Copper Oxide Surfaces Research Articles

Redox-performance correlations in Ag–Cu–Mg–Al, Ce–Cu–Mg–Al, and Ga–Cu–Mg–Al hydrotalcite derived mixed metal oxides

Ag-, Ce- and Ga-promoted Cu–Mg–Al hydrotalcite derived mixed metal oxides were obtained by standard coprecipitation, followed by calcination. The obtained Ag(Ce, Ga)–Cu–Mg–Al–Ox mixed metal oxides were characterized with respect to their crystalline structure (XRD, TEM), texture (BET), surface acidity (NH3-TPD), redox properties (H2-TPR), chemical surface composition (XPS), and tested in the selective catalytic oxidation of ammonia into nitrogen and water vapour (NH3-SCO). The loading of Agy-, Cey-, or Gay–Cu5–Mg66−y–Al29 (y=0–1) had a clear effect on the catalytic performances. For materials with low metal loadings (y≤0.25), the redox properties determined the catalytic performances in NH3-SCO. The formation of easily reducible CuOx played a crucial role for enhanced catalytic activity at lower temperatures, with a drop in the selectivity to N2 at higher temperatures. Higher metal loading (y≥0.5) led to the formation of surface and bulk copper oxide species, and other aggregated metal oxide phases, which enhanced the catalytic activity for Ag–Cu–Mg–Al–Ox, and diminished activity for Ce(Ga)–Cu–Mg–Al–Ox.

Formation and characterization of infrared absorbing copper oxide surfaces

Copper oxide formation has been investigated to combine the advantages of producing different size and shapes of coatings that possess good light absorbing properties. An aqueous blackening solution was investigated and optimum composition was found as 2.5M NaOH and 0.225M NaClO to form velvet copper oxide films. A two-step oxidation mechanism was proposed for the blackening process by carefully examining the experimental results. Formation of Cu2O was observed until the entire copper surface was covered at first. In the second step, Cu2O surface was further oxidized to CuO until the whole Cu2O surface was covered by CuO. Therefore, blackened copper surfaces consisted of Cu2O/CuO duplex oxides. Characterization of the coatings were performed in terms of microstructure, phase analysis, chemical state, infrared specular and total reflectivity by SEM, XRD, XPS, FTIR and UV–vis spectrophotometry, respectively.

Corrosion and inhibition of copper in hydrocarbon solution on a molecular level investigated using neutron reflectometry and XPS

The behaviour and molecular structure of three organic additives adsorbed from oil onto a copper surface rich in copper (II) oxide is presented. The variation in the ability of these three agents to prevent corrosion of the copper oxide surface by exposure to sulphur is also included. A combination of visible observations, X-ray photoelectron spectroscopy (XPS) and neutron reflection (NR) are used to characterise both the adsorbed layer and underlying copper oxide. The model additives are chosen to represent ‘effective’, ‘intermediate’ and ‘ineffective’ inhibitors for copper.

Insight into reactive oxygen species in persulfate activation with copper oxide: Activated persulfate and trace radicals

Activated persulfate along with trace sulfate and hydroxyl radicals were found to be reactive oxygen species in copper oxide activated persulfate system in this study. Persulfate decomposition study revealed that activated persulfate was predominant instead of sulfate and hydroxyl radicals. The radical scavenger investigation supported this point and led to the surface activation mechanism of persulfate. The p-chloroaniline (PCA) degradation in copper oxide activated persulfate system in neutral and acidic condition provided the evidence of prime heterogeneous persulfate activation and minor homogeneous persulfate activation caused by dissolved copper ion. Electron Paramagnetic Resonance study indicated the generation of sulfate and hydroxyl radicals. Integrating all the experiment facts, it is proposed that activated persulfate was main reactive oxygen species generated on copper oxide surface, along with minor sulfate and hydroxyl radicals produced. Chloride ions showed no effect on PCA degradation in copper oxide activated persulfate system. While bicarbonate ions improved PCA degradation without pH adjustment (initial pH is about 3) and performed an inhibition effect when the initial pH was adjusted to neutral. These findings indicated the potential applicability of copper oxide for in situ chemical oxidation for two reasons: (1) ubiquitous chloride and bicarbonate ions showed no negative effect and (2) the slight radical generation avoid the persulfate consumption by non-pollutant reductive materials.

