Showing papers on "Reaction rate published in 2010"

Fundamental studies of methanol synthesis from CO(2) hydrogenation on Cu(111), Cu clusters, and Cu/ZnO(0001).

Abstract: A combination of experimental and theoretical methods were employed to investigate the synthesis of methanol via CO2 hydrogenation (CO2 + 3H2 → CH3OH + H2O) on Cu(111) and Cu nanoparticle surfaces High pressure reactivity studies show that Cu nanoparticles supported on a ZnO(000) single crystal exhibit a higher catalytic activity than the Cu(111) planar surface Complementary density functional theory (DFT) calculations of methanol synthesis were also performed for a Cu(111) surface and unsupported Cu29 nanoparticles, and the results support a higher activity for Cu nanoparticles The DFT calculations show that methanol synthesis on Cu surfaces proceeds through a formate intermediate and the overall reaction rate is limited by both formate and dioxomethylene hydrogenation Moreover, the superior activity of the nanoparticle is associated with its fluxionality and the presence of low-coordinated Cu sites, which stabilize the key intermediates, eg formate and dioxomethylene, and lower the barrier for the rate-limiting hydrogenation process The reverse water-gas-shift (RWGS) reaction (CO2 + H2 → CO + H2O) was experimentally observed to compete with methanol synthesis and was also considered in our DFT calculations In agreement with experiment, the rate of the RWGS reaction on Cu nanoparticles is estimated to be ∼2 orders of magnitude faster than methanol synthesis at T = 573 K The experiments and calculations also indicate that CO produced by the fast RWGS reaction does not undergo subsequent hydrogenation to methanol, but instead simply accumulates as a product Methanol production from CO hydrogenation via the RWGS pathway is hindered by the first hydrogenation of CO to formyl, which is not stable and prefers to dissociate into CO and H atoms on Cu Our calculated results suggest that the methanol yield over Cu-based catalysts could be improved by adding dopants or promoters which are able to stabilize formyl species or facilitate the hydrogenation of formate and dioxomethylene

Photocatalysis fundamentals revisited to avoid several misconceptions

Abstract: Photocatalysis has presently become a major discipline owing to two factors: (i) the intuition of the pioneers of last 20th century and (ii) the mutual enrichment of scientists arising from different fields: photochemistry, electrochemistry, analytical chemistry, radiochemistry, material chemistry, surface science, electronics, and hopefully catalysis. Since heterogeneous photocatalysis belongs to catalysis, all the bases of this discipline must be respected: (i) proportionality of the reaction rate to the mass of catalyst (below the plateau due to a full absorption of photons); (ii) implication of the Langmuir–Hinshelwood mechanism of kinetics with the initial rate being proportional to the coverages θ in reactants;(iii) conversions obtained above the stoichiometric threshold defined as the maximum number of potential active sites initially present at the surface of a mass m of titania used in the reaction. In addition, one should respect photonics, with the photocatalytic activity, i.e. the reaction rate being (i) parallel to the absorbance of the photocatalyst and (ii) proportional to the radiant flux Φ. In every study, one should determine the quantum yield (QY) (or efficiency), which, although dimensionless, is a “doubly kinetic” magnitude defined as the ratio of the reaction rate r (in molecules converted/second) to the efficient photonic flux (in photons/second) received by the solid. This is an instantaneous magnitude directly linked to the parameters mentioned above, in particular to the concentration. It can vary from a maximum value of ca. 40% in pure liquid phase to very low values (10−2%) in diluted media (pollutants trace eliminations). To establish true photocatalytic normalized tests, the above recommendations must be observed with a real catalytic activity independent of non-catalytic side-reaction. In particular, dye decolorization, especially in the visible, provides an apparent “disappearance” of the dye, due to a limited stoichiometric electron transfer from the photo-excited dye molecule to titania, subsequently compensated by an additional ionosorption of molecular oxygen.The energetics of photocatalysis on TiO2, being based on the energy E of the photons, i.e. E ≥ 3.2 eV, enables one to produce OH radicals, the second best oxidizing agent. The decrease of energy E to the visible may be thermodynamically detrimental for the generation of such highly cracking and degrading species. Concerning solid state chemistry, it is now finally admitted that cationic doping is detrimental for photocatalysis. In conclusion, all these recommendations have to be addressed and experiments have to be operated in suitable conditions before claiming that one deals with a true photocatalytic reaction.

