Sum frequency generation vibrational spectroscopy at solid-gas interfaces: CO adsorption on Pd model catalysts at ambient pressure

Carbon monoxide adsorption on Pd(111) and Pd nanoparticles supported by Al2O3/NiAl(110) was examined by vibrational sum frequency generation spectroscopy from 10(-8) to 1000 mbar, and from 100 to 400 K. Identical CO saturation structures were observed on Pd(111) under ultrahigh vacuum (similar to10(-7) mbar, 95 K) and at high pressure (e.g. greater than or equal to 1 mbar, 190 K) with no indications of pressure-induced surface rearrangements. Special attention was paid to experimental artifacts that may occur under elevated pressure and may be misinterpreted as "high pressure effects". Vibrational spectra of CO on defect-rich Pd(111) exhibited an additional peak that originated from CO bound to defect (step or edge) sites. The CO adsorbate structure on supported Pd nanoparticles was different from Pd(111) but more similar to stepped Pd(111). At low pressure (10(-7) mbar CO) the adsorbate structure depended strongly on the I'd morphology revealing specific differences in the adsorption properties of supported nanoparticles and single crystal surfaces. At high pressure (e.g. 200 mbar CO) these differences were even more pronounced. Prominent high coverage CO structures on Pd(111) could not be established on Pd particles. However, in spite of structural differences between well faceted and rough I'd nanoparticles nearly identical adsorption site occupancies were observed in both cases at 200 mbar CO. Initial tests of the catalytic activity of Pd/Al2O3/NiAl(110) for ethylene hydrogenation at I bar revealed a remarkable activity and stability of the model system with catalytic properties similar to impregnated catalysts. (C) 2002 Elsevier Science B.V. All rights reserved.