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

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.