2.1. Textural and Structural Characterization
Composition and BET surface area of the catalysts are reported in Table 1
. Similar BET surface areas have been found for all catalysts (in the range 17–35 m2
/g). The addition of a single component to the support (CeO2
) does not affect the surface area that remains almost stable, while a small decrease was observed after addition of the two dopants in the Sr-doped materials. In accordance with the BET results, crystal size values are in the range 13–22 nm.
The structural features of all the materials were analysed by powder X-ray diffraction (XRD). Diffraction profiles of ceria-based catalysts (Figure 1
A) exhibit reflections characteristic of a pure fluorite phase, while zirconia-based materials show a monoclinic structure (Figure 1
B). In ceria-based catalysts, two very weak peaks, characteristic of a CuO phase, were also detected at 2θ = 35.5° and 38.8° for CeCu and CeSrCu samples, which is in agreement with other studies in the literature [20
phase was also detected for Sr impregnated materials (CeSr, CeSrFe, and CeSrCu) and it may occur as a consequence of the exposure of the catalysts to ambient air conditions; indeed, the basic oxides such as the oxides of alkaline-earth elements are readily carbonated when exposed in air [56
]. No evidence of the presence of Fe2
or other iron-containing phases was obtained, in accordance with the literature [22
]. XRD features do not indicate formation of any ceria solid solution with copper or iron, suggesting that Fe or Cu are dispersed on the surface. It is known that lower valence ions, such as Fe3+
, do not easily dissolve into the ceria lattice using conventional impregnation methods; dissolution, if present, is limited to a small fraction of the overall loading [22
]. To facilitate formation of solid solution alternative preparation methods are required [60
]. In zirconia-based samples no evidence for any copper or iron phase was found, while formation of SrCO3
phases was observed.
2.2. Reduction Behaviour
In order to characterize the reduction behaviour of the materials, temperature-programmed reduction experiments with H2
-TPR) have been carried out on all samples (Figure 2
). Figure 2
A shows the temperature programmed reduction profiles of ceria and ceria modified catalysts. The reduction feature of pure ceria is well known and it shows the characteristic bimodal profile with two peaks at low (ca. 525 °C) and high (ca. 840 °C) temperature attributable, respectively, to the reduction of small crystallites and/or surface ceria and to the reduction of bulk and large ceria crystallites [62
]. For pure zirconia, a TPR feature of a typical ‘‘non-reducible’’ support was found (Figure 2
B). The reduction profiles of the two supports are affected by the addition of metals.
ZrFe shows two reduction signals that could be ascribed to the existence of free Fe2
on the support surface. The hydrogen reduction profile for Fe2
occurs in two steps, with Fe2
first converting to Fe3
(with maximum at around 340 °C) and then to Fe (with maximum at ca. 550 °C) [63
]. In CeFe, reduction of iron species overlap with the reduction features of ceria and four peaks are distinguished: the first one at around 350 °C was related to the reduction of hematite to magnetite state; the second and third peaks at around 450 °C and 600 °C can be attributed to the surface reduction of Ce4+
and the reduction of Fe2+
to Fe, while the last signal at 825 °C can be associated with the reduction of bulk ceria [65
Materials loaded with Sr evidenced a main peak at around 700–750 °C that could be correlated to the desorption of superficial carbonate species, as confirmed by the analysis of outlet gas composition followed by an online quadrupole mass-spectrometer. For CeSr, the peak overlaps with the bulk reduction feature of ceria.
For the CeSrFe, the H2-TPR profile undergoes some modifications respect CeFe and only three reduction features are detected due to the merging of the first two signals of CeFe. The low temperature peak (375 °C) is correlated to the reduction of Fe3+ to Fe2+ and to the reduction of surface Ce4+. The second signal (600 °C), corresponding to the third peak in CeFe, is not affected by the presence of Sr in the formulation. Moreover, the high temperature peak band (790 °C) is due to the overlapping of bulk reduction of ceria and the desorption of carbonate species from the Sr phase.
ZrSrFe is less affected by Sr addition and the TPR profile is the result of the combination of ZrSr and ZrFe features. The first two peaks are due to the iron oxide reduction (as in the ZrFe sample) while the high temperature signal is due to the surface carbonate adsorbed on Sr (as in ZrSr).
The addition of Cu on ceria modifies the redox properties of both species, as a consequence of the CuO-CeO2
interaction at the oxide interface [59
]. TPR profile of CeCu exhibits three main reduction peaks at 130, 160, and 840 °C. The two low-temperature signals can be assigned to the reduction of CuOx
species and surface Ce4+
. It is well known that bulk CuO reduction takes place at around 315–380 °C [20
] while in our material a shift to lower temperature of the CuOx
reduction features occurs due to metal support interaction. In addition, we can observe that the low reduction signal of ceria at around 525 °C is not present, indicating that the hydrogen spillover process promotes surface ceria reduction at a much lower temperature [67
]. The quantitative analysis of the TPR profile reveals that a part of Ce4+
is reduced at 100–300 °C; the amount of hydrogen consumption in this temperature range (1.10 mmol/gcat
) is larger than that required for the complete reduction of CuOx
species (0.79 mmol/gcat
), in agreement with other studies [20
]. The presence of copper can, therefore, promote the reduction of surface ceria at much lower temperatures. The presence of Sr slightly modifies the shape and position of the low temperature signals, and only one peak is visible in CeSrCu, shifted at a slightly higher temperature. The modification of the reduction profile at low temperature can also be associated to a detrimental effect on the reduction properties of the catalyst, confirmed by the decrease of the H2
consumption at low temperature compared to CeCu (0.83 vs. 1.10 mmol/gcat
materials exhibit more than one reduction peaks correlated to Cu [20
], suggesting the presence of more than one copper oxide species. A great amount of literature data is available for TPR studies of Cu-based catalysts; although shape and position highly depend on the experimental conditions and on the preparation of the catalyst, the shift to lower temperature of the reduction peaks suggests a synergic interaction between CuO and CeO2
. The low temperature peak (typically denoted as peak α) is usually attributed to highly-dispersed CuO species closely interacting with CeO2
(more easily reduced species), while the higher temperature peak (typically denoted as peak β) is assigned to the overlapping of the reduction of larger CuO particles (still highly dispersed and strongly interacting with the support) and surface Ce4+
. Usually a third reduction peak at higher temperature is found in TPR profile of CuO-CeO2
catalyst (not present in our materials) and is correlated with reduction of segregated crystalline CuO [67
The low temperature region of ZrCu sample is comparable to CeCu, indeed, two peaks at 160 °C and 230 °C due to the reduction of copper species are displayed. The main difference between CeCu and ZrCu is the consumption of H2 in the low-temperature range, and that, for the latter, corresponds only to the reduction from Cu2+ to Cu°, as we can expect for a “non-reducible” support like zirconia (0.70 mmol/g). These peaks are found also in the ZrSrCu sample, where a peak at high temperature (710 °C) due to the contribution of adsorbed strontium carbonate is also detected.
