NO Decomposition Activated by Preadsorption of O2 onto Copper

Feb 12, 2015 - Reactions of Ti- and V-Doped Cu Cluster Cations with Nitric Oxide and Oxygen: Size Dependence and Preferential NO Adsorption. Shinichi ...
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NO Decomposition Activated by Preadsorption of O2 onto Copper Cluster Anions Shinichi Hirabayashi† and Masahiko Ichihashi*,‡ †

East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan Cluster Research Laboratory, Toyota Technological Institute: in East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan

‡

S Supporting Information *

ABSTRACT: Reactions of anionic copper cluster dioxides, CunO2− (n = 8, 10, and 12), with NO were studied in the gas phase by using a guided ion beam tandem mass spectrometer. A product ion, Cun−2O4−, is observed under multiple collision conditions together with the single-collision reaction products, CunO2NO− and Cun−1O2NO−. The NO-pressure dependence studies show that Cun−2O4− is formed via adsorption of two NO molecules followed by releasing an N2 molecule, which is clear evidence for NO decomposition. Density functional theory calculations were also performed for the reaction of Cu8O2− with two NO molecules and confirm the production of Cu6O4− under the experimental conditions used.

1. INTRODUCTION Nitric oxide (NO) is a harmful material that is mainly emitted from automotive engines, and its removal is one of the most important environmental problems. Currently, noble metalbased three-way catalysts including Rh, Pd, and Pt are used to clean up exhaust gases from gasoline engines.1 However, these noble metals are scarce and expensive resources, and hence, non-noble-metal alternatives are strongly required. To find the optimal alternatives, it is important to understand catalytic processes at the molecular level. Although practical catalysts consist of metal particles with a variety of sizes, compositions, and structures, metal clusters should play an important role in the catalytic processes.2,3 By using mass spectrometry, clusters of a desired single size and composition can be prepared and examined in the gas phase without the influence of supports. Gas phase studies on reactivity of metal clusters have not only provided fundamental insights into the reaction mechanisms but also led to the understanding of catalytic processes and reactive sites.4−7 Reactions of metal and metal oxide cluster ions with NO have been extensively investigated, but there are only a few observations of NO decomposition with releasing N 2 molecules. Mackenzie and co-workers reported NO decomposition on size-selected Rhn± (n = 6−12, 14−16),8,9 Nbn+ (n = 9, 10),10 and Con+ (n = 5−20)11 using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. A guided ion beam (GIB) study on reactions of Con+ (n = 2−10) with NO by Hanmura et al.12 showed that NO decomposition takes place for n = 5−10, which is confirmed by the subsequent FTICR study.11 These previous studies indicate that dissociative © XXXX American Chemical Society

adsorption of NO is the initial and critical step in NO decomposition. Recently, we investigated the adsorption and reactions of NO on copper and copper oxide cluster ions, CunOm± (n = 3−19, m = 0−9), under single collision conditions by using a GIB tandem mass spectrometer.13 We found that the NO adsorption probabilities of copper cluster anions are dramatically enhanced by the preadsorption of two oxygen atoms in the size range of n ≥ 8. Furthermore, we observed that NO adsorption onto CunO2− (n = 8, 10, and 12) is followed by the release of a Cu atom. This finding suggests that NO adsorbs dissociatively, which was supported by density functional theory (DFT) calculations. Similar metal atom release had been observed in the reactions of small Con+ clusters with NO, where NO decomposition proceeds under multiple collision conditions.11,12 In this further study, we have focused on multiple collision reactions of CunO2− (n = 8, 10, and 12) with NO. The intensities of reaction product ions are measured by varying the NO gas pressure to find evidence of NO decomposition. Furthermore, we have performed DFT calculations to give reaction pathways via the ions observed experimentally. Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: October 9, 2014 Revised: February 3, 2015

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The Journal of Physical Chemistry C

