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The reactivity of small molecules with coadsorbed oxygen on AU(111)

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The reactivity of small molecules with coadsorbed oxygen on AU(111)

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Content THE R E A C T IV IT Y OF S M A L L M O LEC U LES W IT H
CO ADSO RBED O X Y G E N ON A U ( l l l )
by
M ark Lazaga
A Thesis Presented to the
F A C U LTY OF T H E G R A D U A T E SCHOOL
U N IV E R S ITY OF SO UTHERN C A L IF O R N IA
In Partial F u lfillm e n t o f the
Requirements fo r the Degree
M ASTER OF SCIENCE
(Chem istry)
M ay 1992
UMI Number: EP41680
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THE GRADUATE SCHOOL
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LOS ANGELES, CALIFORNIA 9 0 0 0 7 £
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This thesis, w ritten by
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sented to and accepted by the D ean of The
G raduate School, in p a rtia l fu lfillm e n t of the
requirements fo r the degree of
Master Science
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Date....A ^ l U L J 99l
THESIS COMMITTEE
ii
D e d ic a tio n
To Diane
Table of Contents
page
List o f Figures v
1 Introduction 1
2 Adsorption o f H 2O on A u (l 11) and 0 /A u (l 11) 4
2.1 Introduction 4
2.2 Experim ental 4
2.3 Results 7
2.3.1 TPD o f H 2O on clean A u (l 11) 7
2.3.2 TPD o f H2O exposed to atomic oxygen on A u (l 11) 9
2.3.3 XPS o f water on clean and oxygen preadsorbed 14
A u ( l l l )
2.4 Discussion 19
2.5 Conclusion 20
3 Interaction o f Ethylene w ith A u (l 11) 23
and 0 /A u (l 11) Surfaces
3.1 Introduction 23
3.2 Experim ental 24
3.3 Results 25
iv
page
I 3.3.1 TPD o f ethylene exposed to clean A u (l 11) 25
J 3.3.2 TPD o f ethylene on oxygen covered A u ( l l l ) 29
3.3.3 XPS annealing experiments: ethylene on 30
clean A u ( ll 1)
3.4 Discussion 33
3.4.1 Ethylene exposure to clean A u (l 11) 33
3.4.2 Ethylene exposure to oxygen adatoms on A u (l 11) 33
3.5 Conclusion
4 Studies of carbon monoxide and carbon dioxide 37
adsorption on A u (l 11) and 0 /A u (l 11) surfaces
|
4.1 Introduction 37
4.2 Experimental 37
4.3 Results 39
4.3.1 XPS o f CO exposed on A u (l 11) and 0 /A u (l 11) 39
4.3.2 XPS o fC O i exposed on A u ( ll 1) and 0 /A u (l 11) 45
4.4 Discussion 45
4.5 Conclusion 47
L is t o f F igures
page
V
2.1 H 2O TPD spectra follow ing the adsorption
o f H 2O on clean A u (l 11) at 110 K.
8
2.2 H 2O TPD spectra after the adsorption o f H 2O
on A u ( l l l ) at 110 K w ith 0 O = 0.25.
1 1
2.3 H 2O TPD spectra after the adsorption o f H 2O
on A u (l 11) at 110 K w ith 0 O = 0.50.
12
2.4 H 2O TPD spectra after the adsorption o f H 2O
on A u (l 11) at 110 K w ith 0 O= 1.0.
13
2.5 O (ls ) XPS spectra after ozone exposure on clean
A u (l 11) at 300 K.
14
2.6 O (ls ) XPS spectra follow ing 0.12 L H 2O exposure on
A u ( l l l ) at 110 K w ith several oxygen precoverages.
17
2.7 XPS difference spectra o f oxygen spectra, figure 2.5,
subtracted from the spectra o f water adsorbed on
oxygen precovered A u ( lll) , figure 2.6.
18
3.1 C2H 4 TPD spectra follow ing the adsorption o f
ethylene on clean A u (l 11) at 95 K.
26
3.2 C2H 4 TPD spectra follow ing the adsorption o f
ethylene on oxygen covered A u ( l ll) at 95 K.
Ethylene exposures were 2 L.
27
3.3 O 2 TPD spectra follow ing the adsorption o f
ethylene on oxygen covered A u ( l l l ) at 95 K.
Ethylene exposures were 2 L.
28
3.4 C (ls ) XPS spectra fo r ethylene on clean A u ( l ll) 31
VI
page
3.5 C (ls ) difference spectra obtained by subtracting a 32
spectrum o f clean A u (l 11) from each o f the
spectrum in figure 3.4.
4.1 C (Is) XPS spectra after ozone exposure on clean 40
A u ( l l l ) at 300 K.
4.2 C (ls ) XPS spectra after 10 Langm uir CO exposure 41
to oxygen on A u ( l ll) at 100 K.
4.3 O (ls ) XPS spectra after 10 Langm uir CO exposure 42
to oxygen on A u ( l l l ) at 100 K.
4.4 O (ls ) XPS spectra after ozone exposure to clean 43
A u ( l l l ) at 100 K.
4.5 O (ls ) XPS spectra after 10 Langm uir CO2 exposure 44
to oxygen on A u ( l l l ) at 100 K.
1 Introduction
| Im proved understanding o f oxidation reactions on metal
5
surfaces is important for many commercial technologies. These
reactions are involved in many heterogeneous catalytic,
electrochemical and corrosion processes. Furthermore, the
investigation of the elementary steps involved in these reactions at
the gas/metal interface is interesting from a fundamental point of
view in gaining a better understanding of chemical reactions at
interfaces. The development of surface science techniques in the
I past 2 0 years has allowed great access for observing molecular
j interactions at the gas/metal interface.
I Several reports have been published on the reactivity of a
j variety of molecules such as CO, CO2, H 2O, C2H 2, C2H4, H 2CO
and CH3OH with oxygen adatoms on copper and silver surfaces [1-
6 ]. In general, these molecules show no reactivity on the clean
surface, but are easily oxidized on these surfaces when coadsorbed
with oxygen adatoms. A correlation of the reactivity of these
molecules, based on their gas phase acidities, has been established
by Outka and Madix and described in terms of Bronsted acid-base
reactions [7].
