Prep a ra ti o n o f PAMAM- a n d P P I-Me t a l ( S i lv e r , P l a t i n u m ,
a n d P a l l a d i u m ) N a n o c o m p o s i t e s a n d T h e i r C a t a l y t i c
Ac t i v i t i e s f o r R e d u c t i o n o f 4 -N i t r o p h e n o l
Kunio Esumi,* Ryoko Isono, and Tomokazu Yoshimura
Dep ar tm en t of App li ed Ch em is tr y an d In st it u te of Col loi d an d In ter fa ce S cien ce,
Tokyo University of S cience, Kagurazaka, S hinjuk u-ku, T okyo 162-8601, Japan
R eceiv ed Au gu st 6, 20 03 . In Fi n al For m : Octob er 6, 20 03
Dendrimer -metal (silver, platinum, and palladium) nanocomposites are prepared in aqueous solutionscontaining poly(amidoamine) (PAMAM) dendrimers with surface amino groups (generations 3, 4, and 5)or poly(propyleneimine)(PPI)dendrimers with surface aminogroups (generations 2, 3, and 4). The particlesizes oft he metal nan oparticles obtained are almost independent ofthe generation as wella s the concentrat ionof the dendr imer for both the P AMAM an d th e PPI dendrimers; the a verage sizes of silver, platinum , andpalladium nanoparticles are 5.6-7.5, 1.2-1.6, and 1.6-2.0 nm, respectively. It is suggested that thedendrimer -metal n anocomposites are formed by adsorbing the dendr imers on th e meta l nan oparticles.Studies of the r eduction reaction of 4-nitrophenol by these n anocomposites show tha t the rat e consta ntsare very similar between PAMAM and PPI dendrimer -silver nanocomposites, whereas the rate constantsfor t h e P P I d en d r im e r-p la t i n u m a n d -pa lla dium na nocomposites a re gre a ter tha n those for thecorresponding PAMAM dendrimer nanocomposites. In addition, it is found that the rate constants for thereduction of 4-nitrophenol involving all the dendrimer -metal n anocomposites decrease with an increasein t he dendr imer concentrat ions, an d t he catalytic activity of dendrimer -palladium nanocomposites ishighest.
I n t r o d u c t i o n
Recently,dendrimer-metal nan ocomposites have beenintensively prepared and characterized from the stand-point ofcatalyticpr operties1-7 and mulitifilm formation 8-10
using poly(amidoamine) (PAMAM) dendrimers. In thecatalytic reactions, it has been found that the catalyticactivity of PAMAM dendrimer -metal nanocompositesdepends significantly on the generation of the dendrimeras well as the sur face fun ctional groups of th e dendrimer.The cata lytic activities of PAMAM dendrimer -metal
na nocomposites ha ve been cha ra cterized by two differen tnan ocomposite stru ctur es;one is that metal nan oparticlesare formed inside th e dendrimers as higher generationdendrimers (G3, G4) with surface hydroxyl groups areused.1,7 The other is that when using dendrimers withamino or carboxyl groups th e dendrimer-metal nano-comosites are formed by adsorption of dendrimers on themetal nan oparticles.6 Unt il now, only th e catalytic activi-ties for PAMAM dendrimer -metal (gold, platinum , pal-ladium, platinum -palladium) have been studied.
Like PAMAM dendr imers , poly(propyleneimin e) (PP I)dendrimers ha ve interior tertiary amine groups, but th eydo not conta in am ide groups. The size of PPI den drimerswith surface am ino groups is smaller than t ha t of PAMAMdendr imers of th e sam e genera tion. These differences willaffect the formation of metal nanoparticles as well as thestabilization of metal nanoparticles. F urth er, t hus ob-tained dendrimer -met al nan ocomposites are expected toexhibit differen t cat alytic activities with the differen t kindsof the dendrimer.
In t he present s tu dy, we report on the preparat ion of
dendrimer -metal nanocomposites such as silver, plati-num, and palladium in the presence of PAMAM or PPIdendrimers with surface aminogroups.F urth ermore, thecata lyticr eduction of 4-nitr ophenol by these dendrimer -
metal nan ocomposites is also investigated.
