Activation Barrier Research Articles

Understanding Activity and Stability of Ru1-XMxO2 Oxygen Evolution Reaction Catalyst Surfaces (M = Ti, Zr): Theory and Experiments

Proton-exchange membrane water electrolyzers, pivotal for sustainable hydrogen production, encounter challenges in the oxygen evolution reaction (OER) marked by high overpotential and limited electrocatalyst stability. This study delves into the computational exploration of two metal substitutions, titanium (Ti) and zirconium (Zr), within ruthenium oxide (RuO2), aiming to enhance catalytic stability. The investigation encompasses nanoscale titanium-substituted and zirconium-substituted rutile RuO2, (Ru1-xMxO2) and evaluating their atomic and electronic structures, OER activity, and dissolution behavior.In the case of Ti substitution, varying concentrations (x = 0–50 at %) were systematically investigated. Our experiments and theoretical analyses elucidate concentration-dependent effects on the electronic structure, showcasing shifts in the d-band and O2p band centers. Titanium substitution induces electron accumulation and depletion regions on the surface, influencing OER activity and stability. Electrochemical testing corroborates the theory, demonstrating that lower Ti concentrations (12.5 and 20 at %) enhance catalyst stability and mitigate ruthenium dissolution. Specifically, Ti substitution influences reaction pathways, with penta-coordinated Ru sites exhibiting the lowest barriers for OER, while hexa-coordinated Ru sites facilitate lower energetic barriers for dissolution.Simultaneously, the study extends to the substitution of Zr within rutile RuO2. Computational and experimental analyses reveal anisotropic shifts in the rutile unit cell, dependent on Zr concentration. Zr substitution induces alterations in the electronic structure, impacting the surface density of states and electron density distribution. Experimentally, lower OER activity is observed for Zr-substituted RuO2 compared to pure RuO2, consistent with computational averages across all sites. However, specific sites with Zr substitution exhibit higher theoretical OER activity. Dissolution analyses underscore the complexity of interactions, with Zr affecting activation barriers for Ru dissolution, thus influencing OER stability.Notably, the dissolution behavior emerges as a critical aspect of this investigation. In Ti-substituted RuO2, experimental accelerated durability testing showcases enhanced stability and reduced Ru dissolution at low and intermediate Ti concentrations. Computational analyses corroborate these findings, indicating site-specific effects on both OER activity and stability. Conversely, Zr-substituted RuO2 exhibits lower Ru dissolution compared to commercial RuO2, emphasizing the role of Zr in impeding Ru dissolution while contributing to surface restructuring.In conclusion, this comprehensive exploration of Ti and Zr substitutions within ruthenium oxide advances our understanding of their effects on atomic and electronic structures, OER activity, and dissolution dynamics. The intricate interplay between specific sites, metal concentrations, and substituents provides valuable insights for designing high-performance, cost-effective OER nanoscale electrocatalysts. The findings underscore the importance of considering both activity and stability in the pursuit of efficient and durable electrocatalysts for sustainable hydrogen production.

Understanding the Degradation Mechanism for NiMo Composite and Effect of Ionomer on the Activity Towards HER

