Propylene deep oxidation on the Pt(111) surface: temperature programmed studies over extended coverage ranges

Propylene oxidation was studied on the Pt(111) surface over a wide range of reaction stoichiometries using temperature programmed methods. Reaction of propylene with excess oxygen results in complete oxidation to water and ca rbon dioxide, with oxydehydrogenation to form water beginning at 290 K. The initiation of skeletal oxidation occurs after water formation begins, exce pt for the highest propylene coverages. A stable dehydrogenated intermediat e with a C3H5 stoichiometry is formed in the 300 K temperature range during oxidation. Reaction of propylene with substoichiometric amounts of oxygen results in incomplete oxidation with hydrocarbon decomposition dominating a fter depletion of surface oxygen. Increasing oxygen overage results in more complete oxidation. Oxidation processes result in water, carbon dioxide, a nd carbon monoxide, while decomposition results in hydrogen, propylene, and propane desorption with some surface carbon remaining. Propylene-d(6) and selectively labeled propylene-3,3,3-d(3) (CH2CHCD3) experiments indicated i nitial water formation results from oxydehydrogenation of one of the olefin ic hydrogens. At the highest propylene and oxygen coverages studied, we obs erved small amounts of partial oxidation which indicate that the vinyl hydr ogen is removed initially, resulting in the formation of an adsorbed H2CCCH 3 intermediate. The partial oxidation products observed are acetone desorbi ng at 200 K and acetic acid at 320 K. Removal of the first skeletal carbon begins at 320 K, except for the highest propylene coverages. Preadsorption of molecular oxygen limits adsorption of propylene and preadsorption of pro pylene limits molecular oxygen adsorption at 110 K. Similar oxidation mecha nisms are observed following initial adsorption of both molecular and atomi c oxygen, which is expected since molecular oxygen dissociates and/or desor bs well below oxidation temperatures. (C) 1999 Published by Elsevier Scienc e B.V. All rights reserved.