Article pubs.acs.org/JPCA
Stability of Aluminum-Doped Copper Cluster Cations and Their Reactivity toward NO and O2 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: Aluminum-doped copper cluster cations, CunAl+, were produced via an ion sputtering method and analyzed by mass spectrometry. The measured size distributions show that Cu6Al+ and Cu18Al+ are highly stable species, which can be understood in terms of the electronic subshell 1P and 2S closings, respectively. Furthermore, the reactions of size-selected CunAl+ (n = 4−6 and 8−16) with NO and O2 were studied at near thermal energies by using a tandem-type mass spectrometer. The doping of an Al atom improves the reactivity of the clusters toward NO in particular for n = 9, 11, 13, and 15, whereas it does not change the reactivity toward O2 significantly. Consequently, it was found that CunAl+ (n = 9, 11, 13 and 15) are more reactive toward NO than toward O2. The high reactivity of Cu9Al+ toward NO compared to that of Cu10+ is explained in terms of the increase of the adsorption energy and the lowering of the barrier to dissociative adsorption, with the aid of calculations based on density functional theory. Moreover, the multiplecollision reactions of CunAl+ (n = 9, 11, and 13) with NO result in the production of cluster dioxides, Cun−3AlO2+, (i.e., release of N2), which clearly indicates that NO decomposition proceeds on these clusters. explained by the spherical electronic shell model.9 Furthermore, CuAl22− with 68 valence electrons was observed as a magicnumber cluster against O2 etching reactions and this etching resistance was interpreted in terms of a crystal field splitting of subshell levels arising from a geometric distortion of the cluster.15 These studies in refs 9−15 have focused on Cu-doped Al clusters and bimetallic clusters consisting of almost the same number of Cu and Al atoms at most. To our knowledge, however, the previous study on Al-doped Cu clusters, which can be models for local atomic configurations of α-Cu−Al surfaces, has been limited to a recent theoretical work on CO oxidation by small CunAl (n = 1−3) clusters.16 The reactions of single-element copper clusters with NO and O2 have been extensively investigated.17−22 These studies indicate that Cu clusters are more reactive toward O2 than toward NO, irrespective of the cluster size and the charge state. This implies that Cu clusters cannot adsorb nor react with NO molecules efficiently in the presence of excess O2. For example, such a comparative study of reactivity toward NO and O2 is very important for the application to new NOx reduction catalysts of lean-burn engines because the actual conversion efficiency of NOx is affected by the competition between NO and O2 for providing reactive oxygen atoms that react with CO and hydrocarbons.23
1. INTRODUCTION Alloy clusters and nanoparticles have attracted considerable attention owing to their promising applications in various fields such as catalysis, electronics, and magnetics.1 They often display different chemical and physical properties from those of single-element metal clusters and nanoparticles, and these properties may be modified by controlling the size, composition, and structure. Studies of size- and compositionselected bimetallic clusters also provide information on the optimal sizes and composition ratios for new alloy catalysts and fundamental understanding of the catalytic properties,2,3 because the reactivity can be investigated as a function of the number of atoms of each element with mass spectrometric techniques. Bimetallic alloy systems composed of copper and aluminum are of great interest for the chemical properties of their surfaces and free clusters. The adsorption of NO and O2 on the surfaces of the α-Cu−Al alloys, which have compositions below 19.6 at. % Al, has been studied because of their importance in heterogeneous catalysis.4−8 These studies showed that the adsorption rates of these molecules are significantly enhanced by doping a small amount of Al to Cu. On the contrary, for Cu−Al binary clusters,9−16 most of the experimental and theoretical efforts have been devoted to the studies on their electronic structures and stability because both metals exhibit free-electron-like behaviors of the valence electrons. For instance, the stability of clusters such as Cu6Al5+ with 20 valence electrons and Cu11Al10+ with 40 valence electrons was © XXXX American Chemical Society
Received: April 27, 2015 Revised: July 13, 2015
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DOI: 10.1021/acs.jpca.5b04018 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
the product ions, respectively. A partial reaction cross section, σp, for the formation of a given product ion was obtained using Ip σp = σr ∑ Ip (2)
Recently, we revealed that the preadsorption of seemingly harmful oxygen atoms onto copper cluster anions enhances the adsorption probability of NO and promotes the decomposition of NO.22,24 This could be attributable to the contribution of 3d orbitals of the copper atoms. It is thus expected that the doping of p- or d-block metals such as aluminum or transition metals also affect the cluster reactivity drastically. In this study, we investigate the reactivity of aluminum-doped copper cluster cations, CunAl+, toward NO and O2, as well as the size distribution of these clusters. The cluster stability is discussed in terms of the electronic shell structures. The measured reaction cross sections with NO and O2 are compared with each other to examine the specific reactivity toward NO and further evaluated in comparison with those for single-element copper clusters obtained in our previous studies21,22 to extract the role of the doped atom as a promoter.
