Ma. Bollinger et Ma. Vannice, A KINETIC AND DRIFTS STUDY OF LOW-TEMPERATURE CARBON-MONOXIDE OXIDATION OVER AU-TIO2 CATALYSTS, Applied catalysis. B, Environmental, 8(4), 1996, pp. 417-443
Titania-supported gold catalysts however, deactivation is observed Imp
regnated Au-TiO2 is most active after a sequential pretreatment consis
ting of high temperature reduction at 773 K, calcination at 673 K and
low temperature reduction at 473 K (HTR/C/LTR); the activity after eit
her only low temperature reduction or calcination is much lower. A cat
alyst prepared by coprecipitation had much smaller Au particles than i
mpregnated Au-TiO2 and was active at 273 K after either an HTR/C/LTR o
r a calcination pretreatment. Deposition of TiOx overlayers onto an in
active Au powder produced high activity; this argues against an electr
onic effect in small Au particles as the major factor contributing to
the activity of Au-TiO2 catalysts and argues for the formation of acti
ve sites at the Au-TiOx interface produced by the mobility of TiOx spe
cies. DRIFTS (diffuse reflectance FTIR) spectra of impregnated Au-TiO2
reveal the presence of weak reversible CO adsorption on the Au surfac
e but not on the TiO2; however, a band for adsorbed CO is observed on
the pure TiO2. Kinetic studies with a 1.0 wt.-% impregnated Au-TiO2 sa
mple showed a near half-order rate dependence on CO and a near zero-or
der rate dependence on O-2 between 273 and 313 K with an activation en
ergy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and O-
2 adsorbing on TiO2, is consistent with Langmuir-Hinselwood kinetics f
or noncompetitive adsorption, fits partial pressure data well and show
s consistent enthalpies and entropies of adsorption. The formation of
carbonate and carboxylate species on the titania surface was detected
but it appears that these are spectator species. DRIFTS experiments un
der reaction conditions also show the presence of weak, reversible ads
orption of CO2 (near 2340 cm(-1)) which may be competing with CD for a
dsorption sites.