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Bell, Alexis T.

Publications and source records attributed to Bell, Alexis T..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Factors and Dynamics of Cu Nanocrystal Reconstruction under CO 2 Reduction

Cu nanocrystals as catalysts for CO 2 reduction are a subject of considerable contemporary interest, but their stability has not been considered as extensively. In this paper, we report on the reconstruction of Cu nanocrystals during CO 2 reduction and discuss the factors influencing the observed changes with computer-based quantitative analysis and spectroscopic techniques. The timelines of opposing phenomena, sintering and declustering, previously reported separately, are detailed with a focus on two forces affecting the final morphology: Applied potential and reaction intermediates. This intriguing system demonstrates the need for fundamental understanding of catalyst behavior preceding the ability to control its performance.

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DOE OSTI2019

Understanding cation effects in electrochemical CO 2 reduction

Solid–liquid interface engineering has recently emerged as a promising technique to optimize the activity and product selectivity of the electrochemical reduction of CO 2 . In particular, the cation identity and the interfacial electric field have been shown to have a particularly significant impact on the activity of desired products. Using a combination of theoretical and experimental investigations, we show the cation size and its resultant impact on the interfacial electric field to be the critical factor behind the ion specificity of electrochemical CO 2 reduction. We present a multi-scale modeling approach that combines size-modified Poisson–Boltzmann theory with ab initio simulations of field effects on critical reaction intermediates. The model shows an unprecedented quantitative agreement with experimental trends in cation effects on CO production on Ag, C 2 production on Cu, CO vibrational signatures on Pt and Cu as well as Au(111) single crystal experimental double layer capacitances. The insights obtained represent quantitative evidence for the impact of cations on the interfacial electric field. Lastly, we present design principles to increase the activity and selectivity of any field-sensitive electrochemical process based on the surface charging properties: the potential of zero charge, the ion size, and the double layer capacitance.

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DOE OSTI2019

Zeolite-Catalyzed Isobutene Amination: Mechanism and Kinetics

Amination of isobutene with NH 3 was researched over Brønsted acidic zeolites at 1 atm and 453-483 K. To compare catalytic activities over different zeolites, the measured reaction rates are normalized by the number of active sites determined by tert-butylamine temperature-programmed desorption (TPD). Small- and medium-pore zeolites with one-dimensional channels exhibit low activity because of pore blockage by adsorbed tert-butylammonium ions. Yet, turnover frequencies and activation energies are not sensitive to framework identity, as long as the active site is accessible to isobutene and tert-butylamine. Kinetic measurements and FTIR spectroscopy reveal that the Brønsted acid sites in MFI are covered predominantly with tert-butylammonium ions under reaction conditions. The desorption of tert-butylamine is assisted by the concurrent adsorption of isobutene. DFT simulations show that at very low tert-butylamine partial pressures, for example, at the inlet to the reactor, tert-butylamine desorption is rate-limiting. However, at sufficiently high tert-butylamine partial pressures (>0.03 kPa), protonation of isobutene to the corresponding carbenium ion limits the rate of amination.

2-methylpropene↗
DOE OSTI2019

Synthesis of Biomass-Derived Ethers for Use as Fuels and Lubricants

Ethers synthesized from biomass-derived compounds have exceptional properties as fuels, lubricants, and specialty chemicals and can serve as replacements for petroleum-derived products. Recent efforts have identified heterogeneous catalysts for the selective synthesis of ethers from alcohols, aldehydes, ketones, furans, esters, olefins, carboxylic acids, and other molecules derived from biomass. This Review highlights the scope of etherification reactions and provides insights into the choice of catalysts and reaction conditions best suited for producing targeted ethers from the available starting materials. First, the properties of ethers for specific applications and the methods by which synthons for ether synthesis can be obtained from biomass are discussed. Then the progress that has been made on the synthesis of ethers via the following methods is summarized: direct etherification of alcohols; reductive etherification of alcohols with aldehydes or ketones; etherification of furanic compounds, esters, and carboxylic acids; and the addition of alcohols to olefins. Next, the mechanisms of these reactions and catalyst properties required to promote them are discussed, with the goal of understanding how reaction conditions can be tuned to optimize catalyst activity and selectivity towards desired ethers. Finally, the Review closes by examining the tradeoffs between catalyst selectivity, activity, stability, and reaction conditions required to achieve the most economically and environmentally favorable routes to biomass-derived ethers.

