2.1. Interaction of Carbonyl Clusters with Different Supports
The impregnation processes with the cluster solutions of [NEt4]4[Au4Fe4(CO)16], [NEt4][AuFe4(CO)16] and [NEt4][HFe3(CO)11] on the different oxidic supports examined, i.e., TiO2, CeO2 and SBA-15, have been monitored via FTIR in order to better understand these processes and the different phenomena occurring during impregnation. The general procedure followed was: (i) to suspend and stir the support in acetone under nitrogen; (ii) to slowly add to it an acetone solution containing the appropriate amount of cluster; (iii) to stir the mixture overnight under nitrogen; (iv) to remove the solvent in vacuum. The dried powders were then thermally treated as described in the next section. FTIR spectra were recorded (1) on the starting acetone solution of the cluster, (2) on the acetone solution after being in contact overnight with the support, (3) on the dried powder in a nujol mull, and (4) after extraction from the dried powder with CH3CN. Acetonitrile was used instead of acetone in the last stage because of its greater capability to solubilize these metal clusters.
In the case of TiO
2 as the support, FTIR studies (
Figure 1a,b) showed that [NEt
4][AuFe
4(CO)
16] and [NEt
4][HFe
3(CO)
11] are adsorbed without the occurrence of any reaction, whereas significant changes have been observed during the adsorption of [NEt
4]
4[Au
4Fe
4(CO)
16] (
Figure 1c).
Figure 1.
FTIR spectra in the ν(CO) region obtained during catalyst preparation: (a) [NEt4][AuFe4(CO)16]; (b) [NEt4][HFe3(CO)11]; (c) [NEt4]4[Au4Fe4(CO)16]; (1) starting acetone solution of the cluster; (2) acetone solution after being in contact overnight with the support; (3) in vacuum dried powder; (4) CH3CN extraction from the dried powder; (5) sample after thermal treatment in air.
Figure 1.
FTIR spectra in the ν(CO) region obtained during catalyst preparation: (a) [NEt4][AuFe4(CO)16]; (b) [NEt4][HFe3(CO)11]; (c) [NEt4]4[Au4Fe4(CO)16]; (1) starting acetone solution of the cluster; (2) acetone solution after being in contact overnight with the support; (3) in vacuum dried powder; (4) CH3CN extraction from the dried powder; (5) sample after thermal treatment in air.
For all systems, the carbonyl cluster is totally decomposed after thermal treatment, since no ν(CO) band is present in the final spectra. It is clear that the two bimetallic clusters not only have a different Au:Fe composition, but also a rather different chemical behavior. Thus, whereas [NEt4][AuFe4(CO)16] is quite stable and non-reactive, [NEt4]4[Au4Fe4(CO)16] is very reactive, being easily oxidized as indicated by the increased ν(CO), yielding other Au-Fe carbonyl species. Moreover, the final product of its oxidation is [NEt4][AuFe4(CO)16].
Several other Au-Fe carbonyl species exist between these two species and, among these, [Au
5Fe
4(CO)
16]
3− [ν(CO) 1945(s) and 1861 (s) cm
−1], [Au
21Fe
10(CO)
40]
6− [ν(CO) 1982 (s), 1937 (sh), 1889 (sh) cm
−1], [Au
22Fe
12(CO)
48]
6− [ν(CO) 1980 (s), 1925 (sh), 1880 (sh) cm
−1], [Au
28Fe
14(CO)
52]
8− [ν(CO) 1985 (s), 1927 (sh), 1887 (sh) cm
−1], and [Au
34Fe
14(CO)
50]
10− [ν(CO) 1990 (s), 1932 (sh), 1900 (sh) cm
−1] have been recently structurally characterized [
25,
26]. Overall, these brown species are intermediates in the oxidation in solution of [Au
4Fe
4(CO)
16]
4− to eventually give [AuFe
4(CO)
16]
−. All this information can be helpful for fully understanding the fate of [Au
4Fe
4(CO)
16]
4− during impregnation on TiO
2.
