A novel tandem ion mobility method is developed to study the size-dependent kinetics of Zn nanoparticle hydrolysis reaction, which is the second, exothermic step of Zn/ZnO Two-Step Water-Splitting Cycle for hydrogen generation. The experiment consists of two different ion-mobility schemes in series. The first mobility characterization is to size select particles with a differential mobility analyzer (DMA). The second mobility characterization employs an aerosol particle mass analyzer (APM) and measures changes in mass resulting from a controlled hydrolysis reaction of the Zn nanoparticles. The intrinsic reaction rates are determined for particles of different initial mobility sizes. The unsupported single Zn nanoparticles are generated using a condensation-evaporation method. It is found that smaller particles have a higher reaction rate which favors hydrogen generation. Electron microscopic analysis is employed to study the morphology change of oxidized Zn nanoparticles.

The same tandem ion mobility method is also used to study the reaction kinetics for Zn nanoparticles oxidized in air at elevated temperatures. Compared with the hydrolysis reaction, the air oxidation kinetics are much faster while the onset temperature of reaction is higher than hydrolysis reaction for the same starting particle size. The latter may be due to the effect of fast water ionization reaction which happens at the surfaces of Zn nanoparticles.

This paper will give quantitative rates for hydrolysis and oxidation as a function of particle size.