Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES
Letter
Porous Graphene-Confined Fe–K as Highly Efficient Catalyst for CO2 Direct Hydrogenation to Light Olefins Tijun Wu, Jun Lin, Yi Cheng, Jing Tian, Shunwu Wang, Songhai Xie, Yan Pei, Shirun Yan, Minghua Qiao, Hualong Xu, and Baoning Zong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05411 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
Porous Graphene-Confined Fe–K as Highly Efficient Catalyst for CO2 Direct Hydrogenation to Light Olefins Tijun Wu,† Jun Lin,‡ Yi Cheng,† Jing Tian,† Shunwu Wang,† Songhai Xie,† Yan Pei,† Shirun Yan,† Minghua Qiao,*,† Hualong Xu,† Baoning Zong*,§ †
ABSTRACT: We devised iron-based catalysts with honeycombstructured graphene (HSG) as the support and potassium as the promoter for CO2 direct hydrogenation to light olefins (CO2-FTO). Over the optimal FeK1.5/HSG catalyst, the iron time yield of light olefins amounted to 73 µmolCO2 gFe–1 s–1 with high selectivity of 59%. No obvious deactivation occurred within 120 h on stream. The excellent catalytic performance is attributed to the confinement effect of the porous HSG on the sintering of the active sites and the promotion effect of potassium on the activation of inert CO2 and the formation of iron carbide active for CO2-FTO.
24 20 16 12
dV/dd (cm3 g− 1 nm−1)
Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China ‡ Key Laboratory of Nuclear Analysis Techniques, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China § State Key Laboratory of Catalytic Materials and Chemical Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, P. R. China
Adsorbed volume (cm3 g−1)
(a)
0.003
0.001 0.000
8
0
50 100 150 Pore size (nm)
4 0 0.0
0.2 0.4 0.6 0.8 Relative pressure (P/P0)
KEYWORDS: porous graphene, potassium-promoted iron, CO2, hydrogenation, light olefins
500 nm
1.0
(c)
In 2017, the global emissions of CO
reached 41.5 ± 4.4 billion ton. Except for ~55% being absorbed by ocean and land as carbon “sinks”, ~45% of them was left in the atmosphere.1 Physical sequestration of CO2 in non-atmospheric reservoirs is an important way to stabilize the concentration of atmospheric greenhouse gases.2 Alternatively, CO2 is a potential carbon source for the production of organic chemicals by means of photocatalysis,3–5 electrocatalysis using renewable electricity,6,7 and thermochemical catalysis using renewable H2.8–10 Among these chemical approaches, thermochemical, catalytic CO2 hydrogenation is powerful in generating more valuable hydrocarbons other than CO and CH4.11 Analogous to CO hydrogenation, known as Fischer– Tropsch synthesis (FTS), CO2 hydrogenation is highly exothermic.12 However, owing to the chemical inertness of the CO2 molecule,13 its hydrogenation is not efficient on most catalysts. Hence, solving this problem is essential to illuminate the prospect of CO2 hydrogenation in practical applications. Light olefins (C2=–C4=) are key building blocks in chemical industry and currently produced mainly through thermal cracking of unrenewable petroleum-derived naphtha. The rapidly growing demand has aroused recent research interests in sustainably producing light olefins from alternative carbon sources. 10,12,14–17 However, the iron-based catalysts supported on oxide materials showed low activities in CO2 direct hydrogenation to light olefins (CO2-FTO) (Table S1). Comparing with these supports, carbon materials generally bear the merits of high stability in reducing
(b)
22.0 nm
0.002
d = 7.1 ± 1.4 nm 20
10
0
2
(d)
30
Frequency (%)
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 Applied Materials & Interfaces
4
6
8 10
Particle size (nm)
200 nm
100 nm
5 nm
Figure 1. (a) N2 physisorption isotherms and pore size distribution, (b) SEM image, (c) HAADF–STEM image, and (d) TEM image and particle distribution histogram of the as-prepared FeK1.5/HSG catalyst. Inset in d) shows the lattice fringes of the nanoparticle on the catalyst. atmospheres, robustness to water attack, and moderate metal– support interaction, and consequently high degree of reduction and carburization of iron, making them promising supports for iron-based FTS catalysts.18 Recently, we identified that iron-based catalysts supported on few-layer graphene (rGO) were highly active in CO-FTO.15,17 However, 2D nanosheet graphene cannot geometrically restrict the aggregation of the active components supported on. Herein, by exploiting a novel honeycomb-structured graphene (HSG) as the support featuring high porosity19 together with the fascinating intrinsic properties of graphene itself and potassium as the promoter, we fabricated Fe–K/HSG catalysts highly efficient for CO2-FTO. The unique three-dimensional (3D) architecture of HSG effectively impedes the agglomeration of iron carbide nanoparticles (NPs), while its large pores permit free inand out- diffusion of the reactants and the products,respectively.20
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
368 K
300
FeK1.5/HSG
427 K
FeK1.5/HSG
Fe/HSG 400
500
Fe/HSG
600
350
400
200
CO2-TPD
423 K
450
500
80
(a) 150
60
100
40
50
20
0
0
FTY= (µ µ molCO2 gFe− 1 s− 1)
(b)
FTY (µ µ molCO2 gFe− 1 s− 1)
H2-TPD
(a) Intensity (a.u.)
