Download Structural and Chemisorptive Properties of Model Catalysts: Copper

J. Phys. Chem. 1993,97, 683689
683
Structural and Chemisorptive Properties of Model Catalysts: Copper Supported on Si02
Thin Films
Xueping Xu and D. W . Goodman'
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255
Received: September 8, 1992; In Final Form: November 9, 1992
The structural and chemical properties of model silica-supported copper catalysts have been investigated with
infrared reflection-absorption spectroscopy and temperature-programmed desorption using C O as a probe
molecule. The isosteric heats of adsorption of carbon monoxide on the model catalysts were measured at
pressures between 10-8 and 10-3 Torr and temperatures between 180 and 300 K. The model catalysts were
prepared by evaporating copper onto a planar silica thin film (- 100 A) which, in turn, was supported on a
Mo(110) surface. When deposited at 100 K, copper initially forms a two-dimensional structure on silica;
however, annealing induces the ultrathin copper films (<3 ML)to form small clusters. The unannealed copper
films have a significant density of low-coordinated copper sites, whereas the annealed films consist of copper
clusters with structures similar to low-index [( 1 1 l), (1 IO)] and high-index copper planes [(211) or (3 1l)], The
distribution of the facets depends upon the initial copper coverage. The dispersion of the copper on the planar
silica films is high, -0.5, for the annealed films with Bcu 2 ML. Carbon monoxide desorbs in a single peak
centered a t -210 K from the unannealed copper film, but in several peaks centered between 150 and 220 K
from the annealed film. The CO adsorption energy strongly depends on the C O coverage, varying from 17
kcal/mol at the low coverage limit to 10 kcal/mol near C O saturation. The copper structure, the dispersion,
the particle size distribution, and the heats of C O adsorption for the model systems are remarkably similar to
the corresponding values for silica-supported copper catalysts prepared by the ion-exchange method. These
similarities demonstrate that the method of preparation described here using a dispersed metal on an oxide film
produces an excellent model for the analogous supported metal catalyst.
-
-
Introduction
Model catalysts used in conjunction with surface science
techniques have provided valuable new insights into many
fundamental issues of catalysis.' Typically,single-crystal metal
surfaces have served as model catalysts in that transition and
noble metals are the components of many important catalyst
systems. However, practical metal catalysts are usually supported
on high surface area oxides. It is well recognized that there can
be metal-support interactionsthat alter catalytic properties? even
for relatively inert supports such as silica.3 To investigate these
metal-support interactions, oxides have been deposited ontosinglecrystal metal surfacesand their modified reactivities studiedusing
surfacescience techniq~es?,~
The metal catalysts used in industry,
however, are dispersed on the support material; therefore, a
preferred approach to model these systems involves metal
deposition onto the support material. There have been relatively
few investigations of metals on oxide supportsdue to experimental
difficulties associated with surface charging, sample mounting,
and sample heating/cooling.6-8
Recently, we have developed a new approach to modeling
supported metal catalysts that circumvents these experimental
difficulties? This approach utilizes the deposition of metals onto
a silicon dioxide thin film (- 100A), which, in turn, is supported
on a Mo( 110) surface. In addition to supporting the Si02 thin
film, the Mo( 110) substrate provides a metallic reflecting surface
which facilitates the use of infrared reflection-absorption spectroscopy ( I U S ) . The Mo(110) substrate, however, does not
alter the chemical properties of the -100-A Si02 film. The
Si02 film was prepared by evaporating silicon onto Mo(110) in
-1 X 10-5 Torr of oxygen followed by an anneal to 1500 K.'O
Auger electron, X-ray photoelectron, electron energy loss,infrared
reflection-absorption, and thermal desorption spectroscopies show
that the Si02 films have similar electronic and vibrational
properties to those of fused silica, and that these films are stable
to >1600 K . l i J 2 Therefore, the Si02 thin film provides a good
model for a silica support.
