**5. Porous ceramic materials (micro- and nano-materials) for sensing hydrocarbons**

**Substance Porous ceramic materials Reference**

O3

O3

O3

Porous nanoparticles of α-Fe<sup>2</sup>

) Mesoporous SnO<sup>2</sup>

Porous Ga<sup>2</sup>

Porous SiO<sup>2</sup>

Sb2 O5

In2 O3

O) Porous SnO<sup>2</sup>

Porous ZnFe<sup>2</sup>

Porous α-Fe<sup>2</sup>

Porous ZnO-Co<sup>3</sup>

Mesoporous Co<sup>3</sup>

Mesoporous In<sup>2</sup>

Ce-doped In<sup>2</sup>

Pd-Pt-In<sup>2</sup> O3

**Table 2.** Porous ceramic materials for hydrocarbon leaks detection.

Cr<sup>2</sup> O3

O) Porous WO<sup>3</sup>

Vanadium-doped SnO<sup>2</sup>

modified SnO<sup>2</sup>

Porous corundum-type In<sup>2</sup>

O3

O4

Porous indium oxide (In<sup>2</sup>

O4

O4

O3

O3


) Porous Pd-loaded flower-like SnO<sup>2</sup>

O3

nanoflowers and porous nanospheres [64]

Nanoporous SnO<sup>2</sup> [68]

doped with Pd [65]

Porous Ceramic Sensors: Hydrocarbon Gas Leaks Detection

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

63

doped with Pd [66]

O<sup>3</sup> [67]

porous composite ceramics [69]

coated with amorphous microporous Si–B–C–N layers [72]

CuO/p-porous silicon [74]

W- and V-modified mesoporous MCM-41 SiO<sup>2</sup> [76] Au-functionalized porous ZnO nanosheets [77]

Porous ZnO nanosheet-built network film [79] CuO nanostructures with porous nanosheets [80]

Mesoporous nickel oxides nanowires [29]

Macropore and mesopore SnO<sup>2</sup> [92]

Porous ZnO crystals [96]

porous microspheres [94]

OH) Porous hierarchical SnO<sup>2</sup> [89]

micro/nanostructured porous thin film [81]

nanotubes and nanowires [78]

nanosheets [82]

O<sup>3</sup> [83]

hollow nanospheres [84]

hollow polyhedrons heterostructures [86]

nanoneedle arrays [87]


porous nanospheres [90]

thin films [95]

composited nanocrystalline SnO<sup>2</sup> [91]

) nanostructured with Pd [85]

microspheres [93]

films doped with NiO and Au nanocrystals [70]

oxide porous nanofibers [71]

nanospheres [73]

porous nanocomposites [41]

nanosheets [75]

LPG SnO<sup>2</sup>

H<sup>2</sup> Pt-WO<sup>3</sup>

CO SnO<sup>2</sup>

NO<sup>2</sup> Mesoporous In<sup>2</sup>

S Porous In<sup>2</sup>

Methane (CH<sup>4</sup>

H2

Ethanol (C<sup>2</sup>

Methanol (CH<sup>3</sup>

Toluene (C7

Acetone (C<sup>3</sup>

H8

H6

H6

Wherever there is a leak at a hydrocarbon facility (e.g., pipeline), there will be hydrocarbon vapors that may be detected. *In situ* detection of hydrocarbon plumes from leakages involves reading of a variety of environmental variables and combination of techniques that will allow to detect the leak in the air, soil, or water. when a gaseous hydrocarbon leaks takes place and it migrates to the environment; this would require an immediate detection procedure of hydrocarbon vapors in order to take the respectively actions to control any catastrophic accident. This section extensively reviews the recent development of porous ceramic gas sensor materials for hydrocarbon gas leaks including LPG [64, 65], CH<sup>4</sup> [66–68], H<sup>2</sup> [69–71], ethanol [29, 84–88], methanol [89–92], and associated gases such as NO<sup>2</sup> [41, 73–77], H<sup>2</sup> S [78–83], and CO [72]. Basically, the discussion will be focused on the specific overlapped section between three interesting areas such as (1) porous ceramic materials, (2) hydrocarbon gas leaks detection, and (3) sensor materials giving the opportunity to explore the interesting niche area for sensing hydrocarbons and their associated gases leaks by porous ceramic materials. In addition, promising materials for sensitive detection of diverse hydrocarbons and/or their associated gases have been identified and are summarized in **Table 2**.

