Exploring Oxidation Dynamics of Pt3M (M=Ti, V) Polycrystalline Alloys with Ambient-pressure X-ray Photoelectron Spectroscopy

Abstract

In the development of heterogeneous catalysts, alloying foreign elements on host material is a common practice to improve the reactivity as well as to enhance the performance of catalysts influencing catalytic activity, stability, and selectivity. In this thesis, with both ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) and ambient-pressure scanning tunneling microscopy (AP-STM), the surface chemical/electronic dynamics of Pt-3d early transition metal alloy systems are investigated for understanding surface phenomena, surface segregation and oxidation, under catalytic reaction conditions and various oxidation conditions.

First, under oxygen rich CO oxidation conditions, the surfaces of Pt3V and Pt3Ti polycrystalline are investigated. After annealing under UHV condition, Pt3V exhibits enrichment of Pt while Pt3Ti shows mixed Pt and Ti atoms on the surface. As temperature increases, surface segregation and oxidation of the metal atoms take place under elevated pressure condition for CO oxidation, P(O2) = 0.3 mbar and P(CO) = 0.03 mbar. Under ambient pressure condition, Pt skin surface in Pt3V is no longer stable due to segregation of V atoms to the surface, displaying various forms of vanadium oxides on surface. In the case of Pt3Ti, numerous forms of Ti oxides start to appear during oxidation process. In addition, as surface temperature increases, XP spectra of Pt 4f core level show increase in sign of metallic phase of Pt state for both alloy surfaces, suggesting the redistribution of charges of Pt to 3d transition metal and oxides.

Second, we explored chemical states and morphology of Pt3V polycrystalline surface under oxygen environment at elevated temperatures. Under UHV and RT, the formation of a Pt-skin is observed, resulting in the spatial separation between Pt and V. The electronic state of the Pt-skin shows similar characteristics to that of pure Pt. After O2 introduction under RT, both of strong oxygen chemisorption and oxide nucleation occur. In this stage, the dissolution of the Pt skin also takes place, as a consequence of oxygen-induced V surface segregation. The majority of the oxides are highly defective V oxides, but a considerable amount of well-defined oxides, estimated as V2O5, begins to grow. As the temperature rises to 400 K, most chemisorbed oxygen is dissolved into the alloy to form oxides, and surface oxides with less defects appear. Since oxidation creates new V-O bonds, the alloying between Pt and V begins to dissolve, i.e. the charge redistribution between Pt and V occurs. As a result, some Pt start to show the electronic states of pure Pt. When the temperature reaches 600 K, the growth of V2O5 is saturated, indicating the slowdown of oxidation. Our fundamental research on the electronic structure/surface morphology change of Pt-early 3d transition metal under oxidative conditions can offer useful insight about the degradation and corrosion of various Pt-based alloy catalysts.