It is well known that the sulfur included in
automobile exhaust deactivates the exhaust catalysts,
and that the sulfur affects the Pt and Pd catalysts
more than the Rh catalyst. The adsorptive state
of the sulfur atom on polycrystalline noble metals
has been studied with X-ray photoelectron spectroscopy
(XPS). The sulfur atom adsorbed on the Pt, Pd
and Rh staied on the surface in a vacuum up to
800°C. Under low pressure
O2 conditions (10-3`10-1Pa), the sulfur
on the Rh disappeared from the surface at a lower
O2 pressure and temperature than the
sulfur on the Pt and Pd. The reaction of sulfur
and oxygen on the Pd and Rh in 10-6 Pa O2
has been studied by temperature programmed XPS(TPXPS).
On the Pd, the adsorbed sulfur decreased at temperatures
between 200°C and 250°C
for only a low initial sulfur coverage of less
than 0.5 ML. On the Rh, the adsorbed sulfur disappeared
from the surface at about 300°C
irrespective of sulfur coverage. The activation
energy for the SO2 recombination on
the Pd and Rh derived from the Arrhenius plots
were 57}16C32}6kJ/mol, respectively.
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