The interplay between mass transport and the kinetics of CO adsorption/CO electrooxidation was studied on nanostructured electrodes, which consist of regular arrays of catalytically active cylindrical Pt nanodisks supported on a planar glassy carbon (GC) substrate, and are fabricated via Hole-mask Colloidal Lithography. CO adsorption and oxidation were measured under controlled transport conditions in a thin-layer flow cell interfaced to a mass spectrometer. The temporal evolution of the relative COad coverage, the effective sticking coefficient and the dependence of the adsorption rate on the COad coverage were evaluated for CO adsorption at 0.06 V (vs. reversible hydrogen electrode) at systematically varied CO concentrations and Pt nanodisk coverages. Continuous CO oxidation was studied at different electrode potentials under identical mass transport condition. While qualitatively, the characteristics of the adsorption/reaction kinetics do not depend on the transport conditions, the absolute rates of CO adsorption and CO (bulk) oxidation, normalized to the Pt coverage, increase strongly with decreasing Pt coverage on the nanostructured Pt/GC electrodes. This is explained by a gradual transition from one-dimensional planar diffusion (concentration gradients planar to the surface-extended Pt surface) to three-dimensional hemispherical diffusion (nanostructured surfaces). (C) 2011 Elsevier B.V. All rights reserved.

This paper summarises the result of previous experimental investigations of heterogeneous electrocatalytic reactions performed in flow cells which provide an environment with controlled parameters. Measurements of the oxygen reduction reaction in a flow cell with an electrode consisting of an array of Pt nanodisks on a glassy carbon substrate exhibited a decreasing fraction of the intermediate H2O2 in the overall reaction products with increasing density of the nanodisks. A similar result is true for the dependence on the catalyst loading in the case of a supported Pt/C catalyst thin-film electrode, where the fraction of the intermediate decreases with increasing catalyst loading. Similar effects have been detected for the methanol oxidation.In order to give a possible explanation to the observed effect, we present a model of multistep heterogeneous electrocatalytic oxidation and reduction reactions based on an adsorption-reaction-desorption scheme using the Langmuir assumption and macroscopic transport equations. A continuum based model problem in a vertical cross-section of a rectangular flow cell is proposed in order to explain basic principles of the experimental situation. It includes three model species A, B, C, which undergo adsorption and desorption at a catalyst surface, as well as adsorbate reactions from A to B to C. These surface reactions are coupled with diffusion and advection in the Hagen Poiseuille flow in the flow chamber of the cell. High velocity asymptotic theory and a finite volume numerical method are used to obtain approximate solutions to the model. Both approaches show a behaviour similar to the experimentally observed. Working in more general situations, the finite volume scheme was applied to a catalyst layer consisting of a number of small catalytically active areas corresponding to nanodisks. Good qualitative agreement with the experimental findings is established for this case as well.

The interplay between electrocatalytic CO oxidation kinetics and mass transport in a thin-layer flow cell with a polycrystalline platinum electrode was experimentally studied and numerically simulated. The experiments were performed in a flow cell under controlled electrolyte flow. The computations are based on four different models for mass transport, coupled with a three-step reaction mechanism for CO oxidation that includes surface coverage effects. A zero-dimensional model neglecting mass transport effects on the overall reaction rate is applied to verify the effect of the kinetic parameters on the Faradaic current qualitatively. Transport models of increasing complexity, one-, two-, and three-dimensional, are used to analyze the impact of diffusion and convection on the total reaction rate in the flow cell. The numerical simulation, based on a time-resolved three-dimensional transport model coupled with the electrocatalytic kinetics, resolves profiles of temporal and spatial concentrations, velocities, and surface coverages. Mass transfer limitations of the electrocatalytic reaction rate are primarily caused by CO diffusion normal to the electrode. Both the two- and the three-dimensional models can quantitatively predict the Faradaic current as a function of potential measured in the experimental setup.

