PROPYLENE OXIDATION ON COPPER-OXIDE SURFACES - ELECTRONIC AND GEOMETRIC CONTRIBUTIONS TO REACTIVITY AND SELECTIVITY

Cu2O is an efficient catalyst in the partial oxidation of propylene to acrolein, while propylene oxidation on CuO leads to complete combusti on. The interaction of propylene at elevated temperature (>300 K) and elevated pressure (5 Torr) with cuprous and cupric oxide has been inve stigated with core level XPS, resonant photoemission, and temperature- programmed desorption. Reduction of the copper oxide surfaces was exam ined as a function of temperature and revealed that cupric oxide has a greater reactivity toward propylene oxidation than cuprous oxide (E-a = 5.9 versus 11.5 kcal/mol for cuprous oxide (24.7 and 48.1 kT/mol)). This variable temperature: oxidation of propylene was also monitored via core level and resonant photoemission and was found to occur by a similar mechanism on both surfaces. Reaction at lower temperature prod uces a surface intermediate which exhibits carbon Is XPS peaks at 284. 0 and 285.5 eV binding energy in a 2:1 intensity ratio. This is:consis tent with an allyl alkoxide surface species, indicating a reaction mec hanism involving an initial H atom abstraction from propylene followed by rapid oxide insertion. The relative surface reactivities are relat ed to the redox potential of the metal ion and the pK(a) of the proton ated surface oxide. The presence of a significant amount of this surfa ce alkoxide is consistent with a relatively slow alkoxide decompositio n step. This decomposition occurs more readily on the cuprous oxide su rface (E-a (decomposition) = 24.5 kcal/ mol (102.6 kT/mol) versus 28.7 kcal/mol (120.1 kT/mol) on cupric oxide) and involves a hydride elimi nation mechanism. At elevated temperatures a new carbon Is peak at sim ilar to 288 eV binding energy is observed which is consistent with the formation of further oxidized surface species (RCOx). The CuO surface is found to be more reactive in forming these nonselective highly oxi dized products. The observed differences in reactivity, rates of react ion steps, selectivity, and product distribution are addressed and pro vide insight into the factors which influence the reactivity and selec tivity of the copper oxides toward the heterogeneous oxidation of prop ylene.