A Monolayer Carbon Nitride on Au(111) with a High Density of Single Co Sites.

Carbon nitrides that expose atomically dispersed single-atom metals in the form of M-N-C (M = metal) sites are attractive earth-abundant catalyst materials that have been demonstrated in electrocatalytic conversion reactions. The catalytic performance is determined by the abundance of N-doped sites and the type of metal coordination to N, but challenges remain to synthesize pristine carbon nitrides with a high concentration of the most active sites and prepare homogeneously doped materials that allow for in-depth characterization of the M-N-C sites and quantitative evaluation of their catalytic performance. Herein, we have synthesized and characterized a well-defined monolayer carbon nitride phase on a Au(111) surface that exposes an exceedingly high concentration of Co-N4 sites. The crystalline monolayer carbon nitride, whose formation is controlled by an on-surface reaction between Co atoms and melamine on Au(111), is characterized by a dense array of 4- and 6-fold N-terminated pockets, whereof only the 4-fold pocket is found to be holding Co atoms. Through detailed characterization using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory modeling, we determine the atomic structure and chemical state of the carbon nitride network. Furthermore, we show that the monolayer carbon nitride structure is stable and reactive toward the electrocatalytic oxygen reduction reaction in alkaline electrolyte, with a quantitative performance metric that significantly exceeds comparable M-N-C-based catalyst types. The work demonstrates that high-density active catalytic sites can be created using common precursor materials, and the formed networks themselves offer an excellent platform for onward studies addressing the characteristics of M-N-C sites.

Steering carbon dioxide reduction toward C-C coupling using copper electrodes modified with porous molecular films.

Copper offers unique capability as catalyst for multicarbon compounds production in the electrochemical carbon dioxide reduction reaction. In lieu of conventional catalysis alloying with other elements, copper can be modified with organic molecules to regulate product distribution. Here, we systematically study to which extent the carbon dioxide reduction is affected by film thickness and porosity. On a polycrystalline copper electrode, immobilization of porous bipyridine-based films of varying thicknesses is shown to result in almost an order of magnitude enhancement of the intrinsic current density pertaining to ethylene formation while multicarbon products selectivity increases from 9.7 to 61.9%. In contrast, the total current density remains mostly unaffected by the modification once it is normalized with respect to the electrochemical active surface area. Supported by a microkinetic model, we propose that porous and thick films increase both local carbon monoxide partial pressure and the carbon monoxide surface coverage by retaining in situ generated carbon monoxide. This reroutes the reaction pathway toward multicarbon products by enhancing carbon-carbon coupling. Our study highlights the significance of customizing the molecular film structure to improve the selectivity of copper catalysts for carbon dioxide reduction reaction.

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 CoOx/Pt(111) planar model catalyst to analyze the active phase and the adsorbed species at the CoOx/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.

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.

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 (ß-region). As a result of the surface registry, in the ß-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 ß-regions, thus opening avenues to modify the structure and reactivity of transition metal oxides by external stimuli like electric fields.

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.

Cubes on a string: a series of linear coordination polymers with cubane-like nodes and dicarboxylate linkers.

A series of semicrystalline and amorphous one-dimensional (1D) polymeric chains consisting of cubane-like CoII4L4 units (L = S-1,2-bis(benzimidazol-2-yl)ethanol) and dicarboxylates were synthesized and characterized by single crystal diffraction and X-ray total scattering. The polycationic chains are composed of [Co4L4(dicarboxylate)]2+ monomeric units, while one molecular dicarboxylate counterion is balancing the charge of each monomer. The linear compound series has five members, and the crystal structures were solved for [Co4L4(tph)](tph) and [Co4L4(ndc)](ndc), where tph = terephthalate and ndc = 2,6-naphthalenedicarboxylate. Partly crystalline compounds were produced by slow assembly at elevated temperature (over days), while the amorphous compounds were formed by fast precipitation (within minutes). Pair distribution function (PDF) analysis based on X-ray total scattering data reveals the presence of the cubane-like entity in both the amorphous and semicrystalline samples. While the powders are non-porous, precipitation is a fast and versatile method to produce compounds with cubane-like centres with moderate surface areas of 17-49 m2 g-1 allowing for surface chemical reactions. The powders have a high concentration of Lewis base sites as verified by their selective adsorption of CO2 over N2. The use of an amorphous cubane-like polymer for the electrocatalytic oxygen evolution reaction was demonstrated.

Polymeric structure optimization of g-C3N4 by using confined argon-assisted highly-ionized ammonia plasma for improved photocatalytic activity.

