Highly dispersed cobalt Fischer–Tropsch synthesis catalysts supported on γ-Al2O3, CNTs, and graphene nanosheet using chemical vapor deposition

International Journal of Industrial Chemistry (2019) 10:321–333 https://doi.org/10.1007/s40090-019-00195-9 RESEARCH Highly dispersed cobalt Fischer–Tropsch synthesis catalysts supported on γ‑Al2O3, CNTs, and graphene nanosheet using chemical vapor deposition Iraj Kazemnejad1 · Alireza Feizbakhsh1 · Ali Niazi1 · Ahmad Tavasoli2 Received: 15 September 2018 / Accepted: 28 August 2019 / Published online: 26 September 2019 © The Author(s) 2019 Abstract Highly dispersed 15.0 wt% cobalt catalysts were prepared on γ-Al2O3, carbon nanotubes (CNTs), and graphene nanosheet (GNS) using chemical vapor deposition (CVD) procedure. The physico-chemical properties of the catalysts were studied by inductively coupled plasma (ICP), Brunauer–Emmett–Teller (BET), X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and temperature-programmed reduction (TPR) techniques, and the Fischer–Tropsch synthesis (FTS) performance of the catalysts was assessed at 220 °C, 18 bar, H 2/CO = 2 and feed flow rate of 45 ml/min g cat. Based on BET results, Co/GNS catalyst provided highest surface area in comparison to the other catalysts. XRD and FESEM results revealed that CVD method prepared smaller particles on GNS compared to the other supports and resulted in the most dispersed metal particles on GNS according to H 2-chemisorption results. The performance of Co/Al2O3 catalyst prepared by CVD method was compared with conventional 15 wt% Co/Al2O3 catalyst prepared by impregnation method. The Co/Al2O3 catalyst prepared with CVD method showed 5.3% higher %CO conversion and 2.1% lower C5+ selectivity as compared with the Co/Al2O3 catalysts prepared by impregnation method. Among three catalysts prepared by CVD, Co/ GNS showed higher %CO conversion of 78.4% and C5+ selectivity of 70.3%. Co/γ-Al2O3 catalyst showed higher stability. Keywords Chemical vapor deposition · Cobalt · Dispersion · Activity · Selectivity Introduction Fischer–Tropsch synthesis (FTS) is a major process in conversion of gas-to-liquids, coal-to-liquids, and biomass-toliquids. In the FTS, syngas produced from natural gas, coal, or biomass converts to valuable hydrocarbons by hydrogenation of carbon monoxide [1–3]. In the quest for the development of more active catalysts, a rational strategy is to enhance the cobalt dispersion by decreasing the average particle size [4]. In recent years, gas-phase-based methods have attracted a huge attention in catalysts preparation approaches. Chemical vapor deposition (CVD) is mainly an industrial chemical * Alireza Feizbakhsh 1 Department of Chemistry, Central Tehran Branch, Islamic Azad University, Tehran, Iran 2 School of Chemistry, College of Science, University of Tehran, Tehran, Iran process, which includes a substrate exposed to a single- or multi-component volatile precursor (in gaseous phase) in an inert atmosphere at controlled temperature and pressure. Metal organic chemical vapor deposition (MOCVD) is a potential alternative for making stable and active catalysts with well-developed structure [5]. This technique offers several advantages: (1) allow direct and efficient deposition of active metals by reaction between surface groups of the support and vapors of a suitable volatile compound; (2) small, uniform, well-dispersed, nanocrystallites active metal particles can be formed [6]. The low cost and facile deposition process to control the thickness and morphology of the films on different substrates make CVD an attractive method for catalyst preparation [7]. Dry methods (working in gas/vapor phase) may represent a significant improvement over wet methods [8]. Among dry methods, chemical vapor deposition (CVD) is known to generate more active and selective catalysts compared to conventional techniques [9]. Alumina is one of the most common supports for FT catalysts and carbon materials such as carbon nanotubes and graphene have also been recently applied for supporting iron 13 Vol.:(0123456789) 322 International Journal of Industrial Chemistry (2019) 10:321–333 and cobalt catalysts [10–12]. To best of our knowledge, gasphase methods for preparing cobalt catalysts on oxide and carbonous supports and evaluating their performance and stability in FTS have not been studied widely [13]. Cobalt and iron are proposed metal catalysts as the first catalysts for FTS. These catalysts have been employed in industrial processes for hydrocarbon synthesis. Cobalt catalysts are more expensive than iron catalysts, but they are resistant to deactivation. Cobalt-supported catalysts in FTS are usually prepared by impregnation. In this method, cobalt is deposited on porous support in which a dry support is contacted with a solution containing dissolved cobalt precursors. The distribution of cobalt ions on the surface of the support after impregnation is affected by some electrostatic interactions and van der Waals interactions that occur between the nanoparticles and results in the complex particle dynamics manifesting in deposition patterns on the support material [14]. In the previous studies, cobalt precursors have been dispersed on different porous carriers such as SiO2, Al2O3, and TiO2. The concentration, nature, and distribution of hydroxyl groups play an important role in the genesis of the dispersion of supported metal. Porous supports such as silica, alumina, and titanium have different points of zero charges, and should be pretreatment before use [15]. The concentration of hydroxyl groups can be controlled by pretreatment of the support with some organic compounds such as 1-propanol prior to impregnation resulted in higher metal dispersion and better activity in the FT reaction [16–18]. Carbon materials can be used as catalysts support due to special properties including high surface area, high mechanical strength, chemical inertness, and possibility of using in both basic and acidic solutions. Graphene is a one-atom-thick planar sheet of sp2-carbon atoms a crystal lattice. The unique specifications of graphene have attracted tremendous interest both in academics and industry. Among numerous procedures, chemical vapor deposition method has been introduced to synthesize the graphene from graphite flakes [16]. CVD has chemical reactions of gaseous reactants on the vicinity of a heated substrate surface. This method can provide a nanostructured and functionally coated materials with a special structure to use in different areas [19]. The advantage of this procedure is because of the uniform distribution of cobalt nanoparticles on catalyst support and possibility of a narrow distribution for cobalt particle size. Use of different types of cobalt materials could be explained by their low melting point and high vapor pressures [20]. The aim of the present study is to employ CVD technique to investigate the properties, performance, and stability of cobalt catalysts on different supports in FTS. The main focus is to investigate CVD-prepared catalysts on differe (...truncated)