Reaction mechanism and electrochemical characterization of a Sn-Co-C composite anode for Li-ion batteries

A Sn-Co-C composite is prepared by mechanochemical synthesis using tin powder and a cobalt-carbon composite through the pyrolysis of Co (III)-acetylacetonate. The composite is studied as an anode material for Li secondary batteries. The reaction mechanism is investigated using various analytical techniques. Although the composite is initially composed of Co₃Sn₂ as the major phase and CoSn₂ as the minor one, the Co₃Sn₂ transformed into CoSn₂ during the second cycle and remained in this form throughout the following cycles. The Sn-Co-C composite shows an excellent capacity retention of 435 mAh g⁻¹ over 100 cycles.     

A theoretical study of the initial stages of the hydrogenation of Si(111) 7×7

We have used the periodic extended Hückel method to examine the initial stages of the hydrogenation of Si(111) 7×7. Energies, atomic charges and reduced overlap populations are considered, and we discuss the inequivalence of the adatoms. The adatom dangling bond (DB) level positions with respect to E_F are also discussed. On the basis of our results, we propose the following adsorption sequence: first, saturation of the five occupied adatom DBs, then hydrogenation of the remaining atoms by pairs, the hydrogenation of an adatom being coupled with that of a restatom or that of the corner atom. We also show that the atomic charges of the surface become more uniform when the 19 DBs are saturated, and that this modification occurs in discontinuous stages.     

The Reaction Between Soot and NO2 – Investigation on Functional Groups Using TPD-MS

Three different soot samples, two from a Diesel engine and one commercially available, were subjected to a detailed laboratory investigation using temperature programmed desorption mass spectroscopy (TPD-MS) as major technique. Additional information was obtained by nitrogen adsorption (BET) and diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS). Surface changes due to a high temperature reaction with NO₂ as oxidizing agent are of particular interest. Depending on the reaction temperature different functional groups are formed on the surface. An attempt has been made to find an adequate reaction temperature for the conversion with NO₂.     

Enhanced Sulfur Tolerance of Bimetallic PtPd/Al2O3 Catalysts for Hydrogenation of Tetralin by Addition of Fluorine

A series of mono- and bi-metallic Pt-Pd/Al₂O₃ samples with and without F were studied as aromatic hydrogenation catalysts. The effects of changing the order of impregnation of the Pt precursor and F as well as varying the calcination temperature (300–500 °C) were investigated. Temperature programmed reduction (TPR) results demonstrate the presence of a higher fraction of dispersed metal precursor species left on the surface from the impregnation (PtO _x Cl _y ) on the Pt/Al₂O₃ sample calcined at high temperature. The impregnation of F before the Pt precursor significantly decreases the interaction between the metal and the support. However, this decrease is not observed when F is impregnated after the metal precursor. For the bimetallic Pt-Pd catalysts, the sample prepared adding F before the metal show a higher degree of Pt-Pd interaction than either the parent Pt-Pd/Al₂O₃ catalyst or the one prepared with F added later. TPD of ammonia result show the increase in strong acid sites when F is present. Activity tests for tetralin hydrogenation in the presence of 350 ppm dibenzothiophene indicate a better sulfur tolerance for all F-promoted catalysts, especially Pt-Pd.     

The effects of a potassium overlayer on the reaction pathway of CH2 and C2H5 on Rh(111)

The effect of potassium on the reaction pathways of adsorbed CH₂ and C₂H₅ species on Rh(111) was investigated by means of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TDS). Hydrocarbon fragments were produced by thermal and photo-induced dissociation of the corresponding iodo compounds. Potassium adatoms markedly stabilized the adsorbed CH₂ and converted it into C₂H₄, the formation of which was not observed for K-free Rh(111). New routes of the surface reactions of C₂H₅ have been also opened in the presence of potassium, namely its transformation into butane and butene.     

