Electrochemical Analysis of Catalytic and Oxygen Interfacial Transfer Effects on MnO₂ Deposited on Gold Electrodes

Abstract

A theoretical model for obtaining thermochemical and kinetic information on oxygen interfacial transfer in metal oxides deposited on gold electrodes, based in the voltammetry of immobilized particle methodology, is presented. It is applied to MnO2 microparticulate deposits on gold in contact with aqueous NaOH solution using the voltammetric signals for the oxidation of gold to gold oxide monolayer and its subsequent desorptive reduction. Assuming reversibility, voltammetric peak potentials permit to estimate a variation of Gibbs free energy of interfacial oxygen transfer of −32 ± 3 kJ mol–1. Analysis of peak current data based on the Sharp–Hancock formalism of solid-state kinetics was consistent with the diffusive nature of the transfer process and permitted the separation between this effect and the catalytic effect exerted by MnO2 on the electrochemical oxidation of gold.

Synthesis of isomorphically substituted Ru manganese molecular sieves and their catalytic properties for selective alcohol oxidation

“…This experimental fact is in accordance with the initial reaction rates calculated for these catalysts as shown below. Interestingly, a control reaction in the absence of oxygen showed the existence of a residual oxidation activity by the [Ru]-K-OMS2 (2 wt%) catalyst suggesting the plausible involvement of a Mars van Krevelen mechanism, according to which the structural lattice oxygen would participate in the reaction being later reoxidized by molecular oxygen when present (entry 3, Table 2) [15,16] as it has been already pointed out for related octahedral molecular sieves [17][18][19]55]. The catalytic activity of RuO x /K-OMS2 (1 wt%) and K-OMS2 was also studied under inert atmosphere just for comparison and we observed that the extension of benzylic alcohol oxidation was greater for the isomorphically substituted catalyst [Ru]-K-OMS2 (2 wt%) than for catalysts RuO x /K-OMS2 (1 wt%) and K-OMS2 (compare entries 3, 5 and 8 in Table 2).…”

“…To this respect it is important to indicate that the oxidation of alcohols to aldehydes has been extensivelly studied in the presence of manganese molecular sieves [18,19,[53][54][55] as well as ruthenium catalysts although separately [56][57][58]. In general, stoichiometric amounts of high valent metal oxidants have been traditionally used to accomplish this transformation, but they have high toxicity and negative economic implications [59].…”

“…For example, chromium-based reagents are toxic, irritants and corrosive; whereas manganese dioxide (MnO 2 ), being more environment-friendly, has been used over the years as a highly efficient and selective catalyst for oxidation of allylic and benzylic alcohols to aldehydes, amines to imines, oxidation of CO, H 2 O 2 decomposition, oxygen reduction, oligomerization of methane… [19,55,[60][61][62][63][64] Table 2 includes the most important results obtained with K-OMS2, [Ru]-K-OMS2 (2 wt%) as well as other related oxides as catalysts for the sele ctive synthesis of benzaldehyde from benzyl alcohol as model reaction.…”

Redox Hopping in Metal–Organic Frameworks through the Lens of the Scholz Model

“…One of the most critical assumptions of the Scholz model is that the reaction starts at the three-boundary, which has been widely observed and used to explain electrochemical processes in solid-state materials, e.g., metal oxides. − The first observation of the three-phase boundary was demonstrated by Scholz et al through the reduction of lead oxide to lead. In situ atomic force microscopy was applied to monitor the real-time reduction of a single α-PbO particle (Figure a) .…”

Recent Manganese Oxide Octahedral Molecular Sieves (OMS–2) with Isomorphically Substituted Cationic Dopants and Their Catalytic Applications

“…Doping processes are clearly used to tune the structure, morphology and lattice parameters in order to provide novel chemical and physical properties to the solid. In this line, structural defects, active metal species derived from doping [136][137][138] and oxygen vacancies [105,139,140], together with unexpected synergic effects [132,141], will define the true performance of the materials in catalytic reactions.…”