Reactivity of Aluminum and Titanium Oxides under the Conditions of the Synthesis of Strontium and Barium Titanates in Water Fluid Media


Strontium and barium titanates deposited on a porous support (α-Al2O3) are synthesized by the treatment of previously deposited precursors (titanium oxide, strontium nitrate, or barium nitrite) in a water fluid medium at 400°C. The obtained samples are characterized by X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) spectroscopy. It is found that, in the presence of titanium oxide, α-Al2O3 is partially hydrated to form basic aluminum hydroxide AlO(OH) (boehmite), which is not detected by XRD after the treatment of α-Al2O3 in a water fluid. SEM with EDX spectroscopy demonstrates that strontium ions under the conditions of the treatment in a water fluid preferably interact with titanium oxide to form SrTiO3, although the aluminum oxide content in the samples is much higher. The conditions are determined for obtaining systems with different spatial distributions of the supported component inside the granules of the support by varying the procedure of its preliminary impregnation with the titanium oxide precursor.


The development of processes of direct conversion of light alkanes into valuable products and intermediates of organic and petrochemical synthesis is a promising approach to increase the efficiency of using raw hydrocarbons [1]. Producing lower olefins by the thermodynamically unlimited oxidative transformations of lower alkanes, including the oxidative coupling of methane, oxidative dehydrogenation of alkanes C2+ and the oxidative cracking of alkanes C3+, is of special interest.

The possibility of implementing industrial technologies based on these processes is largely determined by the progress of developing efficient catalysts. A number of systems exhibiting catalytic activity in the processes mentioned above were described in the literature (see, e.g., [13]). The most efficient catalysts are mixed oxides that generally do not contain transition metal ions, including alkaline- and rare-earth metal oxides, their combinations of various compositions, or compounds with oxides of other elements (e.g., titanates [4, 5]). These systems have a high level of structural sensitivity, i.e., a strong dependence of the catalytic properties on the phase composition and morphological features of the components and, hence, on the methods of their preparation [6]. The fact that catalysts based on alkaline- and rare-earth metal oxides are either crystalline powders or sintered ceramics is a significant disadvantage. In the former case, they are difficult to use as catalysts in a flow fixed-bed reactor, and in the latter case, it is hard to reach the optimal specific surface area and the developed porosity to ensure the efficiency of the catalytic process. These difficulties could be overcome by supporting them on a formable support. However, this problem has not been solved to date, partly because of the interactions of components that are difficult to control in complex oxide systems. Alkaline- and rare-earth metal oxides themselves are highly reactive to oxides that are typically used as catalyst supports. For example, in the case of SiO2 and Al2O3, at typical temperatures of the processes of oxidation of light alkanes (>600°C), the corresponding silicates and aluminates form quite rapidly, which drastically changes the catalytic performance of the system.

For complex oxides containing alkaline- and/or rare-earth metal oxides, there is a problem of their directed synthesis on an oxide support. For example, for strontium or barium titanates, the production of a supported catalyst can include successive steps of deposition of TiO2 and alkaline-earth metal compounds on an oxide support (SiO2, Al2O3). However, the further production of titanates by thermal synthesis can be accompanied by the uncontrollable formation of alkaline-earth metal silicates and aluminates, ternary compounds, and free TiO2. Moreover, at high synthesis temperatures, there can be additional sintering of the support and a decrease in the porosity and the specific surface area, which inevitably leads to a decrease in the accessibility of the supported active component to the reaction mixture and to a loss of the total activity of the catalytic system.

