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Bimetallic Fe–Co catalysts for CO2 hydrogenation to higher hydrocarbons
Journal of CO2 Utilization 3–4 (2013) 102–106
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Short communication
Bimetallic Fe–Co catalysts for CO2 hydrogenation to higher hydrocarbons Ratchprapa Satthawong a,b, Naoto Koizumi a, Chunshan Song a,c,*, Pattarapan Prasassarakich b a
Clean Fuels & Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA b Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Phyathai, Bangkok 10330, Thailand c PSU-DUT Joint Center for Energy Research, Pennsylvania State University, University Park, PA 16802, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 May 2013 Received in revised form 27 July 2013 Accepted 3 October 2013 Available online 29 October 2013
This paper reports on Fe–Co bimetallic catalysts that are active and selective for synthesis of olefin-rich C2+ hydrocarbons from CO2 hydrogenation. The combination of Fe and a small amount of Co led to a dramatic bimetallic promotion of C2+ hydrocarbons synthesis in CO2 hydrogenation on Fe–Co/Al2O3 catalyst with 15 wt% total metal loading. The addition of K to Fe–Co/Al2O3 catalyst further improved the formation rate of C2+ hydrocarbons as well as their olefin contents, while it suppressed CH4 formation significantly. Olefin-rich C2+ hydrocarbons was successfully synthesized using K-promoted Fe–Co/Al2O3 catalysts with high K loadings (Co/(Co + Fe) = 0.17 atom atom 1, K/Fe 0.5 atom atom 1) using CO2 as a carbon source. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: CO2 hydrogenation Olefin-rich higher hydrocarbons Fe–Co bimetallic catalyst K promotion
1. Introduction Catalytic conversion of CO2 including hydrogenation has attracted great attention as a way for chemical fixation of CO2 in combination with other techniques such as CO2 capture and storage [1–7]. Hydrogenation can turn CO2 into useful chemicals, polymers, materials, liquid fuels and synthetic natural gas, as shown in Scheme 1. In this scheme, H2 used for CO2 conversion should be produced using renewable energy, which may include water electrolysis using electricity generated with solar or wind or other renewable energy, and water splitting using photocatalytic, photoelectrochemical or other photochemical processes. There are established industrial technologies for water electrolysis with energy efficiencies around 70% [8]. Until now, the hydrogenation of CO2 to hydrocarbons has been studied mainly on traditional catalysts for Fischer–Tropsch synthesis (FTS), i.e., Fe, Co, Ni and Ru catalysts. When supported Ni and Ru catalysts are used for this reaction, the main product is CH4; only minor amounts of higher hydrocarbons are observed [9– 12]. Riedel et al. [13] and Gnanamani et al. [14] studied the activities and selectivities of Co and Fe catalysts for the hydrogenation of CO, CO2 and their mixtures. It was demonstrated
* Corresponding author at: Clean Fuels & Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1 8148634466; fax: +1 8148653573. E-mail address: [email protected] (C. Song). 2212-9820/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcou.2013.10.002
that the product composition with the Co-based catalyst shifted from an FTS type to almost exclusively CH4 with increasing partial pressure of CO2 and decreasing partial pressure of CO, while similar product composition was obtained from both CO/H2 and CO2/H2 using Fe catalyst. Different behaviors of these catalysts are explained in terms of different types of the kinetic regime of FTS, namely strong adsorption of CO for Co catalysts and carbide formation for Fe catalysts [13]. Due to thermodynamic constraints, CO partial pressure would be low in CO2/H2 atmosphere, which suppresses strong adsorption of CO over Co surface leading to limited chance of carbon–carbon bond formation. Conversely, Fe carbide phases are stabilized on the surface of alkalized Fe catalyst even under low CO partial pressure which is of substantial importance for FTS. As a consequence of the prior studies including those mentioned above, attention has been paid on improving the catalytic performances of Fe-based catalysts for synthesis of higher hydrocarbons by the addition of promoters such as K and Mn or use of different support materials [15–21]. Most promising Fe catalysts for higher hydrocarbons synthesis from CO2 are K promoted Fe/ Al2O3 catalysts with K contents of up to 0.5 mol-K mol 1 of Fe [16– 19]. However, these catalysts still have low efficiencies for converting CO2. How to achieve substantial activity improvement in synthesis of higher hydrocarbons remains a major challenge. Dramatically different catalytic properties of Co and Fe catalysts in the hydrogenation of CO2 and CO imply importance of controlling coverage of CO2 and hydrogen over active metal surface for carbon–carbon bond formation for C2+ hydrocarbons. The idea behind our work in this paper is that surface
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Scheme 1. Conceptual system for CO2-based sustainable chemicals and fuels.
chemisorption properties for CO2 and H2 could be tailored over bimetallic surface involving Fe and Co by changing their composition for facilitating carbon–carbon bond formation thus leading to increasing higher hydrocarbons. Several research groups have studied bimetallic effect in CO hydrogenation in FTS, which suggested that bimetallic alloy formation was associated with their improved activities and selectivities [22– 31]. Only limited numbers of reports involved bimetallic catalysts [28,32] on the CO2 hydrogenation to hydrocarbons, but only Fe–Co catalysts with high Co/Fe atomic ratios (1–3) were studied [28,32]. It is also worth noting that these bimetallic catalysts exhibit higher CH4 selectivity than conventional K-promoted Fe catalyst [28,32]. Systematic study on the effect of the chemical composition on the CO2 hydrogenation activity and selectivity of the bimetallic catalyst has not been reported. Although the addition of K to Fe/Al2O3 catalyst suppresses CH4 formation in the CO2 hydrogenation [16,18,19], its effect on the activity and selectivity of the Fe–Co bimetallic catalyst is still unknown. The aim of this work is to explore CO2 hydrogenation to higher hydrocarbons over Fe–Co bimetallic catalysts. For this purpose, a series of Fe–Co/Al2O3 catalysts with a wide range of Co/(Co + Fe) atomic ratios (15 wt% total metal loading) were prepared. The effect of the addition of K was also studied by a comparative examination of the catalysts with and without K promoter. Kpromoted Fe–Mn catalyst was prepared and tested for CO2 hydrogenation at the same reaction condition as a reference catalyst.
