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New method for the synthesis of Al2O3–CaO and Al2O3–CaO–CaCO3 systems from a metallic precursor by the sol–gel route

New method for the synthesis of Al2O3–CaO and Al2O3–CaO–CaCO3 systems from a metallic precursor... A series ofbinaryAl O –CaO and Al O –CaO–CaCO systems with Ca/Al molar ratios of 0.05, 0.1, 0.25, 0.5 and 1.0 have been 2 3 2 3 3 synthesised by the sol–gel technique from aluminium isopropoxide and metallic calcium powder. The rate of the metal reaction is used as a limiting factor to control the binary gel formation. The proposed modification of the traditional sol–gel method was used to examine the influence the effect of the metallic form of the second component as an oxide precursor on the form of the final product. By applying acetic acid instead of mineral acid, calcium acetate is formed and then decomposed to calcium carbonate upon thermal processing. During the synthesis of the binary systems, metallic calcium acts both as a precursor of calcium acetate and as a secondary pH modifier of the gel system. Calcination in air at 600 °C did not produce systems containing only oxides and the calcium carbonate phase was still present. Due to particle size reduction, the CaCO to CaO decomposition temperature was lowered. The systems were characterised by X-ray powder diffraction, low-temperature nitrogen adsorption, transmission and scanning electron microscopy (TEM, SEM and SEM/EDS), thermogravimetric analysis (TGA) and FTIR spectra. . . . . Keywords CaO modified alumina Alumina xerogel Mixed oxide/carbonate phases Sol-gel Metallic precursor Introduction the catalytic reaction of partial oxidation of methane (POM). It also contributes to maintaining large surface area of the Al O –CaO systems in the form of cement or oxides have support [4]. Al O –CaO system, thanks to its properties, can 2 3 2 3 excellent flame retardant qualities that are important from be used as high-temperature CO absorbent [5]inthe the point of view of industrial applications, e.g., for produc- reforming process [6] or as an efficient adsorbent for fluorine tion of flooring, mortars resistant to chemicals and concretes, removal form water [7]. Another field of application of this for construction of sewers, production of tile adhesives, or type of oxide is metallurgy. The CaO ability to bind to the non- protective laminates [1–3]. Addition of CaO considerably im- metallic systems allows a better deoxidation of steel [8]. By proves the stability of the Pd/Al O catalyst by reducing car- careful selection of the key parameters of the gel synthesis and 2 3 bon deposit formation and Al O support transformation in processing, including solvent type, precursor concentration, 2 3 water and acid (or base) ratio, gel aging time, initial heat treatment and calcination, it is possible to control the process Electronic supplementary material The online version of this article to obtain materials having properties suitable for a particular (https://doi.org/10.1007/s41779-018-0197-0) contains supplementary application [9]. These variables influence the process of gel material, which is available to authorized users. network formation that determines the structure, physical * Robert E. Przekop properties, thermal stability, porosity and surface area of the rprzekop@amu.edu.pl xerogel [10, 11]. In this work, a new synthesis method has been proposed, in which calcium metal powder is used as alternative source of Centre for Advanced Technologies Adam Mickiewicz University in Poznan, Umultowska 89c, 61-614 Poznan, Poland the second component of the gel matrix. Our previous work has shown that the introduction of a metal precursor could Faculty of Chemistry, Adam Mickiewicz University in Poznan, Umultowska 89b, 61-614 Poznan, Poland lead to a change in the parameters of the final product [12] and to the formation of new (carbonate) intermediate phases Central Laboratory of Batteries and Cells, Institute of Non-Ferrous Metals Division in Poznan, Forteczna 12, 61-362 Poznan, Poland upon thermal treatment [13]. One of the goals of this 680 J Aust Ceram Soc (2018) 54:679–690 procedure was to maintain large initial surface area after mod- ratio of Al(C H O) , water and acetic acid was constant and 3 7 3 ification, which is of importance for application of the systems equal to 1:100:1.2. The sol was refluxed for more than 24 h at as a carrier for metallic phase catalysts or for other purposes. 95 °C (Fig. 1). The introduction of metallic calcium was a fast, pure and After that time, an appropriate amount of metallic calcium efficient method for incorporation of the second component in the form of granules was added in small portions as the of the alumina gel matrix. In addition, the replacement of second oxide component precursor. Table 1 shows sample mineral acids in the sol–gel synthesis, such as hydrochloric labelling, composition and the amount of calcium added to [14, 15], nitric [16, 17] or sulphuric acid [18], by acetic acid each oxide system (per 100 g of aluminium isopropoxide). allows the formation of a carbonate form that is stable over a The resulting mixture was refluxed upon vigorous stirring relatively wide temperature range. for 18 h at 95 °C. The final product was a homogeneous, liquid gel. The gel was poured onto the Petri dish and dried at 65 °C for 72 h to obtain a monolithic xerogel used for further TG investigation. Materials and methods For wet gels up to the Ca/Al 0.5 system, a high degree of homogeneity was obtained after synthesis (Fig. 2a) but for the Materials 1.0 system (with the highest Ca content), the result of phase segregation can be seen (Fig. 2a). This is also reflected in Aluminium isopropoxide (≥ 98%), acetic acid (≥ 99.5%) and morphological parameters and textural properties. Gel drying metallic calcium (granular, 99%) were purchased from Sigma- reveals differences in their homogeneity already in macro- Aldrich and used as received, without any further treatment. scopic terms. Gels of lower Ca content systems, Ca/Al 0.1, 0.25 (Fig. 1b), remain transparent, and for higher concentra- Preparation tions, their turbidity or clear crystallisation is visible (Ca/Al 1.0). The obtained gel was evaporatively dried (90 °C, Al O –CaO binary oxide systems of different molar ratios of 2 3 80 mbar) and the resulting solid was ground in a mortar; Ca to Al: 0.05, 0.1, 0.25, 0.5 and 1.0 were synthesised by sieved to collect two required grain fractions, 0.1–0.2 mm aqueous sol–gel chemistry. The synthesis was based on our and < 0.1 mm; and annealed at 600 °C in air flow for 6 h. previous experience with similar systems [12, 19, 20] but with For the porous structure determination, the grain size fraction a novel synthesis modification concerning the source of the between 0.1 and 0.2 mm of diameter was used. For the XRD second oxide component of the system. Synthesis of oxide and electron microscope analysis, the latter fraction was used. systems based on the sol–gel method is based primarily on the use of metal alkoxides as precursors, while the methods for obtaining metal alkoxides often employ a direct reaction of Characterisation metal (mainly of an alkaline