Binary Neutral Metal Oxide Clusters with Oxygen Radical Centers for

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Binary Neutral Metal Oxide Clusters with Oxygen Radical Centers for Catalytic Oxidation Reactions: From Cluster Models Toward Surfaces Melanie Nößler,† Roland Mitrić,‡ and Vlasta Bonačić-Koutecký†,§,* †

Insitut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany Fachbereich Physik, Freie Universität Berlin, Arnimallee, 14, D-14195 Berlin, Germany § Interdisciplinary Center for Advanced Sciences and Technology (ICAST), University of Split, Meštrovićevo Šetalište 45, 2100 Split, Croatia ‡

ABSTRACT: We present theoretical results based on DFT calculations of the reactivity of binary neutral stoichiometric Zrn‑1ScO2n clusters with oxygen radical centers toward CO and acetylene with the aim to build a bridge between clusters and surface models. Following the concept of the same total valence electron count, the studied series of binary clusters mimics cationic stoichiometric (ZrO2)n+ species characterized also by the presence of an oxygen radical center. In the case of Zr11ScO24, representing a section of the (ZrO2)n bulk, in addition to the oxygen radical center, the presence of neighboring Zr atoms effectively promotes oxidation reactions toward CO and acetylene, showing the importance of the surrounding of active centers.

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INTRODUCTION Recent economic and environmental requirements led to increased activity directed toward development of alternative fuels and reduction of pollutants and of the energetic requirements for chemical production. In order to achieve these three goals, research in the field of heterogeneous catalysis became of increasing importance.1−6 An ideal catalyst should have a tailored surface to convert reactants directly into the desired products without producing byproduct. A combinatorial method of preparing and testing different materials is commonly used for the development of heterogeneous catalysts.7,8 This approach is inherently inefficient and the knowledge about exact mechanisms of catalytic reactions and the active sites might significantly improve the efficiency of catalyst design. Surface chemistry methods have proven to be valuable in characterizing the surface structure of the catalyst materials as well as for identification of intermediates.2−6 However, the identification of structure−reactivity relationships is difficult due to the presence of different active sites as well as other surface inhomogeneities. In order to design more efficient catalysts, determination of the structure−reactivity relationship is mandatory and can be achieved by cluster studies.9−22 Moreover, cluster-like surface chemical bonds involving surface metal and next nearest neighbors were identified to enable catalytic reactions.5 Therefore, the identification of molecular-level interactions occurring within a heterogeneous catalytic process can provide a method of optimizing the route to production of the desired chemical species. Thus, a considerable theoretical and experimental effort by us10,12,18−22 and others15−17,23−26 has been undertaken in © 2012 American Chemical Society

recent years with the aim to gain molecular level understanding of important catalytic reactions. The resulting findings allowed us to reveal mechanisms of oxidation reactions of CO and small organic molecules catalyzed by transition-metal oxide clusters containing oxygen radical centers depending on the size, composition, and charge state. In particular, our common studies with the experimental group of A. W. Castleman Jr. led to identification of radical centers for the cationic zirconium and titanium oxide clusters of stoichiometric composition (ZrO2)n+ and (TiO2)n+ that are responsible for the oxidation reactions with CO, ethylene and acetylene.18,22 By adding an oxygen atom with a full octet of valence electrons (O2−) to the stoichiometric cationic clusters, a series of anionic zirconium oxide clusters (ZrnOn+1)− with radical centers has also been produced.19 However, their reactivity toward hydrocarbons is inferior with respect to cationic species. In addition to charged species, which are suitable for the gas phase experimental investigation, we proposed to design neutral metal oxide clusters with oxygen radical centers by replacing one Zr atom by an atom that has one less or one more electron (e.g., scandium or niobium) to mimic cationic and anionic clusters with radical centers. We succeeded to show the validity of the same total valence electron count (termed isoelectronic) concept for the reactivity of neutral ZrScO4 and ZrNbO5 species with oxygen radical centers that are responsible for the oxidation reactions.20 In this contribution, we present theoretical results based on DFT calculations on the reactivity of the neutral stoichiometric Received: February 16, 2012 Revised: April 2, 2012 Published: April 23, 2012 11570

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Zrn‑1ScO2n (n = 4−7,12) clusters toward CO and acetylene with the aim to build a bridge between the cluster and surface models. The series of clusters with n = 4−7 serves to study the influence of binary metal oxide cluster size on the reactivity, and the Zr11ScO24 species corresponding to the section of the (ZrO2)n bulk allows to study the influence of the next nearest neighbors on oxidation reactions.

