Lu2SiO5 by single‐crystal X‐ray and neutron diffraction

The structure of dilutetium silicon pentaoxide, Lu₂SiO₅, has isolated ionic SiO₄ tetrahedral units and non‐Si‐bonded O atoms in distorted OLu₄ tetrahedra. The OLu₄ tetrahedra form edge‐sharing infinite chains and double O₂Lu₆ tetrahedra along the c axis. The edge‐sharing chains are connected to the O₂Lu₆ double tetrahedra by isolated SiO₄ units. The structure has been determined by neutron diffraction.     

Crystal Structure and Magnetic Properties of a New Iron(III) Silicophosphate Fe3P5SiO19 Studied by Neutron Diffraction and Mössbauer Techniques

Fe₃P₅SiO₁₉ has been prepared by solid state reaction of Fe(PO₃)₃, FePO₄, and SiO₂ at 1000°C. The structure has been determined from a single crystal through direct methods and difference Fourier synthesis and refined to R=0.052. The unit cell is hexagonal, space group P6₃, with a=14.4804(8) Å, c=7.4256(2) Å, and Z=4. The three-dimensional framework is built up from [Fe₂O₉] units of two faces sharing octahedra and Si₂O₇ disilicates linked by PO₄ tetrahedra. Fe₃P₅SiO₁₉ is isotypic with V₃P₅SiO₁₉. Fe₃P₅SiO₁₉ is antiferromagnetic below T_N=35 K. The magnetic structure has been determined by means of powder neutron diffraction methods: the magnetic moments are antiferromagnetically coupled inside the [Fe₂O₉] units, in agreement with the Goodenough rules. These units are linked to each other through several Fe-O-P-O-Fe super-superexchange pathways and form antiferromagnetic [001] rows. The moment direction lies in the (001) plane (μ_Fe=4.56(5) μ_B at 2 K). There is a competition between the intra- and interunits interactions which all are antiferromagnetic and cannot be simultaneously satisfied without frustration. Mössbauer spectra are fitted with two doublets and two sextuplets in the paramagnetic and antiferromagnetic states, respectively. Their rather high isomer shifts are explained by the inductive effect. The magnetic interactions are discussed.     

H_6P_2W_(18)O_(62)/SiO_2:用模板剂制备及光催化降解二甲酚橙的性能

分别采用非离子表面活性剂(C_3H_6OC_2H_4O)_x(P123),阴离子表面活性剂C_(12)H_(25)NaO_4S(SDS)和C_(18)H_(29)NaO_3S(SDBS)作为模板剂,通过溶胶-凝胶再结合程序升温溶剂热一步法制备了一系列固载杂多酸光催化材料-H_6P_2W_(18)O_(62)/SiO_2。通过傅立叶-红外光谱(FTIR)、X-射线衍射(XRD)、电感耦合等离子体原子发射光谱(ICP-AES)、氮气吸附-脱附测定、透射电子显微镜(TEM)以及扫描电子显微镜配合X-射线能量色散谱仪(SEM-EDS)等测试手段对不同模板剂作用下合成产物进行了对比表征分析。结果表明,不同模板剂作用下的系列固载杂多酸-H_6P_2W_(18)O_(62)/SiO_2产物中母体多酸的Dawson基本结构均未发生明显变化,但固载后比表面积显著不同,其中,经模板剂P123和SDS作用合成的H_6P_2W_(18)O_(62)/SiO_2(P123)和H_6P_2W_(18)O_(62)/SiO_2(SDS)的比表面积高达916和634m~2·g~(-1),且显示为有序介孔材料。以二甲酚橙为模型分子,在微波场作用下,该系列固载多酸光催化性能研究结果显示,它们的光催化活性可被微波显著增强,其中,采用P123作用合成产物的光催化活性最高,60 min内对二甲酚橙的降解率达99%以上。     

