Characterization of cobalt clusters in ion-exchanged zeolite under various pretreatment conditions

Abstract

Characterization of cobalt metal clusters supported on NaY zeolite was carried out by means of ferromagnetic resonance spectroscopy, $H_2$-consumption and such probe reactions as hydrogenation of carbon monoxide and hydrogenolysis of ethane. The catalysts were prepared by ion exchange method, impregnation method and other special methods. Various pretreaments were carried out in different atmospheres and at different temperatures in order to change the locations and loadings of cobalt metal clusters. In the case of the cobalt ion-exchanged zeolites of cobalt loading less than 5.4 wt%, FMR spectra of the catalysts revealed the existence of very small cobalt metal clusters with spherical type indicated by symmetrical FMR line shapes and low intensities. The catalysts showed very low activities in CO hydrogenation reactions and no activities in $C_2H_6$ hydrogenolysis reactions. These results represented that the very small cobalt metal particles were formed in sodalite cage and were atomically dispersed without destruction of zeolite framework, after reduction in hydrogen atmosphere at 500℃. In the case of cobalt ion exchanged catalyst with 7 wt% loading, the activities in CO hydrogenation and ethane hydrogenolysis reaction were large. FMR spectrum was asymmetric and consisted of two peaks after 2 h of reduction, one of which was symmetric with narrow line width and another was asymmetric and broad. Considering the migration of cobalt ions exchanged to the inner cage by vacuum or $H_2$ treatments, these facts indicated that a portion of cobalt ions would be located in sodalite units or hexagonal prisms and rest of them in the supercage or on the exterior surface of zeolite were reduced easily, which was resulting in the large catalytic activities in both reactions and made the FMR line shape asymmetric. But even in this case the size of metal particles was expected to be small because the chemisorption of $H_2$ and TEM failed to detect the cobalt metal particles,...