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  • Boosting of Soot Combustion on Alkaline Mn/ZrO 2 Nanostructures
    Top. Catal. (IF 2.226) Pub Date : 2020-01-25
    R. Camposeco, S. Castillo, N. Nava, R. Zanella

    Abstract An important decrease in the soot combustion temperature was observed by employing potassium and manganese on ZrO2 as support; significantly boosted soot combustion was obtained with the K/ZrO2 and Mn–K/ZrO2 catalysts for T50 at 295 and 240 °C, respectively. The enhanced catalytic activity by using the Mn–K/ZrO2 catalyst could be attributed to the alkaline metal ion, which improved the catalyst/soot contact. XPS results revealed the presence of surface vacancies, where the addition of manganese improved the lattice oxygen and concentration of OH groups on the Mn–K/ZrO2 catalyst at low temperatures; the presence of surface vacancies was facilitated by the interaction between potassium and manganese. The Mn–K/ZrO2 catalyst showed resistance to deactivation for five cycles.

    更新日期:2020-01-26
  • Hydrogen Release from Decalin by Catalytic Dehydrogenation over Supported Platinum Catalysts
    Top. Catal. (IF 2.226) Pub Date : 2020-01-25
    E. A. Martynenko, Al. A. Pimerzin, A. A. Savinov, S. P. Verevkin, A. A. Pimerzin

    Abstract Pt catalysts supported on the commonly used supports Al2O3, SiO2 and on mesostructured silicates SBA-15 and MCM-48 were examined in the dehydrogenation of decalin, that is the promising liquid organic hydrogen carrier (LOHC). Synthesized catalysts were studied by low-temperature nitrogen adsorption, temperature programmed reduction, high-resolution transmission electron microscopy. The catalysts are characterized by various dispersion of platinum. The activity of catalysts in decalin dehydrogenation decreases in the following order Pt/MCM-48 > Pt/SBA-15 > Pt/SiO2 > Pt/Al2O3. Pt/MCM-48 catalyst allows to release hydrogen in the decalin amount of 5.7 wt% and can be considered the promising LOHC hydrogen storage technology. Graphic Abstract

    更新日期:2020-01-26
  • Dispersed Nickel-Based Catalyst for Enhanced Oil Recovery (EOR) Under Limited Hydrogen Conditions
    Top. Catal. (IF 2.226) Pub Date : 2020-01-24
    O. Morelos-Santos, A. I. Reyes de la Torre, J. A. Melo-Banda, A. M. Mendoza-Martínez, P. Schacht-Hernández, B. Portales-Martínez, I. Soto-Escalante, J. M. Domínguez-Esquivel, M. José-Yacamán

    Abstract Heavy crude oil enhanced into a lighter oil by hydrocracking process with nickel nanoparticles (Ni NPs) as catalysts. Ni NPs were synthesized by colloidal method (chemical reduction of metal salts). The reduction of nickel nitrate hexahydrate ((Ni(NO3)2·6H2O) was done using sodium borohydride (NaBH4) as a reducing agent in presence of polyvinylpyrrolidone which worked as a protective and stabilizing agent; ethanol was used as a solvent. The properties of the nanoparticles were investigated by FT-IR, TEM, XRD, and HRTEM. These techniques confirmed the formation of Ni NPs with an average size of 10 nm and a tetragonal structure. The experiments were carried out in a batch reactor at 45 Kgf cm−2 (initial H2 pressure), 380 °C, stirring speed of 500 rpm and reaction time of 1 h. In all cases, the increase in the concentration of Ni nanoparticles improved the physical and chemical properties of heavy crude oil, even in limited hydrogen conditions; these properties were measured in terms of viscosity, SARA analysis, sulfur and nitrogen removal, and chromatographic analysis of gaseous products. The asphaltenes conversion was of 26.31% and moderate sulfur removal was achieved, these results are promising for EOR application. Graphic Abstract

    更新日期:2020-01-26
  • High Photocatalytic Activity Under Visible Light of Sandwich Structures Based on Anodic TiO 2 /CdS Nanoparticles/Sol–Gel TiO 2
    Top. Catal. (IF 2.226) Pub Date : 2020-01-24
    A. A. Rempel, Yu. V. Kuznetsova, I. B. Dorosheva, A. A. Valeeva, I. A. Weinstein, E. A. Kozlova, A. A. Saraev, D. S. Selishchev

    Abstract In order to increase the catalytic activity of titanium dioxide (TiO2) under visible light, a method for the synthesis of a complex sandwich catalyst based on nanotubular and sol–gel TiO2 and colloidal cadmium sulfide (CdS) was developed. The use of titanium dioxide in the form of nanotubes allowed increasing the specific surface area of the catalyst, which is one of the main factors in increasing the efficiency of the catalysis process. At the same time, the presence of CdS nanoparticles made it possible to widen the spectral range of irradiation used. Using the methods of X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy a comprehensive characterization of the synthesized samples was carried out. In addition, their chemical and phase composition, as well as the size of the nanoparticles and their morphology, were established. The photocatalytic activity was determined under the visible light and ultraviolet of nanostructures in reactions involving the oxidation of acetone in the gas phase.

    更新日期:2020-01-24
  • Chloroform Hydrodechlorination on Palladium Surfaces: A Comparative DFT Study on Pd(111), Pd(100), and Pd(211)
    Top. Catal. (IF 2.226) Pub Date : 2020-01-13
    Lang Xu, Saurabh Bhandari, Jiming Chen, Jonathan Glasgow, Manos Mavrikakis

    Palladium has been shown to be an effective catalyst for chloroform hydrodechlorination, which serves as a promising treatment method for industrial chloroform waste. To investigate the structure sensitivity of this chemistry on Pd surfaces, we performed a density functional theory (DFT, GGA-PW91) study of the chloroform hydrodechlorination reaction on three distinct facets: Pd(111), Pd(100), and Pd(211). Based on the DFT results, the binding strengths of most surface intermediates generally increase in the following order: Pd(111) < Pd(100) < Pd(211). On all three Pd facets, methane is formed as the preferred reaction product through a pathway in which CHCl3* is fully dechlorinated to CH* first, and then hydrogenated to CH4. We constructed potential energy diagrams (PED) and compared the reaction energetics for chloroform hydrodechlorination on the three Pd facets. We propose that the competition between the desorption of chloroform and its initial dechlorination to form CHCl2* likely determines the hydrodechlorination activity of the catalyst. On Pd(111), the desorption of chloroform is energetically favored over its dechlorination while the dechlorination barriers are lower than the desorption barriers on Pd(100) and Pd(211). On the other hand, Pd(100) and Pd(211) bind atomic chlorine stronger and also catalyze the formation of atomic carbon effectively; both are potential site-blocking species. Our results suggest that the more open facets and step edge sites of a Pd nanoparticle may carry higher intrinsic activity towards chloroform hydrodechlorination than the close-packed facets, yet these under-coordinated sites are more prone to catalyst poisoning.

    更新日期:2020-01-13
  • Absorption of Hydrocarbons on Palladium Catalysts: From Simple Models Towards Machine Learning Analysis of X-ray Absorption Spectroscopy Data
    Top. Catal. (IF 2.226) Pub Date : 2020-01-11
    Oleg A. Usoltsev, Aram L. Bugaev, Alexander A. Guda, Sergey A. Guda, Alexander V. Soldatov

    Abstract Element selectivity and possibilities for in situ and operando applications make X-ray absorption spectroscopy a powerful tool for structural characterization of catalysts. While determination of coordination numbers and interatomic distances from extended spectral region is rather straightforward, analysis of X-ray absorption near-edge structure (XANES) spectra remains a highly debated and topical problem. The latter region of spectra is shaped depending on the local 3D geometry and electronic structure. However, there is no straightforward procedure for the unambiguous extraction of these parameters. This work gives a critical vision on the amount of information that can be practically extracted from Pd K-edge XANES spectra measured under in situ and operando conditions, in which adsorption of reactive molecules at the surface of palladium with further formation of subsurface and bulk palladium carbides are expected. We investigate how particle size, concentration of carbon impurities, and their distribution in the bulk and at the surface of palladium particles affect Pd K-edge XANES features and to which extend they should be implemented in the theoretical model to adequately reproduce experimental data. Then, we show how the step-by-step increasing the complexity of the theoretical model improves the agreement with experiment. Finally, we suggest a set of formal descriptors relevant to possible structural diversity and construct a library of theoretical spectra for machine-learning-based analysis of the experimental data.

    更新日期:2020-01-13
  • Effect of Gold Nanoparticles on MnO x /TiO 2 Nanostructures for Improving the CO Oxidation at Low Temperature
    Top. Catal. (IF 2.226) Pub Date : 2020-01-03
    Roberto Camposeco, Salvador Castillo, Noel Nava, Juan Carlos Medina, Rodolfo Zanella

    Gold nanoparticles dispersed on MnOx/TiO2 were used to perform the oxidation of carbon monoxide at low temperatures. Remarkable dispersion of gold nanoparticles under hydrogen treatment was obtained, mainly in Au–MnOx/TiO2 catalysts with 2 wt% Au and 5 wt% Mn, where the Au nanoparticles displayed average sizes of 1–3 nm. The addition of Au nanoparticles to the MnOx/TiO2 catalysts promoted both the CO oxidation from 0 °C and stability at room temperature. This behavior could be attributed to the synergistic interactions between Au0/Au1+ and Mn3+/Mn4+ on the Au–MnOx/TiO2 catalyst surface.

