BN Catalyst on FischerâTropsch

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Support effect of the Fe/BN catalyst on FischerTropsch performances: the role of surface B-O defect Jianghong Wu, Liancheng Wang, Xi Yang, Baoliang Lv, and Jiangang Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04864 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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Industrial & Engineering Chemistry Research

Support Effect of the Fe/BN Catalyst on Fischer-Tropsch Performances: the Role of Surface B-O Defect Jianghong Wu ,†,‡ Liancheng Wang ,*,† Xi Yang,† Baoliang Lv ,† and Jiangang Chen *,†

†

State Key Laboratory of Coal Conversion, Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

‡

Shanxi Institute of Energy, Jinzhong 030600, China

*corresponding author's email：[email protected], [email protected] ABSTRACT: BN nanosheets (BN) have drawn great attention in heterogeneous catalysis recently and their defects have a profound effect on the key performance of the catalyst, but it has been overseen and little addressed. Here, three Fe/BN nanosheets (Fe/BN) catalysts were prepared BN with varied B-O surface defects. Fischer-Tropsch synthesis, a key industrial process, was taken as an example. It is found that the rich B-O defects not only resulted in the decreased crystal size and low crystallinity of the Fe2O3 but also showed enhanced interaction with the active component and support. As a consequence, the three Fe/BN catalysts showed dramatic difference in catalytic performance. Here, our studies reported here could be a guide for the rational design of the highly efficient hydrogenation BN-based catalyst. KEYWORDS: B-O defect, BN, support, Fischer-Tropsch synthesis

1. INTRODUCTION The support has a conspicuous effect on dispersion, sizes and reduction degree of the active phase, which plays a crucial role on the key performance indicators, including activity, selectivity and stability. The Tb-doped CeO2 support could stabilize the Cu+ and improve their CO conversion of the doped catalyst, which displayed remarkable 1

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difference1. Mo adsorption on two γ-alumina and α-alumina planar substrates also displayed obvious difference on model hydrodesulphurization (HDS) catalysis2. Also, the ceria added into supported vanadia catalyst can stabilize the reduced states in oxidative dehydrogenation by accommodating electrons in localized f-states3. When it came to the activated carbon (AC) and graphite case, the supported Ru catalysts presented different oxide forms. The AC supported Ru catalyst showed better dispersion and the higher conversion of the glycerol hydrogenolysis, while the graphite case made the availability of Ru higher on the surface, which formed deep hydrogenolysis products4. It also indicated that surface coordination/defects of the supports caused the steric effects that promoted the selectivity of some hydrogenation reactions5. These support effects are also prominent in Fischer-Tropsch synthesis (FTS) and industrial catalytic reactions likewise. As the two key parameters of catalysts, the activity and the product distribution of the FTS6 can both be tuned by the support. For example, the β-SiC and carbon nano-fiber support benefited a higher activity, but showed inferior stability to that of Fe/α-Al2O3. The β-SiC and SiO2 support showed high C2-4 olefins selectivity, while the CH4 fraction was more than 30%. In light of the formation of ternary oxides, the decreased reduction degree of the iron catalyst could hinder the formation of the active carbides7,8. Fortunately, some tactics was adopted using support with weaker interaction to improve reduction degree or using auxiliary K, Mn promoter. Among those, the carbon support has made much progress over several decades. For example, Moussa et. al found that the Graphite supported iron catalyst shown higher FTS activity than that of CNTs, because the former catalysts with less defects benefited the formation of the active iron carbides9. Coville et. al used two types of carbon supported Co catalysts. The Co at inner CNT support yielding lighter hydrocarbons in compared with that of carbon spheres (CS) was attributed to the faster diffusion rate of the H2 over CO. That is because the inside H2 rich atmosphere and the inner catalyst favored the hydrogenation other than carbon chain-growth10. Still, the loss of supports at harsh temperatures and the inevitable deposited carbon blocked its recycled usage in FTS. 2


