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Adsorption and decomposition of NO2 on Pt(100)
Surface Science 152/153 (1985) 5966602 North-H[~llalld. Amsterdam
596
ADSORPTION
AND DECOMPOSITION
Udo SCHWALKE,
Horst NIEHUS
OF NO2 ON Pt(100)
and George
COMSA
Institut ftir C;rerlzflichenforschung und Vukuumphysrk, K~rnforschungsanluR Jiilich GmbH. P.O. Bou 1913. D - 5170Jiilich, Fed. Rep. of Germany
Received 27 March 1984; accepted for publication 11 May 1984
Electron stimulated desorption (ESD) in combination with LEED, TPD and AES measurements has been used to investigate the adsorption and decomposition of NC& on Pt(lOO). The two modifications of the Pt(lOO) surface (unreconstructed (1 x 1) and reconstructed (5 x 20) phase) were found to play an important role in the adsorption and decomposition behavior of NO, on Pt(100). It is shown that the decomposition of molecularly adsorbed NOz on the reconstructed surface does not follow simple dissociation kinetics. On the basis of our results vve suggest an autocatalytic reaction in which the substrate phase transition (5 x 20) --f (1 X 1)accelerates the NO, decomposition.
The clean Pt(lOO) surface is known to expose two main phases. The unreconstructed (1 x 1) phase is metastable and transforms into a quasi-hexagonal structure upon heating above 400 K [l]. This superstructure is known as the (5 x 20) LEED structure (reconstructed surface) according to the size of the coincidence mesh. The two modifications differ in their reactivity [Z], the (1 x 1) form being the more reactive one. It has been shown that adsorption of NO [3] and CO [4], on the (5 X 20) Pt(lOO) surface removes the reconstruction. Thus such an adsorbate-induced rearrangement of the Pt(lOO) surface changes its reactivity, which in its part may feed-back to the adsorbate. Recently, Ertl et al. [S] have shown that the CO-induced phase transition is the driving force for the oscillatory behavior of the CO oxidation on Pt(lOO). The adsorption and dissociation of NO, on well characterized Pt surfaces is yet scarcely investigated. Two recently published studies of NO,/Pt(lll) [6,7] are in part controversial. Dahlgren and Hemminger [6] reported that NO, is molecularly adsorbed on Pt(ll1) at 120 K and starts to dissociate into NO,,,, and Ocad) at - 240 K. In contrast, Segner et al. [7] found no evidence for molecularly adsorbed NO, on Pt(l11) even at surface temperatures as low as 120 K. Both investigations agree that NOz decomposes into NO(,,, and Ocird). By means of ESD, LEED, TPD and AES we investigated the adsorption
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
U. Schafke
et al. / NO, on
Pt(loO)
597
and decomposition of NO2 on Pt(lOO). We have shown recently that NO(,,, and Ocad, can be straightforwardly discriminated due to their drastically different threshold energies for Of emission [8]. The threshold for 0” emission from O/Pt(lOO) is < 50 eV (valence band ESD process) and from NO/ Pt(lOO) - 565 eV (core-level ESD process). Neither Nt nor NO+ desorption is observed from NO/Pt(lOO). It should be noted that the core-level ESD signals were found to be to a good approximation proportional to the molecular coverage [8,9] and are not created by minority species with large ESD cross sections. It will be shown that the dissociation kinetics of NO, on Pt(100) is strongiy influenced by the surface phase transition (5 x 20) -+ (1 x 1) during decomposition. 2. Experimental The experiments were carried out in the UHV system, described previously in detail [83. In brief, the system is equipped with an CMA Auger electron spectrometer, a LEED/ESDIAD device, a quadrupole mass spectrometer, a gas dosing system and a goniometer mounted LEED/ESD detector. The quadrupole was used for TPD measurements, for analyzing ESD ions and for monitoring the gas composition during NO2 exposure. To obtain high purity NO, exposures, the gas dosing system had to be rinsed several times with NO, (NO2 purity 98%, Messer Griesheim) before starting the experiments. With the doser opening in front of the quadrupole the intensity ratio of masses 