1.2 Characterization of Monometallic Systems

The SBET and total pore volume (obtained at 0.95 of P/P°) of calcined Co(x)/SiO2 and Fe(x)/SiO2 catalysts are shown in Table 1. As shown, the SBET decreases slightly with the Co and Fe loading in both systems. This result suggests that Co and Fe species were highly dispersed into the pores of the silica substrate and that pore blockage was almost absent.

In Table 1, the cobalt and iron particle average size estimated from TEM micro­graphs of Co(x)/SiO2 and Fe(x)/SiO2 catalysts are summarized. In general, both catalyst systems present a broadening in the metal particles size distribution upon increasing Co and Fe loading and the average particle was found to increase gradu­ally with metals loading from 37 to 52 nm and from 29 to 40 nm for Co and Fe, respectively. Also, Table 1 shows that Fe catalysts display slightly lower metal par­ticles size comparing to Co counterpart.

TPR profiles of the oxide precursors for both catalytic systems are given in Fig. 1 and show that the reduction process of the Co(x)/SiO2 catalysts occurs in two distinct stages, while for the Fe(x)/SiO2 catalysts occurs according to the three char­acteristic steps of the reduction of Fe2O3 species. For the Co(x)/SiO2 catalysts the first peak centred at 350°C is ascribed to the transformation of Co3O4 to CoO, whereas the second stage centred to 406°C represents the reduction of CoO to Co [20, 21]. The relative intensity and width of the second reduction peak increases with Co-loading in a higher extent than the first peak, suggesting a higher reduction degree of CoO to metallic Co with an increase of the average diameter of Co3 O4 particles, in agreement with results reported previously by Martinez et al. [22] . This is also in agreement with those results obtained by TEM. On the other hand, in Fig. 1b, the first peak centred at 420°C in the Fe(x)/SiO2 catalysts is related to the transformation of Fe2O3 to Fe3O4, the second peak centred to 615°C represents the reduction of Fe3O4 to FeO and the third peak centred around 720°C corresponds to

the transformation of FeO to metallic Fe [23, 24]. The intensities of the three peaks gradually increase with the Fe loading. Figure 1b also shows that the position of the maximum reduction of three peaks does not change significantly as iron loading increases. This observation suggests that iron species are homogeneously dispersed on the surface of the silica carrier.

XPS results of reduced Fe(x)/SiO2 and Co(x)/SiO2 catalysts are summarized in Table 2. XP spectra showed that all catalysts in the Fe 2p and Co 2p region display the doublet corresponding to Fe 2p3/2-Fe 2p1/2 and Co 2p3/2-Co 2p1/2 for iron and cobalt species, respectively. Table 2 summarizes the most intense peaks of each doublet. Thus, the peak at 707.3 eV represents a signal due to metallic iron (2p3/2) [25] and the peak at710.5.3 eV is due to iron oxide (2p3/2) [26]. Due to no satellite line is observed somewhere around 719.0 eV indicative of the presence of Fe3+ ions, it is inferred that the iron oxides responsible for the peak around 710.5 eV in the reduced catalysts comes from partially reduced iron oxides, such as Fe3O4 (magne­tite) species. The relative intensities of the two Fe 2p components (peaks at 707.3 and 710.5 eV) are also included in Table 2 (in parentheses). It can be seen that the fraction of metallic iron determined on the surface region of these catalysts is much lower than the fraction of Fe oxides. In addition, the fraction of reduced iron to metallic state (peak at 707.3 eV) increases upon increasing the iron loading in the catalysts. This behaviour suggests that at low Fe content, the ionic Fe species strongly interact with the SiO2 surface and therefore are difficult to reduce to the metal state under the experimental conditions of this work. On the contrary, in the catalysts with higher Fe loadings a higher proportion of tridimensional iron oxide structures are developed and therefore they can be easily reduced to zero valent

oxidation state. On the other hand, the most intense Co2p3/2 peak was fitted to two components: one at 778.0 eV belonging to metallic cobalt [27] and at 780.0 eV originated from cobalt oxide [28]. The relative intensities of the two Co 2p peaks were calculated and the values obtained are also showed in Table 2. It can be seen that the proportion of Co metallic phase increases gradually with Co-loading whereas the cobalt oxide phase follows an opposite trend. This behaviour suggests that at low Co content Co+2 interacts strongly with the SiO2 support and is not completely reduced to metallic cobalt. Conversely, at higher Co-loading, cobalt is present in the form of larger particles which are easier to reduce.

In order to examine the extent of dispersion of the active phase over the silica surface, the Fe/Si and Co/Si atomic ratio were calculated (see Table 2). The variation of the Fe/Si and Co/Si atomic surface ratio as a function of metal-loading of the catalysts is shown in Fig. 2. Clearly, the Fe phase appears as rather large crystallites (around 32 nm) up to 15% of Fe. This trend is similar to that found with Co(x)/SiO2 catalysts, where the activity increased almost linearly as Co-loading increases, reaching the maximum at about 20 wt% Co (around of 41 nm) and then levelled off. The observed deviation from linearity above 15% of Fe and 20% of Co suggests the formation of large segregated crystalline particles mainly on the external surface of the silica particles. The higher Fe/Si ratios were observed at higher Fe loading, especially for Fe(20)/SiO2 and Fe(25)/SiO2 catalysts. These results are likely due to the presence of a high density iron oxide particles and therefore keep less exposed the silica surface to incident photons. Similar behaviour was observed for Co(25)/ SiO2 and Co(30)/SiO2 catalysts.