Chemisorption on metal films on oxide surfaces

439

Chemisorption on metal films on oxide surfaces Charles T Campbell During the past year, great advances have been made in the preparation of smoother and more defect-free oxide surfaces. Metal particles have been prepared on these surfaces which are extremely well-defined with respect to particle thickness, lateral dimension, shape, electronic structure and energy. Chemisorption and catalytic reactions have been proven to depend on particle thickness, size and separation, and the nature of the oxide support surface; and these effects have been explained. Address Chemistry Department, University of Washington, Seattle, WA 981951700, USA Current Opinion in Solid State & Materials Science 1998, 3:439-445 Electronic identifier: 1359-0288-003-00439

been truly impressive. The particles are usually vapor deposited, but spin coating from liquid solutions has recently been demonstrated [3”]. This will enable the processes (such as metal salt precipitation, calcination and reduction) occurring in industrial catalyst preparation from liquid solutions to also be studied. Scanning tunneling microscopy (STM), electron microscopy (EM), spot-profile analysis in low energy electron diffraction (SPALEED) and helium diffraction have all been used to provide unprecedented detail about the structure of metal films and particles [ 1l ,2’*-So’,6’,7*‘,8’,9]. Many characterization techniques, including STM and SPA-LEED, require highly conductive samples. Methods to prepare thin, highly ordered and crystalline films of oxides, such as Al,O,, on metal supports have permitted a whole new range of catalytically interesting oxides to be studied with these techniques [3’*,4**,7’*].

0 Current Chemistry Ltd ISSN 1359-0288

Introduction films and particles dispersed across the solid surfaces of oxide supports form the basis for a wide variety of catalyst and chemical sensor systems. As such, the chemisorption of gases on the surfaces of these complex solids has been the subject of intensive research for many years; however, during the past year the progress towards a fundamental understanding of these systems has been dramatic, due to the recent development of improved methods for structurally and electronically characterizing these systems and for preparing oxide surfaces with a more homogenous distribution of sites and more well-defined metal films or particles. For example, it is now possible to prepare metal particles on relatively defect-free singlecrystalline oxide surfaces, where both the thickness and the lateral dimension (i.e. parallel to the surface) are known and can be controlled with almost atomic accuracy. Thus, the most exciting developments have been associated with the necessity to refer to more than just their ‘diameter’ such that their chemisorption, catalytic, and electronic properties are now being correlated not only with their lateral diameter, but also with their thickness and even their shape. Also important has been the new development of an ability to measure the energetic stability of these metal particles, because this opens up the possibility to correlate their chemisorption properties with their energetics. Metal

A number of very important reviews has recently [l*,Z**-5*‘,6*,7*‘] on this subject.

Preparation and characterization defined metal particles

appeared

of well-

The progress in preparing more well-defined metal particles on model supports, often single crystalline oxides, has

A beautiful example of a well-defined system is shown in Figure 1, that is an STM image of Rh particles on a highly ordered Al,03 thin film (from Franketa/. [lO”]). Here, the left-hand image (Figure la) shows two-dimensional Rh particles with an average thickness of only 0.3 nm (i.e. one Rh atom) and a lateral dimension (parallel to the surface) of -2 nm. The right-hand image (Figure lb) shows threedimensional Rh particles obtained at a much, much higher Rh coverage and at a higher temperature, where the Rh particles now nucleate in rows along the linear defect (domain boundaries) of the oxide substrate, bur surprisingly do not coalesce. Beyond the first few percent of a monolayer (ML), the late transition metals cluster into two-dimensional (ZD) and three-dimensional (3D) particles on such oxide surfaces [Z”]. At room temperature or below, very thin metal islands can be maintained with a very large aspect ratio (i.e. much, much wider parallel to the surface than thick) [ lo”,1 1’,12’,13,14’]. The metal-metal bond distances in these particles are nearly the same as those in the bulk metal [43,14-l, but in the smallest (thinnest) islands they are slightly smaller (by up to 10%) [15”]. The in-plane geometry of the ultrathin islands tends to be disordered, but with a tendency toward close-packing of the atoms in the surface planer [4”,5’*,10’~,13,14’,15”]. Xu eta/. [ 16”] have seen evidence for pseudomarphic growth in tiny Pd clusters on TiO,(llO) using STM. They observed what they assigned to be Pd tetramers with surprisingly long Pd-Pd distance of -0.65 nm in both the and directions. (Note that the text says 0.35 nm for direction, but that has been corrected to 0.65 nm [DW Goodman, personal communication]). However, it is not clear whether there may be more Pd atoms hidden below (or within) the four bright spots in the beautiful STM image of this tetrameric structure. Surface hydroxyls

440

Surface science

Figure 1 (a)

STM Image of Rh particles

(b)

ordered

AI,Os

on a highly

thin film. (a) A Rh deposit

0.01 nm average thickness, deposited 80 nm x 80 nm. The line protrusions

of

at 90K,

(indicated by the white arrow) are antiphase domain boundanes of the oxide film. (b) A 0.5 nm Rh film deposited at 300K, 100 nm x 100 nm. Reproduced permission from [lo”].

on the oxide persion

surface

have been

of vapor-deposited

metal

shown

to increase

particles

the dis-

[ 12’1.

