selective reduction of NO_x with hydrocarbons over Co/MFI prepared by sublimation of CoBr_2 and other methods

摘要

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 CoCb (at 700°C) or CoBr2 (at 600°C) onto H/MFI. The catalysts were tested for the reduction of NOx with C~ or iSO-C4HIO in excess 02. Below 425°C the SUB catalysts show the highest NOx reduction activity with C~ or iSO-C4HIO. Above 425°C, the best performance is given by WIE. Below the temperature of maximum N2 yield, a mixture of Fe/FER and WIE is superior to either catalyst. Addition of 10% H20 to the feed drastically decreases the N2 yield in NOx reduction with C~, but increases the activity with iSO-C4HIO under some conditions. Permanent damage of the zeolite lattice as a potential cause for the adverse effect of H20 in the tests with C~ is eliminated, as the original activity is fully restored after calcination. A 100 h test with a wet iSO-C4HIO feed shows excellent stability with a SUB catalyst prepared from CoBr2. Characterization by XRD, H2- TPR, and FTIR reveals that WIE contains isolated Co2+ and (C0-0H)+ 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 NOy chemisorption complex is most rapid on these catalysts. No oxidation states between Coo and Co2+ are detectable; the one-step reduction of Co2+ to CoO clusters could be a cause for the unique propensity of Co/MFI to reduce NOx with CH_4.