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Enhancement of Ammonia Dehydrogenation by Introduction of Oxygen onto Cobalt and Iron Cluster Cations Shinichi Hirabayashi,† Masahiko Ichihashi,*,‡ and Tamotsu Kondow‡,§ East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan, and Cluster Research Laboratory, Toyota Technological Institute in East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan ReceiVed: September 24, 2010; ReVised Manuscript ReceiVed: NoVember 8, 2010

Reactions of oxygen-chemisorbed cobalt and iron cluster cations (ConOm+ and FenOm+; n ) 3-6, m ) 1-3) with an NH3 molecule have been investigated in comparison with their bare metal cluster cations at a collision energy of 0.2 eV by use of a guided ion beam tandem mass spectrometer. We have observed three kinds of reaction products, which come from NH3 chemisorption with and without release of a metal atom from the cluster and dehydrogenation of the chemisorbed NH3. Reaction cross sections and branching fractions are strongly influenced by the number of oxygen atoms introduced onto the metal clusters. Oxygen-chemisorbed metal clusters with particular compositions such as Co4O+, Co5O2+, and Fe5O2+ are extremely reactive for NH3 dehydrogenation, whereas Co4O2+ and Fe4O2+ exhibit high reactivity for NH3 chemisorption with metal release. The enhancement of dehydrogenation for specific compositions can be interpreted in terms of competition between O-H and neighboring Co-H (or Fe-H) formation. 1. Introduction Transition metals and their oxides are widely used as heterogeneous catalysts.1 For example, iron catalysts are employed for ammonia synthesis, and vanadium oxide, for sulfuric acid production. Since practical catalysts consist of small metal particles with a variety of sizes, compositions, and structures, clusters with a small number of atoms should be considered as models of the active sites.2,3 The reactivity studies of metal clusters isolated in the gas phase provide useful knowledge of the catalytic mechanism at the reaction sites and would also be helpful for the design of a novel catalyst having higher activity and selectivity. The reactivity of transition metal clusters including oxygen has been extensively studied in the gas phase.2-10 Most of these studies have focused on the metal oxide clusters, which refer to fully oxidized clusters or clusters including more oxygen atoms than metal atoms. One of the reasons for this interest is that the stable oxides are readily available as bulk materials and widely used as catalysts, and recent investigations have extended to the oxidations of CO and hydrocarbons.4-6 On the other hand, the reactivity of poorly oxidized metal clusters, such as oxygen-chemisorbed metal clusters, should be investigated in comparison with the corresponding bare metal clusters as well as the geometric and electronic structures because their reactivity can change drastically with the number of chemisorbed oxygen atoms. The oxygen chemisorption improves the reactivity of metal clusters in some cases and suppresses it in others. The improvement of the reactivity has been shown in the reactions of vanadium group metal octamers with D2 and N27-9 and the reactions of Agn+ with N2.10 In the latter reaction, it was found that the preadsorbed O2 molecules increase the positive charge * Corresponding author. E-mail: [email protected]. † Genesis Research Institute, Inc. ‡ Toyota Technological Institute. § Deceased May 25, 2009.

on Ag atoms, and then N2 molecules can adsorb efficiently onto Agn+(O2)m. As a result, the mean number of N2 molecules on Agn+(O2)m was significantly larger than that on Agn+. On the other hand, it has been observed that the chemisorption of an oxygen atom suppresses the reactivity of vanadium group trimers with D2 and N2,7-9 and Crn+ (n ) 2-4) with C2H4.2 Ammonia synthesis with metal catalysts is one of the most important industrial processes for nitrogen fixation.11 For example, iron is widely used as a catalyst in the industrial ammonia synthesis, and cobalt particles supported on carbon have been reported to be a promising catalyst.12,13 On the metal catalysts, the ammonia synthesis proceeds via the stepwise hydrogenation of the atomic nitrogen, and the dehydrogenation of NH3 proceeds inversely via the same reaction steps. Therefore, the studies on the dehydrogenation of NH3 can also provide valuable information on catalytic ammonia synthesis. The NH3 dehydrogenation on metal clusters such as Fen+,14-19 Agn+,15 Ptn(,20-22 and Aun+15 has been studied in the gas phase, and Fe4+ and Pt2+ exhibited high reactivity. These reaction mechanisms have been confirmed as stepwise dehydrogenation by recent theoretical studies.19,21,23 The effect of the oxygen introduction to Fe2+ has also been investigated, and it was revealed that the dehydrogenation of NH3 on Fe2O2+ accompanies the hydrogenation of the atomic oxygen and that a H2O molecule is released finally.24 Here, we investigate the reactions of oxygen-chemisorbed metal cluster cations (ConOm+ and FenOm+; n ) 3-6, m ) 0-3) with NH3 by measuring the absolute reaction cross sections. It is shown that the reactivity of ConOm+ and FenOm+ depends significantly on the compositions, that is, the numbers of metal and oxygen atoms, and that some specific clusters exhibit extremely high reactivity for the dehydrogenation of NH3. 2. Experimental Section The present study was performed by use of a tandem mass spectrometer equipped with a reaction cell and radio frequency

