Surface Oxygen Ions Research Articles

Characterization of well dispersed copper species on the surface of ZnO by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was employed to characterize Cu(II) species on the surface of calcined CuO-ZnO systems with Cu/Zn atomic ratios ⪯ 0.10. Such solids are of interest as precursors of Cu-ZnO catalysts. It is shown that well dispersed Cu(II) species on the surface of ZnO have unique spectroscopic properties. For the Cu(2p 3/2) transition the intensity of the satellite structure, typical of bulk Cu(II) compounds, is strongly reduced or even absent in the more dilute samples. To explain this result, it is suggested that a stronger Cu-O covalent bond than in bulk CuO is formed at the interface between the surface oxygen ions of ZnO and the supported copper species. The strong interaction between Cu(II) species and the support leads on reduction to the stabilization of Cu(I) species at the periphery of metallic copper particles which could play a role in the reaction mechanisms of CO hydrogenation at the surface of Cu-ZnO catalysts.

Combined infrared spectroscopy, chemical trapping, and thermoprogrammed desorption studies of methanol adsorption and decomposition on ZnAl 2O 4 and Cu/ZnAl 2O 4 catalysts

It is shown that FT-IR spectroscopy permits discrimination to be made between methoxy (methanol) and formate species adsorbed on ZnAl 2O 4 and CuZnAl 2O 4 catalysts. These species were found to be less stable on copper than on ZnAl 2O 4. The presence of reduced copper promotes methanol transformation into formates and then into C0 2: (i) FT-IR results show that copper formate formation from methanol adsorption occurs even at room temperature and that surface oxygen ion participates in its formation; (ii) chemical trapping experiments demonstrate that increasing copper percentage destabilizes formate species, while TPD experiments correlatively indicate an accelerated transformation of formate into CO 2. Formyl species are detected by chemical trapping only at the end of the reaction and are therefore assumed not to participate in the decomposition reaction.

Surface electronic structure and chemisorption on corundum transition-metal oxides: alpha -Fe2O3.

Ultraviolet photoemission spectroscopy has been used to study the electronic structure of both nearly stoichiometric, well-ordered \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ surfaces and surfaces containing point defects. The interaction of ${\mathrm{O}}_{2}$, ${\mathrm{H}}_{2}$O, ${\mathrm{H}}_{2}$, and ${\mathrm{SO}}_{2}$ with both types of surfaces has also been investigated. The energy levels of the five 3d electrons on the ${\mathrm{Fe}}^{3+}$ ions overlap those of the filled O 2p orbitals, leading to a complex valence band about 10 eV wide. Oxygen-vacancy surface defects produced by ${\mathrm{Ar}}^{+}$-ion bombardment result in a conducting surface layer containing both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{0}$ ions. The well-ordered \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$(0001) surface is relatively inert with respect to all four molecules studied, with exposures greater than ${10}^{3}$ L (1 L\ensuremath{\equiv}${10}^{\mathrm{\ensuremath{-}}6}$ Torr sec) necessary before any chemisorption could be seen. For large exposures, the ${\mathrm{O}}_{2}$-surface interaction gives rise to a negative adsorbed species. ${\mathrm{H}}_{2}$O adsorbs dissociatively on both well-ordered and defect surfaces, resulting in adsorbed ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ ions. ${\mathrm{SO}}_{2}$ appears to bond primarily to surface oxygen ions, yielding a complex similar to ${\mathrm{SO}}_{4}$${\mathrm{}}^{2\mathrm{\ensuremath{-}}}$.

ChemInform Abstract: Relaxation in ZnO (1010), (0001), and (0001) Surfaces and the Adsorption of CO.

AbstractAn atom superposition and electron delocalization molecular orbital study using cluster models shows surface zinc and oxygen ions relax into the (0001), (0001), and (1010) surfaces.

