Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides

Cui, Yi; Shao, Xiang; Baldofski, Matthias; Sauer, Joachim; Nilius, Niklas; Freund, Hans‐Joachim

doi:10.1002/anie.201305119

Cited by 81 publications

(99 citation statements)

References 32 publications

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“…Previous results have indicated that a Mo-doped CaO film has donor character and provides excess electrons to be transferred into adsorbates with high electron affinity [9,10]. As the oxide electronic structure has been monitored here, we can now prove this correlation for the example of O 2 adsorption.…”

Section: Interplay Between Electronic Structure and Adsorption Behsupporting

confidence: 71%

“…This conclusion has been corroborated by several earlier experiments that shall be summarized here [8]. First, Mo-doped CaO features a pronounced donor character, which leads to the formation of negatively charged Au atoms and to the activation of O 2 molecules towards superoxo oxygen upon adsorption [9,10]. The required charge transfer is possible only for Mo ions that have not yet reached their highest oxidation state, while Mo 5+ and Mo 6+ species are unsuitable donors [22,23].…”

Section: B Evolution Of the Mo Core-level Spectra During Cao Annealingsupporting

confidence: 62%

“…As recently demonstrated, traces of high-valence Mo incorporated into a CaO matrix drastically affect the growth behavior of electronegative metals and the adsorption and activation of molecules, e.g., O 2 , on the oxide surface [9,10,11]. In both case, the excess electrons from the Mo dopants were made responsible for the enhanced activity of the originally inert material.…”

Section: Introductionmentioning

confidence: 94%

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Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

et al. 2015

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The electronic structure of CaO films of 10-60 monolayer thickness grown on Mo(001) has been investigated with synchrotron-mediated x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Upon annealing or reducing the thickness of the film, a rigid shift of the CaO bands to lower energy is revealed. This evolution is explained with a temperature-induced diffusion of Mo ions from the metal substrate to the oxide and their accumulation in the interface region of the film. The Mo substitutes divalent Ca species in the rocksalt lattice and is able to release electrons to the system. The subsequent changes in the Mo oxidation state have been followed with high-resolution XPS measurements. While near-interface Mo transfers extra electrons back to the substrate, generating an interface dipole that gives rise to the observed band shift, near-surface species are able to exchange electrons with adsorbates bound to the oxide surface. For example, exposure of O 2 results in the formation of superoxo species on the oxide surface, as revealed from STM measurements. Mo interdiffusion is therefore responsible for the pronounced donor character of the initially inert oxide, and largely modifies its adsorption and reactivity behavior.

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Section: Interplay Between Electronic Structure and Adsorption Behsupporting

confidence: 71%

Section: B Evolution Of the Mo Core-level Spectra During Cao Annealingsupporting

confidence: 62%

Section: Introductionmentioning

confidence: 94%

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Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

et al. 2015

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“…[9] Defects will definitely fulfill a key function with respect to the activation of either methane or oxygen by changing the electronic structure of the wide band gap magnesium oxide particularly with regard to facilitate the transfer of electrons between the solid surface and the adsorbed molecules, which undergo a redox reaction. [10][11][12][13][14][15] The present work addresses relations between the nature and abundance of morphological surface defects such as steps and corners on pure magnesium oxide and its reactivity in the oxidative coupling of methane.…”

Section: Li-doped Mgo Was Discovered By Lunsford Et Al As An Active mentioning

confidence: 99%

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

Schwach

Hamilton

Thum

et al. 2015

Journal of Catalysis

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The role of surface structure and defects in the oxidative coupling of methane (OCM) was studied over magnesium oxide as a model catalyst. Pure, nano-structured MgO catalysts with varying primary particle size, shape and specific surface area were prepared by sol–gel synthesis, oxidation of metallic magnesium, and hydrothermal post treatments. The initial activity of MgO in the OCM reaction is clearly structure-sensitive. Kinetic studies reveal the occurrence of two parallel reaction mechanisms and a change in the contribution of these pathways to the overall performance of the catalysts with time on stream. The initial performance of freshly calcined MgO is governed by a surface-mediated coupling mechanism involving direct electron transfer between methane and oxygen. The two molecules are weakly adsorbed at structural defects (steps) on the surface of MgO. The proposed mechanism is consistent with high methane conversion, a correlation between methane and oxygen consumption rates, and high C₂H₄ selectivity after short times on stream. The water formed in the OCM reaction causes sintering of the MgO particles and loss of active sites by degradation of structural defects, which is reflected in decreasing activity of MgO with time on stream. At the same time, gas-phase chemistry becomes more important, which includes the formation of ethane by coupling of methyl radicals formed at the surface and the partial oxidation of C₂H₆. The mechanistic concepts proposed in this work (Part I) will be substantiated in Part II by spectroscopic characterization of the catalysts (Schwach et al. [1])

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“…The transition metal (TM) elements introduce new states in the gap of CaO or MgO that change the reactivity of the oxide surface. In particular, the TM atom can donate some of its valence electrons to other defects present in the structure or to adsorbed species like Au atoms and clusters or O 2 molecules with formation of negatively charged adsorbates [80][81][82][83]. The position of the defect states in the band gap of the ionic oxide (CaO or MgO) could critically depend on the level of theoretical treatment used, as discussed above.…”

Section: The Thermochemistry Problemmentioning

confidence: 99%

First Principles Calculations on Oxide-Based Heterogeneous Catalysts and Photocatalysts: Problems and Advances

Pacchioni

2014

Catal Lett

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Density functional theory (DFT) has become an essential complement of experiments to interpret, rationalize, and understand structures, spectroscopic data, microscopy analyses, etc. of relevance in heterogeneous catalysis and photocatalysis. However, one of the major goals of theory in catalysis remains the prediction of reaction enthalpies and entropies, of transition state structures, and the identification of reaction mechanisms. While accurate theoretical methods are currently available to study the thermochemistry of molecular systems, this is much less so when reactions involve solid surfaces and in particular oxide materials. The problem stems from the approximate nature of the exchange-correlation functionals used in all DFT approaches. Attempts to improve the accuracy of reaction energies is at the core of the efforts made in the past 30 years to generate new functionals. In this review we will discuss some recent advances in the theoretical description of oxides and their surfaces in heterogeneous catalysis and photocatalysis. In particular, we will focus on two problems: (1) the determination of an oxide band gap and the proper alignment of the occupied and unoccupied levels with respect to the vacuum level, an aspect relevant for the red-ox properties of the material, and (2) the calculation of reaction energies at oxide surfaces. To this end, we will discuss and comment on the performance of current implementations of exchange-correlation functionals (standard GGA, GGA?U, hybrid functionals, meta-GGA, etc.) or alternative approaches (like the quasiparticle GW method), in predicting band alignment and chemical reactivity at oxide surfaces. The role of dispersion forces will be also briefly discussed.

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Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides

Cited by 81 publications

References 32 publications

Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

First Principles Calculations on Oxide-Based Heterogeneous Catalysts and Photocatalysts: Problems and Advances

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