Acoustic behavior of the incommensurate state of Rb2Zncl4 by brillouin scattering?

Temperature dependence of elastic stiffness constant C₁₁ was studied from 60 to -100°C by Brillouin scattering and refractive index measurements. It is found that C₁₁ decreases with decreasing temperature over the incommensurate phase and its temperature gradient becomes more or less flat at several degrees above the incommensurate-commensurate transition point T_c, where the dis-commensurate structure appears. This unusual temperature dependence of C₁₁ seems to be attributable to the incommensurately modulated structure of Rb₂ZnC1₄.     

The homogeneous gas phase oxidation of methane and the retarding effect of basic/inert surfaces

The homogeneous gas phase O₂-based oxidation of methane was studied in the temperature range, from 500°C to 750°C at methane partial pressures ranging from 3 bar to 40 bar. At the lower end of the temperature range methanol, formaldehyde, and CO represent the main products, while at temperatures exceeding 650° C/C-coupled products, C₂H₆, C₂H₄, C₃H₆ and C₃H₈ predominate. The change in selectivity as function of the temperature is well explained based on a free radical chain mechanism with degenerate branching, initiated by the gas phase reaction, CH₄+O₂→CH^·₃+HO^·₂. Bringing in basic catalysts known to catalyze the system at low methane partial pressures, in the reactor e.g. SrCO₃, BaCO₃, and 7% Li/MgO resulted in reduced rates of methane and oxygen conversions, and only minor changes in the selectivity to coupled products were observed.     

Thermodynamic investigation of the CaO---Al2O3---CaSO4---H2O system at 50°C and 85°C

The CaO---Al₂O₃---CaSO₄---H₂O system has been investigated at 50°C and 85°C, by calculations of the equilibrium solubility surfaces of AH₃, C₃AH₆, CH, ettringite, monosulphoaluminate and gypsum. The results indicate that monosulphoaluminate becomes a stable phase at elevated temperature. Since ettringite also remains stable at 85°C, new invariant points are created, for which calculated aqueous phase compositions are given.     

Reactivity of steam in exhaust gas catalysis III. Steam and oxygen/steam conversions of propane on a Pd/Al2O3 catalyst

A 1% Pd catalyst (38% dispersion) was prepared by impregnating a γ-alumina with palladium acetylacetonate dissolved in acetone. The behaviour of this catalyst in oxidation and steam reforming (SR) of propane was investigated. Temperature-programmed reactions of C₃H₈ with O₂ or with O₂ + H₂O were carried out with different stoichiometric ratios S(S =[O₂]/5[C₃H₈]). The conversion profiles of C₃H₈ for the reaction carried out in substoichiometry of O₂ (S < 1) showed two discrete domains of conversion: oxidation at temperatures below 350°C and SR at temperatures above 350°C. The presence of steam in the inlet gases is not necessary for SR to occur: there is sufficient water produced in the oxidation to form H₂ and carbon oxides by this reaction. Contrary to what was observed with Pt, an apparent deactivation between 310 and 385°C could be observed with Pd in oxidation. This is due to a reduction of PdO_x into Pd⁰, which is much less active than the oxide in propane oxidation. Steam added to the reactants inhibits oxidation while it prevents the reduction of PdO_x into Pd⁰. Compared to Pt and to Rh, Pd has a higher thermal resistance: no deactivation occurred after treatment up to 700°C and limited deactivation after treatment up to 900°C, provided that the catalyst is maintained in an oxygen-rich atmosphere during the cooling.     

Phase transitions in R2 SnCl6-type crystals (R = NH3 C2H3, NH3C2H5 and N(CH3)4)

The thermal dilatation in (NH₃ ·CH₃) SnCl₆, (NH₃ · C₂H₅) SnCl₆ and [N(CH₃)] SnCl₆ was measured, and as the results it has turned out that (NH₃ ⁶·C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnCl₆ undergo the first order transitions at 128 K and 158 K, respectively. The low temperature phases of (NH · C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnC1₆ are found to be monoclinic and tetragonal, respectively, No phase transition was observed in (NH₃ ·CH₃)₂ SnCl₆ down to 77 K.     

