I n d . E n g . Chem. Res. 1995,34, 2364-2370
2364
A Solid-state Electrochemical Study of Adsorbed Oxygen Species in the Light-Off Phenomenon of CO Oxidation over Platinum Catalysts Chung-Liang Chang and Ta-Jen Huanp Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C.
A solid-state electrochemical method was applied to determine the forms of adsorbed oxygen species on the surface of catalysts by the theory of statistics and electrochemistry. The forms of adsorbed oxygen species on the platinum surface were demonstrated to play a significant role in the light-off property of CO oxidation. On the contrary, the forms of adsorbed oxygen species on CuO had no influence on CO oxidation. The adsorbed 02-ion predominantly formed on the platinum surface at 200 "C was suggested as a crucial form in the light-off phenomenon. A competitive backdonation model was proposed to reasonably illustrate the mechanism of the light-off phenomenon of CO oxidation over platinum catalysts.
Introduction Oxygen plays a fundamental role in catalysis because it is a component of the most generally used type of catalysts, i.e., oxides, and it is the reactant in one of the most significant forms of catalytic reaction, i.e., oxidation. Regarding the research involving carbon monoxide oxidation, noble metal catalysts have received considerable attention for use in automotive emission control (Voltz et al., 1973; Kummer, 1980, 1986); their oxidation activities increase with increasing oxygen concentration and are inhibited by carbon monoxide (Langmuir, 1922; Voltz et al., 1973). A specific lightoff phenomenon on a platinum catalyst was observed at a specific temperature; this study (Schlatter et al., 1973) indicated that the activity of CO oxidation on platinum catalysts has a dramatic increase a t about 250 "C. A further investigation (Summers and Hegedus, 1978) explored the effects of poisioning and sintering on the properties of the light-off phenomenon on platinum and palladium-platinum catalysts. Nevertheless, so far there is no investigation on the mechanism or reason for the light-off phenomenon. In practice, it has been verified that the forms of adsorbed oxygen species have a significant relation with the catalytic activity of several catalytic reactions. Regarding the reactivity of adsorbed oxygen species on zinc oxide, CO oxidation with two different adsorbed oxygen species was studied by ESR (electron spin resonance) (Sancier, 1967), and 0 2 - ion was assumed to be the slowly reactive species. In another paper (Morrison and Bonnelle, 19721, it was suggested that the low-temperature form of adsorbed oxygen, i.e., 0 2 - , was probably the active species for CO oxidation. In addition, on the SnO2 surface, 0 2 - was identified as the major source of adsorbed oxygen ions up to 400 "C and the reactivity of CO with 0-at 125 "C was found to be about 20 times higher than that with 0 2 - by electrical conductance measurement (Lantto and Romppainen, 1987). Ding et al. (1994) indicated that the ratio of electrophilic oxygen species 0- and 0 2 - to lattice oxygen on the surface was crucial for C2 selectivity of oxidative coupling of methane over Ce4+-dopedBasWO6 catalysts. The identification and reactivity of adsorbed oxygen species on the surface of oxides were recently reviewed (Che and Tench, 1982,1983). Oxygen adsorbed on the MgO surface has two forms, i.e., 0 3 - and 0-, as
* Author to whom correspondence
should be addressed.
