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The attention toward single-atom catalysts (SACs) for electrochemical processes is growing at an impressive pace. Electronic structure calculations play an important role in this race by providing proposals of potentially relevant catalysts based on screening studies or on the identification of descriptors of the chemical activity. So far, almost all of these predictions ignore a crucial aspect in the design of a catalyst: its stability. We propose a simple yet general first-principles approach to predict the stability of SACs under working conditions of pH and applied voltage. This is based on the construction of a thermodynamic cycle, where part of the information is taken from experiment and the rest from density functional theory (DFT) calculations. In particular, we make use of the formalism of Pourbaix diagrams to investigate the stability of SACs in reductive or oxidative conditions and we identify those that show a pronounced tendency to dissolve or to form other chemical species. Applying the procedure to four transition metal atoms, Cr, Mn, Fe, and Co, and to three supports, N-doped graphene, carbon nitride, and covalent organic frameworks, we show that a key factor in determining the final stability is the binding energy of the free metal atom to the support. The results show that several potentially very good catalysts in key electrochemical reactions are, in fact, dramatically prone to dissolution or transformation in other chemical species, suggesting that every prediction of the SAC’s catalytic activity should be accompanied by a parallel investigation of the stability.

Single atom catalysis is relatively new frontier in catalysis with potential application in several critical chemical processes, such as water splitting reactions. Single atom catalysts (SACs) are analogues of coordination chemistry compounds, opening the way to the formation of unconventional intermediates compared to extended metal or oxide catalyst surfaces. In this work we show by means of density functional theory (DFT) calculations that the formation of unconventional adsorbates on SACs in the oxygen evolution reaction (OER) leads to complex scaling relationships, that differ from the scaling relations observed on extended surfaces. The evidence of new scaling relations directly impacts the expected catalytic activity and provides a further example of the importance of including in the modelling species beyond those that form on classical electrodes.

Cross-coupling reactions are of great importance in chemistry due to their ability to facilitate the construction of complex organic molecules. Among these reactions, the Ullmann-type C−O coupling between phenols and aryl halides is particularly noteworthy and useful for preparing diarylethers. However, this reaction typically relies on homogeneous catalysts that rapidly deactivate under harsh reaction conditions. In this study, we introduce a novel heterogeneous catalyst for the Ullmann-type C−O coupling reaction, comprised of isolated Cu atoms chelated to a tetraethylenepentamine-pyrrole ligand that is immobilized on graphite nanoplatelets. The catalytic study reveals the recyclability of the material, and demonstrates the crucial role of the pyrrole linker in stabilizing the Cu sites. The work expands the potential of single-atom catalyst nanoarchitectures and underscores the significance of ligands in stabilizing metals in cationic forms, providing a novel, tailored catalyst for cross-coupling chemistries.

For more than half a century, the industrial production of methanol in the world has been based on a heterogeneous Cu/ZnO/Al2O3 catalyst where Cu nanoparticles are dispersed on an alumina support with ZnO particles as promoters. The reaction starts with syngas, a mixture of CO, CO2, and hydrogen, and the hydrogenation of carbon dioxide is an essential part of the process. Despite the technological importance and the high number of studies dedicated to identifying the structure of the functioning catalyst, there are still several open questions on the nature of the active sites and phases. Over the years, various hypotheses have been formulated. The most accredited versions concern the formation of CuZn alloys, the migration of ZnO over the Cu nanoparticles which become covered by an ultrathin oxide layer, and the role of the Cu-ZnO interface and, in particular, of the presence of Cu+ ions. The role of alumina as a promoter has also been widely discussed, suggesting that the oxide provides acidic sites where intermediates are stabilized or that Al dopants enter the ZnO phase, altering its properties. The reality, as discussed in this brief overview, is that all these mechanisms are probably contributing but under different conditions of temperature, pressure, and reagent composition. This results in a dynamic catalyst that makes unambiguous identification of active sites difficult, if not impossible. Less controversial is the reaction mechanism, although even in this case the path followed by the reaction is a delicate function of various conditions, including the composition of the reaction mixture and the presence of water or other contaminants.

This work evaluates the feasibility of alkaline hydrogen evolution reaction (HER) using Pt single-atoms (1.0 wt %) on defect-rich ceria (Pt1/CeOx) as an active and stable dual-site catalyst. The catalyst displayed a low overpotential and a small Tafel slope in an alkaline medium. Moreover, Pt1/CeOx presented a high mass activity and excellent durability, competing with those of the commercial Pt/C (20 wt %). In this picture, the defective CeOx is active for water adsorption and dissociation to create H* intermediates, providing the first site where the reaction occurs. The H* intermediate species then migrate to adsorb and react on the Pt2+ isolated atoms, the site where H2 is formed and released. DFT calculations were also performed to obtain mechanistic insight on the Pt1/CeOx catalyst for the HER. The results indicate a new possibility to improve the state-of-the-art alkaline HER catalysts via a combined effect of the O vacancies on the ceria support and Pt2+ single atoms.

Although visible-light-driven dual photoredox catalysis, a method that combines photoabsorbers and transition metals, has become a powerful tool to conduct coupling reactions, resource economical and scalability issues persist, owing to the use of catalysts and light absorbers that exploit critical raw materials (such as iridium complexes), and are homogeneous in nature. Here we report the merger of metallic single-atom and photoredox catalysis, in the form of a Ni atom-supported carbon nitride material, for the C–O coupling of carboxylic acids and alkyl halides. This operationally straightforward system, composed of only earth-abundant components, exhibits a wide functional group tolerance. Additionally, short reaction times, facile recovery and high catalyst stability make this method highly attractive for industrial applications.

