Browsing by Subject "Formaldehyde"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen Access
    Towards understanding the catalytic bond-breaking sequences of polyol oxidation on PD(111) single crystal model catalysts
    (2023-09) Sadak, Ömer Faruk
    Understanding the bond-breaking sequences of catalytic polyol oxidation on transition metal catalysts is critical for the chemical transformation of biomass derived chemical feedstock into value-added products which may also offer new alternatives to fossil fuel-based commodity chemicals. In the current work, oxidation of ethylene glycol on an atomically well-defined Pd(111) single crystal planar model catalysts was investigated via temperature programmed desorption (TPD) technique under ultra-high vacuum (UHV) conditions. Presence of surface oxygen atoms was found to promote the formation of formaldehyde (H2CO) and carbon dioxide as the most prominent catalytic oxidation products. Enhancement in formaldehyde generation was observed upon increasing the ethylene glycol-to-oxygen ratio. Our results indicate that the activation of C-C bonds was primarily facilitated by atomic oxygen, preceding the complete dehydrogenation of the C2HxOz surface species. The formation of H2CO was mainly attributed to the most unstable surface species in terms of C-C bond scission, namely -OCH2CO- and -OCH2CHO-. Other surface species such as -OCHCHO- and -OCHCO- led to additional decomposition products such as CO rather than formaldehyde.