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.

We report the first study of the hydrolysis of tetraethyl orthosilicate (TEOS) in an aqueous solution of N,N,N-trimethyl-1-adamantammonium (TMAda) hydroxide, the clear sol precursor for the preparation of the high-silica zeolite SSZ-13 (CHA). The initial stages of the hydrolysis of TEOS were monitored by quantitative 29 Si and 27 Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). 29 Si NMR allowed quantitative characterization of Si in nanoparticles and dissolved oligomers and measuring the average Si−O−Si connectivity. The average Si connectivity increases when hydrolysis advances, and at a [Si]/[TMAdaOH] ratio of one, nanoparticles are detected. The average connectivity of nanoparticles reached 3.1. This is similar to what has been observed during TEOS hydrolysis with other organic bases, i.e., tetrapropylammonim hydroxide (TPAOH) and tetrabutylammonium hydroxide (TBAOH) used for silicalite-1 and silicalite-2 syntheses, confirming that it is a general phenomenon independent of the structure of the organocation. 27 Al NMR shows that the connectivity of Al increases as well with increasing [Si]/[TMAdaOH] ratio. Aluminum atoms are in tetrahedral coordination to four silicate units SiO 4 4− and are located exclusively in the nanoparticles.

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