Active state of model cobalt foil catalyst studied by SEM, TPR/TPO, XPS and TG

Four states of the cobalt foil catalyst, corresponding to different redox t reatment and activity, were defined: oxidised, reduced, active and deactiva ted. They were investigated by scanning electron microscopy (SEM), temperat ure-programmed reduction (TPR), temperature-programmed oxidation (TPO), X-r ay photoelectron spectroscopy (XPS) and thermogravimetric (TG) methods and in the hydrogenation of ethylene used as a test reaction. Particular emphas is was laid on the study of the active state, achieved after the catalyst r eduction at moderate temperatures. It was shown that the catalyst preactiva ted by a series of redox cycles is built of a cobalt oxide layer of a chara cteristic size and dispersion, which is stuck to the metallic bulk. Reducti on at a moderate temperature, prolonged even to several hours, converts onl y a small fraction of the oxide layer into metallic Co. XPS, TPR and TPO me thods distinguished various states of oxygen and cobalt on the surface of t he activated or partially activated samples. The results were interpreted i n terms of the mechanism of autocatalytic reduction. The deactivation was a ssociated with the structural reconstruction of the surface, taking place e ither in the reaction mixture during the hydrogenation of ethylene or in hy drogen atmosphere. Formation of the inactive carbon deposit was experimenta lly excluded. (C) 2001 Elsevier Science BN. All rights reserved.