Controlled selectivity for palladium catalysts using self-assembled monolayers

Abstract: The selective reaction of one part of a bifunctional molecule is a fundamental challenge in heterogeneous catalysis and for many processes including the conversion of biomass-derived intermediates. Selective hydrogenation of unsaturated epoxides to saturated epoxides is particularly difficult given the reactivity of the strained epoxide ring, and traditional platinum group catalysts show low selectivities. We describe the preparation of highly selective Pd catalysts involving the deposition of n-alkanethiol self-assembled monolayer (SAM) coatings. These coatings improve the selectivity of 1-epoxybutane formation from 1-epoxy-3-butene on palladium catalysts from 11 to 94% at equivalent reaction conditions and conversions. Although sulphur species are generally considered to be indiscriminate catalyst poisons, the reaction rate to the desired product on a catalyst coated with a thiol was 40% of the rate on an uncoated catalyst. Interestingly the activity decreased for less-ordered SAMs with shorter chains. The behaviour of SAM-coated catalysts was compared with catalysts where surface sites were modified by carbon monoxide, hydrocarbons or sulphur atoms. The results suggest that the SAMs restrict sulphur coverage to enhance selectivity without significantly poisoning the activity of the desired reaction.

Glyoxal processing by aerosol multiphase chemistry: towards a kinetic modeling framework of secondary organic aerosol formation in aqueous particles

Abstract: . This study presents a modeling framework based on laboratory data to describe the kinetics of glyoxal reactions that form secondary organic aerosol (SOA) in aqueous aerosol particles. Recent laboratory results on glyoxal reactions are reviewed and a consistent set of empirical reaction rate constants is derived that captures the kinetics of glyoxal hydration and subsequent reversible and irreversible reactions in aqueous inorganic and water-soluble organic aerosol seeds. Products of these processes include (a) oligomers, (b) nitrogen-containing products, (c) photochemical oxidation products with high molecular weight. These additional aqueous phase processes enhance the SOA formation rate in particles and yield two to three orders of magnitude more SOA than predicted based on reaction schemes for dilute aqueous phase (cloud) chemistry for the same conditions (liquid water content, particle size). The application of the new module including detailed chemical processes in a box model demonstrates that both the time scale to reach aqueous phase equilibria and the choice of rate constants of irreversible reactions have a pronounced effect on the predicted atmospheric relevance of SOA formation from glyoxal. During day time, a photochemical (most likely radical-initiated) process is the major SOA formation pathway forming ∼5 μg m−3 SOA over 12 h (assuming a constant glyoxal mixing ratio of 300 ppt). During night time, reactions of nitrogen-containing compounds (ammonium, amines, amino acids) contribute most to the predicted SOA mass; however, the absolute predicted SOA masses are reduced by an order of magnitude as compared to day time production. The contribution of the ammonium reaction significantly increases in moderately acidic or neutral particles (5 Glyoxal uptake into ammonium sulfate seed under dark conditions can be represented with a single reaction parameter keffupt that does not depend on aerosol loading or water content, which indicates a possibly catalytic role of aerosol water in SOA formation. However, the reversible nature of uptake under dark conditions is not captured by keffupt, and can be parameterized by an effective Henry's law constant including an equilibrium constant Kolig = 1000 (in ammonium sulfate solution). Such reversible glyoxal oligomerization contributes Sensitivity tests reveal five parameters that strongly affect the predicted SOA mass from glyoxal: (1) time scales to reach equilibrium states (as opposed to assuming instantaneous equilibrium), (2) particle pH, (3) chemical composition of the bulk aerosol, (4) particle surface composition, and (5) particle liquid water content that is mostly determined by the amount and hygroscopicity of aerosol mass and to a lesser extent by the ambient relative humidity. Glyoxal serves as an example molecule, and the conclusions about SOA formation in aqueous particles can serve for comparative studies of other molecules that form SOA as the result of multiphase chemical processing in aerosol water. This SOA source is currently underrepresented in atmospheric models; if included it is likely to bring SOA predictions (mass and O/C ratio) into better agreement with field observations.

