Dendrimer-Encapsulated Nanoparticle Precursors to Supported Platinum Catalysts

Lang, Hainan; May, R. Alan; Iversen, Brianna L.; Chandler, Bert D.

doi:10.1021/ja0364120

Cited by 276 publications

(316 citation statements)

References 26 publications

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“…Their catalytic ability has been studied extensively both as colloidal suspensions [2] and as oxide-or carbon-supported NPs [3,4]. The polymer mediated method of metal colloid synthesis can yield transition metal NPs with narrow particle size distribution and has the potential to serve as model systems for the elucidation of particle size effects in heterogeneous catalysis [5,6].…”

Section: Introductionmentioning

confidence: 99%

Dependence of Gas-Phase Crotonaldehyde Hydrogenation Selectivity and Activity on the Size of Pt Nanoparticles (1.7–7.1 nm) Supported on SBA-15

2008

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Running title: Influence of particle size on selectivity during crotonaldehyde hydrogenation 2 AbstractThe selectivity and activity for the hydrogenation of crotonaldehyde to crotyl alcohol and butyraldehyde was studied over a series of Pt nanoparticles (diameter of 1.7, 2.9, 3.6, and 7.1 nm). The nanoparticles were synthesized by the reduction of chloroplatinic acid by alcohol in the presence of poly(vinylpyrrolidone) (PVP), followed by encapsulation into mesoporous SBA-15 silica. The rate of crotonaldehyde hydrogenation and selectivity towards crotyl alcohol both increase with increasing particle size. , The selectivity towards crotyl alcohol increased from 13.7 % to 33.9 % (8 Torr crotonaldehyde, 160 Torr H 2 and 353 K), while the turnover frequency increases from 2.1 × 10 -2 s -1 to 4.8 × 10 -2 s -1 with an increase in the particle size from 1.7 nm to 7.1 nm. The decarbonylation pathway to form propene and CO is enhanced over the higher proportion of coordinatively unsaturated sites on the smaller nanoparticles. The apparent activation energy remains constant (~ 16 kcal mol -1 for the formation of butyraldehyde and ~ 8 kcal mol -1 for the formation of crotyl alcohol) as a function of particle size. In the presence of 130 -260 mTorr CO, the reaction rate decreases for all products with a CO reaction order of -0.9 for crotyl alcohol and butyraldehyde over 7.1 nm Pt particles; over 1.7 nm Pt particles, the order in CO is -1.4 and -0.9, respectively. Hydrogen reduction at 673 K after calcination in oxygen results in increased activity and selectivity relative to reduction at either higher or lower temperature; this is discussed with regards to the incomplete removal and/or change in morphology of the polymeric surface stabilizing agent, poly(vinylpyrrolidone) used for the synthesis of the Pt nanoparticles..3

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Section: Introductionmentioning

confidence: 99%

Dependence of Gas-Phase Crotonaldehyde Hydrogenation Selectivity and Activity on the Size of Pt Nanoparticles (1.7–7.1 nm) Supported on SBA-15

2008

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“…Both of these values are consistent with studies of other supported Pt catalysts prepared from DENs and with traditionally prepared Pt catalysts. 27,28,31 All of the bimetallic catalyst spectra ( Figure 1) were red-shifted 10-15 cm -1 from the high-coverage value. Infrared spectra attributable to CO bound to Cu sites (ν CO between 2180 and 2100 cm -1 ) were not observed, indicating that surface Cu is present as Cu 0 (vide infra).…”

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confidence: 97%

Tuning Supported Catalyst Reactivity with Dendrimer-Templated Pt−Cu Nanoparticles

