Photocatalytic materials based on silica-titania (SiO2-TiO2) were synthesized by sol-gel and dip-coating method. TEOS and titanium butoxide were used as precursors of the silica-titania, respectively. A thin film with anatase phase was obtained on the surface of the support. The effect of variables as dispersion mechanism, immersion time, and number of treatment cycles were studied. The materials were characterized using X-ray diffraction, scanning electron microscopy, energy dispersion scanning, and N2 adsorption-desorption. The highest crystallinity of TiO2 on silica, high specific surface area in TiO2-SiO2 materials, and thin film formation were obtained by using a stirring plate and minimum immersion time. The so synthesized catalyst allowed the production of formaldehyde from the photocatalyzed methanol oxidation in a packed-bed reactor.
Among the advanced oxidation processes (AOPs), heterogeneous photocatalysis has been widely applied in the degradation of organic compounds, hydrogen production from water, reduction of heavy metals, and selective oxidation reactions [
The most studied semiconductor in the field of heterogeneous photocatalysis is titanium dioxide (TiO2) due to its high oxidative capacity, nontoxicity, low cost, high chemical and physical stability, corrosion resistance, and chemical inertness [
There are several methods for the preparation of TiO2 thin films, and the physicochemical properties strongly depend on the selected method [
This work aims to elucidate the effect on morphological, textural, and structural properties of three important variables involved in the synthesis of TiO2 coated SiO2 monoliths. These variables are the mechanism to disperse Ti alkoxide species in an appropriate medium allowing the hydrolysis-condensation processes, dipping time, and number of coating cycles (dip + drying + calcination). In order to evaluate the photocatalytic activity of the synthesized catalysts, the photooxidation of methanol was carried out in a bench-scale continuous-flow packed-bed reactor. This reaction was elected because of its industrial importance and because it is a consecutive reaction, whose selectivity towards intermediate compounds may help to prove the application of TiO2 in selective oxidation processes instead of the organic compounds mineralization. This process could offer the advantage being performed at mild temperature and pressure conditions unlike other existing processes that occur at relatively high temperatures.
Silica dioxide monoliths were synthesized by sol-gel method. The synthesis of the silica support was performed using tetraethyl orthosilicate (TEOS) [Si(OC2H5)4] as alkoxide precursor of the Si sol.
First, the ethanol was added into a beaker and it was maintained under continuous stirring until the temperature reached 60°C. At this point, alkoxide was added and mixed into the beaker for 15 min. After this time, a water and nitric acid solution (1 : 0.0012 molar ratio) was added and the mixture was kept under stirring and keeping the temperature constant for one hour. The molar ratio of water : ethanol : TEOS was of 16 : 4 : 1, respectively.
After that, 2.5 ml of the resulting sol were poured into one container to begin the aging process. This was repeated several times to obtain various monoliths. The container lids were previously drilled to allow solvent diffusion. During this step, alkoxide groups are removed by acid- or base-catalyzed hydrolysis reactions, and link networks O-Si-O are formed in subsequent condensation reactions involving hydroxyl groups [
The obtained monoliths were then dried from room temperature to 100°C during 14 hours with a slow heating profile to eliminate the solvent. The drying was performed in an Isotemp Vacuum Oven programmable stove model 282 A. The drying treatment was slow to lead the formation of open pores. The drying profile was as follows: 1 h at 40°C, 2.5 h at 50°C, 13 h at 60°C, 2.5 h at 70°C, 3.5 h at 80°C, 2.5 h at 90°C, and 27 h at 100°C. This procedure was performed in order to keep the structure, since a fast drying profile could cause a structure collapse causing cracking of the monolith.
Finally, to provide the monoliths with the appropriate structural and mechanical properties, they were calcined from room temperature (25°C) to 550°C for 6 h at a heating rate of 2.5°C/min using a Jelrus muffle with 2 steps. Amorphous silica compounds without a defined crystalline phase are found at this temperature.
