# Perspectives of Hydrogen Generation in Cavitation-Jet Hydrodynamic Reactor
## 3 Results and Discussions
### Hydrogen from Al in Cavitation-Jet Apparatus  
The potential of hydrogen production via the formation of slightly ionized plasma in water flow, induced by cavitation within a hydrodynamic tube, was established earlier in [45]. Experimental results support the viability of safely producing hydrogen in cavitation streams and highlight the need for further investigation. To enhance hydrogen production during cavitation-jet processing of water, various metals or alloys with differing affinities for liquid media were utilized in the CJR design. Notably, components of the CJC such as the screw, resonator-splitter, and flow reflector, which receive liquid from the centrifugal pump, are constructed from aluminum. This is because aluminum is a widely accepted metal that can act as an inexpensive hydrogen source [28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48]. Most cavitation bubbles rapidly collapse under environmental pressure shortly after their formation. At the end of this collapse, they momentarily generate extremely high pressures within the vapor-gas mixture inside the bubble. When a bubble is near a solid surface, it forms a microscopic cumulative jet from the liquid during the collapse, which erodes the oxidized layer on the aluminum surface [49; 50; 51]. The activation of aluminum, rendering it chemically reactive with water, is linked to the disruption of the dense oxide layer on its surface, achievable through various methods. Researchers [30; 46] have observed hydrogen gas production during the wet cutting of aluminum and its alloys. This occurrence results from a chemical reaction between the freshly exposed aluminum surface and water, a reaction that is highly dependent on the surface's freshness and is both safe and manageable. The significantly negative standard redox potential (E\({}_{0}\) = \(-\)1.66 V) of aluminum renders it a superior reducing agent, which can generate hydrogen gas when it comes into contact with water. This corrosion process uniquely does not involve the production of CO\({}_{2}\). Such reactions can be initiated using an alkali or under neutral conditions as well as at high temperatures or through the reaction of aluminum with alcohol [48]. Due to its low equivalent weight, aluminum is an outstanding potential source of hydrogen by weight. However, it is surpassed by sodium borohydride, which can produce 2.4 L of hydrogen gas per gram of substance through hydrolysis. Aluminum and magnesium yield 1.245 L and 0.95 L per gram, respectively, while their corresponding hydrides, AlH\({}_{3}\) and MgH\({}_{2}\), generate 2.24 g:L\({}^{-1}\) and 1.88 g:L\({}^{-1}\), respectively [49]. During the intense collapse, temperature and pressure increase significantly, initiating chemical interactions between the solid surface of the cavitation-jet chamber and the activated water molecules within the cavitation bubbles. The reactions discussed in this study are presumed to occur within the bulk volume of bubbles, owing to water's volatility and the hydrophilic characteristics of the active radicals generated in the cavitation-jet chamber. Earlier, the authors [52] showed that under the cavitation conditions, HO\({}^{\bullet}\) and H\({}^{\bullet}\) radicals are produced by the pyrolysis of water molecules (H\({}_{2}\)O \(\rightarrow\) HO\({}^{\bullet}\) + H\({}^{\bullet}\)). The HO\({}^{\bullet}\) radicals have a strong oxidation capacity, with a redox potential of 2.80 eV. The primary outcome of the cavitation-jet processing of water is the generation of highly oxidizing active particles (HO\({}^{\bullet}\)) and peroxide compounds (H\({}_{2}\)O\({}_{2}\)), which possess a high oxidation potential concerning metals. The chemical reactions that occur in aqueous media for pure aluminum to produce hydrogen and other by-products are as follows:  
1.