Thermal and calorimetric investigations of titania–silica composites

Abstract

The paper presents the studies of effects of TiO₂ amount differentiation on the structural and thermal properties of titania–silica complex oxides prepared by chemical vapor deposition of TiCL₄ onto the Si-40, Si-60, and Si-100 silica gels. The mesoporous materials, characterized by a varied pore structure with highly developed surface and large pore volume, were obtained. The porous structure of materials under investigations was characterized by the low-temperature nitrogen adsorption–desorption method as well as by power spectral density calculated from the calorimetric investigations of water confined in the pores. Moreover, the thermodesorption of water using the quasi-isothermal thermogravimetry was used to characterize their thermal and surface properties. The adsorbed water layers and the concentration of weakly and strongly bound water as well as the surface free energy on the adsorbent–water interfaces were calculated. It was stated that the increase of titania content causes a gradual decrease of specific surface area and has a significant effect on the porous structure formation. The water thermodesorption from the surface proceeds in few stages because of the porosity created by TiO₂. The decrease in the total surface free energy (ΔG_Σ) can be observed with the increasing TiO₂ content. The largest ΔG_Σ value at the adsorbent/strongly bound water interface is exhibited by the adsorbents of Si-100 series. The lowering of the freezing/melting points of water contained in the pores of the studied materials is strongly connected with their porous structure.

Introduction

Complex oxides, especially those based on silica as the mineral matrix, play a significant role in the adsorption and catalytic processes (Gonzalez et al., 1997; Miller and Ko, 1997; Miller and Lakshmi, 1998; Thoma and Nenoff, 2000). TiO₂–SiO₂ complex oxides (TS) are essential. Incorporation of the transition metal (in the form of elements, ions or oxides, particularly titanium) (Corma, 1997; Corma et al., 1997; Tanev et al., 1994) into the silica gel structure can cause changes in the porous structure of the complex oxides and lead to creation of additional, special catalytic properties. These materials are considered as a substitute for pure TiO₂, due to the superior mechanical strength and thermal stability of such highly dispersive composites including TiO₂.

One of the most common methods used in the process of titania–silica complex oxide (TS) preparation is the chemical vapor deposition (CVD) method. As a result, one can observe separated phases of TiO₂ and SiO₂ with a clear-cut phase boundary because one oxide is formed on another. The activity of such oxides in various processes is affected by different factors, e.g. porous structure, chemical nature of surface functional groups, and their natural distribution and concentration (Crişan et al., 2008). The surface and sorption properties of such materials depend largely on the amount of bound and physically adsorbed water. The water confined in the pores changes its physicochemical properties because of the interactions with the surface. Thus, by studying the phase transitions of water bound to the surface of such materials, one can obtain the additional information about their porous structure and surface nature.

The aim of this study was the application of differential scanning calorimetry (DSC) cryoporometry and quasi-isothermal thermogravimetry (Q-TG) analysis for characterization of the structural and thermal properties of titanium–silica composites of different TiO₂ contents.

Experimental

Materials

The adsorbents were prepared on the basis of the initial silica gels: Si-40, Si-60, and Si-100 by CVD of TiCl₄ followed by thermal modification. The reaction cycles of TiO₂ deposition onto the silica surfaces were repeated 1 to 4 times in order to differentiate the TiO₂ loading. The TiO₂ contents of the obtained materials were from 2.3 to 19.2 wt%. They were designed as TS-40 _x , TS-60 _x , and TS-100 _x , where x denotes the number of CVD cycles. The details of the preparation process were described in Gun’ko et al. (2002).

