Growth and Structure of Tb₂O₃(111) Films on Pt(111)

We investigated the formation and structure of Tb₂O₃(111) thin films grown on Pt(111) using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Tb₂O₃(111) films adopt an oxygen-deficient cubic fluorite structure, wherein the Tb cations form an approximately (1.32 × 1.32) lattice in registry with the Pt(111) substrate. STM shows that terbia film growth follows the Stranski–Krastanov mechanism, in which Tb₂O₃(111) initially forms a well-connected wetting layer up to a Tb₂O₃ coverage of ∼2 ML (monolayer), whereas multilayer Tb₂O₃ islands develop as the coverage increases thereafter. The Tb₂O₃(111) wetting layer yields a sharp (3 × 3) LEED pattern relative to the primary Tb spots that we attribute to diffraction from a 3/4 coincidence lattice formed at the Tb₂O₃(111)–Pt­(111) interface. STM further shows that oxygen vacancies are randomly distributed within the lattice of the Tb₂O₃(111) wetting layer. The (3 × 3) LEED pattern diminishes during the transition to island growth but re-emerges as the islands ripen at Tb₂O₃ coverages beyond about 5 ML. We attribute the re-emergent LEED pattern to a (3 × 3) superstructure of oxygen vacancies within the Tb₂O₃(111) islands, the formation of which is likely mediated by the (3 × 3) Tb₂O₃–Pt­(111) coincidence lattice. The (3 × 3) structure persists to Tb₂O₃ coverages of at least 10 ML and remains stable after annealing to temperatures up to 1100 K. An implication of this study is that Tb₂O₃(111) surfaces with well-defined, ordered oxygen vacancies can be stabilized to better study the role of structure and oxygen vacancies on the chemical reactivity of TbO_<i>x</i> surfaces

Influence of Water on the Catalytic Oxidation of Ethane on IrO₂(110)

Gaseous H2O can strongly influence the performance of solid catalysts in applications of alkane oxidation. In the present study, we investigated the influence of H2O on the catalytic oxidation of C2H6 on IrO2(110) thin films using measurements of reaction rates and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) performed with synchrotron radiation. We find that the IrO2(110) surface is highly active and selective for catalyzing the complete oxidation of C2H6 at temperatures between 550 and 650 K and that adding H2O in small quantities (<1%) to the gaseous reactant mixtures significantly lowers the catalytic activity without altering the selectivity for CO2 production. AP-XPS measurements performed in about 0.10 Torr of H2O and temperatures between 500 and 600 K show that H2O adsorption on IrO2(110) produces high-coverage mixtures of H2O and HO in which HO groups are the majority species. We present evidence that H2O molecules preferentially incorporate into HO-H2O aggregates on IrO2(110) when HO groups are abundant. AP-XPS further shows that high coverages of HO and COx surface species are generated on IrO2(110) during the catalytic oxidation of C2H6. Introducing H2O into the gas phase causes the coverage of COx surface species to significantly decrease, while the coverages of H2O and HO increase. Based on this behavior, we conclude that H2O suppresses the catalytic oxidation of C2H6 on IrO2(110) at 550–650 K by outcompeting C2H6 and O2 for Ir adsorption sites in addition to deactivating surface O atoms toward C2H6 dehydrogenation by converting them to HO groups. Our results provide molecular-level insights for understanding the adsorption of H2O on IrO2(110) as well as its influence on both the selectivity and activity of IrO2(110) in catalyzing the oxidation of light alkanes

Catalytic Oxidation of Methane on IrO₂(110) Films Investigated Using Ambient-Pressure X‑ray Photoelectron Spectroscopy

