Structure and dynamic behaviour of atoms and molecules at catalyst model surfaces

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 28, 135–141 (1999)

Structure and Dynamic Behaviour of Atoms andMolecules at Catalyst Model Surfaces

Shushi Suzuki, Ken-ichi Fukui, Hiroshi Onishi and Yasuhiro Iwasawa*Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

This paper attempts to review our recent work dealing with TiO2(110)-(1× 1) single crystals and to presentnew scanning tunnelling microscopy (STM) images of the adsorbed state and condensation reaction ofpyridine at 350 K. Combined efforts of STM and non-contact atomic force microscopy (AFM) providedthe detail of atom-resolved pictures of TiO2(110) surfaces.In situ STM observation documented the weakand mobile adsorption and the site-specific adsorption of pyridine, and the unexpected condensation ofpyridine physisorbed on the surface in the presence of pyridine ambient at 350 K. Copyright 1999 JohnWiley & Sons, Ltd.

KEYWORDS: STM; AFM; TiO2.110/; visualization of surface reactions and adsorbed states; pyridine; acetate ion

INTRODUCTION

Metal oxides find application in a variety of technologieswhere surface science is critical to success, includingcatalysis, gas sensors, photoelectrolysis, electronic ceram-ics, semiconductor devices, pigments, cosmotics, etc.Metal oxides are tractable materials not only for spec-troscopic techniques such as fourier transform infraredspectroscopy Raman, x-ray absorption fine structure, etc.but also for techniques that might be disrupted by chargingeffects, including XPS, scanning tunnelling microscopy(STM), etc., and special devices for sample preparationand careful interpretation of the spectra are essential.1,2

Understanding and controlling oxide surfaces are the keyissues for the development of industrial oxide catalysts,but oxide surfaces are in general heterogeneous and com-plicated and hence have been little studied so as to putthem on a scientific basis by traditional approaches.

The most important objective in the characterization ofmetal oxide surfaces requires a depth of knowledge similarto that available in homogeneous catalysis. Recently, thecharacterization of oxide surfaces at atomic and molecularlevel has received a great boost from the development ofa variety of sophisticated techniques, including STM andatomic force microscopy (AFM), which provide valuablesupport for the mostly empirical approach to catalystdesign.

A highly detailed picture of a reaction mechanismevolvesin situ studies.1 It is now known that adsorptionof molecules from the gas phase can seriously influencethe reactivity of adsorbed species at oxide surfaces.3 Insitu observation of adsorbed molecules on metal oxide

* Correspondence to: Y. Iwasawa, Department of Chemistry, Grad-uate School of Science, The University of Tokyo, Hongo, Bunkyo-ku,Tokyo 113-0033, Japan.E-mail: [email protected]

Contract/grant sponsor: Japan Science and Technology Corpora-tion (JST).

surfaces is a crucial issue in molecular-scale understand-ing of catalysis. The transport of adsorbed species oftencontrols the rate of surface reactions.

Among the properties of oxide surfaces acid–base reac-tivity is important; its control is important in industrialapplications and also is interesting in catalytic surfacescience.4 However, there is little atomic or molecular-scale information on the structure of acidic oxide surfaces,which should be uncovered scientifically. In practice, theinherent compositional and structural inhomogeneity ofoxide surfaces makes the problem of identifying the essen-tial issues for the catalytic performance of different sitesof oxide surfaces extremely difficult. In order to reducethe level of complexity, a common approach is to studymodel catalysts such as single-crystal oxide surfaces andepitaxial oxide flat surfaces.

Scanning tunnelling microscopy has particularly greatpotential forin situ chemical studies. Although our presentknowledge of the atomic structure of catalyst surfacesis largely limited to those structures that are stable inultrahigh vacuum before and after reaction, STM mayprovide an insight into both adsorbate and catalyst surfacestructurein situ during the reaction. The following issuesto be characterized by STM may be most relevant to thecharacterization of catalysts and catalysis:

(1) Identification of structural characteristics of the vari-ety of non-equivalent surface sites and observation ofsite specificity to reactivity.

(2) Study of how the reaction at one particular surfacesite affects the local activity of neighbouring sites.

