Engineering topics

Kropf, A. Jeremy

Publications and source records attributed to Kropf, A. Jeremy.

At least 37 records · Page 2

DOE OSTI2023

Structural and reactive evolution of oxidatively grafted Pd catalysts on MnO 2 for the low-temperature oxidation of CO

We report Isolated Pd atoms supported on high surface area MnO 2 , prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) oxidation of CO (7.7 kPa O 2 , 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO 2 in facilitating redox turnovers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis

Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing the cost for green hydrogen production. Here we report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Isotope effect in formaldehyde steam reforming on Pt/m-ZrO 2 : Insight into chemical promotion by alkalis

Infrared spectroscopy, temperature programmed reaction mass spectrometry (TP-reaction/MS), and catalyst testing were used to investigate alkali promotion of dehydrogenation selectivity during formaldehyde steam reforming (FSR) on Pt/m-ZrO 2 . In a preferred pathway, formaldehyde reacts with water forming hydrogen and formate, followed by forward formate decomposition to CO 2 and H 2 . Alkali-doping of 2 wt% Pt/m-ZrO 2 increases catalyst basicity, which weakens the formate C-H bond promoting formate dehydrogenation / decarboxylation. Promotion by alkali in FSR was observed through a formate nu(CH) band shift to lower wavenumbers in infrared spectroscopy, and through a decrease in the normal isotope effect in switching from H-to D-labeled formalde-hyde in TP-reaction/MS.

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DOE OSTI2023

Kinetic and X-ray Absorption Spectroscopic Analysis of Catalytic Redox Cycles over Highly Uniform Polymetal Oxo Clusters

Metal–organic framework materials (MOFs) offer an opportunity for investigating catalytic properties of polymetal oxo clusters that are highly well defined and uniform in nature, in contrast to other classes of catalysts that may exhibit a propensity toward active site heterogeneity. We report herein a kinetic and X-ray absorption spectroscopy (XAS) analysis of the two-electron oxidation of CO over divalent metal sites in MIL-100(M = Fe, Cr) (MIL = Materials of Institut Lavoisier) materials carrying μ 3 -oxo bridged trimers, and connect observations about the kinetic relevance of redox steps to density functional theory (DFT) predictions published previously. The high degree of uniformity evident from in situ titration measurements leads to a congruence in mechanistic inferences made from steady-state catalytic, transient stoichiometric, isotopic exchange, and isotopic tracer data that all point to a sequential mechanism comprised of separate oxidation and reduction half-cycle steps conjoined by an active oxygen intermediate. In situ XAS data reinforce mechanistic conclusions derived from kinetic analysis, and suggest that the active oxygen intermediate may be more appropriately characterized as an iron-oxyl (Fe 3+ –O – ) rather than an iron-oxo (Fe 4+ =O 2– ) species. The Cr analogue of MIL-100 exhibits contrasting rate features that can be rationalized using an identical sequence of steps as MIL-100(Fe), but with a highly dissimilar set of kinetic parameters that can also be validated using transient stoichiometric experiments. The larger coverages of active oxygen intermediates on MIL-100(Cr) are consistent with predictions from prior DFT studies that suggest more stable and less reactive active oxygen species for metals with lower d-electron counts, and point to metal identity as a lever for precisely controlling the kinetic relevance of oxidation and reduction half-cycles in catalytic redox sequences over polymetal oxo clusters. Importantly, the results presented point to the utility of developing broadly applicable structure–catalytic property relationships over MOF nodes specifically, and highly uniform catalysts more generally.

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DOE OSTI2023

Intermetallic alloy structure–activity descriptors derived from inelastic X-ray scattering

Synchrotron spectroscopy and Density Functional Theory (DFT) are combined to develop a new descriptor for the stability of adsorbed chemical intermediates on metal alloy surfaces. This descriptor probes the separation of occupied and unoccupied d electron density in platinum and is related to shifts in Resonant Inelastic X-ray Scattering (RIXS) signals. Simulated and experimental spectroscopy are directly compared to show that the promoter metal identity controls the orbital shifts in platinum electronic structure. The associated RIXS features are correlated with the differences in the band centers of the occupied and unoccupied d bands, providing chemical intuition for the alloy ligand effect and providing a connection to traditional descriptions of chemisorption. The ready accessibility of this descriptor to both DFT calculations and experimental spectroscopy, and its connection to chemisorption, allow for deeper connections between theory and characterization in the discovery of new catalysts.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Silica Supported Organometallic Ir I Complexes Enable Efficient Catalytic Methane Borylation

