Activating Nonreducible Oxides via Doping

Abstract

Nonreducible oxides are characterized by large band gaps and are therefore unable to exchange electrons or to form bonds with surface species, explaining their chemical inertness. The insertion of aliovalent dopants alters this situation, as new electronic states become available in the gap that may be involved in charge-transfer processes. Consequently, the adsorption and reactivity pattern of doped oxides changes with respect to their nondoped counterparts. This Account describes scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS) experiments that demonstrate the impact of dopants on the physical and chemical properties of well-defined crystalline oxide films. For this purpose, MgO and CaO as archetypical rocksalt oxides have been loaded either with high-valence (Mo, Cr) or low-valence dopants (Li). While the former generate filled states in the oxide band gap and serve as electron donors, the latter produce valence-band holes and give rise to an acceptor response. The dopant-related electronic states and their polarization effect on the surrounding host material are explored with XPS and STM spectroscopy on nonlocal and local scales. Moreover, charge-compensating defects were found to develop in the oxide lattice, such as Ca and O vacancies in Mo- and Li-doped CaO films, respectively. These native defects are able to trap the excess charges of the impurities and therefore diminish the desired doping effect. If noncompensated dopants reside in the host lattice, electron exchange with surface species is observed. Mo ions in CaO, for example, were found to donate electrons to surface Au atoms. The anionic Au strongly binds to the CaO surface and nucleates in the form of monolayer islands, in contrast to the 3D growth prevailing on pristine oxides. Charge transfer is also revealed for surface O2 that traps one Mo electron by forming a superoxo-species. The activated oxygen is characterized by a reinforced binding to the surface, an elongated O-O bond length, and a reduced barrier for dissociation, and represents an important intermediate for oxidation reactions. The charge-transfer processes described here are quenched if Li is inserted into the oxide lattice, neutralizing the effect of the extra electrons. The specific behavior of doped oxides has been explored on a mechanistic level, i.e. on thin-film model systems at ultrahigh vacuum and low temperature. We believe, however, that our results are transferrable to realistic conditions and doping might thus develop into a powerful method to improve the performance of nonreducible oxides in surface-catalyzed reactions.

Inverse Oxide/Metal Catalysts in Fundamental Studies and Practical Applications: A Perspective of Recent Developments

“…For example, in the case of Cu/ZnO catalysts for methanol synthesis, the doping of the ZnO support with Al 3+ and Ga 3+ cations facilitates the formation of an inverse ZnO/Cu catalyst (Figure 16) and eventually enhances the catalytic activity 22. In inverse oxide/metal systems, there may be situations in which there is not a sharp oxide-metal interface and atoms of the metal substrate migrate into the lattice of the oxide overlayer 71. This migration, or doping, can also be used to enhance the reactivity of the oxide overlayer 71.…”

“…In inverse oxide/metal systems, there may be situations in which there is not a sharp oxide-metal interface and atoms of the metal substrate migrate into the lattice of the oxide overlayer 71. This migration, or doping, can also be used to enhance the reactivity of the oxide overlayer 71. This effect is particular useful when working with nonreducible oxides which by themselves have a tendency to a low chemical activity 71.…”

Well-Ordered In Adatoms at the In2O3(111) Surface Created by Fe Deposition

“…For relatively noble metals (e.g., the coinage elements Ag, Au, and Pt, Pd) deposition at ambient temperatures usually results in aggregation into small nanoclusters that may nucleate at special sites such as surface defects. While some charge transfer between the overlayer and the metal oxide does occur for these elements (and can lead to interesting effects [1,2]), the substrate itself is generally not affected. This situation changes when the deposited metal has a high affinity for oxygen; it can react with the surface, which leads to a re-arrangement of the local atomic structure.…”

Characterization of O−-Centers on Single Crystalline MgO(001)-Films

“…The recent literature provides numerous examples, which illustrate the importance of doping for different aspects of the surface chemistry of oxides both from a theoretical as well as an experimental point of view (e.g. [34,[72][73][74]). …”