E x p e r i m e n t a l S e c t i o n
Materials. PAMAM dendrimers were prepared according tot h e l it erat u re.11 PPI dendrimers were purchased from AldrichCo. AgNO3, H 2PtCl4, and Na 2PdCl4 were pur chased from ShowaChemical Co., Wako Pur e Chemicals Co., and Kan to ChemicalsCo., r espectively. 4-Nitrophenol was obtained from KantoChemicals Co. All other chemicals were of analytical grade.Mil li -Q wat er was u s ed i n al l ex p eri m en t s. Th e p h ys i calchara cteristics of the PAMAM and P PI dendr imers a re given in
Table 1.12
P r e p a r a t i o n o f D e n d r i m e r -M e t a l N a n o c o m p o s i t e s . Th epreparation of metal nanoparticles in an aqueous solution wascarried out by the chemical reduction of a met al salt -dendrimermixtur e with sodium borohydride. For a t ypical experiment , 0.2cm 3 of freshly prepar ed 20 mmol dm -3 metal salt was added to19.7 cm 3 of dendrimer aqueous solutions of various concentra-
* Author to whom correspondence should be addressed.(1) Zhao,M.;Crooks,R. M. Ad v. Ma ter. 1999, 11 ,217; An gew . Ch em .,
In t Ed . 1999, 38 , 364.(2) Rahim, E. H .; Kamounah , F. S.; Frederiksen, J.; Christensen, J .
B. Nano Lett. 2002, 1, 449.(3) Ooe, M.; Mura ta, M.; Mizugaki, T.; Ebitani, K.; Kaneda , K. N a n o Let t . 2002, 2, 999.
(4) Scott, R. W.; Dat ye, A. K.; Crooks, R. M. J. Am . Chem. S oc. 2003,125 , 3708.
(5) Esum i, K.; Miyamoto, K.; Yoshimur a, T . J . Colloid In terface Sci.2002, 254 , 402.
(6) Esumi, K.; Sa toh, K.; Suzuk i, A.; Torigoe, K. Sh ikizai Kyokaishi2000, 73 , 434.
(7) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938.(8) He, J .-A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C.;
Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A.Chem. Mater. 1999, 11 , 3268.
(9) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mullen, K.;Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551.
(10) Esumi,K.;Akiyama, S.;Yoshimura, T. La ng m ui r 2003 , 19 ,7679.
(11) Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.; Kallos, G.;Martin, S.; Roeck, J .; Ryder, J .; Smith, P. Polym. J . 1985, 17 , 117.
(12) Crooks, R. M.; Chechik, V.; Lemon, B. I.; Sun, Li.; Yeung, L. K.;Zhao, M. In Metal Nanoparticles, Synthesis, Characterization, an d
Ap pl ica ti ons ; Feldheim, D. L., Foss, C. A., Eds.; Marcel Dekker: NewYork, 2002; Chapter 11.
tions, and t he solutions were stirr ed for 1 h. Th en, 0.1 cm
3
of 0.4mol dm -3 freshly prepa red ice-cold sodium borohydride wa squickly added to t he solutions under stirring for 30 min. Themetal nanoparticles obtained were analyzed by transmissionelectr on microscopy (TEM) a nd UV-visible a bsorption spec-troscopy. TEM observat ion wa s performed for t he sa mples driedon carbon-coated copper grids. A Hitachi H -9000 NAR tra nsm is-sion electron microscope was operated at an accelerating voltageof 200 kV and direct magnification of 200 000×. T h e s i z edistribution of the metal nanoparticles was determined from atleast about 200 particles.
C a t a l y t i c R e d u c t i o n o f 4 - N i t r o p h e n o l . I n t h e s t a n da r dquar tz cuvette with a 1-cm pat h length, 1.4 cm3 ofwa ter , 0.3 cm3
of dendr imer -meta l na nocomposites solution, and 0.3 cm 3 of 2mm ol dm -3 4-nitrophenol were taken.Then,addition and propermixing of 1 cm 3 of aqueous 0.03 mol dm -3 sodium borohyride tothe rea ction mixtures caused the decrease in the inten sity of the
peak of 4-nitr ophenol. The a bsorption spectra wer e recorded ona U V-visible spectrophotometer (Hewlett-Packard 8453A)every5 s in t he ra nge of 190-820 nm at 15 °C.
The concentrations of both the PAMAM and the PPI den-drimers r eferred t o that of the surface amino groups.
R e s u l t s a n d D i s c u s s i o n
Before reporting the catalytic activity for dendrimer -
metal nanocomposites, the dendrimer -metal nanocom-posites obtained have been characterized.
