Single-Atom Catalysis: Insights from Model Systems.

The field of single-atom catalysis (SAC) has expanded greatly in recent years. While there has been much success developing new synthesis methods, a fundamental disconnect exists between most experiments and the theoretical computations used to model them. The real catalysts are based on powder supports, which inevitably contain a multitude of different facets, different surface sites, defects, hydroxyl groups, and other contaminants due to the environment. This makes it extremely difficult to determine the structure of the active SAC site using current techniques. To be tractable, computations aimed at modeling SAC utilize periodic boundary conditions and low-index facets of an idealized support. Thus, the reaction barriers and mechanisms determined computationally represent, at best, a plausibility argument, and there is a strong chance that some critical aspect is omitted. One way to better understand what is plausible is by experimental modeling, i.e., comparing the results of computations to experiments based on precisely defined single-crystalline supports prepared in an ultrahigh-vacuum (UHV) environment. In this review, we report the status of the surface-science literature as it pertains to SAC. We focus on experimental work on supports where the site of the metal atom are unambiguously determined from experiment, in particular, the surfaces of rutile and anatase TiO2, the iron oxides Fe2O3 and Fe3O4, as well as CeO2 and MgO. Much of this work is based on scanning probe microscopy in conjunction with spectroscopy, and we highlight the remarkably few studies in which metal atoms are stable on low-index surfaces of typical supports. In the Perspective section, we discuss the possibility for expanding such studies into other relevant supports.

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.

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.

IrO_{2} Surface Complexions Identified through Machine Learning and Surface Investigations.

A Gaussian approximation potential was trained using density-functional theory data to enable a global geometry optimization of low-index rutile IrO_{2} facets through simulated annealing. Ab initio thermodynamics identifies (101) and (111) (1×1) terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate and exhibit the theoretically predicted (1×1) periodicity and x-ray photoelectron spectroscopy core-level shifts. The obtained structures are analogous to the complexions discussed in the context of ceramic battery materials.

Atomic-Scale Studies of Fe3 O4 (001) and TiO2 (110) Surfaces Following Immersion in CO2 -Acidified Water.

Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water and studied its interaction with TiO2 and Fe3 O4 surfaces. Herein, we describe a simple approach to vary the pH through the partial pressure of CO2 ( p C O 2 ) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO2 (110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3 O4 (001)-( 2 × 2 )R45° surface begins to dissolve at a pH 4.0-3.9 ( p C O 2 =0.8-1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe3 O4 (001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced and contains a significant accumulation of bicarbonate (HCO3 - ) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3 )2 ), which precipitate onto the surface when the water evaporates.

Order-disorder phase transition of the subsurface cation vacancy reconstruction on Fe3O4(001).

We present surface X-ray diffraction and fast scanning tunneling microscopy results to elucidate the nature of the surface phase transition on magnetite (001) from a reconstructed to a non-reconstructed surface around 720 K. In situ surface X-ray diffraction at a temperature above the phase transition, at which long-range order is lost, gives evidence that the subsurface cation vacancy reconstruction still exists as a local structural motif, in line with the characteristics of a 2D second-order phase transition. Fast scanning tunneling microscopy results across the phase transition underpin the hypothesis that the reconstruction lifting is initiated by surplus Fe ions occupying subsurface octahedral vacancies. The reversible near-surface iron enrichment and reduction of the surface to stoichiometric composition is further confirmed by in situ low-energy ion scattering, as well as ultraviolet and X-ray photoemission results.

Adsorption and reaction of methanol on Fe3O4(001).

The interaction of methanol with iron oxide surfaces is of interest due to its potential in hydrogen storage and from a fundamental perspective as a chemical probe of reactivity. We present here a study examining the adsorption and reaction of methanol on magnetite Fe3O4(001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The methanol desorption profile from Fe3O4(001) is complex, exhibiting peaks at 140 K, 173 K, 230 K, and 268 K, corresponding to the desorption of intact methanol, as well as peaks at 341 K and 495 K due to the reaction of methoxy intermediates. The saturation of a monolayer of methanol corresponds to ∼5 molecules/unit cell (u.c.), which is slightly higher than the number of surface octahedral iron atoms of 4/u.c. We probe the kinetics and thermodynamics of the desorption of molecular methanol using inversion analysis. The deconvolution of the complex desorption profile into individual peaks allows for calculations of both the desorption energy and the prefactor of each feature. The initial 0.7 methanol/u.c. reacts to form methoxy and hydroxy intermediates at 180 K, which remain on the surface above room temperature after intact methanol has desorbed. The methoxy species react via one of two channels, a recombination reaction with surface hydroxyls to form additional methanol at ∼350 K and a disproportionation reaction to form methanol and formaldehyde at ∼500 K. Only 20% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway.

