1. INTRODUCTION

Nanotechnology is one of the most promising emerging technologies for efficient, economical and environmentally friendly in the production of value-added goods or products. The worldwide turnover of nanotechnological applications alone has reached an estimated sum of USD 6.6 billion in 2015. To achieve a long-term sustainable development in socio-economic front, it is utmost necessary to consider the transformation of readily available resources into useful products without causing any ecological disturbances. In this respect, chemical industries play a vital role in transforming the raw materials into desired products that we use in our day-to-day life. It is very important to understand that how the chemical industry has

touched our primary facets of life namely, food, health and energy sectors [1]. It is also equally important to observe that our recycling industries start minimising the usage of virgin starting materials to finish their end products.

Nanoparticles are describedas materials which are sized between 1 and 100 nm. Due to this extremely small size, the properties of nanoparticles such as physical, chemical, optical, electrical and magnetic are significantly different from the conventional materials. Because of theseexceptional properties, the nanomaterials are widely used in many industrial applications. Due to their smaller size and larger surface area, nanoparticles show strong adsorption capacities and reactivity in chemical transformation of reactants to products. In addition, the high mobility of nanomaterials in solution, makes them more useful in water and wastewater treatment.

2. TYPES OF NANOPARTICLES

The most extensively studied nanoparticles are the zero-valent nanoparticles, metal oxide nanoparticles and nanocomposites.

2.1. ZERO-VALENT NANOPARTICLES

In recent years, various zero-valent metal nanoparticles such as Fe, Zn, Al, Ag and Ni have drawn a wide research interest. They are highly toxic to microorganisms and have a strong bacterial effect against viruses, bacteria and fungi [2]. As a good anti-microbial agent, the Ag NPs have been successfully used in water and wastewater treatment. Direct application of Ag NPs might cause some problems, such as their tendency to aggregate in aqueous media that gradually reduces their efficiency during long-term use. The Ag NPs attached to an inert filter material have been quite promising for water disinfection due to their high antibacterial activity and cost-effectiveness. The Ag NPs sheets show antibacterial properties towards suspensions of E Coli and E faecalis and deactivate them during filtration through the sheet [3].

2.2. METAL OXIDES NANOPARTICLES

Metal oxides nanoparticles are of technological importance in environmental remediation because of their capability of generating charge carriers when stimulated with required amount of energy. The promising arrangement of electronic structure, light absorption properties and charge transport characteristics have made the metal oxides nanoparticles (MO NPs) as an efficient photocatalyst [4].

Among many MO NPs, titanium dioxide nanoparticles (TiO₂ NPs), zinc oxide nanoparticles (ZnO NPs), Ceria nanoparticles (CeO NPs) and so on have been used in photocatalytic degradation of volatile organic pollutants. In presence of light, photocatalyst gradually oxidises the pollutants/contaminants into low molecular weight intermediate products and eventually transform them into CO₂, H₂O and anions such as NO₃^-. PO₄^3- and Cl^-, TiO₂ NPs have been extensively studied for its high photocatalytic activity and stability. The large band gap energy of TiO₂ NPs (3.2 eV) requires near UV light / Visible light irradiation to dissociate charge carriers (e^- + h⁺) within the particles. The electrons (e^-) from the conduction band (CB) are promoted to the valence band (VB) and thereby leaving the hole (h⁺) in the CB. In this way, there is a formation of an electron-hole pair, as shown in Figure 2.2. The photogenerated pair

(e^-/h⁺) can reduce and/or oxidise a pollutant/contaminant adsorbed on the surface of the TiO₂ NPs.The hole either oxidises the pollutant directly or reacts with water to produce hydroxyl radicals (OH*). Whereas, the electron present in the CB reduces the oxygen adsorbed (from the atmosphere) on the surface of the TiO₂ NPs. This photo-mechanism, as described above, is illustrated as shown in equations 1-3:

TiO₂ NPs + hﬠà TiO₂*à e^-_CB + h⁺_VBà recombination à heat (1)

O₂ + e^-_CBà O₂^- + H₂O à OH^- + HO₂* à HO₂* + HO₂* àH₂O₂ (2)

h⁺_VB + OH^-à OH* + Pollutant/Contaminant à degraded products (3)

