2.1. Preliminary Photocatalytic Studies and Characterization of the Commercial TiO2 Samples
In order to establish a correlation between the photocatalyst features and the photocatalytic activity, a preliminary screening was performed on the commercial samples by investigating the catalytic performance in the photo degradation of ethylbenzene (EB) and the results are shown in Figure 1
Very different EB conversions were observed for the three materials. In particular, P25 Degussa, which is usually regarded as the reference commercial photocatalyst, showed the lowest conversion (7%). On the contrary, Kronos VLP and Mirkat 211 reached 35% and 44% conversion, respectively. As already discussed, the activity of TiO2
is related to various factors. Among these, the crystalline phase and the surface area are determinant [12
]. Therefore, N2
physisorption, SEM and XRD analyses were performed on the commercial samples to investigate such features.
It is worth noting that the reaction here considered (EB photo-oxidation) occurs through a heterogeneous catalytic process that strongly depends on the surface area of the catalyst. The photocatalytic degradation of pollutants is based on two steps and the first one foresees the absorption of VOCs on the semiconductor surface (see Paragraph 2.2). As reported in Table 1
, the BET surface area values and the pore volume obtained for Kronos VLP and Mirkat 211 commercial materials are very similar (above 200 m2
/g), whilst P25 has a specific surface area of 52 m2
/g and a significantly lower pore volume.
However, the sample characterized by the highest surface area did not provide the greatest photoactivity, indicating that this parameter, though being extremely important, is not sufficient to provide satisfactory activity. First of all, according to the SEM images reported in sections a and b of Figure S1
, both Kronos VLP and Mirkat 211 samples are made of non-homogeneous particles with size of several tens of nanometers, whilst large agglomerates of particles that are characterized by jagged edges were observed in the case of P25 (section c of the same Figure). This morphology is consistent with the presence of a certain fraction of rutile crystal phase (see XRD patterns) that is generally characterized by granules larger than those belonging to anatase one [33
The presence of the anatase crystal phase in all the commercial catalysts was confirmed by XRD, as shown in Figure 2
. Moreover, weak peaks ascribable to the rutile phase were also observed in the pattern of P25 (red line), in agreement with SEM findings. The peaks related to Kronos VLP (blue) and Mirkat 211 (green line) commercial samples are not well resolved and this feature indicates the presence of very small titania nanocrystals (according to the estimated particle sizes reported in Table 1
The electronic properties of the commercial materials were investigated by DRUV-Vis spectroscopy and the measurements were carried out in air and at room temperature. The results are shown in Figure 3
, in which the spectrum of bare SBA-15 is also reported for comparison purposes.
The band-gap values of Kronos VLP (blue line) and Mirkat 211 (green line) were graphically extrapolated and it was found that close to the reference value reported for anatase (3.2 eV) [34
]: A band gap of 3.21 eV and 3.25 eV was obtained respectively (see insert Figure 3
). On the contrary, P25 showed a band gap of 3.15 eV, possibly due to the presence of rutile, whose band gap is lower than anatase (3.0 eV vs. 3.2 eV).
Basing on the overall findings, performing catalysts must fulfill (i) high surface area to improve EB adsorption and (ii) high crystallinity (anatase phase) to increase the photodegradation activity. However, the surface area of the investigated samples was not high enough also to provide suitable insulation properties [35
]. For this reason, silica–titania materials represent a promising option to further improve the surface area and, as a consequence, both EB adsorption and insulation properties.
2.2. High Surface Area TiO2/SBA-15 Materials
The photocatalytic activity of the composite systems was investigated, and the results are shown in Figure 4
. The combination of titania and silica positively influenced the photocatalysis, since all TiO2
/SBA-15 materials show conversion values higher than those related to the bare commercial ones. Such synergic effect is particularly evident in the case of Mirkat211 based sample, that reaches the highest conversion (70%) and for the P25/SBA-15 composite where EB conversion increased around for times.
The results indicate that the use of an ordered mesoporous silica as support effectively improved the catalytic activity, possibly due to an improved TiO2 dispersion, which might enhance the interaction among the catalyst and photons.
