We have built an end-station for near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) at the Photon Factory. This NAP-XPS end-station has been applied to operando measurements for functional material surfaces to understand the mechanisms of the functional surface processes under working conditions. Here we introduce two application studies concerning NAP-XPS based operando observations for functional material surfaces : (1) Rh catalyst for NO conversion and (2) PtRh thin-film sensor for selective detection of H₂ under coexistence of NH₃. In the former study, the catalytic reaction rates measured by mass spectrometry are directly correlated to coverages of surface species under working conditions, which enables to perform kinetic analyses to understand the mechanisms behind the catalytic reactions. In the latter study, the resistivity changes of the thin film under gas exposure were measured and correlated to surface chemical states of the sensor, which unveils the mechanisms for the selective detection of H₂.

In photoelectron spectroscopy in a gas atmosphere, the charging of an insulator sample is relaxed (environmental charge compensation). Here, we show that the environmental charge compensation effect becomes more remarkable when the distance (d) between the sample and the aperture cone increases. The large d dependence can be attributed to that electron scattering is largely suppressed by the aperture cone with vacuum inside at small d values. Moreover, photoelectron intensities obtained at various d values were found to be reproduced by replacing d with d＋do (do is a constant) in the Beer-Lambert law. The residual gas in the pre-lens is considered to be the cause of do.

X-ray Photoelectron Spectroscopy (XPS) is widely used as one of the primary techniques for investigating the surface chemical composition of materials. In recent years, there has been a significant increase in the demand to observe the electronic states associated with chemical reactions under operating conditions close to real environments, such as in fuel cells and catalytic materials. To address this need, Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) instruments have been developed and continuously improved. This article introduces atmospheric pressure photoelectron spectroscopy at pressures of several bars and provides data examples. Additionally, we present software technology designed to visualize spatiotemporal-resolved data expected to be obtained in the future using this measurement technique.

Hydrogenation of carbon dioxide (CO₂) on copper/zinc oxide-based catalysts was investigated using near-ambient pressure X-ray photoelectron spectroscopy (AP-XPS). An initial hydrogenation step of CO₂ to formate was studied using Zn-deposited single-crystal Cu surfaces. CO₂ activation on the model catalysts leads to the formation of carbonates (CO₃) on the surface in the presence of near-ambient pressure CO₂ and hydrogen (H₂). The addition of a small amount of water to the feed gas enhances the hydrogenation of CO₂ into formate. This indicates that hydroxyl produced through dissociative adsorption of water is a source for the CO₂ hydrogenation observed under the present reaction conditions. We also performed operando spectroscopy of an industrial catalyst for methanol synthesis at higher pressure to reveal the chemical states of the catalyst and adsorbed intermediates during the catalytic reaction. In-situ formation of active catalytic sites was detected by operando hard X-ray photoelectron spectroscopy. Analysis of the reaction products in the presence of CO₂ and H₂ gases shows the formation of methanol and CO. The rate of methanol synthesis correlates with the dynamic changes in the chemical states of the catalyst, and the reaction intermediates adsorbed on the catalyst.

Near ambient pressure photoemission spectroscopy that uses hard X-rays (NAP-HAXPES) has been performed at BL36XU in SPring-8 and HAXPES measurements under atmospheric pressure have been achieved using a small diameter aperture recently. The main target of measurement using this system has been the operando measurement of fuel cell electrodes. We measured Pt/C and Pt₃Co/C nanoparticle catalyst electrodes and obtained results on sulfur poisoning of the catalyst surface due to bias voltage changes. Recently, this system was moved to the upgraded HAXPES beamline “HAXPES II” (BL46XU). The beamline is publicly available, and it is expected to be used by users not only in Japan but also all over the world to promote HAXPES measurements on various samples and environments.