The active sites over commercial copper/zinc oxide/aluminum oxide (Cu/ZnO/AlO) catalysts for carbon dioxide (CO) hydrogenation to methanol, the Zn-Cu bimetallic sites or ZnO-Cu interfacial sites, have recently been the subject of intense debate. We report a direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis. By combining x-ray photoemission spectroscopy, density functional theory, and kinetic Monte Carlo simulations, we can identify and characterize the reactivity of each catalyst. Both experimental and theoretical results agree that ZnCu undergoes surface oxidation under the reaction conditions so that surface Zn transforms into ZnO and allows ZnCu to reach the activity of ZnO/Cu with the same Zn coverage. Our results highlight a synergy of Cu and ZnO at the interface that facilitates methanol synthesis via formate intermediates.

The electronic properties of Pt nanoparticles deposited on CeO(2)(111) and CeO(x)/TiO(2)(110) model catalysts have been examined using valence photoemission experiments and density functional theory (DFT) calculations. The valence photoemission and DFT results point to a new type of "strong metal-support interaction" that produces large electronic perturbations for small Pt particles in contact with ceria and significantly enhances the ability of the admetal to dissociate the O-H bonds in water. When going from Pt(111) to Pt(8)/CeO(2)(111), the dissociation of water becomes a very exothermic process. The ceria-supported Pt(8) appears as a fluxional system that can change geometry and charge distribution to accommodate adsorbates better. In comparison with other water-gas shift (WGS) catalysts [Cu(111), Pt(111), Cu/CeO(2)(111), and Au/CeO(2)(111)], the Pt/CeO(2)(111) surface has the unique property that the admetal is able to dissociate water in an efficient way. Furthermore, for the codeposition of Pt and CeO(x) nanoparticles on TiO(2)(110), we have found a transfer of O from the ceria to Pt that opens new paths for the WGS process and makes the mixed-metal oxide an extremely active catalyst for the production of hydrogen.

The adsorption and activation of a CO 2 molecule on cubic d-MoC(001) and orthorhombic b-Mo 2 C(001) surfaces have been investigated by means of periodic density functional theory based calculations using the Perdew-Burke-Ernzerhof exchange-correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme. The DFT results indicate that an orthorhombic b-Mo 2 C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO 2 and carbon monoxide formation, whereas on a b-Mo 2 C(001) C-terminated polar surface or on a d-MoC (001) nonpolar surface the CO 2 molecule is activated yet the C-O bond prevails. Experimental tests showed that Mo-terminated b-Mo 2 C(001) easily adsorbs and decomposes the CO 2 molecule. This surface is an active catalyst for the hydrogenation of CO 2 to methanol and methane. Although MoC does not dissociate C-O bonds on its own, it binds CO 2 better than transition metal surfaces and is an active and selective catalyst for the CO 2 + 3H 2 -CH 3 OH + H 2 O reaction. Our theoretical and experimental results illustrate the tremendous impact that the carbon/metal ratio has on the chemical and catalytic properties of molybdenum carbides. This ratio must be taken into consideration when designing catalysts for the activation and conversion of CO 2 .

The results of core-level photoemission indicate that Ni-CeO 2 (111) surfaces with small or medium coverages of nickel are able to activate methane at 300 K producing adsorbed CH x and CO x (x = 2,3) groups. Calculations based on density-functional 1 theory predict relatively low activation energy of 0.6−0.7 eV for the cleavage of the first C−H bond in the adsorbed methane molecule. Ni and O centers of ceria work in a cooperative way in the dissociation of the C−H bond at room temperature where a low Ni loading is crucial for the catalyst activity and stability. The strong electronic perturbations in the Ni nanoparticles produced by the ceria support of varying nature such as stoichiometric and reduced, result in a drastic change in their chemical properties towards methane adsorption and dissociation as well as the DRM reaction. The coverage of Ni had a drastic effect on the ability of the system to dissociate methane and catalyze the dry reforming process.

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH → CHOH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.

The role of the interface between a metal and oxide (CeO x −Cu and ZnO−Cu) is critical to the production of methanol through the hydrogenation of CO 2 (CO 2 + 3H 2 → CH 3 OH + H 2 O). The deposition of nanoparticles of CeO x or ZnO on Cu(111), θ oxi < 0.3 monolayer, produces highly active catalysts for methanol synthesis. The catalytic activity of these systems increases in the sequence: Cu(111) < ZnO/Cu(111) < CeO x /Cu(111). The apparent activation energy for the CO 2 → CH 3 OH conversion decreases from 25 kcal/mol on Cu(111) to 16 kcal/mol on ZnO/Cu(111) and 13 kcal/mol on CeO x /Cu(111). The surface chemistry of the highly active CeO x −Cu(111) interface was investigated using ambient pressure X-ray photoemission spectroscopy (AP-XPS) and infrared reflection absorption spectroscopy (AP-IRRAS). Both techniques point to the formation of formates (HCOO −) and carboxylates (CO 2 δ−) during the reaction. Our results show an active state of the catalyst rich in Ce 3+ sites which stabilize a CO 2 δ− species that is an essential intermediate for the production of methanol. The inverse oxide/metal configuration favors strong metal−oxide interactions and makes possible reaction channels not seen in conventional metal/oxide catalysts.

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.

Studies with a series of metal/ceria(111) (metal=Co, Ni, Cu; ceria=CeO ) surfaces indicate that metal-oxide interactions can play a very important role for the activation of methane and its reforming with CO at relatively low temperatures (600-700 K). Among the systems examined, Co/CeO (111) exhibits the best performance and Cu/CeO (111) has negligible activity. Experiments using ambient pressure X-ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO (111) at temperatures as low as 300 K-generating CH and CO species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co /CeO (111), and to only 0.05 eV on Co /CeO (111). At 700 K, under methane dry reforming conditions, CO dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CH formed on the Co /CeO (111) catalyst recombines to yield ethane or ethylene.