    更新日期:2020-01-04
  • In Situ XPS and MS Study of Methane Oxidation on the Pd–Pt/Al 2 O 3 Catalysts
    Top. Catal. (IF 2.226) Pub Date : 2020-01-02
    Igor A. Chetyrin, Andrey V. Bukhtiyarov, Igor P. Prosvirin, Alexander K. Khudorozhkov, Valerii I. Bukhtiyarov

    Abstract The bimetallic Pd–Pt/Al2O3 supported catalysts with different Pd/Pt ratios have been prepared and studied in methane oxidation reaction. Comparison of their catalytic properties with those of monometallic Pd/Al2O3 and Pt/Al2O3 samples has shown that the bimetallic catalysts with a relatively low Pt loading (less than 0.4 mol%) are more active than the monometallic ones indicating the synergy effect. An increase in Pt loading above 0.5 mol% decreases the catalytic activity, so that these catalysts become less active than the monometallic Pd/Al2O3 catalysts, but still keep an activity higher than that of the least active monometallic Pt/Al2O3 sample. The chemical states of the palladium and platinum for all the catalysts produced depending on the reaction conditions have been investigated with in situ XPS. It has been shown that even at room temperature the reaction mixture affects the surface composition of the catalysts, which does not change much at higher reaction temperatures. The data have led us to suggestion that activity in the total methane oxidation is promoted by formation of Pd2+, surface fraction of which depends on Pt content in the bimetallic catalysts.

    更新日期:2020-01-04
  • Spatial and Temporal Self-organization During CO Oxidation Over Ni
    Top. Catal. (IF 2.226) Pub Date : 2020-01-02
    Alexei G. Makeev, Nickolai V. Peskov, Marina M. Slinko, Victor Yu. Bychkov, Vladimir N. Korchak

    Abstract Oscillatory behavior during CO oxidation over a Ni foil in a continuous-flow catalytic reactor has been discovered in the reaction mixture containing CO excess at atmospheric pressure in the temperature range of 500–630 °C. The oscillations are accompanied by propagation of the oxidation and reduction fronts which were recorded by a photo–video camera. Dark fronts of the oxidized state appear periodically in the upstream part of the catalyst and propagate to the downstream region in the direction of the gas flow. Reduction fronts move in the opposite direction. A mathematical model has been developed to simulate the oscillatory dynamics. The developed model simulates experimental data almost quantitatively. The origin of oscillations is connected with periodic oxidation and reduction of the nickel surface. It is shown that a precursor-mediated adsorption of CO on Ni surface is required to simulate the oscillations under reducing conditions.

    更新日期:2020-01-02
  • Effect of La Addition on the Performance of Three-Way Catalysts Containing Palladium and Rhodium
    Top. Catal. (IF 2.226) Pub Date : 2020-01-02
    Aleksey A. Vedyagin, Roman M. Kenzhin, Mikhail Yu. Tashlanov, Evgeny A. Alikin, Vladimir O. Stoyanovskii, Pavel E. Plyusnin, Yury V. Shubin, Ilya V. Mishakov, Mikhail Yu. Smirnov, Alexander V. Kalinkin, Valerii I. Bukhtiyarov

    Three-way catalysts containing palladium and/or rhodium were prepared using γ­Al2O3 doped with lanthanum oxide as a support. All the samples were obtained by an incipient wetness impregnation of the support with an aqueous solution of nitrates. In order to investigate the metal–support interaction, the support was additionally calcined at 800 °C before the impregnation procedure. Characterization of the support thermally treated within a range of 600–1000 °C by low-temperature nitrogen absorption, X-ray diffraction analysis, and electron paramagnetic resonance spectroscopy has revealed that the treatment conditions strongly affect the textural properties, the phase composition and the concentration of electron-donor sites on the surface of the support. Deposition of metals by the impregnation of initial support with a joint solution of Pd and Rh nitrates has led to formation of small Pd–Rh alloyed nanoparticles with strong metal–metal interaction, which was confirmed by a testing reaction of ethane hydrogenolysis. No alloy formation was observed in the case of mechanical mixing of the separately prepared Pd-only and Rh-only catalysts as well as in the case of preliminary calcined support impregnated with a joint solution of Pd and Rh nitrates. Bimetallic Pd–Rh catalyst of alloyed type was shown to be the most promising in terms of catalytic performance and thermal stability.

    更新日期:2020-01-02
  • Zinc Addition Influence on the Properties of Pd/Sibunit Catalyst in Selective Acetylene Hydrogenation
    Top. Catal. (IF 2.226) Pub Date : 2020-01-01
    Daria V. Glyzdova, Tatyana N. Afonasenko, Evgeny V. Khramov, Natalya N. Leont’eva, Mikhail V. Trenikhin, Igor P. Prosvirin, Andrey V. Bukhtiyarov, Dmitry A. Shlyapin

    Abstract A series of bimetallic Pd–Zn catalysts supported on the carbon material Sibunit was synthesized for selective hydrogenation of acetylene. XRD, XPS, XAFS and TEM data revealed that the structure and dispersion of bimetallic particles in Pd–Zn/Sibunit samples depend on the molar ratio Pd:Zn. The active component of the Pd–Zn(1:0.25)/Sibunit sample is a substitutional solid solution. In the catalysts with Pd:Zn ≤ 1, the interaction proceeds more completely with the transformation of FCC lattice of the solid solution into the tetragonal structure of intermetallic phase with the composition close to PdZn, whereas an excess of zinc forms the individual ZnO phase. It was found that an increase in the zinc content in Pd–Zn/Sibunit catalysts from Pd:Zn = 1:0 to 1:4 leads to dispergation of the deposited mono- and bimetallic particles from dav = 7.2 to 2.0 nm. The formation of the PdZn intermetallic compound with increasing the zinc content from Pd:Zn = 1:0 to 1:1 is accompanied by a gradual increase in selectivity to ethylene from 42% for Pd/Sibunit to 67% for Pd–Zn(1:1)/Sibunit (the reaction temperature is 95 °C), and also by a decrease in activity. The introduction of a zinc excess (Pd:Zn < 1), on the contrary, decreases the hydrogenation selectivity to ~ 62%, presumably owing to a high dispersion of bimetallic particles.

    更新日期:2020-01-01
  • The Effect of Polymer Addition on Base Catalysed Glycerol Oxidation Using Gold and Gold–Palladium Bimetallic Catalysts
    Top. Catal. (IF 2.226) Pub Date : 2019-12-17
    Laura Abis, Nikolaos Dimitritatos, Meenakshisundaram Sankar, Simon J. Freakley, Graham J. Hutchings

    The oxidation of glycerol represents both a viable route to catalytic upgrading of biomass and has become a model reaction for catalytic polyol oxidation. Gold and gold–palladium nanoparticle catalysts prepared by colloidal methods involving polymer additives have been extensively studied. However, the effect of residual polymer at the catalyst surface on reaction pathways has not been decoupled from particle size effects. We show that when using catalysts prepared without polymer stabilisers the addition of either polyvinyl alcohol or polyvinylpyrrolidone to the reaction changes the reaction rate and results in a change in reaction selectivity. We conclude that the polymer additive has a significant effect on the reaction pathway and that these systems should be considered as a metal surface–polymer interface catalytic systems and properties should not be rationalised solely based on nanoparticle size.

    更新日期:2019-12-18
  • Pairwise Parahydrogen Addition Over Molybdenum Carbide Catalysts
    Top. Catal. (IF 2.226) Pub Date : 2019-12-14
    Dudari B. Burueva, Andrey A. Smirnov, Olga A. Bulavchenko, Igor P. Prosvirin, Evgeny Yu Gerasimov, Vadim A. Yakovlev, Kirill V. Kovtunov, Igor V. Koptyug

    Herein, we have shown that the phase composition of molybdenum carbide catalysts has a pronounced effect on the pairwise hydrogen addition selectivity in the gas-phase propyne hydrogenation with parahydrogen. Molybdenum carbide catalysts were prepared using either the Pechini method or temperature-programmed reduction with CH4/H2 carburizing gas mixture. The structures of carbide catalysts were characterized by high-resolution transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. It was found that molybdenum carbide prepared by the Pechini method predominantly contains a face-centered-cubic MoC1−x phase, while the TPR method yields a hexagonal-close-packed Mo2C phase. By varying the gas hourly space velocity of carburizing gas mixture, the defected phase can be produced. Computer modeling for XRD patterns was used to identify the phase composition of Mo2C catalysts. All the catalysts were found to be active in pairwise hydrogen addition; however the hcp-Mo2C phase exhibits a higher contribution of pairwise H2 addition providing ~ 150-fold proton NMR signal enhancement.

    更新日期:2019-12-17
  • Selective Ethylene Dimerization into 2-Butenes Using Homogeneous and Supported Nickel(II) 2-Iminopyridine Catalysts
    Top. Catal. (IF 2.226) Pub Date : 2019-11-29
    Artem A. Antonov, Nina V. Semikolenova, Igor E. Soshnikov, Evgenii P. Talsi, Konstantin P. Bryliakov

    In this work, we report a series of novel nickel(II) dibromide and dichloride complexes with 2-iminopyridine and 2-iminoquinoline ligands bearing electron-withdrawing substituents (F, Cl, CF3), that have demonstrated high ethylene dimerization activity [up to 19.2 × 106 g of oligomers·(mol Ni)−1 h−1] in the presence of MAO or Et2AlCl, affording predominantly a mixture of 1-butene and cis- and trans-2-butene (C4 selectivity varies from 89 to 100%; 2-butenes selectivity in the C4 fraction approaches 96%). The effects of ligand substituents and the cocatalyst nature on the activity and selectivity of the nickel(II) complexes in ethylene dimerization have been established. Several nickel complexes were supported on silica-alumina and the resulting heterogeneous catalysts were probed towards ethylene dimerization. These catalysts also afford 2-butenes, with the activity and C4 selectivity being comparable to that of homogeneous catalysts.

