The adsorption of and reaction of NO2 on Ag(1 1 1)-p(4 × 4)-O and formation of surface nitrate

The adsorption and reaction of nitrogen dioxide on the Ag(1 1 1)-p(4 × 4)-O surface has been investigated with RAIRS, TPRS and STM. At 300 K NO₂ initially reacts with the oxygen overlayer to form nitrate in p(3 × 3) and p(4 × 4) structures, which convert to a new p(3 × 3) at saturation coverage. Surface pitting during nitrate adsorption is suggestive of the incorporation of silver atoms into the NO₃ structure. With heating NO₃ decomposes into NO₂ and O at 396 K and 497 K, and oxygen desorbs at 578 K.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

Adsorption and reactions of CH2I2 on Ru(001) surface

The adsorption and dissociation of CH₂I₂ were studied at 110 K with the aim of generating CH₂ species on the Ru(001) surface. The methods used included X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and work function measurements. Adsorption of CH₂I₂ is characterized by a work function decrease (0.96 eV at monolayer), indicating that adsorbed CH₂I₂ has a positive outward dipole moment. Three adsorption states were distinguished: a multilayer (T_p=200 K), a weakly bonded state (T_p=220 K) and an irreversibly adsorbed state. A new feature is the formation of CH₃I, which desorbs with T_p=160 K. The adsorption of CH₂I₂ at 110 K is dissociative at submonolayer, but molecular at higher coverages. Dissociation of the monolayer to CH₂ and I proceeded at 198–230 K, as indicated by a shift in the I(3d_5/2) binding energy from 620.6 eV to 619.9 eV. A fraction of adsorbed CH₂ is self-hydrogenated into CH₄ (T_p=220 K), and another one is coupled to di-σ-bonded ethylene, which — instead of desorption — is converted to ethylidyne at 220–300 K. Illumination of the adsorbed CH₂I₂ initiated the dissociation of CH₂I₂ monolayer even at 110 K, and affected the reaction pathways of CH₂.     

Chloromethane surface chemistry on Fe3O4(1 1 1)-(2 × 2): A thermal desorption spectrometry comparison of CCl4, CBr2Cl2, and CH2Cl2

The surface chemistry of CBr₂Cl₂ on the Fe₃O₄(1 1 1)-(2 × 2) selvedge of single-crystal α-Fe₂O₃(0 0 0 1) has been investigated using temperature programmed reaction and desorption (TPR/D) measurements. The spectra obtained in this case show that strong chemisorption occurs and that a series of adsorbed halogenated reaction products are present. By comparison, studies of the adsorbed phase of CH₂Cl₂ show that only physisorption occurs. The TPR/D spectra of CBr₂Cl₂ show that dissociative formation of CCl₂ followed by its reaction with lattice oxygen is central to the monolayer reaction chemistry in this chloromethane. The branching ratios of the various desorbed products are compared with those obtained from CCl₄ adsorbed on the same (2 × 2) surface.     

Coadsorption of CO and C6H6 on Co(0 0 0 1)

We have studied the influence of CO on the adsorption of benzene on the Co(0 0 0 1) surface using LEED, XPS, TDS and work function measurements. CO was found to reduce the benzene adsorption, but even at saturation CO exposure no complete blocking was observed. Thermal desorption of the coadsorbed layer featured CO and H₂ peaks indicating partial dehydrogenation of benzene and retaining of the CO bond. Ordered LEED structures were found with all coverages: Pre-adsorption of CO led to patterns already seen for pure carbon monoxide adsorption. Pre-adsorption of benzene showed the known structure of pure benzene also with small CO exposures, but higher CO exposures yielded a mixture of and patterns.     

