Nanoporous Oxides and Nanoporous Composites

*Dong Duan, Haiyang Wang, Wenyu Shi and Zhanbo Sun*

### **Abstract**

Nanoporous oxides, such as cupric oxide (CuO), nickelous oxide (NiO), titanium dioxide (TiO2), cobaltosic oxide (Co3O4), and cerium oxide (CeO2), and noble-metal-based nanoporous composites, such as silver (Ag) ligaments loaded with CeO2, TiO2, zirconium dioxide (ZrO2) or NiO and palladium (Pd) ligaments loaded with TiO2 or ZrO2, are described in the chapter. Oxide-based nanoporous composites, such as Au loaded on CuO and CeO2 or platinum (Pt) loaded on TiO2, are also summarized. The structures, microstructures, and microstructure parameters of these materials are reviewed. The performance of the noble-based nanoporous composites is presented, including the catalytic oxidation of methanol and ethanol. Environmental protection applications, such as catalytic oxidation of carbon monoxide (CO) for the oxide-based nanoporous composites, have also been developed. Applications of rare earth elements in nanoporous materials are also reviewed.

**Keywords:** precursor alloy, dealloying, nanoporous oxides, nanoporous composites, rare earth elements

#### **1. Introduction**

Research on nanoporous materials prepared by dealloying originated from work with nanoporous noble metals. During dealloying, the active metals in precursor alloys composed of corrosion-resistant noble metals, such as Ag and gold (Au), and cheaper metals, such as manganese (Mn), zinc (Zn), aluminum (Al), and magnesium (Mg), were selectively removed. Simultaneously, ligaments of corrosion-resistant noble metals and pores were formed [1]. At the nanoscale, both the preparation and the performance of noble metals have unparalleled advantages due to the excellent catalytic activity and chemical stability. However, the high price and the scarce resources of noble metals have prompted investigation of new paths to reduce their usage, resulting in the formation of alloy nanoporous materials containing less noble metals [2].

Certain metals, such as copper (Cu), nickel (Ni), and cobalt (Co), can remain in dilute acid or alkali solutions and maintain acceptable catalytic properties. These metals are potential substitutes for noble metals. Therefore, the preparation of cheaper nanoporous metals, such as Cu, was investigated [3]. However, it was found that the nanoscale ligaments inevitably oxidized in air or corrodents due to small scale and large surface effects after dealloying, which means that the materials must be kept in a special environment. These materials cannot be used in ideal

environmental conditions, which will inevitably lead to their limited use. Certain nanooxides, such as CuO, NiO, cobaltic oxide (Co2O3), and CeO2, have acceptable catalytic activities. In acid and alkali solutions, certain stable metals, such as Cu, Ni, and Co, can form nanoporous oxides after further oxidation [4–6]. These metals could be used to produce nanoporous composites with excellent catalytic performance after they are loaded with noble metals.

Research [7] has shown that the interface between the noble metal and loaded oxides and the interaction between the two phases can significantly enhance the catalytic activity at the nanoscale. Certain oxides, such as CeO2, could prevent poisoning and failure of noble metals when loaded on the surface of noble metals. To improve the performance of noble metals, the proper amount of oxides should be loaded on the ligament surface of the noble metal. The direct solution mechanism is to infiltrate the target metal saline solution into the nanoporous metals. The metal ions are transformed into oxides after drying or dewatering. However, the added matter concentrates on the surface layer of the sample during drying of the aqueous solution, causing little oxide to be distributed in the inner region. In 2013, Li et al. added cerium (Ce) to the Al-Ag precursor alloy. With the removal of Al elements during corrosion, Ce compounds are formed during the formation of nanoporous Ag. After heat treatment in air or oxygen (O2), CeO2 is formed and loaded on the surface of Ag ligaments to create a nanoporous composite material of Ag ligament loaded with CeO2 [8]. This material's performance was significantly better than that of pure nanoporous Ag. Nanoporous Ag loaded with TiO2 [9] and ZrO2 [10] and nanoporous Pd loaded with NiO [11] and TiO2 [12] have also been prepared.

