Supported metal catalysts, consisting of metal nanoparticles of a few nm in diameter supported on high-surface-area materials, mostly oxides, play a key role in heterogeneous catalysis. While initially, the support was assumed to be catalytically inert and mainly served to stabilize and separate the active metal nanoparticles, this picture changed with time. Already in the late 1970s, Tauster introduced the concept of 'strong metal -support interactions (SMSIs)', according to which the catalytic activity of the catalyst can be modified (and mainly lowered) by partial overgrowth of the metal surface by a thin layer of the partly reduced oxide. In 2012, Campbell introduced 'electronic metalsupport interactions (EMSIs)' as another fundamentally different type of metal -support interactions (MSIs), based on work by Bruix et al. In this case, the adsorption / reaction behavior of the metal surface sites was modified by charge transfer between support and metal NP (for recent overviews see ). Furthermore, it turned out that in addition to other physical parameters of the catalyst such as the size of the metal nanoparticles, the chemical potential of the metal atoms in these particles, or their oxophilicity, also the reducibility of the support plays an important if not decisive role. It allows to tune the charge transfer to the metal nanoparticle and thus their reactivity via formation of O-vacancies or even oxidation reactions to proceed via a Marsvan Krevelen reaction mechanism. Details of the charge transfer process and its impact on the adsorption and reaction characteristics as well as of their range, however, are still under debate. These aspects are topic of the present study, where we investigated O-vacancy formation and their impact on the CO adsorption characteristics on Ru/ZrO2(111) model catalysts. . Recently, we had investigated the CO2 methanation reaction on a Ru/ZrO2 catalyst in a combined experimental and theoretical approach, including also extensive density functional theory (DFT) based calculations on a Ru/ZrO2(111) model catalyst system. In that work we had demonstrated that the formation of surface O-vacancies can by enhanced and stabilized by the presence of metal nanoparticles (NPs), in this case of a Ru nanorod: The formation of a surface O-vacancy at the interface, i.e., directly underneath the Ru nanorod, was found to lead to a significant transfer of (negative) charge to neighboring Ru atom(s) at the interface and furthermore also to an increase of the CO adsorption energy on these sites. In contrast, metal atoms further apart from the O-vacancy and the interface, e.g., in the second or third layer of the three-layer Ru nanorod, were little affected by these O-vacancies. These results indicated that charge transfer from the O-vacancy to the metal is essentially limited to the metal atoms directly at the metal-oxide interface. This is in contrast to the commonly published statement of a charge transfer to the (entire) metal nanoparticle, which was concluded, e.g., from shifts of the vibrational features of the adsorbed species detected experimentally. This has considerable consequences on the fundamental understanding of the role of O-vacancies in catalytic reactions on metal catalysts supported on reducible oxides, as only a small fraction of the accessible metal surface atoms will be modified, namely those at the interface perimeter of the metal nanoparticles. The other ones, which depending on the shape and size of the NPs may often represent the majority of surface atoms, maintain their typical character as essentially charge-neutral metal atoms. Considering the close correlation between local adsorption strength and catalytic activity, which is well known as the Sabatier principle, these interface perimeter sites may thus represent (more) active sites in a catalytic reaction. Thus, the introduction of vacancies allows to tune the reactivity of the metal surface sites at the interface. While these general principles had been clarified in the previous calculations, supported also by experimental findings, several important aspects are still open. These include first the question whether the limitation to a strictly local effect is true also for O-vacancy formation, or whether O-vacancies closely aside or deeper underneath the metal nanorod can also be stabilized by the metal nanoparticle/nanorod (see Figure and). One could imagine that the formation of several O-vacancies (vacancy clusters), either aside the Ru nanorod in the topmost layer or reaching from the metal-oxide interface to deeper layers, could result in an enhanced charge transfer from the more remote O-vacancy. These points are essential for clarifying whether the formation of O-vacancies slightly off from the metal-oxide interface is energetically favorable as well, or whether in these cases the energetics are rather similar to Ovacancy formation in the absence of metal NPs. A second important question deals with the effect of O-vacancies on the CO adsorption behavior, in particular on the CO adsorption energy and the vibrational properties. The CO adsorption energy is not only highly relevant, e.g., for all catalytic reactions where it serves as descriptor for the activity of the respective catalyst.