Electrocatalysis with Atomically Defined Model Systems: Metal

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Electrocatalysis with Atomically Defined Model Systems: Metal–Support Interactions between Pt Nanoparticles and CoO(111) in Ultrahigh Vacuum and in Liquid Electrolytes 3

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Firas Faisal, Manon Bertram, Corinna Stumm, Tobias Wähler, Ralf Schuster, Yaroslava Lykhach, Armin Neitzel, Tomáš Skála, Nataliya Tsud, Klára Beranová, Kevin Charles Prince, Vladimír Matolín, Olaf Brummel, and Jörg Libuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/ acs.jpcc.8b05594 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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Pt(0.09 ML)/Co3O4(111)/Ir(100) The Journal of Physical Chemistry

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a) IRAS Pt NPs

b) IRAS 2nd CO uptake

1st CO uptake O C

O C

O C

O C

ΔT

O C

O C

Co3O4 Ir(100)

CO dosage [L] 0.003 0.006 0.012 0.025 0.05 0.09 0.19 0.38 0.75 1.5 3.0 6.0 12.5 25.0 50.0 0

Pt 2079

COt

2+

COt

2114 + Pt

Co 2146 2129

ΔR/R 0.1 %

Co2+ 2145

Pt0 2078

ΔR/R 0.1 %

Paragon Plus2300 Environment2200 2100 2000 1900 2300 2200 2100 2000ACS 1900 -1 Wavenumber [cm ] Wavenumber [cm-1]

Pt/Co3O4(111)/Ir(100)

The Journal of Physical Chemistry

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after heating

b) IRAS before heating Pt coverage [ML] CO dosage 0.75 L

a) IRAS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 2300 56 57

1882

2167

2150 2114 2x 2078 CO dosage 3 L

0.0 0.09 0.14 0.20 0.27 0.45 1.36 2.73

2162 2147

1864 2x 2095

1876

1857

2114 2089 2100

CO dosage 50 L

Co2+

COb 1864 +

Pt

COt 2114 ΔR/R 1% 2102

Co2+ Pt0

0

Pt

O C

O C

O C

O O C C

COb 1857

COt ΔR/R 1% 2100

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O C

O C

O C

O O C C

2200 2100 2000 1900 1800 1700 2300 2200 2100 2000 1900 1800 1700 Wavenumber [cm-1] Wavenumber [cm-1]

Pt/Co3O4(111)/Ir(100) The Journal of Physical Chemistry b) TPD m/z = 44 (CO2) a) TPD m/z = 28 (CO) CO on Pt pCO arb. u. pCO2 arb. u.

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Pt coverage [ML] 2+

CO on Co

CO2

2.73 1.36 0.45 0.27 0.20 0.14 0.09 0.00

ACS Paragon Plus Environment 100 200 300 400 500 100 200 300 400 500 Temperature [K] Temperature [K]

Pt/Co3O4(111)/Ir(100) a) SR-PES h=180 eV

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80


Pt 4f

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b) SR-PES h=180 eV

Pt 4f

Pt0 Pt Pt coverage [ML] 2.0 1.50 1.00 0.67 0.50 0.33 0.25 0.17

Pt0 annealing Temperature [K]

Pt

300 350 400 450 500 550 600 650

0.12

700

0.07

750

0.02

800

0.0

850

ACS Paragon Environment 78 76 74 72 70 68 Plus 80 78 76 74 72 70 Binding Energy [eV] Binding Energy [eV]

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Pt/Co O (111)/Ir(100) 1st vs.4th cycle EC-IRRAS applied potential 1 [V]RHE 2 a) 0.36 ML Pt 3 0.33 1993 2025 4 3 4 Page 5 of 44 The Journal of Physical Chemistry

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 b) 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 c) 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 2200 54 55

0.705

0.68 ML Pt 2012

0.33 2008

0.705

2.73 ML Pt

0.33

2021 2020

ΔR/R 0.5 % ACS Paragon Plus Environment

0.705

2100 2000 1900 1800 Wavenumber [cm-1]

0.36 ML Pt

Pt NPs

OO O O CC C C

Po

ten

tia

2.73 ML Pt

ΔR/R 0.1 %

2200 2100 2000 1900 -1 Wavenumber [cm ]

Relative band intensity COt

Cycles

0.68 ML Pt

ΔR/R 0.1 %

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Co3O4 Ir(100)

Cycles

ΔR/R 0.1 %

Cycles

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Pt/Co3O4(111)/Ir(100) The Journal of Physical Chemistry O O COt a) EC-IRRAS CC

lc

yc

lin

g

b) EC-IRRAS

100 % 80 %

Pt(111) 2.73ML Pt

60 %

0.68 ML Pt

40 %

0.36 ML Pt

1 (p) 2 (p) 3 (s) 4 (p) Potential cycle (polarization)


Pt/Co3O4(111)/Ir(100) Pt/Co3O4(111)/Ir(100)The Journal of Physical Chemistry

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Integrated peak area COt + COb [arb. u.]

