Preparation of Porous SiO 2 -TiO 2 -NiO Synthesized by the Sol-Gel Process and its Characterization

An experimental strategy was developed to obtain Si-Ti-Ni transparent sols via the sol-gel process. The sol was prepared from Si(OEt) 4 , Ti(OBu) 4 and Nickel(II) acetylacetonate. The chelating agent (2,4 pentanedione, acacH) was employed to stabilize Ti precursor in order to control their chemical reactivity, avoiding precipitation. A prehydrolyzed tetraethyl orthosilicate (TEOS) sol was the Si source. The sol was characterized by UV-Vis and Fourier Transform Infrared Spectroscopy (FTIR) spectroscopies. The solids treated were characterized by FTIR. Assignments of the simultaneous formation of the Si-O-Ti and Si-O-Ni bonds were done. The sol was polymerized at room temperature (293 K) to obtain gels and these were dried and treated at 673, 773 and 873 K in air. The characterization techniques were, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), 29 Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The surface area of the solids was determined by N 2 adsorption/desorption isotherms. The corresponding average pore diameter was evaluated using the methods BJH, DA and HK. These models were used because all together cover the full range of the pore size. The purpose of producing porous polymeric system of SiO 2 -TiO 2 -NiO is to combine the properties of each of the oxides in a single material for use in catalytic reactions.


Res Dev Material Sci
Copyright © Mendoza-Serna R RDMS.000810. 13 (2).2020 materials for bone implants because of their excellent corrosion, wear resistance, biocompatibility, mechanical properties, and high strength to weight ratio [10], one SiO 2 film coating on the surface of TiNi can reduce the release of Ni ion to the human body [11]. Ni-based catalysts, with or without metal promoters or modifiers, are extensively studied with the objective or replacing much more expensive noble metals. The catalytic properties of the Ni/TiO 2 -SiO 2 system have been reported in the literature for CO 2 reformation from methane to synthesis gas [12]. Therefore, we consider that the synthesis and characterization of a porous and homogeneous SiO 2 -TiO 2 -NiO material has multiple potential applications.

Characterization techniques
A Perkin Elmer λ10 spectrophotometer was used to obtain the UV-Vis spectra in the 200-450nm region. Quartz cells were used and EtOH was the solvent.
The FTIR spectra of the sols were obtained using a Varian 640 IR spectrophotometer in the 4000-500cm -1 region. The FTIR study of xerogels and oxides calcined was performed in the 2000-500cm -1 region.
The X-ray diffractograms were obtained using D8 Bruker Advance equipment applying CuKα radiation (λ=1.5406Å) ranging from 5° to 70° to determine the possible crystalline structure of the treated samples.
The morphologies of the samples were observed by using a scanning electron microscope (JEOL, JSM 6510) operating at 20kV.
The analysis of the samples in regard to the temperature that the material supports was through the application of Thermogravimetry with a team brand Perkin Elmer, model TGA400, with software Pyris; calibrating the equipment with the metals Alumel, Perkalloy and Fierro and initiating the heating at 303K up to a temperature of 1173K and with a heating ramp of 10K/min, in a nitrogen atmosphere. Calorimetry Scanning Difference (DSC) was applied, with a Mettler Toledo equipment, Model DSC1, with STAR software version 14.0; calibrating the equipment with three methods: for temperature adjustment, heat flow and total calibration; and initiating the heating at 123K up to a temperature of 773K and with a heating ramp of 10K/min, in a nitrogen atmosphere.
The N 2 adsorption/desorption isotherms of the xerogels treated at 673, 773 and 873K, were measured at 77K in a Belsorp-mini II equipment with software version 2.4.0. The samples were degassed with Helium at 573K for 12 hrs. The specific area of solids were calculated applying the BET method in the region of the relative pressure of 0.05 <P/Po<0.5.
The acquisition of the 29 Si spectrum was carried out in a Jeol 600MHz Nuclear Magnetic Resonance spectrophotometer model ECZ600R, using the pulse sequence Single pulse. For this, about 60mg of the sample was packed in a 3.2mm diameter rotor. The spectrum of 29 Si was acquired at 298K, with a relaxation time of 5 seconds, a pulse of 90° (10μs), 1024 scans and rotating at 15KHz.

