Controlled Reduction of LaFexMnyMozO 3 / Al 2 O 3 Composites to Produce Highly Dispersed and Stable Fe 0 Catalysts : a Mössbauer Investigation

In this work, controlled reduction of perovskites supported on Al 2 O 3 was used to prepare thermally stable nanodispersed iron catalysts based on Fe/La 2 O 3 /Al 2 O 3 . The perovskites composites LaFe 0.90 Mn 0.08 Mo 0.02 O 3 (25, 33 and 50 wt (%)) /Al 2 O 3 and LaFe 0.90 Mn 0.1 O 3 (25 wt (%)) /Al 2 O 3 were prepared and characterized by XRD, BET, TPR, SEM and Mössbauer spectroscopy. XRD for unsupported perovskite showed the formation of a single phase perovskite structure. The Mössbauer spectra of the perovskites were fitted with hyperfine field distribution model for the perovskite. Supported perovskites on Al 2 O 3 showed a decrease of the hyperfine field in respect to unsupported perovskite, due to decrease of particle size and dispersion of the Fe specimens on the support. Also showed broaden lines and relaxation effects due to the small particle size. To produce the Fe catalyst, the composite perovskite(25%)/Al 2 O 3 was reduced with H 2 at 900, 1000 and 1100 °C for 1 hour. XRD data indicated the formation of Fe catalyst with particles sizes of ca. 35 nm. The Mössbauer spectrum showed the formation of metallic iron and doublets corresponding to species of octahedric Fe and Fe sites dispersed on Al 2 O 3 . These catalysts showed improved stability towards sintering even upon treatment at 1000 and 1100 °C under H 2 .

Metallic iron catalysts have been investigated for many different applications, e.g.permeable reactive barriers 1 , reduction of organochloro 2 , and Fenton chemistry 3,4 .More recently, supported iron catalysts have been intensively investigated for the synthesis of carbon nanotubes by chemical vapor deposition.For these catalysts the control of the particle size and the stability towards sintering are key factors for the catalytic performance.Different strategies have been explored recently to prepare uniformly distributed metallic catalysts for the synthesis of carbon nanotubes.These include work on zeolite templates [5][6][7][8][9][10] , the decomposition of soluble precursors-based molecular clusters containing Fe and Mo 10,11 or metal carbonyl complexes [Fe(CO) 5 ] and [Mo(CO) 6 ] embedded in a long-chain carboxylic acid/ amine mixture 12 , and the controlled reduction of different oxides or solid oxide solutions 13 .
In this work, we have investigated a new approach to produce stable and highly dispersed Fe 0 catalysts.The Fe particles are formed by the reduction of the precursors LaFe x Mn y Mo z O 3 perovskites dispersed on the surface of an Al 2 O 3 support (Figure 1).This catalyst has been recently investigated for the synthesis of single wall carbon nanotube showing promising results 14 .
This preparation method shows several potential advantages: i) should allow a good control of the metallic particle size distribution, ii) should improve the thermal stability to the catalyst due to a strong matrix insulation effect (La 2 O 3 and Al 2 O 3 should keep Fe o particles well separated), iii) the precursor elements and stoichiometry can be adjusted to produce catalysts with different metals and different ratios, iv) the metals present in the precursor, e.g.Fe and Mo, should be homogeneously distributed throughout the oxide and produce well-defined metallic particles, and v) the dispersion of the metal particle can be further controlled by the reduction conditions.

