Near-surface Alloys Research Articles

Theoretical Insights into H2 Activation and Hydrogen Spillover on Near-Surface Alloys with Embedded Single Pt Atoms

Theoretical Insights into H₂ Activation and Hydrogen Spillover on Near-Surface Alloys with Embedded Single Pt Atoms

Oxidation of NiCr and NiCrMo Alloys at Low Temperatures

Oxidation of Ni-Cr and Ni-Cr-Mo was studied in operando with near ambient pressure x-ray photoelectron spectroscopy in the Cabrera-Mott regime. The oxidation temperature was 200°C—a severely diffusion-limited regime. The near-surface alloy is Cr-enriched after the reduction of native oxide in vacuum, and especially so for Ni-15Cr-6Mo. Mo-cations are integrated into the oxide and Mo(VI) dominates at the surface. The surface chemistry-driven promotion of chromia by Mo predicted by theory is negated by the limited surface diffusion of reactants. Preoxidation processing is proposed to control the oxide properties for the use of Ni-Cr superalloys at low temperatures.

DFT study on ORR catalyzed by bimetallic Pt-skin metals over substrates of Ir, Pd and Au

Bimetallic Pt-skin catalyst is a class of near-surface alloy (NSA) that owns a high degree of control over composition. Herein, density functional theory (DFT) is used to calculate the energetics of oxygen reduction reaction (ORR) on Pt-skin over Ir, Pd and Au substrates. A Brønsted-Evans-Polanyi (BEP) relationship can be determined for the oxygen molecule dissociation. The binding energy of both atomic oxygen and hydroxyl radical is found to correlate well with the d band center of Pt-skin atoms. Their catalytic activities show the volcano relationship as the positions of each substrate in the periodic table. The effect of surface strain, band structure and charge transfer on the d band center is well studied, and it can be found that the surface strain effect plays a dominant role for all Pt-skin catalysts. Ir substrate makes the d band center of Pt-skin go far away from the Fermi level, while Au substrate makes it move towards the Fermi level. Being different from both Ir and Au, Pd substrate makes the d band center of Pt-skin comparable with the monometallic Pt.

Process and catalyst improvements for the dry reforming of methane

Catalyst and process improvement strategies for dry reforming of methane (DRM) are shown by enhancing the performance of an existing Ni/Al2O3 catalyst and adding oxygen to the feed. Alumina-supported Ni-Co catalyst, Ni/Co molar ratio = 3, was synthesized and calcined at 500 and 850 °C to take advantage of the possible strong metal-support interactions. The freshly reduced catalysts contained Ni-Co alloys. Higher CH4 and CO2 conversions, H2 and CO yields and H2/CO ratio during DRM and oxidative dry reforming of methane (ODRM) were achieved at 600 and 750 °C and two space-velocities for Ni3Co-500 due to a higher effective dispersion of 4.9 %. Significant carbon formation and activity loss occurred during DRM. Increasing the reaction temperature and/or with O2 co-feed increased the CH4 conversion. The outlet concentrations were stable, and carbon formed was significantly less. Changes in the crystallite size and near-surface alloy composition in the spent catalyst were detected.

Modeling Anion Poisoning during Oxygen Reduction on Pt Near-Surface Alloys

Electrolyte effects play an important role in the activity of the oxygen reduction reaction (ORR) of Pt-based electrodes. Herein, we combine a computational model and rotating disk electrode measurements to investigate the effects from phosphate anion poisoning for the ORR on well-defined extended Pt surfaces. We construct a model including the poisoning effect from phosphate species on Pt(111) and Cu/Pt(111) based on density functional theory simulations. By varying the subsurface Cu content of the Cu/Pt(111) alloy, we tune the *OH binding energies on the surface by means of ligand effects, and as a result, we tune the ORR activity. We have investigated the effect of adsorbed phosphate species at low overpotentials when tuning *OH binding energies. Our results display a direct scaling relationship between adsorbed *OH and phosphate species. From the model, we observe how the three-fold binding sites of phosphate anions limit the packing of poisoning phosphate on the surface, thus allowing for *OH adsorption even when poisoned. Our work shows that, regardless of surface site blockage from phosphate, the trend in the catalytic oxygen reduction activity is predominantly governed by the *OH binding.