46/30 corresponds to the literature value 0.37 [lo]. No increase in mass 28 was observed. The NO, dose was evaluated by measuring the pressure in a calibrated volume of known size filled with NO,. The pressures in the volume were between lOA2 and 10e4 mbar and could be measured with high accuracy by means of a spinning rotor gas friction gauge [ll]. LEED intensity measurements were performed with the goniometer driven LEED/ESD detector consisting of a shielded channeltron in conjunction with a retarding field energy analyzer. Preparation procedure and characte~zation of the clean Pt(lOO) surface has already been described in ref. [8]. 3. Results and discussion 3.1. Adsorption of NO, on the reconstructed Pt(lO0) surface at 300 K versus adsorption at 200 K The reconstructed Pt(100) surface was exposed at both 300 and 200 K sample temperature to a NO, dose of 1.5 X IOr molecules/cm2. The behavior of the AES peaks at 390 and 518 eV corresponding to the N,, and OKvV
598
U. Schwalke
et al. / NO2 on Pt(lOO)
transitions during exposure, indicates similar sticking probabilities at the two temperatures. The ESD spectra as well as LEED patterns taken after adsorption at 300 and 200 K were radically different suggesting however a different adsorption behavior, i.e. dissociative and molecular adsorption, respectively. Upon adsorption at 300 K characteristic high threshold 0’. 02+ and low threshold O+ ESD signals were observed. They are indicative of the presence of NOcad) and Ocad) species, respectively [8]. Neither NO:, NO+ nor N + signals were detectable. As a result of the adsorption the reconstruction was lifted. a (1 x 1) LEED pattern being observed. Upon adsorption at 200 K no significant high threshold O+, O’+ ions nor low threshold O+ are observed in the ESD spectra [12] but only N + ion desorption (threshold energy - 460 eV) occurs. This indicates that neither NO, NO(,,, nor Ocadl sP ecies are present and suggests that the non-dissociated molecule is characterized by the emission of high threshold Nt ESD ions. The still present (5 x 20) LEED pattern clearly indicates that the reconstruction of the Pt(lOO) surface is not lifted by NO, adsorption at 200 K. The ESD results obtained upon NO, adsorption of 200 K show further no dissociation products (neither NO(,,, nor O(,,,) in contrast to NO, adsorption at 300 K. In fact the experimental finding that the reconstruction is not lifted upon NO, adsorption at 200 K confirms the absence of NO(,,,. This is because of the it has been measured that even at 200 K, NOcad) lifts the reconstruction Pt(lOO) surface and the characteristic high threshold O+, O*+ ESD signals can be observed [13]. Further support is obtained by electron induced dissociation of the NO, molecules at 200 K: electron bombardment results in the appearance of the O+ ESD signals characteristic of the presence of NO,,,, and Ocad) and of the (1 x 1) LEED pattern (lifting of the reconstruction). We thus conclude that the NO, adsorption on the reconstructed Pt(lOO) surface is molecular at 200 K and dissociative at 300 K. In addition, we measured that on the highly reactive non-reconstructed Pt(lOO) surface the NO, adsorption is almost completely dissociative even at 200 K. The lack of O+ ESD ions (only N+ ion emission) from molecularly adsorbed NO, seems at first sight surprising, in particular if we assume that the molecule is bonded via the N atom to the Pt surface. However, if we further assume that the O-N-O angle retains its gas phase value (- 134”) the O-bonding angle (N-O direction with respect to surface normal) probably surpasses the critical cut-off angle introduced by Clinton [14]. Accordingly, the trajectories of the O+ ions are so strongly bend towards the surface that no O+ ions are able to escape. It is of practical importance that molecularly adsorbed NO, is characterized by the exclusive emission of (high threshold energy) N+ ESD ions: we are now (N+ ions) from its dissociation products NO{,,, ~~~h’O,h~~~~~~+~~~~’ ions) and 0, (ad) (low threshold Ot ions). This correlation will be used in the following section.
