The (111) surface of copper (Cu), its most compact and lowest energy surface, became unstable when exposed to carbon monoxide (CO) gas. Scanning tunneling microscopy revealed that at room temperature in the pressure range 0.1 to 100 Torr, the surface decomposed into clusters decorated by CO molecules attached to edge atoms. Between 0.2 and a few Torr CO, the clusters became mobile in the scale of minutes. Density functional theory showed that the energy gain from CO binding to low-coordinated Cu atoms and the weakening of binding of Cu to neighboring atoms help drive this process. Particularly for softer metals, the optimal balance of these two effects occurs near reaction conditions. Cluster formation activated the surface for water dissociation, an important step in the water-gas shift reaction.

The chemical structure of a Cu(111) model catalyst during the CO oxidation reaction in the CO+O2 pressure range of 10-300 mTorr at 298-413 K was studied in situ using surface sensitive X-ray photoelectron and adsorption spectroscopy techniques [X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS)]. For O2:CO partial pressure ratios below 1:3, the surface is covered by chemisorbed O and by a thin (∼1 nm) Cu2O layer, which covers completely the surface for ratios above 1:3 between 333 and 413 K. The Cu2O film increases in thickness and exceeds the escape depth (∼3-4 nm) of the XPS and NEXAFS photoelectrons used for analysis at 413 K. No CuO formation was detected under the reaction conditions used in this work. The main reaction intermediate was found to be CO2(δ-), with a coverage that correlates with the amount of Cu2O, suggesting that this phase is the most active for CO oxidation.

Ambient-pressure X-ray photoelectron spectroscopy (APXPS) and high-pressure scanning tunneling microscopy (HPSTM) were used to study the structure and chemistry of model Cu(100) and Cu(111) catalyst surfaces in the adsorption and dissociation of CO2. It was found that the (100) face is more active in dissociating CO2 than the (111) face. Atomic oxygen formed after the dissociation of CO2 poisons the surface by blocking further adsorption of CO2. This "self-poisoning" mechanism explains the need to mix CO into the industrial feed for methanol production from CO2, as it scavenges the chemisorbed O. The HPSTM images show that the (100) surface breaks up into nanoclusters in the presence of CO2 at 20 Torr and above, producing active kink and step sites. If the surface is precovered with atomic oxygen, no such nanoclustering occurs.

Bimetallic and multi-component catalysts often exhibit superior activity and selectivity compared with their single-component counterparts. To investigate the origin of the composition dependence observed in the catalytic activities of CoPd bimetallic catalysts, the compositional and structural evolution of monodisperse CoPd alloy nanoparticles (NPs) were followed under catalytic CO oxidation conditions using ambient pressure X-ray spectroscopy (AP-XPS) and transmission electron microscopy (TEM). It was found that the catalysis process induced a reconstruction of the catalysts, leaving CoO x on the NP surface. The synergy between Pd and CoO x coexisting on the surface promotes the catalytic activity of the bimetallic catalysts. Such synergistic effects can be optimized by tuning the Co/Pd ratios in the NP synthesis and reach a maximum at compositions near Co 0.26 Pd 0.74 , which exhibits the lowest temperature for complete CO conversion. Our combined AP-XPS and TEM studies provide a direct observation of the bimetallic NPs surface evolution under catalytic conditions and its correlation to catalytic properties. Recent advances in in situ/operando surface characterization 28-32 have made it possible to study catalyst surfaces and their interaction with gas-phase reactants under reaction conditions. One example is the development of ambient-pressure x-ray photoelectron spectroscopy (AP-XPS), which makes this traditionally vacuum-requiring surface analysis tool operational under reactant gases up to Torr pressures. 28,33 Herein, we combined an in situ AP-XPS investigation of monodisperse CoPd NPs under CO oxidation conditions with in situ and ex situ (scanning) transmission electron microscopy [(S)TEM] and electron energy loss spectroscopy (EELS), as well as ex situ x-ray absorption spectroscopy. This integrated study reveals the surface/structure evolution and bimetallic synergy of NP catalysts in action. We observed that the atomic surface composition of the CoPd alloy NPs transformed during the oxidation/reduction pre-treatments. At 200 and 300°C, exposure to CO drives Pd atoms to migrate to the surface, whereas O 2 exposure does the opposite. Such reversible reactant-driven surface segregation however, becomes less prominent with increasing Co content and is eventually negligible in the case of Co 0.52 Pd 0.48 NPs, where the NPs with highest Co content the surface becomes completely covered by CoO x as corroborated by STEM-EELS mapping. The observed segregation behavior in CoPd NPs suggested that Pd and CoO x coexist on the catalyst surface both exposed to the reactant gases, at least for NPs Co content below 50%. The Pd/CoO x coexistence contributes to the promotion of the CO oxidation kinetics. This mechanism explains the trend of the catalytic properties of five NP catalysts of different compositions from pure Pd to Co 0.52 Pd 0.49 , among which the Co 0.26 Pd 0.74 shows the lowest temperature for complete conversion of CO to CO 2. This work highlights the benefits of using well