    更新日期:2019-12-11
  • NMR Investigation into the Influence of Surface Interactions on Liquid Diffusion in a Mesoporous Catalyst Support
    Top. Catal. (IF 2.226) Pub Date : 2019-11-25
    Neil Robinson, Carmine D’Agostino

    Pulsed field gradient NMR diffusion measurements provide a non-invasive measure of the mass transport (self-diffusion) characteristics of liquids confined to porous catalyst materials. Here we explore the ability of this technique to probe the diffusive behaviour of a series of short-chain primary alcohols within a mesoporous catalyst support material; through the comparison of our results with highly surface-sensitive NMR relaxation data, we show that the evaluation of bulk-pore diffusion dynamics may provide a simple and indirect method to access and explore surface interaction phenomena occurring at the catalyst-liquid interface.

    更新日期:2019-12-11
  • Partial Oxidation of Dimethoxymethane to Syngas Over Supported Noble Metal Catalysts
    Top. Catal. (IF 2.226) Pub Date : 2019-11-24
    S. D. Badmaev, N. O. Akhmetov, V. A. Sobyanin

    Catalytic partial oxidation (PO) of dimethoxymethane (DMM) to syngas was investigated in a fixed-bed continuous flow reactor under ambient pressure and at 250–500 °C over Pt-, Rh- and Ru-supported on Ce0.75Zr0.25O2–δ catalysts. The Pt catalyst was found to be the most active and selective. It provided complete conversion of DMM to gas mixture containing ˃ 60 vol% of H2 and CO at 400 °C, GHSV = 10,000 h−1 using a reaction mixture: DMM:O2:N2 = 28.6:14.3:57.1 (vol%).

    更新日期:2019-12-11
  • Inductive Heating Assisted-Catalytic Dehydrogenation of Tetralin as a Hydrogen Source for Downhole Catalytic Upgrading of Heavy Oil
    Top. Catal. (IF 2.226) Pub Date : 2019-11-23
    Abarasi Hart, Mohamed Adam, John P. Robinson, Sean P. Rigby, Joseph Wood

    The Toe-to-Heel Air Injection (THAI) combined with a catalytic add-on (CAPRI, CATalytic upgrading PRocess In-situ) have been a subject of investigation since 2002. The major challenges have been catalyst deactivation due to coke deposition and low temperatures (~ 300 °C) of the mobilised hot oil flowing over the catalyst packing around the horizontal well. Tetralin has been used to suppress coke formation and also improve upgraded oil quality due to its hydrogen-donor capability. Herein, inductive heating (IH) incorporated to the horizontal production well is investigated as one means to resolve the temperature shortfall. The effect of reaction temperature on tetralin dehydrogenation and hydrogen evolution over NiMo/Al2O3 catalyst at 250–350 °C, catalyst-to-steel ball ratio (70% v/v), 18 bar and 0.75 h−1 was investigated. As temperature increased from 250 to 350 °C, tetralin conversion increased from 40 to 88% while liberated hydrogen increased from 0.36 to 0.88 mol based on 0.61 mol of tetralin used. The evolved hydrogen in situ hydrogenated unreacted tetralin to trans and cis-decalins with the selectivity of cis-decalin slightly more at 250 °C while at 300–350 °C trans-decalin showed superior selectivity. With IH the catalyst bed temperature was closer to the desired temperature (300 °C) with a mean of 299.2 °C while conventional heating is 294.3 °C. This thermal advantage and the nonthermal effect from electromagnetic field under IH improved catalytic activity and reaction rate, though coke formation increased.

    更新日期:2019-12-11
  • Role of Metal/Oxide Interfaces in Enhancing the Local Oxide Reducibility
    Top. Catal. (IF 2.226) Pub Date : 2018-10-11
    Philomena Schlexer, Antonio Ruiz Puigdollers, Gianfranco Pacchioni

    Oxide reducibility is an important property in catalysis by metal-oxides. The reducibility of an oxide can be substantially modified when an interface is created between the oxide and a metal. Here we discuss two types of interfaces. One consists of gold nanoparticles deposited on anatase TiO2 or tetragonal ZrO2 (101) surfaces; these are traditional direct catalysts (metal deposited on an oxide). The second example consists of a metal support, Pt or a Pt3Zr alloy, where a ZrO2 nanofilm is deposited; this is representative of an inverse catalyst (oxide on metal). We designed models of these systems and analyzed by means of first principle calculations a key descriptor of the oxide reducibility, the cost of formation of an oxygen vacancy. We show that this cost is dramatically reduced when the oxide is interfaced with the metal. The effect on catalytic reactions is analyzed by computing the energy profiles for the CO oxidation reaction on Au/TiO2 and Au/ZrO2 model catalysts. Despite the very different nature of the two oxide supports, reducible for TiO2 and non-reducible for ZrO2, the same Au-assisted Mars–van Krevelen mechanism is found, with similar barriers.

    更新日期:2019-12-11
  • Professor Frank S. Stone, 1925–2018: The Life of a Solid State and Catalytic Chemist
    Top. Catal. (IF 2.226) Pub Date : 2019-11-13
    Martyn V. Twigg

    With the passing of Frank Sidney Stone on 5 March 2018 in Dorset, England, the solid state chemistry and the catalysis scientific communities lost a highly respected elder statesman. Always a gentleman, he was pleasant, unassuming and very knowledgeable with what might be described as a serious sense of humour that made his speeches especially entertaining. He was born and based throughout his life in the Bristol area of England. During World War II he was an undergraduate at the University of Bristol and after completing a PhD he was appointed an Assistant Lecturer and his career continued with an appointment as a Lecturer in 1951 and Reader in the mid-1960s. Perhaps because he was not promoted further in 1972 he moved to the nearby University of Bath as Professor of Chemistry. Here he held several important positions including that of Pro-Vice-Chancellor (1984 to 1987) and he formally retired in 1991. Then as Emeritus Professor he maintained a university office continuing as the long serving European Editor of the Journal of Catalysis until 1996. Stone’s first research involved copper and copper oxides and throughout his c career he worked with metal oxides and he did much to incorporate modern solid-state science into traditional heterogeneous catalysis that underpinned important advances made during the latter half of the twentieth century.

    更新日期:2019-12-11
  • Regioselective Biocatalytic Transformations Employing Transaminases and Tyrosine Phenol Lyases
    Top. Catal. (IF 2.226) Pub Date : 2018-10-06
    Lía Martínez-Montero, Joerg H. Schrittwieser, Wolfgang Kroutil

    Regioselective reactions allow the differentiation between two or more chemically identical reactive centers within the same molecule. They are highly desirable transformations in organic synthesis, as they avoid additional chemical operations and sophisticated protection/deprotection strategies. In this context, enzymes, which present exquisite selectivity and reactivity, have been widely employed as catalysts in numerous regioselective transformations. This review focuses on two recently developed biocatalytic processes that present outstanding regioselectity: the transaminase-catalyzed asymmetric amination of di- and triketo compounds, and the stereoselective C–C coupling between phenol derivatives, ammonia and pyruvate for the synthesis of tyrosine analogues, catalyzed by tyrosine phenol lyases. Additionally, elegant and straightforward cascades that have combined the aforementioned biotransformations with other enzymatic and/or chemocatalytic processes are compiled in this contribution. Overall, this review aims to provide a general view of the synthetic possibilities that two relatively recently described regio- and stereoselective biotransformations can provide.

    更新日期:2019-12-11
  • Influence of Composition and Preparation Method on the Continuous Performance of Sn-Beta for Glucose-Fructose Isomerisation
    Top. Catal. (IF 2.226) Pub Date : 2018-11-19
    Luca Botti, Ricardo Navar, Søren Tolborg, Juan S. Martinez-Espin, Daniele Padovan, Esben Taarning, Ceri Hammond

    The stability, activity and selectivity of various Sn-Beta catalysts are investigated to identify how the composition of the catalyst, in addition to its method of preparation, impact its ability to continuously isomerise glucose to fructose. Increasing the Sn loading in post-synthetically prepared catalysts leads to a decrease of both activity and stability. Accordingly, materials containing dilute amounts of Sn appear to be most suitable for continuous operation. Furthermore, the method of preparation has a profound impact on the overall performance of the catalyst. In fact, preparation of Sn-Beta by hydrothermal synthesis results in improvements of both activity and stability, with respect to the post-synthetic preparation of an otherwise-analogous material. The improved resistance of hydrothermal Sn-Beta is attributed, through a combination of operando UV–Vis, TPD-MS and vapour adsorption isotherms, to its greater resistance to deactivation by methanol (the reaction solvent). Complementary 119Sn CPMG MAS NMR experiments also indicate the presence of different Sn sites in the hydrothermal material, which, alongside the presence of a less adsorptive siliceous matrix, may be intrinsically less prone to solvent interaction than those present in post-synthetic Sn-Beta.

    更新日期:2019-12-11
  • A Perspective on Counting Catalytic Active Sites and Rates of Reaction Using X-Ray Spectroscopy
    Top. Catal. (IF 2.226) Pub Date : 2018-10-13
    S. A. Kondrat, J. A. van Bokhoven

    Identification of active sites and phases in heterogeneous catalysts and the understanding of the reaction mechanism remain highly challenging. In most catalysts, the existence of a multitude of surface species, which are dynamic in relation to reaction conditions, presents a challenge of distinguishing those that are involved in the catalytic cycle from those which are spectators. The emergence of the field of single-site catalysts potentially eliminates these issues, although it can be argued that these systems remain dynamic and that multiple speciation, each a candidate for the active site, often remains a consideration. A perspective on how X-ray spectroscopy and characterization tools in general, can be used to correlate the number of active sites and the rate of their formation, in single-site and redox catalyst systems, is presented. The importance of observing proportionality between spectra features and the reaction rate, to differentiate between active sites and spectator species is discussed. Performing characterisation under catalyticly relevant conditions on structures that are demonstrably representative of actual catalysts is essential.

