Structural and chemical states of palladium in Pd/Al2O3 catalysts under self-sustained oscillations in reaction of CO oxidation

Pd/Al2O3 supported catalysts, differed in the initial structural and chemical states of palladium, were examined for the possible existence of self-sustained oscillations in the reaction of CO oxidation under temperature-programmed and isothermal modes using plug flow reactor at ambient pressure and CO and O-2 concentration of 0.2 and 1.0 vol%, respectively. The supported samples after the action of reaction mixture were characterized using the HRTEM and XPS methods. The catalysts, calcined at relatively low temperatures 450 degrees C, cannot induce the self-sustained oscillation phenomena in the reaction of CO oxidation. The calcination at 800 degrees C yields highly dispersed particles of Pd-0 and PdO in the catalysts with sizes varying between 2-3 and 10-20 nm, respectively. In this case self-sustained oscillations (SO) of the CO oxidation appeared in the region of middle CO conversions. In the catalyst calcined at 1000 degrees C, PdO is formed as large particles with sizes 20-100 nm. During prolonged self-sustained oscillations under isothermal conditions with varying CO concentrations in the reaction mixture, particles with core-shell structures are formed, where the cores are PdO and the shells are a Pd-0 clusters ordered along a specific direction due to an epitaxy on PdO. Calcination at 1200 degrees C results in the formation of large Pd-0 particles with sizes >= 200 nm. During prolonged self-sustained oscillations with varying CO concentrations under isothermal conditions, the reaction mixture causes destruction of the above particles to yield palladium particles with core-shell structures; in this case the cores are Pd-0, and the shells are PdO layers. (C) 2011 Elsevier B.V. All rights reserved.