CO-Induced Dimer Decay Responsible for Gem-Dicarbonyl Formation on a Model Single-Atom Catalyst.

The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh1 adatoms and minority Rh2 dimers on Fe3O4(001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh1 sites results exclusively in stable Rh1CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh1(CO)2 gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh2 dimers via an unstable Rh2(CO)3 intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis.

Tailoring Secondary Coordination Sphere Effects in Single-metal-site Catalysts by Surface Immobilization of Supramolecular Cages.

Controlling the coordination sphere of heterogeneous single-metal-site catalysts is a powerful strategy for fine-tuning their catalytic properties but is fairly difficult to achieve. To address this problem, we immobilized supramolecular cages where the primary- and secondary coordination sphere are controlled by ligand design. The kinetics of these catalysts were studied in a model reaction, the hydrolysis of ammonia borane, over a temperature range using fast and precise online measurements generating high-precision Arrhenius plots. The results show how catalytic properties can be enhanced by placing a well-defined reaction pocket around the active site. Our fine-tuning yielded a catalyst with such performance that the reaction kinetics are diffusion-controlled rather than chemically controlled.

Water agglomerates on Fe3O4(001).

Determining the structure of water adsorbed on solid surfaces is a notoriously difficult task and pushes the limits of experimental and theoretical techniques. Here, we follow the evolution of water agglomerates on Fe3O4(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH-H2O bonds drive the formation of partially dissociated water dimers at low coverage, but a surface reconstruction restricts the density of such species to one per unit cell. The dimers act as an anchor for further water molecules as the coverage increases, leading first to partially dissociated water trimers, and then to a ring-like, hydrogen-bonded network that covers the entire surface. Unraveling this complexity requires the concerted application of several state-of-the-art methods. Quantitative temperature-programmed desorption (TPD) reveals the coverage of stable structures, monochromatic X-ray photoelectron spectroscopy (XPS) shows the extent of partial dissociation, and noncontact atomic force microscopy (AFM) using a CO-functionalized tip provides a direct view of the agglomerate structure. Together, these data provide a stringent test of the minimum-energy configurations determined via a van der Waals density functional theory (DFT)-based genetic search.

Electrochemical Polarization Dependence of the Elastic and Electrostatic Driving Forces to Aliovalent Dopant Segregation on LaMnO3.

Segregation of aliovalent dopant cations is a common degradation pathway on perovskite oxide surfaces in energy conversion and catalysis applications. Here we focus on resolving quantitatively how dopant segregation is affected by oxygen chemical potential, which varies over a wide range in electrochemical and thermochemical energy conversion reactions. We employ electrochemical polarization to tune the oxygen chemical potential over many orders of magnitude. Altering the effective oxygen chemical potential causes the oxygen nonstoichiometry to change in the electrode. This then influences the mechanisms underlying the segregation of aliovalent dopants. These mechanisms are (i) the formation of oxygen vacancies that couples to the electrostatic energy of the dopant in the perovskite lattice and (ii) the elastic energy of the dopant due to cation size mismatch, which also promotes the reaction of the dopant with O2 from the gas phase. The present study resolves these two contributions over a wide range of effective oxygen pressures. Ca-, Sr-, and Ba-doped LaMnO3 are selected as model systems, where the dopants have the same charge but different ionic sizes. We found that there is a transition between the electrostatically and elastically dominated segregation regimes, and the transition shifted to a lower oxygen pressure with increasing cation size. This behavior is consistent with the results of our ab initio thermodynamics calculations. The present study provides quantitative insights into how the elastic energy and the electrostatic energy determine the extent of segregation for a given overpotential and atmosphere relevant to the operating conditions of perovskite oxides in energy conversion applications.

Dual role of CO in the stability of subnano Pt clusters at the Fe3O4(001) surface.

Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe3O4(001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt1-CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase.

Local Structure and Coordination Define Adsorption in a Model Ir1 /Fe3 O4 Single-Atom Catalyst.

Single-atom catalysts (SACs) bridge homo- and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus strongly influence how reactants adsorb. Now, atomically resolved scanning-probe microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3 O4 (001) support. The two- and five-fold-coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square-planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism.

A multi-technique study of CO2 adsorption on Fe3O4 magnetite.

The adsorption of CO2 on the Fe3O4(001)-(2 × 2)R45° surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO2 binds most strongly at defects related to Fe2+, including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages,CO2 adsorbs at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (2 × 2)R45° unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO2 ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO2 multilayers (≈88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements.

