Presence Of Subsurface Oxygen Research Articles

In Situ Infrared Spectroscopic Evidence of Enhanced Electrochemical CO2 Reduction and C-C Coupling on Oxide-Derived Copper.

The reaction mechanism of CO2 electroreduction on oxide-derived copper has not yet been unraveled even though high C2+ Faradaic efficiencies are commonly observed on these surfaces. In this study, we aim to explore the effects of copper anodization on the adsorption of various CO2RR intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) on metallic and mildly anodized copper thin films. The in situ SEIRAS results show that the preoxidation process can significantly improve the overall CO2 reduction activity by (1) enhancing CO2 activation, (2) increasing CO uptake, and (3) promoting C-C coupling. First, the strong *COO- redshift indicates that the preoxidation process significantly enhances the first elementary step of CO2 adsorption and activation. The rapid uptake of adsorbed *COatop also illustrates how a high *CO coverage can be achieved in oxide-derived copper electrocatalysts. Finally, for the first time, we observed the formation of the *COCHO dimer on the anodized copper thin film. Using DFT calculations, we show how the presence of subsurface oxygen within the Cu lattice can improve the thermodynamics of C2 product formation via the coupling of adsorbed *CO and *CHO intermediates. This study advances our understanding of the role of surface and subsurface conditions in improving the catalytic reaction kinetics and product selectivity of CO2 reduction.

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte‐Driven Nanostructuring

Production of multicarbon products (C2+) from CO2 electroreduction reaction (CO2RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO2RR catalysts that are highly selective for C2+ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO2 into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO3 solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm−2 for C2+ products at −0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C2+ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu+ species, and the adsorbed halides.

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte‐Driven Nanostructuring

AbstractProduction of multicarbon products (C2+) from CO2 electroreduction reaction (CO2RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO2RR catalysts that are highly selective for C2+ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO2 into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO3 solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm−2 for C2+ products at −0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C2+ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu+ species, and the adsorbed halides.

Nature, Stability and Distribution of Subsurface Oxygen in Copper Electrodes during the Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO2RR) allows the storage of energy in readily available chemicals such as ethylene and other hydrocarbons, while contributing to the abatement of CO2. Copper is the only pure metal able to perform CO2RR with appreciable activity and selectivity towards multi-carbon products, due to optimal binding of the key intermediate CO,1 especially when the electrode is nanostructured and derived from an oxide.2 Oxide-derived copper (OD-Cu) electrodes exhibit activity and ethylene selectivity higher than pristine copper during the carbon dioxide reduction reaction (CO2RR), and the presence of residual subsurface oxygen in OD-Cu is associated with such improvement.3 In order to establish a structure-activity relationship for the catalyst during reaction, operando investigation is necessary. Using in situ x-ray photoelectron spectroscopy (XPS), quasi in situ electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) and quasi in situ positron annihilation spectroscopy (PAS), we show that oxygen is primarily concentrated in an amorphous 1-2 nm thick layer on the OD-Cu surface (see Figure), it is stable during CO2RR for up to 1 hour at -1.15 V vs RHE and is associated with a high density of defects in the OD-Cu structure.4 Corroborated with density functional theory (DFT) calculations on copper nanoclusters we show that both the low-coordination of the amorphous OD-Cu surface and the presence of subsurface oxygen that withdraws charge from the copper d-band selectively enhance the binding energy of CO without altering that of other reaction intermediates, therefore breaking the scaling relation between d-band center vs CO binding energy.4,5 (1) Peterson, A. A.; Nørskov, J. K. Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3 (2), 251–258. (2) Roberts, F. S.; Kuhl, K. P.; Nilsson, A. High Selectivity for Ethylene from Carbon Dioxide Reduction over Copper Nanocube Electrocatalysts. Angew. Chemie 2015, 127, 5268–5271. (3) Eilert, A.; Cavalca, F.; Roberts, F. S.; Osterwalder, J.; Liu, C.; Favaro, M.; Crumlin, E. J.; Ogasawara, H.; Friebel, D.; Pettersson, L. G. M.; et al. Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction. J. Phys. Chem. Lett. 2017, 8 (1), 285–290. (4) Cavalca, F.; Ferragut, R.; Aghion, S.; Eilert, A.; Diaz-Morales, O.; Liu, C.; Koh, A. L.; Hansen, T. W.; Pettersson, L. G. M.; Nilsson, A. Nature and Distribution of Stable Subsurface Oxygen in Copper Electrodes During Electrochemical CO 2 Reduction. J. Phys. Chem. C 2017, acs.jpcc.7b08278. (5) Liu, C.; Lourenço, M. P.; Hedström, S.; Cavalca, F.; Diaz-Morales, O.; Duarte, H. A.; Nilsson, A.; Pettersson, L. G. M. Stability and Effects of Subsurface Oxygen in Oxide-Derived Cu Catalyst for CO 2 Reduction. J. Phys. Chem. C 2017, acs.jpcc.7b08269. Figure 1