In previous studies, we have found that the initial sticking
probability of copper on silica film is significantly less than unity,
-0.6 at 90 K and less than 0.1 at a surface temperature of >400
K.13 During temperature-programmed desorption,copper desorbs
from the silica film in two peaks: a metallic copper sublimation
peak at -9W1200 K and a peak at 1200-1400 K derived from
a strongly adsorbed copper species! The initial desorption energy
for copper sublimation strongly depends on the effective copper
coverage, ranging from -60 kcal/mol for 0.5monolayers (ML),
Bcu = 0.5,to 75 kcal/mol for 8cU= 7 ML, and approaching the
value of the copper sublimation energy of 79.3 kcal/mol14 for
higher copper coverages. The strongly adsorbed copper species,
which is partially oxidized, corresponds to an effective copper
coverage less than 0.1 ML. Carbon monoxide does not adsorb
on the partially oxidized copper species at 90 K under ultrahighvacuum ~ n d i t i o n s . ~
In this work, we have investigated the structural and chemisorptive properties of model silica-supported copper catalysts,
whose industrial counterparts are used in methanol synthesis and
water gas shift reactions.15 Infrared reflection-absorption spectra
of adsorbed carbon monoxide has been used to address the
structure of the supported copper on the silica film, as well as to
measure the isosteric heats of carbon monoxide adsorption on
these model copper catalysts. The results are compared to the
corresponding measurements on copper single-crystal surfaces
and silica-supported copper catalysts. Scanning tunneling microscopic (STM) studies of the copper/silica/Mo( l 10) model
system have been described elsewhere.I6
Experimental Section
The experiments were performed in an ultra-high-vacuum
chamber equipped with an Auger electron and quadrupole mass
spectrometersand a contiguous high-pressureinfrared cell. This
0022-3654/58/2091-0683S04.00/0 Q 1993 American Chemical Society
Xu and Goodman
684 The Journal of Physical Chemistry, Vol. 97, No. 3, I993
TPD, W/CU/SD~/MO(~~O)
@ TPD,
e(cu)-1.1ML
CO/Cu/SiO
Annealed to 700 K
-
J
CO/CUlSi(lWA
e(cu)
15 ML
\
,/
15ML
7ML
AI
n
CL
F 10-
I/
I=
I
I
I
I
8
I A
0
,
I
I
,
l
l
l
5-
1.1 ML
\
/
l
t
l
l
?
1
,
100
300
500
700
900
Annealing Temperature (K)
Figure 2. Integrated TPD area for CO on silica-supported copper as a
function of the preannealing temperature. The experimental conditions
were the same as those in Figure 1.
1 .o
1 Cu/SiO, (100A)/Mo(I IO)
1o---
3 R I F . d I ( 1 ML
I4Gj-3.4 ML
Annealed to 700 K
.-0
~
0 S(Cu)-15ML
o,8[OS-
2
4 0 4
0,
Anne
0.01
0
I
I
I
2
4
6
I
8
I
I
I
10
12
14
Copper Coverage (ML)
Figure 3, Dispersion of copper on silica as a function of the effective
copper coverage. The inset shows the dispersion as a function of the
annealing temperature for three copper films.
Figure 1b shows the CO TPD spectra as a function of copper
coverage, subsequent to an anneal to 700 K followed by a
saturation CO exposure at 100 K. At low copper coverages, CO
desorbs in two peaks with peak maxima located at 160 and 180
K. As the copper coverage is increased, a new CO TPD feature
develops at -210 K.
The copper films nucleate into 3D clusters upon annealing.
The saturation coverage of CO, determined by TPD, decreases
with an increase in the sample annealing temperature (Figure 2).
The copper surface area (based on the CO TPD peak area)
decreases approximately by a factor of 2 as the film is annealed
from 100 to 900 K. In addition, the pronounced changes in the
CO TPD spectra (Figure la) and in the IR reflection-absorption
spectra (described below) indicatesthedegreeof structural change
taking place in the supported copper upon heating.
The dispersion of the copper (Figure 3) was determined using
temperature-programmeddesorption. The total amount of copper
can be accurately defined by desorbing the Cu from silica and
referencing its corresponding TPD peak area to the Cu TPD
peak area obtained from 1 ML of copper on Mo(llO).I3 The
surface copper also can be measured with CO TPD, assuming the
saturation coverage of CO on Cu to be -0.5 ML.'a-20 A planar,
multilayer copper film was used to establish the reference for the
copper CO TPD area. An atomic force microscopic examination
of a 30 A thick copper film (annealed to 700 K) shows it to be
continuous.16 The copper surface area for the 30-A film was
-
ReSultS
A. Temperature-F'rogrammedDesorption of Carboo Monoxide.
Figure 1 shows temperature-programmed desorption spectra for
CO adsorbed on the model silica-supported copper catalysts.
Copper was deposited onto the silica film at 100 K and annealed
to the indicated temperatures. The sample was then cooled to
90 K and CO adsorbed to saturation. For all preparations, CO
desorbs from the supported copper in the temperature range 140240 K. Figure l a shows the CO TPD from a copper film (1.1
ML) as a function of the annealing temperature. A single CO
TPD peak is observed at -210 K for the unannealed copper
sample. For the annealed (500-900 K) sample, CO TPD exhibits
two peaks at 160 and 180 K. The CO TPD spectra for other
copper coverages (0.5-60 ML) are qualitatively similar to those
shown in Figure la. The unannealed copper samples exhibit a
single CO TPD peak at -220 K, whereas the TPD for the
annealed films shows several peaks between 160 and 210 K. The
peak desorption temperature of CO from the as-deposited copper
film is higher than that from the annealed samples.
Silica-Supported Copper Catalysts
00-6
0.25%
I
L
I
A
2099
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 685
CO/CU/S~O~
(1
e(cu)-2.2 ML
Ts-90 K
-PI
A
1
c o / C u / S i ~(I, 00 A)
Cu deposited at 100 K
I
2106~-C1l
41
I fi
I--
I
2200
, 2108
, 2150
2072
700K
~
-
2100
2050
,900Ki
2000 1950
Frequency (cm -')
Figure4. Infrared reflection-absorptionspectra of CO on silica-supported
copper as a function of the preannealing temperature. Copper was
deposited a t 100 K and annealed to the indicated temperatures, followed
by CO adsorption to saturation at 90 K.
k
&
'?
j:
0.5 ML
0.3 ML
2091 -
2200
2150
2100
2050
2000
1950
Frequency (cm-')
Figure 5. IRAS of CO on silica-supported copper as a function of copper
coverage. Copper was deposited a t 100 K, followed by CO adsorption
to saturation.
