Amino Acid Electrosynthesis with Oxygen Vacancy-Mediated CeO2 Nanocrystals: Facet Effect and Catalytic Mechanism

Abstract

Amino acids are widely used in food, pharmaceuticals, and agrochemicals, presenting significant societal demand, and the artificial synthesis of amino acids is an important yet challenging task. Through electrocatalytic C–N coupling, the synthesis of amino acids from biomass α-keto acids and waste nitrate under mild aqueous conditions has become a green and alternative strategy. Rare-earth-based materials, due to their unique 4f orbitals and tunable crystal facets, often serve as potential resource-rich catalysts. However, their structure–performance relationship in C–N coupling for amino acids synthesis remains unclear. Therefore, eight rare-earth-based catalysts were screened in this work and CeO₂ was chosen as an appropriate model catalyst for the mechanism investigation on the electrosynthesis of alanine. Four CeO₂ nanomaterials with distinct morphologies and crystal facets were synthesized, among which CeO₂ nanorods (CeO₂-NRs) exposing the (110) facet exhibited the highest oxygen vacancy (O_v) concentration and optimal electrosynthetic performance for alanine. A series of control experiments, electrochemical characterizations, in situ electrochemical attenuated total reflection Fourier transform infrared spectroscopy (in situ ATR-FTIR), online electrochemical differential mass spectrometry (DEMS), quasi in situ electron paramagnetic resonance (quasi in situ EPR) experiments, combined with density functional theory (DFT) calculations indicated that the synthesis pathway for alanine involved the reduction of NO₃^– to produce ^*NH₂OH in situ, which nucleophilically attacked the carbonyl group of pyruvate to form the key intermediate species, oxime, then underwent further amination to generate alanine. The key step responsible for the performance difference of four CeO₂ nanocrystals lay in the reduction amination of pyruvate oxime (PO), confirming the (110) facet with more O_v exposure facilitated the cleavage of the N–O bond in pyruvate oxime (^*OOC(H₃C)C═N–OH→^*OOC(H₃C)C═N), while also lowering the energy consumption for the hydrogenation of the C═N bond (^*OOC(H₃C)C═NH→^*OOC(H₃C)CNH₂). This innovative strategy not only provides a new route for the valorization of biomass and waste nitrate but also offers valuable guidance for the design of more efficient rare-earth-based catalysts in this field.