Supporting Online Material for

Transcription

1 Supporting Online Material for Identification of Active Gold Nanoclusters on Iron Oxide Supports for CO Oxidation Andrew A. Herzing, Christopher J. Kiely,* Albert F. Carley, Philip Landon Graham J. Hutchings* *To whom correspondence should be addressed. (C.J.K.); (G.J.H.) This PDF file includes: Materials and Methods Figs. S1 to S6 Published 5 September 2008, Science 321, 1331 (2008) DOI: /science

2 Materials and Methods (i) Catalyst Preparation All catalysts were prepared by co-precipitation. Dilute aqueous solutions of HAuCl 4 3H 2 O and Fe(NO 3 ) 3 9H 2 O containing the calculated amounts of Au and Fe required to give the desired loading on Fe 2 O 3 were mixed together under stirring at 353 K. Na 2 CO 3 (0.25 mol l 1 ) was added drop-wise until a ph 8.2 was attained. The resulting precipitate was then recovered by filtration and washed with hot de-ionized water (353K, 1 l). The material was then dried at 393K for 16 hrs in either a tube furnace under static air and a ramp rate of 15 o C or a GC oven under flowing air (i.e. in the latter case the drying temperature was reached before inserting the sample into the oven). (ii) Catalytic Testing Catalyst samples (50 mg) were evaluated for CO oxidation using a fixed-bed reactor (i.d. = 3 mm). The standard test conditions were as follows: CO (flow rate, 0.5 ml min 1 ), He (4.5 ml min 1 ), and O 2 (50 ml min 1 ), with total gas space velocity of 66,000 ml of gas (g of catalyst) 1 h 1 fed to the reactor (held at 293 K) using mass flow controllers. The product gases were analyzed by on-line gas chromatography, and the conversion of CO and formation of CO 2 were both quantified. After being used in the reactor, the catalysts were stored at room-temperature in sealed containers before spectroscopic analysis. Under such conditions, the state of the catalyst surface will be unaffected; in particular, the oxidation state of the gold particles will remain unchanged.

3 (iii) Electron Microscopy Samples of each catalyst were prepared for examination by TEM by dispersing the catalyst powder in high-purity ethanol. A drop of the suspension was then allowed to evaporate on a holey-carbon film supported by a 300-mesh copper TEM grid. Atomic-resolution, high-angle annular darkfield (HAADF) scanning transmission electron microscopy (STEM) was carried out using a JEOL 2200FS TEM/STEM and an FEI Titan TEM/STEM, both equipped with CEOS spherical aberration correctors. All STEM-HAADF images were medium low-pass filtered using a 3x3 kernel in order to reduce high-frequency noise. Note: Atomic percentages for the various Au species were generated under the rough assumption that the small clusters contain 10 atoms and the larger particles are 5nm, hemi-spherical particles which contain 1900 atoms. This is an extreme case in terms of the at% of Au present in the clusters because many have been observed which almost certainly contain less than 10 atoms, and many larger particles have been observed that have exceeded 5 nm in diameter. (iv) X-ray Photoelectron Spectroscopy X ray photoelectron spectra (XPS) were measured on a Kratos Axis Ultra DLD spectrometer using a monochromatic Al K α X-ray source ( W) and an analyser pass energy of 160eV (survey scans) or 40eV (detailed scans). Samples were mounted using double-sided adhesive tape, and binding

4 energies are referenced to the C(1s) binding energy of adventitious carbon contamination taken to be 284.7eV. Comment on the detection limit of XPS: We may estimate the detection limit of XPS for Au by consideration of Figure S4.1 which shows the Au(4f) spectra for the two Au/Fe2O3 catalyst samples. A signal intensity of approximately 10% of that shown would be difficult to discriminate from the baseline noise, and this would correspond (assuming the signal is proportional to the gold content) to a detection limit of ca 0.3 at% Au. The individual gold atoms comprise at most 2 % of the total gold content (2.9 at%) which corresponds to 0.06 at%, well below our XPS detection limits, especially when we are looking for 2% of a much more dominant signal. (v) Other Characterization Methods Surface areas were determined by nitrogen adsorption at -196 C using the BET method. Au loadings were determined using a Varian 55B AA spectrometer.

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6 Figure S.1 Additional STEM-HAADF images of the highly-active Au/FeOx catalyst (Sample B).

7 Figure S.2 Additional STEM-HAADF images of the inactive Au/FeO x catalyst (Sample A).

8 a b Figure S.3 Schematic diagrams of potential clusters consisting of (a) 0.5nm bi-layer containing 10 atoms and (b) nm planar monolayer structures consisting of only 3 or 4 atoms.

9 Figure S Fe(3s) and Au(4f) XPS spectra for the two catalysts dried at 120 o C. A slight shift of the Au(4f) peak towards higher binding energy in the catalyst dried under static air indicates the presence of Au 3+ in this sample. Figure S.4.2 O(1s) XPS spectra from the two catalysts dried at 120 o C. Both samples exhibit a peak at 530.4eV typical of oxygen (O 2- ) and a shoulder at 531.8eV. The latter is much more pronounced in the sample dried under static air, and most likely arises from a combination of hydroxyl and carbonate species.

10 Figure S C(1s) XPS spectra from the two catalysts dried at 120 o C which clearly indicates the presence of similar amounts of carbonate species in both catalysts.

11 Figure S.5.1 Representative aberration-corrected HAADF images from the catalyst calcined at 400 o C (Sample C).

12 Figure S.5.2 Representative aberration-corrected HAADF images from the catalyst calcined at 550oC (Sample D).

13 Figure S.5.3 Representative aberration-corrected HAADF images from the catalyst calcined at 600oC (Sample E).

14 Fe(3s) Figure S.6.1 Comparison of Au(4f) and Fe(3s), X-ray photoelectron spectra from the Au/Fe 2 O 3 catalysts calcined at (a) 400 o C, (b) 550 o C and (c) 600 o C. Figure S.6.2 Comparison of Fe(2p), X-ray photoelectron spectra from the Au/Fe 2 O 3 catalysts calcined at (a) 400 o C, (b) 550 o C and (c) 600 o C.

15 Figure S.6.3 Comparison of O(1s), X-ray photoelectron spectra from the Au/Fe 2 O 3 catalysts calcined at (a) 400 o C, (b) 550 o C and (c) 600 o C. Figure S.6.4 Comparison of C(1s), X-ray photoelectron spectra from the Au/Fe 2 O 3 catalysts calcined at (a) 400 o C, (b) 550 o C and (c) 600 o C.