F. Gao et al. / Journal of Catalysis 268 (2009) 115–121
121
sures, the NO
NO
pressures lower than ꢀ0.04 Torr, the CO
while at higher NO pressures, the CO formation is inhibited.
2
/NO ratio rose to ꢀ0.07. As is clear from Fig. 5a and b, at
From a more practical point of view, the current study together
2
2
formation is promoted
with our previous studies on the CO + O reaction [30,31] demon-
2
2
2
strates an unusual high activity of PdAu at relatively low tempera-
The surface oxygen coverage increase, caused by reaction (12),
can be examined qualitatively using PM-IRAS since the adsorption
of oxygen concurrently alters the CO band intensity and frequency.
tures (e.g., <500 K) suggesting that the addition of PdAu to
traditional TWCs could in principle resolve the so-called ‘‘cold
start” problem. Furthermore, PdAu alloys may be advantageous
compared with pure Pd in high-temperature operation since the al-
loy is more difficult to oxidize due to electron transfer from Pd to
Au [46].
As displayed in Fig. 6a, although adding 1 Torr of NO to the CO + O
2
mixture has no apparent effect on the CO signal intensity, upon add-
ing 2 Torr of NO to the gas mixture, the CO signal intensity decreases
significantly, consistent with a significant oxygen coverage. Note
that this corresponds with the CO
Fig. 5a. It is noteworthy that the surface oxygen remains chemi-
sorbed so long as the gas-phase NO pressure remains low. This is
apparent from post-reaction AES showing that the surface is essen-
tially free of oxygen (data not shown) upon the addition of 64 Torr of
NO to theCO + O mixture. Thisis duetoa well-knownphenomenon,
2
2
formation rate data shown in
Acknowledgments
2
We gratefully acknowledge the support for this work by the
Department of Energy, Office of Basic Energy Sciences, Division of
Chemical Sciences, Geosciences, and Biosciences (DE-FG02-95ER-
14511), and the Robert A. Welch Foundation.
namely ‘‘electron-stimulated-desorption (ESD)” of chemisorbed oxy-
gen from a Pd surface [44]. In contrast, oxygen builds up in the sub-
surface region upon Pd oxidation and can be detected easily with
AES. Following the addition of NO to greater than 6 Torr, post-reac-
tion AES data (bottom curve, Fig. 6b) indeed showed clearly a large
oxygen feature, consistent with the formation of bulk Pd oxides.
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1