in a rotary evaporator for 2 h followed by drying in the electric oven at 383 K overnight. This sample was then impregnated with the mixed solution of Fe and Co nitrates followed by drying and calcination under the same conditions mentioned above. The catalysts prepared in this work are denoted as Fe–Co(X)/K(Y)/ Al2O3, where X and Y represent the Co/(Co + Fe) and K/Fe atomic ratios, respectively. For comparison, a K-promoted Fe–Mn/Al2O3 catalyst was prepared as a reference catalyst by a wet coimpregnation method using Fe(NO3)39H2O (Aldrich, 99.99%), KMnO4 (Sigma–Aldrich, 99.0%) and KNO3 (Aldrich, 99.99%) as precursors by following the preparation procedure in the literature [19]. To evaluate the effect of the preparation methods, Fe–Co/Al2O3 catalyst was also prepared by the same method as Fe–Mn–K/Al2O3 catalyst. Loadings of Fe, Co, Mn and K of prepared catalysts are listed in Table 1. 2.2. Activity test Hydrogenation of CO2 to hydrocarbons was carried out in a high-pressure fixed-bed flow reactor system. About 200 mg of the catalyst was charged into a stainless steel reactor (internal diameter = 6 mm) with amorphous SiO2 as a diluent. The catalyst was reduced under a 50 mL (STP) min 1 H2 flow (purity >99.995%) at 673 K for 2 h, and then allowed to cool down to ambient temperature before CO2 hydrogenation. The feed gas, 24 vol% CO2/ 72 vol% H2/4 vol% Ar (purity >99.99995%), then fed into the system with GHSV of 3600 mL (STP) g 1 h 1 at 1.1 MPa, and the catalyst bed was heated at a ramp rate of 2 K min 1 to 573 K for the activity
2. Experimental 2.1. Preparation of catalysts Fe–Co bimetallic catalysts were prepared by a pore-filling incipient wetness impregnation method using gamma-alumina (Sasol PURALOX TH 100/150, BET surface area = 150 m2 g 1, average pore diameter = 22 nm, pore volume = 1.0 mL g 1) as a support. Fe and Co precursors were added dropwise to dried alumina using the aqueous solution containing both Fe(NO3)39H2O (Aldrich, 99.99%) and Co(NO3)26H2O (Sigma–Aldrich, 98%). Concentrations of Fe and Co in the impregnating solution were adjusted to obtain desired Co/(Co + Fe) atomic ratios with maintaining total metal (Fe + Co) loading at 15 wt% as support weight basis when total volume of the added solution was equivalent to 90% of pore volume of the support. The impregnated sample was dried at 333 K in a rotary evaporator for 2 h, dried in an electric oven at 383 K for 3 h in ambient air, followed by calcination at 673 K for 2 h under flowing dry air. Monometallic catalysts, Fe/Al2O3 and Co/Al2O3, were also prepared using the same procedure. The K promoted catalysts were prepared by a two-step impregnation method; K2CO3 (Sigma–Aldrich, 99.0%) aqueous solution was impregnated onto the alumina support by the pore-filling method in the first step and dried at 333 K
Table 1 Metal loadings of unpromoted and K-promoted Fe–Co/Al2O3 catalysts and a Fe– Mn–K/Al2O3 catalyst. Catalyst
Loading/wt% (support weight basis) Fe
Co
K
Mn
Fe Fe–Co(0.10) Fe–Co(0.17) Fe–Co(0.17)a Fe–Co(0.25) Fe–Co(0.50) Co
15.0 13.4 12.4 12.4 11.1 7.3 –
– 1.6 2.6 2.6 3.9 7.7 15.0
– – – – – – –
– – – – – – –
Fe/K(0.3) Fe–Co(0.10)/K(0.3) Fe–Co(0.17)/K(0.3) Fe–Co(0.17)/K(0.5) Fe–Co(0.17)/K(1.0) Fe–Co(0.25)/K(0.3) Fe–Co(0.50)/K(0.3) Co/K(0.3) Fe–Mn–Ka
15.0 13.4 12.4 12.4 12.4 11.1 7.3 – 17.0
– 1.6 2.6 2.6 2.6 3.9 7.7 15.0 –
3.2 2.8 2.6 4.3 8.7 2.3 1.5 3.0 8.0b
a b
Prepared by a wet co-impregnation method. K loading from KNO3.
