High loading TiO2 and ZrO2 nanocrystals ensembles inside the mesopores of SBA-15: preparation, texture and stability

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Abstract

TiO2 (30–80 wt.%) and ZrO2 (48–75 wt.%) were inserted inside the pores of SBA-15 mesostructured silica host by chemical solution decomposition (CSD) or internal hydrolysis (IH) of the corresponding alkoxides. Both methods yielded composites with 85–94% TiO2 crystallinity (anatase). In case of ZrO2, CSD yielded >95% crystallinity (tetragonal phase), while IH gave an amorphous ZrOx-phase that does not crystallize up to 1073 K. The guest Ti(Zr)-oxide phases did not block the SBA-15 pores, and their surface was fully accessible for nitrogen adsorption. Calcination in air of TiO2/SBA-15 and ZrO2/SBA-15 (CSD) composites up to 1073 K did not change the nanocrystals structure and slightly increased the domain size derived from XRD data from 5.0–8.5 to 6–10 nm for TiO2 (IH and CSD) and from 4.5 to 6.5 nm for ZrO2 (CSD). After the same treatment the crystals domain size of bulk reference TiO2 increased to >100 nm with full conversion to rutile polymorph and of reference bulk ZrO2––to 20–25 nm with partial conversion to monoclinic modification. Thorough characterization of the texture, structure, location and dispersion by HRTEM, SAXS, EDS, SEM, XRD, N2-adsorption methods allowed evaluation of the assembling mode of TiO2 and ZrO2 inside SBA-15 nanotubes: amorphous layer, ensemble of small 4–5 nm crystals (TiO2-IH and ZrO2-CSD) or single large 8.5 nm crystals (TiO2-CSD).

Introduction

Ordered mesostructured silicas (OMS) with adjustable pore size and close to maximal possible specific surface area [1], [2] provide wide opportunities for synthesis of advanced materials with different functionalities serving as catalysts, chemical sensors, materials with wholesome optical and magnetic properties [3], [4], [5]. Various guest phases (GP) such as metals, pure and mixed metal oxides, sulfides, heteropoly-compounds and others could serve as “functionalization source” being assembled in the pore system of OMS as nanoparticles with distinct structure. The unique pore geometry and high surface area of OMS hosts allows substantially higher loading of GP relative to the traditional porous supports like silica-gel or activated alumina––up to 70 wt.% even filling only 50% of the pore volume (Table 1). The properties of embedded GP/OMS composite materials are determined, besides GP loading, by location of GP nanoparticles ensemble inside the hosts pore system, the particles assembling mode with minimal pores blocking and by the crystallinity of target GP. Yet the simultaneous control of all these parameters is a very complicated practical problem. Thus only few examples of successful preparations of GP/OMS materials were reported that substantially excel in performance the corresponding GP embedded in traditional matrixes or the bulk GP materials. Among them, preparation of catalytic materials by insertion the Fe2O3 in MCM-41 [6], WS2 [7], SO4–ZrO2 [8], Pt–TiO2 [9] and phosphorotungstic heteropolyacid [10] in SBA-15 matrixes were reported. Silicon clusters [11] and CdS nanoparticles [12] in oriented OMS films demonstrated unique photoluminescence performance. Super magnetic behavior of iron nanoparticles inside the OMS matrix was detected in [13].

The crystalline titania is widely used as a basis for oxidation [14], selective catalytic reduction [15] and photo catalysts [16]. Stabilized TiO2 nanocrystals are used as efficient gas sensors [17] and high performance photovoltaics [18]. Tetragonal zirconia modified with sulfate anions forms a highly acidic catalytic phase with an excellent performance in a variety of acid-catalyzed reactions [19], [20]. Zirconia-based air-fuel ratio sensors are widely used for automotive applications [21]. For the most of these purposes, especially in catalytic applications, the performance of crystalline phases exceed by far those of amorphous phases [14], [15], [19], [20]. Furthermore, application of nanocrystals with high surface area yields better performance. However, due to high sintering ability it is very difficult to prepare TiO2 and ZrO2 phases as thermally stable nanocrystals with high surface area [22]. Therefore stabilizing them as GP inside the OMS matrixes received much attention in the last years. It is reflected by tens of publications related to TiO2/OMS [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35] and ZrO2/OMS [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52] composites.