CO/Cu/SiO2(100 )
-50% larger than for a 70-A film (annealed to 700 K), whereas
the surface area for the 70-A film was the same as that for 150-A
copper film (annealed to 700 K). Accordingly, the 150-A copper
film was considered to be flat and to have an atomic density of
-1.4 X lOI5 atoms/cm2.21 Any roughness in the thick copper
film would lead to an underestimation of the copper dispersion.
Sincethe highest dispersion calculated was 0.83 for an unannealed
1.1-MLcopper film, the error in the copper dispersion is less than
20%.
Figure 3 shows the copper dispersion as a function of the
effective copper coverage for copper films deposited at 100 K and
annealed to 700 K. The dispersion is high (0.5) for copper
coverages less than 1.5 ML and gradually decreaseswith increase
in the copper coverage. The dispersion of the unannealed film
isevenhigher, -0.8 fora 1.1-MLcopperfilmat 100K(theinset
of Figure 3), suggesting that copper wets silica at 100 K.
Annealing the film decreases the copper dispersion, consistent
with the formation of larger 3D copper clusters.
B. Infrared Reflection-Absorption Spectra (W).The
structure of copper on silica was investigated with IRAS using
CO as a probe molecule. Figure 4 clearly shows the structural
changes in the supported copper (2.2 ML) as a function of the
anneal temperature. On the as-deposited (100 K) copper film,
CO exhibits an IR absorption band centered at 2099 cm-I with
a small shoulder on the low-frequency side of the peak. Upon
annealing to 300-500 K, the 2099-cm-l band shifts to 2097 cm-I
and a new band appears at 2070 cm-I. Further heating to 700900 K results in a splitting of the 2097-cm-1 feature into two
peaks at 2108 and 2094 cm-I. These CO absorption bands are
attributed to several distinct atop CO adsorption sites, since
bridging and hollow adsorptionsites on Cu( 111) have CO stretch
frequencies in the range 1814-1834 cm-1.22.23
The structure of the supported copper also depends on the
copper coverage. Figure 5 shows the IRAS for saturation CO
on theunannealedcopper samplesas a function of copper coverage.
At very low effective copper coverages (0.3 ML), two CO IR
absorption bands are observed at 2091 and 2049 cm-I. As the
coverage increases, the high-frequency band gradually blue-shifts
to 2106 cm-1 concomitantly with an increase in its intensity,
whereas the intensity of the 2049-cm-1 band remains essentially
Frequency (cm - l )
Figure 6. IRAS of CO on silica-supported copper as a function of copper
coverage. Copper wasdepositedat 100Kand annealed to 900 K, followed
by C O adsorption to saturation at 90 K.
unchanged. At higher copper coverages (>2.2 ML), the lowfrequency band appears as an asymmetric tail. The full width
at half-maximum (fwhm) of the CO feature is -20 cm-I, much
larger than the intrinsic - 5 cm-l width of CO on single crystal
copper surfaces.20
Figure 6 further demonstrates the effects of copper coverage
and the anneal temperature on the structure of the supported
copper. The CO IRAS spectra (Figure 6) for the annealed (900
K) copper films are dramatically different from the unannealed
films (Figure 5 ) . On low-coverage copper films (<3.4 ML), CO
exhibits three resolved IR absorption bands at -2108,2094, and
2076 cm-I. The lowest frequency CO band gradually shifts from
2062 cm-1 on a 0.3-ML copper sample to 2076 cm-1on a 3.4-ML
copper film. More apparent, the intensity of the 2094-cm-l band
Xu and Goodman
686 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
TABLE I: Literature Values of CO Stretching Frequencies
on Copper SingleCrystal Surfaces and Films
surface
low 8co
Cu(ll1)
Cu(ll0)
Cu(100)
Cu(211)
Cu(311)
Cu(755)
Cufilms
2080
2088
2079
2109/2095
2109/2093
2111
2102
a
frequency (cm-I)
medium 8coa saturn Bco
2074
2093
2085
2100
2102
2098
2102
2070
2094
2088
2110
2104
2106
2102
1I
0.03%
ref
A
CO/Cu(O.5 ML)/Si02(100A)
Annealed to 500 K
Ts-90 K
CO Exposure
A
100 L
-
18,22-24
19,24-26
20,24,27
24
24
24
24,28
CO coverage at maximum surface potential.
grows with the copper coverage, whereas the 21 10-cm-l band
remains approximately the same and gradually appears as a
shoulder on the 2094-cm-1 feature. On a medium-coverage (1 5
ML) copper film, the 2094-cm-1 CO absorption band has
disappeared, replaced by an intense IR absorption at 2102 cm-I
and two small unresolved shoulders at 2087 and 2068 cm-I. The
fwhm for the 2102 cm-1 band is -14 cm-1, narrower than that
for the unannealed film. On the high-coverage (>30 ML) copper
samples, CO exhibits three resolved IR absoprtion features at
2102,2087, and 2076 cm-I.