Nature and Distribution of Stable Subsurface Oxygen in Copper Electrodes During Electrochemical CO2 Reduction

Oxide-derived copper (OD-Cu) electrodes exhibit higher activity than pristine copper during the carbon dioxide reduction reaction (CO2RR) and higher selectivity toward ethylene. The presence of residual subsurface oxygen in OD-Cu has been proposed to be responsible for such improvements, although its stability under the reductive CO2RR conditions remains unclear. This work sheds light on the nature and stability of subsurface oxygen. Our spectroscopic results show that oxygen is primarily concentrated in an amorphous 1–2 nm thick layer within the Cu subsurface, confirming that subsurface oxygen is stable during CO2RR for up to 1 h at −1.15 V vs RHE. Besides, it is associated with a high density of defects in the OD-Cu structure. We propose that both low coordination of the amorphous OD-Cu surface and the presence of subsurface oxygen that withdraws charge from the copper sp- and d-bands might selectively enhance the binding energy of CO.

Direct graphene growth on (111) Cu2O templates with atomic Cu surface layer

This work explores nucleation and epitaxy of graphene on crystalline Cu2O templates formed via self-assembly and surface reduction of Cu2O nanocrystallites on the cubic textured (100) orientation Cu (CTO-Cu) and polycrystalline Cu (poly-Cu) substrates, respectively. It has been found that the presence of sub-surface oxygen causes the reconstruction of Cu surface due to the formation of oriented Cu2O nanocrystallites at a low H2 gas flow. Self-assembly of the Cu2O nanocrystallites into a textured surface template provides direct nucleation sites for graphene growth after the oxygen-sublattice on the template surface is reduced. The atomic Cu surface layer provides advantages of high graphene growth rate due to the catalytic role of Cu and in-plane alignment of graphene nuclei. It is particularly important that the Cu2O crystallites have predominantly (111) orientation aligned to each other in the plane of the (100) CTO-Cu substrates, which allows epitaxy of graphene with much lower defect density as compared to that in the poly-Cu case. Since Cu2O (111) templates may be developed on lattice matched (100) surfaces of other dielectric materials, this self-assembly approach provides a promising pathway for large-scale, transfer free graphene epitaxy on nonmetallic surfaces.

Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd(111)

The formation of palladium subsurface oxygen, i.e., Pd(111) saturated with oxygen below the surface, has been demonstrated. This compound is much more stable than palladium surface oxide or bulk palladium oxide, which easily reacts with CO at elevated temperatures. The catalytic activity of an atomically clean Pd(111) surface and of Pd(111) affected by subsurface oxygen (coverage Θ = 0.00–0.46 ML) was studied during the CO oxidation reaction. The measurements were performed for a temperature range of 353–523 K, partial pressures of reactants in the range of 10–7–10–5 mbar, and different subsurface oxygen content. A distinct, previously not observed feature in the dependency of the reaction rate vs temperature was found. Furthermore, it was found that subsurface oxygen reduces the catalytic activity of the Pd(111) surface significantly, which is attributed to reduction of the sticking coefficient of CO and oxygen. In the temperature range applied for investigation of catalytic CO oxidation, the rate of the formation of subsurface oxygen is negligible. However, the formation of this phase starts to be pronounced above ∼600 K. The presence of subsurface oxygen was confirmed by the KLL Auger transition excited by monochromatic X-ray radiation, not by electron beam. It was observed that the classical Auger measurement with an electron beam destroys the subsurface oxygen compound. The combination of the CO titration with Auger peak intensity measurements allows the quantitative analysis of the subsurface oxygen. The problem of subsurface oxygen detection and its impact on the adsorption of CO and oxygen on Pd(111) are discussed. It was suggested that subsurface oxygen and surface oxide can be manufactured simultaneously. Then the obtained activity in the CO oxidation reaction will be a result of two competitive effects, i.e., promoting from surface oxide and inhibiting from subsurface oxide. This might be a source of the discrepancy in results on the influence of surface oxide on the CO oxidation reaction measured by different researchers.

Oxygen assisted reconstructions of rhodium and platinum nanocrystals and their effects on local catalytic activity of hydrogenation reactions

• Morphological changes of single nanoparticles occur during exposure to reactive gases. • The reconstructions are imaged operando by field emission techniques. • On rhodium, reconstructions have only a limited influence on the structure sensitivity of the O 2 –H 2 reaction at T > 500 K. • On platinum, the NO 2 –H 2 reaction occurs first on {0 1 1} facets, whereas (0 0 1) plane becomes active as the reconstruction proceeds. The reconstruction of rhodium and platinum crystals of some tens of nanometres diameter was investigated during the ongoing hydrogenation of oxygen atoms resulting from the dissociation of O 2 and NO 2 species. Field ion and field emission electron microscopies (FIM and FEM) were used to characterise the apex of tip samples before, during and after the catalytic reactions. On rhodium samples, the exposure of less than 10 Langmuir of O 2 is sufficient to induce significant morphological changes. At higher exposures, the presence of subsurface oxygen causes surface reconstructions illustrated with atomic resolution by FIM at 50 K. The same pattern is also visible at 505 K in the presence of H 2 and O 2 during water production. Upon the decrease of H 2 pressure, surface oxidation shows a strong sensitivity to the local surface initiated along the 〈0 0 1〉 zone lines. On platinum, the kinetic instabilities of the NO 2 –H 2 reaction are followed by FEM at 390 K starting from a hemispherical tip sample. The instabilities are expressed as surface explosions occurring randomly in time, but synchronised over {0 1 1} facets. These instabilities expand along the 〈0 0 1〉 lines over the (0 0 1) pole and exhibit self-sustained kinetic oscillations. The analysis of the tips by FIM after the reaction shows dark regions over the {1 1 3} facets, suggesting the extension of those to the detriment of vicinal ones. A well-controlled field evaporation procedure reveals that these regions appear dark due to the presence of surface oxygen. Structural reconstructions are observed but do not lead to the drastic morphological changes suggested by the FIM and FEM patterns. Nanoparticle dynamics must be accounted in models describing the non-linear features of catalytic reactions and more generally included in the description of catalytic properties of nanosized particles.