The detection effectivity of hydrocarbons gases by porous ceramic sensor materials (e.g., metal-oxide semiconductor—MOS) have a great influence on the chemical composition of the ceramics, the doping type by specific elements, the porosity type, how morphology-affect the gas sensing properties, etc. According to the information in **Table 2**, the most used porous ceramic materials for hydrocarbon sensing application are (1) the porous tin oxide (SnO<sup>2</sup> ) which have been applied to detect LPG, methane, H<sup>2</sup> , NO<sup>2</sup> , ethanol, methanol, toluene, etc.; (2) the porous indium oxide (In2 O3 ) that shows a great potential to sensing ethanol, methanol and other associated gases to the hydrocarbon field such as NO<sup>2</sup> , H<sup>2</sup> S, etc.; and (3) the porous zinc oxide (ZnO) that has been used to sensing ethanol, acetone, NO<sup>2</sup> , H<sup>2</sup> S, etc. On the other hand, the catalyst elements mostly used as doping materials in order to improve the sensing properties of the porous ceramics are palladium (Pd) [65, 66, 85], gold (Au) [77], tungsten (W) [76, 95], vanadium (V) [71], cerium (Ce) [90], platinum (Pt) [69], and/or combinations thereof [70, 76, 91]. The Pd could be used to explain the principle of operation of these doping elements which is based on the fact that Pd is a catalytic metal that dissociates the ambient gas to ions. These travel by diffusion to the metal-oxide interface where an electrically polarized layer is formed (according to the ambient gas used). This layer stimulates a change in the electrical characteristics of the MOS device, and hence a sensing mechanism is established [97]. In addition, other compounds such as Sb2 O5 [41], NiO [70], and graphene [88] have been also evaluated as doping or as secondary materials in advanced composite sensors.

There are only few papers devoted to investigation of sensor properties of porous ceramic materials toward low temperature at high humidity sensing conditions. As humidity is a permanent environmental factor, its control and measurement are particularly important not only for human comfort but also for many industries and technologies. It has been found that the ambient humidity plays a crucial role in the response of the porous ceramic materials based sensor to different hydrocarbons and their associated gases at room temperature. Therefore,


**Table 2.** Porous ceramic materials for hydrocarbon leaks detection.

**5. Porous ceramic materials (micro- and nano-materials) for sensing** 

rials for hydrocarbon gas leaks including LPG [64, 65], CH<sup>4</sup>

84–88], methanol [89–92], and associated gases such as NO<sup>2</sup>

gases have been identified and are summarized in **Table 2**.

which have been applied to detect LPG, methane, H<sup>2</sup>

O3

zinc oxide (ZnO) that has been used to sensing ethanol, acetone, NO<sup>2</sup>

and other associated gases to the hydrocarbon field such as NO<sup>2</sup>

(2) the porous indium oxide (In2

In addition, other compounds such as Sb2

Wherever there is a leak at a hydrocarbon facility (e.g., pipeline), there will be hydrocarbon vapors that may be detected. *In situ* detection of hydrocarbon plumes from leakages involves reading of a variety of environmental variables and combination of techniques that will allow to detect the leak in the air, soil, or water. when a gaseous hydrocarbon leaks takes place and it migrates to the environment; this would require an immediate detection procedure of hydrocarbon vapors in order to take the respectively actions to control any catastrophic accident. This section extensively reviews the recent development of porous ceramic gas sensor mate-

[72]. Basically, the discussion will be focused on the specific overlapped section between three interesting areas such as (1) porous ceramic materials, (2) hydrocarbon gas leaks detection, and (3) sensor materials giving the opportunity to explore the interesting niche area for sensing hydrocarbons and their associated gases leaks by porous ceramic materials. In addition, promising materials for sensitive detection of diverse hydrocarbons and/or their associated

The detection effectivity of hydrocarbons gases by porous ceramic sensor materials (e.g., metal-oxide semiconductor—MOS) have a great influence on the chemical composition of the ceramics, the doping type by specific elements, the porosity type, how morphology-affect the gas sensing properties, etc. According to the information in **Table 2**, the most used porous ceramic materials for hydrocarbon sensing application are (1) the porous tin oxide (SnO<sup>2</sup>

hand, the catalyst elements mostly used as doping materials in order to improve the sensing properties of the porous ceramics are palladium (Pd) [65, 66, 85], gold (Au) [77], tungsten (W) [76, 95], vanadium (V) [71], cerium (Ce) [90], platinum (Pt) [69], and/or combinations thereof [70, 76, 91]. The Pd could be used to explain the principle of operation of these doping elements which is based on the fact that Pd is a catalytic metal that dissociates the ambient gas to ions. These travel by diffusion to the metal-oxide interface where an electrically polarized layer is formed (according to the ambient gas used). This layer stimulates a change in the electrical characteristics of the MOS device, and hence a sensing mechanism is established [97].

