Low Reaction Barrier Research Articles

Surface Modifications of Ti2 CO2 for Obtaining High Hydrogen Evolution Reaction Activity and Conductivity: A Computational Approach.

The identification of hydrogen evolution reaction (HER) catalysts with high conductivity and high activity is the top priority in the development of renewable energy. Oxygen-terminated Ti2 C (Ti2 CO2 ), as one of the typical MXenes, shows good HER Gibbs free energy (ΔGH ) at low hydrogen coverage, whereas the large band gap of approximately 1.0 eV limits its conductivity capability. By doping phosphor (P) or sulfur (S) to partially substitute the surficial O, the average free energy (ΔGHa ) at various hydrogen coverages has been draggd to approach zero, and the conductivity is also significantly improved by narrowing the band gap to lower than 0.3 eV. Partial charge analysis indicates that doping P or S on the surface induces the diffusion of electrons on oxygen from O 2px to O 2pz , resulting in O 2pz reaction with H 1s. The facial overlapping of H 1s with O 2pz will strengthen the binding strength, hence lowering ΔGH . The energy shift toward Fermi level of Ti 3d after P or S doping contributes to the reduced band gap and high conductivity. Surficial O or P vacancies are created to evaluate the vacancy effect on HER performance, which not only improves ΔGHa and conductivity but also leads to a low reaction barrier of H2 O splitting (<0.2 eV). The armchair nanoribbon (ANR) displays improved HER activity by P-doping at 50% ratio. Our research shows that modification of end-group in MXenes can effectively improve HER catalytic activity and conductivity, which is expected to promote their potential applications in electrocatalysis and energy conversion.

Fluorination of graphene oxide at ambient conditions

Fluorination is a feasible method to modify the properties of graphene. However, the fluorination methods still meet different limitations such as inconvenient, low-yield, high-cost, etc. Here, a room-temperature method at ambient conditions was presented to synthesize fluorinated graphene oxide (FGO). The obtained FGO had the F content of ca. 28.6 at%. The mechanism of the reaction at ambient conditions was discussed by theoretical calculations, which showed that the oxygen groups facilitated to generate stable CF bonds by overcoming a low reaction barrier. This work provides an easy synthesis of FGO, broadening the applications of functionalized graphene, for example, increasing the magnetic inducing efficiency to ca. 1.03 × 10−2 μB/F.

Shape-modulated synthesis of mullite SmMn2O5 nanostructures with fast sensing response to acetone

Development of semiconductor sensing materials with fast response requires low reaction barrier between gas species and materials surface, hence contributing to real-time gas monitoring. Herein, mullite SmMn2O5 nanofibers (SNFs), nanotube-in-tubes (NT@NTs), and porous nanotubes (PNTs) have been achieved, via optimized electrospinning technique and annealing procedure. The formation mechanism for present shapes of SmMn2O5 nanostructures was discussed. With respect to the sensing properties to acetone, three sensors prototypes employed with SmMn2O5 NT@NTs, SNFs and PNTs were built and in detail studied by taking SmMn2O5 SNFs sensor as an example. Delightfully, the sensor exhibited the response and recovery time as short as 3 s and 8 s at 300 °C, respectively. The intrinsic high catalytic activity of SmMn2O5 lowers the sensing reaction barrier, which might contribute to the reaction kinetics thereby the fast sensing behavior. This work provides synthetic strategies towards one-dimensional (1D) mullite families and the novel material for acetone gas detection would facilitate further development of gas sensing.

Evidence for competing proton-transfer and hydrogen-transfer reactions in the S1 state of indigo

Indigo is a blue dye molecule that has been used since antiquity, although it is better known today for its use in blue jeans. Indigo has previously been shown to exhibit remarkable photostability due to fast excited state dynamics mediated by an excited state intramolecular proton transfer. Study of this process is complicated by the fact that the photophysics of indigo is very sensitive to the environment. In order to disentangle the intrinsic photodynamics of indigo from the effects contributed by the environment, we studied indigo in a molecular beam using resonance enhanced multiphoton ionization. We obtained excited state lifetimes of individual vibronic bands with pump-probe spectroscopy ranging from 60 ps to 24 ns. We have mapped a barrier to relaxation at about 700 cm−1, beyond which fast excited state dynamics dominate. Below this barrier two decay processes compete and mode-specific relaxation occurs with certain vibronic bands near the origin relaxing faster than others, or exhibiting different partitioning between the two relaxation channels. Computational studies at the ADC(2)/MP2/cc-pVDZ level indicate that two low-barrier reaction paths exist in the S1 state of indigo, one corresponding to proton transfer, the other to hydrogen transfer. In both cases the charge distribution changes drastically upon de-excitation. These data provide a sensitive probe of the potential energy landscape, responsible for the response to absorption of light. The results may help in understanding the photostability that preserves the blue color of indigo dyes.

