2.2.1. Characterization of the Catalysts’ Crystallinity by Raman Spectroscopy, X-ray Diffraction (XRD), and Helium Picnometry
The crystalline structure of the catalysts was studied, and Figure 3
shows the X-ray diffractograms.
These diffractograms include the characteristic peaks attributed to the fluorite structure of ceria (JCPDS 34-0394), and are similar for the two catalysts. The cell parameters and the crystallite sizes, compiled in Table 1
, were obtained from these diffractograms, and are also equal for both Ce-Pr-3DOM and Ce-Pr-ref. The crystallite sizes are 10 nm and the cell parameter is 0.5429 nm, this value being in agreement with the cell parameters reported in the literature for materials of similar composition [13
The density of the two catalysts, which was determined by helium picnometry (Table 1
), is also the same. This is additional evidence of the similar structure of the primary crystals of both Ce-Pr-3DOM and Ce-Pr-ref.
The conclusions shown by XRD and He picnometry are supported by Raman spectroscopy. Figure 4
shows the Raman spectra of the fresh and used catalysts, all of them being qualitatively similar.
A single band centered at 462 cm−1
is shown, which is the F2g
band of the typical fluorite structure of ceria. In this structure, cerium cations are located at the corners and center of the faces of a cubic unit cell and oxygen anions are placed in the octahedral positions. The F2g
band is related to the oxygen anions’ vibration around the octahedral position [14
The maximum of the F2g
band is located at 462 cm−1
in all spectra, and this value is slightly lower to that reported for pure ceria (465 cm−1
]). This shift towards low values can be related to the partial substitution of Ce4+
cations (0.097 nm) by Pr3+
cations (0.113 nm), the latter being larger than the former. Note that both cerium and praseodymium can adopt the +3 and +4 oxidation states, but praseodymium is reduced more easily than cerium [17
]. Therefore, the Raman spectra in Figure 4
provide evidence of Ce-Pr solid solution formation in both catalysts.
In addition to the F2g
band at 465 cm−1
, ceria also presents two low-intensity bands around 260 and 595 cm−1
], and our spectra in Figure 4
show bands at 194 and 572 cm−1
that could be consistent with these two features. Our spectra also show a band at 1145 cm−1
that has been assigned to surface oxygen species. The small band around 572 cm−1
has been related to oxygen vacancies, and relevant differences in the intensity of the vacancies bands have been detected for the two catalysts. The intensity ratio of the vacancies (572 cm−1
) and F2g
) bands are compiled in Table 2
for all spectra.
The relative intensity of the vacancies band is higher for the Ce-Pr-3DOM catalyst than for the reference catalyst (Ce-Pr-ref), and the vacancies’ population is preserved during the catalytic tests. These results point out that the formation of the Ce-Pr solid solution into the PMMA template enhances the formation of oxygen vacancies. A more detailed analysis of the presence of 3+ and 4+ cations on the catalysts and their reducibility was carried out by X-ray photoelectron spectroscopy (XPS) and Temperature-programmed reduction with H2 (H2-TPR), respectively, and the results are discussed later on.
As a summary, it can be concluded that according to the Raman spectroscopy, XRD, and helium density results, the utilization of the PMMA template does not affect the crystalline features (type of phases, crystallite sizes, and cell parameters) of the catalysts, which seem to be related to the chemistry and thermal history of the synthesis process. However, evidences of different populations of vacant sites have been provided by Raman spectroscopy, suggesting that the PMMA template affects this parameter. These results allow the interpretation of the differences in catalytic activity, paying attention only to the porous structure of the catalysts and to the chemical properties of their surface, and ruling out differences in the structure of the primary crystals.
