Utilization of shape-controlled nanoparticles as catalysts with enhanced activity and selectivity

Abstract

Shape-controlled nanoparticles with well-defined facets can be used as heterogeneous catalysts with enhanced activity and selectivity. The surface crystalline structure has a significant effect on the surface reaction, and shape control can be a way to obtain a desirable surface structure to improve the catalytic properties of nanoparticles. The shape of the nanoparticle can be formed by controlling the nucleation and overgrowth steps. Surface-capping agents are typically used to prevent aggregation of the nanoparticles during the overgrowth, but the subsequent treatment for their removal should be performed carefully. The extent of surface cleanness and the type of organic remnant can yield different catalytic properties. The surface agents, however, can also contribute to modulating the electronic structure or oxidation state of the surface, inducing improved catalytic activity and durability. Examples showing enhancements in the activity and selectivity of shape-controlled nanoparticles with well-defined facets are presented in this review, including electrocatalytic reactions, coupling reactions of organic compounds, water-gas shift reactions, CO oxidation, reforming reactions, and photocatalytic reactions. The well-defined facets control the adsorption of reactants to the surface, bond cleavage at the surface, desorption of products from the surface, and degree of surface-poisoning, resulting in enhanced activity and selectivity. However, the issues of shape preservation and mass production should be addressed further to apply the shaped nanoparticles in practical applications.

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1. Introduction

Shaped nanoparticles have been actively investigated in recent years due to their unique properties in optical, magnetic, or catalytic applications.^1–8 This review will in particular address the catalytic applications of shape-controlled nanoparticles. The first question to be raised on this topic would be why the shape matters for catalytic applications. The heterogeneous catalysis community has examined the single crystalline surface for decades to elucidate the role of surface properties in the activity and selectivity of the catalytic reaction. By cutting a bulk crystal in a certain direction, single crystalline surfaces with distinct structures could be prepared. In cases of face-centered cubic (fcc) metals such as Pt, Pd, and Rh, the (100) surface has a square atomic arrangement, whereas the (111) surface has a hexagonal atomic arrangement. This different atomic arrangement affects the adsorption of reactants onto the surface or the bond cleavage during the surface reaction. For example, the Pt(111) surface showed several times higher activity than the Pt(100) surface for aromatization of hexane to benzene or heptane to toluene.⁹ The Pt(111) surface produced benzene after hydrogenolysis of methylcyclopentane, while the Pt(100) surface produced no benzene, but generated many fragments with fewer numbers of carbons instead.¹⁰ Electrocatalytic reactions are good probes to evaluate the effect of the surface structure. For electrocatalytic oxidation of formic acid, the Pt(100) surface produces no current in a forward scan due to severe surface poisoning, while the backward scan showed a high current resulting from the oxidation of the surface-poisoning species. However, the Pt(111) surface showed little surface poisoning with a much lower current in the backward scan (Fig. 1).¹¹

Fig. 1 Formic acid electro-oxidation on Pt(100), Pt(111), Pt(110), and polycrystalline Pt surfaces. Strong structure-dependence is observed. Reproduced from ref. 11 with permission from the PCCP Owner Societies.

When nanoparticles are synthesized with a particular shape, they can have a well-defined surface structure. For example, the cubic shape of fcc nanoparticles has (100) facets, and tetrahedral, octahedral, and icosahedral fcc nanoparticles have (111) facets. Truncated octahedra have both (100) and (111) facets. The shaped nanoparticles with a distinct surface atomic arrangement would show different catalytic properties. Better catalysts with higher activity and selectivity can potentially be developed by modulating the shapes of nanoparticles.

In this review, I will mainly discuss about the synthesis of shape-controlled nanoparticles with well-defined facets and their applications as catalysts. The facets on the shaped nanoparticles can be compared to single-crystalline surfaces or theoretical model surfaces to elucidate the reaction mechanism. Synthesis of shaped nanoparticles, especially for catalytic applications, will be introduced briefly. The catalytic applications of shaped nanoparticles are severely affected by surface-capping agents. Therefore, I will discuss various points about surface-capping agents, such as how to remove them, how the agents can aid the surface reaction, and the unique roles of the agents. Then, several examples will be listed, showing the shape effect presenting the enhancement of catalytic activity and selectivity. Lastly, important remaining issues related to the use of shaped nanoparticles in practical applications will be discussed.

