The catalytic and photon-induced oxidation of NO₂ on anatase TiO₂ has been studied and compared with the surface nitrate species obtained after adsorption of HNO₃. Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface–NO₃^– species, where NO₂ bonds to the lattice oxygen, and freely adsorbed NO₃^– ions. These products can be obtained either by dissociative adsorption of HNO₃ or by catalytic/photocatalytic oxidation of NO₂, which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface–NO₃^– species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N₂O₅) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO₂ adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO₂ crystal. In particular, a stable surface–NO₃^– and NO₃^– ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO₂ and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO₂ particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N₂O during continued UV light illumination.

中文翻译：

---

NO2 在锐钛矿 TiO2 上的吸附和氧化：协同硝酸盐相互作用和光子刺激反应

研究了锐钛矿 TiO _{2上 NO 2}的催化氧化和光子诱导氧化，并与吸附 HNO ₃后获得的表面硝酸盐物种进行了比较。使用原位漫反射红外傅里叶变换光谱 (DRIFTS)、密度泛函理论 (DFT) 和程序升温脱附 (TPD) 的组合，表明在所有反应系统中都获得了相同的产物，但它们的形成速率不同显著地。表面反应产物被确定为表面-NO ₃ ^-物质的组合，其中 NO ₂与晶格氧键合，并自由吸附 NO ₃ ^-离子。这些产物可以通过HNO _{3的解离吸附或通过NO 2}的催化/光催化氧化获得，这由紫外光促进。揭示了一种协同反应机制，该机制涉及二齿硝酸盐的重新定向，该二齿硝酸盐推出相邻的质子化晶格氧以形成表面 NO ₃物质和末端 OH 基团^。这些表面物质的热稳定性已经通过 TPD 和同时原位DRIFTS 测量进行了研究，该测量揭示了在 430 °C 左右的主要解吸峰 ( m / z = 46)，这归因于硝酸盐通过五氧化二氮 (N ₂ O ₅) 形成。在约 185 °C 时发现较弱和较宽的 TPD 峰，这归因于以压缩配置键合的硝酸盐物质的解吸。实验结果可以通过所鉴定的硝酸盐产物的稳定性变化来解释，这在很大程度上取决于表面化学环境和表面覆盖率。DFT结果表明，中间体NO _{2吸附物和最终硝酸盐反应产物的稳定化是通过由TiO 2}晶体介导的双功能电荷交换机制发生的。特别是，稳定的表面^——NO _3——^和NO _3——离子对配置已确定。这种机制解释了 NO ₂的热氧化和光致氧化及其热稳定性和不同的形成速率，从而产生高光致氧化反应速率。_{我们的研究结果提供了对 TiO 2}颗粒上硝酸盐表面产物的结构和化学稳定性及其形成机制的深入了解，这对于理解它们在持续的紫外光照射下催化转化为有害化合物 HONO 和 N _{2 O 具有重要意义。}