09 BIOMASS FUELS↗
DOE OSTI2019

Towards membrane-electrode assembly systems for CO 2 reduction: a modeling study

Membrane-electrode assemblies (MEAs) are an attractive cell design for the electrochemical reduction of CO 2 because they exhibit low ohmic loss and high energy efficiency. We describe here the development and application of a multiphysics model to investigate the fundamental limitations of two MEA designs: one with gaseous feeds at both the anode and cathode (full-MEA), and the other with an aqueous anode feed (KHCO 3 or KOH exchange solution) and a gaseous cathode feed (exchange-MEA). The total current density for the three cases follows the order: KOH-MEA > KHCO 3 -MEA > full-MEA. This trend is established by examining the distribution of the applied voltage. We show that the main charge-carrying species are carbonate anions for an MEA that uses an anion-exchange membrane (AEM). The amount of CO 2 consumed but not converted to CO decreases with increasing current densities above 100 mA cm –2 for a full-MEA, but converges to 50% for exchange-MEAs. The full-MEA becomes limited by ohmic resistance as the membrane dehydrates with increasing cell temperature, and eventually becomes limited due to water mass transport. The exchange-MEAs can maintain membrane hydration and the local ion concentration at the anode, but are limited by salt precipitation at the cathode, as well as a higher tendency to flood. Lastly, we explore the effects of temperature and discuss the possibility of increasing water supply to the full-MEA to improve its performance at elevated temperatures. The MEA model and the understanding of MEA performance for the electrochemical reduction of CO 2 presented in this study should help guide the design of next-generation CO 2 reduction cells.

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DOE OSTI2019

Influence of Atomic Surface Structure on the Activity of Ag for the Electrochemical Reduction of CO 2 to CO

This work was undertaken to elucidate the facet-dependent activity of Ag for the electrochemical reduction of CO 2 to CO. To this end, CO2 reduction was investigated over Ag thin films with (111), (100), and (110) orientations prepared via epitaxial growth on single-crystal Si wafers with the same crystallographic orientations. Thus, this preparation technique yielded larger area electrodes than can be achieved using single-crystals, which enabled the electrocatalytic activity of the corresponding Ag surfaces to be quantified in the Tafel regime. The Ag(110) thin films exhibited higher CO evolution activity compared to the Ag(111) and Ag(100) thin films, consistent with previous single-crystal studies. Density functional theory calculations suggest that CO 2 reduction to CO is strongly facet-dependent, and that steps are more active than highly coordinated terraces. This is the result of both a higher binding energy of the key intermediate COOH as well as an enhanced double-layer electric field stabilization over undercoordinated surface atoms located at step edge defects. As a consequence, step edge defects likely dominate the CO 2 reduction activity observed over the Ag(111) and Ag(100) thin films. The higher activity observed over the Ag(110) thin film is then related to the larger density of undercoordinated sites compared to the Ag(111) and Ag(100) thin films. Our conclusion that undercoordinated sites dominate the CO 2 reduction activity observed over close-packed surfaces highlights the need to consider the contribution of such defects in studies of single-crystal electrodes.

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DOE OSTI2019

Explaining the Incorporation of Oxygen Derived from Solvent Water into the Oxygenated Products of CO Reduction over Cu

The electrochemical reduction of CO and CO 2 over Cu makes a variety of multicarbon products. Interestingly, recent isotope experiments have suggested that the oxygen atoms contained in the multicarbon alcohols produced over Cu are derived from solvent water. This observation has brought into question many of the presented reaction mechanisms by which these multicarbon alcohols are produced over Cu. Yet, these surprising experimental observations are likely the result of isotopic scrambling between transiently produced carbonyl-containing intermediate reaction products, such as acetaldehyde, with solvent water and not another mechanism. The existence of such carbonyl-containing intermediate reaction products is supported by both experimental and theoretical studies. Moreover, theoretical calculations support the notion that the reversible hydration of these carbonyl-containing species is facile in the vicinity of the Cu surface.

