Growing Nanoscale Model Surfaces to Enable Correlation of Catalytic

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Perspective

Growing Nanoscale Model Surfaces to Enable Correlation of Catalytic Behavior Across Dissimilar Reaction Environments Daniel D. Robertson, and Michelle L Personick Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04595 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Chemistry of Materials

Growing Nanoscale Model Surfaces to Enable Correlation of Catalytic Behavior Across Dissimilar Reaction Environments Daniel D. Robertson† and Michelle L. Personick* Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459, United States *Email: [email protected] †

Current address: Department of Chemistry & Biochemistry, University of California Los Angeles, Los Angeles, California 90095, United States

Abstract The development of catalytic materials that can effectively facilitate currently challenging chemical transformations requires enhanced understanding of reaction mechanisms and insight into the structure and composition of optimal active site geometries. New methods for translating fundamental insights obtained in highly-controlled environments to industrially viable catalysts that function under dramatically different operating conditions can accelerate the catalyst design process. This Perspective highlights the application of noble metal nanoparticles with well-defined surfaces as nanoscale experimental models that open up opportunities to correlate fundamental reactivity and catalytic performance across reaction environments of increasing complexity. Recent advances in synthetic control over both nanoparticle shape and composition allow for the generation of specific active site geometries of interest on materials that can be stable in both ultrahigh vacuum and elevated pressure gas-phase environments, and potentially in solution-phase and electrocatalytic systems as well. Coupled with significant recent developments in surface science techniques, including operando spectroscopy methods, the use of nanoscale model surfaces represents a promising approach to establishing principles of reactivity and catalytic behavior in these diverse environments.

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1. Introduction The ability to rationally design catalytic materials using principles derived from fundamental surface science studies has long been a goal in the field of catalysis.1-4 Through the intelligent choice of material structure and composition, this approach promises to accelerate development of catalysts that can facilitate currently challenging chemical transformations and address an ever-increasing need for the sustainable utilization of energy resources. An ongoing challenge in the broader implementation of this strategy, however, is the difficulty of translating fundamental insights obtained under well-controlled conditions at low reactant coverages on single crystalline model surfaces to the design of functional catalytic materials operating at ambient pressures and higher reactant coverages.5-6 Directly understanding the principles of reactivity for existing “working” catalysts can also be challenging, as the vast majority of commercial catalysts are supported metal nanoparticles under 10 nm in diameter because of their high surface area to volume ratio and the ability to add functionality through an active support. The difficulty of controlling particle shape and composition in this size regime results in materials that are inherently complex from a fundamental perspective: the combination of varied surface facets, an inhomogeneous dispersion of bimetallic sites, and metal-support interactions limits efforts to understand their catalytic behavior at a mechanistic level, even when these catalysts are examined under model reaction conditions. Studying single crystal model surfaces under operating catalytic conditions is similarly impractical, as these macroscopic materials often do not have a sufficiently high surface area or density of active sites to drive a significant number of turnovers under standard reaction conditions. To address the challenge of bridging these regimes, it is necessary to develop model catalytic materials that have sufficient catalytic activity but also have well-defined and


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relatively uniform surface structures so that they can realistically offer insights under both model and operating conditions. To this end, larger nanoparticles with well-defined shapes and compositions show significant potential as a transitional model system to link reactivity studies carried out under ultrahigh vacuum (UHV) model conditions with those conducted under atmospheric conditions. In the last two decades, the maturation of nanoparticle synthesis for materials with length scales of around 50 to 200 nm has enabled a precise degree of control over both surface structure and composition that remains challenging for sub-10 nm particles.7-11 In particular, the strategic choice of reaction conditions in colloidal synthesis techniques promotes growth of monodisperse nanoparticles that uniformly expose specific chosen surface facets. These well-defined nanocrystal facets offer a near analog to model single crystal surfaces in terms of their ordered atomic arrangement, with the advantage of being localized on materials with sufficient surface area to operate as catalysts with a non-negligible turnover rate. Importantly, syntheses for such nanomaterials are also highly versatile because of the wide parameter space of possible growthdirecting agents and reaction conditions. As a result, colloidal syntheses have opened up new opportunities to tailor nanoparticle structure and composition to match surface features of interest analyzed in fundamental studies. Recently, this strategy has seen success in addressing the challenge of bridging dissimilar materials and pressure regimes because it provides a means to generate specific active sites on the surfaces of functional catalyst materials. Doing so allows researchers to extrapolate experimental observations from model surfaces at low reactant coverages to functional materials under conditions of catalytic turnover, or to test computationally predicted active sites that are not accessible using macroscopic single crystal substrates.



In this Perspective, we examine contributions to research in this area and highlight the potential of colloidally synthesized nanoparticles as experimental models that can contribute to understanding catalytic behavior and reactivity in both fundamental and functional contexts. A primary goal of this Perspective is to provide foundational background information regarding both surface science studies of catalytic reactivity and principles of nanomaterials synthesis for shaped noble metal nanoparticles to encourage collaboration at the interface between these two fields. We begin by exploring the major features of a catalytic material that can influence its activity and selectivity, and follow by establishing how these characteristics can be controlled synthetically in noble metal nanoparticles. We then show how a collective understanding of these principles enables the design of nanoparticle catalysts that can test the results of fundamental studies in operating catalytic conditions. Notably, the capacity to finely control both shape and composition of these catalysts has provided new depth to this strategy by enabling the modeling of bimetallic active sites with specific structural arrangements. Finally, we explore the outlook for the use of well-defined noble metal nanoparticles as catalytic materials that can bridge pressure regimes as a way to realize a more complete understanding of surface reactivity, and briefly discuss some challenges to this approach, including the dynamics of catalyst structure and the presence of residual adsorbates from colloidal syntheses. 2. Materials Parameters that Influence Catalyst Behavior At a basic level, a heterogeneous catalyst provides a surface that stabilizes reaction intermediates in order to facilitate an alternate energetic pathway for the reaction. The new mechanism of the reaction on the surface of the catalyst depends on the strength and orientation with which species chemisorb to that surface.12 As a result, the rate at which the reaction proceeds and the distribution of products that form are determined by these binding interactions between


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the catalyst surface and the specific chemical species involved. Importantly, optimal reaction rates and high selectivity are both achieved when species chemisorb to the catalyst surface with intermediate strength, so that the reaction is not limited by the availability of adsorbed reactants nor by the desorption of products.13-14 When reaction products, in particular, bind too strongly and accumulate on the catalyst surface, they can poison the catalyst by inhibiting further adsorption of reactants, or they can undergo unselective side reactions to form unwanted byproducts. 15-17 Modifying the properties of a catalyst that affect these binding interactions—primarily surface structure and composition—can be used to tune binding strength of reactive species and thus to influence catalytic performance for a particular chemical transformation. In the following section, we briefly discuss these factors and present some guiding principles to describe how they influence catalytic activity and selectivity. 2.1 The Relationship Between Catalyst Structure and Reactivity The surfaces of metal crystals expose specific crystal facets (Figure 1), and differences in the arrangement of atoms on these facets can lead to differing chemical and physical interactions with reactant species. Fabricated bulk single crystals used in fundamental surface science are cut strategically to expose desired surface facets, while surface facets on nanoparticles are determined by the shape of the particle, which forms during a bottom-up growth process. These crystal facets vary in energy based on the degree of coordination of their surface atoms, with surfaces containing more undercoordinated atoms—which have less than the maximum number of nearest neighbors— having higher energies than fully coordinated surfaces. For example, the most thermodynamically stable facets for face-centered cubic (fcc) metals are the {111} facets, in which the surface atoms are hexagonally close packed and each surface atom has the maximum coordination number (9coordinate) for an fcc metal.9 The coordination of surface atoms is especially important in catalysis,



because atoms that are undercoordinated have unfilled valence shells and thus are more reactive to incoming molecules. As a result, the reactivity of a crystal facet tends to increase proportionally to its surface energy.

