J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1307-1312 '*O Tracer Studies of CO Oxidation with 0, on MOO, Part 2.-Active Sites for CO Oxidation with 0, and for Oxygen Isotope Exchange between CO, and MOO, Yasuo lizuka," Haruyuki Tanigaki, Masakazu Sanada, Junji Tsunetoshi, Naruki Yamauchi and Shigeyoshi Arai Depa rtmen t of Chemistry and Ma te ria Is Techno logy , Kyoto Ins titu te of Technology, Matsugasaki , Sakyo-ku, Kyoto, 606,Japan Oxygen isotope exchange between C 1802and MOO, has been examined in connection with the catalytic oxida- tion of C l60with "02on MOO,. Oxide ions at mobile and immobile sites on MOO,, which oxidize CO in the catalytic oxidation, were found to work also as active sites in the oxygen isotope exchange between C 1802and Mo "03.The observed time dependences of the yields of C 1802,C '*O'6Oand C '602in the exchange agreed satisfactorily with those calculated from the mobile and immobile site model.In the calculation, the number of immobile sites was estimated to be 1.8% of the total surface lattice oxide ions. The exchange rate at immobile sites was 13% of the total exchange rate. Catalytic oxygen isotope exchange on metal oxides has fre- quently been investigated by using l80tracer technique. Miura et al. examined the oxidation of CO with on Bi, Ca, Ni and La molybdate catalysts by measuring l80atom fractions in the product CO,.l The isotope mixing between C02 and surface oxygen occurred very rapidly over these catalysts. Therefore, they concluded that the observed l80 atom fraction in CO, does not give direct information about the active oxygen species on the surfaces of these catalysts.' Chaudhri et al.studied the oxidation of CO on nickel oxides by using the l80tracer technique.' They calculated the amount of exchangeable or reactive oxygen in the catalyst on the assumption of equilibrium between the product CO, and exchangeable or reactive oxygen atoms.' In general, oxygen isotope exchange between CO, and the metal oxide proceeds much more rapidly than does the cata- lytic oxidation of CO.'-' Muzykantov et al. studied the isotopic exchange of oxygen between CO, and MOO, in the range 573-873 K.4 The rate for oxygen exchange per unit surface area of MOO, at 753 K is 20 times larger than the rate for the catalytic CO oxidation with 0, on MOO, mea- sured in our laboratory.' Morikawa and Amenomiya studied oxygen isotope exchange between C'802 and y-alumina in the range 300-973 K.6 They calculated the number of exchangeable oxygen atoms on the surface from the material balance of l6O, by assuming equilibrium between oxygen atoms in the gas phase and the exchangeable oxygen atoms on the surface.6 Peri studied oxygen isotope exchange between C1802 and several high-area oxides such as silica, y-alumina, silica-alumina and ~eolite.~ He found that these oxides have some unusually exchangeable surface oxide ions at low concentrations and the order of activities of these cata- lysts for cracking of n-heptane agrees with that for the esti- mated numbers of exchangeable oxide ions.7 This suggests that the active sites for oxygen exchange between CO, and these oxides are involved in the n-heptane cracking reaction.In the previous paper, on the catalytic oxidation of CO with "0,on MOO,, we attempted to determine the numbers of active oxide ions from the variation of the iso- topic composition of the product CO, with time.' There are two kinds of "0 atoms on active sites. l80atoms taken up into mobile sites during the catalysis immediately diffuse into the bulk and are replaced by l60oxide ions from the bulk. As a result, only C1602 is produced at mobile sites. On the other hand, "0 atoms taken up into immobile sites do not diffuse and can be recovered by the CO reduction step, resulting in the formation of C'80160.Furthermore, we confirmed that the isotope composition of the product CO, is not changed by its contact with the surface lattice oxide ions of MOO, in the catalysis. This finding leads us to the conclusion that the observed isotopic composition of the product CO, must reflect the isotopic composition of active