Single-atom catalysts (SACs) are heterogenous catalysts with elements in common with coordination compounds. We discuss some fundamental elements required for the successful computational modeling of SACs for electrocatalytic applications. The first two aspects are the role played by the exchange-correlation functional adopted within a given DFT approach and the role of the local coordination of the active transition metal atom. Next, we discuss new intermediates that can form on SACs and that are not present on extended metal electrodes and how to model solvation, with particular emphasis on the fact that on SACs water can not only act as a solvent but also as a ligand. Finally, we discuss challenges related to the inclusion of pH and voltage in the models and some open issue concerning the rational design of new SACs.

Electronic structure calculations represent an essential complement of experiments to characterize single-atom catalysts (SACs), consisting of isolated metal atoms stabilized on a support, but also to predict new catalysts. However, simulating SACs with quantum chemistry approaches is not as simple as often assumed. In this work, the essential factors that characterize a reliable simulation of SACs activity are examined. The Perspective focuses on the importance of precise atomistic characterization of the active site, since even small changes in the metal atom's surroundings can result in large changes in reactivity. The dynamical behavior and stability of SACs under working conditions, as well as the importance of adopting appropriate methods to solve the Schrödinger equation for a quantitative evaluation of reaction energies are addressed. The Perspective also focuses on the relevance of the model adopted. For electrocatalysis this must include the effects of the solvent, the presence of electrolytes, the pH, and the external potential. Finally, it is discussed how the similarities between SACs and coordination compounds may result in reaction intermediates that usually are not observed on metal electrodes. When these aspects are not adequately considered, the predictive power of electronic structure calculations is quite limited.

Single Atom Alloys (SAAs) represent one of the most promising classes of heterogeneous catalysts. They are based on isolated transition metal (TM) atoms stabilized in a host metal matrix. Among others, SAAs are widely studied for processes involving hydrogen, such as water splitting or hydrogenation reactions. On a metal surface the H2 molecule forms either weakly physisorbed species (H2phys) or dissociates with formation of chemisorbed H atoms (H*). In electrochemical processes, the adsorption free energy of the H* intermediate is normally used as a descriptor of the catalyst reactivity. Recently, it has been shown that on Single Atom Catalysts (SAC) embedded in carbon-based, sulfide, or oxide supports other species can form, where two H atoms are stably bound to the SAC, forming dihydrogen or dihydride (H*H*) surface species reminiscent of the classical Kubas's transition metal complexes in coordination chemistry. In this work we show, based on density functional theory calculations, that dihydrogen and dihydride complexes can also form on SAAs and, depending on the nature of the host metal, can be even more stable than two separated chemisorbed H* species (2H*). These new stable intermediates have not been considered so far in the analysis of the mechanism and kinetics of the reaction. The work shows that the possible formation of dihydrogen and dihydride complexes is not limited to SACs but is also valid for SAAs and needs to be considered in the study of hydrogen-based reactions on these systems.

In this work we present a computational study of the nature of a Single Atom Catalyst (SAC) consisting of a Pt1 atom anchored on a C3N4 support, and of its reactivity in the water splitting semi-reactions, the Hydrogen Evolution (HER) and Oxygen Evolution (OER) Reactions. The work is motivated by the intense research in designing catalytic materials for water splitting characterized by a low amount of noble metal species, maximization of active phase, and stability of the catalyst. C3N4-based SACs are promising candidates. The results indicate that the chemistry of a single atom is complex, as it can be anchored to the support in different ways resulting in a different stability. The reactivity of the most stable structure in HER and OER has been considered, finding that Pt1@C3N4 is more reactive than metallic platinum. Furthermore, unconventional but stable intermediates can form that differ from the intermediates usually found on extended catalytic surfaces. The work highlights the importance of considering the complex chemistry of SACs in view of the analogies existing with coordination chemistry compounds.

Single Atom Catalysts (SACs) bridge homo- and heterogenous catalysis and are promising for several chemical processes of interest, including water splitting. SACs can form reaction adducts that do not likely form on conventional metal catalysts. Besides the typical supporting matrices made by carbon-based materials, Covalent Organic Frameworks (COFs) are gaining attention because of the possibility to design the hosting cavity to stably bind the active metal site. We performed a density functional theory (DFT) study of a set of SACs made by transition metal atoms embedded in a recently synthesized COF material. We explored their reactivity in Hydrogen and Oxygen Evolution Reactions (HER and OER, respectively). SACs@COF can form several intermediates with no counterpart on the classical metal electrodes, with important implications on the reaction mechanism. The results are useful for the design of novel catalytic materials and for the identification of interpretative/predictive activity descriptors.

RuO2 and IrO2 are among the most active catalysts for the Oxygen Evolution Reaction (OER). Recently, it was demonstrated that the catalytic surface of these oxides plays a role in the reaction, where a hydrogen bond with a neighbor OH group stabilizes an unconventional −OO intermediate (−OO-H), prior to O2 evolution. Quantum chemical calculations neglecting solvation effects indicated that this intermediate is more stable than the conventional −OOH, and that deprotonation of the stabilizing −OH is the rate limiting step for OER on RuO2(110) and RuO2(100). In this work, we investigate the role of water molecules on the relative stability of −OOH and −OO-H oxygenates on RuO2 (110) by means of density functional theory calculations combined with ab initio Molecular Dynamics simulations (AIMD). We show that the two intermediates participate in a hydrogen bonding network with water to a similar extent, but leading to different interfacial water structures, with possible implications on interfacial proton dynamics and reaction kinetics. Moreover, −OOH can spontaneously convert to −OO-H through a process mediated by water, demonstrating the critical role of explicitly including water in the model. This study provides further mechanistic insights on the role of the oxide surface chemistry in the OER mechanism and highlights the importance of explicitly treating the catalyst/water interfaces including dynamical aspects to assess the stability and the interconversion mechanism of key surface species, since the adoption of static solvation approaches tends to overestimate the energetic difference between −OOH and −OO-H reaction intermediates.

