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.

The adsorption of CO on Pt(111) was studied by picosecond infrared-visible sum frequency generation (SFG) vibrational spectroscopy in a pressure range from 10 -7 to 500 mbar and in a temperature range of 160-400 K. At low pressure the experiments were complemented by TPD, LEED, and AES. Terminally bonded (on-top) CO was the only species observed between 160 and 400 K, independent of gas pressure. The CO stretching frequency was blue-shifted by about 15 cm -1 with increasing pressure (up to 2097 cm -1 ), but no evidence for high-pressure CO species or surface roughening was found. The influence of defects was also investigated. CO adsorption on a defective (nonannealed) Pt( 111) surface yielded peaks that were slightly broadened but otherwise identical to the defect-free surface. At 160 K, a second peak at 2085 cm -1 evolved above 50 mbar of CO. TPD revealed that under these conditions residual (contaminant) water adsorbs on the surface. The coadsorption of water and CO red-shifted the terminal CO peak by about 15 cm -1 , resulting from the substrate-mediated interaction of CO and water.

Infrared-visible sum frequency generation vibrational spectroscopy is applied for the first time to monitor CO stretching vibrations on alumina supported Pd nanoparticles in a pressure range from 10(-7) to 200 mbar. The adsorption behavior of Pd aggregates with 3 and 6 nm mean size is dominated by surface defects and two different adsorption sites (twofold bridging and on-top) were identified. The CO adsorption site occupancy on Pd nanocrystals is mainly governed by the gas phase pressure while the structure of the particles and their temperature have a smaller influence.

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