<i>In Situ</i> Studies of Carbon Monoxide Oxidation on Platinum and Platinum–Rhenium Alloy Surfaces

CO oxidation has been investigated by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on Pt(111), Re films on Pt(111), and a Pt–Re alloy surface. The Pt–Re alloy surface was prepared by annealing Re films on Pt(111) to 1000 K; scanning tunneling microscopy, low energy ion scattering, and X-ray photoelectron spectroscopy studies indicate that this treatment resulted in the diffusion of Re into the Pt(111) surface. Under CO oxidation conditions of 500 mTorr O₂/50 mTorr CO, CO remains on the Pt(111) surface at 450 K, whereas CO desorbs from the Pt–Re alloy surface at lower temperatures. Furthermore, the Pt–Re alloy dissociates oxygen more readily than Pt(111) despite the fact that all of the Re atoms are initially in the subsurface region. Mass spectrometer studies show that the Pt–Re alloy, Re film on Pt, and Pt(111) all have similar activities for CO oxidation, with the Pt–Re alloy producing ∼10% more CO₂ than Pt(111). The Re film is not stable under CO oxidation conditions at temperatures ≥450 K due to the formation and subsequent sublimation of volatile Re₂O₇. However, the Pt–Re alloy surface is more resistant to oxidation and therefore also more stable against Re sublimation

Interactions of Hydrogen, CO, Oxygen, and Water with Molybdenum-Modified Pt(111)

Modification of Pt group catalysts by molybdenum is known to improve catalyst performance in a number of important chemical reactions. To investigate fundamental mechanisms responsible for the promoting effect of Mo, temperature-programmed desorption (TPD) and low energy electron diffraction (LEED) experiments were performed to examine the adsorption of O₂, D₂, CO, and water on Pt(111) modified with submonolayer quantities of molybdenum. Auger electron spectroscopy (AES) was used to detect and quantify the Mo coverage and X-ray photoelectron spectroscopy (XPS) was employed in conjunction with density functional theory (DFT) calculations to identify Mo species present following various surface treatments. The state of Mo on the surface was found to vary depending on prior surface treatment. Treatment with oxygen resulted in a surface molybdenum oxide, whereas treatment with hydrogen resulted in a reduced bimetallic surface. XPS results indicate that high pressures of oxygen create a higher valent oxide than what is created under ultrahigh vacuum. Oxidized Mo appeared to block Pt surface sites without significantly altering the behavior of species adsorbed on Pt. Reduced surfaces, on the other hand, were shown to decrease yield and desorption temperature for both D₂ and CO. Isotopic TPD studies provided evidence of water dissociation on the reduced Mo modified surface, with a maximum extent of water dissociation occurring at intermediate Mo coverages

Nucleation, Growth, and Adsorbate-Induced Changes in Composition for Co–Au Bimetallic Clusters on TiO₂

The nucleation, growth, and CO-induced changes in composition for Co–Au bimetallic clusters deposited on TiO₂(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. STM experiments show that the mobility of Co atoms on TiO₂(110) is significantly lower than of Au atoms; for equivalent or lower coverages of Co, the number of clusters is higher and the average cluster height is smaller than for Au deposition. Consequently, bimetallic clusters are formed by first depositing the less mobile Co atoms, followed by the addition of the more mobile Au atoms. Furthermore, the reverse deposition of Au followed by Co results in clusters of pure Co coexisting with clusters that are Au-rich. For clusters with a total coverage of 0.25 ML, the cluster density increases and average cluster height decreases as the fraction of Co is increased. Annealing to 800 K results in cluster sintering and an increase of ∼3–5 Å in average height for all compositions. LEIS experiments indicate that the surfaces of the bimetallic clusters are 80–100% Au for bulk Au fractions greater than 50%, but Co and Au coexist at the surfaces when there are not enough Au atoms available to completely cover the surfaces of the clusters. After heating to 800 K, pure Co clusters become partially encapsulated by titania, and for bimetallic clusters, the Co is selectively encapsulated at the cluster surface. The desorption of CO from the bimetallic clusters demonstrates that the presence of the CO adsorbate induces diffusion of Co to the cluster surface, but the extent of this diffusion is less than what is observed in the Ni–Au and Pt–Au systems. Density functional theory calculations confirm that for a 50% Co/50% Au bimetallic structure: the surface is predominantly Au in the absence of CO; CO induces diffusion of Co to the cluster surface; and this CO-induced diffusion is less extensive on Co–Au than on the Ni–Au and Pt–Au surfaces

