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Topics in Catalysis Vol. 14, No. 1-4, 2001 71
Modeling heterogeneous catalysts: metal clusters on planar oxide
supports
C.C. Chusuei, X. Lai,K. Luo and D.W. Goodman∗
Department of Chemistry, PO Box 30012, Texas A&M University, College Station, TX 77842-3012, USA
Model catalysts consisting of Au and Ag clusters of varying size have been prepared on single crystal TiO2(110) and ultra-thin films of
TiO2,SiO
2and Al2O3. The morphology, electronic structure, and catalytic properties of these Au and Ag clusters have been investigated
using low-energy ion scattering spectroscopy (LEIS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS)
and scanning tunneling microscopy (STM) and spectroscopy (STS) with emphasis on the unique properties of clusters <5.0 nm in size.
Motivating this work is the recent literature report that gold supported on TiO2is active for various reactions including low-temperature
CO oxidation and the selective oxidation of propylene. These studies illustrate the novel and unique physical and chemical properties of
nanosized supported metal clusters.
KEY WORDS: metal clusters; activity; quantum size effects; thin film oxides; Au; Ag; TiO2;SiO
2;Al
2O3; TPD; XPS; LEIS; STM; STS
1. Introduction
In its bulk form, Au has typically been regarded to be
chemically inert as compared to other platinum group met-
als and received little attention as a catalyst. However, re-
cent findings have shown that Au clusters when deposited as
finely dispersed, small particles (<5 nm in diameter) on re-
ducible metal oxides such as TiO2,Fe
2O3and Co3O4[1–3]
dramatically enhance a number of industrially relevant reac-
tions. Reactions that occur on TiO2supports include low-
temperature CO oxidation [1,2,4–21], hydrogenation and
partial oxidation of hydrocarbons [2,3,21–23] and the se-
lective oxidation of propylene to propylene oxide [3,24].
Similarly, small Ag clusters on metal oxides have been
shown to promote selective oxidation of ethylene to ethyl-
ene oxide [25–27] and methanol to formaldehyde [28–30].
The catalytic properties of these small noble metal clusters
vary widely depending on their particle size. Unsupported,
nanosized metal particles are typically highly reactive and
known to agglomerate (limiting their practical application to
catalysis), but can be stabilized on solid metal oxide matri-
ces. The oxides most commonly used to support Au parti-
cles (in addition to the aforementioned TiO2) include SiO2
[14,16,21,31,32] and Al2O3[21,33]. Ag particles supported
on TiO2[34–38], SiO2[39–41] and Al2O3[27,33,42–49]
have also been examined, showing cluster size-dependent
catalytic activity. Although considerable effort has been di-
rected to understanding activity observed in the high surface
area catalyst systems, much remains to be understood re-
garding chemical interactions at the atomic level. Such in-
vestigations regarding structure size dependency on activity
and selectivity are still in their infancy. Fundamental insight
into the nature of the interaction of metal clusters with the
oxide support is a necessary prerequisite to fine-tuning and
improving catalytic performance.
∗To whom correspondence should be addressed.
Significant progress has been made in the past several
decades using ultrahigh vacuum (UHV) surface sensitive
probes, capable of providing detailed information about sur-
face composition and structure [50–53]. However, since
bulk metal oxides (ubiquitousin heterogeneous catalyst sys-
tems) are insulators, charging problems typically hamper
their analysis. In our laboratory, these difficulties have been
circumvented by synthesizing ultra-thin film metal oxides
via “hot” filament deposition on planar supports under back-
ground O2pressure, using them as model planar supports
for metal particle deposition to mimic the “real-world” sys-
tems. In addition to using single crystal TiO2(110) [36,37],
thin films of TiO2[34,35,38,54], SiO2[55–59] and Al2O3
[60–62] have been synthesized on single crystal refractory
metal substrates, making them amenable for surface analy-
sis. These films (∼50 Å thick) are thin enough to surmount
the insulating problems associated with charged particles
and yet substantive enough to retain the bulk chemical and
electronic properties of the metal oxides. Since flat surfaces
are used, these systems are also suitable for scanning probe
microscopy to study morphology. In addition, difficulties re-
lated to inhomogeneous sample heating, which complicates
the use of temperature-programmed desorption (TPD), are
also circumvented by the thin-film oxide synthesis proce-
dure.
In this review, a summary of recent studies in this labo-
ratory are described involving model catalysts of nanosized
Au and Ag clusters supported on single crystal TiO2(110)
and ultra-thin films of TiO2,Al
2O3and SiO2. These sys-
tems have been investigated using low-energy ion scatter-
ing spectroscopy (LEIS), TPD, X-ray photoelectron spec-
troscopy (XPS) and scanning tunneling microscopy (STM)
and spectroscopy (STS) specifically to establish correlations
among physical, chemical and catalytic properties.
1022-5528/00/1200-0071$18.00/0 2000 Plenum Publishing Corporation
72 C.C. Chusuei et al. / Modeling heterogeneous catalysts
2. Experimental
ATiO
2(110) single crystal (Commercial Crystal Labora-
tories) had been chosen as the oxide support chiefly due to
its suitability for atomically-resolved STM and STS analy-
sis [63]. The n-type semiconductor was found to be suffi-
ciently conductive for STM and electron spectroscopy after
cycles of Ar+bombardment and annealing to 700–1100 K.
The TiO2(110) crystal was mounted onto a tantalum sample
holder. The temperature was monitored using an optical py-
rometer (OMEGA OS3700), the temperature scale of which
was calibrated from voltages from a W-5% Re/W-26% Re
thermocouple glued to the edge of the crystal using high-
temperature ceramic adhesive (AREMCO). The TiO2(110)
crystal could be resistively heated to 1500 K (or electron
beam heated to 2400 K) and cooled via a liquid nitrogen
(LN2) reservoir thermally attached to the sample using Cu
leads. TPD experiments were carried out using a line-of-
sight quadrupole mass spectrometer (QMS). In order to re-
duce the TiO2(110) sample (making it conductive and hence
suitable for STM/STS), the crystal was annealed to 1000 K
for several hours in UHV. Stoichiometric TiO2(110)-(1 ×1)
was prepared by Ar+bombardment (1.0 kV), followed by
annealing to 1000 K in 4 ×10−6Torr O2pressure for 10 min
[64]. The TiO2(110)-(1 ×2) reconstructed surface was pre-
pared by annealing to 1300 K; low-energy electron diffrac-
tion (LEED) and STM verified both the (1 ×1) and (1 ×2)
structures. A low binding energy shoulder in the Ti 2p3/2
XP spectrum (indicative of surface reconstructed Ti3+)pro-
vided an additional diagnosis of the (1 ×2) structure [65].
Thus, the effects of both surface structures on metal clus-
ter strong-metal–support interaction were investigated using
LEIS.
Thin film TiO2had been deposited onto Mo(110), sup-
ported by 99.9+% pure Ta wire. Similarly, the SiO2and
Al2O3supports were epitaxially grown as thin films de-
posited onto Mo(110) and Re(0001) substrates, respectively.
W-5%Re/W-26% Re thermocouplewires were spot-welded
onto the metal supports in order to measure temperature. The
chemical compositions of these oxide supports were verified
by XPS and their surface structures by LEED.
Metal dosers used to deposit the noble metal clusters of
interest (Au and Ag) were constructed by wrapping high
purity wires (99.99%) of the metals around 0.010 diameter
tungsten filaments (H&R Cross), which were then resistively
heated in vacuum by passing current through the filament
wires. These wires were melted and thoroughly outgassed
to remove impurities prior to use. The Ag and Au fluxes, re-
spectively, were calibrated via integrated TPD peak areas of
the metals (depositing them onto a Re(0001) substrate) and
also using the metal-to-substrate Auger intensity “break”to
denote the first monolayer equivalent (ML) [66]. A 1 ML
surface coverage was defined as 1.39 ×1015 atoms/cm2.
