Richards-Henderson Nicole K, Anderson Crisand, Anastasio Cort, Finlayson-Pitts Barbara J
Department of Chemistry, University of California, Irvine, CA 92697-2025, USA.
Phys Chem Chem Phys. 2015 Dec 28;17(48):32211-8. doi: 10.1039/c5cp05325k. Epub 2015 Nov 18.
The photochemistry of nitrate ions in bulk aqueous solution is well known, yet recent evidence suggests that the photolysis of nitrate may be more efficient at the air-water interface. Whether and how this surface enhancement is altered by the presence of different cations is not known. In the present studies, thin aqueous films of nitrate salts with different cations were deposited on the walls of a Teflon chamber and irradiated with 311 nm light at 298 K. The films were generated by nebulizing aqueous 0.5 M solutions of the nitrate salts and the generation of gas-phase NO2 was monitored with time. The nitrate salts fall into three groups based on their observed rate of NO2 formation (R(NO2)): (1) RbNO3 and KNO3, which readily produce NO2 (R(NO2) > 3 ppb min(-1)), (2) Ca(NO3)2, which produces NO2 more slowly (R(NO2) < 1 ppb min(-1)), and (3) Mg(NO3)2 and NaNO3, which lie between the other two groups. Neither differences in the UV-visible spectra of the nitrate salt solutions nor the results of bulk-phase photolysis studies could explain the differences in the rates of NO2 production between these three groups. These experimental results, combined with some insights from previous molecular dynamic simulations and vibrational sum frequency generation studies, show that cations may impact the concentration of nitrate ions in the interface region, thereby directly impacting the effective quantum yields for nitrate ions.
硝酸盐离子在大量水溶液中的光化学性质是众所周知的，但最近有证据表明，硝酸盐在气-水界面的光解可能更有效。目前尚不清楚不同阳离子的存在是否以及如何改变这种表面增强效应。在本研究中，将含有不同阳离子的硝酸盐盐水薄膜沉积在特氟龙腔室的壁上，并在298 K下用311 nm光照射。通过雾化0.5 M硝酸盐盐水溶液来制备薄膜，并随时间监测气相NO₂的生成。根据观察到的NO₂生成速率（R(NO₂)），硝酸盐可分为三组：（1）RbNO₃和KNO₃，它们很容易产生NO₂（R(NO₂) > 3 ppb min⁻¹）；（2）Ca(NO₃)₂，产生NO₂的速度较慢（R(NO₂) < 1 ppb min⁻¹）；（3）Mg(NO₃)₂和NaNO₃，其生成速率介于另外两组之间。硝酸盐溶液的紫外-可见光谱差异以及体相光解研究结果均无法解释这三组之间NO₂生成速率的差异。这些实验结果，结合先前分子动力学模拟和振动和频产生研究的一些见解，表明阳离子可能会影响界面区域中硝酸根离子的浓度，从而直接影响硝酸根离子的有效量子产率。