A Review of the Water Gas Shift Reaction Kinetics

Abstract: The world’s progression towards the Hydrogen economy is facilitating the production of hydrogen from various resources. In the carbon based hydrogen production, Water gas shift reaction is the intermediate step used for hydrogen enrichment and CO reduction in the synthesis gas. This paper makes a critical review of the developments in the modeling approaches of the reaction for use in designing and simulating the water gas shift reactor. Considering the fact that the rate of the reaction is dependent on various parameters including the composition of the catalyst, the active surface and structure of the catalyst, the size of the catalyst, age of the catalyst, its operating temperature and pressure and the composition of the gases, it is difficult to narrow down the expression for the shift reaction. With different authors conducting experiments still to validate the kinetic expressions for the shift reaction, continuous research on different composition and new catalysts are also reported periodically. Moreover the commercial catalyst manufacturers seldom provide information on the catalyst. This makes the task of designers difficult to model the shift reaction. This review provides a consolidated listing of the various important kinetic expressions published for both the high temperature and the low temperature water gas shift reaction along with the details of the catalysts and the operating conditions at which they have been validated.

Non-Oxidative Vanadium-Catalyzed CO Bond Cleavage: Application to Degradation of Lignin Model Compounds

Abstract: Lignocellulosic biomass has recently received great interest as a renewable source of fuel and chemicals.[1] Among the three major components of non-edible lignocellulose (cellulose, hemicellulose, and lignin), extensive efforts have been made to convert cellulose to ethanol and other biofuels. In contrast, research on the conversion of lignin has been limited to its removal from biomass either to enhance the accessibility of chemicals and enzymes to other components of biomass or to prevent photo-yellowing of paper and pulp. Despite the fact that lignin corresponds up to 30% of the weight and 40% of the energy content of lignocellulosic biomass, few novel processes aimed at producing high value compounds have been reported. Recently, several reports inspired by the pulp bleaching process have been published regarding the mechanism and product distribution of enzymatic and chemical oxidation reactions.[2] Using dimeric lignin model compounds (e.g. 1) containing a β-O-4 linkage that represents the most common substructure in lignin,[3] aromatic aldehydes were obtained as the main products in low yield. Although these methods show promises for selective conversion of lignin, fundamentally new catalytic processes need to be developed to fully realize lignin's potential as a chemical feedstock. In addition, thorough understanding of the mechanism of these processes is necessary to successfully achieve high selectivity. Aiming to develop a novel method to selectively convert lignin to highly functionalized aromatic compounds, we explored various homogeneous vanadium complexes for the conversion of 1 (Table 1).[4] Most of the vanadium catalysts tested yielded benzylic alcohol oxidation product 4 as the major product[5] in addition to small amounts of C–O bond cleavage products 2 and 3 (entries 2–7). In spite of the low yield, the formation of 2 distinguishes this reaction from previous reports: not only is 2 a novel product, but it is also a redox-neutral transformation. Excited by this new reactivity, we explored other vanadium catalysts and found that tridendate Schiff base ligands favor C–O bond cleavage over benzylic oxidation (entries 8–11). Higher selectivity for C–O bond cleavage was observed when ligands with larger bite angles were employed (entries 8 vs. 9 and 10 vs. 11).[6] The increased reactivity of catalyst 11 compared to 9 (entry 11 vs. 9) may be attributed to its tBu substituents, which enable intermediates from 11 to remain as catalytically active monomeric species instead of forming insoluble aggregates.[7] Thus, through subtle changes in the ligand structure, the reactivity of the vanadium(V)-oxo catalyst was tuned away from simple alcohol oxidation toward the cleavage of the β-O-4 carbon-oxygen bond. Table 1 Ligand effects on degradation of lignin model compound 1. To understand the role of oxygen in the formally non-oxidative process, the reaction was performed under anaerobic conditions with catalyst 11. After 24 h, the same products were obtained as those under aerobic conditions, albeit with lower conversion, along with pale purple precipitate (Scheme 1). The collected precipitate exhibited the same analytical properties as independently prepared V(IV) complex 12.