Gas-Phase Reactions of Copper Oxide Cluster Cations with Ammonia

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Gas-Phase Reactions of Copper Oxide Cluster Cations with Ammonia: Selective Catalytic Oxidation to Nitrogen and Water Molecules Shinichi Hirabayashi, and Masahiko Ichihashi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03017 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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The Journal of Physical Chemistry

Gas-Phase Reactions of Copper Oxide Cluster Cations with Ammonia: Selective Catalytic Oxidation to Nitrogen and Water Molecules

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

Submitted to The Journal of Physical Chemistry.

1 ACS Paragon Plus Environment

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ABSTRACT: Reactions of copper oxide cluster cations, CunOm+ (n = 3–7; m ≤ 5), with ammonia, NH3, are studied at near thermal energies using a guided ion beam tandem mass spectrometer.

The single collision reactions of specific clusters such as Cu4O2+,

Cu5O3+, Cu6O3+, Cu7O3+, and Cu7O4+ give rise to the release of H2O after the NH3 adsorption efficiently and result in the formation of CunOm−1NH+. clusters commonly have the Cu average oxidation numbers of 1.0–1.4.

These CunOm+ On the other

hand, the formation of CunOm−1H2+, i.e., the release of HNO, is dominantly observed for Cu7O5+ with a higher Cu oxidation number.

Density functional theory

calculations are performed for the reaction, Cu5O3+ + NH3 → Cu5O2NH+ + H2O, as a typical example of the H2O release.

The calculations show that this reaction occurs

almost thermoneutrally, consistent with the experimental observation. Further, our experimental studies indicate that the multiple collision reactions of Cu5O3+ and Cu7O4+ with NH3 lead to the production of Cu5+ and Cu7O+, respectively.

This

suggests that the desirable NH3 oxidation to N2 and H2O proceeds on these clusters.


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1. Introduction Ammonia (NH3) can harm human health, and its emission into the atmosphere has become an important problem.

Selective catalytic oxidation (SCO) of NH3 to N2 and

H2O is one of the most desirable and potential methods to eliminate NH3:1 2NH3 + 3/2O2 → N2 + 3H2O.

(1)

This catalytic reaction has been involved in a wide field of applications.

For instance,

NH3 is used as a reducing agent in selective catalytic reduction (SCR) of nitrogen oxides (NOx) in the exhaust gas emitted from mobile diesel engines, and excess NH3 slip can be lowered in an NH3 oxidation catalyst placed downstream of the SCR catalyst.2,3

Recently, NH3 has also received attention as an energy reservoir

alternative to fossil fuels, and its catalytic combustion has been investigated as a promising technique.4,5

Noble metals (e.g. platinum) are known to be highly active as

the NH3 oxidation catalysts, but do not possess high selectivity toward the formation of N2.1,2,6

The formation of undesirable by-products such as NO and N2O is a problem

derived from the use of noble metal based catalysts, as well as their high costs are. Therefore, we require the development of non-noble metal based catalysts capable of converting NH3 to N2 with high efficiency and selectivity. Several decades ago, Il'chenko investigated the NH3-SCO by use of various transition metal oxides and revealed that CuO exhibits high N2 selectivity albeit relatively low catalytic activity.6

Since then, copper-based materials including oxides,

ion-exchanged zeolites, and modified clays have been extensively explored as promising catalysts for the SCO of NH3 to N2.3

Later, Mayer et al. studied the NH3

oxidation on copper oxides with in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy by using Cu nanoparticles and polycrystalline foil.7,8

They

found that the gradual reduction of the catalyst surface from CuO to Cu2O correlates



with the switching in the reaction product from NO to N2.

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This means that the Cu

oxidation state of copper oxide influences the selectivity of the final products in the NH3 oxidation.

Recently, Hinokuma et al. extensively investigated the catalytic NH3

oxidation on supported copper oxide nanoparticles, and indicated that the particle size and the Cu oxidation state are critical determinants of the catalytic activity and selectivity.9,10

However, further microscopic studies are indispensable for a

fundamental understanding of the composition, geometry, and Cu oxidation state of the active species in the copper oxide catalysts for the NH3-SCO. Elucidation of the active sites on the surfaces of metal oxide catalysts needs an approach at a molecular level.