Co on Fe3O4(001): Towards precise control of surface properties.

A novel approach to incorporate cobalt atoms into a magnetite single crystal is demonstrated by a combination of x-ray spectro-microscopy, low-energy electron diffraction, and density-functional theory calculations. Co is deposited at room temperature on the reconstructed magnetite (001) surface filling first the subsurface octahedral vacancies and then occupying adatom sites on the surface. Progressive annealing treatments at temperatures up to 733 K diffuse the Co atoms into deeper crystal positions, mainly into octahedral ones with a marked inversion level. The oxidation state, coordination, and magnetic moments of the cobalt atoms are followed from their adsorption to their final incorporation into the bulk, mostly as octahedral Co(2+). This precise control of the near-surface Co atoms location opens up the way to accurately tune the surface physical and magnetic properties of mixed spinel oxides.

An Atomic-Scale View of CO and H2 Oxidation on a Pt/Fe3 O4 Model Catalyst.

Metal-support interactions are frequently invoked to explain the enhanced catalytic activity of metal nanoparticles dispersed over reducible metal oxide supports, yet the atomic-scale mechanisms are rarely known. In this report, scanning tunneling microscopy was used to study a Pt1-6/Fe3O4 model catalyst exposed to CO, H2, O2, and mixtures thereof at 550 K. CO extracts lattice oxygen atoms at the cluster perimeter to form CO2, creating large holes in the metal oxide surface. H2 and O2 dissociate on the metal clusters and spill over onto the support. The former creates surface hydroxy groups, which react with the support, ultimately leading to the desorption of water, while oxygen atoms react with Fe from the bulk to create new Fe3O4(001) islands. The presence of the Pt is crucial because it catalyzes reactions that already occur on the bare iron oxide surface, but only at higher temperatures.

Bottom-Up Synthesis of Platinum Dual-Atom Catalysts on Cerium Oxide.

We present here the synthesis and performance of dual-atom catalysts (DACs), analogous to well-known single-atom catalysts (SACs). DACs feature sites containing pairs of metal atoms and can outperform SACs due to their additional binding possibilities. Yet quantifying the improved catalytic activity in terms of proximity effects remains difficult, as it requires both high-resolution kinetic data and an understanding of the reaction pathways. Here, we use an automated bubble counter setup for comparing the catalytic performance of ceria-supported platinum SACs and DACs in ammonia borane hydrolysis. The catalysts were synthesized by wet impregnation and characterized using SEM, HAADF-STEM, XRD, XPS, and CO-DRIFTS. High-precision kinetic studies of ammonia borane hydrolysis in the presence of SACs show two temperature-dependent regions, with a transition point at 43 °C. Conversely, the DACs show only one regime. We show that this is because DACs preorganize both ammonia borane and water at the dual-atom active site. The additional proximal Pt atom improves the reaction rate 3-fold and enables faster reactions at lower temperatures. We suggest that the DACs enable the activation of the water-O-H bond as well as increase the hydrogen spillover effect due to the adjacent Pt site. Interestingly, using ammonia borane hydrolysis as a benchmark reaction gives further insight into hydrogen spillover mechanisms, above what is known from the CO oxidation studies.

Why Teflon is so slippery while other polymers are not.

Polytetrafluoroethylene [PTFE (Teflon)] is a uniquely slippery polymer, with a coefficient of friction that is an order of magnitude lower than that of other polymers. Though known as nonsticky, PTFE leaves a layer of material behind on the substrate while sliding. Here, we use contact-sensitive fluorescent probes to image the sliding contact in situ: We show that slip happens at an internal PTFE-PTFE interface that has an unusually low shear strength of 0.8 MPa. This weak internal interface directly leads to low friction and enables transfer of the PTFE film to the substrate even in the absence of strong adhesion.

Femtosecond Laser-Induced Emission of Coherent Terahertz Pulses from Ruthenium Thin Films.

We demonstrate emission of electromagnetic pulses with frequencies in the terahertz (THz) range from ruthenium thin films through a second-order nonlinear optical process. Ruthenium deposited on different substrates showed different THz emission properties. We provide evidence that for Ru on glass above a certain power threshold, laser-induced oxidation occurs, resulting in an increased slope of the linear dependence of the THz electric field amplitude on pump power. The THz electric field is mainly polarized parallel to the sample surface, pointing in the same direction everywhere. In contrast to Ru on glass, the electric field amplitude of the THz pulses emitted by Ru on sapphire and on CaF2 shows a simple single linear dependence on pump power, and it is polarized orthogonal to the sample surface. In this case, thermal oxidation in an oven enhances the emission and introduces an additional polarization component along the sample surface. This component also points in the same direction everywhere on the surface, similar to the as-deposited Ru on glass. Although the precise THz generation mechanism remains an open question, our results show a strong correlation between the emission strength and the degree of oxidation. Furthermore, the results highlight the importance of the interfaces, i.e., both the choice of the substrate and the chemical composition of the top surface in THz emission experiments. Knowledge of the state of the sample surface is therefore crucial for the interpretation of THz emission experiments from (nonmagnetic) metal surfaces.

