In this study, the mechanism and kinetics of C<sub>3</sub>H<sub>8</sub> dehydrogenation and cracking are examined over Ga/H-MFI catalysts prepared via vapor-phase exchange of H-MFI with GaCl<sub>3</sub>. The present study demonstrates that [GaH]<sup>2+</sup> cations are the active centers for C<sub>3</sub>H<sub>8</sub> dehydrogenation and cracking, independent of the Ga/Al ratio. For identical reaction conditions, [GaH]<sup>2+</sup> cations in Ga/H-MFI exhibit a turnover frequency for C<sub>3</sub>H<sub>8</sub> dehydrogenation that is 2 orders of magnitude higher and for C<sub>3</sub>H<sub>8</sub> cracking, that is 1 order of magnitude higher than the corresponding turnover frequencies over H-MFI. C<sub>3</sub>H<sub>8</sub> dehydrogenation and cracking exhibit first-order kinetics with respect to C<sub>3</sub>H<sub>8</sub> over H-MFI, but both reactions exhibit first-order kinetics over Ga/H-MFI only at very low C<sub>3</sub>H<sub>8</sub> partial pressures and zero-order kinetics at higher C<sub>3</sub>H<sub>8</sub> partial pressures. H<sub>2</sub> 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<sub>3</sub>H<sub>8</sub> and H<sub>2</sub> 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<sub>3</sub>H<sub>8</sub> proceed over Ga/H-MFI via reversible, heterolytic dissociation of C<sub>3</sub>H<sub>8</sub> at [GaH]<sup>2+</sup> sites to form [C<sub>3</sub>H<sub>7</sub>-GaH]<sup>+</sup>-H<sup>+</sup> cation pairs. The rate-determining step for dehydrogenation is the β-hydride elimination of C<sub>3</sub>H<sub>6</sub> and H<sub>2</sub> from the C<sub>3</sub>H<sub>7</sub> 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<sub>2</sub> inhibits both dehydrogenation and cracking over Ga/H-MFI via reaction with [GaH]<sup>2+</sup> cations to form [GaH<sub>2</sub>]<sup>+</sup>-H<sup>+</sup> cation pairs.
We introduce a semiempirical method to correct the systematic equilibrium lattice parameters underestimation present in first principles calculations based on the local density approximation. The method consists in performing calculations under a negative pressure such that the calculated equilibrium volume matches the experimentally observed one. We find that elastic properties obtained under these conditions are typically in better agreement with experiment. We also observe that the negative pressure which needs to be applied to crystalline compound can be reliably interpolated by taking the concentration-weighted average of the pressures determined from pure crystals made of each of the elements present in the compound. In a large class of materials, the knowledge of one pressure per element is thus sufficient to correct most of the bias in lattice constants and elastic properties. We finally propose a simple model of the nonlocal contribution to the exchange-correlations energy that is able to explain the observed linear dependence between the required negative pressure and concentration.
We have studied stability of lithium-manganese oxides using density functional theory in the local density and generalized gradient approximation (GGA). In particular, the effect of spin-polarization and magnetic ordering on the relative stability of various structures is investigated. At all lithium compositions the effect of spin polarization is large, although it does not affect different structures to the same extent. At composition ${\mathrm{LiMnO}}_{2},$ globally stable Jahn-Teller distortions could only be obtained in the spin-polarized GGA approximation, and antiferromagnetic spin ordering was critical to reproduce the orthorhombic ${\mathrm{LiMnO}}_{2}$ structure as ground state. We also investigate the effect of magnetism on the Li intercalation potential, an important property for rechargeable Li batteries.
The electrochemical reduction of CO 2 reaction (CO 2 RR) offers an attractive means for converting the carbon content of CO 2 , released from stationary source or the atmosphere, into fuels and chemicals [1].Utilization of CO 2 captured from the atmosphere and energy provided by electricity sourced from wind or solar energy is particularly appealing because it enable sustainable production of carbon-containing products.The most attractive type of CO 2 electrolyzer for commercial application is a membrane electrode assembly (MEA), as shown in Fig. 1a [2,3].This device consists of an anion-exchange membrane (AEM) sandwiched between two gas diffusion electrodes (GDE), each of which comprises a gas diffusion layer (GDL) and a catalyst layer (CL).The cathode CL contains metal nanoparticles,
Read moreHydrogen produced by wind- or solar energy-driven electrochemical splitting of water could be used to store renewable electrical energy or to reduce biomass or CO2 to carbon-containing fuels. The potential required for the splitting of water is larger than the thermodynamic potential due to the insufficient activity of the catalysts required for the two half reactions involved in water splitting—the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). The OER and HER occur at the anode and cathode, respectively, of the electrochemical cell. Since the overpotential for the OER can be nearly an order of magnitude larger than that for the HER, considerable attention has been devoted to finding and developing highly active OER catalysts, and in particular those based on earth-abundant elements. To date this goal has been best met with catalysts based on oxides and oxyhydroxides of Ni and Fe for alkaline electrolysis. This chapter reviews the current understanding of such catalysts and examines the role of catalyst synthesis method and percentage of Fe content on catalyst performance. Particular attention is given to the role of Fe3+ cations exchanged into the lattice of NiOOH in enhancing the OER activity of the host material. This issue is discussed from both experimental and theoretical perspectives with the aim of identifying how and why the additions of Fe3+ cations enhance catalyst performance. The chapter ends with a brief overview of recent efforts aimed at identifying elements other than Fe that can be added to Ni oxide to enhance its OER activity and elements that can be added to NiFe oxyhydroxides to further enhance their OER activity.
