Effect of Pd Nanoparticle Size on the Catalytic Hydrogenation of Allyl Alcohol

Abstract

We report that the rate of hydrogenation of allyl alcohol is a function of the diameter of the Pd nanoparticles (1.3-1.9 nm) used to catalyze the reaction. Furthermore, kinetic data indicate that this effect is electronic in nature for particles having diameters <1.5 nm, but for larger particles it depends primarily on their geometric properties. This is a significant finding, because it represents a particle size effect for hydrogenation over unsupported Pd nanoparticle catalysts in a size range that has not been widely studied.Dendrimer templates have been used to exert a high degree of control over the size, composition, and structure of catalytically active nanoparticles in the <3 nm size range. 1 Because of this, dendrimer-encapsulated nanoparticles (DENs) are well-suited for studying the effect of particle size on catalytic function. 1 We have previously briefly noted a qualitative link between the size of DENs and their catalytic properties. 2,3 This paper represents our first quantitative analysis of the origin of these trends.The procedure used to prepare Pd DEN catalysts has been previously reported. 2 For the experiments reported here, the total moles of Pd used for each hydrogenation reaction was maintained constant for all experiments, but the Pd/dendrimer ratio was varied to yield DENs having different sizes. Specifically, sixth-generation, hydroxyl-terminated polyamidoamine dendrimers (G6-OH) were used to synthesize Pd DENs containing an average of 55, 100, 147, 200, or 250 Pd atoms (G6-OH(Pd n ), where n is the average number of atoms per particle). These particles have measured (by TEM, Figures S1 and S2, Supporting Information) and calculated (in parentheses) diameters of 1.3 (1.2), 1.4 (1.4), 1.5 (1.6), 1.7 (1.8), and 1.9 (1.9) nm, respectively. 4,5 Alkene hydrogenation occurs via the Horiuti-Polanyi mechanism, which involves dissociative adsorption of H 2 onto the catalyst surface, followed by stepwise hydrogenation of the CdC double bond. 6 Here, the rate of hydrogenation of allyl alcohol was determined by measuring hydrogen uptake. 7 Briefly, 20.0 mL of the catalyst solution (3.0 µmol of Pd) was transferred to a Schlenk flask. The system was sealed and purged with H 2 for 10 min and then stirred for an additional 10 min. Allyl alcohol was added, and differential H 2 pressure measurements were obtained every 10 s for 10 min. 7 Turnover frequencies (TOFs, mol H 2 /mol active siteh) were determined from the slope of plots of turnover (mol H 2 /mol catalyst) versus time. Figure 1a shows that the rate of hydrogen consumption increases significantly as the measured particle diameter increases. However, this relationship does not correlate to a simple change in the total number of surface Pd atoms. This is because the total amount of Pd used in all these experiments is fixed, and therefore the number of catalyst particles present is smaller for the larger particles. This point is illustrated in Figure 1b. Here, the total number of Pd atoms present at particular locations on cuboctahedra are plotte...

Recent Advances in the Liquid‐Phase Synthesis of Metal Nanostructures with Controlled Shape and Size for Catalysis

“…Dendrimer templates have been also used to exert a high degree of control over the size of catalytically active nanoparticles in the ,3 nm range (51,52). Specifically, sixth-generation, hydroxyl-terminated polyamidoamine Nanocatalysis with Controlled Shape and Size dendrimers (G6-OH) at different Pd/dendrimer ratio were used to produce Pd nanoparticles of 1.3 ¡ 0.3, 1.4 ¡ 0.3, 1.5 ¡ 0.4, 1.7 ¡ 0.3, and 1.9 ¡ 0.2 nm.…”

“…The advances in colloidal preparation of metal nanostructures open new opportunities in the study of structure-sensitive catalytic reactions (6,24,51). Nanostructures of various shapes and sizes have become a powerful tool in revealing the reaction active sites due to a predominance of definite surface atoms on some nanostructures.…”

Formation and growth of palladium nanoparticles inside porous poly(4-vinyl-pyridine) monitored by operando techniques: The role of different reducing agents

“…As discussed in the introduction, alcohols are often used as solvents in reactions catalyzed by Pdbased catalysts [33], or they may be used even as reactants [34][35][36][37][38][39][40][41]. Therefore, it is important to clarify if they are innocent toward the catalyst, or if they may react with it and eventually lead to its final deactivation.…”

“…In other cases, alcohols are used as reactants. For example, alcohol hydrogenation, oxidation and cross-coupling reactions are also catalyzed by palladium-based catalysts, either homogeneous or heterogeneous [34][35][36][37][38][39][40][41]. In this respect, the homogeneous Pd(OAc)2/pyridine system is by far one of the most efficient and selective catalyst for oxidation of alcohols to aldehydes.…”

Ligand-assisted preparation of highly active and stable nanometric Pd confined catalysts for deep catalytic oxidation of toluene

“…Recent work suggested that highly dispersed Pd nanoparticles are extraordinarily active and may exhibit unique properties in a wide range of important chemical processes [2,14,15]. Various strategies were developed for incorporating metal nanoparticles into mesoporous silica, which could be generally classified into three groups: (1) chemical vapor deposition of metal precursors into the silica frameworks [16,17]; (2) wet impregnation or ion exchange of the appropriate metal salt into the host silica, followed by thermal treatment and reduction procedure [18,19]; and (3) synthesis of the mesoporous materials around the pre-prepared metal nanocrystals [20].…”