[8] The EPR spectrum of the dark purple reaction mixture under aerobic conditions with 11 after 30 minutes also indicated the presence of V(IV) species. The V(IV) complex 12 is insoluble in various organic solvents and stable under air in the solid state. However, a heterogeneous reaction mixture with 12 turns into a dark purple solution under aerobic reaction conditions and furnishes a similar result to that with 11: >95% conversion, 80% 2, 58% 3, and 1% 4 (Scheme 2). These results indicate that the V(V) catalyst is reduced to V(IV) species during the reaction, but oxygen is not essential for the catalyst turnover, although it does increase the reaction rate. The facile interconversion between 11 and 12 allows one to employ the high reactivity of a homogeneous catalyst during the reaction and then easily recover the vanadium catalyst as an insoluble complex after the reaction by simply controlling the reaction atmosphere. Scheme 1 Degradation of 1 under anaerobic conditions. Scheme 2 Recovery of the reactivity of V(IV) complex 12 under aerobic conditions. To gain further insight on the mechanism of the non-oxidative C–O bond cleavage, we performed the reaction with various derivatives of 1 (Scheme 3). To test the possibility of oxidation of 1 to ketone 4 followed by reductive cleavage, 4 was subjected to the reaction conditions with or without benzylic alcohol 13. In both cases, 4 was recovered in high yield without any evidence of degradation to 2 or 3. The corresponding ketone 14 was obtained when 13 was added to reproduce the catalytic species formed after oxidation of 1 to 4. The lack of reactivity of 4 under the reaction conditions eliminates the possibility of a sequential oxidation and cleavage process of 1. Additionally, this experiment suggests that a V(III)–V(V) cycle,[9] which has been proposed for some vanadium-catalyzed aerobic alcohol oxidation,[4a,4c,4d] is likely not operational for this transformation. When the benzylic hydroxy group was replaced by a methoxy group (1a), the reaction proceeded with only 12% conversion, to afford conjugated aldehyde 15 as the product, indicating the importance of ligand exchange on 11 with the benzylic hydroxy group. In contrast, methyl ether 1b was converted to 2 and 3 with only slightly diminished yield and selectivity compared to 1. When the aryloxy group was absent (1c), only the corresponding ketone 16 was observed. Scheme 3 Reactivity study of analogues of 1. Based on these data, we propose a one-electron process as the most plausible mechanism (Figure 1). After ligand exchange on the vanadium complex with the benzylic hydroxy group, the benzylic hydrogen is abstracted to generate the ketyl radical, which eliminates the aryloxy radical.[10] The elimination of the hydroxy group from the resulting enolate produces enone 2 and a vanadium(IV) complex (17a or 17b), which is re-oxidized to vanadium(V) by the aryloxy radical.[11] Degradation of lignin via a ketyl radical had been suggested by several groups as a mechanism responsible for photo-yellowing of paper.[12] However, this hypothesis was later disputed because the ketyl radical generated by various methods reacted readily with oxygen to produce the corresponding ketone, and only a small amount of fragmentation products was formed by secondary photolysis of this ketone.[13] The high yield and good selectivity observed for β-O-4 cleavage of vanadium-catalyzed reaction indicates a dramatic reactivity change of the ketyl radical by coordination to vanadium. Figure 1 Plausible mechanism for vanadium-catalyzed non-oxidative cleavage of 1. Formation of the insoluble V(IV) complex 12 and only a slightly lower turnover number under anaerobic conditions suggests that regeneration of a V(V) complex by the aryloxy radical competes with formation of an insoluble V(IV) aggregate. Molecular oxygen probably accelerates the reaction by increasing the effective concentration of catalytically active species by converting insoluble 12 to soluble V(V) species. Although the vanadium-catalyzed non-oxidative degradation of 1 proceeds with high efficiency and selectivity in CH3CN, it is desirable to utilize solvents without nitrogen atoms for application to biofuel synthesis to prevent formation of NOx. When the reaction was performed in EtOAc on 1 mmol scale, the desired products were obtained with higher yields and selectivity than reactions run in CH3CN (Scheme 4).[14] Moreover, a more complex trimeric model compound 18[2f, 15] underwent clean C–O cleavage to provide three monomeric compounds, demonstrating the possibility of application of this reaction to more complex systems (Scheme 5).[16] Scheme 4 Conversion of 1 on 1 mmol scale in a solvent without nitrogen atoms. Scheme 5 Conversion of trimeric lignin model compound 18. In conclusion, we have demonstrated that changes in the ancilliary ligands can divert the reactivity of vanadium-oxo complexes from the typical alcohol oxidation to an unprecedented C-O bond cleavage reaction. This novel reactivity has been applied to a vanadium-catalyzed non-oxidative C–O bond cleavage reactions of dimeric lignin model compounds that produces aryl enones as unprecedented degradation products. This transformation is proposed to proceed via a ketyl radical generated by hydrogen atom transfer to a vanadium(V) complex. Oxygen is not essential for the reaction, although it increases the reaction rate. The novel reactivity of this transformation combined with good selectivity for a highly functionalized aryl enone demonstrates a great potential of lignin as a chemical feedstock. Further mechanistic studies and applications to biomass degradation are underway.