Also impressive is the ability to characterize these metal particles electronically, even on a particle-by-particle basis. Old errors in the interpretation of X1’S (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy) peak positions, associated with a lack of understanding of final-state effects, have been rectified, and it now appears. at least to this author, that metal adatoms are nearly charge neutral if they are both late transition metals and at coverages beyond a few percent of a monolayer [2”,17.18’]. This is consistent with strong metal-metal bonding which drives them to form 2D and 3D islands. Scanning tunneling spectroscopy has shown a moderate band gap for small particles, which decreases as the particle size increases [16”,1c),ZO]. A size dependence in the electronic structure is also seen in dispersion effects by angular-resolved photoemision [Zl’] and from observations of other properties [Z]. An adsorption microcalorimcter has been developed, which enables the heats of metal adsorption to bc directly mcasued on single-crystal oxide surfaces as a detailed function of coverage [23,24”,2.5]. From this, the metal/oxide adhcsion energy can also be determined, an exllmple of which is shown in Figure 2. Here, a pulsed beam of Pb vapor impinges on a thin film of iLlgO( l(M), grown epitaxially on an ultrathin hlo(100) single crystal, and the transient heating rise associated with the adsorption of each 0.Wmonolayer p~llse of I’b is recorded with a pyroelectric heat detector. Similar results have also been seen for Pb and (:LI on several oxide surfaces (DJ Bald, DA Starr, J hlusgrove. CT Campbell, unpublished data). Such results promise to reveal how metal-oxide bond strengths depend on both the nature of the metal and that of the oxide support. In Figure 2, the very low initial heat of adsorption of I% on hlgO( 100) is a combination of two effects: firstly, the metal particles are very small, so fewer nearest neighbors are present than in bulk Pb; and secondly, the downward bonding

with

of the metal to higO is much weaker than the dowrnvard bonding between, for example, the metal atoms in the topmost atomic layer of bulk Pb(ll1) and the metal atoms in its second layer. ‘I’hc relative contributions from these t\vo effects can be separated by repeating such mcasuremcnts at lower temperatures, where it should be possible to gro\\ large 2D metal islands. The strength of bonding at the metal/oxide interhcc mllst dramatically influence metal particle dispersion (i.e. the fraction of the metal atoms exposed at the particle surfaces). the resistance of highly dispersed catalysts to particle thickening and Ostwald ripening, and the chemical reactivity of ultrathin metal particles (see below). As late transition mctal particles on oxides irreversibly thicken and Ostwald ripen when annealed in a vacuum, the existence of high dispersions in a vacuum is only due to kinetic limitations [Z**]. This is also proven by Figure 2 and other such data (IIJ Bald, DA Starr, J hlusgrove, Cl’ Campbell. unpublished data). Nevertheless, correlations between high dispersions and energetic stability are to be expected, because kinetics generally correlate with reaction energctics. ‘I’here have also been advances in theoretical understanding of these electronic, structural and energetic properties [26’,27-331.

Adsorption particles

and catalysis on metal films and

It has long been expected that tiny metal particles \zould be more aggressive in the chemisorption of small molecules. because the average coordination number of the metal atoms is smaller. This has indeed been frequentl) seen in the past year (see below): howc\,er, there is another effect equally important to consider. As shown by Figure Z and from other such results ([24”]; I>J Bald, DA Starr, J hlusgrove, Cl Campbell, unpublished data), late transition metals bond much more weakly to typical oxide supports than to themselves. This has a strong influence on

Chemisorption on metal films on oxide surfaces Campbell

how molecules chemisorb, because the weaker downward bonding in 2D metal islands (like those in Figure la) renders their metal atoms effectively more coordinately unsaturated. Thus, the chemisorption properties of such 2D islands tend to resemble those of coordinatively unsaturated bulk planes, such as fcc( 110) [Z”], even though their inplane geometry tends to be disordered and flatter but more closely-packed than that of fcc( 110) (see above).

441

Figure 2

Pb / MgO(100)

180

The chemisorption properties of Pd particles have been characterized in detail. In previous years, the vibrational modes of adsorbed CO have been used to characterize the particle morphology, based on the assignment of certain C-O stretch frequencies to CO at specific low-index bulklike Pd facets. Recently, parts of these assignments have been challenged based on correlations between the C-O frequencies and particle structures determined by SPA-LEED and STM [34”]. Whereas bulk Pd or Rh surfaces do not readily dissociate CO, small particles of Pd or Rh supported on oxides do [ 10”,35’,36,37”,38,39]. Surprisingly, it was found that the dissociation probability during CO TPD (temperature programmed desorption) maximized at a particle size of -1000 atoms [10”,35’]. The lower rate on smaller particles was attributed to the fact that these smaller particles were in the form of ZD (or otherwise very thin) islands, such that the step sites needed for CO dissociation were not present. Particle size effects were found to be negligible with respect to the heat of CO adsorption versus coverage on Pd on SiO,/Si( 100) [40]. The adsorption of CO has also been shown to change the metal particle morphology resulting in an increase in metal particle dispersion [37”,41’] for which a thermodynamic explanation has been offered [Z”]. It has been found that the adsorption rate of CO depends on metal particle sizes and their average separation. This is because CO diffuses across the oxide surface in a weakly held state, which acts as a precursor to chemisorption on the metal particles [1’,.5”] (and whose adsorption strength depends on the nature and extent of reduction of the oxide substrate 1421). Therefore, the metal particle size and number density also influence the kinetics of catalytic CO oxidation [ 1’,5”,43,44”]. Recently, model catalysts have been fabricated on silica films on Si wafers using photolithography in which nanoscale metal particles are present in a regular array and thus separated by equal distances [3”,45,46’]. The adsorption of CO and HZ has been studied on such arrays [46’]. These systems should allow the particle separation and particle size to be addressed in a very controlled way, whereby each parameter can be independently varied. A new CO adsorption state on small particles has been indirectly suggested by a transient CO, peak seen after closing the CO valve in the CO+O, reaction on Pd particles on MgO( 100) [44”], but it has not been ruled out that