Although such oxidation reactions on copper and silver have
been extensively studied, little has been reported on the reactivity
j of oxygen on gold, largely due to the low reactivity of gold. In
this study we investigate the interactions of several small
molecules, i.e. H 2O, C2H 4, CO, and CO2, on clean A u ( l ll)
surfaces and w ith coadsorbed oxygen on A u ( lll) , 0 / A u ( lll) ,
under ultrahigh vacuum conditions. Our results show that water
and ethylene are nondissociatively adsorbed on the clean and
oxygen coadsorbed surface at 100 K. W ater was not observed to
react w ith oxygen on A u ( ll 1) to form hydroxyl species on the
surface. Ethylene was unreactive w ith coadsorbed oxygen at
temperature up to 400 K and pressures as high as 5 x 10-9 torr.
Carbon monoxide exposed to oxygen adatoms on A u ( ll 1) does not
form a surface carbonate species. A new surface species was
observed when carbon dioxide was exposed to oxygen on A u ( lll) .
The reactivity o f these molecules w ith oxygen coadsorbed on
A u (l 11) is shown to be less than on copper and silver surfaces.
References
1. I.E. Wachs and R.J. M adix, Surface Sci. 76(1978)531.
2. T.E. Felter, W .H . Weinberg, G .Y. Lastushkina, P.A. Zhdau and
G .K. Hrbek, J. A ppl. Surface Sci. 16(1983)351.
3. M . Bow ker and R.J. M adix, Surface Sci. 95(1980)190
4. I.E. Wachs and R.J. M adix, J. Appl. Surface Sci. 1(1978)303.
5. M . Bowker, M . Barteau and R.J. M adix, Surface Sci.
92(1980)528.
j
6 . A. Spitzer and H. Luth, Surf. Sci. 120(1982)376.
i 7. D. Outka and R.J. M adix, J. Am. Chem. Soc. 109(1987)1708.
C hapter 2
4
Adsorption of H 2 O on A u ( lll) and 0 /A u ( lll)
2.1 Introduction
The interactions o f water w ith metal surfaces are o f interest
fo r catalytic, electrochemical and corrosion processes.
Fundamental surface science studies have revealed much atomic-
level detail about these interactions [1]. However, few detailed
studies are available on the interactions o f water w ith gold
surfaces or w ith coadsorbed species on gold.
In this chapter, we present results fo r water adsorbed on clean and
oxygen precovered A u ( lll) , using principally temperature programmed
desorption (TPD) and X -ray photoelectron spectroscopy (XPS). We
determine the interactions o f water w ith oxygen fo r a variety o f water
exposures and oxygen coverages under ultrahigh vacuum conditions
(U H V ). In addition, XPS results o f oxygen on A u ( l l l ) w ill be presented
fo r the first time.
2.2 Experimental
The experiments were performed in a stainless steel U H V
chamber equipped w ith PHI 120° low energy electron diffraction
(LE E D ) optics, a PHI glancing incidence electron gun fo r Auger
electron spectroscopy (AES), a PH I dual anode (M g and A l) X -ray
source, a PHI spherical capacitance analyzer (SCA) and a U T I
100C mass spectrometer. Temperature programmed desorption
was carried out w ith the crystal in front o f the quadrupole mass
spectrometer (Q M S) using a linear temperature ramp o f 5 K/s; up
to eight masses were monitored simultaneously by computer
control o f the mass analyzer. Data was acquired w ith an DBM X T
computer.
The A u (l 11) crystal was mounted between two N i posts by
0.010 inch Ta wires. The N i posts were inserted into a Cu block
which is attached to a liquid nitrogen cooled stainless steel probe.
The sample could be cooled to 95 K or resistively heated. Sample
temperatures were monitored w ith a chrom el-alum el
thermocouple inserted firm ly into a hole drilled in the side o f the
crystal.
The Au( 111) crystal was cleaned by Ar+ ion sputtering
(Ep= 1 k V ) in itia lly w ith the sample at 300 K then 700 K and
fin a lly annealing at 1000 K. Small amounts o f carbon could be
removed by exposure to ozone. Surface purity was checked by
AES. LE E D gave a hexagonal pattern o f broad spots and the
hexagonal cluster surrounding these integral order spots was not
resolved, indicating poor long range order.
A tom ic oxygen was cleanly adsorbed on the A u (l 11)
surface by exposure to ozone at 300 K. The ozone was prepared
using a Polymetrics ozone generator. Oxygen at one atmosphere
pressure was passed through the generator and the effluent gas
trapped in a bottle containing silica gel suspended in an
ethanol/dry ice bath at -80 °C. This process was continued until
the silica gel was saturated w ith ozone producing a deep blue color
(approximately 20 minutes). Since the trapped gas m ixture is
largely unreacted oxygen, the ozone was purified by slow ly
j cooling the bath to -110 °C and pumping out the oxygen. The bath
i
j was then warmed to -80 °C and the cycle repeated. Ozone was
| dosed to the sample through a collim ated beam doser at 300 K.
|
j TPD spectra o f ozone exposed to clean A u (l 11) was consistent
j w ith that o f Parker and Koel [2]. 8.1 Mohm nanopure distilled
water was degassed by repeated freeze-pump-thaw cycles.
Ozone coverages were determined by temperature
programmed desorption. Ozone was exposed to the gold crystal
u n til a saturation coverage is reached, and the area o f the
j desorption peak was measured. This was assigned to 1.0
monolayer oxygen coverage. Know ing this area allowed other
coverages to be determined by comparison and repeated TPD was
carried out u n til the proper exposure was determined. W ater was
dosed through a stainless steel tube w ith the sample rotated away
from the doser. Exposures o f all gases are uncorrected fo r ion
gauge sensitivities and doser enhancements.