2.2.2. Characterization of the Catalysts’ Surface by Scanning electron microscopy (SEM), Mercury Intrusion Porosimetry, and N2 Adsorption
The morphology of the catalysts was studied by SEM microscopy, and representative images are shown in Figure 5
together with one image of the PMMA template. The reference catalyst (Figure 5
a; Ce-Pr-ref), prepared without the template, shows a flat surface without evident porosity. Monodisperse spheres of PMMA with about 200 nm diameter are observed in Figure 5
c, which are packed in regular planes forming the colloidal crystal, and the catalyst prepared by infiltration into the PMMA template (Figure 5
b; Ce-Pr-3DOM) shows the opposite morphology. Large spherical macropores with roughly 80 nm diameter are arranged in the same way as the PMMA spheres. This shrinkage from 200 to 80 nm (60%) is quite similar to that reported by other authors for other 3DOM-ceria catalysts [20
]. The inset in Figure 2
b shows that the spherical macropores are linked by smaller windows, which are necessary for the gases to diffuse into the solid structure during the catalytic reactions.
These important differences in the SEM images are consistent with differences in the meso and macroporosity measured by mercury intrusion porosimetry (Figure 6
). The reference catalyst Ce-Pr-ref has almost negligible area in the range of meso and macropores, while the catalyst prepared by infiltration into the PMMA template shows two well-defined peaks in the macropore range. The area of these meso and macropores was estimated from the mercury intrusion curves (Table 1
), being 41 m2
/g for Ce-Pr-3DOM while only 3 m2
/g for Ce-Pr-ref.
In addition, the catalyst Ce-Pr-3DOM also presents much more developed microposority than the reference catalyst Ce-Pr-ref, as deduced from the N2
adsorption-desorption isotherms included in Figure 7
The shape of the Ce-Pr-3DOM isotherm is Type IV, according to the IUPAC classification [21
]. This isotherm combines N2
adsorption at very low partial pressures with a certain adsorption in the whole range of partial pressures and a well-defined hysteresis loop. This type of isotherm is attributed to solids with both micro and mesopores. The shape of the hysteresis loop also suggests the presence of macropores, in agreement with the mercury intrusion results, since the adsorption near saturation pressure does not reach horizontal values but keeps increasing.
On the contrary, the N2
adsorption-desorption isotherm of the Ce-Pr-ref catalyst shows little N2
adsorption in the whole range of partial pressures and the hysteresis loop is almost negligible, evidencing that this solid has a very poor porosity. In agreement with the shape of the isotherms, the BET specific surface area of Ce-Pr-3DOM (51 m2
/g; data shown in Table 1
) is also much higher to that of Ce-Pr-ref (24 m2
Additional information about the porosity of both solids is obtained by comparing the BET specific surface areas with the areas determined by mercury intrusion porosimetry (Table 1
). The BET surface areas include the whole porosity, while the surface area determined by mercury intrusion porosimetry provides information about the meso and macropores. The areas of Ce-Pr-3DOM (41 and 51 m2
/g for mercury intrusion and N2
adsorption areas, respectively; Table 1
) confirm that this solid combines micro, meso, and macropores, with the most area lying in the macropore range.
According to these surface characterization results, it could be argued that the better catalytic activity of Ce-Pr-3DOM with regards to Ce-Pr-ref is related to its higher surface area, since it has more surface available for CO to be adsorbed and oxidized. However, different attempts have been performed in order to normalize the reaction rate per square meter of catalyst surface (considering BET surface areas and areas in different ranges determined by mercury intrusion) and all of them have failed. This leads us to think that, probably, it is partially true that the higher surface area leads to higher catalytic activity, but surface area differences alone are not enough to explain the differences in activity. In other words, if the only reason for the better catalytic activity of Ce-Pr-3DOM with respect to Ce-Pr-ref was its higher surface area, much higher differences in the CO oxidation rates would be expected. Raman spectroscopy evidenced the higher population of vacant sites on the Ce-Pr-3DOM catalyst and, as demonstrated in the next section, the nature of the surface must also be taken into account. The surface of the Ce-Pr-3DOM catalyst seems to be more efficient than that of Ce-Pr-ref, that is, the formation of the Ce-Pr solid solution into the PMMA porosity framework has benefits in catalytic activity with respect to crystallization without the template, in addition to the improved porosity.
