Methanol and water compete for the same adsorption sites on In₂O₃(111)

Abstract

Indium oxide, a transparent conductive oxide, has been discovered to be a highly selective catalyst for methanol synthesis by CO2 hydrogenation. The large surface unit cell of In2O3(111) exhibits a variety of Lewis acid and base sites for methanol and water adsorption. We compare the adsorption of both molecules under UHV conditions by employing AFM, STM, XPS, and TPD, together with DFT calculations. For medium coverages up to 9 molecules per surface unit cell we find a similar behavior: Initially, at room temperature, both molecules dissociate and protonate the most reactive O sites of the unit cell. When both molecules are dosed consecutively, methanol readily replaces water, while water is less efficient to substitute methanol. Below 300 K, both methanol and water adsorb molecularly in less-favored regions of the unit cell, still occupying the same sites and thus forming the same structural motifs. However, at coverages beyond 9 molecules per unit cell significant difference are found due to the ability of water to form and receive two hydrogen bonds instead of only one in the case of methanol.