Monte Carlo simulations of surface segregation to discover new hydrogen separation membranes

Surface compositions play a predominant role in the efficiency and lifetime of membranes and catalysts. The surface composition can change during operation due to segregation, thus controlling and predicting the surface composition is essential. Computational modelling can aid in predicting alloy stability, along with designing surface alloys and near-surface alloys that can outperform existing materials. In this work, a computational model to predict surface segregation in ternary alloys is developed. The model, based on Miedema's semi-empirical model and Monte Carlo simulations, enables to predict long- and short-range ordering in the surface and subsurface layers. It is used to screen a vast range of alloy compositions to design a novel ternary Pd-based material for H2 separation membranes. The addition of specific amounts of Cu and Zr to Pd is expected to reduce poisoning and enhance the permeability as compared to pure Pd.

Influence of the Current Regime during Electrodeposition in a Cr(III)-Containing Fe-Cr-Ni Electrolyte on the Near-Surface pH, Alloy Composition, and Microcrack Behavior

This study shows the electrodeposition of Fe-Cr-Ni alloys using the advantages of a stepped direct-current deposition in a Cr(III)-containing electrolyte, and its influence on the pH value. The resulting coatings are uniform and free of microcracks, with a Cr content around 30% and thicknesses above 10 µm. The influence of the current mode (direct current, stepped direct current, pulsed current) on pH development is investigated and correlated with the arising microstructure and alloy composition. Considering the current flow, it can be stated that pauses interrupt high overvoltage and restrain the pH increase at the cathode. The associated formation of chromium hydrides and their deposition onto the chromium layer, leading to cracking, is thus reduced. In this work, direct evidence for this theory and a suggestion for near-surface pH measurement during electrodeposition are presented.

Structure-Controlled Chemical Properties of PdZn Near-Surface Alloys

Structure-Controlled Chemical Properties of PdZn Near-Surface Alloys

Selective Epoxidation of 1,3-Butadiene on AgCu Near-Surface Alloys

Selective Epoxidation of 1,3-Butadiene on AgCu Near-Surface Alloys

Modeling Temperature-, Humidity-, and Material-Dependent Kinetics of the Oxygen Reduction Reaction

We propose a mathematical model that examines the temperature-, humidity-, and material-dependent oxygen reduction reaction (ORR) activity. We extended a conventional theoretical ORR micro-kinetic model by considering the temperature dependencies of the free energies of the adsorbed intermediate species and the solvent reorganization energic barrier. This model was validated by experimental analyses: Temperature- and material-dependent activities were experimentally measured by rotating disk electrode tests using Cu/Pt (111) near-surface alloy catalysts, while humidity-dependent activity was examined by fuel cell tests using a mesoporous carbon as the catalyst support, which reduces the effect of ionomer poisoning. Both the theoretical and experimental results showed that the activities are lowered with increasing temperature on catalysts with weak OH binding energy, whereas the opposite trend was observed on catalysts with strong OH binding energy. The results also showed that the activity on pure Pt, whose OH binding energy is strong, increases with a decrease in the relative humidity. These trends are reasonably explained from the shift in the thermodynamics of the limiting steps. Further calculations over a wider range of temperature and relative humidity suggested that the optimal OH binding strength (catalyst material) and catalytic activity strongly depend on operating conditions.

A universal picture for ejecting atoms on metallics

The surface-site stability of transition metals and alloys, which can be measured by the ejection energy, is essential in corrosion and electrocatalysis. However, the complex geometric and alloying effects of alloys prohibit the accurate estimation of ejection energies. Herein we introduce the geometric and electronic gradients and cohesive properties of surface sites to determine the ejection energy for transition metals, single-atom alloys, binary alloys, near-surface alloys, and high-entropy alloys. This predictive model reveals not only the electronic and geometric determinants of interatomic bonding in alloys but also the range-reaching characters of alloying and geometric variation, which outlines explicitly the commonality and difference between different alloys in bonding. Our model uncovers a novel physical picture for the interatomic bonding of alloys, provides a simple way to estimate the alloying effect, and is helpful for modulating the stability of alloy surfaces.

