Highly selective CO₂-to-CH₄ conversion via multisun-assisted photocatalysis enabled by dual cocatalyst interfaces

Solar-driven conversion of low-concentration CO₂ into useful chemicals and fuels offers a way to turn even dilute carbon sources into renewable energy carriers [1], [2], [3]. Yet, this remains challenging due to limited CO₂ availability and slow reaction kinetics [4], [5], [6]. TiO₂ stands out as one of the best photocatalysts due to its cost-effectiveness, sustainability, and photostability; however, its usefulness in CO₂ reduction is curtailed by a wide bandgap, rapid charge recombination, and limited understanding of its surface catalytic activity [7]. Extensive research has explored different cocatalysts to improve the performance of TiO₂, focusing on metal nanoparticles (NPs) such as Pt, Ag, Au, Pd, and Cu, as well as their bimetallic combinations. Fewer studies have investigated non-metal cocatalysts [8]. Most work highlights the role of metal NPs in enhancing charge separation and catalytic activity.

Two-dimensional transition metal dichalcogenides (TMDs) such as MoS₂ and MoSe₂ have emerged as promising nonprecious materials owing to their unique optical and electronic properties [9]. MoSe₂ has gained attention for energy applications due to its high electrical conductivity, and its easy-to-form Se–Mo–Se layered structure [10]. MoSe₂ has been explored as a cocatalyst for photocatalytic H₂ production, showing potential to replace Pt because of its favorable Se–H_ads bond energy (273 kJ/mol) compared to Pt–H (251 kJ/mol) [11], [12]. DFT studies by Nørskov et al. also indicate its capability for CO₂ reduction [13]. With a narrow band gap (1.1–1.8 eV), MoSe₂ enhances visible-light absorption and facilitates CO₂ photoreduction. Previous studies employed MoSe₂ in a heterostructure formation with another semiconductor for photocatalytic CO₂ reduction in a liquid phase using triethanolamine (TEOA) as a sacrificial agent [14], [15]. Similarly, MoSe₂ with 1T-phase dominance can also be used as a cocatalyst for photocatalytic CO₂ reduction [16], depending on the synthesis approach of the catalyst. The quantum-confinement effect enables an indirect-to-direct band-gap transition in bulk-to-nanolayered MoSe₂. Therefore, exfoliation of bulk MoSe₂ into multi-layered structures can be possible, in which the weakly bonded two-dimensional (2D) atomic layers (i.e., nanosheets) can be separated [17]. These multilayers can be combined with other semiconductors to make them useful materials for photocatalytic CO₂ reduction reactions. Most MoSe₂-based photocatalysts reported to date are binary composites but still fall short of fully exploiting this material's synergistic potential. Thus, further exploration of their properties and CO₂ reduction performance is highly desirable. Constructing a ternary hybrid architecture by integrating additional active components offers a promising route to enhanced efficiency. Anchoring Pt NPs as co-catalysts can significantly boost activity through their localized surface plasmon resonance (LSPR) effect, which broadens visible-light absorption and improves charge separation [18]. In addition, they provide highly active adsorption sites, which are crucial for gas-phase CO₂ reduction, where efficient CO₂ activation is essential [19].

Based on these insights, we designed a Pt/TiO₂-MoSe₂ (Pt/TM) ternary photocatalyst to improve low-concentration CO₂ photoreduction. Although MoSe₂ alone shows limited activity, integrating Pt NPs and MoSe₂ nanosheets on TiO₂ creates a dual co-catalyst system that enhances charge separation and CO₂ activation. Pt acts as an electron trap and multi-electron transfer center, while MoSe₂ edge sites and Se vacancies provide Lewis basic sites for strong CO₂ adsorption and activation, enabling efficient and selective CO₂ reduction. The Pt_1.5%-TM catalyst exhibited superior CO₂-to-CH₄ conversion activity under 5-sun illumination, with ≈ 98% CH₄ selectivity with a trace of C₂H₆, and 24-hour stability. This enhanced performance under multi-sun irradiation stems from increased photon flux, which generates more reactive electrons and holes, thereby accelerating charge utilization, CO₂ activation, and CH₄ desorption. In situ EXAFS, TRPL, operando spectroscopies, isotopic labeling, and DFT calculations revealed the charge-transfer dynamics and the CO₂ reduction pathway responsible for enhanced performance.