*2.5.1. How to design and synthesize catalyst of the HDC*


#### *2.5.2. Core-shell catalyst*

Nanometer catalysts because of their nanosize effect may be an efficient heterogeneous catalyst- model which are widely used in many different catalytic reaction, such as, catalysis, biological-

**Figure 12.** The advantages of core-shell nanoparticles.

**Figure 13.** STEM micrograph of Pd/Al2 O3 catalyst (a) and TEM micrographs of Pd/AC (b) and Rh/AC catalysts(c).


**Table 2.** The kinetics of hydrodechlorination of 4-chlorophenol on Pd/Al2 O Pd/AC, Rh/AC. 3,

medicine, material chemistry, sensors and so on [30]. As a nanomaterials, core-shell structure catalyst materials can be synthesized through the grafting reaction method step by step.

Because of the advantages that core-shell nanomaterials own (**Figure 12**) precious metals (Pd, Rh, Au…) can be loaded on core-shell nanoparticles by stronger chemical bonds, such as coordination bond or covalent bonds.

 Elena Diaz et al. [31] studied the kinetics of hydrodechlorination of 4-chlorophenol on alumina and activated carbon supported Pd and Rh catalysts. The hydrodechlorination of 4-chlorophenols based on Pd and Rh on γ-alumina and activated carbon was investigated in continuously stirred basket reactors (20–40°C and 1 bar). For 4-chlorophenol, the reaction rate shows a first-order dependence. All catalysts are effective in removing 4-chlorophenol. Phenol, cyclohexanone and cyclohexanol were identified as reaction products (**Figure 13**). The hydrogenation of 4-chlorophenol to phenol was in the range of 146–478 L/kgcat h for Pd/Al2 O3 , 179–713 L/kgcat h for Pd/AC, and 44–172 L/kgcat h for Rh/AC. In all cases, the kl

constant shows a much larger value than k2 , indicating that the formation of phenol is superior to cyclohexanone as the first reaction step (**Table 2**).

Wu et al. [32] designed and synthesized Fe3 O4 @SiO2 @Pd-Au catalyst (**Figure 14**). Fe3 O4 @SiO2 @ Pd-Au was synthesized with the reduction of Pd2+ and Au3+. The amine-modified silica is coated on the outer layer of magnetic Fe3 O4 nanoparticles to be a carrier for Pd-Au nanoparticles, where in the amine acts as a bridge connecting the Pd-Au nanoparticles to the support, making it highly dispersible, the magnetic properties of Fe3 O4 . allows the catalyst to be recycled. The performance of the catalyst was evaluated by hydrodechlorination of 4-chlorophenol (25°C, atmospheric pressure, a certain amount of 4-CP, 0.05 g NaOH, 0.5 g catalyst). The results showed that Pd nanoparticles have higher activity in HDC of 4-CP then the Au

**Figure 14.** Synthesis procedure of Fe3 O4 @SiO2 @Pd-Au catalyst.

**Figure 15.** 4-CP HDC conversions on different catalysts.-

**Figure 16.** Design and synthesize the Pd/Fe3 O4 @C catalyst.

**Figure 17.** Controllable design of tunable nanostructures inside metal–organic frameworks.

nanoparticle, the Pd-Au alloy increases the conversion significantly and achieves a complete conversion of HDC within 20 min, much faster than the Pd metal catalyst (**Figure 15**).

Li et al. [33]., studies a high efficient Pd nanocatalysts (Pd/Fe<sup>3</sup> O4 @C) applied in HDC. Pd nanoparticles in magnetic carbon shell can effectively improve the catalytic activity, separation and reusable. Catalyst was synthesized by Fe3 O4 nanoparticles as a core, and then a layer of carbon layer on the outside package, finally by APTES modified carbon layer improving Pd Application of Heterogeneous Catalysts in Dechlorination of Chlorophenols 59 http://dx.doi.org/10.5772/intechopen.79134

**Figure 18.** MPC utilized as a catalyst support to fabricate Au and Pd NP-based nanocatalysts.

loaded (**Figure 16**). Pd/Fe3 O4 @C can be recycled at least five times without obvious loss activity. Pd/Fe3 O4 @C is not only used in the aqueous solution of 4-chlorophenol hydrogenation dechlorination but also be used for reduction of 4-nitrophenol.

Porous materials are widely used in catalysis, gas adsorption, separation and other fields because of their large surface area and structure controllable. Metal organic frameworks (MOFs) materials are a kind of regular porous material formed by DCC chemistry by metal and organic ligand [34]. Controllable design of adjustable nanostructures in metal-organic frameworks is in **Figure 17**.

Noble metal nanoparticles (NMNPs) have attracted attention as the activity center. But NMNPs were easy blocking pores of active carbon, and leaching from carbon nanotubes, and graphene faces in the process of catalytic. In order to solve these problems, Dong used metal organic framework (MOF) to synthesize the porous carbon (MPC) which can provides a large surface area and pore (**Figure 18**) not only can make the active center (Pd NPs) scattered on it well, but also has paramagnetic behavior that the catalyst can be easily recycled [35].