(3) Structural transformation and chemical modificationof the surface caused by adsorbates and chemicalreactions.

(4) Detailed information about the mechanism of thesurface chemical reaction.

(5) Surface diffusion and surface mobility.

In the present paper site-specific adsorption of pyridineis used as a probe molecule for identifying the acidicproperty of a TiO2(110)-(1ð 1) surface and condensationreaction of adsorbed pyridine on the TiO2 surface in the

CCC 0142–2421/99/130135–07 $17.50 Received 30 November 1998Copyright 1999 John Wiley & Sons, Ltd. Accepted 16 December 1998

136 S. SUZUKIET AL.

presence of gas-phase pyridine at 350 K, characterizedby STM. We have succeeded in visualizing where andhow the adsorption and unexpected reaction of the organicmolecule proceed at the TiO2.110/ surface.

EXPERIMENTAL

The experiments were performed with an ultra-high vacuum-compatible scanning tunnelling microscope(JEOL JSTM-4500VT) equipped with an ArC gun andLEED-AES optics. A polished TiO2.110/ wafer of dimen-sions 6.5ð1ð0.25 mm3 (Earth Chemicals) was annealedin air at 1100 K for 1 h. Nickel film was then depositedon a side of the wafer for resistive heating on a micro-scope stage in the microscope. A small input power (2 V,1 A) was sufficient to heat the deposited wafer at 1200 K.The surface temperature of the crystal was monitoredwith an IR radiation thermometer (Chino). After cyclesof ion sputtering (3 keV, 0.3µA) and vacuum annealingat 900 K, the surface consisted of large flat terraces with asharp.1ð1/ LEED pattern. The.1ð1/ surface was cooledto room temperature and then exposed to acetic acid orpyridine (research grade, Wako Pure Chemicals) vapour.Electrochemically etched Pt–Ir or W tips were used forSTM imaging of acetate or pyridine admolecules, respec-tively. Constant current topographies of the acetic acid-and pyridine-exposed surfaces were recorded continuouslyat 16.6 or 18 s per frame. A small tunnelling current.It/of 0.05 nA was employed to monitor the adsorbed sur-face, where the relatively smallIt enabled us to observestable STM images of the surfaces in pyridine ambient.

The AFM experiments were performed in an ultra-high vacuum-compatible atomic force microscope (JEOLJAFM4500XT) equipped with an ArC gun and LEEDoptics. Stiff silicon cantilevers withfo D 260–290 kHzand k D 26–32 N m�1 (NANOSENSORS) were used asthe force sensor.

RESULTS AND DISCUSSION

The STM and non-contact AFM imagesof TiO2(110)-(1× 1)

Figure 1(a) is an STM image of a TiO2.110/ surface thatresolves atomically each Ti atom. The image was recordedwith a sample bias ofC1 V. The positive bias suggeststhat unoccupied states of five-coordinated Ti atoms causethe bright contrasts. The exposed Ti atoms make one-dimensional rows along the [001] direction with a sepa-ration of 0.65 nm from each other. The adjacent Ti–Tidistance in the rows is 0.3 nm. The STM image coincideswith the surface structure model for TiO2.110/ (Fig. 1(c)).The black circles are the exposed Ti atoms traced bySTM. The bridging oxygen atoms are located closer tothe tip by 0.11 nm. In spite of the physical protrusion ofthe bridging oxygen atoms, STM images the surface Tiatoms, i.e. electronic structure effects cause the apparentcorrugation to be reversed from naive expectations.5,6

Scanning tunnelling microscopy cannot provide anyimage of the protruding oxygen rows. We succeeded inprobing the surface at atomic resolution by non-contact

Figure 1. (a) An STM image of TiO2(110)-(1 ð 1); 10 ð 10 nm2

(b) A non-contact AFM image of TiO2(110)-(1 ð 1); 30 ð 30 nm2;fo D 270 kHz; Ao ¾ 30 nm; f ¾ 80 Hz. (c) A structural model(top view and side view).

