Selective reduction of NOx with hydrocarbons over Co/MFI prepared by sublimation of CoBr2 and other methods

Abstract Co/MFI catalysts were prepared by various methods, including wet-ion exchange (WIE), either as such or in combination with impregnation (IMP), solid-state ion exchange (SSI), and sublimation (SUB) of CoCl 2 (at 700°C) or CoBr 2 (at 600°C) onto H/MFI. The catalysts were tested for the reduction of NO x with CH 4 or iso-C 4 H 10 in excess O 2 . Below 425°C the SUB catalysts show the highest NO x reduction activity with CH 4 or iso-C 4 H 10 . Above 425°C, the best performance is given by WIE. Below the temperature of maximum N 2 yield, a mixture of Fe/FER and WIE is superior to either catalyst. Addition of 10% H 2 O to the feed drastically decreases the N 2 yield in NO x reduction with CH 4 , but increases the activity with iso-C 4 H 10 under some conditions. Permanent damage of the zeolite lattice as a potential cause for the adverse effect of H 2 O in the tests with CH 4 is eliminated, as the original activity is fully restored after calcination. A 100 h test with a wet iso-C 4 H 10 feed shows excellent stability with a SUB catalyst prepared from CoBr 2 . Characterization by XRD, H 2 -TPR, and FTIR reveals that WIE contains isolated Co 2+ and (Co–OH) + ions that are only reducible at 700°C. SUB catalysts show additional TPR peaks at low temperature, including a feature at 220–250°C, ascribed to multinuclear Co oxo-ions. The formation of an NO y chemisorption complex is most rapid on these catalysts. No oxidation states between Co 0 and Co 2+ are detectable; the one-step reduction of Co 2+ to Co 0 clusters could be a cause for the unique propensity of Co/MFI to reduce NO x with CH 4 .