with the homogeneous models using transition metal β-diketiminato complexes, showing that both heterogeneous and homogeneous catalysis of CO₂ reduction display the same energetic trend as a function of metal.