Characterization and catalytic study of PtGe/Al2O3 catalysts prepared by organometallic grafting

Ge was added to 1% Pt/Al2O3 catalyst by controlled surface reaction of Ge(n-C4H9)4 in amounts corresponding nominally to 1/12, 1/8, 1/2, 1, or 2 monolayers. These PtGe/Al2O3 catalysts were characterized by FTIR of CO, TEM, H2 chemisorption, and EXAFS as well as tested in catalytic reactions, that is, transformation of hexane, benzene and cyclohexene in the presence of excess hydrogen. Loading of Ge in amounts of 1/12–1/2 monolayers resulted in catalysts with “bimetallic surface.” Loading of 1/12 monolayer of Ge resulted in randomly deposited Ge atoms on the surface of Pt. It hardly affected the catalytic behavior as compared with the Ge-free parent catalyst; 1/8 monolayer of Ge was still located on Pt as single atoms (as shown by EXAFS), but Ge selectively poisoned high coordination sites, active in benzene hydrogenation. This reaction was completely suppressed here, whereas this catalyst was most active in cyclohexene transformation. Pt with 1/8 and 1/2 monolayers of Ge transformed hexane with high selectivity into saturated C6 products and formed hardly any benzene. The formation of cyclohexane from hexane was also observed, not typical for monofunctional Pt catalysts. Adding 1–2 monolayers of Ge caused a new type of interaction between Pt and Ge containing sites that adsorbed CO but did not adsorb hydrogen. A solid solution of PtGe may have arisen here, creating “bulk bimetallic catalysts” with somewhat more surface Pt atoms not interacting with Ge. These catalysts behaved similarly in hydrocarbon transformations as the original parent catalyst. The possible reaction mechanism of hexane transformation is discussed in detail, in terms of thermodynamic limitations of benzene formation and possible surface species.