    更新日期:2019-12-11
  • The Development of a N 2 O Abatement Catalyst: from Laboratory Scale to Plant Testing
    Top. Catal. (IF 2.226) Pub Date : 2018-11-10
    Øystein Nirisen, David Waller, David M. Brackenbury

    Nitrous oxide is a powerful greenhouse gas with a global warming potential stated to be between 265 and 310. The production of nitric acid is the largest source of nitrous oxide from the chemical process industries, and it equates to circa 50% of the total greenhouse gas emissions from nitric acid production. This paper describes the successful development of a catalyst for the decomposition of nitrous oxide in the ammonia burner, from laboratory, pilot and plant-scale testing. This catalyst is capable of reducing nitrous oxide emissions by more than 90%, with no significant modifications to plant operation.

    更新日期:2019-12-11
  • Novel Catalytic Reactive Distillation Processes for a Sustainable Chemical Industry
    Top. Catal. (IF 2.226) Pub Date : 2018-10-05
    Anton A. Kiss

    Reactive distillation (RD) is a great process intensification concept taking advantage of the synergy created when combining (catalyzed) reaction and separation into a single unit, which allows the concurrent production and removal of products. This feat improves the productivity and selectivity, reduces the energy usage, eliminates the need for solvents, and leads to highly-efficient systems with improved sustainability metrics (e.g. less waste and emissions). This paper provides an overview of the key features of RD processes, with emphasis on novel catalytic/reactive distillation processes that can make a difference at large scale and pave the way for a more sustainable chemical process industry that is more profitable, safer and less polluting. These examples include the production of: acrylic and methacrylic monomers, unsaturated polyesters resins, di-alkyl ethers, fatty esters, as well as other short alkyl esters (e.g. by enzymatic reactive distillation). The main drivers for such new RD applications are: economical (large reduction of costs and energy use), environmental (lower CO2 emissions, no or reduced waste) and social (improved safety and health due to lower reactive content, reduced footprint and run away sensitivity). Hence RD technology strongly contributes to all three pillars of sustainability in the chemical process industry. Nonetheless, the potential of RD technology has not been fully tapped yet, and there is still undergoing research to improve it further by various means: e.g. ultrasound or microwave assisted RD, use of high-gravity fields (HiGee), internally heat integration, cyclic operation, or coupling RD with other operations such as membrane separations.

    更新日期:2019-12-11
  • Reaction Engineering for the Industrial Implementation of Biocatalysis
    Top. Catal. (IF 2.226) Pub Date : 2019-03-01
    John M. Woodley

    Biocatalytic processes (using one or more enzymes) for the production of chemicals is an important potential route to more sustainable manufacturing and today have found application in several industries, but most notably in the pharmaceutical sector. For high-priced pharmaceuticals, the development of new processes is primarily dependent upon enzyme development. However, the wider application of biocatalytic processes towards lower-priced chemicals will demand reaction engineering to be considered, alongside biocatalyst development. Bioreaction engineering should include an evaluation of the thermodynamics and reaction kinetics, as well as the stability of the biocatalyst. In this brief article, tools to assist in the collection and evaluation of reaction engineering data will be discussed, and illustrated using biological oxidation as an example.

    更新日期:2019-12-11
  • Ru-Based Catalysts for H 2 Production from Ammonia: Effect of 1D Support
    Top. Catal. (IF 2.226) Pub Date : 2018-11-10
    Zhigang Hu, Julien Mahin, Sukanya Datta, Tamsin E. Bell, Laura Torrente-Murciano

    This study reveals the effect of the catalytic 1D supports (carbon, ceria, alumina and titanate) for ruthenium particles on the low temperature release of hydrogen from ammonia. While the state-of-art literature presents Ru/carbon nanotubes (CNT) as the most active catalyst, we found in this work that ruthenium supported on ceria nanorods (Ru/CeO2) catalyst exhibited activity over 8 times higher than the Ru/CNT counterpart system. This enhanced activity is believed to be related to a strong metal-support interaction on the Ru/CeO2 catalysts promoting the formation of small (~ 3 nm) Ru particles. Addition of sodium as a promoter leads to the formation of smaller Ru particle sizes in addition to the modification of the electronic environment of Ru, enhancing the ammonia decomposition activity at low temperatures. This effect is particularly noticeable in the Ru–Na/CNT catalysts, facilitated by the high conductivity of the support, allowing distant electronic modification of the Ru active sites. This work provides novel insights in designing catalysts for hydrogen production from ammonia in our effort to enable the long-term energy storage in chemical bonds.

    更新日期:2019-12-11
  • Multi-dimensional Crystal Structuring of Complex Metal Oxide Catalysts of Group V and VI Elements by Unit-Assembling
    Top. Catal. (IF 2.226) Pub Date : 2018-11-07
    Satoshi Ishikawa, Zhenxin Zhang, Toru Murayama, Norihito Hiyoshi, Masahiro Sadakane, Wataru Ueda

    Four new porous crystalline complex metal oxide families based on group V and VI elements, all of which are synthesized through unit-assembling, are introduced along with crystal structure formation mechanism. Polyoxometalates (POMs) are utilized as building units and assembled for constructing microporous complex metal oxides. Assembling of MoVO-POMs having pentagonal units of [Mo6O21] with {VO} linkers under hydrothermal conditions forms microporous orthorhombic (NH4)4[Mo30V4O106] {VO}6 oxide and trigonal (NH4)3[Mo19.5V1.5O69] {VO}6 oxide. Assembling of ε-Keggin Mo-POMs with bismuth ions as a linker under a hydrothermal condition produces a cubic (NH4)4[Mo9.4V3.6O40] {Bi}2 crystal with cages and channels with the diameter sizes of 0.77 and 0.34 nm, respectively. One dimensional anionic tungstosellenate molecular wire building block, [SeW6O21]2−n, is first formed by linear connection of hexagonal tungstosellenate POM units [SeW6O27] and then linked with the CoII ion to form a crystalline microporous materials, (NH4)0.4 [SeW6O21] {Co(OH)}1.3. [W4O16] building blocks are orderly connected with {VO} linkers to form a microporous framework (K1.5(NH4)0.2H0.3[W4O16]{VO}3) with a pore diameter of 0.43 nm which is fully opened and is accessible to small molecules. These new porous crystalline complex metal oxides showed high catalytic performance for alkane oxidation, aldehyde oxidation, alcohol oxidation, H2O2 oxidation, NH3-SCR, and so on.

    更新日期:2019-12-11
  • Sulfur-Tolerant, Exsolved Fe–Ni Alloy Nanoparticles for CO Oxidation
    Top. Catal. (IF 2.226) Pub Date : 2018-10-05
    Evangelos I. Papaioannou, Dragos Neagu, Wan K. W. Ramli, John T. S. Irvine, Ian S. Metcalfe

    Metallic nanoparticles exsolved at the surface of perovskite oxides have been recently shown to unlock superior catalytic activity and durability towards various chemical reactions of practical importance. For example, for the CO oxidation reaction, exsolved Ni nanoparticles in oxidised form exhibit site activities approaching those of noble metals. This is of particular interest for the prospect of replacing noble metals with earth-abundant metal/metal oxide catalysts in the automotive exhaust control industry. Here we show that for the CO oxidation reaction, the functionality of exsolved Ni nanoparticles can be further improved when Fe is co-exsolved with Ni, as Fe–Ni alloy nanoparticles, eventually forming mixed oxide nanoparticles. As compared to the Ni nanoparticles, the alloy nanoparticles exhibit higher site activities, greatly improved durability over 170 h of continuous testing and increased tolerance towards sulphur-based atmospheres. Similarly to the single metal nanoparticles, the alloys demonstrate outstanding microstructural stability and high tolerance towards coking. These results open additional directions for tailoring the activity and durability of exsolved materials for the CO oxidation reaction and beyond.

    更新日期:2019-12-11
  • Aerobic Oxidation of Benzyl Alcohol in a Continuous Catalytic Membrane Reactor
    Top. Catal. (IF 2.226) Pub Date : 2018-10-20
    Achilleas Constantinou, Gaowei Wu, Baldassarre Venezia, Peter Ellis, Simon Kuhn, Asterios Gavriilidis

    A catalytic membrane reactor with a Au–Pd catalyst, impregnated at the inner side of the membrane, was studied in the catalytic oxidation of benzyl alcohol in flow. The reactor comprised of four concentric sections. The liquid substrate flowed in the annulus created by an inner tube and the membrane. The membrane consisted of 3 layers of α-alumina and a titania top layer with 5 nm average pore size. Oxygen was fed on the outer side of the membrane, and its use allowed the controlled contact of the liquid and the gas phase. Experiments revealed excellent stability of the impregnated membrane and selectivities to benzaldehyde were on average > 95%. Increasing the pressure of the gas phase and decreasing liquid flowrates and benzyl alcohol concentration resulted in an increased conversion, while selectivities to benzaldehyde remained constant and in excess of 95%.

