Article pubs.acs.org/JPCA
Oxidation of CO and NO on Composition-Selected Cerium Oxide 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
‡
S Supporting Information *
ABSTRACT: The collisional reactions of composition-selected cerium oxide cluster cations, CenOm+ (n = 2−6; m ≤ 2n), with CO and NO have been investigated under single collision conditions using a tandem mass spectrometer. At near-thermal energy, oxidation of CO and NO is observed only for the stoichiometric clusters, CenO2n+ (n = 3−5), and the cross sections for the NO oxidation are found to be larger than those for the CO oxidation. In addition, the collision-energy dependence of the reaction cross sections reveals that the CO oxidation has a small activation barrier, whereas the NO oxidation is a barrierless process. These experimental findings are supported by density functional theory calculations.
1. INTRODUCTION Cerium oxide (ceria) is a key component of catalytic converters in automobiles, where it can act as an oxygen buffer by capturing and releasing oxygen during redox reactions.1 This oxygen storage/release capability is associated with the facile change of the oxidation states of cerium atoms.2 Recently, the catalytic activity of ceria has been found to be strongly influenced by the shape and the surface morphology of the particles as well as the size.3−7 For example, ceria nanorods which predominantly expose the (110) and (100) surfaces show enhanced catalytic activity for oxidation of CO, compared with ceria nanoparticles that mainly expose the (111) surface.3,5,6 In addition, it has been revealed that ceria nanorods with rougher surfaces show higher catalytic activity for CO oxidation.5 Thus, studies on the reactions of isolated ceria clusters are crucial to understand the processes occurring on nanosized ceria materials at the atomic level and to elucidate the active sites and the catalytic reaction mechanisms. Reactivity and structures of cerium oxide clusters have been experimentally investigated by several groups.8−13 He and coworkers have reported that CO oxidation proceeds on CenO2n+ (n = 4−6 and possibly n = 2 and 3) and CenO2n+1− (n = 4−21) in a fast flow reactor.9,11 In their experiments, however, the parent cluster ions were not mass-selected before the reactions, and no information on the reaction mechanisms and energetics has been obtained experimentally. Instead, based on the density functional theory (DFT) calculations, they have revealed that the active site of the cation is the CeO2 moiety where an unpaired electron is distributed, whereas that of the anion is a single terminal oxygen atom. Recently, we have studied the reactions of compositionselected cerium oxide cluster cations, CenOm+ (n = 2−6; m ≤ 2n), © 2013 American Chemical Society
with an O2 molecule and found that the observed reaction pathway significantly changes with the average oxidation number (AON) of Ce atoms in the cluster.12 Oxidation by O2 gives rise to the decomposition of the cluster with AON < 3 while simple O2-attachment occurs for the cluster with AON ∼ 3. We have also suggested that the O2 molecule added to the cluster with AON ∼ 3 is not bound as strongly as the bridging O atoms between Ce atoms. So far, CO oxidation has been accomplished in the gas phase at near-thermal energies by various metal oxide clusters,14−26 as well as ceria clusters. On the other hand, reaction studies with NO are limited to only a few metal oxide clusters. Castleman and co-workers have studied the reactions of NinOm± with NO, and shown that NO adsorbs onto NinOm± and that the adsorbed NO converts to NO2 on NinOm−.27,28 Xie et al. have reported that NO oxidation proceeds with releasing NO2 on ConOm.24 In the present study, we investigate the reactions of composition-selected CenOm+ (n = 2−6; m ≤ 2n) with CO and NO, respectively, under single collision conditions. The reaction cross sections are measured as functions of the cluster composition (n,m) and the collision energy. The measured collision-energy dependence shows a contrasting behavior for the energy barrier in oxidation of CO and NO. The reaction mechanisms derived experimentally are supported by DFT calculations. This result gives an insight on the catalytic reactivity of ceria clusters. Received: June 27, 2013 Revised: August 26, 2013 Published: September 10, 2013 9005
dx.doi.org/10.1021/jp406339z | J. Phys. Chem. A 2013, 117, 9005−9010
The Journal of Physical Chemistry A
Article
2. EXPERIMENTAL SECTION All experiments were performed using a tandem mass spectrometer equipped with a reaction cell and radio frequency ion guides. The details of the experimental setup have been described before.29 Cerium oxide clusters were produced by sputtering four separate targets of CeO2 (99.9% purity, 30 mm × 30 mm, 3-mm thick, Kojundo Chemical Laboratory Co., Ltd.) with 8.5-keV xenon-ion beams using an ion gun (CORDIS Ar25/ 35c, Rokion Ionenstrahl-Technologie). Sputtered cluster cations were extracted by a series of ion lenses and admitted into an octopole ion beam guide (OPIG) passing through a cooling cell of 290 mm in length. In this cooling cell, the cluster ions were thermalized by collisions with He gas of 10−2 Torr at room temperature. To produce oxygen-rich cluster ions, a small amount of O2 gas was added to the cooling cell. Our method combined with ion sputtering and the following O2-reaction produces specific cerium oxide clusters with high abundances as reported in our previous study.12 The relative abundance of the clusters is significantly different from that produced by laser ablation of a cerium metal disk in the gas mixture of He and O2.9 The cluster ions were mass-selected by the first quadrupole mass filter (QMF), and the composition-selected cluster ions (parent ions) were admitted to an OPIG passing through the reaction cell of 100 mm in length, where they were allowed to react with a reactant molecule (CO or NO). When the pressure of the reactant gas was less than 2 × 10−4 Torr, it was corroborated that the intensities of the product ions increase proportionally with the pressure and that there is no formation of ions which result from multiple collisions. Thus the reaction experiments were carried out in the range of 1 × 10−4−2 × 10−4 Torr, corresponding to single collision conditions. The translational energy of the parent cluster ions in the reaction cell was measured by the retarding potential method using the OPIG and converted to the collision energy, Ecol, in the center-of-mass frame. A typical spread of the collision energy was 0.4 eV in full width at halfmaximum (fwhm). Parent and product ions coming from the reaction cell were then mass-analyzed with 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. Reaction cross sections, σ, were obtained from the ratio of the intensities of the intact parent ions and the product ions.
