Article,
Interactions between oxygen and carbon monoxide on a Pd(111) surface
Surf. Sci., (1978)
DOI: 10.1016/0039-6028(78)90101-2
Abstract
Adsorption, mutual interactions and transient product formation between
oxygen and carbon monoxide on a Pd(111) surface were studied by means
of LEED, UPS and isothermal as well as temperature-programmed desorption
spectrometry at temperatures between 200 and 550 K. CO adsorption
at 200 K was found to cause the formation of additional periodic
overlayer structures above theta = 0.5, apart from the already known
surface structures at theta < 0.5. Saturation is reached with a hexagonal
close-packed layer at theta = 0.66 (Image ). Depending on the sequence
of gas exposure and on the partial coverages of COad andOad, different
stages of coadsorption (island formation, �compression� of the 2
� 2-Oad layer into a sqrt-3 � sqrt-3 R 30� structure, growth of a
mixed phase with 2 � 1 periodicity etc.) may be distinguished, which
influence the electronic properties and the reactivity of the adsorbates.
At high surface concentrations CO2 formation proceeds with an appreciable
rate already at 200 K,whereas at low coverages temperatures above
400 K are needed. The results demonstrate that no appropriate description
of the kinetics of catalytic CO2 formation over the whole range of
surface concentrations may be achieved in terms of simple rate laws
involving coverage-independent rate constants (and activation energies)
and mere surface concentrations of the reactants. There is on the
other hand no indication for a reaction path involving the collision
of a gaseous CO molecule with an adsorbed oxygen atom (Eley-Rideal
mechanism), i.e. the reaction proceeds always with chemisorbed CO
via Oad + COad to CO2 (Langmuir-Hinshelwood mechanism).
