- Email: [email protected]
Reaction sites working in steady-state CO oxidation on a stepped Pt(113) surface
18 September 1998
Chemical Physics Letters 294 Ž1998. 419–424
Reaction sites working in steady-state CO oxidation on a stepped Ptž 113/ surface Gengyu Cao a , Yoshiyuki Seimiya b, Yuichi Ohno a , Tatsuo Matsushima
a,)
a
b
Catalysis Research Center, Hokkaido UniÕersity, Sapporo 060-0811, Japan Graduate School of EnÕironmental Earth Science, Hokkaido UniÕersity, Sapporo 060-0811, Japan Received 23 March 1998; in final form 3 August 1998
Abstract Reaction sites working in CO oxidation at the steady state were studied over PtŽ113. s Žs.2Ž111. = Ž001. through analysis of the angular and velocity distributions of the desorbing product CO 2 . The site preference was found to switch from the Ž111. site to Ž001. sharply in the boundary between the active region and the inhibited zone above critical CO pressures. A large difference was found in the translational energy of CO 2 produced on each site. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction This Letter reports that the preference of reaction sites in the course of catalytic CO oxidation on a stepped PtŽ113. s Žs.2Ž111. = Ž001. surface switches sharply in the boundary between the active region and the inhibited one above critical CO pressures, as predicted in previous AR-TDS Žangle-resolved thermal desorption spectroscopy. work w1x. Very fast CO 2 molecules were found on the Ž001. sites probably modified by absorbed oxygen. Spatial distributions of the desorbing products provide the structural information on the reaction sites when the molecules are repulsively desorbed because their orientation is fairly preserved in the collimation angle of the desorption and their symmetry in the anisotropy of the distribution w2,3x. This method was successfully applied to the CO oxidation )
Corresponding author. E-mail: [email protected]
on platinum metals during thermal desorption procedures. On PtŽ113., it was concluded that reactive CO 2 desorption occurs mainly on Ž111. terraces when COŽa. is dominant, but on Ž001. facets at high oxygen coverages w1x. The latter was explained by the localization of CO on step-platinum atoms. Thus, opposite site preference was predicted at temperatures high enough to yield COŽa. with a high mobility. This also leads to the view that the working site switches around critical CO pressures in the CO oxidation at the steady state, because the surface species change drastically in the boundary region w4,5x. Furthermore, the Ž001. site was easily modified by absorbed oxygen above 400 K w6x. A survey of working sites at the steady state will bring suitable conditions for energy partition studies in the reactive desorption events on individual sites w3x. Most of the energy measurements of the desorbing product CO 2 have been performed by MMB Žmodulated molecular beam.-TOF Žtime-of-flight.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 9 6 - 3
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w7–11x and also by IR Žinfrared chemi-luminescence and infrared absorption spectroscopy. under steadystate conditions w12–19x. Both the translational and internal energies are higher than those expected from the surface temperature and therefore are able to provide information of the reaction dynamics and site structures. However, the experimental conditions used so far were not always suitable for this purpose. In MMB work, chopping of reactant beams may cause the oxygen and CO coverage changes throughout critical conditions for kinetic and dynamic measurements w20x. In the IR experiments, the pressure of the nearly equi-molar reactant mixtures was kept around 10y2 Torr, probably in the boundary region w17–19x. Moreover, most of such measurements were performed in non-angle-resolved form although the translational energy depended strongly on the desorption angle w2,8–11,21–23x.
2. Experiments The principle behind the apparatus is shown in Fig. 1. The apparatus consisted of three chambers for the reaction, a chopper and an analyzer w24x. The reaction chamber had LEED Žlow-energy electron diffraction. and XPS ŽX-ray photo-electron spectroscopy. optics, an Arq gun, a mass spectrometer and a gas-handling system for back-filling of CO and
oxygen gases. A chopper disk in the second chamber had slots of equal width Ž1 mm = 6 mm. ordered in a pseudo-random sequence Žwith a double sequence of 255 elements each. w25x. A time resolution of 20 ms was obtained at a rotation rate of 98.04 Hz. The arrival time to the ionizer of a second mass spectrometer in the analyzer was registered on a multichannel scaler ŽOrtec, Turbo-MCS. running synchronously with the chopper disk. The flight path between the chopper blade and the ionizer measured 377 mm and the ion drift time in the mass spectrometer was determined in separate experiments. The first slit was in a rectangular shape Ž1.5 mm = 6 mm. and the second was a drift tube of 50 mm long and 4 mm inner diameter. A PtŽ113. crystal Žsupplied by MaTeck Germany. was mounted on top of the manipulator and rotated to change the desorption angle Žpolar angle, u .. It was cleaned by the standard procedures and annealed in vacuo up to 1150 K w1x. The LEED pattern at this stage showed a sharp Ž1 = 1. structure. The desorption angle was varied in a plane perpendicular to the surface trough, where its sign was defined as positive into the w111x direction side.
