Temperature-dependent Kinetic Studies Research Articles

Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters - pH- and Temperature-Dependent Kinetic Studies.

The atmospheric α-pinene oxidation leads to three carboxylic acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). In this study, the OH radical kinetics in the aqueous phase of these carboxylic acids were investigated at different temperatures and pH values of solutions. Activation parameters and the corresponding atmospheric lifetimes of the acids in the troposphere were derived. The overall second-order rate constants for the individual speciation forms of the acids (AH and A- for NPA; AH2, AH- and A2- for PA; and AH3, AH2-, AH2- and A3- for MBTCA) were determined. At 298 K, the rate constants for reactions of protonated forms (AHx) of NPA, PA, and MBTCA with •OH, were (1.5 ± 0.2) × 109 L mol-1 s-1, (2.4 ± 0.1) × 109 L mol-1 s-1, and (4.1 ± 0.6) × 108 L mol-1 s-1, respectively. For the fully deprotonated forms (Ax-) of studied acids, the second-order rate constants were (2.2 ± 0.2) × 109 L mol-1 s-1, (2.8 ± 0.1) × 109 L mol-1 s-1, and (10.2 ± 0.7) × 108 L mol-1 s-1 at 298 K, respectively. It was found that the reactions of NPA and PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction rate depends on pH more strongly at elevated temperatures (>298 K). The atmospheric lifetimes of the acids considered due to their reactivity with •OH were calculated for different model scenarios at a temperature of 283 K and pH = 2 in the aqueous phase. For this purpose, liquid water content (LWC) was used for aerosols and clouds under storm conditions and at various aqueous-phase concentrations of OH radicals. The lifetimes decreased with increasing LWC (from 10-12 m3 m-3 in aerosol to 10-5 m3 m-3 in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. Besides, the aerosol systems appeared less effective in removing PA and NPA, with lifetimes ranging from hundreds of days to tens and hundreds of hours, respectively. Clouds were more effective, with lifetimes ranging from tens of hours to a single second or less. MBTCA, which dissolves better in water, was effectively removed in all systems, with the longest lifetime of approximately 90 min.

Effects of Ionic Liquids on the Nucleofugality of Bromide.

The nucleofugality of bromide was measured in solvent mixtures containing ionic liquids. The solvolysis rate constants of the bromides of well-defined electrofuges were determined in mixtures containing different proportions of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide in ethanol. Temperature-dependent kinetic studies allowed an explanation of the observed solvent effects in different mixtures in terms of interactions in solution. Using the solvolysis data, the nucleofugality of bromide in these systems was determined. Likewise, nucleofugality data for bromide were determined in mixtures containing high proportions of seven further ionic liquids. These data allowed quantification of the effects of both varying the amount of ionic liquid and the nature of ionic liquid components on the nucleofugality of bromide. Importantly, ionic liquid mixtures were shown to affect the nucleofugality in a manner similar to chloride, providing a method for predicting the effects of ionic liquids on other electrofuges. Further, the ionic liquids were shown to move the transition state earlier along the reaction coordinate, meaning that there is less charge development in the transition state.

Temperature-Dependent Kinetic Study of the Reactions of Hydrogen Atoms with H2S and C2H4S

A discharge-flow reactor combined with modulated molecular beam mass spectrometry technique was employed to determine the rate constants of H-atom reactions with hydrogen sulfide and thiirane. The rate constants for both reactions were determined at a total pressure of 2 Torr from 220 to 950 K under pseudo-first-order conditions by monitoring either consumption of H atoms in excess of H2S (C4H4S) or the molecular species in excess of atomic hydrogen. For H + H2S reaction, a suggested previously strong curvature of the Arrhenius plot was confirmed: kl = 8.7 × 10−13 × (T/298)2.87 × exp(−125/T) cm3 molecule−1 s−1 with a conservative uncertainty of 15% at all temperatures. Non-Arrhenius behavior was also observed for the reaction of H-atom with C2H4S, with the experimental rate constant data being best fitted to a sum of two exponential functions: k2 = 1.85 × 10−10 exp(−1410/T) + 4.17 × 10−12 exp(−242/T) cm3 molecule−1 s−1 with an independent of temperature uncertainty of 15%.

Halide Ion Mixing across Colloidal 2D Ruddlesden-Popper Perovskites: Implication of Spacer Ligand on Mixing Kinetics.