FTIR studies indicate that the oxidation of [Au
4Fe
4(CO)
16]
4− occurs in two different steps during catalyst preparation, as can be inferred from
Figure 1c, showing the comparison of the FTIR spectra in the ν(CO) region of: (1) [NEt
4]
4[Au
4Fe
4(CO)
16] in acetone [ν(CO) 1931 (s) and 1861 (s) cm
−1]; (2) the acetone solution after being in contact with TiO
2 [ν(CO) 1982 (s) 1940 (m), 1913(w) and 1881 (s) cm
−1]; (3) the powder (in nujol mull) after removal in vacuum of the solvent [ν(CO) 2000 (sh) 1978 (s) and 1917(m) cm
−1]; (4) the CH
3CN extraction from the dried powder [ν(CO) 2000 (sh) 1986 (s) and 1914 (m) cm
−1] (nominal composition of the powder 4 wt.% Au).
Similar experiments have been carried out with different cluster loadings, showing the dependence of oxidation on the cluster: TiO2 ratio.
As soon as the acetone solution of [Au4Fe4(CO)16]4− comes into contact with TiO2, the cluster reacts, resulting in more oxidized species such as [Au5Fe4(CO)16]3− or mixtures of the above-mentioned brown compounds, depending on the cluster: TiO2 ratio; the more cluster is added, the less oxidation is observed. The latter species contain an Au:Fe ratio higher than the precursor [Au4Fe4(CO)16]4− and the excess Fe is released in the form of [HFe(CO)4]− as indicated by ν(CO) at 1,881 cm−1. The oxidant is likely to be TiO2 and, therefore, Ti(IV) is partially reduced, as indicated by the fact that the solid turns from white to grey. This is also in agreement with the fact that the degree of oxidation of the cluster increases by increasing the amount of TiO2 (lower Au wt.%). Further reaction then occurs when the clusters are forced into closer contact with TiO2 by removal of the solvent in a vacuum, as indicated by the changes in the FTIR spectra.
The carbonyl species present at this point on the solid support can be extracted with a polar solvent such as CH3CN, and this makes it easier to compare their FTIR features with the ones reported for the Au-Fe clusters studied in solution. This analysis confirms that the species formed on the solid support at the end of the deposition process are oxidized clusters such as brown [Au21Fe10(CO)40]6−, [Au22Fe12(CO)48]6−, [Au28Fe14(CO)52]8−, [Au34Fe14(CO)50]10−, and green [AuFe4(CO)16]−. The brown species are favored by a high load of cluster (4–6 wt.%), whereas the latter increases by lowering the amount of cluster (1–2 wt.%). Oxidation of [HFe(CO)4]− is observed, also, yielding mainly [HFe3(CO)11]− [ν(CO) 2000 (s) cm−1], as also confirmed by independent experiments where [HFe(CO)4]− has been directly deposited on TiO2 following the same procedure.
On the contrary, it was observed that no chemical reaction occurs during any phase of the impregnation process of [NEt
4][AuFe
4(CO)
16] on TiO
2 (
Figure 1a). This salt displays a strong ν(CO) band in acetone at 2018 cm
−1, which is also maintained after adsorption on TiO
2 both in solution and in the solid state. Moreover, the intact starting cluster can be recovered from the solid by extraction with CH
3CN. These results are independent from the cluster loading.
Nevertheless, a careful inspection of the FTIR spectra of supported [NEt4][AuFe4(CO)16] before thermal decomposition, in addition to minor changes on the terminal ν(CO) region, indicates the presence in the solid state of edge-bridging carbonyls with typical ν(CO) at 1826 and 1778 cm−1, which are negligible in CH3CN solution. This difference be accounted for by the following equilibrium (1) between the two structural isomers [AuFe4(CO)16]− (all terminal CO) and [AuFe4(CO)14(μ-CO)2]− (two bridging CO).
