The Study On Ceria-containing Oxides Catalysts Towards Soot Oxidation With The Presence Of SO₂

The catalytic combustion technology is a solution for soot abatement and it also presents challenges. Firstly, SO₂ emission from fuel is still a problem for diesel engine; Secondly, the exhaust temperature is always below 300-400℃; Thirdly, the solid-solid-gas reaction type of soot combustion via catalyst, requires a good contact between soot and catalyst. Therefore, the sulfur tolerance, activity at lower temperatures, and oxygen mobility of catalysts play important roles in soot oxidation. As promosing catalysts system, MOx-CeO₂ (M-Ce-O, M=Fe, Cu, Zn, Co, Ni and Mn), with the emphasis of CuO-CeO₂, were selected to study the mechanism of soot oxidation with the presence of SO₂ in TPO atmosphere. The characterization techniques including XRD, BET, FTIR, H2-TPR, O₂-TPD, NH3-IR and XPS, were applied to explore the activity-affecting factors. For example, the sulfur species deposited on catalyst surface, especially its effect on surface properties including oxygen species, redox couple, thus affecting activity, were investigated. The results and conclusions are as follows:(I)The study on the activity factors for MOx-CeO₂ emphasically for CuO-CeO₂ towards soot oxidation. It was found that the addition of TMO(transient metal oxides) into CeO₂ could cause the formation of solid solution due to the interaction of TMO and ceria, such as Cuz²⁺Ce1-z4+O₂-z2-â–¡z in the case of copper incorporated into ceria; A slight increase of specific surface area(SSA) and decrease of the average crystalline size were observed; The presence of solid solution could lead to the generation of more oxygen vacancies(oxygen storage capacity, OSC), active species such as oxygen species and Ce4+-O₂--Cu+ redox couple, as well as solid surface acid. The improvement of catalytic activity could benefit from all of those parameters, generally called as activity factors. The activity sequence of MOx-CeO₂ prepared with coprecipitation method(CP) was Ni2O3-CeO₂>MnO₂-CeO₂>CuO-CeO₂>Fe2O3-CeO₂>CeO₂> CoO-CeO₂>ZnO-CeO₂.The addition of Ni, Mn, Cu and Fe could cause the formation of solid solution of M-Ce-O, thus more oxygen vacancies and oxygen species(according to the total hydrogen consumption, THC) are generated. Another example is the superiority at activity for CuO-CeO₂(CP) compared with CuO-only or CeO₂–only catalyst, due to the effect of those above activity factors. Additional examples are CuO-CeO₂(CA) with the activity order of Cu0.05Ce0.95>Cu0.2Ce0.8>Cu0.6Ce0.4≈Cu0.8Ce0.2>Cu0.4Ce0.6>CuO, and the superiroty of the activity of Cu0.05Ce0.95-CA compared with that of Cu0.05Ce0.95-CP. These differences were also resulted from those above activity factors.(II)The study on the correlation of SO₂-activity factors-catalytic activity for soot oxidation. It was found that the concentration of SO₂ in TPO atmosphere increasing to some level could resulted in the deposition of sulfur species on catalyst surface, SO₂, SO42- and SO32-, included; accompanied by the variation of the activity factors, such as Ce⁴⁺-O₂^--M^x+ especially Ce⁴⁺-O₂^--Cu⁺ redox couple, and solid acidity; thus affecting the activity. For example, 0.03%SO₂ could deactivate coprecipitation synthesized Fe-Ce-O, Ni-Ce-O and Mn-Ce-O, however it could enhance the activity of Cu-Ce-O and Co-Ce-O, and no significant effect of SO₂ on Zn-Ce-O or CeO₂ was observed. As another example, FTIR, BET, NH3-IR and XPS reveal that the low amount of SO₄^2-/SO₃^2- on the surface of Cu50Ce50(CP), with the increase of Br?nsted acidity, result in the accelerated catalytic activity; >0.03%SO₂ could lead to the growth of the amount of SO₄^2-/SO₃^2-, causing the contamination of the active sites and deactivate the catalyst. An additional example was that the activity of Cu0.05Ce0.95(CA) was promoted with the presence of lower concentration of SO₂(0.02%). This activity enhancement was ascribed to the increase of the amount of surface Ce4+ and superoxide(O₂-), or due to the enrichment of Ce-O-Cu(Ce4+-O₂--Cu+) from these two kinds of species. In the case of higher concentration of SO₂(0.03%), lattice oxygen(O₂-) and O₂- transformed to O-, the latter was probably presence in sulfates. Moreover, in the range of 0%-0.04%SO₂, the turning points of activity for Cu0.05Ce0.95-CP and Cu0.05Ce0.95-CA were 0.03% and 0.02%SO₂, respectively. The signals for both two Cu0.05Ce0.95 catalysts at ¹385 cm-1 in FTIR, were assigned as Ce4+-O₂--Cu+ redox couples on the surface of solid, responsible for the active sites of these two catalysts, corresponding to the highest activities. In the case of higher concentration of SO₂(0.04%), sulfur species including sulfates were found on the surface of Cu0.05Ce0.95-CP and Cu0.05Ce0.95-CA, followed with the inhibition of catalytic activity.As mentioned above, (i)the formation of solid solution in M'Ox-CeO₂(M'=Ni, Mn,Cu, Fe), such as Cuy²⁺Ce1-y4+O₂-y2-â–¡y in the case of CeO₂ corporated with CuO, caused their higher catalytic activities towards soot oxidation when compared to CeO₂-only. The formation of solid solution could favor the generation of more activity factors including oxygen vacancies, acitive oxygen species, and redox couple. (ii) The above activity factors varied due to the introduction of SO₂, thus affecting the activity. On one hand, the activity could be promoted resulting from the surface solid acid due to the presence of SO₂. The more important is that more sulfur species including sulfates are deposited with the increase of SO₂, followed with the decrease of the amount of surface species such as active oxygen species and redox couple,thus inhibited the oxygen mobility of catalyst and deactivated the catalytic activity. Generally, the effect of SO₂ on the activity of catalysts is determined by the combined action of the so-called active factors and the deposited sulfur species observed in this paper.(iii) In the case of the study of Cu_0.05Ce_0.95-CP/CA, superoxide O₂^- was found to play an important role in soot oxidation, moreover, for the description and interpretation of the redox properties of these two catalysts, the ratio of Ce4+/Ce3+ could be more reliable than active oxygen species O*( O₂^-,O^- and O_<sup>2-).