The effect of Fe2O3 crystal phases on CO2 hydrogenation

Introduction

CO2 hydrogenation for organic chemicals is a worthy study under the background that CO2 used as raw material for chemicals other than discharged into atmosphere would be helpful to abate the greenhouse effect. However, the difficulty to capture CO2 and the cost to supply H2 make most of the researches stayed in laboratory. The concept and trial using seawater as starting materials brings an applicable and profitable scene to CO2 hydrogenation [1-4]. Seawater is a natural absorbent of CO2, from which plenty of CO2 can be captured. Seawater is an unlimited source for H2, too. The above concept becomes accepted to us because the device to transfer solar power into electricity can be constructed on the vast ocean, which would supply enough energy to produce CO2 and H2 from seawater simultaneously. With the concept breakthrough where and how to perform CO2 hydrogenation, active catalysts are the key component to commercialize CO2 hydrogenation.

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The organic chemicals synthesized from CO2 hydrogenation include methane, methanol, methyl acid, dimethyl ether, hydrocarbons and mixed alcohols [5-7]. Among them, hydrocarbons are a good product because it can be upgraded into liquid fuels which are cleaner than the petroleum-based fuels [8]. It is accepted that CO2 is hydrogenated into hydrocarbons by two steps: CO is produced from CO2 by reverse Water-Gas shift (WGS) reaction (Reaction I), then the CO reacts with H2 to synthesize hydrocarbons via Fischer-Tropsch synthesis (FTS) (Reaction II) [9-14].

Fe and Co are commercial catalysts for FTS. Riedel et al. [15] compared the performance of Fe and Co catalysts in the mixtures of CO, CO2, and H2. With increased CO2 and decreased CO content in the feedgas, the product composition shifted from a mixture of mainly higher hydrocarbons to almost exclusively methane for Co catalyst, while Fe catalyst synthesized the same hydrocarbon products from CO2 /H2 as from CO/H2 syngas. Zhang et al. [16] also found that the CO2 hydrogenation products contained about 70% or more methane for supported Co catalyst. These distinctions are partly attributed to that Fe catalyst is active for both of the Reaction I and II [15,17,18].

In order to improve the performance of Fe catalysts in CO2 hydrogenation, the effects of promoter [8,10,13,15,19-24], supporter [15,20,22,24,25], preparation method [8,13,15,21-26] and reducing agent [8] are studied very much. In these studies, iron oxide almost presents in α-Fe2O3 [15,21,26] or Fe3O4 [25] crystal phase in the as-prepared catalysts. Considering that γ-Fe2O3 is one kind of iron oxide as common as α-Fe2O3 [27,28], it is surprising that there are very few reports about the behavior of γ-Fe2O3 in CO2 hydrogenation. Al-Dossary et al. found γ-Fe2O3 coexisted with α-Fe2O3 in the catalysts, but no benefit from γ-Fe2O3 was disclosed [21]. However, it has been confirmed that γ-Fe2O3 is superior to α-Fe2O3 in other catalytic reactions, such as photodecomposition of H2S [29], selective catalytic reduction of NOX with NH3 [30], electroanalysis and ultrasensitive detection of Pb2+ [27], WGS reaction [31] and so on. The lack on the performance of γ-Fe2O3 in CO2 hydrogenation makes it necessary to study Fe catalyst in γ-Fe2O3 phase, not only to supply the knowledge about γ-Fe2O3 in the reaction, but also to find active catalyst to make CO2 profitable.

We has reported the influences of Fe2O3 crystal phases on CO2 hydrogenation [32]. γ-Fe2O3 phase in the catalysts was formed by washing FeAl precipitate with anhydrous ethanol. The catalyst with strong γ-Fe2O3 phase was more active in the reaction than the catalysts with none or weak γ-Fe2O3 phase. In order to avoid the possible promotion of Al on the catalyst activity and prepare the catalyst in pure γ-Fe2O3 phase simultaneously, solid-phase reaction was used recently for catalyst preparation. The effect of Fe2O3 phase on the catalyst reactivity is explored in this work.

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