The isotherms of nitrogen adsorption–desorption for the deposited samples prepared using different methods are presented in Figs. 1, 2, and 3. The parameters of porous structure which were calculated from the adsorption–desorption isotherms are presented in Table 1. One can see that the initial bulk ZrP is micro-mesoporous (sample 1). Its dry mechanochemical deposition in the form of xerogel on aerosil A-380 leads to the increase of specific surface area of the resulted composition (sample 2). However, this sample can be considered as non-porous because initial support (A-380) itself is superfine and non-porous and remains in that state in the process of dry milling [25]. The use of a more coarse dispersed support, namely fumed TiO2, results in formation of rigid porous compositions as a result of dry and wet milling (samples 3, 4). In both cases milled samples contain mesopores with two maxima in the same position which are observed on the PSD curves. It can be seen that the PSD curve, showing the characteristic of the sample prepared by wet milling, has no sharp peaks. At the same time, the fraction of mesopores (column 6) for this sample is small (about 16%) but the content of macropores is prevalent (column 8). Because of this its specific surface area is three times lower compared with that of the sample subjected to dry milling. It should be noted that isotherms of nitrogen adsorption–desorption obtained for the compositions, based on titania, contain a weakly pronounced capillar-condensation hysteresis loop (Fig. 1). Deposition of ZrP in the form of wet hydrogel leads to formation of meso-macroporous compositions with different contents of indicated pores and high value of total pore volume (samples 5–7). Meantime, the sample based on more dispersed A380 possesses maximal value of V me while that prepared from lesser dispersed A50 has a maximal value of V ma. The isotherms obtained for the compositions prepared from ZrP hydrogel relate to type IV and contain a capillar-condensation hysteresis loop of type A (Figs. 1,2). It is noteworthy that the PSD curve for the composition based on A380 has one maximum but those for the samples based on lesser dispersed A50 and TiO2 possess two maxima (Figs. 1, 2). The latter is better than for the sample on the basis of TiO2 (Fig. 1 curve c).
Fig. 1Isotherms of nitrogen adsorption–desorption and curves of PSD for the bulk ZrP (a) and the milled samples based on TiO2: ZrP xerogel air (b), ZrP xerogel water (c), ZrP hydrogel (d)
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Fig. 2Isotherms of nitrogen adsorption–desorption and curves of PSD for the bulk ZrP (a) and the milled samples based on aerosils: ZrP xerogel air on A380 (b), ZrP hydrogel on A380 (c), ZrP hydrogel on A50 (d)
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Fig. 3Isotherms of nitrogen adsorption–desorption and curves of PSD for the bulk ZrP (a) and the samples prepared via the sol–gel method: 10% w/w ZrP at 20 °C (b), 20% w/w ZrP at 20 °C (c), and 200 °C (d)
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Table 1 Influence of synthesis conditions on porous structure resulted compositionsFull size table
It can also be seen that embedding of ZrP moieties in the siliceous framework in the stage of gelation results in decrease of both specific surface area S and volume of sorption pores V s, and micropores V mi (samples 8,9 in comparison with sample 11). In fact, the mesopores volume and size (V me and d PSD, respectively) as well as the total pore volume V Σ are not changed. However, the value of V Σ exceeds that of V s. The latter indicates formation of macropores in the composite samples. Soft HTT in the hydrogel stage unexpectedly leads to the sharp growth of S, V s, V me, and V Σ (sample 10). At the same time, micropores almost disappear. In other words, more open and larger porous, namely mesoporous, structure, which is more accessible for sorbate molecules, is formed. This can be seen from the change of the shape of adsorption–desorption isotherms (Fig. 3) and the curves of PSD that are constructed based on the isotherms (Fig. 3). Indeed, the curves of PSD for untreated samples 8 and 9 have a broad maximum around 2.7 nm, while a distinct peak at 3.7 nm is on the PSD curve for modified sample 10.
