Increased Back-Bonding Explains Step-Edge Reactivity and Particle Size Effect for CO Activation on Ru Nanoparticles

Carbon monoxide is a ubiquitous molecule, a key feedstock and intermediate in chemical processes. Its adsorption and activation, typically carried out on metallic nanoparticles (NPs), are strongly dependent on the particle size. In particular, small NPs, which in principle contain more corner and step-edge atoms, are surprisingly less reactive than larger ones. Hereby, first-principles calculations on explicit Ru NP models (1–2 nm) show that both small and large NPs can present step-edge sites (e.g., B₅ and B₆ sites). However, such sites display strong particle-size-dependent reactivity because of very subtle differences in local chemical bonding. State-of-the-art crystal orbital Hamilton population analysis allows a detailed molecular orbital picture of adsorbed CO on step-edges, which can be classified as <i>flat</i> (η¹ coordination) and <i>concave</i> (η² coordination) sites. Our analysis shows that the CO π-metal <i>d</i>_π hybrid band responsible for the electron back-donation is better represented by an oxygen lone pair on flat sites, whereas it is delocalized on both C and O atoms on concave sites, increasing the back-bonding on these sites compared to flat step-edges or low-index surface sites. The bonding analysis also rationalizes why CO cleavage is easier on step-edge sites of large NPs compared to small ones irrespective of the site geometry. The lower reactivity of small NPs is due to the smaller extent of the Ru–O interaction in the η² adsorption mode, which destabilizes the η² transition-state structure for CO direct cleavage. Our findings provide a molecular understanding of the reactivity of CO on NPs, which is consistent with the observed particle size effect

Adlayer Dynamics Drives CO Activation in Ru-Catalyzed Fischer–Tropsch Synthesis

The first step of the Fischer–Tropsch synthesis (FTS) consists of the carbon monoxide activation at ca. 200 °C on catalyst metal surfaces covered with dense adlayers. Based on first-principles calculations, two main mechanisms for the C–O bond cleavage have been proposed for Ru catalysts: the direct and the hydrogen-assisted routes on step-edges and flat surfaces, respectively. However, commonly used static density functional theory (DFT) methods describe FTS adlayers using nonmobile adsorbed CO (CO*) species, while under reaction conditions the adsorbates diffuse and interact with each other. Here, we use ab initio molecular dynamics (AIMD) simulations on Ru flat and stepped model surfaces covered with CO* and H* to interrogate the effect of adlayer dynamics on the preferred reaction mechanisms. We show that hydrogen-assisted CO activation mechanisms via hydroxyl-carbonyl (COH*) intermediates formed on step-edges are the most favored according to AIMD simulations. Therefore, both step-edges and surface hydrogen play a key role in CO cleavage during Ru-catalyzed FTS at high CO* coverage. Direct comparison with static DFT results reveals that the dynamic adlayer significantly affects the relative stability of reaction intermediates and shows that the mobility of adsorbed molecules modulates the reaction paths, calling for a systematic analysis of reaction networks on complex systems at high coverages using AIMD simulations

Adlayer Dynamics Drives CO Activation in Ru-Catalyzed Fischer–Tropsch Synthesis

The first step of the Fischer–Tropsch synthesis (FTS) consists of the carbon monoxide activation at ca. 200 °C on catalyst metal surfaces covered with dense adlayers. Based on first-principles calculations, two main mechanisms for the C–O bond cleavage have been proposed for Ru catalysts: the direct and the hydrogen-assisted routes on step-edges and flat surfaces, respectively. However, commonly used static density functional theory (DFT) methods describe FTS adlayers using nonmobile adsorbed CO (CO*) species, while under reaction conditions the adsorbates diffuse and interact with each other. Here, we use ab initio molecular dynamics (AIMD) simulations on Ru flat and stepped model surfaces covered with CO* and H* to interrogate the effect of adlayer dynamics on the preferred reaction mechanisms. We show that hydrogen-assisted CO activation mechanisms via hydroxyl-carbonyl (COH*) intermediates formed on step-edges are the most favored according to AIMD simulations. Therefore, both step-edges and surface hydrogen play a key role in CO cleavage during Ru-catalyzed FTS at high CO* coverage. Direct comparison with static DFT results reveals that the dynamic adlayer significantly affects the relative stability of reaction intermediates and shows that the mobility of adsorbed molecules modulates the reaction paths, calling for a systematic analysis of reaction networks on complex systems at high coverages using AIMD simulations

Electronic Structure–Reactivity Relationship on Ruthenium Step-Edge Sites from Carbonyl ¹³C Chemical Shift Analysis

