| 1. |
- Balaji Gopal, Chirranjeevi, et al.
(författare)
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Equilibrium oxygen storage capacity of ultrathin CeO2-δ depends non-monotonically on large biaxial strain
- 2017
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion-electron conducting oxides. However, its effect on oxygen storage capacity is not well established. Here, we fabricate ultrathin, coherently strained films of CeO2-δ between 5.6% biaxial compression and 2.1% tension. In situ ambient pressure X-ray photoelectron spectroscopy reveals up to a fourfold enhancement in equilibrium oxygen storage capacity under both compression and tension. This non-monotonic variation with strain departs from the conventional wisdom based on a chemical expansion dominated behaviour. Through depth profiling, film thickness variations and a coupled photoemission-thermodynamic analysis of space-charge effects, we show that the enhanced reducibility is not dominated by interfacial effects. On the basis of ab initio calculations of oxygen vacancy formation incorporating defect interactions and vibrational contributions, we suggest that the non-monotonicity arises from the tetragonal distortion under large biaxial strain. These results may guide the rational engineering of multilayer and core-shell oxide nanomaterials.
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| 2. |
- Boix, Virginia, et al.
(författare)
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Following the Kinetics of Undercover Catalysis with APXPS and the Role of Hydrogen as an Intercalation Promoter
- 2022
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 12:16, s. 9897-9907
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Tidskriftsartikel (refereegranskat)abstract
- While improved catalytic properties of many surfaces covered by two-dimensional materials have been demonstrated, a detailed in situ picture of gas delivery, undercover reaction, and product removal from the confined space is lacking. Here, we demonstrate how a combination of gas pulses with varying compositions and time-resolved ambient pressure photoelectron spectroscopy can be used to obtain such knowledge. This approach allows us to sequentially form and remove undercover reaction products, in contrast to previous work, where co-dosing of reactant gases was used. In more detail, we study CO and H2 oxidation below oxygen-intercalated graphene flakes partially covering an Ir(111) surface. We show that hydrogen rapidly mixes into a p(2 × 1)-O structure below the graphene flakes and converts it into a dense OH-H2O phase. In contrast, CO exposure only leads to oxygen removal from the confined space and little CO intercalation. Finally, our study shows that H2 mixed into CO pulses can be used as a promoter to change the undercover chemistry. Their combined exposure leads to the formation of OH-H2O below the flakes, which, in turn, unbinds the flakes for enough time for CO to intercalate, resulting in a CO structure stable only in coexistence with the OH-H2O phase. Altogether, our study proves that promoter chemistry in the form of adding trace gases to the gas feed is essential to consider for undercover reactions.
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| 3. |
- Boix, Virginia, et al.
(författare)
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Graphene as an Adsorption Template for Studying Double Bond Activation in Catalysis
- 2022
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:33, s. 14116-14124
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogenated graphene (H-Gr) is an extensively studied system not only because of its capabilities as a simplified model system for hydrocarbon chemistry but also because hydrogenation is a compelling method for Gr functionalization. However, knowledge of how H-Gr interacts with molecules at higher pressures and ambient conditions is lacking. Here we present experimental and theoretical evidence that room temperature O2exposure at millibar pressures leads to preferential removal of H dimers on H-functionalized graphene, leaving H clusters on the surface. Our density functional theory (DFT) analysis shows that the removal of H dimers is the result of water or hydrogen peroxide formation. For water formation, we show that the two H atoms in the dimer motif attack one end of the physisorbed O2molecule. Moreover, by comparing the reaction pathways in a vacuum with the ones on free-standing graphene and on the graphene/Ir(111) system, we find that the main role of graphene is to arrange the H atoms in geometrical positions, which facilitates the activation of the O═O double bond.
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| 4. |
- Cole, Jordan, et al.
(författare)
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In situ XPS of competitive CO2/H2O absorption in an ionic liquid
- 2023
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Ingår i: JPhys Materials. - 2515-7639. ; 6:4
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Tidskriftsartikel (refereegranskat)abstract
- Superbasic ionic liquids (SBILs) are being investigated as potential carbon dioxide (CO2) gas capture agents, however, the presence of H2O in the flue stream can inhibit the uptake of CO2. In this study a thin film of the SBIL trihexyltetradecylphosphonium 1,2,4-triazolide ([P66614][124Triz]) was deposited onto rutile TiO2 (110) using in situ electrospray deposition and studied upon exposure to CO2 and H2O using in situ near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS). The molar uptake ratio of gas in the electrosprayed SBIL (n gas :n IL) was calculated to be 0.3:1 for CO2, 0.7:1 for H2O, and 0.9:1 for a CO2/H2O mixture. NAP-XPS taken at two different depths reveals that the competitive absorption of CO2 and H2O in [P66614][124Triz] varies with sampling depth. A greater concentration of CO2 absorbs in the bulk layers, while more H2O adsorbs/absorbs at the surface. The presence of H2O in the gas mixture does not inhibit the absorption of CO2. Measurements taken during exposure and after the removal of gas indicate that CO2 absorbed in the bulk does so reversibly, whilst CO2 adsorbed/absorbed at the surface does so irreversibly. This is contrary to the fully reversible CO2 reaction shown for bulk ionic liquids (ILs) in literature and suggests that irreversible absorption of CO2 in our highly-structured thin films is largely attributed to reactions at the surface. This has potential implications on IL gas capture and thin film IL catalysis applications.