Reaction of O2 with the boron-terminated TaB2(0 0 0 1) surface

X-ray photoelectron spectroscopy has been used to study the clean TaB₂(0 0 0 1) surface and its reaction with O₂. In agreement with previous studies, XPS indicates that the clean surface is boron terminated. The topmost boron layer shows a chemically shifted B 1s peak at 187.1 eV compared to a B 1s peak at 188.6 eV for boron layers below the surface. The 187.1-188.6 eV peak intensity ratio and its variation with angle between the crystal normal and the detector is well described by a simple theoretical model based on an independently calculated electron inelastic mean free path of 15.7 Å for TaB₂. The dissociative sticking probability of O₂ on the boron-terminated TaB₂(0 0 0 1) surface is lower by a factor of 10⁴ than for the metal-terminated HfB₂(0 0 0 1) surface.     

Thermal decomposition mechanisms of tungsten nitride CVD precursors on Cu(1 1 1)

Chemisorption and thermal decomposition of metallorganic chemical vapor deposition precursors, (t-BuN)₂W(NHBu-t)₂, bis(tert-butylimido)bis(tert-butylamido)tungsten (BTBTT) and (t-BuN)₂W(NEt₂)₂, bis(tert-butylimido)bis(diethylamido)tungsten (BTBDT), on Cu(1 1 1) have been investigated by means of thermal desorption spectroscopy (TDS) and synchrotron-based X-ray photoelectron spectroscopy (SR-XPS) under ultrahigh vacuum conditions. The precursors remain intact upon chemisorption on Cu(1 1 1) at 100 K, and at 300 K both precursors decompose readily via the characteristic hydride abstraction/elimination pathways to produce two stable surface intermediates for each precursor. For BTBTT, one species is W(=NBu-t)₃ and the other is proposed to be a bridged amido complex, [(t-BuN)₂W(μ-NBu-t)]₂. In comparison, a W-imine complex and a W-N-C metallacycle are two intermediates produced from BTBDT. Annealing toward 800 K further decomposes the intermediates and the detectable desorption species are completely derived from the ligands. The desorption products from BTBTT include t-butylamine generated from α-H abstraction, isobutylene from γ-H elimination, acetonitrile from β-methyl elimination, and molecular hydrogen. In addition to these desorption species, BTBDT produces hydrogen cyanide and imine (EtN = CHMe) via β-H elimination, not possible with BTBTT due to the absence of β-H in the ligands. Eventually, tungsten nitrides incorporating oxygen atoms and a small amount of graphitic carbons are formed and the stoichiometry is approximated as WN_1.5O_0.1. Oxygen incorporation, driven by a large oxide formation enthalpy, is sensitively dependent on the moisture exposure in UHV environment.     

The catalytic reduction of NO by H2 on Ru(0 0 0 1): Observation of NHads species

Adsorption of NO and the reaction between NO and H₂ were investigated on the Ru(0 0 0 1) surface by X-ray photoelectron spectroscopy (XPS). Surface composition was measured after NO adsorption and after the selective catalytic reduction of nitric oxide with hydrogen in steady-state conditions at 320 K and 390 K in a 30:1 mixture of H₂ and NO (total pressure = 10⁻⁴ mbar). After steady-state NO reduction, molecularly adsorbed NO in both the linear on-top and threefold coordinations, NH_ads and N_ads species were identified by XPS. The coverage of the NH_ads and N_ads species was higher after the reaction at 390 K than the corresponding values at 320 K. Strong destabilisation of N_ads by O_ads was detected. A possible reaction mechanism is discussed.     

Chlorinated ethenes at Cu(1 1 0): adsorption, dissociation and reactions of trans-1,2-dichloroethene

The adsorption and reactions of trans-1,2-dichloroethene on Cu(1 1 0) have been investigated using reflection absorption infrared spectroscopy, temperature programmed desorption and molecular beam adsorption reaction spectroscopy. These data reveal that the behaviour of the system is critically affected by temperature and can be described in terms of three main regimes: Regime I, which occurs over 85-165 K, represents molecular adsorption in the monolayer and the multilayer where the trans-1,2-dichloroethene adsorbs with its molecular plane oriented largely parallel to the metal surface; Regime II, extending over the temperature range 170-280 K, is dominated by desorption/dechlorination events which are first initiated at 171 K, then poisoned rapidly and re-initiated at the much higher temperature of 263 K, culminating in the creation of adsorbed acetylene and Cl atoms on the surface; Regime III, spanning the temperature range of 280-500 K, is governed by the behaviour of the C₂H₂ molecular intermediate at the surface which, if created at low temperatures, trimerises to form benzene that subsequently desorbs. However, when created at high temperature, direct desorption of C₂H₂ competes effectively with the trimerisation process.     

