Vibrational sum frequency generation studies of the (2×2)→(√19×√19) phase transition of CO on Pt(111) electrodes

Abstract

The potential-dependent (2x2)-3CO-->(radical19x radical19)R23.4 degrees-13CO adlayer phase transition on Pt(111) with 0.1M H(2)SO(4) electrolyte was studied using femtosecond broadband multiplex sum frequency generation (SFG) spectroscopy combined with linear scan voltammetry. Across the phase boundary the SFG atop intensity jumps, and at the same time the SFG spectrum of threefold CO sites is transformed into a bridge site spectrum with a small decrease in integrated SFG intensity. The SFG atop intensity jump and three fold-to-bridge intensity drop are noticeably different from what would be expected for these structures on the basis of coverage alone. This occurs because the SFG signal is sensitive to both the coverage and changes in the local field that result from a changing adlayer structure. We derive an equation that allows us to correct the SFG intensities for these effects using information derived from infrared absorption-reflection spectroscopy (IRAS) and second-harmonic generation (SHG) measurements. With this correction, the SFG results agree well with what would be expected for a transition between perfect adlattices. A small (approximately 20%) discrepancy in the SFG determination of atop coverage is attributed to either a small amount of surface disorder or uncertainties in the SFG, SHG, and IRAS measurements. SFG is also used to examine the reversibility hysteresis and kinetics of the phase transition and its dependence on electrolyte composition. The phase transition is reversible with an approximately 150 mV anodic overpotential and the forward (2x2)-->(radical19x radical19) transition is slower than the reverse. Repeated cycles of phase transition indicate that the 25 microm electrolyte layer used here does not appreciably distort the potential-coverage relationships.

Commensurate monolayers on surfaces: Geometry and ground states

“…One expects lower energy structures to have higher symmetry, although this would require that the Fourier amplitudes be strictly decaying. Although this seems to be the case for some physisorption systems, 17,18 many examples of higher-order commensurate structures exist in the experimental literature on atomic and molecular adsorption, 5,9,11,13,21,22,[27][28][29] where the stability should be mainly determined by the contribution of the high-order Fourier terms. Furthermore, the knowledge of the Fourier amplitudes for any particular system ͑i.e., obtained from DFT calculations͒ could allow better interpretation of the translational symmetry of commensurate monolayers in addition to the fitting of the LEED spectra.…”

New Concepts in the Chemistry and Engineering of Low‐Temperature Fuel Cells

“…Aside from broader use of synchrotron techniques, a substantial amount of the progress in the methodology of fuel cell catalysis research can be attributed to the development of two different methods [213]: metal/surface NMR [or electrochemical NMR (EC-NMR)], ideally suited to study nanoparticles [214,215], and broadband sum frequency generation spectroscopy (BB-SFG), ideally suited to investigate smooth solid/liquid interfaces, including single crystals [216,217]. These two methods are discussed in Sections 19.4.1 and 19.4.2.…”

“…The two illuminating beams are usually in the infrared and visible ranges. However, since the reflected infrared beam is no longer used for data analysis, the thickness of the solution film between the optical window and the working electrode surface can be extended to 25 mm [216,217]; that is, in BB-SFG, all vibrations lying within the spectral region defined by the BBIR pulse spectrum are probed simultaneously [229]. Figure 19.11 shows BB-SFG spectra of CO chemisorbed on Pt.…”

Pathways for the Non‐CO‐Involved Oxidation of Methanol on Pt(111)

“…[3,[12][13][14] The oxidation and phase transition of CO on PtA C H T U N G T R E N N U N G (111) electrodes were also studied experimentally. [15][16][17] The pathway: CH 3 OH!CH 2 OH! HCOH!CO!CO 2 is dominant under typical reaction conditions, [3,14] while there are two oxidation processes for CO, namely, CO + O + H 2 O!CO 2 + H 2 O and CO + 2 OH + H 2 O!…”