sites. However, as shown by Muzykantov et a/., the product CO, must undergo oxygen isotope exchange with surface lattice oxide ions of MOO, during the catalytic oxidation of C0.4 This means that the oxygen isotope exchange between the product CO, and MOO, which proceeds during the cata- lytic oxidation of CO with 1802does not change the isotopic composition of the product CO,.Therefore we decided to examine the oxygen isotope exchange between C"0, and MOO, and to elucidate the relation between active sites for the oxygen isotope exchange and those for the catalytic oxi- dation of CO with 0,. Experimental Analytical-grade MOO, (99.5%)from East Merck Company was used as the catalyst. The specific surface area of MOO, was 0.40 m2g-' as determined by the BET method. CO and 0, obtained commercially were fractionally distilled at liquid-nitrogen temperature for purification. Both C "0 (180-atom fraction, 99.7%) and C 1802 (180-atom fraction 99%) were purchased from Commissariat a L'Energie Atom- ique and used without further purification. The experimental apparatus, which had both circulating and static reaction systems, was described elsewhere.8 MOO, powder (ca.6.0 g) was at first oxidized at 800 K for 3 h with circulating 0, (7.0 kPa). The catalyst was then treated with a mixture of CO and 0, (CO: O2= 2: 1) until it showed steady catalytic activity. After the catalyst had been cooled to the reaction temperature and completely evacuated, a known amount of C1802 was introduced into the static reaction system (225 cm3). The change in the isotopic composition of CO, was followed at regular intervals (usually 15 min) by using a ULVAC MSQ-150 mass spectrometer. The total amount of CO, consumed in mass analyses was so small that the decrease in CO, pressure was negligible. Before each iso- topic experiment, MOO, was treated at 800 K with a mixture of CO and O2 until the amount of C "0l6O in the product C02 was negligible.1308 In order to examine whether l80atoms that occur at active sites through oxygen isotope exchange are involved in the catalytic oxidation of CO with O,, oxygen isotope exchange between Cl80, and MOO, was succeeded by the catalytic oxidation of C l60with 1602 or C l60with 1802. The catalytic oxidation of CO with "0, was followed by oxygen exchange between C1602 and MOO, in order to examine whether I80 atoms taken up into active sites through "0, dissociative adsorption in the catalytic oxida- tion are involved in oxygen exchange between C1602 and MOO,. The catalytic oxidation of C l80with 1602 was per- formed on MOO, to examine the exchange between the product CO, and MOO, which proceeds during the catalytic oxidation.The time dependence of the yield of isotopic species of CO, in the oxygen isotope exchange between Cl80, and MOO, was calculated using an NEC 9801-VX type personal computer. Results Oxygen Isotope Exchange between C 1802 and MOO, MOO, was exposed to C "0, (95.1 Pa) at 753 K in the static system. The total amount of l80atoms in Cl80, corre-sponded to 26% of all surface lattice oxide ions. Oxygen isotope exchange between C and Mo 1603produced C l80I60 and C 1602 during the reaction. Fig. 1 shows the time dependence of the yields of CI8O2, CI80 l60and C 1602 in CO,. C l80 l60 is the primary product in the exchange and CI60, is produced in a secondary step uia c l80l60.A logarithmic plot of the yield of C 1802us. time is shown in Fig. 2. A linear relationship is observed at the beginning of the reaction. Therefore, the rate of decrease in the yield of Cl80, is proportional to the concentration of C1802, at least initially. loo K'*O n s V C 0.6 6 0.4 0.2 0.0 0 30 60 90 120 150 tlmin Fig. 1 Time dependences of the isotopic composition of CO, in the oxygen isotope exchange between C '*02and MOO, at 753 K. The pressure of CO, was 95.1 Pa. Dashed curves were calculated accord- ing to the following two-step irreversible and successive first-order mechanism : c 180, -P c l80l60--+ c 160,. Solid curves were calculated according to the proposed immobile and mobile sites model.The dotted curve shows the time dependence of 6. a,c ,602 ; 0,c l8Ol60;0,c '80,. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.10 0 30 60 90 120 150 r/min Fig. 2 Logarithmic plot of yield of C'80, as a function of time. The specific rate constant calculated from the slope of the straight line was k, = 4.0 x loT2min-'. Oxygen Isotope Exchange between C 1802 and MOO, followed by the Catalytic Oxidation of C l60with 1602 Oxygen isotope exchange between C "0, (47 Pa) and MOO, was continued for 2 h at 753 K. The amount of "0 atoms transferred to the surface was estimated to be 9.6% of all surface lattice oxide ions of MOO, from the material balance of l80in CO, . After the sample had been evacuated for 20 min, the catalytic oxidation of Cl60 with 1602was per- formed for 2 rnin at a total pressure of 1.33 kPa. The catalyst was then evacuated again.This cycle was repeated three times at 60, 120 and 180 min after the exchange reaction in order to examine the change in the l80concentration at the active sites with time. The percentage of C l80l60in CO, produced in each pulse oxidation is shown in Fig. 3 as a function of time elapsed after the exchange reaction. The CO, produced in the first pulse oxidation contained 3.7% C l80l60. This amount did not decrease appreciably throughout the oxidation and evacuation cycles. Oxygen Isotope Exchange between C 1802 and Mo l6O3 followed by the Catalytic Oxidation of CO with 1802 The oxygen isotope exchange between C1802 (95 Pa) and Mo 1603was allowed to continue for 60 rnin at 753 K, then 0 0 0.0 0 60 120 180 t/min Fig.3 Change in the yield of C l8O l6O in the product CO, as a function of time. After completion of the oxygen isotope exchange between C I8O2 and MOO, at 753 K for 2 h, the catalytic oxidation of Cl6O with I6O2 for 2 min was intermittently carried out four times after continuous evacuation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20.0 h ,\" Y -0 9 10.0 0 0.0 IIIII1 I I 20 40 60 80 t/min Fig. 4 Change in the yield of C l80l60in the CO, produced in the catalytic oxidation of C l6O with 1802 performed at 753 K and 1.33 kPa. The oxygen isotope exchange reactions between C"0, and MOO, for 60min were repeated three times prior to this experiment.the CO, was' evacuated. This procedure was repeated three times over the same catalyst in order to transfer l80atoms into MOO, to a large extent. The total amount of "0 atoms transferred to the surface of MOO, reached 50% of all surface lattice oxide ions. Afterwards, the oxidation of Cl60 with 1802 was carried out for 100 min on the same catalyst. The total pressure of the mixture of Cl60 and 180,was main- tained at 1.33 kPa and the temperature of the oxidation was 753 K. The isotopic compositions of the CO, produced were examined at 0, 20, 40 and 60 min after starting the catalytic oxidation. The CO, produced at the beginning of the cataly- sis already contained 18% of C l80l60, as shown in Fig. 4. The yield of C "0 l60did not change appreciably with the progress of the catalysis.Catalytic Oxidation of C l60with lSO2on MOO, followed by Oxygen Isotope Exchange between C "02and the Catalyst The catalytic oxidation of Cl60 with l80,was performed over Mo 1603 for 3 h at 753 K, until the yield of C l80l60 in the product CO, reached saturation, i.e. nearly 13%. The total pressure was maintained at 1.33 kPa throughout. The time dependence of the yield of C l8Ol60is shown on the left-hand side of Fig. 5. After the sample had been evacuated, C1602 (3.72 Pa) was introduced into the reactor, where the amount of l60atoms in CO, corresponded to 1% of the total surface lattice oxide ions. C l80 l60was present. The change in the yield of C l80l60in CO, is shown as a func- tion of time on theright-hand side of Fig.5. The yield grad- ually increased with time up to a maximum and then decreased slightly. The maximum value coincided with the evac -c0+'*02 \b Moo3+ Cl602 saturation level of C l80l60observed in the catalytic oxida- tion of CO with "0,. Oxygen Isotope Exchange between Product C02 and MOO, during the Catalytic Oxidation of C ''0 with 1602 C l80was oxidized with 1602over Mo I6O3at 753 K, main-taining the total pressure of the