Mass-selected-nanoparticles for ammonia synthesis

Currently, ammonia (NH3) production accounts for approximately 2% of global CO₂ emissions. Yet ammonia is essential, supporting nearly half of global food production, and is also emerging as a leading candidate for carbon-neutral shipping fuel. For a transition to be economically feasible the operating pressure and temperature of the synthesis process must be reduced. This, in turn, demands a systematic improvement of ammonia catalysts. Therefore a fundamental understanding of the reaction and promotion mechanism is indispensable. This thesis provides technical and experimental strategies to elucidate reactivity of low surface area model catalysts in ammonia synthesis.

In this thesis, the development of a reactor cell operating at ambient pressure, which is integrated into an ultra-high vacuum (UHV) system, is described. It allows highly sensitive detection of products even on single crystals, that have a very low surface area compared to most catalysts. This is done by coupling a quadrupole mass spectrometer (QMS) to a small volume. The entire setup enables the synthesis, characterisation, and reaction testing of model catalysts without exposure to non-UHV environment. This capability allowed for fundamental studies of promising new catalysts and provided mechanistic insights into their behaviour. 

With the newly developed reaction cell, the promotion effect of two different transition metals (La and Nb) on cobalt was studied for ammonia synthesis. For this, the catalysts, either single crystals or mass-selected Co nanoparticles supported on nitride or oxide films, were synthesised and characterized by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS) before and after reaction. The results show that 3.5 nm diameter Co particles (20% projected surface coverage) on NbN exhibit a turnover frequency (TOF) of 0.17 s⁻¹ at 500 ◦C, with activity attributed to sites at the Co–NbN interface. An even more promising system was found in cobalt promoted by lanthanum. Turnover frequencies of 0.04 s⁻¹ at 350 ◦C for Co on LaN and 0.03 s⁻¹ (irrespective of particle size) for Co on La₂O₃ were measured, both exceeding that of potassium-promoted Fe(111) of 0.02 s⁻¹, a model for the industrial catalyst. Together with isotope labelling experiments experiments of Co single crystals doped with La, it was shown that La migrates from LaN onto the cobalt nanoparticles during reaction and the active phase is a Co surface promoted with La. These findings are consistent with DFT-based modelling in which La and NbN promote Co by quenching its spin and donating electrons to establish a dipole. 

Together, these studies deepen the understanding of promotion mechanisms in ammonia synthesis can support further improvement of ammonia catalysts. Promising activity of Co on LaN and La₂O₃, could act as a starting point for developing more active high surface area catalysts for industrial applications.