7
2.3 Results
I
| 2.3.1 TPD of H 2 O on clean A u (lll)
j Temperature programmed desorption results fo r water
j adsorbed on clean A u ( l l l ) at 110 K are shown in figure 2.1. For
| a 0.60 L (L = Langm uir = 10' 6 torr sec) water exposure two
|
\ peaks exist at 160 and 164 K, the high temperature state exhibits
i
! less than first order desorption kinetics while the low temperature
i
| state exhibits zero order desorption kinetics. For first order
| desorption kinetics the maximum peak temperature remains
j constant w ith changes in surface coverage. I f desorption is less
i
j than first order an increase in exposure w ill result in an increase
| in the m axim um peak temperature. Generally first order
desorption kinetics are observed for monolayer desorption. S till,
! we assign this peak to monolayer desorption since this state
| com pletely fills (at approximately 1 L ) w hile the low temperature
state continues to increase. K inetic orders other than first order
fo r monolayer desorption can be due to hydrogen bonding.
S im ilar desorption behavior for water on Au(110) has been
observed by Outka and M adix [3]. The low temperature state in
figure 2.1 is attributed to m ultilayer desorption. Assuming zero
j order desorption kinetics fo r both the monolayer and m ultilayer,
the activation energy fo r desorption using Redhead analysis [4] is
! calculated to be 10-11 kcal/m ol. The long desorption ta il may be
attributed to hydrogen bonding between water molecules or
m /e = 1 8 signal intensity (arb. units)
H 2 0 exposure _I
(langmuir) :
1.20
0.60
0.24
0.06
0.03
0.00
100 150 200 250 300
temperature (Kelvin)
Figure 2.1 H 2O TPD spectra follow ing the adsorption o f H 2 O on
clean A u (l 11) at 110K.
surface defects, but is not attributed to the pumping speed o f the
chamber since subsequent experiments in this chamber gave sharp
desorption peaks.
2.3.2 TPD of H 2 O exposed to atomic oxygen on A u ( lll)
W ater was exposed to 0.25, 0.50, and 1.0 monolayers
atomic oxygen on A u ( lll) , and the subsequent TPD spectra are
shown in figures 2-4. For these particular experiments three
masses were monitored, H 2 (m/e = 2), H 2O (m/e = 1 8 ) and O 2
(m/e =32). Hydrogen was not observed to desorb at temperatures
up to 700 K . Oxygen desorbed w ith the same quantity and at the
same temperature, 529-542 K, as from a clean gold surface
indicating that oxygen was unreactive on the surface or that water
reacting on the surface to form hydroxyl groups reversibly
desorbed at low er temperatures. XPS results show that water is
nondissociatively adsorbed on the surface at 100 K (as shown
below).
TPD results give evidence for nondissociative adsorption o f
water on oxygen covered A u ( ll 1). For water exposures to both
0.25 and 0.50 monolayer oxygen coverages, i.e. 0 O = 0.25 and
0.50, tw o desorption peaks were observed at 183 K and 167 K .
Figure 2.2 shows that the high temperature state is populated prior
to population o f the low temperature state. Sim ilar results are
observed in figure 2.3 except the low temperature state begins to
f ill at low er water exposures. I f hydroxyl form ation occurred it is
10
reasonable to expect that w ith an increase in oxygen coverage
there would be an increase in hydroxyl form ation, i.e., an increase
in the area o f the high temperature state. This increase is not
observed. H ydroxyl form ation may be less if large coverages o f
oxygen block the adsorption o f water on the gold surface. Results
on Ru(001) show that the ratio o f water bound to adsorbed oxygen
on a 0.25 and 0.50 monolayer oxygen covered surface is 3.5:1 and
1:1, respectively [5]. S im ilar results are observed on Ag(110) [6 ].
The ratio o f water to oxygen indicate that there is a sufficient
amount o f water on the surface to consume the entire amount o f
oxygen by form ing hydroxyl species. Considering these results,
the first order desorption peak at 182 K is attributed to, oxygen-
stabilized nondissociatively adsorbed water. The 30 K increase in
peak desorption temperature, w ith respect to water desorption
from the clean surface, is due to the stabilization o f water on the
surface by hydrogen bonding w ith adsorbed oxygen or electron
transfer through the metal surface. The activation energy for
desorption using Redhead analysis [4] and assuming a pre
exponential factor o f 10*3 s_ 1 fo r first order kinetics and
neglecting changes in activation energy w ith coverage is 10.8
kcal/m ol. The low temperature state displays zero order
desorption kinetics and is ascribed to water physisorbed on either
the adsorbed oxygen or water. The activation energy fo r
desorption is 10-11 kcal/mole.
For water exposures up to 1.2 L on 1.0 monolayer oxygen
precovered A u (l 11) a single desorption peak is observed at 168 K,
m /e = 18' signal intensity (arb. units)
H20 exposure
(langmuir)
1.20
0.60
0.24
0.06
0.03
0.00
100 150 200 250 300
temperature (Kelvin)
Figure 2.2 H2O TPD spectra after the adsorption of H2O on
A u (lll) at 110K with 0 O = 0.25.
m /e = 1 8 signal intensity (arb. units)
1 68
181
H20 exposure
(lang m u i r)
1.20
0.60
0.24
0.12
0.06
0.03
0.00
100 150 200 250 300
temperature (Kelvin)
j
Figure 2.3 H2O TPD spectra after the adsorption of H2O on
A u (lll) at 110K with 0 O = 0.50.
m /e = 1 8 signal intensity (arb. units)
1 68
H20 exposure
(langmuir)
1.20
0.60
0.24
0.12
0.06
0.03
100 1 50 200 250 300
temperature (Kelvin)
Figure 2.4 H2O TPD spectra after the adsorption of H2O on
Au(lll) at 110K with 0 O = 1.0.
~14j
as shown in figure 2.4. A desorption peak at 168 K has been
assigned (above) to water adsorbed on either water or oxygen. For
this experiment, oxygen covers the A u ( l l l ) surface. Therefore
water forms a m ultilayer on the oxygen. The absence o f a high
temperature state indicates that oxygen blocks the direct
| adsorption o f water to the A u ( l l l ) surface. j
; !
J i
| 2.3.3 XPS of water on clean and oxygen preadsorbed A u ( lll) j
i
i
I The XPS spectra o f oxygen adsorbed on clean A u (l 11),
water exposed to clean and oxygen precovered A u ( lll) , and their
difference spectra are shown in figures 2.5, 2.6 and 2.7,
respectively. For atomic oxygen adsorbed on a clean surface, the
|
j binding energy o f the 0(1 s) peak is observed between 529.8 eV j
'
and 530.2 eV. The O (ls ) peak shifts to higher binding energies i
w ith increases in oxygen coverage. Since oxygen atoms are j
competing fo r electron density at the surface it is reasonable to j
I
expect a rise in binding energy. Small peaks at 532.2 eV in figure j
j !
