Search

• The concept of van der Waals interactions through a double-layer nitrogen-doped graphene (M2N8) with axial d orbital modification is introduced. • High throughput DFT screening of 3d, 4d, and 5d transition metals dual atom catalysts (DAC) is performed. • HER taking place within the van der Waals gap of V2N8 and Co2N8, with overpotentials at 0.10 V and 0.16 V. • ORR occurring on Fe2N8 and OER on Ir2N8 surfaces, with overpotentials at 0.39 V and 0.37 V, respectively. • Co2N8 identified as suitable for syngas (CO+H 2) production, displaying the CO 2 RR overpotential of 0.85 VRHE and HER overpotential of 0.16 V. Single- and dual-atom catalysts (SACs and DACs) on single-layer graphene are widely investigated for a wide range of electrochemical reactions. However, the effect of van der Waals interactions on the activity of these catalysts has not been investigated through systematic high-throughput screening. Here we introduce the concept of van der Waals interactions through a double-layer DAC structure which has axial d orbital modification towards enhanced CO 2 reduction reaction (CO 2 RR), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER). We applied density functional theory (DFT) to screen 3d, 4d, and 5d transition metals supported by double-layer nitrogen-doped graphene, denoted as M2N8. We sought catalysts with high thermodynamic and electrochemical stabilities along with low overpotentials for CO 2 RR, ORR, OER, or HER. We find that HER can take place inside the van der Waals gap of V2N8 and Co2N8 leading to overpotentials of 0.10 and 0.16 V. Moreover, ORR and OER can take place on the surface of Fe2N8 and Ir2N8, respectively, leading to overpotentials of 0.39 and 0.37 V. DFT predicts a CO 2 RR overpotential of 0.85 V towards CO on the surface of Co2N8 along with the HER overpotential of 0.16 V inside the van der Waals gap of Co2N8 towards the production of syngas (CO+H 2). This paper provides fundamental insights into the design of advanced multi-layer catalysts by applying the concept of van der Waals interactions for electrochemistry at room temperature. [Display omitted] [ABSTRACT FROM AUTHOR]

We present comprehensive computational and experimental studies on the mechanism of an asymmetric photoredox/Pd dual-catalytic reductive C(sp3)–C(sp3) homocoupling of allylic electrophiles. In stark contrast to the canonical assumption that photoredox promotes bond formation via facile reductive elimination from high-valent metal–organic species, our computational analysis revealed an intriguing low-valent allylpalladium pathway that features tandem operation of Pd(0/II/I)–Pd(0/II/I/II) cycles. Specifically, we propose that (i) the photoredox/Pd system enables the in situ generation of allyl radicals from low-valent Pd(I)-allyl species, and (ii) effective interception of the fleeting allyl radical by the chiral Pd(I)-allyl species results in the formation of an enantioenriched product. Notably, the cooperation of the two pathways highlights the bifunctional role of Pd(I)-allyl species in the generation and interception of transient allyl radicals. Moreover, the mechanism implies divergent substrate-activation modes in this homocoupling reaction, suggesting a theoretical possibility for cross-coupling. Combined, the current study offers a novel mechanistic hypothesis for photoredox/Pd dual catalysis and highlights the use of low-valent allylpalladium as a means to efficiently intercept radicals for selective asymmetric bond constructions.

Preparation of a redox-frustrated high-energy-density energetic material is achieved by gentle protolysis of Mn[N(SiMe3)2]2with the perchlorate salt of the tetrazolamide [H2NtBuMeTz]ClO4(Tz = tetrazole), yielding the Mn6N6hexagonal prismatic cluster, Mn6(μ3-NTztBuMe)6(ClO4)6. Quantum mechanics-based molecular dynamics simulations of the decomposition of this molecule predict that magnetic ordering of the d5Mn2+ions influences the pathway and rates of decomposition, suggesting that the initiation of decomposition of the bulk material might be significantly retarded by an applied magnetic field. We report here experimental tests of the prediction showing that the presence of a 0.5 T magnetic field modulates the ignition onset temperature by +10.4 ± 3.9 °C (from 414 ± 4 °C), demonstrating the first example of a magnetically modulated explosive.