This study was undertaken as part of the development of an alternative approach to the synthesis of catalytic systems designed as simple or complex oxides deposited on supports that are also oxide. In this approach, precursors of the active component are deposited on a preliminarily formed support with a certain (chemical and phase) composition and morphology (pore volume and structure, specific surface area) and then treated in a water fluid medium. Together with a significant decrease in the temperature of the catalyst’s formation, such a treatment enables us to obtain systems differing from those formed by conventional thermal synthesis. This is because heat treatment is characterized by a limited number of factors influencing the reactivity of the components of the system, namely, by temperature and the properties of the surrounding gas phase (composition and pressure). In the synthesis of oxide systems, the properties of the gas atmosphere relatively weakly affect the results of the synthesis, unless we are talking about varying the oxidation state of individual cations that make up the oxides. In other words, the only factor that controls the reactivity is temperature. In the treatment in water fluids, the number of factors affecting the reactivity of the components of the system is much larger. Together with the temperature and redox potential of the medium, such factors also include the density and phase state of the water fluid, as well as use of the homogeneous modifiers (initiators or inhibitors) of the phase formation.

In the literature, there is a large number of examples of the synthesis of complex oxides of various compositions, structures, and morphologies (see, e.g., [720], including titanates [1520]). However, almost all of them describe the production of individual crystalline compounds. For such a synthesis, it is important to activate the initial reactants in the system. In complex multicomponent systems, the optimization problem is complicated by the fact that the selective synthesis of the supported strontium and barium titanates is possible if there is no simultaneous interaction of the precursors with the alumina support. In other words, the selective formation of titanates supported on Al2O3 is possible only if the reactivities of titanium and aluminum oxides under the treatment conditions differ significantly.

In this work, the task is to search for the conditions under which the selective synthesis of strontium and barium titanates of the composition MTiO3 (where M is Sr or Ba) with the perovskite structure supported on aluminum oxide is possible. For this purpose, it was necessary to obtain data on the differences in the reactivity between oxides and hydroxides of aluminum and titanium towards various alkaline-earth metal compounds in a water fluid medium. The precursor salts were strontium nitrate and barium nitrite, which, unlike carboxylic acid salts, cannot form carbonates as the end products of the side reactions. Because barium nitrate is poorly soluble in water and, therefore, cannot be deposited in a sufficiently large amount on a porous support by impregnation, the precursor in the synthesis of barium titanate was barium nitrite; its solubility is well above that of barium nitrate, and the chemical stability is far below that of nitrate [21].


α-Al2O3 samples obtained by the calcination of two commodity forms of γ-Al2O3 at 1150°C in air for 15 h were used as the support. The commodity forms were 0.1–0.2-mm (Sasol, RSA) and 1–2-mm (Sorbis Group, HKC Corp., Hong Kong) spherical granules, hereinafter referred to as SL and SS, respectively. In some of the experiments, a crushed SS support was used (0.1–0.2-mm fraction).

The precursor of titanium oxide was titanium tetraisopropoxide (iso-C3H7O)4Ti (TTIP). To deposit titanium oxide on porous aluminum oxide, weighed samples of the support were placed in weighing bottles and were heated to 100°C to remove the weakly bound moisture. Then, the hot samples were flooded with a TTIP solution in diethyl ether (DEE) in a 1 : 1volume ratio, and the weighing bottles were closed and left at room temperature for 20 min. After that, the organic phase was removed (pumped out with a syringe), and the samples were flooded with a mixture of ethanol, DEE, and water (in the volume ratio of 3 : 1 : 1) and were held for 10–12 h. It was assumed that TTIP in contact with the water-containing solution rapidly hydrolyzes to form titanium hydroxide, which remains in the pores of the support, and isopropanol, which passes into the aqueous–organic solution. After decantation of the liquid phase, the samples were washed twice with DEE, twice with ethanol, and thrice with water, and were dried at 100°C for about 1 h. In order to convert titanium hydroxide into oxide, the samples were heated at 530°C for 1.5 h. The amount of the supported titanium oxide was determined from the difference between the weights of the initial support and the obtained sample.

The initial alkaline-earth metal compounds were strontium nitrate and barium nitrite (both chemically pure, Reakhim). The required amount of salt was calculated from the stoichiometry of the desired titanates MTiO3 (M = Sr or Ba; the atomic ratio M : Ti = 1 : 1) and the data on the titanium oxide content on the support. The calculated amount of the salt was dissolved in water, and the obtained solution was used for incipient wetness impregnation of the TiO2-containing support.

