Electrochemical Infrared Characterization of Carbon-Supported Platinum Nanoparticles:  A Benchmark Structural Comparison with Single-Crystal Electrodes and High-Nuclearity Carbonyl Clusters

Electrode potential-dependent infrared spectra for carbon monoxide dosed onto carbon-supported platinum nanoparticle films, significant as commercial fuel-cell catalysts as well as of fundamental importance, are reported with the aim of elucidating their structure as a function of particle size. The need to acquire absolute unipolar, rather than bipolar, spectra by means of potential-difference infrared tactics for such nanoparticle films is demonstrated, given the broad asymmetric C−O stretching band shapes. For larger particle diameters ( d ≥ 4 nm), the potential-dependent peak stretching frequencies ( ) for saturated CO are closely similar to atop CO on Pt(111) electrodes, indicating a preponderance of 9-coordinate Pt sites. However, for nanoparticle diameters in the range d ≈ 2−4 nm, the values at a given potential, E, redshift sharply with decreasing d, approaching frequencies compatible with those measured at the same surface potential for atop CO in chargeable high-nuclearity Pt carbonyl solutes. The latter, structurally well-characterized, nanoparticles are known to contain predominantly edge- rather than (111) terrace-bound CO. The implication that the nanoparticle size-dependent structural transition is associated with changes in the Pt surface coordination number, consistent with pseudo-spherical packing-density considerations, is supported by comparisons of the − E data for lower CO coverages with corresponding potential-dependent spectra for CO bound to step sites on high-index Pt electrodes. The broad-based value of vibrational measurements at controlled surface potentials for characterizing conducting nanomaterials is pointed out.