Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

Abstract

Direct conversion of methane (CH 4 ) to C 1−2 liquid oxygenates is a captivating approach to lock carbons in transportable value-added chemicals, while reducing global warming. Existing approaches utilizing the transformation of CH 4 to liquid fuel via tandemized steam methane reforming and the Fischer−Tropsch synthesis are energy and capital intensive. Chemocatalytic partial oxidation of methane remains challenging due to the negligible electron affinity, poor C−H bond polarizability, and high activation energy barrier. Transition-metal and stoichiometric catalysts utilizing harsh oxidants and reaction conditions perform poorly with randomized product distribution. Paradoxically, the catalysts which are active enough to break C−H also promote overoxidation, resulting in CO 2 generation and reduced carbon balance. Developing catalysts which can break C−H bonds of methane to selectively make useful chemicals at mild conditions is vital to commercialization. Single atom catalysts (SACs) with specifically coordinated metal centers on active support have displayed intrigued reactivity and selectivity for methane oxidation. SACs can significantly reduce the activation energy due to induced electrostatic polarization of the C−H bond to facilitate the accelerated reaction rate at the low reaction temperature. The distinct metal−support interaction can stabilize the intermediate and prevent the overoxidation of the reaction products. The present review accounts for recent progress in the field of SACs for the selective oxidation of CH 4 to C 1−2 oxygenates. The chemical nature of catalytic sites, effects of metal−support interaction, and stabilization of intermediate species on catalysts to minimize overoxidation are thoroughly discussed with a forward-looking perspective to improve the catalytic performance.

High-Density Cobalt Single-Atom Catalysts for Enhanced Oxygen Evolution Reaction

“…A similar trend was observed in CoML as well; however, R ct was higher, as depicted in Figure S35b. 7 The ICP-OES analysis of the electrolyte after OER at 5, 10 mA cm −2 for 16 h and 500 CV cycles at 100 mV s −1 using the CoMM electrode does not show any trace of Co, corroborating that Co SA species were stably ligated to N-rich carbonaceous framework (Table S8 and Section 4.14 in SI). Furthermore, XPS analysis of the CoMM electrode after OER reveals the absence of any significant changes in C 1s and N 1s spectra; however, Co 2p spectra displayed the evolution of Co 3+ species (CoOOH), suggesting that hypervalent Co 3+ participates in OER (Figure S36 and Section 4.15 in the SI).…”

Selective Methane Oxidation to Acetic Acid Using Molecular Oxygen over a Mono-Copper Hydroxyl Catalyst

“…Acetic acid, an essential bulk chemical widely used in synthesizing vinyl acetate monomer (VAM), acetate esters, acetic anhydride, and cellulose acetate, has a global demand of 6.5 million metric tons (Mt/a) every year. − The industrial production of acetic acid involves three separate energy and capital-intensive processes: (1) high-temperature conversion of methane or coal to syngas via steam methane reforming or coal gasification, respectively; (2) high-temperature catalytic conversion of syngas to methanol; and (3) finally, carbonylation of methanol to acetic acid via the Monsanto or Cativa process. ,,− The single-step chemoselective transformation of methane to acetic acid under mild reaction conditions could be more economical but very challenging due to the intrinsic inertness of methane originating from strong C–H bonds (104 kcal/mol) and low acidity (p K a ∼ 50), low solubility in the solvent, and poor selectivity. − …”

Tailoring of Active Sites from Single to Dual Atom Sites for Highly Efficient Electrocatalysis

“…Nevertheless, a high catalyst loading is necessary to realize a high overall electrocatalytic performance, which can potentially lead to a mass-transport issue. Although there are recent reports about the increased metal loading above 5 wt % in SACs by adopting sophisticated methods in lab scale, − the low metal loading and unsatisfactory metal site density are still the main challenges, and this is a long way from industrial applications, especially for traditional approaches such as wet chemistry based, simple high temperature pyrolysis, and atomic layer deposition. ,− On the other hand, a strong bond between isolated single atoms and supports can also contribute to the stability of SACs, but this strong interaction may also affect the electronic states of single atoms. Such an interaction can induce an electron-donation from metal atoms to supports, which yields more oxidic single atoms and manifestly alters the intrinsic activity.…”