Browsing by Subject "Heterogeneous catalyst"

Filter results by typing the first few letters
Now showing 1 - 3 of 3
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen Access
    N-O activation on precious metal-free metal oxide based NOx removal systems
    (2022-01) Ercan, Kerem Emre
    Elevated operational costs of platinum group metal (PGM) based environmental catalytic systems shift the focus of catalysis research towards cost-effective materials. In search for PGM-free alternative catalytic materials for NOx removal, high catalytic performance and long catalyst lifetime emerge as two important technical challenges. Within the scope of this dissertation, novel B-site mixed perovskites LaCoxMn1-xO3 (x = 0.1-0.9) and Fe and/or Co based CeO2 catalysts were synthesized, investigated and optimized as high performance, PGM-free, and durable catalyst alternatives for NOx removal systems. The perovskite based catalytic architectures can be utilized as diesel oxidation catalyst (DOC) oxidizing NO/CO to NO2/CO2, which can be coupled with selective catalytic reduction (SCR) catalysts to reduce NOx species to N2. On the other hand, Fe/Co based CeO2 systems can be exploited as catalyst candidates in SCR of NOx. In both of these NOx aftertreatment systems, NO activation is required. A simple and reproducible synthetic protocol was utilized to obtain perovskite-based DOC catalysts whose comprehensive structural characterization was carried out via XRD, N2 adsorption-desorption isotherm, ICP-MS, TEM, H2-TPR, ex-situ and in-situ XANES, EXAFS, in-situ FTIR, XPS, and TPD techniques. The oxidative catalytic performance of the perovskites for CO and NO oxidation was determined in flow-mode catalytic activity tests. It was demonstrated that bulk-oxygen vacancies have a strong influence on the redox activity of the B-site mixed perovskites with the ABO3 structure (where A = La, B = Co, Mn) allowing them to efficiently switch between high and low oxidation states in a reversible fashion under relatively moderate redox conditions without requiring elevated temperatures for regeneration, unlike conventional LaMnO3 and LaCoO3-based simple perovskite systems. La1.01Co0.75Mn0.24O2.97 and La1.04Co0.65Mn0.31O2.97 were found to reveal the best NO and CO oxidation performances among the currently investigated perovskites (La1.01Co0.75Mn0.24O2.97, La1.04Co0.65Mn0.31O2.97, La0.97Co1.03O2.91, and La0.97Mn1.03O3.17), which were on par with a conventional precious-metal benchmark catalyst (i.e., 1 wt. % Pt/Al2O3). Influence of Fe and Co loading on monometallic (Fe or Co) or bimetallic (Fe- Co) catalysts with different CeO2 support materials were studied in SCR of NO to N2. The flow-mode NO reduction experiments point out that 4 wt. % Co/CeO2 is the best catalyst in the studied group of catalysts based on its high N2 selectivity at relatively low temperatures. Detailed structural characterization experiments conducted via XRD, N2 adsorption-desorption isotherm, ATR-FTIR, Raman, and in-situ FTIR techniques indicate correlations between catalyst structure and SCR functionality. Our experimental findings indicate that 4 wt. % Co/CeO2 has relatively higher catalytic performance under excess H2(g) concentrations. The NO activation performance of both La1.01Co0.75Mn0.24O2.97 and La1.04Co0.65Mn0.31O2.97 B-site mixed perovskites and 4 wt. % Co/CeO2 were tested under significantly harsh conditions indicating their strong potential to be used not only in mobile applications but also in stationary NOx removal systems.
  • Thumbnail Image
    ItemOpen Access
    One-dimensional copper (II) coordination polymer as an electrocatalyst for water oxidation
    (Wiley-VCH Verlag, 2017) Mishra, R.; Ülker, E.; Karadas, F.
    Although cobalt-based heterogeneous catalysts are the central focus in water oxidation research, interest in copper-based water oxidation catalysts has been growing thanks the great abundance of copper and its biological relevance. Several copper oxides have recently been reported to be active catalysts for water oxidation. In this study, a heterogeneous copper-based water oxidation catalyst that is not an oxide has been reported for the first time. Single-crystal XRD studies indicate that the compound is a one-dimensional coordination compound incorporating copper paddle-wheel units connected through phosphine dioxide ligands. The catalyst exhibits an onset potential of 372 mV at pH 10.2, whereas an overpotential of only 563 mV is required to produce a current density of 1 mA cm−2. In addition to cyclic voltammetric and chronoamperometric studies, an investigation into the effect of pH on the catalytic activity and the robustness of the catalyst using long-term bulk electrolysis (12 h) is presented.