AFM in an ultrahigh vacuum as shown in Fig. 1(b).7

Bright rows parallel to the [001] direction were observedclearly at a regular interval of 0.65 nm. There are oxy-gen defects distributed randomly on the terrace. Oxygendeficiency on the TiO2 surface is induced by vacuumannealing at high temperatures. An atomically resolvednon-contact AFM image reproduced the.1ð 1/ unit at0.65 nmð0.30 nm of the alignment of the bridge oxygenatoms. Oxygen atoms were visualized individually by theuse of non-contact AFM.7

Temperature-jump STM observation of acetateintermediate

The TiO2(110)-(1ð 1) surface was exposed to aceticacid vapour for 3 Langumin in the microscope. Brightspots ordered in a.2ð 1/ periodicity .0.65ð 0.59 nm2/were observed on the exposed surface with a positivesample bias voltage. Figure 2 presents the constant currenttopography of the surface, where the raw image was low-pass filtered and rotated by 45° to align the vertical frameto the [001] axis.8 Diagonal scratches were artifacts alongthe scan direction. Acetic acid is dissociatively adsorbedto produce acetate ion and a surface hydroxyl group at

Surf. Interface Anal. 28, 135–141 (1999) Copyright 1999 John Wiley & Sons, Ltd.

CATALYST MODEL SURFACES 137

Figure 2. A (2 ð 1)-CH3COO� monolayer on TiO2(110)-(1 ð 1);10ð 10 nm2; Vs D C2.0 V; It D 0.3 nA.

room temperature, as expressed by Eqn (1), where Os2�

represents an oxygen ion of the substrate

CH3COOHCOs2� ��! CH3COO�.a/COsH

�.a/ .1/

Reaction of the acetate-covered surface was monitoredby a temperature-jump technique; the input power forresistive heating was increased stepwise by 0.02 W, whileconstant-current topography was determined continuouslyat a scan rate of 16.6 s per frame. The surface tempera-ture typically jumped up by 10–30 K at each incrementin power. Topography of the same area of the surfacecould be recorded sequentially for several minutes fol-lowing a temperature jump, by careful tracking againstthermal drift. In this procedure, we essentially monitoredreaction on the acetate-covered surface activated by thetemperature increments in a quasi-isothermal condition.Surface temperature gradually rose as small as 3–4 Kduring each sequence following the jumps.

According to the thermal desorption study on (011)-faceted TiO2.001/, acetate ions decompose above 520 Kvia net unimolecular dehydration to release ketene(Eqn (2)). The dehydration reaction

CH3COO�.a/ ��! CH2 C .g/COH�.a/ .2/

can be expected on TiO2.110/, because the adsorptionsites of acetate ions (Ti atoms exposed on surface) are five-fold coordinated on both the (110) and (011)-faceted (001)surface. Thus, we monitored the unimolecular decompo-sition reaction on the acetate-covered TiO2.110/ surfaceusing the temperature-jump technique.

Topography of the acetate-covered surface heated at510 K was recorded sequentially. Figure 3(a) shows threeframes (frames 1, 3 and 5) selected from a sequence of 12images (frames 1–12) recorded on the same area.10ð10 nm2/. Acetate ions as small bright spots were observedat different positions, frame by frame, due to surfacemigration. The number of acetate ions did not change

significantly at 510 K because there was no reaction atthis temperature.

The number of acetate ions decreased at 540 K, asshown in Fig. 3(b). We could not distinguish topographyassignable to the hydroxyl ions produced in the reac-tion. Instead, particles larger than acetate appeared onthe surface. The particles did not migrate on the sur-face until they disappeared at higher temperatures. Themigrating acetate ions (typically indicated by smallercircles) and immobile particles (typically indicated bylarger circles) were easily distinguished by animatingthe sequential frames. The particles might be carbona-ceous residuals yielded in a side reaction. Carbonate ionshave been detected as by-products on an acetate-covered(114)-faceted TiO2.001/ surface reacted at 650 K.9 Thickrow-like structures along the [001] axis observed on thesurface in Fig. 3(b) are Ti2O3-added rows, which are asurface-limited phase of titanium oxide inevitably formedon the vacuum-annealed surface.5,10 They are stable atthis temperature and regarded as spectators in the decom-position reaction of acetate ions. The number of acetateions decreased from 145 to 65 over 216 s of the reac-tion. The number of acetate ions in a frame is plottedas a function of frame number. A linear line fitted thedata presented in semi-log mode, indicating first-orderkinetics of acetate conversion with a rate constant of.4š 1/ ð 10�3 s�1. This value agreed with the rate con-stant of 3ð 10�3 s�1 calculated from the microscopicrate law deduced from the thermal desorption study.9

