Effects of Second-Metal (Al, V, Co) Doping on the NO Reactivity of

Article pubs.acs.org/JPCA

Effects of Second-Metal (Al, V, Co) Doping on the NO Reactivity of Small Rhodium Cluster Cations 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

‡

ABSTRACT: Reactions of pure and doped rhodium cluster cations, RhnX+ (n = 2−6; X = Al, V, Co, Rh), with NO molecules were investigated at near-thermal energy using a guided ion beam tandem mass spectrometer. We found that the doping with Al and V increases the total reaction cross section mostly. Under single-collision conditions, Rh2X+ reacts with NO to produce Rh2N+ with release of metal monoxide, XO, whereas RhnX+ (n = 3−6) adsorb NO. For the specific clusters RhnAl+ (n = 3 and 4) and RhnV+ (n = 4−6), the NO adsorption is often accompanied by the release of one Rh atom. In addition, we examined the reactions of Rh5X+ (X = Al, V, Co, Rh) with NO under multiple-collision conditions and observed the cluster dioxide formation and the N2 release, i.e., NO decomposition. Particularly, the V-doping is most effective for the NO decomposition. One possible explanation for the present results is that the formation of a stable dopant metal−oxygen bond directly leads to the increase of NO dissociative adsorption energy and the reduction of the energy barrier between the molecular and dissociative adsorption, thereby encouraging the NO decomposition on the small RhnX+ clusters studied.

1. INTRODUCTION

Gas-phase reaction studies of Rh clusters with NO have been extensively performed.10−15 Andersson et al. measured the adsorption probability of NO onto neutral Rhn (n = 10−50) clusters at room temperature and showed that it increases monotonically from n = 10 to 25 and levels off at larger sizes.10 Mackenzie and co-workers studied the reactions of cationic and anionic Rhn± (n = 6−30) clusters with NO using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.11,12 They found that the rate constant increases smoothly with the cluster size and that the cluster cations have larger rate constants than the corresponding anions though they both show considerably high reactivity. Moreover, the reactions of Rhn+ (n = 6−12, 14−16) resulted in the formation of RhnOm+ (m = even), i.e., the apparent N2 release, which is evidence for the decomposition of NO. The NO decomposition on Rh6+ and Rh7+ was also confirmed by density functional theory (DFT) calculations.13,14 Recently, Mafuné and co-workers investigated the reactions of Rhn+ (n = 4−8) with NO in helium buffer gas at 300−1000 K and observed that Rhn+ (n ≥ 6) gives much higher intensities of the product ions such as Rhn(NO)mO2,4+ at high temperatures (>800 K).15 This result implies that Rh4,5+ have lower reactivity for the NO decomposition in contrast to Rhn+ (n ≥ 6). In comparison to the single-element Rh clusters, little information is available for the reactivity of Rh-based bimetallic

The removal of nitrogen oxides (NOx) from automotive exhaust gas has long been one of the challenging environmental problems.1 Currently, three-way catalytic converters are widely used for reducing NOx and simultaneously oxidizing carbon monoxide and hydrocarbons.2 The commercial three-way catalysts contain precious metal (Rh, Pd, Pt) particles, and rhodium is thus far known to be the most effective element for the reduction of NOx.3,4 However, these precious metals are very expensive and limited in supply, and thus there is a continued need to reduce and minimize the amounts used in the catalytic converters. Commonly, most catalysts consist of metal particles with the diameter ranging from subnanometers to a few hundred nanometers, and the size is one of the crucial factors that determine their catalytic activity.5,6 For instance, it was shown that Rh particles exhibit contrasting size dependences in the reactions of CO + O2 and CO + NO, and the turnover number decreases gradually with decreasing the size in the CO + NO reaction.7 However, it is unclear whether this tendency continues to the single Rh atom. Additionally, the catalytic activity can be improved by doping a Rh particle with a second metal. The catalytic activity for the NO reduction was significantly enhanced by the addition of a small amount of other precious metals such as Ag8 and Ir.9 Furthermore, understanding the effects of the size and the doping with a base metal on the reactivity would be useful for providing an efficient and low-cost NOx reduction catalyst. © XXXX American Chemical Society

Received: November 17, 2016 Revised: February 21, 2017 Published: March 20, 2017 A

