Introduction
One of the major issues of our current energy supply is the huge negative environmental impact caused by our energy dependency on fossil fuels. Consequently, decarbonizing our industrial world to combat climate change probably represents the biggest challenge of our times. Hydrogen has been identified as a possible energy vector a long time ago and led to the introduction of the “hydrogen economy” concept back in the 1970’s [1]. In this concept, the use of hydrogen is presented as a fundamental cornerstone of the energy transition by enabling the generation of clean and sustainable energy. Nowadays, the storage and transport of hydrogen requires hydrogen to be compressed or liquified, which is consuming a significant amount of energy. Thus, with the rise of the hydrogen economy, better tools for the storage and transport of hydrogen have been considered [2, 3]. Among the many potential hydrogen carriers, ammonia is particularly promising and can be stored and transported relatively easily. After transportation via ammonia, the stored hydrogen can be released on-site and on-demand using the ammonia cracking reaction. Ruthenium has been found to be the most active metal for the ammonia cracking reaction [4]. In the present work, various Ru-based catalysts were prepared, characterized, and tested using high throughput testing equipment. In addition, an accelerated ageing procedure was performed to investigate the stability of a selected set of shaped Ru-based catalysts.

Materials and Methods
Various methods of Ru deposition and different supports were used to prepare the Ru catalysts tested in this work. In addition, several promoters were incorporated into the final catalyst formulation using a variety of techniques known to the art. The precious metal content of the catalysts was determined with inductively coupled plasma analysis (ICP) and the particle size distribution was characterized using transmission electron microscopy (TEM). The shaped catalysts were tested for ammonia cracking at 30 bar in a high throughput parallel single-pellet string reactor system. The temperature was varied between 350 °C and 650 °C. For the accelerated ageing study, the temperature was periodically increased to 650 °C, 700 °C and 750 °C.

Results and Discussion
In the first set of high throughput experiments, the fifteen prepared catalysts were tested at 30 bar and 575 °C. The feed composition was 94.7 % NH3, 5 % Ar and 0.3 % H2O. After reaching stable ammonia conversion, the temperature was increased to 650 °C and the samples were aged for 92 h. After this ageing period, the temperature was decreased to the reference temperature of 575 °C to check the activity of the catalysts for ammonia cracking. As can be seen in Figure 1 below, our reference catalyst 241111 shows a very high activity at 575 °C and 650 °C. After the ageing phase and the return to the reference temperature a slight deactivation can be observed, which leads to a decrease in ammonia conversion of about 1 %. Among the newly developed catalysts tested in this first set of experiment, three formulations (240985, 241168 and 241321) show a very high initial activity at 575°C. Moreover, a very high stability with no decrease of the ammonia cracking activity can be observed after the first ageing step at 650 °C. As compared to our reference most of the remaining samples showed lower initial ammonia conversions during the first test at 575 °C and suffered from the ageing step at 650 °C, with a decrease in ammonia conversion up to 20 %. Interestingly, sample 241158 seems to activate during the first step at 575 °C as well as during the ageing at 650 °C. The most promising candidates have been further investigated in a second set of high throughput experiments. In this second trial phase, the chosen catalysts have been periodically aged at 650 °C (3 x 92 h), 700 °C (3 x 15 h) and finally 750 °C (3 x 14 h). Between each ageing step, the temperature was decreased to our reference temperature at 575 °C to check the remaining activity of each catalyst. The results of this ageing study will be presented in this contribution as well as the characterization of the samples before and after ageing.

Significance
The ammonia cracking reaction has been widely studied and the development of highly active and stable catalysts for this reaction is of uttermost interest. Ru catalysts, because of their high activity, will definitively have a role to play in the future of ammonia cracking. To further improve the low temperature activity and stability of our Ru-based catalysts, a high throughput testing was performed. Significant performance improvements could be achieved by applying the right impregnation technique and appropriate dopants. The resulting Ru-based catalysts with a stable performance up to 750 °C can make an important contribution to the successful realization of the ammonia cracking technology on an industrial scale.

References
1. Bockris, J. O. M. J. Hydrogen Energy and Gas Journal 38, 2579 (2013).
2. Aziz, M., Wijayanta, A. T., Nandiyanto, A. B. D. Energies 13, 3062 (2020).
3. Aakko-Saksa, P. T., Cook, C., Kiviaho, Repo, T. J. Of Power Sources 396, 803 (2020).
4. Yin, S. F., Xu, B. Q., Zhou, X. P. et al. Appl. Catal. A: General 277, 1 (2004).