Dual-Pathway CO Sensing Mechanism in Pd-Loaded SnO₂ Nanocrystals: An Operando Spectroscopic Study

Pd-loaded SnO₂ nanocrystals were synthesized via a hot soap method, achieving uniform Pd dispersion and controlled morphology. Structural and surface analyses confirmed well-crystallized SnO₂ with nanoscale Pd species and a porous film architecture. Pd-SnO₂ exhibited significantly enhanced CO sensitivity at low temperatures, with responses up to S = 5300 (S = R_air/R_gas) at 100 °C, whereas the pristine SnO₂ showed minimal response. Operando DRIFTS, UV-Vis, and Raman spectroscopy revealed that the sensing mechanism is temperature-dependent. At low temperatures, CO adsorption on Pd Lewis acid sites dominates, enabling electron donation without CO oxidation. At higher temperatures (∼300 °C), conventional oxygen-mediated CO oxidation occurs. Pyridine DRIFTS confirmed the critical role of Pd-associated acid sites in CO chemisorption. Even under a N₂ atmosphere, the sensors retained high response, confirming the occurrence of the nonoxidative sensing mechanism. These findings demonstrate that enhancing CO adsorption and suppressing CO combustion are key to developing high-performance MOX-based CO sensors.