Constitutive model for plasticity in an amorphous polycarbonate

A constitutive model for describing the mechanical response of an amorphous glassy polycarbonate is proposed. The model is based on an isotropic elastic phase surrounded by an SO(3) continuum of plastic phases onto which the elastic phase can collapse under strain. An approximate relaxed energy is developed for this model on the basis of physical considerations and extensive numerical testing, and it is shown that it corresponds to an ideal elastic-plastic behavior. Kinetic effects are introduced as rate-independent viscoplasticity, and a comparison with experimental data is presented, showing that the proposed model is able to capture the main features of the plastic behavior of amophous glassy polycarbonate

The Dimer Model for k-phase Organic Superconductors

We prove that the upper electronic bands of k-phase BEDT-TTF salts are adequately modeled by an half-filled tight-binding lattice with one site per cell. The band parameters are derived from recent ab-initio calculations, getting a very simple but extremely accurate one-electron picture. This picture allows us to solve the BCS gap equation adopting a real-space pairing potential. Comparison of the calculated superconducting properties with the experimental data points to isotropic s_0-pairing. Residual many-body or phonon-mediated interactions offer a plausible explanation of the large variety of physical properties observed in k-phase BEDT-TTF salts.Comment: 8 pages, 6 PostScript figures, uses RevTe

Reaction intermediates during operando electrocatalysis identified from full solvent quantum mechanics molecular dynamics

Electrocatalysis provides a powerful means to selectively transform molecules, but a serious impediment in making rapid progress is the lack of a molecular-based understanding of the reactive mechanisms or intermediates at the electrode–electrolyte interface (EEI). Recent experimental techniques have been developed for operando identification of reaction intermediates using surface infrared (IR) and Raman spectroscopy. However, large noises in the experimental spectrum pose great challenges in resolving the atomistic structures of reactive intermediates. To provide an interpretation of these experimental studies and target for additional studies, we report the results from quantum mechanics molecular dynamics (QM-MD) with explicit consideration of solvent, electrode–electrolyte interface, and applied potential at 298 K, which conceptually resemble the operando experimental condition, leading to a prototype of operando QM-MD (o-QM-MD). With o-QM-MD, we characterize 22 possible reactive intermediates in carbon dioxide reduction reactions (CO_2 RRs). Furthermore, we report the vibrational density of states (v-DoSs) of these intermediates from two-phase thermodynamic (2PT) analysis. Accordingly, we identify important intermediates such as chemisorbed CO_2 (b-CO_2), *HOC-COH, *C-CH, and *C-COH in our o-QM-MD likely to explain the experimental spectrum. Indeed, we assign the experimental peak at 1,191 cm^(−1) to the mode of C-O stretch in *HOC-COH predicted at 1,189 cm^(−1) and the experimental peak at 1,584 cm^(−1) to the mode of C-C stretch in *C-COD predicted at 1,581 cm^(−1). Interestingly, we find that surface ketene (*C=C=O), arising from *HOC-COH dehydration, also shows signals at around 1,584 cm^(−1), which indicates a nonelectrochemical pathway of hydrocarbon formation at low overpotential and high pH conditions

Catalytic activity of Pt_(38) in the oxygen reduction reaction from first-principles simulations

The activity of truncated octahedral Pt_(38) clusters as a catalyst in the oxygen reduction reaction (ORR) is investigated via first-principles simulations. Three catalytic steps: O_2 dissociation (O_(2ads) → 2_O_(ads)), O hydration (O_(ads) + H_2O_(ads) → 2OH_(ads)), and H_2O formation (OH_(ads) + H_(ads) → H_2O_(ads)) are considered, in which all reactant species are co-adsorbed on the Pt_(38) cluster according to a Langmuir–Hinshelwood mechanism. The minimum structures and saddle points for these different steps are then calculated at the density-functional theory (DFT) level using a gradient-corrected exchange–correlation (xc-)functional and taking into account the effect of the solvent via a self-consistent continuum solvation model. Moreover, first-principles molecular dynamics (AIMD) simulations in which the H_2O solvent is explicitly described are performed to explore dynamic phenomena such as fast hydrogen transfer via meta-stable hydronium-type configurations and their possible role in ORR reaction paths. By comparing the present findings with previous results on the Pt(111) surface, it is shown that in such a nanometer-size cluster the rate-determining-step (rds) corresponds to H_2O formation, at variance with the extended surface in which O hydration was rate-determining, and that the overall reaction barrier is actually increased with respect to the extended system. This is in agreement with and rationalizes experimental results showing a decrease of ORR catalytic activity in the nanometer-size cluster range

Highly Efficient Ni-Doped Iron Catalyst for Ammonia Synthesis from QM-Based Hierarchical High Throughput Catalyst Screening

