Comparative Analysis of Cobalt Oxide Nanoisland Stability and Edge Structures on Three Related Noble Metal Surfaces: Au(111), Pt(111) and Ag(111)

Elucidating the active phases of CoOx films on Au(111) in the CO oxidation reaction

Noble metals supported on reducible oxides, like CoOx and TiOx, exhibit superior activity in many chemical reactions, but the origin of the increased activity is not well understood. To answer this question we studied thin films of CoOx supported on an Au(111) single crystal surface as a model for the CO oxidation reaction. We show that three reaction regimes exist in response to chemical and topographic restructuring of the CoOx catalyst as a function of reactant gas phase CO/O2 stoichiometry and temperature. Under oxygen-lean conditions and moderate temperatures (≤150 °C), partially oxidized films (CoOx<1) containing Co0 were found to be efficient catalysts. In contrast, stoichiometric CoO films containing only Co2+ form carbonates in the presence of CO that poison the reaction below 300 °C. Under oxygen-rich conditions a more oxidized catalyst phase (CoOx>1) forms containing Co3+ species that are effective in a wide temperature range. Resonant photoemission spectroscopy (ResPES) revealed the unique role of Co3+ sites in catalyzing the CO oxidation. Density function theory (DFT) calculations provided deeper insights into the pathway and free energy barriers for the reactions on these oxide phases. These findings in this work highlight the versatility of catalysts and their evolution to form different active phases, both topological and chemically, in response to reaction conditions exposing a new paradigm in the catalyst structure during operation.

Stabilizing Oxide Nanolayer via Interface Confinement and Surface Hydroxylation

Surface hydroxylation over oxide catalysts often occurs in many catalytic processes involving H₂ and H₂O, which is considered to play an important role in elementary steps of the reactions. Here, monolayer CoO and CoOH_x nanoislands on Pt(111) are used as inverse model catalysts to study the effect of surface hydroxylation on the stability of Co oxide overlayers in O₂. Surface science experiments indicate that hydroxyl groups formed on CoO nanoislands produced by deuterium-spillover can enhance oxidation resistance of the Co oxide nanostructures. Theoretical calculation shows that the interfacial adhesion between CoO and Pt is linearly strengthened with the increasing hydroxylation degree of CoO surface. Thus, the interface confinement effect between CoO and Pt can be enhanced by the surface hydroxylation due to the more reduced Co ions and stronger Co-Pt bonding at the CoOH_x/Pt interface.

The cobalt oxidation state in preferential CO oxidation on CoOx/Pt(111) investigated by operando X-ray photoemission spectroscopy

The combination of a reducible transition metal oxide and a noble metal such as Pt often leads to active low-temperature catalysts for the preferential oxidation of CO in excess H₂ gas (PROX reaction). While CO oxidation has been investigated for such systems in model studies, the added influence of hydrogen gas, representative of PROX, remains less explored. Herein, we use ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy on a CoO_x/Pt(111) planar model catalyst to analyze the active phase and the adsorbed species at the CoO_x/Pt(111) interface under atmospheres of CO and O₂ with a varying partial pressure of H₂ gas. By following the evolution of the Co oxidation state as the catalyst is brought to a reaction temperature of above 150 °C, we determine that the active state is characterized by the transformation from planar CoO with Co in the 2+ state to a mixed Co²⁺/Co³⁺ phase at the temperature where CO₂ production is first observed. Furthermore, our spectroscopy observations of the surface species suggest a reaction pathway for CO oxidation, proceeding from CO exclusively adsorbed on Co²⁺ sites reacting with the lattice O from the oxide. Under steady state CO oxidation conditions (CO/O₂), the mixed oxide phase is replenished from oxygen incorporating into cobalt oxide nanoislands. In CO/O₂/H₂, however, the onset of the active Co²⁺/Co³⁺ phase formation is surprisingly sensitive to the H₂ pressure, which we explain by the formation of several possible hydroxylated intermediate phases that expose both Co²⁺ and Co³⁺. This variation, however, has no influence on the temperature where CO oxidation is observed. Our study points to the general importance of a dynamic reducibility window of cobalt oxide, which is influenced by hydroxylation, and the bonding strength of CO to the reduced oxide phase as important parameters for the activity of the system.

