Recent advances in semiconductor-based photocatalysis have substantially improved the degradation of organic molecules in water, fueled by both nanotechnology breakthroughs and novel continuous-flow photoreactor designs. In particular, the coiled flow inverter (CFI) has gained attention for exploiting static mixing to enhance photon utilization and mass transfer. However, the issue of catalyst particle suspension stability—often addressed simply by increasing flow rate—remains largely overlooked. This challenge becomes especially critical in the CFI because its upper flow-rate limit is constrained by the need to maintain static-mixing benefits. In this study, we investigated the visible-light-induced degradation of fluorescein using ZnO-APTMS-Au microparticle catalysts in a CFI. Our findings revealed a reaction-induced destabilization of these microparticles, leading to sedimentation even under conditions initially optimized for stable suspension. Kinetic analysis following a Langmuir–Hinshelwood model yielded a rate constant k = 〈1.0042 ± 102〉×10-5 mol/L min and a binding constant K = 〈1.204 ± 269〉×105 mol/L min. Notably, the accumulation of degradation products on the catalyst surface appeared to foster further agglomeration, thereby exacerbating particle settling. As a result, the photonic efficiency diminished over time, indicating that the reaction chemistry itself can accelerate catalyst suspension loss. These observations underscore the importance of integrating both chemical kinetics and suspension stability considerations into a heterogeneous photoreactor design. Merely tuning the flow rate has limited efficacy in preventing particle sedimentation, and alternative strategies—including surface modifications, optimized reactor geometries, or auxiliary mixing approaches—may be necessary to sustain catalyst dispersion. By establishing a direct link between degradation kinetics and catalyst destabilization, this work highlights the need for holistic design and operation protocols that account for multiphase flow behavior over the entire course of the reaction. In doing so, it contributes to the broader effort of advancing continuous-flow photocatalytic processes for chemical synthesis, water remediation, and other environmental applications.