Certain metals, such as Ce and titanium (Ti), are active elements. However, they do not evolve into ions that are removed in alkaline solutions, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH). Instead, they evolve into solid compounds containing water, and the nanoporous structure is formed after crystallization. Some of them, such as Ce and samarium (Sm), also form nanorods because the crystallization has a clear direction. After high-temperature dehydration, they evolved into rare earth oxide nanorods [5, 13]. The nanorods support each other and form a supporting pore structure or framework with a large number of pores. If noble metals, such as Au and Ag, are added to the precursor alloys in small quantities, the noble metal nanoparticles or atoms are loaded on the surface of the nanorods. These metals presented excellent gas catalytic activity [6, 14].

Focusing on the work of our group, this chapter introduces the recent development of nanoporous oxides and nanoporous composites prepared by dealloying. The design principle of the precursor alloys and preparation process and dealloying methods and effects of the preparation process on the morphologies and physical parameters of nanoporous oxides and nanoporous composites are reviewed.

### **2. Design and preparation of precursor alloys and the dealloying process**

Generally, the more active elements, that is, the elements removed by corrosion in the precursor alloys, are the higher the porosity of the nanoporous material is and the thinner the ligaments are after dealloying. However, if the stable metal content is too low, that is, less than 5 at%, the nanoporous structure may not form. Therefore, a stable metal content of 8–15 at% is recommended [4–14]. In addition, it is important to consider whether the components can form alloys. If the solubility between the components is not good in the melt, it is likely that the alloy will have a serious tendency to macrosegregate, and the ideal nanoporous structure cannot form after dealloying. Some systems, such as the Al-Ti system, have a large temperature difference between the liquidus and the solidus on the Al-rich side (when

**9**

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

structure of the nanoporous Ag was obviously refined.

into thin ribbons at cooling rates of 106

10% Ti is used, the distance between liquidus and the solidus is approximately 585°C), which would result in a serious macrosegregation tendency. Therefore, the Ti content should be minimized if possible. Thus, a eutectic system, such as Al-Cu, is an ideal precursor alloy. For some systems, such as Al-Ce and Al-Pt systems, the temperature difference between the liquidus and solidus is not very large. The melt quenching can be used to prepare precursor alloys with homogeneous composition. In addition, systems with a little or no intermetallic compounds in the precursor alloys are suitable for preparing nanoporous materials by dealloying. Some intermetallic compounds, such as Ag2Al in the Al-Ag system, do not effectively decompose in alkaline solutions at room temperature, resulting in residual Ag2Al and decreased Ag content in the nanoporous structures [8–10]. The properties of the nanoporous structures were less optimal. Certain intermetallic compounds, such as Al2Cu, easily decompose in corrosion fluids, and its content has little effect on nanostructures. If a binary alloy cannot form an ideal nanoporous material, a third component could be added to the binary system. However, the third component should be completely removed in the dealloying. For example, Li et al. added an appropriate amount of rare earth element Ce to Cu-Ag alloys, resulting in significant microstructure refinement of the precursor alloys [15]. After electrochemical dealloy removal in aqueous cupric sulfate (CuSO4), all of the Al and Ce were removed, and the micro-

