Size- and Support-Dependent Evolution of the Oxidation State and

Jun 12, 2014 - Size-selected subnanometer cobalt clusters with 4, 7, and 27 cobalt atoms supported on amorphous alumina and ultrananocrystalline diamo...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCA

Size- and Support-Dependent Evolution of the Oxidation State and Structure by Oxidation of Subnanometer Cobalt Clusters Chunrong Yin,†,¶ Fan Zheng,§,¶ Sungsik Lee,‡,¶ Jinghua Guo,∥ Wei-Cheng Wang,∥,⊥ Gihan Kwon,† Viktor Vajda,# Hsien-Hau Wang,† Byeongdu Lee,‡ Janae DeBartolo,‡ Sönke Seifert,‡ Randall E. Winans,‡ and Stefan Vajda*,†,▽,○,◆ †

Materials Science Division, ‡X-ray Science Division, and ▽Nanoscience and Technology Center, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States § Materials Science Division and ∥Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ⊥ Department of Physics, Tamkang University, Tamsui, Taiwan 250, R.O.C. # Northwestern University, Evanston, Illinois 60208, United States ○ Institute for Molecular Engineering, The University of Chicago, 5801 South Ellis Avenue, Chicago, Illinois 60637, United States ◆ Department of Chemical and Environmental Engineering, Yale University, 10 Hillhouse Avenue, New Haven, Connecticut 06520, United States S Supporting Information *

ABSTRACT: Size-selected subnanometer cobalt clusters with 4, 7, and 27 cobalt atoms supported on amorphous alumina and ultrananocrystalline diamond (UNCD) surfaces were oxidized after exposure to ambient air. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) and near-edge X-ray absorption fine structure (NEXAFS) were used to characterize the clusters revealed a strong dependency of the oxidation state and structure of the clusters on the surface. A dominant Co2+ phase was identified in all samples. However, XANES analysis of cobalt clusters on UNCD showed that ∼10% fraction of a Co0 phase was identified for all three cluster sizes and about 30 and 12% fraction of a Co3+ phase in 4, 7, and 27 atom clusters, respectively. In the alumina-supported clusters, the dominating Co2+ component was attributed to a cobalt aluminate, indicative of a very strong binding to the support. NEXAFS showed that in addition to strong binding of the clusters to alumina, their structure to a great extent follows the tetrahedral morphology of the support. All supported clusters were found to be resistant to agglomeration when exposed to reactive gases at elevated temperatures and atmospheric pressure.

■

support material.2,39,41−44 The catalytic performance of subnanometer clusters can also vary significantly as a function of particle size, with only certain sized clusters having high activity or desired selectivity.2,41 In addition, because practically all atoms of the subnanometer clusters are at the surface and thus accessible to reactants, the small clusters may offer a costeffective usage of the catalytic metals as well. However, because of formidable challenges in the synthesis, stability, and characterization of well-defined catalysts made of ultrasmall particles, the subnanometer size range still remains largely unexplored. For example, the sintering of subnanometer clusters can result in a broad distribution of particle sizes and shapes, which may lead to the alteration of catalysts selectivity

INTRODUCTION The development of highly active, selective, low-cost, and longlife catalysts is being driven for the production of alternative fuel as well as for the production of commodity chemicals using more sustainable processes.1 Small clusters can have properties that are often dramatically different from those observed for bulk materials or larger nanoparticles.2−38 In addition, the properties of atomic subnanometer clusters, such as their structure, oxidation state, and accordingly catalytic performance, may be controlled by the interactions with the support. Moreover, the large fraction of atoms at the interface between the cluster and support may promote distinct reaction pathways as well, including a facile activation of oxygen, which is often the rate-limiting step in oxidative reactions.2,39−42 Several recent advancements in the use of supported subnanometer clusters for catalysis demonstrate that such clusters can be highly active and selective for important chemical reactions, and their catalytic performance is tuned by the choice of the © XXXX American Chemical Society

Special Issue: A. W. Castleman, Jr. Festschrift Received: February 20, 2014 Revised: June 8, 2014

A

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

equivalent of 0.1 atomic monolayer of Co was used for all samples. After deposition, the samples were exposed to air, which led to the partial oxidation of the cobalt clusters termed Co4Ox, Co7Ox, and Co27Ox. Grazing Incidence Small-Angle X-ray Scattering. The GISAXS measurements were performed at the 12-ID-B beamline of the Advanced Photon Source in a specifically designed reaction cell.42,44 GISAXS with a geometry optimized for particle sizes starting above 1 nm was used to monitor possible sintering of clusters during the heat treatment.2 The GISAXS data were collected on a 1475 × 1679 pixel Pilatus 2M detector with 12 keV X-rays as a function of sample temperature. The 2-D GISAXS images were then processed by taking an average of three close cuts in the qy direction for horizontal information and in the qz direction for vertical information. The horizontal cuts were taken 46 pixels away from the horizontal center of the direct beam, and the vertical cuts were taken 20 pixels away from vertical center. Grazing Incidence X-ray Absorption Near-Edge Spectra. The GIXANES measurements were performed at the 12-ID-C and 12-BM beamlines of the Advanced Photon Source, in the same reaction cell as used for GISAXS.42,44 GIXANES data were collected at room temperature under helium, using a four-element Vortex detector mounted perpendicular to the plane of incidence and photon energies scanned between 7.67 and 7.85 keV. The spectra were scanned with 0.25 eV steps between 7.71 and 7.73 keV, while 0.5 eV steps were used for the rest. Cobalt standards were purchased from Sigma-Aldrich in powder form and were compressed and sealed between two layers of Kapton foil to avoid the contamination of reaction cell. The GIXANES data were analyzed using the IFEFFIT interactive software package (with ATHENA and ARTEMIS graphical interfaces).54 Near-Edge X-ray Absorption Fine Structure. In situ Xray absorption experiments were performed at the undulator beamline 7.0.1 of the Advanced Light Source (Berkeley, CA). In X-ray absorption, soft X-ray photons promote electrons from the core level into the empty states. Each absorption event produces Auger and secondary electrons, and the total electron yield (TEY) is proportional to the number of absorption events. Different from X-ray photoelectron spectroscopy (XPS), in the TEY mode the signals are detected through a conductor to the ammeter instead of electrons going through the vacuum and collected by the spectrometer.55 The X-ray absorption at the Co L-edges was recorded with 0.3 eV resolution, and data are presented for the Co L3 (778 eV) and Co L2 (793 eV) edges. A cobalt sample in the UHV chamber is used for energy calibration.

