Low Activation Barrier Research Articles

Metal–semiconductor transition in bilayer graphene by bowl inversion of monofluorosumanene

The bowl-shaped hydrocarbon molecule, monofluorosumanene (C21H11F), can act as a molecular switch to control the carrier density of bilayer graphene by flipping its conformation. Our calculations indicate that monofluorosumanene, in which F atom is located outside the curved C–C network (exo-F molecular conformation), induces electron and hole co-doping of 1.5 × 1013 cm−2 in monofluorosumanene-intercalated bilayer graphene because of a large dipole moment normal to the molecular plane of the monofluorosumanene. The intercalated monofluorosumanene does not affect the electronic structure of bilayer graphene when the F atom is located inside the curved C–C network (endo-F conformation) owing to a small out-of-plane dipole moment. The application of an external electric field across the graphene layers promotes bowl inversion between endo-F and exo-F molecular conformations because of the low activation barrier (approximately 800 meV) between these two conformations and the dipole moment normal to the molecular plane of the exo-F conformation.

Superior electro-catalytic performance achieved by the negatively charged boron atom on BC3/TM/graphene sandwich heterostructures

The sandwich heterostructures (SHSs) are novel two-dimensional materials that hold great potential as efficient electro-catalysts. In this work, we computationally designed the BC3/TM/Gr SHSs by intercalating transition metal atoms into the BC3/graphene heterostructure. After the computational screening, only BC3/Sc/Gr, BC3/Ti/Gr, BC3/Y/Gr and BC3/Zr/Gr are validated as stable SHSs. The electron donation from the intercalated TM atom results in the formation of the negatively charged boron atom (Bδ−) and activation of the BC3 surface, making the BC3/TM/Gr SHSs highly promising as single-atom catalysts (SACs). The BC3/Sc/Gr and BC3/Y/Gr SHSs exhibit potential in carbon dioxide reduction reaction (CO2RR) and carbon monoxide reduction reaction (CORR) electro-catalysis. Particularly, when BC3/Y/Gr SHS serves as CORR electro-catalyst, the step (*CHO→*CHOH) is a potential determining step, with an extremely low limiting potential (UL = −0.10 V). The BC3/Ti/Gr and BC3/Zr/Gr SHSs are suitable as hydrogen evolution reaction (HER) electro-catalysts. Specially, the BC3/Ti/Gr SHS serves as an ideal HER electro-catalyst in acid condition, with close-to-zero adsorption free energy (ΔGH = 0.006 eV) and fairly low overall activation barrier (0.20 eV). By analyzing the electronic properties, the unique adsorption activity of the Bδ− on H atom and unsaturated CO2RR intermediates is elucidated as the origin of excellent catalytic activity of BC3/TM/Gr SHSs, which is modulated by the intercalated TM atom. Our work is instructive to rational design of SACs towards energy conversion based on non-metal elements.

Searching for the Achilles' Heel of Urethane Linkage-An Energetic Perspective.

A sudden increase in polyurethane (PU) production necessitates viable recycling methods for the waste generated. PU is one of the most important plastic materials with a wide range of applications; however, the stability of the urethane linkage is a major issue in chemical recycling. In this work, termination reactions of a model urethane molecule, namely methyl N-phenyl carbamate (MPCate), are investigated using G3MP2B3 composite quantum chemical method. Our main goal was to gain insights into the energetic profile of urethane bond termination and find an applicable chemical recycling method. Hydrogenation, hydrolysis, methanolysis, peroxidation, glycolysis, ammonolysis, reduction with methylamine and termination by dimethyl phosphite were explored in both gas and condensed phases. Out of these chemicals, degradation by H2, H2O2 and CH3NH2 revealed promising results with lower activation barriers and exergonic pathways, especially in water solvation. Implementing these effective PU recycling methods can also have significant economic benefits since the obtained products from the reactions are industrially relevant substances. For example, aniline and dimethyl carbonate could be reusable in polymer technologies serving as potential methods for circular economy. As further potential transformations, several ionizations of MPCate were also examined including electron capture and detachment, protonation/deprotonation and reaction with OH-. Alkaline digestion against the model urethane MPCate was found to be promising due to the relatively low activation energy. In an ideal case, the transformation of the urethane bond could be an enzymatic process; therefore, potential enzymes, such as lipoxygenase, were also considered for the catalysis of peroxidation, and lipases for methanolysis.

