Rational Design of Atomically Precise Catalysts for Parahydrogen Enhanced Nuclear Magnetic Resonance Imaging and Spectroscopy

核磁共振是一种强大的化学和生物分析方法，它通过探测原子核对强磁场的响应来提供关于分子中原子排列的信息。提高核磁共振的灵敏度将使生物医学中的新成像模式在药物发现、蛋白质相互作用、磁共振血管成像、用于肿瘤检测的代谢功能障碍的识别/监测以及脑血流灌注中成为可能。当一个分子包含来自同一氢源分子的两个氢原子时，可以优先检测在这些和其他应用中使用的核磁共振信号。然而，当氢加成在金属表面进行时，反应容易从不同的氢分子转移氢原子，因此没有产生核磁共振信号增强的结果。研究人员建议通过系统地改变催化剂的颗粒大小和反应中心组成来合成催化剂，这种催化剂可以驱动从同一氢源分子中选择性地添加两个氢原子的反应。使用这些新催化剂合成的分子，具有改进的磁共振信号，可以用作各种诊断技术的生物标记物。这一进展将代表着生物医学成像领域的革命性发展，以及对催化剂结构-活性关系的新理解。本研究项目基于这样一种假设，即通过限制氢原子在金属表面的扩散，可以提高烯烃和烯烃氢化反应的对选择性。据预测，选择性地封闭表面位置将减少加氢反应的解离-重组过程所导致的对位邻位反向转化。这些假设将通过合理的设计和使用原子层沉积(ALD)的原子精密催化剂合成来验证。通过小心地将活性金属沉积到具有特定表面的纳米氧化物形状上，可以合成定义明确的催化剂。通过这种方法，结合仔细的催化剂表征，将使用从原子分散物种到金属纳米颗粒的颗粒来确定作为模型体系的丙烯和丙炔加氢反应中颗粒尺寸对活性和成对选择性的影响。最先进的催化剂合成(区域选择性ALD)也将允许封闭特定的金属中心，即梯形中心或协调不饱和的角落和边缘中心。系统的研究方法可以揭示不同活性金属中心的相对重要性，促进结构-活性关系的建立。纳米氧化物形状的使用和适当的催化剂表征方法将导致对电子金属-载体相互作用及其如何影响探测器反应的新理解。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nuclear magnetic resonance (NMR) is a powerful chemical and biological analytical method that provides information about the arrangement of atoms in molecules by probing responses of atomic nuclei to strong magnetic fields. Improving the sensitivity of NMR will enable new imaging modalities in biomedicine for drug-discovery, protein interactions, magnetic resonance angiography, identification/monitoring of metabolic dysfunction for tumor detection, and brain perfusion. NMR signals used in these and other applications can be preferentially detected when a molecule contains two hydrogen atoms from the same hydrogen source molecule. However, when hydrogen addition is performed on metal surfaces the reaction is prone to transfer of hydrogen atoms from different hydrogen molecules, and therefore no NMR signal enhancement results. The investigators propose to synthesize catalysts that drive reactions which selectively add two hydrogen atoms from the same hydrogen source molecule by systematically varying the catalyst particle size and reaction site composition. Molecules synthesized using these new catalysts, with improved magnetic resonance signals, can be used as biomarkers in various diagnostic techniques. This advance will represent a transformative development in the field of biomedical imaging and new understanding of catalyst structure-activity relationships. This research project is based upon the hypothesis that pair-wise selectivity in alkene and alkyne hydrogenation can be increased by restricting hydrogen ad-atom diffusion on the metal surface. Selectively blocking surface sites is predicted to reduce para-ortho back-conversion that results from the hydrogenation reaction's dissociation-recombination process. The hypotheses will be tested through rational design and atomically precise synthesis of catalysts using atomic layer deposition (ALD). Well-defined catalysts will be synthesized through careful deposition of active metal onto nanoparticle oxide shapes with specific surface facets. Through this approach, together with careful catalyst characterizations, the effects of particle size on the activity and pairwise selectivity in the hydrogenation of propene and propyne, as model systems, will be determined using particles ranging from atomically dispersed species to metal nanoparticles. State-of-the-art catalyst synthesis (area-selective ALD) will also allow blocking of specific metal sites, i.e. terrace sites or coordinatively unsaturated corner and edge sites. The systematic approach in the proposed research can reveal the relative importance of different active metal sites and facilitate structure-activity relationships. The use of nanoparticle oxide shapes together with appropriate catalyst characterization methods will lead to a new understanding of electronic metal-support interactions and how they affect the probe reactions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.