Results and Discussion
The UV-Vis of the Si-Ti-Ni sol is shown in Figure 1. The sunlight´s radiation of below 280nm is almost completely absorbed by the atmosphere. Therefore, the UV spectrum suggests that the sample can exhibit photocatalytic activity under atmospheric conditions [15]. The FTIR bands Assignment for Si-Ti-Ni sol ( Figure 2) is given in Table 1. The CH 3 stretching of Si-O-CH 3 was found in the fresh sol at 1273cm -1 and disappeared after thermal treatment at 673K (not shown here), as we expected due to the combustion of the organic groups [16]. The band at 910-960cm -1 is due to the overlapping of vibrations of Si-OH, Si-O-Ti bonds [15] (932cm -1 ) [17] and Si-O-Ni (960cm -1 ) [18], this region widens as a result of heat treatment. The bands in the wave number range up 556cm -1 are attributed to the deformation vibration of Si-O groups containing SiO 4 tetrahedra [19].  O-H stretching [27] C-H stretching [37] CH 3 bending [36] C-O-M stretching [39] O-H stretching in Si-OH [38] Si The Si-Ti-Ni powders analyzed by X-ray diffraction were found to be amorphous even at the temperature of 873K, Figure 3. The intense peak at 2Ɵ=22° indicates that the SiO 2 peak has amorphous nature [20]. No other impurity peaks were present in the Si-Ti-Ni powder. In X-ray diffraction patterns, the characteristic peaks of anatase 2Ɵ=25.6° (101), 38.17° (004), 48.3° (200) and 54.36° (105) is consistent with the values on the JCPDS card (No: 04-0477) [21] or rutile 2Ɵ=27.6° (110) [22], should not be observed for pure TiO 2 the transformation from anatase to rutile phase takes place at 873K. The three diffraction peaks are not observed at 2Ɵ=37.2° (111), 43° (200) and 63° (220) [23], which are identified as reflections of a cubic structure of NiO, respectively [24,25]. Additional peaks from NiO were not observed which signifies that NiO was incorporated into the structure of the SiO 2 -TiO 2 [26].
The scanning electron microscopy images of Si-Ti-Ni treated at 673, 773 and 873K showed a trimodal size distribution, consisting of big, medium and small faceted particles. The SEM of 773 and 873K are presented in Figure 4a & 4b, respectively. The smaller particles of less 1μm, the medium 5μm and the larger ones are 20 and more μm. The morphology is well defined, showing different shapes, smooth and flat surfaces, faces and edges. The samples maintain the shape and sizes at 773 and 873K.  The materials synthesized were dried and later analyzed by thermal gravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). The TGA curve is shown in Figure 5. For a better description of the thermogravimetric study, it is divided into three temperature ranges. 1. In the range of 303 to 453K, a greater weight loss is observed by evaporation of water and alcohols (ethanol and butanol) from silicon and titanium precursors (16%) [27]. Process that is justified by the endothermic peak at 365K present in the DSC (not shown here).
2. The second weight loss observed in the range of 453 to 748K associated to the combustion of alcohols (ethanol and butanol) as well as to groups ethoxide and peroxides of the condensation (7.5%) [28]. The DSC graph of this temperature range only shows one exothermic peak at 698K, which strengthens the aforementioned discussion.
3. Finally, in the temperature range 698 to 773K, a small weight change is observed, corresponding to the continuous dehydroxylation at high temperature of the formed material.
The isotherms for the samples at 673, 773 and 873K are shown in Figure 6. The first two samples exhibit a type I isotherm [29,30], indicating that micro porous material is present along with mesoporous one. The adsorbed volume gradually decrease between 673 and 773K, this temperature range correspond to the elimination of organic residue according the DSC graph. Sintering occurred during this calcining temperature range (773-873K). The sample treated at 873K exhibits characteristics of type IV isotherm that correspond to mesoporous materials [31]. The pore size distribution in the micro porous zone was evaluated using the method proposed by Dubinin-Astakhov (DA) [32], the pore size for the solids treated at 773 and 873K are shown in Figure 7. Horváth-Kawazo [33], the pore size distribution for the solid treated at 773K is shown in Figure 8. And for the solid treated at 873K in Figure  9. The pore size distribution in the mesopore zone was evaluated using the method proposed by Barret, Joiner and Halenda (BJH) [34]. The textural properties for the solids treated at 773 and 873K are shown in Table 2. There is a decrease in adsorbed volume and number of pores as a consequence of the heat treatment, even when at 873K the sintered material still has micropores. In the literature there are reported the calcined NiO at 873K embedded in silica matrix shows a slightly higher BET surface area, 450m 2 /g, compared to that of the silica matrix itself, 400m 2 /g, suggesting that the NiO contributed somehow to increase the adsorption capacity [3]. Others authors reported that the surface area and average pore size of their NiO/TiO 2 /SiO 2 system treated at 773K were 265.3m 2 /g and 12.6nm, respectively [6].    The results for the 29 Si MAS NMR spectra of the Si-Ti-Ni solid sample calcined at 873K is observed in Figure 10. In order to compare the relative amounts of Q 1 , Q 2 , Q 3 and Q 4 species, we considered the chemical range -80 to -89.9ppm for Q1, -90 to -99ppm for Q 2 , -99.1 to -108.9ppm for Q 3 and -109 to -120 for Q 4 [35]. The deconvolution with accuracy of 1% was 3.2 % Q 1 , 13.0 % Q 2 , 49.9 % Q 3 and 33.8 % of Q 4 .

Conclusion
The use of prehydrolysis of TEOS and the chelating agent acacH used to stabilize Ti ensure the homogeneity of the sols, gels, xerogels and oxides of SiO 2 -TiO 2 -NiO obtained up to 873K.
The sample can exhibit photocatalytic activity under atmospheric conditions according to the UV-Vis results. The homogeneity of the material was demonstrated by FTIR with the presence of the characteristic bands of the Si-O-Ni and Si-O-Ti bonds and their stability with respect to thermal treatment, these results indicate that a homogeneous solid structure was produced. The powders analyzed by X-ray diffraction were found to be amorphous even after thermal treatment at 873K. The morphology analyzed by SEM shown that is well defined; the samples maintain the shape and sizes at 773 and 873K. A porous material was obtained that combine micro and mesoporous still at 873K. 29 Si MAS NMR indicate that the sample contain 50 % of specie Q 3 .