For nanoporous oxides, binary precursor alloy systems, such as Al-Cu [3, 4], Al-Ni [5], and Al-Ce [5], can be used. For the noble metal-loaded oxide systems, Al-noble metal-oxide formation elements can be used to form ternary alloy systems, such as Al-Ag-Ce [8] and Al-Pd-Ni [11]. Their noble metal content is approximately 10–15%, and the oxide-forming element content is generally less than 2 at%. In the dealloying, elemental Al is dissolved or removed and the two other elements remain. Using components with higher noble metal content, noble metal would form ligaments. After heat treatment, oxide particles form and load on the surface of the ligaments. In oxide-loading noble metal systems, the precursor alloys are Al-oxide formation element-noble metal ternary systems, such as Al-Cu-Au [4], Al-Ce-Au [14], and Al-Ti-Pt [16]. Their oxide-formation element content is approximately 8–10%, and their noble metal content is less than 2%. In the dealloying, Cu, Ce, and Ti are oxidized into oxides and form ligaments. The noble metals are loaded on the surface of ligaments after forming nanoparticles. Intermediate alloy ingots can be synthesized by melting and powder metallurgy. Melting is the most recommended method. However, the direct dealloying of alloy ingots results in a longer dealloying time due to their larger ingot size. The size of ligaments and holes might be too large. Some systems, such as Cu-Ag and Al-Cu systems, can be made into thin strips by cold rolling. However, other systems, such as the Al-Ti system, do not allow for creating a high-quality precursor by cold rolling due to poor ductility. Thus, it is necessary to adopt melt spinning. Melt spinning is a rapid solidification process. In this method, alloy melts can directly solidify or evolve

–109

the system might not be an optimal precursor alloy for nanoporous materials.

thickness of the thin ribbons can be controlled from 20 to 100 μm, and the width can be adjusted. For most alloys, alloy ribbons with a highly homogeneous distribution of elements can be obtained by melt spinning or melt quenching. Binary Al-Ti and Al-Ce and ternary Al-Ti-Pt and Al-Ce-Au systems could be prepared into high-quality precursor ribbons. If the required thin ribbons cannot be prepared in this manner,

Hydrochloric acid or aqueous KOH or NaOH is often used as dealloying solvents. The dealloying temperature should be room temperature. However, at higher temperatures such as 80°C, nanorod support pore structures can form [5, 6, 14] in the Al-Ce precursor system. Thus, the dealloying temperature is important for products

K/s. Based on the conditions adopted, the

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

*Nanofluid Flow in Porous Media*

mance after they are loaded with noble metals.

environmental conditions, which will inevitably lead to their limited use. Certain nanooxides, such as CuO, NiO, cobaltic oxide (Co2O3), and CeO2, have acceptable catalytic activities. In acid and alkali solutions, certain stable metals, such as Cu, Ni, and Co, can form nanoporous oxides after further oxidation [4–6]. These metals could be used to produce nanoporous composites with excellent catalytic perfor-

Research [7] has shown that the interface between the noble metal and loaded oxides and the interaction between the two phases can significantly enhance the catalytic activity at the nanoscale. Certain oxides, such as CeO2, could prevent poisoning and failure of noble metals when loaded on the surface of noble metals. To improve the performance of noble metals, the proper amount of oxides should be loaded on the ligament surface of the noble metal. The direct solution mechanism is to infiltrate the target metal saline solution into the nanoporous metals. The metal ions are transformed into oxides after drying or dewatering. However, the added matter concentrates on the surface layer of the sample during drying of the aqueous solution, causing little oxide to be distributed in the inner region. In 2013, Li et al. added cerium (Ce) to the Al-Ag precursor alloy. With the removal of Al elements during corrosion, Ce compounds are formed during the formation of nanoporous Ag. After heat treatment in air or oxygen (O2), CeO2 is formed and loaded on the surface of Ag ligaments to create a nanoporous composite material of Ag ligament loaded with CeO2 [8]. This material's performance was significantly better than that of pure nanoporous Ag. Nanoporous Ag loaded with TiO2 [9] and ZrO2 [10] and nanoporous Pd loaded with NiO [11] and TiO2 [12] have also been prepared. Certain metals, such as Ce and titanium (Ti), are active elements. However, they do not evolve into ions that are removed in alkaline solutions, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH). Instead, they evolve into solid compounds containing water, and the nanoporous structure is formed after crystallization. Some of them, such as Ce and samarium (Sm), also form nanorods because the crystallization has a clear direction. After high-temperature dehydration, they evolved into rare earth oxide nanorods [5, 13]. The nanorods support each other and form a supporting pore structure or framework with a large number of pores. If noble metals, such as Au and Ag, are added to the precursor alloys in small quantities, the noble metal nanoparticles or atoms are loaded on the surface of the

nanorods. These metals presented excellent gas catalytic activity [6, 14].