    更新日期:2019-12-11
  • Theoretical Study of the Water–Gas Shift Reaction on a Au/Hematite Model Catalyst
    Top. Catal. (IF 2.226) Pub Date : 2019-05-14
    Silvia A. Fuente, Carolina Zubieta, Ricardo M. Ferullo, Patricia G. Belelli

    Using the density functional theory, the mechanism of the water–gas shift reaction has been investigated employing a model catalyst formed by a Au5 cluster supported on the Fe-terminated (0001) face of hematite (α-Fe2O3), to better understand the role played by the metal–oxide interface in this reaction. Our results indicate that the Au5/hematite model catalyst has a good performance to catalyze the reaction following the so-called adsorptive mechanism. The presence of Au favors the development of the reaction due mainly to the following factors: (i) H2O dissociates very easily at the metal–oxide interface producing OH species; (ii) CO adsorbs strongly on a Au site nearby the position of OH; (iii) the hydroxycarbonyl intermediate (HOCO) is formed at the interface from CO and OH with a low activation barrier; and (iv) after hydrogen releasing, CO2 is desorbed with relative facility from the interface region. The formation of H2 is the stage of the whole reaction that more energy demands; however, this process is favored if one hydrogen atom comes directly from HOCO, instead of from two hydrogen atoms bound to surface oxygen anions.

    更新日期:2019-12-11
  • Oxidative Carboxylation of 1-Decene to 1,2-Decylene Carbonate.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Rebecca V Engel,Raiedhah Alsaiari,Ewa Nowicka,Samuel Pattisson,Peter J Miedziak,Simon A Kondrat,David J Morgan,Graham J Hutchings

    Cyclic carbonates are valuable chemicals for the chemical industry and thus, their efficient synthesis is essential. Commonly, cyclic carbonates are synthesised in a two-step process involving the epoxidation of an alkene and a subsequent carboxylation to the cyclic carbonate. To couple both steps into a direct oxidative carboxylation reaction would be desired from an economical view point since additional work-up procedures can be avoided. Furthermore, the efficient sequestration of CO2, a major greenhouse gas, would also be highly desirable. In this work, the oxidative carboxylation of 1-decene is investigated using supported gold catalysts for the epoxidation step and tetrabutylammonium bromide in combination with zinc bromide for the cycloaddition of carbon dioxide in the second step. The compatibility of the catalysts for both steps is explored and a detailed study of catalyst deactivation using X-ray photoelectron spectroscopy and scanning electron microscopy is reported. Promising selectivity of the 1,2-decylene carbonate is observed using a one-pot two-step approach.

    更新日期:2019-11-01
  • Carbon Monoxide Poisoning Resistance and Structural Stability of Single Atom Alloys.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Matthew T Darby,E Charles H Sykes,Angelos Michaelides,Michail Stamatakis

    Platinum group metals (PGMs) serve as highly active catalysts in a variety of heterogeneous chemical processes. Unfortunately, their high activity is accompanied by a high affinity for CO and thus, PGMs are susceptible to poisoning. Alloying PGMs with metals exhibiting lower affinity to CO could be an effective strategy toward preventing such poisoning. In this work, we use density functional theory to demonstrate this strategy, focusing on highly dilute alloys of PGMs (Pd, Pt, Rh, Ir and Ni) with poison resistant coinage metal hosts (Cu, Ag, Au), such that individual PGM atoms are dispersed at the atomic limit forming single atom alloys (SAAs). We show that compared to the pure metals, CO exhibits lower binding strength on the majority of SAAs studied, and we use kinetic Monte Carlo simulation to obtain relevant temperature programed desorption spectra, which are found to be in good agreement with experiments. Additionally, we consider the effects of CO adsorption on the structure of SAAs. We calculate segregation energies which are indicative of the stability of dopant atoms in the bulk compared to the surface layer, as well as aggregation energies to determine the stability of isolated surface dopant atoms compared to dimer and trimer configurations. Our calculations reveal that CO adsorption induces dopant atom segregation into the surface layer for all SAAs considered here, whereas aggregation and island formation may be promoted or inhibited depending on alloy constitution and CO coverage. This observation suggests the possibility of controlling ensemble effects in novel catalyst architectures through CO-induced aggregation and kinetic trapping.

    更新日期:2019-11-01
  • VOx/Fe2O3 Shell-Core Catalysts for the Selective Oxidation of Methanol to Formaldehyde.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Pip Hellier,Peter P Wells,Diego Gianolio,Michael Bowker

    Efficient oxidation catalysts are important in many current industrial processes, including the selective oxidation of methanol to formaldehyde. Vanadium-containing catalysts have been shown to be effective selective oxidation catalysts for certain reactions, and research continues to examine their applicability to other reactions of interest. Several VOx/Fe2O3 shell-core catalysts with varying VOx coverage have been produced to investigate the stability of VOx monolayers and their selectivity for methanol oxidation. Catalyst formation proceeds via a clear progression of distinct surface species produced during catalyst calcination. At 300 °C the selective VOx overlayer has formed; by 500 °C a sandwich layer of FeVO4 arises between the VOx shell and the Fe2O3 core, inhibiting iron cation participation in the catalysis and enhancing catalyst selectivity. The resulting catalysts, comprising a shell-subshell-core system of VOx/FeVO4/Fe2O3, possess good catalytic activity and selectivity to formaldehyde.

    更新日期:2019-11-01
  • Variation of SMSI with the Au:Pd Ratio of Bimetallic Nanoparticles on TiO2(110).
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Richard Gubó,Chi M Yim,Michael Allan,Chi L Pang,András Berkó,Geoff Thornton

    Au/Pd nanoparticles are important in a number of catalytic processes. Here we investigate the formation of Au-Pd bimetallic nanoparticles on TiO2(110) and their susceptibility to encapsulation using scanning tunneling microscopy, as well as Auger spectroscopy and low energy electron diffraction. Sequentially depositing 5 MLE Pd and 1 MLE Au at 298 K followed by annealing to 573 K results in a bimetallic core and Pd shell, with TiOx encapsulation on annealing to ~ 800 K. Further deposition of Au on the pinwheel type TiOx layer results in a template-assisted nucleation of Au nanoclusters, while on the zigzag type TiOx layer no preferential adsorption site of Au was observed. Increasing the Au:Pd ratio to 3 MLE Pd and 2 MLE Au results in nanoparticles that are enriched in Au at their surface, which exhibit a strong resistance towards encapsulation. Hence the degree of encapsulation of the nanoparticles during sintering can be controlled by tuning the Au:Pd ratio.

    更新日期:2019-11-01
  • Infrared Spectroscopy of Au+(CH4) n Complexes and Vibrationally-Enhanced C-H Activation Reactions.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Alexander S Gentleman,Alice E Green,Daniel R Price,Ethan M Cunningham,Andreas Iskra,Stuart R Mackenzie

    A combined spectroscopic and computational study of gas-phase Au+(CH4) n (n = 3-8) complexes reveals a strongly-bound linear Au+(CH4)2 core structure to which up to four additional ligands bind in a secondary coordination shell. Infrared resonance-enhanced photodissociation spectroscopy in the region of the CH4 a 1 and t 2 fundamental transitions reveals essentially free internal rotation of the core ligands about the H4C-Au+-CH4 axis, with sharp spectral features assigned by comparison with spectral simulations based on density functional theory. In separate experiments, vibrationally-enhanced dehydrogenation is observed when the t 2 vibrational normal mode in methane is excited prior to complexation. Clear infrared-induced enhancement is observed in the mass spectrum for peaks corresponding 4u below the mass of the Au+(CH4) n=2,3 complexes corresponding, presumably, to the loss of two H2 molecules.

    更新日期:2019-11-01
  • Gas-Phase Reactivity Studies of Small Molybdenum Cluster Ions with Dimethyl Disulfide.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Aristeidis Baloglou,Milan Ončák,Christian van der Linde,Martin K Beyer

    Molybdenum sulfide is a potent hydrogen evolution catalyst, and is discussed as a replacement of platinum in large-scale electrochemical hydrogen production. To learn more about the elementary steps of MoS2 production by sputtering in the presence of dimethyl disulfide (DMDS), the reactions of Mox +, x = 1-3, with DMDS are studied by Fourier transform ion cyclotron resonance mass spectrometry and density functional theory calculations. A rich variety of products composed of molybdenum, sulfur, carbon and hydrogen was observed. MoxSy + species are formed in the first reaction step, together with products containing carbon and hydrogen. The calculations indicate that the strong Mo-S bonds are formed preferentially, followed by Mo-C bonds. Hydrogen is exclusively bound to carbon atoms, i.e. no insertion of a molybdenum atom into a C-H bond is observed. The reactions are efficient and highly exothermic, explaining the rich chemistry observed in the experiment.

    更新日期:2019-11-01
  • Mechanism and Catalysis of Oxidative Degradation of Fiber-Reinforced Epoxy Composites.
    Top. Catal. (IF 2.226) Pub Date : 2018-10-06
    Carlos A Navarro,Elyse A Kedzie,Yijia Ma,Katelyn H Michael,Steven R Nutt,Travis J Williams

    Carbon fiber-reinforced polymer (CFRP) materials are widely used in aerospace and recreational equipment, but there is no efficient procedure for their end-of-life recycling. Ongoing work in the chemistry and engineering communities emphasizes recovering carbon fibers from such waste streams by dissolving or destroying the polymer binding. By contrast, our goal is to depolymerize amine-cured epoxy CFRP composites catalytically, thus enabling not only isolation of high-value carbon fibers, but simultaneously opening an approach to recovery of small molecule monomers that can be used to regenerate precursors to new composite resin. To do so will require understanding of the molecular mechanism(s) of such degradation sequences. Prior work has shown the utility of hydrogen peroxide as a reagent to affect epoxy matrix decomposition [1]. Herein we describe the chemical transformations involved in that sequence: the reaction proceeds by oxygen atom transfer to the polymer's linking aniline group, forming an N-oxide intermediate. The polymer is then cleaved by an elimination and hydrolysis sequence. We find that elimination is the slower step. Scandium trichloride is an efficient catalyst for this step, reducing reaction time in homogeneous model systems and neat cured matrix blocks. The conditions can be applied to composed composite materials, from which pristine carbon fibers can be recovered.

