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Nanoporous Au has different properties compared to bulk Au, making it an interesting material for numerous applications. Depending on the preparation procedure, the porosity, thickness, and homogeneity of the NPG films can be tuned. To modify the structure of NPG films in a targeted manner and thus adapt them to specific applications, a fundamental understanding of the structure formation is essential. In this work, we focus on NPG prepared from Au oxide formed during high voltage electrolysis in an alkaline electrolyte on a poly-oriented Au single crystal electrode. These poly-oriented single crystals consist of a single crystalline metal bead, with faces with different crystallographic orientations. Consequently, these POSCs allow screening of the influence of the crystallographic orientation on the structure formation of the Au oxide formed during high voltage electrolysis and the NPG film formed via electrochemical reduction of the Au oxide for different facets in a single experiment. The high voltage electrolysis is performed at current densities between 2.70 and 3.76 A / cm² (300 V) and between 0.24 and 6.39 A / cm² (540 V) with electrolysis times ranging from 100 ms to 30 s. The amount of Au oxide formed is determined by electrochemical measurements and the structural properties are investigated by scanning electron microscopy and optical microscopy. We show that the Au oxide formation is mostly independent of the crystallographic orientation except for thick Au oxide layers. In turn, the macroscopic structure of the NPG films depend on the experimental parameters, the thickness of the Au oxide precursor thickness, and the crystallographic orientation of the substrate. Possible reasons for the frequently observed exfoliation of the NPG films are discussed.

Nanoporous Au (NPG) films often show distinctly different properties than bare Au electrodes, which make them suitable for various applications in (electro)catalysis or (bio)sensing. A great deal of effort has gone into finding suitable preparation techniques that can be used to target structural properties, such as the pore size or the surface-to-volume ratio. Many of the methods described for preparing these NPG films require complex starting materials such as alloys, multiple synthesis steps, lengthy preparation procedures or a combination of these factors. Here we present an approach that circumvents these difficulties, enabling for a rapid and controlled preparation of NPG films starting from bare Au electrodes. One approach is to prepare in a first step a Au oxide film by high voltage (HV) electrolysis in a KOH solution, which in a second step is reduced either electrochemically or in the presence of H₂O₂. The resulting NPG structures as well as their electrochemically active surface areas strongly depend on the reduction procedure, the concentration and temperature of the H₂O₂-containing KOH solution, as well as the applied voltage and temperature during the HV electrolysis. The NPG film can also be prepared directly by applying electrolysis voltages that result in anodic contact glow discharge electrolysis (aCGDE) over an extended period of time. By carefully adjusting the corresponding parameters, the surface area of the final NPG film can be specifically controlled. The structural properties of the electrodes are investigated by means of XPS, SEM and electrochemical methods.

Applying an external potential difference between two electrodes leads to a voltage drop in an ion conducting electrolyte. This drop is particularly large in poorly conducting electrolytes and for high currents. Measuring the electrolyte potential is relevant in electrochemistry, e.g., bipolar electrochemistry, ohmic microscopy, or contact glow discharge electrolysis. Here we study the course of the electrolyte potential during high voltage electrolysis in an electrolysis cell using two reversible hydrogen electrodes as reference electrodes, placed at different positions in the electrolyte. The electrolysis is performed with a Pt working and stainless steel counter electrode in a KOH solution. A computational COMSOL® model is devised which supports the experimentally obtained potential distribution. The influence of the cell geometry on the electrolyte potentials is evaluated. Applying the knowledge of the potential distribution to the formation of a Au oxide surface structure produced during high voltage electrolysis, we find that the amount of oxide formed is related to the current rather than the applied voltage.

It is widely accepted that MeOH and CO electroxdation is enhanced on bimetallic Pt−Ru electrodes, where Ru provides oxygen species at lower overpotential than Pt, decreasing the effect of surface poisoning by CO. By investigating surfaces with different Pt−Ru compositions and structures, the authors show that bimetallic Pt−Ru sites on these electrodes are almost inactive to MeOH oxidation, whereas pure Pt defect sites play an important role. The most prominent and intensively studied anode catalyst material for direct methanol oxidation fuel cells consists of a combination of platinum (Pt) and ruthenium (Ru). Classically, their high performance is attributed to a bifunctional reaction mechanism where Ru sites provide oxygen species at lower overpotential than Pt. In turn, they oxidize the adsorbed carbonaceous reaction intermediates at lower overpotential; among these, the Pt site-blocking carbon monoxide. We demonstrate that well-defined Pt modified Ru(0001) single crystal electrodes, with varying Pt contents and different local PtRu configurations at the surface, are unexpectedly inactive for the methanol oxidation reaction. This observation stands in contradiction with theoretical predictions and the concept of bifunctional catalysis for this reaction. Instead, we suggest that pure Pt defect sites play a more critical role than bifunctional defect sites on the electrodes investigated in this work., publishedVersion