Electrically Tunable Reactivity of Substrate-Supported Cobalt Oxide Nanocrystals

First-row transition metal oxides are promising materials for catalyzing the oxygen evolution reaction. Surface sensitive techniques provide a unique perspective allowing the study of the structure, adsorption sites, and reactivity of catalysts at the atomic scale, which furnishes rationalization and improves the design of highly efficient catalytic materials. Here, a scanning probe microscopy study complemented by density functional theory on the structural and electronic properties of CoO nanoislands grown on Au(111) is reported. Two distinct phases are observed: The most extended displays a Moiré pattern (α-region), while the less abundant is 1Co:1Au coincidental (β-region). As a result of the surface registry, in the β-region the oxide adlayer is compressed by 9%, increasing the unoccupied local density of states and enhancing the selective water adsorption at low temperature through a cobalt inversion mechanism. Tip-induced voltage pulses irreversibly transform α- into β-regions, thus opening avenues to modify the structure and reactivity of transition metal oxides by external stimuli like electric fields.

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.

The Effect of Fe Dopant Location in Co(Fe)OOHx Nanoparticles for the Oxygen Evolution Reaction

The addition of iron (Fe) can in certain cases have a strong positive effect on the activity of cobalt and nickel oxide nanoparticles in the electrocatalytic oxygen evolution reaction (OER). The reported optimal Fe dopant concentrations are, however, inconsistent, and the origin of the increased activity due to Fe dopants in mixed oxides has not been identified so far. Here, we combine density functional theory calculations, scanning tunneling microscopy, and OER activity measurements on atomically defined Fe-doped Co oxyhydroxide nanoparticles supported on a gold surface to establish the link between the activity and the Fe distribution and concentration within the oxyhydroxide phase. We find that addition of Fe results in distinct effects depending on its location on edge or basal plane sites of the oxyhydroxide nanoparticles, resulting in a nonlinear OER activity as a function of Fe content. Fe atom substitution itself does not lead to intrinsically more active OER sites than the best Co sites. Instead, the sensitivity to Fe promoter content is explained by the strong preference for Fe to locate on the most active edge sites of oxyhydroxide nanoparticles, which for low Fe concentrations stabilizes the particles but in higher concentrations leads to a shell structure with less active Fe on all edge positions. The optimal Fe content thereby becomes dependent on nanoparticle size. Our findings demonstrate that synthesis strategies that adjust not only the Fe concentration in mixed oxides but also its distribution within a catalyst nanoparticle can lead to enhanced OER performance.

A versatile electrochemical cell for hanging meniscus or flow cell measurement of planar model electrodes characterized with scanning tunneling microscopy and x-ray photoelectron spectroscopy

We demonstrate the development of a portable electrochemistry (EC) cell setup that can be applied to measure relevant electrochemical signals on planar samples in conjunction with pre- and post-characterization by surface science methods, such as scanning tunneling microscopy and x-ray photoelectron spectroscopy. The EC cell setup, including the transfer and EC cell compartments, possesses the advantage of a small size and can be integrated with standard ultra-high vacuum (UHV) systems or synchrotron end-stations by replacing the flange adaptor, sample housing, and transfer arm. It allows a direct transfer of the pre-characterized planar sample from the UHV environment to the EC cell to conduct in situ electrochemical measurements without exposing to ambient air. The EC cell setup can operate in both the hanging meniscus and flow cell mode. As a proof of concept, using a Au(111) single crystal electrode, we demonstrate the application of the EC cell setup in both modes and report on the post-EC structure and chemical surface composition as provided by scanning tunneling microscopy and x-ray photoelectron spectroscopy. To exemplify the advantage of an in situ EC cell, the EC cell performance is further compared to a corresponding experiment on a Au(111) sample measured by transfer at ambient conditions. The EC cell demonstrated here enables a wealth of future electrocatalysis measurements that combine surface science model catalyst approaches to facilitate the understanding of nano- and atomic-scale structures of electrocatalytic interfaces, the crucial role of catalyst stability, and the nature of low-concentration and atomically dispersed metal (single atom) dopants.