Focusing on the work of our group, this chapter introduces the recent development of nanoporous oxides and nanoporous composites prepared by dealloying. The design principle of the precursor alloys and preparation process and dealloying methods and effects of the preparation process on the morphologies and physical parameters of nanoporous oxides and nanoporous composites are reviewed.

**2. Design and preparation of precursor alloys and the dealloying process**

Generally, the more active elements, that is, the elements removed by corrosion in the precursor alloys, are the higher the porosity of the nanoporous material is and the thinner the ligaments are after dealloying. However, if the stable metal content is too low, that is, less than 5 at%, the nanoporous structure may not form. Therefore, a stable metal content of 8–15 at% is recommended [4–14]. In addition, it is important to consider whether the components can form alloys. If the solubility between the components is not good in the melt, it is likely that the alloy will have a serious tendency to macrosegregate, and the ideal nanoporous structure cannot form after dealloying. Some systems, such as the Al-Ti system, have a large temperature difference between the liquidus and the solidus on the Al-rich side (when

**8**

10% Ti is used, the distance between liquidus and the solidus is approximately 585°C), which would result in a serious macrosegregation tendency. Therefore, the Ti content should be minimized if possible. Thus, a eutectic system, such as Al-Cu, is an ideal precursor alloy. For some systems, such as Al-Ce and Al-Pt systems, the temperature difference between the liquidus and solidus is not very large. The melt quenching can be used to prepare precursor alloys with homogeneous composition. In addition, systems with a little or no intermetallic compounds in the precursor alloys are suitable for preparing nanoporous materials by dealloying. Some intermetallic compounds, such as Ag2Al in the Al-Ag system, do not effectively decompose in alkaline solutions at room temperature, resulting in residual Ag2Al and decreased Ag content in the nanoporous structures [8–10]. The properties of the nanoporous structures were less optimal. Certain intermetallic compounds, such as Al2Cu, easily decompose in corrosion fluids, and its content has little effect on nanostructures. If a binary alloy cannot form an ideal nanoporous material, a third component could be added to the binary system. However, the third component should be completely removed in the dealloying. For example, Li et al. added an appropriate amount of rare earth element Ce to Cu-Ag alloys, resulting in significant microstructure refinement of the precursor alloys [15]. After electrochemical dealloy removal in aqueous cupric sulfate (CuSO4), all of the Al and Ce were removed, and the microstructure of the nanoporous Ag was obviously refined.

For nanoporous oxides, binary precursor alloy systems, such as Al-Cu [3, 4], Al-Ni [5], and Al-Ce [5], can be used. For the noble metal-loaded oxide systems, Al-noble metal-oxide formation elements can be used to form ternary alloy systems, such as Al-Ag-Ce [8] and Al-Pd-Ni [11]. Their noble metal content is approximately 10–15%, and the oxide-forming element content is generally less than 2 at%. In the dealloying, elemental Al is dissolved or removed and the two other elements remain. Using components with higher noble metal content, noble metal would form ligaments. After heat treatment, oxide particles form and load on the surface of the ligaments. In oxide-loading noble metal systems, the precursor alloys are Al-oxide formation element-noble metal ternary systems, such as Al-Cu-Au [4], Al-Ce-Au [14], and Al-Ti-Pt [16]. Their oxide-formation element content is approximately 8–10%, and their noble metal content is less than 2%. In the dealloying, Cu, Ce, and Ti are oxidized into oxides and form ligaments. The noble metals are loaded on the surface of ligaments after forming nanoparticles.

