Commentary pubs.acs.org/accounts
Toward Cost-Effective and Sustainable Use of Precious Metals in Heterogeneous Catalysts Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Younan Xia*,†,§ and Xuan Yang† †
The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States § School of Chemistry & Biochemistry, School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ABSTRACT: Waste not, want not. There is a pressing need to maximize the use of precious metals in catalysts to attain affordable and sustainable products. Recent progress in nanochemistry suggests that it is feasible to put the majority of metal atoms in a catalytic particle to work at optimal activity and selectivity.
P
recious metals, including Pd, Pt, Ir, Rh, and Ru, are key components of many heterogeneous catalysts essential to chemical, pharmaceutical, petroleum, energy, and automobile industries.1 Because of their extremely low contents in the Earth’s crust (typically at a ppb level), very limited supplies, and the ever-increasing demands from various sectors, there is a pressing need to achieve sustainable use of these metals without compromising the catalytic performance and material cost. One attractive strategy is to maximize the mass activity of a catalytic particle by engineering its size, composition, surface structure (i.e., the type of facet and twin boundary), internal structure (solid vs hollow), or a combination of them.2 As illustrated in Figure 1, The mass activity of a catalytic particle is a product of its specif ic surface area and specif ic activity. The specific surface area is directly proportional to the utilization efficiency (or dispersion, the proportion of atoms situated on the surface) of the active atoms. Traditionally, the utilization efficiency is increased by reducing the size of the catalytic particle. For example, the utilization efficiency will be increased from 9.5% to 26% when the edge length of a Pt cubic particle is reduced from 11.7 to 3.9 nm. This explains why commercial catalysts are typically based on tiny particles measuring only a few nanometers across (Figure 2A). Despite the extensive practice of this strategy, it has been challenging to optimize the specific activity of such small particles by engineering their surface structure through facet-controlled synthesis. The large portions of atoms positioned at vertices and edges might be detrimental to certain reactions. Such minuscule particles also tend to aggregate into larger particles, as well as dissolve or detach from the support, during operation. © 2017 American Chemical Society
Figure 1. Schematic illustration of two different strategies for increasing the mass activity, which should be combined to minimize the loading of precious metals in a heterogeneous catalyst.
The poor durability requires the metal to be loaded in large excess. Received: September 16, 2016 Published: March 21, 2017 450
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Accounts of Chemical Research
two atomic layers, the utilization efficiency can, in principle, reach 100%. However, application of such thin sheets to catalysts still faces a number of challenges because the top and bottom faces of the sheet must be capped or stabilized by organic ligands, only hexagonal lattice has been achieved, corresponding to just one type of facet, and it is almost impossible to deposit individual sheets on a catalytic support while having both sides fully exposed. An alternative approach is to deposit the precious metal as conformal shells on nanoscale templates made of another metal or an inorganic material (Figure 2D).7−13 When the shell thickness is reduced to two and one atomic layers, respectively, the utilization efficiency can reach 50% and 100%. As an immediate advantage over the conventional systems, one can work with nanoparticles relatively large in size (typically, >10 nm) to greatly improve the catalytic durability, by hindering aggregation and surface detachment, without compromising the specific surface area. Meanwhile, the specific catalytic activity can still be augmented by engineering the elemental composition and surface structure of the template and by leveraging the possible strain and ligand effects. More significantly, the template can be selectively removed through etching to generate metal nanocages with ultrathin walls and well-defined surface structures (Figure 2E,F).14−16 When the wall thickness is reduced to four atomic layers (or about 1 nm), one should be able to achieve a utilization efficiency as high as 50% by