Characteristics of the support and active phases of Monolithic Co/Al2O3-SO4 catalysts for CH4-SCR
Advancing the chemical engineering fundamentals
Catalysis (T2-13P)
Keywords: SCR, Co/Al2O3, monolithic catalyst, EPR, UV-VIS
Characteristics of the support and active phases of Monolithic Co/Al2O3-SO4 catalysts for CH4-SCR
S.B. Rasmussen1+2, J.C. Martín1, M. Yates1, P. Avila1,
1) Instituto de Catalísis y Petroleochimica, CSIC, C/Marie Curie no 2, Cantoblanco, 28049 Madrid, Spain,
2) Center for Sustainable and Green Chemistry, Dep. Chemistry, Technical University of Denmark, Denmark,
The structural and morphological properties of monolithic CoOx/Al2O3–SO4 and CoOx/Al2O3 catalysts and their activities towards reduction of nitric oxide with methane in the presence of oxygen (lean CH4-SCR of NOx) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NOx was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Significant differences were observed for sulphated and unsulphated samples in the catalytic activities which were related to the textural states, surface acidities and the structural properties of the active species. Treatment with H2SO4, increases the activity towards NOx elimination. This behaviour was attributed not only to an increase in surface acidity but also to the stabilisation of the active Co2+ species, thus avoiding the formation of Co3O4 spinel that is responsible for the strongly adsorbed NOx species. These lead to NO2 formation which increase the rate of the undesired methane oxidation reaction at high temperatures. The coordination chemistry of the active Co2+ species was studied by EPR and UV-VIS-NIR spectroscopy. We found that for the sulphated carrier material, cobalt is stabilised as a tetrahedral Co(II) complex bonded to sulphate ligands. The analogue unsulphated carrier did not exhibit the same stabilising properties, thus the cobalt was found as the mixed valence spinel structure Co3O4 - possibly supplemented by isolated octahedral Co2+ complexes. The EPR spectra of Co2+ on Al2O3-SO4 shows a Kramers’ doublet (S = 1/2) visible at in-situ conditions (T = 760K) split due to spin-orbit coupling from a S = 3/2 ground state. The temperature dependence of the EPR line widths confirms this and thus, that the corresponding zero field splitting parameter D must be negative. The mean g value is equal to 2.3, and the correlation between the electronic transitions and g values can be sufficiently related directly to the free ion spin orbit coupling constant -600 cm-1 of Co2+ - suggesting a coordination sphere consisting of almost pure ionic bonds, as would be expected for sulphate rather than alumina oxide ligands.
Presented Wednesday 19, 13:30 to 15:00, in session Catalysis (T2-13P).