Hydrogen is an essential commodity for achieving cleaner and greener energy.1,2 Due to its high gravimetric energy density, hydrogen is favored as a carbon-neutral fuel.2,3 Moreover, the electrochemical accessibility of hydrogen evolution and oxidation reactions enable hydrogen to be used as a reversible fuel, as in hydrogen battery anodes and unitized reversible fuel cells.4,5 Platinum group metals (PGMs) are excellent catalyst for hydrogen chemistry, as they exhibit negligible activation barrier for HER and HOR, but they are costly and the upstream processes for mining and purification are unsustainable.6 A promising alternative to PGM catalysts would be nonprecious transition metal alloys/composites in form of nanoparticles. These materials are more readily available and lower in cost. Nonetheless, non-precious catalysts developed to date have never met the benchmarks set by PGMs in terms of catalytic activity and stability.Our lab is continuing a long-running effort to develop and improve nickel-molybdenum (Ni–Mo) composites, which stand as one of the strongest candidates for non-PGM HER and the HOR. In prior work, we developed a method to synthesize Ni–Mo nanoparticles supported on oxidized carbon, which increases catalyst dispersion and significantly reduces losses due to interfacial resistivity.7 The catalyst composite has a core@shell structure, where in the core is mainly nickel-rich alloy and the shell is primarily molybdenum-rich oxide.Furthermore, DFT calculations suggest the addition of Mo in the catalyst subsurface weakens hydrogen binding to surface Ni sites, thus increasing the overall activity toward HER/HOR.8 In ongoing work to further develop Ni–Mo/C composites, we are focusing on catalyst degradation under storage and operating conditions. To examine catalyst durability upon extended storage, we measured activity toward hydrogen evolution progressively for a single batch of catalyst powdered retained at room temperature in an aerobic environment over 850 hours. We observed a monotonic decrease in catalytic activity culminating in a >90% decrease in activity. Post-mortem analysis via XRD showed the increased peak intensities corresponding to NiO and MoO3. We further observed that the catalyst remains active over extended periods when stored under a dry, oxygen-free atmosphere. Finally, exposing the degraded catalyst to thermal reduction increased catalytic activity. Together, these results strongly suggest aerobic oxidation as the primary degradation mechanism under shelf-storage, with straightforward mitigations steps readily available by excluding air and water. This finding further suggests that oxide component of the synthesized catalyst is not an active site for hydrogen chemistry, although we speculate that some surface oxide is beneficial for enhancing the rate of water dissociation.9 We also studied the impact of the ionomer chemistry on the catalytic activity of Ni–Mo composites for alkaline anion exchange membrane (AEM) electrolyzer applications. Surprisingly, we observed an ~80% reduction in catalytic activity for Ni–Mo composites formulated into thin film HER catalysts using poly(aryl piperidinium) (PAP) carbonate AEM ionomers relative to control samples based on Na-exchanged Nafion ionomer binders. Further work showed this result was specific to the carbonate form of the ionomer, as HER activity was fully recovered when we used halide forms of the PAP ionomer. Work is ongoing to establish the physicochemical basis of this poisoning effect.Based on these findings, we developed functional AEM electrolyzer assemblies with IrOx anodes and Ni–Mo/C cathodes, where the cathode composition was formulated based on our best results from RDE studies. These MEAs yielded electrolyzer performance properties that were indistinguishable from MEAs we tested using Pt–Ru/C cathodes that were otherwise identical in composition.8 This leads us to conclude that advanced Ni–Mo/C catalyst composites can indeed achieve performance parity with PGM catalysts on the basis of initial catalytic activity, whereas significant work remains to characterize durability under extended operating conditions.

Phase-Field Model of SEI Formation in Li-Metal Batteries

The formation of solid electrolyte interphase (SEI) is critical to the overall performance of lithium batteries. Here we have developed a rather comprehensive phase-field model for predicting the formation and growth of SEI, taking into account of multiple electrochemical/chemical reaction pathways leading to multiple SEI reaction products, diffusion of ionic and molecular species, and electron tunneling. All the SEI products are treated stoichiometric compounds without the typical approximation of parabolic composition dependence in almost all existing phase-field models involving stoichiometric compounds, allowing the direct connection between the phase-field model and atomistic calculations without further approximation. As a model system, we consider a 1-D prototypical half-cell with Li metal anode, liquid ethylene carbonate (EC) electrolyte with 1 molar (M) LiPF6, and SEI products Li2BDC and Li2CO3 under the open-circuit condition. The reduction potentials and activation barriers of the reactions are determined using atomistic simulations while the diffusivities of Li+ and EC in the bulk electrolyte are obtained by performing molecular dynamics simulations. The phase-field simulation is capable of predicting the evolution behavior of SEI products within a wide range of time scales from nanoseconds to a few seconds. It reveals that: (i) a porous Li2BDC layer forms at the anode/electrolyte interface and grows to a stable thickness of several tens of nanometers within ~20 μs; (ii) a porous Li2CO3 layer forms between anode and Li2BDC and grows to ~10 nm by partially consuming the existing Li2BDC layer within ~33 μs; and (iii) Li2CO3 forms a dense layer by filling the pores within ~20 ms. The stage (i) and (ii) are dominated by Li+ diffusion kinetics whereas stage (iii) is controlled by the reaction kinetics. The general computational framework can be extended and applied to the simultaneous evolution of Li-metal deposition and stripping and the SEI evolution.

