Bifunctional Catalysts for the Selective Catalytic Reduction of NO by Hydrocarbons

Novel bifunctional catalysts combining two active phases, typically Cu-ZSM-5 and a modifier, were prepared and tested for the selective catalytic reduction of nitrogen oxides using propylene in order to overcome the hindering effects of water typically seen for single-phase catalysts such as Cu-ZSM-5. The catalysts were made by typical preparation techniques, but parameters could be varied to influence the catalyst. The physical characterization of the materials showed that the modification phase was added strictly to the external surface of the zeolite without hindering any internal surface area. Chemical characterization by temperature-programmed reactions, DRIFTS and x-ray absorption spectroscopy indicated strong interaction between the two phases, primarily producing materials that exhibited lower reduction temperatures. Two improvements in NOx reduction activity (1000 ppm NO, 1000 ppm C₃H₆, 2% O₂, 30,000 hr⁻¹ GHSV) were seen for these catalysts compared with Cu-ZSM-5: a lower temperature of maximum NOx conversion activity (as low at 250°C), and an enhancement of activity when water was present in the system. The use of a second phase provides a way to furthure tune the properties of the catalyst in order to achieve mechanistic conditions necessary to maximize NOx remediation.