! 2.5 are due to background adsorption o f water. j
I
W ater adsorbed on clean A u (l 11) displays an O (ls ) peak j
i
w ith a binding energy o f 533.2 eV, as shown in figure 2.6. This j
result is consistent w ith O (ls ) binding energies o f monolayer
| coverages o f water on other metal surfaces, e.g. 533.3 eV on I
j C u (l 11 )[7] and 532.2 eV on P t(l 11) [8]. The oxygen peak fo r |
| i
m ultilayer water, not shown, is at 534.3 eV. The 1.1 eV shift is
due to fin a l state screening effects.
For water exposures on 0.25 and 0.50 monolayer coverages
j o f preadsorbed oxygen an O (ls ) peak is observed at 532.1
i eV(figure 2.7). The 0.9 eV shift can be attributed to form ation o f
a new species, hydrogen bonding, or changes in the surface
|
j electronic state. The dissociation o f water on oxygen precovered
! metal surfaces to form hydroxyl has been observed in many
studies. On C u ( lll) the O (ls ) peak is at 531.5 eV [7] and on
P t ( l l l ) at 530.5 eV [8], corresponding to 2.0 eV and 1.7 eV
shifts, respectively, when compared w ith water adsorbed on the
clean surface. Also, if water dissociated on the oxygen covered
A u (l 11) surface, the intensity o f the adsorbed oxygen O (ls ) peak
j would dim inish as hydroxyl species are formed. This is not
observed in figure 2.6. Hydrogen bonding between adsorbed water
molecules has been observed to cause 0.2 eV shifts [9]. W ork
function measurements show that charge transfers from the metal
J to oxygen. The increase in electron density on the oxygen may
cause a stronger interaction fo r hydrogen bonding w ith adsorbed
water. It is not expected that hydrogen bonding can fu lly account
fo r the 0.9 eV shift. Thus the 0.9 eV shift is attributed to
I contributions from both hydrogen bonding between water and
!
adsorbed atomic oxygen and changes in the surface electronic
state. It is d iffic u lt to explain a further decrease in binding energy
when water is exposed to a saturation coverage o f oxygen on
A u ( lll) , one explanation is that there may be greater propensity
fo r hydrogen bonding or stabilization o f water through oxygen.
signal intensity (arb. units)
oxygen coverage ~
(monolayers)_
1.00
0.50
0.25
0.00
530.1
530.0
529.8'
528 532 536 540
binding energy (eV)
Figure 2.5 O (ls) XPS spectra after ozone exposure on clean
A u (l 11) at 300K.
signal intensity (arb. units)
" oxygen coverage
- (monolayers)
1.00
0.50
0.25
0.00
530.1
532.2
529.9
533.2
528 532 536 540
binding energy (eV)
Figure 2.6 0(1 s) XPS spectra fo llow ing 0.12 L H 2 O exposure on
A u (l 11) at 11 OK w ith several oxygen precoverages.
ianal intensity (arb.)
oxygen coverage
(monolayers)
1.00
0.50
0.25
0.00
532.3
532.1
533.2
528 532 536 540
binding energy (eV)
Figure 2.7 XPS difference spectra o f oxygen spectra, figure 2.5,
subtracted from the spectra o f water adsorbed on
oxygen precovered A u (l 11), figure 2.6.
2.4 Discussion
O ur TPD and XPS results show that water is
nondissociatively adsorbed on clean A u (l 11) at 100 K . This is in
good agreement w ith previous studies on other Group IB surfaces,
C u ( lll) [7], Ag(110) [10] and Au(110) [3]. Sub-monolayer
desorption o f water on Group IB and P t(l 11) [8] surfaces exhibit
less than first order desorption kinetics. This kinetic behavior has
been an area o f much study. Monte Carlo simulations predict that
water adsorbs in clusters on the surface [11]. Electron energy loss
spectra fo r water adsorbed on Ag(110) indicate that extensive
clustering o f water does occur at 100 K [10]. Electron stimulated
desorption ion angular distribution (E S D IA D ) experiments on
Ru(001) show that at low coverages, less than 0.2 monolayers,
interactions between neighboring molecules are weak, but lateral
interactions increase w ith increasing coverage [12]. TPD o f water
on clean Pt( 111) suggests the form ation o f islands w ith sufficient
exposures [8]. Thus, the complex desorption kinetics are related to
strong lateral interactions and possibly island form ation w hich are
consistent w ith hydrogen bonding. These interactions influence the
surface residence time and activation energy o f desorption w ith
respect to coverage causing complex desorption behavior.
The reactivity o f water on oxygen covered surfaces is
observed to be sim ilar fo r Group IB metal surfaces [5,7,10]. A t
temperatures below 150 K water is nondissociatively adsorbed.
Upon heating the sample, water disproportionates to form
20
adsorbed hydroxyl species. A t higher temperatures hydroxyl
species recombine to desorb as m olecular water. Our XPS results
fo r water adsorbed on oxygen precovered A u (l 11) show that
water is m olecularly adsorbed on the surface at 100 K. The
observed 0.9 eV difference in binding energy between water
j adsorbed on the clean surface and oxygen covered surface is
j attributed to hydrogen bonding and a change in the surface
j electronic state. The O (ls ) 1.1 eV binding energy shift observed
j fo r water exposed to a saturation coverage o f oxygen on A u ( l l l )
I
| is d iffic u lt to explain. TPD results indicate that the activation
energy fo r desorption is low er fo r m ultilayer adsorption than fo r
monolayer adsorption.The decrease in binding energy may be a
j result o f screening effects from water adsorbed directly on the
oxygen monolayer.
TPD results indicate that water does not react w ith adsorbed
oxygen on A u ( l l l ) to produce hydroxl groups on the surface. The
high temperature state is attributed to adsorbed water hydrogen
bonded to atomic oxygen on the A u ( ll 1) surface. The peak at
168 K is assigned to m ultilayer water bound to surface oxygen or
water.