1 Institut fur Oberflachemie und Katalyse, Universitat Ulm, 89081 Ulm 2 School of Chemical Science and Engineering, KTH (Royal Institute of Technology), SE-10044 Stockholm, Sweden 3 Institut fur Oberflachemie und Katalyse, Universitat Ulm, 89081 Ulm 4 Dept. of Applied Physics, Chalmers University of Technology, SE-41296 Gothenburg, Sweden 5 Institut fur Technische Chemie und Polymerchemie, Karlsruher Institut fur Technologie (KIT), D-76131 Karlsruhe 6 Fachbereich Physik, Universitat Konstanz, 78457 Konstanz

Transport effects in the methanol oxidation reaction (MOR) were investigated using nanostructured Pt/glassy carbon (GC) electrodes and, for comparison, a polycrystalline Pt electrode. The nanostructured Pt/GC electrodes, consisting of a regular array of catalytically active cylindrical Pt nanostructures with 55 +/- 10 nm in diameter and different densities supported on a planar GC substrate, were fabricated employing hole-mask colloidal lithography (HCL). The MOR measurements were performed under controlled transport conditions in a thin-layer flow cell interfaced to a differential electrochemical mass spectrometry (DEMS) setup. The measurements reveal a distinct variation in the MOR activity and selectivity (product distribution) with Pt nanostructure density and with electrolyte flow rate, showing an increasing overall activity, reflected by a higher Faradaic reaction current, as well as a pronounced increase of the turnover frequency for CO(2) formation and of the CO(2) current efficiency with decreasing flow rate and increasing Pt coverage. These findings are discussed in terms of the "desorption-readsorption-reaction" model introduced recently (Seidel et al. Faraday Discuss. 2008, 140, 67). Finally, consequences for applications in direct methanol fuel cells are outlined.

Transport effects may not only influence the rate of electrocatalytic reactions, but also the product distribution. The underlying effects were studied under well-defined reaction and transport conditions, using nanostructured Pt/glassy carbon electrodes, which consist of ordered arrays of electrocatalytically active Pt nanostructures on a planar glassy carbon substrate. The effect of varying the density of the Pt nanostructures or the electrolyte flow rate on the reaction characteristics and product distribution was investigated for apparently simple electrocatalytic reactions such as CO oxidation, methanol oxidation and oxygen reduction. It is shown that reducing the Pt coverage leads to an increase of the relative amount of the reaction intermediates. Similar effects are obtained for increasing the electrolyte flow rate. The results are discussed on a molecular scale in terms of the 'desorption - re-adsorption - reaction' model which we had introduced recently (Y.E. Seidel et al., Faraday Discuss. 140, 167 (2008)).

The role and contribution of transport processes in electrocatalytic reactions was investigated in model studies of the oxidation of CO (single-product reaction) and formaldehyde (dual-product reaction), using nanostructured Pt/glassy carbon electrodes with variable Pt loading and defined reactant transport conditions. Nanostructured electrodes with monodispersed, uniformly distributed Pt nanostructures (100–140nm diameter) supported on planar glassy carbon (GC) electrodes with different densities were prepared by Colloidal Lithography (CL) or Hole-Mask Colloidal Lithography (HCL). Transport effects were evaluated by varying the density of the nanostructures and the electrolyte flow. The resulting changes in the transport limited reaction current (CO oxidation – transition from planar to spherical diffusion with decreasing Pt nanostructure density) and in the distribution of the reaction products HCOOH and CO2 (HCHO oxidation), which is probed by differential electrochemical mass spectrometry (DEMS), are discussed focusing on transport effects. The increasing amount of CO2 with decreasing space velocity (higher nanostructure density, lower electrolyte flow) is explained by increasing re-adsorption and further reaction of desorbing reaction intermediates.

Nanostructured, glassy carbon (GC) supported Pt/GC electrodes, with Pt nanostructures (nanodisks) of controlled size (100-140 nm in diameter) and separation homogeneously distributed on a planar GC substrate, were recently shown to be interesting model systems for electrocatalytic reaction studies [M. Gustavsson, H. Fredriksson, B. Kasemo, Z. Jusys, C. Jun, and R. J. Behm, J. Electroanal. Chem., 568, 371 (2004)]. We present here electron microscopy and electrochemical measurements which reveal that the fabrication of these nanostructured electrodes via colloidal lithography, in addition to the intended nanodisks, results in a dilute layer of much smaller Pt nanoparticles (diameter similar to 5 nm) on the GC surface in the areas between the Pt nanodisks. We further demonstrate that by using the developed, related method of hole-mask colloidal lithography (HCL) [H. Fredriksson, Y. Alaverdyan, A. Dmitriev, C. Langhammer, D. S. Sutherland, M. Zach, and B. Kasemo, Adv. Mater. (Weinheim, Ger.), 19, 4297 (2007)], similar electrodes can be prepared which are free from these Pt nanoparticles. The effect of the additional small Pt nanoparticles on the electrochemical and electrocatalytic properties of these nanostructured electrodes, which is significant and can become dominant at low densities of the Pt nanodisks, is illustrated and discussed. These results leave HCL the preferred method for the fabrication of nanostructured Pt/GC electrodes, in particular, of low-density Pt/GC electrodes. (C) 2008 The Electrochemical Society.

