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Plasma-assisted Preparation of Highly Dispersed Cobalt Catalysts for Enhanced Fischer-Tropsch Synthesis Performance Jingping Hong, Juan Du, bo wang, yuhua zhang, chenchao Liu, Haifeng Xiong, fenglou sun, Sufang Chen, and Jinlin Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00960 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Catalysis

Plasma-assisted Preparation of Highly Dispersed Cobalt Catalysts for Enhanced Fischer-Tropsch Synthesis Performance Jingping Hong,*† Juan Du,† Bo Wang, † Yuhua Zhang,† Chenchao Liu,† Haifeng Xiong,* ‡ Fenglou Sun, § Sufang Chen,∥ and Jinlin Li*†

†

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs

Commission & Ministry of Education, South-Central University for Nationalities, Wuhan, Hubei 430073, China. ‡

Department

of

Chemical

and

Biological

Engineering

and

Center

for

Micro-Engineered Materials, University of New Mexico, Albuquerque, NM 87131, USA. §

College of Electronics and Information, South–Central University for Nationalities,

Wuhan 430074, Hubei Province, China. ∥

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

Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei 430073, China.

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ABSTRACT: Glow discharge plasma (GDP) was used for the preparation of Co/TiO2 Fischer-Tropsch synthesis (FTS) catalysts promoted by Pt. This technique was compared with conventional thermal calcination for decomposing cobalt precursor. In this study, Pt was added as a reduction promoter in order to improve the reducibility of Co/TiO2 catalysts because the strong interaction between Co and TiO2 is detrimental to the FTS performances. It was found that plasma treatment can effectively decompose the cobalt precursor at a low temperature in a short time (1-4 h). As compared to thermal calcination, the cobalt species prepared by GDP is highly dispersed on TiO2 support. CoPt/TiO2 catalyst with homogeneously and highly dispersed cobalt particles (~1 nm) was achieved by prolonging plasma treatment from 1 to 4 h. This is difficult to accomplish via conventional calcination method on preparing this loading cobalt catalyst (12 wt.%) without the use of extra chemicals and a complicated process. Furthermore, we found that a short time plasma treatment (< 3 h) not only increased the amount of active sites but also enhanced their specific reactivity (TOF), leading to much higher FT reaction rate.

KEYWORDS: Fischer-Tropsch synthesis, Glow discharge plasma, Cobalt catalyst, Particle size, Active sites, Reactivity, TOF

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1. INTRODUCTION Syngas (CO + H2) is an important linkage between carbon resources (such as coal, natural gas and biomass) and liquid fuels or high-value chemicals via in-direct conversion processes1. Fischer-Tropsch synthesis (FTS) is a representative pathway to convert syngas into ultra-clean liquid fuels2-4. Supported cobalt catalysts are preferred for low-temperature FTS, to obtain long chain hydrocarbons5-10. Cobalt dispersion (or particle size), reducibility and pore structure, significantly affect the catalytic properties (activity, hydrocarbon selectivity and stability). These catalyst structure parameters can be manipulated during catalyst preparation, which involves catalyst impregnation, precursor decomposition and reduction. The most common way for cobalt precursor decomposition in FTS is thermal treatment performed in a muffle furnace. The high calcination temperature used (> 350 oC) usually leads to the aggregation of cobalt species (i.e. reduced cobalt dispersion) and the formation of strong cobalt-support interacted compounds. Furthermore, besides the high energy consumption, thermal calcination process is also time-consuming (> 6 h). Therefore, alternative techniques for efficient precursor decomposition are desired. Plasmas are powerful methods for the synthesis of nano-materials.11-17 Plasma is generated by the application of energy, usually an electric field on a gas, the gas molecules or atoms are then ionized to produce high energetic electrons, positive and negative ions, radicals, and excited species. As for cold plasmas (such as glow discharge plasma, radio frequency discharge and dielectric barrier discharge plasma), the operation temperature is much lower than the electron temperature, even as low as room temperature. Thus, decomposition of catalyst precursor via cold plasmas is not based on the thermal effect but on the impact of energetic species (electrons, ions and radicals) produced in plasma field, making the catalyst presenting novel properties in

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catalysis.18-21 Several cold plasmas have been used to substitute the thermal calcination or reduction processes in catalyst preparation.22-25 For example, argon glow-discharge plasma was used to decompose and/or reduce Pt and Ni-based catalysts for methane conversion and CO oxidation12, 26, a better low-temperature activity as well as enhanced stability in plasma-treated samples were observed.26-27 Radio frequency (RF) plasma was used to prepare Fe-N/C and Co-N/C catalysts for oxygen and hydrogen peroxide reduction, and it was found that the plasma treatment led to chemical and morphological changes and enhanced the performance of the catalysts.19 Recently, the catalyst prepared by dielectric barrier discharge plasma has been shown to present uniform Co particles and similar catalytic reactivity as thermal calcination15, 28. The specific site activity (turnover frequency, TOF) of cobalt FTS catalyst is sensitive to the cobalt particle size when the cobalt particles are smaller than ca. 8 nm that the TOF sharply decreased with decreased particle size.29-31 This is because the smaller cobalt particles have stronger interaction with supports and exhibit lower reducibility, which reduce the number of active sites available. However, catalysts prepared by plasma treatment have smaller metal particle sizes around 3-6 nm. Therefore, our recent work focused on developing a catalyst having both high dispersion (i.e. small particle sizes) and high specific site reactivity, without using additional chemicals or complicated process17,

