Bimetallic nanoparticles containing Au and Pd were synthesized using poly(vinylpyrrolidone) (PVP) as the polymer stabilizer using both co-reduction and sequential reduction strategies. The nanoparticle structures and catalytic activities for the aerobic oxidation of crotyl alcohol to crotonaldehyde at room temperature in the absence of base were investigated. The chemical, structural, and electronic properties of these nanoparticles were investigated using Pd-K-edge and Au-LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and Pd-LIII and Au-LIII edge X-ray absorption near edge structure (XANES) spectroscopy. EXAFS analysis for the sequentially reduced bimetallic systems indicates the presence of significantly Pd-enriched surfaces and Au cores. XANES spectra of the Pd-LIII edges indicated that the sequentially reduced particles showed significant d-charge depletion compared to pure monometallic Pd and co-reduced AuPd nanoparticles. The sequentially reduced nanoparticles with Pd rich surfaces were extremely active for crotyl alcohol oxidation at room temperature in the absence of base, and were quite selective for the formation of crotonaldehyde. A proposed mechanism for the reaction involving the oxidation and re-reduction of Pd on the surface of the particles is postulated based on catalytic activity measurements using sequentially reduced particles and control reactions with Pd2+ salts in the absence and presence of Au, Pd, and Pt nanoparticles.

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A mechanism is herein proposed for the reduction of oxygen on polycrystalline Au in 0.1 M NaOH + 0.9 M NaClO4 aqueous electrolytes, that assumes, in agreement with theoretical arguments put forward rather recently by Ignaczak Ignaczak Nano Energy201626558564, formation of solution-phase superoxide, O2 –, as the initial, rate-determining step, followed by a reversible (Nernstian) one-electron reduction to yield solution-phase peroxide, HO2 –(aq), and not by a second-order dismutation of adsorbed O2 –, as has been postulated by other authors. Also considered in this model is the direct reduction of HO2 –(aq) and O2(aq) to generate OH–(aq). A detailed mathematical analysis of data collected with a rotating ring-disk electrode in O2-saturated and Ar-purged HO2 –(aq)-containing solutions made it possible to determine a unique set of kinetic rate constants for the various steps in the proposed mechanism over a wide potential range.

A method for measuring downstream concentration effects through electrochemical impedance spectroscopy at double channel electrodes is demonstrated. An ac current perturbation is applied at an upstream working electrode and the resulting ac potential response at a downstream sensing electrode is measured. This generator-detector scheme is implemented with a single potentiostat. Experimental data for a reversible redox couple are presented and good agreement is found with numerical simulations. A qualitative explanation of the features is given which lays the foundation for a more rigorous theoretical treatment. Relationships with flow rate and frequency are found that can scale the data to lie on a universal curve.

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