The role of transport and re-adsorption processes on the oxygen reduction reaction (ORR), and in particular on its selectivity was studied using nanostructured model electrodes consisting of arrays of Pt nanostructures of well-defined size and separation on a planar glassy carbon (GC) substrate. The electrochemical measurements were performed under controlled transport conditions in a double-disk electrode thin-layer flow-cell configuration; the model electrodes were fabricated by colloidal lithography techniques, yielding Pt nanostructures of well defined and controlled size and density (diameter: 140 or 85 nm, height: 20 or 10 nm, separation: from 1-2 to more than 10 diameters). The nanostructured model electrodes were characterized by scanning electron microscopy and electrochemical probing of the active surface area (via the hydrogen adsorption charge). The electrocatalytic measurements revealed a pronounced variation of the hydrogen peroxide yield, which increases by up to two orders of magnitude with increasing separation and decreasing size of the Pt nanostructures. Similar, though less pronounced effects were observed upon varying the electrolyte flow and thus the mass transport characteristics. These effects are discussed in a reaction model which includes (i) direct reduction to H(2)O on the Pt surface and (ii) additional H(2)O(2) formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H(2)O(2) molecules on the Pt surface.

The role of mesoscopic mass transport and re-adsorption effects in electrocatalytic reactions was investigated using the oxygen reduction reaction (ORR) as an example. The electrochemical measurements were performed on structurally well-defined nanostructured model electrodes under controlled transport conditions in a thin-layer flow cell. The electrodes consist of arrays of Pt ultra-microelectrodes (nanodisks) of defined size (diameter approximately 100 nm) separated on a planar glassy carbon (GC) substrate, which were fabricated employing hole-mask colloidal lithography (HCL). The measurements reveal a distinct variation in the ORR selectivity with Pt nanodisk density and with increasing electrolyte flow, showing a pronounced increase of the H2O2 yield, by up to 65%, when increasing the flow rate from 1 to 30 microL s(-1). These results are compared with previous findings and discussed in terms of a reaction model proposed recently (A. Schneider et al., Phys. Chem. Chem. Phys., 2008, 10, 1931), which includes (i) direct reduction to H2O on the Pt surface and (ii) additional H2O2 formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H2O2 molecules on the Pt surface. The potential of model studies on structurally defined catalyst surfaces and under well-defined mass transport conditions in combination with simulations for the description of electrocatalytic reactions is discussed.

The stability of nanostructured Pt/glassy carbon (GC) model electrodes upon exposure to a realistic electrochemical/electrocatalytic reaction environment (continuous reaction, continuous electrolyte flow) was studied by microscopic techniques, X-ray photoelectron spectroscopy, and electrochemical measurements. The model electrodes consist of Pt nanostructures with well-defined sizes and regular spacing on planar GC substrates, and were fabricated using colloidal lithography techniques. Additional plasma treatments of the GC substrates prior to Pt deposition were tested to improve the stability of the resulting Pt/GC model electrodes. Both evaporation and sputter deposition were used for Pt-film fabrication. The model catalysts prepared by Pt evaporation were found to be rather unstable. The stability was significantly improved for sputter-deposited Pt films, and Pt sputter deposition on a GC substrate, pretreated first in oxygen plasma and then in Ar plasma, resulted in stable model electrodes with a fully intact layer of Pt nanostructures after the electrocatalytic experiments. (c) 2008 The Electrochemical Society.

Novel, nanostructured, carbon-supported Pt model electrodes with homogeneously distributed Pt nanoparticles of uniform size were fabricated and analyzed with respect to their electrochemical properties. For this purpose, Pt-salt-loaded micelles were deposited on a glassy carbon substrate and subsequently exposed to an oxygen plasma and a H2 atmosphere for removal of the polymer carriers and reduction of the Pt salt. The morphology of the resulting nanoparticles and their electrochemical/electrocatalytic properties were characterized by high-resolution scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and differential electrochemical mass spectrometry for CO electrooxidation. The data demonstrate that this method is generally suited to the production of nanostructured model electrodes with well-defined and independently adjustable particle size and interparticle distance distributions, which are specifically suited for quantitative studies of transport processes in electrocatalytic reactions.