X-ray absorption spectroscopy, x-ray photoelectron spectroscopy, and analytical electron microscopy studies of cobalt catalysts. 2. Hydrogen reduction properties

Abstract

XAS, XPS, AEM, and temperature-programmed reduction were used to characterize the hydrogen reduction of bulk Co{sub 3}O{sub 4} and Co{sub 3}O{sub 4} supported on small (Co/SiO{sub 2}-62) and large (Co/SiO{sub 2}-923) pore size silica. The reduction process proceeded as Co{sub 3}O{sub 4} {yields} CoO {yields} Co, with the CoO intermediate species spectroscopically identified on all three samples. The first step of the reduction, Co{sub 3}O{sub 4} {yields} CoO, occurred near 300{degree}C for all three catalysts. The second step of the reduction, CoO {yields} Co, showed significant differences among the samples, with the ease of reduction decreasing in the order Co{sub 3}O{sub 4} > Co/SiO{sub 2}-62 > Co/SiO{sub 2}-923. The ease of reduction appeared to be correlated to the ease of removing the H{sub 2}O produced during the CoO {yields} Co step. The small pores (ca. 30 {angstrom}) of the Co/SiO{sub 2}-923 sample made H{sub 2}O removal and reduction for that sample the most difficult. Upon reduction of the Co{sub 3}O{sub 4} supported particles to metallic Co the particle size decreased by 30-50% due to removal of oxygen from the Co{sub 3}O{sub 4} particles. For the reduced Co/SiO{sub 2}-62 sample, the Co particles were 90 {angstrom} in size and grouped togethermore » in 0.1-1-{mu}m aggregates. The crystallographic orientation of the individual particles in each aggregate became radomized upon reduction and reoxidation. For the reduced Co/SiO{sub 2}-923 sample the Co particles were 35 {angstrom} in size.« less