КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 5, с. 620-630
УДК 541.128:547.458.5
THE PROPERTIES OF Pd/Au BIMETALLIC COLLOIDAL CATALYSTS STABILIZED BY CHITOSAN AND PREPARED BY SIMULTANEOUS AND STEPWISE CHEMICAL REDUCTION OF THE PRECURSOR IONS
© 2013 M. Adlim1, *, M. A. Bakar2
1FKIP Program Studi Kimia Universitas Syiah Kuala Darussalam Banda Aceh 23111, Provinci Aceh Indonesia 2School of Chemical Science, University Sains Malaysia, 11800, Minden, Penang, Malaysia *E-mail: adlimbandang@yahoo.com, adlim@unsyiah.ac.id Received 28.06.2011
Bimetallic Pd/Au nanoparticles catalysts were prepared with chitosan as a stabilizer. The preparation procedure included mixing or stepwise adding palladium and gold ions in various molar ratios followed by simultaneous or stepwise reduction using either methanol or sodium borohydride (nb) as reducing agents. TEM and UV-Vis characterization showed that the particle size of bimetallic Chi-Pd/Au prepared by simultaneous reduction was smaller than that of the samples prepared by stepwise reduction methods. The particle size varied in the 1 to 24 nm range at all Pd/Au molar ratios of bimetallic compositions. Sodium borohydride was the most effective reducing agent for the preparation of bimetallic Chi-PdcoreAusheU by the stepwise reduction. The catalytic activities of Chi-Pd/Au prepared by either simultaneous or stepwise reductions were generally higher than those of the respective monometallic systems whereas the most active catalysts were prepared by the simultaneous reduction. Shielding the palladium metal colloid with gold sol led to the decrease in catalytic activity. The turnover frequencies (TOF-s) for Chi-Pd/Au-me in catalytic hydrogenation of 1-octene were as high as 20.855 and 89.336 for monometallic and bimetallic catalysts respectively. TOFs for Chi-Pd/Au-nb were in the region between 2.978 and 87.429. The core—shell and alloy formation of the bimetallic Chi-Pd/Au were inferred from the particle size measurements and evaluation of catalytic activity.
DOI: 10.7868/S0453881113050018
Chitosan or poly[P-(1—4)-2-amino-2-deoxy-.D-glucose] is a natural polymer that is biodegradable, nontoxic and has numerous applications in industrial and manufacturing processes [1]. It has been used as a protecting agent for colloidal catalyst preparations [2, 3].
Activity, the metal synergic catalytic effect, selectivity, resistance for deactivation and stability of catalysts based on bimetallic nanoparticles were described in many reports [4, 5]. The morphology of bimetallic nanoparticles also has a significant importance for designing electronic devices [6, 7]. Most studies on the bimetallic catalysts have been focused on the relationship between catalytic activities and the ratio of the metals in bimetallic clusters prepared by simultaneous reduction. Bimetallic core—shell structure of Pd-Au colloids has been prepared with various techniques, i.e. irradiation, calorimetric, sonochemical methods and using synthetic polymer or dendrimers [8—12]. It is interesting to compare catalytic properties of the colloidal alloy and the core—shell Pd/Au colloid in which Pd metal colloid is partially covered by Au sol in chitosan matrix. This comparison can help to elucidate a specific assembling of nanosized particles. The present work describes the effect of the way of reduction on the particle size and catalytic activities of chi-tosan-Pd/Au catalysts. Reduction procedures includ-
ed simultaneous or stepwise reduction of gold-palladium ions followed by progressive covering palladium sol with gold colloid stabilized by chitosan.
EXPERIMENTAL
Materials and Equipment
Sodium borohydride (95%, "Reidel de Haen", Germany), 1-octene (99.99%) and chitosan of medium molecular weight (-400000, "Fluka", Switzerland), PdCl2 (99.99%, "Merck", USA); HAuCl4 • 3H2O (99.5%, "Sigma", USA) and methanol (ACS certified grade, "Systerm", Malaysia) were purchased and used without further purification.
Philips CM 12 transmission electron microscope was used to obtain TEM micrographs for the determination of particle size and particle-size distribution. The particle diameters were measured using a computer softwar "analysis Docu 2.11" ("GmbH", Germany, 1986-1997). The average particle size and particle-size distribution were obtained from at least 300 particles. The results of calculations verified the earlier report [13]. A powder X-ray diffractometer, Siemens model D5000 (Cu^), was used to determine particle crystallinity. Samples preparations for powder
XRD characterization were prepared in a solid by pouring the chitosan-stabilized colloidal metal solution in acetone to obtain gel-like precipitates. The precipitate was washed several times with acetone-water mixture (1 : 1). The precipitate was smeared on a piece of silicon wafer (1 x 1 cm) and dried in vacuum prior to characterization.