a) SR-PES

Integrated Pt 4f intensity [arb. u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Pt

Pt

Pt (111)

PtCO

0

0

0

b) EC-IRRAS

0.5 1 1.5 Pt coverage [ML]

2

0 0.5 1 1.5 2 2.5 3 3.5 Pt coverage [ML]


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

O O CC

+


Pt ,Pt

0

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Co3O4(111) OO O O CC C C

Ir(100)

Potential cycling


Pt

0

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Electrocatalysis with Atomically Defined Model Systems: Metal–Support Interactions between Pt Nanoparticles and Co3O4(111) in Ultrahigh Vacuum and in Liquid Electrolytes Firas Faisal1, Manon Bertram1, Corinna Stumm1, Tobias Wähler1, Ralf Schuster1, Yaroslava Lykhach1, Armin Neitzel1, Tomáš Skála2, Nataliya Tsud2, Klára Beranová3,†, Kevin C. Prince3, Vladimír Matolín2, Olaf Brummel1*, Jörg Libuda1,4 1

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany

2

Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, 18000 Prague, Czech Republic 3

Elettra-Sincrotrone Trieste SCpA and IOM, Strada Statale 14, km 163.5, 34149 Basovizza-Trieste, Italy

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Erlangen Catalysis Resource Center and Interdisciplinary Center Interface Controlled Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany

*corresponding author: Olaf Brummel; [email protected] †

present address: Institute of Physics, Czech Academy of Sciences, Na Slovance 2, CZ-18221 Prague, Czech Republic

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Abstract Electronic metal–support interactions play a key role in the design of heterogeneous catalysts, as they provide a tool for tuning catalytic properties and enhancing catalyst stability. In this work, we explore the role of metal–support interactions in electrocatalysis using a model approach. We investigate the adsorption and reaction behavior of atomically defined Pt/Co3O4 model catalysts in ultrahigh vacuum (UHV) and under electrochemical conditions. The model systems were prepared by physical vapor deposition (PVD) of Pt onto well-ordered Co3O4(111) films on Ir(100), varying the average Pt nanoparticle (NP) size between 10 and 500 atoms per NP. In UHV, the model catalysts were characterized by synchrotron radiation photoelectron spectroscopy (SRPES), temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS). By SRPES, we show that partially oxidized Ptδ+ species are formed at the interface with the Co3O4 support. CO adsorbs weakly on these Ptδ+ sites and can be identified by IRAS at 115 K. Upon heating, CO adsorbed on metallic Pt0, reacts with oxygen released from Co3O4 and gives rise to CO2 between 450 and 500 K. As a result of oxygen depletion, the Ptδ+ at the NP interface is reduced to Pt0. Subsequently, we investigated the adsorption and oxidation of CO under electrochemical conditions on the same Pt/Co3O4 model catalysts. After preparation and characterization in UHV, the model systems were transferred into the electrochemical environment without exposure to ambient conditions. CO adsorption and electrooxidation were performed under conditions where the model system is stable (pH 10, 0.33 to 1.03 VRHE, phosphate buffer). By electrochemical infrared reflection absorption spectroscopy (EC-IRRAS), we show that CO does not adsorb at the partially oxidized Ptδ+ sites in the electrolyte at 300 K. Nevertheless, the Ptδ+ species at the NP/oxide interface is reduced to Pt0 upon repeated experimental cycles. This effect increases with decreasing NP size, in line with the behavior observed under UHV conditions. Our findings suggest that electronic metal–support interactions in metal/oxide catalysts play a very similar role in reactions with gaseous reactants and at the electrified interface.

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1. Introduction With the future transition to a global renewable energy system, electrocatalytic processes will become increasingly important both in energy technology and in chemical production.1-5 Many emerging technologies for energy transformation and storage, such as e.g. fuel cells, electrolyzers, dye-sensitized solar cells, or the production of solar fuels, rely on electrocatalytic transformations. However, the underlying processes are, in general, poorly understood at the microscopic level. Whereas electrochemical surface science has contributed much to our understanding of electrocatalysis on single-crystalline surfaces, achieving the same level of understanding for electrocatalysis by complex nanostructured materials has remained a great challenge up to date.6-8 In classical heterogeneous catalysis, model studies have contributed greatly to bridge this socalled “materials gap” between surface science and applied catalysis.9-15 The model catalysts in these studies are complex but atomically defined interfaces prepared with atomic-level control using a surface science approach. Our present work aims at transferring the same model approach that successfully advanced heterogeneous catalysis to the field of electrocatalysis.16 We prepare complex model interfaces in ultrahigh vacuum (UHV) and transfer these systems into liquid electrolytes to study reactions at the electrified interface. The approach of “electrified model catalysis” comes with two major advantages: firstly, the UHV-based synthesis allows us to prepare a very wide range of model systems, way beyond what is possible by classical flame annealing or electrochemical deposition methods.8,