Methods

Adsorption

N₂ adsorption–desorption isotherms were recorded at −196℃ by means of a Micromeritics ASAP 2405N adsorption analyzer. The specific surface area S_BET was calculated using the standard BET equation (Gregg and Sing, 1982) at p/p_o between 0.06 and 0.2 (where p and p_o denote the equilibrium and saturation pressures of nitrogen, respectively). The pore volume V_p was estimated under the relative pressure p/p_o ≈ 0.98. The average pore radius (R_av) was calculated for a model of cylindrical pores R_av = 2V_p/S_BET (Gregg and Sing, 1982). The nitrogen desorption data were used to calculate the pore-size distribution ((IPSD_v, dV = f(R))) using the procedure described in Gun’ko and Mikhalovsky (2004). The contribution of nanopores (V_nano, S_nano for R_p < 1 nm), mesopores (V_meso, S_meso for 1 nm < R_p < 25 nm), and macropores (V_macro, S_macro for R_p > 25 nm) was determined (Gun’ko and Do, 2001; Gun’ko and Turov, 2013; Gun’ko et al., 2013; Skubiszewska-Zięba et al., 2012).

DSC investigations

Differential scanning calorimetry (DSC) examination of interactions between the solids surface and water was carried out using the differential scanning calorimeter (PYRIS Diamond, Perkin Elmer Instruments, USA) at a constant rate of 2℃/min. DSC was calibrated using the ultrapure water (Millipore, Bedford, MA, USA, melting temperature T_m = 0℃) and the standard indium sample (T_m = 156.6℃) using a standard calibration procedure. Before measurements the samples were placed in the aluminum pans. As a reference, an empty aluminum pan was used. Measurements were made in an atmosphere of inert gas (helium). Nitrogen was used as the curtain gas to prevent the system defrost.

Materials under investigation were saturated before the measurement with water vapor in the hermetic vessel for 72 h at room temperature. Then the weighed amount (∼10 mg) was placed in a DSC pan and cooled to −70℃. Melting was recorded at a temperature ranging from −70℃ to + 20℃. The melting thermograms were used to determine the PSD curves and define the temperature (T_max and T_onset) and thermal (ΔH) characteristics of the outgoing processes. The decrease of liquids freezing points in the pores compared to that parameter for the bulk water (thermoporometry DSC) due to the confining effect is observed. Moreover, the degree of pores filled with the liquid medium depends on the nature of the surface of a tested material and is related to the polarity, porosity, and content of the catalyst. It was assumed that the liquid in the pores of R_min melts at T_onset, while the temperature T_max corresponds to the melting of liquid in R_av pores. The mathematical dependencies of the processes were thoroughly described in Brun et al. (1977), Charmas (2013, 2014), Gun’ko and Turov (2013), Kjeldsen and Geiker (2008) and Landry (2005).

Q-TG investigations

Thermodesorption of water from the surface of complex oxides was performed under the quasi-isothermal conditions. The water vapor was adsorbed on the complex oxides surface at 293 K. A portion (ca. 1 g) of adsorbent was placed in the hermetic vessel containing bidistilled liquid water for 72 h. Then the weighed portions (ca. 50 mg) of the adsorbents were placed in a spherical spiral platinum crucible of the Derivatograph C (Paulik, Paulik and Erdey, Hungary). The measurements of programmed water thermodesorption were made in the temperature range of 20–300℃.

Results and discussion

Figure 1 presents the exemplary N₂ adsorption–desorption isotherms (Figure 1(a)) and the curves of pore volume distribution (Figure 1(b)) for the TS oxides prepared on the basis of Si-40. The analysis of the isotherms shape (Figure 1(a), also for the other samples which are not shown here) indicates that the initial silica gels and titania–silica adsorbents are mesoporous materials.

Figure 1.

Low-temperature adsorption–desorption of N₂ (a) and pore volume distribution functions regarding their radius (b) for the initial silica gel Si-40 and silica-titania adsorbents prepared on Si-40. (b*) The PSD obtained based on the water DSC thermogram.

Deposition of TiO₂ layers causes changes in the specific surface area (S_BET) and insignificant decrease in the pores volume (V_p) (Figure 1(b)). This can also be seen by analyzing the structural parameters (S_BET, V_p) of the studied materials included in Table 1. The course of PSD curves indicates that the TiO₂ deposit causes pores volume decrease but does not result in significant changes of their sizes (Table 1), which is confirmed by the calculated contribution of nano-, meso-, and macropores in the studied materials (Table 1). As the contribution of macropores was negligible in all cases, these values were not included in the table. The mesopores (S_meso, V_meso, V_meso/V_p, Table 1) contribute largely to the porous structure. With the increasing TiO₂ content, this contribution diminishes insignificantly. At the same time, the contribution of nanopores (S_nano, V_nano, V_nano/V_p, Table 1) grows slightly. This may be due to the creation of a small amount of nanopores between the crystallites of deposited TiO₂.

