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

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 Fe2O3/Al2O3 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 Fe2O3/Al2O3 was compared with those over Fe2O3, Al2O3, and a cascade combination of both. It was found that the performance of Fe2O3/Al2O3 was superior to that of other catalysts in terms of chain cracking and C–C/C–H bond cleavage. The hydrogen yield with Fe2O3/Al2O3 was 50.53 mmol·gplastic–1, equivalent to more than 70% of hydrogen in plastic. Besides, alkanes/alkenes ranging from C2 to C9 dominated the hydrocarbon products. The analysis of the cycle performance revealed that the reduction pathway of Fe2O3/Al2O3 was different from those of other Fe2O3-containing catalysts, which was also confirmed by temperature-programmed reduction. To investigate the essential role and reaction mechanism with Fe2O3/Al2O3, characterizations of Fe2O3/Al2O3 before and after the reaction were conducted. The grain boundary between Fe2O3 and Al2O3 enhanced the adsorption of gaseous products. More importantly, the catalyst–support interaction to form FeAl2O4 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.