Ion-collision induced molecular growth in polycyclic aromatic hydrocarbon clusters: comparison of C16H10 structural isomers

The interaction of 3 keV Ar+ ions with clusters of two structural isomers of C16H10 polycyclic aromatic hydrocarbons, namely fluoranthene and pyrene, has been studied. Following the collision with the atomic projectile, a rich molecular growth inside of the cluster is observed for both isomers. The observed growth processes include the hydrogenation of the parent molecule and the addition of CHx units. They are more important in the case of collisions with fluoranthene clusters, this can be explained by a more flexible carbon skeleton and lower reaction barriers. Moreover, the evolution of the hydrogenation with the size of the growth products gives information on the growth mechanisms.

Strong affinity of mineral dusts for sulfur dioxide and catalytic mechanisms towards acid rain formation

Mineral dust is an important arena for acid rain formation. Here, p-DFT calculations were conducted to study SO2 adsorption onto gibbsite and possible catalytic mechanisms for acid rain formation. The obtained results clearly indicated that SO2 is preferentially adsorbed onto mineral dusts instead of staying in the gas phase, and two catalytic paths were then posed. Path A with first hydrolysis and then oxidation is kinetically preferred, where SO2 hydrolysis appeared to be the rate-determining step for all the investigated surfaces. Partially dehydrated gibbsite (100) surface has the lowest reaction barrier, and a second water molecule causes acid rain formation to occur facilely at ambient circumstances.

Reactivity in interstellar ice analogs: role of the structural evolution

Context. The synthesis of interstellar complex organic molecules in ice involves several types of reactions between molecules and/or radicals that are usually considered to be diffusion controlled. Aims. We aim to understand the coupling between diffusion and reactivity in the interstellar ice mantle using a model binary reaction in the diffusion-limited regime. Methods. We performed isothermal kinetic laboratory experiments on interstellar ice analogs at low temperatures, using the NH3:CO2:H2O model system where reactants NH3 and CO2 have a low reaction barrier and are diluted in a water-dominated ice. Results. We found that in the diffusion-limited regime, the reaction kinetics is not determined by the intrinsic bulk diffusivity of reactants. Instead, reactions are driven by structural changes evolving in amorphous water ice, such as pore collapse and crystallization. Diffusion of reactants in this case likely occurs along the surface of (tiny) cracks generated by the structural changes. Conclusions. The reactivity driven by the structural changes breaks the conventional picture of reactant molecules/radicals diffusing in a bulk water ice. This phenomenon is expected to lead to a dramatic increase in production rates of interstellar complex organic molecules in star-forming regions.

Mechanistic Insights into the Pore Confinement Effect on Bimolecular and Monomolecular Cracking Mechanisms of N-Octane over HY and HZSM-5 Zeolites: A DFT Study

Bimolecular and monomolecular cracking mechanisms of alkanes simultaneously occur and have a competitive relationship, which strongly influences the product distribution. In this work, the density functional theory (DFT) calculation is first carried out to elucidate two cracking mechanisms in HZSM-5 and HY zeolites. It is found that the overall apparent reaction barrier for the monomolecular cracking reaction at 750 K in the HZSM-5 zeolite is 5.30 kcal/mol, much lower than that (23.12 kcal/mol) for bimolecular cracking reaction, indicating that the monomolecular mechanism is predominant in the HZSM-5 zeolite. In contrast, the bimolecular mechanism is predominant in the HY zeolite because of a lower apparent reaction barrier energy barrier (6.95 kcal/mol) for bimolecular cracking reaction than that (24.34 kcal/mol) for the monomolecular cracking reaction. Moreover, the intrinsic reason for the different mechanisms is further elucidated. The confinement effect can effectively decrease the energy barrier whe...

Cethrene: The Chameleon of Woodward-Hoffmann Rules.

