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Communication

The Influence of Potassium Promotion on the Structural Properties of Cobalt Molybdenum Nitrides in Ammonia Synthesis

by
Paweł Adamski
*,
Wojciech Czerwonko
,
Aleksander Albrecht
and
Dariusz Moszyński
Department of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(8), 1158; https://doi.org/10.3390/catal13081158
Submission received: 21 June 2023 / Revised: 20 July 2023 / Accepted: 25 July 2023 / Published: 27 July 2023
(This article belongs to the Section Catalytic Materials)
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Abstract

:
Cobalt molybdenum nitrides are one of the most promising ammonia-synthesis catalysts of the future. However, the selection of the optimal promoter composition is a challenging task. In this paper, the structure–property relationship of ternary cobalt molybdenum nitrides as ammonia synthesis catalysts promoted with potassium was studied. A series of catalysts containing 0.2–3.5 wt% potassium was analyzed by means of X-ray diffraction, volumetric gas adsorption, and activity tests in ammonia synthesis. The catalysts were subjected to thermal aging in the same catalytical reactor. The influence of the potassium promoter on the thermostability was determined. The observed loss of activity in catalysts with a high potassium content was related to the concentration of Co2Mo3N and Co3Mo3N phases, the mean crystallite sizes, the specific surface area, and the pore size distribution.

Graphical Abstract

1. Introduction

Today, the implementation of green technologies is one of the most important subjects in industrial chemical processes. The environmental impact of ammonia synthesis becomes an issue as important as the overall reduction of production costs. However, even though the hydrogen that can be obtained from novel solid oxide electrolyzer cells (SOEC) is becoming a promising substrate for ammonia synthesis, the classic Haber–Bosch process is still the most widely used method for ammonia synthesis [1]. Unconventional activation of nitrogen with the use of enzymes and photo-, plasma- and electrochemical processes yields lower reaction rates, which makes them too expensive for commercial use [1]. Due to that, the development of conventional catalysts remains an important research area. Several factors, including electronic donation, structure, and stability, should be considered during the design of the new catalysts.
The most common catalyst that is used in industry is an iron catalyst promoted with metal oxides. The addition of structural promoters, especially Al2O3, prevents the sintering of iron crystallites during the reaction, which would lead to the complete deactivation of the catalyst [2]. In addition to promoters that influence the structure, there is a group of materials called the electron promoters, which are claimed to modify the distribution of electron density on the iron surface, leading to an increase in the catalyst activity [3]. Potassium oxide, K2O, is the most commonly used and one of the most important electron promoters for iron catalysts [4]. The activity and structure of fused iron catalysts for ammonia synthesis are significantly affected by the addition of potassium [5,6,7,8,9]. Also, potassium promotion was proven to change the binding energy of reactant molecules on the Fe surface in the case of modern K-promoted Fe/C catalyst obtained through pyrolysis of the Fe-based metal–organic framework (MOF) xerogel [10].