Ethanol, water, titanium butoxide, and diethanolamine (basic catalyst) were used to obtain Ti sols via sol-gel method. Titanium butoxide (Ti[O(CH2)3CH3]4) was dispersed in the ethanol. Immediately, a diethanolamine and water solution was dropped into the volume. It is necessary to maintain a 1 : 1 alkoxide : water molar ratio. Once the solution addition was completed, the agitation was maintained for two hours. After this time, the solution was aged for further two hours without any stirring. Diethanolamine was elected because of its low reactivity during sol-gel process, this makes the hydrolysis reactions slow by favoring the thin film formation [
Finally, SiO2 monoliths were immersed into the Ti sol obtained. In this process, the studied variables were the dispersion mechanism (mechanic or ultrasound) and the residence time into the Ti sol. For the former, a stirring plate and an ultrasonic cleaner were utilized as agitation media. Regarding residence time in the Ti sol, this variable was studied at three levels (half, one, and three hours) for each stirring medium. The number of cycles (immersion-drying-calcination) was also studied in order to establish its relationship with the amount of titania on SiO2.
The monoliths coated with titanium species were dried at room temperature for 24 h and calcined under an air flow at 550°C for 5 hours.
A Bruker Advance 8 diffractometer was employed to carry out the X-ray diffraction analysis and determine the presence of anatase in the synthesized catalysts. The patterns were obtained using CuK
A JEOL JSM-6510LV electron microscope coupled with an energy dispersive X-ray spectrometer was employed to observe the surface morphology of the prepared catalysts and to perform elemental analysis of the catalysts.
Autosorb-1 Quantachrome sorption equipment was employed to determine the specific surface area and average pore diameter of the synthesized samples by using liquid nitrogen (77 K). The pore size distributions and the specific surface areas of the materials were estimated by Dubinin-Astakhov (DA) and Brunauer Emmett–Teller (BET) methodologies, respectively.
The photooxidation of methanol was performed using a bench-scale continuous packed-bed reactor. An eight-watt UV lamp emitting 254 nm waves was placed right in the center of the reactor. 30 monoliths constituted the catalytic bed. Compressed air was used as carrier gas to help methanol to flow through the packed-bed reactor. The air flow was constant at 50 ml/min. The methanol liquid was heated at 65°C in order to vaporize it. The reactor set-up is depicted in Figure
Bench-scale photocatalytic reactor set-up.
Identification of formaldehyde by photooxidation of methanol was carried out according to the following methodology: a proper container with a 2,4-dinitrophenylhydrazine (2,4-dnph) solution was placed instead of the condenser (see Figure
2,4-Dinitrophenylhydrazine general reaction with aldehyde functional groups.
The 2,4-dnph solution was prepared as follows: 2 ml of concentrated sulfuric acid was mixed under stirring with 0.4 gr of 2,4-dnph and 3 ml of water until total dissolution appears. At this point, 10 ml of ethanol at 95% are added to the solution.
The quantitative analysis of formaldehyde after photooxidation of methanol was verified by collecting the condensed reaction product in a container at 4°C and analyzed in a Varian GC 3800 using a 52 CP WAX column (30 m × 0.320 mm).
SiO2 monoliths with a diameter of 15 mm and thickness of 1 mm approximately were obtained following the methodology described in the previous section. Figure
SiO2 monoliths obtained by sol-gel technique.
In total, 8 samples were characterized in order to decide at what conditions the monoliths for the methanol photooxidation should be synthesized. From the 8 monoliths, 7 were coated with TiO2 while the left one was only SiO2. The nomenclature used to name the samples is explained in Table
Nomenclature of synthesized materials (B by ultrasound bath and P by stirring plate).
Sample name | Number of immersion-drying-calcination cycles | Immersion time (hours) |
---|---|---|
3ST12B | 3 | 1/2 |
3ST12P | 3 | 1/2 |
3ST1B | 3 | 1 |
3ST1P | 3 | 1 |
3ST3B | 3 | 3 |
3ST3P | 3 | 3 |
4ST1P | 4 | 1 |
Figure
XRD patterns of TiO2/SiO2 and pure SiO2 samples.
The average crystallite size of samples was estimated using the Scherrer’s equation through the full width at half maximum of the anatase (101) peak (see Table
Average crystallite size of the synthesized materials in Figure
Sample | Average crystallite size (nm) |
---|---|
3ST12P | 19.4 |
4ST1P | 17.0 |
3ST3P | 16.3 |
3ST3B | 13.5 |
Regarding samples 3ST1B and 3ST12B, these were discarded because the ultrasound influenced the structure to an extent that the SiO2 monolith was broken. This may be ascribed to the vibrational movements caused by ultrasonic, causing the structure to become weaker. This is a consequence of the immersion under ultrasound presence during TiO2/SiO2 monoliths preparation as well as the decrease in crystallinity.