The stretch frequenciesfor CO adsorbed on the annealed copper
films are remarkably similar to the atop CO features found on
single-crystal copper surfaces at similar CO saturation conditions
(Table I). CO adsorption on copper single-crystal surfaces has
been extensivelydocumented;ls-20~22-28
thus, using the thud column
of Table I for saturation CO adsorption, the resolved IR bands
in Figure 6 can be assigned to CO adsorption on specific singlecrystal facets. Accordingly, the 2074- and 2087-cm-l bands are
assigned to CO adsorption on the (111) and (100) facets,
respectively,of the thick copper films (>15 ML). The 2102-cm-l
band is attributed to step/edge sites of the polycrystalline copper
film. For very thin films (<15 ML), the copper clusters are
relatively small (<200 A) and thus may not be crystalline.
Nevertheless, the well-defined CO IR absorption bands suggest
that the thin films have surface structural similar to the (1 10)plane (2094 cm-I), the (1 11)-plane (2074 cm-I), and the highindex (211) and (311) planes (2108-2110 cm-I). The IR
absorption band at 2062 cm-I, observed at very low copper
coverages (<0.5 ML), is lower than that for CO on any extended
surfacesand is attributed to a particle size effect (see Discussion).
It has been shown that the CO coverage influences the CO
stretch frequency via a dipole coupling and a chemical shift
mechanism.19,29q30
It is noteworthy that the frequency shift trends
depend upon the specific crystal orientation. Figure 7 shows the
IRAS of CO on a 0.5-ML copper film as a function of the CO
exposure. Thedataareconsistent with (1 11)-likeandother highindex facets composing the majority facets at low copper coverages.
At low exposures (<1 L), two CO IR absorption bands are
observedat 2108and2083cm-I. AstheCOexposureisincreased
(1-14 L), the intensity of the 2108-cm-l feature increases and
gradually red-shifts to 2099 cm-1. The intensity of the 2083-cm-1
band also increases as the peak broadens and red-shifts to 2060
cm-I. A further increase in the CO exposure to 100 langmuirs
causes an abrupt blue-shift of the high-frequency band from 2099
to 2107 cm-I. The frequency shift with coverage of the band at
2099-2108 cm-l on the supported copper is essentially identical
to that shift observed on the high-index Cu(211) and Cu(755)
~urfaces.2~
The IR absorption band for CO on Cu(755) shifts
continuously from 21 11 cm-I at the lowest Cu coverage to 2098
cm-I at the surface potential maximum and then shifts to a higher
frequency (2106 cm-I) with the growth of the compressed CO
str~cture.2~
The IRAS for CO on Cu(211) and Cu(3 11) exhibits
features very similar to those observed for CO on Cu(755), with
the exception of additional bands at -2095 cm-1.24
Figure 8 further illustrates thedependence of the IRAS on the
CO coverage. Carbon monoxide was adsorbed at 90 K on a
2150
2100
2050
2000
Frequency (cm-1)
Figure 7. IRAS of C O on 0.5-ML copper supported on silica as a function
of C O exposure.
190 K
I
2150
2100
I
I
I
2050
2Ooo
Frequency (cm -1)
Figure 8. IRAS of C O on 2.2-ML copper supported on silica as a function
of temperature. Copper was deposited a t 100 K and annealed to 900 K,
followed by C O adsorption to saturation at 90 K. The same was then
warmed to the indicated temperature.
2.2-ML copper film to saturation and the surface temperature
then gradually raised. At 90 K,three resolved CO IR absorption
bands are observed at 2108,2094, and 2074 cm-I. The two highfrequency bands gradually merge into a single peak at 2097 cm-'
during an anneal to 130 K, and the 2074-cm-I band shifts slightly
to 2070 cm-I. The total IR absorbancedecreasesslightly between
90 and 130 K. On increasingthe temperature to 140-1 50 K,the
IR absorbance decreases due to CO desorption. The 2099-cm-1
band red-shifts somewhat to 2097 cm-I, whereas the 2070-cm-1
band blue-shifts slightly to 2077 cm-I. Upon further annealing
the sample to 190 K,the intensity of the high-frequency band
decreases significantly and the peak position gradually shifts to
2 104 cm-I .
Silica-Supported Copper Catalysts
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 607
103 -
5 10'-
h
t
e?
3
v)
v)
e?
I
10.'
10.8
1o-6
1o
.~
1o.4
1o
.~
CO PRESSURE (Torr)
Figure 9. A series of isotherms for CO adsorption onto 2.0-ML copper
supported on silica. IRAS intensity was used to monitor the COcoverage.
