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Heterogeneous Catalysis

Dealloyed Nanoporous Gold Catalysts: From Macroscopic Foams toNanoparticulate ArchitecturesGuangfang Grace Li[a] and Hui Wang*[a]

897ChemNanoMat 2018, 4, 897–908 T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Focus ReviewDOI: 10.1002/cnma.201800161

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Abstract: Dealloyed nanoporous Au membranes and spongy

Au nanoparticles exhibit a set of unique structural features

highly desirable for heterogeneous catalysis and electro-

catalysis. In this Focus Review, we present the state-of-the-art

understanding of the complex mechanisms dictating the

nanoscale porosity evolution during percolation dealloying of

alloys and the structure-composition-performance correla-

tions underpinning the catalytic behaviors of dealloyed

nanoporous Au. We focus on several fundamentally intriguing

but widely debated topics concerning the nature of the

active sites, the dynamic surface reconstruction under

reaction conditions, and the origin of catalytic selectivity

toward certain reactions. We also provide perspectives on

versatile dealloying-based synthetic approaches for precise

architectural tailoring of metallic nanocatalysts as well as

exciting opportunities of harnessing the combined optical

and catalytic properties of dealloyed nanoporous Au to drive

or enhance unconventional interfacial chemical transforma-

tions.

1. Catalysis on Nanoporous Au: A ParadigmShift in Heterogeneous Catalysis

The ever-increasing interest in heterogeneous catalysis by

metallic Au dates back to 1980s, when Haruta and coworkers[1]

discovered that oxide-supported sub-5 nm Au nanoparticles

exhibited surprisingly high catalytic activity toward aerobic CO

oxidation under mild reaction conditions even below 0 8C,whereas Au nanoparticles larger than 5 nm showed diminished

catalytic activity. At first glance, Haruta’s observations appear

counter-intuitive because Au has long been considered an inert

noble metal and is the only one exhibiting an endothermic O2

chemisorption energy among all the late transition metals.[2]

While the detailed mechanisms underpinning the size-depend-

ent activity of Au nanocatalysts have long been a fundamen-

tally intriguing subject under intense debate, consensus has

been reached that the undercoordinated surface atoms, which

become highly abundant when the particle sizes shrink to the

sub-5 nm regime, are indispensable for catalyzing the chemical

transformations.[3] However, the origin of the catalytic activities

of Au nanocatalysts cannot be interpreted solely in the context

of the surface atomic coordinations. To prevent the particle

sintering under reaction conditions, the Au nanoparticles are

typically dispersed on high surface area oxide supports, which

also provide crucial contributions to the overall catalytic

activities due to intricate particle-support and molecule-support

interactions.[4–8] In addition, for sub-5 nm nanoparticles, quan-

tum confinement becomes a predominant effect that modifies

the electronic structures of the materials, which may arguably

influence the catalytic activity of Au as well.[9–10] Therefore,

multiple intertwining effects come into play and synergistically

dictate the size-dependent catalytic behaviors of the interface-

rich, substrate-supported Au nanocatalysts.

The emergence of dealloyed nanoporous Au catalysts, in

the form of either foamy membranes or spongy nanoparticles,

represents a paradigm shift in Au-based heterogeneous

catalysis.[11–12] Bicontinuous nanoporous Au catalysts composed

of interconnected nanoligaments are typically derived from

alloys through nanoporosity-evolving percolation dealloying.[13]

Although the feature sizes of both the ligaments and pores are

typically beyond 5 nm, the dealloyed nanoporous Au exhibits

remarkable catalytic activities comparable to or even surpassing

those of the oxide-supported sub-5 nm Au nanopar-

ticles.[11–12,14–17] Dealloyed nanoporous Au catalysts provide a

unique free-standing materials system that enables detailed

correlation of the intrinsic catalytic activity of Au to the atomic-

level surface structures without complications introduced by

the support materials or quantum confinement effects.

In this Focus Review, we first discuss the complex mecha-

nisms dictating the nanoporosity-evolving percolation deal-

loying of alloys, which serve as the key knowledge foundation

for deliberate structural control of nanoporous Au catalysts.

Then we focus on several fundamentally intriguing but still

controversial issues regarding the structure-property relation-

ships of dealloyed nanoporous Au catalysts, such as the nature

of the catalytically active sites, the dynamic structural remodel-

ing of catalyst surfaces, and the origin of catalytic selectivity.

Through a detailed case study using electrocatalytic alcohol

oxidation as model reactions, we further demonstrate how the

structures and compositions of dealloyed spongy nanoparticles

can be systematically fine-tailored by controlling the dealloying

of Au@Cu alloy nanoparticles, based on which optimal catalytic

performance can be achieved. Finally, we briefly summarize the

state-of-the-art knowledge regarding the catalytic behaviors of

dealloyed nanoporous Au and provide perspectives on new

synthetic approaches that will further enhance our capabilities

to fine-optimize the catalyst structures as well as exciting

opportunities of exploring unconventional interfacial molecular

transformations on the dealloyed nanoporous Au catalysts.