Halide ion exchange seen in metal halide perovskites provide a substantial opportunity to control their halide composition and corresponding optoelectronic properties. Halide ion mixing across colloidal 3D perovskite nanocrystals have been extensively studied while the mixing within colloidal 2D counterparts remain underexplored. In this study, the halide ion exchange kinetics across colloidally stable 2D Ruddlesden-Popper layered bromide (Br) and iodide (I) perovskites using two different spacer ligands such as aromatic phenethylammonium (PEA) versus linear butyammonium (BA) is demonstrated. The halide exchange kinetic rate constant (k), as determined by tracking time-dependent absorbance changes, indicates that Br/I halide mixing in 2D PEA-based perovskites (2.7 × 10-3 min-1 ) occurs at an order of magnitude slower than in 2D BA-based perovskites (3.3 × 10-2 min-1 ). Concentration (≈1mM to 100mM) and temperature-dependent (50 to 80 °C) kinetic studies further allow for the determination of activation barrier for halide ion mixing across the 2D layered perovskites with 75.2 ± 4.4kJ mol-1 (2D PEA) and 57.8 ± 7.8kJ mol-1 (2D BA), respectively. The activation energy reveals that the type of spacer cations plays a crucial role in controlling the halide ion mobility and halide stability due mainly to the internal ligand chemical interaction within 2D structures.

Ionic Cooperativity between Lysine and Potassium in the Lysine Riboswitch: Single-Molecule Kinetic and Thermodynamic Studies.

Functionality in many biological systems, including proteins and nucleic acid structures, including protein and nucleic acid riboswitch structures, can depend on cooperative kinetic behavior between multiple small molecule ligands. In this work, single-molecule FRET data on the Bacillus subtilis lysine riboswitch reveals that affinity for the cognate lysine ligand increases significantly with K+, providing evidence for synergism between lysine/K+ binding to the aptamer and successful folding of the riboswitch. To describe/interpret this more complex kinetic scenario, we explore the conventional 4-state ("square") model for aptamer binding as a function of K+. Extension into this additional dimension generates a novel "cube" model for riboswitch folding dynamics with respect to lysine/K+ binding, revealing that riboswitch folding (kfold) and unfolding (kunfold) rate constants increase and decrease dramatically with K+, respectively. Furthermore, temperature-dependent single-molecule kinetic studies indicate that the presence of K+ entropically enhances the transition state barrier to folding but partially compensates for this by increasing the overall exothermicity for lysine binding. We rationalize this behavior as evidence that K+ facilitates hydrogen bonding between the negatively charged carboxyl group of lysine and the RNA, increasing structural rigidity and lowering entropy in the binding pocket. Finally, we explore the effects of cation size with Na+ and Cs+ studies to demonstrate that K+ is optimally suited for bridging interactions between lysine and the riboswitch aptamer domain. Regulation of lysine production and transport, dictated by the riboswitch's ability to recognize and bind lysine, is therefore intimately tied to the presence of K+ in the binding pocket and is strongly modulated by local cation conditions. The results suggest an increase in lysine riboswitch functionality by sensitivity to additional species in the cellular riboswitch environment.

Kinetic study of plasma assisted oxidation of H2 for an undiluted rich mixture

Non-thermal plasma discharges are considered as an assisted method for supporting conventional thermal processes, because the discharge-generated radicals can be used to benefit them. In this regard, plasma assisted combustion has been investigated extensively, and plasma assisted fuel reforming has expanded its horizons to include plasma assisted chemical synthesis, in accord with a global megatrend—electrification. To scientifically advance this area, plasma-chemical kinetic studies must be conducted, and their mechanisms should consider both electron and gas temperatures to describe electron-induced and thermally-induced chemistry, respectively. Here we present a temperature-dependent plasma-chemical kinetic study for the plasma assisted oxidation of an undiluted rich H2/O2 mixture; it represents our continued effort to establish a foundational plasma assisted reaction mechanism for H2/O2 systems. A plasma-chemical kinetic model (KAUSTKin) and a modified plasma-chemical reaction mechanism were used for the zero-dimensional simulation; a temperature-controlled dielectric barrier discharge reactor was employed for the experiment. Non-linear oxidation behavior was observed in a range of 300–850 K for various discharge powers. To unravel the underlying mechanisms, analysis was broken down to a physical effect, described by the reduced electric field (E/N), and a chemical effect originating from the temperature dependence of the plasma assisted oxidation chemistry. We concluded that (i) the oxidation was initiated by the electron impact dissociation of H2, (ii) E/N governed the degree of H2 dissociation (and thus the degree of oxidation), (iii) the key intermediate was HO2, and (iv) a significant increase in the chain termination reaction, H + HO2 → H2 + O2, suppressed the oxidation (especially in the Negative Temperature Coefficient (NTC)-like regime of 600–725 K). The need to improve existing thermal reaction mechanisms for temperatures below 1000 K was emphasized, and a method for temperature-dependent plasma-chemical kinetic studies to provide a novel and complementary tool for this task was demonstrated.

