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On the role of active phase for Fischer-Tropsch synthesis – experimental evidence of CO activation over single-phase cobalt catalysts Lyu Shuai, Li Wang, Jianghao Zhang, Chen Liu, Junming Sun, Bo Peng, Yong Wang, Kenneth G. Rappe, Yuhua Zhang, Jinlin Li, and Lei Nie ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00834 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018
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Unsupported single-phase Co catalysts for understanding of key elementary step in Fischer-Tropsch synthesis. 171x96mm (300 x 300 DPI)
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On the Role of Active Phase for Fischer-Tropsch Synthesis – Experimental Evidence of CO Activation over Single-phase Cobalt Catalysts Shuai Lyu†¶, Li Wang*†¶, Jianghao Zhang§, Chen Liu†, Junming Sun§, Bo Peng*‡, Yong Wang‡, §, Kenneth G. Rappé‡, Yuhua Zhang†, Jinlin Li† and Lei Nie*†, ‡ † Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, China. ‡ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P. O. Box 999, Richland, WA 99352, USA. § The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA
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ABSTRACT. In order to understand the role of Co catalysts with different phases on FischerTropsch synthesis, single-phase face-centered cubic (fcc) and hexagonal close-packed (hcp) Co were synthesized via a two-step approach, involving the formation of single-phase CoO materials followed by reduction in H2. The physicochemical properties of Co catalysts were thoroughly characterized by XRD, SEM, TEM, TPR and H2 chemisorption. It was found that hcp-Co exhibits higher activity on hydrocarbon formation than fcc-Co in Fischer-Tropsch synthesis. For both catalysts, CO dissociation was suggested as the rate determining step, on which hcp-Co presents ca. 40 kJ mol-1 lower activation energy than fcc-Co, in agreement of reported computational study. As a result, hcp-Co is concluded as a preferable phase for rational catalyst design.
KEYWORDS. Syngas chemistry, Fischer-Tropsch synthesis, Cobalt catalyst, Phase impact, CO dissociation, Wulff construction.
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1. INTRODUCTION Fischer-Tropsch (F-T) synthesis is an approach to produce clean fuels and key chemicals from syngas (gas mixture of CO and H2), which can be derived from natural gas, coal or biomass.1-3 Fe- and Co-based catalysts have been widely applied in industry and intensively studied for decades.4-9 It has been accepted that Co-based catalysts are superior to Fe-based catalysts for F-T synthesis due to their higher activity, durability and selectivity to long-chain hydrocarbons.10-12 However, even with decades of experimental and theoretical studies, the Co-structurecorrelation to F-T synthesis activity remains controversial.13-15 The major challenge of acquiring a convincing fundamental understanding lies on the gap between practical relevant reaction condition and computational studies employing ideal assumptions. Specifically, for the cobaltbased catalysts, Co0 is believed to be the only active species in the reaction. Previous works claimed that Co with hexagonal close-packed (hcp) structure possesses higher catalytic activity than Co with face-centered cubic (fcc) structure.16-18 However, results from those studies are not convincing. For instance, the use of catalyst support and / or promoter leads to ambiguous attribution of active species under F-T synthesis reaction condition. In addition, the presence of either carbide- or oxide-form Co instead of metallic Co causes more paradoxical conclusions.16, 17
In contrast to experimental studies, computational studies have enabled a greater level of
clarity and allowed for insightful understanding of the catalytic system at the molecular level. The similar conclusion was also suggested that hcp-Co has lower activation barrier for CO activation (the rate determining step for F-T synthesis).18 Nonetheless, it was also prospected that a clean catalytic system of Co with identical phase composition and without influences from support or promoter would be necessary for experimental validation of theory predictions.
