Theoretical study of structure sensitivity on ceria-supported single platinum atoms and its influence on carbon monoxide adsorption.

Density functional theory (DFT) calculations explore the stability of a single platinum atom on various flat, stepped, and defective ceria surfaces, in the context of single-atom catalysts (SACs) for the water-gas shift (WGS) reaction. The adsorption properties and diffusion kinetics of the metal strongly depend on the support termination with large stability on metastable and stepped CeO2(100) and (210) surfaces where the diffusion of the platinum atom is hindered. At the opposite, the more stable CeO2(111) and (110) terminations weakly bind the platinum atom and can promote the growth of metallic clusters thanks to fast diffusion kinetics. The adsorption of carbon monoxide on the single platinum atom supported on the various ceria terminations is also sensitive to the surface structure. Carbon monoxide weakly binds to the single platinum atom supported on reduced CeO2(111) and (211) terminations. The desorption of the CO2 formed during the WGS reaction is thus facilitated on the latter terminations. A vibrational analysis underlines the significant changes in the calculated scaled anharmonic CO stretching frequency on these catalysts.

Identifying the Structure of Supported Metal Catalysts Using Vibrational Fingerprints from Ab Initio Nanoscale Models.

Identifying active sites of supported noble metal nanocatalysts remains challenging, since their size and shape undergo changes depending on the support, temperature, and gas mixture composition. Herein, the anharmonic infrared spectrum of adsorbed CO is simulated using density functional theory (DFT) to gain insight into the nature of Pd nanoparticles (NPs) supported on ceria. The authors systematically determine how the simulated infrared spectra are affected by CO coverage, NP size (0.5-1.5 nm), NP morphology (octahedral, icosahedral), and metal-support contact angle, by exploring a diversity of realistic models inspired by ab initio molecular dynamics. The simulated spectra are then used as a spectroscopic fingerprint to characterize nanoparticles in a real catalyst, by comparison with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. Truncated octahedral NPs with an acute Pd-ceria angle reproduce most of the measurements. In particular, the authors isolate features characteristic of CO adsorbed at the metal-support interface appearing at low frequencies, both seen in simulation and experiment. This work illustrates the strong need for realistic models to provide a robust description of the active sites, especially at the interface of supported metal nanocatalysts, which can be highly dynamic and evolve considerably during reaction.

Machine Learning-Based Prediction of Activation Energies for Chemical Reactions on Metal Surfaces.

In computational surface catalysis, the calculation of activation energies of chemical reactions is expensive, which, in many cases, limits our ability to understand complex reaction networks. Here, we present a universal, machine learning-based approach for the prediction of activation energies for reactions of C-, O-, and H-containing molecules on transition metal surfaces. We rely on generalized Bronsted-Evans-Polanyi relationships in combination with machine learning-based multiparameter regression techniques to train our model for reactions included in the University of Arizona Reaction database. In our best approach, we find a mean absolute error for activation energies within our test set of 0.14 eV if the reaction energy is known and 0.19 eV if the reaction energy is unknown. We expect that this methodology will often replace the explicit calculation of activation energies within surface catalysis when exploring large reaction networks or screening catalysts for desirable properties in the future.

Gaussian attractive potential for carboxylate/cobalt surface interactions.

Ligand-decorated metal surfaces play a pivotal role in various areas of chemistry, particularly in selective catalysis. Molecular dynamics simulations at the molecular mechanics level of theory are best adapted to gain complementary insights to experiments regarding the structure and dynamics of such organic films. However, standard force fields tend to capture only weak physisorption interactions. This is inadequate for ligands that are strongly adsorbed such as carboxylates on metal surfaces. To address this limitation, we employ the Gaussian Lennard-Jones (GLJ) potential, which incorporates an attractive Gaussian potential between the surface and ligand atoms. Here, we develop this approach for the interaction between cobalt surfaces and carboxylate ligands. The accuracy of the GLJ approach is validated through the analysis of the interaction of oxygen with two distinct cobalt surfaces. The accuracy of this method reaches a root mean square deviation (RMSD) of about 3 kcal/mol across all probed configurations, which corresponds to a percentage error of roughly 4%. Application of the GLJ force field to the dynamics of the organic layer on these surfaces reveals how the ligand concentration influences the film order, and highlights differing mobility in the x and y directions, attributable to surface corrugation on Co(112̄0). GLJ is versatile, suitable for a broad range of metal/ligand systems, and can, subsequently, be utilized to study the organic film on the adsorption/desorption of reactants and products during a catalytic process.

