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Catalytic reactions of gas phase zirconium oxide clusters with NO and CO revealed by post heating
Chemical Physics Letters 660 (2016) 261–265
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Catalytic reactions of gas phase zirconium oxide clusters with NO and CO revealed by post heating Ken Miyajima, Fumitaka Mafuné ⇑ Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan
a r t i c l e
i n f o
Article history: Received 20 June 2016 In final form 12 July 2016 Available online 14 July 2016 Keywords: Cluster Redox reactions Zirconium oxide Post-heating Catalysis
a b s t r a c t Reactivity of gas phase zirconium oxide clusters (ZrnOm+) toward NO and CO gases was investigated by mass spectrometry in combination with post heating. Reaction of ZrnO2n+x+ with NO gas resulted in the depletion of extremely oxygen-deficient clusters and the formation of oxygen-rich clusters, ZrnO2n+x+ (0 6 x 6 3). Reaction with CO substantially lead to an increase in the amount of ZrnO2n2+ and ZrnO2n1+ clusters and depletion in the amount of ZrnO2n+. The catalytic cycle, achieved by regenerating ZrnO2n+ by the oxidation of ZrnO2n2+ by NO, were discussed in comparison with the reactivity of cerium oxide clusters. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Individual aspects of catalytic processes can be probed at the molecular level by studying the reaction of clusters in the gasphase [1]. The catalytic properties of gas-phase clusters have been investigated by many researchers; the relationships between their reactivity, elemental composition, and stoichiometry and their geometric and electronic structures have been discussed for several systems. Various metal oxide clusters that contain radical oxygen have been studied by Castleman and co-workers and other groups [1–3]. Among the clusters, zirconium oxide clusters have been proposed for various applications in nanotechnology because of their unique, tailorable properties as cluster assembled materials. Castleman’s group reported a series of particularly reactive clusters (ZrO2)x+ (x = 1–4) by combining gas-phase experiments and density functional theory (DFT) calculations [4]. They proposed that this stoichiometric series of cationic zirconium oxide clusters are potential building blocks for a cluster assembled catalyst that would efficiently promote oxidation reactions. In the condensed phase, zirconia is widely used in a variety of technological applications [5–7]. The versatility of zirconia originates entirely from atomic or point defects in the crystal that are created by adding aliovalent oxides. It is well known that small nanoparticles exhibit unique properties, which are characteristic of neither the atomic nor the bulk (condensed phase) regimes. For instance, Grena et al. analyzed the stabilizing effect of surface ⇑ Corresponding author. E-mail address: [email protected] (F. Mafuné). http://dx.doi.org/10.1016/j.cplett.2016.07.028 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
impurities on the structure of ultrasmall ZrO2 nanoparticles (Zr43O86; 1.25 nm) using ab initio calculations [8]. They found that the surface stabilization effects play a crucial role in the structure of ZrO2 nanoparticles, in contrast with other systems (TiO2 and CeO2). However, it is not easy to anticipate the real structure of these nanoparticles when not in isolation (e.g., aggregation, impurities, surfactants, surface relaxation, or reconstruction). Some of these problems can be omitted by producing and analyzing the clusters in vacuum. Several groups have studied zirconium oxide clusters in the gas phase [4,9–17]. Bowen and coworkers reported the preparation of zirconium oxide cluster anions and their photoelectron spectra [9]. In their mass spectrum, oxygen-rich zirconium oxide cluster anions ZrnO2n+2,3,4,5 were produced predominantly. They also determined the adiabatic electron affinity and dissociation energy of ZrO. Von Helden and coworkers measured IR spectra of zirconium oxide clusters and observed clusters with a composition + ZrnO2n1 at high fluencies of the IR irradiation [10]. Bernstein and coworkers studied growth dynamics, stabilities, and structures of small zirconium oxide clusters by mass spectrometry and DFT calculations [11,12]. They observed nonstoichiometric clusters [(ZrO2)n1ZrO]+ at high ionization laser intensities; at lower laser intensities, stoichiometric (ZrO2)n+ appeared preferentially. Among the (ZrO2)n clusters, (ZrO2)5 is a very stable cluster, which has neither a high symmetry nor a closed shell electronic structure. Modeling of the covariance matrix over a wide range of ionization laser intensities indicated that (ZrO2)n neutral clusters absorb two photons of 193 nm radiation to ionize and then the ion can absorb more photons in the case of high laser fluence.
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There have been many reports on the reactivity of zirconium oxide clusters. Johnson et al. studied the reactivity of zirconium oxide cations with CO, C2H4, C2H2, or N2O using a guided-ionbeam mass spectrometer [4]. They identified that a distinct series of stoichiometric cationic zirconium oxide clusters (ZrO2)x+ (x = 1–4) showed O-atom transfer activity towards CO, C2H4, and C2H4. Furthermore, these active stoichiometric species can be regenerated from oxygen-deficient clusters using a suitable oxidizer. This indicated that these species may be used to promote multiple cycles of oxidation and behave as true catalysts. Soon after this work, they produced (ZrO2)x+ and (ZrxO2x+1) (x = 1–4) and performed an experimental and theoretical investigation into the influence of different charge states on the oxidation of CO, C2H2, and C2H4 in zirconium oxide clusters containing radical oxygen centers [2]. Tyo et al. investigated the structure and reactivity of superoxide (O 2 )-containing clusters ZrxO2x+1 (x = 1–3) [14]. + Among such clusters, Zr2O5 exhibited the highest reactivity for the oxidation of C3H6, C4H6, and C2H2. They found that the surrounding environment of a superoxide unit plays an important role in its reactivity. Wu et al. investigated CO adsorption of zirconium oxide cluster ions by means of collision-induced dissociations [15]. They found that most cationic clusters adsorb CO, while only specific anionic clusters adsorb CO. Furthermore, loss of CO and CO2 occurs for ZrxOyCO+ and ZrxO2x+1CO, respectively. Ma et al. generated zirconium oxide cluster anions Zr2Oy (y = 5–8) and studied their reactivity with n-butane [16]. DFT calculations indicated that the highly oxygen-rich cluster Zr2O 8 contains one mononuclear oxygencentered radical (O), leading to its high reactivity toward n-C4H10 oxidation. Recently, they also produced titanium and zirconium oxide cluster anions, (TiO2)nO and (ZrO2)nO (n = 3–25), respectively. The reaction of (ZrO2)nO with CO generated CO adducts (ZrO2)nOCO, which lose CO2 after collisions with the He gas [17]. Their DFT calculations (n = 3–8) revealed that these clusters are atomic radical anion-bonded systems, which are difficult to capture and characterize in the condensed phase. Despite these intensive gas-phase experiments and theoretical works, important factors that govern the redox reactivity have not been fully explained yet, especially when using a mixture of reactant gases. In this study, we report the reactivity of zirconium oxide cluster cations ZrnOm+ toward O2, CO, and NO by mass spectrometry. The variation in the abundance of clusters with the oxidative and reductive gas ratio is discussed.
