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At least 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Calorimetric metal vapor adsorption energies for characterizing industrial catalyst support materials

Many important catalysts and electro-catalysts for energy and environmental technologies involve late transition metal atoms and nanoparticles dispersed across the surface of high-surface-area support materials, for example, oxide nanoparticles. The surface reactivity and long-term stability against deactivation of these materials depends strongly on the strength of bonding of the metal atom and nanoparticles to the support surface. Here, we introduce a novel calorimetry method that implements a LiTaO 3 crystal for heat detection and extends metal adsorption calorimetry to surfaces of nanomaterials that are deposited from liquid solutions, as is typical when preparing industrial catalyst supports. This allows one to measure the strength of metal atom bonding and the adhesion energy of metal nanoparticles to clean surfaces of technologically relevant catalyst supports. We demonstrate that here by measuring the heat of Ag vapor adsorption onto the clean surfaces of high-surface-area TiO 2 anatase powdered support materials (i.e., 5-nm-diameter nanoparticles). These heats of adsorption show a pulse-to-pulse standard deviation of only 9.3 kJ/mol (3.3%) for pulses containing only 1% of a monolayer of Ag vapor, with potential for a 5-fold reduction in noise if used with another metal evaporator. The calorimeter was also proven to work well measuring heats of Ag adsorption on calcium niobate(001) nanosheets deposited as thin films from liquid solution. Here, this calorimeter opens up the possibility of using metal adsorption calorimetry to screen catalyst support materials for the strengths with which they bind metal atoms and metal nanoparticles, offering new opportunities to discover better support materials or for basic scientific study of structure / function relationships with respect to metal / support bond energies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Identifying higher oxygenate synthesis sites in Cu catalysts promoted and stabilized by atomic layer deposited Fe 2 O 3

The conversion of syngas to higher oxygenates is an important step in the pathway towards the long-term sustainable production of fuels and chemicals. Mixed Cu/Fe catalysts combine metals with different CO activation characteristics, a property that makes them promising candidates for higher oxygenate production. However, Cu/Fe catalysts suffer from instability due to sintering and phase separation, and the active sites for higher oxygenate formation on these materials are not well understood. In this work, we use atomic layer deposition (ALD) to modify silica-supported Cu nanoparticles with monolayer precise amounts of Fe 2 O 3 and study the effects on catalyst structure, reactivity, and stability. We demonstrate that the Fe 2 O 3 ALD process is inherently selective towards deposition on the SiO 2 surface versus the Cu nanoparticles. As a result, a sufficiently thick Fe 2 O 3 layer surrounding the Cu nanoparticles acts as a barrier against Cu migration, preventing catalyst deactivation through sintering without covering the Cu. In syngas conversion reactions, we find that higher oxygenate selectivity increases with Fe 2 O 3 loading at low and intermediate ALD cycle numbers. Furthermore, using in situ X-ray absorption spectroscopy, we demonstrate that the Fe near the Cu nanoparticles is reduced to a metallic state under reaction conditions. From these measurements, we identify these metallic Cu/Fe interfaces as the active sites for higher oxygenate formation, which are most abundant at intermediate Fe 2 O 3 ALD cycle numbers. As the Fe 2 O 3 loading increases, higher oxygenate selectivity falls, resulting from encapsulation of the Cu nanoparticles by thick Fe 2 O 3 layers. Overall, the results demonstrate how thickness-controlled ALD Fe 2 O 3 can be used as both a promoter and stabilizer of Cu-based higher oxygenate synthesis catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Reaction paths of methane activation and oxidation of surface intermediates over NiO on Ceria-Zirconia catalysts studied by In-situ FTIR spectroscopy

Methane activation over NiO/Ce 0.82 Zr 0.18 O 2 catalysts synthesized by combustion synthesis is studied by in-situ IR spectroscopy and correlated to the properties and oxygen speciation of the catalyst. Through XRD, H 2 -TPR, and pyridine adsorption followed by FTIR spectroscopy, the reducibility and Lewis acidity of the catalysts are assessed. In-situ FTIR spectroscopy is used to monitor the methane activation on catalyst surfaces. Complex IR features of methane-derived surface species are observed and are attributed to the formation of surface alkyl/alkoxy, aldehyde, formate/carbonate, and aromatic species. A data analysis algorithm is developed to evaluate the evolution of different surface species over time. The formation of formate and carbonate species is driven by adsorbed surface oxygen, while less reactive oxygen species associated with NiO allow for the production of aromatics and alkoxy intermediates. Finally, by tuning the reducibility and Lewis acidity of the catalyst, the selectivity to alkoxy intermediates can be improved.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Core-shell and egg-shell zeolite catalysts for enhanced hydrocarbon processing