Beyond single-crystal surfaces: The GAL21 water/metal force field.

Solvent effects are notoriously difficult to describe for metallic nanoparticles (NPs). Here, we introduce GAL21 which is the first pairwise additive force field that is specifically designed to modulate the near chemisorption energy of water as a function of the coordination numbers of the metallic atoms. We find a quadratic dependence to be most suitable for capturing the dependence of the adsorption energy of water on the generalized coordination number (GCN) of the metal atoms. GAL21 has been fitted against DFT adsorption energies for Cu, Ag, Au, Ni, Pd, Pt, and Co on 500 configurations and validated on about 3000 configurations for each metal, constructed on five surfaces with GCNs varying from 2.5 to 11.25. Depending on the metals, the root mean square deviation is found between 0.7 kcal mol-1 (Au) to 1.6 kcal mol-1 (Ni). Using GAL21, as implemented in the open-source code CP2K, we then evaluate the solvation energy of Au55 and Pt55 NPs in water using thermodynamic integration. The solvation free energy is found to be larger for Pt than for Au and systematically larger than 200 kcal mol-1, demonstrating the large impact of solvent on the surface energetics of NPs. Still, given that the amorphous NPs are both, the most stable and the most solvated ones, we do not predict a change in the preferred morphology between the gas-phase and in water. Finally, based on a linear regression on three sizes of NPs (from 38 to 147), the solvation energy for Au and Pt surface atoms is found to be -5.2 and -9.9 kcal mol-1, respectively.

DockOnSurf: A Python Code for the High-Throughput Screening of Flexible Molecules Adsorbed on Surfaces.

We present the open-source python package DockOnSurf which automates the generation and optimization of low-energy adsorption configurations of molecules on extended surfaces and nanoparticles. DockOnSurf is especially geared toward handling polyfunctional flexible adsorbates. The use of this high-throughput workflow allows us to carry out the screening of adsorbate-surface configurations in a systematic, customizable, and traceable way, while keeping the focus on the chemically relevant structures. The screening strategy consists in splitting the exploration of the adsorbate-surface configurational space into chemically meaningful domains, that is, by choosing among different conformers to adsorb, surface adsorption sites, adsorbate anchoring points, and orientations and allowing dissociation of (acidic) protons. We demonstrate the performance of the main features based on varying examples, ranging from CO adsorption on a gold nanoparticle to sorbitol adsorption on hematite. Through the use of the presented program, we aim to foster efficiency, traceability, and ease of use in research within tribology, catalysis, nanoscience, and surface science in general.

(Dis)Similarities of adsorption of diverse functional groups over alumina and hematite depending on the surface state.

To accelerate the conversion to more sustainable lubricants, there is a need for an improved understanding of the adsorption at the solid/liquid interface. As a first step, the density functional theory computed adsorption energies can be used to screen the ability of additives to cover a surface. Analogously to what has been found in catalysis with the universal scaling relations, we investigate here if a general universal ranking of additives can be found, independently of the surface considered. We divided our set of 25 diverse representative molecules into aprotic and protic molecules. We compared their adsorption over alumina and hematite, which are models of surface oxidized aluminum and steel, respectively. The adsorption energy ranking of our set is not strongly affected by alumina hydration. In contrast, adsorption on hematite is more strongly affected by hydration since all exposed Fe Lewis acid sites are converted into hydroxylated Brønsted basic sites. However, the ranking obtained on hydrated hematite is close to the one obtained on dry alumina, paving the road to a fast screening of additives. In our library, protic molecules are more strongly adsorbed than non-protic molecules. In particular, methyl and dimethyl phosphates are the most strongly adsorbed ones, followed by N-methyldiethanolamine, succinimide, and ethanoic acid. Additives combining these functional groups are expected to strongly adsorb at the solid/liquid interface and, therefore, likely to be relevant components of lubricant formulations.