Plasma in-liquid by means of anodic contact glow discharge electrolysis (aCGDE) is a growing research field allowing the selective modification of the electrode and the electrolyte. The aim of this proof of concept study is to demonstrate that auxiliary electrochemical electrodes placed in the vicinity of the plasma electrode can be modified by aCGDE (ignited at the anode by applying a DC voltage between the driving electrodes). Furthermore, we illustrate in how far such auxiliary electrodes can be used as a probe to detect products (in particular, [Formula: see text], [Formula: see text], and [Formula: see text]) formed in the solution by aCGDE via electrochemical techniques. In this work, aCGDE is achieved by applying a voltage of 580 V to a small Pt wire (plasma electrode) versus a large stainless steel counter electrode. An auxiliary Pt electrochemical working electrode, operated in a three electrode configuration, is placed at different distances from the plasma working electrode. Depending on the distance, we find small changes in the working electrode structure. More importantly, we will show that, in principle, the local [Formula: see text] concentration in the electrolyte can be monitored operando. After aCGDE, the concentration changes with time and depends on the distance from the plasma electrode.

Hydrogen production from electrocatalytic water splitting in electrolyzers is a key process to store excess electric energy produced from intermittent renewable energy sources. For proton exchange membrane (PEM) electrolyzers, carbon supported platinum particles exhibit the highest rates for the hydrogen evolution reaction (HER); however, high Pt costs limit the wide spread use of this technology. By employing a graphene layer grown on a Ru(0001) single crystal as a template for Pt nanocluster (NC) growth, we studied the dependence of the HER activity on the NC size using NCs of different sizes. We provide clear quantitative experimental evidence for a volcano-like relationship between the HER activity and the NC size which has been missing so far. For Pt NCs with very low sizes below 2 nm, we found stunningly improved exchange HER current densities. The highest exchange current density was observed for Pt NCs with an average size of ca. 38 atoms. These Pt38 NCs do not only surpass the Pt-mass-specific activity of commercial Pt electrode materials by well above three orders of magnitude, also their exchange current density is located close to the maximum exchange current density for the HER predicted theoretically for transition metal surfaces. The present work provides a strong stimulus for future research towards technically feasible Pt NC catalysts with cluster sizes in the range of few tens of Pt atoms.

The structural changes of the catalytically relevant materials Pt, Au and Cu upon electrolysis at 50 to 580 V were studied in alkaline electrolytes. Pt is stable under all investigated electrolysis conditions, while Au and Cu form thick oxide films under the same conditions. The Au and Cu oxides can reduce spontaneously (partially for Cu) in the electrolysis solution or by electrochemical reduction, resulting in nanoporous film formation. Applying a voltage to metal electrodes in contact with aqueous electrolytes results in the electrolysis of water at voltages above the decomposition voltage and plasma formation in the electrolyte at much higher voltages referred to as contact glow discharge electrolysis (CGDE). While several studies explore parameters that lead to changes in the I–U characteristics in this voltage range, little is known about the evolution of the structural properties of the electrodes. Here we study this aspect on materials essential to electrocatalysis, namely Pt, Au, and Cu. The stationary I–U characteristics are almost identical for all electrodes. Detailed structural characterization by optical microscopy, scanning electron microscopy, and electrochemical approaches reveal that Pt is stable during electrolysis and CGDE, while Au and Cu exhibit a voltage‐dependent oxide formation. More importantly, oxides are reduced when the Au and Cu electrodes are kept in the electrolysis solution after electrolysis. We suspect that H2O2 (formed during electrolysis) is responsible for the oxide reduction. The reduced oxides (which are also accessible via electrochemical reduction) form a porous film, representing a possible new class of materials in energy storage and conversion studies., publishedVersion