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces II. Effect of coadsorbed oxygen

The effect of coadsorbed oxygen on the thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). I 3d and C 1s XPS data indicated that adsorbed diiodomethane undergoes two sequential C-I bond scission steps to ultimately produce methylene surface species, the same as on clean Ni(1 1 0). Moreover, significant amounts of methane and other heavier hydrocarbons are produced after further thermal activation of those chemisorbed methylene groups. The production of alkanes and alkenes, which is accounted for by a chain growth mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates, is inhibited but not fully blocked by the coadsorbed oxygen. New reaction pathways are also opened up by the presence of oxygen in this system, including a direct coupling of two methylene groups to ethene, the insertion of an oxygen atom into a nickel-methylene group to produce formaldehyde, and a parallel methylene insertion chain growth sequence starting from a CH₂I_ads intermediate to ultimately yield C₃H₅ and C₄H₇ unsaturated gas-phase radicals.     

Adsorption and decomposition of NO on K-deposited Pd(1 1 1)

The adsorption and decomposition of NO on a K-deposited Pd(1 1 1) surface were investigated using X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and temperature-programmed desorption. For the K-deposited Pd(1 1 1) surface, two different NO adsorption sites were observed in addition to the Pd site. On the clean Pd(1 1 1) surface, the adsorption of NO was purely molecular and reversible, but on the K-deposited surface, the adsorbed NO decomposed at two different temperatures, 530 and 610 K. These results indicate that the NO adsorption and decomposition sites were newly created by the deposition of K onto the Pd(1 1 1) surface.     

Towards the preparation of realistic model Ziegler-Natta catalysts: XPS study of the MgCl2/TiCl4 interaction with flat SiO2/Si(1 0 0)

Despite of the wide use of supported Ti based Ziegler-Natta catalysts in the olefin polymerization industry, questions concerning the role of each one of the catalyst components in the polymerization process, have not found a satisfactory answer yet. This is mainly because of the high sensitivity of these systems to oxygen and atmospheric moisture that makes their study in an atomic level rather complicated. Realistic surface science models of the pre-activated SiO₂ supported MgCl₂/TiCl₄ and TiCl₄ Ziegler-Natta catalysts were prepared by spin coating on flat conductive SiO₂/Si(1 0 0) supports under inert atmosphere. This preparation technique resembles the wet chemical impregnation which is the industrial method of the catalyst preparation. XPS analysis showed that the catalyst precursor anchors on the silica surface through bonding of the Ti atoms with surface silanes or siloxanes, while Mg is attached to the Ti through chlorine bridges. Thermal treatment of the catalysts at 723 K leads to total Cl desorption when MgCl₂ is not present while a significant amount of the Ti atoms is reduced to the Ti³⁺ state.     

Adsorption behavior and reaction properties of NO and CO on Rh(1 1 1)

Reactions between NO and CO on Rh(1 1 1) surfaces were investigated using infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption. NO adsorbed on the fcc, atop, and hcp sites in that order, whereas CO adsorbed initially on the atop sites and then on the hollow (fcc + hcp) sites. The results of experiments with NO exposure on CO-preadsorbed Rh(1 1 1) surfaces indicated that the adsorption of NO on the hcp sites was inhibited by preadsorption of CO on the atop sites, and NO adsorption on the atop and fcc sites was inhibited by CO preadsorbed on each type of site, which indicates that NO and CO competitively adsorbed on Rh(1 1 1). From a Rh(1 1 1) surface with coadsorbed NO and CO, N₂ was produced from the dissociation of fcc-NO, and CO₂ was formed by the reaction of adsorbed CO with atomic oxygen from dissociated fcc-NO. The CO₂ production increased remarkably in the presence of hollow-CO. Coverage of fcc-NO and hollow-CO on Rh(1 1 1) depended on the composition ratio of the NO/CO gas mixture, and a gas mixture with NO/CO ? 1/2 was required for the co-existence of fcc-NO and hollow-CO at 273 K.     

