Prototype NO, sensors for use in automotive exhaust can contain Au/Pt elect
rodes that dissociate O-2 for removal by electrochemical pumping, but these
electrodes should not dissociate NO. In these sensors, the NO diffuses to
a second chamber where it is then detected. To understand the process that
occurs at the Au/Pt electrode we have used temperature-programmed desorptio
n to study the dissociation of NO and O-2 On Pt(335) and on partially Au co
vered Pt(335). With a gold coverage of 0.15 ML, the dissociation probabilit
y of NO is decreased by a factor of 5 relative to bare Pt. Increasing Au co
verage to 0.3 ML decreases NO dissociation to an undetectable level. At 0.3
ML Au, the saturation coverage of atomic O is reduced by only 20%. The var
iation of NO dissociation with Au coverage occurs in the range where all of
the Au atoms are at steps, but Au is more effective at decreasing NO disso
ciation than can be explained by a simple site-blocking model. The effect o
f adsorbed O on NO dissociation was also investigated. Both Au and O decrea
se NO dissociation, but An is much more effective. Blocking 40% of the step
sites with O reduces NO dissociation by 70%-more than site-blocking, but m
uch less than a comparable coverage of Au. Our observations show how the Au
/Pt electrode in a NOx sensor is able to dissociate oxygen similar to 10(3)
times more efficiently than MO.