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DOE OSTI2019

Propanol Amination over Supported Nickel Catalysts: Reaction Mechanism and Role of the Support

Ni-supported hydroxyapatite catalyst (Ni/HAP) was characterized and evaluated for propanol amination to propylamine at 423 K. The reaction proceeds via dehydroamination, a process that involves sequential dehydrogenation, condensation, and hydrogenation. Kinetic and isotopic studies indicate that α-H abstraction from propoxide species limits the rate of the dehydrogenation step and hence the overall rate of reaction. The rate of propanol dehydrogenation depends on the composition of the support and on the concentration of Ni sites located at the interface between Ni nanoparticles and the support. Ni/HAP is an order of magnitude more active than Ni/SiO 2 and displays a higher selectivity toward the primary amine. In conclusion, the superior performance of Ni/HAP is attributed to the high density of basic sites on HAP, which are responsible for stabilizing alkoxide intermediates and suppressing the disproportionation and secondary amination of amines.

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DOE OSTI2018

Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCl 3

In this study, the mechanism and kinetics of C 3 H 8 dehydrogenation and cracking are examined over Ga/H-MFI catalysts prepared via vapor-phase exchange of H-MFI with GaCl 3 . The present study demonstrates that [GaH] 2+ cations are the active centers for C 3 H 8 dehydrogenation and cracking, independent of the Ga/Al ratio. For identical reaction conditions, [GaH] 2+ cations in Ga/H-MFI exhibit a turnover frequency for C 3 H 8 dehydrogenation that is 2 orders of magnitude higher and for C 3 H 8 cracking, that is 1 order of magnitude higher than the corresponding turnover frequencies over H-MFI. C 3 H 8 dehydrogenation and cracking exhibit first-order kinetics with respect to C 3 H 8 over H-MFI, but both reactions exhibit first-order kinetics over Ga/H-MFI only at very low C 3 H 8 partial pressures and zero-order kinetics at higher C 3 H 8 partial pressures. H 2 inhibits both reactions over Ga/H-MFI. It is also found that the ratio of the rate of dehydrogenation to the rate of cracking over Ga/H-MFI is independent of C 3 H 8 and H 2 partial pressures but weakly dependent on temperature. Measured activation enthalpies together with theoretical analysis are consistent with a mechanism in which both the dehydrogenation and cracking of C 3 H 8 proceed over Ga/H-MFI via reversible, heterolytic dissociation of C 3 H 8 at [GaH] 2+ sites to form [C 3 H 7 -GaH] + -H + cation pairs. The rate-determining step for dehydrogenation is the β-hydride elimination of C 3 H 6 and H 2 from the C 3 H 7 fragment. The rate-determining step for cracking is C-C bond attack of the same propyl fragment by the proximal Brønsted acid O-H group. H 2 inhibits both dehydrogenation and cracking over Ga/H-MFI via reaction with [GaH] 2+ cations to form [GaH 2 ] + -H + cation pairs.

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DOE OSTI2018

Challenges in Modeling Electrochemical Reaction Energetics with Polarizable Continuum Models

