CeO2 and Ce-Zr mixed oxides with different Ce:Zr ratios were prepared; characterised by Raman spectroscopy, XRD, TEM, N2 adsorption at -196 degrees C, and H2-TPR; and tested for soot oxidation under ...NOx/O2. Among the different mixed oxides, Ce0.76Zr0.24O2 provided the best results. Ce0.76Zr0.24O2 presented greater activity than pure CeO2 for soot oxidation by NOx/O2 when both catalysts were calcined at 500 degrees C (soot oxidation rates at 500 degrees C are 14.9 and 11.4 mcg soot/s, respectively), and the catalytic activity of CeO2 decayed significantly with calcination temperature (from 500 to 1000 degrees C), whereas Ce0.76Zr0.24O2 presented enhanced thermal stability at temperatures as high as 1000 degrees C. In addition, Ce0.76Zr0.24O2 catalysed the reduction of NOx by soot at around 500 degrees C more efficiently than CeO2, thereby contributing to the decreased NOx emission level. The catalytic activity of CeO2 and Ce0.76Zr0.24O2 for soot oxidation by NOx/O2 depended on the textural properties (BET area; crystallite size), but other properties of the oxides, such as redox behaviour and/or enhanced lattice oxygen mobility, also played a significant role. PUBLICATION ABSTRACT
Ce
1−
x
Y
x
O
2 and Ce
0.85−
x
Zr
0.15Y
x
O
2 mixed oxides have been prepared, characterised and tested for catalysed soot oxidation. Ce
0.84Zr
0.15Y
0.01O
2 is the most active catalyst, and higher ...yttrium loading (
x
=
0.05–0.12) is less effective than
x
=
0.01 because yttrium is mainly accumulated at the surface of the particles and hinders the catalytic activity of cerium.
▪
Ce
1−
x
Y
x
O
2 and Ce
0.85−
x
Zr
0.15Y
x
O
2 mixed oxides have been prepared by 1000
°C-nitrates calcination to ensure thermally stable catalysts. The physico-chemical properties of the mixed oxides have been studied by N
2 adsorption at −196
°C, XPS, XRD, Raman spectroscopy and H
2-TPR, and the catalytic activity for soot oxidation in air has been studied by TG in the loose and tight contact modes. Yttrium is accumulated at the surface of Ce
1−
x
Y
x
O
2 and Ce
0.85−
x
Zr
0.15Y
x
O
2, and this accumulation is more pronounced for the former formulation than for the latter, because the deformation of the lattice due to zirconium doping favours yttrium incorporation. Yttrium and zirconium exhibit opposite effects on the surface concentration of cerium; while zirconium promotes the formation of cerium-rich surfaces, yttrium hinders the accumulation of cerium on the surface. For experiments in tight contact between soot and catalyst, all the Ce
1−
x
Y
x
O
2 catalysts are more active than bare CeO
2, and Ce
0.99Y
0.01O
2 is the most active catalyst. The benefit of yttrium doping in catalytic activity of ceria can be related to two facts: (i) the Y
3+ surface enrichment hinders crystallite growth; (ii) the surface segregation of Y
3+ promotes oxygen vacancies creation. High yttrium loading (
x
=
0.12) is less effective than low dosage (
x
=
0.01) because yttrium is mainly accumulated at the surface of the particles and hinders the participation of cerium in the soot oxidation reaction, which is the active component. For the mixed oxides with formulation Ce
0.85−
x
Zr
0.15Y
x
O
2 (operating in tight contact) the effect of zirconium on the catalytic activity prevails with respect to that of yttrium. For experiments in loose contact between soot and catalyst, the catalytic activity depends on their BET surface area, and the catalysts Ce
0.85−
x
Zr
0.15Y
x
O
2 (BET
=
10–13
m
2/g) are more active than the catalysts Ce
1−
x
Y
x
O
2 (BET
=
2–3
m
2/g). In the loose contact mode, the yttrium doping and loading have a minor or null affect on the activity, and the stabilising effect of the BET area due to zirconium doping prevails.
TiO2 and La-doped TiO2 samples with La loading between 0.2% and 2% have been prepared, characterised, and tested in the catalytic oxidation of soot by O2. La doping prevents anatase to rutile phase ...transformation and crystallite size growth when heated at 800°C, as deduced from XRD, Raman spectroscopy and BET surface area measurements. The optimum La loading was found to be 0.2%. The catalytic activity of TiO2 and La-doped TiO2 samples tested mainly depends on crystallite size, the lower the size the better the activity. TiO2 phase composition is not a key factor in the catalytic oxidation of soot.
