| Autor: |
Morkel, Matthias, Rupprechter, Günther, Freund, Hans-Joachim |
| Předmět: |
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| Zdroj: |
Journal of Chemical Physics; 11/22/2003, Vol. 119 Issue 20, p10853-10866, 14p, 6 Graphs |
| Abstrakt: |
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[sub 2] on Pd(111). Sequential dosing as well as various CO/H[sub 2] mixtures was utilized to study intermolecular interactions between CO and H[sub 2]. Preadsorbed CO effectively prevented the dissociative adsorption of hydrogen for CO coverages >=0.33 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[sub 2] were used at 100 K, hydrogen selectively hindered CO adsorption on on-top sites, while above ∼125 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) ∼125 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[sub 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 >=0.5 ML were observed which strongly limit H[sub 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. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR] |
| Databáze: |
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