2.5 Conclusion
W ater is weakly and nondissociatively adsorbed on clean
A u ( l ll) . The desorption kinetics fo r adsorbed water are complex
as a result o f hydrogen bonding. W ater nondissociatively adsorbs
21
on oxygen precovered A u ( l l l ) at 100 K. W ater is stabilized by
oxygen on the surface via hydrogen bonding. The form ation o f
hydroxyl species is not observed.
References
I. T.E. Madey and J.T. Yates, Chem. Phys. L e tt 51(1977)77.
i
I 2. D.H. Parker and B.E. Koel, submitted to Surface Sci.
3. D .A . Outka and R.J. M adix, J. Am. Chem. Soc.
109(1987)1708.
| 4. P.A. Redhead, Vacuum 12(1962)203.
!
j 5. K. Kretzschmar, J.K. Sass, A .M . Bradshaw and S. Hollow ay,
| Surface Sci. 115(1982)183.
|
I 6 . M . Browker, M .A . Bartreau and R.J. M adix, Surface Sci.
| 92(1980)528.
s
I 7. C. Au, J. Breza and M .W . Roberts, Chem. Phys. Lett.
66[2](1979)340.
J 8 . G.B. Fisher and J.L. Gland, Surface Sci. 94(1980)446.
! 9. B .A . Sexton, Surface Sci. 94(1980)435.
i
10. E.M. Stuve, R.J. M adix and B.A. Sexton, Surface Sci.
111(1981)11.
I I . E. Clementi, J. Phys. Chem. 89(1985)4426.
j 12. C. Benndorf, C. Noble, M . Rosenburg and F. Thieme, Surf.
Sci. 111(1981)87.
Chapter 3
Interaction of Ethylene with A u ( lll) and 0 /A u ( lll)
j Surfaces
i
i
| 3.1 Introduction
I The oxidation o f ethylene to ethylene oxide is an important
I
! industrial reaction u tilizin g silver catalysts. There have been
I
j extensive studies on the reactivity o f these catalysts [1-3].
However, questions regarding the chemical nature o f the active
oxygen species still remain. To gain a better understanding o f this
reaction, surface science studies have been conducted under high
vacuum conditions on silver single crystal surfaces [4-7]. But,
under these conditions ethylene combusts, form ing water and a
carbon phase [4] , a surface carbide [5], and a carbonate species
[6].
It is o f interest to see i f A u surfaces m ight exhibit high
selectivity fo r the partial oxidation o f ethylene and propylene. The
principal d iffic u lty o f Au, activating O 2, can be overcome by
using stronger oxidants, i.e. O3, to adsorb oxygen atoms on Au
single crystal surfaces. This study shows that ethylene does not
react on the oxygen covered A u ( l l l ) surface fo r temperatures
between 100 - 400 K and pressures o f 2 x 10' 10 to 5.0 x 10"9 torr.
The observed lack o f reactivity is consistent w ith the gas phase
acidity - reactivity model proposed by Barteau and M adix [ 8].
J
24
3.2 Experimental
Experiments were performed in a stainless steel U H V
chamber equipped w ith LEED, AES, XPS and TPD apparatus,
w hich is described in Chapter 2. The aluminum anode was used in
the dual anode X -ray source to generate A1 K a radiation at 1486.6
eV.
For these experiments ozone was dosed through a collim ated
beam doser at 300 K w ith the crystal rotated in front o f and
approxim ately one centimeter away from the the doser. The ozone
was prepared as described in Chapter 2. Ethylene (Matheson, C.P.
grade) was used w ithout further purification. Ethylene was dosed
through a 1/4 inch stainless steel tube w ith the sample rotated away
from the tube, except during exposures at 300 and 400 K where
the sample was rotated in front o f and approximately one
centimeter away from the dosing tube. Dosing in front o f the
dosing tube causes an increase in partial pressure at the surface o f
the sample. These partial pressures at the sample may be
considerably higher than the pressure measured at the ion gauge.
The dosing pressure is defined as the pressure measured at the ion
gauge, which is the pressure o f the chamber. A ll exposures are
uncorrected fo r doser enhancements. Exposures are uncorrected
fo r differences in ion gauge sensitivity fo r the various molecules.
3.3 Results
25
3.3.1 TPD of ethylene exposed to clean A u ( lll)
Ethylene adsorbed reversibly on clean A u ( l l l ) at 95 K.
Subsequent thermal desorption spectra are shown in figure 3.1.
Desorption o f ethylene from the clean surface was observed at
approxim ately 104 K. In addition, a broad feature was seen
extending to 250 K. No hydrogen (m/e = 2) was evolved from the
surface up to temperatures o f 700 K. AES showed no residual
carbon on the surface after heating.
The reason fo r the broad desorption peak is uncertain.
Outka and M adix have shown that a broad range o f desorption is
typical o f C2 hydrocarbons on Au(110) [8]. They propose that
these molecules are weakly bound and therefore occupy a variety
o f binding sites w ith different activation energies. Consequently
they desorb at different temperatures. We also attribute the broad
desorption feature to defects on the (111) surface. The desorption
ta il to 250 K is assigned to background desorption from the
sample mount. The peak at 104K exhibits first order desorption
kinetics. We can calculate an activation energy fo r desorption o f 6
kcal/m ol fo r the peak at 104 K using Redhead analysis [9] and
assuming first order desorption and a preexponential factor o f
1013 sec-1-
26
C2H4 exposure
(langmuir)
O )
0.064
0.016
0.008
0.004
0 0
C V J
1 00 150 200 250 300
Temperature (Kelvin)
Figure 3.1 C2 H4 TPD spectra following the adsorption of ethylene
on clean Au(l 11) at 95K.
m /e = 2 8 signal intensity (arb. units)
oxygen coverage
(monolayers)
100 150 200 250 300
Temperature (Kelvin)
Figure 3.2 C2 H4 TPD spectra following the adsorption of ethylene
on oxyen covered Au(l 11) at 95K. Ethylene exposures
were 2 Langmuir.
m /e = 3 2 signal intensity (arb. units)
oxygen coverage
(monolayers)
700 600 500 400 300
Temperature (Kelvin)
Figure 3.3 O2TPD spectra following the adsorption of ethylene
on oxyen covered Au(l 11) at 95K. Ethylene exposures
were 2 Langmuir.
