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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Acid-resistant catalyst supports and catalysts

A process for preparing a catalyst comprises coating substantial internal surfaces of porous inorganic powders with titanium oxide to form titanium oxide-coated inorganic powders. After the coating, an extrudate comprising the titanium oxide-coated inorganic powders is formed and calcined to form a catalyst support. Then, the catalyst support is impregnated with a solution containing one or more salts of metal selected from the group consisting of molybdenum, cobalt, and nickel.

Schmidt, Stephen R.↗
DOE OSTI2022

Asymmetrical C–C Coupling for Electroreduction of CO on Bimetallic Cu–Pd Catalysts

Electroreduction of carbon monoxide (CO) possesses great potential for achieving the renewable synthesis of hydrocarbon chemicals from CO 2 . We report here selective reduction of CO to acetate using Cu–Pd bimetallic electrocatalysts. High activity and selectivity are demonstrated for CO-to-acetate conversion with >200 mA/cm 2 in geometric current density and >65% in Faradaic efficiency (FE). An asymmetrical C–C coupling mechanism is proposed to explain the composition-dependent catalytic performance and high selectivity toward acetate. This mechanism is supported by the computationally predicted shift of the *CO adsorption from the top-site configuration on Cu (or Cu-rich) surfaces to the bridge sites of Cu–Pd bimetallic surfaces, which is also associated with the reduction of the CO hydrogenation barrier. Further kinetic analysis of the reaction order with respect to CO and Tafel slope supports a reaction pathway with *CO–*CHO recombination following a CO hydrogenation step, which could account for the electroreduction of CO to acetate on the Cu–Pd bimetallic catalysts. Furthermore, our work highlights how heteroatomic alloy surfaces can be tailored to enable distinct reaction pathways and achieve advanced catalytic performance beyond monometallic catalysts.

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

Dependence of interface energetics and kinetics on catalyst loading in a photoelectrochemical system

Solar hydrogen production by the photoelectrochemical method promises a means to store solar energy. While it is generally understood that the process is highly sensitive to the nature of the interface between the semiconductor and the electrolyte, a detailed understanding of this interface is still missing. For instance, few prior studies have established a clear relationship between the interface energetics and the catalyst loading amount. Here we aim to study this relationship on a prototypical Si-based photoelectrochemical system. Two types of interfaces were examined, one with GaN nanowires as a protection layer and one without. It was found that when GaN was present, higher Pt loading (> 0.1 μg/cm 2 ) led to not only better water reduction (and, hence, hydrogen evolution) kinetics but also more favorable interface energetics for greater photovoltages. In the absence of the protection layer, by stark contrast, increased Pt loading exhibited no measurable influence on the interface energetics, and the main difference was observed only in the hydrogen evolution kinetics. Furthermore, the study sheds new light on the importance of interface engineering for further improvement of photoelectrochemical systems, especially concerning the role of catalysts and protection layers.

14 SOLAR ENERGY↗
DOE OSTI2015

Pt Catalyst Degradation in Aqueous and Fuel Cell Environments studied via In-Operando Anomalous Small-Angle X-ray Scattering

The evolution of Pt nanoparticle cathode electrocatalyst size distribution in a polymer electrolyte membrane fuel cell (PEMFC) was followed during accelerated stress tests using in-operando anomalous small-angle X-ray scattering (ASAXS). This evolution was compared to that observed in an aqueous electrolyte environment using stagnant electrolyte, flowing electrolyte, and flowing electrolyte at elevated temperature to reveal the different degradation trends in the PEMFC and aqueous environments and to determine the relevance of aqueous measurements to the stability of Pt nanoparticle catalyst in the fuel cell environment. The observed changes in the particle size distributions (PSDs) were analyzed to elucidate the extent and mechanisms of particle growth and corresponding mass and active surface area losses in the different environments. These losses indicate a Pt nanoparticle surface area loss mechanism controlled by Pt dissolution, the particle size dependence of Pt dissolution, the loss of dissolved Pt into the membrane and electrolyte, and, to a lesser extent, the re-deposition of dissolved Pt onto larger particles. Based on the geometric surface area loss, mass loss, and mean particle size increase trends, the aqueous environment best reflecting the fuel cell environment was found to be one in which the electrolyte is flowing rather than stagnant. Pt nanoparticle surface area loss resulting from potential cycling can be inhibited by reducing the number of particles smaller than a critical particle diameter (CPD), which was found to be similar to 3.5 to similar to 4 nm, with the CPD dependent on both the cycling protocol (square wave vs triangle wave) and the catalyst environment (fuel cell, aqueous stagnant, aqueous flowing electrolyte, or elevated temperature flowing electrolyte)

