**4.1 Nanoporous Ag-CeO2**

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, and Ce. The ribbons were prepared by melt spinning at 33 m s<sup>−</sup><sup>1</sup> and protected by 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. This may aid in enhancing the longevity performance.

## **4.2 Nanoporous Ag-ZrO2**

After the Ag-ZrO2 was prepared, nanoporous Ag-ZrO2 composite catalysts were created by chemical dealloying of the melt-spun Al-Ag-Zr precursor alloys [10]. The composition of the precursor alloys was Al80−XAg20ZrX (x = 1, 2, 3). The preparation of alloy ribbons and the dealloying method was the same as in the Ag-CeO2 system. During dealloying, the zirconium (Zr) atoms released from the precursory alloys are oxidized into ZrO2 and are loaded on the inner surface of the nanoporous Ag. The dealloyed ribbons exhibit an interpenetrating ligament-channel structure with nanometer-length scales. X-ray photoelectron spectroscopy (XPS) analysis indicated that the binding energy of the Zr 3d core levels decreases slightly with increasing calcination temperature because the Zr4+ in the ZrO2 particles is partially reduced to Zr 3+, suggesting electron transfer occurring from the ZrO2 to the metallic centers and the formation of oxygen vacancies in the thermal treatment. The electrochemical tests demonstrated that the current density peak increased with calcination temperature in a certain range, that nanoporous composites with the optimized ZrO2 content exhibited higher catalytic activity, and that the optimized precursor alloy composition was Al78Ag20Zr2. The oxidation current density increased by 91.3% compared with that of nanoporous Ag. The excellent catalytic activity can be attributed to the interfacial interaction and electron charge transfer between Ag and ZrO2. In addition, nanoporous composites with high specific surface areas and pore volume can enhance their electrocatalytic activities, which can provide more transport channels and activity sites for conductive ions and reactant molecules.

### **4.3 Nanoporous Ag-TiO2**

Nanoporous Ag-TiO2 composites were prepared by dealloying the melt-spun Al-Ag-Ti ribbons in aqueous NaOH [9] in 2014. The composition of the precursor alloys was Al80-XAg20TiX (X = 0.5, 1, 2, 3). The preparation of alloy ribbons and the dealloying method was the same as in the Ag-CeO2 and Ag-ZrO2 systems. The results revealed that TiO2 formed in situ on the Ag ligaments. Ti3+ and Ag+ species co-existed after the dealloyed samples were calcined at 600°C, which significantly influenced the catalytic oxidation of sodium borohydride. The electrochemical results showed that the nanoporous Ag-TiO2 composites significantly promoted the direct oxidation of BH4-superior to pure Ag. The current density of the oxidation peak for the nanoporous Ag-TiO2 electrode prepared from the Al79.5Ag20Ti0.5 alloy increased from 10.91 to 18.13 mA cm<sup>−</sup><sup>2</sup> . With increasing the Ti content from 0.5 to 1.0% in the precursor alloys, the current density of the oxidation peak increased to 28.86 mA cm<sup>−</sup><sup>2</sup> . With further increases in the Ti content, however, the current density of the nanoporous Ag-TiO2 electrode obviously decreased, and the position of the oxidation peak became more positive. The enhanced catalytic activity could be attributed to the strong interfacial effects between the Ag ligaments and TiO2 [9].

Among these Ag-based nanocomposites, the Ag-ZrO2 nanocomposite catalyst exhibited the greatest improvement in the catalytic oxidation of sodium borohydride. The optimized precursor alloy composition was Al78Ag20Zr2. However, Ag-CeO2 may have the best anti-poisoning ability of CO.

#### **4.4 Nanoporous Pd-TiO2**

Nanoporous Pd-TiO2 composite catalysts were prepared by chemical dealloying of melt-spun Al-Pd-Ti precursor alloys [12] in 2014. The precursor alloy was Al85−xPd15Tix (x = 0, 0.3, 0.5, 0.7, 1.0, at%), and the preparation of alloy ribbons was the same as in the Ag-CeO2 and Ag-ZrO2 systems. However, calcination was

**15**

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

diate species and result in superior poisoning tolerance.

promoting methanol oxidation via a surface redox process.