3.3.2 TPD of ethylene on oxygen covered A u ( lll)
Ethylene was exposed to A u ( ll 1) w ith 0 O = 0.1, 0.5 and
1.0. Subsequent TPD spectra are shown in figure 3.2. Ethylene
desorbed at 106 K fo r 0 O = 0.1 and 111 K fo r 0 O = 0.5 and 1.0.
Hydrogen (m/e = 2), carbon (m/e = 12), water (m/e = 18),
ethylene oxide (m/e = 29 and 44), carbon monoxide (m/e = 12,16
and 28), and carbon dioxide (m/e = 12, 16, and 44) were also
| monitored (not shown), but were not observed to desorb upon
| heating the sample to 700 K. AES showed that carbon did not exist
on the surface follow ing TPD. Oxygen was observed to desorb at
the same temperature as on the clean surface (see figure 3.3). The
amount o f oxygen desorbed after ethylene exposure is 10, 94 and
96 percent o f the amount adsorbed prior to ethylene exposure fo r
0.1, 0.5 and 1.0 monolayer coverages, respectively.
CO 2 desorbs from the surface immediately upon exposure
to CO [10]. The oxidation o f CO to CO2 was not monitored. The
absolute amount o f oxygen loss prior to TPD fo r each oxygen
coverage are w ithin 20 percent. Since no reaction products were
observed the oxygen loss is attributed to reactions w ith
background carbon monoxide to form carbon dioxide. Thus
| ethylene is reversibly adsorbed onto the oxygen covered A u ( l l l )
i
j surface. Ethylene is observed to be unreactive on the oxygen
precovered surface and m olecularly desorbs via first order
desorption kinetics. Assuming a typical first order preexponential
30
factor o f 1013 sec-1 the activation energy o f desorption is 6.5
kcal/m ol.
A n additional experiment was carried out to explore the
reactivity o f ethylene w ith oxygen precovered A u ( l l l ) at higher
ethylene exposures, temperatures, and pressures. Ethylene was
exposed on A u ( l l l ) w ith @0 = 0.25 at 300 and 400 K . The system
pressure during the ethylene exposure was 5 x 10‘9 torr. Masses
fo r the same molecules as listed above were monitored during
ethylene exposure. Reaction products were not observed during
ethylene exposure. Products were not observed during subsequent
TPD. Oxygen desorbed at the same temperature as on the clean
surface w ith above 70 percent o f the amount adsorbed p rio r to
ethylene exposure. The reason fo r this loss o f oxygen is described
above. Thus ethylene does not react w ith adsorbed oxygen on the
A u (l 11) surface at temperatures below 400 K at these pressures.
3.3.3 XPS annealing experiments: ethylene on clean A u ( lll)
Heating experiments for ethylene exposed to clean A u (l 11)
were monitored by XPS. C (ls ) XPS spectra fo r these experiments
are shown in figure 3.4. The observed C (ls ) peak at 284.7 eV BE
is w ith in 0.1 eV o f typically observed values fo r alkenes [11]. The
difference spectra, figure 3.5 shows that ethylene does not desorb
from the surface at temperatures up to 115 K. A t 165 K the C (ls)
peak has diminished considerably. Above 200K a C (ls ) peak is not
31
n r — i— r 1 — i— i— i— i i i— i— i— i— i— i— i— i— i— i— i— i— m
to
E
3
-Q
V —
C O
to
c
0)
c o
c
O )
( O
'to
o
C/D
Q .
X
annealing temp
(Kelvin)
clean
i i i » i i i i i i i i i i i » i i i «
295 290 285 280 275
binding energy (eV)
Figure 3.4 C(ls) XPS spectra for ethylene on clean A u (lll)
32
i i j i— i i— i— |— i— i— i— i— |— i— i— i— i— |— i— i— i— i— [
annealing temp
(Kelvin)
284.7
295 290 285 280
binding energy (eV)
275
Figure 3.5 C (ls ) XPS difference spectra obtained by subtracting a
spectrum o f clean A u ( l l l ) from each o f the spectrum.
33
observed. These results indicate that ethylene is adsorbed on the
clean A u (l 11) surface at temperatures below 115 K. A t
temperatures above 165 K ethylene is not observed on the surface.
A comparison o f XPS spectra w ith TPD results on clean A u (l 11)
show that the long tail in the TPD spectra is not due to desorption
from the sample.
3.4 Discussion
3.4.1 Ethylene exposure to clean A u ( lll)
Temperature programmed desorption results show that ethylene
reversibly adsorbs on clean A u ( l l l ) at 100 K. Upon heating the sample,
ethylene is observed to desorb at 104 K.The activation energy o f
desorption is 6 kcal/m ol assuming first order kinetics and a
preexponential factor o f 10i3 sec*1. Hydrogen is not evolved upon
heating the sample from 100 to 700 K. XPS and AES show that residual
carbon was not present on the surface after heating to 700 K . These
results are in good agreement w ith sim ilar experiments carried out on
Au(110) [8]. The lack o f reactivity on clean Au is not surprising as gold
is known to have a low activity fo r hydrocarbon dissociation.
3.4.2 Ethylene exposure to oxygen adatoms on A u ( lll)
Ethylene adsorbs on oxygen preadsorbed A u ( l l l ) surfaces
at 95 K . TPD results show that ethylene desorbs at 111 K and is
unreactive w ith coadsorbed oxygen. The activation energy fo r
ethylene desorption is 6.5 kcal/m ol assuming first order kinetics
and a preexponential factor o f 1013 sec-1. The increase in peak
desorption temperature, w ith respect to TPD o f ethylene on clean
A u ( l ll) , is due to increased interaction w ith Au^+ formed from
i
i oxygen coadsorption. AES show that residual carbon was not
!
| present on the surface after heating to 700 K. Oxygen desorbed
j from the surface prior to thermal desorption, particularly fo r © Q
= 0.1. Experiments were carried out to determine i f ethylene
j reacted w ith surface oxygen and immediately desorbed. In these
experiments ethylene was exposed to A u (l 11) w ith 0 O = 0.25
J w hile H 2O, C2H 4, CO, CO2 and C2H 4O were monitored w ith the
QMS. No products were observed fo r ethylene pressures up to 5 x
10-9 torr, exposures up to 4 L, and sample temperatures as high as
400 K. I f oxidation does occur on the surface the reaction is slow
under these conditions and is below detectable lim its. D uring
ethylene exposure, the CO level was observed to rise in the
chamber. CO 2 desorbs from the surface im m ediately upon
exposure to CO [10]. CO oxidation is strongly exothermic and
may be responsible fo r the titration o f surface oxygen. O ur results
are not conclusive at low oxygen coverages and further
investigation is necessary.