The interaction of CaO and Ca(OH)2 with solvated or gaseous SO2 plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO2 from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO2 with a CaO(001) single crystal. First, the surface chemistry of SO2 was investigated at different temperatures using polarization-resolved IR reflection absorption spectroscopy. Three species were identified, and an in-depth density functional theory study was carried out, which allowed deriving a consistent picture. Unexpectedly, low temperature exposure to SO2 solely yields a physisorbed species. Only above 100 K, the transformation of this weakly bound adsorbate first to a chemisorbed sulfite and then to a sulfate occurs, effectively passivatating the surface. Our results provide the basis for more efficient strategies in corrosion protection of urban infrastructure and in lime-based desulfurization of flue gas.

TiO2 is a relevant catalytic material, and its chemistry in aqueous environment is a challenging aspect to address. Also, the morphology of TiO2 particles at the nanoscale is often complex, spanning from faceted to spherical. In this work, we study the pH- and facet-dependent surface chemistry of TiO2/water interfaces by performing ab initio molecular dynamics simulations with the grand canonical formulation of species in solution. We first determined the acid-base equilibrium constants at the interface, which allows us to estimate the pH at the point of zero charge, an important experimental observable. Then, based on simulated equilibrium constants, we predict the amount of H+, OH-, and adsorbed H2O species present on the surfaces as a function of the pH, a relevant aspect for water splitting semi-reactions. We approximated the complex morphology of TiO2 particles by considering the rutile (110) and (011), and anatase (101), (001), and (100) surfaces.

CO2 adsorption and activation on Cu single atom catalysts and Cu nanoclusters supported on the (110) surface of rutile and on the (101) surface of anatase TiO2 have been investigated by means of first principles electronic structure calculations. The role of oxide reduction associated to the presence of oxygen vacancies has been considered. Five main messages emerge from this study. (1) CO2 activation on Cu/TiO2 nanostructures is surface sensitive, as the rutile and anatase surfaces can exhibit different behaviors; (2) the surface morphology is essential since CO2 is activated only when the molecule can simultaneously bind to at least two active sites, such as a Cu atom on one side and an oxide ion on the other site; (3) Cu atoms on TiO2 are in the +I oxidation state and can bind and activate CO2 via charge transfer from the oxide; (4) on supported Cu clusters CO2 activation occurs mostly at the metal/oxide interface; (5) the presence of O vacancy sites facilitates the spontaneous dissociation of CO2 to CO, or increases the electron density of the metal catalyst, two effects that can influence the mechanism of CO2 reduction to methanol or other chemicals.

The electrochemical reduction of CO2 on single atom catalysts (SAC) has emerged as a highly promising yet intricate process, requiring an in-depth understanding of each elementary step of the reaction. Most of the theoretical studies pertaining the screening of new efficient catalysts neglect important effects such as the capability of the catalytic site to bind and activate CO2, the occurrence of competing reaction paths via formation of different isomers in the intermediate steps, and the role of water. In this work we will show that these are key aspects of the CO2 reduction reaction (CO2RR). Employing density functional theory, we investigated a series of Transition Metal atoms (Sc-Cu, Mo-Ag, W-Au) embedded in nitrogen doped graphene (TM@4N-Gr) and their activity in the CO2 reduction. Our results show that water is not only solvating the reaction intermediates, but it acts as a ligand itself, competing with CO2 when binding to the catalytic site. This coordination chemistry effect largely affects the stability of the chemical intermediates. Furthermore, only a small fraction of the investigated SACs can bind and activate CO2, and that in most cases the reaction proceeds via formation of the OCHO intermediate and not of the COOH one, as often assumed.

In this tutorial review we report the state-of-the-art of the modeling approaches of Single Atom Catalysts (SAC) for water splitting and fuel cells reactions. The discussion applies for Hydrogen Evolution Reaction (HER), Oxygen Reduction Reaction (OER), Hydrogen Oxidation Reaction (HOR), and Oxygen Reduction Reaction (ORR). The main scope of this work is to underline the relevant aspects of SACs modelling. On the one hand, the review could help computational chemists aiming to start the study of SACs. On the other hand, experimentalists could find the critical analysis of DFT results of interest to understand better the strengths and weaknesses of simulations, and how to interpret computational results. After an introductory section of SACs, we start by briefly presenting the state-of-the-art methodologies. Then, we analyze the critical aspects for a reliable prediction of the electronic properties, and we discuss the robustness of the structural models and ways to validate them. Furthermore, we discuss the main approaches to predict catalytic activity and selectivity, which is the final goal of the computational catalysis. We conclude this review with a critical analysis of the current challenges in the field, and the main limitations of the modeling approaches that are described.