During use of the dosers for metal deposition, the overall
UHV system pressure did not exceed 2.0×10−10 Torr. The
deposition rates used for Au and Ag dosing were 0.083 and
0.166 ML/min, respectively. Pressures in the HPC region,
isolated from the STM-UHV system via gate valve, were
measured with a Baratron gauge.
Three UHV systems were used to carry out the deposition
of Au and Ag onto the planar metal oxide supports and the
subsequent surface spectroscopic analysis. The first cham-
ber was equipped with XP and Auger electron (AES) spec-
troscopies, LEIS, TPD and LEED capabilities. XPS data
were collected using a Mg Kαanode (hν =1253.6eV)
operated at 300 W and 15 kV (Perkin–Elmer PHI 04-500 X-
ray source) and a concentric hemispherical analyzer (PHI,
SCA 10–360) with an incident angle of ca. 30◦from the
surface normal. LEIS data were acquired using He+pro-
jectiles with a beam energy of 650 eV and a scattering in-
cident angle of 45◦from the surface normal. To mini-
mize surface damage, only one sweep (across the kinetic en-
ergy range) per set of scans was acquired. A second UHV
chamber (also equipped with XPS, AES and LEED) was
used to acquire the STM and STS data (Omicron STM-1).
STM data were typically collected with sample biases of
1–2 V and tunneling currents of 0.5–2.0 nA. All STM
and STS data were obtained at 298 K. A constant current
topographic (CCT) mode was employed for STM imag-
ing. Highly ordered pyrolytic graphite (0001) was used to
calibrate the scaling of the STM micrographs. The STM
tip was made from tungsten wire (0.020 in diameter, H&R
Cross) and prepared by electrolytic etching [67]. In or-
der to remove the oxide overlayer formed from the etch-
ing, the tip was dipped in HF solution for 30 s, rinsed
in deionized H2O, rapidly transferred into UHV, and then
heated to 1000 K prior to use. The base pressure of these
first two chambers were both ∼1×10−10 Torr after bake-
out.
An elevated high pressure reactor cell (HPC) had been
combined with the STM chamber to carry out reaction kinet-
ics at elevated pressure. Using the HPC, described elsewhere
[68,69], reactions of the prepared model catalyst surfaces
(Ag/TiO2(110) and Au/TiO2(110)) with gas mixtures of CO
and O2were carried out and subsequently studied with STM
and STS. The model catalysts were transferred from the
HPC to UHV using differentially-pumped sliding seals. Re-
search grade CO was purified by storing it at liquid-N2tem-
perature; O2was used as received. The CO : O2(2 : 1) mix-
tures were premixed prior to use. The third UHV system
(having a base pressure of ∼6×10−10 Torr after bakeout)
was equipped with AES, TPD, an HPC reactor and a Var-
ian 3400CX series gas chromatograph (GC) with a thermal
conductivity detector (TCD) and Hyesept D, 40 foot col-
umn.
3. Results and discussion
3.1. Finite size effects and catalytic activity
Figure 1(A) shows a CCT–STM micrograph of 0.25 ML
Au deposited onto single crystal TiO2(110)-(1 ×1) [22].
The deposition was performed at 300 K, followed by an-
nealing the TiO2surface to 850 K for 2 min. Only the Ti
C.C. Chusuei et al. / Modeling heterogeneous catalysts 73
(A)
(B)
Figure 1. (A) A CCT–STM image of a 0.25 ML Au deposited onto
TiO2(110)-(1×1) pr epared just prior to a CO : O2reaction. The sample had
been annealed to 850 K for 2 min; (B) STS data acquired for Au clusters
of varying sizes on the TiO2(110)-(1 ×1). An STS of the TiO2substrate,
having a wider band gap than the Au clusters, is also shown as a point of
reference.
cations are imaged in the STM; the O atoms are not seen
[63]. The inter-atomic distances between the [001] rows are
separated by ∼0.65 nm, which can be observed along the
terraces corresponding to the length of the unit cell along
the [110] direction of unreconstructed TiO2(110)-(1 ×1).
Three-dimensional (3D) Au clusters, imaged as bright pro-
trusions, have average diameters of ∼2.6 and ∼0.7 nm
height (corresponding to 2–3 atoms thick) and preferen-
tially nucleate on the step edges. Quasi-two-dimensional
(2D) clusters are characterized by heights of 1–2 atomic lay-
ers. Previous annealing studies revealed that the Au clus-
ters form large microcrystals with well-defined hexagonal
shapes [36].
Figure 1(B) shows STS taken at various points on the
surface. The ST spectra had been acquired by position-
ing the STM tungsten tip at a desired point and interrupt-
ing the STM feedback loop. In the figure, the tunneling
current (I) as a function bias voltage (V) across the tip is
measured. The I–Vcurves are thus correlated with vari-
ous Au clusters of varying sizes on the TiO2surface (fig-
ure 1(A)). The length of the observed “plateaus”at the zero
tunneling current (figure 1(B)) denotes the band gap (along
the bias voltage axis) of electrons tunneling between the va-
lence and conduction bands between the surface (imaged
Au particles) and the tungsten tip. The electronic charac-
ter of these clusters vary between that of a metal and a non-
metal, depending on size. With increase in size, the clus-
ters gradually adopt metallic character with an enhanced
density of states at the Fermi level, characterized by the
more abrupt slopes in the STS for larger clusters. Note
that the cluster with the 2.5 nm ×0.7 nm size has a larger
band gap than that of the 5.0 nm ×2.5 nm cluster (fig-
ure 1(B)); smaller Au clusters have a non-metallic charac-
ter resulting in significant band gaps whereas larger clusters
have bulk-like metallic properties giving rise to essentially
no band gaps. The 4.0 nm ×1.5 nm and 3.0 nm ×1.0 nm
clusters, having intermediate-sized band gaps, also follow
this cluster size–band gap dependency. Similar changes in
the band gap as a function of cluster size also have been
observed in STM/STS analysis of Fe clusters epitaxially
grown on GaAs(110) [70]. In these studies, Fe clusters with
volumes of ∼1nm
3(85 Fe atoms) displayed fully metal-
lic characteristics while volumes ∼0.15 nm3(13 atoms)
were also found to be non-metallic. Also, the same trends
were observed for ErP islands (20–50 nm in size) grown
on InP(001) surfaces [71]. Semi-metallic behavior denoted
by narrower band gaps measured by STS I–V curves was
observed for thick ErP islands (>3.4 nm) while (relatively
wide) semi-conducting band gaps were observed for thin-
ner ones (<3.4 nm). A more detailed discussion of quantum
sizes effects and their relationship with structure-sensitive
activity (due to differences in metal cluster island size) will
be published elsewhere [72]. Thus, the electronic structure
of the adsorbed Au particles can play an important role in
the activity of Au on the surface.
In this present study, a correlation is observed between
Au cluster size and catalytic activity for the oxidation
of CO on Au/TiO2(110)-(1 ×1). Figure 2(A) shows a
plot of activity for CO oxidation (expressed as (product
molecules)/((total Au atoms on surface sites) ×(second)) or
turnover frequency(TOF)) at 350 K as a function of Au clus-
ter size supported on the TiO2(110)-(1 ×1) substrate [22].
