Studies of the Kinetics of Reaction Between Iron Catalysts and Ammonia—Nitriding of Nanocrystalline Iron with Parallel Catalytic Ammonia Decomposition

Analysis of two parallel chemical reactions was performed using a flow differential tubular reactor with thermogravimetric measurement and analysis of the gas phase composition. The nitriding rate of the iron ammonia synthesis catalyst and the ammonia decomposition rate were investigated at 350–550 °C. Various gas-phase nitriding potentials were applied. Phase composition was analysed by X-ray diffraction. From a comparison with Lehrer diagram, the critical nitriding potentials for nanoiron were found to be higher than that for bulk materials. The rates of nitriding and ammonia decomposition on iron and various nitrides were determined. Ammonia decomposition was the most rapid on α-Fe and the slowest on γ′-Fe₄N. Results were interpreted on the basis of the adsorption range model and values of kinetics and thermodynamic parameters were assessed. A new method for the determination of crystallite mass distribution, using the results of iron catalyst nitriding process rate measurements, was proposed.     

Long-Range Diffusion of K Promoter on an Ammonia Synthesis Catalyst Surface—Ionization of Excited Potassium Species in the Sample Edge Fields

The potassium ions K⁺which diffuse out to the surface from the K promoted ammonia synthesis iron catalyst material do not desorb from there but diffuse rapidly along the surface until they reach the edges of the sample. This is shown by angular distributions of the ion emission at the normal operating temperatures of 900–1100 K. The ionic emission at low field strengths, of the order of 2–50 V cm⁻¹, has a minimum or even a zero signal in the direction of the surface normal. Instead of desorbing from the surface, the ions interact strongly with the surface and give electronically excited states K^*on the surface, which diffuse rapidly along the surface over a distance of several millimeters to the edges of the sample. A detailed model is proposed for this process, based on recent kinetic results. At the edges, ions are formed in the stronger electric field just outside the surface giving lobes along the surface. With the highest field strengths used, these lobes are transformed into strongly peaked distributions at 45–70° from the normal, with a strong minimum in the normal direction. From the open surface, only clusters K_nand neutral atoms K can be emitted. Trajectory calculations show that ions, which are emitted from the edges of the sample with higher then thermal energy, appear in the experimentally observed angular range. Their excess energy may be derived from the work function difference between the catalyst sample and its Ta holder.     

Studies on the promotion of nickel—alumina coprecipitated catalysts: II. Lanthanum oxide

Two series of lanthanum promoted nickel—alumina catalysts have been prepared by coprecipitation of the metal nitrates, using potassium carbonate. The molar ratio between nickel and the sum of aluminium and lanthanum was kept constant at 2.5 or 9.0 within each series. The calcination and reduction of these samples were studied by thermogravimetry and their structures before and after calcination and reduction were examined by X-ray diffraction. The methanation activities of the final catalysts were determined by differential scanning calorimetry. The results showed clearly that the methanation of carbon monoxide over nickel—alumina catalysts is enhanced by the presence of La₂O₃. With low percentages of lanthanum, the promoter is built into the precursor structure during the coprecipitation process. This is a meta-stable situation; phase separation occurs during hydrothermal treatment. In both series there was an optimum amount of lanthanum at which the activity per gram of nickel reached a maximum. The optimum specific activity of a lanthanum promoted nickel—alumina catalyst was twice as large as that of the unpromoted material. Above these optimum values, the activity per gram of nickel decreased because of two effects: an increase in the nickel particle sizes and an increase in the amounts of potassium remaining from the precipitation step. Alumina is needed to stabilize the nickel crystallites against sintering. The promoting action of La₂O₃ is slightly higher after reduction at 400°C than after reduction at 600°C. Lanthanum increased the amount of carbon monoxide which was adsorbed slowly; the amount of carbon monoxide which was rapidly adsorbed, however, was not altered. The increase in activity was accompanied by an increase in the apparent activation energy.     