<i>In Situ</i> Ambient Pressure X‑ray Photoelectron Spectroscopy Studies of Methanol Oxidation on Pt(111) and Pt–Re Alloys

For methanol oxidation reactions, Pt–Re alloy surfaces are found to have better selectivity for CO₂ production and less accumulation of surface carbon compared to pure Pt surfaces. The unique activity of the Pt–Re surface is attributed to the increased ability of Re to dissociate oxygen compared to Pt and the ability of Re to diffuse gradually to the surface under reaction conditions. In this work, the oxidation of methanol was studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and mass spectrometry on Pt(111), a Pt–Re surface alloy, and a Re film on Pt(111) as well as Pt(111) and Pt–Re alloy surfaces that were preoxidized before reaction. Methanol oxidation conditions consisted of 200 mTorr of O₂/100 mTorr of methanol at temperatures ranging from 300 to 550 K. The activities of all of the surfaces studied are similar in that CO₂ and H₂O are the main oxidation products, along with formaldehyde, which is produced below 450 K. For reaction on Pt(111), there is a change in selectivity that favors CO and H₂ over CO₂ at 500 K and above. This shift in selectivity is not as pronounced on the Pt–Re alloy surface and is completely absent on the oxidized Pt–Re alloy surfaces and oxidized Re film. AP-XPS results demonstrate that Pt(111) is more susceptible to poisoning by carbonaceous surface species than any of the Re-containing surfaces. Oxygen-induced diffusion of Re to the surface is believed to occur at elevated temperatures under reaction conditions, based on the increase in the Re/Pt ratio upon heating; density function theory (DFT) calculations confirm that there is a thermodynamic driving force for Re atoms to diffuse to the surface in the presence of oxygen. Furthermore, Re diffuses to the surface when the Pt–Re alloy is exposed to O₂ at 450 K before methanol oxidation, and consequently this surface has the highest CO₂ production at temperatures below that required for Re diffusion during methanol reaction. Although the oxidized Re film also exhibits high selectivity for CO₂ production and minimal carbon deposition, this surface is unstable due to the sublimation of Re₂O₇; in contrast, the Pt–Re alloy is more resistant to Re sublimation since the majority of Re resides in the subsurface region

Oxygen-Promoted Methane Activation on Copper

The role of oxygen in the activation of C–H bonds in methane on clean and oxygen-precovered Cu(111) and Cu₂O­(111) surfaces was studied with combined in situ near-ambient-pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy. Activation of methane at 300 K and “moderate pressures” was only observed on oxygen-precovered Cu(111) surfaces. Density functional theory calculations reveal that the lowest activation energy barrier of C–H on Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site, four-centered mechanism, which stabilizes the required transition-state intermediate by dipole–dipole attraction of O–H and Cu–CH₃ species. The C–H bond activation barriers on Cu₂O­(111) surfaces are large due to the weak stabilization of H and CH₃ fragments

Key Structure–Property Relationships in CO₂ Capture by Supported Alkanolamines

Heterogeneous interfaces exhibit remarkable material properties resulting from their structural motifs, the judicious placement of functional chemical groups, etc. It has been a long-standing challenge to manipulate and design interface structures at the atomic level to achieve new functionalities. Here, we demonstrate that by modifying the length of the backbone in alkanolamines one can control the packing density of organic monolayers adsorbed on rutile TiO₂ and the interaction strength between their amine functional group and the substrate. As a result, we observed strikingly different activities in CO₂ capture by the amine functional group of different alkanolamines on TiO₂(110). Synchrotron photoelectron spectroscopy at near-ambient CO₂ pressures showed that adsorbed 2-amino-1-ethanol (monoethanolamine, MEA) is inactive, whereas the amine group in 3-amino-1-propanol (3AP)/TiO₂(110) readily reacts with and captures CO₂. Our results suggest that the geometry of the interface plays a decisive role in the reactivity of adsorbed functionalized organic molecules, such as solid-supported alkanolamines for CO₂ capture