**2. Porous ceramic materials: synthesis methods and characterization**

Among these materials, porous ceramics have played an important role because of their intrinsic physicochemical properties. They have also been widely used to satisfy diverse needs for sensing devices, and consistent results have been obtained in the field of atmospheric sensors, i.e., temperature, humidity, and presence of hydrocarbon gases sensors. In addition, the management of risk in oil and gas industrial installations is a priority task, especially when exploration and exploitation activities of the petroleum and gas sector in remote places and fragile environments are increasing rapidly. In particular, due to the nature of the working fluids in the oil industry, the risks associated to plant operations are considered as a high-risk activity. The risk management activity begins at the study and understanding of the hazards involved in a particular industrial activity, in order to establish security zones and secure

On the other hand, the current tendency of porous ceramic materials used in hydrocarbon leaks detection is shown in **Figure 1a**. The technological advances in this field indicate

with 23% and by zinc oxide (ZnO) that represents the 18%. These three metal oxides are the most common porous ceramic materials that have been using for sensing hydrocarbon

ties. In addition, diverse elements are adding to the ceramic porous materials in order to improve their sensing capabilities. **Figure 1b** shows that palladium (Pd) with 32% is the most used element for doping the porous ceramic sensors for hydrocarbon leaks detection, followed by platinum (Pt), and vanadium (V) with 16%. Other elements such as cerium (Ce), gold (Au), tungsten (W), etc., were also identified as important secondary compounds of these kinds of sensing materials. These statistic data were generated based on the stateof-the-art developed for this specific book chapter and will be explained extensively in the

**Figure 1.** (a) Percentage distribution of the most used porous ceramic materials for sensing hydrocarbons and (b)

percentage of elements most used for doping the porous ceramic sensor materials.

S, CO, toluene, among others. However, several researchers have been

O3

and/or associated gases such as liquefied petroleum gas (LPG), methane, H<sup>2</sup>

), followed by indium oxide (In<sup>2</sup>

), cobaltic oxide (Co<sup>3</sup>

) as potential porous ceramics with sensing proper-

, NO<sup>2</sup>

O4

O3 )

, ethanol,

) tungsten

procedures to be followed.

50 Recent Advances in Porous Ceramics

methanol, acetone, H<sup>2</sup>

trioxide (WO<sup>3</sup>

following sections.

that 31% of these materials are based on tin oxide (SnO<sup>2</sup>

testing additional oxides such as chromium oxide (Cr<sup>2</sup>

), and silicon dioxide (SiO<sup>2</sup>

A porous material is identified by the presence of channels, holes, or interstices. Ceramic materials with either ordered or disordered porosity in different size ranges have attracted the attention as sensing materials in the fields of hydrocarbon leaks detection and other important gases such as CO, CO<sup>2</sup> , H<sup>2</sup> , NH<sup>3</sup> , NO<sup>x</sup> , SO<sup>x</sup> , and H<sup>2</sup> S. Typical classification of porous materials is given by IUPAC depending on the pore size as follows: micropores (less than 2 nm), mesopores (2–50 nm), and macropores (more than 50 nm). The application and performance of porous materials in such fields lies in their physical, morphological, and textural properties. For example, high-specific surface area is of great importance, especially to interact with gases. Pore properties, such as pore size, porosity, and pore shape, also strongly influence on the desired performance of the material. Through different synthesis methods, the formation of porous ceramic materials is possible. Typical methods that are widely practiced are sol-gel synthesis, wet synthesis, impregnation, co-precipitation, and hydrothermal synthesis. Hydrothermal synthesis is the most common method to obtain a variety of nanostructured materials with different shapes, such as spheres, roads, wires, sheets, tubes, and so on. Others are sponge method, foam method, leaching, sintering of particles, emulsion templating, gel casting, and injection molding. More sophisticated methods have been developed to produce complex ceramic materials, for example, solution-combustion method. Many kinds of materials have received attention regarding gas sensors field. In the following text, a description of some relevant porous materials, their synthesis methods, and main characterization that determine their potential application in hydrocarbon and gas sensors is presented.