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS All experiments were carried out using a GIB tandem mass spectrometer, and the details of the experimental setup and data analysis method have been described previously.12−14 Copper clusters were produced by ion sputtering of four separate Cu plates with 8.5-keV Xe ion beams emitted from an ion gun (CORDIS Ar25/35c, Rokion Ionenstrahl-Technologie). After extracted by a series of ion lenses, copper cluster anions were admitted into an octopole ion beam guide (OPIG) passing through a 290-mm-long cooling cell. A small amount of O2 gas (∼10−4 Torr) was supplied into the cooling cell together with He gas (∼10−2 Torr) to prepare O2-preadsorbed copper cluster anions, CunO2−. The resulting CunO2− were cooled down to room temperature by the collisions with the He atoms and mass-selected by the first quadrupole mass filter (QMF). Then these parent ions were admitted into another OPIG passing through a 100-mm-long reaction cell, where they were allowed to react with NO molecules. The NO gas was introduced into the reaction cell through a stainless steel tube equipped with a cold trap of an ethanol/liquid-nitrogen mixture at ca. 170 K to eliminate impurities such as nitrogen dioxide and water. The NO pressure in the reaction cell was regulated using a variable leak valve in the range from 5 × 10−5 to 5 × 10−3 Torr, which covers both single and multiple collision conditions. The translational energy of the parent cluster ions in the reaction cell was measured by the retarding potential method using the OPIG and converted to the collision energy, Ecol, in the center-of-mass frame. A typical spread of the collision energy was 0.4 eV in full width at half-maximum (fwhm). All the measurements were performed at the initial collision energy of 0.2 eV to examine the reactions under a near-thermal condition. The collision energy dependence of the reaction cross sections for Cu8O2− has been measured under single collision conditions.13 Unreacted parent ions and product ions were mass-analyzed with the second QMF and then collected by a series of ion lenses to a secondary electron multiplier coupled with an ion conversion dynode. Signals from the secondary electron multiplier were processed in a pulse counting mode. DFT calculations were performed with the Gaussian 09 program package15 and optimized structures were obtained at the BPW91/LanL2DZ level.16,17 The same computational method was successfully applied to our previous study on the adsorption and dissociation of an NO molecule on Cu8O2−.13 Initially, the geometric structures of Cu8O2− were optimized with a doublet spin state, and three isomers were obtained. The vibrational frequency analysis confirmed that the lowest-energy isomer obtained represents a local minimum in energy because it has no imaginary vibrational frequency. The quartet states of these isomers were also calculated, and it was found that they are located significantly higher in energy than the doublet states. Thus, the obtained lowest-energy isomer of Cu8O2−, which is the same as that shown in our previous study,13 was used as the reactant in the energy diagram.

Figure 1. Mass spectra obtained in the reactions of Cu8O2− with NO under (a) a single collision condition (NO pressure of 2 × 10−4 Torr) and (b) a multiple collision condition (NO pressure of 4 × 10−3 Torr). For comparison, (c) shows a mass spectrum obtained in the multiple collision of Cu8O2− with Xe (Xe pressure of 4 × 10−3 Torr). The intensities are normalized to the total ion intensity. The initial collision energy of NO or Xe is 0.2 eV.

445, very slightly in Figure 1a and clearly in Figure 1b. This peak is assignable to Cu7− (m/z = 444.9) and/or Cu6O4− (m/z = 445.3). In the single collision reactions with NO, Cu6O4− cannot be produced from Cu8O2− and hence the peak at m/z ≈ 445 in Figure 1a is assigned to Cu7− exclusively. The formation of Cu7− is probably attributed to the collision-induced dissociation (CID) of Cu8O2− in the high-energy tail of the translational energy distribution. The formation of Cu6O4− at a high NO pressure is confirmed by the mass spectra measured at a relatively high resolution, where the isotopes of 63 Cux65Cu6−xO4− are clearly distinguished from those of 63 Cux65Cu7−x− (see Figure 2). Furthermore, this assignment is supported by comparing the mass spectrum in Figure 1b with that measured in the CID experiment of Cu8O2− by Xe (Figure 1c) because the intensity of the m/z = 445 peak in the multiple collision reactions with NO is considerably higher than that of Cu7− in Cu8O2− + Xe.

3. RESULTS Figure 1 (panels a and b) shows typical mass spectra obtained in the reactions of Cu8O2− with NO at the initial collision energy of 0.2 eV under single and multiple collision conditions, respectively. Two product species, Cu 8 O 2 NO − and Cu7O2NO−, appear dominantly as described in our recent study.13 Additionally, another peak is also observed at m/z ≈

Figure 2. (a) High-resolution mass spectrum of Cu6O4− produced in the reaction of Cu8O2− with NO under a multiple collision condition (NO pressure of 4 × 10−3 Torr) at the initial collision energy of 0.2 eV. (b) Typical mass spectrum of Cu7− produced by the ion sputtering method. The thick red lines show simulated mass spectra of Cu6O4− and Cu7− using the natural abundances of copper isotopes. B