Effect of the EMIM Cation on the CO2 Reduction Reaction on Gold

The electrochemical CO2 reduction reaction (CO2RR) is emerging as a promising method to produce carbon neutral chemical feedstocks. Additives or co-catalysts in the electrolyte can lower the activation barrier for CO2RR, yielding faster conversion rates at lower voltages. Recently, electrocatalytic CO2 reduction mediated by ionic liquids (IL) has been reported but with conflicting accounts to whether the ILs benefit CO2RR or adversely, the competing hydrogen evolution reaction (HER).We report the effect of added IL EMIM-Cl, specifically EMIM+, in aqueous solutions on the electrochemical CO2RR and competing HER on gold. We show the CO2 reduction current increases by ~1.5x with the highest amount of added EMIM+ relative to the pure electrolyte. Differential pulse voltammetry reveals an additional large change in the reduction current that only appears with both CO2 and EMIM+ present. Lastly, we use advanced electrochemical analysis methods to determine the reaction kinetics and preliminary evidence shows that the standard heterogeneous rate constant increases for the reaction at large overpotentials while the HER is suppressed at low overpotentials with EMIM+ present.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Phosphine Oxide‐Containing Gold(III) Complexes with Tunable Emission Color and Thermally Enhanced Luminescence Behavior

AbstractA series of phosphine oxide‐containing gold(III) complexes with tunable emission colors spanning from sky‐blue to near‐infrared region is reported. This is accomplished by the switching of the excited state characters from intraligand to ligand‐to‐ligand charge transfer through the replacement of the auxiliary ligand from aryl to nitrogen‐based ligands. In addition to high photoluminescence quantum yields in both solution and solid‐state thin films, these complexes exhibit large radiative decay rate constants of the order of 106 s−1, much larger than those commonly found for other gold(III) complexes. The origin of such enhanced performance is believed to be arising from the occurrence of both thermally activated delayed fluorescence and thermally stimulated delayed phosphorescence processes within the emitters. This is probed by ns‐ and fs‐transient absorption spectroscopy, time‐resolved, and temperature‐dependent emission spectroscopy. In particular, the direct observation of the upconversion processes and the determination of the activation barriers are achieved in the variable‐temperature fs‐transient absorption spectroscopic studies. Solution‐processed organic light‐emitting devices with satisfactory external quantum efficiencies of up to 15.2% are achieved, which could be ascribed to the presence of thermally activated delayed fluorescence and/or thermally stimulated delayed phosphorescence processes.

Temperature Effects and Activation Barriers in Aqueous Proton-Uptake Reactions.

Aqueous proton transfer reactions are fundamental in biology and chemistry, yet kinetics and mechanisms of strong base-weak acid reactions are not well understood. In this work, we present a temperature-dependent reaction kinetic study of the water-soluble photobase actinoquinol, in the presence and absence of succinimide, a weak acid reaction partner. We study the temperature dependence of the reaction and connect the observed dynamics to the reaction's thermodynamics. We find that actinoquinol reacts in associated complexes with water/succinimide, creating an intermediate complex that can undergo either dissociation to create products, or reverse proton transfer within the complex to recreate the initial reactants. We find that the intermediates' formation is energetically unfavorable with both reaction partners, which impacts the net reaction rates. We also find that the net reaction rate is additionally strongly influenced by the competition between the dissociation of the intermediates and their reverse reaction.