2 5

160 170

195.2 kJ/mol

.g

150

g

140

+I? ,”

130

i

120

X 110 100 90

“Y

0

12

3

4

5

6

Coverage I Pb ML Current Op~mon anSohd State

7

8

& Mater&

9

Science

Calorimetrically measured heat of Pb adsorption as a detailed function of Pb coverage on a clean, 3 nm thick MgO(lO0) thin film grown on Mo(lO0). Note that the heat of adsorption at high coverages is just the sublimation energy (AH& of bulk Pb, within the calorimeter’s absolute accuracy (< 2%). DJ Bald, DA Starr, J Musgrove, CT Campbell, unpublished data.

this peak could have been an artifact which resulted from the increasingly peaked angular distribution of CO, expected to accompany the increasing oxygen coverage during this transient. The dissociation of NO is also more rapid on smaller Pd particles. This causes the steady-state coverage of N(ad) to be higher on small particles during the reaction NO+CO, thus poisoning the net catalytic rate [47”,48]. Oxygen dissociatively adsorbed on small Pd, Pt and Rh particles supported on a-Al,O,(OOOl), and on tiny metal particles (-2 nm) oxygen builds up a much higher coverage than on larger particles or bulk metal surfaces. The oxygen desorption temperature is higher on the smaller metal particles for Pt and Pd [49’]. Ceria supports are particularly interesting in oxidation catalysis, because several experiments have provided evidence for a very interesting additional oxygen species, which is directly involved in the oxidation. All supports show reactions wherein the oxidant is chemisorbed oxygen atoms adsorbed on the metal (Pd, Pt or Rh) particles, but partially-reduced ceria shows additional oxidation activity due to an oxygen species associated with the ceria

442

Surface science

Figure 3

Steady-state,

Figure 4

low conversion

CO oxidation

rates versus CO pressure

at 512K and 0.3 torr 0, over model Pd/ceria-zirconia catalysts. The total Pd loading was -two monolayers, in the form of Pd particles of -2.5 nm diameter. The different symbols correspond to different temperatures of precalclnation of the model ceria-zirconia support (before Pd deposition): (%) symbols correspond

n, 570K;

+, 1170K; 0, 1270K ; q,l 470K. The to Pd on an ultrathln ceria film vapor-

deposlted onto a zirconia support, with permtssion from [51 “I.

with no precalcination.

Reproduced

0.0

(SO,S1”,52’,53’]. ,4n example, which nicely addresses the oxygen storage components in three-way automotive catalysts is shown in Figure 3 [Sl”]. Here, the rate of CC) oxidation at 51SK is plotted versus the CO partial pressure for vapor-deposited Pd particles on ceria-zirconia model supports. The line with a slope of -1 (i.e. the samples with an order in CO of -1) is what one measures for similar I’d particles on an inert support, such as alumina or zirconia alone, or for the ceria-zirconia if it had been calcined at too high a temperature. hluch higher rates with a zero slope were observed for the ceria-zirconia support pre-calcined to < 127OK, and the additional activity was associated with oxygen from the ceria lattice. ‘This additional, zero-order mechanism dies if the support is pre-annealed in air to -127OK. On ceria alone, such deactivation OCCLUS by 117OK, that is the zirconia component stabilizes the catalyst. By correlating this with X-ray diffraction measurements of the support crystallite sizes, the additional activity seems to require small ceria crystallites, which are maintained to higher temperatures when Zr is also present. The adsorption of NO has also been studied on Rh particles supported on ceria [S3’]. Gold supported on reducible oxides exhibits interesting activity in low-temperature CO oxidation [Sill. Gold particles on chromia dissociate H,S more readily than bulk gold surfaces [SS”]. Adsorbed CO [%‘I and oxygen (VA Bond&, SC Parker, C’I’ Campbell, unpublished data) bind more strongly to small Au particles on Ti02(l 10) than to bulk Au surfaces. The adsorption of formic acid and CO has been studied on 2D and 3D Cu particles on the Zn-terminated face of

I 200

I

I 300

400

Temperature

I 500

ML

I 600

I

-I

700

[KI

Desorption of benzene in TPD (temperature programmed desorption) following a 70 Langmuir exposure of acetylene at 150K to Pd films vapor-deposited onto a 3.0 nm AI,Os thin film on Mo(l 10). Each curve corresponds to a different average thickness of the Pd film, in monolayers (ML). These annealed Pd films were in the form of Pd particles according to STM and TEM, with average diameters of -1.5-2 nm for the 0.5 ML film, 2-2.5 nm for 1 .O ML, 4-5 nm for 2.0 ML, and 7-8 nm for 4.0 ML. Note the low production temperature for benzene (230K) from large particles. Reproduced with permission from [58”].

