Poisoning and promoting effects of additives on the catalytic behavior of metal clusters

Abstract

Extended Hückel calculations are used to explain the mechanisms by which additives such as Cu, Ag, Cl, and S affect the catalytic behavior of Pt or Ag clusters. The complete oxidation of ethylene over Pt Al 2O 3 , the oxidation of CO over Pt Al 2O 3 , and the ethylene epoxidation over supported silver catalysts are used as examples. It is found that Cu or Ag, unlike S, change mainly the net atomic population (the localized electrons around each atom) and very slightly the bond population among the atoms of the Pt clusters. The increased electron density at the surface of Pt clusters due to the electron transfer from Cu or Ag facilitates the electron transfer from the cluster to the π ∗ antibonding MO of adsorbed O 2. As a result, the dissociative chemisorption of O 2 over Cu- or Ag-treated Pt catalyst is enhanced and the reaction rates for the complete oxidation of C 2H 4 and the oxidation of CO are increased. The electronic effect of Cu on the electron redistribution in the clusters is stronger than that of Ag. Sulfur inhibits these reactions because it is a strong electron acceptor and the competition for electrons, as well as the destabilization of the MOs of the cluster, reduces the dissociative chemisorption of O 2. The inhibition effect of sulfur overcomes the promoting effect of Cu or Ag. Two forms of chemisorbed O 2 molecule, normal and parallel to the surface of a Ag (111) cluster, have been assumed in the calculations. For both forms, the coadsorption of chlorine results in a significant energy shift of the O 2 dissociation curves toward higher energies, thus inhibiting the chemisorption of O 2 and its dissociation. As a consequence, a loss in overall activity occurs, while the selectivity for epoxidation is higher. In the presence of chlorine, the O 2 adsorbed normal to the surface seems to be more stable than the O 2 adsorbed parallel to the surface. In the absence of chlorine, the former form becomes less stable than the latter if the elongation of the OO bond is larger than 1.4 Å.