Size-Dependent Oxidation State and CO Oxidation Activity of Tin

Nov 27, 2017 - (2, 11) Tin oxide has two oxidation states, SnO with Sn(II) and SnO2 with .... The charge states of the tin atoms and the composition o...
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Size Dependent Oxidation state and CO Oxidation Activity of Tin Oxide Clusters Yusuke Inomata, Ken Albrecht, and Kimihisa Yamamoto ACS Catal., Just Accepted Manuscript • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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Size Dependent Oxidation State and CO Oxidation Activity of Tin Oxide Clusters Yusuke Inomata† , Ken Albrecht†,‡, Kimihisa Yamamoto†,‡,* †

Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, Yokohama 226-8503, Japan

‡

JST-ERATO, Yamamoto Atom Hybrid Project, Yokohama 226-8503, Japan

ABSTRACT: The CO oxidation reaction is industrially important reaction, however, the practical catalysts are limited to noble metals. In this paper, we report a systematic study of the CO oxidation ability on cheap and less noble tin oxide clusters with the aim of quantitatively understanding the active sites. We synthesized size-controlled tin oxide clusters in mesoporous silica using the dendrimer templating method, employing dendritic phenylazomethine with a tetraphenylmethane core (TPMG4) as the template molecule. The clusters had different sizes depending on the added amount of SnCl2 as a precursor to TPMG4. The synthesized tin oxide clusters contained not only stable tetravalent Sn(IV), but also metastable divalent Sn(II) due to the structural stability and had a size-dependent composition. The CO oxidation activity of the tin oxide clusters increased with decreasing the cluster size depending on the Sn(II)/ Sn(IV) ratio. We also found the correlation between the Sn(II) fraction and the CO oxidation activity, clearly indicating that the partially reduced Sn(II) acted as the active site for the CO oxidation in the tin oxide clusters. These knowledge give a clue to design a highly active CO oxidation catalyst with base metal oxides. KEYWORDS:

heterogeneous

catalysts,

CO

oxidation,

INTRODUCTION The CO oxidation reaction is one of the most simple oxidation reaction but it is industrially important to exclude the poisoning by CO from reformed gas and exhaust gas catalysts.1 Base metal oxides are generally used as support materials for metal catalysts, but are also found to be effective catalysts themselves alternating widely used noble metal-based catalysts in some cases, especially as heterogeneous oxidation catalysts.2–10 Tin oxide based catalysts, in particular, have attracted much attention due to their physical and chemical stability, low cost and great abundance.2,11 Tin oxide has two oxidation states, SnO with Sn(II) and SnO2 with Sn(IV) (Figure S1),12 and the latter SnO2 is found to be effective catalysts for CO oxidation owing to its low oxygen vacancy formation energy and the surface is easily reduced from Sn(IV) to Sn(II) depending on the oxygen partial pressure or the atmosphere.13–15 The CO oxidation over the surface of tin oxide proceeds based on the Mars van Kreveren mechanism,16 in which the reduction and the oxidation of tin atoms occur (Sn(IV)↔Sn(II)).11 The common understanding is that

tin

oxide,

metal

oxide

clusters,

dendrimers

partially reduced Sn(II) species of SnO2-x, including various size, crystallographic planes, morphology and surface compositions, are the active sites,2,11,17–25 and the Sn(II) species act as an electron donor and transfer an electron to oxygen molecules adsorbed on the surface to generate active oxygen species attacking CO molecules in the reaction cycle. However, previous reports lack the quantitative evidence on the active sites for the CO oxidation reaction of tin oxide, and the relationship between surface stoichiometries and oxidation activity is obscure. In recent years, metal oxide clusters deposited on supports have gained much attention to provide a fundamental insight into the mechanism of heterogeneous catalysis. 7,26–28 Recent studies have shown that metal oxide clusters have different stoichiometries depending on the size and the synthetic conditions and sometimes have a high density of low coordinated or defect sites.7,26,27 Combined with the nature, we can study the CO oxidation activity of tin oxide systematically by investigating correlation between the reactivity and size-dependent stoichiometry of tin oxide clusters (i.e. Sn(II) to Sn(IV) ratio).

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Scheme 1. Schematic illustration of size-selective synthetic method of tin oxide clusters loaded in MPS using the dendrimer templating approach. Tin oxide clusters with a narrow size distribution can form in the MPS pore depending on the added equivalent of SnCl2 to TPMG4.