We demonstrate that the electrocyclic (EC) ring-closure of cethrene in solution proceeds in a conrotatory mode both thermally and photochemically. The facile photochemical EC process promises that cethrene can serve as an efficient chiroptical switch operated solely by light. As for the thermally activated EC reaction, a low reaction barrier and a solvation effect on the EC rate indicate that the C2-symmetric pathway predicted by DFT calculations might not be the correct mechanism. Instead, we argue that the molecular symmetry decreases along the reaction coordinate as a consequence of the low-energy singlet excited state in this diradicaloid molecule, which might lead to a lower activation energy in accord with that determined through kinetic studies. Cethrene, therefore, represents a thought-provoking molecular chameleon of the Woodward-Hoffmann rules that puts our chemical concepts and intuition to test.

A computational investigation of the sulphuric acid‐catalysed 1,4‐hydrogen transfer in higher Criegee intermediates

AbstractCriegee intermediates (CIs) are formed during the ozonolysis of unsaturated hydrocarbons in the troposphere. The fate of CIs is of critical importance to tropospheric oxidation chemistry, particularly in the context of radical and secondary organic aerosol formation. Using the high‐level ab initio G4(MP2) method, we investigate the 1,4 hydrogen shift reaction in CIs formed from ozonolysis of two common biogenic hydrocarbons: isoprene and α‐pinene. We consider the uncatalysed reaction, as well as the reaction catalysed by a water molecule and by sulphuric acid. We show that sulphuric acid is a very effective catalyst, leading to a barrierless tautomerization relative to the free reactants and to very low reaction barrier heights relative to the reactant complexes. In particular, we obtain reaction barrier heights of Δ = 24.5 (isoprene CI) and 8.4 (α‐pinene CI) kJ mol−1 relative to the reactant complexes. Given the reaction of OH radicals with SO2 in the troposphere can ultimately yield sulphuric acid, these findings may have significant consequences for current atmospheric chemical models for regions of high sulphur concentrations.

N-doped ZnO nanosheets: towards high performance two dimensional catalysts

Recently, catalytic activity of atomically thin two dimensional (2D) materials has attracted great interest. In this paper, via first principles calculations, we show for the first time that N-doped 2D one-atom-thick ZnO nanosheets exhibit high catalytic activity towards CO oxidation. A pristine 2D ZnO nanosheet is chemically inert and as a result, CO and O2 molecules do not chemically bind on the nanosheet. Our calculations predict that the N doping activates the ZnO sheet, leading to strong CO and O2 adsorptions. We further show that the CO oxidation catalyzed by the N-doped 2D ZnO sheet has a low reaction barrier around 0.5 eV. Besides high catalytic activity, the N-doped 2D ZnO sheet also demonstrates intriguing electronic and magnetic properties. These findings provide new opportunities for the future development of high performance 2D catalysts.

A DFT study of H2O dissociation on metal‐precovered Fe (100) surface

The H2O adsorption and dissociation on the Fe (100) surface with different precovered metals are studied by density functional theory. On both kinds of metal‐precovered surface, H2O molecules prefer adsorb on hollow sites than bridge and top sites. The impurity energy difference is proportional to the adsorption energy, but the adsorbates are not sensitive to the adsorption orientation and height relative to the surface. The Hirshfeld charge analysis shows that water molecules act as an electron donor while the surface Fe atoms act as an electron acceptor. The rotation and dissociation of H2O molecule occur on the Co‐ and Mn‐precovered surfaces. Some H2O molecules are dissociated into OH and H groups. The energy barriers are about 0.5 to 1.0 eV, whose are consistence with the experimental data. H2O molecules can be dissociated more easily at the top site on Co‐precovered surface 1 than that at bridge site on Mn‐precovered surface 2 because of the lower reaction barrier. The dispersion correction effects on the energies and adsorption configurations on Co‐precovered surface 1 were calculated by OBS + PW91. The dispersion contributions can improve a bit of the bond energy of adsorbates and weaken the hydrogen bond effect between adsorption molecules a little.

