Infrared Spectroscopic Study of CO Adsorption and Electro-oxidation on Carbon-Supported Pt Nanoparticles: Interparticle versus Intraparticle Heterogeneity Fre ´ de ´ ric Maillard, † Elena R. Savinova,* ,†,‡ Pavel A. Simonov, ‡ Vladimir I. Zaikovskii, ‡ and Ulrich Stimming † Technische UniVersita ¨t Mu ¨nchen, Department of Physics E19, James-Franck-Str. 1, D-85748 Garching, Germany, and BoreskoV Institute of Catalysis, Pr. Akademika LaVrentieVa 5, 630090 NoVosibirsk, Russian Federation ReceiVed: May 15, 2004; In Final Form: August 26, 2004 In this paper, we use Fourier transform infrared (FTIR) spectroscopy and stripping voltammetry at saturation and submonolayer CO coverages to shed light on the influence of size on the CO adsorption and electro- oxidation on Pt nanoparticles. Pt nanoparticles supported on low surface area (∼1m 2 g -1 ) carbon (Sibunit) are used throughout the study. The vibrational spectra of adsorbed CO are dominated by interparticle heterogeneity (contribution of particles of different size in the range from 0.5 to 5 nm) rather than intraparticle heterogeneity (contribution of different adsorption sites). CO stripping voltammetry exhibits two peaks separated by approximately 0.25 V (at 0.02 V s -1 ), which are attributed to the CO oxidation from “large” (∼3.6 nm) and “small” (∼1.7 nm) Pt nanoparticles. Using stepwise oxidation, we were able to separate the contributions of “large” and “small” nanoparticles and obtain their infrared and voltammetric “fingerprints”. Considerable differences are observed between “large” and “small” nanoparticles in terms of (i) the vibrational frequencies of adsorbed CO molecules (ii) their vibrational coupling, and (iii) CO oxidation overpotential. 1. Introduction Investigation of particle size effects attracts an increasing attention of the surface science and electrochemical communities (see, e.g., review articles in Catalysis & Electrocatalysis at Nanoparticle Surfaces 1 ). CO monolayer oxidation is one of the most widely explored surface reactions at both the solid/gas and the electrified solid/liquid interfaces. Considerable differ- ences have recently been observed between CO oxidation at metal nanoparticles and at extended surfaces, both at the solid/ gas 2-6 as well as the solid/liquid interfaces. 7-10 In the case of electrochemical CO oxidation, the overpotential increases considerably as compared to extended surfaces, and the reaction kinetics changes with a decrease in the particle size below approximately 3 nm. The reason for the reduced catalytic activity of metal nanoparticles versus extended surfaces is not clear yet. The most extensively discussed hypothesis ascribes slower CO oxidation on nanoparticles to the high ratio of low coordinated sites (edges and corners), which adsorb both CO and the second reaction partner, oxygen containing species, stronger. 2-6,11 Recently, Guerin et al. 12 observed two voltammetric peaks upon CO oxidative stripping from commercial Pt/Vulcan catalysts and ascribed the peak at more negative potentials to the CO oxidation on terraces while attributing the more positive one to the CO oxidation on particle edges. Zdhanov and Kasemo 13 simulated a CO stripping voltammogram from a nanometer-sized supported Pt crystallite exhibiting (111) and (100) facets and showed that the voltammetric peak will indeed split into two if CO diffusion between the facets is hindered. On the other hand, Friedrich et al. 7 attributed multiple peaks in CO stripping voltammograms from Pt colloidal particles im- mobilized on a Au substrate to CO oxidation on nanoparticles of different size. It is therefore important to clarify whether the (i) intraparticle or (ii) interparticle heterogeneity is dominating the behavior of nanosized metal particles. The first hypothesis considers a nanoparticle as a sum of the adsorption sites (high coordinated terrace and low coordinated edge) it comprises. If this is true, the behavior of nanoparticles is likely to be similar to that of high index single crystals with high step density. The second hypothesis is based on the assumption that a decrease of particle size does not only influence the contribution of different facets and increase the ratio between high and low coordination sites on its surface, but due to electronic effects also changes the properties of a particle as a whole. This means that a Pt atom with the coordination number 9, belonging to a (111) facet of a 1 nm particle, will have properties different from a Pt atom with the same coordination number but belonging to a (111) facet of, say, a 5 nm particle. At present, the first hypothesis dominates in the surface science and catalysis, 2-6 as well as in the electrochemical, 11,12 communities. It appears surprising that very few studies take into account the particle size distribution when analyzing the behavior of supported metal electrocatalysts. Meanwhile, size distribution is an immanent property of a statistical ensemble of nanometer- size metal particles, and it must be taken into consideration. IR spectroscopy has proven to be useful in shedding light on the mechanism of CO oxidation on Pt single crystalline surfaces both at solid/gas 14,15 and at electrified solid/liquid interfaces. 16-18 IR spectra of CO adsorbed on metal surfaces at saturation coverage are dominated by dipole-dipole coupling of the adsorbed molecules, resulting in blue shifts of the absorption bands and intensity borrowing from low frequency to high frequency vibrations. Decreasing the density of dipoles (i.e., decreasing the adsorbate coverage) lifts vibrational coupling and * Corresponding author. Tel: +7 3832 34 25 63. Fax: +7 3832 34 30 56. E-mail: elensav@catalysis.ru. † Technische Universita ¨t Mu ¨nchen. ‡ Boreskov Institute of Catalysis. 17893 J. Phys. Chem. B 2004, 108, 17893-17904 10.1021/jp0479163 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/26/2004