Contribution of pyrolytic gas medium to the fabrication of co-impregnated biochar

Co-impregnated biochars were fabricated by pyrolysis of spent coffee grounds (SCG) or glucose as carbon (C) sources under N₂ and CO₂. The formed biochar samples were characterized with FE-SEM, TEM/EDS, XRD, TGA, Raman, XPS, and BET, and further used as catalytic medium for the reduction of p-nitrophenol in the presence of NaBH₄. The physicochemical characteristics of biochar significantly changed with the types of C precursor and flow gas stream. The biochar from pyrolysis of SCG under N₂ gas stream showed good dispersion of Co nanoparticles (10-20nm) in carbon matrix as compared to that produced under CO₂ stream, but BET surface area was very small (4.5 m² g^-1) due to pore blockage by thermal degradation products. SCG-derived biochar formed in the presence of CO₂ possessed highly porous structure due to CO₂-assisted C volatilization. In contrast, for glucose-derived biochar, significant reduction in porosity of biochar occurred with excess devolatilization of C under CO₂ conditions, while the pyrolysis under N₂ flow produced porous biochar. These contrasting results demonstrated the significance of structure crystallinity and thermal degradability of C precursors. The catalytic performance of biochar was better correlated with the external surface area rather than microporosity.