Oxygen Reduction Reaction on Palladium-Cobalt Alloy Catalysts for Polymer Electrolyte Fuel Cells

L'activité de la réaction de réduction de l’oxygène (RRO) en milieu acide sur les alliages Pd-Co a été étudiée dans ce travail. Les électro-catalyseurs ont été synthétisés par deux techniques : a) La technique de dépôt physique en phase vapeur et b) la technique de réaction de pulvérisation à ultrasons qui a été développée pour la première fois dans notre laboratoire pour préparer directement des électro-catalyseurs supportés sur du tissu ou des feuilles de carbone (GDE) pour des applications de type PEMFC. Les variations des propriétés électrochimiques telles que les quantités de charge liée à : i) l’adsorption/désorption de d'hydrogène, ii) la formation ou la réduction de la couche d’oxyde en fonction de la composition de l’alliage Pd-Co ont été mises en évidence pour la première fois dans ce travail. Les travaux ont été faits sur des couches minces de Pd-Co préparés par un procédé de pulvérisation sous vide cathodique (PVD). Même si un catalyseur à couche mince ne peut pas être utilisé directement comme électrode de PEMFC fonctionnelle parce que sa surface active est faible, le procédé de pulvérisation cathodique est très utile car la composition chimique de l'alliage et l’aire de surface de l'électrode peuvent être contrôlées avec facilité pour assurer des études fondamentales en électro-catalyse. C’est pour cela, les propriétés électrochimiques fondamentales en électro-catalyse ont été réalisées sur ces couches minces de catalyseurs à base d’alliage de Pd-Co. Ainsi, les quantités de charge des processus électrochimiques sur les alliages de Pd-Co indiqués ci-dessus ont été corrélées aux activités électro-catalytiques de la réaction de réduction de l’oxygène (RRO) en milieu acide. Les bonnes activités électro-catalytiques de la RRO sur des alliages binaires de Pd-Co obtenues dans ce travail sont en accord avec les résultats présentés dans les études antérieures. L’activité électro-catalytique de la RRO sur ces alliages augmente dans le sens suivant : Pd16Co84 < Pd42Co58 < Pd < Pd50Co50 < Pd75Co25 < Pd65Co35. L'alliage Pd65Co35 donne l’activité de la RRO la plus élevée suivie par les alliages Pd75Co25 et Pd50Co50. Aucunes améliorations d'activité évidentes n'ont été obtenues pour Pd42Co58 et Pd16Co84 qui ont un contenu de Co plus élevé. Une corrélation linéaire entre les charges d’adsorption/désorption de d'hydrogène et les activités électro-catalytiques de la RRO a été obtenue sur ces alliages de Pd-Co. Les catalyseurs ayant une vi teneur en Co plus élevée ont des activités en RRO plus faibles. Par contre aucune corrélation linéaire n’a été observée entre les caractéristiques de l’activité de la RRO et la quantité de charges liée à la formation ou la réduction des oxydes sur les catalyseurs. Les meilleures activités électro-catalytiques pour le RRO on été identifiées sur les alliages ayant un ratio atomique optimisé entre Pd et Co aux alentours de Pd: Co = 3:1. La méthode de pulvérisation à ultrasons a été développée pour la première fois dans le cadre de ce travail pour la préparation de catalyseurs déposés à l’échelle nanométrique directement sur des supports de feuilles de tissus de carbone commerciaux utilisés (GDE) dans la fabrication des assemblages membranes électrodes pour les piles PEMFC dans notre laboratoire. Cette approche répond bien aux critères d’obtention de catalyseurs de fines particules ayant de grande surface sur la poudre de carbone comme matériaux de cathode pour les piles de type PEMFC. Ceci est d’autant plus pertinent qu’aucune des études réalisées avant n’a réussi à obtenir des particules fines de Pd-Co ayant des dimensions qui soient comparables à celles du catalyseur existant à base de platine à support de carbone (φ2-4 nm). Par conséquent, la mise au point de la méthode de synthèse de catalyseur à base de particules fines de Pd-Co est nécessaire pour mieux utiliser ce catalyseur pour les applications des piles PEMFC. En utilisant cette méthode développée dans ce travail, les catalyseurs de Pt, de Pd et Pd-Co à support de carbone ont été synthétisés et caractérisés pour leur activité en RRO en milieu acide. L’imagerie TEM des échantillons préparés indiquent que la taille des particules dominante de 2,5-4,5 nm. Pour les catalyseurs obtenus. Ce qui montre que cette technique est très utile pour la préparation de catalyseurs à support de carbone à l'échelle nanométrique. La diffraction des rayons-X (DRX) sur les échantillons fabriqués de Pt a montré que les pics de diffraction sont ceux d’une structure cubique à faces centrées (cfc) de Pt. La DRX descatalyseurs synthétisés de Pd et Pd-Co par réaction de pulvérisation ultrasonique a également indiqué des pics de diffraction d’un système cristallin cubique face centré (CFC). Tous les diffractogrammes des échantillons sont similaires à celui de Pd cfc, mais les pics d'origine Co ne peuvent être observés car le système Pd-Co est une solution solide de substitution, de sorte que certains atomes de Pd sont remplacés par des atomes de cobalt. Ce qui constitue un atout car le système de solution solide est