550
Temperature (K)
G SG SG SG SG /HSG HS /H /H /H /H .5/ .5 Fe K5 K2 K1 K0 K1 Fe Fe Fe Fe Fe
Figure 2. (a) The H2-TPD profiles and (b) CO2-TPD profiles of the Fe/HSG (black line) and FeK1.5/HSG (red line) catalysts. Potassium pronouncedly improves the amount and strength of CO2 adsorbed on the catalyst on one hand; on the other hand, it also promotes the formation of iron carbide, the active phase for CO2 hydrogenation. These beneficial characteristics of HSG and potassium endow the Fe–K/HSG catalysts with high activity and C2=–C4= productivity in CO2-FTO. The HSG support was synthesized according to the protocol raised by Hu and co-workers with some modifications.19 The preparation details of HSG and the Fe–K/HSG catalysts with the nominal iron loading of 20.0 wt% and different amounts of potassium promoter (0–5.0 wt% with respective to iron and HSG) are described in the Supporting Information; the preliminary physicochemical properties of the as-prepared catalysts are compiled in Table S2. Figure 1 displays the texture, morphology, and microstructure of the representative FeK1.5/HSG catalyst promoted with 1.5% of potassium. N2 physisorption verified the mesoporosity of the catalyst with the most probable pore diameter of around 22.0 nm (Figure 1a). Scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) readily visualized the 3D porous structure, as shown in Figures 1b and 1c, respectively. Moreover, in these images macropores of ~50–70 nm in diameter were also observed. The porous framework of HSG homogeneously accommodated the magnetite NPs that appear as bright dots in Figure 1c and as black particles in the TEM image in Figure 1d. The assignment of the magnetite phase is based on the X-ray diffraction (XRD) pattern in Figure S1 and the high-resolution TEM (HRTEM) image inserted in Figure 1d. Figure 2 presents the temperature-programmed desorption (TPD) profiles of the H2 and CO2 reactants on the Fe/HSG and FeK1.5/HSG catalysts. Potassium promotion almost doubled the adsorption capacity of H2 from 27 µmol gcat–1 to 53 µmol gcat–1, while the desorption temperature was virtually unaffected (Figure 2a). On the other hand, as a basic promoter, potassium not only drastically increased the adsorption capacity of the acidic CO2 molecule from 74 µmol gcat–1 to 160 µmol gcat–1, but also pronouncedly shifted the desorption temperature from 368 K to 423 K (Figure 2b). The enhanced adsorption capacities of both reactants would improve the hydrogenation rate of CO2 in terms of the law of mass action. Moreover, the elevated desorption temperature of CO2 reflects its stronger interaction with the catalyst evoked by potassium, which is conducive to the activation of the chemically stable CO2 and consequently the enhancement of its reactivity. The catalytic performances of the Fe/HSG and Fe–K/HSG catalysts in CO2-FTO were evaluated at 613 K and 20 bar, which are typical reaction conditions for CO-FTO on iron-based catalysts,14 except for using a H2/CO2 mixture gas of 3/1 by volume as the reactants. Figure 3a presents the effects of potassium on the cata-
70 60
Selectivity (%)
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 2 of 7
(b)
CH4 C2=-C4= C2p-C4p C5+
50 40 30 20 10 0 G G SG SG SG SG HS HS /H 5/H 1/H 2/H .5/ .5/ Fe eK eK eK K0 K1 F F F e e F F
