Ibbotson, D. E.

Taylor, J. L.

Weinberg, W. H.

English

Caltech Authors - Main

Summary Abstract: Carbon monoxide oxidation 
catalysis over lr{11 O)a> 
W. H. Weinberg,bl D. E. Ibbotson, and J. L. Taylorcl 
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 
(Received II August 1980; accepted 29 October 1980) 
PACS numbers: 82.65.Jv, 82.80.Ms 
The heterogeneously catalyzed reaction of CO and 0 2 to form 
C02 over Ir(llO) has been studied through measurements of 
both the transient kinetics of the various elementary reactions 
that might limit the steady state rate1 as well as the steady state 
rates under various conditions. 2 The elementary reactions that 
are relevant include the adsorption of CO, the adsorption of 
oxygen, the desorption of CO, and the oxidation of CO via the 
Langmuir-Hinshelwood mechanism. Mass spectrometry was 
used to develop rate expressions for these elementary reactions 
under transient conditions and to measure the steady state 
rates of C02 production. 
Transient Experiments 1: The nonreactive coadsorption 
and subsequent reaction of CO and oxygen on Ir(llO) under 
transient conditions focus on the following: (1) The depen-
dence of the rate expressions for CO desorption and CO oxi-
dation upon the composition of the adlayer, and (2) the 
competitive adsorption of CO and oxygen. Several thermal 
desorption spectra of CO, 0 2, and C02 were obtained by 
flashing coadsorbed layers of CO and oxygen from clean and 
oxidized Ir(110) at a constant heating rate. Activation energies 
and pre-exponential factors for CO desorption and CO oxi-
dation were determined from the thermal desorption spectra 
by varying the initial coverage of one reactant. The coad-
sorbed layers were formed by sequential exposure of the 
reactants. During the exposures, the surface temperature was 
300 K, where the oxidation reaction is negligibly slow. Hence, 
the adsorption kinetics of oxygen (CO) on a surface partially 
covered with CO (oxygen) could be measured by a knowledge 
of the adsorption kinetics of the first component and a mass 
balance for the second component from the thermal desorp-
tion spectra. The results of this study are as follows. The ki-
netics of CO oxidation and CO desorption may be modeled 
accurately by Arrhenius expressions. The activation energy 
for CO desorption, which depends on the coverages of both 
CO and oxygen, may vary from 10 to 36 kcal mol-1, but 
maintains near 16 kcal moi-l where the steady state rate of 
CO oxidation is rapid, i.e. on a surface partially covered with 
oxygen near 600 K. The pre-exponential factor associated with 
this activation energy is 106 s-1. The activation energy for CO 
oxidation via the Langmuir-Hinshelwood mechanism is 
relatively insensitive to the composition of the adlayer and 
is approximately equal to 12 kcal mol-1. The pre-exponential 
factor for the oxidation reaction is 3 X 10-9 or 3 X 10-10 cm2 
s-1, for surface temperatures near 400 and 550 K, respectively. 
This variation as well as the small value of the pre-exponential 
factor indicates diffusion of the reactants on the surface, rather 
than the adatom-admolecule reaction, may limit the oxidation 
rate on the surface. 
The initial probability of adsorption of CO on clean or ox-
idized Ir( 110) at 300 K is unity, regardless of the coverage of 
chemisorbed oxygen, indicating oxygen does not block sites 
for CO chemisorption. However, if sufficient oxygen is 
chemisorbed, the adsorption kinetics of CO are of the Lang-
muir type and are second order in the coverage of CO. Both 
chemisorbed oxygen and CO block sites for oxygen chemi-
sorption so that the adsorption kinetics of oxygen are of the 
Langmuir type, vary as second order in the coverages of CO 
and oxygen, but also depend on the structure of the chemi-
sorbed layer. If the sites occupied by CO are accounted for, 
the initial probability of adsorption of oxygen at a surface 
temperature of 300 K is 0.27 on the clean surface, the same 
as found in the absence of CO; but it is 0.035 on the oxidized 
surface, which is an order of magnitude smaller than the value 
obtained in the absence of CO. 
Steady State Experiments 2: The steady state rates of C02 
production from CO and 0 2 were measured over Ir(llO) for 
surface temperatures between 300 and 1000 K, for partial 
pressures of the reactants between 10-8 and 3 X w-6 Torr, 
and for partial pressure ratios (P col P o2) between 0.1 and 10. 
Rate expressions developed from transient reactions were 
combined into a model that both qualitatively and quantita-
tively predicts trends in the steady state measurements. 
However, the following four phenomena limit the applica-
bility of this model. (1) The activation energy for CO de-
sorption depends markedly upon the composition of the ad-
layer. (2) Diffusion in the adlayer may limit the rates of CO 
desorption and CO oxidation, but this effect can be included 
empirically in the rate expression. (3) The catalyst surface is 