and deterioration of catalytic activity during operation. Another challenge is the identification of the oxidation state of the working catalyst, which may affect activity, but also selectivity through different binding and activation of the reaction species from reactants to intermediates to final products.41,45 Oxidized subnanometer cobalt clusters have emerged as promising candidates for a number of chemical transformation reactions, such as the economically attractive oxidative dehydrogenation.40,44 As demonstrated for the dehydrogenation of cyclohexene, the catalytic performance of oxidized subnanometer cobalt clusters strongly depends on the support material used as well as on the reaction conditions.39,42 In this study, grazing incidence X-ray absorption near-edge spectra (GIXANES) and near-edge X-ray absorption fine structure (NEXAFS) techniques were employed to determine the evolution in size-selected 4, 7, and 27 atom cobalt clusters of the oxidation state and coordination with cluster size on ultrananocrystalline diamond (UNCD) and amorphous alumina support. This support pair was chosen to be representative of carbon- and oxide-based supports, thus offering very different cluster−support interactions.46 Also, UNCD possesses excellent mechanical, chemical, and electrical properties that can be tailored for specific applications, especially when high chemical inertness and robustness is expected from the support material.47−49 Amorphous alumina films50 produced by atomic layer deposition are of industrial relevance and have been proven as excellent supports for stabilizing subnanometer-41,43,51,52 and nanometer-sized clusters2,46,50 under realistic reaction conditions of temperature and pressure, as shown by grazing incidence small-angle X-ray scattering (GISAXS).

■

EXPERIMENTAL METHODS Support Material. The UNCD-coated silicon wafers were purchased from Advanced Diamond Technologies. The wafers consisted of a 300 nm thick UNCD film deposited on a doped silicon wafer (UNCD, 25 Aqua DoSi).53 The use of doped silicon wafer as a base support facilitated conductivity for flux measurements of the charged clusters landing on the support during cluster deposition, which was used to determine the amount of deposited metal and surface coverage. For alumina support, approximately three monolayer (ML) thick alumina layer was prepared by atomic layer deposition41 on the top of naturally oxidized doped silicon wafer, yielding an amorphous surface with RMS roughness of ∼0.69 nm.50 Atomic Force Microscopy. The roughness and morphology of UNCD support was characterized with atomic force microscopy (AFM) using of a Veeco/Digital Instruments Nanoscope Dimension 3100 and controller IV. Tapping mode was applied with a Si cantilever from Nanosensors (tapping frequency ∼300 kHz and force constant ∼100 N/m). Averaged particle size was estimated with software Scion Image (from NIH Image) on tapping mode phase images. Deposition of Size-Selected Clusters. The size-selected cluster deposition method was used to prepare the narrow sizedistribution of clusters on UNCD and aluminum oxide. The detailed description of the method can be found elsewhere.2,41,51,52 In brief, a molecular beam of cobalt cluster ions was prepared in a laser ablation source. After passing through ion optics, the mass selected, positively charged, cobalt clusters were deposited on the support. For the three cluster sizes chosen in this study (Co4, Co7, and Co27), the size distribution was 1, 2, and 4 atoms around the main size, respectively (i.e., 4 ± 1, 7 ± 2, and 27 ± 4). A coverage

■

RESULTS AND DISCUSSION Morphology of the UNCD and Alumina Support. Figures S1a and S1b (see Supporting Information) show the morphology of the UNCD and alumina supports imaged by AFM and scanning-tunneling microscope (STM). In the case of the UNCD surface, the apparent size of the particles ranges from ∼20 nm for the individual diamond crystallites to ∼70 nm for the aggregates, yielding an average diameter of 24 nm. The UNCD surface RMS roughness is ∼9.9 nm. In comparison, the RMS roughness of the alumina film is ∼0.69 nm.50 GIXANES Characterization of the Oxidized Clusters. The GIXANES spectra on Co K-edge of UNCD- and aluminasupported Co4Ox, Co7Ox, and Co27Ox clusters are shown in Figure 1a,b, indicating a dominant Co2+ composition, B

