| AVS 65th International Symposium & Exhibition | |
| Fundamental Discoveries in Heterogeneous Catalysis Focus Topic | Wednesday Sessions |
| Session HC+SS-WeM |
| Session: | Mechanisms and Reaction Pathways of Heterogeneously Catalyzed Reactions |
| Presenter: | Matthew Marcinkowski, Pacific Northwest National Laboratory |
| Authors: | M.D. Marcinkowski, Pacific Northwest National Laboratory N. Doudin, Pacific Northwest National Laboratory R.S. Smith, Pacific Northwest National Laboratory B.D. Kay, Pacific Northwest National Laboratory Z. Dohnalek, Pacific Northwest National Laboratory |
| Correspondent: | Click to Email |
Single atom catalysts offer potential improvements in reactivity, selectivity, and decreased catalyst cost, but are often difficult to characterize and stabilize. Iron oxides are frequently used as catalyst supports, and recent studies have shown that Fe3O4(001) magnetite can stabilize isolated atoms of many transition metals.1 This property makes the (001) surface of magnetite ideal for model studies of the catalytic activity of supported single atoms. In this study, we examine the adsorption and reaction of methanol on Fe3O4(001) and Pd/Fe3O4(001) surfaces using a combination of scanning tunneling microscopy, temperature programmed desorption, and x-ray photoelectron spectroscopy. We find that a monolayer of methanol saturates on Fe3O4(001) at a coverage of 5.8 x 1014 molecules/cm2, which corresponds to one methanol per every surface Fe3+ ion. The majority of methanol desorbs molecularly by 280 K. Above 300 K, methanol dissociates to form methoxy and hydroxyl species.2 The maximum coverage of methoxy that can be achieved is 1.2 x 1014 molecules/cm2. The methoxy species form an ordered layer adsorbed on the Fe3+ sites above the subsurface cation vacancies. At elevated temperatures, the methoxy species react via one of two processes: recombining with hydroxyl to desorb as additional molecular methanol at 350 K, or further dehydrogenation to produce formaldehyde above 500 K. Production of formaldehyde is limited to 2.3 x 1013 molecules/cm2. On single Pd atoms on Fe3O4(001), the production of formaldehyde above 500 K is suppressed while a new formaldehyde channel is observed at 300 K. Increasing the Pd atom concentration increases the magnitude of the low-temperature channel. Interestingly, the total amount of formaldehyde produced in these two channels remains constant throughout the range of Pd concentrations studied, indicating that the methoxy species are formed on the Fe3O4 substrate and Pd atoms only facilitate the conversion of methoxy species to formaldehyde. However, the appearance of the lower temperature reaction channel for formaldehyde shows that the Pd atoms lower the reaction barrier to dehydrogenate methoxy to formaldehyde by almost a factor of two.
1. R. Bliem et al. Phys. Rev B92, 075440 (2015)
2. O. Gamba et al. Top. Catal.60, 420 (2017)