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Thermal Desorption Spectroscopy Study of the Adsorption and Reduction of NO by Cobalt Cluster Ions under Thermal Equilibrium Conditions at 300 K Kohei Koyama, Satoshi Kudoh, Ken Miyajima, and Fumitaka Mafuné* Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan

Downloaded by SUNY UPSTATE MEDICAL UNIV on September 5, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.jpca.5b05320

S Supporting Information *

ABSTRACT: Adsorption of NO molecules on gas phase cobalt cluster ions, Con+ (n = 4−9), was investigated in thermal equilibrium with He gas at 300 K. The Con+ clusters, contrary to the isolated clusters in a vacuum, adsorbed NO without undergoing significant dissociation. Thermal desorption spectroscopy of Con+(NO)m indicated that Con+ clusters with n = 4−6 and n = 7−9 can have four and six adatoms chemisorbed, respectively. Reduction of NO occurred, releasing N2 molecules, to form Con+Ok(NO)m−k (k = 2, 4, ...). The reaction mechanism involved the exchange of chemisorbed N atoms with the O atom in NO bound to the clusters. The reactivity of Con+ (n = 4−9) exhibited periodic n dependence, and Co6+ and Co9+ was similar to the case of the isolated Co16+ clusters holding up to eight adatoms reported by Anderson et al. (J. Chem. Phys. 2009, 130, 10992−11000).

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INTRODUCTION The catalytic reduction of NO represents one of the most important processes in environmental chemistry. NO reduction by transition metals has been the subject of intensive research over the past few decades. However, there is much to this chemistry that remains unknown. In particular, a thorough understanding of the reaction mechanism is lacking. To this end, the use of a gas-phase cluster to study this reaction is preferable, as atoms and molecules involved in the reaction are well separated and the number of reactions possible is limited. Rhodium is a popular catalyst used in applications such as three-way catalytic converters in automobiles. Mackenzie and his co-workers reported reduction of NO on isolated Rhn± in the gas phase using mass spectrometry.1−3 For small clusters (n < 17), N2 was produced after the adsorption of two NO molecules. The result was interpreted in terms of dissociative chemisorption of NO, generating N and O atoms that were able to migrate on the surface of the cluster. It was suggested that the reduction was likely driven by heat generated upon NO chemisorption onto Rhn± in a vacuum. Cobalt, a member of the same chemical group as Rh, is also expected to show reactivity to NO. Collisional reactions of NO with Con+ (n = 2−10) were studied by measuring the absolute cross sections for ionic products created in a beam-gas geometry at different collision energies and initial Con+ internal temperatures.4 Chemisorption of NO on Con+ was observed producing Con+(NO) and Con−1+(NO). Similar dissociation reaction upon chemisorption of NO was observed by Anderson et al.5 In addition, Anderson et al. studied the reactions of Con+ with NO under multiple collision conditions by storing clusters for extended periods, and found that NO was reduced producing molecular nitrogen. © XXXX American Chemical Society

Klaassen et al. elucidated dissociative chemisorption of NO on Co3+ and Co4+ on the basis of a collision-induced dissociation study.6 They investigated the collisional reaction of ConN16O+ with an isotope-labeled 18O2 and observed displacement reactions forming ConO2+. It was shown that 16O was present in significant quantities in both Co3O2+ and Co4O2+. The presence of 16O was due to NO undergoing dissociative chemisorption on Co3+ and Co4+.7 The dissociative chemisorption of two NO molecules on Con+ (n = 5, 6 and 7) was clearly shown by Hanmura et al. using the DFT calculation. They also showed that the CoN2 fragment may be released upon adsorption of two NO molecules on Co5+, forming Co4+O2.8 Several previous studies showed that chemisorption of even a single NO molecule was accompanied by dissociation reaction of Con+, because the adsorption energy exceeded the bond dissociation energy D(Con−1+−Co):9 The adsorption energy was distributed in the vibrational degree of freedom, which caused dissociation of the clusters. Taking into account previous studies, we wanted to know whether the dissociation could be prevented by removing the internal energy from Con+, and whether N2 could be formed upon coadsorption of two NO molecules on Con+ in a thermal equilibrium at 300 K. We therefore examined the reaction of Con+ with NO in thermal equilibrium in He at 300 K. This process involved the introduction of Con+ into a reaction gas cell filled with He and NO.10,11 Collisions between the clusters and He were expected to result in a good thermal equilibrium with the reaction gas cell maintained at 300 K. In addition, Con+ containing adsorbed Received: June 4, 2015 Revised: August 27, 2015