pressure inside the valve was kept constant at 1.1 105 Pa by using a pressure controller. The typical total gas density inside the reaction gas cell was estimated to be 1018 molecules cm3. The residence time of clusters in the reaction gas cell was estimated to be 70 ls. After passing the cluster ions through the reaction gas cell, they were introduced into an extension tube. The extension tube was heated to 300–1000 K using a resistive heater and was monitored using thermocouples (type K). The residence time of the cluster ions and the density of the He gas in the extension tube were estimated to be 100 ls and 1017 molecules cm3, respectively. Thermal equilibrium between the clusters and the wall of cluster source was achieved by collisions with the He carrier gas well before expansion under vacuum. A mixture of clusters and He carrier gas was entered into the second chamber through a skimmer. Positively charged cluster ions were orthogonally extracted by a pulsed voltage for the TOF mass analysis.
3. Results and discussion 3.1. Production of oxygen-deficient zirconium oxide clusters Fig. 1a shows a mass spectrum of zirconium oxide clusters produced directly by the laser ablation of a zirconia rod with
2. Experimental methods A detailed explanation of the experimental setup has been described elsewhere [18,19]. Briefly, zirconium oxide clusters were prepared by laser ablation and were detected using a reflectronequipped time-of-flight mass spectrometer (TOF-MS) after introducing the clusters into a reaction gas cell. A sintered zirconia rod (ZrO2; 99.9%, Rare Metallic, Co. Ltd.) was set downstream of the supersonic source from a solenoid pulsed valve. The rod (5 mm in diameter, 30 mm long) was irradiated with focused laser pulses (f = 200 mm) with a pulse energy of 1.5 mJ for the ZrO2 rod at 532 nm to generate plasma. The evaporated zirconium oxides were cooled in a cylindrical channel (6 mm diameter) using He gas (>99.99995%, 0.5 MPa) from the valve. The clusters were passed through a reaction gas cell (2 mm diameter, 60 mm long, kept in room temperature) and an extension tube (4 mm diameter, 120 mm long) with a resistive heater for post-heating before expansion into the first vacuum chamber; they were then introduced into the differentially pumped second chamber through a skimmer. Reactant gases (CO (>99.95%), NO, and O2 (>99.9%)) were injected inside the reaction gas cell using another solenoid pulsed valve to examine the chemical reactivity of the clusters. The total
Fig. 1. (a–b) Mass spectrum of zirconium oxide clusters ZrnOm+ (n = 5 and 6) produced by the laser ablation of a zirconia target. Red plot is the raw data, and black and blue plots show the processed experimental mass intensity and simulated one which have been segmented in 0.9994 amu steps. Residual error is given in the bottom. (c–d) Abundance of ZrnO2n+x+ (n = 5–7) obtained by deconvolution of raw data before and after the reaction with O2 gas. Black and light blue plots indicate the intensity of ZrnO2n+x+ and ZrnO2n+x(H2O)+ clusters, respectively. The horizontal axis, x, indicates the deviation of the oxygen atom number from the stoichiometric compositions. The vertical broken lines indicate the position of x = 2 and 0 and are shown as a guide for eyes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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O2-undoped He carrier gas. As a zirconium atom comprises a variety of isotopes (90Zr 51.45 ± 0.40%, 91Zr 11.22 ± 0.05%, 92 Zr 17.15 ± 0.08%, 94Zr 17.38 ± 0.28%, and 96Zr 2.80 ± 0.09%) [20], a zirconium oxide cluster provides many mass peaks in the mass spectrum. In order to fairly evaluate abundances of the clusters, deconvolution of the mass peaks was required. Using homemade softwares based on a protocol described in the Supporting Material, we estimated the abundances. As the residual error was minimal, we consider that the processes worked reasonably well (see Fig. 1b). As shown in Fig. 1c, oxygen-deficient zirconium oxide clusters exhibited a smooth, wide, and gradual distribution of ZrnOm+ (0 6 m 6 2n + 1). Prominent peaks were assigned to nearstoichiometric clusters, ZrnO2n1+ or ZrnO2n2+ where the number of oxygen atoms is one or two less than the stoichiometric composition. In order to express the cluster’s stoichiometric compositions in a simple way, deviation from stoichiometric oxygen number, defined as x = m2n, will be used hereafter. The high reactivity of oxygen-deficient clusters with oxygen was evidently observed as the changes of the cluster abundance distribution (see Fig. 1(d)): Oxygen-deficient clusters, ZrnO2n+x+ (2n 6 x 6 1), decreased + considerably, whereas ZrnO2n+ and ZrnO2n+1 were predominantly generated. In this respect, the chemistry of oxygen-deficient zirconium oxide clusters is similar to that of oxygen-deficient cerium oxide clusters [21].