Promoting Atoms into Delocalized Long-Living Magnetically Modified State Using Atomic Force Microscopy.

We report on a low-temperature atomic force microscropy manipulation of Co atoms in ultrahigh vacuum on an oxidized copper surface in which the manipulated atom is kept delocalized above several surface unit cells over macroscopic times. The manipulation employed, in addition to the ubiquitous short-range tip-generated chemical forces, also long-range forces generated via Friedel oscillations of the metal charge density due to Co nanostructures prearranged on the surface by lateral manipulation. We show that our manipulation protocol requires mechanical control of the spin state of the Co atom.

Electrochemical Reduction of CO2 on Copper Oxidized By Electrochemical Methods

The electrochemical oxidation of copper surface in alkaline solution results in the formation of different products, which are a function of scan rate and concentration of OH- ions. It has been found that, the electro-oxidation results in oxide film on copper surface consist of Cu2O, Cu(OH)2 and CuO. The scanning electron microscopy (SEM) images of copper foils oxidized by linear sweep voltammetry at different scan rate show a systematic growth of oxides on the copper surface. At low scan rate growth of oxide takes place resulting in needle like structures projecting outwards from copper surface. Further, the density of these needles of oxides decreases as the scan rate increases. A similar behaviour was also observed in the chronoamperometry experiments performed at different potential and for different time duration. The length and the density of needles increase with increase in time when the applied potential was in potential region of peak B. On the other hand, at more negative potential and low concentration OH- ions instead of needles, a layer of oxide particles were observed. The x-ray photoelectron spectroscopy (XPS) analysis of oxidized copper surface shows the presence of Cu2O, Cu(OH)2and CuO in the oxide layer and the percentage of CuO increases as the density of needles increases on the surface. It has found that the catalytic activity of the copper obtained by reducing it from its oxides is more than that of the original polycrystalline copper foil for CO2reduction. The oxidation of copper surface can be achieved either by annealing in air or oxygen, or by electrochemical oxidation. Further, electrochemical oxidation of copper foil can be performed either by potential sweep or potential step techniques. Here, the oxidation of copper surface was carried out by chronoamperometry (CA), linear sweep voltammetry (LSV) as well as cyclic voltammetry (successive oxidation and reduction cycles) in Ar saturated 0.5 M KOH solution. The copper foil was oxidized by CA at -0.1 V for 30 minutes and LSV at scan rate of 0.5 mV/s from -1.2 to 0.5 V in Ar saturated 0.5 M KOH. The five cycles of CVs were carried out at a scan rate of 0.5 mV/s in Ar saturated 0.5 M KOH between -1.2 to 0.5 V. The SEM images shows that the needle like growth of oxides gets reduced back to copper in first few minutes of the electrochemical reaction when applied potential is more negative than hydrogen evolution. These in turn have been used for electrochemical reduction of CO2. The main product of CO2 reduction in aqueous phase was found to HCOO- and the gas phase analysis revealed predominantly hydrogen molecule. Faradaic efficiency for formic acid on copper foil oxidized by CV was higher than that for copper foil oxidized by the CA and LSV. The reason this observation may be the amount of active copper produced on the polycrystalline copper from the reduction of the copper oxides. This has been verified from the amount of charge transferred during the oxidation of copper which is highest for the CVs as with each cycle the peak current of both the peaks observed during oxidation of copper increases. During oxidation of copper foil by CV in fifth cycle a total charge of ~0.70 C/cm2 charge was transferred, whereas, during LSV a total of ~0.45 C/cm2 and during CA at total of ~0.30 C/cm2 of charge was transferred. Thus indicating more oxidation during CV than LSV and CA, and hence more amount of active copper will be produced on the copper surface resulting in increased faradaic efficiency of formic acid. Figure 1