Ru-core Pt-shell nanoparticles show an enhanced electrocatalytic activity for the methanol oxidation reaction (MOR) compared to the respective monometallic or most bimetallic alloy systems. Due to their ill-defined structure, however, a simple interpretation of these findings is challenging. In the present work we present results of a systematic study on the influence of the Pt film thickness of structurally well-defined PtX-ML/Ru(0001) model electrodes with varying Pt film thickness (1.1–5.5 monolayers (ML)) on their MOR performance. The electrodes are prepared and structurally characterized by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. The electrochemical/-catalytic properties of the electrodes are evaluated in an electrochemical flow cell, which is attached to the UHV system and combined with a mass spectrometer in a differential electrochemical mass spectrometry (DEMS) set-up, allowing for online detection of volatile reaction products/intermediates. We demonstrate that the MOR activity of PtX-ML/Ru(0001) increases with increasing Pt film thickness and that thicker Pt films (>2.5 ML) are significantly more active than Pt(111) and thinner Pt films, whereas the selectivity for CO2 formation remains essentially constant. We explain this trend in the activity via the Sabatier principle, where the higher activities of the Pt film electrodes with thicker Pt films compared to both Pt(111) and electrodes with thinner Pt films are due to a lower binding strength of adsorbed species than on Pt(111), due to vertical electronic ligand effects (Ru – Pt interactions) and compressive strain effects in the pseudomorphic films, and due to an increasing binding strength with increasing Pt film thickness, due to decreasing electronic ligand effects. In this picture the Pt5.5-ML/Ru(0001) electrode exhibits the best value of the binding strength and thus the highest rates, while for electrodes with lower/higher binding strength the rates are lower. Consequences of these findings for bimetallic PtRu nanoparticle catalysts are discussed.

The enhanced activity of various bimetallic catalysts/electrodes in electrocatalytic reactions is usually ascribed to a bifunctional mechanism. This seems to neglect other effects arising from electronic modifications and further geometric effects, which are known to strongly affect the interaction of these surfaces with adsorbed species. In this work we elucidate the role of electronic and geometric effects on the activity of structurally well-defined PtRu electrodes in the bulk CO oxidation reaction, using model electrodes prepared and structurally characterized on an atomic scale by scanning tunneling microscopy under ultrahigh vacuum (UHV) conditions. Samples include PtxRu1-x/Ru(0001) surface alloys with varying Pt content consisting of a Ru(0001) substrate and a PtxRu1-x alloy surface layer as well as a Ru(0001) electrode covered by a pseudomorphic Pt monolayer (x = 1), and Pt(111) for comparison. Comparing the structural/chemical properties and the relative abundance of specific PtnRum ensembles on the electrode surfaces with the respective bulk CO electro-oxidation activity enabled us to identify Pt1Ru3 ensembles as the most active ensembles under the present conditions. The importance of electronic and geometric ensemble effects are discussed. The results demonstrate that the reactivity of bimetallic surfaces is much more complex than described by the classical bifunctional mechanism, which is proposed as a general feature.

While numerous reference electrodes suitable for aqueous electrolytes exist, there is no well‐defined standard for non‐aqueous electrolytes. Furthermore, reference electrodes are often large and do not meet the size requirements for small cells. In this work, we present a simple method for fabricating stable 3D‐printed micro‐reference electrodes. The prints are made from polyvinylidene fluoride, which is chemically inert in strong acids, bases, and commonly used non‐aqueous solvents. We chose six different reference systems based on Ag, Cu, Zn, and Na, including three aqueous and three non‐aqueous systems to demonstrate the versatility of the approach. Subsequently, we conducted cyclic voltammetry experiments and measured the potential difference between the aqueous homemade reference electrodes and a commercial Ag/AgCl‐electrode. For the non‐aqueous reference electrodes, we chose the ferrocene redox couple as an internal standard. From these measurements, we deduced that this new class of micro‐reference electrodes is leak‐tight and shows a stable electrode potential., The lack of stable reference electrodes (REs) for electrochemical measurements in small cell arrangements is an often‐neglected problem. Herein we present an easily applicable procedure for the assembly of stable and cheap micro‐REs, which are highly adaptable due to 3D‐printed housings; 3 aqueous and 3 non‐aqueous micro‐REs based on Ag, Cu, Zn, and Na. After fabrication, their stability and leak‐tightness are shown by electrochemical measurements.

