Designing FeO@graphite@C Nanocomposites Based on Humins as Efficient Catalysts for Reverse Water-Gas Shift

Acid-catalyzed conversion of biomass into bio-based platform chemicals such as levulinic acid and 5-hydroxymethylfurfural is an important route in biorefineries, which has attracted much attention in recent years. Such a route however unavoidably yields massive recalcitrant byproducts called humins, which are now broadly considered as waste and are limited to combustion, causing unfavorable energy and environmental processes. Therefore, the development of a value-added utilization approach for such humin byproducts is crucial for making the biorefineries economical and environmentally viable. In this work, we present a starting point for valorization of humins via the preparation of carbon-based iron oxide nanocomposites of FeO@graphite@C by using the humins as carbon resources and material templates via a facile synthesis strategy. The as-prepared catalyst is capable of promoting the reverse water-gas shift reaction and reaching a high CO₂ conversion ratio with excellent CO selectivity (> 99%) at 500-700 °C, enabling an efficient utilization of waste CO₂. The unique graphite-capsuled FeO structure of FeO@graphite@C was found to be the origin of its excellent catalytic activity toward CO₂ reduction into CO, which shifts electrons from the graphite layer to FeO, reconstructing the Fe electron structure. This strengthened the electrophilic attack ability toward CO₂ and weakened the bond with the derived CO* species of the Fe active sites, associated with the excellent CO₂ conversion and CO selectivity.