Comparative Study of Ba, Cs, K, and Li as Promoters for Ru/La2Ce2O7-Based Catalyst for Ammonia Synthesis

Vieri, Hizkia Manuel; Badakhsh, Arash; Choi, Sun Hee

doi:https://doi.org/10.1155/2023/2072245

International Journal of Energy Research

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Abstract Introduction Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 2072245 | https://doi.org/10.1155/2023/2072245

Comparative Study of Ba, Cs, K, and Li as Promoters for Ru/La₂Ce₂O₇-Based Catalyst for Ammonia Synthesis

Hizkia Manuel Vieri,^1,2Arash Badakhsh,^1,3and Sun Hee Choi^1,2

Academic Editor: Sanjay Basumatary

Received15 Nov 2022

Revised05 Apr 2023

Accepted02 May 2023

Published13 May 2023

Abstract

Ammonia is one of the promising carriers for hydrogen and a critical ingredient in many industries including fertilizers and pharmaceuticals. In the KAAP process, ruthenium- (Ru-) based catalysts showed 10-20 more activity compared with iron- (Fe-) based catalysts. The modifications that are applied to Ru-based catalysts revolve around changing the material of its support and/or promoters. This study compares the performance of a Ru-based catalyst for ammonia synthesis supported by La₂Ce₂O₇ using barium (Ba), cesium (Cs), potassium (K), and lithium (Li) as promoters. Based on structural, physicochemical, adsorption, and electronic state analysis, the Cs-promoted catalyst is expected to perform best among all the promoted catalysts, while our findings suggest that the K-promoted catalyst performed the best in the actual catalytic reaction. This result will affect the development of Ru/La₂Ce₂O₇-based catalysts, especially in ammonia synthesis at different temperatures and pressures.

1. Introduction

Hydrogen (H₂) is one of the most promising energy storage materials. However, the low density of H₂ makes its transport difficult. The conversion of hydrogen to ammonia (NH₃) is a promising solution for its easy transportation. Hydrogen is first produced by water electrolysis and then reacted with nitrogen to form ammonia. After being transported, ammonia is dehydrogenated back into hydrogen and nitrogen [1]. The Haber-Bosch process is most commonly used in the industry to produce ammonia with the help of an iron- (Fe-) based catalyst [2]. The drawback of this process is that the strong bonding energy of the nitrogen triple bond requires very high temperatures and pressures to obtain high ammonia yields (>450°C and >20 MPa) [3]. In 1972, Aika et al. pioneered the use of a Ru catalyst for ammonia synthesis, which showed high activity under mild conditions (600 Torr pressure and 350°C temperature) [4].

Various catalyst supports have also been coupled with Ru. Ru/La₂Ce₂O₇ (Ru/LCO) catalysts have been reported to be stable and easy to synthesize. Additionally, they have higher catalytic activity than other established catalysts such as Cs⁺/Ru/MgO and Ba²⁺/Ru/activated carbon [5]. The addition of Ce can act as a nest to form strong interactions between transition metals (Ru) and Ce through strong metal-support interaction (SMSI), increasing the number of donated electrons and reducing the water vapor content [6–8]. During ammonia synthesis, Ru catalysts can be inhibited by H₂. The addition of La to a Ru catalyst supported by zeolite significantly eliminates H₂ inhibition or poisoning [9]. The donated electrons can easily break the N₂ triple bonds.

Promoters are used for further enhancing the catalytic activity. They can be classified into structural and electronic promoters [10]. Considerable research has already been conducted on using alkali metal, alkali earth metal, and rare earth metal as promoters for the catalysts used in ammonia synthesis [11]. In addition to being commonly used as promoters, alkali metals are recognized for their ability to improve the activity, selectivity, and stability of heterogeneous catalysts in many important chemical processes [12]. K and Cs can increase the chemisorption ability of the Ru surface, while Li can facilitate nondissociative ammonia synthesis [13–15]. The influence of Ba on increasing the electron-donating ability of Ru has also been reported [3].