Sm4S3[Si2O7] und NaSm9S2[SiO4]6: Zwei Sulfidsilicate mit dreiwertigem Samarium

Sm₄S₃[Si₂O₇] and NaSm₉S₂[SiO₄]₆: Two Sulfide Silicates with Trivalent Samarium The sulfide silicates Sm₄S₃[Si₂O₇] and NaSm₉S₂[SiO₄]₆ are obtained as light yellow transparent crystals by the reaction of Sm, Sm₂O₃, S, and SiO₂ with fluxing SmCl₃ or NaCl, respectively, in suitable molar ratios in fused evacuated silica tubes (850 °C, 7 d). Tetragonal crystals of Sm₄S₃[Si₂O₇] (I4₁/amd; Z = 8; a = 1186.4(1); c = 1387.0(2) pm) with ecliptically conformed [Si₂O₇]^6–‐groups of corner sharing [SiO₄]‐tetrahedra are formed. These double tetrahedra as well the sulfide anions (S^2–) coordinate two crystallographically independent metal cations. They provide coordination numbers of 8 + 1 (5 S^2– and 3 + 1 O^2–) for Sm1 and 9 (3 S^2– and 6 O^2–) for Sm2. NaSm₉S₂[SiO₄]₆ crystallizes hexagonally (P6₃/m; Z = 1; a = 975.32(9); c = 676.46(7) pm) in a modified bromapatite‐type structure. The coordination spheres about the two crystallographically different Sm³⁺ cations are built up by oxygen atoms of the orthosilicate units ([SiO₄]^4–) and sulfide anions (S^2–). As a result, Sm1 and Sm2 have coordination numbers of 9 and 8, respectively. Na⁺ and (Sm1)³⁺ occupy the position 4 f in a molar ratio of 1 : 3 whereas the lower coordinated (Sm2)³⁺ occupies the 6 h position.     

Zwei Chloridsilicate des Yttriums: Y3Cl[SiO4]2 und Y6Cl10[Si4O12]

Two Chloride Silicates of Yttrium: Y₃Cl[SiO₄]₂ and Y₆Cl₁₀[Si₄O₁₂] The chloride‐poor yttrium(III) chloride silicate Y₃Cl[SiO₄]₂ crystallizes orthorhombically (a = 685.84(4), b = 1775.23(14), c = 618.65(4) pm; Z = 4) in space group Pnma. Single crystals are obtained by the reaction of Y₂O₃, YCl₃ and SiO₂ in the stoichiometric ratio 4 : 1 : 6 with ten times the molar amount of YCl₃ as flux in evacuated silica tubes (7 d, 1000 °C) as colorless, strongly light‐reflecting platelets, insensitive to air and water. The crystal structure contains isolated orthosilicate units [SiO₄]^4– and comprises cationic layers {(Y2)Cl}²⁺ which are alternatingly piled parallel (010) with anionic double layers {(Y1)₂[SiO₄]₂}^2–. Both crystallographic different Y³⁺ cations exhibit coordination numbers of eight. Y1 is surrounded by one Cl^– and 7 O^2– anions as a distorted trigonal dodecahedron, whereas the coordination polyhedra around Y2 show the shape of bicapped trigonal prisms consisting of 2 Cl^– and 6 O^2– anions. The chloride‐rich chloride silicate Y₆Cl₁₀[Si₄O₁₂] crystallizes monoclinically (a = 1061,46(8), b = 1030,91(6), c = 1156,15(9) pm, β = 103,279(8)°; Z = 2) in space group C2/m. By the reaction of Y₂O₃, YCl₃ and SiO₂ in 2 : 5 : 6‐molar ratio with the double amount of YCl₃ as flux in evacuated silica tubes (7 d, 850 °C), colorless, air‐ and water‐resistant, brittle single crystals emerge as pseudo‐octagonal columns. Here also a layered structure parallel (001) with distinguished cationic double‐layers {(Y2)₅Cl₉}⁶⁺ and anionic layers {(Y1)Cl[Si₄O₁₂]}^6– is present. The latter ones contain discrete cyclo‐tetrasilicate units [Si₄O₁₂]^8– of four cyclically corner‐linked [SiO₄] tetrahedra in all‐ecliptical arrangement. The coordination sphere around (Y1)³⁺ (CN = 8) has the shape of a slightly distorted hexagonal bipyramid comprising 2 Cl^– and 6 O^2– anions. The 5 Cl^– and 2 O^2– anions building the coordination polyhedra around (Y2)³⁺ (CN = 7) form a strongly distorted pentagonal bipyramid.     

