**4. Outlook and future trends in the field**

Further developments in the field of X-ray spectroscopy applied to catalysis science are nowadays focused in multiple aspects. Most importantly, improvements in the experimental time resolution are necessary for deep insight knowledge of many chemical processes. For this, a range of complementary X-ray sources is necessary due to timescale changes in the chemical environment of the atomic species that extend over a wide time range from femtoseconds up to a few seconds. Therefore, a lot of expectation is associated with new, large-scale X-ray facilities being under development worldwide. A new synchrotron sources, like MAXIV in Sweden, are expected to deliver ultrabright X-ray pulses with pulse durations from tens of picoseconds down to sub-picoseconds times. Recently developed fourth-generation X-ray sources – X-ray free electrons lasers – pushed the time limit down to femto-second timescales. The two operating XFELs (LCSL, USA and SACLA, Japan) as well as those under construction (EuXFEL, Germany and SwissFEL, Switzerland) provide a unique opportunity to access timescales of charge-transfers and the following bond-breaking and bond-making mechanism. On the other hand, in order to fully explore capabilities of those new X-ray sources, further developments on X-ray spectroscopy techniques is necessary together with progress on sample delivery/treatments schemes for in situ investigations. Finally, further advancement and evolution in the field of X-ray spectrometers and detectors will be crucial in order to fully explore opportunities provided by the subpicoseconds and femtoseconds X-ray probes.
