THE EFFECT OF Ti ON THE CATALYTIC PROPERTIES OF Pt/Al_2O_3 I . THE HYDROGENOLYSIS OF CYCLOPENTANE
Yang Xi-yao, Fu Xian-zhi and Pang Li(Department of Chemistry, Beijing University)
The hydrogenolysis of cyclopentane and the CS2-poisoning method have been used to study the effect of small amount of additive of Ti(0-5%wt) on the catalytic properties of Pt/Al2O3.The experimental facts have been found as follows: (1) The hydrogeno-lysis of cyclopentane on Pt/Al2O3 and Pt-Ti/Al2O3 takes place at clouble atom sites. (2) The density of active centers increases first and then decreases with the increase of the amount of Ti.(3)The activity of hydrogenolysis decreases with the increase of the amount of Ti, however, there is a little changing when the amount of Ti is in the range of 0.25 to 1.0%wt, and if the amount of Ti is higher than 1.0%, the activity will decrease rapidly again. (4) The activation energy,E,and preexponential factor,A0,compensating each other, increase with the increase of the amount of Ti.According to the facts given above, the following discussions have been proposed: (1) The diminish of the activity of hydrogenolysis and the density of active center is the result of strong interaction (i. e. electronic ligand effect) between Pt and Ti4+or Ti0, which are formed during the reduction by H2.(2) The compensation effect between E and A0 verifies , the electronic ligand effect given above. According to equation A0=Ncke S/R,and the fact that Nc decreases and A0 increases with the increase of the amount of Ti, it is obviously that the increase of A0 is the result of the increase of S. Such increase of S signifies the enhancement of the mobility of reaction molecules changed from adsorption state to transition complex state on the surface of catalyst. Probably such enhancement is the result of the electronic ligand effect of Ti4+ on Pt atoms, which may weaken the adsorption strength and reduce the activity of catalysts.(3) When the amount of Ti is in the range of 0.25-1.0%wt, on the surface of A12O3, Ti4+ which have not been reduced might be existed as the surface ions performing the function of core for the dispersion of Pt, Ti4+ ions and adjacent Pt atoms interact each other making Pt atoms more electron-deficient, therefore, the activity of Pt is strengthened. I lowever, the counteraction between Ti4+ and Ti4+ or Ti0 causes a little change of the activity of catalyst.Based upon the facts and the discussions given above, it has been suggested that there may be different valence state of Ti:Ti4+, Ti4 or Ti0 in the catalysts. They all exert influence on the catalytic properties of Pt through the electronic ligand effect. Ti4 strengthens the activity of Pt, but Ti4+ or Ti0 weakens it.