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In conventional approaches for the synthesis of supported Co catalysts, Co precursor was used in wetness impregnation or homogeneous deposition precipitation, forming Co3O4 with either spinel structure or CoO with cubic structure after calcination.19, 20 After reduction by H2, cobalt oxide transforms into metallic Co, presenting in both hcp and fcc forms. Extensive research has been conducted to control the crystallinity of metallic Co that could be applied as heterogeneous catalysts.21-28 However, metallic Co is unstable at nanoscale, being easily oxidized without using surfactants. By far, the reported smallest single-phase Co nanoparticle is larger than 100 nm, 29 which is too large to be practically considered for F-T synthesis catalysis.30, 31 Other efforts on phase control of metallic Co nanoparticle either involve the use of heteroatom, or fail to achieve the stable single-phase particle which is required for a steady-state catalytic test .31-34
In this work, we report the first direct experimental evidence of phase effects of Co catalyst for F-T synthesis. This work was enabled by an easy-to-handle and reliable method to prepare single-phase Co catalysts (both hcp and fcc phases) from cobalt oxides nanocrystal, with the phase transition during reduction elucidated by in-situ X-ray diffraction experiments. The results of kinetic measurement well fit the established theoretical model, providing meaningful fundamental understanding for further catalyst development.
2. EXPERIMENTAL SECTION 2.1. Chemicals Benzylamine (> 99.99%) and Cobalt (III) acetylacetonate (Co(acac)3, > 99.99%) were purchased from Aladdin. All the chemicals were used as received.
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2.2. Preparation of the catalysts The synthesis of Co nanoparticles comprises two steps. The first step is the synthesis of CoO nanocrystals with different phases (hexagonal and cubic), via approaches reported previously.35 The second step is the reduction of CoO nanocrystals under H2, forming single-phase metallic Co particles (hcp-Co from hexagonal CoO, while fcc-Co from cubic CoO).
For the synthesis of hexagonal CoO (denoted as h-CoO), 40 g benzylamine and 0.5 g Co(acac)3 were mixed in a 100 mL round-bottom flask connected to a reflux unit, followed by vigorous stirring with a speed of 900 rpm in a preheated oil bath (filled with methyl silicone oil) at 195 °C for 1 h. After cooling to room temperature, the precipitate was collected by centrifuging (4000 rpm), then thoroughly rinsed with ethanol for more than 5 times and dried at 40 °C.
For the synthesis of cubic CoO (denoted as c-CoO), 40 g benzylamine and 1.5 g Co(acac)3 were mixed in a 100 mL round-bottom flask connected to a reflux unit, followed by vigorous stirring (900 rpm) in the same oil bath as for h-CoO at room temperature for 5 min, then heated up to 180 °C at a rate of 2 °C min-1 and maintained at 180 °C for another 2 h. After cooling to room temperature, the precipitate was collected by centrifuging (4000 rpm), then thoroughly rinsed with ethanol for more than 5 times and dried at 40 °C.
The reduction of synthesized h-CoO and c-CoO was conducted in a tube furnace under H2 atmosphere. Under continuous H2 flow (30 mL min-1), the CoO samples were kept at room temperature for 1 h, then the furnace was heated up to 350 °C with a temperature ramp of 2 °C min-1 and maintained at 350 °C for 3 h under the same H2 flow, before cooling down to room
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temperature. For the catalysts used for catalytic test or in-situ characterization, the reduction was performed in the reactor or measurement cell. For the catalysts used for characterization, a passivation step is performed by flowing O2 / N2 gaseous mixture (volume ratio 1 / 99, flow rate 10 mL min-1) through the catalysts at room temperature for 1 h.
2.3. Characterization In-situ X-ray diffraction (XRD) was performed on an Advance D8 diffractometer accompanied with a Cu-Kα radiation source to understand the phase transformation during reduction of CoO materials. Under continuous H2 flow (30 mL min-1), the CoO samples were kept at room temperature for 1 h, then the furnace was heated up to 350 °C with a temperature ramp of 2 °C min-1 and maintained at 350 °C for 3 h. The XRD signal was automatically recorded during the whole temperature ramp with a scanning rate of 0.05° s-1.