considéré comme étant stable en milieu acide [1]. vii Aussi nous avons développé dans ce travail une nouvelle technique utilisant la méthode de réaction de pulvérisation à ultrasons pour réaliser dans un même processus la synthèse du catalyseur à support de carbone nanométrique qui est ensuite déposé directement sur l’électrode à diffusion de gaz (GDE) et formé ainsi un GDE catalysé. En utilisant du papier carbone comme comme support de réaction de la pulvérisation à ultrasons tel que décrit ci-dessus, le catalyseur supporté sur de la poudre de carbone à support est directement déposé sur le papier carbone et formé ainsi le GDE catalysé qui peut ainsi être utilisée dans les piles PEFC. Cette technique permet d'économiser certaines étapes importantes comme par exemple la mise en forme de la poudre de catalyseur (préparation de la pâte de catalyseur et son étalement) et de la fabrication du GDE et conduit à un moindre coût de la PEMFC. Les améliorations obtenues sur l'activité électro-catalytique de la réaction de réduction de l’oxygène en milieu acide sur les catalyseurs de Pd-Co synthétisés par le procédé de réaction de pulvérisation à ultrasons ont été confirmées. Ce résultat est en bon accord avec l’amélioration de l'activité de la RRO de la couche mince synthétisée par PVD. Pour un potentiel donné, le courant de la RRO de Pd3Co1/C et Pd2Co1/C étaient presque les mêmes. Ensuite ce courant diminue de Pd5Co1/C, Pd1Co1/C à Pd/C. Les résultats ont aussi montré que les catalyseurs à base de Pd ont autour de 45mV/dec de pente de Tafel et de courant d’échange normalisée à la masse de Pd qui est égale à 10-11 mA mg-1. Il est supposé que la cinétique de RRO dans cette région de potentiel est la même pour Pd et les catalyseurs à base de Pd-Co¸ et l'ajout de Co dans le Pd a peu d’effet sur la cinétique de RRO. Nous concluons que la méthode de réaction de pulvérisation à ultrasons avec transducteur piézoélectrique est applicable pour la synthèse du catalyseur à alliage binaire de Pd-Co ou de Pt. L’effet de l’addition de Co au Pd pour former l’alliage Pd-Co sur l’amélioration de l'activité électro-catalytique de la RRO par rapport à celle de RRO sur Pd seul a été confirmé. Cette étude contribue à l'avancement des connaissances sur les propriétés électro catalytiques fondamentales de la RRO sur les alliages de Pd-Co. En particulier la relation entre la composition de l’alliage en Co et ses activités électro-catalytiques pour la RRO a été établie et une composition optimum de l’alliage qui conduit à une meilleure activité de la RRO a été déduite. Des corrélations ont aussi étudiés réalisées entre la RRO et la quantité de charge liée à viii l’adsorption/désorption de l’hydrogène et la formation et à la réduction de l’oxyde sur l’électro-catalyseur. En outre, la méthode de réaction de pulvérisation à ultrasons, développée dans cette étude, contribue de manière significative et intéressante du point de vue industrielle à un processus intégré de préparation des catalyseurs (alliages de Pd-Co ou Pt) et de fabrication d’électrodes à diffusion de gaz (GDE) catalysées pour les piles PMFC. En plus cette méthode de synthèse mise au point dans ce travail a permis de disposer de catalyseurs ayant des particules de petite taille (φ2-4nm) en alliage Pd-Co ou de Pt qu’il n’a pas été possible d’obtenir de manière homogène et reproductible par les autres méthodes conventionnelles de préparation. Ces résultats sont d’une grade importance pour le développent commercial de la technologie des piles PEMFC parce qu’en plus de permettent l’obtention de particules de tailles optimisées, ils permettent de réduire les étapes de fabrication des assemblages membranes électrodes (MEA) comme par exemple l’étape de mise en forme du catalyseur de la poudre à la pâte et son étalement sur le GDE dans le cas des méthodes conventionnelles. Alors que dans l’approche de ce travail c’est un dépôt direct du catalyseur sur le GDE au fur et à mesure qu’il est fabriqué. En plus, notre approche permet de réduire leur coût de fabrication et augmente la fiabilité du MEA suite à la réduction des étapes de sa fabrication. ---------- The Oxygen Reduction Reaction (ORR) activity in acid medium on Pd-Co was studied in this work. The catalysts were synthesized by two techniques; physical vapor deposition technique and ultrasonic spray reaction technique. The last technique was developed for the first time in our laboratory for the supported electro catalyst preparation and direct deposition onto the carbon paper or gas diffusion electrode the for PEMFC applications. The electrochemical properties such as the amount of hydrogen adsorption/desorption, the oxide formation/reduction of Pd-Co alloy catalyst have not been sufficiently studied before. Therefore these electrochemical properties were investigated by using the Pd-Co thin films prepared by sputtering method. A thin film catalyst cannot be directly used as an electrode of working PEMFCs, however the sputtering method is very useful since the chemical composition of alloy and surface area of the electrode can be controlled easily. Thus the fundamental electrochemical properties such