O5

There are only few papers devoted to investigation of sensor properties of porous ceramic materials toward low temperature at high humidity sensing conditions. As humidity is a permanent environmental factor, its control and measurement are particularly important not only for human comfort but also for many industries and technologies. It has been found that the ambient humidity plays a crucial role in the response of the porous ceramic materials based sensor to different hydrocarbons and their associated gases at room temperature. Therefore,

evaluated as doping or as secondary materials in advanced composite sensors.

, NO<sup>2</sup>

) that shows a great potential to sensing ethanol, methanol

, H<sup>2</sup>

, H<sup>2</sup>

[41], NiO [70], and graphene [88] have been also

[66–68], H<sup>2</sup>

[41, 73–77], H<sup>2</sup>

[69–71], ethanol [29,

, ethanol, methanol, toluene, etc.;

S, etc.; and (3) the porous

S, etc. On the other

S [78–83], and CO

)

**hydrocarbons**

62 Recent Advances in Porous Ceramics

ceramic metal oxides are found to be a good choice as humidity sensing materials due to their properties such as high mechanical strength, good physical and chemical stabilities, fast response and recovery times, and wide range of operating temperatures. Semiconducting metal oxide sensors are the widely studied chemiresistive sensors. Recently, nanostructures of semiconductor metal oxides have received considerable interest in the fabrication of humidity and gas sensors due to their high surface-to-volume ratio of atoms, excellent surface reactivity, and the ability to tailor their surface and charge transport properties. Hence, they are considered as ideal candidates as humidity sensors. SnO<sup>2</sup> nanowires, ZrO<sup>2</sup> nanorods, Al2 O3 nanowires, TiO<sup>2</sup> nanotubes, BaTiO<sup>3</sup> nanofibers, ZnSnO<sup>3</sup> nanocubes, among others constitute the recently explored nanostructured metal oxides to this effect [98].

in comparation to the In2

hand, hydrogen sulfide (H<sup>2</sup>

In2 O3

idea, In2

line In2 O3

of the In<sup>2</sup>

on the In2

of 24.8% for H<sup>2</sup>

H2

O3

performances to H<sup>2</sup>

selective sensing to H<sup>2</sup>

porous film where H<sup>2</sup>

O3

O3

proposed a design of an In<sup>2</sup>

shows an ultra-high response value to H<sup>2</sup>

film for the H<sup>2</sup>

the ambient humidity-induced H<sup>2</sup>

O3

ticles have a positive effect on the sensing mechanism of In<sup>2</sup>



colloidal template. This sensing material was used for H<sup>2</sup>

O3

as a sensing material for many gases including H<sup>2</sup>

ated its sensing properties for CO and NO<sup>2</sup>

countries and showed previously.

S detection under environmental operational conditions.

between 100 and 200°C. These sensing materials used for H<sup>2</sup>

nanocuboids sensor. The results also indicate that Pd nanopar-

S) is a toxic, flammable, colorless, and malodorous gas that has

been getting strong attention in the industrial gases sensing field for several years. Therefore,

perature) have been much studied and received superior attentions [99]. In the same order of

nanostructured orderly porous thin film produced by solution-dipping monolayer organic

induced enhanced sensing performance. The results also indicate that the sensing mechanism

of the chemisorbed oxygen and adsorption of water, or (3) even formation of water thin film

Zinc oxide (ZnO) is one of the most auspicious materials for a large number of applications because of its physicochemical properties, where its chemical sensitivity permits to be used

Accordingly, Al-Salmana et al. [100] produced ZnO nanostructure deposited on the PET and quartz substrates, and found that ZnO sensor based on PET substrate has a sensitivity valor

a response of 224. Otherwise, ZnO sensor based on quartz substrate showed high sensitivity (96.29%) at 100°C, with an increase until 99.95% at 250°C and a response value of 2254. The ZnO gas sensors based on PET and quartz substrates showed high sensitivity, stability, and recovery to the initial value of the sensor signal when they were operating at temperatures

nanostructures were stable over several cycles and had a fast response at different operating temperatures. On the other hand, Wagner et al. [101] produced mesoporous ZnO and evalu-

at a relative humidity of 50%. It is well known that the long-term permit-exposure values for the human race without health damage are about 30 ppm for CO and 5 ppm for the NO<sup>2</sup>