The treatment with water fluid was carried out in 14–17-mL autoclaves. The equipment and procedure of the treatment were described previously [1820, 22, 23]. A sample was placed in a stainless-steel inner container, and water was poured into the autoclave outside the container to avoid the sample coming into contact with liquid water. The amount of the water poured into the autoclave was calculated from the fact that the density of the water fluid formed by the complete evaporation of this water should be 0.1 g/cm3 (i.e., e.g., at a volume of the autoclave of 15 cm3, 1.5 g of water was loaded into it). If the samples contained Sr(NO3)2 and Ba(NO2)2, then, to accelerate their decomposition (reduction), concentrated (25 wt %) aqueous ammonia was used instead of distilled water. To ensure the identity of the treatment conditions, several autoclaves with different samples were simultaneously placed in a SNOL furnace. The autoclaves were heated in two modes: either they were placed in the furnace preheated to the given temperature (400°C), or they were first put into a cold furnace, after which heating was started.

The obtained samples were studied by X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) spectroscopy. The physicochemical analysis procedures were described in detail previously [23, 24].


Effect of Treatment in a Water Fluid Medium on Various Modifications of Aluminum Oxides

Table 1 presents the conditions of the treatment of samples in water fluid media and their initial and final compositions.

Table 1.   Chemical and phase compositions of the initial samples, the conditions of the treatment in water fluid, and the phase composition of the treated samples (according to XRD data)

The formation of hydrated forms of oxides can be considered a sign of their activation in a water fluid. In the case of aluminum oxide, they can be various crystalline polymorphs of Al(OH)3 (e.g., gibbsite or bayerite) or AlO(OH) (boehmite). As the data in Table 1 show, in the treatment in a water fluid medium, samples that have similar compositions but contain different forms of Al2O3 behave differently. In the case of γ‑Al2O3, AlO(OH) (boehmite) forms, which, with increasing treatment time, at 400°C gradually transforms to corundum (α-Al2O3).

Under the same conditions, no changes were observed in the phase composition of the α-Al2O3. It could be concluded from this that the treatment in a water fluid medium leads to the transition of the γ‑form of aluminum oxide to corundum, the process is irreversible, and AlO(OH) (boehmite) is an intermediate compound. The transformation occurs at a much lower temperature than in air, where this process requires holding for a long time at temperatures above 1100°C.

Estimation of the thermodynamic parameter of the reactions

$$\begin{gathered} {\text{Al(OH}}{{{\text{)}}}_{{\text{3}}}} \to {\text{AlO(OH)(boehmite)}} + {{{\text{H}}}_{{\text{2}}}}{\text{O}}, \\ \Delta G_{{298}}^{^\circ } = + 17.4\,\,{{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {{\text{mol}}}}} \right. \kern-0em} {{\text{mol}}}}{\text{,}} \\ \end{gathered} $$
$$\begin{gathered} {\text{2AlO(OH)(boehmite)}} \to \alpha {\text{-A}}{{{\text{l}}}_{2}}{{{\text{O}}}_{3}} + {{{\text{H}}}_{{\text{2}}}}{\text{O}}, \\ \Delta G_{{298}}^{^\circ } = + 9.6\,\,{{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {{\text{mol}}}}} \right. \kern-0em} {{\text{mol}}}}{\text{,}} \\ \end{gathered} $$
$$\begin{gathered} {\text{2AlO(OH)(boehmite)}} \to \gamma {\text{-A}}{{{\text{l}}}_{2}}{{{\text{O}}}_{3}} + {{{\text{H}}}_{{\text{2}}}}{\text{O}}, \\ \Delta G_{{298}}^{^\circ } = + 27.9\,\,{{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {{\text{mol}}}}} \right. \kern-0em} {{\text{mol}}}}{\text{,}} \\ \end{gathered} $$

