RESUMO
The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past â¼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.
RESUMO
A key question for the BaO-based NOx storage/reduction catalyst system is the morphological evolution of the catalyst particles during the uptake and release of NOx. Notably, because the formed product during NOx uptake, Ba(NO3)2, requires a lattice expansion from BaO, one can anticipate that significant structural rearrangements are possible during the storage/reduction processes. Associated with the small crystallite size of high-surface area gamma-Al2O3, it is difficult to extract structural and morphological features of Ba(NO3)2 supported on gamma-Al2O3 by any direct imaging method, including transmission electron microscopy. In this work, by choosing a model system of Ba(NO3)2 particles supported on single-crystal alpha-Al2O3, we have investigated the structural and morphological features of Ba(NO3)2 as well as the formation of BaO from Ba(NO3)2 during the thermal release of NOx, using ex-situ and in-situ TEM imaging, electron diffraction, energy dispersive spectroscopy (EDS), and Wulff shape construction. We find that Ba(NO3)2 supported on alpha-Al2O3 possesses a platelet morphology, with the interface and facets being invariably the eight [111] planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)2 and alpha-Al2O3, indicating that the interfacial energy is lower than the Ba(NO3)2 surface free energy. In fact, Wulff shape constructions indicate that the interfacial energy is approximately 1/4 of the [111] surface free energy of Ba(NO3)2. The orientation relationship between Ba(NO3)2 and the alpha-Al2O3 is alpha-Al2O3[0001]//Ba(NO3)2[111] and alpha-Al2O3(1-210)//Ba(NO3)2(110). Thus, the results clearly demonstrate dramatic morphology changes in these materials during NOx release processes. Such changes are expected to have significant consequences for the operation of the practical NOx storage/reduction catalyst technology.
RESUMO
The hydrogenation, employing hydrogen gas, of cinnamaldehyde was performed using Pd-black and Raney Ni catalysts at 298+/-3 K in a water-cooled (jacketed) reaction vessel. Sampling at pre-determined time intervals and GC/MS analysis yielded time-dependent product state distribution information. A kinetic modeling of the data revealed that cinnamaldehyde was both hydrogenated directly to the final product benzenepropanol, as well as a fraction being converted to the intermediate benzenepropanal, where the latter was subsequently hydrogenated to benzenepropanol. Comparing the ultrasound-assisted and blank (stirred) experiments revealed that a higher maximum relative concentration of the intermediate benzenepropanal was formed in the ultrasound experiments compared to the stirred experiment. The activity of the ultrasound experiments compared to blank were 9-fold and 20-fold greater for the Pd-black and Raney Ni catalysts, respectively. Finally, an application of the Bell-Evans-Polanyi principle to yield an estimate of the ratio of rate coefficients for benzenepropanal and benzenepropanol formation was performed by considering chemical group energy differences and surface adsorption energy differences in the first mechanistic step of hydrogenation.