There are many types of MO NPs photocatalysts used for various reactionsthat have been reported in the literature. The ZnO NPs and CeO₂ NPs have emerged as efficient candidates because of their unique photocatalytic properties [5]. CeO₂ NPs with different oxidation states and various crystalline structure have been explored for different applications. A remarkable property of CeO₂ NPs is the number of effective redox Ce⁴⁺/Ce³⁺ sites and their ability to exchange oxygen [6,7]. In the recent years, there is a growing interest in the use of iron oxides nanoparticles (Fe₂O₃ / Fe₃O₄NPs) for the removal of heavy metals due to their simplicity and availability [8]. Another class of new materials, namely, carbon nanomaterials (CNMs) show an exceptional behaviour due to its unique structures and electronic properties. There are several forms of CNMs, such as carbon nanotubes (CNTs), carbon fibres, and nanoporous carbon. Among them, CNTs have been tried in many applications [9] .

2.3. NANOCOMPOSITES

It is interesting to note that every nanomaterial has its own drawback. For example, zero-valent nanoparticles have the disadvantage of aggregation, oxidation and separation difficulty from the reaction products after the reaction gets completed. The light absorption of TiO₂ and ZnO NPs is limited to UV light region due to their high band gap energies. Nanofiltration membranes are troubled by the membrane fouling.

In recent years, the synthesis of nanocomposites has become the most active subject in the field of nanomaterials. Numerous studies have been made throughout the world. For example, via chemical deposition of zero-valent metal nanoparticles on carbon nanotubes (CNT)s, a novel nanoscale adsorbent was prepared. However, CNT have a limitation due to their low volume production and high costs. CNTs have shown an extraordinary potential for effective removal of anions from wastewater. Besides, due to their unique magnetic property, the adsorbent can be separated easily from the products by using the magnetic field [10].

3. SUPPORTED METAL ANOCATALYSTS

Nanocatalyst are widely exploited in many industries, particularly the chemical, pharmaceuticals, agrochemicals and petrochemicals sectors. These catalysts should withstand the conditions within the chemical reactor during the course of the reaction without undergoing attrition and/or degradation, Although, some catalysts can be used in bulk or non-supported form, many involve materials that cannot be used directly without the aid of an additional material, termed as catalyst support. They are many reasons why a support is being used. It can be either to impart stability or to minimise agglomeration or to reduce the cost by dilution.

3.1. ACTIVATED CARBON SUPPORTED GOLD NANOCATALYST (Au/AC)

A variety of gold-based nanocatalysts with different particle sizes of gold and deposited over different supports like Al₂O₃, SnO₂, CeO₂, TiO₂ and Activated Carbon (AC) have been reported for a variety of reactions, such as oxidation, hydrogenolysis, hydration and so on [11-14]. To illustrate, an industrially important reaction like oxidation of glycerol to glyceric acid on a nanocatalyst containing gold (Au) supported on Activated Carbon (Au/AC) is discussed here as a model reaction [15]. Glycerol is derived from bio-sustainable resources. Glycerol is widely used as an intermediate in various agro- and pharmaceutical industries [16]. The three hydroxyl groups associated with glycerol molecule give rise to variety of products. By controlling the reaction parameters, either the primary or secondary hydroxyl group, one could selectively oxidise glycerol into different end products. Amongst them, glyceric acid is most useful in pharmaceutical industries.

3.1.1. SYNTHESIS OF Au/AC NANOCATALYST

A series of Au/AC nanocatalysts was synthesised by homogeneous deposition-precipitation (HDP) method. Activated Carbon (AC) was used as an active catalysts support, since it is highly microporous, excellent stability and longer life under reaction conditions. According to this HDP method, the solution mixture containing an aqueous solution of gold (III) chloride hydrate (HAuCl₄. 3H₂O) with desired gold content and urea [CO(NH₂)₂] were stirred in a beaker with gradual heating until 95^oC for 6 h. The gradual heating helps to decompose urea into ammonia which gets precipitated homogeneously in the solution as the pH shift towards basic conditions (pH = 6 to 8). The support, AC was now added to the solution mixture while stirring. The freshly prepared 0.1 M sodium borohydride (NaBH₄) was mixed to the above solution mixture to reduce the gold ion particles over the AC support. The precipitate formed was filtered and washed several times with deionised water to ensure that no chloride ions were left on the catalyst surface. Finally, the catalyst samples were first dried in an air oven at 100^oC and then calcined at 400^oC for 3 h in N₂ atmosphere.

