Harald Wallander

The dissociation of NO is a critical step in its catalytic reduction to N₂, which is key to automotive exhaust treatment. Here, we examine the role of Rh atomic steps in the NO dissociation reaction under 0.05 mbar NO. We use a Rh crystal sample curved around the (111) direction and ambient-pressure X-ray photoelectron spectroscopy to probe different Rh surfaces subject to the very same reaction conditions. At the dissociation onset, this approach allows us to quantitatively determine the NO species involved in the reaction, and to rationally assess the process in terms of diffusion and dissociation probability at terraces and steps. At a higher temperature we trigger surface oxidation, which begins preferentially on flat Rh(111) and B-type stepped surfaces, as compared to A-type stepped surfaces. Surface X-ray diffraction performed on single crystal samples reveals similar oxide structures at the atomic scale, but while B-type Rh(553) and Rh(111) surfaces do not reconstruct, A-type Rh(223) facet exhibits faceting. These findings underscore the structural sensitivity of NO dissociation and its potential impact on Rh-catalyzed NO reduction.

We explore the effect of Rh steps on the NO dissociation process at 0.05 mbar NO, which is a crucial step of the NO reduction reaction. By using a curved Rh(111) crystal and ambient-pressure X-ray photoemission spectroscopy, data demonstrate a lower temperature onset for NO dissociation at stepped surfaces compared to Rh(111), and suggest that dissociation occurs predominantly at the lower side of the step. We also observe that surface oxidation arises at a lower temperature for Rh(111) and its B-type vicinals, with respect to A-type stepped surfaces. Surface X-ray diffraction was employed to characterize the Rh surface oxides. At the atomic scale, these were found structurally similar in flat Rh(111), A-type Rh(223) and B-type Rh(553), but they exhibited different growth behavior depending on the substrate orientation. These results showcase the structural sensitivity of NO dissociation, which is likely to affect the reactivity of these Rh surfaces under operando NO reduction conditions.

We have used grazing incidence X‐ray absorption near edge spectroscopy (XANES) to investigate the behavior of monolayer FeO films on Pt(111) under near ambient pressure CO oxidation conditions with a total gas pressure of 1 bar. Spectra indicate reversible changes during oxidation and reduction by O and CO at 150 °C, attributed to a transformation between FeO bilayer and FeO trilayer phases. The trilayer phase is also reduced upon heating in CO+O , consistent with a Mars‐van‐Krevelen type mechanism for CO oxidation. At higher temperatures, the monolayer film dewets the surface, resulting in a loss of the observed reducibility. A similar iron oxide film prepared on Au(111) shows little sign of reduction or oxidation under the same conditions. The results highlight the unique properties of monolayer FeO and the importance of the Pt support in this reaction. The study furthermore demonstrates the power of grazing‐incidence XAFS for in situ studies of these model catalysts under realistic conditions.

Nickel (Ni)-promoted Molybdenum (Mo)-based catalysts are used for hydrotreatment processes in the chemical industry where the catalysts are exposed to high-pressure H₂ at elevated temperature. In this environment, the catalyst transforms into the active phase, which involves the reduction of the oxide. Here, we report on the first in situ study on the reduction of alumina supported Ni- and Mo-based catalysts in 1 mbar H₂ using ambient-pressure X-ray photoelectron spectroscopy (APXPS). The study confirms that mixing Ni and Mo lowers the reduction temperature of both Ni- and Mo-oxide as compared to the monometallic catalysts and shows that the MoO₃ reduction starts at a lower temperature than the reduction of NiO in NiMo/Al₂O₃ catalysts. Additionally, the reduction of Ni and Mo foil was directly compared to the reduction of the Al₂O₃-supported catalysts and it was observed that the reduction of the supported catalysts is more gradual than the reduction of the foils, indicating a strong interaction between the Ni/Mo and the alumina support.

Heterogeneous catalyst surfaces are dynamic entities that respond rapidly to changes in their local gas environment, and the dynamics of the response is a decisive factor for the catalysts’ action and activity. Few probes are able to map catalyst structure and local gas environment simultaneously under reaction conditions at the timescales of the dynamic changes. Here we use the CO oxidation reaction and a Pd(100) model catalyst to demonstrate how such studies can be performed by time-resolved ambient pressure photoelectron spectroscopy. Central elements of the method are cyclic gas pulsing and software-based event-averaging by image recognition of spectral features. A key finding is that at 3.2 mbar total pressure a metallic, predominantly CO-covered metallic surface turns highly active for a few seconds once the O₂:CO ratio becomes high enough to lift the CO poisoning effect before mass transport limitations triggers formation of a √5 oxide.