Impurity and alloying effects on interfacial reaction layers in Pb-free soldering

Abstract: The objective of this review is to study the effect of minor alloying and impurity elements, typically present in electronics manufacturing environment, on the interfacial reactions between Sn and Cu, which is the base system for Pb-free soldering. Especially, the reasons leading to the observed interfacial reaction layers and their microstructural evolution are analysed. The following conclusions have been reached. Alloying and impurity elements can have three major effects on the reactions between the Sn-based solder and the conductor metal: Firstly, they can increase or decrease the reaction/growth rate. Secondly, additives can change the physical properties of the phases formed (in the case of Cu and Sn, ɛ and η). Thirdly they can form additional reaction layers at the interface or they can displace the binary phases that would normally appear and form other reaction products instead. Further, the alloying and impurity elements can be divided roughly into two major categories: (i) elements (Ni, Au, Sb, In, Co, Pt, Pd, and Zn) that show marked solubility in the intermetallic compound (IMC) layer (generally take part in the interfacial reaction in question) and (ii) elements (Bi, Ag, Fe, Al, P, rare-earth elements, Ti and S) that are not extensively soluble in IMC layer (only change the activities of species taking part in the interfacial reaction and do not usually participate themselves). The elements belonging to category (i) usually have the most pronounced effect on IMC formation. It is also shown that by adding appropriate amounts of certain alloying elements to Sn-based solder, it is possible to tailor the properties of the interfacial compounds to exhibit, for example, better drop test reliability. Further, it is demonstrated that if excess amount of the same alloying elements are used, drastic decrease in reliability can occur. The analysis for this behaviour is based on the so-called thermodynamic–kinetic method.

Tailored ligand acceleration of the Cu-catalyzed azide-alkyne cycloaddition reaction: practical and mechanistic implications.

Abstract: Tris(heterocyclemethyl)amines containing mixtures of 1,2,3-triazolyl, 2-benzimidazoyl, and 2-pyridyl components were prepared for ligand acceleration of the copper-catalyzed azide−alkyne cycloaddition reaction. Two classes of ligands were identified: those that give rise to high reaction rates in coordinating solvents but inhibit the process when used in excess relative to copper and those that provide for fast catalysis in water and are not inhibitory in excess. Several “mixed” ligands were identified that performed well under both types of conditions. Kinetics measurements as a function of ligand:metal ratio and catalyst concentration were found to be consistent with an active Cu2L formulation. Since strongly bound chelating agents are not always the most effective, achieving optimal rates requires an assessment of the potential donor molecules in the reaction mixture. Simple rules are provided to guide the user in the choice of effective ligands and reaction conditions to suit most classes of substrate...

NOx photocatalytic degradation employing concrete pavement containing titanium dioxide

Abstract: In the present work the degradation of nitrogen oxides (NOx) by concrete paving stones containing TiO2 to be applied in road construction is studied. A kinetic model is proposed to describe the photocatalytic reaction of NOx (combining the degradation of NO and the appearance and disappearance of NO2) in a standard laminar flow photoreactor irradiated with UV lamps employing only NO as the contaminant source. In addition, the influences of several parameters that can affect the performance of these stones are investigated, such as NO inlet concentration, reactor height and flow rate. The kinetic parameters present in the NO and NO2 reaction rate are estimated employing experimental data obtained in the photoreactor. The obtained model predictions employing the determined kinetic constants are in good agreement with the experimental results of NO and NO2 concentration at the reactor outlet.

