# Perspectives of Hydrogen Generation in Cavitation-Jet Hydrodynamic Reactor
## 3 Results and Discussions
### Hydrogen from Al in Cavitation-Jet Apparatus
The data of the gas chromatograph (Figure 6) shows that the resulting dry gas has a composition of hydrogen 59.97% vol., oxygen 9.15%, and nitrogen 30.87% by volume (carbon monoxide and methane content not defined due to technical limitations of the Chromos 1000 gas chromatograph due to the lack of detectors for their detection). Figure 5: Dependence of concentration of four metals on temperature and time of cavitation–jet processing of demineralized water (**a**) and aqueous solution of 0.1 wet % NaOH (**b**). The XRD data (Figure 8a) indicate that the cavitation treatment of demineralized water mainly results in the formation of two polymorphic forms of calcium carbonate, namely calcite and aragonite, which manifest as a white suspension in the liquid. However, the production of hydrogen during cavitation was minimal and scarcely detectable by the hydrogen sensors. Figure 6: Accumulation of gaseous products as a function of temperature during cavitation–jet processing of a 0.1% solution in a cavitation–jet reactor. Figure 7: Hydrogen accumulation data from gas chromatography during the cavitation–jet processing of 0.1% solution of NaOH in CJR. The SEM images (Figure 8b) illustrate a significant difference in the morphology of polymorphic calcite crystals. Aragonite crystals resemble needles and have a structure akin to a flower, whereas calcite crystals appear as blocky, cubic particles [53; 54]. It is possible to identify the repeating elements of individual aragonite and calcite crystals as well as their aggregates, which are reflected in both their internal (bI) and external structures (bII). The single element of the skeletal crystal structure is a needle-like sub-individual aragonite elongated along the axis and with cubic sub-individual calcite. The initially formed crystal of AG is aligned with the growth of similar crystals. The edge of one crystal grows in conjunction with the next oriented crystal. Additionally, the subsequent generation of lateral aragonite crystals also forms on the side plane of the cubic calcite crystal, creating an aragonite-calcite aggregate. Thus, from the data presented (Figures 6 and 7), it can be concluded that during the cavitation-jet processing of a polar liquid (water), a number of plasma-chemical reactions occur, the nature of which cannot be explained from the viewpoint of classical thermodynamics. In principal, each cavitation bubble can be considered as a plasmochemical microreactor providing highly energetic processes at almost room temperature of the bulk solution. The photons and the \"hot\" particles produced inside the bubble enable exciting the nonvolatile species in solutions, thus increasing their chemical reactivity [39]. The cavitation event also gives rise to acoustic microstreaming or the formation of miniature eddies that enhance the mass and heat transfer in the liquid and also cause velocity gradients that results in shear forces. In heterogeneous sonochemistry, the mechanical effects of cavitation resulting from the erosion action of microjets formed during asymmetric collapse of bubbles at the vicinity of interfaces are also important [55]. It is believed that various reaction products may be formed at the boundary of the aluminum-water phase boundary during high-speed liquid flow through the elements of the cavitation-jet chamber. The Venturi tube with a screw facilitates the processes depicted in the reaction formulae (a, b, and c). As the accelerated gas-vapor mixture moves to the Laval nozzle, the carbon dioxide dissolved in the water is reduced to carbon monoxide following the outlined scheme. \[\text{CO}_{2}+\text{H}_{2}\rightarrow\text{CO}+\text{H}_{2}\text{O}\]  
Figure 8: XRD (**a**), SEM (**bI,bII**) and EDA (**bIII**) data of the suspension formed during the cavitation–jet treatment of demineralized water. The colors of the lines indicate the element: calcium—brown line, carbon—turquoise line, oxygen—green line.