Competing reaction pathways in the decomposition of 2-propanol over V-doped Co3O4(111) model catalysts: a mechanistic study.

Abstract

We present a mechanistic study on selective alcohol decomposition over model cobalt oxide-based catalysts: pristine Co₃O₄(111)/Au(111) and Co₃O₄(111) containing different amounts and forms of vanadium oxide. These two types of model catalysts were prepared and investigated with respect to their structural and catalytic properties following a rigorous surface science approach under well-defined ultra-high vacuum (UHV) conditions employing a combination of scanning-tunnelling microscopy (STM), molecular-beam techniques, infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Upon vanadium deposition, three types of V-containing structures were identified on Co₃O₄(111): V atoms embedded into the Co₃O₄ lattice, and triangular two-dimensional (2D) and three-dimensional (3D) truncated triangular VO_x islands, whose relative abundance strongly depends on the amount of deposited V. Employing CO as a sensitive probe for adsorption sites, V atoms in different oxidation states were identified, including V³⁺, V⁴⁺ and V⁵⁺ involved in vanadyl (V⁵⁺O) groups. The latter species were shown to participate in dissociation of 2-propanol to the propoxy reaction intermediate, likely serving as a H acceptor. The propoxy intermediate was found to dissociate to acetone on pristine Co₃O₄(111) and Co₃O₄(111) containing low V coverages, while increasing V loading leads to an alternative reaction pathway towards propene proceeding via C-O bond scission. Both the adsorbed reaction intermediate preceding propene formation and the gaseous product propene evolving above 430 K in TPD were detected for this reaction pathway. We discuss the possible reaction mechanisms of both competing reaction routes and connect them to the structure of different types of VO_x species.