Developing structure-performance relationships with the underlying goal of optimizing known zeolite catalysts involves the manipulation of their physicochemical properties. Here, we systematically assessed the impact of mesoscopic gradients in acid site concentration, which has generally received little attention in the design of zeolite catalysts for hydrocarbon upgrading. A series of core–shell MEL-type zeolites were synthesized with catalytically active ZSM-11 cores and passivated silicalite-2 shells of varying thickness. Our findings revealed that ZSM-11@silicalite-2 particles with ultrathin shells (<10 nm) have enhanced mass transport, characteristic of relatively smaller particles, compared to the corresponding ZSM-11 core. Additionally, catalytic testing using the methanol-to-hydrocarbon (MTH) reaction showed that core–shell zeolites exhibit longer lifetimes, higher total turnovers, and an unexpected promotion of the aromatic cycle in the hydrocarbon pool mechanism. Time-resolved acid titration of core and core–shell catalysts confirmed that the siliceous shell introduces a hydrophobic exterior that impacts molecular diffusion. In comparison, prepared MFI core-shells (ZSM-5@silicalite-1) showed similar enhancement in catalyst performance. Moreover, we prepared egg-shell configurations of each zeolite, silicalite-2@ZSM-11 and silicalite-1@ZSM-5, comprised of an inert core and catalytically active shell. This inverse design of the egg-shell created pseudo nanosheets with total turnovers that were markedly higher than their homogeneous counterparts. Collectively, this study demonstrated that mesoscopic gradients in acid concentration via the design of core–shell and egg-shell zeolites significantly improve catalyst performance over conventional analogues for hydrocarbon upgrading.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Bimetallic NiCu catalysts supported on a Metal-Organic framework for Non-oxidative ethanol dehydrogenation

Non-oxidative ethanol dehydrogenation is a promising route to produce acetaldehyde and hydrogen from sustainable feedstock. Bimetallic NiCu catalysts have shown high efficiency and selectivity for this chemical transformation. In this study, we leverage the high porosity and uniform catalyst deposition sites on a Zr-based metal–organic framework (MOF) catalyst support, NU-1000, to understand how the changes in Cu:Ni ratio affects the reactivity for non-oxidative ethanol dehydrogenation. We found that increasing the Ni 2+ concentration significantly reduces the activation energy of the reaction due to the role of Ni 2+ in suppressing the onset of Cu reduction. Furthermore, this study illustrates how MOFs can be used as catalyst supports to fine-tune the catalyst compositions and understand their effect on the overall catalytic performances.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Synthesis and application of different phthalocyanine molecular sieve catalyst for oxidative desulfurization

M{sub 2}(PcAN){sub 2} (M=Fe, Co, Ni, Cu, Zn and Mn) anchored onto W-HZSM-5 (M{sub 2}(PcAN){sub 2}–W-HZSM-5) or the M{sub 2}(PcTN){sub 2} doping W-HZSM-5 (M{sub 2}(PcTN){sub 2}/W-HZSM-5) were prepared and their catalytic performances were tested for oxidative desulfurization in the presence of oxygen. Thiophene (T), benzothiophene (BT), and dibenzothiophene (DBT) were considered as sulfur compounds. Among zeolite-based catalysts, the Cu{sub 2}(PcAN){sub 2}–W-HZSM-5 and Cu{sub 2}(PcTN){sub 2}/W-HZSM-5 showed superior desulfurization performance and the activity of selectivity followed the order: T>BT>DBT. The effects of phthalocyanine concentration were studied by UV–Vis and calcination temperature was obtained by TG-DSC for Cu{sub 2}(PcTN){sub 2}/W-HZSM-5. Catalysts were characterized by EA, IR, XRD, SEM, TEM, ICP, and N{sub 2} adsorption. Reaction time, temperature and the amount of catalyst were investigated as the important parameters for optimization of the reaction. Furthermore, a possible process of oxidative desulfurization and the reaction products were proposed. - Graphical abstract: The ODS reaction schematic shows the reaction mechanism of ultra-deep desulfurization. The sulfur compounds are oxidized to their corresponding sulfoxides or sulfones through the use of oxygen and catalysts. The reaction process of ultra-deep desulfurization. - Highlights: • A kind of novel catalyst for deep desulfurization was synthesized. • Cu{sub 2}(PcAN){sub 2}–W-HZSM-5 exhibits excellent catalytic performance for desulfurization. • The reaction conditions that affect desulfurization efficiency are investigated. • The reaction process of model sulfur compounds is proposed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Preparation of amorphous copper - chromium oxides catalysts for selective oxidation of cyclohexane