The donation of electrons to the Ru surface by alkali metals under the H₂ atmosphere can facilitate the adsorption of N₂ by promoting the donation of electron density from Ru to the antibonding orbital of N₂ [4, 16–18]. In an experiment employing Ru/Ba/La_1.75Ce_1.75O₃ and Ru/ZrH₂, Ba acted as an electronic promoter [3, 19]. Conversely, other studies on Ru/MgO and Ru/Si₃N₄ showed that Ba acted as a structural promoter rather than an electronic promoter [20, 21]. Additionally, Gao et al. reported the role of Ba as a structural promoter [12]. Therefore, we report a comparative study of Li, K, Cs, and Ba as promoters for Ru/La₂C₂O₇-supported catalysts. The order of electronegativity of the promoters is . In this study, the order of the activity ranks was obtained as , which is different from the ranks reported by Raróg et al. [22] and Forni et al. [23].

The optimum amounts of Ba, Cs, K, Li, and Li-Ba have been reported to be 7 wt%, 4 wt%, 10 wt%, 7.6 wt%, and 12 wt% (5 wt% Ba, 7.6 wt% Li), respectively [3, 15, 24, 25]. However, a comparative study of promoters generally utilizes the same amount of each [19]. Therefore, in this study, 4 wt% of each promoter was used. The promoter addition can be performed before or after Ru impregnation into the support [3, 13, 25, 26]. Previous work has shown that exsolved Ru on catalysts prepared using a high-intensity ball milling method exhibit greater durability than the catalysts prepared in this study. However, given the fact that impregnated Ru on still has greater catalytic activity and metal dispersion, we choose to use the traditional wet-impregnation method to prepare our catalyst [27]. In this study, we will add that after Ru impregnation. The promoter composition also needs to be optimized to understand its interaction not only with Ru but also with the support. We focused on the impact of promoter type and composition on the morphological and chemical characteristics over the promoted Ru/LCO, which were thoroughly characterized using X-ray diffraction (XRD), CO chemisorption, temperature-programmed reduction/desorption (TPR/TPD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), and transmission electron spectroscopy (TEM).

2. Methods

2.1. Preparation of Catalysts

Ru₃(CO)₁₂ was used as the Ru precursor, owing to its stronger metal-support interactions than other precursors [28]. A stoichiometric amount of La(NO₃)·6H₂O (99.999%, Sigma-Aldrich) and Ce(NO₃)·6H₂O (99%, Sigma-Aldrich) were dissolved in a 3 M ammonia solution (Junsei) and coprecipitated for 12 h to make La₂Ce₂O₇. The precipitated sample was filtered with distilled water to remove the ammonia solution. The filtered samples were then mixed with various promoters. CeO₂ and La₂O₃ were also made with a similar method to compare their structure with La₂Ce₂O₇ perovskite. Additionally, CeO₂ and La₂O₃ powder were also mixed to form CeO₂-La₂O₃ composite oxide.

For the Ba²⁺-promoted catalyst, Ba(OH)₂ powder was added to the filtered sample to obtain a specific amount of barium. Subsequently, the mixtures of promoter and support were dried in an oven at 80°C for 12 h and then calcined at a ramping rate of 5°C/min using a muffle oven for 5 h at 700°C. The resulting powder was impregnated with Ru₃(CO)₁₂ to obtain 5 wt% Ru. Ru/K/LCO, Ru/Li/LCO, and Ru/Cs/LCO were prepared by adding the corresponding precursors, as described for the Ru/Ba/LCO catalyst. KOH, LiOH, and CsCO₃ powders were added to the filtered samples to obtain 4 wt% lithium, 4 wt% potassium, and 4 wt% cesium for the K⁺-promoted and Cs⁺-promoted catalysts, respectively.

CeO₂ and La₂O₃ were also made with a similar method to compare their structure with La₂Ce₂O₇ perovskite.

