Abstract
A study has been made of the effect of the activation temperature on the properties of Rh supported on MgO of various purities. Hydrogen chemisorption, ethane hydrogenolysis, and cyclohexane dehydrogenation were used as tests for metal-support interactions. When either a 98 or 99.5% pure MgO was used as support for Rh, we observed, respectively, a 3- or 20-fold decrease in H 2 chemisorption capacity and a 3 or 5 order of magnitude suppression in ethane hydrogenolysis activity after a high temperature reduction (773 K) relative to a low temperautre reduction (523 K). The effect was reversible by oxidation at 673 K followed by reduction at low temperature in H 2 and was dependent on the loading of Rh when the dispersion of the metal was kept about constant, i.e., the lower the loading, the more profound the effect. However, little effect of reduction temperatures was observed on H 2 adsorption or ethane hydrogenolysis when a 99.999% pure MgO was used as support for Rh. The difference may be attributed to the impurities in the support, and in particular, sulfate is suspected to be the precursor of sulfide poisoning which is responsible for the apparent metal-support interaction in the Rh MgO system.
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