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Review
. 2023 May 10;123(9):6233-6256.
doi: 10.1021/acs.chemrev.2c00424. Epub 2022 Oct 5.

Heterogeneous M-N-C Catalysts for Aerobic Oxidation Reactions: Lessons from Oxygen Reduction Electrocatalysts

Jason S Bates  1 , Mathew R Johnson  1 , Fatemeh Khamespanah  1 , Thatcher W Root  2 , Shannon S Stahl  1
Affiliations
Review

Heterogeneous M-N-C Catalysts for Aerobic Oxidation Reactions: Lessons from Oxygen Reduction Electrocatalysts

Jason S Bates et al. Chem Rev. .

Abstract

Nonprecious metal heterogeneous catalysts composed of first-row transition metals incorporated into nitrogen-doped carbon matrices (M-N-Cs) have been studied for decades as leading alternatives to Pt for the electrocatalytic O2 reduction reaction (ORR). More recently, similar M-N-C catalysts have been shown to catalyze the aerobic oxidation of organic molecules. This Focus Review highlights mechanistic similarities and distinctions between these two reaction classes and then surveys the aerobic oxidation reactions catalyzed by M-N-Cs. As the active-site structures and kinetic properties of M-N-C aerobic oxidation catalysts have not been extensively studied, the array of tools and methods used to characterize ORR catalysts are presented with the goal of supporting further advances in the field of aerobic oxidation.

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Figures

Figure 1.
Figure 1.
O2 accepts protons and electrons in the oxygen reduction reaction and aerobic oxidation catalysis (left). In the latter case, the protons and electrons derive from the bonds of molecules (SubH2), such as the O–H and C–H bonds of alcohols (right).
Figure 2.
Figure 2.
Overview of (a) synthetic methods and (b) structural features of M-N-C catalysts.
Figure 3.
Figure 3.
Representation of electrochemical and thermochemical reactions involving O2 reduction. (a) Relevance of O2 reactivity to fuel cells and aerobic oxidation which feature different sources of protons and electrons: H+/e− or chemical bonds in SubH2, and (b) two potential reaction pathways for thermochemical reactivity of O2 in aerobic oxidations. Adapted from Ref. . Copyright 2022 American Chemical Society.
Figure 4.
Figure 4.
Inner-sphere and independent half-reaction modes of aerobic oxidation reactivity exhibited by hydroquinone substrates. Aqueous-phase hydroquinone oxidation (a) conditions and (b) kinetic parameters. (c) ISR reactivity of slurry-phase M-N-C and (d) IHR reactivity of Nafion-bound M-N-C. Adapted from Refs. and . Copyright 2022 American Chemical Society.
Figure 5.
Figure 5.
Aerobic oxidation of alcohols to aldehydes and ketones (a) without exogenous base, and (b) with exogenous base. Percentages reflect yield of the specified product.
Figure 6.
Figure 6.
Oxidation of alcohols to carboxylic acids over M-N-C. Percentages reflect yield of the specified product.
Figure 7.
Figure 7.
Oxidative alcohol coupling to form esters. Percentages reflect yield of the specified product.
Figure 8.
Figure 8.
Synthesis of nitriles from alcohols and ammonia, (a) in organic solvent and (b) in aqueous solvent. Percentages reflect yield of the specified product.
Figure 9.
Figure 9.
Aerobic oxidation of alcohols to amides over M-N-C. Percentages reflect yield of the specified product.
Figure 10.
Figure 10.
Dehydrogenation of N-heterocycles developed by Iosub and Stahl (a) and Beller and coworkers (b). Percentages reflect yield of the specified product.
Figure 11.
Figure 11.
Comprehensive description of factors that influence kinetic behavior of M-N-C catalysts. (a) Contributions of transport, site quantity, accessibility, and intrinsic reactivity to overall rates, (b) substrate concentration profile in bulk solution, at the particle surface, and within a catalyst particle under transport-controlled conditions, and (c) systematic studies across bulk metal content in ORR. Panel (c) is adapted with permission from Ref. . Copyright 2007 Elsevier.
Figure 12.
Figure 12.
Methods for active site quantification in Fe-N-C catalysts reported in the ORR literature: (a, b) irreversible poisons and (c, d) ex situ chemical titrations. Panels (a) and (b) are adapted from Ref. . Copyright 2016 Nature Publishing Group. Panels (c) and (d) are adapted from Ref. . Copyright 2018 American Chemical Society.
Figure 13.
Figure 13.
Commonly used approaches to characterize metal sites in M-N-C materials: (a-d) in situ XANES Δμ technique,, (e, f) EXAFS,– (g, h) Mössbauer spectroscopy, (i) XPS, (j, k) electron microscopy,, (l) XRD, , and (m) model complexes. The EXAFS spectra in (e) and (f) are normalized to their maximum intensity for comparative purposes. The XRD patterns in (l) are normalized to their maximum intensity for comparative purposes. Panels (a) and (b) are adapted from Ref. . Copyright 2013 American Chemical Society. Panels (c) and (d) are adapted from Ref. . Copyright 2015 American Chemical Society. Panels (e) and (f) contain spectra adapted with permission from Refs. ,–. Copyright 2019 American Chemical Society, 2019 Wiley-VCH, 2019 Royal Society of Chemistry, 2019 American Chemical Society, 2020 Wiley-VCH, 2020 American Chemical Society. Panels (g) and (h) are adapted with permission from Ref. . Copyright 2016 Elsevier. Panel (i) is adapted with permission from Ref. . Copyright 2016 Elsevier. Panel (j) is adapted with permission from Ref. . Copyright 2017 American Association for the Advancement of Science. Panel (k) is adapted with permission from Ref. . Copyright 2011 American Association for the Advancement of Science. The top trace in panel (l) is adapted with permission from Ref. . Copyright 2017 American Association for the Advancement of Science. The bottom trace in panel (l) is adapted from Ref. . Copyright 2021 American Chemical Society.
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