2.2. NH₃ Synthesis

Before testing, the catalyst samples were reduced in a tubular furnace under 10% H₂/Ar gas at 450°C for 1 h. The reducing time was set based on the experiment done by Ogura et al. [5], and the temperature was set by our optimization (Figure S1). Subsequently, 100 mg of the sample was placed inside a tubular reactor (ID: 3/8) and packed with quartz wool and Al₂O₃ beads ( mm). The reaction was performed first at 1 MPa constant pressure in the temperature range of 300–400°C and then at 400°C constant temperature with pressures of 0, 0.1, 0.3, 0.5, and 1 MPa. The reactor was filled with 25% N₂/75% H₂ gas at a flow rate of 90 NmL. The NH₃ synthesis rate was determined using the titration method, with the effluent gas flowing into a 0.1 M H₂SO₄ solution (95–98%, Sigma-Aldrich). The change in the solution conductivity (Orion Star™ A212 conductivity benchtop meter) indicated the ammonia synthesis rate. The detail of the NH₃ synthesis rate calculation is explained in Figures S22–S23. NH₃ synthesis experiments were conducted using a gas mixture of 25% N₂ and 75% H₂ flow rate to prevent the contribution of the reverse reaction which results in NH₃ concentration way below its thermodynamic equilibrium concentration.

2.3. Characterization

The nanoscale morphology of Ru/LCO catalysts was analyzed by using high-resolution transmission electron microscopy (HRTEM) bright-field imaging, and the distribution of elements was verified through scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS) mapping of TEM-Talos. The TEM machine was operated at an accelerating voltage of 200 kV with a magnification of around 1,300,000x. The size of surface Ru particles was determined through the use of ImageJ software.

The SmartLab SE instrument (Rigaku, Tokyo, Japan) was utilized to obtain wide-angle X-ray diffraction (XRD) patterns within the range of 10-90°. The instrument was set to operate at 30 kV and 20 mA and with a scan speed of 3°/minute. It relied on Cu Kα radiation with a wavelength of 1.54 Å as the X-ray source.

To measure the chemisorption of CO, a 10% CO/He pulsed injection mixture was used with a BELCAT-M catalyst chemisorption analyzer equipped with a thermal conductivity detector (TCD). Prior to the measurements, about 70 mg of catalyst powder that had undergone reduction at 450°C for 1 h under 10% H₂/Ar was subjected to a heating process. This involved heating the powder to 150°C at a rate of 10°C per minute for 30 minutes in 5% H₂/Ar. The reduced powder was then cooled to 50°C before a pulse of CO was charged to initiate CO chemisorption. To calculate the dispersion of Ru metal, CO pulses were repeatedly injected into the tube until there is negligible change in peak area and height for three consecutive peaks. The dispersion was determined by summing up the adsorbed CO and assuming a stoichiometry factor of 1 ().

A BELCAT-M catalyst chemisorption analyzer was used to conduct temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) experiments. Prior to the H₂-TPR experiments, around 70 mg of catalyst powder was pretreated with pure Ar gas at 300°C for 1 h to remove any impurities or water from the surface. The temperature was then lowered to 30°C, and the TCD baseline was stabilized before switching to 10% H₂/Ar gas. The temperature was gradually raised to 650°C at a rate of 5°C/min and maintained for 15 minutes. For CO₂-TPD experiment, the samples were pretreated at 500°C for 2 h in pure He gas before exposure with 10% CO₂/He for 1 h. After removing CO₂ and stabilizing the TCD signal by flowing pure He for 1 h, the CO₂ TPD was conducted, and the temperature was gradually raised from 50°C to 900°C at a rate of 10°C/min.

The specific surface areas of the catalysts were determined using the Brunauer-Emmett-Teller (BET) method, which involved pretreating the samples under vacuum at 70°C for 2 h, followed by further degassing at 200°C for approximately 12 h until the pressure dropped below 60-70 mTorr. After cooling down to room temperature, the tube which contains the samples was then transferred to the analysis port to be filled with N₂, and the amount of N₂ adsorbed was measured using a Micromeritics ASAP 2420 instrument. The N₂ adsorption isotherms were measured at a liquid nitrogen temperature of 77 K, with an equilibrium time of 10 seconds.

