MECHANISM OF SULFUR POISONING ON SUPPORTED NOBLE-METAL CATALYST - THE ADSORPTION AND TRANSFORMATION OF SULFUR ON PALLADIUM CATALYSTS WITH DIFFERENT SUPPORTS

A series of supported palladium catalysts (Pd/Al2O3, Pd/MgO and Pd/TiO2) were prepared by the impregnating method and treated with H2S, H-2 + O2 or O2, among which H2S is used as a poison and H-2 + O2 or O2 are as purging atmospheres. The S2- species in the supports was introduced by means of mechanically mixing Na2S with the supports or catalysts. X-ray photoelectron spectroscopy (XPS) was employed to determine the changes in the chemical states of oxygen, palladium and sulfur in the catalysts before and after the treatment, while infrared (IR) spectroscopy was used to measure the SO42- group produced in the catalysts and supports. The results show that on MgO and TiO2 carriers whose acidities are weak, there exist two kinds of oxygen species, one is the lattice oxygen, the other one is the active species of oxygen. The latter can oxidize the S2- into SO42- even at room temperature in air. Because of the weak acidities and smaller specific surface area of MgO and TiO2, the S2- is liable to adsorb on the catalysts and to transform into SO42-. But for the case of Al2O3 SUPport its acidity is rather strong, and its surface oxygen species under the experimental conditions is not so active as that in MgO and TiO2 carries. The poison H2S on the Al2O3 support only experiences a process of physical adsorption-desorption. In Pd/Al2O3 catalyst, the negatively charged sulfur ions are not so easily adsorbed and transformed as those in Pd/MgO and Pd/TiO2. It is also implied that the properties of the carriers are related to the ability of self-regeneration of the corresponding catalysts. Pd/Al2O3 catalyst is more able to self-regenerate than Pd/MgO and Pd/TiO2 catalyst