Biomimetic Catalyst for Solar CO₂ Methanation: Harnessing Natural Pigments in a Halloysite–Kojic Acid Composite

Introduction
This study presents a biomimetic approach to enhance the photoconversion of CO₂ into methane. A catalyst based on halloysite (HNT) and kojic acid (K) was developed and further improved with xanthopterin, a natural pigment found in oriental wasp wings. The integration of this component significantly increased both efficiency and selectivity, highlighting the potential of nature-inspired strategies for sustainable solar-driven CO₂ conversion.

Methods
The synthesis involved functionalizing HNTs with chlorokojic acid, then complexing with CuCl₂ and reducing to Cu(I) with ascorbic acid. The resulting HNT-K/Cu(I) was combined with xanthopterin or riboflavin in DMSO and purified. Antenna loading was quantified by UV analysis, functionalization confirmed with FTIR and TGA, and optical properties evaluated by UV-DRS.

Results
Catalysts lacking Cu(I), including HNT, K, HNT-K, and Xant:HNT-K, showed negligible activity, confirming the essential role of cuprous ions. Even with Cu(I), activity remained low unless kojic acid was present, demonstrating its importance in stabilizing the active sites. The best-performing system, Xant:HNT-K/Cu(I), achieved 40.9% CO₂ conversion with 85.1% methane selectivity, far surpassing HNT-K/Cu(I). This performance arises from the synergistic effect of Cu(I), kojic acid, and xanthopterin. Compared with other catalysts that yield higher conversion but more CO, this system favored the formation of methane. It also remained stable over four cycles.

Conclusions
The combination of HNTs, kojic acid, and xanthopterin yields an efficient, selective, and stable catalyst for CO₂ photoreduction to methane. These results highlight the promise of biomimetic strategies and natural materials for designing sustainable CO₂ conversion systems.