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The Journal of Physical Chemistry C In order to estimate the net intensity of Cu6O4− in the m/z = 445 peak, we assume the contribution of Cu7− to the m/z = 445 peak as follows: (1) the m/z = 445 peak observed at the NO pressure of 2 × 10−4 Torr (Figure 1a) is due to only Cu7− and (2) the relative intensity of Cu7− increases linearly with the NO pressure since it does with the Xe pressure in the CID. The relative intensity of Cu6O4− thus obtained is plotted as a function of the NO pressure in Figure 3a. These results indicate

Figure 4. Mass spectra obtained in the reactions of (a) Cu10O2− and (b) Cu12O2− with NO under multiple collision conditions. The NO pressures are 5 × 10−3 and 6 × 10−3 Torr for Cu10O2− and Cu12O2−, respectively. The initial collision energy is 0.2 eV.

Figure 3. (a) Relative intensities of the parent ion and the major product ions (Cu8O2NO−, Cu7O2NO−, and Cu6O4−) in the reaction of Cu8O2− with NO as a function of the NO pressure. (b) Pressure dependence in the reaction of Cu8− with NO. The initial collision energy is 0.2 eV. The dashed lines represent the first- and secondorder dependences on the NO pressure for eye guide.

that the intensity of Cu6O4− increases quadratically with the pressure of NO, whereas those of Cu8O2NO− and Cu7O2NO− increase linearly. Note that the intensities of the single-collision reaction products essentially deviate from linear dependences at high NO pressures, and the sum of their intensities gradually converges to one if no subsequent reactions. The deviations observed may result from the essential one and the subsequent reaction. Mass spectra for multiple-collision reactions of Cu10O2− and Cu12O2− with NO are shown in Figure 4. Similarly to the reaction of Cu8O2−, the peaks assignable to Cun−2O4− are observed for Cu10O2− and Cu12O2−, in addition to the singlecollision products, CunO2NO− and Cun−1O2NO−. Figure 5 shows the NO pressure dependences of the relative intensities of the parent and major product ions. The relative intensity of Cu8O4− produced in the reaction of Cu10O2− + NO is estimated in the similar way assumed in Cu8O2− + NO, while there is no observation of Cu11− in the reaction of Cu12O2− + NO under single collision conditions. Each intensity of Cun−2O4− (n = 10 and 12) shows a roughly second-order

Figure 5. Relative intensities of the parent ion and the major product ions (CunO2NO−, Cun−1O2NO−, and Cun−2O4−) in the reactions of CunO2− with NO as a function of the NO pressure [(a) n = 10 and (b) n = 12]. The initial collision energy is 0.2 eV. The dashed lines represent the first- and second-order dependences on the NO pressure for eye guide.

dependence on the NO pressure and deviates slightly at a high NO pressure probably because of subsequent reactions, for example, the formation of Cun−2O4NO−. Consequently, the experimental observations indicate that Cun−2O4− are formed via the reactions of CunO2− (n = 8, 10, and 12) with two NO molecules and suggest that the following C

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The Journal of Physical Chemistry C NO decomposition proceeds with releasing neutral species corresponding to two Cu atoms and an N2 molecule: Cu nO2− + 2NO → Cu n − 2O4 − + [2Cu, N2] (n = 8, 10, and 12)

(1)

where the square bracket is used to indicate the ambiguity of the neutral products. It is apparent from Figures 3a and 5 that the probability of NO decomposition is higher for n = 8 than for n = 10 and 12 by comparing the relative intensities of Cun−2O4− with one another at the same NO pressure. In addition, a small amount of CunO2(NO)2− appears for n = 10 and 12 at only the highest NO pressure employed (∼5 × 10−3 Torr) as shown in Figure 4. Under the experimental conditions employed, limited number of collisions, no buffer gas, and short reaction time (∼100 μs), collisional and radiative cooling would not be effective. Actually, the formation of Cun−1O2NO− is not suppressed even at high NO pressures. As shown in Figure 3b, a bare Cu8− cluster does not adsorb an NO molecule as efficiently as its dioxide Cu8O2− does, and there is no evidence for the decomposition of NO on Cu8− in the pressure range studied. Similarly, neither Cu10− nor Cu12− react readily with an NO molecule.13 However, CunO2− can adsorb NO molecules dissociatively to produce the anionic cluster tetroxides. Clearly, the activity of the anionic copper clusters contrasts with those of other transition metal clusters where the bare clusters are oxidized sequentially by NO molecules.8−12

