Browsing by Subject "Au(111)"

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    Acetaldehyde partial oxidation on the Au (111) model catalyst surface: C-C bond activation and formation of methyl acetate as an oxidative coupling product
    (Elsevier, 2015) Karatok, M.; Vovk, E. I.; Shah, A. A.; Turksoy, A.; Ozensoy, E.
    Partial oxidation of acetaldehyde (CH3CHO) on the oxygen pre-covered Au(111) single crystal model catalyst was investigated via Temperature Programmed Desorption (TPD) and Temperature Programmed Reaction Spectroscopy (TPRS) techniques, where ozone (O3) was utilized as the oxygen delivery agent providing atomic oxygen to the reacting surface. We show that for low exposures of O3 and small surface oxygen coverages, two partial oxidation products namely, methyl acetate (CH3COOCH3) and acetic acid (CH3COOH) can be generated without the formation of significant quantities of carbon dioxide. The formation of methyl acetate as the oxidative coupling reaction product implies that oxygen pre-covered Au(111) single crystal model catalyst surface can activate C-C bonds. In addition to the generation of these products; indications of the polymerization of acetaldehyde on the gold surface were also observed as an additional reaction route competing with the partial and total oxidation pathways. The interplay between the partial oxidation, total oxidation and polymerization pathways reveals the complex catalytic chemistry associated with the interaction between the acetaldehyde and atomic oxygen on catalytic gold surfaces.
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    Nature of oxygen species on Au(111) and Ag(111) model catalysts and their role in O-H, C-H, C-C, N-H bond activation
    (2017-10) Karatok, Mustafa
    Metal-catalyzed heterogeneous oxidation reactions have high importance for the large-scale production of the commodity chemicals vastly used in the chemical industry. Controlling the selectivity in such processes to increase the product yield and minimize the production of undesired byproducts requires a molecular level understanding of the bond activation mechanisms. Thus, understanding the nature of oxygen species in various bond cleavage processes is critical. In the current work, nature of oxygen species was studied on the planar Au(111) and Ag(111) single crystal model catalyst surfaces via x-ray photoelectron spectroscopy (XPS), temperature programmed desorption/ temperature programmed reaction spectroscopy (TPD/TPRS), low energy electron diffraction (LEED) and infrared reflection absorption spectroscopy (IRAS) techniques under ultra-high vacuum (UHV) conditions. Ozone (O3) was utilized as the oxygen delivery agent providing atomic oxygen to the reacting surface. Various oxygen species were determined on both Au(111) and Ag(111) model catalysts and their role in O-H, C-H, C-C and N-H bond activation was investigated by using probe molecules such as methanol, acetaldehyde and ammonia. Three different oxygen species such as atomic oxygen (Oa), subsurface oxygen (Osub) and surface oxide (Oox) were determined on Au(111) single crystal. Oxygen accumulation on Au(111) surface at 140 K for O<1.0 MLE of oxygen coverage resulted in the surface atomic oxygen (Oa) formation while 2D surface oxide (Oox) started to grow for O>1.0 MLE of oxygen coverage at the same temperature. It was also shown that oxygen atoms dissolved (Osub) into the bulk of the Au(111) single crystal when oxygen was accumulated at 473 K. Atomic oxygen species (Oa) on Au(111) was found to be very active for the cleavage of O-H and C-H bonds in methanol; C-C bond in acetaldehyde; N-H bond in ammonia molecules. Surface oxide (Oox) overlayer was also active for methanol oxidation, however it showed very high selectivity towards CO2. Dissolved oxygen atoms (Osub) revealed almost no activity in methanol oxidation reactions on Au(111). In a similar manner, three different oxygen species were determined on the Ag(111) surface such as surface atomic oxygen (Oa), surface oxide (Oox) and bulk-like oxide (Obulk) species. Disordered atomic oxygen (Oa) and surface oxide (Oox) overlayers prepared at 140 K on Ag(111) for O 0.2 MLE were found to be very active for O-H and CH bond cleavage producing formaldehyde as the dominant product. Increasing oxygen quantity for both oxygen species (0.7 MLE O 1.3 MLE) resulted mostly CO2 formation. Oa ( O < 1.10 MLE) was also found to be highly active in N-H bond cleavage for ammonia and selective to N2 as the dominant product. On the other hand, ordered p(5×1) and c(4×8) surface oxide (Oox) overlayers on Ag(111) prepared 473 K were found to be almost entirely inactive for N-H cleavage. Extreme oxygen exposures on Ag(111) ( O > 1.93 MLE ) at 140 K led to bulk-like silver oxide (Obulk) species with poor N2 selectivity in ammonia oxidation and increasing extent of formation of toxic pollutants such as NO and N2O.