Er4F2[Si2O7][SiO4]: Das erste Selten‐Erd‐Fluoridsilicat mit zwei verschiedenen Silicat‐Anionen

Er₄F₂[Si₂O₇][SiO₄]: The First Rare‐Earth Fluoride Silicate with Two Different Silicate Anions By the reaction of Er₂O₃ with ErF₃ and SiO₂ at 700 °C in sealed tantalum capsules using CsCl as flux (molar ratio 5 : 2 : 3 : 20), the compound Er₄F₂[Si₂O₇][SiO₄] (triclinic, P 1; a = 648.51(5), b = 660.34(5), c = 1324.43(9) pm, α = 87.449(8), β = 85.793(8), γ = 60.816(7)°; V_m = 148.69(1) cm³/mol, Z = 2) is obtained as pale pink platelets or lath‐shaped single crystals. It consists of disilicate anions [Si₂O₇]^6– in eclipsed conformation, ortho‐silicate anions [SiO₄]^4– and isolated [Er₄F₂]¹⁰⁺ units comprising two edge‐shared [Er₃F] triangles. Er³⁺ is surrounded by 7 + 1 (Er1) or 7 (Er2–Er4) anionic neighbors, respectively, of which two are F^– in the case of Er1 and Er4 but only one for Er2 and Er3. The other ligands recruit from oxygen atoms of the different oxosilicate groups. The crystal structure can be described as simple rowing up of the three building groups ([SiO₄]^4–, [Er₄F₂]¹⁰⁺, and [Si₂O₇]^6–) along [001]. The necessity of a large excess of fluoride for a successful synthesis of Er₄F₂[Si₂O₇][SiO₄] will be discussed.     

Two Pyrophosphates with Large Birefringences and Second-Harmonic Responses as Ultraviolet Nonlinear Optical Materials

Two new pyrophosphates nonlinear optical (NLO) materials, Rb₃PbBi(P₂O₇)₂ ( I ) and Cs₃PbBi(P₂O₇)₂ ( II ), were successfully designed and synthesized. Both compounds exhibit large NLO effects and birefringences. Material I presents the scarce case of possessing the coexistence of large birefringence (0.031 at 1064 nm and 0.037 at 532 nm) and second harmonic generation (SHG) response (2.8× potassium dihydrogen phosphate (KDP)) in ultraviolet NLO phosphates and its SHG is the largest in the phase-matching (PM) pyrophosphates. Both I and II have three-dimensional (3D) crystal structures composed of corner-shared RbO₁₂ (CsO₁₁), RbO₁₀ (CsO₁₀), BiO₆, PbO₇ (PbO₆) and P₂O₇ groups, in which P₂O₇ and PbO₇ (PbO₆) units form an alveolate [PbPO]_∞ skeleton frame. Theoretical calculations reveal that the P−O, Bi−O and Pb−O units are mainly responsible for the moderate birefringence and large SHG efficiency of I .     

Two Pyrophosphates with Large Birefringences and Second‐Harmonic Responses as Ultraviolet Nonlinear Optical Materials

Two new pyrophosphates nonlinear optical (NLO) materials, Rb₃PbBi(P₂O₇)₂ ( I ) and Cs₃PbBi(P₂O₇)₂ ( II ), were successfully designed and synthesized. Both compounds exhibit large NLO effects and birefringences. Material I presents the scarce case of possessing the coexistence of large birefringence (0.031 at 1064 nm and 0.037 at 532 nm) and second harmonic generation (SHG) response (2.8× potassium dihydrogen phosphate (KDP)) in ultraviolet NLO phosphates and its SHG is the largest in the phase‐matching (PM) pyrophosphates. Both I and II have three‐dimensional (3D) crystal structures composed of corner‐shared RbO₁₂ (CsO₁₁), RbO₁₀ (CsO₁₀), BiO₆, PbO₇ (PbO₆) and P₂O₇ groups, in which P₂O₇ and PbO₇ (PbO₆) units form an alveolate [PbPO]_∞ skeleton frame. Theoretical calculations reveal that the P?O, Bi?O and Pb?O units are mainly responsible for the moderate birefringence and large SHG efficiency of I .     