32-33

. In this study, we used the

technique of glow discharge plasma to prepare a series of CoPt/TiO2 catalysts, and compared their FTS reactivity to the counterpart prepared using thermal calcination. Different from previous studies using SiO217, 32 or Al2O316, 33 as supports, low surface area TiO2 was used as support in this work because it is more difficult to prepare highly dispersed Co catalyst on TiO2. Since a strong metal-support interaction existed

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between Co and TiO2, which leading to the formation of unreduced compounds, Pt was added as a reduction promoter in order to improve the reducibility of Co/TiO2. Various characterizations were applied to understand the effect of plasma treatment on cobalt surface structure and on the properties of cobalt active sites. 2. EXPERIMENTAL SECTION 2.1 Catalyst preparation. CoPt/TiO2 catalysts were prepared by incipient wetness co-impregnation method, using Degussa P25 TiO2 as support, Co(NO3)26H2O and Pt(NO2)2(NH3)2 as precursors. The cobalt and platinum loadings are 12 wt.% and 0.5 wt.%, respectively. The impregnated catalyst was placed in air at ambient temperature for 12 h and dried in an oven at 110 oC for 12h. After that, the sample was divided into two parts, one part was calcined at 210 oC in a muffle furnace in air for 5h, and denoted as CoPt/TiO2-C; the other part was treated by glow discharge plasma (GDP), which was generated by a high-voltage AC transformer with output power of 330W, and a duty factor (the percentage of plasma working time over total treating time in one pulse cycle) of 20%. During the GDP treating process, the sample temperature was measured by a thermocouple that was placed inside the sample holder for the precise measurement of temperature. The temperature was always kept at 210 oC. Air was used as the plasma generating gas and the pressure of system was maintained at 100 Pa through the adjustment of air flow rate. The detailed setup description can be found in our previous work32. The obtained catalysts were named as CoPt/TiO2-PN, where N indicated the plasma treating duration, with unit of hour. 2.2 Catalyst characterization. N2 adsorption-desorption isotherms were recorded using a Quanta chrome Autosorb-1-C-TCD-MS instrument. The samples were degassed at 150 oC for 3 h under vacuum. The specific surface areas of the samples were calculated with the Brunauer-Emmett-Teller (BET) equation. X-ray diffraction

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(XRD) patterns were recorded on a Bruker Advanced D8 diffractor meter with a Cu Kα radiation source (λ = 0.154056 nm). The scans were taken at the 2θ range from 20o to 80o with a step size of 0.02o. The X-ray source was operated at 40 kV and 40 mA. Raman spectra were performed on a Confocal Renishaw RM-1000 instrument with Ar ion laser of wavelength 514.5 nm. The laser power was adjusted at 7 mW with an exposure of 30 s after 3 accumulations. The FTIR spectra of the catalysts were recorded on a Nicolet NEXUS-470 Spectrometer (500-4000 cm-1) using KBr pellets. Transmission electron microscopy (TEM) of pre-reduced catalyst was performed on a FEI Tecnai G2-20 microscope (200 kV), and scanning transmission electron microscopy (STEM) of catalyst was performed using a JEOL 2010F microscope. An electron probe diameter of 0.2 nm was scanned over the specimen, and electrons scattered at high angles were collected to form the images. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG Mutilab 2000 spectrometer with monochromatic Al Kα (1486.6 eV) radiation source. Binding energies were calibrated using the C 1s peak (284.6 eV) as the reference. Hydrogen temperature-programmed reduction (H2-TPR) was conducted on a Zeton Altamira AMI-200 apparatus. A gaseous mixture of 5% H2 in N2 was used as reductant with a flow rate of 30 mL/min. The dispersion and crystallite size of cobalt were measured by hydrogen temperature programmed desorption (H2-TPD) and oxygen titration, using the Zeton Altamira AMI-200 unit. The catalysts were reduced at 330 oC (same temperature as catalyst activation) for 12 h and cooled down to 50 oC in a hydrogen flow, and then purged with argon. H2-TPD was performed from 50 -330 o

C with a heating rate of 10 oC/min and held at 330 oC under flowing argon until TCD

signal returned to the baseline. Subsequently, the reduced catalyst was re-oxidized at 330 oC, by purging with oxygen pulses until no further consumption of O2 was

