Methane adsorption and dissociation on mechanochemically activated iron-particle surfaces

Experimental and theoretical studies of the interfacial reactions between methane and mechanochemically activated iron particles were investigated using a milling technique and a discrete variational molecular orbital calculation. The particles were milled in a stainless steel vessel under a methane atmosphere. Scanning electron microscope (SEM) images revealed that the particles welded together to form larger particles and subsequently fractured into smaller homogeneous particles. The inner pressure of the vessel decreased with increasing milling time, indicative of methane adsorption on the particle surfaces and hydrogen was detected by gas chromatog-raphy. Two possible reactions promoted by milling are postulates: (i) fixation of adsorptive methane on the iron particles, and (iij methane conversion to hydrogen on the naturally oxidized iron. A bond overlap population (BOP) calculation was performed to evaluate the molecular orbital interactions between the methane and iron-particle surfaces. The covalent BOP exhibited a maximum positive value at a distance of 0.15 nm between the hydrogen atom of tetrahedral vertex and the iron atom facing each other at their interface, indicating methane adsorption on the iron particle surfaces. The BOP values between the hydrogen atoms of tetrahedral plane (not facing the iron) and carbon atom of methane decreased slightly with decreasing distance, indicating a dissociative adsorption state and weakened strength of the covalent C-H bond. These phenomena would be affected primarily by the strain in the iron particles and the forces between the methane and the near-surface iron and oxygen atoms. Therefore, methane adsorption and dissociation at the interface are consistent with the mechanochemical conversion of methane to hydrogen on the mechanochemically activated iron-particle surfaces.