**4. Application of PGMs based nanocatalysts**

#### **4.1 Environmental decontamination**

PGMs nanocatalysts have become a new class of environmental remediation materials that could provide affordable solutions to some of the environmental challenges. At nanoscale, PGMs particles have high surface areas and surface reactivity. As such, they provide more flexibility for in situ applications. PGMs nanocatalysts have proven to be an excellent choice for the transformation and decontamination of a wide variety of common environmental contaminants, such as organochlorine pesticides and chlorinated organic compounds. The major consumer of PGMs is the automobile industry. PGMs such as Pt, Rh and Pd are used as catalysts in the automobile industry in order to reduce the level of unburnt hydrocarbons, carbon monoxide (CO) and nitrogen oxide present in the exhaust gases. Generally, a typical automobile converter contains 0.04% Pd, 0.005–0.007%

**47**

reduction of BrO3<sup>−</sup>

**4.3 Antimicrobial**

*Platinum Group Metal Based Nanocatalysts for Environmental Decontamination*

and 0.08% Pt and Rh supported on a base [18]. Iridium has become the new entrant in this application area. A typical example is the introduction of iridium-containing catalytic converters in their direct injection engines by Mitsubishi of Japan [19]. A typical example is the reduction of nitrogen oxides to nitrogen used in car exhaust systems for abatement of emissions from petrol/rich-burn engines. There is a huge number of reports on use of Rhodium as it is the most effective element in

Jianbing et al., in their study, investigated the catalytic ozonation of dimethyl phthalate (DMP) in aqueous solution and DBP precursors in natural water using Ru/AC. These two kinds of organics are both recalcitrant to biodegradation and will

Ru/AC was an active nanocatalysts in the catalytic ozonation of dimethyl phthalate and had the ability to complete mineralize the DMP in a semi-batch experiment. On the other hand, the total organic carbon (TOC) removals were stable around 75% for a duration of 42 h and no trace of Ru was observed from the reactor in the continuous experiments of Ru/AC catalyzed ozonation of DMP. Consequently, Ru/ AC catalyzed ozonation was found to be more efficient than ozonation alone for

<sup>−</sup> and ClO4

<sup>−</sup> are toxic and they are perva-

<sup>−</sup> [32]. Five activated carbon supported

−.

−

−)

was achieved

− than

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

cause severe hazards to human health.

**4.2 Water treatment**

Oxyanions, such as BrO3

have used PGMs for the reduction of BrO3

When catalyst loading 0.1 g L<sup>−</sup><sup>1</sup>

the conversion of nitrous oxides to nitrogen [20, 21].

TOC removals in the natural water treatment [22].

<sup>−</sup> ClO3

than the other PGMs/C catalysts used for the reduction of BrO3

<sup>−</sup>, NO3

sive in drinking water. These oxyanions originate from both anthropogenic and natural sources. Furthermore, they are also produced during water treatment processes such as ozonation, desalination, electrochemical treatment and chlorination [23–26]. These ions have mutagenic, endocrine disrupting and carcinogenic effects [27, 28]. Ion exchange and reverse osmosis cannot completely degrade these oxyanions [28, 29]. Thus, it would be preferable to apply destructive treatment technologies based on nanocatalysts for sustainable drinking water treatment processes [30]. Pd-based heterogeneous catalysis has garnered momentous attention as a potential solution for reduction of these oxyanions and for other highly oxidized contaminants such as halogenated and nitro organics [31]. Chen et al.,

on metal namely (with a 5 wt % Pd, Pt, Rh, Ru and 1 wt% for Ir) with a M/C of 0.1 g L catalysts loading were used for the catalytic reduction of bromated (BrO3

**Figure 4**. From **Figure 4**, it is noticeable that Rh/C was significantly more active

in approximately 5 minutes. Rh/C had a much higher activity than most supported metal catalyst reported in literature when compared with metal mass- normalized basis. While each metal catalyst dispersion differs, it was vital to compare the activity of metals hydrogenation using the initial turnover frequency values (TOFo) [33–35]. Additionally, Rh/C had the best performance compared to the other four catalysts at pH 7.2. On the other hand, Ir/C had a slightly lower apparent reactivity with BrO3

that of Pd/C but was the second highest performance. Therefore, the incorporation of the two PGMs catalysts namely Rh and Ir led to the highest activity for the catalytic

Microbial contamination and growth on the surfaces are risks to human health. The chemicals used to tackle microbials such as detergents, alcohols and chlorine are very aggressive and hence not environmentally friendly besides being

was used, a reduction of 1 mM BrO3

under condition that are suitable for the water treatment systems.