2. Synthetic methods for catalytic applications

The preparation of shaped nanoparticles has been discussed for years. Several excellent review papers have already covered this issue,^12–16 so synthetic methods will be explained very briefly here, especially considering the applications of these nanoparticles as heterogeneous catalysts. Wet colloidal chemistry has been most commonly used to explore various shapes. Colloidal metallic nanoparticles can be synthesized by reducing metal precursors dissolved in solvents in the presence of organic surface-capping agents. First, nuclei are formed, and subsequently, these nuclei overgrow into particular shapes. Controlling the nucleation step would result in a monodispersed distribution of the nanoparticles. The overgrowth step can be modulated by (i) organic capping agents, (ii) inorganic capping agents, or (iii) reducing agents. Organic capping agents, typically used in colloid synthesis to prevent nanoparticle aggregation, can be adsorbed on a specific facet more strongly to increase the likelihood that the facet will survive. Shaped Pt nanoparticles were first created by El Sayed group by controlling the ratio of the Pt precursor (NaPtCl₄) and the organic capping agents (sodium polyacrylate).¹⁷ When the ratio was 1 : 1, cubic nanoparticles were obtained, but the ratio of 1 : 5 induced a tetrahedral shape. They explained that polyacrylate is adsorbed on the Pt(111) surface more strongly during the overgrowth step, so when more polyacrylate is used, the Pt(111) surface survives by inducing a tetrahedral shape. Recently, organic capping agents were found not only to modulate the shape but also to modulate the crystalline structure of the resulting nanoparticles. Ni hourglass-shaped nanoparticles were synthesized with a hexagonal close packed (hcp) crystalline structure, although Ni has a naturally fcc structure.¹⁸

Foreign metal ions, halide ions, or even gaseous additives such as CO can also aid in shaping. Shaped Pt nanoparticles with a highly uniform shape and a small size were synthesized by using Ag or Co as additives.^19,20 Increased amounts of Ag ions showed good shape evolution from cubes to cuboctahedra to octahedra. Different types of halide ions enabled the synthesis of highly uniform PtPd cubes and octahedra.²¹ NaI induced a cubic shape, and NaCl induced an octahedral shape. Bromide ions were also reported to induce a cubic shape in Rh nanoparticles.²² Gaseous CO or CO generated at the point-of-use was also used to induce various shapes for Pt, Pt alloys, Pd, and Au.^23,24

When the shape effect is evaluated using shaped nanoparticles in catalytic applications, several conditions should be considered: (i) the differently shaped nanoparticles should have similar sizes and high purity of shape, (ii) the same organic capping agents should be used, and (iii) the shaping agents should not contaminate the surface. The size is known to affect the catalytic properties significantly because smaller nanoparticles have more atoms on the edges with lower coordination numbers. The atoms at the edges typically have higher activity. When the sizes of shaped nanoparticles are different, it may be difficult to determine whether the altered catalytic property results from the different shapes or the different sizes. Fig. 2 shows fractions of atoms located at the terrace or at the edge for Pt cubes of various sizes. The shaped Pt nanoparticles typically have a size of 5–10 nm, and the fraction of atoms at the terrace changes from 3.4% to 0.9%. The ratio of edge to terrace decreases significantly from 0.13 (3.4%/26.3%) to 0.06 (0.9%/14.8%) as the particle size increases from 5 nm to 10 nm. The size effect should therefore be carefully considered when the shape effect is evaluated. Additionally, if there are many impurities with other shapes, the shape effect will be blurred. Recently, Kang et al. devised a way to purify shaped Pt nanocrystals by forming superlattices.²⁵

Fig. 2 Fraction of atoms located at terrace or edge of Pt cubes with various sizes. Size effect should also be considered carefully when shape effect is evaluated, because catalytic property of small nanoparticles would be affected significantly by more edge atoms with lower coordination number.

The organic capping agents affect the surface property significantly.²⁶ When different organic capping agents are used to synthesize different shapes, the shape effect may be concealed by the effect of the organic capping agents. Preparation of different shapes of nanoparticles using the same organic capping agents is desirable to provide an exact comparison of the shape effects. When inorganic capping agents are used, the surface of the shaped nanoparticles is often contaminated. For example, when Ag ions were used to induce cubic, cuboctahedral, and octahedral Pt nanoparticles, the Pt surface was severely contaminated by residual Ag, resulting in diminished catalytic activity.²⁷ The different activity resulted from different amounts of Ag and was not the result of the shape. The residual Ag could be removed further by selective etching.²⁸