Cobalt Nanoparticles Supported Active Carbon from Chitosan Biopolymer Using Thermal Method: Synthesis, Characterization, and Hydrogen Production

AbstractActivated carbon based Cobalt nanoparticles (Co@AC NPs), considered in the context of hydrogen energy, which is a renewable and sustainable energy, were synthesized by the hydrothermal method, and their catalytic activities were tested. For this, hydrogen production tests were carried out with the help of sodium borohydride (NaBH4) methanolysis of Co@AC NPs synthesized by the thermal method. Ultraviolet–visible spectroscopy (UV–Vis), Fourier transmission spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray diffraction (XRD) characterization tests were performed. According to the TEM characterization result, it has been observed that the NPs have a spherical shape and an average size of 2.52 ± 0.92 nm. Then, using the catalytic studies, it was observed that hydrogen production’s reusability is found to be 86% . The activation energy (Ea), enthalpy (∆H), and entropy (∆S) values were found to be 20.28 kJ⋅mol−1, 17.74 kJ⋅mol−1, and −125.97 J⋅mol−1 K−1, respectively. The obtained values have yielded excellent results and guide future sustainable and renewable hydrogen energy studies by reducing costs, ensuring environmental sustainability by avoiding the formation of undesirable by-products, and producing hydrogen from NaBH4 through its high catalytic properties.

Co-Pyrolysis of Mushroom Residue Blended with Pine Sawdust/Wheat Straw for Sustainable Utilization of Biomass Wastes: Thermal Characteristics, Kinetic/Thermodynamic Analysis, and Structure Evolution of Co-Pyrolytic Char

Co-pyrolysis technology is considered to be one of the most promising methods for the sustainable utilization of biomass wastes, as it can realize waste reduction and convert wastes into high-value-added products with little impact on the environment. The evaluation of thermal characteristics and product properties is necessary for understanding this technique. In this paper, thermal characteristics and kinetic and thermodynamic analysis during the co-pyrolysis of mushroom residue (MR) with pine sawdust (PS) or wheat straw (WS) were investigated in a TGA. The carbon structure and surface textures of co-pyrolytic char were explored using Raman spectroscopy and a scanning electron microscope. As the PS or WS mass ratio increased, the devolatilization index increased obviously, indicating that volatile release was promoted and concentrated. Weak interactions were observed between 250 and 400 °C during the co-pyrolysis process, which primarily affected the mass transfer, resulting in a change in the thermal decomposition temperatures and rates. The interactions had no prominent influence on the volatiles’ yields. The non-additive performance of average activation energies for the blends was observed due to the interactions, and the lowest average activation energy was obtained when the PS or WS mass ratio was 50%. The lower average pre-exponential factor of the blends indicated the reduced complicacy of the pyrolysis reaction. The relatively small deviation between the activation energy and enthalpy change (4.94–5.18 kJ·mol−1) signified the energy sensitivity of product formation. PS promoted the formation of small aromatic rings (<6 fused rings) in co-pyrolytic chars, whereas WS favored the production of larger rings (≥6 fused rings). The surface textures of the co-pyrolytic chars became porous, and the greater fractal dimensions of the surface morphology for the co-pyrolytic chars indicated that the char surface became irregular and rough.

Investigation of Huisgen's Noble metal catalyst click reaction mechanism for the synthesis of 1,4-disubstituted 1,2,3-triazoles

The current study delves into the mechanistic intricacies of azide-alkyne cycloaddition reactions (MAAC) catalyzed by transition metals (M = Cu, Ag, and Au) bound to N-heterocyclic carbene (NHC) ligands, utilizing Density Functional Theory (DFT) calculations at the MN12-L/Def2-SVP level of theory, with Def2-TZVP for metal atoms, to shed light on the 1,4-regioisomer formation. A detailed comparison between mono- and bi-nuclear catalytic pathways for these metal complexes is presented. Our findings underscore the nuanced role of the metal identity in dictating the reaction's course, with a notable preference for a stepwise mechanism across the board, except in the mononuclear pathway of Au and Ag, which follows a concerted mechanism. The activation energies, analyzed in toluene solvent, underscore the efficiency of the binuclear pathway for all metals, showcasing significantly lower activation barriers (18.76 kcal/mol for Au and Ag, and 8.47 kcal/mol for Cu) compared to the mononuclear route. This distinction highlights the potential for optimizing catalytic performance through careful selection of metal-NHC combinations, providing valuable insights for the design of efficient catalysts in click chemistry applications. The study not only reaffirms the superior efficacy of the binuclear pathway but also enriches our understanding of metal-catalyzed MAAC reactions, contributing to the advancement of catalysis science and offering avenues for the development of novel synthetic strategies in organic chemistry.