making use of both inner and outer surfaces of each nanocage. If the thickness can be further reduced down to two atomic layers without breaking the cage structure, the utilization efficiency will reach 100% for a structure-insensitive reaction! For a structure-sensitive reaction, the utilization efficiency can be increased to approach 100% by optimizing the type of facet on the side faces. In our opinion, such an ambitious goal to achieve 100% utilization efficiency for the active metal atoms in a catalytic particle is qualified as one of the Holy Grails in Chemistry, which will have major implications for a large number of areas, including catalysis, surface science, materials chemistry, energy, and nanotechnology. It will also enable us to realize cost-effective and sustainable use of some of the rarest and most expensive resources on the Earth. A number of recent reports suggest that it is indeed feasible to produce those structures with all the active metal atoms presented on the surface. In one approach, electrochemical deposition was explored to deposit a monolayer of Cu on Pd nanoparticles, and galvanic replacement was then used to transform the Cu monolayers into other precious metals such as Pt.7 While this approach is versatile in working with different metals, it is nontrivial to ensure the formation of a monolayer only. It is also challenging to increase the thickness beyond a monolayer. Due to the involvement of electrodes, it may be difficult to scale up the production volume. Alternatively, a solution-phase method was recently developed for the conformal deposition of various precious metals on facetcontrolled Pd nanocrystals to generate Pd@MnL (M = Pt, Ir, Rh, and Ru) core−shell structures, in which the number (n) of atomic layers in the shell can be readily controlled from 1 to 6.8−13 Owing to the involvement of epitaxial, conformal growth, the facets on the Pd@MnL nanocrystals should be identical to those on the original Pd templates. The particle size can be varied by simply using Pd nanocrystals with different dimensions.
Figure 2. Schematic illustrations of different types of nanostructures with atoms at extremely high utilization efficiency: (A) cuboctahedral particle of a few nanometers in size, typically found in a conventional heterogeneous catalyst, (B) nanoframe, (C) nanosheet of two atomic layers thick, (D) core−shell structure with the shell of only one monolayer thick, (E) cubic nanocage with a wall thickness of two atomic layers, and (F) octahedral nanocage with a wall thickness of two atomic layers.
The durability issue can be addressed by switching to nanoframes, three-dimensional, highly open structures comprising multiple ridges as thin as a few nanometers (Figure 2B).3−5 In a sense, each ridge can be considered as a linear aggregate of nanoparticles fused tougher through lattice attachment. Except for the very small portion of atoms located at the corners, all other atoms should have essentially identical catalytic properties. Most precious metals (as well as alloys among them and with other transition metals) can be prepared as nanoframes through the selective removal of a sacrificial component, which can be the more reactive metal in an alloy or the particles serving as sacrificial template for the site-selected deposition of the precious metal. Remarkably, the three-dimensional architecture allows for the loading of metal at a high level without compromising the separation between adjacent particles. As an immediate advantage over the conventional systems, catalysts based on metal nanoframes have been demonstrated to be less susceptible to sintering, dissolution, and detachment due to their enlarged dimensions and enhanced interactions with the support. In a notable example, Pt3Ni nanoframes with an extraordinary mass activity of 5.7 A mgPt−1 toward the oxygen reduction reaction have been reported.4 However, it is very challenging to optimize the catalytic activity and selectivity of nanoframes by controlling the type of facet exposed on the surface. A different strategy for increasing the utilization efficiency of a precious metal is to assemble the atoms into sheets as thin as a few monolayers (Figure 2C).6 For such a sheet consisting of 451
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Commentary