As part of an extensive effort to explore the function of Au/ZnO catalysts in the synthesis of methanol from CO2 and H2, we have systematically investigated the temperature dependent growth, structure formation, and surface intermixing of Zn on the herringbone reconstructed Au(111) surface and the thermal stability of the resulting surfaces by scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). After Zn deposition at low temperatures, at about 105 K (STM) or below (XPS), we observed nucleation and two-dimensional growth of Zn islands mainly at the elbow sites of the Au(111) herringbone reconstruction. This results in local perturbations of the reconstruction pattern of the Au(111) substrate, which can create additional nucleation sites. XPS data indicate that Zn dissolution into deeper layers is kinetically hindered under these conditions, while local exchange with the Au surface layer, in particular at the elbow sites during nucleation, cannot be excluded. Zn deposition at room temperature, in contrast, results in near-surface alloy formation with a strongly distorted pattern of the herringbone reconstruction and condensation of the Zn and exchanged Au adatoms at ascending steps, together with some loss of Zn into deeper layers. Upon annealing, Zn atoms diffuse to lower layers and eventually to the Au bulk, and the surface successively regains its original Au(111) herringbone structure, which is almost reached after 500 K annealing. Compared with previous reports on the growth of other metals on Au(111), Zn shows a rather high tendency for intermixing and near-surface alloy formation.

Employing Pt(111) supported 2D Pt-core Au-shell model catalysts, we demonstrate that 2D core-shell surfaces prepared under ultrahigh vacuum (UHV) conditions constitute excellent model systems to determine the activity of step sites in electrocatalysis, especially because UHV-scanning tunneling microscopy (STM) enables control of the quality of narrow step modifications with high accuracy on such systems. As verified with STM, cyclic voltammetry (CV), and temperature-programmed desorption (TPD) measurements, this approach allows us (i) to increase the step density by homoepitaxial growth of monolayer high islands on the respective single crystal and (ii) to modify the step sites for adsorption of reactants by selective deposition of a guest metal. Herein, STM imaging in combination with electrochemical characterization provides a direct control to ascertain a selective modification of the entire steps. Comparing the electrocatalytic activity of 2D core-shell systems with and without the shell enables us to identify the activity of step sites for electrocatalytic reactions, as demonstrated for the bulk CO electro-oxidation.

This special collection on Interface Phenomena is dedicated to R. Jürgen Behm on the occasion of his retirement and 70th birthday. Jürgen Behm's research over the past 40 years has addressed a wide variety of interface processes in the fields of growth, corrosion, heterogeneous catalysis, electrocatalysis, and batteries.

As part of an extensive study of the interaction between Zn and Au in Zn/Au(111) model systems, we have systematically investigated the low-temperature (LT) nucleation and growth behavior of Zn on the Au(111) surface as well as the thermal stability of the resulting structures toward sintering, intermixing, and dissolution by scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). Zn deposition at LT, at 105 K (STM) or 80 K (XPS), leads to nucleation and two-dimensional growth of Zn islands mainly at the elbows of the Au(111) herringbone reconstruction, with a slight preference for island formation at pinched-in (pi) rather than bulged-out (bu) elbows. Local surface intermixing during LT Zn deposition leads to local perturbations of the Au(111) herringbone reconstruction, which results in the formation of additional nucleation sites (edge sites). At higher coverages (>0.11 ML), island coalescence sets in. Testing the thermal stability by annealing experiments, we find the structures to be stable up to about 200 K, while at higher temperatures, 2D sintering, intermixing, and dissolution set in, with subtle differences between pi- and bu-elbow islands. This indicates largely comparable activation barriers for the underlying (surface-)diffusion and exchange processes. Upon annealing to 330 K, all island structures dissolved. Compared with previous reports on the growth of other metals on Au(111), Zn shows a rather high tendency for intermixing and can be considered to be typical of metal deposition systems with comparable barriers for 2D Zn detachment/sintering and intermixing/bulk diffusion.

Disentangling the potential dependent surface(electro)chemical processes can be an arduous task, in particular for ill-defined electrode surfaces or when several reactions either coincide or occur without electron transfer. Employing ultrahigh vacuum surface preparation, electrochemical measurements and in situ spectroscopic / structural characterisation we have investigated the potential dependent surface processes proceeding on a Ru(0001) single crystal electrode in the absence and presence of specifically adsorbing anions in acid electrolyte. Online differential electrochemical mass spectrometry (DEMS) measurements clearly identify anodic H2 formation in both cases, a recently reported phenomenon (Scott et al. Catal. Sci. Technol. 10 (2020) 6870), suggesting that this can be induced not only by OH adsorption, but also by other strongly adsorbing anions such as bisulfate. Furthermore, operando surface X-ray diffraction (SXRD) measurements demonstrate that the composition of the adlayer depends significantly on the potential history of the electrode, where strongly adsorbed hydroxyl species lead to pronounced hysteresis effects. Such complex processes are proposed to be characteristic for strongly interacting electrodes in general, not only for Ru.

