Surface composition of Pd–Fe catalysts supported on silica

Abstract

Pd–Fe/SiO₂ bimetallic catalysts, prepared by sequential wet impregnation, with a constant 2 wt.% Pd loading and with atomic ratios Fe/Pd = 0.5–2.1, and monometallic samples have been studied by quantitative chemisorption and by FTIR spectroscopy of adsorbed CO and NO. The FTIR results indicate that on the examined bimetallic samples CO is selectively adsorbed on Pd^o surface sites and NO is mainly adsorbed on Fe²⁺ surface sites; furthermore, readsorption and coadsorption experiments reveal that Pd and Fe are interdispersed on an atomic level. The quantitative chemisorption data show that the CO/Pd ratio on all the bimetallic samples is lower than on the pure Pd sample (<0.2 CO/Pd). The NO/Fe ratio shows a maximum for the sample with Fe/Pd = 1 (NO/Fe = 0.58), significantly higher than the NO/Fe ratio on both Fe/SiO₂(NO/Fe = 0.05) and Pd/SiO₂(NO/Pd = 0.16) samples. The Pd–Fe/SiO₂ samples show a significant enhancement of catalytic activity in the hydrogenation of the 2,4-dinitrotoluene by addition of iron up to Fe/Pd = 1. The apparent relationship between the catalytic activity of the samples and the iron exposed at the surface, revealed by NO adsorption, is discussed.