CO and O2(1:5 mixtureofCO:O
2) had been reacted on
the previously prepared Au(0.25 ML)/TiO2(110)-(1×1) sur-
faces at 40 Torr [5,22,23]. Thin-film TiO2epitaxially grown
74 C.C. Chusuei et al. / Modeling heterogeneous catalysts
Figure 2. (A) The activity for CO oxidation at 350 K as a function of Au
cluster size supported on TiO2(110)-(1 ×1) assuming total dispersion of
the Au. A 1:5 CO:O2mixture had been used at a total pressure of 40
Torr. Activity is expressed as (product molecules)/((total Au atoms) ×(sec-
ond)); (B) cluster band gaps measured by STS as a function of Au cluster
size supported on TiO2(110)-(1 ×1). The band gaps were obtained while
the corresponding topographic scan was acquired on various Au coverages
ranging from 0.2 to 4.0 ML. (") Two-dimensional (2D) clusters, (E)3D
clusters, two-atom layers in height, (P) 3D clusters, three-atom layers or
greater in height; (C) relative population of Au clusters (two-atom layers
in height) that exhibit a band gap of 0.2–0.6 eV, as measured by STS of
Au/TiO2(110)-(1 ×1).
on a Mo(100) substrate [73] followed by Au cluster deposi-
tion was used for reaction kinetics in the GC-UHV system.
These kinetic studies were carried out in parallel with the
UHV-STM Au/TiO2(110)-(1×1) single crystal experiments.
The product (CO2) was extracted from the reactor with a
vacuum syringe, compressed and then analyzed with a GC.
For each point in figure 2(A), a particular Au cluster size
was prepared then subjected to the CO2:O
2reaction. The
cluster sizes of the Au particles and coverage of the surface
sites (identical to that of the TiO2(110)-(1×1) single crystal)
obtained from the parallel STM imaging experiments were
used to calculate the TOF. The activity of the Au/TiO2cat-
alyst exhibits a maximum 1.90 TOF at an average Au clus-
ter diameter of ∼3.5 nm, decreasing with larger diameter.
Figure 2(B) shows a plot of the STS band gaps measured
over the same cluster size regime used for the CO: O2re-
actions (figure 2(A)). Maximum catalytic activity occurs in
concert with the metal-to-nonmetal transition. The average
Au cluster size at the divergence from metallic character is
3.5 nm in diameter and 1.0 nm in height, corresponding to
approximately 300 atoms per cluster. Figure 2(C) shows a
histogram indicating the relative distribution of the Au clus-
ter sizes ranging from 2.0 to 4.0 nm in diameter at maximum
catalytic activity, corresponding to STS measured band gaps
from 0.2 to 0.6 V. Two-atom-thick clusters (with diameters
between 2.5 and 3.0 nm) are characteristic of those optimally
active for CO oxidation.
These above studies thus demonstrate that cluster elec-
tronic properties play a crucial role in defining the catalytic
reactivity of small clusters [22]. There has been consider-
able interest, theoretically and experimentally, in studying
these size-dependent changes in electronic structure. In re-
cent years, XP and ultraviolet photoelectron (UPS) spectro-
scopies have provided many examples of cluster size depen-
dent electronic modifications from the discrete energy levels
of free atoms to the continuous, k-dependent energy bands
of bulk metals [74–77]. These methods (with typical spatial
resolutions in the µm scale) are not useful for analyzing the
metal particles within the nanometer range; however, STM
and STS can be utilized.
3.2. Effects of O2exposure on admetal cluster size and
distribution
Ambient pressures of O2, ubiquitous in “real world”cat-
alyst preparation conditions,is an important variable to con-
sider as it may affect the admetal’s ability to wet the sur-
face and thereby alter particle size and distribution. From
aBrønsted linear free energy interpretation of TPD data of
Au adsorbed on TiO2(110), Bondzie et al. [78] postulated
that small clusters of Au are able to dissociatively adsorb O2
(at 10−5Torr) more readily than large ones. Further inves-
tigations [22,23,37] show that smaller Au clusters become
larger with an increase in O2pressure, effectively reducing
catalytic activity. For example, after a 10 Torr O2exposure
to 0.25 ML Au deposited onto TiO2(110)-(1×1) for 120 min
[22,37], the Au cluster density decreased and the cluster size
increased. These changes are attributed to sintering of the
Au particles via a ripening process possibly involving AuOx;
however, thermodynamic data regarding Au oxide formation
is lacking. Thus, this mechanism cannot be confirmed. Sim-
ilar thermodynamic data, however, exists for Ag oxide for-
mation.
In an experiment analogous to those described for the
Au/TiO2system, various cluster sizes of Ag on TiO2(110)-
(1 ×1) have been deposited and studied by STM and STS.
The metal-to-nonmetal transition for Ag (figure 3) occurs at
a slightly larger cluster size (∼3.0–5.0 nm diameter) than
that of Au (∼2.0–4.0 nm diameter). STM and STS had
been carried out on the Ag/TiO2system before and after
high pressure O2exposure. Figure 4(A) shows a CCT–
C.C. Chusuei et al. / Modeling heterogeneous catalysts 75
Figure 3. Size-dependent metal-to-nonmetal transitions for Au and Ag clus-
ters on TiO2(110)-(1 ×1). The transition size regimes are ∼2.0–4.0 nm in
diameter for Au clusters and ∼3.0–5.0 nm in diameter for Ag clusters.
STM image of 2.0 ML of Ag (the approximate coverage
where the metal-to-nonmetal transition is observed) vapor
deposited on a clean TiO2(110)-(1 ×1) single crystal sur-
face. The features displayed in the micrograph correspond
to Ti+cations. Direct tunneling into (or out of) the oxy-
gen sites is unlikely since the O 2p state is ∼3eVbelow
the Fermi level, beyond the operational range of the micro-
scope. The 3D homogeneous hemispherical Ag clusters are
observed on both the flat terraces and step edges. These par-
ticles have an average cross-sectional area of ∼4.8 nm ×
2.6 nm (diameter ×height), corresponding to 1900 atoms
per cluster. After the Ag/TiO2surface had been prepared,
the substrate was transferred into the HPC reactor and ex-
posed to 10 Torr O2at 298 K for 120 min and then trans-
ferred back into the UHV for STM and STS. The resulting
image is shown in figure 4(B). A bimodal distribution of Ag
clusters is evident on the TiO2(110)-(1 ×1) surface after
the O2exposure. Comparing with the original micrograph
(figure 4(A)), some clusters enlarge while others diminish
in size (figure 4(B)). In addition, there is a 5–15% increase
in cluster density, indicative of Ag cluster redispersion on
the surface. A histogram of the Ag (figure 5) shows a trans-
formation from a unimodal dispersion with a mean cluster
diameter of 5 nm to a bimodal distribution with mean clus-
ter diameters of ∼3.5 and ∼6.8 nm. The single distribu-
tion has cluster sizes ranging from 2.0 to 6.5 nm; the bi-
modal distribution has one size domain ranging from 1.0 to
5.0 nm in diameter while the second domain ranged from
5.0to11nm.Thesmallerclustersinthe1.0–5.0 nm domain
have a higher density and narrower distribution with an av-
erage size of ∼3.0 nm ×∼1.1 nm (∼260 atoms/cluster).
The larger Ag cluster domain with the lower density and
broader size distribution had an average size of ∼6.7 nm ×
∼3.1 nm (∼4200 atoms/cluster). STS band gap measure-
ments showed the 1.0–5.0 nm domain electronic structure
to be nonmetallic and the 5.0–11.0 nm domain to be fully
metallic. The calculated Ag cluster volumes (from STM
data) before and after O2exposure were the same (within
±10% error).
The redispersion of the Ag clusters is a ripening process
attributed to two possible mechanisms: migration and co-
alescence of the metal atoms on the surface or intercluster
and/or vapor phase transport. Regarding the second possi-
bility, reduction of the total surface free energy by interclus-
ter transport occurs such that the larger clusters grow at the
expense of the smaller ones [79]; some clusters increase in
size while others decrease, leading to the bimodal distribu-
tion. The data are more consistent with an Ostwald ripening
mechanism than of simple coalescence. With O2exposure
the following reaction is plausible:
2Ag(s) +1
2O2(g) →Ag2O(s)
and is thermodynamically favorable at 298 K. The standard
free energy of formation (G)ofAg
2Ois−11.2 kJ/mol, al-
lowing an estimate of the equilibrium partial pressure of O2
required for the above reaction to be 0.094 Torr, far lower
that than the pressure used in the data of figures 4 and 5.