2. Nanoporosity-Evolving PercolationDealloying

Percolation dealloying of alloys involves selective etching of the

less-noble constituents, entangled with the structural rear-

rangement of the nonleachable more-noble components.[18] A

prototypical binary alloy system of particular interest has been

macroscopic Au@Ag alloy membranes, which transform through

[a] G. G. Li, Prof. H. WangDepartment of Chemistry and BiochemistryUniversity of South Carolina631 Sumter Street, Columbia, South Carolina 29208, United StatesE-mail: wang344@mailbox.sc.edu

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percolation dealloying into a unique 3D solid/void bicontinuous

structure consisting of Au-rich nanoligaments that are hierarchi-

cally interconnected (Figure 1A and 1B).[13] The nanoporosity

evolution during percolation dealloying involves a series of

intriguing structure-rearranging processes that dynamically

interplay over multiple length- and time-scales both in the bulk

of alloy matrices and at the evolving solid/electrolyte inter-

faces.[18] According to a surface-diffusion continuum model

(Figure 1C) proposed by Erlebacher and coworkers,[13,18] the

percolation dealloying of an Au@Ag alloy is initiated upon

dissolution of an Ag surface atom at the solid/electrolyte

interface. As the interfacial dissolution of Ag atoms further

proceeds, the undercoordinated Au atoms left behind rapidly

agglomerate into Au-rich patchy islands through surface

migration. Therefore, the alloy surface at the dealloying frontier

is essentially composed of Au-rich surface-passivating domains

and patches of undealloyed material still exposed to the

etchant. Further dealloying results in a branched pore channel

network, which eventually evolves into the bicontinuous

spongy structures through a ligament and pore coarsening

process.

The dealloying behaviors of bimetallic alloys are composi-

tion-dependent. For example, a macroscopic Au@Ag alloy

membrane may selectively undergo either surface atomic

dealloying or nanoporosity-evolving percolation dealloying,

depending on the atomic fraction of Ag in the alloys and the

detailed experimental conditions under which the dealloying

occurs.[13] The onset potential of percolation dealloying is

termed as critical potential, Ec.[19] At potentials below Ec, only

surface dealloying occurs at the top-most atomic layer of the

alloy materials. Therefore, the current remains close to zero as

the potential increases until reaching Ec, above which the

current increases drastically (Figure 1D) due to percolation

dissolution of Ag from the alloy matrices. Ec is essentially a

function of Au/Ag stoichiometric ratio (Figure 1D and 1E). A

bulk binary alloy with a compositional formula of A1-pBp (A and

B represent the nonleachable and leachable constituents,

respectively, and p is atomic fraction of B) has a characteristic Ecexpressed as[20]

EcðpÞ ¼ EeqðpÞ þ 4gB=elecWA

nFx, ð1Þ

where gB/elec is the interfacial free energy of B exposed to the

electrolyte, WA is the molar volume of A, n is the number of

electrons transferred upon oxidation of one B atom, F is the

Faraday constant, and x is the local radius of the surface where

a cylindrical pit is created upon dealloying. E-c is the critical

potential of the bulk alloy. E-eq refers to the onset potential for

surface dealloying, also known as the equilibrium potential.

Under certain dealloying conditions, almost all bimetallic alloys

feature a characteristic threshold p known as the parting limit,

above which percolation dealloying occurs. The parting limits

of Au@Ag and Au@Cu alloys were determined to be ~55–60 atomic% of Ag[13,19,21–22] and ~70 atomic% of Cu,[23–25] respec-

tively, in acidic etching environments at room temperature.

Nanoparticles of alloys exhibit size-dependent critical

potentials that are negatively shifted relative to those of their

bulk analogs with the same compositional stoichiome-

tries.[19,22,26] The critical potential, Ec(p,r), of a spherical A1-pBp alloy

nanoparticle with a radius of r is given by[26]

Ecðp; rÞ ¼ EcðpÞ@ gAlloyðWAÞ þ fAlloyðW_

A @ Wh iÞh i

> 2

nFr

. -, ð2Þ

where gAlloy and fAlloy represent the free energy and the stress at

the alloy/electrolyte interface, respectively. is the partial molar

volume of A in the alloy, and Wh i is the average molar volume

of the alloy. Ec and E-c become virtually equivalent when r is

larger than ~5 nm, because the maximum values of gAlloy and

fAlloy are ~2 and ~6 Jm@2,[26] respectively. Equation (2) essentially

reflects the surface curvature-dependent Gibbs-Thomson ef-

fects, which diminish at a length scale larger than 10 nm.

However, pronounced negative shifts of Ec have been exper-

imentally observed on Au@Ag alloy nanoparticles over a size

regime far broader than that predicted by equation (2), ranging

from tens of millivolts for ~100 nm sized particles up to

hundreds of millivolts for 10–15 nm sized particles, because of

additional complication arising from the microstructural ef-

Guangfang Grace Li received her B.S.and M.S. both in chemistry from Wu-han Institute of Technology in China in2009 and 2012, respectively. Since 2013,she has been a graduate studentmajoring in Physical Chemistry at theUniversity of South Carolina. Her Ph.D.work, supervised by Hui Wang, hasbeen focusing on the structural trans-formations and the electrocatalyticproperties of complex multimetallicnanostructures.

Hui Wang received his B.S. (Chemistry)with honors from Nanjing University inChina in 2001 and Ph.D. (PhysicalChemistry) from Rice University in 2007.His Ph.D. work, supervised by Naomi J.Halas, focused on tunable plasmonicnanostructures and plasmon-enhancedspectroscopies. He did postdoctoralresearch on single-molecule spectro-scopy under the tutelage of Paul F.Barbara at the University of Texas atAustin. He joined the faculty ofChemistry and Biochemistry at theUniversity of South Carolina as atenure-track Assistant Professor in 2010and was promoted to Associate Profes-sor with tenure in 2016. His independ-ent research has been focusing on thestructure-property relationships of com-plex nanostructures and the mecha-nisms of catalytic molecular transfor-mations at the nanoparticle-moleculeinterfaces.

899ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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fects.[22] Alloy nanoparticles may undergo dealloying-driven

structural transformations that are more versatile than those of

their bulk and thin film counterparts displaying a planar surface

to the electrolyte. For example, Au0.23Ag0.77 alloy nanoparticles

smaller than 10 nm transform into core-shell nanoparticles each

of which is composed of an alloy core and an Au shell

(Figure 2A–2C), whereas Au0.23Ag0.77 alloy nanoparticles larger

than 20 nm evolve into spongy nanoparticles (Figure 2D–2F)

under otherwise identical electrochemical dealloying condi-

tions. Alloy nanoparticles substantially larger than ~20 nm

typically undergo nanoporosity-evolving morphological

changes involving both ligament pinch-off and void bubble

formation during percolation dealloying (Figure 2G),[27–28] analo-

gous to their macroscopic bulk counterparts with the same

compositions. The percolation dealloying of alloy nanoparticles

enables controlled introduction of nanoscale porosity to a large

variety of substrate-supported or free-standing Au nanostruc-

tures (Figure 2H–2L).[19,28–32]

3. Structure-Property RelationshipsUnderpinning the Catalytic Behaviors

Dealloyed nanoporous Au possesses large surface-to-volume

ratios, highly abundant undercoordinated surface atoms, con-

ductive skeletal frameworks, and fully accessible open surface

structures, all of which are highly desired for heterogeneous

catalysis and electrocatalysis. The exact nature of the catalyti-

cally active sites, however, still remains elusive because of the

overwhelmingly complicated surface structures of dealloyed

nanoporous Au and lack of a unified mechanism broadly

applicable to a diverse set of reactions. Here we focus on

several intensively debated critical issues regarding the roles of

the undercoordinated Au surface atoms and the residual less

noble elements in dictating the activity, durability, and

selectivity of the dealloyed nanoporous Au catalysts.

3.1. Nature of Active Sites

It has become increasingly evident that the catalytic activity of

dealloyed nanoporous Au toward a variety of important

oxidation and hydrogenation reactions[11–12,14–17] originates from

the highly abundant undercoordinated atoms on the locally

curved ligament surfaces.[12,33–36] As illustrated in Figure 3A, the

surface atoms can be categorized into three types located at

terraces, step edges, and kink sites, respectively, in the order of

decreasing atomic coordination numbers (ACNs). The latter two

represent the undercoordinated surface atoms that are catalyti-

cally active.[12] Density Functional Theory (DFT) calculations

suggest that Au becomes catalytically active for activation of

molecular oxygen only when the surface atoms are under-

coordinated.[37] As revealed by high-resolution transmission

electron microscopy (HRTEM) images, the complex 3D surfaces

of dealloyed Au membranes (Figure 3B–3D)[33] and dealloyed

spongy Au nanoparticles (Figure 3E)[23] are enclosed by high

densities of various types of undercoordinated atoms at steps

and kinks, which may serve as the primary active sites for

catalysis.

Figure 1. (A) Cross-sectional and (B) plan view scanning electron microscopy (SEM) images of nanoporous Au foam made by selective dissolution of Ag fromAg@Au alloys immersed in nitric acid. Reprinted with permission from ref [13]. Copyright 2001, Springer Nature. (C) Kinetic Monte Carlo simulations of atomic-scale structural evolution of an Au@Ag alloy membrane during percolation dealloying: (a) the planar surface prior to dealloying, (b) vacancy island nucleation,(c) surface roughening, (d) formation of a branched pore channel network, and (e) pore and ligament coarsening. Ag and Au atoms are represented by greyand yellow spheres, respectively. Reprinted with permission from ref [18]. Copyright 2018, Cambridge University Press. (D) Composition-dependent current-potential behaviors of Ag@Au alloys dealloyed in 0.1 M HClO4+0.1 M Ag+. The Au atomic% of each Ag@Au alloy sample was labeled in the figure. (E)Comparison of experimental (line) and simulated (triangles) critical potentials. The zero of overpotential has been set equal to the onset of dissolution of puresilver both in simulation and in experiment. Reprinted with permission from ref [13]. Copyright 2001, Springer Nature.

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Recent experimental observations, however, strongly sug-

gest that the undercoordinated Au surface atoms alone are

unlikely to be capable of activating molecular O2 and thus

efficiently catalyzing the CO oxidation.[11–12,38–41] For example,

high-index faceting {211} Au surface constitutes a prototypical

step-terrace structure with high density of steps. Although Au

{221} facet shows strong affinity for CO chemisorption, it

exhibits plain reactivity toward CO oxidation.[42] Friend and

coworkers also observed that dealloyed nanoporous Au

catalysts highly active for selective methanol oxidation were

not necessarily active for CO oxidation,[43–45] in spite of the fact

that both reactions required O2 activation. Another surprising

observation made by Tao and coworkers shows that pretreating

nanoporous Au with ozone, which is usually viewed as an

efficient way to eliminate undercoordinated surface sites,

drastically enhances the activity for cyclohexene oxidation.[46]

All these observations coherently point to a more complicated

underlying structure-property relationship that cannot be fully

elucidated solely based on the surface atomic coordination.

