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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...

The dry reforming of methane (DRM), i.e., the reaction of methane and CO 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 MgAlO matrix derived via a hydrotalcite-like precursor were synthesized. Importantly, the textural features of the catalysts, i.e., the specific surface area (172-178 m/g), pore volume (0.51-0.66 cm/g), and particle size (5.4-5.8 nm) were kept constant. Bimetallic, NiFe 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.

Sn-β zeolites prepared using different recipes feature very different catalytic activity for aqueous phase glucose isomerization, suggesting the presence of different active sites. A systematic study of the morphology and atomic-level structure of the materials using DNP NMR spectroscopy in combination with first principles calculations allows for the discrimination between potential sites and 2 leads to a proposal of specific structural features that are important for high activity. The results indicate that the materials showing highest activity posses a highly hydrophobic, defect-free zeolite framework. Those materials show so-called closed and associated partially hydrolyzed Sn(IV)-sites in the T6 and T5/T7 lattice position. On the other hand post-synthetically synthesized Sn-b samples prepared in two steps via dealumination and subsequent solid-state ion-exchange from Al-b show significant lower activity which is associated with a hydrophilic framework and/or a lower accessibility and lattice position of the Sn sites in the zeolite crystal. Further we provide a method to distinguish between different Sn sites based on NMR cartography using chemical shift and chemical shift anisotropy as readily measurable parameters. This cartography not only allows identifying the nature of the active sites (closed, defect-open and hydrolyzed-open), but also their position within the BEA framework.

Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mössbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-β zeolite. This approach unambiguously shows the presence of framework Sn(IV)-active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework).

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.

Significance The Phillips catalyst—CrO x /SiO 2 —produces 40–50% of global high-density polyethylene, yet several fundamental mechanistic controversies surround this catalyst. What is the oxidation state and nuclearity of the active Cr sites? How is the first Cr–C bond formed? How does the polymer propagate and regulate its molecular weight? Here we show through combined experimental (infrared, ultraviolet-visible, X-ray near edge absorption spectroscopy, and extended X-ray absorption fine structures) and density functional theory modeling approaches that mononuclear tricoordinate Cr(III) sites immobilized on silica polymerize ethylene by the classical Cossee–Arlman mechanism. Initiation (C–H bond activation) and polymer molecular weight regulation (the microreverse of C–H activation) are controlled by proton transfer steps.

In this contribution, realistic amorphous SiO2 models of 2.1 × 2.1 nm with silanol densities ranging 1.1-7.2 OH per nm(2) are obtained by means of ab initio calculations via the dehydroxylation of a fully hydroxylated silica surface. The dehydroxyation process is considered to take place via direct condensation of adjacent silanol groups and silica migration steps. The latter reconstructions are needed in order to obtain highly dehydroxylated silica surfaces with favorable energetics and without the formation of defects. The obtained surface phase diagram of different silica models as a function of temperature and PH2O is able to correctly describe the silanol density under different conditions, and the IR spectroscopic signatures of the silanols are in qualitative agreement with the experiment. The amorphous silica models presented here have a high degree of heterogeneity as found from the big variability obtained in the energetics of the dehydroxylation steps. It was also found that the resulting average Si-O distance of the newly formed siloxane bridges serves as a descriptor of the strain introduced in the silica surface. All these factors can be crucial in order to simulate the activity of catalysts grafted onto silica with different silanol densities, especially the one containing ca. 1 OH per nm(2), which can serve as a model for the SiO2 surface pretreated under high vacuum and at 700 °C.