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DOI: 10.1021/acs.jpca.5b05320 J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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COMPUTATIONAL DETAILS To estimate the adsorption energies of N and O atoms on Co6+, DFT calculations were performed using the Gaussian09 program.15 The 6-311+G(d) basis set was used to describe all atoms.16,17 Becke’s three-parameter hybrid density functional18 with the Lee−Yang−Parr correlation functional19 (B3LYP) was used for all calculations. The geometrical structures of the dissociatively chemisorbed Co6NO+ and Co6(NO)2+ reported by Hanmura et al.8 were adopted as the initial structures, and the structures were reoptimized by the DFT calculations.

NO molecules was introduced into an extension tube elevated to a temperature of 300−700 K. The reactions were monitored by mass spectrometry.


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EXPERIMENTAL SECTION

Gas phase cobalt clusters, Con+ (n = 4−9), were prepared in a cluster source and, after introduction into a reaction gas cell and an extension tube (Figure S1), were detected using a reflectron-equipped time-of-flight mass spectrometer.12,13 A Co metal rod (Nilaco Co., Ltd., 99.9%) was set downstream of the He gas flow from a solenoid pulsed valve (General Valve). The rod was irradiated with focused laser pulses from a Nd:YAG laser (ca. 10 mJ per pulse, Continuum Surelite II) at 532 nm, forming Co clusters above the rod. The clusters were cooled in a cylindrical channel (6 mm diameter) using He gas (>99.99995%). The clusters were passed through a reaction gas cell (2 mm diameter, 60 mm long), where reactant gas, nitric oxide (NO), diluted by He, was injected by using the other solenoid pulsed valve to examine the chemical reactivity of the clusters. The concentration of NO in the valve varied by changing the mixing ratio of NO and He gases while keeping the pressure at 1 bar. Mass flow controllers and a pressure controller automatically scanned the concentration of NO. The amount of NO introduced into the vacuum chamber through the valve was monitored by a residual gas analyzer combined with a mass spectrometer (MKS e-Vision2). Direct measurement of the number density of NO in the reaction gas cell was quite hard, so that it was estimated by observing the reaction, Nbn+ + O2, under the identical experimental condition, whose rate constant is well-known.14 The number density of NO was 3.0 × 1013 molecules/cm3 at the mixing ratio of 10%. The clusters were then passed through an extension tube (6 mm diameter, 116 mm long), in which the temperature was varied in the range 300−1000 K using a resistive heater. The temperature was monitored with multiple thermocouples. The residence time of the cluster ions and the density of the He gas in the extension tube were estimated to be ∼100 μs and ∼1017 molecules cm−3, respectively. Hence, thermal equilibrium of the clusters was achieved through collisions with the He carrier gas well before expansion into the vacuum. The composition changes of the cluster ions resulting from reaction and thermal desorption were monitored by mass spectrometry. During the mass analysis, the cluster ions gained 3.5 keV kinetic energy in the acceleration region. After traveling in a 1 m field-free region, the ions’ direction was reversed by the reflectron and detected using a Hamamatsu double-microchannel plate detector. Signals from the detector were amplified with a preamplifier (Stanford Research Systems SR445A) and digitized using an oscilloscope (LeCroy LT374). The TOF spectra, accumulated for typically 500 sweeps, were sent to a computer for data storage and analysis. The mass resolution (m/Δm) of our mass spectrometer was high (∼1000 at m = 500) enough to identify a H atom that could be involved in the clusters such as Co6+O(H2O). Note that H2O existed in the vacuum chamber as an impurity. However, it was still difficult to distinguish Co6+O2 (m/z = 385.60) from Co4+(NO)5 (m/z = 385.76) with the mass resolution capability of the instrument. Hence, we used isotope-labeled 15NO to check the mass assignment of the ion peaks (Figure S2).

