Zeolites — Versatile materials for gas sensors
Introduction
Zeolites are a family of inorganic, crystalline materials. They consist of interconnected aluminosilicate building blocks, i.e., AlO4 or SiO4 tetrahedra, which form 3-dimensional (3D) frameworks with linked channel systems and well-defined micro- and mesopores. At the same time, this high degree of open porosity gives rise to an exceptionally high-surface area. The database of the International Zeolite Association (IZA) comprises structural information and crystallographic data of more than 170 framework types.1 To date, only about 17 of these structures are of commercial interest [1].
In addition to the 3D framework of one particular zeolite, the chemical composition is crucial in defining its specific properties. Generally, the following sum formula applies [2]Mey/mm+[(SiO2)x·(AlO2−)y]·zH2O.Here, Me denotes a cation with the charge m.
As shown in Eq. (1), a negative charge is introduced into the framework with each tetrahedrally coordinated aluminum ion Al3+. This charge needs to be compensated by y/m cations Mem+, which are electrostatically bound to the host framework and mobile along the channels [3]. As a consequence, zeolites with a high aluminum content are highly polar materials, potential ion conductors, and excellent ion exchangers. Furthermore, the aluminum ions act as highly acidic sites that can catalyze a number of chemical reactions. An important parameter for zeolites is the ratio of silicon to aluminum atoms in the lattice, x/y, which indicates the content of mobile cations as well as the amount of acidic centers per unit cell. In addition to their intrinsic components, zeolites can also be modified in a post-synthesis step by incorporating catalytically active metal clusters such as Pt, Fe, or Cu.
Due to the characteristic properties outlined above, zeolites are well-established in a number of industrial applications, ranging from low-end to high-end. They are used for example as ion exchangers for water purification [4], [5], as drying agents or absorbents for organic vapors [6], as molecular sieves and separation membranes [7], [8], [9], [10], or as catalyst beds in the petrochemical industry or in the synthesis of fine chemicals (e.g. [2] and references therein, [11], [12], [13]).
Ongoing research within the field of zeolites aims at opening up even more sophisticated applications by better exploiting the high potential of this materials family [14]. The field of heterogeneous catalysis has witnessed an intense research activity in the development of zeolite membrane reactors [15], [16], [17], [18]. Here, classical approaches use the zeolite membrane as a mere filter to eliminate inhibitors or to promote the reaction by continuously removing the product. More novel concepts aim at utilizing the channel and pore structure of the membrane to control the residence time or to sterically reduce unwanted side-reactions.
For analytical chemistry, zeolites are also high-potential candidates [19]. Zeolite-modified electrodes, a subgroup of the chemically modified electrodes, are used for electroanalytical purpose in solutions. In this case, conventional electrodes are covered with a zeolite material, thus combining the advantages of ion-exchange voltammetry with size and shape selectivity. For a detailed discussion of these devices, the reader is referred to review Ref. [20] and references therein.
Due to their unique property spectrum, zeolites are of high interest in the field of gas sensing. Their adsorptivity, high-surface area and porosity, presence of mobile ions, and catalytic activity make them attractive candidates for numerous applications as chemical sensors. As shown in Fig. 1, gas sensor devices based on zeolites can be divided into two major groups depending on the respective role of the framework material. On one hand, the zeolite can act as the main functional element. This is the case for sensor principles relying directly on conductive, adsorptive, or catalytic properties of one specific zeolite and its interaction with the surrounding atmosphere. The second group encompasses devices using zeolites as auxiliary elements. This includes coated systems (sensitive material coated onto a zeolite and vice versa) as well as host–guest set-ups with the active material encapsulated within the zeolite framework.
While other reviews dealing with zeolite-based gas sensors classify the devices according to the gases to be detected [21], [22], the present paper highlights the various underlying sensor principles presented in Fig. 1. Particular focus is laid on devices where the zeolite itself acts as the sensitive element (left-hand part of Fig. 1). The corresponding mechanisms and examples are discussed in detail in 2 Ionic conductivity, 3 Catalytic activity, 4 Adsorptivity. The respective section titles denote the main property of the zeolite material responsible for gas sensitivity. In Section 5, various sensor concepts with the zeolite as an auxiliary element (right-hand part of Fig. 1) are briefly summarized.
To denote the various zeolites, the present paper follows the framework type codes consisting of three capital letters, which is assigned by the IZA. If applicable, the nature of the mobile ion is given in front of the three letter code. Hence, a zeolite beta compensated by protons is denoted as H BEA.
Section snippets
Ionic conductivity
Due to their large band-gap of several eV, no electronic conductivity is observed with zeolites in general [3]. However, since mobile cations are present within the zeolite framework that may hop from one binding site to the next, zeolites exhibit ionic conductivity (e.g. review Ref. [23]). General aspects on ionic conductivity of zeolites have been investigated for several decades. The thermal activation energy of conduction and the specific conductivity depend on the nature of the mobile
Catalytic activity
As already indicated in the Introduction section, zeolites – in particular the silicon-rich members – are promising candidates for heterogeneous catalysis. Highly selective catalysts can be synthesized within the family by controlling (a) the amount of aluminum atoms and hence the strength and number of active acidic centers, (b) the accessibility of these centers within the framework, and (c) the presence of additional active noble metal clusters. This high degree of selectivity is of
Adsorptivity
One intrinsic property of zeolite frameworks is their selective adsorption behavior resulting in a measurable mass change. As a consequence, a great variety of gas sensor studies investigate the application of zeolite layers onto piezoelectric microbalances or surface acoustic wave (SAW) devices. In both cases, the mass change is transformed into a frequency shift of the piezoelectric resonator.
A number of corresponding devices studying a large variety of framework types and analytes are
Zeolites as an auxiliary phase
As indicated in Fig. 1, the uses of zeolites as an auxiliary phase in gas sensing are divided into three major subgroups. They can act as
- •
filter layers (either catalytic or size-restrictive) to enhance selectivity of a sensitive film
- •
high-surface templates
- •
host structures to immobilize gas-sensitive species.
The first application group, i.e., zeolite filters, is of particular interest for conventional n- or p-type semiconducting sensor materials. One major drawback of these devices is their lack of
Conclusion
During the past decades, the versatile family of zeolites has opened up new resources for chemical gas sensors. In the face of the growth in the number of studies dedicated to zeolites, this group of porous materials is expected to play an ever increasing role in the design of next-generation sensors. Although many groups still focus on using zeolites as mere auxiliary elements, such as filters and supports, increasing effort is dedicated to exploit the intrinsic properties of this material
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