Very low temperature CO oxidation over colloidally deposited gold nanoparticles on Mg(OH) 2 and MgO

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1 Supporing Information Very low temperature CO oxidation over colloidally deposited gold nanoparticles on Mg(OH) 2 and MgO Chun-Jiang Jia, Yong Liu, Hans Bongard, Ferdi Schüth* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, Germany. schueth@mpi-muelheim.mpg.de Details of experiments 1. Catalyst preparation Preparation of the mesoporous MgO as starting support for gold colloid deposition. Mesoporous MgO as support for gold catalyst was prepared by calcining Mg(OH) 2 nanopower from Aldrich (99%) at 400 C for 5 h in static air (muffle oven, heating rate: 5 C min 1 ). The XRD and N 2 adsorption results of the as-prepared mesoporous MgO are shown in Figure S1. Synthesis of the gold supported catalyst. Synthesis of the Au/Mg(OH) 2 catalyst. The colloidal deposition method used here has been extensively described elsewhere [1]. The colloidal gold solutions were prepared using PVA (poly vinyl alcohol, MW from Aldrich, 80% hydrolyzed) as protecting agents to achieve narrow particle size distributions. In a typical preparation, the protecting agent was added (Au/PVA) 1.5:1 mg mg 1 ) to a 100 mg L 1 aqueous gold solution (as HAuCl 4 Alfa-Aesar, 99.99%) at room temperature under vigorous stirring. The obtained solution was then left under stirring for 10 min. Following rapid injection of an aqueous solution of NaBH N from Aldrich, 97% purity (Au/NaBH 4 1:5 mol mol 1 ), led to formation of a dark orange-brown solution, indicating the formation of the gold sol. The mesoporous MgO support was then added to the colloidal gold solution under stirring and kept in contact (for 5 h) until total adsorption (1 wt % of gold on the support) occurred, indicated by decoloration of the solution. All steps previously described were carried out under exclusion of light by covering all containers with a layer of aluminum foil. The solids were collected by vacuum filtration followed by washing the solid with 2 L of doubly distilled water to remove all the dissolved species (e.g., Na +, Cl ). It should be noted that, although the starting support is MgO, the resulting catalyst is Au/Mg(OH) 2 with a gold loading of ca. 0.7 wt%, and not Au/MgO with the nominal 1 wt% loading gold, due to the hydrolysis of the MgO in aqueous gold colloid solution. Finally, the solids were dried under a vacuum of 10 2 S1

2 mbar in a desiccator where P 2 O 5 (Aldrich 97% purity grade) as a drying agent was scattered on the bottom. Before catalytic testing, the catalyst powders were pressed, crushed, and sieved to mesh. Synthesis of the Au/Mg(OH) 2 -II catalyst. For comparison, replacing mesoporous MgO with Mg(OH) 2 nanopower (Aldrich, 99%) as starting support for gold deposition, another Au/Mg(OH) 2 catalyst was synthesized, labelled Au/Mg(OH) 2 -II. Synthesis of the Au/MgO catalyst. The Au/MgO catalyst was prepared by calcining the above synthesized Au/Mg(OH) 2 at 375 C for 3 h. The catalyst was also dried under a vacuum of 10 2 mbar in a desiccator where P 2 O 5 (Aldrich 97% purity grade) as a drying agent was scattered on the bottom. Before catalytic testing, the catalyst powders were pressed, crushed, and sieved to mesh. Reference catalyst of Au/TiO 2. Au/TiO 2 (1 wt% gold loading) as a reference catalyst for CO titration experiments was kindly provided by AuTEK Company. 2. Characterization The powder X-ray diffraction (XRD) patterns were recorded on a Stoe diffractometer operating in reflection mode with Cu Kα radiation. The nitrogen sorption measurements were performed on an ASAP 2010 (Micromeritics) at 77 K. High resolution scanning electron microscope (HR-SEM) images and scanning transmission electron microscope (STEM) images of the catalyst were taken on a Hitachi S-5500 ultrahigh resolution cold field emission scanning microscope at an acceleration voltage of 30 kv. The morphological characterization for the gold colloid before deposition on the support was performed with a Hitachi HF2000 microscope equipped with a cold field emission gun at the electronic acceleration voltage of 200 kv. The energy dispersive X-ray analysis (EDX) was conducted on a Hitachi S-3500N scanning electron microscope. 3. Catalytic reaction of CO oxidation The activities of the catalysts for CO oxidation were measured in a plug flow reactor using 50 mg for Au/Mg(OH) 2 and 35 mg for Au/MgO catalyst (the amount of the supported gold is kept the same as 0.35 mg) in a gas mixture of 1% CO in air (1% CO, 20% O 2 and 79% N 2, from AIR LIQUIDE, % purity) at a flow rate of 67 ml min 1, corresponding to a space velocity of 80,000 ml h 1 g 1 cat for Au/Mg(OH) 2 and 114,000 ml h 1 g cat 1 for Au/MgO, respectively. The operation temperature was controlled with a thermocouple and could be adjusted in the range of 100 to 400 C. Temperature data in the catalytic test are always referred to the value measured with a second thermocouple placed in the catalyst bed. Before measurement, the catalysts were activated in the reaction gas with the temperature ramped with a rate of 2 C min 1 from 40 to 275 C, through which the adsorbed polymer agents on the gold particle were removed S2

