TiO2–MCM-41 for the photocatalytic abatement of NOx in gas phase
Introduction
Titanium dioxide is one of the most common materials in everyday life. It has been widely used as white pigment in paints, cosmetics and foodstuffs and in recent years photocatalytic processes using TiO2 have been applied to important problems of environmental interest like purification of water and air. TiO2 is a semiconductor with a band gap energy Eg = 3.2 eV (this value is that of the anatase form); when this material is irradiated with photons (e.g. sunlight) the electron–hole pair that is created may separate and the resulting charge carriers might migrate to the surface where they can react with adsorbed water and oxygen to produce radical species. These attack any adsorbed organic molecule and can lead to complete decomposition into CO2 and H2O [1]. Compared with traditional advanced oxidation processes the technology of photocatalysis is known to have some advantages, such as ease of setup and operation at room temperatures, no need for post-processes, low consumption of energy and consequently low costs, high degradation efficiency in removing organic pollutants even at ultra low concentrations. For the realization of these practical applications, development of highly active photocatalysts is keenly desired. Based on the kinetic investigation of photocatalytic reactions, TiO2 nanoparticles in the anatase crystal form having both high crystallinity and large surface area [2] must exhibit higher photocatalytic activity. This last property should increase the amount of surface-adsorbed substrate to enhance the capture of photogenerated electron and positive hole. The control of the final features of the oxide can be achieved by several approach among that: sol–gel synthesis [3], by employing surfactants [4], [5] and more recently there has been increasing interest in introducing titanium into high surface area support materials. Good candidates for supports are mesostructured silicas because of their high surface areas, ordered frameworks, and narrow pore size distribution [6], [7], [8], [9].
Several approaches can be adopted for the synthesis of high area mesoporous silica/titania systems that can be divided in two classes:
- (i)
introduction of TiO2 during the formation of the silica material (one-pot synthesis) [10], [11], [12].
- (ii)
introduction of TiO2 in a pre-synthesized silica support by applying post-synthesis methods: acid-catalysed sol–gel method, chemical solution deposition, multistep deposition [9], [10], [11], [12], [13], [14], [15].
The first approach permits an accurate control of the final properties of the titania/silica catalyst by varying the variables involved in the synthesis process but at the same time requests a careful control of the reactivity of the Ti- and Si-precursors that have to be adjusted to each other. This is the most critical step (very hard in some cases) of the synthesis process and the successfully realization of the final composite (TiO2/SiO2) is strictly dependent to it. Post-modification is a more practical pathway for to obtain silica/titania catalysts but often does not permit an accurate control on the incorporation of the TiO2 particles at the different location within the porous structure, namely inside or outside of the mesoporous channel, whereby a possible blocking of the pore apertures can occur. Obtaining the required full information about the actual position of TiO2 nanoparticles is usually a very difficult task [9]. In any case, the review of literature works suggests that an ideal and general approach for the synthesis of supported nanotitania does not exist. On the contrary, ad hoc preparation methods must be chosen in order to account for the specific requirements of final applications.
The twofold goal of this work is to establish first a new preparation method to produce titania/silica catalysts characterized by high surface area and ordered pore structure for photocatalytic applications. Such an aim can be accomplished by supporting the TiO2 particles on MCM-41 mesoporous silica via an incipient wetness impregnation using Ti(iOPr)4 as a precursor. Particular attention has to be paid to the influence of the TiO2 amount on the chemical–physical properties of the final systems. In fact, different Ti loadings in the catalysts may induce changes not only in the textural characteristics but also in the surface chemical properties of the TiO2/SiO2 composites. As a second goal, the catalytic behaviour of the investigated systems will be tested towards the photocatalytic abatement of NOx in gas phase. This part of the study will be focussed, in particular, on the preliminary assessment of the critical parameter involved in the catalytic process.
Section snippets
Synthesis
The MCM-41 support was prepared as reported in a previous work [16]. The surfactant was removed from the as-synthesized MCM-41 silica by calcination in air at 540 °C for 6 h.
The silica support was activated with TiO2 by incipient wetness impregnation by using an alcoholic solution of Ti(iOPr)4. The obtained Ti/MCM-41 composites were dried at 80 °C for 12 h and finally calcined at 500 °C in flowing air (30 mL/min) for 3 h. Five samples with increasing Ti/Si ratio (9 wt%, 17 wt%, 23 wt%, 28 wt%, 33 wt%) have
Optimization of working conditions
A preliminary study was carried out in order to determine the optimal working conditions for the catalytic test. In particular, the high activity of the catalysts required the minimization of the contact time between the catalytic bed and the reagent species. This condition is necessary to allow an effective evaluation of the performance of the investigated samples. This aim was pursued by choosing the best compromise between the power of irradiation, the amount of catalyst and the feed mixture
Conclusions
This work demonstrated that the incorporation of titanium on MCM-41 by incipient wetness impregnation is a reliable method for the synthesis of silica-supported titania nanoparticles (size down to about 7 nm) with the retention of the mesoporous structure of the support (ordered structure and high surface area). The NOx oxidation, employed as test reaction, was strongly influenced by the degree of TiO2 loading and good performances could be achieved by an optimal compromise between surface area,
Acknowledgement
The authors want to thank Mrs. Tania Fantinel for the excellent technical assistance.
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