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RESULTS Figure 1 shows the mass spectra of Con+ before and after reaction with NO in He gas at 300 K. In the present experiment

Figure 1. Mass spectra of Co6+ before and after the reaction with NO at room temperature in He gas. The concentration of NO was (a) 0%, (b) 1%, (c) 3%, and (d) 20%. Asterisks in (d) show ion peaks assignable to a series of Co4+Ok(NO)m and Co5+Ok(NO)m (k = 0, 2, 4, ...; m = 1, 2, 3, ...). The number density of NO in the reaction gas cell was estimated to be 3.0 × 1013 molecules/cm3 at the mixing ratio of 10%.

Con+ (n = 4−9) was observed, but the mass range in the vicinity of Co6+ and Co7+ is magnified for ease of viewing in Figure 1. After reaction with NO, ion peaks corresponding to Co6+(NO) appeared first. As the concentration of NO in the reaction gas cell increased, the number of NO molecules added to Co6+, m, gradually increased forming Co6+(NO)m (m = 2, 3, ...). In addition, ion peaks assignable to Co6+O2 appeared, suggesting N2 was released from the clusters. As the concentration of NO increased further, Co6+O2(NO) was detected. Note that trace amounts of peaks corresponding to Co6+O and Co6+N appeared in the mass spectrum. As these peaks were observed without NO in the reaction gas cell, we interpreted this observation as resulting from the generation of clusters by laser ablation of preadsorbed NO or O2 on the Co rod surface. Indeed, the signal intensity of Co6+O and Co6+N did not change significantly as the concentration of NO in the gas cell changed. The reaction processes of the clusters were examined by measuring the signal intensities of Co6+Ok(NO)m (k = 0, 2; m = 0, 1, 2, ...) as a function of NO concentration in the reaction gas cell, as shown in Figure 2. The intensity of the Co6+ signal decreased and the signal for Co6+(NO) increased as the concentration of NO increased. These results indicated that Co6+(NO) was formed by the adsorption of NO molecules on the naked cluster in He gas according to eq 1, whose rate constant ranges in ∼1.0 × 10−9 cm3s−1 (Figure S3), B


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The intensity of the Co6+O2(NO)m signal also increased as the concentration of NO increased. This concentration dependence showed that the intensity of the Co6+O2(NO)m signal leveled off at higher concentrations. If Co6+O2(NO)m formed by the stepwise adsorption of NO molecules by Co6+O2, one would expect that Co6+O2 would decay as the concentration of NO increased, similar to that observed for Co6+(NO). The trend toward intensity leveling of the Co6+O2(NO)m signal indicated that this compound was the final product in a series of reactions. Hence, we can consider that adsorption of NO molecules by Co6+O2 is the minor process in the formation of Co6+O2(NO)m. Instead, it is likely that Co6+O2(NO) was formed from Co6+(NO)3 by the release of N2, as shown in eq 6. Co6+(NO)3 → Co6+O2 (NO) + N2