It should be added that the above-described patterns of change of the porous structure largely coincide with those previously obtained for the heteropolycompounds/oxide supports compositions [26, 27].
In accordance with the X-ray fluorescent analysis, bulk and deposited ZrP has the atom ratio Zr/P = 2:1, which corresponds to the compound Zr(HPO4)2. The XRD data show that all supported samples synthesized based on silica are X-ray amorphous. At the same time, diffractograms of the samples based on fumed titania contain reflexes attributed to anatase and rutile, i.e., they relate only to support. In fact, intensity of the reflections of support and their ratios somewhat differ for the prepared compositions. Therefore, all deposited samples contain amorphous Zr(HPO4)2. It should be added that initial bulk ZrP is also X-ray amorphous. The DT- and TG analyses indicate thermal stability of the amorphous component up to 800 °C. The DTA and TG curves obtained for the deposited samples show only the final effect of adsorbed water removal in the temperature range 20–200 °C and have no effects related to phase transformations.
The FTIR spectra depicted in Figs. 4, 5, and 6 confirm the presence of ZrP in the structure of compositions. Thus, the spectrum of bulk ZrP contains a. and b. centered around 425, 510, 595, 710, and 1038 cm−1 (Fig. 4, curve a) which are attributed to deformation and stretching vibrations of P–O bonds in PO4 3− [4, 28, 29]. The embedding of ZrP into the siliceous framework during the sol–gel synthesis results in shift of a.b. 510 to 528 cm−1 and a.b. 1038 cm−1 toward 980 cm−1 (Fig. 4, curve b). The latter may be a consequence of the interaction between ZrP and the surface groups of support in narrow pores the content of which is the largest in this sample. The changes of position of indicated a.b. are smaller and lesser for samples prepared at 200 °C (curve c): thus, a.b. 1038 cm−1 shifts to 1000 cm−1. The latter may be because of the fact that the hydrothermally modified sample possesses larger pores as mentioned above.
Fig. 4FTIR spectra for bulk ZrP (a) and the deposited samples prepared via the sol–gel method at 20 °C (b), 200 °C (c)
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Fig. 5FTIR spectra for the milled deposited samples based on aerosils: ZrP xerogel air on A380 (a), ZrP hydrogel on A380 (b), and A50 (c)
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Fig. 6FTIR spectra for the bulk ZrP (a) and the milled samples based on TiO2: ZrP xerogel air (b), ZrP xerogel water (c), ZrP hydrogel (d)
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In contrast, interaction of deposited ZrP with non-porous fumed oxides obviously is minimal. At least, this interaction does not appear in spectra of the samples synthesized via milling (Figs. 5, 6): the shift of a. and b. which are characteristic of ZrP is not observed. Moreover, these bands are overlapped by the intense absorption which is attributed to supports. This can be explained by larger porous structure of the matrices, which are formed from fumed oxides compared with the samples prepared via the sol–gel route. This can be clearly seen in Table 1 (column 9).
Very important parameters characterizing the electric double layer are isoelectric point pHiep and point of zero charge pHpzc. Assuming that the potential of diffuse layer equals the zeta potential, the concentration of surface groups negatively charged in pHiep point equals of surface groups which are positively charged. pHiep of bulk and some supported ZrP as a function of pH in the 0.001 M aqueous NaCl solution is depicted in Figs. 7, 8. The values of pHiep determined for all samples are collected in Table 2. As can be seen a zeta potential reduces with the pH increase in all cases. It has been also found that nature of support and procedure of ZrP deposition have great influence on pHiep value. The differences between the samples are expressed as in the course of zeta potential–pH depencence and in the position of the isoelectric point.