The Oxygen Evolution Reaction (OER) is a key part of water splitting. On metal and oxide surfaces it usually occurs via formation of three intermediates, M(OH), M(O), and M(OOH) (also referred to as OH*, O*, and OOH* species where * indicates a surface site). The last step consists of O2 release. So far, it has been generally assumed that the same path occurs on single atom catalysts (SACs). However, the chemistry of SACs may differ substantially from that of extended surfaces and is reminiscent of that of coordination compounds. This raises the question of whether on SACs the OER follows the classical mechanism or not. Using a DFT approach, we studied a set of 30 SACs made by ten metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Pd and Pt) anchored on three widely used 2D carbon-based materials, graphene, nitrogen-doped graphene and carbon nitride. In none of the cases examined the most favourable reaction path is the conventional one. In fact, in all cases other intermediates with higher stabilities form: M(OH)2, M(O)(OH), M(O)2, and M(O2) (OH* OH*, O* OH*, O* O*, O2* according to standard nomenclature). Therefore, the common assumption that on SACs the OER proceeds via formation of OH*, O*, and OOH* intermediates is not verified. Predictions of new catalysts based on the screening of large number of potential structures can lead to completely incorrect conclusions if these additional intermediates are not taken into consideration.

The influence of metallic and oxide phases coexisting on surfaces is of fundamental importance in heterogeneous catalysis. Many reactions lead to the reduction of the oxidized areas, but the elucidation of the mechanisms driving these processes is often challenging. In addition, intermediate species or designed organic ligands increase the complexity of the surface. In the present study, we address the thermal reduction of a copper oxide overlayer grown on Cu(111) in the presence of N-heterocyclic carbene (NHC) ligands by means of scanning tunneling microscopy (STM) and density functional theory (DFT). We show that the NHC ligands actively participate in the copper oxide reduction, promoting its removal at temperatures as low as 470 K. The reduction of the oxide was tracked by employing scanning tunneling spectroscopy (STS), providing a chemical identification of metallic and oxide areas at the nanometric scale.

In this brief perspective we analyze the present status of the field of defect engineering of oxide surfaces. In particular we discuss the tools and techniques available to generate, identify, quantify, and characterize point defects at oxide surfaces and the main areas where these centers play a role in practical applications.

The electronic properties of N-doped ZnO/Cu(111) bilayer films have been studied by using the DFT+U method including dispersion contributions and compared to that of free-standing ZnO bilayer films. The substitution of a lattice O ion in free-standing ZnO bilayer by a N atom results in oxidation state of N of -2, while the oxidation state of N becomes -3 when it replaces a lattice O in a ZnO/Cu(111) bilayer film. The extra negative charge of N in N:ZnO/Cu(111) compared to freestanding N:ZnO is due to an electron transfer from the Cu support. The difference in oxidation state of the N dopant is demonstrated by the analysis of the Bader charge, of the magnetization, and of the density of states. The changes in electronic properties of the doped ZnO films were evaluated by adsorbing CO and H2 molecules. The results show that the adsorption energies are significantly enhanced in N-doped ZnO/Cu(111) compared to the unsupported N:ZnO bilayer counterpart. This reflects the effect of the ZnO/Cu(111) interface in improving adsorption and possibly catalytic properties and the role of the N-dopant in activating the oxide film.

In this brief account dedicated to the memory of Professor Renato Ugo I will trace the history of 40 years of research in cluster chemistry. I will describe how my research activity started in the early 1980s with studies inspired by the vision of Renato Ugo on the electronic structure of gas-phase Li clusters and their interaction with hydrogen molecules. Since then, research on cluster chemistry and physics has done enormous progress, thanks to the development of new experimental techniques, of advanced theoretical methods, and a continuous growth of computing power. The structure and properties of both naked clusters and ligand-stabilized organometallic clusters have been elucidated with the final aim to create strong links between cluster science, surface chemistry, and heterogenous catalysis. This resulted in a better understand of catalysis by supported nanoparticles and in the possibility to design new nanostructured catalysts with desired properties.

In the present work, we have studied the effect of iso-valent dopants on the chemical reactivity of the stoichiometric SnO2 (110) surface. Si, Ge, Pb, Ti, Zr, Hf, and Ce ions have been placed at the Sn lattice sites (substitutional doping) in bulk, sub-surface, and surface positions. The results show that surface or sub-surface sites are clearly preferred over the bulk ones. Next, we studied the effect of the iso-valent dopants on the adsorption and decomposition reaction of HCOOH molecule. On this basis, four paths with desorption of: (a) H2, (b) CO, (c) H2O, or (d) CO2 can occur. Comparing the results of this decomposition reaction on the SnO2 surface with the same process on rutile TiO2 (110) having the same structure and morphology, reveals that SnO2 is considerably more reactive in all reaction steps of the formic acid decomposition.

The characterization of catalytic oxide surfaces is often done by studying the properties of adsorbed probe molecules. The 31P NMR chemical shift of adsorbed trimethylphosphine, P(CH3)3 or TMP, has been used to identify the presence of different facets in oxide nanocrystals and to study the acid-base properties of the adsorption sites. The NMR studies are often complemented by DFT calculations to provide additional information on TMP adsorption mode, bond strength, etc. So far, however, no systematic study has been undertaken in order to compare on the same footing the chemical shifts and the adsorption properties of TMP on different oxide surfaces. In this work we report the results of DFT+D (D = dispersion) calculations on the adsorption of TMP on the following oxide surfaces: anatase TiO2(101) and (001), rutile TiO2(110), tetragonal ZrO2(101), stepped ZrO2(134) and (145) surfaces, rutile SnO2(110), (101) and (100), wurtzite ZnO(101̄0), and cubic CeO2(111) and (110). Beside the stoichiometric surfaces, also reduced oxides have been considered creating O vacancies in various sites. TMP has been adsorbed on top of variously coordinated Lewis acid cation sites, with the aim to identify, also with the support of machine learning algorithms, trends or patterns that can help to correlate the 31P chemical shift with physico-chemical properties of the oxide surfaces such as adsorption energy, Bader charges, cation-P distance, work function, etc. Some simple correlation can be found within the same oxide between the 31P chemical shift and the adsorption energy, while when the full set of data is considered the only correlation found is with the net charge on the TMP molecule, a descriptor of the acid strength of the adsorption site.