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detected by the TCD. The detailed description of how to calculate the cobalt catalyst dispersion and reduction degree has been reported previously34. 2.3 Catalyst evaluation. Fischer-Tropsch synthesis (FTS) reactions were carried out in a fixed-bed stainless-steel reactor (id=12 mm). The catalyst was mixed with the same-sized inert silicon carbide particles to improve the temperature distribution, with a weight ratio of 2:5. After reducing the sample at atmospheric pressure in flowing H2 at 330 oC for 10 h, the temperature of catalyst bed was cooled down to 100 oC in flowing H2. Next, the system pressure was increased to 1.0 MPa using syngas with a H2:CO ratio of 2:1. The gas flow was set at a space velocity of 4 SL g−1 h−1 and the reaction temperature was slowly increased to 210 oC. The reaction products were collected after more than 80 h of operation, to achieve a good mass balance at close to a steady state. Liquid products and wax were collected through a cold trap keeping at 0 oC and a hot trap keeping at 120 oC, respectively. Gaseous products were analyzed on line by an Agilent Micro GC 3000A gas chromatograph (GC). The oil and wax fractions were analyzed by flame ionization detector (FID) with an Agilent GC7890 and Agilent GC6890, respectively. The mass balance and carbon balance of the reaction were both in the range of 96 % to 102 %. The cobalt-time yield is an expression of FT reaction rate, which expresses in moles of converted CO per second divided by the total amount of cobalt (in moles) loaded into the reactor. 3. RESULTS AND DISCUSSION N2 adsorption-desorption isotherms and pore size distribution curves of the calcined and plasma-treated CoPt/TiO2 catalysts are shown in Figure S1. For this type of low surface area material, the pores are referred to the voids derived from the packed nanoparticles (5-40 nm) as revealed by TEM (see below). According to the IUPAC classification, both the TiO2 support and CoPt/TiO2 catalysts prepared under

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different conditions show type IV isotherms with nonstandard hysteresis loop (Figure S1a), indicating the presence of irregular mesopores. Indeed, pore sizes of both support and catalysts show broad distribution curves and the curve peaks center at ca. 32 nm (Figure S1b). As shown in Table 1, impregnation of TiO2 with cobalt and platinum results in a considerable decrease of BET surface area and pore volume. In comparison with TiO2 support, the decreased amplitude of surface area in plasma-treated samples was in the range of 10-13%, similar to the value of cobalt loading, therefore, the drop of surface area is attributed to the effect of support dilution because of the presence of cobalt and platinum species. The much higher magnitude of the surface area drop (~29 %) in the calcined CoPt/TiO2 catalyst suggests despite of the effect of support dilution, pore plugging with cobalt particles contributes to the decreased surface area more significantly. XRD patterns of calcined and plasma-treated CoPt/TiO2 catalysts are shown in Figure 1. For comparison, the XRD pattern of pure TiO2 support is also presented. All the samples present typical diffraction peaks assigned to anatase titania and a few weak diffraction peaks corresponding to rutile titania in the two-theta range of 30-80 o, illustrating that the crystalline phase of TiO2 support is mainly in anatase as well as a small fraction of rutile. Diffraction peaks attributed to Co3O4 are only found in the XRD pattern of calcined CoPt/TiO2-C catalyst, indicating that cobalt precursor got decomposed at 210 oC35-36. However, no diffraction peak of any cobalt species was observed in plasma-treated catalysts, indicating a higher dispersion or weaker crystallinity of cobalt species. The particle size of CoPt/TiO2-C catalyst estimated using Scherrer equation is 17.1 nm. Increasing the calcination temperature from 210 o

C to 330 oC had no apparent effect on the cobalt particle size, as shown in Figure S2. In order to achieve the direct insight on cobalt dispersion of plasma-treated 8


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samples, TEM and STEM measurements were performed. The representative TEM/STEM images of calcined and plasma-treated CoPt/TiO2 catalysts are shown in Figure 2 and Figure S3. Figure 2a clearly illustrates that large cobalt particles are adhered to the surface of TiO2 support, the particle size distribution is wide, and the average particle size is ca. 12.2 nm. Note that before TEM measurements, the samples were pre-reduced under the same reduction conditions as FTS reaction and the samples for XRD

