Study on solid-state chemistry of the ZnO/Fe3O4/H2O system for H2 production at 973–1073 K

The reaction mechanism of H2 production with the ZnO/Fe3O4/H2O system at 973–1073 K was studied from the view point of solid-state chemistry. This reaction proceeds with nearly 100% yield via metal zinc the vaporization/deposition process between ZnO particles and Fe3O4 particles. The vaporized metal zinc from ZnO by ZnO=Zn(g)+1/2O2 seems to deposit on the surface of the Fe3O4 particle, and to form Zn2+ by the reaction with Fe3+ ions in Fe3O4. The Fe3+ ions will be immediately changed to Fe2+ ions by charge-hopping in the spinel structure. This causes the successive transportation of zinc atoms to Fe3O4 and the successive accumulation of Fe2+ and Zn2+ ions in the spinel structure. The accumulation of the Fe2+ ions gives a higher potential to react with O2, which is generated in the metal zinc vaporization process, and the total reaction equation can be written as: xZn+1/2xO2+Fe3O4=(3+x)/3(Znx/(3+x)Fe3/(3+x))3O4−δ); δ=x/(3+x). This indicates that the oxygen-deficient spinel-type compound is formed. The value of δ will increase, and the OD spinel-type compound will have a potential to decompose H2O to generate H2 gas.