**Abstract**

Research and development in chemical engineering is currently focused on design of highly active and selective catalytic systems for process intensification. In recent years, there has been growing interest in the use of catalysts based on nanosized metal particles to improve catalytic processes. Among the many metal catalysts, platinum group metals (PGMs) have received greater attention because of their physical and catalytic properties. They have found applications in a wide range of chemical conversion and environmental decontamination reactions due to their chemical stability and enhanced catalytic reactivity in the nano range. This chapter reviews some of the major innovative applications of PGM nanocatalysts for catalytic environmental decontamination.

**Keywords:** nanocatalyst, decontamination, platinum group metal, organic transformation, water treatment

### **1. Introduction**

Industrialization and rapid population growth has resulted in energy shortages and environmental contamination which has raised concern of a potential global crisis. For sustainable human society development, technologies for environmental decontamination need urgent attention. Among numerous available technologies, catalysis has gained considerable attention because of the diverse potentials in energy and environmental applications. Generally, catalysts for environmental applications are based on less expensive materials that will not cause secondary environmental pollution [1]. The major advantage of environmental catalysis is the chemical conversion of pollutants into non-hazardous and less toxic products. Pollutants can be degraded and transformed efficiently through homogenous or heterogeneous oxidation and reduction processes under ambient conditions or conditions in which external energy such as light may be required [2]. Therefore, the present book chapter aims to provide analysis in the use of PGMs nanocatalysts in the recent development and appraise their potential applications in environmental decontamination.

#### **1.1 Nanotechnology and the environment**

Nanotechnology refers to the research and development of materials at the atomic, molecular or macromolecular scale. Materials at nanoscale find applications in a myriad of areas, such as magnetic and optoelectronic, biomedical, pharmaceutical, cosmetic, energy, electronic, catalytic, and environmental domains. Because of the

potential of nanotechnology, there has been a worldwide increase in investment in nanotechnology research and development [3]. The unique properties of materials and their stupendous performance at nanoscale are the main reason for the increased growth in this area. Controlled assembly of nanoparticles has been proposed as one of the most ways to achieve the target technologies [4]. Nanotechnology has immense potential in environmental decontamination through the use of materials such as adsorbents and nanocatalysts. Therefore, it is necessary to develop novel processes for the fabrication of nanomaterials that can be used as the basis for the development of highly efficient new technologies for solving environment challenges.

#### **1.2 Platinum group metals**

Platinum group metals (PGMs) including iridium (Ir), osmium (Os), platinum (Pt), palladium (Pd), rhodium (Rh) and ruthenium (Ru) have high resistance to corrosion and oxidation in moist air, unlike most base metals [5]. Their precious nature derives from their rarity in Earth's crust. It has been reported by Grabowska et al., that PGMs such as platinum and palladium allow the extension of light absorption of semiconductors such as TiO2 into the visible region [6].

Recently, Dozzi et al. has investigated the catalytic effect of PGMs metal for the degradation of formic acid using Pt, Au doped TiO2 [7]. On the other hand, Kisch et al. modified TiO2 with chloride complexes of Pt for the photocatalytic degradation of 4-chlorophenol [8]. The presence of PGMs nanocatalysts gave rise to an enhanced charge carrier separation hence improving the photocatalytic performances of the modified photocatalyst systems. According to Yoon et al., the surface of PGMs serves as visible light absorbing sensitizers and centers of charge separation [9]. The possibility of designing and applying nanosized PGM catalysts has been demonstrated in a number of studies [9–12]. This has resulted in interest in considering their application in environmental remediation, water treatment, chemical transformation and microbial disinfection. A priority task of present day science is to solve the problem of environmental contamination of planetary resources which requires development of efficient technologies employing nanocatalysts. PGMs catalysts have already found applications in a number of industrially important R&D niche areas, including environmental cleanup. The effectiveness of the PGM nanocatalysts depends on a number of factors, including their size, morphology of the particles and their packaging into usable devices and systems. In most of these applications, the cost of PGMs cannot be ignored. Platinum has been reported to be a very good catalyst but a trade-off has to be established between efficacy and cost. In the nano-range the amount of the catalyst is usually kept very low while huge improvements in efficiency are realized. However, there is still need for more effort in design of viable systems for environmental decontamination using PGM based nanocatalysts. Potential up-scaling of these devices and systems is currently an area of increasing research interest.

