Carbon-Based Electrocatalysts for Hydrogen and Oxygen Evolution Reactions

Carbon-Based Electrocatalysts for Hydrogen and Oxygen Evolution
Reactions

 Lulu Zhang, Jin Xiao, Haiyan Wang, and Minha Shao

ACS Catal. 2017, 7, 7855-7865

http://pubs.acs.org/doi/pdf/10.1021/acscatal.7b02718

Abstract:

 Hydrogen and oxygen evolution reactions (HER and OER) are important for many electrochemical systems. Besides traditional noble-metal-based catalysts, carbon-based materials have been found to be effective for catalyzing these reactions. Various carbon structures doped with heteroatoms (N, S, P, B, and transition metals) and graphitic-layer-encapsulated metal and compound particles have shown good activities toward HER and OER at universal pHs. In this Perspective, recent research on the development of carbon-based electrocatalysts for HER and OER, as well as their challenges and opportunities are discussed.

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Europium Oxybromide Catalysts for Efficient Bromine Looping in Natural Gas Valorization

Europium Oxybromide Catalysts for Efficient Bromine Looping in Natural Gas Valorization

Vladimir Paunovic ́+, Ronghe Lin+, Matthias Scharfe, Amol P. Amrute, Sharon Mitchell, Roland Hauert, and Javier Pérez-Ramírez*

Angew. Chem. Int. Ed. 2017, 56, 9791-9795
http://onlinelibrary.wiley.com/doi/10.1002/anie.201704406/abstract

Abstract:
 

The industrialization of bromine-mediated natural gas upgrading is contingent on the ability to fully recycle hydrogen bromide (HBr), which is the end form of the halogen after the activation and coupling of the alkanes. Europium oxybromide (EuOBr) is introduced as a unique catalytic material to close the bromine loop via HBr oxidation, permitting low-temperature operation and long lifetimes with a stoichiometric feed (O2:HBr=0.25)—conditions at which any catalyst reported to date severely deactivates because of excessive bromination. Besides, EuOBr exhibits unparalleled selectivity to methyl bromide in methane oxybromination, which is an alternative route for bromine looping. This novel active phase is finely dispersed on appropriate carriers and scaled up to technical extrudates, enhancing the utilization of the europium phase while preserving the performance. This catalytic system paves the way for sustainable valorization of stranded natural gas via bromine chemistry.

Mechanistic Basis for Efficient, Site-Selective, Aerobic Catalytic Turnover in Pd-Catalyzed C–H Imidoylation of Heterocycle-Containing Molecules

Key world: PdO2 adduct, CH activation

Stephen J. Tereniak and Shannon S. Stahl* 

Department of Chemistry, University of Wisconsin—Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/jacs.7b07359

Abstract

A recently reported Pd-catalyzed method for oxidative imidoylation of C–H bonds exhibits unique features that have important implications for Pd-catalyzed aerobic oxidation catalysis: (1) The reaction tolerates heterocycles that commonly poison Pd catalysts. (2) The site selectivity of C–H activation is controlled by an N-methoxyamide group rather than a suitably positioned heterocycle. (3) A Pd0 source, Pd2(dba)3 (dba = dibenzylideneacetone), is superior to Pd(OAc)2 as a precatalyst, and other PdII sources are ineffective. (4) The reaction performs better with air, rather than pure O2. The present study elucidates the origin of these features. Kinetic, mechanistic, and in situ spectroscopic studies establish that PdII-mediated C–H activation is the turnover-limiting step. The tBuNC substrate is shown to coordinate more strongly to PdII than pyridine, thereby contributing to the lack of heterocycle catalyst poisoning. A well-defined PdII–peroxo complex is a competent intermediate that promotes substrate coordination via proton-coupled ligand exchange. The effectiveness of this substrate coordination step correlates with the basicity of the anionic ligands coordinated to PdII, and Pd0 catalyst precursors are most effective because they selectively afford the PdII–peroxo in situ. Finally, elevated O2 pressures are shown to contribute to background oxidation of the isonitrile, thereby explaining the improved performance of reactions conducted with air rather than 1 atm O2. These collective results explain the unique features of the aerobic C–H imidoylation of N-methoxybenzamides and have important implications for other Pd-catalyzed aerobic C–H oxidation reactions.

How to Control Inversion vs Retention Transmetalation between Pd^II–Phenyl and Cu^I–Alkyl Complexes: Theoretical Insight

Hong Zheng^†, Kazuhiko Semba^‡, Yoshiaki Nakao^‡ , and Shigeyoshi Sakaki^*† 

^† Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishi-hiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan

^‡ Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/jacs.7b04383

Abstract

Transmetalation between Pd(Br)(Ph^A)(PCyp₃)₂ (Ph = phenyl, Cyp = cyclopentyl) and Cu(C^aHMePh^B)(NHC) (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene) is an important elementary step in recently reported catalytic cross-coupling reaction by Pd/Cu cooperative system. DFT study discloses that the transmetalation occurs with inversion of the stereochemistry of the C^aHMePh^B group. In its transition state, the C^aHMePh^B group has almost planar structure around the C^a atom. That planar geometry is stabilized by conjugation between the π* orbital of the Ph^B and the 2p orbital of the C^a. Another important factor is activation entropy (ΔS°^‡); retention transmetalation occurs through Br-bridging transition state, which is less flexible than that of the inversion transmetalation because of the Br-bridging structure, leading to a smaller activation entropy in the retention transition state than in the inversion transition state. For C^aHMeEt group, transmetalation occurs in a retention manner. In the planar C^aHMeEt group of the inversion transition state, the C^a 2p orbital cannot find a conjugation partner because of the absence of π-electron system in the C^aHMeEt. Transmetalation of C^aHMe(CH═CH₂) occurs in a retention manner because the vinyl π* is less effective for the conjugation with the C^a 2p because of its higher orbital energy than the Ph π*. The introduction of electron-withdrawing substituent on the Ph^B is favorable for inversion transmetalation. These results suggest that the stereochemistry of the C^a atom in transmetalation can be controlled by electronic effect of the C^aHMeR (R = phenyl, vinyl, or alkyl) and sizes of the substituent and ligand.