clusters were heated in the extension tube (T = 673 K) and the mass spectra were compared to those of the unheated ones to assess the NO release (see Fig. 2). If the NO molecules were adsorbed weakly on ZrnO2n+x(NO)k+ (3 6 x 6 0; 0 6 k 6 2) clusters, detachment of NO and regeneration of bare clusters should occur on heating. However, regeneration of the bare oxygendeficient zirconium oxide clusters was not observed. Moreover, the obtained distribution was substantially the same, except for reduction in the intensity of di-NO or tri-NO adducts, which can therefore be considered to be weakly bound species. In contrast, the first NO molecule is considered to be strongly bound to the zirconium oxide cluster. Production of ZrnO2n+ by the reaction with NO gas was evidenced by the fact that the total abundance of Zr5O+10 and Zr5O10(NO)+ is greater than the initial abundance of Zr5O+10 before the NO reaction. This trend has also been observed for clusters with 4 6 n 6 10. The increase in the oxygen atom number, i.e., the production of oxygen-rich clusters, is interpreted as resulting from oxidation by NO. Oxygen transfer reactions are considered to occur between oxygen-deficient ZrnO2n+x+ clusters and two NO molecules + to form oxygen-rich ZrnO2n+x+2 in the following manner:
Zrn Oþ2nþx þ NO ! Zrn O2nþx ðNOÞþ
ðx 6 1Þ
Zrn O2nþx ðNOÞþ þ NO $ Zrn O2nþx ðNOÞþ2 ! Zrn Oþ2nþxþ2 þ N2 : ðx 6 1Þ
3.2. Adsorption and reaction of NO molecules on
ZrnO2n+x+
Oxygen-deficient zirconium oxide clusters, readily reacted with NO gas. Fig. 2(a) shows cluster abundance change before and after reaction with NO gas without post-heating. The mass spectral changes occurring owing to the reaction with NO can be summarized as the (i) production of oxygen-rich clusters from oxygendeficient clusters by extraction of an oxygen atom from NO; (ii) formation of mono-NO adducts, ZrnO2n+x(NO)+, for limited compositions of 3 6 x 6 0 (no significant intensity of ZrnO2n+1(NO)+ was observed); and (iii) formation of di- and tri-NO adducts of the form ZrnO2n+x (NO)+2,3. For example, the amount of bare Zr5O+7,8,9 and Zr6O+9,10,11 clusters decreased markedly after the reaction with NO, as shown in Fig. 2(a–d). The most abundant products were mono-NO clusters with compositions of 3 6 x 6 0. In order to examine how strongly the NO molecules were bound to the zirconium oxide clusters,
Fig. 2. Abundance of bare and NO adducts of ZrnO2n+x+ clusters (n = 5, 6) before and after the reaction with NO in He ((a–b) 7 and (c–d) 30 Torr in 800 Torr): (a–b) without and (c–d) with post-heating at T = 673 K. The horizontal axis, x, indicates the deviation of the oxygen atom number from the stoichiometric compositions.
ð1Þ
ð2Þ
The range shown in parentheses is the number of oxygen atoms in the reactant zirconium oxide clusters. The first attachment of NO to zirconium oxide clusters occurs at low NO concentration, while sufficient amounts of di-NO or tri-NO adducts do not form even after increasing the NO concentration. This implies that the major reaction pathway is not sequential NO attachment. The second NO molecule attachment triggers the N2 release process. The N2 release reaction readily occurs at room temperature because this reaction is exothermic and has low activation barrier. It should be noted that the intensity distribution of ZrnO2n+x(NO)+ shows a reproducible dip at x = 2 (see Fig. 2(a–d)). This finding suggests that a faster rate of ZrnO2n2(NO)+ toward NO forming ZrnO2n+ + N2 (Eq. (2)) compared with the reaction rate observed for related compositions ZrnO2n1,2n3(NO)+. 3.3. Adsorption and reaction of CO molecules on ZrnO2n+x+ Fig. 3(a–b) shows the abundance of ZrnO2n+x+ clusters produced by the post heating after reaction with CO diluted by He: (a) n = 5 and (b) n = 6. When the oxygen-deficient zirconium oxide clusters ZrnO2n+x+ (n = 4–7) were reacted with CO gas at room temperature, the amount of bare clusters decreased and multiple-CO adducts formed: ZrnO2n+x(CO)23+ clusters were formed only in extremely oxygen-deficient cases such as x = 5. In order to release the weakly bound CO molecules, clusters were heated in the extension tube set at T = 673 K. The amount of CO adducts was drastically decreased (almost zero within error) and some near-stoichiometric zirconium oxide clusters appeared again after post-heating. Hence, the binding between CO and these zirconium oxide clusters is not strong; the binding energies were estimated to be marginally less than 1 eV [22]. Comparing the cluster abundance distributions, it is evident that the amount of stoichiometric clusters ZrnO2n+ decreased and that of oxygen-deficient clusters ZrnO2n1+ and ZrnO2n2+ increased (blue up-pointing arrows in Fig. 3(a)). The CO oxidation reaction is consistent with the results by Castleman and co-workers that showed that ZrnO2n+ (2 6 n 6 5) is reactive toward CO: ZrnO2n+ + CO ? ZrnO2n1+ + CO2 [4]. In addition, we found that ZrnO2n1+ is also reactive toward CO. Reactions with CO are summarized as