characterized by ESR (Tench et al., 1972; Tench, 1972). A chemical method (Bielanski and Najbar, 1972) was designed to determine the mean number of elementary electric charges per adsorbed oxygen atom. The temperature-programmed desorption spectrum of oxygen adsorbed on zinc oxide (Tanaka and Blyholder, 1972) has two peaks with maxima at 180-190 "C and 285295 "C, respectively. The low-temperature peak was identified as 0 2 - , and the high-temperature peak was probably due to 0- by ESR. Another study (Chon and Pajares, 1969) suggested that oxygen is chemisorbed primarily as 0 2 - between 100 and about 180 "C and as 0- above around 230 "C on zinc oxide. In the present work, a solid-state electrochemical method is applied to determine the forms of adsorbed oxygen species on surfaces of catalysts. This method improves the traditional SEP (solid electrolyte potentiometry) technique (Wagner, 1970; Lintz and Vayenas, 1989) which was used for measurement of oxygen activity in order to determine the adsorbed states of oxygen. SEP had been used t o study the mechanism of catalytic reactions (Vayenas and Saltsburg, 1979; Okamot0 et al., 1983). In fact, the chemical and physical phenomena on electrode surfaces belong t o the field of surface science. The electrochemical potential used in electrochemistry is equivalent to the Fermi level in physics (Reiss, 1985). The contact potential from contact between molecules and solid surfaces is the fundamental mechanism to form an electromotive force in an electrochemical system. In this work, electrochemistry is combined with the Boltzmann approximation to derive equations with respect to the forms and states of adsorbed oxygen species. The relations of adsorbed oxygen species and temperature on platinum and copper oxide catalysts provide an explanation for the light-off phenomenon. A competitive backdonation model was proposed to clarify the mechanism of the light-off phenomenon.
Theory In this section, we derive the equations for the determination of the states of adsorbed oxygen species. Consider the following electrode reaction,
At thermodynamic equilibrium, the sum of electrochemical potentials of the adsorbed oxygen and the electron
0888-588519512634-2364$09.00/0 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995 2366 is equal to that of adsorbed oxygen ions (Brett, 1993). The equilibrium of the electrochemical potential as expressed by
p(0,)
+ p ( e - ) = p(O,-)
(2)
is thus obtained. Combining the electrochemical potential with the thermodynamic activity, eq 2 can be rewritten as
p"(0,)
+ k T ln(aoz)+ ,ii(e-) = p"(O,-) + k T ln(aOz-) (3)
From eq 3, we obtain p(e-) = k T ln(K)
+ k T ln(aOz_laoz)
The activity of adsorbed oxygen species can be used as the mean number of particles at the Ek energy level (Reiss, 1985; Clark, 1974). Thus, differentiating eq 9 with respect to nk gives dnk = dUk = dii(e-)/kT
(10)
The combination of eqs 5 and 10 gives d%-
dii =
Eb )'( Oz
k T exp@(e-)/kTl= 1/kT
(11)
Therefore, for electrode reaction 1, the relation of the oxygen activity and the Fermi level (electrochemical potential) from eq 11 is
(4)
p(e-)= kT In(l/aoz)+ kT ln(K)
(12)
where In our system, the temperature is kept constant so that the second term of eq 12 is a constant. Since oxygen is an ideal gas at 1atm and above 150 "C (Perry et al., 19841, Le., ao, = PO,,eq 12 can be written as
p"(02-)- p"(0,) = k T ln(K) Equation 4 is rewritten as ao,- = (l/K)ao, exp&(e-)/kTl
p(e-)= k T 1n(l/Poz)+ C
(5)
The electrochemical potential used in electrochemistry has been demonstrated to be equivalent to the Fermi level used in physics (Brett, 1993; Reiss, 1985; Lin et al., 1993). The surface electrostatic potential affects the Fermi level by the relation (Guggenheim, 1929, 1930; Bard and Faulkner, 1980; Reiss, 19851,
The relation between the redox potential of the electrode and the Fermi level is (Reiss, 1985; Brett, 1993)
so that
kT Vredox= -p(e-)/e = -ln(Po,) + C e
When oxygen is chemisorbed on surfaces with charge transfer, a change of &, has a direct influence on the Fermi level. In order to describe the influence of adsorbed oxygen ions on the Fermi level, the Fermi-Dirac statistics is applied to derive the relation. From the grand canonical ensemble, the Fermi-Dirac statistical expression (McQuarrie, 1976) is nk =
1
+
1 exp[(rk - p(e-))/kTl
(7)
If a system coincides with the condition (Reif, 1965)
(Nl'V)1/3 >> (h2/3mkT)112 the Fermi-Dirac statistics approaches the Boltzmann approximation. In our system where the temperature is above 150 "C, ("P3 is equal t o 38 x which is value of (h2/3mkT)",. much larger than the 0.16 x Therefore, the Boltzmann approximation can be used to describe the behavior of adsorbed oxygen species in our syvtem and eq 7 can be simplified to the Boltzmann approximation, nk = exp[@(e-) - E J ~ T I
(8)
At high enough temperature, the @(e-) - ck)/kT term can be assumed t o be much smaller than 1(McQuarrie, 1976). Hence, eq 8 as expanded by Taylor's expansion can be simplified to
nk = 1
+ @(e-) - c,)/kT
where other expansion terms are neglected.
