Effect of potassium promoter on phase transformation during H₂ pretreatment of a Fe₂O₃ Fischer Tropsch synthesis catalyst precursor

Highlights

• H₂ reduction behaviors of Fe₂O₃ based catalysts are investigated by in situ XRD.

• Potassium promoter notably inhibits the reduction from α-Fe₂O₃ to Fe₃O₄.

• The formation of α-Fe crystallite is accelerated or unchanged during reduction.

• Potassium effect on FTS performance of the Fe₂O₃ based catalysts is studied.

Abstract

Effect of potassium promoter on phase transformation during the isothermal and temperature programmed reduction of the α-Fe₂O₃ catalyst were systematically investigated by in situ XRD. The reduction pathway of Fe₂O₃ based catalysts follows the same sequence of α-Fe₂O₃ → Fe₃O₄ → α-Fe during the isothermal reduction, in contrast to that of α-Fe₂O₃ → Fe₃O₄ → FeO/ α-Fe→ α-Fe during the temperature programmed reduction. The potassium promoter notably inhibits the reduction from α-Fe₂O₃ to Fe₃O₄ due to the enhanced strength of Fe-O bonds, whereas it either accelerates or has no effect on the formation rate of α-Fe crystallites during reduction. The potassium promoter increases the CO conversion during FTS, while enhances C₅+, olefins, CO₂ selectivity, and decreases CH₄ selectivity.

Introduction

Iron based catalyst is widely used in Fischer Tropsch synthesis (FTS) for the production of hydrocarbons from coal, biomass or natural gas derived syngas (CO/H₂) [1,2]. However, these catalysts are usually activated in H₂, CO or syngas before use to form α-Fe or various iron carbides depending on pretreatment gases [[3], [4], [5], [6], [7], [8], [9]]. During FTS, even though the initial activity of H₂ pretreated catalyst is lower than that of CO or syngas pretreated catalysts, H₂ pretreated catalyst results in a relative stable or even increasing FTS activity with time on stream [[10], [11], [12], [13], [14]]. In terms of iron based catalyst stability, it is concurred that the transformation of α-Fe to various iron carbides coincides with the increase of FTS activity for the H₂ pretreated catalysts [15,16], while the CO or syngas pretreated catalysts often deactivate with time on stream, with corresponding buildup of inactive carbon species on catalyst surface [[10], [11], [12], [13],17,18]. In addition, the oxidation of the iron carbides also correlates with the deactivation of the iron based catalysts [19,20].Therefore, the focus of this part of our in situ XRD work was on the iron phase transformation during the H₂ reduction and its effect on the catalyst performance.

Typically, potassium promoter increases the catalyst surface basicity, promotes the CO dissociation and inhibits H₂ dissociation, which can be used in iron based catalysts to increase their FTS activity and the selectivity to long chain hydrocarbon and olefins [[21], [22], [23], [24]]. However, potassium promoter is another key factor that can modify the activation behaviors and iron phase distribution of the working catalysts [25,26]. It is reported that the addition of potassium can favor the formation of FeC_x nuclei on iron oxide surface, promote the carburization rate and carburization extent of iron based catalysts [25,27,28]. During FTS, a low amount of potassium is enough to promote the FTS activity and stability [29,30], due to the transformation of iron carbides to the inactive iron oxide is inhibited substantially by potassium promoter [26]. However, further increase of the potassium content tends to notably enhance the deactivation rate, coinciding with the conversion of χ-Fe₅C₂ to ε’-Fe_2.2C and the formation of inactive carbon species surrounding the catalysts surface which prevent the contact of catalysts surface with the gaseous reactants [26,31]. Even though numerous studies on the effect of H₂ pretreatment and potassium promoter on the FTS performance have been carried out, few were in situ studies on the effect of potassium during the dynamic phase transformation under hydrogen pretreatment [32,33].

In our previous study, the carburization behaviors of the un-promoted and potassium promoted Fe₂O₃ catalysts during pretreatment in 2% CO/He or 2% CO/8% H₂/He at 270 °C were systematically investigated by in situ XRD. It was observed that the carburization followed a consecutive pathway of α-Fe₂O₃ → Fe₃O₄ → χ-Fe₅C₂ and/or θ-Fe₃C [34]. Further details suggest that the carburization of α-Fe₂O₃ catalysts is a bidirectional process of oxygen removal against carbon diffusion in the iron oxide crystal lattice. In order to study the carburization process of the α-Fe₂O₃ catalysts, the carburization process is divided into two process: (1) the outward lattice oxygen removal which corresponding to the reduction of the α-Fe₂O₃ catalysts by H₂; (2) the inward carbon diffusion which corresponding to the carburization of the α-Fe catalysts by CO.

This work presents an in situ XRD study to trace the reduction behaviors of α-Fe₂O₃ based catalysts by 10% H₂/He at both isothermal reduction (270 °C) and temperature programmed reduction processes. In addition, the effect of potassium on the reduction behaviors of α-Fe₂O₃ catalysts was also investigated systematically. The relative abundance of individual crystallite was collected during the reduction process to illustrate the effect of potassium promoter on the reduction behaviors of α-Fe₂O₃ catalyst. Particular attention was paid to the influence of potassium on the reduction rate of α-Fe₂O₃ catalyst.