Lead-Free Vacancy-Ordered K ₂ MCl ₆ (M = Sn, Ti, Pd, Pt) Double Perovskites as Stable Photocatalysts for Solar H ₂ Evolution and CO ₂ Reduction

Lead-free vacancy-ordered double perovskites K 2 MCl 6 (M = Sn 4+ , Ti 4+ , Pd 4+ , Pt 4+ ) were systematically investigated as potential photocatalysts for H 2 evolution and CO 2 reduction using first-principles DFT calculations combined with AIMD simulations. The calculated negative formation energies, favorable tolerance factors, phonon spectra, and AIMD results confirm the structural and dynamical stability of all compounds. Electronic structure calculations reveal band gaps ranging from 2.46 to 3.21 eV, with K 2 PdCl 6 exhibiting the narrowest band gap and visible-light absorption up to 503 nm. The calculated electron effective masses are lower than hole effective masses, suggesting favorable charge separation and reduced recombination. Furthermore, the band-edge positions indicate suitable redox potentials for photocatalytic water splitting and CO reduction. Comparative analysis with representative photocatalysts highlights the promising photocatalytic characteristics of the investigated chloride systems. Among them, K 2 PdCl 6 exhibits the most favorable combination of visible-light absorption, carrier transport, and band alignment, making it a promising candidate for solar-energy conversion applications.