Continuing our efforts to elucidate the working principle of Au/ZnO catalysts for the synthesis of green methanol from CO2 and H2, we present a systematic X-ray photoelectron spectroscopy (XPS) study on the interaction of bimetallic Zn/Au(111) surfaces with O2 and NO2 (O*), on the oxidation of Zn atoms in these bimetallic model systems and on the formation ZnOx species or thin films on these surfaces. At room temperature, we find little adsorption and essentially no oxidation of Zn atoms alloyed into the Au(111) near-surface region upon exposure to O2 or NO2, and accordingly no Zn de-alloying. LT interaction of O2 or NO2 with Zn submonolayer structures leads to higher oxygen and / or NO2 coverages. Formation of oxidic ZnOx species / structures occurs, however, only after annealing above RT, both after O2 and NO2 exposure. This involves partial conversion of adsorbed species into ZnOx. ZnO/Au(111) thin films, especially thicker ones, can be efficiently prepared by reactive deposition of Zn in O2 or NO2, followed by annealing. RT deposition in NO2 gives a higher fraction of (incompletely) oxidized ZnOx compared to O2, Fully oxidized ZnO is formed after subsequent annealing, upon thermally activated conversion of adsorbed Oad/OH species into oxidic oxygen and dissolution of remaining metallic Zn atoms into the Au bulk. Overall, the study provides detailed insights into the formation of ZnOx thin films on Au(111).

In a joint experimental and theoretical study, we investigated the formation and morphology of PtCu/Ru(0001) bimetallic surfaces grown at room and higher temperatures under UHV conditions. We obtained the PtCu/Ru(0001) surfaces by deposition of Pt atoms on a previously created Cu/Ru(0001) structure which includes only one Cu monolayer. Bimetallic surfaces prepared at different Pt coverages are investigated using STM imaging, revealing the existence of reconstruction lines and Cu islands. Although primarily created Cu islands continue growing in size by increasing Pt coverage, a continuous formation of new Cu islands is observed. This leads to an atypical exponential increase of the island density as well as to an atypical behavior of the average number of atoms per island for low Pt coverages. Although coalescence of the islands is observed for high Pt coverages, the island density remains almost constant in that regime. In order to understand the trends observed in the experiments, we study the stability of these surfaces, atom adsorption, and adatom diffusion using periodic density functional theory calculations. On the basis of the experimental observations and the first-principles calculations, we suggest a model that includes exchange of Pt adatoms with Cu surface atoms, Pt and Cu adatom diffusion, and attractive (repulsive) interactions between Cu (Pt) adatoms with substitutional Pt surface atoms, which explains the main trends in island formation and growth observed in the experiment.

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.

As part of an extensive study of the performance of structurally well-defined AgxPt1–x/Pt(111) monolayer surface alloys in the O2 electroreduction we have systematically investigated the electrochemical properties and the stability of these electrodes in 0.5 M H2SO4 under mass controlled conditions in a flow cell setup, varying the surface Ag content over a wide range. The surface alloys were prepared and structurally characterized on an atomic scale by scanning tunneling microscopy (STM) under UHV conditions, and a transfer system allowed electrochemical measurements without intermediate contact to air. Cyclic voltammetry (CV) measurements showed distinct changes in the hydrogen and (bi)sulfate sorption behavior of the surface alloys with increasing Ag surface content. The electrochemical stability of the electrode surfaces was tested by CV in 0.5 M sulfuric acid supporting electrolyte with a stepwise increase of the potential limit up to 0.95 V (vs RHE). CVs as well as STM measurements revealed that sur...