Zeolites are versatile materials that have found industrial applications in several fields such as water purification, catalysis, adsorption, and more recently in sensors for different hydrocarbons and gases [1–3]. Synthetic zeolites by hydrothermal method are obtained from Si and Al sources dissolved in water into an autoclave. Here, the growth of crystals is developed under high pressure and temperature in a closed system by controlling reaction temperature and time, precursors, and chemical composition of reaction mixture [4]. Novel strategy to synthesize nanozeolite LTA (Linde Type A) in its sodium form by hydrothermal method was reported by Anbia et al. [5]. In this report, pore size of 6–7 nm and BET surface areas around 500 m2 /g were calculated by N<sup>2</sup> adsorption-desorption analysis. Novel approach of *in-situ* hydrothermal synthesis was applied to glass fibers coated with zeolite for chemical sensors toward ethane and propane [6]. SEM images showed homogeneous layer of zeolite crystals and XRD patterns confirmed the zeolite type structure.

ZnO has been successfully loaded into mesoporous ZSM-5 zeolites by simple wet impregnation method, which consists on the immersion of sample in solution of the corresponding metal oxide at determined concentration, temperature, and stirring [7]. In this way, sensors based on ZnO particles have been fabricated toward CO, H<sup>2</sup> , and H<sup>2</sup> S, showing high preference to ethanol detection [8]. Additionally, zeolite has been used as layer support and overlay onto metal oxides as filters for modified gas sensors. In this regard, the sorption and catalytic properties of zeolites can improve the response of sensor and make it sensitive or insensitive to specific species. Layers of different porous zeolites such as silicalite, zeolite A, ZSM-5, LTA onto metal oxides (SnO<sup>2</sup> , WO<sup>3</sup> , and Cr<sup>2</sup> O3 ) have been assessed [9–12]. Seeding process and screen printing deposition seem to be common methodologies for fabrication of zeolite films onto metal oxides. The results of this investigation indicated an excellent discriminatory behavior when zeolite overlays were used, making the sensor more selective to specific gases even humidity or mixture-gas environments were tested. Nevertheless, in all cases, the sensor sensitivity was very dependent on the zeolite structure.

formed by Pluronic 123 and cetyltrimethylammonium bromide (CTAB) as template and co-template, respectively, induced the co-assembling of hydrolyzed silicate species from TEOS to synthesize spherical SBA-15 mesoporous silica with potential application in gas adsorption processes (**Figure 2**) [19]. Silica particles were not obtained when Pluronic 123/CTAB molar ratio was less than 0.31. After synthesis, samples were calcined for surfactant and co-surfactant elimination. Samples calcined at 540°C showed narrow pore size distribution with an average

Mesoporous MCM-41 silica was synthesized by hydrothermal method at several pH values [20]. Well-ordered hexagonal mesoporous structure was validated by TEM (transmission electron microscopy) images and XRD patterns which showed the characteristic reflection peaks indexed to the planes [100, 110, 200] of this type of silica. Yang et al. [21] synthesized hierarchical porous wheat-like silica particles by sol-gel method and co-hydrothermal aging. Bimodal

tion measurements was achieved through controlling the templates ratio and pH solution. Microwave-assisted hydrothermal methodology has allowed the preparation of mesoporous silica particles in shorter reaction time with similar structural and textural properties to those

On the other hand, sol-gel method has been also applied for silica synthesis with no hydrothermal conditions. Spherical mesoporous MCM-48 silica have been successfully obtained at room temperature conditions from TEOS [23]. High structural ordering of mesoporous evidenced by XRD patterns and TEM images was achieved by varying the reaction time, surfactant/TEOS, and water/ ethanol ratios. Uniform spherical MCM-48 silica particles with high surface area (900–1800 m<sup>2</sup>

and average pore diameter of 2 nm were obtained by modulating reaction conditions and initial gel composition [24]. SEM and HR-TEM images of *in-situ* amino-functionalized MCM-48 mesoporous silica are shown in **Figure 3**. Spherical particles with well-ordered pore structure were

