Total oxidation of propane using nanocrystalline cobalt oxide and supported cobalt oxide catalysts

Abstract

Supported and unsupported nanocrystalline cobalt oxides have been shown to be extremely efficient catalysts for the total oxidation of propane. Total conversion with a high stability has been achieved at reaction temperatures as low as 250°C. In the present work, a comparison between the catalytic performance of bulk and alumina-supported nanocrystalline cobalt oxide catalysts has been made. The influence of crystallite size, nature of the support (alpha, gamma and mesoporous alumina) and cobalt loading, has been probed. Unsupported cobalt oxide catalysts were more active than any supported cobalt oxide catalysts. The catalytic activity was mainly dependent on the crystallite size, decreasing with an increase in the crystallite size. Whilst the TOF of supported catalysts increases with both a decrease in the surface area of the support and an increase of the cobalt content. Only those catalysts with a cobalt content of 50wt% achieve a catalytic activity per cobalt atom similar to that of unsupported Co3O4 prepared under similar conditions. High surface area alumina was shown to be a less efficient support especially at low Co-loadings due to the dispersion of cobalt on the surface leading to the formation of inactive Co–O–Al species. The low surface area support (α-Al2O3) presents a low capacity to isolate the cobalt oxide species resulting in a lower concentration of inactive Co–O–Al species and, therefore, the highest activity among supported catalysts. In conclusion, two parameters seem to determine the catalytic activity of nanocrystalline cobalt oxide: these are the presence of inactive Co–Al bonds (prevalent in supported catalysts) and also the size of the active Co3O4 particles.