The catalytic oxidation of CH4 over IrO2(110) films grown on Ir(100) was investigated using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at total pressures near 1 Torr. The IrO2(110) films undergo negligible reduction during catalytic CH4 oxidation in reactant mixtures with as much as 95% CH4 and temperatures from ca. 500–650 K, demonstrating that IrO2(110) can catalyze the oxidation of CH4 over a wide range of temperatures and mixture compositions. High coverages of OH groups and oxidized C-containing species formed on the IrO2(110) surfaces during CH4 oxidation, including excess OH groups bound directly to the initially, coordinatively unsaturated Ir atoms. The formation of excess OH groups demonstrates that O-rich IrO2(110) surfaces were maintained even under highly CH4-rich conditions and provides evidence that the dissociative adsorption of O2 is more facile than CH4 activation and conversion to adsorbed intermediates on IrO2(110). Extensively oxidized surface species with a CHyO2 stoichiometry preferentially formed under all reaction conditions studied. The conversion of CH4 to the CHyO2 surface species became optimal at an intermediate composition of the reactant mixture (∼90% CH4), consistent with a site competition between CH4 and O2 during their initial adsorption as well as a high oxidation activity of chemisorbed O atoms on IrO2(110). These results provide quantitative information about the identities and coverages of adsorbed species that form during the catalytic oxidation of CH4 on IrO2(110). Such knowledge is essential for validating first-principles models of the reaction kinetics for this system and ultimately gaining insights needed to optimize the performance of IrO2 catalysts for the oxidation of light alkanes

Adsorption of NO on FeO_<i>x</i> Films Grown on Ag(111)

We used temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) to characterize the adsorption of NO on crystalline iron oxide films grown on Ag(111), including a Fe₃O₄(111) layer, an FeO(111) monolayer, and an intermediate FeO_<i>x</i> multilayer structure. TPD shows that the NO binding energies vary significantly among the Fe cation sites present on these FeO_<i>x</i> surfaces, and provides evidence that NO binds more strongly on Fe²⁺ sites than Fe³⁺ sites. The NO TPD spectra obtained from the Fe₃O₄(111) layer exhibit a dominant peak at 380 K, attributed to NO bound on Fe²⁺ sites, as well as a broad feature centered at ∼250 K that is consistent with NO bound on Fe³⁺ sites of Fe₃O₄(111) as well as NO adsorbed on a minority FeO structure. The NO TPD spectra obtained from the monolayer FeO(111) film exhibits a prominent peak at 269 K. After growing FeO_<i>x</i> multilayer islands within the FeO(111) monolayer, we observe a new NO TPD feature at ∼200 K as well as diminution of the sharp TPD peak at 269 K. We speculate that these changes occur because the multilayer FeO_<i>x</i> islands expose Fe³⁺ sites that bind NO more weakly than the Fe²⁺ sites of the FeO monolayer. RAIR spectra obtained from the NO-covered FeO_<i>x</i> surfaces exhibit an N–O stretch band that blueshifts over a range from about 1800 to 1840 cm^–1 with increasing NO coverage. The measured N–O stretching frequency is only slightly red-shifted from the gas-phase value, and lies in a range that is consistent with atop, linearly bound NO on the Fe surface sites. In contrast to the NO binding energy, we find that the N–O stretch band is relatively insensitive to the NO binding site on the FeO_<i>x</i> surfaces. This behavior suggests that π-backbonding occurs to similar extents among the adsorbed NO species, irrespective of the oxidation state and local structural environment of the Fe surface site

Microscopic Investigation of H₂ Reduced CuO_<i>x</i>/Cu(111) and ZnO/CuO_<i>x</i>/Cu(111) Inverse Catalysts: STM, AP-XPS, and DFT Studies

Understanding the reduction mechanism of ZnO/CuOx interfaces by hydrogen is of great importance in advancing the performance of industrial catalysts used for CO and CO2 hydrogenation to oxygenates, the water-gas shift, and the reforming of methanol. Here, the reduction of pristine and ZnO-modified CuOx/Cu(111) by H2 was investigated using ambient-pressure scanning tunneling microscopy (AP-STM), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT). The morphological changes and reaction rates seen for the reduction of CuOx/Cu(111) and ZnO/CuOx/Cu(111) are very different. On CuOx/Cu(111), perfect “44” and “29” structures displayed a very low reactivity toward H2 at room temperature. A long induction period associated with an autocatalytic process was observed to enable the reduction by the removal of chemisorbed nonlattice oxygen initially and lattice oxygen sequentially at the CuOx–Cu interface, which led to the formation of oxygen-deficient “5–7” hex and honeycomb structures. In the final stages of the reduction process, regions of residual oxygen species and metallic Cu were seen. The addition of ZnO particles to CuOx/Cu(111) opened additional reaction channels. On the ZnO sites, the dissociation of H2 was fast and H adatoms easily migrated to adjacent regions of copper oxide. This hydrogen spillover substantially enhanced the rate of oxygen removal, resulting in the rapid reduction of the copper oxide located in the periphery of the zinc oxide islands with no signs of the reduction of ZnO. The deposited ZnO completely modified the dynamics for H2 dissociation and hydrogen migration, providing an excellent source for CO2 hydrogenation processes on the inverse oxide/metal system

