Daniel E. Resasco

Metal–Support Interaction: Group VIII Metals and Reducible Oxides

Publisher Summary This chapter discusses the interaction of small metal particles with reducible oxide supports, and focuses on Group VIII (or Group 8-10) metals supported on TiO 2 . The strong suppression in catalytic activity and chemisorption capacity observed on TiO 2 -supported catalysts after reduction at high temperatures can be, at least partially, explained by a geometric blocking of sites by TiO 2 species. The dominant mechanism of formation of TiO 2 species on metal particles is migration during the high-temperature reduction, but some deposition of metal precursors during preparation may also occur. This picture finds an analog in the structure of the bimetallic clusters observed in Group VIII-Group Ib catalysts, in the sense that the structure sensitive reactions, e.g., alkane hydrogenolysis, are most affected whereas only modest effects are observed for structureinsensitive reactions, e.g., hydrogenation-dehydrogenation. The migration of reduced species from the support is accompanied by the formation of metal-Ti bonds, which provide the thermodynamic driving force for the migration. The metal-support bonding is not identical to that in the intermetallic compounds because of the associated oxygen that imparts cationic character to the Ti and Group VIII metal partner; in other words, it may be analogous to the Group VIII-Ti bonding that persists on intermetallic compounds surfaces after oxygen chemisorption.

A model of metal-oxide support interaction for Rh on TiO2

Abstract Using the Rh/TiO 2 catalyst system, a simple model for the different kinds of interaction that occur following a low- and a high-temperature reduction of Group VIII noble metals supported on TiO 2 , was developed. The model proposes a delocalized transfer of charge from Rh to TiO 2 after low-temperature reduction and a localized (chemical bonding) transfer of charge from the support to Rh after a high-temperature reduction. In addition, it is suggested that migration of a reduced species of the support is responsible, in part, for the depression of hydrogenolysis activity following a high-temperature reduction. The experimental evidences for the latter comes from comparing H 2 and H atom reduction, the kinetics of the onset of strong-metal-support interaction, and a strong analogy between group VIII-group Ib and group VIII-TiO 2 interaction effects on structure-sensitive and insensitive reactions. The kind of charge transfer is based on observed effects on kinetic parameters and previously reported spectroscopic experiments.

The effect of silica support texture and anion of impregnating solution on Ru dispersion and on RuCu interaction

Three sets of Ru and Ru-Cu catalysts were prepared by different methods and were placed on different types of silica supports which have been used by various groups of experimentalists. The initial reduction temperature (773 and 723/sup 0/K) and the volume of solution/g of support were judged not to be critical preparation variables in this study - the ethane hydrogenation activity. Little evidence exists for Ru-Cu interaction in the H/sub 2/ chemisorption. The anions of the impregnating solution and the texture do not directly affect Cu-Ru interactions, but may do so indirectly by their effect on the dispersion of Ru. Apparently nitrate complexes of Ru undergo stronger ion exchange on silica than do chloride containing complexes. The resulting better dispersion of Ru with its accompanying segregation decreases the likelihood of Ru-Cu interaction. This interpretation is undergoing further study. 2 figures, 1 table.

Comparison of hydrogenation and hydrogenolysis on unsupported and silica-supported Rh–V2O3 and Pt–V2O3

Several Rh and Pt catalysts where V2O3 is used as support or a promoter have been characterized by H2 chemisorption, benzene hydrogenation and reactions of n-butane. The chemisorption and activity effects of different degrees of metal–oxide interaction induced by low- and high-temperature reduction are compared with those observed with similar catalysts where TiO2 was the support (promoter). The nature of the metal–oxide interaction and how it differs for the same metal interacting with V2O3 and TiO2 are discussed.

Indirect effect of the strong metal-support interaction on the metal-metal inter-action in Rh-Ag/TiO2 catalysts

Abstract An indirect effect of support on the kind of bimetallic interaction has been observed on Rh-Ag catalysts supported on TiO 2 compared to SiO 2 . This effect occurs at significantly lower temperatures than the direct Strong Metal-Support Inter- action (SMSI) between Rh and TiO 2 . The main feature of this indirect effect is a pronounced decay in the H 2 chemisorption capability and activity for ethane hydrogenolysis with time when the catalysts are left in flowing H 2 at 523 K. This behavior can be rationalized in terms of a modification of the energetics of the metal-metal interaction, which may cause the Ag to spread over Rh particles. Hydrogenolysis activity and H 2 chemisorption capacity can be regenerated by an oxidation-reduction treatment.

Nitrogen chemisorption on Rh induced by a strong metal–support (TiO2) interaction

Nitrogen chemisorption, which does not normally occur on Rh, can be effected by oxidation followed by reduction at 773 K when Rh is well dispersed on TiO2.