| 1. |
- Edström, Helen, et al.
(författare)
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Alloying and Oxidation of PdAu Thin Films
- 2024
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Ingår i: Thin Solid Films. - 0040-6090. ; 790
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Tidskriftsartikel (refereegranskat)abstract
- The relation between catalytic activity and the presence of oxides on the catalyst’s surface has proven very complex, especially in the case of methane oxidation over Pd. While the metallic Pd surface and a thin, but at least two atomic layers thick, oxide film has been found catalytically active, a single-layer surface oxide or a too thick oxide film are both low-active. Unfortunately, under reaction conditions, the oxide tends to grow thick and deactivate. The reason for this deactivation is believed to be exposure of the PdO(100) surface, which does not have any suitable active sites for the methane adsorption and activation, in contrast to PdO(101), which is exposed for thin films. In an attempt to limit the thickness of the oxide film, and hence stabilise the active PdO orientation, we have investigated the oxidation of thin PdAu films. The effects of different mixtures (25%, 50%, and 75% Au, respectively) and treatments on the oxidation and orientation of the alloy and oxide films were investigated. As intended, PdAu turned out to be significantly more difficult to oxidise compared to pure Pd. This effect was even stronger than expected. Depending on the amount of Au present in the alloy, the orientation of the oxide is affected. At lower Au concentration, the desired (101) oxide orientation is favoured, while higher Au concentration favours the low-active (100) oxide orientation. Thus, PdAu might be a good candidate for methane oxidation if the Au concentration is low, probably below 25%. The larger lattice constant of Au compared to Pd might also affect the oxide orientation, so another choice of alloying material, e.g. PdPt, may also work to stabilise PdO(101).
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| 2. |
- Edström, Helen, et al.
(författare)
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Alloying and oxidation of PdAu thin films
- 2024
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Ingår i: Thin Solid Films. - 0040-6090. ; 790
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Tidskriftsartikel (refereegranskat)abstract
- The relation between catalytic activity and the presence of oxides on the catalyst's surface has proven very complex, especially in the case of methane oxidation over Pd. While the metallic Pd surface and a thin, but at least two atomic layers thick, oxide film has been found catalytically active, a single-layer surface oxide or a too thick oxide film are both low-active. Unfortunately, under reaction conditions, the oxide tends to grow thick and deactivate. The reason for this deactivation is believed to be exposure of the PdO(100) surface, which does not have any suitable active sites for the methane adsorption and activation, in contrast to PdO(101), which is exposed for thin films. In an attempt to limit the thickness of the oxide film, and hence stabilise the active PdO orientation, we have investigated the oxidation of thin PdAu films. The effects of different mixtures (25%, 50%, and 75% Au, respectively) and treatments on the oxidation and orientation of the alloy and oxide films were investigated. As intended, PdAu turned out to be significantly more difficult to oxidise compared to pure Pd. This effect was even stronger than expected. Depending on the sample temperature and the amount of Au present in the alloy, the orientation of the oxide is affected. At lower temperatures and Au concentration, the desired [301] oxide orientation (corresponding to the (101) planes being parallel to the sample surface) is favoured, while higher Au concentration favours the low-active [100] oxide orientation. Thus, PdAu might be a good candidate for methane oxidation if the Au concentration is low, probably below 25%. The larger lattice constant of Au compared to Pd might also affect the oxide orientation, so another choice of alloying material, e.g. PdPt, may also work to stabilise PdO[301]. Independent of the alloy composition, the sample temperature during oxide growth should be optimised in order to achieve an active oxide film.
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| 3. |
- Gustafson, Johan, et al.
(författare)
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The Role of Oxides in Catalytic CO Oxidation over Rhodium and Palladium
- 2018
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 8:5, s. 4438-4445
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Tidskriftsartikel (refereegranskat)abstract
- Catalytic CO oxidation is a seemingly simple reaction between CO and O2 molecules, one of the reactions in automotive catalytic converters, and the fruit-fly reaction in model catalysis. Surprisingly, the phase responsible for the catalytic activity is still under debate, despite decades of investigations. We have performed a simple but yet conclusive study of single crystal Rh and Pd model catalysts, resolving this controversy. For Rh, the oxygen-covered metallic surface is more active than the oxide, while for Pd, thin oxidefilms are at least as active as the metallic surface, but a thicker oxide is less active. Apart from resolving a long-standing debate, our results pinpoint important design principles for oxidation catalysts as to prevent catalytic extinction at high oxygen exposures.
