X-ray photoelectron spectroscopic study of unsupported cobalt-molybdenum sulfide catalysts

Abstract

X-Ray photoelectron spectroscopic (XPS) measurements have been made on unsupported, fully sulfided, cobalt-molybdenum catalysts which had previously operated catalytically until attainment of steady activity. Total sulfur content and intrinsic activity for hydrogenolysis of thiophene, for hydrogenation of cyclohexene and for isomerization of cyclohexane are also reported. The entire range of concentration 0.0 < r < 1.00, where r = Co (Co + Mo) , has been investigated. There exists a good correlation between XPS and catalytic activity measurements. Two concentration domains can be distinguished. In the low cobalt concentration range there is an increase in the binding energy of the Mo 3d 5 2 , Mo 3d 5 2 , and S 2p 3 2 − 1 2 levels. In the range of concentration where the catalysts exhibit a strong synergy for hydrogenolysis and isomerization, the binding energy of these levels decreases. Furthermore, the sulfur content is maximum. The formation of a reduced species with a charge between Mo 4+ and Mo 3+, which could be involved in the active sites, has been postulated.