Catalysis is a surface phenomenon which involves only metal atoms exposed on the surface of metal particles. Tiny metal particles are shown to exhibit properties which are significantly different from those of bulk metals. Gold particles provide the best example to illustrate this point. For example, while large gold particles are completely inert for many reactions, gold particles with sizes below 3 nm are very active for CO oxidation.221-223 In general, metal particles with sizes in the nanometer range have higher dispersion and hence provide larger amount of active sites on which the reaction can occur.224 The concentration of low coordinated sites, such as steps and kinks, is relative abundant in nanomaterials and these sites usually provide low activation barriers for the adsorption of reactants.66,225,226
Conventional catalyst preparation techniques involve impregnation of metal salts (wet impregnation, incipient wetness) followed by thermal treatments and often result in wide size distributions of metal particles.227 When used for preparations of bimetallic systems, these traditional techniques yield catalysts containing both monometallic and
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bimetallic particles with varying compositions. The use of well-defined organometallic cluster complexes as precursors helps to overcome these problems.228 Molecular cluster complexes typically have well-defined structures that can be retained throughout catalyst preparation steps.229 Unfortunately, limited number of such compounds is commercially available. Furthermore, the advantage of their structural uniformity can be lost after the activation step on which different treatments at elevated temperatures are used to remove ligands.57 The synthesis of colloidal metal nanoparticles has made it possible to synthesize monodisperse and nearly uniform metal nanoparticles with dimensions in the 1-10 nm range.230-232 These colloidal nanoparticles can be used as heterogeneous catalysts in the same way as molecular clusters.1,202,203 Their preparation process involves the reduction of metal salts in the presence of stabilizers and the following immobilization of the colloidal solution onto the surface of a porous oxide support.233-235
Among such stabilizers, Poly(amidoamine) (PAMAM) dendrimers have recently attracted a lot of attention because these macromolecules offer a variety of functional groups and have interior voids which can be used to trap and protect metal nanoparticles in solution until they are delivered to a support.8,70,233 These features offer an opportunity to control the architecture and the size of metal nanoparticles in solution and maximize the uniformity of active metal sites in supported catalytic materials. While the synthesis of metal-dendrimer nanocomposites incorporating noble metals has been reported in literature, a limited number of reports describing the preparation of such nanocomposites from base metals is available.91 Furthermore, little is known about the complexation of the base metal cations with PAMAM dendrimers, the strength of metal- dendrimer interactions, and how these interactions can be effected by preparation conditions.75
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Despite the advantages of using dendrimers as nanoparticle stabilizers, the uptake of metal cations Mn+, where Mn+ = Cu(II), Fe(III), Au(III), Ni(II), Co(II), Mn(II), Zn(II) and Ag(I), by dendrimers with non-complexing terminal groups, such as PAMAM-OH, is still not well understood. From a more fundamental point of view, little is known of the effects of dendrimer tertiary amine/amide and terminal groups’ chemistry on the metal cation binding capacity and selectivity of PAMAM dendrimers in aqueous solutions.236 Moreover, it has been reported in literature that although it is not possible to prepare Ag particles inside PAMAM-OH dendrimer by direct reduction of interior cations, stable dendrimer-encapsulated Ag particles can be prepared by a metal displacement reaction.78,236 In this approach, dendrimer encapsulated nanoparticles (DENs) prepared from a particular metal such as Cu, can be exchanged with Ag as long as the latter metal is more noble than the former. Thus, key unanswered questions are whether the attainment of the Cu0-Ag+ displacement reaction depends on the solution pH and the successful reduction of Cu2+ cations inside thedendrimer solution.
