| The simplicity of direct NO decomposition to N2 and O 2 provides an attractive alternative compared to the complex selective catalytic reduction (SCR) reaction in controlling the emission of NO from automobiles and coal-fired power plants. However, unambiguous knowledge of the reaction intermediates and the catalytic sites on which they adsorb as well as their role in the NO decomposition mechanism on Cu-ZSM-5 would provide a basis for the design and development of a novel catalyst capable of operating with high activity under all reaction conditions.; A catalytic process involves adsorption of the reactant onto the catalyst surface, reaction on the catalyst surface, and desorption of products from the surface. Understanding the sequence and identity of the species involved in the adsorption, reaction, and desorption processes is vital for catalyst design and development. In situ infrared (IR) technique has emerged as a powerful tool in not only identifying the structure of the reaction intermediates but also in monitoring the dynamics of reaction intermediates under reaction conditions. The objective of this study is to develop reaction mechanisms for the NO and N2O decomposition over Cu-ZSM-5.; IR and mass spectrometer (MS) studies on Cu-ZSM-5 reveal Cu2+ (NO), Cu+(NO), bridging Cu2+(NO 3--), and NO+ as the major adsorbates and N2, N2O, O2, and NO2 as the products during NO decomposition. Transient step and pulse results indicate that the sequence of adsorbate formation is: Cu+(NO), Cu 2+(NO), NO+ > Cu2+(NO3 --). NO decomposition is envisioned to occur in the following steps: (a) NO adsorbs on Cu+ as Cu+(NO), (b) NO on Cu+(NO) dissociates to produce Cu+-N and Cu2+O--, (c) Cu+(NO) reacts with Cu+-N to form N2O and Cu2+O --, (d) N2O adsorbs onto Cu+ to produce N2 and Cu2+O--, (e) Cu 2+O-- adsorbs NO and interacts with adsorbed O to produce Cu2+(NO3--), (f) Cu 2+(NO3--) decomposes to NO, O 2, and Cu+, and (g) Cu2+O -- reduces to Cu+ producing O2.; IR studies have identified that the major adsorbates during N2O decomposition on Cu-ZSM-5 as Cu+-N2O, Cu +-ON2, and Cu2+(NO3-- ). N2O TPR and transient step studies have revealed the sequence of formation of the observed adsorbates as: Cu+-N 2O, Cu+-ON2 > Cu2+(NO 3--). The N2O decomposition reaction over Cu-ZSM-5 is expected to occur as indicated in the following steps: (a) adsorption of N2O on Cu+ via N as Cu+-N 2O and via O as Cu+-ON2, (b) dissociation of Cu+-ON2 to produce Cu2+O -- and N2, (c) reaction of Cu2+O -- with Cu+-ON2 to form Cu2+ O2-- and Cu2+(NO 3--), and (d) formation of O2 by reduction of Cu2+O2-- to Cu + and Cu2+O-- to Cu+. N2O step studies at 298 K revealed that N2O adsorbs onto Cu-ZSM-5 as Cu+-N2O and Cu+-ON 2 (step (a)). (Abstract shortened by UMI.)... |