We have systematically investigated the structure and structure formation of two-dimensional PtCu monolayer surface alloys on Ru(0001) as model systems for bimetallic PtCu catalysts and surfaces by scanning tunneling microscopy (STM). The surface alloys were prepared by deposition of Pt and Cu on Ru(0001) and thermal intermixing; different procedures were developed and tested to produce bimetallic surfaces with homogeneous structure, including also a homogeneous distribution of the different surface species, while at the same time intermixing with the Ru(0001) substrate should be inhibited. STM imaging revealed that for Pt concentrations below 65% surface alloys with homogeneous distribution could be formed, while at higher concentrations in the mixed phase, up to 82%, pure Pt or Pt-rich surface areas were formed as well. At Pt contents of 0.20 Pt x Cu 1 -x /Ru(0001) surface alloys were pseudomorphic, while lower Pt contents resulted in triangular dislocation line patterns. Also at x Pt > 0.65 line structures were observed, but of different nature. The distribution of surface atoms in the mixed phase was evaluated from STM images with chemical contrast, the related short-range order parameters were determined. The resulting structures and their energetics, the influence of different deposition and annealing procedures and the suitability of these surfaces as model systems for studies of the surface chemistry of bimetallic PtCu surfaces are discussed.

Graphene, a single atomic layer of graphite, has been the focus of recent intensive studies due to its novel electronic and structural properties. Metals grown on graphene also have been of interest because of their potential use as metal contacts in graphene devices, for spintronics applications, and for catalysis. All of these applications require good understanding and control of the metal growth morphology, which in part reflects the strength of the metal–graphene bond. Also of importance is whether the interaction between graphene and metal is sufficiently strong to modify the electronic structure of graphene. In this review, we will discuss recent experimental and computational studies related to deposition of metals on graphene supported on various substrates (SiC, SiO 2 , and hexagonal close-packed metal surfaces). Of specific interest are the metal–graphene interactions (adsorption energies and diffusion barriers of metal adatoms), and the crystal structures and thermal stability of the metal nanoclusters.

The surface structure formation and the activity of alloy surfaces in (electro-)catalysis strongly depend on the interaction and order of the atoms in the bulk and on the surface, and thus knowledge of the exact position and chemical identity of individual atoms is of fundamental interest. Scanning tunneling microscopy (STM) imaging allows for real space imaging of ordered and especially disordered alloy surfaces, providing insights in the exact position of different atoms by means of chemical resolution, which were and are not accessible with other surface science techniques. In order to understand STM imaging with chemical resolution, in this work first the fundamental experimental aspects and theoretical concepts of STM imaging are summarized. Focusing on STM imaging on an atomic scale level of alloy surfaces, different concepts which would allow for the differentiation of different types of atoms are discussed. The relevance of STM imaging with chemical resolution of alloy surfaces is finally presented on some selected examples from the literature, with respect to structure formation and the structure–activity relationship in (electro-)catalysis. Finally, some current experimental and technical limitations in STM imaging with chemical resolution are summarized.

C uis the only monometallic electrocatalyst to produce highly reduced products from CO2 selectively because of its intermediate binding of CO. We investigate the performance of polycrystalline Cu for the electroreduction of CO in alkaline media (0.1 M KOH) at low overpotentials (−0.4 to −0.6 V vs RHE). We find that polycrystalline Cu is highly active at these potentials. The overall CO reduction rates are comparable to those of the nanostructured forms of the material, albeit with a distinct product distribution. While nanostructured forms of Cu favor alcohols, polycrystalline Cu produces greater amounts of C2 and C3 aldehydes, as well as ethylene.

Single- and polycrystalline Cu electrodes serve as model systems for the study of the electroreduction of CO2, CO and nitrate, or for corrosion studies; even so, there are very few reports combining electrochemical measurements with structural characterization. Herein we investigate both the electrochemical properties of polycrystalline Cu and single crystal Cu(100) electrodes in alkaline solutions (0.1 M KOH and 0.1 M NaOH). We demonstrate that the pre-treatment of the electrodes plays a crucial role in the electrochemical properties of the electrodes. We perform scanning tunneling microscopy, X-ray photo electron spectroscopy and cyclic voltammetry on Cu(100) electrodes prepared under UHV conditions; we show that the electrochemical properties of these atomically well-defined electrodes are distinct from electrodes prepared by other methods. We also highlight the significant role of residual oxygen and electrolyte convection in influencing the electrochemical properties.