Metal and metal oxide cluster ions isolated in the gas

phase can be used to understand the active sites of heterogeneous catalysts, owing to their well-controlled size, composition, and charge state by using mass spectrometric techniques.11,12

Such studies can also provide useful information on the design of

new catalysts.

In fact, reactions of some transition metal oxide clusters, MnOm+/0,

with NH3 have been investigated at near thermal energies by several research groups.13-17

Our previous studies showed that FenOm+ and ConOm+ (n = 3–6, m = 1–3)

clusters mainly exhibit H2 desorption after the NH3 adsorption.17

As an elemental

step of reaction (1), the formation of MnOm–1NH+ with releasing an H2O molecule was reported in the reactions of specific clusters, Fe2O2+ (refs 13 and 14) and Mo3O9+ (ref 15).

Further, the multiple collision reaction of Mo3O9+ with NH3 results in the

formation of Mo3O7(NH)2+ and Mo3O7N2+ accompanied with the release of two H2O molecules.15

However, either of the production or desorption of N2 have not been

demonstrated. In the present study, we investigate the gas-phase reactions of copper oxide cluster cations, CunOm+ (n = 3–7; m ≤ 5), with NH3.

First, the reaction cross sections


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measured under single collision conditions are discussed in terms of the cluster composition and average oxidation number (AON) of Cu atoms in the cluster. indicates the activity and the selectivity in the initial step.

This

Next, the multiple collision

reactions are carried out to examine the further formation of H2O, N2, and other products such as NOx involved in the NH3 oxidation.

The structures of the reactive

CunOm+ clusters are explored by collision-induced dissociation (CID).

Density

functional theory (DFT) calculations are also employed to elucidate the geometric structure and reaction mechanism of Cu5O3+ as a typical example.

2. Experimental Section The present reaction experiments were performed using a guided ion beam tandem mass spectrometer described in detail previously.18

Briefly, copper clusters

were efficiently produced by simultaneous sputtering of four separate copper targets with 8.5 keV beams of xenon ions generated by an ion source (CORDIS Ar25/35c, Rokion Ionenstrahl-Technologie).

The sputtered cluster cations were deflected by a

repeller plate and extracted from the target chamber by a series of electrostatic lenses. Then, the cluster cations were guided by an octopole ion guide (OPIG) into a cooling cell (290 mm length), where they were thermalized to room temperature by collisions with helium gas (~10−2 Torr).

To prepare copper oxide clusters, a small amount of O2

gas was additionally introduced into the target chamber or the cooling cell by using a variable leak valve.

The thermalized cluster ions were guided by an OPIG into the

first quadrupole mass filter (QMF) to select the cluster ions of interest.

The cluster

ions were further guided by an OPIG into the reaction cell (100 mm length), where they were allowed to collide with reactant gas (NH3, ND3, or Xe) introduced through a variable leak valve.

A spinning rotor gauge (SRG-2, MKS) was used to monitor the 5 ACS Paragon Plus Environment


pressure of the reactant gas in the reaction cell.

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The translational energy of the

projectile cluster ions in the reaction cell was measured by the retarding potential method using the OPIG there and converted to the collision energy, Ecol, in the center-of-mass frame. Unreacted cluster ions and product ions were guided into the second QMF, where they were analyzed.

Detection of the ions was performed using 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 determined under single collision conditions from

σr =

k BT I + ∑ I p , ln Pl I

(2)

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 achieve the mass-independent high transmittance of the ions.

For the assignment

of the product ions, if necessary, the mass spectra were measured at a higher mass resolution.

The partial reaction cross section, σp, for the formation of a given product

ion was obtained by the following equation:

σp =σr

Ip

∑I

,

(3)

p

where Ip/ΣIp represents the branching fraction for the product ion of interest.


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3. Results 3.1. Single collision reactions of CunOm+.

All the experiments under single

collision conditions were carried out at Ecol = 0.2 eV, which corresponds to a near thermal energy.

Copper oxide cluster cations, Cu3O1,3+, Cu4O1–4+, Cu5O1–4+, Cu6O3,5+,

and Cu7O1–5+, are studied, and it is found that all but Cu3O+ are reactive toward NH3. First, we focus our attention on the product ions having the same number of copper atoms as the parent cluster ions, since these product ions could be involved in the NH3-SCO.