For an average cluster diameter of 5.0 nm (r=2.5nm),
the G298(r) value decreases to −22.7 kJ/mol, taking into
account the effect of the cluster curvature on the free en-
ergy. The driving force for Ag cluster oxidation is high at
room temperature and likely leads to Ostwald ripening. De-
tailed calculations of these thermodynamic considerations
are shown elsewhere [37,72]. The intrinsic electronic prop-
erties suggest that certain sizes of Ag clusters are more re-
active to O2molecules than others, i.e., certain Ag clus-
ter sizes would undergo Ostwald ripening more rapidly and
eventually deplete. Other Ag cluster sizes would experi-
ence ripening more slowly due to reduced kinetics with
O2to form Ag2O. Clearly, the relative size-dependent re-
activity to O2also contributes to the bimodal distribution
of Ag.
To further study the effect of the substrate on Ostwald
ripening of the Ag clusters, a different oxide support had
been selected for comparison with TiO2(110). Ultra-fine
particles of Ag supported on Al2O3has attracted recent in-
terest for the oxidation of CO and reduction of NO showing
high conversions (90%) of NO and CO on the high sur-
face area catalyst systems [80]. Al2O3thin films were pre-
pared on a Re(0001) single crystal. Ag was then deposited
via Ag metal evaporator and then followed by a 10 Torr O2
exposure. The chemical compositional and surface struc-
tures of Al2O3thin films grown heteroepitaxially on refrac-
76 C.C. Chusuei et al. / Modeling heterogeneous catalysts
Figure 4. CCT–STM images (100 ×100 nm2, 2.0 V, 1.0 nA) of 2.0 ML Ag/TiO2(110). (A) Fresh 2.0 ML Ag/TiO2(110)-(1×1); (B) 2.0 ML Ag/TiO2(110)-
(1 ×1) after exposure to 10 Torr O2for 120 min at 298 K.
tory metal substrates have been well-characterized [60–62].
Compared to TiO2,Al
2O3is an irreducible and unrecon-
structed surface and relatively free of surface defects. Fig-
ure 6 shows a series of O2exposed Ag–support CCT–STM
images. Image (A), shown for comparison, is an image of
2.0 ML of Ag deposited onto a cleaned TiO2crystal. Im-
age (B) is a CCT–STM of the same Ag coverage onto an
Al2O3thin film. The Ag clusters on the Al2O3,ascom-
pared to the TiO2, are slightly larger and have a smaller
cluster density. Interestingly, the metal oxide support with
higher defect density (TiO2) results in smaller cluster sizes
at a higher dispersion. (This effect of defect density on ad-
metal dispersion and particle size will be further addressed
in section 3.3.) After the Ag/Al2O3surface had been ex-
posed to 10 Torr O2for 120 min at 298 K in the HPC,
image (C) was obtained. Instead of a bimodal distribution,
prevalent for the Ag/TiO2(110) system, a relatively homoge-
neous distributionof Ag on the Al2O3is evident. Ag cluster
C.C. Chusuei et al. / Modeling heterogeneous catalysts 77
Figure 5. Size distribution of 2.0 ML Ag/TiO2(110)-(1 ×1) before (top)
and after (bottom) a 10.00 Torr exposure for 120 min at 298 K. A bimodal
distribution results after the O2treatment.
ripening, forming the bimodal distribution, is not observed
until higher O2pressures are employed. There is an ob-
vious substrate effect of the oxide support on the admetal
cluster distribution and Ostwald ripening. Ag is apparently
more stable and resistant to ripening when supported on
Al2O3than on TiO2(110) due to its comparatively defect-
free surface. It can be concluded that the greater number
of surface defects/vacancies on the TiO2(110) is responsi-
ble for the relative ease of Ostwald ripening occurring at
10 Torr O2. The relatively defect-free Al2O3support leads
to reduced admetal reactivity with O2, a requirement for
ripening.
3.3. Cluster dispersion on TiO2(110)-(1 ×2) versus
TiO2(110)-(1 ×1)
As alluded to in section 3.2, the structure of the underly-
ing metal oxide substrate supportalso has an effect on metal
cluster dispersion on deposited Au and Ag clusters. Fac-
tors influencing noble metal dispersion are important consid-
erations for addressing practical catalyst preparation issues
since higher dispersions on oxide supports generally leads to
increased catalytic activity [81,82]. TiO2(110)-(1 ×1) and
TiO2(110)-(1×2) were chosen as the substratesfor the study
since their surface structures are well characterized, in par-
ticular with respect to surface defects that can be produced
on each surface via ion bombardment and heating [65,83–
85]. Nanosized Au and Ag metal clusters had been deposited
onto TiO2(110)-(1 ×1) and TiO2(110)-(1 ×2) at 300 K, re-
spectively, using metal evaporators and their growths were
monitored with LEIS. Surface coverages had been checked
with AES and TPD. The (1 ×2) surface has relatively more
defect sites than the (1 ×1) surface; the defects serve as
nucleation sites for metal cluster growth. Hence, metal de-
position on a rough (1 ×2) surface would likely differ from
the (1 ×1) surface metal cluster nucleation and growth be-
havior.
LEIS (in conjunction with STM) is ideally suited for
probing the surface compositional structure of adsorbed
metal clusters due to its high surface sensitivity (capable of
probing the top 0.1–0.2 nm of the substrate). Plotting the
integrated LEIS peak areas (arbitrary units) as a function
of adsorbed metal clusters (ML units) has proven to be an
effective diagnostic tool for distinguishing various admetal
growth modes: layer-by-layer (Frank van der Merwe, FM),
3D cluster growth (Volmer–Weber, VW) or an initial mono-
layer, followed by 3D cluster growth (Stranski–Krastanov,
SK) mode. Au, Pd and Ag admetals have been found to nu-
cleate and grow in a 3D fashion when adsorbed onto thesin-
gle crystal TiO2(110) surface at coverages 1 ML. However,
the metal grows as quasi-2D clusters at coverages <1ML,
as shown by both LEIS [64] and STM imaging [36]. An FM
or SK growth is characterized by a linear increase in the inte-
grated LEIS peak areas as a function of increasing adsorbate
surface coverage, reaching a plateau upon completion of the
first monolayer. The substrate intensity simultaneously de-
creases with increasing admetal intensity, which is then fully
attenuated above 1 ML. This effect has been observed for
Fe [86] and Hf [87] clusters deposited onto single crystal
TiO2(110) at 160 K and room temperature, respectively. It
should be noted that after the 1 ML coverage, the substrate
signal cannot be seen and hence FM and SK modes can-
not be readily distinguished from LEIS data alone. During
VW growth, on the other hand, the LEIS intensity increases
with a smooth non-linear function at increasing metal cov-
erages since the substrate is still exposed. The non-linear
increase continues even after a few monolayers of admetal
are deposited. This same growth behavior had been ob-
served for VW growth of Cu [88] and Pt [89] in TiO2(110).
Based on LEIS and XPS characterization, Campbell and co-
workers have proposeda kinetic model for Au cluster growth
on TiO2(110) [90]: one-atom, 2D thick Au islands initially
form until a critical coverage is reached [91], after which
the growth switches to 3D islanding. From LEIS data, this
critical coverage was found to be independent of the metal
vapor flux, but increased with decreasing adsorption tem-
perature and increasing defect density of the substrate. The
underlying metal substrate surface structure was thus clearly
shown to affect the growth behavior of the deposited admet-
als. To further investigate the phenomenon in this current
study, differencesbetween TiO2(110)-(1×1) and TiO2(110)-
(1 ×2) with respect to Au particle dispersion are exam-
ined.
