Physicochemical characterization of the samples
XRD patterns show reflections that are typical of copper oxide for Cu/KIL-2 (Figure 1 a), but not for the Cu/FeKIL-2 sample, probably because of the formation of small copper oxide nanoparticles. Copper oxide with a particle size of approximately 100 nm is determined for Cu/KIL-2 (Figure 1 a) with calculations from the Scherrer equation, which is based on the selected diffraction peaks of the corresponding XRD pattern.
Figure 1. a) XRD patterns with a 01-080-1916 reference for copper oxide, b) nitrogen physisorption data, and c) UV/Vis spectra for the studied samples.
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Gas physisorption is extensively used in the characterization of porous solids, particularly for evaluation of their specific surface area, total pore volume, pore size distribution as well as surface properties. The shape of the isotherm obtained from these adsorption measurements is important because it can reveal the kind of porosity (micro, meso, or macroporosity) present in the sample. Nitrogen adsorption isotherms for KIL-2, FeKIL-2 and Cu/FeKIL-2 materials are shown in Figure 1 b, whereas structural parameters determined on the basis of these isotherms are listed in Table 1. KIL-2 and FeKIL-2 samples exhibit adsorption isotherms typical for KIL-2 and FeKIL-2 materials,30–32 that is, with relatively narrow hysteresis loops of type IV.26 The FeKIL-2 sample undergoes a capillary condensation step at higher relative pressures than the KIL-2 sample, which indicates that larger mesopores are present in the FeKIL-232 material compared to the KIL-2 support. It can be clearly observed that the presence of copper oxide nanoparticles in KIL-2 and FeKIL-2 supports leads to a marked change in the shape of the hysteresis loop. The hysteresis loop of copper(II) oxide loading samples is widening, less intensive and closing down at lower p/p0 values compared to the original supports, which indicates that the pores are partially narrowed with copper oxide nanoparticles. The loading of copper oxide nanoparticles into KIL-2 and FeKIL-2 supports also leads to a decreased specific surface area, pore volume and pore diameter (Table 1). Therefore, it can be concluded that the copper oxide nanoparticles are dispersed inside the KIL-2 and FeKIL-2 matrices.
Table 1. Structural parameters of the catalysts determined from nitrogen adsorption isotherms.
|Sample||Copper oxide [wt %]||SBET[a] [m2 g−1]||VT[b] [cm3 g−1]||Vmeso[c] [cm3 g−1]||wBJH[d] [nm]|
Pore size distribution of the different mesoporous materials was determined by using the Barrett–Joyner–Halenda (BJH) model, which is widely used for this type of samples.28 Although this model often underestimates pore sizes,29 it is appropriate for comparative purposes. The pore size distributions determined from the adsorption isotherms are displayed in Figure 2 b (inset). It can be observed that the maxima characteristics of the mesopores of KIL-2 and FeKIL-2 supports are more intense and broader in comparison to their copper oxide loaded samples. They show average pore diameters of 19.9 and 21.6 nm, respectively. The maxima characteristics of copper oxide impregnated materials are shifted to lower pore size values (14.8 nm for Cu/KIL-2 and 16.1 nm for Cu/FeKIL-2).
Figure 2. TEM micrographs of a) Cu/KIL-2 and b) Cu/KIL-2 with copper oxide crystallites (dark spots) located on the surface of the KIL-2 silica matrix, c) SAED pattern of Cu/KIL-2 with selected reflections with the corresponding marked crystal lattices, d) close-up TEM micrograph of Cu/KIL-2, e) TEM micrograph of Cu/FeKIL-2 with copper oxide crystallites (dark spots) located on the surface of the FeKIL-2 silica matrix, f) close-up TEM image of Cu/FeKIL-2.
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A TEM micrograph of the Cu/KIL-2 sample shows a porous texture with interparticle porosity typical for KIL-2 silica materials (Figure 2 a). Agglomerated silica nanoparticles with an estimated size of 10 nm form voids and cavities with a broad size distribution. The porous silica matrix contains individual crystallites of copper oxide. The relatively large thickness of the crystallites makes the TEM observation somewhat difficult, however an approximate size of 100 nm could be estimated (Figure 2 b). This is in agreement with the calculations from the Scherrer equation, which is based on the selected diffraction peaks of the corresponding XRD pattern.