(9)
(13)
(15)
Other electrode reactions with different adsorbed oxygen species have similar relations between the redox potential and the oxygen activity as shown in Table 1. When a plot of Vredox vs Po, can be fitted to a specific equation in Table 1, the form of the adsorbed oxygen species is thus determined from the corresponding electrode reaction.
Experimental Procedure Apparatus. The experimental apparatus is shown in Figure 1. A solid electrolyte tube was used as the reactor. This reactor was composed of a one-end-closed 8 mol % yttria-stabilized zirconia (YSZ) tube which was 145 mm in length, 25.4 mm in inner diameter, and 3 mm in wall thickness. A potentiostat (Pine RDE4) was used as a power supply for oxygen pumping, and a digital multimeter (HP34401A1 with a 10 GQ input resistance was employed to record the redox potential of the working electrodes. Electrode Preparation. Copper paste and zinc oxide paste prepared in a laboratory were deposited on the inner surface of the tube as a working electrode. The sample was then heated from room temperature with a heating rate of 5 "C/min to 750 "C and kept at 750 "C for 90 min. Platinum paste (Engelhand A11211 was deposited on the inner and outer surfaces of the YSZ tube as the working and reference electrodes, respectively. The samples were heated from room temperature with a heating rate of 5 "C/min to 400 "C, kept at 400 "C for 2 h, and then heated to 820 "C and held for 20 min. The reference electrode was exposed to the atmosphere, and the working electrode served as catalyst for reactions.
2366 Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995 Table 1. Equations for Characterizationof the Adsorbed Oxygen Species electrode reaction
carbon monoxide balanced with argon (99.9995%,purity) a t 180-215 "C. The total flow rate was 200 mIJ min. The gas compositions were analyzed by gas chromotography.
equation
Results and Discussion
Potent iostat
Multimeter
-4
8
Figure 1. Scheme of reaction apparatus: 1, Pt lead; 2, thermocouple; 3, gas inlet; 4, gas outlet; 5 , cooling water inlet; 6, cooling water outlet; 7, reactor (YSZ tube); 8, furnace; 9, working electrode; 10, insulator; 11, reference electrode; 12, Pt lead.
Redox Potential Measurement. Before the measurement, argon gas (99.9995%,purity) was passed to purge the reactor €or 30 min. A high-input-resistance multimeter (HP34401A) was employed to record the redox potential of a working electrode relative to the Pt reference electrode which was exposed to air. The working electrode was exposed to an oxygen partial pressure from 0.01 to 0.08 atm which was balanced with argon (99.9995%, purity) with a flow rate of 200 mIJ min as controlled by mass flow meters. The measurement of the redox potential required 2 h to attain a stable value. The measurement temperature range was 170-370 "C. Activity Measurement. Activity measurement of CO oxidation on a fresh platinum electrode was carried out with a reactant composition of 2% oxygen and 2%
Forms of Adsorbed Oxygen Species. From the equations in Table 1, the adsorbed oxygen form on various catalyst surfaces is obtained from the best fits of the data as shown in Figure 2 and the results are listed in Table 2. For example, a plot of Vredox vs 1n(Po2)'" on the ZnO surface as shown in Figure 2 parts a and b is best fitted by eq 17, that is, the curve fitting is linear, and the slope is equal to kTIe. Hence, the form of the adsorbed oxygen species is predominantly 0- on the ZnO surface when the temperature is over 235 "C. On the other hand, at a lower temperature such as 170 "C, the adsorbed oxygen species is predominantly 0 2 on the ZnO surface. According to previous studies by ESR and Hall effect measurements on ZnO (Tanaka and Blyholder, 1972; Chon and Pajares, 1969; Morrison and Bonnelle, 1972), if the temperature is over 225 "C, oxygen is adsorbed in the high-temperature form identified as 0-; if the temperature is 175 "C or less, oxygen is chemisorbed in the low-temperature form identified as 0 2 - . Therefore, the characterization of adsorbed oxygen species by our equations as derived by electrochemistry is consistent with that by ESR and Hall