Near-Surface Alloys of PtRh on Rh(111) and Pt(111) Characterized by STM

The use of Pt–Rh catalysts is central in a number of industrial processes, and to explain their performance it is essential to have a solid understanding of their nanoscopic surface structure. Here we use scanning tunneling microscopy to investigate the growth, elemental nanostructuring, and reconstruction behavior of various Pt–Rh near-surface alloys (NSA) on fcc Rh(111) and Pt(111). We document the formation of a novel lamellar island reconstruction atop Pt/Rh(111) islands as well as network reconstructions on Rh/Pt(111), similar to those previously observed for Pt/Pt(111). The extended roadmap for preparation of PtRh NSAs allows comparison with other homo/heteroepitaxial metal/fcc surfaces and provides a facile guideline for producing tailor-made model catalytic surfaces.

Label-free electrochemical immunosensor based on PdCuPt/PPY/DCSC as a signal amplification platform for sensitive detection of cardiac troponin I

Monitoring cardiac troponin I (CTnI) by immunosensor is of great significance in the diagnosis of acute myocardial infarction (AMI). The core of extending immunosensor technology to micro-detection is the excellent performance of immunosensor, emphasizing that the constructed immunosensor needs to have high sensitivity. For this reason, we synthesized a kind of trimetallic PdCuPt nanoparticles (NPs) with excellent catalytic performance. Due to the combination of the electronic and geometric effect originating from the formation of near surface alloy (NSA) with special ultra-small polyhedral spherical structure, PdCuPt NPs exhibit excellent catalytic performance, which provides a good foundation for achieving high sensitivity. In the light of the unique hollow window structure of the dice-shaped cobalt square cage (DCSC), we combined DCSC with PdCuPt NPs by polypyrrole (PPY) as a signal amplification platform (PdCuPt/PPY/DCSC) to further amplify the response signal. The introduction of PPY into DCSC (PPY/DCSC) can enhance conductivity to accelerate electron transfer and improve mechanical stability. The hierarchical porous structured PPY/DCSC can not only accelerate the mass transfer but also promote the adhesion of PdCuPt NPs and improve the dispersion. In view of the above-mentioned signal amplification platform, the fabricated label-free electrochemical immunosensor shows ultra-high sensitivity, wide dynamic range (50 fg/mL~1 μg/mL) and low limit of detection (LOD, 16.03 fg/mL). The introduction of this sensitive electrochemical immunosensor provides a reference for clinical sensitive detection of CTnI.

Mechanistic insights into the oxidation of catalytically relevant AgCu near‐surface alloy interfaces

AbstractRecent heterogeneous catalysis studies have demonstrated that synergy between Ag and Cu can lead to more selective partial oxidation chemistries. We performed a series of scanning tunneling microscope experiments to gain a better understanding of the AgCu system under oxidative conditions. These experiments were carried out by exposing sub‐monolayer coverages of Ag on Cu(111), in the form of a near‐surface alloy (NSA), to range of oxygen exposures and temperatures. This enabled us to study the initial stages of oxidation of well‐defined Ag/Cu interfaces with atomic resolution and thereby understand the dynamic response of the AgCu NSA to oxygen environments. At low oxygen exposures, oxidation was observed on exposed Cu terraces and at the interface between the AgCu NSA and Cu(111). Higher oxygen exposure led to the segregation of Cu atoms up through the Ag layer and the appearance of surface adsorbed oxygen. Significant phase segregation of Cu was then observed at higher oxygen exposures at elevated temperatures, evidenced by the formation of Cu oxide patches within and on the top of the Ag layer. These results provide a more detailed picture of how AgCu NSAs interact with, and restructure in response to, oxygen.

Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces.