Both these isomers were structurally characterized, and an equilibrium constant K~10
−2 in solution was established [
27]. In the present case, interaction with the TiO
2 support likely shifts the equilibrium toward the [AuFe
4(CO)
14(μ-CO)
2]
− isomer after deposition.
FTIR investigation was also carried out during preparation of the Fe-TiO2 catalysts employing [NEt4][HFe3(CO)11] as precursor, showing the occurrence of no reaction.
Very similar results were obtained on CeO2 and SBA-15, and therefore they will not be discussed any further.
2.2. Effect of Catalyst Thermal Treatment
Cluster decomposition was investigated by thermal analyses in order to better understand how this process can be influenced by temperature and atmosphere composition. Then the conditions for the thermal treatment of the samples were optimized in order to set the parameters and basically avoid or minimize Au sintering. In order to verify the cluster decomposition processes and then understand what the best conditions for catalyst thermal treatment are, a thermal study was carried out on bulk clusters by means of TGA and DSC analyses. The information obtained, on the extent of the weight losses and the temperatures at which they occur, can be useful in understanding (i) how the clusters decompose due to the effect of the temperature and (ii) what products arise from their decomposition. Performing the analyses in air and in nitrogen also helps understand the effect of the atmosphere composition. The TGA analyses of [NEt
4][HFe
3(CO)
11] (
Figure 2a) show that the decomposition process seems to take place in a single step in air, while two different processes can be distinguished under nitrogen. In both cases the results agree with quantitative formation of iron oxides.
XRD analyses on bulk samples decomposed under similar experimental conditions indicate Fe3O4 as the more likely product. Nevertheless, for this cluster salt, the temperature at which a constant weight is reached depends on whether the TGA is performed in air (approximate 60 °C) or under nitrogen (approximate 275 °C). It is well known, in fact, that when iron is in a very disperse form, as in this case, it is highly pyrophoric; then the decomposition/oxidation of the iron carbonyl cluster in air is very fast. Thus, the combustion of the iron cluster salt in air seems to be more efficient than its thermal decomposition under nitrogen.
Figure 2.
TGA analyses of bulk cluster samples: (
a) [NEt
4][HFe
3(CO)
11]; (
b) [NEt
4]
4[Au
4Fe
4(CO)
16]; (
c) [NEt
4][AuFe
4(CO)
16]. (
) in air and (
) under nitrogen.
Figure 2.
TGA analyses of bulk cluster samples: (
a) [NEt
4][HFe
3(CO)
11]; (
b) [NEt
4]
4[Au
4Fe
4(CO)
16]; (
c) [NEt
4][AuFe
4(CO)
16]. (
) in air and (
) under nitrogen.
The same analyses were performed for the bi-metallic cluster [NEt
4]
4[Au
4Fe
4(CO)
16]. In
Figure 2b it can be seen how the weight loss is similar in air and under nitrogen, and the decomposition occurs at approx. 150 °C in a single step. In this case, the final weight is 55% of the initial one; this value is lower than expected if the bi-metallic cluster decomposes to Au + Fe
2O
3 (67%) or Au + Fe
3O
4 (66%). Since it is extremely unlikely that the observed discrepancy is due to a gold loss from the bulk, probably some iron is lost in the form of the volatile Fe(CO)
5.
Different results have been obtained for [NEt
4][AuFe
4(CO)
16] decomposition (
Figure 2c). Indeed, for this cluster the molar ratio Fe/Au is higher than in [NEt
4]
4[Au
4Fe
4(CO)
16] and the oxidation state of Au is different; both these factors influence the reactivity of the cluster and its decomposition. The samples in air as well as under nitrogen reach a constant weight, which is approx. 35% of the starting one be lost in order to justify the final weight.
Regarding the TGA profile obtained in air, it is rather likely that the first process occurring at approx. 100 °C is the decarbonylation of the cluster, followed by the thermal combustion of the organic cation and iron oxidation.
Calorimetric data reported in
Figure 3 confirmed this hypothesis, indicating that a strong exothermic phenomenon occurs in air at temperatures higher than 120 °C, while only rather feeble endothermic processes occur under nitrogen at ~100 °C, 150 °C and 200 °C.