– – – – – – – – 12.0
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104
evaluation. An Agilent 3000 micro GC was used for the online analysis of gas products including Ar, CO, CH4 and CO2 while the gas-phase hydrocarbon products were analyzed online with SRI 8610C GC-FID. Liquid hydrocarbons were collected in an ice trap, and then analyzed with GC–MS (Shimadzu, QP-5000) after the reaction. CO2 conversions and space–time yields of gas-phase products (STYs; mmol of products formed per g-catalyst per second) reported in this paper were calculated using the data obtained at 15–16 h on stream. Product selectivities were calculated based on molar amounts of the products accumulated during 15–16 h on stream. 3. Results and discussion 3.1. Bimetallic effect in CO2 hydrogenation CO2 hydrogenation activities and selectivities of Fe–Co bimetallic catalysts with wide range of Co/(Co + Fe) atomic ratios were investigated at 573 K and 1.1 MPa. Fe–Co bimetallic catalysts with low Co/(Co + Fe) atomic ratios (0.25) showed stable CO2 conversions and selectivities after 3–4 h on stream, and no deactivations of the activity and selectivity were observed for over 50 h. Time-on-stream stabilities of CO2 conversions and product yields on K-promoted Fe–Co bimetallic catalysts are shown in Fig. 1 as examples. The catalysts with higher atomic ratios (0.5) exhibited different time-dependent behaviors; CO2 conversions over these catalysts slightly decreased with time on stream, however, decreasing rates were less than 0.4% per hour at 15–16 h on stream. Furthermore, these catalysts produced CH4 selectively and their product selectivities did not change with time-on-stream. Fig. 2 shows the change in CO2 conversion over Fe–Co/Al2O3 catalysts as a function of the Co/(Co + Fe) atomic ratio. CO2 conversion over Fe/Al2O3 catalyst with 15 wt% Fe loading was 12% at this reaction condition. Similar activity was reported by Riedel et al. [13]; Fe/Al2O3 catalyst with 20 wt% Fe loading showed 23% CO2 conversion at the same reaction temperature and pressure but nearly half of GHSV (1900 mL g 1 h 1). The combination of Fe and a small amount of Co (Co/(Co + Fe) atomic ratio = 0.17) increased the conversion almost twice compared to Fe/Al2O3 catalyst. The conversion increased almost linearly with further increase in the Co/(Co + Fe) atomic ratio, and reached 50% at the Co/(Co + Fe) atomic ratio of unity, namely Co/Al2O3 catalyst. Fig. 3A and B show the impact of Co content on space–time yields (STYs) of C2–C7 hydrocarbons and CH4 over Fe–Co/Al2O3 catalysts, respectively. As illustrated in these figures, the STY of C2– C7 hydrocarbons showed a maximum at the Co/(Co + Fe) atomic
Fig. 2. CO2 conversion over Fe–Co(X)/K(0.3)/Al2O3 and Fe–Co(X)/Al2O3 catalysts as a function of Co/(Co + Fe) atomic ratio.
ratio of 0.17, while that of CH4 increased linearly with an increase of the Co/(Co + Fe) atomic ratio up to 0.50. The maximum STY of C2–C7 hydrocarbons is evidently much higher than the simple sum of those over monometallic catalysts which indicates that the combination of Fe and a small amount of Co leads to significant bimetallic promotion of C2+ hydrocarbons synthesis in CO2 hydrogenation. Furthermore, the STYs of CH4 and C2–C7 hydrocarbons increased at similar rates (2–3 folds) at small Co/(Co + Fe) atomic ratio (0.17). These results reveal that Fe–Co bimetallic formulation with a low Co content enhances the CO2 conversion without losing ability for carbon–carbon bond formation for higher hydrocarbons. Hence, our results clearly demonstrate importance of Fe-rich Fe–Co combination for C2+ hydrocarbons formation through CO2 hydrogenation which has never been reported previously. High activity and selectivity of the bimetallic catalyst suggest that combining Fe and Co makes the surface adsorption properties for CO2 and H2 suitable for facilitating the carbon chain growth thus leading to increasing higher hydrocarbons. It should be noted that the optimum Co/(Co + Fe) atomic ratios were reported to 0.5–0.75 for CO hydrogenation using Fe–Co bimetallic catalysts [22,27]; the optimum catalyst composition for CO2 hydrogenation cannot be simply predicted from these previous studies. Different optimum ratios for CO and CO2 hydrogenation indicate different site requirements for these two reactions. As shown in Fig. 3, the STY of C2–C7 hydrocarbons was significantly suppressed in the case of high Co/(Co + Fe) atomic ratios. Only small amounts of C2+ hydrocarbons were observed in
Fig. 1. Time-on-stream profiles of CO2 conversion and product yields over Fe–Co(0.17)/K(0.3)/Al2O3 (A) and Fe–Co(0.17)/K(1.0)/Al2O3 (B) catalysts.
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105
Fig. 3. Effect of Co/(Co + Fe) atomic ratio on STYs of C2–C7 hydrocarbons (A) and CH4 (B) over Fe–Co bimetallic catalysts. Table 2 CO2 conversion and product selectivities of unpromoted and K-promoted Fe–Co/Al2O3 in comparison to monometallic catalysts. Catalyst
CO2 conversion (%)
Selectivity/C-mol%
O/P
Alphaa
+c
CO
CH4
C2
Fe Fe–Co(0.10) Fe–Co(0.17) Fe–Co(0.17)b Fe–Co(0.25) Fe–Co(0.50) Co
12.1 20.3 25.2 25.7 26.8 33.1 48.8
49 28 13 12 10 1 2
41 33 44 44 54 87 97
10 39 43 44 36 12 1
0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.27 0.37 0.36 0.40 0.31 0.04 0.00
Fe/K(0.3) Fe–Co(0.10)/K(0.3) Fe–Co(0.17)/K(0.3) Fe–Co(0.17)/K(0.5) Fe–Co(0.17)/K(1.0) Fe–Co(0.25)/K(0.3) Fe–Co(0.50)/K(0.3) Co/K(0.3) Fe–Mn–Kb
27.0 35.8 33.7 33.6 31.0 32.0 50.3 60.6 19.4
21 13 14 13 18 38 1 1 50
15 16 18 19 13 15 63 96 7
64 71 68 68 69 47 36 3 43
1.1 0.6 0.7 1.5 5.2 0.2 0.0 0.0 5.1
0.55 0.57 0.52 0.50 0.53 0.40 0.20 0.10 0.51
a b c
Alpha (chain growth probability) were calculated from molar fractions of C1–C7 hydrocarbons. Prepared by a wet co-impregnation method. Including small amounts of alcohols.