Figure 1. Examples of noble metal nanoparticles with well-controlled shapes defined by specific surface facets: (A) {100}-faceted cube; (B) {111}-faceted octahedron; (C) {110}-faceted rhombic dodecahedron; and (D) high-index {hkl}-faceted concave cube, where one or more of the Miller indices (h, k, or l) is greater than 1. Modified with permission from reference 18. Copyright 2014 Cambridge University Press. Beyond this general trend, however, the interaction of specific molecules with the atomic structure of the catalyst surface is highly complex and varies depending on the chemical species and reaction conditions involved.12, 19 The influence of surface structure on catalytic behavior can be broadly divided into two categories: electronic effects and geometric effects.12 For monometallic materials, electronic effects of surface structure on reactivity are a consequence of the differing coordination number of surfaces atoms in various facets as well as at step edges and terraces. Transition metal atoms with lower coordination numbers—such as those at step sites, edges, and defects—generally bond more strongly to adsorbates as a consequence of their 6 ACS Paragon Plus Environment

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electronic structure (specifically, higher d-band centers).20-21 The identity of the metal atom of interest also influences its electronic structure, and, therefore, its binding affinity and reactivity.22 Geometric effects, in contrast, result from preferential orientations and bonding configurations of an intermediate as a function of the atomic arrangement of the catalyst surface. For example, the alignment of key reaction intermediates with the atomic spacing of a certain surface facet can lead to significantly increased activity or selectivity on that facet compared to on others. 23-25 Often, electronic and geometric effects contribute concurrently to the mechanism of a reaction on a particular surface, and their relative influences on the reaction pathway can sometimes be difficult to deconvolute. An example of this structure sensitivity was demonstrated by Somorjai et al., who showed that the hydrogenation of benzene over {100} platinum (Pt) cubes produced only cyclohexane, but the same reaction over Pt cuboctahedra containing both {111} and {100} facets produced a mix of cyclohexane and cyclohexene (Figure 2).26-27 In this case and many others, surface structure features that influence the orientation of adsorbed reactants can have controlling effects on reaction selectivity, especially in cases where reactants contain two or more diverse functional groups.28 The selective hydrogenation of α,β-unsaturated aldehydes offers a schematic example of the importance of binding orientation for product distribution (Scheme 1). The relative stability of the olefin-bound conformation of the reactant compared to that of the carbonyl-bound conformation determines whether an aldehyde or an unsaturated alcohol is produced, respectively. The topography of the catalytic surface influences the stability of these bound intermediates, and thus, controls the outcome of the reaction.



Figure 2. Illustrative example of differences in catalytic selectivity over metal nanoparticle catalysts with different shapes. (A) Model of the {100} surface facet exposed by Pt cubic nanoparticles and (B) model of the {111} surface facet exposed by the truncated corners of Pt cuboctahedral nanoparticles. (C) TEM image of Pt nanocubes and schematic selectivity of benzene hydrogenation over Pt nanocubes. (D) TEM image of Pt cuboctahedra and schematic selectivity of benzene hydrogenation over Pt cuboctahedra. Hydrogenation on {100} facets (purple surfaces) yields cyclohexane, while reaction on {111} facets (blue surfaces) produces both cyclohexane and the partial hydrogenation product, cyclohexene. Scale bars: 50 nm. Modified with permission from references 9 (A,B) and 27 (C,D). Copyright 2013 and 2007 American Chemical Society. Scheme 1. Simplified Representation of the Effect of Adsorbate Binding Orientation on Product Selectivity in the Partial Hydrogenation of ,-Unsaturated Aldehydesa

a

Reproduced with permission from reference 28. Copyright 2015 American Chemical Society.

2.2 Synergistic Effects on Catalysis in Bimetallic Materials The addition of a second metal to a catalyst to generate an alloy can change its behavior in subtle or dramatic ways, depending on the relative amounts of the two metals and the type of alloying that results. The influence of an auxiliary metal on the binding of reactants and other


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catalytic properties is again attributed to the combination of geometric effects, which result from alterations in lattice structure due to the lattice mismatch between different components, and electronic effects, which result from the donation of electron density from one metal to another.12, 20, 29-31

Importantly, these effects depend not only on the amount of the second metal, but also on

the atomic structural configuration and overall architecture of the alloy. Dilute, core-shell, mixed, intermetallic, and phase-segregated alloys are all possible in bimetallic materials, with each having different outcomes with respect to catalyst performance. Dilute alloys, in which the material is largely composed of one metal but also contains a relatively small amount (< 10%) of another, often retain the catalytic behavior of the primary metal while improving its functionality or adding new capabilities. For instance, the dilution of a highly active metal in a less active host metal can be used to promote greater selectivity. By tuning the composition of the alloy, activity or selectivity can be improved by increasing or decreasing the abundance of the more active metal, respectively. For example, gold (Au) shows exceptional selectivity and stability for selective oxidation and hydrogenation reactions when pre-covered with adsorbed oxygen (O) or hydrogen (H) atoms or in the presence of a suitable support material, but Au lacks the ability to independently activate molecular reactants such as O2 and H2 due to its low reactivity.32-34 In contrast, the energetic barriers to O2 and H2 dissociation are much lower on silver (Ag) and palladium (Pd), respectively, but these materials are non-selective, leading to combustion or over-hydrogenation. Alloying a dilute amount of Ag or Pd with Au allows the resulting material to maintain the weak binding and favorable selectivity of Au while simultaneously granting the catalyst the ability to dissociate molecular O2 or H2.7, 17, 35-38 Even in catalytic materials which have a near 50:50 bulk ratio of a more active and a less active metal, the formation of a dilute surface alloy shell can lead to similar behavior. Hutchings



and others have reported that AuPd alloys exhibit enhanced catalytic performance in the selective oxidation of alcohols,39-40 polyols,41-42 and methane43 relative to monometallic Pd materials.44 They show that these materials, despite having a 50:50 overall Au:Pd composition, develop a Pdrich shell around a Au-rich core due to surface segregation during the calcination pretreatment. The increased reaction rate on the bimetallic surfaces of these nanoparticles has been attributed primarily to a decrease in adsorbate binding strength on the Pd-rich Au-Pd alloy surface.42 Although DFT calculations by Medlin and coworkers showed that the electronic contribution of added Au actually increased the d-band center of Pd slightly, experimental work by the same group indicated that geometric ensemble effects from the lack of three-fold Pd hollow sites on the AuPd alloy surface were responsible for the overall decrease in adsorbate binding strength. Because the more stable hollow sites were so few, key intermediates on the AuPd surface were required to shift to less stable bridge sites, which weakened their interaction with the catalyst surface. When a less expensive metal is used as the diluent for a more precious active metal, dilute alloy catalysts can also considerably decrease material cost with minimal detriment to, or even improvement of, catalytic performance. This approach has shown potential for Pt- and Pd-based catalysts, which are highly active for a number of hydrogenation and oxidation reactions, but have seen relatively limited commercial application because of their high cost.45 Single atom alloys of these metals, in which individual atoms of Pt or Pd are dispersed in a more inert host metal, such as copper (Cu), have been shown to function more effectively than bulk Pd or Pt in a wide variety of selective hydrogenations.46-50 Notably, beyond just advantages in material cost in these systems, dilute alloys—of Pt in particular—offer resistance to poisoning by carbon monoxide (CO), which is an ongoing challenge in the field of Pt-based catalysis.51-52


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Core-shell alloys, in which a shell of one metal surrounds a core of another, are similar to dilute alloys in that they provide an active surface of one metal while allowing the second metal to influence the first’s catalytic behavior. Whereas dilute alloys typically utilize the minor component to provide emergent catalytic behavior, core-shell particles rely on the core to influence the existing catalytic behavior of the shell.53-55 Importantly, synergistic effects still exist even when the core metal is not accessible to reactants as long as the shell is relatively thin. Fundamental computational studies have explored the promising catalytic behavior of near-surface-alloys (NSAs)—two dimensional analogs of the core-shell structure in which one or two monolayers of one metal cover more layers of another.2, 31, 56 For example, Mavikrakis and coworkers found that the unique synergistic effects of certain metal combinations in NSAs produced catalytic surfaces that bound H atoms relatively weakly while simultaneously permitting facile H2 activation, two key attributes for potential selective hydrogenation catalysts.57-59 In addition to modification of catalytic behavior through bimetallic effects, core-shell particles again decrease material cost when a catalytically active shell covers a less expensive core, which comprises the bulk of the catalyst. Mixed bimetallic alloys in which the amounts of each metal are relatively similar offer surfaces whose catalytic behavior can be varied significantly by tuning over a wide range of compositions. At different regimes of relative composition, behavior from one metal can dominate as in a dilute alloy, or the material may exhibit properties unlike either of its components.3, 13 Often, the performance of a material as a function of composition follows a volcano plot, where maximum activity and/or selectivity is reached at some intermediate composition fraction. Yang et al. demonstrated this concept experimentally by testing bimetallic nanoparticles with a variety of Au:Cu ratios for the electrochemical reduction of carbon dioxide (CO 2) and found that the production of CO reached its maximum at 25% Cu in the nanoparticles (Figure 3A,B).60 This



Au3Cu alloy maximized activity because the electronic and geometric influence of the Au on Cu weakened the binding of the CO product enough to allow it to desorb readily without compromising the Cu’s affinity for the critical COOH intermediate. Specifically, the addition of Au weakened the binding strength of the carbon-metal bond for both the CO and COOH intermediates via electronic effects resulting from the lower-lying d-band of Au relative to Cu (Figure 3C). Based solely on electronic effects, however, monometallic Au would be predicted to be the most active catalyst for the conversion of CO2 to CO. The differential stabilization of CO and COOH on the Au3Cu alloy catalysts was proposed to be a result of geometric effects, in which Cu atoms adjacent to a Au-C bond further stabilized COOH relative to CO by forming a bond with the oxygen end of COOH (Figure 3D). Alternatively, a second metal can provide additional functionality, similarly to the case of dilute bimetallic alloys. For example, studies by Davis et al. identified a bifunctional mechanism for glycerol hydrogenolysis on PtRe alloy catalysts, in which partially oxidized rhenium (Re) serves as a Brønsted acid site and promotes activity and selectivity on the bimetallic catalyst.61-62 The first step of the reaction is an acid-catalyzed dehydration that takes place on Re, followed by hydrogenation of the unsaturated intermediate on neighboring metallic Pt. The oxophilicity of Re opens up reactive pathways on the bimetallic catalyst that are not accessible on more noble monometallic Pt catalysts.