This research group indicated that their mesoporous sensing materials can be used for the detection of these gases under concentration bellow of the legal thresholds register in most

surface produced by the effect of the ambient humidity. This research team also

, NO<sup>2</sup> , O<sup>2</sup> , CH<sup>3</sup> CH<sup>2</sup>

gas tested at room temperature with an increase until 99.53% at 200°C with

The results indicated that the humidity has a great effect on the sensing properties of the In<sup>2</sup>

thin films, induced by spray pyrolysis technique, showed a good sensitivity to H<sup>2</sup>

temperature of 50°C [99]. Wang et al. [99] reported the sensing properties of In<sup>2</sup>

nanoparticle film synthetized by hydrothermal method and studied its gas sensing

S at different temperatures above125°C. Such film exhibited a strong and

S at 268°C among the normal gas molecules. In addition, nanocrystal-

S was not detected in ambient without humidity. This sensing material

S at room temperature is due to three potential effect such as (1)

porous thin film-based sensor array with potential uses for the

S hydrolyzation, (2) the hydrolyzation-induced desorption

O3

nanocuboids. On the other

O3

65

O3

S at a lower

micro/

O3

, and LPG [100].

.

S (especially at low working tem-

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

S detection at room temperature.

S gas at 150°C. Also, the nanostructured In<sup>2</sup>

Porous Ceramic Sensors: Hydrocarbon Gas Leaks Detection

S at room temperature with a significant humidity-

OH, NH<sup>3</sup>

gas detection and based on ZnO

detection in a concentration range of 2–10 ppm

Tin oxide (SnO<sup>2</sup> ) is the most versatile oxide used as porous ceramics for sensing hydrocarbon leaks and/or their associated gases. For instance, Wagner et al. [66] produced mesoporous SnO<sup>2</sup> doped with Pd species to be exposed to different gas mixtures at high temperature (600°C) and simulate long term usage. The Pd oxidation state was directly associated to the resistive change of the SnO<sup>2</sup> sensor at different concentrations of methane gas. An important reduction of Pd(II) to Pd(0) was registered for samples evaluated at 5000 ppm of methane in air. The resistive response is affected by the temperature 300°C or 600°C evaluated during the test, and the type of gas used, i.e., synthetic air, pure N<sup>2</sup> , etc. On the other hand, Waitz et al. [68] reveals that mesoporous SnO<sup>2</sup> synthesized by structure replication (nanocasting) from ordered mesoporous KIT-6 silica shows a high thermal stability with no structural loss up to 600°C and only minor decrease in specific surface area by 18% at 800°C. In particular, the samples turn out to be much more stable than porous SnO<sup>2</sup> materials prepared by sol-gel-based synthesis procedures for comparison. The thermal stability facilitates the utilization of the materials as sensors for combustible gases showing promising behavior for the methane (CH<sup>4</sup> ) sensing methodologies. In addition, Ho et al. [64] synthetized SnO<sup>2</sup> with two different microstructures: (1) hierarchical SnO<sup>2</sup> flowers assembled by numerous one-dimensional tetragonal prism nanorods, and (2) SnO<sup>2</sup> sphere architectures formed by numerous smaller particles. The results show that the nanoflowers exhibited higher sensitivities to ethanol than the nanospheres, whereas the typical responses of these sensors to H<sup>2</sup> and LPG indicated that the porous spheres demonstrated better sensing performance than the hierarchical flowers [64]. This research is a main example about how the material's microstructure/morphology could affect the sensing properties of the same material; the results demonstrated the great effect that the microstructures could have on the final gas sensing properties of this metal oxide.