nature) with the corresponding alcohol according to the reaction: The samples synthesised in this work were characterised using the following techniques. Me þ nROH→MeðÞ OR þ n=2H ð1Þ In the proposed method of synthesis, apart from the hydro- lysis and condensation reactions of the alumina precursor, direct reactions of metallic calcium in a system containing acid (2) and water (3) have the dominant role, giving respec- tively acetate and hydroxide, which undergo further reactions. Me þ nRCOOH→MeðÞ COO þ n=2H ð2Þ Me þ nH O→MeðÞ OH þ n=2H ð3Þ 2 2 Aluminium isopropoxide was used as a precursor of the starting alumina, while acetic acid was applied as a moderator of hydrolysis and condensation rates (regulation of pH). The reaction took place in a 2-l glass reactor upon stirring under reflux. The precursor of alumina (100 g of fine powder) was slowly added and hydrolysed in 880 cm of water at 75 °C and after 2 h of stirring, the resulting slurry was peptized with 35 g Fig. 1 Schematic procedure of Ca/Al gel synthesis (33.5 cm ) of glacial acetic acid. In all preparations, the molar J Aust Ceram Soc (2018) 54:679–690 681 Table 1 Composition and Sample name Al O Ca/Al 0.05 Ca/Al 0.1 Ca/Al 0.25 Ca/Al 0.5 Ca/Al 1.0 labelling of the systems and the 2 3 amount of added metallic calcium Relative molar amount of Al 1 1 1 1 1 1 Relative molar amount of Ca 0 0.05 0.10 0.25 0.50 1 Mass of added Ca [g] 0 0.98 1.96 4.90 9.80 19.59 X-ray powder diffraction analysis Porous structure TheX-raypowderdiffraction(XRPD)measurementswerecarried The porous structure was determined by the low-temperature out using a Philips PW1050 diffractometer working in the θ–2θ nitrogen adsorption measurements carried out on an Autosorb geometry with Ni-filtered CuK radiation. The following mea- iQ Station 2 (Quantachrome Instruments) in standard analysis surement conditions were applied: 2θ 5–100°, voltage 35 kV, cur- mode, using 200–300 mg of material with the grain size fraction rent 20 mA, scan step 0.040° at 1° per minute. The positions of between 0.1 and 0.2 mm. Prior to nitrogen adsorption, all sam- reflections were calculated by the Philips APD program. ples were outgassed for about 10 h at 350 °C at 0.4 Pa till constant weight. Both adsorptive and desorptive branches of the isotherm were recorded in the range of p/p 0–1.0. Reports Thermal analysis were provided by Quantachrome ASiQwin software (version 2.0). Distribution of pore area and pore volume was calculated Thermal transformation of unprocessed gel samples was car- using the de Boer t-method and BJH method. Pore volume and ried out on a NETZSCH TG 209 F1 Libra thermogravimetric pore diameter were found from the adsorptive branch of the apparatus. A 5-mg sample was placed in an alumina crucible −1 isotherm using the BJH method, and surface area was calculated (85 μl volume) and heated at a rate of 20 °C min up to using the BET method. 1000 °C. For all experiments, the fraction of the grain size below 0.1 mm was used. The TG traces were recorded in air atmosphere (20 ml/min flow) with a resolution of 0.1 μg. No FTIR measurements drying under vacuum or at elevated temperature was applied. FTIR measurements were carried out using a Thermo Scientific Nicolet iS50 FTIR spectrometer. One milligram was mixed with SEM, TEM and EDS (energy-dispersive X-ray spectroscopy) 200 mg KBr and pressed at 10 t press into the form of discs. analysis Surface morphology of the oxide systems was depicted by a scanning electron microscope (Quanta 250 FEG scanning Results and discussion electron microscope) which was operated at 5 kV) with EDAX system for EDS analysis, and their structures were Modification of alumina gel network employing a metallic pre- characterised by transmission electron microscopy (JEOL cursor of calcium oxide may have numerous implications. It is 200 CX which was operated at 80 kV). EDS maps of element known that the reactivity of alkoxide precursor can be changed overlay were made with a resolution of 0.3 μm. and controlled by various parameters, such as the type and Fig. 2 Gel photos before (a)and after drying at 65 °C, 24 h (b) 682 J Aust Ceram Soc (2018) 54:679–690 Table 2 Values of pH of reported [22] and have been reported γ-Al(O)OH pH values PZC System composition pH the gels after synthesis of ca. 7.4 [22]) and can attract positively charged calcium species in a basic solution. Al O [Al(O)OH] 3.39 2 3 Ca/Al 0.05 4.11 X-ray powder diffraction Ca/Al 0.1 4.64 Ca/Al 0.25 5.25 The XRPD patterns of the samples after the synthesis (before Ca/Al 0.5 6.06 calcination) are shown in Fig. 3a. The presence of crystalline Ca/Al 1.0 10.25 phases of Ca(OH) , bohemite and calcium acetate was identi- fied in the samples; they disappear after calcination and ther- concentration of precursor, temperature, water and acid or base mal decomposition of precursors [23]. content, solvent type. Also, other parameters related to further The calcined samples (6 h at 600 °C under continuous oxygen steps of preparation influence the gel forming process: gel ageing flow) are amorphous and only minor amounts of any crystalline timeanddryingmethod[12, 21]. In this process, thanks to its phases have been found in them (Fig. 3b). After thermal treat- strong basicity, calcium can act as a pH regulator and therefore ment, the intensity of the diffraction peaks decreases with in- can significantly affect the further process of solid gel formation creasing content of calcium oxide and only a wide peak assigned and transformation, also upon drying and annealing. to the low-crystallinity γ-Al O phase in the range of 2θ =20– 2 3 The rate of reaction of metallic calcium granules (in compar- 40° is observed [7] with small single peaks of CaCO and CaO ison to other precursors) with gel solution may be an obstacle to crystalline phase for the systems Ca/Al 0.25 to 1.0. proper process control and result in heterogeneities. Also, the The positions of two broad maxima suggest that the system proposed method employing metallic calcium was used as an Ca/Al 0.05 contains higher amount of low-crystallinity γ-Al O 2 3 alternative to traditional methods of co-precipitation (e.g., of car- phase. This effect is diminished in further compositions, not only bonates) or carrier impregnation with salts (e.g., nitrates) which because of the lower content of this component but also as a often lead to a considerable decrease in the carrier surface area. result of interaction with the calcium phase. However, after cal- Table 2 shows the pH values of the gels after the synthesis. cination in air at 600 °C, the calcium carbonate phase was still With the addition of calcium, the acidity of the solution decreases present. This fact can be explained by the use of acetic acid continuously in a series of samples and their pH increases to a during the synthesis and formation of calcium acetate that is high value for the Ca/Al 1.0 system. The pH of the solution transformed into carbonate, and its decomposition to CaO re- changes the net surface charge of sol particles that depends on quires higher temperature. Depending on the calcination temper- the point of zero charge of the oxide components. Alumina par- ature, the systems