46 Search Results

Insertion reactions play a central role in the catalytic synthesis of ethanol and higher alcohols. X-ray photoelectron and absorption spectroscopies have been used to follow migratory CO insertion and C─C coupling in a cis-[Ru(2,2′-bipyridine)₂(CO)(CH₃)]⁺ complex heated in a vacuum or exposed to CO. Heating of the Ru complex in a vacuum to temperatures above 50 °C induced spontaneous migration of CO into the Ru─CH₃ bond to yield a ─COCH₃ ligand. In conclusion, after adding CO to the background gas, the CO insertion reaction was seen at room temperature, opening the door for the synthesis of ethanol and more energy densemore » liquids.« less

In this article, the direct conversion of carbon dioxide (CO₂) into methanol (CH₃OH) via low-temperature hydrogenation is crucial for recycling anthropogenic CO₂ emissions and producing fuels or high value chemicals. Nevertheless, it continues to be a great challenge due to the trade-off between selectivity and catalytic activity. For CO₂ hydrogenation, In₂O₃ catalysts are known for their high CH₃OH selectivity. Subsequent studies explored depositing metals on In₂O₃ to enhance CO₂ conversion. Despite extensive research on metal (M) supported In₂O₃ catalysts, the role of In-M alloys and M/In₂O₃ interfaces in CO₂ activation and CH₃OH selectivity remains unclear. In this work, we havemore » examined the behavior of In/Au(111) alloys and InO_x/Au(111) inverse systems during CO₂ hydrogenation using synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and catalytic tests in a batch reactor. Indium forms alloys with Au(111) after deposition. The In-Au(111) alloys display high reactivity towards CO₂ and can dissociate the molecule at room temperature to generate InO_x nanostructures. At very low coverages of In (≤ 0.05 ML), the InO_x nanostructures are not stable under CO₂ hydrogenation conditions and the active In-Au(111) alloys produces mainly CO and little methanol. An increase in indium coverage to 0.3 ML led to stable InOx nanostructures under CO₂ hydrogenation conditions. These InO_x/Au(111) catalysts displayed a high selectivity (~ 80 %) towards CH₃OH production and an activity for CO₂ conversion that was at least 10 times larger than that of plain In₂O₃ or Cu(111) and Cu/ZnO(000$$\overline{1)}$$ benchmark catalysts. The results of AP-XPS show that InO_x/Au(111) produces methanol via methoxy intermediates. Inverse oxide/metal catalysts containing InOx open up a possibility for improving CO₂ → CH₃OH conversion in processes associated with the control of environmental pollution and the production of high value chemicals.« less

A combination of several in-situ techniques (XRD, XAS, AP-XPS, E-TEM) was used to explore links between the structural and chemical properties of a Cu@TiO_x catalyst under CO₂ hydrogenation conditions. The active phase of the catalyst involved an inverse oxide/metal configuration, but the initial core@shell motif was disrupted during the pre-treatment in H₂. As a consequence of strong metal-support interactions, the titania shell cracked and Cu particles migrated from the core to on top of the oxide with the simultaneous formation of a Cu-Ti-O_x phase. The generated Cu particles had a diameter of 20-40 nm and were decorated by small clustersmore » of TiO_x (< 5 nm in size). Results of in-situ XAS and XRD and images of E-TEM showed a very dynamic system, where the inverse oxide/metal configuration promoted the reactivity of the system towards CO₂ and H₂. At room temperature, CO₂ oxidized the Cu nanoparticles (CO_2,gas → CO_gas + O_oxide) inducing a redistribution of the TiO_x clusters and big modifications in catalyst surface morphology. The generated oxide overlayer disappeared at elevated temperatures (> 180 °C) upon exposure to H₂, producing a transient surface that was very active for the reverse water-gas shift reaction (CO₂ + H₂ → CO + H₂O) but was not stable at 250 °C. When oxidation and reduction occurred at the same time, under a mixture of CO₂ and H₂, the surface structure evolved toward a dynamic equilibrium that strongly depended on the temperature. Neither CO₂ nor H₂ can be considered as passive reactants. In the Cu@TiO_x system, morphological changes were linked to variations in the composition of metal-oxide interfaces which were reversible with temperature or chemical environment and affected the catalytic activity of the system. Finally, the present study illustrates the dynamic nature of phenomena associated with the trapping and conversion of CO₂.« less

