Hydrothermal Carbon-Coated TiO2 as Support for Co-Based Catalyst

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Hydrothermal carbon coated TiO2 as support for Co-based catalyst in Fischer–Tropsch synthesis Chengchao Liu, Yu He, Liang Wei, Yuhua Zhang, Yanxi Zhao, Jingping Hong, Sufang Chen, Li Wang, and Jinlin Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03887 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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ACS Catalysis

The HTC layers on TiO2 with enrichment functional groups (e.g. COOH and COH) are beneficial for cobalt dispersion 265x105mm (96 x 96 DPI)

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Hydrothermal carbon coated TiO2 as support for Co-based catalyst in

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Fischer-Tropsch synthesis

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Chengchao Liu, Yu He, Liang Wei, Yuhua Zhang, Yanxi Zhao, Jingping Hong, Sufang Chen, Li

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Wang, and Jinlin Li*,

†

†

†

‡

†

†

†

§

†

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†

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of

Education, South-Central University for Nationalities, Wuhan 430074, China. ‡

Department College of Chemistry and Materials Science, Institution Guangxi Teachers Education

University, Nanning 530001, China. §

Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemical Engineering

and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, China.

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Information of corresponding author

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Tel.: +86 27 67843016

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*E-mail address: [email protected]

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ACS Catalysis

ABSTRACT

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A range of cobalt catalysts, prepared using TiO2 enwrapped with a hydrothermal carbon (HTC) layer, was

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used as a support for the Fischer-Tropsch Synthesis (FTS). The effects of the HTC coating thickness and

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annealing process of HTC-TiO2 on the structure and performance of the cobalt catalysts were investigated. The

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HTC coating with enrichment functional groups (e.g. COOH and COH) generated a strong interaction with the

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Co species during catalyst preparation, and finally enhanced cobalt oxide dispersion. Furthermore, the

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protection of the surface of TiO2 by a HTC coating improved the reducibility of the cobalt catalysts. The Co

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particle size was correlated with the thickness of HTC coating and the HTC-TiO2 support annealing process. It

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was found that the HTC-TiO2 support with a coating thickness of 8 nm and without an annealing process gave

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catalysts with a cobalt particle size of 8.6 nm and optimal FTS performance. The corresponding Co/8C-TiO2

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catalyst exhibited an activity 2.4 times higher than that of a Co/TiO2 catalyst, and showed 85% selectivity to

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C5+ products under realistic FTS reaction conditions (210 oC, 1.0 MPa). The synergic effect of the HTC coating

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and the TiO2 core had benefits on the catalytic properties of the Co/C-TiO2 catalysts; the HTC coating

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improved dispersion and reducibility of cobalt species, while the existence of the TiO2 core enhanced the

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stability of the catalyst structure in the reaction.

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KEYWORDS: Fischer-Tropsch synthesis; hydrothermal carbon; cobalt nanoparticles; synergic effect;

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metal-support interaction

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1. INTRODUCTION

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Fischer-Tropsch synthesis (FTS) is an alternative route used to convert natural gas, coal or biomass into

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environmentally friendly fuels and valuable chemicals via syngas (CO + H2).1 Supported cobalt catalysts are of

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great interest for their applications in FTS due to their unique catalytic properties. Tremendous effort has been

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devoted to the fabrication of supported cobalt catalysts and the exploration of their catalytic properties from

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both scientific and industrial perspectives.2,3 Cobalt dispersion, reducibility and stability are the key parameters

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which affect catalytic performance. For cobalt metal particles larger than 6~8 nm, optimal FT reaction rates

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have been observed.4,5 Previous researchers have revealed that the physical and chemical properties of the

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support, such as surface area, micro- and meso-porosity, morphology, strong metal-support interactions (SMSI)

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and chemical composition have a significant influence on the catalytic performance of supported cobalt-based

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FTS catalysts.6-11 Various oxide support materials such as TiO2, SiO2, Al2O3, CeO2, ZrO2 and so on, have been

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used to disperse and stabilize the Co nanoparticles.10,12-15 Unfortunately, highly dispersed cobalt particles

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interact strongly with an oxide carrier during thermal activation treatments, resulting in the formation of mixed

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cobalt-support compounds (i.e., cobalt silicates in the case of Co/SiO2 catalysts) or strong metal-support

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interactions (e.g. cobalt titanates in the case of Co/TiO2 catalysts), which are difficult to reduce and are inactive

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in the CO hydrogenation reaction.9,11,16

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The performance of supported cobalt catalysts can be improved by promotion with noble metals (Pt, Ru, and

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Re).17,18 The high cost and rarity of noble metals however, can undermine the economic efficiency of the FT

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technology. The structure and performance of supported cobalt catalysts can also be optimized by using carbon

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materials as catalytic supports. Different types of carbonaceous materials such as carbon nanotubes (CNT),

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activated carbon (AC), and carbon nanofibers (CNF) have been tested as supports for cobalt catalysis.19,20

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These catalysts demonstrated a high turnover frequency and a high selectivity to C5+ hydrocarbons in FT

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synthesis due to the high dispersion and reducibility of cobalt species. Carbon is a good support for use in 3


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catalysis research due its limited reactivity, lower metal-support interaction and adjustable texture. However,

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these novel carbon materials are much more expensive than oxide supports that are typically used in catalysis.