This agreement demonstrates that the unimolecular reac-tion of the acetate intermediates is monitored properly bythe temperature-jump method. It is to be noted that themacroscopic (mean-field) kinetics is reproduced by themicroscopic kinetics of individual molecules at the oxidesurface when the surface reaction proceeds by the uni-molecular reaction mechanism. Detailed inspection of thetopography of (110) terraces in Fig. 3(b) found the regularlines of slight contrast that were parallel to the [001] axisand separated from each other by 0.6 nm.8 This indicatesthat the (110) terraces maintained the.1 ð 1/ structurein the presence of the acetate ions at 540 K because theperiodic line was assigned to the uncovered Ti row ofthe surface.5,10 – 12

Site-specific adsorption of pyridine

Pyridine is a typical Lewis base molecule and is oftenemployed as a probe molecule to examine the acidicproperty of oxide surfaces. From the acid–base point ofview, the highly ionized Ti atoms exposed to the (110)surface are assumed to be Lewis acid sites favourable forpyridine chemisorption, making the bonding between Ti4C

and the nitrogen atom of pyridine. The single coordinationvacancy of the fivefold-coordinated Ti4C reserves spaceaccessible by a Lewis base molecule. It is still difficultto identify where the molecules of a particular state areadsorbed on the surface with inhomogeneity. We showthat most of the pyridine molecules adsorb on particularsites of the surface, with the aromatic ring parallel to thesurface.

Figure 4 shows two sequential constant-current STMimages of pyridine trapped at the step sites. White parti-cles located on several sites along the steps were assignedto individual pyridine molecules. The rows of the fivefold-coordinated Ti4C atoms were resolved as white lines

Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 28, 135–141 (1999)

138 S. SUZUKIET AL.

Figure 3. (a) Sequential constant-current STM images .10 ð 10 nm2/ of an acetate-covered TiO2.110/ surface jumped at 510 K;Vs D C1.7 V; It D 0.2 nA. This surface density corresponds to 58% of the population of the .2ð 1/ monolayer of acetate. (b) Sequentialconstant-current STM images .10ð 10 nm2/ of an acetate-covered TiO2.110/ surface jumped at 540 K; Vs D C1.7 V; It D 0.2 nA.

of slight contrast in the topography of the upper ter-race. The step-bonded pyridine molecules were alwaysadsorbed on the Ti rows. The step-bonded molecules wereimaged completely, whereas the terrace-adsorbed pyridinemolecules showed fragmented images. When an adsorbedmolecule migrates quickly at the surface, its whole shapecannot be imaged by STM.

The steps on which pyridine molecules can adsorb runparallel to the [11 2] and [1 12] directions. Similar activitywas observed on the steps parallel to [11 3], [1 1 4] and[1 1 5] directions. In contrast, steps of [11 1], [1 1 0]and [001] directions were not active for the adsorption.Steps along the [001] and [110] directions contain sixfold-coordinated Ti atoms and fivefold-coordinated Ti atoms,similar to the terrace. The inactivity of the [11 1] stepis interesting, because fourfold-coordinated Ti atoms are

exposed. The Ti atoms at the active [11 2] step andthe inactive [11 1] step are situated in an equivalentstate, in the sense of local coordination number aroundTi atoms. A possible interpretation of the site-specificpyridine adsorption is the steric hindrance around theadsorption site. A fourfold-coordinated Ti atom at the[1 1 1] step has two fivefold-coordinated Ti neighboursat the step, which are accompanied and coordinated withoxygen atoms. At the [11 2] step, one of the neighbours ismissing. The extra oxygen atoms may obstruct effectiveapproach of pyridine to the fourfold-coordinated site atthe [1 1 1] step. The missing Ti atom may perturb theelectronic states of the adsorption site through the bridgingoxygen atom in-between. This step-direction-dependentaffinity is beyond the traditional, simple coordinationvacancy argument.13



Figure 4. Two STM images .15 ð 15 nm2/ of pyridine-exposedTiO2(110)-(1ð1) at an interval of 91.5 s; Vs D C2.5 V; It D 0.05 nA.

