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Abstract. The NH 4 Fe(HPO 4 ) 2 and its deuterated form, ND 4 Fe(DPO 4 ) 2 , were investigated in detail from powder neutron diffraction data allowing a precise determination of the hydrogen positions and low temperature magnetic structures. Below TC = 17.82 K they order with a ferrimagnetic structure and the magnetic moments lying in the crystallographic plane ac. As the temperature is lowered to Tt = 3.52 K the system undergoes a magnetic phase transition to an equal moment antiphase structure characterized by the propagation vector close to k AF ≈ (1/16, 0, 1/16) and a magnetic moment for the Fe 3+ ions of 4.8 µ B at 1.89 K. The low symmetry of this crystal structure and the complex pattern of competing superexchange pathways are responsible for the existence of this double magnetic phase transition.

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“…Cell parameters reported from single-crystal x-ray experiments on NH 4 Fe(HPO 4 ) 2 by Yakubovich were used as starting point [6] (see Fig. 1).…”

Section: Resultsmentioning

confidence: 99%

Powder neutron diffraction investigation of the crystal and magnetic structures of NH₄Fe(HPO₄)₂and its deuterated form

Alfonso

Piqué

Trobajo

et al. 2011

J. Phys.: Conf. Ser.

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“…The formation of a 2 -hydrocarbons at elevated pressures on the Co/SiO 2 ·Zr(IV) catalyst proceeds through the following mechanism [8]: (VII) ZCH 2 CH 3 + ZCH 2 ® ZCH 2 CH 2 CH 3 + Z (chain growth), (VIII) ZCH 2 CH 2 CH 3 ® HZCH 2 CHCH 3 (chain termination), (IX) HZCH 2 CHCH 3 + ZH ® CH 3 CH 2 CH 3 + 2Z (propane formation), (X) HZCH 2 CHCH 3 ® CH 2 CHCH 3 + ZH (propylene formation), (XI) ZCH 2 CH 2 CH 3 + ZCH 2 ® Z(CH 2 ) 3 CH 3 ¼ (further chain growth).…”

Section: Resultsmentioning

confidence: 99%

Secondary reactions of ethylene and propylene in the Fischer-Tropsch synthesis on cobalt-aluminum and cobalt-chromium catalysts

2008

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The only secondary reaction of olefins in the Fischer-Tropsch synthesis on Co/SiO 2 ·Al(III) and Co/SiO 2 ·Cr(III) with 2.7·10 -6 and 4.0·10 -6 g-ion/m 2 cation concentration in the support matrix, respectively, is dimerization of ethylene to give a four-carbon intermediate, which participates in the growth of C-C bond chains according to the classical FT synthesis mechanism. When the chromium concentration is increased by a factor of 1.7, secondary ethylene reactions occurring on the acid sites of the catalyst are also observed. When the aluminum concentration is increased by a factor of 3.4, we observe the oligomerization of ethylene and propylene. The secondary reactions of ethylene mainly result in linear hydrocarbons, while oligomerization involving propylene gives a significant amount of branched hydrocarbons.The secondary reactions of olefins in the Fischer-Tropsch (FT) synthesis on cobalt catalysts have been studied in the past few decades [1]. The interest in these reactions has arisen primarily in attempts to control the selectivity of the hydrocarbon synthesis toward specific products. Thus, these reactions hold great theoretical and practical importance in the study of the FT synthesis. The secondary reactions of olefins may proceed not only on cobalt but also on the acid sites of the catalysts [2]. Such sites may be metal cations (Lewis sites (L sites)) or related Brönsted acid sites [3].In the present work, we studied the effect of the surface concentration of aluminum and chromium cations on the secondary reactions of olefins in the FT synthesis on Co/SiO 2 ·Al(III) and Co/SiO 2 ·Cr(III) catalysts. EXPERIMENTALThe experiments were carried out in a flow reactor at 5.0 MPa and 530 K. The volumetric rate for the introduction of synthesis gas (H 2 : CO = 2.5 ± 0.1) was 1.2 ± 0.1 h -1 .Silicon dioxide was obtained by adding a sodium silicate solution to neutralize aqueous sulfuric acid to pH 6.5-7.0. After syneresis, the silica hydrogel was ground and washed with distilled water to remove the sodium sulfate side-product (monitoring by reaction with 5% aqueous BaCl 2 ). The washed silica gel was subjected to steam treatment at water bath temperature over 7 h. The chromium and aluminum forms of silica gel for the preparation of samples 1 were obtained by direct ion exchange through treatment of the sodium form by chromium nitrate and aluminum sulfate at 373 K with subsequent 0040-5760/08/4402-0121