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COMPUTATIONAL The structural properties of the neutral Zrn‑1ScO2n (n = 4−7) and Zr11ScO24 clusters and their reactivity were studied using DFT with the hybrid B3LYP functional.27−29 For the Zr and Sc atoms the triple-ζ-valence-plus-polarization (TZVP) atomic basis sets combined with the Stuttgart group relativistic effective core potentials were employed.30−33 For the C, O, N, and H atoms the TZVP basis sets were used.34 Our previous studies of the reactivity of transition metal oxides have shown that such a combination of hybrid density functionals with triple-ζ quality basis sets allows the accurate prediction of the reaction energetics and mechanisms.10,12,18,19 All structures presented were fully optimized using gradient minimization techniques and stationary points were characterized as minima or transition states by calculating the vibrational frequencies. Moreover, from the energy profiles based on energies obtained from DFT calculations the reaction mechanisms were deduced.

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RESULTS AND DISCUSSION Following the concept of the isoelectronic effects, the Zrn‑1ScO2n (n = 4−7,12) series of species characterized by the presence of an oxygen radical center as shown in Figure 1

Figure 2. The energy profiles in eV for the oxidation of CO by (a) Zr3ScO8, (b) Zr4ScO10, (c) Zr5ScO12, and (d) Zr6ScO14. E‡ labels the transition state.

oxidation involves the charge transfer from the formed CO2 subunit to the cluster requiring relatively small barriers E‡ of approximately 0.3 to 0.86 eV which are easy to overcome due to excess energy available from the initially formed stable complexes. After formation of complexes with linear CO2 subunits, the desorption of CO2 molecules requires a relatively small amount of energy so that oxidation reactions for all four clusters proceed exothermically (ΔE = 0.9 − 1.2 eV). In summary, due to similar stabilization of the complex as well as similar energies of reaction barriers the CO oxidation reactions are not substantially influenced by the studied cluster sizes. The situation is more subtle in the case of acetylene oxidation since the energies of the formation of initial complexes and the barriers for hydrogen transfer are very similar as can be seen from the calculated energy profiles shown in Figures 3 and 4. The mechanism of oxidation of acetylene according to eq 2

Figure 1. Optimized ground state structures for Zrn‑1ScO2n, n = 4−7 and for Zr11ScO24. The radical oxygen centers are indicated by an arrow. The gray isosurfaces indicate localized spin density. The Sc atom is labeled by gray color.

mimics stoichiometric cationic clusters. In the case of n = 4−7 clusters, the oxygen radical center in the optimized structures is bound to the Zr atom which is in addition coordinated with the other three oxygen atoms. In contrast, in the case of Zr11ScO24 the coordination of the Zr atom to which the radical center is attached increases. Moreover, the presence of a not fully coordinated additional Zr atom indicates that the neighborhood of the reactive center may be important for the stability of the complexes built with CO or C2H2 molecules. This is a decisive factor to overcome energetically the barriers for the oxidation reactions, in particular for the hydrogen transfer necessary for the oxidation of acetylene. The calculated energy profiles for the CO oxidation by the series of the Zrn‑1ScO2n (n = 4−7) clusters according to eq 1 shown in Figure 2 exhibit common features. Zrn − 1ScO2n + CO → Zrn − 1ScO2n − 1 + CO2

Zrn − 1ScO2n + C2H 2 → Zrn − 1ScO2n − 1 + C2H 2O

(2)

involves formation of a strong bond of one carbon atom to the radical oxygen center and weaker binding of both carbon atoms to the Zr atom. Stabilization of complexes is slightly larger for clusters with n = 3,5 (1.8−1.83 eV) than for species with n = 4,6 (1.71−1.74 eV) (cf. Figures 3 and 4). In the next step, the hydrogen atom is transferred from the oxygen-bound carbon atom to the other carbon atom of acetylene involving transition states with energies ranging from 1.72 to 1.77 eV for all the four cluster sizes. However, the barriers are slightly higher than the energies available from formation of complexes in the case of clusters with n = 4,6 as shown in Figure 4.

