Plasma-catalytic reforming of toluene over Ni/HZSM-5 catalysts: Synergistic effect and reaction mechanism

Tar removal is a critical barrier to commercializing biomass gasification, necessitating efficient and cost-effective treatment methods. In this study, toluene was employed as a model compound of tar, and a highly synergistic non-thermal plasma (NTP)-coupled Ni/HZSM-5 catalyst system was established for the reforming of toluene into syngas. The performance data demonstrated a remarkable synergy between NTP and the Ni/HZSM-5 catalyst at low temperatures ranging from ambient to 525 °C, with the maximum toluene conversion rate reaching approximately 1.3 times that of the NTP-only system and about 7.6 times that of the catalyst-only system. The distribution of syngas and other C1 products was effectively tailored by adjusting discharge power, reforming temperature, and Ni loading on the catalyst. Optimal conditions included a discharge power of 75 W, a reforming temperature of 525 °C and an appropriate Ni loading of 15 wt%, leading to the highest toluene conversion rate of 97.58 %, H₂ selectivity of 42 %, and CO selectivity of 31.72 %, respectively. Conversely, lower discharge power (30 W) proved more favorable for achieving higher energy efficiency (6.31 g/kWh). The catalytic performance analysis indicated that catalysts with higher chemisorbed oxygen content, better reducibility, more medium acidic sites, and fewer strong acidic sites exhibited stronger synergy with plasma and demonstrated excellent stability. Furthermore, the direct identification of active species and by-products provided valuable mechanistic insights into the reaction pathway for toluene reforming using the NTP-Ni/HZSM-5 system. These findings offer new perspectives on the synergistic effect in NTP-Ni/HZSM-5, advancing the development of effective and economical tar reforming processes for the bioenergy industry.