As a way to preserve the catalytically active surface, the reduction rate was controlled in the presence of the same organic capping agents. Pt cubes and cuboctahedra were synthesized by controlling the pH of the reducing solution.²⁹ The same activity for a structure-insensitive reaction confirmed that the platinum surface was catalytically clean and active and was not affected by surface adsorbates. The same concept was also applied to synthesize shaped Pd nanocrystals. The pH of the solution was controlled by NO₂. When NO₂ was not added, Pd cubes were synthesized. However, the addition of NO₂ significantly lowered the reduction rate, resulting in Pd octahedra (Fig. 3).³⁰ The electrochemical reduction method can also be considered as a way to control the reducing rate. When the metal precursor was reduced at programmed potentials, crystals of various shapes could be synthesized.^31,32 The first Pt nanocrystals with high-index facets were also prepared by the electrochemical method.³³

Fig. 3 Pd nanoparticles with cubic, cuboctahedral, and octahedral shapes. Shape evolution was observed by slowing down a reduction rate. The rate was controlled by adding different amounts of NO₂ species. The various shapes showed the different electro-oxidation signals. The right bottom figure shows cyclovoltammograms for formic acid oxidation of Pd cubes and Pd octahedra. Reproduced from ref. 30 with permission.

Similar methods can be applied to synthesize shape-controlled metal oxide nanoparticles.³⁴ Interestingly, metal oxide nanoparticles could be synthesized without surface-capping agents.³⁵ Hydrothermal methods are typically applied by using autoclaves under a controlled temperature and pressure. Mostly, metal precursors turn into metal hydroxide nuclei, then the hydroxide is further oxidized to form metal oxide nanoparticles. Controlling the formation of hydroxide nuclei or the transformation from hydroxide to oxide can enable shape-control of metal oxide nanoparticles. Cubic, octahedral, and rod shapes of CeO₂ nanoparticles were obtained by using different amounts of NaOH or Na₃PO₄ at various temperatures.^36,37 The small nanocrystals can grow directed by the crystallographic structure without the aid of organic surface-capping agents. Selective adsorption of organic capping agents such as polyvinylpyrrolidone (PVP) or cetyltrimethylammonium bromide (CTAB), which are widely used for shape control of metallic nanoparticles, can also induce metal oxide nanoparticles with well-defined facets.^38,39 Good shape evolution from cubes to octahedra was observed by increasing the concentration of PVP for Cu₂O nanocrystals.³⁹ Polyhedral 50-facet Cu₂O microcrystals partially enclosed by (311) high-index facets were prepared by controlling the concentration of OH⁻ and the volume ratio of polar organic solvents to water.⁴⁰ Electrochemical method was also actively explored to realize various surface structures of Cu₂O.⁴¹ TiO₂ nanocrystals with high-index (105) facets were prepared by a high-temperature gas-phase oxidation.⁴² Disk and rod-shaped hexagonal ZnO crystals were synthesized with different ratios of (100) nonpolar facets to (001) polar facets by varying Zn precursor concentration in the presence of hexamethylenetetramine.⁴³ MgO nanocubes were also synthesized by thermal evaporation of Mg metal powder at high temperature (1190 °C) in the presence of oxygen.⁴⁴

3. Surface-capping agents

3.1. Removal of surface-capping agents

When the shaped nanoparticles are used as catalysts, the catalytic reaction occurs at the surface. So, preparing a clean or at least catalytically active surface is very important. Immediately after preparation of the shaped nanoparticles, they are usually washed with copious amounts of solvents and stored while dispersed in the solution. Although free surface-capping agents can be removed from the washing process, the surface of the shaped nanoparticles is tightly capped by the organic molecules, which is the reason that the nanoparticles are stably dispersed in the solution. These tightly bonded organic agents should be removed prior to the catalytic reaction. When the shaped nanoparticles were used for an electrocatalytic reaction, the nanoparticles often showed no current at all without post-treatment, indicating that the organic agents prevent electron transfer between the electrode and the metallic nanoparticles. Various methods, such as solvent cleaning,⁴⁵ oxidation/reduction,⁴⁶ UV irradiation,^47,48 thermal annealing,⁴⁹ acetic acid washing,⁵⁰ plasma treatment,⁵¹ and electrochemical imposition of high voltage⁵² have been applied to remove the organic capping agents.

