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Spin counting in dipolar coupled spin 1/2 systems by multiple quantum NMR spectroscopy in solids: Development of a single quantum propagator and its application to heterogeneous catalysis

机译：固体中多量子核磁共振波谱法研究偶极耦合自旋1/2系统中的自旋计数：单量子传播剂的开发及其在非均相催化中的应用

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摘要

The ideas behind the development and detection of multiple quantum coherence in ensembles of dipolar coupled spin 1/2 systems are developed. An rf pulse sequence for a single quantum propagator which creates multiple quantum coherence in steps of single quanta was developed and examined to for use as a tool for spin counting in solids. Experimental details concerning the manipulations necessary to produce multiple quantum NMR spectra are presented. Experimental results from time-resolved multiple quantum NMR studies using the propagator are shown and discussed for materials with a uniform distribution of spins (adamantane) and for a system of isolated spin clusters with finite numbers of spins (liquid crystal);The multiple quantum (MQ) spin counting was applied to identify reaction intermediates in heterogeneous catalysis. Spin counting was shown to yield an upper limit of n = 6 for strongly bound, rigid intermediates CmHn in the reaction of ethylene over Ru/SiO2. The results eliminated the possibility of major components of such intermediates being polymer-like coke precursors. A precursor to 2-butene such as Ru-CH2-CH=CH-CH2-Ru was proposed as one possible intermediate which is in accord with the experimental observations;Another application of multiple quantum spin counting was performed in order to characterize the local structure of silica in supported Ru/SiO2 catalysts. Cab-O-Sil HS5 used as the support in silica supported ruthenium (Ru/SiO2) catalysts was characterized via single and multiple quantum (MQ) 1H NMR spectroscopy. The samples were studied both in the presence and the absence of ruthenium. Single quantum spin counting of protons on a silica support with and without ruthenium metal indicated that the total number of hydroxyl groups decreased significantly with increasing reduction temperature over the range of 350-530∘C. Two different components shown in static 1H NMR were found to reveal homogeneous and inhomogeneous broadening via spectral hole burning experiments. 1H MQ-NMR spin counting, based on the number of MQ coherences observed, showed the existence of small clusters of proton spins on the silica surface. The maximum measured cluster size was 6-7, or less spins. Segments of silica resembling the 100 face of cristobalite on the surface are postulated to be possible sites for the clusters. The clusters in pure silica became smaller as the reduction temperature increased over 500∘C. No such change was detected in the presence of ruthenium.
机译：发展了开发和检测偶极耦合自旋1/2系统的集合体中的多个量子相干性的思想。开发并检查了用于单个量子传播器的rf脉冲序列，该序列在单个量子步中创建了多个量子相干性，并被用作固体自旋计数的工具。提供了有关产生多个量子NMR光谱所需的操作的实验细节。展示并讨论了使用传播子的时间分辨多量子NMR研究的实验结果，这些材料具有自旋均匀分布的材料（金刚烷）和具有有限数目的自旋的孤立的自旋簇系统（液晶）; MQ）自旋计数用于鉴定多相催化中的反应中间体。对于乙烯在Ru / SiO2上的反应，对于强结合的刚性中间体CmHn，自旋计数显示出n = 6的上限。结果消除了这种中间体的主要成分是聚合物状焦炭前体的可能性。提出了一种2-丁烯的前体，如Ru-CH2-CH = CH-CH2-Ru，作为一种可能的中间体，这与实验观察结果相符；为了表征局部结构，还进行了多量子自旋计数的另一种应用负载Ru / SiO2催化剂中的二氧化硅含量通过单量子和多量子（MQ）1H NMR光谱对在二氧化硅负载的钌（Ru / SiO2）催化剂中用作载体的Cab-O-Sil HS5进行了表征。在有或没有钌的情况下都对样品进行了研究。在含和不含钌金属的二氧化硅载体上对质子进行单量子自旋计数表明，在350-530∘C范围内，随着还原温度的升高，羟基总数显着下降。通过光谱孔燃烧实验发现，静态1 H NMR中显示的两种不同组分显示出均相和不均相加宽。基于观察到的MQ相干数量，1 H MQ-NMR自旋计数表明在二氧化硅表面上存在小簇质子自旋。测得的最大簇尺寸为6-7，或更少的旋转。推测在表面上类似于方石英的100面的二氧化硅片段可能是这些簇的位置。随着还原温度超过500∘C，纯二氧化硅中的簇变得更小。在钌存在下未检测到这种变化。

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    Hwang, Sonjong;

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  • 年度 1994
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  • 正文语种 en
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