Axial-time mapping: A diagnostic method to reveal concealed long-term catalyst deactivation mechanism in CO₂ 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 CO₂ 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-Fe₅C₂ and Fe₇C₃ in a CO-rich environment, whereas longer t_c downstream—where H₂O accumulates—favors reoxidation to Fe₃O₄. Over time, residual Fe₃O₄ is further carburized, enriching carbide phases, suppressing CO₂ activation, and enabling unconverted CO₂ 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.