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Figure 3. Example of the synergistic effects of bimetallic composition on catalyst performance. (A) Relative turnover rates for CO production in the electrocatalytic reduction of CO2 on Aucontaining bimetallic nanoparticle catalysts relative to monometallic Cu and (B) CO mass activity for the same AuCu bimetallic nanoparticles in the same electrocatalytic reaction. (C) Surface valence band photoemission spectra for a series of AuCu bimetallic catalysts, where the white line indicates the d-band center. (D) Schematic representation of the proposed mechanism for CO2 reduction on different AuCu alloys. Carbon is represented by grey circles, oxygen by red circles, and hydrogen by white circles. The relative binding strength of different intermediates is indicated by the thickness of the wedged bonds. Relative product distributions are indicated by the thicknesses of the colored arrows, with the colors representing pathways to different products: blue for formate, red for CO, and green for hydrocarbons. Production of CO is maximized on the Au3Cu catalyst (A,B) due to a combination of electronic (C) and geometric effects (D). Modified from reference 60 by permission from Springer Nature, copyright 2014. The possibilities for tuning reactivity in mixed alloys extend beyond just the fraction of each component metal, as the mixing pattern of metals in the alloy can influence catalytic



performance as well. In testing PdCu particles for electrochemical CO2 reduction, for example, Yamauchi and Kenis et al. noted that intermetallic ordered alloy, disordered alloy, and phaseseparated catalysts generated different distributions of products because of geometric effects resulting from the variation in relative abundance of specific alloy active site configurations.63 The prevalence of Cu-Cu bonding in the phase-separated particles facilitated coupling of adsorbed CO intermediates to form C2 products such as C2H4 and C2H5OH, while intermetallic Pd-Cu sites in the mixed alloy arrangements favored CH4 production. In general, intermetallic alloy materials with well-defined atomic ordering of the component metals have shown distinct, and often enhanced, catalytic behavior with respect to activity and stability in comparison to disordered mixed alloy materials, particularly for electrocatalytic reactions.64-65 In a follow-up study to their work on varying Au:Cu ratios described above, Yang and coworkers compared the behavior of disordered and intermetallic AuCu nanoparticles with fixed 1:1 Au:Cu compositions and similar sizes (~7 nm) for the CO2 electroreduction reaction.66 The ordered AuCu intermetallic particles were more selective for CO2 reduction to CO, while the disordered AuCu particles exhibited higher selectivity toward H2 evolution. Based on DFT studies, the authors attributed the improved activity of the ordered AuCu particles to a compressively strained three-atom-thick Au overlayer observed experimentally by aberration-corrected scanning transmission electron microscopy (STEM) and x-ray absorption spectroscopy (XAS). This surface enrichment of compressively-strained Au, which is a specific consequence of the smaller lattice constant of the intermetallic material, led to an intermediate binding strength of the key intermediate for CO2 reduction, COOH, compared to the binding strength of COOH on pure Au or the disordered AuCu particles. Enhancement of catalytic activity and durability due to a strained overlayer on an intermetallic core had also been observed previously for Pt3Co intermetallic particles with two-to-three-atom-thick Pt shells, which


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were employed as catalysts for the oxygen reduction reaction.67 The direct comparison of the catalytic performance of disordered alloys with their intermetallic analogues is a relatively new area of research due to the challenges of generating disordered and intermetallic particles with the same size and shape, and continuing advances in materials synthesis will facilitate the further expansion of this field.64-65 3. Colloidal Syntheses of Noble Metal Nanoparticles While a variety of techniques can be used to synthesize noble metal nanoparticles, colloidal syntheses are unparalleled in their capacity for synthesis of monodisperse, well-defined particle shapes.9,

11, 68-69

This structural monodispersity is key for establishing structure-function

relationships, such as in the use of nanoparticles as experimental models to realize surfaces of interest for catalysis. Here, we discuss strategies for controlling shape and composition during colloidal nanoparticle synthesis with a focus on seed-mediated methods. The general framework of any colloidal nanoparticle synthesis involves solution-phase reduction of metal salt precursors to induce nucleation and growth of solid metal nanostructures. Typically, the solution in which these processes occur contains, at minimum, a metal salt precursor, a reducing agent, and a surfactant or polymer capping agent that prevents particle aggregation, although a variety of morphology-directing additives can also be used. Colloidal syntheses can be divided into two main categories based on the mechanism by which particles are initially nucleated. Homogeneous nucleation occurs when no seed or template is used to facilitate the process, and typically requires strong reductants and/or high temperatures in order to produce particles with uniform shapes and sizes. Polyol syntheses, for example, in which high boiling point alcohols are employed as both the solvent and the reductant, have seen significant success in using homogenous nucleation to produce monodisperse metal nanostructures.70-72 Alternatively, seed-mediated syntheses separate



particle growth into two discrete steps: a nucleation phase, during which seeds are produced, and a growth phase in which metal is reduced onto the seed particles to yield the final nanoparticle colloid.9, 73-75 In the former, seed generation is induced via reduction of the metal salt with a strong reducing agent such as NaBH4, which promotes formation of small, monodisperse seed particles through the rapid simultaneous nucleation of large number of nanoparticles in solution. Once the strong reducing agent has degraded, these seeds are injected into a reaction solution that contains additional metal salt, a weak reducing agent, a stabilizing capping agent, and morphology-directing additives to initiate growth of larger nanoparticles. Importantly, the use of a weak reducing agent in this second step promotes growth onto the seeds instead of further homogeneous nucleation and provides a slower reaction rate so that manipulation of growth conditions leads to a meaningful influence on the morphology of the resulting nanoparticles. 3.1 Shaped Nanoparticles with Tailored Surface Facets For both one-step and seed-mediated methods, the conditions of growth determine the final shape and crystallinity of the resulting nanoparticles.9, 11 As discussed in the previous section, different nanoparticle shapes expose specific surface facets that differ in atomic arrangement and energy. In direct analogy to the case of catalytic behavior, differences in the energies of these surface facets affect their reactivity toward the deposition of metal atoms during nanoparticle growth, as well as their stability toward other dynamic processes that take place during particle formation, such as dissolution and rearrangement. During particle synthesis, the strategic choice of reaction conditions energetically favors expression of a certain facet and thereby promotes a high yield of the corresponding nanoparticle shape that is bound by that facet. The mechanisms for tuning reaction conditions to achieve shape control can largely be divided into two primary pathways: selective surface passivation and kinetic control.9, 69 The former involves the use of


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capping agents, or species that bind to the surface of the growing nanoparticle, to influence the relative stabilities of particular surface facets and direct particle growth, while the latter controls nanoparticle formation through the manipulation of reaction rate. At the most fundamental level, surface passivating species present in solution form bonds with atoms at the particle surface and thus decrease the growing particle’s surface energy. Beyond just a general stabilization of the surface, however, capping agents can also play an important role in controlling particle shape through the selective passivation of specific surface facets. As metal is reduced onto the surface of a growing nanoparticle, it must displace any adsorbed molecules, so the strength with which these species bind to a facet affects how quickly that facet grows. Consequently, the use of a capping agent that strongly passivates a particular facet during synthesis results in a particle shape entirely bound by that facet (Figure 4). There are a wide variety of potential additives to use as capping agents for selective surface passivation, including, most commonly: surfactants or polymers, halide anions, and other metal ions, each with different mechanistic effects on particle growth. Bromide anions, for example, whether added in a salt such as NaBr or along with a surfactant such as cetyltrimethylammonium bromide (CTA-Br), have been found to promote the formation {100} surface facets in several different metal systems.76-78 Another common capping agent is polyvinylpyrrolidone (PVP)—a non-ionic polymer which stabilizes particles against agglomeration due to its size—which has been frequently employed in polyol syntheses of Ag nanocubes and nanowires, where it plays an important role in passivating {100} surfaces as well.79 The addition of citrate to particle growth conditions, in contrast to both PVP and bromide, tends to stabilize {111} metal surfaces much more strongly than it does {100} because the threefold symmetry of citrate’s carboxyl groups aligns well with the lattice symmetry of the {111} surface.69, 80



Figure 4. Schematic representation of nanoparticle growth directed by selective surface passivation. A selective surface passivating agent—shown here with an affinity for the purple surface facet—binds to the surface of a particle enclosed by a mixture of purple and blue facets. During particle growth, an incoming metal atom must first displace the surface passivating agent on the purple facet before depositing, which slows deposition on this surface relative to metal atom deposition on the blue facet. As a result, growth perpendicular to the blue facet occurs faster than growth perpendicular to the purple facet. As new purple facets form, they are passivated by the surface passivating agent. The final result is a particle bound entirely by the selectively passivated facet. The second pathway for directing nanoparticle growth, kinetic control, involves tuning the rate of metal ion reduction in order to govern which facets are present at the surface of the particle. The rate of reduction largely determines whether metal atoms are deposited at thermodynamically stable sites with lower absolute energy, or at easily accessible sites at which the activation energy of metal deposition is lowest. Slow rates of reduction favor the former because they allow metal atoms to diffuse across the surface until they reach low-energy equilibrium states.11, 69 When the reaction rate is fast, however, atoms are deposited at sites where reduction is most facile, and have little time to rearrange. Under such circumstances, deposition is likely to occur at sites with a high degree of undercoordination, such as the corners or step edges of a particle, because the favorability of increasing the coordination of those atoms leads to a low barrier to bond with them. Experimentally, the reduction rate can be altered using several parameters, including the concentration and reducing strength of the reductant, the pH of the solution, the reaction


