Fig. 1. H₂O formation rate for H₂ oxidation on Pd–Au alloy as a function of Pd composition. The rate was calculated on a per Pd basis using the bulk Pd ratio. The original data were obtained from Ref. [9].

Fig. 2. Left panel: LEISS spectra of 5 ML Pd/5 ML Au/Mo(1 1 0) as a function of annealing temperature. LEISS spectra were collected at 300 K after the sample was annealed to the specified temperature. Right panel: surface concentration of Au and Pd of 5 ML Pd/5 ML Au/Mo(1 1 0) as a function of annealing temperature. The sample was annealed at each temperature for 20 min.

Fig. 3. Surface concentration of various Pd–Au alloys on Mo(1 1 0) measured by LEISS compared to the corresponding bulk concentration. The sample was annealed at 800 K for 20 min. Results from Pd–Au alloy were displayed together for comparison [28].

Fig. 4. IRAS spectra for CO adsorption on: (a–c) Pd(1 1 1) at 500, 200 and 100 K in 1 × 10⁻⁶ Torr CO; (d) Pd(1 0 0) with saturated CO coverage; (e) Pd/SiO₂ with saturated CO coverage. The original data were obtained from Ref. [29].

Fig. 5. IRAS of CO on 5 ML Pd/5 ML Au/Mo(1 1 0) as a function of CO exposure. (A) Annealed at 600 K for 20 min, (B) annealed at 800 K for 20 min.

Fig. 6. TPD of CO on: (a) 10 ML Pd/Mo(1 1 0), and (b) 5 ML Pd/5 ML Au/Mo(1 1 0) annealed at 800 K for 20 min.

Fig. 7. Vinyl acetate (VA) formation rates as a function of Pd coverage. The TOF's are computed with respect to the (1 × 1) surface unit. The VA synthesis was carried out at 453 K, with acetic acid, ethylene and O₂ pressures of 4, 8 and 2 Torr, respectively. Total reaction time is 3 h. The error bars are based on background rate data. Inserts show the structures for Au(1 0 0)-c(2 × 2)-Pd and Au(1 1 1)-R30°-Pd, where the maximum number of surface Pd monomers occur for the Pd/Au(1 0 0) and Pd/Au(1 1 1) surfaces, respectively.

Fig. 8. IRAS spectra for CO adsorption at 90 K on 4 ML Pd/Au(1 0 0) and 4 ML Pd/Au(1 1 1); Pd was deposited at 90 K with a subsequent anneal at 300 and 600 K, each for of 10 min.

Fig. 9. Vinyl acetate (VA) formation rates as a function of Pd coverage. The TOF's are computed with respect to the Pd atom concentration. The VA synthesis was carried out at 453 K, with acetic acid, ethylene and O₂ pressures of 4, 8 and 2 Torr, respectively. Total reaction time is 3 h. The error bars are based on background rate data. The inserts show Pd monomers and monomer pairs on the Au(1 0 0) and Au(1 1 1).

Fig. 10. The ratio of the number of Pd monomers to the number of total surface Pd atoms as a function of Pd coverage. The original data were obtained from Ref. [37].

Fig. 11. Schematic for VA synthesis from acetic acid and ethylene. The optimized distance between the two active centers for the coupling of surface ethylene and acetate species to form VA is 3.3 Å. With lateral displacement, coupling of an ethylene and acetate species on a Pd monomer pair is possible on the Au(1 0 0), but unlikely on the Au(1 1 1).

Fig. 12. HREELS spectra of: (A) acetic acid adsorption on (a) Pd/Au(1 1 1) with only isolated Pd sites, (b) Pd/Au(1 1 1) with continuous Pd sites, and (c) 1 ML Pd/Au(1 1 1) after VA synthesis reaction for 3 h. (B) Ethylene adsorption on (a) Pd/Au(1 1 1) with only isolated Pd sites, and (b) Pd/Au(1 1 1) with continuous Pd sites. Acetic acid was dosed into the chamber with a background pressure of 2 × 10⁻⁸ Torr and cooling the sample from 350 to 210 K. Ethylene was dosed into the chamber with a background pressure of 2 × 10⁻⁸ Torr, then cooling the sample from 300 to 170 K.

Fig. 13. TPD of ethylene (C₂D₄) from supported Pd and Pd–Au alloy clusters.