Content uploaded by Beytullah Afsin
Author content
All content in this area was uploaded by Beytullah Afsin on May 10, 2019
Content may be subject to copyright.
The role of a dioxygen precursor in the selective formation of imide NH(a) species at a Cu(110) surface
Abstract
The coadsorption of dioxygen and ammonia results in a highly selective oxy-dehydrogenation reaction to form just chemisorbed imide NH(a) species at a Cu(110) surface at 298 K. The latter have been characterised by both core-level, N(1s), and vibrational (HREEL) spectroscopy. The role of a transient dioxygen precursor O2-(s) is discussed.
... Roberts' argued [4] that using such dynamic surface science methods is necessary to gain information on relevant intermediates in catalytic reactions, and also to explore possible reaction pathways that ''static'' surface science experiments would not detect. The validity of this approach has already been demonstrated with the imide species on Cu(110) [48,49] but has yet to be exploited in the case of the hydroxyls. ...
From the earliest studies of heterogeneous catalysis, it was apparent that water plays a more important role in many systems than simply acting as a solvent. Its wide ranging effects have attracted increasing attention in recent years and was the topic of Prof. M.W. Roberts’ final paper. The present review explores some of the latest work on water in reactions ranging from CO oxidation to Fischer–Tropsch catalysis, the different mechanisms proposed for its role are discussed and compared.
IntroductionScanning Tunneling MicroscopyAtomic Force MicroscopyConclusion
References
The reactivity of water at Ag(111), Zn(0001) and Cu(111) surfaces studied by X-ray photoelectron and high resolution electron spectroscopies at low temperatures (80–295 K) is reviewed. Emphasis is given to the coadsorption approach in revealing mechanisms for oxygen–water reactions with comparisons made with previous studies and the models proposed for ammonia–oxygen reactions at metal surfaces. Transient dioxygen–water precursor states result in facile and extensive surface hydroxylation with implications for catalytic oxidation. The orientation of the hydroxyl states with respect to the surface normal are sensitive to the oxygen–water ratio and for high oxygen ratios (200:1) an unreactive oxide overlayer forms at Cu(111) and Zn(0001). The significance of weakly adsorbed complexes, dioxygen–water and carbon dioxide–water, is discussed and, provided they are not kinetically controlled, are models for reactions at high pressures and temperatures with insights to the mechanisms of “real” catalytic reactions. Although scanning tunnelling microscopy has revealed “complex and unpredictable” water structures at low temperatures at metal surfaces, their reactivity and surface dynamics have received less attention.
Graphical Abstract
Density functional theory calculations are presented for adsorption and dissociation of CH4 on clean and oxygen atom pre-adsorbed metal surfaces (Cu, Ag, Au, Ni, Pd, Pt, Ru, Rh, Os, Ir, and Mo). The total energy change and the activation barrier have been calculated for the direct and the oxygen-assisted cleavage of the C–H bonds. Our results indicate that pre-adsorbed oxygen promotes the CH4 dissociation process on IB group metal surfaces, but inhibits the dissociation process on transition metal surfaces. A good Brønsted–Evans–Polanyi correlation for CH4 dissociation on clean and atomic oxygen pre-adsorbed metal surfaces is found, which is helpful to reveal the nature of CH4 dissociation. From the analysis of activation barrier, we expect our work can provide a clear understanding of the nature of CH4 dissociation.
Water dissociation on clean and oxygen-preadsorbed transition metal surfaces was investigated by the DFT-GGA method. The total energy change and the reaction barrier were calculated with respect to the direct and oxygen-assisted cleavage of O-H bonds of water. The calculated results showed periodic trends for water dissociation on both clean and oxygen-preadsorbed surfaces. On clean surfaces, the chemical activity for water dissociation increases in the order of Au < Ag < Cu < Pd < Rh < Ru < Ni; on oxygen-preadsorbed surfaces, the inducing effect decreases in the above order with a strongly facilitating O-H bond cleavage on the inactive metals An, Ag, and Cu; a moderate inducing effect on Pd and Rh; and little effect on Ru and Ni surfaces. We found an interesting relationship between the adsorption energy of oxygen atom on the given metal surfaces and the difference between the activation barrier of clean metal surfaces and that of oxygen-preadsorbed metal surfaces. That is, the more strongly the oxygen atoms bound to the metal surface, the less promoting effect they have on the water O-H bond scission. Our results are in general agreement with previous experimental observations. (c) 2006 Elsevier Inc. All rights reserved.
The role of basicity in the surface reactions of amines has been investigated in an STM/XPS study of the oxidation of methylamine, ethylamine and 2,2,2-trifluoroethylamine at a Cu(110) surface. Methylamine and ethylamine are strongly basic molecules and for sub monolayer concentrations of oxygen form an amine/oxygen complex characterised by a (3×1) structure. The complex rapidly decomposes at room temperature to form water and an imide which desorbs via β-elimination. In contrast, 2,2,2-trifluoroethylamine has a relatively low basicity and its reactions with surface oxygen do not involve a stable intermediate.
Dissociative adsorption of oxygen on Cu(110) forms isolated, added CuO rows at low coverages and p(2 × 1)-O islands at high coverages. The reactivity of both types of species for ammonia oxydehydrogenation has been investigated quantitatively by in situ imaging with an atomic-resolution scanning tunneling microscope. We find that oxygen in both forms is reactive with ammonia. At low coverages all oxygen adatoms are consumed readily, while at high precoverages reaction proceeds from the boundaries of oxygen islands. Though the reaction can be initiated from both the end and the side of the CuO rows, the reactivity appears to be anisotropic, being higher along the [001] direction. The reactivity of transient oxygen species formed during the exposure of the surface to a mixture of ammonia and oxygen is estimated to be comparable to that of the reactivity of preadsorbed oxygen at low coverages.
