Nanosized Sn-MFI zeolite for selective detection of exhaust gases
Graphical abstract
Introduction
Over the past decades, many efforts have been made in order to develop filtration/adsorption devices as well as systems able to convert harmful molecules into harmless ones [1], [2], [3] There is an increasing demand for sensors able to selectively detect at ppm levels gaseous pollutants such as CO, CO2 and NOx gases, which are continuously produced from combustion engines. Various sensor devices have been developed based on different material including semiconducting metal oxides, conductive polymer-based composite and porous materials, and various transductions platforms such as electrical, optical mass and heat transducers. [4], [5], [6] Semiconducting metal oxide materials such as SnO2, WO3, TiO2, ZrO2, ZnO and their corresponding doped hybrid materials have been extensively used for the CO, CO2 and NOx gas sensing [4], [5], [6]. Among them, the SnO2 based semiconductor devices have attracted high research interest, mainly because they display high sensitivities at low cost [7], [8], [9], [10]. However, reported gas sensors based on SnO2 materials usually operated at high temperature (200–400 °C) and have a huge lack of selectivity [11].
To enhance selectivity of devices based on tin oxide materials, their incorporation into zeolite structures, which are known to provide high selectivity towards various molecules [12], [13], [14], has been envisaged. Zeolites are a class of microporous inorganic crystals, which contain molecular sized voids (pores) within their crystal structures. Because of their unique size and shape of the zeolite pores and their large specific surface areas, zeolites can discriminate between molecules based on size and shape selectivity. As a sensing layer, zeolites are very favorable due to their extremely high thermal stability and chemical resistance [15]. In addition, the recent preparation of nanosized zeolites with enhanced accessibility to their micropores by reduction of zeolite particle sizes going from micron to nanometer sized dimensions [16], [17], [18], [19], affords enhanced physical properties such as increased specific surface area and decreased diffusion path lengths. This offers additional advantages towards the extraordinary performance of zeolite materials for catalytic, sorption, membrane and sensing applications [20], [21].
All of the previously reported Sn-doped materials synthesized by a direct synthesis approach possess large particle sizes in the range of 1–5 μm [22], [23], [24]. While post-synthetic introduction of Sn in zeolites was reported to lead to invariable loss of their crystallinity [22], [23], [24]. Recently, hierarchical Sn-MFI zeolite nanosheets were successfully prepared and used as catalysts for glucose and lactose isomerization [25].
Herein, we describe the synthesis of highly crystalline nanosized tin containing MFI (Sn-MFI) zeolite nanocrystals in alkaline media under hydrothermal conditions. The Sn-MFI crystals have a mean particle diameter below 100 nm, and the Sn introduced is about 3 wt%. The Sn-MFI materials demonstrated high detection towards CO, CO2, NO and NO2 gases in contrast to pure silicalite-1 (Si-MFI) sample.
Section snippets
Materials preparation
The following chemicals were used without further purification: tetra-n-propylammonium hydroxide (TPAOH, 20 wt% water solution, Alfa Aesar), tetraethylorthosilicate (TEOS, 98%, Aldrich) and tin tetrachloride pentahydrate (SnCl4. 5H2O, Aldrich). Syntheses were carried out in 100 cm3 polypropylene bottle (PP bottle) at autogenous pressure without agitation.
The following gel compositions were used for the hydrothermal synthesis of Sn-MFI and Si-MFI materials after optimization: Sn-MFI: SiO2: 0.01SnO2
Materials characterization
Fig. 1 shows the powder X-ray diffraction (XRD) patterns of the calcined Sn-MFI sample and its corresponding silica analog (Si-MFI). Both samples are highly crystalline and the XRD peaks match well with the standard MFI-phase [26]. Only the Bragg peaks corresponding to the MFI-type zeolite are present in the Sn-MFI pattern and no signatures for any other crystalline or amorphous phases are observed, indicating that the Sn-MFI sample possess high phase purity. The main peaks corresponding to
Conclusions
In this study, the preparation of nanosized tin containing MFI type zeolite crystals following a hydrothermal synthesis is reported. The prepared MFI nanocrystals exhibit well-ordered microporous structure, high specific surface area and high porosity. The Sn-MFI materials assembled as self-supported pellet and thin films are further employed for detection of NO2, NO, CO and CO2 at room temperature using in situ infrared spectroscopy. The Sn-MFI material shows high sensitivity, high capacity
Acknowledgement
The authors acknowledge the financial support from the Region of Basse-Normandie (TARGED ANR-13-BS08-0002-01), co-financer of TARGED project.
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Equally contributed.