2.1 Catalysts preparation

Fe₂O₃/SiO₂, Fe₂O₃/TiO₂ and Fe₂O₃/MgO systems were prepared by the incipient wetness impregnation method. The 30 wt.% Fe₂O₃/support catalysts were prepared by impregnation of SiO₂, TiO₂ or MgO with an appropriate concentration of Fe(NO₃)₃.9 H₂O. After drying at 80 °C overnight, the resulting solid was calcined in air at 400 °C for 2.5 h [12].

Bulk Fe₂O₃ catalyst was synthesized by the co-precipitation method. The starting reagents of high purity FeSO₄.7 H₂O and Fe₂(SO₄)₃.5 H₂O with a molar ratio (1/1) were dissolved in water to which 100 mL of NH₄OH was added as a precipitant agent. The mixture was vigorously stirred for 30 min. The resulting powder was washed several times with deionized water, and dried at 110 °C. Subsequently, the powder was calcined at 400 °C in air (33 mL/min) with a rate of 5 °C/mn for 2.5 h [9].

2.2 Apparatus and procedure

X-ray powder diffraction patterns were obtained with a Philips PW 1050/81 automated powder goniometer, equipped with diffracted-beam graphite monochromatic Cu (K_α) radiation.

H₂− TPR profiles were determined using a laboratory-made apparatus equipped with a TCD detector. The catalyst sample (200 mg) was loaded into a U-shape quartz reactor, then heated at 200 °C under argon flow with a rate of 5 °C/min for 1 h and cooled to room temperature. Next, the sample was submitted to a flow of 5% H₂/Ar from room temperature to 800 °C. A cold trap was placed downstream of the sample to capture any water that evolved from catalyst prior to entering the TCD detector.

Catalysts were tested under atmospheric pressure between 350 and 450 °C after being activated in situ under a flow of 48% H₂/N₂ using an amount of catalyst of 250 mg for all runs. After reduction, the reacting gases (H₂O/CO = 4.4) were introduced into the reactor at a total flow rate of 33 mL/min. Before each analysis, the effluent passed through a water-trap at 0 °C to remove the water of the reaction. The gas composition was analyzed by a TCD chromatograph containing two 4 m carbosieve B columns (1/8 inch, 100 to 200 meshes).

In situ DRIFT studies were performed on a Nicolet Magna IRTF spectrometer with OMNIC software. The reaction was performed in a high temperature Spectratech cell equipped with a ZnSe window. The sample (∼50 mg) was first treated in a flow of (∼35 mL/min) H₂ at 400 °C for 30 min (high-temperature treatment) before each experiment. The heating rate during the treatment was 10 °C/min. After this treatment, the sample was cooled under He and then put in contact with the reacting mixture CO/H₂O at increasing temperature (25–450 °C). Unless stated otherwise, infrared (IR) spectra were recorded against a background of the sample at the reaction temperature under flowing He. IR spectra were recorded with co-addition of 64 scans in single-beam spectra or absorbance spectra, by applying a resolution of 4 cm⁻¹.

3.1 XRD and H₂− TPR measurements

The X-ray phases of Fe₂O₃-only and Fe₂O₃-promoted freshly calcined samples are given in Table 1. α-Fe₂O₃ hematite is the mainly crystalline phase in Fe₂O₃, Fe₂O₃/SiO₂ and Fe₂O₃/TiO₂. For the system doped with MgO, at higher Fe₂O₃ loading (30 wt.%) the band characteristic of MgO decreased while new bands assigned to MgFe₂O₄ are formed [9]. This indicates that iron oxide is in strong interaction with MgO, leading to a mixed oxide formation.

