<p dir="ltr">Insights from research into the natural photosynthetic processes are applied to inform the rational design of inorganic catalysts. The study of these synthetic systems – artificial photosynthesis – will lead towards the development of a device able to absorb light, convert and store the energy in the form of chemical bonds. The water-splitting reaction, a bottleneck of the photosynthetic process, is a key barrier to overcome in this endeavor. Thus, the focused study of water-oxidation catalysts able to facilitate this difficult reaction is performed, in order to develop a green-energy solution in the form of an artificial photosynthesis system.</p>

<p>Although Cu-SSZ-13 zeolites are used commercially in diesel engine exhaust after-treatment for abatement of toxic NO_x pollutants via selective catalytic reduction (SCR) with NH₃, molecular details of its active centers and mechanistic details of the redox reactions they catalyze, specifically of the Cu(I) to Cu(II) oxidation half-reaction, are not well understood. A detailed understanding of the SCR reaction mechanism and nature of the Cu active site would provide insight into their catalytic performance and guidance on synthesizing materials with improved low temperature (< 473 K) reactivity and stability against deactivation (e.g. hydrothermal, sulfur oxides). We use computational, titration, spectroscopic, and kinetic techniques to elucidate (1) the presence of two types of Cu²⁺ ions in Cu-SSZ-13 materials, (2) molecular details on how these Cu cations, facilitated by NH₃ solvation, undergo a reduction-oxidation catalytic cycle, and (3) that sulfur oxides poison the two different types of Cu²⁺ ions to different extents at via different mechanisms. </p><p><br></p> <p> </p> <p>Copper was exchanged onto H-SSZ-13 samples with different Si:Al ratios (4.5, 15, and 25) via liquid-phase ion exchange using Cu(NO₃)₂ as the precursor. The speciation of copper started from the most stable Cu²⁺ coordinated to two anionic sites on the zeolite framework to [CuOH]⁺ coordinated to only one anionic site on the zeolite framework with increasing Cu:Al ratios. The number of Cu²⁺ and [CuOH]⁺ sites was quantified by selective NH₃ titration of the number of residual Brønsted acid sites after Cu exchange, and by quantification of Brønsted acidic Si(OH)Al and CuOH stretching vibrations from IR spectra. Cu-SSZ-13 with similar Cu densities and anionic framework site densities exhibit similar standard SCR rates, apparent activation energies, and orders regardless of the fraction of Z₂Cu and ZCuOH sites, indicating that both sites are equally active within measurable error for SCR. </p><p><br></p> <p> </p> <p>The standard SCR reaction uses O₂ as the oxidant (4NH₃ + 4NO + O₂ -> 6H₂O + 4N₂) and involves a Cu(I)/Cu(II) redox cycle, with Cu(II) reduction mediated by NO and NH₃, and Cu(I) oxidation mediated by NO and O₂. In contrast, the fast SCR reaction (4NH₃ + 2NO + 2NO₂ -> 6H₂O + 4N₂) uses NO₂ as the oxidant. Low temperature (437 K) standard SCR reaction kinetics over Cu-SSZ-13 zeolites depend on the spatial density and distribution of Cu ions, varied by changing the Cu:Al and Si:Al ratio. Facilitated by NH₃ solvation, mobile Cu(I) complexes can dimerize with other Cu(I) complexes within diffusion distances to activate O₂, as demonstrated through X-ray absorption spectroscopy and density functional theory calculations. Monte Carlo simulations are used to define average Cu-Cu distances. In contrast with O₂-assisted oxidation reactions, NO₂ oxidizes single Cu(I) complexes with similar kinetics among samples of varying Cu spatial density. These findings demonstrate that low temperature standard SCR is dependent on Cu spatial density and requires NH₃ solvation to mobilize Cu(I) sites to activate O₂, while in contrast fast SCR uses NO₂ to oxidize single Cu(I) sites. </p><p><br></p> <p> </p> <p>We also studied the effect of sulfur oxides, a common poison in diesel exhaust, on Cu-SSZ-13 zeolites. Model Cu-SSZ-13 samples exposed to dry SO₂ and O₂ streams at 473 and 673 K. These Cu-SSZ-13 zeolites were synthesized and characterized to contain distinct Cu active site types, predominantly