This research project is dedicated to a systematic investigation of the evidence of metal cation/dendrimer complexation, the determination of the complexation time, as well as the effects of metal cation acidity, the solution pH and metal cation/dendrimer loading on the extent of binding (EOB) and fractional binding (FB) of metal cations Mn+ to EDA core, fourth generation hydroxyl terminated PAMAM dendrimers (G4OH). Furthermore, we will focus on the Cu0-Ag+ intradendrimer displacement reactions followed by variations of the solution pH at different stages of the preparation procedure, with an overall goal to gain a better understanding of the intradendrimer displacement reaction preparation technique. The following specific objectives are proposed for this project:
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1. Determination of the complexation time of the Mn+/G4OH system and effect of the
metal cation concentration on the Mn+/G4OH complexation time. The
complexation step involves interactions between the metal cations and the tertiary amine and amide groups of the dendrimer, which will be monitored in the present work by using UV-vis spectroscopy. Data available in literature suggest that cations of most metals do not complex with dendrimers instantly and UV-vis spectroscopy is used most often to examine the complexation kinetics.88 Therefore, the UV-vis absorbance of the band that is responsible for the specific Mn+/G4OH interaction will be recorded over time in order to establish the optimal time for the completion of the Mn+/G4OH complexation reaction. Finally, the effect of Mn+/G4OH concentration on the complexation time will be addressed by changing the metal-to-dendrimer ratios, followed by characterization by UV-vis spectroscopy.
2. Investigation of the strength/weakness of the metal cation - G4OH interaction.
To achieve this goal, Mn+/G4OH solutions with different molar ratios will be prepared, complexed at the appropriate amount of time and their solution pH will be recorded. The Mn+/G4OH solutions will be subsequently dialyzed using benzoylated cellulose tubing to remove the uncomplexed metal cations. The percentage of the metal cations retained inside the dialysis sack over the initial amount of Mn+ used (Fractional Binding - FB), as well as the experimentally obtained Mn+/G4OH molar ratio (Extent of Binding – EOB) will be evaluated as a function of dialysis time. Both FB and EOB will give us an indication of the strength of Mn+/G4OH interaction. The concentration of Mn+ cations retained
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inside the dialyzed solution will be measured using Atomic Absorption Spectroscopy (AAS).
3. Significance of interior functional groups of the G4OH dendrimer in the
complexation process. In order to address this goal the EOB of G4OH-(Mn+)30 will be compared with the corresponding EOB of G4NH2-(Mn+)30 solutions after 1 day of dialysis. Comparing the EOB of metal cations with dendrimers containing complexing and non-complexing terminal groups, such as G4NH2 and G4OH, respectively, will give us an indicating for the degree of participation of the interior functional groups of the dendrimer in the complexation process.
4. Determination of the stage throughout the DENs preparation procedure that
dialysis should be applied. It has been reported in literature that dialysis can be
applied prior or after treatment with NaBH4 of the Mn+/G4OH complexed solutions.88,189 In order to determine when it is most appropriate to perform the dialysis step, G4OH-(Mn+)30 solutions will be prepared for Ag+, Co2+, Ni2+, Cu2+, and Mn2+ and dialyzed before the performance of the reduction step. The obtained FB will be compared with the FB of the reduced solution, undergoing dialysis for the same period of time. This data will allow us to understand whether the dialysis procedure should be applied prior or after treatment with NaBH4 of the Mn+/G4OH solutions.
5. Analysis of the effect of pH on the ability of G4OH dendrimers to complex with
metal cations in aqueous solution. In order to investigate the effect of
concentration of metal cations on the binding ability of G4OH dendrimer, the pH of Cu2+/G4OH and Fe3+/G4OH solutions will be fixed to 6.1 and 7.0, respectively, for varying metal/dendrimer molar ratios. The resulting solutions
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will be dialyzed and the EOB/FB of Cu2+, Fe3+/G4OH with and without pH adjustments will be compared. It is expected, that increase in the solution pH will increase the amount of available binding sites in the interior of the dendrimer (deprotonation), leading to subsequent increase of the metal cation uptake by G4OH dendrimers.
6. Fundamental understanding of the Cu0-Ag+ intradendrimer displacement
reactions. In order to achieve this goal Cu2+/G4OH solutions with different molar ratios will be prepared, followed by appropriate pH adjustments and treatment with NaBH4, in order to obtain (ideally) reduced G4OH-(Cu0) nanocomposites. After the completion of the reduction step, Ag+ cations will be added in the G4OH-(Cu0) solution. Assuming that the intradendrimer displacement reactions are taking place, Ag, being nobler than Cu, will be capable of displacing it. The resulting solutions will be dialyzed and the concentration of Ag and Cu will be recorded (AAS) as a function of dialysis time. Finally, the influence of the initially used Cu2+/G4OH molar ratio as well as the amount of Ag+ added in the solution on the experimentally obtained Ag+/G4OH molar ratio, will be examined systematically.