**3.4 Nanoporous TiO2**

In 2018, Shi et al. prepared nanoporous TiO2 by dealloying an Al-Si-Ti precursor alloy and calcining [16]. The Al61Si30Ti9 precursor alloy was prepared from pure Al, Ti, and silicon (Si) by arc-melting. The melt-spun ribbons were obtained by melt spinning at 33 m s<sup>−</sup><sup>1</sup> and protected by 0.05 MPa Ar. The as-quenched ribbons were dealloyed in aqueous 10% NaOH at 80°C. Subsequently, the ribbons were immersed in aqueous 3% (by weight) hydrogen chloride (HCl) at an ambient temperature for 7 h. The samples were then calcined at 400°C for 2 h. The results show that nanoporous TiO2 exhibits a laminated sheet morphology, and the irregular sheets show a pore-ligament structure. The structure had large specific surface areas, small pore diameters, and large pore volumes. The measurements also revealed that nanoporous TiO2 had favorable photocatalytic performance in degradation of methyl orange (MO).

#### **3.5 Rare earth oxides**

Due to the special electronic structure of rare earth elements, which have unique physical and chemical properties, and the continuous development of nanotechnology, the preparation of nanoporous rare earth oxides has attracted increasing attention [29, 30]. Rare earth oxides such as CeO2 have heterogeneous catalytic ability and excellent storage/deoxygenation capacity, which is optimal for supporting noble metal nanoparticle carriers [31, 32]. Nanoporous rare earth oxides have a wide range of applications in energy, chemical, environmental, and magnetic materials.

In 2017, Zhang et al. reported a CeO2 nanorod framework synthesized via dealloying Al-Ce alloys coupled with calcination treatment [5]. The precursor alloy was Al90Ce10, and ribbons were prepared by melt spinning at 33 m s<sup>−</sup><sup>1</sup> and protected by 0.05 MPa Ar. The precursor alloy ribbons were dealloyed in 20 wt% NaOH solution at 80°C. The dealloyed samples were heat treated at 400°C in O2. After dealloying, the CeO2 particles grew into nanorods with a diameter of approximately 20 nm. The nanorods piled up into frameworks containing pores. The nanorods and pores were

**13**

*Nanoporous Oxides and Nanoporous Composites DOI: http://dx.doi.org/10.5772/intechopen.82028*

surface area [33].

rous materials.

performance are reviewed.

**4.1 Nanoporous Ag-CeO2**

homogeneously distributed and formed the framework structure. The material presented favorable CO catalytic oxidization properties: the reaction temperature for catalyzing 50% of CO was approximately 300°C, and the temperature for completely converting CO was 440°C. In 2018, Wang et al. reported that the CeO2 nanorod framework also exhibits a high-specific capacitance and superior charge/ discharge stability, which are mainly ascribed to its high-Brunauer-Emmett-Teller

In 2018, Duan et al. prepared nanorod samarium oxide (Sm2O3) by dealloying Al-Sm alloy and calcining in O2 [13]. The preparation is the same as in the CeO2 framework [5]. Similar to CeO2, Sm2O3 also exhibits uniform nanorods, measuring approximately 20–30 nm in diameter, and a framework structure. The loose nanorod framework structure provides efficient transport of reactive gases and sufficient space for the activation of molecular oxygen, which improves catalytic performance. The Sm2O3 nanorod framework shows good performance for the catalytic oxidation of CO. The 50% CO conversion temperature was approximately

The above results show that a reasonable precursor alloy design and a combination of dealloying and calcination processes can be used to generate nanoporous oxide structures with ideal structural parameters. These materials have favorable catalytic properties and can act as a support for noble metals or oxides, providing good conditions for the development of higher performance nanopo-

Nanoporous composites in which the noble ligaments are loaded with metallic oxides have recently been investigated. These composites include nanoporous Ag-TiO2, Ag-CeO2, Pd-NiO, and Pd-TiO2 and exhibit improved catalytic properties compared with pure nanoporous nobles. In this section, their structures and

Nanoporous Ag-CeO2 ribbons have been prepared through dealloying meltspun Al79.5Ag20Ce0.5 alloy in aqueous 5 wt% NaOH, followed by calcining in air [8]. Precursor ingots were obtained by melting the alloys composed of pure Al, Ag,

0.05 MPa Ar. The melt-spun ribbons were dealloyed in aqueous 5 wt% NaOH at room temperature. The dealloyed ribbons were calcined for 2 h at 300–700°C. The SEM analyses indicated that the morphologies were not evaluated due to the addition of Ce in the precursor alloy. However, the CeO2 particles were distributed on the surface of the Ag ligaments, and a composite with CeO2 loaded on the nanoporous Ag was obtained. The electrochemical tests revealed that the nanoporous Ag-CeO2 catalyst exhibited enhanced activity for the direct oxidation of sodium borohydride. The Raman analysis indicated that the intensity of the peak in the 2LO band (CeO2) is stronger than that in F2g and D bands (CeO2) [8]. Therefore, the superior catalytic activity is attributed to the enhancement of the interfacial interaction between Ag and CeO2. This research provided a novel method for preparing noble metal-oxide nanocomposites with high catalytic performance. The results also indicated that the structural stability of Ag-CeO2 ribbons with a homogeneous pore/grain structure is improved due to the formation of the Ag-CeO2 composite.

and protected by

and Ce. The ribbons were prepared by melt spinning at 33 m s<sup>−</sup><sup>1</sup>

This may aid in enhancing the longevity performance.

