Catalytic, Spectroscopic, and Theoretical Studies of Fe4S4-Based Coordination Polymers as Heterogenous Coupled Proton-Electron Transfer Mediators for Electrocatalysis

Iron-sulfur clusters play essential roles in biological systems, and thus synthetic [Fe4S4] clusters have been an area of active research. Recent studies have demonstrated that soluble [Fe4S4] clusters can serve as net H atom transfer mediators, improving the activity and selectivity of a homogeneous Mn CO2 reduction catalyst. Here, we demonstrate that incorporating these [Fe4S4] clusters into a coordination polymer enables heterogeneous H atom transfer from an electrode surface to a Mn complex dissolved in solution. A previously reported solution-processable Fe4S4-based coordination polymer was successfully deposited on the surfaces of different electrodes. The coated electrodes serve as H atom transfer mediators to a soluble Mn CO2 reduction catalyst displaying good product selectivity for formic acid. Furthermore, these electrodes are recyclable with a minimal decrease in activity after multiple catalytic cycles. The heterogenization of the mediator also enables the characterization of solution-phase and electrode surface species separately. Surface enhanced infrared absorption spectroscopy (SEIRAS) reveals spectroscopic signatures for an in situ generated active Mn-H species, providing a more complete mechanistic picture for this system. The active species, reaction mechanism, and the protonation sites on the [Fe4S4] clusters were further confirmed by density functional theory calculations. The observed H atom transfer reactivity of these coordination polymer-coated electrodes motivates additional applications of this composite material in reductive H atom transfer electrocatalysis.

Advances in heterogeneous single-cluster catalysis

Heterogeneous single-cluster catalysts (SCCs) comprising atomically precise and isolated metal clusters stabilized on appropriately chosen supports offer exciting prospects for enabling novel chemical reactions owing to their broad structural diversity with unparalled opportunities for engineering their properties. Although the pioneering work revealed intriguing performance trends of size-selected metal clusters deposited on supports, synthetic and analytical challenges hindered a thorough understanding of surface chemistry under realistic conditions. This Review underscores the importance of considering the cluster environment in SCCs, encompassing the development of robust metal-support interactions, precise control over the ligand sphere, the influence of reaction media and dynamic behaviour, to uncover new reactivities. Through examples, we illustrate the criticality of tailoring the entire catalytic ensemble in SCCs to achieve stable and selective performance with practically relevant metal coverages. This expansion in application scope transcends from model reactions to complex and technically relevant reactions. Furthermore, we provide a perspective on the opportunities and future directions for SCC design within this rapidly evolving field.

A new look at the structure of the neutral Au18 cluster: hollow versus filled golden cage

The geometry of the neutral Au₁₈ gold cluster was probed by a combination of quantum chemical calculations and far-infrared multiple photon dissociation (FIR-MPD) spectroscopy of a Kr messenger complex. Two low-lying isomers are identified to potentially contribute to the experimental IR spectrum, both being derived from a star-like Au₁₇ structure upon capping with one extra Au atom either inside (18_1) or outside (18_5) the star. In particular, the present detection of structure 18_1 by DFT computations where a golden cage encapsulates an endohedral Au atom, is intriguing as a stable core-shell isomer has, to our knowledge, never been found before for such small neutral gold clusters. DFT and local coupled-cluster (DLPNO and PNO-CCSD(T)) computations indicate that both Au₁₈ isomers are close to each other, within ∼3 kcal mol^-1, on the energy scale. Although the exact energy ordering is again method-dependent and remains, at present, inconclusive, the most striking spectral signatures of both isomers are related to vibrational modes localized at atoms capping the inner pentaprism sub-structure that result in prominent peaks centered at ∼80 cm^-1, close to the most prominent experimental feature found at 78 cm^-1. The calculated IR spectra of both core-shell and hollow isomers are very similar to each other and both agree comparably well with the experimental FIR-MPD spectra of the Au₁₈Kr_1,2 complexes.

