Effect of Ni–Co morphology on kinetics for Fischer–Tropsch reaction in a fixed-bed reactor

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Highlights

  • For the first time, NiCo2O4 nanowires were prepared by simple template-free hydrothermal method.

  • Kinetics experiments were performed over single FCC phase of NiCo2O4 catalyst with different morphologies.

  • Mechanism of FTS depends on morphology of catalysts.

  • Our experimental results support both CO dissociation routes over FCC-NiCo catalyst and show it depends on the catalyst morphology.

Abstract

In order to investigate the effect of catalyst morphology on the kinetics modeling and mechanism of Fischer–Tropsch Synthesis (FTS), experiments were conducted on single-phase face-centered cubic NiCo2O4 catalysts with different morphologies in a fixed-bed reactor under identical operating conditions. A simple template-free hydrothermal method is presented for the preparation of NiCo2O4 nanowires (NiCo-NWs). In addition, hierarchical NiCo2O4 hollow microspheres (NiCo-HMS) and hierarchical NiCo2O4 microspheres (NiCo-MS) are prepared by hydrothermal method. Characterization of catalysts was carried out using BET, SEM, EDX, XRD and TEM techniques. The findings revealed that CO dissociation is closely dependent on the morphology of catalysts, and proceeds via H-assisted route over NiCo-NWs, but direct CO dissociation route occurs over NiCo-HMS and NiCo-MS. The kinetics models of all three catalysts were able to correctly predict the experimental rate data. The obtained kinetics parameters for FTS catalytic process over NiCo-NWs, NiCo-HMS and NiCo-MS are in good agreement with the literature reports.

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.

References (87)

  • R. Paulose et al.

    Nanostructured nickel oxide and its electrochemical behaviour—A brief review

    Nano-Struct Nano-Objects

    (2017)
  • A.N. Pour et al.

    Effect of nano-particle size on product distribution and kinetic parameters of Fe/Cu/La catalyst in Fischer–Tropsch synthesis

    J Nat Gas Chem

    (2010)
  • R. Agrawal et al.

    Effect of phase and size on surface sites in cobalt nanoparticles

    Catal Today

    (2018)
  • M. Rahmati et al.

    Effect of different alumina supports on performance of cobalt Fischer–Tropsch catalysts

    J Catal

    (2018)
  • A. Mosayebi et al.

    Detailed kinetic study of Fischer–Tropsch synthesis for gasoline production over Co Ni/hzsm-5 nano-structure catalyst

    Int J Hydrog Energy

    (2017)
  • N. Moazami et al.

    Modelling of a fixed bed reactor for Fischer–Tropsch synthesis of simulated n 2 -rich syngas over Co/Sio 2: hydrocarbon production

    Fuel

    (2015)
  • QianW. et al.

    The comprehensive kinetics of Fischer–Tropsch synthesis over a Co/Ac catalyst on the basis of Co insertion mechanism

    Chem Eng J

    (2013)
  • C.G. Visconti et al.

    Detailed kinetics of the Fischer–Tropsch synthesis over co-based catalysts containing sulphur

    Catal Today

    (2010)
  • S. Mousavi et al.

    Statistical investigation of macro kinetics for iron and cobalt based Fischer–Tropsch synthesis: mechanistic and kinetic implications

    J Nat Gas Chem

    (2016)
  • TengB.-.T. et al.

    A comprehensive kinetics model of Fischer–Tropsch synthesis over an industrial Fe–Mn catalyst

    App Catal A-Gen

    (2006)
  • YangJ. et al.

    Reaction mechanism of Co activation and methane formation on Co Fischer–Tropsch catalyst: a combined DFT, transient, and steady-state kinetic modeling

    J Catal

    (2013)
  • N. Moazami et al.

    A comprehensive study of kinetics mechanism of Fischer–Tropsch synthesis over cobalt-based catalyst

    Chem Eng Sci

    (2017)
  • LiuX.Y. et al.