Figure 6. Potential energy diagram for Cu8O2− + 2NO → Cu6O4− + 2Cu (or Cu2) + N2. The numbers represent the energies (in eV) of the species with respect to the energy of the initial state. The relative free energies at 298 K (in eV) are also given in the parentheses.

having the lower spin multiplicities (singlet or doublet) for NO decomposition with the releases of two Cu atoms (or a Cu dimer) and an N2 molecule. The optimized structures and their energies with respect to the initial state, Cu8O2− + 2NO are also indicated in this figure. The reaction pathways of the clusters in the higher spin states (triplet or quartet states) are shown in Figure S1 of the Supporting Information. The structures of Cu8O2− and the molecularly adsorbed species, Cu8O2(NO)−, are the same as those obtained in our previous study,13 though they are shown from a different view to clarify the positions of N and O atoms. On the other hand, as shown in Figure 6, we found a lower-energy isomer of the dissociatively adsorbed species, Cu8O2(N)(O)− (−2.89 eV), than the isomer obtained previously (−2.53 eV).13 In the adsorption process of the first NO molecule, the calculations show that Cu8O2(N)(O)− has lower energy than Cu8O2(NO)− (−2.12 eV). The reaction pathway branches off from the formation of Cu8O2(N)(O)−: one goes through the formation of Cu8O4N2− and the other goes through Cu7O4N2−. In the former reaction pathway, the two intermediate states, Cu8O2(N)(O)(NO)− and Cu8O4(N2)−, are shown to have quite low energies. The final state, Cu6O4− + Cu2 + N2, is also energetically more stable than the initial state by 2.87 eV, which indicates that this reaction pathway is highly exothermic. On the other hand, the latter reaction pathway involves the release of a Cu atom from Cu8O2(N)(O)− in an early stage. Although the reaction intermediate state, Cu7O2(N)(O)− + Cu + NO, has the energy almost equal to that of the initial state, the experimental observation of this product ion can be explained by considering the initial internal energy of Cu8O2− and the collision energy. The final state, Cu6O4− + 2Cu + N2, in the reaction via Cu7O2(N)(O)− is energetically lower than the initial state by 0.71 eV. Although the transition states were not calculated here, Figure 6 suggests that NO decomposition can proceed on

4. DISCUSSION The single collision reactions of CunO2− dominantly give the two product species, CunO2NO− and Cun−1O2NO−,13 and the latter ion should be produced by the unimolecular dissociation of the former ion. In addition, the NO pressure dependence of the product ion intensities indicates that CunO2− reacts with two NO molecules to form Cun−2O4− and [2Cu, N2]. These experimental findings suggest that NO decomposition proceeds as follows: Cu nO2− + NO → Cu nO2 NO−

(2)

Cu nO2 NO− → Cu n − 1O2 NO− + Cu

(3)

Cu nO2 NO− + NO → Cu n − 2O4 − + [2Cu, N2]

(4)

Cu n − 1O2 NO− + NO → Cu n − 2O4 − + [Cu, N2]

(5)

Similar reaction processes were proposed in the reactions of other transition metal clusters, such as Rhn±, Nbn+, and Con+ with NO, though only small Con+ clusters often accompany the release of one or two metal atoms in the formation of cluster dioxides.8−12 We have not determined which pathway (4) or (5) is favored in the Cun−2O4− formation yet, but it would be interesting to investigate the reactions of CunO2NO− or Cun−1O2NO−, which is prepared as a parent ion, with NO selectively. In fact, similar experiments were carried out for the reactions of Con+ + NO.12 To examine the above reaction pathways, the structures and energies of the reactants, intermediates, and products in the reaction of Cu8O2− + 2NO → Cu6O4− + [2Cu, N2] were calculated by the DFT method. For all species, the vibrational frequency analysis was performed in order to confirm that they have local minimum energies on the potential energy surface. Figure 6 represents the reaction pathways involving the clusters D

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The Journal of Physical Chemistry C Cu8O2− via the formation of Cu7O4N2− as well as via Cu8O4N2− on the premise of no appreciable energy barrier along the reaction coordinate. We also explored stable geometries of Cu6O4− in the doublet, quartet, and sextet separately and then found more than ten isomers. The obtained structures of the three lowest-energy isomers in doublet states are shown in Figure 7, and the most

been reported in theoretical studies for the dissociative adsorption of NO on Cu(111) and Cu2O(111) surfaces by Kasai and co-workers.18,19 They found that the energy of the transition state toward the NO dissociation on the Cuterminated Cu2O(111) surface is significantly lower than those on the Cu(111) and O-terminated Cu2O(111) surfaces. Furthermore, it was shown that the energy levels of the d orbitals of the outermost Cu atoms in the Cu-terminated Cu2O(111) surface shift toward the Fermi level by their interaction with the subsurface O atoms and that these d orbitals facilitate the NO dissociation.