the product stream with high Co loading catalysts, and CH4 formation accounted for 90–99% of the total hydrocarbons product. These results are similar to the previous work by Park et al. [32], in which they reported that Fe–Co/Al2O3 catalyst with 5 wt% metal loading each for Fe and Co showed higher CO2 conversion, but lower selectivity for C2–C5 hydrocarbons compared to 10 wt% Fe/Al2O3 catalyst. The conversions and selectivities of all monometallic and bimetallic catalysts are summarized in Table 2. Fe–Co(0.17)/Al2O3 catalyst exhibited significantly lower CO selectivity than monometallic Fe catalyst with more than 4 times higher C2+ selectivity, while CH4 selectivity barely changed. There were no olefins observed in the gas phase hydrocarbons with these monometallic and bimetallic catalysts. CH4 selectivities of the unpromoted Fe– Co/Al2O3 bimetallic catalysts were relatively high (30–40%) even at the low Co/(Co + Fe) atomic ratios compared to those in typical FTS. However, these bimetallic catalysts, in particular monometallic Co catalyst, are still useful for fuel production from CO2 hydrogenation because CH4 can be used as synthetic natural gas.
higher CO2 conversion regardless of the Co/(Co + Fe) ratios. The addition of K also improved the STY of C2–C7 hydrocarbons over bimetallic catalysts as displayed in Fig. 3A, while it suppressed CH4 formation when the Co/(Co + Fe) atomic ratio was smaller than 0.5 (Fig. 3B). Further increasing K loading to Fe–Co(0.17)/Al2O3 did not cause clear changes in the C2–C7 STY as illustrated in Fig. 4, while CH4 STY decreased linearly with increasing K loading. These results
3.2. Effect of K addition The effect of K addition on the activities and selectivities of Fe– Co bimetallic catalysts were then studied. Fig. 2 compares CO2 conversion over K-promoted (K/Fe = 0.3 atom atom 1) and unpromoted Fe–Co bimetallic catalysts. K-promoted catalyst showed
Fig. 4. Effects of K/Fe atomic ratio on the STYs of CH4 and C2–C7 hydrocarbons and the olefin to paraffin ratio of C2–C4 hydrocarbons on Fe–Co(0.17)/K(Y)/Al2O3.
106
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results are important for developing both new catalysts and fundamental understanding of the mechanism involved in activation of CO2 and H2 towards carbon–carbon bond formation for higher hydrocarbons. More detailed investigations on these catalysts are currently underway. Acknowledgements This research is supported in part by the Pennsylvania State University through Penn State Institutes of Energy and the Environment. The authors also wish to thank the Thailand Research Fund and Graduate School of Chulalongkorn University through the Royal Golden Jubilee Ph.D. Program Scholarship to RS at PSU. Fig. 5. GC–MS total ion chromatogram of liquid products from CO2 hydrogenation on Fe–Co(0.17)/K(1.0)/Al2O3 catalyst.
demonstrate that the addition of K to Fe–Co/Al2O3 catalysts enhances the chain propagation of hydrocarbons. Table 2 also shows product selectivities of K-promoted Fe–Co bimetallic catalysts with different Co/(Co + Fe) atomic ratios. Selectivity for C2+ hydrocarbons and chain growth probability of gas-phase hydrocarbons were enhanced notably by the addition of K to Fe–Co bimetallic catalyst, while CH4 selectivity decreased drastically. CH4 selectivity of Fe–Co(0.17)/K(Y)/Al2O3 catalysts (Y 0.3) were relatively similar to that of Fe/K/Al2O3 with 15 wt% Fe loading, but the former catalysts yielded more C2+ hydrocarbons selectivity compared to the latter one. Furthermore, these Fe–Co bimetallic catalysts showed higher CO2 conversions and C2+ selectivities than K-promoted Fe–Mn catalyst with higher Fe loading as shown in Table 2. Since the CO2 hydrogenation activities and selectivities of Fe–Co(0.17)/Al2O3 catalyst were hardly dependent on the preparation methods, namely pore-filling incipient wetness impregnation and wet co-impregnation methods (Table 2), our results reveal higher effectiveness of Fe–Co–K combination than Fe–Mn–K for CO2 hydrogenation to C2+ hydrocarbons under the conditions employed. The addition of K also increased olefin to paraffin ratio (O/P) of C2+ hydrocarbons as shown in Table 2. Olefin contents increased with increasing K loading, and olefins predominated in C2+ hydrocarbons when the catalyst was heavily alkalized (K/ Fe 0.5 atom atom 1). O/P reached approximately 5 at the K/Fe atomic ratio of unity (see also Fig. 4). Fig. 5 illustrates GC–MS total ion chromatogram of liquid products from CO2 hydrogenation over Fe–Co(0.17)/K(1.0)/Al2O3 catalyst. Linear alpha olefins were main products with carbon numbers up to 27. The main carbon number range of this liquid product was approximately corresponding to jet fuel. Small amount of alcohols were also observed in the product. 4. Conclusion By combining Fe and a small amount of Co on an alumina support, we have discovered a strong bimetallic promotion of C2+ hydrocarbons formation from CO2 hydrogenation on Fe–Co/Al2O3 catalysts. Olefin-rich C2+ hydrocarbons were synthesized using Fe– Co bimetallic catalysts in the presence of a K promoter. Kpromoted bimetallic Fe–Co catalysts with desired compositions also show significant advantages in synthesis of higher hydrocarbons over K-promoted Fe/Al2O3 and Fe–Mn/Al2O3 catalysts with higher Fe loadings under the same reaction conditions. These
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Short communication
Bimetallic Fe–Co catalysts for CO2 hydrogenation to higher hydrocarbons Ratchprapa Satthawong a,b, Naoto Koizumi a, Chunshan Song a,c,*, Pattarapan Prasassarakich b a
Clean Fuels & Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA b Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Phyathai, Bangkok 10330, Thailand c PSU-DUT Joint Center for Energy Research, Pennsylvania State University, University Park, PA 16802, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 May 2013 Received in revised form 27 July 2013 Accepted 3 October 2013 Available online 29 October 2013