Al O –Ca(CH COO) (< 320 °C), Al O – 2 3 3 2 2 3 ticles are charged negatively at the pH higher than the PZC of γ- CaCO (430–650 °C) or Al O –CaO (> 700 °C) can be obtained 3 2 3 Al O (pH =8.2 [7], but also values 6.2 to 7.9 have been 2 3 PZC in the same way. Fig. 3 XRPD patterns of Ca/Al systems: a before calcination and b after calcination at 600 °C in oxygen atmosphere J Aust Ceram Soc (2018) 54:679–690 683 Fig. 4 Thermogravimetric analysis of Ca/Al systems. a TG traces of reference materials. b DTG tracesofreference materials. c TG traces of Ca/Al systems. d DTG traces of Ca/Al systems Thermal analysis With respect to the undoped Al O , the obtained binary 2 3 oxide systems do not exhibit a higher complexity of thermal Thermograms of the analysed systems are shown in Fig. 4. decomposition and desorption processes. As the calcium con- For alumina gel, three characteristic areas of mass loss effects tent increases, the thermograms become similar to the calcium are observed. They are related to the dehydration, dehydrox- acetate thermal profile (Fig. 4b). In the temperature range of ylation and oxidation process of organic compounds bound to 370–570 °C, the dominant transformation is the decomposi- the gel structure (Fig. 4a). On the basis of previous works [12, tion of calcium acetate according to the equation [24]: 21], the first endothermic mass loss was interpreted as corre- CaðÞ CH COO →CaCO þðÞ CH CO ð4Þ sponding to the removal of physisorbed and internally trapped 3 3sðÞ 3 ðÞ v 2sðÞ 2 water in gel. This effect is responsible for the loss of < 10% of In this temperature range (III), also calcium hydroxide un- the total weight of the gel dried at room temperature. As a dergoes decomposition (> 512 °C). Above 680 °C, an addi- result of further heating, the decomposition and desorption tional weight loss associated with the decomposition of calci- of the organic compounds combined with their oxidation fol- um carbonate takes place: lows. This exothermic effect occurs in the temperature range of 390–530 °C (range (III), Table 3). Also, Ca(OH) is CaCO →CaO þ CO ð5Þ 3sðÞ ðÞ s 2gðÞ decomposed to CaO (Fig. 3 XRPD patterns). Table 3 Quantitative mass loss of System composition Ca/Al 0.05 Ca/Al 0.1 Ca/Al 0.25 Ca/Al 0.5 Ca/Al 1.0 the Ca/Al systems during thermogravimetric analysis Temperature range [°C] Sample mass loss [%] I30–200 6.25 9.1 9.3 7.41 5 II 200–370 13 11.8 13.8 8.6 8.42 III 370–570 16.9 20.38 17 22 20.4 IV 570–800 2 3.45 6.7 9.2 18.8 Total mass loss 38.15 44.73 46.8 47.21 52.62 684 J Aust Ceram Soc (2018) 54:679–690 Fig. 5 SEM micrographs of Ca/ Al gel monoliths before calcination at the Al/Ca molar ratio: a 0.1, b–d 0.25, e, f 1.0 Both effects (III, IV) are shifted to lower temperatures the other methods for the synthesis of binary oxide systems based on an alumina matrix is the fact that a stable calcium (430 and 720 °C) when compared to that for the reference substances and literature data (500 and 810 °C) [25]. This is carbonate phase is obtained in the range 500–630 °C. The above observations are confirmed by spectroscopic analy- an effect of particle size reduction [26]. In quantitative −1 terms, the process of gel decomposition after its synthesis sis of structural bands in the range 1600–400 cm .Itis worthnotingthatAl O and CaO have no catalytic effect results in more volatile degradation products with increas- 2 3 ing calcium content. The most important difference from on the kinetics of CaCO calcination [26]. 3 J Aust Ceram Soc (2018) 54:679–690 685 SEM, TEM and EDS analyses system, the ratio of Ca to Al obtained from the EDS map of uncalcined systems is more than twice (0.25) higher than that The SEM micrographs of the uncalcined samples’ surfaces resulting from the theoretical composition and indicates the con- (Fig. 5) show the formation of two distinct phases during gela- centration of calcium ions on the surface as well as for the sys- tion: the amorphous phase of alumina/calcium acetate (Fig. 5a– tems Ca/Al 0.25 (Ca/Al = 0.49) and Ca/Al 0.5 (Ca/Al = 0.74). d) consisting of densely packed spherical domains of different For the Ca/Al 1.0 system, the ratio of Ca to Al is many times size (Ca/Al 0.1 system: mainly approximately 10 μmand small higher and equal to 43, which points to the almost complete amount of ca. 0.25 μm) and microcrystalline phase of calcium coverage of the surface layer with calcium precursor (acetate). acetate forming bundles of long (~ 25 μm), thin (ca. 50 nm), Thus, the binary systems are not fully homogeneous but they are parallel fibres (Fig. 5a, b, e) present in the structure of the material also microcrystalline or highly amorphous. The location and before and after thermal treatment [26]. The formation of the two orientation of the two phases is not clear: EDS data suggest phases is independent of the calcium to aluminium ratio (Fig. 5a– increasing effect of alumina surface decoration with calcium f). Another significant observation is the formation of a calcium component; however, SEM images indicate the presence of the acetate-rich phase under the surface of the alumina enriched latter component under alumina surface or between layers and phase (Fig. 5b). The EDS elemental analysis of the surface also planes of the alumina matrix. shows the presence of two phases (Fig. 6). Figure 8 shows TEM micrographs of calcined samples. The The SEM micrographs of the materials with higher calcium amorphous alumina structure is visible, but it is difficult to indi- content after calcination at 600 °C show two phases (Fig. 7a–c) cate the presence of the calcium phase, which is separated from as observed for the samples before calcination (Fig. 5). The the Al O matrix and should be visible as Brods^ of CaO crystals. 2 3 calcination changes the structure of alumina transforming it into a smooth, continuous and amorphous phase (Fig. 7a), but the Porous structure—low temperature nitrogen calcium carbonate phase remains structurally similar to the cal- adsorption–desorption cium acetate phase. The surface texture of the two calcined sys- tems is typical of alumina (Fig. 7a) yet very different for high Surface area, pore volume and pore diameter are present- calcium content (Fig. 7b) that is less porous, and the microcrys- ed in Table 4. Plots of isotherms, pore volume distribution talline calcium carbonate phase is present. For the Ca/Al 0.1 and pore area distribution are shown in Fig. 9. Fig. 6 EDS elements maps of Ca/ Al gel monoliths before calcination at the Al/Ca molar ratio: a 0.1, b 0.25, c 0.5, d 1.0 686 J Aust Ceram Soc (2018) 54:679–690 Fig. 7 SEM micrographs of Ca/ Al samples calcined at 600 °C at the Al/Ca molar ratio: a–c 1.0, d– f 0.25 Binary systems based on alumina in a wide range of CaO p/p 0.5–1, which is characteristic of metal oxide mesoporous content have large surface area (~ 180–320 m /g), which de- materials. The shape of hysteresis loops is similar for the ox- creases almost linearly with the content of the second compo- ides with Ca/Al ratio higher than 0.05. The obtained systems nent (having much lower specific surface area) to about 70 m / contain mainly small mesopores (< 10–15 nm) which bring g for the Ca/Al 1.0 system. The