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.

近期研究表明，负载于绝缘氧化物载体上的过渡金属纳米颗粒可利用可见光光子通量驱动多种化学转化。针对这类现象，学界围绕光驱动催化反应的机理尚存争议：一说认为是催化床的平衡热升温效应，另一说则认为是由吸附质的瞬时电荷转移所介导的光化学机制。本研究证实，针对低氨气分压（约0.01 bar）下钌/三氧化二铝（Ru/Al₂O₃）催化剂上的氨气（NH₃）分解反应，可见光辐照（440–635 nm光子，辐照功率密度0–5.5 W·cm⁻²）对氨气分解速率的提升效应，完全可通过催化床的光致热升温效应予以解释。该结论得到了多组条件下催化速率测量数据的支撑：涵盖有无辐照、不同波长以及不同共进料氢气用量的实验条件，并通过置于催化床内的热电偶获取局部温度数据予以验证。进一步研究证实，铜钌/三氧化二铝（CuRu/Al₂O₃）体系在光驱动氨气分解过程中表现出非热机制。最终，本研究成功区分了Ru/Al₂O₃催化剂上低压氨气分解反应的热贡献与非热贡献，该实验体系可作为有效对照系统，用于验证光驱动催化中区分热效应与光化学贡献的实验方法。