# Noble Metal-Based Heterogeneous Catalysts for Electrochemical Hydrogen Evolution Reaction
## 4 Noble Metal-Based Heterogeneous Catalysts
### Alloying Strategy
#### 4.1.4. Intermetallic Compound  
The alloy can reduce the cost of precious metals and improve the catalytic activity of precious metals, but the disordered structure of a solid solution alloy makes its stability poor. Intermetallic compounds mainly refer to alloy compounds that combine metals and metals or metals and nonmetals (such as H, B, N, S, P, C, Si, etc.) according to a certain atomic stoichiometric ratio to form a crystal lattice different from the original single component. Thermodynamically, solid solution alloys have a higher entropy due to their greatly disordered atomic arrangement, which is more favorable than ordered intermetallic compounds at high temperature. However, under certain compositions and temperatures, the strong d-d interaction between transition metal atoms can provide the required enthalpy change for the alloy system, resulting in a decrease in Gibbs free energy and forming ordered intermetallic junction physical properties. These strong d-d interactions have better stability than disordered solid solution alloys with similar composition and morphology, thus attracting much attention [79]. The synthesis of intermetallic compounds can be divided into two types: annealing after liquid phase synthesis and direct liquid phase synthesis. The difference between the above two methods is the annealing step and the formation mechanism of the ordered structure. The annealing after liquid phase synthesis needs high-temperature treatment under a reduced gas atmosphere. When the annealing temperature is higher than 500 \({}^{\circ}\)C, the size and shape of nanostructures are unevenly distributed during the transition from disorder to order, which makes it difficult to accurately control the size and shape of the nanostructures of intermetallic compounds [80]. The direct liquid phase synthesis needs a seed to grow material. During this process, it is hard to control the size of the catalyst. Therefore, developing a synthetic strategy that can accurately control the size and shape of intermetallic compounds is a challenge. [PERSON] et al. [81] used a mesoporous silica template to synthesize a well-defined nanomaterial through nanomaterial space limiting. This method allows precise control of the size and shape of nanostructures by converting disordered alloy Pt\({}_{3}\)Co nanowires (D-Pt\({}_{3}\)Co-NWs) into ordered intermetallic compound Pt\({}_{3}\)Co nanowires (O-Pt\({}_{3}\)Co-NWs) without agglomeration (Figure 6a,b). O-Pt\({}_{3}\)Co NWs have higher HER activity than O-Pt\({}_{3}\)Co NWs and Pt/C catalysts in alkaline conditions (Figure 6c). The enhancement of the O-Pt\({}_{3}\)Co NWs HER kinetic is achieved through alloying and atomic ordering regulation of the hydrogen binding energy. Figure 6: (**a**) Atomic-resolution HAADF-STEM image of O-Pt\({}_{3}\)Co NWs; (**b**) HER polarization curves of Pt/C, D-Pt\({}_{3}\)Co NWs, and O–Pt\({}_{3}\)Co NWs in N\({}_{2}\)–saturated 0.1 M KOH electrolyte at a rotation speed of 2500 rpm and a scan rate of 2 mV s\({}^{-1}\); (**c**) Corresponding Tafel plots of HER polarization curves [81]. Copyright 2019 American Chemical Society. Although most research on intermetallic compounds has focused on materials with polymetallic components, some researchers have focused on alloying products formed between metals and non-metals. For example, [PERSON] et al. [82] studied the critical role of electron interaction between metal and boron in surface hydrogenation adsorption and catalytic activity using a combination of theory and experiments. The result shows that the adsorption of hydrogen atoms on the surface of intermetallic compounds is weaker than that on the corresponding pure metal surface. This is due to the strong hybridization between the d-orbital of transition metal and the sp-orbital of boron, which changes the d-band properties.