Redistribution dynamics of ultrathin vanadium oxide layers under catalytic conditions and activation of diffusion by surface acoustic waves

Abstract

The reaction-induced redistribution of vanadium oxide supported on noble metal single crystal surfaces (inverse model catalysts), and the influence of surface acoustic waves (SAW) on the composition of a bimetallic Rh/Pt surface are studied. Previously, the movement and coalescence of macroscopic, two-dimensional vanadium oxide islands on Rh(111) during catalytic methanol oxidation was explained with a polymerization / depolymerization mechanism. To investigate how general this mechanism is, the reaction dynamics of vanadium oxide on Rh(111), Rh(110) and Pt(111) are investigated in a number of catalytic reactions. Island formation and coalescence are observed in ammonia, CO, and methanol oxidation on VOx/Rh(111) in the 0.0001 mbar range. Exchanging oxygen by NO as oxidizing agent results in an inverse pattern, i. e. holes in a dense vanadium oxide film instead of vanadium oxide islands surrounded by bare Rh surface. Spectroscopic LEEM reveals, that NO influences the width of the interface vanadium oxide island / bare Rh(111), thus indicating a change in the line tension. The line tension possibly explains the complementary types of pattern formation. Indications for a Rh surface oxide under reaction conditions are found. In the 0.0001 mbar range on Rh(111), oscillating vanadium oxide islands occur. A tentative mechanism is proposed, based on phase transitions inside the vanadium oxide islands, which result from gradients in the oxygen coverage. With near ambient pressure LEEM, turbulent redistribution dynamics are observed during methanol oxidation at 0.02mbar. On VOx/Rh(110) island formation occurs, but no island coalescence is seen. Instead, a wealth of chemical wave pattern is found: traveling interface modulations (TIMs), traveling wave fragments and target pattern, as well as chemical waves propagating over both, the bare Rh(110) substrate and macroscopic vanadium oxide islands. TIMs are explained by a mechanism based on the reversible creation of surface defects at the interface. The system VOx/Pt(111) is characterized by the reversible diffusion of V into the Pt bulk under reaction conditions. As a consequence, no pattern formation occurs. A strong effect of the metallic support on the behavior of VOx catalysts is demonstrated by the different types of pattern formation in VOx/Rh(111), VOx/Rh(110), and VOx/Pt(111). In addition to different types of pattern formation, also the selectivity and catalytic activity is strongly influenced by the support. Whereas formaldehyde is the main product in catalytic methanol oxidation on VOx/Rh(111), no formaldehyde production is detected on VOx/Rh(110) and VOx/Pt(111). The influence of SAWs on the diffusive intermixing of a Rh/Pt surface is investigated by laterally resolved X-ray spectroscopy. The results are compared to Auger spectroscopy measurements on the thermal diffusion of Rh into the Pt bulk on a Pt(100) single crystal and on polycrystalline Pt. At 445 K, a SAW-induced intermixing of Pt and Rh is detected. In thermal diffusion experiments, the onset of Rh diffusion into the Pt bulk is found to occur around 500 to 550 K. The experiments are a first step towards verifying the working hypothesis, that structural defects caused by SAWs are the main reason for a SAW-induced increase in catalytic activity reported in literature.