Intermediate alloy ingots can be synthesized by melting and powder metallurgy. Melting is the most recommended method. However, the direct dealloying of alloy ingots results in a longer dealloying time due to their larger ingot size. The size of ligaments and holes might be too large. Some systems, such as Cu-Ag and Al-Cu systems, can be made into thin strips by cold rolling. However, other systems, such as the Al-Ti system, do not allow for creating a high-quality precursor by cold rolling due to poor ductility. Thus, it is necessary to adopt melt spinning. Melt spinning is a rapid solidification process. In this method, alloy melts can directly solidify or evolve into thin ribbons at cooling rates of 106 –109 K/s. Based on the conditions adopted, the thickness of the thin ribbons can be controlled from 20 to 100 μm, and the width can be adjusted. For most alloys, alloy ribbons with a highly homogeneous distribution of elements can be obtained by melt spinning or melt quenching. Binary Al-Ti and Al-Ce and ternary Al-Ti-Pt and Al-Ce-Au systems could be prepared into high-quality precursor ribbons. If the required thin ribbons cannot be prepared in this manner, the system might not be an optimal precursor alloy for nanoporous materials.

Hydrochloric acid or aqueous KOH or NaOH is often used as dealloying solvents. The dealloying temperature should be room temperature. However, at higher temperatures such as 80°C, nanorod support pore structures can form [5, 6, 14] in the Al-Ce precursor system. Thus, the dealloying temperature is important for products with specific morphologies. In dealloying, the smaller components in the precursor alloys are distributed in the ligaments by mechanical mixing. During subsequent heat treatment, the ligament begins to crystallize or crystallizes further. The loaded elements cannot be fixed or dissolved in the ligaments, and they are forced to diffuse to the surface of the ligament. This promotes the formation of nanoparticles and atomic load on the ligament surface. As a result, composite materials from loading the ligament surface with other particles evolve well [4]. Regardless of whether the noble metal is loaded on oxides or the oxides are loaded on noble metals, designing the proper composition of precursor alloys is critical. Notably, the binding between the load and the carrier in the nanoporous composite prepared by dealloying is similar to that of metallurgical binding or a semi-embedding structure [4]. Compared with other methods, they have a stronger binding force, which is conducive to the stability of structures and stronger interface effects.

Heat treatment or calcination is an indispensable procedure in preparation of nanoporous oxides and nanoporous composites. During heat treating, some elements are oxidized into oxides and the desired structures can be formed [4, 5, 8, 9, 13, 14]. In certain composites, such as Co3O4 [17] and CeO2 loaded with Au [14], interface and crystalline defects occurred. This can be useful to enhance the catalytic properties. The calcination technologies indicated that the choice of atmosphere should be optimized according to the materials. This approach provides a starting point for researchers. However, certain composites, such as nanoporous Pd loaded with oxides [11, 12, 18], are not suitable for calcination because the nano Pd easily oxidizes at high temperatures.

Rare earth elements are important metallurgical additives in the industry. In the past 5 years, rear earth elements, such as Ce, have been used in research examining nanoporous materials [5, 6, 8, 14, 15]. The Ce in melt-spun Cu-Ag-Ce alloys can observably refine the microstructures of precursors. The nanoporous structures were obviously fine after electrochemistry dealloying, and all Cu and Ce were removed [15]. The most compounds with rare earth elements could be decomposed during dealloying. Some rare earth elements, such as lanthanum (La), Ce, neodymium (Nd), and Sm, could not be removed by NaOH and KOH solutions at up to 100°C. When their contents are lower, that is, less than 2%, they can be loaded by ligaments of noble metals, resulting in improved catalytic properties [8]. In alkali liquors, these elements can form compounds with water and crystallize. The crystallization has specific directions, which results in the formation of nanorods at temperatures above 80°C. The compounds with water can transform into rare earth oxidations after dehydration, and a framework structure constructed by support nanorods forms [5, 8, 14]. Rare earth oxidation nanorods are important carriers loaded by nobles and other oxidations because they have special interactions with other matter. The composites present excellent catalytic properties. In 5% nitric acid (HNO3) solution, the CeO2 can be removed. Ultrafine nanoporous Pt with excellent properties was also achieved [19].