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

edge peaks, along with the position and height of the white line, are listed in Table 1 for the cobalt cluster samples and for powder samples of CoO, Co3O4, and CoAl2O4. The low intensity of the pre-edge peak from the clusters indicates low degree of local mixing of 3d and 4p states, which reflects the localized nature of the electronic state of the subnanometer Co clusters. The position of the pre-edge peaks and of the white line shows close resemblance with CoO and so does the height of the white line for the UNCD-supported clusters. However, the white line of the alumina-supported clusters is significantly higher than that of the bulk standards, which presumably reflects unusually strong cluster−surface interactions (or high degree of 1s to 4p transition). Using spectra of a set of macroscopic standards showed in Figure 1c, a linear composition fitting was performed at the edge of the spectra. Two examples of fits, for 27-atom clusters supported on alumina and UNCD, are shown in Figure 2a,b, respectively. As previously described, on the basis of visual comparison of the general shape of spectra of subnanometer Co clusters with those of macroscopic reference standards, the shape of the spectra of Co clusters more closely resembles the spectrum of the CoO standard. However, as shown in the examples in Figure 2, in the case of alumina-supported Co27 clusters, the edge feature seems closer to that of the CoAl2O4 standard, while in the case of the UNCD supported clusters, the best fit is obtained with a dominant Co(OH)2 component. The best-fit results for all six samples are summarized in Table 2 and visualized in Figure 3. Compared to the fitting results from conventional bulk samples, the residual errors from the fitting models are higher due to the low degree of multiple scattering, the imhomogeneities of the surface as a result of support-cluster contact, and the significant structural differences between subnanometer size clusters and crystalline reference standards.57 The linear combination fitting provides a general trend as a function of the size of the subnanometer clusters and the effect of the support, not a precise quantitative analysis. Alumina-supported cobalt oxide clusters show a high degree of resemblance with tetrahedral CoAl2O4, which shows similar trends in the NEXAFS results shown later. The main finding from the linear combination fit results can be summarized as follows. Alumina-supported clusters show a Co2+ phase for all cluster sizes, while in the UNCD-supported clusters, a cluster-size-independent Co0 component and a sizedependent Co3+ component are added to it. A common feature observed on both supports was the formation of cobalt hydroxide on the 27-atom clusters. The dominant CoAl2O4 component identified in the alumina-supported clusters is indicative of very strong interactions between the cluster and the support, which may affect cluster structure and its binding to the surface atoms of the support, as predicted, for example, for bare and oxidized Pd clusters on periodic alumina and MgO supports58 or demonstrated experimentally.59,60

Figure 1. Co K-edge GIXANES data with enlarged pre-edge as inset obtained on UNCD (a) and alumina- (b) supported cobalt clusters with 4 (blue), 7 (green), and 27 (red) atoms, indicating oxidized Con clusters. The reference spectra of standards are shown in panel c.

resembling the main features of the reference macroscopic CoO standard shown in Figure 1c, as previously reported.39,42,44,56 The GIXANES spectra of the Co4Ox, Co7Ox, and Co27Ox clusters on either alumina or UNCD closely resemble each other, which is indicative of a similar oxidation state for all three clusters on the given support. The positions of pre-edge in all cluster samples were ∼7710.0 eV, which is much closer to CoO (7709.8 eV) than Co3O4 (7709.3 eV). (See Table 1.) A similar trend was observed for white lines. The white lines of clusters are all ∼7725.0 eV and are closer to CoO (7726.3 eV) compared with 7729.6 eV for Co3O4. The comparison with the XANES spectra of bulk standards (Figure 1c) indicates the presence of a dominant CoO phase. Noted is the broad spectral feature of the subnanometer clusters, which can be caused by the size of the ultrasmall particles as well as by a distribution of structures and clusters binding to various sites of the support. However, the spectra of the different cluster sizes on UNCD and alumina are slightly different, which is indicative of a support effect on the oxidation state of the clusters. The pre-edge features shown in the insets of Figures 1a,b exhibit variations between samples. The positions of the pre-

Table 1. Energy Positions of Pre-Edge and White Line Intensity of Co in UNCD and Alumina-Supported Co4Ox, Co7Ox, and Co27Ox Clusters and Three Powder Standards: CoO, Co3O4, and CoAl2O4. UNCD pre-edge (eV) white line position (eV) white line height

Al2O3

Co4Ox

Co7Ox

Co27Ox

Co4Ox

Co7Ox

Co27Ox

CoO powder

Co3O4 powder

CoAl2O4 powder

7710.0 7725.3 1.27

7709.5 7724.8 1.29

7710.5 7725.2 1.22

7710.4 7724.7 1.79

7710.4 7724.9 1.79

7710.3 7725.3 1.68

7709.8 7726.3 1.28

7709.3 7729.6 1.38

7709.5 7727.4 1.48

C

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

Figure 2. (a) XANES data (solid line) and linear combination analysis fit result (dotted line) of Co27Ox/Al2O3 and (b) Co27Ox/UNCD. See Table 2 for the parameters of the best fit.