Experimental Procedures
Two composites were prepared: LaFe 0.90 Mn 0.10 O 3 (25 wt (%))/ Al 2 O 3 and LaFe 0.90 Mn 0.08 Mo 0.02 O 3 (25, 33 and 50 wt (%))/ Al 2 O 3 .Alumina nanopowder Aldrich (<50 nm particle size and 35 m 2 .g - ) was used.The perovskites were prepared by the reaction of 0.5 mol of citric acid (CA) dissolved in 2 mol of water at 60 °C, followed by the addition of 1 mmol of La(NO 3 ) 3 .6H 2 O and different proportions of the other metals such as Fe(NO 3 ) 3 .9H 2 O, Mn(NO 3 ) 2 .4H 2 O and Mo(acac) 2 O 2 (acac acethylacetonate) in order to produce the desired stoichiometry.The mixture was stirred for about 2 hours, until a clear orange solution of the stable metal-CA complexes is obtained.After the complete dissolution, 400 mmol of ethylene glycol (EG) followed by the addition of the Al 2 O 3 nanopowder in amounts to obtain composites with 25, 33 and 50 wt (%) of perovskite on alumina.The suspension was continuously stirred while the temperature was slowly increased to 90 °C.This step removes the excess of water and allows the polyesterification reaction between CA and EG to be further activated.The prolonged heating at 90 °C occurs for 7 hours and resulted in a viscous orange mass 15 .This resin was then treated at 400-450 °C in air for 2 hours for the charring.The final product, a dark brown powder was ground and then calcined at 800 °C in air for 6 hours.
The powder XRD data were obtained in a Rigaku model Geigerflex equipment using Co Kα radiation scanning from 10 to 80° (2θ) at a scan rate of 4° min -1 .Silicon was used as an external standard.
The crystallite sizes were determined by Scherrer's equation through the width of the Bragg reflection at half maximum.The surface area was determined by nitrogen adsorption using the BET method with a 22 cycles N 2 adsorption/desorption in an Autosorb 1 Quantachrome instrument.The transmission Mössbauer spectroscopy experiments were carried out in a spectrometer CMTE model MA250 with a 57 Co/Rh source at room temperature using α-Fe as a reference.The TPR (temperature programmed reduction) analysis was performed in a CHEM BET 3000 TPR using H 2 (8% in N 2 ) with heating rate of 10 °C min -1 .The H 2 consumption was obtained after calibration of the TPR system using a CuO standard.Scanning electron microscopy (SEM) analysis was done using a Jeol JSM 840A.
The reduction of the perovskites was carried out in a quartz tube of 40 mm diameter with a batch of catalyst (50 mg) placed in the central part and heated to the reduction temperatures in the range of 900, 1000 and 1100 °C at 10 °C min -1 under an H 2 /Ar flow (100/800 mL.min -1 ) for 1 hour.The system was cooled down to room temperature under a constant H 2 /Ar flow.The reduced catalyst still under H 2 flow at room temperature was completely immersed and covered with dichlorobenzene liquid to prevent oxidation by O 2 from the air.
The main peaks in Figure 2 are related to the perovskite structure, i.e.LaMO 3 (2θ = 26.0,37.3, 46.2, 54.0, 61.0 and 67.6°).A small shift in Figure 2 of the XRD peaks to lower diffraction angles have been observed due of the incorporation of the Mo into the structure.Analyses of the lattice parameters of the unsupported LaFe 0.90 Mn 0.10 O 3 and LaFe 0.90 Mn 0.08 Mo 0.02 O 3 perovskite showed crystal structures best fitted to a pseudo-cubic arrangement 16,17 .Figure 3 shows the XRD patterns for the LaFe 0.90 Mn 0.08 Mo 0.02 O 3 perovskite supported on Al 2 O 3 at different concentrations of perovskite.We note that lower perovskite contents produce larger peaks related to small particle size and a good dispersion on the support.Data of surface area, crystallite size and   lattice parameter for the synthesized perovskite are shown in Table 1.The crystallite size found for the unsupported LaFe 0.90 Mn 0.08 Mo 0.02 O 3 was 202 Å.On the other hand, for the supported perovskites significantly smaller crystallites were obtained, i.e. 174, 165 and 149 Å for LaFe 0.90 Mn 0.08 Mo 0.02 O 3 50, 33 and 25 wt(%), respectively (Table 1).
BET surface area slowly decreases from 24 to 23 and 21 m 2 g -1 as the perovskite content increases.This results indicate that the prepared perovskite should have small surface area compared to the Al 2 O 3 (ca.35 m 2 .g - ) and contributes to decrease the composite surface area.
Mössbauer spectra of the samples (Figure 4 and 5) were fitted with a hyperfine field distribution model for octahedric Fe 3+ coordination in the structure of the perovskite.It is possible to observe that the supported perovskites (Figure 4b, 4c, 4d and 5b) show broaden lines relaxation effects, central doublets and higher range of field distribution, compared with unsupported perovskite.These effects are related to the small crystallite size and Fe 3+ specimens highly dispersed on the support.
The Hyperfine parameters obtained from the Mössbauer spectra for the pure perovskites and supported on Al 2 O 3 are shown in Table 2.The hyperfine parameters for the supported perovskites indicate the presence of two signals for octahedric Fe 3+ specimens coordination and a decrease of the hyperfine field.These results suggest that the support promotes a strong dispersion of the Fe 3+ specimens.The Mössbauer parameters (Table 2) of the supported perovskites 50, 33 and 25 wt% in perovskite content (Figure 4b, 4c and 4d) showed that the relative sub-spectral area of sextets were 80, 69 and 61% respectively, suggesting that higher alumina contents cause a higher dispersion of the Fe 3+ species.The values of the quadrupole shift close to 0 corroborate with the pseudo-cubical symmetry proposal for these perovskites.