%nO(OOOl) [57”], to make a comparison with earlier results on the O-terminated face of %nO. On these two faces, the oxide’s surface site separation and symmetry is the same, with the only difference being that one face has a hexagonal array of Zn ions, and the other has oxygen ions. In spite of this rather dramatic difference, the chemisorption behavior of the 2D and 31) Cu particles are almost the same on these two faces. ‘I-his suggests that the bonding mechanism between Cu and the oxide support is neither strongly covalent nor ionic in character, hut instead it may simply be due to an interaction between the polarizability of the metal particle and the hladelung potential and polarizability of the oxide. When larger molecules such as benzene are involved, small par&es can show low activity due to the lack of a sufficiently large metal area to satisfy the reaction’s ensemble requirement. An example, involving the

Chemisorption bn metal films on oxide surfaces Campbell

of acetylene to benzene is shown in Figure 4. This reaction only proceeds on large enough Pd particles when dispersed on thin film alumina, and the selectivity is only high when the particles are dominated by Pd( 111) facets [58”]. The necessity for Pd( 111) facets is associated with the fact that benzene desorbs at a lower temperature from this closest-packed facet, and thus competitive dissociation processes are less likely to consume the benzene product during the TPD. The adsorption of ethylene has been studied with vibrational spectroscopy (HREELS [high-resolution electron energy loss spectroscopy]) on 3-monolayer-thick Pt films on single-crystalline Zr02(100) and ZnO(OOOl), and it was found that similar forms of adsorbed ethylene are produced as those seen on bulk Pt surfaces [59].

cyclotrimerization

It has been shown through kinetic models of small metal particles, whose surfaces are composed of a set of welldefined crystal facets, that the rate of a catalytic reaction, such as CO or H, oxidation is not always simply an areaweighted sum of the rates from bulk crystal surfaces of all the facets [60]. Differences arise due to adsorbate diffusion between facets, which changes the steady-state surface concentrations of adsorbed intermediates. The influence of supported particle and facet sizes on catalytic reaction rates have also been theoretically studied using model mechanisms and Monte Carlo methods [61’]. The interaction of simple molecules with alkali metals adsorbed on singlecrystal oxide surfaces has also been studied [62-65, 66’1. Other studies of model Cu/ZnO catalysts have been performed [67,68].

Conclusions There has been a dramatic improvement in our ability to prepare and characterize model catalysts based on welldefined metal particles on planar oxide supports. While a few chemisorption measurements have been made on some of the best characterized systems, only the simplest molecules have thus far been studied (mainly CO, with some 0, and NO). It is now possible to study more complex molecules on better defined particles on supports, which is needed so that a general picture can be formed of how both particle size and average separation and the nature of the support affect chemisorption and catalytic properties. The-work so far indicates (hat the thinnest and smallest particles are the most aggressive chemically, unless the particles are too small to present a sufficiently large ensemble. This is due to their high effective degree of coordinative unsaturation, resulting from extremely weak bonding to the oxide below and the lack of nearest neighbors when present as tiny particles.

References and recommended

443

reading

Papers of particular interest, published within the annual period of review, have been highlighted as: l of special interest -0 of outstanding interest

1. .

Henry CR, Chapon C, Giorgio S, Goyhenex C: Size effects in heterogeneous catalysis: a surface science approach. In Chemisorption and Reactivity on Supported Clusters and Thin Films. Edited by Lambert RM, Pacchioni G. Amsterdam: Kluwer Academic Publishers; 1997:117. This briefly reviews how nucleation and growth of vapor-deposited metal particles on clean oxides can be used to obtain well-defined cluster sizes and shapes, and how these properties manifest themselves in size- and shape-dependent electronic structure, chemisorption properties and catalytic activity. (Contains 50 relevant references.) 2. Campbell CT: Ultrathin metal films on oxide surfaces: structural, -0 electronic and chemisorptive properties. Surf Sci Rep 1997, 227. An extensive review (with 286 references), which particularly addresses the structure, growth kinetics, electronic and chemisorption properties of 2D versus 3D metal islands. 3. Gunter PLJ, Niemantsverdriet JW: Surface science approach to l* modeling supported catalysts. Catal Rev-&i Eng 1997,39:77. Methods for preparing model planar support oxides, with well-defined and evenly-separated metal particles on them, are extensively reviewed. Sintering and redispersion, adsorption and desorption of small molecules, and catalytic activity are also reviewed. (With 424 references.) 4. Freund H-J: Adsorption of gases on complex solid surfaces. lAngew Chem lnt Ed Engl1997, 36:452. Excellent and extensive review of this author’s own ground-breaking research in the preparation of well-defined metal particles on single-crystalline oxide surfaces, their characterization in terms of structural and electronic properties, and their reactions with and chemisorption of simple gases (mainly CO on Pd and Rh on alumina). (Contains 224 references.) 5. Henry CR: Surface studies of supported model catalysts. Surf Sci * Rep 1998, in press. A much more extensive review but similar in subject to this author’s earlier review [lo]. This is excellent reading for students and experts alike. l

6. .

