MODIFICATION OF THE PROPERTIES AND SULFUR RESISTANCE OF A PD SIO2 CATALYST BY LA ADDITION/

The effect of LaCl3 on the properties and deactivation of Pd/SiO2 cata lysts by thiophene poisoning in the hydrogenation of ethylbenzene has been studied. Gas chemisorption and TPR measurements, respectively, sh ow that Pd dispersion increases with the ratio La/Pd and that La hinde rs palladium reduction. The presence of Pd-n+ species on Pd/SiO2 was e videnced by XPS, but the incorporation of lanthanum decreases the conc entration of Pd-n+. Deactivation results show that La improves the Pd sulfur resistance, which goes through a maximum for the catalyst with La/Pd = 2 atomic ratio. This effect cannot be attributed to the increa se in Pd dispersion produced by La, a decorating effect of Pd by La sp ecies, or sulfur adsorption by La. From the mechanism for metal poison ing by thiophene at 493 K, which involves the hydrogenolysis of the su lfur compound on the metal sites, an alternative explanation of the re sults is suggested. It seems that LaCl3 essentially plays a diluent ro le, disrupting the ensembles of palladium atoms required for the break ing of the C-S bond. XPS and TPR results, which indicate the occurrenc e of a weak Pd-La interaction, support this hypothesis. Thus, the cata lysts containing lanthanum are more sulfur resistant, with a maximum f or La/Pd = 2. For this sample, XPS suggests that a slight charge trans fer from Pd to La takes place and therefore the electronic effect of P d-n+ is added to the geometric effect produced by La3+. (C) 1995 Acade mic Press, Inc.