The photocatalytic oxidation of methanol on a rutile TiO2(110) surface was studied by means of thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The combined TDS and XPS results unambiguously identify methyl formate as the product in addition to formaldehyde. By monitoring the evolution of various surface species during the photocatalytic oxidation of methanol on TiO2(110), XPS results give direct spectroscopic evidence for the formation of methyl formate as the product of photocatalytic cross-coupling of chemisorbed formaldehyde with chemisorbed methoxy species and clearly demonstrate that the photocatalytic dissociation of chemisorbed methanol to methoxy species occurs and contributes to the photocatalytic oxidation of methanol. These results not only greatly broaden and deepen the fundamental understanding of photochemistry of methanol on the TiO2 surface but also demonstrate a novel green and benign photocatalytic route for the synthesis of esters directly from alcohols or from alcohols and aldehydes.

The interaction of atomic hydrogen and H 2 O with stoichiometric and partially reduced CeO 2 (111) thin films deposited on a Cu(111) substrate was investigated by temperature programmed desorption and X-ray photoelectron spectroscopy. On stoichiometric CeO 2 (111) surface, the adsorption of atomic H(g) leads to the formation of surface hydroxyl (OH(a)) and H 2 O(a) as well as the reduction of Ce 4+ into Ce 3+ . On reduced CeO 2 (111) surfaces, the stability of OH(a) was enhanced by the presence of oxygen vacancies. Upon heating, surface hydroxyls undergo two competing reaction pathways: one is the associative desorption of OH(a) releasing H 2 O and creating oxygen vacancies (OH(a) + OH(a) → H 2 O(g) + O lattice + O vacancy ), and the other one is to produce H 2 via OH(a) + OH(a) → H 2 (g) + 2O lattice . The presence of oxygen vacancies in CeO 2 favors the reaction pathway of H 2 formation. At 115 K, reversible dissociation and molecular adsorption of H 2 O occur on stoichiometric CeO 2 (111) surface, but irreversible dissociation of H 2 O occurs on reduced CeO 2 (111) surfaces. These results deepen the fundamental understanding of the influence of oxygen vacancies on the reactivity of surface hydroxyls and water on CeO 2 surface.

By rational design of the FeO(111)/Pt(111) inverse model catalyst and the control experiments, we report for the first time direct experimental evidence for the interfacial CO(ads) + OH(ads) reaction to produce CO(2) at the Pt-oxide interface at low temperatures, providing deep insights into the reaction mechanism and active site of the important low-temperature water-gas shift and preferential CO oxidation reactions catalyzed by Pt/oxide nanocatalysts at the molecular level.

We have investigated the adsorption of water on a Co(0001) surface by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At 130 K, the interaction between adsorbed water (H 2 O(a)) and Co(0001) is quite weak, and water adsorption forms an intact, nonwetting, and three-dimensional layer on Co(0001). The monolayer and multilayer of H 2 O(a) could be distinguished by XPS, and they desorb molecularly upon heating but only giving a single water desorption peak. An ordered p(2 × 2) LEED pattern was observed at low exposures of water. At room temperature, water adsorbs dissociatively on Co(0001), forming chemisorbed atomic oxygen (O(a)) and atomic hydrogen (H(a)). Upon heating, H(a) recombines into H 2 desorbing from the surface, whereas O(a) remains on the surface. The interaction of water with Co( 0001) is greatly influenced by the presence and nature of the oxygen species on Co(0001). Water adsorption on Co(0001) precovered with 0.45 ML O(a) at 130 K forms a mixture layer of OH(a) and H(a) via the reaction of 2H 2 O(a) + O(a) f 3OH(a) + H(a). However, on oxide-like Co(0001) surface, the water decomposition is completely passivated even at room temperature. These results broaden our fundamental understanding of water interaction with metal surfaces and provide insights into the water-involved catalytic reactions catalyzed by cobalt. † Part of the "D. Wayne Goodman Festschrift".

The reactivity of surface hydroxyls on FeO(111) monolayer films on Pt(111) with different oxygen vacancy concentrations has been investigated by means of X-ray photoelectron spectroscopy, thermal desorption spectroscopy, low energy electron diffraction, and density functional theory calculations. Surface hydroxyls on the FeO(111) monolayer films undergo two types of surface reactions: one type is surface reactions to form H2O and create oxygen vacancies; the other is surface reactions to form H2. Surface reactions to form H2O and create oxygen vacancies are preferred for surface hydroxyls on the stoichiometric FeO(111) monolayer film but get suppressed with the increasing of the oxygen vacancy concentration on the FeO(111) monolayer film. On the FeO0.67(111) monolayer film, surface hydroxyls prefer surface reactions to form H2. The accompanying DFT calculation results demonstrate that the thermodynamically favorable reaction between two OH(a) switches from the surface reaction to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to the surface reaction to form H2 on the FeO0.75(111) monolayer film. These results reveal a novel concept of oxygen vacancy-controlled reactivity of surface hydroxyls in which the thermodynamically favorable reactions switch from reactions to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to those to form H2 on the partially reduced FeO0.75(111) monolayer film. The interplay between oxygen vacancies and surface hydroxyls that both exert great influence on the physical chemistry and reactivity of oxide surface will greatly deepen the fundamental understanding of the relevant heterogeneous catalytic reaction systems involving transitional metal oxides.

We have comprehensively investigated the reactivity of hydroxyls on FeO x (111) monolayer islands with different amounts of oxygen vacancy concentrations grown on Pt(111) by means of X-ray photoelectron spectroscopy, temperature-programmed desorption/reaction spectroscopy, and low energy electron diffraction. Hydroxyls on FeO x (111) monolayer islands are capable of oxidizing CO(a) on Pt(111) at the FeO x (111)-Pt(111) interface at low temperatures and such an interfacial oxidation of CO by hydroxyls to produce CO 2 is not suppressed by either excess CO(a) or excess H(a) on FeO x (111)/Pt(111) inverse model catalyst surface. However, the reactivity of hydroxyls is controlled by the oxygen vacancy concentration in FeO x (111) monolayer islands. With the increase of oxygen vacancy concentration, reaction pathways of hydroxyls on FeO x (111) monolayer islands to produce H 2 O are thermodynamically suppressed, which thus opens other hydroxyls-involved reaction pathways including the interfacial oxidation of CO to produce CO 2 . These results greatly deepen the fundamental understanding of the reaction mechanism and catalytically active structure for low temperature WGS and PROX reactions catalyzed by oxide supported Pt nanocatalysts.

No abstract

We have studied the interaction of ethylene on Co(0001) in detail by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). At 130 K, the chemisorption and decomposition of C2H4 were found to depend on the vacant sites available on Co(0001). C2H4 chemisorbs dissociatively at low exposures but both dissociatively and molecularly at large exposures. Upon heating, some C2H4(a) desorbs molecularly from the surface, releasing vacant surface sites that result in the simultaneous decomposition of other C2H4(a). C2H2(a) is the surface intermediate for the decomposition of C2H4(a) on Co(0001). At elevated temperatures, C2H2(a) simultaneously undergoes the direct dehydrogenation and the cyclopolymerization–dehydrogenation to form surface C2 cluster and graphitic carbon on Co(0001). At 500 K, C2H4 directly decomposes on Co(0001), forming surface atomic carbon. These results provide novel information on the chemisorption and decomposition of C2H4 on Co(0001) and the nature of resulted carbon species, greatly deepening our fundamental understanding of the relevant heterogeneous catalytic reactions catalyzed by Co-based catalysts and the growth of graphene on Co surfaces by chemical vapor deposition.