Kinetic Study for Plasma Assisted Cracking of NH3: Approaches and Challenges.

Ammonia is considered as one of the promising hydrogen carriers toward a sustainable world. Plasma assisted decomposition of NH3 could provide cost- and energy-effective, low-temperature, on-demand (partial) cracking of NH3 into H2. Here, we presented a temperature-dependent plasma-chemical kinetic study to investigate the role of both electron-induced reactions and thermally induced reactions on the decomposition of NH3. We employed a plasma-chemical kinetic model (KAUSTKin), developed a plasma-chemical reaction mechanism for the numerical analysis, and introduced a temperature-controlled dielectric barrier discharge reactor for the experimental investigation using 1 mol % NH3 diluted in N2. As a result, we observed the plasma significantly lowered the cracking temperature and found that the plasma-chemical mechanism should be further improved to better predict the experiment. The commonly used rates for the key NH3 pyrolysis reaction (NH3 + M ↔ NH2 + H + M) significantly overpredicted the recombination rate at temperatures below 600 K. Furthermore, the other identified shortcomings in the available data are (i) thermal hydrazine chemistry, (ii) electron-scattering cross-section data of NxHy, (iii) electron-impact dissociation of N2, and (iv) dissociative quenching of excited states of N2. We believe that the present study will spark fundamental interest to address these shortcomings and contribute to technical advancements in plasma assisted NH3 cracking technology.

The effects of ionic liquids on the ethanolysis of a chloroacenaphthene. Evaluation of the effectiveness of nucleofugality data to predict reaction outcome.

The reaction of a chlorobenzene in mixtures containing ethanol and eight different ionic liquids was investigated in order to understand the effects of varying proportions and constituent ions of an ionic liquid on the rate constant of the process. The results were found to be generally consistent with previously studied reactions of the same type, with small proportions of an ionic liquid resulting in a rate constant increase compared to ethanol and large proportions causing a rate constant decrease. Temperature dependent kinetic studies were used to interpret the changes in reaction outcome, particularly noting an entropic cost on moving to high proportions of ionic liquid, consistent with organisation of solvent around the transition state. While attempts to use empirical solvent parameters to correlate outcome with the ionic liquid used were unsuccessful, use of recently acquired nucleofugality data for chloride and estimations for the electrofuge allowed for excellent prediction of the effects of ionic liquids, with rate constants quantitatively predicted in systems containing both different proportions of ionic liquid (mean absolute error (MAE) log(k1) = 0.11) and different ionic liquids (MAE log(k1) = 0.33). Importantly, this demonstrates the ready application of these quantitative reactivity parameters.

Effects of Ionic Liquids on the Nucleofugality of Chloride.

The nucleofugality of chloride has been measured in solvent mixtures containing ionic liquids for the first time, allowing reactivity in these solvents to be put in context with molecular solvents. Using well-described electrofuges, solvolysis rate constants were determined in mixtures containing different proportions of ethanol and the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide; the different solvent effects observed as the mixture changed could be explained using interactions of the ionic liquid with species along the reaction coordinate, determined using temperature dependent kinetic studies. The solvolysis data allowed determination of the nucleofugality of chloride in these mixtures, which varied with the proportion of salt in the reaction mixture, demonstrating quantitatively the importance of the amount of ionic liquid in the reaction mixture in determining reaction outcome. Nucleofugality data for chloride were determined in seven further ionic liquids, with the reactivity shown to vary over more than an order of magnitude. This outcome illustrates that the components of the ionic liquid are critical in determining reaction outcome. Overall, this work quantitatively extends the understanding of solvent effects in ionic liquids and demonstrates the potential for such information to be used to rationally select an ionic liquid to control reaction outcome.