as the amount of hydrogen adsorption/desorption, oxide formation/reduction and oxide reduction peak position on thin films of Pd-Co alloy, Pd and Pt catalysts were determined and their correlations to ORR catalytic activities in acid medium were studied. Enhancements of the catalytic activities for ORR by Pd-Co binary alloys were found to be in agreement with results obtained in previous studies. The electro catalytic performance of the ORR on the various electrodes studied here is: in the order Pd16Co84 < Pd42Co58 < Pd < Pd50Co50 < Pd75Co25 < Pd65Co35. This result clearly shows that alloying Pd with a specific composition of Co enhances significantly the electro-catalytic properties of the ORR on Pd-Co alloys in comparison to Pd alone. Pd65Co35 alloy exhibited the highest ORR activity followed by Pd75Co25 and Pd50Co50 alloys. No obvious activity enhancements were found for Pd42Co58 and Pd16Co84, which have higher Co content. A linear correlation between hydrogen charges and ORR catalytic activities on Pd-Co alloys was obtained. Catalysts having more Co content have lower ORR activities. With regard to the correlation between the amount of oxide formation/reduction and ORR activities, no linier correlation was found. However the results of this work indicated that the optimized atomic ratio between Pd and Co for ORR in acid medium was found to be around Pd:Co = 3:1. Ultrasonic spray reaction method was developed for the first time in our laboratory for carbon supported nano-scale catalyst for PEMFC application. Fine catalyst particles supported on high surface area carbon powder are required to apply the catalyst as the PEMFC cathode materials for x the commercialization, but none of the studies done before were able to successfully obtain the Pd-Co fine particles which are comparable with the existing carbon supported platinum catalyst (φ2-4nm). Therefore the establishment of the catalyst synthesis method for Pd-Co fine particles are required to use the catalyst for PEMFCs. By using this method established in this study, carbon supported Pt, Pd and Pd-Co catalysts were synthesized and characterised for ORR activity. TEM images indicate that this technique is very useful for preparing carbon supported nano-scale catalysts having the dominant particle size of 2.5-4.5 nm. XRD showed diffraction peaks consistent with face-centered cubic (fcc) structure for Pt. XRD of the synthesized Pd and Pd-Co catalysts by ultrasonic spray reaction also indicated fcc crystal system. All diffractograms of the samples are similar to the structure of fcc Pd, but the Co-origin peaks cannot be found. The Pd-Co system is substitutional solid solutions where some Pd atoms are replaced by Co atoms. The solid solution system is considered to be stable in acid media [1]. We developed this new technique of the ultrasonic spray reaction method not only for nano-scale carbon supported catalyst synthesis but also fabricate directly catalyzed GDE. By using carbon paper as a filter of ultrasonic spray reaction method as described above, carbon supported catalyst are directly deposited onto the carbon paper. It directly forms catalyzed GDE. This technique saves some step of the GDE fabrication and will leads lower cost of PEMFC. Catalytic activity enhancements of ORR for the synthesized Pd-Co catalysts by ultrasonic spray reaction method are confirmed. This result has good agreement with the ORR activity enhancement of the thin film synthesized by PVD. For a given potential, the ORR current of Pd3Co1/C and Pd2Co1/C were almost the same and show the highest values among the values of all the other alloys. After, the value of the current decreases from Pd5Co1/C to Pd1Co1/C and Pd/C. All Pd based catalysts have around 45mV/dec of tafel slope and Pd mass corrected exchange current around 10-11 mA mg-1. It is assumed that the ORR kinetics in this potential region are the same among the Pd and Pd based catalysts and the addition of Co into Pd have small effect on the ORR kinetics. We conclude that ultrasonic spray reaction method with piezoelectric transducer is applicable for Pd-Co binary alloy catalyst synthesis and the activity enhancement effect caused by alloying with Co was confirmed on the synthesized catalyst by this technique. xi This study contributes to the advancement of the knowledge of fundamental properties of Pd-Co alloy catalysts and their correlations to the oxygen reduction reaction activities. In addition, the ultrasonic spray reaction method developed for the first time in this study introduces a new easy and low cost Membrane Electrode Assembly preparation significantly to an industrial fabrication process of method of preparation of catalyzed gas diffusion electrodes for PEMFCs. The synthesis method of small particle size (φ2-4nm) Pd-Co alloy catalysts were not possible to be obtained by using the existing conventional methods. However this work has achieved the synthesis of the fine particle catalysts supported on carbons by using the ultrasonic spray reaction method. These results will contribute significantly in the development of PEMFCs based on Pd-Co alloy catalysts in the future

