Ultrahigh vacuum and high-pressure coadsorption of CO and H2 on Pd-(111):: A combined SFG, TDS, and LEED study

Sum frequency generation (SFG) vibrational spectroscopy was carried out in conjunction with thermal desorption spectroscopy, low-energy electron diffraction, and Auger electron spectroscopy to examine the coadsorption of CO and H-2 on Pd(111). Sequential dosing as well as various CO/H-2 mixtures was utilized to study intermolecular interactions between CO and H-2. Preadsorbed CO effectively prevented the dissociative adsorption of hydrogen for CO coverages greater than or equal to0.33 ML. While preadsorbed hydrogen was able to hinder CO adsorption at low temperature (100 K), hydrogen was replaced from the surface by CO at 150 K. When 1:1 mixtures of CO/H-2 were used at 100 K, hydrogen selectively hindered CO adsorption on on-top sites, while above similar to125 K no blocking of CO adsorption was observed. The observations are explained in terms of mutual site blocking, of a CO-H phase separation, and of a CO-assisted hydrogen dissolution in the Pd bulk. The temperature-dependent site blocking effect of hydrogen is attributed to the ability (inability) of surface hydrogen to diffuse into the Pd bulk above (below) similar to125 K. Nonlinear optical SFG spectroscopy allowed us to study these effects not only in ultrahigh vacuum but also in a high-pressure environment. Using an SFG-compatible ultrahigh vacuum-high-pressure cell, spectra of 1:10 CO/H-2 mixtures were acquired up to 55 mbar and 550 K, with simultaneous gas chromatographic and mass spectrometric gas phase analysis. Under reaction conditions, CO coverages greater than or equal to0.5 ML were observed which strongly limit H-2 adsorption and thus may be partly responsible for the low CO hydrogenation rate. The high-pressure and high-temperature SFG spectra also showed indications of a reversible surface roughening or a highly dynamic (not perfectly ordered) CO adsorbate phase. Implications of the observed adsorbate structures on catalytic CO hydrogenation on supported Pd nanoparticles are discussed. (C) 2003 American Institute of Physics.