The following reaction formulae give the observed product ions without

the release of copper atoms: CunOm+ + NH3 → CunOmNH3+,

(4)

CunOm+ + NH3 → CunOm−1NH+ + H2O,

(5)

CunOm+ + NH3 → CunOm−1H2+ + HNO,

(6)

CunOm+ + NH3 → CunOm−2NH3+ + O2,

(7)

CunOm+ + NH3 → CunOm−2+ + NH3 + O2,

(8)

CunOm+ + NH3 → CunOm−3NH+ + H2O + O2.

(9)

Reactions (4)–(6) are defined as “simple NH3 adsorption”, “H2O release”, and “HNO (nitroxyl) release”, respectively.

Both reactions (7) and (8) are defined as “O2

release” though the latter reaction does not occur so frequently in most cases since it is attributable to the CID at Ecol = 0.2 eV.

Reaction (9) is observed only for Cu4O4+,

where the formation of Cu4ONH+ may take place via Cu4O2NH3+: Cu4O4+ + NH3 → Cu4O2NH3+ + O2 → Cu4ONH+ + H2O + O2.

(10)

It is because that Cu4O2NH3+ is the dominant product in Cu4O4+ + NH3 with O2 release [reaction (7)]. reaction (5).

The intermediate Cu4O2NH3+ can dissociate to Cu4ONH+ and H2O like Hence, reaction (9) can be also classified into “O2 release”.

The

neutral products are not directly observed in this experiment, and thus assumed from 7 ACS Paragon Plus Environment


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the mass difference between the reactants and the product ions. The other reactions, which are accompanied with the Cu release, are observed for some specific clusters. Both Cu4O+ and Cu5O+ react with NH3 to produce Cu3ONH3+, and the reaction of Cu7O3+ + NH3 results in the formation of Cu6O3NH3+: CunOm+ + NH3 → Cun−1Om+ + NH3 + Cu

[(n,m) = (4,1), (7,3)],

Cu5O+ + NH3 → Cu3ONH3+ + Cu2 (or 2Cu).

(11) (12)

In the reaction of Cu7O5+ with NH3, Cu6O4H3+ is observed as a minor product, which suggests that the following reaction occurs: Cu7O5+ + NH3 → Cu6O4H3+ + [Cu, NO].

(13)

We only observe the release of both Cu and NO, not either, in this reaction.

Figure 1. Typical mass spectra of Cu5O3+ obtained (a) without and (b) with mass selection by the first QMF.

The mass spectra measured in the reactions of

mass-selected Cu5O3+ with (c) NH3 and (d) ND3 at the pressure of 2×10−4 Torr are shown.

The selectivity of the reaction pathways strongly depends on the composition of 8 ACS Paragon Plus Environment

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the reactant cluster, CunOm+.

For instance, as shown in Figure 1, three product ions,

Cu5O3NH3+ [reaction (4)], Cu5O2NH+ [reaction (5)], and Cu5O2H2+ [reaction (6)], are observed in the reaction of Cu5O3+ with NH3, and these assignments are confirmed by On the other hand, the reaction of Cu5O4+ with

the isotope experiments with ND3.

NH3 results in the formation of Cu5O2NH3+ [reaction (7)] and Cu5O2+ [reaction (8)], while the oxygen-poor clusters, Cu5O+ and Cu5O2+, exhibit only the simple NH3 adsorption [reaction (4)]. The total reaction cross sections of CunOm+ at Ecol = 0.2 eV are shown in Figure 2 as a function of the number of oxygen atoms, m. increase with the increase of m.

The reaction cross section tends to

The deviated large cross section of Cu4O+ comes

from the Cu release [reaction (11)], probably due to the small bond dissociation energy of Cu–Cu3O+.

By contrast, no apparent correlation is recognized between the total

cross section and the number of copper atoms, n.

Among the clusters studied here,

Cu6O5+, Cu7O4+, and Cu7O5+ exhibit particularly large total reaction cross sections of roughly 100 Å2, which are comparable to the collision cross section (108 Å2) calculated by the trajectory theory.19

Figure 2. Total cross sections for the reactions of CunOm+ with NH3 at Ecol = 0.2 eV as



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a function of the number of oxygen atoms, m.

Figure 3 shows the partial cross sections for the H2O release, HNO release, and O2 release as a function of the AON of Cu atoms in the CunOm+ cluster, which is defined as (2m + 1)/n.