Molecularly intact and dissociative adsorption of water on clean Cu(1 1 0): A comparison with the water/Ru(0 0 1) system

An X-ray photoelectron spectroscopy (XPS) study was undertaken of the water/Cu(1 1 0)-system finding non-dissociative adsorption on clean Cu(1 1 0) at temperatures below 150 K. Thermally induced dissociation of D₂O is observed to occur above 150 K, similar to the H₂O/Ru(0 0 1) system, with an experimentally derived activation barrier of 0.53-0.56 eV which is very close in magnitude to the derived activation barrier for desorption of 0.50-0.53 eV. X-ray and electron induced damage to the water overlayer was quantified and used to rationalize the results of a recent XPS study of the water/Cu(1 1 0)-system where partial dissociation was observed already at 90 K.     

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces I. Clean and hydrogen-covered

The thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Diiodomethane was chosen as a precursor for the formation of methylene surface species. I 3d and C 1s XPS data indicated that, indeed, adsorbed diiodomethane undergoes the C-I bond dissociations needed for that transformation, and detection of iodomethane production in TPD experiments pointed to the stepwise nature of those reactions. Significant amounts of methane are produced from further thermal activation of the chemisorbed methylene groups. This involves surface hydrogen, both coadsorbed from background gases and produced by dehydrogenation of some of the adsorbed diiodomethane, and can be induced at temperatures as low as about 160 K, right after the C-I bond breaking steps. Unique to this system is the detection of significant amounts, up to 10% of the total CH₂I₂ adsorbed, of heavier hydrocarbons, including ethene, ethane, propene, propane, and butene. Deuterium labeling experiments were used to provide support for a mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates. Facile subsequent β-hydride elimination and reductive elimination with coadsorbed hydrogen account for the formation of ethene and ethane, respectively, while a second and third methylene insertions lead to C₃ and C₄ production. Based on the final product distribution, the methylene insertion was estimated to be approximately 20 times slower than the following hydrogenation-dehydrogenation reactions. Normal kinetic isotope effects were observed for most of the hydrogenation and dehydrogenation reactions involved.     

In situ XPS study of Pd(1 1 1) oxidation at elevated pressure, Part 2: Palladium oxidation in the 10 mbar range

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 0.4 mbar O₂. The in situ XPS data were complemented by ex situ TPD results. A number of oxygen species and oxidation states of palladium were observed in situ and ex situ. At 430 K, the Pd(1 1 1) surface was covered by a 2D oxide and by a supersaturated O_ads layer. The supersaturated O_ads layer transforms into the Pd₅O₄ phase upon heating and disappears completely at approximately 470 K. Simultaneously, small clusters of PdO, PdO seeds, are formed. Above 655 K, the bulk PdO phase appears and this phase decomposes completely at 815 K. Decomposition of the bulk oxide is followed by oxygen dissolution in the near-surface region and in the bulk. The oxygen species dissolved in the bulk is more favoured at high temperatures because oxygen cannot accumulate in the near-surface region and diffusion shifts the equilibrium towards the bulk species. The saturation of the bulk “reservoir” with oxygen leads to increasing the uptake of the near-surface region species. Surprisingly, the bulk PdO phase does not form during cooling in 0.4 mbar O₂, but the Pd₅O₄ phase appears below 745 K. This is proposed to be due to a kinetic limitation of PdO formation because at high temperature the rate of PdO seed formation is compatible with the rate of decomposition.     