1
8
$1
10'6I
4.0
I
co/cu/siq(io0 A)/Mo(~io)
3.5
/
I
I
4.5
I
5.5
5.0
I
6.0
1000/T ( K - ' )
B(Cu)-O.B ML
%I-
\
\
\
/
I
Figure 11. Adsorption isosteres for C O on 2.0-ML copper supported on
silica at three CO coverages.
18
L
O(Cu)=l5 ML
Integrated CO TPD Area (Arbitrary Unit)
Figure 10. Relationship between the IRAS intensity for CO on copper/
silica (100 A)/Mo(I 10) and the amount of CO adsorbed for two copper
films. The amount of CO was determined by temperature-programmed
desorption and was not normalized to the copper surface area.
c
a
10
I
The shift of the 2070 cm-1 band with temperature (coverage)
in Figure 8 is consistent with this band corresponding to CO
adsorption onto Cu( 111)-like facets. The IR absorption band
for CO on Cu( 111) appears at 2078 cm-l at very low coverages,
red-shifts slightly to 2074 cm-I at CO coverage of 0.33 ML,and
decreases to 2070 cm-1 at a saturation CO coverage of 0.52
ML.18.22The bands at 2094 and 2108 cm-I are derived from two
majority facets, as is demonstrated in Figure 6. As the
temperature is increased (such that the CO coverage decreases
slightly), the IR bands for CO adsorption onto the high-index
facets abruptly red-shifts 10 ~ m - ' , 2causing
~
these two bands
to merge into one.
C. Isosteric Heats of Adsorption of CO. The isosteric heats
of CO adsorption on the model silica-supportedcopper catalysts
were measured using infrared reflection-absorption as a monitor
of CO coverage. The isosteric heats of adsorption of CO on
Cu(100) with CO pressure up to 10 Torr have been measured
previously using IRAS.3' Figure 9 shows the adsorption isotherms
of CO on an annealed copper film (2 ML)supported on silica.
The data were collected in the pressure range of 10-8-10-3 Torr
and in the temperature range of 175-300 K. The isotherms are
expressed as a plot of integratd IR absorbance versus the
logarithm of the CO pressure.
The IR reflection-absorption intensity depends on both CO
coverage and the copper cluster size. Figure 10 shows the
relationship between the intensity of the CO IR band and the
amount of CO adsorbed (measured by the CO TPD area). The
IR intensity is not linear with CO TPD area for the copper
coverages larger than 1 ML.The CO coveragesfor the adsorption
isotherms are determined from IR intensities and calibrated by
-
0.0 0.1
I
0.2
0.3
I
0.4
I
I
I
I
I
0.5
0.6
0.7
0.8
0.9
.o
CO Coverage e()/,
Figure 12. Isosteric heats of adsorption for C O on silica-supported copper
as a function of the C O coverage for two different copper coverages.
the IR/TPD curves similar to those in Figure 10 for the specific
copper coverages.
The isosteric heats of adsorption ( M a d s ) were obtained from
the slope of the isosteric plots of log (Pco) versus 1/ T a t constant
CO coverage (Figure 1l), according to the Clausius-Clapeyron
equation:
Mads
= 2+303Rd(log
PC,)/d(
/ nI.9
Figure 12 shows the isosteric heats of adsorption of CO on the
model silica-supported copper catalysts as a function of CO
coverage. Data for two copper coverages are shown, one for a
relative thick polycrystalline copper film (1 5 ML)and the other
for a 2-ML copper film annealed to 700 K. The isosteric heats
of adsorption decreases with an increase in the CO coverage, in
agreement with previous work on single-crystal
Within experimentalerror, the heats of adsorption do not change
with copper coverage at the same CO coverage.
Discussion
The data presented in the previous section clearly establish
that the structure of copper supported on silica depends on both
the copper coverage and the annealing temperature. The
dispersion of the unannealed copper is very high, -0.8 for
monolayer films (Figure 3),indicating that the copper initially
Xu and Goodman
688 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
forms two-dimensional structures during deposition on silica at
100 K. Annealing inducesthe formation of 3D Cu clusters, whose
surface area and dispersion decrease with the annealing temperature (Figure 2). Furthermore, the annealed copper films
have several distinctive CO IR absorption bands corresponding
to CO adsorption onto various copper crystalline facets, whereas
the unannealed films exhibit a single band.
- The single IR absorption band at -2100 cm-I for the
unannealed copper films indicates that the morphologies of the
unannealed films are quite similar. Temperature-programmed
desorption show only one CO desorption peak on the unannealed
copper films, whereas several peaks are seen for the annealed
films. However, dipole coupling between CO molecules tends to
transfer the IR absorption intensity of the low-frequency features
to the high-frequency ones when these dipoles are adjacent to
each ~ t h e r . ~ The
~ . ~magnitude
'
of the intensity transfer inversely
depends on the separation in distance and frequency between the
dipoles.33 The low-frequency tails in Figure 5 demonstrate that
the CO adsorption sites are not all identical on the unannealed
copper films.