Densification mechanisms of alumina,aluminosilicates and aluminoborosilicates gels,glasses and ceramics

The thermal evolution of gels, glasses and ceramics of various more or less refractory compositions (Al₂O₃, 3Al₂O₃2SiO₂, 7Al₂O₃3SiO₂, Al₂O₃2SiO₂, Al₂O₃2SiO₂0.7B₂O₃, Al₂O₃2SiO₂2B₂O₃, Al₂O₃2SiO₂6B₂ O₃) have been studied by dilatometry, DTA, and helium density measurements. Comparison is made for materials prepared by rapid (powder) or by very slow gelation (optically clear monoliths). The influence of atmosphere sintering (air, H₂, vacuum) is reported. Densification and kinetic laws are discussed.Also at LASIR, CNRS, 2 rue Henry Dunant, 94320 Thiais, France.     

Preparation and Electrical Properties of Lnx(SiO4)6O(1.5x?12) (Ln: Nd, La) with Apatite Structure

In this study, Ln_x(SiO₄)₆O_(1.5x–12) (Ln: Nd, La) materials as electrolytes for solid oxide fuel cells (SOFC) were prepared by the sol-gel process. It has been reported that the apatite structure of Ln_x(SiO₄)₆O_(1.5x–12) shows higher ionic conductivity than yttrium-stabilized zirconium oxide at the working temperature of the SOFC. Ln₁₀(SiO₄)₆O₃ is a major component of the Ln_x(SiO₄)₆O_(1.5x–12) system. Ln₁₀(SiO₄)₆O₃ consists of Ln_9.33(SiO₄)₆O₂ and a small amount of Ln₂SiO₅. It has been proposed that the ionic conductivity of Ln_x(SiO₄)₆O_(1.5x–12) decreases with increasing Ln₂SiO₅ with non-apatite structure. The object of the present study was to bring about this decrease by generating Ln₂SiO₅ in the system.Precursor solutions for synthesis of the powder were prepared using tetraethoxysilane (Si(OC₂H₅)₄) and neodymium acetate monohydrate (Nd(CH₃COO)₃·H₂O) or lanthanum acetate monohydrate (La (CH₃COO)₃·H₂O) as raw materials and acetic acid (CH₃COOH), 2-methoxyethanol (C₂H₅OCH₂CH₂OH), and triethanolamine (N(CH₂CH₂OH)₃) as solvents. To obtain the powder, the solution was dried and heat-treated at 600 °C for 2 h. Disks made from the powder were heat-treated at temperatures between 1100 and 1500 °C for 10 h. The results of an XRD investigation indicate that almost all diffraction peaks of these samples could be assigned to Ln_9.33(SiO₄)₆O₂. The sample with x = 10.00 included a small amount of Ln₂SiO₅. The ionic conductivity of this latter sample was higher than that of other samples with similar values of x (x = 9.33 and 10.67).     

The vibrational spectra of strontium and barium diarsenates

The infrared and laser-Raman spectra of crystalline Sr₂As₂O₇ and Ba₂As₂O₇ are reported and discussed. The principal force constants for the As₂O₇^4? ion are calculated using a simplified molecular model.     

Anhydrous Silicophosphoric Acid Glass: Thermal Properties and Proton Conductivity

Anhydrous silicophosphoric acid glass with an approximate composition of H₅Si₂P₉O₂₉ was synthesized and its thermal and proton-conducting properties were characterized. Despite exhibiting a glass transition at 192 °C, the supercooled liquid could be handled as a solid up to 280 °C owing to its high viscosity. The glass and its melt exhibited proton conduction with a proton transport number of ∼1. Although covalent O−H bonds were weakened by relatively strong hydrogen bonding, the proton conductivity (4×10⁻⁴ S cm⁻¹ at 276 °C) was considerably lower than that of phosphoric acid. The high viscosity of the melt was due to the tight cross-linking of phosphate ion chains by six-fold-coordinated Si atoms. The low proton conductivity was attributed to the trapping of positively charged proton carriers around anionic SiO₆ units (expressed as (SiO_6/2)²⁻) to compensate for the negative charges.     