(1)

The binding of CO to the radical oxygen center stabilizes the formed complexes for all considered cluster sizes by similar amounts of energy (∼1.7 eV). The mechanism of the CO 11571

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Figure 3. Energy profile in eV for the oxidation of C2H2 by (a) Zr3ScO8 and (b) Zr5ScO12.

In contrast, for species with n = 3,5 the energies of barriers for hydrogen transfers are slightly below the energies available from initial complex formations (cf. Figure 3). Notice that the quantitative values of barriers lie within the accuracy of the method. The resulting complexes after the hydrogen transfer has occurred are slightly more stable for species with n = 3,5 than for those with n = 4,6. However, in the former cases, an additional small amount of energy is needed to break the carbon−zirconium bonds. Nevertheless, the cleavage of ethenone requires between 1.02 and 1.3 eV. Altogether the reactions are exothermic as can be seen from Figures 3 and 4, although the barriers for species with n = 4,6 might prevent oxidation reactions while in the case of clusters with n = 3,5 the oxidation of acetylene is more likely to occur. This means that for a given cluster size, the balance between stability of the initial complex and the height of the barriers for the hydrogen transfer will determine whether oxidation of acetylene will proceed or not.

Figure 4. Energy profile in eV for the oxidation of C2H2 by (a) Zr4ScO10 and (b) Zr6ScO14.

In contrast to the above-described cases, the Zr11ScO24 representing an optimized section of the (ZrO2)n bulk offers an opportunity to neighboring Zr atoms close to the Zr-bound oxygen radical center to interact with CO and C2H2 as well. As shown in Figures 5 and 6, this is particularly important for the oxidation of acetylene. In the case of the reaction with CO, the formed complex involves CO bridging the oxygen radical center with the carbon atom and one of the neighboring Zr atoms that is not fully coordinated with the oxygen atom. The stability of this complex is 2.33 eV and the barrier for breaking the Zr−O radical bond is 1 eV (cf. Figure 5). The resulting complex in which CO2 is bound to the neighboring Zr atom does not need much energy 11572

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atoms promotes effectively oxidation reactions toward CO and acetylene, showing the importance of the surrounding of active centers. Notice that the Sc atom present in the binary metal oxides plays the role of mimicking the removal of one electron, analogous to the pure cationic stoichiometric zirconium oxide clusters. This effect might also be achieved by the presence of suitable defect centers at surfaces. Thus, the presented findings open a new route for proposing the construction of efficient catalysts and stimulate new experiments in which creation of radical centers might be induced during the interaction of metal oxide thin films [e.g., (ZrO2)x or (TiO2)x] with the reactants promoting industrially important oxidation reactions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 5. Energy profile in eV for oxidation of CO by Zr11ScO24.

ACKNOWLEDGMENTS R.M. acknowledge the support by the DFG in the framework of the Emmy Noether program. M.N. thanks to the Fazit-Stiftung for financial support.

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REFERENCES

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Figure 6. Energy profile in eV for oxidation of C2H2 by Zr11ScO24.

to cleave this subunit. Altogether, the oxidation reaction is exothermic and the energy barrier does not prevent the process. In the case of acetylene oxidation by the Zr11ScO24 cluster, the presence of a not fully coordinated Zr atom in the neighborhood of the oxygen radical center plays a particularly important role. The formation of a considerably stable complex by 2.26 eV arises from binding of acetylene carbon atoms to the radical oxygen center and to the lower coordinated Zr atom. For hydrogen transfer it is necessary to overcome the barrier of 1.37 eV (cf. Figure 6). For the cleavage of ethenone 1.77 eV is needed, which is available due to the formed complex. Consequently, the oxidation reaction is exothermic by 0.8 eV. This shows clearly that the next-nearest neighbors of the oxygen-radical center in the Zr11ScO24-cluster contribute substantially to the stabilization of the initial complex as well as to the lowering of the barrier for hydrogen transfer in comparison to the other studied cluster models which are not sections of the (ZrO2)n bulk.

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CONCLUSIONS In summary, in the case of the cluster models representing a section of zirconium oxide bulk such as Zr11ScO24, in addition to the radical oxygen center the presence of neighboring Zr 11573

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