Alkali oxides are typically used as promoters of heterogeneous catalysts for the water–gas shift (WGS; H₂O + CO → H₂ + CO₂) reaction. On Au(111), CsO_x exhibits diverse nanostructures at varying coverages, as revealed by scanning tunneling microscopy. Clusters of cesium oxide (Cs₂O₂) nucleate at elbow sites of the Au(111) herringbone when θ_Cs is less than 0.1 ML. Subsequently, these clusters transform into two-dimensional (2D) islands (Cs₂O, Cs₂O₂, CsO₂) as the cesium coverage increases (θ_Cs > 0.1 ML). Both types of CsO_x nanostructures enable the WGS process on Au(111). The highest activity was seen for the cesium oxide clusters whichmore » facilitated the partial dissociation of water and binding of CO. The CO_ads and OH_ads groups were not strongly bound and probably reacted to yield a short-lived HOCO intermediate that led to gaseous H₂ and CO₂. The 2D islands of CsO_x also enabled the WGS but their efficiency was reduced due to the formation of cesium hydroxide compounds (limiting mobility of OH groups) and the generation of CO₃ and C species (blocking of active centers). The fact that the performance of the CsO_x/Au(111) catalysts changed dramatically with variations in the chemical properties of the CsO_x nanostructures indicates that the alkali oxide was an integral part of the active phase, playing a central role in the activation and conversion of the reactants. To attach the label of “promoter” to CsO_x is a simplification that does not help in the design and optimization of catalysts for C1 chemistry. In conclusion, to achieve a rational design, one must consider the structural and chemical properties of the alkali oxide.« less

The reactivity and reaction mechanism of a Pt single atom supported on ceria can be tuned by changing its local electronic and atomic structure through Gd doping.

Understanding the reduction mechanism of ZnO/CuO_x interfaces by hydrogen is of great importance for advancing the performance of industrial catalysts for CO₂ hydrogenation to methanol. Here, the reduction of pristine and ZnO-modified CuO_x/Cu(111) by H₂ was investigated using ambient pressure scanning tunnelling microscopy (AP-STM), ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT). The morphological changes and reaction rates seen for the reduction of CuO_x/Cu(111) and ZnO/CuO_x/Cu(111) are very different. On CuO_x/Cu(111), perfect "44" and "29" structures displayed a very low reactivity towards H₂ at room temperature. A long induction period associated with an autocatalytic process was observedmore » to enable the reduction by the removal of chemisorbed non-lattice oxygen initially and lattice oxygen sequentially at the CuO_x-Cu interface, which led to formation of oxygen deficient "5-7" hex and honeycomb structures. In the final stages of the reduction process, regions of residual oxygen species and metallic Cu were seen. The addition of ZnO particles to CuO_x/Cu(111) opened new reaction channels. On the ZnO sites, the dissociation of H₂ was fast and H adatoms easily migrated to adjacent regions of copper oxide. This hydrogen spillover substantially enhanced the rate of oxygen removal, resulting in the rapid reduction of the copper oxide located in the periphery of the zinc oxide islands with no signs for the reduction of ZnO. The deposited ZnO completely modified the dynamics for H₂ dissociation and hydrogen migration, providing an excellent source for CO₂ hydrogenation processes on the inverse oxide/metal system.« less