Catalytic C–H borylation is an attractive method for the conversion of the most abundant hydrocarbon, methane (CH 4 ), to a mild nucleophilic building block. However, existing CH 4 borylation catalysts often suffer from low turnover numbers and conversions, which is hypothesized to result from inactive metal hydride agglomerates. Herein we report that the heterogenization of a bisphosphine molecular precatalyst, [(dmpe)Ir(cod)CH 3 ], onto amorphous silica dramatically enhances its performance, yielding a catalyst that is 12-times more efficient than the current standard for CH 4 borylation. The catalyst affords over 2000 turnovers at 150 °C in 16 h with a selectivity of 91.5% for mono- vs diborylation. Higher catalyst loadings improve yield and selectivity for the monoborylated product (H 3 CBpin) with 82.8% yield and >99% selectivity being achieved with 1255 turnovers. X-ray absorption and dynamic nuclear polarization-enhanced solid-state NMR spectroscopic studies identify the supported precatalyst as an Ir I species, and indicate that upon completion of catalysis, multinuclear Ir polyhydrides are not formed. This is consistent with the hypothesis that immobilization of the organometallic Ir species on a surface prevents bimolecular decomposition pathways. Importantly, immobilization of the homogeneous Ir I fragment onto amorphous silica represents a unique and simple strategy to improve the TON and longevity of a CH 4 borylation catalyst.

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DOE OSTI2023

CO 2 Hydrogenation: Na Doping Promotes CO and Hydrocarbon Formation over Ru/m-ZrO 2 at Elevated Pressures in Gas Phase Media

Sodium-promoted monoclinic zirconia supported ruthenium catalysts were tested for CO 2 hydrogenation at 20 bar and a H 2 :CO 2 ratio of 3:1. Although increasing sodium promotion, from 2.5% to 5% by weight, slightly decreased CO 2 conversion (14% to 10%), it doubled the selectivity to both CO (~36% to ~71%) and chain growth products (~4% to ~8%) remarkably and reduced the methane selectivity by two-thirds (~60% to ~21%). For CO 2 hydrogenation during in situ DRIFTS under atmospheric pressure, it was revealed that Na increases the catalyst basicity and suppresses the reactivity of Ru sites. Higher basicity facilitates CO 2 adsorption, weakens the C–H bond of the formate intermediate promoting CO formation, and inhibits methanation occurring on ruthenium nanoparticle surfaces. The suppression of excessive hydrogenation increases the chain growth probability. Decelerated reduction during H 2 -TPR/TPR-MS and H 2 -TPR-EXAFS/XANES at the K-edge of ruthenium indicates that sodium is in contact with ruthenium. A comparison of the XANES spectra of unpromoted and Na-promoted catalysts after H 2 reduction showed no evidence of a promoting effect involving electron charge transfer.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Iron redox behavior and oxygen reduction activity of Fe-N-C electrocatalysts in different electrolytes

The iron redox behavior and oxygen reduction reaction (ORR) activity of Fe-N-C ORR electrocatalysts synthesized by a variety of techniques were investigated as a function of the identity of the electrolyte anion (bisulfate/sulfate or perchlorate) at a constant pH. In situ X-ray absorption spectroscopy data support the assignment of the redox peaks in the voltammograms to the Fe 3+ /Fe 2+ redox couple. It was found that for a given Fe-N-C catalyst, there is a correlation between the Fe redox couple peak potential and the ORR activity in perchloric acid electrolyte, but not in sulfuric acid electrolyte. While a higher Fe redox couple potential (≥ 110 mV higher) was observed in perchloric acid electrolyte, a higher ORR activity was obtained in sulfuric acid electrolyte. Here, the higher ORR activity observed in sulfuric acid than perchloric acid was correlated with the higher peak current and larger faradaic charge for the Fe redox couple. A study of the Fe redox behavior using a cavity microelectrode, eliminating the impact of ionomer, showed that the interaction of H 2 SO 4 with Fe-N-C is stronger than that of HClO 4 and that Fe redox in both electrolytes is a reversible surface electrochemical reaction.

