Removal and Utilization of Capping Agents in Nanocatalysis

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Removal and Utilization of Capping Agents in Nanocatalysis Zhiqiang Niu, and Yadong Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4022479 • Publication Date (Web): 03 Sep 2013 Downloaded from http://pubs.acs.org on September 6, 2013

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Chemistry of Materials

Removal and Utilization of Capping Agents in Nanocatalysis Zhiqiang Niu1, and Yadong Li1,2* 1

2

Department of Chemistry, Tsinghua University, Beijing 100084, China

State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China *

Corresponding author. E-mail: [email protected] Abstract

Capping agents are frequently used in colloidal synthesis to inhibit nanoparticles over-growth and aggregation as well as to control the structural characteristics of the resulted nanoparticles in a precise manner. Study of the effect of the residual capping agents on particle surface has unveiled various adverse and favorable behaviors in catalytic applications. In essence, while the capping agents usually act as a physical barrier to restrict the free access of reactants to catalytic nanoparticles, they can also be utilized to promote catalytic performance of nanocrystals. Due to the complexity of these effects, a general survey of capping agents in nanocatalysis is therefore necessary. This short review starts from a brief introduction of common capping agents in nanoparticle synthesis and their adverse impact on heterogeneous catalysis. Next, representative progresses in capping agent removal and surfactant-free synthesis for obtaining surface-clean nanocatalysts are summarized. Lastly, we discuss the recent advance in utilizing the capping agent effect including chiral modification, molecular recognition, adsorption regulation, surface crowding, and charge transfer at the metal-organic interface and so on to improve the catalytic performance of

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nanocatalysts. Keywords: nanoparticles, catalysis, capping agents, activity, selectivity 1. Introduction Catalysis has changed and is still changing our world. The vital role of catalysis in modern society is shaped by transportation fuel supply, vehicle emissions control, production of low-toxic pesticide, artificial fertilizer, high-strength polymers, various pharmaceuticals, as well as many other chemicals. It is estimated about 35% of global GDP (gross domestic product) and over 90% of chemical products in worldwide rely on catalysis.1 A majority of industrial heterogeneous catalysts are small nanoparticles (less than 20 nm) that are highly dispersed on solid supports with high surface area. The classic preparation of these catalysts on an industrial scale usually includes three steps, namely, introduction of metal precursors on supports by impregnation or precipitation, calcination in oxidative atmosphere and reduction in hydrogen flow.2 The catalysts produced in this way are generally poor-defined with varied facets and sizes, which conceals the genuine catalytically active species. Therefore, the development of new catalysts with improved activity, selectivity, or durability, is most often based on try-and-error approach rather than rational design. In the past five decades great attempts have been made in using single crystal surface as a model system to understand the nature of heterogeneous catalysts. A combination of surface analysis techniques, such as low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), sum frequency generation


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spectroscopy (SFG), scanning tunneling microscope (STM) and so on has shown to be powerful tools.3 For instance, the catalytic mechanism of ammonia synthesis over iron (Haber-Bosch process) was detailed at molecular level by Gerhard Ertl, a pioneering study that has won him the 2007 Nobel Prize in Chemistry. Reaction mechanisms pictured by surface chemistry provide us valuable information in the quest of better catalysts. Nevertheless, this kind of information is typically obtained from single crystal surface in ultrahigh vacuum, different from the actual reaction conditions. Considering colloidal nanoparticles are closer mimics of real catalysts, much attention has been put on nanocatalysis to bridge the gap between laboratory discovery and industrial realization.4 The development of colloidal synthesis endows us with the ability to precisely tailor the structural characteristics (e.g., size,5,6 shape,7-9 composition10-12) of nanoparticles. And by changing only one characteristic at a time, a versatile platform is built to study and disclose the structure-property relationships of nanoparticle catalysts, just as what have done in the study of benzene hydrogenation over Pt with well-defined shapes13 and oxidative-addition-promoted

leaching

mechanism

of

Suzuki

reaction

over

monodispersed Pd with poor crystallinity.14 Colloidal nanocrystals are synthesized and stabilized in solution with the help of organics or polymers which could bind on particle surface. These binding molecules are often denoted as either surfactants, ligands or capping agents in literatures. Figure 1 shows the structures of representative capping agents in nanoparticle synthesis, such as oleylamine (OAm), oleic acid (OA), trioctylphosphine (TOP), dodecanethiol,



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cetyltrimethylammonium bromide (CTAB), poly(N-vinyl-2-pyrrolidone) (PVP), polyvinyl alcohol (PVA), Poly(amido amine) (PAMAM), just to name a few. As for catalytic application, the capping agents at the shell of nanocrystals are just like “organic armor” blocking reactants to approach the metal surface. Moreover, the presence of capping agents also brings extra complexity to the system, such as uncertain coverage density of capping molecules, non-covalent interaction between capping molecules and reactants, as well as charge transfer at the organic-metal interface. In this regard, an increasing research interest has been focused on the weakness and strength of capping agents on catalysis. In this short review, we first briefly discuss the key role of capping agents in the colloidal synthesis of nanocrystals and their adverse impact on catalysis. For obtaining surface-clean nanocatalysts, we summarize various physical and chemical methods for the removal of capping agents as well as the surfactant-free syntheses of nanoparticles. While the “organic armor” outside the metal core is unfavorable in many catalytic cases, with quantitative analysis and elaborate design, capping agents can be also utilized to promote catalytic performance of nanocrystals, which will be thoroughly discussed in the last section. We conclude this review by proposing several crucial issues to be addressed in the future with regard to the removal and utilization of capping agents.