Electron transfer between anatase TiO2 and an O2 molecule directly observed by atomic force microscopy.

Activation of molecular oxygen is a key step in converting fuels into energy, but there is precious little experimental insight into how the process proceeds at the atomic scale. Here, we show that a combined atomic force microscopy/scanning tunneling microscopy (AFM/STM) experiment can both distinguish neutral O2 molecules in the triplet state from negatively charged (O2)- radicals and charge and discharge the molecules at will. By measuring the chemical forces above the different species adsorbed on an anatase TiO2 surface, we show that the tip-generated (O2)- radicals are identical to those created when (i) an O2 molecule accepts an electron from a near-surface dopant or (ii) when a photo-generated electron is transferred following irradiation of the anatase sample with UV light. Kelvin probe spectroscopy measurements indicate that electron transfer between the TiO2 and the adsorbed molecules is governed by competition between electron affinity of the physisorbed (triplet) O2 and band bending induced by the (O2)- radicals. Temperature-programmed desorption and X-ray photoelectron spectroscopy data provide information about thermal stability of the species, and confirm the chemical identification inferred from AFM/STM.

Electrochemical Stability of the Reconstructed Fe3 O4 (001) Surface.

Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)-oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X-ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between -0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm-2 and strongly affects the OER kinetics. We attribute this to a stabilization of the Fe3 O4 bulk by the reconstructed surface. At more negative potentials, a gradual and largely irreversible lifting of the reconstruction is observed due to the onset of oxide reduction.

Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water.

Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g., dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the ultrahigh vacuum-compatible dosing of liquids to explore the stability of the Fe3O4(001)-(√2 × âˆš2)R45° surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy and low-energy electron diffraction data show that extensive hydroxylation causes the surface to revert to a bulklike (1 × 1) termination. However, scanning tunneling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM studies leads us to believe that a similar structure exists at the solid-electrolyte interface during immersion at pH 7.

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.

Surface point defects on bulk oxides: atomically-resolved scanning probe microscopy.

Metal oxides are abundant in nature and they are some of the most versatile materials for applications ranging from catalysis to novel electronics. The physical and chemical properties of metal oxides are dramatically influenced, and can be judiciously tailored, by defects. Small changes in stoichiometry introduce so-called intrinsic defects, e.g., atomic vacancies and/or interstitials. This review gives an overview of using Scanning Probe Microscopy (SPM), in particular Scanning Tunneling Microscopy (STM), to study the changes in the local geometric and electronic structure related to these intrinsic point defects at the surfaces of metal oxides. Three prototypical systems are discussed: titanium dioxide (TiO2), iron oxides (Fe3O4), and, as an example for a post-transition-metal oxide, indium oxide (In2O3). Each of these three materials prefers a different type of surface point defect: oxygen vacancies, cation vacancies, and cation adatoms, respectively. The different modes of STM imaging and the promising capabilities of non-contact Atomic Force Microscopy (nc-AFM) techniques are discussed, as well as the capability of STM to manipulate single point defects.

Adsorption of water at the SrO surface of ruthenates.

Although perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Sr(n+1)Ru(n)O3(n+1) (n = 1, 2) using low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighbouring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions. The OH stemming from the original molecule stays trapped at Sr-Sr bridge positions, circling the surface OH with a measured activation energy of 187 ± 10 meV. At higher coverage, dimers of dissociated water assemble into one-dimensional chains and form a percolating network where water adsorbs molecularly in the gaps. Our work shows the limitations of applying surface chemistry concepts derived for binary rocksalt oxides to perovskites.

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.

Carbon monoxide-induced adatom sintering in a Pd-Fe3O4 model catalyst.

The coarsening of catalytically active metal clusters is often accelerated by the presence of gases, but the role played by gas molecules is difficult to ascertain and varies from system to system. We use scanning tunnelling microscopy to follow the CO-induced coalescence of Pd adatoms supported on the Fe3O4(001) surface at room temperature, and find Pd-carbonyl species to be responsible for mobility in this system. Once these reach a critical density, clusters nucleate; subsequent coarsening occurs through cluster diffusion and coalescence. Whereas CO induces the mobility in the Pd/Fe3O4 system, surface hydroxyls have the opposite effect. Pd atoms transported to surface OH groups are no longer susceptible to carbonyl formation and remain isolated. Following the evolution from well-dispersed metal adatoms into clusters, atom-by-atom, allows identification of the key processes that underlie gas-induced mass transport.

Charge trapping at the step edges of TiO(2) anatase (101).

A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the TiO2 anatase (101) surface are trapped at step edges. Consequently, steps act as preferred adsorption sites for O2 . In density functional theory calculations electrons localize at clean step edges, this tendency is enhanced by O vacancies and hydroxylation. The results show the importance of defects for the wide-ranging applications of titania.