APXPS – Lightsources.org

Catching “Hydrogen Spillover” onto a Catalytic Surface

2025/05/272025/06/06

Researchers uncovered the precise mechanism of hydrogen spillover (H₂ splitting and migration) onto a catalytic surface by watching it happen under various conditions at the Advanced Light Source (ALS).

The research lays the foundation for designing more efficient catalysts and storage materials essential for next-generation hydrogen energy technologies.

Hydrogen on the move

The splitting and migration of molecular hydrogen (H₂) over a catalytic surface (a process known as “hydrogen spillover”) is a fundamental yet elusive phenomenon in catalysis that affects a wide range of uses, from hydrogenation (which can be used to upgrade or purify crude oil components) to energy storage (when bonded to a metal, hydrogen can be stored in the solid state). Despite its importance, direct experimental evidence capturing the real-time mechanistic steps of hydrogen spillover remains scarce.

In particular, tungsten oxide (WO₃), a widely used catalytic material, exhibits dynamic interactions with hydrogen, yet the precise nature of these interactions has been a subject of long-standing debate, especially for distinguishing the chemical dynamics occurring on the surface from those in the bulk.

This research was driven by the need to resolve these ambiguities using ambient-pressure x-ray photoelectron spectroscopy (APXPS), which provides direct spectroscopic evidence of the spillover process as it unfolds. By integrating experimental observations with theoretical models, the researchers unlocked a comprehensive understanding of how hydrogen interacts with reducible oxide surfaces and influences their catalytic properties.

Operando APXPS at the ALS

This study focused on WO₃ thin films “decorated” with Pt metal clusters that facilitate hydrogen activation and dissociation. To directly visualize the stepwise evolution of hydrogen spillover on WO₃, the researchers employed APXPS at ALS Beamline 9.3.2, a technique pioneered at the ALS and uniquely suited for studying solid–gas interfaces in real time under realistic (“operando”) reaction conditions.

APXPS detected the oxidation states of tungsten and the presence of surface hydrogen species as the samples were exposed over time to hydrogen gas at various temperatures. The tunable incident photon energy allowed selective analysis of different elements (including differentiating between various hydrogen species—molecular, protonic, or hydride-like) at variable depths, enabling the researchers to track hydrogen-induced changes with high precision. The ability to collect real-time spectra while exposing the sample to hydrogen enabled the detection of intermediates that would be difficult to observe with other methods.

Furthermore, by combining the APXPS experimental observations with first-principles-based microkinetic modeling and simulations, the researchers gained a comprehensive understanding of the reaction mechanisms underlying hydrogen spillover.

Read more on ALS website

Image: Artistic depiction of a tungsten trioxide (WO₃) surface (purple/red) “decorated” with a platinum nanocluster (metallic gray). Green arrows trace the evolution of hydrogen (white) from gas form (H₂) to dissociation into H⁺ on the platinum, to spillover (migration) onto the WO₃ surface, and, at elevated temperatures, desorption as water vapor (H₂O) and diffusion into the bulk.

First direct measurement of elusive Donnan potential

2023/01/192023/02/09

Scientific achievement

At the Advanced Light Source (ALS), researchers performed the first direct measurement of the Donnan electrical potential, which arises from an imbalance of charges at membrane-solution interfaces.

Significance and impact

Considered unmeasurable for over a century, the Donnan potential is relevant to a wide range of fields, from cell biology to energy storage and water desalination.

A breakthrough with great potential

The Donnan electrical potential arises from an imbalance of charges at the interface of a charged membrane and a liquid, and for more than a century it stubbornly eluded direct measurement. Many researchers had even written off such a measurement as impossible. Now, using ambient-pressure x-ray photoelectrion spectroscopy (APXPS) at the ALS, scientists directly measured the Donnan potential for the first time.

The ability to probe the characteristics of this potential at membrane-solution interfaces could yield new insights in biology, energy science, and materials science. For example, the Donnan potential plays a critical role in biological functions ranging from muscle contractions to neural signaling. Energy storage and water purification using ion exchange membranes (IEMs) are also important applications involving the Donnan potential.

