Characterization of Al₂O₃-Supported Manganese Oxides by Electron Spin Resonance and Diffuse Reflectance Spectroscopy

Abstract

Alumina-supported manganese oxides, used as catalysts for the selective catalytic reduction of NO, were characterized by combined electron spin resonance and diffuse reflectance spectroscopies. Upon impregnation of the acetate precursor solution, the [Mn(H 2 O) 6 ] 2+ complex interacts strongly with surface hydroxyls of the γ-Al 2 O 3 . Evidence was obtained that this anchoring reaction proceeds at a Mn/OH ) 1/2 ratio up to 4.5 wt % Mn loading, leading to a highly dispersed oxidic manganese layer. At higher loadings, the precursor complex is deposited on the surface concurrently. Upon drying at 383 K, part of the manganese is oxidized to higher oxidation states (Mn 3+ and Mn 4+ ), while a further increase in (average) oxidation state takes place upon calcination at 573 K. After calcination, the manganese species are present as a mixture of Mn 2+ , Mn 3+ , and Mn 4+ . At low loadings (<1 wt %), approximately equal amounts of these three oxidation states are present, whereas Mn 3+ becomes the predominant species at higher loadings. ESR reveals that at low loadings, almost all the manganese is present as isolated species, while at 4.5 wt % Mn loading, still more than 70% of the manganese is isolated. The decrease of the fraction of isolated manganese species at higher loadings is accompanied by a decreased selectivity toward N 2 production in the selective catalytic reduction of NO. The fraction Mn 2+ is present in an axially distorted octahedral coordination.

Relationship Between the Structure of Manganese Oxides on Alumina and Catalytic Activities for Benzene Oxidation with Ozone

“…EXAFS and Raman spectroscopy coupled with complementary ab initio calculations suggested that the structure of the manganese in MnOx/c-Al 2 O 3 was likely a four coordinate mononuclear species [19]. ESR studies revealed that all the manganese is present as isolated species at low loading levels, while the fraction of the isolated Mn species decreased with increasing the Mn loadings [20]. These results are in good agreement with our finding obtained from XRD, Raman, and EXAFS studies and clarified the structures of manganese oxides on alumina support.…”

Synthesis, characterization and magnetic investigation of (NH4)0.5Mn1.25(H2O)2[BP2O8]·0.5H2O

“…Since the growth takes place via the formation of these polygonal crystals it is hard to estimate a grain size for this system. EPR is able to detect paramagnetically isolated species and gives information about the coordination of isolated sites, therefore it can be used to detect Mn 2+ and Mn 4+ , whereas Mn 3+ is usually not detected due to complete splitting of energy levels (no ground-state degeneracy) [28,29]. Normally, it is expected that any electron paramagnetic (EPR) spectrum, coming from the transitions between the electronic magnetic states (|Ms> |Ms 1>), to split into 6 hyperfine components separated about 90-110 G from each other.…”

Estudio del mecanismo de reacción de la oxidación de isopropanol sobre óxidos mixtos de Mn-Ti mediante espectroscopía FT-IR y cromatografía de gases

“…Los óxidos mixtos de Mn y Ti se prepararon por un méto-do de impregnación húmeda, partiendo de disoluciones acuosas de la sal precursora, Mn(CH 3 COO) 2 . 2H 2 O, conteniendo las cantidades requeridas de Mn para obtener los óxidos mixtos denotados como XMn-TiO 2 (con X = 0.5, 1.0, 1.5 y 2.0, fracciones de la monocapa teórica) y añadiendo posteriormente el soporte TiO 2 anatasa (Tioxide, 129 m 2 /g).…”