U. Sch~alke et al. / NO, on Pt~~~~
599
3.2. Decomposition of NO, on Pt(l00)
In fig. 1 the measured data of ESD, LEED and thermal desorption versus temperature are shown. Prior to all shown experiments the surface was initially
29 ESD -------___
-IOK____ _. ‘\
/ (11 N’
\
\I / ,I II ,t
T
(21 o+
TEMPERATURE IKl Fig. 1. Temperature dependence of the ESD, LEED and TPD signals. The reconstructed Pt(lOO) surface was exposed to 1.5 x lOI NO, molecules/cm* at 200 K in all cases; the surface was subsequently heated with a rate of 4 K/s. All ordinates are in arbitrary units. (A(1)) High threshold N’ ESD signal characterizing NO (ad); (A(2)) High threshold O+ ESD signal characterizing NO(,); (B) Integrated intensity of the (20) LEED beam; (C(1)) TPD spectrum of mass 46 (NO,); (C(2)) TPD spectrum of mass 30 (NO).
600
U. SchwaIke
et d
/ NO,
on Pt(100)
dosed with 1.5 x 10” NO, molecules/cm2 at 200 K. Subsequently the temperature was raised linearly with a heating rate of - 4 K/s. The decomposition of NO, is studied by monitoring the high threshold N + ESD signal (characteristic of molecularly adsorbed NO,), fig. 1A. curve 1. and the high threshold Ot ESD signal (- concentration of molecular adsorbed NO(,,,), fig. lA, curve 2. The striking feature visible in both plots is the steep variation of the signals between 285 and 295 K. It demonstrates that within 10 K all NO, molecules present on the surface dissociate resulting in a steep increase of the NO,;,,, concentration. Simultaneously the reconstruction of the Pt(lOO) surface is abruptly lifted as it is obvious in fig. 1B. The integral intensity of the (20) LEED beam increases steeply in the same temperature interval marking the (5 x 20) -+ (1 X 1) phase transition. The rough explanation of the unusual dissociation behavior (within - 10 K) is now straightforward: with increasing temperature NO, starts to dissociate on the initially unreconstructed (5 X 20) surface resulting in NOcad) molecules and Oca,iJ. The newly created NO,,,, molecules lift the reconstruction more or less locally. hence creating reactive (1 X 1) domains which in turn lead to rapid dissociation of neighboring NO, molecules creating additional new NOcad) molecules, and so on. Let us now look in more detail to the evolution of this autocatalytic reaction. The O+ signal in fig. lA, curve 2, i.e. the NO,,,,, creation from dissociating NO, starts to increase slowly around 270 K. It is not before a certain critical NO,,,, concentration is reached, that the NOz dissociation process accelerates (becomes autocatalytic). We have recently observed that below a critical initial NO, concentration (leading to a subcritical NOcad) concentration) neither the surface phase transformation. nor the autocatalytic NO, dissociation occurs [15]. This is consistent with the recent finding of Thiel et al. [16] that the phase transition (5 x 20) + (1 x 1) is rapid only for CO coverages larger than 5%. The N+ signal in fig. 1A. curve 1. i.e. the concentration of NO, molecules. starts to decrease slowly already below 250 K. This is due to the thermal desorption of NO1 molecules which sets in just above 230 K. fig. lC, curve 1. As shown above around 270 K NO, starts also to dissociate, fig. 1A. curve 2. This enhances additionally the decrease of the N4 signal. Finally above 285 K the autocatalytic dissociation of NO, makes the N’ signal to vanish rapidly. The shape of the NO_, TPD curve in fig. lC, curve 1, is unusual. but consistent with the other observations. The left wing of the curve is normal, while the right wing decreases more rapidly than usual. This abrupt decrease of the NO, desorption rate is a consequence of the autocatalytic reaction resulting in the dramatic decrease of NOZcad) concentration. The location of the NO, peak demonstrates again that the steep decrease of the N+ signal in fig. 1A. curve 2, is not due to NO, desorption but to its dissociation. Finally. we discuss the NO TPD curve presented in fig. 1C. curve 2. It