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Fig. 3. Abundance of bare and CO adducts of ZrnO2n+x+ clusters (n = 5, 6) after the reaction with CO 80 Torr in He 800 Torr. The temperature of extension tube was set to 673 K. Blue arrows change of intensity by the reaction with CO. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Zrn Oþ2n þ CO ! Zrn Oþ2n1 þ CO2
ð3Þ
Zrn Oþ2n1 þ CO ! Zrn Oþ2n2 þ CO2
ð4Þ
3.4. Reactions of ZrnO2n+x+ clusters with a mixture of CO and NO In the previous sections, the oxidation and reduction of zirconium oxide clusters by NO and CO gases have been discussed. By combining these two reactions, it is expected that the oxidation of CO by NO will proceed on zirconium oxide clusters that cycle between ZrnO2n+ and ZrnO2n2+. In this section, experimental results obtained by reacting the clusters with a mixture of CO and NO are discussed. As shown in Fig. 4, we observed the reactions of ZrnO2n+x+ with pure CO gas, CO-rich, nearly equal, NO-rich gas mixtures, pure NO gas, and He gas as a reference. When the concentration of CO and NO is nearly equal (see Figs. 4(c) and S6), the most prominent mass peaks are three bundled peaks assigned to NO adduct clusters, ZrnO2n+x(NO)+ (3 6 x 6 1). Mass spectral analysis revealed that the bare zirconium oxide clusters (ZrnO2n+x+ with 1 6 x 6 1) are almost nonexistent. This distribution is similar to the NO-only experiment (case (a)), except for the lack of ZrnO2n(NO)+; ZrnO2n (NO)+ mostly disappears because it is reduced readily by CO. At pCO:pNO = 10:1, the formation of CO-adducts was evident (blueish plots in Fig. 4(d)). Decreasing the concentration of CO and increasing that of NO (pCO:pNO = 1:1) significantly reduced the formation of CO-adducts, and the total amount of NO adducts was greater than that of CO-containing products, indicating the faster rate of adduct formation with NO compared to that with CO for oxygendeficient zirconium oxide clusters. For the reaction of zirconium oxide and CO, Wu et al. reported that the rate constants of 1.0 1010 and 1.1 1010 cm3 molecule1 s1 for the reactions Zr3O+7 + CO and Zr3O 7 + CO, respectively [15]. Therefore, the reaction rate for ZrnO2n+x+ + NO (5 < x) in this study is expected to be in the order of 109 cm3 molecule1 s1, which is close to the rate of collisions.
Ce10O+20, and Ce11O+22 and the other CenO2n+ clusters exhibited slow CO oxidation [23]. It is an important observation that no further reduction of CenO2n1+ clusters occurs because of the high oxygen affinity of CenO2n2+, in contrast to zirconium oxide clusters.
Cen Oþ2n þ CO ! Cen Oþ2n1 þ CO2 Cen Oþ2n1 þ CO ! Cen Oþ2n2 þ CO2
ðn ¼ 5; 10; and 11Þ
ð5Þ ð6Þ
3.5. Comparison with cerium oxide clusters In a previous study, we found that oxygen-deficient cerium oxide clusters were oxidized by NO until the ratio of the Ce and O atoms reached about 2:3 [21]. In comparison, the zirconium oxide clusters oxidize up to Zr:O 1:2, and hence, the number of oxygen atoms involved in cerium oxide clusters is smaller. Cerium oxide clusters are rather inert toward CO except for extremely oxygen-poor cerium clusters (CenOm+, m/n < 3/2), which have a high oxygen affinity and readily abstract an O-atom from CO [21]. Some cerium oxide clusters can oxidize CO, such as Ce5O+10,
Fig. 4. Abundance of the reaction product ZrnO2n+x+ clusters (n = 5, 6, and 7) with various ratios of CO and NO mixture gas diluted by He with post-heating at T = 673 K. The vertical broken lines indicate the position of x = 2 and 0 and are shown as a guide for eyes.
K. Miyajima, F. Mafuné / Chemical Physics Letters 660 (2016) 261–265
The resultant CenO2n1+ clusters, Ce5O+9, Ce10O+19, and Ce11O+21, are not oxidized by NO. Thus, these findings suggest that, at least in the gas-phase, cerium oxide clusters cannot catalyze CO oxidation in the presence of the coexisting NO. In other words, the reaction of CO with zirconium oxide clusters differs from that with cerium oxide because one-oxygen-less clusters ZrnO2n2+ can be produced; these can be used to regenerate oxygen-richer clusters, ZrnO2n+, by oxidation with NO. Therefore, oxygen-deficient zirconium oxide clusters can play an important role as an NO reduction and a CO oxidation catalyst as
Zrn Oþ2n þ 2CO ! Zrn Oþ2n2 þ 2CO2
ð7Þ
265
Acknowledgements This work was supported by JSPS KAKENHI Grant Numbers 25248004, 24550010; additional funding for cluster research was provided by the Genesis Research Institute, Inc. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.07. 028. References
Zrn Oþ2n2
þ 2NO !
