Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites.

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

Deciphering the Structure of Tungstate and Molybdate Complexes with Glucose, Mannose, and Erythrose.

Combining nuclear magnetic resonance (NMR), X-ray absorption spectroscopy near-edge structure (XANES), and density functional theory (DFT), we elucidate the structures of tungstate and molybdate with sugars of interest in the conversion of biomass to platform chemicals (glucose, mannose, and erythrose). We highlight a number of complexes, including one nearly isostructural structure that is formed with each metal-sugar combination. We also emphasize the singular reactivity of erythrose that undergoes retro-aldolization at room temperature.

Elucidation of Metal-Sugar Complexes: When Tungstate Combines with d-Mannose.

The control of metal-sugar complexes speciation in solution is crucial in an energy transition context. Herein, the formation of tungstate-mannose complexes is unraveled in aqueous solution using a multitechnique experimental and theoretical approach. 13C nuclear magnetic resonance (NMR), as well as 13C-1H and 1H-1H correlation spectra, analyzed in the light of coordination-induced shift method and conformation analysis, were employed to characterize the structure of the sugar involved in the complexes. X-ray absorption near edge structure spectroscopy was performed to provide relevant information about the metal electronic and coordination environment. The calculation of 13C NMR chemical shifts for a series of tungstate-mannose complexes using density functional theory (DFT) is a key to identify the appropriate structure among several candidates. Furthermore, a parametric study based on several relevant parameters, namely, pH and tungstate concentration, was carried out to look over the change of the nature and concentrations of the complexes. Two series of complexes were detected, in which the metallic core is either in a ditungstate or a monotungstate form. With respect to previous proposals, we identify two new species. Dinuclear complexes involve both α- and ß-furanose forms chelating the metallic center in a tetradentate fashion. A hydrate form chelating a ditungstate core is also revealed. One monotungstate complex appears at high pH, in which a tetrahedral tungstate center is bound to α-mannofuranose through a monodentate site at the second deprotonated hydroxyl group. This unequalled level of knowledge opens the door to structure-reactivity relationships.

What Can We Learn from First Principles Multi-Scale Models in Catalysis? The Role of the Ni/Al2O3 Interface in Water-Gas Shift and Dry Reforming as a Case Study.

Computational first principles models based on density functional theory (DFT) have emerged as an important tool to address reaction mechanisms and active sites in metal nanoparticle catalysis. However, the common evaluation of potential energy surfaces for selected reaction steps contrasts with the complexity of reaction networks under operating conditions, where the interplay of adsorbate populations and competing routes at reaction conditions determine the most relevant states for catalyst activity and selectivity. Here, we discuss how the use of a multi-scale first principles approach combining DFT calculations at the atomistic level with kinetic models may be used to understand reactions catalyzed by metal nanoparticles. The potential of such an approach is illustrated for the case of Al2O3-supported Ni nanoparticle catalysts in the water-gas shift and dry reforming reactions. In these systems, both Ni nanoparticle (metal) as well as metal/oxide interface sites are available and may play a role in catalysis, which depends not only on the energy for critical reaction steps, as captured by DFT, but also on the reaction temperature and adsorbate populations, as shown by microkinetic modelling and experiments.

CO2 Hydrogenation on Cu/Al2 O3 : Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst.

Selective hydrogenation of CO2 into methanol is a key sustainable technology, where Cu/Al2 O3 prepared by surface organometallic chemistry displays high activity towards CO2 hydrogenation compared to Cu/SiO2 , yielding CH3 OH, dimethyl ether (DME), and CO. CH3 OH formation rate increases due to the metal-oxide interface and involves formate intermediates according to advanced spectroscopy and DFT calculations. Al2 O3 promotes the subsequent conversion of CH3 OH to DME, showing bifunctional catalysis, but also increases the rate of CO formation. The latter takes place 1) directly by activation of CO2 at the metal-oxide interface, and 2) indirectly by the conversion of formate surface species and CH3 OH to methyl formate, which is further decomposed into CH3 OH and CO. This study shows how Al2 O3 , a Lewis acidic and non-reducible support, can promote CO2 hydrogenation by enabling multiple competitive reaction pathways on the oxide and metal-oxide interface.

Isolated Zr Surface Sites on Silica Promote Hydrogenation of CO2 to CH3OH in Supported Cu Catalysts.