Structural modifications during electrochemical measurements on well defined Pt modified Ru(0001) electrode surfaces, which were prepared and characterized under ultrahigh vacuum (UHV) conditions, and the influence of the restructuring on the CO oxidation reaction have been investigated in a set-up combining surface preparation and scanning tunneling microscopy characterization under UHV conditions (UHV-STM) and electrochemical flow cell measurements. Bare Ru(0001) and Pt monolayer island modified Ru(0001) electrodes with different Pt coverages were investigated, together with a Pt 0.3 Ru 0.7 /Ru(0001) monolayer surface alloy for comparison. Comparing bulk CO oxidation measurements performed upon cycling in base electrolyte (0.5 M H 2 SO 4 ) to 0.90 V RHE with similar measurements performed after potential cycling to 1.05 V RHE , we find pronounced differences in the current–voltage characteristics, with a distinct new peak at low potentials in the positive-going scan in the latter case, which is centered at 0.67 V RHE . STM imaging performed before and after the electrocatalytic measurements revealed a distinct restructuring of the Pt monolayer island modified Ru(0001) surfaces upon potential cycling to 1.05 V RHE , while cycling to 0.90 V RHE maintains the original structure and morphology of the bimetallic surface. In contrast, for the bare Ru(0001) electrode, restructuring of steps is observed already upon potential cycling to 0.9 V RHE . Implications of these findings on the electrochemical stability of the electrodes as well as on the mechanistic understanding of the CO oxidation reaction on bimetallic PtRu electrode surfaces and on the activity of different mono- and bimetallic nanostructures are discussed.

The activity of structurally well defined Au x Pt 1− x /Pt(111) monolayer surface alloys with varying Au surface contents for the oxygen reduction reaction (ORR) was investigated by flow cell measurements under defined mass transport conditions, using a newly designed electrochemical flow cell attached to a ultra high vacuum (UHV) system. Surface alloy electrodes with well known concentrations and distributions of surface atoms were prepared under UHV conditions and characterized by scanning tunnelling microscopy (STM). Surface alloys with small amounts of Au (⩽24%) were found to exhibit reaction characteristics similar to Pt(111), transforming O 2 to H 2 O, but did not show a measurable lowering of the ORR overpotential compared to Pt(111), while higher Au surface contents (>24%) led to an increasing shift to higher overpotentials. For Au rich surfaces (>50%), the catalytic properties of the surface alloys shift towards that of polycrystalline Au, with an increasing tendency for H 2 O 2 formation. The kinetic currents determined experimentally are compared with simulated kinetic current – potential profiles based on the distribution of different trimer ensembles on the surface derived from STM imaging and on the adsorption energies of reaction intermediates taken from existent density functional theory (DFT) calculations (J. Greeley and J.K. Norskov, J. Phys. Chem. 113 (2009) 4932), using different approaches for averaging over the inhomogeneous surface. It is demonstrated that the modified procedure introduced by us recently ( j k averaging ) yields good agreement for these inhomogeneous surfaces. Potentials and limits of this approach are discussed.

The electrocatalytic activity of different, structurally well defined bimetallic PtRu surfaces in the oxygen reduction reaction was investigated by a combination of scanning tunnelling microscopy and electrochemical measurements performed under controlled mass transport conditions in a flow cell. We compare the effect of pseudomorphic Pt cover layers, mimicking the situation in a core–shell Pt/Ru nanoparticle, and of mixed PtxRu1−x monolayer surface alloys, reflecting the situation in an alloyed nanoparticle. The results unambiguously demonstrate that these bimetallic surfaces can reach activities well in excess of that of Pt(111), both for the film surfaces and the surface alloys, by optimizing the Pt surface content (surface alloys) or the Pt film thickness (film surfaces). The results are compared with simulated kinetic current–potential profiles based on existent density functional theory calculations (Greeley and Norskov, J Phys Chem C 113:4932, 2009; Lischka et al., Electrochim Acta 52:2219, 2007) revealing very good agreement in trends. Potential and limits of this approach are discussed.

Atomistic lattice-gas models for thermodynamically and kinetically directed assembly are applied to Ru nanocluster formation on a monolayer of graphene supported on Ru(0001) at 309 K. Nanocluster density, mean size, height distribution, and spatial ordering are analyzed by kinetic Monte Carlo simulations. Both models can reproduce the experimental data, but additional density functional theory analysis favors the former.