D e n d r i m e r-Silve r N anoc om pos ite s . Figure1showsrepresentative UV spectra of the PPI dendrimer (G4)-
AgNO 3 aqueous solution before and after reduction withsodium borohydride. The TEM image and particle sizedistribution of the PP I dendrimer-silver na nocompositesobtained a re also shown in F igure 1. After the reduction,a typical plasmon band of silver nanoparticles appears ataround 400 nm. All of the other systems for both thePAMAM and th e PPI dendr imers show spectra similar tothat of the PPI dendrimer (G4). It should be mentionedthat the particle sizes of silver nanoparticles obtainedusing both the P AMAM and the P PI dendrimers ran gebetween 5 and 7 nm , despite different generations of thedendrimers.
D e n d r i m e r-P l a t i n u m N a n o c o m p o s i te s . Figure 2shows representat ive UV-v is ib le s p ect r a of t h e P P Idendrimer (G4)-H 2PtCl6 aqu eous solution before an d afterreduction with sodium borohydride. The TEM image andpart icle size distribut ion oft he PP I dendrimer -platinum
na nocomposites are also given in Figure 2. The absorptionband at 260 nm, assigned to platinum ions, disappearsafter the reduction, and a broad band is observed in awide wavelength region, indicating the format ion of platinum nan oparticles. Similar spectra are obtained forall the other systems. The particle sizes of platinumnanoparticles obtained using both the PAMAM and thePPI den drimers r ange between 1.2 and 1.6 nm, which isa l m os t i n d ep en d e n t of t h e g en e r a t ion a s w el l a s t h econcentr ation ofth e dendrimer. It is noteworth y tha t stableplatinum nanoparticles could be obtained using PP Idendrimers (G3 an d G4) at their lower concentrationscompared to PAMAM dendr imers (G3 an d G4).
D e n d r i m e r-P a l l a d i u m N a n o c o m p o s i t e s . Figure 3shows representat ive UV-visible spectra of the PPI
dendrimer (G4)-Na 2PdCl4 aqueous solution before an dafter redu ction with sodium borohydride. The TEM imageand particle size distribution of the PPI dendrimer -
palladium n an ocomposites ar e alsogiven in Figur e 3.Afterthe reduction, the intensity of a broad absorption band
T a b l e 1 . P h y s i c a l P r o p e r t i e s o f P A MA M a n d P P I
D e n d r i m e r s U s e d
m olecu la r w eigh t d ia m e te r (n m )
generationsurfacegr ou p P AMAM P P I P AMAM P P I
Figure 1. (a) UV-visible spectr a of the PP I dendr imer (G4)-
AgNO3 aqu eous solution before an d after r eduction with sodiumborohydride: [AgNO3] ) 0.2 mmol dm -3; [NaBH 4] ) 2 mm oldm -3;[dendrimer] ) 5 mm ol dm -3. (b) TEM microgragh a nd (c)particle size distribution ofsilver nanoparticles after reduction.
23 8 L an gm u ir, Vol. 20, N o. 1, 2004 E su m i et al.
increases and the solution turns to clear dark brown,suggesting the formation ofpalladium na noparticles. Thepart icle sizes of palladium nan oparticles obtained u singboth PAMAM and PPI dendrimers range between 1.6 and
2.0 nm , which are almost independent oft he concentr at ionas well as the generation of the dendrimer. Similarly tothe platinum nanoparticles, s table palladium nanopar-
Figure 2. (a) UV-visible spectra of the PPI dendrimer (G4)-
H 2PtCl6 aqu eous solution before an d after red uction with sodiumborohydride: [H2PtCl6] ) 0.2 mmol dm -3; [NaBH 4] ) 2 mmoldm -3; [dendrimer] ) 5 mmol dm -3. (b) TEM micrograph and(c) particle size distribution of platinum nanoparticles afterreduction.
Figure 3. (a) UV-visible spectr a of the PP I dendr imer (G4)-
Na 2PdCl4 aqueous solution before and after reduction withborohydride: [Na 2PdCl4] ) 0.2 mmol dm -3; [NaBH 4] ) 2 mmoldm -3; [dendrimer ] ) 10 mmol dm -3. (b) TEM micrograph and(c) par ticle size distr ibution of palladium nan oparticles a fterreduction.
