Surface Science 598 (2005) 96–103

www.elsevier.com/locate/susc

Size selectivity for CO-oxidation of Ag nanoparticles onhighly ordered pyrolytic graphite (HOPG)

Dong Chan Lim, Ignacio Lopez-Salido, Young Dok Kim *

Department of Physics, University of Konstanz, Universitaetsstrasse 10, D-78457 Konstanz, Germany

Received 15 June 2005; accepted for publication 25 August 2005Available online 21 October 2005

Abstract

The Ag nanoparticles on HOPG were oxidized using atomic oxygen environments under ultrahigh-vacuum condi-tions, and the nature of the oxidized Ag particles, and their interactions with CO were studied using X-ray photoelec-tron spectroscopy (XPS). The oxygen uptake of a smaller Ag nanoparticle is significantly higher than that of a largerparticle and bulk-like Ag. For the Ag nanoparticles larger than 3 nanometers (nm), two different oxygen species wereidentified, one of which readily reacts with CO to form CO2 at room temperature. Based on our XPS studies, the cat-alytically active oxygen species is assigned to be in Ag2O and/or AgO. For the smaller Ag nanoparticles, only a singleoxygen species can form upon reacting with oxygen atoms in the gas phase, which is inert towards CO-oxidation. Thenature of the catalytically inert oxygen species is discussed. The formation of different oxide species as a function ofparticle size can be responsible for the size selectivity in chemical reactions catalyzed by Ag nanoparticles.� 2005 Elsevier B.V. All rights reserved.

Keywords: Ag; Oxygen; XPS; CO-oxidation; Nanoparticle

1. Introduction

Oxidation of metal has drawn particular atten-tion due to its implication for the corrosion process[1]. Moreover, the interaction of oxygen with metalsurfaces has been suggested to be one of the mostimportant elementary steps in heterogeneous catal-ysis. It has been previously proposed that the for-

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2005.08.030

* Corresponding author.E-mail address: young.kim@uni-konstanz.de (Y.D. Kim).

mation of the oxygen-rich phases is related to thepoisoning of catalyst surfaces; however, oxide sur-faces can be catalytically active [2]. Ru becomesRuO2 under high-pressure conditions, which facili-tates heterogeneously catalyzed reactions such asCO-oxidation [3]. It has been in debate about thenature of the catalytically active species of the Agcatalysts for the ethylene epoxidation, and (surface)Ag-oxide was recently suggested to participatein the ethylene epoxidation [4–8]. Oxide speciescannot be easily prepared under high-vacuum

ed.

D.C. Lim et al. / Surface Science 598 (2005) 96–103 97

conditions, yet they can readily form under high-pressure conditions. Therefore, enhanced catalyticactivities of oxide surfaces have been suggested tobe responsible for the so-called ‘‘pressure gap’’ insurface science [9].

Catalytic activity can drastically change as afunction of particle size [10–17]. Au, which isknown to be inert as bulk, can become catalyti-cally extraordinarily active as the particle sizebecomes smaller than about 5 nanometers (nm)[10–12]. For other metals like Ag, similar sizeselectivity in catalysis was reported: for the propyl-ene partial oxidation and the low-temperature CO-oxidation, the Ag nanoparticles smaller thanabout 5 nm in diameter were shown to be as activeas the Au nanoparticles, whereas for the ethyleneepoxidation, only Ag particles larger than about30 nm can catalyze the reaction [14–17]. Geometricand electronic factors of metal nanoparticles aswell as metal-support interactions have beeninvoked to explain the size selectivity in heteroge-neous catalysis [12]. In the present work, we showdifferent chemical behaviors of the Ag nanoparti-cles with various sizes deposited on sputteredhighly ordered pyrolytic graphite (HOPG) sur-faces. The choice of the HOPG samples as sub-strate allows us to more easily detect oxygensignals on Ag nanoparticles, compared to the caseof using an oxide substrate, which has a largebackground oxygen signal [15,16]. In general, anincrease of the oxygen coverage on metal yieldsthe formation of chemisorbed atomic oxygen !dissolved oxygen in the bulk + surface oxide!increase of the thickness of the oxide layer (gener-ally Ag2O for Ag) [18,19]. Here, we provideevidence for the formation of different oxygen spe-cies depending on the Ag particle size, resulting inthe size selectivity of the catalytic activity of CO-oxidation. Ag2O/AgO, which is active for CO-oxidation, can form only in the Ag nanoparticleslarger than 3 nm in diameter.