Studies on the promotion of nickel—alumina coprecipitated catalysts: III. Cerium oxide

Three series of cerium-promoted nickel—alumina catalysts with different nickel-to-aluminium ratios each containing different amounts of cerium have been prepared and characterized. The calcination and reduction behaviour were found not to be altered by the presence of cerium. Part of the promoter was found to separate during the precipitation process as poorly crystalline CeO₂, the amount of which was largely determined by the drying temperature. This phase separation process was accompanied by a partial change in the valence state of the cerium. The effect of cerium on the nickel particle sizes was very small. Cerium enhances the activity of coprecipitated nickel—alumina catalysts in the carbon monoxide methanation reaction. This enhancement is accompanied by an increased apparent activation energy. Cerium- and lanthanum-promoted materials are compared with one another and it is concluded that although both promoters behave differently in determining the catalyst structure, their behaviour in the carbon monoxide methanation reaction is very similar and the specific activities of both types of material are nearly equal.     

Studies on the promotion of nickel—alumina coprecipitated catalysts: I. Titanium oxide

A series of TiO₂-promoted nickel—alumina catalysts has been prepared and characterized. The promoter was added in various proportions to a calcined coprecipitated nickel—alumina material by adsorption of the acetylacetonate complex of titanium, followed by further calcination and reduction. The structure of the resultant materials was similar to that of the unpromoted coprecipitated nickel—alumina. The chemisorption properties of the catalyst and its behaviour in the CO/H₂ reaction were characteristic of a strong metal-support interaction (SMSI) reported in the literature for Ni/TiO₂. The strong adsorption of both carbon monoxide and hydrogen were suppressed while the activity for carbon monoxide hydrogenation was increased, the activation energy being lowered. The higher activity, however, was relatively unstable under reaction conditions.     

Synthesis and Characterization of Titania Nanoparticles on the Surface of Microporous Perlite Using Sol–Gel Method: Influence of Titania Precursor on Characteristics

Titania colloidal nanoparticles have been successfully fabricated by a very simple and inexpensive sol–gel spin-coating method on the surface perlite granules. This was achieved by adjustment of the sol–gel parameters such as the precursors, spin-coating time and heating processes. Five samples were prepared using different titania e.g. Millennium, Degussa P25, titania nanoparticle, sol–gel derived titania and rutile nanoparticles precursors. Titania nanoparticles were grown on perlite granules by the controlled immobilization of titania. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were employed to investigate the growth and morphology of the titania coating on perlite granules. The results showed surface modification of perlite granules with uniform coating of titania on supports and confirmed that the coatings are composed of aggregated crystallites of 11–61 nm in diameter, good compositional uniformity and good adherence of the fabricated titania layer on perlite granules. The synthesis of adsorptive supports for immobilization of powdery photocatalysts would show significant improvements in both practical use and degradation efficiency of photocatalysis in environmental protection.     

Adsorption to the Surface of Hemozoin Crystals: Structure-Based Design and Synthesis of Amino-Phenoxazine β-Hematin Inhibitors

In silico adsorption of eight antimalarials that inhibit β-hematin (synthetic hemozoin) formation identified a primary binding site on the (001) face, which accommodates inhibitors via formation of predominantly π-π interactions. A good correlation (r²=0.64, P=0.017) between adsorption energies and the logarithm of β-hematin inhibitory activity was found for this face. Of 53 monocyclic, bicyclic and tricyclic scaffolds, the latter yielded the most favorable adsorption energies. Five new amino-phenoxazine compounds were pursued as β-hematin inhibitors based on adsorption behaviour. The 2-substituted phenoxazines show good to moderate β-hematin inhibitory activity (<100 μM) and Plasmodium falciparum blood stage activity against the 3D7 strain. N¹,N¹-diethyl-N⁴-(10H-phenoxazin-2-yl)pentane-1,4-diamine ( P2a ) is the most promising hit with IC₅₀ values of 4.7±0.6 and 0.64±0.05 μM, respectively. Adsorption energies are predictive of β-hematin inhibitory activity, and thus the in silico approach is a beneficial tool for structure-based development of new non-quinoline inhibitors.     

