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Translational energy of desorbing CO2 in steady state CO oxidation on a stepped platinum (133) surface
Surface Science 415 (1998) L988–L992
Surface Science Letters
Translational energy of desorbing CO in steady state CO 2 oxidation on a stepped platinum (133) surface Y. Seimiya a, G. Cao a, Y. Ohno a, T. Yamanaka a, T. Matsushima a,*, K. Jacobi b a Catalysis Research Center, Hokkaido University, Sapporo 060, Japan b Fritz-Haber Institute of Max-Planck Society, Faradayweg, Berlin, D-14195, Germany Received 1 April 1998; accepted for publication 7 July 1998
Abstract Velocity distributions of desorbing product CO in CO oxidation on Pt(133) were analyzed in both the active and inhibited 2 regions at the steady state. The translation temperature is high in the active region where the reaction is in the first order with respect to CO. It decreases sharply with increasing CO pressure when the reaction shifts to the inhibited region where CO retards the reaction. The translational energy of CO increases linearly with the surface temperature. © 1998 Elsevier Science B.V. All 2 rights reserved. Keywords: Carbon dioxide; Carbon monoxide; Molecule–surface interaction; Oxidation; Platinum; Stepped single crystal surfaces
1. Introduction The velocity of desorbing product provides dynamic information on surface processes. Combined with internal energy measurements, its determination will yield the partitioning of energy in the reactive desorption [1,2]. This paper reports, for the first time, the velocity of desorbing product CO in both the active and the inhibited regions 2 commonly observed for CO oxidation on platinum metals [3,4]. The translational temperature decreased quickly when the kinetics were switched from the active region to the inhibited one. The velocity distribution was determined over Pt(133)=(s)3(111)×(111) in a constant flow of reactant gases by a cross-correlation time-of-flight ( TOF ) technique. * Corresponding author. Fax: (+81) 11 706 3695; e-mail: [email protected]
Modulated molecular beams have been used for velocity measurements of product CO at surface 2 temperatures above about 500 K because, below this value, the surface is eventually covered by CO and the reaction is retarded at the steady state [5]. Chopping of incoming beams may cause the oxygen and CO coverages to change through the conditions critical for kinetics [3,4]. Angleresolved thermal desorption (ARTDS ) combined with the TOF technique is useful for dynamic studies at low temperatures [1]. However, neither temperature dependence nor dynamics at the steady state has been studied. A few reports are found on the surface temperature dependence of the velocity of product CO . Poehlmann, Schmitt, Hoinkes and Wilsch 2 (PSHW ) reported that the total kinetic energy was described as the sum of an excess energy E ex and 2kT , where k is the Boltzmann constant and s
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Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
T the surface temperature [6 ]. Sibener’s group s argued that the energy increased with a slope of 8.7k over Rh(111) [7]. For reaction conditions reported to date, it is uncertain whether the kinetics was in the active or the inhibited region. It is well known that the amounts of the surface species change drastically in the boundary region, and the velocity is sensitive to the reactant coverages [1,3,4].
2. Experimental The apparatus consisted of three chambers, i.e. a reaction, a chopper and an analyzer chamber. The principle behind the apparatus is shown in Fig. 1. The first chamber had LEED and XPS optics, an Ar+ gun, a quadrupole mass spectrometer and a gas-handling system. A turbo-molecular pump of 1500 l/s pumped it out. The second was pumped out with a turbo pump and a cryo-panel below 40 K (with a pumping rate of about 7000 l/s). The chopper disk had slots of equal width (1 mm×6 mm) distributed in a pseudorandom sequence [8]. The time resolution of 20 ms was obtained at a rotation rate of 98.04 Hz. The trigger position was determined from curve fitting to a Maxwellian distribution of an effusive Ar
Fig. 1. Principle of the apparatus for velocity measurements at the steady state. S, sample crystal; RL, reverse view LEED; S1, S2, slit; XPS, electron energy analyzer; IG, ion gun; QM 1 and 2, quadrupole mass spectrometer; C, random chopper; M, motor; PC, photocell for trigger; CP, cryogenic plate; I, ionizer.