We provide the rational design of dual atom catalysts (DACs) supported on nitrogen-doped graphene for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) through high-throughput computational screening of M1M2N6-DAC systems, where M1and M2represent Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, or Pt metals. We predict that FeRuN6-DAC at the summit of the volcano plot exhibits a low theoretical ORR overpotential (ηORR) of 0.24 V and a low theoretical OER overpotential (ηOER) of 0.19 V. The low ηORRand ηOERresult from the catalytic performance of the Fe site being tuned to electronic properties that facilitate adsorption and desorption of the OH* intermediate. Inspired by these hybrid density functional theory (DFT) computational and machine learning (ML) results, we synthesized FeRuN6-DAC, FeN4-SAC, and RuN4-SAC and characterized them using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and in-situelectron spin resonance (ESR). Our in-situESR spectroscopy signifies that the spin of the Fe active site increases with increasing applied potential due to the increase in the concentration of OH* intermediate on Fe. We verified experimentally the predicted catalytic performances, finding that FeRuN6-DAC leads to an experimental ORR overpotential of 0.29 V with a Tafel slope of 104 mVdec−1and an OER overpotential of 0.27 V with a Tafel slope of 124 mVdec−1. The rechargeable Zinc-air battery setup was fabricated with FeRuN6-DAC in place of the cathode, showing a maximum power density of 0.45 W/cm2at the current density of 0.44 A/cm2and good stability after 120 cycles. According to our findings, we demonstrate that DFT-guided strategies are useful for designing advanced DACs applicable to ORR, OER, and Zinc-air battery applications.

We report a combined experimental/theoretical approach to studying heterogeneous gas/solid catalytic processes using low-pressure pulse response experiments achieving a controlled approach to equilibrium that combined with quantum mechanics (QM)-based computational analysis provides information needed to reconstruct the role of the different surface reaction steps. We demonstrate this approach using model catalysts for ammonia synthesis/decomposition. Polycrystalline iron and cobalt are studied via low-pressure TAP (temporal analysis of products) pulse response, with the results interpreted through reaction free energies calculated using QM on Fe-BCC(110), Fe-BCC(111), and Co-FCC(111) facets. In TAP experiments, simultaneous pulsing of ammonia and deuterium creates a condition where the participation of reactants and products can be distinguished in both forward and reverse reaction steps. This establishes a balance between competitive reactions for D* surface species that is used to observe the influence of steps leading to nitrogen formation as the nitrogen product remains far from equilibrium. The approach to equilibrium is further controlled by introducing delay timing between NH3and D2which allows time for surface reactions to evolve before being driven in the reverse direction from the gas phase. The resulting isotopic product distributions for NH2D, NHD2, and HD at different temperatures and delay times and NH3/D2pulsing order reveal the role of the N2formation barrier in controlling the surface concentration of NHx* species, as well as providing information on the surface lifetimes of key reaction intermediates. Conclusions derived for monometallic materials are used to interpret experimental results on a more complex and active CoFe bimetallic catalyst.

Electrochemical CO2reduction over Cu could provide value-added multicarbon hydrocarbons and alcohols. Despite recent breakthroughs, it remains a significant challenge to design a catalytic system with high product selectivity. Here we demonstrate that a high selectivity of ethylene (55%) and C2+products (77%) could be achieved by a highly modular tricomponent copolymer modified Cu electrode, rivaling the best performance using other modified polycrystalline Cu foil catalysts. Such a copolymer can be conveniently prepared by a ring-opening metathesis polymerization, thereby offering a new degree of freedom for tuning the selectivity. Control experiments indicate all three components are essential for the selectivity enhancement. A surface characterization showed that the incorporation of a phenylpyridinium component increased the film robustness against delamination. It was also shown that its superior performance is not due to a morphology change of the Cu underneath. Molecular dynamics (MD) simulations indicate that a combination of increased local CO2concentration, increased porosity for gas diffusion, and the local electric field effect together contribute to the increased ethylene and C2+product selectivity.