2.3. Catalytic Tests
summarizes the results of soot combustion studies carried out under O2
atmosphere in terms of peak-top temperature (T
p). A representative oxidation profile is shown in Figure 4
for the CeSrCu sample.
atmosphere (Figure 3
A) all of the catalyst formulation are slightly active compared to the oxidation of soot without a catalyst, displaying T
p in the range 580–595 °C with the exception of CeSr, Zr, and ZrCu, which show temperatures of oxidation higher than 600 °C.
Several studies investigated the promotional effects of copper and iron in soot combustion catalysts under O2
]. The mechanism of reaction commonly proposed is correlated to the presence of Mx
particles and the ability of the metal to be reduced to M(n−1)+
and then reoxidized to Mn+
, producing active oxygen species that can easily react with soot [45
The addition of NO in the reaction mixture (Figure 3
B) causes a decrease in the temperature of combustion for all catalysts (Tp lower than 560 °C). The addition of Cu and Fe results in a beneficial effect on soot combustion compared to bare supports, while the introduction of Sr has a detrimental influence on the performances. The fact that Sr acts as an efficient NOx
], with the formation of stable nitrates species, is likely to cause a loss of soot combustion activity, in agreement with previous literature results [15
With transition metals and Sr containing formulations, the oxidation is shifted at temperatures lower than 500 °C, suggesting a synergic effect of the two components. The best performances are obtained with Cu-Sr combination, with a Tp of 468 and 482 °C for Zr and Ce sample, respectively, compared to 501 °C and 496 °C for SrFeZr and SrFeCe.
As shown in Figure 4
when soot oxidation is carried out with a mixture of NO/O2
, an enhancement in catalytic combustion was found, compared to the reaction performed under oxygen environment and the reason could be ascribed to two different mechanisms that could take part in a NO/O2
atmosphere: (i) from one side soot can be oxidized by active oxygen species (O*) that are generated over Cu- or Fe-containing materials, i.e., transition metals exhibit the capability to cycle between two states of oxidation contributing to soot combustion [45
]; (ii) on the other hand, the NOx
-assisted mechanism can improve the catalytic activity forming NO2
, a more oxidant and mobile species [71
In the case of NOx
-assisted reaction it is important that the catalyst can promote the oxidation of NO to NO2
at low temperature. If NO2
is formed at temperature lower than that of soot oxidation, it can participate to reaction and offer an alternative route in the combustion of particulate. CeCu is the most active catalyst in NO oxidation as shown in Figure 5
, with a Tm lower than 400 °C. All of the other samples containing Cu and Fe exhibit a Tm in the range 425–470 °C, with the exception of pure zirconia, CeSr and ZrSr samples that are only moderately active in NO oxidation (T
m is higher than 500 °C). In the presence of NO, Cu- and Fe-loaded ceria and zirconia promote soot combustion at lower temperature, showing that catalysts that are more efficient in NO oxidation are more active in soot combustion. This can be clearly seen in Figure 6
, where the T
p vs. Tm temperatures are reported and a correlation between soot combustion (T
p) and NO oxidation (T
m) temperatures can be found for both CeO2
- and ZrO2
With the only exception of CeCu catalyst, Figure 6
A shows a tendency of soot oxidation temperature against NO oxidation activity (i.e., the highest is the NO oxidation activity, the highest is the soot combustion temperature). The best catalysts for soot oxidation are the formulations doped with both transition and alkaline-earth metals (in particular CeSrCu and ZrSrCu), which suggests a synergic effect between Cu and Sr, independently from the support. It is well known that, in the presence of NO, alkali-earth metals are mainly involved in the storage of NOx
]. Table 2
adsorption/desorption properties obtained from NOx
During the NOx-TPD measurements all of the materials investigated release a significant amount of NOx species, where the amount released with Ce-based catalysts is higher if compared to Zr-based formulations. The incorporation of Sr further enhances the desorption of NOx indicating that when Sr is present in the formulation nitrite/nitrate species can be efficiently stored on the sample and then released when increasing the temperature.
To summarize, the transition metal is favourably involved in the oxidation of NO to NO2, while strontium is involved in the storage of NOx species. The nitrite/nitrate species stored on strontium, when the temperature increases, start to decompose, releasing NOx that can easily react with particulate-forming CO and CO2. At this point, the presence of a transition metal is crucial to recycling NO to NO2. When Sr is added to the catalyst, the NOx involved in soot oxidation can originate from the oxidation of NO present in the reaction mixture and from the nitrates stored on the system, positively influencing the particulate combustion activity.