where Ip/ΣIp represents the branching fraction for the product ion of interest.
3. RESULTS 3.1. Size Distributions of Cun+ and CunAl+. Figure 1a shows a typical mass spectrum of cluster cations produced by
2. EXPERIMENTAL SECTION The experimental apparatus used here mainly consists of a cluster ion source, cooling and reaction cells, three octopole ion guides (OPIGs), two quadrupole mass spectrometers (QMSs), and an ion detector, as described elsewhere.25 Clusters were produced by cosputtering four separate targets with accelerated xenon ion beams from the ion gun (CORDIS Ar25/35c, Rokion Ionenstrahl-Technologie). Here, three copper plates and one aluminum plate were used as targets. The sputtered cluster cations were extracted by a series of ion lenses and admitted into the 290 mm long cooling cell having an OPIG filled with He gas (>10−2 Torr) at room temperature. To measure the size distributions of the thermalized cluster cations, they were mass-analyzed with the first QMS and then detected by a secondary electron multiplier equipped with an ion conversion dynode, via the OPIGs and the second QMS in the ion guide mode. In the reaction experiments, the thermalized cluster cations were mass-selected by the first QMS and admitted into an OPIG passing through the 100 mm long reaction cell. In this reaction cell, the mass-selected cluster ions (parent cluster ions) were allowed to react with NO or O2 under single or multiple collision conditions. The single collision conditions were achieved below the NO (or O2) pressure of ∼2 × 10−4 Torr. The translational energy of the parent cluster ions in the reaction cell was measured by the retarding potential method using the OPIG passing through the reaction cell without reaction gas. The collision energy, Ecol, in the center-of-mass frame was obtained from the translational energy, and a typical spread of Ecol was estimated to be 0.4 eV in the full width at half-maximum (fwhm) (Figure S1, Supporting Information). Unreacted parent ions and product ions were mass-analyzed with the second QMS and then detected. The mass resolution of the second QMS was set to be relatively low to achieve the mass-independent high transmittance in the measurement of the reaction cross sections shown below. The total reaction cross section, σr, was derived from σr =
I + ∑ Ip kBT ln Pl I
Figure 1. (a) Typical mass spectrum obtained by cosputtering of three Cu and one Al targets. The peaks of CunAl+ extracted from panel (a) are shown in panel (b), where the intensity was magnified by a factor of 10 relative to (a).