3.1.2. CHARACTERISATION OF Au/AC NANOCATALYST

These freshly prepared nanocatalyst were characterised by different techniques to understand their structural and textural properties of the nanocatalysts. Some of the techniques employed were as follows: X-ray diffraction (XRD), Transmission electron microscopy (TEM) andN₂ adsorption-desorption(BET)measurements one can find the available specific surface area, pore-size and pore-volume of the sample materials.

Figure [2] shows the XRD patterns of as prepared Au/AC catalysts. The two broad peaks observed at the 2θ values of 25º and 44º show the two diffraction planes, (002) and (100) respectively, associated to AC support. The sharp peaks observed at the 2q values of 38.2º, 44.5º, 64.6º and 77.6° are of Au nanoparticles [17]. Also, the major diffraction peak of AC

remains unchanged in all Au/AC catalysts, thus confirming that nature of the support is not altered by the homogeneous deposition precipitation (HDP) method. The intense nature of diffraction peaks corresponding to Au confirms that it has a highly crystalline structure.

The size distribution of 1 wt% Au nanoparticles over AC support is shown in Figure [3]. The mean diameter of the particles calculated from the images were in the range of 6 to 8 nm.The data also reveal that the size of Au nanoparticles is larger than the average pore diameter of AC support. This confirms that the active metal particles are more available on the surface of the catalyst for the oxidation reaction.

The nitrogen adsorption/desorption is a unique way to investigate the textural properties of surface of the catalysts such as surface area, average pore diameter, and pore volume of the porous materials. The BET surface area for pure AC and various Au/AC catalysts are reported in Table [1]

Table.1. Textural properties of activated carbon and various Au/AC nanocatalysts

Au loading (wt%)	Au content^[a] (wt%)	Surface area^[b] (m²/g)	V_t (cc/g)	D_BJH (nm)	Binding Energy (eV)
Au loading (wt%)	Au content^[a] (wt%)	Surface area^[b] (m²/g)	V_t (cc/g)	D_BJH (nm)	4f_5/2	4f_7/2
0.0	0.00	1340	0.88	3.98	---	---
0.5	0.48	1278	0.86	3.88	87.27	83.80
1.0	0.84	1210	0.82	3.71	87.24	83.72
2.0	1.65	1160	0.78	3.65	87.22	83.65
^[a]Gold content measured by ICP-OES. ^[b]BET method. V_t : total pore volume. D_BJH : average pore diameter calculated by BJH adsorption method.

From Table [1], it can be observed that the specific surface area of Au/AC catalysts decreases with increase in Au loading. This may be due to higher Au content; the smaller size nanoparticles of Au can block the pores of the support and causes the surface area to decrease.Figure. [4] shows the XPS analysis of Au present in 1 wt% Au/AC catalyst and the corresponding data calculated are given in Table [1]. The binding energy of Au 4f_7/2 and Au

4f_5/2 observed at 84.0 eV and 87.7 eV, respectively, were significantly different from Au⁺ 4f_7/2 (84.6 eV) and Au³⁺ 4f_7/2 (87.0 eV). This confirms that the binding energy values are corresponding to Au nanoparticles which are in pure metallic state [14]. The XPS results also confirm that Au nanoparticles on the surface of the support AC are in zero valence state. The absence of the peaks at the binding energies of 84.6 eV (4f_7/2) and 87.0 eV (4f_7/2) which represent the two cationic form of Au, namely Au⁺and Au³⁺ oxidation states, respectively may be responsible for the formation of zero valence state of Au nanoparticles on the surface of the catalyst.

