Mechanistic-assisted design of catalysts for nitric oxide emission control.

摘要

Nitric oxide (NO) is one of the major air pollutants. The U.S. Environmental Protection Agency (EPA) has implemented stringent rules and regulations to control its emission. Catalytic removal of NO has been considered to be the most cost effective and feasible approach. This dissertation focuses on mechanistic study and catalyst design of two NO removal reactions: direct NO decomposition on Tb-Pt/{dollar}rm Alsb2Osb3{dollar} and NO reduction with CO on supported Rh catalysts.; Mechanistic information is essential in designing an effective catalyst for NO-CO reaction and NO decomposition. Results of the previous and present studies have established that NO adsorbs on reduce rhodium sites {dollar}rm (Rhsp{lcub}o{rcub}){dollar} as anionic NO (Rh-{dollar}rm NOsp-rbrack {dollar} and on oxidized rhodium sites {dollar}rm (Rhsp+){dollar} as cationic NO (Rh-{dollar}rm NOsp+rbrack .{dollar} The former is the active adsorbate which dissociates to form adsorbed oxygen and adsorbed nitrogen; the latter is a spectator. Since reduced Rh sites have been found to be more active than oxidized Rh sites for NO-CO reaction, suppression of the transformation of {dollar}rm Rhsp{lcub}o{rcub}{dollar} sites to less active {dollar}rm Rhsp+{dollar} sites should allow maintenance of high catalyst activity.; One approach to prohibit the transformation of {dollar}rm Rhsp{lcub}o{rcub}{dollar} to {dollar}rm Rhsp+{dollar} is the silanation of {dollar}rm Rh/SiOsb2{dollar} with trimethylchorosilane. The silanation approach was tested in this study under NO-CO environment. Results of this study show that silanation fails to prevent the formation of {dollar}rm Rhsp+{dollar} in NO-CO environment. Adsorbed oxygen resulted from NO dissociation oxidizes {dollar}rm Rhsp{lcub}o{rcub}{dollar} to {dollar}rm Rhsp+.{dollar}; Promoting the dissociation of adsorbed NO species may also increase the activity of the catalyst for NO-CO reaction. MnO has been known to act as an oxophilic promoter which promotes CO dissociation. Since NO and CO have similar molecular orbitals with an exception of the lone electron for NO, MnO may also promote NO dissociation. Infrared temperature-programmed reaction studies over Mn-{dollar}rm Rh/SiOsb2{dollar} reveal that MnO does not show any promotion effect on adsorbed NO dissociation. Thus, the interaction of MnO with CO cannot be extended to the interaction of MnO with NO.; Extensive literature search concludes that the low NO decomposition activity of a catalyst is due to its inability to desorb oxygen from NO dissociation sites, suggesting that promoting oxygen desorption may increase NO conversion. We have identified terbium oxide as a promoter which possesses the ability to desorb oxygen at low temperatures. Results of the study of NO decomposition on Tb-promoted {dollar}rm Pt/Alsb2Osb3{dollar} catalysts shows that Tb oxide promotes desorption of oxygen from dissociated NO at 593 K which is significantly lower than the reported oxygen desorption temperatures for Pt catalysts. Desorbed oxygen is produced from decomposition of chelating bidentate nitrato which may be resulted from the reaction of adsorbed oxygen on Pt and adsorbed NO on Tb oxide. Promotion of oxygen desorption is an effective approach for enhancement of NO decomposition activity. This study points out a new direction for the development of NO decomposition catalysts. The potential of the Tb-{dollar}rm Pt/Alsb2Osb3{dollar} catalyst for the practical application has to further be determined under the effect of air, {dollar}rm Hsb2O,{dollar} and {dollar}rm SOsb2{dollar} under the steady-state flow condition.; This study demonstrates that fundamental understanding of the mechanism of the reaction is essential for designing and developing effective catalysts for NO decomposition and NO reduction with CO.