As described above, Co6+Ok(NO)m (k = 0, 2; m = 0, 1, 2, ...) was formed in He at 300 K, when the concentration of NO in the reaction gas cell was high enough. To examine how strongly NO was bound to Con+, the clusters were heated in an extension tube elevated to 773 K after the initial preparation in He at 300 K. Figure 3 shows plots displaying the intensities of Con+Ok(NO)m for different size, n, at 300 and 773 K. For n = 6, the clusters involved a number of NO molecules at 300 K, whereas Co 6 + , Co 6 + (NO), Co 6 + (NO) 2 , Co 6 + O 2 , and Co6+O2(NO) were the only remaining species after passing through the extension tube heated at 773 K. Among others, Co6+(NO)2 and Co6+O2(NO) were dominant. Evidently, up to four atoms, either N or O, adsorbed strongly on Co6+. Similarly, for n = 9, Co 9 + (NO) m ≤ 3 , Co 9 + O 2 (NO) m ≤ 2 , and Co9+O4(NO)m≤1 having fewer than six adatoms, are most thermally stable (Figures S4 and S5). To examine the thermal desorption processes more closely, the intensities of Co6+Ok(NO)m (k = 0, 2) were measured as a function of the extension tube temperature (Figure 4).12 It was found Co6+(NO)m≥3 decreased and Co6+(NO)2 increased in their signal intensities in the vicinity of 500−700 K. These concomitant changes were mainly caused by the release of NO molecules into the gas phase as shown in eq 7.

Figure 2. Intensity ratios of Co6+Ok(NO)m (k = 0, 2) as a function of concentration of NO in the reaction gas cell: (a) Co6+(NO)m (m = 0− 4); (b) Co6+O2(NO)m (m = 0−4). The number in the figure represents the number of NO molecules, m, in the cluster. Solid lines are drawn as a guide to the eye. The number density of NO in the reaction gas cell was estimated to be 3.0 × 1013 molecules/cm3 at the mixing ratio of 10%.

Co6+ + NO → Co6+(NO)

(1)

As the concentration of NO increased, more NO molecules were adsorbed according to eqs 2−4. Co6+(NO) + NO → Co6+(NO)2

(2)

Co6+(NO)2 + NO → Co6+(NO)3

(3)

Co6+(NO)3 + NO → Co6+(NO)4

(4)

+

In addition, Co6 O2 began to appear in synchronization with the formation of Co6+(NO)2. Considering the composition of the clusters, Co6+O2 was likely produced from Co6+(NO)2 by releasing N2 according to eq 5. Co6+(NO)2 → Co6+O2 + N2

(6)

Co6+(NO)m → Co6+(NO)m − 1 → ... → Co6+(NO)2

(5)

(7)

For k = 2, Co6+O2(NO)m≥2 decreased in the same temperature range, which resulted in the increase of Co6+O2(NO) as shown in eq 8.

In other words, the reaction pathway splits for Co6+(NO)2 leading to Co6+O2, or Co6+(NO)3 by the adsorption of one NO molecule.

Figure 3. Intensity maps of Con+NmOk for n = 4−9 produced after passing through the extension tube heated at 300 and 773 K. The gray scale displays intensity of the cluster in each cell, and the darker cell corresponds to the higher intensity. C