This paper reports on Fe–Co bimetallic catalysts that are active and selective for synthesis of olefin-rich C2+ hydrocarbons from CO2 hydrogenation. The combination of Fe and a small amount of Co led to a dramatic bimetallic promotion of C2+ hydrocarbons synthesis in CO2 hydrogenation on Fe–Co/Al2O3 catalyst with 15 wt% total metal loading. The addition of K to Fe–Co/Al2O3 catalyst further improved the formation rate of C2+ hydrocarbons as well as their olefin contents, while it suppressed CH4 formation significantly. Olefin-rich C2+ hydrocarbons was successfully synthesized using K-promoted Fe–Co/Al2O3 catalysts with high K loadings (Co/(Co + Fe) = 0.17 atom atom 1, K/Fe 0.5 atom atom 1) using CO2 as a carbon source. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: CO2 hydrogenation Olefin-rich higher hydrocarbons Fe–Co bimetallic catalyst K promotion
1. Introduction Catalytic conversion of CO2 including hydrogenation has attracted great attention as a way for chemical fixation of CO2 in combination with other techniques such as CO2 capture and storage [1–7]. Hydrogenation can turn CO2 into useful chemicals, polymers, materials, liquid fuels and synthetic natural gas, as shown in Scheme 1. In this scheme, H2 used for CO2 conversion should be produced using renewable energy, which may include water electrolysis using electricity generated with solar or wind or other renewable energy, and water splitting using photocatalytic, photoelectrochemical or other photochemical processes. There are established industrial technologies for water electrolysis with energy efficiencies around 70% [8]. Until now, the hydrogenation of CO2 to hydrocarbons has been studied mainly on traditional catalysts for Fischer–Tropsch synthesis (FTS), i.e., Fe, Co, Ni and Ru catalysts. When supported Ni and Ru catalysts are used for this reaction, the main product is CH4; only minor amounts of higher hydrocarbons are observed [9– 12]. Riedel et al. [13] and Gnanamani et al. [14] studied the activities and selectivities of Co and Fe catalysts for the hydrogenation of CO, CO2 and their mixtures. It was demonstrated
* Corresponding author at: Clean Fuels & Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1 8148634466; fax: +1 8148653573. E-mail address: [email protected] (C. Song). 2212-9820/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcou.2013.10.002
that the product composition with the Co-based catalyst shifted from an FTS type to almost exclusively CH4 with increasing partial pressure of CO2 and decreasing partial pressure of CO, while similar product composition was obtained from both CO/H2 and CO2/H2 using Fe catalyst. Different behaviors of these catalysts are explained in terms of different types of the kinetic regime of FTS, namely strong adsorption of CO for Co catalysts and carbide formation for Fe catalysts [13]. Due to thermodynamic constraints, CO partial pressure would be low in CO2/H2 atmosphere, which suppresses strong adsorption of CO over Co surface leading to limited chance of carbon–carbon bond formation. Conversely, Fe carbide phases are stabilized on the surface of alkalized Fe catalyst even under low CO partial pressure which is of substantial importance for FTS. As a consequence of the prior studies including those mentioned above, attention has been paid on improving the catalytic performances of Fe-based catalysts for synthesis of higher hydrocarbons by the addition of promoters such as K and Mn or use of different support materials [15–21]. Most promising Fe catalysts for higher hydrocarbons synthesis from CO2 are K promoted Fe/ Al2O3 catalysts with K contents of up to 0.5 mol-K mol 1 of Fe [16– 19]. However, these catalysts still have low efficiencies for converting CO2. How to achieve substantial activity improvement in synthesis of higher hydrocarbons remains a major challenge. Dramatically different catalytic properties of Co and Fe catalysts in the hydrogenation of CO2 and CO imply importance of controlling coverage of CO2 and hydrogen over active metal surface for carbon–carbon bond formation for C2+ hydrocarbons. The idea behind our work in this paper is that surface
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Scheme 1. Conceptual system for CO2-based sustainable chemicals and fuels.
chemisorption properties for CO2 and H2 could be tailored over bimetallic surface involving Fe and Co by changing their composition for facilitating carbon–carbon bond formation thus leading to increasing higher hydrocarbons. Several research groups have studied bimetallic effect in CO hydrogenation in FTS, which suggested that bimetallic alloy formation was associated with their improved activities and selectivities [22– 31]. Only limited numbers of reports involved bimetallic catalysts [28,32] on the CO2 hydrogenation to hydrocarbons, but only Fe–Co catalysts with high Co/Fe atomic ratios (1–3) were studied [28,32]. It is also worth noting that these bimetallic catalysts exhibit higher CH4 selectivity than conventional K-promoted Fe catalyst [28,32]. Systematic study on the effect of the chemical composition on the CO2 hydrogenation activity and selectivity of the bimetallic catalyst has not been reported. Although the addition of K to Fe/Al2O3 catalyst suppresses CH4 formation in the CO2 hydrogenation [16,18,19], its effect on the activity and selectivity of the Fe–Co bimetallic catalyst is still unknown. The aim of this work is to explore CO2 hydrogenation to higher hydrocarbons over Fe–Co bimetallic catalysts. For this purpose, a series of Fe–Co/Al2O3 catalysts with a wide range of Co/(Co + Fe) atomic ratios (15 wt% total metal loading) were prepared. The effect of the addition of K was also studied by a comparative examination of the catalysts with and without K promoter. Kpromoted Fe–Mn catalyst was prepared and tested for CO2 hydrogenation at the same reaction condition as a reference catalyst.