Figure 3. (a) Iron time yields to hydrocarbons and light olefins, termed as FTY (black circle) and FTY= (red bar), respectively, and (b) product selectivities on the Fe/HSG and Fe–K/HSG catalysts. Reaction conditions: 0.15 g catalyst, T = 613 K, P = 20 bar, H2/CO2 = 3 by volume, and time on stream of 24 h. Data are reported at CO2 conversion levels between 43–50% by adjusting the space velocity; about 15−45% of CO2 was converted to CO. lytic activity, which is expressed as moles of CO2 converted to hydrocarbons per gram iron per second (Fe time yield to hydrocarbons, termed as FTY). On the Fe/HSG catalyst, the FTY was 81 µmolCO2 gFe–1 s–1, which increased with the content of potassium and maximized at 177 µmolCO2 gFe–1 s–1 on the FeK1/HSG catalyst. It is noteworthy that all the HSG-supported catalysts are more active than the iron-based catalysts in the literature also evaluated in CO2-FTO (Table S1). In particular, the FTY on the FeK1/HSG catalyst is about one order of magnitude higher than those on conventional iron-based catalysts and more than 1.5 times of those reported recently on the novel In–Zr/SAPO-34 physically mixed catalyst under identical reaction conditions21 and In2O3-β + SAPO-3413 and ZnZrO/SAPO physically mixed catalysts16 evaluated at even higher temperatures (Table S1). Figure 3b shows the distributions of the hydrocarbons in CO2FTO. The Fe/HSG catalyst gave a low C2=–C4= selectivity of 14% and a high selectivity to C2–C4 paraffins of 27%. Promoting 0.5% and 1.0% of potassium markedly improved the C2=–C4= selectivity to above 30%. Further raising potassium to 1.5% drastically boosted the C2=–C4= selectivity to 59%. Meanwhile, the selectivity to C2–C4 paraffins dropped to 11%. The C2=–C4= selectivity increased slightly at higher potassium contents and reached 61% at 5.0% of potassium, a value higher than those reported previously on the iron-based catalysts (Table S1). With the increment in the content of potassium, the CH4 selectivity decreased steadily from 59% to 23%. It is apparent that potassium not only suppresses the over-hydrogenation of light olefins, but also increases the chaingrowth probability, which conjunctively improve the C2=–C4= selectivity at the expense of CH4 and C2–C4 paraffins.
2 ACS Paragon Plus Environment
Page 3 of 7
ε,-Fe2.2C
χ-Fe5C2(C)
χ-Fe5C2(C)
χ-Fe5C2(B)
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 Applied Materials & Interfaces
χ-Fe5C2(B)
χ-Fe5C2(A)
Fe(II) (h)
χ-Fe5C2(A)
Fe(II) (l)
Exp.
Fe(III)
Fitted
(a) -10
Fitted
Exp.
(b) -5
0
5
10
-10
-5
0
5
10
Velocity (mm s−1)
Figure 4. 57Fe Mössbauer spectra of (a) the Fe/HSG and (b) FeK1.5/HSG catalysts after 24 h on stream in CO2-FTO. With the values of FTY and the C2=–C4= selectivity, the iron time yield to C2=–C4= (FTY=, moles of CO2 converted to light olefins per gram Fe per second) was figured out, as also illustrated in Figure 3a. On the Fe/HSG catalyst, the FTY= was only 11 µmolCO2 gFe–1 s–1. The FTY= increased with the addition of potassium and maximized at 73 µmolCO2 gFe–1 s–1 on the FeK1.5/HSG catalyst, which is the highest C2=–C4= productivity via CO2-FTO as far as we are aware of among the iron-based catalysts (Table S1) and comparable to the best values documented so far on the In2O3-β + SAPO-3410 and ZnZrO/SAPO catalysts.16 However, the FeK1.5/HSG catalyst, as well as the In2O3-β + SAPO-3410 and ZnZrO/SAPO catalysts,16 also showed high CO selectivity (44%, 45%, and 47%, respectively). Hence, how to lower CO selectivity while retaining high C2=–C4= productivity deserves further investigation in catalyst design for CO2-FTO. To evidence the superiority of the HSG support in CO2-FTO, we prepared various FeK1.5 catalysts using conventional SiO2, Al2O3, TiO2, and ZrO2 as the supports. The details for the preparation of these control catalysts and their XRD patterns (Figure S2) are given in the Supporting Information. Among these catalysts, the FeK1.5/TiO2 catalyst displayed the highest FTY and FTY= of 33 µmolCO2 gFe–1 s–1 and 19 µmolCO2 gFe–1 s–1, respectively (Table S3), but they were still far lower than the