Promoting Role of Isolated Surface Hydroxyls on Selective Dehydrogenation of 2-Propanol to Acetone over Co ₃ O ₄ Catalyst: A Mechanistic Study

In this study, the catalytic decomposition of 2-propanol to acetone was investigated over Co3O4 catalysts, with a specific focus on the role of water and its derivatives in the reaction mechanism and the overall catalytic activity of this process. Two types of catalysts were addressed in this study: the powdered materials investigated under ambient pressure conditions and their model Co3O4(111) counterparts studied in ultrahigh vacuum (UHV) following a rigorous surface science approach. Pretreatment of both types of catalysts with water at elevated temperatures was shown to result in a substantial increase in the acetone formation rate. Results obtained by a combination of scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRAS) revealed that pretreatment of the Co3O4(111) model surface with water at elevated temperatures leads to the formation of isolated hydroxyls (OsH) involving a lattice oxygen atom (Os). In contrast, water deposition at lower temperatures results in the formation of an extended, partly dissociated OH/H2O network layer exhibiting short-range order. The isolated OsH groups obtained after high-temperature pretreatment with water were observed to play a key role in H abstraction, both from molecular 2-propanol and from the propoxy reaction intermediate, leading to efficient formation of the target product acetone. In contrast, on the surface depleted of isolated OsH species, both H abstraction steps become strongly inhibited. Importantly, dissociation of both 2-propanol and the propoxy reaction intermediate takes place only when the hydroxyl groups are present as isolated OsH species, which are not integrated into an extended, partly dissociated OH/H2O network: if the OH/H2O network was formed prior to the reaction, both H abstraction steps were observed to be nearly completely suppressed despite the presence of substantial amounts of adsorbed 2-propanol. The role of the isolated OsH groups can most likely be attributed to their function as hydrogen acceptors for H atoms leaving from either 2-propanol or the propoxy intermediate. Our results also suggest that the isolated OsH group appears to be a substantially more efficient hydrogen acceptor than the adsorbate-free lattice oxygen Os.