Figure 3.
DSC analyses of bulk [NEt
4][AuFe
4(CO)
16] (
) in air and (
) under nitrogen.
Figure 3.
DSC analyses of bulk [NEt
4][AuFe
4(CO)
16] (
) in air and (
) under nitrogen.
In order to optimize thermal treatment conditions for bi-metallic catalysts, a Fe0.6Au2-Ti sample was thermally treated at 400 °C in three different gas atmospheres: air, nitrogen and hydrogen.
Au crystallites, as determined from XRD line broadening, were found to significantly sinter during calcination in air (
Figure 4), but did not greatly change after thermal treatment under N
2 and H
2.
Figure 4.
Effect of the thermal treatment conditions on Au particles size for Fe0.6Au2-Ti sample.
Figure 4.
Effect of the thermal treatment conditions on Au particles size for Fe0.6Au2-Ti sample.
These results indicate an appreciable agglomeration of gold particles upon calcination in air, confirming the hypothesis that the cluster transformation in the presence of an oxidizing atmosphere could lead to uncontrolled decomposition with rapid temperature rise, causing gold nanoparticles sintering, but underscore the possibility of directing the size of gold particles by controlling cluster oxidation/decomposition. Thus the supported bi-metallic catalysts were thermally treated under nitrogen while the iron catalysts were calcined in air, because the performances of the latter materials are not affected by a similar treatment [
28].
The effect of the thermal treatment temperature on Au/FeOx-Ce performances was also investigated. In this case the Fe2.3Au2-Ce sample was prepared as usual, but after drying in air (Fe2.3Au2-Ce-D sample), one part of the sample was treated under nitrogen at 200 °C (Fe2.3Au2-Ce-D-200 sample), while the other part was characterized and used as is. These two samples were then compared with Fe2.3Au2-Ce thermally treated at 400 °C in nitrogen (Fe2.3Au2-Ce-D-400 sample).
The average size of gold particles, evaluated through the reflection at 2θ 38.2° (
Table 1), indicated the achievement of a higher dispersion by thermally treating the catalyst at 200 °C and 400 °C than with simple drying. Moreover, as also observed for TiO
2 supported samples, raising the temperature to 400 °C under nitrogen does not lead to any detrimental effect on gold particle size.
Table 1.
Surface area and Au crystal size for Fe2.3Au2-Ce samples depending on thermal treatment temperature.
Table 1.
Surface area and Au crystal size for Fe2.3Au2-Ce samples depending on thermal treatment temperature.
Catalyst | Drying conditions | Thermal treatment conditions | Surface Area (m2/g) | Au particle size (nm) |
---|
Fe2.3Au2-Ce-D | Air at 100 °C | - | - | 7.7 |
Fe2.3Au2-Ce-D-200 | Air at 100 °C | N2 at 200 °C | 64 | 4.6 |
Fe2.3Au2-Ce-D-400 | Air at 100 °C | N2 at 400 °C | 73 | 4.8 |
A beneficial effect due to the higher thermal treatment temperature in N2 was also observed on the surface area; this effect was probably due to the fact that at 200 °C the cationic part of the cluster salt (tetra-ethyl ammonium) was only partially decomposed, blocking part of support porosity. In fact, the TGA-DTA in nitrogen of [NEt4][AuFe4(CO)16] salt showed a significant weight loss after 200 °C.
Despite the absence of any chemical reaction during cluster salts impregnation on different supports, ICP analysis on the catalysts indicated that the real metal content differs significantly from the nominal one. In particular, the actual Fe loading is considerably lower than the nominal one for the catalysts prepared from all cluster salts (
Figure 5), whereas the gold content is almost as expected.
Figure 5.
ICP Fe loading
vs. nominal Fe content for titania catalysts prepared using, respectively, (
)[NEt
4][HFe
3(CO)
11]; (
)[NEt
4][AuFe
4(CO)
16] and (
)[NEt
4]
4[Au
4Fe
4(CO)
16] cluster salts.