Nano ZrP2O7 Catalyzed Multicomponent Reaction for an Easy Access of 4H-pyrans and 1,4-dihydropyridines

ZrP₂O₇ nanoparticle catalyzed one-pot synthesis of 4H-pyran scaffolds installing a one-pot three-component coupling reaction of an aldehyde, malononitrile, and ethyl acetoacetate. Also the synthesis of 1,4-dihydropyridines was investigated by using aldehydes, ethyl acetoacetate and ammonium acetate as utilizing nano structured ZrP₂O₇ as an efficient catalyst in ethanol. The attractive features of this process are mild reaction conditions, reusability of the catalyst, short reaction times, easy isolation of the products, and excellent yields.     

Pd nanoparticle enhanced re-oxidation of non-stoichiometric TiO2: STM imaging of spillover and a new form of SMSI

We have used STM imaging in situ to demonstrate two fundamental steps in catalytic processes on model catalysts at elevated temperature. We show that Pd nanoparticles on sub-stoichiometric TiO₂(110) dissociatively adsorb O₂ at 673 K which spills over onto the support where further reaction takes place. The spillover oxygen re-oxidises the surface by removing Tiⁿ⁺ interstitial ions trapped in the crystal lattice, preferentially re-growing TiO₂ around and over the particles. The identification of the metal enhanced re-oxidation mechanism may have important and general consequences for the understanding of catalysis and gas sensing.     

NO2 inhibits the catalytic reaction of NO and O2 over Pt

The rate equation for the overall reaction of NO and O₂ over Pt/Al₂O₃ was determined to be r=k_f[NO] ^1.05±0.08[O₂]^1.03±0.08[NO₂]^0.92±0.07(1-), with k_f as the forward rate constant, =([NO₂]/K[NO][O₂]^1/2), and K as the equilibrium constant for the overall reaction. An apparent activation energy of 82 kJ mol^–1 ± 9 kJ mol^–1 was observed. The inhibition by the product NO₂ makes it imperative to include the influence of NO₂ concentration in any analysis of the kinetics of this reaction. The reaction mechanism that fits our observed orders consists of the equilibrated dissociation of NO₂ to produce a surface mostly covered by oxygen, thereby inhibiting the equilibrium adsorption of NO, and the non-dissociative adsorption of O₂, which is the proposed rate determining step.     

Reduction of NO2 stored in a commercial lean NO x trap at low temperatures

Isothermal storage of NO₂ and subsequent reduction with different reducing agents (H₂, CO or H₂ + CO) in a lean NO _x trap catalyst was tested by Temperature Programmed Desorption (TPD) and Temperature Programmed Reduction (TPR) experiments at temperatures representative of automotive “cold-start” conditions (<200 °C) using a commercial NO _x trap catalyst. Results from the TPR experiments revealed that no reduction of stored NO₂ to N₂ was observed at 100–180 °C, and at 200 °C 10% reduction only of NO₂ to N₂ was measured. A special affinity of H₂ to form NH₃ was observed during the reduction of stored NO₂. The formation of NH₃ increases with increasing amount of stored NO₂ and decreases with increasing storage temperature. Direct relation exists between the amount of adsorbed and/or stored NO₂ and the formation of H₂O and NH₃.     

XPS analysis of wet chemical etching of GaAs(110) by Br2–H2O: comparison of emersion and model experiments

We have applied photoelectron spectroscopy to investigate the surface composition after different surface treatments involving Br₂–H₂O mixtures in order to study wet chemical etching. Emersion experiments from Br₂–H₂O solution are compared with model experiments, in which Br₂–H₂O adsorbate and coadsorbate mixtures react with clean GaAs(110) surfaces. Our results indicate that Ga- and As-bromides formed initially are hydrolyzed to form the respective oxides. Without addition of Br₂, only slight oxidation of the surface takes place. There is an enrichment of Ga due to loss of As both in adsorption as well as in emersion experiments. Since in emersion experiments only a final situation is analyzed, the relative influence of surface reactivity and subsequent solvation effects cannot be distinguished easily, while model experiments give clear information on reaction products formed intermediately. However, model experiments differ in environment and temperature from the real solid–liquid interface. The presented results demonstrate that a combination of emersion and model experiments provide valuable insight into the mechanism of wet chemical etching on a microscopic level.     

