Standing-wave photoelectron spectroscopy of multi-layer structures, as pioneered by Prof. Chuck Fadley and his group in the past two decades, proved to be a very powerful technique for probing solid/solid, but also solid/liquid and solid/gas interfaces. Its superior depth selectivity and non-destructive nature were crucial to answer key questions in problems spread over several scientific fields, such as emergent phenomena at complex oxide interfaces [1], artificial multiferroics [2], adsorption mechanisms in liquids [3], corrosion [4], and electrocatalysis [5].These achievements were only possible thanks to innovative approaches both in experiments and analyses, including development of X-ray optical simulations package [6] and its coupling with the black-box optimizer [7]. In this talk I will introduce these tools and highlight their applications to answer diverse scientific problems.

[1] S. Nemsak et al., Physical Review B93 (24), 245103 (2016).

[2] H. P. Martins et al., arXiv preprint arXiv:2012.07993.

[3] S. Nemsak et al., Nature Communications5, 5441 (2014).

[4] O. Karslioglu et al., Faraday Discussions180, 35 (2015).

[5] C. Baeumer et al., Nature Materials20, 674 (2021).

[6] S.-H. Yang et al., Journal of Applied Physics113, 073513 (2013).

[7] O. Karslioglu et al., Journal of Electron Spectroscopy and Related Phenomena230, 10 (2019).

Charles Fadley was a pioneer in the field of x-ray photoelectron spectroscopy (XPS) for over 5 decades and I had the pleasure to interact with him frequently since my time at Uppsala University in the 1980s. In the last part of his scientific life he brought forward hard x-ray XPS as an important tool to investigate interfaces. In the following contribution, I would like to honor him with a new direction of hard x-ray XPS that can play an important role for the development of a sustainable society to address climate change and in particular to mitigate CO₂ emissions.

Some of the most essential catalytic reactions for our energy society is to reduce CO₂ to hydrocarbons and alcohols to be used as fuels and base chemicals for the chemical industry. In order to convert a fossil dependent chemical industry and instead use sustainable hydrogen as a feedstock we need to develop new catalysts. Furthermore, the catalytic reduction of N₂ to ammonia has been considered as one of the most important discoveries during the 20^th century to produce fertilizers for a growing population. Despite an enormous effort in studying these catalytic reactions we are still lacking experimental information about the chemical state of the catalytic surface and the adsorbates existing as the reaction is turning over.

X-ray photoelectron spectroscopy (XPS) is a powerful surface sensitive technique that can provide almost all essential chemical information and it has been developed to operate also in a few mbar of pressure with great success for probing oxidation catalytic reactions. Unfortunately, this pressure regime is too low for the hydrogenation reactions to turn over. However, utilizing the hard x-ray XPS technique can bring the pressure up by many orders of magnitude in an unique instrumentation¹.

Here I will present how Fischer-Tropsch, methanol and ammonia synthesis reactions on single crystal metal surfaces have been probed during operando conditions in the pressure range 100 mbar-1 bar using the new XPS system built at Stockholm University¹ and permanently located at the PETRA III synchrotron in Hamburg. The instrument can vary the incidence angle of the X-rays allowing it to be either surface or bulk sensitive.

¹P. Amann, D. Degerman, M. T. Lee, J. D. Alexander. M. Shipilin, H. Y. Wang, F. Cavalca, M. Weston, J. Gladh, M. Blom, M. Björkhage, P. Löfgren, C. Schlueter, P. Loemker, K. Ederer, W. Drube, H. Noei, J. Zehetner, H. Wentzel, J. Ålund and A. Nilsson, Rev. Sci. Instrum. 90, 103102 (2019).