It is noteworthy that, at low effective copper coverages (<1
ML), the unannealed copper films exhibit two resolved CO IR
band at 2050 and 209 1 cm-' (Figure 5). This indicates that there
are at least two types of small copper islands on the silica surface
at copper coverages less than 1 ML. Otherwise, dipole coupling
and intensity transfer could lead to only one CO band.33 As the
copper coverage is increased, the small islands are merged into
an extended smooth surface, and intensity transfer causes the
2049-cm-I band to appear as a tail (Figure 5). On the basis of
CO stretch frequencies, we propose that copper atoms are initially
aggregated into (1 1 1)-like islands (2050 cm-I) and islands with
a surface similar to the high-index planes during deposition of
Cu onto silica at 100 K.
Annealing thecopper films causes the formation of 3D clusters.
The well-resolved CO IR absorption bands suggest that there are
severaltypes of isolated copper clusters for the annealed ultrathin
(0.3-3.4 ML) copper films (Figure 6). If the annealed film is
continuous, IR intensity transfer will diminish the resolution,
producing a singleband. Indeed, scanning tunneling microscopic
studies revealed several unique cluster structures for a 1 .O-ML
film.I6 For a medium coverage film (-15 ML), atomic force
microscopy showed that the surface consisted of a continuous
copper grain of -200A.16 IRAS for CO on this surface exhibits
a large peak at 2102 cm-1 with two unresolved shoulder at low
frequencies (Figure 6). The CO vibrational frequencies at 2074,
2094, and 2 108 cm-I suggest that the copper clusters have surface
structures resembling (1 1 l), (1 lo), and other high-index planes
[(211) and (311)] of copper single crystals (Table I). Indeed,
a copper cluster with a (3 1 1) surface structure was observed with
STM.16
The distribution of copper clusters for the annealed film varies
with coverage (Figure 6). At very low copper coverages (<0.5
ML), the annealed films mainly consist of (1 1 1)-like and (21 1)/
(31 1)-like facets. As the copper coverage is increased, a (1 10)like facet grows and becomes the major facet at an effective
coverage of 3.4 ML. On increasing the Cu coverage to 15 ML,
the copper film become continuous,I6 and the IRAS spectrum
exhibits a CO feature corresponding to adsorption on step/edge
sites (2102 cm-I). The adsorption bands corresponding to (1 11)
and (100) facets (2074 and 2087 cm-I) only appear as shoulders
due to intensity transfer to the 2102 cm-1 band. With a further
increase in the copper coverage to 65 ML, the intensity of the
2076- and 2087-cm-l bands shows the growth of the (1 11) and
(100) facets. At this coverage, the (1 11) or (100) facet is large
enough such that the IR intensity transfer to step/edge sites no
longer dominates.
The nature of the preferential crystallinefacet growth of copper
on silica is not well understood. Minimization of the surface free
-
energy of the metal particles and the substrate atomic structure
are the important contributing factors. For the thick copper film
(65 ML), the majority facets are (11 1 ) and (100) due to their
lower surface free energy. For the low copper coverages (small
particle sizes), the structure of the silica substrate may play a
role. Although the silica film is macroscopically disordered, the
silica surface structure on a small scale may be ordered, and may
resemble certain copper facets, servingas a template for nucleation
of copper atoms.
CO on evaporated copper films has been studied previously by
IRAS.24J8 However, only a single band at -2102 cm-1 was
observed in the earlier studies, in contrast to this work where
several well-resolved IR bands were detected. This discrepancy
is attributed to the method of preparation of the copper films. In
the previous work, copper films were evaporated onto the substrate
at 1300 K. Figure 4 clearly shows that annealing to 700 K is
required in order to produce several resolved IR bands. In
addition, the copper coverage plays an important role in
determining the IR spectra of adsorbed CO.
It is noteworthy that a CO IR absorption band at 2050-2060
cm-I is observed for ultrathin copper (0.3 ML) on silica. This
band has a much lower frequency than that for CO on singlecrystal copper surfaces. It is proposed that this unusual lowfrequency band at 2050-2060 cm-] is due to the fact that dipole
coupling on very small particles is less than that on extended
surfaces, whereas the chemical interaction remains essentially
the same. It is well recognized that the shift of the CO vibrational
frequency with respect to the CO coverage has two components,
a dipole shift and a chemical
On copper single crystals
and on copper fiims, the dipole coupling and chemical shift
virtually cancel each other, resulting in a small CO frequency
shift (<lo cm-I) with coverage. For example, the dipole shift is
26 and 53 cm-1, and the chemical shift is -27 and - 6 3 cm-l for
CO on Cu( 1 11) at coverages of 0.33 and 0.52 ML, respectively.18
Calculations have shown that the dipole shift decreases with the
size of the CO islands on the extended Cu( 1 1 1 ) surface, from 25
cm-l for a d 3 X d 3 infinite CO overlayer, to 18 cm-' for a -27
A diameter CO ( d 3 X d 3 ) island on Cu( 1 1 l), to 9 cm-l for a
-9 A diameter CO island.34 Therefore, if the copper clusters
on silica are sufficientlysmall, thedipolecoupling and the resulting
shift should be smaller than that on the extended surface. That
is, the CO island sizes are limited by the copper clusters. As a
result, the CO stretch frequency decreases with smaller cluster
sizes. Scanning tunneling microscopic studies have revealed that
a -0.6-ML film has a particle size of 10-30 & I 6 in the range
where the dipole shift is minimal. The effect of particle size on
the chemical shift should be small compared to the dipole shift,
since the chemical interactionis relativelyshort range. In contrast,
dipole coupling interactions are characteristically long range.