"Magic" Sinter-Resistant Cluster Sizes of Ptn Supported on Alumina

Subnano cluster catalysts, while highly promising due to unique activity, selectivity, and atom-efficiency, are limited in wider applications, as they are prone to deactivation via sintering. Even size-selection, which was previously shown to reduce sintering of nanoparticles, cannot reduce the sintering of highly fluxional subnano clusters due to their inherent isomeric diversity. Here, we use a combination of theory and experiment to show that Pt clusters on Al₂O₃ exhibit size-dependent sintering resistance. We furthermore show that Pt₄/Al₂O₃ and Pt₇/Al₂O₃ are "magic" sinter-resistant cluster sizes. Their stability is attributed to the greater degree of bulk-like crystallinity of the dominant isomers. In addition, we identify different spatial signatures characteristic of the sintering of clusters with differing sintering stabilities.

Reactive Molecular Dynamics Simulations of the Depolymerization of Polyethylene Using Graphene-Oxide-Supported Platinum Nanoparticles

While plastic materials offer many benefits to society, the slow degradation and difficulty in recycling plastics raise important environmental and sustainability concerns. Traditional recycling efforts often lead to materials with inferior properties and correspondingly lower value, making them uneconomical to recycle. Recent efforts have shown promising chemical pathways for converting plastic materials into a wide range of value-added products, feedstocks or monomers. This is commonly referred to as "chemical recycling". Here, we use reactive molecular dynamics (MD) simulations to study the catalytic process of depolymerization of polyethylene (PE) using platinum (Pt) nanoparticles (NPs) in comparison to PE pyrolysis (thermal degradation). We apply a simple kinetic model to our MD results for the catalytic reaction rate as a function of temperature, from which we obtain the activation energy of the reaction, which shows the that the Pt NPs reduce the barrier for depolymerization. We further evaluate the molecular mass distribution of the reaction products to gain insight into the influence of the Pt NPs on reaction selectivity. Our results demonstrate the potential for the reactive MD method to help the design of recycling approaches for polymer materials.

Size-dependent phase transitions boost catalytic activity of sub-nanometer gold clusters

The characterization and identification of the dynamics of cluster catalysis are crucial to unraveling the origin of catalytic activity. However, the dynamical catalytic effects during the reaction process remain unclear. Herein, we investigate the dynamic coupling effect of elementary reactions with the structural fluctuations of sub-nanometer Au clusters with different sizes using ab initio molecular dynamics and the free energy calculation method. It was found that the adsorption-induced solid-to-liquid phase transitions of the cluster catalysts give rise to abnormal entropy increase, facilitating the proceeding of reaction, and this phase transition catalysis exists in a range of clusters with different sizes. Moreover, clusters with different sizes show different transition temperatures, resulting in a non-trivial size effect. These results unveil the dynamic effect of catalysts and help understand cluster catalysis to design better catalysts rationally.

Autonomous Reaction Network Exploration in Homogeneous and Heterogeneous Catalysis

Autonomous computations that rely on automated reaction network elucidation algorithms may pave the way to make computational catalysis on a par with experimental research in the field. Several advantages of this approach are key to catalysis: (i) automation allows one to consider orders of magnitude more structures in a systematic and open-ended fashion than what would be accessible by manual inspection. Eventually, full resolution in terms of structural varieties and conformations as well as with respect to the type and number of potentially important elementary reaction steps (including decomposition reactions that determine turnover numbers) may be achieved. (ii) Fast electronic structure methods with uncertainty quantification warrant high efficiency and reliability in order to not only deliver results quickly, but also to allow for predictive work. (iii) A high degree of autonomy reduces the amount of manual human work, processing errors, and human bias. Although being inherently unbiased, it is still steerable with respect to specific regions of an emerging network and with respect to the addition of new reactant species. This allows for a high fidelity of the formalization of some catalytic process and for surprising in silico discoveries. In this work, we first review the state of the art in computational catalysis to embed autonomous explorations into the general field from which it draws its ingredients. We then elaborate on the specific conceptual issues that arise in the context of autonomous computational procedures, some of which we discuss at an example catalytic system.

GRAPHICAL ABSTRACT

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11244-021-01543-9.

