Influence of K-doping on a Pd/SiO2–Al2O3 catalyst

This work is devoted to understand the effect of K-doping (from K2CO3) on the reducibility, sintering, and surface properties of Pd metal nanoparticles deposited on a peculiar high surface area SiO2–Al2O3 (SA) support. The effect of K-doping on the Pd/SA catalyst is systematically investigated by means of several complementary techniques (TPR, TEM, CO chemisorption and FTIR spectroscopy), trying to separate the effects of the doping loading (spanning the large 0.25–20.3 K/Pd atomic ratio interval), of the activation temperature (up to 823 K) and of the activation atmosphere (air or H2). It is shown that the presence of dopant, already at the lowest loading, causes an important increase of the PdO reduction temperature. The sulphur poisoning released from the support in the undoped sample during the thermal treatments in H2 atmosphere is inhibited in the presence of K2CO3. The dramatic loss of metal area measured by CO chemisorption for samples H2-treated at high temperature is not justified by the small particle sintering observed by TEM, and is assigned to the progressive particle encapsulation due to the mobilization of the support by reaction between carbonate and support itself. FTIR spectroscopy of adsorbed CO, allowing to probe the nature of the Pd surface available for adsorption, confirms the hypothesis.