Accounts of Chemical Research Palladium is supposed to be the best substrate (other than Pt) for achieving epitaxial growth of Pt due to a very small lattice mismatch of only 0.77% between these two metals. However, the high surface energy (2.30, 2.73, and 2.82 J m−2 for {111}, {100}, and {110} facets, respectively) and interatomic bond energy (307 kJ mol−1) of Pt tend to push the deposition toward an island growth mode (i.e., the Volmer−Weber mode).17 By performing the deposition at a proper ratio between the deposition rate and surface diffusion rate, layer-by-layer deposition of Pt atoms have been achieved for various types of Pd nanoscale templates, including cubes, octahedra, icosahedra, decahedra, plates, rods, and wires. Experimentally, the deposition can be conducted with a mild reducing agent in an aqueous system so the Pt precursor is introduced in one shot. Alternatively, the deposition can be conducted with a strong reducing agent in a polyol at a relatively higher temperature. In this case, the Pt precursor has to be titrated dropwise to avoid homogeneous nucleation for the Pt atoms. With adequate surface diffusion, the deposited atoms are able to form a thin, uniform coating on the Pd template in a layer-by-layer fashion (Figure 3A).18 Two typical examples are shown in Figure 3B−E. When Ru was used for coating, the Ru atoms would adopt a face-centered cubic (fcc) crystal structure, as dictated by the Pd template, rather than the conventional hexagonal close-packed (hcp) structure of bulk Ru.13 The interiors of core−shell structures, however, are still occupied by another precious metal, (for example, Pd), which will contribute to a large portion of the materials cost associated with a catalyst. A logical solution is to selectively remove the Pd template after Pt deposition, generating a nanocage made of Pt only. When conducted appropriately, the facets present on the surface of a template can still be well preserved during the Pt coating and Pd etching processes to engineer the activity or selectivity of the catalyst or both. Indeed, the Pd cores could be selectively etched away using an aqueous etchant based on FeCl3/KBr or HNO3 at an elevated temperature.14,15 During the deposition of Pt atoms, some Pd atoms tend to be incorporated into the Pt overlayers because of intermixing or co-reduction. Upon contact with the etchant, the Pd atoms in the outermost layer will be oxidized to generate vacancies on the surface. At a high enough density, the vacancies will merge to form large channels, through which the underlying Pd atoms are continuously etched away. Figure 4 shows transmission electron microscopy (TEM) images of Pt nanocages with different types of facets on the surface, including those with twin boundaries. Oxygen reduction has been used as a model reaction to characterize and compare the catalytic activities of core−shell nanocrystals and nanocages. Compared to commercial Pt/C, the catalytic activities of Pd@PtnL (n = 1−6) core−shell nanocrystals were all greatly enhanced relative to a commercial Pt/C catalyst. For Pd@Pt2−3L octahedra, the specific and mass activities were as high as 0.74 mA cm−2 and 0.47 A mgPt−1, respectively.9 Due to the presence of twin boundaries, the specific and mass activities of
[email protected] icosahedra were further increased to 1.36 mA cm−2 and 0.64 A mgPt−1.10 Compared to the core−shell nanocrystals, nanocages have much higher utilization efficiency because the Pt atoms on the inner surface can also participate in the reaction. For Pt icosahedral nanocages, they showed a remarkably high mass activity of 1.28 mgPt−1, which represents the highest value ever reported for ORR catalysts based on pure Pt.15 In addition to the great
Figure 3. (A) Schematic illustration showing the two major steps involved in the deposition of Pt atoms on a Pd cubic seed: (1) deposition of Pt atoms at the corner sites and (2) diffusion of the deposited Pt atoms across the surface, where Vdeposition and Vdiffusion represent the rates for atom deposition and surface diffusion, respectively. At Vdeposition < Vdiffusion, the Pt atoms can quickly spread across the entire surface of a Pd seed, generating a Pd@PtnL core−shell nanocube with well-controlled number of Pt atomic layers. At Vdeposition > Vdiffusion, most of the deposited Pt atoms will stay at the originally deposited sites, generating a Pd−Pt core−frame nanocube. (B, C) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of an individual Pd@Pt3L core−shell nanocube. (D, E) HAADF-STEM images of an individual
[email protected] core−shell icosahedron. Panels A−C adapted with permission from ref 8. Copyright 2014 American Chemical Society. Panels D and E adapted with permission from ref 10. Copyright 2015 Nature Publishing Group.
