Study of CO, NO, and H2 adsorption on model Pd/alpha-Al2O3(0001)
catalysts
Surface Science, (1995)DOI: 10.1016/0039-6028(94)00663-6
Abstract
We have examined the adsorption of CO, NO, and H2 on alpha-Al2O3(0001)-supported,
Pd particles using temperature-programmed desorption (TPD) in order
to determine the effect of particle size on the desorption rates.
For vapor deposition of Pd at 295 K, one obtains layer-by-layer growth
of the Pd film. The film coalesces into a relatively uniform distribution
of particles above not, vert, similar 600 K, as shown by TEM, with
the particle sizes depending on the metal coverage. TPD curves for
CO from all particle sizes are very similar to curves reported for
bulk metals, with a main desorption feature at not, vert, similar
490 K and a second peak at not, vert, similar 375 K; however, the
relative amount of CO desorbing from the low-temperature feature,
assigned to linear CO, increases significantly for very small particles
(not, vert, similar 1.6 nm). For NO, more than half of the molecules
desorb intact from large particles (> 5 nm), with the remainder forming
N2O and N2 at not, vert, similar 550 K and leaving adsorbed oxygen.
The fraction of molecules which dissociate during TPD increases significantly
for small particles and only N2 is observed as a product above 600
K. For H2 at 120 K, desorption occurs in two regions on large particles,
between 250 and 400 K for surface hydrogen and at not, vert, similar
180 K for hydrogen from the bulk. The only significant difference
on small particles is the absence of the 180 K state. These results
are discussed in terms of their implications for Pd particle-size
effects in catalysis.