CO organization at ambient pressure on stepped Pt surfaces: first principles modeling accelerated by neural networks

Step and kink sites at Pt surfaces have crucial importance in catalysis. We employ a high dimensional neural network potential (HDNNP) trained using first-principles calculations to determine the adsorption structure of CO under ambient conditions (T = 300 K and P = 1 atm) on these surfaces. To thoroughly explore the potential energy surface (PES), we use a modified basin hopping method. We utilize the explored PES to identify the adsorbate structures and show that under the considered conditions several low free energy structures exist. Under the considered temperature and pressure conditions, the step edge (or kink) is totally occupied by on-top CO molecules. We show that the step structure and the structure of CO molecules on the step dictate the arrangement of CO molecules on the lower terrace. On surfaces with (111) steps, like Pt(553), CO forms quasi-hexagonal structures on the terrace with the top site preferred, with on average two top site CO for one multiply bonded CO, while in contrast surfaces with (100) steps, like Pt(557), present a majority of multiply bonded CO on their terrace. Short terraced surfaces, like Pt(643), with square (100) steps that are broken by kink sites constrain the CO arrangement parallel to the step edge. Overall, this effort provides detailed analysis on the influence of the step edge structure, kink sites, and terrace width on the organization of CO molecules on non-reconstructed stepped surfaces, yielding initial structures for understanding restructuring events driven by CO at high coverages and ambient pressure.

Doping Platinum with Germanium: An Effective Way to Mitigate the CO Poisoning

The vulnerability towards CO poisoning is a major drawback affecting the efficiency and long-term performance of platinum catalysts in fuel cells. In the present work, by a combination of density functional theory calculations and mass spectrometry experiments, we test and explain the promotional effect of Ge on Pt catalysts with higher resistance to deactivation via CO poisoning. A thorough exploration of the configurational space of gas-phase Pt_n ⁺ and GePt_n-1 ⁺ (n=5-9) clusters using global minima search techniques and the subsequent electronic structure analysis reveals that germanium doping reduces the binding strength between Pt and CO by hindering the 2π-back-donation. Importantly, the clusters remain catalytically active towards H₂ dissociation. The ability of Ge to weaken the Pt-CO interaction was confirmed by mass spectrometry experiments. Ge can be a promising alloying agent to tune the selectivity and improve the durability of Pt particles, thus opening the way to novel catalytic alternatives for fuel cells.

Structure, Energetics, and Thermal Behavior of Bimetallic Re-Pt Clusters

Bimetallic Re-Pt is a widely used catalyst in petroleum reforming to obtain high-octane gasoline, but experimental and theoretical information of such systems at the subnanometer scale-namely, as cluster aggregates-is currently lacking. Thus, in this work, we performed a density functional theory-based global optimization study to determine the physicochemical properties of the most stable Re-Pt gas-phase clusters up to six atoms for all compositions. Our results indicate that in these putative global minima (GM) geometries, Re atoms tend to aggregate, while most Pt atoms remain separated from each other. This is even observed in Pt-rich clusters-an indication of the strength of the Re-Re and Re-Pt bonds over pure Pt-Pt ones-due to a strong, directional hybridization of the Re half-filled 5d and the nearly full Pt 5d states. We observe that doping monometallic Pt clusters even with a single Re atom increases their binding energy values and widens the bimetallic cluster highest occupied molecular orbital-lowest unoccupied molecular orbital gap. As catalysis occurs at elevated temperatures, we explore the concept of cluster fluxionality for Re-Pt minima in terms of the calculated isomer occupation probability, P(T). This allows us to quantify the abundance of GM and low-energy isomer configurations as a function of temperature. This is done at size 5 atoms due to the wide isomer observed variety. Our calculations indicate that for pure Re₅, the P(T) of the GM configuration substantially decreases after 750 K. Especially, for Re₄Pt₁, the GM is the dominant structure up to nearly 700 K when the second-energy isomer becomes the stable one. Although no ordering changes are seen for Re₃Pt₂, Re₂Pt₃, and Re₁Pt₄, we do observe a structural transition-between the GM and the second isomer-for pure Pt₅ above 1000 K. We expect this type of combined first-principles analysis to add to the overall, continuous understanding of the stability and energetics of ultrafine and highly-dispersed Re-Pt petroleum-reforming catalysts and the scarce available information on this particular bimetallic system.

