Devlin J Paul, Gulluru Dheeraj B, Buch Victoria
Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, USA.
J Phys Chem B. 2005 Mar 3;109(8):3392-401. doi: 10.1021/jp0456281.
This FTIR study focuses on solid-state chemistry associated with formation and interconversion of the ionic HX (X = Cl, Br) hydrates. Kinetic data are reported for conversions of ice nanocrystal arrays exposed to the saturation pressure of the acids in the 110 approximately 125 K range. The product is amorphous acid dihydrate in the case of HBr, and amorphous monohydrate for HCl. The rate-determining step is identified as HX diffusion through the hydrate product crust toward the interfacial reaction zone, rather than diffusion through ice, as commonly believed. Slowing of the conversion process is thus observed with increasing thickness of the crust. The diffusion coefficient (D(e)) and activation energy values for HX diffusion through the hydrates were evaluated with the help of the shrinking-core model. Hydrate crystallization occurs as a separate step, upon heating above 130 K. Subsequently, rates of reversible transitions between crystal di- and monohydrates were observed upon exposure to acid vapor and acid evacuation. In conversion from di- to monohydrate, the rate slows after fast formation of several layers; subsequently, diffusion through the product crust appears to be the rate-controlling step. The activation energy for HBr diffusion through crystal dihydrate is found to be significantly higher than that for the amorphous analogue. Conjecture is offered for a molecular mechanism of HX transport through the crystal hydrate, based on (i) spectroscopic/computational evidence for the presence of molecular HX bonded to X(-) in each of the ionic hydrate phases and (ii) the relative E(a) values found for HBr and HCl diffusion. Monte Carlo modeling suggests acid transport to the reaction zone along boundaries between "nanocrystallites" generated by multiple hydrate nucleation events at the particle surfaces. The reverse conversion, of crystalline monohydrate particles to the dihydrate phase, as well as dihydrate to trihydrate, displays nearly constant rate throughout the particle conversion; suggesting desorption of HX from the particle surface as the rate-limiting factor. Like for D(e), the activation energies for desorption were found to be approximately 20% greater for HCl than HBr for related hydrate phases.
这项傅里叶变换红外光谱（FTIR）研究聚焦于与离子型HX（X = Cl、Br）水合物的形成及相互转化相关的固态化学。报告了在110至125 K范围内，暴露于酸的饱和压力下的冰纳米晶体阵列转化的动力学数据。对于HBr，产物是无定形酸二水合物；对于HCl，产物是无定形一水合物。速率决定步骤被确定为HX通过水合物产物外壳向界面反应区的扩散，而不是像通常认为的那样通过冰扩散。因此，随着外壳厚度增加，观察到转化过程变慢。借助缩核模型评估了HX通过水合物的扩散系数（D(e)）和活化能值。水合物结晶在加热至130 K以上时作为一个单独步骤发生。随后，在暴露于酸蒸气和抽空酸时，观察到晶体二水合物和一水合物之间可逆转变的速率。从二水合物转化为一水合物时，在快速形成几层后速率减慢；随后，通过产物外壳的扩散似乎是速率控制步骤。发现HBr通过晶体二水合物扩散的活化能明显高于无定形类似物的活化能。基于（i）在每个离子水合物相中存在与X(-)键合的分子HX的光谱/计算证据，以及（ii）HBr和HCl扩散的相对E(a)值，对HX通过晶体水合物的分子机制提出了推测。蒙特卡罗模拟表明，酸沿着颗粒表面多次水合物成核事件产生的“纳米微晶”之间的边界传输到反应区。结晶一水合物颗粒向二水合物相的反向转化，以及二水合物向三水合物的转化，在整个颗粒转化过程中显示出几乎恒定的速率；表明HX从颗粒表面解吸是限速因素。与D(e)一样，发现相关水合物相中HCl解吸的活化能比HBr大约高20%。