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Moreover, these carbon materials prepared by high temperature carbonization must be treated in acid and/or

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base solutions to introduce functionalized groups before anchoring metal or metal oxide particles, which is

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environmentally unfriendly. Thus, the use of carbon supports may not be suitable for the preparation of FT

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catalysts on an industrial scale.

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A carbon material, termed a hydrothermal carbon (HTC), which is prepared by hydrothermal carbonization

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of sugars or other biomass derived compounds, offers a cheap and environmentally friendly method to produce

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carbon materials.21,22 In addition, one important advantage offered by HTC materials over conventional carbons

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and oxide supports is the rich functionalized surface that is offered by these carbon materials (Figure 1).23

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These surface oxide functional groups have shown reactivity towards various inorganic precursors. For

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example, based on their redox behavior, they could reduce in situ and stabilize metal ions by forming very

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small metal nanostructures.23,24 Indeed, HTCs have been utilized as catalyst supports for the deposition of metal

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nanoparticles.25,26 The main drawback of using the metal catalysts supported on HTC is that they have

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insufficient mechanical strength, a low chemical stability and an inability to be regenerated in an oxidizing

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atmosphere. It appears that these materials may not be suitable for use under harsh conditions in a continuous

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FT reactor. However, HTC carbon has been shown to coat on oxide nanoparticles giving a homogenous

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distribution of particles with a thickness of only several nanometers on the outer surface of high surface area

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oxide nanoparticle.27,28 Preliminary have observed that such a HTC-oxide nanocomposite with a rich

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functionalized surface had advantages compared to conventional oxide for the preparation of cobalt catalysts

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with high cobalt dispersion and reducibility which exhibited enhanced hydrocarbon productivity in FT

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synthesis.29 In the past few years, many functional carbon-oxide hybrid materials have been produced via

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different processes and these materials have shown great potential in many catalytic fields.30-34 For example, 4


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Cheng et al. reported that introduction of activated carbon on porous silica as supports during catalyst

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impregnation and drying processes, and with subsequent carbon removal led to a dramatic enhancement of the

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cobalt dispersion and catalytic performance in Fischer-Tropsch synthesis.34 However, further study on the

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effects of carbonaceous structures still needs to be carried out in more detail in order to realize the goal of a

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controlled catalyst synthesis-catalyst activity correlation.

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In the present study, we have prepared a series of cobalt catalysts supported on HTC coated TiO2, pure TiO2

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and HTC sphere supports. Conventional impregnation method was used to investigate the effect of the

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functional surface layer of the TiO2 on the FTS performance of a Co loaded catalyst. The study allowed for an

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investigation of the synergic effect of the HTC coating on the TiO2 core as determined by studies of supported

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cobalt FT catalysts. Commercial P25 was used as the titanium oxide precursor, to demonstrate the applicability

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of our approach.

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2. EXPERIMENTAL SECTION

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2.1. Support and catalyst preparation. The synthesis procedure to make C-TiO2 is based on the hydrothermal

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synthesis,27 partially modified in order to enhance carbon layer uniformity. In a typical synthesis, 30 g of

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glucose and 5 g of P25 (TiO2, Degussa) were added into 250 ml distilled water under ultra-sonication (100W,

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40 kHz). After sonication for 30 min in a water bath at a temperature of 35 °C, the solution was transferred to a

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500 mL autoclave and heated to 180 oC and held at this temperature for 10 h. Finally, the resultant dark

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colloidal solution was separated by filtration, and washed with deionized water and ethanol repetitively. The

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obtained dark products were dried in air at 120 °C overnight and served as the catalyst support. The sample was

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named C-TiO2. When C-TiO2 was annealed in N2 (800 oC for 3 h) it was called aC-TiO2.

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The thickness of the carbon layer was affected by the amount of glucose added (adjusted mass ratio of

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glucose to P25 was 2:1, 4:1, 6:1 and 8:1). The C-TiO2 supports with different carbon layers were denoted as 5


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nC-TiO2, where n are 3, 5, 8, or 12 representing the average thickness of the carbon layers (corresponding to

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the four ratios used). In order to investigate the role of HTC on the activity and the selectivity of the catalysts

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without the interference from TiO2, HTC spheres with large diameter (180 nm - 220 nm) were used as a support

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to prepare cobalt catalysts. The HTC spheres denoted as CS were obtained as reported in previous papers.35

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All of the supported catalysts were prepared by impregnation of the supports with aqueous solutions of

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cobalt nitrate. The catalysts were dried at 110 oC for 12 h, and then calcined in static N2 (for the carbon content

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supports) or air (for the oxide supports) at 350 oC for 6 h. The academic cobalt content in the catalysts was 15

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wt. %. The reference supported catalysts are designated as Co/S where S is the support used.