Table 2. Results from Linear Combination Composition Fit of Co in UNCD and Alumina Supported Co4Ox, Co7Ox, and Co27Ox Clustersa

a

sample

Co0 (foil) [%]

CoO [%]

Co(OH)2 [%]

Co2O3 [%]

Co3O4 [%]

CoAl2O4 [%]

Co4Ox (UNCD) Co4Ox (Al2O3) Co7Ox (UNCD) Co7Ox (Al2O3) Co27Ox (UNCD) Co27Ox (Al2O3)

10 0 7 0 9 0

30 0 34 0 17 0

30 0 32 0 62 19

30 0 27 0 12 0

0 0 0 0 0 0

0 100 0 100 0 82

Estimated uncertainty is higher than 5%.

that is that the overall white line shape Co4Ox/UNCD shows more a Oh feature because there is the Co0 part that makes both the 778.7 and 780 eV peaks higher. Should one virtually take away the contribution of Co0, one would see that Co4Ox/ UNCD shows more Oh feature than Co4Ox/Al2O3. This agrees with the very low intensity of the K pre-edge, in general, as shown in insets in Figures 1a, b. Transition at preedge is much more favorable for no inversion symmetry as in Co2+ Td than that in Co2+ Oh. The pre-edge for clusters on Al2O3 support is slightly more pronounced than on UNCD. The following quantitative least-squares fitting with bulk references also supports this observation. This implies that substrate may induce a structural change of the Co clusters via strong cluster−support interactions. The least-squares fitting method was used to analyze the NEXAFS spectra with four references that represent the known oxidation states or structures.55 The Co0, Co2+ Oh, and Co3O4 references are from ref 62. Co2+ Td reference is from ref 61. As an example, Figure 4c shows the fitted NEXAFS spectrum obtained for the UNCD-supported Co4Ox clusters, and the results are summarized in Table 3. It is important to note that the reference spectra used for the fitting are from bulk materials,61,62 but the subnanometer clusters may have different spectral features from the bulk.63 However, the component fitting used here is still useful to compare the oxidation states of different samples as a function of cluster size and support. Because the measured signals of the clusters are weak on samples with low coverage, the background subtraction cannot make both the pre-edge and

Figure 3. Plot with the results of linear composition fitting.

NEXAFS Characterization. The NEXAFS spectra of the UNCD and alumina-supported clusters are shown in Figure 4. One noticeable difference between the two substrates is that in the photon energy range between 778.7 and 780.0 eV the spectra of clusters on the Al2O3 substrate more resemble the spectrum of Co2+ Td, while clusters on the UNCD support have a larger component of Co2+ Oh.55,61,62 For example, if comparing based on Co4Ox/Al2O3, the main difference between the Td and Oh structure is that the peak difference between 778.7 and 780 eV is larger for Td than for Oh. We see similarly large difference between the two peaks in Co4Ox/ Al2O3, which is more like the case in Td. For Co4Ox/UNCD, the difference between the two peaks is smaller. The reason for D

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

Figure 4. NEXAFS spectra on Co L2 and L3 edge of the Co4Ox, Co7Ox, and Co27Ox clusters on UNCD (a) and Al2O3 support (b). The Co L3 edge absorption spectrum of UNCD-supported Co4Ox clusters and fit with the reference spectra (c). The resulting Co0, Co2+ Oh, and Co2+ Td fit components are shown in the bottom half of the plot; the Co3O4 component was zero and is not shown in the plot. (An offset was applied on the curves for better viewing.) The Co0, Co2+ Oh, and Co3O4 references are from ref 62. Co2+ Td reference is from ref 61.

Table 3. Fractions of Oxidation States of Co in UNCD and Alumina-Supported Co4Ox, Co7Ox, and Co27Ox Clusters, Average Valence, and Number of Oxygen Atoms Per Clustera sample

Co0 [%]

Co2+ Oh [%]

Co4Ox (UNCD) Co4Ox (Al2O3) Co7Ox (UNCD) Co7Ox (Al2O3) Co27Ox (UNCD) Co27Ox (Al2O3)

34 (5) 0 (10) 8 (7) 4 (10) 4 (8) 11 (12)

23 (7) 10 (7) 38 (5) 0 (9) 27 (6) 12 (11)

Co2+ Td [%] 43 90 54 96 68 77

(6) (11) (6) (14) (8) (15)

Co3+ [%] 0 0 0 0 0 0

average valence [%]

(2) (2) (1) (2) (1) (2)

1.31 2.00 1.84 1.92 1.91 1.78

(0.19) (0.27) (0.16) (0.34) (0.20) (0.38)

oxygen coordination [number of O atoms per cluster] 2.6 (0.4) 4.0 (0.5) 6.4 (0.6) 6.7 (1.2) 25.9 (2.7) 24.1 (5.1)

a

Average valence is the weighted sum of component valence. The weighting parameter is the percentage of that valence oxygen coordination and is estimated by Co atoms in the cluster multiplied by the average valence and divided by 2 (assuming valence −2 for the oxygen.).

post-edge flat. Therefore, the fitting was only performed around the L3-edge, where the most characteristic features are present.55,61,62 The following major findings can be made. At first glance, no Co3+ states are identified in these clusters by NEXAFS, in contrast with XANES. We note that some deviations in the analysis may be caused by the fact that not the same standards were used. With the exception of Co4Ox on UNCD, the average valence of Co in all clusters is close to 2. This finding is corroborated by density functional theory (DFT) calculations,56 where we found that Co4 cluster after landing on θalumina support and subsequently exposed to the air transforms into Co4O4 composition. We observed that the Co4 clusters deposited on the UNCD substrate contain significantly larger fraction of the Co0 component (34%) than the other samples, for example, Co27 clusters on UNCD (4%). We hypothesize that the different fractions reflect differences in the type(s) of binding of the Co atoms within the subnanometer clusters as well as in the support (i.e., due to pronounced differences in the effect of the support on these length scales), which may considerably differ in the bindings typical in larger particles and macroscopic (powder) reference standards. For Co27 clusters, the composition difference on different supports (i.e., the difference between the fractions of components for alumina- and UNCD-supported clusters listed in Table 3) is 15% for Co2+ Oh and 9% for Co2+ Td, which are much smaller than the difference for smaller clusters. For Co4 clusters, the difference is as large as 34% in Co0 and 47% in