Dry reforming of methane over single-atom Rh/Al2O3 catalysts prepared by exsolution.

Single-atom catalysts often show exceptionally high performance per metal loading. However, the isolated atom sites tend to agglomerate during preparation and/or high-temperature reaction. Here we show that in the case of Rh/Al2O3 this deactivation can be prevented by dissolution/exsolution of metal atoms into/from the support. We design and synthesise a series of single-atom catalysts, characterise them and study the impact of exsolution in the dry reforming of methane at 700-900 °C. The catalysts' performance increases with increasing reaction time, as the rhodium atoms migrate from the subsurface to the surface. Although the oxidation state of rhodium changes from Rh(iii) to Rh(ii) or Rh(0) during catalysis, atom migration is the main factor affecting catalyst performance. The implications of these results for preparing real-life catalysts are discussed.

Understanding the Oxidative Properties of Nickel Oxyhydroxide in Alcohol Oxidation Reactions.

The NiOOH electrode is commonly used in electrochemical alcohol oxidations. Yet understanding the reaction mechanism is far from trivial. In many cases, the difficulty lies in the decoupling of the overlapping influence of chemical and electrochemical factors that not only govern the reaction pathway but also the crystal structure of the in situ formed oxyhydroxide. Here, we use a different approach to understand this system: we start with synthesizing pure forms of the two oxyhydroxides, ß-NiOOH and γ-NiOOH. Then, using the oxidative dehydrogenation of three typical alcohols as the model reactions, we examine the reactivity and selectivity of each oxyhydroxide. While solvent has a clear effect on the reaction rate of ß-NiOOH, the observed selectivity was found to be unaffected and remained over 95% for the dehydrogenation of both primary and secondary alcohols to aldehydes and ketones, respectively. Yet, high concentration of OH- in aqueous solvent promoted the preferential conversion of benzyl alcohol to benzoic acid. Thus, the formation of carboxylic compounds in the electrochemical oxidation without alkaline electrolyte is more likely to follow the direct electrochemical oxidation pathway. Overoxidation of NiOOH from the ß- to γ-phase will affect the selectivity but not the reactivity with a sustained >95% conversion. The mechanistic examinations comprising kinetic isotope effects, Hammett analysis, and spin trapping studies reveal that benzyl alcohol is oxidatively dehydrogenated to benzaldehyde via two consecutive hydrogen atom transfer steps. This work offers the unique oxidative and catalytic properties of NiOOH in alcohol oxidation reactions, shedding light on the mechanistic understanding of the electrochemical alcohol conversion using NiOOH-based electrodes.

Electronic and structural properties of crystalline and amorphous (TaNbHfTiZr)C from first principles.

High entropy materials (HEMs) are of great interest for their mechanical, chemical and electronic properties. In this paper we analyse (TaNbHfTiZr)C, a carbide type of HEM, both in crystalline and amorphous phases, using density functional theory (DFT). We find that the relaxed lattice volume of the amorphous phase is larger, while its bulk modulus is lower, than that of its crystalline counterpart. Both phases are metallic with all the transition metals contributing similarly to the density of states close to the Fermi level, with Ti and Nb giving the proportionally largest contribution of states. We confirm that despite its great structural complexity,2×2×2supercells are large enough for reliable simulation of the presented mechanical and electronic properties by DFT.

CO oxidation by Pt2/Fe3O4: Metastable dimer and support configurations facilitate lattice oxygen extraction.

Heterogeneous catalysts based on subnanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we use a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)-based calculations to study CO oxidation by a model Pt/Fe3O4(001) "single-atom" catalyst. We demonstrate that (PtCO)2 dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyze a reaction involving the oxide support to form CO2. Pt2 dimers produce one CO2 molecule before falling apart into two adatoms, releasing the second CO. Olattice extraction only becomes facile when both the Pt-dimer and the Fe3O4 support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature.

