Article,
The adsorption of carbon monoxide on TiO2(110) supported palladium
Surf. Sci., 360 (1-3): 61 - 73 (1996)
DOI: DOI: 10.1016/0039-6028(96)00591-2
Abstract
Palladium overlayers deposited on TiO2(110) by metal vapour deposition
have been investigated using LEED, XPS and FT-RAIRS of adsorbed CO.
Low coverages of palladium (<3 ML) deposited at 300 K adsorb CO exclusively
in a bridged configuration with a band (B1 at 1990 cm-1) characteristic
of CO adsorption on Pd(110) and Pd(100) surfaces. When annealed to
500 K, XPS and LEED indicate the nucleation of Pd particles on which
CO adsorbs predominantly as a strongly bound linear species which
we associate with edge sites on the Pd particles (L* band at 2085
cm-1). Both bridged and linear CO bands are exhibited as increases
in reflectivity at the resonant frequency, indicating the retention
of small particle size during the annealing process. Palladium overlayers
of intermediate coverages (10-20 ML) deposited at 300 K undergo some
nucleation during growth, and adsorbed CO exhibits both absorption
and transmission bands in the B1 (1990 cm-1) and B2 (1940 cm-1) regions.
The latter is associated with the formation of Pd(111) facets. Highly
dispersed Pd particles are produced on annealing at 500 K. This is
evidenced by the dominance of transmission bands for adsorbed CO
and a significant concentration of edge sites, which accommodate
the strongly bound linear species at 300 K. Adsorption of CO at low
temperature also allows the identification of the constituent faces
of Pd and the conversion of Pd(110)/(100) facets to Pd(111) facets
during the annealing process. High coverages of palladium (100 ML)
produce only absorption bands in FT-RAIRS of adsorbed CO associated
with the Pd facets, but annealing these surfaces also shows a conversion
to Pd(111) facets. LEED indicates that at coverages above 10 ML,
the palladium particles exhibit (111) facets parallel to the substrate
and aligned with the TiO2(110) unit cell, and that this ordering
in the particles is enhanced by annealing.