H₂− TPR results of the four catalysts are reported in Fig. 1 and their peak maxima are summarized in Table 1. In all cases, three peaks of hydrogen consumption in a temperature range of 25–800 °C are observed [13,14]. For Fe₂O₃-only, the lower reduction temperature region (446 °C) of Fe₂O₃ is attributed to the reduction of Fe₂O₃ to Fe₃O₄ while the higher temperature region (606–684 °C) is credited to the reduction of Fe₃O₄ to FeO and Fe. The low temperature peaks of Fe₂O₃/TiO₂ and Fe₂O₃/MgO shifted to 340 and 368 °C respectively. On the other hand, MgO− promoted catalyst involved a shift of the reduction temperature to lower values as depicted in Fig. 1, curve c. XRD studies showed that MgFe₂O₄ crystalline phase is present in Fe₂O₃/MgO. Indeed, the presence of this phase improves the reducibility of Fe₂O₃/MgO. More detailed studies were reported elsewhere [9].

3.2 Catalytic properties in CO + H₂O reaction

The only products of the WGS reaction were found to be carbon dioxide and hydrogen, no methane being detected. No reaction occurred on every support alone. The WGS reaction conversion of the tested catalysts (unsupported and supported Fe₂O₃) was expressed by the conversion of CO to CO₂, measured at 450 °C, and expressed per surface area unit, as presented in Table 1. The specific activity appears to span over a large range, the most active system (Fe₂O₃/MgO) displaying a rate 100 times higher more than the least active (Fe₂O₃/SiO₂). The four tested systems were ranked as follows: Fe₂O₃/MgO >> Fe₂O₃/TiO₂ > Fe₂O₃ >> Fe₂O₃/SiO₂. The activation energy was estimated to be 97.6 kJ/mol in the temperature range of 350–450 °C over the Fe₂O₃/MgO catalyst. This value is in accord with the value reported by Rethwisch et al. (106 kJ/mol) [1].

As can be seen in Table 1 and Fig. 2, a direct relationship can be observed between catalytic activity and acid/base properties of our catalysts: Fe₂O₃/MgO >> Fe₂O₃/TiO₂ > Fe₂O₃ >> Fe₂O₃/SiO₂. Basic properties of catalysts are given here in terms of dehydrogenation of isopropanol: CH₃-CHOH-CH₃ → CH₃-CO-CH₃ + H₂ at a temperature reaction of 250 °C [12]. The dependence found that oxides which are basic were more active than those which are acidic or amphoprotic and can be explained in terms of the higher activity of surface species like formates [1]. It is suggested, by these authors, that formate species readily formed and dehydrogenated to provide CO₂ and H₂.

One can see that the increase in intensity of the 2420–2330 cm⁻¹ band (Fig. 3b–d), as well as the decrease of formate species bands with raising the temperature to 450 °C illustrated the decomposition of adsorbed HCOO species to CO₂ and H₂ [11]. This decomposition is facilitated by the weaker metal-oxygen bond in the basic oxides. In the opposite, on an acidic sample, such as Fe₂O₃/SiO₂, no formate intermediates were detected and the intrinsic rate was the lowest of the four systems.