Most of the clusters having AONs of 1.0–1.4 (Cu4O2+, Cu5O3+,

Cu6O3+, Cu7O3+, and Cu7O4+) give rise to the H2O release [reaction (5)].

On the other

hand, the HNO release [reaction (6)] occurs almost only in the reaction of Cu7O5+, which has AON of ~1.6, and the cross section is considerably large (91.0 Å2).

The O2

release [reactions (7)–(9)] is observed for all the clusters with AONs > 1.7 (Cu3O3+, Cu4O3+, Cu4O4+, Cu5O4+, and Cu6O5+).

Figure 3. Cross sections for (a) H2O release [reaction (5)], (b) HNO release [reaction (6)], and (c) O2 release [reactions (7)–(9)] in the reactions of CunOm+ with NH3 at Ecol = 0.2 eV as a function of the average oxidation number (AON) of Cu atoms in the cluster.

The corresponding cluster compositions (n,m) are denoted on the upper 10 ACS Paragon Plus Environment

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abscissa axis and near the symbols.

3.2. Multiple collision reactions of CunOm+.

To investigate the multimolecular

reactions of NH3 on CunOm+, reactivity experiments are performed by increasing the NH3 pressure. Cu4O2+, Cu5O3+, Cu6O3+, Cu7O3+, and Cu7O4+ (AONs = 1.0–1.4), and Cu7O5+ (AON ~ 1.6) are surveyed because of their high reactivity toward the H2O release or the HNO release, which should be relevant to the NH3 oxidation, under single collision conditions.

Figure 4a shows the relative intensities of the unreacted

parent cluster ion and product ions in the reactions of Cu5O3+ with increasing pressure of NH3 up to 1×10−3 Torr.

Together with the single collision products (Cu5O3NH3+,

Cu5O2NH+, and Cu5O2H2+), different product ions, Cu5O(NH)2+, Cu5ONH3+, Cu5OH2+, and Cu5+, appear gradually with the increase of the NH3 pressure.

The intensities of

these emerging products exhibit roughly second-order dependences on the pressure, which indicates that two NH3 molecules are involved in the formation. pathways for these product ions are proposed in Figure 4b.

The reaction

The detection of Cu5OH2+

and Cu5+ strongly suggests that Cu5O3+ can oxidize NH3 to N2 in the multiple collision reactions.

A similar result is obtained for Cu7O4+, as shown in Figure 5.

The

reactions of Cu7O4+ with multiple NH3 molecules result in the formation of the product ions such as Cu7O3H2+, Cu7O2H2+, and Cu7O+, which is considered as evidence for the oxidation of NH3 to N2.

On the other hand, no product ions involving the N2 release

appear in the multiple collision reactions of the other four clusters, Cu4O2+, Cu6O3+, Cu7O3+, and Cu7O5+, in the NH3 pressure range studied here (Figure S1).

The release

of multiple H2O molecules takes place for all the studied clusters having AONs of 1.0–1.4.



Figure 4. (a) Relative intensities of the parent and product ions in the reactions of Cu5O3+ with NH3 as a function of the NH3 pressure. eV.

The initial collision energy is 0.2

The dotted and dashed lines represent the slopes for first- and second-order

dependences, respectively, as eye guide.

The proposed reaction pathways for the

formation of the product ions are given in (b).

The probable structures of Cu5O2NH+

are discussed in section 4 on the basis of DFT calculations.


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Figure 5. (a) Relative intensities of the parent and product ions in the reactions of Cu7O4+ with NH3 as a function of the NH3 pressure. eV.

The initial collision energy is 0.2

The dotted and dashed lines represent the slopes for first- and second-order

dependences, respectively, as eye guide.

The proposed reaction pathways for the

formation of the product ions are given in (b).

3.3. Collision-induced dissociation of CunOm+.

To obtain information on the

geometric structures and the bond strengths of copper oxide cluster cations, CID studies were performed for Cu4O2+, Cu5O3+, Cu6O3+, Cu7O3+, Cu7O4+ (AONs = 1.0–1.4), and Cu7O5+ (AON ~ 1.6).

Figure 6 shows mass spectra obtained in the

collisions of these clusters with xenon atoms at the collision energy of 5 eV under single collision conditions.

Specific fragments such as Cu3O+, Cu4O2+, Cu5O2+, and

Cu6O3+ are commonly observed, which indicates that these fragments are relatively 13 ACS Paragon Plus Environment


stable.