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| 4. |
- Hagman, Benjamin
(författare)
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Dissociative Adsorption of CO2 on Copper: The Role of Steps
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- CO2 chemistry has received significant interest in recent time, due to the greenhouse effects of CO2 emissions and the resulting climate change. CO2 reduction reactions, such as methanol synthesis and reverse water-gas shift, is providing a route for recycling of CO2 and thus limiting the CO2 emissions. These reactions are commonly performed over Cu-based catalysts, making the interaction of CO2 and Cu on the atomic scale of tremendous importance for a fundamental understanding and, as a consequence, the development of new and more efficient catalysts.This thesis presents results on the adsorption and dissociation of CO2 and the initial oxidation of both the low-index Cu(100) and the vicinal Cu(911) surface. Due to the inertness of CO2, techniques with the possibility to measure at elevated pressures are necessary. Hence, the main methods used in this thesis are Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Surface X-Ray Diffraction (SXRD).It was found that the oxidation of Cu(100) starts by forming a p(2×2) structure that transforms to a p(2√2×√2)R45 missing row structure as the oxygen coverage increases. The core-level shifts of the O 1s core-level is shown to be a fingerprint of the Cu coordination number of the absorbed oxygen. The results on the adsorption of CO2 on Cu(100) showed that the increase of oxygen coverage from CO2 dissociation is constant in the range of 0-0.25 ML (MonoLayers, 1 ML = 1.53×1015 cm−2). After 0.25 ML the dissociation is still constant, but with a lower dissociation rate, until the oxygen coverage saturates at 0.50 ML. Results from DFT calculations show that CO2 dissociation on terraces cannot explain the constant dissociation rate as the adsorbed oxygen drastically affects the stability of adsorbed CO2. However, steps were found to both lower the dissociation barrier and separate the products, lowering the probability for recombination. Furthermore, the active site was kept available by oxygen diffusion away from the steps, leading to a constant number of reaction sites. Thus, the inclusion of atomic steps on Cu(100) is necessary to explain the experimental findings.To validate the findings from Cu(100), the vicinal Cu(911), which has closed-packed (111) steps each 11.5 Å, was investigated. It was found that the initialoxidation of the surface proceeds by faceting into (410), (401), and (100) facets. As a consequence, the steps will reform from being closed packed to the more open (110) steps. As Cu2O starts to grow on the surface the (410) and (401) facets disappear. Instead, the (911), (311), and (100) facets are present. Well-ordered oxide is seen to grow on the (311) facets with the orientation of Cu2O(110) || Cu(311). The results on the dsorption of CO2 on Cu(911) show that although the stepped surface facilitates the adsorption, the rate of the increase of atomic oxygen is not faster compared to on Cu(100). This is most likely due to the short terraces where recombination of CO and O takes place more readily. The (110) steps are showed to be able to adsorb CO2 even with the presence of oxygen, hence showing the importance of the (110) steps for the reaction.
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| 5. |
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| 6. |
- Hagman, Benjamin, et al.
(författare)
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Oxygen induced faceting of Cu(911)
- 2022
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 715
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of copper is essential for several fields, such as corrosion, catalytic methanol synthesis, and CO2 electroreduction. However, the understanding of the oxidation of copper under various conditions is not complete. Here, we study the oxidation of the vicinal Cu(911) surface by O2 with in-situ Surface X-ray Diffraction. It was found that the surface facets to (410), (401), and (100) which are stable in the parameter range of T = 25–400 °C and p = 10−10–10−5 mbar O2. The (410) and (401) facets are present until the surface is further oxidized to Cu2O, at 10−5 mbar and above. These results further the knowledge on the oxidation of copper and its surfaces, which may be of importance for a wide range of applications.
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| 7. |
- Hagman, Benjamin, et al.
(författare)
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Steps Control the Dissociation of CO2 on Cu(100)
- 2018
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 140:40, s. 12974-12979
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Tidskriftsartikel (refereegranskat)abstract
- CO2 reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO2 on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO2 dissociation provides an understanding that can be used in the design of future CO2 reduction catalysts.
