Al2O3 Catalyst by a

J. J. Lecomte, S. Haydar, P. Granger, L. Leclercq, G. Leclercq, and J. P. Joly*. Université Claude Bernard Lyon 1, Laboratoire d'Application de la Ch...
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9266

Langmuir 2003, 19, 9266-9270

Investigation of Oxygen Interaction with a Pt-Rh/Al2O3 Catalyst by a Differential Temperature-Programmed Desorption Method J. J. Lecomte,‡ S. Haydar,†,§ P. Granger,‡ L. Leclercq,‡ G. Leclercq,‡ and J. P. Joly*,† Universite´ Claude Bernard Lyon 1, Laboratoire d’Application de la Chimie a` l’Environnement, UMR 5634, 43 boulevard du 11 Novembre 1918, 69622, Villeurbanne Cedex, France, and Universite´ des Sciences et Technologies de Lille, Laboratoire de Catalyse, UMR 8010, Baˆ t. C3, 59655, Villeneuve d'Ascq Cedex, France Received March 18, 2003. In Final Form: July 23, 2003 The interaction between gaseous oxygen and a catalyst Pt-Rh supported on γ-Al2O3 (specific surface area: 100 m2 g-1) was investigated by means of a differential desorption technique called intermittent temperature-programmed desorption (ITPD). Experiments were carried out under conventional secondary vacuum (P ≈ 10-4 Pa). Essentially, three desorption steps R, β1, and β2 were observed, occurring around 350, 650, and 750 K, respectively. Steps β1 and β2 stem from the desorption of oxygen strongly and dissociatively adsorbed on the metallic particles. Desorption step R is better observed after oxygen adsorption at ambient temperature; it corresponds to weakly bonded oxygen. The high values of the frequency factors strongly suggest that no readsorption occurred when oxygen desorbed through steps β1 and β2. Desorption activation energies for steps β1 and β2 were estimated at 219 and 305 kJ/mol, respectively.

Introduction The characterization of oxygen species adsorbed on noble metals has been the subject of numerous studies reported in the literature1,2 either to clarify the mechanism of catalytic oxidation reactions or to improve the performances of noble metal-based catalysts. Most of these investigations have been carried out with single crystals,1 whereas only few results concern metallic catalysts dispersed on supports despite their practical interest.3,4 Three-way platinum-rhodium supported catalysts provide a typical example of the lack of information on this subject. Additionally, the interaction of oxygen with noble metals is usually questionable and may lead to controversial assumptions in catalytic oxidation mechanisms. Theoretical calculations showed that either atomic or molecular oxygen species can react with CO to form CO2,5 while the diffusion of oxygen into the bulk usually provides a less reactive subsurface oxygen species.6 In a recent study dealing with the kinetics of the CO + O2 reaction between 470 and 560 K,7 we found that under CO rich conditions, that is, under a reductive gas mixture, a molecular oxygen species adsorbed on Pt-Rh/Al2O3 is the most likely reactive species involved. Thus, more information about the nature * Corresponding author. Tel.: +33 472 44 85 60; fax: +33 472 44 81 14; e-mail.: [email protected]. † Universite ´ Claude Bernard Lyon 1. ‡ Universite ´ des Sciences et Technologies de Lille. § Present address: Universite ´ Libanaise, Faculte´ des Sciences II, De´partement de Chimie, B.P. 90656, Fanar, Lebanon. (1) Masel, R. I. Principles of adsorption and reaction on solid surfaces; John Wiley & Sons: New York, 1996; p 137. (2) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1994. (3) Jaeger, N. I.; Jourdan, A. L.; Schulz-Ekloff, G. J. Chem. Soc., Faraday Trans. 1991, 87, 1251. (4) Putna, E. S.; Vohs, J. M.; Gorte, R. J. Surf. Sci. 1997, 391, L1178. (5) Eichler, A.; Hafner J. Surf. Sci. 1999, 433, 58. (6) Comelli, G.; Dhanak, V. R.; Kiskinova, M.; Pangher, N.; Paolucci, G.; Prince, K. C.; Rosei, R. Surf. Sci. 1992, 260, 7. (7) Granger, P.; Lecomte, J.-J.; Leclercq, L.; Leclercq G. Appl. Catal., A 2001, 218, 257.