280°C, and 100% CO conversion occurred at approximately 360°C.

**4. Noble ligaments loaded with metallic oxides**

*Nanoporous Oxides and Nanoporous Composites DOI: http://dx.doi.org/10.5772/intechopen.82028*

*Nanofluid Flow in Porous Media*

In 2011, Xu et al. employed a simple fabrication method for Co3O4 nanosheets through dealloying Al-Co alloy in alkaline solutions [17]. The precursor alloy was Al95Co5, and ribbons were prepared by melt spinning. The microstructure was a hierarchical flower-like aggregate structure with the typical size at the micron scale, where each nanoflower is composed of many irregular interlaced nanoslices with thicknesses as small as 6 nm, which is a typical porous structure. The results indicated that Co3O4 nanosheets exhibited excellent catalytic activity for CO oxidation at ambient temperature, the reaction temperature for catalyzing 50% of the CO was approximately 80°C, and the temperature for completely converting the CO was 140°C. Calcination in an O2 atmosphere was essential to achieve high CO oxidation activity for these nanostructures, which allowed for the generation of active species for surface reactions. In addition, the calcination temperature significantly affected the catalytic activity; 300°C was a more favorable calcination temperature than 200 or 450°C, considering the optimum balance between the active reaction species and the surface area upon calcination. In addition, Co3O4 nanosheets showed good time-onstream catalytic stability. It is expected that many other useful metal oxide materials can be fabricated similarly. Due to the evident advantages of simple processing, nearly perfect yield, and low fabrication cost, these functional nanoma-

In 2018, Shi et al. prepared nanoporous TiO2 by dealloying an Al-Si-Ti precursor alloy and calcining [16]. The Al61Si30Ti9 precursor alloy was prepared from pure Al, Ti, and silicon (Si) by arc-melting. The melt-spun ribbons were obtained by melt

dealloyed in aqueous 10% NaOH at 80°C. Subsequently, the ribbons were immersed in aqueous 3% (by weight) hydrogen chloride (HCl) at an ambient temperature for 7 h. The samples were then calcined at 400°C for 2 h. The results show that nanoporous TiO2 exhibits a laminated sheet morphology, and the irregular sheets show a pore-ligament structure. The structure had large specific surface areas, small pore diameters, and large pore volumes. The measurements also revealed that nanoporous TiO2 had favorable photocatalytic performance in degradation of methyl

Due to the special electronic structure of rare earth elements, which have unique physical and chemical properties, and the continuous development of nanotechnology, the preparation of nanoporous rare earth oxides has attracted increasing attention [29, 30]. Rare earth oxides such as CeO2 have heterogeneous catalytic ability and excellent storage/deoxygenation capacity, which is optimal for supporting noble metal nanoparticle carriers [31, 32]. Nanoporous rare earth oxides have a wide range of applications in energy, chemical, environmental, and magnetic materials. In 2017, Zhang et al. reported a CeO2 nanorod framework synthesized via dealloying Al-Ce alloys coupled with calcination treatment [5]. The precursor alloy was

0.05 MPa Ar. The precursor alloy ribbons were dealloyed in 20 wt% NaOH solution at 80°C. The dealloyed samples were heat treated at 400°C in O2. After dealloying, the CeO2 particles grew into nanorods with a diameter of approximately 20 nm. The nanorods piled up into frameworks containing pores. The nanorods and pores were

Al90Ce10, and ribbons were prepared by melt spinning at 33 m s<sup>−</sup><sup>1</sup>

and protected by 0.05 MPa Ar. The as-quenched ribbons were

and protected by

terials may lead to applications in various catalytic processes.

**3.3 Nanoporous Co3O4**

**3.4 Nanoporous TiO2**

spinning at 33 m s<sup>−</sup><sup>1</sup>

orange (MO).

**3.5 Rare earth oxides**

**12**

homogeneously distributed and formed the framework structure. The material presented favorable CO catalytic oxidization properties: the reaction temperature for catalyzing 50% of CO was approximately 300°C, and the temperature for completely converting CO was 440°C. In 2018, Wang et al. reported that the CeO2 nanorod framework also exhibits a high-specific capacitance and superior charge/ discharge stability, which are mainly ascribed to its high-Brunauer-Emmett-Teller surface area [33].

In 2018, Duan et al. prepared nanorod samarium oxide (Sm2O3) by dealloying Al-Sm alloy and calcining in O2 [13]. The preparation is the same as in the CeO2 framework [5]. Similar to CeO2, Sm2O3 also exhibits uniform nanorods, measuring approximately 20–30 nm in diameter, and a framework structure. The loose nanorod framework structure provides efficient transport of reactive gases and sufficient space for the activation of molecular oxygen, which improves catalytic performance. The Sm2O3 nanorod framework shows good performance for the catalytic oxidation of CO. The 50% CO conversion temperature was approximately 280°C, and 100% CO conversion occurred at approximately 360°C.

The above results show that a reasonable precursor alloy design and a combination of dealloying and calcination processes can be used to generate nanoporous oxide structures with ideal structural parameters. These materials have favorable catalytic properties and can act as a support for noble metals or oxides, providing good conditions for the development of higher performance nanoporous materials.
