Browsing by Subject "Dehydrogenation"

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    Effects of Bronsted and Lewis bases on formic acid dehydrogenation selectivity of Pd(111) single crystal model catalyst
    (2020-06) Karakurt, Bartu
    Formic acid (FA) is an environmentally friendly hydrogen-based energy vector that can be obtained from renewable biomass feedstocks. However, catalytic decomposition of FA involves two different competing chemical pathways called dehydration and dehydrogenation. Thus, molecular level studies focusing on the selective catalytic FA dehydrogenation are essential for establishing structure-reactivity relationships which can be used in order to increase the catalytic dehydrogenation selectivity. In the current work, effects of Bronsted and Lewis bases on catalytic FA dehydrogenation selectivity were studied under ultra-high vacuum (UHV) conditions on an atomically well-defined Pd(111) single crystal model catalyst surface by using temperature programmed desorption/temperature programmed reaction spectroscopy (TPD/TPRS), X-Ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) techniques. Doubly-deuterated FA (DCOOD) was used as the FA source, while ammonia and manganese oxide were chosen as the model Bronsted and Lewis bases. Adsorption and subsequent surface decomposition reaction of DCOOD on Pd(111) showed that model catalyst was not totally selective towards dehydrogenation. Functionalizing the Pd(111) surface with ammonia suppressed the FA dehydration and boosted the dehydrogenation pathway, where positive influence of ammonia on FA dehydrogenation selectivity decayed when ammonia coverage was greater than 1 ML. A boost in hydrogen generation was observed in the catalytic FA dehydrogenation on manganese oxide-deposited Pd(111) surface (at sub-monolayer manganese oxide regime) as compared to that of a clean Pd(111) model catalyst. It was found out that manganese oxide can enhance FA dehydrogenation by acting as a promoter and/or catalytically contributing to the reaction depending on the oxidation state composition.
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    ItemOpen Access
    MnOx-Promoted pdAg alloy nanoparticles for the additive-free dehydrogenation of formic acid at room temperature
    (American Chemical Society, 2015) Bulut, A.; Yurderi, M.; Karatas, Y.; Say, Z.; Kivrak H.; Kaya, M.; Gulcan, M.; Ozensoy, E.; Zahmakiran, M.
    Formic acid (HCOOH) has a great potential as a safe and a convenient hydrogen carrier for fuel cell applications. However, efficient and CO-free hydrogen production through the decomposition of formic acid at low temperatures (<363 K) in the absence of additives constitutes a major challenge. Herein, we present a new heterogeneous catalyst system composed of bimetallic PdAg alloy and MnOx nanoparticles supported on amine-grafted silica facilitating the liberation of hydrogen at room temperature through the dehydrogenation of formic acid in the absence of any additives with remarkable activity (330 mol H2·mol catalyst-1·h-1) and selectivity (>99%) at complete conversion (>99%). Moreover this new catalytic system enables facile catalyst recovery and very high stability against agglomeration, leaching, and CO poisoning. Through a comprehensive set of structural and functional characterization experiments, mechanistic origins of the unusually high catalytic activity, selectivity, and stability of this unique catalytic system are elucidated. Current heterogeneous catalytic architecture presents itself as an excellent contender for clean hydrogen production via room-temperature additive-free dehydrogenation of formic acid for on-board hydrogen fuel cell applications.
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    ItemOpen Access
    Trimetalic heterogeneous catalyst for dehydrogenation of formic acid with enhanced CO tolerance
    (2017-09) Perşembe, Elif
    Hydrogen energy is considered to be a promising alternative for the sustainable and environmentally friendly solution of the global energy problem. One of the major obstacles of hydrogen energy applications is to maintain safe and efficient storage of hydrogen which can also be achieved chemically using suitable carrier materials. Formic acid (HCOOH, FA) can be utilized as a hydrogen carrier due to its low molecular weight (46 g/mol) and high hydrogen density (%4.4 weight). FA is a stable, non-flammable, and non-toxic biomass side-product rendering it a perfect candidate for an alternative hydrogen vector. Design of novel heterogeneous catalysts which can substitute the existing homogeneous catalytic systems may allow overcoming catalyst isolation and recovery costs and associated logistical problems hindering their applications in on-board operations. FA can be catalytically decomposed via dehydrogenation and dehydration reactions. Selective dehydrogenation of FA is crucial because, the production of CO from dehydration mechanism can suppress the activity of the catalyst by blocking/poisoning the precious metal sites. Consequently, development of CO-resistant, selective, catalytically active, and reusable heterogeneous catalysts has a great significance. In the current work, a new material that can produce H2(g) from FA under ambient conditions in the absence of additives with high CO-poisoning tolerance will be introduced, which is comprised of Pd-based trimetallic active centers functionalized with Ag and Cr in addition to amine-functionalized MnOx promoters dispersed on a SiO2 support surface. A novel trimetallic FA dehydrogenation catalyst was prepared and studied using analytical, ex-situ and in-situ spectroscopic techniques and compared to the results obtained for monometallic, bimetallic and active site-free counterparts. Trimetallic catalysts were found to reveal superior catalytic activity and stability compared to all of the currently investigated catalysts. Structural and catalytic properties of the trimetallic catalysts were investigated as a function of metal loadings. Structural characterization of the synthesized materials was carried out by Raman spectroscopy, Inductively-Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray Diffraction (XRD), Brunauer, Emmett and Teller (BET) Specific Surface Area Analysis, Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM), Scanning Transmission Electron Microscopy (STEM), and STEM/Energy Dispersive X-Ray (EDX), High-Angle Annular Dark Field (HAADF)/STEM. In addition, interaction of the catalyst surfaces with reactants and products were also monitored via in-situ FTIR spectroscopy for functional characterization. Detailed in-situ FTIR spectroscopic experiments were also performed using HCOOD, DCOOH and DCOOD in order to understand the nature of the adsorbed species, products and catalytic inhibitors.