In this work, amorphous copper-chromium catalysts with different Cu/Cr molar ratios have been prepared by a sol-gel method and were used in the oxidation of cyclohexane to KA-oil. The catalysts have been characterized by a variety of analytical techniques including XRD, XPS, Raman, FT-IR, BET, TEM and SEM. XRD results confirmed that the formation of amorphous structure. Here, the XPS and Raman results indicated that most of Cu species existed in well-dispersed tetrahedral coordination before calcination. After calcination, the tetrahedrally coordinated Cu ions are transformed into octahedral coordination and the activity of catalyst is significantly reduced. The results show that the tetrahedrally coordinated Cu ions play the synergistic catalytic role with Cr ions. Interestingly, oxidation of cyclohexane is more inclined to produce cyclohexanol with uncalcined catalyst due to the presence of the tetrahedrally coordinated Cu ions; however, the appearance of surface lattice oxygen makes the reaction more conducive to cyclohexanone after calcination. After optimizing conditions, Cu-Cr oxide catalyst with the molar ratio 1:1 shows the highest activity, which obtains 62.2% yield and 92.4% selectivity for KA-oil using H 2 O 2 as oxygen source.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Selective catalytic reduction of NO by ammonia and NO oxidation Over CoO x /CeO 2 catalysts

The selective catalytic reduction of NOsub>x with NH 3 (NH 3 -SCR) at low temperatures is still a major challenge in environmental catalysis research. Metal oxide based catalysts have attracted great attention due to their excellent low-temperature activity. In this study, CeO 2 -supported cobalt (Co)-oxide with variable Co/Ce weight ratios were prepared by the incipient-wetness impregnation method. The effect of Co loading as well as the effect of temperature on catalytic behaviors have been studied for the NO reduction and NO oxidation reactions. Thorough characterization by means of BET, XRD, and Raman has shown the topological model of the catalyst and interpretation of the observed reactivity trends. The activity test of CoO x /CeO 2 catalysts showed that the addition of Co enhanced the activity of NO reduction as well as that of NO oxidation. The most active catalyst with 10 wt% Co loading showed 70% NO reduction and 85% NO oxidation activity. Catalyst characterization results indicated that Co was monodispersed on ceria surface with the low Co loading amount, while Co 3 O 4 was formed on ceria with high Co loading amounts that affected the overall catalytic activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Observing gas-catalyst dynamics at atomic resolution and single-atom sensitivity

Transmission electron microscopy (TEM) has become an indispensable technique for studying heterogeneous catalysts. Specifically, advancements of aberration-corrected electron optics and data acquisition schemes have made TEM capable of delivering images of catalysts with sub-Ångström resolution and single-atom sensitivity. Parallel developments of differentially pumped electron microscopes and of gas cells enable in situ observations of catalysts during the exposure to reactive gas environments at pressures of up to atmospheric levels and temperatures of up to several hundred centigrade. Here, we outline how to take advantage of the emerging state-of-the-art instrumentation and methodologies to study surface structures and dynamics to improve the understanding of structure-sensitive catalytic functionality. The idea of using low electron dose-rates in TEM in conjunction with in-line holography and aberration-correction at low voltage (80kV) is introduced to allow maintaining atomic resolution and sensitivity during in situ observations of catalysts. Benefits are demonstrated by exit wave reconstructions of TEM images of a nanocrystalline Co 3 O 4 catalyst material acquired in situ during their exposure to either a reducing or oxidizing gas environment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

