Charles Kappenstein

Effect of doping elements on the thermal stability of transition alumina

Abstract The thermal stability of porous transition alumina at high temperature is a crucial challenge for several applications. To delay the θ to α-alumina transition temperature (1180°C), the synthesis by sol–gel method and the introduction of doping element as nitrate salt M(NO 3 ) x (M=Ba, Mg, Pr, La and Ce) have been investigated. Over three preparation procedures, the introduction of the doping element through the sol–gel process gave the higher BET surface area. Additional thermal analysis and powder X-ray diffraction studies have shown that only a few amount (1 mol percent) of doping element is sufficient to increase the θ→α-Al 2 O 3 phase transition temperature up to 1315°C. The most promising samples are (Al 2 O 3 ) 0.98 Ba 0.02 and (Al 2 O 3 ) 0.99 Pr 0.01 . Even after 5 h at 1200°C, they maintain a rather good specific area (32 m 2 /g) in relation with the incomplete and very slow transformation of θ into α-alumina.

Dealumination of zeolites II. Kinetic study of the dealumination by hydrothermal treatment of a NH4NaY zeolite

The hydrothermal treatment of a (NH4)0.8Na0.2Y zeolite was carried out in a flow reactor in order to determine quantitatively the effect of the operating conditions (time, temperature, water pressure) on the dealumination of this zeolite. X-ray diffraction was used to follow the process of dealumination and to characterize the variations of the crystallinity. The enrichment of the outer surface in Al was measured by X-ray photoelectron spectroscopy (XPS). First the temperature of the zeolite was raised rapidly to the temperature chosen for the hydrothermal treatment (260 to 820°C) under nitrogen flow. During this rise in temperature there was a significant dealumination caused by water physically adsorbed or liberated by dehydroxylation. This dealumination is accompanied by a partial collapse of the zeolite framework. After the ceiling temperature was reached, the zeolite was submitted to a flow of water vapor. Two periods could be distinguished, namely an initial one in which dealumination was rapid, and a second one in which it was slow. Only the aluminum atoms with which NH4+ (and not Na+) are associated could be extracted from the framework. For both periods the kinetic orders with respect to these aluminum atoms and to water are equal to 1; the apparent activation energy is about 20 kJ mol−1 for the rapid dealumination and 70 for the slow process. The first period would correspond to the dealumination of the protonic zeolite and the second to that of the zeolite exchanged by cationic aluminum species. Under high water pressure (>50 kPa) there is a relationship between zeolite dealumination and subsequent migration of extra-framework aluminum species to the outer surface of the crystallites. However, these processes are independent. Indeed during the self-steaming of the zeolite, dealumination occurs without enrichment in Al of the outer surface.

Hydrogen Peroxide Decomposition on Various Supported Catalysts Effect of Stabilizers

The decomposition of hydrogen peroxide (H 2 O 2 ) has been studied on various catalysts (platinum supported on silica; silver, iridium, platinum-tin or manganese oxides supported on alumina). The experiments were performed using two reactors: 1) a conventional constant pressure reactor for the determination of the volume increase vs time using diluted H 2 O 2 solutions; 2) a constant volume reactor to measure the pressure increase using more concentrated solutions. The first reactor leads to the determination of the kinetic order of the reaction, to the comparison of the activities of the different samples, and to the characterization of the influence of some stabilizers of H 2 O 2 solutions on the catalytic activity. Two kinetic orders were found, depending on the catalyst: a zero order and a first order. The shape of the catalysts samples is an important parameter, with powders always being more reactive than grains and pellets. The catalyst activities are sorted as follows: Pt-Sn/Al 2 O 3 < Ir/Al 2 O 3 < Pt/SiO 2 < MnO x /Al 2 O 3 < Ag/Al 2 O 3 . The presence of pyrophosphate stabilizer leads to a loss of activity mainly as a result of passivation in the case of MnO x -supported samples, whereas the presence of stannate increases slightly the activity of silver and displays no influence on manganese samples.

