Operando Elucidation of Electrocatalytic and Redox Mechanisms on a 2D Metal Organic Framework Catalyst for Efficient Electrosynthesis of Hydrogen Peroxide in Neutral Media.

The practical electrosynthesis of hydrogen peroxide (H2O2) is hindered by the lack of inexpensive and efficient catalysts for the two-electron oxygen reduction reaction (2e- ORR) in neutral electrolytes. Here, we show that Ni3HAB2 (HAB = hexaaminobenzene), a two-dimensional metal organic framework (MOF), is a selective and active 2e- ORR catalyst in buffered neutral electrolytes with a linker-based redox feature that dynamically affects the ORR behaviors. Rotating ring-disk electrode measurements reveal that Ni3HAB2 has high selectivity for 2e- ORR (>80% at 0.6 V vs RHE) but lower Faradaic efficiency due to this linker redox process. Operando X-ray absorption spectroscopy measurements reveal that under argon gas the charging of the organic linkers causes a dynamic Ni oxidation state, but in O2-saturated conditions, the electronic and physical structures of Ni3HAB2 change little and oxygen-containing species strongly adsorb at potentials more cathodic than the reduction potential of the organic linker (Eredox ∼ 0.3 V vs RHE). We hypothesize that a primary 2e- ORR mechanism occurs directly on the organic linkers (rather than the Ni) when E > Eredox, but when E < Eredox, H2O2 production can also occur through Ni-mediated linker discharge. By operating the bulk electrosynthesis at a low overpotential (0.4 V vs RHE), up to 662 ppm of H2O2 can be produced in a buffered neutral solution in an H-cell due to minimized strong adsorption of oxygenates. This work demonstrates the potential of conductive MOF catalysts for 2e- ORR and the importance of understanding catalytic active sites under electrochemical operation.

Defect-mediated ferromagnetism in correlated two-dimensional transition metal phosphorus trisulfides.

Controlling the magnetic spin states of two-dimensional (2D) van der Waals (vdW) materials with strong electronic or magnetic correlation is important for spintronic applications but challenging. Crystal defects that are often present in 2D materials such as transition metal phosphorus trisulfides (MPS3) could influence their physical properties. Here, we report the effect of sulfur vacancies on the magnetic exchange interactions and spin ordering of few-layered vdW magnetic Ni1−xCoxPS3 nanosheets. Magnetic and structural characterization in corroboration with theoretical calculations reveal that sulfur vacancies effectively suppress the strong intralayer antiferromagnetic correlation, giving rise to a weak ferromagnetic ground state in Ni1−xCoxPS3 nanosheets. Notably, the magnetic field required to tune this ferromagnetic state (<300 Oe) is much lower than the value needed to tune a typical vdW antiferromagnet (> several thousand oersted). These findings provide a previously unexplored route for controlling competing correlated states and magnetic ordering by defect engineering in vdW materials.

The Impact of 5-Hydroxymethylfurfural (HMF)-Metal Interactions on the Electrochemical Reduction Pathways of HMF on Various Metal Electrodes.

5-Hydroxymethylfurfural (HMF), which can be derived from lignocellulosic biomass, is an important platform molecule that can be used to produce valuable biofuels and polymeric materials. Electrochemical reduction of HMF is of great interest as it uses water as the hydrogen source and achieves desired reduction reactions at room temperature and ambient pressure. Hydrogenation and hydrogenolysis are two important reactions for reductive HMF conversion. Therefore, elucidating key characteristics of electrocatalysts that govern the selectivity for hydrogenation and hydrogenolysis is critical in rationally developing efficient and selective electrocatalysts. In this study, combined experimental and computational investigations are used to demonstrate how the adsorption energy of HMF on metal surfaces and the resulting changes in the intramolecular bond lengths of adsorbed HMF directly impact the reduction pathways of HMF. These results make it possible to rationally understand a general trend in the behaviors observed when using various metal electrodes for HMF reduction.

Micki: A python-based object-oriented microkinetic modeling code.

We have developed a flexible, general-purpose microkinetic modeling code, Micki, to analyze complex, heterogeneously catalyzed chemical reactions based upon first-principles calculations. This Python-based code is modular and object oriented, framing the development of microkinetic models in familiar chemical terms. We also present novel approaches, incorporated into Micki, to describe diffusion limited reactions, multidentate bindings, thermodynamically consistent lateral interactions, and Brønsted-Evans-Polanyi estimates of changes in barrier heights. Micki has built-in modules for subsequent analysis of microkinetic models, including degree of rate control and rate order. As a demonstration of the power and flexibility of the code, we build a microkinetic model for the water-gas shift reaction and compare to previously published experimental results and microkinetic models, showing that Micki can quantitatively reproduce experimental turnover frequencies with minimal empirical optimization.