The presence of the monoclinic copper oxide phase (Cc, a=4.69 Å, b=3.43 Å, c=5.13 Å, β=99.65°) in the CuKIL-2 sample, which is already indicated by XRD analysis, is finally confirmed by the selected area electron diffraction method (SAED). The SAED pattern of the copper oxide crystallites with the selected maxima and their corresponding crystal planes are shown in Figure 2 c, whereas a comparison of measured and referenced d values of selected crystal planes are shown in Table 2.
Table 2. Comparison of measured and referenced d values of corresponding crystal planes.
| ||Crystallographic plane||Measured [nm]||PDF[a] (No. 01-080-1917) [nm]|
The sizes of the copper oxide crystallites indicate that they are located exclusively on the surface of the KIL-2 silica matrix. The observations at higher magnification did not reveal the presence of any smaller metal oxide nanoparticles that could be located within the voids of KIL-2 matrix(Figure 2 d).
TEM analysis of the Cu/FeKIL-2 sample shows similar textural properties as for the Cu/KIL-2 sample. The FeKIL-2 silica matrix is inhomogenously covered with the copper oxide phase, however individual metal oxide nanoparticles are somewhat smaller (100 nm) and seem to be agglomerated into larger clusters (Figure 2 e). Similarly, as in the case of the Cu/KIL-2 sample, the observations at higher magnification do not reveal the presence of any smaller metal oxide nanoparticles (Figure 2 f).
More information on the oxidation state of copper oxide species and their reducibility was obtained by temperature-programmed oxidation–reduction thermogravimetric analysis (TPO/TPR-TG) experiments (Figure 3). Pretreatment in different media (air and argon) was applied before the TPR experiments. The samples showed different behavior during the pretreatment procedure, which depended on the pretreatment media and the used support (Figure 3).
There were no significant changes in the weight of both samples Cu/KIL-2 and Cu/FeKIL-2 during the pretreatment in air. A low intensity peak is registered at 870 K for both samples; this is probably due to silica matrix dehydroxylation. In contrast, the pretreatment in argon results in significant weight changes, which correspond to 100 % reduction of copper(II) ions to metallic copper on the FeKIL-2 sample. Reduction peaks at 595 and 770 K are registered for Cu/FeKIL-2 pretreated in argon. For the first time the copper autoreduction process is determined on mesoporous materials. We ascribe the copper autoreduction process to the influence of iron(III) ions and the formation of weak Brønsted acid sites in the sample as a result of iron incorporation into the silica structure. The predominant formation of copper(II) ions on this sample is due to the presence of Brønsted acid sites in the iron-containing KIL-2 support and their interaction with the copper precursor during the impregnation process. These copper(II) species can be easily reduced to metallic copper during evacuation at high temperature or under an inert atmosphere [Eq. (1)].(1)
Kuroda et al13 also found that the presence of Brønsted acid sites in the sample that has a high Si/Al ratio is a necessary condition for the formation of copper(I) ions. The reduction profiles of the copper-containing catalysts depend on the support material used and the pretreatment procedure. The copper-containing KIL-2 and FeKIL-2 samples pretreated in air possess clearly defined low-temperature reduction peaks at 530 and 440 K, respectively, which can be attributed to copper(II) to metallic copper reduction. The extent of reduction is 89.33 % for Cu/KIL-2 and 100 % for Cu/FeKIL-2. The narrow reduction peak with a maximum at 430 K, registered for Cu/KIL-2 after pretreatment in argon (Figure 3 d), is related to the more homogenous distribution of copper oxide species on the supports. There are no peaks in the TPR-TG profile of the Cu/KIL-2 sample after pretreatment in argon at 773 K (Figure 3 c), which is due to total copper autoreduction during the pretreatment under an inert atmosphere.
By summarizing the above observations we can conclude that the reduction of copper oxide species is easier on the FeKIL-2 supported sample in comparison to its KIL-2 supported analogue. It is probable that the formation of different types of copper oxide species on the two applied supports and the influence of iron in FeKIL-2 are responsible for the different reduction behavior. This assumption is also supported by diffuse reflectance UV/Vis data, in which the formation of different copper species is registered.