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

and 0.08% Pt and Rh supported on a base [18]. Iridium has become the new entrant in this application area. A typical example is the introduction of iridium-containing catalytic converters in their direct injection engines by Mitsubishi of Japan [19].

A typical example is the reduction of nitrogen oxides to nitrogen used in car exhaust systems for abatement of emissions from petrol/rich-burn engines. There is a huge number of reports on use of Rhodium as it is the most effective element in the conversion of nitrous oxides to nitrogen [20, 21].

Jianbing et al., in their study, investigated the catalytic ozonation of dimethyl phthalate (DMP) in aqueous solution and DBP precursors in natural water using Ru/AC. These two kinds of organics are both recalcitrant to biodegradation and will cause severe hazards to human health.

Ru/AC was an active nanocatalysts in the catalytic ozonation of dimethyl phthalate and had the ability to complete mineralize the DMP in a semi-batch experiment. On the other hand, the total organic carbon (TOC) removals were stable around 75% for a duration of 42 h and no trace of Ru was observed from the reactor in the continuous experiments of Ru/AC catalyzed ozonation of DMP. Consequently, Ru/ AC catalyzed ozonation was found to be more efficient than ozonation alone for TOC removals in the natural water treatment [22].

#### **4.2 Water treatment**

*Nanocatalysts*

**Figure 3.**

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

*FT-IR spectra of (a) the Fe3O4 microspheres, (b) the Fe3O4@C composite, (c) the Fe3O4@C@Pt catalyst* 

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

Xie and co-workers used Fourier Transform infrared spectroscopy (FTIR) to verify the bond vibrations related to functionalities on the surfaces of their synthe-

PGMs nanocatalysts have become a new class of environmental remediation materials that could provide affordable solutions to some of the environmental challenges. At nanoscale, PGMs particles have high surface areas and surface reactivity. As such, they provide more flexibility for in situ applications. PGMs nanocatalysts have proven to be an excellent choice for the transformation and decontamination of a wide variety of common environmental contaminants, such as organochlorine pesticides and chlorinated organic compounds. The major consumer of PGMs is the automobile industry. PGMs such as Pt, Rh and Pd are used as catalysts in the automobile industry in order to reduce the level of unburnt hydrocarbons, carbon monoxide (CO) and nitrogen oxide present in the exhaust gases. Generally, a typical automobile converter contains 0.04% Pd, 0.005–0.007%

nature of bonding between the metals and the supports.

*( reproduced with permission from the Royal Society of Chemistry).*

was conspicuous in the samples containing Pt (**Figure 2**).

**4. Application of PGMs based nanocatalysts**

sized materials (**Figure 3**) [17].

**4.1 Environmental decontamination**

**46**

Oxyanions, such as BrO3 <sup>−</sup> ClO3 <sup>−</sup>, NO3 <sup>−</sup> and ClO4 <sup>−</sup> are toxic and they are pervasive in drinking water. These oxyanions originate from both anthropogenic and natural sources. Furthermore, they are also produced during water treatment processes such as ozonation, desalination, electrochemical treatment and chlorination [23–26]. These ions have mutagenic, endocrine disrupting and carcinogenic effects [27, 28]. Ion exchange and reverse osmosis cannot completely degrade these oxyanions [28, 29]. Thus, it would be preferable to apply destructive treatment technologies based on nanocatalysts for sustainable drinking water treatment processes [30]. Pd-based heterogeneous catalysis has garnered momentous attention as a potential solution for reduction of these oxyanions and for other highly oxidized contaminants such as halogenated and nitro organics [31]. Chen et al., have used PGMs for the reduction of BrO3 <sup>−</sup> [32]. Five activated carbon supported on metal namely (with a 5 wt % Pd, Pt, Rh, Ru and 1 wt% for Ir) with a M/C of 0.1 g L catalysts loading were used for the catalytic reduction of bromated (BrO3 −) **Figure 4**. From **Figure 4**, it is noticeable that Rh/C was significantly more active than the other PGMs/C catalysts used for the reduction of BrO3 −.