CenOm+ + CO → CenOm − 1+ + CO2 [(n , m) = (3, 6), (4, 8), and (5, 10)]
(1)
In addition, a small amount of Ce3O4+ is observed in the reaction of Ce3O6+. The cross sections for CenOm+ + CO at Ecol = 0.2 eV are shown in Figure 1 as functions of AON of Ce atoms in
Figure 1. Cross sections for the reactions of CenOm+ with CO as functions of the average oxidation number of Ce in CenOm+ (the bottom horizontal axis) and the cluster composition (the top horizontal axis). O2 release corresponds to the formation of CenOm−2+. The collision energy is 0.2 eV.
CenOm+ and the composition of CenOm+ (= n,m). Here, AON is obtained as (2m + 1)/n for CenOm+. The reaction cross sections for Ce3O6+ and Ce4O8+ are found to depend significantly on the collision energy, and the results are shown in Figure 2. Note that the collision-energy dependence of Ce5O10+ + CO could not be obtained reliably because of the low intensity of the parent cluster ion. The observed reaction products are CenOm−2+ and/or CenOm−1+ at all of the collision energies studied. In the reaction of Ce3O6+ + CO, both formation cross sections of Ce3O4+ and Ce3O5+ increase with the collision energy, as seen in Figure 2a. The formation of Ce3O4+ dominates throughout the collision energy range studied, and this cross section increases sharply around Ecol = 1 eV and then become constant above Ecol = 3 eV. The formation cross section of Ce3O5+ increases moderately as the collision energy increases. As shown in Figure 2b, the formation cross sections of Ce4O6+ and Ce4O7+ in the reaction of Ce4O8+ + CO also increase with the collision energy, reach a maximum, and then level off or decrease gradually. In contrast to the reaction of Ce3O6+, CenOm−1+ is a dominant product in the reaction of Ce4O8+ at low collision energies, but its cross section is surpassed by that of the CenOm−2+ formation above ∼2.5 eV. The collision-energy dependences of the formation cross section of CenOm−2+ from CenOm+ [(n,m) = (3,6) and (4,8)] + CO agree with those of the CID cross section of CenOm−2+ from CenOm+ + Xe.12 This indicates that CenOm−2+ observed in CenOm+ + CO is formed by CID. 3.2. CenOm+ + NO. Figure 3 shows the cross sections for the reactions of CenOm+ with NO at near-thermal collision energy, Ecol = 0.2 eV, as functions of AON of Ce atoms in CenOm+ and the composition of CenOm+ (= n,m). In an analogous fashion to CenOm+ + CO, CenOm−1+ is observed in the reactions of Ce3O6+, Ce4O8+, and Ce5O10+ with NO, and this species would be produced by the following NO oxidation:
3. RESULTS Here, twelve species of cerium oxide cluster cations including three stoichiometric CenO2n+ clusters are chosen for reaction experiments: Ce2Om+ (m = 2 and 3), Ce3Om+ (m = 3−6), Ce4Om+ (m = 6 and 8), Ce5Om+ (m = 7, 9, and 10), and Ce6O9+. 3.1. CenOm+ + CO. First, the reactions of CenOm+ with CO are examined at near-thermal collision energy, Ecol = 0.2 eV, under single collision conditions. Most clusters give no detectable products, and this implies that their reaction cross sections should be smaller than the detection limit of ∼0.1 Å2. In contrast, Ce3O6+, Ce4O8+, and Ce5O10+ show the formation of CenOm−1+, which is in good agreement with the results in ref 9. In our previous studies on collision-induced dissociation (CID) of Ce3O6+ and Ce4O8+ with Xe, no or negligible dissociation to CenOm−1+ + O was observed at Ecol = 0.2 eV.12 Therefore, the formation of CenOm−1+ from CenOm+ indicates that the following CO oxidation takes place: 9006
dx.doi.org/10.1021/jp406339z | J. Phys. Chem. A 2013, 117, 9005−9010
The Journal of Physical Chemistry A
Article
Figure 4. Cross sections for the formation of CenOm−2+ and CenOm−1+ in the reactions of (a) Ce3O6+ and (b) Ce4O8+ with NO as functions of the collision energy (the bottom horizontal axis) and the total energy (the top horizontal axis). The total energy is the sum of the collision energy and the internal energy of the parent ion.
Figure 2. Cross sections for the formation of CenOm−2+ and CenOm−1+ in the reactions of (a) Ce3O6+ and (b) Ce4O8+ with CO as functions of the collision energy (the bottom horizontal axis) and the total energy (the top horizontal axis). The total energy is the sum of the collision energy and the internal energy of the parent ion.
NO. The cross section for the NO oxidation decreases sharply with the collision energy and is surpassed by that of the CenOm−1+ formation at high energies. The formation cross section of CenOm−2+ increases with the collision energy and then levels off above ∼4 eV. It is most likely that CenOm−2+ is formed by CID with NO, based on the similarity of the collision-energy dependences of the CenOm−2+ formation cross section among CenOm+ + Xe, CO, and NO. Furthermore, as shown in Figure 3, Ce5O7+ adsorbs an NO molecule, and Ce5O7+(NO) is observed. This formation cross section decreases sharply as the collision energy increases, and then falls below the detection limit (