**Figure 3.** SEM (a) and HR-TEM (b) images of in-situ amino-functionalized mesoporous MCM-48 silica synthesized by

at 850°C indicated the structural order and spherical morphology were maintained.

mesoporous structure (average pore size of 2–10 nm) determined by N<sup>2</sup>

achieved at 7 h of reaction time with particle size between 200 and 500 nm [25].

/g. The analysis of thermally treated samples

Porous Ceramic Sensors: Hydrocarbon Gas Leaks Detection

http://dx.doi.org/10.5772/intechopen.72315

adsorption-desorp-

/g)

53

pore size of 3 nm and BET surface area of 667 m<sup>2</sup>

obtained by conventional hydrothermal route [22].

sol-gel at room temperature.

The combination of zeolite and conductive polymers has resulted in gas sensors lighter and less expensive with favorable operation on extreme conditions in comparison with metal sensors. Polythiophene (PT), polypyrrole (PPr), polyphenylene (PP), polyphenylenevinylene (PPV), and mainly polyaniline (PANI) have been taken into account for these purposes [13]. PANI/clinoptilolite and PT/zeolite 13X composites for CO sensors have been included by chemical oxidative polymerization of the respective monomer solution in presence of zeolite dispersion to promote the polymer penetration into zeolite pores [14]. The electrical conductivity sensitivity to CO increased significantly when zeolite content increased too, suggesting the higher amount of zeolite pores and surface area, the better interaction with gas molecules.

Since the discovery in 1990, mesoporous silica has had wide and varied field of application in catalysis, sorption, drug delivery, oil and gas industry, sensor fabrication, and so forth. Ordered mesoporous silica molecular sieves are produced widely by sol-gel method and also under hydrothermal conditions using a surfactant (cationic, anionic, or non-ionic) as template and either tetraethyl orthosilicate (TEOS) or sodium silicate as silica source [15, 16]. The first stage of sol-gel process involves the formation of colloidal suspension (sol) and then the gelation of the sol to form a network. Particularly, the synthesis of silica comprising the hydrolysis and condensation of silica source at specific pH conditions (acidic or basic pH) as catalyst of the reaction to form silica particles that precipitate after a nucleation and growth process [17]. The regulation of reaction conditions has been decisive in order to obtain well-ordered pore structure with defined morphology. During synthesis processes, time and temperature aging, pH of solution, type of surfactant, and co-surfactant have been evaluated to investigate the effect of reaction conditions on the structural and textural properties of silica particles [18]. Micelles

**Figure 2.** Proposed mechanism for synthesis of spherical SBA-15.

formed by Pluronic 123 and cetyltrimethylammonium bromide (CTAB) as template and co-template, respectively, induced the co-assembling of hydrolyzed silicate species from TEOS to synthesize spherical SBA-15 mesoporous silica with potential application in gas adsorption processes (**Figure 2**) [19]. Silica particles were not obtained when Pluronic 123/CTAB molar ratio was less than 0.31. After synthesis, samples were calcined for surfactant and co-surfactant elimination. Samples calcined at 540°C showed narrow pore size distribution with an average pore size of 3 nm and BET surface area of 667 m<sup>2</sup> /g. The analysis of thermally treated samples at 850°C indicated the structural order and spherical morphology were maintained.

ZnO particles have been fabricated toward CO, H<sup>2</sup>

, and Cr<sup>2</sup>

**Figure 2.** Proposed mechanism for synthesis of spherical SBA-15.

very dependent on the zeolite structure.