Although the post-treatment enabled the catalytic surface reaction, the catalytic property is often strongly dependent on the specific post-treatment method. When the oleylamine-capped Pt nanoparticles were treated with three different methods of thermal annealing under O₂, acetic acid washing, and UV-ozone treatment, the mass activity for the oxygen reduction reaction differed by a factor of more than three.⁴⁹ These differences possibly come from different residues on the surface after the post-treatment. Baker et al. found that when PVP is removed from the Pt surface, their carbon residues undergo reversible restructuring presenting very different catalytic activity in H₂ and O₂, respectively.⁴⁷ Because the PVP fragments form a porous coating around the Pt in H₂ but collapse to a tightly closed shell in O₂, the Pt nanoparticles showed much higher activity for hydrogenation than oxidation (Fig. 4). In situ IR measurement of tetradecyltrimethylammonium bromide (TTAB)-capped Pt nanoparticles also showed that the ammonium head group remained after thermal treatment above 350 °C, while the tail group was easily removed at ∼200 °C.⁵³ The removal process often degraded the shape of the nanoparticles. When the Pt nanocubes were deposited on the electrode and the cyclovoltammogram was repeated to remove organic capping agents, the cubic shape was often degraded, exposing both Pt(100) and Pt(111) facets.⁵⁴

Fig. 4 A schematic diagram illustrating the effect of remnant organic capping agents on surface catalytic reactions. When PVP-capped Pt nanoparticles were used as catalyst for ethylene hydrogenation and methanol oxidation, the nanoparticles showed 10 times increase in activity for ethylene hydrogenation after UV cleaning, while they presented 3 times decrease for methanol oxidation after the removal. Reproduced from ref. 47 with permission.

3.2. Catalytic reactions aided by surface-capping agents

The remaining organic capping agents do not always have a detrimental effect on the catalytic reaction. Chung et al. intentionally attached oleylamine onto commercial Pt/C electrocatalysts and confirmed that the activity was enhanced for the oxygen reduction reaction due to the modification in the electronic structure.⁵⁵ The interaction of oleylamine and the Pt surface downshifted the d-band center of platinum, resulting in ∼3 times the activity enhancement (Fig. 5). When the thiol groups are immobilized on carbon nanotube supports, the thiol group enables Pt nanoclusters to have a strong interaction with carbon nanotubes by modulating the electronic structure, enhancing the durability significantly.⁵⁶ The surface-capping agents can also prevent surface oxidation, preserving metallic state of the surface. When dodecylamine-capped Pt nanoparticles were deposited on an Fe₃O₄ support and used for preferential oxidation of CO, they showed higher activity than ligand-free Pt deposited on the support due to a retained metallic state of Pt surface.⁵⁷

Fig. 5 Activity enhancement by amine adsorption on commercial Pt/C electrocatalyst. When a proper amount of oleylamine is adsorbed on Pt nanoparticles, they change the electronic structure of Pt surface which d-band center becomes downshifted, resulting in ∼3 times higher mass activity for an oxygen reduction reaction than commercial Pt/C at 0.95 V. Reproduced from ref. 55 with permission.

Cascade or tandem reactions have recently received much attention.^58,59 When the surface-capping agents have the ability to catalyze chemical reactions, the nanoparticles capped by the organic layer can be used as catalysts for a cascade reaction. As an example, the bacterial aminopeptidase from Streptococcus pneumoniae (PepA) self-assembles into a well-defined tetrahedral dodecameric complex with a void space in the center. Very small Pt nanoparticles with a size of 1–2 nm could be synthesized in the void, and the surrounding enzyme acted as a type of a capping agent.⁶⁰ For transformation of glutamic acid p-nitroanilide, the enzyme in the outer shell catalyzed peptide bond cleavage, producing glutamic acid and p-nitroanilide, and Pt nanoparticles inside the void space catalyzed hydrogenation of p-nitroanilide, producing p-phenylenediamine. Finally, glutamic acid and p-phenylenediamine were produced by the cascade reaction.

The nanoparticles are often deposited onto a high surface area support for catalytic applications to make separation and recycling easier. However, if the prepared catalysts (shaped nanoparticles deposited on supports) are used for a liquid phase reaction, the nanoparticles would leave the support easily unless the nanoparticles are linked to the surface by covalent bonding. Surface-capping agents can contribute to fixing the nanoparticles to the support. CuPt nanorods were immobilized on a graphene oxide support by click chemistry, and they were then used for an enzyme-like reaction in water.⁶¹ Because the nanorods were originally synthesized in non-polar solvents, the nanoparticles were not dispersed in water. But after the immobilization using click chemistry of the azide group on nanorods and the alkyne group on graphene oxide, the resulting composite could catalyze the reaction in the aqueous phase. Niu and Li recently provided an excellent review covering removal and utilization of capping agents in nanocatalysis.⁶²