2. Experimental details

All experiments were performed under ultra-high vacuum (UHV) conditions. XPS measure-ments were carried out in an ultrahigh vacuum

(UHV) chamber with a base pressure of 1 ·10�10 mbar using a Cylindrical HemisphericalAnalyzer (CHA, OMICRON). A pass energy of20 eV was used for all measurements presentedhere. The HOPG samples prepared by thescotch-tape pilling method were inserted into theUHV system and outgassed at about 700 K forlonger than 12 h. The cleanness of the HOPG sam-ples was confirmed using XPS (X-rays source:AlKa photon energy = 1486.6 eV). Before Agdeposition, HOPG surfaces were mildly sputteredusing an Ar-ion gun to create defect sites, whichare known to increase the adhesion of ad-metalson the surface [20]. When an HOPG surface wasnot sputtered, the particle density was too low sothat they can hardly be detected using XPS. Wehave used 0.5 kV to accelerate Ar ions for sputter-ing. Ag nanoparticles were grown by evaporatingan Ag rod (purity 99.999% from Alfa Aesar),which was wrapped by tungsten wires. Duringthe Ag evaporation, the HOPG samples were keptat room temperature.

3. Results and discussion

3.1. Oxidation of Ag nanoparticles

In order to mimic the severe oxidizing condi-tions of high-pressure catalytic reactions underUHV conditions with a minor contamination, Agnanoparticles on HOPG were exposed to ‘‘atomicoxygen’’. The atomic oxygen atmosphere was cre-ated by using a clean hot Pt filament located at thebackside of the sample, and backfilling the cham-ber with O2. In this way, vibrationally excited O2

molecules and/or oxygen atoms are produced inthe gas phase [21]. The contamination of the sam-ples by Pt is safely excluded in our XPS analysis.

In Figs. 1–3, the Ag 3d level shifts of the Agnanoparticles (1–6 nm in particle diameter) as afunction of atomic oxygen exposure are summa-rized. In our previous studies using STM andXPS, a positive core level shift and a broadeningof the Ag 3d state with decreasing Ag particle sizeon HOPG were confirmed, indicating that one canestimate the particle size using XPS data [22]. Forsmaller Ag particles, the onset of the chemical shit

380 376 372 368 364

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3 nm

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Fig. 1. Changes of the Ag 3d core levels of the Ag nanoparticleswith a mean size of 1, 2, and 4 nm as a function of atomicoxygen exposure.

380 376 372 368 364

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5 nm

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Fig. 2. Changes of the Ag 3d core levels of the Ag nanoparticleswith a mean size of 5 and 6 nm as a function of atomic oxygenexposure.

0 20 40 60 80 100 120 140

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onset of oxid formation in O 1s

Fig. 3. Changes of the Ag 3d states of the Ag nanoparticleswith various sizes upon exposing to the atomic oxygenatmosphere (summary of the data in Figs. 1–3). For the lasttwo steps of the atomic oxygen exposure for the bulk-like Agand 5-nm sized particles, a Pt filament current of 3.6 A wasused. For the other cases, 3.5 A was used. The partial pressureof O2 in the chamber was 8 · 10�5 mbar during the oxidationexperiments. The results of the mean particle size of 6 nm arenot compared, since a different Pt filament set-up was used forthis sample, and therefore the experimental conditions are notdirectly comparable.

98 D.C. Lim et al. / Surface Science 598 (2005) 96–103

of the Ag 3d state to the lower binding energiesappears at a much lower atomic oxygen exposurecompared to that of larger particles: using thesame amount of atomic oxygen required for thecomplete oxidation of the 1-nm sized Ag particles,the onset of the oxide formation does not appear

for the bulk-like Ag thin film (data not shownhere), implying that the small Ag nanoparticlescan more efficiently be oxidized compared to thebulk-like Ag film (Figs. 2–4). For the Ag nanopar-ticle with mean diameters of 5 and 6 nm, very highatomic oxygen exposures yield positive chemicalshifts, which do not reconcile the Ag-oxide forma-tion (Fig. 2). The origin of the positive shifts of theAg 3d states is not clear: one possible explanationmight be that a charging problem may be caused,when the large Ag nanoparticles are heavily oxi-dized, resulting in the positive core level shifts. Itis important to mention that the Ag(3d)/C(1s)intensity ratios remain constant during the oxida-tion experiments, indicating that a significantchange of the particle size upon oxidation (sinter-ing) can be excluded.

To shed light on the properties of oxygen in theoxidized Ag nanoparticles, the O 1s core levelshifts were studied (Fig. 4). Exposures of Ag nano-particles on HOPG to the atomic oxygen atmo-sphere yield various distinct states in the O 1score level spectra. The O 1s states at the binding

1 nm

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O of HOPGO of Ag

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524 528 532 536

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Ag2O/AgO

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Fig. 4. The O 1s spectra from different Ag nanoparticles obtained after exposing to various amounts of atomic oxygen. The meanparticle diameters are given in the figure.

