Comparative investigation of hydrogen chemisorption and benzene hydrogenation activity of supported rhodium catalysts

Abstract

Samples of rhodium on Al₂O₃(0.5–7.0 wt.% Rh) and Rh on various supports (Al₂O₃, SiO₂, TiO₂, anatase and rutile, ZrO₂ and MgO) containing 1.0 wt.% Rh have been prepared via the incipient impregnation method using RhCl₃·3H₂O. Calcined samples (at 623 K) were reduced at 673 K in hydrogen. Chemisorption of hydrogen at room temperature and hydrogenation of benzene (373–493 K) over these catalysts have been carried out in order to calculate the metal dispersion and its influence on the hydrogenation activity. Rh shows ‘ultra dispersion’ on Al₂O₃ at very low loadings. The dispersion and metal area decrease with metal loadings up to 3 wt.% of Rh on Al₂O₃, as does the turnover number (TON) for benzene hydrogenation. Increasing the loading to >3 wt.% of Rh on Al₂O₃ has no significant effect on the dispersion or the hydrogenation activity. This is an unusual inverse relationship between Rh metal area and benzene hydrogenation activity among catalysts containing 1 wt.% Rh on different supports. The characteristics of the support influence the extent of metal–support interaction and hence the availability of metal on the surface of the support for hydrogen adsorption and hydrogenation activity. The ratio S_metal/S_B.E.T., a measure of the dispersion per unit surface area of the support, is used to explain the TON variation of the catalysts for benzene hydrogenation.