Precisely size-controlled synthetic technique is required for such investigation using cluster as the model system. Tin oxide nanoparticles are generally synthesized by conventional solvothermal method or sol-gel process but these methods give us the particles with wide size distribution.29,30 The mass selection technique in the gaseous phase is a strong tool to provide size-controlled clusters, but it has limitations in terms of the low yield and the applicability of generally used analysis techniques or equipments.7,26,27 As the conventional but strictly sizecontrolled synthetic method, dendrimer templating clusters synthesis can be a good choice.31–33 Juttukonda et al. synthesized size-controlled tin oxide fine particles with the size above 2.5 nm by employing poly(propyleneimine) (PPI) and poly(amidoamine) (PAMAM) dendrimer by the reaction of SnO32- ions and CO2.34 However, the mixed valence state of Sn(II) and Sn(IV) significantly generates below the size of ca. 2.5 nm,35,36 therefore, this method is not suitable for this discussion. We have developed the dendrimer templating cluster synthesis method using dendritic phenylazomethine (DPA), which can synthesize clusters with well-difined size below 2 nm.37–40 Herein, in order to obtain a fundamental understanding of the heterogeneous oxidation catalysis of tin oxide, we report the synthesis of tin oxide clusters with different sizes and Sn(II) to Sn(IV) ratio, and the effect on the CO oxidation activity.

RESULTS AND DISCUSSION Synthetic strategy of tin oxide clusters loaded into mesoporous silica. We synthesized tin oxide clusters using the dendrimer templating method utilizing 4th-generation DPA with a tetraphenylmethane core (TPMG4) as a template molecule for controlling the cluster size (Scheme 1).37,39 TPMG4 is composed of a πconjugated rigid skeleton and the intramolecular imine moiety, a typical Schiff base, can form a complex stepwisely with Lewis acidic species as a precursor of a cluster

Figure 1. (a-c) HAADF-STEM images of Sn12, Sn28 and Sn60 clusters loaded in MPS. (d) High magnification image of Sn60. (e-g) IFFT images of corresponding FFT image of Figure 1a-c masked spots of the ordered pore structure of MPS (shown in Figure S6) for the clarification of the loaded clusters. Size distributions of (h) Sn28 and (i) Sn60 clusters loaded in MPS. The size distribution of the Sn12 clusters was hard to obtain due to the small difference in thecontrast between the clusters and MPS.

(e.g. metal halide).41,42 Owing to the assembling properties, we can synthesize size-controlled clusters depending on the amount of Lewis acids added to TPMG4.37 In this study, the tin oxide clusters (SnN, N is the number of tin atoms in the precursor complex) were synthesized via SnCl2 as a Lewis acid and supported by mesoporous silica (MPS). The detailed synthetic method of the tin oxide clusters is provided in the Supporting Information. The

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synthetic method occurs in four steps: (1) assembling SnCl2 with TPMG4,39 (2) loading the dendrimer complexes into MPS, (3) reduction of precursors by NaBH4 and (4) calcination at 600oC in air for oxidizing and removing the dendrimer. The reaction of precursor and the removal of any dendrimer were confirmed by the disappearance of Cl2p and N1s core level X-ray photoelectron spectroscopy (XPS) spectra (Figure S5). (a)

(EDX) and elemental mapping also showed the homogenous distribution of tin oxide clusters all over the MPS (Figures S6 and S7). For the Sn12 clusters, it was difficult to obtain a sufficiently obvious cluster image to obtain the size distribution. Although the particle images were obscure to identify due to the small difference in the contrast between the clusters and MPS, more consistent images of the clusters were confirmed from the corresponding IFFT images excluded the ordered pore structure of MPS by FFT and IFFT processing (Figures 1e-g and S8).

Charge composition of tin oxide clusters: XPS study. The charge states of the tin atoms and the

(b)

Figure 2. (a) Sn 3d5/2 core level XPS spectra of tin oxide clusters Sn12-60. The gray dots are the raw data and the solid lines are the fitting curves. The black dotted line with colorless and yellow-colored areas correspond to the fitting component for different charge states, Sn(IV) and Sn(II), respectively. The clusters show differently shaped spectra depending on the cluster size. (b) Peak area ratio of Sn(II) peak to Sn(IV) peak calculated from XPS spectra as a function of cluster size. The divalent Sn(II) component significantly increases with decreasing the cluster size.

Characterization of tin oxide clusters. A high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurement showed the uniformly sized tin oxide clusters incorporated along the ordered pores of the MPS (Figure 1a-d). The clusters have different sizes depending on the amount of SnCl2 added to TPMG4 (Figures 1h, 1i and upper right corner of Figure 1a-c). Energy dispersive X-ray spectroscopy

