Collaborative Research: Understanding the Role of Surface Bound Ligands on Metals in H2O2 Direct Synthesis

在化学系化学催化项目的支持下，密歇根大学的Eranda Nikolla教授和科罗拉多大学博尔德分校的Will Medlin教授正在研究用于直接合成过氧化氢的新型多相催化剂。过氧化氢是许多应用的重要产品，包括清洁水和化学品制造中的有效氧化剂。然而，它目前是通过间接方法以工业规模生产的，该方法涉及有机化学品的反应，并且需要大规模才经济。对于分布式制造，期望开发一种直接使氢与氧反应的“更绿色”工艺。然而，需要新的催化剂来加速过氧化氢合成的速率并避免导致氢完全氧化成水的副反应。最近的研究表明，将某些有机涂层应用于常见的负载型金属催化剂可以提高过氧化氢合成产率，但是这些提高性能的方式尚不清楚。 Nikolla和Medlin教授及其团队将系统地改变有机金属界面的性质，以确定与高过氧化氢产量相关的分子特征。他们还将进行反应动力学研究，以了解反应物和有机涂层之间的相互作用如何特别导致提高速率。该研究将由一个多机构团队进行，其中包括与国际和国家实验室合作伙伴的合作。该项目的教育部分将包括研究生和本科生的培训和交流计划，以及与研究问题相关的新在线教育工具的开发。在该奖项下，密歇根大学的Eranda Nikolla教授和科罗拉多大学博尔德分校的Will Medlin教授正在研究近地表环境如何影响负载金属催化剂上的过氧化氢直接合成。有机配体广泛用于合成可用作明确定义的催化剂的金属纳米晶体。虽然在许多情况下期望除去配体，但在其它情况下，配体的保留可导致期望的催化剂性能。其中已经发现配体效应产生主要选择性改进的一个重要实例是在Pd催化剂上由H2和O2直接合成过氧化氢。虽然已经有一些迹象表明，配体的功能通过（i）封闭连续的表面位点负责O2活化和（ii）促进质子穿梭吸附O2，涂层提高H2 O2直接合成的选择性的基本步骤机制知之甚少。为了开发过氧化氢合成的结构-反应性关系，Nikolla和Medlin教授将系统地改变（a）配体的结构和密度，（B）金属纳米颗粒的大小、形状和组成，以及（c）溶剂性质。所制备的材料将进行深入的表征，并用于动力学研究，以了解选择性催化的起源。该项目将涉及纳米颗粒合成，配体保护催化剂的表征，反应动力学研究以及有机改性金属表面化学的计算研究。虽然该项目将强调过氧化氢的直接合成，但这里开发的原理可以外推到不同的催化反应，特别是因为直接合成的基本步骤及其不希望的副反应在许多化学中广泛重要。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

With the support of the Chemical Catalysis program in the Division of Chemistry, Professor Eranda Nikolla of the University of Michigan and Professor Will Medlin of the University of Colorado Boulder are studying new heterogeneous catalysts for the direct synthesis of hydrogen peroxide. Hydrogen peroxide is an important product for numerous applications including clean water and as an efficient oxidant in chemicals manufacturing. However, it is currently produced at industrial scale via an indirect process that involves reactions of organic chemicals and requires large scales to be economical. For distributed manufacturing, it is desirable to develop a “greener” process that directly reacts hydrogen with oxygen. However, new catalysts are needed to accelerate the rate of hydrogen peroxide synthesis and avoid side reactions that lead to complete hydrogen oxidation to water. Recent work has suggested that the application of certain organic coatings to common supported metal catalysts can improve hydrogen peroxide synthesis yields, however the way these improve performance is not understood. Professors Nikolla and Medlin and their teams will systematically vary the properties of the organic-metal interface to identify molecular features associated with high hydrogen peroxide yields. They will also conduct reaction kinetic studies to understand how interactions between the reactants and organic coatings specifically lead to enhanced rates. The research will be carried out by a multi-institutional team that includes collaboration with international and national laboratory partners. The educational component of the project will include training and exchange programs for graduate and undergraduate students and development of new online educational tools related to the research problem.Under this award, Professors Eranda Nikolla of the University of Michigan and Will Medlin of the University of Colorado Boulder are studying how the near-surface environment influences direct hydrogen peroxide synthesis on supported metal catalysts. Organic ligands are widely used in the synthesis of metal nanocrystals that can be employed as well-defined catalysts. While in many cases it is desirable to remove the ligands, in other instances the retention of ligands can lead to desirable catalyst performance. One important example in which ligand effects have been found to yield major selectivity improvements is the direct synthesis of hydrogen peroxide from H2 and O2 over Pd catalysts. While there have been some indications that the ligands function via (i) blocking of contiguous surface sites responsible for O2 activation and (ii) promotion of proton shuttling to adsorbed O2, the elementary-step mechanisms by which the coatings enhance selectivity in H2O2 direct synthesis are poorly understood. To develop structure-reactivity relations for hydrogen peroxide synthesis, Professors Nikolla and Medlin will systematically vary (a) the structure and density of ligands, (b) the metal nanoparticle size, shape, and composition, and (c) the solvent properties. The prepared materials will be characterized in depth and utilized in kinetic studies to understand the origins of selective catalysis. The project will involve collaborative efforts in nanoparticle synthesis, characterization of ligand-protected catalysts, reaction kinetic studies, and computational studies of chemistry at organic-modified metal surfaces. Although the project will emphasize the direct synthesis of hydrogen peroxide, the principles developed here can be extrapolated to diverse catalytic reactions, particularly since the elementary steps in direct synthesis and its undesired side reactions are broadly important in many chemistries.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.