D.C. Lim et al. / Surface Science 598 (2005) 96–103 99

energy regime above 530 eV are partially attrib-uted to the oxygen species in the pure HOPG,since a peak at 533 eV with a shoulder at 531 eVcan be detected in the O 1s state, when a bare sput-tered HOPG is exposed to the atomic oxygenatmosphere (not shown here). It is not clear, howoxygen atoms are bound to carbon. Most likely,defect sites may interact with atomic oxygen, ora small amount of water existing in our vacuumsystem can also adsorb on the HOPG surface orin the subsurface regime. There is no change ofthe C 1s peak upon atomic oxygen exposure, i.e.we cannot observe that the graphite structure is

destroyed by atomic oxygen (due to the CO orCO2 formation), even though this cannot be com-pletely excluded. The amount of O on a bare sput-tered HOPG surface detected here is estimated tobe below 10% of a monolayer.

For the 1- and 3-nm sized Ag nanoparticles, asingle peak can be found centered at 531–532 eV,respectively, upon oxidation (when the O/HOPGsignal is subtracted), whereas for the larger parti-cles (>4 nm), two O 1s states at 529 eV and531 eV can be observed at the initial stage of theatomic oxygen exposures. The O 1s state at529 eV can be assigned to the formation of Ag2O

100 D.C. Lim et al. / Surface Science 598 (2005) 96–103

based on the previous results on Ag bulk crystals,even though the AgO formation cannot be com-pletely excluded. Note that Ag2O and AgO showthe nearly same binding energies of the O 1s state[19,23,24]. Moreover, both AgO and Ag2O shownegative Ag 3d core level shifts with respect tothe metallic Ag [23,24]. The O 1s state at 531–532 eV has been generally assigned to the dissolvedoxygen in the bulk, or atomically bound oxygen onmetallic Ag surfaces in previous studies [15,16,19].One can actually attribute the O 1s states at 531–532 eV in Fig. 4 partially to the dissolved oxygenspecies; however, considering that the appearanceof the O 1s state at 531–532 eV accompanies alarge Ag 3d core level shift, the O 1s state at about531–532 eV cannot be completely rationalized bytaking only the formation of the dissolved/subsur-face oxygen species into account (Figs. 2,3). Notethat the formation of the dissolved oxygen speciesin metallic Ag does not accompany an Ag 3d corelevel shift [19].

For the Ag nanoparticles showing two O 1sstates centered at 529 eV and 531 eV at the initialstage of the oxidation (particle size >4 nm), a fur-ther oxidation decreases the intensity of the O 1sstate at 529 eV, and concomitantly the O 1s statecentered at 531 eV increases (Fig. 4). Our resultindicates that the O 1s state at 529 eV correspond-ing to the Ag2O/AgO formation can be trans-formed into a new species upon oxidation,identified by the O 1s states at 531–532 eV. Thetransformation of Ag-oxide to the dissolved oxy-gen species is not likely, and thus the peak at531–532 eV is not attributed solely to the dis-solved/subsurface oxygen species. The nature ofthe new oxygen species showing a peak at�532 eV is a subject of future studies.

Previously, it was suggested that the existenceof the ‘‘ionic’’ oxygen is required for the ethyleneepoxidation. The ‘‘ionic’’ oxygen was suggestedto be the atomically bound oxygen in the Ag(I)-oxide-like structures [4,5,8,15,16]. In the Ag bulkcrystals and larger Ag particles, Ag2O is stableunder oxidizing conditions, whereas under similarconditions, Ag2O of smaller nanoparticles can fur-ther transform into other Ag-oxides, in which theformal oxidation state of Ag is probably higherthan (I), i.e. the surfaces of the small Ag nano-

particles are easily passivated under oxidizingconditions. This result reconciles the decreasedcatalytic activities of the Ag nanoparticles smallerthan 30 nm for ethylene epoxidation. Similar O 1sspectra to those of ours were observed after ex-posing Ag nanoparticles to a high-pressure O2

(100 Pa) at 470 K, suggesting that the experimentalresults in the present work could be also relevantunder real catalytic conditions [15,16].

3.2. Reduction of the oxidized Ag nanoparticles

by CO

In Fig. 5, the O 1s spectra of the partially oxi-dized Ag nanoparticles with a mean diameter of4 nm before and after an exposure of 1000 L(Langmuir = 1 · 10�6 mbar · 1 s) of CO are shown.The O 1s states above 530 eV do not change uponCO exposure, whereas the O 1s state at 529 eVdecreases in intensity after exposing the sampleto CO. In the Ag 3d state, also, it is evident thatthe Ag nanoparticles are initially oxidized (nega-tive core level shift), and then partially reducedby CO (positive core level shift). Obviously, theO 1s state at 529 eV, which is assigned to Ag2O/AgO, can readily react with CO to CO2, whereasthe other oxygen species are inert.