Read moreAl based alloy powders (Al₈₅Ni₅Y₆Co₂Fe₂) are produced by spray atomization method. High energy ball milling is done to modify the surface topology and particle size for better electrochemical performance. X ray diffraction (XRD), differential scanning calorimeter (DSC), scanning electron microscope (SEM) and transmission electron microscope (TEM) were conducted to characterize the microstructure of the alloys after ball milling. It is found that 5 hours ball milling gives the minimum crystallization and structure change. Thin film sample is also deposited on stainless steel substrate by pulsed laser deposition (PLD) method for electrochemical test. The capacity and reversibility for different samples are compared and discussed. A capacity of 200mAh/g is obtained for the battery with thin film sample as anode and a capacity of 140mAh/g is obtained for that with electrode from powder sample. Both of the batteries give up to 94% capacity retention after 20 cycles.
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Read moreWe show that cluster expansions (CE), previously used to model solid-state materials with binary or ternary configurational disorder, can be extended to the protein design problem. We present a generalized CE framework, in which properties such as energy can be unambiguously expanded in the amino-acid sequence space. The CE coarse grains over nonsequence degrees of freedom (e.g., side-chain conformations) and thereby simplifies the problem of designing proteins, or predicting the compatibility of a sequence with a given structure, by many orders of magnitude. The CE is physically transparent, and can be evaluated through linear regression on the energies of training sequences. We show, as example, that good prediction accuracy is obtained with up to pairwise interactions for a coiled-coil backbone, and that triplet interactions are important in the energetics of a more globular zinc-finger backbone.
Read moreThe local environment and short-range ordering of Li(Ni0.5Mn0.5)O-2, a potential Li-ion battery positive electrode material obtained via an ion-exchange route from Na(Ni0.5Mn0.5)O-2, were investigated by using a combination of Li-6 Magic Angle Spinning (MAS) NMR spectroscopy and neutron Pair Distribution Function (PDF) analysis, associated with Reverse Monte Carlo (RMC) calculations. Li-6 MAS NMR experiments on Li(Ni0.5Mn0.5)O-2 showed that there are almost no Li ions in the transition metal layers. Neutron diffraction data for the precursor Na(Ni0.5Mn0.5)O-2 indicated that there is no Na/ Ni disorder and that the material is perfectly layered. Neutron PDF analysis of Li(Ni0.5Mn0.5)O-2 and Na(Ni0.5Mn0.5)O-2 revealed differences in the local transition metal arrangements between those present in the ion-exchanged material and its precursor, and those found in the cathode material synthesized directly from hydroxide starting materials. Large clusters of 3456 atoms were built to investigate cation ordering. Reverse Monte Carlo results, for both the Na and Li-containing compounds, showed a non- random distribution of Ni and Mn cations in the transition metal layers: in the first coordination shell, Ni atoms are on average close to more Mn ions than predicted based on a random distribution of these ions in the transition metal layers. Analysis of the number of Ni/Ni, Mn/Mn and Ni/Mn pairs in the second coordination shell revealed that the Ni and Mn cations show a clear preference for ordering in zigzags rather than in chains.
Read moreWe argue that surface segregation can be substantially modified by the presence of adsorbates and present a first-principles method that allows us to equilibrate segregation and adsorption simultaneously on surfaces with fixed topology. The method is based on a cluster expansion theory to write the state of the system in terms of adsorbate and surface layer occupation variables. This model can be parametrized with density functional theory calculations and equilibrated at finite temperature with Monte Carlo simulation. The method is applied to surface ordering and segregation at a (111) surface of ${\mathrm{Pt}}_{(1\ensuremath{-}x)}{\mathrm{Ru}}_{x}$ alloys in the presence of adsorbing oxygen. While Pt segregates under vacuum conditions, the strong binding between oxygen and Ru couples the segregation energy of the Ru to the oxygen chemical potential. As a result, we find that variations in oxygen chemical potential can dramatically alter the segregation and surface ordering tendency of dilute Ru in Pt.
Read moreA detailed analysis of the formation energies for alkali, earth-alkali, and transition-metal hydrides is presented. The hydriding energies are computed for various crystal structures using density functional theory. The early transition metals are found to have a strong tendency for hydride formation which decreases as one goes to the right in the transition-metal series. A detailed analysis of the changes in band structure and electron density upon hydride formation has allowed us to understand the hydriding energy on the basis of three contributions. The first is the energy to convert the crystal structure of the metal to the structure formed by the metal ions in the hydride (fcc in most cases). In particular, for metals with a strong bcc preference such as V and Cr, this significantly lowers the driving force for hydride formation. A second contribution, which for some materials is dominant, is the loss of cohesive energy when the metal structure is expanded to form the hydride. This expansion lowers the cohesive energy of the metal and is a significant impediment to form stable hydrides for the middle to late transition metals, as they have high cohesive energies. The final contribution to the hydride formation energy is the chemical bonding between the hydrogen and metal in which it is inserted. This is the only contribution that is negative and hence favorable to hydride formation.
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