Co2+ Td, while for Co7Ox clusters, the difference is as large as 38% in Co2+ Oh and 42% in Co2+ Td. Our recent studies show that amorphous alumina possesses tetrahedral sites46 and that the structure of small clusters of oxidized palladium can be strongly influenced by the morphology of the alumina support.58 Thus, the prevalence of Co2+ Td appears to be induced through strong interactions with the support. We note that the strong support effects on alumina reflected also via the dominating cobalt-aluminate component obtained from the composition analysis of the XANES data. This exemplified that the support effects are more pronounced for small clusters, illustrating the origin of tenability of subnanometer clusters in catalyst design. All three alumina-supported Co clusters possess a larger Co2+ Td (77−96%) and smaller Co2+ Oh (around 10%) component compared with the Co clusters on the UNCD substrate, thus indicating some support effect on the structure of the subnanometer cobalt (sub)oxide clusters. Regarding the average valence, for Al2O3 support, it increases as follows: Co27Ox (1.78) < Co7Ox (1.92) < Co4Ox (2.00). This order is in good agreement with the intensive white line for small cluster observed in GIXANES, as shown in Figure 1. This implies the direct correlation among the degree of oxidation, the intensity of white line, and average valence. Finally, the average valence of Co on UNCD decreases as follows: Co27Ox (1.91) > Co7Ox (1.84) > Co4Ox (1.31). This is in the opposite order compared with clusters on Al2O3. This may hint at a difference in electron donation for Al2O3 and UNCD, which plays an important role E

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

Figure 5. Horizontal cuts of the GISAXS images recorded between temperatures 25 and 350 °C of the Co4Ox and Co27Ox cluster-based sample on the UNCD and surface exposed to propane, oxygen, and water vapor seeded in helium. An offset was applied on the curves for clarity. There was no change observed in the vertical cuts either (not shown here).The unchanged patterns indicate no formation of nanosized aggregates, implicating sinter-resistant clusters. The apparently missing data points around 0.9 nm−1 arise from the physical border area between the elements of the detector, where no readout takes place.

tunability of the oxidation state and structure in these clusters via cluster size and support effects provides a new way of tailoring the activity and selectivity of the subnanometer clusters and makes them excellent candidates for the design of new classes of catalysts on the atomic scale, down to the tailoring of single-site catalysts.64,65

in fundamental understanding and atomic control of highly active and selective catalysts. The comparison of the results from GIXANES and NEXAFS analysis shows similar trends on the evolution of the composition of the oxidized clusters with their size and the support. We note, however, that the absolute numbers differ. This is primarily caused by different (macroscopic) standards used at fitting; however, some effects from the UHV versus atmospheric pressure helium environments in NEXAFS and GIXANES experiments, respectively, cannot be fully excluded either. GISAXS Characterization. Typical vertical and horizontal cuts of the GISAXS images recorded in the temperature range of 25 to 350 °C in the presence of oxygen, propane, and water vapor are illustrated in Figure 5 for the smallest and largest Co4 and Co27 clusters on UNCD. The changes in the GISAXS patterns were very minor for all investigated samples, thus indicating highly stable sinter-resistant subnanometer clusters under the applied conditions. In addition to the alumina and UNCD support, the sinter-resistance of the cobalt clusters was also confirmed in various gas environments on titania, zincoxide, and iron-oxide supports (not shown here).39,42,56 The studies found the subnanometer clusters sinter-resistant at temperatures up to 350 °C. An exception was observed in the case of the MgO-supported clusters, when a solid Co−Mg−O solution was formed.39,42

■

ASSOCIATED CONTENT

S Supporting Information *

AFM and STM images of the UNCD and alumina support, select fits of XANES spectra, and references with full authors list. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ¶

C.Y., F.Z. and S.L. contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Drs. J. W. Elam and J. A. Libera for providing the alumina-coated silicon chips and Dr. L. A. Curtiss, Dr. G. A. Ferguson, Prof. J. E. Greeley, and Dr. P. Zapol for discussions on cluster−support interactions. The U.S. Department of Energy, BES-Materials Sciences, and BES-Scientific User Facilities (Advanced Photon Source) under Contract DE-AC02-06CH11357 supported the work performed at Argonne National Laboratory with UChicago Argonne LLC, the operator of Argonne National Laboratory. The Director, Office of Energy Research, and Office of Basic Energy Sciences of the U.S. Department of Energy under Contract DE-AC0205CH11231 supported the work performed at Lawrence Berkeley National Laboratory, including the work performed

■

SUMMARY The investigation of subnanometer cobalt oxide clusters CoxOy (with main sizes x = 4, 7, and 27) on UNCD and alumina supports using combined X-ray techniques has revealed that the degree of the oxidation of the clusters, their structure, and their binding to the support is strongly affected not only by the composition of the support but also by its morphology. The clusters have proven to be sintering resistant under gas mixtures at atmospheric pressure and elevated temperatures. The F