Role of Temperature in the Growth of Silver Nanoparticles Through a Synergetic Reduction Approach

Abstract: This study presents the role of reaction temperature in the formation and growth of silver nanoparticles through a synergetic reduction approach using two or three reducing agents simultaneously. By this approach, the shape-/size-controlled silver nanoparticles (plates and spheres) can be generated under mild conditions. It was found that the reaction temperature could play a key role in particle growth and shape/size control, especially for silver nanoplates. These nanoplates could exhibit an intensive surface plasmon resonance in the wavelength range of 700–1,400 nm in the UV–vis spectrum depending upon their shapes and sizes, which make them useful for optical applications, such as optical probes, ionic sensing, and biochemical sensors. A detailed analysis conducted in this study clearly shows that the reaction temperature can greatly influence reaction rate, and hence the particle characteristics. The findings would be useful for optimization of experimental parameters for shape-controlled synthesis of other metallic nanoparticles (e.g., Au, Cu, Pt, and Pd) with desirable functional properties.

Catalytic hydroconversion of tricaprylin and caprylic acid as model reaction for biofuel production from triglycerides

Abstract: There is strong interest in the production of fuels from triglycerides of biological origin. In this work tricaprylin (TC) and caprylic acid (CA) were used as model compounds to study the catalytic hydroconversion process of triglycerides to acyclic aliphatic hydrocarbons. Supported metal and metal oxide catalysts, such as palladium on activated carbon (Pd/C), and promoted molybdena–alumina (Ni,Mo/γ-Al2O3) were used. The reaction was found to proceed in consecutive steps: hydrogenolysis (HYS) of TC to CA and propane, followed by hydrodeoxygenation (HDO) of the CA intermediate. The overall reaction rate was governed by the rate of the HDO reaction. Two distinct HDO routes were distinguished: (i) hydrodecabonylation and (ii) reduction of oxygen. Over Pd/C the prevailing reaction route of CA hydroconversion was the decarbonylation giving mainly C7 alkane and CO, whereas the HDO over the Ni,Mo/γ-Al2O3 catalysts proceeded in consecutive H2 addition and dehydration steps giving predominantly C8= alkenes, C8 alkanes and water. In reaction route (ii) alcohol and traces of aldehyde were detected as acid-to-alkane intermediates. Results suggest that reaction route (i) passes through formic acid intermediate that, in the presence of H2, rapidly decomposes to CO and H2O.

Catalysis by Au@pNIPAM Nanocomposites: Effect of the Cross-Linking Density

Abstract: Gold nanoparticles encapsulated in a thermoresponsive microgel (pNIPAM) were used as catalysts in the electron-transfer reaction between hexacyanoferrate(III) and borohydride ions. The thermosensitive pNIPAM network can act as a “nanogate” that can be opened or closed to a certain extent, thereby controlling the diffusion of reactants toward the catalytic core. Interestingly, the crosslinker density plays an important role, because it defines the thermal response of the Au@pNIPAM system and, in turn, the extent of the volume change and therefore the polymeric density. The catalytic activity of the encapsulated gold nanoparticles is thus affected both by temperature and by the composition of the shell. A mathematical model reproducing the key features of the temperature-controlled catalysis by our thermosensitive nanoparticles confirms the effect of diffusion rate through the shell on the actual reaction rate.

Pathways for Oxygen Incorporation in Mixed Conducting Perovskites: A DFT-Based Mechanistic Analysis for (La, Sr)MnO3−δ

Abstract: An extensive set of DFT calculations on LaMnO3 slabs has been generated and used as a basis to identify the most probable reaction mechanism for oxygen incorporation into (La, Sr)MnO3−δ cathode materials. MnO2[001] is found to be the most stable surface termination under fuel cell operation conditions (high temperature, high pO2, cubic unit cell). Chemisorption leading to the formation of O2−, O22−, and O− atop Mn is exothermic, but due to the negative adsorption entropy and electrostatic repulsion the levels of coverage of molecular oxygen adsorbates are low (in the few percent range). Under typical solid oxide fuel cell conditions, a mechanism in which the encounter of O− with a surface oxygen vacancy at the surface is rate-determining exhibits the fastest rate. The variation of the reaction rate and preferred mechanism(s) with adsorbate and point defect concentrations is discussed.