The evolution of model Rh/Fe 3 O 4 (001) catalysts in hydrogen environments

Single metal atoms dispersed on oxides are a new emerging class of catalysts owing to their unique electronic and chemical properties. Here, in this study, we have prepared a series of model single-atom catalysts possessing well-characterized Rh sites that include Rh adatoms (Rh ad ), mixed surface layers with octahedrally-coordinated Rh (Rh oct ), as well as metallic Rh clusters and nanoparticles (Rh met ) on Fe 3 O 4 (001). Using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we investigated the activity of such model systems towards H2 and their stability in reducing environments. Our results show that the atomically dispersed Rhad and Rhoct species do not activate H 2 , which would result in the formation of surface hydroxyls on Fe 3 O 4 (001). In contrast, the presence of Rh met in H 2 results in the formation of hydroxyls and subsequent etching of the Fe 3 O 4 (001) at higher temperatures (≥ 500 K) due to water formation via the Mars-van Krevelen mechanism. Additionally, such surface etching leads to the release of the Rh oct from the surface lattice and their sintering to Rh met . To bridge the material gap between the surface science models and high surface area catalysts, we perform parallel studies on powder Rh/Fe 3 O 4 catalysts. The XPS characterization shows remarkable similarities between these systems. Further, our surface science studies provide an atomistic picture of the behavior of high surface area catalysts in the H 2 atmosphere.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Dynamics of precatalyst conversion and iron incorporation in nickel-based alkaline oxygen evolution reaction catalysts

The efficiency of alkaline water electrolyzers is limited by the oxygen evolution reaction (OER). The design of improved OER catalysts requires understanding of material changes induced by the electrolyte under oxidizing potentials. We compare four Ni-based thin-film precatalysts—Ni, NiO, Ni(OH) 2 , and NiS x —in 0.1 M KOH with and without Fe impurities. Precatalyst conversion to the active oxyhydroxide catalysts and their OER performance are induced and followed using cyclic voltammetry. Without Fe electrolyte impurities, the precatalysts convert at different rates to a similar, modestly active NiOOH catalyst. Added Fe impurities are incorporated concurrently with the oxyhydroxide formation leading to active Ni 1-x Fe x OOH catalysts. The NiS x and Ni(OH) 2 precatalysts rapidly convert to oxyhydroxides both with and without Fe, while conversion of Ni and especially NiO is slowed down by Fe impurities. Choice of the precatalyst and presence of Fe impurities are key factors in designing active Ni 1-x Fe x OOH OER catalysts for electrolyzers.

alkaline electrolysis↗
DOE OSTI2020

Atomic-Scale Structural Mapping of Active Sites in Monolayer PGM-Free Catalysts by Low-Voltage 4D-STEM

Two-dimensional (2D) materials have attracted a large amount of attention in both basic and applied fields, and scanning transmission electron microscopy (STEM) is often uniquely well-suited for characterizing the atomic-scale structure of these materials [1-4]. As a result, STEM is poised to significantly impact progress on platinum group metal (PGM)-free catalysts, which are currently under intense development to enable low-cost, commercially viable hydrogen fuel cells [5]. While recent advancements have resulted in fuel cell performance comparable to Pt catalysts by some measures [6], cell durability remains a significant challenge, limiting practical applications [7]. Catalytically active sites in PGM-free materials are proposed to be FeN4 complexes embedded in a graphene lattice (Fig. 1b) within layered or other larger materials, but this is still under debate largely due to the range of potential actives sites predicted by computational methods and lack of methods for directly validating these models [5]. Fundamental insights into the atomic structure and resulting degradation pathways of proposed active sites are therefore needed to fully understand and control cell performance and durability [6].2D materials typically make ideal samples for STEM, but those within PGM-free catalysts present additional challenges since these materials are often defect-rich, with a high density of edges, dopant atoms, etc., which significantly increase susceptibility to beam damage at standard operating voltages. This makes analysis of potential FeN4 active sites particularly challenging, since a large proportion of Fe exists at edge sites where beam-induced atomic displacements can prohibit high-resolution structural characterization [6]. Conventional dark-field imaging compounds this problem by producing less signal for a given dose and being less sensitive to light elements than dose-efficient phase contrast imaging techniques such as those enabled by four-dimensional (4D)-STEM [8-10] (Fig. 1a). Consequently, active site structural analysis is often left to methods such as low-resolution imaging combined with quantum chemical calculations [6], which hinders accurate determination of reaction and degradation mechanisms.Here, we demonstrate direct atomic-scale structural mapping of FeN4 sites by performing low-voltage 4D-STEM on a model PGM-free catalyst system with many exposed monolayer regions. To accomplish this, we pair a 30 keV aberration-corrected probe with a fast pixelated detector that has optimal performance at low beam voltages [11]. This enables us to simultaneously image light and heavy elements with high signal-to-noise by center-of-mass analysis (Fig. 1c) while minimizing beam-induced atomic displacements at sensitive sites. The monolayer nature of these materials additionally allows for experimental validation by direct comparison with multislice simulations [12] of model structures (Fig. 1d-e). This work demonstrates how low-voltage 4D-STEM will provide new insights into the atomic-scale structure and degradation mechanisms of active sites in PGM-free catalysts, facilitating the development of low-cost hydrogen fuel cells and other energy conversion technologies in the future [13].