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

(23) Jiang, H. L.; Umegaki, T.; Akita, T.; Zhang, X. B.; Haruta, M.; Xu, Q. Bimetallic Au-Ni Nanoparticles Embedded in SiO2 Nanospheres: Synergetic Catalysis in Hydrolytic Dehydrogenation of Ammonia Borane. Chem.Eur. J. 2010, 16, 3132−3137. (24) Johnson, G. E.; Mitric, R.; Bonacic-Koutecky, V.; Castleman, A. W. Clusters as model systems for investigating nanoscale oxidation catalysis. Chem. Phys. Lett. 2009, 475, 1−9. (25) Kemper, P.; Kolmakov, A.; Tong, X.; Lilach, Y.; Benz, L.; Manard, M.; Metiu, H.; Buratto, S. K.; Bowers, M. T. Formation, deposition and examination of size selected metal clusters on semiconductor surfaces: An experimental setup. Int. J. Mass Spectrom. 2006, 254, 202−209. (26) Kurak, K. A.; Anderson, A. B. Selenium: A Nonprecious Metal Cathode Catalyst for Oxygen Reduction. J. Electrochem. Soc. 2010, 157, B173−B179. (27) Landman, U.; Yoon, B.; Zhang, C.; Heiz, U.; Arenz, M. Factors in gold nanocatalysis: oxidation of CO in the non-scalable size regime. Top. Catal. 2007, 44, 145−158. (28) Libuda, J.; Schauermann, S.; Laurin, M.; Schalow, T.; Freund, H. J. Model studies in heterogeneous catalysis. From structure to kinetics. Monatsh. Chem. 2005, 136, 59−75. (29) Lu, J.-L.; Gao, H.-J.; Shi, D.-X.; Shuikhutdinov, S.; Freund, H. J. Heterogeneous catalysis on an atomic scale. Wuli 2007, 36, 370−376. (30) Morkel, M.; Rupprechter, G.; Freund, H. J. Finite size effects on supported Pd nanoparticles: Interaction of hydrogen with CO and C2H4. Surf. Sci. 2005, 588, L209−L219. (31) Nossler, M.; Mitric, R.; Bonacic-Koutecky, V.; Johnson, G. E.; Tyo, E. C.; Castleman, A. W. Generation of Oxygen Radical Centers in Binary Neutral Metal Oxide Clusters for Catalytic Oxidation Reactions. Angew. Chem., Int. Ed. 2010, 49, 407−410. (32) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Hakkinen, H.; Barnett, R. N.; Landman, U. When gold is not noble: Nanoscale gold catalysts. J. Phys. Chem. A 1999, 103, 9573−9578. (33) Santra, A. K.; Min, B. K.; Goodman, D. W. Ag growth on Mo(112)-O-a and MoO2 surfaces. J. Vac. Sci. Technol., B 2002, 20, 1897−1900. (34) St. Clair, T. P.; Goodman, D. W. Metal nanoclusters supported on metal oxide thin films: bridging the materials gap. Top. Catal. 2000, 13, 5−19. (35) Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Kay, B. D. Methane adsorption and dissociation and oxygen adsorption and reaction with CO on Pd nanoparticles on MgO(100) and on Pd(111). Surf. Sci. 2005, 591, 90−107. (36) Uzun, A.; Dixon, D. A.; Gates, B. C. Prototype Supported Metal Cluster Catalysts: Ir4 and Ir6. ChemCatChem. 2011, 3, 95−107. (37) Yong Sun, W.; Young Seok, K.; Kryliouk, O.; Anderson, T. Growth mechanism of catalyst- and template-free InN nanorods. Phys. Status Solidi C 2008, 1633−8. (38) Zihao, Z.; Feng, G.; Goodman, D. W. Deposition of metal clusters on single-layer graphene/Ru(0001): Factors that govern cluster growth. Surf. Sci. 2010, L31−8. (39) Lee, S.; Di Vece, M.; Lee, B.; Seifert, S.; Winans, R. E.; Vajda, S. Support-dependent Performance of Size-selected Subnanometer Cobalt Cluster-based Catalysts in the Dehydrogenation of Cyclohexene. ChemCatChem. 2012, 4, 1632−1637. (40) Lee, S.; Lee, B.; Seifert, S.; Winans, R. E.; Di Vece, M.; Vajda, S., In situ real time GIXAS/GISAXS study of oxidative dehydrogenation of cyclohexene on size selected subnanometer cobalt cluster catalysts. Abstr. Pap. Am. Chem. Soc. 2011, 241. (41) Lee, S.; Molina, L. M.; López, M. J.; Alonso, J. A.; Hammer, B.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Pellin, M. J.; et al. Selective Propene Epoxidation on Immobilized Au6−10 Clusters: The Effect of Hydrogen and Water on Activity and Selectivity. Angew. Chem. 2009, 121, 1495−1499. (42) Lee, S.; Vece, M. D.; Lee, B.; Seifert, S.; Winans, R. E.; Vajda, S. Oxidative dehydrogenation of cyclohexene on size selected subnanometer cobalt clusters: improved catalytic performance via evolution of cluster-assembled nanostructures. Phys. Chem. Chem. Phys. 2012, 14, 9336−9342.

at the Advanced Light Source. V.V. acknowledges support of his research at the Advanced Photon Source provided by Argonne’s Undergraduate Summer Program.

