The role of oxygenated species in the catalytic self-coupling of MeOH on O pre-covered Au(111)

The oxidation of alcohols plays a central role in the valorisation of biomass, in particular when performed with a non-toxic oxidant such as O2. Aerobic oxidation of methanol on gold has attracted attention lately and the main steps of its mechanism have been described experimentally. However, the exact role of O and OH on each elementary step and the effect of the interactions between adsorbates are still not completely understood. Here we investigate the mechanism of methanol oxidation to HCOOCH3 and CO2. We use Density Functional Theory (DFT) to assess the energetics of the underlying pathways, and subsequently build lattice kinetic Monte Carlo (kMC) models of increasing complexity, to elucidate the role of different oxygenates. Detailed comparisons of our simulation results with experimental temperature programmed desorption (TPD) spectra enable us to validate the mechanism and identify rate determining steps. Crucially, taking into account dispersion (van der Waals forces) and adsorbate-adsorbate lateral interactions are both important for reproducing the experimental data

Elucidating the Stability and Reactivity of Surface Intermediates on Single-Atom Alloy Catalysts

Doping isolated single atoms of a platinum-group metal into the surface of a noble-metal host is sufficient to dramatically improve the activity of the unreactive host yet also facilitates the retention of the host’s high reaction selectivity in numerous catalytic reactions. The atomically dispersed highly active sites in these single-atom alloy (SAA) materials are capable of performing facile bond activations allowing for the uptake of species onto the surface and the subsequent spillover of adspecies onto the noble host material, where selective catalysis can be performed. For example, SAAs have been shown to activate C–H bonds at low temperatures without coke formation, as well as selectively hydrogenate unsaturated hydrocarbons with excellent activity. However, to date, only a small subset of SAAs has been synthesized experimentally and it is unclear which metallic combinations may best catalyze which chemical reactions. To shed light on this issue, we have performed a widespread screening study using density functional theory to elucidate the fundamental adsorptive and catalytic properties of 12 SAAs (Ni-, Pd-, Pt-, and Rh-doped Cu(111), Ag(111), and Au(111)). We considered the interaction of these SAAs with a variety of adsorbates often found in catalysis and computed reaction mechanisms for the activation of several catalytically relevant species (H₂, CH₄, NH₃, CH₃OH, and CO₂) by SAAs. Finally, we discuss the applicability of thermochemical linear scaling and the Brønsted–Evans–Polanyi relationship to SAA systems, demonstrating that SAAs combine weak binding with low activation energies to give enhanced catalytic behavior over their monometallic counterparts. This work will ultimately facilitate the discovery and development of SAAs, serving as a guide to experimentalists and theoreticians alike

Controlling Hydrocarbon (De)Hydrogenation Pathways with Bifunctional PtCu Single-Atom Alloys

The conversions of surface-bound alkyl groups to alkanes and alkenes are important steps in many heterogeneously catalyzed reactions. On the one hand, while Pt is ubiquitous in industry because of its high activity toward C-H activation, many Pt-based catalysts tend to overbind reactive intermediates, which leads to deactivation by carbon deposition and coke formation. On the other hand, Cu binds intermediates more weakly than Pt, but activation barriers tend to be higher on Cu. We examine the reactivity of ethyl, the simplest alkyl group that can undergo hydrogenation and dehydrogenation via β-elimination, and show that isolated Pt atoms in Cu enable low-temperature hydrogenation of ethyl, unseen on Cu, while avoiding the decomposition pathways on pure Pt that lead to coking. Furthermore, we confirm the predictions of our theoretical model and experimentally demonstrate that the selectivity of ethyl (de)hydrogenation can be controlled by changing the surface coverage of hydrogen

Tuning the Product Selectivity of Single-Atom Alloys by Active Site Modification

There is widespread interest in developing catalysts with uniform active sites that consist of single atoms, thereby simplifying the reaction mechanism and improving product selectivity. We examine experimentally how CO can be used to modify the active sites on a strong binding single-atom alloy and examine how this in turn impacts product selectivity for a reaction that has two different pathways. Specifically, we find that CO can be used to selectively block isolated Rh atom active sites in a RhCu(111) model single-atom alloy catalyst surface and promote the dehydrogenation pathway for adsorbed ethyl groups by suppressing the hydrogenation pathway

