Intermetallic compounds with their peculiar combination of crystal and electronic structures are perfect candidates for the development of novel catalytic materials for heterogeneous catalysis with improved properties. Investigation of phase equilibria, conditions of the intermetallic compounds and solid solutions formation in the system Ga-M-T, especially Ga-M-Pd, with emphasis on application in catalysis is relevant.
Інтерметалічні сполуки з їхньою особливою комбінацією кристалічної та електронної структур є ідеальними кандидатами для розробки нових матеріалів для гетерогенного каталізу з покращеними властивостями. Дослідження фазових рівноваг, умов утворення інтерметалічних сполук і твердих розчинів у системах Ga-M-T, особливо Ga-M-Pd, з акцентом на застосуванні в каталізі є актуальним.
By means of X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy, the phase equilibria in the systems Al-Ga-Pd, Ga-Sn-Pd and Ga-Sb-Pd were determined, and isothermal sections of the phase diagrams were constructed for the first time (Al-Ga-Pd at 600°C for ≥ 50 at.% Pd, Ga-Sn-Pd and Ga-Sb-Pd at 500°C in the whole concentration range).
In the Al-Ga-Pd system, four continuous solid solutions between isotypic binary compounds were found: Al1-xGaxPd (structure type FeSi), (Al1-xGax)3Pd5 (Rh5Ge3), Al1-xGaxPd2 (Co2Si), and (Al1-xGax)2Pd5 (Pd5Ga2), all with 0 ≤ x ≤ 1.
In the Ga-Sn-Pd system, one ternary palladide forms: Ga2+x+ySn4-xPd9 (x = 0.72, y = 0.06 from X-ray diffraction); its crystal structure is derived from the Ti2Ni type. Application of the Quantum Theory of Atoms In Molecules (QTAIM) allowed us to estimate the effective charges of the atoms. The charges obtained for the Pd atoms (-0.40, -0.60) are very close to the values obtained for Pd in GaPd (-0.50), whereas the positive charges for Ga and Sn (+0.70 and +0.85) are slightly higher than for Ga in GaPd. The distribution of the Electron Localizability Indicator (ELI) allowed detecting two-center Pd-Sn and Pd-Ga interactions. In Ga2+x+ySn4-xPd9 these are distributed isotropically, which may interfere with the formation of separate active sites on the surface, thereby reducing the catalytic activity.
For the solid solution Ga1-xSnxPd2 (0 ≤ x ≤ 1; Co2Si-type structure), the influence of the electronic structure on the catalytic properties in the reaction of acetylene semi-hydrogenation (maximum catalytic activity at the nominal composition x = 0.28) was studied. The method of Coherent Potential Approximation (CPA) was used to calculate the electronic structure of Ga1-xSnxPd2 from X-ray photoelectron spectroscopy for different values of x around the value for maximum catalytic activity. The dependence of the activity on the center of gravity of the Pd d-band shows a sharp maximum.
Three new ternary phases were discovered in the Pd-rich part of the Ga-Sb-Pd system:
Ga1-xSbxPd2 (Fe2P-type structure) with x = 0.5-0.7 at 500 °C, and two tetragonal phases: Ga0.62(3)Sb0.38(3)Pd3 (Pt3Ga), Ga0.61(3)Sb0.39(3)Pd3 (own structure type).
