Chemical looping steam methane reforming using Al doped LaMnO3+δ perovskites as oxygen carriers

Chemical looping steam methane reforming (CLSMR) is capable of co-production of high-quality syngas and pure hydrogen, and it is important to develop appropriate oxygen carriers for this process. In this work, LaMn1-xAlxO3+δ (x = 0, 0.1, 0.3, 0.5, 0.7) perovskites were investigated as oxygen carriers for CLSMR by means of characterizations and fixed-bed tests. The characterization and test results suggested that the substitution of Al leaded to more surface active sites and higher symmetry of crystal structure, which facilitates the activation of methane molecule on the surface and the formation of the oxygen vacancy in the bulk of the oxygen carrier particles, increasing the release rate of selective oxygen and the yield of syngas. The yield of CO2 declined with the Al doped proportion due to the decrease of the mount of Mn4+ and surface absorbed oxygen. The substitution of Al cations could stabilize the crystal structure and prevent the destruction of perovskite structure. No carbon formed on LaMn1-xAlxO3+δ with x from 0 to 0.5 and a long period of partial oxidation was achieved to produce high-quality syngas with the H2/CO ratio of 2 and pure hydrogen, while carbon deposition occurred on the LaMn0.3Al0.7O3+δ oxygen carrier. LaMn0.5Al0.5O3+δ possessed the best performance with CO selectivity of 96.4%, the CO yield of 1.70 mmol·g−1, the H2 yield of 3.32 mmol·g−1 in the reduction stage and the H2 yield of 1.98 mmol·g−1 in the oxidation stage on average in 20 cyclic redox tests. LaMn0.5Al0.5O3+δ exhibited good thermal stability and cyclic performance. It can be deduced that LaMn0.5Al0.5O3+δ perovskite is a potential oxygen carrier for cyclic CLSMR.