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The AONs of these stable fragments are also located in the range of 1.0–1.4.

Figure 6. Mass spectra obtained by CID of CunOm+ with Xe at the pressure of 2×10−4 Torr and the collision energy of 5 eV.

Numbers (n,m) denote CunOm+.

4. Discussion The reaction selectivity of CunOm+ toward NH3 is found to be very sensitive to the AON of Cu atoms in the cluster, as shown in Figure 3.

The H2O release occurs on the

clusters having AONs of 1.0–1.4, which can be represented as Cu(I)-rich oxide clusters. In addition, the multiple collision reactions of some of these clusters, Cu5O3+ and Cu7O4+, provide evidence for the release of one N2 molecule and three H2O molecules (see Figures 4 and 5).

These results suggest that a Cu(I)-rich oxide cluster is regarded

as an active site of the copper oxide catalysts for the beneficial NH3-SCO.

This

finding corresponds to the oxidation-number dependent selectivity for the catalytic NH3 oxidation on copper oxides in the previous studies with in situ NEXAFS


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spectroscopy.7,8

These studies showed that the main reaction product changes from

NO to N2 with the gradual progress of the reduction of CuO to Cu2O on their surfaces. The present study also indicates that the formation of HNO is dominant for Cu7O5+ having a slightly higher AON than the Cu(I)-rich oxide clusters.

Zawadzki

proposed that adsorbed nitroxyl, NHO(ad), is formed as an intermediate via the reaction of NH(ad) with O(ad) in the NH3-SCO.20

Then NHO(ad) reacts with NH(ad)

to produce N2(g) and H2O(g) while the association of two NHO(ad) species results in the desorption of N2O and H2O.

Therefore, it is possible that N2 or N2O is produced

on Cu7O5+ if NHO(ad) can react with NH(ad) or other NHO(ad) before the desorption. The N2 release in Cu5O3+ + NH3 and Cu7O4+ + NH3 may proceed via the reaction between NHO(ad) and NH(ad). The release of O2 dominates the reactions of CunOm+ having high AONs of ~2. Possibly these clusters consist of an O2 moiety and a stable cluster such as Cu3O+, Cu4O2+, Cu5O2+, or Cu6O3+.

Our CID studies (see Figure 6) and the previous

studies21–24 imply that these clusters are stable.

Furthermore, in the CID of Cu3O3+,

the O2 desorption is dominantly observed even at a significantly low collision energy of 0.2 eV (Figure S2), which strongly suggests a molecular adsorption of O2 on Cu3O+. Thus, the clusters having AONs of ~2 can change easily to the stable oxides [Cu(I)-rich oxide clusters] with the O2 release. The comparison of the partial cross sections of Cu5O3+ with NH3 and ND3 reveals the reaction dynamics of adsorbates on the clusters.

As shown in Figures 1c and 1d,

the relative intensity of Cu5O3ND3+ is higher than that of Cu5O3NH3+, while the relative intensity of Cu5O2D2+ is almost equal to that of Cu5O2H2+.

In this

measurement, the intensity of Cu5O2ND+ is unclear in Cu5O3+ + ND3 because the mass number of Cu5O2ND+ coincides with that of Cu5O3+.

However, if we assume that the



Page 16 of 25

sum of the intensities of Cu5O3ND3+, Cu5O2D2+, and Cu5O2ND+, is equal to that of Cu5O3NH3+, Cu5O2H2+, and Cu5O2NH+, the intensity of Cu5O2ND+ turns out to be lower than that of Cu5O2NH+.

Thus, the formation cross section of Cu5O2ND+ can be

estimated as ~86% of that of Cu5O2NH+.

This indicates that the release of D2O in the

reaction with ND3 is less efficient than the release of H2O in the reaction with NH3. Similar kinetic isotope effect is observed in the reactions of Cu7O3+ and Cu7O4+.

It is

suggested that the hydrogen migration on CunOm+ is the rate-determining step in the H2O release.

Figure 7. Optimized structures of the lowest-energy isomer (a) and the second-lowest energy (almost equal-energy) isomer (b) of Cu5O2+ determined by BPW91/LanL2DZ calculations, together with the energy difference.

The structure of a transition state

(TS) between these isomers is also shown.

The structural information of Cu5O3+, which is one of the clusters showing the highest reactivity toward the H2O release, can be obtained from its CID fragmentation pattern.