in a rotary evaporator for 2 h followed by drying in the electric oven at 383 K overnight. This sample was then impregnated with the mixed solution of Fe and Co nitrates followed by drying and calcination under the same conditions mentioned above. The catalysts prepared in this work are denoted as Fe–Co(X)/K(Y)/ Al2O3, where X and Y represent the Co/(Co + Fe) and K/Fe atomic ratios, respectively. For comparison, a K-promoted Fe–Mn/Al2O3 catalyst was prepared as a reference catalyst by a wet coimpregnation method using Fe(NO3)39H2O (Aldrich, 99.99%), KMnO4 (Sigma–Aldrich, 99.0%) and KNO3 (Aldrich, 99.99%) as precursors by following the preparation procedure in the literature [19]. To evaluate the effect of the preparation methods, Fe–Co/Al2O3 catalyst was also prepared by the same method as Fe–Mn–K/Al2O3 catalyst. Loadings of Fe, Co, Mn and K of prepared catalysts are listed in Table 1. 2.2. Activity test Hydrogenation of CO2 to hydrocarbons was carried out in a high-pressure fixed-bed flow reactor system. About 200 mg of the catalyst was charged into a stainless steel reactor (internal diameter = 6 mm) with amorphous SiO2 as a diluent. The catalyst was reduced under a 50 mL (STP) min 1 H2 flow (purity >99.995%) at 673 K for 2 h, and then allowed to cool down to ambient temperature before CO2 hydrogenation. The feed gas, 24 vol% CO2/ 72 vol% H2/4 vol% Ar (purity >99.99995%), then fed into the system with GHSV of 3600 mL (STP) g 1 h 1 at 1.1 MPa, and the catalyst bed was heated at a ramp rate of 2 K min 1 to 573 K for the activity
2. Experimental 2.1. Preparation of catalysts Fe–Co bimetallic catalysts were prepared by a pore-filling incipient wetness impregnation method using gamma-alumina (Sasol PURALOX TH 100/150, BET surface area = 150 m2 g 1, average pore diameter = 22 nm, pore volume = 1.0 mL g 1) as a support. Fe and Co precursors were added dropwise to dried alumina using the aqueous solution containing both Fe(NO3)39H2O (Aldrich, 99.99%) and Co(NO3)26H2O (Sigma–Aldrich, 98%). Concentrations of Fe and Co in the impregnating solution were adjusted to obtain desired Co/(Co + Fe) atomic ratios with maintaining total metal (Fe + Co) loading at 15 wt% as support weight basis when total volume of the added solution was equivalent to 90% of pore volume of the support. The impregnated sample was dried at 333 K in a rotary evaporator for 2 h, dried in an electric oven at 383 K for 3 h in ambient air, followed by calcination at 673 K for 2 h under flowing dry air. Monometallic catalysts, Fe/Al2O3 and Co/Al2O3, were also prepared using the same procedure. The K promoted catalysts were prepared by a two-step impregnation method; K2CO3 (Sigma–Aldrich, 99.0%) aqueous solution was impregnated onto the alumina support by the pore-filling method in the first step and dried at 333 K
Table 1 Metal loadings of unpromoted and K-promoted Fe–Co/Al2O3 catalysts and a Fe– Mn–K/Al2O3 catalyst. Catalyst
Loading/wt% (support weight basis) Fe
Co
K
Mn
Fe Fe–Co(0.10) Fe–Co(0.17) Fe–Co(0.17)a Fe–Co(0.25) Fe–Co(0.50) Co
15.0 13.4 12.4 12.4 11.1 7.3 –
– 1.6 2.6 2.6 3.9 7.7 15.0
– – – – – – –
– – – – – – –
Fe/K(0.3) Fe–Co(0.10)/K(0.3) Fe–Co(0.17)/K(0.3) Fe–Co(0.17)/K(0.5) Fe–Co(0.17)/K(1.0) Fe–Co(0.25)/K(0.3) Fe–Co(0.50)/K(0.3) Co/K(0.3) Fe–Mn–Ka
15.0 13.4 12.4 12.4 12.4 11.1 7.3 – 17.0
– 1.6 2.6 2.6 2.6 3.9 7.7 15.0 –
3.2 2.8 2.6 4.3 8.7 2.3 1.5 3.0 8.0b
a b
Prepared by a wet co-impregnation method. K loading from KNO3.
