School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.
Phys Chem Chem Phys. 2019 Jan 30;21(5):2325-2336. doi: 10.1039/c8cp06889e.
The production of gas-phase hydroperoxyl radicals, HO2, is observed directly from sub-micron airborne TiO2 nanoparticles irradiated by 300-400 nm radiation. The rate of HO2 production as a function of O2 pressure follows Langmuir isotherm behaviour suggesting O2 is involved in the production of HO2 following its adsorption onto the surface of the TiO2 aerosol. Reduction of adsorbed O2 by photogenerated electrons is likely to be the initial step followed by reaction with a proton produced via oxidation of adsorbed water with a photogenerated hole. The rate of HO2 production decreased significantly over the range of relative humidities between 8.7 and 36.9%, suggesting competitive adsorption of water vapour inhibits HO2 production. From the data, the adsorption equilibrium constants were calculated to be: KO2 = 0.27 ± 0.02 Pa-1 and KH2O = 2.16 ± 0.12 Pa-1 for RH = 8.7%, decreasing to KO2 = 0.18 ± 0.01 Pa-1 and KH2O = 1.33 ± 0.04 Pa-1 at RH = 22.1%. The increased coverage of H2O onto the TiO2 aerosol surface may inhibit HO2 production by decreasing the effective surface area of the TiO2 particle and lowering the binding energy of O2 on the aerosol surface, hence shortening its desorption lifetime. The maximum yield (i.e. when [O2] is projected to atmospherically relevant levels) for production of gas-phase HO2, normalised for surface area and light intensity, was found to be at a RH of 8.7% for the 80% anatase and 20% rutile formulation of TiO2 used here. This yield decreased to as the RH was increased to 22.1%. Using this value, the rate of production of HO2 from TiO2 surfaces under atmospheric conditions was estimated to be in the range 5 × 104-1 × 106 molecule cm-3 s-1 using observed surface areas of mineral dust at Cape Verde, and assuming a TiO2 fraction of 4.5%. For the largest loadings of dust in the troposphere, the rate of this novel heterogeneous production mechanism begins to approach that of HO2 production from the gas-phase reaction of OH with CO in unpolluted regions. The production of gas-phase OH radicals could only be observed conclusively at high aerosol surface areas, and was attributed to the decomposition of H2O2 at the surface by photogenerated electrons.
气相过氧羟自由基（HO2）的生成直接观察到亚微米级空气传播的 TiO2 纳米粒子在 300-400nm 辐射照射下生成。HO2 的生成速率随 O2 压力的变化符合朗缪尔等温线行为，表明 O2 参与了 HO2 的生成，其是在吸附到 TiO2 气溶胶表面后发生反应的。被光生电子还原的吸附氧可能是初始步骤，随后与光生空穴氧化吸附水产生的质子发生反应。HO2 的生成速率在相对湿度为 8.7%至 36.9%的范围内显著下降，表明水蒸气的竞争吸附抑制了 HO2 的生成。根据数据，计算得到吸附平衡常数为：KO2=0.27±0.02Pa-1 和 KH2O=2.16±0.12Pa-1（RH=8.7%），当 RH 为 22.1%时，吸附平衡常数降低为 KO2=0.18±0.01Pa-1 和 KH2O=1.33±0.04Pa-1。TiO2 气溶胶表面上 H2O 的覆盖率增加可能通过降低 TiO2 颗粒的有效表面积和降低 O2 在气溶胶表面上的结合能来抑制 HO2 的生成，从而缩短其解吸寿命。在本文中使用的 80%锐钛矿和 20%金红石配方的 TiO2，当[O2]被投影到大气相关水平时，归一化到表面积和光强，气相 HO2 的最大产率（即）发现是在 RH=8.7%时。当 RH 增加到 22.1%时，这个产率下降。使用这个值，根据在佛得角观测到的矿物尘表面积，估算了在大气条件下 TiO2 表面产生 HO2 的速率，假设 TiO2 占比为 4.5%。在对流层中尘埃的最大负荷下，这种新的非均相生成机制的速率开始接近未受污染地区中 OH 与 CO 的气相反应产生 HO2 的速率。只有在高气溶胶表面积时，气相 OH 自由基的生成才能被明确观察到，这归因于光生电子在表面分解 H2O2。