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beam at room temperature. The arrival times at the ionizer of a second mass spectrometer in the third chamber were registered on a multi-channel scaler running synchronously with the chopper blade. The flight path between the chopper blade and the ionizer measured 377 mm. The fight time was corrected by using an ion drift time of 38 ms determined in separate experiments. the first slit (S1) is in a rectangular shape (1×6 mm2) and the second (S2) in a circle of diameter 4 mm. The pressure in the analyzer was kept below 1×10−9 Torr even when the reaction chamber was filled to 1.5×10−4 Torr. A platinum (133) crystal (supplied by MaTeck, Germany) was rotated to change the desorption angle (polar angle, h). It was cleaned by the standard procedure and flashed in vacuo up to 1200 K. The LEED pattern showed a sharp (1×1) structure at this stage.
3. Results Two regions, an active and an inhibited one, characterize the kinetics of CO oxidation at the steady state. The angle-resolved CO production 2 rate monitored at h=0° (the normal direction) is shown in Fig. 2a as a function of CO pressure (P ) at a fixed O pressure and two different CO 2 surface temperatures. DCO is the CO mass spec2 2 trometric signal after subtraction of the background. The rate was negligible below 450 K and increased steeply with increasing temperature until it decreased again at higher temperatures. At a fixed surface temperature, the CO pressure dependence was characterized by a sharp transition at a certain CO pressure [3,4]. Below this pressure, the reaction was first-order in CO and independent of the surface temperature, whereas above this pressure it was negative order with respect to CO and sensitive to the surface temperature. The former is named the ‘‘active region’’ and the latter the ‘‘inhibited region’’. The critical CO pressure increased with increasing O pressure and also 2 with the surface temperature. The reaction rate is limited by CO adsorption below the critical pressure, whereas the rate is controlled by dissociative adsorption of oxygen above it [3]. In the inhibited
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Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
The CO desorption in the normal direction was 2 analyzed in TOF measurements. Typical velocity distributions are shown in Fig. 3. The distribution was fitted to a modified Maxwellian form (the resultant speed ratio is less than 1.1) drawn by the solid curves. It largely shifts from a Maxwellian form at the surface temperature drawn by the broken lines. The mean translational energy, E, is shown in a temperature unit as T = E/2k.
E This value decreased quickly with increasing CO pressure near the critical value (in the boundary between the active and inhibited regions), as plotted in Fig. 2b. The translational energy depended on the surface temperature. The results in the active region are shown in Fig. 4. The energy increased with a slope of 2k as E= E +2kT . The value of ex s
Fig. 2. (a) Variation of CO production rate with CO pressures 2 at different surface temperatures and a fixed O pressure. (b) 2 Translational temperature of CO in the transition region. The 2 vertical broken lines indicate the transition in kinetics.
region, adsorbed CO prevents oxygen from adsorbing, yielding negative reaction orders. In this region with 1×10−4 Torr O , the surface 2 absorbed oxygen significantly in an inactive form and was slowly deactivated. The activity could be recovered by heating above 1200 K to remove inactive oxygen. This oxygen was not removed by CO exposure. Angular distributions of desorbing CO were 2 different in the two regions and also depended on the pressure range. In the active region at 1×10−4 Torr O , the CO desorption distribution 2 2 showed the form cos4 h when the angle was varied in a plane perpendicular to the step edge. It broadened to cos2 h in the inhibited region. On the other hand, the desorption was collimated at 10° off the surface normal in the step-up direction below 5×10−6 Torr. In fact, the (111) terrace on the present surface declines 22.0° from the bulk surface plane.
Fig. 3. Typical velocity distributions of desorbing CO at 650 K 2 in (a) active, (b) boundary, and (c) inhibited region. The mean translational energy in the temperature unit was determined by curve fitting to modified Maxwellian forms shown by the solid curve. The broken curves show a Maxwellian form at the surface temperature.
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Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
Fig. 4. Variation of the translational energy with surface temperatures in the active region.
E was estimated to be 0.24 eV, less than the ex value of 0.40 eV on Pt(111) reported by PSHW [6 ]. The energy accommodation may proceed more rapidly on an open surface. The translational energy showed similar temperature dependence in the inhibited region.