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temperature, the availability of metal ions, and the binding strength of adsorbates on the particle surface.9 With a variety of species to use as surface passivating agents, and a number of ways to manipulate reaction rate, the interplay of these two pathways during particle growth enables a wealth of opportunities for shape control in colloidal nanoparticle synthesis. Notably, the addition of a certain molecule to the growth solution can sometimes change multiple different growthcontrolling parameters at once. Consider, for example, the reduction of HAuCl4 by ascorbic acid, a weak reducing agent, onto pre-synthesized Au seeds in a solution of CTA-Br.81 The bromide from the CTA-Br not only passivates the surface of the growing particle, but also slows the reaction rate by displacing chloride ligands on the Au precursor and lowering the reduction potential of the Au ion complex, thereby making the Au precursor more difficult to reduce. At relatively low concentrations of ascorbic acid, the slow rate of reduction enables rearrangement of the growing particles to form thermodynamically stable {111}-faceted octahedra. With a greater amount of ascorbic acid added to the growth solution, this rearrangement plays less of a role, and instead, the selective passivation of bromide on {100} surfaces results in {100}-Au nanocubes. As the ascorbic acid concentration in the growth solution is increased further to a large excess, the fast reaction rate overcomes the passivating influence of the bromide and leads to the formation of high index {221}-faceted trisoctahedra. Even in this relatively simple example, the competing influence of the selective surface passivation and kinetic control pathways at different growth conditions offers tunable control over particle shape through the variation of only one or two experimental parameters (Figure 5).81-85 In more complex systems of particle growth, the wide parameter space of different chemical tools for influencing the expression of specific facets opens up numerous



possibilities for the synthesis of monodisperse, uniformly shaped nanoparticles with a broad range of well-defined low- and high-energy surfaces as well as controlled defect structures.86-89

Figure 5. Graphical representation of the complex interplay between ascorbic acid and bromide ions in controlling Au nanoparticle shape based on collected data from the literature. Colored regions correspond to the reaction conditions where each particle shape is observed to form: octahedra (blue), cubes (red), trisoctahedra (purple), and rhombic dodecahedra (green). The x-axis is increasing bromide concentration and the y-axis is increasing ratio of reducing agent (ascorbic acid) to Au ion precursor. Axes are on a log scale to facilitate the representation of many reaction conditions on a single graph. Colored regions and boundaries are included to guide the eye, as more data points are needed to establish firm boundaries. In addition, there are some minor experimental differences between the studies shown, such as the use of surfactants with slightly different headgroups (cetyltrimethylammonium vs. cetylpyridinium), which may slightly shift the region of formation for a particular nanoparticle shape. 3.2 Control of Composition and Alloy Structure in Bimetallic Nanoparticles Shape in bimetallic nanoparticle synthesis is largely manipulated through the same pathways of kinetic control and selective surface passivation used for monometallic nanoparticles, but with the added requirement of accounting for differences in structure and reactivity between metals. In particular, lattice mismatch between two metals, disparity in the reduction potentials of their ions, and/or high cohesive energy of one or both metals tend to generate additional challenges during bimetallic particle growth, as they can lead to immiscibility or poor alloying. Typically, homogeneous alloying is best achieved through rapid, simultaneous reduction of both components, 20 ACS Paragon Plus Environment

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which can be accomplished through the use of strong reducing agents and high temperatures.90 Unfortunately, particle shape and uniformity are difficult to control under these conditions, since the need for fast reduction leaves few opportunities to meaningfully alter the mechanisms of growth. Still, shaped mixed alloy particles can be obtained through the judicious choice of capping agents that bind strongly enough to influence the relative stabilities of surface facets even in harsh reduction conditions. For example, Yu et al. achieved simultaneous reduction of Pd and Cu to form 1:1 PdCu alloyed particles using elevated temperatures (200°C) and found that the choice of an appropriate concentration of trioctylphosphine capping agent led to a high yield of monodisperse {100} PdCu nanocubes.91 Alternatively, shaped bimetallic nanoparticles with other compositional architectures can be synthesized by utilizing the differences between metal species to develop specific configurations of alloying.92-93 In the AuPt bimetallic system, for instance, which is complicated by a severe lattice mismatch as well as a significant difference in reduction potentials of the Au and Pt ions, we recently reported the successful synthesis of shaped Au@Pt core-shell particles with smooth, well-defined AuPt alloy surfaces using the difference in precursor reduction potentials as a way to develop the core-shell architecture.94 By manipulating the reduction rate of Au ions through the addition of halide anions, the facile modification of particle shape, composition, and shell thickness was achieved (Figure 6A). This method enables the generation of core-shell particles with a range of surface facets, including those most commonly studied in fundamental surface science experiments: {111}, {100}, and {110}. In addition, we have also shown that additives such as iodide ions can also be used to differentially tune the relative reduction rates of two metal ion precursors, such as Pd and Au or Pd and Cu, in co-reduction



reactions to generate more complex nanoparticles with high energy stepped surfaces (Figure 6B,C).95-96

Figure 6. Examples of methods for controlling the shapes of bimetallic nanoparticles by tuning the relative rates of reduction of two metal ion precursors in a co-reduction reaction. (A) The shape of Au@Pt core-shell nanoparticles can be tuned by adjusting the rate of reduction of the Au precursor through the addition of bromide ions according to the well-studied principles of monometallic Au nanoparticle formation. The Au precursor reduces more rapidly than the Pt precursor, which slowly forms a Pt shell. (B) The rate of reduction of Pd2+ can be increased by introducing low micromolar concentrations of iodide ions. This shifts the particle morphology from alloyed AuPd rhombic dodecahedra to AuPd tetradecapods, where the Pd grows out from the edges to form nanoparticles with both convex and concave features. (C) Similarly, the addition of low micromolar concentrations of iodide ions differentially tunes the rate of reduction of Pd and Cu ion precursors. In the absence of iodide, alloyed PdCu tripods and rods form from the coreduction of Pd2+ and Cu2+ precursors. The addition of iodide ions accelerates the rate of reduction of Pd ions while concurrently slowing the rate of reduction of Cu ions, yielding terraced particles whose steps are stabilized by underpotentially deposited copper. Scale bars: (A) 100 nm, (B,C) 200 nm. Modified with permission from references 94 (A), 95 (B), and 96 (C). Copyright 2018 and 2017 Wiley VCH (A,B) and 2018 Royal Society of Chemistry (C). Another technique to develop compositional architecture in shaped bimetallic particles is to strategically employ the synergistic oxidation-reduction processes that occur between metals as a means to overcome differences in relative nobility. One such process is underpotential deposition, which enables the reduction of up to a monolayer of metal onto an existing surface of another metal at a potential more positive than the former’s Nernst potential.97-98 Under normal 22 ACS Paragon Plus Environment

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circumstances, reduction cannot occur unless the applied potential is more negative than the Nernst potential of the metal being reduced. In the case of underpotential deposition, however, the presence of a mild ionic interaction between the metal and the existing surface stabilizes deposition of the metal atoms and facilities their reduction, even when the reducing agent would not normally be strong enough to do so. As a result, this phenomenon allows for the controlled introduction of a second, less noble metal at the surface of a nanoparticle composed of a more noble metal provided that the reducing agent is chosen appropriately.99-101 Underpotential deposition can be carried out in two steps to add a second metal to preformed monometallic nanoparticles, or in a single simultaneous bimetallic reduction step, where the underpotentially deposited metal acts as a passivating agent to stabilize undercoordinated step sites (Figure 7A). The latter has been explored with a high degree of success in the synthesis of dilute Ag-Au alloy shapes, where the addition of increasing concentrations of Ag ions to the bimetallic growth solution results in the formation of particles with increasing densities of step edges (Figure 7B,C).81, 100, 102 In a solution of CTA-Cl, low Ag concentrations result in thermodynamically favored shapes such as {111}bound octahedra, while the influence of higher amounts of Ag in the growth solution leads to the production of particles with kinetically-trapped high-index surfaces, such as {720}-bound concave cubes.100 When bromide is present during similar growth conditions, the slower rate of Au ion reduction and the slight destabilization of underpotentially deposited Ag by the bromide anions yields {730}-bound tetrahexahedra.103-104



