Structures of the and adlayers, adapted from Villegas and Weaver (Ref. 4) and Tolmachev et al. (Ref. 7). In the lattice every atop site is surrounded by six threefold sites at a distance of 1.15d. All atop (coverage 0.25) and threefold sites (coverage 0.50) are equivalent. In the slightly lower density lattice, a single atop site is surrounded by six near-atop neighbors and six pairs of near-bridge dimers. Each near-atop site has three nearby near-bridge sites at 1.32d and 1.5d. The atop coverage is (0.368) and the bridge coverage is (0.316).
Schematic of the spectroscopic and electrochemical system (Ref. 1). The electrochemical cell had a Teflon spacer sandwiched between the Pt(111) electrode and window to provide an electrolyte layer of the known and controlled thickness. The dichroic beam combiner (DBC) combined the broadband IR and narrowband visible pulses. A CCD detector acquired broadband multiplex SFG spectra. The potentiostat and the CCD were synchronized by a common trigger.
SFG spectra Pt(111) with -thick CO-saturated electrolyte. (A) Spectrum of atop CO in the and adlayers. The broadband IR (BBIR) pulses were tuned to maximize atop intensity. The multiple-bonded transitions were not seen. (B) The BBIR pulses were tuned to maximize the intensities of bridge or threefold CO. In this case the much more intense atop transitions could also be seen despite being in the wings of the BBIR pulse spectrum.
Cyclic voltammogram of the clean Pt(111) electrode with a meniscus configuration with a electrolyte in the SFG cell. Scan rate was .
SFG spectra from CO on Pt(111) before repolishing with a -thick CO-saturated electrolyte, using a scan rate of . Each displayed spectrum resulted from a acquisition. At above the atop intensity jumped and threefold sites were converted to bridge sites. At , bridge and threefold CO coexist.
SFG spectra from CO on Pt(111) after repolishing, with a -thick CO-saturated electrolyte and a scan rate of . Each displayed spectrum resulted from a acquisition. Above the atop intensity jumped and threefold sites were converted to bridge sites. At , bridge and threefold CO coexist.
Analysis of atop SFG spectra from CO on Pt(111) with a -thick electrolyte, with a scan rate. Each displayed data point was the average of three or five spectra. The atop spectra were fit to Eq. (5) to extract the amplitude, frequency, and width. The phase transition resulted in a jump in atop amplitude.
SFG amplitude of atop CO as a function of the scan potential on a Pt(111) electrode in CO-saturated and solutions. Replacing with shifted both the phase transition and CO oxidation to lower potentials.
Reversibility and hysteresis in the phase transition on a Pt(111) electrode in a CO-unsaturated electrolyte observed via SFG of atop CO. An overpotential of is needed to reverse the phase transition. The different symbols represent two complete cyclic scans of the potential; the filled symbols represent the initial scan and the open symbols the second scan.
Kinetics of the phase transition on a Pt(111) electrode in both CO-saturated and CO-unsaturated electrolyte, observed via SFG of atop CO. The frequency shift data in B and E indicate that a new potential is established on the electrode within . The forward transformation is much slower than the reverse. There are minimal differences between the first and second cycle, indicating minimal change in the electrolyte composition during the kinetic measurements.
SFG, SHG, and IRAS intensity ratios for adlayer transition.
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