Temperature-programmed desorption study of the interactions of H2, CO and CO2 with LaMnO3

Abstract

Surface interactions of H₂, CO and CO₂ with the perovskite-type oxide LaMnO₃ have been studied by temperature-programmed desorption (t.p.d.) and i.r. spectroscopy. A t.p.d. desorption peak of H₂ at 355–360 K, which increases in intensity with increasing reduction temperature of the oxide (T_r), is assigned to molecular adsorption of H₂ on reduced manganese sites (Mnⁿ⁺, n≈ 2). CO adsorption yielded t.p.d. peaks of CO and CO₂. A peak of CO at 473 K (for oxidized LaMnO₃) associated with a carbonate group and peaks at 360–395 K, 540–550 K and 773–800 K (for reduced LaMnO₃) associated with linear and bridged CO species adsorbed on Mnⁿ⁺ ions were observed. A very wide CO₂ desorption peak at 473 K and tail centred at 773 K (oxidized LaMnO₃) are associated with monodentate and bidentate carbonates interacting with Mn³⁺. CO₂ adsorption yielded t.p.d. peaks of CO₂ at 345–385 K and at 540–665 K whose intensity decreased and increased, respectively, with T_r. These are associated with monodentate and bidentate carbonates, respectively, interacting with reduced sites of manganese or La³⁺. Detection of bands at ca. 2900 cm^–1 in the i.r. spectrum obtained after CO + H₂ adsorption, the appearance of new CO desorption features at 570 K and above 860 K, and the detection of a new H₂ desorption peak at 770–785 K in the t.p.d. spectra obtained after CO–H₂ or H₂–CO adsorptions suggest decomposition of an oxygenated species formed by interaction of CO and H₂ adsorbed on the same adsorption centre.