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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Transition metal carbide catalysts for biomass conversion: A review

The increasing demand for sustainable energy resources has initiated the investigation of biomass conversion over a wide range of catalysts. Among those, transition metal carbides have been extensively studied which demonstrated distinct reactivity and/or selectivity from transition or noble metals in a variety of chemical reactions. In this review, we summarize recent advances in the synthesis of transition metal carbides and their applications in biomass conversion, particularly focusing on the catalytic conversions of (hemi)cellulose, lignin and some typical platform chemicals to fuels or fine chemicals involving C-C, C-O-C and C-O-H bonds cleavages. Perspectives regarding the future research directions on the improvement of transition metal carbide catalysts and detailed reaction mechanism studies are also presented.

Pang, Jifeng↗
DOE OSTI2016

Highly active Pt/MoC and Pt/TiC catalysts for the low-temperature water-gas shift reaction: Effects of the carbide metal/carbon ratio on the catalyst performance

We present that Pt/MoC and Pt/TiC(001) are excellent catalysts for the low-temperature water-gas shift (WGS, CO + H 2 O → H 2 + CO 2 ) reaction. They exhibit high-activity, stability and selectivity. The highest catalytic activities are seen for small coverages of Pt on the carbide substrates. Synergistic effects at the metal-carbide interface produce an enhancement in chemical activity with respect to pure Pt, MoC and TiC. A clear correlation is found between the ability of the Pt/MoC and Pt/TiC(001) surfaces to partially dissociate water and their catalytic activity for the WGS reaction. Finally, an overall comparison of the results for Pt/MoC and Pt/Mo 2 C(001) indicates that the metal/carbon ratio in the carbide support can have a strong influence in the stability and selectivity of WGS catalysts and is a parameter that must be taken into consideration when designing these systems.

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

A Theoretical Study on Reaction Mechanisms and Kinetics of Thiophene Hydrodesulfurization over MoS 2 Catalysts

In the present work, thiophene hydrodesulphurization (HDS) over the Mo-edge, the S-edge, and the Mo-S connection edge of MoS 2 catalyst with 50% sulfur coverage was studied using first-principles based microkinetic modeling. Two parallel HDS routes, i.e., direct desulfurization (DDS) and hydrogenation (HYD) were taken into account. It has been found that the major reaction route of thiophene HDS on the Mo- and the Mo-S edges is temperature dependent. In the low temperature range of 500~600 K, the HYD route is dominant, leading to the C 4 H 8 formation. As the temperature increases, the DDS route becomes competitive with the HYD route. At the temperature above 650 K, the DDS route will be the dominant HDS reaction route on the Mo- and the Mo-S edges. While the DDS route that leads to the formation of C4H6 is the major thiophene HDS reaction route on the S-edge in the entire temperature range of 500~750 K. The microkinetic modeling results show the overall HDS activity on the S-edge is lower than it on the Mo- and the Mo-S edges. The Mo-S edge also provides a preferential reaction pathway, which facilitates 2-hydrothiophene migration from the Mo-edge to the S-edge, followed by remaining elementary steps with lower activation barriers in the DDS route. The work was financially supported by the National Natural Science Foundation of China (No. 21476012, 21571012, 21407007 and 91534201). The computing time was granted by a scientific theme user proposal in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), which is a U.S. Department of Energy national scientific user facility located at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. Part of computing time was provided the National Energy Research Scientific Computing Center (NERSC). D. Mei is supported by the US Department of Energy, Office of Sciences, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences.

Thiophene, hydrodesulphurization, MoS2, DFT, micro↗
DOE OSTI2017

Catalysts by Design: The Power of Theory

Theoretical design of effective catalysts, in conjunction with the identification of guiding design principles and strategies, is a Holy Grail in Chemistry. Although further progress will benefit from additional computational advances, theoretical studies have already enhanced the design of molecular electrocatalysts, photocatalysts, and enzymes. This paper comments on tools, design principles, and remaining challenges.

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

Rational Design of a Catalyst for the Selective Monoborylation of Methane

Combined computational and experimental studies elucidate the mechanism and suggest rational design and optimization strategies of a bis(phosphine)-supported iridium-catalyst for methane monoborylation. The activation of the C–H bond in methane via oxidative addition using tris(boryl) iridium(III) complexes bearing bis-chelating supporting ligands is modeled computationally. This model shows that the use of the soft Lewis base ligand such as 1,2-bis(dimethylphosphino)ethane (dmpe) lowers the activation barrier of the rate-determining step as it facilitates polarization of the metal-center, lowering the barrier of the oxidative addition to afford a seven-coordinate iridium(V) intermediate. The experimental optimization of this reaction using high-throughput methods shows that up to 170 turnovers can be achieved at 150 °C (500 psi) within 16 h using bis(pinacolato)diboron, a well-defined homogeneous and monomeric catalyst (dmpe)Ir(COD)Cl that is readily available from commercial precursors, with selectivity for the monoborylation product. High-boiling cyclic aliphatic solvents decalin and cyclooctane also prove suitable for this reaction, while being inert toward borylation. In accordance with the lower calculated activation barrier, catalytic turnover is also observed at 120 °C with up to 50 turnovers over the course of 4 days in cyclohexane solvent. Here, it was found that the borylation of methane is only achieved via one catalytic cycle, and buildup of pinacolborane, a side-product from methane borylation with bis(pinacolato)diboron, inhibits catalytic activity.

