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

Abstract

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 CO 2 conversion ratio with excellent CO selectivity (> 99%) at 500−700 °C, enabling an efficient utilization of waste CO 2 . The unique graphite-capsuled FeO structure of FeO@graphite@C was found to be the origin of its excellent catalytic activity toward CO 2 reduction into CO, which shifts electrons from the graphite layer to FeO, reconstructing the Fe electron structure. This strengthened the electrophilic attack ability toward CO 2 and weakened the bond with the derived CO* species of the Fe active sites, associated with the excellent CO 2 conversion and CO selectivity.

Mechanistic Insight into Hydrocarbon Synthesis via CO2 Hydrogenation on χ-Fe5C2 Catalysts

“…Despite great efforts having been made over the last decades, CO 2 concentration control and efficient conversion remain challenging . The catalytic hydrogenation of CO 2 using H 2 derived from renewable energy sources (such as solar, wind, and biomass) to produce hydrocarbons and higher oxygenates is an important research direction in the field of heterogeneous catalysis and CO 2 chemistry and has also become one of the promising approaches in energy storage. − The advances in energy-efficient CO 2 conversion can potentially alleviate CO 2 emissions, reduce the dependence on nonrenewable energy resources, and minimize the negative environmental effect from traditional fossil fuels. − …”

Mechanistic Understanding of Hydrocarbon Formation from CO2 Hydrogenation over χ-Fe5C2(111) and the Effect of H2O and Transition Metal Addition

“…The increase of CO 2 levels in the atmospheric environment is mainly caused by the combustion of fossil fuels (oil, natural gas, and coal), and the resulting resource depletion and greenhouse effect are of increasing concern worldwide. − Converting CO 2 into a range of chemicals and fuels is an effective approach to address the abovementioned challenge. − Importantly, hydrogenation of CO 2 to olefins (C 2+ = ) is one of the promising ways to reduce the dependence of the production of valuable chemicals on fossil fuels while mitigating greenhouse gas emissions. − Fe-based catalysts have been widely used in Fischer–Tropsch synthesis (FTS), which are found to be also appealing for CO 2 hydrogenation due to their activity for the reverse water–gas shift (RWGS) reaction and FTS. − However, the nature of active phases in Fe-based CO 2 hydrogenation catalysts are still under debate because the evolution of Fe phases under reaction conditions is complicated. , …”

“…lation results, the optimal energetic pathways for CH4 and C 2 H 4 formation are screened out, as plotted in Figure 9a. Hydrogenation of CO 2 * to form a CH 2 * active intermediate is the hinge to the generation of hydrocarbon products.…”