Preparations of Colloidal Catalysts
Bimetallic Pd/Au metal colloid stabilized by chito-san is designated as Chi-Pd/Au. Methanol and sodium borohydride used as reducing agents are denoted by letters "me" and "nb", correspondingly. The bimetallic samples prepared by simultaneous reduction of the monometallic species taken at various molar ratios and employing various reducing agents are signified as Chi-Pd1Au1.5-me and Chi-Au1Pd1.5-nb. The bimetallic samples prepared by the stepwise metal reduction are labeled as Chi-Pd1_coreAu1_shell-nb or Chi-Au1_corePd1.5_shell-me. The subscript (numerical number) such as 1, 1.5 stands for molar ratios of each monometallic species. The molar fraction of the first metal in the bimetallic composition was held constant (2.8 x 10-5 mole) and is indicated by the numerical index "1". The molar fraction of the second metal was related to the fraction of first metal as l. The designation Chi-Pd1Au15-nb means, for instance, that the first metal is Pd and the mole content of Au is 1.5 times that of Pd.
Monometallic catalysts. Samples Chi-Pd-me, Chi-Pd-nb, Chi-Au-me and Chi-Au-nb were prepared according to earlier reports [3]. The amount of chitosan in each experiment was 60 mg and total volume of the solution was 45 mL. The concentrations of monometallic component were 1.4 x 10-5, 2.8 x 10-5 and 4.2 x 10-5 mole. Typically, 10 mL of solution containing 6 mg/mL of chitosan was diluted with 6 mL of a 1.5% (by volume) solution of aqueous acetic acid and 22.5 mL of methanol. Using a micropipette, a 1.24 mL (or 2.8 x 10-5 mole) of palladium chloride stock solution was added into the solution. The palladium was reduced either by reflux or by adding 5 mL of aqueous NaBH4, the amount of which was in a 25-fold molar excess to the metal. The pH of the solution was adjusted by adding a few drops of concentrated acetic acid and water to maintain the pH at around pH 4 and the total volume of 45 mL.
Bimetallic catalysts prepared by simultaneous reduction. Similar to monometallic preparative procedures and according to an earlier report [14], bimetallic Chi-Pd/Au was prepared by holding the molar content of one metal constant, while varying the content of other metals. For instance, Chi-Pd1Au15 was prepared by diluting 10 mL of 6 mg/mL of chitosan in 5 mL of a 1.5% (by volume) solution of aqueous acetic acid and 22.5 mL of methanol. A portion containing 1.24 mL (or 2.8 x 10-5 mole) of palladium chloride
stock solution was quickly mixed with 0.42 mL (or 4.2 x 10-5 mole) of gold stock solution and added to the chitosan solution. The bimetallic ions in solution were reduced either by reflux for 5 h or by adding 5 mL of NaBH4, the amount ofwhich was in a 25-fold molar excess to the amount of bimetallic ions. The pH of solution was adjusted by adding a few drops of concentrated acetic acid and water to maintain pH at around pH 4 and the total volume of 45 mL.
Bimetallic catalysts prepared by stepwise reduction. Stepwise preparations are similar to simultaneous reduction method except that the first metal was reduced prior to addition and reduction of the second metal. Typically, Chi-Pd1_coreAu15shell was prepared by diluting 10 mL solution of chitosan (6 mg/mL) in 5 mL of a 1.5% (by volume) solution of aqueous acetic acid and 22.5 mL of methanol. First, 1.24 mL (or 2.8 x x 10-5 mole) of palladium chloride stock solution was added into the chitosan solution. The palladium was reduced either by reflux for 5 h or by adding sodium borohydride (0.0224 g in 2.5 mL). For reflux method the Chi-Pd colloid was cooled down before addition of gold metal. First, 0.42 mL (or 4.2 x 10-5 mole) of gold stock solution was added to the chitosan solution-palladium colloid and reduced by reflux or addition of sodium borohydride (0.0357 g in 2.5 mL). The solution pH was adjusted at pH 4 by adding a few drops of concentrated acetic acid and water to maintain a total volume of 45 mL.
Catalytic Test on Hydrogenation of 1-Octene
Hydrogenations of 1-octene either with monometallic or bimetallic Pd/Au catalysts were performed at atmospheric pressure and 30°C in a closed 50 mL glass reactor as described in a previous study [2]. Exactly 5.3 mL solution of the colloidal catalyst was diluted with methanol (44.7 mL) and fed into the reactor. Hydrogen gas was fed several times to eliminate air and then the catalyst was activated for 60 min with vigorous stirring. The reaction was started with the injection of 0.5 mL of 1-octene (3.1 x 10-3 mole). The hydrogen consumption was monitored with a graduated gas burette. The reaction mixtures were sampled at different intervals and the samples containing catalyst were separated with a membrane filter prior to gas chromatography analysis (Hitachi G-3000) with FID at 20 m long carbo-wax capillary column (HP-20 M) maintained at 50°C.
RESULTS AND DISCUSSION
Spectrophotometry Studies
UV-Vis spectra of monometallic Chi-Pd and Chi-Au
were recorded and the observed spectrum of Chi-Au-me has a sharp maximum band at 525 nm as shown in Fig. 1. The broader bands of colloidal gold were observed in the spectra of the Chi-Au
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