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2.2. Support and catalyst Characterization. The morphology of the samples was characterized using TEM

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(FEI Tecnai G2 20, 40 kV). The textural properties of the prepared supports and catalysts were determined via

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N2 physisorption at 196 oC, using a Quantochrome Autosorb-1 instrument. XRD patterns were obtained on a

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Bruker-D8 Powder Diffractometer with Cu Kα radiation (k = 1.54056 Å ), 40 mA, 40 kV, scanning speed

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2˚·min-1. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a VG Multilab 2000 spectrometer

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(Thermo Electron Corporation) with Al Kα radiation as the exciting source (300 W). The C 1s peak of carbon

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(284.6 eV) was used as a reference for estimating the binding energy.36 Raman spectra were recorded on a

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Thermo Fischer DXR equipped with a diode laser of excitation 532 nm (laser serial number: AJC1200566).

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Spectra were obtained at a laser output power of 1 mW (532 nm), and a 0.2 s acquisition time with 900

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lines·mm-1 grating (Grating serial number: AJG1200531) in the wavenumber range of 50-3500 cm−1. The cobalt

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catalyst loadings were determined by ICP-AES using a Perkin-Elmer Plasma 400 ICP Emission spectrometer.

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H2-TPR measurements were carried out using an AMI-200 unit. A calcined catalyst was placed in a U-shaped

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quartz tubular reactor, and treated in 10 vol. % of hydrogen in argon at a constant flow rate 30 mL·min-1, from

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room temperature to 800 oC (hold 30 min) at a heating rate 10 oC·min-1 with a TC detector. Before the analysis, 6


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the samples were calcined at 150 oC in an argon atmosphere for 1 h to drive away traces of moisture. Hydrogen

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chemisorption and oxygen titration measurements were also performed using the Zeton Altamira AMI-200 unit.

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The catalysts were reduced in flowing H2 at 450 oC, for 10 h, then cooled to 100 oC, under a hydrogen gas flow

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and purged with argon and held at this temperature under flowing Ar, in order to desorb the remaining

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chemisorbed H2. The reduced catalyst was re-oxidized at 450 oC, by purging it with oxygen pulses until no

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further consumption of O2, was detected by a TCD located downstream. The methods to calculate the cobalt

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catalyst dispersion and reduction degree have been reported previously.37

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2.3. Catalyst Evaluation. The FTS performance of the catalysts was tested in a stainless steel fixed-bed reactor

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with inner diameter of 12 mm. Catalyst (0.5 g) was mixed with inert SiC particles (5 g) and loaded into a

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reactor for all the reaction tests. All the catalysts were activated with flowing H2 at 450 oC, 0.10 MPa, and 6

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NL·h−1·g−1 for 10 h. The reaction conditions were maintained at 210 oC, 1.0 MPa, 2 SL·g−1·h−1, and H2/CO = 2.

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During the reaction, the wax and liquid products were condensed in two consecutive traps maintained at 100 oC

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and 0 oC, respectively. The reactant and effluent gas composition were monitored online using an Agilent

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MicroGC 3000A gas chromatograph. The product selectivity was calculated based on the carbon mass balance.

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3. RESULTS AND DISCUSSION

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3.1. Effect of the functional groups of HTC coating on catalysts properties. Figure 2a clearly shows that the

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commercial TiO2 consist of spherical-like particles, with an diameter of 15-35 nm. For prepared C-TiO2 (Figure

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2b) and aC-TiO2 (Figure 2c), a uniform carbon layer of approximately 8 nm and 4 nm in thickness was formed

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around the TiO2 particles, respectively. The bulk composition of the supports was determined by XRD analysis

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(Figure S1). Titanium oxide in all the supports had the phases of both rutile and anatase, different treatments

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had almost no effect on the phase composition of TiO2. No peaks belong to the carbon were observed,

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indicating an amorphous structure for the coated materials.

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XPS characterization was conducted to analyze the surface composition of the samples, as shown in Figure

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S2. The C 1s peak of carbon (284.6 eV) was used as a reference for estimating the binding energy.36 For the

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pure TiO2, the peak located at 463.9 eV corresponds to the Ti 2p1/2 and the peak located at 458.2 eV was

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assigned to Ti 2p3/2.38 In comparison with pure TiO2, the peaks for Ti 2p in the aC-TiO2 supports shows lower

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intensity after addition of the 4 nm carbon layer on the TiO2 surface. For the C-TiO2 with 8 nm HTC layer, the

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peaks of Ti 2p were difficult to detect, indicating that the TiO2 particles had been well coated by the HTC. For

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pure TiO2, only an oxygen signal at 529.4 eV was detected and this is attributed to lattice O (Ti-O-Ti) in TiO2.27

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In the HTC containing samples, the broad peaks for O 1s between 536.0 eV to 528.0 eV were assigned to O

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atoms in oxygen-containing functional groups and Ti-O-Ti. Figure 3 shows the O 1s signal of C-TiO2 and

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aC-TiO2 could be fitted with five contributions. The five peaks of 530.0, 530.8, 532.0, 532.8 and 533.6 eV can

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be attributed to Ti-O-Ti (lattice O), C=O (ketone or carbonyl), O=C-OH or O=C-OR (carboxyl or esters),

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C-O-C (epoxy) and C-OH (hydroxyl) species, respectively.21,36 When compared with C-TiO2, the proportion of

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the O 1s peaks at 532.0, 532.8 and 533.6 eV for aC-TiO2 decreased relative to C-TiO2 demonstrating that the

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hydroxyl, epoxide, and carboxyl groups have been effectively diminished after annealing in N2 at 800 oC for 6

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h (Table S1).

