The transformation of CO2 into alcohols or other hydrocarbon compounds is challenging because of the difficulties associated with the chemical activation of CO2 by heterogeneous catalysts. Pure metals and bimetallic systems used for this task usually have low catalytic activity. Here we present experimental and theoretical evidence for a completely different type of site for CO2 activation: a copper-ceria interface that is highly efficient for the synthesis of methanol. The combination of metal and oxide sites in the copper-ceria interface affords complementary chemical properties that lead to special reaction pathways for the CO2→CH3OH conversion.

The high performance of Au-CeO2 and Au-TiO2 catalysts in the water-gas shift (WGS) reaction (H2O + CO-->H2 + CO2) relies heavily on the direct participation of the oxide in the catalytic process. Although clean Au(111) is not catalytically active for the WGS, gold surfaces that are 20 to 30% covered by ceria or titania nanoparticles have activities comparable to those of good WGS catalysts such as Cu(111) or Cu(100). In TiO(2-x)/Au(111) and CeO(2-x)/Au(111), water dissociates on O vacancies of the oxide nanoparticles, CO adsorbs on Au sites located nearby, and subsequent reaction steps take place at the metal-oxide interface. In these inverse catalysts, the moderate chemical activity of bulk gold is coupled to that of a more reactive oxide.

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.

Currently, the primary source of hydrogen is steam reforming from hydrocarbons. The reformed fuel contains 1-10 % CO.[1, 2] The water gas shift reaction (WGS, CO + H 2 O! H 2 + CO 2 ) and preferential CO oxidation (2 CO + O 2 ! 2 CO 2 ) processes are critical in providing clean hydrogen for polymer electrolyte membrane fuel cells and other industrial applications.[1-3] Conventional WGS catalysts (e.g., Cu on zinc oxide) meet the needs of large stationary applications. [4] Improved air-tolerant, cost-effective WGS catalysts for lower-temperature processing are needed to enable mobile fuel-cell applications. [1][2][3]5] Recently, Au/CeO 2 and Cu/CeO 2 materials were reported to be very promising catalysts for the lower-temperature WGS reaction. [6,7] There is no generally accepted picture for the role of ceria and the metal in the WGS reaction. The existence and role of reduced metal nanoparticles versus cationic metal centers (e.g., Au d+ ) are debated. [5][6][7] Very recent in situ measurements by near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy show that Cu d+ and Au d+ species are not stable under the typical conditions of the WGS reaction.