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Modeling the Kinetics of Sulfur Oxides Adsorption-desorption on a Pt/γ-Al2O3 Catalyst as Diesel Oxidation Catalyst (DOC)

Farzi, Amir Hossein | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54032 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Hamzeh Louyan, Tayyebeh
  7. Abstract:
  8. Sulfur oxides are categorized as one of the most important poisoning and activity-reducer groups acting on the catalytic converters present in the exhaust treatment systems. In the present work, interactions of the SOX species with diesel oxidation catalysts (DOCs) are investigated by implementing a kinetic modeling approach. Experimental data obtained from a catalyst of this type, i.e., Pt/γ-Al2O3, was employed and a multi-step microkinetic model was developed in Matlab 2018b® to proxy the poisoning effect of the adsorption of surface sulfur species and their desorption behavior. The microkinetic model was run to describe the individual SO2 and SO3 adsorption-desorption process using the reactor feed and reaction conditions reported in the experimental data set. For SO2, the model managed to follow experimental results at 157 centigrade degrees and the SO2 inlet concentration of 50 ppm, verifying the accuracy of the model. After the validation, the effect of the changes in the inlet temperature on the behavior of SO2 adsorption-desorption on the catalyst was studied. It was observed that in the case of increase in the temperature and availability of the higher levels of energy, sulfate species formed on the Pt sites and also on the alumina support, have higher contribution compared to other surface species. To evaluate the SO2-DOC interactions over typical operational temperatures of DOCs, we presented the model’s output at 200, 400, and 600oC and observed the meaningful domination of sulfate species at higher temperatures. Due to the higher desorption barrier for these sulfate species, that result can be interpreted as more severe poisoning of the catalyst, including both Pt sites and support. In the next section, to investigate the impact of varying SO2 inlet concentration on the adsorption-desorption behavior, SO2 inlet concentrations of 3 ppm, 20 ppm, and 67 ppm (corresponding to 50 ppm, 300 ppm and 1000 ppm sulfur in the diesel fuel, respectively) were introduced to the model and results were reported for each case. Modeling results for the 20 and 67 ppm inlet concentration indicated similar trends, whereas, in the case of 3 ppm, corresponding to the exhaust gas of Euro-IV diesel fuel, quite different ratios of surface species were obtained. In that case, we observed an infinitesimal occupancy of the DOC’s surface by more stable and poisonous sulfates species. In contrast with all previous observations, the dominating surface species was surface sulfite (-SO3) which can partly originate from SO2 readsorbtion at the beginning of the desorption process. In the second part of this work, a mechanism for SO3 adsorption-desorption over DOCs was proposed based on previous studies in the literature and a kinetic model with a structure similar to the SO2 model was developed to describe adsorption-desorption properties of this species. Comparing the modeling results with the experimental data indicated that the model prediction underestimated the experimental adsorption capacity. However, the SO3 model was capable to qualitatively describe the occupancy of the surface sites and showed a dominance of SO3 coverage on the surface of the catalyst. Finally, different approaches were proposed to increase the accuracy of the developed SO3 adsorption-desorption model
  9. Keywords:
  10. Kinetic Model ; Disel Oxidation Catalyst (DOC) ; Sulfur Oxides ; Gas Refine ; Exhaust Gases ; Pt/γ-Al2O3 Catalyst ; Sulfur Oxides Adsorption-Desorption Mechanism ; Sulfur Dioxide Oxidation Kinetic

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