■

REFERENCES

(1) Okada, T.; Kaneko, M. Molecular Catalysts for Energy Conversion; Springer: Berlin, 2009; p 434. (2) Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Meyer, R. J.; Redfern, P. C.; et al. Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects. Science 2010, 328, 224−228. (3) Abbet, S.; Ferrari, A. M.; Giordano, L.; Pacchioni, G.; Hakkinen, H.; Landman, U.; Heiz, U. Pd1/MgO(100): a model system in nanocatalysis. Surf. Sci. 2002, 514, 249−255. (4) Campbell, C. T. The active site in nanopaticle gold catalysis. Science 2004, 306, 234−235. (5) Campbell, C. T.; Parker, S. C.; Starr, D. E. The effect of sizedependent nanoparticle energetics on catalyst sintering. Science 2002, 298, 811−814. (6) Choudhary, T. V.; Goodman, D. W. Oxidation catalysis by supported gold nano-clusters. Top. Catal. 2002, 21, 25−34. (7) Chretien, S.; Buratto, S. K.; Metiu, H. Catalysis by very small Au clusters. Curr. Opin. Solid State Mater. Sci. 2007, 11, 62−75. (8) Fierro-Gonzalez, J. C.; Gates, B. C. Catalysis by gold dispersed on supports: the importance of cationic gold. Chem. Soc. Rev. 2008, 37, 2127−2134. (9) Fierro-Gonzalez, J. C.; Kuba, S.; Hao, Y. L.; Gates, B. C. Oxideand zeolite-supported molecular metal complexes and clusters: Physical characterization and determination of structure, bonding, and metal oxidation state. J. Phys. Chem. B 2006, 110, 13326−13351. (10) Frank, M.; Baumer, M.; Kuhnemuth, R.; Freund, N. J. Adsorption and reaction of ethene on oxide-supported Pd, Rh, and Ir particles. J. Vac. Sci. Technol., A 2001, 19, 1497−1501. (11) Freund, H. J. Clusters and islands on oxides: from catalysis via electronics and magnetism to optics. Surf. Sci. 2002, 500, 271−299. (12) Freund, H. J.; Baumer, M.; Kuhlenbeck, H. Catalysis and surface science: What do we learn from studies of oxide-supported cluster model systems? Adv. Catal. 2000, 45, 333−384. (13) Freund, H. J.; Baumer, M.; Libuda, J.; Kuhlenbeck, H.; Risse, T.; Al-Shamery, K.; Hamann, H. Metal aggregates on oxide surfaces: Structure and adsorption. Cryst. Res. Technol. 1998, 33, 977−1008. (14) Freund, H. J.; Ernst, N.; Risse, T.; Hamann, H.; Rupprechter, G. Models in heterogeneous catalysis: Surface science quo vadis? Phys. Status Solidi C 2001, 187, 257−274. (15) Fujitani, T.; Nakamura, I.; Akita, T.; Okumura, M.; Haruta, M. Hydrogen Dissociation by Gold Clusters. Angew. Chem., Int. Ed. 2009, 48, 9515−9518. (16) Gao, F.; Wood, T. E.; Goodman, D. W. The Effects of Water on CO Oxidation over TiO2 Supported Au Catalysts. Catal. Lett. 2010, 134, 9−12. (17) Gates, B. C. Supported metal-clusters: synthesis, structure and catalysis. Chem. Rev. 1995, 95, 511−522. (18) Goodman, D. W. Catalysis by metals- from small clusters to extended single crystals. Abstr. Pap. Am. Chem. Soc. 1995, 209, 55PHYS. (19) Guzman, J.; Gates, B. C. Supported molecular catalysts: metal complexes and clusters on oxides and zeolites. Dalton Trans. 2003, 3303−3318. (20) Heiz, U.; Abbet, S.; Sanchez, A.; Schneider, W. D.; Hakkinen, H.; Landman, U. Chemical reactions on size-selected clusters on surfaces. Phys. Chem. Clusters, Proc. Nobel Symp. 2001, 117, 87−98. (21) Huang, J. H.; Akita, T.; Faye, J.; Fujitani, T.; Takei, T.; Haruta, M. Propene Epoxidation with Dioxygen Catalyzed by Gold Clusters. Angew. Chem., Int. Ed. 2009, 48, 7862−7866. (22) Jiang, H. L.; Akita, T.; Ishida, T.; Haruta, M.; Xu, Q. A. Synergistic Catalysis of Au@Ag Core-Shell Nanoparticles Stabilized on Metal-Organic Framework. J. Am. Chem. Soc. 2011, 133, 1304−1306. G

