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

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 COCl₂ (at 700°C) or CoBr₂ (at 600°C) onto H/MFI. The catalysts were tested for the reduction of NO_x with CH₄ or iso-C₄H₁₀ in excess O₂. Below 425°C the SUB catalysts show the highest NO_x reduction activity with CH₄ or iso-C₄H₁₀. 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% H₂O to the feed drastically decreases the N₂ yield in NO_x reduction with CH₄, but increases the activity with iso-C₄H₁₀ under some conditions. Permanent damage of the zeolite lattice as a potential cause for the adverse effect of H₂O in the tests with CH₄ is eliminated, as the original activity is fully restored after calcination. A 100 h test with a wet iso-C₄H₁₀ 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 (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 Co⁰ and Co²⁺ are detectable; the one-step reduction of C0²⁺ to Co⁰ clusters could be a cause for the unique propensity of Co/MFI to reduce NO_x with CH₄.