NH3 interaction with catalytically modified nano-WO3 powders for gas sensing applications

Nanocrystalline powders of WO 3 , pure and with catalytic additives such as copper and vanadium, for ammonia gas detection are analyzed in detail. Material was annealed at two different temperatures (400 and 700 ° C) and catalytic additives were introduced in two different concentrations (0.2 and 2%)in order to study the gas sensor performances of these WO 3 -based materials. Crystalline structure characterization shows that a mixture of triclinic and monoclinic structure was present in the materials analyzed. Additive characterization reveals that catalytic metals were located as cations in the matrix lattice. Thick-film gas sensors based on pure WO 3 show an abnormal sensor response, which is attributed to a complex process originated by the oxidation of ammonia to NO. On the other hand, catalyzed WO 3 -based gas sensors show a more direct and simple sensor response. Interaction of ammonia with WO 3 was studied by diffuse reflectance infrared spectroscopy. Only pure WO 3 presented a W=O overtone band decrease and some nitrosil bands. In this case, NH 3 would react with the surface oxygen of terminal W=O bonds and would lead to the formation of NO. Catalyzed WO 3 avoided this reaction and so the unselective catalytic oxidation of NH 3 , improving sensor response. Influence of introduced additives on ammonia oxidation and thus on sensor response is discussed.

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