**6. Conclusions**

*Nanofluid Flow in Porous Media*

**5.3 Nanoporous Pt-TiO2 nanocomposites**

processes.

large pore volumes.

g<sup>−</sup><sup>1</sup>

metal and the oxide.

98.06 m<sup>2</sup>

performance.

The formation process of the Au-CeO2 nanorod catalyst can be deduced as: during the formation of the porous CeOx framework, the Au atoms may mix with the nanorods. After the nanorods are calcined in O2, CeOx is oxidized to CeO2 NF, whereas the Au atoms diffuse to the surface of the nanorods and form NPs/clusters or nanoparticles, which are in situ supported and immobilized on the surface of the CeO2 nanorods. The dispersion of Au NPs or clusters on CeO2 NF could create many nanoscale contact interfaces and prevent NP/cluster sintering and migration of active Au species during calcination and catalytic reaction. This result was significantly different from those of other preparation methods, in which the NP aggregation typically leads to unexpected activity loss due to the collapse of the nanostructure during the annealing and catalytic

Nanoporous TiO2 and Pt-TiO2 composites were prepared by dealloying meltspun Al-Si-Ti and Al-Si-Ti-Pt alloys, respectively [16]. The preparation of alloy ribbons was the same as in the Au-CuO system. The as-quenched ribbons were dealloyed in aqueous 10 wt% NaOH at 80°C. After the samples were dried in air, the ribbons were immersed in aqueous 3 wt% HCl at an ambient temperature for 7 h. The samples were then calcined at 400°C for 2 h. The mechanism of formation of the structure is as follows: Al and Si are etched away in the NaOH solution, and Ti and Pt are retained. During dissolution, the remaining Ti and Pt move freely along the interface between the alloy and the dissolution medium. The Ti reacts with NaOH to form insoluble Na-titanate (Ti + NaOH → Na-titanate). The Pt-Na-titanate reorganizes into a three-dimensional network exhibiting an open porosity. After acid treatment, there is no significant evolution in the microstructure morphology in the H-titanate (pt-Na-titanate + H<sup>+</sup> → Pt-H-titanate + Na<sup>+</sup>

Nanoporous Pt-TiO2 can be obtained through the calcination of Pt-H-titanate. Measurement of the structural parameters indicated that the nanoporous Pt-TiO2 nanocomposites exhibit large specific surface areas, small pore diameters, and

The performance measurements revealed that nanoporous TiO2 showed favorable photocatalytic performance in the degradation of methyl orange (MO). Using Pt-TiO2 nanocomposites, approximately 98% of the MO solution degraded in 50 minutes. The significant enhancement of the photocatalytic performance of Pt-TiO2 was a result of the large specific surface area of approximately

the band gap of TiO2 and expanded the absorption range. With a high work function, Pt enhances the Schottky barrier effect; the Schottky barrier formation at the interfaces of Pt-TiO2 composites reduced the rate of electron-hole (e-h<sup>+</sup>

pair recombination, which contributed to the improvement of the photocatalytic

and their supported noble metals have the following advantages: (1) unique stable structure, which results in stability of the catalyst, (2) refined and homogeneous distribution of nanopores provides a larger specific surface area for the catalyst, exposing more active sites and effectively promoting catalytic performance, (3) loaded noble metals avoid agglomeration and evenly disperse on oxide surfaces or ligaments, which improves the availability of noble metals and enhances catalytic performance, and (4) the tight interaction between the supported noble metal and the oxide enhances the synergy between the noble

Noble metal-modified nanoporous oxides prepared by the dealloying method

and synergistic effect between TiO2 and metallic Pt, which reduced

).

)

**18**

By the reasonable design of precursors and adoption of dealloying process, the nanoporous oxides, such as CuO, NiO, TiO2, Co3O4, and CeO2, noble-metal-based nanoporous composites, such as Ag ligaments loaded with CeO2, TiO2, ZrO2, or NiO, and Pd ligaments loaded with TiO2 or ZrO2 could be obtained. Adding the noble into the melt-spun Al-M (M = Ce, Cu, and Ti) ribbons and dealloying in NaOH or KOH solutions, oxide-based nanoporous composites, such as Au loaded on CuO and CeO2 or Pt loaded on TiO2, have been achieved. The catalytic performances of the noblebased nanoporous composites, including the catalytic oxidation of methanol and ethanol, could be improved obviously comparing with single nanoporous nobles. The catalytic oxidation activities of CO for the oxide-based nanoporous composites have also been increased much. The rare earth elements could play an important role in the nanoporous materials. Sometimes, they could be the key metals for the nanoporous composites with high properties.