reaction mixture at 1.33 kPa. All CO, was expected to be C l80l60immediately after its formation at active sites, because C l8O is oxidized with l60 oxide ions on the catalyst, at least, at the beginning. C 1602 may be produced in the subsequent oxygen isotope exchange between C l80l60and l60oxide ions on the surface.The yields of C l80l60,C 1602 and C 1802 in the product CO, are plotted against time in Fig. 6. CO, produced at the beginning of the oxidation contained ca. 80% C l80l60and ca. 20% C 1602, suggesting that ca. 80% of C l80l60pro-duced in this catalysis left the catalyst bed without first trans- ferring its l80atom to the surface. On the other hand, 20% of the product C'80160 exchanged its l80atom with surface oxide ions before leaving the catalyst bed. The yield of Cl80 l60increased slightly with time, while that of C 1602decreased. A small amount of C "0,appeared after 60 min. Discussion Oxygen Isotope Exchange between C lSO2and MOO, and Diffusion of "0 Atoms from the Active Sites The primary process in oxygen isotope exchange between C1802 and MOO, is the formation of C'80160 and the secondary exchange process between C l80l60and MOO, gives C 1602, as seen in Fig.1. The decrease in the C 1802 yield, i.e. the concentration of C 180,fits a first-order kinetic law at the beginning of the exchange, as shown in Fig. 2. The first-order kinetics may be applied to the secondary exchange between C180160 and MOO,. Therefore, the oxygen isotope exchange may proceed, at least initially, according to the following irreversible and successive two-step process : c 180, k c 180 160 LCW, where k (4.0 x lo-, min-') denotes the specific rate for the first step calculated from the slope of the straight line in Fig.2. The specific rate for the second step is assumed to be half 0 60 120 180 240 300 360 420 480 t/min Fig. 5 Change in the yield of C "0l60in CO, as a function of elapsed time. The catalytic oxidation of CO with "0,was carried out at 753 K for 3 h and 1.33 kPa. Subsequently, the oxygen isotope exchange between C 1602 and MOO, was performed. The pressure of CO, was 3.72 Pa. The amount of oxygen atoms in CO, corres-ponded to 1% of all surface lattice oxide ions of the catalyst. -(c) Fig. 6 Change in the isotopic composition of CO, produced in the catalytic oxidation of C l8O with 1602 with time. (a) C l8Ol60, (b) c lag,, (c) c 180,. of that for the first step, because each C l80l60molecule has only one l80atom. The dashed curves in Fig.1 show the time dependences of the yields of C 1802, C l80 l60 and C 1602 calculated according to the abovementioned simple model.' The dashed curves agree well with the observed data in the initial part of the exchange reaction. However, the observed yields of C 1802and C l80l60gradually deviate upwards from the calculated curves and that of Cl60, decreases as the exchange proceeds. This irreversible and successive first-order two-step model assumes that the surface of MOO, is always covered with only l60oxide ions during the exchange. In other words, all l80oxide ions transferred from either C 1802 or C l80l60 to the surface immediately diffuse into the bulk to be replaced by l60oxide ions. Consequently, Fig. 1 shows that a large proportion of the l80atoms transferred from C1802 and C'80160 probably diffuse into the bulk of the catalyst, while a small fraction of the l80atoms seem to remain on the surface, as the exchange proceeds.Active Sites for Oxygen Isotope Exchange between CO, and MOO,and for CO Oxidation with 0, on MOO, Fig. 3 shows that the CO, produced in the first pulse oxida- tion of C l60with 1602 after the exchange between C 1802 and Mo 1603contains 3.7% C l80l60.The CO, produced in the same procedure after 180 min contains almost the same amount of C l80l60. Our previous study showed that l80atoms taken up at immobile sites during the catalytic oxidation do not diffuse and can be recovered in the CO reduction step. Therefore, C'80160 is produced only at immobile sites.The production of C l80l60in the catalytic oxidation of C l60with 1602shown in Fig. 3 therefore clearly indicates that a part of l80atoms