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 on to 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 kneed to acquire absolute unipolar, rather than bipolar, spectra by means of potential-difference infrared tactics for such nanoparticle films is dem onstrated, given the broad asymmetric C-O stretching band shapes. For large r particle diameters (d greater than or equal to 4 nm), the potential-depen dent peak stretching frequencies (v(CO)(P)) for saturated CO are closely si milar to atop CO on Pt(111) electrodes, indicating a preponderance of 9-coo rdinate Pt sites. However, for nanoparticle diameters in the range d approx imate to 2-4 nm, the v(CO)(P) values at a given potential, E, redshift shar ply with decreasing d, approaching frequencies compatible with those measur ed at the same surface potential for atop CO in chargeable high-nuclearity Pt carbonyl solutes. The latter, structurally well-characterized, nanoparti cles are known to contain predominantly edge- rather than (111) terrace-bou nd CO. The implication that the nanoparticle size-dependent structural tran sition is associated with changes in the Pt surface coordination number, co nsistent with pseudo-spherical packing-density considerations, is supported by comparisons of the v(CO)(P)-E data for co lower CO coverages with corre sponding potential-dependent spectra for CO bound to step sites on high-ind ex Pt electrodes. The broad-based value of vibrational measurements at cont rolled surface potentials for characterizing conducting nanomaterials is po inted out.