Zrn Oþ2n
þ N2
ð8Þ
4. Conclusion We studied the reactions of zirconium oxide cluster cations with O2, CO, and NO gas. Without reactant gases, abundant ZrnO2n1+ and ZrnO2n+ (4 6 n 6 10) were formed in addition to the formation of other oxygen-deficient clusters, ZrnO2n+x+ + (4 6 x 6 2). ZrnO2n+ and ZrnO2n+1 were generated by reaction with O2 gas. The mass spectral changes of the reaction of oxygen-deficient clusters with NO can be summarized as follows: production of oxygen-rich clusters coinciding with the depletion of bare oxygen-deficient zirconium oxide clusters by the abstraction of two O-atoms from two NO molecules. The reaction with CO showed that the ZrnO2n+ and ZrnO2n1+ clusters act as oxidizers for CO. When the gas mixtures of CO and NO were employed, oxidation of CO by NO proceeds on zirconium oxide clusters that cycle between ZrnO2n+ and ZrnO2n2+. In contrast to the zirconium oxide clusters, cerium oxide clusters cannot catalyze CO oxidation in the presence of the coexisting NO. This methodology can be applied to study substituted zirconium oxides such as CenZrmOx+. On the basis of these findings, we propose that future studies should be performed to deduce the effect of the doping of a second component.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Catalytic reactions of gas phase zirconium oxide clusters with NO and CO revealed by post heating Ken Miyajima, Fumitaka Mafuné ⇑ Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan
a r t i c l e
i n f o
Article history: Received 20 June 2016 In final form 12 July 2016 Available online 14 July 2016 Keywords: Cluster Redox reactions Zirconium oxide Post-heating Catalysis
a b s t r a c t Reactivity of gas phase zirconium oxide clusters (ZrnOm+) toward NO and CO gases was investigated by mass spectrometry in combination with post heating. Reaction of ZrnO2n+x+ with NO gas resulted in the depletion of extremely oxygen-deficient clusters and the formation of oxygen-rich clusters, ZrnO2n+x+ (0 6 x 6 3). Reaction with CO substantially lead to an increase in the amount of ZrnO2n2+ and ZrnO2n1+ clusters and depletion in the amount of ZrnO2n+. The catalytic cycle, achieved by regenerating ZrnO2n+ by the oxidation of ZrnO2n2+ by NO, were discussed in comparison with the reactivity of cerium oxide clusters. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Individual aspects of catalytic processes can be probed at the molecular level by studying the reaction of clusters in the gasphase [1]. The catalytic properties of gas-phase clusters have been investigated by many researchers; the relationships between their reactivity, elemental composition, and stoichiometry and their geometric and electronic structures have been discussed for several systems. Various metal oxide clusters that contain radical oxygen have been studied by Castleman and co-workers and other groups [1–3]. Among the clusters, zirconium oxide clusters have been proposed for various applications in nanotechnology because of their unique, tailorable properties as cluster assembled materials. Castleman’s group reported a series of particularly reactive clusters (ZrO2)x+ (x = 1–4) by combining gas-phase experiments and density functional theory (DFT) calculations [4]. They proposed that this stoichiometric series of cationic zirconium oxide clusters are potential building blocks for a cluster assembled catalyst that would efficiently promote oxidation reactions. In the condensed phase, zirconia is widely used in a variety of technological applications [5–7]. The versatility of zirconia originates entirely from atomic or point defects in the crystal that are created by adding aliovalent oxides. It is well known that small nanoparticles exhibit unique properties, which are characteristic of neither the atomic nor the bulk (condensed phase) regimes. For instance, Grena et al. analyzed the stabilizing effect of surface ⇑ Corresponding author. E-mail address: [email protected] (F. Mafuné). http://dx.doi.org/10.1016/j.cplett.2016.07.028 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
impurities on the structure of ultrasmall ZrO2 nanoparticles (Zr43O86; 1.25 nm) using ab initio calculations [8]. They found that the surface stabilization effects play a crucial role in the structure of ZrO2 nanoparticles, in contrast with other systems (TiO2 and CeO2). However, it is not easy to anticipate the real structure of these nanoparticles when not in isolation (e.g., aggregation, impurities, surfactants, surface relaxation, or reconstruction). Some of these problems can be omitted by producing and analyzing the clusters in vacuum. Several groups have studied zirconium oxide clusters in the gas phase [4,9–17]. Bowen and coworkers reported the preparation of zirconium oxide cluster anions and their photoelectron spectra [9]. In their mass spectrum, oxygen-rich zirconium oxide cluster anions ZrnO2n+2,3,4,5 were produced predominantly. They also determined the adiabatic electron affinity and dissociation energy of ZrO. Von Helden and coworkers measured IR spectra of zirconium oxide clusters and observed clusters with a composition + ZrnO2n1 at high fluencies of the IR irradiation [10]. Bernstein and coworkers studied growth dynamics, stabilities, and structures of small zirconium oxide clusters by mass spectrometry and DFT calculations [11,12]. They observed nonstoichiometric clusters [(ZrO2)n1ZrO]+ at high ionization laser intensities; at lower laser intensities, stoichiometric (ZrO2)n+ appeared preferentially. Among the (ZrO2)n clusters, (ZrO2)5 is a very stable cluster, which has neither a high symmetry nor a closed shell electronic structure. Modeling of the covariance matrix over a wide range of ionization laser intensities indicated that (ZrO2)n neutral clusters absorb two photons of 193 nm radiation to ionize and then the ion can absorb more photons in the case of high laser fluence.
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There have been many reports on the reactivity of zirconium oxide clusters. Johnson et al. studied the reactivity of zirconium oxide cations with CO, C2H4, C2H2, or N2O using a guided-ionbeam mass spectrometer [4]. They identified that a distinct series of stoichiometric cationic zirconium oxide clusters (ZrO2)x+ (x = 1–4) showed O-atom transfer activity towards CO, C2H4, and C2H4. Furthermore, these active stoichiometric species can be regenerated from oxygen-deficient clusters using a suitable oxidizer. This indicated that these species may be used to promote multiple cycles of oxidation and behave as true catalysts. Soon after this work, they produced (ZrO2)x+ and (ZrxO2x+1) (x = 1–4) and performed an experimental and theoretical investigation into the influence of different charge states on the oxidation of CO, C2H2, and C2H4 in zirconium oxide clusters containing radical oxygen centers [2]. Tyo et al. investigated the structure and reactivity of superoxide (O 2 )-containing clusters ZrxO2x+1 (x = 1–3) [14]. + Among such clusters, Zr2O5 exhibited the highest reactivity for the oxidation of C3H6, C4H6, and C2H2. They found that the surrounding environment of a superoxide unit plays an important role in its reactivity. Wu et al. investigated CO adsorption of zirconium oxide cluster ions by means of collision-induced dissociations [15]. They found that most cationic clusters adsorb CO, while only specific anionic clusters adsorb CO. Furthermore, loss of CO and CO2 occurs for ZrxOyCO+ and ZrxO2x+1CO, respectively. Ma et al. generated zirconium oxide cluster anions Zr2Oy (y = 5–8) and studied their reactivity with n-butane [16]. DFT calculations indicated that the highly oxygen-rich cluster Zr2O 8 contains one mononuclear oxygencentered radical (O), leading to its high reactivity toward n-C4H10 oxidation. Recently, they also produced titanium and zirconium oxide cluster anions, (TiO2)nO and (ZrO2)nO (n = 3–25), respectively. The reaction of (ZrO2)nO with CO generated CO adducts (ZrO2)nOCO, which lose CO2 after collisions with the He gas [17]. Their DFT calculations (n = 3–8) revealed that these clusters are atomic radical anion-bonded systems, which are difficult to capture and characterize in the condensed phase. Despite these intensive gas-phase experiments and theoretical works, important factors that govern the redox reactivity have not been fully explained yet, especially when using a mixture of reactant gases. In this study, we report the reactivity of zirconium oxide cluster cations ZrnOm+ toward O2, CO, and NO by mass spectrometry. The variation in the abundance of clusters with the oxidative and reductive gas ratio is discussed.
