1 / 5 Supporting Information for The Influence of Size-Induced Oxidation State of Platinum Nanoparticles on Selectivity and Activity in Catalytic Methanol Oxidation in the Gas Phase Hailiang Wang, Yihai Wang, Zhongwei Zhu, Andras Sapi, Kwangjin An, Griffin Kennedy, William D. Michalak, Gabor A. Somorjai* Department of Chemistry, University of California, Berkeley, California 94720, United States Materials Sciences and Chemical Sciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Email: somorjai@berkeley.edu Synthesis of 1, 2, 4 and 6 nm Pt nanoparticles 1 nm sized PVP-stabilized Pt nanoparticles were generated by UV-induced solution process at room temperature. In a typical synthesis, 10 mm aqueous H 2 PtCl 6 solution and 100 mm aqueous PVP (average M w ~29,000) solution were mixed with the volume ratio of 1:1. The yellow mixed solution was then stored in darkness for 24 hours and transferred into a quartz tube tightly sealed with a rubber stopper. The tube was purged with Argon for 3 times and inserted into a Rayonet RPR-200 UV reactor. The reaction took place under 254 nm UV irradiation with energy density of ~30 mw/cm 2. The solution turned brown after 1 hour, which represented the formation of PVP-capped Pt nanoparticles. To synthesize 2 nm Pt nanoparticles, 0.1 g of NaOH was dissolved in 5 ml of ethylene glycol and the solution was added to another 5 ml of ethylene glycol solution containing 0.08 g of H 2 PtCl 6 xh 2 O (~0.2 mmol). The mixture was heated to 160 o C in an oil bath and held at that temperature for 2 h under Argon atmosphere. The resulting nanoparticles were precipitated with 2 ml of 2 M HCl solution and
2 / 5 re-dispersed in ethanol with 0.1 g of PVP (M w = 55,000). The nanoparticles were repeatedly washed by precipitating with hexane, centrifuging, and re-dispersing in ethanol before use. To synthesize 4 nm Pt nanoparticles, 50 mg of H 2 PtCl 6 6H 2 O was dissolved in 5 ml of ethylene glycol and the solution was mixed with another 5 ml of ethylene glycol solution containing 220 mg of PVP (M w = 29,000). The mixture was heated to 160 o C in an oil bath and held at that temperature for 1 h under Argon atmosphere. The resulting nanoparticles were precipitated with 40 ml of acetone and re-dispersed in 10 ml of ethanol. The nanoparticles were repeatedly washed by precipitating with hexane, centrifuging, and re-dispersing in ethanol before use. To synthesize 6 nm Pt nanoparticles, 80 mg of platinum (II) acetylacetonate (~0.2 mmol) and 55 mg of PVP (M w = 55,000) were dissolved in 5 ml of ethylene glycol. The solution was then heated to 200 o C in an oil bath and held at that temperature for 5 min under Argon atmosphere. Cooled to room temperature, the resulting nanoparticles were precipitated with 45 ml of acetone and re-dispersed in 10 ml of ethanol. The nanoparticles were repeatedly washed by precipitating with hexane, centrifuging, and re-dispersing in ethanol before use. TEM imaging of Pt nanoparticles TEM samples were prepared by drop-casting ethanol or water suspensions of Pt nanoparticles on film-coated copper grids. TEM imaging was performed on a Hitachi H-7650 transmission electron microscope operated at 120 kv and a JEOL 2100 transmission electron microscope operated at 200 kv. Figure 1a, e, f, g and h were obtained with samples deposited on ultrathin carbon film (4-5 nm) coated 400-mesh copper grids (Electron Microscopy Sciences) and imaged with the JEOL 2100 transmission electron microscope operated at 200 kv. Figure 1b, c and d were obtained with samples deposited on formvar resin film (30-75 nm) coated 200-mesh copper grids (Electron Microscopy Sciences) and imaged with the Hitachi H-7650 transmission electron microscope operated at 120 kv.
3 / 5 XPS measurements XPS study of the Pt nanoparticles with various sizes was performed with a PHI 5400 XPS system equipped with Al X-ray source (incident photon energy of 1486.7 ev), and the samples were prepared by drop-casting the ethanol or water suspensions of the Pt nanoparticles onto Si substrates. The binding energy of the obtained XPS spectra was calibrated with respect to the Si 2p peak located at 99.3 ev. XPS spectra were quantitatively analyzed by deconvoluting Voigt-type line-shapes, preceded by subtracting the Shirley-type background. DRIFTS study of CO adsorbed on Pt nanoparticles Pt nanoparticles with various sizes were drop-casted onto Si substrates from their ethanol or water suspensions. The samples were then sealed in a quartz tube with CO gas flowing under ambient conditions for more than 12 hours before the DRIFTS spectra were taken with a Thermo Nicolet 6700 FT-IR spectrometer with integrated diffuse reflectance optics (Foundation Series by Thermo Spectra-Tech). Catalysis test The nanoparticles were drop-casted from their ethanol or water suspensions onto Si substrates to prepare the catalysts. The reactions were carried out in a batch mode reactor equipped with a boron nitride substrate heater (Momentive) and a metal bellows circulation pump (Senior Aerospace, MB-25) for gas mixing. The reactor was typically filled with 10 Torr of methanol, 50 Torr of oxygen and 710 Torr of helium at the temperature of 60 C unless otherwise noted. Total conversion of methanol was kept below 10% in order to assess the initial reaction rates. The products were detected by a gas chromatography (HP 5890, column HayeSep T) with a thermal conductivity detector. Number of active sites for each catalyst was measured from an ethylene hydrogenation reaction with 100 Torr of H 2 and 10 Torr of ethylene at 25 o C
4 / 5 following the methanol oxidation reaction. A turnover frequency of 11.7 molecules site -1 s -1 for ethylene hydrogenation on Pt surface was used to calculate the number of Pt active sites for the nanoparticle catalysts. Figure S1. Size distributions of the four Pt nanoparticle samples measured from TEM images.
5 / 5 Figure S2. Pt 4f XPS spectra for Pt nanoparticles with average sizes of (a) 1 nm, (b) 2 nm, (c) 4 nm and (d) 6 nm.