A major challenge in the modeling of electrochemical phenomena is the accurate description of the interface between an electrolyte and a charged conductor. Polarizable continuum models (PCM) have been gaining popularity because they offer a computationally inexpensive method of modeling the electrolyte. In this Perspective, we discuss challenges from using one such model which treats the ions using a linearized Poisson–Boltzmann (LPB) distribution. From a physical perspective, this model places charge unphysically close to the surface and adsorbates, and it includes excessively steep ramping of the dielectric constant from the surface to the bulk solvent. Both of these issues can be somewhat mitigated by adjusting parameters built into the model, but in doing so, the resultant capacitance deviates from experimental values. Likewise, hybrid explicit-implicit approaches to the solvent may offer a more realistic description of hydrogen bonding and solvation to reaction intermediates, but the corresponding capacitances also deviate from experimental values. These deviations highlight the need for a careful adjustment of parameters in order to reproduce not only solvation energies but also other physical properties of solid–liquid interfaces. Continuum approaches alone also necessarily do not capture local variations in the electric field from cations at the interface, which can affect the energetics of intermediates with substantial dipoles or polarizability. Finally, since the double-layer charge can be varied continuously, LPB/PCM models provide a way to determine electrochemical barriers at constant potential. However, double-layer charging and the atomic motion associated with reaction events occur on significantly different timescales. As a result, we suggest that more detailed approaches, such as the modified Poisson–Boltzmann model and/or the addition of a Stern layer, may be able to mitigate some but not all of the challenges discussed.

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DOE OSTI2018

Reaction mechanism of the selective reduction of CO 2 to CO by a tetraaza [Co II N 4 H] 2+ complex in the presence of protons

The tetraaza [Co II N 4 H] 2+ complex ( 1 ) is remarkable for its ability to selectively reduce CO 2 to CO with 45% Faradaic efficiency and a CO to H 2 ratio of 3 : 2. We employ density functional theory (DFT) to determine the reasons behind the unusual catalytic properties of 1 and the most likely mechanism for CO 2 reduction. The selectivity for CO 2 over proton reduction is explained by analyzing the catalyst's affinity for the possible ligands present under typical reaction conditions: acetonitrile, water, CO 2 , and bicarbonate. After reduction of the catalyst by two electrons, formation of [Co I N 4 H] + –-CO 2 - is strongly favored. Based on thermodynamic and kinetic data, we establish that the only likely route for producing CO from here consists of a protonation step to yield [Co I N 4 H] + –CO 2 H, followed by reaction with CO 2 to form [Co II N 4 H] 2+ –CO and bicarbonate. This conclusion corroborates the idea of a direct role of CO 2 as a Lewis acid to assist in C–O bond dissociation, a conjecture put forward by other authors to explain recent experimental observations. The pathway to formic acid is predicted to be forbidden by high activation barriers, in accordance with the products that are known to be generated by 1 . Calculated physical observables such as standard reduction potentials and the turnover frequency for our proposed catalytic cycle are in agreement with available experimental data reported in the literature. The mechanism also makes a prediction that may be experimentally verified: that the rate of CO formation should increase linearly with the partial pressure of CO 2 .

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DOE OSTI2018

Effect of Alcohol Structure on the Kinetics of Etherification and Dehydration over Tungstated Zirconia

We report that linear and branched ether molecules have attracted recent interest as diesel additives and lubricants that can be produced from biomass-derived alcohols. In this study, tungstated zirconia was identified as a selective and green solid acid catalyst for the direct etherification of primary alcohols in the liquid phase, achieving ether selectivities of >94 % for C 6 –C 12 linear alcohol coupling at 393 K. The length of linear primary alcohols (C 6 –C 12 ) was shown to have a negligible effect on apparent activation energies for etherification and dehydration, demonstrating the possibility to produce both symmetrical and asymmetrical linear ethers. Reactions over a series of C 6 alcohols with varying methyl branch positions indicated that substituted alcohols (2°, 3°) and alcohols with branches on the β-carbon readily undergo dehydration, but alcohols with branches at least three carbons away from the -OH group are highly selective to ether. A novel model compound, 4-hexyl-1dodecanol, was synthesized and tested to further demonstrate this structure–activity relationship. Lastly, trends in the effects of alcohol structure on selectivity were consistent with previously proposed mechanisms for etherification and dehydration, and help to define possible pathways to selectively form ethers from biomass-derived alcohols.