Persaud R, Madey TE: Growth, structure and reactivity of ultrathin metal films on TiO, surfaces. In The Chemical Physics of Solid Surfaces and Heterogenous Catalysis. Edited by King DA, Woodruff DP. Amsterdam: Elsevier; 1997,8:407. This brief review highlights the author’s important studies of the structural and electronic properties of metal films on TiO,, mainly the (1 10) face, with brief mention of gas adsorption on a few of these metal films. (With 116 references.) 7. ..

Rainer DR, Goodman DW: Metal clusters on ultrathin oxide films: model catalysts for surface science studies. / Molec Catal 1998, in press. An extensive review, mainly of work from the Goodman group. 8. .

Kizuka T, Tanaka N: Atomic process of epitaxial growth of Au on MgO studied by cross-sectional time-resolved high resolution electron microscopy (THRTEM). Phys Rev El 1997,56:R10079. A gold cluster nucleates at steps on MgO(001). A cluster of ~60 atoms fluctuates rapidly in structure and in its orientational relationship to the MgO, at room temperature. As the cluster grows from 90 to 1400 atoms, it fluctuates from a tetragonal pyramid with all (111) faces, to a (lOO)-truncated version of the pyramid with up to 15% of the area being (100) facets. 9.

Henry CR, Meunier M: Helium diffraction study of the nucleation and growth of supported metal clusters. Mater Sci Eng A 1996, 21712161239.

10. *

Frank M, Andersson S, Libuda J, Stempel S, Sandell A, Brena B, Giertz A, Bruhwiler PA, Baumer M, Mattensson N, Freund H-J: Particle size dependent CO dissociation on alumina-supported Rh: a model study. Chem Phys Lett 1997,279:92.

l

This is similar to a related later paper [35-l but less complete, it has nice STM images showing 2D islands of Rh (average height = 0.3 nm). 11.

Adcnowledgements The author wishes to acknowledge the National Science Foundation the Department of Energy, Office of Basic Energy Sciences for support this work, and H-J Freund, M Frank, S Stemple, RJ Gorte DW Goodman for permission to use Figures from their work. He thanks them as well as CR Henry, TE Madey and JA Rodriguez providing in-press manuscripts.

and of and also for

Martin D, Creuzet F, Jupille J, Borensztein Y, Gadenne P: 2D and 3D silver adlayers on TiO,(l IO) surfaces. Surf Sci 1997, 377-379:958. Evidence is provided for the formation of 2D islands with a diameter >I 0 atoms, from Surface Differential Reflectance, following the energy of the Mie resonance. 12. .

Libuda J, Frank M. Sandell A, Andersson S, Bruhwiler PA, Baumer M, Martensson N, Freund H-J: Interaction of rhodium with hydroxylated alumina model substrates. Surf SC; 1997, 384:106.

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Musolino V, Selloni A, Car R: First principles study of adsorbed Cu, (n = I-4) microclusters on MgO(l00): structural and electronic properties. J Chem Phys 1998,108:5044.

29.

Neyman KM, Vent S, Pacchioni G, Rosch N: Adsorption of isolated Cu. Ni, and Pd atoms on various sites of MgO(001): density functional studies. I/ Nuovo Cimento 1997, 19:1743.

14.

Murray PW, Shen J, Condon NG, Pang SJ, Thornton G: STM study of Pd growth on TiO,(lOO)-(1 x 3). Surf SC; 1997, 38O:L455. kmall Pd particles are imaged with STM at atomic resolutions.

30.

Pacchloni G, Rosch N: Supported nickel and copper clusters MgO(l00): a first-principles calculation on the metal/oxide interface. J Chem Phys 1996, 104:7329.

15. ..

31.

Yudanov I, Pacchionl G, Neyman K, Rosch N: Systematic density functional study of the adsorption of transition metal atoms on the MgO(001) surface. J Phys Chem B 1997,101:2786.

32.

Yudanov I, Vent S, Neyman K, Pacchioni G, Rosch N: Adsorption of Pd atoms and Pd, clusters on the MgO(001) surface: a density functional study. Chem Phys Letf 1997, 275:245.

33.

loannides T, Veryklos X: Charge transfer in metal catalysts supported on doped TiO,: a theoretical approach based on metalsemiconductor contact theory. J Catal 1996, 161:560.

Rhodium reacts more strongly with the hydroxylated the Rh islands to spread (i.e. higher dispersion). 13.

oxide surface causing

Chun WJ, Asakura K, lwasawa Y: Surface structure analysis of dispersed metal sites on single crystal metal oxides by means of polarization-dependent total-reflection fluorescent EXAFS. Appl Surf Sci 1996, 100/l 01 :I 43.