oxidized at temperatures where the steady state rate of CO 
oxidation is rapid, i.e., a second type of atomically adsorbed 
oxygen forms irreversibly near 700 K. (4) Hysteresis in the rate 
of C02 production with temperature occurs in the steady state 
reaction, which cannot be attributed solely to oxidation of the 
catalyst. 
The experimental results are as follows. Below 600 K, where 
the oxidation rate increases with surface temperature, the rate 
is inversely proportional to the partial pressure of CO, but it 
is proportional to the square of the partial pressure of oxygen. 
The unusual variation in the rate with the oxygen pressure 
may be associated with the diffusion of chemisorbed oxygen 
on the surface. If the steady state rate is measured below 600 
K as the surface temperature is increased, the apparent acti-
vation energy and pre-exponential factor for the steady state 
rate on clean Ir(llO) are 14 ± 2 kcal moi-1 and 2 X 1()25±1 
620 J. Vac. Sci. Techno!., 18(2), March 1981 0022-5355/81/020620-02$01.00 © 1981 American Vacuum Society 620 
Weinberg, Ibbotson and Taylor: Summary Abstract: Carbon monoxide oxidation 
molecules cm-2 Torr-1 s-1. However, if the rate is measured 
as the temperature is decreased, these two quantities vary 
considerably due to hysteresis in the oxidation rate. Above 600 
K, where the steady state rate decreases with surface tem-
perature, the oxidation rate is proportional to the partial 
pressure of CO, but it is independent of the partial pressure 
of oxygen. Regardless of whether the surface is clean or oxi-
dized and whether the surface temperature is being increased 
or decreased, the apparent activation energy and pre-expo-
nential factor for the overall rate coefficient are -4 ± 2 kcal 
mol-1 and 2 X 1019± 1 molecules cm-2 Torr-1 s-1. The tem-
perature associated with the maximum oxidation rate at 
steady state occurs near the intersections of the asymptotic 
curves above and below 600 K, and it is inversely proportional 
to the logarithm of the ratio of the CO partial pressure to the 
oxygen partial pressure. 
•)Supported by the National Science Foundation under Grant No. CHE77-
14976. 
b)Camille and Henry Dreyfus Foundation Teacher-Scholar. 
c)Present address: Proctor and Gamble Co., Miami Valley Laboratories, 
Cincinnati, Ohio 45247. 
1J. L. Taylor, D. E. Ibbotson, and W. H. Weinberg, Surf. Sci. 90, 37 
(1979). 
2J. L. Taylor, D. E. Ibbotson, and W. H. Weinberg, J. Catalysis 62, 1 
(1980). 
Summary Abstract: Decomposition of NO on clean Pt 
near atmospheric pressuresa> 
L. D. Schmidt and M. J. Mummeyb) 
Department a/Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 554455 
(Received 19 September 1980; accepted 18 November 1980) 
PACS numbers: 82.30.Lp, 82.65.Jv, 82.80.Pv, 82.20.Pm 
Nitric oxide decomposition on clean Pt is studied at pressures 
and temperatures comparable to those in the automotive 
catalytic converter. A high pressure reaction cell inside a UHV 
chamber is used to clean the surface before reaction and de-
termine its cleanliness and adsorbate coverages after reaction. 
The objectives of this research are to examine kinetics on a 
well defined surface and to determine which binding states 
of adsorbates are involved in high pressure reaction. 
The Pt ribbon is cleaned by heating in 02 and vacuum as 
determined by AES. It is then sealed in a 35 cm3 stainless steel 
cup through which NO and 0 2 are passed at pressures be-
tween 10-4 (1.3 X 10-2 Pa) and 1000 Torr. Reaction rates are 
measured in either batch or flow configurations. After reac-
tion the pressure is reduced to 10-9 Torr, and the ribbon is 
examined by AES and TPD of NO, Nz, 02, and N20. In an-
other set of experiments TPD is carried out at high pressures 
under reaction conditions to determine the coverages during 
reaction. 
The reaction rate was measured for NO pressures between 
10-4 and 30 Torr, temperatures between 500 and 1400 K, and 
rates between 1014 and 1018 molecules cm-2s- 1. Over the 
entire range of temperatures and pressures the rate attained 
a steady state within a few seconds and could be fit quanti-
tatively by a Langmuir-Hinshelwood rate expression, 
kRKNoPNo r = -----'"----'-'-"'----'--'-=----
1 + KNoPNo + Ko2Po2 
to within an accuracy of ±20%. The reaction rate coefficient 
kR and adsorption equilibrium constants KNo and K02 were 
found to have Arrhenius temperature dependences 
kR(molecules cm-2s-1) = 8 X 1019 exp[-13 500/RT] 
KNo(Torr-1) = 7 X 10-4 exp[8250/RT] 
and 
Ko2(Torr-
1) = 1.6 exp[9550/RT] 
with energies in cal mol-1. 
The simple interpretation of this rate expression is the 
unimolecular decomposition of NO with strong competitive 
adsorption of oxygen. The heats of adsorption of NO and 0 2 
are 8.5 and 10 kcal mol-1 respectively, and the P0~ depen-
dence suggests that the inhibiting state of oxygen is molecu-
larly adsorbed. 
The adsorption parameters measured in reaction of NO are 
621 J. Vac. Sci. Techno!., 18(2), March 1981 0022-5355/81/020621-02$01.00 © 1981 American Vacuum Society 621 