The relationship between the IR absorption intensity and the
CO coverage further supports the premise that dipole coupling
is smaller for small copper particles (Figure 10). For small
particles (low copper coverages), the IR absorbance is approximately linear with the CO coverage. However, for extended
surfaces, the slope of the IR intensity versus CO TPD is much
smaller at high CO coverages than at low CO coverages (Figure
10). The IR peak intensity is related to the dipole coupling by35
In (z/z,,)a a,e/(i
+ a,q2
where a"and a,are the vibrational and electronic polarizabilities,
respectively, and T is the sum of the dipole coupling. Clearly,
when the dipole coupling is small, the IR absorbance is
proportional to the CO coverage.
The isosteric heats of adsorption of CO on supported copper
(Ocu > 2 ML, annealed to 700 K) strongly depend on the CO
coverage. The heats of adsorption are 16-17 kcal/mol for Bco
< 0.1 and decrease to 14-15 kcal/mol for 0.1 < 8 ~ <0 0.3. At
high CO coverages, the isosteric heats of adsorption further
Silica-Supported Copper Catalysts
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 689
-
decrease to 10 kcal/mol. These values are in good agreement
with values for single-crystal copper surfaces. For example, the
isosteric heats of adsorption, measured with work function, are
14 kcal/mol on Cu( 1
and 12 kcal/mol on Cu(1 11)l8
at low CO coverages and decrease to -9 kcal/mol at saturation
coverages. On Cu( loo), the adsorption energy is 17 kcal/mol
at the lowest CO coverage, decreases to 13-14 kcal/mol at dco
= 0.1-0.4, and falls to -10 kcal/mol at saturation CO
co~erage.~IJ*
The high adsorption energy on Cu(100) at the low
coverage limit has been attributed to CO adsorption onto defect
sites.31 Therefore, the high heats of adsorption at low CO
coverages are attributed to CO adsorption on step/edge sites of
the supported copper, whereas the low values at a saturation CO
coverage are attributed to repulsive interactions between CO
molecules.
The model system, Cu/Si02(100 A)/Mo( 1lo), is an excellent
low surface area model for high surface area silica-supported
copper catalysts. The model Cu catalysts are shown to contain
both low-index planes [( 11l), (1 lo), and (loo)] and high-index
planes [(211) or (31 l)]. The sizes of the copper clusters in the
most highly dispersed model systems are on the order of 10-30
A (ec, < l),l6 In highly dispersed silica-supported copper
catalysts, prepared by ion-exchangetechnique, the typical particle
sizes are from 10 to 30 A . 3 6 1 3 ~Both low- and high-index copper
planes are believed to be present on the high surface area copper
~atalysts.3~
Furthermore,the heats of CO adsorption on the model
systems are approximately the same as those found on the more
realistic silica-supportedcopper ~ a t a l y s t s . 3In
~ ~both
~ ~ systems,
the CO adsorption energy decreases with an increase in the CO
coverage, from 14 to 16 kcal/mol at low CO coverages to less
than 10 kcal/mol at high CO coverages.
-
-
-
Conclusion
Copper initially forms two-dimensional structures when deposited on silica at 100 K; however, annealing induces twodimensional copper to form small, three-dimensional clusters.
The unannealed copper films have a significant density of lowcoordinated copper sites, whereas the annealed films consist of
copper clusters with structures similar to low-indexcopper planes
[ ( l l l ) , (llO)] and high-index planes [(211) or 311)]. At high
copper coverages (- 15 ML), copper clusters of -200 A coalesce
to form an extended film. The distribution of cluster facets is
strongly dependent upon the copper coverage. The dispersion of
annealed (700 K) copper on the flat silica film is high, -0.5, for
,,e C 2. Carbon monoxide desorbs in a single peak centered at
-210 K from the unannealed copper film but in several peaks
centered between 150 and 220 K from the annealed film. The
CO adsorption energy strongly depends upon the CO coverage,
varying from 17 kcal/mol at the low coverage limit to 10
kcal/mol at saturation CO coverage. The copper structure, the
high dispersion, the particle size, and the heats of CO adsorption
for the model system are remarkably similar to those for silicasupported copper catalysts prepared by the ion-exchange method,
-
-
demonstrating that this method of dispersing a metal on a thin
oxide film yields an excellent model for a supported metal catalyst.
Acknowledgment. We acknowledge with pleasure the support
of this work by the Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Sciences.
References and Notes
(1) See, for example: (a) Friend, C. M.; Xu,X . Annu. Rev. Phys. Chem.