10.1021/jp109118d  2010 American Chemical Society Published on Web 11/19/2010

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ion guides, which has been described in detail elsewhere.25 A brief description of the setup and method employed is given here. Clusters were produced with the sputtering of four separate metal (cobalt or iron) targets by 15-keV xenon ions from an ion gun (Rokion Ionenstrahl-Technologie, CORDIS Ar25/35c). The metal cluster cations thus produced were extracted by ion lenses and admitted to an octopole ion guide (OPIG) passing through a cooling cell of 290 mm in length. In this cooling cell, the cluster ions were cooled down by more than 100 collisions in He gas of 10-2 Torr at a temperature of 300 or 100 K. This number of collisions is sufficient to thermalize the cluster ions on the basis of the fact that Al2+ and Al11+ have been shown to be thermalized by 50 and 100 collisions with He atoms, respectively.26 In the experiments of the oxygenchemisorbed cluster ions, a small amount of O2 gas was added to the cooling cell. Then, cluster ions of interest (parent cluster ions) were mass-selected by the first quadrupole mass filter (QMF) and admitted to an OPIG passing through the reaction cell (OPIG-R), where they were allowed to react with NH3 molecules. The pressure in the reaction cell was adjusted in the range of 5 × 10-5-2 × 10-4 Torr, where single collision conditions were fulfilled. The translational energy of the parent cluster ions in the reaction cell was measured by the retarding potential method using the OPIG-R and converted to the collision energy in the center-of-mass frame. A typical spread of the collision energy was 0.2 eV. Parent and product ions coming from the reaction cell were mass-analyzed by the second QMF and detected by a secondary electron multiplier in combination with an ion conversion dynode. Electric signals from the detector were processed in a pulse counting mode. A total absolute reaction cross section, σr, was obtained as

∑ Ip

k BT I + σr ) ln Pl I

Ip

∑ Ip

typical mass spectrum in the reaction of Co4O+ with an NH3 molecule. Three peaks assigned to Co3O+(NH3), Co4O+(NH), and Co4O+(NH3) appear clearly in the mass spectrum, together with that of the intact parent ion. Similarly, for all the cluster ions (MnOm+; M ) Co and Fe) studied, we observed no other product ions than MnOm+(NH3), Mn-1Om+(NH3), and MnOm+(NH). This indicates that the following three reactions take place:

MnOm+ + NH3 f MnOm+(NH3) (simple NH3 chemisorption) (3) f Mn-1Om+(NH3) + M (NH3 chemisorption with metal release) (4) f MnOm+(NH) + H2

(dehydrogenation)

(5)

(1)

where kB is the Boltzmann constant; P and T are the pressure and the temperature of the NH3 gas, respectively; l () 120 mm) is the effective path length of the reaction region; and I and ΣIp represent the intensity of the intact parent ion that passed through the reaction region and the sum of the intensities of the product ions, respectively. In this measurement, the second QMF was set to work at a relatively low mass resolution (m/∆m ) ∼50) to achieve the mass-independent high transmittance of ions. Statistical errors in the reaction cross sections thus obtained were estimated to be typically 20-30%. A partial reaction cross section, σp, for the formation of a given product ion was obtained by

σp ) σr

Figure 1. Typical mass spectrum of parent and product ions in the reaction of Co4O+ with NH3 at a collision energy of 0.2 eV. The internal temperature of the parent cluster is 300 K.