(Keynote) Radiation Induced Corrosion of Copper for Spent Nuclear Fuel

The influence of radiation on various forms of corrosion is a complex and largely unexplored issue. In principle, radiation can influence physicochemical processes in the whole interfacial region which trigger corrosion: in the gas, liquid and solid phases, and in the interfaces in-between. This presentation highlights results and conclusions from a recently completed doctoral study on radiation induced corrosion of copper (ref. 1). The storage of nuclear waste in Sweden is planned to be based on copper canisters with cast iron inserts and surrounded by bentonite which will be deposited at a depth of 500 meters in ground water saturated granitic bedrock. Gamma radiation from the fuel will penetrate the copper canisters and be absorbed by the surrounding bentonite clay. When water in the clay absorbs gamma radiation aqueous radiolysis will occur. Two oxidants from aqueous radiolysis, hydrogen peroxide (H2O2) and the hydroxyl radical (HO∙), have significantly higher standard reduction potentials than copper, and are therefore thermodynamically capable of initiating corrosion of the canisters. The aim of the present work was to explore the effect of total gamma dose on the corrosion behavior of copper under well controlled conditions. A series of experiments were performed, in which samples of copper taken from a canister received a number of different total doses. Three different dose rates of gamma radiation were used and the experiments were performed in pure, anoxic, water, in air of 60 to 100 % relative humidity, and in humidified argon. The total doses of gamma radiation used were within the range relevant for a geological deep repository for spent nuclear fuel. Qualitative characterization of the corrosion products formed on the copper surface was performed using IRAS, XPS, SEM and AFM, quantitative characterization of corrosion products by cathodic reduction, and trace elemental analysis of copper on all solutions by inductively coupled plasma atomic emission spectroscopy, ICP-AES. The overall conclusion is that gamma radiation causes significantly enhanced corrosion of copper in anoxic aqueous solution in comparison to non-irradiated samples. The main corrosion product formed on the copper surface during gamma irradiation is cuprite (Cu2O), while only a small fraction consists of Cu(II) compounds. The thickness of the oxide layer, after irradiation at dose rates of 370 and 770 Gyh-1and total doses of 35.5 and 74 kGy, was measured to be 50–100 nm (refs 2 and 3). Dissolution of copper during irradiation depends on the total absorbed dose, also on the nature of the pre-formed oxide. Complementary exposures were performed in which the kinetics and mechanisms of the stable radiolysis product H2O2 reacting with copper and copper oxides were explored. The results clearly show that aqueous radiolysis only can account for very small fraction of the radiation induced corrosion of copper (ref. 4). Further studies were performed in which the influence of pre-grown copper oxides was explored. Radiation-induced corrosion of copper is significantly more effective on pre-oxidized copper than on polished copper, and the total amount of oxidized copper exceeds the amount expected solely from radiation chemistry of water by more than three orders of magnitude. Based on the results a mechanism has been proposed in which the HO∙ radical is the main radiolytic oxidative species driving the corrosion process. When taking all results from different exposures into consideration the major mechanism found so far to explain the dramatically enhanced radiation induced corrosion of copper is proposed to be an enhanced chemical yield of HO∙ interacting with the copper oxide surface and an increased reactive copper oxide surface area (ref. 5).

Cyclohexane oxidative dehydrogenation over copper oxide catalysts

Here, we report on structure-reactivity trends for cyclohexane oxidative dehydrogenation (ODH) with silica supported copper oxide catalysts as a function of surface structure. Copper oxide was supported on mesostructured KIT-6 silica at low surface densities <0.2Cu/nm2 using copper (II) nitrate, ammonium and sodium copper (II) ethylenediaminetetraacetate, and a hexanuclear copper (I) siloxide complex. Copper oxide surface structures were characterized by X-ray absorption spectroscopy as well as ambient and in situ diffuse reflectance UV–visible (DRUV–vis) spectroscopy to determine trends in copper oxide nuclearity. DRUV–vis spectroscopy identifies three copper species based on Cu2+ ligand to metal transfer (LMCT) bands at 238, 266, and >300nm as well as Cu+ LMCT bands at 235, 296, and 312nm. Counterintuitively, EXAFS analysis shows that the multinuclear precursor leads to fewer average Cu–Cu interactions than syntheses with mononuclear copper salt precursors. Turnover frequency and selectivity to benzene increase with decreasing copper oxide nuclearity, and thus the multinuclear precursor leads to the highest turnover frequency and benzene production. This work shows the variety of surface species that exist even at extremely low copper surface densities, control of which can improve reactivity of an atypical ODH catalyst up to rates comparable to benchmark vanadia catalysts.