Although compositionally Au-rich, the dealloyed nano-

porous catalysts inevitably contain residual less-noble elements,

most commonly Ag and Cu, that cannot be completely

removed during the nanoporosity-evolving percolation deal-

loying.[11–12,33,38,40,47–50] The retention and distribution of the less

noble elements in the dealloyed nanoporous structures are

influenced by the rates of atomic dissolution and the potential

under which the dealloying occurs.[49–50] The residual Ag may

either remain fully alloyed with Au[33,47] or segregate at the

ligament surfaces to form localized patchy islands (Figure 3F–

3I).[48] The presence of residual Ag remarkably enhances the

catalytic activities of the dealloyed nanoporous Au toward

certain oxidation reactions,[40–41] though the quantitative corre-

lation between the amount of residual Ag and the overall

catalytic activity still remains open to further scrutiny. For

catalytic CO oxidation, it is highly likely that adsorbed oxygen is

Figure 2. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Ag0.77Au0.23 nanoparticles(nominally 4 nm in diameter) (A) prior to dealloying and (B) after electrochemical dealloying at 1.4 V (vs. NHE). (C) Elemental map of a 4.8 nm diameter Ag0.77

Au0.23 nanoparticle dealloyed at 1.4 V (vs. NHE). Blue: dark field image intensity; Magenta: Ag signals from electron energy loss spectroscopy (EELS).Representative HAADF-STEM images of ~40 nm diameter Ag0.77Au0.23 nanoparticles (D) before dealloying and after dealloying for 6 h at (E) 0.54 V and (F)0.74 V. Reprinted with permission from ref [19]. Copyright 2014, American Chemical Society. (G) Kinetic Monte Carlo simulations modeling the nanoporosityevolution during electrochemical dealloying of a nanoparticle. At the early stages, the surface becomes roughened and porosity begins to evolve on thesurface. As dealloying time increases (to the right and down), porosity fully penetrates into the particle and the average feature size increases as aconsequence of ligament and pore coarsening. Reprinted with permission from ref [28]. Copyright 2018, Cambridge University Press. (H) Planar view and (I)cross-sectional SEM images of a substrate-supported dealloyed nanoporous Au island. Reprinted with permission from ref [29]. Copyright 2016, AmericanChemical Society. (J) Transmission electron microscopy (TEM) image of spongy Au nanoparticles encapsulated in a SiO2 shell synthesized through dealloying ofSiO2-coated Au@Ag alloy nanoparticles. Reprinted with permission from ref [30]. Copyright 2016, American Chemical Society. (K) TEM image of spongy Aunanoparticles synthesized through dealloying of Au@Cu alloy nanoparticles. Reprinted with permission from ref [31]. Copyright 2016, Wiley-VCH. (L) TEMimage of Au nanotubes with nanoporous walls synthesized through dealloying of Ag@Ag@Au alloy core-shell nanowires. Reprinted with permission from ref[32]. Copyright 2009, Springer Nature.

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associated with Ag or Ag@Au sites rather than undercoordi-

nated Au surface atoms.[12] By introducing Ag atoms to the Au

surfaces, the binding affinity of molecular oxygen to the

catalyst surfaces increases while the energy barrier of O2

dissociation decreases, both of which facilitate O2 activation.

Theoretical analysis reveals that the substitution of surface Au

atoms with Ag on the {321} model surface significantly reduces

the O2 dissociation barrier,[40,51] which is in line with experimen-

tal observations. Residual Cu in dealloyed nanoporous Au also

exhibits similar effects as residual Ag on catalytic enhancement

for aerobic oxidation reactions.[52] The detailed surface atomic

configurations involving the residual Ag or Cu specifically

required for catalytic enhancements, however, still remains

unclear at this point. Interestingly, the roles of residual less

noble elements vary drastically from reaction to reaction. For

chemoselective hydrogenation reactions under low pressures,

the catalytic activity decreases with the increase in the fraction

of residual Ag, which exhibits an opposite trend to the aerobic

oxidation reactions.[53] For electrocatalytic oxidation reactions

that does not involve O2, no clear correlation between the

residual less noble elements and catalytic activity has been

observed so far.[23,54] The exact nature of the active sites on the

dealloyed nanoporous Au for various reactions is still a

fundamentally intriguing topic under intense debate and well-

worthy of further investigations.

3.2. Dynamic Surface Structural Remodeling Under ReactionConditions

The nanoligament surfaces may undergo dynamic structural

rearrangements to evolve into thermodynamically more stable

Figure 3. (A) Schematic illustration of surface atoms at terrace, step edge, and kink sites with various characteristic atomic coordination numbers (ACNs). (B)Bright-field TEM image and (C) 3D tomographic reconstruction by electron tomography of dealloyed nanoporous Au foams. (D) TEM image of a nanopore.The electron diffraction pattern (inset) shows that the incident direction is [01(1]. The labelled squares, b, c, d, and e, indicate the areas imaged by high-resolution HAADF-STEM in panels D-b, D-c, D-d, and D-e, respectively. Reprinted with permission from ref [33]. Copyright 2012, Springer Nature. (E) HAADF-STEM image of an individual nanoporous Au0.97Cu0.03 particle dealloyed from a Au0.19Cu0.81 alloy nanoparticle. High-resolution HAADF-STEM images showingthe atomic-level structures of regions a and b are shown in panels E-a and E-b, respectively. The insets in panels E-a and E-b are the FFT patterns of theregions labeled as i, ii, and iii, respectively. In the high-resolution HAADF-STEM images, the crystalline domains were projected along the [01(1] zone axis.Reprinted with permission from ref [23]. Copyright 2016, American Chemical Society. (F) HAADF-STEM image and (G) elemental distribution of Au (red signal)and Ag (green signal) of dealloyed nanoporous Au with 11 at% residual Ag. (H) HAADF-STEM image and (I) elemental distribution of Au (red signal) and Ag(green signal) of dealloyed nanoporous Au with 8 at% residual Ag. Reprinted with permission from ref [48]. Copyright 2017, Elsevier.

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but catalytically less active surface structures during the

catalytic reactions, resulting in deterioration of catalytic activity.

