## 3 Nanocomposite Catalysts
### Cold Atmospheric Pressure Plasmas Ex Situ Approach
Up until now, the synthesized with the aid of CAPP metallic nanostructures were stabilized by using sodium citrate [70], sucrose [71], fructose [64; 67; 79; 85], gelatin [73; 74; 75; 77; 82; 84], sodium dodecyl sulfate (SDS) [68; 69; 83], hexadecyltrimethylammonium chloride (CTAC) [62], polyvinyl alcohol [63], poly(vinylpyrrolidone) [65; 81], pectins [78; 83], and (11-mercaptoethanol)-N,N,N-trimethylammonium chloride (TMA) alkanethiol [80]. On the other hand, when a stabilizer is not added to the NPs' precursor solution, the resultant nanostructures might be well dispersed in the solution because of their electrostatic stabilization [73; 86]. The possibility of the application of CAPP-based technology in the nanomaterials' synthesis without addition of either reducing or capping agents enables us to produce the uncoated nanomaterials [72; 75; 76; 77; 86]. This vital feature led to the possibility of incorporating the obtained nanostructures within the polymeric matrix, in order to produce nanocomposites of defined properties [72; 76]. However, such an ex situ approach suffers from diffusional limitations that inorganic NPs immobilized in the polymeric network. Hence, the expected yield of stabilization was expected to be not sufficient. For this reason, the synthesis of NCs with NPs generated in CAPPs  
Figure 8: Developed CAPP-based system for continuous production of nanofluids—as an example AuNPs synthesis is shown. (**1**) High-Voltage inputs (+/-); (**2**) a flowing liquid solution, being a precursor to AuNPs; (**3**) tungsten electrode; (**4**) dc-APGD; (**5**) graphite electrode covering quartz capillary; and (**6**) sample reservoir. was carried out by developing a new _in-ex_ situ approach [87]. The uniqueness of this method relies on introducing the suspensions of NPs of defined granulometric properties within a water phase containing hydrophilic monomers, which are further subjected to the free radical polymerization, resulting in the formation of a unique NC, as displayed in Figure 9.  
The optimization of the procedure was done on the material, resulting from the polymerization-driven immobilization of AuNPs in the vinylbenzyl trimethyl ammonium chloride (VBTAC) and N,N-methylenebis(acryl amide) (MBA), VBTAC-co-MBA copolymer [87]. Although the procedure seemed to be facile, the difficulties that have been met involved the anion-exchange reaction between strong base functionalities of VBTAC and the unreacted AuNPs' precursor (AuCl\({}_{4}\)-). The process resulted in the production of excessive HCl, which digested the AuNPs that should be immobilized within the hydrogel [87]. Hence, the synthesis was altered by forcing the anion-exchange on VBTAC first, using an equivalent amount of NaOH. This approach allowed us to exchange excessive Cl\({}^{-}\) anion located on VBTAC and neutralize it prior to introducing AuNPs. As displayed in Figure 10, only the sample treated with NaOH prior to the immobilization of AuNPs appeared with the characteristic ruby-red color, and as confirmed, this was successfully synthesized NC [87]. Figure 9: Simplified synthetic route for obtaining hydrogel nanocomposite containing AuNPs. Figure reproduced from Reference [87], with the permission of MDPI, under the Creative Commons Attribution License. The so-obtained material was tested for its ability to increase heat-exchange rate, in an arrangement simulating cooling systems of modern electronic devices. The AuNP-loaded NC was allowed to increase its cooling rate by 300%, which complies with other reports in the literature that provide similar materials based on polyaniline [88], poly(N-isopropylacrylamide) [89], and stearic acid [90]. The research on NCs for heat-management systems was also tested by using PtNPs produced in the developed-by-us CAPP-based reaction-discharge system, utilizing dc-APGD as a CAPP source.