3. Results 3.1. Kinetic studies
Fig. 1. Schematic view of the apparatus for velocity measurements at the steady state ŽSssample crystal; RL s reverse view LEED; S1s first slit; S2 ssecond slit; XPSs X-ray photo-electron energy analyzer; IG s ion gun; QM1 and QMS2 s quadrupole masss pectrometer; C s random chopper; M s motor; PC s photocell for trigger; CPs cryogenic plate; I s ionizer..
The steady-state CO 2 formation rate was monitored in angle-resolved form with the analyzer mass spectrometer. It was sensitive to reactant pressures and surface temperature. The rate was negligible below 400 K and increased rapidly to a maximum with increasing temperature before decreasing again at higher values. The CO pressure dependence was characterized by sharp transitions at certain PCO values. This jump became large at lower temperatures. Below these critical pressures, the reaction was first-order to CO and independent of the surface temperature, whereas above them it was of negative-orders to CO and sensitive to the temperature. Hereafter, the former region is named the ‘active region’ and the latter with PCO higher than the critical values, the ‘inhibited region.’ The results at 1 = 10y4 Torr O 2 are
G. Cao et al.r Chemical Physics Letters 294 (1998) 419–424
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reaction, while in the inhibited region the other contributes ; 60%. 3.3. Velocity distribution Typical velocity distributions of desorbing CO 2 in the normal direction at different CO pressures are shown in Fig. 3. The distributions largely shifted from a Maxwellian form at the surface temperature shown by the dashed lines. The translational temperature defined as T² E : s ² E :r2 k was derived from curve fitting to a modified Maxwellian form, where ² E : is the mean translational energy and k the Boltzmann constant w26x. This modified form is written as f Ž Õ . s Õ 3 expwyŽ Õ y Õ 0 . 2ra 2 x, where Õ is the velocity of a molecule, Õ 0 is the stream velocity and a is a width parameter. The normalized speed ratio, defined as Ž² Õ 2 :r² Õ :2 y 1.1r2rŽ32r9p y 1.1r2 , where ² Õ : is the mean velocity of the molecule Fig. 2. Steady-state CO 2 formation rate as a function of PCO at a fixed PO 2 and different temperatures. The inserted numbers indicate the slope Žor the reaction order with respect to CO.. Two inserted graphs in polar coordinates show the angular distribution of CO 2 in the active and the inhibited regions at 555 K.
shown in Fig. 2, where the CO 2 signal was determined in the normal direction. Similar kinetics were observed in a wide range of O 2 pressure. 3.2. Angular distribution The angular distribution of desorbed CO 2 was measured in a plane perpendicular to the step edge, because the distribution along the trough was simply collimated in the normal direction w1x. The distribution was different in the two regions. The desorption collimated sharply around u s q228 in the active region. The distribution curve also showed a shoulder around u s y208. However, in the inhibited region, the latter component was enhanced and the other suppressed. The results, presented in polar coordinates, are shown in Fig. 2. Each distribution curve was well deconvoluted into two simple power functions of cosine of the desorption angle, i.e. cos9 Ž u y 22. and cos 5 Ž u q 20. in the active region, and cos 4 Ž u y 22. and cos9 Ž u q 20. in the inhibited region. In the active region, the component collimated at u s q228 contributes ; 70% to the total
Fig. 3. Velocity distributions of CO 2 desorbing in the surface normal direction at PO 2 s1=10y4 Torr and 555 K with different PCO . The solid curves were obtained by curve fitting. The vertical bars show their peak positions. Each T² E : value is also inserted. The dashed lines show a Maxwellian distribution at 555 K of the surface temperature.