La2Si2O7 im I—Typ: Gemäß La6[Si4O13][SiO4]2 kein echtes Lanthandisilicat

I‐Type La₂Si₂O₇: According to La₆[Si₄O₁₃][SiO₄]₂ not a Real Lanthanum Disilicate In attempts to synthesize lanthanum telluride silicate La₂Te[SiO₄] (from La, TeO₂, SiO₂ and CsCl, molar ratio: 1 : 1: 1 : 20, 950 °C, 7 d) or fluoride‐rich lanthanum fluoride silicates (from LaF₃, La₂O₃, SiO₂ and CsCl, molar ratio: 5 : 2 : 3 : 17, 700 °C, 7 d) in evacuated silica tubes, colourless lath‐shaped single crystals of hitherto unknown I‐type La₂Si₂O₇ (monoclinic, P2₁/c; a = 726.14(5), b = 2353.2(2), c = 1013.11(8) pm, β = 90.159(7)°) were found in the CsCl‐flux melts. Nevertheless, this new modification of lanthanum disilicate does not contain any discrete disilicate groups [Si₂O₇]^6‐ but formally three of them are dismutated into one catena‐tetrasilicate ([Si₄O₁₃]^10‐ unit of four vertex‐linked [SiO₄]^4‐ tetrahedra) and two ortho‐silicate anions (isolated [SiO₄]^4‐ tetrahedra) according to La₆[Si₄O₁₃][SiO₄]₂. This compound can be described as built up of alternating layers of these [SiO₄]^4‐ and the horseshoe‐shaped [Si₄O₁₃]^10‐ anions along [010]. Between and within the layers the high‐coordinated La ³⁺ cations (CN = 9 ‐ 11) are localized. The close structural relationship to the borosilicates M₃[BSiO₆][SiO₄](M = Ce ‐ Eu) is discussed and structural comparisons with other catena‐tetrasilicates are presented.     

Zwei neue Silicat-Chloride mit zweiwertigem Europium: LiEu3[SiO4]Cl3 und Li7Eu8[SiO4]4Cl7

Two New Silicate-Chlorides with Divalent Europium: LiEu₃[SiO₄]Cl₃ and Li₇Eu₈[SiO₄]₄Cl₇ LiEu₃[SiO₄]Cl₃ was prepared by reaction of LiCl with Eu₂SiO₄ and Li₇Eu₈[SiO₄]₄Cl₇ from Li with Eu₂O₃, SiO₂ and LiCl. The crystal structures of LiEu₃[SiO₄]Cl₃ (Pmna, a = 946.95(13); b = 699.52(8); c = 1 368.0(2) pm; Z = 4; R1 = 0.0325, R2_w = 0.0642) and Li₇Eu₈[SiO₄]₄Cl₇ (P2₁/c; a = 851.85(5); b = 948.62(7); c = 1 679.0(2) pm; β = 96.221(8)°; Z = 2; R1 = 0.0352, R2_w = 0.0744) were determined from four-circle diffractometer data. LiEu₃[SiO₄]Cl₃ contains [Li(SiO₄)₂] units and LiCl₆ octahedra while in Li₇Eu₈[SiO₄]₄Cl₇ larger ?lithosilicate”? groups are found. In both structures, the Eu²⁺ ions are coordinated mostly eightfold by O^2? and Cl^? ligands.     

Synthesis and Disassembly/Reassembly of Giant Ring‐Shaped Polyoxotungstate Oligomers

The disassembly and reassembly of giant molecules are essential processes in controlling the structure and function of biological and artificial systems. In this work, the disassembly and reassembly of a giant ring‐shaped polyoxometalate (POM) without isomerization of the monomeric units is reported. The reaction of a hexavacant lacunary POM that is soluble in organic solvents, [P₂W₁₂O₄₈]^14?, with manganese cations gave the giant ring‐shaped POM [{γ‐P₂W₁₂O₄₈Mn₄(C₅H₇O₂)₂(CH₃CO₂)}₆]^42?. This POM is a hexamer of manganese‐substituted {P₂W₁₂O₄₈Mn₄} units, and its inner cavity was larger than any of those previously reported for ring‐shaped polyoxotungstates. It was disassembled into monomeric units in acetonitrile, and the removal of the capping organic ligands on the manganese cations led to reassembly into a tetrameric ring‐shaped POM, [{γ‐P₂W₁₂O₄₈Mn₄(H₂O)₆}₄(H₂O)₄]^24?.     