**4.5 Nanoporous Pd-NiO**

Al84.7Pd15Ni0.3 was 289.0 mA mg<sup>−</sup><sup>1</sup>

**4.6 Nanoporous Pd-ZrO2**

not adopted to prevent oxidation of the Pd. The structure analysis revealed that the nanoporous Pd-TiO2 composites exhibited a bicontinuous interpenetrating ligament-pore structure. The XPS analysis revealed that the Pd 3d spectrum of the nanoporous Pd-TiO2 composites presented a slightly negative shift to lower binding energy compared to that of nanoporous Pd. The electrochemical measurements showed that the composites observably enhanced the electrocatalytic performance toward methanol/ethanol oxidation when the Ti content in the precursor alloys was 0.3–0.7 at%. Among these composites, the nanoporous Pd-TiO2 composite dealloyed from Al84.5Pd15Ti0.5 exhibited the largest catalytic activity, which was triple and double that of methanol and ethanol oxidation, respectively, compared with that of nanoporous Pd. This enhancement can be attributed to the synergistic effect between Pd and TiO2. However, when the Ti content of the precursor alloys is 1 at%, the catalytic activity will obviously decline. These results imply that an electronic interaction between Pd and TiO2 occurs in the nanoporous Pd-TiO2 composites. The reduction of binding energy decreases the chemisorption ability of the adsorbate during electrochemical reactions. It will reduce the adsorption of CO-like interme-

Nanoporous Pd-NiO composites were prepared for methanol oxidation in alkaline media by one-step dealloying from melt-spun Al-Pd-Ni precursor alloys [11]. The precursor alloy was Al85−xPd15Nix (x = 0, 0.1, 0.3, 0.5, 0.7, 1.0 at%). The preparation of alloy ribbons was the same as in the Pd-TiO2 systems. The structure, morphology, composition, and electrocatalytic activities of the composites were characterized. The results demonstrated that the composites exhibited a uniform bicontinuous and interpenetrating three-dimensional nanoporous structure. Pore channels that were less than 10 nm ran throughout the ribbons. The XPS analysis revealed that the nanoporous Pd-NiO composites significantly increased compared to that of nanoporous Pd due to the reduction of the PdO content in the Pd-NiO composites. The nanoporous Pd-NiO composites possessed better electrocatalytic performance compared to that of nanoporous Pd, and the composite dealloyed from Al84.7Pd15Ni0.3 showed the highest catalytic activity of the catalytic oxidation of methanol; the mass activity of the mesoporous Pd-NiO composite from

porous Pd. The improvement in the electrocatalytic properties was attributed to the increased electrochemically active specific surface areas and the synergistic effect between Pd and NiO. The reduction of the PdO content in the nanoporous Pd-NiO composites means that there are more active sites (Pd atoms) in the composites involved in the electrocatalytic process. There is also a synergistic effect between Pd and NiO. Oxidative states of Ni in the nanoporous Pd-NiO composites serve as oxophilic sites for the oxygen source required for the surface removal of CO and

Composition-controllable three-dimensional mesoporous Pd-ZrO2 composites

were synthesized through simple one-step dealloying of melt-spun Al-Pd-Zr ribbons [18]. The precursor alloy was Al85−xPd15Zrx (x = 0, 0.5, 1, 2, 3 at%). The preparation of alloy ribbons was the same as in the Pd-TiO2 systems. The Zr atoms in the precursor alloy transformed into approximately 3 nm ZrO2 nanoparticles after dealloying without being calcined. The ZrO2 particles embedded in the Pd ligaments. The XPS analysis revealed that the Pd 3d spectrum of the dealloyed

, which was approximately 4 times that of nano-

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

*Nanofluid Flow in Porous Media*

**4.2 Nanoporous Ag-ZrO2**

**4.3 Nanoporous Ag-TiO2**

increased from 10.91 to 18.13 mA cm<sup>−</sup><sup>2</sup>

Ag-CeO2 may have the best anti-poisoning ability of CO.

