100% Conversion of CO2–CH4 with Non-Precious Co@ZnO Catalyst in Hot Water

Abstract

The combination of solar energy and natural hydrothermal systems will innovate the chemistry of CO2 hydrogenation; however, the approach remains challenging due to the lack of robust and cost-effective catalytic system. Here, Zn which can be recycled with solar energy-induced approach was chosen as the reductant and Co as catalyst to achieve robust hydrothermal CO2 methanation. Nanosheets of honeycomb ZnO were grown in situ on the Co surface, resulting in a new motif (Co@ZnO catalyst) that inhibits Co deactivation through ZnO-assisted CoOx reduction. The stabilized Co and interaction between Co and ZnO functioned collaboratively toward the full conversion of CO2–CH4. In situ hydrothermal infrared spectroscopy confirmed the formation of formic acid as an intermediate, thereby avoiding CO formation and unwanted side reaction pathways. This study presents a straightforward one-step process for both highly efficient CO2 conversion and catalyst synthesis, paving the way for solar-driven CO2 methanation.

1. Introduction

Access to affordable and reliable energy that is produced with minimal negative environmental impacts represents a global challenge. To this end, the conversion of carbon dioxide (CO2), which is the main greenhouse gas, into hydrocarbons using solar energy is critical for minimizing these environmental issues. The photocatalytic conversion of CO2 over semiconductors has been widely developed and reported [1, 2]; however, the efficiency of the conversion is limited by the kinetic restrictions of multiple e−/H+ transfer processes and ability of the semiconductors to activate thermodynamically stable CO2 [3, 4]. In contrast, the cascade approach for CO2 conversion by solar energy has sparked interest in exploring novel methods for fuel generation. This approach involves a two-step process in which active reductants such as H2 or metals are initially produced via a solar energy-driven reaction, followed by the conversion of CO2 utilizing the reductive power of hydrogen or active metals [5, 6]. These two-step solar-chemical approaches employ diverse and robust reductants as the driving force, leading to higher reaction rates, flexibility in the selection of non-noble metal-based catalysts, and the potential for large-scale industrial applications.

In pioneering work by A. Steinfeld, the heat obtained from concentrated solar radiation was used to drive multiple cycles of metal oxide redox pairs, including Zn/ZnO, Ce2O3/CeO2, FeO/Fe3O4, and SnO/SnO2, potentially offering an abundant selection of reductants for CO2 conversion [7–12]. Further insights into metal-initiated CO2 reduction are gained from abiogenic organic synthetic processes. In bio- and geo-chemistry, abiogenic organic compounds are formed from CO2 reduction in the mantle and submarine hydrothermal vents, in which serpentinization delivers a substantial amount of hydrogen from the interaction between high-temperature water and earth-abundant metals [13–16]. By mimicking this natural phenomenon, we demonstrated efficient CO2 reduction under hydrothermal conditions using metals Zn, Fe or Al as reductants [17–19], with Zn showing the highest CO2 reduction efficiency [20, 21], thereby standing as a promising reductant for deeper CO2 reduction processes, such as CO2 methanation (CO2 + 4H2 → CH4 + 2H2O, Sabatier reaction). Since methane (CH4) is an ideal energy carrier and hydrocarbon fuel can be conveniently distributed using well-developed gas pipeline infrastructure [22–28], the CO2–CH4 cycle could potentially close the carbon fuel emission cycle. Thus, we consider CH4 production ideal for energy storage and grid integration, providing a practical approach to utilizing CO2, especially in the context of renewable energy.

In abiogenic organic synthesis, the origin of life also lies in the unique catalytic role of common earth-abundant rocks [29–32]. For instance, the acetyl-CoA pathway, which is the only known exergonic autotrophic CO2 fixation pathway, is actively catalyzed by