Homogeneous catalysis for the nitrogen fuel cycle

Catalytic Processes to Accelerate Decarbonization in a Net-Zero Carbon World

Reducing carbon dioxide emissions is one of the critical challenges to mitigate global climate change, which is having detrimental impacts on society and the environment. Fossil fuel combustion in transportation, power generation, and industrial processes is the dominant contributor to carbon emissions. Over the past decades, sustainable solutions and strategies have been investigated and developed to enable decarbonization. Catalysis plays an essential role to address this global challenge by increasing energy efficiency, reducing carbon emissions, capturing carbon dioxide, and utilizing clean energy sources to displace fossil fuels. In this Review, the role of catalysis in reducing energy demand was discussed, enhancing process efficiency, displacing carbon-intensive feedstocks and products, and therefore, reducing carbon emissions. Recent advances in catalyst development were summarized, focusing on applications to enhance industrial processes efficiency and enable utilization of clean energy sources. Emerging approaches in catalysis were reviewed, including the manufacture of iron and steel, direct air capture of CO₂ , production of ethylene, ammonia, and sustainable aviation fuels, plastic recycling, and the synthesis of biobased plastics. The Review was concluded with suggested research directions to achieve a carbon net-zero world.

Concluding remarks: discussion on natural and artificial enzymes including synthetic models

This paper overviews the final remarks lecture delivered (by K. D. K.) at the end of this bioinorganic chemistry Faraday Discussion, held online for a worldwide audience from January 31 - February 3, 2022. This paper provides discussion in six sections: (1) the Introductory lecture, from Ed Solomon, emphasized past and present uses of advanced spectroscopic methods and theoretical approaches to elucidate metalloenzyme active site structure, physical properties and function. (2) The discussion topics are divided into groups having similar research themes, as seen from this author's perspective. Emphasis is given to the non-heme iron group of articles with dioxygen activation research. (3) Small molecule activation (e.g., N2, CO2 and O2 reduction; CH4 or H2O oxidation) is widely covered in this discussion; this authors' view of the important reactions in bioinorganic chemistry is discussed. (4) We discuss current practice and vision for employing materials chemistry to widely apply to electrocatalytic methods to effect small molecule activation (as above) to fulfill societal energy demands. (5) A discussion is given on the topic of synthetic models and the approach utilized therein. (6) New research on the authors' synthetic modeling is presented; preliminary results are given in the area of copper mediated peroxide activation.

A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph₂PC₆H₄NH)(NH₃)]⁺ ([1-NH₃]⁺), which processes electrocatalytic ammonia oxidation to N₂ and H₂ at a mild potential. Through establishing elementary e^-/H⁺ conversions with the ferric ammine, a formal Fe(IV)-amido species, [1-NH₂]⁺, together with its conjugated Lewis acid, [1-NH₃]²⁺, was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [1-NH₂]⁺ induces the reaction of the parent amido unit with NH₃. The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

Manganese-Catalyzed Ammonia Oxidation into Dinitrogen under Chemical or Electrochemical Conditions*

Earth-abundant metal-catalyzed oxidative conversion of ammonia into dinitrogen is a promising process to utilize ammonia as a transportation fuel. Herein, we report the manganese-catalyzed ammonia oxidation under chemical or electrochemical conditions using a manganese complex bearing (1S,2S)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine. Under chemical conditions using oxidant, up to 17.1 equivalents of N₂ per catalyst are generated. Also, mechanistic studies by stoichiometric reactions reveal that a nucleophilic attack of ammonia on manganese nitrogenous species occurs to form a nitrogen-nitrogen bond leading to dinitrogen. Moreover, we conduct density functional theory (DFT) calculations to confirm the plausible reaction mechanism. In addition, this reaction system is applicable under electrochemical conditions. The catalytic reaction proceeds with 96 % faradaic efficiency (FE) in bulk electrolysis to give up to 6.56 equivalents of N₂ per catalyst.

Plasmon-enabled N2 photofixation on partially reduced Ti3C2 MXene

Benefiting from the superior conductivity, rich surface chemistry and tunable bandgap, Ti₃C₂ MXene has become a frontier cocatalyst material for boosting the efficiency of semiconductor photocatalysts. It has been theoretically predicted to be an ideal material for N₂ fixation. However, the realization of N₂ photofixation with Ti₃C₂ as a host photocatalyst has so far remained experimentally challenging. Herein, we report on a sandwich-like plasmon- and an MXene-based photocatalyst made of Au nanospheres and layered Ti₃C₂, and demonstrate its efficient N₂ photofixation in pure water under ambient conditions. The abundant low-valence Ti (Ti^(4-x)+) sites in partially reduced Ti₃C₂ (r-Ti₃C₂) produced by surface engineering through H₂ thermal reduction effectively capture and activate N₂, while Au nanospheres offer plasmonic hot electrons to reduce the activated N₂ into NH₃. The Ti^(4-x)+ active sites and plasmon-generated hot electrons work in tandem to endow r-Ti₃C₂/Au with remarkably enhanced N₂ photofixation activity. Importantly, r-Ti₃C₂/Au exhibits ultrahigh selectivity without the occurrence of competing H₂ evolution. This work opens up a promising route for the rational design of efficient MXene-based photocatalysts.

Triple hydrogen atom abstraction from Mn–NH3 complexes results in cyclophosphazenium cations

All three H atoms of the NH₃ ligand of [Mn(depe)₂(CO)(NH₃)]⁺ are abstracted by an organic radical, giving a rare cyclophosphazenium cation; computations suggest that insertion of NH_x into a Mn–P bond provides a strong thermodynamic driving force.