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3
A G ENERAL S TRATEGY TO A TOMICALLY D ISPERSED
P RECIOUS M ETAL C ATALYSTS
This chapter includes the published contents:
Kim, J. H.; Shin, D.; Lee, J.; Baek, D. S.; Shin, T. J.; Kim, Y.-T.; Jeong, H. Y.; Kwak, J. H.; Kim, H.; Joo, S. H. ACS Nano 2020, 14, 1990–2001. DOI: 10.1021/acsnano.9b08494. Reproduced with permission. Copyright © 2020 American Chemical Society.
3.1.INTRODUCTION
Precious-metal-based heterogeneous catalysts have been mainstays in modern chemical industries1–3 including energy conversion, fine chemical production, and automotive exhaust gas abatement. However, precious metals such as Pt, Ir, and Rh are prohibitively expensive and scarce.
Furthermore, widely used nanoparticle (NP)-based heterogeneous catalysts can exploit only a small fraction of the total metal atoms. The inherent limitations of NP-based catalysts have triggered a recent drive toward atomically dispersed catalysts or single-atom catalysts,4–38 which can maximize atom efficiency and provide unusual catalytic activity13,15–25 and selectivity26–32 for a diverse set of thermo-, photo-, and electro-catalytic reactions. Owing to their extraordinary catalytic properties, atomically dispersed catalysts have received tremendous attention recently as a research frontier in the field of catalysis.
The most straightforward and widely used method for synthesizing atomically dispersed precious metal catalysts is an impregnation–thermal activation method. However, this method is only effective with very low metal loadings;39,40 high metal loadings often lead to agglomeration into NPs owing to the high surface energy of the atomically dispersed sites.4 Hence, considerable efforts have been made toward developing new synthetic strategies for atomically dispersed precious metal catalysts, including thermal conversion of metal-organic frameworks,19,25,31,38,41 defect engineering strategy,42,43 nanopore confinement,30,44,45 photochemical reduction,45,46 atomic layer deposition,28,47,48 galvanic replacement,49,50 and high-temperature redispersion of NPs.51–53 Nevertheless, there is still a lack of generic, robust preparation methods that can afford atomically dispersed catalysts of a wide range of precious metals.5 More importantly, the mechanisms underlying the formation of atomically dispersed precious metal catalysts have mostly remained elusive, hindering their rational design.
In this work, we propose a generalized preparation method for carbon-supported atomically
dispersed precious metal catalysts. The synthetic strategy consists of “trapping” precious metal precursor molecules on a heteroatom-doped carbonaceous layer coated on a carbon support and then
“immobilizing” them during thermal activation via coating with a SiO2 protective layer. By combining X-ray absorption spectroscopy (XAS) and mass spectrometry (MS) during thermal activation processes, we found that the strong interaction between the metal precursors and the support provides stable, isolated anchoring of metal precursors during the impregnation step and retards precursor decompositions during the activation step, which assists the conversion of the precursor into an isolated single-atom site. The SiO2 protective layer could mitigate agglomeration of the isolated metal sites during the thermal activation step, thus maximizing the density of atomically dispersed sites. Through this “trapping-and-immobilizing” strategy, a series of atomically dispersed precious metals (Os, Ru, Rh, Ir, and Pt) were prepared.