Temperature-dependent kinetic study of the photo-oxidation reaction of vinyl butyrate with Cl atoms and fate of the formation of alkoxy radicals

RR-GC-FID technique was used to investigate the rate coefficient (k) for the reaction of vinyl butyrate (VB) with Cl atoms in the temperature range of 268 and 363 K at 760 Torr of N2. The k at 298 K was measured to be k298KVB+Cl=2.57±0.68×10-10 cm3 molecule−1 s−1. To comprehend the reaction pathway, and kinetic parameters associated with the title reaction were assessed computationally using CVT/SCT/ISPE method at CCSD(T)//MP2/6-311++G(d,p) level of theory. Product analysis was performed using GC–MS and GC-IR. Cumulative Atmospheric Lifetime (CAL), Radiative Forcing (RF), Global Warming Potential (GWP) and Photochemical Ozone Creation Potential (POCP) were estimated.

In Situ and Real-Time ATR-FTIR Temperature-Dependent Adsorption Kinetics Coupled with DFT Calculations of Dimethylarsinate and Arsenate on Hematite Nanoparticles.

Temperature-dependent kinetic studies of the adsorption of critical pollutants onto reactive components in soils and removal technologies provide invaluable rate information and mechanistic insight. Using attenuated total internal reflection Fourier transform infrared spectroscopy, we collected in situ spectra as a function of time, concentration, and temperature in the range of 5-50 °C (278-323 K) for the adsorption of arsenate (iAs) and dimethylarsinate (DMA) on hematite nanoparticles at pH 7. These experimental data were modeled with density functional theory (DFT) calculations on the energy barriers between surface complexes. The Langmuir adsorption kinetic model was used to extract values of the fast (<5 min) and slow (6-10 min) observed adsorption rate, initial rate constants of adsorption and desorption, Arrhenius parameters, effective activation energies (ΔEa), and pre-exponential factors (A). The trend in the kinetic parameters correlated with the type of surface complexes that iAs and DMA form, which are mostly bidentate binuclear compared to a mix of outer sphere and monodentate, respectively. The observed initial adsorption rates were found to be more sensitive to changes in the aqueous concentration of the arsenicals than slow rates. On average, iAs adsorbs 2.5× faster and desorbs 4× slower than dimethylarsinate (DMA). The ΔEa and A values for the adsorption of iAs bidentate complexes are statistically higher than those extracted for outer-sphere DMA by a factor of 3. The DFT results on adsorption energies and ΔEa barriers are consistent with the experimental data and provide a mechanistic explanation for the low ΔEa values observed. The presence of defect sites with under-coordinated Fe atoms or exchangeable surface water (i.e., Fe-OH2 groups) lowers activation barriers of adsorption. These results suggest that increasing organic substitutions on arsenate at the expense of As-O bonds decreases the effective energy barrier for complex formation and lowers the number of collisional orientations that result in binding to the hematite surface.

Temperature-Dependent Kinetic Study of the Reaction of Hydroxyl Radicals with Hydroxyacetone

The kinetics of the reaction of OH radicals with hydroxyacetone has been investigated as a function of temperature at (2.0 - 2.1) Torr total pressure of helium and over an extended temperature range, T = 250 - 830 K, and as a function of pressure at T = 301 K in the pressure range 1.0-10.4 Torr. The rate constant of the reaction OH + CH3C(O)CH2OH  products (1) was measured using both absolute (from the kinetics of OH consumption in excess of hydroxyacetone) and relative rate method, k1 = 4.7×10-22×T3.25exp(1410/T) cm3molecule-1s-1 at T = 250-830 K. The present data combined with selected previous temperature dependent studies of reaction (1) yield k1 = 4.4×10-20×T2.63exp(1110/T) cm3molecule-1s-1, which is recommended from the present work at T = 230-830K (with conservative uncertainty of 20% at all temperatures). k1 was found to be pressure independent in the pressure range from 1.0 to 10.4 Torr of He at T = 301 K. The present results are compared with previous experimental and theoretical data.