transferred from C 1802in the exchange must be taken up by immobile active sites. In other words, a certain part of oxide ions at immobile sites act both in CO oxidation with 0, and in oxygen isotope exchange between C 1802and MOO,. On the other hand, the results of Fig. 4 show that the per- centage of C l80l60in the product CO, already reaches saturation at the beginning of the catalytic oxidation of CO with 1802. Prior to the oxidation, however, MOO, was repeatedly exposed to C "0,. This shows that all oxide ions at immobile sites contribute to both CO oxidation with 0, and oxygen isotope exchange with CO,.However, a large part of l80atoms transferred from C1802diffuse into the bulk, as shown in Fig. 1. Therefore, oxide ions at mobile sites are expected also to work as active sites both in the CO oxi-dation with 0,and the oxygen isotope exchange with CO, . Fig. 5 shows that the yield of C l80l6O in CO, introduced into the reactor after the catalytic oxidation of CO with 1802 gradually increased up to a maximum which coincided with the saturation level of C I80 l60observed in the preceding catalysis. If CO, exchanges its 0 atoms only with l80oxide ions at immobile sites, the percentage of C 180'60must become much larger than the saturation level, i.e. nearly 13% because the amount of oxygen atoms in dosed C02 is only 1% of the total surface lattice oxide ions, while the number of '80-oxidized immobile sites was 1.8% of the total surface lattice oxide ions of If CO, exchanges its 0 atoms with surface lattice oxide ions more readily than those at mobile and immobile sites, the yield of C l80I60 must show a maximum much lower than the saturation level.Therefore, we conclude that (1) oxide ions at mobile sites, as well as those at immobile sites, work as active sites in oxygen isotope exchange with CO, and (2) surface lattice oxide ions other than immobile and mobile sites do not exchange l80atoms with CO, nor do they oxidize CO in catalytic oxidation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Reaction Model of Oxygen Isotope Exchange between C "0, and MOO, and the Number of Immobile Sites Oxygen isotope exchange between C "0, and oxide ions at mobile and immobile sites may proceed as follows.After isotope exchange, l80atoms at mobile sites are immediately replaced with l60oxide ions diffusing from the bulk. As a result, mobile sites are constantly covered with l60oxide ions during the exchange reaction. The oxygen isotope exchange between C "0, and oxide ions at mobile sites can be regarded as an irreversible and successive first-order two- step reaction. On the other hand, l80atoms at immobile sites do not diffuse into the bulk and therefore can again undergo oxygen exchange with CO,. In other words, the oxygen isotope exchange at immobile sites is considered to be a reversible and successive first-order reaction.All immobile sites are covered with l60oxide ions before the beginning of the exchange reaction. The coverage ratio of immobile sites with l80oxide ions may vary with time. When "0 oxide ions at immobile sites cause an exchange reaction with C'80160 or Cl60, in the gas phase, C1802 or C l80l60is formed. Therefore, the yields of C "0, and C l80l60deviate upwards from the dashed curves, while that of Cl6O, decreases with the progress of the exchange reaction (Fig. 1). l80 atoms can be recovered from immobile sites as C l80l60or C "0,. They can, of course, exchange again with l60oxide ions at mobile sites. This means that l80 oxide ions at immobile sites move to mobile sites gradually through CO, in the gas phase and then diffuse into the bulk irreversibly.This irreversible transfer of "0 atoms from immobile sites to mobile sites can explain the slow decrease in the yield of C l80l60in dosed CO, on the right-hand side of Fig. 5. If 8 is the coverage ratio of immobile sites with "0 oxide ions, the oxygen isotope exchange reaction between C I8O2 and oxide ions at mobile and immobile sites can be expressed as follows: immobile sites (2) where k, denotes the specific rate of oxygen isotope exchange at mobile sites and k, denotes the