The development of selectivity controllable catalysts for the synthesis of light olefins from syngas (STO) remains an important goal of chemical industry. [1, 2] Herein, we report that Mn-modulated γ-Fe2O3 nanorods fabricated by a simple aqueous precipitation method are broadly effective STO catalysts, due to the formation of more active iron carbide species at the metal-promoter interface, responding to the morphology effect and crystallographic phase effect in heterogeneous catalysis. Most importantly, the structure evolution of iron oxide with different crystal phase (α-Fe2O3 and γ-Fe2O3) was established by in situ synchrotron radiation XPS, X-ray absorption fine structure (XAFS) and Mössbauer spectra. The results show that the formation of active iron carbides is highly dependent on the initial crystal phase of iron oxide and metal-promoter interaction. This finding enriches the fundamental understanding of active sites in STO reaction and may guide the design of more active, selective, and stable catalysts.
Materials and Methods
The α-Fe2O3 and γ-Fe2O3 nanorods were prepared by an aqueous precipitation method. The γ-Fe2O3 nanorods were prepared by heating the β-FeOOH precursor in PEG at 200 ℃ to reflux for 24 h under a stream of nitrogen. The resulting solid was washed with water and ethanol, followed by drying at 50 ℃ for 12 h under vacuum. The α-Fe2O3 nanorods were prepared by calcining the β-FeOOH precursor at 500 ℃ for 5 h. For preparation of Mn promoted Fe2O3 nanorods catalysts, in order to improving the dispersion of supported Mn,  an ethylene glycol solution of Mn(NO3)2·4H2O was impregnated onto the Fe2O3 nanorods, followed by drying at 473 K under vacuum. The obtained catalysts were characterized by SEM, HRTEM, XRD, N2-physisorption, XAFS, in-situ XPS, H2-TPR, and Mössbauer spectroscopy. The FTS reaction was carried out in a continuous-flow-type fixed-bed reactor under the temperature of 593 K and a pressure of 1.0 MPa for 50 hrs.
Results and discussion
Herein, we take one-dimensional (1D) pure phase Fe2O3 nanorods as model catalyst to explore the effect of initial iron crystal phase on the iron phase evolution during the STO reaction. Different from the traditional supported or bulk Fe nanoparticles catalysts with complicated composition and many mesopores or micropores, here pure phase Fe2O3 nonporous 1D nanorods was prepared successfully to avoid the influence of other factors such as diffusion limitation and farraginous crystal facets. The catalysts were tested in the FTS at 320 ºC, 1 MPa, H2/CO ratio of 1, and W/F=5 gcat h mol-1. The α-Fe2O3 nanorods catalyst shows a lower activity than γ-Fe2O3 nanorods catalyst due to the lower reducibility, as confirmed by H2-TPR result; however, it exhibited much higher light olefins selectivity and olefin/paraffin ratio for C2-C4 compared with the γ-Fe2O3 nanorods catalyst during the FTO reaction. Note that the particle size, texture, morphology and chemical component of two samples (α-Fe2O3 and γ-Fe2O3) are similar. The reaction result suggesting that there could be some essential differences in the electronic structure of the catalysts, which would affect the evolution of Fe phase from initial iron oxide to iron carbides during reduction and/or reaction process. Furthermore, the inclusion of Mn in the catalyst formulation leads to significant changes in the FTS activity and olefins selectivity, especially for the γ-Fe2O3 catalysts. The best catalytic performance was obtained with the 0.5Mn/γ-Fe2O3 catalyst, which had a high catalytic activity (57.1 %), a low methane selectivity (11.7 %), a high light olefin selectivity (61.2 %), and a high O/P ratio (O/P = 6.88), which is even superior to 0.5Mn/α-Fe2O3 catalyst (51.6 %).
Figure 1. a) Fourier-transformed EXAFS spectra at the Fe K-edge of the a-Fe2O3 and g-Fe2O3 nanorods catalysts as-prepared, after reduced, and after STO reaction. b) Fourier-transformed EXAFS spectra at the Mn K-edge of the a-Fe2O3 and g-Fe2O3 nanorods catalysts after reduced. c) X-ray absorption near edge spectra (XANES) at Mn K-edge of 0.5Mn/g-Fe2O3 nanorods catalysts as-prepared, after reduced, and after STO reaction.
To reveal the nature of active sites that favors the formation of light olefins, we resorted to multiple characterization techniques to investigate the structure of Fe catalyst. The result indicates that the formation of θ-Fe3C is easier from α-Fe2O3 phase than that from γ-Fe2O3, as pure α-Fe2O3 phase has the lower μc due to the lower carburization degree. However, when Mn was added, both χ-Fe5C2 and θ-Fe3C was discerned in α-Fe2O3 and γ-Fe2O3 based samples. Moreover, the content of θ-Fe3C phase for Mn/γ-Fe2O3 increased to 21.0 %, which is even higher than Mn/α-Fe2O3. It seemed that the presence of Mn impeded cementite decomposition, probably by decreasing H2 chemisorption. Meanwhile, the electronic state of surface iron species should be influenced via the electronic transfer between iron support and Mn, as confirmed by in situ XPS. The extended X-ray absorption fine structure (EXAFS) studies clarified that the MnOx on γ-Fe2O3 nanorods possessed less Mn-Mn coordination than on the corresponding α-Fe2O3 nanorods (Fig. 1), also suggesting that the highly dispersed MnOx particles located on the γ-Fe2O3 nanorods had more intimate contact with iron oxide.  This would contribute to enhancing the promotional effects of manganese and result in the formation of more θ-Fe3C phase, which has a positive effect on the formation of light olefins.
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