Upcycling hazardous waste into high-performance Ni/η-Al₂O₃ catalysts for CO₂ methanation

Transforming hazardous and difficult-to-process waste materials, like spent Ni-MH batteries and aluminium foil, into nanocatalysts (NCts) provides a sustainable solution for resource management and reducing environmental impact. This study demonstrates a novel approach by extracting nickel sulfate (NiSO₄·xH₂O) from battery waste and subsequently converting it into Ni(OH)₂ hydrogel precursors using l-glutamic acid. Waste aluminium foil was processed into alumina (Al₂O₃), and combined with Ni(OH)₂ to synthesize Ni/η-Al₂O₃ NCts with 4% and 8% Ni loading. Characterization through XRD/SAED, STEM/EFTEM, and EELS revealed a disordered cubic structure of η-Al₂O₃, with well-dispersed Ni particles, making it effective for CO₂ hydrogenation. The 8-Ni/η-Al₂O₃ exhibited the best catalytic performance, with CH₄ selectivity of 99.8% and space time yield (STY) of 80.3 mmol_CH g_cat ^-1 h^-1 at 400 °C. The CO₂ methanation mechanism over Ni/η-Al₂O₃ NCts was further explored using operando DRIFTS aligned with GC + MS. The operando investigation suggested a preferential associative CO₂ methanation pathway, involving sequential adsorption and hydrogenation of CO₂ to hydrogen carbonates on Ni/η-Al₂O₃, and their transformation into formate and methoxy intermediates leading to methane. Finally, to complete the upcycling/recycling loop, the spent Ni/η-Al₂O₃ NCts were recycled into Ni and Al precursors. These findings underscore the potential of upcycling waste materials for synthesizing sustainable, high-performance NCts, and offer insights into the CO₂ methanation mechanism.