Exploring the Mechanistic Landscape of Nitric Oxide Oxidation and Ammonia Selective Catalytic Reduction of Nitric Oxide on Cu-Zeolites Via Kinetic and Spectroscopic Characterization

Atish Anil Parekh, Purdue University

Abstract

The effect of each standard SCR reactant (NO, NH3 and O 2) on the Cu(I)-Cu(II) redox chemistry and the reaction rates was studied via steady state operando XAS experiments. Systematically changing the feed concentration of one of the NO, NH3 and O 2 reactants at a time showed that while NH3 and O2 participated in the reduction of Cu(II) to Cu(I) and re-oxidation of Cu(I) to Cu(II), respectively, NO was involved in both parts of the redox cycle. Together, NO and NH3 acted as the co-reductants (473 K) for Cu(II) to Cu(I) reduction via the NO assisted dissociation of a N-H bond in a Cu-bound NH3 molecule, while the combination of NO and O2 (473 K) completed the catalytic cycle re-oxidizing Cu(I) to Cu(II). We studied the re-oxidation of Cu(I) to Cu(II) in the standard SCR mechanism by O2 and NO2 titration experiments. Two Cu-SSZ-13 catalysts with the same Cu:Al ratio (0.08-0.09) and structurally equivalent exchanged Cu2+ ions charge-compensated by a pair of framework Al atoms but different Si:Al ratio (4.5, 15) were reduced with NO and NH3 (473 K) to Cu(I). Following this reduction, both catalysts were oxidized either in 10% O2 or 90 ppm NO2 under isothermal conditions. Oxidation with O2 followed second order kinetics in the instantaneous Cu(I) fraction for both catalysts, suggesting the involvement of two Cu(I) moieties for O2 oxidation. Further, a smaller second order rate constant (1.79 min-1) and a greater final Cu(I) fraction (0.26) for the low Al (Si:Al = 15) catalyst compared to the corresponding values (8.16 min -1 and 0.15) for the high Al (Si:Al = 4.5) catalyst implied an underlying dependence of the Cu(I) oxidation with O2 on the Al distribution and hence, the proximity of Cu ions. In contrast, oxidation with NO2 was a first order process with identical rate constants of 0.8 min -1 for both catalysts, demonstrating that NO2 oxidation was independent of the Al distribution or Cu proximity, and occurred on isolated Cu(I) ions. Thus, standard SCR, which involves oxidation with O2, is limited by the pairing ability of Cu ions at dilute Cu or Al contents and hence, controlled by the oxidation half-cycle. Fast SCR, on the other hand, proceeds via oxidation with NO2, engaging all the Cu ions in the catalyst independent of its location or concentration within the zeolite. Additionally, a second type of isolated Cu species, [CuOH]+ ions charge-compensated at isolated Al sites, are exchanged in catalysts with dilute Al contents (i.e. high Si:Al) following the saturation of paired Al sites with Cu2+ ions, which are thermodynamically preferred over isolated Al sites during Cu ion exchange. NH3 titration differentiated between the two sites and showed that two protons were replaced per exchanged Cu2+, whereas one proton was replaced per exchanged [CuOH] +a ion. Further, reduction of each Cu2+ generated an additional proton, whereas [CuOH]+ ions did not generate extra protons. FTIR spectra on a series of samples with Si:Al = 15 detected the O-H vibration associated with [CuOH]+ ions at 3651 cm -1, and showed a quantitative increase in its peak area with Cu loading beyond the saturation limit of Cu2+ (Cu:Al = 0.1, Si:Al = 15). In situ oxidizing (20% O2, 673 K) or reducing (He, 673 K or 3.5% H2, 523 K) treatments for two representative samples consisting of exclusively Cu2+ or predominantly [CuOH] + (i.e. 80% of the total exchanged Cu) ions exposed differences in their chemical behavior and showed that [CuOH]+ ions are more reducible compared to Cu2+. The two Cu species, however, are indistinguishable for low temperature (473 K) standard SCR catalysis based on the measured apparent kinetics (Eapp, apparent orders for NO, NH3 , O2 and turnover rates per Cu), steady state Cu(I)-Cu(II) fractions from operando XAS spectra and DFT energetics for the standard SCR pathway on both sites when operating in a kinetic regime that is not limited by the re-oxidation of Cu(I) to Cu(II). DFT calculations rationalized these observations by showing that solvation of Cu by NH3 under reaction conditions nullified the differences between the two types of Cu species. (Abstract shortened by ProQuest.)

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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