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doi:10.1016/j.susc.2004.02.008 
Copyright © 2004 Elsevier B.V. All rights reserved.
Sum frequency generation and density functional studies of CO–H interaction and hydrogen bulk dissolution on Pd(1 1 1)
Received 14 November 2003;
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Abstract
CO–H interaction and H bulk dissolution on Pd(1 1 1) were studied by sum frequency generation (SFG) vibrational spectroscopy and density functional theory (DFT). The theoretical findings are particularly important to rationalize the experimentally observed mutual site blocking of CO and H and the effect of H dissolution on coadsorbate structures. Dissociative hydrogen adsorption on CO-precovered Pd(1 1 1) is impeded due to an activation barrier of 
2.5 eV for a CO coverage of 0.75 ML, an effect which is maintained down to 0.33 ML CO. Preadsorbed hydrogen prevented CO adsorption at 100 K, while hydrogen was replaced from the surface by CO above 125 K. The temperature-dependent site blocking of hydrogen originates from the onset of hydrogen diffusion into the Pd bulk around 125 K, as shown by SFG and theoretical calculations using various approaches. When Pd(1 1 1) was exposed to 1:1 CO/H2 mixtures at 100 K, on-top CO was absent in the SFG spectra although hydrogen occupies only threefold hollow sites on Pd(1 1 1). DFT attributes the absence of on-top CO to H atoms diffusing between hollow sites via bridge sites, thereby destabilizing neighboring on-top CO molecules. According to the calculations, the stretching frequency of bridge-bonded CO with a neighboring bridge-bonded hydrogen atom is redshifted by 16 cm−1 when compared to bridging CO on the clean surface. Implications of the observed effects on hydrogenation reactions are discussed and compared to the C2H4–H coadsorption system.
Author Keywords: Author Keywords: Sum frequency generation; Density functional calculations; Palladium; Carbon monoxide; Hydrogen molecule; Alkenes; Low index single crystal surfaces; Catalysis; Vibrations of adsorbed molecules
Article Outline
- 1. Introduction
- 2. Methods
- 3. Results and discussion
- 3.1. CO adsorption and H2 adsorption on Pd(1 1 1)
- 3.2. H adsorption on CO-precovered Pd(1 1 1)
- 3.3. CO adsorption on H-precovered Pd(1 1 1)
- 3.4. Adsorption of CO–H2 mixtures
- 3.5. Hydrogen bulk dissolution
- 3.6. Implications on hydrogenation reactions
- 4. Conclusions
- Acknowledgements
- References
