Oxidation of Ultrathin FeO(111) Grown on Pt(111): Spectroscopic Evidence for Hydroxylation

Periodic Arrays of Metal Nanoclusters on Ultrathin Fe-Oxide Films Modulated by Metal-Oxide Interactions

Rational design of highly stable and active metal catalysts requires a deep understanding of metal-support interactions at the atomic scale. Here, ultrathin films of FeO and FeO_2-x grown on Pt(111) are used as templates for the construction of well-defined metal nanoclusters. Periodic arrays of Cu clusters in the form of monomers and trimers are preferentially located at FCC domains of FeO/Pt(111) surface, while the selective location of Cu clusters at FeO₂ domains is observed on FeO_2-x /Pt(111) surface. The preferential nucleation and formation of well-ordered Cu clusters are driven by different interactions of Cu with the Fe oxide domains in the sequence of FeO₂-FCC > FeO-FCC > FeO-HCP > FeO-TOP, which is further validated by density functional theory calculations. It has been revealed that the p-band center as a reactivity descriptor of surface O atoms determines the interaction between metal adatoms and Fe oxides. The modulated metal-oxide interaction provides guidance for the rational design of supported single-atom and nanocluster catalysts.

Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy

We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O₂, with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O₂-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co₃O₄ under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.

Water dissociation on mixed Co-Fe oxide bilayer nanoislands on Au(111)

We investigate the hydroxylation behaviour of mixed Co-Fe oxide nanoislands synthesized on a Au(111) surface under exposure to water vapour at vacuum conditions. The pure Co and Fe bilayer oxides both become hydroxylated by water exposure in vacuum conditions, albeit to a very different extent. It is however an open question how mixed oxides, exposing sites with a mixed coordination to Fe and Co, behave. By forming surface O species with a mixed Fe/Co coordination, we can investigate the nature of such sites. By means of scanning tunnelling microscopy and x-ray photoelectron spectroscopy, we characterize a series of Co-Fe oxides samples with different Fe contents at the atomic scale and observe a scaling of the hydroxylation degree with the amount of Fe inside the Co-Fe oxides. Our results indicate that the Fe dopants within the Co-Fe oxides have opposing effects on edge and basal plane sites modifying the maximum hydroxylation degree of pure cobalt oxide, perturbing the original binding sites of H, releasing the absorbed H or blocking the diffusion pathway of H.

Synergism of Iron and Platinum Species for Low-Temperature CO Oxidation: From Two-Dimensional Surface to Nanoparticle and Single-Atom Catalysts

CO oxidation is one of the most studied reactions in heterogeneous catalysis. It is present in air cleaning and automotive emission control. It also participates in the removal of CO from streams of hydrogen used in fuel cells. Because of the competitive adsorption of CO and O₂ over active sites, the use of Pt-based catalysts for low-temperature CO oxidation remains a challenge. Recently, great progress has been made with catalysts containing Pt-Fe species because of the contribution of Fe species to O₂ activation. The structure-activity relationship and reaction mechanisms have been investigated with various Pt-Fe catalysts. In this Perspective, we give a summary of the recent advances of low-temperature CO oxidation over Pt-Fe catalysts with a focus on the synergistic effect of Pt and Fe species in the CO and O₂ activation of catalytic reactions. Future prospects for the preparation of highly effective Pt-Fe catalysts are also proposed.

2D oxides on metal materials: concepts, status, and perspectives

Two-dimensional oxide-on-metal materials: concepts, methods, and link to technological applications, with 5 subtopics: structural motifs, robustness, catalysis, ternaries, and nanopatterning.

A mini review of in situ near-ambient pressure XPS studies on non-noble, late transition metal catalysts

The rich surface chemistry of Fe, Co, Ni and Cu during heterogeneous catalytic reactions from the perspective of NAP-XPS studies.

Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

Model systems are very important to identify the working principles of real catalysts, and to develop concepts that can be used in the design of new catalytic materials.

The Surface Science of Catalysis and More, Using Ultrathin Oxide Films as Templates: A Perspective

Surface science has had a major influence on the understanding of processes at surfaces relevant to catalysis. Real catalysts are complex materials, and in order to approach an understanding at the atomic level, it is necessary in a first step to drastically reduce complexity and then systematically increase it again in order to capture the various structural and electronic factors important for the function of the real catalytic material. The use of thin oxide films as templates to mimic three-dimensional supports as such or for metal particles as well as to model charge barriers turns out to be appropriate to approach an understanding of metal-support interactions. Thin oxide films also exhibit properties in their own right that turn out to be relevant in catalysis. Thin oxide film formation may also be used to create unique two-dimensional materials. The present perspective introduces the subject using case studies and indicates possible routes to further apply this approach successfully.