Temperature dependent kinetic study of the gas phase reaction of ozone with 1-penten-3-ol, cis-2-penten-1-ol and trans-3-hexen-1-ol: Experimental and theoretical data

C5 and C6 unsaturated alcohols are a family of biogenic volatile organic compounds (BVOC) emitted in the atmosphere from vegetation in response to their injuries. In this work, we report the first temperature dependent rate constants for the reaction of three unsaturated alcohols: 1-penten-3-ol (1P3OL), cis-2-penten-1-ol (c-2P1OL) and tran-3-hexen-1-ol (t-3H1OL) with ozone in a pyrex reactor coupled to a FTIR spectrometer and to a GC/MS at four different temperatures (273, 298, 313 and 333 K), at atmospheric pressure, using a relative method. The Arrhenius expressions obtained are (cm3 molecule−1 s−1):The studied reactions were also explored theoretically using computational methods based on quantum chemical theory characterizing the intermediates, transition states and the subsequent formation of reaction products. The rate constants were calculated at room temperature employing a modified transition-state theory (MTST). Both results obtained experimentally and theoretically, were discussed in terms of structure-reactivity relationships and temperature dependence. Tropospheric lifetimes of the investigated species with respect to ozone are calculated. The obtained results showed that these species are not persistent towards ozonolysis process.

Oxidative Degradation Kinetics and Mechanism of Two Biodiesel Constituents (Ethyl-2-Methyl Propionate (E2MP) and Ethyl-2,2-Dimethyl Propionate (E22DMP)) Initiated by Cl Atoms

The oxidative reaction kinetics and mechanism of Ethyl-2-methyl propionate (E2MP) and Ethyl-2, 2-dimethyl propionate (E22DMP) initiated by Cl atoms were investigated both experimentally and computationally. Temperature dependent kinetic studies on the reaction of E2MP and E22DMP with Cl atoms were measured using relative rate technique over the temperature range of 268-363 K and at 760 Torr. The rate coefficients of both the reactions at 298 K were measured (using IUPAC recommended reference rate coefficients) to be k_(E2MP+Cl)^(298 K)= (7.19±1.60) × 10-11 cm3 molecule-1 s-1 and k_(E22DMP+Cl)^(298 K)= (5.87±1.33) × 10-11 cm3 molecule-1 s-1 respectively. In addition to complement our experimental results, the kinetics were computed at CCSD(T)/MG3S//BHandHLYP/6-311+G(d,p) level of theory over the temperature range of 200-400 K using Canonical Variational Transition State theory (CVT) in conjunction with Small Curvature Tunneling (SCT) and Interpolated Single Point Energy (ISPE). Furthermore, the product deg...

Ground state chemistry under vibrational strong coupling: dependence of thermodynamic parameters on the Rabi splitting energy

Abstract Vibrational strong coupling (VSC) is currently emerging as a tool to control chemical dynamics. Here we study the impact of strong coupling strength, given by the Rabi splitting energy (ħΩR), on the thermodynamic parameters associated with the transition state of the desilylation reaction of the model molecule 1-phenyl-2-trimethylsilylacetylene. Under VSC, the enthalpy and entropy of activation determined from the temperature-dependent kinetic studies varied nonlinearly with the coupling strength. The thermodynamic parameters of the noncavity reaction did not show noticeable variation, ruling out concentration effects other than the enhanced ħΩR for the changes observed under VSC. The difference between the total free energy change under VSC and in noncavity was relatively smaller possibly because the enthalpy and entropy of activation compensate each other. This thermodynamic study gives more insight into the role of collective strong coupling on the transition state that leads to modified dynamics and branching ratios.

Three- and two-site heteropolyoxotungstate anions as catalysts for the epoxidation of allylic alcohols by H2O2 under biphasic conditions: Reactivity and kinetic studies of the [Ni3(OH2)3(B-PW9O34){WO5(H2O)}]7−, [Co3(OH2)6(A-PW9O34)2]12−, and [M4(OH2)2(B-PW9O34)2]10− anions, where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)

