As rising star materials, single-atom and dualatom catalysts have been widely reported in the electrocatalysis area. To answer the key question: single-atom and dual-atom catalysts, which is better for electrocatalytic urea synthesis? we design two types of catalysts via a vacancy-anchorage strategy: single-atom Pd 1 À TiO 2 and dual-atom Pd 1 Cu 1 À TiO 2 nanosheets. An ultrahigh urea activity of 166.67 mol urea mol Pd À 1 h 1 with the corresponding 22.54 % Faradaic efficiency at À 0.5 V vs. reversible hydrogen electrode (RHE) is achieved over Pd 1 Cu 1 À TiO 2 , which is much higher than that of Pd 1 À TiO 2 . Various characterization including an in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and theoretical calculations demonstrate that dual-atom Pd 1 Cu 1 site in Pd 1 Cu 1 À TiO 2 is more favorable for producing urea, which experiences a CÀ N coupling pathway with a lower energy barrier compared with Pd

The catalytic activities are generally believed to be relevant to the electronic states of their active center, but understanding this relationship is usually difficult. Here, we design two types of catalysts for electrocatalytic urea via a coordination strategy in a metal–organic frameworks: CuIII-HHTP and CuII-HHTP. CuIII-HHTP exhibits an improved urea production rate of 7.78 mmol h−1 g−1 and an enhanced Faradaic efficiency of 23.09% at − 0.6 V vs. reversible hydrogen electrode, in sharp contrast to CuII-HHTP. Isolated CuIII species with S = 0 spin ground state are demonstrated as the active center in CuIII-HHTP, different from CuII with S = 1/2 in CuII-HHTP. We further demonstrate that isolated CuIII with an empty $${d}_{{{\text{x}}^{2}\text{-y}}^{2}}^{0}$$ d x 2 -y 2 0 orbital in CuIII-HHTP experiences a single-electron migration path with a lower energy barrier in the C–N coupling process, while CuII with a single-spin state ($${d}_{{{\text{x}}^{2}\text{-y}}^{2}}^{1}$$ d x 2 -y 2 1 ) in CuII-HHTP undergoes a two-electron migration pathway.