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CO adsorption on Ni(100) and Pt(111) studied by infrared–visible sum frequency generation spectroscopy: design and application of an SFG-compatible UHV–high-pressure reaction cell

Infrared–visible sum frequency generation (SFG) surface vibrational spectroscopy was applied to monitor CO stretching vibrations on Ni(100) and Pt(111) in the range from submonolayer coverages up to 200 mbar. Since SFG can operate in a pressure range from ultrahigh vacuum (UHV) to ambient conditions, it is particularly suited for in situ studies of adsorbates at elevated pressure or during a catalytic reaction. At high coverages, a compressed overlayer was formed on Ni(100) at 100 K that can be modeled by a coincidence structure. On Pt(111), terminally bonded (on-top) CO was the only species observed at 230 K, independent of gas pressure. At low pressure the SFG spectra were complemented by LEED, AES and TPD. The experiments were carried out in an SFG-compatible elevated pressure reactor that is attached to a UHV surface analysis chamber. After preparation and characterization in UHV, model catalysts can be transferred in vacuo into the reaction cell. The reactor is separated from the UHV chamber by an arrangement of differentially pumped spring-loaded teflon seals and can be pressurized to 1 bar without degrading the vacuum in the UHV analysis system.

Complex model catalysts under UHV and high pressure conditions : CO adsorption and oxidation on alumina-supported Pd particles

Abstract The growth of metal particles on ordered oxide surfaces provides a strategy to prepare well-defined model systems for supported catalysts, which can by easily studied by most surface-science techniques. Here, we focus on Palladium particles grown on an ordered Al2O3 film on NiAl(110), a system which has previously been characterized in detail with respect to its structural, electronic and adsorption properties. In this contribution, we will provide several examples, showing how adsorption and reactivity phenomena on these systems can be addressed over a pressure range from ultrahigh vacuum (UHV) to near atmospheric pressure. In the low pressure region, we apply a combination of molecular beam methods and in-situ infrared reflection absorption spectroscopy (IRAS). For CO adsorption, angular resolved scattering and sticking coefficient measurements and structural information allow us to quantify different adsorption channels including reverse spillover effects. The coverage dependent kinetics of CO oxidation is derived and discussed in comparison with the single crystal kinetics. The adsorption of CO on alumina supported Pd aggregates at low and high pressure, i.e. from 10−7–200 mbar, is examined by IR–VIS sum frequency generation (SFG) vibrational spectroscopy. At low pressure, the CO adsorption site distribution (bridged vs. on-top) depends on the particle surface structure and temperature, but under reaction conditions, the site occupancy is mainly governed by the CO pressure. The impact of these results on the extrapolation of UHV data to high pressure catalysis is discussed.

Sum Frequency Generation Study of CO Adsorption on Palladium Model Catalysts

In the present contribution CO adsorption on palladium aggregates was examined with IR-visible sum frequency generation vibrational spectroscopy. Supported Pd nanoparticles of different mean size and surface roughness were prepared by electron beam evaporation on an ordered Al2O3 thin film formed on NiAl(110). SFG spectra were acquired for a wide range of CO pressures (from 1 × 10—7 to 200 mbar) at 190 to 300 K. At low pressure, evidence for a particle structure dependence of the CO adsorption geometry was found, with on-top CO sites significantly occupied on defective particles. On the other hand, under a high pressure regime, discrepancies between rough and smooth well-faceted particles were mostly reduced, with the relative fraction of on-top CO nearly equal for both cases. As a reference model, adsorption on well-defined Pd(111) single crystals was monitored as a function of CO pressure up to 1 bar and contrasted to the work done on aggregates. Under ultrahigh vacuum the SFG spectra on large Pd particles closely resembled the Pd(111) response. However, for pressures higher than 1 mbar at 190 K twofold bridge bonded sites were no longer populated on the Pd(111) surface while the bridge bonded occupancy was significant on Pd aggregates for all pressures studied.