The trimetallic phosphopolyoxotungstate anions [Ni3(OH2)3(B-PW9O34){WO5(H2O)}]7− and [Co3(OH2)6(A-PW9O34)2]12− have been studied as epoxidation catalysts for oxygen transfer from 30% H2O2 to a range of allylic alcohols under biphasic conditions (1,2-dichloroethane/H2O) at 15 °C. The reaction mechanism involves coordination of an allylic alcohol at an M(II) site in each case, prior to transfer of a peroxy oxygen from an adjacent W(O2) site. The latter is formed from a terminal W = O unit by reaction with H2O2. Evidence of W(O2) formation was obtained through IR studies. The W(O2) group forms the epoxide by transfer of an oxygen atom to the CC bond of the coordinated allylic alcohol. Kinetic studies using 3-methyl-2-buten-1-ol as the allylic alcohol substrate have been modelled with all three metal sites catalytically active. The reaction involves an autocatalysis mechanism involving an induction period, which can be rationalised by proposing not only coordination of the allylic alcohol to M(II), but also the product hydroxy epoxide, both through their –OH groups. The autocatalysis is generated by formation of the W(O2) group adjacent to a coordinated hydroxy epoxide, which competes with coordination of allylic alcohol. The mechanism requires some twenty-one steps involving just the generic steps listed above, with all three metal sites catalytically active. Temperature-dependent kinetic studies and subsequent Eyring analyses have shown that the Co(II)-containing catalyst is the most active of the two. Analogous studies of the epoxidation of 3-methyl-2-buten-1-ol by the two-site [M4(OH2)2(B-PW9O34)2]10− ions as catalysts, where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II), at 15 °C gave an order of reactivity of Cu(II) > Ni(II) > Zn(II), Co(II), Mn(II), which mostly mimics the natural order of stability constants (the Irving-Williams series), suggesting that the formation of the allylic alcohol complexes play a dominant role in this series of related complex anions, with greater replacement of water by allylic alcohol leading to greater reactivity.

Iridium-Based Catalysts for Electro-Oxidation of Ammonia in Alkaline Media

Ammonia is an attractive carbon-free liquid fuel for alkaline membrane fuel cells. While ethanol has higher energy density than ammonia, its product, CO2, reacts with OH- to form carbonate in alkaline media, which can cause kinetic loss and cell failure. Thus, carbon-free is an advantage of ammonia.[1, 2] In the 6-electron ammonia oxidation reaction (AOR), 2NH­3 +6OH- = N2 + 3H2O + 6e-, formation of the N-N bond is a rate-limiting step with rather high activation barrier. Most of research focused on Pt-based catalysts because Pt is essential for high current peak at ambient temperature. Ir is known for its low onset potential for AOR. As our recent temperature-dependent kinetic study showed, alloy PtIr/C is currently the best commercial catalyst for AOR at 60 oC and the temperature enhancement factor between 60 and 25 oC is in the order of Ir (5.5) > IrPt (4.2) > Pt (2.0).[3] Thus, developing Ir-based catalysts holds the promise for improving anode performance in fuel cells operating at elevated temperatures. In this talk, we show that Fe nitride can promote the AOR activity on Ir, Pt, and IrPt catalysts. Three carbon-supported catalysts, Ir4Fe2Nx/C, Ir3Pt1Fe2Nx/C, and Pt3Fe2Nx/C, were prepared by reducing and nitriding metal chloride precursors on Vulcan XC-72 carbon in a tube furnace with mixed hydrogen and ammonia gases. We dispersed the catalysts on gas diffusion electrodes with a loading of platinum group metals (PGM) of 100 μg/cm2 and plot the AOR polarization curves with the currents normalized to PGM loadings in Figure 1. Even though Pt3Fe2Nx/C achieved the highest peak current density, its activity at low overpotential range (0.35- 0.5 V) is much lower than those containing Ir. The compression of the Ir lattice induced by the small alloyed Fe atoms results in slightly higher AOR peak current density of Ir4Fe2Nx/C than Ir/C, while the peak potentials remain unchanged. As the Pt: platinum-group-metal ratio increases from 0 in Ir/C and Ir4Fe2N/C to 100% in Pt3Fe2Nx/C, the peak potential increase gradually, which correspond well with the peak position for Ir3Pt1Fe2Nx/C catalysts which located between Ir/C and IrPt/C. When comparing AOR activity of Ir3Pt1Fe2Nx/C with Ir4Fe2Nx/C, substitution of 1/4 Ir atoms in Ir4Fe2Nx/C with Pt atoms leads to an optimal activity for AOR among the tested samples as a consequence of combining the low onset potentials of Ir4Fe2Nx/C and high peak current density of Pt3Fe2Nx/C. More detailed results of this work will be discussed in the presentation. Figure 2 shows the X-ray diffraction patterns of these three catalysts in comparison with two commercial catalysts of Ir/C and IrPt/C. There is no peak corresponding to any Fe or Fe nitride structures, indicating that Fe is well alloyed with Ir and/or Pt. It appears that AOR activity on Ir-based catalysts prefers neither expansion or contraction of Ir lattice. The diffraction peak for Ir3Pt1Fe2Nx/C locates at the same angle as the Ir/C which may be as an integrated effect from Pt (bigger than Ir) and Fe (smaller than Ir) atoms. Acknowledgements This research was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000805 and by the Division of Chemical Sciences, Geosciences and Biosciences Division, US Department of Energy under contract DE-SC0012704.

