Axial-time mapping: A diagnostic method to reveal concealed long-term catalyst deactivation mechanism in CO ₂ hydrogenation

An axial-time mapping (ATM) with dual time axes—catalyst–reactant contact time ( t c ) and time on stream (TOS)—is developed to clarify long-term deactivation mechanisms of iron catalysts in CO 2 hydrogenation. Treating t c and TOS as spatially distributed variables, ex situ datasets reconstruct axial phase gradients and performance shifts. Short t c at the reactor inlet promotes carburization to χ-Fe 5 C 2 and Fe 7 C 3 in a CO-rich environment, whereas longer t c downstream—where H 2 O accumulates—favors reoxidation to Fe 3 O 4 . Over time, residual Fe 3 O 4 is further carburized, enriching carbide phases, suppressing CO 2 activation, and enabling unconverted CO 2 to bypass the upper bed, thereby reducing the effective t c . This leads to downstream migration of the CO formation zone and spatial separation of active and inactive regions. The t c -TOS–resolved approach provides a sensitive and practical diagnostic tool for detecting and mitigating deactivation under industrial conditions. The ATM framework offers a generalizable strategy for probing phase evolution and deactivation pathways in complex heterogeneous catalytic systems.