U. Sckwalke et al. / NO, on
Pt~~#}
601
consists of a low temperature shoulder and an extremely narrow peak (FWHM - 5 K) centered at - 292 K. The shoulder represents an artifact: it is due to the cracking of the desorbed NO, molecules (fig. lC, curve 1) in the ionizer of the quadrupole. The location of the sharp peak is independent of the NO, coverage [15] suggesting first order desorption kinetics. In the presence of a surface phase transition it is of course impossible to have real first order kinetics. This is evident from a peak shape analysis (according to ref. [17]) which yields in an “activation energy” of 82 kcal/mol and the unrealistically high pre-exponent in excess of 106’ s-l. The coincidence of both, the NO TPD shape and location of the derivative of the integral of the NO+it creation rate (fig. lA, curve 2) suggests that NO desorption is reaction rate limited by NO, dissociation. However, the location of the peak (- 292 K) at a much lower temperature than the “normal” NO desorption temperature (Tr - 500 K) suggests in addition that the NO desorption is caused or at least influenced by the rearrangement of the Pt atoms during the phase transition. This in fact opens a promising way to study the dynamics of phase transformation.
4. Conclusions
It is demonstrated that the high threshold N” ESD signal can be used to monitor the molecularly adsorbed NO*. So it is possible to discriminate between Nq,,,, and the dissociation products NO,,,, and O(=+. By combining ESD measurements with other analytical methods we have found that NO, adsorbs molecular on the reconstructed Pt(lOO) surface at 200 K. At this temperature the reconstruction is not lifted upon NO, adsorption. The thermal dissociation of NO,(,,, into NO(,,, and Ocad, occurs within an extremely narrow temperature interval (< 10 K) around 295 K. Simultaneously a (5 x 20) -+ (1 x 1) surface phase transition occurs within the same narrow temperature interval. This transition leads to highly reactive domains which in turn accelerates the NO2 decomposition resulting in an autocatalytic reaction.
References [l] K. Heinz. E. Lang, K. Strauss and K. Miiller, Appl. Surface Sci. 11/12 (1982) 611.
[Z] H.P. Bonzel and G. Comsa, Le Vide 189 (1977) 130. [3] H.P. Bonzel and G. Pirug, Surface Sci. 62 (1977) 45. [4] C.. Helms, H.P. Bonzel and S. Kelemen, J. Chem. Phys. 65 (1976) 1773. [5] G. Ertl, P.R. Norton and J. Rustig, Phys. Rev. Letters 49 (1982) 177. [6] D. Dahlgren and J.C. Hemminger, Surface Sci. 123 (1982) L739. [7] J. Segner, W. Vielhaber and G. Ertl, Israel J. Chem. 22 (1982) 375. [S] U. SchwaIke, H. Niehus and G. Comsa, Surface Sci. 137 (1984) 23. [9] J.E. Houston and T.E. Madey. Phys. Rev. B26 (1981) 554.
602
U. Schwdke
et al. / NO_, on Pt(lOO)
[lo] A. Cornu and R. Massot, Compilation of Mass Spectra Data (Heyden, London, 1975). [ll] G. Comsa, J.K. Fremerey, B. Lindenau. G. Messer and P. Rohl. J. Vacuum Sci. Technol. 17 (1980) 642. [12] Very weak O+ ESD signals are actually observed (around 1% of the signals observed upon adsorption at 300 K). They are probably due to a residual NO, decomposition in the dosing system and/or to a possible dissociative NO, adsorption at defect sites. [13] U. Schwalke, PhD Thesis, to be published. [14] W.L. Clinton, Surface Sci. 112 (1981) L791. [15] U. Schwalke, H. Niehus and G. Comsa, to be published. [16] P.A. Thiel, R.J. Behm, P.R. Norton and G. Ertl. J. Chem. Phys. 78 (1983) 744X. [17] D. Edwards, Surface Sci. 54 (1976) 1.
