Effect of sulphuric acid pretreatment concentration on the behaviour of CoOX/γ-Al2O3-SO4 monolithic catalysts in the lean CH4-SCR process

The pretreatment of conformed γ-alumina monolithic supports with sulphuric acid solutions induces an increase in the activity of CoOX/γ-Al2O3 catalysts for the selective catalytic reduction of nitrogen oxides employing methane as the reductant. The improvement in the activity was related to the acid strength of the acid used in the pretreatment step, where both the activities and selectivities were found to be greatest when the monolithic supports were pretreated with a 2.5 M H2SO4 solution. The UV–vis-NIR, XRD spectroscopies, zero point charge measurements and TGA studies demonstrated that the active cobalt phase was stabilised as a tetrahedral Co(II) complex, possibly associated to sulphate ligands. UV–vis-NIR spectroscopy further revealed that pretreatment with 2.5 M H2SO4 led to the lowest ratios of crystalline Co3O4, explaining the enhanced activity and higher selectivity, since this compound is known to promote the undesirable direct methane combustion with oxygen. Furthermore, TGA analysis combined with zeta-potential measurements, lead to the conclusion that the acidity, important for the activation of methane, increased on the catalyst surface with increasing acid concentration up to 2.5 M H2SO4, whereafter formation of Al2(SO4)3 took place, that lowered the catalytic activity due to the formation of non-selective cobalt oxide clusters.