Fe-Based Nano-Materials in Catalysis

Abstract

The role of iron in view of its further utilization in chemical processes is presented, based on current knowledge of its properties. The addition of iron to a catalyst provides redox functionality, enhancing its resistance to carbon deposition. FeO_x species can be formed in the presence of an oxidizing agent, such as CO₂, H₂O or O₂, during reaction, which can further react via a redox mechanism with the carbon deposits. This can be exploited in the synthesis of active and stable catalysts for several processes, such as syngas and chemicals production, catalytic oxidation in exhaust converters, etc. Iron is considered an important promoter or co-catalyst, due to its high availability and low toxicity that can enhance the overall catalytic performance. However, its operation is more subtle and diverse than first sight reveals. Hence, iron and its oxides start to become a hot topic for more scientists and their findings are most promising. The scope of this article is to provide a review on iron/iron-oxide containing catalytic systems, including experimental and theoretical evidence, highlighting their properties mainly in view of syngas production, chemical looping, methane decomposition for carbon nanotubes production and propane dehydrogenation, over the last decade. The main focus goes to Fe-containing nano-alloys and specifically to the Fe⁻Ni nano-alloy, which is a very versatile material.

Keywords: CO2 utilization; carbon; chemical looping; dehydrogenation; hydrocarbon conversion; nano-alloys; role of iron.

Figure 1

Use of iron/iron oxides throughout mankind [6,7,8].

Figure 2

Schematic illustration of mixed CeO₂–Fe₂O₃ samples, based upon ICP composition, XRD patterns, STEM, EDX, and EELS. Obtained from [65].

Figure 3

Oxygen storage capacity of MgFe_xAl_2−xO₄ materials as a function of the Fe₂O₃ content. Note that when Fe₂O₃ loading is less than 30 wt %, it is completely incorporated into the spinel structure without separate Fe₂O₃ phases. : iron incorporated in spinel structure; : separate Fe₂O₃ phase. Obtained from [81].

Figure 4

Crystallite size of Fe₂O₃ and MgFe_xAl_2−xO₄ phases in the samples, as calculated based on XRD using the Scherrer equation. As-prepared: (□) MgFe_xAl_2−xO₄ and (Δ) Fe₂O₃; (■) MgFe_xAl_2−xO₄ and (▲) Fe₃O₄ after 5 isothermal redox cycles of H₂/CO₂ at 1023 K. Obtained from [81].

Figure 5

CO yield in CO₂ to CO conversion as a function of isothermal H₂-CO₂ redox cycles for MgFe_xAl_2−xO₄ and Fe₂O₃/MgFe_xAl_2−xO₄ with (●) 10 wt % Fe₂O₃ (MgFe_0.14Al_1.86O₄) and (▲) 90 wt % Fe₂O₃. Each cycle (16 min) is composed of 4 min H₂ (5% in Ar), 4 min He, 4 min CO₂ (100%) and 4 min He at 1123 K. All the gas flows were 1.1 NmL/s. Obtained from [81].

Figure 6

Schematic representation of the shrinking core model in a MgFe_xAl_2−xO₄ crystallite. Top right inset: Observed and calculated conversion profile of Fe³⁺ based on pre-edge fitting of QXANES spectra from MgFe_0.14Al_1.86O₄. Obtained from [85].

Figure 7

Conversion and product distribution of the Ru_xFe/TiO₂ catalysts, during anisole hydrodeoxygenation (HDO), in a stainless-steel batch reactor. Reaction conditions: 10 wt % anisole (40 mL), catalyst (1.0 g), 200 °C and 10 bar H₂ for 3 h. Obtained from [94].

Figure 8

Phase diagram of the bimetallic Fe–Ni system. Obtained from [99].

Figure 9

Full XRD scans of MgAl₂O₄, as-prepared, reduced and re-oxidized 8 wt %Ni-5 wt %Fe/MgAl₂O₄ (1 mL/s of 10%H₂/He mixture or CO₂ at a total pressure of 101.3 kPa and 1123 K). The NiFe₂O₄ phase cannot be distinguished due to overlapping with Fe₂O₃. Reproduced from [17].

Figure 10

EDX element mapping of 8 wt %Ni-5 wt %Fe/MgAl₂O₄. (A) After H₂-reduction (1 mL/s of 5%H₂/Ar mixture at a total pressure of 101.3 kPa and 1123 K). (B) After CO₂ oxidation (1 mL/s of CO₂ at a total pressure of 101.3 kPa and 1123 K). Red and green colors correspond to Fe and Ni elements respectively. Obtained from [17].

Figure 11

Schematic diagram of Fe–Ni nano-alloy formation and decomposition, depending on the applied environment. Obtained from [17].