Breaking Continuously Packed Bimetallic Sites to Singly Dispersed on Nonmetallic Support for Efficient Hydrogen Production.

We have synthesized Pt1Zn3/ZnO, also termed 0.01 wt %Pt/ZnO-O2-H2, as a catalyst containing singly dispersed single-atom bimetallic sites, also called a catalyst of singly dispersed bimetallic sites or a catalyst of isolated single-atom bimetallic sites. Its catalytic activity in partial oxidation of methanol to hydrogen at 290 °C is found to be 2-3 orders of magnitude higher than that of Pt-Zn bimetallic nanoparticles supported on ZnO, 5.0 wt %Pt/ZnO-N2-H2. Selectivity for H2 on Pt1Zn3/ZnO reaches 96%-100% at 290-330 °C, arising from the uniform coordination environment of single-atom Pt1 in singly dispersed single-atom bimetallic sites, Pt1Zn3 on 0.01 wt %Pt/ZnO-O2-H2, which is sharply different from various coordination environments of Pt atoms in coexisting PtxZny (x ≥ 0, y ≥ 0) sites on Pt-Zn bimetallic nanoparticles. Computational simulations attribute the extraordinary catalytic performance of Pt1Zn3/ZnO to the stronger adsorption of methanol and the lower activation barriers in O-H dissociation of CH3OH, C-H dissociations of CH2O to CO, and coupling of intermediate CO with atomic oxygen to form CO2 on Pt1Zn3/ZnO as compared to those on Pt-Zn bimetallic nanoparticles. It demonstrates that anchoring uniform, isolated single-atom bimetallic sites, also called singly dispersed bimetallic sites on a nonmetallic support can create new catalysts for certain types of reactions with much higher activity and selectivity in contrast to bimetallic nanoparticle catalysts with coexisting, various metallic sites MxAy (x ≥ 0, y ≥ 0). As these single-atom bimetallic sites are cationic and anchored on a nonmetallic support, the catalyst of singly dispersed single-atom bimetallic sites is different from a single-atom alloy nanoparticle catalyst. The critical role of the 0.01 wt %Pt in the extraordinary catalytic performance calls on fundamental studies of the profound role of a trace amount of a metal in heterogeneous catalysis.

Effect of substitution on thermal decomposition of formaldehyde: A theoretical study

Unimolecular decomposition of organic compounds is strongly influenced by substituted functional group. The present work focusses to explore the energetics and kinetics of unimolecular decomposition of formaldehyde and chloro- substituted formaldehydes. Dissociation of molecular systems are observed to compete between molecular elimination reaction and bond dissociation reaction. It is observed that degradation of formaldehyde and formyl chloride ruled by 3-centered concerted mechanism produced H2 and HCl fragments respectively. On the other hand, Cl2 elimination reaction is generated via formation of isohalon structure. Despite of presence of isohalon intermediate, molecular elimination in phosgene required higher energy as compared to formyl chloride. Ease of molecular elimination in case of formyl chloride is attributed to favourable electrostatic interaction between hydrogen and chlorine atom, which assist in lower activation barrier. Further, the kinetic analysis suggested the applied temperature and pressure play a key role in formation of dominant elimination channel during decomposition reaction.

