Boreskov, G. K.

Felter, T. E.

Weinberg, W. H.

Zhdan, P. A.

English

Caltech Authors - Main

L. D. Schmidt and M. J. Mummey: Summary Abstract: Decomposition of NO 
quite different than those measured at low pressure. When 
the NO was pumped out after reaction, the surface was cov-
ered by a monolayer of NO which had several binding states, 
the most tightly bound having a heat of adsorption of ""30 kcal 
mol-1. Oxygen adsorption on the Pt ribbon was also found to 
have a heat of adsorption of ,.,.,50 kcal mol-1 as expected. 
TPD and AES were only carried out after cooling the rib-
bon to 300 K and pumping out gases to <l0-8 Torr, and the 
states observed in reaction should be so weakly bound that 
they desorb upon pumpout. 
Another set of experiments was carried out with TPD under 
reaction conditions at pressures between w-4 and w-2 Torr. 
The ribbon temperature and reaction rate were allowed to 
reach steady state after which the surface was heated to desorb 
all species and the TPD spectra were recorded. All species 
desorption properties were in agreement with those obtained 
at w-s Torr. These experiments also yielded equilibrium 
coverages of NO vs temperature. Isobars between 10-4 and 
w-3 Torr yielded a heat of adsorption of NO of ,..,.,30 kcal 
mol-l under reaction conditions, in agreement with UHV 
results. 
These results appear to indicate that the tightly bound states 
of NO and atomic oxygen are not those observed in reaction. 
The reaction data is consistent with weakly bound species, 
essentially physically adsorbed, being reactive. However, TPD 
shows that the tightly bound state of NO is present during 
reaction. The explanation which appears to agree with all data 
is that reaction involves a weakly bound precursor interme-
diate species. We have shown that the kinetics should then 
exhibit the properties of the precursor state only. Apparently 
reaction of NO proceeds through a weakly bound state which 
is inhibited by a weakly bound state of oxygen. Recent results 
in this laboratory on the reaction between NO and CO con-
firm that weakly bound states may also be reactive. 
•)This work partially supported by NSF under grant No. DMR75-02627. 
b)Monsanto Corporation, St. Louis, MO. 
Summary Abstract: Oxidation of carbon monoxide 
over Ag (111) by preadsorbed active oxygen studied 
by XPS and upsa> 
T. E. Felter and W. H. Weinbergbl 
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 
P. A. Zhdan and G. K. Boreskov 
Institute of Catalysis, The Academy of Sciences of the U.S.S.R., Novosibirsk, U.S.S.R. 630090 
(Received 5 September 1980; accepted 31 December 1980) 
PACS numbers: 81.60.Hv, 79.60.Gs, 82.65.Nz 
Recently, the oxidation of adsorbed carbon monoxide has 
attracted attention as a simple surface chemical reaction 
amenable to various surface-sensitive techniques. For ex-
ample, CO oxidation over Ir(lll),l-3 Ir(ll0),4 Pt,5,6 Rh,7,8 
Ag(ll0),9 Pd, 10-12 and Cu(ll0)13 have been studied, usually 
by mass spectrometry,2-11 but also by XPS, 1·3 molecular 
beams, 12 LEED, lO and work function and ellipsometry mea-
surements.13 In the present work, the kinetics of CO oxidation 
over Ag(l11) was studied by monitoring XPS intensities. The 
electronic structure is revealed by XPS and UPS and is pre-
sented elsewhere in a more comprehensive paper. 14 
The VG ESCA-3 hemispherical electron spectrometer, with 
UV and x-ray (1486.6) sources, 1 calibration, sample prepa-
ration, in situ cleaning by Ar+ bombardment, annealing, and 
temperature control have all been described previously. 15 
Oxygen adsorbs on Ag(1ll) at 300 K provided a hot (1500 
K) Pt filament is nearby. The filament was a 9 em length of 
0.5 mm diameter very high purity wire that was tightly coiled 
and placed "line of sight," 3.5 em from the front face of the 
Ag(lll) sample. The adsorbed oxygen exhibits an asym-
metrical 0 1s line at 528.5 eV with a full width at half maxi-
mum of approximately 1.6 eV and valence levels at 1.8 and 
approximately 5 eV, and is denoted active oxygen because 
it reacts readily with incident CO molecules. Probably the hot 
Pt excites, e.g. triplet -singlet rather than dissociates the 
oxygen. Apparently, the excited (unlike the ground state) 
molecule adsorbs with a high sticking coefficient, S, on 
Ag(lll), where it dissociatively adsorbs. With the filament 
off, S ""0. On Ag(llO), Wachs and Madix16 observed a simi-
lar, but much less dramatic effect due either to pronounced 
622 J. Vac. Sci. Technol., 18(2), March 1981 0022-5355/81/020622-03$01.00 © 1981 American Vacuum Society 622 
623 T. E. Felter eta/.: Summary Abstract: Oxidation of carbon 623 
Ag (111)/00 +CO 
FIG. 1. Absolute XPS coverage determination for oxygen14.l5.l7 during re-
action with CO. 
