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

Increasingly stringent regulations to reduce emissions of nitrogen oxides (NOx) from exhausts of heavy-duty diesel engines has set the stage to delve into a detailed investigation of engine after-treatment catalysts in order to understand the chemistry during their operation and design the next generation of catalytic formulations to meet future requirements. Small-pore Cu- and Fe-exchanged SSZ-13 catalysts with chabazite (CHA) topology are able to sustain high catalytic rates for selective catalytic reduction (SCR) even after exposure to harsh hydrothermal conditions present in diesel exhaust. Probing the redox behavior and the active site requirements for standard SCR on Cu-SSZ-13 catalysts using a combination of infrared (FTIR) and x-ray absorption (XAS) spectroscopies, kinetic measurements and density functional theory (DFT) calculations forms the basis for this dissertation

Mechanistic Study of NO Oxidation on Copper Zeolites

The objective of this work was to identify the nature of the active intermediate for the NO oxidation reaction on copper (Cu) exchanged ZSM-5 zeolite. A combination of Diffuse Reflectance (DRIFTS) and Operando Transmission Fourier Transform Infrared Spectroscopy (FTIR) was used for this purpose. In situ DRIFTS studies revealed that three types of nitrates, 1626 cm-1, 1600 cm-1 and 1565 cm -1, were formed only on the surfaces of those catalysts that were active for NO oxidation. Inactive samples did not show any nitrate formation. The total amount of stored nitrates and the NO oxidation reaction rate, both followed similar trends with varying amounts of copper loading and water content in the feed stream. Hence, nitrates were important intermediates for the NO oxidation reaction. In addition, diamond dust was shown to be a good scatterer for future experiments with dark coloured samples that absorb infrared radiation. A custom-designed operando FTIR reactor with very small internal volume was used for transmission infrared spectroscopy experiments. NO oxidation was used as a simple probe reaction to validate the fast switching FTIR - MS system. The apparent activation energies measured in this novel FTIR reactor and in a separate plug flow reactor (PFR) were very similar. Space velocity was varied by changing the total flow rate and the amount of sample loaded into the reactor, to prove that there were no external mass transfer limitations. These results confirmed that the chemistry for the NO oxidation reaction was consistent in both reactors, thus allowing correlation of results from the two. This setup was used for steady state kinetic measurements along with transient isotope switching experiments. The latter allowed us to follow the isotopic label simultaneously on the catalyst surface using FTIR, and in the effluent phase using a mass spectrometer (MS), furnishing important information about the reaction mechanism. 14N 16O→15N18O switching experiments on multiple Cu/ZSM-5 samples showed that the peak area of the bridged/bidentate nitrate observed at 1626 cm-1 tracked with the 15N containing NO2 product. Thus, it was shown that nitrates are relevant intermediates, for the NO oxidation reaction and that the bridged/bidentate nitrate at 1626 cm -1 is a likely intermediate

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NO_<i>x</i> with NH₃ are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein’s rule. Further, exchanged Cu^II ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu^IIOH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH₃ under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13