Chapter 181 The metals and alloys in catalysis

Abstract

This chapter presents the properties of rare earths alloyed with transition metals. Rare-earth oxides have been found to be very good catalysts in many reactions. Another possible way of using rare earths is as alloys with d transition metals which are known to be active in catalysis. Alloying transition metals with a rare earth is known to modify their electronic properties and therefore their reactivity with surface reactants. In addition, several intermetallic compounds of rare earths and transition metals have the property to absorb hydrogen reversibly, a property which can be used in many hydrogenation or dehydrogenation reactions. However, it was observed that during several catalytic reactions (methanation, alcohol or ammonia synthesis) the intermetallic compounds decompose into transition-metal particles and rare-earth oxide or hydride to form more active catalysts. This catalytic activity can be improved by an appropriate pretreatment under hydrogen or oxidizing reactants at high temperature. Intermetallic compounds are therefore used as precursors of new active catalysts which have been in some cases compared to transition metals supported on rare-earth oxides or classical supports like SiO 2 or Al 2 O 3 .