    更新日期:2019-11-01
  • Nonlinear Optical Methods for Characterization of Molecular Structure and Surface Chemistry.
    Top. Catal. (IF 2.226) Pub Date : 2018-06-30
    Patrik K Johansson,Lars Schmüser,David G Castner

    The principles, strengths and limitations of several nonlinear optical (NLO) methods for characterizing biological systems are reviewed. NLO methods encompass a wide range of approaches that can be used for real-time, in-situ characterization of biological systems, typically in a label-free mode. Multiphoton excitation fluorescence (MPEF) is widely used for high-quality imaging based on electronic transitions, but lacks interface specificity. Second harmonic generation (SHG) is a parametric process that has all the virtues of the two-photon version of MPEF, yielding a signal at twice the frequency of the excitation light, which provides interface specificity. Both SHG and MPEF can provide images with high structural contrast, but they typically lack molecular or chemical specificity. Other NLO methods such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) can provide high-sensitivity imaging with chemical information since Raman active vibrations are probed. However, CARS and SRS lack interface and surface specificity. A NLO method that provides both interface/surface specificity as well as molecular specificity is vibrational sum frequency generation (SFG) spectroscopy. Vibration modes that are both Raman and IR active are probed in the SFG process, providing the molecular specificity. SFG, like SHG, is a parametric process, which provides the interface and surface specificity. SFG is typically done in the reflection mode from planar samples. This has yielded rich and detailed information about the molecular structure of biomaterial interfaces and biomolecules interacting with their surfaces. However, 2-D systems have limitations for understanding the interactions of biomolecules and interfaces in the 3-D biological environment. The recent advances made in instrumentation and analysis methods for sum frequency scattering (SFS) now present the opportunity for SFS to be used to directly study biological solutions. By detecting the scattering at angles away from the phase-matched direction even centrosymmetric structures that are isotropic (e.g., spherical nanoparticles functionalized with self-assembled monolayers or biomolecules) can be probed. Often a combination of multiple NLO methods or a combination of a NLO method with other spectroscopic methods is required to obtain a full understanding of the molecular structure and surface chemistry of biomaterials and the biomolecules that interact with them. Using the right combination methods provides a powerful approach for characterizing biological materials.

    更新日期:2019-11-01
  • Toward Asymmetric Synthesis of Pentaorganosilicates.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Leon J P van der Boon,Shin-Ichi Fuku-En,J Chris Slootweg,Koop Lammertsma,Andreas W Ehlers

    Introducing chiral silicon centers was explored for the asymmetric Rh-catalyzed cyclization of dihydrosilanes to enantiomerically enriched spirosilanes as targets to enable access to enantiostable pentacoordinate silicates. The steric rigidity required in such systems demands the presence of two naphthyl or benzo[b]thiophene groups. The synthetic approach to the expanded spirosilanes extends Takai's method (Kuninobu et al. in Angew Chem Int Ed 52(5):1520-1522, 2013) for the synthesis of spirosilabifluorenes in which both a Si-H and a C-H bond of a dihydrosilane are activated by a rhodium catalyst. The expanded dihydrosilanes were obtained from halogenated aromatic precursors. Their asymmetric cyclization to the spirosilanes were conducted with [Rh(cod)Cl]2 in the presence of the chiral bidentate phosphane ligands (R)-BINAP, (R)-MeO-BIPHEP, and (R)-SEGPHOS, including derivatives with P-(3,5-t-Bu-4-MeO)-phenyl (DTBM) groups. The highest enantiomeric excess of 84% was obtained for 11,11'-spirobi[benzo[b]-naphtho[2,1-d]silole] with the DTBM-SEGPHOS ligand.

    更新日期:2019-11-01
  • Catalysis of the Oxygen Evolution Reaction by 4-10 nm Cobalt Nanoparticles.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Edward Locke,Shan Jiang,Simon K Beaumont

    Electrolysis of water is key technology, not only for clean energy production, but to ensure a continued supply of hydrogen beyond fossil resources, essential to the manufacture of many chemical goods other than fuels. Cobalt nanomaterials have been widely identified as a promising candidate for the anode (oxygen evolution) reaction in this process, but much work has focused on applied materials or electrode design. Given the importance of oxidation state changes Co(III) → Co(IV) in the accepted reaction mechanism, in this work we look at size effects in small (4-10 nm) cobalt nanoparticles, where the ease of oxidation for lower cobalt oxidation states is known to change with particle size. To discriminate between geometric and chemical effects we have compared the catalysts in this study to others in the literature by turnover frequency (widely used in other areas of catalysis), in addition to the more commonly employed performance metric of the overpotential required to produce a current density of 10 mA cm-2. Comparisons are drawn to key examples of using well defined nanomaterials (where the surface are of cobalt sites can be estimated). This has enabled an estimated intrinsic turnover rate of ~ 1 O2 molecule per surface Co atom per second at an overpotential of 500 mV in the oxygen evolution reaction under typical alkaline reaction conditions (pH 14.0) to be identified.

    更新日期:2019-11-01
  • α-Thio Carbocations (Thionium Ions) as Intermediates in Brønsted Acid-Catalyzed Reactions of Enone-Derived 1,3-Dithianes and 1,3-Dithiolanes.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Christoph Brenninger,Thorsten Bach

    Abstract Evidence was collected for the intermediate formation of thionium ions in Brønsted acid-catalyzed [2 + 2] photocycloaddition and electrophilic addition reactions to enone dithianes and dithiolanes. Low-temperature NMR studies helped to elucidate the structure and configuration of the thionium ions and thus support previous and current results obtained by UV/Vis spectroscopy. Graphical Abstract

    更新日期:2019-11-01
  • Discovery and Investigation of Mutase-like Activity in a Phenylalanine Ammonia Lyase from Anabaena variabilis.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Nicholas J Weise,Fabio Parmeggiani,Syed T Ahmed,Nicholas J Turner

    The effect of extended reaction times on the regio- and enantioselectivity of the phenylalanine ammonia lyase (PAL)-catalysed amination of a subset of cinnamate derivatives was investigated. This was done using a PAL from the cyanobacterium Anabaena variabilis and incubation in a concentrated ammonia buffer. Whilst early time point analyses revealed excellent selectivities to give mostly the well-documented (S)-α-amino acid products, subsequent accumulation of other regio-/stereo- isomers was seen. For many para-substituted substrates, the β-regioisomer, a previously-unreported product with this enzyme class, was found to become more abundant than the α-, after sufficient incubation, with slight preference for the (R)-enantiomer. Although attempts to tune the selectivity of the PAL toward any of the three side products were largely unsuccessful, the results provide insight into the evolutionary history of this class of enzymes and reinforce the prominence of the toolbox of specific and selective cinnamate-aminating enzymes.

    更新日期:2019-11-01
  • Electrochemical Promotion of CO Oxidation on Na-Promoted Pt/YSZ: Interaction Between Multiple Promoting Species.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Efstratios Stavrakakis,Danai Poulidi

    The combined promotional effect of electrochemically-supplied O2- and chemically-supplied Na+ promoters, was studied for the case of CO oxidation on Pt/YSZ. Four different sodium coverages (0.16, 1.6, 8 and 40%) were loaded onto the catalyst surface and the catalytic behaviour was compared with a nominally 'clean' catalyst under a wide range of reactants' ratios under open-circuit and polarised conditions. Sodium generally increased oxygen adsorption by lowering the work function of the catalyst. However, sodium promoted the catalytic rate only at coverages up to 1.6% and worked synergistically with O2- promoting species to an increased overall promotion of the catalytic rate. At higher sodium coverages, i.e. θNa ≥ 8%, the catalytic behaviour was strongly affected by the interactions between the sodium species, the catalyst, the reactants and oxygen ions promoting species. The postulated formation of stable sodium oxide species on the catalyst pores reduced the active catalytic area which resulted in poisoning the catalytic rate and suppressing EPOC effect, respectively. It is suggested that these stable sodium oxide species which also induced a permanent EPOC effect by oxygen storage, were formed by the migrated oxygen ions.

    更新日期:2019-11-01
  • Hydrogen Generation from Additive-Free Formic Acid Decomposition Under Mild Conditions by Pd/C: Experimental and DFT Studies.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Felipe Sanchez,Davide Motta,Alberto Roldan,Ceri Hammond,Alberto Villa,Nikolaos Dimitratos

    Safe and efficient hydrogen generation and storage has received much attention in recent years. Herein, a commercial 5 wt% Pd/C catalyst has been investigated for the catalytic, additive-free decomposition of formic acid at mild conditions, and the experimental parameters affecting the process systematically have been investigated and optimised. The 5 wt% Pd/C catalyst exhibited a remarkable 99.9% H2 selectivity and a high catalytic activity (TOF = 1136 h-1) at 30 °C toward the selective dehydrogenation of formic acid to H2 and CO2. The present commercial catalyst demonstrates to be a promising candidate for the efficient in-situ hydrogen generation at mild conditions possibiliting practical applications of formic acid systems on fuel cells. Finally DFT studies have been carried out to provide insights into the reactivity and decomposition of formic acid along with the two-reaction pathways on the Pd (111) surface.