Mechanistic insights into carbon–carbon coupling on NiAu and PdAu single-atom alloys

Carbon–carbon coupling is an important step in many catalytic reactions, and performing sp³–sp³ carbon–carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups, producing ethane from methyl iodide. Herein, we extend this study to NiAu SAAs and find that Ni atoms in Au are active for C–I cleavage and selective sp³–sp³ carbon–carbon coupling to produce ethane. Furthermore, we perform ab initio kinetic Monte Carlo simulations that include the effect of the iodine atom, which was previously considered a bystander species. We find that model NiAu surfaces exhibit a similar chemistry to PdAu, but the reason for the similarity is due to the role the iodine atoms play in terms of blocking the Ni atom active sites. Specifically, on NiAu SAAs, the iodine atoms outcompete the methyl groups for occupancy of the Ni sites leaving the Me groups on Au, while on PdAu SAAs, the binding strengths of methyl groups and iodine atoms at the Pd atom active site are more similar. These simulations shed light on the mechanism of this important sp3–sp3 carbon–carbon coupling chemistry on SAAs. Furthermore, we discuss the effect of the iodine atoms on the reaction energetics and make an analogy between the effect of iodine as an active site blocker on this model heterogeneous catalyst and homogeneous catalysts in which ligands must detach in order for the active site to be accessed by the reactants

Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water

International audienceThe kinetic stability of any material in water relies on the presence of surface weak spots responsible for chemical weathering by hydrolysis. Being able to identify the atomistic nature of these sites and the first steps of transformation is therefore critical to master the decomposition processes. This is the challenge that we tackle here: combining experimental and modeling studies we investigate the stability of alumina in water. Exploring the reactivity of shape-controlled crystals, we identify experimentally a specific facet as the location of the weak spots. Using biased ab initio molecular dynamics, we recognize this weak spot as a surface exposed tetra-coordinated Al atom and further provide a detailed mechanism of the first steps of hydrolysis. This understanding is of great importance to heterogeneous catalysis where alumina is a major support. Furthermore, it paves the way to atomistic understanding of interfacial reactions, at the crossroad of a variety of fields of research

Mechanistic insights into carbon–carbon coupling on NiAu and PdAu single-atom alloys

Carbon–carbon coupling is an important step in many catalytic reactions, and performing sp³–sp³ carbon–carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups, producing ethane from methyl iodide. Herein, we extend this study to NiAu SAAs and find that Ni atoms in Au are active for C–I cleavage and selective sp³–sp³ carbon–carbon coupling to produce ethane. Furthermore, we perform ab initio kinetic Monte Carlo simulations that include the effect of the iodine atom, which was previously considered a bystander species. We find that model NiAu surfaces exhibit a similar chemistry to PdAu, but the reason for the similarity is due to the role the iodine atoms play in terms of blocking the Ni atom active sites. Specifically, on NiAu SAAs, the iodine atoms outcompete the methyl groups for occupancy of the Ni sites leaving the Me groups on Au, while on PdAu SAAs, the binding strengths of methyl groups and iodine atoms at the Pd atom active site are more similar. These simulations shed light on the mechanism of this important sp3–sp3 carbon–carbon coupling chemistry on SAAs. Furthermore, we discuss the effect of the iodine atoms on the reaction energetics and make an analogy between the effect of iodine as an active site blocker on this model heterogeneous catalyst and homogeneous catalysts in which ligands must detach in order for the active site to be accessed by the reactants

Comportement du combustible en situation accidentelle - Le programme Phébus

L'installation Phébus est destinée à étudier le comportement du combustible des réacteurs à eau sous pression dans des conditions simulées de refroidissement en circonstances accidentelles. Après avoir présenté l'installation, les auteurs décrivent les programmes d'essais qui y sont engagés