Influence of support acid pretreatment on the behaviour of CoOx/γ-alumina monolithic catalysts in the CH4-SCR reaction

The effect of treatment with different mineral acids (H₂SO₄, H₃PO₄, HNO₃ and HCl) on the activity of monolithic CoO_x/γ-Al₂O₃ catalysts in the reduction of nitric oxide with methane in the presence of oxygen (CH₄-SCR of NO_x) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NO_x was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Depending on the nature of the acid used, significant differences were observed in the catalytic activities which were related to the textural states, surface acidities and the nature of the detected species. The best results were obtained after treatment with H₂SO₄, which increased the activity towards NO_x elimination compared to the other catalysts. 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 NO_x species that lead to NO₂ formation which increase the rate of the undesired methane oxidation reaction at high temperatures.