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

(43) Deng, W.; Lee, S.; Libera, J. A.; Elam, J. W.; Vajda, S.; Marshall, C. L. Cleavage of the C-O-C bond on size-selected subnanometer cobalt catalysts and on ALD-cobalt coated nanoporous membranes. Appl. Catal., A 2011, 393, 29−35. (44) Lee, S.; Lee, B.; Seifert, S.; Vajda, S.; Winans, R. E. Simultaneous measurement of X-ray small angle scattering, absorption and reactivity: A continuous flow catalysis reactor. Nucl. Instrum. Methods Phys. Res., Sect. A 2011, 649, 200−203. (45) Cao, A.; Lu, R.; Veser, G. Stabilizing metal nanoparticles for heterogeneous catalysis. Phys. Chem. Chem. Phys. 2010, 12, 13499− 510. (46) Cheng, L.; Yin, C.; Mehmood, F.; Liu, B.; Greeley, J.; Lee, S.; Lee, B.; Seifert, S.; Winans, R. E.; Teschner, D.; et al. Reaction Mechanism for Direct Propylene Epoxidation by Alumina-Supported Silver Aggregates: The Role of the Particle/Support Interface. ACS Catal. 2013, 4, 32−39. (47) Gruen, D. M. Nanocrystalline diamond films. Annu. Rev. Mater. Sci. 1999, 29, 211−259. (48) Shenderova, O. G., Deiter Ultrananocrystalline Diamond: Synthesis, Properties, And Applications; William Andrew, Inc.: Norwich, NY, 2006; p 600. (49) Kwon, G.; Ferguson, G. A.; Heard, C. J.; Tyo, E. C.; Yin, C.; DeBartolo, J.; Seifert, S.; Winans, R. E.; Kropf, A. J.; Greeley, J.; et al. Size-Dependent Subnanometer Pd Cluster (Pd4, Pd6, and Pd17) Water Oxidation Electrocatalysis. ACS Nano 2013, 7, 5808−5817. (50) Molina, L. M.; Lee, S.; Sell, K.; Barcaro, G.; Fortunelli, A.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Pellin, M. J.; et al. Sizedependent selectivity and activity of silver nanoclusters in the partial oxidation of propylene to propylene oxide and acrolein: A joint experimental and theoretical study. Catal. Today 2011, 160, 116−130. (51) Vajda, S.; Winans, R. E.; Elam, J. W.; Lee, B.; Pellin, M. J.; Seifert, S.; Tikhonov, G. Y.; Tomczyk, N. A. Supported gold clusters and cluster-based nanomaterials: characterization, stability and growth studies by in situ GISAXS under vacuum conditions and in the presence of hydrogen. Top. Catal. 2006, 39, 161−166. (52) Winans, R. E.; Vajda, S.; Ballentine, G. E.; Elam, J. W.; Lee, B.; Pellin, M. J.; Seifert, S.; Tikhonov, G. Y.; Tomczyk, N. A. Reactivity of supported platinum nanoclusters studied by in situ GISAXS: clusters stability under hydrogen. Top. Catal. 2006, 39, 145−149. (53) Carlisle, J.; Advanced Diamond Technologies, Inc., 2011−2012. http://www.thindiamond.com/index.php?cID=145. (54) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537−541. (55) Zheng, F.; Alayoglu, S.; Guo, J.; Pushkarev, V.; Li, Y.; Glans, P.A.; Chen, J.-l.; Somorjai, G. In-situ X-ray Absorption Study of Evolution of Oxidation States and Structure of Cobalt in Co and CoPt Bimetallic Nanoparticles (4 nm) under Reducing (H2) and Oxidizing (O2) Environments. Nano Lett. 2011, 11, 847−853. (56) Ferguson, G.; Yin, C.; Kwon, G.; Lee, S.; Greeley, J.; Zapol, P.; Lee, B.; Seifert, S.; Winans, R. E.; Vajda, S.; Curtiss, L. Stable subnanometer cobalt oxide clusters on ultrananocrystalline diamond and alumina supports: Oxidation state and the origin of sinteringresistance. J. Phys. Chem. C 2012, 116, 24027−24034. (57) Peters, S.; Peredkov, S.; Balkaya, B.; Ferretti, N.; Savci, A.; Vollmer, A.; Neeb, M.; Eberhardt, W. Inner-shell photoionization spectroscopy on deposited metal clusters using soft x-ray synchrotron radiation: An experimental setup. Rev. Sci. Instrum. 2009, 80, 8. (58) Heard, C. J.; Vajda, S.; Johnston, R. L. Support and Oxidation Effects on Subnanometer Palladium Nanoparticles. J. Phys. Chem. C 2014, 118, 3581−3589. (59) Watanabe, Y.; Wu, X.; Hirata, H.; Isomura, N. Size-dependent catalytic activity and geometries of size-selected Pt clusters on TiO2(110) surfaces. Catal. Sci. Technol. 2011, 1, 1490−1495. (60) Mao, B.-H.; Chang, R.; Lee, S.; Axnanda, S.; Crumlin, E.; Grass, M. E.; Wang, S.-D.; Vajda, S.; Liu, Z. Oxidation and reduction of sizeselected subnanometer Pd clusters on Al2O3 surface. J. Chem. Phys. 2013, 138, 214304−7.

(61) Kobayashi, M.; Hidai, S.; Niwa, H.; Harada, Y.; Oshima, M.; Horikawa, Y.; Tokushima, T.; Shin, S.; Nakamori, Y.; Aoki, T. Co oxidation accompanied by degradation of Pt-Co alloy cathode catalysts in polymer electrolyte fuel cells. Phys. Chem. Chem. Phys. 2009, 11, 8226−8230. (62) Morales, F.; de Groot, F. M. F.; Glatzel, P.; Kleimenov, E.; Bluhm, H.; Hävecker, M.; Knop-Gericke, A.; Weckhuysen, B. M. In Situ X-ray Absorption of Co/Mn/TiO2 Catalysts for Fischer−Tropsch Synthesis. J. Phys. Chem. B 2004, 108, 16201−16207. (63) Bazin, D.; Rehr, J. J. Limits and Advantages of X-ray Absorption Near Edge Structure for Nanometer Scale Metallic Clusters. J. Phys. Chem. B 2003, 107, 12398−12402. (64) Thomas, J. M. Heterogeneous catalysis: Enigmas, illusions, challenges, realities, and emergent strategies of design. J. Chem. Phys. 2008, 128, 182502. (65) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgley, P. A. High-Performance Nanocatalysts for Single-Step Hydrogenations. Acc. Chem. Res. 2002, 36, 20−30.

H

dx.doi.org/10.1021/jp501817u | J. Phys. Chem. A XXXX, XXX, XXX−XXX