Ammonium Dawsonite Research Articles

Terahertz time domain spectroscopy of hydrothermally synthesized boehmite and ammonium dawsonite nanostructures

The frequency dependent optical and dielectric properties of boehmite (AlOOH) and ammonium aluminum carbonate hydroxide (AACH) nanostructures, prepared at different temperatures via hydrothermal synthesis, were studied by using terahertz time domain spectroscopy (THz-TDS). The complex refractive indices, absorption coefficients and complex dielectric constants were determined and compared for different synthesis temperatures. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were also performed to confirm the formation of boehmite and AACH. Scanning electron microscopy (SEM) was performed to study the morphology of nanostructures. The study reveals the formation of AACH at higher temperatures and decrease in refractive index with increasing synthesis temperature. The higher absorption coefficient and lower refractive index and dielectric constant were observed for AACH than for boehmite.

Synthesis and Sintering of Magnesium Aluminate Spinel Nanopowders Prepared by Precipitation Method using Ammonium Hydrogen Carbonate as a Precipitant

Magnesium aluminate spinel (MgAl2O4) is widely used in many engineering applications due to its high melting point (2135°C), high mechanical strength, chemical inertness, and good optical properties. Precipitation method is recognized as a convenient and cost-effective method for the synthesis of nanopowders. In this present work, MgAl2O4 nanopowders were prepared by precipitation method using ammonium hydrogen carbonate as a precipitant. The precipitated precursors were a mixture of ammonium dawsonite (NH4Al (OH)2CO3·H2O) and hydrotalcite (Mg6Al2(CO3)(OH)16·4H2O). After calcining at 1100°C for 2 hours, The MgAl2O4 nanopowders with particle size of 20-170 nm were obtained. The sinterability of the MgAl2O4 nanopowders was evaluated by sintering compacts of the MgAl2O4 nanopowders at temperature of 1300-1650°C for 2 hours. The relative density of the sintered MgAl2O4 ceramics reached about >97% of theoretical density after sintering at 1500°C for 2 hours. The Vicker’s hardness of the sintered ceramics reached a value of 1414 HV (13.9 GPa) after sintering at 1650°C for 2 hours.

Alumina/YAG 20 vol% composites prepared by the dawsonite thermal decomposition

The co-precipitation of alumina/YAG precursor from the aqueous solution of aluminum and yttrium nitrates with ammonium carbonate results in ammonium dawsonite (NH4Al(OH)2CO3) with yttrium built into its structure. The decomposition of dawsonite at elevated temperatures leads to the formation of γ-Al2O3. The δ- and θ-Al2O3 modifications appear at higher and higher temperatures. A full transformation to α-Al2O3 and YAG occurs at the temperatures higher than 1230°C. The starting powder for sintering experiments was prepared using the co-precipitated precursor calcination at 600°C. Seeding of such a powder with 5wt% α-Al2O3 proved to be necessary in order to receive the dense composite material at 1500°C. Such a material shows much lower wear susceptibility as compared to the dense pure alumina polycrystal. The unseeded powder led to the high porosity samples and showed symptoms of a discontinuous grain growth.

Dynamic study of ammonium dawsonite doped with yttrium transformation at elevated temperatures

Co-precipitation of alumina/YAG precursor from aluminum and yttrium nitrates solution with ammonium carbonate results in dawsonite (NH4Al(OH)2CO3). Its crystallographic parameters differ from the compound precipitated without the yttrium additive. It indicates that yttrium ions become incorporated into the dawsonite structure. The DSC/TG and X-ray measurements show decomposition of dawsonite at elevated temperature resulting in γ-Al2O3 which transforms to δ and θ modifications at still higher temperatures. The full transformation to α-Al2O3 and YAG occurs at temperatures higher than 1,230 °C.

Preparation of Lithium Aluminum Layered Double Hydroxide from Ammonium Dawsonite and Lithium Carbonate

ABSTRACTAluminum lithium hydroxide carbonate hydrate, also known as Al/Li layered double hydroxide or Al-Li hydrotalcite-like compound [Al2Li(OH)6]2CO3•nH2O, was prepared by reaction of lithium carbonate with ammonium dawsonite [NH4Al(OH)2CO3]. The reaction of ammonium dawsonite with a lithium carbonate satured solution at different temperatures and lithium carbonate concentrations was studied. The obtained solids were characterized by differential thermal analysis (DTA), thermogravimetry (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). By this method, crystalline Li/Al LDH [Al2Li(OH)6]2CO3·3H2O can be obtained at 60 °C and 4 h reaction time.