crystallographic specificity, or inadvertent excitation by other 
hot filaments or the V aclon pump. All filaments were off and 
oil diffusion pumps were employed in the present work. 
Transient kinetics measured in the spectrometer during 
exposure of Ag(ll1)/0 to CO at an estimated110-7 Torr (1.33 
X 10-5 Pa) are shown in Fig. 1 for various substrate temper-
atures, T, between 180 and 400 K. The ordinate is the 0 1s 
intensity for active oxygen at 528.5 eV, scaled according to 
the absolute coverage determination of Zhdan et a/. 14·15·17 
Evidently, [0], the active oxygen coverage is depleted by 
impinging CO, either by conversion to another adsorbed state 
with different 0 1s binding energy, or by removal by 
chemical reaction. Supplemental XPS experiments rule out 
the former and we assume the removal product to be C02(g). 
The removal rate increases with decreasing T, but it is always 
at least three times smaller than the incident rate of CO im-
pingement. 
Since supplemental experiments also showed no significant 
carbon 1s intensity, clearly a second CO removal mechanism 
dominates over C02(g) production by at least a factor of two. 
The simplest is, of course, first order CO desorption. Thus, the 
mass balance equation (assuming S = 1 for CO) is 
Pco -d[O] Y k = -d- + vco[CO] exp(-Eco/kT). (1) 
21r TM t 
For our coverage regime, the Langmuir-Hinshelwood 
mechanism is a reasonable starting point1-14,16: 
-d[O] , , 
~ = Vco2[CO][O] exp(-Eco2/kT) 
={Rate of C~2(g)}, (2) 
formation 
where the prime indicates rate constants for the rate limiting 
reaction step-C02 desorption being undoubtedly instan-
taneous at these temperatures. Substituting Eq. (2) into Eq. 
(1) and rearranging, gives 
-d[O] { Pco v[O] exp(-E/kT) } 
----;jt = y27rkTM v[O] exp(-E/kT) + 1 ' 
(3) 
where v = v~02/ vco and E = E~o2 - Eco. 
J. Vac. Sci. Technol., Vol. 18, No.2, March 1981 
Since the incident rate of CO impingement is at least three 
times smaller than the rate of C02 formation, the latter is less 
than half the rate of CO desorption. Therefore, their ratio (the 
first term in the denominator) is small compared to unity and 
Eq. ( 3) then becomes 
f =-In (y21rkTM X -d[O]) = ,!;__- ln[O]v, (4) 
Pco dt kT 
which is plotted in Fig. 2 for t = 0, along with a straight line 
least squares fit wherein E = -1.4 kcal/mole and v = 10-17 
cm2. The linearity of Fig. 2 further corroborates that Eq. (4) 
is a suitable approximation to Eq. (3). A recent study by 
Bowker et al. for CO oxidation over Ag(ll0)9 gives these 
values to be -1.1 kcal/mole and 1.5 X 10-17 cm2.18 Note that 
E is known to a greater accuracy ("'0.1 kcal/mole) than is 
possible for either of the difference terms defining it. The 
negative sign forE indicates C02 formation is energetically 
favored over CO desorption. Nevertheless, CO desorption is 
kinetically favored, i.e. more frequent because C02(g) pro-
duction depends on [CO] which is small since 180 K exceeds 
the CO desorption temperature, T d· In other words, since the 
CO is weakly chemisorbed,21 the residence time at the rela-
tively warm temperatures employed here (~180 K) is too 
small to allow for significant reaction with adsorbed 
oxygen. 
For a sufficiently low temperature, however, the first term 
in the denominator of Eq. (3) dominates so that the rate of CO 
incidence flux is equal to that of C02(g) production. In this 
regime f = 0, a horizontal line through the origin in Fig. 2. 