pressure inside the valve was kept constant at 1.1 105 Pa by using a pressure controller. The typical total gas density inside the reaction gas cell was estimated to be 1018 molecules cm3. The residence time of clusters in the reaction gas cell was estimated to be 70 ls. After passing the cluster ions through the reaction gas cell, they were introduced into an extension tube. The extension tube was heated to 300–1000 K using a resistive heater and was monitored using thermocouples (type K). The residence time of the cluster ions and the density of the He gas in the extension tube were estimated to be 100 ls and 1017 molecules cm3, respectively. Thermal equilibrium between the clusters and the wall of cluster source was achieved by collisions with the He carrier gas well before expansion under vacuum. A mixture of clusters and He carrier gas was entered into the second chamber through a skimmer. Positively charged cluster ions were orthogonally extracted by a pulsed voltage for the TOF mass analysis.
3. Results and discussion 3.1. Production of oxygen-deficient zirconium oxide clusters Fig. 1a shows a mass spectrum of zirconium oxide clusters produced directly by the laser ablation of a zirconia rod with
2. Experimental methods A detailed explanation of the experimental setup has been described elsewhere [18,19]. Briefly, zirconium oxide clusters were prepared by laser ablation and were detected using a reflectronequipped time-of-flight mass spectrometer (TOF-MS) after introducing the clusters into a reaction gas cell. A sintered zirconia rod (ZrO2; 99.9%, Rare Metallic, Co. Ltd.) was set downstream of the supersonic source from a solenoid pulsed valve. The rod (5 mm in diameter, 30 mm long) was irradiated with focused laser pulses (f = 200 mm) with a pulse energy of 1.5 mJ for the ZrO2 rod at 532 nm to generate plasma. The evaporated zirconium oxides were cooled in a cylindrical channel (6 mm diameter) using He gas (>99.99995%, 0.5 MPa) from the valve. The clusters were passed through a reaction gas cell (2 mm diameter, 60 mm long, kept in room temperature) and an extension tube (4 mm diameter, 120 mm long) with a resistive heater for post-heating before expansion into the first vacuum chamber; they were then introduced into the differentially pumped second chamber through a skimmer. Reactant gases (CO (>99.95%), NO, and O2 (>99.9%)) were injected inside the reaction gas cell using another solenoid pulsed valve to examine the chemical reactivity of the clusters. The total
Fig. 1. (a–b) Mass spectrum of zirconium oxide clusters ZrnOm+ (n = 5 and 6) produced by the laser ablation of a zirconia target. Red plot is the raw data, and black and blue plots show the processed experimental mass intensity and simulated one which have been segmented in 0.9994 amu steps. Residual error is given in the bottom. (c–d) Abundance of ZrnO2n+x+ (n = 5–7) obtained by deconvolution of raw data before and after the reaction with O2 gas. Black and light blue plots indicate the intensity of ZrnO2n+x+ and ZrnO2n+x(H2O)+ clusters, respectively. The horizontal axis, x, indicates the deviation of the oxygen atom number from the stoichiometric compositions. The vertical broken lines indicate the position of x = 2 and 0 and are shown as a guide for eyes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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O2-undoped He carrier gas. As a zirconium atom comprises a variety of isotopes (90Zr 51.45 ± 0.40%, 91Zr 11.22 ± 0.05%, 92 Zr 17.15 ± 0.08%, 94Zr 17.38 ± 0.28%, and 96Zr 2.80 ± 0.09%) [20], a zirconium oxide cluster provides many mass peaks in the mass spectrum. In order to fairly evaluate abundances of the clusters, deconvolution of the mass peaks was required. Using homemade softwares based on a protocol described in the Supporting Material, we estimated the abundances. As the residual error was minimal, we consider that the processes worked reasonably well (see Fig. 1b). As shown in Fig. 1c, oxygen-deficient zirconium oxide clusters exhibited a smooth, wide, and gradual distribution of ZrnOm+ (0 6 m 6 2n + 1). Prominent peaks were assigned to nearstoichiometric clusters, ZrnO2n1+ or ZrnO2n2+ where the number of oxygen atoms is one or two less than the stoichiometric composition. In order to express the cluster’s stoichiometric compositions in a simple way, deviation from stoichiometric oxygen number, defined as x = m2n, will be used hereafter. The high reactivity of oxygen-deficient clusters with oxygen was evidently observed as the changes of the cluster abundance distribution (see Fig. 1(d)): Oxygen-deficient clusters, ZrnO2n+x+ (2n 6 x 6 1), decreased + considerably, whereas ZrnO2n+ and ZrnO2n+1 were predominantly generated. In this respect, the chemistry of oxygen-deficient zirconium oxide clusters is similar to that of oxygen-deficient cerium oxide clusters [21].