Cobalt and molybdenum compounds are studied intensively in ammonia synthesis [11,12]. Chemical looping ammonia synthesis over the Mo-Mo2N couple gives a three times higher ammonia productivity than conventional thermocatalytic synthesis over Mo2N [11]. The associative and dissociative concerted mechanism of ammonia synthesis over Co/CeN results in extraordinary activity under mild reaction conditions [12]. There is an increasing interest focusing on ternary cobalt molybdenum nitrides (COMON) catalysts, which exhibit high activity in the ammonia synthesis process [13,14]. Their activity is particularly influenced by the concentration of alkali promoters [15,16,17]. In analogy to iron catalysts, potassium is associated with the electronic effects influencing the adsorbates that interact with the active surface [18,19]. Alkali metals are good electron donors in catalytic systems, significantly changing the behavior of catalysts [19]. In the ammonia synthesis process, the alkali metal atoms modify the electronic character of the active surface by either increasing the dissociation rate of N2 [20] or influencing the coverage of NHx species on the metal surface and promoting the desorption process of these species from the material [21,22]. Regarding COMON catalysts, Kojima and Aika [23] reported that, during the process, the surface of Co3Mo3N is covered by intermediate surface NHx species, which are in equilibrium with ammonia and hydrogen. They also suggested that the nature of the Co3Mo3N surface was changed by the addition of alkali promoters, changing the adsorption of NHx from NH2 to NH.
However, the impact of potassium admixture on the structural properties of catalysts should not be neglected. In our recent study, we have described the properties of potassium-promoted cobalt molybdenum nitrides [17]. We have determined that the activity of the COMON catalyst in ammonia synthesis is under 450 °C and 10 MPa increases when potassium is added up to 1.5 wt%. A further increase in the promoter concentration results in a decrease in the activity of the promoted catalyst below that of the unpromoted catalyst. We have observed a similar trend for the specific surface area and phase composition of COMON catalysts. Also, we have realized that the properties of the studied COMON catalysts, after prolonged thermal treatment, are connected to the potassium admixture. In previous work, we have suggested that the loss in activity after thermal treatment is most probably associated with the collapse of the porous structure of the potassium-promoted catalysts. However, the mechanism responsible for the promoted catalyst deactivation process and the role of potassium in it remained unclear. In this work, we present a further investigation of the potassium-promoted COMON system. We have conducted additional pore size distribution and mean crystallite sizes analysis of the same samples. We have correlated the results from the new analyses to the already presented results of cobalt molybdenum nitrides concentrations, specific surface areas, and activity in ammonia synthesis to disentangle the catalyst deactivation mechanism.

