Efficient Methane Conversion to Hydrogen by the Force-Activated Oxides on Iron Particle Surfaces

Abstract

Mechanochemically activated iron oxide formation on α-iron (Fe) microparticles for the efficient reaction with methane (CH4) was investigated using a simple milling process to successfully clarify the efficient CH4 adsorption as well as the C–H bonding dissociation. First, the oxides with different oxidation degrees (i.e., disordered or ordered iron oxides) were formed on the Fe by mechanochemical milling under an oxygen atmosphere. The surface analyses by X-ray diffraction, Raman, and X-ray photoelectron spectroscopy (XPS) revealed that the surface iron oxides consisted of two phases (α-Fe2O3 and Fe3O4), and the Fe2O3-like structure was dominantly grown on the near-surface of the disordered iron oxides during the initial oxidation time (0.08 and 0.80 h). Furthermore, the signals from an electron spin resonance analysis suggested dangling bondings on the disordered oxides, indicating the successful formation of the active O sites. Second, the mechanochemical reaction of the resultant iron oxide surfaces with CH4 was investigated. As a result, the efficient CH4 adsorption as well H2 generation were prominently observed on the disordered oxides, indicating an effectively dissociative adsorption. XPS analysis of the resultant particles revealed that the C–H bonding dissociation dominantly occurred on the near-surface oxygen atoms. Furthermore, using a molecular orbital calculation analysis, the activated O atoms with dangling bondings in the disordered iron oxides were found to affect the C–H dissociation of the adsorbate molecules (e.g., CH4 and CH3•) and exhibit an efficient H2 generation. On the other hand, the O atoms in the ordered lattice could not induce the C–H dissociation of CH3• to produce the equilibrium adsorption/desorption states and lower H2 generation amount, suggesting the importance of both the force-induced activated O atoms and closest interatomic distances. Therefore, we first achieved the efficient mechanochemical decomposition of CH4 and CH3• to generate H2 by the disordered iron oxide surfaces on α-Fe particles.