Pt is the best catalyst for the oxygen reduction reactions (ORRs), but it is far too slow. Huang and co-workers showed that dealloying 5 nm Ni7Pt3nanowires (NW) led to 2 nm pure Pt jagged NW (J-PtNW) with ORRs 50 times faster than Pt/C. They suggested that the undercoordinated surface Pt atoms, mechanical strain, and high electrochemically active surface area (ECSA) are the main contributors. We report here multiscale atomic simulations that further explain this remarkably accelerated ORR activity from an atomistic perspective. We used the ReaxFF reactive force field to convert the 5 nm Ni7Pt3NW to the jagged 2 nm NW. We applied quantum mechanics to find that 14.4% of the surface sites are barrierless for Oads+ H2Oads→ 2OHads, the rate-determining step (RDS). The reason is that the concave nature of many surface sites pushes the OH bond of the H2Oadsclose to the Oads, leading to a dramatically reduced barrier. We used this observation to predict the performance improvement of the J-PtNW relative to Pt (111). Assuming every surface site reacts independently with this predicted rate leads to a 212-fold enhancement at 298.15 K, compared to 50 times experimentally. The atomic structures of the active sites provide insights for designing high-performance electrocatalysts for ORR.

The limited selectivity of existing CO2reduction catalysts and rising levels of CO2in the atmosphere necessitate the identification of specific structure–reactivity relationships to inform catalyst development. Herein, we develop a predictive framework to tune the selectivity of CO2reduction on Cu by examining a series of polymeric and molecular modifiers. We find that protic species enhance selectivity for H2, hydrophilic species enhance formic acid formation, and cationic hydrophobic species enhance CO selectivity. ReaxFF reactive molecular dynamics simulations indicate that the hydrophilic/hydrophobic modifiers influence the formation of surface hydrides, which yield formic acid or H2. These observations offer insights into how these modifiers influence catalytic behavior at the non-precious Cu surface and may aid in the future implementation of organic structures in CO2reduction devices.

The graph representation of complex materials plays a crucial role in the field of inorganic and organic materials investigations for developing data-centric materials science, such as those using graph neural networks (GNNs). However, the currently prevalent GNN models are primarily employed for investigating periodic crystals and organic small molecule data, yet they still encounter challenges in terms of interpretability and computational efficiency when applied to polymer monomers and organic macromolecules data. There is still a lack of graph representation of organic polymers and macromolecules specifically tailored for GNN models to explore the structural characteristics. The Polymer-unit Graph, a novel coarse-grained graph representation method introduced in study, is dedicated to expressing and analyzing polymers and macromolecules. By incorporating the Polymer-unit Graphinto the GNN models and analyzing the organic semiconductor (OSC) materials database, it becomes possible to uncover intricate structure–property relationships involving branched-chain engineering, fluoridation substitution, and donor–acceptor combination effects on the elementary structure of OSC polymers. Furthermore, the Polymer-unit Graphenables visualizing the relationship between target properties and polymer units while reducing training time by an impressive 98% and minimizing molecular graph representation models. In conclusion, the Polymer-unit Graphsuccessfully integrates the concept of Polymer-unitinto the field of GNNs, enabling more accurate analysis and understanding of organic polymers and macromolecules.

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

We demonstrate the design of graphene-supported dual atom catalysts (DACs) for the four-electron oxygen reduction reaction (ORR), by utilizing the non-bonding interaction of counterpart metals (M) that synergistically tune the electronic properties and catalytic activity of the Fe active site in FeMN6-DAC and FeMN8-DAC systems, where M stands for Fe, Co, Ni, Cu, and Zn. More specifically, for Fe-M distances below 15 Å, the non-bonding interaction is significant, making the system act as the DAC. We predicted that FeNiN6-DAC and FeNiN8-DAC exhibit a low ORR overpotential (ηORR) of 0.28 V and 0.47 V, respectively, which are at the summits of volcano plots. This low ηORRoriginates from the high Bader charge transfer coupled with high spin density at the Fe site in both the FeNiN6-DAC and FeNiN8-DAC systems, which weakens the adsorption of OH* intermediate while enhancing its desorption to H2O. Guided by these density functional theory (DFT) computational results, we synthesized FeCoN8-DAC and FeNiN8-DAC along with N-doped graphene and confirmed their structures with scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR). We verify experimentally the catalytic activities and find that FeNiN8-DAC has the low experimental overpotential of 0.39 V with a Tafel slope of 47 mVdec−1. Based on these results, we propose a DFT-guided strategy to tune the charge transfer and spin population of the active site toward designing DACs for electrochemical ORR.

We apply the machine learning (ML) tool to calculate the Gibbs free energy (ΔG) of reaction intermediates rapidly and accurately as a guide for designing porphyrin- and graphene-supported single-atom catalysts (SACs) toward electrochemical reactions. Based on the 2105 DFT calculation data from the literature, we trained a support vector machine (SVR) algorithm. The hyperparameters were optimized using Bayesian optimization along with 10-fold cross-validation to avoid overfitting. Based on the Shapley Additive exPlanation (SHAP) and permutation methods, the feature importance analysis suggests that the most important parameters are the number of pyridinic nitrogen (Npy), the number of d electrons (θd), and the number of valence electrons of reaction intermediates. Inspired by this feature importance analysis and the Pearson correlation coefficient, we found a linear dependent, simple, and general descriptor (φ) to describe ΔGof reaction intermediates (e.g., ΔGOH*= 0.020φ – 2.190). Using the trained SVR algorithm, ΔGOH*, ΔGO*, ΔGOOH*, ΔGOO*, ΔGH*, ΔGCOOH*, ΔGCO*, and ΔGN2*intermediates are predicted for the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER), and the CO2reduction reaction (CO2RR). The SVR model predicts an ORR overpotential of 0.51 V and an HER overpotential of 0.22 V for FeN4-SAC. Moreover, we used the SVR algorithm for high-throughput screening of SACs, suggesting new SACs with low ORR overpotentials. This strategy provides a data-driven catalyst design method that significantly reduces the costs of DFT calculations while providing the means for designing SACs for electrocatalysis and beyond.

In recent years, two-dimensional metal–organic frameworks (2D MOF) have attracted great interest for their ease of synthesis and for their catalytic activities and semiconducting properties. The appeal of these materials is that they are layered and easily exfoliated to obtain a monolayer (or few layer) material with interesting optoelectronic properties. Moreover, they have great potential for CO2reduction to obtain solar fuels with more than one carbon atom, such as ethylene and ethanol, in addition to methane and methanol. In this paper, we explore how a particular class of 2D MOF based on a phthalocyanine core provides the reactive center for the production of ethylene and ethanol. We examine the reaction mechanism using the new grand canonical potential kinetics (GCP-K) or grand canonical quantum mechanics (GC-QM) computational methodology, which obtains reaction rates at constant applied potential to compare directly with experimental results (rather than at constant electrons as in standard QM). We explain the reaction mechanism underlying the formation of methane and ethylene. Here, the key reaction step is direct coupling of CO into CHO, without the usual rate-determining CO–CO dimerization step observed on Cu metal surfaces. Indeed, the 2D MOF behaves like a single-atom catalyst.

While the complex 7 × 7 structure that arises upon annealing the Si(111) surface is well-known, the mechanism underlying this unusual surface reconstruction has remained a mystery. Here, we report molecular dynamics simulations using a machine-learning force field for Si to investigate the Si(111)-7 × 7 surface reconstruction from the melt. We find that there are two possible pathways for the formation of the 7 × 7 structure. The first path arises from the growth of a faulted half domain from the metastable 5 × 5 phase to the final 7 × 7 structure, while the second path involves the direct formation of the 7 × 7 reconstruction. Both pathways involve the creation of dimers and bridged five-membered rings, followed by the formation of additional dimers and the stabilization of the triangular halves of the unit cell. The corner hole is formed from the joining of several five-member rings. The insertion of atoms below the adatoms to form a dumbbell configuration involves extra atom diffusion or rearrangement during the evolution of triangular halves and dimer formation along the unit cell boundary. Our findings may provide insights for manipulating the surface structure by introducing other atomic species.

The activity of truncated octahedral Pt38clusters as a catalyst in the oxygen reduction reaction (ORR) is investigated viafirst-principles simulations. Three catalytic steps: O2dissociation (O2ads→ 2Oads), O hydration (Oads+ H2Oads→ 2OHads), and H2O formation (OHads+ Hads→ H2Oads) are considered, in which all reactant species are co-adsorbed on the Pt38cluster 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 viaa self-consistent continuum solvation model. Moreover, first-principles molecular dynamics (AIMD) simulations in which the H2O solvent is explicitly described are performed to explore dynamic phenomena such as fast hydrogen transfer viameta-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 H2O 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.