3.3. In situ shaping: shaped nanoparticles synthesized without surface-capping agents

Although surface-capping agents can sometimes help surface catalytic reactions, their absence is usually preferred. The shaped nanoparticles are typically deposited on high surface area supports, subsequently, the surface-capping agents are removed by the methods listed in Section 3.1. Because the capping agents enable dispersion in the solution, the removal of the capping agents should be performed after the nanoparticles are deposited on the supports. We devised a way to synthesize the shaped nanoparticles directly on the support. When the nanoparticles are nucleated on the supports, the surface-capping agents are unnecessary because the nanoparticles do not have to be dispersed in the solution. Then, the overgrowth of the nanoparticles can be controlled using shaping agents. Cysteamine could enable anchoring of Pt nuclei on the carbon support. Pt was confirmed to be nucleated on the support and overgrown into cubic nanoparticles with an average size of 7.9 nm. The cysteamine also acted as a shaping agent inducing the cubic shape. The cubic Pt/C catalysts showed an electronic signal just after washing, whereas the shaped nanoparticles synthesized with surface-capping agents typically show the electronic signal after the activation process that repeatedly imposes high potentials. The cubic Pt/C showed higher activity for the oxygen reduction reaction compared with cubic Pt nanoparticles synthesized with oleylamine, presumably due to a cleaner Pt surface. Additionally, the cubic Pt/C presented enhanced stability because migration of Pt nanoparticles was prevented due to anchoring (Fig. 6).⁶³ This method could be extended into other types of supports such as mesoporous silica.⁶⁴

Fig. 6 In situ shaped Pt nanocubes on carbon black support. Pt was directly nucleated on carbon supports by anchoring agents, then overgrew into a cubic shape. Because the nanoparticles do not have to be dispersed in solution, organic capping agents were not used. As a result, the cubic Pt/C was clean enough to show electronic signal without any special post-treatment. It showed enhanced activity and durability for an oxygen reduction reaction due to cleaner surface and anchoring. Reproduced from ref. 63 with permission.

4. Catalytic properties enhanced by shape-control

4.1. Activity enhancement

The most impressive example of enhanced activity by shape-control would be the realization of Pt₃Ni(111) facets. In 2007, Stamenkovic et al. reported that the Pt₃Ni(111) surface would have ∼90 times higher activity than commercial Pt/C for an oxygen reduction reaction.⁶⁵ They prepared a single crystalline surface with different structures and compositions, and discovered that the Pt₃Ni(111) surface would have all-Pt skin at the topmost layer and the underlying Ni would modulate the electronic structure of Pt, thereby weakening the strength of the oxygen adsorption leading to enhanced activity for the oxygen reduction reaction. Since then, much effort has been concentrated on achieving a Pt₃Ni(111) surface by using shape-controlled nanoparticles.^66–68 Zhang et al. reported the synthesis of first Pt₃Ni nano-octahedra and their utilization for the oxygen reduction reaction.⁶⁹ However, the mass activity was not very good (0.11 A per mg_Pt at 0.9 V (vs. RHE)). The mass activity of commercial Pt/C is known to be ∼0.1 A per mg_Pt. Wu et al. reported the synthesis of Pt₃Ni octahedra using gaseous reducing agents, and the mass activity was 0.44 A per mg_Pt.²⁴ Pt₃Ni icosahedra with (111) facets synthesized in the same research group showed a higher mass activity of 0.62 A per mg_Pt.⁷⁰ Cui et al. reported the synthesis of PtNi octahedra, and they reported an activity of 1.45 A per mg_Pt.⁷¹ Choi et al. presented the synthesis of Pt_2.5Ni octahedra, and their mass activity was 3.3 A per mg_Pt.⁷² Most recently, Chen et al. reported that Pt₃Ni nanoframes show a record high mass activity of 5.7 A per mg_Pt.⁷³ The size, the exact composition, and degree of surface cleanness affect the mass activity, but it is clear that the shape-control has contributed to significant enhancement of the mass activity of platinum for the oxygen reduction reaction, which is a bottleneck reaction for fuel cell commercialization (Fig. 7).

Fig. 7 Mass activity enhancement by shape-control of Pt–Ni alloy nanoparticles for an oxygen reduction reaction (octa: octahedra, icosa: icosahedra). Comparing to ∼0.1 A per mg_Pt, which is a typical mass activity of commercial Pt/C catalyst, the mass activity has increased rapidly up to 5.7 A per mg_Pt so far.