Reaction path identification and validation from molecular dynamics simulations of hydrocarbon pyrolysis

AbstractCreation of complex chemical mechanisms for hydrocarbon pyrolysis and combustion is challenging due to the large number of species and reactions involved. Reactive molecular dynamics (RMD) enables the simulation of thousands of reactions and the discovery of previously unknown components of the reaction network. However, due to the inherent imprecision of reactive force fields, it is necessary to verify RMD‐obtained reaction paths using more accurate methods such as Density Functional Theory (DFT). We demonstrate a method for identification and confirmation of reaction pathways from RMD that supplement an established mechanism, using the example of benzene formation from n‐heptane and iso‐octane pyrolysis. We establish a validation workflow to extract reaction geometries from RMD and optimize transition states using the Nudged‐Elastic‐Band method on semi‐empirical and quantum mechanical levels of theory. Our findings demonstrate that the widely recognized ReaxFF parameterization, CHO2016, can identify known pathways from a established soot formation mechanism while also indicating new ones. We also show that CHO2016 underestimates hydrogen migration barriers by up to as compared to DFT and can lower activation barriers significantly for spin‐forbidden reactions. This highlights the necessity for validation or potentially even reparametrization of CHO2016.

Tuning Pd species via electronic metal-support interaction for methane combustion

Utilizing catalytic combustion to convert methane (CH4) into CO2 and H2O stands as one of the most effective approaches for mitigating unburnt CH4 emissions from natural gas engines. Supported Pd catalysts have been extensively researched for their role in low-temperature CH4 combustion, with their catalytic activity greatly influenced by metal-support interactions. Surface interaction Pd phases, as a special type of Pd species originating from metal-support interactions on supported Pd catalysts, show controversial catalytic performance in CH4 combustion. Moreover, the impact of electronic metal-support interactions (EMSI, which refers to metal-support interactions associated with electron transfer) remains unclear. Hence, we opted for Ce-Zr solid solutions with different Ce:Zr molar ratios as supports and synthesized a range of supported Pd catalysts with varying EMSI intensities. Characterization revealed that as the oxygen vacancy concentration on the support increased, electron transfer weakened, leading to a higher Pd-O-Ce content, resulting in a lower CH4 activation barrier and better catalytic performance. This study offers a promising approach for regulating EMSI and active Pd species on supported catalysts in practical applications.

P-Bridging Asymmetry Diatomic Catalysts Sites Drive Efficient Bifunctional Oxygen Electrocatalysis for Zinc-Air Batteries.

Rechargeable zinc-air batteries (ZABs) rely on the development of high-performance bifunctional oxygen electrocatalysts to facilitate efficient oxygen reduction/evolution reactions (ORR/OER). Single-atom catalysts (SACs), characterized by their precisely defined active sites, have great potential for applications in ZABs. However, the design and architecture of atomic site electrocatalysts with both high activity and durability present significant challenges, owing to their spatial confinement and electronic states. In this study, a strategy is proposed to fabricate structurally uniform dual single-atom electrocatalyst (denoted as P-FeCo/NC) consisting of P-bridging Fe and Co bimetal atom (i.e., Fe-P-Co) decorated on N, P-co-doped carbon framework as an efficient and durable bifunctional electrocatalyst for ZABs. Experimental investigations and theoretical calculations reveal that the Fe-P-Co bridge-coupling structure enables a facile adsorption/desorption of oxygen intermediates and low activation barrier. The resultant P-FeCo/NC exhibits ultralow overpotential of 340mV at 10mA cm-2 for OER and high half-wave potential of 0.95V for ORR. In addition, the application of P-FeCo/NC in rechargeable ZABs demonstrates enhanced performance with maximum power density of 115mW cm-2 and long cyclic stability, which surpass Pt/C and RuO2 catalysts. This study provides valuable insights into the design and mechanism of atomically dispersed catalysts for energy conversion applications.

Multifunctional asymmetric bi-ligand iron chelating agents towards low-cost, high performance, and stable zinc-iron redox flow battery

Zinc‑iron flow batteries hold great potential as stationary storage due to their attractive cost and abundance of materials; however, they still suffer from precipitation problems. This work demonstrates a novel asymmetric iron complex with bi-ligand from pyridine and disodium 4,5-dihydroxy-1,3-benzenedisulfonate (TIRON) to synergically reduces precipitation, improves the mass transport of the redox-active species, and lowers the electron transfer resistance. The low activation barrier of mixed complexes calculated via the Density Functional Theory (DFT) explains the ease of redox reaction, which is reflected in the low overpotential voltage in the Fe//Fe symmetrical cell. Moreover, no change in Raman and Fourier-transform infrared spectroscopy peaks after symmetrical cell cycling confirms the stability of the mixed complexes catholyte. Contrary to other works, our strategy requires a lower molar ratio of complexes per iron, while still yields a superior capacity of 6.81 Ah/L. From its outstanding capacity utilization, the overall cost of the electrolytes for the mixed complexes system is only $26.3/kWh, which is lower than the non-complex iron system at $42.9/kWh. Additionally, the mixed complexes battery can be operated stably for >200 cycles. Our perspective on eco-friendly and low-cost catholyte design accelerates development towards reliable stationary storage with high performance and stability.

