Role of cerium as a chemical vector for the design of new generation of ammonia synthesis catalysts

Abstract

The industrial production of ammonia has been carried out by the Haber−Bosch process for more than a century. However, a small-scale decentralized process is on high demand, either for the use of ammonia as raw material in fertilizers industry or for its growing interest as an energy vector for hydrogen storage. Particularly, the rising interest of green ammonia (i.e., ammonia produced from renewables), has motivated a massive research effort during the last two decades concerning new materials which lead to circumvent the main handicap of the iron-based catalysts used in the Haber−Bosch process: the current requirements of high temperatures and pressures that hinder the coupling of ammonia reaction systems with renewable resources (mainly, water electrolysis). As discussed in the Chapter 1 from the Section II of this document, there is a wide variety of materials suitable for boosting the traditional ammonia synthesis process, thus leading to a successful operation at mild conditions. However, most of these catalysts are on an incipient research state or present some drawbacks (e.g., complex synthesis methods) which hinder their scalability. Thus, the search of more suitable, attainable and stable catalysts is required for an effective transition towards a green ammonia thermocatalytic synthesis scenario. This doctoral Thesis is focused on the role of cerium (Ce) as a support for 3rd generation ammonia synthesis materials, since its versatile properties allow this metal to be a chemical platform for the design of efficient catalysts. In the Chapter 1 of this Thesis, a review of the state-of-the-art is done, in which the historical evolution of the most relevant 1st, 2nd and 3rd generation catalysts for ammonia synthesis is presented. Furthermore, the fundamentals of this reaction are unveiled with a particular focus on the metal-support interactions. The results from this work led to the rational design of the catalysts presented in the following Chapters. In the Chapter 2, the experimental results of the activity of CeNix alloys are shown. This work, carried out in the MDX research Center for Element Strategy, International Research Frontiers Initiative, Tokyo Institute of Technology, Japan, highlighted the versatility of cerium by the good performance of the CeNi2 alloy, whose fundamental key relies on the formation of a CeN surface layer over the original alloy, acting as a second active center for the N2 dissociation and activation steps. Despite the use of a non-noble metal like Ni, an activation energy as low as 55.3 kJ mol-1 was achieved for CeNi2 bulk particles. In the Chapter 3, an experimental work of catalysts made of Ru/CeO2 and Ru/CeO2-Al2O3 is presented. It was demonstrated that a very simple impregnationcalcination method led to the synthesis of a high surface cerium oxide support for Ruthenium (Ru), creating a catalyst with low crystallinity and good electronic promotion between metal and support, derived from the formation of surface oxygen vacancies, typical from the reduction of cerium oxide (Ce4 +→Ce3 +). Further enhancement in the kinetic mechanism can be found by the structural promotion of alumina. Activation energies as low as 44.8 kJ mol-1 were obtained. In the Chapter 4, the activity of Ru/CeO2-La2O3 catalysts is shown. In this case, it was demonstrated that the activity of the original Ru/CeO2 catalyst can be enhanced by the addition of La to the oxide lattice, since the crystal structure of ceria can be disrupted by the formation of Ce-La solid solutions. As a result, there is a decrease in the crystallinity of the oxydes and a higher number of structural defects is obtained. Thus, it was observed that a superior generation of surface oxygen defects boosts the electron promotion of the support towards the metal. The optimum catalyst was made of a 50% of Ce in molar bases and its apparent activation energy was as low as 34.1 kJ mol-1. The results presented in the present Thesis demonstrate that cerium can be a key element for the design of catalysts for green ammonia thermocatalytic synthesis, either in the form of cerium nitride in metallic alloy complexes with Ni or in the form of ceria as a support for Ru. Furthermore, the performance of the latter can be further enhanced by structural promotion with Al2O3 or by functional promotion by enhanced formation of oxygen vacancies using La2O3, which resulted in a better electron transfer towards Ru. Certainly, the versatility of cerium and its wide margin to design better performing catalysts can play a key role in the transition of ammonia synthesis towards the industrial application as both green hydrogen storage energy carrier and raw material for decentralized small plants of green fertilizers.

En abierto se puede consultar la parte no embargada de la Tesis Doctoral.