using the published data [21, 2527] demonstrated that the conversion of AlO(OH) (boehmite) to α-Al2O3 (corundum) is preferred to the transformation into γ‑Al2O3. However, the fact that, in the samples treated in a water fluid, no Al(OH)3 phases (gibbsite or bayerite) were detected indicates that solely thermodynamic estimation is insufficient to predict the directions of the transformations. The fact that the completion of the exothermic process

$$\begin{gathered} \gamma {\text{-A}}{{{\text{l}}}_{2}}{{{\text{O}}}_{3}} \to \alpha {\text{-A}}{{{\text{l}}}_{2}}{{{\text{O}}}_{3}}, \\ \Delta H_{{298}}^{^\circ } \approx - 19\,\,{{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {{\text{mol}}}}} \right. \kern-0em} {{\text{mol}}}}{\text{,}}\,\,\,\,\Delta G_{{298}}^{^\circ } \approx - 18\,\,{{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {{\text{mol}}}}} \right. \kern-0em} {{\text{mol}}}}{\text{,}} \\ \end{gathered} $$

in air requires holding the samples for a long time at high temperatures (>1100°C) and includes the formation of numerous intermediate forms also shows that the transformations of aluminum oxides that require the rearrangement of the crystal structure are controlled by kinetic factors. The main factor is obviously the low mobility of the structural elements, which is caused by the strength of the Al–O bonds. The thermodynamic stability can be characterized by the destabilization temperature Td, which, for the endothermic reactions of the decomposition (ΔS° > 0), can be found from the condition

$$\Delta {{G}^{^\circ }} = \Delta {{H}^{^\circ }} - {{T}_{{\text{d}}}}\Delta {{S}^{^\circ }} = 0$$


$${{T}_{{\text{d}}}} = {{\Delta {{H}^{^\circ }}} \mathord{\left/ {\vphantom {{\Delta {{H}^{^\circ }}} {\Delta {{S}^{^\circ }}}}} \right. \kern-0em} {\Delta {{S}^{^\circ }}}},$$

where ΔH° and ΔS° are the enthalpy and entropy changes, respectively, in this case, in the reaction of dehydration.

The observed temperature of decomposition of AlO(OH) (>550 K [21], for the complete decomposition, a temperature several hundred degrees higher is required) is much higher than Td, which is 365 and 490 K for reactions (2) and (3), respectively. Furthermore, the primary product is not α-Al2O3 but γ-Al2O3. All these facts suggest that these thermal transformations of the oxide compounds of aluminum take place under kinetic rather than thermodynamic control. In a dense water fluid medium at a relatively low temperature, the kinetic limitations are significantly weakened because of the formation of highly hydrated forms of oxides, in which the mobility of the structural elements is much higher (see, e.g., [6, 7, 19, 27]).

Note, however, that, in this study, we could observe the state of a sample only outside the transformation conditions (room temperature, air atmosphere with a low water vapor content), which prevented us from reliably judging the changes in the phase composition of samples under the reaction conditions.

There is evidence [30] that the hydration of corundum is possible at high temperatures only at water fluid densities close to 1 g/cm3. On the other hand, attention is drawn to the presence of traces of boehmite after the treatment of α-Al2O3 samples, which initially contained variously hydrated titanium oxide. This result made us consider the possible chemical mechanism of the conversion in more detail.

According to the XRD data (Table 1), no compounds (mixed oxides) of aluminum and titanium were formed under our experimental conditions. Individual γ-Al2O3 is easily hydrated to boehmite, which then gradually transforms into the thermodynamically stable form of anhydrous aluminum oxide—corundum (α-Al2O3). As for titanium hydroxides, we failed to find reliable data on their thermodynamic parameters. However, according to the available published data, they are generally much less stable than the hydrated forms of aluminum oxides: the loss of the water of a varying composition of hydroxide TiO2 · nH2O begins and can completely end even during drying in air or even during the boiling of an aqueous suspension (see, e.g., [30]). Under these conditions, boehmite is still quite stable. Nonetheless, under the action of a dense water fluid even at a much higher temperature (400°C), water is likely to be bound by titanium oxide to yield hydrated forms, and their sequential transformation gives the low-temperature form of TiO2, anatase, which is equilibrium at the treatment temperature. The fact that this is accompanied by a partial conversion of corundum to boehmite may indicate the following points:

