Redox Processes of Manganese Oxide in Catalyzing Oxygen Evolution and Reduction: An

Manganese oxides with rich redox chemistry have been widely used in (electro)catalysis in applications of energy and environmental consequence. While they are ubiquitous in catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), redox processes occurring on the surface of manganese oxides are poorly understood. We report valence changes at OER- and ORR-relevant voltages of a layered manganese oxide film prepared by electrodeposition. X-ray absorption spectra were collected in situ in O[subscript 2]-saturated 0.1 M KOH using inverse partial fluorescence yield (IPFY) at the Mn L[subscript 3,2]-edges and partial fluorescence yield (PFY) at the O K-edge. Overall, we found reversible yet hysteretic Mn redox and qualitatively reproducible spectral changes by Mn L[subscript 3,2]IPFY XAS. Oxidation to a mixed Mn[superscript 3+/4+] valence preceded the oxygen evolution at 1.65 V vs RHE, while manganese reduced below Mn[superscript 3+] and contained tetrahedral Mn[superscript 2+] during oxygen reduction at 0.5 V vs RHE. Analysis of the pre-edge in O K-edge XAS provided the Mn-O hybridization, which was highest for Mn[superscript 3+](e[subscript g][superscript 1]). Our study demonstrates that combined in situ experiments at the metal L- and oxygen K-edges are indispensable to identify both the active valence during catalysis and the hybridization with oxygen adsorbates, critical to the rational design of active catalysts for oxygen electrocatalysis.National Science Foundation (U.S.) (Grant DGE-1122374

Reversibility of Ferri-/Ferrocyanide Redox during Operando Soft X-ray Spectroscopy

The ferri-/ferrocyanide redox couple is ubiquitous in many fields of physical chemistry. We studied its photochemical response to intense synchrotron radiation by in situ X-ray absorption spectroscopy (XAS). For photon flux densities equal to and above 2 × 1011 s–1 mm–2, precipitation of ferric (hydr)oxide from both ferricyanide and ferrocyanide solutions was clearly detectable, despite flowing fast enough to replace the solution in the flow cell every 0.4 s (flow rate 1.5 mL/min). During cyclic voltammetry, precipitation of ferric (hydr)oxide was promoted at reducing voltages and observed below 1011 s–1 mm–2. This was accompanied by inhibition of the ferri-/ferrocyanide redox, which we probed by time-resolved operando XAS. Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in situ experiments

Golden single-atomic-site platinum electrocatalysts

Bimetallic nanoparticles with tailored structures constitute a desirable model system for catalysts, as crucial factors such as geometric and electronic effects can be readily controlled by tailoring the structure and alloy bonding of the catalytic site. Here we report a facile colloidal method to prepare a series of platinum–gold (PtAu) nanoparticles with tailored surface structures and particle diameters on the order of 7 nm. Samples with low Pt content, particularly Pt 4 Au 96 , exhibited unprecedented electrocatalytic activity for the oxidation of formic acid. A high forward current density of 3.77 A mg Pt −1 was observed for Pt 4 Au 96 , a value two orders of magnitude greater than those observed for core–shell structured Pt 78 Au 22 and a commercial Pt nanocatalyst. Extensive structural characterization and theoretical density functional theory simulations of the best-performing catalysts revealed densely packed single-atom Pt surface sites surrounded by Au atoms, which suggests that their superior catalytic activity and selectivity could be attributed to the unique structural and alloy-bonding properties of these single-atomic-site catalysts

Aqueous Cu(II)–Organic Complexation Studied in Situ Using Soft X‑ray and Vibrational Spectroscopies

In situ aqueous solutions containing copper–ligand mixtures were measured at the Cu L-edge using X-ray absorption near edge structure (XANES) and with attenuated total reflectance infrared (ATR-FTIR) spectroscopies. Copper complexation with environmentally relevant ligands such as EDTA, citrate, and malate provided a bridge between spectroscopic studies and general environmental behavior and will allow for future study of complex environmental samples. XANES results show that the lowest unoccupied molecular orbital (LUMO) energy is governed by the ligand field strength and is related to Lewis acid/base properties of the ligand functional groups. Complementary ATR-FTIR studies confirmed the importance of water molecules in the structure of these Cu–ligand complexes and provided in-depth structural analysis to support the XANES data. Copper–malate is shown to have a 5/6-O-ring structure, and Cu–ethylenediaminetetraacetate has pentadentate coordination. Cu L-edge XANES also revealed direct Cu–N coordination in these aqueous solutions with amide functional groups

Synchrotron Radiation Calibration method at the N K-edge using interstitial nitrogen gas in solid-state nitrogen-containing inorganic compounds