Figure 7 shows stackplots of LEIS intensity at various
Au cluster surface coverages (in ML units of the admetal)
on both (A) TiO2(110)-(1 ×1) and (B) TiO2(110)-(1 ×2)
surfaces. The Au clusters had been evaporated onto the
TiO2(110) at 300 K. For coverages of 1–5 ML (for the
(1 ×1) surface)), Au cluster diameters are in the 3.5–5.5 nm
range; the Au cluster sizes deposited are thus near the metal–
78 C.C. Chusuei et al. / Modeling heterogeneous catalysts
Figure 6. CCT–STM images showing O2exposure effects on 2.0 ML Ag/Al2O3/Re(0001). (A) Freshly exposed 2.0 ML Ag on TiO2(110) shown as a
reference (2.0 V, 1.0 nA); (B) freshly exposed 2.0 ML Ag/Al2O3/Re(0001) (2.0 V, 0.26 nA); (C) 2.0 ML Ag/Al2O3/Re(0001) exposed to 10 Torr O2for
120 min; (D) 2.0 ML Ag/Al2O3/Re(0001) exposed to 1000 Torr for 120 min.
nonmetal transition region (where increased catalytic activ-
ity would be predicted). LEIS spectra of both TiO2single
crystal substrates at 270, 460 and 580 eV denote He+scat-
tering from O, Ti and Au sites, respectively. Note that in
each case, the Au attenuates the TiO2substrate layer as it
grows; the Ti and O intensities decrease as Au increases. In
comparing the two LEIS stackplots, a relatively greater at-
tenuation of the Ti and O by Au on the (1 ×2) surface than
on the (1 ×1) is apparent for the same amount of Au sur-
face coverage, indicating a greater density of islanding on
the (1 ×2) surface. Figure 8 shows a plot of the integrated
LEIS peak areas of the Au adsorbed divided by those of the
underlying substrateTi and O atoms of the (1×1) and (1 ×2)
substrates. Since the chemical composition of the substrate
remains unchanged as Au coverage increases, the substrate
provides a way of normalizing the adsorbed Au LEIS inten-
sity. Plots of the Au/Ti and Au/O ratios versus Au coverage
further accentuates the various Au cluster dispersions found
for the (1 ×1) and (1×2) surfaces. Note the non-linearity of
the Au growth versus coverage; both the (1 ×1) and (1 ×2)
surfaces exhibit VW crystal growth characteristics. The up-
ward curvature for the (1 ×2) surface is steeper than that of
the (1 ×1); this is particularly pronounced between 2 and
5 ML, indicating that the Au clusters are more completely
dispersed on the (1 ×2). Clearly, the higher dispersion is
surface structure related. The inter-atomic spacing as meas-
ured by STM between the Ti atoms in the [001] direction
is 0.3–0.65 nm on the (1 ×1) whereas it is ∼1.3nmonthe
(1×2) [63]. Also, STM shows that the bare (1×1) surface as
compared with the (1 ×2) has a larger numberof terraces and
fewer isolated cluster structures [38]. Given the tendency of
metal clusters to preferentially nucleate at step edge defects
rather than flat terrace defect sites, a greater density of 3D
islanding is expected for the (1 ×2) at a similar coverage.
C.C. Chusuei et al. / Modeling heterogeneous catalysts 79
Figure 7. Stackplots of LEIS spectra at Au cluster coverages ranging from
0 to 5 ML. Intensities of signals from O and Ti were multiplied by 2 for
clarity. (A) Au cluster growth on TiO2(110)-(1 ×1); (B) Au cluster growth
on TiO2(110)-(1 ×2).
A combination of greater distances between the Ti rows in
conjunction with the inherently greater surface roughness re-
sults in greater dispersion of the 3D islands (figure 8).
Differences have also been seen for Ag cluster growth
(unreacted) on TiO2(110)-(1×1) and TiO2(110)-(1×2). The
3D Ag crystallites are more widely dispersed on the (1 ×2)
surface than on the (1 ×1) structure [38]. The LEIS uptake
curves of the He+scattered Ag intensity relative to Ti and O
on the (1×1) and (1×2) surfaces (not shown) also increased
with smooth non-linear functions, but the increase for the
(1×2) is greater (similar to that for Au cluster growth). The
degree of the curvaturefor the (1 ×2) versus (1 ×1) is also
Figure 8. Plot of Au/O and Au/Ti ratios taken from integrated LEIS peak
areas of Au cluster growth on TiO2(110)-(1 ×1) and TiO2(110)-(1 ×2).
Au coverages ranged from 0 to 5.0 ML. Higher Au/O and Au/Ti ratios on
the (1 ×2) surface denotes a wider cluster dispersion.
greater. The same factors influencing dispersion in for Au
cluster adsorption also apply for Ag cluster adsorption on the
TiO2(110) surface; namely (1) more defects (roughness) and
(2) increased spacing between the Ti rows on the (1×2). The
cluster sizes (comparing Ag and Au) differ slightly, but the
overall growth trends (as measured by LEIS) on the (1 ×1)
versus (1 ×2) surfaces are the same. STM analysis show
that at a 0.5 ML Ag exposure, metal cluster diameters (with
heights in parentheses) on the (1 ×1) has a diameter of 3.5–
4nm(1.0–1.5 nm); for the (1 ×2), it is 2.5–4nm(0.8–
1 nm) [36]. The Ag clusters disperse to a greater degree on
the (1 ×2) surface, further verifying the conclusions drawn
from LEIS.
The dissimilar admetal dispersion on the (1 ×1) ver-
sus (1 ×2) titania structures has implications for metal sur-
face restructuring in defining catalytic activity. According
to the chemisorption induced surface restructuring model
(CISRM), proposed by Somorjai and coworkers [92–95],
surface metallic atoms do not remain in the same surface
locations as for the bare surface but move to new positions
in response to chemical changesin the environment. During
chemisorption, these displacements strengthen the chemical
bond of the adsorbate to the surface, provoking a local sur-
face strain and hence promote catalytic reactions. Substrate
atom rearrangement is more pronounced at steps, open sur-
faces and low coordination sites. It is inferred from our STM
and LEIS results that for TiO2(110), surface Ti atoms dur-
ing chemisorption/catalytic reactions (involving nanosized
noble metal clusters) prefer a rearrangement to a structure
much more like TiO2(110)-(1 ×2) than TiO2(110)-(1 ×1)
80 C.C. Chusuei et al. / Modeling heterogeneous catalysts
structure. The (1 ×2) configuration is more amenable
for increased metal cluster dispersion and subsequent activ-
ity.
3.4. Metal oxide support effects on cluster electronic
structure
XPS is a useful tool for probing the electronic structure
of interactions between the adsorbed metal particles and the
underlying metal oxide support. Binding energy (BE) shifts
due to final state effects are a result of screening of elec-
trons from core level vacancies created by the photoemis-
sion process. As electrons relax to screen the hole, the emit-
ted photoelectron is ejected with increased kinetic energy
(hence lower BE). If a given adsorbed cluster is sufficiently
small such that screening of its photoelectrons is less that
that of the bulk substrate, a higher BE would result. Ini-
tial state effects may originate from a variety of factors such
as (1) interfacial chemical reactions, (2) BE differences be-
tween surface and bulk atoms or (3) nucleation on various
defect sites can also result in cluster size dependent core
level shifts. Self-consistent field (SCF) calculations of the
core ionization potentials (from the XPS 4f core levels) of
small Pt clusters deposited on SiO2substrates show that the
BE shifts and line-broadening depend chiefly on cluster size
and cluster–substrate interactions [96]. Furthermore, SCF
analysis shows that the core level shifts were not due to elec-
trostatic effects of the unit positive charge remaining on the
ionized cluster.
