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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Vapor-Phase Stabilization of Biomass Pyrolysis Vapors Using Mixed-Metal Oxide Catalysts

Mixed-metal oxides possess a wide range of tunability and show promise for catalytic stabilization of biomass pyrolysis products. For materials derived from layered double hydroxides, understanding the effect of divalent cation species and divalent/trivalent cation stoichiometric ratio on catalytic behavior is critical to their successful implementation. In this study, four mixed-metal oxide catalysts consisting of Al, Zn, and Mg in different stoichiometric ratios were synthesized and tested for ex-situ catalytic fast pyrolysis (CFP) using pine wood as feedstock. The catalytic activity and deactivation behavior of these catalysts were monitored in real-time using a lab-scale pyrolysis reactor and fixed catalyst bed coupled with a molecular beam mass spectrometer (MBMS), and data were analyzed by multivariate statistical approaches. In the comparison between Mg-Al and Zn-Al catalyst materials, we demonstrated that the Mg-Al materials possessed greater quantities of basic sites, which we attributed to their higher surface areas, and they produced upgraded pyrolysis vapors which contained less acids and more deoxygenated aromatic hydrocarbons such as toluene and xylene. However, detrimental impacts on carbon yields were realized via decarbonylation and decarboxylation reactions and coke formation. Given that the primary goals of catalytic upgrading of bio-oil are deoxygenation, reduction of acidity, and high carbon yield, these results highlight both promising catalytic effects of mixed-metal oxide materials and opportunities for improvement.

09 BIOMASS FUELS↗
DOE OSTI2013

CO2 Reduction on Supported Ru/Al2O3 Catalysts: Cluster Size Dependence of Product Selectivity

The catalytic performance of a series of Ru/Al2O3 catalysts with Ru content in the 0.1-5% range was examined in the reduction of CO2 with H2. At low Ru loadings (≤0.5 %) where the active metal phase is highly dispersed (mostly atomically) on the alumina support CO is formed with high selectivity. With increasing metal loading the selectivity toward CH4 formation increases, while that for CO production decreases. In the 0.1% Ru/Al2O3 catalyst Ru is mostly present in atomic dispersion as STEM images obtained from the fresh sample prior to catalytic testing reveal. STEM images recorded form this same sample following temperature programmed reaction test clearly show the agglomeration of small metal particles (and atoms) into 3D clusters. The clustering of the highly dispersed metal phase is responsible for the observed dramatic selectivity change during elevated temperature tests: dramatic decrease in CO, and large increase in CH4 selectivity. Apparent activation energies, estimated from the slopes of Arrhenius plots, of 82 kJ/mol and 62 kJ/mol for CO and CH4 formation were determined, respectively, regardless of Ru loading. These results suggest that the formation of CO and CH4 follow different reaction pathways, or proceed on active centers of different nature. Reactions with CO2/H2 and CO/H2 mixtures (under otherwise identical reaction conditions) reveal that the onset temperature of CO2 reduction is about 150 ºC lower than of CO reduction. We thank Dr. Feng Gao for carrying out the H2 chemisorption measurements on all the Ru/Al2O3 catalysts discussed in this work. The catalyst preparation and catalytic measurements were supported by a Laboratory Directed Research and Development (LDRD) project, while the TEM work was supported by the Chemical Imaging Initiative at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US Department of Energy by Battelle under contract number DE-AC05-76RL01830. JHK also acknowledges the support of this work by the 2013 Research Fund of UNIST (Ulsan National Institute of Science and Technology, Ulsan, Korea).

CO/CH4 selectivity↗
DOE OSTI2014

Use of H2S to Probe the Active Sites in FeNC Catalysts for the Oxygen Reduction Reaction (ORR) in Acidic Media

H2S has been used as a probe molecule both in an “in situ” poisoning experiment and in intermediate-temperature heat-treatment steps during and after the preparation of FeNC catalysts in an attempt to analyze its effect on their ORR activity. The heat treatments were employed either on the ball-milled precursor of FeNC or after the Ar-NH3 high temperature heat treatments. ORR activity of the H2S-treated catalysts was seen to be significantly lower than the sulfur-free catalysts, whether the sulfur exposure was during a half-cell testing, or as an intermediate-temperature exposure to H2S. The incorporation of sulfur species and interaction of Fe with sulfur were confirmed by characterization using XPS, EXAFS, TPO, and TPD. This study provides crucial evidence regarding differences in active sites in FeNC versus nitrogen-containing carbon nanostructured (CNx) catalysts.