2. Results and Discussion

Catalytic activity in relation to the potassium concentration is presented in Table 1 [17]. The rate of ammonia synthesis in the case of the COMON catalyst varies with the change in the total concentration of potassium in the material. A general trend is notable: the catalytic activity increases with the increasing potassium concentration up to the maximum of 1.3 wt% of potassium. The drop in the catalytic activity is observed if the potassium concentration exceeds 1.3 wt%. After the thermostability test, the catalytic activity of the potassium-free COMON catalyst increases slightly. This behavior corresponds to the Kojima and Aika [24] observation of substantial activation induced by the short treatment under a (N2 + H2) gas mixture. It may be a result of a complete reduction of a small part of the catalyst, which was not reduced after the initial activation procedure applied before the first activity test. The thermal aging of promoted catalysts with up to 1.5 wt% of potassium results in a decrease in catalytic activity. For the catalysts with a higher concentration of promoter, this trend is reversed and their specific surface catalytic activity is slightly increased.
The XRD results confirmed that two phases are present in the materials after ammonolysis of the precursors: Co3Mo3N (PDF 04-008-1301) and Co2Mo3N (PDF 04-010-6426) [17]. The exemplary XRD pattern of the sample with 1.3 wt% of potassium after the activity tests is presented in Figure S1. The concentration of both phases, determined with the use of Rietveld refinement, varies with the addition of a potassium promoter and with the applied thermal treatment. In Figure 1, the values of the ratio (samples after activity tests combined with overheating compared to samples after initial ammonolysis) are related to the concentration of potassium present in the samples. The phase composition of the potassium-free catalyst is virtually unchanged. The conversion of the Co2Mo3N phase into Co3Mo3N is negligible for catalysts with a small admixture of potassium but increases linearly with the potassium concentration. The highest (17 wt%) decrease of the initial concentration of the Co2Mo3N phase was observed for the catalyst with the highest content of potassium. That implies that the excessive potassium concentrations promote the formation of Co3Mo3N during ammonolysis and this effect intensifies as the temperature increases.
The specific surface area, in relation to the potassium concentration, is presented in Table 2 [17]. The exemplary nitrogen sorption-desorption isotherms for the nonpromoted catalyst, the catalyst containing 1.3 wt% of K, and 3.5 wt% of K are presented in Figure S2. The specific surface area of all of the promoted catalysts was lower in comparison with the unpromoted one. This is a result of potassium which occupies the area between COMON particles. This is in accordance with the previous reports, in which potassium addition resulted in a decrease in the specific surface area and chromium admixture resulted in an increased specific surface area [25]. This is explained based on the double-layer effect, in which it is supposed that the presence of metal–oxygen bonds counteracts the tendency of minimization of the surface energy [8]. The higher metal-to-oxygen ratio in the case of trivalent chromium oxide favors the formation of the developed area, contrary to potassium oxide.
However, this effect does not explain the deactivation mechanism which is observed after catalytical tests for the promoted catalysts. To explain this phenomenon, the dependence of the phase composition on the specific surface was correlated (Figure 2). In the case of the promoted catalysts, the catalysts containing the highest values of the Co2Mo3N phase had the highest specific surface area. Thus, the development of the porous structure is associated with the presence of the Co2Mo3N phase in the material. After all activity tests, the concentration of the Co2Mo3N phase decreases slightly in all catalysts. The loss of specific surface area is greater than what would be expected only by lowering the accessibility of the Co2Mo3N phase (see dotted line in Figure 2). Thus, the concentration of Co2Mo3N is not considered the only factor influencing the development of the porous structure.
Two reasons for the loss of surface area can be considered: the sintering of crystallites induced by the excessive temperature [19] or a collapse of the porous structure [26]. The comparisons of the mean crystallite size evaluated for the catalyst after the ammonolysis and after the activity tests are shown separately for the Co2Mo3N and Co3Mo3N phases in Figure 3. The crystalline structure of COMON catalysts is dependent on the potassium concentration. For all catalysts containing potassium admixtures, the mean crystallite size is larger compared to the reference potassium-free COMON sample. The dependencies of the potassium concentration observed for the identified crystallographic phases differ. It seems that the mean size of Co2Mo3N crystallites is barely influenced by the potassium addition. However, the mean size of Co3Mo3N crystallites is significantly influenced, changing from 40 nm to 60 nm. Regarding the nonpromoted COMON catalyst, the mean crystallite size of Co2Mo3N and Co3Mo3N increases by 5–7 nm after thermal aging. Considering the potassium-promoted samples, the linear regression lines were drawn for the measurements after the ammonolysis and after the activity tests. The general trends observed for Co2Mo3N indicate that its crystallites are not sintering, while the Co3Mo3N crystallites are sintering.