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BNNSs, a structural analogue of the graphene and related materials, are one of the most attractive catalyst supports in recent years. The general consideration of the utilization of BN support is also ascribed to it excellent stability, thermal conductivity and inert nature11-13. The merits of the support can avoid the catalyst running-off and sintering at hot spot. The strong anti-oxidation properties also endow the BN catalyst ability that removes the deposited carbon by simply annealing in ambient atmosphere. Besides, its large surface area and abundant pore structure are supposed to benefit the good mass transfer and adsorption of the reactant matrix (CO and H2)14. Though the reports dealt with the enhanced catalytic activities in the CO oxidation15,16, photo-catalysis17, oxidative desulfurization18, selective oxidation of hydrocarbons19, hydrogenation reaction20and FTS, the defects of those studies were mainly focused on the B or N vacancy. As the main structural defects of h-BN, the B-O spices were wildly existed as defects but less focused on. It must be pointed out that different from neutron B-N pairs, the B-O bond with unpaired electrons could interact with the active phase or even the reactants, therefore a determinate role might be considered in the B-O defects laden BN support7,21. Some literatures have reported oxygen-doped BNNSs can act as an efficient H2 adsorbent, and supported iron catalyst could activate the adsorbed carbon monoxide and hydrogenation theoretically15,22,23. The recent breakthrough on hydrocarbon dehydrogenation for BN-based catalyst also showed the predominate role of B-O groups

19,24-26

. Our previous report has shown that the

N-rich/B vacancy27 and encapsulated28 structure can anchor or confine the active phase and retard the deactivation of the BN support catalyst, the catalyst even showed remarkable stability over 1000 h. Despite those significant progresses has been made, the mostly used supports were still commercial BN, and the knowledge of effect of the B-O defects was still lingering far behind. Herein, we firstly studied the effects of the B-O defects on the FTS behavior of the BN supported iron catalyst. Especially, the commercial BN support used due to its easy to get and feasible to be applied in larger scale. To exclude other influence factors, in the process, the Fe/BN catalysts used here were prepared by an incipient-wetness impregnating method. It is found that the B-O surface defects owing 3



high density not only resulted in the decreased crystal size and low crystallinity of the Fe2O3 but also showed enhanced interaction with the active component, and about 100 °C high temperature shift can be observed in H2-TPR. The enhanced interaction of the B-O defects thus could reduce reduction degree and resulted in the poor activity but the better stability.

2. EXPERIMENTAL 2.1 Materials All the reagents were analytical grade and were obtained from Sinopharm Chemical Reagent Co., Ltd without further purification. Fe2O3 with average sizes of 300-500 nm was prepared according to reference27. Commercial BN support was denoted as BNc. N-rich BN support: The support was obtained by a solid state reaction, as described in literature27. Then using diluted acid (2 mol/L) leached overnight. The support is N-rich and was denoted as BNS. O-rich BN support: BN support was obtained by pyrolysis of boric acid and urea in a weight ratio of 1 : 629. The synthesis was carried at 1000 oC for 3 h under 5% H2/N2 atmosphere ramped at 5 oC min-1, as denoted BNP.

2.2. Catalyst Preparation The incipient wetness impregnation method was used to prepare a final catalyst with 20 wt% Fe2O3. The BN support was impregnated in the calculated Fe (NO3)2 solution, then the catalyst precursor was dried at 110 oC for 12 h, followed by calcinating in air at 400 oC for 12 h. Samples obtained thereafter were referred to as Fe/BNX, The nominal loading of Fe in all catalysts was 20 wt%. Before the FTS reaction, Fe/BNX catalyst was ground and sieved to get particles with a size range of 60~80 meshes.

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2.3 Characterizations X-ray powder diffraction (XRD) was used for the structural analysis of catalyst, with a scanning angle (2θ) from 15◦ to 90◦ and a scanning speed of 2◦/min. X-ray photoelectron spectroscopy (XPS) was acquired on an Axis Ultra DLD imaging photoelectron spectrometer, Kratos Analytical Ltd. The source of X-ray was Al Ka, 1486.6 eV with a quartz monochrometer. The calibration of the binding energy was referenced to the peak of graphitic carbon at 284.6 eV. Transmission electron microscopy (TEM) images were taken on a JEOL model 1010 microscope operated at 100 kV. A Micromeritics TriStar II 3020 instrument was performed to analyze nitrogen adsorption and desorption isotherms. The interaction of the catalysts support and active phase was studied by H2 temperature-programmed reduction (H2-TPR). The iron loading of the catalysts was determined by the ICP optical emission spectroscopy (Optima2100DV, PerkinElmer).

2.4. Catalyst Test The performance of FTS was evaluated on a fixed-bed reactor equipped with a stainless steel tubular reactor. Prior to the FTS reaction, the as-prepared catalysts were reduced at 400 oC for 6 h under flowing pure H2 (GHSV=3000 h-1, P=0.2 MPa). After that, the catalysts were cooled down to room temperature, then a flow of syngas (H2/CO=2:1) was fed into fixed-bed and the temperature was increased up to 340 oC. During the FTS process, the pressure of syngas was maintained at 2 MPa. Liquid products and wax were obtained through a cold trap at 0 oC and a hot trap at 130 oC, respectively. The analysis of H2, CO, CO2, CH4 and N2 was performed using a carbon molecular sieve column and a thermal conductivity detector. Hydrocarbons were separated in capillary Porapak-Q column and analyzed using a flame ionization detector (FID). The mass and carbon balance of the reaction both maintained between 95% and 105%.

