Benco L, Bucko T, Hafner J, Toulhoat H
Institut für Materialphysik and Center for Computational Materials Science, Universität Wien, Sensengasse 8, A-1090 Wien, Austria.
J Phys Chem B. 2005 Dec 1;109(47):22491-501. doi: 10.1021/jp0533729.
Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations. The geometries of the adsorption complex, the adsorption energies, stretching frequencies, and the capacity to dissociate the adsorbed molecule are compared for different active sites. The active centers include a Brønsted acid site, a three-coordinated surface Al site, and Lewis sites formed by extraframework cations: Na+, Cu+, Ag+, Zn2+, Cu2+, Ga3+, and Al3+. Adsorption properties of cations are compared for a location of the cation in the five-membered ring. This location differs from the location in the six-membered ring observed for hydrated cations. The five-membered ring, however, represents a stable location of the bare cation. In this position any cation exhibits higher reactivity compared with the location in the six-membered ring and is well accessible by molecules adsorbed in the main channel of the zeolite. Calculated adsorption energies range from 4 to 87 kJ/mol, depending on electronegativity and ionic radius of the cation and the stability of the cation-zeolite complex. The largest adsorption energy is observed for Cu+ and the lowest for Al3+ integrated into the interstitial site of the zeolite framework. A linear dependence is observed between the stretching frequency and the bond length of the adsorbed H2 molecule. The capacity of the metal-exchanged zeolite to dissociate the H2 molecule does not correlate with the adsorption energy. Dissociation is not possible on single Cu+ cation. The best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data.
采用从头算密度泛函理论（DFT）计算研究了丝光沸石中H2的吸附和化学吸附。比较了不同活性位点上吸附络合物的几何结构、吸附能、伸缩频率以及吸附分子的解离能力。活性中心包括一个布朗斯特酸位点、一个三配位表面Al位点以及由骨架外阳离子（Na+、Cu+、Ag+、Zn2+、Cu2+、Ga3+和Al3+）形成的路易斯位点。比较了阳离子在五元环中的位置对阳离子吸附性质的影响。该位置不同于水合阳离子在六元环中的位置。然而，五元环是裸阳离子的稳定位置。在这个位置，任何阳离子与在六元环中的位置相比都表现出更高的反应活性，并且容易被吸附在沸石主通道中的分子接近。计算得到的吸附能范围为4至87 kJ/mol，这取决于阳离子的电负性和离子半径以及阳离子-沸石络合物的稳定性。对于整合到沸石骨架间隙位置的Cu+，观察到最大吸附能，而对于Al3+则观察到最低吸附能。观察到吸附的H2分子的伸缩频率与键长之间存在线性关系。金属交换沸石解离H2分子的能力与吸附能无关。单个Cu+阳离子上不可能发生解离。对于Ga3+、Zn2+和Al3+骨架外阳离子，观察到最佳性能，这与实验数据吻合良好。
