Growth and structural studies of In/Au(111) alloys and InO/Au(111) inverse oxide/metal model catalysts.

Abstract

Indium oxide has received attention as an exciting candidate for catalyzing the CO hydrogenation to methanol due to its high selectivity (>80%). Compared to the extent of research on the activity of indium oxide-based powder catalysts, very little is known about the phenomena associated with the formation of surface alloys involving indium or the growth mechanism for indium oxide nanoparticles. In this report, scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS) were employed to elucidate the growth mode, structure, and chemical state of In/Au(111) alloys and InO/Au(111) inverse model catalysts. Our study reveals distinct morphological differences between In/Au(111) and InO/Au(111), and the InO structure also depends strongly on the preparation conditions. In/Au surface alloy systems with extremely low coverage (0.02 ML) form islands preferentially on the elbow sites of reconstructed Au(111) herringbone, regardless of hexagonally closed packed and face centered cubic stacking. At higher coverage (0.1 ML), the In islands expand over the herringbone in the ⟨110⟩ direction and create two dimensional domain structures over the entire surfaces. Moreover, this 2D domain structure is disturbed by temperature with high dispersion of indium atoms observed during the annealing process. Oxidation of the In/Au(111) surface alloys with O at 550 K produces InO/Au(111) systems which contain various sizes of InO aggregates (from 0.7 nm to 10 nm). On the other hand, InO/Au(111) surfaces prepared by vapor deposition of In at 550 K in an O background exhibit highly dispersed and uniformly small InO particles (∼1 nm). Both InO systems were confirmed to be partially oxidized by XPS.