Our results agree w ith the gas phase acidity - reactivity
correlation proposed by Outka and M adix [8]. Ethylene has a high
A H acid, gas (low acidity) and is predicted to be less reactive to
adsorbed oxygen on gold than molecules w hich have low er
35
A H acid, gas (greater acidity). For water and ethylene the AH acid,
gas are 391 and 416 kcal/m ol, respectively [8]. W ater is unreactive
on the A u ( l l l ) oxygen covered surface (as discussed in Chapter 2)
and therefore it is expected that ethylene also would be unreactive
w ith adsorbed oxygen on the A u ( l l l ) surface. This is observed.
3.5 Conclusion
Ethylene nondissociatively adsorbs on clean and oxygen
covered A u ( l ll) at 95K. The activation energy fo r ethylene
desorption on the clean and oxygen covered surface is 6 and 6.5
kcal/m ol, respectively. No reaction o f ethylene w ith the oxygen
covered A u ( l ll) surface was observed at temperatures up to
7i400 K and pressures o f 5.x lO 9 torr.
References
'36!
1. L.E. Nault, D.W . Bolm er and L.N . Johanson, Ind. Eng. Chem.
Proc. Design Develop. 1(1962)285.
j 2. E.L. Force and A .T. Bell, J. Catal. 40(1975)356.
j 3. K.E. Hayes, Can. J. Chem. 38(1960)2256.
4. G. Rovide, F. Pratesi and E. Ferreri, A ppl. Surface Sci.
5(1980)121.
j 5. I.E. Wachs and G.R. Kelemen, Proc. 7th Intern. Cong, on
i Catal.,Tokyo 1980.
J 6 . C. Bracks, C.P.M. de G rout and F. Biloen, Proc. 4th Intern.
Conf. on Solid Surfaces and 3rd European Conf. on Surface
Sci. Canmer 1980 p. 248.
7. M .A . Barteau and R.J. M adix, Surface Sci. 103(1981)L171.
I
f
8 . D .A . Outka and R.J. M adix, J. Am . Chem. Soc.
109(1987)1708.
9. P.A. Redhead, Vacuum 12(1962)203.
I
j 10. D. W ickham and B.E. Koel, to be published
37
C hapter 4
Studies o f C a rb o n m onoxide and C a rb o n d io xid e
A d s o rp tio n on A u ( l l l ) and 0 / A u ( l l l ) S urfaces
4.1 In tro d u c tio n
i
j The oxidation o f carbon monoxide has been studied under
j ultrahigh vacuum on silver [1-3] and gold [4]. When CO is
|
exposed to oxygen on the A g (l 10) surface at 307 K , CO2 is
form ed and desorbs from the surface in a clean-off reaction.
Exposing the oxygen covered silver to CO2 also produces
carbonate species on the surface. These species desorb at 485 K
upon heating the sample. AES studies on A u ( l l l ) indicate that CO
reacts to titrate away adsorbed oxygen [4]. The kinetics fo r CO
oxidation on the oxygen covered A u ( l l l ) surface were observed
to be complex. In this study X -ray photoelectron spectroscopy
(XPS) has been used to search fo r a carbonate species. The
presence o f a carbonate species may influence the oxidation o f CO
thereby causing complex reaction kinetics w ith oxygen on A u ( l l l )
4.2 E x p e rim e n ta l
Experiments were performed in a stainless steel U H V
chamber equipped w ith LE ED , AES, XPS and TPD apparatus, as
described in detail in chapter 2. An A1 K a emission line (1486.6
eV) was used fo r XPS.
j The A u ( l l l ) crystal was cleaned by 1 keV A r+ sputtering,
j Surface purity was checked by AES. LE ED gave a hexagonal
j pattern o f broad spots and the hexagonal cluster surrounding these
| integral integer spots was not resolved, indicating poor long range
order.
J Exposing the sample at 300K to ozone formed atomic
I oxygen on the surface. Ozone was dosed through a collim ated
| beam doser w ith the gold crystal rotated in front o f the doser. The
preparation o f ozone is described in Chapter 2. Carbon monoxide
j (Matheson, UHP grade) and carbon dioxide (Matheson,C.P. grade)
were used w ithout further purification. CO and CO 2 were dosed
through a 1/4 inch stainless steel tube w ith the sample rotated in
front o f the tube doser.
Oxygen coverages were determined by O2 TPD. Ozone is
exposed to the gold crystal u n til a saturation Oa coverage is
reached, the area o f the desorption peak is measured and assigned
to 1.0 monolayer oxygen coverage [5]. B y know ing this area other
coverages can be determined by comparison. Repeated dosing and
TPD is carried out until the proper Oa coverage was obtained.
J
4.3 Results
j 4.3.1 XPS of CO exposed on A u (lll) and 0 /A u ( lll)
j The O (ls ) XPS spectra fo r oxygen adsorbed on A u ( l l l )
| w ith ©o = 0.5 and 1.0 were in good agreement w ith previous
| results, discussed in Chapter 2. A C (ls ) XPS peak was not
| observed fo llow ing ozone exposure to clean A u (l 11), as shown in
| figure 4.1. The feature from 282 - 290 eV is a satellite peak
J resulting from the X -ray source. C (ls ) and 0(1 s) XPS spectra
taken after a 10.0 L CO exposure to A u ( l l l ) w ith © 0 = 0.0, 0.5
and 1.0 at 100 K is shown in figures 4.2 and 4.3, respectively.
The area o f the O (ls ) peak after exposure to 10.0 L CO at 100 K
is approxim ately 2 percent smaller for both oxygen coverages.
AES studies show that CO oxidizes to form CO 2 at temperatures
| from 250 - 375 K and pressures between 2 - 10 x 10~ 8 torr [4].
