Umamaheswari V, Hartmann Martin, Pöppl Andreas
Faculty of Physics and Geosciences, University of Leipzig, D-04103 Leipzig, Germany.
J Phys Chem B. 2005 Oct 27;109(42):19723-31. doi: 10.1021/jp0532014.
Cu(I)-NO adsorption complexes were formed over Cu-ZSM-5 zeolites prepared by (i) solid-state ion exchange of NH(4)-ZSM-5 with CuCl and (ii) liquid-state ion exchange of ZSM-5 with Cu(CH(3)COO)(2). Electron spin resonance spectroscopy revealed the formation of two different Cu(I)-NO species A and B in both systems, whose spin Hamiltonian parameters are comparable with those already reported for the Cu(I)-NO species formed over 66% Cu(II) liquid-state ion-exchanged Cu-ZSM-5 materials. The population of the species A and B differs for the two systems studied. Formation of species B is more favored in the solid-state ion-exchanged Cu-ZSM-5 when compared to the liquid-state exchanged zeolite. The X-, Q- and W-band electron spin resonance spectra recorded at 6 and 77 K reveal the presence of a rigid geometry of the adsorption complexes at 6 K and a dynamic complex structure at higher temperatures such as 77 K. This is indicated by the change in the spin Hamiltonian parameters of the formed Cu(I)-NO species in both the liquid- and solid-state ion-exchanged Cu-ZSM-5 zeolites from 6 to 77 K. Possible models for the motional effects found at elevated temperatures are discussed. The temperature dependence of the electron spin phase memory time measured by two-pulse electron spin-echo experiments indicates, likewise, the onset of a motional process of the adsorbed NO molecules at temperatures above 10 K. The studies support previous assignments where the NO complexes are formed at two different Cu(I) cationic sites in the ZSM-5 framework and highlight that multifrequency electron spin resonance experiments at low temperatures are essential for reliable determination of the spin Hamiltonian parameters of the formed adsorption complexes for further comparison with Cu(I)-NO complex structures predicted by quantum chemical calculations.
通过以下两种方法制备的Cu-ZSM-5沸石上形成了Cu(I)-NO吸附络合物：(i) 用CuCl对NH(4)-ZSM-5进行固态离子交换；(ii) 用Cu(CH(3)COO)(2)对ZSM-5进行液态离子交换。电子自旋共振光谱显示，在这两种体系中均形成了两种不同的Cu(I)-NO物种A和B，其自旋哈密顿参数与已报道的在66% Cu(II)液态离子交换的Cu-ZSM-5材料上形成的Cu(I)-NO物种的参数相当。所研究的两种体系中物种A和B的数量不同。与液态离子交换的沸石相比，固态离子交换的Cu-ZSM-5中物种B的形成更有利。在6 K和77 K下记录的X波段、Q波段和W波段电子自旋共振光谱表明，在6 K时吸附络合物存在刚性几何结构，而在77 K等较高温度下存在动态络合物结构。这通过液态和固态离子交换的Cu-ZSM-5沸石中形成的Cu(I)-NO物种的自旋哈密顿参数从6 K到77 K的变化得到证明。讨论了在高温下发现的运动效应的可能模型。通过双脉冲电子自旋回波实验测量的电子自旋相位记忆时间的温度依赖性同样表明，在高于10 K的温度下吸附的NO分子开始运动。这些研究支持了先前的观点，即NO络合物在ZSM-5骨架中的两个不同Cu(I)阳离子位点形成，并强调低温下的多频电子自旋共振实验对于可靠确定形成的吸附络合物的自旋哈密顿参数至关重要，以便进一步与量子化学计算预测的Cu(I)-NO络合物结构进行比较。