596
ADSORPTION
AND DECOMPOSITION
Udo SCHWALKE,
Horst NIEHUS
OF NO2 ON Pt(100)
and George
COMSA
Institut ftir C;rerlzflichenforschung und Vukuumphysrk, K~rnforschungsanluR Jiilich GmbH. P.O. Bou 1913. D - 5170Jiilich, Fed. Rep. of Germany
Received 27 March 1984; accepted for publication 11 May 1984
Electron stimulated desorption (ESD) in combination with LEED, TPD and AES measurements has been used to investigate the adsorption and decomposition of NC& on Pt(lOO). The two modifications of the Pt(lOO) surface (unreconstructed (1 x 1) and reconstructed (5 x 20) phase) were found to play an important role in the adsorption and decomposition behavior of NO, on Pt(100). It is shown that the decomposition of molecularly adsorbed NOz on the reconstructed surface does not follow simple dissociation kinetics. On the basis of our results vve suggest an autocatalytic reaction in which the substrate phase transition (5 x 20) --f (1 X 1)accelerates the NO, decomposition.
The clean Pt(lOO) surface is known to expose two main phases. The unreconstructed (1 x 1) phase is metastable and transforms into a quasi-hexagonal structure upon heating above 400 K [l]. This superstructure is known as the (5 x 20) LEED structure (reconstructed surface) according to the size of the coincidence mesh. The two modifications differ in their reactivity [Z], the (1 x 1) form being the more reactive one. It has been shown that adsorption of NO [3] and CO [4], on the (5 X 20) Pt(lOO) surface removes the reconstruction. Thus such an adsorbate-induced rearrangement of the Pt(lOO) surface changes its reactivity, which in its part may feed-back to the adsorbate. Recently, Ertl et al. [S] have shown that the CO-induced phase transition is the driving force for the oscillatory behavior of the CO oxidation on Pt(lOO). The adsorption and dissociation of NO, on well characterized Pt surfaces is yet scarcely investigated. Two recently published studies of NO,/Pt(lll) [6,7] are in part controversial. Dahlgren and Hemminger [6] reported that NO, is molecularly adsorbed on Pt(ll1) at 120 K and starts to dissociate into NO,,,, and Ocad) at - 240 K. In contrast, Segner et al. [7] found no evidence for molecularly adsorbed NO, on Pt(l11) even at surface temperatures as low as 120 K. Both investigations agree that NOz decomposes into NO(,,, and Ocird). By means of ESD, LEED, TPD and AES we investigated the adsorption
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
U. Schafke
et al. / NO, on
Pt(loO)
597
and decomposition of NO2 on Pt(lOO). We have shown recently that NO(,,, and Ocad, can be straightforwardly discriminated due to their drastically different threshold energies for Of emission [8]. The threshold for 0” emission from O/Pt(lOO) is < 50 eV (valence band ESD process) and from NO/ Pt(lOO) - 565 eV (core-level ESD process). Neither Nt nor NO+ desorption is observed from NO/Pt(lOO). It should be noted that the core-level ESD signals were found to be to a good approximation proportional to the molecular coverage [8,9] and are not created by minority species with large ESD cross sections. It will be shown that the dissociation kinetics of NO, on Pt(100) is strongiy influenced by the surface phase transition (5 x 20) -+ (1 x 1) during decomposition. 2. Experimental The experiments were carried out in the UHV system, described previously in detail [83. In brief, the system is equipped with an CMA Auger electron spectrometer, a LEED/ESDIAD device, a quadrupole mass spectrometer, a gas dosing system and a goniometer mounted LEED/ESD detector. The quadrupole was used for TPD measurements, for analyzing ESD ions and for monitoring the gas composition during NO2 exposure. To obtain high purity NO, exposures, the gas dosing system had to be rinsed several times with NO, (NO2 purity 98%, Messer Griesheim) before starting the experiments. With the doser opening in front of the quadrupole the intensity ratio of masses 46/30 corresponds to the literature value 0.37 [lo]. No increase in mass 28 was observed. The NO, dose was evaluated by measuring the pressure in a calibrated volume of known size filled with NO,. The pressures in the volume were between lOA2 and 10e4 mbar and could be measured with high accuracy by means of a spinning rotor gas friction gauge [ll]. LEED intensity measurements were performed with the goniometer driven LEED/ESD detector consisting of a shielded channeltron in conjunction with a retarding field energy analyzer. Preparation procedure and characte~zation of the clean Pt(lOO) surface has already been described in ref. [8]. 