Design and synthesis of amidine-type peptide bond isosteres: application of nitrile oxide derivatives as active ester equivalents in peptide and peptidomimetics synthesis.

Amidine-type peptide bond isosteres were designed based on the substitution of the peptide bond carbonyl (C=O) group with an imino (C=NH) group. The positively-charged property of the isosteric part resembles a reduced amide-type peptidomimetic. The peptidyl amidine units were synthesized by the reduction of a key amidoxime (N-hydroxyamidine) precursor, which was prepared from nitrile oxide components as an aminoacyl or peptidyl equivalent. This nitrile oxide-mediated C-N bond formation was also used for peptide macrocyclization, in which the amidoxime group was converted to peptide bonds under mild acidic conditions. Syntheses of the cyclic RGD peptide and a peptidomimetic using both approaches, and their inhibitory activity against integrin-mediated cell attachment, are presented

L type Ca2+ channel blockers prevent oxaliplatin-induced cold hyperalgesia and TRPM8 overexpression in rats

<p>Abstract</p> <p>Background</p> <p>Oxaliplatin is an important drug used in the treatment of colorectal cancer. However, it frequently causes severe acute and chronic peripheral neuropathies. We recently reported that repeated administration of oxaliplatin induced cold hyperalgesia in the early phase and mechanical allodynia in the late phase in rats, and that oxalate derived from oxaliplatin is involved in the cold hyperalgesia. In the present study, we examined the effects of Ca²⁺channel blockers on oxaliplatin-induced cold hyperalgesia in rats.</p> <p>Methods</p> <p>Cold hyperalgesia was assessed by the acetone test. Oxaliplatin (4 mg/kg), sodium oxalate (1.3 mg/kg) or vehicle was injected i.p. on days 1 and 2. Ca²⁺(diltiazem, nifedipine and ethosuximide) and Na⁺(mexiletine) channel blockers were administered p.o. simultaneously with oxaliplatin or oxalate on days 1 and 2.</p> <p>Results</p> <p>Oxaliplatin (4 mg/kg) induced cold hyperalgesia and increased in the transient receptor potential melastatin 8 (TRPM8) mRNA levels in the dorsal root ganglia (DRG). Furthermore, oxalate (1.3 mg/kg) significantly induced the increase in TRPM8 protein in the DRG. Treatment with oxaliplatin and oxalate (500 μM for each) also increased the TRPM8 mRNA levels and induced Ca²⁺influx and nuclear factor of activated T-cell (NFAT) nuclear translocation in cultured DRG cells. These changes induced by oxalate were inhibited by nifedipine, diltiazem and mexiletine. Interestingly, co-administration with nifedipine, diltiazem or mexiletine prevented the oxaliplatin-induced cold hyperalgesia and increase in the TRPM8 mRNA levels in the DRG.</p> <p>Conclusions</p> <p>These data suggest that the L type Ca²⁺channels/NFAT/TRPM8 pathway is a downstream mediator for oxaliplatin-induced cold hyperalgesia, and that Ca²⁺channel blockers have prophylactic potential for acute neuropathy.</p

