Reactive phase of oxygen on Cu(100) at 100 K studied by HREELS and TPD (original) (raw)
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The Journal of Physical Chemistry B, 1997
The reactivity of oxygen adatoms formed by dissociative adsorption of O 2 on Cu(100) at 100 K for CO oxidation and hydrogen abstraction from water was investigated by means of HREELS and TPD in comparison with the reactivity of adsorbed oxygen in thermally stable phases. The oxygen adatoms formed on the Cu(100) surface by exposure to O 2 at 100 K, designated as as-exposed oxygen, were found to be reactive with coadsorbed CO to yield a CO 2 desorption peak at 125 K in TPD. The as-exposed oxygen atoms are suggested to be more active for CO oxidation than the oxygen atoms prepared above room temperature on Pt and Pd, which are the most active metals for CO oxidation. On annealing the oxygen-as-exposed surfaces to 300 K, a change in the loss feature of ν(Cu-O) was observed, which was indicative of formations of a pseudo c(2×2)-O phase with O(a) in 4-fold hollow sites and a (2×2 2)R45°-O phase comprised of-Cu-O-chains grown along the [001] direction. On these two phases, CO 2 formation in TPD was suppressed by 1 order of magnitude. Isolated oxygen atoms in the as-exposed surface are responsible for the high reactivity for CO oxidation. Hydrogen abstraction from water was also examined as a probe reaction for oxygen adatoms on different phases. The as-exposed surface and the pseudo c(2×2)-O phase were found to be reactive and OH(a) was detected by means of HREELS after subsequent exposure to water at 100 K. In contrast, the (2×2 2)-R45°-O phase in which oxygen atoms were incorporated into-Cu-O-chains was inert.
Surface Science, 1996
The catalytic reactivity of oxygen atoms on unreconstructed Cu(110) surface, designated as as-adsorbed oxygen, was investigated by mass spectrometry, AES, LEED and HREELS. It was found that the catalytic COO 2 reaction proceeded on unreconstructed Cu(ll0) by the as-adsorbed oxygen at 200-230 K, while the reaction did not occur above 250 K. The growth of the (2 × 1)-O structure caused the halt of the reaction above 200 K. CO 2 and N20 formation in CO-NO reaction on Cu(ll0) was also observed at 200 K. In contrast, reactivity of oxygen atoms on Cu(lll) was much lower than the as-adsorbed oxygen on Cu(ll0).
Oxygen adsorption and stability of surface oxides on Cu(111): A first-principles investigation
Physical Review B, 2006
As a first step towards gaining microscopic understanding of copper-based catalysts, e.g., for the lowtemperature water-gas shift reaction and methanol oxidation reactions, we present density-functional theory calculations investigating the chemisorption of oxygen, and the stability of surface oxides on Cu͑111͒. We report atomic geometries, binding energies, and electronic properties for a wide range of oxygen coverages, in addition to the properties of bulk copper oxide. Through calculation of the Gibbs free energy, taking into account the temperature and pressure via the oxygen chemical potential, we obtain the ͑p , T͒ phase diagram of O/Cu͑111͒. Our results show that for the conditions typical of technical catalysis the bulk oxide is thermodynamically most stable. If, however, formation of this fully oxidized surface is prevented due to a kinetic hindering, a thin surface-oxide structure is found to be energetically preferred compared to chemisorbed oxygen on the surface, even at very low coverage. Similarly to the late 4d transition metals ͑Ru, Rh, Pd, Ag͒, sub-surface oxygen is found to be energetically unfavorable.
On-surface and sub-surface oxygen on ideal and reconstructed Cu(100)
Surface Science, 2005
In order to understand the first steps of the Cu(1 0 0) oxidation we performed first principles calculations for on-surface and sub-surface oxygen on this surface. According to our calculations, the adsorption energies for all on-surface site oxygen atoms increase, whereas the energies of the sub-surface atoms decrease with the increasing oxygen coverage. At coverage 1 ML and higher on the reconstructed surface, structures including both on-and sub-surface atoms are energetically more favourable than structures consisting only of on-surface adsorbates. On the ideal (1 0 0) surface this change can be perceived at coverage 0.75 ML.
Adsorption and dissociation of O 2 on the Cu 2 O(1 1 1) surface: Thermochemistry, reaction barrier
The adsorption and dissociation of O 2 on the perfect and oxygen-deficient Cu 2 O(1 1 1) surface have been systematically studied using periodic density functional calculations. Different kinds of possible modes of atomic O and molecular O 2 adsorbed on the Cu 2 O(1 1 1) surface are identified: atomic O is found to prefer threefold 3Cu site on the perfect surface and O vacancy site on the deficient surface, respectively. Cu CUS is the most advantageous site with molecularly adsorbed O 2 lying flatly over singly coordinate Cu CUS –Cu CSA bridge on the perfect surface. O 2 adsorbed dissociatively on the deficient surface, which is the main dis-sociation pathway of O 2 , and a small quantity of molecularly adsorbed O 2 has been obtained. Further, possible dissociation pathways of molecularly adsorbed O 2 on the Cu 2 O(1 1 1) surface are explored, the reaction energies and relevant barriers show that a small quantity of molecularly adsorbed O 2 disso-ciation into two O atoms on the deficient surface is favorable both thermodynamically and kinetically in comparison with the dissociation of O 2 on the perfect surface. The calculated results suggest that the presence of oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of O 2 and can obviously improve the catalytic activity of Cu 2 O, which is in agreement with the experimental observation.
