Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

Establishing catalytic structure−function relationships introduces the ability to optimize the catalyst structure for enhanced activity, selectivity, and durability against reaction conditions and prolonged catalysis. Here we present experimental and computational data elucidating the mechanism for the O 2 reduction reaction with a conductive nickel-based metal−organic framework (MOF). Elucidation of the O 2 reduction electrokinetics, understanding the role of the extended MOF structure in providing catalytic activity, observation of how the redox activity and pK a of the organic ligand influences catalysis, and identification of the catalyst active site yield a detailed O 2 reduction mechanism where the ligand, rather than the metal, plays a central role. More generally, familiarization with how the structural and electronic properties of the MOF influence reactivity may provide deeper insight into the mechanisms by which less structurally defined nonplatinum group metal electrocatalysts reduce O 2 . KEYWORDS: O 2 reduction, electrocatalysis, metal−organic framework, porous catalysts, 2D materials ■ INTRODUCTIONUnderstanding catalytic kinetics and thermodynamics to construct a reasonable reaction mechanism is central for both elucidating the behavior of a given catalyst and gaining predictive power over structure−function relationships. This predictive power aids in efficiently optimizing catalyst performance by systematically tuning the structural and electronic properties of the catalyst. One class of materials that could benefit from mechanism-guided optimization is nonplatinum group metal (non-PGM) electrocatalysts for the O 2 reduction reaction (ORR) to water (4e − reduction) and/or hydrogen peroxide (2e − reduction). Such catalysts typically include abundant transition metals and/or heteroatoms such as N, O, and S doped into a carbonaceous matrix.1−6 Although quite active and stable during ORR, previously reported non-PGM catalysts often consist of amorphous carbon mechanically blended with transition metal macrocycles or other metal and main group heteroatomic sources. These relatively poorly defined materials do not lend themselves to facile mechanistic studies; the inhomogeneous dispersion and irregular orientation of the dopants throughout the carbon matrix engenders structural ambiguity that makes identification, experimental probing, and computational modeling of active sites difficult.Conversely, highly ordered metal−organic frameworks (MOFs) containing well-defined, spatially isolated active sites present an attractive platform for experimental and computational correlation between the chemical and electronic structure of a given catalyst and the electrocatalytic activity and mechanism, a feat that is traditionally restricted to homogeneous molecular systems. We previously showed that the electrically conductive MOF Ni 3 (HITP) 2 (HITP = 2,3,6,7,10, (Figure 1) functions as

The mean-field solutions of electronic excited states are much less accessible than ground state (e.g., Hartree-Fock) solutions. Energy-based optimization methods for excited states, like Δ-SCF (self-consistent field), tend to fall into the lowest solution consistent with a given symmetry-a problem known as "variational collapse." In this work, we combine the ideas of direct energy-targeting and variance-based optimization in order to describe excited states at the mean-field level. The resulting method, σ-SCF, has several advantages. First, it allows one to target any desired excited state by specifying a single parameter: a guess of the energy of that state. It can therefore, in principle, find all excited states. Second, it avoids variational collapse by using a variance-based, unconstrained local minimization. As a consequence, all states-ground or excited-are treated on an equal footing. Third, it provides an alternate approach to locate Δ-SCF solutions that are otherwise hardly accessible by the usual non-aufbau configuration initial guess. We present results for this new method for small atoms (He, Be) and molecules (H, HF). We find that σ-SCF is very effective at locating excited states, including individual, high energy excitations within a dense manifold of excited states. Like all single determinant methods, σ-SCF shows prominent spin-symmetry breaking for open shell states and our results suggest that this method could be further improved with spin projection.

Fragment embedding is one way to circumvent the high computational scaling of accurate electron correlation methods. The challenge of applying fragment embedding to molecular systems primarily lies in the strong entanglement and correlation that prevent accurate fragmentation across chemical bonds. Recently, Schmidt decomposition has been shown effective for embedding fragments that are strongly coupled to a bath in several model systems. In this work, we extend a recently developed quantum embedding scheme, bootstrap embedding (BE), to molecular systems. The resulting method utilizes the matching conditions naturally arising from using overlapping fragments to optimize the embedding. Numerical simulation suggests that the accuracy of the embedding improves rapidly with fragment size for small molecules, whereas larger fragments that include orbitals from different atoms may be needed for larger molecules. BE scales linearly with system size (apart from an integral transform) and hence can potentially be useful for large-scale calculations.

Using a combination of experimental and computational investigations, we assemble a consistent mechanistic model for the oxygen reduction reaction (ORR) at molecularly well-defined graphite-conjugated catalyst (GCC) active sites featuring aryl-pyridinium moieties (N + -GCC). ORR catalysis at glassy carbon surfaces modified with N + -GCC fragments displays near-first-order dependence in O 2 partial pressure and near-zero-order dependence on electrolyte pH. Tafel analysis suggests an equilibrium one-electron transfer process followed by a rate-limiting chemical step at modest overpotentials that transitions to a rate-limiting electron transfer sequence at higher overpotentials. Finite-cluster computational modeling of the N + -GCC active site reveals preferential O 2 adsorption at electrophilic carbons alpha to the pyridinium moiety. Together, the experimental and computational data indicate that ORR proceeds via a proton-decoupled O 2 activation sequence involving either concerted or stepwise electron transfer and adsorption of O 2 , which is then followed by a series of electron/ proton transfer steps to generate water and turn over the catalytic cycle. The proposed mechanistic model serves as a roadmap for the bottom-up synthesis of highly active N-doped carbon ORR catalysts.