Direct correlations between the activity deterioration and

ligament coarsening were observed during electrocatalytic

methanol oxidation (Figure 4A and 4B)[54] and catalytic aerobic

CO oxidation (Figure 4C).[47] The coarsening of nanoligaments

during CO oxidation was mainly dictated by rapid layer-by-layer

diffusion of Au atoms at undercoordinated surface sites (Fig-

ure 4D and 4E) driven by the interactions between the reactant

molecules and the active surface atoms.[47] Because CO

oxidation is an exothermic reaction, locally generated heat on

the catalyst surface may serve as the primary driving force for

the surface reconstruction and ligament coarsening. Interest-

ingly, twin boundaries could function as pinning sites to

surface-diffusing atoms at the propagating front and the

surface atomic diffusion could be effectively hindered by

judiciously introducing planar defects onto the nanoligament

surfaces. The dealloyed nanoporous Au also undergoes sub-

stantial surface reconstruction to form thermodynamically more

stable low-index facets on the ligament surfaces upon exposure

to electrochemical potential cycling,[55] resulting in catalytic

activity decay during electro-oxidation of methanol.

The surface reconstruction and ligament coarsening under

reaction conditions involve not only the surface migration of

Au atoms but also the diffusion of residual Ag atoms both on

the surface and in the bulk of the nanoligaments. Ag

segregation at the ligament surfaces was clearly observed

during catalytic CO oxidation.[47–48] It was recently revealed by

ab initio molecular dynamics (AIMD) calculations and exper-

imentally verified by Auger electron spectroscopy that surface-

adsorbed oxygen drove the migration of subsurface Ag atoms

to the O-rich sites on the stepped model surfaces that

mimicked the active sites on the nanoligament surfaces.[56]

Besides the vanishment of undercoordinated surface atoms, the

formation of phase segregated monometallic Ag islands on the

ligament surface may be another key factor causing the

catalytic activity deterioration, though it still remains unclear

how the dimensions and atomic surface configurations of the

segregated Ag domains affect the overall catalytic activity.

The highly reactive and dynamic nature of the active sites

imply that the dealloyed nanoporous Au catalysts may

constantly undergo surface reconstructions even under steady

state reaction conditions. Achieving long-term catalytic durabil-

ity does not always necessarily require the presence of

structurally stable active sites on the nanoligament surfaces.

Alternatively, the catalytic activity can also be preserved if the

active surface sites are continuously replenished through

dynamic structural rearrangements as a result of the constant

formation and depletion of molecular adsorbates in the

catalytic processes. The dynamic surface reconstruction ob-

served by in situ HRTEM, however, may not precisely reflect the

structural dynamics of the complex 3D surfaces under real

reaction conditions,[57] because in situ HRTEM visualizes essen-

tially a 2D projection of the nanoporous structure. The lack of

3D structural confinement and larger exposure surface to the

reactants make a slice of 2D porous structure on the TEM grid

Figure 4. SEM images of a nanoporous Au electrode measured (A) before and (B) after electrocatalytic oxidation of methanol. Insets in two images show thecolor change of the nanoporous Au electrode before and after the electrocatalytic reaction. Reprinted with permission from ref [54]. Copyright 2007, AmericanChemical Society. (C) Catalytic performance of dealloyed nanoporous Au at 30 8C for aerobic CO oxidation. Insets are SEM images of the nanoporous Ausamples after catalytic reactions at initial conversion rate, and 90, 80, and 70% of the initial conversion rate, as indicated by arrows. (D, E) Twin boundary as apinning site at the propagating front of coarsening. Dotted lines in panels D-i and E-i represent twin boundaries, and circles represent a kink or step. Asshown in panels D-ii, D-iii, E-ii, and E-iii, the {111} plane diffuses away after pointed kinks disappear. Reprinted with permission from ref [47]. Copyright 2014,American Chemical Society.

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more vulnerable to the reaction environments than the 3D

bicontinuous structure. In addition, the specimen inside the

in situ HRTEM chamber is located where the gas phase

reactants directly hit the catalyst surfaces, which may boost

both the catalytic reactions and structural rearrangements.

Furthermore, the influence of electron beam irradiation has not

been completely ruled out, though the catalytic reactions were

observed to be the primary driving force for the surface

reconstruction and ligament coarsening.[33,57]

3.3. Origin of Catalytic Selectivity

Metallic Au-based nanocatalysts have unique capabilities to

catalyze chemoselective oxidation reactions at low temper-

atures and pressures, which distinguishes them from their

counterparts made of other transition metals, such as Pd and

Pt.[11–12] Friend, Baumer, and co-workers discovered that meth-

anol could be selectively oxidized into methyl formate (partial

oxidation product) or CO2 (complete combustion product) on

dealloyed nanoporous Au catalysts, depending on the amount

of residual Ag, surface abundance of O2, and reaction temper-

ature.[14] To achieve high selectivity toward partial oxidation, the

partially oxidized products must adsorb to the Au catalyst

surfaces with sufficiently low affinity, such that they can rapidly

desorb from the catalyst surfaces before being further oxidized.

The selectivity of a nanoporous Au catalyst toward catalytic

oxidation reactions essentially originates from the delicate

balance between its capability to activate surface-adsorbed

oxygen and its weak, dynamic interactions with the partial

oxidation products.