2. Capping Agents in Colloidal Synthesis Colloidal nanoparticles are typically synthesized in solution by reacting


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molecular metal precursors with appropriate reducing agents in the presence of capping agents to stabilize the high-energy surface of the nanoparticles and protect them from aggregation. It should be noted that capping agents paly a versatile role in colloidal synthesis of nanoparticles other than being the stabilizers. For instance, capping agents often act as ligands to form complexes with original metal precursors and thereby affect their reduction kinetics.15-18 An extreme case is that if the reduction of metal-ligand complexes is substantially retarded, the seeds will take a thermodynamically unfavorable plate-like shape through a random hexagonal close packing (rhcp) of atoms, which is a typical case of kinetic controlled growth.8 In the study of OAm-mediated shape evolution of Pd nanocrystals, Pd(acac)2 (metal precursor) first reacted with OAm (capping agent) to form an intermediate complexes expressed as [Pd(II)(acac)x(OAm)y] under room temperature stirring. Formaldehyde was then added to initiate the reduction of [Pd(II) (acac)x(OAm)y], and the reduction rate was gradually slowed down along with the increased amount of OAm coordinated to Pd(II) center. As a result, the final shape of Pd nanocrystals evolves from polyhedrons to kinetically controlled plates.19 Another critical role played by capping agents is their selective adsorption on particular crystallographic planes and therefore induces the anisotropic growth of nanocrystals.20-23 Functional groups in capping agents interact with unsaturated surface atoms through dynamic adsorption and desorption. Their binding affinities to surface are dependent on distinct atom geometries of different facets. When crystal seeds grow into bigger particles, the atom deposition is limited on strongly bound



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facets and promoted on weakly bound ones. As a result, the facets with a slower growth rate will be preserved and exposed in the final products.24 In order to demonstrate this mechanism, Xia and co-workers have designed a rigorous experiment in which the synthesis of silver nanocrystals was performed under identical conditions except for the choice of capping agents (Figure 2).25 They found the use of PVP led to Ag octahedrons enclosed by eight {111} facets while the use of sodium citrate (Na3CA) produced Ag cubes/bars bound by six {100} facets.

3. Adverse Influence of Capping Agents on Catalysis Nanocrystals synthesized by colloidal methods can be defined as a core-shell structure: a hard inorganic core surrounded by a soft organic shell (i.e. capping agents). The catalytic properties of metallic nanocrystals intrinsically derive from the inorganic cores and can be determined by their size,26 shape,27 composition,28 spatial distributions of different elements,29 etc. The outer capping agents ensure the homo-dispersity of nanoparticles in solution, which is beneficial for further processing. But in many cases, the presence of capping agents is troublesome for catalytic investigations. Capping agents act as a physical barrier to restrict the free access of reactants to catalytically active sites on particle surface. Figure 3 shows three typical conformations of capping agents on particle surface, which are closely related to their molecular structures: (1) linear long-chain hydrocarbons (e.g., OAm, OA, CTAB) stand in an array on particle surface with a tilt angle, (2) unbranched polymers (e.g.,


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PVP, PVA) entangle nanoparticles with partial functional groups attached on their surface, and (3) branched polymers and dendrimers (e.g. PAMAM, PEI) encapsulate nanoparticles in their inner void space. Although it remains unclear that how these different conformations influence the access of reactants to catalyst, it is certain that the out layer organics are detrimental to this process. Compelling evidence can be found in the measurement of electrochemical active surface area (EASA), in which the residual capping agents on nanoparticles have to be cleaned up by electrochemical polishing in order to fully expose the active surface.30,31 For reactions to be conducted in solution, the polarity of capping agents has to be considered in solvent selection. Solvation of capping agents means the surrounding of each capping molecule by a shell of solvent molecules. Since the capping agents are dynamically adsorbed and desorbed from the catalyst surface, a fully solvated capping molecule is tightly stretched by solvent molecules and thereby more frequently detach from catalyst surface. On the contrary, poorly solvated capping agents will shrink to a compact structure and stifle catalysts. An awkward situation is frequently encountered when conducting an organic reaction with nanoparticles as the catalyst: the polarity of reactants and substrates is incompatible to that of capping agents outside of catalytic nanoparticles, resulting in the reduced activity.32 Capping agents can also act as poisons to partly or fully dampen the catalytic metal cores. Sulfur is widely recognized as a poison in hydrogenation or dehydrogenation reactions employing reduced metals as the active phase. It has been demonstrated that Pd nanoparticles stabilized by alkanethiols are composed by Pd(0)



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cores and a submonolayer of sulfide species (PdSx).33 Another notable adverse impact is that capping agents are sometimes reactive in catalytic conditions, which not only consumes other reacting substances but also leads to the structural damage of themselves.34 Moreover, the variation of coverage density of capping agents during the washing procedure (see Section 4) may cause reproducibility problems in the catalytic study. In general, capping agents bring extra complexities for the study of nanocatalysis. We still have little picture about their accurate conformation, packing density, interactions with surrounding chemicals, and so on. Access of surface-clean nanoparticles will simplify the problem, which will be discussed in following section.