—at 400°C in a medium of water fluid with a density of 0.1 g/cm3, there is a partial hydration of α-Al2O3;

—the loss of the water of the hydrated form of aluminum hydroxide primarily gives rise to the most stable high-temperature form of Al2O3, corundum (boehmite was not observed even in trace amounts);

—the presence of titanium oxide may lead to the formation of mixed hydrated forms (hydroxides), including both Al3+ ions and Ti4+ ions, the subsequent dehydration of which leads to the formation of anatase, boehmite, and corundum.

Thus, in a water fluid (density 0.1 g/cm3) at 400°C, titanium oxide and α-aluminum oxide are activated (hydrated) in the event that they are both present in the system. Under the action of a water fluid, the kinetic control of the solid-phase transformations weakens, and the products are the phases (crystalline forms) of aluminum and titanium oxides that correspond to their equilibrium state at the treatment temperature.

Synthesis of Alkaline-Earth Metal Titanates in a Water Fluid Medium

The data in Table 1 demonstrate that barium nitrite reacts with titanium oxide (hydroxide) to form various crystalline polyforms of barium titanate during heating to 400°C. As for strontium nitrate, the formation of the corresponding titanate was observed only when it was held in a water fluid at 400°C for 3 h (mode 2). In other words, the rate of the formation of MTiO3 from strontium nitrate is much lower. Noteworthy, the samples showed no presence of phases of alkaline-earth metal aluminates.

Interestingly, as was shown in some experiments, the treatment of Sr(NO3)2 supported on porous α‑Al2O3 in a water fluid leads to the formation of aluminate of the SrAl4O7 composition. This confirms the conclusion above that the formation of corundum from the hydrated forms of aluminum oxide is reversible and probably also occurs to a certain extent in the absence of titanium compounds. In addition, the hydrated form of aluminum oxide also has sufficiently strong reactivity to involve strontium nitrate in the reaction, with strontium nitrate itself being highly stable, which is indicated by its presence in the samples treated in mode 1.

The presented facts suggest that titanium oxide hydrated in water fluid is more reactive to alkaline-earth metal compounds than a similar form of aluminum oxide, especially considering that the amount of the latter in the sample is significantly higher.

It is just as important that the interaction of barium nitrite with titanium oxide and hydroxide (hydrolyzed TTIP) supported on α-Al2O3 gives various crystallographic forms of BaTiO3: in the former case, cubic perovskite, and in the latter case, its rhombohedral, orthorhombic, or tetragonal forms (see Table 1), depending on the treatment conditions. This opens up additional possibilities for producing composite materials with various properties.

In the synthesis of barium titanate, the presence of reflections of BaCO3 in the X-ray powder diffraction patterns of the samples is of interest (Table 1). The remaining fragments of the isopropoxyl groups of the initial TTIP are probably the source of the carbon for the formation of carbonate.

It remains an open question why, in the system simultaneously containing titanium and aluminum oxides, which are activated in a water fluid, alkaline-earth metal titanates are formed rather than aluminates, although there is a large excess of aluminum oxide in the system. The existing data are insufficient to answer this question.

It was previously assumed that, when BaCO3 is used as a precursor in the thermal synthesis of barium titanate, it forms by the diffusion of barium ions into the TiO2 lattice [30, 31]. A similar mechanism in the synthesis in a water fluid from BaO and TiO2 is suggested by the inheritance of the morphology of the initial TiO2 by the obtained BaTiO3 [27]. In this case, it is assumed that the key role in the increase in the mobility of the structural elements of the reactants and, hence, in the formation of the product, is played by the hydration and also hydroxylation of titanium oxide to form acidic OH groups [27]. The answer to the question regarding the extent to which this mechanism can occur under the conditions of the synthesis from alkaline-earth metal salts of a strong acid (nitrates) also requires additional investigation.