The effect of sulfur evolution on the properties of high-temperature carbons—I. Thianthrene and thianthrene-anthracene precursors

The carbonization (525°C) and graphitization (up to 2500°C) of thianthrene, anthracene and a 17% wt thianthrene-83% wt anthracene mixture were investigated. The thianthrene carbons were found to be non-graphitizable and of low density. Upon heat-treatment from 1200–2500°C sulfur was evolved continuously and there was a further decrease in density due to an increase in closed porosity. The carbons produced from the mixture were found to be graphitizable and possessed similar crystal parameters to those carbons derived from anthracene after heat treatment to the same temperature. In contrast to the anthracene carbons, the mixed carbons showed a sharp decrease in density with heat treatment temperature. The principal reduction in sulfur content occurred between 1400–1600°C. The density change appears to be produced through the disruption of crystallite alignment which substantially increases the open pore volume for pores of less than 200 nm diameter. The behavior of the mixed carbon is similar to that reported for high sulfur petroleum cokes which can undergo an irreversible expansion (puffing) during graphitization.     

Two-Phase Calorimetry. II. Studies on the Thermodynamics of Cesium and Strontium Extraction by Mixtures of H+CCD− and PEG-400 in FS-13

Abstract

Thermochemical characterization of the partitioning of cesium and strontium from nitric acid solutions into mixtures of the acid form of chlorinated cobalt dicarbollide (H⁺CCD^?) and polyethylene glycol (PEG-400) in FS-13 diluent has been completed using isothermal titration microcalorimetry and radiotracer distribution methods. The phase transfer reaction for Cs⁺ is a straightforward (H⁺ for Cs⁺) cation exchange reaction. In contrast, the extraction of Sr²⁺ does not proceed in the absence of the co-solvent molecule PEG-400. This molecule is believed to facilitate the dehydration of the Sr²⁺ aquo cation to overcome its resistance to partitioning. The phase transfer reactions for both Cs⁺ and Sr²⁺ are enthalpy driven (exothermic), but partially compensated by an unfavorable entropy. The results of the calorimetry studies suggest that the PEG-400 functions as a stoichiometric phase transfer reagent rather than acting simply as a phase transfer catalyst or phase modifier. The calorimetry results also demonstrate that the extraction of Sr²⁺ is complex, including evidence for both the partitioning of Sr(NO₃)⁺ and endothermic ion pairing interactions in the organic phase that contribute to the net enthalpic effect. The thermodynamics of the liquid-liquid distribution equilibria are discussed mainly considering the basic features of the ion solvation thermochemistry.     

Adsorption of Phenol and 2,4‐Dinitrophenol on Activated Carbon Cloth: The Influence of Sorbent Surface Acidity and pH

Abstract

In this work the effect of the activated carbon cloth surface acidity and pH of the solution on phenols adsorption has been studied. Two phenols, widely different in the terms of their pKa values (phenol and 2,4‐dinitrophenol), have been chosen as the model compounds. It has been shown that phenol adsorption was favored by low pH values of solution and high point of zero charge values of activated carbon cloths. The adsorption of 2,4‐dinitrophenol was promoted at very low pH values of solution and it was less influenced by activated carbon cloth surface acidity.     