As illustrated in Figure 5A, oxidation of methanol by O2 on

Au catalyst surfaces may result in a variety of partially oxidized

products in addition to the thermodynamically favored com-

bustion product, CO2. On fully dealloyed nanoporous Au

catalysts with residual Ag less than 1 atomic%, methyl formate

is exclusively produced at a reactant stoichiometry of 1 volume

% O2+2 volume % CH3OH. The selectivity toward methyl

formate approaches 100% at room temperature, and only

slightly decreases to 97% even at 80 8C (Figure 5B and 5C). An

oxygen-rich reaction condition favors the formation of CO2,

resulting in loss of selectivity toward partial oxidation as the

temperature increases (Figure 5C). Increasing the residual Ag

content also leads to loss of selectivity at reaction temperature

higher than 80 8C, even in an oxygen deficient reactant

atmosphere (2 volume % methanol+1 volume% O2) (Figur-

es 5D and 5E). When the fraction of residual Ag is above 10

atomic%, the catalytic selectivity drastically drops as CO2

becomes the dominate product and methyl formate is no

longer formed in the entire temperature range up to 80 8C.These observations strongly indicate that residual Ag regulates

the abundance of reactive oxygen on the catalyst surfaces and

thus controls the selectivity toward the partial oxidation of

methanol. More recently, it has also been observed that the

residual Ag in dealloyed nanoporous Au plays crucial roles in

chemoselective hydrogenation of C=C, C/C, C=N, and C=O

bonds under mild conditions,[53] though the detailed mecha-

nisms are still open to further investigations.

Methanol oxidation catalyzed by dealloyed nanoporous Au

exemplifies how the strong synergy between the undercoordi-

nated Au surface atoms and residual Ag dictates chemo-

selective catalytic molecular transformations. More quantitative

examination of the structure-composition-performance rela-

tionships will lead to key design principles for achieving

optimal catalytic selectivity on dealloyed nanoporous Au

catalysts.

Figure 5. (A) Proposed mechanism of selective oxidation of methanol on dealloyed nanoporous Au catalysts. Methanol is activated by surface oxygen andbonded at the surface as methoxy. Subsequent deprotonation leads to the aldehyde. Fast reaction of the highly reactive aldehyde with further methoxy leadsto the coupling product methyl formate (HCO2CH3). In the case of excess oxygen, the aldehyde can be further oxidized, resulting in CO2 formation. Thebackground of panel A is an SEM image showing the structure of monolithic nanoporous Au. (B) Conversion % of total oxidation of methanol (to CO2; bluerhombuses) and partial oxidation of methanol (to methyl formate; gray squares) at various reaction temperatures catalyzed by dealloyed nanoporous Au withresidual Ag less than 1 atomic%. (C) Selectivity (fraction of methanol that is converted into methyl formate) at various reaction temperatures on dealloyednanoporous Au with residual Ag less than 1 atomic%. (D) Activity and (E) selectivity of methanol oxidation catalyzed by dealloyed nanoporous Au with2.5 atomic% of residual Ag. Reprinted with permission from ref [14]. Copyright 2010, The American Association for the Advancement of Science.

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4. Dealloying of Alloy Nanoparticles towardElectrocatalysis Optimization: A Case Study ofAlcohol Electro-Oxidation

A general design criterion for structural optimization of

catalysts is to maximize both the surface area-to-volume ratio

and the density of surface active sites. Using the electrocatalytic

methanol oxidation reaction (MOR) as a model reaction, we

recently demonstrated that both the mass-specific surface area

and the density of surface active sites of spongy nanoparticles

could be systematically fine-tuned through kinetically con-

trolled percolation dealloying of Au@Cu alloy nanoparticles.[23,58]

Analogous to that of the macroscopic Au@Ag alloy membranes,

the percolation dealloying of Au@Cu alloy nanoparticles is

essentially dictated by two interplaying structure-transforming

processes, leaching of Cu atoms from the alloy and coarsening

of the nanoligaments. While the leaching of Cu atoms creates

high densities of atomically undercoordinated surface sites and

drives the nanoporosity evolution, ligament coarsening-driven

surface reconstruction causes both the surface area-to-volume

ratio and the fraction of undercoordinated surface atoms to

decrease. On noble metal nanocatalyst surfaces, it is essentially

the undercoordinated surface atoms that serves as the primary

active sites for electrocatalytic oxidation of liquid alco-

hols.[23,54,58–60] Therefore, nanoporous Au catalysts exhibiting the

optimal electrocatalytic performance are expected to possess

thin nanoliamgents enclosed by highly abundant undercoordi-

nated surface atoms, which can be synthetically achieved by

judiciously maneuvering the Cu leaching and ligament coarsen-

ing processes during percolation dealloying of Au@Cu alloy

nanoparticles under kinetically controlled conditions.

Our synthetic approach to spongy Au nanoparticles

involves a stepwise nanoscale alloying-dealloying process as

illustrated in Figure 6A. We started from Au@Cu2O core-shell

nanoparticles whose core and shell dimensions could be

precisely tuned over a broad size range using a seed-mediated

growth method.[61] The Au@Cu2O core-shell nanoparticles first

transformed into Au@Cu bimetallic heteronanostructures

through chemical reduction, and then underwent intraparticle

alloying to form Au@Cu alloy nanoparticles upon thermal

treatment in either a reducing atmosphere,[23] such as H2, or in a

high boiling point-polyol solvent,[62] such as tetraethylene

glycol. The sizes and Cu/Au stoichiometric ratios of the alloy

nanoparticles were essentially predetermined by the core and

shell dimensions of their parental Au@Cu2O core-shell nano-

particles and thereby could be systematically tuned over a

broad range.

The relative rate of Cu leaching with respect to that of the

ligament coarsening can be tuned by choosing different

etchants or varying the etchant concentrations.[23] By kinetically

trapping the partially dealloyed spongy nanoframes (NFs) at

various dealloying stages, both the mass-specific surface area

and the density of surface active sites can be controlled. As the

percolation dealloying proceeds, the electrochemical surface

area (ECSA) first increased due to nanoporosity formation and

then decreased as a consequence of ligament coarsening, while

the specific activity (SA) normalized to the surface area

progressively decreased because of ligament coarsening-driven

surface reconstruction (Figures 5C). The mass activity (MA) of

the dealloyed nanoparticles is essentially determined by both

the ECSA and SA. No clear correlation between the catalytic

activity and amount of residual Cu was observed, possibly

because the electrocatalytic MOR does not involved surface

adsorbed oxygen. Therefore, it is highly likely that the SA is

primarily determined by the density of the undercoordinated

surface atoms rather than the residual Cu on the nanoligament

surfaces. Despite their remarkable initial activities, the dealloyed

NFs underwent activity deterioration over time under the

reaction conditions due to electrochemically induced surface

reconstruction and ligament coarsening (Figure 6D), which

motivated us to further explore new ways to effectively

enhance both the activity and durability of the dealloyed

nanoporous electrocatalysts.