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Fig. 4. Angle dependence of the tanslational energy at PO 2 s1= 10y4 Torr and 555 K in the active region Ž PCO s 3=10y6 Torr., and in the inhibited region Ž PCO s 3=10y5 Torr.. The vertical bars indicate the experimental uncertainty. The upward arrows indicate the site normal directions.
and ² Õ 2 : is the mean square velocity w26x, was close to unity. The resultant temperature was 1740 " 50 K at PCO s 3.0 = 10y6 Torr. With increasing CO pressure, it decreased to 1400 " 70 K at 7.0 = 10y5 Torr of CO. This temperature was also sensitive to the desorption angle. Its angle dependence was similar in both the regions, in contrast to the angular distribution, as shown in Fig. 4. It showed two peaks around u s y25 " 58 and q20 " 58, supporting the two desorption components. The energy in both the regions reached 2200 " 100 K around u s y258 and 1700 " 100 K around u s q208. The energy decreased slightly with increasing PCO . After velocity measurements, the desorption of oxygen was found above 900 K in the subsequent heating, indicating the formation of inactive oxygen w6,27,28x.
4. Discussion 4.1. Site switching CO 2 formation kinetics switches sharply from first-order to negative orders at critical values with increasing CO pressure. This change is caused by a
switching of the rate-determining step from CO adsorption to oxygen dissociation on the condition that the reaction of COŽa. q OŽa. ™ CO 2 Žg. takes place much faster than the adsorption of either CO or oxygen w4,5x. As a result of the high mobility of CO w29x, the reaction is likely to take place on oxygen adsorption sites, although oxygen may slightly move during the activation of the Pt–O bond w30x. On the present surface, there are two kinds of oxygen adsorption sites, on Ž111. terraces and Ž001. steps. AR-TDS work by Yamanaka et al. showed that CO 2 reactive desorption takes place mostly on Ž001. sites when OŽa. ) COŽa. w1x. This condition holds in the active region at the steady state w4,5x. However, the preference of the Ž001. site can be found only in the inhibited region where CO mostly covers the surface, i.e., COŽa. 4 OŽa.. There are large differences in the experimental conditions between TDS and the steady state. In TDS work, both COŽa. and OŽa. are significant and CO 2 desorption is controlled by their reaction and not by the reactant adsorption. Furthermore, the mobility of both reactants is limited at low temperatures. Therefore, their surface distribution each depends on the occupation of the other as well as the binding energy and the activation energy for diffusion. The experimental result of the Ž001. site preference in TDS was explained by localization of CO on step platinum atoms when OŽa. 4 COŽa.. On the other hand, at the steady state above ; 500 K, the CO 2 desorption rate is controlled by the adsorption of either CO or oxygen. The mobility of CO is extremely high w29x and oxygen can move quickly. In the active region, almost all surface species are oxygen, which are distributed on both Ž111. and Ž001. sites. CO can interact with oxygen at different sites. Thus, the site preference is mostly controlled by the reactivity of oxygen and the reaction takes place mostly on the Ž111. terrace because of a less binding energy. In the inhibited region, however, the reaction rate is controlled by oxygen dissociation. Oxygen is immediately removed as CO 2 when it dissociates. The reaction may take place on the dissociation site or nearby. This dissociation occurs preferentially on Ž001. sites w31x. Hence, the CO 2 desorption collimates closely along the Ž001. site normal. Incidentally, the oxygen dissociation process is omitted in TDS work except for a-CO 2 formation w32x.