O3

oxides (SnO<sup>2</sup>

, WO<sup>3</sup>

52 Recent Advances in Porous Ceramics

, and H<sup>2</sup>

) have been assessed [9–12]. Seeding process and screen print-

nol detection [8]. Additionally, zeolite has been used as layer support and overlay onto metal oxides as filters for modified gas sensors. In this regard, the sorption and catalytic properties of zeolites can improve the response of sensor and make it sensitive or insensitive to specific species. Layers of different porous zeolites such as silicalite, zeolite A, ZSM-5, LTA onto metal

ing deposition seem to be common methodologies for fabrication of zeolite films onto metal oxides. The results of this investigation indicated an excellent discriminatory behavior when zeolite overlays were used, making the sensor more selective to specific gases even humidity or mixture-gas environments were tested. Nevertheless, in all cases, the sensor sensitivity was

The combination of zeolite and conductive polymers has resulted in gas sensors lighter and less expensive with favorable operation on extreme conditions in comparison with metal sensors. Polythiophene (PT), polypyrrole (PPr), polyphenylene (PP), polyphenylenevinylene (PPV), and mainly polyaniline (PANI) have been taken into account for these purposes [13]. PANI/clinoptilolite and PT/zeolite 13X composites for CO sensors have been included by chemical oxidative polymerization of the respective monomer solution in presence of zeolite dispersion to promote the polymer penetration into zeolite pores [14]. The electrical conductivity sensitivity to CO increased significantly when zeolite content increased too, suggesting the higher amount of zeolite pores and surface area, the better interaction with gas molecules. Since the discovery in 1990, mesoporous silica has had wide and varied field of application in catalysis, sorption, drug delivery, oil and gas industry, sensor fabrication, and so forth. Ordered mesoporous silica molecular sieves are produced widely by sol-gel method and also under hydrothermal conditions using a surfactant (cationic, anionic, or non-ionic) as template and either tetraethyl orthosilicate (TEOS) or sodium silicate as silica source [15, 16]. The first stage of sol-gel process involves the formation of colloidal suspension (sol) and then the gelation of the sol to form a network. Particularly, the synthesis of silica comprising the hydrolysis and condensation of silica source at specific pH conditions (acidic or basic pH) as catalyst of the reaction to form silica particles that precipitate after a nucleation and growth process [17]. The regulation of reaction conditions has been decisive in order to obtain well-ordered pore structure with defined morphology. During synthesis processes, time and temperature aging, pH of solution, type of surfactant, and co-surfactant have been evaluated to investigate the effect of reaction conditions on the structural and textural properties of silica particles [18]. Micelles

S, showing high preference to etha-

Mesoporous MCM-41 silica was synthesized by hydrothermal method at several pH values [20]. Well-ordered hexagonal mesoporous structure was validated by TEM (transmission electron microscopy) images and XRD patterns which showed the characteristic reflection peaks indexed to the planes [100, 110, 200] of this type of silica. Yang et al. [21] synthesized hierarchical porous wheat-like silica particles by sol-gel method and co-hydrothermal aging. Bimodal mesoporous structure (average pore size of 2–10 nm) determined by N<sup>2</sup> adsorption-desorption measurements was achieved through controlling the templates ratio and pH solution. Microwave-assisted hydrothermal methodology has allowed the preparation of mesoporous silica particles in shorter reaction time with similar structural and textural properties to those obtained by conventional hydrothermal route [22].

On the other hand, sol-gel method has been also applied for silica synthesis with no hydrothermal conditions. Spherical mesoporous MCM-48 silica have been successfully obtained at room temperature conditions from TEOS [23]. High structural ordering of mesoporous evidenced by XRD patterns and TEM images was achieved by varying the reaction time, surfactant/TEOS, and water/ ethanol ratios. Uniform spherical MCM-48 silica particles with high surface area (900–1800 m<sup>2</sup> /g) and average pore diameter of 2 nm were obtained by modulating reaction conditions and initial gel composition [24]. SEM and HR-TEM images of *in-situ* amino-functionalized MCM-48 mesoporous silica are shown in **Figure 3**. Spherical particles with well-ordered pore structure were achieved at 7 h of reaction time with particle size between 200 and 500 nm [25].