It has been demonstrated by Erlebacher and coworkers[63]

that enhanced catalytic durability of dealloyed nanoporous Au

can be achieved by incorporating Pt into the Au@Ag alloy

precursors such that the undercoordinated surface atoms can

be stabilized upon surface accumulation of Pt around the

atomic step edges and kinks during percolation dealloying.

More recently, we found that residual Ag was also capable of

enhancing the electrocatalytic durability of dealloyed nano-

sponge (NS) particles,[58] though Ag was a leachable less-noble

element whose behaviors appeared fundamentally distinct

from those of the non-leachable Pt. The co-leaching of Ag and

Cu from Au@Ag@Cu ternary alloy nanoparticles not only greatly

accelerated Cu leaching, but also effectively suppressed

ligament coarsening. As shown in Figures 6E and 6F, the fully

dealloyed NS particles obtained from dealloying of Au@Ag@Cuternary alloy nanoparticles (denoted as NS-T) exhibited substan-

tially larger specific surface areas, higher densities of surface

active sites, thinner ligaments, and smaller average pore sizes in

comparison to their Ag-free counterparts derived from Au@Cubinary alloy nanoparticles (denoted as NS-B). As a consequence,

the dealloyed NS-T particles exhibited remarkably enhanced

electrocatalytic activities toward a series of alcohol oxidation

reactions (Figures 6G–6 J), including MOR, ethanol oxidation

reaction (EOR), iso-propanol oxidation reaction (i-POR), and

ethylene glycol oxidation reaction (EGOR), in comparison to

those of the Ag-free NS-B particles. The residual Ag in the

dealloyed NS-T particles greatly suppressed the surface recon-

struction during electrocatalytic reactions, enabling the reten-

tion of the superior catalytic activities over much longer periods

(Figure 6K). The insights gained from this case study shed light

on the crucial roles of residual less noble elements in enhancing

the durability of dealloyed metallic electrocatalysts for fuel cell

applications.

5. Summary and Outlook

Nanoporous Au catalysts derived from percolation dealloying of

Au-containing alloys exhibit remarkable catalytic activities

toward a large variety of reactions ranging from industrially

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important CO and alcohol oxidation to synthetically challenging

C@C ring formation.[11–12] Of particular interest to the catalysis

community is the unique capability of dealloyed nanoporous

Au to efficiently catalyze chemoselective reactions under mild

reaction conditions.[14,53] Instead of providing an exhaustive

survey on all catalytic reactions reported in the literature, this

review focuses on several representative chemical and electro-

chemical oxidation reactions to shed light on the structure-

composition-property correlations of the dealloyed nanoporous

Au catalysts. The nanoporosity evolution during percolation

dealloying involves a series of intriguing structure-rearranging

processes, such as interfacial atomic dissolution, surface

reconstruction, atomic interdiffusion, and ligament coarsening.

Erlebacher’s surface-diffusion continuum model[13,18,28] rigorously

interprets how multiple structural remodeling processes syn-

ergistically modulate the nanoporosity evolution. The success

in structure-controlled synthesis through kinetically maneu-

vered percolation dealloying enables systematic invesitigations

of the structure-composition-property relationships underpin-

ning the intriguing catalytic behaviors of the dealloyed nano-

porous Au catalysts. The undercoordinated surface atoms and

the residual less-noble elements have been identified as two

key factors that dicate the catalytic activity, durability, and

selectivity of the dealloyed nanoporous Au catalysts. In situ

environmental HRTEM provides a powerful characterization tool

capable of resolving not only the complex atomic-level surface

structures but also, more importantly, the dynamic restructuring

of the active sites under reaction conditions.[33,47,57] Many

aspects regarding the detailed mechanisms of nanoporous Au-

based catalysis, however, vary significantly from reaction to

reaction and thus still remain ambiguous. The synergy between

undercoordinated surface atoms and redisual less-noble ele-

Figure 6. (A) Scheme illustrating the transformation of Au@Cu2O core@shell nanoparticles (NPs) into Au@Cu alloy NPs and the percolation dealloying of Au@Cualloy NPs. (B) Cyclic voltammetry curves of MOR on Au0.19Cu0.81 alloy NPs and various dealloyed spongy nanoframes (NFs) in 1.0 M methanol and 0.5 M KOH ata potential sweep rate of 10 mVs@1. The NF samples labeled as NF-i, NF-ii, NF-iii, and NF-iv correspond to the samples obtained through dealloying of Au0.19

