Article,
CO Adsorption on Pd Nanoparticles: Density Functional and Vibrational Spectroscopy Studies
J. Phys. Chem. B, 107 (1): 255--264 (2003)
DOI: 10.1021/jp022052b
Abstract
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 Oh 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 19501970 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.
