| 1. |
- Garcia-Martinez, Fernando, et al.
(författare)
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Structure Matters: Asymmetric CO Oxidation at Rh Steps with Different Atomic Packing
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 144:33, s. 15363-15371
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Tidskriftsartikel (refereegranskat)abstract
- Curved crystals are a simple but powerful approach to bridge the gap between single crystal surfaces and nanoparticle catalysts, by allowing a rational assessment of the role of active step sites in gas-surface reactions. Using a curved Rh(111) crystal, here, we investigate the effect of A-type (square geometry) and B-type (triangular geometry) atomic packing of steps on the catalytic CO oxidation on Rh at millibar pressures. Imaging the crystal during reaction ignition with laser-induced CO2 fluorescence demonstrates a two-step process, where B-steps ignite at lower temperature than A-steps. Such fundamental dissimilarity is explained in ambient pressure X-ray photoemission (AP-XPS) experiments, which reveal partial CO desorption and oxygen buildup only at B-steps. AP-XPS also proves that A-B step asymmetries extend to the active stage: at A-steps, low-active O-Rh-O trilayers buildup immediately after ignition, while highly active chemisorbed O is the dominant species on B-type steps. We conclude that B-steps are more efficient than A-steps for the CO oxidation.
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| 2. |
- Pfaff, Sebastian
(författare)
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On the Chemical Romance Between Gas and Surface, and how to Illuminate it
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An increasing number of catalysis experiments are conducted at higher pressures to bridge the pressure gap. This means that we need to measure not only the sample surface but also the gas phase around the sample with spatial resolution. This thesis treats the development and applications of two optical techniques that can be used to image the surface of a model catalyst together with the surrounding gas: planar laser-induced fluorescence (PLIF) and 2D-surface optical reflectance (2D-SOR). The strengths and weaknesses of the techniques are discussed. Examples are used to show how the techniques can be applied to study active model catalysts \textit{in situ} under ambient pressure conditions. PLIF is used to image the CO and CO₂ concentrations around various Pd model catalysts during the CO oxidation reaction. 2D-SOR is used to show the surface oxidation process during the same reaction. We show how PLIF and 2D-SOR can be combined with additional techniques, primarily high-energy surface X-ray diffraction (HESXRD), thermography, mass spectrometry (MS) and polarisation modulation infrared reflection absorption spectroscopy (PMIRRAS).Finally, we show how PLIF and 2D-SOR can be used to bridge the so-called materials gap by taking advantage of the spatial resolution of the techniques to study polycrystalline samples. These samples, which exhibit a plethora of surface structures, can be seen as a step towards more industry-like samples.
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| 3. |
- Pfaff, Sebastian, et al.
(författare)
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Visualizing the Gas Diffusion Induced Ignition of a Catalytic Reaction
- 2022
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; :12, s. 6589-6595
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Tidskriftsartikel (refereegranskat)abstract
- Many surface science experiments within heterogeneous catalysis are now conducted in realistic conditions at higher pressures. At these pressures, localized gas conditions will form throughout the reactor. Understanding these gas conditions and their interaction with the catalyst surface at relevant time scales and with spatial resolution is important. To address this issue, we use a combination of techniques that can resolve the gas and surface composition with enough temporal and spatial resolution to show even very rapid gas–surface interactions. Planar laser-induced fluorescence is used to monitor the gas phase, thermography visualizes the surface temperature, and 2D-surface optical reflectance measurements show oxide growth. By combining these techniques in an operando experiment, we demonstrate that the spatial evolution of the catalytic ignition of CO oxidation over Pd(100) at higher pressures is driven by localized gas conditions, emphasizing the need for 2D gas-phase measurements when studying model catalysts in high-pressure conditions.
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| 4. |
- Rämisch, Lisa, et al.
(författare)
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Infrared surface spectroscopy and surface optical reflectance for operando catalyst surface characterization
- 2022
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 578
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Tidskriftsartikel (refereegranskat)abstract
- Using a new sample environment, the two operando surface diagnostic techniques two-dimensional surface optical reflection (2D-SOR) and polarization-modulated infrared reflection–absorption spectroscopy (PM-IRRAS) have been combined with mass spectrometry (MS) to characterize a catalytic reaction. With both techniques integrated in a custom-built setup, we can correlate molecular information of the adsorbed surface species from PM-IRRAS with information about oxide formation on the sample from 2D-SOR. The new setup was evaluated by performing CO oxidation over a Palladium single crystal Pd(1 0 0) surface. The results reveal that the macroscopic and the microscopic molecular behavior correlate well. When the CO adsorption peak disappears in the PM-IRRAS spectrum, the formation of a well-defined ultra-thin surface oxide is observed in the 2D-SOR trend. We discuss the benefits and limitations of the two techniques as well as their potential for further studies of catalytic reactions at both gas–solid and liquid–solid interfaces.
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