As shown in Figure 6, Cu5O3+ mainly dissociates into Cu5O2+ + O, Cu4O2+ +

CuO, or Cu3O+ + Cu2O2.

The structures of these fragment ions were previously

calculated to be planar-like geometries, which can be described as either (Cu2O)n+ or [(Cu2O)nCu]+.24

Based on this previous result, the structures of Cu5O2+ and Cu5O3+

were investigated at the BPW91/LanL2DZ level25-29 of DFT by using GAUSSIAN 09 program.30

As shown in Figure 7, two isomers (a) and (b) of Cu5O2+ in a singlet state


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are obtained.

Both isomers have almost equal energies and the structure of the isomer

(b) is quite similar to that reported in ref 24.

The energy of the transition state

between these two isomers is so low that the two isomers can convert to each other frequently.

Then, geometry optimizations of Cu5O3+ in singlet and triplet states are

performed by adding an O atom onto these Cu5O2+ isomers.

The obtained

lowest-energy structure of Cu5O3+, which is in a triplet state, is shown in Figure 8. The additional O atom bridges two Cu atoms in the isomer (a) of Cu5O2+.

In the

reaction of Cu5O3+ + NH3 → Cu5O2NH+ + H2O, several candidates for the product ion are assumed and calculated.

Finally, it is found that the production of Cu5ON(OH)+

and Cu5O2(NH)+ is almost thermoneutral whereas that of Cu5O(NHO)+ is endothermic by 1.25 eV, as shown in Figure 8.

These thermoneutral products should be accessible

under the present experimental condition (Ecol = 0.2 eV).

Actually, any product ion of

Cu5O2NH+ is observed in our experiment as shown in Figure 1c.



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Figure 8. Potential energy diagram of the reaction, Cu5O3+ + NH3 → Cu5ON(OH)+ + H2O, calculated at the BPW91/LanL2DZ level.

The optimized structures of the

reactants, reaction intermediates, transition states, and products are given together with the energies (eV) with respect to the initial state.

For the other product ions,

Cu5O2(NH)+ and Cu5O(NHO)+, the structures and relative energies are also shown.

The DFT calculations are further performed to explore a reaction pathway to the formation of Cu5ON(OH)+ and H2O in the reaction of Cu5O3+ with NH3.

The

obtained energies of the reactants, reaction intermediates, transition states, and final products with respect to the initial state are shown in Figure 8, together with their structures.

An NH3 molecule prefers to adsorb on top of a Cu atom of Cu5O3+ with

the adsorption energy of 2.53 eV.

The adsorbed NH3 dissociates into NH2 and H, and

the H atom moves toward an O atom to form an OH moiety on a Cu–Cu bridge site. During this first dehydrogenation step, the structure of the cluster core consisting of Cu atoms changes from a triangular geometry to a square-like one.

An H atom of NH2

further moves to the other bridging O atom, and the second OH moiety is formed. Then the H atom of the NH moiety moves toward an OH moiety to form an H2O molecule adsorbing on top of a Cu atom.

In this step, the structure of the cluster core

consisting of Cu atoms returns to a triangular geometry like Cu3 in Cu5O3+ and the N atom occupies a three-hold hollow site of Cu atoms. desorbs from the cluster to form Cu5ON(OH)+.

Finally, the H2O molecule

Probably, the structural change of the

cluster core occurs because most of the adsorbed species (OH, NH2, and NH) prefer a Cu–Cu bridge site.

The structural flexibility of Cu5O3+ in the reaction may contribute

to the high reactivity toward the H2O release, as well as the optimal oxidation states of the Cu atoms.

As shown in Figure 8, the reaction proceeds via three transition states, 18 ACS Paragon Plus Environment

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TS1, TS2, and TS3, and all the transition states are significantly lower in energy than the initial and the final states.

The energy diagram indicates that the final step, that is,

the desorption of H2O from Cu5ON(OH)(H2O)+, is the rate-determining step under gas phase single-collision conditions.

5. Conclusions The reactions of CunOm+ with NH3 have been studied at near thermal energies. The measured reaction cross sections are found to roughly increase with the number of oxygen atoms, m.

On the other hand, the selectivity among the products depends on

the ratio of m to n, that is, the AON of Cu atoms in the cluster.