PAMAM - and PPI - M eta l N an ocom pos it es L an gm u ir, Vol . 20, N o. 1, 20 04 23 9
ticles are obtained using PP I dendrim ers (G3 and G4) attheir lower concentrations compared to PAMAM den-
drimers (G3 and G4).Tables 2 and 3 show the average particle size and the
standard deviation of the metal n anoparticles with thedendrimer concentrations. I t is found that the averagep a r t icl e s ize s of t h e m e t a l n a n o pa r t i cl es a r e a l m os tindependent oft he concentrat ion oft he dendrimer as wella s t h e g e n e r a t i o n f o r b o t h t h e P A M A M a n d t h e P P Idendrimers. Interestin gly, one can see that monodisperseplatinum and pa lladium n anopart icles are obtained usingPPI dendrimers r ather than PAMAM dendrimers, whereasmonodisperse silver nanoparticles are not obtained forboth PAMAM and PPI dendrimers.
It is essentially important to discuss the structure of dendrimer-metal nanocomposites obtained. In the presentstu dy, becau se the pHs of mixed solutions before reductiona r e a t a r o u n d 6-7 , t h e s u r f a c e a m i n o g r o u p s o f t h edendrimers are pr otonat ed.13 Accordingly,m etal ions su chas PtCl6
2- and PdCl42- are expected to bind preferent ially
to the surface amino groups, resulting in the formationof th e dendrimer -metal complex. By th e redu ction withsodium borohydride, dendrimer -metal nan ocompositeswould be formed by adsorbing th e dendr imer m oleculeson the meta l nanopa rt icles. Alth ough th e Ag+ ions are notexpected to interact s t rongly with the dendrimer, den-drimer molecules would be covered on the silver nano-particles to stabilize the silver nanoparticles after thereduction. However, relatively large silver nanoparticleswith a large standard deviation are only obtained as aresult of a weak int eraction of silver na noparticles with
the dendrimers. These stru ctur es are alsosu pported fromthe results ofFourier tran sform infrared measur ements.14
From the size of dendrimer itself shown in Ta ble 1, it iseasily un derstood th at the m etal na noparticles could notbe formed inside the PPI dendrimers because t he den-drimer sizes ar e similar or smaller than t he nan opart icles.Compar ison of PAMAM and PPI dendrimers shows thatstable platinum an d palladium n an oparticles are obtainedat lower concentra tions when using PPI dendr imers th anwhen using PAMAM dendrimers. From th e fact tha t PP Idendrimers are soluble in water, dimethylformamide, and
dichlorometh an e but PAMAM dendrim ers ar e not solublein dichlorometh ane, it is suggested that t he hydrophobicityofth e PP I dendrimers is greater tha n tha t of the PAMAM
dendrimers. Accordingly, there is a great possibility thatthe PP I dendr imers could be pr eferentially adsorbed onthe metal nanoparticles compared to PAMAM dendrimers,although a competitive adsorption experiment was notcarried out. As a result, stable dendrimer -metal nano-composites could be obtain ed at lower concentr at ions usingPPI dendrimers.
C a t a l y t i c R e d u c t i o n o f 4 - N i t r o p h e n o l . F i g u r e 4shows representative successive UV-visible spectra of the reduction of 4-nitrophenol by PPI dendrimer (G4)-
platinum nanocomposites. Similar spectral changes arealso obtained for all the other dendrimer -metal nano-composite systems. In the absence of any catalysts, thepeak due to 4-nitrophenol at 400 nm remains unaltered.Addition and proper mixing of an aliquot of dendrimer -
a The dots indicate in wh ich cases no data were obta ined as a r esult of the inst ability of the part icles; the dashes indicate in which casesthe experiments were not carried out.
F i g u r e 4 . Successive UV-visible spectra of the reduction of 4-nitrophenol by the PP I dendr imer (G4)-platinum nan ocom-posites: [4-nitrophenol] ) 0.1 mmol dm -3; [platinum] ) 0.01mmoldm -3;[dendrimer] ) 0.25 mm oldm -3;[NaBH 4] ) 10 mmoldm -3.