I The small oxygen loss is attributed to the form ation o f CO 2 which
has desorbed from the surface. In the AES studies oxygen on
A u (l 11) exposures were as high as 90.0 L at 250 K and titrated 90
percent o f the adsorbed oxygen. The small amount o f titrated
oxygen in our experiment may be due to significantly smaller
exposures o f CO. No carbon species were observed on the surface
as indicated by the absence o f a C (ls) peak in 4.2. These results
indicate that no carbonate species is stable on the A u ( l l l ) surface
under these conditions.
40
C O
cz
3
-O
x _
C O
co
c
0 5
O coverage
(monolayers)
1 .00
0.00
t i I i i i i — I
3 0 0 2 9 5 2 9 0 2 8 5 2 8 0 2 7 5 2 7 0
Binding Energy (eV)
Figure 4.1 C (Is) XPS spectra after ozone exposure on clean
A u ( l ll) at 300K.
41
iiii | - n-r- i | n r i |iii i | i i i r |-i r ~ r ~ r
O coverage
(monolayers)
1.00
— iiii Ii i i iIii tilii ti tiJiiliiii-3
3 0 0 2 9 5 2 9 0 2 8 5 2 8 0 2 7 5 2 7 0
Binding Energy (eV)
Figure 4.2 C (ls ) XPS spectra after 10 Langm uir CO exposure to
oxygen on A u (l 11) at 100K.
42
O coverage
(monolayers)
1 .00
0.50
0.00
5 2 0 5 3 0 5 4 0 5 5 0 5 6 0
Binding Energy (eV)
Figure 4.3 O (ls ) XPS spectra after 10 Langm uir CO exposure to
oxygen on A u (l 11) at 100K.
Intensity (arb. units)
oxygen coverage
(monolayers)
1 .00
0.75
0.25
0.00
5 2 0 5 3 0 5 5 0 5 4 0 5 6 0
Binding Energy (eV)
Figure 4.4 O (ls ) XPS spectra after ozone exposure to
clean A u ( l ll) at 100K.
44
oxygen coverage
(monolayers)
< / 3
1 .00
0.75
0.25
0.00
tz
ZJ
0 3
C / 3
< z
03
C
5 6 0 5 5 0 5 4 0 5 3 0 5 2 0
Binding Energy (eV)
Figure 4.5 O (ls) XPS spectra after 10 Langmuir CO2 exposure to
oxygen on Au(l 11) at 100K.
4.3.2 XPS of C O 2 exposed on A u ( lll) and 0 /A u ( lll)
Surfaces
The 0 (1 s) XPS spectra o f © 0 = 0.0, 0.25, 0.75 and 1.0 on
A u ( l l l ) is shown in figure 4.4. The spectra is in good agreement
w ith that observed above and in chapter 2. The small peak at 532.3
eV is due to background adsorption o f water, see Chapter 2. The
XPS spectra in the C (ls ) region prior to CO 2 exposure is sim ilar
to figure 4.1, i.e. no carbon peak is observed. F ollow ing CO2
exposure the 0 (1 s) peak area is observed to dim inish
approxim ately 10 and 15 percent for 0 O = 0.25 and 0.75,
respectively (figure 4.5 ). A commensurate increase at 534.1 eV
is observed. The nature o f this peak is d iffic u lt to explain as
carbon was not observed on the surface, figure 4. One explanation
is that a surface carbonate may be formed, the cross section fo r
carbon is small and may be below detectable lim its. The peak may
also be attributed to background adsorption o f water. Peaks at
534.3 eV have been observed fo r m ultilayer water, as discussed in
Chapter 2 . The 0(1 s) peak remained constant fo r C O 2 exposures
to ©o = 1.0
4.4 Discussion
The oxidation o f CO to form CO2 on A u ( l l l ) does not
exhibit simple first order reaction kinetics fo r either oxygen or
carbon monoxide [4]. This result suggests the form ation o f a
surface intermediate or a competing reaction.The readsorption o f
CO 2 onto surface oxygen to form a carbonate species occurs on
silver under U H V conditions [3],
XPS results indicate that only a small amount o f oxygen is
titrated from the A u ( l l l ) surface at 100 K. AES studies indicate
that CO does oxidize on the oxygen covered surface at 250 - 375
K [4]. CO exposures in the AES study were 9 times greater than in
our study. TPD experiments show that oxygen is titrated when
CO is exposed to oxygen preadsorbed on A u ( l l l ) at 100 K.
Therefore the small amount o f titrated oxygen in our experiment
may be due to significantly smaller exposures o f CO. The C (ls )
XPS spectra showed that carbon is not present on the surface at
100 K. Thus the oxidation reaction kinetics cannot be explained
by the presence o f carbon dioxide or a stable carbonate species on
j the surface.
j XPS results show a decrease in the atomic oxygen 0 (1 s)
peak fo llow ing exposure o f CO2 to the oxygen covered A u ( ll 1)
surface, an equivalent increase in peak area is observed at 534.1
J eV. The nature o f this peak is unknown. One possibility is the
form ation o f a carbonate species, however XPS spectra reveal that
carbon is not present on the surface. The peak at 534.1 eV may be
caused by background adsorption o f water. The 0 (1 s) XPS peak
fo r a water m ultilayer was observed at 534.3 eV in Chapter 2.
4.5 Conclusion
47
XPS results indicate that carbon dioxide and carbonate
species are not present on the surface follow ing the exposure o f
CO to CO2 oxygen adatoms on A u (l 11) at 100 K.
References
1. H .A . Englhardt, A .M . Bradshaw and D. Menzel, Surf. Sci.
40(1973)4.
2. G. M cElhiney, H. Papp and J. Pritchard, Surface
Sci.54(1976)617.
3. M . Bowker, M .A . Barteau and R.J. M adix, Surface Sci.
| 92(1980)528
|
1 4. D. W ickham and B.E. Koel, to be submitted.
|
5. D.H. Parker and B.E. Koel, submitted to Surface Sci.

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Creator Lazaga, Mark (author)

Core Title The reactivity of small molecules with coadsorbed oxygen on AU(111)

Degree Master of Science

Degree Program Chemistry

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