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The thermal stability and amount of carbon in the C-TiO2 and aC-TiO2 samples was analyzed by TG. The

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TGA curves of C-TiO2 showed mass losses of about 47.3% during calcination (Figure 4). The first weight loss

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peak at ca. 250 oC-400 oC was due to the decomposition of hydrocarbons. The second weight loss peak at ca.

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400-550 oC is due to the oxidation of poly-aromatic hydrocarbons.24 Upon annealing in N2 at 800 oC for 3 h,

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the oxidation temperature of the aC-TiO2 increased, indicating that the HTC structure was more stable after

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annealing. 8


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In order to check the carbon structures in the C-TiO2 and aC-TiO2 products, Raman spectroscopy analysis

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was performed (Figure S3). The Raman spectrum of the C-TiO2 is composed of a G band, situated at 1578 cm−1,

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a broad shoulder at 1300-1400 cm−1 containing the D band, and two weak and broad bands situated at 400-600

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cm−1 and 2800-2900 cm−1, relating to C-H bonds. These are characteristic of the presence of sp2 carbon atoms

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in disordered ring structures of polyaromatic hydrocarbons.21,39 For aC-TiO2, the G band narrows and blue

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shifts by 10 cm−1, while the D band is observed at 1350 cm−1, and shows an IG/ID intensity ratio of 1.08. This

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intensity ratio indicates a low disordered sp2 clustering in an amorphous carbon structure.40 The evolution of

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the Raman spectra implies aromatization and/or defect passivation occurred as a result of the removal of

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oxygen-functional groups (e.g. COOH and COH) during annealing, which is in good agreement with the XPS

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and TGA results. As expected, heat treatment of the HTC materials had a significant affected on the surface

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structural properties.

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The XRD patterns of the Co/TiO2, Co/C-TiO2 and Co/aC-TiO2 catalysts are shown in Figure 5. The mean

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particle sizes of the supported cobalt oxide crystallites, estimated by XRD analysis of the intense reflections at

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2θ = 65.3˚ (for Co3O4) and 42.3˚ (for CoO), using the Scherrer equation, are given in Table S2. The size of the

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Co3O4 crystallites on Co/TiO2 is 16.3 nm. The CoO particles of 22.1 nm are formed on the Co/aC-TiO2

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catalysts. The Co/C-TiO2 had a CoO nanoparticles of 9.3 nm, indicating more surface functional groups (−COH

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and –COOH) favor smaller CoO particles formed. The surface area for these supports was in the same level

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(40-60 m2·g-1 (Table 1)), indicating that the effect of surface area on cobalt dispersion could be neglected in this

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study.

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Figure 6 displays the TEM images of the Co/TiO2, Co/C-TiO2 and Co/aC-TiO2 catalysts. It can be seen that

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the catalyst Co/TiO2 contain agglomerates of small cobalt particles (labeled by arrows), which stretch over a

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number of support particles to form islands (Figure 6 (a)). However, the highly dispersed cobalt particles on the

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Co/C-TiO2 (Figure 6 (b)) and Co/aC-TiO2 (Figure 6 (c)) catalysts were obtained, indicating that the HTC layers 9


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are beneficial for cobalt dispersion. The size of cobalt species found for the Co/C-TiO2 are 9.7 ± 3 nm and

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Co/aC-TiO2 are 22.5 ± 5 nm as measured by TEM images (Figure S4), consistent with the XRD results.

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The reduction behavior of the catalysts was studied by H2-TPR (Figure 7). For the Co/TiO2 catalysts, the

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peak located at 325 oC and the peak located at 432 oC can be attributed to the two reduction steps of Co3O4 to

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CoO and CoO to Co0.12,37 A very broad peak was located at 340 oC-700 oC, and a shoulder peak at 439 oC for

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Co/C-TiO2 and 528 oC for Co/aC-TiO2, which can be attributed to the reduction of CoO particle to Co0 and

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carbon gasification.41 The H2-TPR patterns show that the cobalt species of Co/C-TiO2 and Co/aC-TiO2 catalysts

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may complete reduced at T < 450 oC. Thus, after reduction in H2 at 450 oC for 12 h, the main cobalt phase on

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the Co/C-TiO2 and Co/aC-TiO2 catalyst is expected to be metallic cobalt. The H2 chemisorption results show

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that the cobalt dispersion increased in the order Co/aC-TiO2 < Co/TiO2 < Co/C-TiO2, while the average cobalt

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metal particle size decreased (Table 1). As would be expected, the catalysts prepared by impregnation of HTC

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coated TiO2 displayed good reducibility even though it contained smaller cobalt particles. Therefore, the

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protection of the surface of oxide supports by carbon before the deposition of cobalt nanoparticles is essential

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to avoid the formation of a strong metal-oxide interaction, which then allows a high degree of reduction.