09 BIOMASS FUELS↗
DOE OSTI2018

The mechanism and kinetics of methyl isobutyl ketone synthesis from acetone over ion-exchanged hydroxyapatite

The synthesis of methyl isobutyl ketone (MIBK) can be carried out by the condensation of acetone in the presence of hydrogen over a supported metal catalyst. Previous studies have shown that hydroxyapatite is an excellent catalyst for condensation reactions. The present investigation was undertaken in order to elucidate the reaction mechanism and site requirements for acetone coupling to MIBK over a physical mixture of hydroxyapatite and Pd/SiO 2 . The reaction is found to proceed by consecutive aldol addition to form diacetone alcohol (DAA), dehydration of DAA to mesityl oxide (MO), and hydrogenation of MO to MIBK. The products formed by feeding DAA and MO reveal that aldol addition of acetone is rapid and reversible, and that the subsequent dehydration of DAA is rate-limiting. Pyridine and CO 2 titration show that aldol dehydration occurs over basic sites via an E 1cB mechanism. A series of cation-substituted hydroxyapatite samples were prepared by ion-exchange to further investigate the role of acid-base strength on catalyst performance. Characterization of these samples by PXRD, BET, ICP-OES, XPS, CO 2 -TPD, and Raman spectroscopy demonstrated that the exchange procedure used does not affect the bulk properties of hydroxyapatite. Here, DFT calculations reveal that in addition to affecting the Lewis acidity/basicity of the support, the size of the cation plays a significant role in the chemistry: cations that are too large (Ba 2+ ) or too small (Mg 2+ ) adversely affect reaction rates due to excessive stabilization of intermediate species. Strontium-exchanged hydroxyapatite was found to be the most active catalyst because it promoted α-hydrogen abstraction and C–O bond cleavage of DAA efficiently.

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DOE OSTI2018

Modeling gas-diffusion electrodes for CO 2 reduction

CO 2 reduction conducted in electrochemical cells with planar electrodes immersed in an aqueous electrolyte is severely limited by mass transport across the hydrodynamic boundary layer. This limitation can be minimized by use of vapor-fed, gas-diffusion electrodes (GDEs), enabling current densities that are almost two orders of magnitude greater at the same applied cathode overpotential than what is achievable with planar electrodes in an aqueous electrolyte. The addition of porous cathode layers, however, introduces a number of parameters that need to be tuned in order to optimize the performance of the GDE cell. In this study, we develop a multiphysics model for gas diffusion electrodes for CO 2 reduction and used it to investigate the interplay between species transport and electrochemical reaction kinetics. The model demonstrates how the local environment near the catalyst layer, which is a function of the operating conditions, affects cell performance. Finally, we also examine the effects of catalyst layer hydrophobicity, loading, porosity, and electrolyte flowrate to help guide experimental design of vapor-fed CO 2 reduction cells.

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DOE OSTI2018

Standards and Protocols for Data Acquisition and Reporting for Studies of the Electrochemical Reduction of Carbon Dioxide

Objective evaluation of the performance of electrocatalysts for CO 2 reduction has been complicated by a lack of standardized methods for measuring and reporting activity data. In this perspective, we advocate that standardizing these practices can aid in advancing research efforts toward the development of efficient and selective CO 2 reduction electrocatalysts. Using information taken from experimental studies, we identify variables that influence the measured activity of CO 2 reduction electrocatalysts and propose procedures to account for these variables in order to improve the accuracy and reproducibility of reported data. We recommend that catalysts be measured under conditions which do not introduce artifacts from impurities, from either the electrolyte or counter electrode, and advocate the acquisition of data measured in the absence of mass transport effects. Furthermore, measured rates of electrochemical reactions should be normalized to both the geometric electrode area as well as the electrochemically active surface area to facilitate the comparison of reported catalysts with those previously known. We demonstrate that, when these factors are accounted for, the CO 2 reduction activities of Ag and Cu measured in different laboratories exhibit little difference. Furthermore, adoption of the recommendations presented in this perspective would greatly facilitate the identification of superior catalysts for CO 2 reduction arising solely from changes in their composition and pretreatment.