We describe the construction and discuss the performance of a novel combined ultrahigh vacuum (UHV)-electrochemistry set-up, allowing the controlled preparation and structural characterization of complex nanostructured electrode surfaces by high resolution scanning tunnelling microscopy (STM) under UHV conditions on the one hand and, after electrode transfer under clean conditions, electrochemical measurements under continuous, controlled electrolyte mass transport conditions on the other. Electrochemical measurements can be coupled with online product detection, either using an additional collector electrode or by differential electrochemical mass spectrometry (DEMS). The potential of the set-up will be illustrated in two electrocatalytic reactions on complex, but structurally well-defined bimetallic electrode surfaces, O2 reduction on PtxAg1−x/Pt(111) monolayer surface alloys and bulk CO oxidation on Pt monolayer island modified Ru(0001) electrodes. We will particularly demonstrate the importance of structural characterization after the electrochemical measurements for identifying structural modifications induced by the electrochemical environment and thus avoiding misleading conclusions about the structure–activity relationships.

The necessity of the careful structural characterization of model electrodes before and after the electrochemical measurements for a proper mechanistic interpretation is demonstrated for a well-known electrocatalytic system, bulk CO oxidation on PtRu model electrodes. Bimetallic, Pt-monolayer-island-modified Ru(0001) electrodes, which were prepared and characterized by scanning tunneling microscopy under ultrahigh-vacuum conditions, were found to undergo a distinct restructuring when they were potential cycled to 1.05 VRHE , while up to 0.90 VRHE they are stable. The restructuring, which is not evident in base voltammograms, is accompanied by a significant increase in the CO oxidation activity at low potentials (0.5-0.8 V), indicating that it is caused by new active sites created by the restructuring, and not by the PtRu sites that existed in the original surface and that were previously held responsible for the high activity of these bimetallic surfaces in terms of a bifunctional mechanism.

Directed assembly of an array of Ru nanoclusters (NCs) is achieved by deposition of Ru at around room temperature on a single layer of graphene supported on Ru(0001). In this system, directed assembly is guided by the periodic moire structure of the buckled graphene sheet. Behavior is analyzed utilizing both scanning tunneling microscopy and atomistic lattice-gas modeling together with kinetic Monte Carlo simulation. We elucidate the kinetics of NC nucleation and growth, specifically assessing the coverage dependence of the NC density and height distribution. In addition, we provide a detailed characterization of the development of short-range spatial order within the NC array, identifying a tendency for row formation.

The formation and atom distribution in two-dimensional Pd(x)Ag(1-x)/Pd(111) monolayer surface alloys were studied by high resolution scanning tunnelling microscopy (STM) with chemical contrast. From short-range order (SRO) parameters, we calculate preferences for like or unlike nearest neighbours to elucidate the mixing behaviour of the two components for various sub monolayer Ag surface contents. In the regime of low Ag surface contents (40% Ag), the system shows a weak tendency towards phase separation, high Ag coverages (60% Ag) result in a disperse distribution of the atoms in the surface. Effective pair interactions (EPIs) were derived by comparing the measured distribution with distributions obtained using Monte Carlo (MC) simulations. From the EPIs, we derived a function for the mixing energy, which can describe the change from clustering to a disperse distribution. The effects of the resulting surface atom distributions and of the Ag coverage dependent surface mixing/demixing on catalytic reactions are discussed.

We report on results of a detailed scanning tunnelling microscopy study on the formation, size and size distribution, and internal structure of small bimetallic PtRu clusters on a graphene monolayer film supported on a Ru(0001) substrate. These clusters, with sizes around ∼15 (Ru) or ∼40 (Pt) atoms per cluster at the lowest coverage, are interesting model systems for the catalytic behaviour of small metal PtRu particles, for example for application in electrocatalytic oxidation reactions. The clusters were generated by sequential deposition of the two metals at room temperature. The data reveal a distinct influence of the deposition sequence on the cluster formation process, with Ru pre-deposition followed by Pt deposition leading to predominantly bimetallic clusters, possibly with a core–shell-type structure, while the reverse sequence results in co-existent mono- and bimetallic clusters, where the latter are likely to intermix at the interface. The observations are related to the nucleation process of the respective metals on the templated surface, and the 2D growth behaviour of the two metals.

The formation of Pt-Ru nanoclusters (NCs) by sequential deposition of Pt and Ru on a periodically rumpled graphene sheet supported on Ru(0001) is analyzed by atomistic-level modeling and kinetic Monte Carlo simulations. The "coarse-scale" periodic variation of the adsorption energy of metal adatoms across the graphene sheet directs the assembly of NCs to a periodic array of thermodynamically preferred locations. The modeling describes not only just the NC densities and size distributions, but also the composition distribution for mixed NCs. A strong dependence of these quantities on the deposition order is primarily related to different effective mobilities of Pt and Ru on the supported graphene.