Pd/γ-Al2O3 monolithic catalysts for NOx reduction with CH4 in excess of O2: Effect of precursor salt

Palladium monolithic catalysts based on γ-Al2O3 were developed and their activities and selectivities for the selective catalytic reduction of nitrogen oxides employing methane as the reducing agent in oxygen excess (CH4-SCR) for use at pilot plant scale, were determined. The alumina monoliths were sulphated to increase the activity of the support and subsequently impregnated with one of three precursor salts: palladium chloride, nitrate or acetate, respectively. The catalysts were characterised by XRF, XRD, TGA–DTA, H2-TPR, N2 ad/desorption and MIP. Significant differences in the catalytic activities, selectivity and stabilities were found. In the case of nitrate and acetate palladium precursors, multilayer sulphates for both and Al2(SO4) phase for the latter were observed that may inhibit the interaction of the reactants with the active phase, leading to catalysts with reduced activity. Palladium chloride was the most efficient precursor for NOx abatement, not only due to the stability of the surface sulphates that hinder the competitive CH4 oxidation reaction but also to the presence of Cl− ions that seem to play an important role in the reaction. This catalyst had the lowest tendency for the competing hydrocarbon oxidation reaction and also demonstrated the highest stability after 40 h of reaction in the chosen operating conditions.