When Fluxionality Beats Size Selection: Acceleration of Ostwald Ripening of Sub-Nano Clusters

Size selection was demonstrated to suppress Ostwald ripening of supported catalytic nanoparticles. When the supported clusters are subnanometer in size and highly fluxional, such as Pt clusters on the rutile TiO₂ (110) surface, this paradigm breaks down, and the established theory of sintering needs a revision. At temperatures characteristic of catalysis (i.e. 700 K), sub-nano clusters thermally populate many low-energy metastable isomers. As these isomers all have different geometric and electronic structures, and thus, formation and dissociation energies (in lieu of surface energy), Ostwald ripening is not suppressed, despite the size-selection. However, some clusters arise as magic numbers in terms of sintering stability at the ensemble level. Acceleration of sintering by metastable species persists though weakens in polydisperse cluster systems. We propose a competing pathways theory for sintering, which at the atomistic level describes the found size-specific sintering behavior.

Computational Methods in Heterogeneous Catalysis

The unprecedented ability of computations to probe atomic-level details of catalytic systems holds immense promise for the fundamentals-based bottom-up design of novel heterogeneous catalysts, which are at the heart of the chemical and energy sectors of industry. Here, we critically analyze recent advances in computational heterogeneous catalysis. First, we will survey the progress in electronic structure methods and atomistic catalyst models employed, which have enabled the catalysis community to build increasingly intricate, realistic, and accurate models of the active sites of supported transition-metal catalysts. We then review developments in microkinetic modeling, specifically mean-field microkinetic models and kinetic Monte Carlo simulations, which bridge the gap between nanoscale computational insights and macroscale experimental kinetics data with increasing fidelity. We finally review the advancements in theoretical methods for accelerating catalyst design and discovery. Throughout the review, we provide ample examples of applications, discuss remaining challenges, and provide our outlook for the near future.

The Mechanism of the Intramolecular Hydrocarbyl Metathesis within a Planar Triruthenium Cluster: Combining Core Flexibility with Hydride Mobility

The transition metal catalysed formation and cleavage of C-C bonds is of utmost importance in synthetic chemistry. While most of the existing homogeneous catalysts are mononuclear, knowledge of the behaviour of polynuclear species is much more limited. By using computational methods, here we shed light into the mechanistic details of the thermally-induced isomerization of Cp*₃ Ru₃ (μ-H)₂ (μ₃ -η² -pentyne)(μ₃ -pentylidyne) (2) into Cp*₃ Ru₃ (μ-H)₂ (μ₃ -η² -octyne)(μ₃ -ethylidyne) (3), a process that involves the migration of a C₃ fragment between the hydrocarbyl ligands and across the plane formed by the three Ru centres. Our results show this to be a complex transformation that comprises of five individual rearrangements in an A→B→A→B→A order. Each so-called rearrangement A consists of the CH migration from the μ₃ -η² -alkyne into the μ₃ -alkylidine ligand in the other side of the Ru₃ plane. This process is facilitated by the cluster's ability to adopt open-core structures in which one Ru-Ru bond is broken and a new C-C bond is formed. In contrast, rearrangements B do not involve the formation or cleavage of C-C bonds, nor do they require the opening of the cluster core. Instead, they consist of the isomerization of the μ₃ -η² -alkyne and μ₃ -alkylidyne ligands on each side of the triruthenium plane into μ₃ -alkylidyne and μ₃ -η² -alkyne, respectively. Such transformation implies the migration of three H atoms within the hydrocarbyl ligands, and in this case, it is aided by the cluster's ability to behave as a H reservoir. All in all, this study highlights the plasticity of these Ru₃ clusters, whereby Ru-Ru, Ru-C, Ru-H, C-C, and C-H bonds are formed and broken with surprising ease.