Catalytic oxidation of saturated hydrocarbons on multicomponent Au/Al2O3 catalysts: Effect of various promoters

The performance of unpromoted and MO_x-(M: alkali (earth), transition metal and cerium) promoted Au/Al₂O₃ catalysts have been studied for combustion of the saturated hydrocarbons methane and propane. As expected, higher temperatures are required to oxidize CH₄ (above 400 °C), compared with C₃H₈ (above 250 °C). The addition of various MO_x to Au/Al₂O₃ improves the catalytic activity in both methane and propane oxidation. For methane oxidation, the most efficient promoters to enhance the catalytic performance of Au/Al₂O₃ are FeO_x and MnO_x. For C₃H₈ oxidation a direct relationship is found between the catalytic performance and the average size of the gold particles in the presence of alkali (earth) metal oxides. The effect of the gold particle size becomes less important for additives of the type of transition metal oxides and ceria. The results suggest that the role of the alkali (earth) metal oxides is related to the stabilization of the gold nanoparticles, whereas transition metal oxide and ceria additives may be involved in oxygen activation.     

Study of stabilized phases in high alumina cement mortars Part I Hydration at elevated temperatures followed by carbonation

The use of temperatures higher than 30°C during the hydration of 1:3 high alumina cement mortar leads to the formation of the cubic hydrate C₃AH₆, hence avoiding the conversion of CAH₁₀ to C₃AH₆. The subsequent carbonation of C₃AH₆ with CO₂ and thermal treatment leads to the formation of stable carbonated phases. The optimum hydration ages are the minimum necessary times to get C₃AH₆: 3 hours at 80°C, 6 hours at 60°C. The product is in a state we call “semiplastic” in which the next structural transformation - carbonation - occurs more easily than when the specimens have a rigid structure. The effect of a super-water-reducing admixture has also been studied.     

Microstructure and phase development in NiMn2O4 spinel ceramics during isothermal sintering

Studies of the sintering behaviour of NiMn₂O₄ semiconducting ceramics at 1100°C under oxygen atmosphere show the advantage of a powder prepared by thermal decomposition of oxalate mixed crystals NiMn₂ (C₂O₄)₃. 6H₂O at 450°C for the formation of dense ceramics with homogeneous microstructure. The microstructure of the semiconductor ceramics was established by image analysis. SEM and EDX indicate a phase separation related to partial oxygen loss as observed by thermogravimetry and redox analytic measurements. Reoxidation to a single-phase homogeneous microstructure by annealing at 800°C is possible only in porous samples. NiMn₂O₄ is not stable in oxygen at 1100°C. The spinel decomposes into NiO and a Mn-rich spinel matrix Ni_x^IIMn_1−x^IIMn₂^IIIO₄. For producing a reproducible semiconducting ceramics it is necessary to stimulate sintering by separation of NiO. A one-phase spinel is obtained by reoxidation of long duration.     

Selective catalytic reduction of nitrogen oxide by hydrocarbons over mordenite-type zeolite catalysts

The reduction of NO by hydrocarbons such as C₂H₄, C₂H₆, C₃H₆, and C₃H₈ has been investigated over mordenite-type zeolite catalysts including HM, CuHM, NZA (natural zeolite), and CuNZA prepared by an ion-exchange method in a continuous flow fixed-bed reactor. NO conversion over CuNZA catalyst reaches about 94% with 2000 ppm of C₃H₆ at 500°C. As reductants, alkenes seem to exhibit a higher performance for NO conversion than alkanes regardless of the catalysts. No deterioration of the catalytic activity due to carbonaceous deposits for CuNZA was observed above 400°C even after 30 h of on-stream time, but SO₂ in the feed gas stream causes a severe poisoning of the CuNZA catalyst. The effect of H₂O on NO conversion was significant regardless of the catalysts and the reductants employed in this study. However, CuNZA catalyst shows a unique water tolerance with C₃H₆. The reaction path of NO to N₂ is the most important factor for high performance of this catalytic system. NO is directly reduced by a reaction intermediate, C_n′H_m′(O) formed from hydrocarbon and O₂, N₂O is another reaction intermediate which can be easily removed by C_n′H_m′(O).     