The intersection with Eq. (4), at 137 K, is a crude measure of 
T d and agrees with values obtained by TDMS for similar 
systems.9·19·20·21 In particular, for W(100)/Xe, Td = 145 K, 
and vxe = 3 X 1014 s-1 and Exe = 9.3 kcal/mole.l9 If these 
values are appropriate to CO/ Ag(ll1), we estimate v~02 = 
3 X w-3 cm2/s (typical for second order) and E~02 = 7.9 
kcal/mole. Alternatively, by employing isosteric measure-
ments of the adsorption of CO on Ag(lll) (which do not yield 
the pre-exponential)21 where the heat of adsorption was found 
0 
~0 3.0 -.bt.- 1100 
"--' 
2.0 
1.0 
2 3 
_L 
T 
0 
Ag (111)/0
0 
+CO 
-7 
Pco = 10 torr 
E = -1.4kcal/mole 
0 
0 
4 
( ~~-3) 
137K 
~ 
7 "-.. 
FIG. 2. Plot of Eq. (4) (see text) for the data of Fig. 1 at t = 0, 101 = 5.6 X 
1014 atoms/cm2. 
624 T. E. Felter eta/.: Summary Abstract: Oxidation of carbon 624 
to be 6.5 kcal/mole, we estimate E~02 = 5.1 kcal/mole, in 
agreement with Bowker's study 5.2 kcaljmole9 employing 
the same Eco. Since the rate parameters v and E for CO ox-
idation over the Ag(lll) surface (present study) are very 
similar indeed to those over Ag(ll0)9 we conclude that similar 
reaction mechanisms are involved. 
•)Supported by the National Science Foundation under Grant No. CHE77-
10629A02. 
h)Camille and Henry Dreyfus Foundation Teacher-Scholar. 
1P. A. Zhdan, G. K. Boreskov, W. F. Egelhoff, Jr., and W. H. Weinberg, Surf. 
Sci. 61, 377 (1976). 
2V. P. Ivanov, G. K. Boreskov, V. I. Savchenko, W. F. Egelhoff, Jr., and W. 
H. Weinberg, J. Catal. 48, 269 (1977). 
3P. A. Zhdan, G. K. Boreskov, A. I. Boronin, A. P. Schepelin, S. P. Withrow, 
and W. H. Weinberg, Appl. Surf. Sci. 3, 145 (1979). 
4J. L. Taylor, D. E. Ibbotson, and W. H. Weinberg, Surf. Sci. 90, 37 (1979); 
J. Catal. 61, 1 (1980). 
5G. K. Hori and L. D. Schmidt, J. Catal. 38, 335 (1975). 
6H. P. Bonze! and J. J. Burton, Surf. Sci. 52, 223 (1975). 
J. Vac. Sci. Technol., Vol. 18, No.2, March 1981 
7C. T. Campbell, S.-K. Shi, and J. M. White, J. Phys. Chern. 83, 2255 
(1979). 
8H.-I. Lee, G. Praline, and J. M. White, Surf. Sci. 91,581 (1980). 
9M. Bowker, M. A. Barteau, and R. J. Madix, Surf. Sci. 92, 528 (1980). 
10G. Ertl and J. Koch, 5th Proc. Int. Congr. Catal., 969 (1973). 
11T. Matsushima, C. J. Mussett, and J. M. White, J. Catal. 41, 397 (1976). 
12T. Engel and G. Ertl, J. Chern. Phys. 69, 1267 (1978). 
13F. H. P. M. Habraken, G. A. Bootsma, P. Hofmann, S. Hachiche, and A. 
M. Bradshaw, Surf. Sci. 88,285 (1979). 
14T. E. Felter, W. H. Weinberg, G. Ya. Lastushkina, and P. A. Zhdan, (to he 
published). 
15T. E. Felter, W. H. Weinberg, P. A. Zhdan, and G. K. Boreskov, Surf. Sci. 
Lett. 97, L313 (1980). 
161. E. Wachs and R. J. Madix, Surf. Sci. 76, 531 (1978). 
17P. A. Zhdan, (to he published). 
18The frequency factor ratio is obtained from their intercept of 0.002 sec-
onds-! upon division by F = 3 X 1014 molecules/cm2 s, (their calibration 
for the gas flux from their closer) and multiplication by In 10. 
19G. Doyan and G. Ertl, Surf. Sci. 69, 157 (1977), and references therein. 
2°C.-M. Chan, R. Aris, and W. H. Weinberg, Appl. Surf. Sci. 1, 360 
(1978). 
21G. McElhiney, H. Papp, and J. Pritchard, Surf. Sci. 54,617 (1976). 


