Simultaneous alkali/air activation for hierarchical pore development in biochar and its use as catalyst carrier for formic acid dehydrogenation

Abstract

Biochar provides an efficient strategy for making use of biomass residues, because it has shown to be a multifunctional material in energy and environmental applications. However, the underdeveloped porosity of biochar often makes it perform below potential. Herein, a novel activation method, namely simultaneous alkali/air activation, was tested for hierarchical pore development in biochar. The enhanced performance of the as-prepared porous carbon was evaluated by loading of palladium (Pd) for catalyzing formic acid dehydrogenation, a key reaction for the safe storage and transport of hydrogen. It was found that NaHCO₃ was more effective than KOH on mesopore development in biochar when it was activated by air together. The carbon product possessing developed hierarchical pore structure (SA_BET = 1013 m² g⁻¹, SA_meso/SA_BET = 33.0 %) was obtained at 700 °C, using NaHCO₃ as the alkali activator in a mixed air/nitrogen gas flow (30/70 by volume). The hierarchical pore structure of carbon made it a competent carrier of Pd catalyst, because the hierarchical pores not only enhanced the dispersion of Pd nanoparticles, but also served as the fast channels for the reactants to access the active sites intra catalyst particles. Therefore, the fast formic acid dehydrogenation (TOF = 156 h⁻¹) was achieved when using the Pd catalyst loaded on such a hierarchically porous carbon.