4. Surface-Clean Nanocatalysts The so-called “surface-clean” nanoparticles are not truly naked but free of formidable long-chain organics. Surface-clean nanoparticles are stabilized by small molecules which are easy to be displaced by reactants during catalytic reactions. These small molecules include small adsorbates that are intentionally added for exchange of long-chain organics, solvent molecules, solute ions and even gases from the nanoparticle growth or storage surroundings. Two typical ways to obtain surface-clean nanoparticles have been developed and are discussed in detail as the following. Capping Agents Removal. As a routine procedure, freshly lab-prepared colloidal nanoparticles are repeatedly washed by a large amount of solvent and


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collected by centrifugation. Hutchings and co-workers have shown that PVA on Au nanoparticles could be effectively removed through a simple solvent washing procedure.35 However, unsupported nanoparticles of less than 5 nm, a typical size region for solid catalyst, are difficult to precipitate under regular centrifugation during the washing treatment. This phenomenon can be illustrated by Boltzmann distribution.36 A rough mathematic explanation is given as the following. Suppose we have a tube of nanoparticle solution in centrifuge field. Set the potential energy at the bottom of the tube as the zero-point (i.e., ground state). The system energy when a nanoparticle at the height of h above the tube bottom is given by Equation 1: Eh = (m – msol) Gh = ∆ρVGh

(1)

Where m and msol represent the mass of nanoparticle and solvent, respectively; G is the centrifugal acceleration; ∆ρ equals to the density contrast of nanoparticle and solvent; V is the volume of the nanoparticle. According to Boltzmann factor, the probability of a nanoparticle at the height of h can be given by Equation 2: P(Eh) = exp(-βEh) = exp(-β∆ρVGh)

(2)

Where β equals to (kBT)-1, and kB is Boltzmann constant, and T is temperature. The relative concentration of Pd nanoparticles of different sizes at the height of 1 cm above the centrifugal tube at a rotational speed of 5000 rpm is shown in Figure 4A (red line). It is clear that if the volume of a naked nanoparticle is too small, their probability at the height of 1 cm will be exponentially increased. In practice, capping-agents-stabilized small nanoparticles are treated by



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poor/good solvent mixtures (e.g., ethanol/hexane for OAm removal, acetone/water for PVP removal). Specifically, small nanoparticles are first dispersed in good solvent with their capping agents fully solvated and easy to detach from particle surface. When appropriate amount of poor solvent is added, the stretched capping molecules on particle surface shrink and form a more compact structure as depicted in Figure 4B. According to Equation 3, these huddled nanoparticles statistically have better chance to appear at the bottom of the centrifuge tube because of the decreased volume. In other words, small nanoparticles become easier to precipitate by centrifugation with the addition of poor solvent. P(Eh) = exp(-βEh) = exp[-β(m – msol)Gh] = exp[-β(m –ρsolV)Gh]

(3)

Where ρsol represents the density of solvent. Although excess capping agents could be stripped off from nanoparticles by extensive washing, there are always residuals resistant to removal. Three strategies have been developed to deal with this problem. The first strategy is to decompose capping agents into small molecular fragments by heat or light, followed by sweeping them off by gas flow or solvent. Methods including high-temperature thermal annealing and UV–ozone (UVO) irradiation adopt this tactic. In the removal of PVP (M.W. 55 k) from Pt cubes, it has been demonstrated that PVP decomposes at 473– 623 K when calcinated in 20% O2/He, and these Pt nanocubes start to change their shape at 623 K under the vacuum condition as shown by in-situ transmission electron microscopy (TEM) study.37 The removal of PVP is also investigated by UVO irradiation. The UVO irradiation seems to decompose PVP to pyrrolidone monomers


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on Pt nanoparticles, and after the treatment, the morphology of nanoparticles preserves but their surface atoms are partially oxidized.38 UVO irradiation on PVP-capped Pd nanocubes appears to generate COads on particle surface from the decomposition of PVP and no detectable N from PVP was found by X-ray photoelectron spectroscopy (XPS) analysis after a 4h treatment.39 Nikles and co-workers have tried to remove OAm on Pt nanoparticles by both thermal treatment and UVO irradiation.40 Different from PVP, OAm can hardly fall off from Pt under UVO irradiation. But two types of thermal treatments (185 oC in air and 400 oC in H2/Ar) work efficiently in removing OAm. Further comparison of the two thermal treatments indicates that considerable particle sintering occurs at 400 oC in H2/Ar, while treatment at 185 oC in air does not affect the size distribution of the sample. Moreover, no apparent oxidation of Pt was detected by XPS after the mild-temperature annealing. Amiridis and co-workers investigated the thermal decomposition

and

removal

of

PAMAM

dendrimers

from

supported

Pt

nanoparticles.41 Fourier transform infrared (FTIR) spectra indicate that the PAMAM first break into adsorbed carboxylate species at 200 oC and then these carboxylates desorb from the catalyst surface at 450 oC. Their study also shows that the second stage is catalyzed by Pt. It should be noted that the partial decomposition of organic compounds during thermal treatment or UVO irradiation may generate coke and other adsorbed poisoning substances that deactivate the catalyst.42 Subsequent oxidation–reduction cycles are necessary for catalyst activation. Zaera and co-workers treated residual