The CunOm+ clusters

like Cu4O4+ with high AONs of ~2 can release O2 easily, which is attributed to the stability of the fragments, CunOm–2+.

These fragment ions are Cu(I)-rich oxide

clusters having the AONs of 1.0–1.4.

The H2O release following NH3 adsorption

occurs on the Cu(I)-rich oxide clusters such as Cu4O2+, Cu5O3+, Cu6O3+, Cu7O3+, Cu7O4+, and Cu7O5+, while the HNO release is dominantly observed only for Cu7O5+ having a slightly higher AON of ~1.6.

The DFT calculation of Cu5O3+ confirms that

the H2O release can proceed at near thermal energies.

Moreover, the release of one

N2 molecule and three H2O molecules is observed experimentally in the multiple collision reactions of the specific Cu(I)-rich oxide clusters, Cu5O3+ and Cu7O4+.

Our

studies of the gas-phase CunOm+ clusters provide an insight into the essentials of the SCO of NH3 to N2 and H2O on copper-based catalysts.

ASSOCIATED CONTENT Supporting Information Pressure dependence of the intensities of the parent and product ions in the 19 ACS Paragon Plus Environment


reactions of Cu4O2+, Cu6O3+, Cu7O3+, and Cu7O5+ with NH3 (Figure S1), and a mass spectrum obtained by CID of Cu3O3+ with Xe (Figure S2). (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M. Ichihashi).

ORCID Shinichi Hirabayashi: 0000-0003-2810-9713 Masahiko Ichihashi: 0000-0002-4980-1955

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.

REFERENCES (1)

Chmielarz, L.; Jabłońska, M. Advances in Selective Catalytic Oxidation of Ammonia to Dinitrogen: A Review. RCS Adv. 2015, 5, 43408-43431.

(2)

Scheuer, A.; Hauptmann, W.; Drochner, A.; Gieshoff, J.; Vogel, H.; Votsmeier, M. Dual Layer Automotive Ammonia Oxidation Catalysts: Experiments and Computer Simulation. Appl. Catal. B: Environ. 2012, 111-112, 445-455.

(3) Jabłońska, M. Palkovits, R. Copper Based Catalysts for the Selective Ammonia Oxidation into Nitrogen and Water Vapour—Recent Trends and Open Challenges. Appl. Catal. B: Environ. 2016, 181, 332-351. 20 ACS Paragon Plus Environment

Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4)

Hayakawa, A.; Goto, T.; Mimoto, R.; Kudo, T.; Kobayashi, H. NO Formation/Reduction Mechanisms of Ammonia/Air Premixed Flames at Various Equivalence Ratios and Pressures. Mech. Eng. J. 2015, 2, 14-00402.

(5)

Hinokuma, S.; Shimanoe, H.; Matsuki, S.; Kawano, M.; Kawabata, Y.; Machida, M. Catalytic Activity and Selectivities of Metal Oxides and Pt/Al2O3 for NH3 Combustion. Chem. Lett. 2016, 45, 179-181.

(6)

Il’chenko, N. I. Catalytic Oxidation of Ammonia. Russ. Chem. Rev. 1976, 45, 1119-1134.

(7)

Mayer, R. W.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R. Investigation of Ammonia Oxidation over Copper with in situ NEXAFS in the Soft X-Ray Range: Influence of Pressure on the Catalyst Performance. Catal. Lett. 2001, 74, 115-119.

(8)

Mayer, R. W.; Melzer, M.; Hävecker, M.; Knop-Gericke, A.; Urban, J.; Freund, H.-J.; Schlögl, R. Comparison of Oxidized Polycrystalline Copper Foil with Small Deposited Copper Clusters in Their Behavior in Ammonia Oxidation: An Investigation by Means of in situ NEXAFS Spectroscopy in the Soft X-ray Range. Catal. Lett. 2003, 86, 245-250.

(9)

Hinokuma, S.; Matsuki, S.; Kawabata, Y.; Shimanoe, H.; Kiritoshi, S.; Machida, M. Copper Oxides Supported on Aluminum Oxide Borates for Catalytic Ammonia Combustion. J. Phys. Chem. C 2016, 120, 24734-24742.