3.1.3. REACTOR SET-UP AND ACTIVITY STUDY

A fixed-bed vertical down flow reactor was used to carry out the oxidation reaction of glycerol to glyceric acid under normal atmospheric pressure, as shown in Figure 3.1.3.

In a normal run, Ca. 1.5 g of nanocatalyst was packed between layers of quartz wool in the reactor. Initially the samples were activated at 250^oC for 2 h in N₂ atmosphere after which a solution mixture of glycerol: water (10:90 w/w) was fed into the reactor in a stoichiometric quantity (WHSV = 2.52 h^-1) via a syringe pump by passing air along with the reactant at 300^oC. The reaction products were collected at an hourly interval in an ice-trap kept below the reactor. The reaction products were analysed by a gas-chromatograph (GC) having Field-ion detector (FI) and Thermal conductivity detector (TCD). From the GC data, the conversion of glycerol converted and the selectivity of glyceric acid (along with other secondary products) were estimated.

The catalytic activity of the Au/AC nanocatalyst was tested in terms of conversion of glycerol and selectivity for glyceric acid formed. Both glycerol conversion and selectivity for glyceric acid were influenced by the nature of the support material (AC), size of the gold nanoparticles present in the nanocatalyst and the reaction parameters such as feed rate, nanocatalyst weight and the reaction temperature. The 1.0 wt% of Au/AC nanocatalyst showed an optimum performance in terms of conversion and selectivity for glyceric acid. This enhanced nanocatalyst activity is attributed to the higher dispersion of smaller nanoparticles of goldon the surface. The reason for less activity at higher loading of gold is attributed to the formation of larger gold particles on the surface due to agglomeration of smaller gold nanoparticles during reaction. This was confirmed from the XRD and TEM results. When the reaction temperature was varied within a range of 240 to 320^oC, an optimum conversion of glycerol (80%) and selectivity for glyceric acid (82%) was observed at 300^oC. To understand the stability and recyclability of Au/AC nanocatalyst using glycerol oxidation, the fresh Au/AC nanocatalyst were tested for time-on-stream (TOS) study. An optimum conversion was observed for the first 2 h irrespective of gold loading on the surface. But beyond 2 h, the conversion of glycerol started decreasing. However, the selectivity for glyceric acid remained constant.

4. CONCLUSION

In summary, nanoscale science and technology have shown an enormous potential towards economic growth and development. Newer nanostructured materials have developed and tested in many areas, such as medicine, biology, sensing, manufacturing industry and so on. The most extensively studied nanomaterials include metal nanoparticles, metal oxide nanoparticles and nanocomposites. Recently, it is reported that nanomaterials are being used in producing high volume but low-priced chemicals as well as for the low volume but high-priced chemicals. In this paper, the importance of gold nanoparticles deposited over activated carbon support has been cited as an example for the application of a nanocatalyst (Au/AC) for an industrially important reaction, such as oxidation of glycerol to glycericin vapour phase medium. The reaction study suggests that smaller nanoparticles of gold are finely dispersed over the catalyst support, activated carbon by the HDP method. This was confirmed from the XRD, TEM and N₂ adsorption results. The spectroscopic results suggest that nanoparticles of gold are present in a zero-valent state (Au^o). The oxidation reaction suggests that an optimum conversion of glycerol and selectivity of glyceric acid are observed for a 1.0 wt% Au/AC nanocatalyst at a reaction temperature of 300^oC under atmospheric pressure.

In addition to this, even though it is established now that nano gold or nanosilver have been usedin certain specific applications, such for decontamination of water, it is still an open question why these two metals in the nano state has this particular property. These two metals in nano state might be having electronic configuration and unoccupied levels suitable for promoting the oxidation reactions or these two metals in nano state have possibly active sites so that the pollutants are preferentially adsorbed on its surface and thus the electronic structure of the pollutant in the adsorbed state possibly have electronic energy level perturbation facilitating the oxidation reaction. These aspects will become clear with new results emerging in the future.

REFERENCES

[1] K. Locharoenrat, Review Article: Recent Advances in Nanomaterial Fabrication, Journal of Physics, Vol.295, 2014.

