Oxygen chemisorption at Cu(110) at 120 K: dimers, clusters and mono-atomic oxygen states

Three distinct states of oxygen have been observed at a Cu(110) surface at 120 K by scanning tunnelling microscopy (STM): isolated oxygen adatoms; pairs or dimers, separated by about 6 Å and clusters of five or six atoms arranged anisotropically. There is also evidence for oxygen atoms undergoing ballistic motion as might be expected from “hot” oxygen atoms. Such states of oxygen have been central to the mechanistic models proposed earlier, and based on surface spectroscopic studies, for the oxidation of ammonia at copper surfaces under ammonia-rich conditions.     

Adsorption of Copper Ion with Metal Hydroxide from Ammonia Solution

Abstract

The removal of copper(II) from ammonia solution by adsorption on iron (III), aluminum(III), and tin(IV) hydroxide is studied. The effects of experimental parameters such as solution pH and concentration of total ammonia on adsorption are examined both from the change of solution composition and electrical properties of the solid surface. In a moderately high electrolyte concentration, an optimum solution pH is found for the solution composition when the sum of the species fractions of Cu(NH₃)²⁺, Cu(NH₃)₂ ²⁺, and Cu(NH₃)₃ ²⁺ reaches its maximum. The ligand number of aminecopper(II) of the solution is near 2.0 under this optimum condition. The decrease in adsorption in the more basic tetraamminecopper(II) solution is attributed to the competing reaction for copper(II) by the free ammonia in the solution. Adsorption isotherms at various concentrations of total ammonia show decreasing adsorption with high electrolyte concentration as a result of a highly positively charged surface and a reduction of available surface sites. The relative extent of adsorption is discussed for various experimental conditions based on the data and the surface complexation concept.     

Oxidation of NH3 on polycrystalline copper and Cu(1 1 0): a combined FT-IRAS and kinetics investigation

The kinetics of the reaction of oxidation of ammonia on polycrystalline copper, has been investigated, in a re-circulating reactor, and correlated to a characterisation of the catalyst surface at different extent of conversions.

The rapid formation of a nitride or oxynitride phase and its reactivity have been demonstrated. Under oxidising conditions, P_NH3=P_O2, and up to 650 K, dinitrogen is the only product of the reaction; N₂O being formed when T or P_O2 increases further. The correlation of these kinetics results to an in situ characterisation of the same reaction at RT by Fourier-transformed infrared reflection-absorption spectroscopy (FT-IRAS), on a well defined Cu(1 1 0) surface, led to the following conclusion: two reaction pathways contribute to the conversion of ammonia: (i) its decomposition on copper; (ii) the reaction between ammonia molecules and oxygen adsorbed from the gas phase. The major adsorbed species is oxygen; intermediate species are NH₂, NH and possibly HNO formed when the oxygen surface concentration is sufficient. Increasing the pressure of oxygen induces, at high T, the formation of nitrous oxide; N₂O results from an oxidation of the surface copper nitride or from the interaction of two surface HNO intermediates.     

Low temperature CO oxidation on Au(111) and the role of adsorbed water

This paper presents results of an investigation of low-temperature CO oxidation and the role of moisture on an atomic oxygen covered Au(111) surface by employing molecular beam scattering techniques under ultrahigh vacuum (UHV) conditions. The effect of atomic oxygen precoverage on CO oxidation was examined at sample temperatures as low as 77 K. Prompt CO₂ production was observed when the CO beam impinges on the sample followed by a rapid decay of CO₂ production in all cases. At oxygen precoverages above 0.5 ML, CO₂ production decreases with increasing oxygen precoverage primarily due to the decrease in CO uptake. CO oxidation at 77 K goes through a precursor mediated reaction mechanism, where CO is in a precursor or trapped state and oxygen atoms are in a chemisorbed state. The role of adsorbed water was studied by using isotopically labeled water [H ₂ ¹⁸ O] to distinguish the oxygen species from that used in oxygen atom exposures [¹⁶O]. Evidence is presented that shows activated water or OH groups formed from water can directly participate in oxidizing CO on an atomic oxygen covered Au(111) surface.     