Oxidation of carbon monoxide over Ag (111) by preadsorbed active oxygen studied by XPS and UPS

L. D. Schmidt and M. J. Mummey: Summary Abstract: Decomposition of NO 
quite different than those measured at low pressure. When 
the NO was pumped out after reaction, the surface was cov-
ered by a monolayer of NO which had several binding states, 
the most tightly bound having a heat of adsorption of ""30 kcal 
mol-1. Oxygen adsorption on the Pt ribbon was also found to 
have a heat of adsorption of ,.,.,50 kcal mol-1 as expected. 
TPD and AES were only carried out after cooling the rib-
bon to 300 K and pumping out gases to <l0-8 Torr, and the 
states observed in reaction should be so weakly bound that 
they desorb upon pumpout. 
Another set of experiments was carried out with TPD under 
reaction conditions at pressures between w-4 and w-2 Torr. 
The ribbon temperature and reaction rate were allowed to 
reach steady state after which the surface was heated to desorb 
all species and the TPD spectra were recorded. All species 
desorption properties were in agreement with those obtained 
at w-s Torr. These experiments also yielded equilibrium 
coverages of NO vs temperature. Isobars between 10-4 and 
w-3 Torr yielded a heat of adsorption of NO of ,..,.,30 kcal 
mol-l under reaction conditions, in agreement with UHV 
results. 
These results appear to indicate that the tightly bound states 
of NO and atomic oxygen are not those observed in reaction. 
The reaction data is consistent with weakly bound species, 
essentially physically adsorbed, being reactive. However, TPD 
shows that the tightly bound state of NO is present during 
reaction. The explanation which appears to agree with all data 
is that reaction involves a weakly bound precursor interme-
diate species. We have shown that the kinetics should then 
exhibit the properties of the precursor state only. Apparently 
reaction of NO proceeds through a weakly bound state which 
is inhibited by a weakly bound state of oxygen. Recent results 
in this laboratory on the reaction between NO and CO con-
firm that weakly bound states may also be reactive. 
•)This work partially supported by NSF under grant No. DMR75-02627. 
b)Monsanto Corporation, St. Louis, MO. 
Summary Abstract: Oxidation of carbon monoxide 
over Ag (111) by preadsorbed active oxygen studied 
by XPS and upsa> 
T. E. Felter and W. H. Weinbergbl 
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 
P. A. Zhdan and G. K. Boreskov 
Institute of Catalysis, The Academy of Sciences of the U.S.S.R., Novosibirsk, U.S.S.R. 630090 
(Received 5 September 1980; accepted 31 December 1980) 
PACS numbers: 81.60.Hv, 79.60.Gs, 82.65.Nz 
Recently, the oxidation of adsorbed carbon monoxide has 
attracted attention as a simple surface chemical reaction 
amenable to various surface-sensitive techniques. For ex-
ample, CO oxidation over Ir(lll),l-3 Ir(ll0),4 Pt,5,6 Rh,7,8 
Ag(ll0),9 Pd, 10-12 and Cu(ll0)13 have been studied, usually 
by mass spectrometry,2-11 but also by XPS, 1·3 molecular 
beams, 12 LEED, lO and work function and ellipsometry mea-
surements.13 In the present work, the kinetics of CO oxidation 
over Ag(l11) was studied by monitoring XPS intensities. The 
electronic structure is revealed by XPS and UPS and is pre-
sented elsewhere in a more comprehensive paper. 14 
The VG ESCA-3 hemispherical electron spectrometer, with 
UV and x-ray (1486.6) sources, 1 calibration, sample prepa-
ration, in situ cleaning by Ar+ bombardment, annealing, and 
temperature control have all been described previously. 15 
Oxygen adsorbs on Ag(1ll) at 300 K provided a hot (1500 
K) Pt filament is nearby. The filament was a 9 em length of 
0.5 mm diameter very high purity wire that was tightly coiled 
and placed "line of sight," 3.5 em from the front face of the 
Ag(lll) sample. The adsorbed oxygen exhibits an asym-
metrical 0 1s line at 528.5 eV with a full width at half maxi-
mum of approximately 1.6 eV and valence levels at 1.8 and 
approximately 5 eV, and is denoted active oxygen because 
it reacts readily with incident CO molecules. Probably the hot 
Pt excites, e.g. triplet -singlet rather than dissociates the 
oxygen. Apparently, the excited (unlike the ground state) 
molecule adsorbs with a high sticking coefficient, S, on 
Ag(lll), where it dissociatively adsorbs. With the filament 
off, S ""0. On Ag(llO), Wachs and Madix16 observed a simi-
lar, but much less dramatic effect due either to pronounced 
622 J. Vac. Sci. Technol., 18(2), March 1981 0022-5355/81/020622-03$01.00 © 1981 American Vacuum Society 622 
623 T. E. Felter eta/.: Summary Abstract: Oxidation of carbon 623 
Ag (111)/00 +CO 
FIG. 1. Absolute XPS coverage determination for oxygen14.l5.l7 during re-
action with CO. 