When catalyst loading 0.1 g L<sup>−</sup><sup>1</sup> was used, a reduction of 1 mM BrO3 − was achieved in approximately 5 minutes. Rh/C had a much higher activity than most supported metal catalyst reported in literature when compared with metal mass- normalized basis. While each metal catalyst dispersion differs, it was vital to compare the activity of metals hydrogenation using the initial turnover frequency values (TOFo) [33–35]. Additionally, Rh/C had the best performance compared to the other four catalysts at pH 7.2. On the other hand, Ir/C had a slightly lower apparent reactivity with BrO3 − than that of Pd/C but was the second highest performance. Therefore, the incorporation of the two PGMs catalysts namely Rh and Ir led to the highest activity for the catalytic reduction of BrO3<sup>−</sup> under condition that are suitable for the water treatment systems.

#### **4.3 Antimicrobial**

Microbial contamination and growth on the surfaces are risks to human health. The chemicals used to tackle microbials such as detergents, alcohols and chlorine are very aggressive and hence not environmentally friendly besides being

#### **Figure 4.**

*Kinetics of 1 mM BrO3 <sup>−</sup> by 0.1 g L<sup>−</sup><sup>1</sup> M/C catalyst at 1 atm H2, pH 7.2 and 22o C (with 5 wt% metal for Pd, Rh, Ru, and Pt; 1 wt% metal for Ir). (Copyright Chemical Engineering Journal, 313, 2017, 745-752, Ref. [28]).*

**Figure 5.** *UV–vis spectra of bare TiO2 and PdO/TiO2 samples. (Copyright, Chemistry 184 (2006) [34]).*

ineffective for long-term disinfection. Therefore, a myriad of studies are being conducted in order to tackle these challenges. Arcan et al. doped SnO2 and TiO2 with Pd for microbial inactivation of *E. coli, S. aureus* and *S. cerevisiae* [36].

The addition of Pd led to an enhancement in the photocatalytic efficiency observed for the degradation of microorganisms when 1% of Pd was used. In addition, the UV–Vis showed an extension of the absorption edge into the visible range without affection the phase of the catalysts (**Figures 5** and **6**).

#### **4.4 Chemical transformation**

PGM nanoparticles have proven to be efficient heterogeneous and homogeneous catalyst with advantages such as a high specific surface area due to their small

**49**

**Figure 8.**

*Ref. [38]).*

**Figure 6.**

**Figure 7.**

*Commun., 2013, 49, 8160, Ref [10]).*

*Platinum Group Metal Based Nanocatalysts for Environmental Decontamination*

*UV–vis spectra of bare SnO2 and PdO/SnO2 samples. (Copyrights Chemistry 184 (2006) Ref. [34]).*

*Pd/C-catalyzed one-pot synthesis of secondary amines by hydrogenation of nitrocompounds. (Copyright Chem.* 

*Proposed mechanism for the hydrogenation of nitroarenes. (Copyright ChemCatChem, 2009, 1, 210–221,* 

*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*

#### **Figure 6.**

*Nanocatalysts*

**48**

**Figure 5.**

**Figure 4.**

*Kinetics of 1 mM BrO3*

*<sup>−</sup> by 0.1 g L<sup>−</sup><sup>1</sup>*

ineffective for long-term disinfection. Therefore, a myriad of studies are being conducted in order to tackle these challenges. Arcan et al. doped SnO2 and TiO2 with Pd

 *M/C catalyst at 1 atm H2, pH 7.2 and 22o*

*Ru, and Pt; 1 wt% metal for Ir). (Copyright Chemical Engineering Journal, 313, 2017, 745-752, Ref. [28]).*

*C (with 5 wt% metal for Pd, Rh,* 

The addition of Pd led to an enhancement in the photocatalytic efficiency observed for the degradation of microorganisms when 1% of Pd was used. In addition, the UV–Vis showed an extension of the absorption edge into the visible range

PGM nanoparticles have proven to be efficient heterogeneous and homogeneous

catalyst with advantages such as a high specific surface area due to their small

for microbial inactivation of *E. coli, S. aureus* and *S. cerevisiae* [36].

*UV–vis spectra of bare TiO2 and PdO/TiO2 samples. (Copyright, Chemistry 184 (2006) [34]).*

without affection the phase of the catalysts (**Figures 5** and **6**).

