Huang Po-Wei, Tian Nianhan, Rajh Tijana, Liu Yu-Hsuan, Innocenti Giada, Sievers Carsten, Medford Andrew J, Hatzell Marta C
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
School of Molecular Science, Arizona State University, Tempe, Arizona 85281, United States.
JACS Au. 2023 Nov 27;3(12):3283-3289. doi: 10.1021/jacsau.3c00556. eCollection 2023 Dec 25.
Titanium dioxide is the most studied photocatalytic material and has been reported to be active for a wide range of reactions, including the oxidation of hydrocarbons and the reduction of nitrogen. However, the molecular-scale interactions between the titania photocatalyst and dinitrogen are still debated, particularly in the presence of hydrocarbons. Here, we used several spectroscopic and computational techniques to identify interactions among nitrogen, methanol, and titania under illumination. Electron paramagnetic resonance spectroscopy (EPR) allowed us to observe the formation of carbon radicals upon exposure to ultraviolet radiation. These carbon radicals are observed to transform into diazo- and nitrogen-centered radicals (e.g., CHN and CHNH) during photoreaction in nitrogen environment. In situ infrared (IR) spectroscopy under the same conditions revealed C-N stretching on titania. Furthermore, density functional theory (DFT) calculations revealed that nitrogen adsorption and the thermodynamic barrier to photocatalytic nitrogen fixation are significantly more favorable in the presence of hydroxymethyl or surface carbon. These results provide compelling evidence that carbon radicals formed from the photooxidation of hydrocarbons interact with dinitrogen and suggest that the role of carbon-based "hole scavengers" and the inertness of nitrogen atmospheres should be reevaluated in the field of photocatalysis.
二氧化钛是研究最多的光催化材料，据报道它对包括碳氢化合物氧化和氮还原在内的多种反应具有活性。然而，二氧化钛光催化剂与二氮之间的分子尺度相互作用仍存在争议，尤其是在有碳氢化合物存在的情况下。在此，我们使用了几种光谱和计算技术来确定光照下氮、甲醇和二氧化钛之间的相互作用。电子顺磁共振光谱（EPR）使我们能够观察到暴露于紫外线辐射时碳自由基的形成。在氮环境中的光反应过程中，这些碳自由基会转化为重氮和以氮为中心的自由基（例如CHN和CHNH）。相同条件下的原位红外（IR）光谱显示二氧化钛上存在C-N伸缩振动。此外，密度泛函理论（DFT）计算表明，在存在羟甲基或表面碳的情况下，氮吸附和光催化固氮的热力学势垒明显更有利。这些结果提供了令人信服的证据，表明碳氢化合物光氧化形成的碳自由基与二氮相互作用，并表明在光催化领域中应重新评估碳基“空穴清除剂”的作用和氮气氛的惰性。
