Ligand-field theory-based analysis of the adsorption properties of ruthenium nanoparticles

Uncovering the Mechanism of the Hydrogen Poisoning on Ru Nanoparticles via Density Functional Theory Calculations

Although hydrogen plays a crucial role in ammonia synthesis, very little is known about its poisoning of Ru catalysts. In this study, density functional theory calculations of H2 and N2 dissociations, and H atom binding on Ru153 were performed to provide a fundamental understanding of hydrogen poisoning. Because of the kinetic dominance of the H2 dissociation over N2 (vertically or horizontally adsorbed) splitting, the dissociated H atoms block the active sites required for horizontal (less energetically demanding dissociation) N2 adsorption to occur either from the gas phase or after its geometrical transformation from being adsorbed vertically. Additionally, the dissociated H atoms withdraw electrons from the surface, which reduces the ability of the neighboring Ru atoms to donate electrons for N2 activation, hindering its dissociation and suppressing ammonia synthesis.

Adsorption States of N2/H2 Activated on Ru Nanoparticles Uncovered by Modulation-Excitation Infrared Spectroscopy and Density Functional Theory Calculations

The adsorption states of N₂ and H₂ on MgO-supported Ru nanoparticles under conditions close to those of ammonia synthesis (AS; 1 atm, 250 °C) were uncovered by modulation-excitation infrared spectroscopy and density functional theory calculations using a nanoscale Ru particle model. The two most intense N₂ adsorption peaks corresponded to the vertical chemisorption of N₂ on the nanoparticle's top and bridge sites, while the remaining peaks were assigned to horizontally adsorbed N₂ in view of the site heterogeneity of Ru nanoparticles. Long-term observations showed that vertically adsorbed N₂ molecules gradually migrated from the top sites to the bridge sites. Compared to those adsorbed vertically, N₂ molecules adsorbed horizontally exhibited a lower dipole moment, an increased N─N bond distance, and a decreased N─N bond order (i.e., were activated), which was ascribed to enhanced Ru-to-N charge transfer. H₂ molecules were preferentially adsorbed horizontally on top sites and then rapidly dissociated to afford strongly surface-bound H atoms and thus block the active sites of Ru nanoparticles. Our results clarify the controversial adsorption/desorption behavior of N₂ and H₂ on AS catalysts and facilitate their further development.

A combined theoretical/experimental study highlighting the formation of carbides on Ru nanoparticles during CO hydrogenation

Formation of stable carbides during CO bond dissociation on small ruthenium nanoparticles (RuNPs) is demonstrated, both by means of DFT calculations and by solid state ¹³C NMR techniques. Theoretical calculations of chemical shifts in several model clusters are employed in order to secure experimental spectroscopic assignations for surface ruthenium carbides. Mechanistic DFT investigations, carried out on a realistic Ru₅₅ nanoparticle model (∼1 nm) in terms of size, structure and surface composition, reveal that ruthenium carbides are obtained during CO hydrogenation. Calculations also indicate that carbide formation via hydrogen-assisted hydroxymethylidyne (COH) pathways is exothermic and occurs at reasonable kinetic cost on standard sites of the RuNPs, such as 4-fold ones on flat terraces, and not only in steps as previously suggested. Another novel outcome of the DFT mechanistic study consists of the possible formation of μ₆ ruthenium carbides in the tip-B₅ site, similar examples being known only for molecular ruthenium clusters. Moreover, based on DFT energies, the possible rearrangement of the surface metal atoms around the same tip-site results in a μ-Ru atom coordinated to the remaining RuNP moiety, reminiscent of a pseudo-octahedral metal center on the NP surface.

σ-H-H, σ-C-H, and σ-Si-H Bond Activation Catalyzed by Metal Nanoparticles

Activation of H-H, Si-H, and C-H bonds through σ-bond coordination has grown in the past 30 years from a scientific curiosity to an important tool in the functionalization of hydrocarbons. Several mechanisms were discovered via which the initially σ-bonded substrate could be converted: oxidative addition, heterolytic cleavage, σ-bond metathesis, electrophilic attack, etc. The use of metal nanoparticles (NPs) in this area is a more recent development, but obviously nanoparticles offer a much richer basis than classical homogeneous and heterogeneous catalysts for tuning reactivity for such a demanding process as C-H functionalization. Here, we will review the surface chemistry of nanoparticles and catalytic reactions occurring in the liquid phase, catalyzed by either colloidal or supported metal NPs. We consider nanoparticles prepared in solution, which are stabilized and tuned by polymers, ligands, and supports. The question we have addressed concerns the differences and similarities between molecular complexes and metal NPs in their reactivity toward σ-bond activation and functionalization.

Carboxylic acid-capped ruthenium nanoparticles: experimental and theoretical case study with ethanoic acid

Given that the properties of metal nanoparticles (NPs) depend on several parameters (namely, morphology, size, surface composition, crystalline structure, etc.), a computational model that brings a better understanding of a structure-property relationship at the nanoscale is a significant plus in order to explain the surface properties of metal NPs and also their catalytic viability, in particular, when envisaging a new stabilizing agent. In this study we combined experimental and theoretical tools to obtain a mapping of the surface of ruthenium NPs stabilized by ethanoic acid as a new capping ligand. For this purpose, the organometallic approach was applied as the synthesis method. The morphology and crystalline structure of the obtained particles was characterized by state-of-the art techniques (TEM, HRTEM, WAXS) and their surface composition was determined by various techniques (solution and solid-state NMR, IR, chemical titration, DFT calculations). DFT calculations of the vibrational features of model NPs and of the chemical shifts of model clusters allowed us to secure the spectroscopic experimental assignations. Spectroscopic data as well as DFT mechanistic studies showed that ethanoic acid lies on the metal surface as ethanoate, together with hydrogen atoms. The optimal surface composition determined by DFT calculations appeared to be ca. [0.4-0.6] H/Rusurf and 0.4 ethanoate/RuSurf, which was corroborated by experimental results. Moreover, for such a composition, a hydrogen adsorption Gibbs free energy in the range -2.0 to -3.0 kcal mol-1 was calculated, which makes these ruthenium NPs a promising nanocatalyst for the hydrogen evolution reaction in the electrolysis of water.