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3.2. Effect of the HTC coating thickness on catalysts properties. The TEM images of the prepared nC-TiO2

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and CS supports are presented in Figure 8(a)-(e). These images clearly show that nC-TiO2 supports with

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different carbon coatings were successfully prepared by varying the preparation parameters. An increase in the

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mass ratio of glucose/TiO2 from 2 to 8 increased the carbon coating thickness from 3 to 12 nm. The CS has a

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spherical structure with uniform diameter of 180 nm-220 nm (Figure 8(e)). The amount of carbon deposited on

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the four nC-TiO2 supports was determined from TGA data and is shown in Figure 8(f). All samples displayed a

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first weight loss peak at ca. 250 oC-400 oC and showed a second weight loss peak at ca. 400-550 oC indicating

10


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that all samples were coated with HTC. The total weight loss increased from 23.6 % to 56.8 % with increasing

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carbon coating thickness.

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The surface chemical composition of the prepared nC-TiO2 supports was analyzed by XPS. The C 1s and O

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1s XPS spectra are shown in Figure 9. The major peak at 284.6 eV is characteristic of the C 1s carbon shell

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(Figure 9(a)). The intensity of the O 1s peak at 529.9 eV is attributed to O in Ti-O-Ti for the nC-TiO2 supports

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and this decreased remarkably as the carbon coating thickness increased (Figure 9(b)). The spectra displayed

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intense C 1s and O 1s peaks which indicated the presence of carbon and oxygen in the samples. Quantitative

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analysis suggested that the carbon and oxygen concentrations on the nC-TiO2 supports surface are similar to the

9

data for CS (Table S3). This suggests that the pre-coated TiO2 particles with different HTC coverage give

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results with similar surface functional groups for the series nC-TiO2 supports.

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Figure S5 shows the XRD patterns of the prepared Co/nC-TiO2 and Co/CS catalysts in the 2θ range of

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10-80˚. The relatively broad peak observed in the XRD patterns of the catalysts suggests that they comprise of

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very small nanocrystallites. It is noted that, with the increase of the thickness of the carbon coatings, all of the

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CoO peaks became more intense indicating increased crystallinity of the primary CoO particles (Table S2). For

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the Co/3C-TiO2 catalyst, the peak was too broad to be used for the analysis of the CoO particle size; this

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indicates a finely dispersed Co oxide is formed prior to catalyst activation. For the Co/CS catalyst, the CoO

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particle size was the largest at 16.2 nm, and a Co0 phase (15.3 nm) was also observed at 2θ = 45.7˚. This

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indicates that the cobalt species can be reduced by HTC. Similar behavior showing that cobalt species can be

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reduced by carbon during heat treatment in N2 has been reported.41 TEM images shows the crystallite sizes

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varied from 4.0 to 12.2 nm when the corresponding thickness of the carbon coatings changed from 3 nm to

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12 nm (Figure 10 and Figure S6), and with average diameter of 15.5 nm are homogeneously dispersed on

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the Co/CS catalyst (Figure 11). XRD and TEM results indicate that an increase in the carbon coating

23

thickness of nC-TiO2 support results in an increase in the Co crystallite size. 11


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Figure S7 shows the H2-TPR profiles of the Co/nC-TiO2 and Co/CS catalysts. The intensity of the high

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temperature peak at  450 oC increased indicating that total carbon gasification increased. However, increasing

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the HTC thickness of the catalysts from 3 nm to a large value (CS ~ 200 nm), gave no significant change in the

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temperature of the reduction peaks of CoO , indicating that the reducibility of CoO to Co0 was not influenced

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by the CoO particle sizes in these catalysts. This is different from results obtained from cobalt catalysts

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supported on porous oxide materials where the particle size significantly affected the reduction behavior of the

7

cobalt oxide phase.42 Thus, the nC-TiO2 and CS supported cobalt catalysts have a weak interaction between the

8

cobalt oxide particles and the supports, in which cobalt species are easier to reduce even for smaller CoO

9

particles. H2-TPD data for the Co/nC-TiO2 and Co/CS catalysts are summarized in Table 1. It is seen that the

10

thickness of the carbon coatings have a great influence on the cobalt particle size. With an increase in the

11

thickness of the carbon coating, the hydrogen consumption decreased from 281.1 to 108.9 µmol·g-1 and the

12

cobalt dispersion decreased from 22.6 % to 8.3 %. As the degree of reduction almost 100 % (estimated from

13

TPR results), the cobalt particle size of the catalysts increased from 4.5 to 12.3 nm with an increase in the

14

thickness of the carbon coatings (Figure 12), consistent with the results of XRD and TEM data. It should be

15

noted that the Co0 particle size measured from H2-TPD studies of the Co/nC-TiO2 catalysts were silightly larger

16

than those measuered by XRD and TEM. The Co/CS catalysts had very large particle (36.2 nm) indicating

17

sintering of Co0 particles during catalyst reduction.

