Electron transfer assisted the electrosynthesis of formamide via MnO2 crystal-phase engineering

• Green electrosynthesis of formamide from NH 3 and CH 3 OH with high productivity. • Four well-defined MnO 2 electrocatalysts via crystal-phase engineering. • Bridge Ov sites enhanced charge transfer from CH 3 OH to β-MnO 2 through Mn–O bonds. • The CH 3 OH electrooxidation to *CH 2 O as the rate determining step. • Eley-Rideal pathway identified for electrochemical C–N coupling of NH 3 and *CH 2 O. Electrocatalytic synthesis of formamide (FA) from abundant methanol and ammonia is a promising alternative to the traditional energy-consumption thermocatalytic route. However, the sluggish electron transfer between catalyst and adsorbed reactants can result in a poor faraday efficiency (FE) and FA yield. Herein, four well-defined α-, β-, γ-, and δ-MnO 2 electrocatalysts are achieved through a crystal-phase engineering strategy. The β-MnO 2 is thermodynamical in favor of the Ov formation in comparison to the other three crystal phases of MnO 2 . The formed bridge Ov sites on β-MnO 2 presents a positive electric field effect, which induces more charge transfer from CH 3 OH to β-MnO 2 via Mn-O bonds and promotes the CH 3 OH oxidation to the *CH 2 O key intermediate. Subsequently, the C–N coupling reaction was accomplished via the directly attack of *CH 2 O intermediates by NH 3 molecules without chemical adsorption. Lastly, the FA formation mechanism was clearly proposed, in which the conversion of *CH 2 OH into *CH 2 O is the rate determining step. The reaction mechanism contains two steps: (1) Adsorption and oxidation of methanol to *CH 2 O via Ov sites assisted electron transfer; (2) NH 3 attack and C–N coupling via the E-R reaction pathway. Therefore, Ov sites assisted electron transfer via crystal-phase engineering strategy was established to enhance the electrosynthesis of formamide.