*Article* **On Optimal Barium Promoter Content in a Cobalt Catalyst for Ammonia Synthesis**

**Aleksandra Tarka <sup>1</sup> , Magdalena Zybert <sup>1</sup> , Hubert Ronduda <sup>1</sup> , Wojciech Patkowski <sup>1</sup> , Bogusław Mierzwa <sup>2</sup> , Leszek K˛epi ´nski <sup>3</sup> and Wioletta Raróg-Pilecka 1,\***


**Abstract:** High priority in developing an efficient cobalt catalyst for ammonia synthesis involves optimizing its composition in terms of the content of promoters. In this work, a series of cobalt catalysts doubly promoted with cerium and barium was prepared and tested in ammonia synthesis (H2/N<sup>2</sup> = 3, 6.3 MPa, 400 ◦C). Barium content was studied in the range of 0–2.6 mmol gCo −1 . Detailed characterization studies by nitrogen physisorption, SEM-EDX, XRPD, H<sup>2</sup> -TPR, and H<sup>2</sup> -TPD showed the impact of barium loading in CoCeBa catalysts on the physicochemical properties and activity of the catalysts. The most pronounced effect was observed in the development of the active phase surface, a differentiation of weakly and strongly binding sites on the catalyst surface and changes in cobalt surface activity (TOF). Barium content in the range of 1.1–1.6 mmol gCo −1 leads to obtaining a catalyst with the most favorable properties. Its excellent catalytic performance is ascribed to the appropriate Ba/Ce molar ratio, i.e., greater than unity, which results in not only a structural promotion of barium, but also a modifying action associated with the in-situ formation of the BaCeO<sup>3</sup> phase.

**Keywords:** ammonia synthesis; cobalt catalyst; barium; promoter; optimization

### **1. Introduction**

Many industrial processes require the use of catalysts to carry out a reaction at a suitable rate and under desirable conditions. A classic example of heterogeneous catalysis is ammonia synthesis over Fe- or Ru-based catalysts. These metals alone are almost inactive in ammonia synthesis [1–3], but their activity significantly increases in the presence of some compounds. These compounds, added to catalysts in small amounts, are called promoters, and they play a crucial role in heterogeneous catalysis [4]. They improve catalyst properties by enhancing activity, lifespan (long-term stability), and selectivity. Promoters can be divided into structural and electronic promoters, depending on the mode of their action. Structural promoters primarily increase the catalyst's activity by increasing the surface area of an active phase. Electronic (chemical) promoters increase the catalytic activity by modifying the active metal and by increasing the reaction rate per surface area [5,6]. This is a general description, but the function of each promoter is always specific to the particular catalytic system and the particular reaction.

In the case of a fused iron catalyst for ammonia synthesis, aluminum oxide, calcium oxide, and magnesium oxide are typically used as structural promoters [7]. They stabilize the active planes of the metal (role of Al2O3), increase and stabilize the catalyst surface area during reduction (role of CaO and MgO), and increase the catalyst resistance to impurities (role of CaO). Moreover, potassium oxide is used as an electronic promoter. It

**Citation:** Tarka, A.; Zybert, M.; Ronduda, H.; Patkowski, W.; Mierzwa, B.; K˛epi ´nski, L.; Raróg-Pilecka, W. On Optimal Barium Promoter Content in a Cobalt Catalyst for Ammonia Synthesis. *Catalysts* **2022**, *12*, 199. https:// doi.org/10.3390/catal12020199

Academic Editor: Benoît Louis

Received: 30 December 2021 Accepted: 3 February 2022 Published: 6 February 2022

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can increase the rate-limiting step of dissociative nitrogen adsorption [8] or decrease the concentration of produced ammonia adsorbed on the iron surface, and hence make more active sites available for nitrogen [9]. In the case of ruthenium, alkali metals are electronic promoters whose influence is similar to that noted for the iron catalyst [2,10–12]. High activity in ammonia synthesis was also achieved by promoting ruthenium by cesium and barium [13,14].