Ru nanoparticles are highly active catalysts for the Fischer–Tropsch and the Haber–Bosch processes. They show various types of surface sites upon CO adsorption according to NMR spectroscopy. Compared to terminal and bridging η¹ adsorption modes on terraces or edges, little is known about side-on η² CO species coordinated to B₅ or B₆ step-edges, the proposed active sites for CO and N₂ cleavage. By using solid-state NMR and DFT calculations, we analyze ¹³C chemical shift tensors (CSTs) of carbonyl ligands on the molecular cluster model for Ru nanoparticles, Ru₆(η²-μ₄-CO)₂(CO)₁₃(η⁶-C₆Me₆), and show that, contrary to η¹ carbonyls, the CST principal components parallel to the C–O bond are extremely deshielded in the η² species due to the population of the C–O π* antibonding orbital, which weakens the bond prior to dissociation. The carbonyl CST is thus an indicator of the reactivity of both Ru clusters and Ru nanoparticles step-edge sites toward C–O bond cleavage

Contrasting the Role of Ni/Al₂O₃ Interfaces in Water–Gas Shift and Dry Reforming of Methane

Transition metal nanoparticles (NPs) are typically supported on oxides to ensure their stability, which may result in modification of the original NP catalyst reactivity. In a number of cases, this is related to the formation of NP/support interface sites that play a role in catalysis. The metal/support interface effect verified experimentally is commonly ascribed to stronger reactants adsorption or their facile activation on such sites compared to bare NPs, as indicated by DFT-derived potential energy surfaces (PESs). However, the relevance of specific reaction elementary steps to the overall reaction rate depends on the preferred reaction pathways at reaction conditions, which usually cannot be inferred based solely on PES. Hereby, we use a multiscale (DFT/microkinetic) modeling approach and experiments to investigate the reactivity of the Ni/Al₂O₃ interface toward water–gas shift (WGS) and dry reforming of methane (DRM), two key industrial reactions with common elementary steps and intermediates, but held at significantly different temperatures: 300 vs 650 °C, respectively. Our model shows that despite the more energetically favorable reaction pathways provided by the Ni/Al₂O₃ interface, such sites may or may not impact the overall reaction rate depending on reaction conditions: the metal/support interface provides the active site for WGS reaction, acting as a reservoir for oxygenated species, while all Ni surface atoms are active for DRM. This is in contrast to what PESs alone indicate. The different active site requirement for WGS and DRM is confirmed by the experimental evaluation of the activity of a series of Al₂O₃-supported Ni NP catalysts with different NP sizes (2–16 nm) toward both reactions

Cooperativity and Dynamics Increase the Performance of NiFe Dry Reforming Catalysts

The dry reforming of methane (DRM), i.e., the reaction of methane and CO₂ to form a synthesis gas, converts two major greenhouse gases into a useful chemical feedstock. In this work, we probe the effect and role of Fe in bimetallic NiFe dry reforming catalysts. To this end, monometallic Ni, Fe, and bimetallic Ni-Fe catalysts supported on a Mg_<i>x</i>Al_<i>y</i>O_<i>z</i> matrix derived via a hydrotalcite-like precursor were synthesized. Importantly, the textural features of the catalysts, i.e., the specific surface area (172–178 m²/g_cat), pore volume (0.51–0.66 cm³/g_cat), and particle size (5.4–5.8 nm) were kept constant. Bimetallic, Ni₄Fe₁ with Ni/(Ni + Fe) = 0.8, showed the highest activity and stability, whereas rapid deactivation and a low catalytic activity were observed for monometallic Ni and Fe catalysts, respectively. XRD, Raman, TPO, and TEM analysis confirmed that the deactivation of monometallic Ni catalysts was in large due to the formation of graphitic carbon. The promoting effect of Fe in bimetallic Ni-Fe was elucidated by combining operando XRD and XAS analyses and energy-dispersive X-ray spectroscopy complemented with density functional theory calculations. Under dry reforming conditions, Fe is oxidized partially to FeO leading to a partial dealloying and formation of a Ni-richer NiFe alloy. Fe migrates leading to the formation of FeO preferentially at the surface. Experiments in an inert helium atmosphere confirm that FeO reacts via a redox mechanism with carbon deposits forming CO, whereby the reduced Fe restores the original Ni-Fe alloy. Owing to the high activity of the material and the absence of any XRD signature of FeO, it is very likely that FeO is formed as small domains of a few atom layer thickness covering a fraction of the surface of the Ni-rich particles, ensuring a close proximity of the carbon removal (FeO) and methane activation (Ni) sites