Initial Decomposition Mechanism of NH3OH+N5- Crystal under Thermal and Shock Loading: A First-Principles Study.

NH3OH+N5- is a novel energetic material (EM) which has attracted much interest for its promising performances, including high energy density, high density, low sensitivity, and low toxicity. In this study, the initial decomposition mechanism of NH3OH+N5- crystal was investigated under thermal and shock loading by molecular dynamics simulation. First, programmed heating and constant temperature simulations were carried out by molecular dynamics simulation on the basis of density functional theory (DFT-MD). Results indicated that the initial decomposition reactions of NH3OH+N5- could be described by three reactions: proton transfer, ring-opening reaction, and cation decomposition and recombination, and three pathways of ring-opening reaction were found, including the ring-opening of N5-, HN5, and H2N6. The first two reactions are the main pathways that produce N2 molecules. Furthermore, we carried out DFT-MD simulations to study the shock decomposition behaviors of NH3OH+N5-, and three initial steps were proposed: N5-, HN5, and N6 ring-opening. The fewer N5- and HN5 ring-opening reactions were found during the shock simulation, accompanied by a significant change in the N5- bond angle. What's more, the transition states of decomposition reactions were investigated through quantum chemical calculations. The results revealed that the proton transfer reaction exhibits lower activation barriers compared to ring-opening reactions, and proton transfer would accelerate ring-opening reactions. In addition, the ring-opening reaction is the main energy-releasing reaction in the early stages of the decomposition. This work could promote the comprehension of the decomposition mechanism and energy release regularity of N5- ions.

Sodium fluoride enables room-temperature synthesis of dimethyl carbonate

Carbon dioxide (CO2) in the atmosphere of the Earth represents an abundant and largely free source of carbon, which remains heavily underestimated nowadays. The disbalance between the emission and utilization of CO2 puts the sustainable development of humanity at risk because of its possible correlation with global climate change. The valorization of CO2 is an important strategy to attend to the above-mentioned challenges. CO2 can be converted to dimethyl carbonate by grafting to methanol (CH3OH) in the presence of the catalyst. The presently known catalytic solutions require high temperatures and pressures to be employed. Herein, we report sodium fluoride as a new catalyst offering an unprecedentedly low activation barrier, 23 kJ/mol, for the carboxylation of CH3OH. The fluoride anion coordinates the hydroxyl hydrogen of CH3OH thanks to forming a very strong H-bond. Upon the collision with CO2, the proton detaches, and CO2 grafts to CH3O* while becoming the carboxyl group. The reaction finishes by methylating CH3OC(O)O– via the methyl radical originating from another CH3OH molecule (esterification stage). The hydrogen fluoride molecule is an intermediate in this reaction providing the proton and fostering esterification. The subsequent in-lab experiments showed that much milder conditions can be used to synthesize DMC out of CH3OH. Under 80 °C and 40 bar, we obtained an outstanding DMC yield of 21.1 %. However, the synthesis was also possible at lower temperatures giving DMC yields of 16.3 % at 65 °C, 14.7 % at 55 °C, and 10.8 % at 25 °C. The unprecedented possibility to obtain a non-negligible production of an organic carbonate at room temperature represents a cornerstone advance in the field of CO2 valorization and promises huge energy savings. Given the current global amount of the yearly prepared DMC amounting to over one million tons and the demand for this chemical by emerging chemical technologies, the achieved result is deemed to be of paramount importance. The exemplified catalytic mechanism is also interesting in the context of other syntheses, in which a low-energetic production of the methoxy moieties is a cornerstone.