24 0 L an gm u ir, Vol. 20, N o. 1, 2004 E su m i et al.
platinum nanocomposites to the reaction mixture causethe fading and ultimat e bleaching of the yellow color of
the 4-nitrophenol solution. Because the amount of th edendrimer -platinu m na nocomposites added is very small,the absorption spectra of4-nitr ophenolis ha rdly interferedwith by the dendrimer -platinum nanocomposites. Thereduction can be visualized by th e disappeara nce of the400-nm peak with the concomitan t a ppearan ce of a n ewp ea k a t 2 90 n m . T h is p ea k h a s b ee n a t t r ib u t ed t o4-aminophenol. 15 The concentration of the borohydrideion great ly exceeds tha t of4-nitr ophenol and th e cat alysispart icles.Th e excess ofsodium borohydride used in creasest h e p H o f t h e r e a c t i n g s y s t e m , t h e r e b y r e t a r d i n g t h edegradation of the borohydride ions, and the liberatedhydrogen purged out air , thereby checking the aerialoxidation of the reduced product of 4-nitrophenol. Fur-ther more, as t he concentrat ion of sodium borohydride is
very high, i t remains essentially constant during thereaction. Therefore, in t his case pseudo-first-order kineticswith respect to 4-nitrophenol could be used to evaluatethe catalytic rate. A good linear correlation with time,that is , log A versus t ime plot, is obtained for all thesystems stud ied (Figure 5).Her e, A stands for absorbancea t a n y t i m e t . The values of the pseudo-first-order rateconstant (k ) determined from these plots are given inTables 4 and 5. Also, the values of the rate constant withthe concentration of dendrimer are plotted in Figure 6.One can see tha t th e rate consta nts for all the systems aredecreased with an increase in the concentration of thedendrimer for both th e PAMAM and th e PPI dendr imers.It is noteworthy 16 that the dendrimers transform fromstarlike to spherelike entities where th ere is a gradualtransition from G3 to G10 and the distribution of thedendrimer su rface groups is not uniform t hroughout t heinterior of the dendr imer. Accordingly, it seems t hat th eshape ofth e dendrimer is an importan t factor in stabilizingthe m etal na noparticles an d affects t he catalyticr eaction.T h a t i s, t h e e a r ly g en e r a t ion s of P A MAM a n d P P Idendrimers (G2 and G3) possess a highly asymmetric,open, hemispherical shape, while the later generations(G > 4)possess a near ly spherical shape with dense-packedsur faces. These dendr imer sh apes would provide differentdendr imer adsorption layers on th e meta l par ticles, which
affect the catalyticreaction. However, in th e present stu dy,a distinct correlation between the r ate consta nts a nd th egeneration of both th e PAMAM and t he PP I dendrimersis not observed. I t is interesting to note that the rate
const an ts for PP I (G3 and G4)-
palladium nan ocompositesa n d -platinum nanocomposites are greater than thosefor th e corresponding P AMAM (G3, G4, an d G5) nan o-composites at the same dendrimer concentration.
The rate of electron transfer at the metal surface canbe influenced by two steps: (1) diffusion of 4-nit rophen olto the metal surfaces and (2) interfacial electron transferand diffusion of 4-aminophenol away from the surface.The observed rate constant may be expressed as 17
where R is the radius of the metal particles, D i s t h e(15) Pradhan , N.; Pal, A.; Pal, T. Colloids Surf., A 2002, 196 , 247.(16) Caminati, G.; Turro, N. J.; Tomalia, D. A. J . Am. Ch e m. S o c.
1990, 112 , 8515. (17) Gr a t zel, M.; F r a nk, A. J . J. Phys. Chem. 1982, 86 , 2964.
T a b l e 4 . R a t e C o n s t a n t s f o r 4 -N i t r o p h e n o l R e d u c t i o n b y P A MA M D e n d r i m e r-M e t a l N a n o c o m p o s i t e s ( 1 0 -4 s -1) a
a The dashes indicate in which cases the experiments were not carried out.
T a b l e 5 . R a t e C o n s t a n t s f o r 4-N i t r o p h e n o l R e d u c t i o n b y P P I D e n d r i m e r-M e t a l N a n o c o m p o s i t e s ( 1 0 -4 s -1) a
silver pla t in u m pa lla diu mconcn of PP I(mmol dm -3) G2.0 G3.0 G4.0 G2.0 G3.0 G4.0 G2.0 G3.0 G4.0
a The dots indicate in wh ich cases no data were obta ined as a r esult of the inst ability of the part icles; the dashes indicate in which casesthe experiments were not carried out.
F i g u re 5 . Plots of ln A versus time for the reduction of 4-nitrophenol by the PP I dendr imer (G4)-platinum nan ocom-posites obtained: (a) [dendrimer ] ) 5 mmol dm -3; (b) [den-drimer] ) 10 mmol dm -3; (c) [dendrimer ] ) 20 mmol dm -3.