were

in their oxidation

state,

if

we assume

d(Co

metal)=0.75d(Co3O4),37 the average sizes of cobalt particles in CoPt/TiO2-C calculated by TEM (12.2 nm) and XRD (0.75 *17.1= 12.8 nm) are in a good agreement. Fig. 2b and 2c shows that homogeneously and highly dispersed cobalt particles are present on the plasma-treated samples. The cobalt particle size in CoPt/TiO2-P1 is around 4 nm (Figure 2b). While for CoPt/TiO2-P4 catalyst, very fine cobalt particles were observed, the average cobalt particle size was around 1 nm (Figure 2c). It is interesting to find that without the use of a complicated synthetic process or addition of extra chemicals, highly dispersed Co3O4 particles was synthesized by glow discharge plasma technique in a short time. Moreover, cobalt particle size can be manipulated by simply adjusting plasma treating duration. Room temperature Raman scattering spectra of calcined and plasma-treated CoPt/TiO2 catalysts are given in Figure 3. Six Raman bonds at 146 (Eg), 198 (Eg), 394 (B1g), 515 (B2g) and 630 (Eg) cm-1 corresponding to anatase TiO2 and a weak broad band at 469 cm-1 (Eg) attributing to rutile TiO2 are observed under typical Raman active modes38. Consistent with XRD results, the Raman data indicated that TiO2 support was in the form of anatase phase mixed with a small fraction of rutile phase. 9


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Besides the bands attributed to anatase and rutile titania, an additional Raman band at around 670 cm-1 is also present in all four CoPt/TiO2 catalysts, this bond is attributed to the A1g vibration of Co3O4 with spinel structure.39 Raman spectra in the region of 650-700 cm-1 (Figure 3, insert) show that with increasing plasma-treating duration from 1 to 4 h, the Raman band of Co3O4 shifts to higher wavenumber, which is considered as a result of the improved dispersion of cobalt species and the strength of cobalt-titania interaction39-40. To further determine the chemical and structural variations of the CoPt/TiO2 catalysts treated by calcination and glow discharge plasma, FT-IR studies were performed and the results are presented in Figure S4. The bands at 3435 cm-1 and 1628 cm-1 are assigned to the stretching vibration and bending vibration of hydroxyl group41, respectively. After eliminating the influence of support structure, the band intensity at 1388 cm-1, which is assigned to the stretching vibration of N-O group, is significantly higher on CoPt/TiO2-C catalyst than that on the other plasma-treated samples. This indicated that thermal calcination at 210 oC for 5 h in air atmosphere led to some residual nitrate species, while plasma treatment can effectively decompose the catalyst precursors (Co(NO3)26H2O or Pt(NO2)2(NH3)2) at 210 oC even in 1h. XPS is an effective technique to obtain the surface compositions of solid materials. Co 2p3/2 and O 1s XPS spectra of the CoPt/TiO2 catalysts are presented in Figure 4. The asymmetrical Co 2p3/2 signal of each sample can be decomposed into two peaks at binding energy (BE) around 780.0 eV and 781.6 eV, corresponding to Co3+ and Co2+, respectively.42-43 The presence of a weak satellite signal at 786.2 eV in plasma-treated catalysts indicates the presence of the abundant Co2+. Surface compositions of elements (such as cobalt, oxygen and titanium) obtained from

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ACS Catalysis

quantitative analysis of XPS peaks are listed in Table 2. The surface Co/Ti molar ratios in plasma-treated samples (0.60-0.83) were much higher than that of calcined catalyst (0.18), indicating a significant enhancement of surface cobalt dispersion by plasma treatment. As compared to calcined catalyst (1.60), lower molar ratio of Co3+/Co2+ (0.89-1.35) was found in plasma-treated samples, demonstrating that less surface cobalt species were in the form of Co3O4 and the increased amount of CoO or CoTiO3-like compounds after plasma treatment. The XPS spectra of O 1s for catalysts and support were also analyzed and are shown in Figure 4 and Figure S5. The asymmetrical O 1s signals of catalysts were deconvoluted into three peaks at BE of 529.5-530 eV, 530.1-530.7 eV and 530.9-531.6 eV, which are assigned to the surface lattice oxygen of TiO2 (Olatt-Ti), the surface lattice oxygen of CoOx (Olatt-Co) and surface adsorbed oxygen (Oads), respectively44. As shown in Table 2, the surface OLatt-Co/OLatt-Ti molar ratio was similar for the four catalysts. However, the amount of surface adsorbed oxygen species (Oads/OLatt ratio) was firstly decreased and then increased with the extended treatment of plasma. When the plasma treatment time reached 4 h (CoPt/TiO2-P4), the Oads/OLatt ratio reached 0.40, two-fold higher than the other three catalysts, indicating an extremely high dispersion. The XPS spectra of Pt 4f for the catalysts are also studied and shown in Figure S6. Very weak intensity of Pt 4f signal was presented for all four catalysts (Table 2), indicating the high dispersion of Pt. The Co/Pt atom ratio for the calcined Co/TiO2 catalyst was 15.9, the value was increased to ca. 70 after plasma treatment, indicating the enrichment of cobalt species on the outer surface of titania support or an improved Pt dispersion. The aim of the addition of Pt is to maintain the reducibility of highly dispersed cobalt species in plasma catalysts. TPR, TPD & O2 titration were applied to evaluate the reduction properties of the catalysts.