Enhanced Cu activity in catalytic ozonation of clofibric acid by incorporation into ammonium dawsonite

Clofibric acid (CFA) degradation by ozonation in the presence of Cu-dawsonites was studied. Cu-dawsonite materials were synthesized via co-precipitation of Al(NO3)3 and Cu(NO3)2 by (NH4)2CO3 at pH 7.5–8.0. The as-synthesized and the calcined samples were characterized using XRD, FTIR, N2-physisorption, TEM and XPS techniques. The CFA degradation was carried out using the as-synthesized and the calcined Cu-dawsonites, dissolved Cu2+ and CuO supported on Al2O3. The heterogeneous as-synthesized Cu-dawsonite catalysts showed more efficient performance with respect to the other catalysts, despite of significant Cu leaching detected at the end of the reactions. The reactions carried out using the homogeneous dissolved Cu2+ (in the concentration range of the leached Cu) indicated low contribution of dissolved Cu2+ in the mineralization of CFA and its intermediates; the efficiency was mainly devoted to the activity of Cu incorporated in the dawsonite structure. Characterization of Cu-dawsonites revealed a possibility of Cu localization in the NH4+ position, which is higher exposed to leaching than that of Al. The reaction performed at initial pH 10 (pH higher than pHPZC of as-synthesized Cu-dawsonite) resulted to a higher TOC and COD reduction after 2h compared with the reaction at pH 3.7. Also lowest Cu leaching was obtained under these conditions. Enhanced activity of Cu incorporated into the structure of dawsonite in the catalytic ozonation of CFA is probably due to the presence of Cu(I) located in the structure of dawsonite revealed by XPS. Some of the main intermediates have been identified by HPLC–MS suggesting three routes for CFA destruction by ozonation using Cu-dawsonite including, C1–O and C4–Cl bond breaking and aromatic ring cleavage.

Synthesis of Ga2O3–Al2O3 Catalysts by a Coprecipitation Method for CH4-SCR of NO

Synthesis of the γ-Ga2O3–Al2O3 solid solutions by the coprecipitation method was examined. Precipitates were obtained by adding at once an aqueous solution of aluminum and gallium nitrates to an excess base solution (reverse strike precipitation). The precipitates were calcined at 700 °C to give γ-Ga2O3–Al2O3 solid solutions. When an ammonium carbonate solution was used as the precipitant, ammonium aluminum carbonate hydroxide (ammonium dawsonite, AACH)–ammonium gallium carbonate hydroxide (AGCH) solid solutions were obtained. Calcination of the AACH–AGCH solid solutions at 700 °C gave the γ-Ga2O3–Al2O3 solid solutions which exhibited high activity for selective catalytic reduction of NO with methane.

Study of magnesium aluminate spinel formation from carbonate precursors

The aim of the presented work was to study formation of magnesium aluminate spinel precursor powder during the co-precipitation of magnesium and aluminium nitrate with ammonium carbonate and its thermal transformation to spinel powder. After precipitation, the only crystalline phase is NH 4Al(OH) 2CO 3·H 2O (ammonium dawsonite). The second one, Mg 6Al 2(CO 3)(OH) 16·4H 2O (hydrotalcite) appears during ageing. As found by DTA-MS and HT-XRD measurements, co-existence of both phases in very close contact results in their easier decomposition. On the base of the results obtained the scheme of phase transformations of co-precipitated precursor leading to formation of MgAl 2O 4 is proposed.

Hydrothermal synthesis and thermodynamic analysis of dawsonite-type compounds

Dawsonite-type compounds of formula MAl(OH) 2CO 3 ( M=Na,K,NH 4) were hydrothermally synthesized. The influences of the composition of starting materials, reaction temperature, acidity of the reaction medium on the formation, morphology and particle size of MAl(OH) 2CO 3 powders were systematically investigated. The experiment results show that increase in the amount of MHCO 3 favors the formation of MAl(OH) 2CO 3 phase with more slender particle morphology. The higher the reaction temperature, the grosser the particle is. It was also demonstrated that the particle size of the MAl(OH) 2CO 3 phase are most uniform when the pH value of reaction system is 10.3 for sodium dawsonite and potassium dawsonite, but it is 9.79 for that of ammonium dawsonite. The thermodynamics of the formation of dawsonite-type compounds have also been analyzed in details.

Synthesis of Mg–Al spinel powder via precipitation using ammonium bicarbonate as the precipitant

A precursor for Mg‐Al spinel has been synthesized via the precipitation method, using ammonium bicarbonate as the precipitant. The precursor was composed of crystalline ammonium dawsonite hydrate [NH4Al(OH)2CO3 .H2O] and hydrotalcite [Mg6Al2 (CO3)(OH)16 .4H2O] phases. The precursor converted to pure spinel phase at900C via two steps upon calcination: (i) decomposition of hydrotalcite at lower temperatures (400‐800C) and (ii) solid-state reaction between MgO (decomposed from hydrotalcite) and g-Al2O3 (derived from NH4Al(OH)2CO3 .H2O) at higher temperatures (>800C). The eAect of calcination temperature on particle morphology and sinterability of the resultant spinel powders were investigated. # 2001 Elsevier Science Ltd. All rights reserved.