Globally optimal catalytic fields for a Diels-Alder reaction

In a previous paper [M. Dittner and B. Hartke, J. Chem. Theory Comput. 14, 3547 (2018)], we introduced a preliminary version of our GOCAT (globally optimal catalyst) concept in which electrostatic catalysts are designed for arbitrary reactions by global optimization of distributed point charges that surround the reaction. In this first version, a pre-defined reaction path was kept fixed. This unrealistic assumption allowed for only small catalytic effects. In the present work, we extend our GOCAT framework by a sophisticated and robust on-the-fly reaction path optimization, plus further concomitant algorithm adaptions. This allows smaller and larger excursions from a pre-defined reaction path under the influence of the GOCAT point-charge surrounding, all the way to drastic mechanistic changes. In contrast to the restricted first GOCAT version, this new version is able to address real-life catalysis. We demonstrate this by applying it to the electrostatic catalysis of a prototypical Diels-Alder reaction. Without using any prior information, this procedure re-discovers theoretically and experimentally established features of electrostatic catalysis of this very reaction, including a field-dependent transition from the synchronous, concerted textbook mechanism to a zwitterionic two-step mechanism, and diastereomeric discrimination by suitable electric field components.

Ensembles of Metastable States Govern Heterogeneous Catalysis on Dynamic Interfaces

Heterogeneous catalysis is at the heart of the chemical industry. Being able to tune and design efficient catalysts for processes of interest is of the utmost importance, and for this, a molecular-level understanding of heterogeneous catalysts is the first step and indeed a prime focus of modern catalysis research. For a long time, the single most thermodynamically stable structure of the catalytic interface attained under the reaction conditions had been envisioned as the reactive phase. However, some catalytic interfaces continue to undergo structural dynamics in the steady state, triggered by high temperatures and pressures and binding and changing reagents. Among particularly dynamic interfaces are such widely used catalysts as crystalline and amorphous surfaced supporting (sub)nanometallic clusters. Recently, it became clear that this dynamic fluxionality causes the supported clusters to populate many distinct structural and stoichiometric states under catalytic conditions. Hence, the catalytic interface should be viewed as an evolving statistical ensemble of many structures (rather than one structure). Every member in the ensemble contributes to the properties of the catalyst differently, in proportion to its probability of being populated. This new notion flips the established paradigm and calls for a new theory, new modeling approaches, operando measurements, and updated design strategies. The statistical ensemble nature of surface-supported subnanocluster catalysts can be exemplified by oxide-supported and adsorbate-covered Pt, Pd, Cu, and CuPd clusters, which are catalytic toward oxidative and nonoxidative dehydrogenation. They have access to a variety of 3D and quasi-2D shapes. The compositions of their thermal ensembles are dependent on the cluster size, leading to size-specific catalytic activities and the famous "every atom counts" phenomenon. The support and adsorbates affect catalyst structures, and the state of the reacting species causes the ensemble to change in every reaction intermediate. The most stable member of the ensemble dominates the thermodynamic properties of the corresponding intermediate, whereas the kinetics can be determined by more active but less populated metastable catalyst states, and that suggests that many earlier studies might have overlooked the actual active sites. Both effects depend on the relative time scales of catalyst restructuring and reaction dynamics. The catalyst may routinely operate off-equilibrium. Ensemble phenomena lead to surprising exceptions from established rules of catalysis, such as scaling relations and Arrhenius behavior. Catalyst deactivation is also an ensemble property, and its extent of mitigation can be predicted through the new paradigm. These findings were enabled by advances in theory, such as global optimization and subsequent utilization of multiple local minima and pathways sampling as well as operando catalyst characterization. The fact that the per-site and per-species resolution is needed for the description and prediction of catalyst properties gives theory the central role in catalysis research, as most experiments provide ensemble-average information and cannot detect the crucial minority species that may be responsible for the catalytic activity.

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

Synchrotron radiation provides insight into spectroscopy and dynamics in clusters and nanoparticles.