corresponding values on the FeK1.5/HSG catalyst under identical reaction conditions. We also fabricated the FeK1.5 catalysts supported on activated carbon (AC, Vulcan X72) and rGO reduced from graphene oxide (GO, XFNANO) to gain an insight into the effect of the carbonaceous materials on the catalytic performance. Under identical reaction conditions, the AC support also possessing the 3D structure but with micropores and mesopores (Figure S3) led to lower FTY than that of the Fe/HSG catalyst and lower FTY= than that of the FeK0.5/HSG catalyst (Table S3). These results imply that, unlike AC, H2 and CO2 can more easily infiltrate into the mesoporous-macroporous hierarchical framework of HSG and get access to the active sites, and the generated light olefins can more easily escape from the catalyst so as to avoid over-hydrogenation. Furthermore, the TEM image in Figure S4 revealed that the magnetite NPs were unevenly dispersed on AC with obvious agglomeration. This observation can be partly linked to the inability of the small pores in AC to accommodate the relatively large NPs. Besides, comparison of the C 1s spectrum of AC with that of HSG (Figure S5) revealed that there are also less C–O and carboxyl groups on AC acting as the anchorage sites for metal oxide NPs.22 The rGO with the 2D nanosheet morphology was much better than AC as the support in CO2-FTO (Table S3), but the FTY,
FTY=, and C2=–C4= selectivity were still inferior to those on the FeK1.5/HSG catalyst. The TEM characterization revealed that the mean size of the iron-containing NPs increased from 9.8 nm on the as-prepared FeK1.5/rGO catalyst (Figure S6a) to 18.1 nm after 24 h on stream (Figure S6b). For comparison, it increased less significantly from 7.1 nm (Figure 1d) to 13.5 nm in the same time span for the FeK1.5/HSG catalyst (Figure S7a), which signifies that the 3D porous structure of HSG is highly effective in confining the NPs at high reaction temperature required by CO2-FTO, thus affording more active sites for the reaction. In agreement with this interpretation, the particle size difference between the as-prepared Fe/HSG catalyst and the catalyst after 24 h on stream (Figure S8) is also much smaller than that of the FeK1.5/rGO catalyst. The promotion effect of potassium on the activity of the Fe–K /HSG catalyst can be associated with the enhanced adsorption capacities of both H2 and CO2, as manifested in Figure 2 as well as in Figure S9 illustrating the monotonic increase in the amounts of the adsorbed H2 and CO2 with the potassium contents from 0 to 5.0%. Moreover, 57Fe Mössbauer absorption spectroscopy (57Fe MAS) revealed that potassium influenced both the type and quantity of the iron phases formed during CO2-FTO. Figure 4 presents the 57Fe MAS spectra of the Fe/HSG and FeK1.5/HSG catalysts after 24 h on stream and the deconvoluted sub-spectra; the corresponding parameters are listed in Table S4. For the Fe/HSG catalyst, there were two doublets with isomer shift (IS) of 0.96 mm s−1 and quadrupole splitting (QS) of 0.28 mm s−1 and IS of 1.21 mm s−1 and QS of 1.82 mm s−1 due to the Fe(II) species in low- and high-coordination environments, respectively.23–25 For the FeK1.5/HSG catalyst, instead, there was only one doublet with IS of 0.31 mm s−1 and QS of 1.05 mm s−1 attributable to the Fe(III) species,26 which implies that potassium is capable of stabilizing high valence-state iron. The presence of Hägg carbide (χ-Fe5C2) on both catalysts is confirmed by three sextets with hyperfine magnetic fields (H) of 18.1–18.3, 21.7, and 10.5–10.9 T.27 Interestingly, the FeK1.5/HSG catalyst had an additional ε’-Fe2.2C phase with the H of 16.4 T, which usually survives only at