**4.4 Chemical transformation**

*UV–vis spectra of bare SnO2 and PdO/SnO2 samples. (Copyrights Chemistry 184 (2006) Ref. [34]).*

#### **Figure 7.**

*Pd/C-catalyzed one-pot synthesis of secondary amines by hydrogenation of nitrocompounds. (Copyright Chem. Commun., 2013, 49, 8160, Ref [10]).*

#### **Figure 8.**

*Proposed mechanism for the hydrogenation of nitroarenes. (Copyright ChemCatChem, 2009, 1, 210–221, Ref. [38]).*


#### **Table 1.**

*Hydrogenations performed using Pt/C as catalysts (Catal. Sci. Technol., 2014, 2445, Ref. [39]).*

size resulting in a high number of potential catalytic sites [37]. Owing to these phenomena, there has been in the past two decades an increase in the use of metal nanoparticles in catalysis [38, 39]. Rong et al., reported the synthesis of supported Pt NPs and their use as catalysts in the partial hydrogenation of nitroarenes to arylhydroxylamines [40]. The particles were prepared by reduction of H2PtCl6 with NaBH4 in the presence of the carbon support. The hydrogenation of several substituted nitroarenes was performed under soft conditions (10.15°C, 1 bar H2) to favor the formation of hydroxylamines, showing excellent activity and selectivity in this transformation. Pd supported on C has also been successfully used as catalysts for the conversion of nitrobenzenes to secondary amines (**Figure 7**).

The proposed mechanism for the hydrogenation of nitrobenzene was proposed as shown in **Figure 8**. During this process, there is generation of intermediates such as hydroxylamines, azo and azoxy derivatives.

For example, in the case of m-dinitrobenzene, a catalyst containing 2 wt% Pt/C yielded 92.3% of the corresponding hydroxylamine after 190 min of reaction in THF (**Table 1**).

In a study, Zeming et al. used carbon as Pt colloid support for the hydrogenation of arylhydroxylamines. The Pt colloid supported on carbon was an active and selective catalyst for the partial hydrogenation of nitroaromatics with electronwithdrawing substituents to the corresponding N-arylhydroxylamine, indicating an additive-free green catalytic approach for arylhydroxylamine synthesis. Very encouraging results were obtained with N-arylhydroxylamine bearing electronwithdrawing substituents. Since N-arylhydroxylamine can be further converted to highly valuable compounds through several reactions like Bamberger rearrangement, this result will generally contribute to a simpler and greener synthetic methodology of N-arylhydroxylamine derivatives.

#### **5. Future perspectives**

Progress has been reported in the application of nano-PGMs for heterogeneous catalysis reactions. While most of the applications have centered on organic transformations, there is potential for extending the catalytic potential of these metals to other fields such as pollutant degradation and microbial inactivation in water

**51**

**Author details**

**6. Conclusion**

**Acknowledgements**

**Conflict of interest**

tion of this book chapter.

expressed.

Johannesburg, South Africa

provided the original work is properly cited.

Sarre M.K. Nzaba, Bhekie B. Mamba and Alex T. Kuvarega\*

\*Address all correspondence to: kuvarat@unisa.ac.za

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Nanotechnology and Water Sustainability Research Unit, College of Science, Engineering and Technology, University of South Africa, Florida Campus,

*Platinum Group Metal Based Nanocatalysts for Environmental Decontamination*

be done to in environmental catalysis for water treatment.

treatment processes. On their own, PGM at the nanoscale tend to aggregate and thus limited application has been realized. However, the use of supports has greatly enhanced the activity of the PGM nanocatalysts in various fields. Mono and bimetallic systems have been reported on. In environmental decontamination processes there is still need to find the most suitable supports and application devices. While encouraging findings have started appearing in literature, more work still needs to

Several efforts have been devoted to the preparation of PGM nanocatalysts for application in environment decontamination. The high number of literature reports highlights the interest in this family of catalysts for catalytic transformation, both in terms of reactivity and selectivity. There is potential for application of PGMs nanocatalysts for environmental decontamination, water treatment, antimicrobial and chemical transformation. Further studies are necessary to better understand parameters influencing the reactivity as well as enhancing the conversion rates and efficiencies.

Appreciation towards funding received from the University of South Africa (UNISA), the National Research Fund (NRF) and support from the Nanotechnology and Water Sustainability and Research Unit (NanoWS) is highly

The authors declare that there is no conflict of interests concerning the publica-

*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*

treatment processes. On their own, PGM at the nanoscale tend to aggregate and thus limited application has been realized. However, the use of supports has greatly enhanced the activity of the PGM nanocatalysts in various fields. Mono and bimetallic systems have been reported on. In environmental decontamination processes there is still need to find the most suitable supports and application devices. While encouraging findings have started appearing in literature, more work still needs to be done to in environmental catalysis for water treatment.