Impact of Organic Templates on the Selective Formation of Zeolite Oligomers.

Zeolites are essential materials to industry due to their adsorption and catalytic properties. The best current approach to prepare a targeted zeolite still relies on trial and error's synthetic procedures since a rational understanding of the impact of synthesis variables on the final structures is still missing. To discern the role of a variety of organic templates, we perform simulations of the early stages of condensation of silica oligomers by combining DFT, Brønsted-Evans-Polanyi relationships and kinetic Monte Carlo simulations. We investigate an extended reaction path mechanism including 258 equilibrium reactions and 242 chemical species up to silica octamers, comparing the computed concentrations of Si oligomers with 29 SI NMR experimental data. The effect of the templating agent is linked to the modification of the intramolecular H-bond network in the growing oligomer, which produces higher concentration of 4-membered ring intermediates, precursors of the key double-four ring building blocks present on more than 39 known zeolite topologies.

Two-sites are better than one: revisiting the OER mechanism on CoOOH by DFT with electrode polarization.

We uncover the existence of several competitive mechanisms of water oxidation on the ß-CoOOH (10-14) surface by going beyond the classical 4-step mechanism frequently used to study this reaction at the DFT level. Our results demonstrate the importance of two-site reactivity and of purely chemical steps with the associated activation energies. Taking the electrochemical potential explicitly into account leads to modifications of the reaction energy profiles finally leading to the proposition of a new family of mechanisms involving tetraoxidane intermediates. The two-site mechanisms revealed in this work are of key importance to rationalize and predict the impact of dopants in the design of future catalysts.

Strong Affinity of Triazolium-Appended Dipyrromethenes (TADs) for BF4.

Because BF4- is a labile, non- or weakly coordinating anion, it is generally chosen by chemists who do not want the anion to interfere with the associated cation. Herein, we demonstrate that BF4- actually strongly binds to triazole-appended dipyrromethenes (TADs). In particular, HETCOR NMR experiments and DFT calculations were used to rationalize the results observed with anion titrations. Hence, special care should be taken when considering that BF4- is innocent.

Can microsolvation effects be estimated from vacuum computations? A case-study of alcohol decomposition at the H2O/Pt(111) interface.

Converting biomass into sustainable chemicals and energy feed-stocks requires innovative heterogeneous catalysts, which are able to efficiently work under aqueous conditions. Computational chemistry is a key asset in the design of these novel catalysts, but it has to face two challenges: the large reaction networks and the potential role of hydration. They can be addressed using scaling relations such as Brønsted-Evans-Polanyi (BEP) and solvation models, respectively. In this study, we show that typical reaction and activation energies of alcohol decomposition on Pt(111) are not strongly modified by the inclusion of the water solvent as a continuum model. In contrast, adding a single water molecule strongly favours O-H and C-OH scission and it prevents C-O and to a lesser extent C-C scissions. The resulting BEP relationships partially reflect these changes induced by the solvent. Predicting Pt-catalysed alcohol decomposition in water should thus account for the influence of the solvent on thermodynamics and kinetics. In addition, we found that the reaction energies obtained in the presence of an explicit water molecule scale with the ones obtained in a vacuum. Hence, we reveal that vacuum computations in combination with corrections based on our linear regressions are able to capture the important H-bonding effect.

C-H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)-Oxo Complex.

In alkene metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation state can serve as precatalysts. However, the activation mechanism for these latter species-generating alkylidenes-is still an open question. Here, we report the syntheses of tungsten(IV)-oxo bisalkoxide molecular complexes stabilized by pyridine ligands, WO(OR)2py3 (R = CMe(CF3)2 (2a), R = Si(O tBu)3 (2b), and R = C(CF3)3 (2c); py = pyridine), and show that upon activation with B(C6F5)3 they display alkene metathesis activities comparable to W(VI)-oxo alkylidenes. The initiation mechanism is examined by kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the alkene reactant is crucial for initiating alkene metathesis. Deuterium labeling of the allylic C-H group shows a primary kinetic isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the allylic C-H bond and show that formation of the metallacyclobutane from the allyl "hydride" involves a proton transfer facilitated by the coordination of a Lewis acid (B(C6F5)3) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining and yields the metathesis active species.