28.86 mA cm<sup>−</sup><sup>2</sup>

**4.4 Nanoporous Pd-TiO2**

After the Ag-ZrO2 was prepared, nanoporous Ag-ZrO2 composite catalysts were created by chemical dealloying of the melt-spun Al-Ag-Zr precursor alloys [10]. The composition of the precursor alloys was Al80−XAg20ZrX (x = 1, 2, 3). The preparation of alloy ribbons and the dealloying method was the same as in the Ag-CeO2 system. During dealloying, the zirconium (Zr) atoms released from the precursory alloys are oxidized into ZrO2 and are loaded on the inner surface of the nanoporous Ag. The dealloyed ribbons exhibit an interpenetrating ligament-channel structure with nanometer-length scales. X-ray photoelectron spectroscopy (XPS) analysis indicated that the binding energy of the Zr 3d core levels decreases slightly with increasing calcination temperature because the Zr4+ in the ZrO2 particles is partially reduced to Zr 3+, suggesting electron transfer occurring from the ZrO2 to the metallic centers and the formation of oxygen vacancies in the thermal treatment. The electrochemical tests demonstrated that the current density peak increased with calcination temperature in a certain range, that nanoporous composites with the optimized ZrO2 content exhibited higher catalytic activity, and that the optimized precursor alloy composition was Al78Ag20Zr2. The oxidation current density increased by 91.3% compared with that of nanoporous Ag. The excellent catalytic activity can be attributed to the interfacial interaction and electron charge transfer between Ag and ZrO2. In addition, nanoporous composites with high specific surface areas and pore volume can enhance their electrocatalytic activities, which can provide more transport channels and activity sites for conductive ions and reactant molecules.

Nanoporous Ag-TiO2 composites were prepared by dealloying the melt-spun Al-Ag-Ti ribbons in aqueous NaOH [9] in 2014. The composition of the precursor alloys was Al80-XAg20TiX (X = 0.5, 1, 2, 3). The preparation of alloy ribbons and the dealloying method was the same as in the Ag-CeO2 and Ag-ZrO2 systems. The results revealed that TiO2 formed in situ on the Ag ligaments. Ti3+ and Ag+

co-existed after the dealloyed samples were calcined at 600°C, which significantly influenced the catalytic oxidation of sodium borohydride. The electrochemical results showed that the nanoporous Ag-TiO2 composites significantly promoted the direct oxidation of BH4-superior to pure Ag. The current density of the oxidation peak for the nanoporous Ag-TiO2 electrode prepared from the Al79.5Ag20Ti0.5 alloy

1.0% in the precursor alloys, the current density of the oxidation peak increased to

sity of the nanoporous Ag-TiO2 electrode obviously decreased, and the position of the oxidation peak became more positive. The enhanced catalytic activity could be attributed to the strong interfacial effects between the Ag ligaments and TiO2 [9]. Among these Ag-based nanocomposites, the Ag-ZrO2 nanocomposite catalyst exhibited the greatest improvement in the catalytic oxidation of sodium borohydride. The optimized precursor alloy composition was Al78Ag20Zr2. However,

Nanoporous Pd-TiO2 composite catalysts were prepared by chemical dealloying of melt-spun Al-Pd-Ti precursor alloys [12] in 2014. The precursor alloy was Al85−xPd15Tix (x = 0, 0.3, 0.5, 0.7, 1.0, at%), and the preparation of alloy ribbons was the same as in the Ag-CeO2 and Ag-ZrO2 systems. However, calcination was