As a proof of concept, the theoretical maximum specific surface area was calculated based on mean crystallite size values obtained from diffraction data (Figure 4). It was assumed that each crystallite has a spherical shape, there is no contact between the crystallites, and that the only phases in the system are Co2Mo3N and Co3Mo3N in concentrations received from the Rietveld refinement. The results obtained are in the range of magnitude, yet obviously larger due to the second assumption, of the previously presented values from the volumetric method [17]. There is a measurable difference in the relation plotted for the sample after the ammonolysis. The points are scattered, especially for lower concentrations of potassium in samples. This might be due to the large influence of other, most likely amorphous, phases on the total specific surface area value. The plot drawn for the sample after activity tests, on the other hand, fits almost perfectly, suggesting that Co2Mo3N and Co3Mo3N are closer to the whole volume of the material.
The pore size distribution was also determined for the COMON catalyst after the ammonolysis and after the activity tests combined with thermal aging. The results for three samples: a nonpromoted COMON catalyst and catalysts containing 1.3 wt% and 3.5 wt% of potassium admixtures are shown in Figure 5. In the case of the reference sample, a broad peak is observed. It indicates the presence of pores with radii between 10–50 nm with a maximum of their population at around 20 nm. The higher surface area observed for the potassium-free COMON catalyst is attributed to the presence of pores of this size. After the thermostability tests, the peak shape for the population between 10–50 nm is similar but the new population of pores with radii below 5 nm has emerged. This bimodal distribution of pores is also visible in the most active COMON catalyst containing 1.3 wt% of potassium. The bimodal porous structure can be the additional factor behind the activity of the COMON catalysts. Such bimodality of pore size distribution can also be found in the industrial iron catalysts [27]. The sample after ammonolysis contains 1.3 wt% of potassium pores, with radii below 10 nm and between 10–50 nm present. Thermal aging results in a decrease in the larger pores’ total volume and an increase in smaller ones. In the case of the catalyst showing the worst catalytic activity, containing 3.5 wt% of potassium, the area of pore population with a maximum of 20 nm is minor and the material contains mainly pores with a vague maximum of 5 nm. After activity tests, the pore size distribution for the COMON catalyst with 3.5 wt% of potassium is almost flat, indicating an almost complete lack of a porous structure. These results suggest that during the catalytic tests and after the thermostability test, a collapse of the porous structure of the COMON catalysts takes place, especially in the catalysts containing the highest potassium concentration.
Multiple similarities may be found between examined materials and the iron catalysts for ammonia synthesis. The latter material is made of small crystallites of the metallic phase, separated by spacers. The spacers are made of promoters, mostly aluminum, and calcium compounds, and they help to maintain the porous structure of the iron catalysts [28,29]. When these spacers break, the porous structure collapses, which results in the loss of surface area. As shown in our previous work [30,31], during the ammonia synthesis reaction, a part of the Co2Mo3N phase interacts with metallic cobalt to form the Co3Mo3N phase. The presence of metallic cobalt originates from the reduction of cobalt molybdates and intermediate-defected cobalt oxides (further referred to collectively as Co(Mo)Ox), which might prove difficult to identify in the XRD data [30]. Considering that this structure is like the one reported for the iron catalyst for ammonia synthesis, we suggest the same effect for COMON catalysts. The volume of the COMON catalyst is built mainly by Co3Mo3N crystallites, which are presumably separated by the spacers. However, the role of the spacers in the iron catalysts is herein attributed to the structure containing the Co2Mo3N phase and likely the cobalt species. During the transformation of Co2Mo3N into Co3Mo3N, the spacers contract.
The smaller the number of spacers present in the material, the denser the structure formed and the lower the porosity of the material (Figure 6). The loss of the Co2Mo3N phase is accompanied by the collapse of the porous structure of the catalysts. The highest conversion of Co2Mo3N into Co3Mo3N is observed for the catalysts that contain the highest potassium concentrations. The potassium-free catalyst is virtually unaffected by the thermostability tests. Therefore, it is assumed that the Co2Mo3N conversion is induced by the presence of potassium atoms. This effect is most likely due to the enhanced reduction of cobalt oxides [32]. The metallic cobalt created as a result is an intermediate and readily reacts with Co2Mo3N to form Co3Mo3N [30].