adsorption–desorption iso- the largest contribution to the surface area. The average pore therm is type IV with H2 hysteresis loop (except for the Ca/ size remains constant at about 6 nm, and the pore size distri- Al 0.05 system) that is found in the range of relative pressure bution is narrow. J Aust Ceram Soc (2018) 54:679–690 687 Fig. 8 TEM images of Ca/Al sol–gel samples at the Al/Ca molar ratio: a 0.1, b 0.5, c 1.0 The texture of Ca/Al 0.05 system is the most distinctive. In (blockage of pores) [7], it can be assumed that this decrease is addition to small mesopores (average diameter of about 3 nm) mostly the effect of alumina surface coverage with calcium resulting from high alumina content, some larger mesopores component, as suggested by the results of EDS element map- are present, including the ones in the range of 10–20 nm in ping. The H3-type hysteresis loop observed at high values of diameter. This is particularly evident taking into account the p/p indicates the presence of plate-like particle aggregates, surface area and volume distribution curves obtained from the giving rise to the slit-shaped pores in which the capillary con- desorptive branch of the isotherm and the shape of isotherms densation occurs, in addition to bottle-shaped pores with nar- and the hysteresis loop. However, the addition of calcium row necks (the H2 type hysteresis loop) [27]. For all alumina- component causes the disappearance of the smallest pores in based systems, a moderate negative effect of CaO addition on favour of larger mesopores. Their size and distribution does the surface area and pore volume can be observed, with little not change significantly with its amount. Multi-modal distri- impact on their average diameter and distribution. The in- bution of pores suggests complex and not fully homogenous crease in CaO/CaCO content also causes disappearance of structure of the system. The pore volume of the systems de- the smallest pores. creases from 0.65 to 0.17 cm /g with increasing calcium con- tent, similarly to the changes in the surface area. The highest FTIR measurements pore volume of the Ca/Al 0.05 system is associated with a significant volume of larger mesopores having a diameter of Spectroscopic studies have provided information on the compo- about 15 nm, whose presence is also apparent from the H2/H3 sition of the obtained systems after sol–gel synthesis and after mixed-type hysteresis loop of nitrogen adsorption isotherm. calcination (Fig. 10). Uncalcined samples show characteristic For this sample, it is even higher than for unmodified alumina. intense bands assigned to asymmetric and symmetric vibrations Despite the fact that the decrease in pore volume may be of calcium acetate carboxylate groups in the range of 1612– −1 caused by CaO or CaCO particles located in pores of alumina 1350 cm . After calcination, as a result of calcium acetate Table 4 Textural properties of System composition Surface area Average pore diameter Average pore volume Ca/Al systems 2 3 S [m /g] D [nm] D [cm /g] BET BJH BJH Al O 337.2 5.5 0.49 2 3 Ca/Al 0.05 322.0 3.3 0.65 Ca/Al 0.1 272.7 5.8 0.45 Ca/Al 0.25 233.0 5.8 0.42 Ca/Al 0.5 179.0 6.3 0.33 Ca/Al 1.0 67.9 6.8 0.17 From reference [12] 688 J Aust Ceram Soc (2018) 54:679–690 Fig. 9 Isotherms, pore volume distribution and pore area distribution of the Ca/Al systems J Aust Ceram Soc (2018) 54:679–690 689 Fig. 10 FTIR spectra of Ca/Al systems. a Before calcination. b After calcination decomposition, the bands disappeared and broad intense bands analysed in combination with microscopic images. High assigned to symmetric stretching vibration of unidentate carbon- CaCO /CaO content, because of decreasing ratio of Al/Ca, is −1 ate at 1540 cm appeared [28]. No bridged bidentate carbonate responsible for a significant, almost linear decrease in the spe- −1 (1620–1670 cm ) was found. Also, the appearance of a sharp cific surface area and pore volume. Introduction of calcium −1 2− band at 875 cm , assigned to the carbonate ionic group (CO ), component to alumina matrix, as seen from the XRPD patterns, confirms the presence of calcium carbonate after thermal treat- results in stabilisation of dispersion of the binary systems and ment [29]. The carbonate bands are observed for the Ca/Al 0.25, decreases the transformation of amorphous alumina into the γ- −1 0.5 and 1.0 systems. A clear 1166 cm band assigned to the Al O crystalline phase. The Ca/Al 0.05 composition has dis- 2 3 octahedral Al is visible for uncalcined systems [25] and dimin- tinctly different structural and surface properties, similar to ishes with increasing calcium content. This band disappears after those of undoped Al O obtained by the sol–gel method. 2 3 calcination. For low calcium contents, a weak Al–OH band Application of the sol–gel route allows obtaining a highly dis- −1 (1640 cm ) shifted to low-frequency occurs, which vanishes persive but not strictly homogeneous system irrespective of the with decreasing of Ca content [30]. molar ratio of the components. The dispersion and homogene- ity of the system diminishes with the rising calcium content in the system. Acetic acid used as a reagent and pH regulator allows the formation of various chemical forms of calcium in Conclusions the alumina matrix: acetate, carbonate or oxide in the structure of the material. Their formation translates into the final proper- The rate of the reaction of metallic calcium with alumina pre- ties of the products, e.g. texture or acid-base properties. cursor solution has a limited effect on the structure and homo- geneity of the oxides systems. We assume that in these systems, Compliance with ethical standards the pH increase related to the calcium content is a determining Conflict of interest The authors declare that they have no conflict of parameter of alumina gel formation [9, 21]. Both components interest. form a mixture of two separate phases: amorphous alumina and crystalline CaCO /CaO. The surface properties of the mixed Open Access This article is distributed under the terms of the Creative Al O –CaO systems are largely influenced by the CaO and 2 3 Commons Attribution 4.0 International License (http:// CaCO phases which have a clear tendency to concentrate on creativecommons.org/licenses/by/4.0/), which permits unrestricted use, alumina surface in the crystalline form and presumably, they distribution, and reproduction in any medium, provided you give also cause pores blockage, which can be evidenced by disap- appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. pearance of larger pores. It is visible from the textural data 690 J Aust Ceram Soc (2018) 54:679–690 17. Kamiya, K., Hioki, N., Hashimoto, T., Nasu, H.: Formation of α- References alumina around 500°C in alkoxy-derived alumina gels under ambi- ent pressure—effects of starting solution composition and seeding. 1. 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New method for the synthesis of Al2O3–CaO and Al2O3–CaO–CaCO3 systems from a metallic precursor by the sol–gel route

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Springer Journals
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Copyright © 2018 by The Author(s)
