Supplementary information Identification of Active Zr-WO x Clusters on a ZrO 2 Support for Solid Acid Catalysts Wu Zhou 1, Elizabeth I. Ross-Medgaarden 2, William V. Knowles 3, Michael S. Wong 3, Israel E. Wachs 2 and Christopher J. Kiely 1* 1 Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015, USA. 2 Operando Molecular Spectroscopy and Catalysis Laboratory, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA. 3 Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005, USA. * To whom correspondence should be addressed - C.J. Kiely: e-mail: chk5@lehigh.edu This PDF file includes: Figures S1 to S3 References nature chemistry www.nature.com/naturechemistry 1
a b c d Figure S1 Electron beam irradiation effects on the supported tungstate species. (a) and (b): Sequential HAADF scans from a low activity WO 3 /ZrO 2 sample illustrating that although the supported mono- and poly-tungstate species show some evidence of surface diffusion during electron beam irradiation (200keV, 30pA, 48 s/pixel) they have no great tendency to agglomerate into clusters. To aid comparison of (a) and (b), arrows have been added as markers that identify identical points in the two images. (c) and (d): Sequential HAADF scans from a high activity WO 3 /ZrO 2 sample illustrating that the supported Zr-WO x clusters are very stable under the electron beam irradiation conditions employed for imaging experiments (200keV, 30pA, 48 s/pixel). Electron beam irradiation damage and sample stability always need to be considered when using aberration corrected electron microscopy. Our HAADF images were acquired at 200keV, with a nature chemistry www.nature.com/naturechemistry 2
low probe current (30pA) and very short dwell times (40-48 s per pixel). Under these mild conditions only some limited surface diffusion, but not particularly agglomeration, of the monoand poly-tungstate WO x species was observed in sequentially acquired HAADF images (see Figs. S1(a) and (b)). The Zr-WO x cluster species were considerably more stable under the same HAADF imaging conditions (see Figs. S1(c) and (d)). When applying HAADF imaging to other more beam sensitive supported oxide catalyst systems, a reduction in probe current/dwell time and using even lower accelerating voltages S1,S2 could in principle further reduce electron beam induced modification effects. nature chemistry www.nature.com/naturechemistry 3
a b Figure S2 HAADF images of the WZrOH-type catalysts. These images were taken from ZrO 2 particles oriented along a major crystallographic zone axis. Some Zr columns are brighter than their neighbors, which indicate the presence of W atoms on these particular Zr atomic columns. The images have been low-pass filtered to reduce high frequency background noise. Inserts: intensity line profiles from the areas indicated. nature chemistry www.nature.com/naturechemistry 4
a b c d Figure S3 Examples of aberration corrected HAADF imaging on other supported oxideon-oxide catalyst systems. (a, b) 7wt% WO 3 /TiO 2 catalyst; (c) 5wt% WO 3 /SiO 2 catalyst; and (d) 5wt% BaO/SiO 2 catalyst. All of these samples were synthesized via standard incipientwetness impregnation methods followed by high temperature calcination in air. HAADF imaging of these samples was performed as a demonstration of the wider applicability of this technique to the study of supported oxide-on-oxide catalysts. All the images presented have been low-pass filtered in order to reduce high frequency noise. For HAADF z-contrast imaging to be effective there needs to be a significant difference in atomic number between the cations of the oxide support and those in the oxide overlayer. In the main text of this paper we show the effectiveness of this z-contrast in discriminating supported tungstate species on a ZrO 2 support. In Figure S3 we demonstrate that surface tungstate species can also be visualized on other supports (namely TiO 2 in (a) & (b), and SiO 2 in (c)). On each of these supports for the low WO x loading levels employed, the distribution of tungstate species is nature chemistry www.nature.com/naturechemistry 5
seen to be very different. On TiO 2 mono- and poly-tungstate species dominate, whereas on amorphous SiO 2 3-D WO x clusters about 1nm in size are the preferred tungstate morphology. This is a direct consequence of the different wetting interactions encountered by tungstate species on TiO 2 and SiO 2 supports respectively. Furthermore, it is found that W atoms in the mono-tungstate species tend to sit directly above the Ti support atom columns as shown in Fig. S3b, whereas some W atoms within the poly-tungstate species were found to lie-off the Ti columns in order to satisfy the constraints imposed by W-O-W bonding geometry. These observations also suggest that there is strong propensity for bonding between the surface WO x species and the TiO 2 crystalline support, as was encountered for the ZrO 2 support described in the main text. Comparing the images shown in Fig S3(c) and (d) we have the situation where the support material is the same (i.e. SiO 2 ), but now the overlayer species are different (i.e. WO x in (c) and BaO in (d)). Once again we see a strong difference in wetting behavior. BaO species were found to form large two-dimensional network structures on the amorphous SiO 2 surface, as compared to the 1nm 3D clusters formed by WO x, indicating a relatively stronger interaction between BaO and SiO 2 as compared with WO x on SiO 2. The WO x /TiO 2 sample was kindly provided by Dr Sun-Jae Kim and Mr Charles Roberts. The BaO/SiO 2 sample was provided by Mr Kevin F. Doura. References S1. Dellby, N. et al. Atomic-resolution STEM at 60 kv primary voltage. Microsc. Microanal. 14(Suppl 2), 136-137 (2008). S2. Suenaga, K. et al. Visualizing and identifying single atoms using electron energy-loss spectroscopy with low accelerating voltage. Nat. Chem. 1, 415-418 (2009). nature chemistry www.nature.com/naturechemistry 6