Fig. 1. XRD data for Pd-only catalysts. (a and b) Pd(5.0 wt.%)/SiO₂; (c and d) Pd(1.0 wt.%)/SiO₂; (a and c) freshly reduced; (b and d) after reaction [13].

Fig. 2. VA synthesis on Pd(5.0 wt.%)/SiO₂, Pd(1.0 wt.%)/SiO₂ and Pd(1.0 wt.%)–Au(0.4 wt.%)/SiO₂ catalysts. p_O2=7.6 Torr; p_C2H4=57.0 Torr; p_AcOH = 12.0 Torr; the remainder N₂; flow rate: 30–60 ml/min; temperature = 413 K; catalyst weight: 0.1–1.6 g [18].

Fig. 3. Surface vs. bulk compositions for Au–Pd polycrystalline alloys (A) (●) steady-state sputtered surfaces, (○) annealed at 973 K [43], and (B) for Au–Pd thin films on Mo(1 1 0) [45].

Fig. 4. Surface concentrations of Pd and Au of 5 ML Pd/5 ML Au/Mo(1 1 0) (○) and 5 ML Au/5 ML Pd/Mo(1 1 0) (■) as a function of annealing temperatures [45].

Fig. 5. In situ atomic resolution STM images of PdAu alloys electrodeposited on Au(1 1 1) for (A) Pd₀₇Au₉₃ and (B) Pd₁₅Au₈₅. Pd atoms appear larger and, depending on tunneling conditions, brighter or darker than Au atoms. (C) Surface coverages of Pd monomers, dimers, and trimers, as obtained from atomically resolved STM images [54].

Fig. 6. Atomic resolution STM images of Au₃Pd(1 0 0) surface (left). Pd atoms appear larger and brighter as indicated by circles or ellipses. The surface Pd coverage is 14%, in which the relative amounts of monomer, monomer pair, dimer and those ensembles of more than two contiguous Pd atoms are shown in the histogram (right) [44].

Fig. 7. IRAS spectra after CO adsorption at 90 K on 4 ML Pd/Au(1 0 0) and 4 ML Pd/Au(1 1 1). Pd was deposited on Au(1 0 0) and Au(1 1 1) at 90 K and subsequently annealed to 300 and 600 K for 10 min each [12].

Fig. 8. VA formation rate as a function of the Pd coverage on Au(1 0 0) and Au(1 1 1) surfaces. TOFs are calculated with respect to (1 × 1) surface unit. Pd was deposited on the respective surface at 300 K and annealed to 600 K for 10 min. C₂H₄ = 8 Torr, O₂ = 2 Torr, CH₃COOH = 4 Torr, reaction temperature = 453 K, reaction time = 3 h [12].

Fig. 9. Vinyl acetate (VA) formation rates as a function of Pd coverage. The TOFs are computed with respect to the Pd atom concentration. Pd was deposited on the respective surface at 300 K and annealed to 600 K for 10 min. C₂H₄ = 8 Torr, O₂ = 2 Torr, CH₃COOH = 4 Torr, reaction temperature = 453 K, reaction time = 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) [12].

Fig. 10. 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) [12].

Fig. 11. (A) 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 to 800 K for 20 min [45] and [59]. (B) TPD of C₂D₄ from supported Pd and Pd–Au bimetallic clusters [59].

Fig. 12. VA formation rate normalized to per Pd site basis on Sn/4.0 ML Pd/Rh(1 0 0) annealed to 600 K for 60 s. Reaction at 450 K; p_O2=2.0 Torr; p_C2H4=9.0 Torr; p_AcOH = 4.0 Torr; time = 3 h [63].