Electrocatalytic reduction of oxygen and Cr(VI) on Pd nanostructures

AGUIRRE MARIA DEL CARMEN,

Buenos Aires

Workshop; 1st Argentine-German Workshop (A-G nanobio 2017) on Nanotechnology and Nanobiosensors; 2017

INTI_Buenos Aires

Electrocatalytic reduction of Oxygen and Cr(VI) on Pd nanostructuresMaría del Carmen Aguirre 1,21-IFEG-CONICET. Instituto de Física Enrique Gaviola, Universidad Nacional de Córdoba, 5016 Córdoba,Argentina.2-Grupo Ciencias de los Materiales, Facultad de Matemática, Astronomía, Física y Computación.Universidad Nacional de Córdoba, Córdoba, Argentina.*carmenaguirre@famaf.unc.edu.arThis work outlines some important aspects of Pd nanostructures electrochemically built on Ti films and also onto glassy carbon (GC) electrodes. Experimental data related to Pd electrocrystallization is satisfactorily described by a 3D instantaneous nucleation mechanism and a diffusion controlled growth [1,2]. Pd/Ti and Pd/GC systems are explored as electrocatalysts foroxygen reduction reaction (ORR) and in chromium(VI) determination in acid electrolytes and potable water. Pd nanostructures synthesized at +0.1V, -0.3V and -0.9V on Ti films crystallize in an fcc structure, exhibiting nanoflower-like morphologies at more negative synthesis potential. In the cyclic voltammetries (CVs) performed in a O2-saturated 0.1 M H2SO4 solution, Pd+01V/Ti and Pd-0.3V/Ti systems exhibit a broad reduction maximum which may be resolved in two peaks, one at (+0.38 V vs. Ag/AgCl) and a small shoulder at (+0.1V vs. Ag/AgCl). The former peak is ascribable to the 2-electron O2 to H2O2 reduction, while the latter is assigned to the H2O2 to H2O reduction process. Assays performed with H2O2 in the 0.1M H2SO4 solution, point toward a 2-step mechanism and a 4-electron reduction pathway. Instead, the Pd-0.9V/Ti electrocatalyst, exhibiting a higher current density, displays a unique well-defined peak at (+0.48 V vs. Ag/AgCl), due to a single direct pathway involving 4-electron reduction from O2 to H2O. The electrocatalysts' activities indicated by the turnover frequency (TOF, s-1) result in efficiency order: Pd-0.9V/Ti> Pd+01V/Ti > Pd-0.3V/Ti. This demonstrates that palladium nanostructures size (60up to 200 nm) and morphology are key parameters in determining the mechanism of oxygen reduction and electroactivity. Pd/Ti and Pd/GC are highly sensitive electrocatalysts for Cr(VI) reduction in 0.1M H2SO4 as evidenced by the (CV) peak current at (+0.58 V vs Ag/AgCl). It is found that Cr(VI) can undergochemically irreversible reduction in the acidic solution. The dependence of the cathodic current peak (Ip) on the Cr(VI) concentration is linear in the range 0.019-1mM for Pd/Ti, Pd/GC and GC electrodes. Experimental data of Pd/Ti at (5-150) mV/s confirm that the reaction is controlled by diffusion, rather than by surface. This is consistent with a mechanism where the rate-limiting step is the electrochemically reversible transfer of the first electron, and that a total number of 3e- are involved in the Cr(VI) to Cr(III) reduction. Differential pulse voltammetry (DPV) developed with Pd/Ti and Pd/GC allows to build calibration curves with a linear range (0.4 - 7) uM, and a detection limit (LOD) of 0.4 uM in 0.1M H2SO4 and of 0.37uM in buffer pH 4 [3]. The responseto Cr(VI) in presence of interferences Cu(II), Ni(II), Fe(II), Cr(III) and also in contaminated drinkable water is promissory, allowing monitoring in natural environment.[1] B. Sharifker and G.Hills, Electrochim. Acta, 28, no7, (1983) 879-888.[2] M.del C.Aguirre, H.Nuñez-Coavas,L.Fabietti, S.Urreta, J.Phys Chem C, 120, 39, (2016) 22142-154.[3] C.M.Welch, O.Nekrassova, R.G.Compton, Talanta,65(2005)74-80.