Characterization of Fe-Ni/SiO2 nanoparticles after pre-reduction and during reductive activation

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Fe-Ni systems as catalysts for hydrogenation reactions

Since the 1980s, various authors have noted that Fe-Ni bimetallic catalysts are promising candidates for hydrogenation processes, whether based on CO or CO2 (methanation reaction, Fischer-Tropsch synthesis) or on biomass-derived organic molecules. Figure 1.1 exhibits the number of publications for each hydrogenation reactions dealing with supported Fe-Ni bimetallic catalysts.

Methanation reactions

CH4 is the principal component of natural gas, but it can also be produced by hydrogenation of CO or CO2, and Ni-based catalysts are known to be effective for this family of reactions [1-4]. However, monometallic Ni catalysts exhibit a poor distribution of metal particles and deactivate under reaction conditions. A second metal has been effectively used as promoter to optimize the properties of monometallic Ni catalysts, as listed in Figure 1.2. Reactions of CO and CO2 methanation are described by Equations (1) and (2) [5]:
3H2+CO→CH4+H2O ΔH = -206.28 kJ.mol-1 (1)
4H2+CO2→CH4+2H2O ΔH = -164.94 kJ.mol-1 (2)
Tian et al. [1] showed that bimetallic Fe–Ni/γ-Al2O3 exhibited a higher catalytic activity than a monometallic Ni/γ-Al2O3 catalyst. Under industrial operation conditions (3.0 MPa, H2/CO = 3.1), the CO conversion provided by 23.3 wt% Fe25Ni75/γ-Al2O3 reached 100% and the CH4 selectivity exceeded 99% in the 300–450 °C temperature range, compared with 23.3 wt% Ni/γ-Al2O3 (conversion of CO: 100%, selectivity to methane: 85%). Also on alumina, Hwang et al. [2] have investigated a series of mesoporous nickel-M/alumina xerogel (AX) catalysts, where M is a second metal (40 wt% Ni75M25/AX, M = Fe, Co, Ce and La). Both the conversion of CO and yield of CH4 decreased in the order Ni75Fe25/AX > Ni100/AX > Ni75Co25/AX > Ni75Ce25/AX > Ni75La25/AX. This indicates that on mesoporous alumina, Fe also improved the properties of the catalyst compared with monometallic Ni100/AX.
Kustov et al. [6] found that 10 wt% bimetallic catalysts supported on MgAl2O4 with molar compositions Fe25Ni75 and Fe50Ni50 showed significantly higher activities (conversion of CO: 99.5 and 95.4%, respectively) and selectivities to methane (92.9 and 85.3%, respectively) in comparison with monometallic 10 wt% Ni and 10 wt% Fe catalysts (Ni: conversion of CO: 34.6%, selectivity to methane: 94.2%; Fe: conversion of CO: 4.4%, selectivity to methane:
68.1%) (Figure 1.3). The selectivity to methane increased at higher CO conversion, and with a higher Ni loading in the catalysts. The highest catalytic activity and selectivity to methane were observed for catalyst Fe25Ni75 with a 20 wt% total metals loading (Table 1.1). The same result was obtained by Andersson et al. [7], who found that a 10 wt% loading on MgAl2O4 of Fe25Ni75 and Fe50Ni50 (rate of CO removal: 6 and 5.5 mmol CO/mmol metal.s, respectively) exhibited superior activity compared with Fe and Ni (rate: 0 and 2 mmol CO/mmol metal.s, respectively). An explanation has been given by combining catalytic measurements with DFT calculations. Like Co and Ru, the Fe25Ni75 and Fe50Ni50 alloy surfaces are anticipated to be optimum systems in terms of rate of hydrogenation of carbon, and dissociation energy of CO (Ediss), used as the main descriptor. Compared with an optimal value of Ediss = 0.06 eV, the calculated gap ΔEdiss = Ediss – Ediss optimal for Fe25Ni75 and Fe50Ni50 is lower than 0.25 eV, which is smaller than for Fe (0.9 eV) and Ni (0.4 eV).

