Highly Selective CO₂ Conversion to CH₄ by a N-Doped HTiNbO₅/NH₂-UiO-66 Photocatalyst without a Sacrificial Electron Donor

Photocatalytic reduction of CO₂ to value-added chemicals is a promising technology for reducing atmospheric CO₂, but selectively producing a specific product still remains a great challenge. In this study, a Z-scheme heterojunction, N-doped HTiNbO₅/NH₂-UiO-66(Zr) (referred to as NH-NU), is developed to integrate the advantages of semiconductor photocatalysts and porous CO₂ adsorbents for CO₂-to-CH₄ conversion. The NH-NU Z-scheme heterojunctions are fabricated via a simple one-pot solvothermal method, enabling the formation of a tight and uniform interface between the two phases, thereby facilitating the separation and transfer of the photoinduced charge carriers, as confirmed by TEM, EPR, electrochemical studies, and work functions. As a result, the as-prepared photocatalyst demonstrates a significant increase in selectivity for CH₄ production through CO₂ photoreduction, achieving a 10-fold enhancement compared to that of the pristine MOF, NH₂-UiO-66. Moreover, there is no obvious decrease in the photocatalytic activity for CH₄ production across four consecutive cycles. In situ FT-IR spectroscopy and DFT calculations reveal that charge-enriched N-doped NH-NU-3 composites stabilize various C₁ intermediates in multistep elementary reactions, leading to superior selectivity in the CO₂-to-CH₄ conversion process. This work establishes that efficient and selective heterogeneous catalytic processes can be achieved through the stabilization of reaction intermediates by designing suitable Z-scheme heterojunctions.