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| 5. |
- Conti, Giuseppina, et al.
(författare)
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X-ray standing-wave spectroscopy : A powerful method for probing buried interfaces
- 2023
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Ingår i: Encyclopedia of Solid-Liquid Interfaces. - 9780323856690 ; 1-3, s. 1-335
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Bokkapitel (refereegranskat)abstract
- In this chapter we present a non-destructive method for characterizing buried interfaces with high depth resolution combining X-rays standing wave and photoemission electron spectroscopy. This method incorporates the power of the X-rays photoelectron spectroscopy, which is a surface sensitive technique in the nanometer and sub-nanometer regime, with the depth selectivity of X-ray standing wave. This method has been successfully applied to study solid/solid interfaces and, more recently, solid/gas and solid/liquid interfaces, as we demonstrate on several examples. In addition, we also describe a few examples of liquid/solid interface characterization by standing wave X-ray fluorescence relevant for catalysis, energy science and biomedical applications.
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| 6. |
- Eren, Baran, et al.
(författare)
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Contamination Issues in Ambient Pressure Experiments
- 2021
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Ingår i: Ambient Pressure Spectroscopy in Complex Chemical Environments. - Washington, DC : American Chemical Society. - 0097-6156 .- 1947-5918. - 9780841298125 - 9780841298118 ; 1396, s. 267-295
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Bokkapitel (refereegranskat)abstract
- Contamination is the most common and arguably the most significant problem scientists are facing in experimental surface science research that is practiced in the presence of gases. It is fair to say that contamination problems are often worse with ambient pressures compared to conventional experiments in vacuum. It is one of the main reasons for poor reproducibility in this field and in relevant basic and applied research fields like heterogeneous catalysis and electrochemistry. Whilst some type of contaminants are more innocent and only hinder quantitative analysis, some are harmful as they change the outcome of the experiments. In this chapter, the potential sources of contamination are summarized and some solutions are suggested. Examples of commonly observed contaminants such as hydrocarbons, oxygenated hydrocarbons, and adsorbed species of traces gases are presented. The scope of this chapter is restricted to ambient pressure x-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy studies on single crystal surfaces, but similar problems exist on other sample surfaces or with other techniques such as x-ray absorption spectroscopy and sum frequency generation spectroscopy.
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| 7. |
- Gericke, Sabrina Maria, et al.
(författare)
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In Situ H2 Reduction of Al2O3-Supported Ni- and Mo-Based Catalysts
- 2022
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Ingår i: Catalysts. - : MDPI. - 2073-4344. ; 12:7
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Tidskriftsartikel (refereegranskat)abstract
- 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. © 2022 by the authors.
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| 8. |
- Kaiser, Sebastian, et al.
(författare)
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Does Cluster Encapsulation Inhibit Sintering? Stabilization of Size-Selected Pt Clusters on Fe3O4(001) by SMSI
- 2023
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Ingår i: ACS Catalysis. - 2155-5435. ; 13:9, s. 6203-6213
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Tidskriftsartikel (refereegranskat)abstract
- The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal-support interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for subnanometer clusters, where concomitant sintering and alloying might play a significant role. In this article, we explore the encapsulation and stability of size-selected Pt5, Pt10, and Pt19 clusters deposited on Fe3O4(001). In a multimodal approach using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), we demonstrate that SMSI indeed leads to the formation of a defective, FeO-like conglomerate encapsulating the clusters. By stepwise annealing up to 1023 K, we observe the succession of encapsulation, cluster coalescence, and Ostwald ripening, resulting in square-shaped crystalline Pt particles, independent of the initial cluster size. The respective sintering onset temperatures scale with the cluster footprint and thus size. Remarkably, while small encapsulated clusters can still diffuse as a whole, atom detachment and thus Ostwald ripening are successfully suppressed up to 823 K, i.e., 200 K above the Hüttig temperature that indicates the thermodynamic stability limit.
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| 9. |
- Kersell, Heath, et al.
(författare)
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Some Future Perspectives in Ambient Pressure X-ray Spectroscopies : Atmospheric Pressure, Spatially Resolved and Multi-modal Experiments
- 2021
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Ingår i: Ambient Pressure Spectroscopy in Complex Chemical Environments. - Washington, DC : American Chemical Society. - 0097-6156 .- 1947-5918. - 9780841298125 - 9780841298118 ; 1396, s. 333-358
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Bokkapitel (refereegranskat)abstract
- This chapter introduces a selection of novel and emerging concepts in ambient pressure X-ray spectroscopies. Technical considerations and first photoemission experiments performed in and above 1 bar pressure are described. Then, X-ray spectroscopy experiments that provide high lateral (microscopy) and depth (X-ray standing-waves) resolution in ambient environments are discussed. The final part of the chapter introduces recent developments in multimodal instrumentation, such as the combination of ambient pressure X-ray Photoelectron Spectroscopy with Infrared spectroscopy and X-ray scattering.
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| 10. |
- Knudsen, Jan, et al.
(författare)
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Stroboscopic operando spectroscopy of the dynamics in heterogeneous catalysis by event-averaging
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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