Final State Distributions of the Radical Photoproducts from the UV Photooxidation of 2‑Butanone on TiO₂(110)

The UV photooxidation of 2-butanone on TiO₂(110) was studied using pump–probe laser methods and time-of-flight (TOF) mass spectrometry to identify the gas-phase photoproducts and probe the dynamics of the photofragmentation process. A unique aspect of this work is the use of coherent VUV radiation for single-photon ionization detection of gas-phase products, which significantly reduces the amount of parent ion fragmentation as compared to electron impact used in previous studies. The pump–probe product mass spectra showed ions at mass 15 (CH₃⁺) and mass 29 (C₂H₅⁺), which are associated with the primary α-carbon bond cleavage of the adsorbed butanone–oxygen complex, as well other C₂H_<i>x</i>⁺ (<i>x</i> = 2–4) fragments, which could originate from ethyl radical secondary surface chemistry or dissociative ionization. Using two different VUV probe energies, it was possible to show that the fragment ions at mass 27 (C₂H₃⁺) and mass 28 (C₂H₃⁺) are not due to secondary reactions of ethyl radicals on the surface, but rather from dissociative ionization of the ethyl radical parent ion (mass 29). Another photoproduct at mass 26 (C₂H₂⁺) peak is also observed, but its pump–probe delay dependence indicates that it is not associated with nascent ethyl radicals. Pump-delayed-probe measurements were also used to obtain translational energy distributions for the methyl and ethyl radical products, both which can be empirically fit to “fast” and “slow” components. The ethyl radical energy distribution is dominated by the “slow” channel, whereas the methyl radical has a much larger contribution from “fast” fragments. The assignment of the C₂H_<i>x</i> (<i>x</i> = 3, 4) fragments to ethyl (C₂H₅) dissociative ionization was also confirmed by showing that all three products have the same translational energy distributions. The origin of the “fast” and “slow” fragmentation channels for both methyl and ethyl ejection is discussed in terms of analogous neutral and ionic fragmentation processes in the gas phase. Finally, we consider the possible energetic and dynamical origins of the higher yield of ethyl radical products as compared to that for methyl radicals

Electronic Interactions of Size-Selected Oxide Clusters on Metallic and Thin Film Oxide Supports

The interfacial electronic structure of various size-selected metal oxide nanoclusters (M₃O_<i>x</i>; M = Mo, Nb, Ti) on Cu(111) and a thin film of Cu₂O supports were investigated by a combination of experimental methods and density functional theory (DFT). These systems explore electron transfer at the metal–metal oxide interface which can modify surface structure, metal oxidation states, and catalytic activity. Electron transfer was probed by measurements of surface dipoles derived from coverage dependent work function measurements using two-photon photoemission (2PPE) and metal core level binding energy spectra from X-ray photoelectron spectroscopy (XPS). The measured surface dipoles are negative for all clusters on Cu(111) and Cu₂O/Cu­(111), but those on the Cu₂O surface are much larger in magnitude. In addition, sub-stoichiometric or “reduced” clusters exhibit smaller surface dipoles on both the Cu(111) and Cu₂O surfaces. Negative surface dipoles for clusters on Cu(111) suggest Cu → cluster electron transfer, which is generally supported by DFT-calculated Bader charge distributions. For Cu₂O/Cu­(111), calculations of the surface electrostatic potentials show that the charge distributions associated with cluster adsorption structures or distortions at the cluster–Cu₂O–Cu­(111) interface are largely responsible for the observed negative surface dipoles. Changes observed in the XPS spectra for the Mo 3d, Nb 3d, and Ti 2p core levels of the clusters on Cu(111) and Cu₂O/Cu­(111) are interpreted with help from the calculated Bader charges and cluster adsorption structures, the latter providing information about the presence of inequivalent cation sites. The results presented in this work illustrate how the combined use of different experimental probes along with theoretical calculations can result in a more realistic picture of cluster–support interactions and bonding

Influence of Cluster–Support Interactions on Reactivity of Size-Selected Nb_<i>x</i>O_<i>y</i> Clusters

