Tuning local Lewis acidity in transition-metal-doped Nb2C MXenes for enhanced urea electrosynthesis

Abstract

Electrocatalytic C-N coupling for urea synthesis offers a sustainable alternative to conventional industrial processes, yet its efficiency is hindered by challenges of activating inert reactants and controlling the competing reaction pathways. Here, we employ first-principles calculations to systematically investigate a series of transition-metal-doped Nb2C MXenes (TM@Nb2C, TM = 3d/4d metals) as catalysts for urea synthesis from CO and N2. Our findings establish that TM doping is a powerful strategy to modulate the local Lewis acidity of surface metal sites, which in turn governs the adsorption and activation of reactants and intermediates. Through a multi-step screening process based on stability, activity, and selectivity, Cu@Nb2C emerges as a superior catalyst. It exhibits an exceptionally low limiting potential (-0.34 V), a modest kinetic barrier for C-N coupling (0.58 eV), and high selectivity against competing hydrogen evolution and nitrogen reduction. To quantitatively link the catalytic performance to Lewis acidity, we introduce the solvated relative fluoride ion affinity (∆FIA TM@Nb2C solv ) as a robust descriptor, using HF as a probe molecule. A distinct volcanotype relationship is uncovered between this descriptor and the urea synthesis activity.Cu@Nb2C sits precisely at the volcano apex, where its moderate Lewis acidity achieves an optimal balance in the free energy landscape of all elementary steps. This work not only identifies Cu@Nb2C as a promising electrocatalyst but also establishes a novel, generalizable framework for quantifying surface Lewis acidity, offering a new design principle for high-performance urea electrocatalysts and beyond.