18 19

3.3. Fischer-Tropsch synthesis performances for the catalysts. Table 2 shows the FTS performance data

20

for the catalysts. It can be seen that the activity and the selectivity of the catalysts were very sensitive to the

21

support. The following trends are apparent: (1) The CO conversion first increased and then decreased with

22

increasing HTC coating thickness and the C5+ selectivity showed a similar trend to the CO conversion; the CH4

23

selectivity showed the reverse trend; (2) The CO conversion of Co/aC-TiO2 and Co/CS were lower than that of 12


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

1

other catalysts studied under the same conditions. The cobalt time yield was calculated and plotted as a

2

function of the HTC coating thickness for the cobalt catalysts. As shown in Figure 13, a clear volcano-like

3

curve for the cobalt time yield and HTC coating thickness was observed such that the 8 nm HTC coated

4

material gave the highest cobalt time yield. FT reaction rate over the catalyst Co/8C-TiO2 was almost 2.5 times

5

than that of Co/TiO2. The highest C5+ hydrocarbon selectivity of 85.2 % and lowest methane selectivity of 8.9 %

6

were also observed for the Co/8C-TiO2 catalyst. Thus, the preparation of the Co catalyst by the HTC-TiO2

7

support leads to a remarkable increase in FTS performance.

8

The amount of exposed active cobalt metal sites and the intrinsic site reaction rate (TOF) are the key

9

parameters to give the higher catalyst activity and selectivity in the FT synthesis.37 The average cobalt particle

10

size of the catalysts studied is summarized in Table S2. The Co nanoparticle size in the series Co/nC-TiO2

11

catalysts increased with an increase in the thickness of the HTC coating of the nC-TiO2 supports (Figure 12).

12

These catalysts however exhibited similar behavior in reducebility. Thus, with an increase in the Co

13

nanoparticle size, the amount of the exposed active cobalt metal sites decreased. The apparent turnover

14

frequency (TOF) of these cobalt catalysts based on H2 chemisorption has also been calculated (Table 2). As can

15

be seen, the TOF is almost constant for cobalt particles above 8.6 nm (catalyst Co/8C-TiO2) but decreased

16

sharply for the cobalt catalysts with smaller cobalt particles. Similar relationships between cobalt particle size

17

and TOF values have been found by other research groups.4,5 Thus, the 8 nm HTC coating gives the optimal

18

size to produce the best cobalt catalysts.

19

It has been reported that C5+ selectivity can be affected by the amount of water produced at different CO

20

conversions.10 The selectivity to C5+ hydrocarbons displayed high values at a high CO conversion. The effect of

21

cobalt particle size on the hydrocarbon selectivity in FTS has also been investigated in many studies.4,43 The

22

decrease in the size to 8 nm cobalt nanoparticles supported on carbon or oxide supports resulted in lower C5+

23

selectivity and a higher CH4 selectivity. Thus, the various product selectivity observed for the series catalysts 13


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1

ACS Catalysis

can be correlated with the change in cobalt particle size and CO conversion in the catalyst.

2 3

3.4. Origin of the HTC coating effects. Aqueous cobalt nitrate impregnation is the most common catalyst

4

preparation method used to make cobalt FTS catalysts. For impregnation of a support material with a precursor

5

containing solution, the formation of cobalt oxide particles involves two steps: deposition of Co(NO3)2•xH2O

6

during drying [Eq. (1)] and subsequent thermal treatment to convert the precursor into the desired metal oxide

7

[Eq. (2)].

8

Co(NO3)2•6H2O(aq) + oxide (s) → Co(NO3)2•xH2O∙∙∙oxide (s) + yH2O (g)

（1）

9

3Co(NO3)2•xH2O∙∙∙oxide (s) →Co3O4∙∙∙oxide (s) +6NO2(g) + O2 (g)+3xH2O (g)

（2）

10

The aggregation of cobalt nanoparticles during the synthesis of supported catalysts is found to originate in the

11

drying and calcination steps after impregnation of the support with an aqueous cobalt nitrate precursor.

12

Previous report have shown that both the deposition and decomposition of the cobalt nitrate can be crucial steps

13

in the preparation of highly dispersed cobalt catalysts.44 Higher metal dispersions can be obtained by improved

14

deposition processes, such as freeze-drying,45 drying in a N2 flow,46 varying drying methods,47 gas anti-solvent

15

precipitation,48 or by organic assisted impregnation methods.49,50 The size of cobalt oxide crystallites, which

16

form on decomposition of cobalt nitrate, depends on the rate of the nitrate decomposition, as well as crystal

17

nucleation and growth. A more gentle decomposition process, such as decomposition at a lower

18

temperature,51,52 decomposition at a higher space velocity,44 decomposition in the presence of NO,53 or

19

decomposition in a glow discharge,54 could result in enhanced cobalt dispersion. The low salt-support

20

interaction in nitrate-oxide systems may be the reason for the aggregation of the cobalt particles.55 Thus, severe

21

sintering and redistribution upon thermal drying and decomposition would lead to aggregation to form large

22

oxide particles on the surface of the support.