and about the thermal stability of surface oxygen species present at the surface of dispersed catalysts is needed to progress the knowledge of catalytic oxidation and related reactions. Temperature-programmed desorption is a technique extensively used under various experimental conditions to study gas-solid interactions.1 According to experimental conditions, this technique allows thermodynamic parameters such as ∆Hads and ∆Sads to be determined (case of free readsorption) or kinetic parameters such as the activation energy of desorption Ed and the frequency factor ν to be estimated (case without readsorption).8 In addition, the value of these parameters can be extracted from experimental data either by integral or differential methods. Advantages and drawbacks of integral and differential approaches have been discussed in a review by Zhdanov.9 An interesting example of determining enthalpy and entropy of H2 adsorption on Rh/Al2O2 by an integral method has been published by Efstathiou and Bennett.10 The present paper reports an investigation, by means of the so-called intermittent temperature-programmed desorption technique11 (ITPD), of the oxygen species adsorbed on a bimetallic Pt-Rh catalyst supported on γ-alumina. This ITPD technique uses a differential approach, similar to that proposed by Habenschaden and Ku¨ppers.12 Its originality is the use of a saw-tooth heating program to generate a sequence of interrupted desorptions.11,13-15 The lower part of these TPDs occurs at quasi-constant surface coverage and may consequently be interpreted by a standard Arrhenius plot. (8) Cvetanovic, R. J.; Amenomiya, Y. Adv. Catal. 1967, 17, 103. (9) Zhdanov, V. P. Surf. Sci. Rep. 1991, 12, 183. (10) Efstathiou, A. M.; Bennett, C. O. J. Catal. 1990, 124, 116. (11) Joly, J. P. C. R. Acad. Sci. Paris 1982, t. 295 (Se´ rie II), 717. (12) Habenschaden, E.; Ku¨ppers, J. Surf. Sci. 1984, 138, L147. (13) Joly, J. P.; Perrard, A. Appl. Catal., A 1993, 96, 355. (14) Bacchus-Montabonel, M. C.; Joly, J. P. J. Chem. Soc., Faraday. Trans. 1986, 82, 3601. (15) Haydar, S.; Joly J. P. J. Therm. Anal. 1998, 52, 345.

10.1021/la0344635 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/26/2003

Investigation of Oxygen Interaction

Langmuir, Vol. 19, No. 22, 2003 9267 Table 1. Effect of Adsorption Temperature Tads on the TPD of O2 from Pt-Rh/Al2O3a Tads (K) 298 429 530 628

Tmax (K) peak R peak β 340

790 748 734 723

amount of O2 (µmol g-1)

% monolayer

0.78 7.88 9.21 6.96

3.4 35 41 31

a The percentage of monolayer is calculated from the amount of oxygen desorbed up to 873 K and from the metal dispersion, assuming a dissociative adsorption of O2.

Figure 1. Temperature-programmed desorption of oxygen from Pt-Rh/Al2O3 after exposure to 3.2 × 103 Pa O2 at various temperatures Tads. 2: at Tads ) 298 K for 24 h; b: at Tads ) 429 K for 15 min; 9: at Tads ) 530 K for 15 min; [: at Tads ) 628 K for 15 min.