3. Results and discussion 3.1. Adsorption of NO, on the reconstructed Pt(lO0) surface at 300 K versus adsorption at 200 K The reconstructed Pt(100) surface was exposed at both 300 and 200 K sample temperature to a NO, dose of 1.5 X IOr molecules/cm2. The behavior of the AES peaks at 390 and 518 eV corresponding to the N,, and OKvV
598
U. Schwalke
et al. / NO2 on Pt(lOO)
transitions during exposure, indicates similar sticking probabilities at the two temperatures. The ESD spectra as well as LEED patterns taken after adsorption at 300 and 200 K were radically different suggesting however a different adsorption behavior, i.e. dissociative and molecular adsorption, respectively. Upon adsorption at 300 K characteristic high threshold 0’. 02+ and low threshold O+ ESD signals were observed. They are indicative of the presence of NOcad) and Ocad) species, respectively [8]. Neither NO:, NO+ nor N + signals were detectable. As a result of the adsorption the reconstruction was lifted. a (1 x 1) LEED pattern being observed. Upon adsorption at 200 K no significant high threshold O+, O’+ ions nor low threshold O+ are observed in the ESD spectra [12] but only N + ion desorption (threshold energy - 460 eV) occurs. This indicates that neither NO, NO(,,, nor Ocadl sP ecies are present and suggests that the non-dissociated molecule is characterized by the emission of high threshold Nt ESD ions. The still present (5 x 20) LEED pattern clearly indicates that the reconstruction of the Pt(lOO) surface is not lifted by NO, adsorption at 200 K. The ESD results obtained upon NO, adsorption of 200 K show further no dissociation products (neither NO(,,, nor O(,,,) in contrast to NO, adsorption at 300 K. In fact the experimental finding that the reconstruction is not lifted upon NO, adsorption at 200 K confirms the absence of NO(,,,. This is because of the it has been measured that even at 200 K, NOcad) lifts the reconstruction Pt(lOO) surface and the characteristic high threshold O+, O*+ ESD signals can be observed [13]. Further support is obtained by electron induced dissociation of the NO, molecules at 200 K: electron bombardment results in the appearance of the O+ ESD signals characteristic of the presence of NO,,,, and Ocad) and of the (1 x 1) LEED pattern (lifting of the reconstruction). We thus conclude that the NO, adsorption on the reconstructed Pt(lOO) surface is molecular at 200 K and dissociative at 300 K. In addition, we measured that on the highly reactive non-reconstructed Pt(lOO) surface the NO, adsorption is almost completely dissociative even at 200 K. The lack of O+ ESD ions (only N+ ion emission) from molecularly adsorbed NO, seems at first sight surprising, in particular if we assume that the molecule is bonded via the N atom to the Pt surface. However, if we further assume that the O-N-O angle retains its gas phase value (- 134”) the O-bonding angle (N-O direction with respect to surface normal) probably surpasses the critical cut-off angle introduced by Clinton [14]. Accordingly, the trajectories of the O+ ions are so strongly bend towards the surface that no O+ ions are able to escape. It is of practical importance that molecularly adsorbed NO, is characterized by the exclusive emission of (high threshold energy) N+ ESD ions: we are now (N+ ions) from its dissociation products NO{,,, ~~~h’O,h~~~~~~+~~~~’ ions) and 0, (ad) (low threshold Ot ions). This correlation will be used in the following section.
