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Reaction of NO and CO Catalyzed with Small Copper Clusters: Mechanism and Design Júlia Barabás, and Tibor Höltzl J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08349 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 8, 2016

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The Journal of Physical Chemistry

Reaction of N2O and CO Catalyzed with Small Copper Clusters: Mechanism and Design Júlia Barabás1, Tibor Höltzl2* 1

Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Hungary 2 *

Furukawa Electric Institute of Technology, Budapest, Hungary

Corresponding author: Tibor Höltzl, [email protected], +36-1-417-3257

Abstract Highly active catalytic clusters are observed during the reaction mechanism study of the copper cluster Cux (x = 4-15) catalyzed N2O + CO → N2 + CO2 reaction. It was shown that N2O adsorbs on the copper cluster without an activation barrier and the nitrogen-oxygen bond is broken during the next step. The analysis of the chemical bonding showed that the oxide ion formation is an important driving force of the reaction. Among the different clusters Cu12 was the most active as the nitrogenoxygen bond is cleaved without an energy barrier, while the nitrogen molecule is eliminated. It was shown that the resulting copper oxide cluster is reduced easily with carbon-monoxide. The elimination of the thus formed carbon-dioxide is thermodynamically a highly favoured process, even at low temperature. Thus Cu12 cluster is a potentially highly active catalyst at ambient condition for the N2O + CO → N2 + CO2 reaction.

Introduction Recently small metal clusters become an increasingly interesting topic, as their physical and chemical properties are significantly different from those of the corresponding bulk material. One of their main advantages are the improved catalytic properties, namely high efficiency and selectivity, stability and easy recovering. Moreover the catalyst performance can be tuned sensitively by the particle size because the surface geometric structure and the electronic properties of the clusters changes significantly in metal clusters. This opens the way to tailor their catalytic activities by appropriate size selection1,2. Air pollution is among the serious environmental problems and the diesel engines are considered an

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important source of pollutants. The four main pollutant emissions are carbon monoxide (CO), hydrocarbons (HC), particulate matter (PM) and nitrogen oxides (NO x). The pollutant emissions is less than 1% of the diesel exhaust gas while NO x has the highest, more than 50% proportion of this gas3. Since the barrier height of the N2O + CO → N2 + CO2 reaction is relatively high, about 200 kJ/mol4, the catalytic converter, based on a three-way catalyst (TWC) is regarded as a promising technique for decreasing the amount of pollutants in the exhaust gas. The main disadvantage of TWCs is the expensiveness, because they mainly composed of noble metals (Pt, Rh and Pd)5. So there is a challenge to substitute the TWCs for an other catalyst that is less expensive and at least as effective, thus the activity of several catalysts were investigated for the reduction of N 2O by CO. Among these probably the simplest are the metal cations4,6,7,8. Blagojevic et al. examined the catalytic activity for O-atom transport catalysis of the reduction of N2O by CO of 26 different atomic cations using ICP/SIFT tandem mass spectrometer at room temperature. They have found that Ca +, Fe+, Ge+, Sr+, Ba+, Os+, Ir+, Pt+, Eu+ and Yb+ cations are better catalyst for this reaction than the others 6. Wannakao and co-workers studied iron-embedded graphene where the graphene sheet acts as electron donor and acceptor surface depending of the reaction pathway9; furthermore Pornsatitworakul et al. used Fe-porphyrin in their calculations10. Application of gold-based catalysts were found to be also promising11. Metal-oxides cluster ions and surface for catalyst of the N2O + CO → N2 + CO2 reaction were also examined by many researchers12,13,14. As there is a very high demand on catalysts involving non-precious metals, the catalytic activity of copper clusters has attracted considerable interest. It was found that in general a higher reactivity of copper clusters with an odd number of atoms have been reported 15,16,17. In the case of CO there is no dependence of the reaction energy with the even-odd alternation18. Hirabayashi et al. studied the gas-phase catalytic reaction of N2O and CO on isolated copper cluster anions and reported that Cux- can catalyze the reaction betweeen CO and N2O and the Cu7- was the most efficient in the size range of x = 5-16. The catalytic cycle of this reaction is shown in Scheme 119. The interaction of O2 with copper clusters has been also examined and the reactivity of cluster size and morphology dependence was established16. For neutral and anionic Cu20 clusters the reduction of CO by O2 was investigated and by the anionic cluster a lower reaction barrier was found20. It was shown that N2O can be adsorbed on various metal surfaces which in many cases lead to surface oxidation and simultanous N2 elimination. The efficiency of the oxidation depends on the metal21. In the case of copper, the adsorption of N2O is rather dissociative than molecular, but it also 2 ACS Paragon Plus Environment

