Browsing by Author "Toffoli, D."

Now showing 1 - 6 of 6

Open Access
Comparative analysis of reactant and product adsorption energies in the selective oxidative coupling of alcohols to esters on Au (111)
(Springer, 2016) Şenozan, S.; Ustunel, H.; Karatok, M.; Vovk, E. I.; Shah, A. A.; Ozensoy, E.; Toffoli, D.
Gold-based heterogeneous catalysts have attracted significant attention due to their selective partial oxidation capabilities, providing promising alternatives for the traditional industrial homogeneous catalysts. In the current study, the energetics of adsorption/desorption of alcohols (CH3OH/methanol, CH3CH2OH/ethanol, CH3CH2CH2OH/n-propanol) and esters (HCOOCH3/methyl formate, CH3COOCH3/methyl acetate, and CH3COOCH2CH3/ethyl acetate) on a planar Au(111) surface was investigated in conjunction with oxidative coupling reactions by means of temperature programmed desorption (TPD) and dispersion-corrected density functional theory (DFT) calculations. The results reveal a complex interplay between inter-molecular and surface-molecule interactions, both mediated by weak van der Waals forces, which dictates their relative stability on the gold surface. Both experimental and theoretical adsorption/desorption energies of the investigated esters are lower than those of the alcohols from which they originate through oxidative coupling reactions. This result can be interpreted as an important indication in favor of the selectivity of Au surfaces in alcohol oxidative coupling/partial oxidation reactions, allowing facile removal of partial oxidation products immediately after their generation preventing their complete oxidation to higher oxygenates.
Open Access
First-principles investigation of Nox and Sox adsorption on anatase-supported BaO and Pt overlayers
(American Chemical Society, 2012) Hummatov, R.; Gülseren, O.; Ozensoy, E.; Toffoli, D.; Üstünel, H.
We present a density functional theory investigation of the adsorption properties of NO and NO2 as well as SO2 and SO3 on BaO and Pt overlayers on anatase TiO2(001) surface. Monolayers, bilayers, and trilayers of BaO grow without strain-induced large scale reconstructions. While the bilayer and trilayer preserve, to a large extent, the NO2 adsorption characteristics of the clean BaO(100) surface, the effect of the support is evident in SO2 and SO3 adsorption energies, which are somewhat reduced with respect to the clean BaO(100) surface. When a Pt(100) layer is added on the TiO2 surface, four stable adsorption geometries are identified in the case of NO while NO2 is found to adsorb in only two configurations.
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
Instability of a noncrystalline NaO2 film in Na-O2 batteries: The controversial effect of the RuO2 catalyst
(American Chemical Society, 2018) Tovini, M. F.; Hong, M.; Park, J.; Demirtaş, M.; Toffoli, D.; Ustunel, H.; Byon, H. R.; Yılmaz, E.
The unique electrochemical and chemical features of sodium-oxygen (Na-O2) batteries distinguish them from the lithium-oxygen (Li-O2) batteries. NaO2 as the main discharge product is unstable in the cell environment and chemically degrades, which triggers side products' formation and charging potential increment. In this study, RuO2 nanoparticles dispersed on carbon nanotubes (CNTs) are used as the catalyst for Na-O2 batteries to elucidate the effect of the catalyst on these complex electrochemical systems. The RuO2/CNT contributes to the formation of a poorly crystalline and coating-like NaO2 structure during oxygen reduction reaction, which is drastically different from the conventional micron-sized cubic NaO2 crystals deposited on the CNT. Our findings demonstrate a competition between NaO2 and side products' decompositions for RuO2/CNT during oxygen evolution reaction (OER). We believe that this is due to the lower stability of a coating-like NaO2 because of its noncrystalline nature and high electrode/electrolyte contact area. Although RuO2/CNT catalyzes the decomposition of side products at a lower potential (3.66 V) compared to CNT (4.03 V), it cannot actively contribute to the main electrochemical reaction of the cell during OER (NaO2 → Na+ + O2 + e-) because of the fast chemical degradation of the film NaO2 to the side products. Therefore, tuning the morphology and crystallinity of NaO2 by a catalyst is detrimental for the Na-O2 cell performance and it should be taken into account for the future applications.
Open Access
Multiscale self-asssembly of silicon quantum dots into an anisotropic three-dimensional random network
(American Chemical Society, 2016) Ilday, S.; Ilday, F. O.; Hübner R.; Prosa, T. J.; Martin, I.; Nogay, G.; Kabacelik, I.; Mics, Z.; Bonn, M.; Turchinovich, D.; Toffoli, H.; Toffoli, D.; Friedrich, D.; Schmidt, B.; Heinig, K.-H.; Turan, R.
Multiscale self-assembly is ubiquitous in nature but its deliberate use to synthesize multifunctional three-dimensional materials remains rare, partly due to the notoriously difficult problem of controlling topology from atomic to macroscopic scales to obtain intended material properties. Here, we propose a simple, modular, noncolloidal methodology that is based on exploiting universality in stochastic growth dynamics and driving the growth process under far-from-equilibrium conditions toward a preplanned structure. As proof of principle, we demonstrate a confined-but-connected solid structure, comprising an anisotropic random network of silicon quantum-dots that hierarchically self-assembles from the atomic to the microscopic scales. First, quantum-dots form to subsequently interconnect without inflating their diameters to form a random network, and this network then grows in a preferential direction to form undulated and branching nanowire-like structures. This specific topology simultaneously achieves two scale-dependent features, which were previously thought to be mutually exclusive: good electrical conduction on the microscale and a bandgap tunable over a range of energies on the nanoscale. © 2016 American Chemical Society.
Open Access
Nanotribological properties of the h-BN/Au(111) interface: a DFT study
(American Chemical Society, 2019) Baksi, M.; Toffoli, D.; Gülseren, Oğuz; Üstünel, H.
Understanding the quantum-mechanical origins of friction forces has become increasingly important in the past decades with the advent of nanotechnology. At the nanometer scale, the universal Amontons–Coulomb laws cease to be valid, and each interface requires individual scrutiny. Because of the well-known lubricating properties of two-dimensional materials, a significant amount of research has been performed in an effort to understand interfaces they form with one another. However, the interfaces between these two-dimensional materials and metals red from a tribological point of view, important for such applications as friction force microscopy, have yet to be thoroughly investigated. In the current work, we present a detailed density functional theory investigation of the hexagonal BN/Au(111) interface. Because of a good agreement between their characteristic lengths, a high level of commensurability is achieved in a suitably constructed model between the bulk surfaces of the two materials. As a result of our calculations, we find that the corrugation in the potential energy surface and the lateral forces in this interface are low compared to other similar interfaces. The friction coefficient falls rapidly with increasing load down to 0.005 for the largest loads considered. In contrast, Aun clusters (n = 1, 4, 13, and 19) sliding on the h-BN surface exhibit much larger lateral forces, indicating strong size and edge effects. The reduction of energy corrugation in going from the Au4 to the Au19 cluster may already indicate a decreasing trend with increasing size even at this very small scale.