1991, 42, 251. (b) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Reports
1991, 14, 1. (c) Surface Science of Catalysis; Dwyer, D. J., Hoffmann, F.
M., Eds.; American Chemical Society: Washington, DC, 1992.
(2) See, for example: (a) Strong Metal-Support Interactions; Baker, R.
T. K., et al., Eds.; American Chemical Society: Washington, DC, 1986. (b)
Metal-Support Interactions in Catalysis, Sintering and Redispersion; Stevenson, S.A., et al., Eds.; Van Nostrand Reinhold: New York, 1987.
(3) (a) Lamber, R.; Jaeger, N.; Schulz-Ekloff, G. Surf. Sci. 1990,227,
268. (b) Katrib, A.; Petit, C.; Legart, P.; Hilaire, L.; Maire, G. Surf. Sci.
1987,189/190,886. (c) Lamber, R.; Romanowski, W. J . Caral. 1987, 105,
213.
(4) See, for example: Vurens, G. H.; Salmeron, M.; Somorjai, G. A.
Prog. Surf. Sci. 1989, 32, 333.
(5) Williams, K. J.; Boffa, A. B.; Lahtinen, J.; Salmeron, M.; Bell, A.
T.; Somorjai, G. A. Caral. Lett. 1990. 5, 385.
(6) Wu, M. C.; Msller, P. J . Surf. Sci. 1989, 224, 250.
(7) Guo, Q.;Msller, P. J. Surf.Sci. 1991, 244, 228.
(8) Gautier, M.; Duraud, J. P.; Pham Van, L. Surf. Sci. 1991,249,L327.
(9) Xu, X.;He, J. W.; Goodman, D. W. Sug. Sci., in press.
(10) Xu, X.; Goodman, D. W. Appl. Phys. Lea. 1992, 61, 774.
(11) Xu, X.;Goodman, D. W. Surf. Sci., in press.
(12) He, J. W.; Xu,X.;Goodman, D. W. Surf. Sci., in press.
(13) Xu, X.;Goodman, D. W. Appl. Phys. Lett. 1992, 61, 1799.
(14) Hultren, R.;Desai, P. D.;Hawkins, D.T.;Gleiser, M.;Kelley, K. K.
Selected Values of the Thermodynamic Properties offheElements; American
Society for Metals: Metals Park, OH, 1973.
(1 5 ) See, for example: Campbell, I. M. Catalysis at Surfaces; Chapman
and Hall: New York, 1988.
(16) Xu,X.; Vesecky, S.M.; Goodman, D. W. Science 1992. 258, 788.
(17) Leung, L. W. H.; He, J. W.; Goodman, D. W. J . Chem. Phys. 1990,
93, 8378.
(18) Hollins, P.; Pritchard, J. Surf. Sci. 1979, 89,486.
(19) Woodruff, D. P.; Hayden, B. E.; Prince, K.; Bradshaw, A. M. Surf.
Sci. 1982, 123, 397.
(20) Ryberg, R. Surf. Sci. 1982, 114, 627.
(21) The surface atomic density is estimated using the average values of
the (loo), (110) and (111) surfaces.
(22) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M. Surf. Sci. 1985,
155, 553.
(23) Chesters, M. A.; Parker, S. F.; Raval, R. Surf. Sci. 1986, 165, 179.
(24) Pritchard, J.; Catterick, T.; Gupta, R. A. Surf. Sci. 1975, 53, 1.
(25) Hollins, P.; Davies, K. J.; Pritchard, J. Surf. Sci. 1984, 138, 75.
(26) Horn, H.; Hussain, M.; Pritchard, J. Surf. Sci. 1977, 63, 244.
(27) Horn, K.; Pritchard, J. Surf. Sci. 1976, 55, 701.
(28) Dumas, P.; Tobin, R. G.; Richards, P. L. Surf. Sci. 1986, 171, 579.
(29) Mahan, G. D.; Lucas, A. A. J. Chem. Phys. 1978,68, 1344.
(30) Scheffler, M. Surf. Sci. 1979, 81, 562.
(31) Truong, C. M.; Rodriguez, J. A.; Goodman, D. W. Surf. Sci. 1992,
271, L385.
(32) Tracy, J. C. J . Chem. Phys. 1970, 56, 2798.
(33) Browne, V. M.; Fox, S.G.; Hollins, P. Caral. Today 1991, 9, 1.
(34) Hollins, P. Surf. Sci. 1981, 107, 75.
(35) Hollins, P.; Pritchard, J. Chem. Phys. Leu. 1980, 75, 378.
(36) Kohler, M.A.; Curry-Hyde, H. E.; Hughes, A. E.; Sexton, B. A.;
Cant, N. W. J. Caral. 1987, 108, 323.
(37) Kohler, M. A.; Cant, N. W.; Wainwright, M. S.;Trimm, D. L.J .
Carol. 1989, 117, 188.
(38) Monti, D. M.; Cant, N. W.; Trimm, D. L.; Wainwright, M.S.J .
Caral. 1986, 100, 17.