CNx↗
DOE OSTI2022

Dynamical Study of Adsorbate-Induced Restructuring Kinetics in Bimetallic Catalysts Using the PdAu(111) Model System

The dynamic restructuring of bimetallic catalysts plays a crucial role in their catalytic activity and selectivity. In particular, catalyst pretreatment with species such as carbon monoxide and oxygen has been shown to be an effective strategy for tuning the surface composition and morphology. Mechanistic and kinetic understanding of such restructuring are fundamental to the chemistry and engineering of surface active sites but have remained challenging due to the large structural, chemical, and temporal degrees of freedom. Here, we combine time-resolved temperature-programmed infrared reflection absorption spectroscopy, ab initio thermodynamics, and machine-learning molecular dynamics to uncover previously unidentified timescale and kinetic parameters of in situ restructuring in Pd/Au(111), a highly relevant model system for dilute Pd-in-Au nanoparticle catalysts. The key innovation lies in utilizing CO not only as a chemically sensitive probe of surface Pd, but also as an agent that induces restructuring of the surface. Upon annealing in vacuum, as-deposited Pd islands became encapsulated by Au and partially dissolved into the subsurface, leaving behind isolated Pd monomers on the surface. Subsequent exposure to 0.1 mbar CO enabled Pd monomers to repopulate the surface up to 373 K, above which complete Pd dissolution occurred by 473 K, with apparent activation energies of 0.14 and 0.48 eV, respectively. Furthermore, these restructuring processes occurred over the span of ~1000 s at a given temperature. Such a minute-timescale dynamics not only elucidates the fluxional nature of alloy catalysts but also presents an opportunity to fine-tune the surface at moderate temperature and pressure conditions.

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

Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts

Atomically dispersed catalysts such as single-atom catalysts have been shown to be effective in selectively oxidizing methane, promising a direct synthetic route to value-added oxygenates such as acetic acid or methanol. However, an important challenge of this approach has been that the loading of active sites by single-atom catalysts is low, leading to a low overall yield of the products. We report an approach that can address this issue. It utilizes a metal–organic framework built with porphyrin as the linker, which provides high concentrations of binding sites to support atomically dispersed rhodium. It is shown that up to 5 wt% rhodium loading can be achieved with excellent dispersity. When used for acetic acid synthesis by methane oxidation, a new benchmark performance of 23.62 mmol·gcat –1 ·h –1 was measured. Furthermore, the catalyst exhibits a unique sensitivity to light, producing acetic acid (under illumination, up to 66.4% selectivity) or methanol (in the dark, up to 65.0% selectivity) under otherwise identical reaction conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Enhancing Substrate–Metal Catalyst Affinity via Hydrogen Bonding: Pd(II)-Catalyzed β-C(sp 3 )–H Bromination of Free Carboxylic Acids

Achieving sufficient substrate-metal catalyst affinity is a fundamental challenge for developing synthetically useful C–H activation reactions of weakly coordinating native substrates. While hydrogen bonding has been harnessed to bias site selectivity in existing C(sp 2 )–H activation reactions, the potential for designing catalysts with hydrogen bond donors (HBDs) to enhance catalyst substrate affinity, and thereby facilitate otherwise unreactive C(sp 3 )–H activation, remains to be demonstrated. Herein, we report the discovery of a ligand scaffold containing a remote amide motif that can form a favorable meta-macrocyclic hydrogen bonding interaction with the aliphatic acid substrate. The utility of this ligand scaffold is demonstrated through the development of an unprecedented C(sp 3 )–H bromination of α-tertiary and α-quaternary free carboxylic acids, which proceeds in exceedingly high mono-selectivity. Furthermore, the geometric relationship of the NHAc hydrogen bond donor and the coordinating quinoline ligand is crucial for forming the meta-macrocyclophane-like hydrogen bonding interaction, providing a guideline for the future design of catalysts employing secondary interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Perovskite Catalysts for Pure-Water-Fed Anion-Exchange-Membrane Electrolyzer Anodes: Co-design of Electrically Conductive Nanoparticle Cores and Active Surfaces

