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 combustion. The interaction of propylene at elevated temperature (>300 K) and elevated pressure (5 Torr) with cuprous and cupric oxide has been investigated with core level XPS, resonant photoemission, and temperature-programmed desorption. Reduction of the copper oxide surfaces was examined as a function of temperature and revealed that cupric oxide has a greater reactivity toward propylene oxidation than cuprous oxide (Ea = 5.9 versus 11.5 kcal/mol for cuprous oxide (24.7 and 48.1 kJ/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 produces a surface intermediate which exhibits carbon 1s XPS peaks at 284.0 and 285.5 eV binding energy in a 2:1 intensity ratio. This is consistent with an allyl alkoxide surface species, indicating a reaction mechanism involving an initial H atom abstraction from propylene followed by rapid oxide insertion. The relative surface reactivities are related to the redox potential of the metal ion and the pKa of the protonated surface oxide. The presence of a significant amount of this surface alkoxide is consistent with a relatively slow alkoxide decomposition step. This decomposition occurs more readily on the cuprous oxide surface (Ea (decomposition) = 24.5 kcal/mol (102.6 kJ/mol) versus 28.7 kcal/mol (120.1 kJ/mol) on cupric oxide) and involves a hydride elimination mechanism. At elevated temperatures a new carbon 1s peak at ∼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 oxidized products. The observed differences in reactivity, rates of reaction steps, selectivity, and product distribution are addressed and provide insight into the factors which influence the reactivity and selectivity of the copper oxides toward the heterogeneous oxidation of propylene.