HREELS study of photo-induced formation of CO2 anion radical on Rh(111) surface

High-resolution electron loss spectroscopy revealed, probably for the first time, that the illumination of adsorbed CO₂ on K-promoted Rh(111) induces or enhances formation of the CO₂ radical.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Investigation of CO2 Reaction with CaO and an Acid Washed Lime in a Packed-Bed Reactor

In this work, a mathematical model was developed for the prediction of packed-bed reactor behavior for CaO+CO₂ reaction based on the random pore model. A natural limestone and a modified sorbent using acetic acid washing were used for the experiments. The performances of these sorbents were initially determined using a thermogravimeter analyzer. Then, the reaction was accomplished in a packed-bed reactor for obtaining CO₂ breakthrough curves and investigation of model predictions. This model was able to successfully predict the effect of process conditions and solid texture on the breakthrough curves of the packed-bed reactor.     

Nano-sized crystallites of vanadyl pyrophosphate as a highly selective catalyst for n-butane oxidation

Exfoliation-reduction of VOPO₄ · 2H₂O in a mixed alcohol consisting of 2-butanol and ethanol, followed by the thermal treatment in the presence of n-butane, O₂, and He at 663 K for 300 h, produces novel nano-sized crystallites (~ 50 nm) of (VO)₂P₂O₇. The nano-sized (VO)₂P₂O₇ crystallites exhibit a high selectivity to maleic anhydride (~ 84%) for the selective oxidation of n-butane.     

Oxidation of dicyclopentadiene catalyzed by palladium(II) acetate and p-benzoquinone in the presence of inorganic acid

The oxidation of dicyclopentadiene catalyzed by palladium(II) acetate and benzoquinone in the presence of perchloric acid was studied. Tricyclodecenone in high selectivity (85–98%) at a conversion of dicyclopentadiene up to 76% was obtained. The kinetic model assumed the significant inhibition complexation between dicyclopentadiene and tricyclodecenone with the catalytic species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ethylene combustion on unsupported and supported Pd: a comparative study

Ethylene combustion is studied at elevated pressures on Pd(100) and compared with supported Pd catalysts. Specific rates of CO₂ formation were determined on Pd(100), 1 wt% (d_Pd = 2.5 nm) and 5 wt% (d_Pd = 4.2 nm) Pd/SiO₂ catalysts as a function of the O₂/C₂H₄ ratio over the temperature range 428–473 K. Reaction rates and apparent activation energies on these catalysts are particle size and reactant ratio dependent. The surface carbon concentration on Pd(100) depends on the ratio of the reactants and decreases with increasing O₂ pressure. The combustion kinetics is related to the corresponding surface carbon coverage; the highest reaction rate was achieved for relatively low coverage of surface carbon.     

Deactivation and coke formation on palladium and platinum catalysts in vegetable oil hydrogenation

Deactivation of palladium and platinum catalysts due to coke formation was studied during hydrogenation of methyl esters of sunflower oil. The supported metal catalysts were prepared by impregnating γ-alumina with either palladium or platinum salts, and by impregnating α-alumina with palladium salt. The catalysts were reused for several batch experiments. The Pd/γ-Al₂O₃ catalyst lost more than 50% of its initial activity after four batch experiments, while the other catalysts did not deactivate. Samples of used catalysts were cleaned from remaining oil by repeated extractions with methanol, and the amount of coke formed on the catalysts was studied by temperature-programmed oxidation. The deactivation of the catalyst is a function of both the metal and the support. The amount of coke increased on the Pd/γ-Al₂O₃ catalyst with repeated use, but the amount of coke remained approximately constant for the Pt/γ-Al₂O₃ catalyst. Virtually no coke was detected on the Pd/α-Al₂O₃ catalyst. The formation of coke on Pd/α-Al₂O₃ may be slower than on the Pd/γ-Al₂O₃ owing to the carrier’s smaller surface area and less acidic character. The absence of deactivation for the Pt/γ-Al₂O₃ catalyst may be explained by slower formation of coke precursors on platinum compared to palladium.     