effect measurements. As shown in Table 2, 02is identified as the primary adsorbed form on the platinum surface if the temperature is over 200 "C. However, when the temperature is 190 "C or less, the redox potential and oxygen partial pressure data cannot be fitted to any equation in Table 1. This may indicate that on the platinum surface at 190 "C or lower temperature, adsorbed oxygen exists as various forms containing neutral atomic oxygen (Gland, 1980; Gland et al., 1980). The adsorbed forms of oxygen on W A l 2 0 3 catalysts were identified as 0 2 and 0- at 25 "C by ESR (Katzer et al., 1979). Therefore, it is suggested that oxygen is adsorbed as various forms of Oz-, 0-, and neutral atomic oxygen on the Pt surface if the temperature is 190 "C or less. When the temperature is over 200 "C, the probability of charge transfer from Pt to oxygen is dramatically enhanced to form 02-species. However, the adsorbed oxygen species on the CuO surface still exists as the 0- form when the temperature is as high as 370 "C, as shown in Table 2. This shows that the probability of charge transfer from the platinum surface to oxygen is much larger than that from the copper oxide surface t o oxygen. Schlatter et al. (1973) indicated that there are fundamental differences in the behavior of platinum and base metal catalysts for oxidation of carbon monoxide in an automotive exhaust stream. It is thus suggested that the form of adsorbed oxygen species may have a significant effect on the catalytic behavior of Pt and CuO catalysts for CO oxidation. Relation of Redox Potential and Temperature. The relation between the redox potential and the oxygen partial pressure on platinum and copper oxide electrodes is shown in Figures 3 and 4, respectively. As shown in Figure 3, the variation of the redox potential with oxygen partial pressure on the Pt electrode is dramatic when the temperature increases from 190 to 200 "C. This change may depend on the strength of the interaction between the oxygen and the platinum surface.
Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995 2367
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I nc p q Y2
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2.4
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n c,
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Figure 2. A plot of the redox potential vs the logarithm of the oxygen partial pressure at constant temperature.
2.1
2388 Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995
A
-0
4
> vo.on -
.-0
+J
0 a0.017
-
6 U
8
0.037
196
190
EUO
185
EC6
210
216
Tenperature t OC)
Figure 3. Variations of redox potential with oxygen partial pressure on a Pt electrode, ( 0 )190 "C, (A)200 "C, and (W) 210 "C.
-
Figure 5. Variations of redox potential with temperature on a Pt electrode.
I
\. 0.0
0.E
0.04
0.08
196
Oxygen partial pressure ( a h ) Figure 4. Variations of redox potential with oxygen partial pressure on a CuO electrode, ( 0 )190 "C, (0)235 "C, (A)275 "C, (A) 300 "c, and (W) 370 "c. Table 2. Adsorbed Oxygen Species on Various Catalyst Surfaces adsorbed form temp ("0 Pt ZnO CUO 170 190 200 210 235 250 275 300 370
0202-
0202-
00-
0202-
00-
According t o the Nernst equation, the redox potential should increase as the temperature increases. However, as observed from Figure 4 for the CuO electrode, the redox potential decreases as the temperature increases. A possible explanation of this phenomenon is that the increase of the oxygen activity with temperature has a larger contribution to the redox potential than the temperature variable in Nernst equation. Oppositely, the redox potential of the Pt electrode increases with temperature as shown in Figure 5. This indicates that the activity of the adsorbed oxygen species on the platinum surface is much less dependent on temperature than that on the copper oxide surface. The Fermi level can provide an explanation of how temperature affects the redox potential. The increase of temperature can be regarded as an energy supply so that the charge transfer probability from the Pt dn
I85
m6
Temperature C
OC)
216
Figure 6. Variations of CO oxidation rate with temperature on a Pt electrode.