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Figure 5 shows TPR spectra of the CoPt/TiO2 catalysts, two main reduction steps are observed for all the cobalt samples in the peak temperature ranges of 109-144 oC and 310-355 oC, the first peak is assigned to the reduction of platinum species and Co3O4 to CoO. The second reduction peak (310-355 oC) is attributed to the reduction of CoO to metal cobalt

36

. The low reduction temperature of Co3O4 to CoO is

attributed to the promotion effect of Pt. The higher and sharper reduction peak at 100oC for CoPt/TiO2-C catalyst may be due to both hydrogen consumption of Co3O4 to CoO and the decomposition of residual nitrate caused by the low calcination temperature (210 oC). With increasing plasma treatment duration, the reduction temperature (CoO to Co0) slightly shifted to higher temperature region. It should be noted that except CoPt/TiO2-P1 catalyst, a weak peak at T > 370 oC which is assigned to strongly interacted cobalt-titania species appeared in the other three catalysts. Integration data of the TPR peaks are listed in Table S1, similar peak area of CoO to Co reduction (the second peak in profiles) is found in both calcined and plasma treated catalysts. Moreover, the area ratios of peak 2 to peak 1 for these catalysts are similar (2~3). To better understand the reduction behavior and cobalt dispersion of the catalysts, H2-TPD and O2-titration were performed; the results are shown in Table 3 and Table S2. Consistent with TPR, XRD and TEM results, the catalysts present similar reducibility (51-54 %) after reducing in hydrogen at 330 oC for 10 h, Pt seems to be an effective reducibility promoter for cobalt species. We found that after plasma treatment, the particle size of cobalt clusters is remarkably decreased, i.e., the cobalt dispersion is significantly increased. With increasing plasma-treatment duration from 1 to 3 h, the particle size of cobalt species was slightly diminished, and then decreased sharply when the duration time reached 4 h. Cobalt particle sizes measured by

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H2-TPD and O2 titration are in good agreement with the trend that obtained from TEM and XRD results. Detailed analysis of H2-TPD spectra of calcined and plasma-treated CoPt/TiO2 catalysts was carried out since H2 is one of the major reactants for FTS and the amount of H2 desorption is supposed to be in proportion to the number of active sites. As shown in Table 3, the desorption amount of hydrogen in CoPt/TiO2-C catalyst was much lower than plasma-treated counterpart, indicating much less active sites in the calcined catalyst. For the three catalysts treated by plasma (CoPt/TiO2-P1, CoPt/TiO2-P3 and CoPt/TiO2-P4), total desorption amount of hydrogen was nearly similar when the treating time is less than 3h, and then increased 2.7 times as the treatment time is 4h. Since the reducibility of the four catalysts was comparable, the number of active sites was thus proportional to the dispersion of cobalt species. Besides the desorption quantity, different hydrogen desorption species were also observed (Figure 6). The H2-TPD spectra of the catalysts were divided into four peaks: low temperature desorption peak located at below 100 oC, medium temperature desorption peak at 139-214oC, and two high temperature desorption peaks in the region higher than 230 oC, corresponding to sites having different adsorption strengths. After plasma treatment, the desorption peaks were firstly shifted to lower temperature regions, and with increasing plasma treatment duration from 1 to 4 h, all the peaks shifted to higher temperatures, indicating stronger adsorption of hydrogen. Especially for CoPt/TiO2-P4 catalyst, most of the adsorbed hydrogen could not desorb below 210 o

C, these strongly bonded hydrogen may occupy the active sites in the catalysts and

prevent the adsorption of CO during FTS reaction. The catalytic reactivity of the CoPt/TiO2 catalysts was tested in a micro-fixed bed Fischer-Tropsch reactor under the conditions of 1.0 MPa, 210 oC, H2/CO= 2, and

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a gas velocity of 4 SL h-1 g-1. The catalytic performance was evaluated in terms of CO conversion, cobalt-time yield, turnover frequency (TOF) and hydrocarbon selectivities, and the data are listed in Table 4. The evolution of CO conversion with time on stream is presented in Figure 7 and Figure S7, catalysts reach quasi-steady state after reaction of 20 h. We found that catalysts with short plasma treatment duration (< 2 h) showed a significant improvement in catalytic activity, the cobalt-time yield of CoPt/TiO2-P1 increased to 3.9 *10-3 molCO molCo-1 s-1, 2.4 times higher than that on the calcined counterpart. However, further increasing plasma treatment duration led to a decline in FTS activity. For CoPt/TiO2-P4 catalyst, the CO conversion is only 3.4 %. Meanwhile, product selectivities are affected by plasma treatment. Increasing plasma treatment duration, the methane selectivity was increased and the C5+hydrocarbon selectivity was decreased. The activity of cobalt-based catalysts is mainly affected by dispersion and reducibility of cobalt species3, 45. In this work, based on TPR, H2-TPD & O2 titration results, the four CoPt/TiO2 catalysts possessed similar cobalt reducibility, therefore, their catalytic activities were related to the particle size of cobalt species. It suggested that the specific site activity of cobalt catalysts remained stable for cobalt particle size larger than 6-8 nm, and then decreased with diminishing cobalt particles30-31. XRD, STEM, H2-TPD & O2 titration results verified the particle size of cobalt species in three plasma-treated CoPt/TiO2 catalysts were below 6 nm. Cobalt particle size obtained from XRD was calculated using the Scherrer equation from the half-width of the diffraction profile. It is volume mean diameter and small cobalt particles (< 2 nm) are missed because of significant XRD line broadening. Therefore, this method overestimated cobalt particle size9,

