Synergistic Activity of the Fe₂O₃/Al₂O₃ Catalyst for Hydrogen Production through Pyrolysis-Catalytic Decomposition of Plastics

Abstract

Plastic recycling through thermochemical conversion for the production of fuels and chemicals is a promising way for simultaneous waste processing and utilization. In the current study, high-density polyethylene (HDPE) was subjected to pyrolysis and then catalytic upgrading to produce hydrogen in the presence of a novel Fe 2 O 3 /Al 2 O 3 catalyst with a grain boundary. To understand the catalyst–support interaction as well as the resulting synergistic catalytic effects, catalytic pyrolysis of HDPE with supported Fe 2 O 3 /Al 2 O 3 was compared with those over Fe 2 O 3, Al 2 O 3, and a cascade combination of both. It was found that the performance of Fe 2 O 3 /Al 2 O 3 was superior to that of other catalysts in terms of chain cracking and C–C/C–H bond cleavage. The hydrogen yield with Fe 2 O 3 /Al 2 O 3 was 50.53 mmol·g plastic –1, equivalent to more than 70% of hydrogen in plastic. Besides, alkanes/alkenes ranging from C 2 to C 9 dominated the hydrocarbon products. The analysis of the cycle performance revealed that the reduction pathway of Fe 2 O 3 /Al 2 O 3 was different from those of other Fe 2 O 3 -containing catalysts, which was also confirmed by temperature-programmed reduction. To investigate the essential role and reaction mechanism with Fe 2 O 3 /Al 2 O 3, characterizations of Fe 2 O 3 /Al 2 O 3 before and after the reaction were conducted. The grain boundary between Fe 2 O 3 and Al 2 O 3 enhanced the adsorption of gaseous products. More importantly, the catalyst–support interaction to form FeAl 2 O 4 during the pyrolysis reaction, determined by X-ray photoelectron spectroscopy, was responsible for effective proton adsorption and C–H bond cleavage. This study provides an insightful understanding of catalyst transformation during plastic catalytic pyrolysis for hydrogen production.

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