CsOx Nanostructures on Au(111): Morphology- and Size-dependent Activity for the Water–Gas Shift Reaction

Alkali oxides are typically used as promoters of heterogeneous catalysts for the water–gas shift (WGS; H2O + CO → H2 + CO2) reaction. On Au(111), CsOx exhibits diverse nanostructures at varying coverages, as revealed by scanning tunneling microscopy. Clusters of cesium oxide (Cs2O2) nucleate at elbow sites of the Au(111) herringbone when θCs is less than 0.1 ML. Subsequently, these clusters transform into two-dimensional (2D) islands (Cs2O, Cs2O2, CsO2) as the cesium coverage increases (θCs > 0.1 ML). Both types of CsOx nanostructures enable the WGS process on Au(111). The highest activity was seen for the cesium oxide clusters which facilitated the partial dissociation of water and binding of CO. The COads and OHads groups were not strongly bound and probably reacted to yield a short-lived HOCO intermediate that led to gaseous H2 and CO2. The 2D islands of CsOx also enabled the WGS but their efficiency was reduced due to the formation of cesium hydroxide compounds (limiting mobility of OH groups) and the generation of CO3 and C species (blocking of active centers). The fact that the performance of the CsOx/Au(111) catalysts changed dramatically with variations in the chemical properties of the CsOx nanostructures indicates that the alkali oxide was an integral part of the active phase, playing a central role in the activation and conversion of the reactants. To attach the label of "promoter" to CsOx is a simplification that does not help in the design and optimization of catalysts for C1 chemistry. To achieve a rational design, one must consider the structural and chemical properties of the alkali oxide.