3 [1]. The dehydration of Mg(OH) 2 to MgO does not take place at this temperature, as separate TG experiments proved. The XRD pattern of the catalyst after reaction also confirmed the only presence of Mg(OH) 2. For a typical light-off run, in which the temperature was ramped, the reactor was cooled to 40 C prior to each experiment under a flow of N 2 (from AIR LIQUIDE, % purity), which was then replaced by the reaction gas, after the base temperature had been reached. Then, the temperature was ramped with a rate of 2 C/min to the final temperature. The concentrations of CO 2 and CO were analyzed at the outlet of the reactor with nondispersive IR spectroscopy, using two URAS 3E (Hartmann and Braun). For the light-off test at very high space velocities, 10 mg Au/Mg(OH) 2 or 7 mg Au/MgO diluted with 90 mg quartz sand was used in a gas mixture of 1% CO in air at a flow rate of 67 ml min 1, corresponding to a space velocity of 400,000 ml h 1 g 1 cat for Au/Mg(OH) 2 and 570,000 ml h 1 g 1 cat for Au/MgO, respectively. In a typical steady-state experiment, the system was adjusted to the desired temperature under N 2 flow, which was then replaced by the reagent mixture. The activity was continuously monitored during the whole experiment for more than 350 min. The titration experiments were carried out in the same reactor as the CO oxidation activity test using 100 mg catalyst. The catalysts (Au/Mg(OH) 2 and Au/TiO 2 ) were first activated at 300 C in mixed gas of 20% O 2 in N 2 (67 ml min 1 ) for 30 min, then cooled down to the desired temperature and kept in gas flow (20% O 2 in N 2, 67 ml min 1 ) for 10 min. The catalysts were switched to a stream of 1% CO in N 2 (from AIR LIQUIDE, % purity) at a flow rate of 67 ml min 1 at the desired temperature after purging in pure N 2 flow (67 ml min 1 ) for 5 min to remove the additional O 2. Online analysis was performed with non-dispersive IR spectroscopy (URAS 3E), which allows detection of CO and CO 2 in a nitrogen matrix without interference problems. S3

4 Additional Results Figure S1 XRD pattern (a) and N 2 sorption isotherms (b) of the mesoporous MgO prepared by calcining the Mg(OH) 2 nanopower (from Aldrich, 99%) at 400 C for 5 h as the starting support for the gold colloid deposition. S4

5 Figure S2 XRD pattern of the Au/Mg(OH) 2 catalyst after reaction using the mesoporous MgO as starting support for gold colloid deposition. During the gold deposition process, the MgO reacted with the H 2 O to induce the formation of Mg(OH) 2, therefore the resulting catalyst is Au/Mg(OH) 2 with ca. 0.7 wt% loading gold. S5

6 Figure S3 Bright (left column) and dark field (right column) STEM images of Au/Mg(OH) 2 catalyst taken on S-5500 ultrahigh resolution cold field emission scanning microscope. S6

7 Figure S4 TEM images with different magnifications of the gold colloid before deposition on the support. The gold particles mainly vary from 1.5 to 5 nm in size. No gold clusters with sizes below 1 nm were observed, which is consistent with the previous results reported by us [1] and other authors [2]. S7

8 Figure S5 (a) Temperature dependence for Au/TiO 2 (1 wt% gold loading, AuTEK) of (a) the activity for CO oxidation, and (b) (e) CO 2 response for the CO titration at different temperatures (Catalyst: 100 mg, flow rate of 1 % CO in N 2 : 67 ml min 1 ). The reference catalyst Au/TiO 2 exhibits high activity for CO oxidation. The temperature for 50% CO conversion (T 50 ) is 17 C, and full CO conversion is reached at ca. 30 C. The results of the CO titration experiments for Au/TiO 2 catalyst at different temperatures correspond to the temperature dependence of the activity, i.e. at low temperatures the CO 2 response intensity is very small, but increases with increasing temperature. S8

9 Figure S6 XRD pattern of the Au/MgO catalyst (after reaction) obtained through calcination of the Au/Mg(OH) 2 at 375 C for 3 h. S9

10 Figure S7 Bright (left column) and dark field (right column) STEM images of the Au/MgO catalyst taken on S-5500 ultrahigh resolution cold field emission scanning microscope. S10

11 Figure S8 Temperature dependence of the CO conversion for the Au/MgO catalyst at space velocities (SV) of 114,000 ml h 1 g cat 1 (catalyst: 35 mg, reaction gas flow rate: 67 ml min 1 ) and 570,000 ml h 1 g cat 1 (catalyst: 7 mg Au/MgO diluted with 90 mg quartz sand, reaction gas flow rate: 67 ml min 1 ). S11

12 Figure S9 Deactivation of the Au/MgO catalyst at different temperatures: (a) 89 C, (b) 50 C, and (c) 110 C. (catalyst: 35 mg, reaction gas flow rate: 67 ml min 1, space velocity = 114,000 ml h 1 g cat 1 ) S12

13 Figure S10 Temperature dependence of the CO conversion for the Au/Mg(OH) 2 -II catalyst with Mg(OH) 2 nanopowder from Aldrich as starting support. (Catalyst: 50 mg, reaction gas flow rate: 67 ml min 1, Space velocity = 80,000 ml h 1 g cat 1 ) Reference [1] Comotti, M.; Li, W. C.; Spliethoff, B.; Schüth, F. J. Am. Chem. Soc. 2006, 128, 917. [2] Dimitratos, N.; Lopez-Sanchez, J. A.; Anthonykutty, J. M.; Brett, G.; Carley, A. F.; Tiruvalam, R. C.; Herzing, A. A.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2009, 11, S13