A density functional theory study of the adsorption behaviour of CO2 on Cu2O surfaces.

Copper has many applications, particularly in electro-catalysis, where the oxidation state of the copper electrode plays a significant role in the selectivity towards products. Although copper-based materials have clear potential as catalysts in the reduction of CO2 and conversion to products, fundamental understanding of CO2 adsorption and activation on different copper oxide surfaces is still limited. We have used DFT+U methodology to study the surface reconstruction of the three most exposed (111), (110), and (001) surfaces of Cu2O with different possible terminations. Considering several adsorbate geometries, we have investigated CO2 adsorption on five different possible terminations and proposed eight different configurations in which CO2 binds with the surface. Similar to earlier findings, CO2 binds weakly with the most stable Cu2O(111):O surface showing no molecular activation, whereas a number of other surfaces, which can appear in the Cu2O particles morphology, show stronger binding as well as activation of the CO2 molecule. Different CO2 coverages were studied and a detailed structural and electronic charge analysis is presented. The activation of the CO2 molecule is characterized by structural transformations and charge transfer between the surface and the CO2 molecule, which is further confirmed by considerable red shifts in the vibrational frequencies.

Calculations of oxide formation on low-index Cu surfaces.

Density-functional theory is used to evaluate the mechanism of copper surface oxidation. Reaction pathways of O2 dissociation on the surface and oxidation of the sub-surface are found on the Cu(100), Cu(110), and Cu(111) facets. At low oxygen coverage, all three surfaces dissociate O2 spontaneously. As oxygen accumulates on the surfaces, O2 dissociation becomes more difficult. A bottleneck to further oxidation occurs when the surfaces are saturated with oxygen. The barriers for O2 dissociation on the O-saturated Cu(100)-c(2×2)-0.5 monolayer (ML) and Cu(100) missing-row structures are 0.97 eV and 0.75 eV, respectively; significantly lower than those have been reported previously. Oxidation of Cu(110)-c(6×2), the most stable (110) surface oxide, has a barrier of 0.72 eV. As the reconstructions grow from step edges, clean Cu(110) surfaces can dissociatively adsorb oxygen until the surface Cu atoms are saturated. After slight rearrangements, these surface areas form a "1 ML" oxide structure which has not been reported in the literature. The barrier for further oxidation of this "1 ML" phase is only 0.31 eV. Finally the oxidized Cu(111) surface has a relatively low reaction energy barrier for O2 dissociation, even at high oxygen coverage, and allows for facile oxidation of the subsurface by fast O diffusion through the surface oxide. The kinetic mechanisms found provide a qualitative explanation of the observed oxidation of the low-index Cu surfaces.

Catalytic properties of a copper transducer for determining flow parameters in a high-frequency induction plasmatron

The rate coefficients of the elementary stages of the complete system of heterogeneous catalytic recombination of dissociated oxygen on a copper oxide surface are determined on the basis of quantum-mechanics calculations within the framework of cluster models. The coefficients are used to calculate the dependence of the effective coefficient of heterogeneous catalytic recombination of oxygen atoms on the temperature and the partial pressure on a wide range of surface conditions. It is established that it can considerably vary depending on these conditions.

Heat transfer coefficient and critical heat flux during nucleate pool boiling of water in the presence of nanoparticles of alumina, maghemite and CNTs

The characteristics of nucleate pool boiling were investigated with nanofluids in a chamber. Nanoparticles of alumina, maghemite and carbon nanotubes were added separately to distilled water. Particle volumetric concentrations of 0.02% and 0.10% were applied. Experiments with water on the heater copper surface covered with nanoparticles were conducted to evaluate the effect of deposition. Boiling curves, critical heat fluxes and heat transfer coefficients were obtained and compared to reference data for water. The wettability was determined from contact angle measurements. In the experiments with the nanofluids, the nanoparticles were deposited on the test surface. The static contact angle for the water on the copper oxide surface decreased from around 85° to 0° following the deposition of the nanoparticles. Values for the critical heat flux increased from around 26–37% for all test cases. These results are attributed to the complete wetting behavior presented by the nanoparticles deposited on the test surface. Estimates obtained using the Kandlikar (2001) correlation deviated from the experimental values by less than 9%. The heat transfer coefficients were the same or lower in comparison with the reference case.