Water repellent porous silica films by sol-gel dip coating method

The wetting of solid surfaces by water droplets is ubiquitous in our daily lives as well as in industrial processes. In the present research work, water repellent porous silica films are prepared on glass substrate at room temperature by sol-gel process. The coating sol was prepared by keeping the molar ratio of methyltriethoxysilane (MTES), methanol (MeOH), water (H(2)O) constant at 1:12.90:4.74, respectively, with 2M NH(4)OH throughout the experiments and the molar ratio (M) of MTES/Ph-TMS was varied from 0 to 0.22. A simple dip coating technique is adopted to coat silica films on the glass substrates. The static water contact angle as high as 164° and water sliding angle as low as 4° was obtained for silica film prepared from M=0.22. The surface morphological studies of the prepared silica film showed the porous structure with pore sizes typically ranging from 200nm to 1.3μm. The superhydrophobic silica films prepared from M=0.22 retained their superhydrophobicity up to a temperature of 285°C and above this temperature the films became superhydrophilic. The porous and water repellent silica films are prepared by proper alteration of the Ph-TMS in the coating solution. The prepared silica films were characterized by surface profilometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier Transform Infrared (FT-IR) spectroscopy, humidity tests, chemical aging tests, static and dynamic water contact angle measurements.

Characterisation and activity in n-hexane rearrangement reactions of metallic phases on Pt–Sn/Al2O3 catalysts of different preparations

The platinum–tin interactions in Pt–Sn/Al2O3 catalysts were followed through several characterisation methods and modified by using two preparation procedures (1.5 wt% Pt, Sn:Pt=1:1): conventional coimpregnation with H2PtCl6 and SnCl4 (T sample) or by use of the bimetallic precursor [Pt(NH3)4]SnCl6, which was synthesised in the support porosity (N sample). The effects of these interactions on catalytic properties were displayed by the activity and selectivity in n-hexane rearrangement reactions. For both samples platinum and tin are reduced, but they have very different platinum dispersions which are related to different temperature-programmed reduction profiles: 52% for sample T and 4% for sample N. Insitu tin Mossbauer spectroscopy confirms that the majority of tin is reduced, and a minority remains as SnII; air treatment leads to a partial reoxidation of SnII to SnIV, sample N retaining more tin as alloy. X-Ray diffraction displays the simultaneous presence of PtSn, Pt3Sn and Pt with more alloys on sample N; the co-impregnated sample, which has a greater platinum phase, shows a better dispersion of tin (XPS data), in accordance with a high interaction with alumina. The catalytic activity was controlled by the platinum phase; for sample T, the influence of the addition of tin is restricted, whereas the catalyst prepared from the bimetallic precursor exhibits particular properties, attributable to the stabilisation of platinum in smaller ensembles, and the modifying effect of tin was clearly evidenced. The catalytic properties are explained by the distribution and morphology of Pt ensembles present on various faces of Pt–Sn alloys. The lower amount of alloys in sample T can be related to a higher initial activity in C5 ring closure whereas the higher amount of these phases on catalyst N is in accord with a higher turnover frequency, and a good selectivity for the formation of olefins which are transformed into C6 saturated skeletal isomers in longer runs. The results are supplemented by thermodynamic data on the reduction of tin oxides and by the geometric properties of the low-index faces of PtSn and Pt3Sn alloys.

Structural changes of Ce-Pr-O oxides in hydrogen: a study by in situ X-ray diffraction and Raman spectroscopy

Ce–Pr–O mixed oxides prepared by coprecipitation and calcined at 1173 K reveal a non stoichiometric structure with oxygen vacancies in the bulk. In situ XRD and Raman spectroscopy were used to investigate these properties simultaneously and identified a thermal expansion coefficient twice as high as that of pure ceria at room temperature and the presence of a Raman band at 560 cm−1, characteristic of the oxygen vacancies. These results confirm that praseodymium atoms are able to induce oxygen mobility in the bulk at 298 K. This is due to a structure containing a high number of anionic vacancies pre-existing at ambient temperature.

Characterization of the thermal genesis course of manganese oxides from inorganic precursors

Abstract NH4MnO4, Mn3O4 and Mn(NO3)2·6H2O were used as precursor compounds for the thermal genesis (at 150–600°C) of manganese oxides. Thermal events occurring during the genesis course were monitored by means of thermogravimetry and differential thermal analysis, in oxidizing and non-oxidizing atmospheres. Intermediate and final solid-phase products were characterized using X-ray diffractometry and infrared spectroscopy. Model manganese oxides were subjected to similar examinations for reference purposes. The results indicated that NH4MnO4 is almost completely decomposed near 120°C, giving rise to predominantly α-Mn2O3. The presence of K+ contaminant supports an oxidative conversion of α-Mn2O3 into KMn8O16+ at ⩾300°C. In contrast, the genesis of pure α-Mn2O3 from Mn(NO3)2·6H2O is not achieved unless the calcination temperature exceeds 500°C; β-MnO2 was the only detectable intermediate. Mn3O4, obtained at room temperature by the addition of aqueous Mn2+ to ammonia solution, was converted into α-Mn2O3 via the formation and subsequent decomposition of Mn5O8 at ⩾300°C.