Size-selected niobium oxide nanoclusters (Nb₃O₅, Nb₃O₇, Nb₄O₇, and Nb₄O₁₀) were deposited at room temperature onto a Cu(111) surface and a thin film of Cu₂O on Cu(111), and their interfacial electronic interactions and reactivity toward water dissociation were examined. These clusters were specifically chosen to elucidate the effects of the oxidation state of the metal centers; Nb₃O₅ and Nb₄O₇ are the reduced counterparts of Nb₃O₇ and Nb₄O₁₀, respectively. From two-photon photoemission spectroscopy (2PPE) measurements, we found that the work function increases upon cluster adsorption in all cases, indicating a negative interfacial dipole moment with the positive end pointing into the surface. The amount of increase was greater for the clusters with more metal centers and higher oxidation state. Further analysis with DFT calculations of the clusters on Cu(111) indicated that the reduced clusters donate electrons to the substrate, indicating that the intrinsic cluster dipole moment makes a larger contribution to the overall interfacial dipole moment than charge transfer. X-ray photoelectron spectroscopy (XPS) measurements showed that the Nb atoms of Nb₃O₇ and Nb₄O₁₀ are primarily Nb⁵⁺ on Cu(111), while for the reduced Nb₃O₅ and Nb₄O₇ clusters, a mixture of oxidation states was observed on Cu(111). Temperature-programmed desorption (TPD) experiments with D₂O showed that water dissociation occurred on all systems except for the oxidized Nb₃O₇ and Nb₄O₁₀ clusters on the Cu₂O film. A comparison of our XPS and TPD results suggests that Nb⁵⁺ cations associated with NbO terminal groups act as Lewis acid sites which are key for water binding and subsequent dissociation. TPD measurements of 2-propanol dehydration also show that the clusters active toward water dissociation are indeed acidic. DFT calculations of water dissociation on Nb₃O₇ support our TPD results, but the use of bulk Cu₂O­(111) as a model for the Cu₂O film merits future scrutiny in terms of interfacial charge transfer. The combination of our experimental and theoretical results suggests that both Lewis acidity and metal reducibility are important for water dissociation

<i>In Situ</i> Formation of FeRh Nanoalloys for Oxygenate Synthesis

Early and late transition metals are often combined as a strategy to tune the selectivity of catalysts for the conversion of syngas (CO/H₂) to C₂₊ oxygenates, such as ethanol. Here we show how the use of a highly reducible Fe₂O₃ support for Rh leads to the <i>in situ</i> formation of supported FeRh nanoalloy catalysts that exhibit high selectivity for ethanol synthesis. <i>In situ</i> characterizations by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) reveal the coexistence of iron oxide, iron carbide, metallic iron, and FeRh alloy phases depending on reaction conditions and Rh loading. Structural analysis coupled with catalytic testing indicates that oxygenate formation is correlated to the presence of FeRh alloys, while the iron oxide and carbide phases lead mainly to hydrocarbons. The formation of nanoalloys by <i>in situ</i> reduction of a metal oxide support under working conditions represents a simple approach for the preparation bimetallic catalysts with enhanced catalytic properties

Hydrogenation of Carbon Dioxide by Water: Alkali-Promoted Synthesis of Formate

Conversion of carbon dioxide utilizing protons from water decomposition is likely to provide a sustainable source of fuels and chemicals in the future. We present here a time-evolved infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD) study of the reaction of CO₂ + H₂O in thin potassium layers. Reaction at temperatures below 200 K results in the hydrogenation of carbon dioxide to potassium formate. Thermal stability of the formate, together with its sequential transformation to oxalate and to carbonate, is monitored and discussed. The data of this model study suggest a dual promoter mechanism of the potassium: the activation of CO₂ and the dissociation of water. Reaction at temperatures above 200 K, in contrast, is characterized by the absence of formate and the direct reaction of CO₂ to oxalate, due to a drastic reduction of the sticking coefficient of water at higher temperatures

Redox-Mediated Reconstruction of Copper during Carbon Monoxide Oxidation

Copper has excellent initial activity for the oxidation of CO, yet it rapidly deactivates under reaction conditions. In an effort to obtain a full picture of the dynamic morphological and chemical changes occurring on the surface of catalysts under CO oxidation conditions, a complementary set of in situ ambient pressure (AP) techniques that include scanning tunneling microscopy, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situ AP CO oxidation experiments over Cu(111) model catalysts at room temperature. Depending on the CO:O₂ ratio, Cu presents different oxidation states, leading to the coexistence of several phases. During CO oxidation, a redox cycle is observed on the substrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edges and then are reduced and rejoin nearby step edges. IRRAS results confirm the presence of under-coordinated Cu atoms during the reaction. By using control experiments to isolate individual phases, it is shown that the rate for CO oxidation decreases systematically as metallic copper is fully oxidized

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces