## 3 Results and Discussion
### Hydrolysis Activation Study
0.035 mol/g in the limit for samples with the highest proportions of Al NPs. The hydrolysis of the \(\upmu\)Al/Al NPs composite material is a complex process that is kinetically very difficult to model. However, tracking the hydrogen gas production from 50 mg samples as a function of reaction time (Figure 4a) clearly revealed a dual phase bimodal process. Irrespective of mass ratio, a burst of hydro  
Figure 3: Total amount of active Al\({}^{0}\) per unit mass of Al NPs as a function of mass % of Al NPs in the sample. Figure 2: Activated \(\upmu\)Al (as defined in Equation (5)) versus mass % of Al NPs. within ca. 1 min. The rate at which hydrogen was produced during this initial phase is linearly dependent upon the mass % of Al NPs in the sample, confirming first order kinetics (Figure 4b). This is entirely consistent with our previously determined kinetic profile for the hydrolysis of poly(\(N\)-isopropylacrylamide)-capped Al nanoparticles [40]. This was followed by a steadier gas formation phase, which settled at around 0.4 mL/min during the final 3 min of measurement, irrespective of the mass % of Al NPs in the sample. This is to be expected, given the now-dominant surface-area limited hydrolysis of the much larger \(\mu\)Al particles. It is also important that the reaction environment created by the Al NPs hydrolysis (vide infra) affords a stable and consistent supply of hydrogen from the \(\mu\)Al, which can be helpful for maintaining steady fuel generation for a device such as a hydrogen fuel cell. For the 50% Al NPs/50% \(\mu\)Al sample after 3 days sitting in water, analysis by PXRD of the recovered, postreaction residue (Figure 5) determined a crystalline composition that was 94% Al(OH)\({}_{3}\) (s) and 6% elemental Al(s), presumably unreacted \(\mu\)Al. No AlO(OH) was detected in this measurement. The Al(OH)\({}_{3}\) formation is typically expected in most aluminum hydrolysis reactions below 280 \({}^{\circ}\)C [4,51]. Additionally, Al(OH)\({}_{3}\) is an affirmed catalytic promoter of this reaction [44; 45; 46; 47]. In the case of our \(\mu\)Al/Al NPs composite samples, there was rapid consumption of the Al NPs to produce Al(OH)\({}_{3}\) in the initial activation phase of the reaction. This is possible because the oligo(epoxyhexane) cap on the nanoparticles is easily compromised by pure water. The Al(OH)\({}_{3}\) produced by Al NPs hydrolysis then catalytically drives the \(\mu\)Al hydrolysis phase by raising the reaction pH and compromising the micron-scale particle oxide shell. Higher proportions of Al NPs in the composite mixture will clearly generate more Al(OH)\({}_{3}\) during the activation, further elevating the pH, as was observed, and further activating the \(\mu\)Al toward the hydrolysis and ultimately increasing the hydrogen production. The hydrolysis of aluminum nanoparticles to bulk Al(OH)\({}_{3}\) is a highly exothermic process (\(\Delta H^{\circ}=-1094\) kJ/mol) [52]. It is conceivable, therefore, that energy released by this swift reaction could drive pursuant \(\mu\)Al particle hydrolysis. However, monitoring the temperature of the reaction solution using a thermocouple probe recorded a temperature increase of just 5-7 \({}^{\circ}\)C. Thus, it would seem the heat release rate is not reasonably sufficient to initiate degradation of the Al\({}_{2}\)O\({}_{3}\) shell protecting the \(\mu\)Al particles and accelerate the \(\mu\)Al reaction. Most micron-scale and bulk forms of Al do not react appreciably with neutral water without some form of ball milling, chemical activation and/or elevated temperatures (\(\geq\)35 \({}^{\circ}\)C) [1,10,53; 54; 55; 56]. On the other hand, the Al\({}_{2}\)O\({}_{3}\) protective shell is vulnerable to alkaline pH [1]. A sufficient increase in reaction pH might compromise the oxide layer to the extent  
Figure 4: (**a**) Tracking of total H\({}_{2}\) gas volume formed by the hydrolysis of 50 mg-\(\mu\)Al/Al NPs composite samples. (**b**) Dependence of initial H\({}_{2}\) formation rate on mass % of Al NPs. that \(\mu\)Al hydrolysis is expedited.