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Materials Science; Ceramics, Glass, Composites, Natural Materials; Materials Engineering; Inorganic Chemistry
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2510-1560
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2510-1579
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10.1007/s41779-018-0197-0
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Abstract

A series ofbinaryAl O –CaO and Al O –CaO–CaCO systems with Ca/Al molar ratios of 0.05, 0.1, 0.25, 0.5 and 1.0 have been 2 3 2 3 3 synthesised by the sol–gel technique from aluminium isopropoxide and metallic calcium powder. The rate of the metal reaction is used as a limiting factor to control the binary gel formation. The proposed modification of the traditional sol–gel method was used to examine the influence the effect of the metallic form of the second component as an oxide precursor on the form of the final product. By applying acetic acid instead of mineral acid, calcium acetate is formed and then decomposed to calcium carbonate upon thermal processing. During the synthesis of the binary systems, metallic calcium acts both as a precursor of calcium acetate and as a secondary pH modifier of the gel system. Calcination in air at 600 °C did not produce systems containing only oxides and the calcium carbonate phase was still present. Due to particle size reduction, the CaCO to CaO decomposition temperature was lowered. The systems were characterised by X-ray powder diffraction, low-temperature nitrogen adsorption, transmission and scanning electron microscopy (TEM, SEM and SEM/EDS), thermogravimetric analysis (TGA) and FTIR spectra. . . . . Keywords CaO modified alumina Alumina xerogel Mixed oxide/carbonate phases Sol-gel Metallic precursor Introduction the catalytic reaction of partial oxidation of methane (POM). It also contributes to maintaining large surface area of the Al O –CaO systems in the form of cement or oxides have support [4]. Al O –CaO system, thanks to its properties, can 2 3 2 3 excellent flame retardant qualities that are important from be used as high-temperature CO absorbent [5]inthe the point of view of industrial applications, e.g., for produc- reforming process [6] or as an efficient adsorbent for fluorine tion of flooring, mortars resistant to chemicals and concretes, removal form water [7]. Another field of application of this for construction of sewers, production of tile adhesives, or type of oxide is metallurgy. The CaO ability to bind to the non- protective laminates [1–3]. Addition of CaO considerably im- metallic systems allows a better deoxidation of steel [8]. By proves the stability of the Pd/Al O catalyst by reducing car- careful selection of the key parameters of the gel synthesis and 2 3 bon deposit formation and Al O support transformation in processing, including solvent type, precursor concentration, 2 3 water and acid (or base) ratio, gel aging time, initial heat treatment and calcination, it is possible to control the process Electronic supplementary material The online version of this article to obtain materials having properties suitable for a particular (https://doi.org/10.1007/s41779-018-0197-0) contains supplementary application [9]. These variables influence the process of gel material, which is available to authorized users. network formation that determines the structure, physical * Robert E. Przekop properties, thermal stability, porosity and surface area of the rprzekop@amu.edu.pl xerogel [10, 11]. In this work, a new synthesis method has been proposed, in which calcium metal powder is used as alternative source of Centre for Advanced Technologies Adam Mickiewicz University in Poznan, Umultowska 89c, 61-614 Poznan, Poland the second component of the gel matrix. Our previous work has shown that the introduction of a metal precursor could Faculty of Chemistry, Adam Mickiewicz University in Poznan, Umultowska 89b, 61-614 Poznan, Poland lead to a change in the parameters of the final product [12] and to the formation of new (carbonate) intermediate phases Central Laboratory of Batteries and Cells, Institute of Non-Ferrous Metals Division in Poznan, Forteczna 12, 61-362 Poznan, Poland upon thermal treatment [13]. One of the goals of this 680 J Aust Ceram Soc (2018) 54:679–690 procedure was to maintain large initial surface area after mod- ratio of Al(C H O) , water and acetic acid was constant and 3 7 3 ification, which is of importance for application of the systems equal to 1:100:1.2. The sol was refluxed for more than 24 h at as a carrier for metallic phase catalysts or for other purposes. 95 °C (Fig. 1). The introduction of metallic calcium was a fast, pure and After that time, an appropriate amount of metallic calcium efficient method for incorporation of the second component in the form of granules was added in small portions as the of the alumina gel matrix. In addition, the replacement of second oxide component precursor. Table 1 shows sample mineral acids in the sol–gel synthesis, such as hydrochloric labelling, composition and the amount of calcium added to [14, 15], nitric [16, 17] or sulphuric acid [18], by acetic acid each oxide system (per 100 g of aluminium isopropoxide). allows the formation of a carbonate form that is stable over a The resulting mixture was refluxed upon vigorous stirring relatively wide temperature range. for 18 h at 95 °C. The final product was a homogeneous, liquid gel. The gel was poured onto the Petri dish and dried at 65 °C for 72 h to obtain a monolithic xerogel used for further TG investigation. Materials and methods For wet gels up to the Ca/Al 0.5 system, a high degree of homogeneity was obtained after synthesis (Fig. 2a) but for the Materials 1.0 system (with the highest Ca content), the result of phase segregation can be seen (Fig. 2a). This is also reflected in Aluminium isopropoxide (≥ 98%), acetic acid (≥ 99.5%) and morphological parameters and textural properties. Gel drying metallic calcium (granular, 99%) were purchased from Sigma- reveals differences in their homogeneity already in macro- Aldrich and used as received, without any further treatment. scopic terms. Gels of lower Ca content systems, Ca/Al 0.1, 0.25 (Fig. 1b), remain transparent, and for higher concentra- Preparation tions, their turbidity or clear crystallisation is visible (Ca/Al 1.0). The obtained gel was evaporatively dried (90 °C, Al O –CaO binary oxide systems of different molar ratios of 2 3 80 mbar) and the resulting solid was ground in a mortar; Ca to Al: 0.05, 0.1, 0.25, 0.5 and 1.0 were synthesised by sieved to collect two required grain fractions, 0.1–0.2 mm aqueous sol–gel chemistry. The synthesis was based on our and < 0.1 mm; and annealed at 600 °C in air flow for 6 h. previous experience with similar systems [12, 19, 20] but with For the porous structure determination, the grain size fraction a novel synthesis modification concerning the source of the between 0.1 and 0.2 mm of diameter was used. For the XRD second oxide component of the system. Synthesis of oxide and electron microscope analysis, the latter fraction was used. systems based on the sol–gel method is based primarily on the use of metal