Cu0.81 alloy NPs in 0.2, 0.5, 1.0, and 2.0 M Fe(NO3)3 for 2 h, respectively. The sample labeled as NF-v was obtained through dealloying of Au0.19Cu0.81 alloy NPs in2.0 M HNO3 for 1 h. HAADF-STEM images of one representative particle for each NF sample are also shown as the insets. (C) Cu atomic%, mass activities (MAs),electrochemical surface areas (ECSAs), and specific activities (SAs) of Au0.19Cu0.81 alloy NPs and dealloyed NFs. The Cu atomic% was quantified by energydispersive spectroscopy (EDS) and inductively coupled plasma mass spectrometry (ICP-MS). (D) Chronoamperometry curves collected on Au0.19Cu0.81 alloy NPsand dealloyed NFs for MOR at 0.1 V (vs. SCE) and 0.3 V (vs. SCE). Reprinted with permission from ref [23]. Copyright 2016, American Chemical Society. TEMimages of (E) NS-T (dealloyed from Au0.14Ag0.14Cu0.72 ternary alloy NPs) and (F) NS-B (dealloyed from Au0.16Cu0.84 binary alloy NPs). Cyclic voltammetry curves ofNS-T and NS-B in 0.5 M KOH electrolyte solutions containing (G) 1 M methanol, (H) 1 M ethanol, (I) 1 M isopropanol, and (J) 0.25 M ethylene glycol. (K) MAs,SAs, and the ratio of current density after 2 hours to the initial current density (jp,2 h/jp,i) for NS-B and NS-T. Reprinted with permission from ref [58]. Copyright2016, American Chemical Society.

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ments, the driving force for the surface reconstruction under

reaction conditions, and the transition states associated with

the molecular transformations on the active sites are all well-

worthy of further investigations.

We envision that dealloyed spongy nanoparticles will attract

conitnuously increasing attention in the fields of Au-based

heterogeneous catalysis and electrocatalysis because they

exhibit a unique set of advantages over the macroscopic

nanoporous foams in terms of catalytic performance, materials

processablity, and structural tunability. First, the catalytically

active sites on the ligament surfaces are easily accessible by the

reactant molecules when the nanoscale porosity is created

inside a nanoparticle with a finite size, whereas the molecules

must overcome long diffusion distances and convoluted paths

to reach the active sites buried inside the interior of a

macroscopic nanoporous membrane. Second, the pore and

ligament dimensions of a macroscopic nanoporous membrane

vary significantly from location to location, with narrower pore

channels and finer ligaments on the outer surfaces than those

in the interior regions.[64] Such intrinsic structural heterogeneity

significantly complicates the energy landscapes and overall

kinetics of the surface-catalyzed molecular transformations.

Switching from bulk materials to nanoparticulate systems

makes it possible to achieve uniform ligament thickness and

pore sizes. Third, using colloidal nanoparticles as an easily

processable ink allows for straightforward nanoparticle assem-

bly on a large variety of substrates, including the flexible and

microstrucured substrates, for constructing high-performance

but low-cost catalysts for widespread applications. Fourth and

most importantly, alloy nanoparticles exhibit drastically en-

hanced structural diversity and tunability compared to their

bulk counterparts, creating unique opportunities for us to fine-

tailor a series of geometric and compositional parameters.

Taking Au@Cu bimetallic nanoparticles as an example, recent

advances in colloidal syntheses allow one to fine-tailor not only

the size, shape, and composition but also intraparticle atomic

configurations (disordered alloys, ordered intermetallic phases,

intraparticle compositional gradient, and phase segrega-

tion),[61–62,65–67] all of which are crucial factors profoundly

influencing the structural transformations of the nanopartilces

during the percolation dealloying. By coupling the percolation

dealloying with other chemical reactions, such as galvanic

replacement reactions[62] and electrochemical atomic layer

deposition,[34,54,68–69] it becomes possible to incorporate other

catalytically active materials into spongy Au nanoparticles in a

highly controllable manner, enbaling us to further fine-tune the

catalytic properties of the dealloyed nanoparticles at a level of

detail and precision unachievable on those dealloyed bulk

materials.

In addition to their catalytic properties, dealloyed nano-

porous Au also exhibits interesting optical properties that are

dominated by the collective oscillation of free electrons known

as plasmons.[34,70–72] Introduction of nanoporosity to the surface

or the bulk of Au nanoparticles results in greatly enhanced

tunability of the plasmon resonance frequencies over a broad

spectral range and enormous local electric field enhancements

on the nanoligament surfaces exploitable for plasmon-en-

hanced spectroscopies.[29–30,73–75] Benefiting from their unique

combination of catalytic and plasmonic properties, the deal-

loyed spongy Au nanoparticles may serve as a dual-functional

materials platform enabling the use of surface-enhanced Raman

scattering (SERS) as a time-resolving and molecular finger-

printing tool to track detailed interfacial molecular transforma-

tions in real time during catalytic reactions.[76–80] In addition, the

dealloyed spongy Au nanoparticles exhibit interesting photo-

thermal behaviors upon plasmonic excitations.[31] The photo-

thermally generated heat on the local surfaces of the catalysts

can be harnessed to further boost a variety of interfacial

thermal catalytic reactions. Furthmore, it has been recently

observed that the energetic hot electrons generated during

plasmon decay can be harnessed to efficiently drive a series of

interesting photocatalytic reactions along unconventional path-

ways distinct from those involved in the conventional thermal

catalytic reactions and semiconductor-based photocataly-

sis.[81–84] Plasmon-driven photocatalysis is a newly emerging field

full of open questions, challenges, and oppotunities. The

dealloyed spongy Au nanoparticles provide an interesting

materials system that may help us interrogate intriguing but

challenging fundamental questions regarding the detailed

mechanisms of plasmon-mediated chemistry and photochemis-

try.

Acknowledgements

The authors thank the United States National Science Founda-

tion (DMR-1253231 and OIA-1655740), United States Depart-

ment of Energy (DE-SC0016574), and the University of South

Carolina (Startup Funds, ASPIRE@I Track I Award, and SPARC

Award) for funding support.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: gold nanocatalysts · nanoporosity · percolation

dealloying · undercoordinated surface atoms · electrocatalysis

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Manuscript received: April 10, 2018Version of record online: May 30, 2018

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