(2)

where Ip/ΣIp represents the branching fraction to the product ion. Here, the mass resolution of the second QMF was adjusted to distinguish the number of hydrogen atoms of the product ion. In addition, the most intense isotopomer, 56FenOm+, was mass-selected by the first QMF in the measurement of FenOm+. 3. Results All the measurements were performed under single collision conditions at the collision energy of 0.2 eV. Figure 1 shows a

The reaction cross sections of ConOm+ (n ) 3-6, m ) 0-3) are shown as a function of the number of the oxygen atoms in Figure 2a. The total reaction cross section increases significantly when oxygen atoms are introduced to Con+, except for n ) 3. In addition, the branching fractions change drastically with the number of oxygen atoms. The dehydrogenation is quite enhanced on Co4O+, Co5O1,2+, and Co6O2,3+ compared with the corresponding bare metal clusters. In particular, the reactivity of Co5O2+ is almost 300 times as high as that of Co5+. On the other hand, the Co release from Co4O2+ and Co5O+ proceeds readily after the NH3 chemisorption. To consider the energetics, we reduced the internal temperature of Co4O1,2+ from 300 to 100 K and measured the reaction cross sections again. Then, the total reaction cross sections at 100 K are almost equal to those at 300 K, but the partial cross sections and branching fractions depend slightly on the temperature of the clusters. The branching fractions to the dehydrogenation for Co4O+ and to the Co release reaction for Co4O2+ decrease, whereas those to the simple NH3 chemisorption for both the clusters increase with the decrease in the temperature. Figure 2b shows the reaction cross sections of FenOm+ (n ) 3-6, m ) 0-3). In the bare iron clusters studied here, Fe4+ is the most reactive for the NH3 dehydrogenation, and this result is fairly consistent with the previously published ones,15-19 whereas the reaction cross sections of the simple NH3 chemisorption for Fen+ are diminished in comparison with those measured by Liyanage et al.,18 probably because of the difference of the temperature of the clusters. The total reaction cross section increases by the introduction of oxygen atoms onto Fen+,

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Figure 2. Reaction cross sections of simple NH3 chemisorption, NH3 chemisorption with metal release, and dehydrogenation for (a) ConOm+ and (b) FenOm+ (n ) 3-6) as a function of the number of oxygen atoms. The internal temperature of the parent clusters and the collision energy are 300 K and 0.2 eV, respectively.

but only Fe3O3+ has an exceptionally small reaction cross section. The dehydrogenation is highly enhanced on Fe3O+ and Fe5O1,2+, and the Fe release from Fe3O+ and Fe4O2+ occurs efficiently after the NH3 chemisorption. 4. Discussion 4.1. Composition Dependence of ConOm+. The reaction of ConOm+ with an NH3 molecule is considered to proceed along the following processes. The NH3 molecule is attracted to ConOm+ by the charge-dipole interaction, is captured to form the physisorbed species (ConOm+ · · · NH3), and then is chemisorbed on the cluster. The capture cross section is estimated to be 108 Å2 at a collision energy of 0.2 eV by the trajectory method.27 For instance, the measured total reaction cross section of Co5O2+ reaches 93 Å2, indicating that the desorption of the captured NH3 molecule rarely occurs on this cluster. The chemisorbed intermediate, ConOm+(NH3), loses its excess energy with releasing the NH3 molecule, one Co atom (eq 4), or a H2 molecule (eq 5), whereas the intermediate can be observed if its lifetime is longer than the reaction time (∼100 µs), which corresponds to the flight time from the reaction cell to the second QMF. Figure 3 shows a schematic reaction potential for explaining the reaction process for the NH3 chemisorption with the Co release. The enhancement of NH3 chemisorption with the Co release takes place on Co4O2+ and Co5O+. This result indicates that the energies necessary for the Co release from these clusters are smaller than the internal energies of the chemisorbed intermediates. Here, the internal energy is given as the sum of the internal energy of the parent ion, the collision energy, and the chemisorption energy of NH3 on ConOm+. Furthermore, the Co release reaction still dominates for Co4O2+, having an internal

Figure 3. Schematic potential energy curve along the reaction coordinate of the NH3 chemisorption with the Co release reaction between ConOm+ and NH3. The chemisorption energy of NH3 on ConOm+ is represented by Echem. The dashed line shows the potential energy curve, where Echem is smaller than the bond dissociation energy, D(NH3Con-1Om+-Co).

temperature of 100 K, although its branching fraction decreases. Here, the difference in the internal temperature between 300 and 100 K corresponds to ∼0.2 eV. Liu et al. have studied the reactions of Con+ with O2 and obtained the Co-Co bond dissociation energies of Con+, ConO+, and ConO2+.28 Their results indicate that both the bond dissociation energies of Co4O2+ and Co5O+ are larger than that of Co4+, in which no Co release reaction occurs. Probably the adsorption energy of NH3 depends on the composition of the cluster, and Co4O2+ and Co5O+ have NH3 adsorption energies larger than the Co-Co bond dissociation energies (see Figure 3). On the other hand, the dehydrogenation proceeds significantly on Co4O+, Co5O1,2+, and Co6O2,3+. This reaction is still dominant at 100 K, as shown in the result of Co4O+, although its branching fraction decreases slightly. This finding indicates that the