G. Cao et al.r Chemical Physics Letters 294 (1998) 419–424
4.2. Velocity and site The translational energy is maximized around u s y258 and q208. The repulsive force is operative to CO 2 in two-directional ways. Furthermore, the two reaction sites yield CO 2 as a product with highly different energies, in contrast to TDS work w23x. This suggests that one of the sites should be modified at the steady state. The maximum translational energy of CO 2 from the Ž001. site is ; 1000 K higher than that in TDS work Ž1200 K., whereas that from the Ž111. terrace is only 300 K higher than the TDS value Ž1400 K.. The latter difference can be explained by the surface temperature effect. In fact, the translational energy increases with increasing surface temperature w24x. This consideration indicates that there is no difference in the CO 2 produced on the terrace in both TDS and steady-state work. On the other hand, the difference of 1000 K on the Ž001. is too large to be explained only by the surface temperature effect. Furthermore, the collimation angle around u s y208 noticeably shifts from the value in AR-TDS work Žy178 to y158. w1x. This supports the modification of the Ž001. site at the steady state and is reminiscent of the deactivation of this site by absorbed oxygen. This was proposed by Akiyama et al. w6x in their AR-TDS work by titrating inactive oxygen with isotope tracers and comparing with the work-function measurements on PtŽ100. after Rotermund et al. w27x. In fact, the presence of such inactive oxygen was confirmed by the subsequent heating. The site is easily modified by subsurface oxygen at temperatures above 400 K in an oxygen atmosphere. This oxygen deactivates the Ž001. site rather than the Ž111. site. Oxygen may dissociate further on the modified Ž001. site. 4.3. Energy change We propose two possibilities for the enhancement of the CO 2 velocity on the modified Ž001. site, i.e. changes in the energy partition andror the repulsive force. A large amount of the energy is released when CO 2 is formed. The potential energy of the activated state of CO 2 formation on PtŽ111. was estimated to be ; 30 kcalrmol above the vacuum level w20x. This energy can be delivered into the translational and internal modes of CO 2 as well as into surface
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modes. In fact, the CO 2 that leaves holds ; 50% of this transferable energy, while the internal mode receives ; 25% w18x. This energy disposal may change on the modified site because of restricted lattice motions or the distorted activate state of CO 2 . The potential energy of the activated state determines mostly the extent of the repulsive force operative to CO 2 . It should decrease with increasing distance of the activated state from the surface plane because the force between nascent CO 2 and the surface is due to Pauli repulsion. This distance may be reduced on modified sites with distorted structures.
5. Summary The angular and velocity distributions of the desorbing product CO 2 were studied in CO oxidation on PtŽ113. at the steady state. The Ž111. terrace sites are operative in the active region, while in the inhibited region, the Ž001. step sites mostly produce CO 2 . This switching occurs sharply at the critical CO pressures. The Ž001. site producing fast CO 2 is modified probably by subsurface oxygen.
Acknowledgements This work was partly supported by a COE special equipment program in 1996 of the Ministry of Education, Science, Sports and Culture of Japan and also by Grand-in-Aid No. 06403012 for General Scientific Research from the Ministry.
References w1x T. Yamanaka, C. Moise, T. Matsushima, J. Chem. Phys. 107 Ž1997. 8138. w2x T. Matsushima, Heterogeneous Chem. Rev. 2 Ž1995. 51. w3x T. Matsushima, T. Yamanaka, C. Moise, Trends Chem. Phys. 4 Ž1996. 1. w4x T. Engel, G. Ertl, Adv. Catal. 28 Ž1979. 1. w5x T. Matsushima, J.M. White, Surf. Sci. 67 Ž1977. 122. w6x H. Akiyama, C. Moise, T. Yamanaka, K. Jacobi, T. Matsushima, Chem. Phys. Lett. 272 Ž1997. 219. w7x C.A. Becker, J.P. Cowin, L. Wharton, D.J. Auerbach, J. Chem. Phys. 67 Ž1977. 3394.
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w8x J.A. Barker, D.J. Auerbach, Surf. Sci. Rep. 4 Ž1985. 1. w9x L.S. Brown, S.J. Sibener, J. Chem. Phys. 90 Ž1989. 2807. w10x E. Poehlmann, M. Schmitt, H. Hoinkes, H. Wilsch, Surf. Sci. 287,288 Ž1993. 269. w11x J.I. Colonell, K.D. Gibson, S.J. Sibener, J. Chem. Phys. 103 Ž1995. 6677. w12x S.L. Bernasek, S.R. Leone, Chem. Phys. Lett. 84 Ž1981. 401. w13x D.A. Mantell, S.B. Ryali, B.L. Halpern, G.L. Haller, J.B. Fenn, Chem. Phys. Lett. 81 Ž1981. 185. w14x D.A. Mantell, S.B. Ryali, G.L. Haller, Chem. Phys. Lett. 102 Ž1983. 37. w15x H. Uetsuka, K. Watanabe, K. Kunimori, Chem. Lett. Ž1995. 633. w16x L.S. Brown, S.L. Bernasek, J. Chem. Phys. 82 Ž1985. 2110. w17x K. Watanabe, H. Ohnuma, H. Uetsuka, K. Kunimori, Surf. Sci. 368 Ž1996. 366. w18x G.W. Coulston, G.L. Haller, J. Chem. Phys. 95 Ž1991. 6932. w19x D.J. Bald, R. Kunkel, S.L. Bernasek, J. Chem. Phys. 104 Ž1996. 7719. w20x C.T. Campbell, G. Ertl, H. Kuippers, J. Segner, J. Chem. Phys. 73 Ž1980. 5862.