Modulating the Microenvironments of Robust Metal Hydrogen‐Bonded Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution

AbstractHydrogen‐bonded organic frameworks (HOFs) are outstanding candidates for photocatalytic hydrogen evolution. However, most of reported HOFs suffer from poor stability and photocatalytic activity in the absence of Pt cocatalyst. Herein, a series of metal HOFs (Co2‐HOF‐X, X=COOMe, Br, tBu and OMe) have been rationally constructed based on dinuclear cobalt complexes, which exhibit exceptional stability in the presence of strong acid (12 M HCl) and strong base (5 M NaOH) for at least 10 days. More impressively, by varying the ‐X groups of the dinuclear cobalt complexes, the microenvironment of Co2‐HOF‐X can be modulated, giving rise to obviously different photocatalytic H2 production rates, following the −X group sequence of −COOMe>−Br>−tBu>−OMe. The optimized Co2‐HOF‐COOMe shows H2 generation rate up to 12.8 mmol g−1 h−1 in the absence of any additional noble‐metal photosensitizers and cocatalysts, which is superior to most reported Pt‐assisted photocatalytic systems. Experiments and theoretical calculations reveal that the −X groups grafted on Co2‐HOF‐X possess different electron‐withdrawing ability, thus regulating the electronic structures of Co catalytic centres and proton activation barrier for H2 production, and leading to the distinctly different photocatalytic activity.

Crystal Structural Characteristics and Electrical Properties of Novel Sol-Gel Synthesis of Ceramic Bi0.75Ba0.25(FeMn)0.5O3.

In this investigation, our primary objective is to explore the structural, morphological, and electrical characteristics of Bi0.75Ba0.25(FeMn)0.5O3 ceramic material synthesized by the sol-gel method. The prepared sample underwent synthesis through the conventional sol-gel technique. Examination through X-ray diffraction (XRD) unveiled a well-defined rhombohedral structure within the R3´C space group. Moreover, to evaluate the purity and nano-grain morphology, we utilized energy dispersive spectroscopy (EDX) and scanning electron microscopy (SEM). Electrical assessments were carried out over a frequency span of 100 Hz to 1 MHz and temperatures ranging from 200 to 340 K. Employing the correlated barrier hopping (CBH) model, we analyzed the AC conductivity of our specimen. The activation energy, determined from both DC conductivity and impedance spectra, demonstrated close correspondence, suggesting that both conductivity and r laxation processes are influenced by similar factors. Notably, the dielectric properties hold significant importance, potentially rendering our sample suitable for electronic applications. Furthermore, we calculated thermodynamic parameters, such as enthalpy (ΔH), entropy change (ΔS), and free energy of activation (ΔF), offering deeper insights into the material's behavior and conductivity mechanisms.

Influence of Carboxylic Acid Structure on the Kinetics of Polyurethane Foam Acidolysis to Recycled Polyol.

Closed-loop recycling of plastics is needed to bridge the gap between the material demands imposed by a growing global population and the depletion of nonrenewable petroleum feedstocks. Here, we examine chemical recycling of polyurethane foams (PUFs), the sixth most produced polymer in the world, through PUF acidolysis via dicarboxylic acids (DCAs) to release recyclable polyols. Acidolysis enables recycling of the polyol component of PUFs to high-quality materials, and while the influence of DCA structure on recycled PUF quality has been reported, there are no reports that examine the influence of DCA structure on the kinetics of polyol release. Here, we develop quantitative relationships between DCA structure and PUF acidolysis function for ∼10 different DCA reagents. PUF acidolysis kinetics were quantified with ∼1 s time resolution using the rate of carbon dioxide (CO2) gas generation, which is shown to occur concomitantly with polyol release. Pseudo-zeroth-order rate constants were measured as a function of DCA composition, reaction temperature, and DCA concentration, and apparent activation barriers were extracted. Our findings demonstrate that DCA carboxyl group proximity and phase of transport are descriptors of PUF acidolysis rates, rather than expected descriptors like pK a. DCAs with closer proximity acid groups exhibited faster PUF acidolysis rate constants. Furthermore, a shrinking core mechanism effectively describes the kinetic functional form of the kinetics of PUF acidolysis by DCAs. Measurements of acidolysis kinetics for model PUF (M-PUF) and end-of-life PUF (EOL PUF) confirm the applicability of our analysis to postconsumer materials. This work provides insights into the physical and chemical mechanisms controlling acidolysis, which can facilitate the development of efficient closed-loop PUF chemical recycling schemes.

