Bucko T, Hafner J, Benco L
Institut für Materialphysik and Center for Computational Material Science, Universität Wien, Sensengasse, A-1090 Wien, Austria.
J Phys Chem B. 2005 Apr 21;109(15):7345-57. doi: 10.1021/jp050151u.
We present a periodic density-functional investigation of the adsorption and the vibrational spectroscopy of CO in mordenite. Our results highlight a pronounced sensitivity of the strength of the hydrogen bond between the acidic hydroxyl groups and the adsorbed molecule, and hence of the induced red shift of the OH, and the blue shift of the CO stretching mode on the choice of the exchange-correlation functional. The popular Perdew-Wang (PW) gradient-corrected functional strongly overestimates the frequency shifts and the interaction energies. We demonstrate that the revised Perdew-Burke-Ernzerhof (RPBE) functional leads to an improved description of hydrogen bonding. For bridging OH groups, terminal silanol groups and for Lewis sites formed by tricoordinated Al atoms, we predict adsorption energies and frequency shifts in good agreement with experiment. The calculated difference in the binding energies of CO in purely siliceous mordenite and at Brønsted acid sites in the main channel agrees very well with microcalorimetry data. We find that Brønsted acid sites in the small channels (the side-pockets) of mordenite do not adsorb CO, which is adsorbed only in the main channel via the C atom. For these adsorption complexes we find reasonable (though not perfect) agreement of the predicted blue shift of the CO-mode and of the red shift of the OH-mode with experiment. Our prediction that the side-pockets are inaccessible to CO correlates well with the microcalorimetric studies and with experimental observation of the adsorption of O(2), N(2) and H(2) molecules but contradicts the current interpretation of experimental adsorption studies for CO.
我们展示了对丝光沸石中CO吸附和振动光谱的周期性密度泛函研究。我们的结果突出了酸性羟基与吸附分子之间氢键强度的显著敏感性，因此也突出了OH诱导红移以及CO伸缩模式蓝移对交换关联泛函选择的敏感性。流行的Perdew-Wang（PW）梯度校正泛函强烈高估了频移和相互作用能。我们证明，修订后的Perdew-Burke-Ernzerhof（RPBE）泛函能更好地描述氢键。对于桥连OH基团、末端硅醇基团以及由三配位Al原子形成的Lewis位点，我们预测的吸附能和频移与实验结果吻合良好。计算得出的纯硅质丝光沸石中CO与主通道中布朗斯台德酸位点的结合能差异与微量热法数据非常吻合。我们发现，丝光沸石小通道（侧袋）中的布朗斯台德酸位点不吸附CO，CO仅通过C原子在主通道中被吸附。对于这些吸附络合物，我们发现预测的CO模式蓝移和OH模式红移与实验结果有合理（尽管不完美）的吻合。我们预测侧袋对CO不可达，这与微量热法研究以及O(2)、N(2)和H(2)分子吸附的实验观察结果很好地相关，但与目前对CO实验吸附研究的解释相矛盾。