Carbon monoxide oxidation catalysis over Ir(110)

Summary Abstract: Carbon monoxide oxidation 
catalysis over lr{11 O)a> 
W. H. Weinberg,bl D. E. Ibbotson, and J. L. Taylorcl 
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 
(Received II August 1980; accepted 29 October 1980) 
PACS numbers: 82.65.Jv, 82.80.Ms 
The heterogeneously catalyzed reaction of CO and 0 2 to form 
C02 over Ir(llO) has been studied through measurements of 
both the transient kinetics of the various elementary reactions 
that might limit the steady state rate1 as well as the steady state 
rates under various conditions. 2 The elementary reactions that 
are relevant include the adsorption of CO, the adsorption of 
oxygen, the desorption of CO, and the oxidation of CO via the 
Langmuir-Hinshelwood mechanism. Mass spectrometry was 
used to develop rate expressions for these elementary reactions 
under transient conditions and to measure the steady state 
rates of C02 production. 
Transient Experiments 1: The nonreactive coadsorption 
and subsequent reaction of CO and oxygen on Ir(llO) under 
transient conditions focus on the following: (1) The depen-
dence of the rate expressions for CO desorption and CO oxi-
dation upon the composition of the adlayer, and (2) the 
competitive adsorption of CO and oxygen. Several thermal 
desorption spectra of CO, 0 2, and C02 were obtained by 
flashing coadsorbed layers of CO and oxygen from clean and 
oxidized Ir(110) at a constant heating rate. Activation energies 
and pre-exponential factors for CO desorption and CO oxi-
dation were determined from the thermal desorption spectra 
by varying the initial coverage of one reactant. The coad-
sorbed layers were formed by sequential exposure of the 
reactants. During the exposures, the surface temperature was 
300 K, where the oxidation reaction is negligibly slow. Hence, 
the adsorption kinetics of oxygen (CO) on a surface partially 
covered with CO (oxygen) could be measured by a knowledge 
of the adsorption kinetics of the first component and a mass 
balance for the second component from the thermal desorp-
tion spectra. The results of this study are as follows. The ki-
netics of CO oxidation and CO desorption may be modeled 
accurately by Arrhenius expressions. The activation energy 
for CO desorption, which depends on the coverages of both 
CO and oxygen, may vary from 10 to 36 kcal mol-1, but 
maintains near 16 kcal moi-l where the steady state rate of 
CO oxidation is rapid, i.e. on a surface partially covered with 
oxygen near 600 K. The pre-exponential factor associated with 
this activation energy is 106 s-1. The activation energy for CO 
oxidation via the Langmuir-Hinshelwood mechanism is 
relatively insensitive to the composition of the adlayer and 
is approximately equal to 12 kcal mol-1. The pre-exponential 
factor for the oxidation reaction is 3 X 10-9 or 3 X 10-10 cm2 
s-1, for surface temperatures near 400 and 550 K, respectively. 
This variation as well as the small value of the pre-exponential 
factor indicates diffusion of the reactants on the surface, rather 
than the adatom-admolecule reaction, may limit the oxidation 
rate on the surface. 
The initial probability of adsorption of CO on clean or ox-
idized Ir( 110) at 300 K is unity, regardless of the coverage of 
chemisorbed oxygen, indicating oxygen does not block sites 
for CO chemisorption. However, if sufficient oxygen is 