Nature of the Copper-Oxide-Mediated C–S Cross-Coupling Reaction: Leaching of Catalytically Active Species from the Metal Oxide Surface

Copper-oxide-catalyzed cross-coupling reaction is a well-known strategy in heterogeneous catalysis. A large number of applications have been developed, and catalytic cycles have been proposed based on the involvement of the copper oxide surface. In the present work, we have demonstrated that copper(I) and copper(II) oxides served as precursors in the coupling reaction between thiols and aryl halides, while catalytically active species were formed upon unusual leaching from the oxide surface. A powerful cryo-SEM technique has been utilized to characterize the solution-state catalytic system by electron microscopy. A series of different experimental methods were used to reveal the key role of copper thiolate intermediates in the studied catalytic reaction. The present study shows an example of leaching from a metal oxide surface, where the leaching process involved the formation of a metal thiolate and the release of water. A new synthetic approach was developed, and many functionalized sulfides were synthe...

Thiol Adsorption on and Reduction of Copper Oxide Particles and Surfaces.

The adsorption of 1-dodecanethiol at room temperature and at 75 °C on submicron cuprous and cupric oxide particles suspended in ethanol has been investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy. Thiol adsorption occurs in all cases via Cu-S bond formation, with partial dissolution of CuO at 75 °C and formation of a copper-thiolate complex replacement layer. Regardless of temperature, the surface of the CuO particles is essentially completely reduced to either Cu2O or metallic copper, as evidenced by loss of the characteristic Cu(2+) XPS features of dried powder samples. Companion ultrahigh-vacuum studies have been performed by dosing clean, oxygen-dosed, and ozone-treated single crystal Cu(111) with methanethiol (MT) gas at room temperature. In the latter case, the surface corresponds to CuO/Cu(111). XPS confirms MT adsorption in all cases, with an S 2p peak binding energy of 162.9 ± 0.1 eV, consistent with methanethiolate adsorption. Heating of MT-covered Cu(111) and oxygen-dosed Cu(111) leads to decomposition/desorption of the MT by 100 °C and formation of copper sulfide with an S 2p binding energy of 161.8 eV. Dosing CuO/Cu(111) with 50-200 L of MT leads to only partial reduction/removal of the CuO surface layers prior to methanethiolate adsorption. This is confirmed by ultraviolet photoelectron spectroscopy (UPS), which measures the occupied states near the Fermi level. For both the colloidal CuO and single crystal CuO/Cu(111) studies, the reduction of the Cu(2+) surface is believed to occur by formation and desorption of the corresponding dithiol prior to thiolate adsorption.

In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of CuIII Oxides as Catalytically Active Species

Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, in situ Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu2O, Cu(OH)2, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu(OH)2 during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a CuIII oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCuIIIO2 standard, we suggest that the chemical composition of the CuIII oxide is CuO2–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurement...

Copper oxide surfaces modified by alkylphosphonic acids with terminal pyridyl-based ligands as a platform for supported catalysis

Self-assembled monolayer (SAM) films of phosphonates have been successfully formed via reaction of 11-hydroxyundecylphosphonic acid or 4,4′-di(methylenephosphonic acid)-2,2′-bipyridine with the oxide layer of copper via the Tethering by Aggregation and Growth (TBAG) deposition method. The hydroxyl-terminated SAM was further modified with isonicotinic acid or 4,4′-dicarboxy-2,2′-bipyridine through a Steglich esterification reaction. These three surfaces derivatized with pyridyl-based ligands are potential platforms for supported catalysis. As a proof of concept, [Ru(CO)3Cl2]2 was bound to the surfaces through the pyridyl-based ligands to yield tethered analogs of the known carbon dioxide reduction catalyst, [Ru(bpy)(CO)2Cl2]. Surface modification reactions were confirmed through specular reflectance infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Characteristic core binding energies were observed in the XPS analyses for phosphorus (P 2p), nitrogen (N 1s), and ruthenium (Ru 3p and Ru 3d), verifying the presence of the various surface functionalizations. IR and XPS data indicate that the phosphonate binding to the copper surface is tridentate in nature.

