Narita Komei, Kuwabara Takayuki, Sone Koji, Shimizu Ken-Ichi, Yagi Masayuki
Faculty of Education and Human Sciences, Niigata University, 8050 Ikarashi-2, Niigata 950-2181, Japan.
J Phys Chem B. 2006 Nov 23;110(46):23107-14. doi: 10.1021/jp063679n.
Hybridization of (OH(2))(terpy)Mn(mu-O)(2)Mn(terpy)(OH(2)) (terpy= 2,2':6',2' '-terpyridine) (1) and mica clay yielded catalytic dioxygen (O(2)) evolution from water using a CeIV oxidant. The reaction was characterized by various spectroscopic measurements and a kinetic analysis of O(2) evolution. X-ray diffraction (XRD) data indicates the interlayer separation of mica changes upon intercalation of 1. The UV-vis diffuse reflectance (RD) and Mn K-edge X-ray absorption near-edge structure (XANES) data suggest that the oxidation state of the di-mu-oxo Mn(2) core is Mn(III)-Mn(IV), but it is not intact. In aqueous solution, the reaction of 1 with a large excess Ce(IV) oxidant led to decomposition of 1 to form MnO(4-) ion without O(2) evolution, most possibly by its disproportionation. However, MnO(4-) formation is suppressed by adsorption of 1 on clay. The maximum turnover number for O(2) evolution catalyzed by 1 adsorbed on mica and kaolin was 15 and 17, respectively, under the optimum conditions. The catalysis occurs in the interlayer space of mica or on the surface of kaolin, whereas MnO(4-) formation occurs in the liquid phase, involving local adsorption equilibria of adsorbed 1 at the interface between the clay surface and the liquid phase. The analysis of O(2) evolution activity showed that the catalysis requires cooperation of two equivalents of 1 adsorbed on clay. The second-order rate constant based on the concentration (mol g(-1)) of 1 per unit weight of clay was 2.7 +/- 0.1 mol(-1) s(-1) g for mica, which is appreciably lower than that for kaolin (23.9 +/- 0.4 mol(-1) s(-1) g). This difference can be explained by the localized adsorption of 1 on the surface for kaolin. However, the apparent turnover frequency ((kO(2))app/s(-1)) of 1 on mica was 2.2 times greater than on kaolin when the same fractional loading is compared. The higher cation exchange capacity (CEC) of mica statistically affords a shorter distance between the anionic sites to which 1 is attracted electrostatically, making the cooperative interaction between adsorbed molecules of 1 easier than that on kaolin. The higher CEC is important not only for attaining a higher loading but also for the higher catalytic activity of adsorbed 1.
[(OH₂)(三联吡啶)Mn(μ - O)₂Mn(三联吡啶)(OH₂)]³⁺（三联吡啶 = 2,2':6',2'' - 三联吡啶）（1）与云母粘土杂交，使用CeIV氧化剂催化水产生二氧（O₂）。该反应通过各种光谱测量和O₂产生的动力学分析进行表征。X射线衍射（XRD）数据表明，插入1后云母的层间间距发生变化。紫外可见漫反射（RD）和Mn K边X射线吸收近边结构（XANES）数据表明，双μ - 氧代Mn(2)核的氧化态为Mn(III)-Mn(IV)，但并不完整。在水溶液中，1与大量过量的Ce(IV)氧化剂反应导致1分解形成MnO₄⁻离子，且不产生O₂，最有可能是通过其歧化反应。然而，1在粘土上的吸附抑制了MnO₄⁻的形成。在最佳条件下，吸附在云母和高岭土上的1催化O₂产生的最大周转数分别为15和17。催化作用发生在云母的层间空间或高岭土的表面，而MnO₄⁻的形成发生在液相中，涉及吸附的1在粘土表面和液相之间界面处的局部吸附平衡。对O₂产生活性的分析表明，催化作用需要吸附在粘土上的两当量1协同作用。基于每单位重量粘土上1的浓度（mol g⁻¹）的二级速率常数，云母为2.7 ± 0.1 mol⁻¹ s⁻¹ g，明显低于高岭土（23.9 ± 0.4 mol⁻¹ s⁻¹ g）。这种差异可以通过1在高岭土表面的局部吸附来解释。然而，当比较相同分数负载时，1在云母上的表观周转频率（(kO₂)app / s⁻¹）比在高岭土上大2.2倍。云母较高的阳离子交换容量（CEC）在统计学上使1被静电吸引到的阴离子位点之间的距离更短，使得吸附的1分子之间的协同相互作用比在高岭土上更容易。较高的CEC不仅对于实现更高的负载量很重要，而且对于吸附的1的更高催化活性也很重要。