Structural and chemical transformations of ultrathin oxide films on transition metals lie at the heart of many complex phenomena in heterogeneous catalysis, such as the strong metal-support interaction (SMSI). However, there is limited atomic-scale understanding of these transformations, especially for irreducible oxides such as ZnO. Here, by combining density functional theory calculations and surface science techniques, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction, we investigated the interfacial interaction of well-defined ultrathin ZnOxHy films on Pd(111) under varying gas-phase conditions [ultrahigh vacuum (UHV), 5 × 10-7 mbar of O2, and a D2/O2 mixture] to shed light on the SMSI effect of irreducible oxides. Sequential treatment of submonolayer zinc oxide films in a D2/O2 mixture (1:4) at 550 K evoked reversible structural transformations from a bilayer to a monolayer and further to a Pd-Zn near-surface alloy, demonstrating that zinc oxide, as an irreducible oxide, can spread on metal surfaces and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical-grand canonical phase diagram was developed to bridge the gap between UHV conditions and true SMSI environments, revealing that, in addition to surface alloy formation, certain ZnOxHy films with stoichiometries that do not exist in bulk are stabilized by Pd in the presence of hydrogen. Based on the combined theoretical and experimental observations, we propose that SMSI metal nanoparticle encapsulation for irreducible oxide supports such as ZnO involves both surface (hydroxy)oxide and surface alloy formation, depending on the environmental conditions.

Bayesian learning of chemisorption for bridging the complexity of electronic descriptors

Building upon the d-band reactivity theory in surface chemistry and catalysis, we develop a Bayesian learning approach to probing chemisorption processes at atomically tailored metal sites. With representative species, e.g., *O and *OH, Bayesian models trained with ab initio adsorption properties of transition metals predict site reactivity at a diverse range of intermetallics and near-surface alloys while naturally providing uncertainty quantification from posterior sampling. More importantly, this conceptual framework sheds light on the orbitalwise nature of chemical bonding at adsorption sites with d-states characteristics ranging from bulk-like semi-elliptic bands to free-atom-like discrete energy levels, bridging the complexity of electronic descriptors for the prediction of novel catalytic materials.

Dynamic Restructuring Induced Oxygen Activation on AgCu Near-Surface Alloys.

Recent studies have shown that the addition of Cu to Ag catalysts improves their epoxidation performance by increasing the overall selectivity of the bimetallic catalyst. We have prepared AgCu near-surface alloys and used scanning tunneling microscopy to gain an atomistic picture of O2 dissociation on the bimetallic system. These data reveal a higher dissociative sticking probability for O2 on AgCu than on Ag(111), and density functional theory (DFT) confirms that the O2 dissociation barrier is 0.17 eV lower on the alloy. Surprisingly, we find that, after a slow initial uptake of O2, the sticking probability increases exponentially. Further DFT calculations indicate that surface oxygen reverses the segregation energy for AgCu, stabilizing Cu atoms in the Ag layer. These single Cu atoms in the Ag surface are found to significantly lower the O2 dissociation barrier. Together, these results explain nonlinear effects in the activation of O2 on this catalytically relevant surface alloy.

Adlayer growth vs spontaneous (near-) surface alloy formation: Zn growth on Au(111).

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.

Tailored electrocatalysts by controlled electrochemical deposition and surface nanostructuring.

Controlled electrodeposition and surface nanostructuring are very promising approaches to tailor the structure of the electrocatalyst surface, with the aim to enhance their efficiency for sustainable energy conversion reactions. In this highlight, we first summarise different strategies to modify the structure of the electrode surface at the atomic and sub-monolayer level for applications in electrocatalysis. We discuss aspects such as structure sensitivity and electronic and geometric effects in electrocatalysis. Nanostructured surfaces are finally introduced as more scalable electrocatalysts, where morphology, cluster size, shape and distribution play an essential role and can be finely tuned. Controlled electrochemical deposition and selective engineering of the surface structure are key to design more active, selective and stable electrocatalysts towards a decarbonised energy scheme.

Statistical learning goes beyond the d-band model providing the thermochemistry of adsorbates on transition metals

The rational design of heterogeneous catalysts relies on the efficient survey of mechanisms by density functional theory (DFT). However, massive reaction networks cannot be sampled effectively as they grow exponentially with the size of reactants. Here we present a statistical principal component analysis and regression applied to the DFT thermochemical data of 71 C{}_{1}–C{}_{2} species on 12 close-packed metal surfaces. Adsorption is controlled by covalent (d-band center) and ionic terms (reduction potential), modulated by conjugation and conformational contributions. All formation energies can be reproduced from only three key intermediates (predictors) calculated with DFT. The results agree with accurate experimental measurements having error bars comparable to those of DFT. The procedure can be extended to single-atom and near-surface alloys reducing the number of explicit DFT calculation needed by a factor of 20, thus paving the way for a rapid and accurate survey of whole reaction networks on multimetallic surfaces.