Ce–Zr mixed oxides calcined at 1000
°C are more active catalysts for soot oxidation than pure CeO
2 calcined at the same temperature, both in loose and tight contact between soot and catalyst. 1000
...°C sinterised-CeO
2 presents a very low surface area (2
m
2/g), a large crystal size (110
nm) and a lack of surface redox properties. Ce–Zr mixed oxides present higher BET surface areas (typically 17–19
m
2/g), smaller crystal sizes and enhanced redox properties. The Zr molar fraction does not affect appreciably the catalytic activity of Ce–Zr mixed oxides in the range studied (Zr molar fraction from 0.11 to 0.51).
In this work, the uncatalysed and catalysed combustion of two commercial carbon blacks and three diesel soot samples were analysed and related to the physico-chemical properties of these carbon ...materials. Model soot samples are less reactive than real soot samples, which can be attributed, mainly, to a lower proportion in heteroatoms and a higher graphitic order for the case of one of the carbon blacks. Among the diesel soot samples tested, the most relevant differences are the volatile matter/fixed carbon contents, which are directly related to the engine operating conditions (idle or loaded) and to the use of an oxidation catalyst or not in the exhaust. The soot collected after an oxidation catalyst (A-soot) is more reactive than the counterpart virgin soot obtained under the same engine operating modes but before the oxidation catalyst. The reactivity of the different soot samples follows the same trend under uncatalysed and catalysed combustion, the combustion profiles being always shifted towards lower temperatures for the catalysed reactions. The differences between the soot samples become less relevant in the presence of a catalyst. The ceria-zirconia catalysts tested are very effective not only to oxidise soot but also to combust the soluble organic fraction emitted at low temperatures. The most reactive soot (A-soot) exhibits a T{sub 50%} parameter of 450 C when using the most active catalyst. (author)
In this work, the uncatalysed and catalysed combustion of two commercial carbon blacks and three diesel soot samples were analysed and related to the physico-chemical properties of these carbon ...materials. Model soot samples are less reactive than real soot samples, which can be attributed, mainly, to a lower proportion in heteroatoms and a higher graphitic order for the case of one of the carbon blacks. Among the diesel soot samples tested, the most relevant differences are the volatile matter/fixed carbon contents, which are directly related to the engine operating conditions (idle or loaded) and to the use of an oxidation catalyst or not in the exhaust. The soot collected after an oxidation catalyst (A-soot) is more reactive than the counterpart virgin soot obtained under the same engine operating modes but before the oxidation catalyst. The reactivity of the different soot samples follows the same trend under uncatalysed and catalysed combustion, the combustion profiles being always shifted towards lower temperatures for the catalysed reactions. The differences between the soot samples become less relevant in the presence of a catalyst. The ceria-zirconia catalysts tested are very effective not only to oxidise soot but also to combust the soluble organic fraction emitted at low temperatures. The most reactive soot (A-soot) exhibits a T sub(50%) parameter of 450 C when using the most active catalyst.
In this work, the uncatalysed and catalysed combustion of two commercial carbon blacks and three diesel soot samples were analysed and related to the physico-chemical properties of these carbon ...materials. Model soot samples are less reactive than real soot samples, which can be attributed, mainly, to a lower proportion in heteroatoms and a higher graphitic order for the case of one of the carbon blacks. Among the diesel soot samples tested, the most relevant differences are the volatile matter/fixed carbon contents, which are directly related to the engine operating conditions (idle or loaded) and to the use of an oxidation catalyst or not in the exhaust. The soot collected after an oxidation catalyst (A-soot) is more reactive than the counterpart virgin soot obtained under the same engine operating modes but before the oxidation catalyst. The reactivity of the different soot samples follows the same trend under uncatalysed and catalysed combustion, the combustion profiles being always shifted towards lower temperatures for the catalysed reactions. The differences between the soot samples become less relevant in the presence of a catalyst. The ceria–zirconia catalysts tested are very effective not only to oxidise soot but also to combust the soluble organic fraction emitted at low temperatures. The most reactive soot (A-soot) exhibits a
T
50% parameter of 450
°C when using the most active catalyst.
PDF
