## 3 Results and Discussion
### Catalytic Properties of Au-ZrO\({}_{2}\)  
The catalytic efficiency of Au-ZrO\({}_{2}\) was tested for the reduction of 4-nitrophenol, which is a highly toxic pollutant, typically derived from industrial wastes [28]. 4-nitrophenol is typically removed from the wastewater by various processes including adsorption, degradation, and electrochemical treatment, and so on [29]. On the other hand, the catalytic reduction of 4-nitrophenol and other nitro-aromatics to amino-phenols has gained significant interest in the scientific community [30]. The catalytic reduction of nitrophenols to amino-phenols is an important class of reaction due to the significance of amino-phenols as intermediates in various processes [31]. Particularly, 4-aminophenol has several applications such as a starting material and intermediate for various drugs, as a corrosion inhibitor, etc. [32; 33]. Additionally, the 4-nitrophenol reduction is widely used as a model catalytic reaction, due to its well-controlled process which usually yields a single product at mild temperature and ideally requires a catalyst to proceed [34; 35]. The reduction of 4-nitrophenol is usually carried out in the presence of NaBH\({}_{4}\) as a reductant and various metal nanoparticles such as Pd, Pt, Ni, Ag, and Au as catalysts [36]. Among several metal NPs, Au NPs often exhibit superior catalytic properties for this type of reaction due to their unique properties. For instance, the catalytic properties of Au NPs are dependent on their particle size, where smaller size NPs show higher catalytic activities and  
Figure 4: TEM, SEM, and EDX analysis of Au-ZrO\({}_{2}\). (**a**) The dark contrast in the TEM image indicates the presence of gold nanoparticles (**b**) the color mapping of the sample under SEM analysis clearly indicates the presence of Zr and gold in the sample. (**c**) EDX also points towards the presence of Au and Zr in The Au-ZrO\({}_{2}\) Sample. also are active under mild conditions [37; 38]. For example, gold aryl based core-shell nanoparticles demonstrated high catalytic activity under mild conditions towards the reduction of 4-nitrophenol in short reaction time (5 min) [39]. Usually, the catalytic property of smaller size, naked Au NPs is adversely affected by the aggregation of NPs [3]. To address this issue, different types of ligands are applied as stabilizers and/or the NPs are dispersed on various supports to prevent the aggregation and to enhance the surface area of the catalysts [40]. Recently, activated coke was used as a support for anchoring the Au NPs to prepare Au/AC catalysts, which exhibited high catalytic properties during 4-nitropenol reduction [39]. Very recently, a trace amount of Pd was used to improve the catalytic properties of Au@ZrO\({}_{2}\) nanocatalyst. The Pd\({}_{1}\)/Au\({}_{20}\)@ZrO\({}_{2}\) composite consists of gold nuclei surrounded by zirconia shell which is decorated with Pd. The composite was prepared using a distinct ratio of Au:Pd. Although, the as-prepared catalyst demonstrated superior catalytic activity and strong stability towards the reduction of 4-nitropenol, however, the catalyst was prepared by a complicated process which requires several steps [41]. However, in this study, smaller sizes Au NPs are uniformly distributed on the ZrO\({}_{2}\) surface in a single step which enhanced the catalytic properties of the resulting composite. The reaction can be easily tracked by the change in the characteristic absorption peak of 4-nitrophenol at ~400 nm. In the presence of NaBH\({}_{4}\) and catalyst, the absorption band of 4-nitrophenol at ~400 nm, disappear and a new band emerges at a lower wavelength at ~300 nm, which indicates the successful transformation of 4-nitropenol to 4-aminophenol [42]. Although, the transformation of nitro to amino phenol is thermodynamically favored in the presence of NaBH\({}_{4}\), still, a catalyst is required to clear the kinetic barrier caused by the difference of potential between donor and acceptor [43].