w21x T. Matsushima, K. Shobatake, Y. Ohno, K. Tabayashi, J. Chem. Phys. 97 Ž1992. 2783. w22x Y. Ohno, T. Matsushima, H. Uetsuka, J. Chem. Phys. 101 Ž1994. 5319. w23x Y. Ohno, T. Yamanaka, H. Akiyama, Y. Seimiya, T. Matsushima, Appl. Surf. Sci. 121,122 Ž1997. 567. w24x G. Cao, Y. Seimiya, T. Matsushima, J. Mol. Catal. Ž1998. in press. w25x G. Comsa, R. David, B.J. Schumacher, Rev. Sci. Instrum. 52 Ž1981. 789. w26x G. Comsa, R. David, Surf. Sci. Rep. 5 Ž1985. 145. w27x J. Lauterbach, K. Asakura, H.H. Rotermund, Surf. Sci. 313 Ž1994. 52. w28x A. von Oertzen, A. Mikhailov, H.H. Rotermund, G. Ertl, Surf. Sci. 350 Ž1996. 259. w29x J.E. Reutt-Robey, D.J. Doren, Y.J. Chabal, S.B. Christman, Phys. Rev. Lett. 61 Ž1998. 2778. w30x A. Alavi, P. Hu, T. Deutsch, P.L. Silvestrelli, J. Hutter, Phys. Rev. Lett. 80 Ž1998. 3650. w31x A. Rar, T. Matsushima, Surf. Sci. 318 Ž1996. 89. w32x T. Matsushima, Surf. Sci. 127 Ž1983. 403.
Chemical Physics Letters 294 Ž1998. 419–424
Reaction sites working in steady-state CO oxidation on a stepped Ptž 113/ surface Gengyu Cao a , Yoshiyuki Seimiya b, Yuichi Ohno a , Tatsuo Matsushima
a,)
a
b
Catalysis Research Center, Hokkaido UniÕersity, Sapporo 060-0811, Japan Graduate School of EnÕironmental Earth Science, Hokkaido UniÕersity, Sapporo 060-0811, Japan Received 23 March 1998; in final form 3 August 1998
Abstract Reaction sites working in CO oxidation at the steady state were studied over PtŽ113. s Žs.2Ž111. = Ž001. through analysis of the angular and velocity distributions of the desorbing product CO 2 . The site preference was found to switch from the Ž111. site to Ž001. sharply in the boundary between the active region and the inhibited zone above critical CO pressures. A large difference was found in the translational energy of CO 2 produced on each site. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction This Letter reports that the preference of reaction sites in the course of catalytic CO oxidation on a stepped PtŽ113. s Žs.2Ž111. = Ž001. surface switches sharply in the boundary between the active region and the inhibited one above critical CO pressures, as predicted in previous AR-TDS Žangle-resolved thermal desorption spectroscopy. work w1x. Very fast CO 2 molecules were found on the Ž001. sites probably modified by absorbed oxygen. Spatial distributions of the desorbing products provide the structural information on the reaction sites when the molecules are repulsively desorbed because their orientation is fairly preserved in the collimation angle of the desorption and their symmetry in the anisotropy of the distribution w2,3x. This method was successfully applied to the CO oxidation )
Corresponding author. E-mail: [email protected]
on platinum metals during thermal desorption procedures. On PtŽ113., it was concluded that reactive CO 2 desorption occurs mainly on Ž111. terraces when COŽa. is dominant, but on Ž001. facets at high oxygen coverages w1x. The latter was explained by the localization of CO on step-platinum atoms. Thus, opposite site preference was predicted at temperatures high enough to yield COŽa. with a high mobility. This also leads to the view that the working site switches around critical CO pressures in the CO oxidation at the steady state, because the surface species change drastically in the boundary region w4,5x. Furthermore, the Ž001. site was easily modified by absorbed oxygen above 400 K w6x. A survey of working sites at the steady state will bring suitable conditions for energy partition studies in the reactive desorption events on individual sites w3x. Most of the energy measurements of the desorbing product CO 2 have been performed by MMB Žmodulated molecular beam.-TOF Žtime-of-flight.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 9 6 - 3
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G. Cao et al.r Chemical Physics Letters 294 (1998) 419–424
w7–11x and also by IR Žinfrared chemi-luminescence and infrared absorption spectroscopy. under steadystate conditions w12–19x. Both the translational and internal energies are higher than those expected from the surface temperature and therefore are able to provide information of the reaction dynamics and site structures. However, the experimental conditions used so far were not always suitable for this purpose. In MMB work, chopping of reactant beams may cause the oxygen and CO coverage changes throughout critical conditions for kinetic and dynamic measurements w20x. In the IR experiments, the pressure of the nearly equi-molar reactant mixtures was kept around 10y2 Torr, probably in the boundary region w17–19x. Moreover, most of such measurements were performed in non-angle-resolved form although the translational energy depended strongly on the desorption angle w2,8–11,21–23x.