improvement in specific and mass activities, all these novel catalysts exhibited significant improvement in catalytic durability. For example, the mass activity of Pd@Pt2−3L octahedra only dropped by 13% after 10 000 cycles of accelerated durability test.9 In summary, recent progress in nanochemistry has demonstrated the feasibility to have all the precious metal atoms in a catalytic particle presented on the surface with a well-defined and controllable packing by switching to the nanocage structure. With the use of nanocages relatively large in size (e.g., 10−20 nm), both the catalytic activity and selectivity can be optimized without compromising the catalytic durability. As the wall thickness of nanocages approaches two 452
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REFERENCES
(1) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 1996. (2) Ruditskiy, A.; Peng, H.-C.; Xia, Y. Shape-Controlled Metal Nanocrystals for Heterogeneous Catalysis. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 327−348. (3) Xie, S.; Lu, N.; Xie, Z.; Wang, J.; Kim, M. J.; Xia, Y. Synthesis of Pd-Rh Core−Frame Concave Nanocubes and Their Conversion to Rh Cubic Nanoframes by Selectively Etching of the Pd Cores. Angew. Chem., Int. Ed. 2012, 51, 10266−10270. (4) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343, 1339−1343. (5) Park, J.; Wang, H.; Vara, M.; Xia, Y. Platinum Cubic Nanoframes with Enhanced Catalytic Activity and Durability toward Oxygen Reduction. ChemSusChem 2016, 9, 2855−2861. (6) Duan, H.; Yan, N.; Yu, R.; Chang, C.-R.; Zhou, G.; Hu, H.-S.; Rong, H.; Niu, Z.; Mao, J.; Asakura, H.; Tanaka, T.; Dyson, P. J.; Li, J.; Li, Y. Ultrathin Rhodium Nanosheets. Nat. Commun. 2014, 5, 3093. (7) Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Controlling the Catalytic Activity of Platinum-Monolayer Electrocatalysts for Oxygen Reduction with Different Substrates. Angew. Chem., Int. Ed. 2005, 44, 2132−2135. (8) Xie, S.; Choi, S.-I.; Lu, N.; Roling, L. T.; Herron, J. A.; Zhang, L.; Park, J.; Wang, J.; Kim, M. J.; Xie, Z.; Mavrikakis, M.; Xia, Y. Atomic Layer-by-Layer Deposition of Pt on Pd Nanocubes for Catalysts with Enhanced Activity and Durability toward Oxygen Reduction. Nano Lett. 2014, 14, 3570−3576. (9) Park, J.; Zhang, L.; Choi, S.-I.; Roling, L. T.; Lu, N.; Herron, J. A.; Xie, S.; Wang, J.; Kim, M. J.; Mavrikakis, M.; Xia, Y. Atomic Layer-byLayer Deposition of Platinum on Palladium Octahedra for Enhanced Catalysts toward the Oxygen Reduction Reaction. ACS Nano 2015, 9, 2635−2647. (10) Wang, X.; Choi, S.-I.; Roling, L. T.; Luo, M.; Ma, C.; Zhang, L.; Chi, M.; Liu, J.; Xie, Z.; Herron, J. A.; Mavrikakis, M.; Xia, Y. Palladium−Platinum Core−Shell Icosahedra with Substantially Enhanced Activity and Durability towards Oxygen Reduction. Nat. Commun. 2015, 6, 7594. (11) Wang, X.; Vara, M.; Luo, M.; Huang, H.; Ruditskiy, A.; Park, J.; Bao, S.; Liu, J.; Howe, J.; Chi, M.; Xie, Z.; Xia, Y. Pd@Pt Core−Shell Concave Decahedra: A Class of Catalysts for the Oxygen Reduction Reaction with Enhanced Activity and Durability. J. Am. Chem. Soc. 2015, 137, 15036−15042. (12) Xia, X.; Figueroa-Cosme, L.; Tao, J.; Peng, H.-C.; Niu, G.; Zhu, Y.; Xia, Y. Facile Synthesis of Iridium Nanocrystals with WellControlled Facets Using Seed-Mediated Growth. J. Am. Chem. Soc. 2014, 136, 10878−10881. (13) Zhao, M.; Figueroa-Cosme, L.; Elnabawy, A. O.; Vara, M.; Yang, X.; Roling, L. T.; Chi, M.; Mavrikakis, M.; Xia, Y. Synthesis and Characterization of Ru Cubic Nanocages with a Face-Centered Cubic Structure by Templating with Pd Nanocubes. Nano Lett. 2016, 16, 5310−5317. (14) Zhang, L.; Roling, L. T.; Wang, X.; Vara, M.; Chi, M.; Liu, J.; Choi, S.