STRUCTURE AND PROMOTION OF BIMETALLIC CATALYSTS - ACTIVITY AND SELECTIVITY

Activity and selectivity of carbon monoxide hydrogenation can be effectively controlled by using bimetallic catalysts. There are two effects which should be taken into consideration: (1) influencing the mode of CO chemisorption: dissociative vs associative, or (2) affecting the amount and type of surface hydrogen: weakly vs strongly bound to the surface. Associative CO chemisorption is controlled by two factors: either by the formation of an alloy phase as is for PtFe or PdFe where rehybridization of the Pd d-orbital results in a decreased back donation, or by an effect of a charged particles such as Fe3+ or RE ions which operates via a charge polarization. In highly dispersed state, e.g. inside a zeolite cage, similar effect is operative. In both cases oxygenates formation is a significant reaction pathway. The mode of hydrogen chemisorption also affects the selectivity of the reaction. When the activated form of hydrogen is in large proportion, all the processes which require hydrogen are suppressed, consequently olefin formation is the main route of reaction. The factors influencing both hydrogen and CO chemisorption will be considered. Here the paper will be focused on the effect of alloying, metal dispersion and shape selective environment. © 1991 J.C. Baltzer A.G., Scientific Publishing Company.