Clock mutation affects circadian regulation of circulating blood cells

BACKGROUND: Although the number of circulating immune cells is subject to high-amplitude circadian rhythms, the underlying mechanisms are not fully understood. METHODS: To determine whether intact CLOCK protein is required for the circadian changes in peripheral blood cells, we examined circulating white (WBC) and red (RBC) blood cells in homozygous Clock mutant mice. RESULTS: Daytime increases in total WBC and lymphocytes were suppressed and slightly phase-delayed along with plasma corticosterone levels in Clock mutant mice. The peak RBC rhythm was significantly reduced and phase-advanced in the Clock mutants. Anatomical examination revealed hemoglobin-rich, swollen red spleens in Clock mutant mice, suggesting RBC accumulation. CONCLUSION: Our results suggest that endogenous clock-regulated circadian corticosterone secretion from the adrenal gland is involved in the effect of a Clock mutation on daily profiles of circulating WBC. However, intact CLOCK seems unnecessary for generating the rhythm of corticosterone secretion in mice. Our results also suggest that CLOCK is involved in discharge of RBC from the spleen

Bioorganic synthesis of a recombinant HIV-1 fusion inhibitor, SC35EK, with an N-terminal pyroglutamate capping group.

The bioorganic synthesis of an end-capped anti-HIV peptide from a recombinant protein was investigated. Cyanogen bromide-mediated cleavage of two Met-Gln sites across the target anti-HIV sequence generated an HIV-1 fusion inhibitor (SC35EK) analog bearing an N-terminal pyroglutamate (pGlu) residue and a C-terminal homoserine lactone (Hsl) residue. The end-capped peptide, pGlu-SC35EK-Hsl, had similar bioactivity and biophysical properties to the parent peptide, and an improved resistance to peptidase-mediated degradation was observed compared with the non-end-capped peptide obtained using standard recombinant technology

Quantification of the chemical reactivity of molten nitrate salts with heat treatable aluminum alloys

This work explores the conditions for safe heat treatment of aluminum alloys containing lithium and magnesium in molten sodium nitrate (NaNO3) bath furnaces, and conditions where industrial accidents may occur. Using calorimetry coupled to classical thermodynamics, the strength of classical thermodynamics when analyzing thermal curves was demonstrated through a series of small-scale thermal analyses of various aluminum alloys in contact with sodium nitrate. This system was selected to illustrate reactions that may lead to severe and violent heat effect phenomena. Using idealized binary alloys, severe oxidation of magnesium- and lithium-rich aluminum alloy samples were shown to occur near 500 °C, a temperature range dangerously close to the operating temperature of solution heat treatment furnaces in manufacturing processes of heat treatable aluminum alloy sheets used in the aerospace industry. Commercial aluminum alloys AW257, 2198, 2024, and 1050 were also assessed with the same tools. The temperature that needed to be reached for these commercial aluminum alloys to react with molten sodium nitrate was significantly higher than the normal operating temperature of a conventional solution heat treatment furnace

On the application of the factsage thermochemical software and databases in materials science and pyrometallurgy