Anion-exchange-membrane water electrolyzers (AEMWEs) are a possible low-capital-expense, efficient, and scalable hydrogen-production technology with inexpensive hardware, earth-abundant catalysts, and pure water. However, pure-water-fed AEMWEs remain at an early stage of development and suffer from inferior performance compared with proton-exchange-membrane water electrolyzers (PEMWEs). One challenge is to develop effective non-platinum-group-metal (non-PGM) anode catalysts and electrodes in pure-water-fed AEMWEs. We show how LaNiO3-based perovskite oxides can be tuned by cosubstitution on both A- and B-sites to simultaneously maintain high metallic electrical conductivity along with a degree of surface reconstruction to expose a stable Co-based active catalyst. The optimized perovskite, Sr0.1La0.9Co0.5Ni0.5O3, yielded pure-water AEMWEs operating at 1.97 V at 2.0 A cm-2 at 70 °C with a pure-water feed, thus illustrating the utility of the catalyst design principles.

Zhai, Tingting↗
DOE OSTI2025

Active Site Dynamics in Molybdenum-Based Silica-Supported Olefin Metathesis Catalysts: Site Renewal and Decay Beyond the Chauvin Cycle

Heterogeneous olefin metathesis catalysts exhibit low active site densities and unpredictable kinetics due to dynamic active site formation and decay processes. Here, in this study, we establish a quantitative framework that captures active site generation, renewal, and decay in olefin metathesis over silica-supported molybdenum oxide catalysts, enabling a mechanistic explanation of catalytic behavior and strategies to achieve high, stable activity. Steady-state active site titrations reveal that 2,3-dimethyl-butene isomers (4MEs) cofeeding increases active site density by up to 4.3-fold, directly correlating with enhanced metathesis rates. Spectroscopic studies demonstrate that 4MEs facilitate Mo(VI) reduction to Mo(IV) and interact strongly with surface Si–OH groups, generating labile protons that promote active site formation via a 1,2-proton shift mechanism. Kinetic modeling indicates that ethylene acts as a decay promoter, shifting kinetic control away from the Chauvin cycle and suppressing metathesis activity. Comparative studies on catalysts with varying Mo loading reveal that promotion is most effective for dispersed molybdate species, with a decline at higher Mo loadings. These findings provide a unified mechanistic framework for heterogeneous olefin metathesis, offering new strategies to enhance active site accessibility, mitigate deactivation, and optimize catalyst design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Unravelling the Stability Stressors of Atomically Dispersed Fe–N–C Oxygen Reduction Catalysts

Enhancing the catalytic stability of Fe–N–C catalysts for cathodic oxygen reduction in proton-exchange membrane fuel cells (PEMFCs) necessitates an in-depth understanding of their degradation mechanisms. Here, this study identifies key stressors affecting the stability of Fe–N–C catalysts, specifically acidic environment, oxygen (O 2 ), and reactive oxygen species (ROS). Through ex situ/operando experiments, we show that the oxidation of local carbon by acidic environment + O 2 + ROS, along with the demetalation of catalytic FeN x C y sites by O 2 or O 2 + ROS, is the primary factor responsible for the initial fast degradation of Fe–N–C catalysts. The demetalation of FeN x C y sites, influenced by O 2 , in particular by O 2 + ROS, leads to the subsequent gradual degradation of Fe–N–C. Notably, FeN 4 C 12 -type active sites are more susceptible to demetalation than FeN 4 C 10 -type sites in O 2 or O 2 + ROS. Our findings indicate that, besides constructing more stable FeN x C y sites, preventing local carbon oxidation and scavenging of ROS are all critical for maintaining the stability of Fe–N–C catalysts.

Xie, Xiaohong [Pacific Northwest National Laborato↗
DOE OSTI2016

Tracking the Structural and Electronic Configurations of a Cobalt Proton Reduction Catalyst in Water

Time resolved X-ray absorption spectroscopy (X-TAS) has been used to study the light induced hydrogen evolution reaction catalyzed by a highly stable cobalt complex, [Ru(bpy)3]2+ photosensitizer and an equimolar mixture of sodium ascorbate/ascorbic acid electron donor in pure water. XANES and EXAFS analysis of a binary mixture of the octahedral Co(III) pre-catalyst and [Ru(bpy)3]2+ after illumination, revealed in-situ formation of a square pyramidal Co(II) intermediate, with electron transfer kinetics of 51 ns. On the other hand, X-TAS experiments of the complete photocatalytic system in the presence of the electron donor showed the formation of a square planar Co(I) intermediate species within a few nanoseconds followed by its decay in the microsecond timescales. The Co(I) structural assignment is supported by calculations based on density functional theory (DFT). At longer reaction times, we observe the formation of the initial Co(III) species concomitant to the decay of Co(I), thus closing the catalytic cycle. The experimental X-ray absorption spectra of the molecular species formed along the catalytic cycle are modeled using a combination of molecular orbital DFT calculations (DFT-MO) and Finite Difference Method (FDM). These findings allowed us to unequivocally assign the full mechanistic pathway followed by the catalyst as well as to determine the rate limiting step of the process, which consists in the protonation of the Co(I). This study provides a complete kinetics scheme for the hydrogen evolution reaction by a cobalt catalyst, revealing unique information for the development of better catalysts for the reductive side of hydrogen fuel cells.