Figure 7. (A) Schematic representation of the stabilization of stepped Au facets by underpotential deposition of Ag. (B) Au nanoparticle shapes with increasing densities of steps that form in the presence of increasing concentrations of Ag+ in the particle growth solution: {110}-faceted rhombic dodecahedra, {310}-faceted truncated ditetragonal prisms, and {720}-faceted concave cubes. Scale bars: 200 nm. (C) Models of the surface facets of the particles shown in (B), with surface atoms highlighted. (B,C) Modified with permission from reference 100, copyright 2011 American Chemical Society. While underpotential deposition facilitates reduction of a less noble metal during bimetallic particle synthesis, galvanic replacement enables exchange of a less noble sacrificial metal (typically Cu or Ag) with a more noble metal species (commonly Pt, Au, or Pd).105 During particle synthesis, metal atoms from a growing or pre-formed particle can be oxidized by metal ions with a higher reduction potential through a galvanic replacement mechanism to reduce ions of the second metal onto the particle surface. This process is driven by the difference in reduction potentials between the two metal species, and thus can only occur when the sacrificial zero-valent metal is less noble than the metal that it reduces. Careful control of reaction stoichiometry during 24 ACS Paragon Plus Environment

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galvanic replacement enables the generation of dilute bimetallic alloys, including highly dilute single atom alloys.46-48 Additionally, deposition and dissolution sites in such processes are highly dependent on surface capping agents present in solution, so particle morphology can still be influenced through selective surface passivation, even though metal ion reduction occurs through a different mechanism.105-106 For example, Zhang, Yan, et al., utilized sequential galvanic replacement processes directed by bromide anions in a one-pot colloidal synthesis to develop PtCu and PtPdCu high-index concave nanocubes.107 Alternatively, underpotential deposition and galvanic replacement can complement each other to promote alloying,106 as demonstrated by Kuang, Zie, et al., in the synthesis of high-index faceted AuPd hexoctahedra.108 During the particle growth process, Au is reduced first because of the high reduction potential of AuCl4-. The presence of an Au particle surface then allows for reduction of Cu2+ ions through underpotential deposition to form a surface layer of Cu, which can be galvanically replaced by Pd2+ ions to yield the final AuPd alloy particles. In this mechanism, the Cu facilitates formation of the AuPd alloy by making the rates of Au and Pd reduction more similar. Without any Cu present, particles with small heterostructures form instead of the well-defined hexoctahedra. Together, control of metal nanoparticle shape and composition enables the bottom-up generation of well-defined model surfaces suitable for catalytic evaluation under a wide variety of reaction conditions. Through established colloidal synthesis approaches, it is possible to access both analogs of commonly studied single crystal model surfaces—such as {111}, {100}, and {110}—as well as surfaces with higher energy facets. In addition, syntheses for the full range of bimetallic architectures of fundamental interest—dilute bimetallic, core-shell, or homogeneous alloy—are accessible or actively being developed, depending on the metal combination. Overall, the recent rapid expansion in the variety of surface facets and compositions available via colloidal



nanoparticle synthetic techniques presents exciting opportunities to combine the complementary tools and expertise of nanoparticle synthesis and fundamental surface science. 4. Noble Metal Nanoparticles as Experimental Models of Catalytic Active Sites Fundamental studies on catalysts have achieved a detailed understanding of basic reactivity and its relationship to catalytic performance for a variety of systems. Experiments on model surfaces have proven essential both in investigating the surface structure of catalytic materials as well as in elucidating the mechanisms by which chemical reactions proceed on those surfaces. Theoretical calculations have been used to develop an atomic-level description of catalytic behavior by determining the energetics of various processes, including the adsorption of reactants or intermediates at active sites and the interactions between these species on catalytic surfaces. Utilizing these mechanistic insights into catalytic reactivity alongside the synthetic capacity to tune nanoparticle shape and composition offers a promising method for the rational design of catalytic materials. In cases where specific structural configurations have been identified as potential active sites of interest, this combination often enables the direct reproduction of these structures on the surfaces of nanoparticle catalysts. These nanoparticle analogs to macroscopic model surfaces provide a way to re-evaluate fundamental insights under new catalytic conditions, since their well-defined structures can potentially remain stable in a variety of environments. Through this approach, experimental observations on single crystal model surfaces under wellcontrolled UHV environments can be extrapolated to high surface area materials, higher pressures, and higher reactant coverages. In addition, computationally predicted active sites can be experimentally tested, even for structural arrangements that are not readily accessible in bulk single crystal surfaces. In the following section, we discuss several literature examples to demonstrate some major successes of this strategy, starting with earlier studies that focus


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exclusively on either particle shape or composition. We then continue to more recent studies that fully utilize the ability to simultaneously control both shape and composition in noble metal nanoparticles. This development shows promise to extend the scope of this strategy because it opens up the possibility for these materials to model bimetallic active sites with specific structural arrangements in a more controlled and precise manner. 4.1 Shaped Nanoparticles as Experimental Models of Facet-Dependent Reactivity The identification of surface facets with improved reactivity through fundamental studies has significant precedent, dating back to the discovery that Fe{111} is over an order of magnitude more active than {100} or {110} for the limiting nitrogen activation step in the Haber-Bosch process.109 In recent years, the body of work on catalyst surface facets has shifted focus away from the maximization of activity for harsh reactions with high structure sensitivity, and instead towards improving selectivity of reactions with milder conditions, in which surface facets play a more subtle role in influencing the product distribution of complex chemical transformations. Bulk single crystals and/or theoretical calculations are typically used to predict or retroactively understand specific facets or sites that catalyze reactions with improved selectivity, but shaped nanoparticles can play an important role not only in realizing the results in practical catalytic systems, but also in verifying these predictions, especially in realistic conditions when higher pressures or the presence of other adsorbed species can affect the mechanism of the catalytic transformation. This process was demonstrated by Zaera and co-workers, who studied how the atomic packing of different Pt facets influenced the stability of cis olefins relative to their thermodynamically favored trans isomers.110-111 Through density functional theory (DFT) calculations, the authors found that adsorption energies of the isomers of 2-butene on a H-saturated



Pt(111) surface were influenced by a surface reconstruction process that was necessary to accommodate the sterically bulky methyl groups on either side of the olefin. Importantly, the need for this surface reconstruction to facilitate adsorption led to the preferential stabilization of the cis isomer over the normally thermodynamically favored trans isomer due to the increased reconstruction required to accommodate the geometry of the latter on the surface (Figure 8A). From this data, the authors hypothesized that close-packed surface facets like Pt(111) would preferentially stabilize the cis conformation because of the energetic penalty for undergoing surface reconstruction to better accommodate the trans form, while more open surfaces could more easily accommodate the trans conformation because the extent of reconstruction required would be reduced. Using temperature-programmed desorption (TPD) on Pt single crystals in combination with deuterium labeling to monitor which molecules had undergone isomerizations, they confirmed that cis-2-butene had a greater adsorption energy for three different close-packed surfaces regardless of whether the molecule adsorbed as the cis conformation or had undergone isomerization from the trans species, and found the opposite to be true for a more open (110)(2x1) Pt surface. These results were translated to a functional catalytic material by synthesizing uniformly sized {111}-faceted Pt tetrahedra, dispersing them onto a silica support, and calcining them at 475 K to remove organic material left over from synthesis (Figure 8B). The rate of transto-cis isomerization of 2-butene on these tetrahedral particles was found to be nearly twice that of the reverse cis-to-trans reaction, while on catalysts that contained a higher proportion of spherical particles with mixed faceting, the thermodynamically favored cis-to-trans reaction dominated (Figure 8C).


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Figure 8. Illustrative examples of the use of model studies to design catalyst materials with optimal surface structures. Top: (A) Computational studies predicted that cis-2-butene would be stabilized relative to trans-2-butene on a Pt(111) surface due to a surface reconstruction required for the adsorption of the trans isomer. This led to the design of {111}-faceted Pt tetrahedral nanoparticles (B) which exhibited a higher rate for the trans-to-cis isomerization of 2-butene when compared to Pt spheres with mixed facets (C). Scale bars: 10 nm. Bottom: (D) Computational studies predicted that Ag(100) would have a lower activation barrier for the desired ethylene oxide product relative to the barrier for non-selective products in epoxidation of ethylene. Based on this work, {100}faceted Ag cubes were synthesized (E) and showed higher selectivity to ethylene oxide compared to Ag nanowires with a mix of {100} and {111} facets or Ag spheres with primarily {111} facets (F). Scale bars: 500 nm (cubes) and 1 μm (rods). (A,B) Modified from reference 110 by permission from Springer Nature, copyright 2009. (B,C) Modified with permission from reference 111. Copyright 2009 American Chemical Society. (D) Modified with permission from reference 112. Copyright 2008 American Chemical Society. (E,F) Modified with permission from reference 113. Copyright 2010 Wiley-VCH. Linic and co-workers likewise successfully utilized design of nanoparticle shapes informed by surface science to improve selectivity for another challenging chemical transformation, the selective epoxidation of ethylene to form ethylene oxide.112-113 By using DFT calculations to determine the energetics of potential reaction pathways of a critical oxametallacycle intermediate on various Ag surfaces, the authors found that Ag(100) showed a relatively large difference in the activation barriers to formation of the desired ethylene oxide product versus that of unselective 29 ACS Paragon Plus Environment


