Methane combustion and CO oxidation on LaAl1-xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl1-xMnxO3 (x = 0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073K were investigated. XRD analysis showed that all the LaAl1-xMnxO3 samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280-–1180Å and 4–-33 m2 g-1 ranges, respectively. Redox properties and the content of Mn4+ were derived from temperature programmed reduction (TPR) with H2. Two reduction steps are observed by TPR for pure LaMnO3, the first attributed to the reduction of Mn4+ to Mn3+ and the second due to complete reduction of Mn3+ to Mn2+. The presence of Al in the LaAl1-xMnxO3 solid solutions produces a strong promoting effect on the Mn4+ ’ Mn3+ reducibility and inhibits the further reduction to Mn2+. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO3, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.