#### **3. Nanoporous oxides**

Transition metal oxides (TMOs) are important nanoporous materials exhibiting a wide variety of structures and electronic and magnetic properties due to the nature of the outer d states [20]. Nanoscale transition metal oxides exhibit favorable catalytic activity for catalytic oxidation of harmful gases due to the presence of variable valency ions [21–23]. Transition metal oxide semiconductor materials, such as TiO2, have important applications in photocatalytic hydrogen production and photodegradation due to their nontoxicity, high chemical stability, low cost,

**11**

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

ordered nanoporous transition metal oxides.

indicating that its performance is favorable.

contact between the MB and catalysts [27].

capacitance of the NiO reached 1670 F g<sup>−</sup><sup>1</sup>

potential window were 170 and 27.5 kW kg<sup>−</sup><sup>1</sup>

fully catalyzed oxidation of CO was approximately 340°C.

**3.2 Nanoporous NiO**

ribbons in a dilute HF aqueous solution at 0.05 mol L<sup>−</sup><sup>1</sup>

**3.1 Nanoporous CuO**

and light stability [24–26]. Thus, research has been devoted to the creation of stable

In 2015, Zhang et al. reported that nanoporous CuO was prepared by a simple method of dealloying Al-Cu alloy ribbon in alkaline solution and calcining in air [4]. The precursor ingots were obtained by melting alloys composed of pure Al and

0.05 MPa argon (Ar). The as-quenched ribbons were immersed into aqueous 5 wt% NaOH at room temperature. The dealloyed ribbons were calcined at 600°C for 1 h in air. The results showed that the samples are mainly composed of Cu and copper (I) oxide (Cu2O) after dealloying and only CuO could be detected after calcination at 600°C. The prepared CuO exhibits a three-dimensional interpenetrating nanoporous structure with mesoporous properties. The pore size is approximately 25 nm, and the ligament size is approximately 50 nm. The materials have good redox capacity and can be used for the catalytic oxidation of CO. The reaction temperature for catalyzing 50% of CO is approximately 170°C, and the temperature for completely converting CO is 240°C. The long-term stability did not degrade even after 60 h,

In 2017, Li et al. reported the fabrication of a 3D free-standing CuO nanowire array supported by a nanoporous CuO network by dealloying Cu60Zr35Al5 glassy

In 2013, Liang et al. reported that a three-dimensional (3D) nanoporous NiO film was fabricated via a two-step process using an electrochemical route [28]. The dealloying process included electrodeposition of the Ni/Zn alloy film and electrochemical dealloying using a direct-current power source. The NiO film had an irregular 3D interconnected nanosheet structure with open channels. The specific

for the supercapacitor. In addition, the NiO exhibited high performance during a long-term cycling. The maximum specific energy and specific power at the 1.1 V

In 2016, Zhang et al. successfully prepared nanoporous NiO by dealloying Al85Ni15 alloy and calcining in air [5]. The precursor alloys were prepared by similar methods with Al-Cu [4]. The surface and pores of the sample present a nanoporous structure. The pore diameter is approximately 30–50 nm, the pore walls are composed of island-like barriers, and the width is approximately 50 nm. The entire material presents a porous frame structure with a uniform pore size. It achieved good performance in the catalytic oxidation of CO, and the active temperature of

, respectively.

[27]. The CuO nanocomposite exhibits a hierarchical nanostructure containing a well-aligned CuO nanowire array and nanoporous substrate with continuous nanoporosity. The nanoporous CuO exhibits superior degradation performance for methylene blue in the presence of hydrogen peroxide (H2O2) compared to that of commercial CuO nanoparticles. The remarkable catalytic activity and good reusability and stability during degradation make the as-prepared nanocomposite a promising candidate for purifying wastewater with organic dyes. The high degradation efficiency of the nanocomposite mainly results from the uniform nanowire array structure and its high internal surface area, which provide more effective

and protected by

for 12 h at room temperature

at a discharge current density of 1 A g<sup>−</sup><sup>1</sup>

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

and light stability [24–26]. Thus, research has been devoted to the creation of stable ordered nanoporous transition metal oxides.