. With further increases in the Ti content, however, the current den-

species

. With increasing the Ti content from 0.5 to

**14**

not adopted to prevent oxidation of the Pd. The structure analysis revealed that the nanoporous Pd-TiO2 composites exhibited a bicontinuous interpenetrating ligament-pore structure. The XPS analysis revealed that the Pd 3d spectrum of the nanoporous Pd-TiO2 composites presented a slightly negative shift to lower binding energy compared to that of nanoporous Pd. The electrochemical measurements showed that the composites observably enhanced the electrocatalytic performance toward methanol/ethanol oxidation when the Ti content in the precursor alloys was 0.3–0.7 at%. Among these composites, the nanoporous Pd-TiO2 composite dealloyed from Al84.5Pd15Ti0.5 exhibited the largest catalytic activity, which was triple and double that of methanol and ethanol oxidation, respectively, compared with that of nanoporous Pd. This enhancement can be attributed to the synergistic effect between Pd and TiO2. However, when the Ti content of the precursor alloys is 1 at%, the catalytic activity will obviously decline. These results imply that an electronic interaction between Pd and TiO2 occurs in the nanoporous Pd-TiO2 composites. The reduction of binding energy decreases the chemisorption ability of the adsorbate during electrochemical reactions. It will reduce the adsorption of CO-like intermediate species and result in superior poisoning tolerance.

#### **4.5 Nanoporous Pd-NiO**

Nanoporous Pd-NiO composites were prepared for methanol oxidation in alkaline media by one-step dealloying from melt-spun Al-Pd-Ni precursor alloys [11]. The precursor alloy was Al85−xPd15Nix (x = 0, 0.1, 0.3, 0.5, 0.7, 1.0 at%). The preparation of alloy ribbons was the same as in the Pd-TiO2 systems. The structure, morphology, composition, and electrocatalytic activities of the composites were characterized. The results demonstrated that the composites exhibited a uniform bicontinuous and interpenetrating three-dimensional nanoporous structure. Pore channels that were less than 10 nm ran throughout the ribbons. The XPS analysis revealed that the nanoporous Pd-NiO composites significantly increased compared to that of nanoporous Pd due to the reduction of the PdO content in the Pd-NiO composites. The nanoporous Pd-NiO composites possessed better electrocatalytic performance compared to that of nanoporous Pd, and the composite dealloyed from Al84.7Pd15Ni0.3 showed the highest catalytic activity of the catalytic oxidation of methanol; the mass activity of the mesoporous Pd-NiO composite from Al84.7Pd15Ni0.3 was 289.0 mA mg<sup>−</sup><sup>1</sup> , which was approximately 4 times that of nanoporous Pd. The improvement in the electrocatalytic properties was attributed to the increased electrochemically active specific surface areas and the synergistic effect between Pd and NiO. The reduction of the PdO content in the nanoporous Pd-NiO composites means that there are more active sites (Pd atoms) in the composites involved in the electrocatalytic process. There is also a synergistic effect between Pd and NiO. Oxidative states of Ni in the nanoporous Pd-NiO composites serve as oxophilic sites for the oxygen source required for the surface removal of CO and promoting methanol oxidation via a surface redox process.

### **4.6 Nanoporous Pd-ZrO2**

Composition-controllable three-dimensional mesoporous Pd-ZrO2 composites were synthesized through simple one-step dealloying of melt-spun Al-Pd-Zr ribbons [18]. The precursor alloy was Al85−xPd15Zrx (x = 0, 0.5, 1, 2, 3 at%). The preparation of alloy ribbons was the same as in the Pd-TiO2 systems. The Zr atoms in the precursor alloy transformed into approximately 3 nm ZrO2 nanoparticles after dealloying without being calcined. The ZrO2 particles embedded in the Pd ligaments. The XPS analysis revealed that the Pd 3d spectrum of the dealloyed

Al83Pd15Zr2 sample was negatively shifted to a lower binding energy after the addition of Zr in comparison with the Pd 3d spectrum of the dealloyed Al85Pd15. This finding suggests the existence of electronic interactions between Pd and ZrO2. The composites exhibited remarkable catalytic activity and stability for methanol oxidation in an alkaline electrolyte due to the synergistic effect and electronic interactions between Pd and ZrO2 [18]. Among the composites, the Pd-ZrO2 sample dealloyed from the Al85Pd13Zr2 precursor alloy had the highest peak current density, and the mass activity of the mesoporous Pd-ZrO2 composite from Al85Pd13Zr2 was 254.24 mA mg<sup>−</sup><sup>1</sup> , which was approximately 3.6 times that of the pure mesoporous Pd sample. In one study [18], Zr in the form of ZrO2 was embedded in the surface of the Pd ligaments in mesoporous Pd-ZrO2 composites. This arrangement endowed ZrO2 with more convenient and effective absorption toward OH<sup>−</sup> species to more rapidly transform CO-like poisoning species on the surface of the active Pd sites into carbon dioxide or other dissolvable cleansing products. The synergistic effect between Pd and ZrO2 results in more active Pd atoms being released to the surface of the samples for the use in additional methanol oxidation processes.