Experimental Section The preparation and the characterization of the Pt-Rh/γ-Al2O3 catalyst (100 m2 g-1) containing 1 wt % Pt and 0.2 wt % Rh has been described previously.7 The metallic dispersion, obtained from H2 titration, was equal to 0.64. The catalyst samples were in the form of a powder with an average particle size equal to 80 µm. TPD experiments were carried out under vacuum (≈10-4 Pa) with an apparatus that has been described elsewhere.16 The evolution of gaseous oxygen was measured with a mass spectrometer (AEI MS 10) set on mass m/e ) 32 amu. Calibrations were carried out by injecting known amounts of gaseous oxygen into the apparatus under the experimental conditions of TPD. Proportionality between the area under the curve showing the MS response versus time and the amount of oxygen introduced has been checked. Once placed into the TPD cell, the sample was outgassed under vacuum (lower pressure limit approximately equal to 10-4 Pa) up to 723 K. Then, it was treated at a chosen temperature (Tads in the range 429-628 K) under 3.2 × 103 Pa of oxygen for 15 min. Finally, the sample was cooled to room temperature under oxygen and outgassed at this temperature. TPD runs were performed under dynamic vacuum at a heating rate equal to 5 K min-1. A TPD run was also performed after contacting the sample with 3.2 × 103 Pa of oxygen at 298 K for 24 h. Intermittent TPD experiments were carried out with a sample heated in oxygen at 530 K as described above. The saw-tooth heating program results in a sequence of interrupted TPDs. The lower part of each interrupted TPD, recorded at the highest sensitivity of the mass spectrometer, provides desorption rates at quasi-constant coverage. The surface coverage only significantly decreases in the upper part of these interrupted TPDs. During the decreasing temperature intervals, the oxygen desorption rate drops rapidly to a value that is under the sensitivity threshold of the mass spectrometer.

Results Influence of Adsorption Temperature on Adsorbed Oxygen. Figure 1 shows the influence of the adsorption temperature (Tads) on the oxygen TPD profiles. An adsorption of O2 under 3.2 × 103 Pa at 298 K results in the appearance of two desorption peaks, labeled R and β at 340 and 790 K, respectively. This peak R does not appear isolated when the adsorption of O2 is carried out at higher temperatures. In this case, a weak desorption of O2 occurs in the temperature range 300-500 K before the onset of peak β. Table 1 gives the temperatures Tmax of the TPD peaks and the amounts of oxygen desorbed up to 873 K. When expressed as a fraction of monolayer, this amount is lower (16) Joly, J. P.; Perrard, A. Langmuir 2001, 17, 1538.

than surface coverage after oxygen treatments at 429 and 530 K because the oxygen signal did not return to the baseline in these cases (see Figure 1). It is seen that the temperature of the maximum of peak β decreases as Tads increases. Peak β may be assigned to the desorption of some chemisorbed atomic species according to the well-known fact that O2 adsorbs dissociatively on noble metals at room temperature or higher.17-21 The behavior of oxygen TPD profile on changing the adsorption temperature shows that adsorbed oxygen is strongly heterogeneous. The strong increase of the amount of adsorbed oxygen when Tads passes from 298 to 429 K may look surprising because a strong dissociative adsorption of oxygen is known to occur readily on noble metals at room temperature with a very low activation energy of adsorption. A likely explanation of this behavior is that, under our experimental conditions (conventional secondary vacuum with a lower pressure limit approximately equal to 10-4 Pa), the surface of the catalyst is contaminated by CO and H2O, the main constituents of the residual gas in glass apparatus. Indeed, it is quite difficult to thoroughly outgas the sample mainly composed of high surface area γ-alumina (Al203, x H2O) (x < 0.6). In fact, the conditions we used are quite different from those applied in thermal desorption spectroscopy (TDS) studies after Ar sputtering and annealing at high temperature (T > 1000 K) under UHV.17,19,22 In our TPD experiments, raising the adsorption temperature from 298 to 429 K cleaned the metallic surface and consequently liberates available sites for the dissociative adsorption of oxygen. The observation of a weakly adsorbed species (step R) after adsorption at 298 K is more surprising and not consistent with the well-known fact that, on clean Pt or Rh surfaces, O2 does not desorb between 300 and 500 K.17-19,23 Nevertheless, such an observation should not be controversial since our experimental conditions are quite different from those generally employed in TDS studies under ultrahigh vacuum (UHV). First, the surface of the alumina supported bimetallic phase in the catalyst we studied may not be directly compared to well-defined surfaces of monatomic single crystals. Second, as stressed before, we used conventional secondary vacuum in sample treatment and during TPD runs. Third, the partial oxygen pressure employed in this study is several orders of magnitude higher than those currently used in TDS. For instance, Salanov et al.23 used a pressure of O2 equal to or lower than 10-5 Pa. Our experimental conditions may (17) Gabelnick, A. M.; Gland, J. L. Surf. Sci. 1999, 440, 340. (18) McClellan, M. R.; Gland, J. L.; McFeely, F. R. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 213. (19) Wang, H.; Tobin, R. G.; Lambert, D. K.; DiMaggio, C. L.; Fisher G. B. Surf. Sci. 1997, 372, 267. (20) Matsushima, T. J. Catal. 1984, 85, 98. (21) Gorodetskii, V. V.; Nieuwenhuys, B. E.; Sachtler, W. M. H.; Boreskov, G. K. Appl. Surf. Sci. 1981, 7, 355. (22) Bowker, M.; Guo, Q.; Joyner, R. Surf. Sci. 1991, 253, 33. (23) Salanov, A. N.; Savchenko, V. I. Surf. Sci. 1993, 296, 393.

