New Insights into Alkali-Catalyzed Gasification Reactions of Carbon: Comparison of N2O Reduction with Carbon over Na and K Catalysts

Abstract

Catalytic conversion of N2O to N2 over Na- and K-impregnated activated carbon (Na/AC and K/AC) was investigated. K and Na are two representative and most active catalysts for the C–NOx reactions. Carbon samples with different K and Na loadings were characterized by N2 adsorption, thermal decomposition (with TGA), TPD, and CO2 chemisorption at 250°C. CO2 chemisorption at 250°C proved to be effective for the measurement of potassium dispersion but not for sodium. Using N2O as the reactant facilitated the observation and analysis of the reaction mechanism of gas–carbon reactions due to its readiness for dissociative chemisorption and also because the C–N2O reaction is an elementary reaction. Based on isothermal reactions and TPR, potassium was found to be more active in both N2O dissociation and oxygen transfer thus gave rise to a much higher activity. It was found that K/C was an excellent catalyst for N2O decomposition. In the low-temperature range of 150–250°C, a significant amount of N2 was produced on K/C with no carbon gasification. Significant CO2 production occurred only at higher temperatures. The fundamental reason for the difference in activities of the two alkali metals is a combination of three factors. First, K/C is highly active for N2O dissociation. The O atoms thus produced would form active surface intermediates, most likely the epoxy intermediate which significantly weakened the surface C–C bonds for gasification (which is the basis of the unified mechanism of Chen and Yang). Second, the surface phenolate group also weakened the C–C bond to facilitate gasification, but the weakening was more pronounced by the –C–O–K group than by the –C–O–Na group. Third, the dispersion of K2O on carbon was higher than that of Na2O.