Surface properties of wustite based iron-cobalt catalysts for ammonia synthesis reaction

Highlights

• The introduction of cobalt into the wustite-iron catalyst structure for ammonia synthesis.

• Addition of cobalt(II) oxide increased the catalyst activity in the ammonia synthesis reaction.

• Influence of the wustite catalyst reduction temperature on its activity in the ammonia synthesis.

• The promotion of cobalt(II) oxide improved the resistance to overheating.

Abstract

In this work, surface properties of wustite-based iron and iron‑cobalt catalysts were investigated. Measurements of hydrogen temperature-programmed desorption (TPD-H₂) and surface area of active forms of catalysts were conducted after reduction of catalyst precursors (16 h) in pure hydrogen at various temperatures (450, 475, 500, 550 and 600 °C). The amount of cobalt in the catalyst structure and the number of adsorption sites relative to hydrogen were correlated with the activity for ammonia synthesis reaction. The presence of cobalt increased thermal resistance of the catalysts and at the same time their activity in ammonia synthesis reaction.

Introduction

Many scientists believe that everything on catalyst development for ammonia synthesis has already been done and that the only way to improve iron catalyst activity is to change the promoters used. However, the role of the precursor was omitted. This perception resulted from the assumption of a classical volcano-type curve of the catalyst activity versus the ratio of Fe²⁺/Fe³⁺ ions (denoted as R) in a precursor form. According to this assumption, the most active catalyst is obtained as a result of reduction of iron oxide with R equal to 0.5 [1]. This value corresponds to the magnetite form of Fe₃O₄. Increasing or decreasing R causes a decrease in the activity of the obtained iron catalyst. The change in the approach to the precursor form led to Liu Huazhang's team results [1]. The authors studied precursors with R higher than 0.5. They found that precursors with a high proportion of Fe²⁺ ions, called wustite, showed higher activity than traditional iron catalysts.

In many publications the Chinese team proved that the wustite-based catalyst is characterized by higher catalytic activity and more advantageous parameters of the reduction process parallel to almost the same thermostability and resistance to CO poisoning in comparison with the conventional magnetite-based iron and iron‑cobalt catalysts [2].

In the following years of studies on the wustite iron catalyst, many scientist described the influence of the phase composition of the precursor on the catalytic activity and the reduction process [3], the impact of a kind of the oxide precursor on the nitrogen desorption process [4], hydrogen desorption [3,5], the in situ course of reduction of wustite and magnetite catalysts [6], the influence of Fe²⁺/Fe³⁺ ratio and promoter additions on the microstructure of the iron fused catalyst for the ammonia synthesis [7], the influence of Nb₂O₅ addition, as a potential promoter, on the reduction and activity of the wustite catalyst in ammonia synthesis [8]. A peculiar resume of the collected experimental results and general knowledge on ammonia synthesis catalysts was published by Huazhang Liu as a book [9] and a research paper [10].

The distribution of promoters in the catalyst precursor structure affects distribution of promoters in the form of an active iron catalyst for ammonia synthesis. The uniform distribution of promoters over the surface of the active form of a catalyst determines high activity and stability of catalyst structure. The results of research on the distribution of lithium oxide in the catalyst structure [11] support that statement. The etching method [12] allows us to examine how individual promoters are distributed in the oxidized form of the iron fused catalyst. The effect of the molar R (Fe²⁺/Fe³⁺) ratio in iron catalyst precursors on the distribution of promoters tested with the etching method is described in [13]. According to the results presented there, potassium oxide is found entirely in the intergranular spaces regardless of R. Aluminium oxide is present in the iron catalyst precursors in three forms: one directly bound to the grain magnetite or wustite, and two forms located in intergranular spaces, soluble as well as insoluble in HCl. Calcium oxide occurs in intergranular spaces, both in magnetite and wustite grains. It was found that when R increases the concentration of aluminium decreases while the concentration of calcium increases in wustite grains.

In our recent work [14] we stated that the introduction of cobalt into the wustite‑iron catalyst structure increased the activity of the catalyst in the ammonia synthesis reaction. The purpose of the present work is to determine the effect of the addition of cobalt into the iron-based wustite catalyst structure on the distribution of promoters in the precursors, and correlation of the activity of these catalysts in the reaction of ammonia synthesis with hydrogen adsorption on active surface sites.