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Lecomte et al.

lead to results more difficult to interpret than those obtained by TDS under UHV but they might more closely reflect the situation of metal surface during practical threeway catalysis. Determination of Activation Energies of Desorption. The observed oxygen desorption rate from a given adsorption state may be modeled through the following equation taking into account a possible oxygen readsorption,16

dθ ) rd ) dt

( ) ( )

νθn exp 1+

Ed RT

qmka Ea (1 - θ)n exp C RT

(1)

where Ed and Ea denote the activation energy of desorption and of adsorption, respectively. Symbols θ , qm, ν, ka , and R denote the oxygen coverage, the amount of oxygen adsorbed at saturation, the so-called desorption frequency factor, the preexponential factor of the adsorption rate constant, and the ideal gas constant, respectively. n is the kinetic order equal to 1 or 2. C is the conductance of tubing linking the sample cell to the vacuum pump. In addition to a possible readsorption, ν and Ed may be dependent on the coverage1,2 because of some surface heterogeneity or interactions between adsorbed species. Consequently, the evaluation of desorption kinetic parameters from the spectra shown in Figure 1 seems a very difficult task. This difficulty has been overcome experimentally by the use of ITPD, a quasi-constant-coverage differential technique.11,13-15 At constant θ, eq 1 simply becomes:

( ) ( )

Ed a exp RT rd ) Ea 1 + b exp RT

Figure 2. Intermittent temperature-programmed desorption of oxygen from Pt-Rh/Al2O3 after exposure to 3.2 × 103 Pa O2 for 15 min at 530 K. For clarity, only the desorption fluxes during the rising temperature periods are shown.

Figure 3. Arrenius plots of the curves shown in Figure 2.

(2)

where a and b are constant. This equation, in the cases without or with free readsorption, reduces, respectively, to the following Arrhenius-form equations:

( )

rd ) a exp -

Ed RT

(3)

or

rd )

a E exp b RT

(

)

(4)

where E ) Ed - Ea. These equations may be thus used to evaluate Ed (or E) at the considered coverage from a classical Arrhenius plot. Heterogeneity of Oxygen Adsorbed on Pt-Rh/ Al2O3 Catalyst. Figure 2 shows the complete TPD spectrum, after an oxygen adsorption at 530 K under 3.2 × 103 Pa, sliced by the saw-tooth heating program. The precise onsets of the partial TPDs are known with a much better sensitivity than it appears in this figure where the MS response has been largely attenuated. Figure 3 shows the Arrhenius transforms of the partial TPDs. As expected for a desorption at quasi-constant coverage, these transforms are straight lines except in their upper part because of a decrease in surface coverage. The slight discrepancy from linearity sometimes observed in the very lower part of the transforms is due to a shift of the baseline at the highest