DOI: 10.1021/acs.jpca.6b11613 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

achieve the mass-independent high transmittance of ions. For the assignment of the product ions, if necessary, the mass resolution of this QMF was improved to distinguish between nitrogen and oxygen atoms adsorbing on a product ion. A partial reaction cross section, σp, for the formation of a given product ion was obtained using

clusters. Mafuné et al. examined the gas-phase reactions of RhnTam+ (n + m = 6) with NO in helium buffer gas and found that the NO decomposition proceeds even at room temperature by alloying Rh with Ta.16 As a reason, it was suggested by using DFT calculations that the Ta-doping increases the dissociative adsorption energy of NO. A DFT study also reported that Rh3X+ (X = Sc and V) have larger adsorption energies of CO than Rh4+.17 In this study, we widely investigate the reactivity of RhnX+ (n = 2−6; X = dopant metal) and the corresponding Rhn+1+ clusters toward NO at near-thermal energy under singlecollision conditions as well as multiple-collision conditions. Three different types of dopant metals (Al, V, and Co) are chosen as an example of typical metals, early transition metals, and late transition metals, respectively. The reaction cross sections and the efficiencies of the NO decomposition are measured and then compared at each cluster size to obtain an insight into the doping effect on the NO reactivity.

σp = σr

I + ∑ Ip kBT ln Pl I

∑ Ip

(2)

The systematic and the statistical uncertainties of the reaction cross section were approximately estimated to be 30% and 20%, respectively. Note that the uncertainties have little influence on the qualitative trend of the reaction cross sections.

3. RESULTS 3.1. Single-Collision Reactions of RhnX+ (n = 2−6). All the single-collision reactions were performed at a near-thermal energy (Ecol = 0.2 eV). The reactions of Rh2X+ (X = Al, V, Co, Rh) with NO result in a cluster fragmentation and give a unique product ion, Rh2N+, which implies the release of XO as follows:

2. EXPERIMENTAL SECTION The experimental setup has been described in detail previously.18 The instrument consists of a cluster ion source, a cooling cell, two quadrupole mass filters (QMFs), a reaction cell, and a detector, which are connected by octopole ion beam guides. Metal clusters were produced by cosputtering of four separate plates with 8.5 keV beams of xenon ions emitted from an ion gun (CORDIS Ar25/35c, Rokion IonenstrahlTechnologie). In the production of Rh-based bimetallic clusters, one or two plates of a second metal (Al, V, or Co) were used as the targets together with Rh plates. The produced metal cluster cations were guided into the cooling cell (290 mm length) filled with helium gas (>10−2 Torr) at room temperature. The cooled cluster ions were size-selected by the first QMF and then transferred into the reaction cell (100 mm length). Reactant NO gas was introduced through a variable leak valve into the reaction cell, where it was allowed to interact with the clusters. The pressure of NO gas in the reaction cell was monitored using a spinning rotor gauge (SRG2, MKS), and the pressure below 2 × 10−4 Torr was low enough to fulfill single-collision conditions. The translational energy of the parent cluster ions in the reaction cell was measured by the retarding potential method using the octopole ion beam guide there and converted to the collision energy, Ecol, in the center-of-mass frame. A typical spread of Ecol was 0.4 eV in the full width at half-maximum (fwhm). Unreacted cluster ions and product ions were mass-analyzed with the second QMF and detected by a secondary electron multiplier equipped with an ion conversion dynode. Signals from the secondary electron multiplier were processed in a pulse counting mode. The total reaction cross section, σr, was derived under singlecollision conditions from σr =

Ip

Rh 2X + + NO → Rh 2N+ + XO

(X = Al, V, Co, Rh) (3) +

For n = 3−6, two kinds of product ions, RhnXNO and Rhn−1XNO+, are observed, which indicates that the following reactions occur: Rh nX + + NO → Rh nXNO+

(X = Al, V, Co, Rh) (4)

Rh nX + + NO → Rh n − 1XNO+ + Rh

(X = Al, V) (5)