Spatially Confined Assembly of Monodisperse Ruthenium Nanoclusters in a Hierarchically Ordered Carbon Electrode for Efficient Hydrogen Evolution

The redox units of polyaniline (PAni) are used cooperatively, and in situ, to assemble ruthenium (Ru) nanoclusters in a hierarchically ordered carbon electrode. The oxidized quinonoid imine (QI) units in PAni bond Ru complex ions selectively, whereas reduced benzenoid amine (BA) units cannot. By electrochemically tuning the ratio of QI to BA, Ru complexes are spatially confined in the outer layer of hierarchical PAni frameworks. Carbonization of Ru-PAni hybrids induces nucleation on the outer surface of the carbon support, generating nearly monodisperse Ru nanoclusters. The optimized catalyst has a low loading of approximately 2 wt % Ru, but exhibits a mass activity for the hydrogen evolution reaction that is about 6.8 times better than commercial 20 wt % Pt/C catalyst.

Shape, electronic structure and steric effects of organometallic nanocatalysts: relevant tools to improve the synergy between theory and experiment

An integrated package that uses structural, first principles and thermodynamic approaches is expected to play a significant role in advancing our knowledge of nanocatalysts.

Theoretical characterization of the surface composition of ruthenium nanoparticles in equilibrium with syngas

A deeper understanding of the relationship between experimental reaction conditions and the surface composition of nanoparticles is crucial in order to elucidate mechanisms involved in nanocatalysis. In the framework of the Fischer-Tropsch synthesis, a resolution of this complex puzzle requires a detailed understanding of the interaction of CO and H with the surface of the catalyst. In this context, the single- and co-adsorption of CO and H to the surface of a 1 nm ruthenium nanoparticle has been investigated with density functional theory. Using several indexes (d-band center, crystal overlap Hamilton population, density of states), a systematic analysis of the bond properties and of the electronic states has also been done, in order to bring an understanding of structure/property relationships at the nanoscale. The H : CO surface composition of this ruthenium nanoparticle exposed to syngas has been evaluated according to a thermodynamic model fed with DFT energies. Such ab initio thermodynamic calculations give access to the optimal H : CO coverage values under a wide range of experimental conditions, through the construction of free energy phase diagrams. Surprisingly, under the Fischer-Tropsch synthesis experimental conditions, and in agreement with new experiments, only CO species are adsorbed at the surface of the nanoparticle. These findings shed new light on the possible reaction pathways underlying the Fischer-Tropsch synthesis, and specifically the initiation of the reaction. It is finally shown that the joint knowledge of the surface composition and energy descriptors can help to identify possible reaction intermediates.

Predictive morphology, stoichiometry and structure of surface species in supported Ru nanoparticles under H2 and CO atmospheres from combined experimental and DFT studies

CO and H₂ chemisorption stoichiometries of silica supported Ru nanoparticles are understood by combining chemisorption experiments and ab initio calculations.

Surfaces of a colloidal iron nanoparticle in its chemical environment: a DFT description

Describing and understanding surface chemistry on the atomic scale is of primary importance in predicting and rationalize nanoparticle morphology as well as their physical and chemical properties. Here we present the results of comprehensive density functional theory studies on the adsorption of several small organic species, representing the major species (H2, Cl2, HCl, NH3, NH4Cl, and CH3COOH), present in the reaction medium during colloidal iron nanoparticle synthesis on various low-index iron surface models, namely, (100), (110), (111), (211), and (310). All of the tested ligands strongly interact with the proposed surfaces. Surface energies are calculated and ligand effects on the morphologies are presented, including temperature effects, based on a thermodynamic approach combined with the Wulff construction scheme. The importance of taking into account vibrational contributions during the calculation of surface energies after adsorption is clearly demonstrated. More importantly, we find that thermodynamic ligand effects can be ruled out as the unique driving force in the formation of recently experimentally observed iron cubic nanoparticles.

A unified picture of adsorption on transition metals through different atoms

A key issue in catalyst design is understanding how adsorption energies of surface intermediates vary across both different surfaces and various types of adsorbing atoms. In this work, we examine trends in adsorption energies of a wide variety of adsorbates that attach to transition metal surfaces through different atoms (H, C, N, O, F, S, etc.). All adsorption energies, as calculated by density functional theory, have nearly identical dependence on the metal bands (the d-band center and the number of p electrons) and the adsorbates' highest occupied molecular orbital (HOMO) energies. However, the dependence on the adsorbate-surface coupling and the d-band filling varies with the energy of the HOMO. Adsorbates with low HOMOs experience a higher level of Pauli repulsion than those with higher HOMOs. This leads to a classification of adsorbates into two groups, where adsorption energies in each group correlate. Even across the groups, adsorbates with similar HOMO energies are likely to have correlated adsorption energies.