Charge Sensitivity/Bond-Order Analysis of Reactivity Trends in Allyl−[MoO3] Chemisorption Systems: A Comparison between (010) and (100) Surfaces

Abstract

In a search for the molecular mechanism of the selective oxidation of allyl to acrolein on a MoO3 surface the previously reported charge responses and bond multiplicities for the allyl−[(010)-MoO3] chemisorption system are compared with the corresponding results for selected allyl−[(100)-MoO3] structures. The charge sensitivity analysis (CSA) in atomic resolution is used to predict the displacements in atomic electron populations for large clusters at both the polarization (P) and charge transfer (CT) stages. The changes in effective bond orders, generated for small surface clusters, are from the Kohn−Sham (LSDA) difference approach. In contrast to the energetically most favorable “perpendicular” adsorption arrangements of allyl on a smooth (010)-MoO3 surface, the “parallel” orientation of allyl on the rough (100)-MoO3 surface is preferred energetically. It is found that the total (P + CT) CSA charge responses due to adsorption are strongly CT-dominated (chemisorption) in the vertical structures; they are practically of P character (physisorption) in the horizontal complexes. The quantum mechanical bond-order analysis reveals a specific bond-forming−bond-weakening mechanism of substituting the terminal hydrogen of allyl by the singly coordinated lattice oxygen in the perpendicular complexes. A nonspecific bond weakening inside the adsorbate, accompanied by an overall bonding between the allyl π-electrons and the molybdenum atom, is revealed in the parallel complexes. These observations are in good agreement with the experimentally determined activity of the (010) surface and inacivity of the (100) cut of the MoO3 crystal in catalyzing the selective oxidation of allyl to acrolein.