Among these Pd-based nanocomposites, the Pd-NiO nanocomposite catalyst showed the greatest improvement in the catalytic oxidation of methanol. The optimized precursor alloy composition was Al84.7Pd15Ni0.3.

#### **5. Nanoporous oxides loaded by noble metals**

Nanoporous metal oxides have been widely applied in catalysis due to their unique porous support structure and large specific surface area, including in carbon monoxide catalytic oxidation, electrochemical catalytic oxidation of methanol, supercapacitors, and photocatalytic fields. As an intermediary, noble metals are irreplaceable in the field of catalysis. They can effectively reduce catalytic activation energy or promote electron transfer, improving the catalytic performance. Loading noble metals on oxide substrates can improve the utilization rate of noble metals and enhance the catalytic performance of oxides. Nanoporous oxides loaded or modified by noble metals can be prepared by simply dealloying melt-spun alloys and subsequent calcination in air. Nanoporous oxide materials obtained via simple and inexpensive preparation methods have homogeneously distributed pores, structural stability, large specific surface area, and excellent dispersion of supported noble metals. The large specific surface area provides more active sites for catalysis, which can effectively improve the catalytic performance of the materials. The good dispersion of loaded noble metal nanoparticles maximizes the utilization of noble metals. Typical oxide-based porous catalytic materials and catalytic applications are as follows.

#### **5.1 Nanoporous CuO loaded with Au nanoparticles**

Ultrafine nanoporous CuO ribbons loaded by Au nanoparticles were prepared by dealloying melt-spun Al-Cu-Au alloys and calcinating in air [4]. The precursor ingots were obtained by melting alloys composed of pure Al, Cu, and Au. The 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 immersed in aqueous 5 wt% NaOH at room temperature. The dealloyed samples were rinsed with reverse osmosis water and dehydrated alcohol. The dealloyed ribbons were calcined in a muffle furnace at 300–900°C for 1 h.

The formation mechanism of the nanoporous Au-CuO composites is as follows: after the Al-Cu-Au alloys are immersed in NaOH solutions, Al is preferentially etched, and the released Cu atoms in the dealloyed layer are rearranged to form a

**17**

CeO2. The Au<sup>δ</sup><sup>+</sup>

/Au0

not evolve after catalytic testing.

of the dealloyed sample (0.21). Therefore, Au0

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

the aggregation of particles can be effectively prevented.

CO oxidation and can lead to more activated CO than Au0

which are favorable to the catalytic reaction of CO oxidation.

**5.2 CeO2 nanorod framework loaded with Au**

nanoporous structure. The Cu and Au atoms are released simultaneously because the Au atoms are mainly dissolved in the fcc Al. A Cu (Au) solid solution forms due to the existence of unlimited solid solution between Au and Cu according to the Cu-Au binary phase diagram. After the samples are calcined in air, Cu ligaments are oxidized to CuO, and the phase segregation occurs because the Au cannot be dissolved by CuO. During calcination, Au atoms diffuse to the surface of the CuO layers and form Au clusters. With the absorbed O atoms diffusing to the inner Cu and Cu2O, the Au atoms dissolved in the Cu solution diffuse to the Au clusters and form Au particles, leading to a slight growth of Au particles. As a result, a portion of Au particles emboss from the surface of CuO ligaments, where the as-generated Au nanoparticles are partially embedded in CuO ligaments. Due to the unique porous structure, Au nanoparticles are immobilized on the surface of the ligaments, and