Figure 9 (A) and (B) shows two sets of core level BE
peak centers for the Au 4f7/2core level as a function of Au
cluster coverageon (a) the TiO2(110) surface and (b) on thin
film SiO2(∼2.5 nm thick) deposited on a Mo(110) refrac-
tory metal surface. It should be noted that in its bulk form,
SiO2would be non-conductive and hamper electron spec-
troscopic analysis; it therefore, had been deposited as a thin
film. All Au dosings were carried out at 300 K. On the TiO2
surface, a –0.8 eV BE shift is evident from small clusters
(0.02 ML, ∼2 nm diameter), shifting to the bulk value of
Au 4f7/2=84.0 eV with increasing cluster size (6 ML Au
coverage, ∼5 nm diameter; figure 9(A)). Average cluster di-
ameters (within parentheses above) had been measured by
STM from Au fluxes on TiO2(110) [36]; the BE of the 4f7/2
core level typically found for bulk Au is 84.0 eV [97]. For
the Au/SiO2system, the corresponding BE shift is greater
(figure 9(B), −1.6 eV). This lowering from high BE to the
bulk value is to be expected for supported metal clusters as
they increase in size and as the crossover from non-metal to
metal character occurs. As reported in an earlier XPS study
by Mason [98], relative BE shifts of Au clusters (of vary-
ing sizes) supported on SiO2and Al2O3have been shown
to differ as a result of differences in the relative abilities of
the substrate to screen (final state effects) the outgoing pho-
toelectrons. In comparing the relative core level BE shifts,
SiO2(−1.3 eV shift from small cluster to bulk size) was
found to have a slightly greaterscreening ability than Al2O3
(−1.1 eV shift). Differences in the magnitudes of these Au
(A)
(B)
Figure 9. Plots of XPS BE peak centers of the Au 4f7/2core level as a func-
tion of Au cluster coverage (ranging from 0.02 ML to bulk) on TiO2(110)
(A) and SiO2(B) surfaces. The BE shift for SiO2(∼1.6 eV) is more pro-
nounced than for TiO2.
cluster core level BE shifts were interpreted to be a result
of the relative abilities of the metal oxide supports to shield
the final-state hole via extra-atomic relaxation. Accordingly,
the higher Au coverage (20 ML of Au on SiO2,figure 9(B))
required to reach the bulk BE value (as compared to 6 ML
for the Au/TiO2system; figure 9(A)) would signify SiO2’s
greater screening ability. In addition, the XPS linewidth,
which is also dependent upon relaxation, should increase
with a decrease in screening. This is the case in this present
XPS study. The full-width half-maxima (fwhm) of the Au
4f7/2peaks (not shown) increased with decreased Au cover-
age on both oxide surfaces and is thus indicative of final state
contributions. The fwhm at 0.02 ML and bulk Au coverages
are 2.1 and 0.8 eV, respectively, for the Au/SiO2system; for
the Au/TiO2(110) system these values are 1.7 and 1.0 eV,
respectively.
It should be noted, however, that the origin of metal clus-
ter size dependent BE shifts is a controversial subject. The
first observations of core-level shifts with metal cluster size
were attributed to a size dependence from the initial-state
electronic structure [99]. An increase of valence d electrons
(i.e., in Pd) with increasing cluster size were thought to be
the chief cause of the BE shift. Later work presented an
C.C. Chusuei et al. / Modeling heterogeneous catalysts 81
alternative interpretation that shifts in BE were not due to
initial state properties, but rather to variations in final state
relaxation processes [75,100,101]. In these current stud-
ies, there may be initial state contributions to the BE shift
due to differences in Au adsorption to various defect sites.
However, at present there is insufficient data (STM on the
Au/SiO2to compare with the Au/TiO2system) to make spe-
cific comments in this regard. The BE shifts that is observed
from Au deposited on the TiO2and SiO2substrates is likely
a convolution of both initial and final state effects. Perhaps
an interplay exists between the quantum size effect of the
clusters and their interaction with the underlying metal ox-
ide support to give rise to their unique activity. The selection
of the metal oxide support is also an important factor for en-
hanced catalytic activity. Clearly, interactions of the TiO2
and SiO2supports influence the Au cluster electronic struc-
ture.
3.5. Metal cluster sublimation energies
TPD is a useful tool for obtainingdetailed information on
adsorbate–surface bonding and on adsorbate–adsorbate in-
teractions, desorption kinetics and determining binding en-
ergies of metals adsorbed onto surfaces. TPD binding en-
ergy determinations also allow for comparative estimations
of admetal cluster size on different oxide supports. In a se-
ries of TPD spectra taken of Au on SiO2, marked decreases
of (adsorbed) Au cluster binding energies, denoted by the
peak temperature maximum (Tm), is observed and attributed
to varying cluster size. A 3 K/s linear heating rate had been
used to acquire these spectra. Figure 10 shows a family
of TPD spectra taken of the Au clusters deposited onto the
SiO2thin film previously examined by XPS (section 3.4).
The leading edge of the TPD peak maxima shifts to higher
temperatures as Au coverage increases. The inset shows a
plot of the sublimation energy (Esub)asafunctionofAu
coverage, which have been determined using leading edge
analysis [102]. At 0.2 ML, the Esub at ∼50 kcal/mol in-
creases rapidly (with increasing Au coverage) to the bulk
value at ∼90 kcal/mol at 5.0 ML. The decrease in Esub
can be explained by the fact that an atom at the edge of a
small cluster has fewer nearest neighbors than larger ones
and hence desorbs more easily due to decreased surface ten-
sion. Rodriguez et al. [48] used differences in Tmin the
TPD leading edges to show that Ag clusters are larger (and
reduced in density) when the same admetal surface cover-
ages were deposited onto an O/Mo(110) surface as com-
paredtoAl
2O3/Mo(110). This effect of decreased adsor-
bate bond energy that accompanies decreasing cluster size
has been demonstrated from Monte Carlo simulations of a
model 400 atom metal cluster island. The Tmof a family
of the calculated TPD spectra shifted to lower temperatures
with decreasing island size [103]. In another series of TPD
experiments, obtained by Xu et al. [104] to measure bond
energies of Au clusters deposited onto TiO2(001)/Mo(100),
the same Tmincrease in the TPD leading edge (5 K/s lin-
ear heating rate) at larger Au coverages was observed from
Figure 10. A set of TPD spectra of Au (m/e =197) on a 2.5 nm thick SiO2
thin film on Mo(110) at Au cluster coverages ranging from 0.2 to 5.0 ML.
The inset shows a plot of Esub determined from leading edge analysis.
1090 K at 0.2 ML to 1190 K at 2.0 ML. A slow decrease
in LEIS TiO2intensity even at high Au coverages (5.0 ML)
denoting 3D Au cluster growth on the surface was further
verified by STM images obtained from this surface. The
Esub reported for bulk Au in this Au/TiO2study was found
to be at ∼90 kcal/mol, in agreement with current results ob-
served for the Au/SiO2system (figure 10). The Tm’s from
the Au TPD spectra obtained from both oxide supports are
approximately equal. For instance, the Tm’s for Au on SiO2
are at 1100 K at 0.2 ML and 1190 K at 2.0 ML. Differ-
ences in the Tmdetermination of the 0.2 ML coverage (as
compared to 1090 K on the Au/TiO2) may be due to sta-
tistical variations that accompany low intensity TPD peaks.
At coverages less than 2.0 nm thick, the morphology of
the Au exists as a single layer. As the clusters grow to
from 2.0 to 4.0 nm thick, a second quasi-2D layer devel-
ops on top of the first. It is precisely in this region where
maximum catalytic activity is observed for CO oxidation
(figure 2(B), quasi-2D clusters denoted by the diamonds).
