Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is The Royal Society of Chemistry 2015 Supplementary Information AgPd@Pd/TiO 2 nanocatalyst synthesis by microwave heating in aqueous solution for efficient hydrogen production from formic acid Masashi Hattori, a Daisuke Shimamoto, b Hiroki Ago, a and Masaharu Tsuji a * a Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580, Japan; e-mail: tsuji@cm.kyushu-u.ac.jp b Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan S1
EXPERIMENTAL SECTION Chemicals Formic acid (HCOOH, FA; Kanto Chemical Co. Inc., 88%), silver(i) nitrate (AgNO 3 ; Kishida Chemical Co. Ltd., 99.8%), palladium(ii) nitrate (Pd(NO 3 ) 2 ; Kanto Chemical Co. Inc., 97%), polyvinylpyrrolidone (PVP; Wako Pure Chemical Industries Ltd., average molecular weight of 10 k in terms of monomer units), titanium tetraisopropoxide (Wako Pure Chem. Inds. Ltd., 95%), 1,5-pentanediol (Tokyo Chemical Industry Co. Ltd., 97%) and sulfuric acid (Kishida Chemical Co. Ltd., 98%) were used without further purification. Characterizations of nanocatalysts Scanning transmission electron microscopy (STEM, JEM-ARM200F; JEOL) and corresponding energy dispersive X-ray (EDX) spectrometry were applied for detailed microstructure and composition analyses. Amorphous carbon coated copper grids were used as the sample supporters. The average sizes of product particles were determined by measuring more than 100 particles in STEM images. The composition of AgPd@Pd/TiO 2 nanocatalysts was analyzed using atomic absorption spectrometry (AA-7000; Shimadzu Corp.). Before measurement, AgPd@Pd/TiO 2 nanocatalysts were dissolved into sulfuric acid and this solution was filtered for removing TiO 2 nanoparticles which were not completely dissolved into sulfuric acid. The concentrations of Ag and Pd ions in this solution were measured and the Pd/Ag atomic ratio was estimated from the results. Powder X-ray diffraction (XRD) patterns of the samples were measured using Cu Kα radiation operating at 45 kv and 200 ma (SmartLab; Rigaku Corp.). X-ray photoelectron spectrometry (XPS) was conducted using Al Kα radiation (AXIS-165; Shimadzu Corp.). The calibration was carried out using a carbon peak which was derived from tape used for stabilizing samples at XPS measurement before analyzing. Detailed analyses for CO 2, H 2, and CO were performed on a gas chromatograph (GC7100; J-Science). TiO 2 nanoparticle preparation TiO 2 nanoparticles were prepared using the MW-polyol method. First, 3 mmol titanium tetraisopropoxide was added to 50 ml of 1,5-pentanediol. The solution was irradiated in a microwave oven (MW, maximum output power 750 W, μ type; Shikoku Keisoku Kogyo K.K.) at 200 W for 3 min. Then 2 ml distilled water was added to this solution and irradiated at 700 W for 1 h. For the use of TiO 2 as support of metallic nanoparticles, products were obtained by centrifuging the colloidal solution at 22,200 g for 30 min. Then they were redispersed in distilled water. S2
Fig. S1a and 1b show STEM image and the XRD pattern of TiO 2 nanoparticles, respectively. The prepared anatase-type of TiO 2 nanoparticles have average diameter of 10±2 nm. AgPd@Pd/TiO 2 nanocatalyst preparation AgPd@Pd nanocatalysts were formed on TiO 2 using a two-step MW heating method. First, Ag core nanoparticles were formed in the presence of TiO 2 nanoparticles. 15 ml of distilled water containing 300 mg polyvinylpyrrolidone (PVP) and 12.26 mg AgNO 3 were mixed with the colloidal solution of 71.88 mg TiO 2 nanoparticles. The mixed solution was heated at 95 C with MW irradiation at 120 W for 40 min under Ar bubbling. Temperature profile of the reagent solution under MW irradiation is shown in Fig. S2a. In the second step, 2 ml of distilled water containing 8.3 mg Pd(NO 3 ) 2 was added to this solution and heated with MW irradiation at 400 W for 30 min, 1 h or 2 h under Ar bubbling. The solution temperature increased to 100 C heated with MW-irradiation. Temperature profile of the reagent solution under MW irradiation for 30 min is shown in Fig. S2b. Finally, the prepared samples were separated from solution by centrifuging the obtained colloidal solution at 22,200 g for 60 min. They were dispersed in ethanol. Then, PVP was removed from the AgPd@Pd/TiO 2 surface by sonication for 30 min and PVP and AgPd@Pd/TiO 2 were separated by centrifuging at 22,200 g for 60 min. They were dispersed in distilled water and PVP was removed and separated from AgPd@Pd/TiO 2 surface by sonication for 30 min and centrifuging at 22,200 g for 60 min. These processes were repeated twice to remove PVP adequately. Finally, the AgPd@Pd/TiO 2 particles were dispersed in distilled water. We cleaned catalysts by sonication in ethanol and centrifuging (called cleaning process here in after) as described above. To determine an enough number of times of cleaning processes, the hydrogen generation rates of three samples (AgPd@Pd/TiO 2 catalysts washed by cleaning process one time, three times