Redox microenvironment regulation of iron-based oxygen carriers via bimetallic design: Facilitating clean conversion of ammonia as a carbon-free hydrogen carrier

Ammonia (NH 3 ) represents a promising hydrogen carrier, and chemical looping combustion (CLC) offers an efficient strategy for NH 3 utilization. Iron-based oxygen carriers feature natural abundance and favorable redox properties, but their reactivity and stability in ammonia CLC (ACLC) still require enhancement. This work systematically modifies iron-based oxygen carriers with 3d transition metals (Mn, Co, Ni, Cu) to clarify the effects on redox behaviors and ACLC performance. Characterization results confirm that for fresh oxygen carriers, Mn addition preserves the M 2 O 3 -type structure, while Ni, Co and Cu promote the formation of spinel phases. XPS and H 2 -TPR results show that transition metals (Mn, Co, Ni, Cu) modification causes the changes of Fe 2+ /Fe 3+ ratio, lattice oxygen content and reduction temperature, thereby regulating the Fe redox microenvironment. Under ACLC conditions of 800 °C, the Fe 2 O 3 reference achieves 99.6% NH 3 conversion and 99.6% N 2 selectivity, but its NO peak concentration reaches 2252.5 ppm. In contrast, the spinel Ni-Fe oxide exhibites nearly 100% NH 3 conversion (without NH 3 slip), N 2 selectivity up to 99.9%, and a significantly suppressed NO generation (NO peak concentration = 415.0 ppm), corresponding to an 81.6% reduction compared with the Fe 2 O 3 reference. The spinel Cu-Fe oxide achieves outstanding redox stability (NH 3 conversion > 99.9%, N 2 selectivity% > 97%) over 10 cycles. N 2 O is not detected in any of the ACLC tests. These results demonstrate that tailoring the Fe redox microenvironment via Ni and Cu modification enables clean NH 3 conversion with high N 2 selectivity and suppressed NOx formation. Based characterization and ACLC results, two NH 3 conversion pathways are proposed: lattice oxygen-mediated oxidation and surface-mediated dehydrogenation. This study elucidates structure-reactivity relationships and illustrates the rational design of high-performance oxygen carriers for ACLC.