Synthesis and characterization of Fe2O3 containing aluminas by thermal decomposition of modified ammonium dawsonite

Aluminas with Fe2O3 particles finely divided in the Al2O3 matrix were prepared via a crystalline precursor derived from ammonium dawsonite and its calcination. The precursors with Fe content between 1 and 10 wt% were synthesized by reaction of ammonium hydrogen carbonate with mixtures containing aluminium ammonium sulfate and ferric ammonium sulfate. The characterization of the as-synthesized and calcined products by chemical analysis, XRD, EPR, Mossbauer, TEM and adsorption measurements shows that all the Fe3+ ions are incorporated into the structure of dawsonite when the Fe content is low and that the calcined materials with micropores and mesopores (maximum 30–40 A) also contain very small Fe2O3 particles (<10 nm) at higher Fe contents.

A wet-chemical process yielding reactive magnesium aluminate spinel (MgAl 2O 4) powder

Ammonium carbonate was used as the precipitant to synthesize Mg–Al spinel precursors from a mixed solution of magnesium and aluminum nitrates. The precursor, composed of crystalline ammonium dawsonite hydrate [NH 4Al(OH) 2CO 3·H 2O] and hydrotalcite [Mg 6Al 2(CO 3)(OH) 16·4H 2O] phases, transformed to pure spinel at ∼900°C via the decomposition of hydrotalcite at ∼400–800°C and a solid-state reaction between MgO (decomposed from hydrotalcite) and γ-Al 2O 3 (derived from NH 4Al(OH) 2CO 3·H 2O) at ∼800–900°C. Sinterability of the resultant spinel powders was evaluated by the constant-rate-of-heating sintering method. The effect of calcination temperature on particle morphology and sinterability of the resultant spinel powders was investigated. Spinel ceramics of ∼99% dense were produced by vacuum sintering at 1550°C for 2 h from the powder calcined at 1100°C for 2 h.

Characterization of yttrium aluminate garnet precursors synthesized via precipitation using ammonium bicarbonate as the precipitant

Stoichiometric yttrium aluminate garnet (Y3Al5O12) precursors with a wide range of chemical compositions were precipitated by dripping a mixed solution of ammonium aluminum sulfate and yttrium nitrate into ammonium bicarbonate solutions. The resultant precursors were characterized by chemical analysis, infrared spectroscopy, differential thermal analysis/thermogravimetry, x-ray diffractometry, and scanning electron microscopy. The effects of reaction temperature, precipitant concentration, dripping speed of the salt solution, and the molar ratio of precipitant/total-cations on the composition, thermal behavior, and particle morphology of the resultant precursors were investigated. The precipitation conditions that yield carbonate precursors composed of ammonium dawsonite [NH4Al(OH)2CO3] and yttrium normal carbonate [Y2(CO3)3 · 3H2O] were determined.

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH4Al(OH)2CO3·H2O) and hydrotalcite (Mg6Al2(CO3)(OH)16·4H2O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X‐ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.

Particle Size Distribution of Alumina Powders Derived from Ammonium Aluminum Carbonate Hydroxide (Ammonium Dawsonite)

This paper describes a study on particle size distribution of alumina powders obtained from thermal decomposition of ammonium dawsonite. The ammonium dawsonite, an ammonium aluminum carbonate hydroxide, was synthesized from solutions of ammonium alum and ammonium hydrogen carbonate, and thermally decomposed at different temperatures. According to the results of the particle size distribution, the values obtained from bi-modal curves were analyzed for determination of the average grain-size in each model.

97/04102 Study of the synthesis of ammonium dawsonite. Part II. Characterization

97/04102 Study of the synthesis of ammonium dawsonite. Part II. Characterization

Thermal stability of aluminium hydroxycarbonates with monovalent cations

Dawsonite-type compounds of formula MAl(OH) 2CO 3 (M = Na. K, NH 4) as well as a laminar hydrotalcite-type hydroxycarbonate of composition [Al 2Li(OH) 6] 2CO 3·4H 2O. have been hydrothermally synthesized The thermal decomposition of these compounds was monitored by DTA and TG, and the resulting products have been studied by X-ray and IR techniques. Sodium and potassium dawsonites are destroyed at 335°C. yielding a poorly crystalline compound in which part of the overall carbonate is present; the remaining carbonate is lost between 600 and 700°C, yielding NaAlO 2 and KAlO 2, respectively Ammonium dawsonite and lithium hydrotalcite are less stable, their thermal decomposition occurring at about 240°C. The ammonium dawsonite heated at 680°C shows the presence of A1 2O 3 with a poorly ordered structure, while lithium hydrotalcite yields poorly crystalline γ-Al 2O 3 at 500°C and a mixture of γ-LiAlO 2 and LiAl 5O 8 when the compound is heated at higher temperatures ( ~ 1000°C).