Structure and Properties of Ultrathin SiOx Films on Cu(111)

The metal-oxide interface plays a crucial role in catalysis, and it has attracted increasing interest in recent years. Cu/SiO₂, as a common copper-based catalyst, has been widely used in industrial catalysis. However, it is still a challenge to clarify the structures of the interface of Cu-SiO_x and the effect on catalytic performance. Herein, we prepared ultrathin SiO_x films by evaporating Si onto a Cu(111) surface followed by annealing in an O₂ atmosphere, which were characterized by various surface science techniques. The results showed that a SiO_x film could grow nearly layer-by-layer on the Cu(111) surface in the present condition. Both X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) results confirmed the presence of Cu-O-Si and Si-O-Si species. Thermal stability experiments illustrated that the well-ordered silica films were stable under annealing in vacuum. The feature of CO adsorption suggested a CO-Cu^δ+ species induced from the Cu^δ+-O-Si. Low-energy ion scattering spectroscopy (LEIS) and XPS results demonstrated that some Cu₂O appeared on the surface when the 1 ML SiO_x/Cu(111) was exposed in O₂ at 353 K.

Formation of a Ti-Cu(111) single atom alloy: Structure and CO binding

A single atom Ti-Cu(111) surface alloy can be generated by depositing small amounts of Ti onto Cu(111) at slightly elevated surface temperatures (∼500 to 600 K). Scanning tunneling microscopy shows that small Ti-rich islands covered by a Cu single layer form preferentially on ascending step edges of Cu(111) during Ti deposition below about 400 K but that a Ti-Cu(111) alloy replaces these small islands during deposition between 500 and 600 K, producing an alloy in the brims of the steps. Larger partially Cu-covered Ti-containing islands also form on the Cu(111) terraces at temperatures between 300 and 700 K. After surface exposure to CO at low temperatures, reflection absorption infrared spectroscopy (RAIRS) reveals distinct C-O stretch bands at 2102 and 2050 cm^-1 attributed to CO adsorbed on Cu-covered Ti-containing domains vs sites in the Ti-Cu(111) surface alloy. Calculations using density functional theory (DFT) suggest that the lower frequency C-O stretch band originates specifically from CO adsorbed on isolated Ti atoms in the Ti-Cu(111) surface alloy and predicts a higher C-O stretch frequency for CO adsorbed on Cu above subsurface Ti ensembles. DFT further predicts that CO preferentially adsorbs in flat-lying configurations on contiguous Ti surface structures with more than one Ti atom and thus that CO adsorbed on such structures should not be observed with RAIRS. The ability to generate a single atom Ti-Cu(111) alloy will provide future opportunities to investigate the surface chemistry promoted by a representative early transition metal dopant on a Cu(111) host surface.

Tuning the properties of copper-based catalysts based on molecular in situ studies of model systems

Studying catalytic processes at the molecular level is extremely challenging, due to the structural and chemical complexity of the materials used as catalysts and the presence of reactants and products in the reactor's environment. The most common materials used on catalysts are transition metals and their oxides. The importance of multifunctional active sites at metal/oxide interfaces has been long recognized, but a molecular picture of them based on experimental observations is only recently emerging. The initial approach to interrogate the surface chemistry of catalysts at the molecular level consisted of studying metal single crystals as models for reactive metal centers, moving later to single crystal or well-defined thin film oxides. The natural next iteration consisted in the deposition of metal nanoparticles on well-defined oxide substrates. Metal nanoparticles contain undercoordinated sites, which are more reactive. It is also possible to create architectures where oxide nanoparticles are deposited on top of metal single crystals, denominated inverse catalysts, leading in this case to a high concentration of reactive cationic sites in direct contact with the underlying fully coordinated metal atoms. Using a second oxide as a support (host), a multifunctional configuration can be built in which both metal and oxide nanoparticles are located in close proximity. Our recent studies on copper-based catalysts are presented here as an example of the application of these complementary model systems, starting from the creation of undercoordinated sites on Cu(111) and Cu2O(111) surfaces, continuing with the formation of mixed-metal copper oxides, the synthesis of ceria nanoparticles on Cu(111) and the codeposition of Cu and ceria nanoparticles on TiO2(110). Catalysts have traditionally been characterized before or after reactions and analyzed based on static representations of surface structures. It is shown here how dynamic changes on a catalyst's chemical state and morphology can be followed during a reaction by a combination of in situ microscopy and spectroscopy. In addition to determining the active phase of a catalyst by in situ methods, the presence of weakly adsorbed surface species or intermediates generated only in the presence of reactants can be detected, allowing in turn the comparison of experimental results with first principle modeling of specific reaction mechanisms. Three reactions are used to exemplify the approach: CO oxidation (CO + 1/2O2 → CO2), water gas shift reaction (WGSR) (CO + H2O → CO2 + H2), and methanol synthesis (CO2 + 3H2 → CH3OH + H2O). During CO oxidation, the full conversion of Cu(0) to Cu(2+) deactivates an initially outstanding catalyst. This can be remedied by the formation of a TiCuOx mixed-oxide that protects the presence of active partially oxidized Cu(+) cations. It is also shown that for the WGSR a switch occurs in the reaction mechanism, going from a redox process on Cu(111) to a more efficient associative pathway at the interface of ceria nanoparticles deposited on Cu(111). Similarly, the activation of CO2 at the ceria/Cu(111) interface allows its facile hydrogenation to methanol. Our combined studies emphasize the need of searching for optimal metal/oxide interfaces, where multifunctional sites can lead to new efficient catalytic reaction pathways.