Anomalous small-angle X-ray scattering↗
DOE OSTI2018

WO x supported on γ-Al 2 O 3 with different morphologies as model catalysts for alkanol dehydration

The distinctive morphological and surface characteristics of platelet-like γ-Al2O3 were compared to a regular, commercial γ-Al2O3. γ-Al2O3 platelets display dominant (110) surface facets and higher densities of coordinative unsaturated penta-coordinate Al3+ (Al3+penta) sites than regular γ-Al2O3, as measured by solid-state magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Such Al3+penta sites are also the preferred surface anchoring sites for tungsten oxide (WOx) species consistent with NMR analysis indicating that these sites are consumed upon WOx adsorption. The higher Al3+penta density on γ-Al2O3 platelets leads to greater WOx dispersion (or smaller WOx clusters), as demonstrated by scanning transmission electron microscopy and ultraviolet–visible spectroscopy, and WOx species at intermediate WOx surface concentration are the most active for the probe reaction of 2-butanol dehydration. WOx on γ-Al2O3 platelets approaches the highest turnover rates at higher surface densities than WOx on regular γ-Al2O3, yet with similar highest rate values for both series of catalysts. This indicates that different Al2O3 supports mainly affect the dispersion of supported WOx rather than the intrinsic reactivity of individual WOx clusters with similar size.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2023

Synergistic double-atom catalysts of metal-boron anchored on g-C 2 N for electrochemical nitrogen reduction: Mechanistic insight and catalyst screening

The rational design of a novel catalytic center with a sound basis remains both challenging and rewarding for the electrochemical reduction of N 2 (eNRR), which has provided a feasible route for achieving clean and sustainable NH 3 production under ambient conditions. Herein, using density functional theory calculations, we demonstrate that hybrid metal (M)-boron (B) double-atom catalysts (DACs) embedded in g-C 2 N substrate (M-B@C 2 N, M = 3d, 4d and 5d transition metals) can achieve both high catalytic activity and high selectivity in eNRR. The proposed M-B@C 2 N DACs have exhibited impressive feasibility and stability thanks to the resilient and robust C 2 N substrate with abundant pyridinic N atoms distributed among right-sized pore structures. Our results reveal that like the metal center, the embedded B atom can actively involve in Ntriple bondN bond activation via π*-backdonation mechanism concomitant with the substantial charge transfer to adsorbed *N 2 , leading to sizable Nsingle bondN bond elongation. Accordingly, both adsorption energy and Nsingle bondN bond length of *N 2 can be employed as catalytic descriptors for predicting eNRR activity in terms of the limiting potentials (UL). Using high-throughput screening method, we found that six M-B@C 2 N candidates have stood out as the outstanding electrocatalysts for driving eNRR, namely, M = Ti (U L = 0 V), Mo (U L = 0 V), Nb (U L = -0.04 V), W (U L = -0.23 V), Zr (U L = -0.26 V), V (U L = -0.28 V). The underlying origin is attributed to the balanced and constrained N-affinity of M-B dual site working in synergy, which can thus be used as one important guide of catalyst design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Reaction Mechanism for the Conversion of γ-Valerolactone (GVL) over a Ru Catalyst: A First-Principles Study