Copper nanoparticles supported on zirconia (Cu/ZrO2) or related supported oxides (Cu/ZrO2/SiO2) show promising activity and selectivity for the hydrogenation of CO2 to CH3OH. However, the role of the support remains controversial because most spectroscopic techniques provide information dominated by the bulk, making interpretation and formulation of structure-activity relationships challenging. In order to understand the role of the support and in particular of the Zr surface species at a molecular level, a surface organometallic chemistry approach has been used to tailor a silica support containing isolated Zr(IV) surface sites, on which copper nanoparticles (∼3 nm) are generated. These supported Cu nanoparticles exhibit increased CH3OH activity and selectivity compared to those supported on SiO2, reaching catalytic performances comparable to those of the corresponding Cu/ZrO2. Ex situ and in situ X-ray absorption spectroscopy reveals that the Zr sites on silica remain isolated and in their +4 oxidation state, while ex situ solid-state nuclear magnetic resonance spectroscopy and catalytic performances show that similar mechanisms are involved with the single-site support and ZrO2. These observations imply that Zr(IV) surface sites at the periphery of Cu particles are responsible for promoting CH3OH formation on Cu-Zr-based catalysts and provide a guideline to develop selective CH3OH synthesis catalysts.

Isolated Surface Hydrides: Formation, Structure, and Reactivity.

Surface hydrides are ubiquitous in catalysis. However, their structures and properties are not as well-understood as those of their molecular counterparts, which have been extensively studied for the past 70 years. Hydrides isolated on surfaces have been characterized as stable entities on oxide surfaces or in zeolites. They have also been proposed as reaction intermediates in numerous catalytic processes (hydrogenation, hydrogenolysis, etc.). They have also been prepared via surface organometallic chemistry. In this review, we describe their key structural features and spectroscopic signatures. We discuss their reactivity and stability and also point out unexplored areas.

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.

Molecularly Tailored Nickel Precursor and Support Yield a Stable Methane Dry Reforming Catalyst with Superior Metal Utilization.

Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. [{Ni(µ2-OCHO)(OCHO)(tmeda)}2(µ2-OH2)] (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, operando X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition.

Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols.

The mechanism of isopropanol dehydration on amorphous silica-alumina (ASA) was unraveled by a combination of experimental kinetic measurements and periodic density functional theory (DFT) calculations. We show that pseudo-bridging silanols (PBS-Al) are the most likely active sites owing to the synergy between the Brønsted and Lewis acidic properties of these sites, which facilitates the activation of alcohol hydroxy groups as leaving groups. Isopropanol dehydration was used to specifically investigate these PBS-Al sites, whose density was estimated to be about 10-1  site nm-2 on the silica-doped alumina surface under investigation, by combining information from experiments and theoretical calculations.

CO2 -to-Methanol Hydrogenation on Zirconia-Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-Support Interface.

Methanol synthesis by CO2 hydrogenation is a key process in a methanol-based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO2 to methanol still remain unknown. A mechanistic study of the hydrogenation of CO2 on Cu/ZrO2 is presented. Using kinetics, in situ IR and NMR spectroscopies, and isotopic labeling strategies, surface intermediates evolved during CO2 hydrogenation were observed at different pressures. Combined with DFT calculations, it is shown that a formate species is the reaction intermediate and that the zirconia/copper interface is crucial for the conversion of this intermediate to methanol.

Aqueous-Phase Preparation of Model HDS Catalysts on Planar Alumina Substrates: Support Effect on Mo Adsorption and Sulfidation.

The role of the oxide support on the structure of the MoS2 active phase (size, morphology, orientation, sulfidation ratio, etc.) remains an open question in hydrotreating catalysis and biomass processing with important industrial implications for the design of improved catalytic formulations. The present work builds on an aqueous-phase surface-science approach using four well-defined α-alumina single crystal surfaces (C (0001), A (112̅0), M (101̅0), and R (11̅02) planes) as surrogates for γ-alumina (the industrial support) in order to discriminate the specific role of individual support facets. The reactivity of the various surface orientations toward molybdenum adsorption is controlled by the speciation of surface hydroxyls that determines the surface charge at the oxide/water interface. The C (0001) plane is inert, and the R (11̅02) plane has a limited Mo adsorption capacity while the A (112̅0) and M (101̅0) surfaces are highly reactive. Sulfidation of model catalysts reveals the highest sulfidation degree for the A (112̅0) and M (101̅0) planes suggesting weak metal/support interactions. Conversely, a low sulfidation rate and shorter MoS2 slabs are found for the R (11̅02) plane implying stronger Mo-O-Al bonds. These limiting cases are reminiscent of type I/type II MoS2 nanostructures. Structural analogies between α- and γ- alumina surfaces allow us to bridge the material gap with real Al2O3-supported catalysts. Hence, it can be proposed that Mo distribution and sulfidation rate are heterogeneous and surface-dependent on industrial γ-Al2O3-supported high-surface-area catalysts. These results demonstrate that a proper control of the γ-alumina morphology is a strategic lever for a molecular-scale design of hydrotreating catalysts.