U. Sch~alke et al. / NO, on Pt~~~~
599
3.2. Decomposition of NO, on Pt(l00)
In fig. 1 the measured data of ESD, LEED and thermal desorption versus temperature are shown. Prior to all shown experiments the surface was initially
29 ESD -------___
-IOK____ _. ‘\
/ (11 N’
\
\I / ,I II ,t
T
(21 o+
TEMPERATURE IKl Fig. 1. Temperature dependence of the ESD, LEED and TPD signals. The reconstructed Pt(lOO) surface was exposed to 1.5 x lOI NO, molecules/cm* at 200 K in all cases; the surface was subsequently heated with a rate of 4 K/s. All ordinates are in arbitrary units. (A(1)) High threshold N’ ESD signal characterizing NO (ad); (A(2)) High threshold O+ ESD signal characterizing NO(,); (B) Integrated intensity of the (20) LEED beam; (C(1)) TPD spectrum of mass 46 (NO,); (C(2)) TPD spectrum of mass 30 (NO).
600
U. SchwaIke
et d
/ NO,
on Pt(100)
dosed with 1.5 x 10” NO, molecules/cm2 at 200 K. Subsequently the temperature was raised linearly with a heating rate of - 4 K/s. The decomposition of NO, is studied by monitoring the high threshold N + ESD signal (characteristic of molecularly adsorbed NO,), fig. 1A. curve 1. and the high threshold Ot ESD signal (- concentration of molecular adsorbed NO(,,,), fig. lA, curve 2. The striking feature visible in both plots is the steep variation of the signals between 285 and 295 K. It demonstrates that within 10 K all NO, molecules present on the surface dissociate resulting in a steep increase of the NO,;,,, concentration. Simultaneously the reconstruction of the Pt(lOO) surface is abruptly lifted as it is obvious in fig. 1B. The integral intensity of the (20) LEED beam increases steeply in the same temperature interval marking the (5 x 20) -+ (1 X 1) phase transition. The rough explanation of the unusual dissociation behavior (within - 10 K) is now straightforward: with increasing temperature NO, starts to dissociate on the initially unreconstructed (5 X 20) surface resulting in NOcad) molecules and Oca,iJ. The newly created NO,,,, molecules lift the reconstruction more or less locally. hence creating reactive (1 X 1) domains which in turn lead to rapid dissociation of neighboring NO, molecules creating additional new NOcad) molecules, and so on. Let us now look in more detail to the evolution of this autocatalytic reaction. The O+ signal in fig. lA, curve 2, i.e. the NO,,,,, creation from dissociating NO, starts to increase slowly around 270 K. It is not before a certain critical NO,,,, concentration is reached, that the NOz dissociation process accelerates (becomes autocatalytic). We have recently observed that below a critical initial NO, concentration (leading to a subcritical NOcad) concentration) neither the surface phase transformation. nor the autocatalytic NO, dissociation occurs [15]. This is consistent with the recent finding of Thiel et al. [16] that the phase transition (5 x 20) + (1 x 1) is rapid only for CO coverages larger than 5%. The N+ signal in fig. 1A. curve 1. i.e. the concentration of NO, molecules. starts to decrease slowly already below 250 K. This is due to the thermal desorption of NO1 molecules which sets in just above 230 K. fig. lC, curve 1. As shown above around 270 K NO, starts also to dissociate, fig. 1A. curve 2. This enhances additionally the decrease of the N4 signal. Finally above 285 K the autocatalytic dissociation of NO, makes the N’ signal to vanish rapidly. The shape of the NO_, TPD curve in fig. lC, curve 1, is unusual. but consistent with the other observations. The left wing of the curve is normal, while the right wing decreases more rapidly than usual. This abrupt decrease of the NO, desorption rate is a consequence of the autocatalytic reaction resulting in the dramatic decrease of NOZcad) concentration. The location of the NO, peak demonstrates again that the steep decrease of the N+ signal in fig. 1A. curve 2, is not due to NO, desorption but to its dissociation. Finally. we discuss the NO TPD curve presented in fig. 1C. curve 2. It
