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

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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.