Relaxation in zinc oxide (10.lovin.10), (0001), and (000.lovin.1) surfaces and the adsorption of carbon monoxide

An atom superposition and electron delocalization molecular orbital study using cluster models shows surface zinc and oxygen ions relax into the (0001), (0001), and (1010) surfaces. The unsaturated zinc ions present surface states consisting of empty 4s, 4p hybridized orbitals with energy levels beneath the empty 4s, 4p band. The filled bands are O 2p nonbonding, Zn 4s + O 2p bonding, and Zn 3d, in order of increasing ionization potential. A study of CO adsorption to an unsaturated zinc ion on the (1010) surface produces an adsorption energy of 12 kcal/mol, a CO bond length decrease of 0.02 A, and an increase in the CO stretching force constant. The zinc ion unrelaxes to a position above the bulk-like position when CO is coordinated to it. The bonding of CO to zinc consists in closed shell sigma donation interactions of the CO 5sigma orbital with the filled Zn 3d and Zn 4s + O 2p bands with the antibonding counterpart stabilized by the Zn 4s, 4p surface state orbital. This 5sigma donation strengthens the CO bond and transfers charge to the surface. These results support a number of experimental structural, vibrational, and electronic studies in the literature.

Analysis of the electronic structure of incompletely coordinated tetra- and octahedral surface centers on oxide compounds of molybdenum and their complexes with propylene

The model of orbitally stoichiometric clusters and the semiempirical MO-LCAO calculation scheme in the self-consistent charge and configuration approximation have been applied to the investigation of the electronic structure and energy spectrum of incompletely coordinated octahedral (I) and tetrahedral (II) surface centers on molybdenum oxides. It has been shown that centers of type II containing Mo/sup n+/ ions (n = 6, 5, 4) have greater electron-acceptor ability in comparison to Mo/sup 6 +/ centers of type I. The interaction of propylene with centers of type II results in considerable weakening of all the carbon-carbon bonds and some of the carbon-hydrogen bonds, and the interaction with centers of type I results in more moderate weakening of the double bond and one of the C/sup ..cap alpha../-H/sup ..cap alpha../ bonds (owing to the appearance of a hydrogen bond between H/sup ..cap alpha../ and a surface oxygen ion). The nature of the surface Mo centers of oxides in various catalytic conversion of olefins has been discussed.

Ethylene oxidation over lead(II) oxide

A search has been made for the centre of the catalytic epoxidation of ethylene over thin films of PbO. A square of surface oxygen ions is possibly the centre of epoxidation on silver. Similar units are present on 001 PbO. Experiments have shown that on PbO ethylene can be converted to ethylene oxide. Selectivity to ethylene oxide was low, most of the ethylene being converted to CO and CO 2. Possible mechanisms for the oxidation are discussed.

Cu2+-Adsorption Characteristics of Aluminum Hydroxide and Oxyhydroxides

AbstractThe nature of Cu2+ adsorption by boehmite, gibbsite, and noncrystalline alumina was studied over a range of equilibrium pH (4.5–7.5) and Cu2+ concentration (10−3–10−8 M) by electron spin resonance (ESR). Available chemisorption sites at pH 4.5 were the most numerous for noncrystalline alumina (~1 mmole/100 g), less for boehmite, and least for gibbsite as indicated by the relative strength of the rigid-limit ESR signal attributed to Cu2+ adsorbed at discrete sites. The chemisorption process involved immobilization of Cu2+ by displacement of one or more H2O ligands by hydroxyl or surface oxygen ions, with the formation of at least one Cu-O-Al bond. As the pH was raised from 4.5 to 6.0, essentially all of the solution Cu2+ appeared to be adsorbed by the solids. However, the noncrystalline alumina and boehmite chemisorbed much of the total adsorbed Cu2+ (10 mmole/100 g), whereas precipitation or nucleation of Cu(OH)2 in the gibbsite system was indicated. Precipitated Cu2+ was more readily redissolved by exposure to NH3 vapor than chemisorbed Cu2+.

