Hydrogen generation from aluminum using nanoparticle catalysts

PITTSBURGH - Feb. 3, 2018 - PRLog -- .
Introduction
Hydrogen is a clean and renewable energy source, but requires a cost-effective method of storage and transport suggesting a mobile system where the hydrogen is generated directly from water at the point of use. Recently, a H-Al system has attracted considerable attention. H-Al stands for hydrogen-aluminum. However, a passive Al₂O₃ film develops on Al surfaces upon exposure to water that inhibits the hydrogen production. Methods [1,2] directed to the disruption of the Al₂O₃ film have been proposed for hydrogen generation using Al with water.

In the uniform corrosion model, the Al₂O₃ film is thought [1] disrupted by hydration when 22 nm NPs of TiO₂ are added to the water. NPs stand for nanoparticles. The thumbnail shows a TiO₂ NP contacting an Al particle coated with a Al₂O₃ film. Water is shown to dissociate into H⁺ and OH^- ions, the highly reactive OH- ions combining with the Al₂O₃ film to produce the intermediary Al (OH)₃ that disrupts the Al₂O₃ film. In [2], the Al (OH)₃ was directly synthesized into NPs of 50-90 nm diameter x 10 nm thick platelets.

Problem
The uniform corrosion model [1,2] does not explain how the TiO₂  and Al (OH)₃ NPs dissociate water into H⁺ and OH^- ions, the dissociation requiring at least EM radiation in the UV of about 5 eV. Until then, the corrosion model must be considered questionable.

Proposal
The source of hydroxyl OH ^- ions in the uniform corrosion model is the catalytic action of TiO₂ and Al (OH)₃ NPs in producing EM radiation that lowers the activation energy to disrupt the Al₂O₃ film. A similar situation arises in the creation of UV radiation upon the IR heating of silicate NPs in water producing steam without boiling. See PR at https://www.prlog.org/12638659.html

Theory
Certainly, NPs do not naturally emit UV, but TiO₂ and Al (OH)₃ NPs do convert heat from the surroundings into UV radiation if the heat capacity of the NPs somehow vanishes. Classically, NPs have heat capacity, but QM differs as the heat capacity of the atom given by the Planck law does indeed vanish under high EM confinement. QM stands for quantum mechanics. But high EM confinement is inherent in NPs upon absorbing heat because their high S/V ratios confine the heat to the NP surface. S/V stands for surface-to-volume. NP atoms are therefore placed under high EM confinement and their heat capacity vanishes. Lacking heat capacity, the heat may only be conserved by a non-thermal mechanism suggested here as simple QED.

Simple QED differs from the complex theory advanced by Feynman in that heat is conserved in NPs by creating non-thermal EM radiation standing across the NP diameter d as the heat adjusts to the EM confinement bounded by the NP surface. Hence, the EM radiation has half-wavelength λ/2 = d. The speed of light c corrected for the refractive index n of the NP gives the Planck energy E = h(c/n)/λ = hc/2nd. See diverse simple QED applications of the Planck law in nanostructures at http://www.nanoqed.org/, 2010 – 2018.

Applications
The NPs in [1] are titanium dioxide TiO₂ having d = 22 nm and n ≈ 2.5 giving 2nd = 110 nm. The absorption of 110 nm EM radiation in water is high ≈ 10⁵/cm as shown in the above PR on steam produced from IR heating. Water is therefore dissociated and hydroxyl ions are available to disrupt the Al₂O₃ film .

In [2] the NPs are 50-90 nm in diameter x 10 nm thick Al (OH)₃ platelets. Consistent with conserving heat in minimum time, simple QED dissipates EM radiation in the mode of the highest frequency which corresponds to the 10 nm thickness direction. But after synthesis, the platelets are attached by their flat sides to Al particles, and in effect, the high frequency EM radiation is quenched, and therefore the direction of EM dissipation is across the platelet diameter. Taking the average diameter d = 70 nm and n ≈ 1.5 gives 2nd = 210 nm, but as shown in the above PR the absorption in water is low and OH ^- ion production is inefficient. However, at d = 50 nm, absorption is ≈ 1x 10⁴ /cm and water is efficiently dissociated with OH ^- ions available to disrupt the Al₂O₃ film.

Conclusion
The uniform corrosion model that relies on a source of hydroxyl OH^- ions to disrupt the Al₂O₃ film is supported by the catalytic action of TiO₂ and Al (OH)₃ NPs that convert heat in the surroundings into EM radiation to dissociate the water molecules.

The TiO₂  and Al (OH)₃ NPs do not change during the Al–water reaction, indicating that they are in fact true catalysts to assist the reaction of Al with water.

References
[1] H. T. Teng, et al., "Effect of Al(OH)3 on the hydrogen generation of aluminum-water system," Journal of Power Sources 219 ,16-21,2012.
[2] Y. C. Wen, et al., "Kinetics study on the generation of hydrogen from an aluminum/water system using synthesized aluminum hydroxides," Int J Energy Res., 1–10, 2017.