N-Heterocyclic carbenes (NHCs) have superior properties as building blocks of self-assembled monolayers (SAMs). Understanding the influence of the substrate in the molecular arrangement is a fundamental step before employing these ligands in technological applications. Herein, we study the molecular arrangement of a model NHC on Cu(100) and Cu(111). While mostly disordered phases appear on Cu(100), on Cu(111) well-defined structures are formed, evolving from magic-number islands to molecular ribbons with coverage. This work presents the first example of magic-number islands formed by NHC assemblies on flat surfaces. Diffusion and commensurability are key factors explaining the observed arrangements. These results shed light on the molecule-substrate interaction and open the possibility of tuning nanopatterned structures based on NHC assemblies.

We investigated the impact of quantum confinement on the band gap of chalcogenides 2D nanostructures by means of density functional theory. We studied six different systems: MoS2, WS2, SnS2, GaS, InSe, and HfS2 and we simulated nanosheets of increasing thickness, ranging from ultrathin films to ∼10-13 nm thick slabs, a size where the properties converge to the bulk. In some cases, the convergence of the band gap with slab thickness is rather slow, and sizeable deviations from the bulk value are still present with few nm-thick sheets. The results of the simulations were compared with the available experimental data, finding a quantitative agreement. The impact of quantum confinement can be rationalized in terms of effective masses of electrons and holes and system’s size. These results show the possibility of reliably describing quantum confinement effects on systems for which experimental data are not available.

Single atom catalysts (SACs) consist of isolated metal atoms stabilized on a solid support. The name suggests that the catalytic activity is due to the nature of the metal atom, but of course the interaction with the substrate plays a role as well. But what is more important? The metal atom or its surrounding? To answer this question, we have performed numerical experiments based on density functional theory (DFT). 24 transition metal atoms have been incorporated in Nitrogen-doped graphene, and the catalytic activity in the hydrogen evolution reaction (HER) has been studied changing the metal and keeping the N-doped graphene matrix fixed. Then, one specific atom, Pt, has been embedded in the same matrix but the nitrogen neighbors of Pt have been systematically replaced by carbon or oxygen atoms generating more than 20 structures. The HER has thus been studied by keeping the metal center fixed but changing the surrounding. It turns out that the same great variability in chemical behavior can be achieved by changing the active site or the surrounding environment. This shows the importance of the local coordination in determining the catalytic activity. The consequences of this conclusion for modeling studies of SACs are discussed.

In this work, we demonstrate the essential role of the formation of superoxo and peroxo complexes on single-atom catalysts (SACs), an aspect that is often neglected in the study of these systems. By means of density functional theory calculations, we consider a representative set of 11 transition metal atoms (Sc, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, Pd, Pt) anchored on nitrogen-doped graphene, and we show that most of them form stable superoxo and peroxo intermediates when they react with molecular oxygen. Their formation has a direct impact on the oxygen evolution reaction (OER), and we develop the corresponding microkinetic models showing that this step of the reaction cannot be neglected. Depending on the transition metal atom, the inclusion of the superoxo/peroxo complexes in the analysis of the reaction profile can change the kinetics by several orders of magnitude. This work reveals the important role of dioxygen complexes species in the OER and reinforces the notion of SACs as analogs of coordination compounds

The most widely used approach to predict catalytic activity is density functional theory, whose results however depend on the adopted exchange-correlation functional. In this work, the role played by the functional in predicting the activity of single atom catalysts (SAC) in the hydrogen and oxygen evolution reactions (HER and OER) is studied. 16 transition metal (TM) atoms embedded in N-doped graphene are simulated and the performance of the widely adopted Perdew–Burke–Ernzerhof (PBE) functional against the hybrid PBE0 functional is assessed. The PBE+U approach, a computationally less complex way to correct for the self-interaction error in density functional theory, is also considered. The predictions obtained with PBE have a substantial deviation from PBE0 for first row TMs, i.e., 3d systems, while smaller deviations are found for the 4d and 5d series. The PBE+U results represent an improvement with respect to PBE, although some differences from PBE0 remain. This study underlines the importance of the choice of the DFT functional in screening new catalysts and in predicting catalytic activities. The use of PBE appears acceptable for 4d and 5d metals, while in the case of 3d systems PBE+U or PBE0 approaches are recommended, in particular for magnetic ground states.

Tuning the properties of oxide surfaces through the adsorption of designed ligands is highly desirable for several applications, such as catalysis. N-Heterocyclic carbenes (NHCs) have been successfully employed as ligands for the modification of metallic surfaces. On the other hand, their potential as modifiers of ubiquitous oxide surfaces still needs to be developed. Here we show that a model NHC binds covalently to a copper oxide surface under UHV conditions. In particular, we report the first example of a covalent bond between NHCs and oxygen atoms from the oxide layer. This study demonstrates that NHC can also act as a strong anchor on oxide surfaces.

