Abstract:
The study investigated the differences in catalytic performance of biochars derived from different feedstocks in hydrogen production via biomass pyrolysis and reforming. Biochar catalysts were prepared from corn stover, bamboo, pine wood, and chitin, and their catalytic reforming effects on rice husk were evaluated in a two-stage fixed-bed reactor. The focus was on analyzing the impacts of these biochars on the yields of gas-liquid-solid products, gas composition, and H
2 selectivity, while exploring the structure-activity relationship between their physicochemical properties (specific surface area, pore structure, functional groups, and elemental composition) and catalytic performance. This work aims to provide theoretical references for developing efficient and low-cost catalysts for biomass hydrogen production. Biochar catalysts were prepared via pyrolysis at 600℃ under N
2 atmosphere using the four feedstocks. In catalytic experiments, rice husk was pyrolyzed in the first stage at 550℃, and the volatile products were mixed with steam and introduced into the second stage for steam reforming at 800℃ over the biochar catalyst bed. Gas components (H
2, CO, CH
4, CO
2, C
2H
4, C
2H
6) were analyzed offline using gas chromatography (GC) to calculate product yields and H
2 selectivity. The biochars were characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to determine their microstructure, specific surface area, pore structure, surface functional groups, and elemental chemical states. The results showed that all four biochar catalysts effectively promoted tar cracking and increased gas yield, but their catalytic performance varied significantly. In the blank control test without catalysts, the yields of gas, liquid tar, and solid char from rice husk pyrolysis were 198.97 mg/g, 488.31 mg/g, and 312.72 mg/g, respectively. For the biochar-added groups, no significant differences were observed in solid product yields, but all exhibited reduced liquid yields and increased gas yields compared to the blank control. The liquid yields for corn stover char, bamboo char, pine wood char, and chitin char were 321.19 mg/g, 310.02 mg/g, 278.13 mg/g, and 282.54 mg/g, respectively—all significantly lower than the blank control. Their gas yields were 371.66 mg/g, 379.98 mg/g, 412.75 mg/g and 408.46 mg/g, respectively, much higher than the blank control. In terms of H
2 yield and selectivity, all catalysts significantly improved both compared to the blank control (1.41 mmol/g and 17.43%, respectively). Chitin biochar exhibited the best catalytic performance, with H
2 yield and selectivity reaching 12.59 mmol/g and 53.33%, followed by pine wood char (10.74 mmol/g and 46.83%). Corn stover char and bamboo char showed relatively lower activity, with H
2 yields of 8.41 mmol/g and 8.38 mmol/g, and selectivities of 42.69% and 41.32%, respectively. The superior catalytic performance of chitin char was attributed to its large specific surface area (86.85 m
2/g), hierarchical pore structure facilitating mass transfer, and high nitrogen content (6.08 %). The "nitrogen self-doping" effect generated abundant nitrogen-containing active functional groups (pyridinic N and pyrrolic N), promoting deep tar cracking and reforming reactions. The good activity of pine wood char was associated with its well-developed mesoporous network, large specific surface area, and rich surface oxygen-containing functional groups. Although corn stover char contained alkali (earth) metals and abundant surface oxygen-containing functional groups, its high ash content reduced catalytic activity. Bamboo char had a low specific surface area, possibly due to a dense silicon layer on its surface that passivated active sites and limited catalytic activity. This study confirmed that the type of biochar feedstock determines its physicochemical properties and catalytic performance in reforming for hydrogen production. The specific surface area, pore structure, and active sites of biochars are key factors enhancing hydrogen production efficiency. These findings suggest that screening biochar feedstocks can improve catalyst performance, enabling targeted design and regulation, and provide new theoretical and technical support for developing low-cost, high-efficiency catalysts for high-value biomass utilization.