specific rate of oxygen isotope exchange at immobile sites. The rate of oxygen isotope exchange at mobile sites does not vary throughout the exchange reaction. On the other hand, the rate of oxygen isotope exchange at immobile sites depends upon 8 and therefore changes with time. Differential equations which give time dependences of the yields of isotopic species of CO, are expressed using k,, k, and 8 as follows: d[C 1802]/dt = -kl[C 1802]-k,(l -e)[c180,] + k, e/2[c 180 1601 (3) d[C l80'"O]/dt = kl[C '802]-k1/2[C l80160] + k,(i -e)c18023-k,(i -e)/2[c 180 1601 +k, ecc 160,1-k2e/2[c 180 1601 (4) d[C 1602]/dt = k1/2[C l80160] + k,(l -8)/2 x [C l80160] -k,B[C 160,] (5) J.CHEM. SOC. FARADAY TRANS.,1994, VOL. 90 The time dependence of 6 is given by the following differen- tial equation : dO/dt = A/N{k,(l -6)[C "O,] + k,(l -6)/2[C "0 160] -k, e/2[cl80 1601-k, ecc 160,1) (6) where A is the amount of CO, in the reactor and N is the number of immobile sites.We attempted to calculate the time dependences of yields of isotopic species of CO, and 0 by solving eqn. (3)-(6) by a numerical integration method. In this case, the isotopic com- position of CO, before starting the exchange reaction is C"02 = 99% and C"0 l60= 1%. 6 is zero at t = 0. A does not change throughout the reaction. The sum of k, and k, ,i.e. k = 4.0 x lop2min-', can be obtained from the slope of the initial straight line of C "0, presented in Fig. 2. However, N and the values of k, and k, are unknown. There- fore, we assumed various combinations of N and the ratio of k, to k (=k, + k,) and calculated the time dependences of the yields of C1'02, C'80160 and C1602 in CO, to compare them with the observed results. The time interval of numerical integration was 1 s and numerical calculations were accumulated up to 150 min with the aid of a computer.A ratio of k,/k, + k, = 0.13, i.e. k, = 3.48 x lo-, min-' and k, = 0.52 x lo-' min-' and N = 1.8% of the total surface lattice oxide ions gave the best fit with the observed data. The results of the calculations (shown in Fig. 1 by solid curves) agree well with the observed data up to CQ. 90 min and therefore support the proposed reaction model, at least at the beginning of the exchange. Our previous study on the catalytic oxidation of CO with "0, on MOO, showed that the ratio of the rate of pro- duction of CO, at immobile sites to the total CO, pro-duction rate is 0.13 and the number of immobile sites is 1.8% of the total surface lattice oxide ions of MOO,.Thus, both the number of immobile sites which work in the exchange and the ratio of the rate of exchange at immobile sites to that of the whole active sites agree with the corresponding number and the ratio of the rate of oxidation at immobile sites determined in the catalytic oxidation of CO with "0,. This might suggest that the reaction mechanism of oxygen exchange between CO, and oxide ions at mobile and immo- bile sites is closely associated with that of CO oxidation with 0, which proceeds on common active sites. The kinetics and reaction mechanism of oxygen exchange between C 180,and MOO, will be discussed in the next paper in connection with the catalytic oxidation of CO with 0, on MOO,. Fig.1 also shows the variation of 6 with time. The time dependence of 8 is similar to that of the yield of C "0 l60. Note that MOO, prepared by the thermal decomposition of (NH,),Mo,O2, -4H,O at 827 K in an O2 stream has been shown also to possess mobile and immobile active sites for catalytic CO oxidation with "0,. X-Ray diffraction patterns revealed that both MOO, powders have orthorhombic crystal form. However, the MOO, from Merck showed intense dif- fraction peaks at (020), (040) and (060) and had the morphol- ogy of flat hexagonal plates. On the other hand, MOO, prepared by thermal decomposition (1.3 m2 g-') exhibited maximum diffraction at (021) and had the morphology of rec- tangular prisms.Furthermore, we confirmed that WO, and ZrO, , which have different crystal structures from that of MOO,, also possess mobile and immobile active sites for the catalytic oxidation of CO with "0,. Oxygen Isotope Exchange