Methanol synthesis via CO2 hydrogenation is a key step in 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. Here, we present a mechanistic study of the hydrogenation of CO2 on Cu/ZrO2. Using kinetics, in situ IR and NMR spectroscopies and isotopic labeling strategies, we examined the surface intermediates during CO2 hydrogenation at different pressures. Combined with DFT calculations, we show that formate species is the reaction intermediate and that the zirconia/copper interface is a key for its conversion to methanol.The catalytic hydrogenation of carbon dioxide to methanol is a key process in the sustainable methanol-based economy. [1] While copper-based catalysts are highly active for this transformation, [2] their activity and selectivity strongly depend on the support and/or the promoters. Understanding the copper-support interaction -its effect on the activity and product selectivity -has been a very intensive field of research over the last decade. While the reaction mechanisms and the nature of the active sites on Cu/ZnO systems have been extensively investigated, [3] copper supported on zirconia and related materials also exhibits high activity and selectivity in CO2 hydrogenation to methanol (Eq. 1) by minimizing the formation of CO, a byproduct often resulting from the competitive reverse water-gas shift reaction (Eq. 2). [4] CO2 + 3H2 = CH3OH + H2O ∆rH° (500 K) = -62 kJ.mol -1 (1) CO2 + H2 = CO + H2O ∆rH° (500 K) = +40 kJ.mol -1 (2)Although the copper-zirconia interface was proposed to play a key role in the selective formation of methanol, [4c, 4e-g] the active site and the reaction mechanism, including the role of the interface on methanol selectivity, are still not understood. In fact, mechanistic investigations using Diffuse Reflectance IR Fourier Transform spectroscopy (DRIFTS) led to opposite conclusions: formate is an intermediate in methanol formation [4c, 4d] vs. CO2 is first reduced to CO that is in turn hydrogenated to methanol through a carboxyl intermediate. [4f] Herein, by using a combined experimental and computational approach on realistic models, we investigated the reaction mechanism of CO2 hydrogenation to methanol on a Cu/ZrO2 catalyst. Kinetic investigation, in situ and ex situ spectroscopies -FTIR and NMRtogether with isotopic labeling and computational modelling showed that methanol is a primary product formed by the hydrogenation of formate as a reaction intermediate. First, narrowly dispersed copper nanoparticles supported on monoclinic zirconia were prepared by a molecular approach. [5] Grafting of [Cu(O t Bu)]4 on the surface hydroxyl groups of the support ( Figure S1-S2, Scheme S1) followed by a treatment under H2 at 500 °C for 5 h [6] yields smal...

Copper nanoparticles supported on zirconia (Cu/ZrO) or related supported oxides (Cu/ZrO/SiO) show promising activity and selectivity for the hydrogenation of CO to CHOH. 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 CHOH activity and selectivity compared to those supported on SiO, reaching catalytic performances comparable to those of the corresponding Cu/ZrO. 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 ZrO. These observations imply that Zr(IV) surface sites at the periphery of Cu particles are responsible for promoting CHOH formation on Cu-Zr-based catalysts and provide a guideline to develop selective CHOH synthesis catalysts.

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.

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/AlO 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/AlO 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 AlO-supported Ni NP catalysts with different NP sizes (2-16 nm) toward both reactions.

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 site nm on the silica-doped alumina surface under investigation, by combining information from experiments and theoretical calculations.