alkoxides as precursors, while the methods for obtaining metal alkoxides often employ a direct reaction of Characterisation metal (mainly of an alkaline nature) with the corresponding alcohol according to the reaction: The samples synthesised in this work were characterised using the following techniques. Me þ nROH→MeðÞ OR þ n=2H ð1Þ In the proposed method of synthesis, apart from the hydro- lysis and condensation reactions of the alumina precursor, direct reactions of metallic calcium in a system containing acid (2) and water (3) have the dominant role, giving respec- tively acetate and hydroxide, which undergo further reactions. Me þ nRCOOH→MeðÞ COO þ n=2H ð2Þ Me þ nH O→MeðÞ OH þ n=2H ð3Þ 2 2 Aluminium isopropoxide was used as a precursor of the starting alumina, while acetic acid was applied as a moderator of hydrolysis and condensation rates (regulation of pH). The reaction took place in a 2-l glass reactor upon stirring under reflux. The precursor of alumina (100 g of fine powder) was slowly added and hydrolysed in 880 cm of water at 75 °C and after 2 h of stirring, the resulting slurry was peptized with 35 g Fig. 1 Schematic procedure of Ca/Al gel synthesis (33.5 cm ) of glacial acetic acid. In all preparations, the molar J Aust Ceram Soc (2018) 54:679–690 681 Table 1 Composition and Sample name Al O Ca/Al 0.05 Ca/Al 0.1 Ca/Al 0.25 Ca/Al 0.5 Ca/Al 1.0 labelling of the systems and the 2 3 amount of added metallic calcium Relative molar amount of Al 1 1 1 1 1 1 Relative molar amount of Ca 0 0.05 0.10 0.25 0.50 1 Mass of added Ca [g] 0 0.98 1.96 4.90 9.80 19.59 X-ray powder diffraction analysis Porous structure TheX-raypowderdiffraction(XRPD)measurementswerecarried The porous structure was determined by the low-temperature out using a Philips PW1050 diffractometer working in the θ–2θ nitrogen adsorption measurements carried out on an Autosorb geometry with Ni-filtered CuK radiation. The following mea- iQ Station 2 (Quantachrome Instruments) in standard analysis surement conditions were applied: 2θ 5–100°, voltage 35 kV, cur- mode, using 200–300 mg of material with the grain size fraction rent 20 mA, scan step 0.040° at 1° per minute. The positions of between 0.1 and 0.2 mm. Prior to nitrogen adsorption, all sam- reflections were calculated by the Philips APD program. ples were outgassed for about 10 h at 350 °C at 0.4 Pa till constant weight. Both adsorptive and desorptive branches of the isotherm were recorded in the range of p/p 0–1.0. Reports Thermal analysis were provided by Quantachrome ASiQwin software (version 2.0). Distribution of pore area and pore volume was calculated Thermal transformation of unprocessed gel samples was car- using the de Boer t-method and BJH method. Pore volume and ried out on a NETZSCH TG 209 F1 Libra thermogravimetric pore diameter were found from the adsorptive branch of the apparatus. A 5-mg sample was placed in an alumina crucible −1 isotherm using the BJH method, and surface area was calculated (85 μl volume) and heated at a rate of 20 °C min up to using the BET method. 1000 °C. For all experiments, the fraction of the grain size below 0.1 mm was used. The TG traces were recorded in air atmosphere (20 ml/min flow) with a resolution of 0.1 μg. No FTIR measurements drying under vacuum or at elevated temperature was applied. FTIR measurements were carried out using a Thermo Scientific Nicolet iS50 FTIR spectrometer. One milligram was mixed with SEM, TEM and EDS (energy-dispersive X-ray spectroscopy) 200 mg KBr and pressed at 10 t press into the form of discs. analysis Surface morphology of the oxide systems was depicted by a scanning electron microscope (Quanta 250 FEG scanning Results and discussion electron microscope) which was operated at 5 kV) with EDAX system for EDS analysis, and their structures were Modification of alumina gel network employing a metallic pre- characterised by transmission electron microscopy (JEOL cursor of calcium oxide may have numerous implications. It is 200 CX which was operated at 80 kV). EDS maps of element known that the reactivity of alkoxide precursor can be changed overlay were made with a resolution of 0.3 μm. and controlled by various parameters, such as the type and Fig. 2 Gel photos before (a)and after drying at 65 °C, 24 h (b) 682 J Aust Ceram Soc (2018) 54:679–690 Table 2 Values of pH of reported [22] and have been reported γ-Al(O)OH pH values PZC System composition pH the gels after synthesis of ca. 7.4 [22]) and can attract positively charged calcium species in a basic solution. Al O [Al(O)OH] 3.39 2 3 Ca/Al 0.05 4.11 X-ray powder diffraction Ca/Al 0.1 4.64 Ca/Al 0.25 5.25 The XRPD patterns of the samples after the synthesis (before Ca/Al 0.5 6.06 calcination) are shown in Fig. 3a. The presence of crystalline Ca/Al 1.0 10.25 phases of Ca(OH) , bohemite and calcium acetate was identi- fied in the samples; they disappear after calcination and ther- concentration of precursor, temperature, water and acid or base mal decomposition of precursors [23]. content, solvent type. Also, other parameters related to further The calcined samples (6 h at 600 °C under continuous oxygen steps of preparation influence the gel forming process: gel ageing flow) are amorphous and only minor amounts of any crystalline timeanddryingmethod[12, 21]. In this process, thanks to its phases have been found in them (Fig. 3b). After thermal treat- strong basicity, calcium can act as a pH regulator and therefore ment, the intensity of the diffraction peaks decreases with in- can significantly affect the further process of solid gel formation creasing content of calcium oxide and only a wide peak assigned and transformation, also upon drying and annealing. to the low-crystallinity γ-Al O phase in the range of 2θ =20– 2 3 The rate of reaction of metallic calcium granules (in compar- 40° is observed [7] with small single peaks of CaCO and CaO ison to other precursors) with gel solution may be an obstacle to crystalline phase for the systems Ca/Al 0.25 to 1.0. proper process control and result in heterogeneities. Also, the The positions of two broad maxima suggest that the system proposed method employing metallic calcium was used as an Ca/Al 0.05 contains higher amount of low-crystallinity γ-Al O 2 3 alternative to traditional methods of co-precipitation (e.g., of car- phase. This effect is diminished in further compositions, not only bonates) or carrier impregnation with salts (e.g., nitrates) which because of the lower content of this component but also as a often lead to a considerable decrease in the carrier surface area. result of interaction with the calcium phase. However, after cal- Table 2 shows the pH values of the gels after the synthesis. cination in air at 600 °C, the calcium carbonate phase was still With the addition of calcium, the acidity of the solution decreases present. This fact can be explained by the use of acetic acid continuously in a series of samples and their pH increases to a during the synthesis and formation of calcium acetate that is high value for the Ca/Al 1.0 system. The pH of the solution transformed into carbonate, and its decomposition to CaO re- changes the net surface charge of sol particles that depends