Klimenkov M, Nepljko S, Kuhlenbeck H, Baumer M, Schlogl R, Freund H-J: The structure of Pt-aggregates on a supported thin aluminum oxide film in comparison with unsupported alumina: a transmission electron microscopy study. Surf Sci 1997, 391:27. The Pt particles have the same structure on highly-ordered alumina In thinfilm form on NiAI(1 10) as on bulk alumina crystals. Particles are flat and Pt(1 1 1).like, but the smaller ones (-1.5 nm in lateral diameter) have Pt-Pt distances that are -10% less than those in the bulk Pt. When particles are 3 nm in lateral diameter, Pt-Pt distances are the same as in the bulk Pt. 16. ..

Xu C, Lai X, Zajac GW, Goodman DW: Scanning tunneling microscopy studies of the TiO,(ll 0) surface: structure and the nucleation growth of Pd. Phys Rev 6 1997,56:13464. Islands of Pd that are 2.0 nm in diameter but only 0.25 nm thick are clearly seen on TiO,(l lo), using STM. These islands are only one Pd atom thick (I.e. ‘2D’ islands). Tiny Pd islands (like tetramers) appear to be pseudomorphic. Deposition of Pd in CO gas caused the cluster density to decrease and their thickness to increase. 17.

Wu Y, Garfunkel E, Madey TE: Initial stages of Cu growth on ordered AI,O, ultrathin films. J Vat Sci Techno/ A 1996, 14:16621667.

18.

Schlerbaum K-D, Fischer S, Wincott P, Hardman P, Dhanak V, Jones G, Thornton G: Electronic structure of pt overlayers on (1 x 3) reconstructed TiO,(iOO) surfaces. Surf Sci 1997, 391 :196. The absence of charge transfer between Pt and TiO,(l 00) in this study (In contrast to the nonstoichlometrlc surface) is attributed to the high work functlon of the (1 x 3) surface, within metal-semiconductor contact theory. .

19.

Xu C, Oh WS, LIU G, Kim DY, Goodman DW: Characterization of metal clusters (Pd and Au) supported on various metal oxide surfaces (MgO and TiO,). J Vat Sci Techno/ A 1997, 15:1261.

20.

Xu C, Goodman DW: Morphology and local electronic structure of metal particles on metal oxide surfaces: a scanning tunneling microscopic and scanning tunneling spectroscopic study. Chem Phys Lett 1996, 263:13.

21. .

Cai YQ Bradshaw AM, Guo Q, Goodman DW: The size dependence of the electronic structure of Pd clusters supported on AI,O,/Re(OOOl). Surf Sci 1998, 399:L357. Dispersion effects in angle-resolved photoemIssIon spectra along directions both parallel and perpendicular to the substrate surface are observed when Pd particles are 2.5 nm in diameter or larger, but not for smaller particles. 22.

Sandell A, Llbuda J, Bruhwiler PA, Andersson S, Baumer M, Maxwell AJ, Martensson N, Freund H-J: Transition from a molecular to a metallic adsorbate system: core-hole creation and decay dynamics for CO coordinated to Pd. Phys Rev 6 1997, 55:7233.

23.

Stuckless JT, Starr DE, Bald DJ, Campbell CT: Calorimetric measurements of adsorption heats and adhesions energies Pb on Mot1 00). Phys Rev B 1997, 56:13496.

on

34. ..

Walter K, Seiferth 0, Kuhlenbeck H, Baumer M, Freund H-J: lnfared spectroscopic investigation of CO adsorbed on Pd aggregates deposited on an alumina model support. Surf SC; 1998, 399:190. Combining IR spectra with previous SPA-LEED (spot-profile analysis In LEED) and STM studies of the structures of the Pd particles led the authors to reassign the peak at 1970-2000 cm-’ to adsorbed CO bridge--bonded at sites on the edges of Pd(1 11) terraces, whereas It was previously asslgned by numerous others to CO on Pd(lO0) sites. A large discrepancy between relative IR intensities and relative amounts of species present IS attnbuted to Intensity transfer. Both conclusions suggest that using IR intensities of adsorbed CO to estimate particle morphology can be problematic. 35. .

Andersson S, Frank M, Sandell A, Glertz A, Brena B, Bruhwiler PA, Martensson N, Libuda I, Baumer M, Freund H-J: CO dissociation characteristics on size-distributed rhodium islands on alumina model substrates. J Chem Phys 1998, 108:2967. Dissociation of CO IS observed upon heating, v&h the greatest probablllty of occurring on Rh particles Intermediate in size (1000 atoms). Smaller particles are thought to be too flat (often only one atom thick) such that no steps are present to assist CO dissociation.

36

Llbuda J, Frank M, Sandell A, Andersson S, Bruhwiler PA, Baumer M, Martensson N, Freund H-J: Size dependent CO dissociation on Rh particles supported on thin alumina films. In Elementary Processes in Excitations and Reactfons on Solid Surfaces. Edited by Okill A, Kasal H, Makoshi K. Berlin-Heidelberg: Springer-Verlag; 1996, 121:210.

37. ..