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DOE OSTI2018

Characterization of Isolated Ga 3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl 3

Ga/H-MFI was prepared by vapor-phase reaction of GaCl 3 with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl 2 ] + cations in the as-exchanged zeolite are treated in H 2 at 823 K to stoichiometrically remove Cl ligands and form [GaH 2 ] + cations. Subsequent oxidation in O 2 and characterization by IR spectroscopy and NH 3 -temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga 3+ exists predominantly as [Ga(OH) 2 ] + -H + cation pairs and to a lesser degree as [Ga(OH)] 2+ cations at low Ga/Al ratios (~0.1); while both species are associated with proximate cation-exchange sites, calculated free energies of formation suggest that [Ga(OH)] 2+ cations are more stable on cation-exchange sites associated with NNN (next-nearest neighbor) framework Al atoms than on those associated with NNNN (next-next-nearest neighbor) framework Al atoms. Ga K-edge X-ray Absorption Near Edge Spectroscopy (XANES) measurements indicate that, under oxidizing conditions and for all Ga/Al ratios, all Ga species are in the +3 oxidation state and are tetrahedrally coordinated to 4 O atoms. Fourier analysis of Ga K-edge Extended X-ray Absorption Fine Structure (EXAFS) data supports the conclusion that Ga 3+ is present predominantly as [Ga(OH) 2 ] + cations (or [Ga(OH) 2 ] + -H + cation pairs). For Ga/Al ratios ≤0.3, wavelet analysis of EXAFS data provide evidence for backscattering from nearest neighboring O atoms and from next-nearest neighboring framework Al atoms. For Ga/Al > 0.3, backscattering from next-nearest neighboring Ga atoms is also evident, characteristic of GaO x species. Here, upon reduction in H 2 , the oxidized Ga 3+ species produce [Ga(OH)H] + -H + cation pairs, [GaH 2 ] + -H + cation pairs, and [GaH] 2+ cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga 3+ species depends sensitively on temperature, Al-Al interatomic distance, and H 2 and H 2 O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH 2 ] + -H + cation pairs and [GaH] 2+ cations are the predominant species present in Ga/H-MFI reduced above 673 K in 10 5 Pa H 2 and in the absence of water vapor.

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DOE OSTI2018

Computational Modeling of the Nature and Role of Ga Species for Light Alkane Dehydrogenation Catalyzed by Ga/H-MFI

Ga-exchanged H-MFI zeolites are highly active for the dehydrogenation of light alkanes; however, both the nature of the active gallium species and the associated dehydrogenation mechanism have been difficult to establish. In this study, we examine the activity of Ga species in Ga/H-MFI by calculating the free energy landscapes on which all reactions occur. To this end, we use a hybrid quantum mechanics/molecular mechanics model for all electronic structure calculations. Quantum chemical calculations were carried out with a range-corrected functional and a good representation of dispersive interactions. The molecular mechanics part of our approach captures the long-range effects of Coulombic and dispersive interactions due to atoms in the extended framework. The rate-determining TS (RDTS) is identified by analysis of the free energy landscape for each mechanism, using the energetic span model. Our analysis reveals that, for reduced Ga/H-MFI, univalent and divalent gallium hydrides, [GaH 2 ] + and [GaH] 2+ , respectively, are more active for ethane dehydrogenation in comparison to H + sites and Ga + sites. [GaH] 2+ sites consistently emerge as the most active sites for light alkane dehydrogenation, providing significant enthalpic stabilization to C-H cleavage TSs via alkyl and carbenium dehydrogenation routes. In contrast, carbenium-like C-H cleavage TSs occurring on Brønsted acid sites are enthalpically less favorable due to their limited electronic interactions with framework O atoms and H + sites. Activation enthalpy barriers for dehydrogenation determined using the energetic span analysis are in good agreement with those measured experimentally. Accounting for entropy of activation reveals that constrained TSs become less favorable in free energy with increasing chain length. Here, we also find that an increase in enthalpic favorability of the alkyl mechanism is observed with increasing chain length for the TS responsible for the second C-H cleavage step leading to alkene formation.

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