chemisorbed, the adsorption kinetics of CO are of the Lang-
muir type and are second order in the coverage of CO. Both 
chemisorbed oxygen and CO block sites for oxygen chemi-
sorption so that the adsorption kinetics of oxygen are of the 
Langmuir type, vary as second order in the coverages of CO 
and oxygen, but also depend on the structure of the chemi-
sorbed layer. If the sites occupied by CO are accounted for, 
the initial probability of adsorption of oxygen at a surface 
temperature of 300 K is 0.27 on the clean surface, the same 
as found in the absence of CO; but it is 0.035 on the oxidized 
surface, which is an order of magnitude smaller than the value 
obtained in the absence of CO. 
Steady State Experiments 2: The steady state rates of C02 
production from CO and 0 2 were measured over Ir(llO) for 
surface temperatures between 300 and 1000 K, for partial 
pressures of the reactants between 10-8 and 3 X w-6 Torr, 
and for partial pressure ratios (P col P o2) between 0.1 and 10. 
Rate expressions developed from transient reactions were 
combined into a model that both qualitatively and quantita-
tively predicts trends in the steady state measurements. 
However, the following four phenomena limit the applica-
bility of this model. (1) The activation energy for CO de-
sorption depends markedly upon the composition of the ad-
layer. (2) Diffusion in the adlayer may limit the rates of CO 
desorption and CO oxidation, but this effect can be included 
empirically in the rate expression. (3) The catalyst surface is 
oxidized at temperatures where the steady state rate of CO 
oxidation is rapid, i.e., a second type of atomically adsorbed 
oxygen forms irreversibly near 700 K. (4) Hysteresis in the rate 
of C02 production with temperature occurs in the steady state 
reaction, which cannot be attributed solely to oxidation of the 
catalyst. 
The experimental results are as follows. Below 600 K, where 
the oxidation rate increases with surface temperature, the rate 
is inversely proportional to the partial pressure of CO, but it 
is proportional to the square of the partial pressure of oxygen. 
The unusual variation in the rate with the oxygen pressure 
may be associated with the diffusion of chemisorbed oxygen 
on the surface. If the steady state rate is measured below 600 
K as the surface temperature is increased, the apparent acti-
vation energy and pre-exponential factor for the steady state 
rate on clean Ir(llO) are 14 ± 2 kcal moi-1 and 2 X 1()25±1 
620 J. Vac. Sci. Techno!., 18(2), March 1981 0022-5355/81/020620-02$01.00 © 1981 American Vacuum Society 620 
Weinberg, Ibbotson and Taylor: Summary Abstract: Carbon monoxide oxidation 
molecules cm-2 Torr-1 s-1. However, if the rate is measured 
as the temperature is decreased, these two quantities vary 
considerably due to hysteresis in the oxidation rate. Above 600 
K, where the steady state rate decreases with surface tem-
perature, the oxidation rate is proportional to the partial 
pressure of CO, but it is independent of the partial pressure 
of oxygen. Regardless of whether the surface is clean or oxi-
dized and whether the surface temperature is being increased 
or decreased, the apparent activation energy and pre-expo-
nential factor for the overall rate coefficient are -4 ± 2 kcal 
mol-1 and 2 X 1019± 1 molecules cm-2 Torr-1 s-1. The tem-
