Browsing by Author "Karatok, Mustafa"

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Open Access
Formaldehyde selectivity in methanol partial oxidation on silver: effect of reactive oxygen species, surface reconstruction, and stability of intermediates
(American Chemical Society, 2021-05-21) Karatok, Mustafa; Şensoy, M. G.; Vovk, Evgeny I.; Üstünel, H.; Toffoli, D.; Özensoy, Emrah
Selective oxidation reactions on heterogeneous silver catalysts are essential for the mass production of numerous industrial commodity chemicals. However, the nature of active oxygen species in such reactions is still debated. To shed light on the role of different oxygen species, we studied the methanol oxidation reaction on Ag(111) single-crystal model catalyst surfaces containing two dissimilar types of oxygen (electrophilic, Oe and nucleophilic, On). X-ray photoelectron spectroscopy and low energy electron diffraction experiments suggested that the atomic structure of the Ag(111) surface remained mostly unchanged after accumulating low Oe coverage at 140 K. Temperature-programmed reaction spectroscopic investigation of low coverages of Oe on Ag(111) revealed that Oe was active for methanol oxidation on Ag(111) with a high selectivity toward formaldehyde (CH2O) production. High surface oxygen coverages, on the other hand, triggered a reconstruction of the Ag(111) surface, yielding Ag oxide domains, which catalyzes methanol total oxidation to CO2 and decreases the formaldehyde selectivity. This important finding indicates a trade-off between CH2O selectivity and methanol conversion, where 93% CH2O selectivity can be achieved for an oxygen surface coverage of θO = 0.08 ML (ML = monolayer) with moderate methanol conversion, while methanol conversion could be boosted by a factor of ∼4 for θO = 0.26 ML with a suppression of CH2O selectivity to 50%. Infrared reflection absorption spectroscopy results and density functional theory calculations indicated that Ag oxide contains dissimilar adsorption sites for methoxy intermediates, which are also energetically less stable than that of the unreconstructed Ag(111). The current findings provide important molecular-level insights regarding the surface structure of the oxidized Ag(111) model catalyst directly governing the competition between different reaction pathways in methanol oxidation reaction, ultimately dictating the reactant conversion and product selectivity.
Open Access
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.