2006

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The effects of particle composition on heterogeneous catalysis were studied using dendrimer-encapsulated nanoparticles (DENs) as precursors to supported Pt-Cu catalysts. Bimetallic Pt-Cu DENs with varying Pt/Cu ratios were prepared in an anaerobic aqueous solution and deposited onto a high-purity commercial alumina support. The dendrimer template was then thermally removed to yield supported nanoparticle catalysts, which were studied with toluene hydrogenation and CO oxidation catalysis as well as infrared spectroscopy of adsorbed CO. Incorporating Cu into Pt nanoparticles had opposite effects on the two test reactions. Cu acted as a mild promoter for CO oxidation catalysis, and the promoting effect was independent of the amount of Cu present. Conversely, Cu acted as a strong poison for toluene hydrogenation catalysis, and the normalized rate tracked inversely with Cu content. Infrared spectroscopy of the supported nanoparticles indicated that electronic effects (electron donation from Cu to Pt) were minimal for these materials. Consequently, the catalysis results are interpreted in terms of potential structural differences as a function of Cu incorporation and reaction conditions. IntroductionHighly dispersed supported metal nanoparticle catalysts are an important class of industrial materials, as one-third of material U. S. gross national product involves a catalytic process somewhere in the production chain. 1 State of the art heterogeneous catalysts often contain dopant metals, which are incorporated to promote a desired reaction, prevent undesirable side reactions, or enhance catalyst longevity. 2,3 Traditional preparative routes involve impregnating metal salts onto high-surfacearea supports, followed by various thermal activation steps. 3 Most important industrial supports can also be employed as separation media; consequently, chromatographic separation of salt precursors as they pass through the support pore structure is unavoidable. Further, nanoparticle preparation via traditional routes depends on the surface (and even gas phase) mobility of the species present during the thermal treatments. 4,5 These processes vary widely for different metals and are poorly understood, at best. 4,5 Consequently, bimetallic materials prepared by such methods are often complicated, have significant compositional inhomogenaity from one particle to the next, and are difficult to characterize.Recent advances in computational methods are beginning to shed light onto the fundamental properties of bimetallic catalyst systems, and predictive models are beginning to emerge. [6][7][8][9] The potential to controllably tune catalyst properties through the nanoscale design of bimetallic nanoparticles is becoming apparent, and designed bimetallic catalysts with novel behavior for important reactions already appear in the recent literature. [10][11][12] However, the utilization of traditional preparative techniques and the complicated materials that arise from these methods remain a significant hurdle to studying new catalysts, eva...

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“…Furthermore, recent reports that suggest more harsh conditions are required for complete dendrimer removal. 31,36,37 Regardless, Pt 40 proved to be the most robust catalyst and, after oxidative modification with PhICl 2 , maintained a steady rate of reaction, only to decrease as the reaction neared complete consumption of starting material ( Figure 2a). (Figure 2b).…”

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Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles

et al. 2009

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A continuing goal in catalysis is the transformation of processes from homogeneous to heterogeneous. To this end, nanoparticles represent a new frontier in heterogeneous catalysis, where this conversion is supplemented by the ability to obtain new or divergent reactivity and selectivity. We report a novel method for applying heterogeneous catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles. These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl 2 , and catalyze a range of π-bond activation reactions previously only homogeneously catalyzed. Multiple experimental methods are utilized to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously inaccessible in heterogeneous catalysis. Furthermore, our size and capping agent study revealed that Pt PAMAM dendrimer-capped nanoparticles demonstrate superior activity and recyclability compared to larger, polymer-capped analogues.The field of homogeneous catalysis can be characterized as a source of easily tuned, selective catalysts with high activity.Heterogeneous catalysts also offer many advantages, some of which are not displayed by their homogeneous counterparts, including recyclability, ease of separation from the reaction mixture and use in continuous flow processes. It is highly desirable to develop new systems that blend the many advantages of heterogeneous catalysis with the versatility of homogeneous catalysts. [1][2][3] Most efforts approach from the homogeneous side and are comprised of immobilized homogeneous catalyst species that promote reactions already accessible by homogeneous solution-state conditions. 1, 4 We sought to utilize an alternative method wherein heterogeneous catalysts are extended to reactions previously only catalyzed by homogeneous species. Metal nanoparticles (NPs) that serve as heterogeneous catalysts in which the metal particle size and oxidation state can be characterized are well-suited for our study. In addition, it is known that the reactivity and selectivity of monodisperse metal NPs can be altered by changes in their size and shape. [5][6][7][8][9][10][11][12][13][14][15] Although Pd and Au NPs have been demonstrated to catalyze a range of cross-coupling and oxidation/reduction reactions in solution, [16][17][18][19][20][21][22][23][24][25] studies have not yet yielded the control necessary to develop new NP catalysts applicable to a wider variety of reactions.A further challenge involves the specific nature of the catalyst. In particular, the distinction between homogeneous and heterogeneous catalysis is often difficult to determine. This is due to the possibility that metal leaches from a heterogeneous catalyst into solution and acts as the catalytically active species. Thus, it was important for us to consider in our studies the fact that for many of the...

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Dendrimer-Encapsulated Nanoparticle Precursors to Supported Platinum Catalysts

Cited by 276 publications

References 26 publications

Dependence of Gas-Phase Crotonaldehyde Hydrogenation Selectivity and Activity on the Size of Pt Nanoparticles (1.7–7.1 nm) Supported on SBA-15

Dependence of Gas-Phase Crotonaldehyde Hydrogenation Selectivity and Activity on the Size of Pt Nanoparticles (1.7–7.1 nm) Supported on SBA-15

Tuning Supported Catalyst Reactivity with Dendrimer-Templated Pt−Cu Nanoparticles

Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles

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