Above a 4.0 nm thick (1.0 ML) coverage, the clusters form
3D islands. This same 2-to-3D growth behavior had also
been observed from LEIS measurements performed in the
groups of Madey [64] and Campbell [78]. Since Au grows
in a 3D fashion (at coverages 1.0 ML) on both TiO2and
SiO2and from relative surface tension effect arguments (on
outer perimeter cluster atoms), the TPD spectra suggestthat
the Au islands deposited are the same size for both sur-
faces at equivalent Au dosings. It should be noted that this
size comparison from the TPD data is an approximate es-
timation. Detailed STM imaging for Au cluster on SiO2
to verify this result are currently in progress. Nevertheless,
this finding provides further support for the role of cluster–
substrate interactions (independent from cluster size factors)
for influencing the admetal electronic structure and account-
ing for differences in the observed XPS core level shifts
(figure 9).
82 C.C. Chusuei et al. / Modeling heterogeneous catalysts
4. Perspective
Model catalyst systems with surface sensitive methods
are a valuable methodology for probing the electronic and
morphological structure of supported metal clusters. From
detailed STM/STS studies, a physical basis for understand-
ing the enhanced catalytic activities of small, dispersed
metal is developing. Ostwald ripening is apparent at elevated
O2pressures. STM suggests that Al2O3, has fewer defects
than TiO2(110), resulting in increased resistance to cluster
ripening upon exposure to similar O2pressures. The dis-
persions of Ag and Au depend upon the underlying surface
structure (roughness, density of plateaus and defects). Rela-
tive differences in XPS core level BE shifts as a function of
cluster coverage also reveal that the chemical composition of
the underlying oxide support affects its electronic structure;
cluster size estimations from TPD analysis provide further
corroboration of the role of the support.
The above results obtained thus far signify an interplay
of admetal cluster size and the cluster–substrate interactions
responsible for catalytic activity. The use of metal clusters
supported on thin oxide films provides new insights into the
special electronic and chemical properties that govern their
unique catalytic chemistry. Future studies toward a more
in depth understanding of nanostructured supported clusters
will undoubtedly lead to practical catalytic applications of
these interesting materials.
Acknowledgment
We acknowledge with pleasure support of this work by
the Department of Energy, Office of Basic Energy Sciences,
Division of Chemical Sciences and the Robert A. Welch
Foundation. CCC gratefully acknowledges financial support
from the Associated Western Universities, Inc. and the Pa-
cific Northwest National Laboratories operated by Battelle.
References
[1] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet and
B. Delmon, J. Catal. 144 (1993) 175.
[2] M. Haruta, Catal. Today 36 (1997) 153.
[3] T. Hayashi, K. Tanaka and M. Haruta, J. Catal. 178 (1998) 566.
[4] J.-D. Grunwaldt and A. Baiker, J. Phys. Chem. B 103 (1999) 1002.
[5] M. Valden, S. Pak, X. Lai and D.W.Goodman, Catal. Lett. 56 (1998)
7.
[6] J.-D. Grunwaldt, C. Kiener, C. Wögerbauer and A. Baiker, J. Catal.
181 (1999) 223.
[7] Z.M. Liu and M.A. Vannice, Catal. Lett. 43 (1997) 51.
[8] F. Boccuzzi, A. Chiorino, S. Tsubota and M. Haruta, J. Phys. Chem.
100 (1996) 3625.
[9] E.D. Park and J.S. Lee, J. Catal. 186 (1999) 1.
[10] H. Liu, A.I. Kozlov, A.P. Kozlova, T. Shido, K. Asakura and Y. Iwa-
sawa, J. Catal. 185 (1999) 252.
[11] Y. Iizuka, H. Fujiki, N. Yamauchi, T. Chijiiwa, S. Arai, S. Tsubota
and M. Haruta, Catal. Today 36 (1997) 115.
[12] M.A.P. Dekkers, M.J. Lippits and B.E. Nieuwenhuys, Catal. Lett. 56
(1998) 195.
[13] S. Tsubota, T. Nakamura, K. Tanaka and M. Haruta, Catal. Lett. 56
(1998) 131.
[14] Y. Yuan, K. Asakura, H. Wan, K. Tsai and Y. Iwasawa, Catal. Lett. 42
(1996) 15.
[15] G.R. Bamwenda, S. Tsubota, T. Nakamura and M. Haruta, Catal. Lett.
44 (1997) 83.
[16] S.D. Lin, M. Bollinger and M.A. Vannice, Catal. Lett. 17 (1993) 245.
[17] S. Minicò,S.Sciré, C. Crisafulli, A.M. Visco and S. Galvagno, Catal.
Lett. 47 (1997) 273.
[18] M.A. Bollinger and M.A. Vannice, Appl. Catal. B 8 (1996) 417.
[19] N.W. Cant and N.J. Ossipoff, Catal. Today 36 (1997) 125.
[20] K. Fukushima, G.H. Takaoka, J. Matsuo and I. Yamada, Jpn. J. Appl.
Phys., Part I 36 (1997) 813.
[21] M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma and
M. Haruta, Catal. Lett. 51 (1998) 53.
[22] M. Valden, X. Lai and D.W. Goodman, Science 281 (1998) 1647.
[23] M. Valden and D.W. Goodman, Isr. J. Chem. 38 (1998) 285.
[24] M. Haruta, in: 3rd World Congr. Oxidation Catal. (Elsevier, Amster-
dam, 1997) p. 123.
[25] S. Cheng and A. Clearfield, J. Catal. 94 (1985) 455.
[26] V.I. Bukhtiyarov, A.I. Boronin, I.P. Prosvirin and V.I. Savchenko,
J. Catal. 150 (1994) 268.
[27] V.I. Bukhtiyarov, I.P. Prosvirin, R.I. Kvon, S.N. Goncharova and B.S.
Bal’zhinimaev, J. Chem. Soc., Faraday Trans. 93 (1997) 2323.
[28] A.N. Pestryakov and A.A. Davydov, Appl. Catal. A 120 (1994) 7.
[29] A.N. Pestryakov, Catal. Today 28 (1996) 239.
[30] D. Herein, H. Werner, T. Schedel-Niedrig, T. Neisius, A. Nagy, S.
Bernd and R. Schlögl, in: 3rd World Congr. Oxidation Catal. (Else-
vier, Amsterdam, 1997) p. 103.
[31] S. Lin, Catal. Lett. 10 (1991) 47.
[32] Y.A. Kalvachev, T. Hayashi, S. Tsubota and M. Haruta, in: 3rd World
Congr. Oxidation Catal. (Elsevier, Amsterdam, 1997) p. 965.
[33] D.G. van Campen and J. Hrbek, J. Phys. Chem. 99 (1995) 16389.
[34] D. Martin, F. Creuzet, J.Jupille, Y. Borensztein and P. Gadenne, Surf.
Sci. 377–379 (1997) 958.
[35] D. Abriou, D. Gagnot, J. Jupille and F. Creuzet, Surf. Rev. Lett. 5
(1998) 387.
[36] X. Lai, T.P. St. Clair, M. Valden and D.W.Goodman, Prog. Surf. Sci.
59 (1998) 25.
[37] X. Lai, T.P. St. Clair and D.W. Goodman, Faraday Discuss. 114
(1999) 279.
[38] K. Luo, T.P. St. Clair, X. Lai and D.W. Goodman, J. Phys. Chem. B
104 (2000) 3050.
[39] S.R. Seyedmonir, J.K. Plischke, M.A. Vannice and H.W. Young,
J. Catal. 123 (1990) 534.
[40] X. Li and A. Vannice, J. Catal. 151 (1995) 87.
[41] H. Kudo and T. Ono, Appl. Surf. Sci. 121/122 (1997) 413.
[42] G.R. Meima, M.G.J. V. Leur, A.J.V. Dillen and J.W. Geus, Appl.
Catal. 44 (1988) 133.
[43] G.R. Meima, L.M. Knijff, R.J. Vis, A.J. van Dillen, F.R. van Buren
and J.W. Geus, J. Chem. Soc., Faraday Trans. I 85 (1989) 269.
[44] G.R. Meima, M. Hasselaar, A.J. van Dillen, F.R. van Buren and J.W.
