Size-Dependent Oxidation State and CO Oxidation Activity of Tin Oxide Clusters

The CO oxidation reaction is an industrially important reaction; however, the practical catalysts are limited to noble metals. In this paper, we report a systematic study of the CO oxidation ability on cheap and less noble tin oxide clusters with the aim of quantitatively understanding the active sites. We synthesized size-controlled tin oxide clusters in mesoporous silica using the dendrimer templating method, employing dendritic phenylazomethine with a tetraphenylmethane core (TPMG4) as the template molecule. The clusters had different sizes depending on the added amount of SnCl2 as a precursor to TPMG4. The synthesized tin oxide clusters contained not only stable tetravalent Sn­(IV) but also metastable divalent Sn­(II) due to the structural stability and had a size-dependent composition. The CO oxidation activity of the tin oxide clusters increased with decreasing cluster size depending on the Sn­(II)/Sn­(IV) ratio. We also found the correlation between the Sn­(II) fraction and the CO oxidation activity, clearly indicating that the partially reduced Sn­(II) acted as the active site for the CO oxidation in the tin oxide clusters. This knowledge give a clue on how to design a highly active CO oxidation catalyst with base metal oxides.