Fischer-Tropsch synthesis

The Fischer–Tropsch synthesis (FTS), discovered by Fischer and Tropsch in the 1920s, has been paid renewed attention in recent years due to the increase of oil prices, decreasing oil reserves and environmental concerns [14-16]. Through FTS, a large variety of carbon-containing resources, coal, natural gas, biomass, can be converted to chemicals and liquid fuels. The principle of the FTS reaction is the strongly exothermic hydrogenation of carbon monoxide (CO) to paraffins and olefins, according to the following chemical equations (3) and (4) [14]:
(2n+1) H2 + n CO → CnH2n+2 + n H2O (3)
2n H2 + n CO → CnH2n + n H2O (4)
Among transition metals, Fe, Co and Ru are known to be active for C-C coupling, and the first two metals are the ones that are mostly used industrially due to the high cost and low abundance of Ru
In FTS, Fe–Ni catalysts have been found to provide a high catalytic activity, and a different product distribution from monometallic Fe and Ni catalysts. Li et al. [15] showed that Ni-promoted Fe fresh catalysts were mainly composed of α-Fe2O3 and NiFe2O4. The addition of nickel improved the dispersion of iron oxides, which led to an increase of the catalyst surface area, and a decrease of the metal oxide crystallite size. The presence of Ni improved the reduction and carbidization rates of Fe in H2 and CO, respectively, while suppressing the formation of iron carbides. The conversion provided by an unsupported Fe87Ni13 catalyst (50%) was more stable with time on stream than that given by the monometallic Fe catalyst (decrease from 65% to 30%). The selectivity to CH4 and C2-4 were up to 54.9 and 42.1% at a conversion of 44.7% for Fe87Ni13, while monometallic Fe gave 22% of selectivity to CH4 and 50% of selectivity to C2-C4 at a conversion of 33%.

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Hydrodeoxygenation of organic molecules

Hydrogenolysis takes place at higher temperatures than hydrogenation reactions presented in the former section. Leng et al. [29] showed that 20 wt% Fe25Ni75/Al2O3 bimetallic catalysts exhibited an excellent activity and selectivity for the hydrodeoxygenation (HDO) of furfuryl alcohol, benzene alcohol and ethyl oenanthate, at atmospheric pressure and T = 400 °C. The conversions were found to be 100, 95.48 and 97.89%, respectively, and the yield to 2-methylfuran, toluene and heptane were 98.85, 93.49 and 96.11%, respectively. Fang et al. [30] studied the effect of the Fe/Ni atomic ratio for catalysts supported on carbon nanotubes (CNT) on the activity and products distribution in the HDO of guaiacol at 300 °C (Figure 1.8). Monometallic 7 wt% Ni/CNTs exhibited a 79% conversion and 60% selectivity to cyclohexane. A 83.4% selectivity to cyclohexane at 96.8% of conversion was obtained with 7 wt% Fe17Ni83/CNT. The guaiacol was supposed to be preferentially adsorbed on the surface of oxophilic Fe domains. When increasing the Fe content, the activity became lower and HDO became incomplete, as selectivity was shifted to the production of phenol, such as with 7 wt% Fe83Ni17/CNT (conversion of guaiacol: 50%, selectivity to phenol: 77%). The HDO of guaiacol requires the dissociation of H2 on Ni species and the amount of activated hydrogen atoms was considered to be insufficient on Fe-rich particles. The Fe-Ni nanoparticle size and the reduction temperature were also investigated. Increasing the reduction temperature from 400 to 600 °C for Fe-Ni/CNT catalysts increased the size of Fe17Ni83 nanoparticles from 7.7 to 11.2 nm, leading to a decrease of the selectivity to cyclohexane from 85.4 to 55.8%.