5. CONCLUSION We have studied the reactions of NO molecules on specific anionic copper cluster dioxides, CunO2− (n = 8, 10, and 12), under multiple collision conditions. It was found that the formation of Cun−2O4− (i.e., NO decomposition) occurs and that this reaction proceeds through the adsorption of two NO molecules onto CunO2−. DFT calculations show that the reaction of Cu8O2− + 2NO → Cu6O4− + 2Cu (or Cu2) + N2 can proceed exothermically, which is consistent with the experimental observations. Therefore, these results suggest that NO decomposition as well as NO adsorption is promoted by the presence of a few oxygen atoms on anionic copper clusters. Our findings provide new insight into the design and development of copper-based catalysts for NO decomposition.

Figure 7. Optimized structures of Cu6O4− isomers in doublet states. Relative energies are given for the species in doublet and quartet states in square brackets.

stable one is the isomer (a) in a doublet state with an octahedral geometry of Cu6, which is capped by four oxygen atoms. The chairlike isomer (b), which is transiently obtained as the final product in Cu8O2− + 2NO, is higher in energy by ∼1.5 eV. If the structural transformation from the isomer (b) to (a) takes place, the reaction pathways of Cu8O2− + 2NO → Cu6O4− + [2Cu, N2] are more exothermic. The NO adsorption cross sections for CunO2− (n = 8, 10, and 12) are almost equal to one another.13 For n = 10 and 12, however, the Cun−2O4− product is less observed than for n = 8, and CunO2(NO)2− appears additionally at a high NO pressure. These findings suggest that the molecular adsorption of NO is more favorable on Cu10O2− and Cu12O2− energetically or that there is a relatively high-energy transition state between the molecular and the dissociative adsorption. Another explanation is that the two N atoms resulting from the dissociative adsorption of NO molecules cannot migrate over the cluster sufficiently within the flight time (∼100 μs) from the reaction cell to the second QMF, which leads to a reduced probability of the formation and subsequent desorption of N2. Such a situation can be true for larger clusters with not only more possible adsorption sites for N atoms but also more vibrational modes, resulting in the enhanced energy dissipation. It is suggested that the lifetimes of the transient intermediates become longer rapidly with increasing the cluster size. Mackenzie and co-workers proposed a similar interpretation for the reactions of Rhn± (n = 7−30) with NO, where NO decomposition proceeds only for n ≤ 16 (except for n = 13), even in the longer timescale (seconds) of their FT-ICR experiments, while Rhn(NO)2± appears for n ≥ 9.9 Likewise, in the reactions of Con+ (n = 2−30) with NO, the observation of NO decomposition is limited to the small clusters (n = 5− 20).11 As shown in Figure 6, for Cu8O2−, the first and second NO molecules adsorb on bridge or 3-fold hollow sites, including the Cu atoms interacting with one O atom. On the other hand, such adsorption sites can be prepared neither in the small cluster dioxides with less Cu atoms nor in the highly oxidized clusters with more O atoms, as well as bare Cu clusters. Thus, efficient NO decomposition on CunO2− (n = 8, 10, and 12) may be allowed by the existence of reactive Cu atoms partially interacting with O atoms. A similar phenomenon has already

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ASSOCIATED CONTENT

S Supporting Information *

Potential energy diagram involving the clusters in the higher spin states (triplet or quartet) for Cu8O2− + 2NO → Cu6O4− + 2Cu (or Cu2) + N2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Calculations were performed using the Fujitsu PRIMERGY RX300 S7 of the Research Center for Computational Science, Okazaki Research Facilities, National Institutes of Natural Sciences. This work was supported by the Special Cluster Research Project of Genesis Research Institute, Inc. and by JSPS KAKENHI Grant 25390004.

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REFERENCES

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