In order to establish structure-activity relationships for further sustainable green building applications, the morphological, textural, and structural features along with the electronic properties and the hydrophilicity of the materials were deeply investigated. As shown in Figure S2
, sections a, c and e, all the TiO2
/SBA-15 samples exhibit quite similar morphology consisting of a long fibrous macrostructure, extending tens of micrometers, made of short clusters of rods with relatively uniform size. Such kind of morphology is generally known to be exhibited by materials having long-range mesostructure, typical of the siliceous support [34
]. The presence of elongated crystals was observed (see sections b, d and f of the same Figure), confirming the strong influence of the silica matrix on the final morphology of the materials. Moreover, TiO2
nanoparticles can be clearly observed at higher magnification on the silica surface of each system (see Figure 5
In particular, TiO2 nanoparticles (highlighted by green dashed circles) appear as bright particles of rounded shape with jagged edges. These nanoparticles seem to be better dispersed in the case of Mirkat 211- and Kronos VLP-containing samples (sections a and b). On the contrary, given the same TiO2 loading, titania nanoparticle agglomerates with bigger size were observed in the material synthesized with P25 (section c of the same Figure), indicating that P25 favors the constitution of a less dispersed material.
The beneficial effect of TiO2
on SBA-15 external surface only was supported by surface area and porosity data from physisorption analyses. The results are summarized in Table 2
and are compared to those obtained for the SBA-15 alone.
All materials exhibit an irreversible type IV isotherm (Figure S3
, black curve) with a clear type-H2 hysteresis loop that is typical of materials with cylindrical mesopores. In addition, the isotherm displays a sharp inflection occurring at 0.7 < p/p0
< 0.9, corresponding to the capillary condensation in the mesopores, which strongly suggests the presence of pores with size of about 8 nm. The sharpness of the hysteresis indicates a uniform pore size and a narrow distribution. Nonetheless, the composite photocatalysts isotherms (pink, blue, and green curves) did not change significantly compared to the bare SBA-15. Indeed, according to the data reported in Table 2
, the values of surface area and porosity of the TiO2
/SBA-15 samples were comparable to those obtained for SBA-15 of silica, indicating that the textural properties of the ordered mesoporous silica were preserved after impregnation. In particular, the values obtained for BET surface area, the pore size and volume of Mirkat 211/SBA-15 are the most similar to those of bare SBA-15.
The above evidences confirm that titania nanoparticles are located at the outer surface of the siliceous matrix and overall do not fill any pore (this is consistent with the results obtained with SEM images). It is worth noting that if the TiO2
nanoparticles were located within the channels of silica, the resulting material would not have been active, since titania would not be available to be photoactivated and thus to catalyze the degradation of ethylbenzene. Titania nanoparticles size was evaluated by Rietveld refinement and the results are reported in Table 2
. The Mirkat 211/SBA-15 composite contains titania nanoparticles with smaller size than those of the two other samples, in agreement with the trends of the photocatalytic activity. Nevertheless, the differences observed for the textural and structural properties of the composite materials are too contained to justify the differences in the photocatalytic activity.
DRUV-Vis measurements were performed to check possible changes occurred on the electronic properties upon the insertion of titania on the SBA-15 support and the results are shown in Figure 6
The band-gap values of all composite materials were larger than those estimated for the commercial titania samples, as shown in Figure 6
, where the band-gap values of all samples are contrasted. The same observations were made in a recent paper [35
] in which the position of the absorption edge was evaluated for TiO2
catalysts supported on SBA-15 in comparison with commercial TiO2
P25. Indeed, in this case, the band gap values were determined from the extrapolation of the slopes of the modified K-M functions versus energy to zero absorption (indirect allowed transition method, n
= 2). However, the authors found that the TiO2/SBA-15 catalyst revealed an increase in the Eg (3.30 eV), which was ascribed to the smaller size of TiO2 particles in the prepared catalyst supported on SBA-15 [36
]. In particular, in this study the variation of band gap (ΔEg) had not the same extent depending on the different titania precursors, but the following trend was observed: P25 > Kronos VLP > Mirkat 211. A similar trend was observed as for the photocatalytic activity improvement upon the insertion in SBA-15 (P25/SBA-15: +3.6%; Kronos VLP/SBA-15: +1.2% ~Mirkat 211/SBA-15: + 1.6%). These features can be taken as an indication that (i) the electronic properties of the different titania samples underwent to a change upon insertion into the SBA-15 support, and (ii) such effect seems to be more contained in the case of the Mirkat 211/SBA-15 material. It can be proposed that small titania nanoparticles (under the XRD detection limit) are embedded in a “hard electron” material as insulating silica, where these nanoparticles can exploit more efficiently their photocatalytic activity. The interaction between the two materials likely occurs through OH groups, as investigated and discussed in detail in the following.