– – – – – – – – 12.0
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evaluation. An Agilent 3000 micro GC was used for the online analysis of gas products including Ar, CO, CH4 and CO2 while the gas-phase hydrocarbon products were analyzed online with SRI 8610C GC-FID. Liquid hydrocarbons were collected in an ice trap, and then analyzed with GC–MS (Shimadzu, QP-5000) after the reaction. CO2 conversions and space–time yields of gas-phase products (STYs; mmol of products formed per g-catalyst per second) reported in this paper were calculated using the data obtained at 15–16 h on stream. Product selectivities were calculated based on molar amounts of the products accumulated during 15–16 h on stream. 3. Results and discussion 3.1. Bimetallic effect in CO2 hydrogenation CO2 hydrogenation activities and selectivities of Fe–Co bimetallic catalysts with wide range of Co/(Co + Fe) atomic ratios were investigated at 573 K and 1.1 MPa. Fe–Co bimetallic catalysts with low Co/(Co + Fe) atomic ratios (0.25) showed stable CO2 conversions and selectivities after 3–4 h on stream, and no deactivations of the activity and selectivity were observed for over 50 h. Time-on-stream stabilities of CO2 conversions and product yields on K-promoted Fe–Co bimetallic catalysts are shown in Fig. 1 as examples. The catalysts with higher atomic ratios (0.5) exhibited different time-dependent behaviors; CO2 conversions over these catalysts slightly decreased with time on stream, however, decreasing rates were less than 0.4% per hour at 15–16 h on stream. Furthermore, these catalysts produced CH4 selectively and their product selectivities did not change with time-on-stream. Fig. 2 shows the change in CO2 conversion over Fe–Co/Al2O3 catalysts as a function of the Co/(Co + Fe) atomic ratio. CO2 conversion over Fe/Al2O3 catalyst with 15 wt% Fe loading was 12% at this reaction condition. Similar activity was reported by Riedel et al. [13]; Fe/Al2O3 catalyst with 20 wt% Fe loading showed 23% CO2 conversion at the same reaction temperature and pressure but nearly half of GHSV (1900 mL g 1 h 1). The combination of Fe and a small amount of Co (Co/(Co + Fe) atomic ratio = 0.17) increased the conversion almost twice compared to Fe/Al2O3 catalyst. The conversion increased almost linearly with further increase in the Co/(Co + Fe) atomic ratio, and reached 50% at the Co/(Co + Fe) atomic ratio of unity, namely Co/Al2O3 catalyst. Fig. 3A and B show the impact of Co content on space–time yields (STYs) of C2–C7 hydrocarbons and CH4 over Fe–Co/Al2O3 catalysts, respectively. As illustrated in these figures, the STY of C2– C7 hydrocarbons showed a maximum at the Co/(Co + Fe) atomic
Fig. 2. CO2 conversion over Fe–Co(X)/K(0.3)/Al2O3 and Fe–Co(X)/Al2O3 catalysts as a function of Co/(Co + Fe) atomic ratio.
ratio of 0.17, while that of CH4 increased linearly with an increase of the Co/(Co + Fe) atomic ratio up to 0.50. The maximum STY of C2–C7 hydrocarbons is evidently much higher than the simple sum of those over monometallic catalysts which indicates that the combination of Fe and a small amount of Co leads to significant bimetallic promotion of C2+ hydrocarbons synthesis in CO2 hydrogenation. Furthermore, the STYs of CH4 and C2–C7 hydrocarbons increased at similar rates (2–3 folds) at small Co/(Co + Fe) atomic ratio (0.17). These results reveal that Fe–Co bimetallic formulation with a low Co content enhances the CO2 conversion without losing ability for carbon–carbon bond formation for higher hydrocarbons. Hence, our results clearly demonstrate importance of Fe-rich Fe–Co combination for C2+ hydrocarbons formation through CO2 hydrogenation which has never been reported previously. High activity and selectivity of the bimetallic catalyst suggest that combining Fe and Co makes the surface adsorption properties for CO2 and H2 suitable for facilitating the carbon chain growth thus leading to increasing higher hydrocarbons. It should be noted that the optimum Co/(Co + Fe) atomic ratios were reported to 0.5–0.75 for CO hydrogenation using Fe–Co bimetallic catalysts [22,27]; the optimum catalyst composition for CO2 hydrogenation cannot be simply predicted from these previous studies. Different optimum ratios for CO and CO2 hydrogenation indicate different site requirements for these two reactions. As shown in Fig. 3, the STY of C2–C7 hydrocarbons was significantly suppressed in the case of high Co/(Co + Fe) atomic ratios. Only small amounts of C2+ hydrocarbons were observed in
Fig. 1. Time-on-stream profiles of CO2 conversion and product yields over Fe–Co(0.17)/K(0.3)/Al2O3 (A) and Fe–Co(0.17)/K(1.0)/Al2O3 (B) catalysts.
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Fig. 3. Effect of Co/(Co + Fe) atomic ratio on STYs of C2–C7 hydrocarbons (A) and CH4 (B) over Fe–Co bimetallic catalysts. Table 2 CO2 conversion and product selectivities of unpromoted and K-promoted Fe–Co/Al2O3 in comparison to monometallic catalysts. Catalyst
CO2 conversion (%)
Selectivity/C-mol%
O/P
Alphaa
+c
CO
CH4
C2
Fe Fe–Co(0.10) Fe–Co(0.17) Fe–Co(0.17)b Fe–Co(0.25) Fe–Co(0.50) Co
12.1 20.3 25.2 25.7 26.8 33.1 48.8
49 28 13 12 10 1 2
41 33 44 44 54 87 97
10 39 43 44 36 12 1
0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.27 0.37 0.36 0.40 0.31 0.04 0.00
Fe/K(0.3) Fe–Co(0.10)/K(0.3) Fe–Co(0.17)/K(0.3) Fe–Co(0.17)/K(0.5) Fe–Co(0.17)/K(1.0) Fe–Co(0.25)/K(0.3) Fe–Co(0.50)/K(0.3) Co/K(0.3) Fe–Mn–Kb
27.0 35.8 33.7 33.6 31.0 32.0 50.3 60.6 19.4
21 13 14 13 18 38 1 1 50
15 16 18 19 13 15 63 96 7
64 71 68 68 69 47 36 3 43
1.1 0.6 0.7 1.5 5.2 0.2 0.0 0.0 5.1
0.55 0.57 0.52 0.50 0.53 0.40 0.20 0.10 0.51
a b c
Alpha (chain growth probability) were calculated from molar fractions of C1–C7 hydrocarbons. Prepared by a wet co-impregnation method. Including small amounts of alcohols.