clusters were heated in the extension tube (T = 673 K) and the mass spectra were compared to those of the unheated ones to assess the NO release (see Fig. 2). If the NO molecules were adsorbed weakly on ZrnO2n+x(NO)k+ (3 6 x 6 0; 0 6 k 6 2) clusters, detachment of NO and regeneration of bare clusters should occur on heating. However, regeneration of the bare oxygendeficient zirconium oxide clusters was not observed. Moreover, the obtained distribution was substantially the same, except for reduction in the intensity of di-NO or tri-NO adducts, which can therefore be considered to be weakly bound species. In contrast, the first NO molecule is considered to be strongly bound to the zirconium oxide cluster. Production of ZrnO2n+ by the reaction with NO gas was evidenced by the fact that the total abundance of Zr5O+10 and Zr5O10(NO)+ is greater than the initial abundance of Zr5O+10 before the NO reaction. This trend has also been observed for clusters with 4 6 n 6 10. The increase in the oxygen atom number, i.e., the production of oxygen-rich clusters, is interpreted as resulting from oxidation by NO. Oxygen transfer reactions are considered to occur between oxygen-deficient ZrnO2n+x+ clusters and two NO molecules + to form oxygen-rich ZrnO2n+x+2 in the following manner:
Zrn Oþ2nþx þ NO ! Zrn O2nþx ðNOÞþ
ðx 6 1Þ
Zrn O2nþx ðNOÞþ þ NO $ Zrn O2nþx ðNOÞþ2 ! Zrn Oþ2nþxþ2 þ N2 : ðx 6 1Þ
3.2. Adsorption and reaction of NO molecules on
ZrnO2n+x+
Oxygen-deficient zirconium oxide clusters, readily reacted with NO gas. Fig. 2(a) shows cluster abundance change before and after reaction with NO gas without post-heating. The mass spectral changes occurring owing to the reaction with NO can be summarized as the (i) production of oxygen-rich clusters from oxygendeficient clusters by extraction of an oxygen atom from NO; (ii) formation of mono-NO adducts, ZrnO2n+x(NO)+, for limited compositions of 3 6 x 6 0 (no significant intensity of ZrnO2n+1(NO)+ was observed); and (iii) formation of di- and tri-NO adducts of the form ZrnO2n+x (NO)+2,3. For example, the amount of bare Zr5O+7,8,9 and Zr6O+9,10,11 clusters decreased markedly after the reaction with NO, as shown in Fig. 2(a–d). The most abundant products were mono-NO clusters with compositions of 3 6 x 6 0. In order to examine how strongly the NO molecules were bound to the zirconium oxide clusters,
Fig. 2. Abundance of bare and NO adducts of ZrnO2n+x+ clusters (n = 5, 6) before and after the reaction with NO in He ((a–b) 7 and (c–d) 30 Torr in 800 Torr): (a–b) without and (c–d) with post-heating at T = 673 K. The horizontal axis, x, indicates the deviation of the oxygen atom number from the stoichiometric compositions.
ð1Þ
ð2Þ
The range shown in parentheses is the number of oxygen atoms in the reactant zirconium oxide clusters. The first attachment of NO to zirconium oxide clusters occurs at low NO concentration, while sufficient amounts of di-NO or tri-NO adducts do not form even after increasing the NO concentration. This implies that the major reaction pathway is not sequential NO attachment. The second NO molecule attachment triggers the N2 release process. The N2 release reaction readily occurs at room temperature because this reaction is exothermic and has low activation barrier. It should be noted that the intensity distribution of ZrnO2n+x(NO)+ shows a reproducible dip at x = 2 (see Fig. 2(a–d)). This finding suggests that a faster rate of ZrnO2n2(NO)+ toward NO forming ZrnO2n+ + N2 (Eq. (2)) compared with the reaction rate observed for related compositions ZrnO2n1,2n3(NO)+. 3.3. Adsorption and reaction of CO molecules on ZrnO2n+x+ Fig. 3(a–b) shows the abundance of ZrnO2n+x+ clusters produced by the post heating after reaction with CO diluted by He: (a) n = 5 and (b) n = 6. When the oxygen-deficient zirconium oxide clusters ZrnO2n+x+ (n = 4–7) were reacted with CO gas at room temperature, the amount of bare clusters decreased and multiple-CO adducts formed: ZrnO2n+x(CO)23+ clusters were formed only in extremely oxygen-deficient cases such as x = 5. In order to release the weakly bound CO molecules, clusters were heated in the extension tube set at T = 673 K. The amount of CO adducts was drastically decreased (almost zero within error) and some near-stoichiometric zirconium oxide clusters appeared again after post-heating. Hence, the binding between CO and these zirconium oxide clusters is not strong; the binding energies were estimated to be marginally less than 1 eV [22]. Comparing the cluster abundance distributions, it is evident that the amount of stoichiometric clusters ZrnO2n+ decreased and that of oxygen-deficient clusters ZrnO2n1+ and ZrnO2n2+ increased (blue up-pointing arrows in Fig. 3(a)). The CO oxidation reaction is consistent with the results by Castleman and co-workers that showed that ZrnO2n+ (2 6 n 6 5) is reactive toward CO: ZrnO2n+ + CO ? ZrnO2n1+ + CO2 [4]. In addition, we found that ZrnO2n1+ is also reactive toward CO. Reactions with CO are summarized as
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Fig. 3. Abundance of bare and CO adducts of ZrnO2n+x+ clusters (n = 5, 6) after the reaction with CO 80 Torr in He 800 Torr. The temperature of extension tube was set to 673 K. Blue arrows change of intensity by the reaction with CO. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Zrn Oþ2n þ CO ! Zrn Oþ2n1 þ CO2
ð3Þ
Zrn Oþ2n1 þ CO ! Zrn Oþ2n2 þ CO2