ABSTRACT: The discovery of new metallic materials is of prime importance for the development of new technologies in many fields such as electronics, aerial and ground transportation as well as construction. These materials require metals which are obtained from various pyrometallurgical processes. Moreover, these materials need to be synthesized under extreme conditions of temperature where liquid solutions are produced and need to be contained. The design and optimization of all these pyrometallurgical processes is a key factor in this development. We present several examples in which computational thermochemistry is used to simulate complex pyrometallurgical processes including the Hall–Heroult process (Al production), the PTVI process (Ni production), and the steel deoxidation from an overall mass balance and energy balance perspective. We also show how computational thermochemistry can assist in the material selection in these extreme operation conditions to select refractory materials in contact with metallic melts. The FactSage thermochemical software and its specialized databases are used to perform these simulations which are proven here to match available data found in the literature

Development of a Highly Sensitive Method for the Detection of Cryptosporidium parvum Virus Type 1 (CSpV1)

Poster Session

Automated Generation of Radiologic Descriptions on Brain Volume Changes From T1-Weighted MR Images: Initial Assessment of Feasibility

Purpose: To examine the feasibility and potential difficulties of automatically generating radiologic reports (RRs) to articulate the clinically important features of brain magnetic resonance (MR) images.Materials and Methods: We focused on examining brain atrophy by using magnetization-prepared rapid gradient-echo (MPRAGE) images. The technology was based on multi-atlas whole-brain segmentation that identified 283 structures, from which larger superstructures were created to represent the anatomic units most frequently used in RRs. Through two layers of data-reduction filters, based on anatomic and clinical knowledge, raw images (~10 MB) were converted to a few kilobytes of human-readable sentences. The tool was applied to images from 92 patients with memory problems, and the results were compared to RRs independently produced by three experienced radiologists. The mechanisms of disagreement were investigated to understand where machine–human interface succeeded or failed.Results: The automatically generated sentences had low sensitivity (mean: 24.5%) and precision (mean: 24.9%) values; these were significantly lower than the inter-rater sensitivity (mean: 32.7%) and precision (mean: 32.2%) of the radiologists. The causes of disagreement were divided into six error categories: mismatch of anatomic definitions (7.2 ± 9.3%), data-reduction errors (11.4 ± 3.9%), translator errors (3.1 ± 3.1%), difference in the spatial extent of used anatomic terms (8.3 ± 6.7%), segmentation quality (9.8 ± 2.0%), and threshold for sentence-triggering (60.2 ± 16.3%).Conclusion: These error mechanisms raise interesting questions about the potential of automated report generation and the quality of image reading by humans. The most significant discrepancy between the human and automatically generated RRs was caused by the sentence-triggering threshold (the degree of abnormality), which was fixed to z-score &gt;2.0 for the automated generation, while the thresholds by radiologists varied among different anatomical structures

Synthesis and characterization of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics: Understanding the stability of silicon substitution

ABSTRACT: (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ are phases that may form in aerospace and automotive aluminium alloys. The substitution of Zr/Ti in these solid solutions is widely reported in the literature; however, it remains relatively unexplored for Si. In this work, in situ precipitation of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics was performed using Al-Si-Zr-Ti alloys. The precipitation, sedimentation and concentration of numerous intermetallic particles were accomplished by filtrating the residual molten aluminium using a temperature/pressure-controlled vessel adapted with a PoDFA filter. A combination of SEM, TEM, XRD and EMP analysis allowed the identification of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics concentrated within α-FCC matrices of non-Si-doped (sample S2) and Si-doped (samples S4 and S6) alloys. EDS analysis confirmed that Zr and Ti substitute each other in the D0₂₂ and D0₂₃ phases, whereas Si substitutes in Al sites. Acceptance of Si inside the D0₂₃ phase was not expected according to FTlite (FactSage) and TCAL7 (Thermo-Calc) databases. Additionally, Si was found to enhance the formation of (Al,Si)₃(Zr,Ti)-D0₂₂ intermetallics with high Zr-content, contrary to FactSage 7.3 predictions. TEM results showed intermetallic/FCC crystal coherency for samples S2 and S6, implying that these intermetallics acted as nucleation sites for the Al-phase due to their small lattice mismatch. Furthermore, Si site occupancy was calculated for both (Al,Si)Ti-D0₂₂ and (Al,Si)₃Zr-D0₂₃ phases via DFT, showing that sites 2b and 4e are the most favorable for Si occupation, respectively. Finally, a thermodynamic model is derived to describe Si substitution upon solidification. Experimental and numerical examinations indicate that Si substitution preferentially occurs in the D0₂₂ intermetallics compared to the D0₂₃ phase