One-pot three-component regioselective azide-alkyne cycloadditions are central reactions for synthesizing pharmaceuticals and fine chemicals and are also applied for in vivo metabolic labeling biotechnology. Homogeneous catalysts based on copper species coordinated with ancillary ligands are regularly used to perform this reaction, offering superior catalytic activity and selectivity compared to conventional heterogeneous counterparts based on supported copper nanoparticles. However, the challenge of catalyst recovery limits the use of these homogeneous compounds in many large-scale applications. In this work, we report the high catalytic performance of a family of Cu-based single-atom catalysts for triazole synthesis, with an emphasis on the fundamental understanding of the structure and function of the catalyst. The catalysts were prepared via tricyanomethanide polymerization to create a joint electronic structure where the mesoporous graphitic carbon nitride carrier acts as a ligand for the atomically dispersed copper species. The material properties and the precise metal location/coordination (i.e., deposited in the heptazine pore of carbon nitride, substituted in the framework of carbon nitride, hosted in a vacancy, or entrapped in sandwich-like arrangement) were characterized through a battery of spectroscopic and theoretical methods. The catalysts were employed in the synthesis of 1,2,3-triazoles employing azide-alkyne click reaction under base-free conditions. The single-atom Cu catalysts demonstrated improved activity and selectivity compared to the homogeneous reference catalyst. Density functional theory calculations corroborated the results and showed that the reaction proceeds through a barrier given by the activation of the acetylenic moiety on Cu1. The activity of this step was primarily affected by the coordination of the metal with the support. Therefore, understanding the metal coordination in single-atom catalysts is critical to further optimizing

We present an approach to determine from first principles the expected efficiency of semiconductors heterojunctions in solar light absorption and electron-hole pairs generation for photocatalysis and solar cells applications. In a composite material, upon absorption of photons of appropriate wavelength, electrons and holes can migrate towards different components of the junction, thus reducing the electron-hole recombination probability. This occurs when the bands of the two semiconductors are properly aligned. However, a favorable band alignment is not a sufficient condition to have a device able to convert efficiently solar light into chemical or electrical energy. Using the Transfer Matrix Method and Density Functional Theory calculations we evaluate the transmittance and reflectivity of the charge carriers, and we determine two descriptors, the injection efficiency and the maximum conversion efficiency, that allow us to predict in a more realistic way the performance of the device. This method is based on the explicit inclusion of surface, interface, and quantum confinement effects that can be present when the heterojunction is formed. By using the YP/TiO2 (Y = Al, Ga, In) and the CsPbX3/TiO2 (X = Cl, Br, I) systems as examples we show that the approach provides an excellent agreement with the available experimental data.

The description of the band gap of halide perovskites at the level of density functional theory (DFT) has been subject of several studies but still presents significant problems and deviations from experimental values. Various approaches have been proposed, including the use of system-specific hybrid functionals with a variable amount of exact exchange or the explicit inclusion of spin-orbit coupling (SOC) effects. In this work, we present a pragmatic recipe to compute the band gap of halide perovskites with a minimum average error. The recipe is tested on a set of 36 halide perovskites of the type ABX3 [A = Cs, methyl-ammonium (MA), and formamidinium (FA); B = Ge, Sn, and Pb; and X = Cl, Br, and I] for which experimental estimates of the band gap have been reported in the literature. Upon assessment of the accuracy of commonly used DFT functionals and the analysis of their performances based on error and statistical analysis, we suggest a strategy to compute band gaps in halide perovskites with a single functional. This is based on the use of the hybrid HSE06 functional where SOC is included exclusively for Pb-containing compounds. The results are rationalized in terms of the materials' chemical nature and are corroborated by the prediction of their expected efficiencies in solar cells. The calculated efficiencies from band gaps obtained with the proposed approach closely follow the experimental trend, demonstrating the importance of adopting a reliable but material-independent computational strategy to screen new halide perovskite materials for solar energy conversion.

The global warming crisis has sparked a series of environmentally cautious trends in chemistry, allowing us to rethink the way we conduct our synthesis, and to incorporate more earth-abundant materials in our catalyst design. “Single-atom catalysis” has recently appeared on the catalytic spectrum, and has truly merged the benefits that homogeneous and heterogeneous analogues have to offer. Further still, the possibility to activate these catalysts by means of a suitable electric potential could pave the way for a true integration of diverse synthetic methodologies and renewable electricity. Despite their esteemed benefits, single-atom electrocatalysts are still limited to the energy sector (hydrogen evolution reaction, oxygen reduction, etc.) and numerous examples in the literature still invoke the use of precious metals (Pd, Pt, Ir, etc.). Additionally, batch electroreactors are employed, which limit the intensification of such processes. It is of paramount importance that the field continues to grow in a more sustainable direction, seeking new ventures into the space of organic electrosynthesis and flow electroreactor technologies. In this piece, we discuss some of the progress being made with earth abundant homogeneous and heterogeneous electrocatalysts and flow electrochemistry, within the context of organic electrosynthesis, and highlight the prospects of alternatively utilizing single-atom catalysts for such applications.

One of the objectives of electronic structure theory is to predict chemical and catalytic activities. This is a challenging target due to the large number of variables that determine the performance of a heterogeneous catalyst. The complexity of the problem has reduced considerably with the advent of single atom catalysts (SACs) and, in particular, of graphene-based SACs for electrocatalytic reactions such as the oxygen reduction (ORR), the oxygen evolution (OER), and the hydrogen evolution (HER) reactions. In this context we assist with a rapidly growing number of theoretical studies based on density functional theory (DFT) and with proposals of universal descriptors that should provide a guide to the experimentalist for the synthesis of efficient catalysts. In this Perspective we critically analyze some of the current problems connected with the prediction of the activity of SACs: accuracy of the calculations, neglect of important contributions in the models used, physical meaning of the proposed descriptors, not to mention some problems of reproducibility. It follows that the "rational design"of a catalyst based on some of the proposed universal descriptors should be considered with caution.

