CHARGE SENSITIVITY BOND-ORDER ANALYSIS OF REACTIVITY TRENDS IN ALLYL-[MOO3] CHEMISORPTION SYSTEMS - A COMPARISON BETWEEN (010)SURFACES AND (100)SURFACES

In a search for the molecular mechanism of the selective oxidation of allyl to acrolein on a MoO3 surface the previously reported charge res ponses and bond multiplicities for the allyl-[(010)-MoO3] chemisorptio n system are compared with the corresponding results for selected ally l-[(100)-MoO3] structures. The charge sensitivity analysis (CSA) in at omic resolution is used to predict the displacements in atomic electro n populations for large clusters at both the polarization (P) and char ge transfer (CT) stages, The changes in effective bond orders, generat ed for small surface clusters, are from the Kohn-Sham (LSDA) differenc e approach. In contrast to the energetically most favorable ''perpendi cular'' adsorption arrangements of allyl on a smooth (010)-MoO3 surfac e, the ''parallel'' orientation of allyl on the rough (100)-MoO3 surfa ce is preferred energetically. It is found that the total (P + CT) CSA charge responses due to adsorption are strongly CT-dominated (chemiso rption) in the vertical structures; they are practically of P characte r (physisorption) in the horizontal complexes. The quantum mechanical bond-order analysis reveals a specific bond-forming-bond-weakening mec hanism of substituting the terminal hydrogen of allyl by the singly co ordinated lattice oxygen in the perpendicular complexes. A nonspecific bond weakening inside the adsorbate, accompanied by an overall bondin g between the allyl pi-electrons and the molybdenum atom, is revealed in the parallel complexes. These observations are in good agreement wi th the experimentally determined activity of the (010) surface and ina civity of the (100) cut of the MoO3 crystal in catalyzing the selectiv e oxidation of allyl to acrolein.