Table 1.

The structural parameters of initial silica gels and titania–silica complex oxides.

Sample	SBET (m2/g)	Snano (m2/g)	Smeso (m2/g)	Vp (cm3/g)	Vnano (cm3/g)	Vnano/Vp (%)	Vmeso (cm3/g)	Vmeso/Vp (%)	Rp (nm)
Si-40	601.0	6.2	594.7	0.49	0.003	0.5	0.495	99.4	2.29
40-TS1	592.5	36.7	544.4	0.46	0.014	3.0	0.449	96.7	2.38
40-TS2	604.1	40.3	563.7	0.46	0.015	3.3	0.445	96.4	2.38
40-TS3	601.5	45.8	555.7	0.45	0.016	3.7	0.420	96.0	2.32
40-TS4	581.1	51.0	530.0	0.44	0.019	4.3	0.417	95.3	2.35
Si-60	399.3	15.9	383.3	0.72	0.005	0.7	0.709	98.9	4.40
60-TS1	417.8	30.8	387.0	0.69	0.011	1.5	0.685	98.0	4.24
60-TS2	408.5	30.0	378.5	0.69	0.011	1.6	0.674	98.0	4.32
60-TS3	383.4	33.0	350.3	0.64	0.012	1.8	0.624	97.8	4.28
60-TS4	377.5	28.1	349.4	0.62	0.010	1.6	0.612	97.9	4.30
Si-100	299.3	52.3	246.4	1.012	0.018	1.8	0.974	96.2	9.68
100-TS1	295.0	57.6	237.1	0.95	0.020	2.1	0.906	96.6	9.07
100-TS2	277.0	56.2	220.4	0.88	0.020	2.2	0.851	96.1	9.38
100-TS3	273.4	51.7	221.5	0.79	0.019	2.4	0.766	96.1	9.04
100-TS4	232.7	49.5	182.8	0.73	0.017	2.4	0.692	95.3	9.74

Porous structure changes for the study series of adsorbents were also observed from the DSC examination. The analysis of the data obtained from ice melting thermograms showed that for all series of adsorbents a significant decrease of characteristic temperatures T_onset was observed with the increasing TiO₂ content. It was seen that T_onset = −25.97℃ for the silica gel Si-40. This is due to the existence of narrow mesopores of the radius R_p = 2.3 nm in it. Deposition of successive layers of TiO₂ on Si-40 causes temperatures of peaks onset to be shifted down to −49℃ for 40-TS₁ and 40-TS₄ and about −40℃ for 40-TS₂ and 40-TS₃. This is connected with the formation of small amount of nanopores in the studied composites (Table 1).

In the case of the materials prepared based on Si-60 and Si-100, shift of characteristic temperatures is smaller than it was observed for the series Si-40. T_onset for Si-60 is −18.9℃ but for Si-100 it is −9.28℃ due to confinement of water in the pores of larger sizes than in the case of Si-40 (Table 1). Deposition of successive TiO₂ layers does not result in significant changes of complex oxides structure for these series (Si-60, Si-100).

The registered changes of melting process enthalpy (ΔH) for all considered cases are much lower than enthalpy of transition of pure ice melting (334 J/g) (Weast, 1981), which is associated with interactions of water with the pores surface and between the water molecules within the pores. The greatest deviation from the standard value was observed for the series of adsorbents prepared on the basis of Si-40 (from 53.5 to 81.6 J/g). The ΔH values recorded for the series obtained from the silica gel Si-60 are from 106 to 147 J/g but for the adsorbents prepared from Si-100 they are from 187.8 to 215.4 J/g.

The ice melting data (DSC) were used for PSD determination. Figure 1(b*) presents the exemplary PSD curves for the series obtained from Si-40. Good consistency of the analyzed curves with the distribution made based on the N₂ desorption data was observed.