Design of Sulfur Resistant Cobalt Catalysts by Boron Promotion: Atomic Scale Insights

The effect of boron promotion on atomic sulfur formation by hydrogen sulfide dissociation on Co(111), flat surfaces of cobalt nanoparticles, was investigated using Density Functional Theory calculations. The results show that on clean Co(111), hydrogen sulfide dissociation proceeds fast due to low activation barriers, yielding atomic sulfur on the cobalt surfaces. Boron promotion hinders the dissociation of hydrogen sulfide due to increased activation barriers. Furthermore, boron prevents the interaction of sulfur compounds with cobalt surface atoms, as these poisons bind on boron. The findings indicate that boron is an effective promoter that can be used to design sulfur resistant cobalt catalysts.

A DFT-based kinetic Monte Carlo simulation of multiphase oxide-metal thin film growth

Functional thin films of nanoscale metal pillars in oxide or nitride matrices known as vertically aligned nanocomposite (VAN) have gained much interest owing to their unique strain-coupled and highly anisotropic properties. So far, the deposition of these films has been explored mostly experimentally. In this work, a density functional theory (DFT)-based kinetic Monte Carlo simulation model using Bortz–Kalos–Lebowitz algorithm was developed to understand the growth of VAN films deposited by pulsed laser technique on mismatching substrates. The model has been parameterized and applied to understand the kinetics of growth thin films consisting of Au pillars in CeO2 matrix deposited on SrTiO3 substrates. The effects of pulsed laser deposition (PLD) conditions including the pulse frequency, deposition flux, and substrate temperature were explored. The simulations indicate that the Au pillar size and shape exhibit significant dependence on the PLD conditions. Namely, increasing the temperature increases the average pillar size and lowers the pillar density, and vice versa. In addition, the simulations revealed that increasing the deposition rate results in lowering the average pillar size and increasing the density. Particularly, the DFT results suggest that Au pillar size can be tuned during the initial growth of the first monolayer due to the significantly low activation barrier. Our analysis showed that the relationship between the average pillar size and pillar density is influenced by the kinetics. Furthermore, autocorrelation analysis showed that pillars self-organize in quasi-ordered patterns at certain windows of the deposition conditions, which is attributed to the complex nature of the chemical interactions in the system, the kinetics, and the deposition parameters.

NHC‐Supported 2‐Sila and 2‐Germavinylidenes: Synthesis, Dynamics, First Reactivity and Theoretical Studies

Abstract2‐tetrelavinylidenes (C=EH2; E=Si, Ge) are according to quantum chemical studies the least stable isomers on the [E,C,2H] potential energy hypersurface isomerizing easily via the trans‐bent tetrelaacetylenes HE≡CH to the thermodynamically most stable 1‐tetrelavinylidenes (E=CH2). Consequently, experimental studies on 2‐tetrelavinylidenes (C=ER2) and their derivatives are lacking. Herein we report experimental and theoretical studies of the first N‐heterocyclic carbene (NHC) supported 2‐silavinylidene (NHC)C=SiBr(Tbb) (1‐Si: NHC=C[N(Dipp)CH]2, Dipp=2,6‐diisopropylphenyl, Tbb=2,6‐bis[bis(trimethylsilyl)methyl]‐4‐tert‐butylphenyl) and the isovalent 2‐germavinylidenes (NHC)C=GeBr(R) (1‐Ge, 1‐GeMind: R=Tbb, Mind (1,1,3,3,5,5,7,7‐octamethyl‐s‐hydrindacene‐4‐yl)). The NHC‐supported 2‐tetrelavinylidenes were obtained selectively from the 1,2‐dibromoditetrelenes (E)‐(R)BrE=EBr(R) using the diazoolefin (NHC)CN2 as vinylidene transfer reagent. 1‐E (E=Si, Ge) have a planar vinylidene core, a bent‐dicoordinated vinylidene carbon atom (CVNL), a very short E=CVNL bond and an almost orthogonal orientation of the NHC five‐membered ring to the vinylidene core. Quantum chemical analysis of the electronic structures of 1‐E suggest a significantly bent 1‐tetrelaallene and tetrelyne character. NMR studies shed light into the dynamics of 1‐E involving NHC‐rotation around the CVNL−CNHC bond with a low activation barrier. Furthermore, the synthetic potential of 1‐E is demonstrated by the synthesis and full characterization of the unprecedented NHC‐supported bromogermynes BrGe=C(EBr2Tbb)(NHC) (2‐SiGe: E=Si; 2‐GeGe: E=Ge).

