Understanding the pyrolysis-assisted conversion of metal salt-impregnated biomass into biogas and nanocatalyst-coated porous biochar

Slow pyrolysis is the most employed technique to design biochar at high yield. It is particularly suitable for converting impregnated biomass into biochar-immobilized nanocatalysts. The latter exhibits a porous structure compared to pristine biochar. However, this distinct behavior is yet to be understood. To this end, the coupled technique of Thermogravimetric Analysis/Fourier Transform Infrared Spectroscopy (TGA/FTIR) was used to simulate pyrolysis in a tube furnace for biochar production and to analyze syngas. Sugarcane bagasse (SCPB) powder, treated with copper and/or nickel nitrates, was pyrolyzed in a TGA under nitrogen at heating rates from RT to 500 °C for 1 hour. TGA/DTG permitted monitoring the thermal events of SCPB which was found to be exacerbated by the introduction of metals. The syngases, including COx, NOx, HNO3, and H2O, were effectively detected and analyzed by FTIR during the pyrolysis process at varying heating rates of 10, 20, and 30 °C/min. Notably, Ni(NO3)2.6H2O yielded N2O, probably associated with the formation of a biochar porous structure. The pyrolysis activation energies (Ea) of SCPB and SCPB/CuNi were determined using FWO and DAEM models. The maximum weight loss was found to occur at a thermochemical conversion of ~60% into char; and Ea = 234 kJ.mol-1 for SCPB, much higher than 90 kJ.mol-1 obtained for the wet impregnated sample SCPB/nitrates; hence the dramatic effect of the nanometal precursors on the pyrolysis process. XPS analysis demonstrated that TG-biochar and “tubular furnace” biochar exhibit identical surface chemical compositions. Raman spectroscopy brought evidence for graphitic carbon structure as judged from D and G bands in SCPB/CuNi. Microscopic observations confirmed a well-defined porous morphology. From the above, this TG-FTIR study permitted to highlight the central role of copper and particularly nickel in the pyrolysis process. These metals: -significantly lower the pyrolysis activation energy - induce porous structure of the underlying biochar -could be added to the repertoire of activators, dominated by ZnCl2 and FeCl3. -serve as activators