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However, we confidently concluded that Co6+(NO) was formed under our experimental conditions by the simple adsorption of NO molecules onto naked Co6+ in He gas for two reasons: 1. If dissociation occurred, the total intensity of Co6+, Σm{[Co6+(NO)m] + [Co6+O2(NO)m]}, would change significantly after reaction with NO. However, the amplitude of the variation was less than 10% in the NO concentration range 0−5% (Figure S6). Perhaps, dissociation of Co7+ to Co6+, and then from Co6+ to Co5+ was just balanced, so that no net change could be detectable. Nevertheless, the amplitude of the variation was less than 20% for Co4−9+ (Figure S6). The previous work for isolated Con+ in a vacuum showed that the ratio of the dissociation depends on cluster size, n, and 90% of Co7+ and 30% of Co6+ were to become Co6+ and Co5+ after adsorption of NO, respectively.4,5 Hence, from a quantitative perspective, the dissociation was considered to be minimal. 2. Hanmura et al. reported reactions of isolated Con+ in a vacuum in which the reaction of Con+(NO) occurred with one NO molecule, and the reaction of Con+ with two NO molecules.8 The reduction of NO occurred predominantly for Con+(NO) (n = 4−6) with the maximum reaction cross section at n = 5. In contrast, reduction dominated for n ≥ 5 for naked Con+, as the release of one Co atom was accompanied by adsorption of the first NO molecule onto Con+. In the present experiment, the reduction of NO occurred for n ≥ 4. The dependence of reactivity was more consistent with that of the isolated Con+(NO) in a vacuum than Con+. Hence, dissociation following adsorption of NO on Con+ was considered minimal. Anderson et al. succeeded in controlling dissociation of clusters for isolated Con+ (n ≥ 13) in a vacuum by taking advantage of a sufficient number of internal degrees of freedom.5 When the number density of NO gas for the reaction was low, collisions with NO molecules were less frequent so that the clusters had more time to react with multiple NO molecules. In the meantime, the available energy was dissipated by radiative cooling so that dissociation was prevented. In contrast, when the number density of the NO gas was high, accumulation of available energy was greater than radiative cooling, causing dissociation. In our experimental setup, NO was adsorbed onto Con+ in the reaction gas cell filled by He. The gas pressure inside the reaction cell, mostly due to He, was monitored using a pressure gauge. The pressure rose to almost 3 × 103 Pa during pulsing. The number density of He gas was estimated to be 7 × 1017 molecules cm−3, giving a collision frequency of 1.2 × 108 s−1. Because the residence time of the cluster ions in the reaction gas cell was estimated to be ∼70 μs, the clusters were subjected to 8400 collisions. Hence, the adsorption energy of Con+ + NO was eliminated by the surrounding He gas. Adsorption and Reduction of NO by Co6+. The overall reaction scheme of Co6+, determined on the basis of the NO

Figure 4. Thermal desorption curve for Co6+Ok(NO)m (k = 0, 2) as a function of temperature in the extension tube.

Co6+O2 (NO)m → Co6+O2 (NO)m − 1 + NO → ... → Co6+O2 (NO)2 + (m − 2)NO

(8)

Reactions in Preset Thermal Conditions. We assumed that dissociation of Con+ upon adsorption of NO by the cluster, i.e., release of the Co atom, was negligible when the intensity of the signals for Co6+Ok(NO)m (k = 0, 2) were viewed (Figure 2). However, dissociation as Co7+ + NO → Co6+(NO) + Co and Co6+ + NO → Co5+(NO) + Co were possible. Because the cluster ions were introduced into the reaction gas cell and probed simultaneously by mass spectrometry, dissociation was not directly identifiable. In fact, Hanmura et al. showed that, for isolated Co6+, both Co6+NO and Co5+NO were formed after the reaction with NO, as shown in eqs 9 and 10. Co6+ + NO → Co6+NO Co6+ + NO → Co5+NO + Co

(9) (10)

The quantity of Co6+NO produced was no more than 30%.4 Dissociation upon adsorption of NO is possible, because adsorption of NO is exothermic.20 According to DFT calculations, NO is dissociatively chemisorbed onto Co6+ with an adsorption energy of 3.3 eV. The bond dissociation energy of Co5+NO−Co is ∼2.4 eV. Hence, available energy generated by chemisorption likely caused dissociation, releasing a Co atom. Note that the bond dissociation energy of Con+ is low because D(Con−1+−Co) is 2.04, 2.12, 2.84, 3.31, and 2.93 eV for n = 3, 4, 5, 6, and 7, respectively.9 Scheme 1

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Equations 11 and 12 show that Co6+ has a high affinity for oxygen. The adsorption energy of a second NO molecule onto Co6+NO is represented by eq 13.

concentration dependence and thermal desorption, is summarized in Scheme 1. There is a boundary line connecting Co6+(NO)2 and Co6+O2(NO), both of which include four adatoms. A variety of clusters involving a number of adsorbed NO molecules were formed at 300 K, but the clusters located outside this boundary completely disappeared when they were heated to 773 K. Hence, these NO molecules were considered to be weakly adsorbed. In contrast, the clusters on and inside the boundary line, Co6+, Co6+(NO), Co6+(NO)2, Co6+O2, and Co6+O2(NO), remained after heating at 773 K, indicating that the four adatoms had been strongly adsorbed. Figure 5 shows the optimized geometry of selected Co6+Ok(NO)m obtained by DFT calculations. The optimized

