**3.1 Nanoporous CuO**

*Nanofluid Flow in Porous Media*

Pd easily oxidizes at high temperatures.

excellent properties was also achieved [19].

**3. Nanoporous oxides**

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

Heat treatment or calcination is an indispensable procedure in preparation of nanoporous oxides and nanoporous composites. During heat treating, some ele-

[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

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

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,

conducive to the stability of structures and stronger interface effects.

ments are oxidized into oxides and the desired structures can be formed

**10**

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 pure Cu. The ribbons were prepared by melt spinning at 33 m s<sup>−</sup><sup>1</sup> and protected by 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, indicating that its performance is favorable.

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 ribbons in a dilute HF aqueous solution at 0.05 mol L<sup>−</sup><sup>1</sup> for 12 h at room temperature [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 contact between the MB and catalysts [27].