and five times) were compared. As the results, all samples showed almost the same efficiency. Therefore, we think that the PVP on the surface of catalysts was sufficiently removed by cleaning process used in this study. Fig. S3, S4, and S5 show STEM and STEM EDS images of the Ag nanoparticles obtained from reduction of AgNO 3 in the presence of TiO 2 nanoparticles after first MW heating, AgPd@Pd/TiO 2 (1 h), and AgPd@Pd/TiO 2 (2 h), respectively. Fig. S3 shows that spherical Ag nanoparticles with average diameter of 3.0±0.5 nm were formed. Fig. S4 and S5 show that AgPd@Pd nanocatalysts of AgPd@Pd/TiO 2 (1 h) and AgPd@Pd/TiO 2 (2 h) respectively had average diameter of 4.4±0.7 nm and 4.5±1.1 nm and about 0.6-nm-thick and 0.5 nm-thick pure Pd shell. The Pd/Ag atomic ratio in whole AgPd@Pd nanocatalysts of AgPd@Pd/TiO 2 (1 h) and AgPd@Pd/TiO 2 (2 h) was determined as 0.33±0.03 and 0.31±0.01 from STEM EDS analysis. S3
Preparation of bare AgPd@Pd nanoparticles Bare AgPd@Pd bimetallic nanocatalysts were also prepared using MW heating. 15 ml of distilled water containing 300 mg polyvinylpyrrolidone (PVP) and 12.26 mg AgNO 3 was heated at 95 C with MW irradiation at 120 W for 40 min under Ar bubbling. In the second step, 2 ml of distilled water containing 8.3 mg Pd(NO 3 ) 2 was added to this solution and heated with MW irradiation at 400 W for 30 min. The obtained AgPd@Pd nanoparticles were precipitated in acetone, washed by sonication for 30 min, and then dried. These processes were repeated twice. Finally, they were dispersed in distilled water. Hydrogen generation activity of Ag Pd and Ag Pd/TiO 2 nanocatalysts The hydrogen production activity of the prepared samples was examined using the following method: total gas volume from a stirred glass tube containing 20 ml of 0.25 M aqueous formic acid and the prepared sample (metallic catalyst weight of 5.1 mg) was measured using a gas burette. A schematic view of hydrogen production activity measurement system is shown in Fig. S6. The hydrogen gas volume as the production per gram of Ag Pd catalyst per hour was calculated using equations (S1) and (S2). x a = P atm V gas /RTn FA, (S1) Therein, x a is the conversion, P atm stands for the atmospheric pressure, V gas represents the generated volume of gas, R denotes the universal gas constant, T is room temperature (300 K), and n FA is the mole number of formic acid. R hydrogen = V gas /2m metal t, (S2) In this equation, R hydrogen represents the initial rate of hydrogen generation when x a reaches 20%, m metal is the weight of the metallic catalyst, and t is the reaction time when x a reaches 20%. For obtaining average value, measurements were taken at least three times. The H 2, CO 2, and CO gases were measured using a gas chromatograph (GC7100; J-Science): 20 ml of 0.25 M aqueous formic acid and the prepared sample (metallic catalyst weight of 1.02 mg) mixture were stirred for 30 min in a 110 ml glass tube filled with Ar gas. Then the atmosphere in the glass tube S4
was measured. When the H 2, CO 2, and CO concentrations were determined using GC, data were corrected using the standard gas, which was N 2 gas containing 50,000 ppm H 2, CO 2, and CO gases. Typical GC data obtained for Ag 93 Pd 7 @Pd/TiO 2 nanocatalysts, which gave the highest hydrogen generation activity in our present study, is shown in Fig. S8. If CO which reduces the catalytic activity of AgPd@Pd/TiO 2 was detected, a peak will be observed around a retention time of about 8 min. No CO peak was observed, leading us to conclude that the concentration of CO was below the detection limit (<10 ppm). Fig. S1 STEM and XRD pattern of TiO 2 nanoparticles: (a) STEM and (b) XRD. S5
Fig. S2 Temperature profiles of microwave heating process when (a) Ag nanoparticles were formed and (b) Pd shell were formed. Fig. S3 STEM image of Ag nanoparticles synthesized using microwave heating in the presence of TiO 2 nanoparticles. The nanoparticles having strong white contrast is Ag nanoparticles. S6
Fig. S4 STEM and STEM EDS images of the AgPd@Pd/TiO 2 (1 h) nanocatalysts: (a) STEM image, (b) Ag component, (c) Pd component, (d) Ag and Pd components, (e) all components, (f) line analysis data along the red line shown in Figure S4d. S7
Fig. S5 STEM and STEM EDS images of the AgPd@Pd/TiO 2 (2 h) nanocatalysts. (a) STEM image, (b) Ag component, (c) Pd component, (d) Ag and Pd components, (e) all components, and (f) line analysis data along the red line shown in Fig. S5d. S8
Fig. S6 XRD patterns of all AgPd@Pd/TiO 2 nanocatalysts within 2θ = 37 ~40. S9
Fig. S7 Schematic view of hydrogen production activity measurement system: 1 ml silicon oil was added to the surface of the water in the gas burette to prevent dissolution of H 2 to water. S10
Fig. S8 A typical GC graph for Ag 93 Pd 7 @Pd/TiO 2 as FA decomposition catalyst at 27 C. S11
Fig. S9 Plots of ln(r hydrogen ) vs 1000/T for (a) AgPd@Pd/TiO 2 (30 min), (b) AgPd@Pd/TiO 2 (1 h) and (c) AgPd@Pd/TiO 2 (2 h). S12