U. Sckwalke et al. / NO, on
Pt~~#}
601
consists of a low temperature shoulder and an extremely narrow peak (FWHM - 5 K) centered at - 292 K. The shoulder represents an artifact: it is due to the cracking of the desorbed NO, molecules (fig. lC, curve 1) in the ionizer of the quadrupole. The location of the sharp peak is independent of the NO, coverage [15] suggesting first order desorption kinetics. In the presence of a surface phase transition it is of course impossible to have real first order kinetics. This is evident from a peak shape analysis (according to ref. [17]) which yields in an “activation energy” of 82 kcal/mol and the unrealistically high pre-exponent in excess of 106’ s-l. The coincidence of both, the NO TPD shape and location of the derivative of the integral of the NO+it creation rate (fig. lA, curve 2) suggests that NO desorption is reaction rate limited by NO, dissociation. However, the location of the peak (- 292 K) at a much lower temperature than the “normal” NO desorption temperature (Tr - 500 K) suggests in addition that the NO desorption is caused or at least influenced by the rearrangement of the Pt atoms during the phase transition. This in fact opens a promising way to study the dynamics of phase transformation.
4. Conclusions
It is demonstrated that the high threshold N” ESD signal can be used to monitor the molecularly adsorbed NO*. So it is possible to discriminate between Nq,,,, and the dissociation products NO,,,, and O(=+. By combining ESD measurements with other analytical methods we have found that NO, adsorbs molecular on the reconstructed Pt(lOO) surface at 200 K. At this temperature the reconstruction is not lifted upon NO, adsorption. The thermal dissociation of NO,(,,, into NO(,,, and Ocad, occurs within an extremely narrow temperature interval (< 10 K) around 295 K. Simultaneously a (5 x 20) -+ (1 x 1) surface phase transition occurs within the same narrow temperature interval. This transition leads to highly reactive domains which in turn accelerates the NO2 decomposition resulting in an autocatalytic reaction.
References [l] K. Heinz. E. Lang, K. Strauss and K. Miiller, Appl. Surface Sci. 11/12 (1982) 611.
[Z] H.P. Bonzel and G. Comsa, Le Vide 189 (1977) 130. [3] H.P. Bonzel and G. Pirug, Surface Sci. 62 (1977) 45. [4] C.. Helms, H.P. Bonzel and S. Kelemen, J. Chem. Phys. 65 (1976) 1773. [5] G. Ertl, P.R. Norton and J. Rustig, Phys. Rev. Letters 49 (1982) 177. [6] D. Dahlgren and J.C. Hemminger, Surface Sci. 123 (1982) L739. [7] J. Segner, W. Vielhaber and G. Ertl, Israel J. Chem. 22 (1982) 375. [S] U. SchwaIke, H. Niehus and G. Comsa, Surface Sci. 137 (1984) 23. [9] J.E. Houston and T.E. Madey. Phys. Rev. B26 (1981) 554.
602
U. Schwdke
et al. / NO_, on Pt(lOO)
[lo] A. Cornu and R. Massot, Compilation of Mass Spectra Data (Heyden, London, 1975). [ll] G. Comsa, J.K. Fremerey, B. Lindenau. G. Messer and P. Rohl. J. Vacuum Sci. Technol. 17 (1980) 642. [12] Very weak O+ ESD signals are actually observed (around 1% of the signals observed upon adsorption at 300 K). They are probably due to a residual NO, decomposition in the dosing system and/or to a possible dissociative NO, adsorption at defect sites. [13] U. Schwalke, PhD Thesis, to be published. [14] W.L. Clinton, Surface Sci. 112 (1981) L791. [15] U. Schwalke, H. Niehus and G. Comsa, to be published. [16] P.A. Thiel, R.J. Behm, P.R. Norton and G. Ertl. J. Chem. Phys. 78 (1983) 744X. [17] D. Edwards, Surface Sci. 54 (1976) 1.