Temperature Programmed Desorption Study of Water Adsorbed on Metal Oxides. I. Anatase and Rutile

Abstract The adsorbed state of water on TiO2 was studied by temperature programmed desorption (TPD) in the range 0–1000 °C. Pure anatase and rutile samples, the TPD spectra of water from which were not affected by any contaminant, could be obtained by the hydrolysis of titanium oxalate and tetrachloride solutions, respectively. Two desorption peaks were observed for anatase with the peak maxima at 69–127 (I) and 191–256 °C (II), and three for rutile at 32–90 (I), 188–265 (II), and 310–356 °C (III). The sum of adsorbed amounts for peaks II, and III are within a monolayer coverage, that of peak I exceeding it. The enthalpies of adsorption were evaluated to be 36 and 55–69kJ·mol−1 for peaks I and II of anatase, and 36, 64, and 104 kj·mol−1 for peaks I, II, and III of rutile, respectively. From these results, the corresponding adsorbed species of peak I was tentatively assigned to physical adsorption, that of peak II to chemisorption on surface oxygen ion by hydrogen bond, and that of peak III to chemisortpion on Ti4+ by the coordination of oxygen atom of water molecule. The last species may be dissociated into surface OH groups.

The Catalytic Action of TiO2 for the Isomerization of Butenes

Abstract The isomerization of 1-butene or cis-2-butene and the co-isomerization of cis-2-C4H8 and cis-2-C4D8 were carried out at 120 °C over TiO2 calcined in air. It has been found that induction period appeared and the ratio of cis-2-butene to trans-2-butene in the isomerization of 1-butene was unusually high (5–20). The double bond migration and cis-trans isomerization occurred by an intramolecular hydrogen transfer reaction and the kinetic isotope effect (kH⁄kD) was 4–5. The activity was completely or almost lost by poisoning with pyridine or nitric oxide. The isomerization was concluded to proceed by a π-allyl anion mechanism, where the cis-trans interconversion is highly restricted and the C–H bond cleavage is involved in the rate-determining step of the double bond migration. Main active sites were suggested to consist of both Lewis acid sites and basic sites (surface oxygen ion, O2−).

The interaction of caesium with clean ZnO surfaces

The interactions of caesium with the polar (0001)-0 surface and the (1010) prism surface ZnO single crystals have been studied using LEED and Auger spectroscopy. On the clean (1*1) polar surface, formed by ion bombardment/anneal cycles, adsorption of caesium was rapid to give ultimately a well resolved ( square root 3* square root 3) superstructure. The adsorbed caesium ions were found to be very strongly bound and complete removal of the fractional order LEED spots could only be accomplished by several cycles of argon ion bombardment. Electrostatic surface energy calculations suggest that in passing from a (1*1) clean surface to a ( square root 3* square root 3) surface there is a change from a surface state compensated to an ionized caesium compensated surface. With some reservations it also follows from these calculations that the most likely position for the adsorbed caesium ions is inside the 'hole' formed by three surface oxygen ions, and immediately above an underlying zinc ion.

Reaction of halogens with oxide surfaces

Charge transfer reactions at oxide surfaces are discussed for the halogens, chlorine, bromine and iodine as strong electron acceptors. Adsorption of chlorine and bromine on MgO prepared in vacuo is characterised by the appearance of a band at 430 nm in the diffuse reflectance spectrum and an e.s.r. signal with g-factors of 2.0099 and 2.0020. These features are destroyed on heating and oxygen is evolved. Chlorine and bromine react with the lattice oxygen ions to give halide ions corresponding to about 20 and 10% respectively of the surface MgO ion pairs; no evidence of the formation of molecular X–2 ions was obtained. These results can be understood in terms of the initial formation of charge deficient groups of oxygen ions, such as On–n and subsequent release of oxygen. The reaction of bromine with MgO would not be expected purely on a consideration of the bulk heats of formation, and indicates an appreciable concentration of surface oxygen ions with less than 5-fold co-ordination. Iodine, as expected from the low heat of formation of the iodide, does not displace oxygen ions on the surface but some iodide ions are formed. All the halogens are able to displace oxygen adsorbed as O–2 from the surface indicating that this ion is only weakly bonded to the surface.