ð4Þ
3.4. Reactions of ZrnO2n+x+ clusters with a mixture of CO and NO In the previous sections, the oxidation and reduction of zirconium oxide clusters by NO and CO gases have been discussed. By combining these two reactions, it is expected that the oxidation of CO by NO will proceed on zirconium oxide clusters that cycle between ZrnO2n+ and ZrnO2n2+. In this section, experimental results obtained by reacting the clusters with a mixture of CO and NO are discussed. As shown in Fig. 4, we observed the reactions of ZrnO2n+x+ with pure CO gas, CO-rich, nearly equal, NO-rich gas mixtures, pure NO gas, and He gas as a reference. When the concentration of CO and NO is nearly equal (see Figs. 4(c) and S6), the most prominent mass peaks are three bundled peaks assigned to NO adduct clusters, ZrnO2n+x(NO)+ (3 6 x 6 1). Mass spectral analysis revealed that the bare zirconium oxide clusters (ZrnO2n+x+ with 1 6 x 6 1) are almost nonexistent. This distribution is similar to the NO-only experiment (case (a)), except for the lack of ZrnO2n(NO)+; ZrnO2n (NO)+ mostly disappears because it is reduced readily by CO. At pCO:pNO = 10:1, the formation of CO-adducts was evident (blueish plots in Fig. 4(d)). Decreasing the concentration of CO and increasing that of NO (pCO:pNO = 1:1) significantly reduced the formation of CO-adducts, and the total amount of NO adducts was greater than that of CO-containing products, indicating the faster rate of adduct formation with NO compared to that with CO for oxygendeficient zirconium oxide clusters. For the reaction of zirconium oxide and CO, Wu et al. reported that the rate constants of 1.0 1010 and 1.1 1010 cm3 molecule1 s1 for the reactions Zr3O+7 + CO and Zr3O 7 + CO, respectively [15]. Therefore, the reaction rate for ZrnO2n+x+ + NO (5 < x) in this study is expected to be in the order of 109 cm3 molecule1 s1, which is close to the rate of collisions.
Ce10O+20, and Ce11O+22 and the other CenO2n+ clusters exhibited slow CO oxidation [23]. It is an important observation that no further reduction of CenO2n1+ clusters occurs because of the high oxygen affinity of CenO2n2+, in contrast to zirconium oxide clusters.
Cen Oþ2n þ CO ! Cen Oþ2n1 þ CO2 Cen Oþ2n1 þ CO ! Cen Oþ2n2 þ CO2
ðn ¼ 5; 10; and 11Þ
ð5Þ ð6Þ
3.5. Comparison with cerium oxide clusters In a previous study, we found that oxygen-deficient cerium oxide clusters were oxidized by NO until the ratio of the Ce and O atoms reached about 2:3 [21]. In comparison, the zirconium oxide clusters oxidize up to Zr:O 1:2, and hence, the number of oxygen atoms involved in cerium oxide clusters is smaller. Cerium oxide clusters are rather inert toward CO except for extremely oxygen-poor cerium clusters (CenOm+, m/n < 3/2), which have a high oxygen affinity and readily abstract an O-atom from CO [21]. Some cerium oxide clusters can oxidize CO, such as Ce5O+10,
Fig. 4. Abundance of the reaction product ZrnO2n+x+ clusters (n = 5, 6, and 7) with various ratios of CO and NO mixture gas diluted by He with post-heating at T = 673 K. The vertical broken lines indicate the position of x = 2 and 0 and are shown as a guide for eyes.
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The resultant CenO2n1+ clusters, Ce5O+9, Ce10O+19, and Ce11O+21, are not oxidized by NO. Thus, these findings suggest that, at least in the gas-phase, cerium oxide clusters cannot catalyze CO oxidation in the presence of the coexisting NO. In other words, the reaction of CO with zirconium oxide clusters differs from that with cerium oxide because one-oxygen-less clusters ZrnO2n2+ can be produced; these can be used to regenerate oxygen-richer clusters, ZrnO2n+, by oxidation with NO. Therefore, oxygen-deficient zirconium oxide clusters can play an important role as an NO reduction and a CO oxidation catalyst as
Zrn Oþ2n þ 2CO ! Zrn Oþ2n2 þ 2CO2
ð7Þ
265
Acknowledgements This work was supported by JSPS KAKENHI Grant Numbers 25248004, 24550010; additional funding for cluster research was provided by the Genesis Research Institute, Inc. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.07. 028. References
Zrn Oþ2n2
þ 2NO !
Zrn Oþ2n
þ N2
ð8Þ
4. Conclusion We studied the reactions of zirconium oxide cluster cations with O2, CO, and NO gas. Without reactant gases, abundant ZrnO2n1+ and ZrnO2n+ (4 6 n 6 10) were formed in addition to the formation of other oxygen-deficient clusters, ZrnO2n+x+ + (4 6 x 6 2). ZrnO2n+ and ZrnO2n+1 were generated by reaction with O2 gas. The mass spectral changes of the reaction of oxygen-deficient clusters with NO can be summarized as follows: production of oxygen-rich clusters coinciding with the depletion of bare oxygen-deficient zirconium oxide clusters by the abstraction of two O-atoms from two NO molecules. The reaction with CO showed that the ZrnO2n+ and ZrnO2n1+ clusters act as oxidizers for CO. When the gas mixtures of CO and NO were employed, oxidation of CO by NO proceeds on zirconium oxide clusters that cycle between ZrnO2n+ and ZrnO2n2+. In contrast to the zirconium oxide clusters, cerium oxide clusters cannot catalyze CO oxidation in the presence of the coexisting NO. This methodology can be applied to study substituted zirconium oxides such as CenZrmOx+. On the basis of these findings, we propose that future studies should be performed to deduce the effect of the doping of a second component.
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