Baumer M, Frank M, Libuda J, Stempel S, Freund H-J: Growth and morphology of Rh deposits on an alumina film under UHV conditions and under the influence of CO. Surf Sci 1997, 391:204. Excellent structural characterization and SPA-LEED (spot-proflle analysis In LEED) reveal disordered particles, nucleated at defects at a temperature of 300K, but not many at 90K. Exposure to CO causes particle dtsperslon to Increase. It is noted that dispersion of metals on alumina increases as Ag
Stara I, Nehasil V, Matolm V: Influence of substrate structure on activity of alumina supported Pd particles: CO adsorption and oxidation. Surf Sci 1996, 365:69.

for

Stuckless JT, Starr DE, Bald DJ, Campbell CT: Metal adsorption calorimetry and adhesion energies on clean single-crystal surfaces. J Chem Phys 1997,107:5547. The heats of adsorption of metals are measured calorlmetncally for the first time on clean, single-crystalline surfaces. Adsorption and adhesion energies for metals (Pb or Cu) on clean Mo(lO0) and on well-defined surface oxides of Mo(lO0) and W(lO0) are reported.

39.

Ralner DR, Wu MC, Mahon DI, Goodman DW: Adsorption on Pd/AI,O,/Ta(l IO) model catalysts. J Vat So khnol 14:1184.

40.

Voogt EH, Coulier L, Gijzeman OLJ, Geus JW: Adsorption of carbon monoxide on Pd(l11) and palladium model catalysts. J Catal 1997, 169:359.

25.

41. .

24. ..

Stuckless JT, Frei NA, Campbell CT: A novel single crystal adsorption calorimeter and additions for determining metal adsorption and adhesion energies. Rev Sci /n&urn 1998, 69: 2427-2438.

26. .

Heifets E, Zhukovskii YF, Kotomin EA, Causa M: The adhesion nature of the Ag/Mgo(lOO) interface: an ab inifio study. Chem Phys Left 1998, 283:395. An ab initio Hartree-Fock calculation with an exact nonlocal treatment of exchange interactions gives the site registry for Ag atoms (directly above 0 ions) and the bond energy between a 3-layer Ag(lO0) slab and a MgO(lO0) slab (-40 kJ/mol of Ag surface atoms). 27.

Looez N. lllas F: Effect of the Madeluna ootential in the structure anb bonbing of metal-oxide systems:-& on MgO(ll0). J Molec Catal 1997, 119:177.

of CO A 1996,

Wolter K, Selferth 0, Libuda J, Kuhlenbeck H, Baumer M, Freund H-J: IR spectroscopy of a Pd-carbonyl surface compound. Chem Phys Lett 1997, 277:513. The transition from Pd carbonyl compounds to CO adsorbed on large aggregates is traced by depositing Pd in CO gas at 90K and heating. 42.

Matolin V, Stara I: CO diffusion over the alumina support particle model catalysts. Surf Sci 1998, 398:l 17.

of Pd

43.

Becker C, Henry CR: Cluster size dependent kinetics for the oxidation of CO on a Pd/MgO(lOO) model catalyst. Surf SC; 1996, 352-354:457.

44. Becker C, Henry CR: A second CO adsorption state on palladium .. clusters supported on MgO(lO0). Catal Lett 1997, 43:55. A transient CO, productjon peak is seen In CO oxidation after closing the CO gas supply, and postulated to be a second adsorption state for CO.

Chemisorption

45.

Eppler S, Rupprechter G, Guczi L, Somorjai GA: Model catalysts fabricated using electron beam lithography and pulsed laser deposition. J Phys Chem B 1997,101:9973.

46. .

Jacobs PW, Wind SJ, Ribeiro FH, Samorjai GA: Nanometer size platinum particle arrays: catalytic and surface chemical properties. Surf Sci 1997,372:L249. Electron beam lithography is used to write 50 nm Pt particles in a 200 nm lattice on SiOzWi(lOO), and these are studied in CO and H, adsorption and the catalysis of ethylene hydrogenation. 47. *

Rainer DR, Vesecky SM, Koranne M, Oh WS, Goodman DW: The CO+NO reaction over W: a combined study using single-crystal, planar-model-supported, and high surface-area Pd/AIIO, catalysts. J Catal 1997, 167:234. While the smaller particles are more active in NO dissociation, the net reaction is more rapid on larger Pd particles. This is attributed to site poisoning of smaller particles by N(ads). l

48.

Rainer DR, Koranne M, Vesecky SM, Goodman DW: CO + 0, and CO + NO reactions over Pd/AI,O, catalysts. J Phys Chem B 1997, 101 :10769.

Putna ES, Vohs JM, Gorte RJ: Oxygen desorption from a-Al,O,(OOl) supported Rh, Pt, and Pd particles. Surf Sci 1997, 391 :L1178. Both Pd and PI particles show higher desorption temperatures (AH,&for O2 on smaller particles, but this is not so with Rh. Smaller particles adsorb up to three times as much oxygen per unit area (estimated by CO intake). Bunluesin T, Putna ES, Gorte RJ: A comparison of CO oxidation on ceria-supported pt Pd and Rh. Catal Letf 1996,41 :I.

51, .*

Bunluesin T, Gorte RJ, Graham GW: CO oxidation for the characterization of reducibility in oxygen storage components of three-way automotive catalysts. Appl Catal B: Environ 1997, 14:105. As for Pd on ceria, Pd on ceria-zirconia shows an additional mechanism (which adds to the Pd sites’ rate, and uses oxygen from the oxide lattice). This mechanism dies if ceria is calcined above 1 170K, but zirconia addition stabilizes it to higher temperature. Craciun R, Shereck B, Gorte RJ: Kinetic studies of methane steam reforming on ceria-supported pd. Catal Leff 1998, in press. Lore evidence for a really interesting oxygen species associated with the ceria is seen in the rates, which are much, much faster than with a silica support.