either divalent Cu²⁺ ions exchanged at proximal framework Al sites (Z₂Cu), or monovalent CuOH+ complexes exchanged at isolated framework Al sites (ZCuOH). On the model Z₂Cu sample, SCR turnover rates (473 K, per Cu) catalyst decreased linearly with increasing S content to undetectable values at equimolar S:Cu molar ratios, while apparent activation energies remained constant at ~65 kJ mol^-1, consistent with poisoning of each Z₂Cu site with one SO₂-derived intermediate. On the model ZCuOH sample, SCR turnover rates also decreased linearly with increasing S content, yet apparent activation energies decreased monotonically from ~50 to ~10 kJ mol^-1, suggesting that multiple phenomena are responsible for the observed poisoning behavior and consistent with findings that SO₂ exposure led to additional storage of SO₂-derived intermediates on non-Cu surface sites. Changes to Cu²⁺ charge transfer features in UV-Visible spectra were more pronounced for SO₂-poisoned ZCuOH than Z₂Cu sites, while X-ray diffraction and micropore volume measurements show evidence of partial occlusion of microporous voids by SO₂-derived deposits, suggesting that deactivation may not only reflect Cu site poisoning. Density functional theory calculations are used to identify the structures and binding energies of different SO₂-derived intermediates at Z₂Cu and ZCuOH sites. It is found that bisulfates are particularly low in energy, and residual Brønsted protons are liberated as these bisulfates are formed. These findings indicate that Z₂Cu sites are more resistant to SO₂ poisoning than ZCuOH sites, and are easier to regenerate once poisoned. </p>

<p>Selective catalytic reduction (SCR) of NO_xusing NH₃as a reductant (4NH₃+ 4NO + O₂ 6H₂O + 4N₂) over Cu-SSZ-13 zeolites is a commercial technology used to meet emissions targets in lean-burn and diesel engine exhaust. Optimization of catalyst design parameters to improve catalyst reactivity and stability against deactivation (hydrothermal and sulfur poisoning) necessitates detailed molecular level understanding of structurally different active Cu sites and the reaction mechanism. With the help of synthetic, titrimetric, spectroscopic, kinetic and computational techniques, we established new molecular level details regarding 1) active Cu site speciation in monomeric and dimeric complexes in Cu-SSZ-13, 2) elementary steps in the catalytic reaction mechanism, 3) and deactivation mechanisms upon hydrothermal treatment and sulfur poisoning.</p><p>We have demonstrated that Cu in Cu-SSZ-13 speciates as two distinct isolated sites, nominally divalent Cu^IIand monovalent [Cu^II(OH)]⁺complexes exchanged at paired Al and isolated Al sites, respectively. This Cu site model accurately described a wide range of zeolite chemical composition, as evidenced by spectroscopic (Infrared and X-ray absorption) and titrimetric characterization of Cu sites under <i>ex situ </i>conditions and <i>in situ </i>and <i>operando </i>SCR reaction conditions. Monovalent [Cu^II(OH)]⁺complexes have been further found to condense to form multinuclear Cu-oxo complexes upon high temperature oxidative treatment, which have been characterized using UV-visible spectroscopy, CO-temperature programmed reduction and dry NO oxidation as a probe reaction. Structurally different isolated Cu sites have different susceptibilities to H₂and He reductions, but are similarly susceptible to NO+NH₃reduction and have been found to catalyze NO_xSCR reaction at similar turnover rates (per Cu^II; 473 K) via a Cu^II/Cu^Iredox cycle, as their structurally different identities are masked by NH₃solvation during reaction. </p><p><br></p><p>Molecular level insights on the low temperature Cu^II/Cu^Iredox mechanism have been obtained using experiments performed <i>in situ</i>and <i>in operando </i>coupled with<i></i>theory. Evidence has been provided to show that the Cu^II to Cu^Ireduction half-cycle involves single-site Cu reduction of isolated Cu^IIsites with NO+NH₃, which is independent of Cu spatial density. In contrast, the Cu^I to Cu^IIoxidation half-cycle involves dual-site Cu oxidation with O₂to form dimeric Cu-oxo complexes, which is dependent