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carbonaceous deposits on cubic Pt by calcination and reduction at 625 K (1 h for each half cycle under 1 mL/s flow of either O2 or H2).27 FTIR and XPS spectra studies have shown that the hydrocarbon residues are all removed and the oxidized Pt observed in the fresh sample notably become less. The second strategy is to weaken the interaction between the capping agent and nanoparticles, facilitating their detachment. Amine-capped nanoparticles are usually treated with pure acetic acid under gentle heating. The protonated amino-groups will lose their strong affinity to metal surface and become easier to wash off.43 Stamenkovic, Sun, and co-workers treated OAm-capped Pt nanoparticles with three different methods, including thermal annealing at 185 oC in air, acetic acid washing, and UVO irradiation. Using electrocatalytic oxygen reduction reaction (ORR) as a probe, the efficiency of OAm removal was systematically examined, which follows the trend of thermal annealing > acetic acid washing > UVO treatment.30 Another widely used approach for capping agents cleaning is to displace long-chain hydrocarbons by excess small molecules which have competitive capping ability. Since small molecules have much lower boiling point and mass, it is not difficult to get rid of them either by extensive washing or vacuum evaporation. 1-butylamine is frequently used in such ligand exchange reactions to displace OAm.44 Meanwhile, it has been demonstrated that Pd cubes free of PVP and Br− can be achieved by ligand exchange with tert-butylamine.45 It should be noted that acetic acid washing and ligand exchange may cause the leaching of base metals (e.g., Ni, Fe, Pb, Zn, Cu) in alloys when treated in air. Very recently, Zhang and co-workers have


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developed an universal method for the removal of organothiols, thiophene, adenine, rhodamine, halide ions (Br− and I−), and PVP from Au nanoparticles by reacting with hazard-free sodium borohydride.46 Computational studies indicate that these adsorbates can be displaced by hydride generated from sodium borohydride because of its stronger binding affinity to Au nanoparticles. Electrochemical cleaning is also employed to deal with the surfactant on nanoparticles. It was established that colloidal Pt nanoparticles can be cleaned by cycling the electrode between 0.05 and 1.0 V until a stable voltammogram was obtained.47 But this procedure was much less effective for Pd-based nanoparticles because of Pd dissolution during the positive scan. Therefore, Solla-Gullón and Aldaz used a modified cleaning method for PtPd alloy.48 In sulfuric acid solution, the electrode was held at a constant potential of 0.03 V for 3 min. The potential was then cycled between 0.05 and 0.4 V. After repeating this procedure for at least three times, CO was adsorbed on the nanoparticles by holding the electrode at a potential of 0.03 V for 2 min with CO bubbling, followed by CO stripping from the surface. Surfactant-Free Synthesis. Now that the long-chain hydrocarbons could be replaced by small absorbates as demonstrated in ligand exchange treatment, directly using small absorbates as the stabilizer in nanoparticle synthesis would be a more straightforward way. As early as 1987, Klabunde and co-workers employed acetone, ethanol, and other organic solvents to solvate Pd atoms and produced 6–8 nm colloids which are stabilized by solvation effects and electrostatic repulsion.49 Later, Wang and co-workers reported the preparation of a series of “unprotected” metal clusters (Pt, Ru,



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Rh) in ethylene glycol with sodium hydroxide (NaOH).50-52 These “unprotected” nanoparticles are free of usual capping agents but capped by glycol molecules and OH– anions. Along this line, N,N-dimethylformamide (DMF)-protected Au,53-55 Cu,56 Pt,57,58 and Pd59 clusters were also successfully prepared. Notably, Obora and co-workers showed that DMF-capped Pd nanoclusters had an extremely high TON (6.0 × 108) in Suzuki coupling reactions with a catalyst loading of only 10-7 mol%.62 Beyond the size control realized in above examples, very recently the surfactant-free synthesis has been expanded to the shape control of nanoparticles. Octahedral and truncated-octahedral Pt-Ni nanocrystals were prepared in DMF through solvothermal synthesis.60,61 The anisotropic growth of Pt-Ni may be induced by functional groups (-CONMe2 and -CHO) derived from DMF. The as-obtained Pt-Ni nanoparticles display activities of at least 9 times higher than the state-of-art Pt catalyst in ORR. Intentionally adding selective adsorbates, typically gas molecules, into the surfactant-free synthetic system can also tailor the shape of nanoparticles. Oxalate-capped Pt nanocrystals with different shapes (cubes, tetrahedrons, and cubooctahedrons) were synthesized by reducing Pt(II) and Pt(IV) compounds with hydrogen.62 When exposed in air, the oxalate-capped Pt nanoparticles connect and fuse into nanowires, which hinted the facile escape of the small absorbates. Pt nanocubes immobilized on carbon support were prepared using cysteamine or CO as the shape-regulating agent.63,64 Liu, Zhang, and co-workers reported the synthesis of dendritic Pt in the presence of NaOH and ascorbic acid (AA).65 The same group then prepared star-like and concave Pd by introducing potassium bromide (KBr) to the