Conversion of biomass-derived γ-valerolactone (GVL) to valuable chemicals has been studied extensively, and understanding the reaction mechanism is very valuable for improving turnover rates and selectivities. Here, we report first-principles density functional calculations, through which we show in detail the reaction pathways of GVL conversion on a Ru(0001) surface, in good agreement with recent experimental results performed on supported Ru catalysts. We find that (i) GVL undergoes a ring-opening reaction rather easily, and (ii) the rate-limiting step toward the formation of 1,4-pentanediol (1,4-PDO) and 2-pentanol (2-PeOH) is the hydrogenation step. The high energy barrier for this step is caused by a strong interaction between Ru and the unsaturated acyl intermediate that is formed after opening the ring. Among all the primary products, formation of 2-butanol (2-BuOH) has the smallest activation barrier, while the slowest step is C–C bond cleavage in the decarbonylation step. We further show that the same acyl intermediate after ring opening of GVL can also be formed by dehydrogenation of 1,4-PDO with a moderate activation barrier, which suggests that both 2-PeOH and 2-BuOH can also be produced in secondary steps.

Bababrik, Reda M.↗
DOE OSTI2015

Tuning Zr 6 Metal–Organic Framework (MOF) Nodes as Catalyst Supports: Site Densities and Electron-Donor Properties Influence Molecular Iridium Complexes as Ethylene Conversion Catalysts

The Zr 6 nodes of the metal–organic frameworks (MOFs) UiO-66 and UiO-67 are metal oxide clusters of atomic precision and can be used as catalyst supports. The bonding sites on these nodes—that is, hydrogen-bonded H 2 O/OH groups on UiO-67 and non-hydrogen-bonded terminal OH groups on UiO-66—were regulated by modulation of the MOF syntheses. Ir(C 2 H 4 ) 2 (C 5 H 7 O 2 ) complexes reacted with these sites to give site-isolated Ir(C 2 H 4 ) 2 complexes, each anchored to the node by two Ir–Onode bonds. The supported iridium complexes on these sites have been characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and density functional theory calculations. The ethylene ligands on iridium are readily replaced by CO, and the ν CO frequencies of the resultant complexes and those of comparable complexes reported elsewhere show that the support electron-donor tendencies increase in the order HY zeolite << UiO-66 < UiO-67 (= NU-1000) < ZrO 2 < MgO. The sharpness of the IR ν CO bands shows that the degree of uniformity of the support bonding sites decreases in the order ZrO 2 ≈ UiO-67 ≈ NU-1000 < MgO < UiO-66 << HY zeolite. The reactivity of supported Ir(CO) 2 complexes with C2H4 to form Ir(C 2 H 4 )(CO) and Ir(C 2 H 4 ) 2 (CO) is influenced by the support electron-donor properties, with the reactivity increasing in the order MgO = ZrO 2 = NU-1000 (not reactive) < UiO-66 < UiO-67 << HY zeolite. Density functional theory calculations characterizing the complexes supported on NU-1000, UiO-66/67, and HY zeolite concur with the use of the calculated ν CO bands as indicators of electron-donor properties of the supported metal catalysts. Our calculations also show that the reactivity of the supported Ir(CO) 2 complexes with C 2 H 4 is correlated with the electron-donor properties of the iridium center. Lastly, the supported Ir(C 2 H 4 ) 2 samples are precatalysts for ethylene hydrogenation and ethylene dimerization, with the activity for each reaction increasing with increasing electron-withdrawing strength of the support.

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

Selective Oxidation of Acetaldehyde to Acetic Acid on Pd–Au Bimetallic Model Catalysts

Acetic acid is a widely employed reactant in the chemical industry, and it is also used as a food ingredient. Here, we report a catalytic reaction pathway for the gas-phase selective oxidation of acetaldehyde to acetic acid on a Pd–Au(111) heterogeneous model catalyst. On an oxygen precovered Pd–Au surface, acetaldehyde is selectively oxidized to acetic acid from 250 to 340 K. Using FT-IR, the formation of acetate species is detected from 160 to 260 K on this surface, which is a widely known adsorbate derived from acetic acid on metal surfaces. With higher Pd coverages, the acetaldehyde is less selectively oxidized to acetic acid, and near 375 K, CO 2 , H 2 O, CH 4 , and H 2 are evolved, evidence for the decarboxylation of acetate. Finally, in our density functional theory calculations, we confirm that the relative energy difference between the acetate state and the decarboxylated state decreases as Pd ensemble size increases.

