Activation strategies for rice husk biochar: enhancing porosity and performance as a support for Pd catalysts in hydrogenation reactions†

Abstract

Thermolytic transformation via pyrolysis is a sustainable approach for valorizing agricultural and organic waste, yielding biogas, bio-oil and biochar (the gas, liquid and solid fractions, respectively). Among these, biochar stands out as a promising carbon-rich material for catalytic applications due to its surface functional groups (carboxyl, hydroxyl, amino and lactone) which facilitate the anchorage of active metal phases. However, its inherently low surface area and underdeveloped porosity often limit its effectiveness as a catalyst support. To overcome this issue, this study explores the enhancement of biochar properties through activation with various agents. Biochar derived from slow pyrolysis of rice husk, an abundant agricultural waste material, was activated using both physical (steam, CO₂) and chemical agents (KOH, H₃PO₄), yielding materials denoted as ARS, ARC, ARK, and ARP, respectively. These activated biochar materials were then employed as supports for Pd nanoparticles synthesized via a deposition–precipitation method, and tested in benzaldehyde (BAL) hydrogenation evaluating the conversion and yield with respect to benzyl alcohol (BALOH) and toluene (TOL). The catalysts were characterized by N₂ physisorption, CHNS elemental analysis, PZC, SEM, TEM, XPS, Raman spectroscopy, and MP-AES to establish structure–activity relationships. Among the tested samples, Pd/ARK, with a surface area of 2635 m² g⁻¹ and an oxygen content of 0.15%, exhibited the highest catalytic activity (TOF = 3.22 s⁻¹). Moreover Pd/ARK achieved a toluene yield of 77% after 1 h, highlighting its superior performance. These findings demonstrate the potential of tailored biochar activation strategies to enhance catalyst performance, offering a sustainable approach for advanced catalytic applications.