Microscopic Proof of Photoluminescence from Mechanochemically Synthesized 1-Octene-Capped Quantum-Confined Silicon Nanoparticles: Implications for Light-Emission Applications.

Silicon nanoparticles (SiNPs) have been explored intensively for their use in applications requiring efficient fluorescence for LEDs, lasers, displays, photovoltaic spectral-shifting filters, and biomedical applications. High radiative rates are essential for such applications, and theoretically these could be achieved via quantum confinement and/or straining. Wet-chemical methods used to synthesize SiNPs are under scrutiny because of reported contamination by fluorescent carbon species. To develop a cleaner method, we utilize a specially designed attritor type high-energy ball-mill and use a high-purity (99.999%) Si microparticle precursor. The mechanochemical process is used under a continuous nitrogen gas atmosphere to avoid oxidation of the particles. We confirm the presence of quantum-confined NPs (<5 nm) using atomic force microscopy (AFM). Microphotoluminescence (PL) spectroscopy coupled to AFM confirms quantum-confined tunable red/near-infrared PL emission in SiNPs capped with an organic ligand (1-octene). Using micro-Raman-PL spectroscopy, we confirm SiNPs as the origin of the emission. These results demonstrate a facile and potentially scalable mechanochemical method of synthesis for contamination-free SiNPs.

Extreme-Ultraviolet Excited Scintillation of Methylammonium Lead Bromide Perovskites.

Inorganic-Organic lead halide materials have been recognized as potential high-energy X-ray detectors because of their high quantum efficiencies and radiation hardness. Surprisingly little is known about whether the same is true for extreme-ultraviolet (XUV) radiation, despite applications in nuclear fusion research and astrophysics. We used a table-top high-harmonic generation setup in the XUV range between 20 and 45 eV to photoexcite methylammonium lead bromide (MAPbBr3) and measure its scintillation properties. The strong absorbance combined with multiple carriers being excited per photon yield a very high carrier density at the surface, triggering photobleaching reactions that rapidly reduce the emission intensity. Concurrent to and in spite of this photobleaching, a recovery of the emission intensity as a function of dose was observed. X-ray photoelectron spectroscopy and X-ray diffraction measurements of XUV-exposed and unexposed areas show that this recovery is caused by XUV-induced oxidation of MAPbBr3, which removes trap states that normally quench emission, thus counteracting the rapid photobleaching caused by the extremely high carrier densities. Furthermore, it was found that preoxidizing the sample with ozone was able to prolong and improve this intensity recovery, highlighting the impact of surface passivation on the scintillation properties of perovskite materials in the XUV range.

Hf Deposition Stabilizes the Surface Chemistry of Perovskite Manganite Oxide.

Stable composition and catalytic activity of surfaces are among the key requirements for materials employed in energy storage and conversion devices, such as solid oxide fuel cells (SOFCs). Perovskite oxides that serve as cathode in SOFCs suffer from segregation of the aliovalent substitutional cations and the formation of an inert, non-conductive phase at the surface. Here, we demonstrate that the surface of the state-of-the-art SOFC cathode material La0.8Sr0.2MnO3 (LSM) is stabilized against the segregation of Sr at high temperature by submonolayer coverages of Hf. The Hf is vapor-deposited onto the LSM thin film surface by e-beam evaporation. Using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we analyze the surface composition of LSM thin films. Half the LSM surface was kept as-prepared, and half was Hf-modified, for a direct comparison of untreated and Hf-treated regions on the same sample. The formation of a binary SrOx surface species is quantified as descriptor for surface degradation. The onset of Sr segregation is observed at 450 °C on the bare LSM, followed by a substantial advance at 550 °C. Hf-treated regions of the same LSM surface exhibit significantly less Sr surface segregation at 450-550 °C. We interpret this stabilization imparted by Hf to arise from the suppression of the electrostatic attraction of Sr2+ cations to surface oxygen vacancies. Doping the surface layer with Hf, that has a higher affinity to oxygen, reduces this attraction by decreasing the surface oxygen vacancy concentration. In doing so, the use of physical vapor deposition highlights the direct role of the metal species in this system and excludes artifacts that could be introduced via chemical routes. The present work demonstrates this stabilizing effect of Hf on the surface of LSM, broadening the relevance of our prior findings on surface metal doping of other perovskite oxides.

Unraveling CO adsorption on model single-atom catalysts.

Understanding how the local environment of a "single-atom" catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper1, silver1, gold1, nickel1, palladium1, platinum1, rhodium1, and iridium1 species on Fe3O4(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag1-CO) or weaken it (as for Ni1-CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.