Figure 12

(A) Methane conversion over Fe–Ni catalysts with Ni/(Ni + Fe) ratio of 1.0, 0.7 and 0.3 as a function of TOS at 873 K. (B) Long term test of a Fe–Ni catalyst with Ni/(Ni + Fe) ratio of 0.7 during methane decomposition at 923 K. Reproduced from [110].

Figure 13

SEM image of carbon nanotubes produced over a Fe–Ni catalyst with Ni/(Ni + Fe) of 0.7 after 210 h TOS under methane decomposition at 923 K. Obtained from [110].

Figure 14

(A) Rate of methane consumption (mol·min⁻¹·kg⁻¹_cat) as a function of the amount of surface Ni (mmol), during methane dry reforming. ■: Pure Ni; ▲: Ni/(Ni + Fe) = 0.80; ●: Ni/(Ni + Fe) = 0.75; □: Ni/(Ni + Fe) = 0.5; ∆: Ni/(Ni + Fe) = 0.25 and ○: Pure Fe, all supported on Mg_xAl_yO_z and (B) rate of methane consumption (mol·min⁻¹·kg⁻¹_cat) as a function of time-on-stream (TOS) during DRM at 923 K. Reproduced from [101].

Figure 15

Deposited carbon as a function of Ni/(Ni + Fe) ratio along with the SEM micrographs of “spent” catalysts supported on MgAl₂O₄. Temperature 1023 K, CH₄:CO₂ = 1:1, reaction time 4 h. Reproduced from [17].

Figure 16

CH₄ consumption rate (mol_CH4·s⁻¹·kg⁻¹_metals) and the produced CO/H₂ ratio over a bimetallic Fe–Ni catalyst with Ni/(Ni + Fe) = 0.65 during DRM at 1023 K (total pressure of 101.3 kPa and CH₄:CO₂ = 1:1). W_metals/F⁰_CH4 = 0.025 mol_CH4·s⁻¹·kg⁻¹_metals, X_CH4: from 62% to 24%. Reproduced from [116].

Figure 17

Phase diagram showing the equilibrium lines between Fe₃O₄, FeO and Fe as a function of temperature and reduction capacity in presence of: H₂/H₂O; H₂/CO/H₂O/CO₂ (equimolar amount of C and H₂ corresponding with a feed of CH₄ + CO₂); CO/CO₂. Obtained from [49].

Figure 18

Deactivation due to Fe segregation from the Fe–Ni surface alloy during DRM at high temperature (1023 K). Obtained from [116].

Figure 19

Raman spectrum of the spent Fe–Ni catalyst, with Ni/(Ni + Fe) ratio of 0.6 (DRM for 1 h, 1023 K, CH₄:CO₂ = 1.1, total pressure of 101.3 kPa). Blue line: pure graphite as a reference, black line: spent Fe–Ni catalyst, grey line: spent Fe–Ni catalyst after CO₂-TPO up to 950 K, purple line: Spent Fe–Ni catalyst after CO₂-TPO up to 1123 K. Obtained from [118].

Figure 20

(A) HRTEM image of a spent Fe–Ni catalyst with Ni/(Ni + Fe) ratio of 0.6 (after DRM at 1023 K, CH₄:CO₂:He = 1.1:1:1, total pressure of 101.3 kPa, reaction time 1 h). EDX element mapping of (B) carbon, (C) Ni and (D) Fe. Obtained from [118].

Figure 21

EDX element mapping of Fe–Ni. (A) After DRM (1023 K, CH₄/CO₂/He = 1.1/1/1, total pressure of 101.3 kPa, reaction time 1 h). (B) After CO₂ oxidation (1 mL/s of CO₂ at a total pressure of 101.3 kPa and 1123 K). Red, green and blue colors correspond to carbon, Fe and Ni elements respectively. Obtained from [118].

Figure 22

Schematic representation of carbon species removal by CO₂ over Fe–Ni catalyst. C_s: deposited carbon. O_s: surface oxygen, O_L: lattice oxygen. C_m: carbon deposited on metals, C_s: Carbon deposited far from metals, O_s: surface oxygen, O_L: lattice oxygen. The carbon illustration is not corresponding to the real carbon structure. Obtained from [118].

Figure 23

2D in situ XRD pattern during H₂-TPR for Fe–Ni–Pd. Heating rate: 30 K/min, maximum temperature 1123 K, flow rate: 1 NmL/s, 10%H₂/He. Obtained from [116].

Figure 24

EDX element mapping of a Fe–Ni–Pd catalyst supported on MgAl₂O₄. (A) as-prepared (B) reduced (1 NmL/s of 5%H₂/He mixture at a total pressure of 101.3 kPa and 1123 K). Red, green and blue colors correspond to Fe, Ni and Pd elements, respectively. Obtained from [116].