#### **2. Synthesis of PGM nanocatalysts**

Designing a new class of highly selective and active catalytic systems with the use of recent developments in chemistry has become one of the main concerns faced in contemporary engineering. As compared to bulk material, nanoparticles have an increased surface area and high dispersity and, thus provide high reactivity and allow fabrication of efficient catalysts with lower noble metal loading. Controlling the growth, size, and monodispersity of metal nanoparticles is a subject of interest in designing of nanosized catalytic systems. Therefore, different methods have been used for the synthesis of nanocatalysts. For instance, in 2005, Wong et al., successfully

**45**

**Figure 2.**

*(Copyright Carbon 50 (2012) 586-596, Ref [18]).*

**Figure 1.**

*Platinum Group Metal Based Nanocatalysts for Environmental Decontamination*

cal particles of average size between 2 and 5 nm were reported [14].

*TEM image of 3% Pt/CNT particles (Copyright, J. Mol. Catal. A: Chem., Ref. [17]).*

*XRD patterns of (a) GO, (b) Pt/RGO-HH, (c) Pt/RGO-EG, (d) Pt/AC-EG and (e) Pt/MWCNT-EG.* 

**3. Characterization of PGMs based nanocatalysts**

synthesized a bimetallic catalyst using Pd-on-Au NPs through the Turkevich–Frens (citrate reduction) method and obtained particles with an average diameter of about 20 nm [10]. Paula and co-workers synthesized Pd/C-catalyst using one-pot method for synthesis of secondary amines by hydrogenation of nitrocompounds as single starting materials [11]. Additionally, Coleman et al. synthesized and Pt/TiO2 catalyst by the photodeposition method in the presence of a sacrificial organic hole scavenger [12]. Barakat et al., synthesized Pt doped TiO2 catalyst by immobilizing colloidal Pt nanoparticles onto titanium dioxide (rutile) [13]. Kuvarega et al., used a modified sol gel method for the synthesis of nitrogen, PGM co doped TiO2. In these studies spheri-

Many different physico-chemical techniques are used to characterize nanocatalysts among them scanning electron microscopy (SEM) used for the morphology of the materials. Transmission electron microscopy (TEM) is also used for particles size and morphology. Chun-Hua et al. used TEM to analyze 3% Pt nanoparticles supported on CNT particles is shown in **Figure 1**. In another study, Pt catalyst containing 3% metal

*DOI: http://dx.doi.org/10.5772/intechopen.84192*

*Platinum Group Metal Based Nanocatalysts for Environmental Decontamination DOI: http://dx.doi.org/10.5772/intechopen.84192*

synthesized a bimetallic catalyst using Pd-on-Au NPs through the Turkevich–Frens (citrate reduction) method and obtained particles with an average diameter of about 20 nm [10]. Paula and co-workers synthesized Pd/C-catalyst using one-pot method for synthesis of secondary amines by hydrogenation of nitrocompounds as single starting materials [11]. Additionally, Coleman et al. synthesized and Pt/TiO2 catalyst by the photodeposition method in the presence of a sacrificial organic hole scavenger [12]. Barakat et al., synthesized Pt doped TiO2 catalyst by immobilizing colloidal Pt nanoparticles onto titanium dioxide (rutile) [13]. Kuvarega et al., used a modified sol gel method for the synthesis of nitrogen, PGM co doped TiO2. In these studies spherical particles of average size between 2 and 5 nm were reported [14].

## **3. Characterization of PGMs based nanocatalysts**

Many different physico-chemical techniques are used to characterize nanocatalysts among them scanning electron microscopy (SEM) used for the morphology of the materials. Transmission electron microscopy (TEM) is also used for particles size and morphology. Chun-Hua et al. used TEM to analyze 3% Pt nanoparticles supported on CNT particles is shown in **Figure 1**. In another study, Pt catalyst containing 3% metal

#### **Figure 1.**

*Nanocatalysts*

**1.2 Platinum group metals**

potential of nanotechnology, there has been a worldwide increase in investment in nanotechnology research and development [3]. The unique properties of materials and their stupendous performance at nanoscale are the main reason for the increased growth in this area. Controlled assembly of nanoparticles has been proposed as one of the most ways to achieve the target technologies [4]. Nanotechnology has immense potential in environmental decontamination through the use of materials such as adsorbents and nanocatalysts. Therefore, it is necessary to develop novel processes for the fabrication of nanomaterials that can be used as the basis for the development of

