Hydrogen Production from a Combination of the Water-Gas Shift and Redox Cycle Process of Methane Partial Oxidation via Lattice Oxygen over LaFeO_3 Perovskite Catalyst

摘要

A redox cycle process,in which CH_4 and air are periodically brought into contact with a solid oxide packed in a fixed-bed reactor,combined with the water-gas shift (WGS) reaction,is proposed for hydrogen production.The sole oxidant for partial oxidation of methane (POM) is found to be lattice oxygen instead of gaseous oxygen.A perovskite-type LaFeO_3 oxide was prepared by a sol-gel method and employed as an oxygen storage material in this process.The results indicate that,under appropriate reaction conditions,methane can be oxidized to CO and H_2 by the lattice oxygen of LaFeO_3 perovskite oxide with a selectivity higher than 95% and the consumed lattice oxygen can be replenished in a reoxidation procedure by a redox operation.It is suggested that the POM to H_2/CO by using the lattice oxygen of the oxygen storage materials instead of gaseous oxygen should be possibly applicable.The LaFeO_3 perovskite oxide maintained relatively high catalytic activity and structural stability,while the carbonaceous deposits,which come from the dissociation of CH_4 in the pulse reaction,occurred due to the low migration rate of lattice oxygen from the bulk toward the surface.A new dissociation-oxidation mechanism for this POM without gaseous oxygen is proposed based on the transient responses of the products checked at different surface states via both pulse reaction and switch reaction over the LaFeO_3 catalyst.In the absence of gaseous-phase oxygen,the rate-determining step of methane conversion is the migration rate of lattice oxygen,but the process can be carried out in optimized cycles.The product distribution for POM over LaFeO_3 catalyst in the absence of gaseous oxygen was determined by the concentration of surface oxygen,which is relevant with the migration rate of lattice oxygen from the bulk toward the surface.This process of hydrogen production via selective oxidation of methane by lattice oxygen is better in avoiding the deep oxidation (to CO_2) and enhancing the selectivity.Therefore,this new route is superior to general POM in stability (resistance to carbonaceous deposition),safety (effectively avoiding accidental explosion),ease of operation and optimization,and low cost (making use of air not oxygen).