Making waves: Unmasking the polymerization transfer pathway in heterogeneous catalytic ozonation

Heterogeneous catalytic ozonation (HCO) is typically evaluated through pollutant degradation and mineralization, implicitly assuming that aqueous carbon removal primarily reflects its oxidative conversion to CO₂. Here we challenge this mineralization-centered paradigm by showing that polymerization-driven carbon transfer can represent an overlooked carbon fate in HCO systems. A simplified stoichiometric comparison of representative phenol-based HCO systems showed that in some cases, the total O₃ input was insufficient to achieve complete mineralization based on the observed aqueous carbon removal. More importantly, in a MgO/Al₂O₃-catalyzed phenol ozonation system, we directly captured a non-uniform nanoscale carbon shell formed at the catalyst interface. Microscopic, thermal, and spectroscopic analyses distinguished this shell from simple adsorption and revealed structural features consistent with polymeric carbon products. Based on interfacial geometry and thermal analysis, carbon partitioning further provided an estimation that the polymerization pathway accounted for approximately 8.9% of the initial carbon under the studied conditions. These findings demonstrate that HCO can redistribute organic carbon not only to CO₂ and soluble intermediates, but also to interfacial polymeric deposits. This overlooked carbon fate has direct implications for ozone demand, catalyst deactivation, and downstream environmental risk, underscoring the need for HCO evaluation to move beyond mineralization efficiency toward an integrated carbon fate framework.