## 3 Results and Discussion
### Hydrolysis Activation Study
Since the Al(OH)\({}_{3}\) produced by Al NP hydrolysis is capable of promoting such pH-driven \(\mu\)Al hydrolysis to produce more Al(OH)\({}_{3}\), it could be viewed as a catalytic enhancement. Increasing the proportion of Al NPs in the composite material naturally increases the rate of initial Al(OH)\({}_{3}\) formation in the reaction mixture (supported, of course, by the observed increase of the initial rate of formation of the other product, H\({}_{2}\)). We also measured a corresponding increase in the pH of the reaction solution at the end of the 15 min reaction monitoring period (Figure 6). Indeed, with the value of the slope close to 3, the dependence of the (basic) pH on the log(Al NP amount reacting) closely matches the 3:1 hydroxide-to-Al stoichiometry of the Al NP hydrolysis equation, which supports the rapid and complete consumption of the nanoparticles in the initial phase. The measured increase in resulting pH with the increasing Al NP proportion is offset by the reduced fraction of \(\mu\)Al in the sample. The result is the same steady rate of \(\mu\)Al hydrolysis observed in the final reaction phase of Figure 4a for all Al NP mass % samples. \[\text{2}\mu\text{Al }(s)+6\text{H}_{2}\text{O }(l)\overset{\text{ Catalytic Al}(\text{OH})_{3}}{ \rightarrow}2\text{Al}(\text{OH})_{3}\left(s\right)+3\text{H}_{2}\left(g\right) \tag{2}\]0 PXRD pattern for the white solid material recovered after 3 days hydrolysis of the 50% Al NPs/50% \(\mu\)Al composite sample. Peaks labeled with * are due to elemental Al (s) (fcc Al, ICDD #00-004-0787) and all remaining peaks are attributable to Al(OH)\({}_{3}\) (s) (bayerite, ICDD #01-074-1119). The three strongest peaks for each identified phase are labeled with their Miller indices. \[\text{2}\mu\text{Al }(s)+6\text{H}_{2}\text{O }(l)\overset{\text{ Catalytic Al}(\text{OH})_{3}}{ \rightarrow}2\text{Al}(\text{OH})_{3}\left(s\right)+3\text{H}_{2}\left(g\right) \tag{2}\]1 Dependence of reaction solution pH on Al NP mass %. The solution pH measured at the culmination of the hydrogen formation experiments ranged from 8 to 11 for the lowest and highest Al NPs mass % samples, respectively. This basic pH regime is consistent with Al\({}^{3+}\) speciation analysis for the aqueous solutions shown in Figure 7a (and determined by equilibria equations shown in Appendix A), bearing in mind that Al(OH)\({}_{3}\) is a sparingly soluble salt. Thus, while most of the Al(OH)\({}_{3}\) precipitates out of solution, observed as a white solid, the thermodynamically equilibrated composition in solution ranges from ca. 50% Al(OH)\({}_{3}\) (_aq_)/50% (Al(OH)\({}_{4}\)\({}^{-}\) (_aq_) at pH 8 to almost entirely Al(OH)\({}_{4}\)\({}^{-}\) (_aq_) at pH 11. These equilibria systems also confirm that the Al\({}_{2}\)O\({}_{3}\) shell protecting the \(\mu\)Al particles is most likely to be compromised at a pH greater than 8, yielding primarily Al(OH)\({}_{4}\)\({}^{-}\) ions. As a result, the total saturated concentration of aqueous Al species increased by three orders of magnitude as expected in the pH range 8-11 (Figure 7b, Appendix B). This, therefore, is the most likely mode of activation of the \(\mu\)Al toward hydrolysis for this composite material.