[2] S. Prabhu and K. K. Poulose, Silver nanoparticles: mechanism of antimicrobial action, synthesis, medical applications, and toxicity effects, International Nano Letters, Vol.2, No.1, 2012, pp.32.

[3] J. Sondiand B. Salopek-Sondi, Silver nanoparticles as antimicrobial agent: A case study on E. Coli as a model for gram-negative bacteria, Journal of Colloid and Interface Science, Vol.275, No.1, 2004, pp.177-182.

[4] Syed Farooq Adil and Abdullah Al-Mayouf, Metal oxides as photocatalysts, Vol.19, 2015, pp.462-464.

[5] Aracely Hernandez – Ramiraz and Iliana Medina – Ramiraz, Photocatalytic conductors, Springer, 2015, ISBN 978-3-319-10998-5.

[6] M. Farahmandjou, M. Zarinkamar and T. P. Firoozabadi, Synthesis of cerium oxide (CeO₂) nanoparticles using simple CO-precipitation method, Revista Mexicana de Fisica, Vol.62, 2016, pp.496-499.

[7] Pol Reshma and Khandavalli Ashwini, Ceriumoxide nanoparticles: Synthesis, characterisation and study of antimicrobial activity, Journal of Nanomaterials & Molecular Nanotechnology, Vol.6, No.3, 2017.

[8] Y. Lei, F. Chen, Y. Luo and L. Zhang, Three - dimensional magnetic graphene oxide foam / Fe₃O₄ nanocomposite as an efficient absorbent for Ce(VI)removal, Journal of Materials Science, Vol.49, No.12, 2014, pp.4236-4245.

[9] M. M. Khin, A. S. Nair, V. J. Babu, R. Murugan and S. Ramakrishna, A review on nanomaterials for environmental remediation, Energy & Environmental Science, Vol.5, No.8, 2012, pp.8075-8109.

[10] S. J. Tesh,and T. B. Scott, Nano-composites for water remediation: A review, Advance Materials, Vol.26, No.35, 2014, pp.6056-6068.

[11] Ashish Kumar, V. P. Kumar, B. P. Kumar, V. Vishwanathan and K. V. R. Chary, Vapour phase oxidation of benzyl alcohol over gold nanoparticles supported on mesoporous TiO₂, Catalysis Letters, Vol.144, 2014, pp.1450-1459.

[12] Ashish Kumar, V. P. Kumar, V. Vishwanathan and K. V. R. Chary, Synthesis, characterisation and reactivity of Au/MCM-41 catalysts prepared from homogeneous deposition-precipitation (HDP) method for vapour phase oxidation of benzyl alcohol, Materials Research Bulletin, Vol.61, 2014, pp.105-112.

[13] Ashish Kumar, V. P. Kumar, V. Vishwanathan and K. V. R. Chary, Influence of preparation methods of nano Au/MCM-41 catalysts for vapour phase oxidation of benzyl alcohol, Journal of Nanoscience and Nanotechnology, Vol.15, 2015, pp.9944-9953.

[14] Ashish Kumar, V. P. Kumar, A. Srikanth, V. Vishwanathan and K. V. R. Chary, Vapour phase oxidation of benzyl alcohol over nano Au/SBA-15 catalysts: Effect of preparation methods, Catalysis Letters, Vol.146, No.1, 2016, pp.35-46.

[15] Ashish Kumar, R. K. Gautam, Mamta Belwal, R. R. Maurya and V. Vishwanathan, Catalytic vapour phase oxidation of glycerol to glyceric acid over Au/AC nanocatalysts, Journal of Nanoscience and Nanotechnology, 2019 (submitted).

[16] E. G. Rodrigues, M. F. Pereira, X. Chen, J. J. Delgado and J. J. M.Orfao, Influence of activated carbon surface chemistry on the activity of Au/AC catalysts in glycerol oxidation, Journal of Catalysis, Vol.281, No.1, 2011, pp.119-127.

[17] Fang Wang and Gongxuan Lu, The effect of K addition on Au. Activated Carbon for CO selective oxidation in hydrogen-rich gas, Catalysis Letters, Vol.115, No.1-2, 2007, pp.46-51.