Morphological Features and Distribution of the Components in the MTiO3/Al2O3 Samples

The supports used in this study had identical phase compositions (α-Al2O3) but different morphologies, which is determined by the differences between their production technologies. Nonetheless, the main characteristics of the synthesis of the supported alkaline-earth metal titanates were unaffected by these differences.

The efficiency of the catalysts deposited on a porous support depends not only on their chemical and phase compositions but also on the distribution of the components in a granule. The SEM EDX studies of the synthesized MTiO3/Al2O3 samples showed that using the procedure of deposition of the TiO2 precursor (TTIP) on aluminum oxide, which was described in the experimental section gives rise to a titanium oxide crust (Figs. 1a, 1b). This was observed both in the intact granules of the support and in the precrushed ones, and indicates that the formation of such a crust is related not to the morphology of the outer layer of granules, which can be (as, e.g., in the case of the α-Al2O3 SS sample) denser than the inner layers, but to the features of the interaction of the TTIP with the support and its conversion to titanium hydroxide.

Fig. 1.

SEM micrographs of α-Al2O3 samples with deposited TiO2: (a) SL sample and (b) precrushed SS sample.

The observed feature is most likely to be related to the fact that the support granules being impregnated with the TTIP solution in DEE begin to be hydrolyzed by the water adsorbed on aluminum oxide once the TTIP comes into contact with the outer surface of the support. The formed titanium hydroxide constitutes a dense layer on the surface of the granules and prevents the further penetration of the impregnation solution into the granules, and the further hydrolysis of the TTIP is produced by water that is contained in the granules of the support and diffuses to their surface.

This assumption is confirmed by the SEM EDX data on the distribution of titanium inside the support granule after it is split (Fig. 2a; Table 2). They show a decrease in the titanium concentration as the center of the granule is approached. This indicates that the impregnation of the support with the TTIP solution in DEE is accompanied by the hydrolysis of the TiO2 precursor, which prevents its penetration into the granule.

Fig. 2.

Positions of the fields of recording EDX spectra inside the granules of the support (α-Al2O3 SL) after deposition of titanium oxide (a) by the method described in the experimental section and (b) by the procedure including preliminary vacuum heat treatment of the support.

Table 2. Change in the titanium concentration according to the EDX data with depth in a granule of the α-Al2O3 SL sample

A still more convincing confirmation of the assumption given above was obtained by varying the procedure of the deposition of the TTIP on the support and its hydrolysis. In the additional experiments, α-Al2O3 granules were dehydrated by heating at 300°C in a vacuum, while evacuating with a forevacuum pump (~10–2 torr), after which the granules were cooled and flooded with a TTIP solution in DEE without intermediate contact with air. Two hours later, the liquid phase was decanted, and the sample was washed twice with DEE and flooded with 90% ethanol. On the next day, the samples were sequentially washed with DEE, ethanol, and water; dried; and calcined for 1.5 h at 530°C.

Figure 2b and Table 2 present the SEM EDX data. It is seen that the trend in the change in the titanium concentration in a section of a granule is reversed: a higher content of the supported component is observed in the deeper layers of the support granule. This indicates the absence of impediments to the penetration of the TTIP into the granules after the removal of the bound water.