2. Experiments The principle behind the apparatus is shown in Fig. 1. The apparatus consisted of three chambers for the reaction, a chopper and an analyzer w24x. The reaction chamber had LEED Žlow-energy electron diffraction. and XPS ŽX-ray photo-electron spectroscopy. optics, an Arq gun, a mass spectrometer and a gas-handling system for back-filling of CO and
oxygen gases. A chopper disk in the second chamber had slots of equal width Ž1 mm = 6 mm. ordered in a pseudo-random sequence Žwith a double sequence of 255 elements each. w25x. A time resolution of 20 ms was obtained at a rotation rate of 98.04 Hz. The arrival time to the ionizer of a second mass spectrometer in the analyzer was registered on a multichannel scaler ŽOrtec, Turbo-MCS. running synchronously with the chopper disk. The flight path between the chopper blade and the ionizer measured 377 mm and the ion drift time in the mass spectrometer was determined in separate experiments. The first slit was in a rectangular shape Ž1.5 mm = 6 mm. and the second was a drift tube of 50 mm long and 4 mm inner diameter. A PtŽ113. crystal Žsupplied by MaTeck Germany. was mounted on top of the manipulator and rotated to change the desorption angle Žpolar angle, u .. It was cleaned by the standard procedures and annealed in vacuo up to 1150 K w1x. The LEED pattern at this stage showed a sharp Ž1 = 1. structure. The desorption angle was varied in a plane perpendicular to the surface trough, where its sign was defined as positive into the w111x direction side.
3. Results 3.1. Kinetic studies
Fig. 1. Schematic view of the apparatus for velocity measurements at the steady state ŽSssample crystal; RL s reverse view LEED; S1s first slit; S2 ssecond slit; XPSs X-ray photo-electron energy analyzer; IG s ion gun; QM1 and QMS2 s quadrupole masss pectrometer; C s random chopper; M s motor; PC s photocell for trigger; CPs cryogenic plate; I s ionizer..
The steady-state CO 2 formation rate was monitored in angle-resolved form with the analyzer mass spectrometer. It was sensitive to reactant pressures and surface temperature. The rate was negligible below 400 K and increased rapidly to a maximum with increasing temperature before decreasing again at higher values. The CO pressure dependence was characterized by sharp transitions at certain PCO values. This jump became large at lower temperatures. Below these critical pressures, the reaction was first-order to CO and independent of the surface temperature, whereas above them it was of negative-orders to CO and sensitive to the temperature. Hereafter, the former region is named the ‘active region’ and the latter with PCO higher than the critical values, the ‘inhibited region.’ The results at 1 = 10y4 Torr O 2 are
G. Cao et al.r Chemical Physics Letters 294 (1998) 419–424
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reaction, while in the inhibited region the other contributes ; 60%. 3.3. Velocity distribution Typical velocity distributions of desorbing CO 2 in the normal direction at different CO pressures are shown in Fig. 3. The distributions largely shifted from a Maxwellian form at the surface temperature shown by the dashed lines. The translational temperature defined as T² E : s ² E :r2 k was derived from curve fitting to a modified Maxwellian form, where ² E : is the mean translational energy and k the Boltzmann constant w26x. This modified form is written as f Ž Õ . s Õ 3 expwyŽ Õ y Õ 0 . 2ra 2 x, where Õ is the velocity of a molecule, Õ 0 is the stream velocity and a is a width parameter. The normalized speed ratio, defined as Ž² Õ 2 :r² Õ :2 y 1.1r2rŽ32r9p y 1.1r2 , where ² Õ : is the mean velocity of the molecule Fig. 2. Steady-state CO 2 formation rate as a function of PCO at a fixed PO 2 and different temperatures. The inserted numbers indicate the slope Žor the reaction order with respect to CO.. Two inserted graphs in polar coordinates show the angular distribution of CO 2 in the active and the inhibited regions at 555 K.