1/ k obs ) [1/(4π R2)][(1/ k et ) + ( R / D)] (1)
PAMAM - and PPI - M eta l N an ocom pos it es L an gm u ir, Vol . 20, N o. 1, 20 04 24 1
diffusion coefficient , an d k et is the ra te const an t for electr ontransfer. When heterogeneous charge transfer is fasterth an diffusion, eq 1 redu ces to th e Smoluchowski expres-sion:
This assumpt ion could be att ributed to th e higher drivingforce for particle-mediated electron transfer caused bytheir large Fermi level shift in the presence of highlyelectron injecting species such as borohydride ions. It issupposed th at the diffusion of 4-nitrophenol and theparticle size mainly control the rate of the reduction.Because P AMAM or P PI dendr imers with su rface aminogroups are considered t o adsorb on the metal n anopar-ticles, th e adsorbing dendr imers would affect th e diffusionof 4-nitrophenol to the metal nanoparticle surface. Ac-cordingly, it is suggested tha t t he ra te consta nts dependon the coverage of the dendrimer on the metal particle
surface under almost the same particle size. This ideaprovides tha t a s th e coverage by the dendr imer increaseswith an increase in the concentration of the dendrimerthe ra te constant s should be reduced with th e dendrimer
concentration. In addition, because the size of the PPIdendrimer is smaller than t hat of the PAMAM dendr imerat th e sam e generation t he diffusion of 4-nitrophenol tothe metal n anopart icle surface would be m uch faster forthe PPI dendrimer th an th e PAMAM dendrimer, resultingi n t h e h i g h e r r a t e c o n s t a n t s f o r t h e P P I d e n d r i m e r -
platinum and -palladium nanocomposites. Thus, for thedendrimer -platinum and -palladium nanocomposites itseems that the size of the dendrimer (PAMAM or PPI)adsorbed on the metal nanoparticles controls predomi-nantly the reduction of 4-nitrophenol.
F i n a ll y, i t i s i n t er e st i n g t o com p a r e t h e ca t a l yt i cactivities for th e redu ction of 4-nitr ophenol by t he kindof metal of the dendrimer -metal nanocomposites. Theorder of the rat e constant s for both t he PAMAM and t he
Figure 6. Plots oft he ra te constan t for t he reduction of4-nitrophenol bydendr imer-meta l na nocomposites again st th e concentr at ionof the su rface amino group of the dendr imers: (a) dendrimer -silver nanocomposites; (b) dendrimer -platinum nanocomposites;(c) dendrimer -palladium nan ocomposites.
k obs ) 4π DR (2)
24 2 L an gm u ir, Vol. 20, N o. 1, 2004 E su m i et al.
PPI dendrimers is palladium > platinum > silver. Thu s,th e origin oft his order clear ly lies in th e different na tu resof th e dendrimer -metal nanocomposites employed be-cau se the resu lting par ticle sizes and su rface compositionsar e different. Th e reason su ggested for t he low activity of the dendrimer -silver n anocomposites is th at th e silvernanoparticles are often easily oxidized and their particlesizes are larger than the other metal n anoparticles.
C o n c l u s i o n s
S il ve r , p la t i n u m , a n d p a ll a di u m n a n op a r t i cl es a r eprepared in th e presence of PAMAM or PP I dendrim erswith various genera tions an d concentr at ions. The particlesizes of the m etal na noparticles th us obtained a re almostunaltered with th e dendrimer generation as well as thedendrimer concentr ation. The pa rticle sizes and part iclesize distributions of the platinum and palladium nano-particles are much sma ller an d n arrower than those of
th e silver nan opart icles for both t he PAMAM and th e PPIdendrimers.
From the studies of the reduction reaction of 4-nitro-phenol, it is found that the change in the rate constantsfor the dendrimer -silver nanocomposites is not clearlyobserved by the kind of the dendrimer as well as thegeneration, while the rate constants for the PPI den-drimer -p la t i n u m a n d -palladium nanocomposites aresignificantly greater than those for the PAMAM den-drimer -p l a t i n u m a n d -palladium nanocomposites. In
addition, the rat e constan ts for all th e systems decreasewith increasing dendr imer concentra tion. This decreaseis attr ibuted to the increase of the den drimer a dsorptionon the metal nanoparticles. Among the three differentmetal nanocomposites, the PPI dendrimer -palladiumna nocomposites show th e highest catalyt ica ctivity for thereduction of 4-nitrophenol.
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