23

The possible reason for higher cobalt dispersion on oxide with coating of HTC layers is relate to its surface 14


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Page 16 of 27

1

composition. In this case, the formed HTC polymers have a large amount of functional groups (such as

2

hydroxyls (-COH) and carbonyls (-COOH)) on the surface. These groups will strongly interact with the cobalt

3

nitrate during drying [Eq. (3)] and in the decomposition process [Eq. (4)].

4

Co(NO3)2•6H2O(aq) + COH-Cn(s) → Co(NO3)2•xH2O∙∙∙COH-Cn (s) + yH2O (g) or/ and

5

Co(NO3)2•6H2O(aq) + COOH-Cn (s) → Co(NO3)2•xH2O∙∙∙COOH-Cn (s) + yH2O (g)

6

Co(NO3)2•xH2O∙∙∙COH-Cn + OH → CoO∙∙∙Cn (s) +2NO2(g) + CO2 (g)+(x+1)H2O (g) or/ and

7

Co(NO3)2•xH2O∙∙∙COOH-Cn + H → CoO∙∙∙Cn (s) +2NO2(g) + CO2 (g)+(x+1)H2O (g)

8

Cn(s) = nC/TiO2

9

The interaction sites on HTC surface could act as the nucleation sites for cobalt oxide crystallization, and give a

10

high rate of crystal nucleation and a low rate of crystal growth. Thus, addition of HTC layers on the TiO2

11

supports results in a higher CoO dispersion rather than aggregation of the Co3O4 particles. However, the formed

12

cobalt species (CoO and Coo) have a weak interaction with the HTC surface, thus the following calcination,

13

reduction and reaction steps seem to increase the sintering of crystal. Considering the carbon layer, a larger

14

thickness gave a cobalt particle size increase. As the carbon layer thickness increased largest cobalt particles

15

(36.2 nm) formed on the Co/CS catalyst that could be explained by the sintering. The TEM images for the spent

16

catalyst Co/CS showed many large Co metal particles that were aggregated after use (Figure S8 (a)). In addition,

17

the resulting carbon nanostructure was broken down to produce numerous voids. This observation is in

18

accordance with previous report,25 where partially graphitized HTC spheres were perforated after FT synthesis

19

experiments. Due to this breakdown of the HTC structure under this condition, cobalt crystallites were in closer

20

contaxt with the TiO2 core. Therefore, the interaction between cobalt and TiO2 may be created by the formation

21

of Co-TiO2 interfaces between cobalt and TiO2 during the reduction and reaction process (Figure S9). Due to

22

the formation of this interaction, particle sintering to form large cobalt particle was restrained, which resulted in

23

a high-dispersion of cobalt of the Co/C-TiO2 catalyst. This may be the origin of the synergic effect found for 15


（3）

（4）

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1

ACS Catalysis

the hydrothermal carbon coating of the TiO2 core.

2 3

4. CONCLUSIONS

4

In summary, we have developed a hydrothermal carbon (HTC) coated TiO2 structure (C-TiO2) for the

5

synthesis supported cobalt catalysts with well particle distribution, high reducibility and controlled chemical

6

composition. The cobalt particle size can be effectively tuned by controlling the HTC coating thickness and

7

annealing temperature. Compared to pure TiO2 support, pure HTC support (CS) and annealed C-TiO2 support

8

(aC-TiO2), the nC-TiO2 supported cobalt catalysts exhibited enhanced catalytic performance with the higher

9

conversion and C5+ hydrocarbon selectivity. The 8 nm HTC coating is the optimal thickness for the nC-TiO2

10

support to given cobalt catalysts with optimal cobalt particle size (8.6 nm) and FTS performance. We have

11

demonstrated there is a synergistic effect between the HTC coating and the TiO2 core related to the breakdown

12

of the HTC structure during the reduction and reaction process. The HTC structure effectively disperses the

13

CoO and the TiO2 immobilized the cobalt particles. It is expected that this study can provide new avenues for

14

the rational design of highly efficient metal-based catalysts for potential practical applications.

15 16

ASSOCIATED CONTENT

17

Supporting Information.

18

The Supporting Information is available free of charge on the ACS Publications website at DOI:

19

Additional characterization data includes XRD, TEM, XPS, Raman spectra, H2-TPR, support surface area and

20

Co particle size.

21 22

AUTHOR INFORMATION

23

Corresponding Author

24

*E-mail: [email protected]

25

ORCID 16


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Chengchao Liu: 0000-0003-1984-6412

2

Jinlin Li: 0000-0001-7708-129X

3

Notes

4

The authors declare no competing financial interest.

Page 18 of 27

5 6

ACKNOWLEDGMENT

7

The authors are grateful for financial support from the Key Program project of the NSFC and China

8

Petrochemical Corporation Joint Fund (Grant No. U1463210), National Natural Science Foundation of China

9

(Grant Nos. 21473259 and 21403158). We thank Prof. Neil J. Coville for the helpful discussions.

10 11

REFERENCES

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ACS Catalysis

Figures

2 3

Figure 1. Schematic to illustrate a hydrothermal carbon with functional groups.