Indium oxide (In2 O3 ), as a typical n-type semiconductor with a band gap of 3.55–3.75 eV, has been investigated extensively in last decade for its applications in diverse areas. For instance, Gong et al. [85] successfully synthesized porous In<sup>2</sup> O3 nanocuboids on a large scale and the sensors made with them exhibit enhanced sensitivity and stability to reducing gases including H<sup>2</sup> S, acetone, ethanol and methanol vapors, after modified with Pd nanoparticles. The results indicate the existence of abundant pores for the aggregations of particles in the materials. The BET surface area of the materials is 36.2 m<sup>2</sup> g−1. As was mentioned, the In<sup>2</sup> O3 and Pd@In<sup>2</sup> O3 nanocuboids were used to produce two different types of chemical sensors. The sensor fabricated with Pd@In<sup>2</sup> O3 was more stable with a higher response to the reducing vapors in comparation to the In2 O3 nanocuboids sensor. The results also indicate that Pd nanoparticles have a positive effect on the sensing mechanism of In<sup>2</sup> O3 nanocuboids. On the other hand, hydrogen sulfide (H<sup>2</sup> S) is a toxic, flammable, colorless, and malodorous gas that has been getting strong attention in the industrial gases sensing field for several years. Therefore, In2 O3 -based sensing film and its sensing performances to H<sup>2</sup> S (especially at low working temperature) have been much studied and received superior attentions [99]. In the same order of idea, In2 O3 nanoparticle film synthetized by hydrothermal method and studied its gas sensing performances to H<sup>2</sup> S at different temperatures above125°C. Such film exhibited a strong and selective sensing to H<sup>2</sup> S at 268°C among the normal gas molecules. In addition, nanocrystalline In2 O3 -based films had a high response to H<sup>2</sup> S gas at 150°C. Also, the nanostructured In<sup>2</sup> O3 thin films, induced by spray pyrolysis technique, showed a good sensitivity to H<sup>2</sup> S at a lower temperature of 50°C [99]. Wang et al. [99] reported the sensing properties of In<sup>2</sup> O3 micro/ nanostructured orderly porous thin film produced by solution-dipping monolayer organic colloidal template. This sensing material was used for H<sup>2</sup> S detection at room temperature. The results indicated that the humidity has a great effect on the sensing properties of the In<sup>2</sup> O3 porous film where H<sup>2</sup> S was not detected in ambient without humidity. This sensing material shows an ultra-high response value to H<sup>2</sup> S at room temperature with a significant humidityinduced enhanced sensing performance. The results also indicate that the sensing mechanism of the In<sup>2</sup> O3 film for the H<sup>2</sup> S at room temperature is due to three potential effect such as (1) the ambient humidity-induced H<sup>2</sup> S hydrolyzation, (2) the hydrolyzation-induced desorption of the chemisorbed oxygen and adsorption of water, or (3) even formation of water thin film on the In2 O3 surface produced by the effect of the ambient humidity. This research team also proposed a design of an In<sup>2</sup> O3 porous thin film-based sensor array with potential uses for the H2 S detection under environmental operational conditions.

ceramic metal oxides are found to be a good choice as humidity sensing materials due to their properties such as high mechanical strength, good physical and chemical stabilities, fast response and recovery times, and wide range of operating temperatures. Semiconducting metal oxide sensors are the widely studied chemiresistive sensors. Recently, nanostructures of semiconductor metal oxides have received considerable interest in the fabrication of humidity and gas sensors due to their high surface-to-volume ratio of atoms, excellent surface reactivity, and the ability to tailor their surface and charge transport properties. Hence, they are

nanofibers, ZnSnO<sup>3</sup>

leaks and/or their associated gases. For instance, Wagner et al. [66] produced mesoporous SnO<sup>2</sup> doped with Pd species to be exposed to different gas mixtures at high temperature (600°C) and simulate long term usage. The Pd oxidation state was directly associated to the resistive

of Pd(II) to Pd(0) was registered for samples evaluated at 5000 ppm of methane in air. The resistive response is affected by the temperature 300°C or 600°C evaluated during the test, and

porous KIT-6 silica shows a high thermal stability with no structural loss up to 600°C and only minor decrease in specific surface area by 18% at 800°C. In particular, the samples turn out to

dures for comparison. The thermal stability facilitates the utilization of the materials as sensors

nanoflowers exhibited higher sensitivities to ethanol than the nanospheres, whereas the typi-

better sensing performance than the hierarchical flowers [64]. This research is a main example about how the material's microstructure/morphology could affect the sensing properties of the same material; the results demonstrated the great effect that the microstructures could

been investigated extensively in last decade for its applications in diverse areas. For instance,

sensors made with them exhibit enhanced sensitivity and stability to reducing gases includ-

rials. The BET surface area of the materials is 36.2 m<sup>2</sup> g−1. As was mentioned, the In<sup>2</sup>