Transition-Metal Ion-Doped Flower-Like Titania Nanospheres as Nonlight-Driven Catalysts for Organic Dye Degradation with Enhanced Performances.

Titania has recently been identified as a new and effective nonlight-driven catalyst for degradation of organic pollutant with the use of H2O2 as an oxidant; however, either relatively low surface area or lack of diversity in chemical composition largely limits its catalytic performance. In this work, a series of transition-metal ion (Mn2+, Co2+, Ni2+, and Cu2+)-doped titania nanomaterials with regular flower-like morphology, good crystallinity (anatase), and large specific surface areas (71.4–124.4 m2 g–1) were facilely synthesized and utilized as catalysts for methylene blue (MB) degradation in the presence of H2O2 without light irradiation. It was revealed that the doping of transition-metal ions (especially Mn2+) into titania could significantly improve the catalytic efficiency. At 30 °C, 10 mL of MB with a concentration of 50 mg L–1 could be completely degraded within 60–100 min for these doped samples, whereas the removal rate was only 35.1% within 100 min with the use of pure flower-like titania. Temperature-dependent kinetic studies indicated that the presence of transition-metal ion dopants could markedly lower the activation energy and thus resulted in enhanced catalytic performances. Test of reusability exhibited that these doped catalysts could well keep their original catalytic activities after reuse for several cycles, indicating their excellent catalytic durability.

Conformationally Regulated Peptide Bond Cleavage in Bradykinin.

Ion mobility and mass spectrometry techniques are used to investigate the stabilities of different conformations of bradykinin (BK, Arg1-Pro2-Pro3-Gly4-Phe5-Ser6-Pro7-Phe8-Arg9). At elevated solution temperatures, we observe a slow protonation reaction, i.e., [BK+2H]2++H+ → [BK+3H]3+, that is regulated by trans → cis isomerization of Arg1-Pro2, resulting in the Arg1- cis-Pro2- cis-Pro3-Gly4-Phe5-Ser6- cis-Pro7-Phe8-Arg9 (all- cis) configuration. Once formed, the all- cis [BK+3H]3+ spontaneously cleaves the bond between Pro2-Pro3 with perfect specificity, a bond that is biologically resistant to cleavage by any human enzyme. Temperature-dependent kinetics studies reveal details about the intrinsic peptide processing mechanism. We propose that nonenzymatic cleavage at Pro2-Pro3 occurs through multiple intermediates and is regulated by trans → cis isomerization of Arg1-Pro2. From this mechanism, we can extract transition state thermochemistry: Δ G‡ = 94.8 ± 0.2 kJ·mol-1, Δ H‡ = 79.8 ± 0.2 kJ·mol-1, and Δ S‡ = -50.4 ± 1.7 J·mol-1·K-1 for the trans → cis protonation event; and, Δ G‡ = 94.1 ± 9.2 kJ·mol-1, Δ H‡ = 107.3 ± 9.2 kJ·mol-1, and Δ S‡ = 44.4 ± 5.1 J·mol-1·K-1 for bond cleavage. Biological resistance to the most favored intrinsic processing pathway prevents formation of Pro3-Gly4-Phe5-Ser6- cis-Pro7-Phe8-Arg9 that is approximately an order of magnitude more antigenic than BK.

A Revised Mechanism for Human Cyclooxygenase-2

The mechanism of ω-6 polyunsaturated fatty acid oxidation by wild-type cyclooxygenase 2 and the Y334F variant, lacking a conserved hydrogen bond to the catalytic tyrosyl radical/tyrosine, was examined for the first time under physiologically relevant conditions. The enzymes show apparent bimolecular rate constants and deuterium kinetic isotope effects that increase in proportion to co-substrate concentrations before converging to limiting values. The trends exclude multiple dioxygenase mechanisms as well as the proposal that initial hydrogen atom abstraction from the fatty acid is the first irreversible step in catalysis. Temperature dependent kinetic studies reinforce the novel finding that hydrogen transfer from the reduced catalytic tyrosine to a terminal peroxyl radical is the first irreversible step that controls regio- and stereospecific product formation.