3.3 In situ DRIFT study of surface species

Fig. 3 shows the change in DRIFT spectra recorded for the Fe₂O₃/MgO system after reduction and after contact with the CO/H₂O gas mixture at increasing reaction temperatures. The spectrum (b), recorded after exposure of the reduced catalyst to CO/H₂O at 350 °C, showed a very prominent broad OH adsorption band with a maximum at about 3570 cm⁻¹; as expected from H₂O interaction with mild basic OH groups of MgO surface along with a sharp band at 3730 cm⁻¹ typical of isolated basic Mg(OH) groups over the surface. Other OH bands are observed at 3130, and 3040 cm⁻¹ which might be assigned to acidic FeOH groups. Moreover, small bands are seen around 2170–2080 cm⁻¹ and in the 2420–2330 cm⁻¹ region. The 2170–2080 cm⁻¹ bands are characteristic of the CO gaseous while the doublet at the 2400–2330 cm⁻¹ region corresponds to adsorbed and/or gaseous CO₂ rotation-vibration P and R bands. The increase of the latter with temperature attests that the shift reaction does proceed in the DRIFT reactor, in line with the outlet gas analysis. The inlet of CO/H₂O on reduced Fe₂O₃/MgO produces, as well as at 350 °C, bands at 1720, 1630, and 1370 cm⁻¹, which diminished synchronously by increasing temperature from 350 to 450 °C. The synchronic dynamics of these 3 bands tends to indicate that they arose from the same OCO containing species. By observing simultaneously some C-H stretching vibrations at 2850–2700 cm⁻¹ associated with the 1720, 1630, and 1370 cm⁻¹ OCO region bands, we propose, in agreement with Tabakova et al. [15], that the diminution of the band at 2170 cm⁻¹ as well as the decrease in intensity of the OH band at 3570 cm⁻¹ (from 350 to 450 °C) can be correlated with the production of formate species. As a matter of fact, formate species involve symmetric and asymmetric OCO band vibrations on the surface of the catalyst. The asymmetric OCO band is expected at 1580 cm⁻¹ [15,16], and the symmetric one at 1375 cm⁻¹. The vibrations of the OCO bands are accompanied by C-H stretching vibrations at 2850–2700 cm⁻¹.

In other terms, it is proposed that the carbonyl adspecies bonded to iron sites might react with the magnesia basic OH groups located at the metal-support interface to produce formate intermediate species. Formate adspecies are often considered as active intermediates in the water-gas shift reaction over supported metals and also on support alone [1,11,17,18] and more precisely over MgFe₂O₄ in [1] or over pure MgO in [19].

Fig. 4 reports the DRIFT spectra of the reduced Fe₂O₃/TiO₂ system after exposure to the CO/H₂O gas mixture at increasing temperature. At variance with the previously reported system, no significant IR contribution from formate species was observed on the reduced Fe₂O₃/TiO₂ catalyst after being contacted with CO/H₂O. This can be deduced from the absence of vibration characteristic of C-H bonds in the 2850–2700 cm⁻¹ range. In line with this observation, only weak formate bands were observed on TiO₂ by Rethwisch and Dumesic [1]. The broad band at 3410 cm⁻¹ observed essentially at 350 °C, and then decreasing at higher temperature, is characteristic of Ti-OH hydroxyl groups. After contacting Fe₂O₃/TiO₂ at 350 °C with CO/H₂O gases, two new bands are detected in the carbonate/carboxylate species region, specifically at 1630 and 1550 cm⁻¹ [20]. In addition, Fig. 4 (curve a) shows the doublet at 2170–2080 cm⁻¹ characteristic of the CO gaseous together with the doublet at ∼ 2370 and 2320 cm⁻¹ characteristic of adsorbed and/or gaseous CO₂ [20,21]. By increasing temperature the intensities of the bands at 3410 cm⁻¹, at 2080–2170 cm⁻¹, at 1630 and at 1550 cm⁻¹ decreased but the intensity of the bands at 2370–2320 cm⁻¹ increased. These trends indicate that while WGS proceeds, the surface is progressively depleted in adsorbed species while the gaseous CO concentration increases. Therefore, no clear evidence of a WGS route via formate intermediates was obtained for this case of Fe₂O₃/TiO₂ catalyst. The weaker basicity of the TiO₂ surface and/or an inadequate interface between metal and oxide phases might be the origin of this statement.

Reduced Fe₂O₃ and Fe₂O₃/SiO₂ (Fig. 5) catalysts produced spectroscopic features significantly different from those of Fe₂O₃/MgO and Fe₂O₃/TiO₂, since no formate or carbonate bands were observed either on the unsupported system or on the silica supported one. This statement quite agrees with the expected feature that no formate/carbonate species can form on an acid oxide (silica or magnetite), under any conditions, in line with [1,11]. Therefore, it might be inferred that a redox or regenerative shift mechanism is likely to apply for these two cases, as proposed over magnetite in [1]. We note in addition that no sufficiently resolved IR spectra could be recorded for this last case due to sample black color after reduction.