To discover more efficient industrial catalysts for ammonia synthesis via the Haber–Bosch (HB) process, we employed quantum mechanics (QM)-based hierarchical high-throughput catalyst screening (HHTCS) to test a wide group of elements (34) as candidates to dope the Fe(111) catalyst subsurface. The QM free-energy reaction network of HB over Fe(111) yields ten barriers as potentially rate-determining, of which we select four as prototypical, arrange them hierarchically, and define a corresponding set of screening criteria, which we then use to screen candidate catalysts. This leads to two promising candidates (Co and Ni), from which we selected the most promising (Ni) for a complete QM and kinetic study. The kinetic Monte Carlo (kMC) simulations predict a 16-fold increase in HB turn-over frequency (TOF) for the Ni-doped catalyst compared to the pure Fe(111) surface under realistic conditions. The 16-fold increase in HB TOF is a significant improvement and may trigger future experimental studies to validate our prediction. This TOF improvement could lead to similar reaction rates as with pure Fe but at a reaction temperature decreased by 100° from 773 to 673 K and a total reactant pressure decreased by 6 times from 201 to 34 atm. We interpret the reasons underlying this improvement using valence bond and kinetic analyses. We suggest this Ni-doped Fe(111) catalyst as a candidate to reduce the world energy consumption for the HB process while satisfying future needs for energy and environment

Controlling the Shapes of Nanoparticles by Dopant-Induced Enhancement of Chemisorption and Catalytic Activity: Application to Fe-Based Ammonia Synthesis

We showed recently that the catalytic efficiency of ammonia synthesis on Fe-based nanoparticles (NP) for Haber–Bosch (HB) reduction of N₂ to ammonia depends very dramatically on the crystal surface exposed and on the doping. In turn, the stability of each surface depends on the stable intermediates present during the catalysis. Thus, under reaction conditions, the shape of the NP is expected to evolve to optimize surface energies. In this paper, we propose to manipulate the shape of the nanoparticles through doping combined with chemisorption and catalysis. To do this, we consider the relationships between the catalyst composition (adding dopant elements) and on how the distribution of the dopant atoms on the bulk and facet sites affects the shape of the particles and therefore the number of active sites on the catalyst surfaces. We use our hierarchical, high-throughput catalyst screening (HHTCS) approach but extend the scope of HHTCS to select dopants that can increase the catalytically active surface orientations, such as Fe-bcc(111), at the expense of catalytically inactive facets, such as Fe-bcc(100). Then, for the most promising dopants, we predict the resulting shape and activity of doped Fe-based nanoparticles under reaction conditions. We examined 34 possible dopants across the periodic table and found 16 dopants that can potentially increase the fraction of active Fe-bcc(111) vs inactive Fe-bcc(100) facets. Combining this reshaping criterion with our HHTCS estimate of the resulting catalytic performance, we show that Si and Ni are the most promising elements for improving the rates of catalysis by optimizing the shape to decrease reaction barriers. Then, using Si dopant as a working example, we build a steady-state dynamical Wulff construction of Si-doped Fe bcc nanoparticles. We use nanoparticles with a diameter of ∼10 nm, typical of industrial catalysts. We predict that doping Si into such Fe nanoparticles at the optimal atomic content of ∼0.3% leads to rate enhancements by a factor of 56 per nanoparticle under target HB conditions

Reaction mechanism and kinetics for ammonia synthesis on the Fe(211) reconstructed surface

To provide guidelines to accelerate the Haber–Bosch (HB) process for synthesis of ammonia from hydrogen and nitrogen, we used Quantum Mechanics (QM) to determine the reaction mechanism and free energy reaction barriers under experimental reaction conditions (400 °C and 20 atm) for all 10 important surface reactions on the Fe(211) reconstructed (Fe(211)R) surface. These conditions were then used in full kMC modeling for 30 minutes to attain steady state. We find that the stable surface under Haber–Bosch conditions is the missing row 2 × 1 reconstructed surface (211)R and that the Turn Over Frequency (TOF) is 18.7 s^(−1) per 2 × 2 surface site for 1.5 Torr NH_3 pressure, but changes to 3.5 s^(−1) for 1 atm, values close (within 6%) to the ones on Fe(111). The experimental ratio between (211) and (111) rates at low (undisclosed) NH_3 pressure was reported to be 0.75. The excellent agreement with experiment on two very different surfaces and reaction mechanisms is a testament of the accuracy of QM modeling. In addition, our kinetic analysis indicates that Fe(211)R is more active than Fe(111) at high pressure, close to HB industrial conditions, and that (211)R is more abundant than (111) via a steady-state Wulff construction under HB conditions. Thus, at variance with common thinking, we advocate the Fe(211)R surface as the catalytically active phase of pure iron ammonia synthesis catalyst under HB industrial conditions