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3. RESULTS AND DISCUSSION

3.1 Phase Structure of Catalysts

Figure 1. XRD patterns of the prepared catalysts: Fe/BNC (a), Fe/BNS (b), Fe/BNP (c).

Figure 1 gives the XRD patterns of the as-obtained three catalysts. The diffraction peaks of Fe/BNC catalyst can be indexed as two major phases, Fe2O3 (JCPDS card No. 33-0664) and h-BN (JCPDS card No. 34-0421). The mean crystallite size of the Fe2O3 was about 25 nm based on the scherrer's formula. It was found that the (002)hBN plane shifted to the low angels and got wider in case of BNS and BNP. There lattice spacing of (002) was of 3.33, 3.37 and 3.62 Å, respectively. The increased width of (002) plane indicated the decreased crystallinity of BN and the rich defects of support30. It is strange that the Fe2O3 diffraction planes cannot be detected by XRD pattern in Fe/BNS and Fe/BNP catalysts. The main reason might be the ill crystallized iron with imperfect periodic structure that the diffraction is hard to occur, the iron spices therefore could be rather small in those cases. In addition, the BN support phase structure was clearly maintained after iron impregnation and calcination.

3.2 Surface Chemistry Analysis of BN Support XPS is one of the most efficient characterization tools to study the surface properties 6


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of catalyst. Figure 2a shows the full XPS spectra of the three BN support. Three main peaks were observed and were B1s, N1s and O1s signals, respectively. As can be seen, the intensity of O1s peak of BNp, BNs and BNc support decreased in sequence. Their surface oxygen contents were calculated of 22 at%, 11 at%, 1.0 at%, respectively. The sequence of BNp > BNs > BNc coincided with the fact that increased BN crystallinity of the supports, hence, we can roughly correlate the surface defects of the surface oxygen spices. As presented in Figure 2b, the high-resolution B 1s spectra provided the surface boron states in details. The main peak around 190.9 eV was assigned to B-N bonds in h-BN and shoulder at 192.5 eV should be attributed to the B-O bonds attached to the defects or the edges31. It is clear that the shoulder peak get weak in BNs and BNp case. The calculated B: N ratio of the BNp was slight larger than 1, indicating the B rich nature. The B: N ratio of the BNs and BNc was slight less than 1, indicating the N rich environment. It can be expected that the varied surface B-O defects affect the support difference. The N-H related species in N-rich support was basic surface27, while that of B-O was acidic surface, those species might be have different effects in the product selectivity. A schematic diagram of the three BN support was shown in Figure 3.

Figure 2. The XPS spectra of full range scanning for the as-prepared BN products: survey spectra (a), B1s spectrum (b).

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Figure 3. Schematic diagram of BN support: BNc (a), BNs (b) and BNp (c).

3.3 TEM Morphologies of the Fe/BN Catalysts

Figure 4. Typical TEM images of three Fe/BN catalysts: Fe/BNc (a), Fe/BNs (b), Fe/BNp (c). Figure 4 displays the TEM images of the as-prepared three catalysts. The active component nano-Fe2O3 were presented as darker particles, while the BN nanosheets as supports were seen lighter. The morphologies of the three catalysts presented different characters. The commercial BNc with thinness of tens of nanometer supported catalyst possess two type of Fe2O3, as cycled area 1 and 2. The ultra-small nanoparticles attached on the surface of the BNc in area 1, while larger nanoparticles of ~25 nm can be observed at the edge of the BNc, as indicated in area 2. The severe aggregation could be attributed to the high loading (20%), and small specific surface area (17 m2 /g) of the BNc, the deficiency in pore and defects made the dispersion and anchoring all the nanocatalyst hard. As it came to the BNs case, the thinness was 8


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mainly around 4~8 nm and the catalyst were uniformly dispersed about 7 nm. Although the same incipient wetness impregnation (IWI) was used, it is strange that the well crystallized Fe2O3 nano-catalyst can hardly be observed in BNp case, but some small dark domains with below 3 nm observed, as supposed to be iron clusters. Also the film-like structure of the BNp support have changed a bit, it seems collapsed and aggregated. In all, the worse crystallized support is, the smaller size and crystallized catalyst are. Actually, the more surface B-O defects, the stronger interaction between the iron oxides and BN supports therefore better dispersion could be expected, since the dangling B-O bond could anchor active phase.