Substrate engineering of graphene reactivity: towards high-performance graphene-based catalysts

Graphene-based solid-state catalysis is an emerging direction in research on graphene, which opens new opportunities in graphene applications and thus has attracted enormous interests recently. A central issue in graphene-based catalysis is the lack of an effective yet practical way to activate the chemically inert graphene, which is largely due to the difficulties in the direct treatment of graphene (such as doping transition metal elements and introducing particular type of vacancies). Here we report a way to overcome these difficulties by promoting the reactivity and catalytic activity of graphene via substrate engineering. With thorough first-principles investigations, we demonstrate that when introduce a defect, either a substitutional impurity atom (e.g. Au, Cu, Ag, Zn) or a single vacancy, in the underlying Ru (0001) substrate, the reactivity of the supported graphene can be greatly enhanced, resulting in the chemical adsorption of O2 molecules on graphene. The origin of the O2 chemical adsorption is found to be the impurity- or vacancy-induced significant charge transfer from the graphene–Ru (0001) contact region to the 2π* orbital of the O2 molecule. We then further show that the charge transfer also leads to high catalytic activity of graphene for chemical reaction of CO oxidation. According to our calculations, the catalyzed CO oxidation takes place in Eley-Rideal (ER) mechanism with low reaction barriers (around 0.5 eV), suggesting that the substrate engineering is an effective way to turn the supported graphene into an excellent catalyst that has potential for large-scale industrial applications.

CO oxidation on inverse Ce6O12/Cu(111) catalyst: role of copper-ceria interactions.

The surface structures, O2 adsorption, and CO oxidation reaction properties of Ce6O12/Cu(111) have been investigated using density functional theory including on-site Coulomb corrections (DFT + U). Results show that the supported ceria nanoparticles would gain electrons from the Cu(111) surface, and the Ce4+ are reduced to Ce3+. In addition, the oxygens at the interface have been largely activated, resulting in much low formation energy of O vacancies. For the CO oxidation reaction, two possible pathways are investigated, CO reacts with the O2 molecule adsorbed on Ce3+ and the lattice O at the interface, respectively. It has been found that CO reacting with the lattice O atom gives a lower reaction barrier than that of adsorbed O2 on Ce3+. These results are important for further understanding of the role of different active sites on the inverse CeOx/Cu(111) surface structure.

The Mechanism of Rh-Catalyzed Transformation of Fatty Acids to Linear Alpha olefins

Linear alpha olefins (LAOs) are key commodity chemicals and petrochemical intermediates that are currently produced from fossil resources. Fatty acids are the obvious renewable starting material for LAOs, which can be obtained via transition-metal-catalyzed decarbonylative dehydration. However, even the best catalysts that have been obtained to date, which are based on palladium, are not active and stable enough for industrial use. To provide insight for design of better catalysts, we here present the first computationally derived mechanism for another attractive transition-metal for this reaction, rhodium. By comparing the calculated mechanisms and free energy profiles for the two metals, Pd and Rh, we single out important factors for a facile, low-barrier reaction and for a stable catalyst. While the olefin formation is rate limiting for both of the metals, the rate-determining intermediate for Rh is, in contrast to Pd, the starting complex, (PPh3)2Rh(CO)Cl. This complex largely draws its stability from the strength of the Rh(I)–CO bond. CO is a much less suitable ligand for the high-oxidation state Rh(III). However, for steric reasons, rhodium dissociates a bulkier triphenylphosphine and keeps the carbonyl during the oxidative addition, which is less favorable than for Pd. When compared to Pd, which dissociates two phosphine ligands at the start of the reaction, the catalytic activity of Rh also appears to be hampered by its preference for high coordination numbers. The remaining ancillary ligands leave less space for the metal to mediate the reaction.

Can Ultrastrong Coupling Change Ground-State Chemical Reactions?

Recent advancements on the fabrication of organic micro- and nanostructures have permitted the strong collective light-matter coupling regime to be reached with molecular materials. Pioneering works in this direction have shown the effects of this regime in the excited state reactivity of molecular systems and at the same time has opened up the question of whether it is possible to introduce any modifications in the electronic ground energy landscape which could affect chemical thermodynamics and/or kinetics. In this work, we use a model system of many molecules coupled to a surface-plasmon field to gain insight on the key parameters which govern the modifications of the ground-state Potential Energy Surface (PES). Our findings confirm that the energetic changes per molecule are determined by single-molecule-light couplings which are essentially local, in contrast with those of the electronically excited states, for which energetic corrections are of a collective nature. Still, we reveal some intriguing quantum-coherent effects associated with pathways of concerted reactions, where two or more molecules undergo reactions simultaneously, and which can be of relevance in low-barrier reactions. Finally, we also explore modifications to nonadiabatic dynamics and conclude that, for this particular model, the presence of a large number of dark states yields negligible changes. Our study reveals new possibilities as well as limitations for the emerging field of polariton chemistry.