The formation of a titanium oxide layer on the outer surface of the support granules adversely affects the synthesis of the supported alkaline-earth metal titanate. However, such a nonuniform distribution turned out to be a convenient tool to determine the reactivity of the second deposited component, Sr or Ba salt, in relation to titanium and aluminum oxides because the subsequent deposition and treatment in a water fluid gives rise to systems with nonuniform distributions of the alkaline-earth metal. Figure 3a presents the EDX spectra of two regions of the surface of a crushed granule of the α-Al2O3 SS support with the deposited titanium oxide after additional impregnation with a strontium nitrate solution and treatment in a water fluid. Figure 3b shows the regions of the surface of the granule on which spectra 1 and 2 were recorded. It is seen that region 1 is free of a TiO2 crust, whereas region 2 is covered with a TiO2 crust. The presented spectra indicate that the strontium concentration is notably higher in region 2, where the titanium content is almost twice as high.

Fig. 3.

EDX spectra of the outer surface of a crushed granule of the α-Al2O3 SS substrate: (a) in regions 1 (top) and (b) 2 (bottom).

The data presented in Table 3 on the concentrations of Al, Ti, and Sr in regions 1 and 2 enable us to estimate the extent to which strontium preferably binds to titanium oxide in comparison with Al2O3. If it is assumed that strontium binds to aluminum and titanium oxides in constant ratios, regardless of whether this occurs on the outer surface of the granules of the support or within its pores, then the total amount of strontium in the given region of the micrograph can be expressed as

$$\left\langle {{\text{Sr}}} \right\rangle = x\left\langle {{\text{Al}}} \right\rangle + y\left\langle {{\text{Ti}}} \right\rangle ,$$
Table 3. Concentrations of elements in regions 1 and 2 (see Fig. 3) as determined by EDX analysis

where x = Sr : Al and y = Sr : Ti are the ratios in which strontium binds to aluminum and titanium, respectively; and \(\left\langle {{\text{Al}}} \right\rangle \) and \(\left\langle {{\text{Ti}}} \right\rangle \) are the amounts of aluminum and titanium in the given region of the micrograph, respectively. Assuming x and y are constant, we can find them by solving the system of Eqs. (4) for two regions by substituting the values in Table 3 for \(\left\langle {{\text{Sr}}} \right\rangle \), \(\left\langle {{\text{Al}}} \right\rangle \) and \(\left\langle {{\text{Ti}}} \right\rangle \). Solving such a system of equations gives x = ~0.058 and y = ~1.3. We note that the y value is close to the stoichiometric ratio Sr : Ti = 1 : 1 in titanate SrTiO3. Although such an estimate is approximate and somewhat restricted, it is indicative of the preferred binding of strontium to titanate and confirms the XRD data on the absence of aluminates in the samples of the phases.

The obtained data can be summarized as follows. The treatment of systems containing the γ-form of aluminum oxide gives highly reactive boehmite. In systems containing α-Al2O3 (corundum), boehmite is also formed; however, this process depends on which components are additionally present in the system. In particular, it was determined that the predeposition of TiO2 by the hydrolysis of the TTIP stimulates the conversion of a part of the corundum to boehmite in a water fluid medium with a density of 0.1 g/cm3 at 400°C.

The treatment of the binary system α-Al2O3–strontium nitrate gives aluminate (SrAl4O7), which is highly undesirable for the synthesis of the supported titanate. Nonetheless, it was shown that the high reactivity of hydrated titanium oxide with respect to the formation of alkaline-earth metal titanates under the conditions of the treatment in a water fluid medium at temperatures of ≤400°C allows us to synthesize them within spherical granules of porous α-Al2O3. The synthesis conditions under which it is possible to control the distribution of the supported phase with the depth of a support granule were determined.


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This work was supported by the Russian Foundation for Basic Research (project no. 18-29-06055).

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Correspondence to M. Yu. Sinev.

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Translated by V. Glyanchenko

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Lagunova, E.A., Ivakin, Y.D., Sinev, M.Y. et al. Reactivity of Aluminum and Titanium Oxides under the Conditions of the Synthesis of Strontium and Barium Titanates in Water Fluid Media. Russ. J. Phys. Chem. B 14, 1252–1259 (2020).

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  • Keywords: reactivity
  • titanium oxide
  • aluminum oxide
  • hydration
  • water fluid
  • alkaline-earth metal titanates