Tracking life and death of carbon nitride supports in platinum-catalyzed vinyl chloride synthesis.

Abstract

Deactivation of metal-based catalysts for vinyl chloride synthesis via acetylene hydrochlorination is often dictated by indispensable, catalytically-active carbon supports, but underlying mechanisms remain unclear. Carbon nitrides offer an attractive platform for studying them thanks to ordered structure and high N-content, which facilitates coking. Herein, we monitor the life and death of carbon nitride supports for Pt single atoms in acetylene hydrochlorination, demonstrating that specific N-functionalities and their restructuring cause distinct deactivation mechanisms. Varying polymerization and exfoliation degrees in pristine carbon nitrides (i.e., -NH_x termination and N-vacancy concentrations), we establish graphitic and pyridinic N-atoms as C₂H₂ adsorption sites and pyridinic N-vacancies as coking sites through kinetic and spectroscopic analyses. Uniquely suited for probing point defects, operando electron paramagnetic spectroscopy, coupled to simulations, reveals that HCl drives depolymerization, by protonating heptazine-linking graphitic N-atoms, and generates graphitic N-vacancies, forming NH₃. These reduce C₂H₂ adsorption and promote radical polymerization into coke, respectively, without altering Pt atoms. Design guidelines to mitigate deactivation are discussed, highlighting the importance of tracking active functionalities in carbons.