Adsorption states of carbon monoxide on oxygenated Cu(110) faces

Oxygen preadsorption on Cu(110) surfaces strongly reduces the CO desorption peak at 220 K, typical for clean Cu(110) and induces the development of less tightly bound states, which probably correspond to sites on Cu(111) micro-facets, formed in the course of oxygen stimulated surface reconstruction. A smaller part of the CO molecules ( 20%) seems to interact with adsorbed oxygen to give adsorbed CO ₂ ^– which can be stabilized in the presence of CO₂ by formation of van der Waals complexes, e.g. [CO₂ · CO ₂ ^– ]. At increasing temperature this complex decomposes or disproportionates to give desorbing CO and adsorbed CO ₃ ^– . The interpretation is tentative, but some evidence is given to it by TDS from Cu(111), by XPS, STM and SIMS studies and by theoretical calculations.     

In situ STM study of the effect of chlorides on the initial stages of anodic oxidation of Cu(111) in alkaline solutions

In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the mechanisms of growth of passive layers on Cu(111) in NaOH solutions in the presence of chlorides. For [Cl⁻]/[OH⁻]=0.01, the same ordered precursor phase of adsorbed OH is observed in the underpotential region of oxidation as in Cl⁻-free solutions. Atomically resolved images reveal the structure of the reconstructed topmost metal plane and the threefold hollow adsorption site of the hydroxide. The induced reconstruction causes the ejection of Cu atoms that contribute to the observed lateral growth of the terraces and to the formation of 2D Cu ad-islands in the final stages of the adsorption process. For [Cl⁻]/[OH⁻]=0.1, threadlike nanostructures resulting from the reaction of the ejected Cu atoms with chlorides are formed before agglomeration with the 2D Cu ad-islands formed in the final stage of the hydroxide adsorption process. For [Cl⁻]/[OH⁻]=10, the step edges, which are normally the preferential sites of the reaction with hydroxide, are blocked by the formation of non-ordered surface chloride complexes. Hydroxide adsorption still predominates the surface reaction on the terraces but the 2D ad-islands form immediately due to the blocking of the step edges. In the potential range of Cu(I) oxide formation, crystalline Cu(I) oxide layers are formed with a high density of steps and (111) terraces. Their step edges are rougher in the presence of chlorides which indicates a Cl⁻-enhanced localized dissolution reaction of the oxide layers at step edges.     

Methanol synthesis over a Zn-deposited copper model catalyst

Methanol synthesis by the hydrogenation of CO₂ over Zn-deposited polycrystalline Cu was studied using surface science techniques. The Zn sub-monolayer was oxidized by the reaction mixture during the reaction at 523 K, leading to the formation of ZnO species. The kinetic results definitely showed that the ZnO species on the Cu surface promoted the catalytic activity of methanol formation, where the activity of Cu increased by a factor of 6 at the Zn coverage of 0.17. A volcano-shaped curve was obtained for the correlation between the Zn coverage and the catalytic activity, which was very similar to the correlation curve between the oxygen coverage and the specific activity for methanol formation previously obtained for the Cu powder catalysts. The role of ZnO in Cu/ZnO based catalysts was ascribed to the stabilization of Cu⁺ species by the ZnO moieties on the Cu surface.     

In situ characterization of interaction of ammonia with carbon surface in oxygen atmosphere

The adsorption and oxidation of ammonia over carbons differing in the chemical structure of surface functional groups have been investigated by FTIR spectroscopy. The reactions of NH₃ with carbons have been studied both in the presence and in the absence of oxygen. As a result of NH₃ chemisorption, in addition to ammonium salts, there are formed surface amide and imide structures. At the higher temperature surface isocyanate species are formed. Thermal stabilities of surface structures, formed as a result of NH₃ chemisorption have been determined by means of FTIR spectroscopy. The activity and selectivity of carbons for the selective catalytic oxidation (SCO) of NH₃ to N₂ with excess O₂ has been shown by microreactor studies at 295-623 K. Carbon catalysts are very active for NH₃ oxidation. Nitrogen is generally the predominant product of ammonia oxidation. The selectivity to N₂, N₂O and NO is determined by the surface oxygen coverage and reaction temperature. The data obtained indicate that the N₂ is formed via selective catalytic reduction (SCR) between NH_x surface species and NO formed from NH₄⁺ oxidation. This implies that ammonia is activated in the form of NH₄⁺ species for both SCR and SCO processes.     