Hydrogen bonding-mediated dehydrogenation in the ammonia borane combined graphene oxide systems

The dehydrogenation of ammonia borane (AB) adsorbed on three different graphene oxide (GO) sheets is investigated within the ab initio density functional theory. The energy barriers to direct combination the hydrogens of hydroxyl groups and the hydridic hydrogens of AB to release H2 are relatively high, indicating that the process is energetically unfavorable. Our theoretical study demonstrates that the dehydrogenation mechanism of the AB-GO systems has undergone two critical steps, first, there is the formation of the hydrogen bond (O–H–O) between two hydroxyl groups, and then, the hydrogen bond further react with the hydridic hydrogens of AB to release H2 with low reaction barriers.

Near room temperature chemical vapor deposition of graphene with diluted methane and molten gallium catalyst

Direct growth of graphene integrated into electronic devices is highly desirable but difficult due to the nominal ~1000 °C chemical vapor deposition (CVD) temperature, which can seriously deteriorate the substrates. Here we report a great reduction of graphene CVD temperature, down to 50 °C on sapphire and 100 °C on polycarbonate, by using dilute methane as the source and molten gallium (Ga) as catalysts. The very low temperature graphene synthesis is made possible by carbon attachment to the island edges of pre-existing graphene nuclei islands, and causes no damages to the substrates. A key benefit of using molten Ga catalyst is the enhanced methane absorption in Ga at lower temperatures; this leads to a surprisingly low apparent reaction barrier of ~0.16 eV below 300 °C. The faster growth kinetics due to a low reaction barrier and a demonstrated low-temperature graphene nuclei transfer protocol can facilitate practical direct graphene synthesis on many kinds of substrates down to 50–100 °C. Our results represent a significant progress in reducing graphene synthesis temperature and understanding its mechanism.

Controlling defects and secondary phases of CZTS by surfactant potassium

Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material with earth abundant and nontoxic elements. However, the detrimental native defects and secondary phases of CSTS will largely reduce the energy conversion efficiencies. To understand the origin of these problems during the growth of CZTS, we investigated the kinetic processes on CZTS (-1-1-2) surface, using first principles calculations. A surface Zn atom was found to occupy the subsurface Cu site easily due to a low reaction barrier, which may lead to a high ZnCu concentration and a secondary phase of ZnS. These n-type defects may create deep electron traps near the interface and become detrimental to device performance. To reduce the population of ZnCu and the secondary phase, we propose to use K as a surfactant to alter surface kinetic processes. Improvements on crystal quality and device performance based on this surfactant are consistent with early experimental observations.

Glassy nature and glass-to-crystal transition in the binary metallic glass CuZr

The prediction for the stability of glassy material is a key challenge in physical science. Here, we report a theoretical framework to predict the glass stability based on stochastic surface walking global optimization and reaction pathway sampling. This is demonstrated by revealing for the first time the global potential energy surface (PES) of two systems, CuZr binary metallic glass and nonglassy pure Cu systems, and establishing the lowest energy pathways linking glassy/amorphous structures with crystalline structures. The CuZr system has a significant number of glassy structures on PES that are $\ensuremath{\sim}0.045\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{atom}$ above the crystal structure. Two clear trends are identified from global PES in the glass-to-crystal transition of the CuZr system: (i) the local Zr-Cu coordination (nearest neighbor) increases, and (ii) the local Zr bonding environment becomes homogeneous. This allows us to introduce quantitative structural and energetics conditions to distinguish the glassy structures from the crystalline structures. Because of the local Zr-Cu exchange in the glass-to-crystal transition, a high reaction barrier $(g0.048\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{atom})$ is present to separate the glassy structures and the crystals in CuZr. By contrast, the Cu system, although it does possess amorphous structures that appear at much higher energy $(\ensuremath{\sim}0.075\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{atom})$ with respect to the crystal structure, has very low reaction barriers for the crystallization of amorphous structures, i.e. $l0.011\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{atom}$. The quantitative data on PES now available from global optimization techniques deepens our understanding on the microscopic nature of glassy material and might eventually facilitate the design of stable glassy materials.