NHC-Supported 2-Sila and 2-Germavinylidenes: Synthesis, Dynamics, First Reactivity and Theoretical Studies.

2-tetrelavinylidenes (C=EH2; E=Si, Ge) are according to quantum chemical studies the least stable isomers on the [E,C,2H] potential energy hypersurface isomerizing easily via the trans-bent tetrelaacetylenes HE≡CH to the thermodynamically most stable 1-tetrelavinylidenes (E=CH2). Consequently, experimental studies on 2-tetrelavinylidenes (C=ER2) and their derivatives are lacking. Herein we report experimental and theoretical studies of the first N-heterocyclic carbene (NHC) supported 2-silavinylidene (NHC)C=SiBr(Tbb) (1-Si: NHC=C[N(Dipp)CH]2, Dipp=2,6-diisopropylphenyl, Tbb=2,6-bis[bis(trimethylsilyl)methyl]-4-tert-butylphenyl) and the isovalent 2-germavinylidenes (NHC)C=GeBr(R) (1-Ge, 1-GeMind: R=Tbb, Mind (1,1,3,3,5,5,7,7-octamethyl-s-hydrindacene-4-yl)). The NHC-supported 2-tetrelavinylidenes were obtained selectively from the 1,2-dibromoditetrelenes (E)-(R)BrE=EBr(R) using the diazoolefin (NHC)CN2 as vinylidene transfer reagent. 1-E (E=Si, Ge) have a planar vinylidene core, a bent-dicoordinated vinylidene carbon atom (CVNL), a very short E=CVNL bond and an almost orthogonal orientation of the NHC five-membered ring to the vinylidene core. Quantum chemical analysis of the electronic structures of 1-E suggest a significantly bent 1-tetrelaallene and tetrelyne character. NMR studies shed light into the dynamics of 1-E involving NHC-rotation around the CVNL-CNHC bond with a low activation barrier. Furthermore, the synthetic potential of 1-E is demonstrated by the synthesis and full characterization of the unprecedented NHC-supported bromogermynes BrGe=C(EBr2Tbb)(NHC) (2-SiGe: E=Si; 2-GeGe: E=Ge).

Ab initio modelling of photocatalytic CO2 reduction reactions over Cu/TiO2 semiconductors including the electronic excitation effects

Photocatalysis is a promising method for reducing CO2 in an environmentally friendly way. Despite extensive experimental studies, theoretical studies lag behind and often resort to describing catalysts in the excited state, while the reaction mechanism is mostly studied at the ground level. We theoretically calculate a full reaction mechanism of CO2 reduction to CO in the excited state for five surfaces: Cu(111), anatase(110), rutile(101) and Cu/anatase, Cu/rutile using the ΔSCF approach. We show that excited states considerably lower activation barriers, making it necessary in describing the experimental performance. The density functional theory calculations in the excited state are used to construct a microkinetic model, which is used to predict the performance of each catalytic surface. We show that Cu(111) is photocatalytically inactive, TiO2 is only active in the UV range and catalyzes water splitting. Only Cu/rutile is active for CO2 reduction to CO, while Cu/anatase produces more H2.

Covalent Benzenesulfonic Functionalization of a Graphene Nanopore for Enhanced and Selective Proton Transport.