Yoshihara J, Campbell CT: Chemisorption of formic acid and CO on Cu oarticles on the Zn-terminated ZnO(0001) surface. Surf Sci 1998,407:256. Copper atoms in 2D Cu islands resemble the coordinatively-unsaturated Cu atoms of Cu(1 IO) in their chemisorption properties. Thick, annealed Cu islands resemble Cu(1 11) in both chemisorption and structure. Similar results were seen on the oxygen-terminated ZnO(0001) surface. 58. ..

Holmblad PM, Rainier DR, Goodman DW: Particle size effects in the acetylene cyclotrimerixation on planar model AI,Os thin film supported Pd clusters. J Phys Chem B 1997,101:8883. An ensemble requirement is suggested by the lack of benzene production from the smallest Pd particles. Only the large particles with (111) facets enable low-temperature benzene desorption, necessary for a high benzene yield. 59.

Dilara PA, Petrie Wl, Vohs JM: HREELS study of the interaction of ethylene with Pt films supported on ZrO& 00) and ZnO(0001). Appl Surf Sci 1997, 1151243.

60.

Zhdano VP, Kasemo B: Kinetics of rapid heterogeneous on the nanometer scale. J Catal 1997, 170:377.

Liu Z M, Vannice MA: CO and 0, adsorption systems. Catal Left 1997,43:51.

on model Au-TiO,

55. lm

Rodriguez JA, Chaturvedi S, Kuhn M, Ek J v, Diebold U, Robbert PS, Geisler H, Ventrice CA Jr: H,S adsorption on chromium, chromia, and gold/chromia surfaces: photoemission studies. J Chem Phys 1997, 107:9146. This system illustrates how supported metal particles can have a reactivity much larger than that of the corresponding bulk metal. This is explained by the charge transfer seen in SCF (self consistent field) calculations. 56. .

Rainer DR, Xu C, Holmblad PM, Goodman DW: Pd, Cu, and Au particles on AI,O, thin films: an infared reflection absorption

reactions

61. .

McLeod AS, Gladden LF: Relating metal particle geometry to the selectivity and activity of supported-metal catalysts: a Monte Carlo study. J Catal 1998, 173:43. The effects of particle size and spatial distribution are studied on a model reaction, chosen to resemble hydrogenolysis. The influence of low-coordination metal atoms at particle edges becomes important below 5-10 nm in size. 62.

Huang HH, Jiang X, Zou Z, Xu GQ, Dai WL, Fan KN, Deng, JF: The oxidation of potassium on MgO(I 00). Surf Sci 1998, 398:203.

63.

Onishi H, lwasawa Y: Atom-resolved observation of Na ensembles activating CO, adsorption on a TiO& I O)-(1 x 1) surface as the genesis of basic sites. Catal Leti 1996, 38:89.

64.

Nerlov J, Christensen S, Weichel S, Pedersen E, Mijller P: A photoemission study of the coadsorption of CO, and Na on TiO,(ll O)-(1 x 1) and (1 x 2) surfaces: adsorption geometry and reactivity. Surf Sci 1997, 371:321.

65.

Wilde M, Beauport II Al-Shamery K, Freund H-J: Photoinduced processes on alkali covered surfaces: NO desorption from WCr,Os(OOOl). Surf Sci 1997, 390:186.

53. .

54.

445

57. .*

52.

Overbury SH, Huntley DR, Mullins DR, Ailey KS, Radulovic, PV: Surface studies of model supported catalysts: NO adsorption on Rh/CeO,(OOl). J Vat Sci Techno/ A 1997,15(3). The dissociation probability for NO in TPD (temperature programmed desorption) from Rh particles on ceria is higher if the ceria is partially reduced/ defective (by sputtering) than on fully oxidized ceria, as smaller Rh particles probably grow on the sputtered ceria, this may just be the expected particle size effect.

Campbell

spectroscopy study of monometallic and bimetallic planar model supported catalysts. J Vat Sci Technol A 1997,15:1653. Small Au particles exhibit stronger adsorption of CO than bulk Au. Alloy particles of Pd with Au or Cu show CO adsorbed at isolated Pd sites.

49. .

50.

on metal films on oxide surfaces

66. .

Rodriguez JA, Jirsak T, Chaturvedi S, Hrbek J: The interaction of H,S and S, with Cs and Cs/ZnO surfaces: photoemission and molecular-orbltal studies. Surf Sci 1998, in press. Several sulfur-contain9 molecules have been studied by this group on many metal/oxide systems, and the sulfur always prefers to bond to the supported metal than to the oxide, and the metal enhances the reactivity compared to the oxide. 67.

Chaturvedi S, Rodriguez JA, Hrbek J: Reaction of S, with ZnO and Cu/ZnO surfaces: photoemission and molecular orbital studies. J Phys Chem B 1997,101:10860.

68.

Harikumar KR, Rao CNR: Oxidation of methanol on the surfaces of model Cu/ZnO catalysts containing Cur+ and Cue species. Caral Lett 1997,47:265.