3. Materials and Methods

3.1. Catalysts Synthesis

The synthesis of the catalysts is described in detail in [17]. Cobalt nitrate, Co(NO3)2·6H2O (Chempur, Piekary Śląskie, Poland), and ammonium heptamolybdate, (NH4)6Mo7O24·4H2O (Chempur, Piekary Śląskie, Poland), were mixed in the batch reactor. The pH during the precipitation was adjusted to remain at pH = 5.5 with the addition of 25% aqueous ammonia solution (NH3·H2O, Chempur, Piekary Śląskie, Poland). The precipitant collected by vacuum filtration was dried and mortar ground. The obtained purple–blue powder was used for further processing. The impregnation with the use of an aqueous solution of potassium nitrite, KNO3 (Chempur, Piekary Śląskie, Poland), was performed in a rotary vacuum evaporator to obtain the potassium-promoted samples. The potassium concentration in the promoted samples ranged from 0.2 to 3.5 wt%. In the last step, the precursors were activated under the flow of gaseous ammonia (250 cm3/min) at 700 °C for 6 h. After the process, the samples were cooled to room temperature and passivated by exposure to the air.

3.2. Catalytic Activity Tests

Catalytic activity tests were performed on the apparatus described elsewhere [33]. Briefly, a tubular flow reactor supplied with a gaseous mixture of N2 + 3H2 (330 cm3/min) was used. The ammonia concentration in the outlet gas stream was measured with a ULTRAMAT 6 NDIR (nondispersive infrared absorbance) gas analyzer (Siemens, Munich, Germany). The reaction rate constants of the ammonia synthesis were calculated for each of the catalysts with the use of the modified Tiemkin–Pyzhev equation [34]. Cobalt molybdenum nitride containing no alkali admixtures was implemented as a reference sample. The ammonia synthesis process was carried out under the pressure of 10 MPa at 450 °C. After the initial activity tests, the samples were overheated in the same reactor up to 650 °C for 12 h and the activity tests were repeated.

3.3. Catalysts Characterization

Powder X-ray Diffraction (XRD) analysis of the samples was carried out to determine their structural properties. The X’pert PRO MPD Diffractometer (Cu tube) with Bragg–Brentano geometry setup was used (Philips PANalytical, Almelo, Netherlands). The diffraction data were collected in the 2θ range of 10–110°. The diffraction results were compared with reference patterns from the ICDD PDF-4+ database (Newtown Square, PA, United States) [35]. The Rietveld refinement was implemented to define the weight fractions and the average crystallite size of the identified phases. The HighScore Plus software v 3.0e [36] by PANalytical. was used to perform a semiautomatic Rietveld refinement. The structural data needed to perform the Rietveld refinement were extracted from the ICDD database. During the Rietveld refinement, the pseudo-Voigt function was implemented. The silicon powder (325 mesh, Sigma Aldrich, Burlington, MA, United States) was used as a line-broadening standard.
The N2 adsorption-desorption isotherm at 77K and the Brunauer–Emmett–Teller (BET) equation were implemented to determine the specific surface area. The Qiadrasorb SI-Kr/MP (Quantachrome, Boynton Beach, FL, United States) apparatus and QuadraWin software (v 6.0) were used. Pore size distributions of the samples were also calculated with the use of QuadraWin software v 6.0. Before the measurements, all samples were degassed for 6 h at 400 °C under a vacuum.
Materials were tested after the synthesis procedure described in Section 3.1. and after the catalytical activity test with the overheating procedure described in Section 3.2.

4. Conclusions

The addition of the optimal concentration of potassium promoter results in the enhancement of the catalytic and structural properties of cobalt molybdenum nitride catalysts. The decrease of catalytic activity in materials with more than 1.5 wt% of potassium is correlated with the collapse of pores of approximately 20 nm in size. This phenomenon is attributed to the transformation of the Co2Mo3N into the Co3Mo3N phase in the ammonia synthesis reaction conditions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13081158/s1, Figure S1: X-ray diffraction pattern with the Rietveld refinement of the sample promoted with 1.3 wt% of K after catalytic test; Figure S2: nitrogen sorption-desorption isotherms for nonpromoted catalyst, the catalyst containing 1.3 wt% of K, and 3.5 wt% of K.

Author Contributions

Conceptualization; methodology; investigation; writing—original draft preparation; writing—review and editing; visualization; project administration; funding acquisition, P.A. Investigation; writing—original draft preparation; writing—review and editing; visualization, W.C. Investigation; writing—original draft preparation; writing—review and editing A.A. Conceptualization: methodology; investigation; writing—original draft preparation; writing—review and editing; visualization; supervision, D.M. All authors have read and agreed to the published version of the manuscript.