    Self-assembled porous NiCo2O4 hetero-structure array for electrochemical capacitor

    J Power Sources

    (2013)
  • A. Sari et al.

    Intrinsic kinetics of Fischer–Tropsch reactions over an industrial Co–ru/γ-Al2O3 catalyst in slurry phase reactor

    Fuel Process

    (2009)
  • YangJ.H. et al.

    Mass transfer limitations on fixed-bed reactor for Fischer–Tropsch synthesis

    Fuel Process

    (2010)
  • S. Golestan et al.

    Kinetic and mechanistic studies of Fischer–Tropsch synthesis over the nano-structured iron–cobalt–manganese catalyst prepared by hydrothermal procedure

    Fuel

    (2017)
  • QiuW. et al.

    Surface modulation of NiCo2O4 nanowire arrays with significantly enhanced reactivity for ultrahigh-energy supercapacitors

    Chem Eng J

    (2018)
  • ZhouX. et al.

    One-dimensional NiCo2O4 nanowire arrays grown on nickel foam for high-performance lithium-ion batteries

    J Power Sources

    (2015)
  • R. Zennaro et al.

    Kinetics of Fischer–Tropsch synthesis on titania-supported cobalt

    Catal Today

    (2000)
  • MaW. et al.

    Fischer–Tropsch synthesis: support and cobalt cluster size effects on kinetics over Co/Al2O3 and Co/SiO2 catalysts

    Fuel

    (2011)
  • D.M. Marinković et al.

    Synthesis and characterization of spherically-shaped Cao/γ-Al2O3 catalyst and its application in biodiesel production

    Energy Convers Manag

    (2017)
  • MaW. et al.

    Fischer–Tropsch synthesis: kinetics and water effect study over 25%Co/Al2O3 catalysts

    Catal Today

    (2014)
  • M. Ojeda et al.

    CO activation pathways and the mechanism of Fischer–Tropsch synthesis

    J Catal

    (2010)
  • E. Rebmann et al.

    Kinetic modeling of transient Fischer–Tropsch experiments over Co/Al2O3 catalysts with different microstructures

    Catal Today

    (2016)
  • SunY. et al.

    Fischer–Tropsch synthesis in a microchannel reactor using mesoporous silica supported bimetallic Co–Ni catalyst: process optimization and kinetic modeling

    Chem Eng Process

    (2017)
  • SunY. et al.

    Optimization using response surface methodology and kinetic study of Fischer–Tropsch synthesis using SiO2 supported bimetallic Co–Ni catalyst

    J Nat Gas Sci Eng

    (2016)
  • P. Nikparsaa et al.

    Effect of reaction conditions and kinetic study on the Fischer–Tropsch synthesis over fused Co–Ni/Al–O3 catalyst

    J Fuel Chem Technol

    (2014)
  • F. Fazlollahi et al.

    Development of a kinetic model for Fischer–Tropsch synthesis over Co/Ni/Al–O3 catalyst

    J Ind Eng Chem

    (2012)
  • A. Eshraghi et al.

    Kinetics of the Fischer–Tropsch reaction in fixed-bed reactor over a nano-structured Fe–Co–Ce catalyst supported with SiO2

    J Nat Gas Sci Eng

    (2015)
  • T.O. Eschemann et al.

    Deactivation behavior of Co/Tio2 catalysts during Fischer–Tropsch synthesis

    ACS Catal

    (2015)
  • R.A. van Santen et al.

    Monomer formation model versus chain growth model of the Fischer–Tropsch reaction

    J Phys Chem C

    (2013)
  • J.H. den Otter et al.

    Synergistic promotion of Co/SiO2 Fischer–Tropsch catalysts by niobia and platinum

    ACS Catal

    (2016)
  • J.C. Mohandas et al.

    Fischer–Tropsch synthesis: characterization and reaction testing of cobalt carbide

    ACS Catal

    (2011)
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