The experimental [4] results indicate that the composites have large surface areas and high activity and stability. The CO conversion as a function of reaction temperature for the dealloyed Al79.7Cu20Au0.3 ribbons calcined at different temperatures indicated that the CO conversion rate can reach 100.0% at 180°C for samples treated with the optimized process. The X-ray photoelectron spectroscopy (XPS) results indicate that the superior performance at low temperature is ascribed to the presence of Au+1 species and the interfacial interaction between Au nanoparticles and CuO ligaments. The binding energy of Cu 2p in the catalyst prepared from Al79.7Cu20Au0.3 alloy shifts lower due to an interaction between Au and CuOx. Notably, Au+1 is present on the surface of the CuO, which acts as an active site for

ment showed that the valence of Au changes whereas the valence of Cu does not change, indicating that the Au state is vital to maintaining a high CO conversion rate and long-term stability. It is probable that Au and Au+1 sites located at the interface enhance the absorption of CO and O and that the presence of Cu2+ would result in high mobility and reactivity of surface lattice oxygen to participate in the reaction,

CeO2 nanorod frameworks (NFs) with the porous structure loaded with Au were prepared by dealloying melt-spun Al89.7Ce10Au0.3 ribbons and calcination [14]. The preparation of alloy ribbons was the same as that in the Au-CuO system. The as-quenched ribbons were immersed in aqueous 20 wt% NaOH at a room temperature for 2 h and treated at 80°C for 10 h. The dealloyed ribbons were pretreated at 200–800°C for 2 h in pure O2. After the dealloyed sample was calcined at 400°C in O2, the XPS results demonstrated that the Au peaks slightly shifted lower due to the interaction of gold with oxygen vacancies. This phenomenon indicates that Au species interacted with the CeO2 nanorods via charge transfer between Au and

calcined Au-CeO2 due to strong interactions, which can effectively enhance the catalytic activity. The Au-CeO2 nanorod catalyst calcined at 400°C exhibited much higher catalytic activity for CO oxidation than the dealloyed sample or pure CeO2 nanorods [14]. Moreover, its complete reaction temperature was as low as 91°C. The designed Au-CeO2 catalyst possessed extreme sintering resistance and exhibited high performance due to the enhanced interaction between the Au clusters/NPs and CeO2 nanorods during calcination. The CeO2 nanorod framework structure can be retained, and some Au nanoparticles supported on the nanorod CeO2 surfaces did

ratio of calcined Au-CeO2 (0.39) was much higher than that

and Au<sup>δ</sup><sup>+</sup>

. The catalytic measure-

species coexisted in the

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

*Nanofluid Flow in Porous Media*

254.24 mA mg<sup>−</sup><sup>1</sup>

Al83Pd15Zr2 sample was negatively shifted to a lower binding energy after the addition of Zr in comparison with the Pd 3d spectrum of the dealloyed Al85Pd15. This finding suggests the existence of electronic interactions between Pd and ZrO2. The composites exhibited remarkable catalytic activity and stability for methanol oxidation in an alkaline electrolyte due to the synergistic effect and electronic interactions between Pd and ZrO2 [18]. Among the composites, the Pd-ZrO2 sample dealloyed from the Al85Pd13Zr2 precursor alloy had the highest peak current density, and the mass activity of the mesoporous Pd-ZrO2 composite from Al85Pd13Zr2 was

, which was approximately 3.6 times that of the pure mesoporous

Pd sample. In one study [18], Zr in the form of ZrO2 was embedded in the surface of the Pd ligaments in mesoporous Pd-ZrO2 composites. This arrangement endowed ZrO2 with more convenient and effective absorption toward OH<sup>−</sup> species to more rapidly transform CO-like poisoning species on the surface of the active Pd sites into carbon dioxide or other dissolvable cleansing products. The synergistic effect between Pd and ZrO2 results in more active Pd atoms being released to the surface

Among these Pd-based nanocomposites, the Pd-NiO nanocomposite catalyst showed the greatest improvement in the catalytic oxidation of methanol. The