An Eight-Atom Iridium-Aluminum Oxide Cluster IrAlO6+ Catalytically Oxidizes Six CO Molecules

Fundamental understanding regarding oxygen storage capacity involving how and why an active site can buffer a large number of oxygen atoms in redox processes is vital to the design of advanced oxygen storage materials, while it is challenging because of the complexity of heterogeneous catalysis. Herein, we identified that an eight-atom iridium-aluminum oxide cluster IrAlO₆⁺ can transfer all the oxygen atoms to catalytically oxidize six CO molecules. This finding represents a breakthrough in cluster catalysis where at most three oxygen atoms from a heteronuclear metal oxide cluster can be catalytically involved in CO oxidation. We found that oxygen prefers to be stored on aluminum to form an O₃^-• radical in the energetically unfavorable IrAlO₆⁺ isomer and generate the low-coordinated iridium that is pivotal to capturing CO and triggering the catalysis. The powerful electron cycling capability of iridium and the cooperative iridium-aluminum interplay are emphasized to drive the oxygen atom-transfer behavior.

Designing Catalytic Sites on Surfaces with Optimal H-Atom Binding via Atom Doping Using the Inverse Molecular Design Approach

It remains a general challenge to computationally design optimal catalytic structures based on earth-abundant metals for hydrogenation. Here, we demonstrate an effective computational approach based on inverse molecular design to deterministically design optimal catalytic sites on the Cu(100) surface through the doping of Fe and/or Zn, and a stable Zn-doped Cu(100) surface was found with minimal binding energy to H atoms. By the calculations at the level of density functional theory, the optimized catalyst sites are verified to be valid on the Cu(100) surface in an infinite periodic system. We analyze the electronic structure cause of the optimal binding sites using the analysis of the density of states. In addition, we use a Cu₂₉Zn₃ atomic cluster, where such an optimum catalytic site is valid on the Cu(100) surface, to understand the role of doped Zn atoms on lowering the H atom binding energy. We found that in the atomic cluster, the atomic orbitals of surface Zn-atoms show less participation in the binding of H atoms, compared to the atomic orbitals of surface Cu atoms. Our study provides valuable chemistry insights on designing catalytic structures using earth-abundant metals, and it may lead to the development of novel Cu-based earth-abundant alloys in bulk, nanoparticles, atomic clusters, or single-atom catalysts for important catalytic applications such as lignin degradation or CO₂ conversion.

Atomic-scale engineering of indium oxide promotion by palladium for methanol production via CO2 hydrogenation

Metal promotion is broadly applied to enhance the performance of heterogeneous catalysts to fulfill industrial requirements. Still, generating and quantifying the effect of the promoter speciation that exclusively introduces desired properties and ensures proximity to or accommodation within the active site and durability upon reaction is very challenging. Recently, In2O3 was discovered as a highly selective and stable catalyst for green methanol production from CO2. Activity boosting by promotion with palladium, an efficient H2-splitter, was partially successful since palladium nanoparticles mediate the parasitic reverse water-gas shift reaction, reducing selectivity, and sinter or alloy with indium, limiting metal utilization and robustness. Here, we show that the precise palladium atoms architecture reached by controlled co-precipitation eliminates these limitations. Palladium atoms replacing indium atoms in the active In3O5 ensemble attract additional palladium atoms deposited onto the surface forming low-nuclearity clusters, which foster H2 activation and remain unaltered, enabling record productivities for 500 h.

Metal Nanoclusters: New Paradigm in Catalysis for Water Splitting, Solar and Chemical Energy Conversion

A sustainable future demands innovative breakthroughs in science and technology today, especially in the energy sector. Earth-abundant resources can be explored and used to develop renewable and sustainable resources of energy to meet the ever-increasing global energy demand. Efficient solar-powered conversion systems exploiting inexpensive and robust catalytic materials for the photo- and photo-electro-catalytic water splitting, photovoltaic cells, fuel cells, and usage of waste products (such as CO₂ ) as chemical fuels are appealing solutions. Many electrocatalysts and nanomaterials have been extensively studied in this regard. Low overpotentials, catalytic stability, and accessibility remain major challenges. Metal nanoclusters (NCs, ≤3 nm) with dimensions between molecule and nanoparticles (NPs) are innovative materials in catalysis. They behave like a "superatom" with exciting size- and facet-dependent properties and dynamic intrinsic characteristics. Being an emerging field in recent scientific endeavors, metal NCs are believed to replace the natural photosystem II for the generation of green electrons in a viable way to facilitate the challenging catalytic processes in energy-conversion schemes. This Review aims to discuss metal NCs in terms of their unique physicochemical properties, possible synthetic approaches by wet chemistry, and various applications (mostly recent advances in the electrochemical and photo-electrochemical water splitting cycle and the oxygen reduction reaction in fuel cells). Moreover, the significant role that MNCs play in dye-sensitized solar cells and nanoarrays as a light-harvesting antenna, the electrochemical reduction of CO₂ into fuels, and concluding remarks about the present and future perspectives of MNCs in the frontiers of surface science are also critically reviewed.

