Comparative Study on the Formation and Reduction of Bulk and Al2O3-Supported Cobalt Oxides by H2-TPR Technique

A systematic and comparative study of the decomposition procedures of unsupported and Al2O3-supported Co(NO3)2·6H2O, as well as the reduction procedures of bulk Co3O4 and Al2O3-supported Co3O4 was carried out. A series of decomposition products of bulk and Al2O3-supported Co(NO3)2·6H2O, and bulk cobalt oxides (Co3O4 and CoOOH) and Al2O3-supported Co3O4 (Co/Al2O3 catalysts) were prepared by the methods of calcination, deposition-precipitation, and incipient-wetness impregnation. The bulk and supported samples were characterized by different techniques, including XRD, UV−vis−NIR, TG-DTA, gas analyzer, and H2-TPR. The results show that the CoO species first appears in the calcination process of bulk Co(NO3)2·6H2O decomposition, and then CoO is oxidized to form Co3O4. CoO-Al2O3 composite oxide first appears in the calcination process of Al2O3-supported Co(NO3)2·6H2O decomposition. CoO disperses on the surface of Al2O3 support at first and then forms a multilayer with the decomposition of Co(NO3)2. Finally, CoO-Al2O3 composite oxide, which is very difficult to be reduced to metal Co, was oxidized to Co3O4. Particularly, it is thoroughly analyzed that the attribution of each reduction peak in the H2-TPR profiles of a series of Al2O3-supported Co-based catalyst. Moreover, it is a first semiquantitative study on the formation process of CoO in the course of calcination of bulk Co(NO3)2·6H2O and Al2O3-supported Co(NO3)2·6H2O, by introducing parameter α. In addition, the reduction mechanisms of bulk Co3O4 and Al2O3-supported Co3O4 were studied in detail, too. The reduction performance of Al2O3-supported Co3O4 is different from that of the bulk Co3O4, because of the strong interaction between Co3O4 and Al2O3 support. The activation energies for the two transformations from bulk Co3O4 to CoO and from CoO to Co are 80 and 53 kJ/mol, respectively. However, the activation energies for the two transformations from Al2O3-supported Co3O4 to CoO-Al2O3 and from CoO-Al2O3 to Co are 90 and 95 kJ/mol, respectively. So the H2-TPR profile of Al2O3-supported Co3O4 exhibits two apparent peaks, whereas the H2-TPR profile of bulk Co3O4 exhibits the incorporated trend of two peaks.