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

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 (Tr), 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 Tr. 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.