46

. Cobalt particle size from H2-TPD and O2

titration was calculated assuming the cobalt particles are spherical morphology34, 47.

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ACS Catalysis

The H:Co stoichiometry ratio was considered as 1:1 and it is surface mean diameter. Thus, this method may underestimate the Co crystallite size. The cobalt particle size from TEM is number mean and it provides direct evidence for cobalt particle size. In this work, although the three methods had slight discrepancies in the absolute values of the cobalt particle size, they showed the same variation tendency with the plasma treatment. The specific activity (expressed as TOF) of the four catalysts and its relationship with cobalt particle size are shown in Table 4 and Figure 8. The calcined catalysts (210 oC and 330 oC) and the counterpart treated by plasma for 1 h were compared firstly. The cobalt particle sizes of CoPt/TiO2-C210 and CoPt/TiO2-C330 (XRD results) were 12.8 nm and 13.5 nm, which are in the region of dCo> 6-8 nm and is supposed to have higher and stable specific site activity according to the function of cobalt particle size and TOF (grey line in Figure 8a)30-31. However, the plasma-treated CoPt/TiO2-P1 catalyst, in which dCo is 3.8 nm, showed 1.6 times higher TOF than the calcined one (Figure 8a). Characterization results showed plasma treatment not only improves the cobalt dispersion but also modifies the adsorption properties of active sites. Higher cobalt dispersion led to much higher desorption area of hydrogen peaks in H2-TPD, corresponding to larger amount of active sites. The combination effect of both more surface active sites and higher specific site activity resulted in a much higher FTS reaction rate (cobalt-time yield) in CoPt/TiO2-P1 catalyst (Table 4). Our results show that the extended plasma treatment generates cobalt particles with smaller size, especially when the treatment duration reached 4 h, homogeneously and highly dispersed cobalt particles with size of ca.1 nm were produced. Catalysts with smaller cobalt particles possessed more surface active sites, however, their specific activity declined remarkably. This phenomenon could be explained by the different adsorption properties of active sites on catalysts with varied cobalt particle

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sizes. On smaller cobalt particles, CO adsorption energy is higher48, and higher fraction of irreversibly bonded CO is un-dissociative31. Meanwhile, stronger adsorbed H2 was also observed on the longer time plasma-treated catalysts (H2-TPD), both irreversible chemisorbed CO and the occupied active sites strongly absorbed by H2 were not available in the FTS reaction, and thus a much lower FT reaction rate was presented. The selectivity data (Table 4) showed that short time plasma treatment led to a decrease in methane selectivity. However, an increased CH4 selectivity was found on samples with longer plasma treatment and the selectivity to C5+ showed an inverse tendency. Figures 8b and S7 show the comparison of the CH4 selectivity, C5+ selectivity as a function of cobalt particle size for the catalysts prepared by thermal calcination and plasma. Firstly, a linear relationship between cobalt particle size and product selectivities was found on plasma-treated samples (cobalt particle sizes are less than 6 nm). It was reported that the H coverage was higher on smaller cobalt particles31, 49, and the higher H/CO surface ratio may explain the increased CH4 selectivity for CoPt/TiO2-P4 catalyst having small cobalt particles (ca. 1 nm). Secondly, compared to the cobalt catalysts prepared by thermal calcination, the catalysts prepared by plasma have smaller cobalt particles. Given that following the trend of methane selectivity evolution with cobalt particle size reported in literature (grey dotted line in Figure 8b), the cobalt catalysts prepared by plasma would have significantly lower methane selectivity than that prepared by conventional thermal calcination when the cobalt particle sizes prepared by the two methods are similar. Thus, glow discharge plasma could be a potential method for preparing cobalt catalyst having low methane selectivity in FTS.