crystallographic specificity, or inadvertent excitation by other 
hot filaments or the V aclon pump. All filaments were off and 
oil diffusion pumps were employed in the present work. 
Transient kinetics measured in the spectrometer during 
exposure of Ag(ll1)/0 to CO at an estimated110-7 Torr (1.33 
X 10-5 Pa) are shown in Fig. 1 for various substrate temper-
atures, T, between 180 and 400 K. The ordinate is the 0 1s 
intensity for active oxygen at 528.5 eV, scaled according to 
the absolute coverage determination of Zhdan et a/. 14·15·17 
Evidently, [0], the active oxygen coverage is depleted by 
impinging CO, either by conversion to another adsorbed state 
with different 0 1s binding energy, or by removal by 
chemical reaction. Supplemental XPS experiments rule out 
the former and we assume the removal product to be C02(g). 
The removal rate increases with decreasing T, but it is always 
at least three times smaller than the incident rate of CO im-
pingement. 
Since supplemental experiments also showed no significant 
carbon 1s intensity, clearly a second CO removal mechanism 
dominates over C02(g) production by at least a factor of two. 
The simplest is, of course, first order CO desorption. Thus, the 
mass balance equation (assuming S = 1 for CO) is 
Pco -d[O] Y k = -d- + vco[CO] exp(-Eco/kT). (1) 21r TM t 
For our coverage regime, the Langmuir-Hinshelwood 
mechanism is a reasonable starting point1-14,16: 
-d[O] , , ~ = Vco2[CO][O] exp(-Eco2/kT) 
={Rate of C~2(g)}, (2) 
formation 
where the prime indicates rate constants for the rate limiting 
reaction step-C02 desorption being undoubtedly instan-
taneous at these temperatures. Substituting Eq. (2) into Eq. 
(1) and rearranging, gives 
-d[O] { Pco v[O] exp(-E/kT) } 
----;jt = y27rkTM v[O] exp(-E/kT) + 1 ' (3) 
where v = v~02/ vco and E = E~o2 - Eco. 
J. Vac. Sci. Technol., Vol. 18, No.2, March 1981 
Since the incident rate of CO impingement is at least three 
times smaller than the rate of C02 formation, the latter is less 
than half the rate of CO desorption. Therefore, their ratio (the 
first term in the denominator) is small compared to unity and 
Eq. ( 3) then becomes 
f =-In (y21rkTM X -d[O]) = ,!;__- ln[O]v, (4) 
Pco dt kT 
which is plotted in Fig. 2 for t = 0, along with a straight line 
least squares fit wherein E = -1.4 kcal/mole and v = 10-17 
cm2. The linearity of Fig. 2 further corroborates that Eq. (4) 
is a suitable approximation to Eq. (3). A recent study by 
Bowker et al. for CO oxidation over Ag(ll0)9 gives these 
values to be -1.1 kcal/mole and 1.5 X 10-17 cm2.18 Note that 
E is known to a greater accuracy ("'0.1 kcal/mole) than is 
possible for either of the difference terms defining it. The 
negative sign forE indicates C02 formation is energetically 
favored over CO desorption. Nevertheless, CO desorption is 
kinetically favored, i.e. more frequent because C02(g) pro-
duction depends on [CO] which is small since 180 K exceeds 
the CO desorption temperature, T d· In other words, since the 
CO is weakly chemisorbed,21 the residence time at the rela-
tively warm temperatures employed here (~180 K) is too 
small to allow for significant reaction with adsorbed 
oxygen. 
For a sufficiently low temperature, however, the first term 
in the denominator of Eq. (3) dominates so that the rate of CO 
incidence flux is equal to that of C02(g) production. In this 
regime f = 0, a horizontal line through the origin in Fig. 2. 
