Coexisting Cu clusters and Cu-N_X sites in ZIF-8-derived carbon for plasma-catalytic hydrogen evolution from water vapor

Abstract

How different active sites cooperate under non-thermal plasma (NTP) conditions is still poorly understood. Here, Cu-doped ZIF-8-derived carbon carbonized at 700 °C (Cu/Z-700) is shown to host three distinct active centers: metallic Cu clusters, Cu–N coordinated (Cu–NC) moieties and pyridinic N (Py-N) sites. Cu K-edge X-ray absorption spectroscopy reveals that ∼80% of Cu exists as metallic clusters and ∼ 20% as isolated Cuδ+–Nₓ sites. Density functional theory (DFT) calculations suggest a functional division among the three sites: Cu clusters show the strongest Cu–OH binding and the most favorable reaction free energies, Cu–NC sites favor intermediate conversion and product desorption, and Py-N sites promote water adsorption. In a dielectric barrier discharge (DBD), Cu/Z-700 delivers a H₂ production rate of 1.33 mL min−1 from water vapor generated from simulated seawater at a residence time of 3 s, an 11-fold enhancement over plasma alone, with 93.6% activity retention over 20 cycles. Gas-phase product analysis shows that more than 97% of the H₂ originates from water splitting. The results clarify the nature of the active sites in Cu/ZIF-8-derived carbon and their contribution to plasma-catalytic water splitting.