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4. CONCLUSIONS Glow discharge plasma was used to decompose cobalt precursors for CoPt/TiO2 catalysts in a short time at low temperature. We found that plasma treatment improves the cobalt dispersion while maintaining the cobalt reducibility of all catalysts at a similar level. Compared to calcined sample by thermal treatment in air, the short time plasma-treated catalysts (< 3h) presented not only more surface active sites but also higher specific reactivity, leading to much higher FT reaction rates. Further increasing plasma treatment duration (4 h) led to the further decrease in cobalt particle size and the increase in the amount of active sites, whereas their specific reactivity was decreased dramatically. Therefore, a decreased reaction activity was presented. CoPt/TiO2-P1 catalyst presented a high cobalt dispersion and a high specific site activity, showing the highest FT reaction rate of 3.9 *10-3 molCO molCo-1 s-1, and the lowest methane selectivity of 9.7 %. We concluded that cobalt particle size can be effectively regulated by simply changing plasma treating duration at low temperature. Using glow discharge plasma technique for decomposing cobalt precursor, homogeneously dispersed cobalt particles with size around 1.0 nm on TiO2 was achieved. This is not plausible for other treatment methods without use of extra chemicals or complicated process. This technique might be applied as a potential route for the green and clean catalyst preparation in catalysis.

ASSOCIATED CONTENT Supporting information The supporting information is available free of charge on the ACS Publications website at DOI:

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N2 adsorption-desorption isotherms and pore size distribution curves, supplementary TEM images, FT-IR spectra, O 1s and Pt 4f XPS spectra, interpretation data of TPR reduction peaks, CH4 selectivity, C5+ selectivity as a function of cobalt particle size of four CoPt/TiO2 catalysts; XRD patterns of 210 oC and 330 oC calcined CoPt/TiO2 catalysts; CO conversion as a function of time on stream (TOS), H2-TPD and O2 titration data for two calcined and five plasma treated sample. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Jingping Hong) [email protected] (Haifeng Xiong) [email protected] (Jinlin Li) Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We thank the financial support by National Natural Science foundation of China (21203255, 21403158, 21473259), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (BZY14037), Hubei Natural Science Foundation (2018CFB556), Innovation group of Hubei Natural Science Foundation (2018CFA023), the Fundamental Research Funds for the Central Universities (South-Central University for Nationalities, CZY14005) and the Fund for Basic Scientific Research of South-Central University for Nationalities (YZZ12001). H.F. Xiong would like to thank NSF GOALI (grant CBET-1438765) and the Center for Biorenewable Chemicals (CBiRC) supported by NSF (grant EEC-0813570).

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ACS Catalysis

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Figures ♦ Anatase TiO2 ∇

Co3O4

♥ Rutile TiO2 ♦

CoPt/TiO2-P4 CoPt/TiO2-P3

Intensity (a.u.)

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

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CoPt/TiO2-P1 ∇

∇

♦

∇

CoPt/TiO2-C

♥♦♦ ♦ ♦♦

♥♦ ♦ ♥

30

40

50

60

♦

TiO2

70

80

2 Theta (degree)

Figure 1. XRD patterns of calcined and plasma-treated CoPt/TiO2 catalysts

Figure 2. TEM images and particle size distribution of CoPt/TiO2 catalysts: (a) CoPt/TiO2-C, (b) CoPt/TiO2-P1 and (c) CoPt/TiO2-P4.

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♦ ♦ Anatase TiO2 Intensity (a.u.)

♥ Rutile TiO2

Intensity (a.u.)

∇ Co3O4

600

625

650

675

700

-1

Raman shift (cm )

∇ CoPt/TiO2-P4 ♦

♥

♦

♦

CoPt/TiO2-P3 CoPt/TiO2-P1 CoPt/TiO2-C

100

200

300

400

500

600

700

800

-1

Raman shift (cm )

Figure 3. Raman spectra of calcined and plasma-treated CoPt/TiO2 catalysts 780.0

O1s

Co2p3/2 530.1 529.5

781.7 530.9

P4

P4

779.8

530.3

781.6

Intensity (a.u.)

P3

P3

Intensity (a.u.)

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

ACS Catalysis

780.4

530.7

P1

529.5

531.2

530.0

531.6

782.1

P1 530.4

781.6

C

790

785

C

779.8

780

775

533

529.9

531.3

532

531

530

529

528

Bind energy (eV)

Binding energy (eV)

Figure 4. Co 2p3/2 and O 1s XPS spectra of calcined and plasma-treated CoPt/TiO2 catalysts, the black solid curves are experimental plots and red dot lines are the fit curves.

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ACS Catalysis

356 145 327

CoPt/TiO2-P4

TCD signal (a.u.)

128 312

CoPt/TiO2-P3

114 100

CoPt/TiO2-P1

303

CoPt/TiO2-C

100

200

300

400

500

600

700

800

o

Temperature ( C)

Figure 5. TPR spectra of calcined and plasma-treated CoPt/TiO2 catalysts

214 330

97

TCD signal (a.u.)

