The electrooxidation of small organic molecules on

Authors: F J E Scheijen G L Beltramo S Hoeppener T H M Housmans M T M Koper

Publish Date: 2007/06/19

Volume: 12, Issue: 5, Pages: 483-495

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Abstract

The electrocatalytic properties of small platinum nanoparticles were investigated for the oxidation of CO methanol and formic acid using voltammetry chronoamperometry and surfaceenhanced Raman spectroscopy The particles were generated by galvanostatic deposition of platinum on a polished gold surface from an H2PtCl6 containing electrolyte and ranged between 10 and 20 nm in diameter for low platinum surface concentrations 10 and 120 nm for medium concentrations and full Pt monolayers for high concentrations CO stripping and bulk CO oxidation experiments on the particles up to 120 nm in diameter displayed pronounced structural effects The CO oxidation currenttime transients show a current decay for low platinum coverages and a current maximum for medium and high coverages These results were also observed in the literature for particles of 2 to 5nm size and agglomerates of these particles The similarities between the literature and our results despite large differences in particle size and morphology suggest that particle structure and morphology are also very important catalytic parameters Surfaceenhanced Raman spectroscopy data obtained for the oxidation of CO on the Ptmodified Au electrodes corroborate this conclusion A difference in the ratio between CO adsorbed in linear and bridgebonded positions on the Pt nanoparticles of different sizes demonstrates the influence of the surface morphology The oxidation activity of methanol was found to decrease with the particle size while the formic acid oxidation rate increases Again a structural effect is observed for particles of up to ca 120 nm in diameter which is much larger than the particles for which a particle size effect was reported in the literature The particle shape effect for the methanol oxidation reaction can be explained by a reduction in available “ensemble sites” and a reduction in the mobility of CO formed by decomposition of methanol As formic acid does not require Pt ensemble sites decreasing the particle size and thus the relative number of defects increases the reaction rateParticles of nanoscale diameter are of considerable scientific interest due to their different often unexpected catalytic properties compared to bulk electrode materials Moreover they are commonly used in technical electrodes Therefore understanding of their electrochemical properties is an important issue in the development of efficient fuel cell catalysts Especially the electrocatalysis of small organic molecules such as carbon monoxide CO methanol MeOH and formic acid HCOOH is an interesting field of study as these reactions play an important role in fuel cell catalysis 1 2 and they display pronounced sizedependent catalytic properties 3 4 5 6 7 8 9 Due to the fact that platinum is one of the most active dehydrogenation catalysts Pt nanoparticles are of particular significanceIn a recent paper Friedrich et al 9 reported a distinct particle size dependence for the electrooxidation of carbon monoxide on colloidal platinum nanoparticles supported on polycrystalline gold They found that the CO oxidation potential on particles with an average diameter of 3 nm is shifted to higher potentials 300–500 mV when compared to polycrystalline platinum electrodes When the diameter of the colloidal particles is increased from 3 nm to about 10–16 nm the oxidation potential was found to decrease again It was suggested that the origin of this “particle size effect” lies in the geometrical structure of the particles and to a lesser extent in the electronic propertiesSimilar effects were observed by Maillard et al 10 11 12 for platinum nanoparticles ranging from 1 to 4 nm supported on glassy carbon GC They ascribed the size effect to a limited mobility of COads on particles smaller than 2 nm A particlesizedependent diffusion coefficient was assumed which is low for small particles 2 nm and higher for larger particles 3 nm Although Arenz et al 13 also obtained similar results for 1 to 5nm Pt particles supported on carbon they attributed the difference in CO oxidation potential for varying particle sizes to the number of defects present on the surface of a particle As defects are able to dissociate water to form OHads more easily the higher number of defects on larger particles would result in a shift of the CO oxidation peak to lower potentialsFor the oxidation of methanol Park et al 8 reported a particle size dependence for carbonsupported particles in the range of 2–9 nm On particles larger than 4 nm in diameter the recorded methanol oxidation current densities were found to approach those of polycrystalline Pt electrodes The decreasing MeOH oxidation rate for particles smaller than 4 nm was ascribed to a decrease in contiguous Pt terrace sites This particle size dependence of the methanol oxidation reaction was also observed by Frelink et al 6 However they measured a more or less constant reaction rate for particles larger than 45 nm Two possible explanations for the decrease in activity with decreasing particle size were given 1 on smaller particles the coverage of OHads increases thus blocking more empty sites resulting in a decrease in the methanol decomposition rate or 2 smaller particles have fewer preferential adsorption sites for methanol In addition to these findings Cherstiouk et al 7 observed lower methanol oxidation rates for Pt particles in the range of 15–3 nm compared to polycrystalline Pt which they ascribed to stronger poisoning of the nanoparticles as compared to polycrystalline surfaces due to slower oxidation of COThe effect of the particle size in the electrooxidation of formic acid was also investigated by Park et al 8 In contrast to methanol the oxidation rate of formic acid on small particles was found to be considerably higher than on larger particles It was suggested that formic acid unlike methanol does not require Pt ensemble sites to react to CO2 and thus in combination with a lower poisoning rate higher oxidation rates are observed Moreover unlike the oxidation of CO and methanol formic acid does not need an additional oxygen atom ie no reaction with surface oxygencontaining species like OHads is necessary to form carbon dioxide 44

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