Strain-dependence of Te interstitial diffusion in CdTe

While the dominant defects which control non-radiative recombination and long-range interstitial diffusion in CdTe correspond to Cd vacancies and Te anti-sites, the short-range diffusion of Te and Se interstitials between these defects is also of interest, since they both play a role in defect passivation. In addition, since CdTe thin films are typically polycrystalline and may also involve interfaces with materials with different lattice constants, the effects of strain are also of interest. Here we present the results of molecular dynamics (MD) simulations of Te interstitial diffusion in zincblende CdTe for values of the triaxial strain ranging from −2% (compressive) strain to +2.8% (tensile) strain. By carrying out MD simulations of Te interstitial diffusion over a range of temperatures, and then carrying out Arrhenius fits, we have determined the effective activation barrier Ea and prefactor D 0 for each value of the global strain. We find that both Ea and D 0 exhibit non-monotonic behavior, increasing with both compressive and tensile strain. We also present an analysis of the key diffusion pathways for 3 different values of the strain which explains the non-monotonic strain dependence obtained in our simulations. Our results also indicate that in each case, the diffusion of interstitial Te involves a variety of concerted events with a wide range of activation barriers.

Diastereoselective Synthesis of Sulfoxide-Functionalized N-Heterocyclic Carbene Ruthenium Complexes: An Experimental and Computational Study.

The synthesis of sulfoxide-functionalized NHC ligand precursors were carried out by direct and mild oxidation from corresponding thioether precursors with high selectivity. Using these salts, a series of cationic [Ru(II)(η6-p-cymene)(NHC-SO)Cl]+ complexes were obtained in excellent yields by the classical Ag2O transmetallation route. NMR analyses suggested a chelate structure for the metal complexes, and X-ray diffractometry studies of complexes 4 b, 4 c, 4dBArF and 4 e unambiguously confirmed the preference for the bidentate (κ2-C,S) coordination mode of the NHC-SO ligands. Interestingly, only one diastereomer, in the form of an enantiomeric pair, was observed both in 1H NMR and in the solid state for the complexes. DFT calculations showed a possible intrinsic energy difference between the two pairs of diastereomer. The calculated energy barriers suggested that inversion of the sulfoxide is only plausible from the higher energy diastereomer together with bulky substituents. Inverting the configuration at the Ru center instead shows a lower and accessible activation barrier to provide the most stable diastereomer through thermodynamic control, consistent with the observation of a single species by 1H NMR as a pair of enantiomers. All these complexes catalyse the β-alkylation of secondary alcohols. Complex 4dPF6 bearing an NHC-functionalised S-Ad group has been further studied with different primary and secondary alcohols as substrates, showing high reactivity and high to moderate β-ol-selectivities.

Molecular dissociation of nitric oxide (NO) on VO2(010) surface : A DFT study including vdW forces