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previous synthetic system.66

5. Utilization of Capping Agents in Catalysis Organometallic compound, which features metal-carbon bond between a metal and organic ligands, is prominent for its versatile catalytic properties that can be elegantly tailored. The organic ligands coordinated to a metal induce the steric and electronic effect, further directing the access orientation and interaction of reactants with catalyst. Such a precise control over the reaction pathway is partly absent in heterogeneous catalysis. Colloidal nanoparticles resemble organometallic catalysts in terms of their metal-organic core-shell structure. The promoting effect of capping agent in nanocatalysis has been observed in several studies,67-71 but the knowledge deficiency of the behaviors of capping agents on metal surface makes it a challenging task to rationally take advantage of capping agents in nanocatalysis. A few studies have been done in attempts to fill this much-needed gap. Behaviors of Capping Agents on Particle Surface. To identify the binding sites of capping agents on nanoparticle is the first question to be resolved. Somorjai and co-workers have investigated the interaction of PVP with Pt by deep UV-Raman and FTIR spectroscopy, which indicates that PVP molecules stick to particle surface through the formation of C═O─Pt bond.72 It is also pointed out that the conformation of PVP shell can be altered by changing their molecular weight and cross-linking degree (Figure 5A). Huang and co-workers find the binding sites of PVP on Pd nanocrystals are dependent on the particle size and the PVP concentration.34 Similar



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to PVP-capped Pt, PVP molecules stand on small Pd nanoparticles (< 8.4 nm) through the chemisorption of carbonyl oxygen atoms on metal surface (Figure 5B, left). However, PVP molecules crouch on large Pd nanoparticles (> 10.9 nm) with oxygen and nitrogen atoms being attached on the surface (Figure 5B, right). The different capping patterns of PVP strongly affect their structural stability. Standing PVP remains mostly intact, whereas the crouching PVP may undergo hydrolysis because their N─C bonds in N─C═O are greatly weakened when chelated with Pd surface. Moreover, the charge transfer from PVP to Pd renders the metal core electron-enriched, potentially capable of altering its catalytic performance. With the aid of computational chemistry, exploring the molecular conformations of capping agents on metal surface becomes possible. Many chiral ligands-modified metal nanoparticles have been applied in asymmetric catalysis.73-75 Baiker and co-workers investigated the adsorption behavior of cinchona-type ligands with different rotational flexibilities on Pt and Rh by means of attenuated total reflection (ATR)-IR spectroscopy and DFT calculations.76 It is shown that rigid β-isocinchonine (β-iCN) mainly take a tilted orientation when adsorbed on both Pt and Rh, whereas flexible cinchonidine (CD) and cinchonine (CN) predominantly adopt a flat orientation on Pt, both independent of the presence of hydrogen. However, when CD and CN are adsorbed on Rh in the presence of hydrogen, the flat quinoline rings are rapidly hydrogenated and thereby only tilted species are observed. The same group later explored the adsorption geometry of 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) on Pd nanoparticles by comparing their experimental and simulated IR


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spectra.77 It is revealed that BINAP is adsorbed on the Pd {111} surface with its C2 symmetry axis perpendicular to the surface, wherein the interaction mainly arise from a pair of phenyl rings that are parallel to Pd surface but not the two P atoms (Figure 6). Such an interaction is not as strong as expected, especially at elevated temperatures or with other strong co-adsorbates, which further explains why a large amount of BINAP is needed in the asymmetric catalysis. The quantification of coverage density of capping agents is another critical issue to be addressed for nanocatalysis study. Reported quantitative studies of ligand adsorption on metal surface are mainly motivated by biomedical application of Au nano-materials. Zhang and co-workers have determined the binding constant and packing density of mercaptobenzimidazole

on Au nanoparticles

with an

isotope-encoded surface-enhanced Raman spectroscopy (SERS) reference method, by measuring the amount of unbound ligand in the centrifugal supernatant of the Au-ligand mixture and fitting the binding data to the Langmuir adsorption equation.78 In the surface modification of Au nanoparticles by sulfur heterocyclic compounds, Granados, Coronado, and co-workers have demonstrated the surface coverage depends on the interplay between the geometry of ligand molecules and the diameter of nanoparticles on the basis of the signal shifts in localized surface plasmon resonance (LSPR).79 They find the packing density of ligand molecules is almost independent to nanoparticle diameter, but becomes greater along with the chain length of thione increases. The latter observation is explained as a result of enhanced interchain Van der Waals forces with longer thione molecules. An opposite chain