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DOE OSTI2023

Enhancing the activity of Fe-N-C oxygen reduction reaction electrocatalysts by high-throughput exploration of synthesis parameters

Here, the most active class of platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts in acidic electrolytes are those synthesized by heat treatment of iron, carbon, nitrogen precursors (Fe-N-C). Due to the large number of possible precursor compounds, a small fraction of the synthesis variable space has been explored. Correlation of synthesis variables with Fe speciation and ORR activity has been limited. In this work, an automation platform and a multi-port ball-milling were utilized to evaluate the effects of synthesis variables, such as identity of iron precursor, iron loading, and carbon and nitrogen sources on the ORR activity of iron -nitrogen-carbon catalysts in acidic electrolyte. The ORR activity is correlated with catalyst Fe speciation determined using Fe K-edge X-ray absorption spectroscopy (XAFS).

30 DIRECT ENERGY CONVERSION↗

DOE OSTI2022

Electrochemical characterization of evolving ionomer/electrocatalyst interactions throughout accelerated stress tests

Here, the degradation of polymer electrolyte membrane fuel cells (PEMFCs) catalyst layers used for heavy-duty vehicles was examined using a catalyst-specific accelerated stress test (AST). High surface area carbon supported dispersed Pt (Pt/HSC), annealed Pt (a-Pt/HSC) and PtCo (PtCo/HSC) alloy catalysts were examined over the course of 90,000 cycles by measuring changes in mass activity, O 2 transport resistance, electrochemical active surface area (ECSA), catalyst accessibility and ionomer-electrocatalyst interactions. Compared to a-Pt/HSC and Pt/HSC catalyst, the PtCo/HSC showed better initial mass activity, a larger initial mass transport loss, and faster degradation after the first 30k AST cycles, as a large portion of Co leached out during potential cycling. Pt/HSC showed higher initial performance relative to a-Pt/HSC but had faster degradation. STEM characterizations show that the ECSA losses are largely related to Pt dissolution resulting in either catalyst particle growth via the Ostwald ripening mechanism or redeposition in the membrane. Catalyst accessibility measurements showed decreased RH sensitivity for all three samples, while CO impedance measurements revealed a decrease in both Pt-water and carbon-water interactions. This implies that, Pt is either preferentially redepositing on the exterior of the carbon support, or that the ionomer is undergoing morphological changes enabling the enhanced intrusion of mesopores.

25 ENERGY STORAGE↗

DOE OSTI2022

Reverse water-gas shift: Na doping of m-ZrO 2 supported Pt for selectivity control

Reverse water-gas shift (RWGS) is a vital step in producing syngas for the chemical conversion of CO 2 to liquid transportation fuels and chemicals. Na-doping of m-ZrO 2 supported Pt catalysts allowed selectivity control by systematically increasing the ratio of relative rates of r CO /r CH4 . This was achieved by facilitating the formation of formate intermediate species, which precedes CO formation, and by suppressing the metallic Pt 0 active sites responsible for CH 4 formation. Here, a 2.5%Na-2%Pt/m-ZrO 2 catalyst was first tested for the forward water-gas shift (FWGS) reaction and found to have 50% higher CO conversion at 285°C compared to the undoped catalyst. Results of DRIFTS spectroscopy of adsorbed CO confirmed a formate ν(CH) band shift to lower wavenumbers (2870–2802 cm —1 ) with the addition of Na and more rapid forward formate decomposition in steam to H 2 and carbonate species, the precursor to CO 2 . This is consistent with C-H bond breaking being the rate limiting step of a FWGS mechanism occurring at the metal-support junction. Consistent with this, DRIFTS of RWGS in 4%CO 2 + 60%H 2 showed more facile formation of formate for the Na-doped catalyst and, once again, the ν(CH) band was shifted to lower wavenumbers (2874–2803 cm —1 ) with Na-doping. In addition, Na doping resulted in a systematic decrease in the Pt-carbonyl band in DRIFTS of adsorbed CO as well as DRIFTS of in-situ RWGS reaction tests, suggesting that Na blocked a fraction of on-top Pt sites, breaking up ensembles of Pt 0 responsible for methanation. The selectivity of the 2.5%Na-doped catalyst, unlike its undoped counterpart, was remarkably resistant to methanation (e.g., selectivity < 0.2% CH 4 with pressures of up to 20 bar).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Synthesis of platinum nanoparticles on strontium titanate nanocuboids via surface organometallic grafting for the catalytic hydrogenolysis of plastic waste