Geus, J. Chem. Soc., Faraday Trans. I 85 (1989) 1267.
[45] G.R. Meima, L.M. Knijff, A.J. van Dillen and J.W. Geus, J. Chem.
Soc., Faraday Trans. I 85 (1989) 293.
[46] G.R. Meima, R.J. Vis, M.G.J. van Leur, A.J. van Dillen, J.W. Geus
and F. van Buren, J. Chem. Soc., Faraday Trans. I 85 (1989) 279.
[47] C.-F. Mao and M.A. Vannice, Appl. Catal. A 122 (1995) 41.
[48] J.A. Rodriguez, M. Kuhn and J. Hrbek, J. Phys. Chem. B 100 (1996)
18240.
[49] J.-A. Wang, G. Aguilar-Ríos and R. Wang, Appl. Surf. Sci. 147 (1999)
44.
[50] G.A. Somorjai, Chemistry in Two Dimensions: Surfaces (Cornell
Univ. Press, Ithaca, NY, 1981).
[51] G. Ertl and J. Küppers, Low Energy Electrons and Surface Chemistry,
2nd Ed. (VCH, Weinheim, 1985).
[52] D.P. Woodruff and T.A. Delchar, Modern Techniques of Surface Sci-
ence, 2nd Ed. (Cambridge Univ. Press, Cambridge, 1994).
[53] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis (Wi-
ley, New York, 1994).
C.C. Chusuei et al. / Modeling heterogeneous catalysts 83
[54] Q. Guo, W.S. Oh and D.W. Goodman, Surf. Sci. 437 (1999) 49.
[55] J.-W. He, X. Xu, J.S. Corneille and D.W. Goodman, Surf. Sci. 279
(1992) 119.
[56] X. Xu and D.W. Goodman, Appl. Phys. Lett. 61 (1992) 774.
[57] X. Xu and D.W. Goodman, Surf. Sci. 282 (1993) 323.
[58] X. Xu, J.-W. He and D.W. Goodman, Surf. Sci. 284 (1993) 103.
[59] X. Xu, H. Bojkov and D.W. Goodman, J. Vac. Sci. Technol. A 12
(1994) 1882.
[60] P.J. Chen and D.W.Goodman, Surf. Sci. Lett. 312 (1994) L767.
[61] M.-C. Wu and D.W. Goodman, J. Phys. Chem. 98 (1994) 9874.
[62] C. Xu, X. Lai and D.W. Goodman, Faraday Discuss. 105 (1996) 247.
[63] C. Xu, X. Lai, G.W. Zajac and D.W. Goodman, Phys. Rev. B 56
(1997) 13464.
[64] L. Zhang, R. Persaud and T.E. Madey, Phys. Rev. B 56 (1997) 10549.
[65] Q. Guo, S. Lee and D.W. Goodman, Surf. Sci. 437 (1999) 38.
[66] L.C. Feldman and J.W. Mayer, Fundamentals of Surface and Thin
Film Analysis (North-Holland, New York, 1986) ch. 6.
[67] A.J. Melmed, J. Vac. Sci. Technol. B 9 (1991) 601.
[68] R.A. Campbell and D.W. Goodman, Rev. Sci. Instrum. 63 (1992) 172.
[69] J. Szanyi and D.W. Goodman, Rev. Sci. Instrum. 64 (1993) 2350.
[70] P.N. First, J.A. Stroscio, R.A. Dragoset, D.T. Pierce and R.J. Celotta,
Phys. Rev. Lett. 63 (1989) 1416.
[71] L. Bolotov, T. Tshuchiya, A. Nakamura, T. Ito, Y. Fujiwara and Y.
Takeda, Phys. Rev. B 59 (1999) 12236.
[72] X. Lai and D.W. Goodman, Mol. Catal. A, in press.
[73] W.S. Oh, C. Xu, D.Y. Kim and D.W. Goodman, J. Vac. Sci. Technol.
A 15 (1997) 1710.
[74] R. Unwin and A.M. Bradshaw, Chem. Phys. Lett. 58 (1978) 58.
[75] W.F. Egelhoff, Jr. and G.G. Tibbetts, Phys. Rev. B 19 (1979) 5028.
[76] T.L. Barr, Modern ESCA (CRC Press, Boca Raton, 1994) ch. 9.
[77] E.H. Voogt, A.J.M. Mens, O.L.J. Gijzeman and J.W. Geus, Surf. Sci.
350 (1996) 21.
[78] V.A. Bondzie, S.C. Parker and C.T. Campbell, J. Vac. Sci. Technol. A
17 (1999) 1717.
[79] P. Wynblatt, R.A. Dalla Betta and N.A. Gjostein, in: The Physical
Basis for Heterogeneous Catalysis (Plenum, New York, 1974) p. 501.
[80] P. Bera, K.C. Patil, V. Jayaram, M.S. Hegde and G.N. Subbanna,
J. Mater. Chem. 9 (1999) 1801.
[81] G.C. Bond, Heterogeneous Catalysis: Principles and Applications,
1st Ed. (Oxford Univ. Press, Oxford, 1987).
[82] B.C. Gates, Catalytic Chemistry (Wiley, New York, 1992).
[83] W. Göpel, J.A. Anderson, D. Frankel, M. Jaehnig, K. Phillips, J.A.
Schäfer and G. Rocker, Surf. Sci. 139 (1984) 333.
[84] Q. Guo, I. Cocks and E.M. Williams, Phys. Rev. Lett. 77 (1996) 3851.
[85] M.A. Henderson, Surf. Sci. 400 (1998) 203.
[86] J.-M. Pan and T.E. Madey, J. Vac. Sci. Technol. A 11 (1993) 1667.
[87] U. Diebold, J.-M. Pan and T.E. Madey, Surf. Sci. 331/333 (1995) 845.
[88] U. Diebold, J.-M. Pan and T.E. Madey, Phys. Rev. B 47 (1993) 3868.
[89] H.-P. Steinrück, F. Pesty, L. Zhang and T.E. Madey, Phys. Rev. B 51
(1995) 2427.
[90] S.C. Parker, A.W.Grant, V.A. Bondzie and C.T. Campbell, Surf. Sci.
441 (1999) 10.
[91] K.H. Ernst, A. Ludviksson, R. Zhang, J. Yoshihara and C.T.
Campbell, Phys. Rev. B 47 (1993) 13782.
[92] G.A. Somorjai and M.A. van Hove, Catal. Lett. 14 (1988) 433.
[93] R.D. Levine and G.A. Somorjai, Surf. Sci. 232 (1990) 407.
[94] G.A. Somorjai, Langmuir 7 (1991) 3176.
[95] G.A. Somorjai, Surf. Sci. 242 (1991) 481.
[96] F. Parmigiani, E. Kay, P.S. Bagus and C.J. Nelin, J. Electron Spec-
trosc. Relat. Phenom. 36 (1985) 257.
[97] M.P. Seah, Surf. Interface Anal. 14 (1989) 488.
[98] M.G. Mason, Phys. Rev. B 27 (1983) 748.
[99] R.C. Baetzold, M.G. Mason and J.F. Hamilton, J. Chem. Phys. 72
(1980) 366.
[100] W.F. Egelhoff, Jr. and G.G. Tibbetts, Solid State Commun. 29 (1979)
53.
[101] M.K. Bahl, S.C. Tsai and Y.W. Chung, Phys. Rev. B 21 (1980) 1344.
[102] E. Habenschaden and J. Küppers, Surf. Sci. 138 (1984) L147.
[103] R.I. Masel, Principles of Adsorption and Reaction on Solid Surfaces
(Wiley, New York, 1996) pp. 538–540.
[104] C. Xu, W.S. Oh, G. Liu, D.Y. Kim and D.W. Goodman, J. Vac. Sci.
Technol. A 15 (1997) 1261.