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length-dependence was observed by Lämmerhofer and co-workers.80 In their studies, the packing density of thiol-containing ligands on Au nanoparticles was determined by inductively coupled plasma mass spectrometry (ICP–MS). It is found that the ligand density decrease significantly with an increase of the chain length of either hydrophobic mercapto-alkanoic acid or hydrophilic mercapto-(PEG)n-carboxylic acid (Figure 7A). It is postulated that the increments of tilt angle, gauche defects, and entropic contributions for longer ligands lead to their decreased coverage density. As the organic shell of nanoparticles, capping agents not only interact with the metal core but also communicate with other adsorbates from nearby surroundings, such as solvent molecules, reactive substrates, dissolved gases, etc. The interaction between capping agents and these adsorbates can have a profound influence on catalyst property. It has been shown that the binding energy of acetate anions on Pd {111} surface is weakened by the non-specific interaction with water.81 It is reasonable to postulate that capping agents can also change the binding energies of reactive substrates on catalyst surface via different non-covalent interactions. However, experimentally determining the interactions of capping agents with adsorbates is quite challenging. Recently, by using single molecule total internal reflection fluorescence (smTIRF) microscopy, Schwartz and co-workers have investigated the dynamics of a fluorescently labeled dodecanoic acid molecule on n-alkyltriethoxysilanes-modified hydrophobic surface.82 Two characteristic diffusive modes, namely short-lived flying and long-lived crawling, were observed, suggesting the interaction between the probe molecule and the hydrophobic surface could be


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either superficial or intercalated (Figure 7B, left). Moreover, systematically increasing the chain length of n-alkyltriethoxysilanes (from n = 4−18) leads to decreased fraction of short-lived diffusion (Figure 7B, right). Capping Agents-Promoted Catalysis. Although our knowledge of the capping agents’ behaviors on particle surface is still very limited, the qualitative exploration of the promoting roles of capping agents on catalysis has taken one step ahead. Generally, their promoting roles can be classified into five categories: (1) chiral modification, (2) molecular recognition, (3) adsorption regulation, (4) surface crowding effect and (5) charge transfer. It should be noted that the promoting effects of capping agents are sometimes roughly designated as “selective poisoning” in literatures.83,84 This nomenclature is based on the fact that addition of capping agents decreases the activity of a catalyst but causes a great improvement of its selectivity. Cinchona alkaloids such as CD modified Pd and Pt nanoparticles have received extensive study in asymmetric hydrogenation of α-ketoesters.85,86 The origin of the high enantioselectivity (up to 95%) of this catalytic system is believed to be the stoichiometric interaction between the pro-chiral reactant and the anchored chiral modifier to favor specific adsorption geometry of the reactant (Figure 8).87,88 Besides asymmetric hydrogenation, enantioselective allylic alkylation was realized by chiral diphosphite-capped Pd nanoparticles.89 BINAP-modified polymer-incarcerated Rh/Ag nanoparticles were applied to asymmetric 1,4-addition of arylboronic acids to enones.90 Somorjai, Toste, and co-workers recently reported asymmetric olefin cyclopropanation

catalyzed

by

a

chiral

self-assembled


monolayer

(SAM)


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encapsulated Au nanoclusters which were immobilized on mesoporous SiO2 (MCF-17) support.91 Spectroscopic measurements indicate that the improved enantioselectivity is related with the hydrogen bonding network in the chiral SAM. Metalloenzymes are composed of small metallic clusters surrounded by large proteins. Their high catalytic specificity results from the molecular recognition of substrates by periphery proteins. With an analogous metal-organic structure, nanocatalysts have been demonstrated to be capable of mimicking enzyme to some extent (Figure 9).92 In silane alcoholysis reactions, an unprecedented activity enhancement (TOF up to 55000 h-1) of alkanethiol-SAM-capped Au nanoparticles was observed. The hydrophobic interactions between alkanethiol-SAM and substrates increase the apparent concentration of substrates in the vicinity of Au surface, leading to the accelerated reaction rate. And the matching degree between the size of substrates and the interstrand space of alkanethiols decides the reaction selectivity among a large scope of substrates. The molecular recognition in this catalytic system is constructed by the collaboration between hydrophobic interactions and the complementary sizes. In the selective hydrogenation of alkynes, it is found that the competitive adsorption of reactants (alkyne and alkene) and capping agents on Pt-based catalyst determines the activity and selectivity of catalyst.93 Alkene is the desired product in the semi-hydrogenation of alkyne, and how to prevent its further hydrogenation to alkane is a key for accomplishing high selectivity. As shown in Figure 10, when the adsorption energy of capping agents (1-octylamine) on the catalyst surface is higher


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than that of alkenes (4-octene and 3-hexene), the reaction proceeds with high selectivity. However, if the adsorption energy of capping agents is lower than that of desired alkene (1-octene), they do not promote the selectivity of hydrogenation of alkyne to alkene at all. Surface crowding effect refers to the dependence of catalyst properties on the packing density and packing order of capping agents on metal surface.94 It should be noted that surface crowing effect is usually a superficial phenomenon, and the underlying mechanism could be quite different for case to case. Medlin and co-workers have systematically investigated the role of SAM on the selective hydrogenation of unsaturated epoxides.95-97 Compared to uncapped catalyst, n-alkanethiol SAM-capped Pd catalysts exhibit a dramatically enhanced selectivity (from 11 to 94%) in the hydrogenation of epoxybutene to epoxybutane. Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) studies indicate that the metal-sulphur interactions can induce relative change in the adsorption energy of olefin and epoxide moieties and create a kinetic barrier for epoxide ring-opening reactions (Figure 11A). S-dodecylthiosulfate-capped Pd nanoparticles could catalyze both the isomerization and hydrogenation of allyl alcohol, depending on the conformation of the capping agents in different solvents (Figure 11B). S-dodecylthiosulfate adopts outstretched or huddled conformations in nonpolar or polar protic solvents, respectively. The varying steric hindrance near the nanoparticle surface of the two conformations determines the reaction pathways via either Markovnikov or anti-Markovnikov addition to yield propanal and 1-propanol,



