Proton transfer across single-layer graphene proceeds with large computed energy barriers and is therefore thought to be unfavourable at room temperature unless nanoscale holes or dopants are introduced, or a potential bias is applied. Here we subject single-layer graphene supported on fused silica to cycles of high and low pH, and show that protons transfer reversibly from the aqueous phase through the graphene to the other side where they undergo acid–base chemistry with the silica hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defects. Computer simulations reveal low energy barriers of 0.61–0.75 eV for aqueous proton transfer across hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations shut down proton exchange. Unfavourable energy barriers to helium and hydrogen transfer indicate the process is selective for aqueous protons.

Increases in the cost of petroleum combined with growing concerns for CO 2 emissions are prompting researchers worldwide to search for renewable sources of liquid fuel. In this respect, biomass is currently the only sustainable source of organic carbon, and biofuels are currently the only sustainable source of liquid fuels. [1] The challenge with biomass conversion into liquid fuels is that efficient processes are not yet economically viable.A large number of processes for converting biomass into biofuels involve hydrogenation of biomass-derived feedstock in the aqueous phase. These include hydrogenation of biooils, [2,3] hydrogenation of fermentation broths, [4] and alkane production by aqueous phase processing. [5,6] Hydrogenation of organic acids, which are produced in fermentation, results in a 50 % increase in ethanol yield compared to when ethanol is produced directly from fermentation. [7] Upgrading bio-oils by hydrotreating involves hydrogenating a range of functional groups, which also include organic acids. In fact, one of the slowest steps in the hydrotreating of bio-oils is the hydrogenation of organic acids. [3] For these hydrotreating routes to become economically viable, it is critical to design highly active and selective catalysts for aqueous-phase hydrogenation (APH) reactions. So far there has been no systematic study for the APH of acetic acid on monometallic catalysts.Herein we report the findings of a combined experimental and theoretical study on the APH of acetic acid catalyzed by transition metals such as Ru, Rh, Pd, Ni, Cu, Ir, and Pt. Significantly different activities for the conversion of acetic acid have been observed over these metals. High selectivities for ethanol were obtained for several of them; in particular, Ru showed approximately 80 % ethanol selectivity at moderate temperatures. Density functional theory (DFT) calculations suggested that the different activity can be attributed to the intrinsic reactivity of the metals for dissociating acetic acid or acetate to acetyl (CH 3 O), and a simple empirical correlation is identified that may be used to readily estimate the free energy of the transition state (TS) for the rate-limiting step in acetic acid conversion based on the adsorption energies of acetyl and hydroxy groups. These findings offer clues for designing [8][9][10] active and selective transition metal catalysts for the hydrogenation of organic acids and oxygenates in general.The activity of the APH of acetic acid has been measured for Ru/C, Pt/C, Pd/C, Rh/C, Ir/Al 2 O 3 , Raney Ni, and Raney Cu catalysts at temperatures from 110-290 8C, and at 5.17 MPa total pressure. The catalytic activity of each catalyst was measured at acetic acid conversion of less than 25 % in the absence of diffusion limitations (see the Supporting Information for Weisz modulus calculations). Figure 1 shows the turnover frequencies (TOFs) as a function of temperature for the total conversion of acetic acid. The TOFs of acetic acid conversion decrease in the order Ru > Rh % Pt > Pd % Ir > Ni >...

The adaptive kinetic Monte Carlo method uses minimum-mode following saddle point searches and harmonic transition state theory to model rare-event, state-to-state dynamics in chemical and material systems. The dynamical events can be complex, involve many atoms, and are not constrained to a grid-relaxing many of the limitations of regular kinetic Monte Carlo. By focusing on low energy processes and asserting a minimum probability of finding any saddle, a confidence level is used to describe the completeness of the calculated event table for each state visited. This confidence level provides a dynamic criterion to decide when sufficient saddle point searches have been completed. The method has been made efficient enough to work with forces and energies from density functional theory calculations. Finding saddle points in parallel reduces the simulation time when many computers are available. Even more important is the recycling of calculated reaction mechanisms from previous states along the dynamics. For systems with localized reactions, the work required to update the event table from state to state does not increase with system size. When the reaction barriers are high with respect to the thermal energy, first-principles simulations over long time scales are possible.

