Free Energy Barrier For Formation Research Articles

Identifying active sites of boron, nitrogen co-doped carbon materials for the oxygen reduction reaction to hydrogen peroxide

The electrochemical synthesis of hydrogen peroxide (H2O2) from two-electron oxygen reduction reaction (2e- ORR) is a promising alternative for producing chemicals on demand, but its widespread application is still hampered by the low efficiency. Here, we successfully prepared a boron and nitrogen co-doped porous carbon (B/NC) aerogel with a tunable B, N co-doped configuration by the gelation of PVA-graphene, borax and PANI, followed by pyrolysis. Due to a hierarchical porous structure and optimized B, N co-doping, B/NC aerogel showed an excellent electrocatalytic performance for H2O2 production in alkaline solution with a high H2O2 selectivity (94.16%) at positive applied potential (0.6 V vs. RHE), superior than most of the other reported electrocatalysts. Density functional theory (DFT) calculations reveal that the hexagonal boron nitride (hBN) coupled with neighboring pyridinic-N species act as the active sites to lower free energy barrier for formation of HOO* intermediate, thus facilitating H2O2 production. Practically, B 2p electron plays an important role for the adsorption of HOO* intermediates. B and Nco-doping into carbon materials provides an effective and facile method to reasonably construct carbon-based catalysts for electroreduction of O2 to H2O2.

Cascading Time Evolution of Dissipative Structures for Creation of Shish-kebab

External fields imposed on homogeneous systems can drive the systems to form ordered structures of the so-called "dissipative structures", when the fields can effectively reduce entropy and hence the free energy barrier for formation of the ordered structure [1]. We elucidated kinetic pathways, along which: (1) homogeneous polyethylene solutions composed of entangled random coils in quiescent state develop shish-kebabs under flow fields imposed on the solutions [2,3]; (2) Shish-kebabs swollen with solvent develop extended chain crystals under hot-drawings at temperatures close to draw-ratio-dependent melting temperatures [4]. The various dissipative patterns found in the kinetic pathways are summarized schematically in Figure 1. Acknowledgements: The author gratefully acknowledges Drs. H. Murase and Y. Ohta for their collaborations on this work.

Contributions of Long-Range Electrostatic Interactions to 4-Chlorobenzoyl-CoA Dehalogenase Catalysis: A Combined Theoretical and Experimental Study

It is well established that electrostatic interactions play a vital role in enzyme catalysis. In this work, we report theory-guided mutation experiments that identified strong electrostatic contributions of a remote residue, namely, Glu232 located on the adjacent subunit, to 4-chlorobenzoyl-CoA dehalogenase catalysis. The Glu232Asp mutant was found to bind the substrate analogue 4-methylbenzoyl-CoA more tightly than does the wild-type dehalogenase. In contrast, the kcat for 4-chlorobenzoyl-CoA conversion to product was reduced 10000-fold in the mutant. UV difference spectra measured for the respective enzyme-ligand complexes revealed an approximately 3-fold shift in the equilibrium of the two active site conformers away from that inducing strong pi-electron polarization in the ligand benzoyl ring. Increased substrate binding, decreased ring polarization, and decreased catalytic efficiency indicated that the repositioning of the point charge in the Glu232Asp mutant might affect the orientation of the Asp145 carboxylate with respect to the substrate aromatic ring. The time course for formation and reaction of the arylated enzyme intermediate during a single turnover was measured for wild-type and Glu232Asp mutant dehalogenases. The accumulation of arylated enzyme in the wild-type dehalogenase was not observed in the mutant. This indicates that the reduced turnover rate in the mutant is the result of a slow arylation of Asp145, owing to decreased efficiency in substrate nucleophilic attack by Asp145. To rationalize the experimental observations, a theoretical model is proposed, which computes the potential of mean force for the nucleophilic aromatic substitution step using a hybrid quantum mechanical/molecular mechanical method. To this end, the removal or reorientation of the side chain charge of residue 232, modeled respectively by the Glu232Gln and Glu232Asp mutants, is shown to increase the rate-limiting energy barrier. The calculated 23.1 kcal/mol free energy barrier for formation of the Meisenheimer intermediate in the Glu232Asp mutant represents an increase of 6 kcal/mol relative to that of the wild-type enzyme, consistent with the 5.6 kcal/mol increase calculated from the difference in experimentally determined rate constants. On the basis of the combination of the experimental and theoretical evidence, we hypothesize that the Glu232(B) residue contributes to catalysis by providing an electrostatic force that acts on the Asp145 nucleophile.

The kinetics of the internal nitriding of Fe-2 at. pct Al alloy

The kinetics of the precipitation of aluminum nitride on internal nitriding the Fe-2 at. pct Al alloy was investigated for cold-rolled and recrystallized specimens exhibiting “ideally weak” interaction behavior of the solutes Al and N. The kinetic analysis was performed using mass-increase data obtained for thin foils (thickness ⪯0.1 mm) upon nitriding in a NH3/H2 gas mixture at temperatures in the range 803 to 853 K. Activation-energy analysis revealed that precipitation of AlN in the recrystallized specimens is associated with a Gibbs free energy barrier for the formation of a precipitate of critical size; the precipitation rate is controlled by both nucleation and growth. On the other hand, precipitation of AlN in the cold-rolled specimens occurs without a Gibbs free energy barrier for formation of a precipitate of critical size; the precipitation rate is controlled by growth with kinetics governed by volume diffusion of alu-minum. Analysis of the total Gibbs free energy of formation of AlN in the α-Fe matrix showed that in the case of the recrystallized specimens, the formation of incoherent AlN precipitates with a hexagonal crystal structure is favored. In the case of the cold-rolled specimens, containing a high dislocation density, the formation of coherent AlN precipitates with cubic crystal structure is favored, at least in the beginning of precipitation.