Rh4+

+

Exceptionally, and Rh3Co exhibit no product ion at Ecol = 0.2 eV. The simple NO adsorption (reaction 4) is the main reaction of Rh3V+ and RhnX+ (n = 4−6). The RhnXNO+ product in this reaction should be a chemisorbed intermediate and can be detected if its lifetime is longer than the flight time (∼100 μs) from the reaction cell to the second QMF. By contrast, the NO adsorption with Rh release (reaction 5) is observed as the only reaction of Rh3Al+ and the minor reaction of Rh4Al+ and RhnV+ (n = 4−6). Because the Rh release does not occur in the reactions of the single-element Rh clusters at the collision energy used (0.2 eV), the presence of Al and V brings the Rh release from the RhnXNO+ intermediate. Figure 1 shows the reaction cross sections of RhnX+ (X = Rh, Al, V, Co) with NO. Among the triatomic clusters, Rh2Al+ exhibits a relatively large reaction cross section (σr = 10.8 Å2). On the contrary, the reactivity of Rh3X+ is low overall (σr < 4 Å2). Particularly, no product ions are observed for Rh4+ and Rh3Co+, which indicates that, if any, their reaction cross sections are smaller than the detection limit (σr < 0.5 Å2). For n = 4 and 5, the NO adsorption cross sections are significantly enhanced by the substitution of a second metal and both follow the same order of Rhn+1+ < RhnCo+ < RhnAl+ < RhnV+. Finally, all the Rh6X+ clusters have the comparable NO adsorption cross sections of 50−70 Å2. As clearly shown in Figure 2, the total reaction cross section increases rapidly from n = 3 with the cluster size, and the values of Rh5V+ and all Rh6X+ clusters exceed the collision cross section calculated by Langevin− Gioumousis−Stevenson (LGS) model19 (σLGS = 49.2 Å2). Such

(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 the product ions, respectively. In this measurement, the mass resolution of the second QMF was set to be relatively low to B


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Figure 3. Typical mass spectra obtained in the reactions of (a) Rh6+, (b) Rh5Al+, (c) Rh5V+, and (d) Rh5Co+ with NO at the pressure of 2 × 10−3 Torr. The mass numbers are relative to the parent ion. The initial collision energy is 0.2 eV.

mass spectrum obtained in the reaction of Rh6+ with NO at the pressure of 2 × 10−3 Torr and indicates the formation of Rh6O2+ as well as the single-collision product, Rh6NO+. As shown in Figure 4a, the relative intensity of Rh6O2+ exhibits a second-order dependence on the NO pressure, whereas that of Rh6NO+ increases linearly in the low-pressure region (1 × 10−3 Torr). Because the relative intensity of this product is quite small and increases linearly with the NO pressure, this product presumably results from the single-collision reaction of the parent ion in the high-energy tail of the translational energy distribution. In the substitution of Co, Al, and V for Rh, the variety of the product ions increases in this order. The mass spectra obtained for Rh5X+ (X = Al, V, Co) at the NO pressure of 2 × 10−3 Torr are shown in Figure 3b−d. It is clear from Figure 3 that the mass spectrum of Rh5Co+ is quite similar to that of Rh6+. In all the spectra of the doped clusters, Rh5XO2+ is observed together with the single-collision product, Rh5XNO+. In addition, Rh4XO2+ (X = Al and V) appears and Rh5+ is dominantly observed only for the V-doping. The relative intensities of the parent and the notable product ions in the reactions of Rh5X+ (X = Al, V, Co) are shown as a function of the NO pressure in Figure 4b−d. Again, from these NO-pressure dependences, it is suggested that two NO molecules are involved in the formation of Rh5XO2+, Rh4XO2+, and Rh5+. The product, Rh5XO2+,

Figure 2. Total reaction cross sections of RhnX+ (X = Rh, Al, V, Co) with NO at the collision energy of 0.2 eV as a function of the cluster size (n). The dashed line shows the LGS cross section.

high reactivity was previously reported for Rh7+ and larger clusters in FT-ICR measurements.12 3.2. Multiple-Collision Reactions of Rh5X+. As mentioned above, significant enhancements by the doping are found in the NO adsorption cross sections of Rh5X+. Thus, to investigate the subsequent reactions of NO molecules adsorbed on these clusters, the reaction experiments were carried out under multiple-collision conditions. Figure 3a shows a typical C


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Figure 4. NO pressure dependence of the relative intensities of the parent and product ions in the reactions of (a) Rh6+, (b) Rh5Al+, (c) Rh5V+, and (d) Rh5Co+. The initial collision energy is 0.2 eV. The long dashed line, short dashed line, and dotted line represent the slopes for first-, second-, and third-order dependences, respectively, as eye guides.