Figure 4. ([): Sample treated with oxygen at 523 K: variation of the apparent activation energy of desorption with the amount of oxygen extracted from the surface. (0): Result obtained from the onset of peak R (shown in Figure 1) for a sample pretreated with oxygen at 298 K.

sensitivity of the MS. Consequently, these have not been taken into account in the evaluation of the kinetic parameters. The set of values for the apparent activation energy of desorption Eapp (Eapp ) Ed or E) has been calculated from the slope of the lines. Figure 4 shows how Eapp varies when oxygen is progressively depleted from surface by ITPD. A strong variation of Ed from 67 kJ mol-1 to about 167 kJ mol-1 is observed when the initial coverage is decreased of some few percents. The two plateaus observed in the curve at about 219 kJ mol-1 and 305 kJ mol-1 possibly correspond to energetically distinct states, β1 and β2. Intercepts of the lines also provide some information about the kinetics of desorption. The cases without or with desorption have to be distinguished.

Investigation of Oxygen Interaction

Langmuir, Vol. 19, No. 22, 2003 9269

Table 2. Desorption Kinetic Parameters Obtained by ITPD for O2 Previously Adsorbed at 530 K on Pt-Rh/Al2O3a

Discussion

equal to 10-7 s-1 is obtained.

The objective of this study was to investigate oxygen interactions with three-way catalysts in the temperature range 298-873 K, using a differential method to obtain reliable kinetic parameters for modeling purposes. Up to now, most of the investigations dealing with oxygen adsorption on metallic surfaces have been performed with well-defined single crystals, the adsorption pressures of O2 being much lower than that used in our TPD experiments on Pt-Rh/Al2O3. Results obtained under our experimental conditions are complementary to those provided by studies carried out with clean single crystals. Lower Temperature Desorption Step. The low energy corresponding to points a and b in Figure 4 seems surprising. These points have been obtained by ITPD with a sample heated in oxygen at 530 K and cooled to room temperature under oxygen. Oxygen corresponding to this low-energy desorption step evolves between 300 and 400 K. At the highest sensitivity of the MS, it appeared in all the spectra shown in Figure 1. In addition, Eapp has also been calculated by applying the law of Arrhenius to the onset of peak R in Figure 1, obtained after treating the catalyst with oxygen at ambient temperature. The corresponding result has also been reported in Figure 4 (open square). It is seen that the low value of Eapp is confirmed for oxygen desorbing around 340 K. It can then be concluded that points a and b in Figure 4 correspond to desorption step R. To our knowledge, no study on single crystals has mentioned such an observation. Indeed, the formation of molecular O2 adsorbed species on Pt or Rh has been reported by numerous authors but it requires a low adsorption temperature6,17,25 (T ≈ 130 K or lower). In addition, this oxygen desorbs under 250 K or transforms into strongly bonded O species that desorb above 500 K. Nevertheless, such differences should not be controversial since our experimental conditions are quite different from those used to study single crystals. The low value of Eapp (69 kJ mol-1), for the highest oxygen surface coverage studied, denoting a weak adsorption state of oxygen on Pt-Rh/Al2O3, could possibly be assigned to either adsorbed atomic or molecular oxygen species, which does not transform into O species because of the lack or pair of adjacent sites. Figure 1 shows that oxygen desorbs steadily in small amount between 400 and 600 K. This extended temperature range shows that the corresponding activation energy of desorption covers a wide distribution. Corresponding points found by ITPD are denoted c and d in Figure 4. Oxygen desorbing under 500 K is a possible candidate for the oxidation of CO between 470 and 560 K as found in an earlier investigation on the kinetics of this reaction performed in our laboratory7 on the same catalyst as that used in this present study. We found that the key step in the oxidation of CO by O2 involves the reaction between chemisorbed CO and superficial O2 molecules over noble metals. The present study does not allow us to draw a definitive conclusion about the identification of oxygen species active in three-way catalysis but it shows the interest to study adsorption properties of catalysts as close as possible to the real ones, that is, highly dispersed on high surface area supports. Higher Temperature Desorption Steps. We found two desorption steps corresponding to activation energies of desorption of 219 and 305 kJ/mol, respectively. The occurrence of two desorption steps β has been frequently

(24) Granger, P.; Lecomte, J. J.; Leclercq, L.; Leclercq, G. Appl. Catal., A 2001, 208, 369.

