EFFECT OF VARIOUS PRETREATMENTS ON THE STRUCTURE AND PROPERTIES OF RUTHENIUM CATALYSTS

The effect of oxidation (623 K) followed by low-temperature reduction (433 K) (O/LTR) on a very highly dispersed 1% Ru/Al2O3 catalyst previo usly reduced at high temperature (753 K) (HTR1) on the hydrogenolysis of ethane (H-2:C2H6 = 10:1) is to increase the turnover frequency (TOF ) at 433 K by a factor of about 200 with respect to that shown after H TR1. Its effect on the hydrogenolysis of 2,2,3,3-tetramethylbutane is also to increase TOF, but in addition the demethylation (alpha gamma) mode of reaction is suppressed and the extent of deep hydrogenolysis i s increased. EXAFS measurements show that the O/LTR procedure causes f irst migration and coalescence of oxidic species and then formation of aggregates of larger metal particles; these are, however, amorphous t o X-rays. The enhancement of TOF is therefore not explicable by an inc rease in the active area; it is, however, partially or completely nega ted by a second high-temperature reduction (HTR2). Similar effects are observed with Ru powder, Mathematical modeling of the dependence of t he rate of ethane hydrogenolysis on H-2 pressure by a rate expression predicated on the formation of a partially dehydrogenated intermediate indicates that the rate enhancement given by the O/LTR treatment is c hiefly due to an increase in the equilibrium constant defining the deh ydrogenation; its value and that of K-H, which defines the H-2 chemiso rption equilibrium, are both lowered by HTR2. Possible explanations in terms either of surface morphology or of alterations in electronic ch aracter are considered. (C) 1996 Academic Press, Inc.