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| 8. |
- Mehar, Vikram, et al.
(författare)
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Formation of Epitaxial PdO(100) During the Oxidation of Pd(100)
- 2023
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 14:38, s. 8493-8499
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The catalytic oxidation of CO and CH4 can be strongly influenced by the structures of oxide phases that form on metallic catalysts during reaction. Here, we show that an epitaxial PdO(100) structure forms at temperatures above 600 K during the oxidation of Pd(100) by gaseous O atoms as well as exposure to O2-rich mixtures at millibar partial pressures. The oxidation of Pd(100) by gaseous O atoms preferentially generates an epitaxial, multilayer PdO(101) structure at 500 K, but initiating Pd(100) oxidation above 600 K causes an epitaxial PdO(100) structure to grow concurrently with PdO(101) and produces a thicker and rougher oxide. We present evidence that this change in the oxidation behavior is caused by a temperature-induced change in the stability of small PdO domains that initiate oxidation. Our discovery of the epitaxial PdO(100) structure may be significant for developing relationships among oxide structure, catalytic activity, and reaction conditions for applications of oxidation catalysis.
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| 9. |
- Merte, Lindsay R., et al.
(författare)
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Structure of an Ultrathin Oxide on Pt3Sn(111) Solved by Machine Learning Enhanced Global Optimization
- 2022
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Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 61:25, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure-the (4×4) surface oxide on Pt3Sn(111)-based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.
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| 10. |
- Merte, Lindsay R., et al.
(författare)
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Structure of an Ultrathin Oxide on Pt3Sn(111) Solved by Machine Learning Enhanced Global Optimization.
- 2022
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Ingår i: Angewandte Chemie. - : John Wiley & Sons. - 0044-8249 .- 1521-3757. ; 134:25, s. e202204244-
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Tidskriftsartikel (refereegranskat)abstract
- Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure-the (4×4) surface oxide on Pt3Sn(111)-based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.
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| 11. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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Initial oxidation of Cu(100) studied by X-ray photo-electron spectroscopy and density functional theory calculations
- 2018
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 675, s. 64-69
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations and ambient pressure X-ray photoelectron spectroscopy are used to investigate initial oxidation of Cu(100). Surface stability with respect to oxygen coverage is calculated together with O 1s core level shifts. Oxidation of Cu(100) is found to occur via the formation of a p(2 × 2) overlayer (0.25 ML) followed by a reconstructed r2×r2 R45° -O missing-row (MR) structure (0.50 ML). A c(4 × 6) structure with a 0.3 ML coverage is close in stability for intermediate oxygen chemical potentials. The relative stability is found to be weakly dependent on the applied exchange-correlation functional. The calculated shifts in the O 1s binding energy are in good agreement with the measured evolution of the binding energy. The shift to higher O 1s binding energies with increasing oxygen coverage is found to correlate with the charge on neighboring copper atoms. The O 1s core-level shifts here obtained with CO2 as oxidant, are similar to previous measurements of Cu(100) oxidation with O2.
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| 12. |
- Schaefer, Andreas, 1981, et al.
(författare)
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Thermal reduction of ceria nanostructures on rhodium(111) and re-oxidation by CO2
- 2018
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 20:29, s. 19447-19457
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Tidskriftsartikel (refereegranskat)abstract
- The thermal reduction of cerium oxide nanostructures deposited on a rhodium(111) single crystal surface and the re-oxidation of the structures by exposure to CO2 were investigated. Two samples are compared: a rhodium surface covered to ≈60% by one to two O-Ce-O trilayer high islands and a surface covered to ≈65% by islands of four O-Ce-O trilayer thickness. Two main results stand out: (1) the thin islands reduce at a lower temperature (870-890 K) and very close to Ce2O3, while the thicker islands need higher temperature for reduction and only reduce to about CeO1.63 at a maximum temperature of 920 K. (2) Ceria is re-oxidized by CO2. The rhodium surface promotes the re-oxidation by splitting the CO2 and thus providing atomic oxygen. The process shows a clear temperature dependence. The maximum oxidation state of the oxide reached by re-oxidation with CO2 differs for the two samples, showing that the thinner structures require a higher temperature for re-oxidation with CO2. Adsorbed carbon species, potentially blocking reactive sites, desorb from both samples at the same temperature and cannot be the sole origin for the observed differences. Instead, an intrinsic property of the differently sized CeOx islands must be at the origin of the observed temperature dependence of the re-oxidation by CO2.