Two-dimensional oxide films are potentially useful for future technological applications, but also important objects to study model catalyst systems on the more fundamental side. Here we study silica, germania, and mixed silica-germania films supported on a metal single crystal surface Ru(0001). Those mixed films are interesting objects to systematically modify the properties of silica films, which may be used as membranes or covers for model studies in confined space, due to the modification of the rather stiff silica layers by incorporating germanium atoms replacing silicon atoms. Here we report a combined experimental and theoretical study of such layers, where we show how X-ray photoelectron spectroscopy in combination with LEED and I/V LEEM measurements allow us to judge the formation of such mixed films.

We studied the role of iso-valent heteroatoms replacing Ti4+ cations in the lattice of two titania polymorphs, rutile (r-) and anatase (a-) by means of first principles calculations. The r-TiO2(110) and the a-TiO2(101) surfaces have been considered and Ti ions in the bulk, sub-surface, and surface sites have been replaced with Si, Ge, Sn, Pb, Zr, Hf, and Ce ions: surface or sub-surface sites are clearly preferred. Since the dopants have the same number of valence electrons as the replaced Ti atom, they can have only two effects: one is steric, related to the different size of the dopant compared to Ti4+; the other is an orbital effect, due to the energy levels associated to the dopant not present on the pristine surface. Both these effects can modify locally the geometric and electronic structure of the surface, in particular by introducing new states in the band gap. To check the effect of the dopants on the surface reactivity we studied as an example the decomposition of HCOOH which can follow four different paths with desorption of (a) H2, (b) CO, (c) H2O, or (d) CO2. The results show the very different behavior of the two titania polymorphs considered, rutile and anatase: rutile is more reactive and more easily reduced than anatase. For specular reasons, the presence of the dopants has in general more pronounced effects on anatase, as they can deeply modify the surface reactivity and the HCOOH decomposition path.

Understanding the mechanism behind the superior catalytic power of single- or few-atom heterogeneous catalysts has become an important topic in surface chemistry. This is particularly the case for gold, with TiO2 being an efficient support. Here we use scanning tunneling microscopy/spectroscopy with theoretical calculations to investigate the adsorption geometry and local electronic structure of several-atom Au clusters on rutile TiO2(110), with the clusters fabricated by controlled manipulation of single atoms. Our study confirms that Au1 and Au2 clusters prefer adsorption at surface O vacancies. Au3 clusters adsorb at O vacancies in a linear-chain configuration parallel to the surface; in the absence of O vacancies they adsorb at Ti5c sites with a structure of a vertically pointing upright triangle. We find that both the electronic structure and cluster-substrate charge transfer depend critically on the cluster size, bonding configuration, and local environment. This suggests the possibility of engineering cluster selectivity for specific catalytic reactions.

In this chapter, we address the computational description of structural and electronic properties of TiO2. Particular emphasis is given to the key methodological aspects related to density functional theory calculations on titania. In particular, it is necessary to correctly account for the self-interaction error in the adopted functional in order to prevent problems such as the severe underestimation of the TiO2 bandgap and the strong bias toward the full delocalization of excess electrons and holes, leading eventually to calculated results in contrast with the experimental evidence. We hereby review computational results on pristine and reduced titania, discussing the nature and the main features of the intrinsic defects (oxygen vacancies and interstitial Ti). We provide insights on the trapping of photoexcited charge carriers and their separation in TiO2 nanocomposites. The last section concerns the modeling of large and complex titania nanoparticles.

Materials for post Li-ion batteries are attracting an increasing interest. Here we have studied the fundamental properties of multi-valent Al and Ca ions intercalated into V2O5 and MoO3 layered oxides. To this end we performed Density Functional Theory calculations using a Hubbard U correction for the electronic structure of the oxides and including dispersion forces. We simulated very low loadings of the intercalated Al and Ca atoms, and we studied both stoichiometric and reduced oxides. Intercalation of the Al and Ca atoms within the oxide layers (intra-layer) and in the spaces between the layers (inter-layer) has been compared. In V2O5 inter-layer intercalation is preferred, and gives rise to theoretical voltages of 1.54 V for Al and of 3.08 V for Ca; for comparison, the theoretical voltage for a Li-V2O5 battery is 3.51 V. Al can also be incorporated within the V2O5 layers, but with a smaller binding energy, while Ca, bigger, is only stable in the inter-layer sites. The situation is partly different on MoO3. Here in fact Ca is stable only in inter-layer sites, and gives rise to theoretical voltages of 1.65 V. Al, on the contrary, prefers to be incorporated into the MoO3 layers, giving rise to a similar theoretical voltage of 1.60 V. For comparison, a Li- MoO3 battery has a computed voltage of 2.16 V. The reduced forms of V2O5 and MoO3 have also been considered. Here the O atoms that are more easily removed are those of the vanadyl, V=O, and molibdyl, Mo=O, groups. The only case where there an increase of the voltage is for Al and Ca intercalation into reduced V2O5, provided that the Al and Ca ions are near an oxygen vacancy, but not in the vacancy. In MoO3 the formation of oxygen vacancies is detrimental for the intercalation process.