4. Discussion The translational energy of desorbing CO 2 showed a steep decrease in the boundary region. The observed variation with P does not seem to CO be simply explained by the coverage change. The CO coverage increases steeply to an equilibrium level with increasing CO pressure above the critical value. On the other hand, the oxygen coverage is suppressed rather slowly with increasing CO pressure below the critical value and decreased to negligible amounts above it [3,4].
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At first sight, the decrease in the translational energy with CO pressure may be correlated with decreasing oxygen coverage. However, the effect cannot be responsible for a further decrease of T above the critical CO pressure. Therefore, we
E conclude that the local structure of the reaction site may play an important role in affecting the translational energy. Different chemical circumstances may be established around reaction sites in each region. The oxidation of CO on platinum metals is likely to take place on oxygen adsorption sites because of extremely high mobility of CO [9]. In the active region, the site is surrounded by oxygen adatoms located deeply in the surface [10]. The activated state of CO formation may be in close proximity 2 to these oxygen sites, receiving a strong repulsive force. On the other hand, the activated state may be far from the metal plane when CO sitting on platinum atoms surrounds the reaction site. This difference in the repulsion may result in different translational energy. Haller and coworkers reported that the internal energy of desorbing CO over polycrystalline platinum increased more 2 than the increase in the surface temperature, and predicted the reduction of the translational energy at higher temperature [11,12]. This is not consistent with the present observation. A large amount of the energy is released when CO is formed. The potential energy of the acti2 vated state of CO formation on Pt(111) was 2 estimated to be about 30 kcal/mol above the vacuum level [13]. This energy can be delivered into the translational and internal modes of the product CO molecule, and also into surface 2 modes. In fact, leaving CO holds about 50% of 2 this transferable energy, while the internal mode receives about 25% of the energy [11,12]. This energy disposal may change with the surface temperature because of less accommodation at higher temperature, and also with circumstances surrounding sites.
Acknowledgements This work was partly supported by a COE special equipment program in 1996 of the Ministry
SU RF A LE CE TT S ER CIE S N CE
L992
Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
of Education, Science, Sports and Culture, and also Grant-in-Aid No. 06403012 for General Scientific Research from the above Ministry.
References [1] T. Matsushima, Heterogen. Chem. Rev. 2 (1995) 51. [2] H. Uetsuka, K. Watanabe, K. Kunimori, Chem. Lett. (1995) 633. [3] T. Matsushima, D.B. Almy, J.M. White, Surf. Sci. 67 (1977) 89. [4] M. Ehsasi, S. Rezaire-Serej, J.H. Block, K. Christmann, J. Chem. Phys. 92 (1990) 7596.
[5] C.A. Becker, J.P. Cowin, L. Wharton, D.J. Auerbach, J. Chem. Phys. 69 (1977) 3394. [6 ] E. Poehlmann, M. Schmitt, H. Hoinkes, H. Wilsch, Surf. Sci. 287/288 (1993) 269. [7] J.I. Colonell, K.D. Gibson, S.J. Sibener, J. Chem. Phys. 103 (1995) 6677. [8] G. Comsa, R. David, B.J. Schumacher, Rev. Sci. Instrum. 52 (1981) 789. [9] J.E. Reutt-Robey, D.J. Doren, Y.J. Chabal, S.B. Christman, Phys. Rev. Lett. 61 (1998) 2778. [10] K. Mortensen, C. Klink, F. Jensen, F. Besenbacher, I. Stensgaard, Surf. Sci. 220 (1989) L701. [11] D.A. Mantell, K. Kunimori, S.B. Ryali, G.L. Haller, J.B. Fenn, Surf. Sci. 172 (1982) 281. [12] D.A. Mantell, S.B. Ryali, G.L. Haller, J.B. Fenn, J. Chem. Phys. 78 (1983) 4250. [13] C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, J. Chem. Phys. 73 (1980) 5862.