Condensation reaction of pyridine on TiO2(110)under the pyridine ambient at 350 K

When the TiO2(110)-(1ð 1) surface was exposed to apyridine ambient of 1ð10�6 Pa at 350 K, nanometre-sizedparticles appeared randomly on the surface. Figure 5

shows a series of STM topographies recorded over100 min under the pyridine atmosphere. The whitestring structures running along the [001] direction arethe Ti2O3 double-strand rows. Figures 5(a)–(c) weredetermined att D 45, 70 and 90 min, respectively, andthe pyridine vapour was introduced in the microscopechamber at t D 0. The number of nanometre-sizedparticles increased with the reaction time at 350 K. Theaverage topographic height was 0.41 š 0.06 nm andthe average full width at half-maximum (FWHM) ofthe cross-section of the particle was 1.37 š 0.25 nm.Topographies of scratch shapes were observed andassigned to physisorbed pyridine molecules mobile overthe surface at this temperature. As mentioned above, thefivefold-coordinated Ti4C site exposed on the (110)-(1ð1)terrace is incapable of making stable Ti–N bonds withpyridine admolecules.14 The nanometre-sized particles areobviously products of a thermally activated reaction ofpyridine on this single-crystal surface. The dimensionsof the particles observed in Fig. 5 were, however,far larger than those of the trapped single molecule.Hence, we assume that the nanometre-sized particlesare condensation products of a few to several pyridinemolecules. The diameter distribution of the particlesimaged in Fig. 5 increased as a function of reaction time,suggesting that they grew gradually by the condensationreaction of pyridine on the TiO2.110/ surface.15

The process of particle growth was monitored in thesequential zoom images of the surface in Fig. 6. Twosmall particles marked X in Fig. 6(a) migrated alongthe [001] axis in opposite directions to each other inFig. 6(b) and merged into a larger particle labelled Y inFig. 6(c). The merged particle became less mobile andstayed at that position until the image of Fig. 6(f). This isa reaction between adsorbed species. On the other hand,one particle, Z, suddenly appeared in Fig. 6(e) and grewlarger in Fig. 6(f). Gas-phase pyridine molecules shouldparticipate in the growth of Z, because adspecies were notobserved in the neighbourhood of the growing particles(Fig. 6(d)–(f)).

The topographical size of species X (0.22 nm in heightand 0.92 nm in FWHM of cross-section) was closeto that of the physisorbed pyridine molecules immobi-lized and measured at 300 K (0.26 nm in height and1.21 nm in FWHM, as shown in Fig. 7). This suggeststhat X is a direct product from one pyridine molecule.A chemisorbed state of pyridine or a partially dehydro-genated state.C5H5�xN/ is a probable interpretation forX (S. Suzuki, H. Onishi, K. Fukui and Y. Iwasawa,unpublished results).

Figure 5. Sequential STM topographic images .40ð40 nm2/ of TiO2(110)-(1ð1) at 350 K in the presence of pyridine vapour .1ð10�6 Pa/.Pyridine vapour entered the microscope chamber at t D 0; Vs D C2.5 V; It D 0.05 nA.


140 S. SUZUKIET AL.

Figure 6. Sequential STM images .20 ð 20 nm2/ observed on the TiO2.110/ surface at 350 K in the presence of pyridine vapour.1ð 10�6 Pa/. Pyridine vapour entered the microscope chamber at t D 0; Vs D C2.5 V; It D 0.05 nA.

Figure 7. Cross-sections of STM topography of pyridine andspecies X on TiO2.110/.