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

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x0.5 CoPt/TiO -P4 2 144 259

86

CoPt/TiO2-P3

83 139

234

CoPt/TiO2-P1

150 82

30

286

130

230

CoPt/TiO2-C o

Isotherm at530 330 C 630 430 Temperature (oC) 330

730

Figure 6. H2-TPD spectra of calcined and plasma-treated CoPt/TiO2 catalysts, the black solid curves are experimental plots and red dot lines are the fit curves.

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70 60

CO Conversion (%)

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

ACS Catalysis

50

CoPt/TiO2-P1

40

CoPt/TiO2-P3

30

CoPt/TiO2-C

20 10

CoPt/TiO2-P4

0 0

20

40

60

80

100

TOS (h)

Figure 7. CO conversion as a function of time on stream (TOS) on CoPt/TiO2 catalysts

Figure 8. (a) TOF as a function of cobalt particle size over CoPt/TiO2 catalysts, data

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marked in red were from plasma treated samples and that marked in blue were from calcined catalysts, cobalt particle sizes of plasma treated catalysts was based on H2-TPD & O2 titration data (Table S2) and those of calcined catalysts were calculated from the equation d(Co metal)=0.75d(Co3O4, XRD); data obtained from references 10 and 30 were plotted in grey for comparison; (b) relation between cobalt particle size and hydrocarbon selectivity for cobalt catalysts prepared by plasma and thermal calcination, the grey dotted line shows the CH4 evolution trend according to literatures.

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ACS Catalysis

Tables Table 1. Summary of the textural properties of TiO2 support and CoPt/TiO2 catalysts SBET(m2/g)

Samples

Total pore

Average pore

Decreasing amplitude of

volume(mL/g)

diameter(nm)

surface area (%)

TiO2

58.2

0.43

29.6

-

CoPt/TiO2-C

41.4

0.30

29.0

28.9

CoPt/TiO2-P1

50.7

0.31

24.7

12.9

CoPt/TiO2-P3

52.4

0.32

24.8

10.0

CoPt/TiO2-P4

51.7

0.32

25.1

11.2

Table 2. Atomic ratios of calcined and plasma treated CoPt/TiO2 catalysts as obtained from XPS Catalysts

Co3+/Co2+

Oads/OLatt

OLatt-Co/OLatt-Ti

Co/Ti

Co/Pt

CoPt/TiO2-C

1.60

0.20

0.88

0.18

15.9

CoPt/TiO2-P1

1.36

0.17

0.82

0.75

69.7

CoPt/TiO2-P3

0.92

0.20

0.84

0.83

69.0

CoPt/TiO2-P4

0.89

0.40

0.82

0.60

70.8

Table 3. H2-TPD and O2 titration results for various catalysts Catalyst

H2 desorbed

Duncorrected(

duncorrected

O2 uptake

Reducibility

dcorrected

Dcorrected

(µmol/g)

%)

(nm)

(µmol/g)

(%)

(nm)

(%)

CoPt/TiO2-C

104.4

10.5

10.0

698.5

51.9

5.2

20.4

CoPt/TiO2-P1

148.8

14.6

7.0

740.2

54.3

3.8

26.9

CoPt/TiO2-P3

157.1

14.8

6.8

730.0

53.5

3.6

27.8

CoPt/TiO2-P4

405.2

39.8

2.2

690.5

51.3

1.1

73.7

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Table 4. Catalytic performance of CoPt/TiO2 catalystsa CO steady

Cobalt-time

CO initial

Yieldb

state Catalysts

conversion conversion (%)

-3

(10 molCO

(%)

molCo-1 s-1)

Product selectivities ( mol %) TOFc

Activity

-3 -1

lossd

(10 s )

CH4

C2

C3

C4

C5+

(%)

CoPt/TiO2-C

28.6

19.9

1.6

45.3

30.4

12.7

0.7

3.1

1.6

81.9

CoPt/TiO2-P1

66.3

48.2

3.9

73.7

27.3

9.7

0.8

2.7

2.7

84.1

CoPt/TiO2-P3

55.6

39.6

3.2

58.5

28.8

10.1

0.8

3.0

2.7

83.4

CoPt/TiO2-P4

4.8

3.4

0.3

2.0

-

14.9

1.4

3.2

2.4

78.1

a

Reaction conditions: P=1.0 MPa, T=210 oC, H2/CO= 2, GHSV= 4 SL h-1 g-1, CO steady state

conversion and hydrocarbon selectivity were collected at 20-50 h for CoPt/TiO2-P4, whereas 30-100 h for the other 3 catalysts. bCobalt-time yield expresses in moles of converted CO (steady state conversion) per second divided by the total amount of cobalt (in moles) loaded into the reactor. cTOF (turnover frequency) was calculated from CO initial conversion, space velocities and H2 chemisorption data. dActivity loss =(CO initial conversion – CO steady state conversion)/CO initial conversion *100

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