4. DISCUSSION The doping effects can be discussed by comparing the observed products and the reaction cross sections between Rhn+1+ and RhnX+ (X = Al, V, and Co) at each size. The single-collision reactions of Rh3+ and Rh2X+ result in only the formation of Rh2N+ with releasing RhO and XO molecules, respectively. Probably, the results of Rh2X+ indicate that the clusters adsorb NO dissociatively, where N and O atoms prefer the adsorption sites of Rh and the dopant metal (Al, V, Co), respectively. A similar preference has been suggested in the DFT calculation of Rh5TaN2O2+, where each N atom adsorbs on a different Rh site while each O atom binds to a different adsorption site including the Ta atom.16 The reported bond energies of atomic oxygen to metal atoms,20 small metal clusters,21 and metal surfaces22 have also suggested that the oxygen affinity of Rh is lower than those of Al, V, and Co as well as Ta, except that the bond energy of an oxygen atom to a Rh atom (4.20 eV) is slightly larger than that to a Co atom (3.99 eV). In the reaction of Rh2X+ + NO → Rh2N+ + XO, the energetics can be roughly estimated from the bond energies of N−O, Rh−N, X−O, and Rh−X. For example, the difference between the bond energies of VO (6.50 eV) and RhV (3.77 eV) is larger than that between AlO (5.30 eV) and RhAl (3.26 eV).20,23 This means that the heat of reaction for Rh2V+ + NO → Rh2N+ + VO is larger than that for Rh2Al+ + NO → Rh2N+ + AlO. However, these energetics cannot explain the fact that Rh2Al+ has higher reactivity than Rh2V+. Given that the reaction cross sections of Rh2X+ are substantially smaller than the LGS collision cross section, their reactivity should be

results from the sequential adsorption of two NO molecules accompanied by the release of N2, Rh5X + + NO → Rh5XNO+

(X = Al, V, Co)

Rh5XNO+ + NO → Rh5XO2+ + N2

(8) (9)

Rh4XO2+

+

The other secondary products, and Rh5 , can be obtained by the further release of Rh and XO2, respectively, as follows: Rh5XNO+ + NO → Rh4XO2+ + [Rh, N2] (X = Al, V)

Rh5XNO+ + NO → Rh5+ + [XO2 , N2]

(10)

(X = V) (11)

The production of Rh4XO2+ may also occur by the reaction of Rh4XNO+ with NO, Rh4XNO+ + NO → Rh4XO2+ + N2

(X = Al, V) (12)

Because the neutral products, particularly in reactions 10 and 11, cannot be clearly identified in our experiments, they are shown in square brackets. However, molecular nitrogen is probably formed because of its relatively high stability. Thus, it is considered that reactions 9−12 involve the NO decomposition. Much higher NO pressures (≥2 × 10−3 Torr) give rise to the third NO adsorption and the formation of Rh5XO2NO+ (X = Al, V, Co) and Rh4XO2NO+ (X = Al, V). D


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The reaction of Rh6+ + NO has been studied extensively,11,13,15,32,33 and the NO decomposition (i.e., the N2 release) observed in our experiments for Rh5X+ (X = Al, V, Co) should be understood in this context. The NO decomposition on Rh6+ was first observed by Mackenzie and co-workers in the FT-ICR experiments performed at a collision energy lower than ours.11 They suggested that the ensemble of the parent Rh6+ clusters studied consists of two isomers at least, and one has a higher reactivity than the other. Afterward, they identified two nearly isoenergetic isomers having square bipyramid (SBP) and trigonal prism (TP) structures by use of the DFT method.32 The estimated energy barrier between these isomers was relatively low (0.41−0.68 eV). These results, together with the subsequent findings,13,15 give us a possible energy diagram of Rh6+ + NO, as shown in Figure 5. Torres et

