In the torr pressure ranges oxygen-driven structural changes of Ni(111) with scanning tunneling microscopy

Abstract

Extensive studies have been conducted on conventional heterogeneous catalysts to improve their catalytic activity and selectivity. In an effort to reduce the high cost of traditional noble metal catalysts (Pt, Rh, Ru, and Pd), transition metal such as nickel (Ni) has been commonly utilized as an alternative for industrial and fundamental chemical reactions (e.g., CO oxidation). The role of surface oxides on metal surface has long been controversial as a critical factor to determine catalytic activity. As the practical Ni catalysts easily goes through facile oxidation at atmospheric conditions, oxide species on metal surface would participate in actual chemical reactions during catalysis. Therefore, it is important to demonstrate oxygen-driven structural changes of Ni(111) surface in ambient pressure of Torr ranges and how the presence of surface oxide is correlated with enhanced reactivity in certain reaction conditions. In this study, we observed notable difference between irreversibly grown oxide species and active surface oxides on a Ni(111) surface under CO oxidation conditions using ambient-pressure scanning tunneling microscopy (AP-STM) at room temperature. Followed by irreversible growth of grain size clusters (NiO, $Ni_2O_3$, and $NiO_x$) on Ni(111) surface, large randomly-shaped clusters (active oxide species) appeared and gradually covered the oxygen-saturated steps and terraces above few hundreds of mTorr $O_2$ pressure range. The cluster formation under ambient pressure of O2 is a reversible oxidation process. Ni(111) surface was further investigated in CO oxidation condition. Ni atoms are likely to be pulled out from surface during consumption of the surface oxides by incoming CO molecules (reduction process). Such transition or segregation to other surface structures is barely shown in irreversible oxygen-saturated Ni surface. We will discuss the influence of chemical potentials on surface morphology and presence of surface oxides consistent with catalytic reactivity.