    更新日期:2019-11-01
  • Effect of Mass Transport on the Electrochemical Oxidation of Alcohols Over Electrodeposited Film and Carbon-Supported Pt Electrodes.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Vinod Kumar Puthiyapura,Wen-Feng Lin,Andrea E Russell,Dan J L Brett,Christopher Hardacre

    Electrochemical oxidation of four different alcohol molecules (methanol, ethanol, n-butanol and 2-butanol) at electrodeposited Pt film and carbon-supported Pt catalyst film electrodes, as well as the effect of mass transport on the oxidation reaction, has been studied systematically using the rotating disk electrode (RDE) technique. It was shown that oxidation current decreased with an increase in the rotation rate (ω) for all alcohols studied over electrodeposited Pt film electrodes. In contrast, the oxidation current was found to increase with an increase in the ω for Pt/C in ethanol and n-butanol-containing solutions. The decrease was found to be nearly reversible for ethanol and n-butanol at the electrodeposited Pt film electrode ruling out the possibility of intermediate COads poisoning being the sole cause of the decrease and was attributed to the formation of soluble intermediate species which diffuse away from the electrode at higher ω. In contrast, an increase in the current with an increase in ω for the carbon supported catalyst may suggest that the increase in residence time of the soluble species within the catalyst layer, results in further oxidation of these species. Furthermore, the reversibility of the peak current on decreasing the ω could indicate that the surface state has not significantly changed due to the sluggish reaction kinetics of ethanol and n-butanol.

    更新日期:2019-11-01
  • An In Situ XAS Study of the Cobalt Rhenium Catalyst for Ammonia Synthesis.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Karina Mathisen,Karsten Granlund Kirste,Justin S J Hargreaves,Said Laassiri,Kate McAulay,Andrew R McFarlane,Nicholas A Spencer

    A cobalt rhenium catalyst active for ammonia synthesis at 400 °C and ambient pressure was studied using in situ XAS to elucidate the reducibility and local environment of the two metals during reaction conditions. The ammonia reactivity is greatly affected by the gas mixture used in the pre-treatment step. Following H2/Ar pre-treatment, a subsequent 20 min induction period is also observed before ammonia production occurs whereas ammonia production commences immediately following comparable H2/N2 pre-treatment. In situ XAS at the Co K-edge and Re LIII-edge show that cobalt initiates reduction, undergoing reduction between 225 and 300 °C, whereas reduction of rhenium starts at 300 °C. The reduction of rhenium is near complete below 400 °C, as also confirmed by H2-TPR measurements. A synergistic co-metal effect is observed for the cobalt rhenium system, as complete reduction of both cobalt and rhenium independently requires higher temperatures. The phases present in the cobalt rhenium catalyst during ammonia production following both pre-treatments are largely bimetallic Co-Re phases, and also monometallic Co and Re phases. The presence of nitrogen during the reduction step strongly promotes mixing of the two metals, and the bimetallic Co-Re phase is believed to be a pre-requisite for activity.

    更新日期:2019-11-01
  • The Role of Catalyst Support, Diluent and Co-Catalyst in Chromium-Mediated Heterogeneous Ethylene Trimerisation.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    M J Lamb,D C Apperley,M J Watson,P W Dyer

    Sequential treatment of a previously-calcined solid oxide support (i.e. SiO2, γ-Al2O3, or mixed SiO2-Al2O3) with solutions of Cr{N(SiMe3)2}3 (0.71 wt% Cr) and a Lewis acidic alkyl aluminium-based co-catalyst (15 molar equivalents) affords initiator systems active for the oligomerisation and/or polymerisation of ethylene. The influence of the oxide support, calcination temperature, co-catalyst, and reaction diluent on both the productivity and selectivity of the immobilised chromium initiator systems have been investigated, with the best performing combination (SiO2-600, modified methyl aluminoxane-12 {MMAO-12}, heptane) producing a mixture of hexenes (61 wt%; 79% 1-hexene), and polyethylene (16 wt%) with an activity of 2403 g gCr -1 h-1. The observed product distribution is rationalised by two competing processes: trimerisation via a supported metallacycle-based mechanism and polymerisation through a classical Cossee-Arlman chain-growth pathway. This is supported by the indirect observation of two distinct chromium environments at the surface of the oxide support by a solid-state 29Si NMR spectroscopic study of the Cr{N(SiMe3)2}x/SiO2-600 pro-initiator.

    更新日期:2019-11-01
  • Reactions of Dimethylether in Single Crystals of the Silicoaluminophosphate STA-7 Studied via Operando Synchrotron Infrared Microspectroscopy.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Russell F Howe,Suwardiyanto,David J Price,Maria Castro,Paul A Wright,Alex Greenaway,Mark D Frogley,Gianfelice Cinque

    Abstract Synchrotron infrared micro-spectroscopy has been applied to measure in situ the reaction of dimethylether in single crystals of the silicoaluminophosphate STA-7. The crystals are found to contain a uniform and homogeneous distribution of acidic hydroxyl groups. Dimethylether is hydrogen bonded to the hydroxyl groups at low temperatures, but evidence is found for dissociation to form surface methoxy groups above 473 K, and aromatic hydrocarbon pool species above 573 K. From time resolved infrared measurements coupled with MS analysis of evolved products it is concluded that alkene formation occurs via a direct mechanism from reaction of dimethylether with surface methoxy groups. Graphical Abstract

    更新日期:2019-11-01
  • Operando Spectroscopic Studies of Cu-SSZ-13 for NH3-SCR deNOx Investigates the Role of NH3 in Observed Cu(II) Reduction at High NO Conversions.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Alex G Greenaway,Ines Lezcano-Gonzalez,Miren Agote-Aran,Emma K Gibson,Yaroslav Odarchenko,Andrew M Beale

    The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic reduction by ammonia (NH3) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH3-SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH3-SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH3 and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH3-SCR of NOx occurring at Cu six ring sites within the zeolite.

    更新日期:2019-11-01
  • Effect of Particle Size and Support Type on Pd Catalysts for 1,3-Butadiene Hydrogenation.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Donato Decarolis,Ines Lezcano-Gonzalez,Diego Gianolio,Andrew M Beale

    Pd nanoparticles supported on SiO2, Si3N4 and Al2O3 were studied to examine the effect of particle size and support type on the hydrogenation of 1,3-butadiene. Pd nanoparticles were produced using a reverse micelle method resulting in particles with a remarkably small particle size distribution (σ < < 1 nm). The support type and particle size were observed to affect both catalytic activity and product selectivity. All catalysts showed a decrease of their activity with time on stream, paired with an increase in selectivity to butenes (1-butene and cis/trans-2-butene) from a product stream initially dominated by n-butane. In situ XAFS demonstrated a correlation between the formation of palladium hydride and n-butane production in the early stages (~ 1 h) of reaction. The extent of palladium hydride formation, as well as its depletion with time on stream, was dependent on both particle size and support type. Metallic Pd was identified as the species selective towards the production of butenes.

    更新日期:2019-11-01
  • AMnO3 (A = Sr, La, Ca, Y) Perovskite Oxides as Oxygen Reduction Electrocatalysts.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    V Celorrio,L Calvillo,G Granozzi,A E Russell,D J Fermin

    A series of perovskite-type manganites AMnO3 (A = Sr, La, Ca and Y) particles were investigated as electrocatalysts for the oxygen reduction reaction. AMnO3 materials were synthesized by means of an ionic-liquid method, yielding phase pure particles at different temperatures. Depending on the calcination temperature, particles with mean diameter between 20 and 150 nm were obtained. Bulk versus surface composition and structure are probed by X-ray photoelectron spectroscopy and extended X-ray absorption fine structure. Electrochemical studies were performed on composite carbon-oxide electrodes in alkaline environment. The electrocatalytic activity is discussed in terms of the effective Mn oxidation state, A:Mn particle surface ratio and the Mn-O distances.

    更新日期:2019-11-01
  • Solvent Free Synthesis of PdZn/TiO2 Catalysts for the Hydrogenation of CO2 to Methanol.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Hasliza Bahruji,Jonathan Ruiz Esquius,Michael Bowker,Graham Hutchings,Robert D Armstrong,Wilm Jones

    Catalytic upgrading of CO2 to value-added chemicals is an important challenge within the chemical sciences. Of particular interest are catalysts which are both active and selective for the hydrogenation of CO2 to methanol. PdZn alloy nanoparticles supported on TiO2 via a solvent-free chemical vapour impregnation method are shown to be effective for this reaction. This synthesis technique is shown to minimise surface contaminants, which are detrimental to catalyst activity. The effect of reductive heat treatments on both structural properties of PdZn/TiO2 catalysts and rates of catalytic CO2 hydrogenation are investigated. PdZn nanoparticles formed upon reduction showed high stability towards particle sintering at high reduction temperature with average diameter of 3-6 nm to give 1710 mmol kg-1 h of methanol. Reductive treatment at high temperature results in the formation of ZnTiO3 as well as PdZn, and gives the highest methanol yield.

    更新日期:2019-11-01
  • The Chemical Evolution of the La0.6Sr0.4CoO3-δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Alexander K Opitz,Christoph Rameshan,Markus Kubicek,Ghislain M Rupp,Andreas Nenning,Thomas Götsch,Raoul Blume,Michael Hävecker,Axel Knop-Gericke,Günther Rupprechter,Bernhard Klötzer,Jürgen Fleig

    Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.

    更新日期:2019-11-01
  • The Role of Adsorbed and Subsurface Carbon Species for the Selective Alkyne Hydrogenation Over a Pd-Black Catalyst: An Operando Study of Bulk and Surface.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    J J Velasco-Vélez,D Teschner,F Girgsdies,M Hävecker,V Streibel,M G Willinger,J Cao,M Lamoth,E Frei,R Wang,A Centeno,A Zurutuza,S Hofmann,R Schlögl,A Knop-Gericke

    The selective hydrogenation of propyne over a Pd-black model catalyst was investigated under operando conditions at 1 bar making use of advanced X-ray diffraction (bulk sensitive) and photo-electron spectroscopy (surface sensitive) techniques. It was found that the population of subsurface species controls the selective catalytic semi-hydrogenation of propyne to propylene due to the formation of surface and near-surface PdCx that inhibits the participation of more reactive bulk hydrogen in the hydrogenation reaction. However, increasing the partial pressure of hydrogen reduces the population of PdCx with the concomitant formation of a β-PdHx phase up to the surface, which is accompanied by a lattice expansion, allowing the participation of more active bulk hydrogen which is responsible for the unselective total alkyne hydrogenation. Therefore, controlling the surface and subsurface catalyst chemistry is crucial to control the selective alkyne semi-hydrogenation.

