Photocatalytic Methanol Reforming over Cs₂RuX₆ (X = Cl, Br) Double Perovskites: H₂ Evolution and Carbon-Containing Gas Formation

Photocatalytic methanol reforming offers a low-temperature route to hydrogen production, yet controlling product selectivity remains a key challenge. Here, we evaluate Cs₂RuX₆ (X = Cl, Br) double-perovskite photocatalysts for methanol reforming under mild conditions (2 mg catalyst, 25 °C, 1.0 bar Ar, 130 mW·cm⁻² irradiation, 3 h), tracking time-resolved gas evolution to elucidate substrate-loading effects and halide-dependent behaviour. Methanol addition markedly enhances H₂ generation compared with methanol-free conditions for both catalysts. Cs₂RuCl₆ shows the highest H₂ productivity at intermediate methanol loading, reaching ~10 mmol·g_cat⁻¹ after 3 h at ~39.6 mmol methanol, while Cs₂RuBr₆ attains ~7–8 mmol·g_cat⁻¹ under the same loading. Increasing methanol to higher levels (e.g., ~98.9 mmol) reduces H₂ output for both materials, indicating an optimum substrate range rather than a monotonic dependence.

O₂ evolution exhibits a contrasting trend: the presence of methanol generally suppresses O₂ formation, most clearly for Cs₂RuBr₆, consistent with competitive consumption of oxidative equivalents in the reforming network. In parallel, carbon-containing gaseous products are co-generated, revealing strong selectivity changes with methanol loading and catalyst halide. CH₄ formation increases with time and is maximised at intermediate methanol loadings (e.g., ~8 mmol · g_cat⁻¹ for Cs₂RuCl₆ at ~39.6 mmol after 3 h; lower maxima for Cs₂RuBr₆). CO production is also favoured at moderate methanol amounts (up to ~3–4 mmol · g_cat⁻¹ for Cs₂RuCl₆ around ~21.9 mmol), whereas higher methanol loading diminishes CO yields. Notably, C₂H₄ emerges as a dominant carbon product on both catalysts, reaching ~350 μmol · g_cat⁻¹ for Cs₂RuCl₆ and ~270 μmol · g_cat⁻¹ for Cs₂RuBr₆ at ~39.6 mmol methanol after 3 h, highlighting a pronounced substrate-dependent shift towards C–C containing products.

Overall, these results demonstrate that Cs₂RuX₆ perovskites enable methanol reforming at ambient temperature with substantial hydrogen evolution and tunable multi-gas selectivity. The combined halide comparison and substrate-loading optimisation provide practical guidelines for balancing H₂ productivity against carbonaceous by-products in perovskite-based reforming systems.

Looking ahead, the demonstrated loading-dependent trade-off between hydrogen productivity and carbonaceous by-product formation offers a clear lever for process tuning. By identifying an optimum methanol range and highlighting distinct selectivity patterns for Cl versus Br, this study sets a baseline for rational catalyst and reactor optimisation. Future work will couple in situ spectroscopy with mass-balance analysis to map reaction pathways and to suppress undesired products, enabling more efficient solar-to-chemical conversion using perovskite reforming platforms under ambient conditions relevant to deployment.