products, allowing for possible high selectivity on that facet (Figure 8D). 112 They subsequently tested the selectivity of Ag particles with highly uniform shapes (Figure 8E), and verified this prediction with the finding that nanocubes exclusively bound by {100} facets were significantly more selective to ethylene oxide than both pentagonal nanorods bound by a mix of {100} and {111} facets and nanospheres with predominantly {111} facets (Figure 8F).113 Additionally, the authors observed a significant difference in selectivity as a function of size for particles of the same shape, with larger particles catalyzing the reaction with greater selectivity. This increase in selectivity with increasing nanoparticle size was attributed to the higher density of unselective undercoordinated defect sites in the smaller particles, which explained why this correlation was much more pronounced for the nanocubes and nanorods than for the nanospheres, since the mixed faceting of the spheres was already less selective than the well-defined surfaces of the other shapes. Notably, these results on particle size offer an important counterpoint to the notion that smaller catalysts are always better because of their high surface area to volume ratio, as the findings in this study demonstrate that controlling the type of active sites can be equally important as, or even more important than, maximizing the number of active sites. 4.2 Bimetallic Nanomaterials as Experimental Models of Composition-Dependent Behavior As discussed earlier, bimetallic materials can exhibit a wide range of catalytic behavior depending on the extent and type of alloying. With this large parameter space of options to tune reactivity, however, also comes increased complexity. Bimetallic nanoparticles play an especially important role in developing a better fundamental understanding of bimetallic catalysts since the variety of accessible nanoparticle architectures imparts greater synthetic tunability compared to bulk single crystals. These advantages of using nanoparticles as model surfaces are exemplified by a study from Eichhorn and Mavikrakis et al., on the development of a Ru@Pt core-shell


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nanoparticle catalyst for the preferential oxidation of CO in H2 (PROX).114 The PROX reaction is essential for implementation of hydrogen-based fuel cells because CO impurities in most hydrogen feedstocks poison fuel cell catalysts and cannot be realistically removed through other means. However, this reaction is particularly challenging from a catalysis perspective because of the need to oxidize CO completely while simultaneously minimizing oxidation of the H2 feed. Using DFT calculations, the authors identified a near surface alloy of Ru and Pt (Pt*/Ru(0001)) as a catalyst that could sustain the high turnover of pure Pt for O2 activation kinetics, but bind adsorbates like CO more weakly than pure Pt does to prevent deactivation from surface poisoning (Figure 9A).57 To validate this prediction, a sequential polyol method was used to synthesize Ru nanoparticles with Pt surfaces that matched the selected near-surface-alloy (Figure 9B).114 These Ru@Pt coreshell particles were found to be significantly more active and selective than pure Pt, a RuPt mixed alloy, or mixed monometallic Ru and Pt nanoparticles, with the Ru@Pt particles showing complete CO oxidation at a lower temperature and a larger difference in temperature between complete CO oxidation and the H2 oxidation light-off point (Figure 9C). Through further characterization, the authors found that the efficacy of the Ru@Pt particles was the result of a novel H-assisted mechanism that was dominant on Ru@Pt but not significant on monometallic Pt and not available on Ru or mixed alloy RuPt surfaces. The H-assisted mechanism resulted in a significantly lowered activation barrier for oxidation of adsorbed CO relative to the activation barrier for oxidation of adsorbed H atoms, thereby improving selectivity to CO oxidation. This work demonstrates not only the possibility of identifying and producing catalysts with new reactivity due to the unique bimetallic architectures available in nanoparticles, but also the capacity to obtain new fundamental insights using well-defined nanoparticle model surfaces in operating catalytic conditions.



Figure 9. Illustrative examples of the use of model studies to design catalyst materials with optimal bimetallic surface compositions. Top: (A) Computational studies predicted that Pt-terminated PtRu near-surface-alloys would exhibit weakened bonding of CO due to electronic modification of the Pt surface by the Ru core. This computational result was used to design Ru@Pt core-shell nanoparticles (B) with enhanced selectivity for preferential oxidation of CO in hydrogen (PROX) relative to PtRu alloys (C). Scale bars: 20 nm, inset scale bars: 5 nm. Bottom: (D) Temperatureprogrammed desorption and scanning tunneling microscopy experiments on single crystals showed that isolated Pd atoms in a Cu surface significantly lower the barrier for hydrogen dissociation which, in combination with weak binding on Cu, improves selectivity in the hydrogenation of styrene and acetylene relative to monometallic surfaces of either metal. Based on this result, Pd0.18Cu15 single atom alloy nanoparticles were developed (E) which exhibited enhanced selectivity to styrene in the partial hydrogenation of phenylacetylene (F). Scale bar: 25 nm. Modified from references 57 (A) and 114 (B,C) by permission from Springer Nature, copyright 2004 and 2008. (D) Modified from reference 46 with permission from AAAS, copyright 2012. (E,F) Modified with permission from reference 115. Copyright 2013 Royal Society of Chemistry. Flytzani-Stephanopoulos, Sykes, and coworkers likewise utilized nanoparticles to develop specific active site compositions on a functional surface.46, 115 In an initial study, Sykes et al. synthesized and characterized Pd on Cu(111) single atom alloys (Pd/Cu(111))—where single atoms of Pd were dispersed in a single crystal Cu surface—for their capacity to catalyze selective 32 ACS Paragon Plus Environment

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hydrogenation reactions.46 This composition had the dual role of tempering Pd’s high reactivity for activating hydrogen while also decreasing the amount of Pd needed in an effective catalyst. Temperature programmed desorption (TPD) and low-temperature scanning tunneling microscopy experiments showed that the Pd monomers provided sites for H2 uptake and dissociation on the otherwise inactive Cu surface (Figure 9D). Instead of binding the adsorbed H atoms very strongly, however, as pure Pd would, these Pd monomer sites allowed activated H atoms to spill over onto the Cu surface, where they could drive hydrogenation reactions with high selectivity. While these well-defined Pd/Cu(111) surfaces were active for this transformation, it was noted that practical implementation of this single atom alloy approach would require consideration of elevated reaction temperatures, which could cause segregation of the minority metal (Pd) into the bulk or into monometallic clusters, either of which would lead to the loss of the material’s unique catalytic properties. Naturally, nanoparticles offered a method for incorporating analogs of the Pd/Cu(111) single atom alloy model surface in a functional catalytic material that would be stable under operating conditions.115 The authors prepared Pd0.18Cu15 nanoparticles by first synthesizing Cu nanoparticles, loading them onto an γ-Al2O3 support, and then incorporating controlled amounts of Pd through galvanic replacement of Cu with Pd(NO3)2 (Figure 9E). Characterization through xray photoelectron spectroscopy and UV-visible spectroscopy confirmed that Pd atoms were welldispersed on the Cu surface in a similar manner to the bulk single crystal Pd/Cu(111) single atom alloys. Compared to monometallic Pd particles, these Pd0.18Cu15 nanoparticles showed a large increase in selectivity to styrene during the partial hydrogenation of phenylacetylene while retaining a high conversion of starting material (Figure 9F). This accurate recreation of an analog of the Pd/Cu(111) surface in a nanoparticle catalyst verified that the insights obtained under highly



controlled low pressure conditions could indeed be generalized to operating temperatures and pressures, and, in the process, generated a useful a catalytic material with novel properties. The concept of developing paradigms for the design of catalysts from studies of fundamental reactivity in UHV has been extensively pursued by Friend, Madix, and coworkers for selective partial oxidation reactions on Au and (Ag)Au surfaces (AgAu alloys dilute in Ag).4, 116 In particular, they demonstrated a direct correlation between the reactivity of adsorbed intermediates on O-activated Au single crystal surfaces and the catalytic behavior of a (Ag)Au alloy material, ozone-activated nanoporous gold (np(Ag)Au), in the oxygen-assisted coupling of alcohols, a group of reactions in which short-chain primary alcohols are selectively oxidized and coupled to form esters. The mechanisms of oxygen-assisted ester formation on Au were established via fundamental experiments in UHV on Au single crystals that had been pre-covered with adsorbed atomic oxygen using ozone or another source of activated oxygen.17, 117-118 An activated source of oxygen atoms was required for the single crystal studies because bulk Au does not dissociate molecular oxygen at an observable rate. Temperature programmed reaction (TPR) experiments were also carried out on model Au(111) surfaces in UHV to probe the effect of reactant mole fraction on product selectivity in the coupling of dissimilar alcohols.119 For example, co-dosing methanol and ethanol onto an O-activated Au surface can potentially lead to the formation of four ester products, including the two homocoupling products—methyl formate and ethyl acetate—as well as the two mixed coupling products—methyl acetate and ethyl formate. The authors showed that selectivity in the formation of esters from two different alcohols on Oactivated Au is controlled by the competitive adsorption of the reactant molecules as their respective alkoxides (ie. methoxy and ethoxy) on the Au surface.119 Because the equilibrium surface coverage favors the larger alkoxide,120-121 here ethoxy, the product distribution of the esters


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is dominated by ethyl acetate and methyl acetate except at high methanol mole fractions. Notably, no ethyl formate is observed due to the relatively slow rate of formation of the required formaldehyde intermediate from adsorbed methoxy in comparison to the more facile formation of the acetaldehyde intermediate from ethanol.122 The results from this model system were then applied to a significantly more complex material, np(Ag)Au, under ambient pressure conditions in a flow reactor. Similar to the single crystals, np(Ag)Au is highly crystalline, but it has a porous mesoscale structure with 30-100 nm ligaments and incorporates 1-3% Ag which serves to dissociate O2.123-124 Remarkably, the experiments on Au single crystals under well-controlled low pressure conditions predicted, with a near one-to-one correspondence, the distribution of ester products formed from mixtures of methanol and ethanol, methanol and butanol, or methanol and allyl alcohol with varying mole fractions on np(Ag)Au at 150 °C in a stream of flowing reactants (Figure 10).119, 122, 125 This work highlights the potential of experiments on single crystal model surfaces to predict behavior on more complex materials by elucidating the fundamental chemical principles that underlie product selectivity in reactions controlled by a unifying mechanism, and it also represents a successful use of bimetallic materials with larger nanoscale features to test predictions from UHV under a broader range of conditions.



