Surface and catalytic properties of lanthanum-promoted Ni/sepiolite catalysts for styrene hydrogenation

The catalytic behavior for styrene hydrogenation of two series of La-promoted nickel/sepiolite catalysts (xLaNi and NixLa) is reported. Both series were prepared by two-step impregnations, but lanthanum was incorporated first and nickel afterward in NixLa series and the order was reversed in xLaNi samples. The catalysts were characterized by temperature-programmed reduction (TPR), kinetics of reduction by hydrogen, photoelectron spectroscopy (XPS), and infrared spectroscopy of adsorbed CO to obtain information on the nature and distribution of nickel and lanthanum in the oxidic and reduced samples. The kinetics of reduction and XPS data indicate clearly that Ni2+ ions interact strongly with the sepiolite substrate in xLaNi samples and to a lesser extent in NixLa series. The IR spectra of CO adsorbed on the reduced catalysts demonstrate that the promotion of Ni/sepiolite catalyst with lanthana results in a change in the distribution of CO-adsorbed structures. It appears that reconstruction of bridge-bonded species occurs via formation of Ni(100) surfaces. Moreover, CO can be adsorbed in a linear form on Ni atoms which are in contact with oxidic phase. The decrease of the bridged form and relative higher concentration of unreduced nickel species gives an indication that lanthanum is deposited on the nickel particles, leading to a decrease in the probability to have adjacent nickel atoms on the surface. The observed stable maximum in activity for the Ni7.1La catalyst is caused not only by covering by La a larger fraction of the silicate substrate but also by isolating Ni atoms on the surface of Ni crystallites by La2O3 patches deposited on it. (C) 1996 Academic Press, Inc.