The Ag nanoparticles smaller than 3 nm show asingle peak at 531 eV in the O 1s spectrum fromthe very early stage of the atomic oxygen exposure,and CO does not react with oxygen in these smallAg nanoparticles (Fig. 6). This result was observedfor the Ag nanoparticles with various oxygenuptakes, indicating that the 3 nm sized particlesare not good catalysts for the CO-oxidation, inde-pendently of the oxygen coverage.

It is important to mention that the oxidationand reduction cycle of the Ag nanoparticles largerthan 4 nm can be repeated. As it is illustrated inFig. 7, the oxides of Ag nanoparticles reducedby CO can be restored using the atomic oxygenatmosphere: after the initially oxidized Ag nano-particles are reduced, a subsequent atomic oxygenexposure results in the appearance of the 529 eVshoulder in the O 1s core level spectrum with ahigher intensity than that before the reduction.An additional reduction using 1000 L of CO yieldsthe disappearance of the shoulder at 529 eV. When

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2) CO 1000L

1) O exposure

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O exposure

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CO-oxidation on 4nm Ag nanoparticles

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(b)

Fig. 5. (a) Ag 3d and (b) O 1s core level spectra from the Agnanoparticles with a mean size of 4 nm after exposing to theatomic oxygen atmosphere and a subsequent reduction by COat room temperature.

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Fig. 6. (a) Ag 3d and (b) O 1s core level spectra from the Agnanoparticles with a mean size of 3 nm after exposing to theatomic oxygen atmosphere and a subsequent reduction by COat room temperature.

D.C. Lim et al. / Surface Science 598 (2005) 96–103 101

a larger amount of atomic oxygen was usedto leave a single peak at 531 eV without the shoul-der at 529 eV in the O 1s spectrum, the CO oxida-tion does not take place, confirming that onlyAg2O/AgO can react with CO to CO2 in ourexperimental conditions (Fig. 7). In addition tothe change of the O 1s state, the negative and posi-tive Ag 3d core level shifts can be observed by theoxidation and reduction cycle of the Ag nanopar-ticles (Fig. 7).

It is interesting to note that Ag2O/AgO isalmost completely consumed by dosing 1000 L of

CO, suggesting that the CO to CO2 conversionprobability is higher than 0.1% [25]. This value iscomparable to that of the oxygen-rich Ru surfaces(which turned out to be RuO2 layers). Ru has beenshown to be less reactive towards CO-oxidationunder UHV conditions; however, under high-pres-sure conditions, the CO-oxidation reactivity of Ruis higher than those of the Pt-group metals, due tothe formation of the catalytically active oxide spe-cies. It is noteworthy that the CO-oxidation reac-tivity of Ag-oxide nanoparticles is comparable tothat of the catalytically active RuO2 layers.

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CO-oxidation on 6nm Ag nanoparticles

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Fig. 7. The Ag 3d core level and the O 1s core level spectra from the Ag nanoparticles with a mean size of 6 nm after exposing to theatomic oxygen atmosphere and subsequent reduction by CO at room temperature. The reduced particles can be oxidized using atomicoxygen and reduced again using CO, i.e. the oxidation and reduction cycles is reversible.

102 D.C. Lim et al. / Surface Science 598 (2005) 96–103

4. Summary

Increased chemical activities of Ag nanoparti-cles towards the atomic oxygen uptake were foundin the Ag particle diameter range below �10 nm.Moreover, different Ag-oxides were identified forthese Ag nanoparticles, which have not beenobserved for the Ag bulk crystals with similarpreparation methods. For the Ag nanoparticleslarger than 4 nm, Ag2O/AgO can form at the ini-tial stage of the atomic oxygen treatment, whereasfor larger oxygen exposures, Ag2O/AgO trans-forms into a different oxygen species. Among var-ious oxygen species observed during the oxidationof Ag nanoparticles, only the oxygen species iden-tified by the O 1s state at 529 eV (Ag2O/AgO) canreact with CO to CO2. With decreasing Ag particlesize, the interaction of metal with oxygen atomsincreases, which can enhance the reactivity of theCO-oxidation. When the particle size becomessmaller than 3 nm, however, particles are catalyti-cally inactive, since the catalytically active Ag2O/AgO does not form, and only the inert oxygen spe-cies can be found. The CO-oxidation reactivity onAg can be very sensitive to the particle size, whichis due to the different oxide species formed in theAg nanoparticles with various sizes. This kind of

information can be used to find a suitable particlesize for a catalytic reaction.

Acknowledgements

Deutsche Forschungsgemeinschaft (DFG) isacknowledged for the financial support withinthe project SFB 513 A15. G. Gantefor is acknowl-edged for his support for XPS experiments.

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