Figure 10. Graphs of relative selectivity to different ester products in the coupling of dissimilar alcohols, illustrating the highly accurate prediction of product selectivity on np(Ag)Au at 1 atm (D-F) from temperature-programmed reaction experiments on Au single crystal surfaces in ultrahigh vacuum (A-C). Modified with permission from references 119 (A,B) and 125 (C,F). Copyright 2010 and 2016 American Chemical Society. (D,E) Modified from reference 122, copyright 2015, with permission from Elsevier. 4.3 Shaped Bimetallic Nanoparticles as Experimental Models of Complex Active Sites Our group recently contributed to this body of work on selective oxidation over (Ag)Au surfaces through the application of colloidally synthesized (Ag)Au concave cubic nanoparticles as a catalyst for the oxygen-assisted coupling of methanol and as an experimental model surface for the active site in ozone-activated np(Ag)Au.126 Np(Ag)Au requires an initial activating ozone treatment, after which it shows reproducible, robust activity for the oxygen-assisted coupling of methanol while maintaining greater than 99% selectivity to the desired ester product.124 Because the structure of np(Ag)Au is highly complex and changes dynamically during the ozone pretreatment necessary for consistent activation,127 it had been challenging to identify the active site geometry that is responsible for dissociation of O2 to generate adsorbed oxygen atoms that are selective for partial oxidation of alcohols. Theoretical calculations by Kaxiras and coworkers based on insights from surface science on bulk (Ag)Au single crystals were used to identify a stepped 36 ACS Paragon Plus Environment

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{221} geometry in which Ag is enriched near undercoordinated Au step edges as a candidate for this active site in ozone-activated np(Ag)Au (Figure 11A).37 We predicted that {720}-faceted concave cubes synthesized through underpotential deposition of Ag on Au would possess a high density of these sites, since Ag reduced through the mechanism of underpotential deposition localizes at and stabilizes the Au step edges of the particles’ surfaces (Figure 11B).100, 128 Indeed, we found these (Ag)Au concave cube particles to be approximately four times more active per unit mass than ozone-activated np(Ag)Au for the oxidative coupling of methanol and equally selective (>99% to methyl formate), without the need for an activating ozone treatment (Figure 11C). 126 Importantly, the (Ag)Au concave cubes and ozone-activated np(Ag)Au have similar surface areas per unit mass, so the greater activity of the former cannot be attributed solely to an increased total surface area. These results not only yielded a highly effective catalytic material, but also provided experimental validation of the proposed active site from DFT calculations. In particular, this work exemplifies the strengths of shape- and composition-controlled nanoparticle catalysts, since the high energy and complexity of the predicted active site made it difficult to access using single crystal substrates. Moreover, this example demonstrates the greater breadth of potential surface features that can be accurately modeled when particle structure and composition can be tuned simultaneously.



Figure 11. Computational studies of dilute (Ag)Au alloy surfaces under the conditions of oxygenassisted methanol coupling at 150 °C (A), predicted that Ag-rich Au steps could serve as a potential active site for the dissociation of O2 on np(Ag)Au alloy materials. (Ag)Au concave cube nanoparticles (B), whose surface is composed of Au steps rich in Ag due to their method of synthesis, were used to experimentally test this prediction and showed an enhanced rate of conversion of methanol relative to np(Ag)Au while maintaining the same selectivity (99+%) to the desired product, methyl formate (C). Scale bars: 200 nm. (A) Modified with permission from reference 37. Copyright 2016 American Chemical Society. (B) Modified from reference 126 by permission of Springer, copyright 2017. 5. Outlook and Challenges We view the use of shaped nanoparticles to achieve a stepwise correlation of catalytic behavior across pressure regimes as a promising avenue towards obtaining a more detailed fundamental understanding of high surface area, industrially viable materials. Colloidally synthesized nanoparticles with well-defined shapes and compositions show particular potential with respect to this goal because their well-defined surfaces and stability in a variety of environments enables their use in both low-pressure conditions and operating catalytic conditions. One way to facilitate correlation between controlled low-pressure environments and higher pressure catalytic conditions is to use shaped metal nanoparticles directly as substrates for UHV surface science characterization techniques. Schauermann and Freund have carried out extensive mechanistic studies of the hydrogenation of unsaturated hydrocarbons and α,β-unsaturated 38 ACS Paragon Plus Environment

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aldehydes by comparing the reactivity of Pd(111) single crystals and Pd nanoparticles under welldefined UHV conditions using a range of techniques including TPD, molecular beam experiments, and in-situ infrared reflection-absorption spectroscopy.129-133 The Pd nanoparticles used in their work were prepared by physical vapor deposition of Pd onto a planar Fe3O4 support followed by annealing to yield island-shaped particles with an average diameter of 12 nm and surfaces defined primarily by {111} facets with a small proportion of {100} facets. With recent advances in the colloidal synthesis of well-defined nanomaterials, the scope of substrates available for use in this type of approach can be significantly expanded by dropcasting colloidally synthesized nanoparticles on an inert surface. This concept has seen some particularly promising early exploration with the use of shaped metal oxide nanoparticles as model surfaces under UHV conditions,134-135 but the potential contributions of this approach using noble metal nanoparticles remain to be full explored. For example, Vohs et al. used TPD on films of monodisperse anatase TiO2 (A-TiO2) nanocrystals with uniform shape to experimentally identify specific active sites in the thermal and photochemical reactions of methanol on the TiO2 catalyst surface.134-135 Comparison of these results with those reported for bulk single crystal A-TiO2 showed a number of parallels, as expected; however, the study also highlighted a few key characteristics unique to the nanoparticle catalysts, such as their ability to promote the thermal bimolecular coupling of methanol on ATiO2(001), which had not been previously observed on bulk single crystal surfaces. Here, as in other model studies, the monodispersity of the nanocrystals in both size and shape is crucial to developing clear structure-function relationships. When TPD is conducted on polycrystalline nanocrystal powders that are commonly used as industrial catalysts, the diverse variety of adsorption sites leads to broad desorption peaks and prevents clear identification of the relevant



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active structures, whereas these well-defined particles enabled relatively clean TPD data.136 This work on uniformly shaped metal oxide nanoparticle catalysts demonstrates the potential of using shaped nanoparticles in UHV studies as a way to analyze specific differences in reactivity between bulk single crystals and high surface area catalysts. Importantly, this strategy is readily applicable to noble metals with the appropriate choice of nanocrystal materials. An alternative way to directly correlate reactivity across pressure regimes is to systematically vary pressures and reactant coverages using the same material via techniques such as temporal analysis of products (TAP). Reactors designed for use with the TAP approach deliver controlled millisecond pulses of reactants to a catalyst and then measure the product responses at the reactor outlet with high temporal resolution.137 These pulse-response experiments enable the determination of fundamental rate parameters that control the kinetics of catalyst reactivity—such as dissociation probability, active site density, and activation energies—on active catalytic materials.138 Using the TAP technique, Madix and coworkers were able to develop a detailed microkinetic model for the selective oxidation of methanol on Au that accurately predicted the distribution of products (methyl formate, formaldehyde, and CO2) across a range of reactant pressures from 10-8 to 1 bar.139 In particular, the model explained the transition from an unusually high selectivity to formaldehyde observed during the transient pulse conditions (10-5 bar) to >99% selectivity to methyl formate in flow reactor experiments as the result of the dramatic increase in methoxy coverage on the catalyst surface under these respective conditions. Importantly, this work showed that the fundamental mechanism and kinetics of the selective oxidation of methanol on Au are conserved across ten decades of pressure and several hundreds of degrees Kelvin. An additional advantage of shaped noble metal nanoparticles is that they can be, and have been, employed beyond gas phase catalysis in liquid phase41,


140-144

and electrocatalytic

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reactions,145-149 where the influence of solvents on reactant diffusion and binding leads to reactivity that can sometimes be dramatically different from reactivity in the gas phase. Correlating catalytic behavior for the same material across these varying reaction conditions facilitates the identification of fundamental differences and similarities in surface reactivity for the same material composition and structure in dissimilar chemical environments.4, 17, 150 In addition, the versatility of shaped nanoparticles with respect to use in various reaction environments enables the development of catalysts for molecular transformations that are not feasible in the gas phase, or for which electrochemical energy input is required. Analogously to the work highlighted in this Perspective, data from the extensive fundamental surface electrochemistry literature on well-defined electrode surfaces can be used to guide the design of nanoscale electrocatalysts.149, 151 One challenge to the use of nanoparticles—or any catalytic material—to correlate observations across different conditions is the dynamic nature of catalyst surface structure. Elevated temperatures and/or chemisorption of molecules on a catalyst surface, particularly at high reactant coverages, can induce rearrangement of structure and composition.19, 127, 152-157 As a result, the highly controlled morphology that is achieved during synthesis may be partially or completely altered depending on the severity of catalytic operating conditions. This is especially true for bimetallic materials, which have repeatedly been shown to undergo component segregation or migration under oxidizing or reducing environments.127, 153, 157 This challenge can be addressed through either careful control of reaction conditions, or through the use of advanced instrumentation and characterization techniques. For certain reactions, correlation of catalytic behavior across pressure regimes can be carried out by limiting reaction conditions to relatively mild temperatures and low to atmospheric pressures in order to prevent or minimize restructuring.