Strategy for Rapid and High-Purity Monocyclic Polymers by CuAAC “Click” Reactions

Abstract: Cyclization of linear polymers by coupling end-groups together to form monocyclic polymers using the very fast Cu-catalyzed azide/alkyne cycloaddition (CuAAC) “click” reaction has been used for many polymer systems. However, the strategy based on the CuAAC methodology has not been guided by theory and relies on the very slow feed of polymer into a highly dilute reaction mixture of solvent and Cu catalyst. This leads to the production of monocyclic polymer in very low concentrations over long periods of time (>10 h) and at high temperatures (>100 °C). In this work we use the Jacobson−Stockmayer theory to predict the % monocyclic polystyrene (c-PSTY) in a one-pot reaction at 25 °C and find from an empirical relationship based on experimental diffusion-controlled rate coefficients for cyclization and condensation of α,ω-polymer chains that the Jacobson−Stockmayer theory is applicable for the CuAAC reaction. This means the % monocyclic can be predicted from theory and is independent of reaction rate parameter...

The Rapid Formation of La(OH)3 from La2O3 Powders on Exposureto Water Vapor

Abstract: The reactivity of various lanthana powders in air was studied. The materials rapidly hydroxylate to form a stable hydroxide, La(OH)3, at room temperature. Smaller amounts of an oxycarbonate species (La2O2CO3) are also observed following air exposure. This oxycarbonate phase is stable to hydroxylation. All oxide materials synthesized here show rapid reactions so that 24 h of exposure to atmosphere is generally sufficient to cause complete hydroxylation at room temperature. The rate of reaction was related to the crystallite size as determined by XRD. The reaction was found to follow a two-stage kinetic process, a relatively slow surface reaction followed by a rapid bulk reaction. The relevance of the reactivity of these powders is discussed.

Chemical looping combustion of coke oven gas by using Fe2O3/CuO with MgAl2O4 as oxygen carrier

Abstract: Chemical looping combustion (CLC) is a new combustion technology that is clean and highly efficienct. The reactivity of four potential oxygen carrier particles composed of Fe2O3/CuO and MgAl2O4 have been investigated using thermogravimetric analysis (TGA) and a laboratory pressurized circulating fluidized bed system. According to the research, the following findings can be made: first, oxygen carrier particles consisting of 45% Fe2O3 and 15% CuO supported on 40% MgAl2O4 (F45C15M40) were found to be the best oxygen carrier for CLC of coke oven gas (COG). Secondly, results of multicycle reduction–oxidation tests in TGA showed that the reactivity of F45C15M40 oxygen carrier remained high and stable after 15 reduction–oxidation reaction cycles. Thirdly, the reaction temperature and cycle number showed a great effect on the reactivity of the oxygen carrier and this was attributed to the high chemical reaction rate of oxygen carrier and morphological change in CuO at high temperature. The pressurized CLC unit was in continuous operation with fuel for a duration of 15 h. The pressurized circulating fluidized bed system for CLC was found to work well with F45C15M40 oxygen carriers. It showed high reactivity in a fluidized bed reactor and gave a high conversion of the fuel, as well as reasonable crushing strength and resistance toward agglomeration and fragmentation. The maximum fuel conversion reached 92.33%.

Degradation of metronidazole by nanoscale zero-valent metal prepared from steel pickling waste liquor

Abstract: In this study, steel pickling waste liquor was employed to obtain reactive nanoscale zero-valent metal (nZVM) with the purpose of engineering application The degradation of metronidazole reacted with as-prepared nZVM in water was investigated to explore the feasibility of using the nZVM to treat antibiotics in wastewater The synthesized nZVM was characterized by Brunauer–Emmett–Teller (BET) surface analyzer, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectrometer (EDS) The results showed that the nZVM (20–40 nm) with crystalline structure had a BET surface area of 35 m 2 /g XPS and EDS only detected Fe, C and O on the surface, suggesting Ni and Zn distributed inside the core of nanoscale alloy Degradation of metronidazole followed the pseudo-first-order kinetics, and the observed reaction rate constant ( k obs ) could be improved with increasing nZVM dosage, as well as with diminishing initial metronidazole concentration and pH A high reaction rate was observed at reduction potential, indicating that electrons and hydrogen species produced by nZVM were driving forces of reaction The surface area-normalized rate coefficient ( k SA ) for nZVM (0254 L min −1 m −2 ) was 3752 times larger than that for commercial iron powder (667 × 10 −4 L min −1 m −2 ) Several possible pathways of degradation of metronidazole were proposed according to the results of UV–vis spectra and HPLC chromatograms