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

High Resolution Image Download MS PowerPoint Slide In this study, the catalytic decomposition of 2-propanol to acetone was investigated over Co 3 O 4 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 Co 3 O 4 (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 Co 3 O 4 (111) model surface with water at elevated temperatures leads to the formation of isolated hydroxyls (O s H) involving a lattice oxygen atom (O s ). In contrast, water deposition at lower temperatures results in the formation of an extended, partly dissociated OH/H 2 O network layer exhibiting short-range order. The isolated O s H 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 O s H 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 O s H species, which are not integrated into an extended, partly dissociated OH/H 2 O network: if the OH/H 2 O 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 O s H 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 O s H group appears to be a substantially more efficient hydrogen acceptor than the adsorbate-free lattice oxygen O s .