Fig. 1. (a) Graphical illustration of the preparation of Pd nanoparticles supported by Al₂O₃/NiAl(1 1 0) and of Pd model catalysts with various (surface) morphologies: (b) STM image (100 nm × 100 nm, adapted from [45] and [46]) and (c) HRTEM image of Pd nanoparticles, grown at 300 K and 90 K, respectively. The insets show individual particles at higher magnification. Controlling the growth conditions allows preparing Pd particles with different morphology/surface structure, for instance well-faceted truncated cuboctahedra (d) or rougher, defective particles (e). Single crystal surfaces that were used for comparison are also shown: (f) well-ordered Pd(1 1 1) and (g) a “defect-rich” (ion-bombarded) Pd(1 1 1).

Fig. 2. High-pressure surface sensitive techniques: (a) IR–vis sum frequency generation (SFG) vibrational spectroscopy and (b) polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS); adapted from [17] and [47] with permission from The Royal Society of Chemistry.

Fig. 3. (a) Experimental setup combining a UHV surface analysis chamber with a UHV–high-pressure reaction cell optimized for PM-IRAS spectroscopy. Pre- and post-reaction surface analysis under UHV can be performed by XPS, LEED, AES and TPD. The optical setup and the high-pressure reaction cell are shown in (b) and (c), respectively.

Fig. 4. (a) High-pressure SFG spectra of CO adsorbed on 6 nm Pd nanoparticles at 600 mbar and 200 K. (b) Thermal desorption spectra of hydrogen acquired after cooling Pd–Al₂O₃/NiAl(1 1 0) in 2 × 10⁻⁷ mbar H₂ from 300 to 100 K (80 L). The model illustrates that Pd-hydride formation proceeds mainly via particle steps/edges and (1 0 0) facets. (c) High-pressure SFG spectra of a 1:10 CO/H₂ mixture on Pd–Al₂O₃ at 500 K [93].

Fig. 5. Schematic illustration of potential CO hydrogenation routes on Pd catalysts.

Fig. 6. In situ PM-IRAS surface (a) and gas phase (b) spectra characterizing elevated pressure CH₃OH decomposition and oxidation on Pd(1 1 1). Methanol conversion was monitored via gas chromatography (c) and via PM-IRAS (d); adapted from [74] with permission. Copyright (2005) American Chemical Society.

Fig. 7. Suggested mechanism of CH₃OH decomposition and oxidation on Pd catalysts.

Fig. 8. Methanol oxidation on Pd–Al₂O₃/NiAl(1 1 0) (mean particle diameter 6 nm): (a) CH₃OH conversion vs. reaction time, monitored by gas chromatography, and (b) corresponding in situ SFG spectra. (c) Pd 3d XPS spectra acquired before and after high-pressure methanol oxidation indicate a partial oxidation of the Pd particles during the reaction. Because the particles were covered by CO after the reaction, the clean sample was also exposed to CO, responsible for the shift from 334.9 to 335.3 eV; adapted from [126] and [141].

Fig. 9. (a) TPD spectra of CO adsorbed on 3.5 nm Pd particles supported on Nb₂O₅/Cu₃Au(1 0 0) (exposure 30 L CO at 100 K) indicating a 50% loss of CO adsorption capacity upon annealing (as well as changes in the adsorption states). (b) SFG spectra acquired in 10⁻⁶ mbar CO at 110 K, after annealing the model catalysts to the indicated temperatures. The values obtained for the peak position, resonant amplitude, peak width (FWHM) and phase of the spectra are plotted in (c) for on-top and bridge-bonded CO. Metal–support interaction upon annealing to ≥300 K leads to the formation of a mixed “Pd–NbOx” phase, responsible for the irreversible loss of CO adsorption capacity; adapted from [147] with permission from Elsevier.

Fig. 10. Selective 1,3-butadiene hydrogenation on Pd–Al₂O₃/NiAl(1 1 0) at 373 K: turnover frequency (TOF) for the different Pd–Al₂O₃ catalysts as a function of mean particle diameter, normalized by the total number of Pd surface atoms (a), and normalized by the number of Pd atoms on incomplete (1 1 1) facets (b), using a truncated cubo-octahedron as realistic structural model (shown as inset; incomplete layers on side facets are not displayed for graphical reasons). TOF values for Pd(1 1 1) and Pd(1 1 0) single crystals under identical reaction conditions are shown as dashed lines. Reaction conditions: 5 mbar 1,3-butadiene; 10 mbar H₂; Ar added up to 1 bar; adapted from [152] with permission from The Royal Society of Chemistry.