Facts and alternative facts in chemical kinetics: remarks about the kinetic use of activities, termolecular processes, and linearization techniques

A Kinetic Study of the Electron-Transfer Reactions of Nickel(III,II) Tripeptide Complexes with Cyano Complexes of Molybdenum, Tungsten, and Iron

Experimentally measured rate constants, k₁₂^obsd, for the reductions of [Ni(III)tripeptides(H₂O)₂] with Fe(CN)₆^4-, Mo(CN)₈^4-, and W(CN)₈^4- are 10² to 10⁵ times faster than the calculated rate constants with the Marcus theory for outer-sphere electron-transfer processes, k₁₂^calc, even when work terms are considered. This gives rise to a kinetic advantage of k₁₂^obsd/k₁₂^calc = 10²-10⁵, which is consistent with an inner-sphere electron-transfer mechanism via a bridged intermediate. In addition, k₁₂^obsd values are nearly independent of the electrochemical driving force of the reactions. This is consistent with one of the two axial water ligands coordinated to [Ni(III)tripeptides(H₂O)₂] being substituted in the rate-limiting step to form bridged intermediates of the type [(CN)_5or7M-(CN)-Ni^III(tripeptide)(H₂O)]^4- with M = Fe^II, Mo^IV, or W^IV. A limiting rate constant of H₂O replacement from [Ni(III)tripeptides(H₂O)₂] of (5 ± 2) × 10⁷ M^-1 s^-1 at 25.0 °C is observed. Electron paramagnetic resonance spectra of Ni(III) peptide complexes in the presence of Fe(CN)₆^3-, Mo(CN)₈^3-, or IrCl₆^3- provide evidence for the cyanide-bridged intermediates. Substitution-limited electron-transfer reactions could serve as an additional criterion for inner-sphere pathways when atom or group transfer does not occur during electron-transfer and when precursor and successor complexes cannot be observed directly.

Characterization and Comparative Performance of TiO2 Photocatalysts on 6-Mercaptopurine Degradation by Solar Heterogeneous Photocatalysis

The crystallographic properties of two titanium dioxide (TiO2) photocatalysts, P25, and commercial C1-TiO2 reactive grade, were analyzed by X-ray diffraction (XRD) and the band-gap was calculated with UV–Vis spectrometry with integration sphere. Then, their performance was tested in the degradation of 6-mercaptopurine (6-MP) by heterogeneous photocatalysis with solar radiation under different pH conditions and the addition of hydrogen peroxide (H2O2); the degradation efficiency was monitored by UV–Vis spectrophotometry. The XRD analysis showed that both photocatalysts studied have anatase phase, while only P25 contains rutile; the band gap values were lower, in both catalysts, than those reported for catalysts obtained by the sol-gel method. With both photocatalysts, degradation experiments showed efficiency greater than 98% in experiments in the presence of H2O2 regardless of pH. The properties of the photocatalysts, along with the data obtained from the experimentation, helped determine the best semiconductor for the degradation of 6-MP with these operating conditions in this work.

Kinetic study of esterification over structured ZSM-5-coated catalysts based on fluid flow situations in macrocellular foam materials

Kinetics on the structured catalysts exhibited strong dependency on a fluid flow characteristic (e.g., Re number). The results can guide the design of structured catalytic packing in the applications of the reaction distillation (RD) process.