Pavel A. Simonov

Model approach to evaluate particle size effects in electrocatalysis: Preparation and properties of Pt nanoparticles supported on GC and HOPG

Although metal nanoparticles are widely used in electrocatalysis, in particular for the preparation of the electrodes for fuel cells, it is still not fully understood how the size of a metal phase affects its electrochemical reactivity. In this paper, we demonstrate a possible approach to design model electrodes for studying size effects in electrocatalysis. A simple chemical deposition procedure is introduced, which allows reproducible preparation of 1.5/3.0 nm Pt nanoparticles anchored to the surface of glassy carbon (GC) or highly oriented pyrolytic graphite (HOPG). Pt nanoparticles supported on GC are stable versus potential variation and can be used for studying size effects. They are examined in electrochemical reactions relevant to low temperature fuel cells: electrooxidation of adsorbed CO and methanol. It is demonstrated that reactivity of Pt nanoparticles in these reactions is considerably decreased in comparison to bulk polycrystalline Pt.

CO monolayer oxidation at Pt nanoparticles supported on glassy carbon electrodes

CO monolayer oxidation on glassy carbon supported 1–2 nm Pt nanoparticles is studied using potential sweep and potential step methods. The CO stripping peak on the nanoparticles is significantly shifted to positive potentials vs. the corresponding feature at bulk polycrystalline Pt. Current transients at nanoparticulate electrodes are highly asymmetric with a steep rise, maximum at θCO≈0.8–0.9, and a slow decay following t−1/2. The experimental results are compared to the theoretical models of adsorbed CO oxidation described in the literature. A tentative model is suggested to account for the experimental observations, which comprises spatially confined formation of oxygen containing species at active sites, and slow diffusion of CO molecules to the active sites, where they are oxidized. The upper limit of the CO surface diffusion coefficient at Pt nanoparticles is estimated as approximately 4×10−15 cm2 s−1.

On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation

PtRu (1:1) catalysts supported on low surface area carbon of the Sibunit family (S(BET) = 72 m(2) g(-1)) with a metal percentage ranging from 5 to 60% are prepared and tested in a CO monolayer and for methanol oxidation in H(2)SO(4) electrolyte. At low metal percentage small (<2 nm) alloy nanoparticles, uniformly distributed on the carbon surface, are formed. As the amount of metal per unit surface area of carbon increases, particles start coalescing and form first quasi two-dimensional, and then three-dimensional metal nanostructures. This results in a strong enhancement of specific catalytic activity in methanol oxidation and a decrease of the overpotential for CO monolayer oxidation. It is suggested that intergrain boundaries connecting crystalline domains in nanostructured PtRu catalysts produced at high metal-on-carbon loadings provide active sites for electrocatalytic processes.

Electrocatalytic properties of platinum anchored to the surface of highly oriented pyrolytic graphite

Nano-sized particles of platinum deposited on highly oriented pyrolytic graphite (HOPG) electrolessly (average sized = 3.4 nm) or electrochemically (d = 20 nm) and polycrystalline platinum are compared. The dispersed electrodes exhibit special features in the oxidation of ethylene glycol and adsorbed carbon monoxide. In particular, they reduce the overpotential of these processes. Possible reasons for the observed distinctions are discussed. The potential cycling leads to coalescence of nano-sized Pt particles on HOPG. As a result, their size distribution expands, and the distribution maximum shifts towards large sizes (d = 6.4 nm). This seriously complicates use of Pt/HOPG as a model electrode for investigating the size effect.

On the effect of Cu on the activity of carbon supported Ni nanoparticles for hydrogen electrode reactions in alkaline medium

Effects of adding varied amounts of copper to carbon-supported nickel particles on their structure, composition and electrocatalytic activity for the hydrogen oxidation and evolution reactions in alkaline medium have been explored. Ni1-xCux/C catalysts were prepared by the incipient wetness impregnation. Comprehensive characterization of the catalysts included X-ray powder diffraction, X-ray photoelectron spectroscopic, transmission electron microscopic and cyclic voltammetric analyses, while atomistic Monte Carlo simulations have been undertaken to obtain further insights into the structure of the bimetallic NiCu nanoparticles. We found that compared to monometallic Ni, NiCu nanoparticles show lower propensity towards oxidation under ambient conditions. Furthermore, we report that adding Cu allows increasing the surface-weighted electrocatalytic activity, and the specific surface area of Ni1-xCux/C electrodes, both contributing to a ca four-fold enhancement of the mass-weighted activity. The nature of the synergistic interactions between Ni and Cu is proposed on the basis of the analysis of experimental data and Monte Carlo structural modelling results.