Enzymatic systems achieve the catalytic conversion of methane at room temperature under mild conditions. Here, in this study, varying thermodynamic and kinetic parameters, we show that the reforming of methane by water (MWR, CH₄ + H₂O → CO + 3H₂) and the water-gas shift reaction (WGS, CO + H₂O → H₂ + CO₂), two essential processes to integrate fossil fuels toward a H2 energy loop, can be achieved on ZrO₂/Cu(111) catalysts near room temperature. Measurements of ambient-pressure X-ray photoelectron spectroscopy and mass spectrometry, combined with density functional calculations and kinetic Monte Carlo simulations, were used to study the behavior ofmore » the inverse oxide/metal catalysts. The superior performance is associated with a unique zirconia-copper interface, where multifunctional sites involving zirconium, oxygen, and copper work coordinatively to dissociate methane and water at 300 K and move forward the MWR and WGS processes.« less

CO₂ methanation reactivity, reaction mechanism, and surface structure were investigated on a mechanochemically prepared Pd/CeO₂ catalyst (PdAcCeO₂-M), where an oxidative pretreatment (-o) increased methane yield by a factor of two compared to a reductive pretreatment (-h). Methanation rates were maintained for over 48 h and further increased upon oxidative regeneration treatments. The surface species of both PdAcCeO₂-M-o and PdAcCeO₂-M-h were explored via in situ CO₂ and CO hydrogenation DRIFTS, where CO hydrogenation effectively models the dissociative CO₂ mechanism (CO₂ → CO → CH₄). PdAcCeO₂-M-o yielded distinct Pd-CO adsorption and the absence of monodentate carbonate at ~ 1400 cm^-1, while AP-XPSmore » showed that PdAcCeO₂-M-o yielded a unique Pd^δ+ contribution at 335.9 eV. By gaining insights from various in situ spectroscopic techniques, and by breaking the CO₂ hydrogenation mechanism into piecewise steps, a deeper understanding of the direct CO₂ reduction towards methane and CO over mechanochemically prepared Pd/CeO₂ catalysts was obtained.« less

CO₂ hydrogenation to methanol is of great environmental and economic interest due to its potential to reduce carbon emissions and produce valuable chemicals in one single reaction. Compared with the unmodified traditional Cu/ZnO/Al₂O₃ catalyst, an indium oxide (In₂O₃)-based catalyst can double the methanol selectivity from 30–50 to 60–100%. It is worth noting that over catalysts involving various active metals dispersed on indium oxide (M/In₂O₃, M = Pd, Ni, Au, etc.), although the methanol yield is boosted, the selectivity remains similar to that of plain In₂O₃ despite the distinct chemical properties of the added metals. Here, to investigate the phenomena behindmore » this behavior, we used RuO₂/In₂O₃ as a test catalyst. The results of ambient pressure photoelectron spectroscopy, in situ X-ray absorption fine structure, and time-resolved X-ray diffraction indicate that the structure of the RuO₂/In₂O₃ catalyst is highly dynamic in the presence of a reactive environment. Specifically, under CO₂ hydrogenation conditions, Ru clusters facilitate the reduction of In₂O₃ to generate In₂O_3–x aggregates, which encapsulate the Ru systems in a migration driven by thermodynamics. In this way, the Ru^O sites for CH₄ production are blocked while creating RuO_x–In₂O_3–x interfacial sites with tunable metal–oxide interactions for selective methanol production. In an inverse oxide/metal configuration, indium oxide has properties not seen in its bulk phase that are useful for the binding and conversion of CO₂. This work reveals the dynamic nature of In₂O₃-based catalysts, providing insights for a rational design of materials for the selective synthesis of methanol.« less

Defect sites on the oxide supports can be used to anchor and activate "single-atoms" catalysts (SACs). By engineering the anchoring sites for supporting SACs, one can alter their electronic and atomic structures which, in turn, define their activity, selectivity, and stability for catalytic reactions. To create and tune unique sites for Pt SACs on CeO₂ support, in this work, we synthesized a system consisting of CeO₂ decorated on TiO₂ nano-oxides for supporting the Pt SACs and investigated the effect of Pt weight loading. Here, a combination of multiple structural characterization methods including diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-raymore » photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) were employed to characterize the distribution of charge states of single atoms and evaluate the heterogeneity of their binding sites. We have found that the placement of Pt atoms can be tuned on a mixed oxide surface by changing the weight loading of Pt.« less