The adsorption and coadsorption of O and N atomic species, as well as the dissociation of NO molecule on VO2(010) surface have been studied using the density functional theory (DFT) with GGA-PBE as the exchange–correlation, including van der Waals (vdW) corrections of the form PBE+vdW-DF2. We found that the short-bridge (sb) site is the energetically most stable for both N and O atoms. The inclusion of PBE+vdW-DF2 corrections does not change the preferred adsorption site, however it reduces the value of the adsorption energy relative to ordinary GGA-PBE functional. Also, while the GGA-PBE functional results in finite magnetization on the O and N atoms when adsorbed on the VO2 surface, the inclusion of the vdW forces results in zero magnetization of the atoms. In addition, our Löwdin charge transfer analysis shows electron transfer from the VO2(010) surface to the O atom, which is further supported by an accumulation of charge density around the O atom as well as the charge depletion from the V surface atoms. At a lower coverage θ= 0.25 ML, the N atom behaves like a charge acceptor, that is, electrons are transferred from the VO2 surface to it. At a higher coverage θ= 0.50 ML and 0.75 ML, the N atom behaves like a charge donor, i.e., there is a transfer of electronic charges to the VO2 surface. We emphasize that charge transfer between the adsorbate (O and N) and the surface establishes adsorbate-surface bonding which enhances the stability of the adsorbate. Furthermore, our nudged elastic band calculations (NEB) for determining the minimum energy path (MEP) and activation barrier energy (Ea) for NO dissociation into N and O atoms show Ea= 2.18 eV (1.78 eV) as obtained using the GGA-PBE (PBE+vdW-DF2) functionals. Regarding potential application, this study thus contributes to reducing the amount and harmful effect of NO gas in the atmosphere.

How Domain Segregation in Ionic Liquids Stabilizes Nanoparticles and Establishes Long-Range Ordering─A Computational Study.

Due to their physical properties including high thermal stability, very low vapor pressure, and high microwave absorption, ionic liquids have attracted great attention as solvents for the synthesis of nanomaterials, being considered as greener alternatives to traditional solvents. While usual solvents often need additives like surfactants, polymers, or other ligands to avoid nanoparticle coalescence, some ionic liquids can stabilize nanoparticles in dispersion without any additive. In order to quantify how the ionic liquids can affect both the aggregation thermodynamics and kinetics, molecular dynamics simulations were performed to simulate the evolution of concentrated dispersions and to compute the potential of mean force between nanoparticles of both hydrophilic and hydrophobic natures in two imidazolium-based ionic liquids, which differ from each other by the length of the cation alkyl group. Depending on the nature of the nanoparticle, structured layers of the polar and apolar regions of the ionic liquid can be formed close to its surface, and those layers lead to activation barriers for dispersed particles to get in contact. If the alkyl group of the ionic liquid is long enough to lead to domain segregation between the ionic and apolar portions of the solvent, the layered structure around the particle becomes more structured and propagates several nanometers away from its surface. This leads to stronger barriers close to the contact and also multiple barriers at larger distances that result from the unfavorable superposition of solvent layers of opposing nature when the nanoparticles approach each other. Those long-range solvent-mediated forces not only provide kinetic stability to dispersions but also affect their dynamics and lead to a long-range ordering between dispersed particles that can be explored as a template for the synthesis of complex materials.

Phthalazine as a Diene in Diels-Alder Reactions With P- and As-Containing Anionic Dienophiles: Comparison of Possible Reaction Channels.

Phthalazine can behave as a diene in Diels-Alder (DA) cycloadditions, typically at the pyridazine ring, however, its application is somewhat limited because these reactions usually require harsh conditions or sophisticated catalysts. As an unconventional example, phthalazine was reported to undergo cycloaddition with the [PCO]- anion without any catalyst. In this computational study, we scrutinise the mechanism of the DA reactions between phthalazine and the so far known [ECX]- (E: P, As; X: O, S, Se) anions as dienophiles. In principle, the attack of an [ECX]- anion may occur at two different sites of phthalazine, either at the benzene or the pyridazine ring, and both of these possible reaction channels were juxtaposed on the basis of energetic aspects. In all of the investigated cases, the analysis of the energy profiles reveals a clear regioselectivity that favours the attack at the pyridazine ring. As a result, so far unprecedented 2-pnictanaphth-3-olate analogues seem achievable as final products. Comparing the characteristics of these pathways allowed us to clarify the source of this regioselectivity: The pyridazine ring of phthalazine exhibits lower aromaticity than the benzene subring; therefore, in the DA step, the former ring shows a higher affinity toward a dienophile than the latter, leading to lower activation barriers. To further map the electronic and structural features of the cycloaddition steps, the local interactions evolving in the transition states were analysed and compared using global and local descriptors. In most aspects, the characteristics of both pathways were found to be rather similar, in contrast to the markedly differing activation barriers on the two routes.