shown in Fig. 2, where the CO 2 signal was determined in the normal direction. Similar kinetics were observed in a wide range of O 2 pressure. 3.2. Angular distribution The angular distribution of desorbed CO 2 was measured in a plane perpendicular to the step edge, because the distribution along the trough was simply collimated in the normal direction w1x. The distribution was different in the two regions. The desorption collimated sharply around u s q228 in the active region. The distribution curve also showed a shoulder around u s y208. However, in the inhibited region, the latter component was enhanced and the other suppressed. The results, presented in polar coordinates, are shown in Fig. 2. Each distribution curve was well deconvoluted into two simple power functions of cosine of the desorption angle, i.e. cos9 Ž u y 22. and cos 5 Ž u q 20. in the active region, and cos 4 Ž u y 22. and cos9 Ž u q 20. in the inhibited region. In the active region, the component collimated at u s q228 contributes ; 70% to the total
Fig. 3. Velocity distributions of CO 2 desorbing in the surface normal direction at PO 2 s1=10y4 Torr and 555 K with different PCO . The solid curves were obtained by curve fitting. The vertical bars show their peak positions. Each T² E : value is also inserted. The dashed lines show a Maxwellian distribution at 555 K of the surface temperature.
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Fig. 4. Angle dependence of the tanslational energy at PO 2 s1= 10y4 Torr and 555 K in the active region Ž PCO s 3=10y6 Torr., and in the inhibited region Ž PCO s 3=10y5 Torr.. The vertical bars indicate the experimental uncertainty. The upward arrows indicate the site normal directions.
and ² Õ 2 : is the mean square velocity w26x, was close to unity. The resultant temperature was 1740 " 50 K at PCO s 3.0 = 10y6 Torr. With increasing CO pressure, it decreased to 1400 " 70 K at 7.0 = 10y5 Torr of CO. This temperature was also sensitive to the desorption angle. Its angle dependence was similar in both the regions, in contrast to the angular distribution, as shown in Fig. 4. It showed two peaks around u s y25 " 58 and q20 " 58, supporting the two desorption components. The energy in both the regions reached 2200 " 100 K around u s y258 and 1700 " 100 K around u s q208. The energy decreased slightly with increasing PCO . After velocity measurements, the desorption of oxygen was found above 900 K in the subsequent heating, indicating the formation of inactive oxygen w6,27,28x.
4. Discussion 4.1. Site switching CO 2 formation kinetics switches sharply from first-order to negative orders at critical values with increasing CO pressure. This change is caused by a
switching of the rate-determining step from CO adsorption to oxygen dissociation on the condition that the reaction of COŽa. q OŽa. ™ CO 2 Žg. takes place much faster than the adsorption of either CO or oxygen w4,5x. As a result of the high mobility of CO w29x, the reaction is likely to take place on oxygen adsorption sites, although oxygen may slightly move during the activation of the Pt–O bond w30x. On the present surface, there are two kinds of oxygen adsorption sites, on Ž111. terraces and Ž001. steps. AR-TDS work by Yamanaka et al. showed that CO 2 reactive desorption takes place mostly on Ž001. sites when OŽa. ) COŽa. w1x. This condition holds in the active region at the steady state w4,5x. However, the preference of the Ž001. site can be found only in the inhibited region where CO mostly covers the surface, i.e., COŽa. 4 OŽa.. There are large differences in the experimental conditions between TDS and the steady state. In TDS work, both COŽa. and OŽa. are significant and CO 2 desorption is controlled by their reaction and not by the reactant adsorption. Furthermore, the mobility of both reactants is limited at low temperatures. Therefore, their surface distribution each depends on the occupation of the other as well as the binding energy and the activation energy for diffusion. The experimental result of the Ž001. site preference in TDS was explained by localization of CO on step platinum atoms when OŽa. 4 COŽa.. On the other hand, at the steady state above ; 500 K, the CO 2 desorption rate is controlled by the adsorption of either CO or oxygen. The mobility of CO is extremely high w29x and oxygen can move quickly. In the active region, almost all surface species are oxygen, which are distributed on both Ž111. and Ž001. sites. CO can interact with oxygen at different sites. Thus, the site preference is mostly controlled by the reactivity of oxygen and the reaction takes place mostly on the Ž111. terrace because of a less binding energy. In the inhibited region, however, the reaction rate is controlled by oxygen dissociation. Oxygen is immediately removed as CO 2 when it dissociates. The reaction may take place on the dissociation site or nearby. This dissociation occurs preferentially on Ž001. sites w31x. Hence, the CO 2 desorption collimates closely along the Ž001. site normal. Incidentally, the oxygen dissociation process is omitted in TDS work except for a-CO 2 formation w32x.