Co6+NO + NO → Co6+NNOO

ΔE = − 4.6 eV (13)

Then, eq 5 was calculated to be slightly endothermic as shown in eq 14. Co6+NNOO → Co6+OO + N2

ΔE = 0.4 eV

(14)

The adsorption energy of the third NO molecule adsorbed onto Co6+NNOO is shown in eq 15. Co6+NNOO + NO → Co6+NNOO(NO) (15)


ΔE = −0.9 eV

Hence, eq 6 was also found to be exothermic as shown in eq 16. Co6+NNOO(NO) → Co6+NOOO + N2 (16)

ΔE = −3.2 eV

Although the energy barrier in the reaction pathway was not calculated, the energy balance suggested that eq 6 was more exothermic than eq 5. In the optimized structure, the third NO molecule adsorbed molecularly to Co6+NNOO, as represented by the formula Co6+NNOO(NO), indicating that four atoms adsorbed strongly on the surface of Co6+. The adsorption energy of one NO molecule on Co6+O2 was found to be quite exothermic according to the DFT calculation as represented by eq 17.

Figure 5. Optimized geometries of Co6+Ok(NO)m obtained by a DFT calculation are shown with the spin multiplicity, M(=2S + 1). The number given under the chemical formula shows the adsorption energy with respect to the energy of the initial state in eV, Co6+ + (k/ 2)O2 + mNO.

Co6+OO + NO → Co6+NOOO

(17)

However, the concentration dependence shown in Figure 2 suggests that adsorption of NO onto Co6+OO was minimal. It is possible that a high-energy barrier prevented the NO molecule from adsorbing onto Co6+OO. Highlighting the fact that Co6+ was able to possess four chemisorbed atoms, the reduction from Co6+NNOO(NO)m to Co6+NOOO(NO)m−1 is described in a more general form as eq 18.

geometry of Co6+ was octahedral with eight faces. The geometry of Co6+(NO) showed that the first NO dissociatively adsorbed on the surface with both adatoms filling hollow sites. The adsorption energy, defined as the energy of Co6+(NO) minus the sum of the energies of the isolated Co6+ and NO before adsorption, was calculated to be −3.3 eV. The second NO molecule also dissociatively adsorbed on the surface, where all the adatoms sat on nonadjacent hollow sites. The adsorption energy, defined as the energy of Co6+(NO)2 minus the sum of the energies of the separated Co6+(NO) and NO, was calculated to be −4.6 eV. Hence, these four adatoms were so strongly bound to Co6+ that they were not released into the gas phase by heating to 773 K. DFT calculations showed that Co6+ possessed four chemisorbed atoms. In fact, the third and fourth NO molecules were found to adsorb molecularly on the surface with adsorption energies of −0.9 and −1.4 eV, respectively. To examine the energetics of adsorption and reduction, the geometries and energies of Co6+O2, Co6+O2(NO), and Co6+O4 were also calculated. The adsorption energy of O2 on Co6+ is represented by eq 11. Co6+ + O2 → Co6+OO

ΔE = − 5.7 eV

Co6+NNOO(NO)m → Co6+NOOO(NO)m − 1 + N2 (18)