Hydration characteristics of monocalcium aluminate at a low water/solid ratio

The hydration characteristics of calcium monoaluminate were studied using an effective water/aluminate ratio of 0.15 at 20° or 80°C, from a few minutes of two months. The material hydrated at 80°C shows a large shrinkage while at the lower temperature a continuous expansion occurs. The product at 80°C shows a much higher strength than that hydrated at 20°C. The main initial hydration products are 2Ca0, Aℓ₂0₃, 8H₂0 and alumina gel. Microcracks are developed in the products hydrated at 20°C while at the higher temperature a very compact mass results. The data indicate that it is possible to obtain a durable high alumina cement by using a low water/cement ratio and hydrating at higher temperatures, and under these conditions C₃AH₃---C₃AH₆ bond is favoured.     

Crystal structure of poly(tetrafluoroethylene) homo- and copolymers in the high pressure phase

X-ray diffraction measurements are reported for 27°C, pressures to 5.2 GPa and concentrations of CF₃ units to 0.049 CF₃/CF₂. These show both orthorhombic and monoclinic structures to exist under high hydrostatic pressure. It is proposed that shear stresses generated at elastic inhomogeneities in the sample lead to the monoclinic phase. Energy calculations are consistent with this concept. They also indicate that conformational and rotational disorders raise the entropy of the high pressure phase III. Perfluoromethyl branches increase the volume/CF₂ of phase III more than that of the low pressure phase I. At high CF₃ concentrations and pressures, both phases become metrically hexagonal. The volume of transition becomes zero at a concentration near 0.05 CF₃/CF₂ and no transition is observed to a pressure of 5.2 GPa. There appears to be a critical point near 27°C, a CF₃ concentration of 0.05 and a pressure of 3.5 to 5 GPa.     

Potassium–copper and potassium–cobalt catalysts supported on alumina for simultaneous NOx and soot removal from simulated diesel engine exhaust

This paper deals with the activity of bimetallic potassium–copper and potassium–cobalt catalysts supported on alumina for the reduction of NO_x with soot from simulated diesel engine exhaust. The effect of the reaction temperature, the soot/catalyst mass ratio and the presence of C₃H₆ has been studied. In addition, the behavior of two monometallic catalysts supported on zeolite beta (Co/beta and Cu/beta), previously used for NO_x reduction with C₃H₆, as well as a highly active HC-SCR catalyst (Pt/beta) has been tested for comparison. The preliminary results obtained in the absence of C₃H₆ indicate that, at temperatures between 250 and 400 °C, the use of bimetallic potassium catalysts notably increases the rate of NO_x reduction with soot evolving N₂ and CO₂ as main reaction products. At higher temperatures, the catalysts mainly favor the direct soot combustion with oxygen. In the presence of C₃H₆, an increase in the activity for NO_x reduction has been observed for the catalyst with the highest metal content. At 450 °C, the copper-based catalysts (Cu/beta and KCu2/Al₂O₃) show the highest activity for both NO_x reduction (to N₂ and CO₂) and soot consumption. The Pt/beta catalyst does not combine, at any temperature, a high NO_x reduction with a high soot consumption rate.     

Brillouin spectra of partially deuterated triglycine selenate near TC

Brillouin spectra of partially deuterated triglycine selenate (-50%deuteration) have been taken in the vicinity of the transition temperature for longitudinal phonon configurations q ∥ [101] and q ∥ [011]. Very weak anomalies were observed at T_C ≌ 27°C, and the spectra did not show up any feature indicative of the first order transition character reported from earlier dielectric constant and hysteresis loops measurements.     