composition of the tin oxide clusters were determined by XPS measurements. For all the tin oxide cluster sizes, we confirmed the shouldered Sn 3d5/2 core level spectra (Figure 2a). The spectra were de-convoluted into two peaks. The first peaks with the higher binding energy at 486.4 to 486.9 eV are corresponded to the tetravalent Sn(IV) site, which were in close agreement with previously reported values.35,36 The second peaks with the lower binding energy at 483.5 to 484.2 eV were attributed to the reduced Sn(II) caused by an oxygen deficiency, although the values were lower than those of reference SnO particles and reported previously (almost close to Sn(0), Figures S2b and S3b). 43,44 However, it is quite unlikely to exist metal Sn in the clusters considering the calcination temperature. A few reports have described the phenomena as quasimetallic Sn species caused by some electronic interactions.45– 47 We suppose that, in the synthesized clusters, the charge-transfer or the polarization of Sn(II) atoms can be occurred. As for the largest Sn60 cluster, we also confirmed the existence of both Sn(IV) and Sn(II) from temperature programmed reaction measurements using H2 gas (H2-TPR) (Figure S9). The measured H2-TPR spectra showed the peaks assigned to the reduction of Sn(IV) and Sn(II) to Sn(0), respectively. The peak area ratio of Sn(II) to Sn(IV) (Sn(II)/ Sn(IV)) in the clusters increased with the cluster size, suggesting that the smaller clusters contain more divalent Sn(II) sites (Figure 2b). We also measured the XPS spectra of commercially purchased SnO2 fine particles with the size of ca. 10 nm as a reference material, which have a rutile type crystal structure observed by the TEM and the electron diffraction pattern (Figure S2a). The commercial SnO2 fine particles gave only a single sharp spectral line assigned to tetravalent Sn(IV) in a tetragonal rutile structure at 486.7 eV, contrary to the synthesized clusters (Figure S3a). The full width at halfmaximum values of the Sn(IV) for the tin oxide clusters were greater than those for the commercially purchased SnO2 fine particles (Table S1, 1.80-2.20 eV for the clusters and 1.03 eV for the SnO2 fine particles), indicating that the tetravalent tin atoms of the clusters are in various environments (e.g., geometry, bond length, bond angle, coordination number) compared to the reference SnO2 fine particles with an ordered rutile structure.26 The tin atoms in the rutile type SnO2 have a slightly distorted 6-fold octahedral coordination (Figure S1a), but are, presumably, more distorted by the neighboring reduced Sn(II)

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associated with the oxygen vacancies to modify the stability in the case of clusters.13,48,49

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getically or geometrically more favorable than composing coordinatively unsaturated Sn(IV).

(b)

(c)

Figure 3. (a) Typical CO-TPR spectra of tin oxide clusters Sn12-60 for different cluster sizes and support MPS. The supported amount of tin were measured to be 0.23, 0.27, 0.35, 0.58, 0.64 and 1.18 wt% for Sn12, Sn14, Sn18, Sn28, Sn44 and Sn60, respectively, by ICP-AES measurements. CO-TPR measurements were conducted by oxygen adsorption on the clusters followed by a temperature increase in CO/He mixture gas stream. The production of CO2 from the tin oxide clusters was monitored by a mass spectrometer (m/z = 44). The clusters exhibited a CO oxidation activity for all the cluster sizes. The bare MPS as a control sample showed no significant peaks (gray dots), indicating MPS itself is inert to the CO oxidation in this temperature range. (b) The amount of desorbed CO2 from the tin oxide clusters as a function of cluster size. The amount of catalytically produced CO2 was calculated from the peak area of the CO-TPR spectra and supported amount of tin. The CO oxidation activity of the clusters increased with decreasing the cluster size. (c) The amount of desorbed CO2 from tin oxide clusters as a function of the peak area ratio of the Sn(II) peak to the Sn(IV) peak. The CO oxidation activity of the clusters increased and linearly correlated with the 2 composition (R =0.990).

In general, divalent Sn(II) is thermodynamically metastable and oxidized to stable Sn(IV) above ca. 400oC.50–52 However, the peak area ratio of Sn(II) to Sn(IV) calculated from the XPS spectra increased with a decrease in the cluster size even on heating to 600oC as described above, indicating that the smaller clusters contain not only stable tetravalent Sn(IV), but also metastable Sn(II). To elucidate whether or not the Sn(II) came from incomplete oxidation, we investigated the peak area ratio of Sn(II) to Sn(IV) for the smallest Sn12 clusters calcined at various temperatures (Figure S11). The result showed that there was no significant change in the composition of the Sn12 clusters even in the case of heating at 800oC (i.e., there was no further oxidation of Sn(II) to Sn(IV)), suggesting that Sn(II) formed due to the structural stability of the clusters. These results imply that the divalent Sn(II) is essentially included in a small cluster and the clusters maintain stable structures by containing not only the 6fold coordinated Sn(IV), but also the SnO-like 4-fold coordinated Sn(II) sites due to the high curvature of the surface (Figure S1).13 At the region of cluster size, the density of corners or edges significantly increases due to raising the ratio of exposed surface atoms. In general, the atoms at corners or edges are forced to adopt unsaturated coordination.53 In the case of the synthesized tin oxide clusters, however, reducing charge to Sn(II) can be ener-