Direct Amination of Alcohols Catalyzed by Aluminum Triflate: An Experimental and Computational Study.

Among the best-performing homogeneous catalysts for the direct amination of activated secondary alcohols with electron-poor amine derivatives, metal triflates, such as aluminum triflate, Al(OTf)3 , stand out. Herein we report the extension of this reaction to electron-rich amines and activated primary alcohols. We provide detailed insight into the structure and reactivity of the catalyst under working conditions in both nitromethane and toluene solvent, through experiment (cyclic voltammetry, conductimetry, NMR spectroscopy), and density functional theory (DFT) simulations. Competition between aniline and benzyl alcohol for Al in the two solvents explains the different reactivities. The catalyst structures predicted from the DFT calculations were validated by the experiments. Whereas a SN 1-type mechanism was found to be active in nitromethane, we propose a SN 2 mechanism in toluene to rationalize the much higher selectivity observed when using this solvent. Also, unlike what is commonly assumed in homogeneous catalysis, we show that different active species may be active instead of only one.

DFT investigations for the catalytic reaction mechanism of methane combustion occurring on Pd(ii)/Al-MCM-41.

In this work, a theoretical analysis was carried out on the mechanism of methane combustion occurring on single site palladium oxide species [Pd]2+ supported on Al-MCM-41 silica. Single site Pd-oxo and PdO2-superoxo structures were used to represent the active centers. Activation energies for all the elementary steps involved in the oxidation of methane into formaldehyde are presented. The competition of methane/methanol substrates on active sites was examined. It was found that the formation of methanol via the reaction of methane with the superoxo species, formed via the adsorption of O2 on reduced Pd(ii) centers, facilitates the production of the very active Pd-oxo catalytic sites.

Molecular mechanics models for the image charge, a comment on "including image charge effects in the molecular dynamics simulations of molecules on metal surfaces".

We re-investigate the image charge model of Iori and Corni (Iori and Corni, J. Comput. Chem. 2008, 29, 1656). We find that a simple symmetrization of their model allows to obtain quantitatively correct results for the electrostatic interaction of a water molecule with a metallic surface. This symmetrization reduces the magnitude of the electrostatic interaction to less than 10% of the total interaction energy. © 2017 Wiley Periodicals, Inc.

Solvation free energies for periodic surfaces: comparison of implicit and explicit solvation models.

The evaluation of solvation energies is a great challenge. We focus here on an organic molecule chemisorbed at a metal-liquid interface, as a prototypical system, essential in tribology, electrochemistry and heterogeneous catalysis. We compare an established implicit solvation scheme with a strategy based on molecular mechanics (MM) free energy perturbation (FEP) seeded by QM computations. First, we benchmark the approaches against experimental hydration energies of standard (organic) molecules and find acceptable errors in the order of 0.06 eV (1.3 kcal mol-1). Then, the impact of various parameters on the solvation energy of an adsorbate have been assessed on a typical system of interest, levulinic acid adsorbed at a Ru(0001)/water interface. We identify the need for dipole corrections or symmetric slabs when including solvation effects on metallic surfaces. The MM-FEP scheme is revealed to be as reliable as the implicit solvent for water. In the case of levulinic acid, both PCM and MM-FEP agree that the bulk solvation effect is not sufficient to change the adsorption mode from bidentate to mono-dentate, despite the fact that the COOH group is desolvated in the bidentate case. MM-FEP has the great advantage of being more easily generalized to other solvents and to be further improved which will be particularly useful to study solvent and (counter-)ion effects on interfacial reactions.

Modeling the HCOOH/CO2 Electrocatalytic Reaction: When Details Are Key.

Our first principles simulations of the electrooxidation of formic acid over nickel identify the reorientation of the formate intermediate and the desorption of CO2 as the rate-limiting steps. Although they are not associated with an electron transfer, these barriers are strongly modified when the electrochemical potential is explicitly accounted for and when modeling the influence of the solvent. Hence, such a level of modeling is key to understand the kinetic limitations that penalize the reaction.