Moonshiram, Dooshaye↗
DOE OSTI2016

Monomolecular Siloxane Film as a Model of Single Site Catalysts

Achieving structurally well-defined catalytic species requires a fundamental understanding of surface chemistry. Detailed structural characterization of the catalyst binding sites in situ, such as single site catalysts on silica supports, is technically challenging or even unattainable. Octadecyltrioxysilane (OTOS) monolayers formed from octadecyltrimethoxysilane (OTMS) at the air-liquid interface after hydrolysis and condensation at low pH were chosen as a model system of surface binding sites in silica-supported Zn 2+ catalysts. We characterize the system by grazing incidence X-ray diffraction, X-ray reflectivity (XR), and X-ray fluorescence spectroscopy (XFS). Previous X-ray and infrared surface studies of OTMS/OTOS films at the airliquid interface proposed the formation of polymer OTOS structures. According to our analysis, polymer formation is inconsistent with the X-ray observations and structural properties of siloxanes; it is energetically unfavorable and thus highly unlikely. We suggest an alternative mechanism of hydrolysis/condensation in OTMS leading to the formation of structurally allowed cyclic trimers with the six-membered siloxane rings, which explain well both the X-ray and infrared results. XR and XFS consistently demonstrate that tetrahedral [Zn(NH 3 ) 4 ] 2+ ions bind to hydroxyl groups of the film at a stoichiometric ratio of OTOS:Zn ~ 2:1. The high binding affinity of zinc ions to OTOS trimers suggests that the six-membered siloxane rings are binding locations for single site Zn/SiO 2 catalysts. Finally, our results show that OTOS monolayers may serve as a platform for studying silica surface chemistry or hydroxyl-mediated reactions.

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

Poly(oxime–ester) Vitrimers with Catalyst-Free Bond Exchange

Vitrimers are network polymers that undergo associative bond exchange reactions in the condensed phase above a threshold temperature, dictated by the exchangeable bonds comprising the vitrimer. For vitrimers, chemistries reliant on poorly nucleophilic bond exchange partners (e.g., hydroxy-functionalized alkanes) or poorly electrophilic exchangeable bonds, catalysts are required to lower the threshold temperature, which is undesirable in that catalyst leaching or deactivation diminishes its influence over time and may compromise reuse. Here we show how to access catalyst-free bond exchange reactions in catalyst-dependent polyester vitrimers by obviating conventional ester bonds in favor of oxime-esters. Poly(oxime-ester) (POE) vitrimers are synthesized using thiol-ene click chemistry, affording high stretchability and malleability. Finally, POE vitrimers are readily recycled with little degradation of their initial mechanical properties, suggesting exciting opportunities for sustainable plastics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Effects of Polymer Morphology on Solvent and Catalyst Accessibility during Polyethylene and Polystyrene Autoxidation

Efficient catalytic deconstruction of plastics requires facile solvent and catalyst access to polymer substrates to minimize mass transfer effects. Autoxidation using Co(II) acetate, Mn(II) acetate, and a radical carrier in acetic acid is a promising strategy to deconstruct mixed plastic waste, yet the role of polymer morphology in governing solvent and catalyst accessibility remains poorly understood. Here, in situ simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS), complemented by X-ray fluorescence (XRF) imaging and high-pressure differential scanning calorimetry (DSC), were used to elucidate interactions between acetic acid, a Co/Mn catalyst solution, and semicrystalline polyethylene (PE) and amorphous polystyrene (PS) from room temperature to 160 °C. In PE, acetic acid and catalyst access were confined to amorphous regions and cryomilled particle interfaces at room temperature, while crystalline lamellae remained intact after soaking for up to 34 h. Increasing temperature enabled solvent uptake into PE, followed by solvent-assisted softening above 100 °C, and a modest melting-point depression that removed lamellar transport barriers upon melting. Conversely for PS, acetic acid penetrated the glassy polymer without inducing chain mobility until the glass transition was reached, above which the observed structural changes were consistent with enhanced segmental mobility which enabled bulk penetration. These results suggest that polymer morphology and thermally activated physical transitions arising from diffusion and polymer–solvent interactions can influence whether autoxidation of plastics is transport-limited or kinetically controlled, providing a framework for aligning reaction conditions with reaction outcomes.