The standard method of soft X-ray beamline calibration at the N K-edge uses the = 0 peak transition of gas-phase N 2 . Interstitial N 2 gas trapped or formed within widely available solid-state ammonium-and amine-containing salts can be used for this purpose, bypassing gas-phase measurements. Evidence from non-nitrogen-containing compounds (KH 2 PO 4 ) and from He-purged ammonium salts suggest that production of N 2 gas is through beam-induced decomposition. Compounds with nitrate or nitrite as anions produce coincident features and are not suitable for this calibration method

Nitrogen NEXAFS spectra for uncharred OM, PyOM, PyOM after extraction and PyOM toluene extract and C_N mineralization data for uncharred OM and PyOM

This data is shared under a CC BY-NC license (https://creativecommons.org/licenses/by-nc/4.0/); the data are free to use with proper attribution and acknowledgment of the original authors, and may not be used for commercial purposes. Users must also indicate that the conclusions and assumptions made from analysis of these data are not necessarily the view of the original authors, Cornell University, or the project funders (see sponsorship information above).The molecular structure of pyrogenic organic matter (PyOM) is generally considered to exert a dominant control on PyOM-C mineralization, yet similar information is lacking for PyOM-N. In this study, we evaluated how the thermal conversion of organic matter (OM) into PyOM altered the N molecular structure and affected subsequent C and N mineralization. Nitrogen near edge X-ray absorption fine structure (NEXAFS) of uncharred OM, PyOM, PyOM toluene extract, and PyOM after toluene extraction were used to predict PyOM-C and –N mineralization potentials. PyOM was produced from three different feedstocks (e.g. Maize- Zea mays L.; Ryegrass- Lollium perenne L.; and Willow-Salix viminalix L.) each with varying initial N content at three pyrolysis temperatures (350, 500 and 700°C). Mineralization of C and N was measured from incubations of uncharred OM and PyOM in a sand matrix for 256 days at 30°C. This data set provides a library of nitrogen (1s) near-edge X-ray absorption fine structure (NEXAFS) spectra of organic N reference compounds and uncharred OM, PyOM, PyOM toluene extract, and PyOM after toluene extraction used in this study. The dataset also contains the C and N mineralization data used to correlated N molecular structure information to PyOM-C persistence in the environment. The datafiles include processed data and derived quantities.This study was supported in part by the Cornell University Program in Biogeochemisty and Environmental Biocomplexity, National Science Foundation’s Basic Research for Enabling Agricultural Development (NSF-BREAD Grant number IOS-0965336) and the Fondation des Fondateurs. DT acknowledges support from the NSF IGERT Program (DGE-0903371). Research described in this paper was performed at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research

Data From: Co-precipitation Induces Changes to Iron and Carbon Chemistry and Spatial Distribution at the Nanometer Scale

Please cite as: Angela R. Possinger, Michael J. Zachman, James J. Dynes, Tom Z. Regier, Lena F. Kourkoutis, Johannes Lehmann. (2021) Data From: Co-precipitation Induces Changes to Iron and Carbon Chemistry and Spatial Distribution at the Nanometer Scale. [dataset] Cornell University eCommons Repository. https://doi.org/10.7298/6134-h423Data in support of research on: Association of organic matter (OM) with mineral phases via co-precipitation is expected to be a widespread process in environments with high OM input and frequent mineral dissolution and re-precipitation. In contrast to surface area-limited adsorption processes, co-precipitation may allow for greater carbon (C) accumulation. However, the potential sub-micrometer scale structural and compositional differences that affect the bioavailability of co-precipitated C are largely unknown. In this study, we used a combination of high-resolution analytical electron microscopy and bulk spectroscopy to probe interactions between a mineral phase (ferrihydrite, nominally Fe2O3•0.5H2O) and organic soil-derived water-extractable OM (WEOM). In co-precipitated WEOM-Fe, nanometer-scale scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) revealed increased Fe(II) and less Fe aggregation relative to adsorbed WEOM-Fe. Spatially distinct lower- and higher-energy C regions were detected in both adsorbed and co-precipitated WEOM-Fe. In co-precipitates, lower-energy aromatic and/or substituted aromatic C was spatially associated with reduced Fe(II), but higher-energy oxidized C was enriched at the oxidized Fe(III) interface. Therefore, we show that co-precipitation does not constitute a non-specific physical encapsulation of C that only affects Fe chemistry and spatial distribution, but may cause a bi-directional set of reactions that lead to spatial separation and transformation of both Fe and C forms. In particular, we propose that abiotic redox reactions between Fe and C via substituted aromatic groups (e.g., hydroquinones) play a role in creating distinct co-precipitate composition, with potential implications for its mineralization.Funding for this study was provided by the NSF IGERT in Cross-Scale Biogeochemistry and Climate at Cornell University (NSF Award #1069193) and the Technical University of Munich Institute for Advanced Studies. Additional research funds were provided by the Andrew W. Mellon Foundation and the Cornell College of Agriculture and Life Sciences Alumni Foundation. M.J.Z. and L.F.K. acknowledge support by the NSF (DMR-1654596) and Packard Foundation. This work uses research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation under award DMR-1332208. The Cornell Center for Materials Research (CCMR) is funded by NSF MRSEC (DMR-1719875). Research described in this paper was performed at the Canadian Light Source (CLS), which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research