Tuning the metal-support interaction by structural recognition of cobalt-based catalyst precursors.

Controlling the nature and size of cobalt(II) polynuclear precursors on γ-alumina and silica-alumina supports represents a challenge for the synthesis of optimal cobalt-based heterogeneous catalysts. By density functional theory (DFT) calculations, we show how after drying the interaction of cobalt(II) precursor on γ-alumina is driven by a structural recognition phenomenon, leading to the formation of an epitaxial Co(OH)2 precipitate involving a Co-Al hydrotalcite-like interface. On a silica-alumina surface, this phenomenon is prevented due to the passivation effect of silica domains. This finding opens new routes to tune the metal-support interaction at the synthesis step of heterogeneous catalysts.

Enhanced CH3OH selectivity in CO2 hydrogenation using Cu-based catalysts generated via SOMC from GaIII single-sites.

Small and narrowly distributed nanoparticles of copper alloyed with gallium supported on silica containing residual GaIII sites can be obtained via surface organometallic chemistry in a two-step process: (i) formation of isolated GaIII surface sites on SiO2 and (ii) subsequent grafting of a CuI precursor, [Cu(O t Bu)]4, followed by a treatment under H2 to generate CuGa x alloys. This material is highly active and selective for CO2 hydrogenation to CH3OH. In situ X-ray absorption spectroscopy shows that gallium is oxidized under reaction conditions while copper remains as Cu0. This CuGa material only stabilizes methoxy surface species while no formate is detected according to ex situ infrared and solid-state nuclear magnetic resonance spectroscopy.

Selective Hydrogenation of CO2 to CH3 OH on Supported Cu Nanoparticles Promoted by Isolated TiIV Surface Sites on SiO2.

Small and narrowly distributed Cu nanoparticles, supported on SiO2 decorated with isolated TiIV sites, prepared through surface organometallic chemistry, showed significantly improved CO2 hydrogenation activity and CH3 OH selectivity compared to the corresponding Cu nanoparticles supported on SiO2 . These isolated Lewis acid TiIV sites, evidenced by UV/Vis spectroscopy, are proposed to stabilize surface intermediates at the interface between Cu nanoparticles and the support.

Understanding surface site structures and properties by first principles calculations: an experimental point of view!

The characterization of the active sites in heterogeneous catalysts is highly challenging and hinders the rational development and design of better catalysts. One approach to achieve this goal is Surface Organometallic Chemistry (SOMC), which allows generation of well-defined active sites characterized at the molecular level using spectroscopic methods. Due to many advances in the field and the increase of computational power, Computational Chemistry (CompChem) plays a key role in providing an atomistic understanding of these systems and the nature of the active sites. In this perspective, we discuss how CompChem has been essential in helping us to understand a broad range of systems from the surface chemistry of the support materials themselves, to the structure, spectroscopic signatures and activity of single-site catalysts and supported metallic nanoparticles.

Surface Sites in Cu-Nanoparticles: Chemical Reactivity or Microscopy?

Copper nanoparticles are widely used in catalysis and electrocatalysis, and the fundamental understanding of their activity requires reliable methods to assess the number of potentially reactive atoms exposed on the surface. Herein, we provide a molecular understanding of the difference observed in addressing surface site titration using prototypical methods: transmission electron micrscopy (TEM), H2 chemisorption, and N2O titration by a combination of experimental and theoretical study. We show in particular that microscopy does not allow assessing the amount of reactive surface sites, while H2 and N2O chemisorptions can, albeit with slightly different stoichiometries (1 O/2CuS and 1 H2/2.2CuS), which can be rationalized by density functional theory calculations. High-resolution TEM shows that the origin of the observed difference between microscopy and titration methods is due to the strong metal support interaction experienced by small copper nanoparticles with the silica surface.

Composition-dependent surface chemistry of colloidal BaxSr1-xTiO3 perovskite nanocrystals.

BaxSr1-xTiO3 perovskite nanocrystals, prepared by the vapor diffusion sol-gel method and characterized by state of the art surface techniques, display significantly different O-H stretching frequencies and adsorption properties towards CO2 as a function of the alkaline earth composition (Ba vs. Sr). The difference of properties can be associated with the more basic nature of BaO-rich than SrO-rich surfaces.