Atmospheric pressure X-ray photoelectron spectroscopy (XPS) is demonstrated using single-layer graphene membranes as photoelectron-transparent barriers that sustain pressure differences in excess of 6 orders of magnitude. The graphene serves as a support for catalyst nanoparticles under atmospheric pressure reaction conditions (up to 1.5 bar), where XPS allows the oxidation state of Cu nanoparticles and gas phase species to be simultaneously probed. We thereby observe that the Cu(2+) oxidation state is stable in O2 (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO2, N2, O2) in the pressure range 10-1500 mbar including species with low photoionization cross sections (He, H2). Pressure-dependent changes in the apparent binding energies of gas-phase species are observed, attributable to changes in work function of the metal-coated grids supporting the graphene. We expect atmospheric pressure XPS based on this graphene membrane approach to be a valuable tool for studying nanoparticle catalysis.

Ambient-pressure XPS was used to investigate the reactions of CO, H 2 , and their mixtures on Co foils. We found that CO adsorbs molecularly on the clean Co surface and desorbs intact in vacuum with increasing rate until ~90°C where all CO desorbs in seconds. In equilibrium with 100 mTorr gas, CO dissociates above 120°C, leaving carbide species on the surface but no oxides, because CO efficiently reduces the oxides at temperatures ~100°C lower than H 2 . Water as impurities or produced by reaction of CO and H 2 efficiently oxidizes Co even at room temperature. Under 97:3 CO/H 2 mixture and with increasing temperatures, the Co surface becomes more oxidized and covered by hydroxyl groups until ~150°C where surface starts to get reduced, accompanied by carbide accumulation indicative of CO dissociation. Similar trend was observed for 9:1 and 1:1 mixtures but surface reduction begins at higher temperatures.

The interaction of O2 with the Ag(111) surface was studied with scanning tunneling microscopy (STM) in the pressure range from 10 -9 Torr to 1 atm at room temperature and with X-ray photoelectron spectroscopy (XPS) up to 0.3 Torr O2 in the temperature range from RT to 413 K. STM images show that the Ag(111) surface topography is little affected in regions with large flat terraces, except for the appearance of mobile features due to oxygen atoms at pressures above 0.01 Torr. In regions where the step density is high the surface became rough under 0.01 Torr of O2, due to the local oxidation of Ag. Various chemical states of oxygen due to chemisorbed, oxide and subsurface species were identified by XPS as a function of pressure and temperature. The findings from the STM images and XPS measurements indicate that formation of an oxide phase, the thermodynamically stable form at room temperature under ambient O2 pressure, is kinetically hindered in the flat terrace areas but proceeds readily in regions with high step density.

The reaction of CO with chemisorbed oxygen on three low-index faces of copper was studied using ambient pressure X-ray photoelectron spectroscopy (XPS) and high-pressure scanning tunneling microscopy. At room temperature, the chemisorbed oxide can be removed by reaction with gas-phase CO in the 0.01−0.20 Torr pressure range. The reaction rates were determined by measuring the XPS peak intensities of O and CO as a function of time, pressure, and temperature. On Cu(111) the rate was found to be one order of magnitude faster than that on Cu(100) and two orders of magnitude faster than that on Cu(110). The apparent activation energies for CO oxidation were measured as 0.24 eV for O/Cu(111), 0.29 eV for O/ Cu(100), and 0.51 eV for O/Cu(110) in the temperature range between 298 and 473 K. These energies are correlated to the oxygen binding energies on each surface.