NH3-Selective Catalytic Reduction (NH3-SCR) is a widely used technology for NO x reduction in the emission control systems of heavy duty diesel vehicles. Copper-based ion exchanged zeolites and in particular Cu-SSZ-13 (CHA framework) catalysts show both exceptional activity and hydrothermal stability for this reaction. In this work, we have studied the origin of the SCR activity of Cu-SSZ-13 as evidenced from a combination of synchrotron-based and laboratory techniques. Synchrotron-based in situ XAFS/XRD measurements were used to provide complementary information on the local copper environment under realistic NH3-SCR conditions. Crucial then to the catalytic activity of Cu-SSZ-13 is the local environment of the copper species, particularly in the zeolite. Cu-SSZ-13 contains mononuclear Cu2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions. At lower temperatures (with low activity), XAFS and XRD data revealed a conformational change in the local geometry of the copper from a planar form toward a distorted tetrahedron as a result of a preferential interaction with NH3. This process appears necessary for activity, but results in a stymieing of activity at low temperatures. At higher temperatures, the Cu2+ possess a local coordination state akin to that seen after calcination.

Cu chabazite catalysts show remarkable low temperature activity in selective catalytic reduction (SCR) of NO. This high activity is due to the unique character of the zeolite framework that allows only the presence of one type of isolated mononuclear Cu(2+) species. These Cu(2+) species are the active sites for SCR.

A high surface area Co(3)O(4)-SiO(2) nanocomposite catalyst has been prepared by use of activated carbon as template. The Co(3)O(4)-SiO(2) composite, the surface of which is rich in silica and Co(II) species compared with normal Co(3)O(4), exhibited very high activity for CO oxidation even at a temperature as low as -76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H(2)O content ~3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co(3)O(4)-SiO(2) catalyst led to the conclusion that adsorption of H(2)O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions.

We report the structure and stability of hydroxylated (1 h11) and (1 h01) surfaces for a water coverage ranging from 0.25 to 1. For the (1 h11) surface water dissociated at low coverage (θ ) 0.25), whereas both dissociated and molecular species coexisted at higher coverage. For the (1 h01) surface molecular adsorption was only observed at the highest coverages (θ ) 0.75 and θ ) 1). The calculated adsorption energy for (1 h11) ranged from -1.20 to -0.83 eV for θ ) 0.25 and θ ) 1, respectively. For the (1 h01) surface the adsorption energies ranged from -1.50 to -1.21 eV for θ ) 0.25 and θ ) 1, respectively. The 1-, 2-, and 3-fold coordinated hydroxyl groups were present in our models. Their structure, energetics, and vibrational frequencies were calculated by ab initio techniques and agreed with in situ infrared spectroscopic measurements. The simultaneous presence of hydroxyl groups of different coordination was concluded from both theoretical and experimental results.