Microstructural Changes of Supported Intermetallic Nanoparticles under Reductive and Oxidative Conditions: An in Situ X-ray Absorption Study of Pd/Ga2O3

In this work, the structure and stability of Pd-Ga intermetallic nanoparticles under various reactive conditions was investigated by combining in situ X-ray absorption spectroscopy (XAS), FTIR of CO adsorption, and XRD. By in situ XAS we followed in detail the formation of Pd-Ga intermetallic compounds (IMC) upon reduction of Pd/Ga2O3, which was observed to be a rather slow process that depends on the availability of reduced Ga formed by the atomic H provided by Pd. Using crystal structures of a variety of Pd-Ga IMCs, we have identified Pd2Ga as the compound that is formed during reduction at 623 K. In contrast to Pd/Ga2O3, beta-hydride formation did not occur once Pd2Ga particles are formed, as evidenced by the absence of lattice expansion in hydrogen atmosphere. However, XAS revealed that Pd2Ga is not stable in oxygen already at room temperature. Although XRD showed no bulk structural modification, CO adsorption on an oxygen exposed catalyst detected a metallic Pd surface., partly decorated with oxidic Ga. Only in situ XAS provided clear indications on the structural modification occurring upon oxygen exposure, showing that the overall state of the sample is a mixture of Pd or a Ga-depleted IMC and Pd2Ga. Based on these observations, Ga segregation from the surface-near region to the surface, followed by oxidation, was concluded. The intermetallic surface is easily reformed by reduction, due to remaining Pd at the surface activating H-2.