The influence of sucrose and glycerol on the formation and transformation of iron oxides — The implication for soil formation

The influence of sucrose and glycerol on the laboratory transformation of synthetic lepidocrocite and on the synthesis of Fe(III) oxide phases by Fe²⁺ oxidation in solution was studied by powder X-ray diffraction, transmission electron microscopy and by measuring hydrolysis rates. The morphologies of samples were compared with those of similar Fe oxides sampled from organic rich pedogenic environments.The transformation of lepidocrocite to goethite in M KOH at 80°C is inhibited by a sufficient concentration of sucrose or glycerol. Dissolution of the serrated-ended laths of lepidocrocite occurs and the dissolved Fe is recrystallised on the surfaces of the undissolved lepidocrocite, rather than forming goethite as expected in the absence of organic compounds.The influence of sucrose and glycerol on the synthesis of lepidocrocite during the controlled oxidation of Fe(II) chloride solutions at pH 7 depends on the nature of the organic additive and its concentration. The presence of sucrose at 0.06 M concentration retarded lepidocrocite formation and caused ferrihydrite to form. Glycerol at high concentrations (0.15 M) also caused ferrihydrite formation even though its presence almost doubled the reaction time. However, at lower concentrations of glycerol, goethite in association with ferrihydrite were the preferred products.Four natural Fe oxide samples from organic-rich pedogenic environments (placic horizons and surface water precipitates) were examined and found to exhibit morphological features similar to those produced by synthesis in the presence of sucrose and glycerol. This indicates that simple organic compounds in the soil influence the formation of ferrihydrite associated with goethite and poorly crystalline lepidocrocite.     

Studies on the Dehydrogenative Polymerizations of Monolignol β-glycosides. Part 3: Horseradish Peroxidase–Catalyzed Polymerizations of Triandrin and Isosyringin

Abstract

Horseradish peroxidase–catalyzed dehydrogenative polymerizations of the p-hydroxyphenyl monolignol glucoside (triandrin (1P)) and the syringyl monolignol glucoside (isosyringin (1S)) resulted in the formation of water-soluble lignin-like polymers (DHPs). The polymerization of 1P gave highly polymerized DHPs in high yields as did previously reported polymerization of the guaiacyl monolignol glucoside (isoconiferin (1G)). It was shown that the hydrophilic D-glucose units of 1G and 1P contribute to a marked increase in the molecular weights of the resulting DHPs. On the other hand, the homogeneous phase polymerization of 1S, similar to the polymerization of sinapyl alcohol, gave DHPs with extremely low molecular masses in poor yields. Structural characterization indicated that the DHPs from 1P and 1S were lignin-like polymers containing glucosidic units on their sidechains. It was also confirmed that D-glucosyl units introduced onto the γ-position of monolignols do not significantly affect the electrochemical oxidizability and the kinetics of the HRP-catalyzed initial monomer consumption.     

The Mechanism of the Selective NOx Sorption on H3PW12O40·6H2O (HPW)

NO _x absorption/desorption capacities of 12-tungstophosphoric acid hexa-hydrate were measured under representative exhaust gas mixture conditions. The amounts of NO _x absorbed and then desorbed are high and equal to 46 of NO₂ mgg^–1 of HPW. The mechanism of absorption proceeds by substitution of lattice water molecules with formation of a [H⁺(NO₂ ^–,NO⁺)] complex. During the cooling phase and in the presence of water, around 100°C, reverse substitution occurs. Two possibilities to wash-coat HPW on a monolith are presented. The first one consists in a partial substitution of H⁺ by a monovalent cation while the second one consists of supporting HPW on a high surface oxide. The anchorage quality is related to the Brønsted acidity, the best candidates for the role of support are SnO₂ and TiO₂.     

Surface stabilities of 3C–SiC and H2O adsorption on the (110) surface

First-principles calculations and thermodynamics analyses were combined to study the surface stabilities of 3C–SiC and H₂O adsorption on the (110) surface. The stoichiometric (110) surface was predicted to be generally the most stable. Only at the extremely C-poor condition, the nonstoichiometric Si-terminated (100) could become more energetically favored. The adsorption and dissociation of single H₂O molecule on the 3C–SiC (110) were then comparatively investigated. Calculations show that H₂O molecules prefer to partially dissociate into one hydroxyl OH and one H adsorbed at the top-most Si and C sites, respectively, leading to the formation of a hydrogen network on the surface. The calculated equilibrium adsorption diagram further suggested that the 3C–SiC (110) surface can be only either completely clean or fully covered by the partially dissociated species of H₂O, for a wide range of temperature and the partial potential of H₂O.     