Adsorption states of oxygen on Cu(111) at 100–300 K were investigated by means of HREELS. Two molecular species were characterized by different OO stretching frequencies (v(OO)) at 610 cm−1 and 820–870 cm−1, which are assigned to the peroxo-like species (O2−2) adsorbed in a bridged form and the one in a bidentate form bound on an atop site, respectively. The bridged peroxo species is preferred at the low coverage and the atop peroxo species becomes dominant at the higher coverage. In addition to the peaks due to the molecular oxygen, a peak assigned to v(CuO) of atomic oxygen was observed at 370 cm−1 at the high coverage. The frequency of this mode was higher than the frequency reported for Cu(111) exposed to oxygen above 300 K, indicating that the adsorption state of atomic oxygen formed at 100 K is different from that above 300 K. The v(OO) modes became faint after annealing to 170 K because of O2 dissociation. The v(CuO) mode of the atomic oxygen formed at 100 K remained up to 230 K and disappeared after annealing to 300 K. No desorption of O2 was detected on annealing to 300 K. It was also found that vibrational spectra for adsorbed NH3 are influenced by the adsorption states of atomic oxygen on Cu(111).
A low energy pathway for the formation of a thermally stable Pt(111)N(2 × 2) overlayer is available when either dioxygen and ammonia are coadsorbed at Pt(111) or a Pt(111)O overlayer is exposed to ammonia at 295 K, although in the latter case there is also evidence for incomplete dehydrogenation. Synthesis of the nitride overlayer occurs under conditions where ammonia adsorption is negligible (θ < 0.01) and dissociative chemisorption of dinitrogen is not feasible. XPS, VEELS and LEED are used to follow surface nitride formation which is driven by the overall thermodynamics of the reaction.
The exact sequence of events involving surface trapping or precursor states is not understood even for the dissociative chemisorption of simple diatomic molecules at metal surfaces. It was to search for experimental evidence for such states in the Zn(0001)-dioxygen system that coadsorption studies, using the ammonia molecule as a specific probe for oxygen transient states, were initiated. Using a combination of photoelectron and vibrational spectroscopies it was shown that a dioxygen-ammonia surface complex provides a highly efficient low-energy pathway for dioxygen bond cleavage leading to chemisorbed amide, hydroxide and oxide species. Surface species have been identified by both core-level and electron energy-loss spectroscopies with XPS providing quantitative surface concentration data. The kinetics exhibit all the characteristics of a precursor-mediated reaction including the rate increasing with decreasing temperature. In the absence of ammonia, dioxygen bond cleavage is highly inefficient at a Zn(0001) surface. Central to the model developed is the concept of an ammonia molecule undergoing surface diffusion (hopping) acting as a chemical trap for reactive oxygen transients and leading to the formation of strongly chemisorbed species which can be spectroscopically identified. Although the dynamics of molecule-surface interactions are being pursued by molecular-beam studies the strategy developed here provides an insight to the chemistry associated with short-lived surface oxygen trapping states. The results have implications for the energetics of bond breaking, the efficiency of accommodation of molecules and catalysis at metal surfaces, it also highlights the limitations of the more static approach to unravel reaction mechanisms at surfaces in that the Zn(0001) oxide overlayer is unreactive to ammonia; a clear distinction must be made between 'preadsorption' and 'coadsorption'.
The rate of dissociative chemisorption of dioxygen at a Zn(0001) surface at 110 K increases by a factor of nearly 103 when coadsorbed with ammonia; a model based on a precursor state involving an ammonia–dioxygen complex is suggested.
Assignment of O(Is) spectra to hydroxyl species present on a Cu(111) surface has been confirmed by selective reduction of a mixed oxy-hydroxide adlayer by CO. The assignment enables definitive comments to be made on the role of chemisorbed oxygen in inducing the chemisorption, through a hydrogen abstraction mechanism of C2H4 on a Cu(111)-O surface. Spectroscopic observations made with other adsorbates (NH3, H2O, H2S, and HCOOH) are discussed.
Ammonia adsorbs without dissociation on clean Ag(110) with a binding energy of 11 kcal/mol. Coadsorption of ammonia and atomic oxygen at 105 K produces adsorbed hydroxyl groups and NHx species. Coadsorption of ammonia and molecular oxygen leads to the stabilization of molecular oxygen, as is shown by the increase in the desorption peak temperature of dioxygen from 180 to 210 K. The reaction of ammonia with both forms of adsorbed oxygen produces the same products at the same temperatures. Water desorbs in a series of peaks at 310, 340, and 400 K resulting from hydroxyl recombination and hydrogen transfer from NHx species to adsorbed oxygen atoms. NO and N2 desorb together at 530 K. Oxygen recombination at 590 K only occurs following small ammonia doses such that excess oxygen persists on the surface. No hydrogen was seen to desorb under any reaction conditions. Vibrational spectroscopy shows that NH groups persist on the surface at temperatures well into the water desorption peak at 310 K and possibly t
Molecular water adlayers at Ag(111) and Cu(100) surfaces undergo facile hydroxylation on exposure to dioxygen at low temperatures. XP and HREEL spectra distinguish between OH(a), H2(a) and O-2(a) and in the case of Ag(111) surfaces establish that O-2(a) is the reactive species. At Cu(100) the evidence points to a transient O-2(s) species. These results establish that preadsorbed chemisorbed oxygen adatoms are not a pre-requisite for the activation of molecularly adsorbed water and provide further evidence for the high chemical reactivity of dioxygen species present transiently in the disociative chemisorption of oxygen at metal surfaces.