Nanoporous metal oxides have been widely applied in catalysis due to their unique porous support structure and large specific surface area, including in carbon monoxide catalytic oxidation, electrochemical catalytic oxidation of methanol, supercapacitors, and photocatalytic fields. As an intermediary, noble metals are irreplaceable in the field of catalysis. They can effectively reduce catalytic activation energy or promote electron transfer, improving the catalytic performance. Loading noble metals on oxide substrates can improve the utilization rate of noble metals and enhance the catalytic performance of oxides. Nanoporous oxides loaded or modified by noble metals can be prepared by simply dealloying melt-spun alloys and subsequent calcination in air. Nanoporous oxide materials obtained via simple and inexpensive preparation methods have homogeneously distributed pores, structural stability, large specific surface area, and excellent dispersion of supported noble metals. The large specific surface area provides more active sites for catalysis, which can effectively improve the catalytic performance of the materials. The good dispersion of loaded noble metal nanoparticles maximizes the utilization of noble metals. Typical oxide-based porous catalytic materials and catalytic

Ultrafine nanoporous CuO ribbons loaded by Au nanoparticles were prepared by dealloying melt-spun Al-Cu-Au alloys and calcinating in air [4]. The precursor ingots were obtained by melting alloys composed of pure Al, Cu, and Au. The rib-

as-quenched ribbons were immersed in aqueous 5 wt% NaOH at room temperature. The dealloyed samples were rinsed with reverse osmosis water and dehydrated alcohol. The dealloyed ribbons were calcined in a muffle furnace at 300–900°C for 1 h. The formation mechanism of the nanoporous Au-CuO composites is as follows:

after the Al-Cu-Au alloys are immersed in NaOH solutions, Al is preferentially etched, and the released Cu atoms in the dealloyed layer are rearranged to form a

and protected by 0.05 MPa Ar. The

of the samples for the use in additional methanol oxidation processes.

optimized precursor alloy composition was Al84.7Pd15Ni0.3.

**5. Nanoporous oxides loaded by noble metals**

**5.1 Nanoporous CuO loaded with Au nanoparticles**

bons were prepared by melt spinning at 33 m s<sup>−</sup><sup>1</sup>

**16**

applications are as follows.

nanoporous structure. The Cu and Au atoms are released simultaneously because the Au atoms are mainly dissolved in the fcc Al. A Cu (Au) solid solution forms due to the existence of unlimited solid solution between Au and Cu according to the Cu-Au binary phase diagram. After the samples are calcined in air, Cu ligaments are oxidized to CuO, and the phase segregation occurs because the Au cannot be dissolved by CuO. During calcination, Au atoms diffuse to the surface of the CuO layers and form Au clusters. With the absorbed O atoms diffusing to the inner Cu and Cu2O, the Au atoms dissolved in the Cu solution diffuse to the Au clusters and form Au particles, leading to a slight growth of Au particles. As a result, a portion of Au particles emboss from the surface of CuO ligaments, where the as-generated Au nanoparticles are partially embedded in CuO ligaments. Due to the unique porous structure, Au nanoparticles are immobilized on the surface of the ligaments, and the aggregation of particles can be effectively prevented.

The experimental [4] results indicate that the composites have large surface areas and high activity and stability. The CO conversion as a function of reaction temperature for the dealloyed Al79.7Cu20Au0.3 ribbons calcined at different temperatures indicated that the CO conversion rate can reach 100.0% at 180°C for samples treated with the optimized process. The X-ray photoelectron spectroscopy (XPS) results indicate that the superior performance at low temperature is ascribed to the presence of Au+1 species and the interfacial interaction between Au nanoparticles and CuO ligaments. The binding energy of Cu 2p in the catalyst prepared from Al79.7Cu20Au0.3 alloy shifts lower due to an interaction between Au and CuOx. Notably, Au+1 is present on the surface of the CuO, which acts as an active site for CO oxidation and can lead to more activated CO than Au0 . The catalytic measurement showed that the valence of Au changes whereas the valence of Cu does not change, indicating that the Au state is vital to maintaining a high CO conversion rate and long-term stability. It is probable that Au and Au+1 sites located at the interface enhance the absorption of CO and O and that the presence of Cu2+ would result in high mobility and reactivity of surface lattice oxygen to participate in the reaction, which are favorable to the catalytic reaction of CO oxidation.