The hydrogen evolution reaction (HER) has a key role in electrochemical water splitting. Recently a lot of attention has been dedicated to HER from single atom catalysts (SACs). The activity of SACs in HER is usually rationalized or predicted using the original model proposed by Nørskov where the free energy of a H atom adsorbed on an extended metal surface M (formation of an MH intermediate) is used to explain the trends in the exchange current for HER. However, SACs differ substantially from metal surfaces and can be considered analogues of coordination compounds. In coordination chemistry, at variance with metal surfaces, stable dihydride or dihydrogen complexes (HMH) can form. We show that the same can occur on SACs and that the formation of stable HMH intermediates, in addition to the MH one, may change the kinetics of the process. Extending the original kinetic model to the case of two intermediates (MH and HMH), one obtains a three-dimensional volcano plot for the HER on SACs. DFT numerical simulations on 55 models demonstrate that the new kinetic model may lead to completely different conclusions about the activity of SACs in HER. The results are validated against selected experimental cases. The work provides an example of the important analogies between the chemistry of SACs and that of coordination compounds.

The role of the support in tuning the properties of transition metal (TM) atoms is studied by means of density functional theory calculations. We have considered the adsorption of Cu, Ag, Au, Ni, Pd, and Pt atoms on crystalline silica bilayers, either free-standing or supported on Ru(0001) and Pt(111) metal surfaces. These systems have been compared with an hydroxylated SiO2/Si(100) film simulating the native oxide formed on a silicon wafer. The properties of the TM atoms change significantly on the various supports. While the unsupported silica bilayer weakly binds some of the TM atoms studied, the SiO2/Ru(0001) or SiO2/Pt(111) supports exhibit enhanced reactivity, sometimes resulting in a net electron transfer with the formation of charged species. Differences in the behavior of SiO2/Ru(0001) and SiO2/Pt(111) are rationalized in terms of different work functions and metal/oxide interfacial distances. No electron transfer is observed on the SiO2/Si(100) films. Here, the presence of hydroxyl groups on the surface provides relatively strong binding sites for the TM atoms that can be stabilized by the interaction with one or two OH groups. The final aspect that has been investigated is the porosity of the silica bilayer, at variance with the dense SiO2/Si(100) film. Depending on the atomic size, some TM atoms can penetrate spontaneously through the six-membered silica rings and become stabilized in the pores of the bilayer or at the SiO2/metal interface. This study shows how very different chemical properties can be obtained by depositing the same TM atom on different silica supports.

Oxidation catalysis on reducible oxide-supported small metal clusters often involves lattice oxygen. The present work aims at differentiating whether the reaction takes place at the cluster/support interface or on the cluster. To that purpose, we trace the path of lattice oxygen from Fe3O4(001) onto small Pt clusters during the CO oxidation. While oxygen vacancies form on many other supports, magnetite maintains its surface stoichiometry upon reduction thanks to high cation mobility. To investigate whether size-dependent oxygen affinities play a role, we study two specific cluster sizes, Pt5 and Pt19. By separating different reaction steps in our experiment, migrating lattice oxygen can be accumulated on the clusters. Temperature-programmed desorption (TPD) and sophisticated pulsed valve experiments indicate that CO oxidation occurs with this highly reactive oxygen on the Pt clusters. Scanning tunneling microscopy (STM) shows a decrease in the apparent height of the clusters, which density functional theory (DFT) explains as a restructuring following lattice oxygen reverse spillover.

The network structure of a silica bilayer film at a monolayer-bilayer transition and across a supporting metal step edge was studied at the atomic scale by scanning tunneling microscopy. The ring size distribution, ring-ring distances, and height profiles are analyzed across the step edge region. Density functional theory proposes two models to explain the observed network structure: a pinning of the lower layer to the substrate and a carpetlike mode. The results indicate a continuous coverage of the silica bilayer film across the step edge.

Semiconductor heterojunctions are widely applied in solid-state device applications, including semiconductor lasers, solar cells, and transistors. In photocatalysis they are of interest due to their capability to hinder charge carriers' recombination. A key role in the performance of heterojunctions is that of the alignment of the band edges of the two units composing the junction. In this work, we compare the performances of three widely applied approaches for the simulation of semiconductors heterostructures, based on density functional theory calculations with hybrid functionals. We benchmark the band offsets of ten semiconductors heterostructures for which experimental values are available: AlP/GaP, AlP/Si, AlAs/GaAs, AlAs/Ge, GaAs/Ge, GaP/Si, ZnSe/Ge, ZnSe/AlAs, ZnSe/GaAs, and TiO2/SrTiO3. The methods considered are (i) the alternating slabs junction (ASJ), (ii) the surface terminated junction (STJ), and (iii) the independent units (IU) approach.Moreover, two different ways to determine a common reference have been considered, (i) the plane averaged electrostatic potential, and (ii) the energy of the core levels. Advantages, drawbacks and overall performances of each method are discussed. The results suggest that the accuracy in the estimation of the band offsets is ∼0.2 eV when the ASJ method is applied. The STJ approach provides a similar accuracy, while the neglection of any interface effect, as in the IU method, provides only a qualitative estimate of the band offset and can result in significant deviations from the experiment 2021 IOP Publishing Ltd.

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO3/BiVO4 heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO3/BiVO4 interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO4 layer over a WO3 scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO4 growth over the WO3 layer along the BiVO4 {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO3/BiVO4 photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-Absorbing BiVO4 layer, a 75 nm thick BiVO4 layer on WO3 being best performing.