The kinetics of nitric oxide reduction by methanol or methane as well as methane combustion over lanthanum oxid-based catalysts

A family of La₂O₃-based catalysts—pure La₂O₃, 4% Sr-promoted La₂O₃, and 40% La₂O₃ supported on γ-Al₂O ₃—were investigated to explore their potential use in NO _x selective catalytic reduction (SCR) systems. First, a kinetic study detailing NO reduction by CH₃OH over pure La₂O ₃ in the presence and absence of O₂ was performed. The activity was measured under differential reactor conditions in the absence of O ₂ between 623 and 823K with an activation energy of 12 kcal/mol. The rate of N₂ formation was shown to be 75 times higher than when CH₄ is the reductant at 773K, but the selectivity of N₂ vs. N₂O formation was only 33% compared to near unity for CH ₄. A Langmuir-Hinshelwood model assuming two different adsorption sites was proposed to describe the behavior in the absence of O₂, which resulted in a good fit that was thermodynamically consistent. The addition of O₂ enhances the NO reduction activity, but a gas-phase reaction limited the kinetic study in the presence of O₂, and after determining CH₃ONO was a significant product at 300K in a blank reactor, no further kinetic study was performed with CH₃OH.; The study was continued using CH₄ as the reductant, and testing the effects of typical flue gas components—9% CO₂, 2% H ₂O, and 1500 ppm SO₂—on the NO reduction activity of the La₂O₃-based catalysts between 773 and 973K. SO ₂ was shown to completely and irreversibly deactivate La₂O ₃ at 773K, such that no activity was apparent after flowing 1500 ppm SO₂ for 1 h. Both CO₂ and H₂O demonstrated reversible inhibitory effects on all of the catalysts; CO₂ was shown to reversibly change the bulk phase of La₂O₃ to La₂O₂CO₃, but H₂O had no effect on bulk catalyst. In the absence of O₂, a previously proposed Langmuir-Hinshelwood model was adapted to account for CO₂ and H₂O in the site balance and yielded a good fit that met the thermodynamic criteria defined for equilibrium constants.; In the presence of O₂, the direct combustion of CH₄ was significant enough that differential conditions could not be maintained at significant levels of NO reduction; therefore, it was necessary to study NO reduction with an integral method of analysis. This method required an expression describing the changing reactant concentrations in the reactor bed, so it was necessary to develop an expression for CH₄ combustion on each of the catalysts. A Langmuir-Hinshelwood mechanism was proposed for the combustion behavior that was capable of fitting the combustion data well, while also maintaining thermodynamically consistent rate constants. This combustion model was then combined with a previously proposed mechanism for NO reduction in the presence of O₂ that had been adapted to account for CO ₂ and H₂O in the site balance. This rate expression was able to fit the data well over all of the catalyst systems, except the supported La₂O₃ with CO₂ in the feed. It is proposed that the support, γ-Al₂O₃, significantly contributes to the activity under these conditions since its NO reduction activity is unaffected by CO₂. A Langmuir-Hinshelwood model was developed describing the behavior on alumina, and when summed with the rate law from pure La ₂O₃, a relatively good fit is achieved for the supported La₂O₃ data.