Pt/SrTiO 3 (Pt/STO), prepared on small scale by atomic layer deposition (ALD), is a capable heterogeneous catalyst for the selective hydrogenolysis of polyolefins to hydrocarbon oils, providing a promising approach for upcycling plastic waste. However, because deposition by ALD is costly and resource-intensive, a new synthesis of Pt/STO is needed to effectively scale catalyst production and pursue the commercialization of upcycling processes. To that effect, this work details a scalable deposition method for Pt/STO made by surface organometallic chemistry (SOMC) techniques using Pt(II) acetylacetonate or and trimethyl(methylcyclopentadienyl)platinum. The STO support was calcined (550 °C), treated with ozone (200 °C), and finally steamed (200 °C) to afford a clean STO surface populated with only hydroxyl groups. Pt precursors were dissolved in toluene and deposited onto STO. After reduction at 300 °C, the STO support was decorated with 1.0–1.5 nm Pt nanoparticles. The size and loading of these nanoparticles were varied by employing a multi-cycle deposition and oxidation and/or reduction process designed to ALD techniques. In conclusion, these Pt/STO catalysts hydrogenolyzed isotactic polypropylene into liquid products (>95% yield) with average molecular weights of 200–300 Da (~25 carbon atoms) and narrow size distributions at 300 °C and 180 psi H 2 .

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 OSTI2022

Ordered CoPt oxygen reduction catalyst with high performance and durability

A high-performance and durable polymer electrolyte membrane fuel cell cathode catalyst composed of an ordered L10–CoPt core and a thin Pt shell was designed and prepared. Under practical fuel cell membrane electrode assembly testing conditions, the cathode catalyst showed an outstanding initial mass activity of 0.6 A/mgPt, which satisfies the U.S. Department of Energy performance target while also meeting the durability target of less than 40% loss in mass activity after 30,000 accelerated stress test voltage cycles. The high structural stability of the L10–CoPt@Pt-shell catalyst was confirmed by postmortem materials characterization, and the origin of this robustness was revealed by density functional theory calculations, where the barriers for diffusion of Co atoms were observed to be significantly increased in the ordered intermetallic core.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Na Promotion of Pt/m-ZrO 2 Catalysts for the Steam Reforming of Formaldehyde

The decomposition selectivity of formaldehyde during steam reforming was explored using unpromoted and sodium promoted Pt/m-ZrO 2 catalysts, and the Na content was varied (0.5%Na, 1%Na, 1.8%Na, 2.5%Na, and 5%Na). In situ DRIFTS experiments during temperature programmed reaction in flowing H 2 O revealed that formaldehyde is adsorbed at reduced defect sites on zirconia, where it is converted to formate species through the addition of labile bridging OH species. Formate species achieve a maximum intensity in the range of 125–175 °C, where only slight changes in intensity are observed. Above this temperature, the formate decomposition reactivity strongly depends on the Na loading, with the optimum loadings being 1.8%Na and 2.5%Na. CO 2 temperature programmed desorption results, as well as a greater splitting observed between the formate ν asym (OCO) and ν sym (OCO) bands in infrared spectroscopy, indicate greater basicity is induced by the presence of Na. This strengthens the interaction between the formate -CO 2 functional group and the catalyst surface, weakening the formate C-H bond. A shift in the ν(CH) band of formate to lower wavenumbers was observed by addition of Na, especially at 1.8%Na and higher loadings. This results in enhanced decarboxylation and dehydrogenation of formate, as observed in in situ DRIFTS, temperature-programmed reaction/mass spectrometry experiments of the steam reforming of formaldehyde, and fixed bed reaction tests. For example, 2.5%Na addition of 2.5% increased the CO 2 selectivity from 83.5% to 99.5% and the catalysts achieved higher stable conversion at lower temperature than NiO catalysts reported in the open literature. At 5%Na loading, Pt sites were severely blocked, hindering H-transfer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Tuning the Product Distribution of Acetylene Dimerization through Bimetallic Metal–Organic Framework-Supported Nanoporous Systems

