Water-Induced Structural Evolution of LaTMSi Ternary Intermetallic Electrides

Abstract

Ternary intermetallic electrides (LaTMSi) are a class of electride materials that show excellent catalytic performance in hydrogenation reactions due to their unique geometric and electronic structures. Compared with conventional electrides, such as C12A7: e–, LaTMSi show higher structural stability of bulk phase in air and water. However, their catalytic activities decrease gradually in reaction systems with H2O as the product. In the previous literatures, the deactivation behavior was attributed to the fact that H atoms enter into the LaTMSi lattice, occupy the electron sites, and lower the electron-donating ability. As a solution, the catalytic activities were supposed to be recovered by calcinating LaTMSi at high temperatures. Herein, under a H2O-containing atmosphere, TEM observations reveal that an amorphous shell with a thickness of ∼3 nm can form on the surface of the LaCu0.67Si1.33 phase even at room temperature. This amorphous layer is relatively stable at room temperature and can prevent the bulk phase from further structural evolution. However, the amorphous layer will undergo peeling off at a high temperature, thus resulting in a new catalytic active surface. Therefore, high-temperature calcination is an effective method to strip away the deactivated surface on LaTMSi which was caused by occasional exposures to H2O. However, for H2O-containing catalytic systems, frequently alternating reactions with H2O and calcination will lead to complete phase transition from LaTMSi to the corresponding oxides. Taking the LaCu0.67Si1.33 sample as an example, X-ray powder diffraction proves the formation of the CuO2 phase at 500 °C under a H2O atmosphere. Therefore, LaTMSi materials should not be applied as catalysts in H2O-containing systems. In addition, the facile H2O dissociation over the LaCu0.67Si1.33 phase is verified by using various experimental approaches (e.g., cyclic H218O pulse-TPD, high-temperature H218O pulse-TPSR, and IR) and theoretical calculations. Moreover, similar surface dissociation of H2O and phase transition are also observed over LaCoSi and LaNiSi ternary intermetallic electrides.