Fischer–Tropsch Synthesis: Studies on the Effect of Support Doping with Si,Mn and Cr on the Selectivity to Alcohols in Ceria Supported Cobalt Catalysts

Mn and Cr doped CeSi mixed oxides were used as supports for Co and tested for CO hydrogenation. Co/CeSi was found to be more active and significantly more selective to n-alcohols/olefins. An increasing selectivity to n-alcohols and decreasing selectivity to olefins as a function of time on stream was also observed, suggesting a trade-off between those two products. Addition of Mn led to similar behavior, although at slightly lower conversions. Addition of Cr, however, considerably suppressed n-alcohol formation, while it kept selectivities to olefins within a 20–30 % range over more than 250 h of testing, indicating either higher alcohol dehydration activity, or that the presence of Cr ions lowered the hydrogenating activity of Co. The present work indicates that enhanced contact area between Co and the reducible support is likely a key factor for enhancing selectivity to alcohols.     

Surface Modification of Pd/α-Si3N4 Catalysts Through the Solvent Used During Synthesis. Implications on the CO Chemisorption Properties and Catalytic Performances

Palladium catalysts supported on α-Si₃N₄ were prepared by impregnation with Pd(II)-acetate dissolved either in toluene or in water. The mean metal particle size of ~0.5 wt% Pd catalysts was similar (~5 nm) and independent of the way of preparation. Nevertheless, the two catalysts present very different chemisorption behaviour chemisorptive and catalytic properties. Fourier transformed infrared (FTIR) spectra of adsorbed CO at different temperatures (ranging from room temperature to 300 °C) show a very different behaviour for both catalysts. While the CO adsorption states on the Pd/α-Si₃N₄ prepared in toluene are very similar to those generally measured for silica and/or alumina supported palladium catalysts, CO chemisorbs less strongly on Pd/α-Si₃N₄ prepared in water and on different adsorption sites. The Pd/α-Si₃N₄ catalyst obtained by aqueous impregnation is much less efficient for the methane total oxidation. It is less active and less stable: it deactivates strongly after 3 h on stream at 650 °C. The two catalysts present about the same activity for the 1,3-butadiene hydrogenation after stabilisation at 20 °C. But, the catalyst prepared in water shows a much better selectivity to butenes. The results are discussed in terms of the possible migration of silicon atoms from the silicon nitride support to the surface of the palladium particles, when the catalyst is prepared in water. This is not the case when prepared in an organic solvent.     

The effect of sulfur evolution on the properties of high-temperature carbons—II. The addition of Fe2O3

A thianthrene-anthracene mixture has been used to study the influence of Fe₂O₃ addition in suppressing the volume expansion of carbons due to sulfur release on heat treatment. For undoped carbons, the particle density began to decrease above about 1200°C and continued to 2500°C. There was an accompanying sharp reduction in sulfur content between 1400–1600°C. With Fe₂O₃ addition, the sulfur content was higher over the whole temperature range studied. Between 1200–1600°C the carbon density increased slightly, while from 1600–2500°C the density continuously decreased. Below 1600°C, iron sulfide was detected in the cooled reaction mixture, consistent with the presence of an iron-sulfur liquid phase during heat treatment. The liquid phase appears to influence density changes through modifying the mechanisms for sulfur release. Although FeS was no longer evident at higher temperatures, the presence of iron compounds appeared to have an influence in reducing expansion.The addition of Fe₂O₃ did not affect crystal growth and development of structural order during graphitization. Density decreases for doped and undoped carbons are attributed to disruption of crystallite alignment. Porosity measurements showed that the increase in pore volume is due to the formation of pores smaller than 200 nm diameter.