Catalytic activity for total toluene oxidation
The catalytic activities versus the temperature and versus the time on a stream of copper-containing materials that were pretreated in an inert gas or air at 723 K in total toluene oxidation are presented in Figure 4. The pretreatment in different media (air or argon) does not lead to a different catalytic activity with a temperature increase for Cu/FeKIL-2 (Figure 4 b). It is the opposite for Cu/KIL-2, the increase in the temperature leads to a higher catalytic activity in toluene oxidation for this sample after the pretreatment in air at 773 K (Figure 4 a). The use of KIL-2 as a support has a positive effect on the catalytic performances of the Cu/KIL-2 sample. The Cu/FeKIL-2 catalyst shows stable catalytic activity at 653 K after pretreatment in different reaction media (air and argon) (Figure 4 d), whereas the Cu/KIL-2 sample shows stable catalytic activity only in the pretreatment with air, and a trend towards deactivation in the pretreatment with argon (Figure 4 c). The physicochemical characteristics (UV/Vis and TPR data) of the catalysts show the formation of finely dispersed copper oxide species on both supports, which determine their high catalytic activities. Kim19 also found that the presence of highly-dispersed copper species had a positive effect on the catalytic activity in toluene oxidation. The catalytic activity of bulk copper oxide is presented in Figure 4 as a reference. All supported samples show much higher catalytic activity in comparison to bulk copper oxide. The easier oxygen release is an essential step in the oxidation reaction according to the Mars-van-Krevelen mechanism19 and in this relationship the reduction properties of the samples can be used for the interpretation of their catalytic performance. However, the higher extent of copper reduction at lower temperature observed for Cu/FeKIL-2 (Figure 3) does not lead to higher catalytic activity in toluene oxidation. We did not find a direct relationship between the reduction properties and the catalytic activity of the catalysts, which could be assumed from the mechanistic point of view. Additional XRD experiments of the spent catalysts (Figure 5) were applied to indicate the changes of the oxidation state of copper oxide species during the reaction. Redispersion of the copper oxide phase during the reaction on both samples (Cu/KIL-2 and Cu/FeKIL-2) was observed after pretreatment in inert gas. The copper autoreduction in argon on FeKIL-2 results in the formation of more finely dispersed copper(I) and copper (II) species (not detectable by XRD, Figure 5) in the oxidative reaction medium, which are less active. The pretreatment in air of Cu/KIL-2 results in the formation of copper oxide that is 100 nm in size; this possesses the highest catalytic activity in toluene oxidation but lower stability compared to the Cu/FeKIL-2 and Cu/KIL-2 samples pretreated in argon. Our experiments show that finely dispersed copper species are less active, and the formation of nanosized crystalline copper oxide is responsible for the high catalytic activity in total toluene oxidation. The Cu/KIL-2 and Cu/FeKIL-2 catalysts after the pretreatment in argon keep their catalytic activity in 3 reaction cycles. A trend to deactivation is observed only for Cu/KIL-2 after pretreatment in air most probably because of the agglomeration of the formed copper oxide on the KIL-2 support after pretreatment (Figure 5 a (2)). The pretreatment in air of the Cu/FeKIL-2 catalyst doesn′t have a negative influence on its catalytic stability, which can be related to the formation and stabilization of finely dispersed copper(II) oxide and copper(I) oxide species.
Figure 4. Catalytic activity versus temperature on a) Cu/KIL-2 and b) Cu/FeKIL-2 samples and versus time on stream at 653 K on c) Cu/KIL-2 and d) Cu/FeKIL-2 samples after pretreatment in argon or air.
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On the basis of the presented catalytic data we conclude that copper supported on KIL-2 pretreated in air is the better catalyst compared to the copper-supported FeKIL-2 sample with a Fe/Si molar ratio of 0.01 for total toluene oxidation. The presence of iron in the KIL-2 structure leads to autoreduction of copper followed by the redispersion and oxidation of metallic copper in the reaction medium; this results in the formation of finely dispersed copper oxide clusters. The latter species possess low catalytic activity in toluene oxidation.