The influence of CaCl2 on the crystallization of calcium silicates in autoclave hydration of C3S

The influence of CaCl₂ on the autoclave hydration C₃S (5 hours at 190°C) after different precuring times (0–16 hours) at room temperature has been studied. The addition of calcium chloride retards the autoclave hydration of C₃S and prevents completely the formation of the crystalline hydrated silicates (-C₂SH and C₃SH_1.5). This result has been compared with that observed in the ball-mill hydration of C₃S in the presence of CaCl₂, i.e., the complete absence of crystalline afwillite.

Abstract

On a étudié l'influence du CaCl₂ sur l'hydratation en autoclave du C₃S (5 heures à 190°C) après différents temps de conservation (0–16 heures à température ambiante. L'addition de chlorure de calcium retarde l'hydratation en autoclave du C₃S et empêche complétement la formation des silicates cristallins hydratés -C₂SH et C₃SH_1.5. Ce résultat a été comparé avec le phenomene observé en étudiant l'hydration du C₃S dans un moulin à boulets en presence de CaCl₂, c'est à dire la disparition complète du composé crystallin afwillite.     

Catalytic selective reduction of NO with ethylene over a series of copper catalysts on amorphous silicas

Catalytic selective reduction of NO to N₂ was studied comparing a series of Cu-based catalysts (ca. 8 wt.%) supported over amorphous pure and modified silicas: SiO₂, SiO₂-Al₂O₃, SiO₂-TiO₂, SiO₂-ZrO₂. The catalysts were prepared by the chemisorption-hydrolysis method which ensured the formation of a unique copper phase well dispersed over all supports, as confirmed by scanning electron micrographs (SEMs). Temperature-programmed reduction (TPR) analyses confirmed the presence of dispersed copper species which underwent complete reduction at a temperature of about 220°C, independently of the support. It was found that the support affects the extent of NO reduction as well as the selectivity to N₂ formation. Maximum N₂ yield was found in the range 275–300°C. The catalyst prepared over SiO₂-Al₂O₃ was the most active and selective with respect to the other silicas. Competitiveness factors (c.f.’s) as high as 13–20% in the temperature range 200–250°C could be calculated. For all catalysts, the temperature of the N₂ peak maximum did not correspond to that of the maximum C₂H₄ oxidation to CO₂, suggesting the presence of two different sites for the oxidation and the reduction activity. On the catalyst prepared on SiO₂-Al₂O₃, a kinetic interpretation of catalytic data collected at different contact times and temperatures permitted evaluating the ratio between kinetic coefficients as well as the difference between activation energies of NO reduction by C₂H₄ and C₂H₄ oxidation by O₂.     

OCM in a fixed-bed reactor: limits and perspectives

The oxidative coupling of methane (OCM) over a La₂O₃/CaO catalyst was studied in a poly tropic fixed-bed reactor (I.D. = 15 mm, W/F= 0.15 g · s/ml). Reaction conditions for stable operation were determined. (1) A minimum inlet temperature of 580°C was necessary to initiate the reaction. (2) The maximum hot-spot temperature of 1000°C limited the highest oxygen inlet concentration to 20%. The temperature gradients in the bed amounted to 250 K. The influence of the reaction conditions on the C₂₊ selectivity was investigated by testing the effects of temperature (T_inlet = 580–860°C), oxygen concentration (C_O2 = 5–20%) and particle diameter (d_p = 250–350 μm, and pellets of h_p = 4 mm and d_p = 4 mm). The C₂₊ selectivity ran through a maximum with increasing temperature and decreased with rising inlet oxygen concentration. Mass-transfer limitations, which occurred when applying pellets, resulted in a drop of C₂₊ selectivity. Highest C₂₊ yields amounted to 15.5% (X_CH4 = 31%, S ₂₊ = 51%). Distributed feed of oxygen was tested as a means to cope with the high temperature gradients and to increase C₂₊. selectivity. Upon applying this mode of operation, oxygen concentrations up to 30% could be converted. However, no improvement of C₂₊ selectivity and yield compared to cofeed operation was achieved.     