the ion sputtering of Cu/Al targets. A series of small peaks assignable to CunAl+ appears in the mass spectrum, together with intense peaks of Cun+. The intensity of Cun+ drops significantly beyond the sizes of n = 3, 9, and 21, which is essentially consistent with the observation in our previous study where four Cu targets were used.21 For CunAl+, similar stepwise drops in the intensity are observed beyond n = 6 and 18 (Figure 1b). Additionally, even−odd alternations appear clearly for both Cun+ and CunAl+ clusters, where the clusters with an odd number of atoms are more abundant than the adjacent even-numbered ones. 3.2. Reactions of CunAl+ with NO. The reactions of CunAl+ (n = 4−6 and 8−16) with NO were studied at a nearthermal energy (Ecol = 0.2 eV). Under single-collision conditions, CunAlNO+, Cun−1AlNO+, and Cun−2AlNO+ are observed as the reaction products. These ions could be produced by the following reactions:
(1)
where kB is the Boltzmann constant, P and T are the pressure and temperature of the reactant gas, respectively, l (=120 mm) is the effective path length of the reaction region, and I and ΣIp represent the intensity of the unreacted parent ion passing through the reaction region and the sum of the intensities of B
DOI: 10.1021/acs.jpca.5b04018 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Cu nAl+ + NO → Cu nAlNO+
(3)
Cu nAl+ + NO → Cu n − 1AlNO+ + Cu
(4)
Cu nAl+ + NO → Cu n − 2AlNO+ + Cu 2 (or 2Cu)
section26 (σLGS = 49.2 Å2) estimated using the polarizability, α(NO) = 1.70 Å3.27 When a hard-sphere model used by Rosén and co-workers is applied,20 where the geometric sizes of a cluster and a molecule are included, the measured reaction cross section for Cu15Al+ is estimated to be ∼20% of the hardsphere cross section (σHS = 109.8 Å2). Here, the radius of Cu15Al+ is assumed to be equal to that of Cu16. The multiple-collision reactions with NO were also investigated for specific reactive clusters, CunAl+ (n = 9, 11, and 13). Figure 3 shows a typical mass spectrum obtained in
(5)
+
The chemisorbed intermediate, CunAlNO , can be observed if its lifetime is longer than the flight time (∼100 μs) from the reaction cell to the second QMS. Additionally, the dependence on the collision energy was investigated in Ecol = 0.2−0.8 eV for Cu9Al+ as a typical example. The partial cross sections for reactions 3−5 decrease monotonically with the collision energy, which indicates that these reactions are exothermic processes without significant barriers. The branching fractions for reactions 4 and 5 tend to increase with the collision energy whereas that for reaction 3 decreases, though reaction 4 is the main process in Ecol = 0.2−0.8 eV. Figure 2 shows the total and partial reaction cross sections at Ecol = 0.2 eV as a function of the number of copper atoms in a
Figure 3. Typical mass spectrum obtained by the reaction of Cu9Al+ with NO under a multiple collision condition. The NO pressure is ∼3 × 10−3 Torr and the initial collision energy is 0.2 eV.
the reaction of Cu9Al+ with NO at the pressure of ∼3 × 10−3 Torr. In addition to the peaks of the single-collision reaction products, CumAlNO+ (m = 7−9), a peak appears at m/z = 440, which corresponds to Cu6AlO2+, not Cu6AlNO+ (m/z = 438). This assignment is supported by the result of NO pressure dependence experiments at first. As shown in Figure 4, the
Figure 2. (a) Total cross sections for the reactions of CunAl+ (n = 4− 6, 8−16) with NO as a function of the number of copper atoms in the cluster. The total cross sections for Cun+1+ + NO in ref 22 are also plotted for comparison. (b) Partial cross sections are shown for the formation of CunAlNO+, Cun−1AlNO+, and Cun−2AlNO+. The collision energy is 0.2 eV. Figure 4. Relative intensities of the parent ion and the major product ions (Cu9AlNO+, Cu8AlNO+, Cu7AlNO+, and Cu6AlO2+) in the reaction of Cu9Al+ with NO as a function of the NO pressure. The initial collision energy is 0.2 eV. The dashed lines represent the firstand second-order dependences on the NO pressure for eye guide.
+
parent cluster cation (n). CunAl (n = 4−6) have relatively small cross sections, and Cu6Al+ is unreactive with NO at this collision energy at all. On the contrary, a remarkable even−odd alternation is observed in the total reaction cross section for n = 8−16, where the clusters with odd-numbered n are more reactive than the adjacent ones with even-numbered n. At this collision energy, the formation of Cun−1AlNO+ is the main reaction at n = 9, whereas the formation of CunAlNO+ is dominant at n = 11, 13, and 15. The total reaction cross sections for these four clusters are found to be larger than 10 Å2. Cu15Al+ shows the largest cross section of 23.8 Å2, which corresponds approximately to half of the Langevin cross
relative intensity of this product exhibits the second-order dependence on the NO pressure, which indicates that this formation involves the adsorption of two NO molecules. It is not impossible that Cu6AlNO+ is produced in a sequential reaction of Cu9AlNO+ with NO. However, this reaction should be highly endothermic because Cu6AlNO+ is not observed in the single collision reaction of Cu9Al+ + NO even at Ecol = 0.8 C
DOI: 10.1021/acs.jpca.5b04018 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A eV. These findings suggest that the dissociative adsorption of two NO molecules occurs and is followed by the release of three Cu atoms and an N2 molecule as follows: Cu 9Al+ + 2NO → Cu6AlO2+ + [3Cu, N2]
reaction product changes roughly from Cun−2AlO2+ to Cun−1AlO2+ with the increase of n. CunAlO2+, which is a transient intermediate, appears only at n = 16, which is the largest size studied here.