For chemical transformations that require harsher reaction conditions, this strategy becomes more difficult to effectively implement, as the functional structure of the catalyst tends to become more different from that of the material under as-synthesized or low-pressure model conditions. Advances in in situ and operando electron microscopy and surface analysis techniques, however, have provided ways to manage the complexity of catalyst dynamics.158-159 In particular, time-resolved characterization of catalysts in environments that fully or partially recreate working conditions can offer new experimental insights into the operating state of a catalytic material as it undergoes reactant adsorption and activation processes. Changes in catalyst shape and/or surface structure, for instance, can be observed with environmental transmission electron microscopy (ETEM).159-160 Ambient/near-ambient pressure x-ray photoelectron spectroscopy (AP/NAP XPS) and x-ray absorption spectroscopy (XAS), on the other hand, elucidate reactant-induced changes to the electronic structure of the catalyst surface or bulk, respectively, to provide information about oxidation state and/or alloy dispersion.62,

154, 161

To probe interactions with relevant chemical

species, infrared spectroscopy of surface adsorbates, conducted in operando or with marker species such as CO or pyridine, can give additional information about the reactivity of active sites on the catalyst surface as well as the coordination of reactants and intermediates.40, 61 Despite the relatively recent introduction of these techniques, their use has already contributed groundbreaking insights about the dynamics of catalyst structure and composition under near-working conditions.127, 157-159, 162-163 For example, in a recent study, a combination of AP-XPS and in situ E-TEM was used to analyze changes in the surface alloy composition and metal oxidation state of np(Ag)Au in oxidizing and reducing environments and to correlate this information with structural changes, such as the formation of metal oxide islands, that occurred during ozone pretreatment and subsequent activation.127 In their work on PtRe alloys, Davis and


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coworkers used XAS and NAP-XPS to determine that the Re in these catalysts is partially oxidized under the conditions of glycerol hydrogenolysis, an assignment which had previously been ambiguous because the oxidation state of Re changes when exposed to different environments.6162

They then proposed a bifunctional mechanism in which Re serves as a Brønsted acid, as

discussed earlier, and confirmed this hypothesis using diffuse reflectance infrared Fourier transform spectroscopy with pyridine serving as a probe molecule for Brønsted acidity. To understand the stability of non-equilibrium nanoparticle structures, Xia and Mavrikakis studied the real time shape evolution of the high energy surface structure and internal defect structure of Pd concave icosahedra at high temperature (200-600 ºC) in vacuum using high resolution electron microscopy (STEM and TEM) in combination with DFT calculations.160 An equilibrium surface structure evolved between 200 and 400 ºC, while 600 ºC was sufficient to drive bulk reconstruction of internal crystal defects. These and other in situ methods of characterization offer ways to better understand catalyst dynamics in specific operating conditions, especially when complemented and informed by computational studies. The resulting insight into how a catalyst changes from its initially prepared morphology can mitigate the complexity of dealing with a material that is no longer so well-defined, and thereby potentially enable the stepwise correlation of reactivity under UHV with operating conditions even when catalyst restructuring does occur. A specific challenge for the application of colloidally synthesized shaped nanoparticles as experimental models is the removal of ligands and other species that are used to direct nanoparticle growth in solution. Surfactants and other stabilizers, in particular, are typically essential to solution phase particle synthesis to prevent aggregation, while small molecule additives can be necessary to achieve control over shape and structure. Ligands and other adsorbates are often used as tools to manipulate functional catalytic behavior of nanoparticle catalysts.164-169 From a fundamental



perspective, however, these species tend to complicate efforts to understand surface reactivity, since adsorbates leftover from synthesis can influence the interaction between reactant molecules and the catalyst surface.170 Moreover, this influence is dependent on reaction conditions, such as the availability of reactants, and thus can significantly increase the complexity of a system when using the same catalyst under different environments, or when comparing nanoparticle catalysts to bulk single crystal surfaces on which such ligands are not present. Typical methods for preparing nanoparticle catalysts that have been synthesized through colloidal methods involve removal of ligands using washing, thermal treatment, or cleaning with oxidants such as ozone or oxygen plasma. Just as particle morphology can evolve under catalytic operating conditions, however, so too can it change during ligand removal processes: harsh treatments with high temperatures or highly oxidizing environments should be minimized to preserve as-synthesized particle structure and composition. Surfactant-free synthetic processes and nondestructive methods of ligand removal, which are each ongoing avenues of study, are key to the successful application of colloidally synthesized nanoparticles as model catalytic materials in reactions where ligands impact catalytic behavior.171-173 With that said, however, there remains the possibility to utilize ligands and adsorbates creatively as a way to facilitate the catalyst development process. The effects of competitive adsorption and selective binding to active sites, which have been employed to improve the functionality of nanoparticle catalysts, could also be applied to fundamental studies. For example, control over ligand concentration on the catalyst surface could be used to modulate the density of accessible active sites, or the complexity of a nanoparticle catalyst could be decreased by selectively binding adsorbates to specific high energy sites in order to reduce the overall polydispersity of active sites in the material.174-175 Work by Marshall and coworkers, in which


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atomic layer deposition of alumina on Pd nanoparticles was used to selectively block step sites and limit the hydrogenation of furfural to furfural alcohol on these surfaces while still allowing hydrogenation of furfural to furan to occur on terrace sites, exemplifies the goals of this selective passivation approach.175 Thus far, the use of ligands to alter catalyst reactivity in fundamental studies has received less attention, but the prospect of doing so shows promise to add a new dimension to the application of colloidally synthesized nanoparticles to stepwise correlation of catalyst behavior across pressure and coverage regimes. 6. Conclusion Overall, continuing synthetic advances in fine control of metal nanoparticle surface structure and composition together with major developments in in situ and operando characterization techniques have generated exciting opportunities for the elucidation of fundamental structure-activity relationships in catalysis. In particular, the correspondence between surfaces of interest for surface science studies of catalytic mechanisms and the particle shapes and compositions accessible with recent advances in colloidal nanoparticle synthesis present new possibilities to extrapolate insights from fundamental studies across differing pressures and reactant coverages. The potential for using highly uniform nanoparticles with well-defined surfaces as catalysts in conditions ranging from UHV to elevated pressures—as well as in solutionphase and electrocatalytic systems—offers a promising path forward for the development of predictive parameters for the design of high performance catalysts based on enhanced understanding of fundamental reaction mechanisms.



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Conflicts of interest There are no conflicts of interest to declare. Acknowledgements Acknowledgement is made to the donors of The American Chemical Society Petroleum Research Fund for partial support of this research. This work was also supported by start-up funding from Wesleyan University.

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Author Biographies Daniel Robertson received his B.A. degree in Chemistry, Physics, and the College of Integrative Sciences from Wesleyan University in 2018. He is currently pursuing a Ph.D in Chemistry researching electrochemical energy storage under the supervision of Prof. Sarah Tolbert at University of California, Los Angeles. His research interests broadly lie in the development of fundamental structure-function relationships in materials for energy-related applications.

Michelle Personick is an Assistant Professor in the Chemistry Department at Wesleyan University in Connecticut. She received a B.A. degree from Middlebury College in 2009, where she was an undergraduate researcher with Prof. Sunhee Choi. In 2013, Michelle obtained a Ph.D. from Northwestern University under the supervision of Prof. Chad A. Mirkin. From 2013 to 2015, she was a postdoctoral researcher at Harvard University working with Prof. Cynthia M. Friend and coadvised by Prof. Robert J. Madix. She joined the faculty at Wesleyan in 2015 and her research group focuses on developing tailored metal nanomaterials to enable fundamental research toward improved catalysts for resource-efficient chemical synthesis and the clean production of energy. Michelle is a recipient of the Victor K. LaMer Award from the ACS Division of Colloid and Surface Chemistry and the Army Research Office Young Investigator Award.


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Growing Nanoscale Model Surfaces to Enable Correlation of Catalytic

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