Linan Zhou1, 4, Dayne F. Swearer 1,4, Chao Zhang 2,4, Hossein Robatjazi 2,4, Peter Nordlander2, 3, 4, and Naomi J. Halas1, 2, 3, 4.1 Department of Chemistry, 2 Department of Electrical and Computer Engineering, 3 Department of Physics and Astronomy, and 4 Laboratory

for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States.

Funding

Conclusions• CuRu surface alloy exhibits extraordinarily efficient photocatalytic activity towards ammonia decomposition, making

plasmonic photocatalysis promising in hydrogen storage application• Reduction of reaction activation barrier by hot carriers results in the superior performance of photocatalysis compared to

thermocatalysis

Efficient Plasmon-driven Decomposition of Ammonia for Hydrogen Storage

Photocatalytic Ammonia Decomposition

Reference• Schüth, F. et. al., Energy Environ. Sci. 5, 6278-6289 (2012).• Clavero, C. Nat. Photon. 8, 95-103 (2014).• Zhou, L. et. al., Science 362, 69-72 (2018).

2NH3 N2 + 3H2 ΔH°= +91 kJ/mol

250 300 350 400 450 50010-1

100

101

102

103

Photocatalysis Thermocatalysis

H 2 pro

duct

ion

rate

(µm

ol/g

/s)

101

102

103

104

105 TOF (m

ol H2 *m

ol -1 Ru*h -1)

3.2 9.64.8 6.4 8Light intensity (W/cm2)

Surface temperature (oC)

CuRu Surface Alloy as Plasmonic Photocatalyst for ammonia decomposition

• Antenna-Reactor strategy combines the traits of plasmonic metals (antenna) and transition metals (reactor)• We proposed surface alloy to be an efficient antenna-reactor structure given the highly absorptive plasmonic core, intimate

electronic contact between antenna and reactors, and high atomic usage of the reactive reactors• Ammonia decomposition is an unimolecular model reaction suitable for exploring the mechanism of hot-carrier-mediated

chemistry

Plasmonic metals: great light harvesters, weak catalysts

Transition metals: excellent catalysts, poor optical response

Light-dependent Activation Barriers and Reaction mechanism

Ammonia as Hydrogen Storage Medium

• NH3 is a promising medium for COx-free H2 storage with high volumetric and gravimetric capacities• But high temperature is required to efficiently extract out H2 through traditional thermocatalysis• Our Cu-Ru surface alloy plasmonic photocatalyst paves the way to viable hydrogen economy with low energy cost

Plasmonic Photocatalysis via Hot-Carrier-Mediated Chemistry

LUMO

HOMO

Metal Molecule

Scattering Absorption

Hot carriers derived from non-radiative decay of localized surface plasmon resonance can drive chemical reactions by activating molecules through hot-carrier transfer

• Photocatalysis of Cu19.5Ru0.5 exhibits 20-100 times higher rate than that of thermocatalysis heated to similar temperatures in the dark, suggesting the predominant contribution of hot carriers compared to photothermal effect

• The temperatures of photocatalysis denote the highest surface temperatures of the sample pellet under illumination due to photothermal heating. They are measured by infrared camera, as illustrated at the right panel for an example

101

102

1.3 1.4 1.5 1.6

101

102

500 nm

450 nm

550 nm

700 nm

600 nm

H 2 pro

duct

ion

rate

(µm

ol/g

/s)

3.2 W/cm2

1000/T (1/K)

4.0 W/cm2

3.2 W/cm2

2.4 W/cm2

1.6 W/cm2

550 nm

Dark

1.21 eV

Dark

1.21 eV

a

0.1 1101

102

n=-0.04

H 2

pro

duct

ion

rate

(µm

ol/g

/s)

Photocatalysis at 6.4 W/cm2

Thermocatalysis at 400 °C

Ammonia partial pressure (atm)

n=0.88

𝑟𝑟 = 𝑘𝑘 � 𝑃𝑃𝑁𝑁𝑁𝑁3𝑛𝑛

d

c

b

a) Arrhenius plot of apparent activation barriers for different illumination conditions.b) 3D representation of apparent activation barrier 𝐸𝐸𝑎𝑎 𝜆𝜆, 𝐼𝐼 for different wavelengths and intensities.c) Reaction order with respect to ammonia partial pressure (𝑃𝑃𝑁𝑁𝑁𝑁3) in photocatalysis (6.4 W/cm2 white light) and thermocatalysis (427 °C).d) Effect of hot carriers on reaction energetics of ammonia decomposition

IntroductionFor the past several decades, photocatalysis research has almost been limited to semiconductor materials, most of which weakly respond to visible light. In contrast, plasmonic nanostructures based on coinagemetals, albeit with poor catalytic activity, interact strongly with visible light like a nanoantenna and have been proved to be capable in driving chemical reaction via energetic hot carriers excited by light. In thiswork, we demonstrated that bimetallic surface alloys is a superior structure that optimizes the synergy between plasmonic antenna and catalytic reactor. Under optimal illumination condition, Cu-Ru surfacealloy achieves 18% energy efficiency and 33.5% quantum yield for ammonia decomposition reaction (2NH3 N2 + 3H2). Kinetic experiments revealed the phenomena of light-dependent activation barrier andup to 75% reduction in the activation barrier under illumination relative to that obtained from thermocatalysis in the dark has been achieved. The capability of hot carriers in modifying the reaction mechanismby preferentially activating the rate-limiting step is proposed to be responsible for the observed highly efficient light-driven chemical reaction, which is up to 100x greater efficiency compared to that achievedby heating at the same temperature in the dark. Ammonia is an ideal COx-free hydrogen carrier with high capacity, but this application faces challenges due to the high activation barrier of NH3 decomposition.Our work provides a promising alternative route to achieve feasible hydrogen economy with less energy cost and pollution production.

2 nm 400 500 600 700 800

Kube

lka-M

unk

Func

tion

Wavelength (nm)5 nm

Morphology and Optical Property of Cu19.5Ru0.5

• Cu19.5Ru0.5 surface alloy supported on MgO/Al2O3 composite was prepared through co-precipitation method and the subscript represents the atomic fraction of Cu and Ru, respectively, with the remaining as the metal elements in the oxide support

• The size of Cu-Ru surface alloy ranges from 2-10 nm, with average diameter of ~ 5nm• UV-Vis diffuse reflectance spectrum shows an absorption feature at around 560 nm, attributed to plasmonic resonance of Cu core

Sample

20 ˚C

300 ˚C

1 mm