Shaped Pd nanoparticles were often used as catalysts for electrocatalytic formic acid oxidation. Pd cubes with (100) facets showed a much higher activity than Pd octahedra with (111) facets, while the Pd cube surface was more susceptible to surface oxidation.^30,74 Concave bipyramidal Pd nanocrystals bound with (110) facets showed enhanced activity for the same reaction.⁷⁵ When Pd was locally overgrown on a Pt cube, Pd–Pt binary nanoparticles showed enhanced activity for formic acid oxidation because the poisoning on the Pt surface was greatly reduced.⁷⁶ PtPd alloy nanoparticles were also used for nitrobenzene hydrogenation. PtPd cubes showed a higher turnover frequency than PtPd octahedra.²¹ The nanoparticles with high index surfaces were reported to have an enhanced specific activity for various electrocatalytic reactions.^77–80 However, the nanoparticles are usually very large (over 50 nm), and there are many atoms occluded inside the particles. So, the overall mass activity was usually poorer than that of commercial catalysts. Additional reviews about shape-controlled nanoparticles with enhanced activity for electrocatalytic reactions can be found elsewhere.^81–84 Shaped Pd nanoparticles were also tested as catalysts for coupling reactions. Pd nanoparticles with high-index facets showed particularly high activity for the Suzuki coupling reactions.^85,86

Shaped metal oxide nanoparticles were also used to enhance the activity of catalytic reactions. Shaped ceria were used for the water-gas shift reaction (CO + H₂O ⇄ CO₂ + H₂) after Au was deposited.⁸⁷ The oxidation state of deposited Au differed significantly depending on the shape of the ceria. Oxidized Au deposited on ceria-nanorods with (110) facets showed a CO conversion superior to metallic Au deposited on ceria-nanocubes with (100) facets (Fig. 8). Similarly, Au or Co deposited on shaped ceria nanoparticles with (110) facets showed enhanced activity for steam reforming of methanol or ethanol, respectively.^88,89 Soot oxidation on ceria catalysts also showed shape-dependance.⁹⁰ Co₃O₄ nanorods with (110) facets showed high activity for CO oxidation, even at −77 °C.⁹¹ While CO conversion at room temperature was 100% for Co₃O₄ nanorods, the conversion was much lower (below 40%) for unshaped Co₃O₄ nanoparticles. Shaped Cu₂O nanocrystals of cubes with (100) facets, octahedra with (111) facets, and rhombic dodecahedra with (110) facets were also synthesized and used for photocatalytic degradation of methyl orange.⁹² Rhombic dodecahedra showed much higher photocatalytic activity than cubes. The band-gap energy of Cu₂O materials can be controlled by using different shapes.⁹³ When Cu₂O cubes, cylinders, and spheres were compared, the cubes had ∼1 eV smaller band-gap energy than spherically shaped materials. Cu₂O with high-index facets presented higher activity for gas-phase CO oxidation.⁴⁰ Decahedral TiO₂ particles with (110) facets presented higher than or at least comparable to that of P25 for photocatalytic reactions operated at various conditions.⁹⁴ TiO₂ nanorods with a higher aspect ratio was more advantageous for H₂ production by water splitting.⁹⁵ MgO nanopowders or nanosheet showed high catalytic activity for the degradation of volatile organic compounds or condensation of benzaldehyde with acetophenone.^96,97 MgO cubes or ZnO nanobelts were also used as support for Au or Pd deposition, and they showed high activity for CO oxidation.⁴⁴

Fig. 8 Enhanced activity of Au/ceria catalysts prepared with shaped ceria nanoparticles for water-gas shift reaction. When Au was deposited on various shapes of ceria nanoparticles, the ceria shape affected the oxidation state of Au, presenting different CO conversions. Reproduced from ref. 87 with permission.

