Acetic acid was chosen to probe the kinetic behavior of carboxylic acid hyd
rogenation over platinum supported on TiO2, SiO2, eta-Al2O3, and Fe2O3. The
reaction was studied in the vapor phase under conditions of 423-573 K, 100
-700 Torr hydrogen, and 7-50 Torr acetic acid in a differential, fixed-bed
reactor. Product selectivity was strongly dependent on the oxide supports.
Carbon-containing products during hydrogenation at low conversions consiste
d of about: 50% CO and 50% CH4 over Pt/SiO2; 8% ethanol, 4% ethyl acetate,
10% ethane, 40% CH4, 33% CO, and 5% CO2 over Pt/eta-Al2O3; 50% ethanol, 30%
ethyl acetate, and 20% ethane over Pt/TiO2 reduced at either 473 or 773 K;
and about 80% acetaldehyde and 20% ethanol over Pt/Fe2O3. The TiO2-support
ed Pt catalysts were the most active, and both their activities and their t
urnover frequencies were up to two orders of magnitude larger than those fo
r Pt dispersed on SiO2 eta-Al2O3, or Fe2O3. The activity dependence on the
partial pressures of hydrogen, P-H2, and acetic acid, P-A, was determined f
or Pt/TiO2 catalysts at three operating temperatures-422, 445, and 465 K-af
ter reduction at either 473 or 773 K. The apparent reaction order with resp
ect to Hz was found to vary between 0.4 and 0.6, while that with respect to
acetic acid was between 0.2 and 0.4. One reaction model that correlated th
ese data well involves a Langmuir-Hinshelwood-type catalytic sequence that
incorporates dissociative hydrogen and acetic acid adsorption on one type o
f site existing on the Pt surface, but only molecular acetic acid adsorptio
n at another type of site on the oxide surface. Only the latter species on
the titania surface was considered catalytically significant in the formati
on of desired products, i.e., acetaldehyde, ethanol, and ethane. The result
ing rate expression in terms of acetic acid disappearance has the form
r(HOAc) = k(1)P(A)P(H2)(1/2)/[(K2PH21/2 + K3PA/P-H2(1/2)) (1 + K4PA)].
Values of enthalpy and entropy of adsorption obtained from the optimized ra
te for hydrogen on platinum and acetic acid on titania were reasonable and
thermodynamically consistent. (C) 2000 Academic Press.