Chain-like crystal structure and magnetic properties of [catena-bis(1,2- diaminoethane)copper(II)-μ-dicyano-argentate]-dicyanoargentate

Abstract Cu(en) 2 Ag 2 (CN) 4 (en = 1,2-diaminoethane) crystallizes in the orthorhombic space group Pnnm with cell parameters a = 6.316(1), b = 9.018(2), c = 13.199(3) A , Z = 2 . The structure is formed of free linear [Ag(CN) 2 ] − anions and infinite cationic chains [-CU(en) 2 NCAgCN-] − , containing paramagnetic copper atoms bridged by a second kind of linear dicyanoargentate species. The coordination geometry of the copper atoms corresponds to an elongated tetragonal bipyramid with two chelating en molecules in the equatorial positions and N-bonded bridging cyano groups in the axial positions. The Ag…Ag distances exhibit short value of 3.1580(5) A. Weak bifurcated hydrogen bonds of the N(3)H(2)…N(2)(C(2))…H(2)N(3) type, symmetrical about twofold axis, are present (N(3)…N(2) 3.195(3) A). These hydrogen bonds form two symmetrically related infinite 2D patterns in the xz plane at y = 0 and y = 1 2 connecting free dicyanoargentate anions and Cu(en) 2 2− cations. The phase identity between single crystal and bulk sample was evidence by powder X-ray diffractometry and Rietveld profile refinement. The study of magnetic properties by magnetization and specific heat measurements reveals that despite the chain structure the title compound may represent an S = 1 2 3 D magnetic system characterized by a low value of the exchange coupling constant. [ J / k b ] ⪡ 60 mK.

Development and Test of a Miniature Hydrogen Peroxide Monopropellant Thruster

Analysis of present and future missions concluded that a miniaturised hydrogen peroxide monopropellant rocket engine is the optimum solution for the increasing demand for small and low cost propulsion systems for small satellites. The attractiveness of monopropellant thrusters is based on its operational and structural simplicity. Additionally, the utilization of hydrogen peroxide as propellant instead of hydrazine allows the reduction of the overall costs and would qualify such a system as a green propellant propulsion system. The present paper describes the development of a monopropellant thruster utilizing hydrogen peroxide and advanced catalyst beds. The utilization of a monolithic catalyst reduces the pressure loss across the catalyst bed significantly compared to formerly used pellet or gauze catalyst. This allows the use of relative lightweight tank and significantly minimizes the total weight. For Two different catalyst materials have been developed to achieve optimized decomposition. The present paper summarizes the experimental evaluation of the catalysts. Decomposition temperatures of up to 670°C and decomposition efficiencies up to 99% have been achieved. Up to 1.2 kg of hydrogen peroxide has been decomposed by a single catalyst, corresponding to about 1.25 hrs of operation. This is estimated to correspond in vacuum condition to a total delivered total impulse of 1600 Ns. A thrust balance was designed and built. Preliminary thrust measurements under atmospheric conditions have shown that the laboratory model can generate thrust in a range of at least 50 to 550 mN.

Preparation, structure and properties of dicyano silver complexes of the M(en)3Ag2(CN)4 and M(en)2Ag2(CN)4 type

Abstract Complexes of the M(en)3Ag2(CN)4 (M = Ni, Zn, Cd) and M(en)2Ag2(CN)4 (M = Ni, Cu, Zn, Cd) type were prepared and identified by elemental analysis, infrared spectroscopy, measurement of magnetic susceptibility, and X-ray powder diffractometry. The crystal structures of Ni(en)3Ag2(CN)4 (I) and Zn(en)2Ag2(CN)4 (II) were determined by the method of monocrystal structure analysis. Complex I crystallizes in the space group C2/c, a = 1.2639(5), b = 1.3739(4), c = 1.2494(4) nm, β = 113.25(4)°, Dm = 1.86(1), Dc = 1.86 gcm−3 Z = 4, R = 0.0429. The crystal structure of I consists of complex cations [Ni(en)3]2+ and complex anions [Ag(CN)2]−. Complex II crystallizes in the space group I2/m, a = 0.9150(3), b = 1.3308(4), c = 0.6442(2) nm, β = 95.80(3)°, Dm = 2.14(1), Dc = 2.15 gcm−3, Z = 2, R = 0.0334. Its crystal structure consists of infinite, positively charged chains of the [-NCAgCNZn- (en)2]nn+ type and isolated [Ag(CN)2]− anions. The atoms of Ag are positioned parallely to the z axis and the AgAg distance is equal to 0.3221(2) nm.