2.2.3. Characterization of the Chemical Properties of the Catalysts’ Surface by Temperature-Programmed Reduction with H2 (H2-TPR) and X-ray Photoelectron Spectroscopy (XPS).
Finally, the catalysts were characterized by H2
-TPR and XPS, and important differences in the surface properties were noticed. Figure 8
shows the H2
reduction profiles, and three different reduction peaks are distinguished. The high-temperature peak at 750 °C can be assigned to the bulk reduction of the catalysts and the peak at 575 °C to surface reduction. The small peak at 450 °C is not easily assigned, because it could be produced by the desorption of hydroxyls and/or carbonates together with actual reduction events. The main difference in the reduction profiles of the two catalysts is the intensity of the surface reduction peak at 575 °C, which is much more intense for Ce-Pr-3DOM than for Ce-Pr-ref.
These H2-TPR experiments confirm that the surface reducibility of the 3DOM catalyst is much higher than that of Ce-Pr-ref, while bulk reduction is more or less equal for both materials. This is consistent with the previous characterization, where it was confirmed that the primary Ce-Pr mixed oxide crystals are equal, and therefore they are reduced in the same way, while the main differences are in their surfaces.
The catalysts’ surface was characterized by XPS and the Ce/Pr ratio and percentages of +3 cations were estimated (Table 3
). The XPS spectra are included in a Supplementary Materials file
: XPS spectra in the energy range of Ce 3d and Figure S2
: XPS spectra in the energy range of Pr 3d). The Ce/Pr surface ratio is the same for both catalysts (~4.2), and the ratio is lower than the nominal value (9), that is, there is more Pr in the solid solutions’ bulk than on their surface, while the opposite occurs with Ce.
The total percentage of 3+ cations (Ce3+ + Pr3+) are similar for both fresh catalysts (34%), but the contribution of Ce3+ and Pr3+ cations to these total values is different for Ce-Pr-3DOM (fresh) and Ce-Pr-ref (fresh). The percentage of reduced Pr3+ cations is much higher in the Ce-Pr-3DOM catalyst (fresh) (54%) than in the reference counterpart (Ce-Pr-ref (fresh); 38%), while the opposite occurs in the Ce3+ percentage.
The percentages of 3+ cations change during the catalytic tests, as deduced from the XPS results obtained with the used catalysts, and these changes affect the Ce-Pr-3DOM catalyst to a higher extent than they affect the Ce-Pr-ref catalyst. The total percentage of 3+ cations on the Ce-Pr-3DOM catalyst decreases from 34% to 26% during the catalytic tests, an evidence of severe modification of the surface’s oxidation degree, this fact being consistent with the redox mechanisms taking place, while the 3+ cations in the Ce-Pr-ref catalyst only change from 34% to 32%. The higher extent of the surface modification of the Ce-Pr-3DOM catalyst during the catalytic experiments agrees with its higher catalytic activity (Figure 1
and Figure 2
), and suggests that the better performance of the Ce-Pr-3DOM catalyst is not only related with its higher surface area but also with the nature of its surface. The higher reducibility of the Ce-Pr-3DOM catalyst surface observed by H2
-TPR (Figure 8
) is also in line with this observation, that is, it seems that crystallization of the Ce-Pr solid solutions into the PMMA template improves both the surface and the redox properties of the catalyst. Additional information is obtained from the individual percentages of the Ce3+
cations on the catalyst’s surface before and after the catalytic tests. As deduced from the data in Table 3
, the main changes during the catalytic tests affect the oxidation of the Pr3+
cations of the Ce-Pr-3DOM catalyst.
In conclusion, the utilization of the PMMA template for the preparation of Ce-Pr-3DOM improves the porosity of the material, in agreement with the literature, but also the redox properties of the catalyst. The Ce-Pr solid solution prepared with the PMMA template has improved surface reducibility with regards to the counterpart reference material prepared without the template, and improved redox behavior under reaction conditions, that is, it has higher area and this area is reduced and reoxidized more easily. This is mainly attributed to praseodymium cations, which seem to accomplish redox cycles more easily.