3.4 Pore Structure of Three Fe/BN Catalysts

Figure 5. N2 adsorption-desorption isotherms (a) and BJH pore size distribution of the prepared three B-O surface defects catalysts (b).

The porous property of Fe-based catalysts was examined by N2 adsorption-desorption measurement. The Fe/BN (Figure 5a) catalysts exhibited Langmuir type IV isotherms with a H1-type hysteresis loop, indicating a typical meso-porous material.

The

catalysts (Figure 5b) all showed single mesoporous structure. The corresponding texture properties of them are summarized in Table 1. The specific surface area (SSA) of BNc and Fe/BNc were 17 and 72 m2/g, and the catalyst show wider pore size distribution (PSD) than that of pure BNc. Different from BNc case, the SSA and pore value of the other two types of catalyst showed a decrescent trend after the loading of 9



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the catalyst. The SSA dropped from 338.1 to 110.1 m2 g-1 and the pore volume from 0.63 to 0.24 cm3 g-1 in BNs case. While the SSA decreased sharply from 863.0 to 80.5 m2 g-1 and the pore volume decreased from 1.03 to 0.21 cm3 g-1 in BNp case. Here, several reasons could account for the changes in the structural properties. (1) The supported iron contents were 20 wt%, a bit high. Our pure Fe2O3 show a SSA of 77 m2/g, larger than BNc but smaller than BNs and BNp. (2) The supported iron particles could occupy the small pores and filled up partially, which contributed to the SSA largely. (3) The strong interaction between the BNp and active phase of iron-based catalyst can effect or even destroy the pore structure to some degree. Table 1. The textural properties of different BN and Fe/BNx catalyst. Catalyst Fe/BNC Fe/BNS Fe/BNP BNC BNS BNP

SBETa (m2g-1) 72.5 110.1 80.5 17.3 338.1 863.0

Vporeb(cm3g-1) 0.13 0.24 0.21 0.11 0.63 1.03

Dporec (nm) 7.02 8.78 17.1 25.7 7.43 4.3

dFe (nm) 25 ~7 nm 0.95. c Average pore diameter calculated by 4 ×Vpore/SBET. d particle estimated in TEM. b

3.5 Interaction of Support and Active Phase As we know, H2-TPR is a favorable instrument for studying complicated and highly dispersed system. For iron-based catalysts, these TPR peaks can be attributed to the reduction of different iron species in Figure 6. The results can imply that the active component Fe2O3 went through a stepwise reduction, and usually for the interaction between the iron oxides and support32. The three catalysts all showed three main reduction peaks. The first peak (I) could be assigned to the reduction of Fe2O3 to Fe3O4, the second peak (II) corresponded to the subsequent reduction of Fe3O4 to FeO and the third small reduction feature (III) may be attributed to the reduction peak of FeO to Fe. In general, the stronger interaction between the support and catalyst is, the higher reduction temperature is. Here, the first reduction peaks was taken as an 10


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example, which occurred at 396 oC, 412 oC and 516 oC for Fe/BNc, Fe/BNs and Fe/BNp, respectively. These results also implied the Fe/BNp catalyst was more difficult to be reduced than Fe/BNc and Fe/BNs catalyst. Bartolini et al. reported33 that the reducibility of supported species was highly influenced by the particle size of supported species. Smaller particles interacting strongly with the support are more difficult to be reduced, while larger particles could be more easily reduced.

Figure 6. H2-TPR profiles of three Fe/BN catalysts.

3.6 Catalytic Performance of Fe/BN Catalysts 3.6.1 Support Effect on Catalytic Performance The catalytic performance was evaluated for all catalysts and the results were given in Table 2. It was noted obviously that the Fe2O3 of Fe/BNc show the largest particle size, worst dispersion and therefore the least exposed iron atoms at the surface that FTS occurs. It is expected that the more surface iron exposed in case of smaller grain size and better dispersion of the supported Fe/BNs and Fe/BNp catalyst resulted in excellent activities. However, Fe/BNc catalyst showed the superior activity than Fe/BNs and Fe/BNp. The initial CO conversion over those catalysts follows the order: Fe/BNc (95.2%) > Fe/BNs (25.7%) > Fe/BNp (11.5%), corresponding catalytic activity per gram of iron (iron time yield, FTY) of 3.60×10-5, 0.99×10-5 and 0.41×10-5 molCO 11