09 BIOMASS FUELS↗
DOE OSTI2016

Sub-particle reaction and photocurrent mapping to optimize catalyst-modified photoanodes

The splitting of water photoelectrochemically into hydrogen and oxygen represents a promising technology for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled—yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalysing water oxidation also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, especially for nanostructured photoanodes benefiting from small charge-carrier transport distances. In this study, we use super-resolution imaging, operated in a chargecarrier- selective manner and with a spatiotemporal resolution of approximately 30 nanometres and 15 milliseconds, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resolution (about 390 nanometres), the photocurrent associated with water oxidation, and find that the most active sites for water oxidation are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod—even though more improvement in photocurrent efficiency correlates with less reduction in onset potential (and vice versa) at the subparticle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential reduction the optimal sites are the sites with more positive onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte

The development of a low-cost, high-performance platinum-group-metal-free hydroxide exchange membrane fuel cell is hindered by the lack of a hydrogen oxidation reaction catalyst at the anode. Here we report that a composite catalyst, nickel nanoparticles supported on nitrogen-doped carbon nanotubes, has hydrogen oxidation activity similar to platinum-group metals in alkaline electrolyte. Although nitrogen-doped carbon nanotubes are a very poor hydrogen oxidation catalyst, as a support, it increases the catalytic performance of nickel nanoparticles by a factor of 33 (mass activity) or 21 (exchange current density) relative to unsupported nickel nanoparticles. Density functional theory calculations indicate that the nitrogen-doped support stabilizes the nanoparticle against reconstruction, while nitrogen located at the edge of the nanoparticle tunes local adsorption sites by affecting the d-orbitals of nickel. Here, owing to its high activity and low cost, our catalyst shows significant potential for use in low-cost, high-performance fuel cells.

25 ENERGY STORAGE↗
DOE OSTI2016

Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene

There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper + species remain on the surface during the reaction. Furthermore, our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper + is key for lowering the onset potential and enhancing ethylene selectivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Chemoselective single-site Earth-abundant metal catalysts at metal–organic framework nodes

Earth-abundant metal catalysts are critically needed for sustainable chemical synthesis. Here we report a simple, cheap and effective strategy of producing novel earth-abundant metal catalysts at metal–organic framework (MOF) nodes for broad-scope organic transformations. The straightforward metalation of MOF secondary building units (SBUs) with cobalt and iron salts affords highly active and reusable single-site solid catalysts for a range of organic reactions, including chemoselective borylation, silylation and amination of benzylic C–H bonds, as well as hydrogenation and hydroboration of alkenes and ketones. Our structural, spectroscopic and kinetic studies suggest that chemoselective organic transformations occur on site-isolated, electron-deficient and coordinatively unsaturated metal centres at the SBUs via σ-bond metathesis pathways and as a result of the steric environment around the catalytic site. MOFs thus provide a novel platform for the development of highly active and affordable base metal catalysts for the sustainable synthesis of fine chemicals.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Gold-supported cerium-doped NiO x catalysts for water oxidation

Here, the development of high-performance catalysts for the oxygen-evolution reaction (OER) is paramount for cost-effective conversion of renewable electricity to fuels and chemicals. Here we report the significant enhancement of the OER activity of electrodeposited NiO x films resulting from the combined effects of using cerium as a dopant and gold as a metal support. This NiCeO x –Au catalyst delivers high OER activity in alkaline media, and is among the most active OER electrocatalysts yet reported. On the basis of experimental observations and theoretical modelling, we ascribe the activity to a combination of electronic, geometric and support effects, where highly active under-coordinated sites at the oxide support interface are modified by the local chemical binding environment and by doping the host Ni oxide with Ce. The NiCeO x –Au catalyst is further demonstrated in a device context by pairing it with a nickel–molybdenum hydrogen evolution catalyst in a water electrolyser, which delivers 50 mA consistently at 1.5 V over 24 h of continuous operation.

36 MATERIALS SCIENCE↗