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depends on the temperature and on the lattice planes. Nowadays computational chemistry not only enables the detailed understanding of complex reaction mechanism, but also opens the way towards rational design of catalyst 22,23. Examination of the reaction and bond energies of the adducts and the investigation of chemical bonding are highly relevant for the understanding of important aspects of reactivity. However the reaction mechanism of the copper cluster catalyzed N 2O + CO reaction is not yet known. Thus in this article we report on the reactivity of copper clusters, Cu x (x = 4-15) toward N2O and CO with the aim of contributing to the rational catalyst design.

Scheme 1

Methods of computations Density Functional Theory calculations were performed using the Q-Chem 4.3 quantum chemical software package24. Molecules were visualized using the PyMol program25. The BP8626,27 functional in conjunction with the LANL2DZ 28,29 basis set was chosen to perform the preliminary computational study. More accurate calculations for the most stable clusters were carried out with the hybrid functional using the TPSSh 30 functional and DEF2-TZVPP31,32 basis set. The (75,302) integration grid was applied in all cases. Based on the optimized structures, initial geometries for the transition states were located using the Freezing String Method, as implemented in the Q-Chem program 33. Geometries were later refined using conventional eigenvector following method34,35,36,37. Reaction energies reported in this article refer to the energy differences between the sum of the total energies of the products and the reactants, respectively. Activation energies are computed as the differences between the energies of the transition state and the corresponding reactants. 3 ACS Paragon Plus Environment


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Thermochemical computations were performed using the Tamkin library38. Chemical bonding was analyzed using the Localized-Orbital Locator (LOL), computed using the DGrid39,40,41, as well as with Natural Charge and Wiberg bond index 42,43 computed with the NBO 6 program suite. The accuracy of the theoretical level was tested by performing computations using various functionals and basis sets. The details are available in the Supporting Information (Figure S1, Figure S19). Basin hopping global minimum optimization was also performed to find the most stable isomers.

Results and discussion The geometries of the Cux clusters applied in this study (reconstructed after the reported lowest energy structures in reference44) are shown in Figure 1.

Figure 1: Optimized structures of the lowest energy Cux clusters (x = 4-15), calculated with TPSSh functional and DEF2-TZVPP basis set. It is well known that the planar isomers are the most stable up to x = 6, and three-dimensional structures are preferred for copper clusters containing 7 or more copper atoms15,44,45. 1. N2O adsorption on the copper clusters 4 ACS Paragon Plus Environment