G. Cao et al.r Chemical Physics Letters 294 (1998) 419–424
4.2. Velocity and site The translational energy is maximized around u s y258 and q208. The repulsive force is operative to CO 2 in two-directional ways. Furthermore, the two reaction sites yield CO 2 as a product with highly different energies, in contrast to TDS work w23x. This suggests that one of the sites should be modified at the steady state. The maximum translational energy of CO 2 from the Ž001. site is ; 1000 K higher than that in TDS work Ž1200 K., whereas that from the Ž111. terrace is only 300 K higher than the TDS value Ž1400 K.. The latter difference can be explained by the surface temperature effect. In fact, the translational energy increases with increasing surface temperature w24x. This consideration indicates that there is no difference in the CO 2 produced on the terrace in both TDS and steady-state work. On the other hand, the difference of 1000 K on the Ž001. is too large to be explained only by the surface temperature effect. Furthermore, the collimation angle around u s y208 noticeably shifts from the value in AR-TDS work Žy178 to y158. w1x. This supports the modification of the Ž001. site at the steady state and is reminiscent of the deactivation of this site by absorbed oxygen. This was proposed by Akiyama et al. w6x in their AR-TDS work by titrating inactive oxygen with isotope tracers and comparing with the work-function measurements on PtŽ100. after Rotermund et al. w27x. In fact, the presence of such inactive oxygen was confirmed by the subsequent heating. The site is easily modified by subsurface oxygen at temperatures above 400 K in an oxygen atmosphere. This oxygen deactivates the Ž001. site rather than the Ž111. site. Oxygen may dissociate further on the modified Ž001. site. 4.3. Energy change We propose two possibilities for the enhancement of the CO 2 velocity on the modified Ž001. site, i.e. changes in the energy partition andror the repulsive force. A large amount of the energy is released when CO 2 is formed. The potential energy of the activated state of CO 2 formation on PtŽ111. was estimated to be ; 30 kcalrmol above the vacuum level w20x. This energy can be delivered into the translational and internal modes of CO 2 as well as into surface
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modes. In fact, the CO 2 that leaves holds ; 50% of this transferable energy, while the internal mode receives ; 25% w18x. This energy disposal may change on the modified site because of restricted lattice motions or the distorted activate state of CO 2 . The potential energy of the activated state determines mostly the extent of the repulsive force operative to CO 2 . It should decrease with increasing distance of the activated state from the surface plane because the force between nascent CO 2 and the surface is due to Pauli repulsion. This distance may be reduced on modified sites with distorted structures.
5. Summary The angular and velocity distributions of the desorbing product CO 2 were studied in CO oxidation on PtŽ113. at the steady state. The Ž111. terrace sites are operative in the active region, while in the inhibited region, the Ž001. step sites mostly produce CO 2 . This switching occurs sharply at the critical CO pressures. The Ž001. site producing fast CO 2 is modified probably by subsurface oxygen.
Acknowledgements This work was partly supported by a COE special equipment program in 1996 of the Ministry of Education, Science, Sports and Culture of Japan and also by Grand-in-Aid No. 06403012 for General Scientific Research from the Ministry.
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