Spectroscopic and Kinetic Analysis of a New Low-Temperature Methanol Synthesis Reaction

The spectroscopy and kinetics of a new low-temperature methanol synthesis method were studied by using in situ DRIFTS on Cu/ZnO catalysts from syngas (CO/CO₂/H₂) using alcohol promoters. The adsorbed formate species easily reacted with ethanol or 2-propanol at 443 K and atmospheric pressure, and the reaction rate with 2-propanol was faster than that with ethanol. Alkyl formate was easily reduced to form methanol at 443 K and 1.0 MPa, and the hydrogenation rate of 2-propyl formate was found to be faster than that of ethyl formate. 2-Propanol used as promoter exhibited a higher activity than ethanol in the reaction of the low-temperature methanol synthesis.     

Effect of CeO2 and La2O3 on the Activity of CeO2−La2O3/Al2O3-Supported Pd Catalysts for Steam Reforming of Methane

The effect of the addition of CeO₂ or La₂O₃ on the surface properties and catalytic behaviors of Al₂O₃-supported Pd catalysts was studied in the steam reforming of methane. The FTIR spectroscopy of adsorbed CO and the Pd dispersion suggest the partial coverage of Pd⁰ by ceria or lanthana species. This could lead to the formation of an adduct MPd _x O (M = Ce or La) at the surface of the metal crystallites. The addition of ceria or lanthana resulted in an increase of the turnover rate and specific rate for steam reforming of methane. One possible explanation if that the Pd⁰*Pd^δ+O–M interfacial species (M = Ce or La) are oxidized by H₂O or CO₂, promoting the O* transfer to the metal surface. This could facilitate the removal of C* species from the metal surface, resulting in the increase of specific reaction rate and increase of the accessibility of CH₄ to metal active sites.     

Synthesis of Dimethyl Carbonate from Methanol and Carbon dioxide using Potassium Methoxide as Catalyst under Mild Conditions

Pd-supported on WO₃–ZrO₂ (W/Zr atomic ratio=0.2) calcined at 1073 K was found to be highly active and selective for gas-phase oxidation of ethylene to acetic acid in the presence of water at 423 K and 0.6 MPa. Contact time dependence demonstrated that acetic acid is formed via acetaldehyde formed by a Wacker-type reaction, not through ethanol by hydration of ethylene.     

Semiconductive and Redox Properties of Ti and Zr Pyrophosphate Catalysts (TiP2O7 and ZrP2O7). Consequences for the Oxidative Dehydrogenation of n-Butane

The electrical conductivity of titanium and zirconium pyrophosphates used as catalysts in n-butane oxidative dehydrogenation has been measured under oxygen and n-butane at 400 and 500 °C and under subsequent exposures to both gases at the catalytic reaction temperature. The two compounds appeared to be p-type semiconductors under air with positive holes as the main charge carriers but became n-type when contacted with n-butane. If their conductivities are comparable as p-type semiconductors (within one order of magnitude), by contrast, they differ by 3 orders of magnitude when being n-type semiconductors. These results explain the difference in catalytic reaction mechanism encountered on the two solids. The alkane activation was proposed to be related in both cases to the p-type semiconducting properties of the solids, likely through hydrogen abstraction by a surface O^- species, forming a C₄H₉ radical which will similarly undergo a second hydrogen abstraction to form butenes. The changes in activation energy and in selectivity on TiP₂O₇ at higher temperatures (>450 C) are indicative of a change in mechanism, possibly with the transient formation of an alkoxide species.