dominated by the energy barriers between the physisorbed and chemisorbed NO states or more probably between the molecularly and dissociatively chemisorbed states, rather than the thermodynamic stability of the products. The most remarkable effect of doping is to open a new reaction channel (cluster fragmentation) involving the release of a Rh atom in the reactions of specific clusters, RhnAl+ (n = 3 and 4) and RhnV+ (n = 4−6), with NO. Similar cluster fragmentation has been observed for Con+ (n = 4−12),24,25 CunO2− (n = 8, 10, and 12),26 Cu9Al+,27 CunTi+ (n = 4−14),28 CunV+ (n = 5−11),28 and Nbn+ (n = 2−10).29 The observation of the cluster fragmentation indicates that the excess energy generated upon the adsorption of NO is large enough to release metal atoms from the cluster at least. Furthermore, the smaller clusters have the smaller number of the internal degrees of freedom and so metal atoms can be liberated from the cluster in a shorter time. In the reaction of RhnX+ + NO, it is assumed that the Rh release occurs via the dissociative NO adsorption where N and O atoms adsorb on a Rh and dopant metal site, respectively, as mentioned for Rh2X+. Because the adsorption energy of atomic N onto a Rh site of RhnX+ should not depend on the dopant atom significantly, the difference of the internal energy of the cluster having atomic N and O is more sensitive to the bond energy of RhnX+−O. The calculated adsorption energy of O on Rh4V (7.34 eV) is significantly larger than those on Rh4Co (5.51 eV) and Rh5 (5.24 eV),21 which is consistent with the observation of the Rh release from Rh4V+. Actually, considering the bond energies of Rh4V−O, Rh4V−N (5.78 eV),21 and N−O (6.54 eV),20 the excess energy generated upon the dissociative adsorption of NO on Rh4V+ is roughly estimated to be 6.58 eV. If one compares this value with the bond dissociation energy of Rh5+ (∼3 eV),30 one notices that this adsorption energy is quite large. In the same manner, the dissociative adsorption energy of NO onto Rh5+ (4.38 eV) estimated using the bond energies of Rh5−O (5.24 eV) and Rh5−N (5.68 eV)21 exceeds the bond dissociation energy of Rh5+. Because in our experiment Rh5+ does not give Rh4NO+ as a product ion, the obtained adsorption energy may be significantly overestimated. Another possibility for no observation of the Rh release is that the dissociation of NO is hindered by a higher energy barrier and NO adsorbs molecularly on Rh5+. Consequently, the doping of Rh clusters with Al and V should significantly lower the energy barriers between the molecularly and dissociatively chemisorbed states, which allows the dissociation of NO followed by the Rh release. By contrast, there is no observation of the Rh release from any RhnCo+ clusters under the single-collision conditions, as is the case of Rhn+1+. However, it is obvious that the Co-doping slightly increases the reaction cross section (Figure 2). Thus, the substitution of Co for Rh in the cluster has a small influence on the NO reactivity. This is probably due to the fact that a cobalt and a rhodium atom have similar electron configurations, 3d74s2 and 4d85s1, respectively. A DFT study demonstrated that the adsorption energies of O on Rhn+1 (n = 2−4) are enhanced slightly (0.1−0.7 eV) by doping with Co.21 Additionally, it has been suggested that the oxygen adsorption energies on metal surfaces become larger as the d band centers shift up in energy relative to the Fermi level, and the calculated d band centers of Rh and Co/Rh surfaces are located at −1.73 and +0.34 eV, respectively.31 Because both the clusters and the surface show the same tendency that the oxygen adsorption energy is enhanced by doping Rh with Co, our experiments may demonstrate the validity of this theory for the clusters.

Figure 5. Schematic potential energy diagram along the reaction coordinate of Rh6+ + NO → Rh6NO+. SBP and TP denote square bipyramid and trigonal prism, respectively.

al. revealed that the SBP isomer is the most stable and that the dissociative adsorption of NO is energetically more favorable than the molecular adsorption on both the SBP and TP isomers.13 In addition, it was shown that the TP isomer undergoes a structural transition to a SBP structure during the conversion to the NO dissociative adsorption. Mafuné and coworkers found that a quite high transition state is located along the direct pathway connecting the molecular NO to the dissociative NO adsorption on the SBP isomer by using the DFT calculations.15 They also succeeded in the detection of the molecular adsorption of NO on Rh6+ in the helium buffer gas at 300 K. Figure 5 indicates that the SBP isomer adsorbs NO molecularly at first, and by use of the adsorption energy of NO (>1.5 eV)13,15 the dissociation of the adsorbed NO can proceed via an intermediate on the TP isomer. It is thus reasonable that the dissociative NO adsorption occurs on both the isomers having SBP and TP structures at near-thermal energy. However, the NO decomposition (or NO adsorption) competes with the desorption of NO from the cluster, and the branching ratio of the two processes should be affected by the energy of the transition states, as well as the NO adsorption energy. We therefore consider that the substitution of a second metal atom for a Rh atom in Rh6+ increases the dissociative adsorption energy and lowers the energy of the transition states, resulting in the enhancement of the NO adsorption cross section and the NO decomposition efficiency (as mentioned below). E


