Spatially resolved investigation into the coke formation and chemical states of nickel during autothermal reforming of acetic acid over Ni/CeO2–ZrO2 catalysts

Abstract

Autothermal reforming (ATR) is a viable option for reducing coke formation and energy consumption in hydrogen production processes. The space-resolved ATR of acetic acid as a model compound over the Ni/Ce_0.75Zr_0.25O₂ catalyst is performed using a spatial discretization approach by means of separating a reactor into up to 4 reaction zones. The spent catalysts from different zones were further characterized by ex situ XPS and TPO techniques to investigate the Ni oxidation states, coke morphology, and coke combustion. In addition, steam reforming (SR) and partial oxidation (POX) were similarly performed to decouple the effects of steam and oxygen from ATR. By comparing with SR, co-fed oxygen in ATR has significantly decreased the overall amount of coke formation with the implication on the reduced H₂ yield partially due to the CO oxidation. The co-fed oxygen consumed in the frontal section of the catalyst bed resulted in the oxidation of metallic Ni, decreasing the acetic acid conversion of the initial zone of its catalyst bed. Such oxidized Ni species could also be reduced by H₂ in the product stream of the previous zone resulting in a lower H₂ yield. Although oxygen can reduce the overall coke formation, its coke structures have been shifted from filamentous coke to the formation of polymeric, soft, and carbidic cokes. Those types of cokes seemed to be related to the formation of NiO that promotes acetate formation and decomposition. In sum, the presence of oxygen in the part of the catalyst bed results in the differences of the catalytic activity, the oxidation state of Ni, and the pattern of coke formation; this has created two recognizable reaction zones.