Fig. 7The ζ potential–pH dependence for the samples ZrP/A380
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Fig. 8The ζ potential–pH dependence for the samples ZrP/TiO2
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Table 2 Value of pHiep samples of zirconium phosphate Zr(HPO4)2 deposited on different supports via different techniquesFull size table
The value of pHiep for bulk ZrP equal to 3.65 is consistent with literature [30]. For pure supports prepared under the same conditions, as deposited samples, pHiep has the following positions: 2.02, 2.30, 2.61, and 2.53 for milled A380, A50, titania and silica synthesized using the sol–gel technique, respectively. The presented values also correspond to literature data [30, 31]. It is known that mixed (two-component) systems may exhibit isoelectric point values that are intermediate to those of the corresponding pure components [32]. So, in column 4 of Table 2 the \( {\text{pH}}_{\text{iep}}^{\text{ad}} \) values there are presented for the supported samples calculated by the additive rule. From Table 2 one can see that for most deposited samples (except one) pHiep, determined experimentally, is smaller than that for bulk ZrP but larger compared with the \( {\text{pH}}_{\text{iep}}^{\text{ad}} \) values. Only for the compositions prepared through milling of ZrP xerogel with titania (samples 3,4) pHiep and \( {\text{pH}}_{\text{iep}}^{\text{ad}} \) coincide. These samples have a minimum value of pHiep close to pHiep of pure support. The magnitude of difference between pHiep and \( {\text{pH}}_{\text{iep}}^{\text{ad}} \) obviously depends on the extent of coverage of support surface with ZrP. Based on this assumption, one can conclude that the surface of titania is covered with ZrP to the smallest extent. At the same time, the use of non-porous support possessing a maximal specific surface area (namely A380) leads to the increase of pHiep of the formed composition and improved coverage of the support surface. The deposition of ZrP in the form of wet hydrogel brings the same results (samples 5–7). The supported samples prepared using sol–gel procedure display an average effect (samples 8–10).
Figures 9, 10 show the relationships of surface charge density versus pH for the electrolyte concentration 0.001 mol dm−3 for some samples. As one can possibly notice, the surface charge density σ decreases with the pH increase in all cases. However, the surface charge density is rather sharply and uniformly reduced for bulk ZrP. At the same time, this parameter decreases very slowly in the range from pHpzc to pH = 7 for the silica based samples (Fig. 9). Fast jump of σ is observed over this pH value. It is characteristic of SiO2/solution system [33]. The course of surface charge density—pH dependences obtained for the TiO2 based compositions is closer to that for bulk ZrP (Fig. 10).
Fig. 9The surface charge density–pH dependence for the samples ZrP/A380
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Fig. 10The surface charge density–pH dependence for the samples ZrP/TiO2
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On the other hand, the point of zero charge pHpzc is approximately equal to 3.45 for bulk ZrP and all studied deposited samples independent of the nature of support and method of their preparation. The values of pHpzc and pHiep of the ZrP sample are very close(within 0.2 pH unit) that may testify that its surface is free of alkali or acid contaminants [34]. The deposited samples of ZrP on fumed TiO2 show different pHiep for ZrP hydrogel deposited on fumed TiO2 is 0.2 pH unit below pHpzc, whereas for xerogels in air and xerogels in H2O is below pHpzc 0.65 and 0.8 pH unit, respectively. This indicate that deposition of ZrP xerogel in air or in H2O on the surface of the samples there appeared groups of acidic character. However, small increase in the surface charge in the sample of ZrP/fumed TiO2 xerogel/electrolyte interface indicated that surface concentration of these groups is lower than that for sample prepared in H2O. As mentioned above, the surface charge density as a function pH of ZrP deposited on A-380 shows similar characteristic to that of the silica/electrolyte system. The comparison of pHpzc and pHiep for these systems is very difficult because the slope of surface charge as a function of ZrP/A-380 samples is very low (as in the case of silica). However, the shift of pHiep shows that for these samples on the surface, the groups with acidic properties also appear.
Therefore, it can be assumed that in the deposited samples ZrP itself influences on the surface charge more than a support.
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