4.2. Selectivity enhancement

Catalytic applications of shape-controlled nanoparticles have been concentrated mainly on activity enhancement, but recently, the control of selectivity using shaped nanoparticles has received more attention.^98,99 Increasing selectivity would minimize the undesired products, diminishing the burden of separation and purification. The reaction mechanism has been studied on single crystalline surfaces, showing that the surface structure would be a key factor for increasing selectivity.^100–102 The size of the nanoparticles has usually been modulated to control the surface structure for selectivity enhancement.^103,104 Since Bratlie et al. firstly reported that Pt cubes with (100) facets and Pt cuboctahedra with both (100) and (111) facets have distinct selectivity for benzene hydrogenation,¹⁰⁵ various gas-phase reactions such as hydrogenation of furan and pyrrole, alkene cis–trans isomerization, and methylcyclopentane hydrogenolysis were tested, and different selectivity was observed depending on the shape of the nanoparticles used.^106–110 For hydrogenation of ring-type chemicals such as furan and pyrrole, C–N or C–O bonds were more easily broken on Pt(100) facets compared to Pt(111) facets.^106,107 However, Pt(111) facets were more advantageous for isomerization of trans-2-butene to cis-2-butene (the cis-form is preferred in food chemistry).¹⁰⁸ Methylcyclopentane hydrogenolysis on Pt(100) facets of Pt nanocubes produced cracked compounds more extensively, whereas Pt(111) facets on octahedral Pt nanoparticles produced hexane.¹¹⁰ Electrocatalytic hydrogenation of cyclohexenone also showed shape-dependence, producing more partially hydrogenated cyclohexanone on Pt cubes, whereas Pt cuboctahedra produced more fully hydrogenated cyclohexanol.¹¹¹ Recently, we compared the selectivity of Pd cubes and Pd spheres deposited on an alumina support for acetylene hydrogenation.¹¹² Hydrogen could be provided to the reactant adsorbed on the Pd surface more easily on Pd(100) facets by Pd hydride decomposition, and ethylene produced on the surface could leave the surface more easily on Pd(100) facets. So, Pd cubes with an average size of 12.8 nm showed higher selectivity to ethylene than Pd spheres of the same size, while the production of ethane was minimized. These Pd nanocubes showed higher selectivity than conventional Pd catalysts with a much smaller particle size of 5 nm at the same conversion level (Fig. 9). When well-defined ZnO crystals were used as cocatalyst with NiMo in the combined reforming-hydrogenolysis of glycerol, a larger portion of (100) nonpolar facets were more effective to the selective production of 1,2-propanediol.⁴³

Fig. 9 Enhanced selectivity of cubic Pd deposited on alumina for acetylene hydrogenation. When the same size of Pd cubes and Pd spheres were compared, Pd cubes showed higher selectivity of acetylene to ethylene than Pd spheres due to more facile hydrogen supply and weaker adsorption of the ethylene product. Pd cubes showed even higher selectivity than conventionally prepared Pd/Al₂O₃ catalysts at the same conversions. Reproduced from ref. 112 with permission.

5. Preservation of the nanoparticle shape

The most important issue yet to be resolved for catalytic applications is shape stability. When the shaped nanoparticles were stored for a long time, surface oxidation was often observed. In the case of PVP-capped Pd nanocrystals, shape stability was found to follow the trend: cube < cuboctahedra < octahedral ∼ concave cube.¹¹³ Pd(100) facets were the most susceptible to surface degradation. The cubic shape was completely lost after six months of storage under ambient conditions (Fig. 10). Interestingly, the concave cubic shape was very stable. PVP might cap the surface more tightly, preserving the shape. Metal oxide nanoparticles are usually more stable than metallic nanoparticles, but shape degradation can occur after thermal annealing at high temperatures. Ceria cubes or rods underwent shape degradation with rounding of the edges after thermal aging at 800 °C for 4 h.⁹⁰

Fig. 10 Shape degradation of Pd cubes during long-term storage over 6 months. PVP-capped Pd cubes and Pd cuboctahedra turned into spherical shape, while Pd octahedra retained the original shape. Pd(100) facets underwent surface reconstruction through high energy (110) facets, possibly existing as surface defects. Reproduced from ref. 113 with permission.

The chemical reactions usually occur under reducing or oxidizing conditions. When Pt cubes of the same shape with different capping agents (PVP, TTAB, oleylamine) were treated in H₂, O₂, or N₂ at elevated temperatures, the shape started to be degraded above 200 °C.¹¹⁴ The cubic shape was more easily degraded under O₂, compared to H₂ or N₂. Furthermore, to use the shaped nanoparticles as practical catalysts, the shape should be stable during the catalytic reaction. When the colloidal nanoparticles were used for the catalytic reactions in solution, sharp edges with lower coordination numbers were easily dissolved with degraded shapes.^115,116 The leached atoms often acted as catalysts, blurring the shape effect of the nanoparticle catalysts. The dendritic shape of Pt nanoparticles was applied as a cathode catalyst in membrane-electrode assembly (MEA) for a proton exchange membrane fuel cell.^117,118 Although the Pt dendrites initially showed high mass activity for the cathode reaction, the shape was completely degraded after the accelerated degradation test (ADT).