(25) Belton, D. N.; Fisher, G. B.; Schmidt L. D. Surf. Sci. 1990, 233, 12.

plateau

Eapp (kJ/mol)

A (s-1)

amount of oxygen removed by ITPD (% monolayer)

A the highest coverages intermediate higher

69 219 305

107 1013 1017

0∼3 5-25 33-40

a Surface coverages have been assessed from the width of the plateaus in Figure 4.

In the first case, eq 3 provides

ν)

( )

Ed 1 dN 1 exp Ntot dt θn RT

(5)

In the second case, eq 4 provides n

νC E 1 dN (1 - θ) ) exp qmka Ntot dt RT θn

( )

(6)

where dN/dt and Ntot, respectively, denote the desorption flux and the number of species adsorbed at the saturation of this state. Both equations may be gathered into one:

A)

(

)

Eapp 1 dN f(θ) exp Ntot dt RT

(7)

Ntot and dN/dt are experimentally available. The coverage θ of the state concerned can be chosen close to 0.5, so that f(θ) may be considered close to unity only as far as the assessment of the order of magnitude of A is of interest. Table 2 provides the apparent energies (Eapp) and preexponential factors (A) corresponding to the various plateaus in Figure 4. For the intermediate and upper plateau, the values of A agree with those generally obtained for the desorption of light molecules from faces of single crystal under ultrahigh vacuum.9 The high value of A, found in the present work, strongly suggests that there is no oxygen readsorption. In this case, A coincides with the frequency factor ν and the apparent activation energy of desorption Eapp provided by ITPD is equal to the activation energy of desorption Ed. In addition, the fact that the value of A is higher than 1013 s-1 for step β2 indicates that the active complex in the desorption process has a higher mobility than the adsorbed species involved. In contrast, the value of A for the less energetic adsorbed state is much lower than 1013 s-1. This is likely due to the readsorption of oxygen during the TPD run. Thus, in this case, it is likely that the apparent activation energy of desorption Eapp is the heat of adsorption E. On the basis of these results, the rate constant of O2 desorption on Pt-Rh/Al2O3 at 573 K has been assessed using Ed ) 219 kJ mol-1, ν ) 1013 s-1. Under these temperature conditions corresponding to 100% NO and CO conversion in the usual three-way catalysis,24 a low value of the desorption rate constant:

( )

kd ) ν exp -

Ed RT

(8)