Morphology Dependent Reactivity of CsO_<i>x</i> Nanostructures on Au(111): Binding and Hydrogenation of CO₂ to HCOOH

Cesium oxide (CsOx) nanostructures grown on Au(111) behave as active centers for the CO2 binding and hydrogenation reactions. The morphology and reactivity of these CsOx systems were investigated as a function of alkali coverage using scanning tunneling microscopy (STM), ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT) calculations. STM results show that initially (0.05–0.10 ML) cesium oxide clusters (Cs2O2) grow at the elbow sites of the herringbone of Au(111), subsequently transforming into two-dimensional islands with increasing cesium coverage (>0.15 ML). XPS measurements reveal the presence of suboxidic (CsyO; y ≥ 2) species for the island structures. The higher coverages of cesium oxide nanostructures contain a lower O/Cs ratio, resulting in a stronger binding of CO2. Moreover, the O atoms in the CsyO structure undergo a rearrangement upon the adsorption of CO2 which is a reversible phenomenon. Under CO2 hydrogenation conditions, the small Cs2O2 clusters are hydroxylated, thereby preventing the adsorption of CO2. However, the hydroxylation of the higher coverages of CsyO did not prevent CO2 adsorption, and adsorbed CO2 transformed to HCOO species that eventually yield HCOOH. DFT calculations further confirm that the dissociated H2 attacks the C in the adsorbate to produce formate, which is both thermodynamically and kinetically favored during the CO2 reaction with hydroxylated CsyO. These results demonstrate that cesium oxide by itself is an excellent catalyst for CO2 hydrogenation that could produce formate, an important intermediate for the generation of value-added species. The role of the alkali oxide nanostructures as active centers, not merely as promoters, may have broad implications, wherein the alkali oxides can be considered in the design of materials tuned for specific applications in heterogeneous catalysis

Formation of Epitaxial PdO(100) During the Oxidation of Pd(100)

The catalytic oxidation of CO and CH4 can be strongly influenced by the structures of oxide phases that form on metallic catalysts during reaction. Here, we show that an epitaxial PdO(100) structure forms at temperatures above 600 K during the oxidation of Pd(100) by gaseous O atoms as well as exposure to O2-rich mixtures at millibar partial pressures. The oxidation of Pd(100) by gaseous O atoms preferentially generates an epitaxial, multilayer PdO(101) structure at 500 K, but initiating Pd(100) oxidation above 600 K causes an epitaxial PdO(100) structure to grow concurrently with PdO(101) and produces a thicker and rougher oxide. We present evidence that this change in the oxidation behavior is caused by a temperature-induced change in the stability of small PdO domains that initiate oxidation. Our discovery of the epitaxial PdO(100) structure may be significant for developing relationships among oxide structure, catalytic activity, and reaction conditions for applications of oxidation catalysis

Formation of Epitaxial PdO(100) During the Oxidation of Pd(100)

The catalytic oxidation of CO and CH4 can be strongly influenced by the structures of oxide phases that form on metallic catalysts during reaction. Here, we show that an epitaxial PdO(100) structure forms at temperatures above 600 K during the oxidation of Pd(100) by gaseous O atoms as well as exposure to O2-rich mixtures at millibar partial pressures. The oxidation of Pd(100) by gaseous O atoms preferentially generates an epitaxial, multilayer PdO(101) structure at 500 K, but initiating Pd(100) oxidation above 600 K causes an epitaxial PdO(100) structure to grow concurrently with PdO(101) and produces a thicker and rougher oxide. We present evidence that this change in the oxidation behavior is caused by a temperature-induced change in the stability of small PdO domains that initiate oxidation. Our discovery of the epitaxial PdO(100) structure may be significant for developing relationships among oxide structure, catalytic activity, and reaction conditions for applications of oxidation catalysis