The Rh(i) complexes [(FlDAB)Rh(coe)(TFA)] (1) and [(BOZO)Rh(coe)(TFA)] (2) [FlDAB = N,N-bis-(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene, coe = cyclooctene, TFA = trifluoroacetate, BOZO = bis(2-oxazolin-2-yl)] are efficient catalyst precursors for H/D exchange between arenes and DTFA. Catalyst precursor 1 exhibits a TOF of 0.06 s−1at 150 °C for benzene H/D exchange. DFT calculations revealed that H/D exchange through reversible oxidative addition or internal electrophilic substitution of benzene is a viable pathway.

Stimulated by the recent report by Yaghi and co-workers of hexagonal metal−organic frameworks (MOF) exhibiting reversible binding of up to 7.5 wt % at 77 K and 70 bar for MOF-177 (called here IRMOF-2-24), we have predicted additional trigonal organic linkers, including IRMOF-2-60, which we calculate to bind 9.7 wt % H 2storage at 77 K and 70 bar, the highest known value for 77 K. These calculations are based on grand canonical Monte Carlo (GCMC) simulations using force fields that match accurate quantum mechanical calculations on the binding of H 2to prototypical systems. These calculations were validated by comparison to the experimental loading curve for IRMOF-2-24 at 77K. We then used the theory to predict the effect of doping Li into the hexagonal MOFs, which leads to substantial H 2density even at ambient temperatures. For example, IRMOF-2-96-Li leads to 6.0 wt % H 2storage at 273 K and 100 bar, the first material to attain the 2010 DOE target.

Molecular dynamics simulations have been performed using a Reax force field for C/H/Ni systems to study the structural changes of an Ni 100cluster adsorbed on a carbon platelet. Three different edges of a carbon platelet are considered. We find a complete restructuring of the initial structure of the Ni 100clusters adsorbed on the armchair and zigzag edges. Nonetheless, the mean Ni−Ni bond length hardly changes. Several preferential sites on each of the graphite edges are identified. Diffusion of the entire cluster is found both for adsorption on the basal plane and for binding to a hydrogen terminated graphite edge.

It is anticipated that future nanoelectronic devices will utilize carbon nanotubes (CNT) and/or single graphene sheets (SGS) as the low-level on-chip interconnects or functional elements. Here we address the contact resistance of Cu for higher level on-chip interconnects with CNT or SGS elements. We use first-principles quantum mechanical (QM) density functional and matrix Green’s function methods to show that perfect Cu−SGS contact has a contact resistance of 16.3 MΩ for a one square nanometer contact. Then we analyzed possible improvements in contact resistance through incorporation of simple functional groups such as aryl (−C 6H 4−), acetylene (−CC−), carboxyl (−COO−), and amide (−CONH−), on CNT. We find that all four anchors enhance the interfacial mechanical stabilities and electrical conductivity. The best scenario is −COOH functionalized CNT which reduces the contact resistance to the Cu by a factor of 275 and increases the mechanical stability by 26 times.

1,2-Dithiolane is a promising anchor group for attaching molecules to metal electrodes in molecular junction devices. This five-membered cyclic disulfide adsorbs on Au surfaces either in a cyclicfashion (with its disulfide bond intact, via molecular adsorption) or in an acyclicfashion (with its disulfide bond broken, via dissociative adsorption). Our density functional theory calculations show that the dissociative adsorption is slightly preferred, but both are stable. We also report nonequilibrium Green’s function calculations showing that molecular junctions of cyclic and acyclic 1,2-dithiolanes sandwiched between two gold electrodes exhibit essentially the same insulating current–voltage characteristics at moderate bias voltages, despite the significant difference in their states of adsorption.

Using molecular dynamics (MD) simulations, we show that metal−organic frameworks (MOFs) constructed using octahedral Zn4O(CO2)6clusters linked via aromatic carbon ring structures lead to negative thermal expansion (NTE) behavior (from 0 K to melting). We find that MOF-C22 contracts volumetrically by 1.9% over the range of 0 to 600 K, making it one of the best NTE materials (linear expansion coefficient of −11.05 × 10-6K-1compared with −9.1 × 10-6K-1found for ZrW2O8,previously the champion NTE material). Indeed, we designed a new MOF using 2-butynediodate linkers that leads to an even larger NTE of 2.2% (from 0 to 500 K). We show that this NTE behavior arises because thermal motions in the rigid Zn−O clusters and the organic moieties linking them lead to tilting of the linkers by successively larger amounts from their alignment along the unit cell axes, resulting in decreased cell parameters. The MOF materials were developed to provide a large reversible hydrogen-storage capacity leading to as much as 73% free volume. However, the NTE properties suggest other possible applications. Thus, their porous but constrained three-dimensional framework provides a framework onto which other materials might blend to form composites with negligible volume change with temperature. To illustrate this, we incorporated polyethylene polymers into MOF-C10 and found that the volume of the composite is constant within 0.059% over the entire range from 300 to 600 K.

