NO₂ Interactions with MoO₃ and CuO at Atmospherically Relevant Pressures

NO_x concentrations in some geographic regions are harmful to human health. Gas filters to trap NO_x and other toxic chemicals contain metal oxides, including MoO₃ and CuO. These materials are also being investigated for NO_x gas sensors. In a step to understand the fundamental adsorption mechanism in sensors and the effect on binding site availability in gas filters, ambient-pressure X-ray photoelectron spectroscopy (APXPS) was used to study the interaction of NO₂ with polycrystalline MoO₃ and CuO surfaces under pressures up to 0.01 Torr (14 parts per million volume (ppmv)). Density functional theory-based computational modeling was performed to reveal the mechanisms of NO₂ interactions with the MoO₃(010) and CuO(111) surfaces to aid interpretation of the experimental results. With pressure dependence, NO₂ interacts with reduced Mo⁵⁺ atoms generated by oxygen vacancies and abstracts hydrogen atoms from hydroxyl groups on MoO₃ without accumulating N-containing species on the surface; vacancy-induced electronic states in the band gap are also removed, hinting toward an increase in the resistivity of the material. N-containing species begin accumulating on the CuO surface at atmospherically relevant pressures of 140 ppbv. NO₂ only decomposes at oxygen vacancy sites of CuO. The nitrogen species leave the CuO surface upon evacuation, highlighting the importance of in situ surface characterization when studying gas sensing and adsorption mechanisms. These results imply that NO₂ removes hydroxyl and O_vac binding sties on these materials when used in gas filtration and sensing applications. Furthermore, the results show the key role of O_vac sites in the gas sensing mechanism of MoO₃ and highlight the potential of APXPS for further studies of gas sensors.