Antecedentes Investigativos

Accurate characterization of DENs is critical for correlating their structure and function. However, two of the principal attributes of DENs, their small size and their location within the interior of a bulky hydrocarbon framework, make this task difficult. Additionally, as synthesis methods have continued to improve and yield ever more complex DENs, the analytical demands have become correspondingly greater [21].

The structural characteristics of DENs include their crystallinity, degree of disorder, and degree of spatial segregation of the two metals when considering bimetallics. An understanding of surface structure is particularly important, because it can directly influence catalytic reactions. The small size of DENs impedes structural characterization techniques that require long-range order, such as typical X-ray diffraction (XRD). However, Extended X-ray absorption fine structure (EXAFS) and X-ray diffraction-pair distribution function (XRD-PDF) are useful for analyzing even the smallest DENs. EXAFS analysis provides information about the coordination environment of an absorbing atom, including the number, type, and bond distances of neighboring atoms [91, 92]. EXAFS is particularly powerful for analyzing DENs, because the average coordination number drops precipitously as particle size decreases and a higher fraction of atoms reside on the particle surface. For example, surface-to-interior atom ratios for 147- and 55-atom cuboctahedra are 1.7 and 4.2, respectively, and therefore EXAFS is quite sensitive to small changes in the size (and shape) of DENs [93]. XRD-PDF measures the distribution of mass in a sample and can be used to determine atomic structure in amorphous materials.

In addition to providing critical insights into the formation of DENs, nuclear magnetic resonance (NMR) techniques may also be used to understand several aspects of the final nanoparticle architecture. NMR stands out here, because it is able to resolve molecular architectures at the surface of the solid phase nanoparticle [94]. A basic property of any nanoparticle is its size. NMR is a useful tool to measure the hydrodynamic radius of metal nanoparticles and can provide an important complement to traditional nanoparticle sizing techniques, such as electron microscopy and dynamic light scattering (DLS). Similar to DLS, the NMR signal can be used to determine nanoparticle size via analysis of particle diffusion [95]. Specifically, NMR uses pulsed-field gradient (PFG) techniques to extract diffusion coefficients of well-dispersed species

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in solution diffusing according to Brownian motion only. Under these conditions, the hydrodynamic size is calculated by rearranging the Stokes-Einstein Equation (1.11) [96]:

(1.11)

where D is the diffusion coefficient, kB is the Boltzmann constant, T is temperature, η is viscosity of the solvent, and RH is the hydrodynamic radius of the diffusing species. Various characterization techniques are shown in Figure 1.11.

Figure 1.11: Different characterization techniques for nanocatalysts

The average size of nanoparticles can be determined through the use of HRTEM and the images analysed with ImageJ software [97] and compared with calculated size. The calculated size of nanoparticles can be obtained from using Equation 1.12, where n is the total number of atoms of the metal, Vg is the molar volume of the metal involved and r represents the radius of the nanoparticle [32, 98, 99].

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(1.12)

1.8.1 Model reactions

Model reactions are frequently used in kinetic studies because the conversion of reactants to products is clean. There are no by-products formed, and the reaction can be monitored by simple spectroscopic techniques like UV-vis or IR spectroscopy. Despite their ease of execution, the mechanisms of these model reactions are still not fully understood.

Reduction of 4-nitrophenol (4NP) to 4-aminophenol (4AP) using sodium borohydride (BH4-) as

the reducing agent, is one of the widely used model reactions [21, 61, 63, 100]. 4NP and its derivatives are used in the synthesis of pesticides, herbicides and insecticides. The United States of America Environmental Protection Agency classified 4NP as one of the major pollutants and hazardous waste and toxic pollutant. Beyond its scope as a model reaction, the reduction of 4- nitrophenol is also important in pharmaceutical processes, because 4AP is an important intermediate in commercial manufacturing of analgesic and antipyretic substances such as acetanilide, paracetamol and phanacetin.

Catalytic degradation of 4NP is conducted in an aqueous medium by observing the disappearance of the 4NP peak at λ 400 nm and the appearance of the 4AP peak at λ 317 nm in the UV-vis spectrum. There is, however, lack of information regarding the complete mechanism of this reaction. In their extensive study, Saha and co-workers [101], found that both the substrate and the reducing agent adsorb onto the surface of the catalyst before the reaction takes place. Following this step is the conversion to products then finally desorption of the product from the surface. A similar study has been undertaken by Zhang and co-workers [102] and provided similar results. They concluded that the borohydride ion transfers a hydrogen species to the surface of the catalyst and 4NP is adsorbed to the surface as well.

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Scheme 1. 2: Direct route for the reduction of 4-nitrophenol by metallic nanoparticles: in Step A, 4-nitrophenol (4NP) is first reduced to the 4-nitrosophenol and then converted to 4- hydroxylaminophenol (Hx) quickly, which is the only stable intermediate. In Step B, Hx is reduced to the final product, namely 4-aminophenol (4AP).