Platinum group metals (PGMs) including iridium (Ir), osmium (Os), platinum (Pt), palladium (Pd), rhodium (Rh) and ruthenium (Ru) have high resistance to corrosion and oxidation in moist air, unlike most base metals [5]. Their precious nature derives from their rarity in Earth's crust. It has been reported by Grabowska et al., that PGMs such as platinum and palladium allow the extension of light

Recently, Dozzi et al. has investigated the catalytic effect of PGMs metal for the degradation of formic acid using Pt, Au doped TiO2 [7]. On the other hand, Kisch et al. modified TiO2 with chloride complexes of Pt for the photocatalytic degradation of 4-chlorophenol [8]. The presence of PGMs nanocatalysts gave rise to an enhanced charge carrier separation hence improving the photocatalytic performances of the modified photocatalyst systems. According to Yoon et al., the surface of PGMs serves as visible light absorbing sensitizers and centers of charge separation [9]. The possibility of designing and applying nanosized PGM catalysts has been demonstrated in a number of studies [9–12]. This has resulted in interest in considering their application in environmental remediation, water treatment, chemical transformation and microbial disinfection. A priority task of present day science is to solve the problem of environmental contamination of planetary resources which requires development of efficient technologies employing nanocatalysts. PGMs catalysts have already found applications in a number of industrially important R&D niche areas, including environmental cleanup. The effectiveness of the PGM nanocatalysts depends on a number of factors, including their size, morphology of the particles and their packaging into usable devices and systems. In most of these applications, the cost of PGMs cannot be ignored. Platinum has been reported to be a very good catalyst but a trade-off has to be established between efficacy and cost. In the nano-range the amount of the catalyst is usually kept very low while huge improvements in efficiency are realized. However, there is still need for more effort in design of viable systems for environmental decontamination using PGM based nanocatalysts. Potential up-scaling of these devices and

highly efficient new technologies for solving environment challenges.

absorption of semiconductors such as TiO2 into the visible region [6].

systems is currently an area of increasing research interest.

Designing a new class of highly selective and active catalytic systems with the use of recent developments in chemistry has become one of the main concerns faced in contemporary engineering. As compared to bulk material, nanoparticles have an increased surface area and high dispersity and, thus provide high reactivity and allow fabrication of efficient catalysts with lower noble metal loading. Controlling the growth, size, and monodispersity of metal nanoparticles is a subject of interest in designing of nanosized catalytic systems. Therefore, different methods have been used for the synthesis of nanocatalysts. For instance, in 2005, Wong et al., successfully

**2. Synthesis of PGM nanocatalysts**

**44**

*TEM image of 3% Pt/CNT particles (Copyright, J. Mol. Catal. A: Chem., Ref. [17]).*

#### **Figure 2.**

*XRD patterns of (a) GO, (b) Pt/RGO-HH, (c) Pt/RGO-EG, (d) Pt/AC-EG and (e) Pt/MWCNT-EG. (Copyright Carbon 50 (2012) 586-596, Ref [18]).*

**Figure 3.**

*FT-IR spectra of (a) the Fe3O4 microspheres, (b) the Fe3O4@C composite, (c) the Fe3O4@C@Pt catalyst ( reproduced with permission from the Royal Society of Chemistry).*

supported on activated carbon (AC) was prepared and the particle size varied from 8 to 10 nm [15]. In addition, techniques such as X-Ray Photoelectron Spectroscopy (XPS) have been used to give information on the oxidation states of the PGM and the nature of bonding between the metals and the supports.

Raman spectroscopy (RS) and X-ray diffraction (XRD) are conducted to identify the crystalline phases and estimate particle sizes of nanocatalysts. For instance, Renfeng et al. used XRD to identify the Pt in Pt/RGO [16]. The Pt peak was conspicuous in the samples containing Pt (**Figure 2**).

Xie and co-workers used Fourier Transform infrared spectroscopy (FTIR) to verify the bond vibrations related to functionalities on the surfaces of their synthesized materials (**Figure 3**) [17].