The Journal of Chemical Physics, 2008
Surface oxidation of Cu͑100͒ has been investigated by variable temperature scanning tunneling microscopy and quantitative x-ray photoelectron spectroscopy as a function of O 2 pressure ͑8.0 ϫ 10 −7 and 3.7ϫ 10 −2 mbar͒ at 373 K. Three distinct phases in the initial oxidation of Cu͑100͒ have been observed: ͑1͒ the formation of the mixed oxygen chemisorption layer consisting of Cu͑100͒-c͑2 ϫ 2͒-O and Cu͑100͒-͑2 ͱ 2 ϫ ͱ 2͒R45°-O domains, ͑2͒ the growth of well-ordered ͑2 ͱ 2 ϫ ͱ 2͒R45°-O islands, and ͑3͒ the onset of subsurface oxide formation leading to the growth of disordered Cu 2 O. We demonstrate that the ͑2 ͱ 2 ϫ ͱ 2͒R45°-O reconstruction is relatively inert in the low pressure regime. The nucleation and growth of well-ordered two-dimensional Cu-O islands between two ͑2 ͱ 2 ϫ ͱ 2͒R45°-O domains is revealed by time-resolved scanning tunneling microscopy experiments up to 0.5 ML of oxygen. The formation of these islands and their nanostructure appear to be critical to the onset of further migration of oxygen atoms deeper into copper and subsequent Cu 2 O formation in the high pressure regime. The reactivity of each phase is correlated with the surface morphology and the role of the various island structures in the oxide growth is discussed.
The presence of atomic oxygen on catalytic surfaces is essential for initiating the oxidation of hydrogen chloride to produce chlorine via the so-called Deacon process. This process provides molecular chlorine for the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) in combustion. In this paper, the dissociative adsorption of molecular oxygen on the Cu(OO I) surface has been studied using density functional theory. A periodic p(3 X 2) 4 layer slab was adopted to simulate the adsorption of both molecular and atomic oxygen at a number of adsorption sites. We have found that a bridge-bridge configuration is the most stable structure on Cu(OO I) with the Oc molecule adsorbed horizontally. The activation barrier for the dissociative adsorption of 0 2 resulting from this configuration was calculated to be 5.1 kcal/mol, with an equivalent transition temperature of-66 K. This is in good agreement with the experimental value of 40 K obtained under ultra high vacuum conditions. We have also found that a less energetically favourable, vertically oriented, physisorbed structure leads to an almost negligible reaction barrier for the dissociative adsorption of 0 2 on Cu(OOI) (1.5 kcal/mol), with an equivalent transition temperature of-20 K.
Adsorption of atomic and molecular oxygen on Cu(1 0 0)
Catalysis Today, 2005
We have studied the initial stages of the oxidation of the Cu(100) surface using ab initio calculations. Both atomic and molecular oxygen are addressed. We show that subsurface oxygen is not energetically favourable, but gets stabilized by on-surface O. We discuss the adsorption of molecular oxygen using elbow plots, which can be used in order to qualitatively understand the measured
Density functional study of oxygen on Cu(100) and Cu(110) surfaces
Physical Review B, 2010
Using density-functional theory within the generalized gradient approximation, we investigate the interaction between atomic oxygen and Cu͑100͒ and Cu͑110͒ surfaces. We consider the adsorption of oxygen at various on-surface and subsurface sites of Cu͑100͒ for coverages of 1/8 to 1 monolayers ͑ML͒. We find that oxygen at a coverage of 1/2 ML preferably binds to Cu͑100͒ in a missing-row surface reconstruction, while oxygen adsorption on the nonreconstructed surface is preferred at 1/4 ML coverage consistent with experimental results. For Cu͑110͒, we consider oxygen binding to both nonreconstructed and added-row reconstructions at various coverages. For coverages up to 1/2 ML coverage, the most stable configuration is predicted to be a p͑2 ϫ 1͒ missing-row structure. At higher oxygen exposures, a surface transition to a c͑6 ϫ 2͒ added strand configuration with 2/3 ML oxygen coverage occurs. Through surface Gibbs free energies, taking into account temperature and oxygen partial pressure, we construct ͑p , T͒ surface phase diagrams for O/Cu͑100͒ and O/Cu͑110͒. On both crystal faces, oxygenated surface structures are stable prior to bulk oxidation. We combine our results with equivalent ͑p , T͒ surface free energy data for the O/Cu͑111͒ surface to predict the morphology of copper nanoparticles in an oxygen environment.
Stability and Effects of Subsurface Oxygen in Oxide-Derived Cu Catalyst for CO2 Reduction
The Journal of Physical Chemistry C, 2017
Oxide-derived copper (OD-Cu) catalysts are promising candidates for the electrochemical CO 2 reduction reaction (CO 2 RR) due to the enhanced selectivity towards ethylene over methane evolution, which has been linked to the presence of subsurface oxygen (O sb). In this work, O sb is investigated with theoretical methods. Although O sb is unstable in slab models, it becomes stabilized within a "manually" reduced OD-Cu nanocube model which was calculated by self-consistent charge density functional tight binding (SCC-DFTB). The results obtained with SCC-DFTB for the full nanocube were confirmed with subcluster models extracted from the nanocube, calculated with both density functional theory (DFT) and SCC-DFTB. The higher stability of O sb in the nanocube is attributed to the disordered structure and greater flexibility. The adsorption strength of CO on Cu(100) is enhanced by O sb withdrawing electron density from the Cu atom resulting in reduction of the σ-repulsion. Hence, the coverage of CO may be increased, facilitating its dimerization.