Metal-support interactions in Pt/Al2O3 and Pd/Al2O3 catalysts for CO oxidation

摘要

Platinum and palladium catalysts supported on γ-Al2O3 were studied by XRD, UV-vis DRS, HRTEM, TPR-H2, XPS together with measurements of their catalytic properties. The properties of the catalysts denoted as Pt(Pd)/Al2O3(X)-Y (X-the calcination temperature of support, °C; Y- the calcination temperature of cat-alyst,°C) were studied as a function of the temperatures used for calcination of the support and/or the catalyst in oxygen or in a reaction mixture of CO + O2. It was found that the deposition of Pt or Pd on γ-Al2O3 did not alter the structure of the support. Two types of the Pt and Pd particles were typically present on the γ-AI2O3 surface: individual particles with dimensions of 1.5-3 nm and agglomerates about 100 nm in size. In the catalysts calcined at relatively low temperatures (Pt/Al2O3(550)-450), platinum was present in the form of metal clusters. However, in the Pd/Al2O3(550)-450 catalyst, the palladium particles were almost completely decorated with a thin layer of an aluminate phase. These structures are not reduced in hydrogen in the temperature range of -15 to 450 °C, and are stable to treatment in a reaction mixture of CO + O2. Pd deposition on the γ-Al2O3-800 support was found to result in stabilization of the active component in two main forms, Pd° and PdO, with varying degrees of interaction due to the decoration effect. Calcination at the low temperature of 550 °C led to the formation of a so-called "core-shell structure", where a palladium metal core is covered with a thin shell of an aluminate phase. Depending on the calcination temperature of the catalyst in the range of 450-1000°C, the morphological form of the active component was converted from the "core-shell" state to a state consisting of two phases, Pd° and PdO, with a gradual decrease of the Pd°/PdO ratio, weakening the interaction with the support and the growth of palladium particles. Under the action of the reaction mixture, the Pd/Al2O3(800)-(450,600,800,1000) catalysts underwent changes in the Pd°/PdO ratio, which regulates the light-off temperature. After catalyst calcination at the highest temperature used in this study, 1200°C, the palladium particles became much larger due to the loss of the palladium interaction with the support. Only the metal phase of palladium was observed in these catalysts, and their catalytic activity decreases substantially.