Propene oxidation catalysis and electronic structure of M55 particles (M = Pd or Rh): differences and similarities between Pd55 and Rh55

Abstract

Propene oxidation is one of the important reactions that occurs in the presence of a three-way catalyst but its reaction mechanism is unclear. The reaction mechanisms and differences in catalysis between Pd and Rh particles were investigated by DFT calculations employing Pd₅₅ and Rh₅₅ as the model catalysts. The O-attack mechanism, in which the O atom adsorbed on the Pd₅₅ and Rh₅₅ surfaces attacks the CC double bond of propene, needs to overcome a large activation barrier (E_a). On the other hand, C–H bond cleavage of the methyl group of propene easily occurs with moderate E_a; the mechanism initiated by this C–H activation is named H-transfer mechanism. In this mechanism, the next step is allyl alcohol formation, followed by the second C–H bond activation of the CH₂OH species of allyl alcohol, and the final step is proton transfer from OH-substituted π-allyl species to the OH group on the metal surface to yield acrolein and water molecules with the regeneration of M₅₅. The rate-determining step is the second C–H bond activation. Its E_a is 17.4 kcal mol⁻¹ for the reaction on Pd₅₅ and 34.4 kcal mol⁻¹ for the reaction on Rh₅₅. These results indicate that Pd particles are more active than Rh particles in propene oxidation, which agrees with the experimental findings. The larger E_a for Rh₅₅ than that for Pd₅₅ arises from the stronger Rh–OH bond than the Pd–OH bond. The higher energy d-valence band-top of Rh₅₅ than that of Pd₅₅ is the origin of the stronger Rh–OH bond than the Pd–OH bond. Thus, the d-valence band-top energy is an important property for understanding and designing catalysts for alkene oxidation.