Abstract
Polarization-dependent sum frequency generation (SFG) vibrational spectroscopy was employed to examine CO overlayers on Pt(111) and Pd(111) single crystal surfaces at room temperature. Utilizing different polarization combinations (SSP and PPP) of the visible and SFG light allows to determine the molecular orientation (tilt angle) of interface molecules but the analysis of the measured I_{text{ppp}}/I_{text{ssp}} is involved and requires a proper optical interface model. For CO/Pt(111), the hyperpolarizability ratio left( {R={beta _{aac}}/{beta _{ccc}}={beta _{bbc}}/{beta _{ccc}}} right) is not exactly known and varying R in the range 0.1–0.5 yields tilt angles of 40°–0°, respectively. Based on the known perpendicular adsorption of CO on Pt, an exact R-value of 0.49 was determined. Polarization-dependent SFG spectra in the pressure range 10−4 to 36 mbar did not indicate any change of the tilt angle of adsorbed CO. Modeling also indicated a strong dependence of {I_{{text{ppp}}}}/{I_{{text{ssp}}}} on the incidence angles of visible and IR laser beams. Complementing previous low temperature/low pressure data, room temperature CO adsorption on Pd(111) was examined from 10−6 to 250 mbar. The absolute PPP and SSP spectral intensities on Pt and Pd were simulated, as well as the expected {I_{{text{ppp}}}}/{I_{{text{ssp}}}} ratios. Although CO on Pt and Pd should exhibit similar intensities (at high CO coverage), the higher {I_{{text{ppp}}}}/{I_{{text{ssp}}}} ratio for Pd (48 vs. 27 on Pt) renders the detection of adsorbed CO in SSP spectra difficult. The presence or absence of CO species in SSP spectra can thus not simply be correlated to tilted or perpendicular CO molecules, respectively. Careful modeling, including not only molecular and interface properties, but also the experimental configuration (incidence angles), is certainly required even for seemingly simple adsorbate–substrate systems.
Highlights
For many years, the surface science approach to heterogeneous catalysis was restricted to gas pressures of 1 0−6 mbar and below, giving rise to the well-known “pressure gap” problem [1,2,3]
Among the first were sum frequency generation (SFG) vibrational spectroscopy and high pressure scanning tunneling microscopy (HP-STM), both methods and their application to catalytic problems being pioneered by Somorjai and coworkers [4,5,6]
The experiments on Pt(111) and Pd(111) single crystals were performed in a new UHV surface analysis system equipped with an SFG-compatible UHV-high pressure cell [25]
Summary
The surface science approach to heterogeneous catalysis was restricted to gas pressures of 1 0−6 mbar and below, giving rise to the well-known “pressure gap” problem [1,2,3]. Topics in Catalysis (2018) 61:751–762 combinations on the orientation of the surface molecule [19,20,21,22,23] In this context, the molecular adsorbate structures of CO on Pt(111) [23, 24] and Pd(111) [22] have been studied, but results on the orientation (tilt angle) of CO on single-crystal surfaces were somewhat ambiguous. The molecular adsorbate structures of CO on Pt(111) [23, 24] and Pd(111) [22] have been studied, but results on the orientation (tilt angle) of CO on single-crystal surfaces were somewhat ambiguous This is due to the complexity of the orientation analysis, which is based on polarization-dependent SFG spectroscopy, and the analysis/modeling of the observed intensities (which sometimes includes simplifications and/or assumptions that may not be justified). We revisit the benchmark systems of CO/Pt(111) and CO/Pd(111), employing a new SFG setup (UHV to mbar) [25] and discuss the orientation analysis in detail, reflecting analogies and differences to previous studies
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