Effect of Ni–Co morphology on kinetics for Fischer–Tropsch reaction in a fixed-bed reactor
Introduction
Fischer–Tropsch Synthesis (FTS) is a catalytic process that can convert syngas to a wide range of hydrocarbons, and it is a promising way to produce valuable chemicals, lubricants and clean liquid fuels [1], [2], [3], [4]. Iron and cobalt-based catalysts are the most common catalysts in FTS [5], [6], [7], and the latter is more widely used than the former. This can be due to lower reaction temperature, high activity and low activity towards water gas shift reaction [8].
Nanostructured compounds because of their unique chemical and physical properties have attracted much attention and in a wide range of potential applications, such as solar cell, catalysis, oxidation, hydrogen-storage devices, wastewater and remediation of soil and sediment contaminated [9], [10], [11], [12], [13]. Many researchers have developed different methods for preparation of nanomaterial, such as sonochemical [14], hydrothermal [15], solid state [16,17], co precipitation [18], sol–gel [19], and microwave [20]. The hydrothermal technique used to synthesize or recrystallize nanomaterial from aqueous solvents under an autoclave at high pressure and temperature. Furthermore, the morphology of the product can be easily changed by varying hydrothermal reaction conditions [10,15,20].
Several studies have shown that the FT reaction is a process, which depends on the structural properties of the catalyst. This means that under the same reaction conditions, changes in the catalyst properties like morphology, size, and crystallographic structure cause changes in the catalyst activity and distribution of products [8,[21], [22], [23], [24], [25], [26], [27], [28]. Chuan Qin and his coworker [21] investigated the effect of crystal planes of cobalt nano-catalyst on the product distribution and catalyst activity. These crystal planes have different performances under the same conditions. Their findings indicate that the catalyst activity and selectivity can be improved by choosing special crystal planes. Cheng et al. [5] performed FT reaction in a fixed-bed reactor over different cobalt nano-particles (7.2–11.4 nm). Their results showed more uniform small-sized cobalt than larger particles that have high selectivity to heavier hydrocarbons. Kinetics investigation is necessary for optimizing reaction parameters and understanding the reaction mechanism [29]. The Fischer–Tropsch complex network reaction has caused intense debates on determining mechanism and product selectivity of this reaction [30]. Numerous literatures reported the FTS kinetics over cobalt-based catalysts [29,[31], [32], [33], [34], [35], [36], [37], [38]. Kinetics of this process is divided into two separated methods: In the first method, the author's concentration is on the consumption rate of carbon monoxide or syngas; and in the second method, researchers focused on the FTS product distribution [39]. However, no general kinetics model for FT reactions has been reported due to the great dependency of reaction kinetics on operating conditions, pretreatment step, catalyst composition and structural properties [8]. CO adsorption and dissociation is an initial step of FT process; therefore, fundamental understanding of this key step is of great importance [40], [41], [42]. Previous experimental and theoretical studies proposed two pathways for CO activation on the catalyst surface. (i) Chain growth in the FT process starts via the carbide mechanism including direct CO dissociation on the catalyst surface and formation of adsorbed C and O. Then, adsorbed C reacts by hydrogen to produce CHx species [41,43,44]. Chen et al. [45] presented experimental evidence showing that direct CO dissociation on the Co catalyst is more favorable. Thanh Hai Pham et al. [40] used spin-polarized density functional theory (DFT) calculations to study CO activation path on the χ-Fe5C2 (510) catalysts. They reported that direct CO dissociation is preferred compared to H-assisted route. (ii) The other path for CO dissociation is H-assisted CO dissociation on the catalyst surface that proceeds via –COH, –HCO and –H2CO intermediates [41,43]. Experimental work by Moazzami et al. [46] showed that H-assisted CO dissociation route is a preferred path over cobalt-based catalysts. In a theoretical and experimental work, Ojeda et al. [47] proved that CO dissociation on Co (0001) and Fe (110) surfaces proceed via H-assisted route. In a theoretical study over Ni catalyst with different crystal phase, face-centered cubic (FCC) and hexagonal close-packed (HCP), Liu et al. [48] showed H-assisted CO dissociation is kinetically favorable on both crystal phases. While, in another study over Co catalyst, they reported that H-assisted is a preferred route over FCC-Co catalyst, but direct CO dissociation dominates the route for HCP-Co catalyst [49].
The overall goal of the present work is the kinetics and mechanism investigation of Fischer–Tropsch Synthesis over NiCo2O4 catalyst with different morphologies. Kinetics study is conducted over three different morphologies of NiCo2O4 catalysts crystallized in the FCC structure under steady-state and the same reaction conditions. Experimental data were fitted to kinetics models based on Langmuir−Hinshelwood−Hougen−Watson (LHHW) and Eley−Riedel (ER), and parameters estimation was performed by nonlinear regression. The obtained results were compared with those reported for FTS in the literatures.
To the best of our knowledge, the kinetics of Fischer–Tropsch Synthesis in a fixed-bed reactor with different catalyst morphologies has not been carried out so far.
Section snippets
Materials
All chemicals, purchased from Sigma Aldrich, were used without any purification, and one-step hydrothermal method was used to synthesize porous NiCo2O4 catalysts.
Synthesis of porous NiCo2O4 nanowires (NiCo-NWs)
A typical synthesis, Co(NO3)2·6H2O and Ni(NO3)2·6H2O with 2:1 molar ratio were dissolved in 45 mL deionized water. Under constant magnetic stirring, 1.82 g urea was added to the solution, and the resulting solution was stirred at 700 rpm and ambient temperature. After 120 min stirring, the solution was put into a 60 mL Teflon-lined
Catalysts characterization
The NiCo2O4 nanowires, hierarchical NiCo2O4 hollow microspheres and hierarchical NiCo2O4 microspheres were prepared by hydrothermal method. Fig. 1 shows XRD patterns of NiCo2O4 catalysts calcined at 350 °C. All the diffraction peaks of calcined catalysts are observed at 18.9°, 31.1°, 36.7°, 44.6°, 59.1° and 65.1° correspond to the (111), (220), (311), (400), (511) and (440) crystal planes, respectively, of face-centered cubic structure of NiCo2O4 (JCPDS No. 73-1702). The resulting XRD data
Conclusion
The morphology effect on the kinetics of FTS reaction were studied by performing three kinetics runs on NiCo2O4 catalysts. This research was conducted to understand the dominant mechanism of CO activation over different morphologies of NiCo2O4 catalyst. The results revealed the morphology dependency of kinetics modeling and mechanism for Fischer–Tropsch network reaction. CO dissociation on the NiCo2O4 nanowires proceeds via H-assisted route (enol-carbide mechanism) while direct CO dissociation
Acknowledgments
The authors gratefully appreciate University of Sistan and Baluchestan for helping and supporting this research.
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