Elsevier

Journal of CO2 Utilization

Volumes 3–4, December 2013, Pages 102-106
Journal of CO2 Utilization

Short communication
Bimetallic Fe–Co catalysts for CO2 hydrogenation to higher hydrocarbons

https://doi.org/10.1016/j.jcou.2013.10.002Get rights and content

Highlights

  • Bimetallic Fe–Co catalysts can be more effective and selective for CO2 hydrogenation to hydrocarbons.

  • Combining a small amount of Co with Fe leads to a dramatic promotion of selectivity to C2+ hydrocarbons.

  • Adding K to Fe–Co/Al2O3 catalyst improves C2+ selectivity and olefin contents and decreases CH4 selectivity.

  • Olefin-rich C2+ hydrocarbons are synthesized using K-promoted Fe–Co/Al2O3 with high K loadings.

Abstract

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.

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], [2], [3], [4], [5], [6], [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], [10], [11], [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 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], [16], [17], [18], [19], [20], [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], [17], [18], [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 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], [23], [24], [25], [26], [27], [28], [29], [30], [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. K-promoted Fe–Mn catalyst was prepared and tested for CO2 hydrogenation at the same reaction condition as a reference catalyst.

Section snippets

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)3·9H2O (Aldrich, 99.99%) and Co(NO3)2·6H2O (Sigma–Aldrich, ≥98%). Concentrations of Fe and Co in the impregnating solution were adjusted to obtain desired

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

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. K-promoted 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

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.

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