Zachman, Michael↗
DOE OSTI2024

Local Electric Field Effects on Water Dissociation in Bipolar Membranes Studied Using Core–Shell Catalysts

The local electric field strength is thought to affect the rate of water dissociation (WD) in bipolar membranes (BPMs) at the catalyst–nanoparticle surfaces. Here, we study core–shell nanoparticles, where the core is metallic, semiconducting, or insulating, to understand this effect. The nanoparticle cores were coated with a WD catalyst layer (TiO2 or HfO2) via atomic layer deposition (ALD), and the morphology was imaged with transmission electron microscopy. Irrespective of the core material, these core–shell catalysts displayed comparable WD overpotentials at optimal mass loading, despite the hypothesized differences in the electric field strength across the catalyst particle suggested by continuum electrostatic simulations. Substantial atomic interdiffusion between the core and shell was ruled out by X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and diffuse reflectance optical measurements. However, the optimal mass loading of catalyst was roughly 1 order of magnitude higher for the conductive and high dielectric core materials than for the low dielectric insulating cores. These findings are consistent with the hypothesis that electric field screening within the core material focuses the electric field drop between particles such that larger film thicknesses can be tolerated. Collectively, these data support the idea that it is the local electric field at the molecular level that controls proton-transfer rates and that the metal core/dielectric-shell constructs introduced here modulate that field. Further materials and synthetic design may enable optimization of the electric field strength across the proton-transfer trajectory at the material surface.

Sarma, Prasad V↗
DOE OSTI2015

A Facile Molecular Precursor Route to Metal Phosphide Nanoparticles and Their Evaluation as Hydrodeoxygenation Catalysts

Metal phosphides have been identified as a promising class of materials for the catalytic upgrading of bio-oils, which are renewable and potentially inexpensive sources for liquid fuels. Herein, we report the facile synthesis of a series of solid, phase-pure metal phosphide nanoparticles (NPs) (Ni 2 P, Rh 2 P, and Pd 3 P) utilizing commercially available, air-stable metal–phosphine complexes in a one-pot reaction. This single-source molecular precursor route provides an alternative method to access metal phosphide NPs with controlled phases and without the formation of metal NP intermediates that can lead to hollow particles. The formation of the Ni 2 P NPs was shown to proceed through an amorphous Ni–P intermediate, leading to the desired NP morphology and metal-rich phase. This low-temperature, rapid route to well-defined metal NPs is expected to have broad applicability to a variety of readily available or easily synthesized metal–phosphine complexes with high decomposition temperatures. Hydrodeoxygenation of acetic acid, an abundant bio-oil component, was performed to investigate H 2 activation and deoxygenation pathways under conditions that are relevant to ex situ catalytic fast pyrolysis (high temperatures, low pressures, and near-stoichiometric H 2 concentrations). The catalytic performance of the silica-supported metal phosphide NPs was compared to the analogous incipient wetness (IW) metal and metal phosphide catalysts over the range 200–500 °C. Decarbonylation was the primary pathway for H 2 incorporation in the presence of all of the catalysts except NP-Pd 3 P, which exhibited minimal productive activity, and IW-Ni, which evolved H 2 . The highly controlled NP-Ni2P and NP-Rh2P catalysts, which were stable under these conditions, behaved comparably to the IW-metal phosphides, with a slight shift to higher product onset temperatures, likely due to the presence of surface ligands. Most importantly, the NP-Ni 2 P catalyst exhibited H 2 activation and incorporation, in contrast to IW-Ni, indicating that the behavior of the metal phosphide is significantly different from that of the parent metal, and more closely resembles that of noble metal catalysts.

