Low Temperature NH3 Selective Catalytic Reduction of NOx over Substituted MnCr2O4 Spinel-Oxide Catalysts

The present work describes the development of Mn 1– x M x Cr 2 O 4 (M = Mg, Ca; x = 0–0.1) substituted spinel catalysts prepared via gel combustion synthesis for the low-temperature selective catalytic reduction (SCR) of NO x with NH 3 . The catalysts have been characterized by means of X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), field emission scanning electron microscopy (FESEM) and temperature programmed reduction (TPR) analyses, whereas their catalytic activity has been tested in a temperature programmed reaction (TPRe) apparatus over a temperature range of 100–200 °C ( W / F = 0.072 g·s·cm –3 ) in the presence of oxygen. The catalytic activity of alkali earth-metal substituted Mn–Cr spinels is compared with that of the stoichiometric MnCr 2 O 4 in order to assess the effect of the substitutions. All the spinel catalysts have shown both high NO conversion and high selectivity to N 2 . Mn 0.95 Ca 0.05 Cr 2 O 4 has shown the best performance, reaching an NO conversion of 96% and a selectivity to N 2 of 97% at 125 °C. All the catalysts were then deposited by means of a modified dip-coating procedure over cordierite honeycomb monoliths and tested in a lab-scale test rig. The monolith catalyzed with the Mn 0.95 Ca 0.05 Cr 2 O 4 spinel showed the best performance, reaching an NO conversion of 87.4% at 200 °C, with a good selectivity to N 2 (94.8%). When the catalyzed monoliths were also tested in the presence of 10 vol % of water vapor, an activity decrease was observed. The correlation between the reducibility of the B site, determined from TPR experiments, and the spinels' catalytic activity order is discussed.