Oxidation of methanol at copper surfaces

The role of preadsorbed oxygen present at Cu(111), Cu(110) and polycrystalline surfaces in the oxidation of methanol has been investigated by X-ray and electron energy loss spectroscopies. In addition to the well established formation of methoxy species and its subsequent decomposition and desorption as formaldehyde, a second reaction pathway to surface formate is present. The latter is temperature dependent being undetectable at 260 K at a polycrystalline surface but occurs at a significant rate at 295 K and above. The limitations of experimental data for methanol oxidation by temperature programmed desorption and molecular beam techniques are discussed.     

Catalytic CO Hydrogenation: Mechanism and Kinetics from Chemical Transients at Low and Atmospheric Pressures

The hydrogenation of CO over Co model catalysts was studied using relaxation-type methods operating in situ either at atmospheric pressures or under surface science conditions. Emphasis was laid on providing information on the surface composition and on how it changes with time under catalytic reaction conditions. Using pressure forcing in chemical transient kinetics (CTK), the build-up of the steady-state was studied at 503 K and atmospheric pressure to demonstrate that the active catalyst surface is not metallic but covered with carbon, oxygen and hydrogen in excess of a monolayer equivalent. Both build-up and backward transients suggest CO to act as the “monomer” which probably inserts into an O–H bond to form the primary surface complex necessary for hydrocarbon and oxygenate formation. Repetitive electric field pulses (pulsed field desorption mass spectrometry, PFDMS) at low pressures have allowed the CO dissociation kinetics on a nano-sized Co 3D crystal (“tip”) to be monitored in the millisecond time range. No evidence for the occurrence of the Boudouard reaction was obtained in either PFDMS or CTK. Adsorbed CH _x (x = 1–3) species were detected in small amounts demonstrating that CO dissociation is fast compared to carbon hydrogenation. Adsorbed Co-subcarbonyl species, Co(CO) _x were also detected by PFDMS and possibly mediate the necessary surface mobility during the initial restructuring of the catalyst. Surface carbon seems to inhibit Co-subcarbonyl formation.     

Oxygen States at Magnesium and Copper Surfaces Revealed by Scanning Tunneling Microscopy and Surface Reactivity

By a combination of STM and XPS a study of the dynamics of oxygen chemisorption at Mg(0001) at 295?K has revealed oxygen states involving nucleation sites and the development of hexagonal and square lattice structures; the hexagonal structures develop epitaxially with the Mg(0001) surface. There is extensive surface mobility with, at the early stage of chemisorption, oxygen states being observed at steps at the (1×1)-O adlayer and overlapping magnesium atoms. These are active in ammonia and hydrocarbon oxidation whereas at higher oxygen coverage the surface is inactive. The dissociative chemisorption of nitric oxide generates a surface characterized by the hexagonal oxide structure; nitrogen adatoms known to be present from the N(Is) spectra are disordered. The chemisorption of hydrogen chloride at a Cu(110) surface results in a c(2×2)–Cl structure with at high coverage domains 1.8 nm wide. A sub-surface oxygen state at Cu(110), although unreactive to ammonia, undergoes a chemisorption replacement (corrosive chemisorption) reaction with HCl at 295 K.     

Surface oxygen species and their reactivities in the mild oxidation of ethylene on cerium oxide studied by FT-IR spectroscopy

Surface oxidation of ethylene on cerium oxide was studied at mild temperatures (293–473K) by usingin situ FT-IR spectroscopy, and isotopic technique. Ethylene oxidation took place even at around 330 K on a well outgassed cerium oxide independent of the presence of gaseous O₂. At mild temperatures, the surface adsorbed product was mainly formate species. Adsorbed Superoxide (O ₂ ^– ) species was definitely observed at temperatures up to 373 K when gaseous oxygen was present. However isotopic experiment confirmed that the Superoxide was not the active form of oxygen due to the mild oxidation of ethylene. The principal oxygen species participating in the mild oxidation of ethylene was surface lattice oxygen which is supposed to be in O^–-like form created by an outgassing at high temperatures. The mild oxidation of ethylene could be also initiated by surface peroxide (O ₂ ^2– ) and O^– species which were formed via the adsorption of O₂ on a partially reduced cerium oxide.     

