Plasma-assisted catalytic reforming of toluene to hydrogen rich syngas

Abstract

The combination of non-thermal plasma (NTP) and transition metal catalysts is an alternative method for reducing tar derived from biomass gasification, converting it to a mix of mainly H₂ and CO (syngas). In this study, Ni- and Fe-based steam reforming catalysts supported on CaO, SiO₂, γ-Al₂O₃ and ZSM-5 were combined with a coaxial dielectric barrier discharge (DBD) plasma reactor for converting toluene, selected as a simple tar model compound. The catalysts supported by ZSM-5 showed the best potential for increasing toluene conversion, promoting the formation of syngas, reducing the production of undesirable liquid by-products (such as benzene and ethylbenzene) and corroborating the resistance to catalyst deactivation caused by sintering and carbon deposition. Toluene cracking has been investigated in a plasma-alone (PA), a post-plasma catalysis (PPC) and an in-plasma catalysis (IPC) system. The IPC system exhibited the highest toluene conversion efficiency and selectivity for syngas formation, compared with the PA and PPC systems. A maximum toluene conversion of 86.5% was achieved in the IPC system over Ni/ZSM-5, together with a maximum gas selectivity (63.3% for H₂, 18.2% for CO, and 4.6% for CH₄), and a minimum selectivity to the undesirable liquid by-products (2.5% for benzene and 0.66% for ethylbenzene and a low carbon selectivity of 6.7%). In addition, the reaction mechanism was significantly different in the IPC system from those in the PA and PPC system, since the catalyst in the IPC system might influence the discharge properties and the reaction between excited and short lifetime reactive plasma species (OH˙, O˙, …) and toluene and its intermediates. Finally, a mechanism of toluene cracking has been proposed, after analysis of the gas and liquid products and characterization of the fresh and used catalysts.