Fig. 1. SFG spectra acquired at increasing CO exposure of Pd(1 1 1) at 95 K. Approximate coverages are also indicated (see text).

Fig. 2. Effect of annealing on the CO adsorbate structure: SFG spectrum after direct exposure of 10⁻⁶ mbar CO at 95 K (dotted line) and after annealing in 10⁻⁶ mbar CO to 700 K with subsequent cooling to 95 K (full line).

Fig. 3. (a) Mass spectra taken from 1 mbar 4.7 CO using a quartz capillary inlet to a differentially pumped mass spectrometer (1) and after passing CO over a carbonyl absorber cartridge and a cold trap (2). (b) Auger spectra of Pd(1 1 1) (3) and after exposure to 4.7 CO (>200 mbar) for several hours (4) revealing Ni contamination.

Fig. 4. SFG spectra of CO adsorption on Pd(1 1 1) at 190 K from 10⁻⁶ to 100 mbar (a) and from 300 to 1000 mbar (b). The final spectrum at 1 mbar demonstrates the reversibility of the adsorbate structure. Difference spectra are displayed in (c).

Fig. 5. SFG spectra of CO adsorption on defect-rich Pd(1 1 1) at 195 K from 10⁻⁶ to 200 mbar. Compared to the perfect (1 1 1) surface an additional peak at 1990 cm⁻¹ appeared.

Fig. 6. SFG spectra of CO adsorption on Al₂O₃ supported Pd nanoparticles between 10⁻⁷ and 200 mbar CO and at 190 K. Spectra (a) were taken from well-faceted Pd particles with a mean size of 6 nm. A model and an STM image (CCT, 50×50 nm, image adapted from Ref. [53]) are displayed in (b,c). The 10⁻⁷ mbar spectrum of 3.5 nm Pd particles exhibiting rough surfaces and the corresponding model are presented in (d,e).

Fig. 7. Ethylene hydrogenation activity of a Pd/Al₂O₃/NiAl(1 1 0) model catalyst with a mean Pd particle size of 3.5 nm. The reaction was carried out with 50 mbar C₂H₄, 215 mbar H₂ and 770 mbar He at 300 to 350 K. Because the SFG cell was used as a recirculated batch reactor, the conversion increased with time. Turnover frequencies for the different temperatures are also indicated.