Methane-philic ZIF-8 Molecular Enrichment for Selective Methane Sensing over Hydrogen in a Pd@SnO₂ Hybrid...

Methane (CH₄) detection below the lower explosive limit (LEL) concentrations remains fundamentally difficult for chemoresistive metal oxide sensors due to its chemical inertness and poor adsorption reactivity, with most Pd-functionalized metal oxide semiconductor (MOS) sensors preferentially responding to hydrogen (H₂) rather than CH₄. In this work, we report a fundamentally new sensing architecture: an FSP-derived Pd@SnO₂/ZIF-8 (PSZ) hybrid that reverses conventional selectivity and enables CH₄-dominant response for the first time in this material system. The sensor is fabricated using a fully scalable, solid-route approach combining flame spray-deposited SnO₂, controlled Pd surface functionalization, and ZIF-8 formation from FSP-grown ZnO, producing a structurally integrated MOS-Pd-metal-organic framework (MOF) multilayer. The optimized PSZ-15 device delivers a CH₄ response of ∼6 at 400 ppm and 200 °C, corresponding to a relative response of ∼508%, placing its response magnitude among the highest reported for MOS-MOF methane sensors under comparable sub-LEL conditions. PSZ-15 also exhibits response/recovery times of 31/304 s while suppressing H₂ and CO₂ interference. This methane-philic selectivity arises from ZIF-8-mediated physisorptive CH₄ enrichment, catalytic oxidation at Pd, and chemisorbed-oxygen-driven electron release in SnO₂. Complementary density functional theory analysis supports a stronger calculated interaction of CH₄ with the ZIF-8/PSZ model relative to CO₂ and H₂. This study establishes a new paradigm for sub-LEL CH₄ sensing and represents the first demonstration that a Pd-activated metal oxide can be flipped from H₂-selective to CH₄-selective through rational MOF integration.