Figure 5.
ICP Fe loading
vs. nominal Fe content for titania catalysts prepared using, respectively, (
)[NEt
4][HFe
3(CO)
11]; (
)[NEt
4][AuFe
4(CO)
16] and (
)[NEt
4]
4[Au
4Fe
4(CO)
16] cluster salts.
In all cases, this can be explained by assuming that iron is partially lost during the thermal treatment in the form of the volatile Fe(CO)5, whereas gold is completely retained. This hypothesis is corroborated by the presence of an endothermic process at approx. 110 °C in the DSC diagram registered under nitrogen, in fair agreement with Fe(CO)5 evaporation (b.p. 103 °C). Moreover, evolution of Fe(CO)5 during thermal treatment of the as-prepared catalysts has been confirmed by condensation of the gases evolved in a cold trap and their consequent analyses by FT-IR.
With the aim of better controlling the bi-metallic cluster decomposition over the support, avoiding Fe(CO)5 loss, a more detailed study was also carried out on catalyst pretreatment conditions.
In order to reduce the extent of Fe loss, the formation at low temperature of volatile Fe(CO)5 compound should be prevented or at least minimized by using more suitable operative conditions. Thus, with the intention of avoiding the Fe(CO)5 formation during cluster decomposition, a wet air flow was used in the course of Fe2.3Au2-Ce pretreatment at 100 °C, instead of dry air. In this way iron oxidation should occur simultaneously with cluster decomposition, being facilitated by the presence of water, avoiding the escape of Fe(CO)5 from the support surface.
Some of this “as-prepared” sample (Fe
2.3Au
2-Ce-W sample) was characterized and tested without performing any other treatment in order to verify if the wet air pretreatment caused any change in the catalyst properties. Moreover, part of the Fe
2.3Au
2-Ce pretreated in wet air was also thermally treated at 400 °C under nitrogen (Fe
2.3Au
2-Ce-W-400) in order to compare its performances with the standard sample (
Table 2).
Table 2.
ICP results of Fe2.3Au2-Ce samples pretreated under different conditions.
Table 2.
ICP results of Fe2.3Au2-Ce samples pretreated under different conditions.
Catalyst | Drying conditions | Thermal treatment conditions | Nominal Fe loading (wt.%) | Measured Fe loading (wt.%) |
---|
Fe2.3Au2-Ce-D | Air at 100 °C | - | 2.3 | 1.9 |
Fe2.3Au2-Ce-D-400 | Air at 100 °C | N2 at 400 °C | | 1.6 |
Fe2.3Au2-Ce-W | Flowing Wet Air | - | 2.3 |
Fe2.3Au2-Ce-W-400 | Flowing Wet Air | N2 at 400 °C | 2.0 |
These data clearly indicate that iron loss is lower for samples pretreated in wet air, confirming the possibility of limiting the evolution of Fe(CO)5 and consequently reducing the discrepancy between the theoretical and the real iron loading observed for samples treated in dry air.
Wet air pretreatment does not influence iron dispersion on ceria. Nevertheless, a detrimental effect on gold dispersion was observed (
Table 3).
In fact, gold crystallites present on samples pretreated in wet-air are larger than the ones dispersed on samples pretreated in dry air. Moreover, the gold re-dispersion with high temperature observed for this latter system was absent in the material pretreated in wet air.
The worse gold dispersion observed with this last treatment may be related to the fast iron oxidation caused by water, which could lead to higher local temperature, responsible for gold sintering.
Table 3.
Surface area and Au crystal size of Fe2.3Au2-Ce samples pretreated under different conditions.
Table 3.
Surface area and Au crystal size of Fe2.3Au2-Ce samples pretreated under different conditions.