**Figure 3.** SEM (a) and HR-TEM (b) images of in-situ amino-functionalized mesoporous MCM-48 silica synthesized by sol-gel at room temperature.

Due of the high-specific surface area and low density, silica aerogels prepared by sol-gel method have received attention for gas sensing purposes [26]. Nanofibers embedded in hydrophobic silica aerogel synthesized from a sol comprising tetramethyl orthosilicate, methanol, and water in basic medium were investigated by Xiao et al. for acetylene detection [27]. SEM images showed the nanofibers 200 mm long and 0.8 μm diameter well-embedded in the aerogel. The porosity of aerogel allowed the fiber performance as evanescent-field gas sensor. Silica can be used as template or coating for other components such as metal oxides. ZnO nanoparticles coated with mesoporous silica through a simple sol-gel method were reported by El-Nahhal et al. [28]. The change on the displacement of XRD peaks and the elemental analysis by EDX (energy-dispersive X-ray spectroscopy) confirmed the presence of silica. Additional evidence through TEM images was provided, where a worm-like silica structure coated the dark ZnO nanoparticles. Li et al. [29] reported the use of SBA-15 (Santa Barbara Amorphous-15) silica as template for mesoporous NiO nanowires to be assessed as sensor toward ethanol. In this work, SBA-15 silica was prepared by hydrothermal method, after that, NiO nanowires was synthesized by nanocasting method which consisted on the dissolution and dispersion of the NiO precursor (Ni(NO<sup>3</sup> ) 2 ) and the silica particles under stirring and heating. After, the resulting powder was calcined and SBA-15 silica was removed with NaOH aqueous solution. Mesoporous NiO nanowires with high surface area (111 m<sup>2</sup> /g) and average pore size of 3.6 nm were achieved. These characteristics make them more sensitive to ethanol gas in the range of 50–3000 ppm.

aluminum butoxide in propanol/water solution under heating to form a white precipitate. The precipitate was impregnated with the solution of the corresponding metal oxide precursor and the mixture was treated by firing at 700°C to obtain the loaded γ-alumina powder. The metal

detect ethanol, and other VOC such as acetone, ethyl acetate, benzene, toluene, and o-xylene. Porous alumina synthesized via anodic oxidation of aluminum, denoted as porous anodic alumina, has been object of numerous studies as template on which metal oxides are deposited for the development of gas sensor systems. The anodizing process consists of exposure the aluminum specimens to certain voltage conditions in an electrolyte solution. Anodization conditions, such as type of electrolyte, anodizing potential, temperature, and duration of process, determine the morphology and microstructure of porous film. Sharma and Islam [33] found that the increasing voltage caused an increase of pore size due to greater dissolution of the oxide layer. Likewise, the porous structure of alumina provided the nucleation sites for uniform growth of Pd-capped Mg when was deposited on it, and a finer film was obtained as was appreciated in XRD patterns and SEM images. Norek et al. [34] argued that the pores of alumina provided enough space for free expansion of metallic film, avoiding the accumulation

/g) and were able to

55

absorption capacity. In the same way, active layers of

in a range of 5–1000 ppm and operating

Porous Ceramic Sensors: Hydrocarbon Gas Leaks Detection

http://dx.doi.org/10.5772/intechopen.72315

were deposited onto anodic porous alumina by sputter-deposition [35]. The

critical combination of the high quality and reproducible porous structure of alumina film

The complex operational mechanism of porous ceramic sensors for hydrocarbon gases is affected by factors, such as chemical composition, humidity, temperature, morphology, and so on. This mechanism is determined by chemical and electronic interactions between the porous ceramic and the specific gas resulting in a resistance change. The main operational