We investigate a new molecular architecture in which water-soluble dendrimers are grafted onto a linear polymer for application to polymer electrolyte membrane fuel cells (PEMFC). Using full-atomistic molecular dynamics simulations, we examined the nanophase segregation and transport properties in hydrated membranes with this new architecture. In order to determine how the nature of the linear polymer backbone might affect membrane properties, we considered three different types of linear polymers, poly(epichlorohydrin) (PECH), polystyrene (PS), and poly(tetrafluoroethylene) (PTFE), each in combination with the second-generation sulfonic polyaryl ether dendrimer to form PECH-D2, PS-D2, and PTFE-D2. Our simulations show that the extent of nanophase segregation in the membrane increases in the order of PECH-D2 (~20 Å) < PS-D2 (~35 Å) < PTFE-D2 (~40 Å) at the same water content, which can be compared to ~30-50 Å for Nafion and ~30 Å for Dendrion at the same water content. We find that the structure and dynamics of the water molecules and transport of protons are strongly affected by the extent of nanophase segregation and water content of the membrane. As the nanophase-segregation scale increases, the structure in water phase, the water dynamics, and the proton transport approach those in bulk water. On the basis of the predicted proton and water transport rates, we expect that the PTFE-D2 may have a performance comparable with Nafion and Dendrion.

Density functional theory (DFT) with the B3LYP exchange-correlation functional was used to study new linkages between electrodes and molecular switches (alligator-clip compounds) for molecular electronics using Pt electrodes. Starting with the commonly used molecule 3-methyl-1,2-dithiolane (MDTL), which forms a five-membered ring structure in the gas phase, we found the most stable structure of the adsorbed MDTL to be the ring-opened molecule (32.44 kcal/mol) with each S atom bound to a surface bridge position. Afterward we calculated binding energies and geometries for a variety of different compounds:? S/O-based (oxathiolanes), O-based (methanol), N-based (imidazole, 1,2,3-triazole, purine, 2,4-diazapentane), and P-based molecules (methylphosphino, PCH3, 3-methyl-1,2-diphospholane before (MDPL) and after H dissociation (Hdiss-MDPL)). Among these alternative linkage molecules we find that only the P-based compounds lead to much higher binding energies than MDTL. The best compromise between strong surface attachment and mechanical stability provide the MDPL molecules. For the cis-ring-closed structure of MDPL a binding energy of 47.72 kcal/mol was calculated and even 54.88 kcal/mol for the ring-opened molecule. In the case of H-dissociative adsorption, which leads to Hdiss-MDPL, both binding energies increase to 53.74 (ring-closed) and 74.99 kcal/mol (ring-opened). Thus, MDPL provides a much more stable link to the metal surface and might increase the conductance between molecular switch and electrode. In addition, the minor differences in charge and spin-density distribution between MDTL and MDPL might indicate similar properties for the attachment of the molecular switch to either of both alligator-clip compounds.

The more electron-rich, thermally, air, and protic stable, cyclometalated Pt(II)(NNC) trifluoroacetate complex (3) (NNC 3-6-phenyl-4,4‘-di-tert-butyl-2,2‘-bipyridine) was synthesized with the expectation that it would be less susceptible to H2O inhibition than the Pt(bpym)(TFA)2system (bpym 2-2,2‘-bipyrimidine) for the catalytic oxidation of hydrocarbons. Complex 3was found to catalyze the H/D exchange between benzene and trifluoroacetic acid via CH activation but at a rate slower than the Pt(bpym) complex. Experimental and theoretical studies show that while the use of the more electron-rich NNC ligand motif lowered the Hfor substrate coordination relative to the Pt(bpym) system, a larger increase in the barrier for CH cleavage led to an increase in the overall barrier for CH activation.