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Figure 4. Reaction scheme for the dehydrogenation of NH3 on Co4O+. The first reaction step, a f b, represents the formation of NH2 and OH by the interaction between the adsorbed NH3 molecule and the oxygen atom. The following steps, b f c and c f d, show the further dissociation of NH2 and the formation of an H2 molecule proceeding on the metal atoms, respectively.

energies of the transition states for the dehydrogenation are much lower than that of the initial state, ConOm+ + NH3. A possible reaction pathway for the dehydrogenation of NH3 on ConOm+ is presumed by analogy with the mechanism of the reactions on metal oxide ions and oxygen-preadsorbed metal surfaces. For instance, in the reaction of FeO+ with NH3, where dehydration proceeds,29,30 HO-Fe+-NH2 and H2O-Fe+-NH have been proposed as the intermediates.31 Furthermore, theoretical investigations on several metal surfaces, such as copper and platinum, have shown that a preadsorbed atomic oxygen promotes the formation of NH2 and OH groups through abstraction of an H atom from NH3.32-34 Therefore, we consider a plausible reaction scheme of the dehydrogenation on ConOm+ as follows. An NH3 molecule adsorbs on a Co atom, and then one of the H atoms in NH3 is abstracted by an adsorbed atomic oxygen to form NH2 and OH groups. Then it is supposed that the other H atoms of NH2 migrate to the Co atoms and that the formation of an H2 molecule proceeds on the Co atoms. To support this reaction scheme, the following simple statistical model is applied to ConOm+ (n ) 4-6, m ) 1-3), and the probabilities of the NH3 dehydrogenation are estimated. An NH3 molecule can initially adsorb onto one of the on-top Co sites in an equal probability. This is shown schematically for Co4O+ in Figure 4. The Co4+ is considered to have a tetrahedral structure,35 and the O atom occupies a 3-fold hollow site. Then, three Co atoms bond to the O atom, while one Co atom is free from the O atom. The dissociation of the adsorbed NH3 occurs if the NH3 binds to a Co that is also bound to an O atom. Otherwise, the adsorbed NH3 does not dissociate at all, and the following dehydrogenation does not proceed. The probability of this first reaction step is 75%. At the next step, the adsorbed NH2 dissociates to an N atom and two H atoms on the cluster. Then, the three H atoms (including the H atom in OH) are redistributed onto any on-top site of Co, O, or N atoms. If two H atoms of the three adsorb on on-top sites of neighboring Co atoms, then the H atoms combine to produce a H2 molecule, which is released. In the second step, there are 10 possible ways to distribute three H atoms among five sites (the N, the O, and three of the four Co atoms; the on-top site on the other Co is occupied by the N atom). Of these, three do not proceed because the two H atoms are on the N and the O while the remaining H is on one of the Co’s. In all other cases, the formation of H2 proceeds, and this probability of the second step is 70%. As a result, the total probability of the NH3 dehydrogenation is estimated to be 52.5%. Similarly, the probabilities are calculated for the other clusters. In this model, it is assumed from our recent calculations that Co5+ and Co6+ have a trigonal bipyramid and an octahedral structure, respectively.35 Note that two H atoms on Co atoms of Co4Om+ neighbor each other, but it is not always the case for Co5,6Om+. Figure 5 shows the probabilities of the dehydro-

Figure 5. Comparison between the measured cross sections (bars) and the calculated probabilities (circles) of the NH3 dehydrogenation for ConOm+ (n ) 4-6).

