Methane-philic ZIF-8 Molecular Enrichment for Selective Methane Sensing over Hydrogen in a Pd@SnO ₂ Hybrid Architecture

Methane (CH 4 ) 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 (H 2 ) rather than CH 4 . In this work, we report a fundamentally new sensing architecture: an FSP-derived Pd@SnO 2 /ZIF-8 (PSZ) hybrid that reverses conventional selectivity and enables CH 4 -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 SnO 2, 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 CH 4 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 H 2 and CO 2 interference. This methane-philic selectivity arises from ZIF-8-mediated physisorptive CH 4 enrichment, catalytic oxidation at Pd, and chemisorbed-oxygen-driven electron release in SnO 2 . Complementary density functional theory analysis supports a stronger calculated interaction of CH 4 with the ZIF-8/PSZ model relative to CO 2 and H 2 . This study establishes a new paradigm for sub-LEL CH 4 sensing and represents the first demonstration that a Pd-activated metal oxide can be flipped from H 2 -selective to CH 4 -selective through rational MOF integration.