Chemical state and reactivity of supported palladium: II. Support effect and catalyst activation by the reaction mixture in ethylene oxidation

Abstract

The oxidation of ethylene over supported palladium was studied in a steady-state flow microreactor, taking into account the influence of various experimental factors such as the nature of the carrier (aluminas, silica, titania), the kind of precursor (chloride, nitrate, ammine), the preparation mode, the metal loading, and the nature of pretreatments. It is first confirmed that, at low conversion, significant selectivities to acetic acid (AcOH) may be obtained. Clear evidence is presented for a dual state behavior of the catalysts. For freshly reduced samples the activity for total oxidation and the AcOH selectivity are low for the majority of samples below 383 °K and seem to depend upon palladium-carrier interactions. However, after working at high conversion with oxygen-rich mixtures, the activity and the selectivity are enhanced and become identical for all catalysts. The origin of this activation phenomenon by the reaction mixture is discussed; it is proposed that working at high conversion leads to the destruction of palladium-carrier interactions and induces a surface reconstruction.