09 BIOMASS FUELS↗
DOE OSTI2018

Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Noninnocent Ligands by Ni L-Edge and S K-Edge X-ray Absorption

Here, the valence electronic structure of several square planar Ni-centered complexes, previously shown to catalyze the hydrogen evolution reaction, are characterized using S K-edge and Ni L-edge X-ray absorption spectroscopy and electronic structure calculations. Measurement of the atomic Ni 3d and S 3p contributions enables assessment of the metal–ligand covalency of the electron accepting valence orbitals and yields insight into the ligand-dependent reaction mechanisms proposed for the catalysts. The electron accepting orbital of the Ni(abt) 2 (abt = 2-aminobenzenethiolate) catalyst is found to have large ligand character (80%), with only 9% S 3p (per S) character, indicating delocalization over the entire abt ligand. Upon two proton-coupled reductions to form the Ni(abt-H) 2 intermediate, the catalyst stores 1.8 electrons on the abt ligand, and the ligand N atoms are protonated, thus supporting its role as an electron and proton reservoir. The electron accepting orbitals of the Ni(abt-H) 2 intermediate and Ni(mpo) 2 (mpo = 2-mercaptopyridyl- N -oxide) catalyst are found to have considerably larger Ni 3d (46–47%) and S 3p (17–18% per S) character, consistent with an orbital localized on the metal–ligand bonds. This finding supports the possibility of metal-based chemistry, resulting in Ni–H bond formation for the reduced Ni(abt-H) 2 intermediate and Ni(mpo) 2 catalyst, a critical reaction intermediate in H 2 generation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Electronic Structure of Molecular Cobalt Catalysts for H 2 Production Revealed by Multifrequency EPR

Improving the design of hydrogen evolution reaction (HER) catalysts requires a detailed understanding of the catalyst's structure and environment. Here wwe have used multifrequency electron paramagnetic resonance (EPR) spectroscopy (X-band, Q-band, and D-band) in combination with density functional theory (DFT) modeling to elucidate the electronic structure of three Co(II) molecular catalysts for H 2 evolution to determine correlations of electronic structure parameters with catalytic activity and stability. The Co(II) coordination complexes investigated here are representative of two types of H 2 -evolving molecular catalysts: cobaloximes, formed from cobalt coordination to two glyoxime ligands, and cobalt poly(pyridyl) complexes, which commonly have either four or five pyridyl ligands coordinating a single Co(II) ion. These Co(II) complexes were studied in a variety of solvents with a range of polarities and stoichiometric amounts of potential ligands to the cobalt ion to assess the strength and nature of ligand- cobalt interactions. Using magnetic resonance parameters obtained by EPR spectroscopy as reference data, we reconstructed the electronic structure of the Co(II) complexes with the help of DFT modeling. For all three catalysts, we uncovered a linear correlation between the Co(II) A z value and the g x value that is dependent on the strength of the axial ligands. The correlations that we have established between the electronic structure and metal coordination environment provide insight into the structural and electronic factors underpinning the observed trends in HER catalysis activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Active-Site Determination and Mechanistic Insights in a MOF-Supported Polymerization Catalyst

The structural elucidation of catalyst active sites in heterogeneous catalysts when supported by traditional metal oxides remains a challenge despite the advanced characterization techniques that have been developed. The catalyst-deposition-site nonuniformity in metal oxides inhibits clear structural characterization through bulk spectroscopic methods and rules out the use of single-crystal X-ray diffraction. However, for metal-organic framework (MOF)-supported catalysts, the crystallinity and uniform structures of the underlying support enhance our ability to identify the precatalyst and catalytically active sites and open the door to using single-crystal X-ray diffraction coupled with spectroscopy under reaction conditions. Additionally, the use of in situ X-ray absorption spectroscopy identifies the catalytically active site in diethylaluminum chloride (DEAC)-pretreated Cr-SIM-NU-1000 to be a Cr-ethyl when used for ethylene polymerization. Further kinetic experiments elucidate the effects of ethylene pressure, temperature, catalyst loading, and cocatalyst loading, furthering mechanistic knowledge and helping to deconvolute the structure-function relationship.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electrochemical Study of the Energetics of the Oxygen Evolution Reaction at Nickel Iron (Oxy)Hydroxide Catalysts

Iron-doped nickel (oxy)hydroxide catalysts (Fe x Ni 1- x OOH) exhibit high electrocatalytic behavior for the oxygen evolution reaction in base. Recent findings suggest that the incorporation of Fe 3+ into a NiOOH lattice leads to nearly optimal adsorption energies for OER intermediates on active Fe sites. Utilizing electrochemical impedance spectroscopy and activation energy measurements, we find that pure NiOOH and FeOOH catalysts exhibit exceedingly high Faradaic resistances and activation energies 40-50 kJ/mol -1 higher than those of the most active Fe x Ni 1- x OOH catalysts. Furthermore, the most active Fe x Ni 1- x OOH catalysts in this study exhibit activation energies that approach those previously reported for IrO 2 OER catalysts.

Swierk, John R.↗