S, acetone, ethanol and methanol vapors, after modified with Pd nanoparticles. The results indicate the existence of abundant pores for the aggregations of particles in the mate-

nanocuboids were used to produce two different types of chemical sensors. The sen-

flowers assembled by numerous one-dimensional tetragonal prism nanorods, and

), as a typical n-type semiconductor with a band gap of 3.55–3.75 eV, has

was more stable with a higher response to the reducing vapors

O3

sphere architectures formed by numerous smaller particles. The results show that the

) is the most versatile oxide used as porous ceramics for sensing hydrocarbon

sensor at different concentrations of methane gas. An important reduction

synthesized by structure replication (nanocasting) from ordered meso-

nanowires, ZrO<sup>2</sup>

, etc. On the other hand, Waitz et al. [68] reveals

with two different microstructures: (1) hierar-

nanocuboids on a large scale and the

materials prepared by sol-gel-based synthesis proce-

and LPG indicated that the porous spheres demonstrated

nanocubes, among others constitute

nanorods, Al2

) sensing methodolo-

O3 and

O3

considered as ideal candidates as humidity sensors. SnO<sup>2</sup>

the recently explored nanostructured metal oxides to this effect [98].

for combustible gases showing promising behavior for the methane (CH<sup>4</sup>

nanotubes, BaTiO<sup>3</sup>

the type of gas used, i.e., synthetic air, pure N<sup>2</sup>

gies. In addition, Ho et al. [64] synthetized SnO<sup>2</sup>

have on the final gas sensing properties of this metal oxide.

Gong et al. [85] successfully synthesized porous In<sup>2</sup>

O3

be much more stable than porous SnO<sup>2</sup>

cal responses of these sensors to H<sup>2</sup>

O3

nanowires, TiO<sup>2</sup>

64 Recent Advances in Porous Ceramics

Tin oxide (SnO<sup>2</sup>

change of the SnO<sup>2</sup>

that mesoporous SnO<sup>2</sup>

chical SnO<sup>2</sup>

Indium oxide (In2

sor fabricated with Pd@In<sup>2</sup>

(2) SnO<sup>2</sup>

ing H<sup>2</sup>

Pd@In<sup>2</sup> O3 Zinc oxide (ZnO) is one of the most auspicious materials for a large number of applications because of its physicochemical properties, where its chemical sensitivity permits to be used as a sensing material for many gases including H<sup>2</sup> , NO<sup>2</sup> , O<sup>2</sup> , CH<sup>3</sup> CH<sup>2</sup> OH, NH<sup>3</sup> , and LPG [100]. Accordingly, Al-Salmana et al. [100] produced ZnO nanostructure deposited on the PET and quartz substrates, and found that ZnO sensor based on PET substrate has a sensitivity valor of 24.8% for H<sup>2</sup> gas tested at room temperature with an increase until 99.53% at 200°C with a response of 224. Otherwise, ZnO sensor based on quartz substrate showed high sensitivity (96.29%) at 100°C, with an increase until 99.95% at 250°C and a response value of 2254. The ZnO gas sensors based on PET and quartz substrates showed high sensitivity, stability, and recovery to the initial value of the sensor signal when they were operating at temperatures between 100 and 200°C. These sensing materials used for H<sup>2</sup> gas detection and based on ZnO nanostructures were stable over several cycles and had a fast response at different operating temperatures. On the other hand, Wagner et al. [101] produced mesoporous ZnO and evaluated its sensing properties for CO and NO<sup>2</sup> detection in a concentration range of 2–10 ppm at a relative humidity of 50%. It is well known that the long-term permit-exposure values for the human race without health damage are about 30 ppm for CO and 5 ppm for the NO<sup>2</sup> . This research group indicated that their mesoporous sensing materials can be used for the detection of these gases under concentration bellow of the legal thresholds register in most countries and showed previously.

