Supported Metal Single‐Atom Thermocatalysts for Oxidation Reactions

Abstract

The interest in single-metal-atom dispersion in heterogeneous catalysis has existed since the 1950s with spectroscopic investigations of rhodium "single-site" catalysts supported on alumina, and more recently with oxometallate monomers for selective oxidation reactions. A decade ago, advances in electron microscopy intensified the interest of the catalysis community in controlling the dispersion of supported metals down to the single-atom active site level and revealing their intrinsic catalytic properties. Pioneering works in the ever-growing field of single-atom catalysis investigated late transition metals (group 8 to 11) supported on transition metal oxides for the CO oxidation and water-gas shift reactions. Since then, various single-atom catalysts, most often supported on oxides, have been evaluated in a broad panel of thermal oxidation reactions, and have often shown remarkable performances. Besides CO oxidation over oxidesupported noble metals, which represents the major part of the literature, this chapter reviews the main classes of oxidation reactions, including total and selective oxidations of hydrocarbons and oxygenates, on 2 oxide-or carbon-supported catalysts. Throughout the chapter, we emphasize several aspects that we consider important, such as the coordination environment and stability of single metal atoms, as well as their compared performance with supported nanoparticles. Figure 3. a) Aberration-corrected STEM image of Pt atoms (identified by yellow circles) on ca. 5 nm diameter anatase TiO2 particles. Adapted with permission from [72], Copyright 2017 ACS. b) CO probemolecule FTIR spectra of a Pt/TiO2 SAC as a function of temperature during a TPD measurement. c) DFT-calculated structure of CO bound to PtO2 adsorbed at a TiO2(145) step edge. b and c adapted with permission from [73], Copyright 2018 Elsevier. d) Schematic showing the structural transformation of isolated Pt species as a function of environmental conditions. Adapted with permission from [74],Copyright 2019 Springer Nature. e) DFT-calculated reaction pathway for CO oxidation on a Pd/TiO2 SAC that was supported by spectroscopic and kinetic analyses. Adapted with permission from [75],

Identification of Stable Species Formed Under CO Adsorption and Oxidation on Alumina-Supported Single Pt Atoms: Why Nanoparticles Are More Active

“…After a decade of considerable expansion, single-atom catalysis has become a major field of heterogeneous catalysis, especially because supported single atoms can exhibit novel catalytic properties with respect to conventional nanocatalysts (NCs). − Pioneering studies in this field have considered noble metal atoms (Rh, Pd, Pt, Au) supported on transition metal oxides (alumina, magnesia, ceria, iron oxide) for oxidation reactions. − The transposition of mechanisms known for NCs to single-atom catalysts (SACs) is not always obvious due to the lack of multiple metallic adsorption sites. Moreover, depending on the reaction under consideration, it remains unclear whether single atoms (SAs) are more or less active than clusters or nanoparticles (NPs).…”