Transmission electron microscopy (TEM) was carried out on a Tecnai G20 with accelerating voltage of 200 kV. Field emission scanning electron microscopy (FESEM) was performed on a Hitachi SU8000 with accelerating voltage of 10 ~ 15 kV. The information of morphology and particle size distribution was then acquired for the CoO samples before and after reduction. N2 physisorption experiments were conducted on a Quantachrome Autosorb-1-C-MS instrument. The sample was degassed at 200 °C for 6 h, followed by N2 adsorption at -196 °C. The BET surface area was determined via Brunauer-Emmett-Teller (BET) method.
H2 temperature-programmed reduction (TPR) was carried out on a Zeton Altamira AMI-200 instrument. Prior to TPR measurement, 50 mg catalyst was flushed with Ar (> 99.999 %) at
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150 °C for 1 h then cooled down to 50 °C. Subsequently, the temperature was raised from 50 to 800 °C with a ramp of 10 °C min-1 under H2 / Ar (volume ratio 1 / 9, flow rate 30 mL min-1). The temperature was finally maintained at 800 °C for 30 min. The H2 concentration was monitored by a thermal conductivity detector (TCD). H2-TPR profile was recorded as H2 consumption versus reduction temperature.
H2 chemisorption on reduced catalysts was performed on the same instrument used for H2-TPR. 0.10 g CoO samples were first reduced at 350 °C for 3 h in H2 flow (> 99.999 %, 30 mL min-1) then cooled down to 100 °C. The same H2 flow was maintained at 100 °C for another 1 h to saturate the surface active site. Afterwards, Ar was introduced instead of H2 (> 99.999%, 10 mL min-1) for 30 min in order to remove the physisorbed H2. Subsequently, the temperature was elevated from 100 to 450 °C at 10 °C min-1 and maintained at 450 °C for another 2 h under Ar flow. The desorbed H2 was monitored by TCD and used for quantification.
In-situ FT-IR measurement was performed on a Bruker Tensor II spectrometer equipped with a home-made in-situ ATR cell. The sample was deposited and placed on the top of a ZnSe internal reflection element (IRE). Prior to measure, the catalyst deposited on IRE was dried at 90 °C for 1 h. The IR spectra were recorded under FT-synthesis relevant conditions (CO / H2 flow with comparable GHSV as catalytic reaction at 190 °C), between 4000 and 800 cm-1 by averaging 128 scans at a resolution of 4 cm-1, aiming for a higher signal-to-noise ratio.
2.4. Catalytic reaction
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The F-T synthesis was performed in a stainless-steel tube (with inner lining) fixed-bed reactor. Prior to each test, 0.1 g catalyst and 0.5 g quartz sand as dilution were loaded and reduced at 350 °C for 3 h in H2 flow (3.33 mL min-1). The feed flow was a mixture of H2 / CO / N2 (volume ratio 6 / 3 / 1), where N2 was used as an internal standard for quantification. The gas hourly space velocity (GHSV) was maintained as 2.0 L (gcat h)-1, while the pressure was maintained at 1.0 MPa. After that, the catalyst bed was heated to 100 °C and held for 1 h, then slowly heated to reaction temperatures (170, 180, 190, 200 and 210 °C) with a ramp of 0.1 °C min-1. The reaction was performed at each temperature isothermally for 4 h in order to ensure the steady state was reached. During reaction, the wax and liquid products were condensed in two consecutive traps maintained at 80 °C and 0 °C, respectively. The reaction products were analyzed by an online gas chromatography (GC, Agilent 7890B), equipped with both flame ionization detector (FID) and thermal conductivity detector (TCD). In order to separate a wide range of F-T synthesis products, five GC columns were used: two Porapak N pre-columns, 5A molecular sieve packed column for N2, H2, CO and CH4, while HP-Al2O3/KCl capillary column for C1 ~ C8 and the Innowax column for the remaining products. The conversion of CO and selectivity of hydrocarbon (carbon number based) were calculated as follows:36 CO conversion: CH4 selectivity: Cn selectivity (n = 2, 3 and 4): C5+ selectivity: CO2 selectivity:
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Under low conversion (