The quasi-isothermal analysis of water desorption from the surface of studied materials enabled determination of the parameters describing water molecules interactions with the surface of the studied materials. The Q-TG studies of the layers of water adsorbed on the surface of initial silicas and mixed oxides showed that the maximal water adsorption proceeds on the silica gel Si-100 and the adsorbents obtained based on it. This refers to both strongly and weakly bound water. The reason for this is the largest volume of sorption pores of this material compared with that for the adsorbents prepared based on Si-40 (the smallest V_p, Table 1) and Si-60 (the average V_p, Table 1).

The analysis of TG_max (the total amount of the adsorbed water referring to the 1 g of the adsorbent) and C_H2O^max (the total concentration of the adsorbed water) values indicates that the successive layers of TiO₂ deposited on SiO₂ block the hydroxyl surface groups. Therefore, polarity of the obtained adsorbents surface and forces of interactions with H₂O molecules change. In all analyzed series, the maximal TG_max values for the initial silica gel and their gradual decrease for the adsorbents of the increasing number of TiO₂ monolayers are observed (Figure 2(a)).

Figure 2.

Dependence of TG_max on the amount of deposited TiO₂ monolayers for the initial silica gels and TS oxides (a) as well as exemplary dependences of the effective radius of the evaporating drop on the concentration of adsorbed water (b) and dG/dM under the conditions of slight surface filling with adsorbed water (c) for the chosen complex oxide TS-40₃ (dG/dM indicates the change of the free energy during water evaporation from the surface).

The shape of the curve of evaporating drop effective radius dependence on the adsorbed water concentration (Figure 2(b)) indicates that in the initial stage of water thermodesorption (large C_H2O) the effective radius of the evaporating drop is small but with the increasing temperature it grows to the value determined by the radius of the dominating pores R_dom. Further increase in the temperature results in the decrease of adsorbed water concentration and size of evaporating drop effective radius. In all cases, the obtained profiles assume the shape of the Gauss curve. On the curve dG/dM = f(n) (Skubiszewska-Zięba et al., 2012) (Figure 2(c)) there are observed single maxima for TS-40₃ and also the series of Si-40 and Si-60 prepared based on the silica gels, which points out to formation of water of similar structure on the cluster surface. For the adsorbents prepared based on silica gel Si-100, the curves course is more differentiated indicating less ordered structure of water layers.

Conclusions

The titania–silica adsorbents prepared on the basis of silica gels of differentiated porous structure (Si-40, Si-60, and Si-100) were proved to be mesoporous materials. The deposited TiO₂ monolayers cause changes in the specific surface area (S_BET) and pore volume (V_p). It was also shown that the deposition of TiO₂ does not cause important changes of the pore (R_p) sizes.

Reduction of freezing–melting temperatures of the water confined in the pores was also found. The size of these changes depends largely on the type of the initial silica gel and quantity of the deposited TiO₂ layers. Moreover, there was a significant change in the melting enthalpy (ΔH) of ice confined in the pores because of strong interactions of the liquid with the surface of the titanium–silica oxides. The investigations also showed the changes in the surface hydrophilicity.

Following the Q-TG analysis, water clusters of similar structure are formed on the surface of Si-40, Si-60 and composites were obtained on the basis of these gels. For the materials prepared from Si-100 the structure of water clusters is disordered. The total amount of adsorbed water and that poorly adsorbed on the surface is the largest for the silica gel Si-100 and the adsorbents prepared on it. The TiO₂ deposition can block the surface hydroxyl groups. Therefore, polarity of the surface of the obtained adsorbents and force of interactions with H₂O molecules change.

With the increasing amount of TiO₂ monolayers, the amount of strongly bound water (C_H2O^s) increases after deposition of the first monolayers, particularly for the series obtained from the silica gel Si-40, then it decreases with the increasing number of monolayers for each analyzed series of adsorbents. The dependences of total surface free energy at the adsorbent/water ( $\Delta G_{\Sigma }$ ) and surface free energy at the adsorbent/strongly bound water ( $\Delta G_{\Sigma }^S$ ) from the amount of the deposited TiO₂ monolayers are quite similar.