Elemental composition and chemical states of Ru, La, Ce, O, and promoters in the catalysts were analyzed by X-ray photoelectron spectroscopy (XPS) (K-Alpha+, Thermo Scientific) using monochromatic Al Kα radiation. The carbon 1s binding energy was used as a reference to correct the binding energy. The XPS data was fitted using the XPSPEAK41 software.

3. Results and Discussion

3.1. Structural Analysis

The XRD pattern (Figure 1) revealed the formation of the La₂Ce₂O₇ perovskite structure. CeO₂ and La₂O₃ peaks were unified there and different from the XRD pattern of CeO₂-La₂O₃ composite oxide. Similarly, Ru/LCO peaks were observed by Ogura et al. to be broader and shifted towards lower angles compared to the corresponding peaks for Ru/CeO₂ [5]. XRD patterns were also obtained for promoted Ru/LCO with various promoter loadings (Figures S6–S9). Similar to observations from Sato et al., reduced Ba-promoted catalysts formed more amount of BaCO₃ [3]. On the opposite, Cs- and K-promoted Ru/LCO (Figure 2) did not form any kind of impurities, which corresponds well with the XRD peaks of K-promoted Ru/C and Cs-promoted Ru/Al₂O₃ [26, 29]. Based on these results, Cs- and K-promoted Ru/LCO are expected to perform better, owing to no phase impurities in them after promoter loading.

The EDX maps (Figures 3(a), 3(b), 3(e), and 3(f)) of Ru/LCO-promoted catalysts demonstrated the homogeneous dispersion of Ce and La within the oxide support, as well as dispersion of Ru particles on the support. TEM characterization was carried out to investigate the impact of promoter and Ru-promoter interaction on the dispersion of Ru metal. Results depicted in Figures 3(c), 3(d), 3(g), and 3(h) revealed that the K- and Li-promoted catalysts had larger Ru particles. Statistical analysis indicated that the average size of Ru particles in Ba-, Cs-, K-, and Li-promoted catalysts was 2.02, 1.678, 3.33, and 5.19 nm, respectively. Ideally, the optimum Ru particle size for round particles is around 2.1 nm [30]. The result implies that the interaction between the Ru metal and the promoter induced by various chemical compositions of the support plays a crucial role in stabilizing the size of Ru particles. The size of Ru particles over K- and Li-promoted catalysts is relatively large due to the weak Ru-K and Ru-Li interaction at the interface of Ru-K-LCO and Ru-Li-LCO. Conversely, the reason behind the small size of Ru particles on Ba- and Cs-promoted catalysts is explained by the strong Ru-Ba and Ru-Cs interaction [19]. The large Li-promoted catalyst particle size would in turn affect the catalytic performance severely.

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3.2. Physicochemical Analysis

Because the addition of promoters induced the sintering of Ru/LCO and damaged the mesoporous structure of the catalyst, the addition of a nonactive metal to the catalyst is expected to decrease the dispersion and surface area [3, 29]. This is also the case for all catalysts in general. Based on the physicochemical characterization (Table 1), the Ru dispersion of all catalysts decreased with the addition of a promoter. Ba- and Cs-promoted catalysts showed higher Ru dispersion compared to K- and Li-promoted catalysts. This result might be related to the atomic size and molecular weight of the promoters. The order of the promoter’s molecular weight is . That means the order of atomic loading is the opposite for the same promoter weight. Aside from each promoter’s electronic and structural properties, it seemed reasonable why Li-promoted catalysts showed much lower dispersion and specific surface area because it has much higher atomic loading which covers the Ru and LCO.

However, a study conducted on catalysts consisting of Ru and lanthanoid oxide showed that an increase in the atomic number of the lanthanoid element in the oxide led to a decrease in the specific surface area [28]. Moreover, when we tried to compare 4, 7, and 10 wt% loading for different promoters (Figures S10–S13), the dispersion follows the trend of catalyst NH₃ synthesis rate.