Oxygen species adsorbed on oxides. Part 1.—Formation and reactivity of (O–)s on MgO

Nitrous oxide reacts with Fs+(H) centres (surface oxygen ion vacancies with a single trapped electron) to give (O–)s+ ions adsorbed at the anion vacancy. The reaction, Fs+(H)+ N2O →(O–)s++ N2 is quantitative and the reflectance spectrum indicates a band centred at 2 eV which is ascribed to the (O–)s+ centre. Comparison of the optical transition energy calculated from the e.s.r. data with the observed transition shows that the normally accepted value of 135 cm–1 for the spin-orbit coupling of the O– ion is too low and that a value of >200 cm–1 is more appropriate for the solid state. Nitrous oxide also reacts with other surface centres to give an adsorbed species which may be (O2–2)s or (O–2)s. The reaction of (O–)s+ with hydrogen reforms the Fs+(H) centre which then reacts with more nitrous oxide; the reaction can be cycled several times. The reactions of (O–)s+ with O2, CO2, alcohols and olefins are also investigated. To facilitate reference to various types of surface defects and adsorbed species a nomenclature is proposed which is compatible with a recently suggested notation for defects in bulk lattice.

Catalytic activity of amorphous aluminas for oxidation of carbon monoxide: I. Activation by evacuation in vacuo

The catalytic activity of amorphous aluminas in the oxidation of carbon monoxide was correlated with surface structural defects created in the course of evacuation in vacuo (10 −6 torr) between 450 and 800 °C. A kinetic study of the oxidation of carbon monoxide at 450 °C showed that the catalytic activity of amorphous aluminas is determined by the type of surface defects, oxygen or aluminium ion vacancies which predominate and which can be created by appropriate treatments.

Paramagnetic defects associated with hydrogen adsorbed on the surface of magnesium and calcium oxides

The irradiation of MgO and CaO in the form of small particles (100–300Å) leads to the formation of surface paramagnetic centers (S centers) which will react with oxygen. Both S centers and new surface centers (S H centers) with g = 1.9998 are formed if the oxides are γ-irradiated in the presence of hydrogen or deuterium. The S H center shows a hyperfine coupling from one hydrogen nucleus, and both the S and S H centers show a hyperfine interaction with the Mg 25 nuclei of the lattice to give splitting constants of 25.5 and 10.2 gauss, respectively. These surface centers are believed to be intrinsic defects consisting of an electron trapped at a surface anion vacancy and provide a surface equivalent of the well-known bulk defects (F centers). The nearby proton in the S H center suggests that the hydrogen must react with surface oxygen ions during irradiation.

Study of the nature of surfaces with polar molecules II. The adsorption of water on aluminas

Aluminas adsorb a large amount of water from a humid atmosphere. Only part of the water thus adsorbed can be expelled by drying at 120 °C. The remaining water content (referred to as “chemisorbed” water) is proportional to the specific surface area of the sample and amounts to 24.8 mg H 2O/100 m 2. This amount points to a binding of one water molecule to two oxygen atoms in the surface. The surface covered with “chemisorbed” water adsorbs another quantity of water by physisorption. From the adsorption isotherms the monolayer capacity V m can be calculated with the BET equation; V m is also approximately proportional to the specific surface area of the sample and amounts to about 33 mg H 2O/100 m 2. At relative water vapor pressures above about 0.4 capillary condensation in the pores of the aluminas of these experiments occurs. Lauric acid can be adsorbed on the water layer adsorbed on the alumina surface. Up to a certain “critical” water content the lauric acid adsorption decreases with increasing amount of adsorbed water. Above this “critical” value the lauric acid adsorption remains constant, water being displaced from the surface by lauric acid. The “critical” water content is proportional to the specific surface area and amounts to about 50 mg H 2O/100 m 2. So for each “chemisorbed” water molecule another water molecule can be bound by a strong physisorption. From the relation between lauric acid adsorption and specific surface area of rehydrated samples it appears that water molecules “chemisorbed” under the conditions studied probably do not react with oxygen atoms in the surface forming OH groups but rather are bound to the surface oxygen ions by very strong hydrogen bonds.