The surface structure is known to be changed upon chemical adsorption and surface reaction. When CO oxidation occurred on Au nanoparticles, surface reconstruction of Au(100) facets upon CO adsorption was observed by environmental TEM.¹¹⁹ Interestingly, reversible surface change was also observed for bimetallic catalysts. When Ru_0.5Pd_0.5 nanoparticles with 15 ± 2 nm in size were exposed to NO + O₂ oxidizing condition and NO + CO catalytic condition, Ru abundant surface was formed in the oxidizing condition, whereas Pd abundant surface was formed in the catalytic condition.¹²⁰ This change was reversible, indicating that the metallic atoms can move inside the nanoparticle during the chemical reaction.

Although shape preservation is surely a very challenging task, promising results have been reported. When shaped ceria nanoparticles were used for preferential oxidation of CO in excess H₂ (PROX reaction) after copper deposition, the shapes presented different durabilities.¹²¹ Ceria octahedra with (111) facets showed no activity degradation for 100 h, whereas cubes with (100) or rods with (110) facets presented a significant decrease in activity, implying shape degradation during the reaction. More stable facets can be chosen to enhance the durability. Pt tetrahexahedral nanocrystals with (730) facets presented shape stability even after heating at 815 K.³³ The crystals were actually very large above 200 nm, but the preservation of the high index facets indicates that surface stabilization (e.g., via a strong interaction with a surface-capping agent) might be achieved, even for smaller nanoparticles.

6. Summary and outlook

Shape-controlled nanoparticles can be good catalysts with enhanced activity and selectivity. However, the synthetic method should be carefully chosen to apply the nanoparticles in catalytic applications. The nanoparticle surface should be as clean as possible, unless the surface-capping agents have other desired roles in the surface reaction. Controlling the reducing rate while using the same surface-capping agents without inorganic shaping agents can be a good way to synthesize variously shaped nanoparticles for catalytic applications. The reduction rate can be adjusted by changing the pH, using different types of reducing agents, or adding molecules to affect the reduction rate. Because organic surface-capping agents are typically used to prevent aggregation in colloidal solutions, the organic agents should be removed prior to a surface reaction. The removal of the surface-capping agent can have a strong influence on the catalytic activity and selectivity. Different removal methods can leave different amounts of organic remnants on the surface, which can induce changes in activity and selectivity. The organic remnants behave differently in various chemical environments, which also affect the catalytic properties. On the other hand, careful control of the organic surface agents can change the electronic structure or oxidation state of the catalyst surface, inducing enhanced activity and durability. The organic agents themselves can also participate in cascade reactions. Synthetic methods to prepare shaped nanoparticles directly on supports without surface-capping agents were also introduced.

Shaped nanoparticles with well-defined facets showed enhanced activity and selectivity for electrocatalytic reactions, solution-phase reactions, gas-phase reactions, and photocatalytic reactions. Pt₃Ni(111) facets were achieved by octahedral, icosahedral, or nanoframe types of Pt–Ni alloy nanoparticles, and the mass activity for the oxygen reduction reaction was enhanced by up to 57 times compared to commercial Pt catalysts. Shaped Pd nanoparticles showed enhanced activity and better surface-poisoning tolerance for electrocatalytic formic acid oxidation or the Suzuki coupling reaction. Shaped ceria nanocrystals were used for the water-gas shift reaction, CO oxidation, or reforming reactions. Cu₂O nanocrystals with well-defined facets also exhibited shape-dependence for photocatalytic degradation of dye molecules. Differently shaped Pt nanoparticles were tested to confirm the distinct selectivity. Pt(100) facets on cubes generally presented easier bond cleavage of C–O, C–N, of C–C than Pt(111) facets on octahedra or cuboctahedra as evidenced for furan hydrogenation, pyrrole hydrogenation, or methylcyclopentane hydrogenolysis. Pd(100) facets on cubes showed better selectivity for acetylene hydrogenation to ethylene compared to spherical nanoparticles containing Pd(111) facets.

Although shaped nanoparticles demonstrated considerable potential to innovate the heterogeneous catalysis with enhanced activity and selectivity, there are still many issues to solve related to their practical application. Shape degradation has often been observed during long-term storage or during chemical reactions. The way to preserve the shape should be developed to maintain the enhanced activity and selectivity. The mass production of the shaped nanoparticles should also be achieved for practical application. The simple synthetic procedure would be preferred for this purpose. Continuing efforts in nanomaterial synthesis and heterogeneous catalysis will pave the way to the development of better catalysts with unprecedented activity and selectivity for practical applications in sustainable energy and the chemical industry.

Acknowledgements

This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System (2011-0031575) and the Basic Science Research Program (2012R1A1A2040791) through National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.

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