Ammonia synthesis over lanthanoid oxide–supported ruthenium catalysts

A Conceptual Approach for the Design of New Catalysts for Ammonia Synthesis: A Metal-Support Interactions Review

The growing interest in green ammonia production has spurred the development of new catalysts with the potential to carry out the Haber-Bosch process under mild pressure and temperature conditions. While there is a wide experimental background on new catalysts involving transition metals, supports and additives, the fundamentals behind ammonia synthesis performance on these catalysts remained partially unsolved. Here, we review the most important works developed to date and analyze the traditional catalysts for ammonia synthesis, as well as the influence of the electron transfer properties of the so-called 3rd-generation catalysts. Finally, the importance of metal-support interactions is highlighted as an effective pathway for the design of new materials with potential to carry out ammonia synthesis at low temperatures and pressures.

Adsorption and Desorption Behaviors of Ammonia on Zeolites at 473 K by the Pressure-Swing Method

Adsorption and desorption behaviors of ammonia on the zeolites were investigated using the pressure-swing method to utilize the ammonia adsorbent at 473 K. For various (Y, A, L, and mordenite) types of zeolites, the effects of their pore sizes, countercation species, and number of adsorption sites on the ammonia adsorption behavior were evaluated. The adsorption capacity increased with increasing the pore size. The differences in the countercation species and the number of adsorption sites contributed to the ammonia adsorption process on the zeolites. Sodium cations strongly adsorb ammonia molecules. The adsorption/desorption of ammonia expanded and shrunk the zeolite crystal, suggesting that the pores in the zeolite were enlarged by the adsorption of ammonia molecules. Despite repetitive lattice expansion and shrinkage, the morphologies of the zeolite particles were not degraded. These results confirm that the zeolite showed high durability for the adsorption and desorption of ammonia at 473 K.

Splitting of Hydrogen Atoms into Proton-Electron Pairs at BaO-Ru Interfaces for Promoting Ammonia Synthesis under Mild Conditions

Ru catalysts promoted with alkali and alkaline earth have shown superior ammonia (NH₃) synthesis activities under mild conditions. Although these promoters play a vital role in enhancing catalytic activity, their function has not been clearly understood. Here, we synthesize a series of Ba-Ru/MgO catalysts with an optimal Ru particle size (∼2.3 nm) and tailored BaO-Ru interfacial structures. We discover that the promoting effect is created through the separate storage of H⁺/e^- pairs at the BaO-Ru interface. Chemisorbed H atoms on Ru dissociate into H⁺/e^- pairs at the BaO-Ru interface, where strongly basic, nonreducible BaO selectively captures H⁺ while leaving e^- on Ru. The resulting electron accumulation in Ru facilitates N₂ activation via enhanced π-backdonation and inhibits hydrogen poisoning during NH₃ synthesis. Consequently, the formation of intimate BaO-Ru interface without an excessive loss of accessible Ru sites enables the synthesis of highly active catalysts for NH₃ synthesis.

Boosted Activity of Cobalt Catalysts for Ammonia Synthesis with BaAl2O4-xHy Electrides

Electrides are promising support materials to promote transition metal catalysts for ammonia synthesis due to their strong electron-donating ability. Cobalt (Co) is an alternative non-noble metal catalyst to ruthenium in ammonia synthesis; however, it is difficult to achieve acceptable activity at low temperatures due to the weak Co-N interaction. Here, we report a novel oxyhydride electride, BaAl₂O_4-xH_y, that can significantly promote ammonia synthesis over Co (500 mmol g_Co^-1 h^-1 at 340 °C and 0.90 MPa) with a very low activation energy (49.6 kJ mol^-1; 260-360 °C), which outperforms the state-of-the-art Co-based catalysts, being comparable to the latest Ru catalyst at 300 °C. BaAl₂O_4-xH_y with a stuffed tridymite structure has interstitial cage sites where anionic electrons are accommodated. The surface of BaAl₂O_4-xH_y with very low work functions (1.7-2.6 eV) can donate electrons strongly to Co, which largely facilitates N₂ reduction into ammonia with the aid of the lattice H^- ions. The stuffed tridymite structure of BaAl₂O_4-xH_y with a three-dimensional AlO₄-based tetrahedral framework has great chemical stability and protects the accommodated electrons and H^- ions from oxidation, leading to robustness toward the ambient atmosphere and good reusability, which is a significant advantage over the reported hydride-based catalysts.

Co Nanoparticle Catalysts Encapsulated by BaO-La2O3 Nanofractions for Efficient Ammonia Synthesis Under Mild Reaction Conditions

Ruthenium catalysts may allow for realization of renewable energy-based ammonia synthesis processes using mild reaction conditions (<400 °C, <10 MPa). However, ruthenium is relatively rare and therefore expensive. Here, we report a Co nanoparticle catalyst loaded on a basic Ba/La₂O₃ support and prereduced at 700 °C (Co/Ba/La₂O₃_700red) that showed higher ammonia synthesis activity at 350 °C and 1.0-3.0 MPa than two benchmark Ru catalysts, Cs⁺/Ru/MgO and Ru/CeO₂. The synthesis rate of the catalyst at 350 °C and 1.0 MPa (19.3 mmol h^-1 g^-1) was 8.0 times that of Co/Ba/La₂O₃_500red and 6.9 times that of Co/La₂O₃_700red. The catalyst showed ammonia synthesis activity at temperatures down to 200 °C. Reduction at the high temperature induced the formation of BaO-La₂O₃ nanofractions around the Co nanoparticles by decomposition of BaCO₃, which increased turnover frequency, inhibited the sintering of Co nanoparticles, and suppressed ammonia poisoning. These strategies may also be applicable to other non-noble metal catalysts, such as nickel.

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.

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.

Applications of rare earth oxides in catalytic ammonia synthesis and decomposition

Due to their unique structural and electronic properties, rare earth oxides have been widely applied as supports and promoters in catalytic ammonia synthesis and decomposition.