Surface treatment of copper (II) oxide nanoparticles using citric acid and ascorbic acid as biocompatible molecules and their utilization for the preparation of poly(vinyl chloride) novel nanocomposite films

In the present study, solution casting method was used for the preparation of nanocomposite films. Primarily, the surface of copper (II) oxide (CuO) nanoparticles was modified with biosafe molecules such as citric acid and ascorbic acid for better dispersion in the polymer matrix. Then novel nanocomposite films were fabricated by loading various percentages of modified CuO nanoparticles in the poly(vinyl chloride) matrix. The ultraviolet (UV)-blocking effects of nanocomposites and their optical properties were studied using UV-visible spectroscopy. Also, other analyses, including Fourier transform infrared spectroscopy, mechanical tensile test, thermogravimetric analysis, and X-microscopy, were performed for investigating thermal, mechanical, and morphological properties of the hybrid materials.

Reducing the graphene grain density in three steps

The production of large-scale, single crystalline graphene is a requirement for enhancing its electronic, mechanical, and chemical properties. Chemical vapor deposition (CVD) has shown the potential to grow high quality graphene but the simultaneous nucleation of many grains limits their achievable domain size. We report here that ultralow nucleation densities can be achieved through multi-step optimization of the catalyst morphology. First, annealing in a hydrogen-free environment is required to retain a surface copper oxide which decreases the nucleation density. Second, CuO was found to be the relevant copper species for this process and air oxidation of the copper foil at 200 °C maximizes its concentration. Both pre-treatment steps were found to affect the morphology of the catalyst and a direct correlation between nucleation density and surface roughness was found which indicates that the primary role of the oxidation step is the decrease in catalyst roughness. To further enhance this determining parameter, confined CVD was carried out after sample oxidation and hydrogen-free annealing. Each of these three steps reduces the grain density by approximately one order of magnitude resulting in ultralow nucleation densities of 1.23 grains/mm2 and high quality, single-crystalline graphene grains of several millimeter sizes were grown using this method.

Insights into the synergistic role of metal–lattice oxygen site pairs in four-centered C–H bond activation of methane: the case of CuO

Of the three mechanisms for activation of methane on copper and copper oxide surfaces, the under-coordinated Cu–O site pair mediated mechanism on CuO surfaces has the lowest activation energy barriers.

Organics on oxidic metal surfaces: a first-principles DFT study of PMDA and ODA fragments on the pristine and mildly oxidized surfaces of Cu(111).

Metal-organic hybrid materials are ubiquitous and a fundamental understanding of the hybrid-interface is key for the development of these hybrid material systems. In this work, using first-principles density-functional theory (including van der Waals (vdW) corrections), we study the fundamental physico-chemical properties of the molecular fragments of pyromellitic dianhydride oxydianiline (PMDA-ODA) on pristine Cu(111), as well as oxidic p4:O/Cu(111) in order to investigate the effect of mild oxidation of the metal substrate on PMDA-ODA adsorption. Firstly, we report the most favorable adsorption geometries amongst the various surface models and correlate the adsorption behavior with the electronic structure of the molecular fragments and the substrate layer. PMDA adsorbs weakly on both the clean and mildly oxidized copper surface via vdW forces while ODA adsorbs much stronger with a significant charge transfer between the substrates. Here, the oxidic layer is found to reduce the adsorption strength of both fragments and in particular, the ODA molecule interacts with the substrate via additional hydrogen bonding. Finally, our simulated scanning tunneling microscopy (STM) images suggest possible orientations of PMDA and ODA on clean and oxidic Cu surfaces to guide future experiments.