Aerogel derived cation-substituted hexaaluminate catalysts for methane combustion

Catalytic combustion is an effective method for performing high efficiency combustion while minimizing emissions of CO and nitrogen oxides. Cation-substituted hexaaluminates have emerged as very promising materials for catalytic combustion. For the first time, singly and doubly cation-substituted hexaaluminates were synthesized from corresponding aerogel precursors. Aerogels are high surface area and low density materials that are prepared by extracting solvent from gels under supercritical conditions. We used two methods to dry the gels. In the direct method, the gels were dried under conditions where ethanol was supercritical (270{dollar}spcirc{dollar}C and 100 atm) while in the CO{dollar}sb2{dollar} exchange/extraction method, the solvent was first exchanged with liquid CO{dollar}sb2{dollar} then the gels were dried under conditions where CO{dollar}sb2{dollar} was supercritical (55{dollar}spcirc{dollar}C and 100 atm). Gels dried using the direct and CO{dollar}sb2{dollar} exchange/extraction methods were referred to as high temperature and low temperature materials, respectively.; The substitution cations (Mn and Co) seemed to accelerate solid-state phase transformation since the cation-substituted materials transformed to the hexaaluminate phase at lower temperatures than the unsubstituted materials. The substitution cations also promoted sintering at calcination temperatures beyond 1400{dollar}spcirc{dollar} given that the cation-substituted materials had lower surface areas than the unsubstituted materials in this temperature range. Phase transformation pathways for the cation-substituted and unsubstituted materials were different. For the high temperature materials, the cation-substituted materials transformed directly from an amorphous material to the hexaaluminate while BaCO{dollar}sb3{dollar} and BaAl{dollar}sb2{dollar}O{dollar}sb4{dollar} were detected in the unsubstituted materials before they transformed to the hexaaluminate phase. For the low temperature materials, BaCO{dollar}sb3{dollar} and BaAl{dollar}sb2{dollar}O{dollar}sb4{dollar} were present in both the unsubstituted and cation-substituted materials. the BaAl{dollar}sb2{dollar}O{dollar}sb4{dollar} disappeared from the unsubstituted materials after calcination at temperatures higher than 1400{dollar}spcirc{dollar}C. Formation of the carbonates was probably a consequence of exposure to the CO{dollar}sb2{dollar} extraction fluid or CO{dollar}sb2{dollar} in the air.; The cation-substituted materials were much more active than the unsubstituted materials for the combustion of methane. The Mn-substituted materials were not only more active but also more thermally stable than the Co-substituted materials. This indicated that Mn was a better substitution cation than Co. The low temperature materials were much more active than the high temperature materials. A cation-rich hexaaluminate phase in the low temperature was thought to be the cause for this activity enhancement. The Mn-Co-substituted materials were more active than the Mn- and Co-substituted materials. This suggested a synergistic effect. The optimal composition for the Mn-Co-substituted materials was BaMn{dollar}sb{lcub}0.5{rcub}{dollar}Co{dollar}sb{lcub}0.5{rcub}{dollar}Al{dollar}sb{lcub}11{rcub}{dollar}O{dollar}sb{lcub}19{rcub}{dollar}-{dollar}alpha{dollar} when the Ba:M (M is Co and/or Mn) ratio was limited to unity.