An unexpected result was also obtained from the BET surface area analysis (Figures S14–S17). The surface area of the Ba-promoted catalyst increased when the promoter loading increased from 7 wt% to 10 wt%. The surface area of Cs- and K-promoted catalysts also increased with an increase in the promoter loadings. Unlike the dispersion trends, the BET surface area did not correlate well with catalytic performance. According to Zhou et al., the activity of NH₃ synthesis catalysts cannot be solely determined by the BET surface area [19].

Based on CO chemisorption analysis results, 4 wt% Ba-, 7 wt% Cs-, 4 wt% K-, and 7 wt% Li-promoted catalysts are expected to perform better than the catalysts of other promoter loading levels (Figures S10–S13). At 4 wt% loading, the order of activity of each promoter is expected to be . Similarly, from the BET surface area analysis result, 4 wt% Ba-, 10 wt% Cs-, 10 wt% K-, and 7 wt% Li-loading catalysts are expected to perform better than the catalysts of other promoter loading levels (Figures S14–S18). At 4 wt% promoter loading, the order of activity of each promoter is expected to be .

3.3. Adsorption Analysis

Because of strong metal-support interaction (SMSI), catalysts that can adsorb hydrogen at lower temperatures tend to perform better [31]. There is a hydrogen reduction peak at 127, 96, 133, and 155°C for Ba-, Cs-, K-, and Li-promoted catalysts, respectively (Figure 4). That peak can be attributed to the reduction of [32]. Peaks that appear above 200°C are assigned to some species that strongly interacted with LCO support or promoter precursors [33]. Based on this analysis, the order of expected performance of each catalyst is . Despite the presence of a peak at 255°C, which is lower than that of K at 295°C, the Ru/4Li/LCO H₂-TPR result has a more dominant peak at temperatures higher than 650°C, which can be assigned to the reduction of the support material.

Catalysts with higher basicity are more desirable for ammonia synthesis [34–37]. In general, the activity of a catalyst is decreased by the presence of acidic sites due to their ability to withdraw electrons. Strong basicity, on the other hand, has been shown to be a crucial factor for achieving high activity. This is because strong basic sites can transfer their electrons to the surface of Ru particles, which promotes the recombinative desorption of N atoms [38]. The basicity of the promoter solution also alters the mesoporous structure of the catalyst [39]. Figure 5 shows the basicity, as determined by CO₂-TPD. The desorption peaks below 250°C indicate the presence of weak basic sites, while those between 250°C and 500°C represent moderate basic sites [40, 41]. Furthermore, the peaks observed at temperatures exceeding 500°C can be attributed to the existence of highly basic sites. Strong basic sites were observed in all the promoter catalysts except Ru/Cs/La₂Ce₂O₇, which also has two weak basic sites. This result was not in accordance with Ru/Cs-MgO catalysts which showed superbasic sites that showed a desorption peak at 640°C [42].

3.4. Electronic State Analysis

The N₂ dissociation ability of the catalyst is strongly linked to the electronic configuration of the Ru sites. Specifically, the XPS spectra of Ru 3d in Figure 6(a) show three peaks corresponding to RuO₂, RuO₃, and Ru⁰, located at 281.1, 282.5, and 279.7 eV, respectively [43–45]. Contents of various Ru, Cs, and O species for Ru/LCO samples are shown in Table 2. The addition of a promoter promotes the formation of metallic Ru, but the Li-promoted catalyst stands out from the rest with 31.6% of Ru⁰ contents. Considering the overlap between C 1s and Ru 3d_3/2 peaks, the Ru 3p profiles were also analyzed (Figure 6(b)). The Ba-, Cs-, and K-promoted catalysts showed a Ru 3p_3/2 peak within the range of 462.62-462.81 eV, while the Li-promoted catalyst had a peak at 461.62 eV, indicating a weaker interaction between the Ru atoms and LCO [46]. In Figure 6(c), it can be seen that all La 3d spectral curves showed wide doublet peaks. Compared with Ru/LCO, the addition of promoters decreases the La 3d_5/2 binding energy and creates a rich electron surface except for the Cs-promoted catalyst.

