Adsorption of CO on nanosize Pd particles was studied theoretically by density functional method and spectroscopically by means of infrared reflection absorption spectroscopy (IRAS) and sum frequency generation (SFG). A density functional approach was applied to three-dimensional crystallites of about 140 atoms. The model clusters were chosen as octahedral fragments of the face centered cubic (fcc) bulk, exhibiting (111) and (001) facets. Bare and adsorbate-decorated cluster models were calculated with O h symmetry constraints. Various types of adsorption sites were inspected:  3-fold hollow, bridge, and on-top positions at (111) facets; 4-fold hollow and on-top sites at (001) facets; bridge positions at cluster edges; on-top positions at cluster corners; and on single Pd atoms deposited at regular (111) facets. Adsorption properties of the relatively small regular cluster facets (111) and (001) are calculated similar to those of corresponding ideal (infinite) Pd surfaces. However, the strongest CO bonding was calculated for the bridge positions at cluster edges. The energy of adsorption on-top of low-coordinated Pd centers (kinks) is also larger than that for on-top sites of (111) and (001) facets. To correlate the theoretical results with spectroscopic data, vibrational spectra of CO adsorbed on supported Pd nanocrystallites of different size and structure (well-faceted and defect-rich) were measured using IRAS and SFG. For CO adsorption under ultrahigh vacuum conditions, a characteristic absorption in the frequency region 1950−1970 cm-1 was observed, which in agreement with the theoretical data was assigned to vibrations of bridge-bonded CO at particle edges and defects. SFG studies carried out at CO pressures up to 200 mbar showed that the edge-related species was still present under catalytic reaction conditions. By decomposition of methanol leading to the formation of carbon species, these sites can be selectively modified. As a result, CO occupies on-top positions at particle edges and defects. On the basis of the computational data, the experimentally observed differences in CO adsorption on alumina-supported Pd nanoparticles of different size and surface quality are interpreted. Differences between adsorption properties of Pd nanoparticles with a large fraction of (111) facets and adsorption properties of an ideal Pd(111) surface are also discussed.

The adsorption of CO on Pd(111) and on Al2O3-supported Pd nanoparticles was studied by picosecond infrared−visible sum frequency generation (SFG) vibrational spectroscopy in a pressure range from 10-7 to 1000 mbar and in a temperature range of 100−520 K. Under ultrahigh vacuum (UHV), the samples were further characterized by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and temperature-programmed desorption (TPD). Identical high coverage (saturation) CO structures were observed on Pd(111) under UHV conditions (10-7 mbar, 100 K) and at high pressure (e.g., 1 mbar, 190 K). No indications of pressure-induced surface rearrangements of Pd(111) were evident from SFG and LEED. SFG spectra of CO adsorption on “defect-rich” Pd(111) revealed an additional peak that was attributed to adsorption on defect (step or edge) sites. The CO adsorbate structure on supported Pd nanoparticles was found to be different from that on Pd(111) and to be more similar to that on stepped or strongly sputtered Pd(111). At low pressure, the adsorption site occupancy depended on the particle surface structure and temperature. CO preferentially adsorbed in bridge sites on well-faceted Pd particles, while on more defective Pd particles, on-top sites were occupied as well. However, at 200 mbar CO, an adsorption site occupancy was obtained that was nearly independent of the particle surface structure. While the surface structure of the Pd particles remained unchanged upon high-pressure gas exposure, an annealing treatment to 300−400 K was able to order the Pd particle surface. Gas mixtures of CO and hydrogen on Pd(111) showed SFG spectra similar to the pure CO case indicating the absence of a strong interaction between CO and hydrogen.

More than skin deep: In spite of their identical 1:1 surface composition, the geometric and electronic structures of a multilayer and monolayer PdZn surface alloy are different, as are their catalytic selectivities. The CO2 selective multilayer alloy features surface ensembles of PdZn exhibiting a “Zn‐up/Pd‐down” corrugation (see picture). These act as “bifunctional” active sites both for water activation and for the conversion of methanol into CO2. On the monolayer alloy CO and not CO2 is produced.

Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1-7.3 wt %). Collectively, this series of catalysts provides a range of metal particle sizes (1.2-8.5 nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)(111) and (111)(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1 wt % loaded catalysts, with the relative inactivity of the catalyst prepared from palladium(II) chloride attributed to a diminished hydrogen supply due to decoration of edge sites by chlorine originating from the preparative process. It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.

Infrared−visible sum frequency generation (SFG) is a surface-specific vibrational spectroscopy that can operate in a pressure range from ultrahigh vacuum (uhv) to atmospheric pressures. SFG is therefore one of the few surface science techniques that permits atomic scale monitoring of surface species during catalytic reactions at high pressures (around 1 atm) and high temperatures. Using single-crystal surfaces of transition metals, platinum and rhodium, reaction rates can be simultaneously determined by gas chromatography, and correlations between the concentration of adsorbates under reaction conditions and the observed turnover numbers can help to elucidate the reaction mechanism. To bridge the gap to traditional surface science experiments, SFG is also employed under uhv or low pressures. The technique has been successfully applied to the adsorption and oxidation of CO, hydrocarbon conversion such as ethylene hydrogenation and cyclohexene hydrogenation and dehydrogenation on Pt(111). The experiments demonstrate that the key intermediates of high-pressure catalytic reactions are not present under low-pressure (uhv) conditions. Furthermore, the identification of active intermediates and their concentration at ambient conditions allows calculation of turnover frequencies per active surface species rather than simply per surface metal atom.