Simultaneous sulfidation of Mo and Co oxides supported on Au(111)

Here we present the results of a study carried out to investigate the simultaneous sulfidation of Co and Mo oxide nanoparticles on Au(111) as a synthesis strategy to prepare a model catalyst for hydrodesulfurization (HDS). We make use of scanning tunneling microscopy and X-ray photoelectron spectroscopy to track the changes in morphology and chemistry during the synthesis of a mixed Mo and Co oxide precursor and the sulfidation thereafter, to the respective sulfides. We investigated the effects of temperature and the duration of sulfidation on the completeness of the sulfidation process. Our study shows that the formation of MoS₂ with the CoMoS edge (the desired model catalyst) is not affected by the time or the temperature of sulfidation. However, the yield of the Co-promoted MoS₂ slabs is limited by the formation of large clusters due to the spreading of Mo and Co oxide phases upon sulfidation. Complete sulfidation of the mixed oxide precursor to Co-promoted MoS₂ can be accelerated by increasing the sulfidation temperature to 730 K due to the thermally activated nature of Mo oxide sulfidation. Thus, we demonstrate that using a mixed Mo and Co oxide precursor as a starting point for the Co-promoted MoS₂ phase for fundamental catalytic studies is a viable strategy.

Tuning the activities of cuprous oxide nanostructures via the oxide-metal interaction

Despite tremendous importance in catalysis, the design of oxide-metal interface has been hampered by the limited understanding of the nature of interfacial sites and the oxide-metal interaction (OMI). Through construction of well-defined Cu₂O/Pt, Cu₂O/Ag and Cu₂O/Au interfaces, we find that Cu₂O nanostructures (NSs) on Pt exhibit much lower thermal stability than on Ag and Au, although they show the same structure. The activities of these interfaces are compared for CO oxidation and follow the order of Cu₂O/Pt > Cu₂O/Au > Cu₂O/Ag. OMI is found to determine the activity and stability of supported Cu₂O NSs, which could be described by the formation energy of interfacial oxygen vacancy. Further, electronic interaction between Cu⁺ and metal substrates is found center to OMI, where the d band center could be used as a key descriptor. Our study provides insight for OMI and for the development of Cu-based catalysts for low temperature oxidation reactions.

The design of oxide-metal interface for heterogeneous catalysis has been hampered by the limited fundamental understanding. Here, the authors demonstrate that the activities of cuprous oxide nanostructures for CO oxidation can be tuned via the oxide-metal (Cu2O/M, M = Pt, Ag, Au) interaction.

Reversible oxidation and reduction of gold-supported iron oxide islands at room temperature

Monolayer iron oxides grown on metal substrates have widely been used as model systems in heterogeneous catalysis. By means of ambient-pressure scanning tunneling microscopy (AP-STM), we studied the in situ oxidation and reduction of FeO(111) grown on Au(111) by oxygen (O₂) and carbon monoxide (CO), respectively. Oxygen dislocation lines present on FeO islands are highly active for O₂ dissociation. X-ray photoelectron spectroscopy measurements distinctly reveal the reversible oxidation and reduction of FeO islands after sequential exposure to O₂ and CO. Our AP-STM results show that excess O atoms can be further incorporated on dislocation lines and react with CO, whereas the CO is not strong enough to reduce the FeO supported on Au(111) that is essential to retain the activity of oxygen dislocation lines.

Prediction of Stable and Active (Oxy-Hydro) Oxide Nanoislands on Noble-Metal Supports for Electrochemical Oxygen Reduction Reaction

Developing cost-effective oxygen electrocatalysts with high activity and stability is key to their commercialization. However, economical earth-abundant catalysts based on first-row transition-metal oxides suffer from low electrochemical stability, which is difficult to improve without compromising their activity. Here, using density functional theory calculations, we demonstrate that noble-metal supports lead to bifunctional enhancement of both the stability and the oxygen reduction reaction (ORR) activity of metal (oxy-hydro) oxide nanoislands. We observe a significant stabilization of supported nanoislands beyond the intrinsic stability limits of bulk phases, which originates from a favorable lattice mismatch and reductive charge transfer from oxophilic supports. We discover that interfacial active sites (located between the nanoisland and the support) reinforce the binding strength of reaction intermediates, hence boosting ORR activity. Considering that both stability and activity lead to discovery of CoOOH|Pt, NiOOH|Ag, and FeO₂|Ag as viable systems for alkaline ORR, we then use a multivariant linear regression method to identify elementary descriptors for efficient screening of promising cost-effective nanoisland|support catalysts.

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

Although cubic rock salt-CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende-CoO phase is antiferromagnetic with a 3rd type structure in a face-centered cubic lattice and a Néel temperature of T_N (zinc-blende) ≈225 K. Wurtzite-CoO also presents an antiferromagnetic order, T_N (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c-axis but an incommensurate order along the y-axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic-like behavior even at room temperature.