A fundamental understanding of proton transport through graphene nanopores, defects, and vacancies is essential for advancing two-dimensional proton exchange membranes (PEMs). This study employs ReaxFF molecular dynamics, metadynamics, and density functional theory to investigate the enhanced proton transport through a graphene nanopore. Covalently functionalizing the nanopore with a benzenesulfonic group yields consistent improvements in proton permeability, with a lower activation barrier (≈0.15 eV) and increased proton selectivity over sodium cations. The benzenesulfonic functionality acts as a dynamic proton shuttle, establishing a favorable hydrogen-bonding network and an efficient proton transport channel. The model reveals an optimal balance between proton permeability and selectivity, which is essential for effective proton exchange membranes. Notably, the benzenesulfonic-functionalized graphene nanopore system achieves a theoretically estimated proton diffusion coefficient comparable to or higher than the current state-of-the-art PEM, Nafion. Ergo, the benzenesulfonic functionalization of graphene nanopores, firmly holds promise for future graphene-based membrane development in energy conversion devices.

Fast crystal growth in deeply undercooled ZrTi melts.

We investigate the growth of crystals in Zr50Ti50 melts by classical molecular-dynamics simulations with an embedded atom method and a Stillinger-Weber potential model. Both models display fast solidification rates that can be captured by the transition state theory or the Ginzburg-Landau theory at small undercoolings. Fast crystal-growth rates are found to be affected by the pre-existing ordering in liquids, such as the body-centered cubic-like and icosahedral-like structures. The interface-induced ordering unveiled by the crystal-freezing method can explain the rate difference between these two models. However, these orderings fail to rationalize the temperature evolution of the growth rate at deep undercoolings. We correlate the growth kinetics with the detailed dynamical processes in liquids, finding the decoupling of hierarchic relaxation processes when collective motion emerges in supercooled liquids. We find that the growth kinetics is nondiffusive, but with a lower activation barrier corresponding to the structural relaxation or the cage-relative motion in ZrTi melts. These results explore a new relaxation mechanism for the fast growth rate in deeply undercooled liquids.

A First Principles Mechanistic Study of Higher Alcohol Synthesis from Syngas on a Stepped Rhodium Surface

The mechanism of higher alcohol synthesis (HAS) from syngas on a stepped Rhodium surface was explored using first principles calculations based on density functional theory. Results showed that the activation of CO proceeds most energetically feasible via a sequential hydrogenation towards CH2OH, followed by the C–OH bond cleavage yielding CHx species. Because the initial CO hydrogenation step is highly activated, the cascade of elementary steps toward methane formation is highly favored. The formation of C2 oxygenates toward ethanol production is kinetically favored by CO insertion to CH2, or alternatively, by a lower activation barrier CHO insertion to CH3. On the other hand, the C3 species is formed more preferably by CO rather than CHO insertion to a CH3CH2 fragment, indicating the effect of a more extended carbon structure on the reaction mechanism. The overall reaction mechanism for HAS points to a cycle of CO insertion, hydrogenation, and OH elimination steps.Graphical

CO2 reduction using aluminum hydride: Generation of in-situ frustrated Lewis pairs and small molecule activation therein.

CO2 reduction is appealing for the long-term production of high-value fuels and chemicals. Herein, using density functional theory (DFT) based calculations, we study the CO2 reduction pathway to formic acid using aluminum hydride and phosphine derivatives. Our primary focus is on aluminum hydride derivatives, aimed at improving the efficiency of the CO2 reduction process. Substituents with σ-donating properties at the aluminum center are discovered to lower the activation barriers. We demonstrate how di-tert-butylphosphine oxide (LB-O)/di-tert-butylphosphine sulfide (LB-S)/di-tert-butylphosphanimine (LB-N) work together with aluminum hydride to facilitate CO2 reduction process and generate in-situ frustrated Lewis pairs (FLPs), such as FLP-O, FLP-S, and FLP-N. The activation strain model (ASM) analysis reveals the significance of strain energy in determining activation barriers. EDA-NOCV and PIO analyses elucidate the orbital interactions at the corresponding transition states. Furthermore, the study delves into the activation of various small molecules, such as dihydrogen, acetylene, ethylene, carbon dioxide, nitrous oxide, and acetonitrile, using those in-situ generated FLPs. The study highlights the low activation barriers and emphasizes the potential for small molecule activation in this context.