Is There a Discernible Photochemical Effect Beyond Heating for Visible Photon-Mediated NH₃ Decomposition over Ru/Al₂O₃?

Recent research has demonstrated that transition metal nanoparticles on insulating oxide supports can utilize visible photon fluxes to drive a wide range of chemical transformations. These observations have been accompanied by debate on the mechanism of photon-driven catalytic reactions: either equilibrium heating of the catalyst bed or photochemical mechanisms mediated by transient charge transfer to adsorbates. Here, we demonstrate that for ammonia (NH3) decomposition at low NH3 pressure (∼0.01 bar) over Ru/Al2O3 catalysts, the promotion of NH3 decomposition rate by visible photon illumination (0–5.5 W cm–2 of 440–635 nm photons) can be fully explained by photon-induced heating of the catalyst bed. This conclusion is supported by catalytic rate measurements collected under a range of conditions─with and without photo illumination, at different wavelengths, and with varying amounts of cofed H2─and through the use of a thermocouple placed in the catalyst bed to report the local temperature. Further, we can confirm that CuRu/Al2O3 exhibits a non-thermal mechanism in photon-driven NH3 decomposition. Ultimately, the successful distinction of thermal and non-thermal contributions for low-pressure NH3 decomposition on Ru/Al2O3 appears to be an effective control system to validate experimental approaches for distinguishing between thermal and photochemical contributions in photon-driven catalysis.