A high performance barium-promoted cobalt catalyst supported on magnesium-lanthanum mixed oxide for ammonia synthesis

Regulating oxygen vacancies by Zn atom doping to anchor and disperse promoter Ba on MgO support to improve Ru-based catalysts activity for ammonia synthesis

In heterogeneous catalysis, surface defects are widely regarded as an effective means to enhance the catalytic performance of catalysts. In this study, the oxygen vacancy-rich Mg(1-X)ZnXO solid solution support was successfully prepared by doping a small amount of Zn into MgO nanocrystals. Based on this support, Ru/Ba-Mg(1-X)ZnXO catalyst for ammonia synthesis was prepared. Characterization using TEM, EPR, XPS, and DFT calculations confirmed the successful substitution of Zn atoms for Mg atoms leading to the formation of more oxygen vacancies (OVs). N2-TPD, SEM and TEM analyses revealed that a small amount of Zn had minimal influence on the surface morphology and the size of Ru nanoparticles. The abundance of OVs in the support was identified as the primary factor enhancing the catalytic activity. XPS, H2-TPD and kinetics experiment studies further elucidated the mechanism by which OVs promote the reaction, with OVs serving as an anchor point for the promoter Ba on the MgO support and promoted the dispersion of Ba. This anchoring effect not only enhanced the electron density on Ru, favoring the dissociation of the N[triple bond, length as m-dash]N bond, but also mitigated hydrogen poisoning. As a result,the ammonia synthesis rate reached 1.73 mmol g-1 h-1. Furthermore, the CO2-TPD and H2-TPR analyses indicated that Zn doping effectively promotes the metal-support interaction (MSI) and surface alkalinity. The findings of this study offers valuable insights for the design of defective modified catalyst supports.

The Influence of Active Phase Content on Properties and Activity of Nd2O3-Supported Cobalt Catalysts for Ammonia Synthesis

A series of neodymium oxide-supported cobalt catalysts with cobalt content ranging from 10 to 50 wt.% was obtained through the recurrent deposition-precipitation method. The effect of active phase, i.e., metallic cobalt, content on structural parameters, morphology, crystal structure, surface state, composition and activity of the catalysts was determined after detailed physicochemical measurements were performed using ICP-AES, N2 physisorption, XRPD, TEM, HRTEM, STEM-EDX, H2-TPD and XPS methods. The results indicate that the catalyst activity strongly depends on the active phase content due to the changes in average cobalt particle size. With the increase of the cobalt content, the productivity per catalyst mass increases, while TOF maintains a constant value. The TOF is below average only for the catalyst with the lowest cobalt content, i.e., when the average Co particle size is below 20 nm. This is due to the predominance of strong hydrogen binding sites on the surface, leading to hydrogen poisoning which prevents nitrogen adsorption, thus inhibiting the rate-determining step of the process.

Co nanoparticles supported on mixed magnesium-lanthanum oxides: effect of calcium and barium addition on ammonia synthesis catalyst performance

The synthesis of ammonia in the Haber-Bosch process produces millions of tons of ammonia annually needed for producing fertilisers required to feed the growing population. Although this process has been optimised extensively, it still accounts for about 2% of global energy consumption. It is, therefore, desirable to develop an efficient ammonia synthesis catalyst. Over the last decades, many attempts have been made to improve the ammonia synthesis catalyst efficiency under mild conditions. Here, we studied the effect of adding Ca and Ba to the cobalt ammonia synthesis catalyst. The combination of the different experimental results allows concluding that Ca served as an inactive additive, whereas Ba served as an electronic promoter. The Ca addition did not change the textural, structural, and chemisorptive properties of the Ca-doped Co catalyst. On the other hand, the Ba addition had a major effect on the nature of active Co sites. It contributed to the formation of new active sites for hydrogen and nitrogen adsorption and dissociation. Barium addition also contributed to the generation of new basic sites, particularly the strong ones. These unique characteristics were ascribed to the formation of Co(core)-BaO(shell) structures. It is likely that the donation of electrons from BaO to N₂ via Co markedly promoted ammonia synthesis. This catalyst exhibited ammonia synthesis activity 4 times higher than that of the undoped Co catalyst and 2 times higher than that of the industrial Fe catalysts under identical conditions.