First-Principles Study of Carbon Monoxide Oxidation on Ag(111) in Presence of Subsurface Oxygen and Stepped Ag(221)

The role of subsurface oxygen in Ag(111) and the step edge of vicinal Ag(221) on CO oxidation were studied by using density functional theory calculations. For high oxygen coverage, the formation of subsurface oxygen is not only energetically favorable but also kinetically likely. CO oxidation with on-surface atomic oxygen is facile with a barrier of about 0.16 eV, whereas the reaction with subsurface oxygen is hindered because of the significant barrier for diffusion of the subsurface oxygen to the surface from the subsurface region. It is found that the adsorption of molecules (CO, atomic O and O2) is stabilized in the presence of subsurface oxygen, and surface reactivity is enhanced. An energetically favorable reaction pathway for CO oxidation with O2 is identified, with a reaction barrier of 0.23 eV via a unique four-center Ο2···CΟ intermediate. A catalytic cycle for CO oxidation on Ag(111) in the presence of subsurface oxygen is proposed. On Ag(221) surfaces, our calculations show that both the adsor...

Transient oxidation of CO over a catalyst in the presence of subsurface oxygen

We consider a lattice model to study the transient oxidation of CO molecules over a catalytic surface. A precovered surface containing a variable number of monolayers of oxygen is taken at the initial time. When a flux of CO molecules impinges the surface, we observe the formation of CO 2, due to the Langmuir–Hinshelwood mechanism. The flux of CO 2 molecules depends on the number of monolayers of oxygen and on temperature. The temperature enters through the desorption of CO molecules, diffusion of active sites over surface and diffusion of subsurface O atoms towards the topmost layer. We employ Monte Carlo simulations in our calculations, and the (0 0 1) surface and subsurface planes of a simple cubic lattice are considered. Our results explain the main features of the reactive scattering of CO molecules at the oxygen-rich Ruthenium surface observed in the experiments performed by Böttcher and coworkers.

Transient CO adsorption and the catalytic properties of surfaces

We show that in presence of subsurface oxygen, CO oxidation of oxygen precovered Ag~001! modifies the surface in such a way that CO adsorption is stabilized up to crystal temperatures of 160 K. When diffusion is activated new surface oxide phases form, characterized by oxygen both in surface and in subsurface sites and by a high density of states just below the Fermi level. Subsurface oxygen accumulation might therefore be relevant for the understanding of the origin of the pressure gap observed for ethylene epoxidation and other important catalytic reactions.

Monomer–dimer CO–O 2 catalytic reaction on (0 0 1) surface and subsurface of a simple cubic structure

The presence of subsurface oxygen has been observed experimentally in monomer–dimer (CO–O 2 and NO–CO) surface reactions. By the use of Monte Carlo simulation the effect of this subsurface oxygen in CO–O 2 catalytic reaction on (0 0 1) surface and subsurface of a simple cubic structure is studied. We have carried out detailed analysis of the effect of increasing the subsurface neighbours on the phase diagram of the CO–O 2 reaction. It is shown that the presence of subsurface completely eliminates the usual second-order phase transition of this reaction. It is also shown that increase in the number of subsurface neighbours shifts the single-transition point towards higher CO concentration value. When first, second and third subsurface nearest-neighbours are considered, the single-transition point is shifted to the stoichiometric ratio of 2/3.

Electrocatalytic Reduction of Peroxodisulfate in 0.5 M NaOH at Copper Electrodes. A Combined Quartz Microbalance and Rotating Ring/Disc Electrode Investigation

From an electrochemical investigation by means of an electrochemical quartz microbalance, a rotating disc electrode, and a ring/disc electrode, two mechanisms for the reduction of S2O82- became apparent. Besides the well-known outer-sphere cathodic reduction, a catalytic mechanism of S2O82- reduction operates in a potential range between the surface oxide region (≈−0.5 V/SCE) and −1.0 V/SCE. It involves the chemical oxidation of the copper surface to a soluble Cu(I) species. The catalytic mechanism is concluded to result from the specific interaction between S2O82- and the Cu surface modified by the presence of subsurface oxygen.