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As it is shown in Scheme 1, the most interesting is the first step of the proposed catalytic reaction mechanism, where dinitrogen-oxide reacts with the cluster, thus first we study this step in detail. In order to model the addition of the gas phase substrate to the catalyst active site, the initial structures for geometry optimization of the N2O adsorbed clusters were generated by systematic addition of N2O to all possible copper atoms. The resulting clusters were obtained by direct geometry optimization of the initial structure, which mimics the barrierless addition of the reactant. However further steps leading to more stable Cu x-N2O isomers are possible. Therefore basin hopping global minimum optimization was performed in the case of x= 4, 6, 8, 10, where the located structures are different than in the other cases. This analysis indicated indeed that lower energy structures exist in these cases. These are substantially more stable than the original structures, but a relatively small energy barrier of about 20-30 kJ/mol exists between the two isomers as it will be shown later in this paper. Subsequent geometry optimization was performed using the BP86/LANL2DZ method. Optimized geometries for the four lowest energy structure are available in the Supporting Information of this paper. These structures were re-optimized using the more accurate TPSSh/DEF2-TZVPP method. The reaction energies defined as ΔEr = E(Cux-N2O) – E(N2O) – E(Cux) were calculated for the four lowest energy structures of each Cux-N2O clusters (Figure 2). The adsorption of N2O over copper clusters leads to several different types of Cux-N2O adducts, as (i) Linear N2O binds with the nitrogen site (e.g. Cu 4/b, Cu6/b) or with the oxygen site (e.g. Cu 4/c, Cu6/c) (ii) Bent N2O binds as a ring (e.g. Cu8/b, Cu10/b), when one nitrogen and the oxygen atom are simultaneously attached to a copper atom in the cluster. (iii) N2O dissociates first and both oxygen and the N2 moiety binds separately to the cluster. This can be seen in terms of almost all clusters. By the Cu 4, Cu6, Cu8 and Cu10 clusters there is energy barrier to reach this structure (Figure S2). In the case of Cu 4 cluster difficulties encountered when finding the transition state, so the mechanism of the transformation should be more complicate. It is worth mentioning that the dissociation of the N-O bond decreases the energy. (iv) N2O dissociates and only oxygen binds to the cluster, while N2 leaves. It is important to note that in the case of Cu 12-N2O system nitrogen molecule detaches during the adsorption on the cluster surface. This means that the O-N bond of N2O is broken without an energy barrier during the Cu12 + N2O reaction. This is a very promising process, as a necessary condition for a good catalyst is the breaking of the N-O bond and the dissociation of N2. Comparing the reaction energies and the structure of the Cux-N2O clusters it was found that by the 5 ACS Paragon Plus Environment


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most stable isomers the oxygen bonded separately to the copper cluster (iii, iv). Inverse correlation can be observed between the reaction energy and HOMO energy of the clusters: the reaction energy is more exothermic if the HOMO energy is relatively low and vice versa (Figure 3). Figure 3 is augmented with a color bar, which is to guide the eye to show that inverse correlation between HOMO energy and reaction energy holds for Cux clusters when x>7 and approximately for x=4 or 5. Cu6 and Cu7 clusters behave differently than the other cluster sizes, as it is also well visible in Figure 3 (the two color dividing line location differs from the others). Stability of copper clusters was investigated by Jaque et al. 46 computing the second difference of the energy with respect to the cluster size according to the equation Δ2E (x) = E (x+1) + E (x-1) -2E (x). Here x is the number of atoms in the cluster. Δ2E measures the relative energy of the given cluster compared to the neighbouring sizes and positive value indicates an enhanced stability. It was shown earlier in the case of scandium doped copper cluster cations that this quantity also correlates with the abundance of the corresponding system in a mass spectrometry study 47. It follows from these relative stability values (Figure S1) that Cu 6 cluster is less stable, while Cu7 is more stable than the other clusters of similar sizes. An enhanced stability of the reactant cluster leads to a less exoergic reaction, which is in line with the observation on Figure 3 for Cu6 and Cu7.


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Figure 2: Optimized structures and energies of the Cux + N2O → Cux-N2O reaction in decreasing stability order of the Cux-N2O clusters. Cu, yellow brown; O, red; N, blue. (Method: TPSSh/DEF2TZVPP)



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Figure 3: Calculated HOMO and reaction energies of the Cux clusters (Method: TPSSh/DEF2TZVPP). Bars are indicated to guide the eye for the inverse relationship between the HOMO energy and the reaction energy for cluster size x=9-15, while a less clear relationship for x10 and x=7 it occurs readily even at ambient conditions.