On the effect of metal loading on the performance of Co catalysts supported on mixed MgO-La2O3 oxides for ammonia synthesis

Synthesis of ammonia from nitrogen and hydrogen is one of the largest manmade chemical processes, with annual production reaching 170 million tons. The Haber-Bosch process is the main industrial method for producing ammonia, which proceeds at high temperatures (400-600 °C) and pressures (20-40 MPa) using an iron-based catalyst. It is thus highly desirable to develop new catalysts with sufficient activity and stability under mild conditions. In this work, we report cobalt catalysts supported on magnesium-lanthanum mixed oxide with different Co loading amounts synthesised via a simple wet impregnation method. We have found a clear relationship between the ammonia synthesis rate and the Co loading amount. Specifically, the NH₃ synthesis rate increased on increasing cobalt loading and reached a maximum at 40 wt% Co deposition. A further increase in Co loading did not change the activity significantly. Interestingly, the surface-specific activity (TOF) remained almost unchanged regardless of the Co loading amount in the catalysts. It revealed that the resultant ammonia synthesis rate over the studied catalysts did not depend on the size and structure of Co nanoparticles but strongly on the Co loading amount. Finally, it is believed that the use of this type of catalyst will be a starting point toward energy-efficient ammonia production.

Influence of the Support Composition on the Activity of Cobalt Catalysts Supported on Hydrotalcite-Derived Mg-Al Mixed Oxides in Ammonia Synthesis

Recently, catalysts with hydrotalcites and hydrotalcite-derived compounds have attracted particular interest due to their specific properties, mostly well-developed texture, high thermal stability, and favorable acid–base properties. In this work, we report the investigation of ammonia synthesis on barium-promoted cobalt catalysts supported on hydrotalcite-derived Mg-Al mixed oxides with different Mg/Al molar ratios. The obtained catalysts were characterized using TGA-MS, nitrogen physisorption, XRPD, TEM, STEM-EDX, H2-TPD, CO2-TPD, and tested in ammonia synthesis (470 °C, 6.3 MPa, H2/N2 = 3). The studies revealed that the prepared Mg-Al mixed oxides are good candidates as support materials for Co-based catalysts. However, interestingly, the support composition does not influence the activity of Ba/Co/Mg-Al catalysts. The change in Mg/Al molar ratio in the range of 2–5 did not significantly change the catalyst properties. All the catalysts are characterized by similar textural, structural, and chemisorption properties. The similar density of basic sites on the surface of the studied catalysts was reflected in their comparable performance in ammonia synthesis.

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

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/N2 = 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, H2-TPR, and H2-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 BaCeO3 phase.

Boosting the Catalytic Performance of Co/Mg/La Catalyst for Ammonia Synthesis by Selecting a Pre-Treatment Method

The influence of the calcination process on the physicochemical properties and catalytic behavior of the Co/Mg/La catalysts for ammonia synthesis has been investigated. The catalysts were prepared using the different thermal pre-treatment methods prior to the activation, i.e., drying and calcination, and the respective activities for ammonia synthesis were assessed. It was found out that changing from air calcination prior to activation to direct activation of the co-precipitated species led to the different catalytic performances. The most favorable catalytic performance was achieved with Co/Mg/La prepared by calcination in air. Detailed characterization methods, employing e.g., XRPD, H2-TPD, N2-TPD, CO2-TPD, SEM, and TEM, showed that the superior catalytic behavior of this catalyst was attributed to its strong basicity and favorable adsorption properties toward hydrogen and nitrogen.