Surface Science Letters
Translational energy of desorbing CO in steady state CO 2 oxidation on a stepped platinum (133) surface Y. Seimiya a, G. Cao a, Y. Ohno a, T. Yamanaka a, T. Matsushima a,*, K. Jacobi b a Catalysis Research Center, Hokkaido University, Sapporo 060, Japan b Fritz-Haber Institute of Max-Planck Society, Faradayweg, Berlin, D-14195, Germany Received 1 April 1998; accepted for publication 7 July 1998
Abstract Velocity distributions of desorbing product CO in CO oxidation on Pt(133) were analyzed in both the active and inhibited 2 regions at the steady state. The translation temperature is high in the active region where the reaction is in the first order with respect to CO. It decreases sharply with increasing CO pressure when the reaction shifts to the inhibited region where CO retards the reaction. The translational energy of CO increases linearly with the surface temperature. © 1998 Elsevier Science B.V. All 2 rights reserved. Keywords: Carbon dioxide; Carbon monoxide; Molecule–surface interaction; Oxidation; Platinum; Stepped single crystal surfaces
1. Introduction The velocity of desorbing product provides dynamic information on surface processes. Combined with internal energy measurements, its determination will yield the partitioning of energy in the reactive desorption [1,2]. This paper reports, for the first time, the velocity of desorbing product CO in both the active and the inhibited regions 2 commonly observed for CO oxidation on platinum metals [3,4]. The translational temperature decreased quickly when the kinetics were switched from the active region to the inhibited one. The velocity distribution was determined over Pt(133)=(s)3(111)×(111) in a constant flow of reactant gases by a cross-correlation time-of-flight ( TOF ) technique. * Corresponding author. Fax: (+81) 11 706 3695; e-mail: [email protected]
Modulated molecular beams have been used for velocity measurements of product CO at surface 2 temperatures above about 500 K because, below this value, the surface is eventually covered by CO and the reaction is retarded at the steady state [5]. Chopping of incoming beams may cause the oxygen and CO coverages to change through the conditions critical for kinetics [3,4]. Angleresolved thermal desorption (ARTDS ) combined with the TOF technique is useful for dynamic studies at low temperatures [1]. However, neither temperature dependence nor dynamics at the steady state has been studied. A few reports are found on the surface temperature dependence of the velocity of product CO . Poehlmann, Schmitt, Hoinkes and Wilsch 2 (PSHW ) reported that the total kinetic energy was described as the sum of an excess energy E ex and 2kT , where k is the Boltzmann constant and s
0039-6028/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 03 9 - 60 2 8 ( 9 8 ) 0 0 5 4 8- 2
CE N IE SC RS CE E A TT RF LE SU
Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
T the surface temperature [6 ]. Sibener’s group s argued that the energy increased with a slope of 8.7k over Rh(111) [7]. For reaction conditions reported to date, it is uncertain whether the kinetics was in the active or the inhibited region. It is well known that the amounts of the surface species change drastically in the boundary region, and the velocity is sensitive to the reactant coverages [1,3,4].
2. Experimental The apparatus consisted of three chambers, i.e. a reaction, a chopper and an analyzer chamber. The principle behind the apparatus is shown in Fig. 1. The first chamber had LEED and XPS optics, an Ar+ gun, a quadrupole mass spectrometer and a gas-handling system. A turbo-molecular pump of 1500 l/s pumped it out. The second was pumped out with a turbo pump and a cryo-panel below 40 K (with a pumping rate of about 7000 l/s). The chopper disk had slots of equal width (1 mm×6 mm) distributed in a pseudorandom sequence [8]. The time resolution of 20 ms was obtained at a rotation rate of 98.04 Hz. The trigger position was determined from curve fitting to a Maxwellian distribution of an effusive Ar
Fig. 1. Principle of the apparatus for velocity measurements at the steady state. S, sample crystal; RL, reverse view LEED; S1, S2, slit; XPS, electron energy analyzer; IG, ion gun; QM 1 and 2, quadrupole mass spectrometer; C, random chopper; M, motor; PC, photocell for trigger; CP, cryogenic plate; I, ionizer.