Equation 18 is interpreted as the exchange of chemisorbed N atoms for the O atom in NO weakly bound to the cluster. This conclusion was formulated because Co6+ has a greater affinity for an O atom than a N atom, according to DFT calculations. Cluster Size Dependence. As shown in Figure 3, a similar boundary line exists for n = 9 between Co9+(NO)3 and Co9+O4(NO). A variety of Co9+Ok(NO)m (k = 0, 2, 4; m = 0, 1, 2, ...) were initially formed by the reaction of Co9+ with NO in He gas at 300 K, but only Co9+(NO), Co9+(NO)2, Co9+(NO)3, Co 9 + O 2 , Co 9 + O 2 (NO), Co 9 + O 2 (NO) 2 , Co 9 + O 4 , and Co9+O4(NO) were remained after heating at 773 K. Similar to Co6+, the boundary line suggested that Co9+ was able to possess a maximum of six chemisorbed atoms (Figures S4 and S5). In addition, existence of such clear boundary lines indicates that dissociation of cobalt clusters after reaction with NO is minimal as discussed in the formation section. If a cobalt atom was released from Co9+, boundary lines could appear on for n = 8. However, no such line was not observed actually at n = 8 (Figure 3).

(11)

Co6+,

As two oxygen atoms adsorbed dissociatively onto they are shown in the chemical formula as Co6+OO. The adsorption energy of one NO molecule on Co6+ is represented by eq 12. Co6+ + NO → Co6+NO

ΔE = −3.3 eV

ΔE = − 4.6 eV

(12) E


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Figure 6. (a), (b) Intensities of Co5+Ok(NO)m (k = 0, 2) as a function of concentration of NO in the reaction gas cell. As the ion peak of Co5+O2 (m = 0) totally overlaps with that of Co3+(NO)5 in the mass spectrum, the signal intensity of Co5+O2 can be contributed by the signal intensity of Co3+(NO)5. On the basis of the isotope labeled experiments using 15NO, the contribution to the signal by Co3+(NO)5 was found to be negligible below the NO concentration of 18%, whereas it is 50% at the NO concentration of 25%. The number density of NO in the reaction gas cell was estimated to be 3.0 × 1013 molecules/cm3 at the mixing ratio of 10%. (c), (d) Thermal desorption curve for Co5+Ok(NO)m (k = 0, 2) as a function of temperature in the extension tube. (e) Reaction scheme of Co5+ with NO.

shows that Co5+O2(NO)m prepared at 300 K released a number of NO molecules into the gas phase above 600 K, forming Co5+O2(NO). This result suggested that Co5+ was able to have four chemisorbed atoms on the surface whereas the other NO molecules were loosely bound to them. The reaction scheme for n = 5 is summarized in Figure 6e. For n = 7 and 8, Co7+O4(NO) and Co8+O4(NO) were formed after thermal desorption at 773 K, as shown in Figure 3. It is evident that Co7+ and Co8+ were able to possess six chemisorbed atoms. Reaction at Higher Temperature. When the clusters were heated to 1000 K after the reaction with NO, Co6+O4 was found (Figure 7). The intensity of the peak was so low it was difficult to assign an intermediate species leading to Co6+O4. Nevertheless, from the composition, Co6+O4 formed from Co6+(NO)4 as shown in eq 19.

Anderson et al. reported a similar chemistry for n = 16 by collision-induced dissociation experiments.5 NO adsorbed on the surface of Co16+ until Co16+(NO)4 formed, at which point NO reduction, signified by N2 loss, occurred producing Co16+O8. Over the course of the reactions, Co16+(NO)4, Co16+O2(NO)3, Co16+O4(NO)2, Co16+O6(NO), and Co16+O8 were observed in the mass spectrum, corresponding to the boundary line of the eight atoms. Other clusters exhibited a different chemistry. For n = 4 and 5, Co4+O2(NO) and Co5+O2(NO) were formed after the thermal desorption at 773 K, as shown in Figure 3. This result indicated that Co4+ and Co5+ were able to possess four chemisorbed atoms. If a similar reaction mechanism occurred as for n = 6 and n = 9, the formation of Co4+(NO)2 should be observed for n = 4. In fact, Co4+(NO)3,4 appeared at 300 K in the mass spectrum and then disappeared at 773 K. The disappearance of this peak was probably due to release of N2 accelerated by heat to yield Co4+O2(NO). For n = 5, Co5+O2(NO) was formed after thermal desorption at 773 K. The concentration dependence of the intensities of Co5+Ok(NO)m at 300 K in Figure 6a,b exhibits features different from the one for n = 6: It is seen that formation of Co5+(NO)m≥2 was suppressed completely, suggesting that NO reduction occurred in Co5+(NO)2 more readily than further adsorption of NO onto Co5+(NO)2. In addition, the sequential adsorption of NO molecules onto Co5+O2 occurred readily, as manifested by the intensity alternation of Co 5 + O 2 , Co5+O2(NO), and Co5+O2(NO)2 in the higher NO concentration range. The thermal desorption curve in Figure 6c,d