Ti-oxide phase was inserted into MCM-41 [23], [24], MCM-48 [34], SBA-15 [23], [25], [26], [27], [28], [29], [30] and metal (V, Cr, Fe)-substituted MCM-41 [33]. Commonly Ti-alkoxides were used as Ti-phase precursor and they were inserted directly together with the silica source during OMS synthesis [28] or by postsynthetic modification of OMS via grafting [29], [32], [34], [35] or impregnation followed by solvent evaporation [24], [27], [30], [31], [33] or Ti-precursor hydrolysis [23]. However, all the TiO2/OMS samples prepared by postsynthetic grafting or by direct insertion during OMS synthesis did not contain crystalline TiO2 due to isomorphous substitution of surface silicon atoms by titanium leading to location of Ti in the silica framework inside the OMS pore walls, or formation of isolated grafted Ti-species [25], [26], [28], [29], [32], [34], [35]. For the similar reasons, impregnation did not lead to formation of crystalline TiO2 phase at low loadings of Ti-phase (<10 wt.% of TiO2) [24], [27], [30]. Thus, crystalline TiO2 was obtained in TiO2 /OMS composite materials only at high loadings >20 wt.% [23], [24], [30], [31], [33]. The information about the size, dispersion, location and assembling mode of the TiO2 nanocrystals inside OMS in these works is limited and sometimes contradictive. TiO2-anatase film formation was clamed in [30] based on the results of spectroscopic methods and shift of pore diameter to the lower values. Though no crystal domain size (XRD) or particles size (TEM) was provided in [30], the sharp peaks in XRD patterns of high loading 21–57 wt.% TiO2/OMS materials seems to contradict with the TiO2-film formation hypothesis, while shift of pore diameter to the lower values may occur also after insertion of nanocrystals. Significantly different TiO2 crystal sizes were obtained in [31] from XRD (18–20 nm) and TEM (5 nm) measurements of TiO2/OMS composites with Si/Ti = 2.5–10 suggesting that at least part of TiO2 crystals with diameter exceeding the OMS pore size were located outside the OMS pores. The crystal domain size of 5–10 nm measured in [23], [24] for 20–60 wt.% TiO2/SBA-15 (or MCM-41) composites by XRD correlated well with TEM data. However, no data were provided on the content of crystalline TiO2 phase relative to the total Ti loading. This information is very important since the significant part of Ti-phase may substitute silicon atoms in the silica framework as was mentioned above. Small shift of pore size distribution to lower pore diameters obtained for 60 wt.% TiO2/SBA-15 material in [23] suggests that significant part of Ti may substitute the silicon in OMS walls. In addition, the possible pore blocking effect was not clarified both in [23], [24].

The Zr-oxide phase was inserted into OMS commonly as sulfated zirconia [39], [41], [42], [43], [44], [45], [46], [50], [51] or gallium [47], [48] and alumina [49] promoted sulfated zirconia. Insertion of pure ZrO2 was reported as well [36], [37], [38], [40], [52]. Addition of Zr-source together with the silica source during the MCM-41 synthesis did not yield crystalline Zr-oxide phase due to location of Zr atoms in the silica framework inside the pore walls [36], [37]. XRD-amorphous zirconia/OMS composites were obtained at low loadings of 10–26 wt.% ZrO2 by postsynthetic modification of OMS [40], [44], [45], [51], [52]. Some of amorphous Zr-oxide phase exists in such composites as small (2–3 nm) zirconia particles which were visualized by bright-field electron tomography [38]. Increasing the ZrO2 loading beyond 10 wt.% [44] or 22–26 wt.% [51], [52] resulted in formation of crystalline ZrO2 phase detected by XRD, while amorphous Zr-oxide phase was obtained via grafting of Zr(OPr)4 on the OMS pore walls at 50 wt.% ZrO2 loading [46]. No information about the location, dispersion (size) and stability of crystalline ZrO2 phase was provided in [44], [52]. Similarly, no additional information about the ZrO2-phase besides the reflections in XRD patterns was provided in [39], [41], [47]. The catalytic performance was reported in some papers [43], [50] without any data about the state of ZrO2 phase. Part of the Zr-oxide crystals with the average size of 10 nm was detected by TEM outside the MCM-41 pores [48]. Similarly, relatively sharp peaks of crystalline ZrO2 phase after thermal treatment at 600–680 °C of a high loading ZrO2/MCM-41 material together with its pore diameter of 2.2–2.5 nm suggests that a significant part of Zr-phase was located outside the MCM-41 nanotubular mesopores [42], [49], [51]. The pore blocking effect was minimal in [42], [48], [49], [51], but it could be a result of the location of significant part of ZrO2 GP outside the OMS pores.