    更新日期:2019-11-01
  • Investigating the Influence of Fe Speciation on N2O Decomposition Over Fe-ZSM-5 Catalysts.
    Top. Catal. (IF 2.226) Pub Date : 2018-01-01
    Nia Richards,Ewa Nowicka,James H Carter,David J Morgan,Nicholas F Dummer,Stanislaw Golunski,Graham J Hutchings

    The influence of Fe speciation on the decomposition rates of N2O over Fe-ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe-ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N2O decomposition at 600 °C improved to 3.99 × 103 s-1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 103 s-1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N2O decomposition when propane is present. When only N2O is present, low metal loading Fe-ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe3+ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as FexOy, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of FexOy nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.

    更新日期:2019-11-01
  • How Au Outperforms Pt in the Catalytic Reduction of Methane towards Ethane and Molecular Hydrogen.
    Top. Catal. (IF 2.226) Pub Date : 2018-09-11
    José I Martínez,Federico Calle-Vallejo,Pedro L de Andrés

    Within the context of a "hydrogen economy", it is paramount to guarantee a stable supply of molecular hydrogen to devices such as fuel cells. At the same time, catalytic conversion of the environmentally harmful methane into ethane, with a significantly lower Global Warming Potential, turns into a highly desirable challenge. Herein we propose a first-step novel proof-of-concept mechanism to accomplish both tasks simultaneously. For that purpose we provide transition-state barriers and reaction Helmholtz free energies obtained from first-principles Density Functional Theory by taking account vibrations for 2CH4(g) → C2H6(g) + H2(g) to show that molecular hydrogen can be produced by subnanometer Pt38 and Au38 nanoparticles from natural gas. Interestingly, the active sites for the reaction are located on different planes on the two nanoparticles, effectively differentiating the working principle of the two metals. The analysis shows that the complete cycle to reduce CH4 can be performed on Au and Pt with similar efficiencies, but Au requires only half the working temperature of Pt. This substantial decrease of temperature can be traced back to several intermediate steps, but most crucially to the final one where the catalyst must be cleaned from H(⋆) to be able to restart the catalytic cycle. This simple study case provides useful guidelines to capitalize on finite-size effects in small nanoparticles for the design of new and more efficient catalysts. Interestingly, present results obtained for the intermediate steps of the catalytic cycle show an excellent agreement with previous experimental evidence. Finally, we stress the importance of including the final cleaning steps to start a new fresh catalytic cycle.

    更新日期:2019-11-01
  • Polarization-Dependent SFG Spectroscopy of Near Ambient Pressure CO Adsorption on Pt(111) and Pd(111) Revisited.
    Top. Catal. (IF 2.226) Pub Date : 2018-06-29
    Xia Li,Matteo Roiaz,Verena Pramhaas,Christoph Rameshan,Günther Rupprechter

    Polarization-dependent sum frequency generation (SFG) vibrational spectroscopy was employed to examine CO overlayers on Pt(111) and Pd(111) single crystal surfaces at room temperature. Utilizing different polarization combinations (SSP and PPP) of the visible and SFG light allows to determine the molecular orientation (tilt angle) of interface molecules but the analysis of the measured Ippp/Issp is involved and requires a proper optical interface model. For CO/Pt(111), the hyperpolarizability ratio R=βaac/βccc=βbbc/βccc is not exactly known and varying R in the range 0.1-0.5 yields tilt angles of 40°-0°, respectively. Based on the known perpendicular adsorption of CO on Pt, an exact R-value of 0.49 was determined. Polarization-dependent SFG spectra in the pressure range 10-4 to 36 mbar did not indicate any change of the tilt angle of adsorbed CO. Modeling also indicated a strong dependence of Ippp/Issp on the incidence angles of visible and IR laser beams. Complementing previous low temperature/low pressure data, room temperature CO adsorption on Pd(111) was examined from 10-6 to 250 mbar. The absolute PPP and SSP spectral intensities on Pt and Pd were simulated, as well as the expected Ippp/Issp ratios. Although CO on Pt and Pd should exhibit similar intensities (at high CO coverage), the higher Ippp/Issp ratio for Pd (48 vs. 27 on Pt) renders the detection of adsorbed CO in SSP spectra difficult. The presence or absence of CO species in SSP spectra can thus not simply be correlated to tilted or perpendicular CO molecules, respectively. Careful modeling, including not only molecular and interface properties, but also the experimental configuration (incidence angles), is certainly required even for seemingly simple adsorbate-substrate systems.

    更新日期:2019-11-01
  • Adsorption and Reaction of CO on (Pd-)Al2O3 and (Pd-)ZrO2: Vibrational Spectroscopy of Carbonate Formation.
    Top. Catal. (IF 2.226) Pub Date : 2017-12-15
    Karin Föttinger,Waltraud Emhofer,David Lennon,Günther Rupprechter

    γ-Alumina is widely used as an oxide support in catalysis, and palladium nanoparticles supported by alumina represent one of the most frequently used dispersed metals. The surface sites of the catalysts are often probed via FTIR spectroscopy upon CO adsorption, which may result in the formation of surface carbonate species. We have examined this process in detail utilizing FTIR to monitor carbonate formation on γ-alumina and zirconia upon exposure to isotopically labelled and unlabelled CO and CO2. The same was carried out for well-defined Pd nanoparticles supported on Al2O3 or ZrO2. A water gas shift reaction of CO with surface hydroxyls was detected, which requires surface defect sites and adjacent OH groups. Furthermore, we have studied the effect of Cl synthesis residues, leading to strongly reduced carbonate formation and changes in the OH region (isolated OH groups were partly replaced or were even absent). To corroborate this finding, samples were deliberately poisoned with Cl to an extent comparable to that of synthesis residues, as confirmed by Auger electron spectroscopy. For catalysts prepared from Cl-containing precursors a new CO band at 2164 cm-1 was observed in the carbonyl region, which was ascribed to Pd interacting with Cl. Finally, the FTIR measurements were complemented by quantification of the amount of carbonates formed via chemisorption, which provides a tool to determine the concentration of reactive defect sites on the alumina surface.

    更新日期:2019-11-01
  • Recent Advances in the Pauson-Khand Reaction.
    Top. Catal. (IF 2.226) Pub Date : 2017-10-24
    J David Ricker,Laina M Geary

    The Pauson-Khand [2+2+1] cycloaddition of alkynes, alkenes, and carbon monoxide has been a vibrant area of research for more than 40 years. This review highlights recent achievements in the Pauson-Khand reaction, particularly in catalytic and asymmetric variants. Discussion of regioselectivity and advances in substrate scope is also presented.

    更新日期:2019-11-01
  • Surface Spectroscopy on UHV-Grown and Technological Ni-ZrO2 Reforming Catalysts: From UHV to Operando Conditions.
    Top. Catal. (IF 2.226) Pub Date : 2016-12-31
    Kresimir Anic,Astrid Wolfbeisser,Hao Li,Christoph Rameshan,Karin Föttinger,Johannes Bernardi,Günther Rupprechter

    Ni nanoparticles supported on ZrO2 are a prototypical system for reforming catalysis converting methane to synthesis gas. Herein, we examine this catalyst on a fundamental level using a 2-fold approach employing industrial-grade catalysts as well as surface science based model catalysts. In both cases we examine the atomic (HRTEM/XRD/LEED) and electronic (XPS) structure, as well as the adsorption properties (FTIR/PM-IRAS), with emphasis on in situ/operando studies under atmospheric pressure conditions. For technological Ni-ZrO2 the rather large Ni nanoparticles (about 20 nm diameter) were evenly distributed over the monoclinic zirconia support. In situ FTIR spectroscopy and ex situ XRD revealed that even upon H2 exposure at 673 K no full reduction of the nickel surface was achieved. CO adsorbed reversibly on metallic and oxidic Ni sites but no CO dissociation was observed at room temperature, most likely because the Ni particle edges/steps comprised Ni oxide. CO desorption temperatures were in line with single crystal data, due to the large size of the nanoparticles. During methane dry reforming at 873 K carbon species were deposited on the Ni surface within the first 3 h but the CH4 and CO2 conversion hardly changed even during 24 h. Post reaction TEM and TPO suggest the formation of graphitic and whisker-type carbon that do not significantly block the Ni surface but rather physically block the tube reactor. Reverse water gas shift decreased the H2/CO ratio. Operando studies of methane steam reforming, simultaneously recording FTIR and MS data, detected activated CH4 (CH3 and CH2), activated water (OH), as well as different bidentate (bi)carbonate species, with the latter being involved in the water gas shift side reaction. Surface science Ni-ZrO2 model catalysts were prepared by first growing an ultrathin "trilayer" (O-Zr-O) ZrO2 support on an Pd3Zr alloy substrate, and subsequently depositing Ni, with the process being monitored by XPS and LEED. Apart from the trilayer oxide, there is a small fraction of ZrO2 clusters with more bulk-like properties. When CO was adsorbed on the (fully metallic) Ni particles at pressures up to 100 mbar, both PM-IRAS and XPS indicated CO dissociation around room temperature and blocking of the Ni surface by carbon (note that on the partially oxidized technological Ni particles, CO dissociation was absent). The Ni nanoparticles were stable up to 550 K but annealing to higher temperatures induced Ni migration through the ultrathin ZrO2 support into the Pd3Zr alloy. Both approaches have their benefits and limitations but enable us to address specific questions on a molecular level.

    更新日期:2019-11-01
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