Investigating the Crystallization Process of Boron-Bearing Silicate-Phosphate Glasses by Thermal and Spectroscopic Methods

Glasses and devitrificates from the SiO₂–B₂O₃–P₂O₅–K₂O–CaO–MgO system with constant contents of SiO₂ and P₂O₅ network formers, modified by the addition of B₂O₃, were analyzed. All materials were synthesized by the traditional melt-quenching technique. The glass stability (GS) parameters (K_rg, ∆T, K_W, K_H) were determined. The effect of the addition of B₂O₃ on the GS, liquation phenomenon, crystallization process, and the type of crystallizing phases were examined using SEM-EDS, DSC, XRD, and Raman spectroscopy imaging methods. It was observed that the addition of B₂O₃ increased the tendency of the glass to crystallize. Both phosphates (e.g., Ca₉MgK(PO₄)₇, Mg₃Ca₃(PO₄)₄), and silicates (e.g., K₂Mg₅(Si₁₂O₃₀), CaMg(Si₂O₆), MgSiO₃) crystallized in the studied system. The Raman spectrum for the orthophosphate Mg₃Ca₃(PO₄)₄ stanfieldite type was obtained. Boron ions were introduced into the structures of crystalline compounds at high crystallization temperatures. The type of crystallizing phases was found to be related to the phenomenon of liquation, and the order of their occurrence was dependent on the Gibbs free enthalpy.     

Thermischer Zerfall von AlPO4−SiO2-Mischkristallen

Zusammenfassung AlPO₄-10 Mol% SiO₂-Mischkristalle mit stapelfehlgeordneter Cristobalit-Tridymit-Struktur zeigen gegenüber reinem AlPO₄ mit Cristobalit-Struktur eine auffällige Hemmung der thermischen Zersetzung. Dies wird auf die Bildung thermisch stabiler diffusionshemmender SiO₂–P₂O₅-Schmelzhäute zurückgeführt. Zusätze von GeO₂, SnO₂ oder TiO₂ zu AlPO₄ bewirken diesen Hemmungseffekt nicht. Schmelzen von GeP₂O₇, SnP₂O₇ und TiP₂O₇ zerfallen bei 1400°C schnell. Mechanische Gemenge von AlPO₄ und SiO₂ zeigen den Hemmungseffekt nur schwach. AlAsO₄ zerfällt mit und ohne SiO₂-Zusatz rasch oberhalb 1000°C.

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Thermal decomposition of AlPO₄–SiO₂ mixed crystals

AlPO₄-10 mole% SiO₂ solid solutions, representing the stacking-disordered cristobalite-tridymite structure, in contrast to pure AlPO₄ show a remarkable reduction of rate of thermal decomposition. This is brought into connection with the formation of molten surface layers of SiO₂–P₂O₅. Simple mixtures of AlPO₄ with SiO₂ in the form of quartz powder or amorphous silica gel show this effect only scarcely. Additions of GeO₂, SnO₂ or TiO₂ do not show this effect at all. In contrast to melts of the composition SiP₂O₇, melts of GeP₂O₇, SnP₂O₇ and TiP₂O₇ decompose quickly at 1400°C. AlAsO₄ even with the addition of SiO₂ decomposes very rapidly above 1000°C.

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Mit 5 Abbildungen

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Herrn Prof. Dr.H. Nowotny gewidmet.     

Molecular Vibrations and Mean Square Amplitudes. Supplement III Mean Amplitudes of Vibration for Inorganic Six-Atomic Molecules

Review of mean amplitudes of vibration for inorganic six-atomic molecules from spectroscopic calculations and electrondiffraction-data. The work contains new results of spectroscopic mean amplitudes and force constants for a number of molecules, viz.: S₆, N₂O₄, B₂F₄, N₂H₄, N₂F₄, P₂I₄ and SOF₄.     

Synthesis of Nickel Supported Catalysts for Hydrogen Production by Sol-Gel Method

SiO₂ and Al₂O₃ supported Ni catalysts were synthesized in the form of xerogels: the SiO₂ based materials were prepared starting from Ni propionate or glycolate salts and reacting them with tetraethoxysilane (TEOS) in propionic acid, Si(ethylene glycolate) or sodium silicate. The Al₂O₃ supported catalysts were prepared similarly from Ni propionate salts with Al iso-propoxide salts. Narrow metal particles and strong metal support interactions are observed in the sol-gel catalysts. The metal dispersion was higher for Al₂O₃ based materials than the SiO₂ ones and it deeply depends on the Ni precursor for the silica supported Ni. Wet impregnated oxides with similar Ni loading have higher metal surface area than those from sol-gel processing. The influence of surface differences on the catalytic activity of the materials was studied following the CH₄ and CO₂ reaction in dry reforming conditions by pulse reaction tests.