The intersection with Eq. (4), at 137 K, is a crude measure of 
T d and agrees with values obtained by TDMS for similar 
systems.9·19·20·21 In particular, for W(100)/Xe, Td = 145 K, 
and vxe = 3 X 1014 s-1 and Exe = 9.3 kcal/mole.l9 If these 
values are appropriate to CO/ Ag(ll1), we estimate v~02 = 
3 X w-3 cm2/s (typical for second order) and E~02 = 7.9 
kcal/mole. Alternatively, by employing isosteric measure-
ments of the adsorption of CO on Ag(lll) (which do not yield 
the pre-exponential)21 where the heat of adsorption was found 
0 
~0 3.0 -.bt.- 1100 
"--' 
2.0 
1.0 
2 3 
_L 
T 
0 
Ag (111)/00 +CO 
-7 Pco = 10 torr 
E = -1.4kcal/mole 
0 
0 
4 
( ~~-3) 
137K 
~ 
7 "-.. 
FIG. 2. Plot of Eq. (4) (see text) for the data of Fig. 1 at t = 0, 101 = 5.6 X 
1014 atoms/cm2. 
624 T. E. Felter eta/.: Summary Abstract: Oxidation of carbon 624 
to be 6.5 kcal/mole, we estimate E~02 = 5.1 kcal/mole, in 
agreement with Bowker's study 5.2 kcaljmole9 employing 
the same Eco. Since the rate parameters v and E for CO ox-
idation over the Ag(lll) surface (present study) are very 
similar indeed to those over Ag(ll0)9 we conclude that similar 
reaction mechanisms are involved. 
•)Supported by the National Science Foundation under Grant No. CHE77-
10629A02. 
h)Camille and Henry Dreyfus Foundation Teacher-Scholar. 
1P. A. Zhdan, G. K. Boreskov, W. F. Egelhoff, Jr., and W. H. Weinberg, Surf. 
Sci. 61, 377 (1976). 
2V. P. Ivanov, G. K. Boreskov, V. I. Savchenko, W. F. Egelhoff, Jr., and W. 
H. Weinberg, J. Catal. 48, 269 (1977). 
3P. A. Zhdan, G. K. Boreskov, A. I. Boronin, A. P. Schepelin, S. P. Withrow, 
and W. H. Weinberg, Appl. Surf. Sci. 3, 145 (1979). 
4J. L. Taylor, D. E. Ibbotson, and W. H. Weinberg, Surf. Sci. 90, 37 (1979); 
J. Catal. 61, 1 (1980). 
5G. K. Hori and L. D. Schmidt, J. Catal. 38, 335 (1975). 
6H. P. Bonze! and J. J. Burton, Surf. Sci. 52, 223 (1975). 
J. Vac. Sci. Technol., Vol. 18, No.2, March 1981 
7C. T. Campbell, S.-K. Shi, and J. M. White, J. Phys. Chern. 83, 2255 
(1979). 
8H.-I. Lee, G. Praline, and J. M. White, Surf. Sci. 91,581 (1980). 
9M. Bowker, M. A. Barteau, and R. J. Madix, Surf. Sci. 92, 528 (1980). 
10G. Ertl and J. Koch, 5th Proc. Int. Congr. Catal., 969 (1973). 
11T. Matsushima, C. J. Mussett, and J. M. White, J. Catal. 41, 397 (1976). 
12T. Engel and G. Ertl, J. Chern. Phys. 69, 1267 (1978). 
13F. H. P. M. Habraken, G. A. Bootsma, P. Hofmann, S. Hachiche, and A. 
M. Bradshaw, Surf. Sci. 88,285 (1979). 
14T. E. Felter, W. H. Weinberg, G. Ya. Lastushkina, and P. A. Zhdan, (to he 
published). 
15T. E. Felter, W. H. Weinberg, P. A. Zhdan, and G. K. Boreskov, Surf. Sci. 
Lett. 97, L313 (1980). 
161. E. Wachs and R. J. Madix, Surf. Sci. 76, 531 (1978). 
17P. A. Zhdan, (to he published). 
18The frequency factor ratio is obtained from their intercept of 0.002 sec-
onds-! upon division by F = 3 X 1014 molecules/cm2 s, (their calibration 
for the gas flux from their closer) and multiplication by In 10. 
19G. Doyan and G. Ertl, Surf. Sci. 69, 157 (1977), and references therein. 
2°C.-M. Chan, R. Aris, and W. H. Weinberg, Appl. Surf. Sci. 1, 360 
(1978). 
21G. McElhiney, H. Papp, and J. Pritchard, Surf. Sci. 54,617 (1976). 


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Oxidation of carbon monoxide over Ag (111) by preadsorbed active oxygen studied by XPS and UPS

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