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

Better Catalysts through Microscopy: Mesoscale M1/M2 Intergrowth in Molybdenum–Vanadium Based Complex Oxide Catalysts for Propane Ammoxidation

Catalysis research has transformed from the predominantly empirical field to one where it is possible to control the catalytic properties via characterization and modification of the atomic-scale active centers. Many phenomena in catalysis, such as synergistic effect, however, transcend the atomic scale and also require the knowledge and control of the mesoscale structure of the specimen to harness. Our paper, we use our discovery of atomic-scale epitaxial interfaces in molybdenum vanadium based complex oxide catalysts systems (i.e., MoVMO, M = Ta, Te, Sb, Nb, etc.) to achieve control of the mesoscale structure of this complex mixture of very different active phases. We can now achieve true epitaxial intergrowth between the catalytically critical M1 and M2 phases in the system that are hypothesized to have synergistic interactions, and demonstrate that the resulting catalyst has improved selectivity in the initial studies. Finally, we highlight the crucial role atomic scale characterization and mesoscale structure control play in uncovering the complex underpinnings of the synergistic effect in catalysis.

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

Modulating Chemical Environments of Metal–Organic Framework-Supported Molybdenum(VI) Catalysts for Insights into the Structure–Activity Relationship in Cyclohexene Epoxidation

Solid supports are crucial in heterogeneous catalysis due to their profound effects on catalytic activity and selectivity. However, elucidating the specific effects arising from such supports remains challenging. We selected a series of metal–organic frameworks (MOFs) with 8-connected Zr 6 nodes as supports to deposit molybdenum(VI) onto to study the effects of pore environment and topology on the resulting Mo-supported catalysts. As characterized by X-ray absorption spectroscopy (XAS) and single-crystal X-ray diffraction (SCXRD), we modulated the chemical environments of the deposited Mo species. For Mo-NU-1000, the Mo species monodentately bound to the Zr 6 nodes were anchored in the microporous c-pore, but for Mo-NU-1008 they were bound in the mesopore of Mo-NU-1008. Both monodentate and bidentate modes were found in the mesopore of Mo-NU-1200. Cyclohexene epoxidation with H 2 O 2 was probed to evaluate the support effect on catalytic activity and to unveil the resulting structure–activity relationships. SCXRD and XAS studies demonstrated the atomically precise structural differences of the Mo binding motifs over the course of cyclohexene epoxidation. No apparent structural change was observed for Mo-NU-1000, whereas the monodentate mode of Mo species in Mo-NU-1008 and the monodentate and bidentate Mo species in Mo-NU-1200 evolved to a new bidentate mode bound between two adjacent oxygen atoms from the Zr 6 node. This work demonstrates the great advantage of using MOF supports for constructing heterogeneous catalysts with modulated chemical environments of an active species and elucidating structure–activity relationships in the resulting reactions.

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

Reorganization Energies for Interfacial Proton-Coupled Electron Transfer to a Water Oxidation Catalyst

