Article,
The Reactivity of Surface Oxygen Phases on Pd(100) Toward Reduction by CO
J. Phys. Chem. B, (2002)
DOI: 10.1021/jp013395x
Abstract
The reactivities of surface oxygen phases on Pd(100) toward reduction
by CO were characterized using temperature-programmed desorption
and reaction (TPD, TPR), isothermal kinetic measurements, low energy
electron diffraction (LEED), and scanning tunneling microscopy (STM).
When CO was exposed to high oxygen coverages where bulk PdO and a
(sqrt5 � sqrt5)R27� oxygen-induced reconstruction exist on the surface,
a lag was observed before any CO2 was produced. The CO2 formation
rate then increased before falling as the oxygen was depleted. LEED
showed that the rate increased as (2 � 2) domains replaced the (sqrt5
� sqrt5)R27� structure on the surface indicating that the lag was
due to the slow reduction of (sqrt5 � sqrt5)R27� domains. In contrast,
LEED showed that a lower oxygen coverage (5 � 5) reconstruction is
immediately destroyed by exposure to CO. Therefore, the results indicated
that the more oxidized (sqrt5 � sqrt5)R27� surface and PdO are relatively
unreactive toward CO. Further, the temperature dependence of the
reduction rate and the TPR results suggested that the inactivity
of these surfaces is due to their inability to strongly adsorb CO.
STM movies recorded while CO was exposed to oxygen-covered surfaces
showed that the (2 � 2) domains were not reduced until the (sqrt5
� sqrt5)R27� structure was completely removed from the surface. This
indicated that oxygen from neighboring (sqrt5 � sqrt5)R27� domains
replenished the oxygen in the reactive (2 � 2) domains lost by reaction
with CO; thus oxygen transport between these phases is rapid. Similarly,
the sensitivity of the lag in the CO2 production to very small changes
in the amount of bulk PdO could be explained in terms of rapid transport
of oxygen from PdO to (sqrt5 � sqrt5)R27� domains preventing their
reduction to more reactive (2 � 2) structures until all the PdO was
removed.