The small and synthetically easily accessible 7-diethylamino-4-thiocoumarinylmethyl photolabile protecting group has been validated for uncaging with blue light. It exhibits a significant action cross-section for uncaging in the 470-500 nm wavelength range and a low light absorption between 350 and 400 nm. These attractive features have been implemented in living zebrafish embryos to perform chromatic orthogonal photoactivation of two biologically active species controlling biological development with UV and blue-cyan light sources, respectively.

Combining the dimer saddle point searching method and periodic density functional theory calculations, the potential energy surface of methanol decomposition on Cu(110) has been mapped out. Each elementary step in the methanol decomposition reaction into CO and hydrogen occurs via one of three possible mechanisms: O-H, C-H, or C-O bond scission. Multiple reaction pathways for each bond scission have been identified in the present work. Reaction pathway calculations are started from an initial (reactant) state with methanol adsorbed in the most stable geometry on Cu(110). The saddle point and corresponding final state of each reaction or diffusion mechanism were determined without assuming the reaction mechanism. In this way, the reaction paths are determined without chemical intuition. The harmonic pre-exponential factor of each identified reaction is calculated from a normal-mode analysis of the stationary points. Then, using harmonic transition state theory, the rate constant of each identified reaction pathway in the entire reaction network is obtained. The most favorable decomposition route for methanol on Cu(110) is found as follows: CH 3 OH f CH 3 O f CH 2 O. The rate-limiting step in this decomposition route is the dehydrogenation of methoxy to formaldehyde. Our calculations are in agreement with previous experimental observations and theoretical results.

Lantibiotics are a type of ribosomally synthesized and post‐translationally modified peptides (termed lanthipeptides) with often potent antimicrobial activity. Herein, we report the discovery of a new lantibiotic, lexapeptide, using the library expression analysis system (LEXAS) approach. Lexapeptide has rare structural modifications, including N‐terminal (N,N)‐dimethyl phenylalanine, C‐terminal (2‐aminovinyl)‐3‐methyl‐cysteine, and d‐Ala. The characteristic lanthionine moiety in lexapeptide is formed by three proteins (LxmK, LxmX, and LxmY), which are distinct from enzymes known to be involved in lanthipeptide biosynthesis. Furthermore, a novel F420H2‐dependent reductase (LxmJ) from the lexapeptide biosynthetic gene cluster (BGC) is identified to catalyze the reduction of dehydroalanine to install d‐Ala. Our findings suggest that lexapeptide is the founding member of a new class of lanthipeptides that we designate as class V. We also identified further class V lanthipeptide BGCs in actinomycetes and cyanobacteria genomes, implying that other class V lantibiotics await discovery.

Density functional theory calculations predict that small clusters of Pd atoms, containing up to at least four atoms, are highly mobile on the MgO(100) surface with the tetramer having the largest diffusion rate at room temperature--larger than the monomer. Surface vacancies are found, however, to bind the larger clusters strongly enough to trap them. These are important considerations when analyzing the growth and sintering of metal islands on oxide surfaces, in particular, the role of point defects.

Abstract:The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry, in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functional theory (DFT) calculations. The MgO(100) thin films (∼4 nm thick) were grown epitaxially on a 1 µm thick Mo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AES and ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surface with a density of ∼1 × 10 12 islands/cm 2 . Ca adsorbs initially at defect sites with a very high heat of adsorption (∼410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), step sites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreases rapidly with coverage, reaching a minimum of 162 kJ/mol at ∼0.3 ML, where Ca is mainly adding to small 3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at ∼1.2 ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the work function with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreement with calculations showing that Ca donates electron density to the MgO. Light ion sputtering of the MgO-(100) surface generates point defects, but these do not change the heat of adsorption versus coverage, implying that they do not nucleate Ca particles. Oxygen vacancies are a likely candidate; DFT calculations show that F and F+ center vacancies bind Ca more weakly than terrace sites. More extensive sputtering creates extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up to ∼400 kJ/mol, similar to that at the intrinsic defect sites.