Catalytic conversions of atmospheric sulfur dioxide and formation of acid rain over mineral dusts: Molecular oxygen as the oxygen source

Sulfur dioxide (SO₂) ranks as a major air pollutant and is likely to generate acid rain. When molecular oxygen is the oxygen source, the regular surfaces of gibbsite (one of the most abundant mineral dusts) show no reactivity for SO₂ conversions to H₂SO₄, while the partially dehydrated (100) surface with coordination-unsaturated Al sites becomes catalytically effective. Because of the easy availability of molecular oxygen, results manifest that acid rain can form under all atmospheric conditions and may account for the high conversion ratio of atmospheric SO₂. The (100) and (001) surfaces show divergent catalytic effects, and hydrolysis is always the rate-limiting step. Path A (hydrolysis and then oxidation) is preferred for (100) surface, whereas a third path with obviously lower activation barriers is presented for (001) surface, which is non-existent for (100) surface. Atomic oxygen originating from the dissociation of molecular oxygen is catalytically active for (100) surface, while the active site of (001) surface fails to be recovered, suggesting that SO₂ conversions over gibbsite surfaces are facet-controlled. This work also offers an environmentally friendly route for production of H₂SO₄ (one of the essential compounds in chemical industry), directly using molecular oxygen as the oxygen source.

Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species

Understanding the molecular-level properties of electrochemically active ions at operating electrode-electrolyte interfaces (EEI) is key to the rational development of high-performance nanostructured surfaces for applications in energy technology. Herein, an electrochemical cell coupled with ion soft landing is employed to examine the effect of "atom-by-atom" metal substitution on the activity and stability of well-defined redox-active anions, PMo _xW_{12- x}O₄₀^3- ( x = 0, 1, 2, 3, 6, 9, or 12) at nanostructured ionic liquid EEI. A striking observation made by in situ electrochemical measurements and further supported by theoretical calculations is that the substitution of only one to three tungsten atoms by molybdenum atoms in the PW₁₂O₄₀^3- anions results in a substantial spike in their first reduction potential. Specifically, PMo₃W₉O₄₀^3- showed the highest redox activity in both in situ electrochemical measurements and as part of a functional redox supercapacitor device, making it a "super-active redox anion" compared with all other PMo _xW_{12- x}O₄₀^3- species. Electronic structure calculations showed that metal substitution in PMo _xW_{12- x}O₄₀^3- causes the lowest unoccupied molecular orbital (LUMO) to protrude locally, making it the "active site" for reduction of the anion. Several critical factors contribute to the observed trend in redox activity including (i) multiple isomeric structures populated at room temperature, which affect the experimentally determined reduction potential; (ii) substantial decrease of the LUMO energy upon replacement of W atoms with more-electronegative Mo atoms; and (iii) structural relaxation of the reduced species produced after the first reduction step. Our results illustrate a path to achieving superior performance of technologically relevant EEIs in functional nanoscale devices through understanding of the molecular-level electronic properties of specific electroactive species with "atom-by-atom" precision.

Role of sterics in phosphine-ligated gold clusters

This study examined the solution-phase exchange reactions of triphenylphosphine (PPh₃) ligands on Au₈L₇²⁺ (L = PPh₃) gold clusters with three different tolyl ligands using electrospray ionization mass spectrometry to provide insight into how steric differences in the phosphines influence the extent of ligand exchange and the stability of the resulting mixed-phosphine clusters.