Identifying a catalytic center in metal-carbon composites for O3 and persulfate activation: Active site switching between carbon and metal phases

Abstract

This study demonstrated that metal-carbon composites (Me-N-C; Me = Mn, Fe, Co, Ni, and Cu) featuring sulfidated metal nanoparticles encapsulated within an N-doped carbon matrix activated both persulfate and O3, albeit through distinct mechanisms. The carbon phase, exhibiting enhanced electrical conductivity due to the presence of internal metal cores, facilitated non-radical persulfate activation. In contrast, the metallic constituent predominantly converted O3 to hydroxyl radical (center dot OH). This oxidant-dependent shift in the principal catalytic site (or degradation pathway) was substantiated by a mechanistic examination of oxidant activation by Ni-N-C that varied in structural characteristics and chemical compositions. The persulfate activation capability of Ni-N-C rose proportionally with the content of graphitic-N as the key species in the non-radical activation pathway. Conversely, center dot OH yield from O3 correlated strongly with the degree of Ni sulfidation, suggesting that sulfidated Ni functioned as the catalytic center for O3 activation. The active-site switching was further supported by the impact of H2-assisted pyrolysis, which suppressed Ni sulfidation while enriching graphitic-N, thereby enhancing persulfate activation but kinetically retarding O3-to-center dot OH conversion. UV irradiation, preventing surface organic accumulation, and thermal sulfidation, enriching metal sulfide content, effectively regenerated Ni-N-C by targeting the respective catalytic centers for persulfate and O3 activation. DFT calculations indicated that Ni3S2 displayed a preferential tendency to dissociatively adsorb and subsequently activate O3, whereas carbonencapsulated Ni promoted non-radical persulfate activation at the carbon interface. The identification of oxidant-specific catalytic sites provided a pivotal design rationale for developing metal-carbon composites as versatile catalysts for oxidant activation.