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Figure 6: Gibbs free energies (at 1atm pressure and various temperatures in the 200-1000K range) of Cux-N2O → CuxO + N2 reactions. (Methods: energy computation TPSSh/DEF2-TZVPP, frequencies: BP86/LANL2DZ) This clearly indicates that cluster sizes x>10 are necessary to make the reaction feasible at ambient conditions. Thus the systematic study of the dinitrogen-oxide reactivity indicates that Cu12 is a promising catalyst, as the nitrogen-oxygen bond is broken without energy barrier and the nitrogen molecule readily leaves the cluster. This makes it possible to operate the catalyst at low temperature, as it is indicated in Figure 6. Therefore we continue our investigation with the Cu 12 cluster. As Cu7 is an often applied model cluster50 we also include it for comparison. 3. CO adsorption and CO2 dissociation To simulate the third step of the reaction, we examined the CO adsorption on the Cu 7O cluster first. Initial structures for the geometry optimization were generated by systematic addition of the carbon-monoxide molecule to every atoms in the Cu7O cluster. The geometric structure and the calculated reaction energies of the lowest energy Cu7O-CO and Cu12O-CO clusters are shown in the Supporting Information (Figure S16), while Cu12O-CO and Cu12-CO2 clusters were generated by analogy to the corresponding smaller clusters. 13 ACS Paragon Plus Environment


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According to the results it can be concluded that carbon-monoxide attaches with the carbon site to the copper cluster, similarly to the analogous copper clusters reported in reference 15. However, in contrary to the pure clusters, carbon-monoxide tends to form chemical bonds to more than one copper atoms, nevertheless subsequent reaction steps are necessary for the further stabilization and for the formation of carbon-dioxide. The Wiberg bond indices show that the C-O bond is always covalent (Figure 7). During the formation of CO2 from the separated O and CO fragment the charge of the oxygen increases from -1.16 to -0.67 and two formal Cu-O bonds are broken. This indicates that the ionic character of the oxygen decreases, as it is expected for the formation of carbon-dioxide. Moreover the Cu-O and Cu-C bonds become weaker and CO2 leaves the cluster without substantial energy barrier. Indeed, the localized-orbital locators show that the Cu12O-CO bonding is more significant than that at the Cu12-CO2, as the localization domain of the CO fragment spreads more to the copper cluster in the first case. The same happens in the case of the Cu7 cluster (see the Figure S17 in the Supporting Material).


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Figure 7: Localized orbital locators, Wiberg bond indices and natural charges (indicated in squares) of Cu12O-CO and Cu12-CO2 clusters. Cu, yellow brown; O, red; C grey. (Method: BP86/LANL2DZ) 4. Reaction mechanism Based on the previous discussion we are now in a position to set up the whole catalytic reaction mechanism for Cu7 and Cu12 (Figure 8).



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Figure 8: Potential energy surface of Cu7 (a) and Cu12 (b) clusters and energies for the reaction (in bracket, in kJ/mol) without any catalyst (c). The energies were calculated with TPSSh functional and DEF2-TZVPP basis set and represent the energy differences between the initial configuration (Cux + CO + N2O) and the given structure. The transitional structures are marked with dashed lines. Each steps correspond to the steps in Scheme 1. Cu, yellow brown; O, red; N, blue; C, grey. The reaction pathway of the Cu12 cluster is similar to that of the Cu 7 cluster, as they consist of similar steps, but the energy difference is significant: comparing the energy barriers it is well visible that Cu12 is catalytically more active than Cu7. It is noteworthy that the nitrogen-oxygen bond is broken without an energy barrier in the case of Cu12. As it is visible in Figure 8, for Cu12 the relative energies of the intermediates monotonically decrease during the process of the reaction, while in the case of the Cu 7 cluster where the relative