Metal-organic frameworks (MOFs) are receiving increased attention due to their well-defined structures that allow the determination of structure-property relationships. MOFs have been used as heterogeneous catalyst supports in a variety of fashions including for confinement of metal nanoparticles, which have demonstrated enhanced resistance to aggregation, a common issue in amorphous metal oxide supports. Cu and In catalysts were installed in the Zr-based MOF NU-907, being confined within the nanoporous structure. The Cu catalyst is known to, under various conditions, either selectively hydrogenate acetylene to ethylene or generate C4 products such as butenes and 1,3-butadiene, an important feedstock for rubber and adhesives. The addition of indium to the Cu catalyst is intended to serve as a promoter to produce C4 products by decreasing the surface coverage of copper while still allowing for C-C coupling. When employed for acetylene dimerization, InCu-NU-907 shows slightly decreased C4 production overall but enhanced 1,3-butadiene production compared to all other catalysts studied herein. These catalysts were thoroughly characterized by a range of techniques to confirm structural integrity and porosity and probe the nature of the interactions of indium with the Cu nanoparticle active site.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Integrated Experimental and Computational K-Edge X-ray Absorption Near-Edge Structure Analysis of Vanadium Catalysts

X-ray Absorption Near-Edge Structure (XANES) spectroscopy is a powerful tool to reveal key structural and electronic features of isolated catalytic sites, yet insights into molecular structure and more detailed orbital analysis through a combination of experimental and computed XANES analysis are necessary for accurate interpretation of the spectra, especially when significant heterogeneity exits among the catalytic sites. Herein, we present an integrated computational and experimental strategy to determine both primary and secondary bonding interactions within the XANES pre-edge region for organovanadium complexes, which was developed using a series of well-defined molecular vanadium complexes and then applied to the characterization of a supported organovanadium olefin hydrogenation catalyst. Time-dependent density functional theory is used to predict the energy of pre-edge XANES features for a series of vanadium complexes with a variety of oxidation states and local coordination environments. Further, a calibration scheme incorporating different density functionals and basis sets is established, resulting in an optimized scheme that accurately predicts pre-edge energies with a mean absolute error of 0.40 eV. Second-shell coordination (e.g., V---V) effects within XANES are identified through the analysis of the computed dominant orbital contributions for multi-vanadium complexes. Orbital analysis also provided confirmation that the vanadium-hydride formation combined with the heterogeneity of the catalytic active species in ole-fin hydrogenation caused the energy shift and broadening of the pre-edge peak after hydrogen treatment of the silica-supported organovanadium pre-catalyst. This work further elucidates computational XANES simulations and techniques potentially guiding characterization in surface organometallic chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

Nitrogen-coordinated single atom iron sites (FeN 4 ) embedded in carbon (Fe–N–C) are the most active platinum group metal-free oxygen reduction catalysts for proton-exchange membrane fuel cells. Still, current Fe–N–C catalysts lack sufficient long-term durability and are not yet viable for practical applications. Here we report a highly durable and active Fe–N–C catalyst synthesized using heat treatment with ammonia chloride followed by high-temperature deposition of a thin layer of nitrogen-doped carbon on the catalyst surface. We propose that catalyst stability is improved by converting defect-rich pyrrolic N-coordinated FeN 4 sites into highly stable pyridinic N-coordinated FeN 4 sites. The stability enhancement is demonstrated in membrane electrode assemblies using accelerated stress testing and a long-term steady-state test (>300 h at 0.67 V), approaching a typical Pt/C cathode (0.1 mg Pt cm -2 ). The encouraging stability improvement represents a critical step in developing viable Fe–N–C catalysts to overcome the cost barriers of hydrogen fuel cells for numerous applications.

08 HYDROGEN↗