Resumen de Studies on chabazite zeolites for the selective catalytic reduction of NOx
Selective catalytic reduction (SCR) over Cu-SSZ-13 catalyst is one of the most promising methods to withdraw NOx emissions from diesel engines. The process consists in reducing NOx, at the highly oxidative ambient presented in diesel exhaust gases, to produce harmless N2 and H2O. This occurs in the presence of a reducing gas, usually ammonia, and the catalyst. Even though the use of Cu-SSZ-13 catalysts for SCR is a well-studied process, being identified its active sites ([CuOH]1+ and Cu2+) and having a widely accepted reaction mechanism for both sites; several aspects might be studied to improve SCR performance. In this work, we studied if the continuous exposure of Cu-SSZ-13 catalyst to sulfur species, presented as impurities in diesel fuels, may have a poisoning effect on its active sites, therefore decreasing its activity for SCR reaction. Cu-SSZ-13 samples with isolated [CuOH]1+ and Cu2+ sites were saturated with SO2, then used to perform a kinetic study, in-situ UV-vis characterizations and operando XAS characterizations. It was observed by kinetics experiments that sulfur caused a reduction in NO consumption rate in both Cu sites, evidencing a poisoning effect. Only [CuOH]1+ site presented changes in the activation energy, therefore in this site, a change in reaction mechanism was presented. in-situ UV-Vis-NIR characterizations and operando X-ray absorption characterizations were used to analyze how Cu sites where affected by sulfur at reaction conditions, information that was used to propose a reaction mechanism for each site. In addition, we studied how high temperatures may affect the activity of [CuOH]1+ and Cu2+ sites for SCR, and if the presence of iron in the catalyst may improve the catalyst performance at such conditions. SCR catalysts can be exposed to periods of high temperatures during the regeneration of particulate matter filters, located prior to SCR unit in diesel exhaust gas treatment systems, therefore it is important to know if Cu-SSZ-13 sites can withstand such temperature increases. SCR, NH3 oxidation and NO oxidation reactions were tested over Cu-SSZ-13 and Fe-Cu-SSZ-13 catalysts at 200 to 600°C. It was observed that when the temperature was higher than 350°C during SCR experiments, NO consumption rates decreased in Cu-SSZ-13 catalysts with higher intensity in a sample with mostly [CuOH]1+ sites. In opposite, Fe-Cu-SSZ-13 samples maintained higher NO consumption rates. However NH3 oxidation and NO oxidation reactions demonstrated that higher NO rates in Fe-Cu-SSZ-13 samples were not caused by NO reduction to N2 but the result of its oxidation to other nitrogen oxides.
Finally, it was studied the synthesis of SAPO-34, a silicoaluminophosphate with the same chabazite structure of SSZ-13 but potentially with a lower production cost. The main challenge during the synthesis of SAPO-34 consists in obtaining the highest availability of Brönsted acid sites to be exchanged by the active species (usually Cu). This can be affected, among several factors, by the selection of the structure directing agent and by changing the composition of Si, Al, or P during the synthesis. Several silicon composition in the initial synthesis gel along with the use of several structure-directing agents were evaluated. The effect of these changes on the Brönsted acidity of the final SAPO-34 materials was tested by NH3-TPD characterizations, and textural properties were studied by SEM and XRD characterizations. Comparing the materials synthesized in this work, silicon : morpholine molar ratio of 0.6 mol allowed to obtain SAPO-34 without other structures and higher Br¨onsted acidity.