-I.; Park, J.; Herron, J. A.; Xie, Z.; Mavrikakis, M.; Xia, Y. Platinum-based Nanocages with Subnanometer-Thick Walls and WellDefined, Controllable Facets. Science 2015, 349, 412−416. (15) Wang, X.; Figueroa-Cosme, L.; Yang, X.; Luo, M.; Liu, J.; Xie, Z.; Xia, Y. Pt-Based Icosahedral Nanocages: Using a Combination of {111} Facets, Twin Defects, and Ultrathin Walls to Greatly Enhance Their Activity toward Oxygen Reduction. Nano Lett. 2016, 16, 1467− 1471. (16) Sun, X.; Kim, J.; Gilroy, K. D.; Liu, J.; König, T. A. F.; Qin, D. Gold-Based Cubic Nanoboxes with Well-Defined Openings at the Corners and Ultrathin Walls Less Than Two Nanometers Thick. ACS Nano 2016, 10, 8019−8025.
Figure 4. Typical TEM images of (A) cubic, (B) octahedral, (C) icosahedral, and (D) decahedral Pt nanocages. The insets are the corresponding HAADF-STEM images of these nanocages. Panels A and B adapted with permission from ref 14. Copyright 2015 American Association for the Advancement of Science. Panel C adapted with permission from ref 15. Copyright 2016 American Chemical Society. Panel D adapted with permission from ref 11. Copyright 2015 American Chemical Society.
atomic layers, however, one must be concerned about their structural stability. To this end, the size and position (corner vs side face16) of the holes, as well as the elemental composition and lateral dimensions of the nanocages, all need to be optimized in order to maximize their structural and thermal stability. It should be pointed out that the utilization efficiency of active metal atoms can also be increased to 100% by switching to single-atom catalysts.19,20 However, the workings of such a catalyst seem to have a closer connection with a homogeneous rather than heterogeneous catalyst as no electronic band structures should be developed for individual atoms. The preparation and catalytic performance of a singleatom catalyst also critically rely on the presence of a proper environment (equivalent to the ligand shell of a homogeneous catalyst) to trap and stabilize the metal atom. In contrast, all the catalytic systems discussed in this Commentary are built upon the same principles as the conventional heterogeneous catalysts.
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Commentary
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Younan Xia: 0000-0003-2431-7048 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research described in this commentary was supported in part by the NSF (CHE 1505441) and startup funds from the Georgia Institute of Technology. 453
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Accounts of Chemical Research (17) Gilroy, K. D.; Ruditskiy, A.; Peng, H.-C.; Qin, D.; Xia, Y. Bimetallic Nanocrystals: Syntheses, Properties, and Applications. Chem. Rev. 2016, 116, 10414−10472. (18) Xia, X.; Xie, S.; Liu, M.; Peng, H.-C.; Lu, N.; Wang, J.; Kim, M. J.; Xia, Y. On the Role of Surface Diffusion in Determining the Shape or Morphology of Noble-Metal Nanocrystals. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 6669−6673. (19) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. SingleAtom Catalysts: A New Frontier in Heterogeneous Catalysis. Acc. Chem. Res. 2013, 46, 1740−1748. (20) Liu, J.; Lucci, F. R.; Yang, M.; Lee, S.; Marcinkowski, M. D.; Therrien, A. J.; Williams, C. T.; Sykes, E. C. H.; FlytzaniStephanopoulos, M. Tackling CO Poisoning with Single-Atom Alloy Catalysts. J. Am. Chem. Soc. 2016, 138, 6396−6399.
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