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energy of the Cu7O is higher than the energy of the Cu 7-N2O and the same occur at the Cu7O-CO – Cu7-CO2 transformation . The results indicated that in the case of Cu12O-CO energy is needed to bind carbon atom to other copper atoms and so the most stable Cu12O-CO structure is received. In addition to CO2 desorption there can be some transition states due to the dynamic processes, but the activation energy is very low (Figure S18). Considering the N2O + CO → N2 + CO2 reaction the activation energy is much higher (about 190 kJ/mol calculated with TPSSh/DEF2-TZVPP method) without catalyst in the system than with copper cluster catalyst, where the higher barrier is 77 kJ/mol (Cu12), respectively 91 kJ/mol (Cu7) (Figure 8). Moreover in the case of the catalyzed reaction the total energies of all the intermediates and the products are lower than that of the initial compounds. These clearly show the promising catalytic activity of the Cu12 cluster. To gain more information on the catalytic activity of Cu 7 and Cu12 clusters at typical conditions of constant temperature and pressure, we computed the Gibbs-free energies of various reaction steps between 200K and 1000K and at the pressure of 1 atm (Figure 9). It is well visible in the figure that in steps 1 and 3 the Gibbs-free energy increases with the temperature due to the entropy factor, as the N2O and the CO molecules are adsorbed on the clusters, respectively. The reaction corresponding to step 1 is feasible throughout the whole computed temperature range, while this is not the case in step 3, where the reactions are only feasible thermodynamically below ~500K and ~800K for Cu7 and Cu12, respectively. On the other hand the Gibbs-free energies decrease in the case of steps 2 and 4, when the N2 and the CO2 molecules are eliminated, respectively. According to the computations the N2 elimination step for the reaction involving the Cu 7 cluster requires at least ~400K. However the large reaction Gibbs-free energy of step 1 is expected to be sufficient to help the dissociation at lower temperatures. For Cu 12 the N2 elimination is favoured at the whole temperature range. Finally, the elimination of CO 2 is a thermodynamically feasible process for both the Cu7 and the Cu12 clusters.



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1. Cux + N2O → CuX-N2O

2. CuX-N2O → CuxO + N2

3. CuxO + CO → Cux-CO2(a)

4. Cux-CO2 → Cux + CO2

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Figure 9. Gibbs-free energies of the four key steps (c.f. Scheme 1 and Figure 8) of the full reaction mechanism at different temperatures and at pressure of 1 atm. (Methods: energy computation TPSSh/DEF2-TZVPP, frequencies: BP86/LANL2DZ) (a)

As it is shown in Figure 8, this is not an elementary reaction step.

Conclusions The adsorption of N2O on Cux clusters with x = 4-15 and the supposed catalytic cycle have been investigated using quantum chemical methods as well as by thermodynamic computations. It has been shown that N2O adsorbs on the copper cluster without activation energy, but in the case of Cu 4, Cu6, Cu8 and Cu10 clusters there are small reaction barrier leading to the most stable isomers. Over x = 8 an even-odd alternation of the N2O adsorption energy was observed, where even clusters are more reactive. The analysis of the chemical bonding indicated that the formation of the oxide ion on the copper cluster surface is an important driving force for this reaction. In line, the stability 18 ACS Paragon Plus Environment

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of the different Cux-N2O isomers correlate with the feasibility of the oxide ion formation. Examination of CO adsorption shows that CO first binds to copper atoms on the Cu xO cluster and subsequent reaction paths through several transition states lead the Cu x-CO2 system. No energy barrier was observed in the CO2 desorption and the thermodynamic analysis showed that this process is thermodinamically favorable, even at a low temperature. The presented calculations suggest that copper cluster can be a good catalyst of N2O + CO → N2 + CO2 reaction. Among the clusters smaller than 10 constituent atoms Cu 7 is the most promising, but Cu12 and Cu14 clusters are the best according to the thermodynamic analysis and these are effective catalysts even at ambient temperatures, i.e. larger clusters consisting of an even number of copper atoms are all catalytically active in the studied range. Supporting Information Further data on method testing, reaction path of the isomerization of the ring isomers of Cu X-N2O (x=6, 8 or 10), Localized orbital locator profiles of Cu XO and Cux-N2O x=(4-15), optimized structures of Cux-CO2 and Cux-OCO (x=7, 12) systems and XYZ coordinates of all compounds. The Supporting Information is available free of charge on the ACS website. Acknowledgments The authors thank Professor Tamás Veszprémi and Dr. András Olasz for the fruitful discussions and for valuable suggestions regarding the manuscript.



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Reaction of N2O and CO Catalyzed with Small ... - ACS Publications

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