As could be seen from this short review, the location of the TiO2 and ZrO2 nanocrystals as well as the pore blocking effect, especially at close to maximal Ti(Zr)O2 loadings, are almost always not clarified. In addition, nanocrystals ensembles may form additional micropores that should be a function of crystal size and assembling mode––the effect that was never mentioned. There is no information on the thermal stability of the TiO2 nanocrystals inside the OMS nanotubular mesopores.

These information gaps together with the superior catalytic performance of SO4–ZrO2/SBA-15 [8] and Pt–TiO2/SBA-15 [9] materials, prepared by chemical solution decomposition (CSD) and internal hydrolysis (IH) methods, represented a strong motivation to conduct a systematic and comprehensive study of these interesting composites prepared by novel methods as materials in general. The present paper contains the results of investigation of the texture, structure, location, dispersion and thermal stability of TiO2 and ZrO2 phases, prepared by IH and CSD at a wide range of GP loadings including that close to complete filling of mesopores, by HRTEM, EDS, SAXS, SEM, XRD and N2-adsorption methods. This allowed evaluation the assembling mode of TiO2 and ZrO2 phases inside SBA-15 nanotubular mesopores.

Section snippets

Samples preparation

SBA-15 was prepared according to procedure [54] by crystallization from acidic aqueous solution of poly(ethylene glycol)-block-poly(propylene glycol)-block poly(ethylene glycol)-copolymer (Aldrich, Mavg = 5800) and TMOS. This procedure was modified by increasing the duration of the hydrothermal treatment to reduce the microporosity [7].

TiO2 and ZrO2 phases were inserted into SBA-15 by two methods: chemical solution decomposition (CSD) and internal hydrolysis (IH). The CSD of both oxides was

Characterization of parent SBA-15 material

Fig. 1 presents SAXS patterns of the parent SBA-15 material. The high-intensity peak (1 0 0) has a d-spacing of 10.3 nm and the following peaks have d-values consistent with a hexagonal arrangement of the pores with the distance of 11.9 nm between their axes. This confirmed that SBA-15 had a well-defined hexagonal pore structure. Fig. 2 shows the N2-adsorption–desorption isotherms for the same parent SBA-15 material. The narrow pore size distribution, derived from the desorption branch of N2

Conclusions

Chemical solution decomposition and internal hydrolysis methods were used for synthesis of high loading Ti(Zr)O2/SBA-15 composites. Metal oxide guest phases were located exclusively inside the OMS pores with the minimal pore blocking. As opposite to that, significant part of the guest phase and substantial pore blocking were obtained via traditional impregnation–evaporation demonstrating the inappropriateness of this strategy for preparation of high loading embedded GP/OMS composites.

Both CSD

Acknowledgments

This study was supported by the Israel Science Foundation, Center of Excellence (Grant No. 8003). The authors gratefully acknowledge Mr. V. Ezersky, Dr. A.I. Erenburg and Dr. S. Pevzner for HRTEM, XRD and SAXS characterizations, respectively.

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