17-20

Secondly, we can take advantage of the full range of surface science methods to characterize these model interfaces with respect to their electronic and geometric structure before and after they are used in the electrocatalytic environment. In the present work, we scrutinize noble metal catalysts using reducible oxides as a stabilizing support. Such systems are of interest in electrocatalysis for two reasons.21-24 First, the supporting oxide can stabilize small noble metal NPs and, thereby, enhance the dispersion of the precious metal component. For instance, we recently showed that the Pt NPs of a protonexchange-membrane fuel cell catalyst can be stabilized by cerium oxide films under conditions of varying electrode potential.22, 25 Moreover, it is possible to re-disperse the active Pt component by anchoring oxidized Pt species to the oxide matrix. Taking advantage of such anchoring effects, higher noble metal efficiencies can be achieved in comparison to classical supports such as carbon. Along the same lines, several other oxides have been proposed as support materials in electrocatalytic applications, e.g., WOx,26 SnO2,27 TiO2,28 MoOx,29 NbO2,30 or Co3O4.31 3 ACS Paragon Plus Environment

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Besides the aspect of catalyst stability, the catalytic properties of supported NPs themselves may be dramatically influenced by the support. Such support effects have been a longstanding research topic in the field of heterogeneous catalysis (see

14, 32-33

and references

therein). The support may serve as a structural modifier, stabilizing specific particle sizes, shapes or orientations, there may be transport effects at the nanoscale, such as surface diffusion or spillover between adsorbed species on the support and on the active NPs and, finally, there may be chemical interactions ranging from electronic metal–support interactions (EMSI) to strong metal–support interactions (SMSI) on reducible supports. Following the above-mentioned model catalysis approach, all of these effects have been studied in great detail. For instance, we have recently shown that the EMSI between Pt NPs and a reducible support can lead to substantial charge transfer at the NP interface and, thereby, to drastic modifications of the electronic structure of the active NP.14 Also, reverse spillover of oxygen between the support and the supported NPs is a very well-characterized phenomenon that has been shown to play an important role in catalytic transformation of hydrocarbons and hydrocarbon oxygenates, for instance, by preventing catalyst poisoning.15 We may speculate that similar mechanisms may also play a role in electrocatalysis. However, very little is known of such unconventional reaction pathways for electrocatalytic reactions in liquid electrolytes. In the present work, we make use of the model catalysis approach to explore the role of metal–support interactions in electrocatalytic reactions. We scrutinize an atomically defined model catalyst that consists of Pt NPs supported on well-ordered Co3O4(111) films grown on Ir(100).34-36 This material combination is of particular interest, as Pt is the most common electrocatalytic material5, 7 and Co3O4 plays a prominent role both as a support and as an active component both in heterogeneous catalysis37 and electrocatalysis.38-39 In our previous work, we showed that it is possible to transfer the above-mentioned Co3O4(111) film from UHV into the electrolyte and back while preserving its surface chemical composition and long range order.40 We established the experimental procedures and determined the conditions under which such a transfer is possible. In a second study, we investigated Pt NPs supported on the Co3O4(111) film and identified particle-size-dependent changes of the adsorption properties.41 In the present work, we focus on reactivity and explore the role of metal–support interactions at the Pt/Co3O4 interface. As a test reaction we use CO oxidation, which is among the best studied reactions both in surface science and in electrocatalysis (e.g., see

42-46

for UHV studies; and

19, 47-53

for electrocatalytic studies on Pt

single crystals, and 54-59 for studies on NPs). As a unique feature in this work, we are able to follow reactions on identical model catalysts both in UHV and in the electrolyte under 4 ACS Paragon Plus Environment

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potential control. Using surface IR spectroscopy in UHV and in the electrolyte, we probe adsorbed CO to identify the active Pt sites. Thus, we can directly compare IR spectra from “both perspectives” on the same surface. Recently, we reported first results from this study in a brief communication.16 Here, we present the results of a comprehensive study which demonstrates the full potential of the approach. We show that, indeed, very similar phenomena can be observed under both sets of conditions. The metal–support interaction leads to partial oxidation of Pt NPs at the interface. This effect does not only change the adsorption properties of the Pt NPs, but it also leads to activation of oxygen from the support in both the heterogeneously catalyzed and the electrocatalytic reaction.

2. Experimental The measurements in this study were performed with three different experimental set-ups. Infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) data were acquired in a UHV system at the Universität Erlangen-Nürnberg (Germany). The samples used for electrochemical infrared reflection absorption spectroscopy (ECIRRAS) were prepared in a dedicated UHV preparation chamber at the Universität ErlangenNürnberg, which is equipped with a special transfer system (see

16

). The EC-IRRAS

experiments were also carried out at the Universität Erlangen-Nürnberg. In addition, we performed high resolution synchrotron radiation photoelectron spectroscopy (SRPES) at the Materials Science Beamline (MSB), Elettra Synchrotron in Trieste (Italy). Preparation of the Co3O4(111) film and the Pt NPs: The well-ordered Co3O4(111) thin films were prepared on an Ir(100) single crystal (MaTecK, 99.99 %, depth of roughness

Electrocatalysis with Atomically Defined Model Systems: Metal

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