Structural and chemical states of palladium in Pd/Al 2O 3 catalysts under self-sustained oscillations in reaction of CO oxidation

Abstract

Pd/Al 2O 3 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 °C, cannot induce the self-sustained oscillation phenomena in the reaction of CO oxidation. The calcination at 800 °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 °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 °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.