orbital to the adsorbed oxygen increases. The increase of charge transfer probability will shift the Fermi level to a lower state (Cimino and Carra, 1980; Clark, 1974). It was demonstrated that the Fermi level was pushed down as adsorption proceeds (Clark, 1974). When the potential energy of the electron as represented by the Fermi level becomes equal to the potential energy of the electron-accepting level of the adsorbed species, then equilibrium of adsorption is established. The drop of the Fermi level responds directly to the redox potential from eq 14. Figure 5 provides a precise relation between the redox potential of the platinum working-electrode and the temperature at 2% oxygen, which is the same as the oxygen content in the activity measurement in CO oxidation. The dramatic increase of the redox potential as temperature increases from 200 t o 205 "C is probably because the charge transfer between the t'l dn orbital and the adsorbed oxygen causes a considerable drop of the Fermi level. A lower Fermi level in a solid is equivalent to a larger work function, which is defined as the energy required to move an electron from the Fermi level of a solid to a vacuum state (Kittel, 1976). An investigation (Ibach, 1975) indicated that the steps on the F't surface can decrease the active energy of oxygen dissociative adsorption due to work-function decrease. It was observed that the CO oxidation rate on the platinum surface increases dramatically a t around 200 "C, as shown in Figure 6. A comparison between Figures 5 and 6 indicates that the temperature of the light-off phenomenon is consistent with that of the redox potential with
Ind. Eng. Chem. Res., Vol. 34,No. 7,1995 2369 a dramatic shift. Therefore, the dramatic increase of the charge transfer probability between the Pt dnorbital and the adsorbed oxygen molecular orbital is proposed as an explanation for the light-off phenomenon of CO oxidation on Pt catalysts. The detailed mechanism of the light-off phenomenon is explained further in the next section. Adsorbed Oxygen Species and the Light-Off Mechanism. The rate-limiting step of CO oxidation on Pt catalysts had been demonstrated to be the adsorption of oxygen (Schlatter et al., 1973). The rate of CO oxidation increases with increasing oxygen concentration and is inhibited by carbon monoxide (Langmuir, 1922;Voltz et al., 1973). When the temperature was less than 200 "C,the forms of the adsorbed oxygen 0-, or species on the Pt surface were identified as 02-, neutral atomic oxygen and the rate of CO oxidation is low as shown in Figure 6. This indicates that a t a temperature below 200 "C, the adsorbed oxygen species on the platinum surface does not have enough ability to proceed with competitive adsorption with carbon monoxide. On the other hand, the rate-limiting step of CO oxidation on base metal catalysts is the reaction of lattice oxygen and CO with no respect t o adsorbed oxygen; in addition, the oxidation starts a t a lower temperature than that on Pt catalysts (Schlatter et al., 1973). Therefore, the activity of platinum for CO oxidation is less than that of CuO a t low temperatures. However, when the temperature is over 200 "C, the CO oxidation rate on Pt catalysts increases dramatically as shown in Figure 6. Simultaneously, adsorbed oxygen species at the same temperature are predominantly ion with saturated net charge. formed as the 02According to these consequences, it is proposed that adsorbed ion has enough adsorptive ability to the 02compete with CO so that the oxidation rate is dramatically enhanced. On the other hand, the adsorbed oxygen species on CuO is predominantly 02-within a temperature range from 190 t o 275 "C, and then it is transformed into the 0- form between 300 and 375 "C. This result shows that the charge transfer probability from CuO to adsorbed oxygen is far less than that from Pt to adsorbed oxygen. It was demonstrated that CO oxidation on base metal catalysts does not depend on the oxygen concentration (Schlatter et al., 1973). Therefore, adsorbed oxygen species, 0- and 0 2 - , do not have any contribution to the catalytic activity of CO oxidation on CuO catalysts. It is suggested that 0- and 02-may be