Catalyst | Surface Area (m2/g) | Au particle size (nm) |
---|
Fe2.3Au2-Ce-D | - | 7.7 |
Fe2.3Au2-Ce-D-400 | 73 | 4.8 |
Fe2.3Au2-Ce-W | - | 8.5 |
Fe2.3Au2-Ce-W-400 | 70 | 7.1 |
2.3. Characterization of Supported Gold/Iron Catalysts
Typical diffraction patterns of freshly prepared catalysts are shown in
Figure 6. These analyses showed that the support phase (CeO
2 or TiO
2) is predominant; no detectable crystallite formation of any kind of ferric oxide or hydroxide was observed. For metallic Au, broad peaks were detected at about 38° and 44°.
Figure 6.
XRD patterns of bimetallic titania (a) and ceria (b) catalysts prepared with [NEt4][AuFe4(CO)16].
Figure 6.
XRD patterns of bimetallic titania (a) and ceria (b) catalysts prepared with [NEt4][AuFe4(CO)16].
The average size of gold particles is given in
Table 4. The corresponding size after catalytic operation is also shown in the same table. It is evident that the presence of an increasing amount of active phase leads to a slight increase of the average size of gold particles, but these results also indicate that the Au crystal growth can be reasonably controlled by utilizing the optimized thermal treatment conditions in N
2.
Table 4.
Au particle size for fresh and spent Au-FeOx-Ti and Au-FeOx-Ce catalysts.
Table 4.
Au particle size for fresh and spent Au-FeOx-Ti and Au-FeOx-Ce catalysts.
Catalyst | Au particle size Fresh (nm) | Au particle size Spent (nm) |
---|
Fe0.6Au2-Ti | 6.0 | 6.0 |
Fe1.2Au4-Ti | 6.6 | 6.7 |
Fe1.8Au6-Ti | 7.4 | 7.6 |
Fe2.3Au2-Ti | <3.0 | 3.7 |
Fe4.5Au4-Ti | 6.9 | 8.7 |
Fe6.8Au6-Ti | 7.0 | 7.3 |
Fe1.2Au1-Ce | <3.0 | 3.0 |
Fe2.3Au2-Ce | 4.8 | 5.0 |
Fe4.5Au4-Ce | 9.0 | 9.1 |
Moreover, data regarding the degree of gold dispersion after catalytic operations showed only minor changes compared to the corresponding as-prepared samples, indicating a high stability of the prepared catalysts. Similar results have been obtained for FeOx-supported systems with no detectable peaks related to crystallite of any kind of ferric oxides or hydroxide.
The dispersed Au/FeO
x particles on different support were studied by combining structural information from high-resolution TEM imaging and elemental mapping using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDX. As an example, a clear image of gold nanoparticle over support is shown in
Figure 7 with a HRTEM image obtained on Fe
2.3Au
2-Ce catalyst. Unfortunately, the presence of metallic iron or other iron oxides species on the catalysts could not be unambiguously identified on any of studied systems due to the fact that most of the reflections due to gold, ceria and iron crystals superimpose within the experimental precision achieved on the recorded diffraction patterns. However, with EDX mapping it was possible to observe the distribution of even the non-crystalline Fe species, which appeared to be uniformly dispersed all over the different supports [
29,
30]. Thus, as a general conclusion, TEM results were fairly in agreement with the findings of the XRD studies and suggest that iron oxide was well dispersed and not segregated either before or after the catalytic reaction. Moreover, the distribution of the gold particle dimensions, calculated from HAADF images, was also in good agreement with the average Au size estimated by XRD.
Figure 7.
High Resolution TEM image of Fe2.3Au2-Ce sample.
Figure 7.
High Resolution TEM image of Fe2.3Au2-Ce sample.