There are three principal reasons for monitoring hydrocarbon gases: (1) combustible/flammable gas, (2) toxic/irritant gases and (3) oxygen levels control. As is well known, any hydrocarbon leaks is a potential explosive hazard, where to avoid an explosion, atmospheric levels must be maintained below the lower explosive limit (LEL) for each gas, or purged of oxygen. In addition, for a flame to exist, three conditions must be met: (1) a source of fuel (any hydrocarbon source, e.g., methane or gasoline vapors), (2) enough oxygen (greater than 10–15%) to oxidize or burn the fuel, and (3) a source of heat (ignition) to start the process [36]. Moreover, combustion can occur at both extreme low-end and high-end gas concentrations. These extremes are called the lower explosive limit (LEL) also known as lower flammability limit (LFL), and the upper explosive limit (UEL) or upper flammability limit (UFL). Any gas or vapor concentration that falls between these two limits is in the flammable (explosive) range [36]. Therefore, the control and knowledge about the sensors' operational conditions

**3. Porous ceramic materials for sensors. Operational principles for** 

oxide-alumina powders showed high specific surface areas (around 200 m<sup>2</sup>

of stress and resulting in a greater H<sup>2</sup>

significantly influenced the sensor response to H<sup>2</sup>

principles of sensing devices are shown in **Table 1** [36].

WO<sup>3</sup>

and NbO<sup>2</sup>

**gases detection**

temperatures of 20–350°C.

Porous alumina (Al<sup>2</sup> O3 ) is other kind of material that has had an important role as ceramic support of metal oxides for gas sensors due to its insulating properties and inert chemical behavior [30]. A conventional method used to prepare it is by means of solid-phase transformation through thermal decomposition of aluminum hydroxides. Alumina precursors can be synthesized by sol-gel process where an aluminum salt, such as AlCl<sup>3</sup> , AlNO<sup>3</sup> , Al2 (SO<sup>4</sup> )3 , is hydrolyzed to form the corresponding aluminum hydroxide which precipitates [31]. The thermal treatment at different temperatures leads several transition alumina and α-alumina with highly porous vermicular microstructure as can be appreciated in **Figure 4**.

On the other hand, γ-alumina loaded at 20 wt% with various metal oxides (CeO<sup>2</sup> , CuO, Fe<sup>2</sup> O3 , Mn<sup>2</sup> O3 , NiO, and RuO<sup>2</sup> ) were prepared by Hyodo et al. [32] for VOC sensing. The mesoporous γ-alumina was synthesized by microwave-assisted solvothermal method from hydrolysis of

**Figure 4.** SEM images of α-alumina synthesized by sol-gel method from different aluminum salts. (a) Precursor of AlNO<sup>3</sup> ; (b) precursor of AlCl<sup>3</sup> ; (c) precursor of Al<sup>2</sup> (SO<sup>4</sup> )3 .

aluminum butoxide in propanol/water solution under heating to form a white precipitate. The precipitate was impregnated with the solution of the corresponding metal oxide precursor and the mixture was treated by firing at 700°C to obtain the loaded γ-alumina powder. The metal oxide-alumina powders showed high specific surface areas (around 200 m<sup>2</sup> /g) and were able to detect ethanol, and other VOC such as acetone, ethyl acetate, benzene, toluene, and o-xylene. Porous alumina synthesized via anodic oxidation of aluminum, denoted as porous anodic alumina, has been object of numerous studies as template on which metal oxides are deposited for the development of gas sensor systems. The anodizing process consists of exposure the aluminum specimens to certain voltage conditions in an electrolyte solution. Anodization conditions, such as type of electrolyte, anodizing potential, temperature, and duration of process, determine the morphology and microstructure of porous film. Sharma and Islam [33] found that the increasing voltage caused an increase of pore size due to greater dissolution of the oxide layer. Likewise, the porous structure of alumina provided the nucleation sites for uniform growth of Pd-capped Mg when was deposited on it, and a finer film was obtained as was appreciated in XRD patterns and SEM images. Norek et al. [34] argued that the pores of alumina provided enough space for free expansion of metallic film, avoiding the accumulation of stress and resulting in a greater H<sup>2</sup> absorption capacity. In the same way, active layers of WO<sup>3</sup> and NbO<sup>2</sup> were deposited onto anodic porous alumina by sputter-deposition [35]. The critical combination of the high quality and reproducible porous structure of alumina film significantly influenced the sensor response to H<sup>2</sup> in a range of 5–1000 ppm and operating temperatures of 20–350°C.