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beam at room temperature. The arrival times at the ionizer of a second mass spectrometer in the third chamber were registered on a multi-channel scaler running synchronously with the chopper blade. The flight path between the chopper blade and the ionizer measured 377 mm. The fight time was corrected by using an ion drift time of 38 ms determined in separate experiments. the first slit (S1) is in a rectangular shape (1×6 mm2) and the second (S2) in a circle of diameter 4 mm. The pressure in the analyzer was kept below 1×10−9 Torr even when the reaction chamber was filled to 1.5×10−4 Torr. A platinum (133) crystal (supplied by MaTeck, Germany) was rotated to change the desorption angle (polar angle, h). It was cleaned by the standard procedure and flashed in vacuo up to 1200 K. The LEED pattern showed a sharp (1×1) structure at this stage.
3. Results Two regions, an active and an inhibited one, characterize the kinetics of CO oxidation at the steady state. The angle-resolved CO production 2 rate monitored at h=0° (the normal direction) is shown in Fig. 2a as a function of CO pressure (P ) at a fixed O pressure and two different CO 2 surface temperatures. DCO is the CO mass spec2 2 trometric signal after subtraction of the background. The rate was negligible below 450 K and increased steeply with increasing temperature until it decreased again at higher temperatures. At a fixed surface temperature, the CO pressure dependence was characterized by a sharp transition at a certain CO pressure [3,4]. Below this pressure, the reaction was first-order in CO and independent of the surface temperature, whereas above this pressure it was negative order with respect to CO and sensitive to the surface temperature. The former is named the ‘‘active region’’ and the latter the ‘‘inhibited region’’. The critical CO pressure increased with increasing O pressure and also 2 with the surface temperature. The reaction rate is limited by CO adsorption below the critical pressure, whereas the rate is controlled by dissociative adsorption of oxygen above it [3]. In the inhibited
SU RF A LE CE TT S ER CIE S N CE
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Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
The CO desorption in the normal direction was 2 analyzed in TOF measurements. Typical velocity distributions are shown in Fig. 3. The distribution was fitted to a modified Maxwellian form (the resultant speed ratio is less than 1.1) drawn by the solid curves. It largely shifts from a Maxwellian form at the surface temperature drawn by the broken lines. The mean translational energy, E, is shown in a temperature unit as T = E/2k.
E This value decreased quickly with increasing CO pressure near the critical value (in the boundary between the active and inhibited regions), as plotted in Fig. 2b. The translational energy depended on the surface temperature. The results in the active region are shown in Fig. 4. The energy increased with a slope of 2k as E= E +2kT . The value of ex s
Fig. 2. (a) Variation of CO production rate with CO pressures 2 at different surface temperatures and a fixed O pressure. (b) 2 Translational temperature of CO in the transition region. The 2 vertical broken lines indicate the transition in kinetics.
region, adsorbed CO prevents oxygen from adsorbing, yielding negative reaction orders. In this region with 1×10−4 Torr O , the surface 2 absorbed oxygen significantly in an inactive form and was slowly deactivated. The activity could be recovered by heating above 1200 K to remove inactive oxygen. This oxygen was not removed by CO exposure. Angular distributions of desorbing CO were 2 different in the two regions and also depended on the pressure range. In the active region at 1×10−4 Torr O , the CO desorption distribution 2 2 showed the form cos4 h when the angle was varied in a plane perpendicular to the step edge. It broadened to cos2 h in the inhibited region. On the other hand, the desorption was collimated at 10° off the surface normal in the step-up direction below 5×10−6 Torr. In fact, the (111) terrace on the present surface declines 22.0° from the bulk surface plane.
Fig. 3. Typical velocity distributions of desorbing CO at 650 K 2 in (a) active, (b) boundary, and (c) inhibited region. The mean translational energy in the temperature unit was determined by curve fitting to modified Maxwellian forms shown by the solid curve. The broken curves show a Maxwellian form at the surface temperature.
CE N IE SC RS CE E A TT RF LE SU
Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
Fig. 4. Variation of the translational energy with surface temperatures in the active region.
