Mechanism of Producing Metallic Nanoparticles, with an Emphasis on Silver and Gold Nanoparticles, Using Bottom-Up Methods

Bottom-up nanoparticle (NP) formation is assumed to begin with the reduction of the precursor metallic ions to form zero-valent atoms. Studies in which this assumption was made are reviewed. The standard reduction potential for the formation of aqueous metallic atoms—E0(Mn+aq/M0aq)—is significantly lower than the usual standard reduction potential for reducing metallic ions Mn+ in aqueous solution to a metal in solid state. E0(Mn+aq/M0solid). E0(Mn+aq/M0aq) values are negative for many typical metals, including Ag and Au, for which E0(Mn+aq/M0solid) is positive. Therefore, many common moderate reduction agents that do not have significantly high negative reduction standard potentials (e.g., hydrogen, carbon monoxide, citrate, hydroxylamine, formaldehyde, ascorbate, squartic acid, and BH4−), and cannot reduce the metallic cations to zero-valent atoms, indicating that the mechanism of NP production should be reconsidered. Both AgNP and AuNP formations were found to be multi-step processes that begin with the formation of clusters constructed from a skeleton of M+-M+ (M = Ag or Au) bonds that is followed by the reduction of a cation M+ in the cluster to M0, to form Mn0 via the formation of NPs. The plausibility of M+-M+ formation is reviewed. Studies that suggest a revised mechanism for the formation of AgNPs and AuNPs are also reviewed

Plausible Mechanisms of the Fenton-Like Reactions, M = Fe(II) and Co(II), in the Presence of RCO₂^– Substrates: Are OH^• Radicals Formed in the Process?

DFT calculations concerning the plausible mechanism of Fenton-like reactions catalyzed by Fe­(II) and Co­(II) cations in the presence of carboxylate ligands suggest that hydroxyl radicals are not formed in these reactions. This conclusion suggests that the commonly accepted mechanisms of Fenton-like reactions induced oxidative stress and advanced oxidation processes have to be reconsidered

Harnessing dimethyl ether and methyl formate fuels for direct electrochemical energy conversion

In this work, the oxidation of a mixture of dimethyl ether (DME) and methyl formate (MF) was studied in both an aqueous electrochemical cell and a vapor-fed polymer electrolyte membrane fuel cell (PEMFC) utilizing a multi-metallic alloy catalyst, Pt3Pd3Sn2/C, discovered earlier by us. The current obtained during the bulk oxidation of a DME-saturated 1 M MF was higher than the summation of the currents provided by the two fuels separately, suggesting the cooperative effect of mixing these fuels. A significant increase in the anodic charge was realized during oxidative stripping of a pre-adsorbed DME+MF mixture as compared to DME or MF individually. This is ascribed to greater utilization of specific catalytic sites leading to lower energy of the dual-fuel than of the sum of the individual molecules as confirmed by the density functional theory (DFT) calculations. Fuel cell polarization was also conducted using a Pt3Pd3Sn2/C (anode) and Pt/C (cathode) catalysts-coated membrane (CCM). The enhanced surface coverage and active site utilization resulted in providing a higher peak power density by the DME+MF mixture-fed fuel cell (123 mW cm−2 at 0. 45 V) than with DME (84 mW cm−2 at 0.35 V) or MF (28 mW cm−2 at 0.2 V) at the same total anode hydrocarbon flow rate, temperature under ambient pressure

Effect of Sol–Gel Silica Matrices on the Chemical Properties of Adsorbed/Entrapped Compounds

The sol–gel process enables the preparation of silica-based matrices with tailored composition and properties that can be used in a variety of applications, including catalysis, controlled release, sensors, separation, etc. Commonly, it is assumed that silica matrices prepared via the sol–gel synthesis route are “inert” and, therefore, do not affect the properties of the substrate or the catalyst. This short review points out that porous silica affects the properties of adsorbed/entrapped species and, in some cases, takes an active part in the reactions. The charged matrix affects the diffusion of ions, thus affecting catalytic and adsorption processes. Furthermore, recent results point out that ≡Si-O. radicals are long-lived and participate in redox processes. Thus, clearly, porous silica is not an inert matrix as commonly considered

The reaction between the peroxide VO(η²-O₂)(pyridine-2-carboxylate)·2H₂O and Fe^II_aq is not a Fenton-like reaction

<p>The reduction of VO(η²-O₂)(pyridine-2-carboxylate) by Fe(H₂O)₆²⁺ proceeds via formation of the transient complex (pyridine-2-carboxylate)(O)V^V(μ-η²: η²-O₂)Fe^II(H₂O)₃²⁺ that is transformed via intramolecular electron transfer into (pyridine-2-carboxylate)(O)V^IV(μ-η²: η²-O₂)Fe^III(H₂O)₃²⁺. The latter transient reacts with another Fe(H₂O)₆²⁺ to yield 2Fe(H₂O)₆³⁺ + V^VO(OH)(pyridine-2-carboxylate)⁺. These results point out that: (1) V^V does not activate the η² bound peroxide toward the Fenton-like reaction. In this aspect, V^V differs from Fe^III in (H₂O)₅Fe–OOH²⁺ and (2) transients of the type L_mMⁿ(μ-η²: η²-O₂)M′L″_l have to be considered in the reductions of complexes of η²-bound peroxides.</p

Sn-based atokite alloy nanocatalyst for high-power dimethyl ether fueled low-temperature polymer electrolyte fuel cell

Next-generation fuels are defined as those produced from non-food resources. A leading member in this group is dimethyl ether− DME (C2H6O), which is a high-energy, non-toxic gas, produced from a wide range of carbon feedstocks and wastes. We explored the oxidation of DME on a highly active catalyst based on Pt3Pd3Sn2 with an atokite structure in comparison to Pt3Sn and Pd3Sn. Following a comprehensive characterization of the new ternary catalyst by electron microscopy, X-ray diffraction, and photoelectron spectroscopy, the DME anodic reaction was analyzed by electrochemical online mass spectrometry of fuel cell gas emission product and supported by density functional theory (DFT) calculations. Pt3Pd3Sn2 catalyst exhibits optimal binding energy (−0.21 eV) and the lowest activation energy for electrochemical oxidation of DME (48.7 kJ mol−1 at 0.80 V). A few preferred oxidation routes were examined at different potentials corroborating with the identified CO2, formic acid, methanol, and methyl-formate by in-operando online mass spectrometry. Fuel-cell constructed using a Pt3Pd3Sn2/C anode catalyst and commercial Pt/C cathode catalyst, delivered an open circuit voltage of 0.9 V, a peak power density of 220 mW cm−2 at 0.40 V, and a gravimetric power density of 135 mW mgpgm−1 at ambient pressure and 80 °C, which exceeded the highest values reported so far for direct DME fuel cells