computational molecular modeling↗
DOE OSTI2017

Mechanistic Studies of Single-Step Styrene Production Using a Rhodium(I) Catalyst

The direct and single-step conversion of benzene, ethylene, and a Cu(II) oxidant to styrene using the Rh(I) catalyst ( Fl DAB)Rh(TFA)(η 2 -C 2 H 4 ) [ Fl DAB = N,N '-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene; TFA = trifluoroacetate] has been reported to give quantitative yields (with Cu(II) as the limiting reagent) and selectivity combined with turnover numbers >800. Our report details mechanistic studies of this catalytic process using a combined experimental and computational approach. Examining catalysis with the complex ( Fl DAB)Rh(OAc)(η 2 -C 2 H 4 ) shows that the reaction rate has a dependence on catalyst concentration between first- and half-order that varies with both temperature and ethylene concentration, a first-order dependence on ethylene concentration with saturation at higher concentrations of ethylene, and a zero-order dependence on the concentration of Cu(II) oxidant. The kinetic isotope effect was found to vary linearly with the order in ( Fl DAB)Rh(OAc)(η 2 -C 2 H 4 ), exhibiting no KIE when [Rh] was in the half-order regime, and a k H / k D value of 6.7(6) when [Rh] was in the first-order regime. From these combined experimental and computational studies, competing pathways, which involve all monomeric Rh intermediates and a binuclear Rh intermediate in the other case, are presented.

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

Molecular Cobalt Catalysts for O 2 Reduction: Low-Overpotential Production of H 2 O 2 and Comparison with Iron-Based Catalysts

A series of mononuclear pseudo-macrocyclic cobalt complexes have been investigated as catalysts for O2 reduction. Each of these complexes, with CoIII/II reduction potentials that span nearly 400 mV, mediate highly selective two- electron reduction of O2 to H2O2 (93–99%) using decamethylferrocene (Fc*) as the reductant and acetic acid as the proton source. Kinetic studies reveal that the rate exhibits a first- order dependence on [Co] and [AcOH], but no dependence on [O2] or [Fc*]. A linear correlation is observed between log(TOF) vs. E1/2(CoIII/II) for the different cobalt complexes (TOF = turnover frequency). The thermodynamic potential for+ O2 reduction to H2O2 was estimated by measuring the H /H2 open-circuit potential under the reaction conditions. This value provides the basis for direct assessment of the thermodynamic efficiency of the different catalysts and shows that H2O2 is formed with overpotentials as low as 90 mV. These results are compared with a recently reported series of Fe-porphyrin complexes, which catalyze four-electron reduction of O2 to H2O. The data show that the TOFs of the Co complexes exhibit a shallower dependence on E1/2(MIII/II) than the Fe complexes. This behavior, which underlies the low overpotential, is rationalized on the basis of the catalytic rate law.

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

Charge Carrier Activity on Single-Particle Photo(electro)catalysts: Toward Function in Solar Energy Conversion

Understanding the fundamental properties of charge carriers on the surface of semiconductor photo(electro)catalysts is key to the rational design of efficient photo(electro)catalytic devices for sunlight-driven energy conversion. Here high spatial resolution information is always desirable because of the ubiquitous heterogeneity in semiconductor particles. In this Perspective, we review the latest advances in nanoscale imaging and quantitative analysis of charge carrier activities on individual semiconductor particles down to subparticle resolution, covering the approaches of single-molecule super-resolution fluorescence imaging, scanning electron microscopy, and photoluminescence microscopy. We further highlight direct, operando functional assessments of their performances toward the targeted photo(electro)-catalytic processes through single- and subparticle photocurrent measurements. Here, we also discuss the significance of establishing quantitative relations between the desired functions of photo(electro)catalysts and their surface charge carrier activities. These fundamental relations can provide guiding principles for rationally engineering photo(electro)catalytic systems, for example with cocatalysts, for a broad range of applications.

14 SOLAR ENERGY↗
DOE OSTI2018

Crossing the great divide between single-crystal reactivity and actual catalyst selectivity with pressure transients

The quantitative prediction of catalyst selectivity is essential to the design of efficient catalytic processes and requires a detailed knowledge of the reaction mechanism and rate constants. Here we present a study that accurately predicts, using the kinetics and a mechanism derived from fundamental studies on single-crystal gold, the product distribution resulting from the complex reaction network that governs the oxidative coupling of methanol, catalysed by nanoporous gold between 360 and 425 K and for a vast range of pressures. Analysis of the transient product responses to micropulses of methanol over nanoporous gold yields a precise understanding of the marked dependence of selectivity on pressure, surface oxygen coverage and temperature. The key to a high selectivity for methyl formate is the surface lifetime and abundance of the methoxy. In conclusion, this successful microkinetic modelling of catalytic reactions across a wide set of reaction conditions is broadly applicable to predicting catalytic selectivity and provides a pathway to designing more efficient catalytic processes.

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

Insight into the phase evolution of a NiMgAl catalyst from the reduction stage to the post-reaction stage during the dry reforming of methane [Insight into the phase evolution of NiMgAl catalyst from reduction to post-reaction for dry reforming of methane]

Herein, phase evolution of a NiMgAl oxide catalyst at the reduction stage was qualitatively analysed and quantitatively determined by employing the continuous changes in its XRD intensity and TPR information. In conclusion, the stable crystallite size of both the active metal and spinel support was responsible for the long stability of the NiMgAl catalyst without carbon deposition during the DRM reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