# Perspectives of Hydrogen Generation in Cavitation-Jet Hydrodynamic Reactor
## 3 Results and Discussions
### Hydrogen from Al in Cavitation-Jet Apparatus
In the diffusor segment of the Laval nozzle, there is a significant increase in pressure and temperature resulting from the collapse of cavitation bubbles. This phenomenon leads to the creation of highly reactive radicals under extreme conditions. Such occurrences are referred to as \"fragmented\" collapses in the scientific literature, where the temperature and pressure may surpass the critical point for water (647 K and 22.1 MPa), resulting in physical properties that are intermediate between those of a gas and a liquid [50]. The high temperatures and pressures produced during cavitation supply the activation energy required to sever molecular bonds. The majority of studies in the field of environmental sonochemistry have employed the concept of the \"hot spot\" in order to elucidate the experimental results. In accordance with this theory, a sono-chemical reaction is regarded as an exceptionally heterogeneous reaction in which reactive agents and heat are generated from a specific microreactor, designated as the \"cavitation bubble\" [49; 50; 51; 52]. The \"structured hot spot\" model, as illustrated in Figure 3, identifies three distinct zones where chemical reactions are known to occur: (1) a hot gas nucleus; (2) an interphase region exhibiting a radial temperature gradient and local radical density; and (3) the bulk solution at ambient temperature. Consequently, the collapse of bubbles can initiate reactions involving free radicals in various locations: within the collapsing bubble, on its surface, and in the adjacent fluid. The extreme conditions created during bubble collapse can cause bond breakdowns and the dissociation of water, vapors, and gases, leading to the generation of free radicals or excited states. The high temperatures and pressures from cavitation supply the energy necessary to sever these bonds. Newly formed radicals might combine to create novel molecules or disperse into the surrounding fluid, functioning as oxidants. Moreover, free radicals can combust and engage in reactions within the liquid shell encasing the bubble [52]. Figure 9 shows the X-ray diffraction plots of sediments on the solid surface of working part of the CJC formed during cavitation-jet processing of a 0.1% sodium hydroxide solution. During cavitation-jet processing, it is observed that the diffraction peaks of the Al disc and foil disappear, while the presence of Al(OH)\({}_{3}\), the primary by-product of hydrogen generation, increases. The sediments in the first and second sections of CJC are predominantly composed of bayerite (B), sodium and magnesium salts of aluminum oxide (MX and Na\({}_{17}\)Al\({}_{5}\)O\({}_{15}\)), and copper oxides (I and II). In the area of total collapse of cavitation bubbles, that is, on the surface of the reflector, iron carbide is formed in addition to the above-mentioned compounds. This is probably the area where the conversion of synthesis gas into methane occurs. Methane formation from CO and H\({}_{2}\) is a complex catalytic process that involves a large number of consequent and parallel reactions. The process can be carried out at normal and elevated pressures and temperatures with catalysts that are based on transition metal of VIII groups (primarily Fe, Co, and Ru). Many studies have addressed this question, which are summarized in [55] and not covered in this article. It should be noted that an iron reinforcement connects the aluminum reflector with the diffuser part of the Laval nozzle, which plays a role as the catalyst. The X-ray diffraction patterns of the reflector deposits display the characteristic peaks of iron carbide, Fe\({}_{7}\)C\({}_{3}\), which is produced by the reduction of iron oxide (FeO) to carbon monoxide (CO) at temperatures between 527 and 727 \({}^{\circ}\)C [56; 57].