We recently reported the COSMO−SAC−BP model for predicting vapor pressure and its temperature derivative, the enthalpy of vaporization. This COSMO−SAC−BP model, which contains no compound specific parameters, is based on determining three major solvation components:  (i) an electrostatic contribution, calculated using the quantum mechanical COSMO (conductor-like-screening-model) method with a statistical mechanical correction for solution nonideality (deviation from a perfect conductor); (ii) a dispersion contribution, obtained from a mean field treatment; and (iii) a cavity formation contribution determined from thermodynamic perturbation theory. This COSMO−SAC−BP model was previously validated to successfully correlate normal boiling point temperatures and enthalpies of vaporization for 369 molecules. In this present study, we have extended the COSMO−SAC−BP model to describe large and more-complex molecules, including pollutants, herbicides, insecticides, and drugs,. The average absolute deviation in the predicted boiling points of these complex molecules, which spans the range of 266−708 K is 17.8 K, or 3.7%. This is similar to the value of 3.2% that was obtained for the 369 molecules in the earlier study, indicating that this method can be applied well outside the systems used to train the model. More importantly, we report here the predicted the normal boiling temperatures for 10 pesticides for which no experimental data are available. This illustrates the advantage to the COSMO−SAC−BP model:  predicting several properties for a wide variety of molecules simultaneously in a unified framework with few parameters (unlike group contribution methods (or quantitative structure−property relationships).

In this paper we use first principles quantum mechanical methods (B3LYP flavor of density functional theory) to examine the mechanism of selective oxidation and ammoxidation of propene by BiMoOxcatalysts. To do this we use finite clusters chosen to mimic likely sites on the heterogeneous surfaces of the catalysts. We conclude that activation of the propene requires a Bi(V) site while all subsequent reactions involve di-oxo Mo(VI) sites adjacent to the Bi. We find that two such Mo sites are required for the most favorable reactions. These results are compatible with current experimental data. For ammoxidation, we conclude that ammonia activation would be easier on Mo(IV) rather than on Mo(VI). Ammonia would be activated more easily for more reducing condition. Since ammonia and propene are reducing agents, higher partial pressures of them could accelerate the ammonia activation. This is consistent with the kinetic model of ammoxidation proposed by Grasselli and coworkers that imido sites (Mo=NH) are more abundant in higher partial pressures of feed. Our calculations also indicate that allyl groups produced as a result of the hydrogen abstraction from propenes would be adsorbed more easily on imido groups (Mo=NH) than on oxo groups (Mo=O) and that the spectator oxo effect is larger than spectator imido effect. Thus, we propose that the best site for ammoxidation (at least for allyl adsorption) is the imido group of the “oxo–imido” species.

Abstract.: We present a general approach for describing chemical processes (bond breaking and bond formation) in materials using force fields (FF) that properly describe multiple bonds at small distances while describing nonbond (Coulomb and van der Waals) interactions at long distances. This approach is referred to as the generalized extended empirical bond-order dependent FF. In this paper we use the Brenner empirical bond-order dependent FF for the short-range interactions and report applications on the energetics and structures of graphite crystal, dynamics of molecular crystals, and distortions of bucky tubes.

Not Available

In hydrogen production, the anodic oxygen evolution reaction (OER) limits the energy conversion efficiency and also impacts stability in proton-exchange membrane water electrolyzers. Widely used Ir-based catalysts suffer from insufficient activity, while more active Ru-based catalysts tend to dissolve under OER conditions. This has been associated with the participation of lattice oxygen (lattice oxygen oxidation mechanism (LOM)), which may lead to the collapse of the crystal structure and accelerate the leaching of active Ru species, leading to low operating stability. Here we develop Sr–Ru–Ir ternary oxide electrocatalysts that achieve high OER activity and stability in acidic electrolyte. The catalysts achieve an overpotential of 190 mV at 10 mA cm–2and the overpotential remains below 225 mV following 1,500 h of operation. X-ray absorption spectroscopy and 18O isotope-labeled online mass spectroscopy studies reveal that the participation of lattice oxygen during OER was suppressed by interactions in the Ru–O–Ir local structure, offering a picture of how stability was improved. The electronic structure of active Ru sites was modulated by Sr and Ir, optimizing the binding energetics of OER oxo-intermediates.

Not Available