The surface content and the oxidation state of the catalysts were characterized by XPS before and after catalytic tests for all studied materials (
Table 5); increasing the Au/support and Fe/support ratios may be correlated with the increased loading of these elements in the catalysts. However, the Au/Fe ratio was systematically below the bulk ratio determined by ICP. These results suggest that the atoms of iron cover the atoms of gold. Indeed, in this kind of configuration, atoms of gold will be less available for XPS analyses and will thus take less part in the signal/spectra. Concerning the oxidation state of different elements, the binding energy of gold always corresponds to the metallic state (Au°). Moreover, the catalytic test does not seem to have an influence on the oxidation state of gold. The Fe 2p
3/2 peak and the iron binding energy were similar in the presence and in the absence of gold in the catalyst. This peak exhibits a shoulder at about 710 eV and can be fitted with two peaks which, according to their binding energy, are attributed to Fe
3+ (711 eV) and Fe
2+ (709.8 eV) species [
31]. Thus, the iron oxide layer appeared to be composed of a mixture of Fe
2O
3 and FeO or Fe
3O
4. According to the TPR data, the presence at the surface of some amount of iron hydroxide cannot be ruled out either.
Table 5.
Atomic concentration ratios and binding energy of gold and iron in the catalysts.
Table 5.
Atomic concentration ratios and binding energy of gold and iron in the catalysts.
Catalysts | Atomic concentration ratios | Binding Energy (eV) |
---|
Au/Fe XPS | Au/Fe ICP | Au 4f | Fe 2p |
---|
Fe2.3Au2-Ti | 0.12 | 0.52 | 83.9 | 710.5 |
Fe2.3Au2-Ti-s | 0.16 | 0.52 | 83.6 | 710.2 |
Fe4.5Au4-Ti | 0.07 | 0.47 | 84.0 | 710.6 |
Fe4.5Au4-Ti-s | 0.07 | 0.47 | 83.9 | 710.3 |
Fe4.5-Ti | / | / | / | 710.6 |
Fe1.2Au1-Ce | 0.03 | 0.40 | 83.9 | 710.6 |
Fe2.3Au2-Ce | 0.04 | 0.30 | 83.8 | 710.4 |
Fe4.5Au4-Ce | 0.03 | 0.24 | 83.8 | 710.4 |
Temperature programmed reduction is a valuable technique in the understanding of the redox behavior of the catalysts. In scientific literature, a sizable effort has been devoted to the study of the reduction of unsupported and supported iron oxides [
32]. Nevertheless, the nature of the total process is extremely complex and may vary with the physicochemical characteristics of the iron oxide or with the conditions of its reduction [
33].
The reduction of the hydroxylated iron oxide species and that of hematite to Fe
3O
4 were observed for all FeO
x catalysts. Moreover, since the hydrogen consumed was found to be significantly greater than that necessary for the reduction [
29,
30], especially for catalysts at low iron content, it may be suggested that some reduction associated with the supports, whether TiO
2 or CeO
2, might overlap that of iron oxides. It is, in fact, known [
34,
35] that the presence of some metals can influence the reduction of oxygen species in TiO
2, and a strong interaction generally occurs between iron oxides and ceria [
34,
35].
The preparation of the catalysts by utilizing the gold/iron carbonyl cluster leads to the formation of species with very high reducibility. Such as an example we report in
Figure 8 the comparison among H
2-TPR profile of support as is (CeO
2), monometallic Fe
1.2-Ce and Fe
1.2Au
1-Ce samples. In the case of bi-component ceria supported catalyst, gold presence increases iron reducibility and strongly influences CeO
2 reduction. In all reduction profiles obtained for bi-component samples, a peak appears at around 150 °C, confirming that gold/iron carbonyl cluster deposition on ceria leads to the formation of highly reducible iron and ceria species. TPR data reported in literature indicate that gold can cause a decrease in the strength of the surface Fe-O and Ce-O bonds adjacent to gold atoms, thus leading to a higher surface lattice oxygen mobility and therefore to a higher reactivity of these oxygens [
6,
36].
Figure 8.
Comparison among H
2-TPR profile of (
) CeO
2, (
) Fe
1.2-Ce and (
) Fe
1.2Au
1-Ce samples.
Figure 8.
Comparison among H
2-TPR profile of (
) CeO
2, (
) Fe
1.2-Ce and (
) Fe
1.2Au
1-Ce samples.