Characterization of supported nickel catalysts by H/D isotopic exchange

Metal catalysts are usually dispersed onto oxide supports in order to achieve a high metal surface area and to become more stable to sintering during the reduction and the catalytic process. The nature of the support can have a significant effect on the catalytic properties of the metal for certain reactions. In this communication, the results concerning the influence of some oxides of lanthanides on the activity of nickel catalysts in the deuterium exchange reaction between hydrogen and water vapour are reported. For this purpose, the nickel catalysts supported on Yb2O3, Gd2O3, Ho2O3, CeO2, Y2O3 and La2O3 were prepared by coprecipitation method. The resulted precipitate was washed with doubly distilled water in order to remove the Na+ and NO3- ions, calcified in flowing nitrogen at 330 deg. C, and reduced in the flowing hydrogen at 350 deg. C. The total surface area of the samples was measured by krypton adsorption (BET method) and the dispersion of the metallic nickel from the hydrogen uptake measured by the temperature programmed desorption. The total specific area value of the catalyst samples, resulted from the physical adsorption of the krypton, increases with increasing of the atomic number of lanthanide of the support oxide. The metal dispersion, calculated from the hydrogen uptake, is practically independent of the nature of the support oxide. The kinetic measurements show that the specific catalytic activity, i.e. the activity per gram of sample, as well as the intrinsic activity, i.e. the activity per metallic area unit, decrease with increasing of the atomic number of the lanthanide from the support oxide. The results presented in this communication can be interpreted in the range of a Langmuir-Hinshelwood mechanism, in which reaction takes place between hydrogen dissociated on the metallic surface and water molecule activated by support surface. Our hypothesis is that at the metal-support interface new catalytic centres more active than those of the nickel black are created. On the other hand, from the literature data, it is known that the alkalinity of the lanthanide oxides increases with increasing atomic number. In conclusion, from the experimental data presented here, results that the hydroxyl groups from the metal-support interface participate at the activating of the water molecule and the new centres created are more active when the supports are stronger acid. (authors)