Impact of Sn Lewis Acid Sites on the Dehydration of Cyclohexanol.

The impact of Sn on the concentration and strength of acid sites in Al containing zeolites with MFI topology and their catalytic activity for the dehydration of cyclohexanol in the aqueous phase has been investigated. The materials maintain constant Al concentrations and consequently Bro̷nsted acid site (BAS) concentrations, while exhibiting an increasing concentration of Sn Lewis acid sites (LAS). The presence of water alters LAS(Sn), leading to weak BAS(Sn) that increases the concentration of water in the zeolite micropore, while leaving the rate of dehydration of cyclohexanol unchanged. The TOF increases with the concentration of BAS(Al) in close contact with framework LAS(Sn), referred to as BAS(Pair). The increase in the Arrhenius pre-exponential factor, without affecting the activation barrier (E a), leads to the hypothesis that the proximity of both sites allows for a later transition state induced by the polarization of the C-O bond, leading in turn to a higher transition entropy.

Self-Diffusion of Ge in Amorphous Ge x Si1-x Films Studied In Situ by Neutron Reflectometry.

Ge x Si1-x alloys are gaining renewed interest for many applications in electronics and optics, especially for miniaturized devices showing quantum size effects. Point defects and atomic diffusion play a crucial role in miniaturized and metastable systems. In the present work, Ge self-diffusion in sputter deposited amorphous Ge x Si1-x alloys is studied in situ as a function of Ge content x = 0.13, 0.43, 0.8, and 1.0 by neutron reflectometry. The determined Ge self-diffusivities obey the Arrhenius law in the investigated temperature ranges. The higher the Ge content x, the higher the Ge self-diffusivity at the same temperature. The activation enthalpy decreases with x from 4.4 eV for self-diffusion in pure silicon films to about 2 eV self-diffusion in Ge0.8Si0.2 and Ge. The decrease of the activation enthalpy for amorphous Ge x Si1-x is similar to the case of crystalline Ge x Si1-x . Possible explanations are discussed.

Hydrogenation of hexene catalyzed by a ruthenium (II) complex with N‐heterocyclic carbene ligands

AbstractIn this study, we investigated the mechanism of the inactivated hexene hydrogenation reaction catalyzed by a ruthenium (II) complex containing “N‐heterocyclic carbene” (NHC) ligands, specifically SIMes and CBA, using DFT calculations. Our focus was on RuH(OSO2CF3)(CO)(SIMes)(CBA), which exhibits excellent catalytic behavior. We tested the B3LYP‐D3, cam‐B3LYP, and TPSSh functionals. The hydrogenation reaction is initiated by the release of SIMes rather than CBA due to the lower associated dissociation energy. Our findings indicate a reaction mechanism consisting of two consecutive steps, each involving one hydrogen atom migration. The first step, considered as the kinetically limiting transition state, exhibits a Gibbs free activation barrier of 12.9 kcal mol−1. This step involves two asynchronous processes. The first one describes the migration of the ruthenium hydride to the internal carbon of the olefine function, transitioning from π to σ coordination mode, which promotes the formation of a bond between ruthenium and the terminal olefinic carbon. The second process involves the oxidation of ruthenium from Ru(II) to Ru(IV). This oxidation is crucial as it enables the decomposition of the H2 molecule into two hydrogen atoms bonded to the ruthenium atom. The geometrical structures of the Hidden Reaction Intermediate Ru(II) complex and the quasi‐transition state of the second process have been determined by means of the RIRC technique. The second step entails the migration of one of the newly formed hydrides of the Ru(IV) complex to the terminal olefinic carbon, resulting in the release of hexane with a weak activation Gibbs free energy of .8 kcal mol−1. Lastly, we explored the use of dichloromethane as a solvent, considering the PCM model. The presence of the solvent significantly decreases the energy dissociation of SIMes from 17.9 to 9.0 kcal mol−1, providing notable benefits.