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| 13. |
- Schiller, Frederik, et al.
(författare)
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Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal : Spatial Variation of Active and Poisoning Phases in Stationary Conditions
- 2018
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 140:47, s. 16245-16252
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Tidskriftsartikel (refereegranskat)abstract
- Understanding nanoparticle catalysis requires novel approaches in which adjoining crystal orientations can be studied under the same reactive conditions. Here we use a curved palladium crystal and near-ambient pressure X-ray photoemission spectroscopy to characterize chemical species during the catalytic oxidation of CO in a whole set of surfaces vicinal to the (111) direction simultaneously. By stabilizing the reaction at fixed temperatures around the ignition point, we observe a strong variation of the catalytic activity across the curved surface. Such spatial modulation of the reaction stage is straightforwardly mapped through the photoemission signal from active oxygen species and poisoning CO, which are shown to coexist in a transient regime that depends on the vicinal angle. Line-shape analysis and direct comparison with ultrahigh vacuum experiments help identifying and quantifying all such surface species, allowing us to reveal the presence of surface oxides during reaction ignition and cooling-off.
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| 14. |
- Wallander, Harald J., et al.
(författare)
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Oxidation of a Platinum–Tin Alloy Surface during Catalytic CO Oxidation
- 2022
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:14, s. 6258-6266
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the surface composition of a well-ordered Pt3Sn(111) surface during CO oxidation with ambient pressure X-ray photoemission spectroscopy. Oxidation of tin in the surface coincides with the onset of catalytic conversion, and we observe significant differences in the oxidation state and morphology of the oxide formed depending on the gas composition, with an oxygen-rich mixture leading to formation of 2D wetting layers and a CO-rich mixture leading to formation of 3D oxide islands. Spontaneous oscillations in conversion are observed at 300 °C in the oxygen-rich gas mixture and attributed to the combined effects of site blocking by tin oxides and by CO. The results highlight the importance of gas−surface interactions in determining the nature of oxides formed and thus the type and number of interfacial sites under reaction conditions.
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| 15. |
- Zhu, Suyun, et al.
(författare)
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HIPPIE : a new platform for ambient-pressure X-ray photoelectron spectroscopy at the MAX IV Laboratory
- 2021
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Ingår i: Journal of Synchrotron Radiation. - : INT UNION CRYSTALLOGRAPHY. - 1600-5775 .- 0909-0495. ; 28, s. 624-636
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Tidskriftsartikel (refereegranskat)abstract
- HIPPIE is a soft X-ray beamline on the 3 GeV electron storage ring of the MAX IV Laboratory, equipped with a novel ambient-pressure X-ray photoelectron spectroscopy (APXPS) instrument. The endstation is dedicated to performing in situ and operando X-ray photoelectron spectroscopy experiments in the presence of a controlled gaseous atmosphere at pressures up to 30 mbar [1 mbar = 100 Pa] as well as under ultra-high-vacuum conditions. The photon energy range is 250 to 2200 eV in planar polarization and with photon fluxes >1012 photons s-1 (500 mA ring current) at a resolving power of greater than 10000 and up to a maximum of 32000. The endstation currently provides two sample environments: a catalysis cell and an electrochemical/liquid cell. The former allows APXPS measurements of solid samples in the presence of a gaseous atmosphere (with a mixture of up to eight gases and a vapour of a liquid) and simultaneous analysis of the inlet/outlet gas composition by online mass spectrometry. The latter is a more versatile setup primarily designed for APXPS at the solid-liquid (dip-and-pull setup) or liquid-gas (liquid microjet) interfaces under full electrochemical control, and it can also be used as an open port for ad hoc-designed non-standard APXPS experiments with different sample environments. The catalysis cell can be further equipped with an IR reflection-absorption spectrometer, allowing for simultaneous APXPS and IR spectroscopy of the samples. The endstation is set up to easily accommodate further sample environments.
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