Among the alkaline earth metals, barium is of particular attention as a very effective promoter of catalysts for the synthesis of ammonia [15–27]. Its role is significant and has been thoroughly investigated by many researcher groups, but its effect has not been fully explained. Some authors have shown that it is a structural promoter [18–20], whereas others postulate that it exhibits an electronic effect [21–23]. There is also a viewpoint in which the influence of barium may have a mixed character, i.e., both structural and electronic [15,24,27]. A cobalt catalyst doubly promoted with cerium and barium was the subject of our previous research [15,27]. These cobalt–cerium–barium systems exhibited very high activity in ammonia synthesis. The studies revealed that the double promotion of cobalt with Ce and Ba causes an approximately twofold increase in catalyst activity, compared to the cobalt system promoted only with barium, and over tenfold increase in activity compared to the cobalt system doped only with cerium. The particularly beneficial properties of the catalyst result from the synergistic action of the two promoters. Cerium oxide is a structural promoter in cobalt–cerium–barium systems preventing Co particles from sintering during the reaction and stabilizing the active hcp cobalt phase [15,27–29]. Optimal cerium oxide content (1.0 mmol gCo −1 ), i.e., one which provides the most favorable catalytic properties, was determined during our further studies [28]. In the case of barium, although it mainly exhibits an electronic character, structural effects have been observed. However, the most important is the participation in the in-situ formation (under the conditions of catalysts activation) of the BaCeO<sup>3</sup> phase. It is the third promoter with strong electron-donating properties and the ability to differentiate the structure of hydrogen adsorption sites (co-existence of weakly and strongly binding sites) on the active phase surface. However, these observations were carried out only for one catalyst composition (Ce content 1.0 mmol gCo −1 , Ba content 1.4 mmol gCo −1 ) [15,27].

As a continuation of the systematic studies of barium-promoted cobalt catalysts, in this work, we studied ammonia synthesis on doubly promoted cobalt–cerium–barium catalysts of various barium content (in the range of 0–2.6 mmol gCo −1 ). The main goal was to determine the optimal content of the barium promoter, providing the most favorable catalytic properties of the studied CoCeBa systems. Thorough characterization studies of the prepared materials by nitrogen physisorption, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDX), X-ray Powder Diffraction (XRPD), Temperature-Programmed Reduction with hydrogen (H2-TPR), and Temperature-Programmed Desorption of hydrogen (H2-TPD) were used to determine the influence of the barium content on the properties and catalytic performance of the doubly promoted cobalt catalysts in ammonia synthesis.

#### **2. Results and Discussion**

### *2.1. Textural Characteristics (N<sup>2</sup> Physisorption)*

The textural characteristics of the catalyst precursors are summarized in Table 1. A small addition of the barium promoter (0.2 mmol gCo −1 ) results in a decrease of the specific surface area (SBET) of the precursor by about 11% and an over twofold decrease of the total pore volume (VP) (CoCeBa(0.2)), compared to that of the precursor without barium (CoCe). When the barium content in samples is increased to 1.4 mmol Ba gCo −1 , a further decrease in SBET and V<sup>P</sup> values is observed, which is probably a result of filling pores with the barium salt. In the samples containing 1.6 mmol Ba gCo <sup>−</sup><sup>1</sup> and more, changes in textural parameters (SBET, VP) are negligibly small. Selected precursors of small, medium, and high Ba content were reduced in-situ, and their specific surface areas were measured (Table 1, S<sup>R</sup> values). A significant decrease in a specific surface area of the materials is observed

due to reduction. For example, the surface of the CoCe sample decreases after reduction over 11 times, and in the case of CoCeBa(2.6), the specific surface area after reduction is nearly 22-times smaller than before the reduction. For CoCeBa(1.4)e, the specific surface area after reduction was only 5 times lower. This indicates that barium has a beneficial effect when added in an optimal amount and effectively prevents sintering of the grains during reduction. The increase of the specific surface area with an increase of the barium content is observed for samples containing 0.2–1.4 mmol Ba gCo −1 . The S<sup>R</sup> value for the reduced sample promoted by a small amount of barium (CoCeBa(0.2)) is approximately 9% larger than the surface area of the reduced sample without barium (CoCe). The highest S<sup>R</sup> value after reduction is observed for CoCeBa(1.4). Further increase of barium content, i.e., over 1.4 mmol Ba gCo −1 , caused a decrease in the surface area of the reduced samples. The observed effects indicate that barium may behave as a structural promoter. However, there is an optimum content of Ba, which may develop the catalyst surface. After exceeding it, the catalyst grains sinter, resulting in decrease of the specific surface area of the catalysts.

**Table 1.** Chemical composition and textural parameters of the promoted cobalt catalysts.


<sup>1</sup> Values determined based on mass balance after impregnation of the Co3O<sup>4</sup> + CeO<sup>2</sup> sample. <sup>2</sup> Cerium content is constant and equal to 1.1 mmol gCo −1 , the value calculated based on the cerium oxide content in the Co3O<sup>4</sup> + CeO<sup>2</sup> sample determined using TG-MS. <sup>3</sup> SBET–specific surface area estimated based on the BET isotherm model. <sup>4</sup> SR–specific surface area estimated based on the BET isotherm model after hydrogen activation. <sup>5</sup> VP–total pore volume estimated based on the BJH isotherm model.