Article

The Journal of Physical Chemistry A The efficiency of the NO decomposition by Rh5X+ (X = Rh, Al, V, Co) can be examined by comparing the sum of the relative intensities of the product ions (Rh5XO2+, Rh4XO2+, and Rh5+) involved in the N2 release. Figure 6 shows the relative

of a Rh atom at near-thermal energy. This can be attributed to the higher oxygen affinity of Al and V compared to Rh. These clusters form a strong dopant metal−oxygen bond, resulting in the increase of NO adsorption energy and the reduction of the energy barrier between the molecular and dissociative adsorption. Moreover, Rh5V+ exhibits the highest efficiency for the NO decomposition among the Rh5X+ clusters studied here. Our results indicate that the NO reactivity of the small Rh clusters can be improved significantly by doping with an early transition metal atom such as vanadium, even though the pure Rh clusters are deactivated by the size reduction. This finding also suggests that the doping can control the oxygen adsorption energy on the Rh-based bimetallic clusters by design.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M. Ichihashi). ORCID

Figure 6. Comparison of the relative intensities of secondary product ions (Rh5XO2+, Rh4XO2+, and Rh5+) in the reactions of Rh5X+ (X = Rh, Al, V, Co) with NO at the pressure of 2 × 10−3 Torr. The initial collision energy is 0.2 eV.

Masahiko Ichihashi: 0000-0002-4980-1955 Notes

The authors declare no competing financial interest.

−3

intensity for each product ion at the NO pressure of 2 × 10 Torr. The intensity of Rh5XO2+ increases slightly by doping with a second metal, which seems to reflect the NO adsorption cross section. By contrast, Rh4XO2+ is observed only for X = Al and V, and the formation of Rh5+ proceeds significantly for X = V. In particular for Rh5V+, these two products are formed most efficiently. It turns out that the NO decomposition efficiency of Rh5V+ is nearly 1 order of magnitude higher than that of Rh6+ and that the NO decomposition is promoted greatly by the Vdoping. In the reaction of Rh5V+ + NO, the ratio of the Rh5+ formation (i.e., VO2 release) to the sum of the Rh4VO2+ and Rh4VO2NO+ formation (i.e., Rh release) is approximately 2:1, which is almost independent of the NO pressure. It seems that the Rh5VN2O2+ intermediate prefers to dissociate by releasing a VO2 molecule rather than a Rh atom energetically or kinetically. Recently, Mafuné et al. studied the reactions of Rh6−mTam+ (m = 0−6) with NO and revealed that the NO decomposition is induced by alloying Rh clusters with Ta,16 which is also an early transition metal. The DFT calculations of Rh5TaN2O2+ showed that each O atom adsorbs on a different adsorption site around the dopant Ta atom. In the reactions studied here the dopant atom, X, in Rh5X+ (X = Al, V, Co) should work similarly, and presumably the second NO molecule adsorbs on a vacant Rh2X hollow site molecularly and then dissociates to atomic N and O. They also revealed that the efficient NO decomposition on Rh6−mTam+ (m = 1−6) is attributed to the increase of the dissociative adsorption energy of two NO molecules and the lowing of the barrier height for the dissociative adsorption. Thus, it is concluded that the addition of early transition metal atoms such as V and Ta to small Rh clusters increases the oxygen adsorption energy (i.e., NO dissociative adsorption energy) and also reduces the energies of the transition states for the NO decomposition.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Special Cluster Research Project of Genesis Research Institute, Inc.

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5. CONCLUSIONS We have studied the effect of doping with a second metal (Al, V, and Co) on the NO reactivity of the small Rh cluster cations. Specific RhnX+ (n = 3−5; X = Al and V) clusters show significant enhancements in the NO adsorption cross section and mostly open a new reaction channel involving the release F


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Effects of Second-Metal (Al, V, Co) Doping on the NO Reactivity of

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