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

Abstract

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 (NH 3 ) decomposition at low NH 3 pressure (∼0.01 bar) over Ru/Al 2 O 3 catalysts, the promotion of NH 3 decomposition rate by visible photon illumination (0−5.5 W cm −2 of 440−635 nm photons) can be fully explained by photoninduced 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 H 2 �and through the use of a thermocouple placed in the catalyst bed to report the local temperature. Further, we can confirm that CuRu/Al 2 O 3 exhibits a non-thermal mechanism in photon-driven NH 3 decomposition. Ultimately, the successful distinction of thermal and nonthermal contributions for low-pressure NH 3 decomposition on Ru/Al 2 O 3 appears to be an effective control system to validate experimental approaches for distinguishing between thermal and photochemical contributions in photon-driven catalysis.