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respectively. In the selective hydrogenation of carbonyl group of α,β-unsaturated aldehydes catalyzed by amine-capped Pt3Co nanoparticles, the reaction selectivity can be promoted by using amines with longer chain length.98 Previous studies have revealed that the packing density as well as the packing order of capping ligands is proportional to their chain length due to the increased interstrand hydrophobic interactions.79 The packing density and packing order further determine the site accessibility when the reactants are approaching the catalyst surface. Herein, the adsorption of end carbonyl group is more favored than internal ethene group on the dense and highly ordered long-chain amine-capped Pd surface (Figure 11C), resulting in the selective production of cinnamyl alcohol. In another piece of study, dodecylamine (DDA)-capped Pt showed an enhanced selectivity in the selective hydrogenation of acetylene to desired ethylene, compared to ligand-free Pt. It is believed such an enhancement results from the blocking of large Pt ensembles, which are catalytic active for the production of undesired ethane, by the amine molecules.99 The coverage density of capping agents also influences the catalytic reaction through site isolation effect.100 In the selective oxidation of benzyl alcohol to benzaldehyde, chemisorbed capping agents on Au nanoparticles can isolate adsorbed benzaldehyde from nearby benzyl alcohol, suppressing the production of undesired benzyl benzoate (Figure 11D). The charge transfer at metal-organic interface will modify the electronic structures of the catalyst. It has been shown that the d-electron distribution in Au NPs is altered by capping agent selection: dendrimer-capped Au atoms obtain 5d electrons


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due to the weak interaction while thiol-capped Au atoms lose 5d electrons because of the strong interaction.101 Wei, Yao, and co-workers characterized n-alkanethiol (n = 3, 8, and 12)-capped Au nanoparticles by X-ray absorption fine structure.102 With increased chain length, the coordiantions of sulphur atom attached on particle surface gradually get higher while the Au−S bond length is gradually shortened. As a result, a dramatic d charge transfer from gold to sulphur was observed for the longest 12-alkanethiol-capped Au nanoparticles. The modified electronic structures of catalysts further influence their catalytic performance. Au nanoparticles are negatively charged when capped with PVP as an electron donor, and the increased electron density on Au leads to enhanced catalytic activity for aerobic oxidation of alcohol than that of PAA-capped Au nanoparticles (Figure 12).103

6. Summary and Outlook Colloidal nanoparticles, composed of an inorganic core and a shell of organic capping agents, play versatile roles in catalytic investigations because of their well-controlled characteristics and mimics of real catalysts. While plenty of attention has been put on their inorganic cores that determine the intrinsic catalytic properties of nanoparticles, the influence of outside organic shells (e.g., capping agents) is relatively less concerned. Generally, capping agents bring extra complexities and opportunities for the catalytic study of colloidal nanoparticles. To simplify the uncontrollable variables and to enhance their catalytic activity, nanoparticles with “clean surface” are therefore



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favored. Several strategies have been developed to remove the stubborn capping agents, such as extensive washing, UVO irradiation, mild-temperature (< 200 oC) annealing in air, acid or base treatment, and ligand-exchange with small absorbates. Besides, surfactant-free synthesis is a more straightforward way to get “unprotected” nanoparticles. On the other hand, structure analogy between organometallics and colloidal nanoparticles in terms of their metal-organic core-shell configurations implies the unlimited potential of capping agents in nanocatalysis. Several promoting roles of capping agents on catalysis have been established qualitatively, including chiral modification, molecular recognition, surface crowding effect, and charge transfer. Future studies will continue to focus on both the removal and the utilization of capping agents to fulfill specific requirements in nanocatalysis. Nanoparticles with “clean surface” are more close to classical heterogeneous catalysts, and capping agent removal and surfactant-free synthesis will facilitate the reveal of structure-activity relationships of these catalysts. For the convenience of capping agent removal, bio-, photo-, thermal-degradable polymers may be more employed in the colloidal synthesis if they could provide high-quality control.104-108 The scope of surfactant-free synthesis needs great expansion to different metals and alloys with various shapes and sizes. Reducing metal precursors in the presence of solid supports (e.g. porous silicates, carbon materials) by small adsorbing molecules might be a universal synthetic system. Meanwhile, it can be expected that the role of capping agents in catalysis will


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become increasingly important due to their versatile properties. Surface analytical methods with high sensitivity are required to provide more detailed pictures about the behaviors of capping agents at the metal-organic interface. The utilization of capping agents needs a shift from qualitative exploration to quantitative investigation. Laboratory synthesis of new capping agents with elaborate designed functionalities will greatly benefit the development of enzyme-mimic catalytic systems.

Acknowledgement. This work was supported by the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CB932401 and 2011CBA00500), National key Basic Research Program of China (2012CB224802), and the National Natural Science Foundation of China (Grant No. 21221062, 21171105 and 21131004).

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Figure 1. Structures of representative capping agents in nanoparticle synthesis, including long-chain hydrocarbons, polymers, chiral ligands, polycarboxylic acids, polyhydroxy compounds, cationic surfactant, and denrimers.