Funding

The scientific work was financed by the Polish National Centre for Research and Development, grant “Lider”, project No. LIDER/10/0039/L-10/18/NCBR/2019 (Paweł Adamski); and the National Science Centre, Poland, grant “Preludium Bis 1”, project No. 2019/35/O/ST5/02500 (Wojciech Czerwonko).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Influence of potassium concentration on the conversion of Co2Mo3N phase after thermostability test.
Figure 1. Influence of potassium concentration on the conversion of Co2Mo3N phase after thermostability test.
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Figure 2. The dependence of the specific surface area of COMON catalysts on the concentration of the Co2Mo3N phase measured after ammonolysis and after catalytic tests. Lines are given for eye guidance.
Figure 2. The dependence of the specific surface area of COMON catalysts on the concentration of the Co2Mo3N phase measured after ammonolysis and after catalytic tests. Lines are given for eye guidance.
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Figure 3. The comparison of the mean crystallite size for (a) Co2Mo3N phase and (b) Co3Mo3N phase after ammonolysis and after all catalytic tests.
Figure 3. The comparison of the mean crystallite size for (a) Co2Mo3N phase and (b) Co3Mo3N phase after ammonolysis and after all catalytic tests.
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Figure 4. Projected maximum specific surface area values calculated for mixtures of Co2Mo3N and Co3Mo3N phases.
Figure 4. Projected maximum specific surface area values calculated for mixtures of Co2Mo3N and Co3Mo3N phases.
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Catalysts 13 01158 g005 550
Figure 5. Pore size distribution for samples: (a) nonpromoted COMON catalyst, (b) COMON catalyst containing 1.3 wt% of potassium, (c) COMON catalyst containing 3.5 wt% of potassium. Measurements were made after the ammonolysis of the precursor and after all activity tests.
Figure 5. Pore size distribution for samples: (a) nonpromoted COMON catalyst, (b) COMON catalyst containing 1.3 wt% of potassium, (c) COMON catalyst containing 3.5 wt% of potassium. Measurements were made after the ammonolysis of the precursor and after all activity tests.
Catalysts 13 01158 g005
Catalysts 13 01158 g006 550
Figure 6. The proposed mechanism of the collapse of the porous structure of the catalysts.
Figure 6. The proposed mechanism of the collapse of the porous structure of the catalysts.
Catalysts 13 01158 g006
Table
Table 1. The activity of the studied catalysts in the ammonia synthesis reaction measured after ammonolysis and after the thermostability test [17].
Table 1. The activity of the studied catalysts in the ammonia synthesis reaction measured after ammonolysis and after the thermostability test [17].
SampleActivity
(gNH3·gcat−1·h−1)
After AmmonolysisAfter Thermostability Test
Nonpromoted0.991.16
0.2 wt% K0.680.21
0.4 wt% K1.010.78
0.6 wt% K1.320.86
0.8 wt% K1.351.03
1.3 wt% K1.491.29
1.5 wt% K1.381.17
2.9 wt% K0.240.26
3.5 wt% K0.100.12
Table
Table 2. The specific surface area of the studied catalysts measured after ammonolysis and after the thermostability test [17].
Table 2. The specific surface area of the studied catalysts measured after ammonolysis and after the thermostability test [17].
SampleSpecific Surface Area
(m2·g−1)
After AmmonolysisAfter Thermostability Test
Nonpromoted15.515.1
0.2 wt% K9.27.1
0.4 wt% K9.68.9
0.6 wt% K10.28.7
0.8 wt% K10.69.0
1.3 wt% K10.38.3
1.5 wt% K9.98.3
2.9 wt% K5.53.8
3.5 wt% K5.62.2
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Adamski, P.; Czerwonko, W.; Albrecht, A.; Moszyński, D. The Influence of Potassium Promotion on the Structural Properties of Cobalt Molybdenum Nitrides in Ammonia Synthesis. Catalysts 2023, 13, 1158. https://doi.org/10.3390/catal13081158

AMA Style

Adamski P, Czerwonko W, Albrecht A, Moszyński D. The Influence of Potassium Promotion on the Structural Properties of Cobalt Molybdenum Nitrides in Ammonia Synthesis. Catalysts. 2023; 13(8):1158. https://doi.org/10.3390/catal13081158

Chicago/Turabian Style

Adamski, Paweł, Wojciech Czerwonko, Aleksander Albrecht, and Dariusz Moszyński. 2023. "The Influence of Potassium Promotion on the Structural Properties of Cobalt Molybdenum Nitrides in Ammonia Synthesis" Catalysts 13, no. 8: 1158. https://doi.org/10.3390/catal13081158

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