Performance of biochar from different sources in catalytic reforming for hydrogen production

This study aims to investigate the catalytic performance of biochar from the various feedstocks in hydrogen production via biomass pyrolysis and reforming. Biochar catalysts were prepared from four feedstocks, such as corn stover, bamboo, pine wood, and chitin. A two-stage fixed-bed reactor was also used for the catalytic reforming of the rice husk. A systematic evaluation was made to explore the impacts of the biochar on the yields of the gas-liquid-solid products, gas composition, and H₂ selectivity. There was a structure-activity relationship between the physicochemical properties (specific surface area, pore structure, functional groups, and elemental composition) and catalytic performance. Biochar catalysts were prepared via pyrolysis at 600°C under N2 atmosphere. In catalytic experiments, the rice husk was pyrolyzed in the first stage at 550°C, then the volatile products were mixed with the steam, and finally introduced into the second stage for steam reforming at 800°C on the biochar catalyst bed. Gas components (H₂, CO, CH₄, CO₂, C₂H₄, and C₂H₆) were selected to calculate the product yields and H₂ selectivity using gas chromatography (GC). The biochar was characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The parameters were determined after comparison, including the 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 gas yield, but their catalytic performance varied significantly. In the blank control test without catalysts, the yields of gas, liquid tar, and solid biochar from the rice husk pyrolysis were 198.97, 488.31, and 312.72 mg/g, respectively. In the biochar-added groups, no significant difference was observed in the solid product yields. But the liquid yields were reduced for the high gas yields, compared with the blank control. The liquid yields for the corn stover char, bamboo char, pine wood char, and chitin char were 321.19, 310.02, 278.13, and 282.54 mg/g, respectively—all significantly lower than the blank control. The gas yields were 371.66, 379.98, 412.75, and 408.46 mg/g, respectively, much higher than the blank control. In terms of H₂ yield and selectivity, all catalysts were significantly improved, compared with the blank control (1.41 mmol/g and 17.43%, respectively). Chitin biochar also exhibited the best catalytic performance, with the H₂ yield and selectivity of 12.59 mmol/g and 53.33%, followed by the pine wood biochar (10.74 mmol/g and 46.83%). Maize stover char and bamboo char showed relatively lower activity, with the H₂ yields of 8.41 and 8.38 mmol/g, while the selectivity of 42.69% and 41.32%, respectively. The superior catalytic performance of chitin char was attributed to the large specific surface area (86.85 m²/g), hierarchical pore structure to facilitate the mass transfer, and high nitrogen content (6.08%). The "nitrogen self-doping" effect also generated the abundant nitrogen-containing active functional groups (pyridinic N and pyrrolic N), thus promoting deep tar cracking and reforming reactions. The high activity of pine wood biochar was associated with the well-developed mesoporous network, large specific surface area, and rich surface oxygen-containing functional groups. Although the corn stover biochar contained alkali (earth) metals and abundant surface oxygen-containing functional groups, the high ash content reduced the catalytic activity. Bamboo char shared the low specific surface area, possibly due to a dense silicon layer on the surface that passivated active sites and limited catalytic activity. The type of biochar feedstock was selected to determine the physicochemical properties and catalytic performance in the reforming of hydrogen production. The specific surface area, pore structure, and active sites of biochar were the key enhancing factors on the hydrogen production efficiency. The screening of biochar feedstocks can be used to improve the catalyst performance, targeted design, and regulation. The findings can provide the theoretical and technical references to develop highly efficient, low-cost catalysts for high-value biomass utilization.