We examined the temperature dependence of theparticle formation reaction in the presence of pyridinevapour. The nanometre-sized particles were not observedat all on the surface reacted at a higher temperatureof 400 K. Figure 8 presents as STM image of thesurface exposed to a pyridine ambient of 1ð 10�6 Pafor 60 min. This can be interpreted with the restrictedcoverage of adsorbed pyridine at this temperature. Inthermal desorption measurements, the physisorbed state ofpyridine desorbed from the TiO2 surface at 300–350 K14

Hence, the absence of particles on the surface at 400 Kindicates that the physisorbed state was the precursor ofthe reaction at 350 K.

Based on these results, the mechanism of the particleformation reaction is considered. The physisorbed state ofpyridine is required for the particle formation. The product

Figure 8. An STM image .42 ð 42 nm2/ of the TiO2(110)-(1 ð 1)surface under pyridine vapour .1ð 10�6 Pa/ for 60 min at 400 K;Vs D C2.5 V; It D 0.05 nA.

particles were not formed on the surface heated at 400 K,at which temperature the coverage of the physisorbedstate was quite small even in the presence of the pyridineatmosphere of 1ð 10�6 Pa. The physisorbed moleculeswere transformed to a less-mobile species X on the surfaceheated at 350 K. Species X and the physisorbed statewere similar in topographic size. A chemisorbed state ofpyridine or a partially dehydrogenated state.C5H5�xN/ is aprobable assignment for X. As visualized in Fig. 6, speciesX migrated along the Ti� row at a rate of 0.85 nm min�1

at 350 K. When two X species came together on thesurface, they merged into a larger particle Y. In some



cases, particle Z appeared without the aggregation of twoX particles. These processes are summarized as follows

Physisorbed pyridine��! X .3/

X C X ��! Y .4/

X C Physisorbed pyridine��! Z .5/

We also exposed the TiO2 surface to benzene vapourinstead of pyridine. The nonometre-sized particles werenot observed at all on the surface exposed to a benzeneflow of 1ð10�6 Pa at 300, 350 or 400 K for 120 min. Thenitrogen atom in the aromatic ring plays a primary rolein activation of the compound. It is also to be noted thatparticle growth was not observed significantly at the stepsites, indicating that the observed process is a TiO2.110/-surface-limited chemical process.

CONCLUSIONS

(1) We have succeeded in monitoring the reaction ofan acetate-covered TiO2.110/ surface by STM in atemperature-jump method.

(2) We have also succeeded in visualizing where and howthe adsorption and reaction of pyridine proceed at theTiO2.110/ surface.

(3) In situ STM observation documented the mobile andsite-specific adsorption of pyridine at room tempera-ture and the condensation of pyridine in the presenceof pyridine ambient at 350 K.

(4) The observed reaction was unexpected, because pyri-dine molecules interact so weakly with the single-crystal TiO2.110/ surface that most of them arephysisorbed on this surface.

(5) The growth process of the nanometre-sized particleswas monitoredin situ by identifying reaction inter-mediates of different sizes and different mobilities.

(6) The present study demonstrates thatin situ imagingwith STM on a model catalyst is a promising tool toobserve directly dispersed reaction intermediates suchas acetate ions and to discover an unknown reactionsuch as condensation of pyridine on TiO2.

(7) It may be important to monitor the surface chemicalprocess under the ambient gas, where very weaklyadsorbed molecules may contribute to the surfaceprocess. This sort of chemical process may not beobserved in a vacuum.

Acknowledgement

This work has been supported by Core Research for Evolutional Sci-ence and Technology (CREST) of the Japan Science and TechnologyCorporation (JST).

REFERENCES

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C. Naccache, G. Couduier, Y. Ben Taarit and J. C. Vedrine,p. 1. Elsevier, Amsterdam (1984).

5. H. Onishi and Y. Iwasawa, Surf. Sci. Lett. 313, L783 (1994).6. H. Onishi and Y. Iwasawa, Chem. Phys. Lett. 226, 111 (1994).7. K. Fukui, H. Onishi and Y. Iwasawa, Phys. Rev. Lett. 79, 4202

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9. K. S. Kim and M. A. Barteau, J. Catal. 125, 353 (1990).10. H. Onisi, K. Fukui and Y. Iwasawa, Bull. Chem. Soc. Jpn. 68,

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Structure and dynamic behaviour of atoms and molecules at catalyst model surfaces