E was estimated to be 0.24 eV, less than the ex value of 0.40 eV on Pt(111) reported by PSHW [6 ]. The energy accommodation may proceed more rapidly on an open surface. The translational energy showed similar temperature dependence in the inhibited region.
4. Discussion The translational energy of desorbing CO 2 showed a steep decrease in the boundary region. The observed variation with P does not seem to CO be simply explained by the coverage change. The CO coverage increases steeply to an equilibrium level with increasing CO pressure above the critical value. On the other hand, the oxygen coverage is suppressed rather slowly with increasing CO pressure below the critical value and decreased to negligible amounts above it [3,4].
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At first sight, the decrease in the translational energy with CO pressure may be correlated with decreasing oxygen coverage. However, the effect cannot be responsible for a further decrease of T above the critical CO pressure. Therefore, we
E conclude that the local structure of the reaction site may play an important role in affecting the translational energy. Different chemical circumstances may be established around reaction sites in each region. The oxidation of CO on platinum metals is likely to take place on oxygen adsorption sites because of extremely high mobility of CO [9]. In the active region, the site is surrounded by oxygen adatoms located deeply in the surface [10]. The activated state of CO formation may be in close proximity 2 to these oxygen sites, receiving a strong repulsive force. On the other hand, the activated state may be far from the metal plane when CO sitting on platinum atoms surrounds the reaction site. This difference in the repulsion may result in different translational energy. Haller and coworkers reported that the internal energy of desorbing CO over polycrystalline platinum increased more 2 than the increase in the surface temperature, and predicted the reduction of the translational energy at higher temperature [11,12]. This is not consistent with the present observation. A large amount of the energy is released when CO is formed. The potential energy of the acti2 vated state of CO formation on Pt(111) was 2 estimated to be about 30 kcal/mol above the vacuum level [13]. This energy can be delivered into the translational and internal modes of the product CO molecule, and also into surface 2 modes. In fact, leaving CO holds about 50% of 2 this transferable energy, while the internal mode receives about 25% of the energy [11,12]. This energy disposal may change with the surface temperature because of less accommodation at higher temperature, and also with circumstances surrounding sites.
Acknowledgements This work was partly supported by a COE special equipment program in 1996 of the Ministry
SU RF A LE CE TT S ER CIE S N CE
L992
Y. Seimiya et al. / Surface Science 415 (1998) L988–L992
of Education, Science, Sports and Culture, and also Grant-in-Aid No. 06403012 for General Scientific Research from the above Ministry.
References [1] T. Matsushima, Heterogen. Chem. Rev. 2 (1995) 51. [2] H. Uetsuka, K. Watanabe, K. Kunimori, Chem. Lett. (1995) 633. [3] T. Matsushima, D.B. Almy, J.M. White, Surf. Sci. 67 (1977) 89. [4] M. Ehsasi, S. Rezaire-Serej, J.H. Block, K. Christmann, J. Chem. Phys. 92 (1990) 7596.
[5] C.A. Becker, J.P. Cowin, L. Wharton, D.J. Auerbach, J. Chem. Phys. 69 (1977) 3394. [6 ] E. Poehlmann, M. Schmitt, H. Hoinkes, H. Wilsch, Surf. Sci. 287/288 (1993) 269. [7] J.I. Colonell, K.D. Gibson, S.J. Sibener, J. Chem. Phys. 103 (1995) 6677. [8] G. Comsa, R. David, B.J. Schumacher, Rev. Sci. Instrum. 52 (1981) 789. [9] J.E. Reutt-Robey, D.J. Doren, Y.J. Chabal, S.B. Christman, Phys. Rev. Lett. 61 (1998) 2778. [10] K. Mortensen, C. Klink, F. Jensen, F. Besenbacher, I. Stensgaard, Surf. Sci. 220 (1989) L701. [11] D.A. Mantell, K. Kunimori, S.B. Ryali, G.L. Haller, J.B. Fenn, Surf. Sci. 172 (1982) 281. [12] D.A. Mantell, S.B. Ryali, G.L. Haller, J.B. Fenn, J. Chem. Phys. 78 (1983) 4250. [13] C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, J. Chem. Phys. 73 (1980) 5862.