between the Product CO, and MOO, under the Catalytic Oxidation of CO with 0, In the catalytic oxidation of C"0 with 1602 on Mo160, we observed the formation of 20% CI6O2even at an early stage (Fig. 6). This indicates that 20% of C "0 l60 molecules produced initially exchange their "0 atoms with l60oxide ions at mobile and immobile sites before leaving the catalyst bed. This means that "0 oxide ions at immobile sites increase with time during the catalytic oxidation of C lSO with I6O2.This increase results in an increasing opportunity for "0 oxide ions at immobile sites to take part in exchange with the product C l80l60.Thus, the opportunity for the formation of Cl6O, decreases, while that for C"0, increases.Fig. 6 shows slight increases in C "0 l6O and C ''0, with time as well as a slight decrease in C 1602. It is-natural that the CO, produced in the catalytic oxida- tion of Cl60 with "0, undergoes similar oxygen isotope exchange with active oxide ions at both mobile and immobile sites before it leaves the catalyst bed. In this case, the percent- age of C'80160 decreases due to "0 exchange with l60 oxide ions at mobile and immobile sites, while a fraction of the C 1602changes into C l80l6O through oxygen exchange with l80oxide ions at immobile sites.As a result, the decrease and increase in the yield of C "0 l6O concur in the catalyst bed. Fig. 5 shows the variation of C "0 l60with time during the exposure of the catalyst surface to C I6O2 after the cata- lytic oxidation of Cl60 with "0,. The percentage of C l8Ol60increased rapidly at first. However, the rate of the increase diminished with time and finally C "0 l6O reaches to a plateau. The rate of the decrease of C "0 l60was larger with increasing yield of C "0 l60in the gas phase. There- fore, the plateau must result from a dynamic equilibrium between the increase and decrease of Cl80l6O. The dynamic equilibrium lasts for more than 30 min, as shown in Fig. 5. CO, produced in the oxidation of C l60with "0, must undergo a similar oxygen exchange in the catalyst bed to that at the plateau on the right-hand side of Fig.5. In other words, CO, produced at the catalyst surface contains C l80l6O at the same level as CO, in the plateau. Pre- viously, we found that the exposure of the CO, produced at the end of the catalysis to the catalyst surface did not appre- ciably change the percentage of C "0 l60.'The time depen- dence of the yield of C'80160 in CO, produced in the oxidation of Cl6O with "0, was independent of the amount of catalyst.' These experimental facts can be explained satisfactorily by the dynamic equilibrium. It was also reported in the previous paper that C"0, is always negligible in the catalytic oxidation of Cl60 with 1802.8C1*0, is produced by oxygen exchange between C "0 l60and "0 oxide ions at immobile sites. The negligi- ble production of C "0, may be caused by the low content of C "0 l60in the reaction mixture, the short time it spends in the catalyst bed and its possible change into C "0 l60etc.We conclude that the subsequent oxygen isotope exchange does not affect the percentage of C "0 l60in the catalytic oxidation of Cl6O with 1802 on MOO, and the percentage is almost equal to the "0 atom fraction at the active sites. The authors thank Plenary Professor Takuya Hamamura of the Kyoto Institute of Technology for his helpful discussions and advice. References 1 H. Miura, Y.Morikawa and T. Shirasaki, Nippon Kagaku Kaishi, 1975,11,1875. 2 S. Chaudhri, Y.Kera and K. Hirota, Bull. Chem. SOC.Jpn., 1972, 45, 3301. 3 G. W. Keulks and C. C. Chang, J. Phys. Chem., 1970,74,2590. 4 V. S. Muzykantov, K. Ts. Cheshkova and G. K. Boreskov, Kinet. Katal., 1973, 14,432. 1312 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 5 6 7 8 Y. Iizuka, Y. Onishi, T. Tamura and T. Hamamura, J. Catal., 1980,64,437. Y. Morikawa and Y. Amenomiya, J. Catal., 1977,48, 120. J. B. Peri, J. Phys. Chem., 1975,79,1582. Y. Iizuka, J. Chem. SOC.,Faraday Trans., 1994,90,1301. 9 A. A. Frost and R. G. Pearson, Kinetics and Mechanism, John Wiley, New York, 2nd edn., 1953, p. 166. Paoer 3/05655D;Received 20th September, 1993