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Figure 6(d) illustrates the Ce 3d XPS regions that have been deconvoluted for all the samples of Ru/LCO. A pair of 3d_5/2 (880.5 and 884.9 eV) is assigned to Ce³⁺, and the other pair (882.1 and 884.6 eV) is assigned to Ce⁴⁺. The other pairs are ascribed to Ce 3d_3/2 species. There was supposed to be a pair of 3d_3/2 peaks (898.9 and 903.3 eV) which are assigned to Ce³⁺, but we hardly observed it. The percentage of Ce³⁺ in the total Ce contents is 31.2% for Ru/Ba/LCO, indicating that the addition of Ba increased the Ce³⁺ concentration and oxygen vacancies [47]. The O 1s XPS spectra (Figure 6(e)) contain a peak at around 531.4 eV binding energy, which is assumed to be caused by oxygen species adsorbed on the surface (referred to as O_ads). Another peak observed at 529.3 eV is believed to be caused by lattice oxygen (referred to as O_latt) of RuO₃ and/or LCO. The O_latt content was affected by the strong metal-support interaction effect (SMSI), reduction methods, and Ru, La, and Ce oxidation states [48]. Table 2 showed that the addition of promoters to Ru/LCO increases the concentration of surface-adsorbed oxygen. The most significant increase was observed in the Li-promoted catalyst. The XPS results showed that each promoter affects Ru, Ce, La, and O differently in terms of electronic properties.

In Ba-promoted catalysts, the increase in promoter loading increased the oxidation state of La, Ru 3p, and Ba 3d and increased the amount of O_ads. However, some irregularities were observed in Ce 3d and Ru 3d scanning, with 7 wt% Ba loading and 10 wt% Ba loading having the least amount of Ce³⁺ and the highest amount of Ru⁰, respectively (Figure S18). Among the Cs-promoted catalysts, the 4 wt% Cs catalyst had the highest amounts of Ce³⁺, Ru⁰, and O_latt and the lowest oxidation states of Ru 3p and Cs 3d. However, some irregularities were observed in all the scanned components, and the addition of Cs loading did not necessarily increase their corresponding oxidation state (Figure S19). In the K-promoted catalysts, the 7 wt% K-loaded catalyst had the highest amounts of Ce³⁺, Ru⁰, and O_latt and the lowest oxidation state for La 3d and Ru 3p and is therefore expected to perform better than the others (Figure S20). In the Li-promoted catalysts, an increase in the promoter loading increased the oxidation states of La, Ru, and Li and decreased the amounts of Ce³⁺ and O_latt. Based on this result, a loading level of 4 wt% is expected to have the best performance (Figure S21). Generally, the addition of a promoter increases the oxidation state of each catalyst component; however, some irregularities were also observed.

3.5. Performance Test

Based on Figure 7, the performance of the catalysts was evaluated. The extent of reproducibility of activity measurement is visualized in Figures S24–S28. Overall, the reaction test showed that the catalyst activity values still fell below the equilibrium value. The activity test suggested the catalyst activity order for each promoter to be at 400°C and 1 MPa which also showed how fast the reaction reached the equilibrium (Figure 7(a)). It is clearly seen that Li addition greatly decreased the Ru/LCO activity, but the addition of Li can be beneficial if we use the right amount (Figure S5). One of the reasons for such an anomaly is that Li has very low atomic mass which means, as we have discussed in the physicochemical testing part, there are a lot more Li atoms covering Ru and LCO surfaces compared to other promoters. Ba-promoted Ru/LCO has been tested before by Sato et al., yet we still tested that catalyst in order to find where it stands compared to our other samples [3]. The pressure dependence result of the catalyst activity test showed a decrease in the Ba-promoted catalyst with the increase in temperature (Figure 7(b)). In terms of optimal composition, 4 wt% Ba, 7 wt% Cs, 4 wt% K, and 7 wt% Li were observed to exhibit the best performance for each type of promoter (Figures S2–S5). It is also worth noticing that some catalysts were still active at 0 MPa like electrochemical cells do [49]. In fuel cells, reaction at 0 MPa is enabled by the presence of a proton conductor (H⁺). This might also be the case in this study where our catalysts assisted not only N₂ but also H₂ dissociation. The K-promoted catalyst had the highest catalytic performance due to its very high basicity observed on CO₂-TPD. On the other hand, despite also having a high basicity and high surface oxygen concentration, the Li-promoted catalyst still obtains a low activity due to their low Ru dispersion. Therefore, it has a low ability to dissociate N₂.