genation for ConOm+ (n ) 4-6, m ) 1-3) estimated from this model. The clusters with a small number of oxygen atoms, Co4O+, Co5O1,2+, and Co6O2,3+, show high probabilities for the dehydrogenation, and this tendency agrees with the experimental results very well (see Figure 5). Consequently, the proposed reaction process can account for much of the compositiondependent reactivity for the dehydrogenation of NH3 on ConOm+. Furthermore, the geometric structures of Co4O3+ and Co5O3+ may differ from those of the other oxygen-chemisorbed cobalt cluster cations, since photoelectron spectra of Co4O3- and Co5O3- have indicated the presence of peroxo and ozonide isomers.36 The characteristic reactivity of Co4O3+ and Co5O3+ toward the simple NH3 chemisorption possibly results from the existence of the molecularly chemisorbed oxygen. 4.2. Comparison of FenOm+ with ConOm+. The total reaction cross sections of Fen+ increase with the oxygen introduction as well as Con+, but even so, the cross sections of FenOm+ are not necessarily equal to those of ConOm+. For example, Fe4O+, Fe5O+, and Fe6O1-3+ have smaller total cross sections, but Fe3O1,2+ and Fe4O3+ have larger ones than the corresponding ConOm+ have. Only the simple NH3 chemisorption occurs on both Fe4O3+ and Co4O3+, but the cross section of the former is almost twice as large as that of the latter. This difference may be attributed to the difference in the NH3 adsorption energy; a calculation based on the Rice-Ramsperger-Kassel (RRK) theory25 shows that a small change in the energy, ∼0.2 eV, can cause the large difference in the measured cross section between Fe4O3+ and Co4O3+. In particular, there is a significant difference in the branching fraction between Fe5O+ and Co5O+. Although the total cross section of the former is quite small, its dehydrogenation cross section is almost equal to that of the latter. The observation of the simple NH3 chemisorption onto Co5O+ suggests that NH3 adsorbs onto this cluster more strongly than onto Fe5O+. A schematic reaction potential for explaining the reaction process for the dehydrogenation on M5O+ is shown in Figure 6. An ammonia molecule adsorbs onto Fe5O+ to form Fe5O+(NH3), and then the dehydrogenation of the chemisorbed NH3 proceeds. Otherwise, the NH3 molecule desorbs from Fe5O+ due to the small adsorption energy. In addition, the Fe-Fe bond dissociation energy of Fe5O+, D(OFe4+-Fe), is larger than D(OCo4+-Co),28,37 and this also contributes to the suppression of the Fe release in the reaction of Fe5O+ with NH3. On the other hand, the enhancement of the reactivity of Fe4O2+ and Fe5O2+ is quite similar to that of the corresponding Co clusters. Again, the enhancement of the NH3 chemisorption with the Fe release for Fe4O2+ cannot be explained by the Fe-Fe bond dissociation energy of FenOm+.37 It is supposed that Fe4O2+

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Figure 6. Schematic potential energy curve in the NH3 dehydrogenation for M5O+ (M ) Co and Fe). The chemisorption energy of NH3 on M5O+ and the energy barrier for the dehydrogenation are represented by Echem and ∆E, respectively. The solid and dashed lines are applied for Co5O+ and Fe5O+, respectively.

can strongly adsorb an NH3 molecule and that Fe4O2+(NH3) consumes the large excess energy with release of one Fe atom. The theoretical studies show that Fe4O2+ and Co4O2+ have similar geometric structures,38,39 which may provide a suitable site for the chemisorption of NH3. The tendency of the dehydrogenation cross section of FenOm+ is very similar to that of ConOm+, and Fe5O2+ exhibits the largest cross section for this reaction. Therefore, the similar reaction scheme may be applicable for the NH3 dehydrogenation. 5. Conclusion We have investigated the reactions of ConOm+ and FenOm+ (n ) 3-6, m ) 0-3) with NH3 under single collision conditions. The reactivity is influenced by the numbers of metal and oxygen atoms, and the particular clusters such as Co4O+, Co5O2+, and Fe5O2+ are highly reactive for the dehydrogenation of NH3. The probability of the NH3 dehydrogenation was calculated on the assumption of the O-triggered NH3 decomposition and the competition between the OH and Co-H (or Fe-H) formations. This reaction scheme can account for the measured compositiondependent reactivity. Our findings suggest that an extremely effective catalyst of ammonia synthesis can be produced from the specific poorly oxidized cobalt clusters. Acknowledgment. This work was supported by the Special Cluster Research Project of Genesis Research Institute, Inc. References and Notes (1) Rase, H. F. Handbook of Commercial Catalysts: Heterogeneous Catalysts; CRC: Boca Raton, 2000. (2) Hanmura, T.; Ichihashi, M.; Monoi, T.; Matsuura, K.; Kondow, T. J. Phys. Chem. A 2004, 108, 10434–10440. (3) Bo¨hme, D. K.; Schwarz, H. Angew. Chem., Int. Ed. 2005, 44, 2336– 2354. (4) Johnson, G. E.; Reveles, J. U.; Reilly, N. M.; Tyo, E. C.; Khanna, S. N.; Castleman, A. W., Jr. J. Phys. Chem. A 2008, 112, 11330–11340.

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