(6)
4. DISCUSSION Specific copper clusters such as Cu3+, Cu9+, and Cu21+ have been reported as magic-number clusters in previous studies.17,21,28 These clusters have the total numbers of valence electrons of 2, 8, and 20, respectively, which correspond to the closed shells as described by the electronic shell model.29 The same magic-number clusters are also observed in the present mass spectrum (Figure 1a). Doped with an Al atom, the observed size distributions show particular steps at Cu6Al+ and Cu18Al+. The stability of Cu6Al+ is further supported by the results that this cluster is unreactive toward both NO and O2 molecules. Because an aluminum atom has three valence electrons, the total numbers of valence electrons in Cu6Al+ and Cu18Al+ are 8 and 20, respectively, by considering one electron from each Cu atom and the positive charge. These numbers correspond to 1P- and 2S-subshell closings predicted by the electronic shell model.29 A similar result was obtained for Au clusters doped with an Al atom; Au6Al+ and Au18Al+ were observed as stable clusters.30 Further, the high reactivity of CunAl+ clusters with odd-numbered n toward both molecules can be explained by the presence of an unpaired electron. The reaction cross sections of single-element Cu cluster cations with NO and O2 have been reported in our previous studies.21,22 Therefore, we can investigate quantitatively the effect caused by substituting an Al atom for a Cu atom in the cluster. As shown in Figure 2a, the total reaction cross sections of CunAl+ + NO are larger than those of Cun+1+ + NO, in particular for n = 9, 11, 13, and 15. For O2, on the contrary, the reactivity of Cu clusters does not change drastically by the substitution of an Al atom, except for n = 5 (Figure 5a). Figure 6 compares the ratios of the total reaction cross sections with
Cun−3AlO2+,
Similarly, a dioxide cation, is observed in the reactions of CunAl+ (n = 11 and 13), but this formation starts at a higher NO pressure (∼4 × 10−3 Torr). 3.3. Reactions of CunAl+ with O2. In the reactions of CunAl+ (n = 4−6 and 8−16) with O2, three kinds of product ions (CunAlO2+, Cun−1AlO2+, and Cun−2AlO2+) are detected at Ecol = 0.2 eV under single collision conditions. These observations indicate that the following reactions occur: Cu nAl+ + O2 → Cu nAlO2+
(7)
Cu nAl+ + O2 → Cu n − 1AlO2+ + Cu +
+
Cu nAl + O2 → Cu n − 2AlO2 + Cu 2 (or 2Cu)
(8) (9)
Figure 5 indicates the size dependence of the total reaction cross section, together with those of the partial cross sections
Figure 5. (a) Total cross sections for the reactions of CunAl+ (n = 4− 6, 8−16) with O2 as a function of the number of copper atoms in the cluster. The results for Cun+1+ + O2 in ref 21 are also plotted for comparison. (b) Partial cross sections are shown for the formation of CunAlO2+, Cun−1AlO2+, and Cun−2AlO2+. The collision energy is 0.2 eV.
Figure 6. Ratios of the total reaction cross sections of CunAl+ with NO and O2, σr(NO)/σr(O2), at the collision energy of 0.2 eV. The ratios of Cun+1+ obtained from the data in refs 21 and 22 are also plotted for comparison.
for these reactions. The total reaction cross section tends to increase with the number of Cu atoms and also exhibits a clear even−odd alternation in n ≥ 10. For all the clusters studied, the measured total reaction cross sections (