Autocatalytic partial reduction of FeO(1 1 1) and Fe3O4(1 1 1) films by atomic hydrogen

The interaction of atomic hydrogen with thin epitaxial FeO(1 1 1) and Fe₃O₄(1 1 1) films was studied by TDS, XPS and LEED. On the thin, one Fe-O bilayer thick FeO film, partial reduction occurs in two steps during exposure. It ends after removal of 1/4 monolayer (ML) of oxygen with a 2 × 2 pattern appearing in LEED. This FeO_0.75 film is passive against further reduction. The first reduction step saturates after removal of ∼0.2 ML and shows autocatalytic kinetics with the oxygen vacancies formed during reduction causing acceleration. The second step is also autocatalytic and is related with reduction to the final composition and an improvement of the 2 × 2 order. A structure model explaining the two-step reduction is proposed. On the thick Fe₃O₄ film, irregular desorption bursts of H₂O and H₂ were observed during exposure. Their occurrence appears to depend on the film quality and thus on surface order. Because of the healing of reduction-induced oxygen vacancies by exchange of oxygen or iron with the bulk, a change of the surface composition was not visible. The existence of partially reduced oxide phases resistant even to atomic hydrogen is relevant to the mechanism of dehydrogenation reactions using iron oxides as catalysts.     

Reactivity of monolayer V2O5 films on TiO2(1 1 0) produced via the oxidation of vapor-deposited vanadium

The growth, and reactivity of monolayer V₂O₅ films supported on TiO₂(1 1 0) produced via the oxidation of vapor-deposited vanadium were studied using X-ray photoelectron spectroscopy and temperature programmed desorption (TPD). Oxidation of vapor-deposited vanadium in 10⁻⁷ Torr of O₂ at 600 K produced vanadia films that contained primarily V³⁺, while oxidation in 10⁻³ Torr at 400 K produced films that contained primarily V⁵⁺. The reactivity of the supported vanadia layers for the oxidation of methanol to formaldehyde was studied using TPD. The activity for this reaction was found to be a function of the oxidation state of the vanadium cations in the film.     

Infrared reflection absorption study of carbon monoxide adsorption on Cu(1 0 0)-c(2 × 2)-Pd surfaces formed by palladium vacuum-depositions at various temperatures

Using infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD), we investigated carbon monoxide (CO) adsorption and desorption behaviors on atomic checkerboard structures of Cu and Pd formed by Pd vacuum deposition at various temperatures of Cu(1 0 0). The 0.15-nm-thick Pd deposition onto a clean Cu(1 0 0) surface at room temperature (RT) showed a clear c(2 × 2) low-energy electron diffraction (LEED) pattern, i.e. Cu(1 0 0)-c(2 × 2)-Pd. The RT-CO exposure to the c(2 × 2) surfaces resulted in IRRAS absorption caused by CO adsorbed on the on-top sites of Pd. The LEED patterns of the Pd-deposited Cu(1 0 0) at higher substrate temperatures revealed less-contrasted c(2 × 2) patterns. The IRRAS intensities of the linearly bonded CO bands on 373-K-, 473-K-, and 673-K-deposited c(2 × 2) surfaces are, respectively, 25%, 22%, and 10% less intense than those on the RT-deposited surface, indicating that Pd coverages at the outermost c(2 × 2) surfaces decrease with increasing deposition temperature. In the initial stage of the 90-K-CO exposure to the RT surface, the band attributable to CO bonded to the Pd emerged at 2067 cm⁻¹ and shifted to higher frequencies with increasing CO exposure. At saturation coverage, the band was located at 2093 cm⁻¹. In contrast, two distinct bands around 2090 cm⁻¹ were apparent on the spectrum of the 473-K-deposited surface: the CO saturation spectrum was dominated by an apparent single absorption at 2090 cm⁻¹ for the 673-K-deposited surface. The TPD spectra of the surfaces showed peaks at around 200 and 300 K, which were ascribable respectively to Cu-CO and Pd-CO. Taking into account the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the ordered checkerboard structures.