Controlled reduction of the prepared perovskite
Temperature Programmed Reduction (TPR) experiments were performed to investigate the reducibility of the different perovskites (Figure 6).
TPR profile for the unsupported perovskite (Figure 6) shows a set of broad peaks at 300-500 °C probably assigned to the reduction of Fe 3+ , Mo 4+ , Mn 4+ and some Mn 3+ .For the reduction at temperatures up to 500 °C the oxygen is removed from the material but the perovskite structure is still maintained.At temperatures higher 650 °C it is observed the beginning of a H 2 consumption that does not end up to 800 °C.It has been reported previously that LaFeO 3 perovskites are remarkably stable towards reduction [18][19][20] .Therefore, this hydrogen consumption is likely related to the second step reduction of the Fe perovskite leading to the collapse of the perovskite structure with the formation of Fe metal, La 2 O 3 and small amounts of MnO and Mo.
However, this reduction process finishes only at temperatures higher than 1000 °C under the TPR conditions.It is interesting to observe for the composite perovskite 33 wt (%)Al 2 O 3 new reduction peaks at approximately 600, 730 and 880 °C (Figure 6).Also for the perovskite 25 wt (%) on Al 2 O 3 a new and intense reduction peak at 700 °C appears.The presence of these new peaks at lower temperatures compared with pure perovskite suggests that the supported perovskites can be more easily reduced.This is likely related to the smaller particle size which should cause an increase in reactivity.
The influence of the reduction temperature to produce stable and highly disperse Fe 0 catalysts with particle size control was investigated.The reductions were carried out with H 2 for 1 hour at three different temperatures, i.e. 900, 1000 and 1100 °C, using the 25 wt (%) LaFe 0.90 Mn 0.08 Mo 0.02 O 3 perovskite on Al 2 O 3 .
The XRD analyses of the precursor LaFe 0.90 Mn 0.08 Mo 0.02 O 3 (25%)/ Al 2 O 3 are presented in Figure 7.The results show that Fe o (see XRD at 2θ = 52 o ) has sintered to form crystallites with average sizes of 37, 35 and 35 nm for the 900, 1000 and 1100 °C treatments, respectively.It is important to observe that the iron XRD peak is composed of different components as shown by the expanded diffractogram (Figure 7 detail).Therefore, the particle size estimation should be analyzed with caution.
Moreover, the XRD peaks of the perovskite after reduction have shifted by 1.0° toward higher 2θ, which is a clear indication that a new perovskite phase, probably LaAlO 3 was formed (see Figure 1).Literature data showed that La 2 O 3 can react with Al 2 O 3 at temperatures near 900 °C to form LaAlO 3 [ 21 ].This reaction follows the steps: (2) Crystallite size of LaAlO 3 have been estimated by the Scherrer equation to give values of 125, 124 and 126 Å after reduction at 900, 1000 and 1100 °C, respectively.
The reduced perovskites were also studied by SEM. Figure 8, shows that the composites perovskite/Al 2 O 3 significantly changed their morphology after the reductions at different temperatures.At 900 °C the material is composed of particle agglomerates with several spaces within the particles.As the reduction temperature increased the dispersed agglomerates tend to compact and the voids tend to disappear.
Mössbauer spectra of the reduced perovskites are shown in Figures 9 and the analyses of the hyperfine parameters are given in Table 3.After the reduction of the perovskite LaFe 0.90 Mn 0.08 Mo 0.02 O 3 (25%)/ Al 2 O 3 at 900 °C with H 2 (Figure 9b) it can be observed the reduction of Fe 3+ to Fe 2+ and Fe 0 , as showed in the Mössbauer spectra by the presence of one sextet with Bhf = 33.1 T corresponding to metallic iron, two doublets of octahedric Fe 2+ and one doublet of octahedric Fe 3+ coordination.After the reduction of the sample at 1100 °C ( Figure 9c) it is possible to observe an increase in the relative area of Fe 0 and a decreasing of area of Fe 2+ and Fe 3+ , suggesting the reduction of these Fe species.

Conclusion
In this work, composites based on the perovskites LaFe 0.90 Mn 0.10 O 3 and LaFe 0.90 Mn 0.08 Mo 0.02 O 3 supported on Al 2 O 3 were synthesized and used to prepare highly dispersed Fe o particles.XRD, Mössbauer spectroscopy, N 2 adsorption, TPR and SEM showed that the perovskites can be prepared on the Al 2 O 3 surface with particle sizes varying from 150-180 Å. Upon reduction with H 2 under controlled conditions highly dispersed Fe 0 particles are formed.These iron catalysts showed improved stability towards sintering even under severe treatments at 1000 and 1100 °C.The production of highly dispersed and stable Fe catalyst is of considerable interest for special applications such as the synthesis of single wall carbon nanotubes.

Figure 1 .Figure 2 .
Figure 1.Formation of highly dispersed metallic particles upon reduction of the perovskite precursor dispersed on the surface of Al 2 O 3 support.

F i g u re 9 .
M ö s s b a u e r s p e c t r a a t 2 9 8 K o f t h e p e r o v s k i t e LaFe 0.90 Mn 0.08 Mo 0.02O3 (25%)/Al 2 O 3 before and after reduction at 900 and 1100 °C.

Table 1 .
Surface area, average crystallite size and lattice parameter for the synthesized perovskites.

Table 2 .
Hyperfine parameters for the pure perovskites and supported on Al 2 O 3 corresponding to spectra recorded at 298 K.