Due of the high-specific surface area and low density, silica aerogels prepared by sol-gel method have received attention for gas sensing purposes [26]. Nanofibers embedded in hydrophobic silica aerogel synthesized from a sol comprising tetramethyl orthosilicate, methanol, and water in basic medium were investigated by Xiao et al. for acetylene detection [27]. SEM images showed the nanofibers 200 mm long and 0.8 μm diameter well-embedded in the aerogel. The porosity of aerogel allowed the fiber performance as evanescent-field gas sensor. Silica can be used as template or coating for other components such as metal oxides. ZnO nanoparticles coated with mesoporous silica through a simple sol-gel method were reported by El-Nahhal et al. [28]. The change on the displacement of XRD peaks and the elemental analysis by EDX (energy-dispersive X-ray spectroscopy) confirmed the presence of silica. Additional evidence through TEM images was provided, where a worm-like silica structure coated the dark ZnO nanoparticles. Li et al. [29] reported the use of SBA-15 (Santa Barbara Amorphous-15) silica as template for mesoporous NiO nanowires to be assessed as sensor toward ethanol. In this work, SBA-15 silica was prepared by hydrothermal method, after that, NiO nanowires was synthesized by nanocasting method which consisted on the dissolution

> ) 2

heating. After, the resulting powder was calcined and SBA-15 silica was removed with NaOH

pore size of 3.6 nm were achieved. These characteristics make them more sensitive to ethanol

support of metal oxides for gas sensors due to its insulating properties and inert chemical behavior [30]. A conventional method used to prepare it is by means of solid-phase transformation through thermal decomposition of aluminum hydroxides. Alumina precursors can

is hydrolyzed to form the corresponding aluminum hydroxide which precipitates [31]. The thermal treatment at different temperatures leads several transition alumina and α-alumina

γ-alumina was synthesized by microwave-assisted solvothermal method from hydrolysis of

**Figure 4.** SEM images of α-alumina synthesized by sol-gel method from different aluminum salts. (a) Precursor of

(SO<sup>4</sup> )3 .

; (c) precursor of Al<sup>2</sup>

) is other kind of material that has had an important role as ceramic

) were prepared by Hyodo et al. [32] for VOC sensing. The mesoporous

aqueous solution. Mesoporous NiO nanowires with high surface area (111 m<sup>2</sup>

be synthesized by sol-gel process where an aluminum salt, such as AlCl<sup>3</sup>

with highly porous vermicular microstructure as can be appreciated in **Figure 4**.

On the other hand, γ-alumina loaded at 20 wt% with various metal oxides (CeO<sup>2</sup>

) and the silica particles under stirring and

/g) and average

, Al2 (SO<sup>4</sup> )3 ,

, CuO, Fe<sup>2</sup>

O3 ,

, AlNO<sup>3</sup>

and dispersion of the NiO precursor (Ni(NO<sup>3</sup>

O3

gas in the range of 50–3000 ppm.

, NiO, and RuO<sup>2</sup>

; (b) precursor of AlCl<sup>3</sup>

Porous alumina (Al<sup>2</sup>

54 Recent Advances in Porous Ceramics

Mn<sup>2</sup> O3

AlNO<sup>3</sup>
