Modulation of support properties in flower-like Pt/Al2O3 nanosheet catalysts for dehydrogenation of cycloalkanes†

Abstract

Dehydrogenation of cycloalkanes derived from plastic waste presents an attractive approach for hydrogen production and plastic waste valorization. In this study, we developed Pt/Al₂O₃ nanosheet catalysts with tailored support properties by adjusting the calcination temperature. Comprehensive characterization of the catalysts revealed that the support properties, including crystal phase, surface area, acid sites, hydroxyl groups, and defect sites, were modulated by increasing the heat treatment temperature. Consequently, these variations led to differences in Pt particle size, dispersion, and the chemical environment of active sites on the resulting Pt/Al₂O₃ catalysts. The catalytic dehydrogenation of methylcyclohexane exhibited a volcano-like trend in terms of catalytic activities, while the stability of the catalyst showed a concave relationship with increasing calcination temperature. The Pt/Al₂O₃-700 nanosheet catalyst, prepared using a support calcined at 700 °C, exhibited exceptional catalytic activity and stability. It achieved a remarkable hydrogen production rate of 3402 mmol g_Pt⁻¹ min⁻¹ at 350 °C, surpassing most Pt-based catalysts reported in the literature. In addition, this catalyst is also effective for the dehydrogenation of plastic waste derived 1,4-dimethylcyclohexane and 1,3-dimethylcyclohexane. The catalytic activity is strongly influenced by factors such as surface area, Pt particle size, the fraction of surface Pt⁰ species, and the electronic density of surface Pt species. On the other hand, the stability of this catalyst is closely associated with acid sites and hydroxyl groups present on the Al₂O₃ support. The superior performance observed in the Pt/Al₂O₃-700 catalyst can be attributed to its optimal combination of factors including a high surface area, an appropriate particle size of 1.4 nm, a desirable amount of metallic Pt species on the surface, and a moderate electron density of surface Pt species that provide a balance between accessible active sites and toluene desorption. Furthermore, its excellent stability can be attributed to an optimal ratio between acid sites and hydroxyl groups that effectively inhibit coke formation and sintering of Pt nanoparticles. This study emphasizes the importance of regulating a rational support with optimal properties to enhance the catalytic efficiency for cycloalkane dehydrogenation.