Nitrogen speciation and transformations in fire-derived organic matter

Vegetation fires are known to have broad geochemical effects on carbon (C) cycles in the Earth system, yet limited information is available for nitrogen (N). In this study, we evaluated how charring organic matter (OM) to pyrogenic OM (PyOM) altered the N molecular structure and affected subsequent C and N mineralization. Nitrogen near-edge X-ray absorption fine structure (NEXAFS) of uncharred OM, PyOM, PyOM toluene extract, and PyOM after toluene extraction were used to predict PyOM-C and -N mineralization potentials. PyOM was produced from three different plants (e.g. Maize-Zea mays L.; Ryegrass-Lollium perenne L.; and Willow-Salix viminalix L.) each with varying initial N contents at three pyrolysis temperatures (350, 500 and 700 °C). Mineralization of C and N was measured from incubations of uncharred OM and PyOM in a sand matrix for 256 days at 30 °C. As pyrolysis temperature increased from 350 to 700 °C, aromatic CN in 6-membered rings (putative) increased threefold. Aromatic CN in 6-membered oxygenated ring increased sevenfold, and quaternary aromatic N doubled. Initial uncharred OM-N content was positively correlated with the proportion of heterocyclic aromatic N in PyOM (R2 = 0.44; P < 0.0001; n = 42). A 55% increase of aromatic N heterocycles at high OM-N content, when compared to low OM-N content, suggests that higher concentrations of N favor the incorporation of N atoms into aromatic structures by overcoming the energy barrier associated with the electronic and atomic configuration of the C structure. A ten-fold increase of aromatic CN in 6-membered rings (putative) in PyOM (as proportion of all PyOM-N) decreased C mineralization by 87%, whereas total N contents and C:N ratios of PyOM had no effects on C mineralization of PyOM-C for both pyrolysis temperatures (for PyOM-350 °C, R2 = 0.15; P < 0.27; for PyOM-700 °C, R2 = 0.22; P < 0.21). Oxidized aromatic N in PyOM toluene extracts correlated with higher C mineralization, whereas aromatic N in 6-membered heterocycles correlated with reduced C mineralization (R2 = 0.56; P = 0.001; n = 100). Similarly, aromatic N in 6-membered heterocycles in PyOM remaining after toluene extraction reduced PyOM-C mineralization (R2 = 0.49; P = 0.0006; n = 100). PyOM-C mineralization increased when N atoms were located at the edge of the C network in the form of oxidized N functionalities or when more N was found in PyOM toluene extracts and was more accessible to microbial oxidation. These results confirm the hypothesis that C persistence of fire-derived OM is significantly affected by its molecular N structure and the presented quantitative structure-activity relationship can be utilized for predictive modeling purposes

Validating the Scalability of Soft X‑ray Spectromicroscopy for Quantitative Soil Ecology and Biogeochemistry Research

Synchrotron-based soft-X-ray scanning transmission X-ray microscopy (STXM) has the potential to provide nanoscale resolution of the associations among biological and geological materials. However, standard methods for how samples should be prepared, measured, and analyzed to allow the results from these nanoscale imaging and spectroscopic tools to be scaled to field scale biogeochemical results are not well established. We utilized a simple sample preparation technique that allows one to assess detailed mineral, metal, and microbe spectroscopic information at the nano- and microscale in soil colloids. We then evaluated three common approaches to collect and process nano- and micronscale information by STXM and the correspondence of these approaches to millimeter scale soil measurements. Finally, we assessed the reproducibility and spatial autocorrelation of nano- and micronscale protein, Fe­(II) and Fe­(III) densities in a soil sample. We demonstrate that linear combination fitting of entire spectra provides slightly different Fe­(II) mineral densities compared to image resonance difference mapping but that difference mapping results are highly reproducible between among sample replicates. Further, STXM results scale to the mm scale in complex systems with an approximate geospatial range of 3 μm in these samples