transformed into the lattice oxygen of copper oxide which then reacts with the adsorbed CO. A competitive backdonation model using orbital theory is proposed to illustrate the light-off mechanism of CO oxidation on platinum catalysts. The electronic config uration of the oxygen molecule is (ag1s)2(a,ls>2(a,2s)2(~~2~)~(0~2p)~(n~2p)~(n~2p>2. Its two ngantibonding orbitals have two unpaired electrons. An investigation (Gland et al., 1980) indicated that the Pt surface donates electrons into the ng* orbital of oxygen, reducing the 0-0 bond order from two t o one and forming 022adsorbed species. This illustrates that oxygen adsorbs on the platinum surface through a backdonation of Pt dn electrons to the adsorbed oxygen ng* orbital. Blyholder's donation-backdonation model (Blyholder, 1964) is generally used to discuss the chemisorption of CO on metal surfaces. In fact, recent cluster calculations (Bagus et al., 1983) demonstrated that the 2n LUMO (lowest unoccupied molecular orbital) of CO is the essential ingredient in a bond formation for CO adsorp-
tion. With respect t o experiments (Rogozik and Dose, 1986;Lin et al., 1993),they verified by inverse photoemission and FTIR that 2n backdonation for CO adsorption has predominant importance. Therefore, according to the orbital theory of CO and 0 2 adsorption on a metal surface, the competitive model of charge transfer is proposed; that is, CO and 0 2 molecules simultaneously compete to get Pt dn electrons for adsorption. The 02ion is identified as the primary adsorbed oxygen species at temperatures above 200 "C. As the 02ion carries saturated two-valence electrons, Pt must provide more electrons to form an 02ion than other forms of oxygen species. Hence, when the temperature is over 200 "C, Pt backdonates most of its dn electrons to the adsorbed oxygen for forming 02ions. This result will cause the probability of Pt dn backdonation t o the CO 2fl orbital to be dramatically decreased. This decreases the adsorption of CO on the Pt surface so that the Pt surface can provide enough active sites for oxygen adsorption for CO oxidation to proceed. Through this competitive backdonation model, the reaction barrier, i.e., oxygen adsorption is overcome so that the CO oxidation rate dramatically increases. Therefore, CO oxidation on Pt catalysts exhibits the light-off phenomenon. It is concluded that the competitive backnonation model successfully illustrates the lightoff mechanism for CO oxidation on platinum catalysts.
Conclusion The equations derived from electrochemistry using the Boltzmann approximation successfully predicted the forms of adsorbed oxygen species on metal or oxide surfaces. The relation between adsorbed oxygen species and catalytic properties promotes the understanding of the light-off mechanism for CO oxidation on platinum catalysts. A competitive backdonation model is proposed to reasonably illustrate this mechanism. Therefore, solid-state electrochemistry is beneficial in the study of heterogeneous catalysis involving oxygen.
Acknowledgment This work was supported by the National Science Council of the Republic of China under Contract NSC81-0402-E-007-09.
Nomenclature ak = activity of adsorbed oxygen species at the
Ek energy level ao2 = activity of adsorbed molecular oxygen ao,- = activity of adsorbed 0 2 - oxygen species C = a constant e = the charge of one electron e- = electron h = Planck's constant k = the Boltzmann constant K = the equilibrium constant at the standard state m = weight of a particle nk = mean number of adsorbed oxygen species at the Ek energy level N = particle number Poz = oxygen partial pressure T = temperature V = volume Vr&x = redox potential of a working electrode with respect to the reference electrode Ek = energy state of the adsorbed oxygen species
2370 Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995 p = chemical potential p = electrochemical potential ji" = the electrochemical potential at the standard state & = electrostatic potential
Subscript a = adsorbed state
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+
Received for review May 10, 1994 Accepted April 14, 1995@ IE940299S
Abstract published in Advance A C S Abstracts, J u n e 1, 1995. @
