Hyh. Chan et al., HIGH-PRESSURE OXIDATION OF RUTHENIUM AS PROBED BY SURFACE-ENHANCED RAMAN AND X-RAY PHOTOELECTRON SPECTROSCOPIES, Journal of catalysis, 172(2), 1997, pp. 336-345
Surface-enhanced Raman spectroscopy (SERS) combined with X-ray photoel
ectron spectroscopy (XPS) has been utilized to study the oxidation of
ruthenium at ambient pressure (1 atm) and elevated temperatures (25-30
0 degrees C). The SERS probe provides in-situ vibrational information
regarding surface oxide bonding. While the XPS probe necessarily invol
ves ex-situ measurements (i.e., transfer to and from ultrahigh vacuum)
, it provides valuable complementary information on the metal and oxyg
en electronic states. Ruthenium surfaces were prepared by electrodepos
iting ultrathin films (about three monolayers) onto electrochemically
roughened (i.e., SERS-active) gold substrates. Insight into the in-sit
u oxidation process was obtained by probing the changes of surface spe
ciation by SERS upon heating Ru in flowing O-2. A pair of SERS bands a
t 470 and 670 cm(-1) appear in the spectrum acquired for a freshly ele
ctrodeposited film, which are assigned to different stretching modes o
f hydrated RuO2 formed during sample transfer to the gas-phase reactor
. However, a fully reduced Ru surface (i.e., devoid of oxide features)
could be formed by adsorbing a protective CO adlayer in an electroche
mical cell followed by heating to 200 degrees C in vacuum so to therma
lly desorb the CO. While the initially oxidized (i.e., RuO2) surface w
as stable to further oxidation upon heating in O-2, adsorbed atomic ox
ygen was detected at 200 degrees C from the appearance of a SERS band
at 600 cm(-1) and a XPS O(1s) peak at 531.7 eV. In contrast, the highe
r oxides RuO4 and possibly RuO3 were produced only upon thermal oxidat
ion of the fully reduced Ru surface. Evidence for RuO3 formation inclu
des the appearance of a 800 cm(-1) SERS band at 200 degrees C which co
rrelates with the advent of a Ru(3d(5/2)) peak at 282.6 eV. The surfac
e was further oxidized to RuO4 at 250 degrees C, as deduced from the f
ormation of a 875 cm(-1) band and a Ru(3d(5/2)) peak at 283.3 eV. Whil
e RuO3 and RuO4 were exclusively formed at temperatures higher than 25
0 degrees C, RuO2 was produced upon cooling to room temperature, possi
bly via the decomposition of RuO4. (C) 1997 Academic Press.