perature associated with the maximum oxidation rate at 
steady state occurs near the intersections of the asymptotic 
curves above and below 600 K, and it is inversely proportional 
to the logarithm of the ratio of the CO partial pressure to the 
oxygen partial pressure. 
•)Supported by the National Science Foundation under Grant No. CHE77-
14976. 
b)Camille and Henry Dreyfus Foundation Teacher-Scholar. 
c)Present address: Proctor and Gamble Co., Miami Valley Laboratories, 
Cincinnati, Ohio 45247. 
1J. L. Taylor, D. E. Ibbotson, and W. H. Weinberg, Surf. Sci. 90, 37 
(1979). 
2J. L. Taylor, D. E. Ibbotson, and W. H. Weinberg, J. Catalysis 62, 1 
(1980). 
Summary Abstract: Decomposition of NO on clean Pt 
near atmospheric pressuresa> 
L. D. Schmidt and M. J. Mummeyb) 
Department a/Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 554455 
(Received 19 September 1980; accepted 18 November 1980) 
PACS numbers: 82.30.Lp, 82.65.Jv, 82.80.Pv, 82.20.Pm 
Nitric oxide decomposition on clean Pt is studied at pressures 
and temperatures comparable to those in the automotive 
catalytic converter. A high pressure reaction cell inside a UHV 
chamber is used to clean the surface before reaction and de-
termine its cleanliness and adsorbate coverages after reaction. 
The objectives of this research are to examine kinetics on a 
well defined surface and to determine which binding states 
of adsorbates are involved in high pressure reaction. 
The Pt ribbon is cleaned by heating in 02 and vacuum as 
determined by AES. It is then sealed in a 35 cm3 stainless steel 
cup through which NO and 0 2 are passed at pressures be-
tween 10-4 (1.3 X 10-2 Pa) and 1000 Torr. Reaction rates are 
measured in either batch or flow configurations. After reac-
tion the pressure is reduced to 10-9 Torr, and the ribbon is 
examined by AES and TPD of NO, Nz, 02, and N20. In an-
other set of experiments TPD is carried out at high pressures 
under reaction conditions to determine the coverages during 
reaction. 
The reaction rate was measured for NO pressures between 
10-4 and 30 Torr, temperatures between 500 and 1400 K, and 
rates between 1014 and 1018 molecules cm-2s- 1. Over the 
entire range of temperatures and pressures the rate attained 
a steady state within a few seconds and could be fit quanti-
tatively by a Langmuir-Hinshelwood rate expression, 
kRKNoPNo r = -----'"----'-'-"'----'--'-=----
1 + KNoPNo + Ko2Po2 
to within an accuracy of ±20%. The reaction rate coefficient 
kR and adsorption equilibrium constants KNo and K02 were 
found to have Arrhenius temperature dependences 
kR(molecules cm-2s-1) = 8 X 1019 exp[-13 500/RT] 
KNo(Torr-1) = 7 X 10-4 exp[8250/RT] 
and 
Ko2(Torr-1) = 1.6 exp[9550/RT] 
with energies in cal mol-1. 
The simple interpretation of this rate expression is the 
unimolecular decomposition of NO with strong competitive 
adsorption of oxygen. The heats of adsorption of NO and 0 2 
are 8.5 and 10 kcal mol-1 respectively, and the P0~ depen-
dence suggests that the inhibiting state of oxygen is molecu-
larly adsorbed. 
The adsorption parameters measured in reaction of NO are 
621 J. Vac. Sci. Techno!., 18(2), March 1981 0022-5355/81/020621-02$01.00 © 1981 American Vacuum Society 621 


https://authors.library.caltech.edu/32551/1/WEIjvst81.pdf

Carbon monoxide oxidation catalysis over Ir(110)

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main