(10) Hinokuma, S.; Kawabata, Y.; Matsuki, S.; Shimanoe, H.; Kiritoshi, S.; Machida, M. Local Structures and Catalytic Ammonia Combustion Properties of Copper Oxides and Silver Supported on Aluminum Oxides. J. Phys. Chem. C 2017, 121, 4188-4196. (11) Lang, S. M.; Bernhardt, T. M. Gas Phase Metal Cluster Model Systems for 21 ACS Paragon Plus Environment


Heterogeneous Catalysis. Phys. Chem. Chem. Phys. 2012, 14, 9255-9269. (12) Luo, Z.; Castleman, A. W., Jr.; Khanna, S. N. Reactivity of Metal Clusters. Chem. Rev. 2016, 116, 14456-14492. (13) Gehret, O.; Irion, M. P. O-Atom Transfer to Fen+ Clusters (n = 2–10) from O2, N2O and CO2: “Microoxides of Iron”. Chem. Eur. J. 1996, 2, 598-603. (14) Jackson, P.; Harvey, J. N.; Schröder, D.; Schwarz, H. Structure and Reactivity of the Prototype Iron-Oxide Cluster Fe2O2+. Int. J. Mass Spectrom. 2001, 204, 233-245. (15) Fialko, E. F.; Kikhtenko, A. V.; Goncharov, V. B.; Zamaraev, K. I. Molybdenum Oxide Cluster Ions in the Gas Phase: Structure and Reactivity with Small Molecules. J. Phys. Chem. A 1997, 101, 8607-8613. (16) Heinbuch, S.; Dong, F.; Rocca, J. J.; Bernstein, E. R. Experimental and Theoretical Studies of Reactions of Neutral Vanadium and Tantalum Oxide Clusters with NO and NH3. J. Chem. Phys. 2010, 133, 174314. (17) Hirabayashi, S.; Ichihashi, M.; Kondow, T. Enhancement of Ammonia Dehydrogenation by Introduction of Oxygen onto Cobalt and Iron Cluster Cations. J. Phys. Chem. A 2010, 114, 13040-13044. (18) Ichihashi, M.; Hanmura, T.; Yadav, R. T.; Kondow, T. Adsorption and Reaction of Methanol Molecule on Nickel Cluster Ions, Nin+ (n = 3–11). J. Phys. Chem. A

2000, 104, 11885-11890. (19) Su, T. Parametrization of Kinetic Energy Dependences of Ion-Polar Molecule Collision Rate Constants by Trajectory Calculations, J. Chem. Phys. 1994, 100, 4703. (20) Zawadzki, J. The Mechanism of Ammonia Oxidation and Certain Analogous Reactions. Disccss. Faraday Soc. 1950, 8, 140-152. 22 ACS Paragon Plus Environment

Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Irion, M. P.; Selinger, A. Fourier Transform Ion Cyclotron Resonance Studies of Sputtered Metal Cluster Ions: The Chemistry of Cun+ with O2. Chem. Phys. Lett.

1989, 158, 145-151. (22) Gord, J. R.; Bemish, R. J.; Freiser, B. S. Collision-Induced Dissociation of Positive and Negative Copper Oxide Cluster Ions Generated by Direst Laser Desorption/Ionization of Copper Oxide. Int. J. Mass Spectrom. Ion Processes.

1990, 102, 115-132. (23) Aubriet, F.; Poleunis, C.; Chaoui, N.; Maunit, B.; Millon, E.; Muller, J.-F.; Bertrand, P. Laser Ablation and Static Secondary Ion Mass Spectrometry Capabilities in the Characterization of Inorganic Materials. Appl. Surf. Sci. 2002, 186, 315-321. (24) Jadraque, M.; Martín, M. DFT Calculations of CunOm0/+ Clusters: Evidence for Cu2O Building Blocks. Chem. Phys. Lett. 2008, 456, 51-54. (25) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098-3100. (26) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244-13249. (27) Dunning, T. H., Jr.; Hay, P. J. Gaussian Basis Sets for Molecular Calculations. In Modern Theoretical Chemistry, Vol. 3; Methods of Electronic Structure Theory; Shaefer, H. F., III, Ed.; Plenum: New York, 1977; pp 1-28. (28) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys.

1985, 82, 270-283. (29) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J. 23 ACS Paragon Plus Environment


Chem. Phys. 1985, 82, 299-310. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. GAUSSIAN 09, Revision E.01; Gaussian Inc.: Wallingford, CT, 2013.


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TOC Graphic


Gas-Phase Reactions of Copper Oxide Cluster Cations with Ammonia

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