Other kind of porous ceramics materials have been investigated for hydrocarbons gases detection. For instance, Picasso et al. [65] have produced sensors based on nanoparticles of α-Fe<sup>2</sup> O3 doped with different amounts of Pd ranging from 0.1 to 1.0 wt.% for liquefied petroleum gas (LPG) detection. They demonstrated that the sample of Pd-doped sensors showed much higher sensitivity than the undoped one revealing the promotion electronic effect of Pd2+ on the surface reaction. Among all samples, the sensor with 0.75 wt.% Pd presented the highest gas response at 300°C in all gas tested concentrations, likely due to the highest BET surface, well-defined hematite crystalline structure and best surface contact over Pd surface via electronic mechanism. On the other hand, Xiaoqing Li et al. [29] produced mesoporous NiO NWs by using SBA-15 silica as the hard templates with the nanocasting method under the calcination temperature between 550 and 750°C. All results showed that all samples exhibited the best response to ethanol gas. The specific surface area decreased with the increasing calcination temperature, while crystallization degree and bandgap increased. Owing to the suitable specific surface area, crystallization degree and bandgap at the calcination temperature of 650°C, mesoporous NiO NWs-650 exhibited the best gas-sensing performance. For ethanol detection Co<sup>3</sup> O4 nanoneedle arrays were successfully fabricated via a facile two-step approach, including the formation of needle-shaped Co(CO<sup>3</sup> )0.5(OH)·0.11H<sup>2</sup> O followed by thermal conversion to mesoporous Co<sup>3</sup> O4 . The highest sensitivity reached ∼89.6 for 100 ppm ethanol vapor and the optimal working temperature was as low as 130°C [87].

The book chapter mentions the principle of the hydrocarbon leaks and the advances in the production and applications of micro- and nanoporous ceramics for hydrocarbon leaks under diverse environmental conditions (e.g., humidity and low temperatures) that also affects the sensing capability of these materials. Finally, sensing of hydrocarbon leaks at low temperatures and high humidity conditions is clearly identified as niche area that requests future

Dr. Y.A. Perera-Mercado is grateful to the West Houston Center for Science and Engineering Center (WHC) at Houston Community College System (HCCS); and Dr. G. Castruita-de Leon is thankful to Cátedras-CONACYT for the support received from both organizations in order to write this book chapter. Finally, the publication charges for this article have been funded by

a grant from the publication fund of UiT The Arctic University of Norway.

\*Address all correspondence to: yibranpereramercado@gmail.com

\*, Griselda Castruita-de Leon<sup>2</sup>

1 West Houston Center for Science & Engineering (WHC) at Houston Community College

2 CONACYT—Research Center for Applied Chemistry (CIQA), Saltillo, Coahuila, México 3 The Arctic University of Norway, Faculty of Engineering and Technology, Institute of

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, C<sup>2</sup> H6 , C<sup>2</sup> H4 as

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**Author details**

**References**

Yibran A. Perera-Mercado<sup>1</sup>

System (HCCS), Houston, Texas, USA

Industrial Technology, Narvik, Norway

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DOI: 10.1039/c6cs00060f

**Acknowledgements**

In addition, some of the most sensing materials used for humidity conditions are based on metal oxides, spinel- and perovskite-type oxides and/or thereof combination. Basically, the physicochemical properties of these materials allow them to detect humidity in gaseous media. The sensing mechanism of ceramic humidity sensors is based on water adsorption on the ceramic surface. The microstructure of these ceramic materials integrated by grains, porous, and their crystalline or non-crystalline phases support the sensing mechanism process. Hence, these kinds of sensors are based on the mechanical or electrical change due to bulk and/or surface modifications of the sensing materials with water adsorption [102]. Finally, the present state-of-art indicates that there are just a few publications related to the sensing hydrocarbons and/or their associated gases at low temperature and high humidity indicating a great niche for future researches.

#### **6. Conclusion**

In the recent past, a great deal of research efforts were directed toward the development of advanced ceramics porous materials due to their sensing properties and potential application for sensing hydrocarbons and/or their associated gases leaks. Among the various techniques that are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. The oxides that are covered in this study include oxides of aluminum, silicon, bismuth, cerium, chromium, cobalt, copper, indium, iron, nickel, niobium, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or multi-component metal oxides. The cover hydrocarbons and their associated gases are liquefied petroleum gas (LPG), CH<sup>4</sup> , ethanol (C<sup>2</sup> H6 O), methanol (CH<sup>3</sup> OH), acetone (C<sup>3</sup> H6 O), H<sup>2</sup> , NH<sup>3</sup> , CO, H<sup>2</sup> S, NO<sup>x</sup> , among others. The book chapter mentions the principle of the hydrocarbon leaks and the advances in the production and applications of micro- and nanoporous ceramics for hydrocarbon leaks under diverse environmental conditions (e.g., humidity and low temperatures) that also affects the sensing capability of these materials. Finally, sensing of hydrocarbon leaks at low temperatures and high humidity conditions is clearly identified as niche area that requests future research efforts.