Moreover, the synergy between titania and silica can lead to an optimal ratio between the hydrophilic and hydrophobic features of the material here formulated. The hydrophilic–hydrophobic balance of a material affects vapor transmission (that is related with the water vapor migration and condensation in buildings) [38
]. Therefore, also this parameter must be taken in consideration during the formulation of an insulating material used with a structural function in a building or as paint or varnish additive. Finally, after having considered assessed the improvements of performances due to the increase of surface area and TiO2
crystallite sizes, surface hydrophilicity was considered. In order to remove water from the surface and to investigate the hydrophilicity of the materials, that is a crucial parameter for an insulating material, further FTIR experiments were performed on all the composite materials. The spectra were collected in air and outgassed at increasing temperature (80 °C, 100 °C, 120 °C, 150 °C, and 170 °C) up to 200 °C, starting from room temperature (spectra collected immediately and after 30′ outgassing) up to 200 °C, and keeping the temperature for 10′ at each step. The same experiments have also been carried out on the commercial samples as well as on bare SBA-15 for comparison.
Unexpectedly, the comparison among the series of FTIR spectra reported in Figure S4
and related to Mirkat 211 (red lines), SBA-15 (wine lines), and Mirkat 211/SBA-15 (orange lines) obtained upon outgassing the samples at increasing temperature reveals that the addition of titania to SBA-15 produced a modification in the spectrum profiles. Such modification can be ascribed to the change of the refraction index, as a consequence of the change of the size of the nanoparticles of the material, that is occurring at all the temperatures here considered, from r.t. (dashed bold lines, orange curves vs. red curves) up to 200 °C (bold lines, orange curves vs. red curves). The appearance of such feature may be possibly due to the effect of ultrasound irradiation to which the different commercial titania samples were submitted before impregnation of SBA-15. If compared to Kronos VLP/SBA-15 (less pronounced, Figure S5
) and P25/SBA-15 (almost nihil, Figure S6
), this phenomenon is mostly occurring in the case of Mirkat 211/SBA-15. This could be reasonably explained by the original small size and high surface area of the Mirkat material among the commercial titania samples and follows the order: Mirkat 211/SBA-15 > Kronos VLP/SBA-15 >> P25/SBA-15.
In Figure 7
the FTIR spectra collected on Mirkat 211 (green lines), Kronos VLP (black lines) and P25 (blue lines) upon outgassing from room temperature up to 200 °C are shown. Upon increasing the temperature, a gradual decrease in intensity of the broad absorption at about 3280 cm−1
as well as of the peak at 1630 cm−1
, due to the presence of water, is observed in all series of spectra collected on all the bare commercial titania samples.
At room temperature, the hydrophilicity of the samples follows the order: Kronos VLP > Mirkat 211 >>P25, whereas at high temperature (bold lines) the Mirkat 211 sample is able to retain more water molecules than the Kronos VLP sample. It also exposes the highest amount of free OH groups, according to the intensity of the peak at 3672 cm−1 (with a shoulder at 3731 cm−1).
The same experiments performed on the composite samples (Figure 8
) revealed that if compared to the bare commercial titania samples (Figures S4–S6
) the hydrophilicity of these materials resulted enhanced by the presence of the SBA-15 matrix. Moreover, the free OH groups (peak at 3742 cm−1
) observed are those related to the bare silica (see the comparison with the dashed red lines in sections a-c of Figure 7
). However, upon outgassing at increasing temperature, the Mirkat 211/SBA-15 sample has the highest amount of free OH groups, but the lowest amount of OH groups in interaction with water molecules (broad absorption in the 3650–3000 cm−1
These features indicate the occurrence of an interaction between the commercial titania samples and SBA-15, resulting in a different hydrophilicity of the composite materials. Such interaction seems more evident in the case of Mirkat 211/SBA-15, the material that benefited the most from deposition of titania on SBA-15 in ethylbenzene oxidation tests.
Finally, beside the improvement of the photocatalytic activity, the attainment of high surface area materials would be also advantageous in view of enhanced thermal insulation. Following this approach, the comparison among the FTIR transmittance spectra of the P25/SBA-15 (pink line), Kronos VLP/SBA-15 (blue line) and Mirkat 211/SBA-15 materials is shown in Figure S7
. The spectra were normalized to the density of the pellets, therefore the differences observed as for the overall intensity of the spectra can be taken as a measure of the scattering capability of each material. The scattering is closely related to the size of the particles constituting the pellet: The larger the particles, the higher the scattering (given the same applied pressure to make the different pellets). These results are in agreement with the XRD and BET findings.