4 5 6

7 8

Figure 2. TEM images of supports (a) TiO2, (b) C-TiO2, (c) aC-TiO2.

9 10 11

12 13

Figure 3. XPS spectra of O 1s for the samples (a) aC-TiO2 and (b) C-TiO2.

14 15 16 17 20


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1

2 3

Figure 4. (a) TGA and (b) DTG patterns of the supports C-TiO2 and aC-TiO2.

4 5 6 7

8 9

Figure 5. Powder XRD patterns of the catalysts Co/TiO2, Co/C-TiO2 and Co/aC-TiO2.

10 11 12 13 14

15 16

Figure 6. TEM images of catalysts (a) Co/TiO2, (b) Co/C-TiO2 and (c) Co/aC-TiO2.

17 18 19 21


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ACS Catalysis

1

2 3

Figure 7. TPR profiles of the catalysts Co/TiO2, Co/C-TiO2 and Co/aC-TiO2.

4 5 6

7 8

Figure 8. TEM images of supports (a) 3C-TiO2, (b) 5C-TiO2, (c) 8C-TiO2, (d) 12C-TiO2 and (e) CS, and (f) TGA patterns of

9

supports nC-TiO2.

10 11 12 22


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1

2 3

Figure 9. XPS patterns of (a) C1s and (b) O1s for the sample CS and nC-TiO2.

4 5

6 7

Figure 10. TEM images of the catalysts: (a) Co/3C-TiO2, (b) Co/5C-TiO2, (c) Co/8C-TiO2 and (d) Co/12C-TiO2.

8

9 10

Figure 11. (a) TEM image and (b) cobalt particle size of the Co/CS catalyst.

23


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ACS Catalysis

1

2 3

Figure 12. Average cobalt particle as a function of HTC layer thickness for Co/nC-TiO2 catalysts.

4 5 6

7 8

Figure 13. The cobalt time yield and C5+ selectivity on Co/nC-TiO2 and Co/CS catalysts in FTS.

9 10 11 12 13 14 15 16 17 18 19 20 24


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1

Tables

2

Table 1. Support surface area, Co content, H2 chemisorption and O2 titration results for the catalysts.a Support SBET

H2 desorbed Co content (%)

Samples (m2·g-1)

3 4 5 6 7 8 9 10

a

duncorrect (%)b

Reducibility (%)

D correct (nm)c

Co/TiO2

55.9

14.1

57.3

4.8

53.7

11.6

Co/C-TiO2

41.7

14.9

150.8

11.9

--

8.7

Co/aC-TiO2

51.2

14.4

43.9

3.6

--

28.8

3C-TiO2

55.4

14.6

281.1

22.6

--

4.5

5C-TiO2

42.6

14.8

249.9

19.8

--

5.2

8C-TiO2

38.6

15.1

154.8

12.0

--

8.6

12C-TiO2

26.3

15.3

108.9

8.3

--

12.3

CS

32.9

16.2

39.2

2.8

--

36.2

Support surface area was determined by nitrogen adsorption, Co content was determined by ICP analysis, H2 chemisorption performance at 100 oC, reducibility tested by O2 pulse oxidation titration at 450 oC (-- evaluating as 100 based on TPR);

b

Uncorrected catalyst dispersion;

c

Corrected Co diameter.

Table 2. Performances of the catalysts in a fixed bed reactor.a Selectivity (mol%)

CO initial

TOF

CO steady state

Co-time-yield

conversion (%)

(10-3s-1)b

conversion (%)

(10-5molCO∙gCo-1s-1)c

CH4

C2~C4

C5+

Co/TiO2

22.9

16.5

19.7

1.1

13.5

10.5

76.0

Co/C-TiO2

48.6

13.3

42.8

2.4

9.0

5.9

85.1

Co/aC-TiO2

12.5

11.7

11.4

0.6

13.1

8.0

78.9

Co/3C-TiO2

26.8

3.9

20.1

1.1

12.3

7.8

79.9

Co/5C-TiO2

28.2

4.6

21.7

1.2

11.1

7.4

81.5

Co/8C-TiO2

51.3

13.6

44.8

2.5

8.9

5.9

85.2

Co/12C-TiO2

35.5

13.4

32.9

1.8

9.5

5.7

84.8

Co/CS

10.2

10.7

7.6

0.4

10.3

10.8

78.9

Catalyst

11 12 13 14 15

(µmolg-1)

a

Reduction conditions: in pure hydrogen at 450 oC and 1 bar for 10 h. Reaction conditions: H2/CO = 2, 210 oC, 1.0MPa, 2 SL•g−1•h−1, CO steady state conversion and hydrocarbon selectivity were collected at 60 ~100 h;

b

Turnover frequency (TOF) calculated from CO initial conversion, space velocities and H2 chemisorption data;

c

Co-time-yield calculated from CO steady state conversion, space velocities and cobalt content data. 25


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1 2

ACS Catalysis

TOC/GRAPHIC

3

26


Hydrothermal Carbon-Coated TiO2 as Support for Co-Based Catalyst

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