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Figure 2. Anisotropic growth of Ag nanocrystals induced by selective capping. Starting with spherical Ag seeds (A, B), octahedrons with eight {111} facets (C, D) and nanocubes with six {100} facets (E, F) are selectively produced by adding Na3CA and PVP, respectively. Reprinted with permission from ref 28. Copyright 2012 American Chemical Society.


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Figure 3. Spatial conformations of capping agents on nanoparticle (gray ball) surface: (A) linear long-chain hydrocarbons; (B) unbranced polymers; (C) branched polymers and dendrimers. Blue triangle represents the anchoring sites of capping agents.



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Figure 4. (A) The relative concentration of Pd nanoparticles of different sizes at the height of 1 cm above the centrifugal tube at a rotational speed of 5000 rpm. Set the concentration of Pd at the bottom as standard. The radius of centrifuge is 13.5 cm. Pd nanoparticles (ρ = 12.02 g/cm3) are dispersed in hexane (ρ = 0.65 g/cm3) at 25 oC. Red and blue lines represent nanoparticles under centrifuge field and gravity field, respectively. (B) Schematic illustration of the volume contraction of organic shell along with the increase of relative amount of poor solvent. Red and gray ball represent metal core and organic shell, respectively.


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Figure 5. (A) Conformations of PVP attached on Pt core when PVP is of (left) low molecular weight, (middle) high molecular weight, and (right) cross linking. Reprinted with permission from ref 65. Copyright 2006 American Chemical Society. (B) Binding sites of PVP molecules adsorbed on (B, left) a small Pd nanoparticle with their carbonyl O atoms adhere to the surface, as well as on (B, right) a large Pd nanoparticle with their O and N atoms adhere to the surface Pink and blue circles represent O and N atoms, respectively. Reprinted with permission from ref 36. Copyright 2012 American Chemical Society.



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Figure 6. Calculated adsorption geometry of BINAP on Pd {111} surface. A pair of phenyl rings (plane B) is adsorbed on Pd surface with parallel orientation while the other pair (plane A) is tilted above the Pd surface. Note that P atoms have no direct interaction with the surface. The green, gray, white, and orange balls represent Pd, C, H, and P atoms, respectively. Reprinted with permission from ref 70. Copyright 2010 American Chemical Society.


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Figure

7.

(A)

Chain-length-dependent

surface

coverage

of

hydrophobic

mercapto-alkanoic acid (red squares) and hydrophilic mercapto-(PEG)n-carboxylic acid (blue squares) on Au nanoparticles. Reprinted with permission from ref 73. Copyright 2013 American Chemical Society. (B, left) Schematic illustration of two distinct diffusion modes for probe molecule on hydrophobic surface: short-lived flying and long-lived crawling. (B, right) The fraction of short-lived diffusion decreases with the increase of alkyl chain length. Reprinted with permission from ref 75. Copyright 2013 American Chemical Society.



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Figure 8. A possible interaction model between cinchonidine and adsorbed cis pro-(R) methylpyruvate on Pt. Cinchonidine is anchored on metal surface with its quinolone moiety in a tilted adsorption manner. Reprinted with permission from ref 80. Copyright 2004 Elsevier Inc.


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Figure 9. Schematic illustration of metalloenzyme-mimic molecular recognition (encapusulation) mechanism based on hydrophobic interactions in alkanethiol-SAM attached on Au nanoparticles. Reprinted with permission from ref 85. Copyright 2012 Wiley-VCH.



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Figure 10. Schematic illustration of the competitive adsorption effect in promoting the selective hydrogenation of alkynes: (A) calculated adsorption energies of alkenes (desired products) as compared to that of 1-octylamine (capping agent) predict the hydrogenation selectivity of corresponding alkynes; (B) experimental selectivity in the production of alkene from different alkynes verifies the prediction. Reprinted with permission from ref 86. Copyright 2012 American Chemical Society.


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Figure 11. Schematic illustration of surface crowding effects in catalysis: (A) epoxybutene rapidly decomposes via epoxide ring opening on uncapped Pd {111} facet, whereas it desorbs intact on SAM-capped surface due to a kinetic barrier arising from the surface crowding. Reprinted with permission from ref 89. Copyright 2011 American Chemical Society. (B) solvent polarity induces conformation change of S-dodecylthiosulfate on Pd, leading to the isomerization and hydrogenation of allyl alcohol. Reprinted with permission from ref 76. Copyright 2012 American Chemical Society. (C) chain length-dependent site accessibility in selective hydrogenation of



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α,β-unsaturated aldehydes on amine-capped Pd. Reprinted with permission from ref 91. Copyright 2012 Wiley-VCH. (D) site isolation effect of capping agents on Au surface to suppress the production of undesired benzyl benzoate. Reprinted with permission from ref 93. Copyright 2013 Royal Society of Chemistry.


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Figure 12. Schematic illustration of charge transfer from PVP to Au core. Note that the enhanced electron density on Au surface promotes the activation of molecular oxygen. Reprinted with permission from ref 96. Copyright 2009 American Chemical Society.



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A Graphic for Table of Contents


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Removal and Utilization of Capping Agents in Nanocatalysis

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