Highly selective oxidation of methane to methanol has long been challenging in catalysis. Here, we reveal key steps for the pro­motion of this reaction by water when tuning the selectivity of a well-defined CeO2/Cu2O/Cu(111) catalyst from carbon monoxide and carbon dioxide to methanol under a reaction environment with methane, oxygen, and water. Ambient-pressure x-ray photoelectron spectroscopy showed that water added to methane and oxygen led to surface methoxy groups and accelerated methanol production. These results were consistent with density functional theory calculations and kinetic Monte Carlo simulations, which showed that water preferentially dissociates over the active cerium ions at the CeO2–Cu2O/Cu(111) interface. The adsorbed hydroxyl species blocked O-O bond cleavage that would dehydrogenate methoxy groups to carbon monoxide and carbon dioxide, and it directly converted this species to methanol, while oxygen reoxidized the reduced surface. Water adsorption also displaced the produced methanol into the gas phase.

The results of kinetic tests and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) show the important role played by a ZnO-copper interface in the generation of CO and the synthesis of methanol from CO hydrogenation. The deposition of nanoparticles of ZnO on Cu(100) and Cu(111), θ < 0.3 monolayer, produces highly active catalysts. The catalytic activity of these systems increases in the sequence: Cu(111) < Cu(100) < ZnO/Cu(111) < ZnO/Cu(100). The structure of the copper substrate influences the catalytic performance of a ZnO-copper interface. Furthermore, size and metal-oxide interactions affect the chemical and catalytic properties of the oxide making the supported nanoparticles different from bulk ZnO. The formation of a ZnO-copper interface favors the binding and conversion of CO into a formate intermediate that is stable on the catalyst surface up to temperatures above 500 K. Alloys of Zn with Cu(111) and Cu(100) were not stable at the elevated temperatures (500-600 K) used for the CO hydrogenation reaction. Reaction with CO oxidized the zinc, enhancing its stability over the copper substrates.

Because of the abundance of natural gas in our planet, a major goal is to achieve a direct methane-to-methanol conversion at medium to low temperatures using mixtures of methane and oxygen. Here, we report an efficient catalyst, ZnO/ Cu 2 O/Cu(111), for this process investigated using a combination of reactor testing, scanning tunneling microscopy, ambientpressure X-ray photoemission spectroscopy, density functional calculations, and kinetic Monte Carlo simulations. The catalyst is capable of methane activation at room temperature and transforms mixtures of methane and oxygen to methanol at 450 K with a selectivity of ∼30%. This performance is not seen for other heterogeneous catalysts which usually require the addition of water to enable a significant conversion of methane to methanol. The unique coarse structure of the ZnO islands supported on a Cu 2 O/ Cu(111) substrate provides a collection of multiple centers that display different catalytic activity during the reaction. ZnO−Cu 2 O step sites are active centers for methanol synthesis when exposed to CH 4 and O 2 due to an effective O−O bond dissociation, which enables a methane-to-methanol conversion with a reasonable selectivity. Upon addition of water, the defected O-rich ZnO sites, introduced by Zn vacancies, show superior behavior toward methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH 4 → CH 3 OH conversion are different from those engaged in methanol formation without water. The identification of the site-dependent behavior of ZnO/Cu 2 O/Cu(111) opens a design strategy for guiding efficient methane reformation with high methanol selectivity.

A catalyst with a ZnO/Cu configuration plays an important role in the synthesis of methanol from CO 2 hydrogenation. In this study, scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the growth mode of small coverages of ZnO x , θ oxi < 0.3 monolayer, on a Cu(111) substrate. Our results show that the modes of growth, size, and shape of the ZnO nanoparticles are strongly dependent on the Zn deposition temperature. In a set of experiments, Zn was deposited on Cu(111) or CuO x / Cu(111) surfaces at 300 K with subsequent exposure to O 2 at higher temperatures (400−550 K), which exhibited small particles of ZnO (<20 nm in size) on the surface. The deposition of Zn onto CuO x /Cu(111) at elevated temperatures (450−600 K) in an oxygen ambient produced large ZnO islands (300−650 nm in size), which were very rough and spread over several terraces of Cu(111). XPS/Auger spectra showed that all of the preparation conditions stated above led to the formation ZnO/CuO x /Cu(111) surfaces where the oxidation state of zinc was uniform. Catalytic tests showed that all these surfaces were active for the hydrogenation of CO 2 to methanol, but only the systems prepared at 600 K displayed long-term stability under reaction conditions.

The addition of potassium atoms to Cu(111) and Cu/TiO 2 (110) surfaces substantially enhances the rate for water dissociation and the production of hydrogen through the water−gas shift reaction (WGS, CO + H 2 O → H 2 + CO 2 ). In the range of temperatures investigated, 550−625 K, Cu/K/TiO 2 (110) exhibits a WGS activity substantially higher than those of K/Cu(111), Cu(111), and Cu/ZnO(0001̅ ) systems used to model an industrial Cu/ZnO catalyst. The apparent activation energy for the WGS drops from 18 Kcal/mol on Cu(111) to 12 Kcal/mol on K/Cu(111) and 6 Kcal/mol on Cu/K/ TiO 2 (110). The results of density functional calculations show that K adatoms favor the thermochemistry for water dissociation on Cu(111) and Cu/TiO 2 (110) with the cleavage of an O−H bond occurring at room temperature. Furthermore, at the Cu/K/ TiO 2 interface, there is a synergy, and this system has a unique ability to dissociate the water molecule and catalyze hydrogen production through the WGS process. Therefore, when optimizing a regular catalyst, it is essential to consider mainly the effects of an alkali promoter on the metal−oxide interface.

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The dry reforming of methane was systematically studied over a series (2-30 wt%) of Co (~5nm in size) loaded CeO2 catalysts, with an effort to elucidate the behavior of Co and ceria in the catalytic process using in-situ methods. For the systems under study, the reaction activity scaled with increasing Co loading, and a 10 wt% Co-CeO2 catalyst exhibiting the best catalytic activity and good stability at 500 °C with little evidence for carbon accumulation. The phase transitions and the nature of active components in the catalyst were investigated during pretreatment and under reaction conditions by ex-situ/in-situ techniques including X-ray diffraction (XRD) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). These studies showed a dynamical evolution in the chemical composition of the catalysts under reaction conditions. A clear transition of Co3O4 → CoO → Co, and Ce 4+ to Ce 3+ , was observed during the temperature programmed reduction under H2 and CH4. However, introduction of CO2, led to partial re-oxidation of all components at low temperatures, followed by reduction at high temperatures. Under optimum CO and H2 producing conditions both XRD and AP-XPS indicated that the active phase involved a majority of metallic Co with a small amount of CoO both supported on a partially reduced ceria (Ce 3+ /Ce 4+). We identified the importance of dispersing Co, anchoring it onto ceria surface sites, and then utilizing the redox properties of ceria for activating and then oxidatively converting methane while inhibiting coke formation. Furthermore, a synergistic effect between cobalt and ceria and the interfacial site are essential to successfully close the catalytic cycle.

To activate methane at low or medium temperatures is a difficult task and a pre-requisite for the conversion of this light alkane into high value chemicals. Herein, we report the...