Li Xia, Roiaz Matteo, Pramhaas Verena, Rameshan Christoph, Rupprechter Günther
Institute of Materials Chemistry, TU Wien, 1060 Vienna, Austria.
Top Catal. 2018;61(9):751-762. doi: 10.1007/s11244-018-0949-7. Epub 2018 Apr 13.
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 is involved and requires a proper optical interface model. For CO/Pt(111), the hyperpolarizability ratio is not exactly known and varying 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 -value of 0.49 was determined. Polarization-dependent SFG spectra in the pressure range 10 to 36 mbar did not indicate any change of the tilt angle of adsorbed CO. Modeling also indicated a strong dependence of 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 to 250 mbar. The absolute PPP and SSP spectral intensities on Pt and Pd were simulated, as well as the expected ratios. Although CO on Pt and Pd should exhibit similar intensities (at high CO coverage), the higher 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.
采用偏振相关和频振动光谱（SFG）在室温下研究了Pt(111)和Pd(111)单晶表面上的CO覆盖层。利用可见光和SFG光的不同偏振组合（SSP和PPP）可以确定界面分子的分子取向（倾斜角），但对测量结果的分析较为复杂，需要一个合适的光学界面模型。对于CO/Pt(111)，超极化率比值并不确切已知，在0.1 - 0.5范围内变化时，分别得到40° - 0°的倾斜角。基于已知的CO在Pt上的垂直吸附，确定了精确的 值为0.49。在10至36毫巴压力范围内的偏振相关SFG光谱未表明吸附的CO倾斜角有任何变化。建模还表明 强烈依赖于可见光和红外激光束的入射角。作为之前低温/低压数据的补充，研究了室温下10至250毫巴压力范围内CO在Pd(111)上的吸附情况。模拟了Pt和Pd上的绝对PPP和SSP光谱强度以及预期的 比值。尽管在高CO覆盖率下，Pt和Pd上的CO应表现出相似的强度，但Pd的 比值较高（Pt上为27，Pd上为48）使得在SSP光谱中检测吸附的CO变得困难。因此，SSP光谱中CO物种的存在与否不能简单地分别与倾斜或垂直的CO分子相关联。即使对于看似简单的吸附质 - 底物系统，也肯定需要仔细建模，不仅要考虑分子和界面性质，还要考虑实验配置（入射角）。