A combined environmental scanning electron microscopy and Raman microscopy study of methanol oxidation on silver(I) oxide

The techniques of environmental scanning electron microscopy (ESEM) and Raman microscopy have been used to respectively elucidate the morphological changes and nature of the adsorbed species on silver(I) oxide powder, during methanol oxidation conditions. Heating Ag2O in either water vapour or oxygen resulted firstly in the decomposition of silver(I) oxide to polycrystalline silver at 578 K followed by sintering of the particles at higher temperature. Raman spectroscopy revealed the presence of subsurface oxygen and hydroxyl species in addition to surface hydroxyl groups after interaction with water vapour. Similar species were identified following exposure to oxygen in an ambient atmosphere. This behaviour indicated that the polycrystalline silver formed from Ag2O decomposition was substantially more reactive than silver produced by electrochemical methods. The interaction of water at elevated temperatures subsequent to heating silver(I) oxide in oxygen resulted in a significantly enhanced concentration of subsurface hydroxyl species. The reaction of methanol with Ag2O at high temperatures was interesting in that an inhibition in silver grain growth was noted. Substantial structural modification of the silver(I) oxide material was induced by catalytic etching in a methanol/air mixture. In particular, "pin-hole" formation was observed to occur at temperatures in excess of 773 K, and it was also recorded that these "pin-holes" coalesced to form large-scale defects under typical industrial reaction conditions. Raman spectroscopy revealed that the working surface consisted mainly of subsurface oxygen and surface Ag=O species. The relative lack of subsurface hydroxyl species suggested that it was the desorption of such moieties which was the cause of the "pin-hole" formation.

Subsurface oxygen formation on the Pt(110) surface: experiment and mathematical modeling

Abstract Experiments using photo emission electron microscopy (PEEM) reveal that regions on a Pt(110) surface covered by chemisorbed O atoms may be converted into a subsurface O-phase, provided that it is preceded by the interaction of CO initiating the 1 × 2 → 1 × 1 transformation of the surface structure. However, the presence of subsurface oxygen also favors lifting of the surface reconstruction. A mathematical model of this process is developed using parameters derived from previous independent experiments and numerical simulations fitting new data to experimental findings.

In situ Raman studies of the selective oxidation of methanol to formaldehyde and ethene to ethylene oxide on a polycrystalline silver catalyst

The combined techniques of in situ Raman microscopy and scanning electron microscopy (SEM) have been used to study the selective oxidation of methanol to formaldehyde and the ethene epoxidation reaction over polycrystalline silver catalysts. The nature of the oxygen species formed on silver was found to depend critically upon the exact morphology of the catalyst studied. Bands at 640, 780 and 960 cm–1 were identified only on silver catalysts containing a significant proportion of defects. These peaks were assigned to subsurface oxygen species situated in the vicinity of surface dislocations, AgIIIO sites formed on silver atoms modified by the presence of subsurface oxygen and O2– species stabilized on subsurface oxygen-modified silver sites, respectively. The selective oxidation of methanol to formaldehyde was determined to occur at defect sites, where reaction of methanol with subsurface oxygen initially produced subsurface OH species (451 cm–1) and adsorbed methoxy species.Two distinct forms of adsorbed ethene were identified on oxidised silver sites. One of these was created on silver sites modified by the interaction of subsurface oxygen species, and the other on silver crystal planes containing a surface coverage of atomic oxygen species. The selective oxidation of ethene to ethylene oxide was achieved by the reaction between ethene adsorbed on modified silver sites and electrophilic AgIIIO species, whereas the combustion reaction was perceived to take place by the reaction of adsorbed ethene with nucleophilic surface atomic oxygen species. Defects were determined to play a critical role in the epoxidation reaction, as these sites allowed the rapid diffusion of oxygen into subsurface positions, and consequently facilitated the formation of the catalytically active AgIIIO sites.