the product stream with high Co loading catalysts, and CH4 formation accounted for 90–99% of the total hydrocarbons product. These results are similar to the previous work by Park et al. [32], in which they reported that Fe–Co/Al2O3 catalyst with 5 wt% metal loading each for Fe and Co showed higher CO2 conversion, but lower selectivity for C2–C5 hydrocarbons compared to 10 wt% Fe/Al2O3 catalyst. The conversions and selectivities of all monometallic and bimetallic catalysts are summarized in Table 2. Fe–Co(0.17)/Al2O3 catalyst exhibited significantly lower CO selectivity than monometallic Fe catalyst with more than 4 times higher C2+ selectivity, while CH4 selectivity barely changed. There were no olefins observed in the gas phase hydrocarbons with these monometallic and bimetallic catalysts. CH4 selectivities of the unpromoted Fe– Co/Al2O3 bimetallic catalysts were relatively high (30–40%) even at the low Co/(Co + Fe) atomic ratios compared to those in typical FTS. However, these bimetallic catalysts, in particular monometallic Co catalyst, are still useful for fuel production from CO2 hydrogenation because CH4 can be used as synthetic natural gas.
higher CO2 conversion regardless of the Co/(Co + Fe) ratios. The addition of K also improved the STY of C2–C7 hydrocarbons over bimetallic catalysts as displayed in Fig. 3A, while it suppressed CH4 formation when the Co/(Co + Fe) atomic ratio was smaller than 0.5 (Fig. 3B). Further increasing K loading to Fe–Co(0.17)/Al2O3 did not cause clear changes in the C2–C7 STY as illustrated in Fig. 4, while CH4 STY decreased linearly with increasing K loading. These results
3.2. Effect of K addition The effect of K addition on the activities and selectivities of Fe– Co bimetallic catalysts were then studied. Fig. 2 compares CO2 conversion over K-promoted (K/Fe = 0.3 atom atom 1) and unpromoted Fe–Co bimetallic catalysts. K-promoted catalyst showed
Fig. 4. Effects of K/Fe atomic ratio on the STYs of CH4 and C2–C7 hydrocarbons and the olefin to paraffin ratio of C2–C4 hydrocarbons on Fe–Co(0.17)/K(Y)/Al2O3.
106
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results are important for developing both new catalysts and fundamental understanding of the mechanism involved in activation of CO2 and H2 towards carbon–carbon bond formation for higher hydrocarbons. More detailed investigations on these catalysts are currently underway. Acknowledgements This research is supported in part by the Pennsylvania State University through Penn State Institutes of Energy and the Environment. The authors also wish to thank the Thailand Research Fund and Graduate School of Chulalongkorn University through the Royal Golden Jubilee Ph.D. Program Scholarship to RS at PSU. Fig. 5. GC–MS total ion chromatogram of liquid products from CO2 hydrogenation on Fe–Co(0.17)/K(1.0)/Al2O3 catalyst.
demonstrate that the addition of K to Fe–Co/Al2O3 catalysts enhances the chain propagation of hydrocarbons. Table 2 also shows product selectivities of K-promoted Fe–Co bimetallic catalysts with different Co/(Co + Fe) atomic ratios. Selectivity for C2+ hydrocarbons and chain growth probability of gas-phase hydrocarbons were enhanced notably by the addition of K to Fe–Co bimetallic catalyst, while CH4 selectivity decreased drastically. CH4 selectivity of Fe–Co(0.17)/K(Y)/Al2O3 catalysts (Y 0.3) were relatively similar to that of Fe/K/Al2O3 with 15 wt% Fe loading, but the former catalysts yielded more C2+ hydrocarbons selectivity compared to the latter one. Furthermore, these Fe–Co bimetallic catalysts showed higher CO2 conversions and C2+ selectivities than K-promoted Fe–Mn catalyst with higher Fe loading as shown in Table 2. Since the CO2 hydrogenation activities and selectivities of Fe–Co(0.17)/Al2O3 catalyst were hardly dependent on the preparation methods, namely pore-filling incipient wetness impregnation and wet co-impregnation methods (Table 2), our results reveal higher effectiveness of Fe–Co–K combination than Fe–Mn–K for CO2 hydrogenation to C2+ hydrocarbons under the conditions employed. The addition of K also increased olefin to paraffin ratio (O/P) of C2+ hydrocarbons as shown in Table 2. Olefin contents increased with increasing K loading, and olefins predominated in C2+ hydrocarbons when the catalyst was heavily alkalized (K/ Fe 0.5 atom atom 1). O/P reached approximately 5 at the K/Fe atomic ratio of unity (see also Fig. 4). Fig. 5 illustrates GC–MS total ion chromatogram of liquid products from CO2 hydrogenation over Fe–Co(0.17)/K(1.0)/Al2O3 catalyst. Linear alpha olefins were main products with carbon numbers up to 27. The main carbon number range of this liquid product was approximately corresponding to jet fuel. Small amount of alcohols were also observed in the product. 4. Conclusion By combining Fe and a small amount of Co on an alumina support, we have discovered a strong bimetallic promotion of C2+ hydrocarbons formation from CO2 hydrogenation on Fe–Co/Al2O3 catalysts. Olefin-rich C2+ hydrocarbons were synthesized using Fe– Co bimetallic catalysts in the presence of a K promoter. Kpromoted bimetallic Fe–Co catalysts with desired compositions also show significant advantages in synthesis of higher hydrocarbons over K-promoted Fe/Al2O3 and Fe–Mn/Al2O3 catalysts with higher Fe loadings under the same reaction conditions. These
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