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“…The molecular mass distribution of the hydrocarbons formed in the Fischer-Tropsch synthesis at Co/SiO 2 catalysts, which have been attracting increased interest [35][36][37][38][39][40], differs from the Anderson-Schulz-Flory distribution [7,9,16,20,30]. Such a distribution has a low content of the C 2 and sometimes of the C 3 hydrocarbons and shows an increase in the effective probability of chain growth with increase in its length for intermediates containing more than two or three carbon atoms.…”

Section: Resultsmentioning

confidence: 99%

“…Various schemes have been proposed for the mechanism of the process to explain the deviations from the ASF distribution [2][3][4][5][6][7][8][9]. On the basis of such schemes mathematical models establishing a relationship between the concentrations of the individual products and the carbon number were derived.…”

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Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

Yakubovich

Strizhak

2007

Theor Exp Chem

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Kinetic models were used to investigate the Fischer-Tropsch synthesis at increased pressures. The general mechanism proposed for the process at low and increased pressures was based on quantitative analysis of the products obtained with cobalt catalysts at pressures of 2.5-10 MPa using various kinetic models, the previously developed reference point method, and published data.The molecular mass distributions (MMD) of the hydrocarbons formed in the Fischer-Tropsch (FT) synthesis often differ from the Anderson-Schulz-Flory (ASF) distribution [1]. Various schemes have been proposed for the mechanism of the process to explain the deviations from the ASF distribution [2][3][4][5][6][7][8][9]. On the basis of such schemes mathematical models establishing a relationship between the concentrations of the individual products and the carbon number were derived. As kinetic coefficients they contain the probabilities or functions of the probabilities that the reactions occur with the participation of intermediates. They are models of the selectivity, but they make it possible to calculate the absolute rates of all stages of the kinetic scheme, starting from the rate of transformation of CO or H 2 [2]. A whole series of reasons for the deviations from the ASF distribution have been discussed in the literature.The reduced or increased methane content of the products is explained by the fact that the C 1 and C 2+ hydrocarbons are formed at different centers on the surface of the catalyst [10] or as a result of reactions leading to termination of the chain [1]. The deviation from the ASF distribution is sometimes attributed to increase in the probability of growth of the chain of C-C bonds (a) according to Herington scheme [11] with increase in the carbon number [1].The molecular mass distributions of the hydrocarbons often take the form of concave curves with a clearly defined kink [1,12,13]. Anderson approximated such distributions by two straight lines with different slopes in relation to the abscissa axis [13]. The existence of two probabilities of chain growth in the Fischer-Tropsch synthesis (usually a 1 < a 2 ) was thus mentioned first in this paper. Later Huff and Sutterfield [12] proposed an equation describing such distributions. This equation was obtained on the basis of the idea that the experimental molecular mass distributions represent the superimposition of the distributions of the a 1 -and a 2 -hydrocarbons.Initially the existence of two values for the probability of chain growth was explained by the fact that the growth of the C-C bonds takes place at different centers of the catalyst [12, 14]. However, subsequent data indicated that the two values of a arise as a result of simultaneous growth of the chain by two different mechanisms through two types of intermediates [9,[15][16][17][18]. The distributions of the hydrocarbons formed at Fe catalysts usually obey the Huff-Sutterfield equation [1,[11][12][13].(Here, they can often be approximated with sufficient accuracy by two straight lines.) The distributions o...

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Cited by 15 publications

References 4 publications

Powder neutron diffraction investigation of the crystal and magnetic structures of NH₄Fe(HPO₄)₂and its deuterated form

Powder neutron diffraction investigation of the crystal and magnetic structures of NH₄Fe(HPO₄)₂and its deuterated form

Secondary reactions of ethylene and propylene in the Fischer-Tropsch synthesis on cobalt-aluminum and cobalt-chromium catalysts

Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

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Cited by 15 publications

References 4 publications

Powder neutron diffraction investigation of the crystal and magnetic structures of NH4Fe(HPO4)2and its deuterated form

Powder neutron diffraction investigation of the crystal and magnetic structures of NH4Fe(HPO4)2and its deuterated form

Secondary reactions of ethylene and propylene in the Fischer-Tropsch synthesis on cobalt-aluminum and cobalt-chromium catalysts

Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

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Product

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Powder neutron diffraction investigation of the crystal and magnetic structures of NH₄Fe(HPO₄)₂and its deuterated form

Powder neutron diffraction investigation of the crystal and magnetic structures of NH₄Fe(HPO₄)₂and its deuterated form