The reorganization energy (λ) for interfacial electron transfer (ET) and proton-coupled electron transfer (PCET) from a conductive metal oxide (In 2 O 3 :Sn, ITO) to a surface-bound water oxidation catalyst were extracted from kinetic data measured as a function of the thermodynamic driving force. Visible light excitation resulted in rapid excited-state injection (k inj > 10 8 s -1 ) to the ITO, which photo-initiated the two interfacial reactions of interest. The rate constants for both reactions increased with the driving force, −ΔG°, to a saturating limit, k max , with rate constants consistently larger for ET than for PCET. Marcus-Gerischer analysis of the kinetic data provided the reorganization energy for interfacial PCET (0.90 ± 0.02 eV) and ET (0.40 ± 0.02 eV), respectively. The magnitude of k max for PCET was found to decrease with pH, behavior that was absent for ET. Both the decrease in k max and the larger reorganization energy for an unwanted competing PCET reaction from the ITO to the oxidized catalyst showcases a significant kinetic advantage for driving solar water oxidation at high pH. Computational analysis revealed a larger inner-sphere reorganization energy contribution for PCET than for ET arising from a more significant change in the Ru–O bond length for PCET. Extending Marcus-Gerischer theory to PCET by including the excited electron-proton vibronic states and the proton donor-acceptor motion provided an apparent reorganization energy of 1.01 eV. Furthermore, this study demonstrates that the Marcus-Gerischer theory initially developed for ET can be reliably extended to PCET for quantifying and interpreting reorganization energies observed experimentally.

Catalysts Kinetic parameters↗
DOE OSTI2016

Optimizing conditions for utilization of an H 2 oxidation catalyst with outer coordination sphere functionalities

[Ni(PCy2NArginine2)2]2+ ( CyArg ) or [Ni(PCy2NBenzyl2)2]2+ ( CyBn ) were evaluated for H 2 oxidation as a function of temperature, pressure, and solvent. 70 °C and 100 atm H 2 result in a TOF of 1.1 × 106 s -1 and an overpotential of 240 mV for CyArg in water. In methanol the rates were 280 s -1 for CyArg and 80 s -1 for CyBn , demonstrating the importance of water and the outer coordination sphere ( OCS ).

bio-inspired↗
DOE OSTI2015

Single atom catalysts on amorphous supports: A quenched disorder perspective

Phenomenological models that invoke catalyst sites with different adsorption constants and rate constants are well-established, but computational and experimental methods are just beginning to provide atomically resolved details about amorphous surfaces and their active sites. This letter develops a statistical transformation from the quenched disorder distribution of site structures to the distribution of activation energies for sites on amorphous supports. We show that the overall kinetics are highly sensitive to the precise nature of the low energy tail in the activation energy distribution. Our analysis motivates further development of systematic methods to identify and understand the most reactive members of the active site distribution.

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

Stabilization of Pt monolayer catalysts under harsh conditions of fuel cells

We employed density functional theory to explore the stability of core (M = Cu, Ru, Rh, Pd, Ag, Os, Ir, Au)-shell (Pt) catalysts under harsh conditions, including solutions and reaction intermediates involved in the oxygen reduction reaction (ORR) in fuel cells. A pseudomorphic surface alloy (PSA) with a Pt monolayer (Pt{sub 1ML}) supported on an M surface, Pt{sub 1ML}/M(111) or (001), was considered as a model system. Different sets of candidate M cores were identified to achieve a stable Pt{sub 1ML} shell depending on the conditions. In vacuum conditions, the Pt{sub 1ML} shell can be stabilized on the most of M cores except Cu, Ag, and Au. The situation varies under various electrochemical conditions. Depending on the solutions and the operating reaction pathways of the ORR, different M should be considered. Pd and Ir are the only core metals studied, being able to keep the Pt{sub ML} shell intact in perchloric acid, sulfuric acid, phosphoric acid, and alkaline solutions as well as under the ORR conditions via different pathways. Ru and Os cores should also be paid attention, which only fall during the ORR via the *OOH intermediate. Rh core works well as long as the ORR does not undergo the pathway via *O intermediate. Our results show that PSAs can behave differently from the near surface alloy, Pt{sub 1ML}/M{sub 1ML}/Pt(111), highlighting the importance of considering both chemical environments and the atomic structures in rational design of highly stable core-shell nanocatalysts. Finally, the roles that d-band center of a core M played in determining the stability of supported Pt{sub 1ML} shell were also discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