Table of contents :

General introduction
Chapter I Bibliographical study 
Ⅰ.1. Fe-Ni systems as catalysts for hydrogenation reactions
Ⅰ.1.1. Methanation reactions
Ⅰ.1.2. Fischer-Tropsch synthesis
Ⅰ.1.3. Hydrogenation of organic molecules
Ⅰ.1.4. Hydrodeoxygenation of organic molecules
Ⅰ.1.5. Hydroconversion of furfural and 5-hydroxymethyl furfural
Ⅰ.1.6. Conclusions
Ⅰ.2. Formation, structure, size and composition of Fe-Ni nanoparticles
Ⅰ.2.1. Preparation methods
Ⅰ.2.1.1. Incipient wetness impregnation
Ⅰ.2.1.2. Sol-gel method
Ⅰ.2.1.3. Deposition-precipitation
Ⅰ.2.2. Reducibility of the oxidic phases to Fe-Ni nanoparticles
Ⅰ.2.3. Influence of the Ni content on the nanoparticles structure
Ⅰ.3. Conclusions
Chapter Ⅱ Methods: characterization techniques and catalytic test 
Ⅱ.1. Chemical analysis
II.1.1. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)
II.1.2. X-ray fluorescence (XRF)
II.1.3. X-ray Photoelectron Spectroscopy (XPS)
Ⅱ.2. Textural and morphological properties
II.2.1. Nitrogen physisorption
II.2.2. In-situ / X-ray diffraction (XRD)
II.2.3. (High Resolution) Transmission Electron Microscopy ((HR)TEM)
II.2.5. Temperature-programmed reduction (TPR)
Ⅱ.3. Spectroscopic techniques
II.3.1. Attenuated Total Reflection Infrared spectroscopy (ATR-IR)
II.3.2. Mössbauer spectroscopy (MössS)
II.3.3. X-ray absorption spectroscopy (XAS)
II.3.4. Chemometric analysis of X-ray absorption spectra
II.4. Catalytic tests
II.4.1. Materials
Ⅱ.4.2. Experimental set-up
II.4.2.1. SPR system presentation
II.4.2.2. Description of catalysts activation in H2
II.4.2.3. Description of the experimental protocol
Ⅱ.4.3. Analytical methods
Chapter Ⅲ Study of the deposition-precipitation and reduction processes for Fe-Ni/SiO2 catalysts
Ⅲ.1. Introduction
Ⅲ.2. Choice of the experimental parameters for deposition-precipitation and drying
Ⅲ.2.1. Choice of the reactants and strategy
Ⅲ.2.2. Description of the preparation procedure
Ⅲ.2.3. Influence of drying on the solids prepared by DPU
Ⅲ.3. Characterization of Fe-Ni/SiO2 samples after drying
Ⅲ.3.1. Speciation of the metal ions
Ⅲ.3.2. Evolution and composition of the phases deposited during DPU: the Fe50Ni50/SiO2 solid
Ⅲ.3.3. Nature and composition of the phases deposited during DPU: the FexNi(100-x)/SiO2 series
Ⅲ.3.4. Conclusions
Ⅲ.4. The process of reduction for Fe-Ni/SiO2 solids prepared by DPU
Ⅲ.4.1. Speciation of the metals: the Fe50Ni50/SiO2 solid
Ⅲ.4.2. Reducibility of the metals across the FexNi(100-x)/SiO2 series, and composition of the Fe-Ni metal nanoparticles
III.5. Conclusions
Chapter Ⅳ Characterization of Fe-Ni/SiO2 nanoparticles after pre-reduction and during reductive activation
IV.1. Introduction
IV.2. Characterization of the Fe-Ni nanoparticles on system Fe50Ni50/SiO2
IV.3. Characterization of the Fe-Ni nanoparticles across the Fe-Ni/SiO2 series
IV.4. Evolution of the nanoparticles upon activation in H2
IV.5. Conclusions
Chapter Ⅴ Catalytic performances of the Fe-Ni/SiO2 catalysts in the hydrogenation of furfural
Ⅴ.1. Introduction
Ⅴ.2. Choice of the solvent and reaction pathways for the hydrogenation of furfural 155
Ⅴ.3. Choice of the experimental conditions for the hydrogenation of furfural
Ⅴ.3.1. Choice of temperature and hydrogen pressure
Ⅴ.3.2. Choice of the ratio between furfural and the catalyst mass
V.6.1. Comparison of reducibility
Ⅴ.6.2. Comparison of particle size and structure
V.6.3 Comparison of catalytic activity
Ⅴ.7. Conclusions


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