Search

Abstract: The site requirements for the methane total combustion, partial oxidation, and decomposition on LaFeO3 perovskite catalysts during the chemical looping process were addressed by a kinetic study through varying the O content and the crystal size. The exchange between the surface oxygen and lattice oxygen is rapid, and the active sites of Fe associated with the surface oxygen are highly dynamic in nature, which depends on the amount of O in the perovskite during reaction and the O binding energy. Three types of active sites are identified where Fe atoms highly coordinated with O lead to total combustion of methane, Fe moderately coordinated with O leads to partial oxidation, and Fe highly coordinated with vacancy sites leads to carbon formation. The crystal size of LaFeO3 perovskite plays a significant role in determining the O binding energy and their catalytic properties on the partial oxidation of methane by the chemical looping process. Different crystal sizes of perovskites have been obtained using various preparation methods. Larger crystal has a high selectivity to synthesis gas and high capacity of O removal, due to a lower O–Fe bond energy, as indicated by a smaller band gap measured by UV–Vis absorption spectroscopy. Nearly pure synthesis gas was produced by methane partial oxidation using chemical looping on LaFeO3 by tuning the surface O coverage through controlling the oxidation degree in the oxidation step. [Copyright &y& Elsevier]

Abstract: The effect of the crystal size of Ni on the growth of carbon nanofibers (CNFs) was studied in a tapered oscillating element microbalance reactor. Small Ni crystals yield a low growth rate and fast deactivation and thus a low final yield of CNF. Large Ni crystals reduce the growth rate because of low surface area. An optimum growth rate and yield of carbon nanofibers can be achieved on optimally sized Ni crystals (around 34 nm). A model has been proposed for interpreting the kinetic effects of the Ni crystal size based on a detailed mechanism of the carbon nanofiber growth. The reduced coking rate on a small-sized Ni crystal is a result of increased saturation concentration of CNF and thus a low driving force for carbon diffusion through the Ni crystals. As a consequence, the surface coverage of carbon increases, which enhances the formation of encapsulating carbon and thus the deactivation. Both the low initial coking rate and the fast deactivation result in a low yield of carbon nanofibers on small-sized crystals. The results also indicate that hydrogen has a significant effect on the formation of CNF, and an optimum partial pressure of hydrogen exists for the CNF growth. [Copyright &y& Elsevier]

Highlights: [•] CO hydrogenation studied at methanation conditions, 483K, 1.85bar, H2/CO=3–10. [•] Integrated transient and steady-state modeling as a powerful tool for useful kinetic parameters. [•] Combining experiments and DFT investigations of kinetic isotopic effect. [•] Dominating CO activation pathway through hydrogen-assisted CO dissociation. [•] Two carbon pools and two corresponding reaction pathways for CH4 formation. [ABSTRACT FROM AUTHOR]