Figure 7. Intensity maps of Con+NmOk(NO)m at 1000 K for n = 4, 6, and 9. The gray scale displays the intensity of the cluster in each cell, and the darker cell corresponds to the higher intensity. F


The Journal of Physical Chemistry A Co6+(NO)4 → Co6+O4 + 2N2

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(19)

*F. Mafuné. E-mail: [email protected]. Tel: +813- 5454-6597. Notes

Co6+NNOO(NO)(NO) → Co6+OOOO + 2N2

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (A) (No. 25248004) and for Exploratory Research (No. 26620002) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT), and by the Genesis Research Institute, Inc. for the cluster research. FM acknowledges the contribution of Dr. Michał P. Kwiatkowski in the early stage of this experiment.

In addition, Anderson et al. reported similar reactions for n = 16 forming Co16+O8 through the reaction shown in eq 21.

Co16+O6 (NO) + NO → Co16+O6 (NO)2 → Co16+O8 + N2

(21)

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Co4+O4

If a similar reaction proceeds for n = 4, could be formed from Co4+O2(NO)2, which was abundantly formed at 300 K. However, the intensity of the peak for Co4+O4 was weak. It is likely that adsorption sites were fully occupied in Co4+O2(NO). Hence, NO was unable to adsorb onto a site to exchange adsorbed N atoms with O atoms from NO.

REFERENCES

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CONCLUSION The reactivity of Con+ (n = 4−9) with NO was investigated under thermal equilibrium conditions in He at 300 K. The Con+ clusters in He gas, contrary to the isolated clusters in a vacuum, were found to adsorb NO molecules forming Con+(NO)m without any significant dissociation of Con+. This adsorption occurred because of the quick dissipation of adsorption energy from the clusters by collision with He atoms. Although a number of NO molecules were adsorbed, Con+ with n = 4−6 and n = 7−9 were able to have only four and six adatoms chemisorbed on the clusters, respectively. The rest of the NO molecules were found to be weakly adsorbed. The DFT calculations suggested that, for n = 6, the first and second NO molecules dissociated and chemisorbed with an adsorption energy of −3.3 and −4.6 eV, respectively. Reduction of NO was signified by N2 loss from the clusters. N2 loss occurred at 300 K, releasing N2 in the gas phase for all n studied. The reduction was considered to occur within the chemisorbed NO and/or between the chemisorbed NO and weakly adsorbed NO. The reactivity of Con+ (n = 4−9) exhibited periodic n dependence, and Co6+ and Co9+ was similar to the case of the isolated Co16+ clusters holding up to eight adatoms reported by Anderson et al.

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AUTHOR INFORMATION

Corresponding Author

The DFT calculation indicated that eq 19 is exothermic as shown in eq 20.

ΔE = −6.1 eV

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b05320. Schematic diagram of the experimental setup; mass spectrum of Co6+ after reaction with 15NO; rate constant for the adsorption reaction, Con+ + NO →Con+(NO) (n = 3−9) as a function of cluster size, n; intensity of the peaks for Co9+Ok(NO)m (k = 0, 2, 4) as a function of the concentration of NO; intensities of Co9+Ok(NO)m (k = 0, 2, 4) as a function of temperature; the total intensity after the reaction with NO as a function of NO concentration for different n; and Cartesian coordinates and total energies of the Co6+Ok(NO)m clusters obtained by the DFT calculation (PDF) G


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