CO vibrational frequencies on methanol synthesis catalysts: a DFT study

Periodic, self-consistent density functional theory calculations are used to explain the observed decrease in the vibrational frequency of CO on methanol synthesis catalysts under severe reducing conditions (N.-Y. Topsøe, H. Topsøe, J. Mol. Catal. A Chem. 141 (1999) 95–105). Vibrational frequencies for CO on eight different models of the methanol synthesis catalyst surface have been determined. The calculated vibrational frequency of CO on Cu(111) (1/9 ML CO coverage) with 1/9 ML of Zn adatoms shows a decrease of between 15 and 38 cm⁻¹ from the corresponding calculated CO frequency on clean Cu(111) (2073 cm⁻¹). The calculated vibrational frequency of CO on Cu(111) with 1/9 ML of ZnO species shows a decrease of up to 72 cm⁻¹ from the CO stretch frequency on clean Cu(111). These calculated CO vibrational frequency decreases agree with the experimentally measured decrease (ca. 50 cm⁻¹), suggesting that Zn and/or ZnO species may be present in the vicinity of active Cu sites of methanol synthesis catalysts under highly reducing conditions. In addition, CO vibrational frequencies on partially oxidized Cu(111) surfaces are shown to increase from the corresponding frequencies on clean Cu(111), in agreement with experimental results.     

Structural aspects of chemisorption at Cu(110) revealed at the atomic level

We illustrate the impact that scanning tunnelling microscopy (STM) has made on our understanding of chemisorption and catalysis at metal surfaces at the atomic level by considering four examples where information from surface sensitive techniques was also available. The advantages of STM and the limitations of some of the other experimental methods are discussed. (1) Through a combination of STM and X-ray photoelectron spectroscopy (XPS) we have established that a number of distinct oxygen chemisorbed states can exist at a Cu(110) surface. These are metastable and temperature dependent. Furthermore, the presence of chemisorbed sulphur is shown to promote a specific oxygen state – isolated oxygen strings – which are likely to be more chemically reactive than the oxygen overlayer present at Cu(110). In this sense the sulphur is a structural promoter. (2) The oxidation of ammonia under ammonia-rich conditions results in the growth of imide (NH) strings at a Cu(110) surface and this has been followed quantitatively by STM. The reactive surface oxygen state participates in an oxydehydrogenation reaction generating NH-radical species which undergo surface diffusion and result in string growth. (3) Nitric oxide dissociates at Cu(110) to generate a two-phase system of chemisorbed nitrogen and oxygen adatoms. The oxygen is present in a well ordered (2 × 1) structure and the nitrogen in a (3 × 2) structure. The limitations of an earlier LEED study are discussed. (4) Structural aspects of chemisorbed sulphur generated by the dissociative chemisorption of hydrogen sulphide and methyl mercaptan are discussed. In the latter case carbon–sulphur bond cleavage results in the formation of a sulphide overlayer at 450 K with complete removal of carbon as desorbed hydrocarbons. Various sulphur structures have been delineated over a wide temperature range. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reactivity of Hydroxyl Species from Coadsorption of Oxygen and Water on Ni(110) Single-Crystal Surfaces

The formation and reactivity of hydroxyl species originating from coadsorption of water and oxygen on Ni(110) single-crystal surfaces have been studied by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The resulting surface population of hydroxyl intermediates at a given water–oxygen coverage combination was found to be temperature-dependent. This was demonstrated by the differences in hydroxyl coverages determined by TPD and XPS: while the TPD data were determined to mostly reflect the maximum coverages that can be reached for a given set of gas exposures at low temperatures, the XPS results measure the OH coverages formed at the temperature of dosing. Our results indicate that, besides the stoichiometric and reversible H₂O(ads) + O(ads) = 2OH(ads) step, a second water-decomposition reaction on the oxygen-precovered surface deposits additional hydroxyl adsorbates. Depletion of surface oxygen can be induced by thermal reaction with coadsorbed ammonia as well, a result that provides direct evidence for the OH(ads) disproportionation reaction.     

