**1. Introduction**

Catalyst synthesis methods have an influence on mean particle size, particle size distribution, the bulk and surface of catalysts' composition, the oxidation state of catalysts, the extent of catalyst alloying, the distribution of catalyst crystal surfaces, and catalyst morphology [1, 2], and hence on the catalytic activity of the metal

catalysts [3]. The standard by which high-performance catalysts are evaluated includes a uniform composition in the entire nanoparticles, a complete alloying degree, a narrow nanoscale size distribution, and high dispersion on carbon support [4]. Various methods for the synthesis of fuel cell catalysts have been reported in the literature, including micro-emulsion, sputtering, and co-precipitation methods. In this study, we report on the synthesis of catalysts using the impregnation, polyol, modified polyol, and microwave-assisted modified polyol methods.

### **1.1 Impregnation method**

The impregnation approach is frequently utilized for the manufacture of Ptbased catalysts. It is a straightforward chemical preparation process for catalyst synthesis that can create tiny particles in the 3–7 nm range with regulated loading [5, 6]. The impregnation method for the synthesis of platinum-ruthenium (PtRu) includes an impregnation step in which Pt and Ru precursor salts are mixed with the support material, which is typically high-surface-area porous or nanostructured carbon and penetrates into pores. The catalyst support aids in the penetration and wetting of the precursor and the carbon support confines the particle size growth during the reduction step. The chemical reduction can be carried out in the liquid phase with a reductive agent such as Na2S2O3, NaBH4, Na4S2O5, N2H4 or formic acid, or in the gas phase with a reductive agent such as a flowing hydrogen stream at elevated temperatures. The difficulty in adjusting nanoparticle size and distribution is a key limitation of the impregnation process [5, 6]. It has been noted that impregnated catalysts have a tendency to generate inhomogeneous agglomerations of active species at the support boundary, resulting in large-sized particles [1].

Other difficulties include the use of chloride precursors, which could result in chloride poisoning and decreased catalytic activity and stability of the chloride-saltproduced catalyst. Metal nitrate/nitrite salts such as Pt(NH3)2(NO2)2 and RuNO (NO3)x [7], carbonyl complexes such as Ru3(CO)12 [8], and metal sulfite salts such as Na6Pt (SO3)4 and Na6Ru(SO3)4 [9] as metal precursors for Pt and Ru, respectively, have been investigated for impregnation methods that could use chloridefree precursors. When compared to the traditional Cl-containing route, these chloride-free pathways provide improved dispersion and catalytic activity. In this chapter, we report on an impregnation method where NaBH4 was used as the reducing agent and ethylene glycol (EG) as the solvent.

#### **1.2 Polyol reduction method**

The polyol method includes the following common steps: (1) preparation of Ptcontaining colloids; (2) deposition of the colloids onto the support, and (3) chemical reduction of the mixture. The synthesis occurs in an organic or aqueous medium where the metal precursor is reduced chemically in the presence of a protective agent (i.e., NR41, PPh3, PVP, SB12, or PVA). Other colloid methods using several reducing agents, organic stabilizers, or shell-removing approaches have also been developed in recent years. The catalyst is supported with catalyst support to enhance the surface area and the dispersion of the catalyst. To achieve a limited size distribution, the colloidal metal nanoparticles are stabilized by steric hindrance or electrostatic charges. Coating the metal core with organic chain molecules can offer steric stability [10, 11]. The aggregation of charged colloids or adsorbed ions is limited by the electrostatic repulsion of similar charges. The use of protective agents, which may influence the catalytic activity of the nanoparticles, poses a problem for the polyol process, but it may be removed by washing in a suitable solvent or breakdown at temperatures in an inert atmosphere. There are also other

#### *Investigation of Synthesis Methods for Improved Platinum-Ruthenium Nanoparticles… DOI: http://dx.doi.org/10.5772/intechopen.104541*

challenges facing the polyol method, such as that it is time-consuming, complex, and expensive, which causes difficulty in terms of scaling up. The colloidal method prepares catalysts with nanoparticle size and narrow size distribution.

The polyol method that was employed in this study has been extensively explored as a preparation method for Pt [5, 12].

## **1.3 Modified polyol reduction method**

Fievet et al. [13] pioneered the use of EG as both a solvent and a reducing agent. They found that EG may support colloidal metal particles in solution, resulting in a well-distributed solution. EG has a relatively high viscosity, and therefore it prevents Pt from being delivered to reaction sites too quickly, resulting in reduced Pt particle sizes [14]. This method is called the modified polyol method [15]. The modified polyol method is able to effectively synthesize very small and welldispersed metal nanoparticles [2]. However, the synthesis parameters such as the water: EG ratio, the concentration of EG, and the pH of the solution have a great effect on the characteristics of the results [2]. Bimetallic catalysts, metal oxides, and metal sulfides with narrow particle size distributions, controlled compositions, and alloy structures have also been effectively prepared using the modified polyol technique [16]. The modified polyol approach, which uses EG as a reducing agent and solvent, was also used to make catalysts in this study. EG was utilized as both a reducing agent and a solvent for the Pt and Ru precursors in this method. The solution of EG, Pt, and Ru precursor salts was heated to 120–170°C during the reduction phase. EG is decomposed in this step, resulting in the reducing species (CH3CHO-acetaldehyde, Eq. (1)) [17].

$$\text{CH}\_3\text{OCH}\_2\text{OH} \rightarrow \text{CH}\_3\text{CHO} + \text{H}\_2\text{O} \tag{1}$$

$$2\text{CH}\_3\text{CHO} + (\text{PtCl}\_6) + 6\text{OH}^- \rightarrow 2\text{CH}\_3\text{COO}^- + \text{Pt} + 6\text{Cl}^- + 4\text{H}\_2\text{O} \tag{2}$$

As represented in Eq. (2), acetaldehyde converts Pt ions into metallic Pt particles. The main feature of this polyol synthesis is that the acetate can act as a stabilizer for Pt and Ru colloids by forming chelate-type complexes via its carbonyl group. It is therefore unnecessary to use stabilization agents to prevent PtRu particles from agglomerating. As a result of using modified polyol synthesis, carbonsupported catalysts with reduced noble metal sizes and narrow size distribution are achieved.

#### **1.4 Microwave-assisted modified polyol methods**

The microwave synthetic approach was one of the methods employed in this study. A modified polyol method has been reported for this technique, with the deposition and reduction steps taking place in a microwave reactor. Microwave synthetic methodology has been utilized to manufacture catalysts because of its fast, uniform, homogeneous, and instant heating environment, which resulted in rapid reduction and facilitated metal particle nucleation [8, 18]. Microwave heating is a promising technology, with its applications rapidly growing due to its advantages over conventional heating, such as rapid volumetric heating, which increased reaction rates and shortened reaction time; however, to induce crystallization, a post-synthesis heat treatment was required [3]. Under such conditions, a microwave-assisted synthesis method is an appropriate option, with the added benefits of narrow size distribution and high purity [4]. At high pH conditions for depositions, promising results were reported with an average Pt size of 2.7 nm. However, because the reaction takes place in a closed system, the pH cannot be

controlled throughout the duration of the reaction, hence, the entire scope of the reaction under these conditions is unknown [19].