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observed for Pt or Rh surfaces. The consideration of results published on Pt surfaces and especially on Rh surfaces may be interesting because Pt-Rh surfaces are enriched with Rh when exposed to oxygen as stressed by Wouda et al.26 Consequently, the behavior of Pt-Rh surfaces with respect to oxygen might resemble that of Rh surfaces. For example, Solanov and Savchenko27 reported for polycrytalline Rh and for Rh(100) surface activation energies of desorption close to 320 and 200 kJ/mol as surface coverage goes from 0 to 0.6. These values, determined by a differential method similar to ours, are close to those found in the present work although we will see in the following that the assignment on oxygen species involved cannot be retained for our study. Peterlinz et al.28 found an oxygen activation energy of desorption equal to 234 kJ/mol for a Rh(111) surface. Winkler et al.29 found values of Ed in the range 188-207 kJ/mol and 190-205 for the two steps of desorption of oxygen from a Pt(112) surface. McLelland et al.18 found that the heat of desorption of oxygen from a kinked Pt(321) surface of both steps is approximately equal and decreases from 290 kJ/mol to 195 kJ/mol with increasing coverage. These examples show that the values of Ed we found for Pt-Rh/Al2O3 catalyst are, although not directly comparable, compatible with those reported in the literature for Pt or Rh surfaces. The assignment of steps β1 and β2 is a quite difficult task, which requires numerous techniques. Indeed, many different interpretations are found in the literature, involving adsorption on different sites on stepped surfaces,18,19 surface and morphological reconstruction,30 and mutual transformation between oxygen species that have penetrated into the near-surface layer of the metal and oxygen species in surface oxide islands.23,27 In addition, for catalyst particle supported on zeolite3 or R-Al2O3,4 oxygen TPD profiles depend on the size of the metallic particles. Nevertheless, changes in the shape of the TPD profiles shown in Figure 1 allows us to get some information about the involved adsorbed oxygen species. It is indeed seen that the width of peak β decreases on increasing the adsorption temperature Tads; this is accompanied by a reduction of the tail toward higher temperatures. This behavior is different from that described by Solanov and Savchenko23,27 for Rh surfaces. In our case, increasing the temperature of adsorption did (26) Wouda, P. T.; Schmid, M.; Hebenstreit, P. V. Surf. Sci. 1997, 388, 63. (27) Salanov, A. N.; Savchenko, V. I. React. Kinet. Catal. Lett. 1992, 48, 357. (28) Peterlinz, K. A.; Sibener, S. J. J. Phys. Chem. 1995, 99 (9), 2817. (29) Winkler, A.; Gua, X.; Siddiqui, H. R.; Hagans, P. L.; Yates, J. T. Jr. Surf. Sci. 1988, 201, 419. (30) Voss, C.; Kruse, N. Surf. Sci. 1998, 409, 252.

Lecomte et al.

not favor the penetration of oxygen into the metal and its back diffusion at higher temperatures. Finally, the calculation of the rate constant of O2 desorption at the temperature of 573 K corresponding to three-way conditions, when NO and CO are quasicompletely converted into N2 and CO2, leads to a substantially lower value of the oxygen desorption rate constant, that is, kd ) 10-7 s-1, than that previously calculated for the rate constants of the dissociation of NO,31 that is, kd ) 4 × 10-2 s-1. This comparison suggests that under lean conditions (with an oxidative gaseous mixture), when CO is quasi-completely converted, the three-way catalyst surface is covered by strongly chemisorbed O atoms produced either from the dissociation of NO or the dissociative adsorption of O2. As it has been shown in a previous kinetic study on the same catalyst7 that O2, CO, and NO compete for adsorption, this adsorbed oxygen species could further inhibit the rate of the CO + NO reaction. This means that the oxygen surface coverage, usually neglected for establishing the rate of NO reduction or CO oxidation under rich conditions, should be taken into account to derive a rate expression under lean conditions. Conclusions It has been shown by TPD under ordinary secondary vacuum (≈10-4 Pa) that oxygen, previously adsorbed on a Pt-Rh/Al2O3 catalyst under 3.2 × 103 Pa at temperatures ranging from 298 to 628 K, essentially desorbs into three steps: R, β1, and β2. The ITPD differential method, previously used for oxygen desorption from metallic oxides or ammonia desorption from acidic oxides, has been used to estimate the kinetic parameters (apparent activation energy and preexponential factor) of these desorption steps. The desorption occurring around 350 K likely takes place with readsorption and the heat of adsorption is about 69 kJ/mol. Desorption steps occurring around 650 K and 750 K are characterized by activation energies equal to 219 kJ mol-1 and 305 kJ mol-1, respectively, and frequency factors close to 1013 and 1017 s-1, respectively. Although results obtained with single crystal and dispersed metals on a support might differ significantly, the results found for the higher temperature desorption steps are compatible with the results found by TDS in the literature. For oxygen evolved above 500 K, our kinetic results are in favor of a desorption of superficial O species without diffusion in the metal sublayers. LA0344635 (31) Granger, P.; Lecomte, J. J.; Dathy, C.; Leclercq, L.; Leclercq, G. J. Catal. 1998, 175, 194.