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

Nanocrystalline powders of WO3, 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 700degreesC) and catalytic additives were introduced in two different concentrations (0.2 and 2%) in order to study the gas sensor performances of these WO3-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 WO3 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 WO3-based gas sensors show a more direct and simple sensor response. Interaction of ammonia with WO3 was studied by diffuse reflectance infrared spectroscopy. Only pure WO3 presented a W=O overtone band decrease and some nitrosil bands. In this case, NH3 would react with the surface oxygen of terminal W=O bonds and would lead to the formation of NO. Catalyzed WO3 avoided this reaction and so the unselective catalytic oxidation of NH3, improving sensor response. Influence of introduced additives on ammonia oxidation and thus on sensor response is discussed. (C) 2003 The Electrochemical Society.