Read more on the ALS website

Image: Left: Schematic of the x-ray experiment. Right: The presence of fixed ions inside a membrane generates an electrochemical potential gradient (the Donnan potential) that leads to more counter-ions (with charge opposite that of the fixed ions) diffusing from the solution to the membrane relative to co-ions (which have the same charge as the fixed ions).

A revolutionary setup for atomic layer deposition at SPECIES

2020/04/152020/04/15

In a joint project across three universities and MAX IV laboratory, researchers have developed a revolutionary experimental setup for atomic layer deposition.

The new instrument was designed specifically for MAX IV and will allow for observations previously impossible.
SPECIES, one of the soft X-ray beamlines in MAX IV 1.5 GeV storage ring, has added to its portfolio a new cutting-edge instrument. The new experimental setup has been specially developed to use Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) for the study of atomic layer deposition (ALD), a process where thin films of material are grown depositing one layer at a time.

This experimental setup is composed of a custom cell where the ALD process is performed and observed using APXPS. The instrument is the result of an extensive collaboration between the University of Helsinki, world-leading in ALD studies, University of Oulu, Lund University, and MAX IV Laboratory, and funded by the University of Helsinki through the FiMAX consortium.
In February, the team from the University of Helsinki led by professor Mikko Ritala, and from the University of Oulu came to MAX IV for the final experiments and refinement activities on the experimental setup. We talked with the scientists to understand how the cell they have developed allows for unprecedented observations.

>Read more on the MAX IV website

Image credit: Matti Putkonen.

New catalyst resists destructive carbon buildup in electrodes

2020/01/282020/02/10

Key challenges in the transition to sustainable energy include the long-duration storage of cheap, renewable electricity and the electrification of the heavy-freight transportation sector. Both challenges can be met using electrochemical cells. Solid oxide electrolysis cells are capable of highly efficient splitting of steam and CO₂ to produce a synthetic H₂–CO gas mixture (syngas), which can be converted into synthetic hydrocarbon transportation fuels using conventional industrial reactors. However, the efficiency of the process is limited by the risk of destructive carbon deposition inside the cells’ porous solid electrodes. A nickel catalyst is responsible for the carbon growth, but replacing this long-standing conventional catalyst has turned out to be highly challenging.

Now, researchers have used ambient-pressure x-ray photoelectron spectroscopy (APXPS) at ALS Beamlines 9.3.2 and 11.0.2 to probe the mechanisms by which carbon grows on different catalysts during CO₂ electrolysis. Gadolinium-doped cerium oxide (GDC) is known to resist carbon growth, and the ambient-pressure experiments probed the degree and mechanism of this carbon resistance. The experimental data, subsequently confirmed by density functional theory calculations, revealed that the carbon atoms are energetically trapped by various oxygen species on the surface of GDC—a capability entirely lacking for nickel.

Image: Artistic representation of a nickel-based electrode as a broken down fuel pump and of a cerium-based electrode as a new, productive pump. Credit: Cube3D

The quest for atomic perfection in semiconductor devices

2018/06/292018/07/12

A research team, including scientists from MAX IV have reported in Nature Communications that the quest for atomic perfection in semiconductor devices was based on an oversimplified model.

Semiconductors are the fundamental building blocks of all modern electronics. Improvements to these materials could affect everything from the clock on our microwave to supercomputers used to crunch big data. The search to make them better involves looking at atomic level changes in semiconductor materials in order to understand how they could be improved, and even made perfect.

The problem with semiconductors and the way they are manufactured is that they need to be processed with metal contacts and thin insulating layers, and the interface between the semiconductor and these contacts contains a lot of defects which hamper device performance. If scientists can find a way to reduce the defects or eliminate them completely, then semiconductors could conceivably become faster and smaller. The problem is, these defects occur on the atomic scale and are very difficult to measure.

Scientists working at Max Lab, the predecessor to the newly built MAX IV, together with physicists from Lund University used the SPECIES beamline to investigate a common semiconductor synthesis method. Hafnium dioxide was deposited on the surface of indium arsenide and monitored using ambient pressure X-ray photoelectron spectroscopy (APXPS). The scientists were able to monitor the very first atomic layer that was deposited, and monitor the chemical reactions that were occurring as the process was underway.