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, which was thought to be due to the suppression of hydrogen chemisorption on small metal particles.
The cobalt metal particles in other catalysts prepared by impregnation or special method were mainly located on exterior surface of zeolites. The behavior of these catalysts were similar to the cobalt catalyst supported on silica or alumina and the reduction was very easily achieved even though the metal loading and reduction temperature were low.
Ultimately the use of zeolite as a support resulted in the catalyst with high dispersion and more resistance to sintering and higher selectivity to ethylene and propylene rather than methane which was the main product in the case of catalysts using silica or alumina as a support at high temperature. But the utilizations have been restricted because of the difficulty in reduction of cobalt ions. $O_2-H_2$ treatments were carried out to facilitate the reduction of cobalt ions. The degree of reduction and activity of cobalt ion exchanged catalyst increase by these treatments and showing another possibility to control the size and location of cobalt metal particles without much change in zeolite framework.
코발트 이온교환된 지올라이트 촉매에 있어서 코발트 이온의 환원은 그 위치에 크게 영향을 받으며, 특히 5.4 wt % 이하의 낮은 담지량을 가진 촉매는 전처리에 따른 코발트 이온의 지올라이트 내부로의 이동현상 때문에 그 환원이 더욱 어렵다.
낮은 담지량을 가진 촉매는 일산화탄소의 수소화반응에서 올레핀에 대한 높은 활성도를 보이며 전체적인 전화율은 매우 낮고, 에탄의 수첨반응에서 전혀 활성도를 보이고 있지 않다. 이 촉매의 FMR, 수소화학흡착, TEM의 실험결과는 매우 작은 구형의 코발트 금속입자가 지올라이트 내부에 형성되었다는 것을 나타내주고 있으며, 반응결과와 결부시켜 볼 때 이러한 작은 입자들은 sodalite 내부에 원자 분산 상태로 존재함을 알 수 있다.
산소-수소의 반복처리에 의해서 낮은 담지량의 촉매 환원도를 증가시킬 수 있었으며 이때 형성되는 코발트 입자를 수소처리에 의해서만 형성된 코발트 입자와는 다르게 크기가 크고 주로 supercage 내부에 형성되는 것을 반응과 FMR 결과로부터 유추할 수 있었다.
7 wt % 이상의 높은 담지량을 가진 코발트 이온교환된 지올라이트 촉매는 환원시키기가 쉽고 CO 수소화반응에서 메탄에 대한 높은 선택도와 에탄의 수첨반응에서 높은 전화율을 보이고 있으며, FMR 실험결과 1.3 nm 한계 크기를 가지는 코발트 입자들이 supercage 내부에 형성되는 것을 알 수 있었다. 이러한 결과를 특수 제법에 의해 제조된 촉매나 침전법에 의해서 제조된 촉매의 실험결과와 비교해 볼 때, 지올라이트 내부에 형성되는 이러한 입자들은 외부에 형성되는 코발트 입자 또는 실리카나 알루미나에 담지된 크기와 형태가 불균일한 입자들과는 크게 성질이 다르다는 것을 볼 수 있었다.
담지량, 제조과정 그리고 전처리 조건 등의 변화에 의해서 지올라이트내에서의 코발트 입자의 크기와 위치를 조정할 수 있었고, 결과적으로 이러한 조정능력은 많은 반응에 쓰이고 있는 코발트 촉매의 반응특성 파악에 크게 도움이 된다.