Influence of oxidation and reduction conditions upon the morphology of silica-supported polycrystalline silver catalysts

The effect of oxidation and reduction conditions upon the morphology of polycrystalline silver catalysts has been investigated by means of in situ Fourier-transform infrared (FTIR) spectroscopy. Characterization of the sample was achieved by inspection of the νas(COO) band profile of adsorbed formate, recorded after dosing with formic acid at ambient temperature. Evidence was obtained for the existence of a silver surface reconstructed by the presence of subsurface oxygen in addition to the conventional family of Ag(111) and Ag(110) crystal faces.Oxidation at 773 K facilitated the reconstruction of silver planes due to the formation of subsurface oxygen species. Prolonged oxygen treatment at 773 K also caused particle fragmentation as a consequence of excessive oxygen penetration of the silver catalyst at defect sites. It was also deduced that the presence of oxygen in the gas phase stabilized the growth of silver planes which could form stronger bonds with oxygen. In contrast, high-temperature thermal treatment in vacuum induced significant sintering of the silver catalyst.Reduction at 773 K resulted in substantial quantities of dissolved hydrogen (and probably hydroxy species) in the bulk silver structure. Furthermore, enhanced defect formation in the catalyst was also noted, as evidenced by the increased concentration of formate species associated with oxygen-reconstructed silver faces.

An in Situ Fourier Transform Infrared Study of Formic Acid Adsorption on a Polycrystalline Silver Catalyst

Infrared spectra are reported for the adsorption of formic acid on a polycrystalline silver catalyst after various degrees of oxidation. Three distinct chemisorbed species were identified, two of which corresponded to adsorbed formate on Ag(110) and Ag(111) crystal planes and the other to adsorbed formate on a silver site modified by the presence of subsurface oxygen. Moreover, it was discovered that subsurface oxygen species were primarily located in the vicinity of grain boundary defects. Desorption profiles suggested that coadsorbed oxygen moieties could destabilize chemisorbed formate. Because of the invariance in infrared band positions associated with formate species, regardless of whether or not neighbouring atomic oxygen species were present, a kinetic stabiiization mechanism is proposed. The pivotal role of subsurface oxygen species found in the region of grain boundaries, in the mechanism for the selective oxidation of methanol to formaldehyde, is emphasised.

Characterization of the active site for the selective oxidation of methanol to formaldehyde on polycrystalline silver catalyst

Using in situ FTIR spectroscopy to compare the interaction of methyl formate and formic acid with a polycrystalline silver catalyst shows that methyl fromate is dissociatively chemisorbed on conventional silver faces such as (111) and (110) which contained nucleophilic adsorbed oxygen species while in contrast, formic acid is adsorbed not only on (111) and (110) silver planes but also on silver sites modified by the presence of subsurface oxygen; this behaviour supports a previously proposed model of the active site for the selective oxidation of methanol to formaldehyde on silver catalyst.

Mathematical modeling of kinetic oscillations in the catalytic CO oxidation on Pd(110): The subsurface oxygen model

Experimental investigations of the catalytic CO oxidation on a Pd(110) surface revealed that the temporal oscillations in the reaction rate (measured in a range 10−3 Torr<p<1 Torr) can be traced back to a periodic formation and depletion of oxygen in the subsurface region. Oscillations in the reaction rate arise because the variation in the subsurface oxygen concentration modulates the oxygen sticking coefficient and hence the catalytic activity. Based on the proposed reaction mechanism, which has been well supported by experimental data, a set of three coupled differential equations was established describing the variations in the absorbate coverages ΘO and ΘCO and in the subsurface oxygen concentration for the system Pd(110)/CO+O2. Numerical solutions from the mathematical model reproduce the essential qualitative and quantitative features of the experiment. Characteristic features in the experiment that indicate the presence of subsurface oxygen, such as a reversal of the usual clockwise hysteresis in the reaction rate, are also found in the simulation. The model reproduces the existence region for kinetic oscillations in good agreement with the experimental data and it exhibits similar bifurcation behavior to that observed in the experiment. The remaining, mainly quantitative, differences can be traced back to simplifications made in the formulation of the kinetics.