Size dependence of tin oxide clusters on CO oxidation reaction. We performed temperature programmed reaction measurements using CO as reacting gas (CO-TPR) to study the CO oxidation rates of the tin oxide clusters depending on the cluster size. The reactivity toward the CO oxidation was evaluated by detecting CO2 molecules generated by the reaction of flowing CO gas with oxygen adsorbed on the cluster surface. The obtained CO-TPR spectra showed significant CO2 desorption peaks for all the cluster sizes (Figure 3a), which did not appear in the case of only MPS, suggesting that the tin oxide clusters in MPS acted as the catalyst for the CO oxidation. The onset and peak temperatures of the COTPR peaks were about 50 to 60 OC and 125 to 140 OC, respectively, which were lower than the temperatures of the reference SnO2 fine particles, measured under the same conditions (Figure S4). Sun et al. reported the catalytic CO oxidation ability of SnO2 (001) thin sheets with five atomic layers and compared the activity to a thicker layer, nanoparticles and the bulk.2 The author found that the reaction temperatures depend on the thickness, and the charge density contributes to lowering the reaction temperatures. As we already mentioned, the synthesized clusters contain more electron-rich Sn(II) sites compared to larger particles, leading to an increase in the surface charge density and lowering the reaction temperature. The surface energy of the cluster is also assumed to be

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higher than of bulk materials and the high surface energy decreases the activation energy of dissociating oxygen molecules into active atomic oxygens. By plotting the CO oxidation activity of the tin oxide clusters as the amount of produced CO2 molecules per weight, we found that the CO2 production exponentially increased with decreasing the cluster size (Figure 3b). The amount of produced CO2 by the smallest Sn12 clusters was 6.1 times higher than of the largest Sn60 clusters. Assuming that the clusters are spherical, the change in the specific surface area of Sn60 to Sn12 is only 1.7-times, therefore, the increase in the produced amount came not only from the increase in the specific surface area. We also plotted the amount of produced CO2 molecules vs. the composition of the tin oxide clusters (peak area ratio Sn(II)/ Sn(IV) calculated from XPS spectra) (Figure 3c). The produced amount of CO2 molecules had a linear correlation with the composition and increased with the ratio of Sn(II) fraction. The result strongly suggests that divalent Sn(II) is the active site of the tin oxide clusters. Combined with previously reported surface studies of tin oxide, the reaction mechanism of the CO oxidation can be considered to consist of the following steps2,11,18: (1) Oxygen molecules are chemically adsorbed on the Sn(II) sites through electron transfer (Oିଶ ), (2) as the temperature increases, oxygen molecules dissociate into activated atomic oxygen (Oି ), and (3) the activated oxygen species attack the CO molecules to produce CO2. Our result quantitatively revealed that the CO oxidation over the surface of tin oxide depends on the number of the partially reduced Sn(II) sites. We also have to consider where the active Sn(II) sites exist. As we described in the previous section, Sn(II) species are predicted to site on the surface of the clusters. Although we consider that CO reacts with oxygen adsorbed on the outermost surface as suggested in various reports, if the Sn(II) sites exist inside of the cluster, the correlation will not be linear. In other words, the linear correlation between the amount of Sn(II) and the activity implies that the reduced Sn(II) sites exist over the surface of the clusters, which have different local structures such as edges or corners from inner parts of the clusters and ordered crystal facets.

adsorbed on Sn(II) play the role of attacking species. For a further discussion, we will consider the effect of a support and detailed geometries of the clusters. By using sizeselectively synthesized clusters as a model system via the dendrimer templating technique, we believe that we can achieve the systematic study on various catalytic processes mediated by metal oxides and the strategy provides a guidepost for designing tailored catalysts.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Details about the synthetic and experimental methods, additional characterizations of tin oxide clusters and reference studies on commercial SnO2 fine particles.

AUTHOR INFORMATION Corresponding Author *[email protected].

Author Contributions Y.I. performed the experiments and wrote the manuscript. K.A and K.Y conducted the experiments and the writing manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported in part by JSPS KAKENHI Grant Nos. JP15H05757, JP17K14490, and JP17H05146, and by JST ERATO Grant Number JPMJER1503, Japan.

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In summary, we synthesized size-controlled tin oxide clusters loaded into MPS by employing the dendrimer templating method. The tin oxide clusters in a pore of MPS had different sizes depending on the added equivalent of SnCl2 as a precursor to the dendrimer. The synthesized tin oxide clusters unprecedentedly contained both stable tetravalent and metastable divalent Sn sites and more so in a smaller cluster due to the structural stability. The produced amount of CO2 molecules generated by the CO oxidation over the cluster surface increased with decreasing the size of the clusters and linearly correlated with the charge composition of the Sn atoms in the tin oxide clusters. These results reveal the quantitative dependence between the amounts of the divalent Sn(II) fraction and the CO oxidation activity, and oxygen species

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