Fundamental studies on copper zeolites for catalytic NO x abatement

Stringent regulations in mobile NOx emissions have resulted in the development of Standard Selective Catalytic Reduction (SCR) as the dominant NOx abatement technology in lean burn diesel engines. Standard SCR is a reaction of nitric oxide (NO) with ammonia (NH3), in the presence of oxygen (O 2) to form nitrogen (N2) and water (H2O). Copper containing zeolites show commercially viable SCR performance. Cu-SSZ-13 (CHA framework), a member of this family, is a preferred catalyst for SCR applications because it shows exceptional hydrothermal stability in addition to commercially viable SCR performance. Our work focuses on 1) determination of the active sites, and 2) elucidation of the dominant reaction steps on active sites, for standard SCR (at 473 K) and catalytic oxidation of NO (at 550 K), over Cu-SSZ-13. A series of Cu-SSZ-13 catalysts (Si:Al = 4.5) tested for standard SCR kinetics exhibited a linear increase in the rate of nitrogen production (per gram catalyst) with Cu:Al ratio till Cu:Al = 0.2. Separate catalyst characterization tools like Ultra-Violet-Visible-Near Infra-red (UV-Vis-NIR) spectroscopy under ambient conditions, X-ray Absorption Near Edge structure (XANES) of Cu ions during standard SCR, Density Functional Theory (DFT) calculations, and titration of residual acid sites with amine titrants on this series of catalysts established isolated Cu ions ion exchanged at the framework Al sites in six member rings of SSZ-13 as the dominant Cu ion configuration below Cu:Al = 0.2, which we assign as the dominant active sites for low temperature standard SCR (T = 473 K). Above Cu:Al = 0.2, Cu ion clusters (CuxOy) were formed, which stabilized on framework Al sites at favorable distances, in the eight member cage of SSZ-13. CuxOy was active in catalyzing dry NO oxidation to NO2 via oxygen activation mediated by local Cu-Oy-Cu bonds. The density of local Cu-Oy-Cu bonds was quantified from in-situ XANES under NO oxidation and correlated linearly with the rate of NO oxidation, thereby establishing CuxOy as the active sites for NO oxidation. NO oxidation was also explored on catalysts prepared to contain exclusively isolated Cu(II) ions and the Gibbs free energy of reaction intermediates was shown to be higher than the free energy of similar intermediates formed from local Cu-O y-Cu bonds, consistent with the experimental observation of experimentally undetectable rate of NO oxidation on isolated Cu(II) ions in the six member rings of SSZ-13. As a result, dry NO oxidation is proposed as a probe reaction to detect Cu ion clustering in Cu-SSZ-13 formulations. Operando XANES during standard SCR also implicated isolated Cu ions (in the six member SSZ-13 ring) as the active site; however, the +2 oxidation state of Cu is not preserved. Experimental XANES analysis during appropriate reactant cutoff from steady state standard SCR and DFT calculated Gibbs free energy analysis of adsorbates under reaction conditions point toward a Cu ion redox between Cu(I) and Cu(II) to mediate standard SCR. The isolated Cu(II) ion reduction is achieved by both NO and NH3 to make nitrogen, isolated Cu(I) ions, and experimentally detectable proximal Brønsted acidic sites. These proximal acid sites can stabilize ammonium ions during standard SCR catalysis, while NO and O2 oxidize the Cu(I) back to Cu(II) via nitrite (NO2-) intermediate formation, as predicted by DFT. The close proximity between nitrites (on Cu) and ammonium ions (on proximal Brønsted acid sites) enable the formation of nitrogen and water via an intermediate which resembles ammonium nitrite, thereby completing the catalytic cycle. These findings highlight the bi-functional nature of Cu-SSZ-13 displayed by a close proximity of Bronsted acidic sites and redox metal ion centers which work in concert to catalyze the selective reduction of NO with NH3 in the presence of oxygen, to form nitrogen and water