Enhanced redox activity of La0.6Ca0.4FeO3-δ oxygen carrier via nickel and cobalt doping for chemical looping reforming coupled with CO2 splitting

Abstract

Since global warming induced by CO$_2$ emission has caused many critical problems in the earth’s environment over past decades, sustainable and CO$_2$-utilizable energy production technologies have gained a lot of attention. Furthermore, the depletion of petroleum and increased accessibility to shale gas accelerate the need for next-generation energy sources. To address those challenges, chemical looping reforming combined with CO$_2$ splitting is emerging as a promising solution that can remove CO$_2$ while also generate a sustainable energy source known as syngas. This process is conducted by repeating a redox reaction between the catalyst's lattice oxygen and the gas molecules. First, partial oxidation of methane occurs mediated by lattice oxygen, and a mixture of CO and H$_2$ comes out. Then, right after the prior step, CO$_2$ is injected to fill the oxygen vacancies with product of CO. Because the availability of oxygen ion in catalyst largely affects the redox performance, designing the highly active and stable oxygen-carrying catalyst is important. Recently, perovskite oxide takes numerous considerations due to its chemical and structural versatility. However, thus far, many studies have adopted the strontium-containing material as a redox catalyst. Strontium is expensive, rare and, its segregation to the surface under the high temperature leads to permanent activity degradation of catalysts. Inspired by prior studies, herein, a strontium-free perovskite oxide with the following composition of La$_{0.6}$Ca$_{0.4}$Fe$_{0.95}$M$_{0.05}$O$_3$ (M = Ni, Co, Ni-Co) is adopted as an oxygen carrier as well as a redox catalyst. Powder samples undoped or doped were synthesized via sol-gel route, and their crystal structures were analyzed by XRD. To find out the ideal dopant type, CH$_4$-TPR, and CH$_4$ pulse injection studies were performed. It was found that the co-doping strategy clearly enhanced the carbon coking resistivity and CH$_4$ reactivity. Not only with that, the increased CO$_2$ reactivity and phase recoverability were also elucidated through a CO$_2$-TPO with Raman spectra analysis. A catalyst co-doped with nickel and cobalt showed the intensified syngas production exceeding more than 2 times that of a pristine carrier over 50 redox cycles. A Post surface analysis found that these observations resulted from Ni-Co-Fe alloys ex-solved under the CH$_4$ atmosphere, where they remain as Ni-Co alloy nanoparticles after a CO$_2$ injection by selective dissolution of Fe, thereby significantly strengthen the redox activity.