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The pressure dependence test indicated that the rate of ammonia synthesis increased (not proportionally) with Cs-, K-, and Li-promoted catalysts, and not with Ba-promoted catalysts. The Ba-promoted catalyst could be more prone to H₂ poisoning, a phenomenon in which H₂ is strongly bonded to Ru active sites at higher pressure, blocking the main reaction [10]. Sabatier’s principle suggests that the most efficient catalysts are those that bind atoms and molecules with optimal strength. If the binding is too weak, the catalysts will not be able to retain the reactants on their surface, resulting in no reaction. On the other hand, if the binding is too strong, the catalysts will not be able to release the products, which reduces the catalyst activity [50].

4. Conclusions

To increase the reactivity of Ru/LCO catalysts for thermochemical ammonia synthesis, several promoters have been added, and their effect on the morphological and chemical characteristics of the catalysts was experimentally studied in detail. The highlight of our research is illustrated in Figure 8, and the summary of our findings is as the following: Ru/LCO can maintain its structural properties and minimize phase impurities with the addition of promoters. The order of reactivity of promoted Ru/LCO in this study is . Some promoters are also able to increase the catalytic performance despite decreasing the catalyst surface area and Ru dispersion. The atomic mass of each promoter played a key part there. Despite having the second-highest atomic mass among the other promoters, the addition of the K promoter displayed the highest effectiveness for Ru/LCO compared with other promoters used in this study because of its phase purity and very high basicity. It increases the reaction rate by 3 times compared with Ru/LCO and 1.5 times compared with the developed Ru/Ba/LCO. Despite poor performance at lower-pressure synthesis, the Ba promoter can perform well under mild conditions, such as in fuel cells. These findings are important for the development of Ru/LCO-based catalysts for hydrogen storage/production.

Data Availability

The original data can be obtained upon request, and additional supporting information can be found online in the supporting information section located at the end of the article.

Conflicts of Interest

The authors of this manuscript declare that there are no competing interests.

Authors’ Contributions

Hizkia Manuel Vieri and Arash Badakhsh have contributed equally to this work.

Acknowledgments

This work was supported by the National Research Foundation Grant funded by the Korean Government (Ministry of ICT) (Grant No. 2021R1A2C2008662).