on quires higher temperature. Depending on the calcination temper- the point of zero charge of the oxide components. Alumina par- ature, the systems Al O –Ca(CH COO) (< 320 °C), Al O – 2 3 3 2 2 3 ticles are charged negatively at the pH higher than the PZC of γ- CaCO (430–650 °C) or Al O –CaO (> 700 °C) can be obtained 3 2 3 Al O (pH =8.2 [7], but also values 6.2 to 7.9 have been 2 3 PZC in the same way. Fig. 3 XRPD patterns of Ca/Al systems: a before calcination and b after calcination at 600 °C in oxygen atmosphere J Aust Ceram Soc (2018) 54:679–690 683 Fig. 4 Thermogravimetric analysis of Ca/Al systems. a TG traces of reference materials. b DTG tracesofreference materials. c TG traces of Ca/Al systems. d DTG traces of Ca/Al systems Thermal analysis With respect to the undoped Al O , the obtained binary 2 3 oxide systems do not exhibit a higher complexity of thermal Thermograms of the analysed systems are shown in Fig. 4. decomposition and desorption processes. As the calcium con- For alumina gel, three characteristic areas of mass loss effects tent increases, the thermograms become similar to the calcium are observed. They are related to the dehydration, dehydrox- acetate thermal profile (Fig. 4b). In the temperature range of ylation and oxidation process of organic compounds bound to 370–570 °C, the dominant transformation is the decomposi- the gel structure (Fig. 4a). On the basis of previous works [12, tion of calcium acetate according to the equation [24]: 21], the first endothermic mass loss was interpreted as corre- CaðÞ CH COO →CaCO þðÞ CH CO ð4Þ sponding to the removal of physisorbed and internally trapped 3 3sðÞ 3 ðÞ v 2sðÞ 2 water in gel. This effect is responsible for the loss of < 10% of In this temperature range (III), also calcium hydroxide un- the total weight of the gel dried at room temperature. As a dergoes decomposition (> 512 °C). Above 680 °C, an addi- result of further heating, the decomposition and desorption tional weight loss associated with the decomposition of calci- of the organic compounds combined with their oxidation fol- um carbonate takes place: lows. This exothermic effect occurs in the temperature range of 390–530 °C (range (III), Table 3). Also, Ca(OH) is CaCO →CaO þ CO ð5Þ 3sðÞ ðÞ s 2gðÞ decomposed to CaO (Fig. 3 XRPD patterns). Table 3 Quantitative mass loss of System composition Ca/Al 0.05 Ca/Al 0.1 Ca/Al 0.25 Ca/Al 0.5 Ca/Al 1.0 the Ca/Al systems during thermogravimetric analysis Temperature range [°C] Sample mass loss [%] I30–200 6.25 9.1 9.3 7.41 5 II 200–370 13 11.8 13.8 8.6 8.42 III 370–570 16.9 20.38 17 22 20.4 IV 570–800 2 3.45 6.7 9.2 18.8 Total mass loss 38.15 44.73 46.8 47.21 52.62 684 J Aust Ceram Soc (2018) 54:679–690 Fig. 5 SEM micrographs of Ca/ Al gel monoliths before calcination at the Al/Ca molar ratio: a 0.1, b–d 0.25, e, f 1.0 Both effects (III, IV) are shifted to lower temperatures the other methods for the synthesis of binary oxide systems based on an alumina matrix is the fact that a stable calcium (430 and 720 °C) when compared to that for the reference substances and literature data (500 and 810 °C) [25]. This is carbonate phase is obtained in the range 500–630 °C. The above observations are confirmed by spectroscopic analy- an effect of particle size reduction [26]. In quantitative −1 terms, the process of gel decomposition after its synthesis sis of structural bands in the range 1600–400 cm .Itis worthnotingthatAl O and CaO have no catalytic effect results in more volatile degradation products with increas- 2 3 ing calcium content. The most important difference from on the kinetics of CaCO calcination [26]. 3 J Aust Ceram Soc (2018) 54:679–690 685 SEM, TEM and EDS analyses system, the ratio of Ca to Al obtained from the EDS map of uncalcined systems is more than twice (0.25) higher than that The SEM micrographs of the uncalcined samples’ surfaces resulting from the theoretical composition and indicates the con- (Fig. 5) show the formation of two distinct phases during gela- centration of calcium ions on the surface as well as for the sys- tion: the amorphous phase of alumina/calcium acetate (Fig. 5a– tems Ca/Al 0.25 (Ca/Al = 0.49) and Ca/Al 0.5 (Ca/Al = 0.74). d) consisting of densely packed spherical domains of different For the Ca/Al 1.0 system, the ratio of Ca to Al is many times size (Ca/Al 0.1 system: mainly approximately 10 μmand small higher and equal to 43, which points to the almost complete amount of ca. 0.25 μm) and microcrystalline phase of calcium coverage of the surface layer with calcium precursor (acetate). acetate forming bundles of long (~ 25 μm), thin (ca. 50 nm), Thus, the binary systems are not fully homogeneous but they are parallel fibres (Fig. 5a, b, e) present in the structure of the material also microcrystalline or highly amorphous. The location and before and after thermal treatment [26]. The formation of the two orientation of the two phases is not clear: EDS data suggest phases is independent of the calcium to aluminium ratio (Fig. 5a– increasing effect of alumina surface decoration with calcium f). Another significant observation is the formation of a calcium component; however, SEM images indicate the presence of the acetate-rich phase under the surface of the alumina enriched latter component under alumina surface or between layers and phase (Fig. 5b). The EDS elemental analysis of the surface also planes of the alumina matrix. shows the presence of two phases (Fig. 6). Figure 8 shows TEM micrographs of calcined samples. The The SEM micrographs of the materials with higher calcium amorphous alumina structure is visible, but it is difficult to indi- content after calcination at 600 °C show two phases (Fig. 7a–c) cate the presence of the calcium phase, which is separated from as observed for the samples before calcination (Fig. 5). The the Al O matrix and should be visible as Brods^ of CaO crystals. 2 3 calcination changes the structure of alumina transforming it into a smooth, continuous and amorphous phase (Fig. 7a), but the Porous structure—low temperature nitrogen calcium carbonate phase remains structurally similar to the cal- adsorption–desorption cium acetate phase. The surface texture of the two calcined sys- tems is typical of alumina (Fig. 7a) yet very different for high Surface area, pore volume and pore diameter are present- calcium content (Fig. 7b) that is less porous, and the microcrys- ed in Table 4. Plots of isotherms, pore volume distribution talline calcium carbonate phase is present. For the Ca/Al 0.1 and pore area distribution are shown in Fig. 9. Fig. 6 EDS elements maps of Ca/ Al gel monoliths before calcination at the Al/Ca molar ratio: a 0.1, b 0.25, c 0.5, d 1.0 686 J Aust Ceram Soc (2018) 54:679–690 Fig. 7 SEM micrographs of Ca/ Al samples calcined at 600 °C at the Al/Ca molar ratio: a–c 1.0, d– f 0.25 Binary systems based on alumina in a wide range of CaO p/p 0.5–1, which is characteristic of metal oxide mesoporous content have large surface area (~ 180–320 m /g), which de- materials. The shape of hysteresis loops is similar for the ox- creases almost linearly with the content of the second compo- ides with Ca/Al ratio higher than 0.05. The obtained systems nent (having much lower specific surface area) to about 70 m / contain mainly small mesopores (< 10–15 nm) which bring g for the Ca/Al 1.0 system. The adsorption–desorption iso- the largest contribution to the surface area. The average pore therm is type IV with H2 hysteresis loop (except for the Ca/ size remains constant at about 6 nm, and the pore size distri- Al 0.05 system) that is found in the range of relative pressure bution is narrow. J Aust Ceram Soc (2018) 54:679–690 687 Fig. 8 TEM images of Ca/Al sol–gel samples at the Al/Ca molar ratio: a 0.1, b 0.5, c 1.0 The texture of Ca/Al 0.05 system is the most distinctive. In (blockage of pores) [7], it can be assumed that this decrease is addition to small mesopores (average diameter of about 3 nm) mostly the effect of alumina surface coverage with calcium resulting from high alumina content, some larger mesopores component, as suggested by the results of EDS element map- are present, including the ones in the range of 10–20 nm in ping. The H3-type hysteresis loop observed at high values of diameter. This is particularly evident taking into account the p/p indicates the presence of plate-like particle aggregates, surface area and volume distribution curves obtained from the giving rise to the slit-shaped pores in which the capillary con- desorptive branch of the isotherm and the shape of isotherms densation occurs, in addition to bottle-shaped pores with nar- and the hysteresis loop. However, the addition of calcium row necks (the H2 type hysteresis loop) [27]. For all alumina- component causes the disappearance of the smallest pores in based systems, a moderate negative effect of CaO addition on favour of larger mesopores. Their size and distribution does the surface area and pore volume can be observed, with little not change significantly with its amount. Multi-modal distri- impact on their average diameter and distribution. The in- bution of pores suggests complex and not fully homogenous crease in CaO/CaCO content also causes disappearance of structure of the system. The pore volume of the systems de- the smallest pores. creases from 0.65 to 0.17 cm /g with increasing calcium con- tent, similarly to the changes in the surface area. The highest FTIR measurements pore volume of the Ca/Al 0.05 system is associated with a significant volume of larger mesopores having a diameter of Spectroscopic studies have provided information on the compo- about 15 nm, whose presence is also apparent from the H2/H3 sition of the obtained systems after sol–gel synthesis and after mixed-type hysteresis loop of nitrogen adsorption isotherm. calcination (Fig. 10). Uncalcined samples show characteristic For this sample, it is even higher than for unmodified alumina. intense bands assigned to asymmetric and symmetric vibrations Despite the fact that the decrease in pore volume may be of calcium acetate carboxylate groups in the range of 1612– −1 caused by CaO or CaCO particles located in pores of alumina 1350 cm . After calcination, as a result of calcium acetate Table 4 Textural properties of System composition Surface area Average pore diameter Average pore volume Ca/Al systems 2 3 S [m /g] D [nm] D [cm /g] BET BJH BJH Al O 337.2 5.5 0.49 2 3 Ca/Al 0.05 322.0 3.3 0.65 Ca/Al 0.1 272.7 5.8 0.45 Ca/Al 0.25 233.0 5.8 0.42 Ca/Al 0.5 179.0 6.3 0.33 Ca/Al 1.0 67.9 6.8 0.17 From reference [12] 688 J Aust Ceram Soc (2018) 54:679–690 Fig. 9 Isotherms, pore volume distribution and pore area distribution of the Ca/Al systems J Aust Ceram Soc (2018) 54:679–690 689 Fig. 10 FTIR spectra of Ca/Al systems. a Before calcination. b After calcination decomposition, the bands disappeared and broad intense bands analysed in combination with microscopic images. High assigned to symmetric stretching vibration of unidentate carbon- CaCO /CaO content, because of decreasing ratio of Al/Ca, is −1 ate at 1540 cm appeared [28]. No bridged bidentate carbonate responsible for a significant, almost linear decrease in the spe- −1 (1620–1670 cm ) was found. Also, the appearance of a sharp cific surface area and pore volume. Introduction of calcium −1 2− band at 875 cm , assigned to the carbonate ionic group (CO ), component to alumina matrix, as seen from the XRPD patterns, confirms the presence of calcium carbonate after thermal treat- results in stabilisation of dispersion of the binary systems and ment [29]. The carbonate bands are observed for the Ca/Al 0.25, decreases the transformation of amorphous alumina into the γ- −1 0.5 and 1.0 systems. A clear 1166 cm band assigned to the Al O crystalline phase. The Ca/Al 0.05 composition has dis- 2 3 octahedral Al is visible for uncalcined systems [25] and dimin- tinctly different structural and surface properties, similar to ishes with increasing calcium content. This band disappears after those of undoped Al O obtained by the sol–gel method. 2 3 calcination. For low calcium contents, a weak Al–OH band Application of the sol–gel route allows obtaining a highly dis- −1 (1640 cm ) shifted to low-frequency occurs, which vanishes persive but not strictly homogeneous system irrespective of the with decreasing of Ca content [30]. molar ratio of the components. The dispersion and homogene- ity of the system diminishes with the rising calcium content in the system. Acetic acid used as a reagent and pH regulator allows the formation of various chemical forms of calcium in Conclusions the alumina matrix: acetate, carbonate or oxide in the structure of the material. Their formation translates into the final proper- The rate of the reaction of metallic calcium with alumina pre- ties of the products, e.g. texture or acid-base properties. cursor solution has a limited effect on the structure and homo- geneity of the oxides systems. We assume that in these systems, Compliance with ethical standards the pH increase related to the calcium content is a determining Conflict of interest The authors declare that they have no conflict of parameter of alumina gel formation [9, 21]. Both components interest. form a mixture of two separate phases: amorphous alumina and crystalline CaCO /CaO. The surface properties of the mixed Open Access This article is distributed under the terms of the Creative Al O –CaO systems are largely influenced by the CaO and 2 3 Commons Attribution 4.0 International License (http:// CaCO phases which have a clear tendency to concentrate on creativecommons.org/licenses/by/4.0/), which permits unrestricted use, alumina surface in the crystalline form and presumably, they distribution, and reproduction in any medium, provided you give also cause pores blockage, which can be evidenced by disap- appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. pearance of larger pores. It is visible from the textural data 690 J Aust Ceram Soc (2018) 54:679–690 17. Kamiya, K., Hioki, N., Hashimoto, T., Nasu, H.: Formation of α- References alumina around 500°C in alkoxy-derived alumina gels under ambi- ent pressure—effects of starting solution composition and seeding. 1. 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Journal of the Australian Ceramic SocietySpringer Journals

Published: May 10, 2018

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