Co from partial reduction of La(Co,Fe)O3 perovskites for Fischer–Tropsch synthesis

Small Co clusters (d<10 nm) supported over mixed La–Co–Fe perovskites were successfully synthesized. These catalysts were active for Fischer–Tropsch (FT). Depending on the Co to Fe ratios the mixed perovskite exhibited two different forms: the rhombohedral phase of LaCoO₃ is maintained for the mixed perovskite when x>0.5, the orthorhombic phase of LaFeO₃ is found for x<0.5. Interestingly only one of these structures is active for the FT reaction: the orthorhombic structure. This is most likely due to the capacity of this material to maintain its structure even with a high number of cation vacancies. These cations (mostly Co) were on purpose extracted and reduced. Magnetic measurements clearly showed their metallic nature. Rhombohedral Co–Fe mixed perovskites (x≥0.5) cannot be used as precursors for Fischer–Tropsch catalysts: their partial reduction only consists in a complete reduction of Co³⁺ into Co²⁺.

The partial reduction of orthorhombic perovskites (x<0.5) leads to active Fischer–Tropsch (FT) catalysts by formation of a metal phase well dispersed on a cation-deficient perovskite. The FT activity is related to the stability of the precursor perovskite. When initially calcined at 600 °C, a maximum of 8.6 wt.% of Co⁰ can be extracted from LaCo_0.40Fe_0.60O₃ (compared to only 2 wt.% after calcination at 750 °C). The catalyst is then composed of Co⁰ particles of 10 nm on a stable deficient perovskite LaCo_0.05³⁺Fe_0.60³⁺O_2.40. Catalytic tests showed that up to 70% in the molar selectivity for hydrocarbons was obtained at 250 °C, 40% of which was composed of the C₂–C₄ fraction.     

The low-temperature performance of NOx storage and reduction catalyst

The catalytic performance and the behavior of NO_x storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the NO_x storage amount in the lean atmosphere was the same as the NO_x reduction amount from the subsequent rich spike (RS) above the temperature of 400 °C, while the former was greater than the latter below the temperature of 400 °C. This indicated that when the temperature was below 400 °C compared with the NO_x storage stage, the reduction of the stored NO_x is somehow restricted. We found that the reduction efficiencies with the reducing agents decrease in the order H₂ > CO > C₃H₆ below 400 °C, thus not all of the NO_x storage sites could be fully regenerated even using an excessive reducing agent of CO or C₃H₆, which was supplied to the NSR catalyst, while all the NO_x storage sites could be fully regenerated if an adequate amount of H₂ was supplied. We also verified that the H₂ generation more favorably occurred through the water gas shift reaction than through the steam reforming reaction. This difference in the H₂ generation could reasonably explain why CO was more efficient for the reduction of the stored NO_x than C₃H₆, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though promoting the H₂ generation reaction.     

The hydration of calcium dialuminate and its mixtures containing slag

In a paste CA₂ hydrates very slowly at 20°C but the reaction is more rapid at 40°C. At both temperatures the products of hydration comprise C₂AH₈, gibbsite and alumina gel after 7 days. For mixtures of CA₂ + CA the initial hydration at 20°C is dominated by CA. In the presence of ground granulated blastfurnace slag CA₂ or a mixture of CA₂ + CA will form strätlingite on hydration at 40°C within 3 days. Slag addition reduces the amount of heat evolved during the first 24 hours of hydration at both 20° and 40°C.     

The NO selective reduction on the La1−xSrxMnO3 catalysts

La_1−xSr_xMnO₃ (x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La_0.7Sr_0.3MnO₃ calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C₃H₈ could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O₂ concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C₃H₈, leading to a decrease of the NO reduction from 100% to zero at 560 °C.