Figure
SEM image of TiO2 film morphology obtained after 3 treatment cycles using a stirring plate and half an hour of immersion.
SEM image after 3 treatment cycles using stirring plate and 1 hour of immersion.
Figure
SEM image after 3 treatment cycles using stirring plate and 3 hours of immersion.
By comparing Figures
Figure
SEM image of TiO2 film after 4 treatment cycles and 1 hour of immersion under mechanical stirring.
The final percentage in weight gained of TiO2 by the SiO2 monoliths is shown in Table
Final percentage in weight gained of TiO2 on the SiO2 monoliths.
Sample | Percentage in weight gained of TiO2 (%) |
---|---|
3ST12P | 14.3 |
3ST1P | 18.5 |
4ST1P | 17.4 |
Table
Specific surface area and average pore size of the synthesized materials.
Sample | Specific surface area |
Average pore size (Å) |
---|---|---|
SiO2 | 339 | 18 |
3ST12P | 241 | 18 |
3ST1P | 210 | 18 |
3ST3P | 170 | 18 |
4ST1P | 240 | 18 |
It can be said that the synthesis conditions in which better crystallinity of the anatase phase is obtained; the highest specific surface area as well as a better uniformity of the formed film are with half an hour immersion, 3 cycles and stirring plate as dispersion mechanism. Therefore, these conditions were used to synthesize 30 monoliths to pack the bed reactor in order to perform the photocatalytic oxidation of methanol.
The minimum air flow rate to carry the methanol gas through the reactor was established as 50 ml/min. Two sets of experiments by triplicate were performed; one set without catalyst and the other one with the catalyst. In the former case, no formaldehyde was detected by the employed analysis method (2,4-dnph), and therefore the production of formaldehyde by photolysis was discarded. In the latter set of experiments, formaldehyde was identified. The analytical technique for the qualitative analysis of formaldehyde with a 2,4-dnph solution was carried out. The formation of micelles as precipitates is due to the formation of 2,4-dinitrophenylhydrazone indicating the presence of formaldehyde during the photocatalytic reaction. A precipitate indicating the presence of the aldehyde was only observed with the material 3ST12P. This does not mean that the other materials did not have photoactivity but that this could be so high that total methanol oxidation rather than selective oxidation was attained. To determine the amount of formaldehyde formed during photooxidation of methanol, the first condensed reaction product was evaluated by gas chromatography (GC). The final concentration of produced formaldehyde corresponds to a value of 457 micromol/L (13.7 mg/L). This result is superior to those obtained with other titania-silica systems [
SiO2 monoliths coated with thin films of TiO2 anatase phase were successfully prepared by using a dip-coating sol-gel method. The dispersion mechanism, the immersion time, and the number of dip-coating cycles of the SiO2 monoliths into the Ti sol were found to affect both the morphology and crystallinity of the TiO2 deposit. SiO2 monoliths coated with crackle-free TiO2 films were obtained after three dip-coating cycles, with a dip time of 30 minutes. It can also be concluded that mechanical stirring should be preferred over ultrasound dispersion since the former favors the structural stability of the monolith and increases the film crystallinity, while the ultrasound dispersion method leads to monolithic structure breakage and also increases film crackles. Immersion time diminished both TiO2 film and homogeneity. Immersion time and number of cycles also affect the surface area and deposit crystallinity. The surface area of the SiO2-TiO2 materials was decreased when the immersion time increased, which is related to the amount of TiO2 on the SiO2 surface. The highest anatase phase crystallinity and specific surface area were obtained after 3 dip-coating cycles and half an hour of immersion under mechanical stirring. Under these preparation conditions, the attained surface area was 241 m2/g and the crystallite size was 19.4 nm. The weight percentage gained by SiO2 monoliths was 14.3% (TiO2 film).
A formaldehyde concentration of 13.7 mg/L was attained at mild conditions of pressure and temperature in a continuous flow reactor packed with SiO2 monoliths coated with TiO2 anatase films prepared with 3 dip-coating cycles and 0.5 hours of immersion time.
The authors declare that there is no conflict of interests.
The authors are grateful to UAEMex for the financial support through project 4373/2017/CI and to CONACYT (project 269093). R. Regalado would like to thank CONACYT for the financial support and to CCIQS from UAEM for the granted support.