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This work investigates the morphological control of the anisotropic [Zn 2 (NDC) 2 (DABCO)] n MOF (Metal organic framework) and the subsequent adsorption characteristics for CO 2 /CH 4 gas separation. Morphology of the MOF crystals is controlled by the use of modulators. The addition of acetic acid or pyridine successfully produce rod or plate morphologies, respectively, with each morphology possessing a different major surface pore aperture. Single-component equilibrium and kinetic adsorption data for CO 2 and CH 4 were collected. Equilibrium analysis indicates a slight selectivity towards CO 2 whereas kinetic data unexpectedly shows lower diffusion time constants for CO 2 compared to CH 4 . Mass transfer resistances on each species is discussed. Finally, a coating technique termed solution shearing is used to orient different morphologies on substrates as a film. An increase in film orientation is observed for the rod morphology, indicating that this MOF morphology is a promising candidate to create large area, thin-film applications.

Fully ion-exchanged Pd/SSZ-13 model passive NOx adsorbers (PNA) exhibit significant NOx storage capability, achieving NOx-to-Pd ratios of 1, and NOx desorption at higher temperatures, where downstream NOx reduction catalysts are active, under simulated exhaust conditions. However, CO has been found to induce PNA degradation, which limits the potential for practical application. In this study, in an attempt to understand the consequences of limiting CO exposure, we integrated a model Pt/Al2O3 diesel oxidation catalyst (DOC) with a Pd/SSZ-13 PNA and performed NOx adsorption and temperature-programmed desorption (TPD) cycles with the PNA, the DOC, and the DOC+PNA integrated system. Despite the high initial NOx-to-Pd ratio, Pd/SSZ-13 experienced significant degradation over 15 adsorption and desorption cycles when including CO, with the NOx-to-Pd ratio dropping from 0.98 to 0.75. CO oxidation over the DOC+PNA integrated system lights off at significantly lower temperature compared to the PNA, limiting the PNA CO exposure at temperatures above 200 °C. As a result, the durability of the DOC+PNA integrated system is enhanced, and only a 0.02 decrease in NOx-to-Pd ratio was observed over the 15 test cycles. A further benefit with integration of the PNA with the DOC was a lower temperature NOx release, within a more practical temperature window. This is due to the enhanced oxidation activity of the DOC+PNA integrated system and consequently an early onset of NO2 formation, which was found to trigger the low temperature NOx release. Low temperature NOx adsorption and TPD experiments with controlled exposure of CO and NO2 reveal two types of NOx storage mechanisms, one of which is destabilized by the presence of NO2, leading to the evolution of a lower temperature NOx release. Overall, integrating the PNA with a highly active low temperature CO oxidation catalyst was beneficial by lowering the NOx release temperature window and leading to significantly less NOx capacity loss.

Catalysts can undergo structural changes during the reaction, affecting the number and/or the shape of active sites. For example, Rh can undergo interconversion between nanoparticles and single atoms when CO is present in the reaction mixture. Therefore, calculating a turnover frequency in such cases can be challenging as the number of active sites can change depending on the reaction conditions. Here, we use CO oxidation kinetics to track Rh structural changes occurring during the reaction. The apparent activation energy, considering the nanoparticles as the active sites, was constant in different temperature regimes. However, in a stoichiometric excess of O2, there were observed changes in the pre-exponential factor, which we link to changes in the number of active Rh sites. An excess of O2 enhanced CO-induced Rh nanoparticle disintegration into single atoms, affecting catalyst activity. The temperature at which these structural changes occur depend on Rh particle size, with small particle sizes disintegrating at higher temperature, relative to the temperature required to break apart bigger particles. Rh structural changes were also observed during in situ infrared spectroscopic studies. Combining CO oxidation kinetics and spectroscopic studies allowed us to calculate the turnover frequency before and after nanoparticle redispersion into single atoms.

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