Supplementary Materials

Figure S1: reduction temperature optimization. Figure S2: catalytic activity test of Ru/Ba/LCO with different promoter loading: (a) pressure dependence of the ammonia synthesis rate at 400°C; (b) temperature dependence of the ammonia synthesis rate at 1 MPa, each of which had been reduced at 450°C. Figure S3: catalytic activity test of Ru/Cs/LCO with different promoter loading: (a) pressure dependence of the ammonia synthesis rate at 400°C; (b) temperature dependence of the ammonia synthesis rate at 1 MPa, each of which had been reduced at 450°C. Figure S4: catalytic activity test of Ru/K/LCO with different promoter loading: (a) pressure dependence of the ammonia synthesis rate at 400°C; (b) temperature dependence of the ammonia synthesis rate at 1 MPa, each of which had been reduced at 450°C. Figure S5: catalytic activity test of Ru/K/LCO with different promoter loading: (a) pressure dependence of the ammonia synthesis rate at 400°C; (b) temperature dependence of the ammonia synthesis rate at 1 MPa, each of which had been reduced at 450°C. Figure S6: X-ray diffraction patterns of Ru/Ba/LCO with different promoter compositions. Figure S7: X-ray diffraction patterns of Ru/Cs/LCO with different promoter compositions. Figure S8: X-ray diffraction patterns of Ru/K/LCO with different promoter compositions. Figure S9: X-ray diffraction patterns of Ru/Li/LCO with different promoter compositions. Figure S10: NH₃ synthesis rate correlation with Ru dispersion for Ru/Ba/LCO. Figure S11: NH₃ synthesis rate correlation with Ru dispersion for Ru/Cs/LCO. Figure S12: NH₃ synthesis rate correlation with Ru dispersion for Ru/K/LCO. Figure S13: NH₃ synthesis rate correlation with Ru dispersion for Ru/Li/LCO. Figure S14: NH₃ synthesis rate correlation with BET surface area for Ru/Ba/LCO. Figure S15: NH₃ synthesis rate correlation with BET surface area for Ru/Cs/LCO. Figure S16: NH₃ synthesis rate correlation with BET surface area for Ru/K/LCO. Figure S17: NH₃ synthesis rate correlation with BET surface area for Ru/Li/LCO. Figure S18: XPS spectra of Ru/Ba/LCO with different promoter loading. Figure S19: XPS spectra of Ru/Cs/LCO with different promoter loading. Figure S20: XPS spectra of Ru/K/LCO with different promoter loading. Figure S21: XPS spectra of Ru/Li/LCO with different promoter loading. Figure S22: H₂SO₄ conductivity value when measuring NH₃ synthesis rate at 400°C and 0.5 MPa. Figure S23: H₂SO₄ conductivity value when calibrated using NH₃ solution. Figure S24: reproducibility proof of Ru/LCO catalyst: (a) temperature dependence of the ammonia synthesis rate at 1 MPa; (b) pressure dependence of the ammonia synthesis rate at 400°C; each catalyst had been reduced at 450°C. Figure S25: reproducibility proof of Ru/Ba/LCO catalyst: (a) temperature dependence of the ammonia synthesis rate at 1 MPa; (b) pressure dependence of the ammonia synthesis rate at 400°C; each catalyst had been reduced at 450°C. Figure S26: reproducibility proof of Ru/Cs/LCO catalyst: (a) temperature dependence of the ammonia synthesis rate at 1 MPa; (b) pressure dependence of the ammonia synthesis rate at 400°C; each catalyst had been reduced at 450°C. Figure S27: reproducibility proof of Ru/K/LCO catalyst: (a) temperature dependence of the ammonia synthesis rate at 1 MPa; (b) pressure dependence of the ammonia synthesis rate at 400°C; each catalyst had been reduced at 450°C. Figure S28: reproducibility proof of Ru/Li/LCO catalyst: (a) temperature dependence of the ammonia synthesis rate at 1 MPa; (b) pressure dependence of the ammonia synthesis rate at 400°C; each catalyst had been reduced at 450°C. (Supplementary Materials)

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Copyright © 2023 Hizkia Manuel Vieri et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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International Journal of Energy Research

Comparative Study of Ba, Cs, K, and Li as Promoters for Ru/La2Ce2O7-Based Catalyst for Ammonia Synthesis

Abstract

1. Introduction

2. Methods

2.1. Preparation of Catalysts

2.2. NH3 Synthesis

2.3. Characterization

3. Results and Discussion

3.1. Structural Analysis

3.2. Physicochemical Analysis

3.3. Adsorption Analysis

3.4. Electronic State Analysis

3.5. Performance Test

4. Conclusions

Data Availability

Conflicts of Interest

Authors’ Contributions

Acknowledgments

Supplementary Materials

References

Copyright

Comparative Study of Ba, Cs, K, and Li as Promoters for Ru/La₂Ce₂O₇-Based Catalyst for Ammonia Synthesis

2.2. NH₃ Synthesis