on Cu spatial density. Such dual-site oxidation during the SCR Cu^II/Cu^Iredox cycle requires two Cu^I(NH₃)₂sites, which is enabled by NH₃solvation that confers mobility to isolated Cu^Isites and allows reactions between two Cu^I(NH₃)₂species and O₂. As a result, standard SCR rates depend on Cu proximity in Cu-SSZ-13 zeolites when Cu^Ioxidation steps are kinetically relevant. Additional unresolved pieces of mechanism have been investigated, such as the reactivity of Cu dimers, the types of reaction intermediates involved, and the debated role of Brønsted acid sites in the SCR cycle, to postulate a detailed reaction mechanism. A strategy has been discussed to operate either in oxidation or reduction-limited kinetic regimes, to extract oxidation and reduction rate constants, and better interpret the kinetic differences among Cu-SSZ-13 catalysts.</p><p><br></p><p>The stability of active Cu sites upon sulfur oxide poisoning has been assessed by exposing model Cu-zeolite samples to dry SO₂and O₂streams at 473 and 673 K, and then analyzing the surface intermediates formed via spectroscopic and kinetic assessments. Model Cu-SSZ-13 zeolites were synthesized to contain distinct Cu active site types, predominantly either divalent Cu^IIions exchanged at proximal framework Al (Z₂Cu), or monovalent [Cu^IIOH]⁺complexes exchanged at isolated framework Al (ZCuOH). SCR turnover rates (473 K, per Cu) decreased linearly with increasing S content to undetectable values at equimolar S:Cu ratios, consistent with poisoning of each Cu site with one SO₂-derived intermediate. Cu and S K-edge X-ray absorption spectroscopy and density functional theory calculations were used to identify the structures and binding energies of different SO₂-derived intermediates at Z₂Cu and ZCuOH sites, revealing that bisulfates are particularly low in energy, and residual Brønsted protons are liberated at Z₂Cu sites as bisulfates are formed. Molecular dynamics simulations also show that Cu sites bound to one HSO₄^-are immobile, but become liberated from the framework and more mobile when bound to two HSO₄^-. These findings indicate that Z₂Cu sites are more resistant to SO₂poisoning than ZCuOH sites, and are easier to regenerate once poisoned.</p><p><br></p><p>The stability of active Cu sites on various small-pore Cu-zeolites during hydrothermal deactivation (high temperature steaming conditions) has also been assessed by probing the structural and kinetic changes to active Cu sites. Three small-pore, eight-membered ring (8-MR) zeolites of different cage-based topology (CHA, AEI, RTH) have been investigated. With the help of UV-visible spectroscopy to probe the Cu structure, in conjunction with measuring differential reaction kinetics before and after subsequent treatments, it has been suggested that the RTH framework imposes internal transport restrictions, effectively functioning as a 1-D framework during SCR catalysis. Hydrothermal aging of Cu-RTH results in complete deactivation and undetectable SCR rates, despite no changes in long-range structure or micropore volume after hydrothermal aging treatments and subsequent SCR exposure, highlighting beneficial properties conferred by double six-membered ring (D6R) composite building units. Exposure aging conditions and SCR reactants resulted in deleterious structural changes to Cu sites, likely reflecting the formation of inactive copper-aluminate domains. Therefore, the viability of Cu-zeolites for practical low temperature NO_xSCR catalysis cannot be inferred solely from assessments of framework structural integrity after aging treatments, but also require Cu active site and kinetic characterization after aged zeolites are exposed to low temperature SCR conditions.</p>

<p>This dissertation describes the effects of a <i>de novo</i> mutation of CaMKII found in children with learning disabilities and describes its effect on catalytic activity. We develop a malachite green assay for the measurement of CaMKII activation and use it for high-throughput chemical screening to identify CaMKII inhibitors and enhancers. We also propose a new mechanism of regulation of CaMKII activity by ADP.</p><p><br></p>