g-1Fe • s-1 respectively. Obviously, the FTY of the Fe/BNc was 4 and 8 times over the Fe/BNs and Fe/BNp catalyst with the more surface iron exposed. Hence, some reasons could play more pronounced role other than SSA, dispersion and particle size in the FTS process. As we taken the B-O contents into account, a reverse trend can be found in compared with that of FTY, which was shown in Figure 7. The B-O rich support in our case could also play a similar role to that of iron/oxides. The stronger interaction between support and catalyst not only benefited the smaller grain size and better dispersion, but also resulted in the formation of low reducibility of iron species, giving rise to a reduced concentration of active sites. On the other hand, the resulted small iron grains could be poisoned by the irreversible adsorption of CO and the longer residence time and low surface coverage of the CHx, further reducing the ability to activation the CO molecule34. As we take the pure Fe2O3 catalyst into account, its CO conversion was 86.9%, only inferior to Fe/BNc. The corresponding FTY was 0.67×10-5 molCO g-1Fe·s-1 also less active than Fe/BNs and Fe/BNc. It need to be noted that even unsupported iron also existed in BNc case, its FTY was 6 times over pure Fe2O3. One could attribute it to the less exposed active site or aggregation of the active iron species in pure Fe2O3 case. Here, the CH4 selectivity of Fe2O3, Fe/BNc, Fe/BNs, Fe/BNp exhibited an increase trend (14.5% < 22.9% of < 29% of < 56.9%), implying an enhanced hydrogenation ability. A positive correlation between the B-O defect of BN support and the enhanced CH4 selectivity can be then observed. According to Nash et. al’s work, the defect-laden BN solely can

work as hydrogenation catalyst for olefin20. Hence, the as-formed olefin in FTS could be further converted to the paraffin, and yield low O/P ratio. Actually, the negligible olefin/paraffin (O/P) selectivity can be observed in Fe/BN case over pure Fe2O3. As listed in Table 1, the more B-O defects, and the lower O/P shows.

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Figure 7. The relations graph of surface B-O defects of support and FTY for three B-O surface defects catalysts. Table 2. Catalytic performance of Fe@BN catalysts from different BN nanosheets.

Catalyst

Fe/BNC Fe/BNS Fe/BNP Pure Fe2O3

Olefin/

CO

FTY

CO2

conversion

(molCO•

selectivity

(%)

g-1Fe· s-1)

(%)

CH4

C2-4

C5+

in C2-C4

95.2

3.60×10-5

34.7

22.9

50.8

26.3

0.13

25.7

-5

32.5

29.0

20.7

40.3

0.08

-5

9.6

56.9

36.4

6.7

0.06

-5

28.0

14.5

48.7

36.7

0.45

11.5 86.9

0.99×10 0.41×10 0.67×10

Selectivity in hydrocarbon (wt %)

Paraffin

Reaction condition: T=340 oC, P=2 MPa, GHSV=1500 h-1, H2/CO=2. 1.5 mL (1.2 g) catalyst used.

3.6.2 Stability of Catalysts with Different Supports A comparison of catalytic activity as a function of the reaction time over pre-prepared Fe/BN catalysts is shown in Figure 8. Along with time, the CH4 selectivity of all catalysts gradually increased, and the Fe/BNc catalysts showed high CO conversion. The Fe/BNc catalysts exhibited unsatisfactory stability (at 340 oC) which decreased gradually from 95.2% to 88% after 120 h running. The activity of Fe/BNC after lose was still better than the other two catalysts that remain stable. The decreased activity of the Fe/BNc might be related to the deficiency defects that cannot anchor the active 13



phase, while the strong interaction may anchor active component. In all, the activity, selectivity and stability can be tuned in B-O defects rich system, the balance between the two contradictory factors for dispersibility and activity could be one of the most important consideration for reasonable design of a high efficient BN based FTS catalyst in the future.

Figure 8. The CO conversions (a) and CH4 selectivity (b) as a function of time on stream in the FTS process. T=340 oC, P=2 MPa, GHSV=1500 h-1, H2/CO = 2.

4. CONCLUSION In this research, the Fe supported on three different defects-laden BN were systemically investigated their FTS properties. The surface B-O defects of supports not only affected the size and reducibility of iron particles but also significantly influenced the activity, selectivity and stability of the catalysts. High density of surface B-O defects allows smaller iron oxide particles, but it is more difficult to be reduced ensuring lower surface concentration of metallic iron, responsible for catalytic activity in the FTS. The high density surface defects for Fe/BNs and Fe/BNp catalysts can ensure good stability while Fe/BNc with lower density surface defects shows better activity. This result leads us to conclude that appropriate surface B-O defects of supports can be introduced so that can balance the activity and stability for FTS hydrogenation reaction.

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Supporting Information TEM images and FT-IR spectrum of the BN supports .

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