Impacts of electrode potentials and solvents on the electroreduction of CO2: a comparison of theoretical approaches.

Since CO2 is a readily available feedstock throughout the world, the utilization of CO2 as a C1 building block for the synthesis of valuable chemicals is a highly attractive concept. However, due to its very nature of energy depleted "carbon sink", CO2 has a very low reactivity. Electrocatalysis offers the most attractive means to activate CO2 through reduction: the electron is the "cleanest" reducing agent whose energy can be tuned to the thermodynamic optimum. Under protic conditions, the reduction of CO2 over many metal electrodes results in formic acid. Thus, to open the road to its utilization as a C1 building block, the presence of water should be avoided to allow a more diverse chemistry, in particular for C-C bond formation with alkenes. Under those conditions, the intrinsic reactivity of CO2 can generate carbonates and oxalates by C-O and C-C bond formation, respectively. On Ni(111), almost exclusively carbonates and carbon monoxide are evidenced experimentally. Despite recent progress in modelling electrocatalytic reactions, determining the actual mechanism and selectivities between competing reaction pathways is still not straight forward. As a simple but important example of the intrinsic reactivity of CO2 under aprotic conditions, we highlight the shortcomings of the popular linear free energy relationship for electrode potentials (LFER-EP). Going beyond this zeroth order approximation by charging the surface and thus explicitly including the electrochemical potential into the electronic structure computations allows us to access more detailed insights, shedding light on coverage effects and on the influence of counterions.

Molecular adsorption at Pt(111). How accurate are DFT functionals?

Molecular chemisorption at a metal surface is a key step for many processes, such as catalysis, electrochemistry, surface treatment, tribology and friction. Modeling with density functional theory is largely used on these systems. From a detailed comparison with accurate micro-calorimetric data on ten systems (involving ethylene, cyclohexene, benzene, naphthalene, CO, O2, H2, methane, ethane), we study the accuracy, for chemisorption on Pt(111), of five exchange-correlation functionals including one generalized gradient approximation functional (PBE) and four functionals that take into account van der Waals interactions (optPBE-vdW, optB86b-vdW, BEEF-vdW, PBE-dDsC). If the functionals used provide very similar geometries and electronic structures, as shown by projected density of states, they give strikingly different results for the adsorption energy of molecules on Pt(111). Among the set of chemisorption data, the lowest mean absolute deviations (MAD) are obtained with the optPBE-vdW and PBE-dDsC functionals (∼0.2 eV) while PBE and optB86b-vdW give twice larger MAD (∼0.45 eV). BEEF-vdW is intermediate with a MAD of 0.33 eV. For laterally π-bound unsaturated hydrocarbons (cyclohexene, benzene, naphthalene) the PBE and the BEEF-vdW functionals are severally under-bound, while optPBE-vdW and PBE-dDsC provide a good match with experiments. Hence both the incorporation of van der Waals dispersive forces and the choice of the exchange functional have a key influence on the chemisorption energy. Vertically bound ethylidyne and CO are in contrast over-bound with all functionals, the best agreement being obtained with BEEF-vdW. None of the selected functionals hence provides a universally accurate treatment of chemisorption energies.

Toward a Realistic Surface State of Ru in Aqueous and Gaseous Environments.

Identifying the surface species is critical in developing a realistic understanding of supported metal catalysts working in water. To this end, we have characterized the surface species present at a Ru/water interface by employing a hybrid computational approach involving an explicit description of the liquid water and a possible pressure of H2. On the close-packed, most stable Ru(0001) facet, the solvation tends to favor the full dissociation of water into atomic O and H in contrast with the partially dissociated water layer reported for ultra-high-vacuum conditions. The solvation stabilization was found to reach -0.279 J m2, which results in stable O and H species on Ru(0001) in the presence of liquid water even at room temperature. Conversely, introducing even a small H2 pressure (10-2 bar) results in a monolayer of chemisorbed H at the interface, a general trend found on the three most exposed facets of Ru nanoparticles. While hydroxyls were often hypothesized as possible surface species at the Ru/water interface, this computational study clearly demonstrates that they are not stabilized by liquid water and are not found under realistic reductive catalytic conditions.