Methane adsorption and dissociation on iron oxide oxygen carriers: the role of oxygen vacancies

Abstract

We performed ab initio DFT+U calculations to explore the interaction between methane and iron oxide oxygen carriers for chemical looping reaction systems. The adsorption of CH4 and CHx (x = 0-3) radicals on α-Fe2O3(001), and the influence of oxygen vacancies at the top surface and on the subsurface on the adsorption properties of the radicals was investigated. The adsorption strength for CH4 and C radicals at the top of the α-Fe2O3(001) surface in the presence of oxygen vacancies is lower than that on the stoichiometric surface. However, for methyl (CH3), methylene (CH2) and methine (CH) radicals, it is correspondingly higher. In contrast, the oxygen vacancy formation on the subsurface not only increases the adsorption strength of CH3, CH2 and CH radicals, but also facilitates C radical adsorption. We found that oxygen vacancies significantly affect the adsorption configuration of CHx radicals, and determine the probability of finding an adsorbed species in the stoichiometric region and the defective region at the surface. With the obtained adsorption geometries and energetics of these species adsorbed on the surface, we extend the analysis to CH4 dissociation under chemical looping reforming conditions. The distribution of adsorbed CH4 and CHx (x = 0-3) radicals is calculated and analyzed which reveals the relationship between adsorbed CHx radical configuration and oxygen vacancies in iron oxide. Also, the oxygen vacancies can significantly facilitate CH4 activation by lowering the dissociation barriers of CH3, CH2 and CH radicals. However, when the oxygen vacancy concentration reaches 2.67%, increasing the oxygen vacancy concentration cannot continue to lower the CH dissociation barrier. The study provides fundamental insights into the mechanism of CH4 dissociation on iron based oxygen carriers and also provide guidance to design more efficient oxygen carriers.

Near 100% CO selectivity in nanoscaled iron-based oxygen carriers for chemical looping methane partial oxidation

“…7. The data of the previous computational study on CH 4 adsorption on Fe 2 O 3 (001) surface are given by the filled circle 21 . It can be seen that CH 4 adsorption energies dramatically decrease with increasing number, n when the sizes of Fe 2 O 3 nanoparticles are at a relatively small scale.…”

“…The formed CO may further react with surface lattice O atoms to form CO 2 21,22 . For the Fe 40 O 60 nanoparticle, the formation of CO 2 needs to overcome a barrier of 148.9 kJ mol −1 , which is 30.4 kJ mol −1 higher than that of CO 2 formation on Fe 2 O 3 (001) surface.…”

“…Figure 4 The formed CO may further react with surface lattice O atoms to form CO 2 . 21,22 For the Fe 40 O 60 nanoparticle, the formation of CO 2 needs to overcome a barrier of 148.9 kJ/mol, which is 30.4 kJ/mol higher than that of CO 2 formation on In summary, we demonstrate that Fe 2 O 3 nanoparticles embedded in SBA-15 enables near 100% CO selectivity in chemical looping methane partial oxidation, which is so far the highest value in product selectivity observed for chemical looping systems. Moreover, the effective temperature for syngas generation is lowered to 750-935°C, which is over 100°C lower than current state-of-the-art processes.…”

Recent Advances of Oxygen Carriers for Chemical Looping Reforming of Methane

“…Considering that oxygen vacancies is doomed to form as OCs donate lattice oxygen during the reduction of CLR, it was therefore important to decern their effect on the interaction between methane and Fe‐based OCs. It was reported that different oxygen vacancies on the surface had distinct influence on the adsorption strength for CH x (x=0–4) radicals [33] . That is, adsorption strength for CH 4 and C radicals at the top of the α‐Fe 2 O 3 (001) surface in the presence of oxygen vacancies was lower than that on the stoichiometric surface while it was correspondingly higher for CH x (x=1–3) radicals, but oxygen vacancy formation on the subsurface significantly increased that of CH x (x=0–3) radicals.…”

Chemical looping partial oxidation over FeWO /SiO2 catalysts

“…DFT calculations revealed the role of oxygen vacancy concentrations in modulating the oxygen diffusion rate. Specifically, a high oxygen vacancy concentration can lead to a low oxygen diffusion energy barrier and oxidize the carbon deposition in time [34].…”