Adsorption and decomposition of NO on carbon and carbon-supported catalysts

The interactions of NO with carbon and carbon-supported catalysts have been investigated by means Fourier transform infrared spectroscopy. Nitric oxide direct decomposition over carbon-supported catalysts (Cu, Pt) was studied in a temperature ranging from 473 to 623 K. NO conversion increased with increasing reaction temperature in the whole temperature range. The carbon-supported Pt catalyst has a very high activity for the decomposition of NO in the absence of oxygen. As a result of NO chemisorption isocyanate (-NCO) species on the surface of carbon containing Cu were observed. When the reaction temperature was increased, the -NCO band at 2229 cm⁻¹ became more intense.     

An AM1 Study of Decomposition of Ozone on a Cu(110) Surface

The decomposition of ozone on a Cu(l10) surface has been studied using AM1 method. O₃ probably adsorbs on Cu(l10) to decompose into a molecular oxygen and an adsorbed atomic oxygen species, an adsorbed atomic oxygen reacts with O₃ to form an O₄ ^-intermediate, and O₄ ^- decomposes into two oxygen molecules. Intermediates, O₃ ^-, O₄ ^- and O₂ ^- are presumably involved in this process. Adsorbed atomic oxygen species are concluded to be stable on a Cu(l10) surface and to have low reactivity to O₃, which agrees with the experimental results in literature.     

Transient Studies of Direct N2O Decomposition over Pt–Rh Gauze Catalyst. Mechanistic and Kinetic Aspects of Oxygen Formation

For elucidating the mechanistic aspects of oxygen formation during N₂O decomposition over commercial woven Pt–Rh gauze, transient experiments were carried out in the temporal analysis of products (TAP) reactor by pulsing N₂ ¹⁶O over ¹⁸O-pretreated gauze catalyst at temperatures typical of industrial ammonia burners (1073–1273 K). The transient responses of N₂O and the products of its decomposition (O₂ and N₂) were fitted to two different mechanistic models. From the isotopic studies and the fitting of transient experiments, two separate routes of oxygen formation during catalytic N₂O decomposition have been identified. Oxygen is produced via both (i) interaction of N₂O with adsorbed oxygen species formed from N₂O and (ii) recombination of adsorbed oxygen species on the catalyst surface. The relative contribution of these reaction pathways depends on the reaction temperature.     

In situ high temperature FTIR studies of NOx reduction with propylene over Cu/ZSM-5 catalysts

High temperature in situ FTIR has been used to investigate the surface species present on Cu/ZSM-5 during the reduction of NO_x with propylene in a lean environment. Parallels have been observed between adsorbed surface species and catalytic activity for this reaction. Species detected at low temperatures are not representative of those detected at high temperatures where the catalyst is active. An oxidized nitrogen-containing species has been observed at 2580 cm^–1 on Cu during reaction conditions (400°C). In contrast, at low temperatures, where the catalyst is less active, coke and Cu⁺-CO predominated. The effects of Cu weight loading, C/NO ratio, reaction temperature, and catalyst deactivation by steaming have been investigated with IR.     

Methanol synthesis by the hydrogenation of CO2 over Zn-deposited Cu(111) and Cu(110) surfaces

The hydrogenation of CO₂ over Zn-deposited Cu(111) and Cu(110) surfaces was performed at 523 K and 18 atm using a high pressure flow reactor combined with XPS apparatus. It was shown that the ZnO _x species formed on Cu(111) during reaction directly promoted the methanol synthesis. However, no such promotional effect of the Zn was observed for methanol formation on Cu(110). Thus, Zn on Cu(111) acts as a promoter, while Zn on Cu(110) acts as a poison. The activation energy and the turnover frequency are in fairly good agreement with those obtained for Cu/ZnO powder catalysts.