The optimization of the polymeric structure and the modulation of surface amino groups in graphitic carbon nitride (g-CN) are critical but challenging in improving the photoelectric and photocatalytic performances of this polymer semiconductor. Ammonia plasma treatment may provide a fast and useful approach to optimize g-CN materials yet is seriously restricted by the low ionization ability of ammonia. Herein, a confined fast and environmental-friendly ammonia plasma method based on argon-assisted high ionization of NH3 was developed for efficient modification of raw g-CN. Compared with the weakly-ionized pure ammonia plasma which can only introduce amino group onto the surface g-CN, the argon-assisted highly-ionized ammonia plasma treatment obviously contributes to the comprehensively polymeric structure optimization of g-CN, and thus plays a key role in enhancing its light-harvesting and decelerating the recombination of the photogenerated charge carriers. As a result, the argon-assisted highly-ionized ammonia plasma-treated g-CN-Ar+NH3 outperformed the raw g-CN by a 2.5-fold higher photocatalytic reduction of hexavalent chromium and a remarkable 3.8-fold higher photocatalytic H2 evolution activity (up to 957.8 µmol·h-1·g-1) under visible light irradiation. Our findings suggest the great prospects of this novel highly-ionized ammonia plasma treatment method in the controllable modification of semiconductors and polymers.

Structural and electronic properties of Fe dopants in cobalt oxide nanoislands on Au(111).

Mixed metal oxides of earth-abundant 3d transition metals are an interesting class of materials that show interesting magnetic properties and a significant synergistic effect as catalysts for electrochemical oxygen evolution compared to simple unary oxides. However, the exact atomic-scale nature of such mixed oxide phases and the link to their interesting physico-chemical properties are poorly understood. Here, a combination of scanning tunneling microscopy and x-ray photoemission spectroscopy reveals that Fe species embed in a facile way into CoO bilayers on Au(111) resulting in an Fe doped oxide. Density functional theory and the spectroscopic fingerprint from x-ray photoemission spectroscopy reveal that the Fe dopants in the cobalt oxide matrix assume a higher oxidation state than in the structurally corresponding unary bilayer oxide. Furthermore, the substituted Fe is structurally displaced further away from the Au than the metal in either of the corresponding pure unary oxides. Both O and to a smaller extent Co in the nearest coordination shell are also structurally and electronically perturbed. The interesting effects observed in the bilayer binary oxides may enable a better fundamental understanding of the nature of doping of metal oxides, in general, and promotion effects in catalytic applications.

The Structure of the Cobalt Oxide/Au Catalyst Interface in Electrochemical Water Splitting.

The catalytic synergy between cobalt oxide and gold leads to strong promotion of the oxygen evolution reaction (OER)-one half-reaction of electrochemical water splitting. However, the mechanism behind the enhancement effect is still not understood, in part due to a missing structural model of the active interface. Using a novel interplay of cyclic voltammetry (CV) for electrochemistry integrated with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) on an atomically defined cobalt oxide/Au(111) system, we reveal here that the supporting gold substrate uniquely favors a flexible cobalt-oxyhydroxide/Au interface in the electrochemically active potential window and thus suppresses the formation of less active bulk cobalt oxide morphologies. The findings substantiate why optimum catalytic synergy is obtained for oxide coverages on gold close to or below one monolayer, and provide the first morphological description of the active phase during electrocatalysis.

Phase Transitions of Cobalt Oxide Bilayers on Au(111) and Pt(111): The Role of Edge Sites and Substrate Interactions.

Well-characterized metal oxides supported on single crystal surfaces serve as valuable model systems to study fundamental chemical properties and reaction mechanisms in heterogeneous catalysis or as new thin film metal oxide catalysts in their own right. Here, we present scanning tunneling microscopy and X-ray photoelectron spectroscopy results for cobalt oxide nanoislands that reveal the detailed atomistic mechanisms leading to transitions between Co-O bilayer and O-Co-O trilayer, induced by oxidation in O2 and reductive vacuum annealing treatments, respectively. By comparing between two different noble metal substrates, Au(111) and Pt(111), we further address the influence of the substrate. Overall, nanoisland edges act to initiate both the oxidation and reduction processes on both substrates. However, important influences of the choice of substrate were found, as the progress of oxidation includes intermediate steps on Au(111) not observed on Pt(111), where the oxidation on the other hand takes place at a significantly higher rate. During reductive treatment of trilayer, the bilayer structure gradually reappears on Pt(111), but not on Au(111) where the reduction rather results in the appearance of a stacked cobalt oxide morphology. These observations point to strong differences in the catalytic behavior between Au and Pt supported cobalt oxides, despite the otherwise strong structural similarities.